Inductive angular sensor method and system

1. A method ( 400 ) of determining a set of calibration values (Aasym 01 , Aasym 12 , Aasym 20 ) for offset-compensation of an inductive angular sensor arrangement ( 310 ) comprising: a substrate and a target; wherein the substrate comprises at least one transmitter coil (TX) and three receiver coils (Rx 1 , Rx 2 , Rx 3 ) for generating three modulated signals from which three baseband signals (In 0 , In 1 , In 2 ) which are electrically shifted relatively to one another by about 120° can be derived, and

wherein the target is rotatable relative to said substrate; the method comprising the steps of:

a) applying an alternating signal to the at least one transmitter coil (TX);

b) performing at least six times, the steps i) to iv):

i) positioning the target in one of six predefined angular positions (α 30 , α 90 , α 150 , α 210 , α 270 , α 330 ) spaced apart by approximately 50° to 70°;

ii) receiving a first, second and third alternating signal (S 0 , S 1 , S 2 ) from the three receiver coils whilst the target is at or near said predefined angular position;

iii) demodulating the first, second and third alternating signal (S 0 , S 1 , S 2 ) thereby obtaining a first, second and third baseband signal (In 0 , In 1 , In 2 );

iv) calculating a first, second and third pairwise difference signal (D 01 a , D 12 a , D 20 a ) from the baseband signals, and storing the difference signals;

c) initializing a first, second and third correction value (Aasym 01 , Aasym 12 , Aasym 20 );

d) adjusting, at least once, the first, second and third pairwise difference signal (D 01 a , D 12 a , D 20 a ) taking into account the first, second and third correction value (Aasym 01 , Aasym 12 , Aasym 20 ), and adjusting at least once the first, second and third correction value (Aasym 01 , Aasym 12 , Aasym 20 ) based on a sum of squares (SoS 30 , SoS 90 , SoS 150 , SoS 210 , SoS 270 , SoS 330 ) of the adjusted pairwise difference signals.

2. The method of claim 1 , wherein step d) further comprises:

performing at least once, the steps v) to viii):

v) subtracting the first, second and third correction value from the first, second, third pairwise difference signal (D 01 a , D 12 a , D 20 a ) respectively, thereby obtaining corrected difference signals (D 01 , D 12 , D 20 );

vi) determining a sum of squares (SoS 30 , SoS 90 , SoS 150 , SoS 210 , SoS 270 , SoS 330 ) of the corrected difference signals for each of said six predefined angular positions;

vii) estimating an amplitude (Arotor) of the corrected difference signals (D 01 , D 12 , D 20 );

viii) updating the first, second and third correction value (Aasym 01 , Aasym 12 , Aasym 20 ) based on the following formulas:

Aasym01=( SoS 90− SoS 270)/(6*Arotor);

Aasym12=( SoS 210− SoS 30)/(6*Arotor);

Aasym20=( SoS 330− SoS 150)/(6*Arotor).

3. The method according to claim 2 , wherein Arotor is calculated in accordance with the following formula:

Arotor=[max( D 01)−min( D 01)+max( D 12)−min( D 12)+max( D 20)−min( D 20)]/6,

where max(·) means a maximum value, and min(·) means a minimum value.

4. The method according to claim 2 ,

wherein step viii) further comprises: determining how much the correction values have changed in this iteration of step d), and comparing this change to a predefined threshold value;

and if the change is larger than the threshold value, performing another iteration of step d);

and if the change is smaller than the threshold value, continuing with step e).

5. The method according to claim 1 , wherein the sum of squares (SoS 30 , SoS 90 , SoS 150 , SoS 210 , SoS 270 , SoS 330 ) is calculated in accordance with the following formula:

SoS=(D 01 ) 2 +(D 12 ) 2 +(D 20 ) 2 .

6. The method according to claim 1 , wherein step d) is performed a predefined number (N) of times, N being at least 3.

7. The method according to claim 1 ,

wherein step i) comprises: positioning the target in a stationary position; and

wherein step ii) comprises: receiving said alternating signals (S 0 , S 1 , S 2 ) from the receiver coils whilst the target is in said predefined angular position.

