Electric power steering apparatus

1. An electric power steering apparatus for giving steering auxiliary power to a steering mechanism of a vehicle by driving, by introducing d-q coordinates as a rotary coordinate system having a d-axis as a direction of magnetic flux of a rotary field system of the motor and a q-axis perpendicular to the d-axis, a cylindrical permanent magnet synchronous motor which is a driving source in response to a value on the d-q coordinates of a motor driving current command value, the apparatus comprising:

a d-axis current control unit which calculates a d-axis voltage command value for driving the motor by a control operation based on a d-axis component of the motor driving current command value;

a q-axis current control unit which calculates a q-axis voltage command value for driving the motor by a control operation based on a q-axis component of the motor driving current command value;

a coordinate transformation unit which transforms the d-axis voltage command value and the q-axis voltage command value to voltage command values in corresponding phases of the motor; and

a motor driving circuit which drives the motor according to the voltage command values in the corresponding phases, wherein:

control parameters of the d-axis current control unit are set on the basis of the d-axis inductance of the motor; and

control parameters of the q-axis current control unit are set on the basis of the q-axis inductance of the motor.

2. The electric power steering apparatus according to claim 1 , wherein

the control parameter of the d-axis current control unit is set on the basis of the d-axis inductance of the motor and d-axis resistance as a d-axis quantity of the resistance of a motor-driving circuit system comprising the motor and the motor driving circuit;

the control parameter of the q-axis current control unit is set on the basis of the q-axis inductance of the motor and q-axis resistance as a q-axis quantity of the resistance of the motor-driving circuit system; and

the d-axis resistance and the q-axis resistance are determined by a internal resistance of the motor and external resistance including wiring resistance of the motor and the motor driving circuit.

3. The electric power steering apparatus according to claim 2 , wherein:

the d-axis current control unit calculates the d-axis voltage command value by carrying out proportional integral control operations on a deviation of the d-axis component of the motor driving current command value from a d-axis component of the value of current flowing through the motor;

the q-axis current control unit calculates the q-axis voltage command value by carrying out proportional integral control operations on a deviation of the q-axis component of the motor driving current command value from a q-axis component of the value of current flowing through the motor; and

a proportional gain Kdp and an integral gain Kdi as the control parameters of the d-axis current control unit, and a proportional gain Kqp and an integral gain Kqi as the control parameters of the q-axis current control unit are set in according with the following equation:

Kdp=K 1· Ld,

Kdi=K 1· Rd

Kqp=K 2· Lq and

Kqi=K 2· Rq

where Ld=the d-axis inductance, Lq=the q-axis inductance, Rd=the d-axis resistance and Rq=the q-axis resistance with Rd=Rq=R+R′ where R=the internal resistance, R′=the external resistance, and K1 and K2=optional coefficients.

4. The electric power steering apparatus according to claim 1 , wherein a d-axis current detection value is substantially equal to a d-axis current command value.

5. The electric power steering apparatus according to claim 1 , wherein a q-axis current detection value is substantially equal to a q-axis current command value.

6. An electric power steering apparatus that provides auxiliary power to a steering mechanism of a vehicle, the apparatus comprising:

a brushless motor operably connected to steering system of the vehicle;

a sensor connected to the brushless motor, the sensor providing d-q coordinates as a rotary coordinate system having a d-axis as a direction of magnetic flux of a rotary field system of the motor and a q-axis perpendicular to the d-axis; and

a controller operably connected to the motor, the steering system and the sensor, the controller comprising:

a d-axis current control unit which calculates a d-axis voltage command value for driving the motor by a control operation based on an inductance of the d-axis component of the motor;

a q-axis current control unit which calculates a q-axis voltage command value for driving the motor by a control operation based on an inductance of the q-axis component of the motor;

a coordinate transformation unit which transforms the d-axis voltage command value and the q-axis voltage command value to voltage command values in corresponding phases of the motor; and

a motor driving circuit which drives the motor according to the voltage command values in the corresponding phases, wherein the controller matches frequency characteristics of a d-axis current and a q-axis current with a frequency range in which the electric power steering apparatus is used.

7. The electric power steering apparatus according to claim 6 , wherein

the d-axis current control unit which calculates the d-axis voltage command value for driving the motor by a control operation based on an internal and an external resistance of the motor, and

the q-axis current control unit which calculates the q-axis voltage command value for driving the motor by a control operation based on an internal and an external resistance of the motor.

8. The electric power steering apparatus according to claim 6 , wherein

the d-axis voltage command value is calculated by a proportional integral operation based on the d-axis inductance and a d-axis resistance, including an external resistance, and

the q-axis voltage command value is calculated by a proportional integral operation based on the q-axis inductance and a q-axis resistance, including an external resistance.

9. The electric power steering apparatus according to claim 7 , wherein the external resistance comprises at least one of a wiring resistance of the motor and the motor driving circuit of the controller.

10. The electric power steering apparatus according to claim 6 , wherein the controller further comprises:

a current control portion that cancels a deviation of a d-axis current command value from a d-axis current detection value, and cancels a deviation of a q-axis current command value from a q-axis current detection value.

11. An electric power steering apparatus that provides auxiliary power to a steering mechanism of a vehicle, the apparatus comprising:

a brushless motor operably connected to steering system of the vehicle;

a sensor connected to the brushless motor, the sensor providing d-q coordinates as a rotary coordinate system having a d-axis as a direction of magnetic flux of a rotary field system of the brushless motor and a q-axis perpendicular to the d-axis; and

a controller operably connected to the brush less motor, the steering system and the sensor, the controller comprising:

a d-axis current control unit which calculates a d-axis voltage command value for driving the brushless motor by a control operation based on an inductance of the d-axis component of the brushless motor and an internal resistance of the brushless motor and an external resistance;

a q-axis current control unit which calculates a q-axis voltage command value for driving the brushless motor by a control operation based on an inductance of the q-axis component of the brushless motor and an internal of the brushless motor and an external resistance;

a coordinate transformation unit which transforms the d-axis voltage command value and the q-axis voltage command value to voltage command values in corresponding phases of the motor; and

a motor driving circuit which drives the motor according to the voltage command values in the corresponding phases, wherein the controller matches frequency characteristics of a d-axis current and a q-axis current with a frequency range in which the electric power steering apparatus is used.

12. The electric power steering apparatus according to claim 11 , wherein the external resistance comprises at least one of a wiring resistance of the motor and a motor driving circuit of the controller.

13. The electric power steering apparatus according to claim 11 , wherein:

the d-axis current control unit calculates the d-axis voltage command value by carrying out proportional integral control operations on a deviation of the d-axis component of the motor driving current command value from a d-axis component of the value of current flowing through the motor;

the q-axis current control unit calculates the q-axis voltage command value by carrying out proportional integral control operations on a deviation of the q-axis component of the motor driving current command value from a q-axis component of the value of current flowing through the motor; and

a proportional gain Kdp and an integral gain Kdi as the control parameters of the d-axis current control unit, and a proportional gain Kqp and an integral gain Kqi as the control parameters of the q-axis current control unit are set in according with the following equation:

Kdp =K 1· Ld,

Kdi =K 1· Rd

Kqp =K 2· Lq and

Kqi =K 2· Rq

where Ld=the d-axis inductance, Lq=the q-axis inductance, Rd=the d-axis resistance and Rq=the q-axis resistance with Rd=Rq=R+R′ where R=the internal resistance, R′=the external resistance, and K1 and K2=optional coefficients.