Compumotor Step Motor & Servo Motor Systems and Controls

1996/1997 

Compumotor 

Step Motor 

& Servo Motor 

Systems and Controls

STEPMOTOR& 

SERVOMOTOR 

SYSTEMSAND 

CONTROLSSTE 

PMOTOR&SERV 

OMOTORSYST 

EMSANDCON 

TROLSSTEPM 

OTOR&SERVOM 

OTORSYSTEM 

SANDCONTRO 

COMPUMOTOR 

THE COMPLETE SOURCE FOR 

PROGRAMMABLE MOTION CONTROL 

A Broad Range of Capabilities 

If you have a motion control problem, 

turn to Compumotor. We offer 

a comprehensive line-up of product 

technologies, plus we provide the 

technical support to ensure timely, 

expert assistance in product selection, 

installation, programming, 

training and troubleshooting. From 

hands-on technical seminars to 

local Automation Technology Centers 

to factory-trained field application 

engineers—solutions to your 

motion control problems are only a 

phone call away. 

A Basis to Objectively Recommend 

the Best Solution 

A full spectrum of motion control 

technologies allows us to objectively 

recommend the best solution 

to your specific problem. We offer: 

• Stepper motor and servo systems 

• Half/full step and microstepping 

• Digital and analog drives 

• Open loop and closed loop systems 

• Brushed and brushless motor/drives 

• Position, velocity and torque control 

servos 

• Stand alone and peripheral controls 

• Absolute and incremental feedback 

• Encoder and resolver feedback 

servos 

• Direct drive rotary motors and motors 

attached to gearboxes, tables, etc. 

• Linear motors and motors mounted 

to leadscrews, belt drives, etc. 

So, if you’re looking for the final 

word on motion control, turn to the 

company that has all the possibilities— 

Compumotor.

P 

A POWERFUL LINE-UP OF PROGRAMMABLE CONTROLS 

Engineering Reference 

and Application 

Solutions 

Step Motor Systems 

Compumotor provides 

thorough technical data and 

support for every product. In 

Section A, you’ll find motor 

and drive technology 

definitions, how-to application 

information, formulas, and 

application examples. A 

special Motor Sizing and 

Selection Software disk 

compliments this catalog and 

is available to help you 

determine the optimum motor 

for your application. And if you 

have any other questions 

about products or services, 

call your local, factory-trained 

applications engineer or our 

Applications Engineering 

department at our toll-free 

factory line: 1-800-358-9070. 

Servo Systems 

Compumotor’s servo systems 

offer power and diversity in a 

multitude of form factors. 

Compumotor offers a wide 

range of digital and analog 

servo drives for all your motion 

needs, as well as convenient 

packaged servo systems. 

Compumotor’s powerful servo 

controllers operate standalone 

or in an AT-bus structure 

and are easily interfaced to 

PLCs, PCs or other factory 

equipment. 

B1-B146 

Compumotor pioneered 

microstepping techniques to 

electronically improve the 

smoothness and resolution of 

step motors. The leadership 

continues with a complete 

range of products, from full 

step and microstepping 

systems to packaged singleand 

multi-axis drive/indexer 

systems and powerful singleand 

multi-axis indexers. 

C1-C160 

An Introduction to 


Page 

Worldwide Support .......... 2-3 

Technical Support Team .. 4-5 

Seminars ......................... 6-7 

Support Software ............ 8-9 

Servo vs. Stepper 

Motor Selection ........... 10-11 

Servo Systems ............ 12-13 

Stepper Systems ......... 14-15 

Custom Products .............. 16 

A1-A96 

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1

INNOVATIVE SOLUTIONS AND UNRIVALLED SUPPORT 

The Foundation for 

Continued Leadership 

Manufacturing 

Workcells 

Our corporate mission is to 

become the leading supplier of 

electronic motion control 

equipment worldwide. 

Compumotor’s merger in May 

1986, with Parker Hannifin 

Corporation has helped 

provide the technical and 

financial resources necessary 

to fulfill that role. With a 70- 

year history of successfully 

supplying motion control 

components and systems, 

Parker Hannifin enhances 

Compumotor’s foundation for 

continued leadership. 

Our Strategy for Your 

Success 

Our strategy for success is 

simple: provide our customers 

with a competitive advantage. 

We do so by offering 

continuous product 

innovation, complete 

solutions, and unrivalled 

technical support. 

Our manufacturing workforce 

consists of efficient workcells, 

all dedicated to producing a 

product family. Each workcell 

member has a voice in 

streamlining every 

manufacturing process. The 

workcell includes a member 

from our Marketing, 

Applications Engineering, 

Customer Service, and 

Manufacturing departments— 

customer feedback and 

special requests can be 

discussed with those actually 

making your product. This 

level of communication gives 

us the flexibility to alter 

manufacturing steps to furnish 

you with the exact product 

you want. Our manufacturing 

workforce is completely crosstrained 

to work on several 

different workcells. Crosstraining 

is yet another method 

we use to efficiently respond 

to your order and overall 

product demand. 

Internal Manufacturing 

Parker Hannifin has invested in 

state-of-the-art surface mount 

and automated insertion 

machines to guarantee a 

prompt response to orders. 

Unlike many other companies, 

Parker Hannifin has the 

flexibility to build any product 

in any quantity (based on 

demand) without relying on an 

outside turn-key vendor to 

build our boards. If your 

manufacturing growth requires 

more Parker Hannifin product, 

we can grow with you! Our 

manufacturing philosophy is 

simple: 

• Authorize JIT-based vendors 

to provide raw materials 

with low lead times 

• Reduce our dock-to-stock 

time required to receive the 

raw parts before we can 

build products 

• Build all boards internally 

with state-of-the-art surface 

mount and PCA equipment 

• Provide consistent lead 

times to our customers 

regardless of product mix 

2

COMPUMOTOR 

Quality Products 

At Parker Hannifin, producing 

quality products is our number 

one priority. Our products are 

designed with high quality 

standards and are 

manufactured with state-ofthe-art 

equipment and 

production methods. Before 

any product reaches our 

customers, it must pass a 

rigorous set of hardware and 

software tests. JIT (Just-intime) 

manufacturing and DFM 

(Design-for-Manufacturability) 

methods lend themselves to 

creating the necessary 

flexibility to readily 

accommodate your special 

needs. As an example of these 

manufacturing principles in 

action, many of our products 

have earned UL recognition. 

In addition to adhering to our 

own rigorous standards, 

Parker Hannifin is dedicated to 

meeting existing quality 

requirements established by 

the industry. The ISO-9000 

international quality standard 

involves a supplier’s internal 

production processes and 

services. ISO-9000 is a 

standard credential that 

verifies that a supplier has a 

quality process in place. Due 

to the emphasis of ISO-9000 

in Europe, Parker Hannifin has 

already achieved the ISO- 

9000 standard at its Digiplan 

division. Many of the quality 

practices performed at 

Digiplan have been adopted at 

the Compumotor division. 

Two-Year Warranty 

It’s one thing to promise 

reliability, quality and service; 

and quite another to 

guarantee it—especially in a 

global marketplace. That’s 

why we offer a two-year 

warranty on our entire line of 

motors, drives, encoders, and 

controllers. 

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TOTAL SUPPORT FROM CONCEPT TO IMPLEMENTATION 

Local Product 

Availability 

and Service— 

Around the World 

At Parker Hannifin, we 

understand the demands of 

the global marketplace. 

Throughout North America, 

Europe and the Pacific Rim, 

our motion control products 

are delivered and supported 

through a comprehensive 

network of Automation 

Technology Centers (ATCs). 

ATCs serve the industrial 

needs unique to each region. 

Parker Hannifin’s 

Electromechanical 

Territory Manager 

Everyone promises service, 

but Compumotor has the 

people to assure timely, expert 

support. For example, Parker 

Hannifin employs a competent 

and motivated team of 

degreed, factory-trained field 

application engineers who are 

ready to offer you assistance 

in product selection, 

installation, product/system 

programming and 

troubleshooting. 

Authorized Automation 

Technology Centers 

Your local independent 

Automation Technology Center 

has been factory-trained to 

offer you expert service and 

advice. The network includes 

90 organizations and more 

than 125 outlets throughout 

the world. In addition to those 

services offered by traditional 

distributors, these 

organizations specialize in the 

application of high technology 

automation equipment. Parker 

Hannifin works cooperatively 

with its authorized ATCs to 

recruit, hire, and train degreed 

engineers for positions with 

ATC organizations. ATCs offer 

local product availability, 

product demonstrations, 

complementary products and 

services, programming 

assistance, system integration, 

and in-depth customer 

seminars. 

Engineering Support 

Tools to Make Your Job 

Easier 

Years of experience have 

culminated in a vast 

assortment of engineering 

support tools that help to 

simplify the sizing, selection, 

and installation process, 

design a system to custom 

application requirements, and 

troubleshoot existing 

installations. A few of these 

tools include: 

• Motor Sizing and Selection 

Software 

• Application Programming 

Software 

• Application success stories 

• Product installation videos 

• In-depth handbooks on 

subjects such as feed-tolength 

• The consolidated 

engineering reference, 

Section A of this catalog 

• A customer newsletter 

4

ASSISTANCE AT YOUR FINGERTIPS 

1-800 Applications 

Engineering Assistance 

When you have urgent 

questions, expert answers are 

only a phone call away. A 

team of engineers is ready to 

take your call from 6:00 a.m. 

to 5:00 p.m. PST. These 

engineers have practical field 

experience and are prepared 

to provide you with application 

and product assistance 

throughout the stages of your 

project and for the life of the 

product. Just call 1-800-358- 

9070. Outside the U.S. call 

707-584-7558. 

E-Mail 

In addition to the 1-800 

number, you can call 

Compumotor via the Internet. 

Designed as a question and 

answer forum, leave us 

messages, requests for 

literature, or send and retrieve 

files. Compumotor’s E-mail 

system is available 24 hours a 

day, 7 days a week. To reach 

our system simply punch in: 

tech_help@cmotor.com 

Fax 

Bulletin Board 

Use your modem to access a 

wide variety of sample 

programs, CAD drawings, 

support software, and even a 

message interface. To reach 

Compumotor’s Bulletin Board, 

dial 707-584-4059. 

S 

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P 

O 

Compumotor offers a fax 

service to customers to 

answer questions and review 

short programs. Answers will 

be faxed or phoned back 

within 24 hours. To send us a 

fax dial 707-584-3793. 

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S 

SEMINARS: PROVIDING THE TOOLS TO MAKE INFORMED DECISIONS 

Motion Control 

Technology Training at 


Customers can attend training 

courses at Compumotor. 

Training courses are available 

on a variety of technical topics 

as well as product-specific 

instruction. The courses are 

designed to give attendees 

hands-on, practical training 

with experienced engineers. In 

many cases, the actual 

product design engineers will 

conduct the training. This 

program provides our 

customers with a unique 

opportunity to develop a 

better understanding of 

application design, 

development, and 

programming. Participants will 

also develop a better 

understanding of Parker 

Hannifin and its commitment 

to quality products and 

service. 

N 

A 

Local and In-House 

Presentations Bring the 

Leading Edge to You 

Compumotor assures you 

have access to the latest 

information by conducting 

over 100 local seminars and 

product workshops annually. If 

you need an in-house 

presentation, talk to your 

Automation Technology Center 

or field application engineer. 

We can customize programs 

to your specific requirements. 

R 

S 

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E 

More Factory Training 

for Customers 

6

PROBLEM SOLVING WORKSHOPS & SEMINARS TO ADDRESS YOUR NEEDS 

Come With a Problem, 

Leave With a Solution 

Since 1986, our Automation 

Technology Centers have 

conducted motion control 

seminars for over 11,000 

attendees. At our seminars, 

you’ll be briefed not only on 

the basics of programmable 

motion control, but also on the 

newest, most innovative 

technologies in the industry— 

and given the facts to evaluate 

them for your applications. 

Seminars are more than 

informational programs— 

they’re problem-solving 

sessions that address your 

needs. So bring a motion 

control problem. You can 

expect to leave with a Parker 

Hannifin solution. 

You’ll Take Away More 

Than Ideas 

Attendees to our seminars get 

more than answers. They 

receive presentation materials, 

article reprints, support 

software, assignments with 

solutions, videos and specific 

application ideas. 

Contact your local 

Automation Technology 

Center or Electromechanical 

Territory Manager for 

upcoming seminars. 

Workshops 

Compumotor offers full-day 

workshops that guide users 

through the installation and 

application of its most popular 

products. Compumotor’s 

workshops empower 

attendees with the following 

information. 

• Relevant technology issues 

• Application design tips 

• Basics of motion control 

hardware and software 

• Helpful troubleshooting tips 

The workshops are designed 

to help you get the most out 

of your new Compumotor 

product in the least amount of 

time. These workshops are 

designed by Compumotor and 

organized, scheduled, and 

conducted by its authorized 

ATC network. 

Use Motor Sizing & 

Selection Software for 

the Right Product, 

Every Time 

A wide range of applications 

can be solved equally well by 

more than one motor. 

However, some applications 

are particularly appropriate for 

each motor type. 

Compumotor’s Motor Sizing 

and Selection software 

package is designed to help 

users easily identify the 

appropriate motor size and 

type for a motion application. 

7

WITH SUPPORT SOFTWARE, THERE’S NO MORE GUESS WORK 

Motion Architect ® 

Software Does the Work 

for You... Configure, 

Diagnose, Debug 

Compumotor’s Motion 

Architect is a Microsoft ® 

Windows-based software 

development tool for 6000 

Series products that allows 

you to automatically generate 

commented setup code, edit 

and execute motion control 

programs, and create a 

custom operator test panel. 

The heart of Motion Architect 

is the shell, which provides an 

integrated environment to 

access the following modules. 

• System Configurator—This 

module prompts you to fill in 

all pertinent set-up 

information to initiate 

motion. Configurable to the 

specific 6000 Series 

product that is selected, the 

information is then used to 

generate actual 6000- 

language code that is the 

beginning of your program. 

• Program Editor—This 

module allows you to edit 

code. It also has the 

commands available 

through “Help” menus. A 

user’s guide is provided on 

disk. 

• Terminal Emulator—This 

module allows you to 

interact directly with the 

6000 product. “Help” is 

again available with all 

commands and their 

definitions available for 

reference. 

• Test Panel—You can 

simulate your programs, 

debug programs, and check 

for program flow using this 

module. 

Because Its Windows, 

You Already Know How 

to Use It 

Motion Architect ® has been 

designed for use with all 6000 

Series products—for both 

servo and stepper 

technologies. The versatility of 

Windows and the 6000 Series 

language allow you to solve 

applications ranging from the 

very simple to the complex. 

Motion Architect comes 

standard with each of the 

6000 Series products and is a 

tool that makes using these 

controllers even more 

simple—shortening the project 

development time 

considerably. A value-added 

feature of Motion Architect, 

when used with the 6000 

Servo Controllers, is its tuning 

aide. This additional module 

allows you to graphically 

display a variety of move 

parameters and see how 

these parameters change 

based on tuning values. 

Using Motion Architect, you 

can open multiple windows at 

once. For example, both the 

Program Editor and Terminal 

Emulator windows can be 

opened to run the program, 

get information, and then 

make changes to the program. 

On-line help is available 

throughout Motion Architect, 

including interactive access to 

the contents of the 

Compumotor 6000 Series 

Software Reference Guide. 

8

SOLVING APPLICATIONS FROM SIMPLE TO COMPLEX 

Servo Control is Yours 

with Servo Tuner 

Software 

Compumotor combines the 

6000 Series servo controllers 

with Servo Tuner software. 

The Servo Tuner is an add-on 

module that expands and 

enhances the capabilities of 

Motion Architect ® . 

Motion Architect and the 

Servo Tuner combine to 

provide graphical feedback of 

real-time motion information 

and provide an easy 

environment for setting tuning 

gains and related system 

parameters as well as 

providing file operations to 

save and recall tuning 

sessions. 

Draw Your Own Motion 

Control Solutions with 

Motion Toolbox 

Software 

Motion Toolbox is an 

extensive library of LabVIEW® 

virtual instruments (VIs) for 

icon-based programming of 

Compumotor’s 6000 Series 

motion controllers. 

When using Motion Toolbox 

with LabVIEW, programming 

of the 6000 Series controller is 

accomplished by linking 

graphic icons, or VIs, together 

to form a block diagram. 

Motion Toolbox’s has a library 

of more than 150 command, 

status, and example VIs. All 

command and status VIs 

include LabVIEW source 

diagrams so you can modify 

them, if necessary, to suit your 

particular needs. Motion 

Toolbox also comes with a 

WIndows-based installer 

and a comprehensive user 

manual to help you gut up and 

running quickly. 

CompuCAM Software 

for Computer-Aided 

Motion Applications 

CompuCAM is a Windowsbased 

programming package 

that imports geometry from 

CAD programs, plotter files, 

or NC programs and 

generates 6000 code 

compatible with 

Compumotor’s 6000 Series 

motion controllers. Available 

for purchase from 

Compumotor, CompuCAM is 

an add-on module which is 

invoked as a utility from the 

menu bar of Motion Architect. 

From CompuCAM, run your 

CAD software package. Once 

a drawing is created, save it 

as either a DXF file, HP-GL 

plot file or G-code NC 

program. This geometry is 

then imported into 

CompuCAM where the 6000 

code is generated. After 

generating the program, you 

may use Motion Architect 

functions such as editing or 

downloading the code for 

execution. 

Motion Builder 

Software for Easy 

Programming of the 

6000 Series 

Motion Builder revolutionizes 

motion control programming. 

This innovative software allows 

programmers to program in a 

way they are familiar with—a 

flowchart-style method. 

Motion Builder decreases the 

learning curve and makes 

motion control programming 

easy. 

Motion Builder is a Microsoft 

Windows-based graphicaldevelopment 

environment 

which allows expert and 

novice programmers to easily 

program the 6000 Series 

products without learning a 

new programming language. 

Simply drag and drop visual 

icons that represent the 

motion functions you want to 

perform. 

Motion Builder is a complete 

application development 

environment. In addition to 

visually programming the 6000 

Series products, users may 

configure, debug, download, 

and execute the motion 

program. 

Windows is a registered trademark of Microsoft Corporation. Motion Architect is a 

registered trademark of Parker Hannifin Corporation, Compumotor Division. Motion 

Toolbox is a trademark of Snider Consultants, Inc. CompuCAM is a trademark of 

Parker Hannifin Corporation, Compumotor Division. 

9

SERVO VERSUS STEPPER... WHAT YOU NEED TO KNOW 

Motor Types and Their 

Applications 

Short, rapid, repetitive 

moves 

The following section will give 

you some idea of the 

applications that are 

particularly appropriate for 

each motor type, together with 

certain applications that are 

best avoided. It should be 

stressed that there is a wide 

range of applications which 

can be equally well met by 

more than one motor type, 

and the choice will tend to be 

dictated by customer 

preference, previous 

experience or compatibility 

with existing equipment. 

A helpful tool for selecting the 

proper motor for your 

application is Compumotor’s 

Motor Sizing and Selection 

software package. Using this 

software, users can easily 

identify the appropriate motor 

size and type. 

High torque, low speed 

continuous duty applications 

are appropriate to the step 

motor. At low speeds it is very 

efficient in terms of torque 

output relative to both size 

and input power. 

Microstepping can be used to 

improve smoothness in lowspeed 

applications such as a 

metering pump drive for very 

accurate flow control. 

High torque, high 

speed 

continuous duty applications 

suit the servo motor, and in 

fact a step motor should be 

avoided in such applications 

because the high-speed 

losses can cause excessive 

motor heating. 

are the natural domain of the 

stepper due to its high torque 

at low speeds, good torqueto-inertia 

ratio and lack of 

commutation problems. The 

brushes of the DC motor can 

limit its potential for frequent 

starts, stops and direction 

changes. 

Low speed, high 

smoothness 

applications 

are appropriate for 

microstepping or direct drive 

servos. 

Applications in 

hazardous 

environments 

or in a vacuum may not be 

able to use a brushed motor. 

Either a stepper or a brushless 

motor is called for, depending 

on the demands of the load. 

Bear in mind that heat 

dissipation may be a problem 

in a vacuum when the loads 

are excessive. 

The Right Motor/Drive Technology for Your Application 

Step Motors 

Full/Half/Mini 

Microstepping 

Brushless Servos* 

Speed 

revs/min 0 180 360 720 1080 1440 1800 3000 6000 

revs/sec 0 3 6 12 18 24 30 50 100 

Series Page Torque (oz-in) 

PDS & PDX Series C55 910 850 650 426 270 170 130 70 

PK130 C67 4500 5000 3300 3000 2200 1450 1000 750 

ZETA4 C17 400 400 380 350 200 150 125 100 

ZETA6104 C17 400 400 380 350 200 150 125 100 

S & SX C37 1900 1900 1800 1500 1000 750 600 350 

LN C51 85 60 35 10 

APEX Series B17 8400 8300 8200 8100 8000 7900 4700 3300 1500 

TQ10 B39 25 324 323 322 318 319 318 313 114 

BLH & BLHX B47 1850 1850 1850 1850 1850 1850 1850 430 70 

Z Series B61 18000 17825 17650 17270 16800 11300 11100 7320 1300 

Direct Drive 

Brushless Servos* 

Series Page Speed (revs/sec) 0 1.0 2.0 3.0 4.0 

Dynaserv B80 Torque (ft-lb) 370 330 50 50 40 

* The torque values indicated are peak values. Please refer to the product section for full technical data. 

10 SERVO 

VS.

SELECTING THE MOTOR THAT SUITS YOUR APPLICATION 

The flow chart seen below will guide you to the 

recommended drive technology. 

Start at the top left corner and proceed by answering the given 

questions until you end at the recommended drive technology. 

You can then proceed to the stepper or servo sections of the 

catalog to determine what type of controller is required for your 

application. 

Start Here 

S E R V O S 

& S T E P P E R & S E R V O S 

Will a stepper meet 

the torque/speed 

requirements? 

Yes 

Do you need to run 

continuously at 

speeds above 

2000 rpm? 

No 

Do you need to 

control torque? 

No 

Will a brush 

servo meet 


requirements? 

Yes 

Must the motor 

EITHER 

1) Be maintenancefree 

2) Operate in any 

environment 

No 

No 

Yes 

Will a brushless 

servo meet 


requirements? 

Yes 

Use a brushless 

servo. 

No 

Higher 

torque/speed 

technology. 

& S T E P P E R S & 

No 

Does the load 

change rapidly 

during operation? 

No 

Are there any 

other brush 

servos in the 

system? 

Yes 

Use a brush servo. 

No 

If there are other 

brushless motors, it may 

be better to be consistent 

with this one. 

Otherwise use a brush servo. 


detect position 

loss OR measure 

actual load 

position to correct 

for backlash? 

Yes 

Is rapid settling 

important? 

Yes 

Is there a hybrid 

servo which meets 


requirements? 

Yes 

No 

No 

Is low-speed 

smoothness 

important? 

No 

Yes 

No 

Use a microstepping 

with encoder 

feedback? 

Is quiet operation 

important? 

Yes 

Try a hybrid 

servo with 

encoder feedback 

if necessary. 

Really quiet? 

Yes 


important? 

Yes 

No 

No 

No 

Use a stepper 

Use a microstepping, 

hybrid servo, or 

direct drive servo 

STEPPER 

11

COMPUMOTOR: YOUR SERVO CONTROL SPECIALIST 

Powerful Controllers 

Designed with the User 

in Mind 

Compumotor’s servo 

controllers offer power and 

diversity with the capability of 

supporting multi-axis 

applications, in 1-, 2-, or 4- 

axis configurations. Operating 

stand-alone or with a host 

computer, these controllers 

incorporate the leading 

processor technology in the 

industry. Support for I/O, 

operator interface, and 

communications is standard in 

all of the controllers. Other 

complex operations including 

multi-axis following can be 

achieved with very little effort. 

Tuning Your Servo 

Servo tuning—one of the more 

challenging aspects of servo 

motion control—is supported 

with powerful software tools 

offered by Compumotor. 

Tuning modules graphically 

depict actual versus 

programmed motion and 

performance, greatly reducing 

the time required to tune the 

servo system. 

Extensive servo application 

experience allows us to 

provide useful and easy-to-use 

front-end software to our 

customers. These programs 

reduce labor-intensive set-up 

and programming tasks. 

Examples of these programs 

include: 

• Motion Architect ® , which is 

used to set up, test, and 

communicate with the 6000 

Series of controllers, 

• Servo Tuner, which 

combines with Motion 

Architect to provide 

graphical feedback of move 

information and makes 

servo tuning a snap, 

• Xware, a terminal emulation 

software package, used 

with our combination 

controller drive packages. 

S 

E 

R 

Systems Made Simple 

by Design 

At Compumotor, we have 

designed each system with 

the user in mind. Our goal is to 

provide you with everything 

that is required to get your 

Compumotor servo system up 

and running quickly. This 

implies that every unit shipped 

from our factory includes the 

necessary cables, 

documentation, and software. 

Because the needs of every 

user is different, drives and 

controllers are either 

packaged with a power supply 

or provided separately to suit 

the needs of the specific 

system. System wiring can 

often be nightmarish with 

other controllers and drives. 

However, Compumotor 

provides screw-terminal 

connections to make wiring 

straight forward and troublefree. 

At Compumotor, we take 

“easy-to-use” seriously, 

because we know you are 

serious about saving valuable 

project development time. 

V 

O 

The Power of the 

6000 Series 

S 


6000 Series products with 

Motion Architect—and servo 

control never looked so good. 

The 6000 family provides 1 to 

4 axes of servo control, in 

stand-alone or AT-bus-based 

systems as well as single axis 

packaged drive/controller 

units. All of the products 

accept incremental encoder 

feedback, and add valuable 

servo tuning capability to the 

package through Motion 

Architect’s optional Servo 

Tuner Module. Position-based 

following is standard on all 

6000 Series products. 

12

FLEXIBLE AND EASY TO USE CONTROLLER AND DRIVE COMBINATIONS 

Controller and Drive 

Combinations— 

Excellent Options for 

Single-Axis 

Applications 

An industry leader in 

innovation, Compumotor 

introduced the X Series—the 

first system that combined 

controller and drive electronics 

in one package. The result? 

The most versatile, costeffective, 

single-axis motion 

control systems available. But 

we have not stopped there. 

Compumotor has now 

combined the 6000 servo 

controller with the APEX drive 

to provide the easiest to use 

packaged servo system on the 

market. 

The APEX Series: 

Drives or Complete 

Packaged Systems 

Compumotor’s competitively 

priced APEX family of servo 

products includes both servo 

drives and complete packaged 

servo systems. APEX drives 

were designed for use with 

servo controllers, in torque or 

velocity mode. Flexibility is the 

word to describe these drives, 

since they mix and match with 

all of Compumotor’s controls. 

These analog input drives are 

available for single or multiaxis 

applications, offer many 

motor options, and come in a 

variety of power ranges. 

APEX packaged controller/ 

drive systems offer 

tremendous value by saving 

both space and money. These 

systems marry Compumotor’s 

6000 Series of controllers with 

the APEX family of drives— 

resulting in a single-axis 

controller and servo drive in 

one package that uses 

Compumotor’s front end 6000 

software tools for quick and 

easy operation. 

State of the Art 

Precision with the 

Dynaserv 

If high accuracy and 

repeatability are required, 

Dynaserv is the answer. By 

utilizing advanced resolver and 

encoder techniques, the 

Dynaserv has accuracies to 

±30 arc-sec and repeatabilities 

up to ±2 arc-sec. The 

resolution is also astounding— 

with values up to 1,024,000 

steps/rev. 

When it comes to loads, 

Dynaserv has a sophisticated 

servo algorithm allowing 

controllability of extremely 

large loads. The proprietary 

cross-roller bearing design can 

carry over 4 tons in 

compression and 296 ft-lbs of 

overhung load. 

High Speed Capability 

with the BL Series 

The BL Series is a costeffective 

solution in a wide 

variety of brushless servo 

applications. With high-speed 

capability in excess of 10,000 

rpm, the system offers 

industry-standard analog input, 

position feedback from the 

built-in incremental encoder, 

torque and velocity monitor 

outputs, and rack compatible 

design. The BL Drive operates 

in torque or velocity amplifier 

mode, and can be supplied 

with an integral positioner to 

accept motion control 

commands via an RS-232C 

link. 

Pre-engineered AC 

Brushless Servo 

Systems 

There’s no need to mix and 

match components with 

Compumotor’s digital AC 

brushless servo systems— 

they’re completely preengineered 

for optimum 

performance. That means 

easy set-up and low 

maintenance. Compumotor 

brushless servo systems offer 

high torque per motor size and 

weight, rapid acceleration and 

smoother machine 

operation—all in one proven, 

pre-engineered package. 

Pretested speed, torque and 

acceleration performance 

assures that every system 

meets your design 

requirements. Each single axis 

model includes everything 

needed—motor, drive, 

resolver, cables and feedback. 

6000 Series and X version 

controller systems include a 

controller integral to the drive 

package. 

Turn to the B Section 

for more information 

and complete product 

specifications on 

Compumotor’s wide 

selection of servo 

motor systems. 

13

FROM POWERFUL AND COMPACT FULL OR MINISTEPPING SYSTEMS... 

Ministepping Motor 

Drives 

The PDX Series drives 

combine built-in RS-232C 

indexers with advanced 

ministepping techniques for 

output resolutions of up to 

4000 steps/revolution. The 

PDX Series drives provide a 

unique level of functionality in 

a compact package, and offer 

an excellent cost/performance 

ratio. The ministepping 

capability offers improved 

smoothness over conventional 

full and half step drives. The 

drive’s ability to run directly 

from local supply voltages 

virtually anywhere in the world 

simplifies the design of 

equipment built for export, 

making the units ideal for 

OEMs and system integrators. 

Low-cost, Slow-speed 

Rotation 

The PDS Series offers 

completely self-contained lowcost 

motion control for 

applications that require a 

combination of good dynamic 

performance and smooth 

slow-speed rotation. Digiplan’s 

PDS Series features built-in 

intelligent switch-mode power 

supply that allows direct online 

operation from any AC 

supply in the range of 95V to 

265V without the need for 

adjustment. PDS drives are 

available with 3A and 5A 

outputs, and a 70V DC bus 

maximizes high-speed torque. 

High Resolution and 

Smooth Control Make 

the Job Easier 

Microstepping, a technique 

pioneered by Compumotor, 

offers precise positioning 

exceptional smoothness at 

very slow speeds. 

Compumotor’s precision 

microstepping electronics offer 

more than smooth, low-speed 

operation. They also provide 

smooth acceleration and 

deceleration which eliminates 

damaging vibration, shock, 

overshoot, and ringing. 

T 

E 

P 

25,000 Steps/Rev 

Standard 

Compumotor’s 25,000 steps 

per revolution has become the 

industrial standard for 

microstepping. A wide range 

of resolutions are available— 

from 2,000 to 100,000 steps 

per revolution for closed loop 

applications requiring submicron 

resolutions. 

While microstepping’s 

increased positional resolution 

isn’t necessary for all 

applications, its low velocity 

ripple and resonance control 

assures smooth machine 

operation that’s unattainable 

with conventional full step 

motors and many servo 

systems. 

S 

P 

E 

R 

S 

14

...TO SMOOTH AND PRECISE MICROSTEPPING SYSTEMS 

ZETA: Revolution in 

Microstepping 

Technology 

Compumotor’s innovative 

leadership continues with the 

introduction of the ZETA 

Series—a true revolution in 

microstepping technology. 

The ZETA drive incorporates 

patentable techniques known 

as active damping and 

electronic viscosity. The result 

is higher throughput in a 

smaller package system—and 

all at a reduced cost. 

The ZETA Series combines the 

benefits of a smaller footprint, 

with increased throughput by 

reducing settling time and 

decreasing motor vibration. 

The user has selectable 

damping to optimize 

performance, and reduce 

audible noise. This 

combination of innovative 

features makes the ZETA drive 

the most cost-effective and 

highest performing 

microstepping systems 

available today. 

Indexer and Drive 

Combinations: Excellent 

Options for Single Axis 

Applications 

Compumotor’s ZETA6104 and 

SX Series combine the 

functions of an indexer and 

microstepping drive in one 

compact system. Each model 

is capable of storing multiple 

move motion programs in 

battery backed memory. 

Programs can be selected in a 

variety of ways including BCD 

switches, programmable 

controllers or a computer via a 

RS-232C interface. 

The ZETA6104 combines the 

flexibility of the 6000 Series 

controls with the revolutionary 

design features of the ZETA 

drive. As a member of the 

6000 Series, the ZETA6104 

offers all the capabilities of the 

6000 language and the benefit 

of Motion Architect 

development software and 

other 6000 Series-compatible 

software packages. 

The SX uses a high-level 

programming language which 

evolved from Compumotor’s 

X-language. This popular 

language provides both easeof 

use and the ability to 

program complex motion. 

UL-Recognized 

Microstepping Systems 

Today’s laboratory and factory 

automation products face 

increasingly stringent 

performance and safety 

criteria. To meet these 

standards, our ZETA Series 

and S Series microstepping 

systems are certified as UL 

Recognized Components 

under the UL508 safety 

standard covering Industrial 

Control Equipment. 

Open or Closed Loop 

Compumotor offers open- or 

closed-loop pre-engineered 

microstepping systems with a 

complete range of motors. 

Both incremental and absolute 

encoder feedback is available. 

Powerful Indexers 

Designed with the User 

in Mind 

Compumotor’s indexers offer 

the power and diversity of 

supporting multi-axis 

applications, in 1-, 2-, or 4- 

axis configurations. These 

indexers incorporate the 

leading processor technology 

in the industry and operate 

stand-alone or with a host 

computer. Support for I/O, 

operator interface, and 

communications is standard in 

all of the indexers. Other 

complex operations including 

multi-axis following can be 

achieved almost effortlessly. 

The Power of the 6000 

Series 


6000 Series with Motion 

Architect, and microstepping 

control is better than ever 

before. The 6000 family 

provides 1 to 4 axes of control 

in stand-alone or AT-busbased 

systems as well as one 

and two axis packaged drive/ 

indexer units. All of the 

products accept incremental 

encoder feedback in order to 

detect stalls, verify position, 

and correct for positioning 

errors. 

Turn to the C Section 

for more information 

and complete product 

specifications on 

Compumotor’s wide 

selection of stepper 

motor systems. 

15

CUSTOM PRODUCTS... JUST GIVE US A CALL 

Adaptability 

Compumotor will enhance 

standard catalog products by 

adding software and hardware 

features for your unique 

application requirements. 

Connectability 

Out of the box component 

installation can be even easier 

with custom cables, modified 

motors and customer specific 

interfaces. 

C 

Complete Subsystems 

Compumotor can provide 

custom packaging, private 

labeling or several 

components integrated into a 

single part number to save 

engineering and production 

time. From your initial concept 

through custom product 

completion, Compumotor is 

your source for application 

specific flexibility. 

If you don’t find what you’re 

looking for in this catalog, 

contact your Automation 

Technology Center or your 

Compumotor Electromechanical 

Territory Manager 

for application-specific motion 

control solutions involving 

customized: 

• Motors 

• Controls 

• Drives 

U 

• Absolute encoders 

• Software 

• Hardware 

• Cabling 

S 

T 

O 

M 

I 

Z 

E 

! 

16

Table 

Drill 

Head 

Engineering 

Reference and 

Application 

Solutions 

Rotating Nut 

Ballscrew 

Motor 

Transfer 

Machine 

Drive/Controller 


Drive 

Drive 


Joystick 

Indexer 

Circuit Board 

Rotary Indexer 

PLC 

Programmable 

Logic Controller 

Controller 

Drive 

A1

Motor Overview Technologies 

Introduction 

Motion control, in its widest sense, could relate to 

anything from a welding robot to the hydraulic 

system in a mobile crane. In the field of Electronic 

Motion Control, we are primarily concerned with 

systems falling within a limited power range, 

typically up to about 10HP (7KW), and requiring 

precision in one or more aspects. This may involve 

accurate control of distance or speed, very often 

both, and sometimes other parameters such as 

torque or acceleration rate. In the case of the two 

examples given, the welding robot requires precise 

control of both speed and distance; the crane 

hydraulic system uses the driver as the feedback 

system so its accuracy varies with the skill of the 

operator. This wouldn’t be considered a motion 

control system in the strict sense of the term. 

Our standard motion control system consists of 

three basic elements: 

Fig. 1 Elements of motion control system 

Host 

Computer 

or PLC 

The motor. This may be a stepper motor (either 

rotary or linear), a DC brush motor or a brushless 

servo motor. The motor needs to be fitted with 

some kind of feedback device unless it is a stepper 

motor. 

Fig. 2 shows a system complete with feedback to 

control motor speed. Such a system is known as a 

closed-loop velocity servo system. 

Fig. 2 Typical closed loop (velocity) servo system 

Controller 

High-Level 

Commands 

Indexer/ 

Controller 

Command 

Signals 

Drive 

Drive 


Hybrid Stepper 

DC Servo 

Brushless 

Servo Linear 

Stepper 

Tachometer 

Velocity Feedback 


The drive. This is an electronic power amplifier that 

delivers the power to operate the motor in response 

to low-level control signals. In general, the drive will 

be specifically designed to operate with a particular 

motor type – you can’t use a stepper drive to 

operate a DC brush motor, for instance. 

The control system. The actual task performed by 

the motor is determined by the indexer/controller; it 

sets things like speed, distance, direction and 

acceleration rate. The control function may be 

distributed between a host controller, such as a 

desktop computer, and a slave unit that accepts 

high-level commands. One controller may operate 

in conjunction with several drives and motors in a 

multi-axis system. 

We’ll be looking at each of these system elements 

as well as their relationships to each other. 

