The General Principle of using a Servo Motor for Position Control

Servo motors are typically employed in closed-loop position control; for example let us consider the following application, where the angular position of the output shaft is intended to follow the reference voltage (ϴ_ref).

The potentiometer mounted on the output shaft provides a feedback voltage proportional to the actual position of the output shaft. The voltage from this potentiometer must be a linear function of angle, and must not vary with temperature; else the accuracy of the system will be affected.

The feedback voltage – representing the actual angle of the shaft, is subtracted from the reference voltage – representing the desired position and the resulting position error signal is amplified and used to drive the motor so as to rotate the output shaft in the desired direction. When the output shaft reaches the target position, the position error becomes zero, no voltage is applied to the motor, and the output shaft remains at rest. Any effort to physically move the output shaft from its target position immediately creates a position error and a restoring torque is applied by the motor.

Related: How Servo Motors are used in Process Control

The tachometer feedback is used in addition to the main position feedback to help improve the dynamic performance of the system.