Vehicle speed control system and method
A vehicle speed control system operable to cause a vehicle to operate in accordance with a target speed value, the system being further operable automatically to control cross-axle locking means of an axle of the vehicle to cause an increase in resistance to relative rotation of wheels of the axle. Thus, the speed control system may be operable automatically to command the cross-axle locking means to increase the resistance to relative rotation of wheels of the axle without a driver being required to intervene to command assumption of this condition.
1. A vehicle speed control system operable to cause a vehicle to operate in accordance with an automatic target speed value, the system being further operable to cause an axle of the vehicle to assume a cross-axle lock condition in which relative rotation of wheels of the axle with respect to one another is substantially prevented,
wherein the system is operable to cause the axle to assume the cross-axle lock condition when it is determined that:
(a) a cross-axle surface mu delta condition exists, being a condition in which a difference in coefficient of friction, mu, between respective wheels of an axle and a corresponding driving surface exceeds a prescribed value, and
(b) a wheel experiencing the lower surface coefficient of friction is rotating faster than the wheel experiencing the higher surface coefficient of friction by more than a prescribed amount.
2. A vehicle speed control system according to claim 1 wherein the prescribed amount is a substantially fixed value.
3. A vehicle speed control system according to claim 1 wherein the prescribed amount is a value determined in dependence on one or more parameters.
4. A vehicle speed control system according to claim 3 wherein the value is determined from a look-up table or by using an algorithm.
5. A vehicle speed control system according to claim 1 operable to cause the axle to assume the cross-axle lock condition in further dependence at least in part on a steering angle of the vehicle.
6. A vehicle speed control system according to claim 5 operable to cause the axle to assume the cross-axle lock condition only if the steering angle is less than a prescribed value.
7. A vehicle speed control system according to claim 1 operable to cause the axle to assume the cross-axle lock condition independently of a steering angle of the vehicle.
8. A vehicle speed control system according to claim 7 operable to cause the cross-axle lock condition to be assumed provided conditions (a) and (b) are met and the difference in wheel speeds exceeds that which would be expected for a cornering condition of the vehicle, regardless of the instant steering angle.
9. A vehicle speed control system according to claim 1 operable to apply a brake to the wheel experiencing the lower surface coefficient of friction when conditions (a) and (b) are met.
10. A vehicle speed control system according to claim 9 operable to apply the brake to the wheel experiencing the lower surface coefficient of friction when conditions (a) and (b) are met, the system being operable subsequently to cause the axle to assume the cross-axle lock condition in dependence on a rate of progress of the vehicle over ground following application of the brake.
11. A vehicle speed control system according to claim 10 operable to determine whether to cause the axle to assume the cross-axle lock condition in dependence on speed of the vehicle over ground or rate of acceleration of the vehicle over ground.
12. A vehicle speed control system according to claim 1 operable to cause the vehicle to maintain the automatic target speed value during application of brake and/or an increase in resistance to relative rotation of wheels of an axle by means of the cross-axle locking means.
13. A vehicle comprising a vehicle speed control system according to claim 1 .
14. A method of controlling a vehicle comprising:
causing a vehicle to operate in accordance with an automatic target speed value; and
causing an axle of the vehicle to assume a cross-axle lock condition in which relative rotation of wheels of the axle with respect to one another is substantially prevented, the method comprising causing the axle to assume the cross-axle lock condition when it is determined that;
(a) a cross-axle surface mu delta condition exists, being a condition in which a difference in coefficient of friction, mu, between respective wheels of an axle and a corresponding driving surface exceeds a prescribed value, and
(b) a wheel experiencing the lower surface coefficient of friction is rotating faster than the wheel experiencing the higher surface coefficient of friction by more than a prescribed amount.
15. A method according to claim 14 comprising causing the axle to assume the cross-axle lock condition in further dependence at least in part on a steering angle of the vehicle.
16. A method according to claim 15 wherein the method comprises causing the axle to assume the cross-axle lock condition only if the steering angle is less than a prescribed value.
17. A method according to claim 14 comprising applying a brake to the wheel experiencing the lower surface coefficient of friction when conditions (a) and (b) are met, subsequently causing the axle to assume the cross-axle lock condition in dependence on a rate of progress of the vehicle over ground following application of the brake.
18. A method according to claim 14 comprising causing the vehicle to maintain the automatic target speed value during application of brake and/or an increase in resistance to relative rotation of wheels of an axle by means of the cross-axle locking means.