Road-surface friction-coefficient estimating device

1. A road-surface friction-coefficient estimating device for a vehicle, for estimating a friction coefficient of a road surface on which the vehicle is traveling, comprising:

a vehicle-speed detecting unit configured to detect a vehicle speed;

a vehicle-wheel slip-angle detecting unit configured to detect a slip angle of a wheel;

a restoring-speed setting unit configured to set a restoring speed at which a road-surface friction coefficient is to be restored to a preliminarily set value based on the vehicle speed and the slip angle of the wheel; and

a road-surface friction-coefficient estimating unit configured to estimate the road-surface friction coefficient by allowing a previously estimated value of the road-surface friction coefficient to be restored to the set value at the restoring speed, wherein the road-surface friction-coefficient estimating unit includes:

a presumptive rack-thrust-force estimating portion configured to estimate a presumptive rack thrust force presumably being generated in actuality;

a reference rack-thrust-force estimating portion configured to estimate a reference rack thrust force expected to be generated;

a tire-force estimating portion configured to estimate a tire force acting on the wheel; a friction-circle utilization-rate calculating portion configured to calculate a friction-circle utilization rate on the basis of the tire force; and

a road-surface friction-coefficient estimating portion configured to set the friction-circle utilization rate as the road-surface friction coefficient when an absolute value of deviation between the presumptive rack thrust force and the reference rack thrust force is above a preliminarily set threshold value, and to estimate the road-surface friction coefficient by allowing a previously estimated value of the road-surface friction coefficient to be restored to the set value at the restoring speed when the absolute value of deviation is below the threshold value.

2. The road-surface friction-coefficient estimating device according to claim 1 , wherein the restoring speed decreases as the vehicle speed increases.

3. The road-surface friction-coefficient estimating device according to claim 1 , wherein the restoring speed decreases as the slip angle of the wheel becomes higher.

4. The road-surface friction-coefficient estimating device according to claim 1 , wherein the preliminarily set value is 1.0.

5. The road-surface friction-coefficient estimating device according to claim 1 , wherein the preliminarily set threshold value is adjusted in value in accordance with an absolute value of a lateral acceleration of the vehicle.

6. A road-surface friction-coefficient estimating device for a vehicle, for estimating a friction coefficient of a road surface on which the vehicle is traveling, comprising:

a vehicle speed detecting unit configured to detect a vehicle speed;

a vehicle wheel slip angle detecting unit configured to detect a slip angle of a wheel;

a change rate setting unit configured to set a change rate of a road-surface friction coefficient based on the vehicle speed and the slip angle of the wheel; and

a road-surface friction-coefficient estimating unit configured to estimate the road-surface friction coefficient by increasing a previously estimated value of the road-surface friction coefficient at the change rate to provide for restoration of the estimated road-surface friction coefficient to a preliminary set estimated road-surface friction coefficient value, wherein the road-surface friction-coefficient estimating unit includes;

a presumptive rack-thrust-force estimating portion configured to estimate a presumptive rack thrust force presumably being generated in actuality;

a reference rack-thrust-force estimating portion configured to estimate a reference rack thrust force expected to be generated;

a tire-force estimating portion configured to estimate a tire force acting on the wheel; a friction-circle utilization-rate calculating portion configured to calculate a friction-circle utilization rate on the basis of the tire force; and

wherein the road-surface friction-coefficient estimating unit is configured to set the friction-circle utilization rate as the road-surface friction coefficient when an absolute value of deviation between the presumptive rack thrust force and the reference rack thrust force is above a preliminarily set threshold value, and to estimate the road-surface friction coefficient by allowing a previously estimated value of the road-surface friction coefficient to be restored by way of the change rate determined to the preliminary set estimated road surface friction coefficient value when the absolute value of deviation is below the threshold value, and wherein the presumptive rack-thrust-force estimating portion is configured to estimate a presumptive rack thrust force presumably being generated in actuality both during a turning of a steering wheel from an initial steering wheel setting and during an hysteresis return of the steering wheel toward the initial steering wheel setting.

