Taking a brief look at ... motor position feedback

When used in positioning applications, position feedback is used to control the position of the motor (such as for a servo motor) or to confirm that the motor has arrived at the desired position (step motor). Servo motors such as DC brush or brushless DC devices require position feedback to maintain their position at all times. These motors are positioned to a certain location or they are commanded to move through a certain path by continuously comparing the rotor position using the position feedback device (“the actual position”) with the desired instantaneous position “the target position” and generating a motor command which is amplified by the motor drive circuit. This process is called servo control and typically uses a stabilization filter such as a PID (proportional, integral, derivative) to generate the motor command value.

Step motors operate on a completely different principle. As long as power is provided to the step motor coils the position will not change. By continuously “rotating” the phases through one electrical cycle after another the motor is driven to move a precise number of steps or microsteps. Each quarter electrical cycle is referred to as a step and step motor controllers perform high speed step and direction generation to create continuous motion at a constant or accelerating velocity. The direction signal is used by the amplifier to determine the phase sequence. Changing the direction signal results in a reversal of motor direction.

Because the step motor does not require a feedback device to position itself most applications using this type of motor do not utilize a position encoder. Nevertheless there is an increasing trend toward use of position feedback devices even in step motor applications because of the additional safety provided.

There are many types of encoders available. The following table provides a summary:

Common Commutation Techniques