METHOD AND APPARATUS FOR ROAD SURFACE FRICTION ESTIMATION BASED ON THE SELF ALIGNING TORQUE

• US 20110130974A1
• Filed: 09/07/2010
• Published: 06/02/2011
• Est. Priority Date: 09/09/2009
• Status: Abandoned Application

• Current Assignee: GM Global Technology Operations LLC (General Motors Company)
• Original Assignee: GM Global Technology Operations Incorporated (General Motors Company)
• Inventors: GHONEIM, Youssef, YNGVE, Simon
• Application Number:
  US12/876,965
• Publication Number:
  US 20110130974A1
• Time in Patent Office: Days
• US Class Current: 702/41
• CPC Class Codes: B60W 2520/26 Wheel slip
• CPC Class Codes: B60W 2540/18 Steering angle
• CPC Class Codes: B60W 40/068 Road friction coefficient

Abstract

A method and an apparatus are disclosed for estimating a road surface friction between a road surface and a tire of a vehicle. The method includes, but is not limited to computing, in a slope estimation step, a slope estimate k_sl for a slope of a linear region of a self aligning torque function that is defined by a self aligning torque as a function of a slip angle. The method further includes, but is not limited to deriving a first estimate μ_sl of a road friction coefficient from the slope estimate k_sl, and deciding, in a linearity estimation step, whether a current slope k_op is within the linear region of the self aligning torque function. If it is decided in the linearity estimation step that the current slope k_op is within the linear region of the self aligning torque function, the first estimate μ_sl of the road friction coefficient is output as a second estimate μ_cont of the road friction coefficient.

Claims

1. A method for estimating a road surface friction between a road surface and a tire of a vehicle, comprising the steps of:

computing in a slope estimation step, a slope estimate k_sl for a slope of a linear region of a self aligning torque function, the self aligning torque function being defined by a self aligning torque as a function of a slip angle;

deriving a first estimate μ

_sl of a road friction coefficient μ

from the slope estimate k_sl;

deciding, in a linearity estimation step, whether a current slope k_op is within the linear region of the self aligning torque function; and

outputting the first estimate μ

_sl of the road friction coefficient as a second estimate μ

_cont of the road friction coefficient if it is decided in the linearity estimation step that the current slope k_op is within the linear region of the self aligning torque function.

2. The method according to claim 1, further comprising the step of halting the computation of the slope estimate k_sl if it is decided in the linearity estimation step that the current slope k_op is not within the linear region of the self aligning torque function.

3. The method according to claim 1, wherein the linearity estimation step comprises a computation of a time derivative of the self aligning torque and of the time derivative of the slip angle.

4. The method according to claim 1, wherein in the linearity estimation step it is decided that the current slope k_op is within a nonlinear region of the self aligning torque function if k_op falls below a lower threshold k_op_threshold_low and it is decided that the current slope k_op is within the linear region of the self aligning torque function if the current slope k_op rises above an upper threshold k_op_threshold_high, wherein k_op_threshold_low<

k_op_threshold_high.

5. The method according to claim 1, wherein the slope estimation step comprises a computation of a quotient from the self aligning torque and the slip angle.

6. The method according to claim 1, wherein the slope estimation step comprises computing estimates of one or more observation variables by an update formula of a Kalman filter.

7. The method according to claim 1, wherein the linearity estimation step comprises computing estimates of one or more observation variables by an update formula of a Kalman filter.

8. The method according to claim 7, wherein the one or more observation variables are given by a time derivative of the self aligning torque and the time derivative of the slip angle.

9. The method according to claim 1, wherein the slope estimation step and the linearity estimation step are executed as computational threads.

10. The method according to claim 1, further comprising the steps of:

comparing the second estimate μ

_cont of the road friction coefficient to a lower limit;

comparing the second estimate μ

_cont of the road friction coefficient to an upper limit;

outputting as a final estimate μ

_SAT of the road friction coefficient the second estimate μ

_cont if the second estimate is within a range defined by the upper limit and the lower limit and outputting the lower limit if the second estimate μ

_cont is less than the lower limit and outputting the upper limit if the second estimate μ

_cont is greater than the upper limit.

