Vehicle speed control system and method

1. A vehicle speed control system, comprising:

an electronic controller operable to cause a vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle, the controller being operable to detect a leading wheel step event in which a leading wheel encounters and clears an obstacle presenting an abrupt increase in surface gradient,

the controller being operable in dependence on the detection of said leading wheel step event to cause brake torque to be applied against drive torque in anticipation of a corresponding trailing wheel step event, the controller being operable to cause said brake torque to be applied after the leading wheel clears the obstacle, and before a trailing wheel has travelled a sufficient distance to reach a location at which said leading wheel experienced the leading wheel step event, the controller being operable to maintain said brake torque at least until the trailing wheel has travelled the sufficient distance to reach the location at which the leading wheel experienced said leading wheel step event.

2. A system according to claim 1 operable to increase the amount of drive torque applied to the one or more wheels to compensate at least in part for the increase in brake torque applied in response to detection of the leading wheel step event.

3. A system according to claim 1 operable to detect a trailing wheel step event.

4. A system according to claim 3 operable to decrease the amount of brake torque applied against drive torque once a trailing wheel step event has been detected.

5. A system according to claim 1 operable to limit a speed of the vehicle to a prescribed step speed limit value after a leading wheel step is detected and before a trailing wheel reaches the location at which the leading wheel step event was detected, the system being operable to lift the limit on speed when the trailing wheel has passed the location at which the leading wheel step event was detected.

6. A system according to claim 5 wherein the step speed limit value is determined at least in part in dependence on one or more characteristics of terrain over which the vehicle is travelling.

7. A system according to claim 6 wherein the one or more characteristics are determined at least in part by reference to a currently selected driving mode of the vehicle.

8. A system according to claim 7 wherein the control system comprises a plurality of subsystem controllers each operable in a plurality of subsystem function modes, the system being operable to cause the vehicle to operate in one of a plurality of driving modes, wherein in each mode each of the sub-systems is set to the function mode most appropriate to those conditions.

9. A system according to claim 1 operable to detect the leading wheel step event at least in part in dependence on detection of an abrupt increase in rate of deceleration of the vehicle consistent with a leading wheel step event.

10. A system according to claim 9 operable to detect the leading wheel step event at least in part in dependence on detection that a rate of deceleration of the vehicle consistent with a leading wheel step event exceeds a prescribed value over a prescribed period.

11. A system according to claim 1 operable to detect the leading wheel step event at least in part in dependence on detection of an abrupt increase in rate of deceleration of the vehicle consistent with a leading wheel step event.

12. A vehicle comprising a system according to claim 1 .

13. A vehicle according to claim 12 having four wheels, the vehicle being operable in a four wheel drive mode in which each of the four wheels are driven by a powertrain of the vehicle.

14. A vehicle according to claim 13 further operable in a two wheel drive mode in which only two wheels of the vehicle are driven by the powertrain.

15. A method of controlling a vehicle comprising:

causing the vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle;

detecting a leading wheel step event in which a leading wheel encounters and clears an obstacle presenting an abrupt increase in surface gradient; and

causing brake torque to be applied against drive torque in dependence on detection of said leading wheel step event in anticipation of a corresponding trailing wheel step event, including:

causing said brake torque to be applied after the leading wheel clears the obstacle, and before a trailing wheel has travelled a sufficient distance to reach a location at which said leading wheel experienced the leading wheel step event; and

maintaining said brake torque at least until a trailing wheel has travelled the sufficient distance to reach the location at which said leading wheel experienced the leading wheel step event.

16. A vehicle speed control system, comprising:

an electronic controller operable to cause a vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle,

the controller being operable to detect a leading wheel step event in which a leading wheel encounters and clears an obstacle presenting an abrupt increase in surface gradient, the controller being operable in dependence on the detection of said leading wheel step event to limit a speed of the vehicle to a prescribed step speed limit value after said leading wheel step event is detected and after the leading wheel clears the obstacle, and before a trailing wheel reaches a location at which the leading wheel step event was detected, the controller being operable to lift the limit on speed when the trailing wheel has passed the location at which the leading wheel step event was detected.

17. A system according to claim 16 wherein the step speed limit value is determined at least in part in dependence on one or more characteristics of terrain over which the vehicle is travelling.

18. A system according to claim 17 wherein the one or more characteristics are determined at least in part by reference to a currently selected driving mode of the vehicle.

19. A method of controlling a vehicle comprising:

causing the vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle;

detecting a leading wheel step event in which a leading wheel encounters and clears an obstacle presenting an abrupt increase in surface gradient; and

causing brake torque to be applied against drive torque in dependence on detection of said leading wheel step event in anticipation of a corresponding trailing wheel step event in which a trailing wheel encounters a corresponding obstacle, including:

causing said brake torque to be applied after the leading wheel clears the obstacle, and before said trailing wheel has travelled a sufficient distance to reach a location at which said leading wheel experienced the leading wheel step event; and

maintaining said brake torque at least until said trailing wheel has travelled the sufficient distance to reach the location at which the leading wheel experienced the leading wheel step event.

20. A non-transitory, computer-readable storage medium storing instructions thereon that when executed by one or more electronic processors causes the one or more electronic processors to carry out the method of claim 15 .

21. An electronic controller for a vehicle having a storage medium associated therewith storing instructions that when executed by the controller control the vehicle in accordance with the method of:

causing the vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle;

detecting a leading wheel step event in which a leading wheel encounters and clears an obstacle presenting an abrupt increase in surface gradient; and

causing brake torque to be applied against drive torque in dependence on detection of said leading wheel step event in anticipation of a corresponding trailing wheel step event in which a trailing wheel encounters a corresponding obstacle, including:

causing said brake torque to be applied after the leading wheel clears the obstacle, and before said trailing wheel has travelled a sufficient distance to reach a location at which said leading wheel experienced the leading wheel step event; and

maintaining said brake torque at least until said trailing wheel has travelled the sufficient distance to reach the location at which the leading wheel experienced the leading wheel step event.