Table of Contents 

Motor Applications 

Step Motor Technology 

Linear Step Motor Technology 

Common Questions Regarding Step Motors 

DC Brush Motor Technology 

Brushless Motor Technology 

Hybrid Servo Technology 

Direct Drive Motor Technology 

Step Motor Drive Technology 

Microstepping Drive Technology 

Analog and Digital Servo Drives 

Brushless Servo Drive Technology 

Servo Tuning 

Feedback Devices 

Machine Control 

Control System Overview 

Understanding I/O Modules 

Serial & Parallel Communications 

Electrical Noise Symptoms & Solutions 

Emergency Stop 

System Selection Considerations 

Motor Sizing and Selection Software 

System Calculations – Move Profiles 

System Calculations – Leadscrew Drives 

System Calculations – Direct Drives 

System Calculations – Gear Drives 

System Calculations – Tangential Drives 

System Calculations – Linear Motors 

Glossary of Terms 

Technical Data 

Application Examples 

A3 

A4 

A9 

A12 

A13 

A17 

A20 

A21 

A23 

A29 

A31 

A34 

A36 

A39 

A45 

A46 

A48 

A51 

A52 

A54 

A55 

A57 

A58 

A60 

A63 

A64 

A65 

A66 

A68 

A71 

A72 

A2

Motor Technologies 

Application Areas of Motor Types 

The following section gives some idea of the 

applications that are particularly appropriate for 

each motor type, together with certain 

applications that are best avoided. It should be 

stressed that there is a wide range of 

applications that can be equally well met by 

more than one motor type, and the choice will 

tend to be dictated by customer preference, 

previous experience or compatibility with 

existing equipment. 

Cost-conscious applications will always be 

worth attempting with a stepper, as it will 

generally be hard to beat the stepper’s price. 

This is particularly true when the dynamic 

requirements are not severe, such as “setting” 

type applications like positioning a guillotine 

back-stop or a print roller. 

High-torque, low-speed, continuous-duty 

applications are also appropriate for step 

motors. At low speeds, it is very efficient in 

terms of torque output relative to both size and 

input power. Microstepping can improve lowspeed 

applications such as a metering pump 

drive for very accurate flow control. 

High-torque, high-speed, continuous-duty 

applications suit the servo motor, and in fact, a 

step motor should be avoided in such 

applications because the high-speed losses can 

cause excessive motor heating. A DC motor 

can deliver greater continuous shaft power at 

high speeds than a stepper of the same frame 

size. 

Short, rapid, repetitive moves are the natural 

domain of steppers or hybrid servos due to their 

high torque at low speeds, good torque-toinertia 

ratio and lack of commutation problems. 

The brushes of the DC motor can limit its 

potential for frequent starts, stops and direction 

changes. 

Low-friction, mainly inertial loads can be 

efficiently handled by the DC servo provided the 

start/stop duty requirements are not excessive. 

This type of load requires a high ratio of peak to 

continuous torque and in this respect the servo 

motor excels. 

Very arduous applications with a high 

dynamic duty cycle or requiring very high 

speeds may require a brushless motor. This 

solution may also be dictated when 

maintenance-free operation is necessary. 

Low-speed, high-smoothness applications 

are appropriate for microstepping or direct drive 

servos. 

Applications in hazardous environments or in 

a vacuum may not be able to use a brush 

motor. Either a stepper or a brushless motor is 

called for, depending on the demands of the 

load. Bear in mind that heat dissipation may be 

a problem in a vacuum when the loads are 

excessive. 

Start Here 

Will a stepper meet 


requirements? 

Yes 

Do you need to run 

continuously at 

speeds above 

2000 rpm? 

No 


control torque? 

No 

Does the load 

change rapidly 

during operation? 

No 


detect position 

loss OR measure 

actual load 

position to correct 

for backlash? 

No 

Is low-speed 

smoothness 

important? 

No 


important? 

No 

Use a stepper 

No 

Yes 

Yes 

Yes 

Will a brush 

servo meet 


requirements? 

Yes 

Must the motor 

EITHER 

1)Be maintenance- 

free 

2) Operate in any 

environment 

No 

Are there any 

other brush 

servos in the 

system? 

Yes 

Use a brush servo. 

Is rapid settling 

important? 

No 

Use a microstepping 

with encoder 

feedback? 


important? 

No 

Use a microstepping, 

hybrid servo, or 

direct drive servo 

No 

Yes 

No 

Yes 

Yes 

Will a brushless 

servo meet 


requirements? 

Yes 

Use a brushless 

servo. 

If there are other 

brushless motors, it may 

be better to be consistent 

with this one. 

Otherwise use a brush servo. 

Is there a hybrid 

servo which meets 


requirements? 

Yes 

Try a hybrid 

servo with 

encoder feedback 

if necessary. 

Really quiet? 

No 

No 

Higher 

torque/speed 

technology. 

No 

Yes 

A Engineering Reference 

A3


Stepper Motors 

Stepper Motor Benefits 

Stepper motors have the following benefits: 

• Low cost 

• Ruggedness 

• Simplicity in construction 

• High reliability 

• No maintenance 

• Wide acceptance 

• No tweaking to stabilize 

• No feedback components are needed 

• They work in just about any environment 

• Inherently more failsafe than servo motors. 

There is virtually no conceivable failure within the 

stepper drive module that could cause the motor to 

run away. Stepper motors are simple to drive and 

control in an open-loop configuration. They only 

require four leads. They provide excellent torque at 

low speeds, up to 5 times the continuous torque of 

a brush motor of the same frame size or double the 

torque of the equivalent brushless motor. This often 

eliminates the need for a gearbox. A stepper-driven 

system is inherently stiff, with known limits to the 

dynamic position error. 

Permanent Magnet (P.M.) Motors. The tin-can or 

“canstack” motor shown in Fig. 1.1 is perhaps the 

most widely-used type in non-industrial 

applications. It is essentially a low-cost, low-torque, 

low-speed device ideally suited to applications in 

fields such as computer peripherals. The motor 

construction results in relatively large step angles, 

but their overall simplicity lends itself to economic 

high-volume production at very low cost. The axialair 

gap or disc motor is a variant of the permanent 

magnet design which achieves higher performance, 

largely because of its very low rotor inertia. 

However this does restrict the applications of the 

motor to those involving little inertia. (e.g., 

positioning the print wheel in a daisy-wheel printer). 

Stepper Motor Disadvantages 

Stepper motors have the following disadvantages: 

• Resonance effects and relatively long settling 

times 

• Rough performance at low speed unless a 

microstep drive is used 

• Liability to undetected position loss as a result of 

operating open-loop 

• They consume current regardless of load 

conditions and therefore tend to run hot 

• Losses at speed are relatively high and can cause 

excessive heating, and they are frequently noisy 

(especially at high speeds). 

• They can exhibit lag-lead oscillation, which is 

difficult to damp. There is a limit to their available 

size, and positioning accuracy relies on the 

mechanics (e.g., ballscrew accuracy). Many of 

these drawbacks can be overcome by the use of 

a closed-loop control scheme. 

Note: The Compumotor Zeta Series minimizes or 

reduces many of these different stepper motor 

disadvantages. 

There are three main stepper motor types: 

• Permanent Magnet (P.M.) Motors 

• Variable Reluctance (V.R.) Motors 

• Hybrid Motors 

Variable Reluctance (V.R.) Motors. There is no 

permanent magnet in a V.R. motor, so the rotor 

spins freely without “detent” torque. Torque output 

for a given frame size is restricted, although the 

torque-to-inertia ratio is good, and this type of motor 

is frequently used in small sizes for applications such 

as micro-positioning tables. V.R. motors are seldom 

used in industrial applications (having no permanent 

magnet). They are not sensitive to current polarity 

and require a different driving arrangement than the 

other motor types. 

Fig. 1.2 Variable reluctance motor 

Fig. 1.1 “Canstack” or permanent magnet motor 

N 

S 

N S 

S N 

N 

S 

N 

S 

N 

S 

N 

S 

N 

S 

N 

S 

N 

S 

N 

S 

Stator cup A 

Rotor 

Coil A 

Coil B 

Stator cup B 

Output shaft 

Courtesy Airpax Corp., USA 

A4


Hybrid Motors. The hybrid motor shown in Fig. 1.3 

is by far the most widely-used stepper motor in 

industrial applications. The name is derived from the 

fact that it combines the operating principles of the 

other two motor types (P.M. & V.R.). Most hybrid 

motors are 2-phase, although 5-phase versions are 

available. A recent development is the “enhanced 

hybrid” motor, which uses flux-focusing magnets to 

give a significant improvement in performance, 

albeit at extra cost. 

Fig. 1.3 Hybrid stepper motor 

Housing 

Prelubricated 

Bearing 

Rotor 

Non-magnetic 

Stainless 

Steel Shaft 

Stator 

The operation of the hybrid motor is most easily 

understood by looking at a very simple model that 

will produce 12 steps per rev. (Fig. 1.4). 

Fig. 1.4 Simple 12 step/rev hybrid motor 

The rotor of this machine consists of two pole 

pieces with three teeth on each. In between the 

pole pieces is a permanent magnet that is 

magnetized along the axis of the rotor, making one 

end a north pole and the other a south pole. The 

teeth are offset at the north and south ends as 

shown in the diagram. 

The stator consists of a shell having four teeth that 

run the full length of the rotor. Coils are wound on 

the stator teeth and are connected together in 

pairs. 

With no current flowing in any of the motor 

windings, the rotor will take one of the positions 

shown in the diagrams. This is because the 

permanent magnet in the rotor is trying to minimize 

the reluctance (or “magnetic resistance”) of the flux 

path from one end to the other. This will occur 

when a pair of north and south pole rotor teeth are 

aligned with two of the stator poles. The torque 

tending to hold the rotor in one of these positions is 

usually small and is called the “detent torque”. The 

motor shown will have 12 possible detent positions. 

If current is now passed through one pair of stator 

windings, as shown in Fig. 1.5(a), the resulting north 

and south stator poles will attract teeth of the 

opposite polarity on each end of the rotor. There 

are now only three stable positions for the rotor, the 

same as the number of rotor teeth. The torque 

required to deflect the rotor from its stable position 

is now much greater, and is referred to as the 

“holding torque”. 

Fig. 1.5 Full stepping, one phase on 


1A 

(a) 

(b) 

N 

2A 

N 

N 

S 

N 

2B 

N 

S 

S 

N 

N 

S 

N 

S 

N 

N 

S 

S 

N 

S 

S 

N 

S 

S 

S 

(c) 

(d) 

S 

1B 

S 

N 

N 

S 

S 

N 

S 

N 

S 

S 

N 

N 

S 

N 

N 

By changing the current flow from the first to the 

second set of stator windings (b), the stator field 

rotates through 90° and attracts a new pair of rotor 

poles. This results in the rotor turning through 30°, 

corresponding to one full step. Reverting to the first 

set of stator windings but energizing them in the 

opposite direction, we rotate the stator field 

through another 90° and the rotor takes another 

30° step (c). Finally, the second set of windings are 

energized in the opposite direction (d) to give a 

third step position. We can now go back to the 

first condition (a), and after these four steps the 

rotor will have moved through one tooth pitch. This 

simple motor therefore performs 12 steps per rev. 

Obviously, if the coils are energized in the reverse 

sequence, the motor will go round the other way. 

A5


If two coils are energized simultaneously (Fig. 1.6), 

the rotor takes up an intermediate position since it 

is equally attracted to two stator poles. Greater 

torque is produced under these conditions because 

all the stator poles are influencing the rotor. The 

motor can be made to take a full step simply by 

reversing the current in one set of windings; this 

causes a 90° rotation of the stator field as before. In 

fact, this would be the normal way of driving the 

motor in the full-step mode, always keeping two 

windings energized and reversing the current in 

each winding alternately. 

Fig. 1.6 Full stepping, two phase on 

N 

S 

S 

N 

N 

S 

N 

S 

N 

S 

S 

N 

S 

N 

S 

N 

N 

S 

S 

N 

N 

S 

S 

N 

N 

S 

N 

S 

S 

N 

N 

S 

S 

N 

S 

N 

N 

S 

S 

N 

motor and drive characteristics). In the half-step 

mode, we are alternately energizing two phases 

and then only one as shown in Fig. 1.9. Assuming 

the drive delivers the same winding current in each 

case, this will cause greater torque to be produced 

when there are two windings energized. In other 

words, alternate steps will be strong and weak. 

This does not represent a major deterrent to motor 

performance—the available torque is obviously 

limited by the weaker step, but there will be a 

significant improvement in low-speed smoothness 

over the full-step mode. 

Clearly, we would like to produce approximately 

equal torque on every step, and this torque should 

be at the level of the stronger step. We can achieve 

this by using a higher current level when there is 

only one winding energized. This does not overdissipate 

the motor because the manufacturer’s 

current rating assumes two phases to be energized 

(the current rating is based on the allowable case 

temperature). With only one phase energized, the 

same total power will be dissipated if the current is 

increased by 40%. Using this higher current in the 

one-phase-on state produces approximately equal 

torque on alternate steps (see Fig. 1.10). 

Fig. 1.8 Full step current, 2-phase on 

1 2 3 4 

By alternately energizing one winding and then two 

(Fig. 1.7), the rotor moves through only 15° at each 

stage and the number of steps per rev will be 

doubled. This is called half stepping, and most 

industrial applications make use of this stepping 

mode. Although there is sometimes a slight loss of 

torque, this mode results in much better 

smoothness at low speeds and less overshoot and 

ringing at the end of each step. 

Fig. 1.7 Half stepping 

Phase 1 

Phase 2 

Fig. 1.9 Half step current 

1 2 3 4 5 6 7 8 

S 

N 

S 

S 

N 

N 

S 

N 

S 

S 

N 

S 

S 

N 

N 

S 

N 

S 

N 

S 

N 

S 

S 

N 

Phase 1 

N 

N 

Current Patterns in the Motor Windings 

When the motor is driven in its full-step mode, 

energizing two windings or “phases” at a time (see 

Fig. 1.8), the torque available on each step will be 

the same (subject to very small variations in the 

Phase 2 

Fig. 1.10 Half step current, profiled 

1 2 3 4 5 6 7 8 

Phase 1 

Phase 2 

A6


We have seen that energizing both phases with 

equal currents produces an intermediate step 

position half-way between the one-phase-on 

positions. If the two phase currents are unequal, the 

rotor position will be shifted towards the stronger 

pole. This effect is utilized in the microstepping 

drive, which subdivides the basic motor step by 

proportioning the current in the two windings. In this 

way, the step size is reduced and the low-speed 

smoothness is dramatically improved. Highresolution 

microstep drives divide the full motor step 

into as many as 500 microsteps, giving 100,000 

steps per revolution. In this situation, the current 

pattern in the windings closely resembles two sine 

waves with a 90° phase shift between them (see 

Fig. 1.11). The motor is now being driven very much 

as though it is a conventional AC synchronous 

motor. In fact, the stepper motor can be driven in 

this way from a 60 Hz-US (50Hz-Europe) sine wave 

source by including a capacitor in series with one 

phase. It will rotate at 72 rpm. 

Fig. 1.11 Phase currents in microstep mode 

Phase 1 Current: Zero 

Phase 2 Current: Zero 

Standard 200-Step Hybrid Motor 

The standard stepper motor operates in the same 

way as our simple model, but has a greater number 

of teeth on the rotor and stator, giving a smaller 

basic step size. The rotor is in two sections as 

before, but has 50 teeth on each section. The halftooth 

displacement between the two sections is 

retained. The stator has 8 poles each with 5 teeth, 

making a total of 40 teeth (see Fig. 1.12). 

Fig. 1.12 200-step hybrid motor 

+ 

- 

+ 

- 

If we imagine that a tooth is placed in each of the 

gaps between the stator poles, there would be a 

total of 48 teeth, two less than the number of rotor 

teeth. So if rotor and stator teeth are aligned at 12 

o’clock, they will also be aligned at 6 o’clock. At 3 

o’clock and 9 o’clock the teeth will be misaligned. 

However, due to the displacement between the 

sets of rotor teeth, alignment will occur at 3 o’clock 

and 9 o’clock at the other end of the rotor. 

The windings are arranged in sets of four, and 

wound such that diametrically-opposite poles are 

the same. So referring to Fig. 1.12, the north poles 

at 12 and 6 o’clock attract the south-pole teeth at 

the front of the rotor; the south poles at 3 and 9 

o’clock attract the north-pole teeth at the back. By 

switching current to the second set of coils, the 

stator field pattern rotates through 45°. However, to 

align with this new field, the rotor only has to turn 

through 1.8°. This is equivalent to one quarter of a 

tooth pitch on the rotor, giving 200 full steps per 

revolution. 

Note that there are as many detent positions as 

there are full steps per rev, normally 200. The 

detent positions correspond with rotor teeth being 

fully aligned with stator teeth. When power is 

applied to a stepper drive, it is usual for it to 

energize in the “zero phase” state in which there is 

current in both sets of windings. The resulting rotor 

position does not correspond with a natural detent 

position, so an unloaded motor will always move by 

at least one half step at power-on. Of course, if the 

system was turned off other than in the zero phase 

state, or the motor is moved in the meantime, a 

greater movement may be seen at power-up. 

Another point to remember is that for a given 

current pattern in the windings, there are as many 

stable positions as there are rotor teeth (50 for a 

200-step motor). If a motor is de-synchronized, the 

resulting positional error will always be a whole 

number of rotor teeth or a multiple of 7.2°. A motor 

cannot “miss” individual steps – position errors of 

one or two steps must be due to noise, spurious 

step pulses or a controller fault. 


Stator 

Rotor 

A7


Bifilar Windings 

Most motors are described as being “bifilar wound”, 

which means there are two identical sets of 

windings on each pole. Two lengths of wire are 

wound together as though they were a single coil. 

This produces two windings that are electrically and 

magnetically almost identical – if one coil were to be 

wound on top of the other, even with the same 

number of turns, the magnetic characteristics 

would be different. In simple terms, whereas almost 

all the flux from the inner coil would flow through 

the iron core, some of the flux from the outer coil 

would flow through the windings of the coil 

underneath. 

The origins of the bifilar winding go back to the 

unipolar drive (see Drive Technologies section, 

page A23). Rather than have to reverse the current 

in one winding, the field may be reversed by 

transferring current to a second coil wound in the 

opposite direction. (Although the two coils are 

wound the same way, interchanging the ends has 

the same effect.) So with a bifilar-wound motor, the 

drive can be kept simple. However, this 

requirement has now largely disappeared with the 

widespread availability of the more-efficient bipolar 

drive. Nevertheless, the two sets of windings do 

give us additional flexibility, and we shall see that 

different connection methods can be used to give 

alternative torque-speed characteristics. 

If all the coils in a bifilar-wound motor are brought 

out separately, there will be a total of 8 leads (see 

Fig. 1.13). This is becoming the most common 

configuration since it gives the greatest flexibility. 

However, there are still a number of motors 

produced with only 6 leads, one lead serving as a 

common connection to each winding in a bifilar 

pair. This arrangement limits the motor’s range of 

application since the windings cannot be connected 

in parallel. Some motors are made with only 4 

leads, these are not bifilar-wound and cannot be 

used with a unipolar drive. There is obviously no 

alternative connection method with a 4-lead motor, 

but in many applications this is not a drawback and 

the problem of insulating unused leads is avoided. 

Fig. 1.13 Motor lead configurations 

4-lead 5-lead 6-lead 8-lead 

Occasionally a 5-lead motor may be encountered. 

These are not recommended since they cannot be 

used with conventional bipolar drives requiring 

electrical isolation between the phases. 

Looking at the motor longitudinal section (Fig. 1.14), 

we can see the permanent magnet in the rotor and 

the path of the flux through the pole pieces and the 

stator. The alternating flux produced by the stator 

windings flows in a plane at right angles to the 

page. Therefore, the two flux paths are at right 

angles to each other and only interact in the rotor 

pole pieces. This is an important feature of the 

hybrid motor – it means that the permanent magnet 

in the rotor does not “see” the alternating field from 

the windings, hence it does not produce a 

demagnetizing effect. Unlike the DC servo motor, it 

is generally impossible to de-magnetize a stepper 

motor by applying excess current. However, too 

much current will damage the motor in other ways. 

Excessive heating may melt the insulation or the 

winding formers, and may soften the bonding 

material holding the rotor laminations. If this 

happens and the laminations are displaced, the 

effects can be the same as if the rotor had been 

de-magnetized 

Fig. 1.14 Longitudinal section through single 

stack motor 

Fig. 1.14 also shows that the rotor flux only has to 

cross a small air gap (typically 0.1mm or 0.004") 

when the rotor is in position. By magnetizing the 

rotor after assembly, a high flux density is obtained 

that can be largely destroyed if the rotor is 

removed. Stepper motors should therefore not be 

dismantled purely to satisfy curiosity, since the 

useful life of the motor will be terminated. 

Because the shaft of the motor passes through the 

center of the permanent magnet, a non-magnetic 

material must be used to avoid a magnetic shortcircuit. 

Stepper shafts are therefore made of 

stainless steel, and should be handled with care. 

Small-diameter motors are particularly vulnerable if 

they are dropped on the shaft end, as this will 

invariably bend the shaft. 

To produce a motor with a higher torque output, 

we need to increase the strength of both the 

permanent magnet in the rotor and the field 

produced by the stator. A stronger rotor magnet 

can be obtained by increasing the diameter, giving 

us a larger cross-sectional area. However, 

increasing the diameter will degrade the 

acceleration performance of the motor because 

the torque-to-inertia ratio worsens (to a first 

approximation, torque increases with diameter 

squared but inertia goes up by the fourth power). 

Nevertheless, we can increase torque output 

without degrading acceleration performance by 

A8


Air 

Gap 

adding further magnet sections or “stacks” to the 

same shaft (Fig. 1.15). A second stack will enable 

twice the torque to be produced and will double the 

inertia, so the torque-to-inertia ratio remains the 

same. Hence, stepper motors are produced in 

single-, two- and three-stack versions in each 

frame size. 

Fig. 1.15 Three-stack hybrid stepping motor 

As a guideline, the torque-to-inertia ratio reduces by 

a factor of two with each increase in frame size 

(diameter). So an unloaded 34-size motor can 

accelerate twice as rapidly as a 42-size, regardless 

of the number of stacks. 

Linear Stepping Motors 

Fig. 1.16 Linear stepping motor 

{ 

Phase A 

Electromagnet 

{ 

Forcer 

Permanent 

Magnet 

N 

Phase B 

Electromagnet 

A 1 A 2 B 1 B2 

Pole Faces 

S 

{ 

{ 

Field Windings 

Platen 

Platen Teeth 

The linear stepper is essentially a conventional 

rotary stepper that has been “unwrapped” so that it 

operates in a straight line. The moving component 

is referred to as the forcer and it travels along a 

fixed element or platen. For operational purposes, 

the platen is equivalent to the rotor in a normal 

stepper, although it is an entirely passive device 

and has no permanent magnet. The magnet is 

incorporated in the moving forcer together with the 

coils (see Fig. 1.16). 

The forcer is equipped with 4 pole pieces each 

having 3 teeth. The teeth are staggered in pitch 

with respect to those on the platen, so that 

switching the current in the coils will bring the next 

set of teeth into alignment. A complete switching 

cycle (4 full steps) is equivalent to one tooth pitch 

on the platen. Like the rotary stepper, the linear 

motor can be driven from a microstep drive. In this 

case, a typical linear resolution will be 12,500 steps 

per inch. 

The linear motor is best suited for applications that 

require a low mass to be moved at high speed. In a 

leadscrew-driven system, the predominant inertia is 

usually the leadscrew rather than the load to be 

moved. Hence, most of the motor torque goes to 

accelerate the leadscrew, and this problem 

becomes more severe the longer the travel 

required. Using a linear motor, all the developed 

force is applied directly to the load and the 

performance achieved is independent of the length 

of the move. A screw-driven system can develop 

greater linear force and better stiffness; however, 

the maximum speed may be as much as ten times 

higher with the equivalent linear motor. For 

example, a typical maximum speed for a linear 

motor is 100 in/sec. To achieve this with a 10-pitch 

ballscrew would require a rotary speed of 6,000 

rpm. In addition, the linear motor can travel up to 

12 feet using a standard platen. 

How the Linear Motor Works 

The forcer consists of two electromagnets (A and B) 

and a strong rare earth permanent magnet. The 

two pole faces of each electromagnet are toothed 

to concentrate the magnetic flux. Four sets of teeth 

on the forcer are spaced in quadrature so that only 

one set at a time can be aligned with the platen 

teeth. 

The magnetic flux passing between the forcer and 

the platen gives rise to a very strong force of 

attraction between the two pieces. The attractive 

force can be up to 10 times the peak holding force 

of the motor, requiring a bearing arrangement to 

maintain precise clearance between the pole faces 

and platen teeth. Either mechanical roller bearings 

or air bearings are used to maintain the required 

clearance. 

When current is established in a field winding, the 

resulting magnetic field tends to reinforce 

permanent magnetic flux at one pole face and 

cancel it at the other. By reversing the current, the 

reinforcement and cancellation are exchanged. 

Removing current divides the permanent magnetic 

flux equally between the pole faces. By selectively 

applying current to phase A and B, it is possible to 

concentrate flux at any of the forcer’s four pole 

faces. The face receiving the highest flux 

concentration will attempt to align its teeth with the 

platen. Fig. 1.17 shows the four primary states or 

full steps of the forcer. The four steps result in 

motion of one tooth interval to the right. Reversing 

the sequence moves the forcer to the left. 


A9


Repeating the sequence in the example will cause 

the forcer to continue its movement. When the 

sequence is stopped, the forcer stops with the 

appropriate tooth set aligned. At rest, the forcer 

develops a holding force that opposes any attempt 

to displace it. As the resting motor is displaced from 

equilibrium, the restoring force increases until the 

displacement reaches one-quarter of a tooth 

interval. (See Fig. 1.18.) Beyond this point, the 

restoring force drops. If the motor is pushed over 

the crest of its holding force, it slips or jumps rather 

sharply and comes to rest at an integral number of 

tooth intervals away from its original location. If this 

occurs while the forcer is travelling along the platen, 

it is referred to as a stall condition. 

Fig. 1.17 The four cardinal states or full steps of 

the forcer 

Phase A 

N 

S 

Phase B 

Step Motor Characteristics 

There are numerous step motor performance 

characteristics that warrant discussion. However, 

we’ll confine ourselves to those traits with the 

greatest practical significance. 

Fig. 1.18 illustrates the static torque curve of the 

hybrid step motor. This relates to a motor that is 

energized but stationary. It shows us how the 

restoring torque varies with rotor position as it is 

deflected from its stable point. We’re assuming that 

there are no frictional or other static loads on the 

motor. As the rotor moves away from the stable 

position, the torque steadily increases until it 

reaches a maximum after one full step (1.8°). This 

maximum value is called the holding torque and it 

represents the largest static load that can be 

applied to the shaft without causing continuous 

rotation. However, it doesn’t tell us the maximum 

running torque of the motor – this is always less 

than the holding torque (typically about 70%). 

Fig. 1.18 Static torque-displacement 

characteristic 

A 1 Aligned 

Direction of MMF due 

to electromagnet 

N 

S 

Flux Lines 

Torque 

Clockwise Counter Clockwise 

Stable 

Max 

Torque 

4 Motor Steps 

Unstable 

Stable 

Angle 

B 2 Aligned 

As the shaft is deflected beyond one full step, the 

torque will fall until it is again at zero after two full 

steps. However, this zero point is unstable and the 

torque reverses immediately beyond it. The next 

stable point is found four full steps away from the 

first, equivalent to one tooth pitch on the rotor or 

1/50 of a revolution. 

A Aligned 

2 

N 

N 

S 

S 

Although this static torque characteristic isn’t a 

great deal of use on its own, it does help explain 

some of the effects we observe. For example, it 

indicates the static stiffness of the system, (i.e., 

how the shaft position changes when a torque load 

is applied to a stationary motor). Clearly the shaft 

must deflect until the generated torque matches the 

applied load. If the load varies, so too will the static 

position. Non-cumulative position errors will 

therefore result from effects such as friction or outof-balance 

torque loads. It is important to 

remember that the static stiffness is not improved 

by using a microstepping drive—a given load on the 

shaft will produce the same angular deflection. So 

while microstepping increases resolution and 

smoothness, it may not necessarily improve 

positioning accuracy. 

B Aligned 

1 

A10


Under dynamic conditions with the motor running, 

the rotor must be lagging behind the stator field if it 

is producing torque. Similarly, there will be a lead 

situation when the torque reverses during 

deceleration. Note that the lag and lead relate only 

to position and not to speed. From the static 

torque curve (Fig. 1.18), clearly this lag or lead 

cannot exceed two full steps (3.6°) if the motor is to 

retain synchronism. This limit to the position error 

can make the stepper an attractive option in 

systems where dynamic position accuracy is 

important. 

When the stepper performs a single step, the 

nature of the response is oscillatory as shown in 

Fig. 1.19. The system can be likened to a mass that 

is located by a “magnetic spring”, so the behavior 

resembles the classic mass-spring characteristic. 

Looking at it in simple terms, the static torque curve 

indicates that during the step, the torque is positive 

during the full forward movement and so is 

accelerating the rotor until the new stable point is 

reached. By this time, the momentum carries the 

rotor past the stable position and the torque now 

reverses, slowing the rotor down and bringing it 

back in the opposite direction. The amplitude, 

frequency and decay rate of this oscillation will 

depend on the friction and inertia in the system as 

well as the electrical characteristics of the motor 

and drive. The initial overshoot also depends on 

step amplitude, so half-stepping produces less 

overshoot than full stepping and microstepping will 

be better still. 

Fig. 1.19 Single step response 

Angle 

Time 

Attempting to step the motor at its natural 

oscillation frequency can cause an exaggerated 

response known as resonance. In severe cases, 

this can lead to the motor desynchronizing or 

“stalling.” It is seldom a problem with half-step 

drives and even less so with a microstepper. The 

natural resonant speed is typically 100-200 full 

steps/sec. (0.5-1 rev/sec). 

Under full dynamic conditions, the performance of 

the motor is described by the torque-speed curve as 

shown in Fig. 1.20. There are two operating ranges, 

the start/stop (or pull in) range and the slew (or pull 

out) range. Within the start/stop range, the motor can 

be started or stopped by applying index pulses at 

constant frequency to the drive. At speeds within this 

range, the motor has sufficient torque to accelerate 

its own inertia up to synchronous speed without the 

position lag exceeding 3.6°. Clearly, if an inertial load 

is added, this speed range is reduced. So the start/ 

stop speed range depends on the load inertia. The 

upper limit to the start/stop range is typically between 

200 and 500 full steps/sec (1-2.5 revs/sec). 

Fig. 1.20 Start/stop and slew curves 

Holding 

Torque 

Torque 

Start/ 

Stop 

Range 

Start/Stop Curve 

Slew 

Range 

Steps per second 

Slew Curve 

To operate the motor at faster speeds, it is 

necessary to start at a speed within the start/stop 

range and then accelerate the motor into the slew 

region. Similarly, when stopping the motor, it must 

be decelerated back into the start/stop range 

before the clock pulses are terminated. Using 

acceleration and deceleration “ramping” allows 

much higher speeds to be achieved, and in 

industrial applications the useful speed range 

extends to about 3000 rpm (10,000 full steps/sec). 

Note that continuous operation at high speeds is 

not normally possible with a stepper due to rotor 

heating, but high speeds can be used successfully 

in positioning applications. 

The torque available in the slew range does not 

depend on load inertia. The torque-speed curve is 

normally measured by accelerating the motor up to 

speed and then increasing the load until the motor 

stalls. With a higher load inertia, a lower 

acceleration rate must be used but the available 

torque at the final speed is unaffected. 


A11


Common Questions and Answers 

About Step Motors 

1. Why do step motors run hot? 

Two reasons: 1. Full current flows through the 

motor windings at standstill. 2. PWM drive 

designs tend to make the motor run hotter. 

Motor construction, such as lamination 

material and riveted rotors, will also affect 

heating. 

2. What are safe operating temperatures? 

The motors have class B insulation, which is 

rated at 130°C. Motor case temperatures of 

90°C will not cause thermal breakdowns. 

Motors should be mounted where operators 

cannot come into contact with the motor case. 

3. What can be done to reduce motor heating? 

Many drives feature a “reduce current at 

standstill” command or jumper. This reduces 

current when the motor is at rest without 

positional loss. 

4. What does the absolute accuracy specification 

mean? 

This refers to inaccuracies, non-cumulative, 

encountered in machining the motor. 

5. How can the repeatability specification be 

better than that of accuracy? 

Repeatability indicates how precisely a 

previous position can be re-obtained. There 

are no inaccuracies in the system that affect a 

given position, returning to that position, the 

same inaccuracy is encountered. 

6. Will motor accuracy increase proportionately 

with the resolution? 

No. The basic absolute accuracy and 

hysteresis of the motor remain unchanged. 

7. Can I use a small motor on a large load if the 

torque requirement is low? 

Yes, however, if the load inertia is more than 

ten times the rotor inertia, cogging and 

extended ringing at the end of the move will be 

experienced. 

8. How can end of move “ringing” be reduced? 

Friction in the system will help damp this 

oscillation. Acceleration/deceleration rates 

could be increased. If start/stop velocities are 

used, lowering or eliminating them will help. 

9. Why does the motor stall during no load 

testing? 

The motor needs inertia roughly equal to its 

own inertia to accelerate properly. Any 

resonances developed in the motor are at their 

worst in a no-load condition. 

10. Why is motor sizing important, why not just go 

with a larger motor? 

If the motor’s rotor inertia is the majority of the 

load, any resonances may become more 

pronounced. Also, productivity would suffer as 

excessive time would be required to accelerate 

the larger rotor inertia. Smaller may be better. 

11. What are the options for eliminating 

resonance? 

This would most likely happen with full step 

systems. Adding inertia would lower the 

resonant frequency. Friction would tend to 

dampen the modulation. Start/stop velocities 

higher than the resonant point could be used. 

Changing to half step operation would greatly 

help. Ministepping and microstepping also 

greatly minimize any resonant vibrations. 

Viscous inertial dampers may also help. 

12. Why does the motor jump at times when it's 

turned on? 

This is due to the rotor having 200 natural 

detent positions. Movement can then be ±3.6° 

in either direction. 

13. Do the rotor and stator teeth actually mesh? 

No. While some designs used this type of 

harmonic drive, in this case, an air gap is very 

carefully maintained between the rotor and the 

stator. 

14. Does the motor itself change if a microstepping 

drive is used? 

The motor is still the standard 1.8° stepper. 

Microstepping is accomplished by 

proportioning currents in the drive (higher 

resolutions result). Ensure the motor’s 

inductance is compatible. 

15. A move is made in one direction, and then the 

motor is commanded to move the same 

distance but in the opposite direction. The 

move ends up short, why? 

Two factors could be influencing the results. 

First, the motor does have magnetic hysteresis 

that is seen on direction changes. This is in the 

area of 0.03°. Second, any mechanical 

backlash in the system to which the motor is 

coupled could also cause loss of motion. 

16. Why are some motors constructed as eightlead 

motors? 

This allows greater flexibility. The motor can be 

run as a six-lead motor with unipolar drives. 

With bipolar drives, the windings can then be 

connected in either series or parallel. 

17. What advantage do series or parallel 

connection windings give? 

With the windings connected in series, lowspeed 

torques are maximized. But this also 

gives the most inductance so performance at 

higher speeds is lower than if the windings 

were connected in parallel. 

18. Can a flat be machined on the motor shaft? 

Yes, but care must be taken to not damage 

the bearings. The motor must not be 

disassembled. Compumotor does not warranty 

the user’s work. 

19. How long can the motor leads be? 

For bipolar drives, 100 feet. For unipolar 

designs, 50 feet. Shielded, twisted pair cables 

are required. 

20. Can specialty motors, explosion-proof, 

radiation-proof, high-temperature, lowtemperature, 

vacuum-rated, or waterproof, be 

provided? 

Compumotor is willing to quote on most 

requirements with the exception of explosion 

proof. 

21. What are the options if an explosion-proof 

motor is needed? 

Installing the motor in a purged box should be 

investigated. 

A12


DC Brush Motors 

The history of the DC motor can be traced back to 

the 1830s, when Michael Faraday did extensive 

work with disc type machines (Fig. 1.21). 

Fig. 1.21 Simple disc motor 

Magnet 

N 

Conductive Disc 

Brush 

S 

Practical Considerations 

The problem now is that of using this force to 

produce the continuous torque required in a 

practical motor. 

To achieve maximum performance from the motor, 

the maximum number of conductors must be 

placed in the magnetic field, to obtain the greatest 

possible force. In practice, this produces a cylinder 

of wire, with the windings running parallel to the axis 

of the cylinder. A shaft is placed down this axis to 

act as a pivot, and this arrangement is called the 

motor armature (Fig. 1.23). 

Fig. 1.23 DC motor armature 

Resultant 

Field Due to 

Armature 

Current 

Shaft 


This design was quickly improved, and by the end 

of the 19th century the design principles of DC 

motors had become well established. 

About that time; however, AC power supply 

systems came into general use and the popularity 

of the DC motor declined in favor of the less 

expensive AC induction motor. More recently, the 

particular characteristics of DC motors, notably high 

starting torque and controllability, have led to their 

application in a wide range of systems requiring 

accurate control of speed and position. This 

process has been helped by the development of 

sophisticated modern drive and computer control 

systems. 

Principles 

It is well known that when a current-carrying 

conductor is placed in a magnetic field, it 

experiences a force (Fig. 1.22). 

Fig. 1.22 Force on a conductor in a 

magnetic field 

Magnetic Field (B) 

Stator Field 

Armature 

Direction 

of Current 

Into Page 

As the armature rotates, so does the resultant 

magnetic field. The armature will come to rest with 

its resultant field aligned with that of the stator field, 

unless some provision is made to constantly 

change the direction of the current in the individual 

armature coils. 

Commutation 

The force that rotates the motor armature is the 

result of the interaction between two magnetic 

fields (the stator field and the armature field). To 

produce a constant torque from the motor, these 

two fields must remain constant in magnitude and 

in relative orientation. 

Fig. 1.24 Electrical arrangement of the armature 

2 

Conductor 

Carrying 

Current (I) 

(Into Page) 

Current 

In 

1 3 

Out 

Force (F) 

6 

4 

Force on Conductor F = I x B 

5 

The force acting on the conductor is given by: 

F = I x B 

where B = magnetic flux density and I = current 

If this single conductor is replaced by a large 

number of conductors (i.e., a length of wire is 

wound into a coil), the force per unit length is 

increased by the number of turns in the coil. This is 

the basis of a DC motor. 

