Patent ID: 7481294

Claim:
An electric power steering system comprising: a steering shaft which turns along with a steering wheel operated by a driver; a steering mechanism which connects the steering shaft to steered wheels of a vehicle, and which converts a turning motion of the steering shaft into a linear motion, thereby steering the steered wheels of the vehicle; an electric motor which applies assisting force for assisting a turning operation of the steering wheel; a steering angle sensor which detects a turning angle of the steering shaft; and a drive control unit which controls driving of the electric motor based on the turning angle detected by the steering angle sensor, wherein the drive control unit comprises: an acting force detecting portion that detects acting force that acts on the steering mechanism; a straight-running state determining portion that determines whether the vehicle is running straight based on whether the acting force continues to be detected for a predetermined time while the detected acting force is smaller than a reference value that is set, in advance, for determining whether the vehicle is running straight; a steering angle midpoint computing portion that calculates an average value of plural angles detected by the steering angle sensor in the predetermined time, and calculates a control steering angle midpoint, which indicates a reference point that is used when the steering angle sensor detects the turning angle of the steering shaft, by using the calculated average value, when the straight-running state determining portion determines that the vehicle is running straight; and a steering angle midpoint storing portion that updates the control steering angle midpoint calculated by the steering angle midpoint computing portion, and stores the updated steering angle midpoints, wherein the straight-running state determining portion comprises: a storage state checking portion that checks a storage state of the control steering angle midpoint that is stored in the steering angle midpoint storing portion; and a predetermined time changing portion that changes the predetermined time based on the storage state of the control steering angle midpoint, which is checked by the storage state checking portion.