Control Step Motors in Closed-Loop

Most step-motor-based motion systems operate in open loop for low cost and provide the only motion technology inherently capable of position control without feedback. However, for certain applications that require added reliability, safety, or product quality assurance, closed-loop control offers an alternative

Here are some methods to obtain closed-loop control of step motors:

Step verification

The simplest position control, uses a low-count optical encoder to 'count' number of steps moved. A simple circuit compares commanded versus measured steps, verifying that the step motor has moved to the desired position.

Back emf

A sensorless detection method, uses the step motor's back electromotive force (emf) signals to measure and control velocity. When 'back emf' voltage drops below detection levels, the 'closed-loop' control shifts to standard open loop for the final positioning move.

Full servo control

Refers to full-time use of a feedback device on the step motor—encoder, resolver, or other feedback sensor—to more precisely control step motor position and torque.

Other methods

These include variants of back emf control based on motor parameter measurement and software techniques, used by some manufacturers. Here the stepper drive monitors and measures the motor windings and uses voltage and current information to improve step-motor control. Active damping uses this information to damp oscillations at speed, allowing more usable torque output and reducing torque lost to mechanical vibrations. Encoderless stall detect uses the information to detect loss of synchronous speed.