This is achieved by constructing the armature as a 

series of small sections connected in sequence to 

the segments of a commutator (Fig 1.24). Electrical 

connection is made to the commutator by means of 

two brushes. It can be seen that if the armature 

rotates through 1/6 of a revolution clockwise, the 

current in coils 3 and 6 will have changed direction. 

As successive commutator segments pass the 

brushes, the current in the coils connected to those 

segments changes direction. This commutation or 

switching effect results in a current flow in the 

A13


armature that occupies a fixed position in space, 

independent of the armature rotation, and allows 

the armature to be regarded as a wound core with 

an axis of magnetization fixed in space. This gives 

rise to the production of a constant torque output 

from the motor shaft. 

The axis of magnetization is determined by the 

position of the brushes. If the motor is to have similar 

characteristics in both directions of rotation, the 

brush axis must be positioned to produce an axis of 

magnetization that is at 90° to the stator field. 

DC Motor Types 

Several different types of DC motor are currently 

in use. 

Iron cored. (Fig. 1.25). This is the most common 

type of motor used in DC servo systems. It is made 

up of two main parts; a housing containing the field 

magnets and a rotor made up of coils of wire 

wound in slots in an iron core and connected to a 

commutator. Brushes, in contact with the 

commutator, carry current to the coils. 

Fig. 1.25 Iron-cored motor 

Commutator 

Brushes 

Stator Magnets 

Rotor Winding 

Moving coil. There are two principle forms of this 

type of motor. 1. The “printed” motor (Fig. 1.26), 

using a disc armature. 2. The “shell” type armature 

(Fig. 1.27). 

Since these types of motors have no moving iron in 

their magnetic field, they do not suffer from iron 

losses. Consequently, higher rotational speeds can 

be obtained with low power inputs. 

Fig. 1.26 Disc-armature “printed” motor 

Motion 

Brushless. The major limiting factor in the 

performance of iron-cored motors is internal 

heating. This heat escapes through the shaft and 

bearings to the outer casing, or through the airgap 

between the armature and field magnets and from 

there to the casing. Both of these routes are 

thermally inefficient, so cooling of the motor 

armature is very poor. 

Fig. 1.28 Brushless motor 

Backiron 

Return 

Path 

Stator 

Lam Teeth 

N 

In the brushless motor, the construction of the iron 

cored motor is turned inside out, so that the rotor 

becomes a permanent magnet and the stator 

becomes a wound iron core. 

The current-carrying coils are now located in the 

housing, providing a short, efficient thermal path to 

the outside air. Cooling can further be improved by 

finning the outer casing and blowing air over it if 

necessary (to effectively cool an iron-cored motor, it 

is necessary to blow air through it.) The ease of 

cooling the brushless motor allows it to produce a 

much higher power in relation to its size. 

The other major advantage of brushless motors is 

their lack of a conventional commutator and brush 

gear. These items are a source of wear and 

potential trouble and may require frequent 

maintenance. By not having these components, the 

brushless motor is inherently more reliable and can 

be used in adverse environmental conditions. 

To achieve high torque and low inertia, brushless 

motors do require rare earth magnets that are 

much more expensive than conventional ceramic 

magnets. The electronics necessary to drive a 

brushless motor are also more complex than for a 

brush motor. A more thorough explanation of 

brushless motors is provided on page A17. 

S 

Magnets 

S 

N 

Windings 

Motion 

Armature 

(Hollow cup, shaped 

conductor array) 

Permanent magnet 

(8 pole) 

Fig. 1.27 Shell-armature motor 

Air gap 

Core 

S 

S 

Magnet pole 

Flux path 

Losses in DC Motors 

DC motors are designed to convert electrical power 

into mechanical power and as a consequence of 

this, during periods of deceleration or if externally 

driven, will generate electrical power. However, all 

the input power is not converted into mechanical 

power due to the electrical resistance of the 

armature and other rotational losses. These losses 

give rise to heat generation within the motor. 

Diagrams courtesy of Electro-Craft Ltd. 

A14


Motor losses can be divided into two areas: Those 

that depend on the load and those that depend on 

speed (Fig. 1.29). 

Fig. 1.29 Losses in a DC motor 

Winding 

losses 

Iron 

losses 

Load 

Friction 

losses 

Motor losses 

Brush 

losses 

Speed 

Short-cut 

circuit losses 

Winding losses. These are caused by the electrical 

resistance of the motor windings and are equal to 

I 2 R (where I = armature current and R = armature 

resistance). 

As the torque output of the motor increases, I 

increases, which gives rise to additional losses. 

Consideration of winding losses is very important 

since heating of the armature winding causes an 

increase in R, which results in further losses and 

heating. This process can destroy the motor if the 

maximum current is not limited. Furthermore, at 

higher temperatures, the field magnets begin to 

lose their strength. Hence, for a required torque 

output the current requirement becomes greater. 

Brush contact losses. These are fairly complex to 

analyze since they depend upon several factors that 

will vary with motor operation. In general, brush 

contact resistance may represent a high proportion 

of the terminal resistance of the motor. The result of 

this resistance will be increased heating due to I 2 R 

losses in the brushes and contact area. 

Iron losses. Iron losses are the major factor in 

determining the maximum speed that may be 

attained by an iron-cored motor. These fall into two 

categories: 

• Eddy current losses are common in all 

conductive cored components experiencing a 

changing magnetic field. Eddy currents are 

induced into the motor armature as it undergoes 

changes in magnetization. These currents are 

speed-dependent and have a significant heating 

effect at high speeds. In practice, eddy currents 

are reduced by producing the armature core as a 

series of thin, insulated sections or laminations, 

stacked to produce the required core length. 

• Hysteresis losses are caused by the resistance 

of the core material to constant changes of 

magnetic orientation, giving rise to additional heat 

generation, which increases with speed. 

Friction losses. These are associated with the 

mechanical characteristics of the motor and arise 

from brush friction, bearing friction, and air 

resistance. These variables will generate heat and 

will require additional armature current to offset this 

condition. 

Short circuit currents. As the brushes slide over 

the commutator, the brush is in contact with two 

commutator segments for a brief period. During this 

period, the brush will short out the coil connected 

to those segments (Fig. 1.30). This condition 

generates a torque that opposes the main driving 

torque and increases with motor speed. 

Fig. 1.30 Generation of short-circuit currents 

All these losses will contribute heat to the motor 

and it is this heating that will ultimately limit the 

motor application. 

Other Limiting Considerations 

Torque ripple. The requirement for constant torque 

output from a DC motor is that the magnetic fields 

due to the stator and the armature are constant in 

magnitude and relative orientation, but this ideal is 

not achieved in practice. As the armature rotates, 

the relative orientation of the fields will change 

slightly and this will result in small changes in torque 

output called “torque ripple” (Fig. 1.31). 

Fig. 1.31 Torque ripple components 

Torque 

Commutator 

Winding 

Brush 

Torque Ripple 

Steady Torque 

O/P 

Time 

This will not usually cause problems at high speeds 

since the inertia of the motor and the load will tend 

to smooth out the effects, but problems may arise 

at low speeds. 

Motors can be designed to minimize the effects of 

torque ripple by increasing the number of windings, 

or the number of motor poles, or by skewing the 

armature windings. 


A15


Demagnetization. The permanent magnets of a 

DC motor field will tend to become demagnetized 

whenever a current flows in the motor armature. 

This effect is known as “armature reaction” and will 

have a negligible effect in normal use. Under high 

load conditions, however, when motor current may 

be high, the effect will cause a reduction in the 

torque constant of the motor and a consequent 

reduction in torque output. 

Above a certain level of armature current, the field 

magnets will become permanently demagnetized. 

Therefore, it is important not to exceed the 

maximum pulse current rating for the motor. 

Mechanical resonances and backlash. It might 

normally be assumed that a motor and its load, 

including a tachometer or position encoder, are all 

rigidly connected together. This may, however, not 

be the case. 

It is important for a bi-directional drive or positioning 

system that the mechanics are free from backlash, 

otherwise, true positioning will present problems. 

In high-performance systems, with high 

accelerations, interconnecting shafts and couplings 

may deflect under the applied torque, such that the 

various parts of the system may have different 

instantaneous velocities that may be in opposite 

directions. Under certain conditions, a shaft may go 

into torsional resonance (Fig. 1.32). 

Fig. 1.32 Torsional oscillation 

Shaft 

Load Motor Tach 

Back emf 

As described previously, a permanent magnet DC 

motor will operate as a generator. As the shaft is 

rotated, a voltage will appear across the brush 

terminals. This voltage is called the back 

electromotive force (emf) and is generated even 

when the motor is driven by an applied voltage. The 

output voltage is essentially linear with motor speed 

and has a slope that is defined as the motor voltage 

constant, K E 

(Fig. 1.33). K E 

is typically quoted in 

volts per 1000 rpm. 

Fig. 1.33 Back-emf characteristic 

Output 

volts 

Motor Equations 

Unlike a step motor, the DC brush motor exhibits 

simple relationships between current, voltage, 

torque and speed. It is therefore worth examining 

these relationships as an aid to the application of 

brush motors. 

The application of a constant voltage to the 

terminals of a motor will result in its accelerating to 

attain a steady final speed (n). Under these 

conditions, the voltage (V) applied to the motor is 

opposed by the back emf (nK E 

) and the resultant 

voltage drives the motor current (I) through the 

motor armature and brush resistance (R s 

). 

The equivalent circuit of a DC motor is shown in 

Fig. 1.34. 

Fig. 1.34 DC motor equivalent circuit 

v 

I 

R s 

R L V g 

R s 

= motor resistance 

L = winding inductance 

V g 

= back emf and 

R L 

represents magnetic losses. 

The value of R L 

is usually large and so can be 

ignored, as can the inductance L, which is generally 

small. 

If we apply a voltage (V) to the motor and a current 

(I) flows, then: 

V = IR s 

+ V g 

but V g 

= nK E 

so V = IR s 

+ nK E 

(1) 

This is the electrical equation of the motor. 

If K T 

is the torque constant of the motor (typically in 

oz/in per Amp), then the torque generated by the 

motor is given by: 

T = IK T 

(2) 

The opposing torque due to friction (T F 

) and viscous 

damping (K D 

) is given by: 

T M 

= T F 

+ nK D 

If the motor is coupled to a load T L 

,then at 

constant speed: 

T = T L 

+ T F 

+ nK D 

(3) 

Equations (1), (2) and (3) allow us to calculate the 

required current and drive voltage to meet given 

torque and speed requirements. The values of K T 

, 

K E 

, etc. are given in the motor manufacturer’s data. 

L 

Shaft speed 

A16


Brushless Motors 

Before we talk about brushless motors in detail, 

let’s clear up a few points about terminology. The 

term “brushless” has become accepted as referring 

to a particular variety of servo motor. Clearly a step 

motor is a brushless device, as is an AC induction 

motor (in fact, the step motor can form the basis of 

a brushless servo motor, often called a hybrid 

servo, which is discussed later). However, the socalled 

“brushless” motor has been designed to have 

a similar performance to the DC brush servo 

without the limitations imposed by a mechanical 

commutator. 

Within the brushless category are two basic motor 

types: trapezoidal and sine wave motors. The 

trapezoidal motor is really a brushless DC servo, 

whereas the sine wave motor bears a close 

resemblance to the AC synchronous motor. To fully 

explain the difference between these motors, we 

must review the evolution of the brushless motor. 

Fig. 1.35 Conventional DC brush motor 

Brushless Motor Operation 

To turn this motor into a brushless design, we must 

start by eliminating the windings on the rotor. This 

can be achieved by turning the motor inside out. In 

other words, we make the permanent magnet the 

rotating part and put the windings on the stator 

poles. We still need some means of reversing the 

current automatically – a cam-operated reversing 

switch could be made to do this job (Fig. 1.36). 

Obviously such an arrangement with a mechanical 

switch is not very satisfactory, but the switching 

capability of non-contacting devices tends to be 

very limited. However, in a servo application, we 

will use an electronic amplifier or drive which can 

also be used to do the commutation in response to 

low-level signals from an optical or hall-effect 

sensor (see Fig. 1.37). This component is referred 

to as the commutation encoder. So unlike the DC 

brush motor, the brushless version cannot be 

driven by simply connecting it to a source of direct 

current. The current in the external circuit must be 

reversed at defined rotor positions. Hence, the 

motor is actually being driven by an alternating 

current. 

Fig. 1.37 Brushless motor 


N 

S 

+ 

- 

Commutator 

A simple conventional DC brush motor (Fig. 1.35) 

consists of a wound rotor that can turn within a 

magnetic field provided by the stator. If the coil 

connections were made through slip rings, this 

motor would behave like a step motor (reversing the 

current in the rotor would cause it to flip through 

180°). By including the commutator and brushes, 

the reversal of current is made automatically and 

the rotor continues to turn in the same direction. 

N 

S 

Commutation 

Encoder 

Drive 

Fig. 1.36 “Inside out” DC motor 

N 

S 

Reversing Switch 

Going back to the conventional brush motor, a 

rotor consisting of only one coil will exhibit a large 

torque variation as it rotates. In fact, the 

characteristic will be sinusoidal, with maximum 

torque produced when the rotor field is at right 

angles to the stator field and zero torque at the 

commutation point (see Fig. 1.38). A practical DC 

motor has a large number of coils on the rotor, 

each one connected not only to its own pair of 

commutator segments but to the other coils as 

well. In this way, the chief contribution to torque is 

made by a coil operating close to its peak-torque 

position. There is also an averaging effect produced 

by current flowing in all the other coils, so the 

resulting torque ripple is very small. 

+ 

- 

A17


Fig. 1.38 3-phase brushless motor 

A1 

B2 

C2 

A1 

Fig. 1.41 Position of rotor at commutation point 

Stator 

Field 

A1 

C1 

A2 

B1 

C1 

C2 

B2 

A2 

B1 

B2 

C1 

N 

N 

S 

S 

C2 

B1 

We would like to reproduce a similar situation in the 

brushless motor; however, this would require a 

large number of coils distributed around the stator. 

This may be feasible, but each coil would require its 

own individual drive circuit. This is clearly 

prohibitive, so a compromise is made. A typical 

brushless motor has either two or three sets of coils 

or “phases” (see Fig. 1.38). The motor shown in Fig. 

1.38 is a two-pole, three-phase design. The rotor 

usually has four or six rotor poles, with a 

corresponding increase in the number of stator 

poles. This doesn’t increase the number of 

phases—each phase has its turns distributed 

between several stator poles. 

Fig. 1.39 Position-torque characteristic 

C1 

B2 

C1 

A2 

A1 

I 

C2 A2 

B2 

A1 

N 

N 

S 

S 

C2 

B1 

B1 

Stator 

Field 

A2 

Torque 

+ 

- 

0° 

90° 

180° 

Direction of Rotor 

Field Relative 

to Stator Field 

C1 

I 

A1 

C2 A2 

B2 

B1 

Fig. 1.40 Stator field positions for different 

phase currents 

B2 

C1 

S 

S 

A1 

N 

N 

C2 

B1 

Stator 

Field 

A2 

Stator 

Field 

S 

N 

Rotation 

A1 

60° 

120° 

C1 

C2 A2 

B2 

I 

B1 

Stator 

Field 

Average Lag = 90° 

Rotor 

Field 

The torque characteristic in Fig. 1.39 indicates that 

maximum torque is produced when the rotor and 

stator fields are at 90° to each other. Therefore, to 

generate constant torque we would need to keep 

the stator field a constant 90° ahead of the rotor. 

Limiting the number of phases to three means that 

we can only advance the stator field in increments 

of 60° (Fig. 1.40). This means we must keep the 

stator field in the same place during 60° of shaft 

rotation. So we can’t maintain a constant 90° 

torque angle, but we can maintain an average of 

90° by working between 60° and 120°. Fig. 1.41 

shows the rotor position at a commutation point. 

When the torque angle has fallen to 60°, the stator 

field is advanced from 1 to 2 so that the angle now 

increases to 120°, and it stays here during the next 

60° of rotation. 

A18

The Trapezoidal Motor 

With a fixed current level in the windings, the use of 

this extended portion of the sinusoidal torque 

characteristic gives rise to a large degree of torque 

ripple. We can minimize the effect by manipulating 

the motor design to “flatten out” the characteristic – 

to make it trapezoidal, (Fig. 1.42). In practice, this is 

not very easy to do, so some degree of non-linearity 

will remain. The effect of this tends to be a slight 

“kick” at the commutation points, which can be 

noticeable when the motor is running very slowly. 

Fig. 1.42 Trapezoidal motor characteristic 

60° 

Torque ripple resulting from non-linearity in the 

torque characteristic tends to produce a velocity 

modulation in the load. However, in a system using 

velocity feedback the velocity loop will generally 

have a high gain. This means that a very small 

increase in velocity will generate a large error signal, 

reducing the torque demand to correct the velocity 

change. So in practice, the output current from the 

amplifier tends to mirror the torque characteristic 

(Fig. 1.43) so that the resulting velocity modulation 

is extremely small. 

Fig. 1.43 Current profile in velocity-controlled 

servo 

Current 

The Sine Wave Motor 

In the sine wave motor (sometimes called an AC 

brushless servo), no attempt is made to modify the 

basic sinusoidal torque characteristic. Such a motor 

can be driven like an AC synchronous motor by 

applying sinusoidal currents to the motor windings. 

These currents must have the appropriate phase 

displacement, 120° in the case of the three-phase 

motor. We now need a much higher resolution 

device to control the commutation if we want 

smooth rotation at low speeds. The drive needs to 

generate 3 currents that are in the correct 

relationship to each other at every rotor position. So 

rather than the simple commutation encoder 

generating a handful of switching points, we now 

need a resolver or high-resolution optical encoder. 

In this way, it’s possible to maintain a 90° torque 

60° 

(Torque) 

angle very accurately, resulting in very smooth lowspeed 

rotation and negligible torque ripple. A 

simplified explanation of why the sine wave motor 

produces constant torque is given in the next 

section. 

The drive for a sine wave motor is more complex 

than for the trapezoidal version. We need a 

reference table from which to generate the 

sinusoidal currents, and these must be multiplied by 

the torque demand signal to determine their 

absolute amplitude. With a star-connected threephase 

motor, it is sufficient to determine the 

currents in two of the windings—this will 

automatically determine what happens in the third. 

As previously mentioned, the sine wave motor 

needs a high-resolution feedback device. However, 

this device can also provide position and velocity 

information for the controller. 

Why constant torque from a sine wave 

motor? 

To understand this, it’s easier to think in terms of a 

two-phase motor. This has just two sets of 

windings that are fed with sinusoidal currents at 90° 

to each other. If we represent shaft position by an 

angle θ, then the currents in the two windings are of 

the form Isinθ and Icosθ. 

Going back to our original motor model, you’ll 

remember that the fundamental torque 

characteristic of the motor is also sinusoidal. So for 

a given current I, the instantaneous torque value 

looks like: 

T = I K T 

sinθ 

Where K T 

is the motor torque constant 

By making the motor current sinusoidal as well, and 

in phase with the motor torque characteristic, the 

torque generated by one phase becomes: 

T 1 

= (I sinθ) K T 

sinθ 

= I K T 

sin 2 θ 

Similarly, the torque produced by the other phase 

is: 

T 2 

= I K T 

cos 2 θ 

The total torque is: 

T 1 

+ T 2 

= I K T 

(sin 2 θ + cos 2 θ) 

but: sin 2 θ + cos 2 θ = 1 for any value of θ 

therefore: T 1 

+ T 2 

= IK T 

So for sinusoidal phase currents with a constant 

amplitude, the resultant torque is also constant and 

independent of shaft position. 

For this condition to remain true, the drive currents 

must accurately follow a sine-cosine relationship. 

This can only occur with a sufficiently high 

resolution in the encoder or resolver used for 

commutation. 



A19


The Hybrid Servo 

In terms of their basic operation, the step motor 

and the brushless servo motor are identical. They 

each have a rotating magnet system and a wound 

stator. The only difference is that one has more 

poles than the other, typically two or three polepairs 

in the brushless servo and 50 in the stepper. 

You could use a brushless servo as a stepper – not 

a very good one, since the step angle would be 

large. But by the same token, you can also use a 

stepper as a brushless servo by fitting a feedback 

device to perform the commutation. Hence the 

“hybrid servo”, so called because it is based on a 

hybrid step motor (Fig. 1.44). These have also been 

dubbed ‘stepping servos’ and ‘closed-loop 

steppers’. We prefer not to use the term ‘stepper’ 

at all since such a servo exhibits none of the 

operating characteristics of a step motor. 

The hybrid servo is driven in precisely the same 

fashion as the brushless motor. A two-phase drive 

provides sine and cosine current waveforms in 

response to signals from the feedback device. This 

device may be an optical encoder or a resolver. Since 

the motor has 50 pole pairs, there will be 50 electrical 

cycles per revolution. This conveniently permits a 50- 

cycle resolver to be constructed from the same rotor 

and stator laminations as the motor itself. 

A hybrid servo generates approximately the same 

torque output as the equivalent step motor, 

assuming the same drive current and supply 

voltage. However, the full torque capability of the 

motor can be utilized since the system is operating 

in a closed loop (with an open-loop step motor, it is 

always necessary to allow an adequate torque 

margin). The hybrid servo system will be more 

expensive than the equivalent step motor systems, 

but less costly than a brushless servo. As with the 

step motor, continuous operation at high speed is 

not recommended since the high pole count results 

in greater iron losses at high speeds. A hybrid servo 

also tends to run quieter and cooler than its step 

motor counterpart; since it is a true servo, power is 

only consumed when torque is required and 

normally no current will flow at standstill. Lowspeed 

smoothness is vastly improved over the 

open-loop full step motor. 

It is worth noting that the hybrid servo is entirely 

different from the open-loop step motor operated in 

‘closed loop’ or ‘position tracking’ mode. In position 

tracking mode, an encoder measures the load 

movement and final positioning is determined by 

encoder feedback. While this technique can provide 

high positioning accuracy and eliminates 

undetected position loss, it does not allow full 

torque utilization, improve smoothness or reduce 

motor heating. 

Fig. 1.44 Hybrid servo motor with resolver feedback 

Stator 

Rotor 

Two MS Style 

Connectors 

Position Feedback 

Device Rotor 

Housing 

Bearing 

Position Feedback 

Device Stator 

A20

Direct Drive Motors 

Motor Construction and Operation 

Direct drive systems couple the system’s load 

directly to the motor without the use of belts or 

gears. In some situations, brushed or brushlesss 

servo motors may lack adequate torque or 

resolution to satisfy some applications’ needs. 

Therefore, mechanical means, such as gear 

reduction systems to increase torque and 

resolution, are used to meet system requirements. 

The Dynaserv Direct Drive can provide very high 

torque in a modest package size and solves many 

of the performance issues of the gear reducer. All in 

a system that is as easy to use as a stepping 

motor. 

Fig. 1.45 below shows the construction of the 

Dynaserv DM Series direct drive motor compared 

to a conventional motor with a gear reducer. The 

gear reducer relies on large amounts of frictional 

contact to reduce the speed of the load. This 

gearing effectively increases torque and resolution 

but sacrifices speed and accuracy. The direct drive 

motor is brushless and gearless so it eliminates 

friction from its power transmission Since the 

feedback element is coupled directly to the load, 

system accuracy and repeatability are greatly 

increased and backlash is eliminated. 

Fig. 1.45 Construction comparison 

The motor contains precision bearings, magnetic 

components and integral feedback in a compact 

motor package (see Fig. 1.46). The motor is an 

outer rotor type, providing direct motion of the 

outside housing of the motor and thus the load. 

The cross roller bearings that support the rotor 

have high stiffness, to allow the motor to be 

connected directly to the load. In most cases, it is 

not necessary to use additional bearings or 

connecting shafts. 

Fig. 1.46 

Hub 

Expanded motor view— 

Dynaserv Model DM 

Encoder Plate 

LED Kit 

Clamp Ring 

Housing Kit 

Retaining 

Ring 

Stator Core 

Rotor Core 

Core 


Housing 

PDA Kit 

Encoder 


Gear 

Reducer 

Conventional Motor 

Encoder 

DC/AC 


Encoder PCB 

Slit Plate 

Stator 

Element 

Rotor Core 

Bearing 

Direct Drive Motor 

Rotating 

Element 

Stator Core 

The torque is proportional to the square of the sum 

of the magnetic flux (Ø m 

), of the permanent magnet 

rotor and the magnetic flux (Ø c 

), of the stator 

windings. See Fig. 1.47. High torque is generated 

due to the following factors. First, the motor 

diameter is large. The tangential forces between 

rotor and stator act as a large radius, resulting in 

higher torque. Secondly, a large number of small 

rotor and stator teeth create many magnetic cycles 

per motor revolution. More working cycles means 

increased torque. 

Fig. 1.47 Dynaserv magnetic circuit 

Rotor 

Φ m 

Φ c 

Φ m 

Stator A 

Excitation 

Coil 

Permanent 

Magnet 

Stator B 

T 

A21


Direct Drive Motor Advantages 

High Precision 

Dynaserv motors eliminate the backlash or 

hysteresis inevitable in using any speed reducer. 

Absolute positioning of 30 arc-sec is typical with a 

repeatability of ±2 arc-sec. 

Faster Settling Time 

The Dynaserv system reduces machine cycle times 

by decreasing settling times. This result is realized 

because of the “gearless” design and sophisticated 

“I-PD” control algorithm. 

High Torque at High Speed 

The torque/speed curve of the various Dynaserv 

models is very flat. This results in high acceleration 

at high speeds (4.0 rps) with good controllability. 

Optimum Tuning 

Dynaserv systems offer the user a tuning mode that 

simplifies the setting of optimum parameters for the 

actual load. Turning on the “test” switch on the 

front panel of the drive produces a test signal. 

Using an oscilloscope, the gain settings are quickly 

optimized by adjusting the digital switches and 

potentiometers on the front panel. 

Clean Operation 

The Dynaserv system is brushless and gearless, 

which results in a maintenance-free operation. With 

preparation, the Dynaserv can operate in class 10 

environments. 

Smooth Rotation 

The very low velocity and torque ripple of the 

Dynaserv contribute to its excellent speed 

controllability with a more than 1:1,000 speed ratio. 

Fig. 1.48 Dynaserv velocity/torque ripple 

Speed Ripple 

(DM1150A) 

Torque Ripple 

(DM1015A) 

20 

Ripple (%) 

15 

10 

5 

3 

Conditions 

• Load 30 x Rotor Inertia 

• Rotation: CW 

• Speed Mode 

Torque (N • m) 

15.3 

15 

14.7 

5 

5% 

0.2 0.4 0.6 0.8 1.0 1.2 

Revolution (rps) 

0 

0° 

90° 180° 270° 360° 

Rotational Angle (degrees) 

A22

Stepping Motor Drives 

The stepper drive delivers electrical power to the 

motor in response to low-level signals from the 

control system. 

The motor is a torque-producing device, and this 

torque is generated by the interaction of magnetic 

fields. The driving force behind the stator field is the 

magneto-motive force (MMF), which is proportional 

to current and to the number of turns in the 

winding. This is often referred to as the amp-turns 

product. Essentially, the drive must act as a source 

of current. The applied voltage is only significant as 

a means of controlling the current. 

Input signals to the stepper drive consist of step 

pulses and a direction signal. One step pulse is 

required for every step the motor is to take. This is 

true regardless of the stepping mode. So the drive 

may require 200 to 101,600 pulses to produce one 

revolution of the shaft. The most commonly-used 

stepping mode in industrial applications is the halfstep 

mode in which the motor performs 400 steps 

per revolution. At a shaft speed of 1800 rpm, this 

corresponds to a step pulse frequency of 20kHz. 

The same shaft speed at 25,000 steps per rev 

requires a step frequency of 750 kHz, so motion 

controllers controlling microstep drives must be 

able to output a much higher step frequency. 

Fig. 2.1 Stepper drive elements 

Step 

Direction 

Translator 

Stepper Drive 

Elements 

Switch 

Set 

Phase 2 

Phase 1 


The simplest type of switch set is the unipolar 

arrangement shown in Fig. 2.2. It is referred to as a 

unipolar drive because current can only flow in one 

direction through any particular motor terminal. A 

bifilar-wound motor must be used since reversal of 

the stator field is achieved by transferring current to 

the second coil. In the case of this very simple 

drive, the current is determined only by the motor 

winding resistance and the applied voltage. 

Fig. 2.2 Basic unipolar drive 

1A 

1B 

TR1 TR2 TR3 TR4 

Such a drive will function perfectly well at low 

stepping rates, but as speed is increased, the 

torque will fall off rapidly due to the inductance of 

the windings. 

V+ 

0V 

2A 

Drive Technologies 

2B 



Direction 

The logic section of the stepper drive is often 

referred to as the translator. Its function is to 

translate the step and direction signals into control 

waveforms for the switch set (see Fig. 2.1). The 

basic translator functions are common to most 

drive types, although the translator is necessarily 

more complex in the case of a microstepping drive. 

However, the design of the switch set is the prime 

factor in determining drive performance, so we will 

look at this in more detail. 

A23


Inductance/Water Analogy 

For those not familiar with the property of 

inductance, the following water analogy may be 

useful (Fig. 2.3). An inductor behaves in the same 

way as a turbine connected to a flywheel. When the 

tap is turned on and pressure is applied to the inlet 

pipe, the turbine will take time to accelerate due to 

the inertia of the flywheel. The only way to increase 

the acceleration rate is to increase the applied 

pressure. If there is no friction or leakage loss in the 

system, acceleration will continue indefinitely for as 

long as the pressure is applied. In a practical case, 

the final speed will be determined by the applied 

pressure and by friction and the leakage past the 

turbine blades. 

Fig. 2.3 Inductance water analogy 

Pressure Equivalent 

to Applied Voltage 

I 

1-Way 

Valve 

Tap 

Kinetic Energy 

of Flywheel 

Equivalent to 

Energy Stored 

in Magnetic Field 

Water Flow 

Equivalent 

to Current 

Turbine 

Higher Pressure 

Causes Flywheel 

to Accelerate 

More Rapidly 

Voltage 

(Pressure) 

Reverse Pressure 

When Flow 

Interrupted 

Current 

(Flow) 

Applying a voltage to the terminals of an inductor 

produces a similar effect. With a pure inductance 

(i.e., no resistance), the current will rise in a linear 

fashion for as long as the voltage is applied. The 

rate of rise of current depends on the inductance 

and the applied voltage, so a higher voltage must 

be applied to get the current to rise more quickly. In 

a practical inductor possessing resistance, the final 

current is determined by the resistance and the 

applied voltage. 

Once the turbine has been accelerated up to 

speed, stopping it again is not a simple matter. The 

kinetic energy of the flywheel has to be dissipated, 

and as soon as the tap is turned off, the flywheel 

drives the turbine like a pump and tries to keep the 

water flowing. This will set up a high pressure 

across the inlet and outlet pipes in the reverse 

direction. The equivalent energy store in the 

inductor is the magnetic field. As this field 

collapses, it tries to maintain the current flow by 

generating a high reverse voltage. 

By including a one-way valve across the turbine 

connections, the water is allowed to continue 

circulating when the tap is turned off. The energy 

stored in the flywheel is now put to good use in 

maintaining the flow. We use the same idea in the 

recirculating chopper drive, in which a diode allows 

the current to recirculate after it has built up. 

Going back to our simple unipolar drive, if we look 

at the way the current builds up (Fig. 2.4) we can 

see that it follows an exponential shape with its final 

value set by the voltage and the winding resistance. 

To get it to build up more rapidly, we could increase 

the applied voltage, but this would also increase the 

final current level. A simple way to alleviate this 

problem is to add a resistor in series with the motor 

to keep the current the same as before. 

A24

R-L Drive 

The principle described in the Inductance/Water 

Analogy (p. A24) is applied in the resistance-limited 

(R-L) drive see Fig. 2.4. Using an applied voltage of 

10 times the rated motor voltage, the current will 

reach its final value in one tenth of the time. If you 

like to think in terms of the electrical time constant, 

this has been reduced from L/R to L/10R, so we’ll 

get a useful increase in speed. However we’re 

paying a price for this extra performance. Under 

steady-state conditions, there is 9 times as much 

power dissipated in the series resistor as in the 

motor itself, producing a significant amount of heat. 

Furthermore, the extra power must all come from 

the DC power supply, so this must be much larger. 

R-L drives are therefore only suited to low-power 

applications, but they do offer the benefits of 

simplicity, robustness and low radiated interference. 

Fig. 2.4 Principle of the R-L drive 

L 

R 

V 

I 

V 

I 

V 

R 

Bipolar Drive 

An obvious possibility is the simple circuit shown in 

Fig. 2.6, in which two power supplies are used 

together with a pair of switching transistors. 

Current can be made to flow in either direction 

through the motor coil by turning on one transistor 

or the other. However, there are distinct drawbacks 

to this scheme. First, we need two power supplies, 

both of which must be capable of delivering the 

total current for both motor phases. When all the 

current is coming from one supply the other is 

doing nothing at all, so the power supply utilization 

is poor. Second, the transistors must be rated at 

double the voltage that can be applied across the 

motor, requiring the use of costly components. 

Fig. 2.6 Simple bipolar drive 

V+ 

TR1 

TR2 

0V 



L R R I 

2V 

Unipolar Drive 

Fig. 2.5 Basic unipolar drive 

V+ 

1A 

1B 

2A 

2V 

I 

2V 

2R 

2B 

The standard arrangement used in bipolar motor 

drives is the bridge system shown in Fig. 2.7. 

Although this uses an extra pair of switching 

transistors, the problems associated with the 

previous configuration are overcome. Only one 

power supply is needed and this is fully utilized; 

transistor voltage ratings are the same as that 

available for driving the motor. In low-power 

systems, this arrangement can still be used with 

resistance limiting as shown in Fig. 2.8. 

Fig. 2.7 Bipolar bridge 

V- 

V+ 

TR1 TR2 TR3 TR4 

TR1 

TR3 

0V 

A drawback of the unipolar drive is its inability to 

utilize all the coils on the motor. At any one time, 

there will only be current flowing in one half of each 

winding. If we could utilize both sections at the 

same time, we could get a 40% increase in ampturns 

for the same power dissipation in the motor. 

To achieve high performance and high efficiency, 

we need a bipolar drive (one that can drive current 

in either direction through each motor coil) and a 

better method of current control. Let’s look first at 

how we can make a bipolar drive. 

TR2 

TR4 

0V 

Fig. 2.8 Bipolar R-L drive 

V+ 

TR1 

TR3 

TR2 

TR4 

0V 

A25


Recirculating Chopper Drive 

The method of current control used in most stepper 

drives is the recirculating chopper (Fig. 2.9). This 

approach incorporates the four-transistor bridge, 

recirculation diodes, and a sense resistor. The 

resistor is of low value (typically 0.1 ohm) and 

provides a feedback voltage proportional to the 

current in the motor. 

Fig. 2.9 Recirculating chopper drive 

+ + 

TR1 

TR3 

TR1 

TR3 

TR2 

D1 

D2 

TR4 

TR2 

D1 

D2 

TR4 

Vs 

Vs 

Rs 

Rs 

– – 

Injection 

Recirculation 

Motor current 

Current is injected into the winding by turning on 

one top switch and one bottom switch, and this 

applies the full supply voltage across the motor. 

Current will rise in an almost linear fashion and we 

can monitor this current by looking across the 

sense resistor. When the required current level has 

been reached, the top switch is turned off and the 

stored energy in the coil keeps the current 

circulating via the bottom switch and the diode. 

Losses in the system cause this current to slowly 

decay, and when a pre-set lower threshold is 

reached, the top switch is turned back on and the 

cycle repeats. The current is therefore maintained at 

the correct average value by switching or 

“chopping” the supply to the motor. 

This method of current control is very efficient 

because very little power is dissipated in the 

switching transistors other than during the transient 

switching state. Power drawn from the power 

supply is closely related to the mechanical power 

delivered by the shaft (unlike the R-L drive, which 

draws maximum power from the supply at 

standstill). 

A variant of this circuit is the regenerative chopper. 

In this drive, the supply voltage is applied across 

the motor winding in alternating directions, causing 

the current to ramp up and down at approximately 

equal rates. This technique tends to require fewer 

components and is consequently lower in cost, 

however, the associated ripple current in the motor 

is usually greater and increases motor heating. 

A26

Regeneration and Power Dumping 

Like other rotating machines with permanent 

magnets, the step motor will act as a generator 

when the shaft is driven mechanically. This means 

that the energy imparted to the load inertia during 

acceleration is returned to the drive during 

deceleration. This will increase the motor current 

and can damage the power switches if the extra 

current is excessive. A threshold detector in the 

drive senses this increase in current and 

momentarily turns off all the bridge transistors 

(Fig. 2.10). There is now a path for the regenerated 

current back to the supply capacitor, where it 

increases the supply voltage. During this phase, the 

current is no longer flowing through the sense 

resistors, so the power switches must be turned on 

again after a short period (typically 30µS) for 

conditions to be reassessed. If the current is still too 

high, the drive returns to the regenerative state. 

Fig. 2.10 Current flow during regeneration 

The circuit of a simple power dump is shown in 

Fig. 2.11. A rectifier and capacitor fed with AC from 

the supply transformer provide a reference voltage 

equal to the peak value of the incoming AC. Under 

normal conditions this will be the same as the drive 

supply voltage. During excess regeneration, the 

drive supply voltage will rise above this reference, 

and this will turn on the dump transistor connecting 

the 33-ohm resistor across the power supply. 

When the supply voltage has decreased sufficiently, 

the transistor is turned back off. Although the 

instantaneous current flowing through the dump 

resistor may be relatively high, the average power 

dissipated is usually small since the dump period is 

very short. In applications where the regenerated 

power is high, perhaps caused by frequent and 

rapid deceleration of a high inertia, a supplementary 

high-power dump resistor may be necessary. 

Fig. 2.11 Power dump circuit 



+V 

HV 

Power 

dump 

circuit 

Power 

supply 

capacitor 

AC 

in 

D1 

R2 

1K 

D2 

TR1 

R5 

R6 

33Ω 

10W 

100K 

R3 

A small increase in supply voltage during 

regeneration is acceptable, but if the rise is too 

great the switches may be damaged by overvoltage 

rather than excessive current. To resolve 

this problem, we use a power dump circuit that 

dissipates the regenerated power. 

