Control apparatus for motor-driven power steering system of motor vehicle

Abstract

A control apparatus for an electric motor-driven power steering system includes a main CPU for arithmetically determining the steering assist torque to be generated by the electric motor and a driving direction thereof on the basis of outputs of a torque sensor and the vehicle speed sensor, a subsidiary CPU for arithmetically determining the driving direction of the motor on the basis of outputs of the torque sensor and the vehicle speed sensor and compare the driving direction signal supplied from the main CPU with a signal of the driving direction determined by the subsidiary CPU to generate a second driving direction signal. A coincidence/discrepancy signal representing coincidence or discrepancy between the motor driving direction signal and the steering torque signal is also supplied to the subsidiary CPU from the main CPU. The subsidiary CPU decides whether the motor driving direction signal coincides with a steering direction signal generated by the subsidiary CPU on the basis of the coincidence/discrepancy signal and the torque sensor signal. In dependence on the decision result, an enable/inhibit signal for enabling or inhibiting supply of the assist torque signal is issued. Each of the main and subsidiary CPUs includes a signal transfer circuit for allowing a signal indicative of the result of the decision to be transferred between the CPUs, and a circuit for interrupting supply of the driving control information to the motor controller when the diagnostic result indication signal indicates occurrence of abnormality in the main or subsidiary CPU.