Effected by sensor measurement noise, system noise and load disturbance torque, dynamic performance of low speed compound direct-drive permanent magnet synchronous machine (PMSM) decreased and speed fluctuation increased, which result in creeping phenomenon and unstable working state of servo system directly driven by PMSM. A load torque observer based on extended kalman filter (EKF) is proposed here, which gives correct estimation of rotor position, rotor speed and load torque through electromagnetic torque and encoder signal. A control strategy using estimated load torque for forward feedback compensation is also discussed with no need of extra hardware equipment. Simulation results showed that PMSM can operate steadily at the speed of 2.5r/min with less speed fluctuation only using a low resolution encoder, which verified the effectiveness and practicability of the method mentioned in this paper.