8. The method according to claim 1 ,

wherein step i) comprises: allowing or causing the target to rotate; and

wherein step ii) comprises: receiving said alternating signals (S 0 , S 1 , S 2 ) from the receiver coils whilst the target is in the vicinity of said predefined angular position.

9. The method according to claim 8 , further comprising offset-compensation of said inductive angular sensor arrangement as part of a position sensor system ( 300 ) that further comprises an angular sensor device ( 320 ) having a non-volatile memory ( 324 );

the method further comprising the steps of:

storing the calibration values (Aasym 01 , Aasym 12 , Aasym 20 ) in the non-volatile memory ( 324 ) of the angular sensor device ( 320 ) during a calibration phase;

reading these calibration values from said non-volatile memory ( 324 ), and using them for offset-compensation during normal use of said angular sensor device ( 320 ).

10. The method according to claim 1 ,

wherein the at least one transmitter coil (TX) is substantially circular; and/or

wherein the at least one transmitter coil (TX) is part of an LC oscillator circuit.

11. The method according to claim 1 , wherein the receiver coils are anti-winded.

12. The method according to claim 1 , wherein the transmitter coil is excited with an alternating signal having a frequency in the range from 1 to 20 MHz.

13. An angular sensor device ( 320 ) comprising:

means for applying an alternating signal to the at least one transmitter coil (TX);

a demodulator circuit ( 323 ) configured for demodulating three modulated signals (S 0 , S 1 , S 2 ) into three baseband signals (In 0 , In 1 , In 2 );

a non-volatile memory ( 324 ) for storing at least three calibration values (Aasym 01 , Aasym 12 , Aasym 20 );

a processing circuit ( 326 ) connected to an output of said demodulator circuit ( 323 ), and connected to said non-volatile memory ( 324 ), and configured for performing the following steps during a calibration phase:

performing at least six times:

i) allowing or causing the target to rotate to position the target in one of six predefined angular positions (α 30 , α 90 , α 150 , α 210 , α 270 , α 330 ) spaced apart by approximately 50° to 70°;

ii) receiving a first, second and third alternating signal (S 0 , S 1 , S 2 ) from the three receiver coils whilst the target is at or near said predefined angular position;

iii) demodulating the first, second and third alternating signal (S 0 , S 1 , S 2 ) thereby obtaining a first, second and third baseband signal (In 0 , In 1 , In 2 );

iv) calculating a first, second and third pairwise difference signal (DOla, D 12 a , D 20 a ) from the baseband signals, and storing the difference signals;

initializing a first, second and third correction value (Aasym 01 , Aasym 12 , Aasym 20 );

adjusting, at least once, the first, second and third pairwise difference signal (D 01 a , D 12 a , D 20 a ) taking into account the first, second and third correction value (Aasym 01 , Aasym 12 , Aasym 20 ), and adjusting at least once the first, second and third correction value (Aasym 01 , Aasym 12 , Aasym 20 ) based on a sum of squares (SoS 30 , SoS 90 , SoS 150 , SoS 210 , SoS 270 , SoS 330 ) of the adjusted pairwise difference signals; and

storing the calibration values (Aasym 01 , Aasym 12 , Aasym 20 ) in the non-volatile memory ( 324 ) of the angular sensor device ( 320 ) during a calibration phase.

14. An angular sensor system ( 300 ) including the angular sensor device ( 320 ) according to claim 13 and comprising:

a substrate comprising at least one transmitter coil (TX) and three receiver coils (Rx 1 , Rx 2 , Rx 3 ), configured for generating three modulated signals from which three baseband signals (In 0 , In 1 , In 2 ) which are electrically shifted relatively to one another by about 120° can be derived;

a target ( 312 ) rotatable relative to said substrate;

the angular sensor device ( 320 ) connected to said at least one transmitter coil (TX) and to said receiver coils (Rx 1 , Rx 2 , Rx 3 ).

15. The angular sensor system ( 300 ) according to claim 14 , wherein the substrate has a rotational symmetry of 120° divided by the number (N) of lobes of the target.