Machine design, tập 84, số 02, 2012

February 9, 2012
A Penton Media Publication
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ANALYZING TRADEOFFS
BETWEEN SERVOS AND
STEPPER MOTORS,
page 42
TAMING ELECTRICAL NOISE
IN MOTION SYSTEMS, page 50
MORE MANUFACTURERS WANT
TO “MAKE IT IN AMERICA,”
page 64
HOW TO USE THINWALLED
BUSHINGS THE RIGHT WAY, page 72







































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VOLUME 84
ISSUE 2
FEBRUARY 9, 2012

MOTION CONTROL

MOTION CONTROL

How

Thermograms of a stepper and an
equivalent servomotor show a difference
in temperature after 10 min of repeatedly
executing a move profile lasting a bit less
than 3 sec. The servomotor maintains a
relatively low 30°C temperature while the
stepper motor reaches 70°C.

SERVOS
and

STEPPERS

stack up

FEATURES

Speed and torque needs are often the
factors that determine whether servos
or steppers are the best approach.

servos and
42 How
steppers stack up
Sometimes the extra expense of
servomotors pays dividends that
stepper motors just can’t provide.

50

Engineers who work in motion control often start out using a relatively simple approach involving stepper motors to synthesize moves.
Steppers are appealing in that their controllers can be simple and inexpensive, at least for undemanding tasks. And most steppers run open
loop, needing no position or velocity feedback to hit their targets.
The problem comes when the application demands appreciable
torque and relatively fast speeds. Stepper motors can handle a range of
torques and speeds that overlap those of more-expensive servomotors,
but the economics for the stepper approach often don’t work out.
It is useful to examine some of the key differences between stepper

and servomotors as a means of understanding which applications each
type of motor fits. Steppers and servos are both synchronous motors. In
both cases, the rotation period exactly equals an integral number of ac
cycles. Both motor technologies employ a rotor with permanent magnets and a stator with coiled windings. Both systems operate by applying a dc voltage to the stator windings in a specific pattern that results
in movement of the rotor, and both technologies are capable of position
and speed control.
Two key differentiators between stepper and servo systems involve
the use of a feedback device and the complexity of the amplifier electronics. Servos, by definition, are closed-loop systems utilizing a feedback device. Steppers are open-loop systems with no feedback.

Authored by:

Scott Carlberg
Yaskawa America Inc.
Waukegan, Ill.
Edited by Leland Teschler


Key points:
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that can give servomotors an
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Resources:
Yaskawa America Inc., www.yaskawa.
com

Basics of servos, http://www.
electricmotors.machinedesign.com/
guiEdits/Content/bdeee4a/bdeee4a_1.
aspx
Thermal safety margins for
servos, />article/thermal-safety-margins-forservomotors-0622
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http://www.
electricmotors.
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com/guiEdits/
SiteMap_Articles.
aspx

The amplifier electronics in a servosystem are typically much more
complex than those of a stepper system. While a stepper amplifier simply sends full-rated current to each
winding set, a servoamplifier regulates the current levels it delivers to
the motor windings. In other words,
a servosystem produces only the current the application needs.
Because current is proportional to
torque, the control loop in the servoamplifier that regulates current is called the torque loop.
The servoamplifier typically also employs velocity and
position-control loops. The point of these feedback loops
is to handle loads that vary within the design parameters.
The servosystem might boost the torque supplied, say,
if necessary to keep the servosystem moving at its programmed velocity. In contrast, a stepper system has no
feedback and no control loops. So the stepper motor will
stall when torque demand exceeds available torque at any

given speed.
Stepper and servosystems also perform differently because of variations in their motor designs. Stepper motors
have a large number of poles and a winding inductance exceeding that of servomotors. As a result, the torque available from a stepper motor drops off much more quickly
with rising speed than with a servomotor, given the same
dc-bus voltage. This behavior can be seen in graphs comparing typical stepper and servosystems. In the nearby
example, both motors are of similar size, about 2.3-in.

square. The servo is slightly longer because of the added
feedback device.
Another inherent disadvantage of stepper motors is
that they exhibit two distinct regions of instability because
the motor behaves as a spring-mass system. One is at low
speeds, typically between 100 and 300 full steps/sec or
30 to 90 rpm. It results from excitation at the natural frequency of the motor. When the motor operates in this region, there will be a large velocity ripple, a potential for lost
steps, and a significant error in the system’s final position.
There is also a midrange instability that results from
interactions between the drive electronics and the motor.
This instability typically arises at the speed where motor
output torque is half its full running torque. Midrange
instability can lead to a stalled motor and, like low-speed
instability, velocity ripple and loss of steps that cause inaccurate positioning.
A technique called microstepping can minimize lowspeed instability by dividing each step into smaller incre-

42

Eliminating EMI in
motion systems
A logical approach to EMI/RFI
elimination offers the best chance of
getting it right the first time.

