Patent ID: 8662242

Claim:
A vehicle steering system, comprising: a variable transmission ratio mechanism that is able to change a transmission ratio between a first shaft on a steering wheel side and a second shaft on a steered wheel side by adding rotation of a motor to rotation of the first shaft and transmitting a sum of the rotation of the motor and the rotation of the first shaft to the second shaft; a lock mechanism that mechanically restricts the rotation of the motor; a torque detecting unit that detects a torque that acts on the first shaft or the second shaft; and a control unit that restricts the rotation of the motor with use of the lock mechanism when a rate of change in the detected torque exceeds a predetermined rate; wherein, when a value lower than a lower limit of reverse input torque that is able to rotate the motor in a direction opposite to a regular rotation direction against driving force of the motor is set as a torque determination value, if the rate of change in the detected torque exceeds the predetermined rate, the control unit estimates a period of time until the detected torque reaches the torque determination value, based on the rate of change in the torque, and restricts the rotation of the motor with the use of the lock mechanism on condition that the estimated period of time is shorter than or equal to a predetermined period of time.