Abstract: In view of the effects of the torque fluctuation on the performance of electric power steering system in the process of switching between i_d=0 control and flux-weakening control in permanent magnet synchronous motor, a load torque observer is designed to compensate the output torque of flux-weakening control in speed loop, and a torque smooth switching module is added during mode switching, so the problem of manipulation torque fluctuation caused by the abrupt change of torque in mode switching process is solved. The results of both simulation and test show that through the load feed-forward compensation by load torque observer, the robustness of system to load disturbances is enhanced, and the torque smooth switching module added well suppress the fluctuation of manipulation torque in mode switching process, so the hand feeling of driver on the heavy turning of steering wheel and torque fluctuation is improved

Key words: EPS, flux-weakening control, load torque observer, torque smooth switching module