Servo motor is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servo motors. Servo motors are not a specific class of motor although the term servo motor is often used to refer to a motor suitable for use in a closed-loop control system. Servo motors are used in applications such as robotics, CNC machinery or automated manufacturing.
The motor is paired with some type of encoder to provide position and speed feedback. In the simplest case, only the position is measured. The measured position of the output is compared to the command position, the external input to the controller. If the output position differs from that required, an error signal is generated which then causes the motor to rotate in either direction, as needed to bring the output shaft to the appropriate position. As the positions approach, the error signal reduces to zero and the motor stops.
The very simplest servo motors use position-only sensing via a potentiometer and bang-bang control of their motor. In bang-bang control the motor always rotates at full speed or is stopped. This type of servo motor is not widely used in industrial motion control, but it forms the basis of the simple and cheap servos used for radio-controlled models.
More sophisticated servo motors measure both the position and also the speed of the output shaft. They may also control the speed of their motor, rather than always running at full speed. Both of these enhancements, usually in combination with a PID control algorithm, allow the motor to be brought to its commanded position more quickly and more precisely, with less overshooting.