Offset calibration of a semi-relative steering wheel angle sensor

1. A method for determining an actual steering wheel angle θ of a vehicle steering mechanism, the vehicle steering mechanism having an initial steering wheel angle θ 1 when the vehicle enters an ignition-on state, the vehicle steering mechanism having a center steering wheel angle θ 0 when the vehicle is heading straight-forward, the method comprising the steps of:

determining a semi-relative steering wheel angle offset Δθ representing the difference between θ 1 and θ 0 ;

storing Δθ in erasable programmable read-only memory when said vehicle enters an ignition-off state;

measuring a relative steering wheel angle θ R from a semi-relative steering wheel angle sensor during the ignition-off state;

retrieving Δθ from the erasable programmable read-only memory when said vehicle enters an ignition-on state;

continuing to measure the relative steering wheel angle θ R from the semi-relative steering wheel angle sensor during the ignition-on state; and

determining said steering wheel angle θ based on θ R and Δθ.

2. The method of claim 1 , further comprising the step of detecting if power to the semi-relative steering wheel angle sensor was terminated.

3. The method of claim 2 , further comprising the step of re-determining an initial semi-relative steering wheel angle offset Δθ when it is detected that power to the semi-relative steering wheel angle sensor was terminated.

4. The method of claim 1 , said vehicle including a sensor cluster including a yaw rate sensor, further comprising the step of determining if a failure has occurred with the sensor cluster.

5. The method of claim 4 , further comprising the step of re-determining an initial semi-relative steering wheel angle offset Δθ when it is detected that a failure has occurred with the sensor cluster.

6. The method of claim 1 , wherein the semi-relative steering wheel angle sensor is powered independently from the vehicle ignition.

7. The method of claim 1 , wherein θ=θ R −Δθ.

8. The method of claim 1 , wherein the measuring step includes measuring the relative steering wheel angle θ R throughout the ignition-off state.

9. A method of determining an actual steering wheel angle θ of a steering mechanism in a vehicle, the vehicle including an electrical control unit in communication with a calibration module, a sensor module and an erasable programmable read-only memory, the calibration module including a semi-relative steering wheel angle sensor reading the relative movement of the steering mechanism, the sensor module including a yaw rate sensor, the method comprising the steps of:

reading a last known semi-relative steering wheel angle offset value Δθ(t n ) from the eraseable programmable read-only memory to the electrical control unit;

determining whether power to calibration module has been terminated

determining whether a failure has occurred with the sensor module; and

setting the last known semi-relative steering wheel angle offset value Δθ(t n ) as an initial semi-relative steering wheel angle offset value Δθ(t 0 ) if power to the calibration module was not terminated and no failure occurred with the sensor module.

10. The method of claim 9 , further comprising the step of activating a vehicle dynamic controller when the initial semi-relative steering wheel angle offset value Δθ(t 0 ) has been determined.

11. The method of claim 10 , further comprising the step of measuring a semi-relative steering wheel angle θ R from said semi-relative steering wheel angle sensor.

12. The method of claim 10 , further comprising the step of determining whether the calibration module outputs valid data.

13. The method of claim 9 , further comprising the step of calculating an initial semi-relative steering wheel angle offset value Δθ(t 0 ) if power to the calibration module was terminated or if a failure occurred with the sensor module.

14. The method of claim 13 , wherein the step of calculating Δθ(t 0 ) includes the steps of:

measuring a relative steering wheel angle θ R from a semi-relative steering wheel angle sensor;

identifying a center steering wheel angle θ 0 of the steering mechanism representing the vehicle being headed straight-forward and stable; and

setting Δθ(t 0 )=θ R when the steering mechanism has been identified as being at θ 0 .

15. The method of claim 14 , wherein the vehicle being headed straight-forward is determined only when the CAN BUS is functional, the YRS signal is valid, and the Semi-RSAS signal is valid.

16. The method of claim 14 , wherein the vehicle being stable is determined by calculating the amplitudes of predetermined vehicle natural frequencies using an on-line frequency detection method.

17. The method of claim 9 , further comprising the steps of:

determining whether communication between the electrical control unit and the sensor module is functional; and

determining whether communication between the electrical control unit and the calibration module is functional.

18. A method for determining a steering wheel angle θ of a steering mechanism for a vehicle, the method comprising the steps of:

measuring a relative steering wheel angle θ R from a semi-relative steering wheel angle sensor during both ignition-off and ignition-on states of said vehicle;

retrieving a semi-relative steering wheel angle offset Δθ from the erasable programmable read-only memory when said vehicle enters an ignition-on state; and

determining said steering wheel angle θ based on θ R and Δθ.

19. The method of claim 18 , further comprising the step of storing the semi-relative steering wheel angle offset Δθ to erasable programmable read-only memory when said vehicle enters an ignition-off state.

20. The method of claim 18 , said vehicle including a sensor cluster including a yaw rate sensor, further comprising the steps of re-determining Δθ if the semi-relative steering wheel angle sensor looses power or a failure occurs with the sensor cluster.

21. The method of claim 18 , further comprising the step of powering the semi-relative steering wheel angle sensor independently from the vehicle ignition.

22. The method of claim 18 , wherein the measuring step includes measuring the relative steering wheel angle θ R throughout the ignition-off state.