Patent ID: 8775027

Claim:
An electric power steering system comprising: an assist mechanism that gives an assist torque from a motor to a vehicle steering mechanism; a control portion that controls driving of the motor based on an assist command value; and a vehicle state detecting portion that detects a skid of the vehicle, wherein the control portion includes: a first assist component computing portion that computes a first assist component based on a steering torque transmitted to the vehicle steering mechanism; a steered-angle command value computing portion that computes, based on the steering torque, a steered-angle command value that serves as a target value for a steered angle of steered wheels; a second assist component computing portion that computes a second assist component by performing a steered-angle feedback control that matches the steered angle of the steered wheels with the steered-angle command value; an assist command value computing portion that computes the assist command value based on a value obtained by adding the second assist component to the first assist component; and an adjustment portion that decreases, when the skid of the vehicle is detected by the vehicle state detecting portion, an absolute value of the second assist component included in the assist command value, as compared to when no skid of the vehicle is detected, the adjustment portion includes: a gain computing portion that computes a gain based on vehicle speed such that: (1) the gain is set at 1 when the vehicle speed is at a predetermined speed or lower, and (2) the gain is decreased linearly in a range from 1 to 0 when the vehicle speed is increased above the predetermined speed, a gain switching portion that corrects the gain when the skid of the vehicle is detected by increasing the gain until the gain has a value of 1.