11. The method according to claim 10, wherein the upper limit is derived from a maximum available road friction μ

_max and the lower limit is derived from a minimum available road friction μ

_min, a first derivation of the upper limit comprises a computation of a forget function of the maximum available road friction μ

_max and a second derivation of the lower limit comprises a computation of the forget function of the minimum available road friction μ

_min and the forget function is defined such that a difference between the lower limit and the upper limit increases with time.

12. A computer readable medium embodying a computer program product, said computer program product comprising:

a program for estimating a road surface friction between a road surface and a tire of a vehicle program, the program configured to;

compute in a slope estimation step, a slope estimate k_sl for a slope of a linear region of a self aligning torque function, the self aligning torque function being defined by a self aligning torque as a function of a slip angle;

derive a first estimate μ

sl of a road friction coefficient μ

from the slope estimate k_sl;

decide, in a linearity estimation step, whether a current slope k_op is within the linear region of the self aligning torque function; and

output the first estimate μ

_sl of the road friction coefficient as a second estimate μ

_cont of the road friction coefficient if it is decided in the linearity estimation step that the current slope k_op is within the linear region of the self aligning torque function.

13. The computer readable medium embodying the computer program product of according to claim 12, said program further configured to halt the computation of the slope estimate k_sl if it is decided in the linearity estimation step that the current slope k_op is not within the linear region of the self aligning torque function.

14. The computer readable medium embodying the computer program product of according to claim 12, wherein the linearity estimation step comprises a computation of a time derivative of the self aligning torque and of the time derivative of the slip angle.

15. The computer readable medium embodying the computer program product of according to according to claim 12, wherein in the linearity estimation step it is decided that the current slope k_op is within a nonlinear region of the self aligning torque function if k_op falls below a lower threshold k_op_threshold_low and it is decided that the current slope k_op is within the linear region of the self aligning torque function if the current slope k_op rises above an upper threshold k_op_threshold_high, wherein k_op_threshold_low<

k_op_threshold_high.

16. The computer readable medium embodying the computer program product of according to claim 12, wherein the slope estimation step comprises a computation of a quotient from the self aligning torque and the slip angle.

17. The computer readable medium embodying the computer program product of according to according to claim 12, wherein the slope estimation step comprises computing estimates of one or more observation variables by an update formula of a Kalman filter

18. The computer readable medium embodying the computer program product of according to according to claim 12, wherein the linearity estimation step comprises computing estimates of one or more observation variables by an update formula of a Kalman filter.

19. The computer readable medium embodying the computer program product of according to according to claim 18, wherein the one or more observation variables are given by a time derivative of the self aligning torque and the time derivative of the slip angle.

20. The computer readable medium embodying the computer program product of according to according to claim 12, wherein the slope estimation step and the linearity estimation step are executed as computational threads.

21. The computer readable medium embodying the computer program product of according to according to claim 12, the program further configured to:

compare the second estimate μ

_cont of the road friction coefficient to a lower limit;

compare the second estimate μ

_cont of the road friction coefficient to an upper limit; and

output as a final estimate μ

_SAT of the road friction coefficient the second estimate μ

_cont if the second estimate is within a range defined by the upper limit and the lower limit and outputting the lower limit if the second estimate μ

_cont is less than the lower limit and outputting the upper limit if the second estimate μ

_cont is greater than the upper limit.

22. The computer readable medium embodying the computer program product of according to according to claim 21, wherein the upper limit is derived from a maximum available road friction μ

_max and the lower limit is derived from a minimum available road friction μ

_min, a first derivation of the upper limit comprises a computation of a forget function of the maximum available road friction μ

_max and a second derivation of the lower limit comprises a computation of the forget function of the minimum available road friction μ

_min and the forget function is defined such that a difference between the lower limit and the upper limit increases with time.