MOTION CONTROL

All factory equipment requires some degree of maintenance, monitoring, and replacement, but some need
much more than others. One group of high-maintenance
components are gearboxes. They are traditionally high
wear items, thanks to their usual task: converting highspeed, low-torque power from electric motors into the
low-speed, high-torque power needed by machinery.
Still, gearboxes remain widespread and
popular because, as one
of the tried and
true rules of
thumb
in the

the most
58 Getting
out of gearboxes
Here are some of the basics engineers
should know before specifying
gearboxes.

power-transmission industry says, “Speed is cheap, torque
is expensive.” And relying on motors to generate the torque
required by many loads (called direct-drive technology) is
usually more expensive than generating the torque with
a motor/gear reducer combination. So most engineers
specify gearboxes in their designs. Plus, gearboxes can
eliminate other mechanical components, such as bearings,
belts, chains, and pulleys, thus simplifying and bringing

down the cost of motion-control applications.
Engineers should also use the most-efficient
gearbox that meets their application’s needs.
Such gearboxes use less energy, which
saves money and the environment. It also
One of Stober’s largest helical gears
is 295 mm in diameter and is used for
high-speed stages. Stober’s smallest,
less than 200 mm in diameter, are used
in planetary gearboxes.

Authored by:

Adam Mellenkamp
Product Manager
Stober Drives Inc.
Maysville, Ky.
Edited by Stephen J. Mraz


Resources:

64

Why it makes sense to
manufacture in the U. S.
Manufacturing in the U.S. on an uptick
for good reason.

truth about thin72 The

walled bushings
Thicker isn’t necessarily better. Here are
some common misconceptions about
thin-walled bushings.

Getting
the

most
out of

Stober Drives Inc., www.stober.com
For info on another
planetary-gear drive unit,
scan this code or go to:
/>article/small-light-rackand-pinion-drive-savesenergy-1208

Engineers should know
some gearing basics before
specifying gearboxes.

GEARBOXES

58
Access our Reader Service
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in the pages of MACHINE DESIGN.
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2

MACHINE DESIGN.com

FEBRUARY 9, 2012


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Servo Systems











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For configuration, tuning and diagnostics, use the drive’s
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www.automationdirect.com/servos or www.sureservo.com

AutomationDirect
Price/Part Number

Allen-Bradley
Price/Part Number

$484.00

$1,050.40

SVA-2040

2098-DSD-005

100W Servo Motor
with connectorized Leads

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$536.31

SVL-201

TLY-A130T-HK62AA

Breakout Board Kit for
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SVC-PFL-010

2090-CPBM6DF-16AA03

Configuration Software

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SV-PRO*

2098-UWCPRG

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*SureServo Pro software is FREE when downloaded and is also available for $9 on a CD

Complete 1-axis 100W System

$978.00

$2,052.71

All prices are U.S. list prices, AutomationDirect prices are from October 2011 Price List. The Allen-Bradley 100W system
consists of part numbers shown in table above with prices from www.rockwellautomation.com/en/e-tools 8/31/11.

www.automationdirect.com
Go online or call to get complete information,
request your free catalog, or place an order.







RS# 103


ON THE COVER
A 295-mm helical gear and
a 20-mm version, both from
Stober Drives Inc.

DEPARTMENTS
8 EDITORIAL

More manufacturing jobs, but no renaissance

10 EDITORIAL STAFF
12 LETTERS
16 SCANNING FOR IDEAS
Rugged hollow-shaft encoder can be user programmed
Fingerprint-activated switch provides biometric protection

20 REPORTER’S NOTEBOOK
30 LOOKING BACK
36 COMMENTARY
Mandating innovation

38 VANTAGE POINT
Death knell for lead-acid batteries— Dr. Peter Harrop

40 INVENTOR’S CORNER
Technique molds large, hollow, monolithic composites

78 BERKE ON SAFETY
Lessons of hospital accidents

80 FE UPDATE
CFD software calculates underwater speed for a small sub

84 SOFTWARE PRODUCTS
86 PRODUCT SPOTLIGHT
Gears & mechanical drives

93
106
107

108
110
110
111
4

PRODUCTS
BUSINESS INDEX
AD INDEX
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BACKTALK
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