Motor driven power steering system

Abstract

A power steering system uses an electric motor to produce an auxiliary torque which is added to the torque applied to the steering wheel of vehicle by the driver. The system includes first detecting mechanism for detecting the rotating direction and rotating torque value applied to a steering shaft by a driver and producing a steering torque signal for responding to the steering torque applied to the driver, control circuit connected to the first detecting mechanism for controlling the energization of an electric motor according to the steering torque signal, coupling mechanism for coupling the motor to the steering shaft, releasing mechanism for releasing the connection between the motor and the coupling mechanism, converting mechanism for converting a part of rotating force of the motor into axial thrust force and installed in the coupling mechanism, said detecting mechanism for detecting the direction of the axial thrust force, and comparing circuit for comparing the output responding to the rotating direction of steering shaft in the first detecting mechanism with the output in the second detecting mechanism and generating a driving signal for driving the releasing means when the compared result is different from a preset condition. This permits the driver to rotate the steering shaft freely without a load of the motor when the motor is locked.