Patent ID: 8204647

Claim:
An electric power steering system of a vehicle comprising: an electric motor that generates output power for power-assisting steering operation of a steering wheel of the vehicle; a torque sensor including a plurality of output generating parts configured to generate a plurality of electric output signals corresponding to a torque applied to the steering wheel; a control unit configured to control the steering operation of the electric motor in accordance with at least one of the electric output signals of the torque sensor; wherein: the control unit includes a difference detection part, a failure checking part and a failing device specifying part; the difference detection part is configured to detect a difference between rotation speeds of a left steered wheel and a right steered wheel of the vehicle; the failure checking part is configured to check whether any one of the plurality of output generating parts is in failure; the failing-device specifying part is configured to specify a failing-device based on the rotation speed difference and the plurality of electric output signals; the control unit is configured to store a characteristic line defining a relation between the torque and the electric output signal with respect to each of the plurality of output generating parts; and the failing device specifying part is configured to check whether the steering wheel is steered in a clockwise direction or in a counter-clockwise direction based on the difference of the rotation speed, estimate a present numerical value range of the electric output signal of each of the plurality of output generating parts based on a direction of steering of the steering wheel and the characteristic line, and check whether an actual value of the electric output signal is within an estimated present numerical value range.