Linear and non-linear identification of the longitudinal tire-road friction coefficient
A method of estimating a tire-road friction coefficient includes determining when the slope of a tire characteristic curve relating a utilized longitudinal friction of a tire to longitudinal slip of the tire is linear and non-linear. When the slope of the tire characteristic curve is linear, then the tire-road friction coefficient is estimated by correlating the slope of the tire characteristic curve to the tire-road friction coefficient. When the slope of the tire characteristic curve is non-linear, indicating that the tire is near or at saturation, then the tire-road friction coefficient is estimated by calculating a current utilized longitudinal friction of the tire.
1. A method of estimating a longitudinal tire-road friction coefficient for a vehicle, the method comprising:
estimating a longitudinal stiffness of a tire;
determining if the tire is within a pre-determined range of saturation;
correlating the longitudinal stiffness of the tire to an estimated value of the tire-road friction coefficient to estimate the tire-road friction coefficient when the tire is not within the pre-determined range of saturation; and
calculating a current utilized longitudinal friction of the tire to estimate the tire-road friction coefficient when the tire is within the pre-determined range of saturation.
2. A method as set forth in claim 1 wherein estimating the longitudinal stiffness of the tire includes intermittently calculating a longitudinal slip of the tire over time.
3. A method as set forth in claim 2 wherein estimating the longitudinal stiffness of the tire includes intermittently calculating a longitudinal force on the tire over time.
4. A method as set forth in claim 3 wherein calculating the longitudinal force on the tire includes referencing an engine torque to a correlated value of the longitudinal force on the tire.
5. A method as set forth in claim 4 wherein referencing an engine torque includes measuring an engine torque.
6. A method as set forth in claim 4 wherein referencing an engine torque includes estimating the engine torque from a model.
7. A method as set forth in claim 3 wherein calculating the longitudinal force on the tire includes calculating the longitudinal force on the tire from a longitudinal acceleration of the vehicle.
8. A method as set forth in claim 7 wherein referencing the longitudinal acceleration of the vehicle includes measuring the longitudinal acceleration of the vehicle.
9. A method as set forth in claim 3 wherein estimating the longitudinal stiffness of the tire includes intermittently calculating a vertical force on the tire over time.
10. A method as set forth in claim 9 wherein estimating the longitudinal stiffness of the tire includes intermittently calculating a utilized longitudinal friction of the tire over time.
11. A method as set forth in claim 10 wherein the utilized longitudinal friction is calculated by dividing the longitudinal force on the tire by the vertical force on the tire.
12. A method as set forth in claim 10 wherein estimating the longitudinal stiffness of the tire includes calculating an initial slope of a tire characteristic curve that relates the utilized longitudinal friction of the tire to the longitudinal slip of the tire over time.
13. A method as set forth in claim 12 wherein identifying the initial slope of the tire characteristic curve includes filtering the tire characteristic curve to estimate a value of the initial slope.
14. A method as set forth in claim 12 wherein determining if the tire is within the pre-determined range of saturation includes continuously monitoring the slope of the tire characteristic curve over time to determine when the slope of the tire characteristic curve falls within the pre-determined range of saturation.
15. A method as set forth in claim 14 wherein continuously monitoring the slope of the tire characteristic curve over time includes tracking a first derivative of the tire characteristic curve.
16. A method as set forth in claim 15 wherein determining if the tire is within the pre-determined range of saturation includes filtering the first derivative of the tire characteristic curve.
17. A method as set forth in claim 1 further comprising defining the pre-determined range of saturation for the tire.
18. A method as set forth in claim 1 wherein correlating the longitudinal stiffness of the tire to an estimated value of the tire-road coefficient includes referencing a look-up table correlating the longitudinal stiffness of the tire to empirically derived values of the tire-road friction coefficient for the tire.
19. A method as set forth in claim 1 wherein calculating a current utilized longitudinal friction of the tire to estimate the tire-road friction coefficient includes dividing a current longitudinal force on the tire by a current vertical force on the tire to calculate the current utilized tire-road friction coefficient.