Motor position calibration

1. A method of determining an electrical angle offset of a motor position sensor mounted to a multi-phase electric motor, the method comprising:

rotating a rotor of the multi-phase electric motor at a motor velocity using an external drive system;

measuring a motor phase voltage of each phase of the multi-phase electric motor, the motor phase voltage comprising a back-EMF generated by the rotation of the motor;

converting the motor phase voltage to a DQ reference frame using as a position of the multi-phase electric motor a value representing a motor electrical position plus a position offset to form a DQ motor phase voltage;

calculating the electrical angle offset from the DQ motor phase voltages by adjusting the position offset until a D-axis component of the DQ motor phase voltage is zero, at which point it can be determined that the position offset is equal to an offset of the motor position sensor relative to an electrical angle of the multi-phase electric motor corresponding to the electrical angle offset.

2. The method according to claim 1 , wherein the motor phase voltages are first converted to an aP reference frame prior to conversion to the DQ reference frame.

3. The method according to claim 2 , further comprising the step of normalising the motor phase voltages to the motor velocity to form normalised motor phase voltage prior to conversion into the DQ motor phase voltage.

4. The method according to claim 3 , wherein the position offset is used in the step of converting the normalised motor phase voltage to the DQ reference frame to form the DQ motor phase voltage.

5. The method according to claim 4 , wherein the step of calculating the electrical angle offset from the DQ motor phase voltages is performed by a closed loop controller.

6. The method according to claim 5 , wherein the closed loop controller is a PI controller.

7. The method according to claim 3 , wherein the step of calculating the electrical angle offset from the DQ motor phase voltages is performed by using an inverse tan function.

8. The method according to claim 7 , wherein the step of calculating the electrical angle offset from the DQ motor phase voltages includes an application of a time lag correction to take into account the motor velocity when performing the calculation.

9. The method according to claim 8 , wherein the time lag correction is applied by filtering a motor velocity signal indicative of the motor velocity with a time lag filter coefficient.

10. The method according to claim 9 , wherein the motor velocity is a constant velocity.

11. The method according to claim 10 , wherein the motor velocity is above a minimum motor velocity.

12. The method according to claim 11 , wherein the motor velocity is below a maximum motor velocity.

13. The method according to claim 12 , further comprising the step of removing DC offsets from the motor phase voltages.

14. The method according to claim 13 , wherein, during operation of the method, a drive stage controller of the multi-phase electric motor is disabled.

15. The method according to claim 14 , wherein the electrical angle offset is filtered with an offset filter coefficient.

16. The method of claim 1 , further comprising:

determining a rotation direction of the of the multi-phase electric motor; and

selecting one of a first parameter or a second parameter from two different parameters based on the determined rotation direction, wherein the electrical angle offset is calculated based on one of the selected first or second parameters.

17. The method of claim 16 , wherein the first parameter is selected in response to determining that the multi-phase electric motor is rotating in a first direction and the second parameter is selected in response to determining that the multi-phase electric motor is rotating in a second direction.