Patent ID: 8738232

Claim:
An electric power steering system comprising: a motor that applies a steering assist force to a steering system of an automobile; a current sensor that detects motor current flowing through the motor; a rotation angle sensor that detects a rotation angle of the motor; a torque sensor that detects steering torque to be transmitted to the steering system; a first control unit that controls the motor based on the motor current, the rotation angle, and the steering torque; a second control unit that, when a fault occurs in the first control unit, controls the motor based on the rotation angle and the steering torque in place of the first control unit, wherein the rotation angle sensor outputs a sin signal and a cos signal corresponding to the rotation angle; and the second control unit comprises: a coordinate converter section that converts the sin signal and the cos signal input from the rotation angle sensor into a three-phase sin signal; a phase converter section that converts the three-phase sin signal to a corresponding digital phase signal; a torque converter section that converts a torque signal input from the torque sensor to a corresponding digital torque signal; and a multiplier section that multiplies the digital phase signal by the digital torque signal to thereby calculate a motor drive signal.