To address the issue of high-frequency chattering in the system caused by the use of sign functions in the full order sliding mode observer(SMO)for sensorless control of permanent magnet synchronous motors(PMSM)

adaptive-switching-function-based full order SMO(ASFB-FOSMO)was proposed. Firstly

according to the mathematical model of PMSM

a full order sliding mode observer was established. Secondly

the designed piecewise square root function was used as the switching function

and the gain of the switching function is adjusted according to the real-time speed of PMSM. The above method can reduce the chattering of the system and ensure the observer performance

so as to effectively improve the performance of rotor position and speed observation error. Then

phase locked loop(PLL)was used to obtain the rotor position and speed

avoiding the high-frequency noise interference introduced by differential operation. Considering the dynamic process of motor speed change

a third-order extended state observer(TOESO)was used to enhance the anti-interference ability and dynamic response performance of PLL. Finally

feasibility and effectiveness of the proposed method were verified by simulation and experiment.