Patent ID: 8684129

Claim:
An electric power steering system comprising: a steering mechanism; a motor that applies a steering operation assist force to the steering mechanism; a reduction mechanism that has a drive gear rotationally driven by the motor and a driven gear attached at the steering mechanism and meshing with the drive gear; a controller that controls the motor; and a steering angle detector that detects a steering angle, wherein in a predetermined no-steering-operation range in which the motor does not apply the steering operation assist force, the controller causes the motor to: apply a slight rotational torque in a first rotational direction, the slight rotation force is not sufficient to drive the driven gear; switch a direction of the slight rotational torque from the first rotational direction to a second rotational direction under a given condition; and reverse the direction of the slight rotational torque between the first rotational direction and the second rotational direction each time an integral value of steering angles detected by the steering angle detector in the predetermined range reaches a set value.