Friday, July 20, 2018

What’s the difference between servo and closed-loop stepper motors?

Servo and stepper motors have similar construction and share the same fundamental operating principle. Both motor types incorporate a rotor with permanent magnets and a stator with coiled windings, and both are operated by energizing, or applying a dc voltage to, the stator windings, which causes the rotor to move. However, this is where the similarities between servo and stepper motors end.

Drive methods for stepper motors
Stepping motors have 50 to 100 poles and are two-phase devices, where servo motors have between four and 12 poles and are three-phase devices. Stepper motor drives generate sine waves with a frequency that changes with speed … but with an amplitude that is constant.

Open-loop Stepper Diagram

Servo drives, on the other hand, produce sine waves with variable frequency and amplitude, allowing them to control both speed and torque.

Closed-loop Servo Diagram

Control methods for stepper motors
Traditional closed-loop stepper motor move when they receive a command to advance a certain number of pulses, which correlate to a distance. Steppers are considered open-loop systems because they lack a feedback mechanism to verify that the target position has been reached. Servo motors also move on receipt of a command signal from their controller.

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