Vehicle steering

1. A method of steering a vehicle in a speed control mode, and comprising substantially continually:

detecting steering angle;

calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and

controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn;

wherein the speed of rotation of the vehicle wheels is controlled by variation of driving torque thereof, and

wherein the variation of driving torque comprises increasing or reducing the driving torque to one output of at least one of a differential gear and a driveline clutch.

2. A method according to claim 1 , and further comprising

determining a current rate of turn of the vehicle; and

controlling said speed of rotation such that the current rate of turn approaches the theoretical rate of turn.

3. A method according to claim 1 operable when the vehicle is travelling at one of: a low speed; in a low transmission range; and in low friction conditions.

4. A method according to claim 1 , wherein the speed of rotation of a steering road wheel is controlled.

5. A method according to claim 1 further comprising receiving a user input of a target speed at which the vehicle is intended to travel and causing the vehicle to operate in accordance with the target speed.

6. A method according to claim 1 , further comprising:

suspending operation of the vehicle in the speed control mode in the event that the current speed is determined to be in excess of a predetermined threshold speed.

7. A method according to claim 1 , further comprising:

controlling the vehicle to operate in a highway cruise control mode wherein controlling the vehicle to operate in the highway cruise control mode comprises maintaining the vehicle speed at speeds above the predetermined threshold speed, and

suspending operation of the highway cruise control mode on receiving the slip detection output signal.

8. A method according to claim 1 further comprising:

detecting the nature of the terrain over which the vehicle is travelling;

determining whether the target speed is appropriate for the nature of the terrain over which the vehicle is travelling; and

maintaining the vehicle at the target speed by applying torque to the at least one of the plurality of wheels only if the target speed is determined to be appropriate.

9. A control system for steering a vehicle in a speed control mode, the system including a processor having inputs of steering angle and rotational speed of steering wheels, the processor being operable when in the speed control mode to repeatedly calculate theoretical vehicle rate of turn with respect to steering angle, and to control individually vehicle wheel speed to achieve said theoretical rate of turn, wherein the speed of rotation of vehicle wheels is controlled by variation of driving torque thereof, and variation of driving torque comprises increasing or reducing the driving torque to one output of at least one of a differential gear and a driveline clutch.

10. A system according to claim 9 , wherein said processor is further operable to compare theoretical vehicle rate of turn with actual vehicle rate of turn and to control individually vehicle wheel speeds such that actual vehicle rate of turn approaches theoretical vehicle rate of turn.

11. A system according to claim 9 wherein the processor is operable to:

receive a user input of a target speed at which the vehicle is intended to travel; and

cause the vehicle to operate in accordance with the target speed by applying torque to the at least one of the plurality of wheels.

12. A system according to claim 11 wherein the processor is operable to:

detect a slip event between any one or more of the wheels and the ground over which the vehicle is travelling when the vehicle is in motion and provide a slip detection output signal in the event thereof; and

maintain the vehicle at the target speed independently of the slip detection output signal.

13. A system as described in claim 12 , wherein the processor is operable to maintain the vehicle speed at the target speed by:

determining a current speed at which the vehicle is travelling;

comparing the current speed with the target speed and providing an output indicative of the difference between the current speed and the target speed; and

evaluating the torque to be applied to at least one of the vehicle wheels in dependence on the output.

14. A system as described in claim 13 , wherein the processor is operable to command the application of torque to at least two wheels of the vehicle substantially simultaneously.

15. A system as described in claim 12 , wherein the processor is further operable to:

inhibit operation of the vehicle speed control system in the event that the current speed is determined to be in excess of a predetermined threshold speed.

16. A system as described in claim 15 , wherein the predetermined threshold speed is a first, lower threshold speed, the processor being further operable to:

compare the current vehicle speed with a second, higher threshold speed and, if the current vehicle speed is less than the second, higher threshold speed, hold the vehicle speed control system in a wait state and initiate vehicle speed control only once the current vehicle speed is reduced to below the first, lower threshold speed.

17. A system as described in claim 15 , further comprising a highway cruise control system which is operable to maintain vehicle speed at speeds above the predetermined threshold speed.

18. A system as described in claim 17 , wherein the highway cruise control system is operable to suspend operation of the system on receiving the slip detection output signal.

19. A system as described in claim 9 , further operable to:

detect the nature of the terrain over which the vehicle is travelling;

and wherein the processor is operable to determine whether the target speed is appropriate for the nature of the terrain over which the vehicle is travelling; and

maintain the vehicle at the target speed by applying torque to the at least one of the plurality of wheels only if the target speed is determined to be appropriate.

20. A vehicle incorporating the system of claim 9 .

21. A method according to claim 1 , wherein the calculated theoretical rate of turn of the vehicle appropriate to the detected steering angle comprises an expected rate of turn of the vehicle for the detected steering angle in the absence of wheel slip.