Now commonly used servo motor is ac servo motor, the axial end in motor with photoelectric encoder, by examining the rotor Angle to frequency conversion control.
目前常用的伺服电机是交流伺服电机，在电机的轴端装有光电编码器，通过检测转子角度用以变频控制。
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Position detection of rotor in hybrid stepping motor servo system can use traditional outside position sensor or sensorless scheme based on theoretical algorism, but there are many problems produced.
The exact rotor position is required while a permanent magnet brushless servo motor (PMBSM) is under commutation and control. Howere, position sensors have some fatal defects.