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from the Motor Types department...
Servo Motor vs Step Motor
Motion control, servos, steppers, etc. topic
Posted by Okan on 28 October, 2005 - 11:10 pm
Hi,

I have some questions about electrical motors that are mainly used in factory automation. What are the main similarities and differences between servo motors and step motors? Can servo motors be also controlled like step motors? For which cases do we prefer step motors instead of servo motors and vice versa?

Thanks for your help,
Okan


Posted by Curt Wilson on 2 November, 2005 - 6:06 pm
The stepper motors and brushless servo motors commonly used in factory automation operate on fundamentally the same principle. Both are technically called permanent-magnet synchronous motors (PMSMs). However, there are significant differences in the design operation and in the typical mode of operation (although both can be operated either in open-loop or closed-loop fashion).

Typical brushless servo motors have 2, 4, 6, or 8 magnetic poles on the rotor, whereas the most common stepper motor design used in factory automation has 100 poles. It is electromagnetically "geared down" compared to the servo motors, so it has higher torque capability, but lower speed range.

Fundamentally, you use a stepper motor when you can, to save the cost of the feedback sensor and its processing electronics. You must have high confidence that the motor will hold its desired position no matter what you ask it to do in the application. This is not always easy to ensure, and can take a lot of development time to demonstrate, and it does put outer bounds on your performance.

You use a servo motor when you want a very high degree of confidence that the system will closely track what you ask it to do, particularly in high-performance cases. You do not need to spend nearly the time characterizing the system to make sure that it can work under all possible conditions -- you really just need to check the boundary cases.

It is technically possible to operate a brushless servo motor as a low-pole-count stepper motor, but you get the worst of both worlds in this case, so it is not widely done. (This is different from the new "sensorless vector" algorithms, which calculate rotor angle from the electrical properties without a position sensor, and so are really servo algorithms.)

It is also possible to operate a stepper motor with an encoder or resolver mounted as a high-pole-count servo motor. While not that common, you do see this occasionally in low-speed applications to eliminate mechanical gearing.

Curt Wilson
Delta Tau Data Systems


Posted by OJ on 4 November, 2005 - 11:55 am
Steppers are good for small load applications, servo dc motors good up to 1 hp, and brushless can be up to 50,000 hp. Steppers are usually open loop, count steps and may be has stop sensor. Probably cheaper. They are commonly used in printers. Steppers can be choppy and servos are smooth. Servos are closed loop controlled with feedback sensors, great for, as Curt pointed out, for tracking closely like in precision machinery, e.g. CNC.

OJ


Posted by Davis Gentry on 5 November, 2005 - 10:18 am
I would comment that small load applications are not limited to steppers - I have used Micromo and Nanomotion (and other) motors for segment sizes below one inch, with resolutions of below 1 nm. In these applications a stepper would not provide sufficient resolution.

Davis Gentry
Senior Applications Engineer
Delta Tau Data Systems


Posted by Anonymous on 26 December, 2006 - 12:33 pm
Hi,
The main difference is Stepper motor can operate with or without feedback(Encoders,Resolvers).But the exact position will not be known by the controller. In case of Servo motor feedback must be used and also accuracy is obtained.Most of the big machines operate on Servo's.

Bye


Posted by Anonymous on 26 December, 2006 - 3:14 pm
Hi,
Some other differentiating factors between steppers and servos involve:

- Continuous speed/torque profiles
- Peak/intermittent torque
- Power & thermal management

For an equivalent bus voltage, servos will usually have a flatter speed/torque profile than steppers. Steppers typically have a torque profile that falls off rapidly at roughly ~20 revs/sec where servos have a torque profile that extends, typically, to >50 revs/sec. (these are not exact figures but just a generalization). For this reason, applications requiring significant torque at higher speeds are generally best solved with servos. Applications requiring high torques at low or zero speeds can be easily and sometimes more efficiently solved with steppers.

In addition to different shape of the speed/torque profiles for continuously available torque, servos systems usually have an "intermittent" torque range, a trait that can be very useful for brief periods of acceleration. Servo motors can sometimes provide a peak torque that is several times the available continuous torque at a given speed. Available power and thermal management will definitely provide a basis for determining the available peak torque in a given servo system.

Power & thermal management is another consideration. Steppers are typically driven in a constant current mode. This can create a significant amount of heat in both the motor and the drive electronics. And, if the drive electronics are such that the stepper motor is driven in a constant current mode, your system require more electrical power input than is needed... such as in situations where low torque is actually required from the motor.

As these issues relate to servos... a servo motor is generally going to be more efficient. Granted, specific inductances, resistances, pole count, etc., definitely affect system efficiency but primary difference between servo vs. stepper is that servos will use only what current is needed to produce a required torque. While it is possible to drive a stepper in this fashion, this is an inherent mode in servos systems.

Hope this information helps!

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