C1 

R1 

R4 

TR2 

0V 

A27


Stepper Drive Technology Overview 

Within the various drive technologies, there is a 

spectrum of performance. The uni-polar resistancelimited 

(R-L) drive is a relatively simple design, but it 

lacks shaft power performance and is very inefficient. 

A uni-polar system only uses half of the motor 

winding at any instant. A bi-polar design allows 

torque producing current to flow in all motor 

windings, using the motor more efficiently, but 

increasing the complexity of the drive. A bi-polar R-L 

drive improves shaft performance, but is still very 

inefficient—generating a lot of wasted heat. An 

alternative to resistance-limiting is to control current 

by means of chopper regulation. ␣A chopper 

regulator is very efficient since it does not waste 

power by dropping voltage through a resistor. 

However, good current control in the motor is 

essential to deliver optimum shaft power. Pulse width 

modulation (PWM) and threshold modulation are two 

types of chopper regulation techniques. PWM 

controls the average of the motor current and is very 

good for precise current control, while threshold 

modulation controls current to a peak level. 

Threshold modulation can be applied to a wider 

range of motors, but it does suffer greater loss of 

performance than PWM when the motor has a large 

resistance or long motor cables are used. Both 

chopper regulation techniques can use recirculating 

current control, which improves the power 

dissipation in the motor and drive and overall system 

efficiency. As system performance increases, the 

complexity and cost of the drive increases. 

Stepper drive technology has evolved—being driven 

by machine builders that require more shaft power in 

smaller packages, higher speed capability, better 

efficiency, and improved accuracy. One trend of the 

technology is towards microstepping, a technique 

that divides each full step of the motor into smaller 

steps. This is achieved electronically in the drive by 

proportioning the current between the motor 

windings. The higher the resolution, the more 

precision is required in the current control circuits. In 

its simplest form, a half-step system increases the 

resolution of a standard 1.8° full-step motor to 400 

steps/rev. Ministepping drives have more precise 

current control and can increase the resolution to 

4,000 steps/rev. Microstep drives typically have 

resolutions of 50,000 steps/rev, and in addition to 

improved current control, they often have 

adjustments to balance offsets between each phase 

of the motor and to optimize the current profile for 

the particular motor being used. 

Full-Step and Half-Step Systems 

Full-step and half-step systems do not have the 

resolution capability of the ministepping or 

microstepping systems. However, the drive 

technology is not as complex and the drives are 

relatively inexpensive. Full-step and half-step systems 

will not have the same low-speed smoothness as 

higher resolution systems. 

An inherent property of a stepper motor is its lowspeed 

resonance, which may de-synchronize a 

motor and cause position loss. Full-step and halfstep 

drives are more prone to resonance effects and 

this may limit their application in low-speed systems. 

Full-step and half-step systems can be operated at 

speeds above the motor’s resonant speed without 

loss of synchronization. For this reason, full-step and 

half-step systems are normally applied in high-speed, 

point-to-point positioning applications. In these types 

of applications, the machine designer is primarily 

concerned with selecting a motor/drive system 

capable of producing the necessary power output. 

Since power is the product of torque and speed, a 

high-torque system with low-speed capability may 

not produce as much power as a low-torque, highspeed 

system. Sizing the system for torque only may 

not provide the most cost-effective solution, selecting 

a system based on power output will make the most 

efficient use of the motor and drive. 

Step motor systems typically require the motor to 

accelerate to reach high speed. If a motor was 

requested to run instantaneously at 3000 rpm, the 

motor would stall immediately. At slow speeds, it is 

possible to start the motor without position loss by 

applying unramped step pulses. The maximum speed 

at which synchronization will occur without ramping is 

called the start/stop velocity. The start/stop velocity is 

inversely proportional to the square-root of the total 

inertia. The start/stop capability provides a benefit for 

applications that require high-speed point-to-point 

positioning—since the acceleration to the start/stop 

velocity is almost instantaneous, the move-time will 

be reduced. No additional time is required to 

accelerate the motor from zero to the start/stop 

velocity. While the move-time can be reduced, it is 

generally more complicated for the controller or 

indexer to calculate the motion profile and implement 

a start/stop velocity. In most applications, using start/ 

stop velocities will eliminate the need to run the motor 

at its resonant frequency and prevent desynchronization. 

Velocity 

Time 

Velocity 

Time 

Ministep Systems 

Applications that require better low-speed 

smoothness than a half-step system should 

consider using a microstepping or ministepping 

solution. Microstepping systems, with resolutions 

of 50,000 steps/rev, can offer exceptional 

smoothness, without requiring a gear-reducer. 

Ministepping systems typically do not have wavetrimming 

capability or offset adjustment to achieve 

the optimum smoothness, but offer a great 

improvement over full-step and half-step systems. 

Ministepping systems have resolutions between 

1,000 and 4,000 steps/rev. 

The motor is an important element in providing 

good smoothness. Some motor designs are 

optimized for high-torque output rather than 

smooth rotation. Others are optimized for 

smoothness rather than high torque. Ministepping 

systems are typically offered with a motor as a 

“packaged” total solution, using a motor that has 

been selected for its premium smoothness 

properties. 

Ministep systems are sometimes selected to 

improve positional accuracy. However, with an 

open-loop system, friction may prevent the 

theoretical unloaded accuracy from being achieved 

in practice. 

A28

Microstepping Drives 

As we mentioned earlier, subdivision of the basic 

motor step is possible by proportioning the current 

in the two motor windings. This produces a series 

of intermediate step positions between the onephase-on 

points. It is clearly desirable that these 

intermediate positions are equally spaced and 

produce approximately equal torque when the 

motor is running. 

Accurate microstepping places increased demands 

on the accuracy of current control in the drive, 

particularly at low current levels. A small phase 

imbalance that may be barely detectable in a halfstep 

drive can produce unacceptable positioning 

errors in a microstep system. Pulse-width 

modulation is frequently used to achieve higher 

accuracy than can be achieved using a simple 

threshold system. 

The phase currents necessary to produce the 

intermediate steps follow an approximately 

sinusoidal profile as shown in Fig. 2.12. However 

the same profile will not give the optimum response 

with all motors. Some will work well with a 

sinusoidal shape, whereas others need a more 

filled-out or trimmed-down shape (Fig. 2.12). So a 

microstep drive intended to operate with a variety 

of motors needs to have provision for adjusting the 

current profile. The intermediate current levels are 

usually stored as data in an EPROM, with some 

means of selecting alternative data sets to give 

different profiles. The change in profile may be 

thought of in terms of adding or subtracting a 

third-harmonic component to or from the basic sine 

wave. 

Fig. 2.12 Microstep current profile 

Due to this dependence on motor type for 

performance, it is usual for high-resolution 

microstep systems to be supplied as a matched 

motor-drive package. 

The Stepper Torque/Speed Curve 

We have seen that motor inductance is the factor 

that opposes rapid changes of current and 

therefore makes it more difficult to drive a stepper 

at high speeds. Looking at the torque-speed curve 

in Fig. 2.13, we can see what is going on. At low 

speeds, the current has plenty of time to reach the 

required level and so the average current in the 

motor is very close to the regulated value from the 

drive. Changing the regulated current setting or 

changing to a drive with a different current rating 

will affect the available torque accordingly. 

Fig 2.13 Regulated and voltage-limited regions 

of the torque-speed curve 

Average 

Current 

During 

Pulse 

Torque 

Regulated Region 

Speed 


Voltage-Limited 

Region 

Drive with 

Higher Supply 

Voltage 


Sinewave Filled out Trimmed 

In the case of high-resolution microstep drives 

producing 10,000 steps per rev or more, the best 

performance will only be obtained with a particular 

type of motor. This is one in which the stator teeth 

are on a 7.5° pitch, giving 48 equal pitches in 360°. 

In most hybrid steppers, the stator teeth have the 

same pitch as the rotor teeth, giving equal 

increments of 7.2°. This latter arrangement tends to 

give superior torque output, but is less satisfactory 

as a microstepper since the magnetic poles are 

“harder” – there is no progressive transfer of tooth 

alignment from one pole to the next. In fact, with 

this type of motor, it can be quite difficult to find a 

current profile that gives good static positioning 

combined with smooth low-speed rotation. An 

alternative to producing a 7.5°-pitch stator is to 

incorporate a slight skew in the rotor teeth. This 

produces a similar effect and has the benefit of 

using standard 7.2° laminations throughout. 

Skewing is also used in DC brush motors as a 

means of improving smoothness. 

As speed increases, the time taken for the current 

to rise becomes a significant proportion of the 

interval between step pulses. This reduces the 

average current level, so the torque starts to fall off. 

As speed increases further, the interval between 

step pulses does not allow the current time to reach 

a level where the chopping action can begin. Under 

these conditions, the final value of current depends 

only on the supply voltage. If the voltage is 

increased, the current will increase more rapidly and 

hence will achieve a higher value in the available 

time. So this region of the curve is described as 

“voltage limited”, as a change in the drive current 

setting would have no effect. We can conclude that 

at low speeds the torque depends on the drive 

current setting, whereas at high speeds it depends 

on the drive supply voltage. It is clear that highspeed 

performance is not affected by the drive 

current setting. Reducing the current simply 

“flattens out” the torque curve without restricting 

the ability to run at high speeds. When performance 

is limited by the available high-speed torque, there 

is much to be said for running at the lowest current 

that gives an adequate torque margin. In general, 

dissipation in motor and drive is reduced and lowspeed 

performance in particular will be smoother 

with less audible noise. 

A29


With a bipolar drive, alternative possibilities exist for 

the motor connections as shown in Fig. 2.14. An 

8-lead motor can be connected with the two halves 

of each winding either in series or in parallel. With 

a 6-lead motor, either one half-winding or both 

half-windings may be connected in series. The 

alternative connection schemes produce different 

torque-speed characteristics and also affect the 

motor’s current rating. 

Fig. 2.14 Series & parallel connections 

1A 1B 2A 2B 1A 1B 2A 2B 

Fig. 2.15 Series & parallel torque/speed curves 

Torque 

Series 

Series 

Speed 

Parallel 

Parallel 

Compared with using one half-winding only, 

connecting both halves in series requires the drive 

current to flow through twice as many turns. For the 

same current, this doubles the “amp-turns” and 

produces a corresponding increase in torque. In 

practice, the torque increase is seldom as high as 

100% due to the non-linearity of the magnetic 

material. Equally, the same torque will be produced at 

half the drive current when the windings are in series. 

However, having doubled the effective number of 

turns in the winding means that we have also 

increased the inductance by a factor of 4. This 

causes the torque to drop off much more rapidly as 

speed is increased, and as a result, the series 

mode is most useful at low speeds. The maximum 

shaft power obtainable in series is typically half that 

available in parallel (using the same current setting 

on the drive). 

Connecting the two half-windings of an 8-lead 

motor in parallel allows the current to divide itself 

between the two coils. It does not change the 

effective number of turns and the inductance 

therefore remains the same. So at a given drive 

current, the torque characteristic will be the same 

for two half-windings in parallel as for one of the 

windings on its own. For this reason, “parallel” in 

the context of a 6-lead motor refers to the use of 

one half-winding only. 

As has already been mentioned, the current rating 

of a step motor is determined by the allowable 

temperature rise. Unless the motor manufacturer’s 

data states otherwise, the rating is a “unipolar” 

value and assumes both phases of the motor are 

energized simultaneously. So a current rating of 5A 

means that the motor will accept 5A flowing in each 

half-winding. 

When the windings of an 8-lead motor are 

connected in parallel, half of the total resistance is 

produced. For the same power dissipation in the 

motor, the current may now be increased by 40%. 

Therefore, the 5A motor will accept 7A with the 

windings in parallel, giving a significant increase in 

available torque. Conversely, connecting the 

windings in series will double the total resistance 

and the current rating is reduced by a factor of 1.4, 

giving a safe current of 3.5A for our 5A-motor in 

series. 

As a general rule, parallel is the preferred 

connection method as it produces a flatter torque 

curve and greater shaft power (Fig. 2.15). Series is 

useful when high torque is required at low speeds, 

and it allows the motor to produce full torque from 

a lower-current drive. Care should be taken to avoid 

overheating the motor in series since its current 

rating is lower in this mode. Series configurations 

also carry a greater likelihood of resonance due to 

the high torque produced in the low-speed region. 

A30

DC Brush Motor Drives 

Linear and Switching Amplifiers 

Linear amplifiers – this type of amplifier operates in 

such a way that, depending on the direction of 

motor rotation, either TR1 or TR2 will be in series 

with the motor and will always have a voltage (V) 

developed across it (Fig. 2.16). 

This characteristic is the primary limitation on the 

use of linear amplifiers (since there will always be 

power dissipated in the output stages of the 

amplifier). To dissipate this power, large transistors 

and heat sinks will be required, making this type of 

amplifier unsuitable for use in high power systems. 

However, the linear amplifier does offer the benefit 

of low radiated electrical noise. 

Fig. 2.16 Linear servo amplifier 

TR1 

TR2 

Switching amplifiers – this amplifier is the most 

commonly used type for all but very low-power 

requirements and the most commonly used 

method of output control is by pulse width 

modulation (PWM). 

Power dissipation is greatly reduced since the 

output transistors are either in an “on” or an “off” 

state. In the “off” state, no current is conducted and 

so no power is dissipated. In the “on” state the 

voltage across the transistors is very low (1-2 volts), 

so that the amount of power dissipated is small. 

Such amplifiers are suitable for a wide range of 

applications (including high power applications). 

The operation of a switching or chopper amplifier is 

very similar to that of the chopping stepper drive 

already described. Only one switch set is required 

to drive a DC brush motor, making the drive 

simpler. However, the function of a DC drive is to 

provide a variable current into the motor to control 

the torque. This may be achieved using either 

analog or digital techniques. 

+ Ve 

Analog and Digital Servo Drives 

Unlike stepper drives, amplifiers for both brush and 

brushless servo motors are either analog or digital. 

The analog drive has been around for many years, 

whereas the digital drive is a relatively recent 

innovation. Both types have their merits. 

- Ve 

V 

M 

Overview – The Analog Drive 

In the traditional analog drive, the desired motor 

velocity is represented by an analog input voltage 

usually in the range ±10 volts. Full forward velocity 

is represented by +10v, and full reverse by -10v. 

Zero (D) 

volts represents the stationary condition and 

intermediate voltages represent speeds in 

proportion to the voltage. 

The various adjustments needed to tune an analog 

drive are usually made with potentiometers. With a 

little experience, this can usually be performed 

quite quickly, but in some difficult applications it 

may take longer. Repeating the adjustments on 

subsequent units may take the same time unless 

there is an easy way of duplicating the potentiometer 

settings. For this reason, some proprietary 

drives use a plug-in “personality card” that may be 

fitted with preset components. However, this not 

only increases the cost but may preclude the 

possibility of fine tuning later. 

Overview – The Digital Drive 

An alternative to the analog system is the digitallycontrolled 

drive in which tuning is performed by 

sending data from a terminal or computer. This leads 

to easy repetition between units and, since such 

drives are invariably processor-based, facilitates 

fully-automatic self tuning. The input signal to such a 

drive may also be an analog voltage but can equally 

take the form of step and direction signals, like a 

stepper drive. 

Digital drives are used more in conjunction with 

brushless servo motors than with DC brush motors. 

Such drives are almost wholly digital with the 

exception of the power stage that actually delivers 

current to the motor. Velocity feedback is derived 

either from an encoder or resolver and again is 

processed as digital information. It becomes logical 

to incorporate a position controller within such a 

drive, so that incoming step and direction signals 

can be derived from a conventional stepper-type 

indexer. Equally, the positioner may be controlled 

by simple commands using a high-level motion 

control language – see the X-code products in this 

catalog. 

A Comparison of Analog and Digital Drives 

The analog drive offers the benefit of lower cost 

and, in the case of a drive using tach feedback, 

very high performance. The wide bandwidth of the 

brush tach allows high gains to be used without 

inducing jitter at standstill, resulting in a very “stiff” 

system. 

The digital drive, while more costly, is comparatively 

easy to set up and adjustments can be quickly 

repeated across several units. Automatic self-tuning 

can be a distinct advantage where the load 

parameters are unknown or difficult to measure. 

The digital drive also offers the possibility of 

dynamic tuning – sometimes vital where the load 

inertia changes dramatically during machine 

operation. Such an option is clearly out of the 

question with a potentiometer-adjusted drive. 



A31


Analog DC Drive Operation 

The elements of an analog velocity amplifier are 

shown in Fig. 2.17. The function of the system is to 

control motor velocity in response to an analog 

input voltage. 

Fig. 2.17 Elements of an analog servo system 

Velocity 

Control 

Signal 

A 

Torque 

Control 

Signal 

B 

Drive 

Amplifier 

Torque Feedback Loop 

Velocity Feedback Loop 

Motor velocity is measured by a tach generator 

attached to the motor shaft. This produces a 

voltage proportional to speed that is compared with 

the incoming velocity demand signal, and the result 

of this comparison is a torque demand. If the speed 

is too low, the drive delivers more current, which in 

turn creates torque to accelerate the load. Similarly, 

if the speed is too high or the velocity demand is 

reduced, current flow in the motor will be reversed 

to produce a braking torque. 

This type of amplifier is often referred to as a fourquadrant 

drive. This means that it can produce 

both acceleration and braking torque in either 

direction of rotation. If we draw a diagram 

representing direction of rotation in one axis and 

direction of torque in the other (see Fig. 2.18), you 

will see that the motor can operate in all four 

“quadrants”. By contrast, a simple variable-speed 

drive capable of running only in one direction and 

with uncontrolled deceleration would be described 

as single-quadrant. 

Fig. 2.18 Four-quadrant operation 

M 

R 

T 

The velocity amplifier in Fig. 2.17 has a high gain so 

that a small velocity difference will produce a large 

error signal. In this way, the accuracy of speed 

control can be made very high even when there are 

large load changes. 

A torque demand from the velocity amplifier 

amounts to a request for more current in the motor. 

The control of current is again achieved by a 

feedback loop that compares the torque demand 

with the current in the motor. This current is 

measured by a sense resistor R, which produces a 

voltage proportional to motor current. This inner 

feedback loop is frequently referred to as a torque 

amplifier since its purpose is to create torque in 

response to a demand from the velocity amplifier. 

The torque amplifier alone may be used as the 

basis of a servo drive. Some types of position 

controller generate a torque output signal rather 

than a velocity demand, and there are also 

applications in which torque rather than speed is of 

primary interest (winding material onto a drum, for 

instance). Most analog drives can be easily 

configured either as velocity or torque amplifiers by 

means of a switch or jumper links. In practice, the 

input signal is still taken to the same point, but the 

velocity amplifier is bypassed. 

Torque 

CW 

Braking 

CCW 

Accelerating 

CW 

CCW 

CW 

Torque 

Accelerating 

CCW 

Braking 

CW 

CCW 

A32

Fig. 2.19 Digital servo drive 


Tuning 


Direction 

RS-232C 

Microprocessor 

D to A 

Converter 

PWM 

Control 

Amplifier 

M 

Encoder 

E 


Digital Servo Drive Operation 

Fig. 2.19 shows the components of a digital drive 

for a servo motor. All the main control functions are 

carried out by the microprocessor, which drives a 

D-to-A convertor to produce an analog torque 

demand signal. From this point on, the drive is very 

much like an analog servo amplifier. 

Feedback information is derived from an encoder 

attached to the motor shaft. The encoder generates 

a pulse stream from which the processor can 

determine the distance travelled, and by calculating 

the pulse frequency it is possible to measure 

velocity. 

The digital drive performs the same operations as 

its analog counterpart, but does so by solving a 

series of equations. The microprocessor is 

programmed with a mathematical model (or 

“algorithm”) of the equivalent analog system. This 

model predicts the behavior of the system. In 

response to a given input demand and output 

position. It also takes into account additional 

information like the output velocity, the rate of 

change of the input and the various tuning settings. 

The tuning of a digital servo is performed either by 

pushbuttons or by sending numerical data from a 

computer or terminal. No potentiometer 

adjustments are involved. The tuning data is used 

to set various coefficients in the servo algorithm and 

hence determines the behavior of the system. Even 

if the tuning is carried out using pushbuttons, the 

final values can be uploaded to a terminal to allow 

easy repetition. 

In some applications, the load inertia varies 

between wide limits – think of an arm robot that 

starts off unloaded and later carries a heavy load at 

full extension. The change in inertia may well be a 

factor of 20 or more, and such a change requires 

that the drive is re-tuned to maintain stable 

performance. This is simply achieved by sending 

the new tuning values at the appropriate point in the 

operating cycle. 

To solve all the equations takes a finite amount 

of time, even with a fast processor – this time is 

typically between 100µs and 2ms. During this 

time, the torque demand must remain constant 

at its previously-calculated value and there will 

be no response to a change at the input or 

output. This “update time” therefore becomes a 

critical factor in the performance of a digital 

servo and in a high-performance system it must 

be kept to a minimum. 

A33


Brushless Motor Drives 

The trapezoidal drive 

Fig. 2.20 shows a simplified layout of the drive for a 

three-phase trapezoidal motor. The switch set is 

based on the familiar H-bridge, but uses three bridge 

legs instead of two.The motor windings are 

connected between the three bridge legs as shown, 

with no connection to the star point at the junction of 

the windings. By turning on the appropriate two 

transistors in the bridge, current can be made to flow 

in either direction through any two motor windings. 

At any particular time, the required current path 

depends on rotor position and direction of rotation, 

so the bridge transistors are selected by logic driven 

from the commutation encoder. 

A PWM recirculating chopper system controls the 

current in the same way as in the DC brush drive 

described previously. The required current 

feedback information is provided by sense 

resistors connected in series with two of the motor 

leads. The voltage signals derived from these 

resistors must be decoded and combined to 

provide a useful current reference, and the circuit 

that does this also uses the commutation encoder 

to determine how to interpret the information. In 

fact, this is not a simple process because the 

relatively small feedback voltage (about 1V) must 

be separated from the large voltage excursions 

generated by the chopping system (240V in the 

case of a typical high-power drive). 

The input stages of the brushless drive follow the 

same pattern as a conventional analog brush drive 

(using a high-gain velocity amplifier that generates 

the torque demand signal). Velocity feedback can 

be derived in a number of ways, but it is clearly 

desirable to use a brushless method in conjunction 

with a brushless motor. Some motors incorporate 

a brushless tach generator that produces 

multi-phase AC outputs. These signals have to 

be processed in a similar way to the current 

feedback information using additional data from a 

tach encoder. Again, this is not a particularly 

straightforward process and it is difficult to obtain 

a smooth, glitch-free feedback signal. A more 

satisfactory alternative is to use a high-resolution 

optical encoder and convert the encoder pulse 

frequency to an analog voltage. The encoder 

can also be used as the feedback device for a 

position controller. 

Fig. 2.20 Simplified trapezoidal brushless servo drive 

Velocity 

Input 

Velocity Amp 

+ 

- 

Torque 

Amp 

Logic 

PWM & 

Circuit 

+ 

Velocity 

Feedback 

- 

Current 

Sense 

Selector 


Communication 

Encoder 

A34

The Sine Wave Drive 

Sine wave brushless motors can be two- or threephase, 

and the drive we'll look at is for the twophase 

version (see Fig. 2.21). This uses two H- 

bridges to control current in the two motor 

windings, and the power section of this drive 

closely resembles a pair of DC brush drives. By 

contrast with the previous example, this drive uses 

a digital processor-based control section that takes 

its input in the form of step and direction signals. 

We need to generate currents in the two motor 

windings that follow a sine and cosine pattern as 

the shaft rotates. The drive shown in Fig. 2.21 uses 

a brushless resolver and a resolver-to-digital 

converter (RDC) to detect the shaft position. From 

this, we will get a number that can be fed to a 

reference table to determine the instantaneous 

current values for that particular shaft position. Bear 

in mind that the reference table will only indicate 

relative currents in the two windings—the absolute 

values will depend on the torque demand at the 

time. So the processor must multiply the sine and 

cosine values by the torque demand to get the final 

value of current in each phase. The resulting 

numbers are fed to D-to-A converters that produce 

an analog voltage proportional to demanded 

current. This is fed to the two PWM chopper 

amplifiers. 

Commutation information for a sine wave drive may 

also be derived from an absolute or incremental 

optical encoder. An incremental encoder will be less 

expensive for the same resolution, but requires 

some form of initialization at power-up to establish 

the required 90° torque angle. 

A “pseudo-sine wave” drive using feedback from a 

low-resolution absolute encoder can offer a costeffective 

alternative. The pseudo sine wave drive 

gives superior performance to the trapezoidal drive 

at lower cost than the standard high-resolution sine 

wave system. 



Fig. 2.21 Two-phase sine wave brushless drive 


D-to-A 

Converter 

PWM 

Control 

H - Bridge 

Micro- 

processor 

Direction 

D-to-A 

Converter 

PWM 

Control 

H - Bridge 


Resolver 

to Digital 

Converter 

Resolver 

A35


Tuning a Servo System 

Any closed-loop servo system, whether analog or 

digital, will require some tuning. This is the process 

of adjusting the characteristics of the servo so that 

it follows the input signal as closely as possible. 

Why is tuning necessary? 

A servo system is error-driven, in other words, there 

must be a difference between the input and the 

output before the servo will begin moving to reduce 

the error. The “gain” of the system determines how 

hard the servo tries to reduce the error. A high-gain 

system can produce large correcting torques when 

the error is very small. A high gain is required if the 

output is to follow the input faithfully with minimal 

error. 

Now a servo motor and its load both have inertia, 

which the servo amplifier must accelerate and 

decelerate while attempting to follow a change at 

the input. The presence of the inertia will tend to 

result in over-correction, with the system oscillating 

or “ringing” beyond either side of its target (Fig. 3.1). 

This ringing must be damped, but too much 

damping will cause the response to be sluggish. 

When we tune a servo, we are trying to achieve the 

fastest response with little or no overshoot. 

Fig. 3.1 System response characteristics 

Output 

Underdamped 

Response 

Critical Damping 

Overdamped Response 

Time 

In practice, tuning a servo means adjusting 

potentiometers in an analog drive or changing gain 

values numerically in a digital system. To carry out 

this process effectively, it helps to understand 

what’s going on in the drive. Unfortunately, the 

theory behind servo system behavior, though well 

understood, does not reveal itself to most of us 

without a struggle. So we’ll use a simplified 

approach to explain the tuning process in a typical 

analog velocity servo. Bear in mind that this 

simplified approach does not necessarily account 

for all aspects of servo behavior. 

A Brief Look at Servo Theory 

A servo is a closed-loop system with negative 

feedback. If you make the feedback positive, you 

will have an oscillator. So for the servo to work 

properly, the feedback must always remain 

negative, otherwise the servo becomes unstable. In 

practice, it’s not as clear-cut as this. The servo can 

almost become an oscillator, in which case it 

overshoots and rings following a rapid change at 

the input. 

So why doesn’t the feedback stay negative all the 

time? 

To answer this, we need to clarify what we mean by 

“negative”. In this context, it means that the input 

and feedback signals are in antiphase. If the input is 

driven with a low frequency sinewave, the feedback 

signal (which will also be a sinewave) is displaced in 

phase by 180°. The 180°-phase displacement is 

achieved by an inversion at the input of the 

amplifier. (In practice it’s achieved simply by 

connecting the tach the right way round – connect 

it the wrong way and the motor runs away.) 

The very nature of a servo system is such that its 

characteristics vary with frequency, and this 

includes phase characteristics. So feedback that 

starts out negative at low frequencies can turn 

positive at high frequencies. The result can be 

overshoot, ringing or ultimately continuous 

oscillation. 

We’ve said that the purpose of servo tuning is to 

get the best possible performance from the system 

without running into instability. We need to 

compensate for the characteristics of the servo 

components and maintain an adequate stability 

margin. 

What determines whether the system will be stable 

or not? 

Closed-loop systems can be difficult to analyze 

because everything is interactive. The output gets 

fed back to the input in antiphase and virtually 

cancels it out, so there seems to be nothing left to 

measure. The best way to determine what’s going 

on is to open the loop and then see what happens. 

Fig. 3.2 Closed-loop velocity servo 

Velocity 

Input 

Fig. 3.3A Measuring open-loop characteristics 

Oscillator 

Scope 

+ - 

Servo 

Amplifier 

+ - 

Feedback Signal 


Tach 


Tach 

A36


Measuring the open-loop characteristic allows us to 

find out what the output (and therefore the 

feedback) signal will be in response to a particular 

input. We need to measure the gain and phase 

shift at different frequencies, and we can plot the 

results graphically. For a typical servo system, the 

results might look like this: 

Fig. 3.3B Open-loop gain and phase characteristics 

+30 

+20 

+10 

db 0 

Gain 

We’ve said that a problem can occur when there is 

a phase shift of 180° round the loop. When this 

happens, the open-loop gain must be less than one 

(1) so that the signal fed back is smaller than the 

input. So here is a basic requirement for a stable 

system: 

The open-loop gain must be less than unity 

when the phase shift is 180°. 

When this condition is only just met (i.e., the phase 

shift is near to180° at unity gain) the system will ring 

after a fast change on the input. 

Fig. 3.4 Underdamped response 

Ringing at 

unity-gain 

frequency 


-10 

Frequency 

Input 

Output 

0 

-90 

-180 

Phase 

Shift 

The gain scale is in decibels (dB), which is a 

logarithmic scale (a 6dB decrease corresponds to a 

reduction in amplitude of about 50%). The 0dB line 

represents an open-loop gain of one (unity), so at 

this frequency the input and output signals will have 

the same amplitude. The falling response in the gain 

characteristics is mainly due to the inertia of the 

motor itself. 

The phase scale is in degrees and shows the phase 

lag between input and output. Remember that the 

feedback loop is arranged to give negative 

feedback at low frequencies, (i.e., 180° phase 

difference). If the additional phase lag introduced by 

the system components reaches 180°, the 

feedback signal is now shifted by 360° and 

therefore back in phase with the input. We need to 

make sure that at no point do we get a feedback 

signal larger than the original input and in phase 

with it. This would amount to positive feedback, 

producing an ever-increasing output leading to 

oscillation. 

Fortunately, it is possible to predict quite accurately 

the gain and phase characteristics of most servo 

systems, provided that you have the necessary 

mathematical expertise and sufficient data about 

the system. So in practice, it is seldom necessary to 

measure these characteristics unless you have a 

particular stability problem that persists. 

Characteristics of a Practical 

Servo System 

Typical open-loop gain and phase characteristics of 

an unloaded drive-motor-tach system will look 

something like Fig. 3.5. 

Fig. 3.5 Characteristics of a practical system 

Gain 

Phase 

+dB 

0 

-dB 

0 

-180° 

-360° 

B 

Crossover frequency 

40–300 Hz typical 

Shaft resonance 

2 kHz typical 

The first thing we notice is the pronounced spike in 

the gain plot at a frequency of around 2kHz. This is 

caused by shaft resonance, torsional oscillation in 

the shaft between the motor and the tach. Observe 

that the phase plot drive dramatically through the 

critical 180° line at this point. This means that the 

loop gain at this frequency must be less than unity 

(0dB), otherwise the system will oscillate. 

The TIME CONSTANT control determines the 

frequency at which the gain of the amplifier starts to 

roll off. You can think of it rather like the treble 

control on an audio amplifier. When we adjust the 

time constant control, we are changing the highfrequency 

gain to keep the gain spike at 2kHz just 

below 0dB. With too high a gain (time constant too 

low) the motor will whistle at about 2kHz. 

A37


The second point of interest is the CROSSOVER 

FREQUENCY, which is the frequency at which the 

gain curve passes through 0dB (unity gain). This 

frequency is typically between 40 and 300Hz. On 

the phase plot, ß (beta) is the phase margin at the 

crossover frequency. If ß is very small, the system 

will overshoot and ring at the crossover frequency. 

So ß represents the degree of damping – the 

system will be heavily damped if ß is large. 

The DAMPING control increases the phase margin 

at the crossover frequency. It operates by applying 

lead compensation, sometimes called acceleration 

feedback. The compensation network creates a 

phase lead in the region of the crossover frequency, 

which increases the phase margin and therefore 

improves the stability. 

Increasing the damping also tends to reduce the 

gain at the 2kHz peak, allowing a higher gain to be 

used before instability occurs. Therefore, the time 

constant should be re-adjusted after the damping 

has been set up. 

What’s the effect of adding load inertia? 

An external load will alter both the gain and phase 

characteristics. Not only will the overall gain be 

reduced owing to the larger inertia, but an 

additional gain spike will be introduced due to 

torsional oscillation between the motor and the 

load. This gain spike may well be larger than the 

original 2kHz spike, in which case the motor will 

start to buzz at a lower frequency when the time 

constant is adjusted. 

Fig. 3.6 Characteristics of a system with inertial load 

Gain 

Phase 

+dB 

0 

-dB 

0 

-180° 

-360° 


plus load 

Motor alone (no load) 

The amplitude of this second spike will depend on 

the compliance or stiffness of the coupling 

between motor and load. A springy coupling will 

produce a large gain spike; this means having to 

reduce the gain to prevent oscillation, resulting in 

poorer system stiffness and slower response. So, 

if you’re after a snappy performance, it’s important 

to use a torsionally-stiff coupling between the 

motor and the load. 

A38

Tachometers 

A permanent magnet DC motor may be used as a 

tachometer. When driven mechanically, this motor 

generates an output voltage that is proportional to 

shaft speed (see Fig. 4.1). The other main 

requirements for a tachometer are that the output 

voltage should be smooth over the operating range 

and that the output should be stabilized against 

temperature variations. 

Small permanent magnet DC “motors” are 

frequently used in servo systems as speed sensing 

devices. These systems usually incorporate 

thermistor temperature compensation and make 

use of a silver commutator and silver loaded 

brushes to improve commutation reliability at low 

speeds and at the low currents, which are typical of 

this application. 

To combine high performance and low cost, DCservo 

motor designs often incorporate a 

tachometer mounted on the motor shaft and 

enclosed within the motor housing (Fig. 4.1). 

Fig. 4.1 Tachometer output characteristics 

Output 

Volts 

Shaft Speed 

Fig. 4.2 Motor with integral tachometer 

Tachometer 

Fig. 4.3 Principle of optical encoder 

An incremental encoder generates a pulse for a 

given increment of shaft rotation (rotary encoder), or 

a pulse for a given linear distance travelled (linear 

encoder). Total distance travelled or shaft angular 

rotation is determined by counting the encoder 

output pulses. 

An absolute encoder has a number of output 

channels, such that every shaft position may be 

described by its own unique code. The higher the 

resolution the more output channels are required. 

The Basics of Incremental Encoders 

Since cost is an important factor in most industrial 

applications, and resetting to a known zero point 

following power failure is seldom a problem, the 

rotary incremental encoder is the type most favored 

by system designers. Its main element is a shaft 

mounted disc carrying a grating, which rotates with 

the grating between a light source and a masked 

detector. The light source may be a light emitting 

diode or an incandescent lamp, and the detector is 

usually a phototransistor or more commonly a 

photo-voltaic diode. Such a simple system, 

providing a single low-level output, is unlikely to be 

frequently encountered, since quite apart from its 

low output signal, it has a DC offset that is 

temperature dependent, making the signal difficult 

to use (Fig. 4.4). 

Fig. 4.4 Encoder output voltage 

V 

Collimated 

Light Source 

Output 

Voltage 

Grating 

Mask 

Feedback Servo Devices Tuning 

Detector 



Optical Encoders 

In servo control systems, where mechanical 

position is required to be controlled, some form of 

position sensing device is needed. There are a 

number of types in use: magnetic, contact, 

resistive, and optical. However, for accurate 

position control, the most commonly used device is 

the optical encoder. There are two forms of this 

encoder – absolute and incremental. 

Optical encoders operate by means of a grating, 

that moves between a light source and a detector. 

When light passes through the transparent areas of 

the grating, an output is seen from the detector. 

For increased resolution, the light source is 

collimated and a mask is placed between the 

grating and the detector. The grating and the mask 

produce a shuttering effect, so that only when their 

transparent sections are in alignment is light 

allowed to pass to the detector (Fig. 4.3). 

DC 

Offset 

Shaft 

Rotation 

In practice, two photodiodes are used with two 

masks, arranged to produce signals with 180° 

phase difference for each channel, the two diode 

outputs being subtracted so as to cancel the DC 

offset (Fig. 4.5). This quasi-sinusoidal output may 

be used unprocessed, but more often it is either 

amplified or used to produce a square wave output. 

Hence, incremental rotary encoders may have sine 

wave or square wave outputs, and usually have up 

to three output channels. 

A39

Feedback Servo Tuning Devices 

Fig. 4.5 Output from dual photodiode system 

V 1 

V 1 

0 

A two-channel encoder, as well as giving position 

of the encoder shaft, can also provide information 

on the direction of rotation by examination of the 

signals to identify the leading channel. This is 

possible since the channels are normally arranged 

to be in quadrature (i.e., 90° phase shifted: 

Fig. 4.6). 

For most machine tool or positioning applications, a 

third channel known as the index channel or Z 

channel is also included. This gives a single output 

pulse per revolution and is used when establishing 

the zero position. 

Fig. 4.6 Quadrature output signals 

90 Phase Shift (± Tolerance) 

DC 

Offset 

Output 1 

Output 2 

Combined 

Output (1-2) 

Shaft 

Rotation 

Channel A 

Channel B 

Fig. 4.6 shows that for each complete square wave 

from channel A, if channel B output is also 

considered during the same period, four pulse 

edges will occur. This allows the resolution of the 

encoder to be quadrupled by processing the A and 

B outputs to produce a separate pulse for each 

square wave edge. For this process to be effective, 

however, it is important that quadrature is 

maintained within the necessary tolerance so that 

the pulses do not run into one another. 

Square wave output encoders are generally 

available in a wide range of resolutions (up to about 

5000 lines/rev), and with a variety of different output 

configurations, some of which are listed below. 

TTL (Transistor-Transistor Logic) – This is a 

commonly available output, compatible with TTL 

logic levels, and normally requiring a 5 volt supply. 

TTL outputs are also available in an open-collector 

configuration which allows the system designer to 

choose from a variety of pull-up resistor value. 

CMOS (Complimentary Metal-Oxide 

Semiconductor) – Available for compatibility with 

the higher logic levels, it normally used with CMOS 

devices. 

Line driver – These are low-output impedance 

devices designed for driving signals over a long 

distance, and are usually used with a matched 

receiver. 

