Control method of electric power steering system based on phase compensation

The physical and mathematic model of electric power steering system with three lumped masses was studied by dynamics analysis and control theory, and the general block-diagram of the system was established; the outer loop property of the system was analyzed, then the general block-diagram was transformed into its equivalent counterpart, and steering control system was designed according to the system requirement; the transfer function of controlled plant, the block-eiagraph, Bode plot and Nyquist plot of the open loop system and the stability of the system were studied, and the testbed test of the system was carried out. It is pointed that the block-diagraph may be expressed by a two-loop diagraph with an inner loop including a feedback channel of back electromotive force and an outer one, back electromotive force may be ignored when a fast proportional-integral(PI) controller in inner loop was used, the gain and phase margin of the system are negative without phase compensation, the system is unstable, while phase compensation may stabilize the system. Analysis result indicates that a lead corrector with phase compensation around low frequency resonance is an effective method to stabilize the system and restrain the system vibration.