Electric power steering system

An electric power steering system comprising:

• a steering force assist device that provides assist force for assisting a steering operation, to a steering system, using a motor as a driving source;
• and
• a controller that controls the operation of the steering force assist device, wherein
• the controller includes
• a control signal generator that generates a motor control signal so as to produce a motor torque corresponding to the assist force, and
• a driving circuit that supplies driving electric power based on the motor control signal, to the motor, wherein
• the control signal generator calculates a current command value representing a target assist force, based on a detected steering torque, and performs a current feedback computation so that an actual current value follows the current command value, thereby to create the motor control signal, wherein
• the driving circuit includes switching arms that are connected to each other in parallel, each of the switching arms comprising a pair of switching devices connected in series and operable to be turned on or off based on the motor control signal, wherein
• the controller has two independent control signal generators that perform the same current feedback computation as the control signal generator, and the two switching arms corresponding to motor terminals operate independently of each other based on the motor control signals generated by the respective control signal generators, so that the driving electric power is supplied to the motor, and wherein
• the controller has an abnormality determining unit that determines that an abnormality occurs when a current deviation of an actual current value from the current command value exceeds a predetermined threshold value in at least one of the control signal generators.