Electric power steering system

摘要

The CPU (11) calculates axial force matching external force that acts on a rack bar (34) by combining steering torque MT that acts on a steering shaft (33) with assist torque AT generated by an electric motor (30). The CUP (11) then determines whether a vehicle is running straight based on a steering angle ¸, a vehicle speed V, and the axial force F. Thus, even in a state where a control steering angle midpoint is deviated and a driver applies torque so as to return a turning position of a steering wheel (31) to a neutral position, whether the vehicle is running straight can be accurately determined by making a determination by using the axial force F that acts on the rack bar (34). The CPU (11) can set a considerably accurate control steering angle midpoint by calculating a smoothed steering angle midpoint by using an instantaneous steering angle midpoint in an accurately determined straight-running state.