Estimating surface friction coefficients using rear-wheel steering excitations

1. A computing system for estimating surface friction coefficients, wherein the computing system comprises:

at least one processor; and

a memory comprising instructions that, when executed, cause the at least one processor to:

cause excitation of a first rear wheel of a vehicle using a first lateral force that causes the first rear wheel to initiate turning in a first lateral direction;

determine one or more slip angles that result from excitation of the first rear wheel;

identify a surface friction coefficient, the surface friction coefficient being based at least in part on a relationship between changes in magnitude of the first lateral force and the one or more slip angles; and

cause maneuvering of the vehicle to be based at least in part on the surface friction coefficient.

2. The system of claim 1 , wherein the instructions further cause the at least one processor to:

determine a second lateral force to excite a second rear wheel; and

provide a control signal configured to maintain a force balance between at least the first lateral force and the second lateral force.

3. The system of claim 2 , wherein to determine the second lateral force, the instructions further cause the at least one processor to:

determine the second lateral force based on a target steering angle of the second wheel as a product of a target steering angle of the first wheel multiplied by a control gain.

4. The system of claim 2 , wherein the instructions further cause the at least one processor to:

increase at least one of: the first lateral force or the second lateral force, until a slip angle threshold is satisfied.

5. The system of claim 1 , wherein the instructions further cause the at least one processor to:

determine whether the one or more slip angles satisfy a slip angle threshold;

in response to determining that the one or more slip angles satisfy the slip angle threshold, determine a peak magnitude of the first lateral force; and

transmit, to a remote server, slip angle data including at least one of the one or more slip angles and the peak magnitude of the first lateral force; and

receive, from the remote server, the surface friction coefficient.

6. The system of claim 1 , wherein the instructions further cause the at least one processor to:

determine whether a threshold amount of slip angle data has been received, wherein the threshold amount of slip angle data is a minimum amount of slip angle data required to allow extrapolation of a relationship trend that approximates at least one known tire model curve;

cease excitation of the first rear wheel.

7. The system of claim 1 , wherein the instructions further cause the at least one processor to:

determine a driving mode of the vehicle from at least one of: a cornering mode or a straight driving mode; and

determine, based at least in part on the driving mode, directionality of the first lateral force.

8. The system of claim 1 , wherein, when in a cornering mode, the instructions further cause the at least one processor to:

determine a reference lateral force value; and

determine, based at least in part on the reference lateral force value, a second lateral force to be applied to a second rear wheel.

9. The system of claim 1 , wherein the instructions further cause the at least one processor to:

determine a longitudinal force to apply to the first rear wheel;

cause excitation of the first rear wheel of the vehicle using the longitudinal force; and

determine, based at least in part on a combined-slip effect from the longitudinal force and the first lateral force, the surface friction coefficient.

10. The system of claim 1 , wherein the first rear wheel comprises a rear-most wheel relative to at least one other rear wheel of the vehicle.

11. An apparatus for estimating surface friction coefficients, the apparatus comprising:

means for causing excitation of a first rear wheel of a vehicle using a first lateral force that causes the first rear wheel to initiate turning in a first lateral direction;

means for determining one or more slip angles that result from excitation of the first rear wheel;

means for identifying a surface friction coefficient, the surface friction coefficient being based at least in part on a relationship between changes in magnitude of the first lateral force and the one or more slip angles; and

means for causing maneuvering of the vehicle to be based at least in part on the surface friction coefficient.

12. A method for estimating surface friction coefficients, the method comprising:

causing excitation of a first rear wheel of a vehicle using a first lateral force that causes the first rear wheel to initiate turning in a first lateral direction;

determining one or more slip angles that result from excitation of the first rear wheel;

identifying a surface friction coefficient, the surface friction coefficient being based at least in part on a relationship between changes in magnitude of the first lateral force and the one or more slip angles; and

causing maneuvering of the vehicle to be based at least in part on the surface friction coefficient.

13. The method of claim 12 , further comprising:

determining a second lateral force to excite a second rear wheel; and

providing a control signal configured to maintain a force balance between at least the first lateral force and the second lateral force.

14. The method of claim 13 , further comprising:

determining the second lateral force based on a target steering angle of the second wheel as a product of a target steering angle of the first wheel multiplied by a control gain.

15. The method of claim 13 , further comprising:

increasing at least one of: the first lateral force or the second lateral force, until a slip angle threshold is satisfied.

16. The method of claim 12 , further comprising:

determining whether the one or more slip angles satisfy a slip angle threshold;

in response to determining that the one or more slip angles satisfy the slip angle threshold, determining a peak magnitude of the first lateral force;

transmitting, to a remote server, slip angle data, including at least one of the one or more slip angles, and the peak magnitude of the first lateral force; and

receiving, from the remote server, the surface friction coefficient.

17. The method of claim 12 , further comprising:

determining whether a threshold amount of slip angle data has been received, wherein the threshold amount of slip angle data is a minimum amount of slip angle data required to allow extrapolation of a relationship trend that approximates at least one known tire model curve; and

responsive to determining that the threshold amount of slip angle data has been received, ceasing excitation of the first rear wheel.

18. The method of claim 12 , further comprising:

determining a driving mode of the vehicle from at least one of: a cornering mode or a straight driving mode; and

determining, based at least in part on the driving mode, directionality of the first lateral force.

19. The method of claim 12 , further comprising, when in a cornering mode:

determining a reference lateral force value; and

determining, based at least in part on the reference lateral force value, a second lateral force to be applied to a second rear wheel.

20. The method of claim 12 , further comprising:

determining a longitudinal force to apply to the first rear wheel;

causing excitation of the first rear wheel of the vehicle using the longitudinal force; and

determining, based at least in part on a combined-slip effect from the longitudinal force and the first lateral force, the surface friction coefficient.