System to measure wheel liftoff

1. A method of determining wheel departure from a road comprising:

measuring the wheel speed of four wheels;

calculating a resonance frequency of the four wheels based at least in part on wheel speed;

calculating variations in the amplitude of the resonance frequency;

comparing the variations in the amplitude of the resonance frequency with a threshold;

indicating a complete lift-off condition of a wheel from the road if the variations exceed a threshold, the wheel being lifted off of a contact surface leaving an air gap between the wheel and the contact surface in the lift-off condition; and

determining and providing a wheel lift status condition that provides whether the complete lift-off condition is no-wheel lift, single-wheel lift, two-wheel lift or single-wheel lift. with impending two-wheel lift.

2. The method of claim 1 wherein the measuring provides a signal that is subsequently filtered in a filter.

3. The method of claim 2 wherein the filter employs a second order spring-mass-damper model of the wheel to generate parameters of interest.

4. The method of claim 3 wherein the parameters of interest are input data to calculate the resonance frequency.

5. The method of claim 2 wherein the filter is a Recursive Least Squares (RLS) filter.

6. The method of claim 2 wherein the filter is a Kalman filter.

7. The method of claim 2 wherein the signal is transmitted to a pre-filter before the filter to remove noise and unwanted information.

8. The method of claim 1 , further comprising correlating the resonance frequency to a vertical force on the wheel.

9. The method of claim 1 , wherein the at least one wheel comprises four wheels, and further comprising the step of providing a wheel lift status condition.

10. The system of claim 1 , further comprising the step of providing a second order spring-mass-damper model of the wheel, and wherein the step of calculating the resonance frequency includes calculating the resonance frequency based on the wheel speed and the second order spring-mass-damper model.

11. A system for determining wheel departure from a road comprising:

a sensor that measures the wheel speed of four wheels; and

a controller that calculates a resonance frequency of the wheels based at least in part on the wheel speed, calculates variations in the amplitude of the resonance frequency, compares the variations with a threshold to determine whether a complete lift-off condition of the wheel from the road occurred, and whether the complete lift-off condition is no-wheel lift, single-wheel lift, two-wheel lift or single-wheel lift with impending two-wheel lift and indicates the specific lift-off condition if the variations exceed the threshold.

12. The system of claim 8 further comprising a filter that filters a signal associated with the wheel speed.

13. The system of claim 12 wherein the filter employs a second order spring-mass-damper model of the wheel to generate parameters of interest.

14. The system of claim 13 wherein the parameters of interest are input data to calculate the resonance frequency.

15. The system of claim 12 wherein the filter is a Recursive Least Squares (RLS) filter.

16. The system of claim 12 wherein the filter is a Kalman filter.

17. The system of claim 12 further comprising a pre-filter wherein the signal is transmitted to a pre-filter to remove noise and unwanted information.

18. The system of claim 11 , the controller correlating the resonance frequency to a vertical force on the wheel.

19. The system of claim 11 , further comprising an indicator that provides a wheel lift status condition, the wheel lift status condition being provided as at least one of the following: no-wheel lift, single-wheel lift, two-wheel lift, and single-wheel lift with impending two-wheel lift.

20. The system of claim 11 , the system being a component of a dynamic stability control system.