Patent ID: 7275619

Claim:
An electric power steering system providing a steering assist by applying a power of an electric motor to a steering mechanism extended from a steering member to steerable road wheels, the electric power steering system comprising: a first rotary shaft coupled to said steering member; a second rotary shaft connected to said first rotary shaft via a torsion bar; a first rotation detection section comprising one or more detector units for outputting a signal corresponding to a rotation of said first rotary shaft; a second rotation detection section comprising one or more detector units for outputting a signal corresponding to a rotation of said second rotary shaft; and a controller detecting respective rotational angles of said first and second rotary shafts and a torque on said torsion bar based on the output signals from said first and second rotation detection sections, and providing a driving control of said electric motor, said controller checking each of plural detector units of said first and second rotation detection sections to make determination on the occurrence of a failure based on the output signals thereof, and providing the driving control of said electric motor as shifting the driving control, based on results of the determination, between a torque control mode using a detected value of said torque and a steering angle control mode using a detected value of either one of the rotational angles of said first and second rotary shafts; and wherein said controller manages history information on the occurrence of failure with respect to each of said plural detector units, and selects either said torque control mode or said steering angle control mode based on said results of the determination and said history information.