10 tips for servos and steppers…a simple guide

What are the basic application differences between servos andsteppers? Where would you choose one over the other? Thisshort 10 point guide, offers a simple non technical explanationof the differences between these positioning technologies andattempts to de-mystify the subject, putting straight manymisconceptions.

By Nigel DawsonEdrives Product Managernigel_dawson@festo.com

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1. Torque comparison ofsteppers and servosMany people believe steppermotors do not have the torquecapacity of servo motors. This isnot necessarily true. The nominaltorque of a stepper motor is verysimilar to a servo for acomparable frame size. Of coursea servo has an additional timedependant peak toque rating andtheir speed curve is considerablymore flexible and higherperformance than a stepper. If astepper solution is correctlysized, it offers a real low costalternative to a servo.

2. Good applications forsteppersServo motors, despite manyclaims, are not always the bestperformance choice forapplications. The structure of astepper motor means thatpositions are discrete steps in itsrevolution cycle. If an applicationrequires very good stand stillstability, for instance positioninga vision system, a stepper willalways be better than a servo.Servo motors resonatebackwards and forwards atstandstill, giving them theirpositioning characteristics;stepper motors stay very stableat standstill.

Positioning of vision systems is an idealapplication for stepper motors

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3. When is a servo not a servo?The term servo, often refers to asystem with closed loopfeedback, hence a servo motor isreally only a servo when coupledto the drive to close the loop.This also means that any motortype can be built into a servosystem to offer a cost optimisedservo system. Such low costservos utilise stepper motors togive all the functionality of aservo at 50% of the cost withspeed being the only drawback.

An example of a servo drive, designedspecifically for encoder enabledstepper motors

4. What to watch on a stepperapplicationCertain characteristics of astepper motor necessitatecareful selection on applications.The torque curve of a stepper isvery short with high RPMsleading to a drop off in torquequite quickly. Very low speeds onsteppers also offer low torque;therefore vertical applicationswhere the motor holds a loadstationary are often not suitablefor steppers. Applications thatrequire very smooth steadyspeed are also difficult withsteppers due to the resonancecreated by the discrete steps;this however can often beimproved by micro stepping orclosing the loop.

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5. What is micro stepping?How can a motor with only 200steps offer 200,000 positions inone revolution? The steppingdriver that gives the motor thecorrect power and phases for it’stask is the product that containsthe micro stepping function. Byvarying the current between 2poles in the motor we can affectthe rotor position between thetwo poles. If the driver gives 50%of the current to each pole, therotor will be exactly half waybetween the poles, thereforeoffering half steps. Many driverscan offer 1/1000 steps, thereforeincreasing the resolutionconsiderably. Be careful, microstepping has a negative effect onthe torque curve for steppermotors, reducing the torque byas much as 30%.

6. “Torque the torque”One major advantage of servomotors is their ability to controltheir torque in the application.Steppers motors operateconstantly at full torque,therefore making it impossible toutilise them in applicationswhere force or torque control isrequired. If a servo motor isconnected to a ballscrew drive,the application can offer preciseforce control for pushing aproduct or press fitting productstogether in assembly. However,closed loop stepper systems dohave the ability to offer torquecontrol.

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7. Size IS everythingServo motors and stepper motorsare sized in EXACTLY the sameway. Care must be taken toensure that the torquerequirements for the steppermotor during acceleration (themost power hungry phase of themovement) lies within thenominal torque curve of motor.

Servo Motors have moreflexibility as the accelerationtorque generally can lie in thepeak torque curve of the servomotor and only the Root MeanSquare torque of the overall cycleneeds to lie within the nominaltorque curve.

Stepper motor torque curves showing single profile for threealternative voltages for the same motor

Servo motor torque curves showing the peak and nominal(sustainable) torques

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8. Sizes for all applicationsRemember, the majority ofstepper and servo motormanufacturers offer the sameframe size motor in differinglengths. This feature allowsdifferent torques and inertiavalues to be obtained by thesame size motor, allowing agreater degree of freedom inselection. It also allows greaterperformance without increasingthe motor size and therefore thecost significantly, resulting is asolution that is more costeffective, lighter and with smallerinstallation space. By keepingthe frame size smaller andincreasing the length, any costsof additional gearboxes arereduced significantly too.

9. Don’t use a sledgehammer tocrack a nut.Many servo motors are attachedto mechanical actuators such asbelt drives to convert the rotarymotion to linear motion. Oftenstepper motors are ignored forthis application on thepresumption that they are tooslow. However, belt drivesgenerally have long traveldistances per revolution, withmany medium sized belt drivesmoving 100mm per rev.

For a relatively fast movement of2 metres/sec., the motor wouldhave to rotate at only 1200 rpm,well within the scope of a goodquality stepper motor. A servomotor with 12000 rpmperformance, would offer a topspeed of 20 metres/second, farfaster than any belt drive cantravel. Don’t over specify theapplication when you do nothave to!

Different length motors with the sameframe size offer significant advantages

Servo motors on belt drives are oftenoversized on speed

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10. Power equals performanceOn both stepper systems andservo systems, the more currentyou have, the more torque isgenerated by the motor. If a100mm frame size servo motor isthe only size that will fit theapplication space, then using a3 phase 400V AC drive will offersignificantly more torque than a240V AC variant. Similarly with astepper drive you have the choiceof varying the DC voltage to thedrive, many systems work on 24VDC, 48V DC or 72V DC, obviouslya 72V power supply will offer anincrease in performance on 24V.

One advantage of a stepper driveis the amount of torque that canbe generated from a simple 48VDC power supply, meaning thedrive is running at a safe supplyvoltage, adding the benefit ofsafe and easy maintenance.

Examples of 24V and 48V DC powersupplies for use with stepper drives

Festo Ltd

Applied Automation CentreCaswell RoadBrackmills Trading EstateNorthamptonNN4 7PY

Tel: (0800) 626422Fax: (01604) 667001email: info_gb@festo.comwww.festo.com/gb