Patent ID: 7694777

Claim:
An electric power steering apparatus, comprising: a steering mechanism that steers a steering wheel in response to a steering operation of a driver's steering wheel; an electric motor that is fitted to the steering mechanism, that produces a steering assist torque used to assist the steering operation of the driver's steering wheel, and that is formed of a salient-pole permanent magnet motor; a rotational angle sensor that rotates in accordance with rotation of the electric motor to detect a rotational angle of the electric motor; a rotational angle estimation unit that estimates the rotational angle of the electric motor using a salient-polarity of the electric motor; a sensor malfunction determination unit that determines whether a malfunction has occurred in the rotational angle sensor; a rotational angle information obtaining unit that obtains information concerning the rotational angle of the electric motor from the rotational angle sensor when it is determined that there is no malfunction in the rotational angle sensor, and that obtains the information concerning the rotational angle of the electric motor from the rotational angle estimation unit when it is determined that a malfunction has occurred in the rotational angle sensor; a motor control unit that controls the rotation of the electric motor based on the information concerning the rotational angle obtained by the rotational angle information obtaining unit; and an assist control unit that calculates a target steering assist torque based on the steering operation of the driver's steering wheel, that provides the motor control unit with a command to supply electricity to the electric motor to cause the electric motor to produce the steering assist torque based on the calculated target steering assist torque, and that changes a control manner for causing the electric motor to produce the steering assist torque between a control process executed when the sensor malfunction determination unit determines that a malfunction has occurred in the rotational angle sensor and a control process executed when the sensor malfunction determination unit determines that there is no malfunction in the rotational angle sensor; wherein the assist control unit includes a pulsatile torque addition unit that adds a pulsatile torque to the steering assist torque produced by the electric motor, when the sensor malfunction determination unit determines that a malfunction has occurred in the rotational angle sensor, to maintain the rotational angular speed of the electronic motor above a predetermined valve.