Electric power steering apparatus

1. An electric power steering apparatus which transmits a driving force of an electric motor to a steering mechanism for steering assist in a vehicle, the apparatus comprising:

a motor driver which drives the electric motor according to a target value;

a torque sensor which detects a steering torque applied to a steering member for steering the vehicle;

a basic assist section which determines a basic assist value according to the steering torque detected by the torque sensor;

a target value determining section which determines the target value by shifting the basic assist value along a steering torque coordinate axis to a modified assist value, whereby the modified assist value acts as the target value; and

a shift amount determining section which determines an amount of the shift from the basic assist value to the modified assist value on the basis of the steering torque detected by the torque sensor;

wherein the shift amount determining section sets the amount of the shift from the basic assist value to the modified assist value at zero in a forward stroke steering where the steering member is moved away from a steering angle midpoint, and

sets the amount of the shift from the basic assist value to the modified assist value at a value such that the target value has an increased absolute value of the modified assist value in a steering holding position and in a return stroke steering operation where the steering member is moved toward the steering angle midpoint.

2. An electric power steering apparatus which transmits a driving force of an electric motor to a steering mechanism for steering assist in a vehicle, the apparatus comprising:

a motor driver which drives the electric motor according to a target value;

a torque sensor which detects a steering torque applied to a steering member for steering the vehicle;

basic assist section which determines a basic assist value according to the steering torque detected by the torque sensor;

a target value determining section which determines the target value by shifting the basic assist value along a steering torque coordinate axis to a modified assist value, whereby the modified assist value acts as the target value; and

a shift amount determining section which determines an amount of the shift from the basic assist value to the modified assist value on the basis of the steering torque detected by the torque sensor;

wherein the shift amount determining section computes the shift amount for the steering holding operation and the return stroke steering operation on the basis of the steering torque detected by the torque sensor when the forward stroke steering is switched to the steering holding operation or to the return stroke steering operation.

3. An electric power steering apparatus which transmits a driving force of an electric motor to a steering mechanism for steering assist in a vehicle, the apparatus comprising:

a motor driver which drives the electric motor according to a target value;

a torque sensor which detects a steering torque applied to a steering member for steering the vehicle;

a basic assist section which determines a basic assist value according to the steering torque detected by the torque sensor;

a target value determining section which determines the target value by shifting the basic assist value along a steering torque coordinate axis to a modified assist value, whereby the modified assist value acts as the target value; and

a shift amount determining section which determines an amount of the shift from the basic assist value to the modified assist value on the basis of the steering torque detected by the torque sensor;

wherein the shift amount determining section determines the shift amount so that an absolute value of the shift amount is reduced as an absolute value of the steering torque increases in a range greater than a predetermined threshold value.

4. The electric power steering apparatus as set forth in claim 1 ,

further comprising a steering speed detecting section which detects a steering speed of the steering member, and

wherein the shift amount determining section variably sets a shift direction and a shift amount of the basic assist value according to a direction and an absolute value of the steering speed.

5. The electric power steering apparatus as set forth in claim 1 ,

further comprising a vehicle speed detecting section which detects a traveling speed of the vehicle, and

wherein the shift amount determining section comprises a vehicle speed adaptive shift amount determining section which variably sets the amount of the shift from the basic assist value to the modified assist value according to the vehicle speed detected by the vehicle speed detecting section.