Apparatus and method for monitoring magnet flux degradation of a permanent magnet motor

1. A method in a vehicle having an electric motor, the method comprising:

measuring, using one or more sensors, a motor current measurement, a motor reference voltage or measured voltage, and an estimated temperature from the electric motor;

determining, using a motor controller, a reference flux linkage estimate λ pm,ref ) from a lookup table based on the motor current measurement and the estimated temperature;

determining, using the motor controller, a back-EMF (electromotive force) estimate based on the motor current measurement, the motor voltage and the estimated temperature;

determining, using the motor controller, an observed flux linkage estimate (λ pm,obs ) from the back-EMF estimate;

determining, using the motor controller, a magnetic strength as a ratio of the observed flux linkage estimate to the reference flux linkage estimate; and

setting a diagnostic trouble code indicating that a rotor magnet in the electric motor is not performing to operational standards when the magnetic strength is below a threshold level.

2. The method of claim 1 , wherein determining a reference flux linkage estimate (λ pm,ref ) from a lookup table based on the motor current measurement and the estimated temperature comprises determining the reference flux linkage estimate using a pre-defined flux lookup table and a temperature coefficient.

3. The method of claim 1 , wherein determining a reference flux linkage estimate (λ pm,ref ) comprises selecting an entry from a lookup table wherein a d-axis current (i d ) is equal to zero and a q-axis current (i q ) is equal to the amplitude of a stator excitation current (I ss ).

4. The method of claim 3 , wherein determining a reference flux linkage estimate (λ pm,ref ) comprises multiplying the selected entry from the lookup table by a temperature coefficient (B r coeff) wherein the B r coeff is calculated using a measured magnet material property from the rotor magnet.

5. The method of claim 1 , wherein determining the back-EMF estimate based on the motor current measurement, the motor voltage and the estimated temperature comprises:

calculating a flux increment based on the motor current measurement, the motor voltage, and the estimated temperature;

calculating a flux error by differencing an estimated flux and a measured flux; and

determining the back-EMF estimate using the calculated flux increment and the calculated flux error.

6. The method of claim 1 , wherein determining the observed flux linkage estimate (λ pm,obs ) using the back-EMF estimate comprises determining the observed flux linkage estimate using a d-axis back-EMF voltage estimate in the synchronously rotating reference frame (ê r ds ) and a q-axis back-EMF voltage estimate in the synchronously rotating reference frame (ê r qs ).

7. The method of claim 6 , wherein the observed flux linkage estimate (λ pm,obs ) is determined by λ pm,obs =√{square root over ((ê r ds ) 2 +(ê r qs ) 2 )}/ω e wherein ω e is the electrical angular speed.

8. The method of claim 1 , wherein determining the magnetic strength as a ratio of the observed flux linkage estimate to the reference flux linkage estimate comprises multiplying the ratio by a slew rate factor to reduce the impact of high speed transient flux changes.

9. A system comprising:

one or more sensors, configured to measure a motor current measurement, a motor reference voltage or measured voltage, and an estimated temperature from an electric motor;

a flux observer module comprising one or more processors configured by programming instructions encoded on non-transient computer readable media, the flux observer module configured to:

determine a reference flux linkage estimate (λ pm,ref ) from a lookup table based on the motor current measurement and the estimated temperature;

determine a back-EMF (electromotive force) estimate based on the motor current measurement, the motor voltage and the estimated temperature; and

determine an observed flux linkage estimate (λ pm,obs ) from the back-EMF estimate; and

a magnet strength module comprising one or more processors configured by programming instructions encoded on non-transient computer readable media, the magnet strength module configured to:

determine a magnetic strength as a ratio of the observed flux linkage estimate to the reference flux linkage estimate; and

cause a diagnostic trouble code to be set indicating that a rotor magnet in the electric motor is not performing to operational standards when the magnetic strength is below a threshold level.

10. The system of claim 9 , wherein the flux observer module is configured to determine a reference flux linkage estimate ( 80 pm,ref ) using a pre-defined flux lookup table and a temperature coefficient.

11. The system of claim 9 , wherein to determine a reference flux linkage estimate (λ pm,ref ) the flux observer module is configured to select an entry from a lookup table wherein a d-axis current (i d ) is equal to zero and a q-axis current (i q ) is equal to the amplitude of a stator excitation current (I ss ).

12. The system of claim 11 , wherein to determine a reference flux linkage estimate (λ pm,ref ) the flux observer module is configured to multiply the selected entry from the lookup table by a temperature coefficient (B r coeff), wherein the B r coeff is calculated using a measured magnet material property from the rotor magnet.

13. The system of claim 9 , wherein to determine the back-EMF estimate the flux observer module is configured to:

calculate a flux increment based on the motor current measurement, the motor voltage, and the estimated temperature;

calculate a flux error by differencing an estimated flux and a measured flux; and

determine the back-EMF estimate using the calculated flux increment and the calculated flux error.

14. The system of claim 9 , wherein to determine the observed flux linkage estimate (λ pm,obs ) using the back-EMF estimate, the flux observer module is configured to determine the observed flux linkage estimate using a d-axis back-EMF voltage estimate in the synchronously rotating reference frame (ê r ds ) and a q-axis back-EMF voltage estimate in the synchronously rotating reference frame (ê r qs ).

15. The system of claim 14 , wherein the flux observer module is configured to determine the observed flux linkage estimate (λ pm,obs ) by λ pm,obs =√{square root over ((ê r ds ) 2 +(ê r qs ) 2 )}/ω e wherein ω e is the electrical angular speed.

16. The system of claim 9 , wherein to determine the magnetic strength as a ratio of the observed flux linkage estimate to the reference flux linkage estimate the magnet strength module is configured to multiply the ratio by a slew rate factor to reduce the impact of high speed transient flux changes.

17. A vehicle, comprising:

an electric permanent magnet motor;

one or more sensors, configured to measure a motor current measurement, a motor reference voltage or measured voltage, and an estimated temperature from the electric motor; and

a controller comprising one or more processors and non-transient computer readable media encoded with programming instructions, the controller configured to:

determine a reference flux linkage estimate (λ pm,ref ) from a lookup table based on the motor current measurement and the estimated temperature;

determine a back-EMF (electromotive force) estimate based on the motor current measurement, the motor voltage and the estimated temperature;

determine an observed flux linkage estimate (λ pm,obs ) from the back-EMF estimate;

determine a magnetic strength as a ratio of the observed flux linkage estimate to the reference flux linkage estimate;

set a diagnostic trouble code in the vehicle indicating that a rotor magnet in the electric motor is not performing to operational standards when the magnetic strength is below a threshold level.

18. The vehicle of claim 17 , wherein to determine a reference flux linkage estimate (λ pm,ref ) the controller is configured to select an entry from a lookup table wherein a d-axis current (i d ) is equal to zero and a q-axis current (i q ) is equal to the amplitude of a stator excitation current (I ss ).

19. The vehicle of claim 17 , wherein to determine a reference flux linkage estimate (λ pm,ref ) the controller is configured to multiply the selected entry from the lookup table by a temperature coefficient (B r coeff), wherein the B r coeff is calculated using a measured magnet material property from the rotor magnet.

20. The vehicle of claim 17 , wherein the controller is configured to determine the observed flux linkage estimate (λ pm,obs ) by λ pm,obs =√{square root over ((ê r ds ) 2 +(ê r qs) 2 )}/ω e wherein ê r ds is a d-axis back- EMF voltage estimate in the synchronously rotating reference frame, ê r qs is a q-axis back-EMF voltage estimate in the synchronously rotating reference frame, and ω e is an electrical angular speed.