Electric power steering apparatus

1. An electric power steering apparatus that performs steering assist by transmitting a driving force produced by an electric motor to a steering mechanism, comprising:

a current detection circuit for detecting a current flowing through the electric motor to output a motor current detection signal, the current detection circuit including a variable gain amplification circuit whose gain can be changed, the current detection circuit outputting the motor current detection signal amplified by the variable gain amplification circuit; and

a microcomputer, having an analog-to-digital conversion port to which the motor current detection signal outputted by the current detection circuit is inputted and also having a gain change section for changing the gain of the variable gain amplification circuit, for controlling the electric motor on the basis of the received motor current detection signal through the analog-to-digital conversion port,

wherein the gain change section determines the gain of the variable gain amplification circuit such that the smaller a motor current represented by the motor current detection signal received from the analog-to-digital conversion port is, the larger the gain is.

2. The electric power steering apparatus according to claim 1 , wherein the microcomputer additionally has a motor current value conversion section that converts a digital value from the analog-to-digital conversion port depending on the gain determined by the gain change section, and generates a motor current value in which the change in gain in the variable gain amplification circuit is compensated for.

3. An electric power steering apparatus that performs steering assist by transmitting a driving force produced by an electric motor to a steering mechanism, comprising:

a current detection circuit for detecting a current flowing through the electric motor to output a motor current detection signal, the current detection circuit including a variable gain amplification circuit whose gain can be changed, the current detection circuit outputting the motor current detection signal amplified by the variable gain amplification circuit;

a microcomputer, having an analog-to-digital conversion port to which the motor current detection signal outputted by the current detection circuit is inputted and also having a gain change section for changing the gain of the variable gain amplification circuit, for controlling the electric motor on the basis of the received motor current detection signal through the analog-to-digital conversion port;

a steering torque sensor for sensing the steering torque applied to an operation member for steering a vehicle,

wherein the microcomputer determines a target current value corresponding to the steering torque sensed by the steering torque sensor, and carries out feedback control of the electric motor such that the motor current detection signal received from the analog-to-digital conversion port coincides with the target current value, and

wherein the gain change section determines the gain of the variable gain amplification circuit such that the smaller the absolute value of the steering torque sensed by the steering torque sensor is, the larger the gain is.

4. The electric power steering apparatus according to claim 3 , wherein the microcomputer additionally has a motor current value conversion section that converts a digital value from the analog-to-digital conversion port depending on the gain determined by the gain change section, and generates a motor current value in which the change in gain in the variable gain amplification circuit is compensated for.

5. A electric power steering apparatus, that performs steering assist by transmitting a driving force produced by an electric motor to a steering mechanism, comprising:

a current detection circuit for detecting a current flowing through the electric motor to output a motor current detection signal, the current detection circuit including a variable gain amplification circuit whose gain can be changed, the current detection circuit outputting the motor current detection signal amplified by the variable gain amplification circuit;

a microcomputer, having an analog-to-digital conversion port to which the motor current detection signal outputted by the current detection circuit is inputted and also having a gain change section for changing the gain of the variable gain amplification circuit, for controlling the electric motor on the basis of the received motor current detection signal through the analog-to-digital conversion port;

a steering torque sensor for sensing a steering torque applied to an operation member for steering a vehicle, and

a vehicle speed sensor for sensing the vehicle speed of a vehicle,

wherein the microcomputer determines a target current value corresponding to the steering torque sensed by the steering torque sensor and the vehicle speed sensed by the vehicle speed sensor, and carries out feedback control of the electric motor such that the motor current detection signal received from the analog-to-digital conversion port coincides with the target current value,

wherein the microcomputer additionally has a motor current value conversion section that converts a digital value from the analog-to-digital conversion port depending on the gain determined by the gain change section, and generates a motor current value in which the change in gain in the variable gain amplification circuit is compensated for, and

wherein the gain change section determines the gain of the variable gain amplification circuit such that the higher the vehicle speed sensed by the vehicle speed sensor is, the larger the gain is.