Complementary outputs – Outputs derived from 

each channel give a pair of signals, 180° out of 

phase. These are useful where maximum immunity 

to interference is required. 

Noise problems 

The control system for a machine is normally 

screened and protected within a metal cabinet. An 

encoder may be similarly housed. However, unless 

suitable precautions are taken, the cable 

connecting the two can be a source of trouble due 

to its picking up electrical noise. This noise may 

result in the loss or gain of signal counts, giving rise 

to incorrect data input and loss of position. 

Fig. 4.7 Corruption of encoder signal by noise 

+ Ve Noise Pulse 

Fig. 4.7, shows how the introduction of two noise 

pulses has converted a four-pulse train into one of 

six pulses. 

A number of techniques are available to overcome 

problems due to noise. The most obvious resolution 

is to use shielded interconnecting cables. 

However, since the signals may be at a low level 

(5 volts) and may be generated by a highimpedance 

source, it may be necessary to go to 

further lengths to eliminate the problem. 

The most effective way to resolve the problem is 

to use an encoder with complementary outputs 

(Fig. 4.8) and connect this to the control system by 

means of shielded, twisted-pair cable. 

The two outputs are processed by the control 

circuitry so that the required signal can be 

reconstituted without the noise. 

Fig. 4.8 Complementary output signals 

Noise Spike 

Channel A 

- Ve Noise Pulse 

Channel A 

A 

A40


If the A signal is inverted and is fed with the A signal 

into an OR gate (whose output depends on one 

signal or the other being present), the resultant 

output will be a square wave (Fig. 4.9). 

Fig. 4.9 Reduction of noise in a complementary 

system 

Channel A 

Inverted A 

The simple interconnection of encoder and 

controller with channel outputs at low level may be 

satisfactory in electrically “clean” environments or 

where interconnections are very short. In cases 

where long interconnections are necessary or 

where the environment is “noisy”, complementary 

line driver outputs will be needed, and 

interconnections should be made with shielded, 

twisted-pair cable. 

Factors Affecting Accuracy 

Slew rate (speed) – An incremental rotary encoder 

will have a maximum frequency at which it will 

operate (typically 100KHz), and the maximum 

rotational speed, or slew rate, will be determined by 

this frequency. Beyond this, the output will become 

unreliable and accuracy will be affected. 

Consider a 600-line encoder rotated at 1rpm (gives 

an output of 10Hz). If the maximum operating 

frequency of the encoder is 50KHz, its speed will be 

limited to 5000 times this (i.e., 50KHz • 10Hz = 

5000 rpm). 

If an encoder is rotated at speeds higher than its 

design maximum, there may be conditions set up 

that will be detrimental to the mechanical 

components of the assembly. This could damage 

the system and affect encoder accuracy. 

Quantization error – All digital systems have 

difficulty, interpolating between output pulses. 

Therefore, knowledge of position will be accurate 

only to the grating width (Fig. 4.10). 

Fig. 4.10 Encoder quantization error 

Quantization Error = F (1, 2N) (N = no. of lines/disk rotation) 

Quantization error F(1,2N) (N = # of lines/disk 

rotation) 

OR 

Quantization Error 

Eccentricity 

This may be caused by bearing play, shaft run out, 

incorrect assembly of the disc on its hub or the hub 

on the shaft. Eccentricity may cause a number of 

different error conditions. 

a) Amplitude Modulation – In a sine wave encoder, 

eccentricity will be apparent as amplitude 

modulation (Fig. 4.11). 

Fig. 4.11 Amplitude modulation caused by 

eccentricity 

b) Frequency modulation – As the encoder is 

rotated at constant speed, the frequency of the 

output will change at a regular rate (Fig. 4.12). If 

viewed on an oscilloscope, this effect will appear as 

“jitter” on the trace. 

Fig. 4.12 Encoder frequency modulation 

Nominal Frequency (f ) 1 

Increased Frequency (f ) 2 

Nominal Signal 

Level 

Signal Amplitude 

c ) Inter-channel jitter – If the optical detectors for 

the two encoder output channels are separated by 

an angular distance on the same radius, then any 

“jitter” will appear at different times on the two 

channels, resulting in “inter-channel jitter”. 

Environmental Considerations 

Like electrical noise, other environmental factors 

should be considered before installing an optical 

encoder. 

In particular, temperature and humidity should be 

considered (consult manufacturers’ specifications). 

In environments contaminated with dust, oil vapor 

or other potentially damaging substances, it may be 

necessary to ensure that the encoder is enclosed 

within a sealed casing. 


A41


Mechanical Construction 

Shaft encoder (Fig. 4.13). In this type of encoder, 

which may be either incremental or absolute, the 

electronics are normally supported on a substantial 

mounting plate that houses the bearings and shaft. 

The shaft protrudes from the bearings on the 

“outside” of the encoder, for connection to the 

rotating system, and on the “inside”, so that the 

encoder disc may be mounted in the appropriate 

position relative to the light source and detector. 

The internal parts are covered by an outer casing, 

through which the interconnecting leads pass. 

Fig. 4.13 Shaft encoder 

Interconnecting 

Leads 

The kit encoder will usually be less robust than the 

shaft encoder, but this need not be a problem if the 

motor is mounted so that the encoder is protected. 

If this cannot be done, it will normally be possible to 

fit a suitable cover over the encoder. 

A typical kit encoder will include a body, on which 

will be mounted the electronic components and a 

hub and disc assembly for fitting to the shaft. 

Some form of cover will also be provided, mainly to 

exclude external light and provide some mechanical 

protection. 

Linear encoder. These encoders are used where it 

is required to make direct measurement of linear 

movement. They comprise a linear scale (which 

may be from a few millimeters to a meter or more in 

length), and a read head. They may be incremental 

or absolute and their resolution is expressed in lines 

per unit length (normally lines/inch or lines/cm). 

Cover 

Mounting Plate 

Shaft 

For use in extreme environmental or industrial 

conditions, the whole enclosure may be specified to 

a more substantial standard (heavy duty) with 

sealed bearings and sealing between the mounting 

plate and cover. Also the external electrical 

connections may be brought out through a high 

quality connector. 

Modular or kit encoder (Fig. 4.14). These are 

available in a number of forms, their principal 

advantage being that of reduced cost. 

Fig. 4.14 Modular encoder 

Cover 

Hub and Disc 

Interconnecting 

Leads 

Electronics 

Body 

Since many servo motors have a double-ended 

shaft, it is a simple matter to fit a kit encoder onto a 

motor. 

A42


Basics of Absolute Encoders 

An absolute encoder is a position verification device 

that provides unique position information for each 

shaft location. The location is independent of all 

other locations, unlike the incremental encoder, 

where a count from a reference is required to 

determine position. 

Fig. 4.15 Absolute disk 

Fig. 4.17 Absolute encoder output 

Bit 

1 

1 

0 

1 

Current 

Position 

1011 = Decimal 11 

The disk pattern of an absolute encoder is in 

machine readable code, usually binary, grey code 

or a variety of grey. The figure above represents a 

simple binary output with four bits of information. 

The current location is equivalent to the decimal 

number 11. Moving to the right from the current 

position, the next decimal number is 10 (1-0-1-0 

binary). Moving to the left from the current position, 

the next position would be 12 (1-1-0-0). 

Fig. 4.18 Multi-turn absolute encoders 

High Resolution 

Main Disk 


Fig. 4.16 Incremental disk 

Bearing 

In an absolute encoder, there are several concentric 

tracks, unlike the incremental encoder, with its 

single track. Each track has an independent light 

source. As the light passes through a slot, a high 

state (true “1”) is created. If light does not pass 

through the disk, a low state (false “0”) is created. 

The position of the shaft can be identified through 

the pattern of 1’s and 0’s. 

The tracks of an absolute encoder vary in slot size, 

moving from smaller at the outside edge to larger 

toward the center. The pattern of slots is also 

staggered with respect to preceding and 

succeeding tracks. The number of tracks 

determines the amount of position information that 

can be derived from the encoder disk – resolution. 

For example, if the disk has ten tracks, the 

resolution of the encoder would usually be 1,024 

positions per revolution or 2 10 . 

For reliability, it is desirable to have the disks 

constructed of metal rather than glass. A metal disk 

is not as fragile, and has lower inertia. 

Seals 

Additional Turns 

Stages 

Gearing an additional absolute disk to the primary 

high-resolution disk provides for turns counting, so 

that unique position information is available over 

multiple revolutions. 

Here is an example of how an encoder with 1,024 

counts per revolution becomes an absolute device 

for 524,288 discrete positions. 

The primary high-resolution disk has 1,024 discrete 

positions per revolution. A second disk with 3 

tracks of information will be attached to the highresolution 

disk geared 8:1. The absolute encoder 

now has 8 complete turns of the shaft or 8,192 

discrete positions. Adding a third disk geared 8:1 

will provide for 64 turns of absolute positions. In 

theory, additional disks could continue to be 

incorporated. But in practice, most encoders stop 

at or below 512 turns. Encoders using this 

technique are called multi-turn absolute encoders. 

This same technique can be incorporated in a rack 

and pinion style linear encoder resulting in long 

lengths of discrete absolute locations. 

Advantages of Absolute Encoders 

Rotary and linear absolute encoders offer a number 

of significant advantages in industrial motion control 

and process control applications. 

No Position Loss During Power Down or 

Loss of Power 

An absolute encoder is not a counting device like an 

incremental encoder, because an absolute system 

reads actual shaft position. The lack of power does 

not cause the encoder lose position information. 

A43


Whenever power is supplied to an absolute system, 

it can read the current position immediately. In a 

facility where frequent power failures are common, 

an absolute encoder is a necessity. 

Operation in Electrically Noisy Environments 

Equipment such as welders and motor starters 

often generate electrical noise that can often look 

like encoder pulses to an incremental counter. 

Electrical noise does not alter the discrete position 

that an absolute system reads. 

High-speed Long-distance Data Transfer 

Use of a serial interface such as RS-422 gives the 

user the option of transmitting absolute position 

information over as much as 4,000 feet. 

Eliminate Go Home or Referenced Starting 

Point 

The need to find a home position or a reference 

point is not required with an absolute encoding 

system since an absolute system always knows its 

location. In many motion control applications, it is 

difficult or impossible to find a home reference 

point. This situation occurs in multi-axis machines 

and on machines that can't reverse direction. This 

feature will be particularly important in a “lights-out” 

manufacturing facility. Significant cost savings is 

realized in reduced scrap and set-up time resulting 

from a power loss. 

Provide Reliable Position Information in 

High-speed Applications 

The counting device is often the factor limiting the 

use of incremental encoders in high-speed 

applications. The counter is often limited to a 

maximum pulse input of 100 KHz. An absolute 

encoder does not require a counting device or 

continuous observation of the shaft or load location. 

This attribute allows the absolute encoder to be 

applied in high-speed and high-resolution 

applications. 

Resolvers 

A resolver is, in principle, a rotating transformer. 

If we consider two windings, A and B (Fig. 4.19), 

and if we feed winding B with a sinusoidal voltage, 

then a voltage will be induced into winding A. If we 

rotate winding B, the induced voltage will be at 

maximum when the planes of A and B are parallel 

and will be at minimum when they are at right 

angles. Also, the voltage induced into A will vary 

sinusoidally at the frequency of rotation of B so that 

EOA = Ei Sinø. If we introduce a third winding (C), 

positioned at right angles to winding A, then as we 

rotate B, a voltage will be induced into this winding 

and this voltage will vary as the cosine of the angle 

ø, so that EOC = Ei Cosø 

Fig. 4.19 Resolver principle 

Winding B 

Referring to Fig 4.20, we can see that if we are able 

to measure the relative amplitudes of the two 

winding (A & C) outputs at a particular point in the 

cycle, these two outputs will be unique to that 

position. 

Fig. 4.20 Resolver output 

1 Electrical Cycle 

The information output from the two phases will 

usually be converted from analog to digital form, for 

use in a digital positioning system, by means of a 

resolver-to-digital converter (Fig. 4.21). Resolutions 

up to 65,536 counts per revolution are typical of 

this type of system. 

Fig. 4.21 Resolver-to-digital converter 

In addition to position information, speed and 

direction information may also be derived. The 

resolver is an absolute position feedback device. 

Within each electrical cycle, Phase A and Phase B 

maintain a constant (fixed) relationship. 

The excitation voltage E i 

may be coupled to the 

rotating winding by slip rings and brushes, though 

this arrangement is a disadvantage when used with a 

brushless motor. In such applications, a brushless 

resolver may be used so that the excitation voltage is 

inductively coupled to the rotor winding (Fig. 4.22). 

Fig. 4.22 Brushless resolver 

Stator 

Phase 1 

Sine 

Multiplier 

Cosine 

Multiplier 

Up/Down 

Counter 

Digital Output 

(Shaft Angle) 

Integrator 

Voltage 

Control 

Oscillator 

E Cos i 

360° 

E i Sin 

Phase 

Comparitor 

DC Signal 

(Velocity) 

Integrator 

Winding A 

q 

Ei 

Stator 

Phase 2 

Rotor 

E OA 

A44

Machine Control 

Many industrial designers are concerned with 

controlling an entire process. Motion control is one 

important and influential aspect of complete 

machine control. The primary elements of machine 

control include: 

Fig. 5.1 Primary machine control elements 

Mainframes 

MIS 

SPC 

Networks 

Operator Interface 

Displays 

Keyboards 

Touchscreens 

Motion Control 

Machine 

Control 

Servos 

Steppers 

Hydraulics 

Switches 

Indicators 

Readout 

Actuators 

Digital I/O 

Analog I/O 

Sensors 

Gauges 

Meters 

Data Acquisition 

Proportional Valves 

Motion Control: For precise programmable load 

movement using a servo motor, stepper motor, 

or hydraulic actuators. Feedback elements are 

often employed. 

Analog and Digital I/O: For actuation of an 

external process, devices such as solenoids, 

cutters, heaters, valves, etc. 

Operator Interface: For flexible interaction with 

the machine process for both setup and on-line 

variations. Touchscreens, data pads, and 

thumbwheels are examples. 

Communications Support: For process 

monitoring, diagnostics and data transfer with 

peripheral systems. 

There are many different machine control 

architectures that integrate these elements. Each 

results in varying levels of complexity and 

integration of both motion and non-motion 

elements. PLC-based, bus-based and integrated 

solutions are all commercially available. Your 

selection of a machine control strategy will often be 

based on performance, total application cost, and 

technology experience. 

PLC-based Control 

The PLC-based architecture is utilized for I/O 

intensive control applications. Based upon banks of 

relays that are scanned, or polled, by a central 

processor, the PLC provides a low-cost option for 

those familiar with its ladder logic programming 

language. Integration of the motion, I/O, operator 

interface, and communication are usually supported 

through additional cards that are plugged in its 

backplane. 

The addition of scanning points decreases the 

polling rate of any individual point, and can thus 

lead to lower machine response. Because PLCs 

have not historically concentrated on motion 

control, plug-in indexers or those that communicate 

over BCD are preferable. Because these indexer 

boards often include their own microprocessor, 

they prevent slow polling rates, but incorporate a 

separate programming language. In general, this 

compromise is acceptable for all but the most 

complicated motion/machine applications. 

Bus-based Systems 

Bus-based machine control systems are common 

in today's industrial environment. STD, VME and 

PC-AT are only a few of the numerous options. 

Most of these options can operate through a 

standard operating system (DOS, OS/2, OS/9) that 

can be used to program add-on cards for I/O, 

motion and communication interfaces. Flexible 

graphical operator interfaces remain one of the 

computer's major advantages. 

Some successful examples of bus-based machine 

control applications include gear grinding and 

dressing. PCB placement machines, hard disk 

manufacturing, and automotive glass bending. 

Wherever intensive communications or data 

processing are required, the benefits of the bus 

structure can be realized. 

There are some disadvantages to the bus-based 

machine control system that relate to the amount of 

integration between the motion and I/O structure. 

Separate cards are required for each, resulting in a 

need for software integration of different 

programming languages. Motion control operations, 

such as servo loops, should be polled and updated 

on a more immediate basis than auxiliary I/O or the 

operator interface. The programmer must develop 

this polling hierarchy to thread the system together. 

Integrated controllers 

A more integrated approach to machine control 

uses a stand-alone architecture that builds in the 

same essential elements of I/O, motion, operator 

interface, and communication. This approach uses 

a single software and hardware platform to control 

an entire machine application. The polling of servo 

loops, I/O points, and the operator interface are 

handled internally, invisible to the user. A common 

software language is provided to integrate the 

motion and I/O actuation. This pre-tested approach 

allows a typical machine control application to be 

developed with a minimum of effort and cost. 

The total application cost is the major consideration 

when selecting an integrated machine controller. 

While the initial hardware cost is typically higher 

than other solutions, the software investment and 

maintenance of a single language is an overriding 

and positive factor. Software development and 

maintenance costs for any machine control 

application can dwarf the initial hardware expense. 

The integrated approach offers a more economical 

solution. 

Control Systems 


A45


Control System Overview 

The controller is an essential part of any motion 

control system. It determines speed, direction, 

distance and acceleration rate – in fact all the 

parameters associated with the operation that the 

motor performs. The output from the controller is 

connected to the drive’s input, either in the form of 

an analog voltage or as step and direction signals. 

In addition to controlling one or more motors, many 

controllers have additional inputs and outputs that 

allow them to monitor other functions on a machine 

(see Machine Control, p. A45). 

Controllers can take a wide variety of forms. Some 

examples are listed below. 

Standalone – This type of controller operates 

without data or other control signals from external 

sources. A standalone unit usually incorporates a 

keypad for data entry as well as a display, and 

frequently includes a main power supply. It will also 

include some form of nonvolatile memory to allow it 

to store a sequence of operations. Many controllers 

that need to be programmed from a terminal or 

computer can, once programmed, also operate in 

standalone mode. 

Bus-based – A bus-based controller is designed to 

accept data from a host computer using a standard 

communications bus. Typical bus systems include 

STD, VME and IBM-PC bus. The controller will 

usually be a plug-in card that conforms to the 

standards for the corresponding bus system. For 

example, a controller operating on the IBM-PC bus 

resides within the PC, plugging into an expansion 

slot and functioning as an intelligent peripheral. 

PLC-based – A PLC-based indexer is designed to 

accept data from a PLC in the form of I/O 

communication. Typically, the I/O information is in 

BCD format. The BCD information may select a 

program to execute, a distance to move, a time 

delay, or any other parameter requiring a number. 

The PLC is well suited to I/O actuation, but poorly 

suited to perform complex operations such as math 

and complicated decision making. The motion 

control functions are separated from the PLC's 

processor and thus do not burden its scan time. 

X Code-based – X Code is a command language 

specifically developed for motion control and 

intended for transmission along an RS-232C link. 

Controllers using this language either accept realtime 

commands from a host computer or execute 

stored sequences that have been previously 

programmed. The simplicity of RS-232C 

communication allows the controller to be 

incorporated into the drive itself, resulting in an 

integrated indexer/drive package. 

X Code Programming 

X Code has been designed to allow motion control 

equipment to be programmed by users with little or 

no computer experience. Although the language 

includes more than 150 commands, depending on 

the product, it is only necessary to learn a small 

percentage of these to write simple programs. 

Most command codes use the initial letter of the 

function name, which makes them easy to 

remember. Here are some examples of frequently 

used commands. 

V – velocity in revs/sec 

D – distance in steps 

A – acceleration rate in revs/sec 2 

G – go; start the move 

T – time delay in seconds 

A typical command string might look like this: 

V10 A50 D4000 G T2 G 

This would set the velocity to 10 revs/sec, 

acceleration to 50 revs/sec 2 and distance to 4000 

steps. The 4000-step move would be performed 

twice with a 2-second wait between moves. 

Please refer to specifications of X Code products 

for a list of all the available X Code commands. 

Single-axis and Multi-axis Controllers 

A single-axis controller can, as the name implies, 

only control one motor. The controller in an 

integrated indexer/drive comes into this category. 

However, such units are frequently used in systems 

using more than one motor where the operations 

do not involve precise synchronization between 

axes. 

A multi-axis controller is designed to control more 

than one motor and can very often perform 

complex operations such as linear or circular 

interpolation. These operations require accurate 

synchronization between axes, which is generally 

easier to achieve with a central controller. 

A variant of the multi-axis controller is the 

multiplexed unit, which can control several motors 

on a time-shared basis. A printing machine having 

the machine settings controlled by stepper motors 

could conveniently use this type of controller when 

the motors do not need to be moved 

simultaneously. 

Hardware-based Controllers 

Control systems designed without the use of a 

microprocessor have been around for many years 

and can be very cost-effective in simpler 

applications. They tend to lack flexibility and are 

therefore inappropriate where the move parameters 

are continually changing. For this reason, the 

hardware-based controller has now given way 

almost exclusively to systems based on a 

microprocessor. 

A46


Fig. 5.2 Processor-based controller 

XCode 

Commands 

Inputs 

RS-232C 

Communications 

Interface 

I/O 

Interface 

Nonvolatile 

RAM 

Microprocessor 

Program 

Memory ROM 

Programmable 

Pulse 

Generator 


Direction 

Output 

to Drive 


Outputs 

Processor-based Controllers 

The flexibility offered by a microprocessor system 

makes it a natural choice for motion control. 

Fig. 5.2 shows the elements of a typical step and 

direction controller that can operate either in 

conjunction with a host computer or as a standalone 

unit. 

All the control functions are handled by the 

microprocessor whose operating program is 

stored in ROM. This program will include an 

interpreter for the command language, which may 

be X Code for example. 

X Code commands are received from the host 

computer or terminal via the RS-232C 

communications interface. These commands are 

simple statements that contain the required speed, 

distance and acceleration rate, etc. The processor 

interprets these commands and uses the 

information to control the programmable pulse 

generator. This in turn produces the step and 

direction signals that will control a stepper or servo 

drive. 

The processor can also switch outputs and 

interrogate inputs via the I/O interface. Outputs 

can initiate other machine functions such as 

punching or cutting, or simply activate drive panel 

indicators to show the program status. Inputs may 

come from sources such as operator pushbuttons 

or directional limit switches. 

When the controller is used in a standalone mode, 

the required motion sequences are programmed 

from the host and stored in nonvolatile memory 

(normally battery-backed RAM). These sequences 

may then be selected and executed from switches 

via the I/O interface or from a separate machine 

controller such as a PLC. 

A47


Understanding Input and Output Modules 

Most motion controllers/indexers offer 

programmable inputs and outputs to control and 

interact with other external devices and machine 

elements. 

Programmable Output Example 

After indexing a table to a preset position, energize 

a programmable output to activate a knife that will 

cut material on the table. 

Programmable Input Example 

After indexing a table in a pick and place 

application, the indexer waits for an input signal 

from a robot arm, signaling the indexer that a part 

has been located on the table. 

The primary reason for using I/O modules is to 

interface 5VDC logic signals from an indexer to 

switches and relays on the factory floor, which 

typically run on voltage levels ranging from 24VDC 

to 220VAC. Solid-state I/O modules are essentially 

a relay, utilizing light emitting diode (LED) and a 

transistor along with a signal conditioning circuit to 

activate a switch. These I/O modules isolate (no 

direct connection) the internal microprocessor 

circuitry of an indexer from oversized DC and AC 

voltages. The lack of a physical connection 

between the indexer and external devices, protects 

the indexer from hazardous voltage spikes and 

current surges. 

DC Input and Output Modules 

As with all DC devices, this is a polarized, + and – 

input module. Since current will flow in only one 

direction, care must be taken to observe these 

polarities during installation. 

DC input modules typically feature an input signal 

conditioning circuit. This circuit requires the input to 

remain on/off for a minimum of 5 milliseconds 

before recognizing the switch. This eliminates a 

short voltage spike or “de-bounce” contact closure 

less than 5 milliseconds in duration. However, a 0.1 

microfarad, ceramic disc capacitor across the 

actual switching contacts is still recommended to 

prevent switch bounce that can be as long as 10- 

80 milliseconds. 

Fig. 5.3 Typical DC input connection diagram 

10 to 24VDC 

Floating 

Source 

+ 

- 

10 to 24 mA 

Switch #1 

Screw 

Terminals 

1K 

Coupling 

LED 

DC Input Operational Sequence 

As switch #1 closes, current flows through the 

limiting resistor (1K ohm), and then into the LED. 

The light issued by the LED due to this forward 

current flow in turn simulates the photo transistor. 

Hence the term “opto” or optically isolated. The 

phototransistor then drives the base of the second 

transistor to a high level, bringing its output, or 

collector, to a low level. 

Fig. 5.4 Typical DC output connection diagram 

+ 

- 

4KV 

Isolation Barrier 

Photo 

Transistor 

Signal 

Conditioning 

Optional 

Circuit Board 

Indicating 

LED 

Logic Signal 

Ground 

+5VDC 

The operation of the DC output model is similar to 

the DC input module. A 5VDC signal from an 

indexer is used to activate an LED. The output of 

the module is defined as open collector. 

Fig. 5.4 represents a typical DC output schematic. 

Note the diode across the relay coil. These should 

always be installed to eliminate the leading inductive 

kick caused by the relay. A typical part number for 

such a diode is 1N4004. Failure to provide this 

protection can cause noise problems or the 

destruction of the output device. 

+5VDC 

Indicator 

Amplifier 

(+) 

(-) 

Freewheeling 

Diode 

Load 10 

(solenoid) 

+ 

- 

(equivalent 

circuit) 

Logic LED Photo 

Transistor 

Output 

Transistor 

Screw 

Terminals 

2.4 Amps 

24VDC 

A48


AC Input and Output Modules 

AC modules are not polarized devices. This makes 

it virtually impossible to install a unit backwards. 

AC input modules operate like DC input modules 

with the addition of a bridge rectifier to change AC 

Fig. 5.5 Typical AC input connection diagram 

Pushbutton 

8mA 

Rectifying 

Bridge 

LED 

voltage to DC levels. AC input modules also include 

transient protection to filter out spikes from the AC 

line (caused by lightning strikes, arc welders, etc.). 

Signal 

Conditioning 

Indicator 

Logic Signal 

+5VDC 


Screw 

Terminals 

14K 

Photo 

Transistor 

120 VAC 

60Hz 

Power Line 

Ground 

AC output modules feature a Triac power device as 

its output. A Triac output offers three distinct 

advantages. 

1. Zero voltage turn on eliminates in-rush currents 

to the load. 

2. Zero voltage turn off eliminates inductive kick 

problems. 

3. A snubber across the output. 

Fig. 5.6 Zero voltage turn on and off 

A B C 

The module will only turn on or off at points A, B, or 

C; where the voltage is zero. 

AC output modules do have leakage current, which 

may “turn-on” small current loads. To solve 

potential problems, add a parallel resistor across 

the load, 5K, 5W for 120VAC and a 10K, 10W for 

240VAC. 

Fig. 5.7 Typical AC output connection diagram 

+5VDC 

Indicator 

Zero 

Voltage 

Circuit 

C 

R 

Load 

(motor) 

(equivalent 

circuit) 

Logic LED Photo 

Transistor 

4KV Isolation Barrier 

Triac 

Snubber 

Screw 

Terminals 

120VAC 

60Hz 

Power Line 

A49


Serial and Parallel Communications 

Serial and parallel communications are methods of 

transferring data from a host computer to a 

peripheral device such as a Compumotor indexer. 

In the case of a Compumotor indexer, the data 

consist of parameters such as acceleration, 

Fig. 5.8 Serial Communications 

Start bit 

Parity bit 

Data bits 

Time 

(baud rate) 

Stop bits 

Serial 

Serial communication transmits data one bit at a 

time on a single data line. Single data bits are 

grouped together into a byte and transmitted at a 

predetermined interval (baud rate). Serial 

communication links can be as simple as a 3-line 

connection; transmit (Tx), receive (Rx) and ground 

(G). This is an advantage from a cost standpoint, 

but usually results in slower communications than 

parallel communications. Common serial interfaces 

include RS-232C, RS-422, RS-485, RS-423. 

Troubleshooting 

Procedure for troubleshooting 3-wire RS-232C 

communication. 

1. Verify that the transmit of the host is wired to the 

receive of the peripheral, and receive of the host 

is wired to the transmit of the peripheral. Note: 

Try switching the receive and transmit wires on 

either the host or peripheral if you fail to get any 


2. Some serial ports require handshaking. You can 

establish 3-wire communication by jumpering 

RTS to CTS (usually pins 4 and 5) and DSR to 

DTR (usually pins 6 and 20). 

3. Configure the host and peripheral to the same 

baud rate, number of data bits, number of stop 

bits, and parity. 

4. If you receive double characters (e.g., typing “A” 

and receiving “AA”), your computer is set to half 

duplex mode. Change to full duplex mode. 

5. Use DC common or signal ground as your 

reference, NOT earth ground. 

6. Cable lengths should not exceed 50 ft. unless 

you are using some form of line driver, optical 

coupler, or shield. As with any control signal, be 

sure to shield the cable to earth ground at one 

end only. 

7. To test terminal or terminal emulation software 

for proper 3-wire communication, unhook the 

peripheral device and transmit a character. An 

echoed character should not be received. If a 

character is received, you are in half duplex 

mode. Jumper the host’s transmit and receive 

lines and send another character. You should 

receive the echoed character. If not, consult the 

manufacturer of the host’s serial interface for 

proper pin outs. 

velocity, move distance, and move direction 

configured in ASCII characters. Both 

communication techniques are generally bidirectional 

allowing the host to both transmit and 

receive information from a peripheral device. 

Fig. 5.9 Parallel Communications 

Parallel 

Parallel communication requires handshaking and 

transmits data one byte (8 bits) at a time. When 

data are transferred from the host processor to a 

peripheral device, the following steps take place. 

1. The host sets a bit on the bus signalling to the 

peripheral that a byte of data has been sent. 

2. The peripheral receives data and sets a bit on 

the bus, signalling to the host that data have 

been received. 

The advantage of communicating in parallel vs. 

serial is faster communications. However, since 

parallel communications require more 

communication lines, the cost can be higher than 

serial communications. 

Parallel bus structures include: 

0 

1 

0 

0 

0 

0 

0 

1 

Data bus 

Signals 

A = 0100 

1 = 0011 

IEEE-488, IBM PC, VME, MULTIBUS, Q and STD. 

Troubleshooting 

Procedure for troubleshooting parallel 


IEEE-488 

IBM PC 

VME Bus 

STD Bus 

Multi Bus 

0001 

0001 

1. Make certain the address setting of the 

peripheral device is configured properly. 

2. Confirm that multiple boards are not set to the 

same address (and each board is sealed 

properly into a slot). 

3. Verify that peripheral subroutines to reset the 

board, write data, and read data work properly. 

Follow the handshaking procedure outlined in 

the device’s user manual. 

Note: Compumotor bus-based indexers come 

complete with a diskette that includes pretested 

programs to verify system functions and 

routines for simple user program development. 

A50

Serial and Parallel Communications 

ADDRESS: Multiple devices are controlled on the 

same bus, each with a separate address or unit 

number. This address allows the host to 

communicate individually to each device. 

ASCII: American Standard Code for Information 

Interchange. This code assigns a number to each 

numeral and letter of the alphabet. In this manner, 

information can be transmitted between machines 

as a series of binary numbers. 

BAUD RATE: Number of bits transmitted per 

second. Typical rates include 300; 600; 1,200; 

2,400; 4,800; 9,600, 19,200. This means at 9,600 

baud, 1 character can be sent nearly every 

millisecond. 

DATA BITS: Since the ASCII set consists of 128 

characters, computers may transmit only 7 bits of 

data. Most computers do, however, support an 8- 

bit extended ASCII character set. 

DCE: Data Communications Equipment transmits 

on pin 3 and receives on pin 2. 

DTE: Data Terminal Equipment. Transmits on pin 2 

and receives on pin 3. 

FULL DUPLEX: The terminal will display only 

received or echoed characters. 

HALF DUPLEX: In half duplex mode, a terminal will 

display every character transmitted. It may also 

display the received character. 

HANDSHAKING SIGNALS: 

RTS: Request To Send DTR: Data Terminal Ready 

CTS: Clear To Send IDB: Input Data Buffer 

DSR: Data Set Ready ODB: Output Data Buffer 

ASCII Table 

DEC HEX GRAPHIC 

000 00 NUL 

001 01 SOH 

002 02 STX 

003 03 ETX 

004 04 EOT 

005 05 ENQ 

006 06 ACK 

007 07 BEL 

008 08 BS 

009 09 HT 

010 0A LF 

011 0B VT 

012 0C FF 

013 0D CR 

014 0E SO 

015 0F S1 

016 10 DLE 

017 11 DC1 

018 12 DC2 

019 13 DC3 

020 14 DC4 

021 15 NAK 

022 16 SYN 

023 17 ETB 

024 18 CAN 

025 19 EM 

026 1A SUB 

027 1B ESC 

028 1C FS 

029 1D GS 

NULL MODEM: A simple device or set of 

connectors that switches the receive and transmit 

lines a 3-wire RS-232C connector. 

PARITY: An RS-232C error detection scheme that 

can detect an odd number of transmission errors. 

SERIAL POLLING: Method of checking the status 

of the IEEE-488 device. By reading the status byte, 

the host can determine if the device is ready to 

receive or send characters. 

START BITS: When using RS-232C, one or two 

bits are added to every character to signal the end 

of a character. 

TEXT/ECHO (ON/OFF): This setup allows received 

characters to be re-transmitted back to the original 

sending device. Echoing characters can be used to 

verify or “close the loop” on a transmission. 

XON/XOFF: Two ASCII characters supported in 

some serial communication programs. If supported, 

the receiving device transmits an XOFF character to 

the host when its character buffer is full. The XOFF 

character directs the host to stop transmitting 

characters to the device. Once the buffer empties, 

the device will transmit an XON character to signal 

the host to resume transmission. 


DEC HEX GRAPHIC DEC HEX GRAPHIC DEC HEX GRAPHIC DEC HEX GRAPHIC 

117 75 u 

118 76 v 

119 77 w 

120 78 x 

121 79 y 

122 7A z 

123 7B { 

124 7C I 

125 7D } 

126 7E ~ 

127 7F DEL 

030 IE RS 

031 1F US 

032 20 SPACE 

033 21 ! 

034 22 " 

035 23 # 

036 24 $ 

037 25 % 

038 26 & 

039 27 ' 

040 28 ( 

041 29 ) 

042 2A * 

043 2B + 

044 2C , 

045 2D - 

046 2E . 

047 2F / 

048 30 0 

049 31 1 

050 32 2 

051 33 3 

052 34 4 

053 35 5 

054 36 6 

055 37 7 

056 38 8 

057 39 9 

058 3A : 

059 3B ; 

060 3C < 

061 3D = 

062 3E > 

063 3F ? 

064 40 @ 

065 41 A 

066 42 B 

067 43 C 

068 44 D 

069 45 E 

070 46 F 

071 47 G 

072 48 H 

073 49 I 

074 4A J 

074 4B K 

075 4C L 

076 4D M 

077 4E N 

078 4F O 

080 50 P 

081 51 Q 

082 52 R 

083 53 S 

084 54 T 

085 55 U 

086 56 V 

087 57 W 

088 58 X 

089 59 Y 

090 5A Z 

091 5B [ 

092 5C / 

093 5D ] 

094 5E V 

095 5F - 

096 60 ’ 

097 61 a 

098 62 b 

099 63 c 

100 64 d 

101 65 e 

102 66 f 

103 67 g 

104 68 h 

105 69 i 

106 6A j 

107 6B k 

108 6C l 

109 6D m 

110 6E n 

111 6F o 

112 70 p 

113 71 q 

114 72 r 

115 73 s 

116 74 t 


A51


Electrical Noise . . . 

Sources, Symptoms and Solutions 

Noise related difficulties can range in severity from 

minor positioning errors to damaged equipment 

from runaway motors crashing blindly through limit 

switches. In microprocessor controlled equipment, 

the processor is constantly retrieving instructions 

from memory in a controlled sequence. If an 

electrical disturbance occurs, it could cause the 

processor to misinterpret an instruction, or access 

the wrong data. This is likely to be catastrophic to 

the program, requiring a processor reset. Most 

Compumotor indexers are designed with a 

watchdog timer that shuts down the system if the 

program is interrupted. This prevents the more 

catastrophic failures. 

Sources of Noise 

Being invisible, electrical noise can be very 

mysterious, but it invariably comes from the 

following sources: 

• Power line disturbances 

• Externally conducted noise 

• Transmitted noise 

• Ground loops 

Some common electrical devices generate 

electrical noise. 

• Coil driven devices: conducted and power line 

noise 

• SCR-fired heaters: transmitted and power line 

noise 

• Motors and motor drives: transmitted and power 

line noise 

• Welders (electric): transmitted and power line 

noise 

Power line disturbances are usually easy to solve 

due to the wide availability of line filtering equipment 

for the industry. Only the most severe situations call 

for an isolation transformer. Line filtering equipment 

is required when other devices connected to the 

local power line are switching large amounts of 

current, especially if the switching takes place at 

high frequency. Corcom and Teal are two 

manufacturers of suitable power line filters. 

Also, any device having coils is likely to upset the 

line when it is switched off. Surge suppressors such 

as MOVs (General Electric) can limit this kind of 

noise. A series RC network across the coil is also 

effective, (resistance; 500 to 1,000 Ω, capacitance; 

0.1 to 0.2µF). Coil-driven devices (inductive loads) 

include relays, solenoids, contactors, clutches, 

brakes and motor starters. 

Fig. 5.10 Typical RC Network 

AC or DC 

R 

Inductive 

Load 

Externally Conducted Noise 

Externally conducted noise is similar to power line 

noise, but the disturbances are created on signal 

and ground wires connected to the indexer. This 

kind of noise can get onto logic circuit ground or 

into the processor power supply and scramble the 

program. The problem here is that control 

equipment often shares a common DC ground that 

may run to several devices, such as a DC power 

supply, programmable controller, remote switches 

and the like. When some noisy device, particularly a 

relay or solenoid, is on the DC ground, it may cause 

disturbances within the indexer. 

The solution for DC mechanical relays and 

solenoids involves connecting a diode backwards 

across the coil to clamp the induced voltage “kick” 

that the coil will produce. The diode should be rated 

at 4 times the coil voltage and 10 times the coil 

current. Using solid-state relays eliminates this 

effect altogether. 

