Servomotors

A servomotor (or servo motor or simply servo)[1] is a rotary or linear actuator that allows for precise control of angular or linear position, velocity, and acceleration in a mechanical system.[1][2] It constitutes part of a servomechanism, and consists of a suitable motor coupled to a sensor for position feedback and a controller (often a dedicated module designed specifically for servomotors).

Servomotors are not a specific class of motor, although the term servomotor is often used to refer to a motor suitable for use in a closed-loop control system. Servomotors are used in applications such as robotics, CNC machinery, and automated manufacturing.

Mechanism

A servomotor is a closed-loop servomechanism that uses position feedback (either linear or rotational position) to control its motion and final position. The input to its control is a signal (either analog or digital) representing the desired position of the output shaft.

The motor is paired with some type of position encoder to provide position feedback (and potentially also speed feedback in more sophisticated designs). The controller compares the measured position with the desired position to generate an error signal, which when fed back causes the motor to rotate in the direction needed to bring the shaft to the desired position. The error signal reduces to zero as the desired position is approached, stopping the motor.

Simple servomotors use position-only sensing via a potentiometer and bang-bang control of their motor; the motor only rotates at full speed or is stopped. This type of servomotor is not widely used in industrial motion control, but it forms the basis of the simple and cheap servos used for radio-controlled models.[3]

More sophisticated servomotors make use of an absolute encoder (a type of rotary encoder) to calculate the shaft's position and infer the speed of the output shaft.[4] A variable-speed drive is used to control the motor speed.[5] Both of these enhancements, usually in combination with a PID control algorithm, allow the servomotor to be brought to its commanded position more quickly and more precisely, with less overshooting.

Servodrives

A servo drive receives a command signal from a control system, amplifies the signal, and transmits electric current to a servo motor in order to produce motion proportional to the command signal. Typically, the command signal represents a desired velocity, but can also represent a desired torque or position. A sensor attached to the servo motor reports the motor's actual status back to the servo drive. The servo drive then compares the actual motor status with the commanded motor status. It then alters the voltage, frequency or pulse width to the motor so as to correct for any deviation from the commanded status.

In a properly configured control system, the servo motor rotates at a velocity that very closely approximates the velocity signal being received by the servo drive from the control system. Several parameters, such as stiffness (also known as proportional gain), damping (also known as derivative gain), and feedback gain, can be adjusted to achieve this desired performance. The process of adjusting these parameters is called performance tuning.

Although many servo motors require a drive specific to that particular motor brand or model, many drives are now available that are compatible with a wide variety of motors..