a servo motor is an engine that controls the operation of the mechanical components in the servo system. servo motor can make the control speed, position accuracy is very accurate, can be converted to torque and speed voltage signals to drive the control object. servo motor rotor speed by the control signal input, and can respond quickly, in the automatic control system that is used for the implementation of components, and has small electromechanical time constant, high linearity, initiating voltage low characteristic, the received signal is converted into the motor shaft angular velocity or displacement output. dc and ac servo motor is divided into two major categories, the main feature is that when the signal voltage is zero without rotation, speed with the increase of torque and uniform decline.