Fig. 5.11 Coil Suppression Methods 

DC 

AC or DC 

Diode 

Varistor (MOV) 

Multiple devices on the same circuit should be 

grounded together at a single point. 

Furthermore, power supplies and programmable 

controllers often have DC common tied to Earth 

(AC power ground). As a rule, it is preferable to 

have indexer signal ground or DC common floating 

with respect to Earth. This prevents noisy 

equipment that is grounded to Earth from sending 

noise into the indexer. The Earth ground 

connection should be made at one point only as 

discussed in “Ground Loops” on p. A53. 

In many cases, optical isolation may be required to 

completely eliminate electrical contact between the 

indexer and a noisy environment. Solid-state relays 

provide this isolation. 

C 

A52


Transmitted Noise 

Transmitted noise is picked up by external 

connections to the indexer, and in severe cases, 

can attack an indexer with no external connections. 

The indexer enclosure will typically shield the 

electronics from this, but openings in the enclosure 

for connection and front panel controls may “leak”. 

As with all electrical equipment, the indexer chassis 

should be scrupulously connected to Earth to 

minimize this effect. 

When high current contacts open, they draw an 

arc, producing a burst of broad spectrum radio 

frequency noise that can be picked up on an 

indexer limit switch or other wiring. High current and 

high voltage wires have an electrical field around 

them, and may induce noise on signal wiring 

(especially when they are tied in the same wiring 

bundle or conduit). 

When this kind of problem occurs, consider 

shielding signal cables or isolating the signals. A 

proper shield surrounds the signal wires to intercept 

electrical fields, but this shield must be tied to Earth 

to drain the induced voltages. At the very least, 

wires should be run in twisted pairs to limit straight 

line antenna effects. 

Most Compumotor cables have shields tied to 

Earth, but in some cases the shields must be 

grounded at installation time. Installing the indexer 

in a NEMA electrical enclosure ensures protection 

from this kind of noise, unless noise-producing 

equipment is also mounted inside the enclosure. 

Connections external to the enclosure must be 

shielded. 

Even the worst noise problems, in environments 

near 600 amp welders and 25kW transmitters, have 

been solved using enclosures, conduit, optical 

isolation, and single-point ground techniques. 

Ground Loops 

Ground Loops create the most mysterious noise 

problems. They seem to occur most often in 

systems where a control computer is using 

RS-232C communication. Garbled transmission 

and intermittent operation symptoms are typical. 

The problem occurs in systems where multiple 

Earth ground connections exist, particularly when 

these connections are far apart. 

Example 

Suppose a Model 500 is controlling an axis, and the 

limit switches use an external power supply. The 

Model 500 is controlled by a computer in another 

room. If the power supply Common is connected to 

Earth, ground loop problems may occur (most 

computers have their RS-232C signal common tied 

to Earth). The loop starts at the Model 500’s limit 

switch ground, goes to Earth through the power 

supply to Earth at the computer. From there, the 

loop returns to the Model 500 through RS-232C 

signal ground. If a voltage potential exists between 

power supply Earth and remote computer Earth, 

ground, current will flow through the RS-232C 

ground creating unpredictable results. 

The way to test for and ultimately eliminate a ground 

loop is to lift or “cheat” Earth ground connections in 

the system until the symptoms disappear. 

Defeating Noise 

The best time to handle electrical noise problems is 

before they occur. When a motion system is in the 

design process, the designer should consider the 

following system wiring guidelines (listed by order of 

importance). 

1. Put surge suppression components on all 

electrical coils: resistor/capacitor filters, MOVs, 

Zener and clamping diodes. 

2. Shield all remote connections and use twisted 

pairs. Shields should be tied to Earth at one 

end. 

3. Put all microelectronic components in an 

enclosure. Keep noisy devices outside. Monitor 

internal temperature. 

4. Ground signal common wiring at one point. 

Float this ground from Earth if possible. 

5. Tie all mechanical grounds to Earth at one point. 

Run chassis and motor grounds to the frame, 

frame to Earth. 

6. Isolate remote signals. Solid-state relays or opto 

isolators are recommended. 

7. Filter the power line. Use common RF filters 

(isolation transformer for worst-case situations). 

A noise problem must be identified before it can be 

solved. The obvious way to approach a problem 

situation is to eliminate potential noise sources until 

the symptoms disappear, as in the case of ground 

loops. When this is not practical, use the above 

guidelines to troubleshoot the installation. 

References 

Information about the equipment referred to may be 

obtained by calling the numbers listed below. 

• Corcom line filters (312) 680-7400 

• Opto-22 optically isolated relays (714) 891-5861 

• Crydom optically isolated relays (213) 322-4987 

• Potter Brumfield optically isolated relays 

(812) 386-1000 

• General Electric MOVs (315) 456-3266 

• Teal power line isolation filters (800) 888-8325 


A53


Stopping in an Emergency 

For safety reasons, it is often necessary to 

incorporate some form of emergency stop system 

into machinery fitted with stepper or servo motors. 

There are several reasons for needing to stop 

quickly. 

• To prevent injury to the operator if he makes a 

mistake or operates the machinery improperly. 

• To prevent damage to the machine or to the 

product as a result of a jam. 

• To guard against machine faults. You should 

consider all the possible reasons for stopping to 

make sure that they are adequately covered. 

How should you stop the system? 

There are several ways to bring a motor to a rapid 

stop. The choice depends partly on whether it is 

more important to stop in the shortest possible time 

or to guarantee a stop under all circumstances. For 

instance, to stop as quickly as possible means 

using the decelerating power of the servo system. 

However, if the servo has failed or control has been 

lost, this may not be an option open to you. In this 

case, removing the power will guarantee that the 

motor stops; but if the load has a high inertia, this 

may take some time. If the load is moving vertically 

and can back-drive the motor, this introduces 

additional complications. In extreme cases where 

personal safety is at risk, it may be necessary to 

mechanically lock the system even at the expense 

of possible damage to the machine. 

Emergency Stop Methods 

1. Full-torque controlled stop. 

Applying zero velocity command to a servo amplifier 

will cause it to decelerate hard to zero speed in 

current limit, in other words, using the maximum 

available torque. This will create the fastest possible 

deceleration to rest. In the case of a digital servo 

with step and direction inputs, cutting off the step 

pulses will produce the same effect. 

The situation is different for a stepper drive. The 

step pulse train should be decelerated to zero 

speed to utilize the available torque. Simply cutting 

off the step pulses at speeds above the start-stop 

rate will de-synchronize the motor and the full 

decelerating torque will no longer be available. The 

controller needs to be able to generate a rapid 

deceleration rate independent of the normal 

programmed rate, to be used only for overtravel 

limit and emergency stop functions. 

2. Disconnect the motor. 

Although this method is undoubtedly safe, it is not 

highly recommended as a quick-stop measure. The 

time taken to stop is indeterminate, since it 

depends on load inertia and friction, and in highperformance 

systems the friction is usually kept to a 

minimum. Certain types of drives may be damaged 

by disconnecting the motor under power. This 

method is particularly unsatisfactory in the case of a 

vertical axis, since the load may fall under gravity. 

3. Remove the AC input power from the drive. 

On drives that incorporate a power dump circuit, a 

degree of dynamic braking is usually provided when 

the power is removed. This is a better solution than 

disconnecting the motor, although the power 

supply capacitors may take some time to decay 

and this will extend the stopping distance. 

4. Use dynamic braking. 

A motor with permanent magnets will act as a 

generator when driven mechanically. By applying a 

resistive load to the motor, a braking effect is 

produced that is speed-dependent. Deceleration is 

therefore rapid at high speeds, but falls off as the 

motor slows down. 

A changeover contactor can be arranged to switch 

the motor connections from the drive to the 

resistive load. This can be made failsafe by ensuring 

that braking occurs if the power supply fails. The 

optimum resistor value depends on the motor, but 

will typically lie in the 1-3 ohms range. It must be 

chosen to avoid the risk of demagnetization at 

maximum speed as well as possible mechanical 

damage through excessive torque. 

5. Use a mechanical brake. 

It is very often possible to fit a mechanical brake 

either directly on the motor or on some other part of 

the mechanism. However, such brakes are usually 

intended to prevent movement at power-down and 

are seldom adequate to bring the system to a rapid 

halt, particularly if the drive is delivering full current 

at the time. Brakes can introduce friction even when 

released, and also add inertia to the system – both 

effects will increase the drive power requirements. 

What is the best stopping method? 

It is clear that each of the methods outlined above 

has certain advantages and drawbacks. This leads 

to the conclusion that the best solution is to use a 

combination of techniques, ideally incorporating a 

short time delay. 

We can make use of the fact that a contactor used 

for dynamic braking will take a finite time to drop 

out, so it is possible to de-energize the contactor 

coil while commanding zero speed to the drive. This 

allows for a controlled stop to occur under full 

torque, with the backup of dynamic braking in the 

event that the amplifier or controller has failed. 

WARNING! – there is a risk that decelerating a 

servo to rest in full current limit could result in 

mechanical damage, especially if a high-ratio 

gearbox is used. This does not necessarily ensure a 

safe stop, be sure that the mechanism can 

withstand this treatment. 

A mechanical brake should also be applied to a 

vertical axis to prevent subsequent movement. An 

alternative to the electrically-operated brake is the 

differential drag brake, which will prevent the load 

from falling but creates negligible torque in the 

opposite direction. 

A54

System Selection System Considerations 

Calculations 

Application Considerations 

Accuracy 

An accuracy specification defines the maximum 

error in achieving a desired position. Some types of 

accuracy are affected by the application. For 

example, repeatability will change with the friction 

and inertia of the system the motor is driving. 

Accuracy in a rotary motor is usually defined in 

terms of arcminutes or arcseconds (the terms 

Stepper Accuracy 

There are several types of performance listed under 

Compumotor’s motor specifications: repeatability, 

accuracy, relative accuracy, and hysteresis. 

Repeatability 

The motor’s ability to return to the same position 

from the same direction. Usually tested by moving 

the motor one revolution, it also applies to linear 

step motors moving to the same place from the 

same direction. This measurement is made with the 

motor unloaded, so that bearing friction is the 

prominent load factor. It is also necessary to ensure 

the motor is moving to the repeat position from a 

distance of at least one motor pole. This 

compensates for the motor’s hysteresis. A motor 

pole in a Compumotor is 1/50 of a revolution. 

Accuracy 

Also referred to as absolute accuracy, this 

specification defines the quality of the motor’s 

mechanical construction. The error cancels itself 

over 360° of rotation, and is typically distributed in a 

sinusoidal fashion. This means the error will 

gradually increase, decrease to zero, increase in the 

opposite direction and finally decrease again upon 

reaching 360° of rotation. Absolute accuracy 

causes the size of microsteps to vary somewhat 

because the full motor steps that must be traversed 

by a fixed number of microsteps varies. The steps 

can be over or undersized by about 4.5% as a 

result of absolute accuracy errors. 

Relative Accuracy 

Also referred to as step-to-step accuracy, this 

specification tells how microsteps can change in 

size. In a perfect system, microsteps would all be 

exactly the same size, but drive characteristics and 

the absolute accuracy of the motor cause the steps 

to expand and contract by an amount up to the 

relative accuracy figure. The error is not cumulative. 

arcsecond and arcminute are equivalent to second 

and minute, respectively). There are 1,296,000 

seconds of arc in a circle. For example, an 

arcsecond represents 0.00291 inches of 

movement on a circle with a 50-foot radius. This is 

equivalent to about the width of a human hair. 

Servo & Closed-Loop Stepper Accuracy 

Repeatability, accuracy and relative accuracy in 

servos and closed-loop stepper systems relate as 

much to their feedback mechanisms as they do to 

the inherent characteristics of the motor and drive. 

Servos 

Compumotor servos use resolver feedback to 

determine their resolution and position. It is 

essentially the resolution of the device reading the 

resolver position that determines the highest 

possible accuracy in the system. Digiplan servos 

use encoder feedback to determine their resolution 

and position. In this case, it is the encoder’s 

resolution that determines the system’s accuracy. 

The positional accuracy is determined by the drive’s 

ability to move the motor to the position indicated 

by the resolver or encoder. Changes in friction or 

inertial loading will adversely affect the accuracy 

until the system is properly tuned. 

Closed-Loop Steppers 

Compumotor closed-loop stepper systems use an 

encoder to provide feedback for the control loop. 

The encoder resolution determines the system’s 

accuracy. When enabled, the controlling indexer 

attempts to position the motor within the specified 

deadband from the encoder. Typically, this means 

the motor will be positioned to within one encoder 

step. To do this satisfactorily, the motor must have 

more resolution than the encoder. If the step size of 

the motor is equal to or larger than the step size of 

the encoder, the motor will be unable to maintain a 

position and may become unstable. In a system 

with adequate motor-to-encoder resolution, the 

motor is able to maintain one encoder step of 

accuracy with great dependability. This is a 

continuous process that will respond to outside 

events that disturb the motor’s position. 


Hysteresis 

The motor’s tendency to resist a change in 

direction. This is a magnetic characteristic of the 

motor, it is not due to friction or other external 

factors. The motor must develop torque to 

overcome hysteresis when it reverses direction. In 

reversing direction, a one revolution move will show 

hysteresis by moving the full distance less the 

hysteresis figure. 

A55

Selection System Calculations Considerations 

Application Considerations 

Load characteristics, performance requirements, 

and coupling techniques need to be understood 

before the designer can select the best motor/drive 

for the job. While not a difficult process, several 

factors need to be considered for an optimum 

solution. A good designer will adjust the 

characteristics of the elements under his control – 

including the motor/drive and the mechanical 

transmission type (gears, lead screws, etc.) – to 

meet the performance requirements. Some 

important parameters are listed below. 

Torque 

Rotational force (ounce-inches) defined as a linear 

force (ounces) multiplied by a radius (inches). When 

selecting a motor/drive, the torque capacity of the 

motor must exceed the load. The torque any motor 

can provide varies with its speed. Individual speed/ 

torque curves should be consulted by the designer 

for each application. 

Inertia 

An object’s inertia is a measure of its resistance to 

change in velocity. The larger the inertial load, the 

longer it takes a motor to accelerate or decelerate 

that load. However, the speed at which a motor 

rotates is independent of inertia. For rotary motion, 

inertia is proportional to the mass of the object 

being moved times the square of its distance from 

the axis of rotation. 

Friction 

All mechanical systems exhibit some frictional force, 

and this should be taken into account when sizing 

the motor, as the motor must provide torque to 

overcome any system friction. A small amount of 

friction is desirable since it can reduce settling time 

and improve performance. 

Torque-to-Inertia Ratio 

This number is defined as a motor’s rated torque 

divided by its rotor inertia. This ratio is a measure of 

how quickly a motor can accelerate and decelerate 

its own mass. Motors with similar ratings can have 

different torque-to-inertia ratios as a result of 

varying construction. 

Load Inertia-to-Rotor Inertia Ratio 

For a high performance, relatively fast system, load 

inertia reflected to the motor should generally not 

exceed the motor inertia by more than 10 times. 

Load inertias in excess of 10 times the rotor inertia 

can cause unstable system behavior. 

Torque Margin 

Whenever possible, a motor/drive that can provide 

more motor torque than the application requires 

should be specified. This torque margin 

accommodates mechanical wear, lubricant 

hardening, and other unexpected friction. 

Resonance effects, while dramatically reduced with 

the Compumotor microstepping system, can cause 

the motor’s torque to be slightly reduced at some 

speeds. Selecting a motor/drive that provides at 

least 50% margin above the minimum needed 

torque is good practice. 

Velocity 

Because available torque varies with velocity, 

motor/drives must be selected with the required 

torque at the velocities needed by the application. 

In some cases, a change in the type of mechanical 

transmission used is needed to achieve the 

required performance. 

Resolution 

The positioning resolution required by the 

application will have an effect on the type of 

transmission used and the motor resolution. For 

instance, a leadscrew with 4 revolutions per inch 

and a 25,000-step-per-revolution motor/drive 

would give 100,000 steps per inch. Each step 

would then be 0.00001 inches. 

Duty Cycle 

Some motor/drives can produce peak torque for 

short time intervals as long as the RMS or average 

torque is within the motor’s continuous duty rating. 

To take advantage of this feature, the application 

torque requirements over various time intervals 

need to be examined so RMS torque can be 

calculated. 

Solving Duty Cycle Limitation Problems 

Operating a motor beyond its recommended duty 

cycle results in excessive heat in the motor and 

drive. The drive cycle may be increased by 

providing active cooling to the drive and the motor. 

A fan directed across the motor and another 

directed across the drive’s heatsink will result in 

increased duty cycle capability. 

In most cases, it is possible to tell if the duty cycle is 

being exceeded by measuring the temperature of 

the motor and drive. Refer to the specifications for 

individual components for their maximum allowable 

temperatures. 

Note: Motors will run at case temperatures up to 

100°C (212°F)—temperatures hot enough to burn 

individuals who touch the motors. 

To Improve Duty Cycle: 

• Mount the drive with heatsink fins running 

vertically 

• Fan cool the motor 

• Fan cool the drive 

• Put the drive into REMOTE POWER SHUTDOWN 

when it isn’t moving, or reduce current 

• Reduce the peak current to the motor 

(if possible) 

• Use a motor large enough for the application 

A56

Motor Sizing and System Selection Calculations Software 

A wide range of applications can be solved 

effectively by more than one motor type. However, 

some applications are particularly appropriate for 

each motor type. Compumotor’s Motor Sizing and 

Selection Software package is designed to help 

you easily identify the proper motor size and type 

for your specific motion control application. 

This software helps calculate load inertias and 

required torques—information that is reflected 

through a variety of machine transmissions and 

reductions, including leadscrews, gears, belts, and 

pulleys. This software then produces graphs of the 

results, allowing you to select the proper motor 

from a comprehensive, detailed database of more 

than 200 motor models. 

IBM ® PC-compatible, Motor Sizing & Selection 

software also generates a number of applicationspecific 

reports, including profiles and speed/ 

torque curves that are based on user-provided 

information. This advanced graphics package is 

VGA/EGA compatible and allows data entry with 

either a mouse or keypad. 

Contact your local Automotive Technology Center 

to obtain a copy. 


A57

System Calculations 

Move Profile 

Before calculating torque requirements of an 

application, you need to know the velocities and 

accelerations needed. For those positioning 

applications where only a distance (X) and a time 

(S) to move that distance are known, the 

trapezoidal motion profile and formulas given below 

are a good starting point for determining your 

requirements. If velocity and acceleration 

parameters are already known, you can proceed to 

one of the specific application examples on the 

following pages. 

Move distance X in time S. 

Assume that: 

1. Distance X/4 is moved in time S/3 (Acceleration) 

2. Distance X/2 is moved in time S/3 (Run) 

3. Distance X/4 is moved in time S/3 (Deceleration) 

The graph would appear as follows: 

V 

The acceleration (a), velocity (v) and deceleration (d) 

may be calculated in terms of the knowns, X and S. 

( 4 ) 

( 3 ) 

2 

X 

X 

a = -d = 2X = = 2 x 9 = 

t 2 

2 

S S 2 

v = at = 4.5X x S = 

S 2 3 

Example 

You need to move 6" in 2 seconds 

4.5 (6 inches) inches 

a = -d = = 6.75 

(2 seconds) 2 second 2 

v = 1.5 (6 inches) = 4.5 

(2 seconds) 

1.5X 

S 

inches 

second 

4.5X 

S 2 

Velocity 

a 

d 

0 S/3 2S/3 S 

time 

S/3 S/3 S/3 

Common Move Profile Considerations 

Distance: ______________ Inches of Travel ________________________ revolutions of motor 

Move Time: ____________________________________________________ seconds 

Accuracy: ______________________________________________________ arcminutes, degrees or inches 

Repeatability: ___________________________________________________ arcseconds, degrees or inches 

Duty Cycle 

on tme: ____________________________________________________ seconds 

off time: ___________________________________________________ seconds 

Cycle Rate: ____________________________________________________ sec. min. hour 

Motor/Drive Selection 

Based on Continuous Torque Requirements 

Having calculated the torque requirements for an 

application, you can select the motor/drive suited 

to your needs. Microstepping motor systems 

(S Series, Zeta Series OEM650 Series, LN Series) 

have speed/torque curves based on continuous 

duty operation. To choose a motor, simply plot total 

torque vs. velocity on the speed/torque curve. This 

point should fall under the curve and allow 

approximately a 50% margin for safety. An S106- 

178 and an S83-135 curve are shown here. 

Note: When selecting a ZETA Series product, a 

50% torque margin is not required. 

Example 

Assume the following results from load calculations: 

T 

F = 25 oz-in Friction torque 

T 

A = 175 oz-in Acceleration torque 

TT = 200 oz-in Total torque 

V = 15 rev/sec Maximum velocity 

You can see that the total torque at the required 

velocity falls within the motor/drive operating range 

for both motors by plotting T T 

. 

oz-in 

1500 

1200 

900 

600 

300 

TT = 200 

0 

106-178 

83-135 

0 10 20 30 40 50 

RPS (Vmax) 

The S83-135 has approximately 250 oz-in available 

at V max (25% more than required). The S106-178 

has 375 oz-in available, an 88% margin. 

In this case, we would select the S106-178 

motor/drive to assure a sufficient torque margin 

to allow for changing load conditions. 

A58

Motor/Drive Selection 

Based on peak torque requirements 

Servo-based motor/drives have two speed/torque 

curves: one for continuous duty operation and 

another for intermittent duty. A servo system can 

be selected according to the total torque and 

maximum velocity indicated by the continuous duty 

curve. However, by calculating the root mean 

square (RMS) torque based on your duty cycle, you 

may be able to take advantage of the higher peak 

torque available in the intermittent duty range. 

T RMS 

= 

Where: 

M 

Ti 2 ti 

ti 

M 

• Ti is the torque required over the time interval ti 

• means “the sum of” 

M 

Example 

Assume the following results from your load 

calculations. 

T F 

= 25 oz-in Friction Torque 

T A 

= 775 oz-in Acceleration Torque 

T T 

= 800 oz-in Total Torque 

V max 

= 20 rps Maximum Velocity 

Duty Cycle 

Index 4 revs in 0.3 seconds, dwell 0.3 seconds 

then repeat. 

If you look at the S106-178 speed/torque curve, 

you’ll see that the requirements fall outside the 

curve. 

T 1 

= 

T 2 

= 

T 3 

= 

T 4 

= 

t 1 

= 

t 2 

= 

t 3 

= 

t 4 

= 

T RMS 

= 

= 

Torque reqired to accelerate the load from 

zero speed to maximum speed (T F 

+ T A 

) 

Torque required to keep the motor moving 

once it reaches max speed (T F 

) 

Torque required to decelerate from max 

speed to a stop (T A 

- T F 

) 

Torque required while motor is sitting still at 

zero speed (Ø) 

Time spent accelerating the load 

Time spent while motor is turning at 

constant speed 

Time spent decelerating the load 

Time spent while motor is at rest 

T RMS 

= 447 oz. in. 

T 12 

t 1 

+ T 2 

2 

t 2 

+ T 3 

2 

t 3 

+ T 4 

2 

t 4 

t 1 

+ t 2 

+ t 3 

+ t 4 

(800) 2 (.1) + (25) 2 (.1) + (750) 2 (.1) + (0) 2 (.3) 

(.1) + (.1) + (.1) + (.3) 



Motion Profile 

20 rps 

Now plot T RMS 

and T T 

vs. T max 

on the speed/torque 

curve. 

The drawing below resembles the 

speed/torque curve for the Z606 motor. 

1800 

0 

t1 

0.1 0.2 0.3 0.4 0.5 0.6 t 

t2 t3 t4 

T T (800) 

600 

T RMS(506) 

10 20 30 40 50 60 

The Z606 motor will meet the requirements. 

RMS torque falls within the continuous duty 

cycle and total torque vs. velocity falls within 

the intermittent range. 

How to Use a Step Motor 

Horsepower Curve 

Horsepower (HP) gives an indication of the motor’s 

top usable speed. The peak or “hump” in a 

horsepower curve indicates a speed that gives 

maximum power. Choosing a speed beyond the peak 

of the HP curve results in no more power: the power 

attained at higher speeds is also attainable at a lower 

speed. Unless the speed is required for the 

application, there is little benefit to going beyond the 

peak as motor wear is faster at higher speeds. 

Applications requiring the most power the motor can 

generate, not the most torque, should use a motor 

speed that is just below the peak of the HP curve. 

Torque 

oz-in (N-m) 

(HP) 

175 (1.22) 

.175 

Torque 

140 (1.98) 

.140 

105 (.73) 

.105 

70 (.49) 

.070 

Horsepower 

35 (.24) 

.035 

0 

0 

0 6 12 18 24 30 

Speed-rps 

Power 

A59


Leadscrew Drives 

Leadscrews convert rotary motion to linear motion 

and come in a wide variety of configurations. 

Screws are available with different lengths, 

diameters, and thread pitches. Nuts range from the 

simple plastic variety to precision ground versions 

with recirculating ball bearings that can achieve very 

high accuracy. 

The combination of microstepping and a quality 

leadscrew provides exceptional positioning 

resolution for many applications. A typical 10-pitch 

(10 threads per inch) screw attached to a 25,000 

step/rev. motor provides a linear resolution of 

0.000004" (4 millionths, or approximately 0.1 

micron) per step. 

A flexible coupling should be used between the 

leadscrew and the motor to provide some damping. 

The coupling will also prevent excessive motor 

bearing loading due to any misalignment. 

Microscope Positioning 

Application Type: X/Y Point to Point 

Motion: Linear 

Description: A medical research lab needs to 

automate their visual inspection process. Each 

specimen has an origin imprinted on the slide with 

all other positions referenced from that point. The 

system uses a PC-AT Bus computer to reduce data 

input from the operator, and determines the next 

data point based on previous readings. Each data 

point must be accurate to within 0.1 microns. 

Machine Objectives 

• Sub-micron positioning 

• Specimen to remain still during inspection 

• Low-speed smoothness (delicate equipment) 

• Use PC-AT Bus computer 

Motion Control Requirements 

• High resolution, linear encoders 

• Stepper (zero speed stability) 

• Microstepping 

• PC-AT Bus controller 

Compumotor Solution: Microstepping motors 

and drives, in conjunction with a precision ground 

40 pitch leadscrew table, provide a means of submicron 

positioning with zero speed stability. 

Conventional mechanics cannot provide 0.1 micron 

accuracies without high grade linear encoders. It is 

necessary for the Compumotor Model AT6400 

indexer, which resides directly on the computer 

bus, to provide full X, Y, Z microscope control and 

accept incremental encoder feedback. 

Microstepping 

motors 

Encoders 

A60

Other Leadscrew Drive Applications 

Precision Grinder 

2.50 17.1807 18.4079 5.9059 oz-in 2 5.00 274.8916 294.5267 94.4940 oz-in 2 

A bearing manufacturer is replacing some 

• XY Plotters 

equipment that finishes bearing races. The old • Facsimile transmission 

equipment had a two-stage grinding arrangement 

• Tool bit positioning 

where one motor and gearbox provided a rough cut 

and a second motor with a higher ratio gearbox • Cut-to-length machinery 

performed the finishing cut. The designer would like • Back gauging 

to simplify the mechanics and eliminate one motor. • Microscope drives 

He wants to use a single leadscrew and exploit the • Coil winders 

wide speed range available with microstepping to 

perform both cuts. This will be accomplished by 

• Slides 

moving a cutting tool mounted on the end of the • Pick-and-Place machines 

leadscrew into the workpiece at two velocities; an • Articulated arms 

initial velocity for the rough cut and a much reduced 

final velocity for the finish cut. 

The torque required to accelerate the load and 

overcome the inertia of the load and the rotational 

inertia of the leadscrew is determined to be 120 ozin. 

The torque necessary to overcome friction is 

measured with a torque wrench and found to be 40 

oz-in. A microstepping motor with 290 oz-in of 

torque is selected and provides adequate torque 

margin. 

This grinder is controlled by a programmable 

controller (PC) and the environment requires that 

the electronics withstand a 60°C environment. An 

indexer will provide the necessary velocities and 

accelerations. The speed change in the middle of 

the grinding operation will be signaled to the PC 

with a limit switch, and the PC will in turn program 

the new velocity into the indexer. Additionally, the 

indexer Stall Detect feature will be used in 

conjunction with an optical encoder mounted on 

the back of the motor to alert the PC if the 

mechanics become “stuck.” 

Leadscrew Application Data 

Inertia of Leadscrews per Inch 

Diameter 

Diameter 

In. Steel Brass Alum. 

In. Steel Brass Alum. 

0.25 0.0017 0.0018 0.0006 oz-in 2 2.75 25.1543 26.9510 8.6468 oz-in 2 

0.50 0.0275 0.0295 0.0094 oz-in 2 3.00 35.6259 38.1707 12.2464 oz-in 2 

0.75 0.1392 0.1491 0.0478 oz-in 2 3.25 49.0699 52.5749 16.8678 oz-in 2 

1.00 0.4398 0.4712 0.1512 oz-in 2 3.50 66.0015 70.7159 22.6880 oz-in 2 

1.25 1.0738 1.1505 0.3691 oz-in 2 3.75 86.9774 93.1901 29.8985 oz-in 2 

1.50 2.2266 2.3857 0.7654 oz-in 2 4.00 112.5956 120.6381 38.7047 oz-in 2 

1.75 4.1251 4.4197 1.4180 oz-in 2 4.25 143.4951 153.7448 49.3264 oz-in 2 

2.00 7.0372 7.5399 2.4190 oz-in 2 4.50 180.3564 193.2390 61.9975 oz-in 2 

2.25 11.2723 12.0774 3.8748 oz-in 2 4.75 223.9009 239.8939 76.9659 oz-in 2 



Coefficients of Static Friction Materials 

(Dry Contact Unless Noted) µS 

Steel on Steel 0.58 

Steel on Steel (lubricated) 0.15 

Aluminum on Steel 0.45 

Copper on Steel 0.22 

Brass on Steel 0.19 

Teflon on Steel 0.04 

Leadscrew Efficiencies 

Efficiency (%) 

Type High Median Low 

Ball-nut 95 90 85 

Acme with metal nut* 55 40 35 

Acme with plastic nut 85 65 50 

* Since metallic nuts usually require a viscous 

lubricant, the coefficient of friction is both speed 

and temperature dependent. 

A61


Leadscrew Drives 

Vertical or Horizontal Application: 

ST – Screw type, ball or acme ST = 

e – Efficiency of screw e = % 

µ S 

– Friction coefficient µ S 

= 

L – Length ofscrew L = inches 

D – Diameter of screw D = inches 

p—Pitch p = threads/inch 

W – Weight of load W = lbs. 

F—Breakaway force F = ounces 

Directly coupled to the motor? yes/no 

If yes, CT – Coupling type 

If no, belt & pulley or gears 

Radius of pulley or gear 

inches 

Gear: Number of teeth – Gear 1 

Number of teeth – Gear 2 

Weight of pulley or gear 

ounces 

Weight of belt 

ounces 

Leadscrew Formulas 

The torque required to drive load W using a 

leadscrew with pitch (p) and efficiency (e) has the 

following components: 

T Total 

= T Friction 

+ T Acceleration 

F 

T Friction 

= 

2πpe 

Where: 

F = frictional force in ounces 

p = pitch in revs/in 

e = leadscrew efficiency 

F = µ s 

W for horizontal surfaces where µ s 

= 

coefficient of static friction and W is the weight of 

the load. This friction component is often called 

“breakaway”. 

Dynamic Friction: F = µ D 

W is the coefficient to use 

for friction during a move profile. However, torque 

calculations for acceleration should use the worst 

case friction coefficient, µ s 

. 

T Accel 

= 1 (J Load 

+ J Leadscrew 

+ J Motor 

) ω 

g 

t 

ω = 2πpv 

J Load 

= w ; J Leadscrew 

= πLρR 4 

(2πp) 2 2 

Where: 

T = torque, oz-in 

ω = angular velocity, radians/sec 

t = time, seconds 

v = linear velocity, in/sec 

L = length, inches 

R = radius, inches 

ρ = density, ounces/in 3 

g = gravity constant, 386 in/sec 2 

The formula for load inertia converts linear inertia 

into the rotational equivalent as reflected to the 

motor shaft by the leadscrew. 

Problem 

Find the torque required to accelerate a 200-lb steel 

load sliding on a steel table to 2 inches per second 

in 100 milliseconds using a 5 thread/inch steel 

leadscrew 36 inches long and 1.5 inches in 

diameter. Assume that the leadscrew has an Acme 

thread and uses a plastic nut. Motor inertia is given 

as 6.56 oz-in 2 . In this example, we assume a 

horizontally oriented leadscrew where the force of 

gravity is perpendicular to the direction of motion. 

In non-horizontal orientations, leadscrews will 

transmit varying degrees of influence from gravity to 

the motor, depending on the angle of inclination. 

Compumotor Sizing Software automatically 

calculates these torques using vector analysis. 

1. Calculate the torque required to overcome 

friction. The coefficient of static friction for steel-tosteel 

lubricant contact is 0.15. The median value of 

efficiency for an Acme thread and plastic nut is 

0.65. Therefore: 

(16 oz) 

F = µ s 

W = 0.15 (200 lb) 

lb 

= 480 oz 

F 480 oz 

T Friction 

= 2πpe = 2π 

x 

5 rev x 0.65 = 23.51 oz-in 

rev in 

2. Compute the rotational inertia of the load and the 

rotational inertia of the leadscrew: 

J Load 

= W = 200 lb x 16 oz 

(2πp) 2 

(2π5) 2 lb 

= 3.24 oz-in 2 

in 2 

J = Leadscrew 

πLρR 4 = π (36 in) (4.48 (0.75 in) 4 

oz) 

2 2 

in 3 

= 80.16 oz-in 2 

3. The torque required to accelerate the load may 

now be computed since the motor inertia was 

given: 

1 

ω 

T Accel 

= g (J Load 

+ J Leadscrew 


) t 

ω = 2π 

e 

sec) = 

5 

( in)( 2 in 

20π 

sec 

20π 

1 

. 

= (4.99 + 80.16 + 6.56(oz-in 2 )) sec 

386 in/sec 2 0.1sec 

= 149 oz-in 

T Total 

= T Friction 

+ T Accel 

T Total 

= 23.51 oz-in + 149 oz-in = 172.51 oz-in 

A62

Directly Driven Loads 

There are many applications where the motion 

being controlled is rotary and the low-speed 

smoothness and high resolution of a Compumotor 

system can be used to eliminate gear trains or other 

mechanical linkages. In direct drive applications, a 

motor is typically connected to the load through a 

Direct Drive Formulas 

R 

L 

flexible or compliant coupling. This coupling 

provides a small amount of damping and helps 

correct for any mechanical misalignment. 

Direct drive is attractive when mechanical simplicity 

is desirable and the load being driven is of 

moderate inertia. 

R 2 

R 1 

L 

5.96 



R – Radius R = inches 

R(1) – Inner radius R(1) = inches 

R(2) – Outer radius R(2) = inches 

L – Length L = inches 

W – Weight of disc W = ounces 

ρ – Density/Material ρ = ounces/inch 3 

g – Gravity constant g = 386 in/sec 2 

Solid Cylinder (oz-in 2 ) 

Where: 

Inertia: J Load 

= WR 2 

a = angular acceleration, radians/sec 2 

2 

ω 2 

= final velocity, radians/sec 

Where weight and radius are known 

ω 1 

= initial velocity, radians/sec 

Inertia (oz-in 2 ) J Load 

= πLρR 4 

t = time for velocity change, seconds 

2 

J = inertia in units of oz-in 2 

Where ρ, the material density is known 

The angular acceleration equals the time rate of 

Weight W = πLρR 2 

change of the angular velocity. For loads 

accelerated from zero, ω 

Inertia may be calculated knowing either the weight 

1 

= 0 and a = ω 

T 

and radius of the solid cylinder (W and R) or its 

Total 

= 1 (J Load 


) ω t 

g 

t 

density, radius and length (ρ, R and L.) 

T Total 

represents the torque the motor must deliver. 

The torque required to accelerate any load is: The gravity constant (g) 

in the denominator 

T (oz-in) = Ja 

represents acceleration 

a = 

ω 2 

- ω 1 = 2π (accel.) for Accel. in rps 2 due to gravity (386 in/ 

sec 2 R 

t 

) and converts 

L 

inertia from units of ozin 

2 to oz-in-sec 2 . 

Hollow Cylinder 

w 

J Load 

= 2 (R 2 + 1 R2 ) 2 

Where W, the weight, is known 

or πLρ 

J Load 

= 2 (R 4 – 2 R4 ) 1 

Where ρ, the density, is known 

W = 1πLρ (R 2 – 2 R2 ) 1 

T = g (J Load 


) 

R 2 

1 

ω 

t 

L 

Problem 

Calculate the motor torque required to accelerate a 

solid cylinder of aluminum 5" in radius and 0.25" 

thick from rest to 2.1 radians/sec (0.33 revs/sec) in 

0.25 seconds. First, calculate J Load 

using the 

density for aluminum of 1.54 oz/in 3 . 

J Load 

= πLρR4 = π x 0.25 x 1.54 x 5 4 

= 378 oz-in 2 

2 

2 

Assume the rotor inertia of the motor you will use is 

37.8 oz-in 2 . 

T Total 

= 1 (J Load 


) x ω g 

t 

= 

1 

x (378 + 37.8) x 

2.1 

386 

0.25 

= 9.05 oz-in 

R 

5.96 

A63


Gear Drives 

Traditional gear drives are more commonly used 

with step motors. The fine resolution of a 

microstepping motor can make gearing 

unnecessary in many applications. Gears generally 

have undesirable efficiency, wear characteristics, 

backlash, and can be noisy. 

Gears are useful, however, when very large inertias 

must be moved because the inertia of the load 

reflected back to the motor through the gearing is 

divided by the square of the gear ratio. 

In this manner, large inertial loads can be moved 

while maintaining a good load-inertia to rotor-inertia 

ratio (less than 10:1). 

