Calibrating position sensor readings

1. A method of correcting a position reading from a position sensing arrangement, the position sensing arrangement comprising a disc having a magnetic ring with magnetic pole pairs and a magnetic sensor assembly comprising a magnetic sensor array configured to detect the magnetic pole pairs of the magnetic ring, the method comprising:

for each pole pair of the magnetic ring, taking a calibration pole pair position reading with the magnetic sensor array, and generating a pole pair correcting function by comparing the calibration pole pair position reading with a model pole pair position reading;

averaging the pole pair correcting functions of the pole pairs of the magnetic ring to generate an average pole pair correcting function for the magnetic ring;

taking a position reading with the magnetic sensor array, the position reading comprising a plurality of pole pair position readings; and

generating a corrected position reading by deducting the average pole pair correcting function from each pole pair position reading.

2. A method as claimed in claim 1 , wherein the calibration pole pair position reading is multi-bit.

3. A method as claimed in claim 1 , wherein the pole pair correcting function comprises a periodically oscillating function.

4. A method as claimed in claim 3 , wherein the pole pair correcting function comprises a sinusoidal function.

5. A method as claimed in claim 1 , comprising, for each pole pair, generating the pole pair correcting function by fitting a curve to the calibration pole pair position reading, and deducting a line representing the model pole pair position reading from the fitted curve.

6. A method as claimed in claim 5 , comprising fitting the curve to the calibration pole pair position reading using a method of least squares.

7. A method as claimed in claim 1 , wherein the disc comprises a further magnetic ring with magnetic pole pairs, and the magnetic sensor assembly comprises a further magnetic sensor array configured to detect the magnetic pole pairs of the further magnetic ring, the method comprising:

for each pole pair of the further magnetic ring, taking a further calibration pole pair position reading with the further magnetic sensor array, and generating a further pole pair correcting function by comparing the further calibration pole pair position reading with the model pole pair position reading;

averaging the further pole pair correcting functions of the pole pairs of the further magnetic ring to generate an average further pole pair correcting function for the further magnetic ring;

taking a further position reading with the further magnetic sensor array, the further position reading comprising a plurality of pole pair position readings; and

deducting the average further pole pair correcting function from each pole pair position reading to generate a corrected further position reading.

8. A method as claimed in claim 7 , wherein the further pole pair correcting function comprises a periodically oscillating function.

9. A method as claimed in claim 8 , wherein the further pole pair correcting function comprises a sinusoidal function.

10. A method as claimed in claim 7 , comprising generating the further pole pair correcting function by fitting a curve to the further calibration pole pair position reading, and deducting a line representing the model pole pair position reading from the fitted curve.

11. A method as claimed in claim 10 , comprising fitting the curve to the further calibration pole pair position reading using a method of least squares.

12. A method as claimed in claim 1 , comprising:

for each pole pair of the magnetic ring, generating a corrected calibration pole pair position reading by deducting the pole pair correcting function from the calibration pole pair position reading;

generating a revolution correcting function by comparing the corrected calibration pole pair position readings for the magnetic ring with model revolution position readings; and

generating the corrected position reading by:

deducting the average pole pair correcting function from each pole pair position reading; and

deducting the revolution correcting function from the position reading.

13. A method as claimed in claim 12 , wherein the revolution correcting function comprises a periodically oscillating function.

14. A method as claimed in claim 13 , wherein the revolution correcting function comprises a sinusoidal function.

15. A method as claimed in claim 12 , comprising generating the revolution correcting function by fitting a curve to the corrected calibration pole pair position readings, and deducting a line representing the model revolution position readings from the fitted curve.

16. A method as claimed in claim 15 , comprising fitting the curve to the corrected calibration pole pair position readings using a method of least squares.

17. A method as claimed in claim 7 , the method comprising:

for each pole pair of the magnetic ring, generating a corrected calibration pole pair position reading by deducting the pole pair correcting function from the calibration pole pair position reading;

generating a revolution correcting function by comparing the corrected calibration pole pair position readings for the magnetic ring with model revolution position readings; and

generating the corrected position reading by:

deducting the average pole pair correcting function from each pole pair position reading; and

deducting the revolution correcting function from the position reading, the method further comprising:

for each pole pair of the further magnetic ring, generating a further corrected calibration pole pair position reading by deducting the further pole pair correcting function from the further calibration pole pair position reading;

generating a further revolution correcting function by comparing the further corrected calibration pole pair position readings for the magnetic ring with further model revolution position readings; and

generating the corrected further position reading by:

deducting the average further pole pair correcting function from each pole pair position reading; and

deducting the further revolution correcting function from the position reading.

18. A method as claimed in claim 17 , wherein the further revolution correcting function comprises a periodically oscillating function.

19. A method as claimed in claim 18 , wherein the further revolution correcting function comprises a sinusoidal function.

20. A method as claimed in claim 17 , comprising generating the further revolution correcting function by fitting a curve to the corrected calibration pole pair position readings, and deducting a line representing the model revolution position readings from the fitted curve.