Patent ID: 6915194

Claim:
A vehicle steering control system that determines a steering angle to be applied to a wheel steering shaft according to an operation angle to be applied to a steering wheel shaft for steering and an operation status of a vehicle, and rotates and drives the wheel steering shaft by a steering shaft driving motor constituted by a three-phase brushless motor so as to obtain the steering angle, comprising: a steering wheel shaft angle detecting portion for detecting an angular position of the steering wheel shaft, a steering shaft angle detecting portion for detecting an angular position of the wheel steering shaft, an operation status detecting portion for detecting an operation status of the vehicle, a steering control portion for determining a target angular position of the wheel steering shaft based on the detected angular position of the steering wheel shaft and the detected operation status of the vehicle, and controlling a rotation speed of the steering shaft driving motor based on a difference between the angular position of the wheel steering shaft and the target angular position by a PWM control method so as to control an operation of the steering shaft driving motor such that the angular position of the wheel steering shaft approaches toward the target angular position, wherein the PWM control method does not perform switching in a state in which a first terminal is connected to a first pole of a DC power supply, and performs switching in a state in which a second terminal is connected to a second pole of the DC power supply, wherein one of conducting terminals of a pair of two phase coils of the steering shaft driving motor that are simultaneously energized is made the first terminal while the other is made the second terminal, terminal voltage detecting means for separately detecting a terminal voltage of respective phase coil of the steering shaft driving motor, and failure determining means for determining a failure based on results obtained from a calculation performed to determine whether V 1 +V 3 coincides with 2*V 2 within a predetermined allowable range, wherein detected values of the terminal voltages of three phase conducting terminals u, v, and w of the steering shaft driving motor are made Vu, Vv, and Vw, respectively, and values when the detected values Vu, Vv, and Vw are arranged in order from largest to smallest voltage are made V 1 , V 2 , and V 3 , respectively, provided that V 1 ≧V 2 ≧V 3 .