Gear Driven Loads 

R – Radius R = inches 

R(1) – Radius gear #1 R(1) = inches 

R(2) – Radius gear #2 R(2) = inches 

N(1) – Number of teeth G#1 N(1) = 

N(2) – Number of teeth G#2 N(2) = 

G – Gear ratio N(1) G = 

N(2) 

W – Weight of load W = ounces 

W(1) – Weight G#1 W(1) = ounces 

W(2) – Weight G#2 W(2) = ounces 

L – Length L = inches 

F – Friction F = 

BT – Breakaway torque BT = ounce/inches 

N1 

R 

G1 

R1 

Gears 

W 

R2 

G2 

N2 

Gear Drive Formulas 

J Load 

= 

or 

J Load 

= 

J Gear1 

= 

J Gear2 

= 

W Load 

2 

πL Load 

ρ Load 

2 

W Gear1 

2 

W Gear2 

2 

2 

N 

R 

( 

Gear 2 

N Gear 1 

) 

2 Load 

2 

N 

R 4 Gear 2 

Load ( N Gear 1 

) 

2 

N 

R 

( 

Gear 2 

N Gear 1 

) 

2 Gear1 

R 2 Gear2 

T Total 

= 1 (J Load 

+ J Gear1 

+ J Gear2 


) 

g 

ω 

t 

R 

Where: 

W 

G1 

R1 

N1 

R2 

G2 

N2 

Gears 

J = inertia, oz-in (gm-cm 2 ) “as seen by the motor” 

T = torque, oz-in (gm-cm) 

W = weight, oz (gm) 

R = radius, in. (cm) 

N = number of gear teeth (constant) 

L = length, in (cm) 

ρ = density, oz/in 3 (gm/cm 3 ) 

ω = angular velocity, radians/sec @ motor shaft 



A64

Tangential Drives 

W 


R– Radius 

R 

R = inches 

W – Weight (include weight of belt or chain) W = ounces 

W(P) – Weight of pulley or material W(P) ounces 

F – Breakaway force F = ounces 

V – Linear velocity V = inches/sec 

CT – Coupling type CT = 

SL – Side load SL = 


Tangential Drive Formulas 

T Total 

= T Load 

+ T Pulley 

+ T Belt 

+ T Motor 

+ T Friction 

T Total 

= 

1 

(J Load 

+ J Pulley 

+ J Belt 


) 

ω 

+T 

g 

t Friction 

J Load 

= W L 

R 2 

W p 

R 2 

J Pulley 

= 2 (Remember to multiply by 2 

if there are 2 pulleys.) 

J Belt 

= W B 

R 2 

T Friction 

= FR 

Problem 

What torque is required to accelerate a 5-lb load to 

a velocity of 20 inches per second in 10 

milliseconds using a flat timing belt? The motor 

drives a 2-inch diameter steel pulley 1/2-inch wide. 

The timing belt weighs 12 oz. Load static friction is 

30 ozs. Motor rotor inertia is 10.24 oz-in. 2 

J Load 

= W L 

R 2 = 5 lb x 16 oz x (1 in) 2 = 80 oz-in 2 

lb 

J Pulley 

= 

2(πLρR 4 ) 

2 

= 7.04 oz-in 2 

= π x 0.5 in x 

J Belt 

= W B 

R 2 = 12 oz (1 in) 2 = 12 oz-in 2 

(4.48 oz/in 3 ) (1 in) 4 

ω = V R 

Where: 

T = torque, oz-in (gm-cm) 

ω = angular velocity, radians/sec 


W L 

= weight of the load, oz 

W P 

= pulley weight, oz 

W B 

= belt or rack weight, oz 

F = frictional force, oz (gm) 

R = radius, in (cm) 

V = linear velocity 


ρ = density, oz/in 3 

T Friction 

= F x R = 30 oz x 1 in = 30 oz-in 

V in 1 rad 

ω = 

R 

= 20 

sec 

x 

1 in 

= 20 

1 

20 

T Total 

= 386 (80 + 7.04 + 12 + 10.24) .01 + 30 

T Total 

= 596.2 oz-in 

rad 

sec 

A65


Linear Step Motors 

There are many characteristics to consider when 

designing, selecting and installing a complete 

motion control system. The applications data 

worksheet and the application considerations 

detailed below will help determine if a linear motor 

system is recommended for a given application. A 

linear motor, when properly specified, will provide 

the optimum performance and the greatest 

reliability. 

Application Data Worksheet #1 

Application: Single Axis √ Multi-Axis X-Y Gantry 

Description of system operation: A part is moved in and out of a 

machine very quickly. The part comes to rest at the same point in the 

machine each time. An operator sets this distance with a thumbwheel 

switch. 

Step 1: Total mass to be accelerated 

Mtotal = Mload (10.0) + Mforcer (2.0) = 12.0 lbs. 

Step 2: Acceleration rate 

A. Average velocity = move distance 

move time 

= (40 inches) 

(1.0 sec) 

= 40.0 in/sec 

B. Maximum velocity 

(Based on trapezoidal move profile) 

Vmax = 1.33 x Vavg (40.0 in/sec) 

= 53.2 in/sec 

Velocity 

(in/sec) 

Sketch the proposed mechanical configuration: 

53.2 

40 

Vmax 

Vave 

4 lbs. 

1/4 1/4 1/4 1/4 

Time (sec) 

1. Motor Sizing AXIS 1 

A. Weight of payload (lbs) 10.0 

B. Fixed forces, if any (lbs) 0 

C. Known move distance (in) 40 

time (sec) 1.0 

D. Angle from horizontal (degrees) 0 

2. Total length of travel (inches) 40 

3. Desired repeatability (in) 001 

4. Desired resolution (in) .0005 

5. Necessary settling time after move 

100 ms to within .001 inches 

6. Life expectancy: 

Percent duty cycle 20% 

Estimated number of moves/year 200,000 

7. Is the center of gravity significantly changed? no 

8. What is the environment? clean [√] dirty [ ] 

Specifics 

9. Operating temperature range 65° to 85°F 

10. Can air be available? yes 

20" 

The Solution 

Actual and assumed factors that contribute to the 

solution are: 

1. Force (F) = mass (M) x acceleration (A) 

Note: mass units are in pounds 

2. Acceleration due to gravity 

(1g=386 inches/sec 2 ) 

3. L20 forcer weighs 2.0 lb. 

4. Attractive force between L20 forcer and platen 

= 200 lbs. 

5. Trapezoidal velocity profile: 

Accel time = 1.0 sec/4 

= 0.250 sec. 

Vmax = 1.33 x Vavg 

0 1.0 

1.0 

4 

C. Minimum acceleration rate 

A = Vmax (53.2 in/sec) = 212.8 in/sec 2 

Accel. time (.250 sec) 

A = Minimum acceleration (212.8 in/sec 2 ) 

386 in/sec 2 per 1 G 

= 0.551 g's 

Step 3: Calculate maximum acceleration rate of 

L20 (using constant acceleration indexer). 

Based on the speed/force curve below, the L20 

has 14.0 lbs of force at 53.2 in/sec (Vmax). 

Force lbs (kg) 

20 

(9.08) 

16 

(7.26) 

12 

(6.45) 

8 

(3.83) 

4 

(1.82) 

0 

Amax = 

Force vs. Speed L-L20 

20 

(50.8) 

14.0 lbs 

40 

(101.6) 

53.2 ips 

60 

(152.4) 

Speed in (cm) 

80 

(203.2) 

100 

(254.0) 

Step 4: Non-damped safety margin 

If all available force could be used, the maximum 

calculated acceleration rate: 

Force (14.0 lb) 

Mtotal (12.0 lbs.) 

= 1.16 g's 

The calculated acceleration rate should be reduced 

by 50% (100% non-damped safety margin) netting 

an acceleration rate for the L20 of 0.58 g's. The 

application requires a 0.55 g's acceleration rate. 

The L20 meets the requirements of this application. 

A66


Velocity Ripple 

Velocity ripple is most noticeable when operating 

near the motor’s resonant frequency. Rotary 

stepping motor’s have this tendency as well, but it 

is usually less noticeable due to mechanical losses 

in the rotary-to-linear transmission system, which 

dampens the effects. Velocity ripple due to 

resonance can be reduced with the electronic 

accelerometer damping option (-AC). 

Platen Mounting 

The air gap between the forcer and the platen 

surface can be as small as 0.0005 inches. Properly 

mounting the platen is extremely important. When 

held down on a magnetic chuck, the platen is flat 

and parallel within its specifications, however, in its 

free state, slight bows and twists may cause the 

forcer (L20) to touch the platen at several places. 

Compumotor recommends mounting the platen 

using all its mounting holes on a ground flat piece of 

steel, such as an I-beam, U-channel or tube. 

Environment 

Due to the small air gap between the forcer and 

platen, care should be taken to keep the platen 

clean. A small amount of dirt or adhesive material 

(such as paint) can cause a reduction in motor 

performance. When appropriate, mounting the 

motor upside down or on its side will help keep 

foreign particles off the platen. Protective boots that 

fold like an accordion as the motor travels can also 

be used to assist in keeping the platen clean. 

Linear Step Motors 

Life Expectancy 

The life of a mechanical bearing motor is limited by 

wearing of the platen surface over which the 

bearings roll. Factors that affect wear and life of a 

mechanical bearing system include: 

A. High velocities – Life is inversely proportional to 

velocity cubed. Increasing velocity raises the 

temperature of the platen due to eddy current 

losses in the solid platen material. (In normal 

high-speed, high duty cycle operation over a 

small piece of platen, the platen can become 

almost too hot to touch.) 

B. Load on the forcer – Load has some effect on 

the life expectancy of the linear motor. Users are 

urged to adhere to the load specifications for 

each motor. 

Yaw, Pitch and Roll 

In applications such as end effector devices or 

where the load is located far from the motor’s 

center of gravity, the stiffness characteristics of the 

forcer must be considered. Moment producing 

forces tend to deflect the forcer, and if strong 

enough, will cause the motor to stall or be removed 

from the platen. Yaw, pitch and roll specifications 

are used to determine the maximum torque you can 

apply to the forcer. 

Accuracy 

In linear positioning systems, some applications 

require high absolute accuracy, while many 

applications require a high degree of repeatability. 

These two variables should be reviewed to 

accurately evaluate proper system performance. 

In the “teach mode”, a linear motor can be 

positioned and subsequently learn the coordinates 

of any given point. After learning a number of points 

in a sequence of moves, the user will be concerned 

with the ability of the forcer to return to the same 

position from the same direction. This scenario 

describes repeatability. 

In a different application, a linear motor is used to 

position a measuring device. The size of an object 

can be measured by positioning the forcer to a 

point on the object. Determining the measured 

value is based on the number of steps required to 

reach the point on the object. System accuracy 

must be smaller than the tolerance on the desired 

measurement. 

Open-loop absolute accuracy of a linear step motor 

is typically less than a precision grade leadscrew 

system. If a linear encoder is used in conjunction 

with a linear motor, the accuracy will be equivalent 

to any other transmission system. 

The worst-case accuracy of the system is the 

sum of these errors: 

Accuracy = A + B + C + D + E + F 

A = 

B = 

C = 

D = 

E = 

F = 

Cyclic Error – The error due to motor 

magnetics that recurs once every pole pitch 

as measured on the body of the motor. 

Unidirectional Repeatability – The error 

measured by repeated moves to the same 

point from different distances in the same 

direction. 

Hysteresis – The backlash of the motor 

when changing direction due to magnetic 

non-linearity and mechanical friction. 

Cumulative Platen Error – Linear error of 

the platen as measured on the body of the 

motor. 

Random Platen Error – The non-linear 

errors remaining in the platen after the linear 

is disregarded. 

Thermal Expansion Error – The error 

caused by a change in temperature 

expanding or contracting the platen. 


A67

System Glossary Calculations 

of Terms 

Absolute Positioning 

Refers to a motion control system 

employing position feedback devices 

(absolute encoders) to maintain a given 

mechanical location. 

Absolute Programming 

A positioning coordinate referenced 

wherein all positions are specified relative 

to some reference, or “zero” position. This 

is different from incremental programming, 

where distances are specified relative to 

the current position. 

AC Servo 

A general term referring to a motor drive 

that generates sinusoidal shaped motor 

currents in a brushless motor wound as to 

generate sinusoidal back EMF. 

Acceleration 

The change in velocity as a function of 

time. Acceleration usually refers to 

increasing velocity and deceleration 

describes decreasing velocity. 

Accuracy 

A measure of the difference between 

expected position and actual position of a 

motor or mechanical system. Motor 

accuracy is usually specified as an angle 

representing the maximum deviation from 

expected position. 

Ambient Temperature 

The temperature of the cooling medium, 

usually air, immediately surrounding the 

motor or another device. 

ASCII 

American Standard Code for Information 

Interchange. This code assigns a number 

series of electrical signals to each numeral 

and letter of the alphabet. In this manner, 

information can be transmitted between 

machines as a series of binary numbers. 

Bandwidth 

A measure of system response. It is the 

frequency range that a control system can 

follow. 

BCD 

Binary Coded Decimal is an encoding 

technique used to describe the numbers 0 

through 9 with four digital (on or off) signal 

lines. Popular in machine tool equipment, 

BCD interfaces are now giving way to 

interfaces requiring fewer wires – such as 

RS-232C. 

Bit 

Abbreviation of Binary Digit, the smallest 

unit of memory equal to 1 or 0. 

Back EMF 

The voltage produced across a winding of 

a motor due to the winding turns being cut 

by a magnetic field while the motor is 

operating. This voltage is directly 

proportional to rotor velocity and is 

opposite in polarity to the applied voltage. 

Sometimes referred to as counter EMF. 

Block Diagram 

A simplified schematic representing 

components and signal flow through a 

system. 

Bode Plot 

A graph of system gain and phase versus 

input frequency which graphically 

illustrates the steady state characteristics 

of the system. 

Break Frequency 

Frequency(ies) at which the gain changes 

slope on a Bode plot (break frequencies 

correspond to the poles and zeroes of the 

system). 

Brushless DC Servo 

A general term referring to a motor drive 

that generates trapezoidal shaped motor 

currents in a motor wound as to generate 

trapezoidal Back EMF. 

Byte 

A group of 8 bits treated as a whole, with 

256 possible combinations of one’s and 

zero’s, each combination representing a 

unique piece of information. 

Commutation 

The switching sequence of drive voltage 

into motor phase windings necessary to 

assure continuous motor rotation. A 

brushed motor relies upon brush/bar 

contact to mechanically switch the 

windings. A brushless motor requires a 

device that senses rotor rotational position, 

feeds that information to a drive that 

determines the next switching sequence. 

Closed Loop 

A broadly applied term relating to any 

system where the output is measured and 

compared to the input. The output is then 

adjusted to reach the desired condition. In 

motion control, the term describes a 

system wherein a velocity or position (or 

both) transducer is used to generate 

correction signals by comparison to 

desired parameters. 

Critical Damping 

A system is critically damped when the 

response to a step change in desired 

velocity or position is achieved in the 

minimum possible time with little or no 

overshoot. 

Crossover Frequency 

The frequency at which the gain intercepts 

the 0 dB point on a Bode plot (used in 

reference to the open-loop gain plot). 

Daisy-Chain 

A term used to describe the linking of 

several RS-232C devices in sequence 

such that a single data stream flows 

through one device and on to the next. 

Daisy-chained devices usually are 

distinguished by device addresses, which 

serve to indicate the desired destination for 

data in the stream. 

Damping 

An indication of the rate of decay of a 

signal to its steady state value. Related to 

settling time. 

Damping Ratio 

Ratio of actual damping to critical 

damping. Less than one is an 

underdamped system and greater than 

one is an overdamped system. 

Dead Band 

A range of input signals for which there is 

no system response. 

Decibel 

A logarithmic measurement of gain. If G is 

a system’s gain (ratio of output to input), 

then 20 log G = gain in decibels (dB). 

Detent Torque 

The minimal torque present in an 

unenergized motor. The detent torque of a 

step motor is typically about 1% of its 

static energized torque. 

Direct Drive Servo 

A high-torque, low-speed servo motor with 

a high resolution encoder or resolver 

intended for direct connection to the load 

without going through a gearbox. 

Duty Cycle 

For a repetitive cycle, the ratio of on time 

to total cycle time. 

On Time 

Duty cycle = 

(On Time + Off Time) 

Efficiency 

The ratio of power output to power input. 

Electrical Time Constant 

The ratio of armature inductance to 

armature resistance. 

Encoder 

A device that translates mechanical motion 

into electronic signals used for monitoring 

position or velocity. 

Form Factor 

The ratio of the RMS value of a harmonic 

signal to its average value in one halfwave. 

Friction 

A resistance to motion. Friction can be 

constant with varying speed (Coulomb 

friction) or proportional to speed (viscous 

friction). 

Gain 

The ratio of system output signal to system 

input signal. 

Holding Torque 

Sometimes called static torque, it specifies 

the maximum external force or torque that 

can be applied to a stopped, energized 

motor without causing the rotor to rotate 

continuously. 

A68

System Glossary Calculations of Terms 

Home 

A reference position in a motion control 

system derived from a mechanical datum 

or switch. Often designated as the “zero” 

position. 

Hybrid Servo 

A brushless servo motor based on a 

conventional hybrid stepper. It may use 

either a resolver or encoder for 

commutation feedback. 

Hysteresis 

The difference in response of a system to 

an increasing or a decreasing input signal. 

IEEE-488 

A digital data communications standard 

popular in instrumentation electronics. This 

parallel interface is also known as GPIB, or 

General Purpose Interface Bus. 

Incremental Motion 

A motion control term that describes a 

device that produces one step of motion 

for each step command (usually a pulse) 

received. 

Incremental Programming 

A coordinate system where positions or 

distances are specified relative to the 

current position. 

Inertia 

A measure of an object’s resistance to a 

change in velocity. The larger an object’s 

inertia, the larger the torque that is 

required to accelerate or decelerate it. 

Inertia is a function of an object’s mass 

and its shape. 

Inertial Match 

For most efficient operation, the system 

coupling ratio should be selected so that 

the reflected inertia of the load is equal to 

the rotor inertia of the motor. 

Indexer 

See PMC. 

I/O 

Abbreviation of input/output. Refers to 

input signals from switches or sensors and 

output signals to relays, solenoids etc. 

Lead Compensation Algorithm 

A mathematical equation implemented by 

a computer to decrease the delay 

between the input and output of a system. 

Limits 

Properly designed motion control systems 

have sensors called limits that alert the 

control electronics that the physical end of 

travel is being approached and that 

motion should stop. 

Logic Ground 

An electrical potential to which all control 

signals in a particular system are 

referenced. 

Mechanical Time Constant 

The time for an energized DC motor to 

reach 2/3rds of its set velocity. Based on a 

fixed voltage applied to the windings. 

Mid-range Instability 

Designates the condition resulting from 

energizing a motor at a multiple of its 

natural frequency (usually the third orders 

condition). Torque loss and oscillation can 

occur in underdamped open-loop 

systems. 

Microstepping 

An electronic control technique that 

proportions the current in a step motor’s 

windings to provide additional intermediate 

positions between poles. Produces 

smooth rotation over a wide speed range 

and high positional resolution. 

Open Collector 

A term used to describe a signal output 

that is performed with a transistor. An 

open collector output acts like a switch 

closure with one end of the switch at 

ground potential and the other end of the 

switch accessible. 

Open Loop 

Refers to a motion control system where 

no external sensors are used to provide 

position or velocity correction signals. 

Opto-isolated 

A method of sending a signal from one 

piece of equipment to another without the 

usual requirement of common ground 

potentials. The signal is transmitted 

optically with a light source (usually a Light 

Emitting Diode) and a light sensor (usually 

a photosensitive transistor). These optical 

components provide electrical isolation. 

Parallel 

Refers to a data communication format 

wherein many signal lines are used to 

communicate more than one piece of data 

at the same time. 

Phase Angle 

The angle at which the steady state input 

signal to a system leads the output signal. 

Input 

Phase Angle 

Output 

Phase Margin 

The difference between 180° and the 

phase angle of a system at its crossover 

frequency. 

PLC 

Programmable logic controller; a machine 

controller that activates relays and other I/ 

O units from a stored program. Additional 

modules support motion control and other 

functions. 

PMC 

Programmable motion controller, 

primarily designed for single- or multiaxis 

motion control with I/O as an 

auxiliary function. 

Pole 

A frequency at which the transfer 

function of a system goes to infinity. 

Pulse Rate 

The frequency of the step pulses applied 

to a motor driver. The pulse rate 

multiplied by the resolution of the motor/ 

drive combination (in steps per 

revolution) yields the rotational speed in 

revolutions per second. 

PWM 

Pulse Width Modulation. A method of 

controlling the average current in a 

motors phase windings by varying the 

on-time (duty cycle) of transistor 

switches. 

Ramping 

The acceleration and deceleration of a 

motor. May also refer to the change in 

frequency of the applied step pulse train. 

Rated Torque 

The torque producing capacity of a 

motor at a given sped. This is the 

maximum torque the motor can deliver 

to a load and is usually specified with a 

torque/speed curve. 

Regeneration 

Usually refers to a circuit in a drive 

amplifier that accepts and drains energy 

produced by a rotating motor either 

during deceleration or free-wheel 

shutdown. 

Registration Move 

Changing the predefined move profile 

that is being executed, to a different 

predefined move profile following receipt 

of an input or interrupt. 

Repeatability 

The degree to which the positioning 

accuracy for a given move performanced 

repetitively can be duplicated. 

Resolution 

The smallest positioning increment that 

can be achieved. Frequently defined as 

the number of steps required for a 

motor’s shaft to rotate one complete 

revolution. 

Resolver 

A feedback device with a construction 

similar to a motor’s construction (stator 

and rotor). Provides velocity and position 

information to a drive’s microprocessor 

or DSP to electronically commutate the 

motor. 


A69

Glossary of Terms 

Resonance 

Designates the condition resulting from energizing a 

motor at a frequency at or close to the motor’s 

natural frequency. Lower resolution, open-loop 

systems will exhibit large oscillations from minimal 

input. 

Ringing 

Oscillation of a system following a sudden change 

in state. 

RMS Torque 

For an intermittent duty cycle application, the RMS 

Torque is equal to the steady- state torque that 

would produce the same amount of motor heating 

over long periods of time. 

T RMS 

= 

Where: 

M M 

(Ti 2 ti) 

ti 

Ti = Torque during interval i 

ti = Time of interval i 

RS-232C 

A data communications standard that encodes a 

string of information on a single line in a time 

sequential format. The standard specifies the 

proper voltage and time requirements so that 

different manufacturers devices are compatible. 


A system consisting of several devices which 

continuously monitor actual information (position, 

velocity), compares those values to desired 

outcome and makes necessary corrections to 

minimize that difference. 

Slew 

In motion control, the portion of a move made at a 

constant non-zero velocity. 

Static Torque 

The maximum torque available at zero speed. 

Step Angle 

The angle the shaft rotates upon receipt of a single 

step command. 

Stiffness 

The ability to resist movement induced by an 

applied torque. Is often specified as a torque 

displacement curve, indicating the amount a motor 

shaft will rotate upon application of a known 

external force when stopped. 

Synchronism 

A motor rotating at a speed correctly corresponding 

to the applied step pulse frequency is said to be in 

synchronism. Load torques in excess of the 

motor’s capacity (rated torque) will cause a loss of 

synchronism. The condition is not damaging to a 

step motor. 

Torque 

Force tending to produce rotation. 

Torque Constant 

K T 

= The torque generated in a DC motor per unit 

Ampere applied to its windings. 

K T 

= T oz-in 

A amp 

Simplified for a brushless motor at 90° commutation 

angle. 

Torque Ripple 

The cyclical variation of generated torque at a 

frequency given by the product of motor angular 

velocity and number of commutator segments or 

magnetic poles. 

Torque-to-Inertia Ratio 

Defined as a motor’s holding torque divided by the 

inertia of its rotor. The higher the ratio, the higher a 

motor’s maximum acceleration capability will be. 

Transfer Function 

A mathematical means of expressing the output to 

input relationship of a system. Expressed as a 

function of frequency. 

Triggers 

Inputs on a controller that initiate or “trigger” the 

next step in a program. 

TTL 

Transistor-Transistor Logic. Describes a common 

digital logic device family that is used in most 

modern digital electronics. TTL signals have two 

distinct states that are described with a voltage – a 

logical “zero” or “low” is represented by a voltage of 

less than 0.8 volts and a logical “one” or “high” is 

represented by a voltage from 2.5 to 5 volts. 

Voltage Constant 

K E 

= The back EMF generated by a DC motor at a 

defined speed. Usually quoted in volts per 1000 

rpm. 

Zero 

A frequency at which the transfer function of a 

system goes to zero. 

A70

Rotary Inertia Conversion Table 

Don’t confuse mass-inertia with weight-inertia: mass inertia = wt. inertia 

g 

To convert from A to B, multiply by entry in Table. 

Technical Data 

B lb-ft-s 2 

A kg-m 2 kg-cm 2 g-cm 2 kg-m-sec 2 kg-cm-sec 2 g-cm-sec 2 oz-in 2 oz-in-s 2 lb-in -2 lb-in-s 2 lb-ft 2 (slug-ft -2 ) 

kg-m 2 1 10 4 10 7 0.10192 10.1972 1.01972-10 4 5.46745-10 4 1.41612-10 2 3.41716-10 3 8.850732 23.73025 0.73756 

kg-cm 2 10 -4 1 10 3 1.01972-10 -5 1.01972-10 -3 1.01972 5.46745 1.41612-10 -2 0.341716 8.85073-10 -4 2.37303-10 -3 7.37561-10 -5 

g-cm 2 10 -7 10 -3 1 1.01972-10 -8 1.01972-10 -6 1.01972-10 -3 5.46745-10 -3 1.41612-10 -5 3.41716-10 -4 8.85073-10 -7 2.37303-10 -6 7.37561-10 -8 

kg-m-s 2 9.80665 9.80665-10 4 9.80665-10 7 1 10 2 10 5 5.36174-10 5 1.388674-10 3 3.35109-10 4 86.79606 2.32714-10 2 7.23300 

kg-cm-s 2 9.80665-10 -2 9.80665-10 2 9.80665-10 5 10 -2 1 10 3 5.36174-10 3 13.88741 3.35109-10 2 0.86796 2.327143 7.23300-10 -2 

g-cm-s 2 9.80665-10 -5 0.980665 9.80665-10 2 10 -5 10 -3 1 5.36174 1.38874-10 -2 0.335109 8.67961-10 -4 2.32714-10 -3 7.23300-10 -5 

oz-in 2 1.82901-10 -5 0.182901 1.82901-10 2 1.86506-10 -6 1.86506-10 -4 0.186506 1 2.59008-10 -3 6.250-10 -2 1.61880-10 -4 4.34028-10 -4 1.34900-10 -5 

oz-in-s 2 7.06154-10 -3 70.6154 7.06154-10 4 7.20077-10 -4 7.20077-10 -2 72.00766 3.86089-10 2 1 24.13045 6.250-10 -2 0.167573 5.20833-10 -3 

lb-in 2 2.92641-10 -4 2.92641 2.92641-10 3 2.98411-10 -5 2.98411-10 -3 2.98411 16 4.14414-10 -2 1 2.59008-10 -3 6.94444-10 -3 2.15840-10 -4 

lb-in-s 2 0.112985 1.12985-10 3 1.12985-10 6 1.15213-10 -2 1.152126 1.15213-10 3 6.17740-10 3 16 3.86088-10 2 1 2.681175 8.3333-10 -2 

lb-ft 2 4.21403-10 -2 4.21403-10 2 4.21403-10 5 4.29711-10 -3 0.429711 4.297114-10 2 2.304-10 3 5.96755 144 0.372971 1 3.10809-10 -2 

lb-ft-s 2 

(slug ft 2 ) 1.35583 1.35582-10 4 1.35582-10 7 0.138255 13.82551 1.38255-10 4 7.41289-10 4 192 4.63306-10 3 12 32.1740 1 


Torque Conversion Table 

To convert from A to B, multiply by entry in Table. 

B 

A N-m N-cm dyn-cm kg-m kg-cm g-cm oz-in ft-lbs in-lbs 

N-m 1 10 2 10 7 0.1019716 10.19716 1.019716-10 4 141.6119 0.737562 8.85074 

N-cm 10 -2 1 10 5 1.019716-10 -3 0.1019716 -3 1.019712-10 2 1.41612 7.37562-10 -3 8.85074-10 -2 

dyn-cm 10 -7 10 -5 1 1.019716-10 -8 1.01972-10 -6 1.01972-10 -3 1.41612-10 -5 7.37562-10 -8 8.85074-10 -7 

kg-m 9.80665 9.80665-10 2 9.80665-10 7 1 10 2 10 5 1.38874-10 3 7.23301 86.79624 

kg-cm 9.80665-10 -2 9.80665 9.80665-10 5 10 -2 1 10 3 13.8874 7.23301-10 -2 0.86792 

g-cm 9.80665-10 -5 9.80665-10 -3 9.80665-10 2 10 -5 10 -3 1 1.38874-10 -2 7.23301-10 -5 8.679624-10 -4 

oz-in 7.06155-10 -3 0.706155 7.06155-10 4 7.20077-10 -4 7.20077-10 -2 72,0077 1 5.20833-10 -3 6.250-10 -2 

ft-lbs 1.35582 1.35582-10 2 1.35582-10 7 0.1382548 13.82548 1.382548-10 4 192 1 12 

in-lbs 0.112085 11.2985 1.12985-10 6 1.15212-10 -2 1.15212 1.15212-10 3 16 8.33333-10 -2 1 

Densities of Common Materials 

Material oz/in 3 gm/cm 3 

Aluminum (cast or hard-drawn) 1.54 2.66 

Brass (cast or rolled 60% CU; 40% Zn) 4.80 8.30 

Bronze (cast, 90% CU; 10% Sn) 4.72 8.17 

Copper (cast or hand-drawn) 5.15 8.91 

Plastic 0.64 1.11 

Steel (hot or cold rolled, 0.2 or 0.8% carbon) 4.48 7.75 

Hard Wood 0.46 0.80 

Soft Wood 0.28 0.48 

Calculate Horsepower 

Horsepower = Torque x Speed 

16,800 

Torque = oz-in 

Speed = revolutions per second 

* The horsepower calculation uses the torque 

available at the specified speed 

1 Horsepower = 746 watts 

Most tables give densities in lb/ft 3 . To convert to oz/in 3 

divide this value by 108. To convert lb/ft 3 to gm/cm 3 

divide by 62.5. The conversion from oz/in 3 to gm/cm 3 

is performed by multiplying oz/in 3 by 1.73. 

Reference: Elements of Strength of Materials, 

S. Timoshinko and D.H. Young, pp. 342-343. 

A71


Summary of Application Examples 

Feed-to-length 

Applications in which a continuous web, strip, or 

strand of material is being indexed to length, most 

often with pinch rolls or some sort of gripping 

arrangement. The index stops and some process 

occurs (cutting, stamping, punching, labeling, 

etc.). 

Application No. 

Page 

1: BBQ Grill-Making Machine .................... A73 

2: Film Advance .......................................... A74 

3: On-the-Fly Welder .................................. A75 

X/Y Point-to-point 

Applications that deal with parts handling 

mechanisms that sort, route, or divert the flow of 

parts. 


Page 

4: Optical Scanner ...................................... A76 

5: Circuit Board Scanning........................... A77 

Metering/Dispensing 

Applications where controlling displacement and/ 

or velocity are required to meter or dispense a 

precise amount of material. 


Page 

6: Telescope Drive ...................................... A78 

7: Engine Test Stand .................................. A79 

8: Capsule Filling Machine .......................... A80 

Indexing/Conveyor 

Applications where a conveyor is being driven in a 

repetitive fashion to index parts into or out of an 

auxiliary process. 


Page 

9: Indexing Table ........................................ A81 

10: Rotary Indexer ........................................ A82 

11: Conveyor ................................................ A83 

Contouring 

Applications where multiple axes of motion are 

used to create a controlled path, (e.g., linear or 

circular interpolation). 


Page 

12: Engraving Machine ................................. A84 

13: Fluted-Bit Cutting Machine ..................... A85 

Tool Feed 

Applications where motion control is used to feed a 

cutting or grinding tool to the proper depth. 


Page 

14: Surface Grinding Machine ...................... A86 

15: Transfer Machine .................................... A87 

16: Flute Grinder ........................................... A88 

17: Disc Burnisher ........................................ A89 

Winding 

Controlling the process of winding material around 

a spindle or some other object. 


Page 

18: Monofilament Winder.............................. A90 

19: Capacitor Winder ................................... A91 

Following 

Applications that require the coordination of motion 

to be in conjunction with an external speed or 

position sensor. 


Page 

20: Labelling Machine ................................... A92 

21: Window Blind Gluing .............................. A93 

22: Moving Positioning Systems ................... A94 

Injection Molding 

Applications where raw material is fed by gravity 

from a hopper into a pressure chamber (die or 

mold). The mold is filled rapidly and considerable 

pressure is applied to produce a molded product. 


Page 

23: Plastic Injection Molding ......................... A95 

Flying Cutoff 

Applications where a web of material is cut while 

the material is moving. Typically, the cutting device 

travels at an angle to the web and with a speed 

proportional to the web. 


Page 

24: Rotating Tube Cutting ............................ A96 

A72


1. BBQ Grill-Making Machine 

Application Type: Feed-to-Length 


Application Description: A manufactuer was 

using a servo motor to feed material into a 

machine to create barbeque grills, shopping carts, 

etc. The process involves cutting steel rods and 

welding the rods in various configurations. 

However, feed-length was inconsistent because 

slippage between the drive roller and the material 

was too frequent. Knurled nip-rolls could not be 

used because they would damage the material. 

The machine builder needed a more accurate 

method of cutting the material at uniform lengths. 

The customer used a load-mounted encoder to 

provide feedback of the actual amount of material 

fed into the cutting head. 

Machine Objectives: 

• Compnesate for material slippage 

• Interface with customer’s operator panel 

• Smooth repeatable operation 

• Variable length indexes 


Motion Control Requirements: 

• Accurate position control 

• Load-mounted encoder feedback 

• High-speed indexing 

• XCode language 

Application Solution: By using the global 

position feedback capability of the BLHX drive, the 

machine builder was able to close the position 

loop with the load-mounted encoder, while the 

velocity feedback was provided by the motormounted 

encoder and signal processing. The twoencoder 

system provides improved stability and 

higher performance than a single load-mounted 

encoder providing both position and velocity 

feedback. The load-mounted encoder was 

coupled to friction drive nip-rollers close to the cut 

head. 

Product Solutions: 

Controller/Drive 

BLHX75BN 


ML3450B-10 


Spool 

Motor and 

Drive Roll 

Nip-Roll and 

Load Mounted 

Encoder 

Cutting Head 

BLHX150BN 

Servo Drive 

A73


2. Film Advance 



Tangential drives consist of a pulley or pinion 

which, when rotated, exerts a force on a belt or 

racks to move a linear load. Common tangential 

drives include pulleys and cables, gears and 

toothed belts, and racks and pinions. 

Tangential drives permit a lot of flexibility in the 

design of drive mechanics, and can be very 

accurate with little backlash. Metal chains should 

be avoided since they provide little or no motor 

damping. 

Application Description: A movie camera is 

being modified to expose each frame under 

computer control for the purpose of generating 

special effects. A motor will be installed in the 

camera connected to a 1/2-inch diameter, 2-inch 

long steel film drive sprocket and must index one 

frame in 200 milliseconds. The frame spacing is 38 

mm (1.5"). 

Machine Requirements: 

• Index one frame within 200 milliseconds 

• Indexer must be compatible with BCD interface 

• Fast rewind and frame indexing 


• Little to no vibration at rest—∴ Stepper 

• Minimum settling time 

• Preset and slew moves 

Application Solution: 

In this application, the move distance and time are 

known, but the required acceleration is not known. 

The acceleration may be derived by observing 

that, for a trapezoidal move profile with equal 

acceleration, slew and deceleration times, 1/3 of 

the move time is spent accelerating and 1/3 of the 

total distance is travelled in that time (a trapezoidal 

move). 

It is determined that the acceleration required is 

107.4 rps 2 at a velocity of 7.166 rps. Assume that 

the film weighs 1 oz. and total film friction is 10 ozin. 

The rotor, sprocket, and film inertia is calculated 

to be 0.545 oz-in/sec 2 . Solving the torque formula 

indicates that the motor for this application must 

provide 11.9 oz-in to drive the film and pulley (refer 

to Direct Drive Formulas on p. A63). 

An indexer is selected to be connected to a BCD 

interface in the camera electronics. Preset and 

Slew modes on the indexer are then controlled by 

the camera electronics to provide fast rewind and 

frame indexing. 


Drive/Indexer 

SX 


S57-51-MO 

Drive/Indexer 


A74


3. On-the-Fly Welder 



Description: In a sheet metal fabrication process, 

an unfastened part rides on a conveyor belt 

moving continuously at an unpredictable velocity. 

Two spot-welds are to be performed on each part, 

4 inches apart, with the first weld 2 inches from the 

leading edge of the part. A weld takes one second. 

Machine Objectives 

• Standalone operation 

• Position welder according to position and 

velocity of each individual part 

• Welding and positioning performed without 

stopping the conveyor 

• Welding process must take 1 second to 

complete 

Motion Control Requirements 

• Programmable I/O; sequence storage 

• Following 

• Motion profiling; complex following 

• High linear acceleration and speed 


This application requires a controller that can 

perform following or motion profiling based on a 

primary encoder position. In this application, the 

controller will receive velocity and position data 

from an incremental encoder mounted to a roller 

on the conveyor belt carrying the unfastened parts. 

The conveyor is considered the primary drive 

system. The secondary motor/drive system 

receives instructions from the controller, based on 

a ratio of the velocity and position information 

supplied by the primary system encoder. The linear 

motor forcer carries the weld head and is mounted 

on an overhead platform in line with the conveyor. 

Linear motor technology was chosen to carry the 

weld head because of the length of travel. The 

linear step motor is not subject to the same linear 

velocity and acceleration limitations inherent in 

systems converting rotary to linear motion. For 

Indexer 

example, in a leadscrew system, the inertia of the 

leadscrew frequently exceeds the inertia of the 

load and as the length of the screw increases, so 

does the inertia. With linear motors, all the force 

generated by the motor is efficiently applied 

directly to the load; thus, length has no effect on 

system inertia. This application requires a 54-inch 

platen to enable following of conveyor speeds over 

20 in/sec. 