7. The road surface friction-coefficient estimating device of claim 1 wherein said vehicle-speed detecting unit comprises a vehicle speed sensor.

8. The road surface friction-coefficient estimating device of claim 6 wherein said vehicle-speed detecting unit comprises a vehicle speed sensor.

9. A vehicle, comprising:

the road-surface friction-coefficient estimating device of claim 1 ; and

a plurality of wheels, and

wherein said vehicle-speed detecting unit comprises wheel speed sensors.

10. A vehicle, comprising:

the road-surface friction-coefficient estimating device of claim 6 ; and

a plurality of wheels, and

wherein said vehicle-speed detecting unit comprises wheel speed sensors.

11. A road-surface friction-coefficient estimating device, comprising:

a vehicle-speed detecting unit configured to detect a vehicle speed;

a vehicle-wheel slip-angle detecting unit configured to detect a slip angle of a wheel;

a restoring-speed setting unit configured to set a restoring speed at which a road-surface friction coefficient is to be restored to a preliminarily set value based on the vehicle speed and the slip angle of the wheel;

a road-surface friction-coefficient estimating unit configured to estimate the road-surface friction coefficient by allowing a previously estimated value of the road-surface friction coefficient to be restored to the set value at the restoring speed; and

a vehicle control unit which is in communication with said road surface friction-coefficient estimating unit and receives said road-surface friction coefficient upon output by said road-surface friction-coefficient estimating unit, wherein the road-surface friction-coefficient estimating unit includes:

a presumptive rack-thrust-force estimating portion configured to estimate a presumptive rack thrust force presumably being generated in actuality;

a reference rack-thrust-force estimating portion configured to estimate a reference rack thrust force expected to be generated;

a tire-force estimating portion configured to estimate a tire force acting on the wheel;

a friction-circle utilization-rate calculating portion configured to calculate a friction-circle utilization rate on the basis of the tire force; and

wherein the road-surface friction-coefficient estimating unit is configured to set the friction-circle utilization rate as the road-surface friction coefficient when an absolute value of deviation between the presumptive rack thrust force and the reference rack thrust force is above a preliminarily set threshold value, and to estimate the road-surface friction coefficient by allowing a previously estimated value of the road-surface friction coefficient to be restored to the set value at the restoring speed when the absolute value of deviation is below the threshold value, and wherein the presumptive rack-thrust-force estimating portion is configured to estimate a presumptive rack thrust force presumably being generated in actuality both during a turning of a steering wheel from an initial steering wheel setting and during an hysteresis return of the steering wheel toward the initial steering wheel setting.

12. The road-surface friction-coefficient estimating device of claim 11 wherein the vehicle control unit is a torque distribution control unit.

13. The road-surface friction-coefficient estimating device of claim 1 wherein the restoring-speed setting unit has accessible reference information for use in determining the restoration speed to be set, and which reference information is configured as to have the restoring speed decrease as the vehicle speed increases and to have the restoring speed decrease as the slip angle increases.

14. The road-surface friction-coefficient estimating device of claim 13 wherein the accessible reference information includes a stored data map of information.

15. The road-surface friction-coefficient estimating device of claim 6 wherein the change rate setting unit has accessible reference information for use in determining the change rate to be set, and which reference information is configured as to have the change rate decrease as the vehicle speed increases and to have the change rate decrease as the slip angle increases.

16. The road-surface friction-coefficient estimating device of claim 11 wherein the restoring-speed setting unit has accessible reference information for use in determining the restoration speed to be set, and which reference information is configured as to have the restoring speed decrease as the vehicle speed increases and to have the restoring speed decrease as the slip angle increases.

17. The road-surface friction-coefficient estimating device of claim 1 wherein the presumptive rack-thrust-force estimating portion is configured to estimate a presumptive rack thrust force presumably being generated in actuality both during a turning of a steering wheel from an initial steering wheel setting and during a hysteresis return of the steering wheel toward the initial steering wheel setting.