Road friction coefficient estimation using steering system signals

1. A method comprising:

computing, by a controller of a steering system that includes at least a motor: a first rack force value and a second rack force value, wherein, the first rack force value is calculated, based on a vehicle speed, steering angle, and a road friction coefficient value, using a bicycle model and wherein the second rack force value is calculated, based on motor angle, using a spring model;

calculating a model rack force value based on the sum of the first rack force value and the second rack force value;

determining, by the controller, a difference between the model rack force value and a load rack force value, wherein the load rack force value is estimated using a state observer model based at least one of a steering motor torque signal and a steering velocity signal;

updating, by the controller, the road-friction coefficient value using the difference that is determined; and

selectively controlling the motor of the steering system based on at least the updated road-friction coefficient value.

2. The method of claim 1 , wherein the load rack force value is an estimated value of tire load at a rack.

3. The method of claim 1 , wherein the bicycle model includes a non-linear bicycle vehicle model.

4. The method of claim 1 , wherein the road-friction coefficient value is updated based on an update flag that is set based on one or more steering system signals and vehicle signals.

5. The method of claim 1 , wherein the road-friction coefficient value is a first road-friction coefficient value, and updating the road-friction coefficient value comprises:

computing an update factor based on the difference between the model rack force value and the load rack force value; and

computing a second road-friction coefficient value by adding the update factor into the first road-friction coefficient value.

6. The method of claim 1 , further comprising:

computing a handwheel torque reference value based on the updated road friction coefficient value, the handwheel torque reference value is summed with an input torque from an operator to the steering system.

7. The method of claim 6 , wherein an overlay torque that is opposite in direction to the input torque is generated in response to determining that based on the updated road-friction coefficient value, the road condition has become, at least one of: wet and icy.

8. The method of claim 7 , further comprising, transmitting the updated road friction coefficient value to an advanced drive assist system.

9. The method of claim 1 , further comprising, changing a maneuver of the steering system based on a difference between the road-friction coefficient value and the updated road-friction coefficient value being above a predetermined threshold.

10. A steering system comprising:

a motor; and

a controller configured to:

compute a first rack force value and a second rack force value,

wherein, the first rack force value is calculated, based on a vehicle speed, steering angle, and a road friction coefficient, using a bicycle model, and the second rack force value is calculated, based on motor angle, using a spring model;

calculate a model rack force value based on the sum of the first rack force value and the second rack force value;

determine a difference between the model rack force value and a load rack force value, wherein the load rack force value is estimated using a state observer model based at least one of a steering motor torque signal and a steering velocity signal;

update the road-friction coefficient value using the difference that is determined; and

selectively control the motor based on at least the updated road-friction coefficient value.

11. The system of claim 10 , wherein the load rack force value is an estimated value of tire load at a rack.

12. The system of claim 10 , wherein the bicycle model includes a non-linear bicycle model.

13. The system of claim 10 , wherein the road-friction coefficient value is updated based on an update flag that is set based on one or more steering system signals and vehicle signals.

14. The system of claim 10 , wherein the road-friction coefficient value is a first road-friction coefficient value, and updating the road-friction coefficient value comprises:

computing an update factor based on the difference between the model rack force value and the load rack force value; and

computing a second road-friction coefficient value by adding the update factor into the first road-friction coefficient value.

15. The system of claim 10 , wherein the controller is further configured to:

compute a handwheel torque reference value based on the updated road friction coefficient value, the handwheel torque reference value is summed with an input torque from an operator to the steering system.

16. The system of claim 10 , wherein an overlay torque is generated based on the updated road-friction coefficient value.

17. A computer program product comprising a non-volatile memory device that has one or more computer executable instructions stored thereon, the computer executable instructions when executed by a processor cause the processor to:

compute a first rack force value and second a second rack force value, wherein, the first rack force value is calculated, based on vehicle speed, steering angle and a road friction coefficient value, using a bicycle model, and the second rack force value is calculated, based on motor angle, using a spring model;

calculate a model rack force value by adding the first rack force value and the second rack force value;

determine a difference between the model rack force value and a load rack force value, wherein the load rack force value is estimated using a state observer model based at least one of a steering motor torque signal and a steering velocity signal;

update the road-friction coefficient value using the difference that is determined; and

selectively control a motor of a steering system based on at least the updated road-friction coefficient value.

18. The computer program product of claim 17 , wherein the load rack force value is an estimated value of tire load at a rack.

19. The computer program product of claim 17 , wherein the road-friction coefficient value is updated based on an update flag that is set based on one or more steering system signals and vehicle signals.

20. The computer program product of claim 17 , wherein the road-friction coefficient value is a first road-friction coefficient value, and updating the road friction coefficient value comprises:

computing an update factor based on the difference between the model rack force value and the load rack force value; and

computing a second road-friction coefficient value by adding the update factor into the first road-friction coefficient value.

21. The computer program product of claim 17 , wherein the instructions further cause the processor to:

compute a handwheel torque reference value based on the updated road friction coefficient value, the handwheel torque reference value is summed with an input torque from an operator.

22. The computer program product of claim 17 , wherein an overlay torque is generated based on the updated road-friction coefficient value.