Application Process 

1. A sensor mounted on the weld head detects 

the leading edge of a moving part and sends a 

trigger pulse to the controller. 

2. The controller receives the trigger signal and 

commands the linear motor/drive to ramp up to 

twice the speed of the conveyor. This provides 

an acceleration such that 2 inches of the part 

passes by the weld head by the time the weld 

head reaches 100% of the conveyor velocity. 

3. The controller changes the speed ratio to 1:1, 

so the weld head maintains the speed of the 

conveyor for the first weld. The weld takes 1 

second. 

4. The following ratio is set to zero, and the 

welder decelerates to zero velocity over 2 

inches. 

5. The controller commands the linear forcer to 

repeat the same acceleration ramp as in step␣1 

above. This causes the weld head to position 

itself, at an equal velocity with the conveyor, 4 

inches behind the first weld. 

6. Step 3 is repeated to make the second weld. 

7. Once the second weld is finished, the controller 

commands the linear forcer to return the weld 

head to the starting position to wait for the next 

part to arrive. 


Indexer Drive Motor Encoder 

Model 500 L Drive PO-L20-P54 -E 


Microstepping 

Drive 

Linear Motor 

Spot Welds 

Encoder 

(Mounted 

to Conveyor) 

Weld Head 

A75


4. Optical Scanner 

Application Type: X-Y Point-to-Point 

Motion: Rotary 

Application Description: A dye laser designer 

needs to precisely rotate a diffraction grating under 

computer control to tune the frequency of the 

laser. The grating must be positioned to an angular 

accuracy of 0.05°. The high resolution of the 

microstepping motor and its freedom from 

“hunting” or other unwanted motion when stopped 

make it ideal. 


• System must precisely rotate a diffraction 

grating to tune the frequency of the laser 

• PC-compatible system control 

• Angular accuracy of 0.05º 

• IEEE-488 interface is required 


• High resolution—∴ Microstepper 

• Little to no vibration at rest—∴ Stepper 

• No “hunting” at the end of move—∴ Stepper 

• Limited space is available for motor—∴ small 

motor is required 


The inertia of the grating is equal to 2% of the 

proposed motor’s rotor inertia and is therefore 

ignored. Space is at a premium in the cavity and a 

small motor is a must. A microstepping motor, 

which provides ample torque for this application, is 

selected. 

The laser’s instrumentation is controlled by a 

computer with an IEEE-488 interface. An indexer 

with an IEEE-488 interface is selected. It is 

mounted in the rack with the computer and is 

controlled with a simple program written in BASIC 

that instructs the indexer to interrupt the computer 

at the completion of each index. 


Indexer Drive Motor 

Model 4000 LN Drive LN57-51 

Drive 

Model 4000 


A76


5. Circuit Board Scanning 

Application Type: X-Y Point-to-Point 


Application Description: An Original Equipment 

Manufacturer (OEM) manufactures X-Ray Scanning 

equipment used in the quality control of printed 

circuit boards and wafer chips. 

The OEM wants to replace the DC motors, 

mechanics and analog controls with an automated 

PC-based system to increase throughput and 

eliminate operator error. The host computer will 

interact with the motion control card using a “C” 

language program. The operator will have the 

option to manually override the system using a 

joystick. 

This machine operates in an environment where 

PWM (pulse width modulation) related EMI 

emission is an issue. 


• 2-Axis analog joystick 

• Joystick button 

• Travel limits 

• Encoder feedback on both axes 

Display Requirements: 

• X and Y position coordinates 

Operator Adjustable Parameters: 

• Dimensions of sample under test 

• (0,0) position—starting point 


• AT-based motion controller card 

• Replace velocity control system (DC motors) and 

mechanics with more accurate and automated 

positioning scheme 

• Manual Joystick control 

• Continuous display of X & Y axis position 

• User-friendly teach mode operations 

• Low EMI amplifiers (drives) 


The solution of this application uses the existing 

PC by providing a PC-based motion controller and 

the AT6400 to control both axes. A microstepping 

drive is used because its linear amplifier 

technology produces little EMI. The PC monitor is 

the operator interface. 

A “C” language program controls the machine. 

Machine operation begins with a display to the operator 

of a main menu. This main menu lets the operator 

select between three modes: Automated Test, Joystick 

Position and Teach New Automated Test. 

In Automated Test mode, the PC displays a menu 

of preprogrammed test routines. Each of these 

programs has stored positions for the different test 

locations. This data is downloaded to the controller 

when a test program is selected. The controller 

controls the axes to a home position, moves to 

each scan position, and waits for scan completion 

before moving to the next position. 

In Joystick Position mode, the controller enables 

the joystick allowing the operator to move in both 

X and Y directions using the joystick. The AT6400 

waits for a signal from the PC to indicate that the 

joystick session is over. 

When Teach mode is selected, the PC downloads a 

teach program to the controller (written by the user). 

After the axes are homed, the controller enables the 

joystick and a “position select” joystick button. The 

operator then jogs axes to a position and presses 

the “position select” button. Each time the operator 

presses this “position select” button, the motion 

controller reads this position into a variable and 

sends this data to the PC for memory storage. 

These new position coordinates can now be stored 

and recalled in Automated Test mode. 


Controller Drive Motor Accessories 

AT6400-AUX1 LN Drive LN57-83-MO -E 

Daedal X-Y Table 

Joystick 



Drive 

Drive 


Indexer 

Joystick 

A77


6. Telescope Drive 

Application Type: Metering/Dispensing 


Traditional gear drives are more commonly used 

with step motors. The fine resolution of a 

microstepping motor can make gearing 

unnecessary in many applications. Gears generally 

have undesirable efficiency, wear characteristics, 

backlash, and can be noisy. 

Gears are useful, however, when very large inertias 

must be moved because the inertia of the load 

reflected back to the motor through the gearing is 

divided by the square of the gear ratio. 

In this manner large inertial loads can be moved 

while maintaining a good load inertia-to-rotor 

inertia ratio (less than 10:1). 

Application Description: An astronomer 

building a telescope needs to track celestial events 

at a slow speed (15°/hour) and also slew quickly 

(15° in 1 second). 


• Smooth, slow speed is required–∴ microstepper 

• High data-intensive application–∴ bus-based 

indexer 

• Future capabilities to control at least 2 axes of 

motion 

• Visual C++ interface 


• High resolution 

• Very slow speed (1.25 revolutions per hour)— 

microstepping 

• AT bus-based motion controller card 

• Dynamic Link Library (DDL) device driver must 

be provided with indexer. This helps Windows 

programmers create Windows-based 

applications (i.e., Visual C++) to interface with 

the indexer 


A 30:1 gearbox is selected so that 30 revolutions 

of the motor result in 1 revolution (360°) of the 

telescope. A tracking velocity of 15°/hour 

corresponds to a motor speed of 1.25 revs/hour or 

about 9 steps/sec. on a 25,000 steps/rev. Moving 

15° (1.25 revolutions) in 1 second requires a 

velocity of 1.25 rps. 

The inverse square law causes the motor to see 1/ 

900 of the telescope’s rotary inertia. The equations 

are solved and the torque required to accelerate 

the telescope is 455 oz-in. The step pulses 

required to drive the motor are obtained from a 

laboratory oscillator under the operator’s control. 



AT6200-AUX1* S Drive S106-178 

* To control up to four axes, refer to the AT6400. 

Drive 

Computer 

(Indexer installed 

in a PC) 


A78


7. Engine Test Stand 



Application Description: A jet engine 

manufacturer is building a test facility for making 

operational measurements on a jet engine. The 

throttle and three other fuel flow controls need to 

be set remotely. While the application only calls for 

a rotary resolution of 1 degree (1/360 rev.), the 

smoothness and stiffness of a microstepping 

system is required. 

Motor speeds are to be low and the inertias of the 

valves connected to the motors are insignificant. 

The main torque requirement is to overcome valve 

friction. 


• Low wear 

• Remote operation 



• Motor velocity is low 

• High stiffness at standstill 

• Slow-speed smoothness 

• Four axes of control 

• Homing function 


Each valve is measured with a torque wrench. 

Two valves measure at 60 oz-in and the other two 

measure at 200 oz-in. Two high-power and two 

low-power microstepping motor/drives systems 

are selected. These choices provide 

approximately 100% torque margin and result in a 

conservative design. 

The operator would like to specify each valve 

position as an angle between 0° and 350°. 

Home position switches are mounted on the test 

rig and connected to each indexer to allow for 

power-on home reference using the indexer’s 

homing feature. 



AT6400* S Drive S57-102 

* A standalone indexer could also be used 

(instead of a bus-based indexer), refer to the 

Model 4000. 





Drive 


Drive 

Drive 

Computer 

(Indexer installed in a PC) 

Drive 

A79


8. Capsule Filling Machine 



Application Description: The design requires a 

machine to dispense radioactive fluid into capsules. 

After the fluid is dispensed, it is inspected and the 

data is stored on a PC. There is a requirement to 

increase throughput without introducing spillage. 


• Increase throughput 

• No spilling of radioactive fluid 

• Automate two axes 

• PC compatible system control 

• Low-cost solution 

• Smooth, repeatable motion 


• Quick, accurate moves 

• Multi-axis controller 

• PC bus-based motion control card 

• Open-loop stepper if possible 

• High-resolution motor/drive (microstepping) 


The multi-axis indexer is selected to control and 

synchronize both axes of motion on one card 

residing in the IBM PC computer. An additional 

feature is the integral I/O capability that’s 

necessary to activate the filling process. The 

horizontal axis carrying the tray of capsules is 

driven by a linear motor. The simple mechanical 

construction of the motor makes it easy to apply, 

and guarantees a long maintenance-free life. The 

vertical axis raises and lowers the filling head and 

is driven by a microstepping motor and a 

leadscrew assembly. A linear motor was also 

considered for this axis, but the fill head would 

have dropped onto the tray with a loss of power 

to the motor. Leadscrew friction and the residual 

torque of the step motor prevents this occurrence. 



AT6200 Axis 1: ZETA Drive S57-51 

Axis 2: ZETA Drive PO-L20-P18 

Top View 

Filling Heads 

Tray of 

Empty Capsules 

Hose 

Full 

Capsules 

Side View 

Platen 

Linear Motor 

Motor with Leadscrew 

Computer 

(Indexer 

installed 

in a PC) 

Drive 

Drive 

A80

9. Indexing Table 

Application Type: Indexing/Conveyor 


Application Description: A system is required 

to plot the response of a sensitive detector that 

must receive equally from all directions. It is 

mounted on a rotary table that needs to be 

indexed in 3.6° steps, completing each index 

within one second. For set-up purposes, the table 

can be positioned manually at 5 rpm. The table 

incorporates a 90:1 worm drive. 


• Low-EMI system 

• Repeatable indexing 

• Remote operation 

• Table speed of 5 rpm 


• Jogging capability 

• Sequence select functionality 

• Capable of remote drive shutdown 


The maximum required shaft speed (450 rpm) is 

well within the capability of a stepper, which is an 

ideal choice in simple indexing applications. 

Operating at a motor resolution of 400 steps/rev, 

the resolution at the table is a convenient 36,000 

step/rev. In this application, it is important that 

electrical noise is minimized to avoid interference 

with the detector. Two possible solutions are to 

use a low-EMI linear drive or to shut down the 

drive after each index (with a stepper driving a 

90:1 worm gear there is no risk of position loss 

during shutdown periods). 



Model 500 LN Drive LN57-102 

* The SX drive/indexer and PK2 drive are other 

products that have been used in these types of 

applications. 



Detector 

Radiation 

Source 

Drive 

Indexer 


Rotary 

Stage 

A81


10. Rotary Indexer 

Application Type: Indexing Conveyor 


Application Description: An engineer for a 

pharmaceutical company is designing a machine 

to fill vials and wants to replace an old style 

Geneva mechanism. A microstepping motor will 

provide smooth motion and will prevent spillage. 

The indexing wheel is aluminum and is 0.250-inch 

thick and 7.5" in diameter. Solving the equation for 

the inertia of a solid cylinder indicates that the 

wheel has 119.3 oz-in 2 . The holes in the indexing 

wheel reduce the inertia to 94 oz-in 2 . The vials 

have negligible mass and may be ignored for the 

purposes of motor sizing. The table holds 12 vials 

(30° apart) that must index in 0.5 seconds and 

dwell for one second. Acceleration torque is 

calculated to be 8.2 oz-in at 1.33 rps 2 . A triangular 

move profile will result in a maximum velocity of 

0.33 rps. The actual torque requirement is less 

than 100 oz-in. However, a low load-to-rotor 

inertia ratio was necessary to gently move the vials 

and fill them. 


• Smooth motion 

• PLC control 

• Variable index lengths 



• Sequence select capability 

• I/O for sequence select 

• Programmable acceleration and deceleration 


The index distance may be changed by the 

engineer who is controlling the machine with a 

programmable controller. Move parameters will be 

changing and can therefore be set via BCD inputs. 

The indexer can be “buried” in the machine and 

activated with a remote START input. 


Drive Indexer 


SX Drive Indexer* S83-135 

* The 6200, AT6200, and Model 500 are other 

indexer products that have been used in these 

types of applications. 

PLC 

Programmable 

Logic Controller 

Controller 

Drive 

A82


11. Conveyor 

Application Type: Indexing/Conveyor 


Tangential drives consist of a pulley or pinion 

which, when rotated, exerts a force on a belt or 

racks to move a linear load. Common tangential 

drives include pulleys and cables, gears and 

toothed belts, and racks and pinions. 

Tangential drives permit a lot of flexibility in the 

design of drive mechanics, and can be very 

accurate with little backlash. Metal chains should 

be avoided since they provide little or no motor 

damping. 

Application Description: A machine vision 

system is being developed to automatically inspect 

small parts for defects. The parts are located on a 

small conveyor and pass through the camera’s 

field of view. The conveyor is started and stopped 

under computer control and the engineer wants to 

use a system to drive the conveyor because it is 

necessary for the part to pass by the camera at a 

constant velocity. 

It is desired to accelerate the conveyor to a speed 

of 20 inches/sec. in 100 milliseconds. A flat timing 

belt weighing 20 ozs. is driven by a 2-inch diameter 

aluminum pulley 4 inches wide (this requires a 

motor velocity of 3.2 rps). The maximum weight of 

the parts on the pulley at any given time is 1 lb. 

and the load is estimated to have an inertia of 2 ozin 

2 . Static friction of all mechanical components is 

30 oz-in. The required motor toque was 

determined to be 50.9 oz-ins (refer to Direct Drive 

Formulas on p. A63). 


• Computer-controlled system 

• High accuracy 

• Low backlash 


• Accurate velocity control 

• Linear motion 


• AT bus-based motion control card 


A computer controls the entire inspection 

machine. A bus-based compatible indexer card 

was selected. A microstepping motor/drive system 

that supplied 100 oz-in of static torque was also 

chosen to complete the application. 



PC21* S Drive S57-83 

* The AT6200 and AT6400 are other PC-based 

indexer products that are often used in these 

types of applications. 



Drive 

Computer 

(Indexer installed 

in a PC) 

A83


12. Engraving Machine 

Application Type: Contouring 


Application Description: An existing engraving 

machine requires an upgrade for accuracy beyond 

0.008 inches, capability and operating 

environment. Using a personal computer as the 

host processor is desirable. 


• Positional accuracy to 0.001 inches 

• Easy-to-use, open-loop control 

• CNC machining capability 

• Interface-to-digitizer pad 

• Compatibility with CAD systems 



• Microstepping 

• G-Code compatibility 

• IBM PC compatible controller 


A four-axis motion controller resides on the bus of 

an IBM compatible computer, allowing full 

integrated control of four axes of motion. Axes 3 

and 4 are synchronized to prevent table skew. 

CompuCAM’s G-Code package allows the user to 

program in industry-standard machine tool 

language (RS274 G-Code) or to import CAD files 

with CompuCAM-DXF. Open-loop microstepping 

drives with precision leadscrews give positional 

accuracies better than the desired ±0.001 inch. 

This simple retrofit to the existing hardware greatly 

improved system performance. 


Indexer Drives Motor 

AT6400* S Drives S83-135 

* The Model 4000 (standalone) and AT6450 are 

servo controller products that have also been 

used in these types of applications. 

IBM PC with Indexer 

Drives 

Digitizer Pad 


Axis 2 

Axis 1 

Axis 3 



Axis 4 


A84


13. Fluted-Bit Cutting Machine 

Application Type: Contouring 


Application Description: The customer 

manufactures a machine that cuts a metal cylinder 

into fluted cutting bits for milling machines. The 

machine operation employed a mechanical cam 

follower to tie the bit’s rotation speed to the 

traverse motion of the bit relative to the cutting 

tool. The cut depth was manually adjusted using a 

hand crank. 

This arrangement was acceptable when the 

company had a bit for the cam they wanted to 

grind. Unfortunately, custom prototype bits made 

of titanium or other high-tech metals required that 

they make a cam before they could machine the 

bit, or do those parts on a $10,000 CNC screw 

machine. Both of these alternatives were too 

expensive for this customer. 


• Machine must be capable of making lowvolume 

custom bits as well as high-volume 

standard bits—an be economical for both 

processes. 

• Quick set-up routine 

• Operator interface for part entry 



• Four axes of coordinated motion 

• 2 axes of linear interpolation 

• Math capabilities 


Controlled by a multi-axis step and direction 

controller, microstepping motors and drives are 

attached to four axes for smooth, programmable 

motion at all speeds. 

• Axis 1: Alignment 

• Axis 2: Chamfer (cutting depth) 

• Axis 3: Traverse 

• Axis 4: Rotation 

To allow for the flexibility required to cut a bit at a 

desired pitch, the traverse and rotation axes (axes 

3 and 4) are synchronized along a straight line. 

The controller’s linear interpolation allows this 

functionality. Both the alignment and chamfer axes 

(axes 1 and 2) remain stationary during the cutting 

process. 

The controller’s operator input panel and math 

capabilities allow the operator to enter the bit 

diameter, desired pitch, depth, and angle. Using 

these part specifications, the controller generates 

all motion profiles and stores them in nonvolatile 

battery-backed RAM. Programming is 

accomplished with the controller’s menu-driven 

language. The typical process is as follows: 

1. Axis 1 aligns the center line of the bit to the 

cutting tool. 

2. Axis 2 lowers the cutting tool to the desired 

cutting depth (chamfer). 

3. Axis 3 traverses the bit along the cutting tool. 

4. While axis 3 traverses, axis 4 rotates the bit to 

create the desired pitch. 


Indexer Drives Motor 

Model 4000* S Drives S83-135 

* The Model AT6400 and AT6450 are other 

controllers that have been used in these types 

of applications. 

Indexer 


Drive 

Drive 

Drive 


(Axis 2 - Chamfer) 

Cutting 

Tool 


(Axis 4 - 

Rotation) 

Drive 

Bit 


(Axis 3 - 

Traverse) 


(Axis 1 - Alignment) 

A85


14. Surface Grinding Machine 

Application Type: Tool Feed 


Application Description: A specialty machine 

shop is improving the efficiency of its surface 

grinding process. The existing machine is sound 

mechanically, but manually operated. Automating 

the machine will free the operator for other tasks, 

which will increase overall throughput of the 

machine shop. 


• Allow flexibility to machine various parts 

• Easy set up for new parts 

• Automate all three axes 

• Keep operator informed as to progress 

• Low-cost solution 

• High-resolution grinding 


• Nonvolatile memory for program storage 

• Teach mode 

• Multi-axis controller 

• Interactive user configurable display 


• High resolution motor/drive (microstepping) 


A four-axis motion controller with a userconfigurable 

front panel is required for this 

application. An indexer with a sealed, backlit 

display would be ideal for the application’s 

industrial environment (machine shop). The 

controller’s Teach mode and sizable nonvolatile 

memory allows for easy entry and storage of new 

part programs. Microstepping drives, which plenty 

of power, resolution, and accuracy are selected 

instead of more expensive closed-loop servo 

systems. The operator utilizes the controller’s jog 

function to position the grinding head at the proper 

“spark off” height. From this point, the controller 

takes over and finishes the part while the operator 

works on other critical tasks. Increasing the parts 

repeatability and throughput of the process 

justified the cost of automating the machine. 



Model 4000* S Drive S83-93 

* The AT6400 PC-based indexer has also been 

used to solve similar applications. 


Safety 

Guard 

Grinding 

Wheel 

Control Panel 

Motors 

Indexer 

A86


15. Transfer Machine 



Application Description: A stage of a transfer 

machine is required to drill several holes in a 

casting using a multi-head drill. The motor has to 

drive the drill head at high speed to within 0.1" of 

the workpiece and then proceed at cutting speed 

to the required depth. The drill is then withdrawn at 

an intermediate speed until clear of the work, then 

fast-retracted and set for the next cycle. The 

complete drilling cycle takes 2.2 seconds with a 

0.6-second delay before the next cycle. 

Due to the proximity of other equipment, the length 

in the direction of travel is very restricted. An 

additional requirement is to monitor the machine 

for drill wear and breakage. 


• Limited length of travel 

• Limited maintenance 

• Monitor and minimize drill damage 

• High-speed drilling 


• Packaged drive controller 

• Complex motion profile 

• High speed 

• High duty cycle 


The combined requirements of high speed, high 

duty cycle and monitoring the drill wear all point to 

the use of a servo motor. By checking the torque 

load on the motor (achieved by monitoring drive 

current), the drilling phase can be monitored (an 

increased load during this phase indicates that the 

drill is broken). 

This type of application will require a ballscrew 

drive to achieve high stiffness together with high 

speed. One way of minimizing the length of the 

mechanism is to attach the ballscrew to the 

moving stage and then rotate the nut, allowing the 

motor to be buried underneath the table. Since 

access for maintenance will then be difficult, a 

brushless motor should be selected. 



APEX6152 

Drill 

Head 


606 Motor 


Table 

Rotating Nut 

Ballscrew 



A87


16. Flute Grinder 



Application Description: A low-cost machine 

for grinding the flutes in twist drills requires two 

axes of movement—one moves the drill forwards 

underneath the grinding wheel, the other rotates 

the drill to produce the helical flute. At the end of 

the cut, the rotary axis has to index the drill round 

by 180° to be ready to grind the second flute. The 

linear speed of the workpiece does not exceed 0.5 

inches/sec. 


• Two-axis control 

• Low cost 

• Easy set-up and change over of part programs 

• Smooth, accurate cutting motion 


• Two-axis indexer 

• Linear interpolation between axes 

• Nonvolatile program storage 

• Flexible data pad input 

• Moderate speeds 

• Programmable I/O 

Grinding Wheel 


This is a natural application for stepper motors, 

since the speeds are moderate and the solution 

must be minimum-cost. The grinding process 

requires that the two axes move at accurately 

related speed, so the controller must be capable 

of performing linear interpolation. The small 

dynamic position error of the stepper system 

ensures that the two axes will track accurately at 

all speeds. 


Operator 

Controller Drive Motor Interface 

6200* S Drive S83-135 RP240 

* The Model 4000-FP has also been used to 

solve similar applications. 

Axis 


Twist 

Drill 

Rotary 

Head 

Drive 

X-Axis 


Axis Drive 

X-Axis 

Drive 

Operator Interface 

Controller 

Drive 

A88


17. Disc Burnisher 



Application Description: Rigid computer discs 

need to be burnished so that they are flat to within 

tight tolerances. A sensor and a burnishing head 

move together radially across the disc. When a 

high spot is sensed, both heads stop while the 

burnishing head removes the raised material. The 

surface speed of the disc relative to the heads 

must remain constant, and at the smallest 

diameter, the required disc speed is 2400 rpm. 

The machine operates in a clean environment, and 

takes approximately one minute to scan an 

unblemished disk. 


• High-speed burnishing 

• Surface speed of disc relative to the heads must 

remain constant 

• Clean environment—∴ no brushed servo 

motors 


• Variable storage, conditional branching and 

math capabilities 

• Linear interpolation between the head axes 

(axes #1 and #2) 

• Change velocity on-the-fly 

• Programmable inputs 


The drive for the disc requires continuous 

operation at high speed, and a brushless solution 

is desirable to help maintain clean conditions. The 

natural choice is a brushless servo system. The 

speed of this axis depends on head position and 

will need to increase as the heads scan from the 

outside to the center. To successfully solve this 

application, the multi-axis indexer requires variable 

storage, the ability to perform math functions, and 

the flexibility to change velocity on-the-fly. 

The sense and burnishing heads traverse at low 

speed and can be driven by stepper motors. 

Stepper motors—since the sense and burnishing 

heads need to start and step at the same time, 

linear interpolation is required. 


Controller Drive #1 Drive #2 Drive #3 

Model 4000* S Drive S Drive Z Drive 

Motor #1 Motor #2 Motor #3 

S83-93 S83-93 Z60 

* The AT6400 PC-based indexer has also been 



Axis 1 

Sensing Head 

Axis 2 

Burnishing Head 



Disc 

Axis 3 

Disc Drive Motor 

Drive 

Multi Axis 

Controller (4000) 

Drive 

Drive 

A89


18. Monofilament Winder 

Application Type: Winding 


Application Description: Monofilament nylon is 

produced by an extrusion process that results in 

an output of filament at a constant rate. The 

product is wound onto a bobbin that rotates at a 

maximum speed of 2000 rpm. The tension in the 

filament must be held between 0.2 lbs. and 0.6 lbs 

to ensure that it is not stretched. The winding 

diameter varies between 2" and 4". 

The filament is laid onto the bobbin by a ballscrewdriven 

arm, which oscillates back and forth at 

constant speed. The arm must reverse rapidly at 

the end of the move. The required ballscrew speed 

is 60 rpm. 


• Controlled tension on monofilament 

• Simple operator interface 

• High throughput 


• 2 axes of coordinated motion 

• Linear interpolation 

• Constant torque from motor 


The prime requirement of the bobbin drive is to 

provide a controlled tension, which means 

operating in Torque mode rather than Velocity 

mode. If the motor produces a constant torque, 

the tension in the filament will be inversely 

proportional to the winding diameter. Since the 

winding diameter varies by 2:1, the tension will fall 

by 50% from start to finish. A 3:1 variation in 

tension is adequate, so constant-torque operation 

is acceptable. (To maintain constant tension, 

torque must be increased in proportion to winding 

diameter.) 

This requirement leads to the use of a servo 

operating in torque mode (the need for constantspeed 

operation at 2000 rpm also makes a 

stepper unsuitable). In practice, a servo in Velocity 

mode might be recommended, but with an 

overriding torque limit, the programmed velocity 

would be a little more than 2000 rpm. In this way, 

the servo will normally operate as a constanttorque 

drive. However, if the filament breaks, the 

velocity would be limited to the programmed 

value. 

The traversing arm can be adequately driven by a 

smaller servo. 



6250* BL30 ML2340 

* The AT6450 PC-based servo controller and the 

APEX20/APEX40 servo controllers have also 

been used in this type of application. 

Bobbin 

Torque 


Drive 


Drive 

Controller 

A90


19. Capacitor Winder 

Application Type: Winding 


Application Description: The customer winds 

aluminum electrolytic capacitors. Six reels, two 

with foil (anode and cathode) and four with paper, 

are all wound together to form the capacitor. After 

winding the material a designated number of turns, 

the process is stopped and anode and cathode 

tabs are placed on the paper and foil. The tabs 

must be placed so that when the capacitor is 

wound, the tabs end up 90° (±0.1°) from each 

other. This process is repeated until the required 

number of tabs are placed and the capacitor 

reaches its appropriate diameter. 

The previous system used a PLC, conventional DC 

drives, and counters to initiate all machine 

functions. DIP switches were used to change and 

select capacitor lengths. Lengthy set-up and 

calibration procedures were required for proper 

operation. In addition, material breakage was 

common, resulting in extensive downtime. An 

operator had to monitor the machine at all times to 

constantly adjust the distances for accurate tab 

placement. 


• Constantly monitor the linear feed length of the 

paper and foil and calculate the constantly 

changing capacitor circumference as a function 

of that length 

• A complete motion control package is required 

to eliminate the need for a PLC and separate 

motion cards 

• Reduce time and complexity of set-up (too much 

wiring in previous system) 

• Reduce machine downtime caused by material 

breakage 


• Following 

• Two axes of coordinated motion 

• Math capability 

• AT-based control card 


Precise motion control of the material feed axes 

demands closed-loop servo commands. Actuation 

of external cylinders and solenoids requires both 

analog and digital I/O. A flexible operator interface 

is needed for diagnostics and other alterations of 

machine function. Motion, I/O, and an operator 

interface should be provided with a machine 

controller. 

The first motorized axis (mandril) pulls all six 

materials together and feeds an appropriate 

distance. An encoder is placed on this motor as 

well as on the materials as they are fed into the 

mandril. The controller constantly compares the 

two encoders to get an exact measurement of 

linear distance, and compensates for material 

stretching. 

When the linear distance is achieved, the first 

motor comes to an abrupt stop while a second 

axis places a tab. The controller then initiates a 

cold weld (pressure weld) of the tab onto the 

paper and foil. 

To avoid material breakage, constant tension is 

applied to each of the six reels via air cylinders. 

Sensors are installed on all axes so that if a break 

occurs, the controller can stop the process. 

A computer makes this process easy to use and 

set up. PC/AT-based support software allows the 

user to build his controller command program. 

The operator sets the diameter of the appropriate 

capacitor, the operating speed and the number of 

capacitors (all via the keyboard). After this 

process, the machine runs until a malfunction 

occurs or it has completed the job. 


Controller Drive Motor Accessories 

AT6250* BL30 ML2340 -E Encoder 

* The 6250 standalone 2-axis servo controller and 

APEX20/APEX40 servo drives have also been 



Drive 

Encoder 

Input 

Drive 


Output 

Tab Feeder 

Axis Motor 

Anode 

Tab 

Reel 

Cathode 

Tab 

Reel 

Anode 

Foil Reel 

Spindel 

Axis Motor 

and Encoder 

Paper 

Reel 

Paper 

Reel 

Opto I/O Rack 

Capacitor 

Wound Onto 

Spindle 

Encoder 

Paper 

Reel 

I/O to Limits, 

Cylinders and 

Solenoids 

Computer 

(Indexer installed in a PC) 

Paper 

Reel 

Cathode 

Foil Reel 

A91


20. Labelling Machine 

Application Type: Following 


Application Description: Bottles on a conveyor 

run through a labelling mechanism that applies a 

label to the bottle. The spacing of the bottles on 

the conveyor is not regulated and the conveyor 

can slow down, speed up, or stop at any time. 


• Accurately apply labels to bottles in motion 

• Allow for variable conveyor speed 

• Allow for inconsistent distance between bottles 

• Pull label web through dispenser 

• Smooth, consistent labelling at all speeds 


• Synchronization to conveyor axis 

• Electronic gearbox function 

• Registration control 

• High torque to overcome high friction 



Velocity 

Primary Axis 


A motion controller that can accept input from an 

encoder mounted to the conveyor and reference 

all of the speeds and distances of the label roll to 

the encoder is required for this application. A 

servo system is also required to provide the 

torque and speed to overcome the friction of the 

dispensing head and the inertia of the large roll of 

labels. A photosensor connected to a 

programmable input on the controller monitors the 

bottles’ positions on the conveyor. The controller 

commands the label motor to accelerate to line 

speed by the time the first edge of the label 

contacts the bottle. The label motor moves at line 

speed until the complete label is applied, and then 

decelerates to a stop and waits for the next bottle. 


Controller 

APEX6152* 


APEX604 

* The ZXF single-axis servo controller has also 

been used in these types of applications. 

Secondary Axis 

Registration Input 

Time 

Start Photocell 


Encoder 


A92


21. Window Blind Gluing 



Application Description: A window blind 

manufacturer uses an adhesive to form a seam 

along the edge of the material. It is critical that the 

glue be applied evenly to avoid flaws; however, the 

speed that the material passes beneath the 

dispensing head is not constant. The glue needs 

to be dispensed at a rate proportional to the 

varying speed of the material. 


• Allow for varying material speed 

• Dispense glue evenly 

• Allow for multiple blind lengths 


• Synchronization to material speed 

• Velocity following capabilities 

• Sequence storage 


A step and direction indexer/follower and a 

microstepping motor/drive are used to power a 

displacement pump. The indexer/follower is 

programmed to run the motor/drive at a velocity 

proportional to the primary velocity of the material, 

based on input from a rotary incremental encoder. 

This assures a constant amount of glue along the 

length of the material. 

When the start button is depressed, the glue will 

begin dispensing and can be discontinued with the 

stop button. If a new speed ratio is desired, FOR 

can be changed with either the front panel 

pushbutton, thumbwheels, or with the RS-232C 

serial link. 

Program 

Two following commands are used. 

FOR 

FOL 

Sets the ratio between the secondary 

motor resolution and the primary 

encoder resolution 

Sets the ratio of the speed between 

the primary and secondary motor 

One input will be configured to start motion, a 

second input will be used to stop motion. The 

motor has 10000 steps/revolution. The encoder 

that is placed on the motor pulling the material 

has 4000 pulses/revolution. It is desired to have 

the motor dispensing the glue turning twice as 

fast as the encoder sensing the material. 

FOR2.5 

FOL2ØØ 

Set the motor to encoder ratio 

The following speed ratio is 200% or 

twice as fast 

A1Ø Set acceleration to 10 rps 2 

AD1Ø Set deceleration to 10 rps 2 

MC 


The controller is placed in Continuous 

mode 



SXF Drive/Controller* S57-102 

* The Model 500 single-axis controller and the 

S Drive have also been used in these types of 

applications. 



Encoder 


A93


22. Moving Positioning System 



Application Description: In a packaging 

application, a single conveyor of boxes rides 

between 2 conveyors of product. The product 

must be accurately placed in the boxes from 

alternate product conveyors without stopping the 

center conveyor of boxes. The line speed of the 

boxes may vary. When the product is ready, the 

controller must decide which box the product can 

be placed into and then move the product into 

alignment with the moving box. The product must 

be moving along side of the box in time for the 

product to be pushed into the box. 


• Reliable product packaging on the fly 


• Multiple product infeeds 

• Continuous operation without stopping the box 

conveyor 



• Sequence storage 

• Complex following capabilities 

• Moving positioning system functionality 

• Multitasking 


A standalone multiple-axis controller provides the 

control for this application. The controller can 

perform motion profiling based on an external 

encoder that is mounted on the center conveyor of 

boxes. The two product conveyors are driven by 

servo motors for high speeds and accelerations. 

The controller looks for a product ready signal 

from a sensor mounted on the product infeed 

conveyor and then makes a move based on the 

status of the boxes on the box conveyor and the 

status of the product on the other product 

conveyor. The controller is multitasking the control 

of the two product conveyors and the external 

encoder input, as well as a sensor input to monitor 

the status of the boxes. Thus the controller can 

instantaneously decide into which box the product 

should be placed and where that box is located. 

The controller then accelerates the product into 

alignment with the appropriate box in time for the 

product to be completely placed in the box, and 

continues to monitor the other rest of the product 

and box positions. 


Controller Drive Motor Encoder 

Model 500 L Drive L20 -E Encoder 

Product 

Product 

Infeed 

Box 

Conveyor 

Product 

Product 

Infeed 

Product 

Synchronization 

Product 

Synchronization 

Controller Drive 

Drive Drive Drive 

A94


23. Plastic Injection Molding 

Application Type: Injection Molding 


Application Description: A manufacturer of 

injection molding machines wants a system that 

will close a molding chamber, apply pressure to 

the molding chamber for 5 seconds and then open 

the mold. This action needs to be synchronized 

with other machine events. When the molding 

chamber is open the motor must be ‘parked’ at a 

designated position to allow clearance to remove 

the molded part. The manufacturer would like an 

electronic solution (this is the only hydraulic axis on 

the current machine). 


• Electronic solution 

• Computer-controlled solution 

• 4000N (900lbs.) force 


• Position and torque control 

• Serial link to computer and other drives 

• Ability to change pressure and dwell 

Application Solution: A BLHX75BP brushless 

servo drive with an ML345OB-25 motor and an 

ETS8O-BO4LA Electro-Thrust Electric Cylinder 

were used. The motor drives the rod inside the 

cylinder and extends/retracts the top molding 

chamber. During this portion of the machine cycle, 

the servo drive must control the position of the 

motor. When the top molding chamber closes on 

the bottom molding chamber, a pressure must be 

applied. While pressure is being applied to the 

mold the position of the motor is not important. 

However, the motor must control the pressure on 

the molding chamber by applying a torque from 

the motor. A regular positioning servo can only 

apply torque by generating a position error—trying 

to control torque through position is not very 

accurate and can create instabilities. The BLHX 

servo was chosen because it can switch between 

position control and torque control on-the-fly 

without instability or saturation and then, while in 

torque control mode, directly controls motor 

torque. 



Controller/Drive Motor Actuator 

BLHX75BN ML3450B-10 -ET580-BO4LA 

ML3450B-25 


Electric 

Cylinder 

Drive 

Top Mold 

Chamber 

Bottom Mold 

Chamber 

"HOME" Position 

"PARK" Position 

"CLOSE" Position 

A95


24. Rotating Tube Cutter 

Application Type: Flying Cutoff 


Application Description: Metal tubing feeds off 

of a spool and needs to be cut into predetermined 

lengths. A rotating blade mechanism is used to cut 

the tube, and the blade mechanism must spin 

around the tube many times in order to complete 

the cut. The throughput of this machine must be 

maximized, so the tubing cannot be stopped while 

this cut is being made. Therefore, to make a clean 

cut on the tube, the blade must move along with 

the tube while the cut is being performed. 



• Move cutting mechanism with the tubing to 

make the cut without stopping 

• Simple user interface to set different tube 

lengths 

• High accuracy on cut 



• Program storage 

• Position following 

• High acceleration and speed 

Coil of Tube 


A single-axis servo controller/drive was chosen to 

solve this application. An external encoder 

monitors the tube output and sends this 

information back to the servo system. The servo 

system tracks the length of the tube that is being 

fed past the cutting blade. Once the appropriate 

amount of material has been fed past the blade, 

the servo accelerates the cutting device up to the 

speed of the tube, sends an output to start the 

cutter, and then follows the tube speed exactly. 



APEX6152 


APEX610 

Encoder 

Cutting 

Mechanism 


RP240 

Leadscrew 

Table 

Controller/Drive 

A96

Compumotor Step Motor & Servo Motor Systems and Controls

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