ac servo motor
The servo motor is a closed-loop servomechanism that uses position feedback in order to control its rotational speed and position. The control signal is the input, either analog or digital, which represents the final position command for the shaft.
Servo motor works on the PWM ( Pulse Width Modulation ) principle, which means its angle of rotation is controlled by the duration of pulse applied to its control PIN. Basically servo motor is made up of DC motor which is controlled by a variable resistor (potentiometer) and some gears.