**4.2. Sensorless drives for Permanent Magnet Synchronous Motors (PMSMs) in AFVs**

Due to the characteristic of getting rid of sensors and reduction of cost and systematic complexity, the sensorless drives for AFV motors have attracted great interest of the auto industry. In a sensorless drive, the motor speed could be accurately estimated through some sophisticated observer and some other electrical parameters (currents, voltages, or flux) as well. Sliding mode control has been widely used for motor control systems because of its insensitive characteristics to disturbances of motor parameters. With the same property, sliding mode observers (SMOs) have been adopted by researchers and the industry in the last decade applied to the motor drives for precise estimation of speed and flux. It is more robust to the operating conditions and systematic uncertainties. However, it has to be noted that imperfect implementation of the switching leads to chattering, which is a major drawback of SMO. Thus the following section will introduce a new fixed-boundary-layer sliding mode observer for AFV motor drives, considering reduction of chattering and guaranteeing estima‐ tion precision in a wide speed range.
