Servo motors are a common component of automated control systems and are also used in robotics. They can be linear or rotary activated and allow for precision control of acceleration, velocity and either linear or angular positioning. Servo motors form closed-loop control systems where the actuator is combined with a sensor that supplies position feedback. They also require quite a sophisticated controller, which is frequently a dedicated module specifically for use with such motors.
In its most basic form a servo motor consists of a motor, sensor and a control circuit. As the motor rotates the sensor changes its resistance or value so that the control circuit knows the direction and position of the motor. More sophisticated servo motors will use an encoder for position sensing. The measured position of the motor is compared against the required position set by the controller which is usually set by an external source such as a PLC or external input. If there is a difference in these positions, the controller will move the motor to the desired position.
Servo motors are available in both AC and DC voltages. AC servos can handle higher currents than a DC servo motor. AC servos are used for applications were high speeds, and heavy loads are to be moved and are usually more expensive than a DC servo motor. DC servo motors are not designed for higher currents and are commonly used for smaller applications especially in electronics assembly such as manufacturing printed circuit boards.