Electric power steering apparatus

An electric power steering apparatus comprising:

• a vehicle speed sensor for detecting a vehicle speed;
• a torque sensor for detecting a steering torque by a vehicle driver;
• a motor having a plurality of wiring sets and outputting torque when supplied with power;
• a reduction gear for transmitting output torque of the motor to a steering shaft as an assist torque, while amplifying the output torque of the motor and reducing output rotation of the motor;
• and
• a control section for controlling rotation of the motor;
• wherein the control section includes:
• a plurality of systems of power converter apparatuses provided corresponding to the plurality of the winding sets for supplying power to the motor;
• a drive circuit for driving the power converter apparatuses in accordance with a commanded assist current;
• a failure detection means for detecting a failure in the power converter apparatus or the winding set of any one of the plurality of the systems;
• a power supply stopping means for stopping supply of power to the power converter apparatus of the failed power system detected by the failure detection means;
• a map storage means for storing at least a normal-state vehicle speed responsive map defining a T-I characteristic of a relationship between the steering torque and the assist current, the normal-state vehicle speed responsive map being used to determine the T-I characteristic in accordance with the vehicle speed when the vehicle speed is normally detected by the vehicle speed sensor;
• an assist current calculation means for calculating the assist current based on the steering torque detected by the torque sensor and the vehicle speed detected by the vehicle speed sensor, the assist current calculation means being configured to, when a failure is detected by the failure detection means, set a first current limit value as an assist current limit value in a one-system failure-state map to be used for the power converter apparatus of each normal system except the failed system equal to an assist current limit value in a vehicle speed detection failure-state map to be used when there is a failure in detecting the vehicle speed by the vehicle speed sensor, the vehicle speed detection failure-state map defining the T-I characteristic independently of the vehicle speed;
• and
• an assist current command means for commanding an assist current command value to the drive circuit based on the assist current calculated by the assist current calculation means.