AC servo system based on the processing of signal in different ways, can be divided into analog servo, digital analog hybrid servo and full digital servo; if used according to the servo motor is different, can be divided into two types: one is made of permanent magnetic synchronous motor servo system, including Fang Boyong magnet synchronous motor ( brushless DC machine ) servo system and permanent magnet synchronous motor servo system; another is to use squirrel-cage asynchronous motor servo system. The two difference is that the permanent magnet synchronous motor servo system needs to use the pole position sensor and induction motor servo system containing a slip frequency calculation. If the microprocessor realization of software servo control, can lead to permanent magnet synchronous motor and the squirrel cage type asynchronous motor using the same set of servo amplifier.