Patent ID: 7681463

Claim:
An electric power steering system for a vehicle having a steering wheel and steered wheels, the system comprising: a torque sensor including a torsion bar connecting an input shaft and an output shaft relatively rotatably, an input shaft rotation angle sensor for detecting an input shaft rotation angle indicative of a rotation angle of the input shaft, and an output shaft rotation angle sensor for detecting an output shaft rotation angle indicative of a rotation angle of the output shaft, the input shaft being coupled to the steering wheel and the output shaft being coupled to the steered wheels; a motor for generating steering assisting power; and a motor control unit configured to control rotation of the motor based on at least two angles among three angles, which are the input shaft rotation angle, the output shaft rotation angle and a twist angle corresponding to a difference between the input shaft rotation angle and the output shaft rotation angle; wherein the motor control unit includes: torque direction determination means for determining, based on the two angles among the three angles, whether torque is applied to the torque sensor from an input shaft side or an output shaft side; assist control means for calculating a twist angle of the torsion bar based on the two angles and driving the motor in a direction to reduce twisting of the torsion bar thereby to generate steering assisting power, when the torque is determined as being applied from the input shaft side; and transfer control means for driving the motor to reduce an amount of reducing the twist of the torsion bar to be less than an amount of reducing the twist of the torsion provided by the assist control means, so that the torque applied from the output shaft side is transferred to the steering wheel, when the torque is determined as being applied from the output shaft side; wherein the transfer control means is configured to drive the motor in a direction to increase the twist of the torsion bar, when the torque is determined as being applied from the output shaft side.