Servo motor is known as executive motor, in the automatic control system as an executive component, the received electrical signal into the motor shaft angular displacement or angular velocity output. Servo motors are divided into DC and AC servo motors. Their main characteristics are that when the signal voltage is zero, there is no self rotation phenomenon, and the speed decreases with the increase of torque. AC servo motor performance is better, because AC servo is sine wave control, torque ripple is small, while brushless DC servo is trapezoidal wave control. But brushless DC servo control is relatively simple and cheap. With the rapid development of permanent magnet AC servo drive technology, DC servo system is facing the crisis of being eliminated. Since the 1980s, with the development of integrated circuit, power electronics and AC variable speed drive technology, permanent magnet AC servo drive technology has made outstanding development. Famous electrical manufacturers in various countries have successively launched new series products of AC servo motor and servo driver. AC servo system has become the main development direction of modern high-performance servo system, which makes DC servo system face the crisis of being eliminated.