Steerable vehicle

1. A steerable, self-propelled vehicle including an agricultural tractor, the steerable self-propelled vehicle comprising:

a steering mechanism having a rotatable steering column that is connected to control a steering angle of one or more ground-engaging members of the vehicle, the vehicle including

a first steering servomechanism acting on the steering mechanism in a first location, the first steering servomechanism having at least a first steering assistance characteristic;

a second steering servomechanism acting on the steering mechanism in a second location, the second servo mechanism having a second steering assistance characteristic; and

a programmable controller for causing the first and second steering assistance characteristics to influence the steering of the vehicle in dependence on one or more control commands generated in the controller; and

a human-machine interface (HMI) configured to receive driver input commands selecting two or more programmed modes of operation of the steering servomechanisms,

wherein the HMI includes respective switches for selecting between programmed modes that control steering during movement of the tractor respectively in a field headland and in other locations, and sequentially selecting sub-modes following selection of a said programmed mode, and

wherein the sub-modes available following selection of a headland operation include a damped steering sub-mode, a maximal assistance sub-mode, an automatic steering centring sub-mode, and a lock-to-lock assistance sub-mode, and the sub-modes available following selection of other location operation include an on-road sub-mode, a damped steering sub-mode, and a maximal assistance sub-mode.

2. A steerable self-propelled vehicle according to claim 1 that is driver-operated and includes one or more steering input members operatively connected to the steering column, wherein the controller generates control commands in dependence on one or more driver inputs via at least one steering input member.

3. A steerable self-propelled vehicle according to claim 1 , wherein the first steering servomechanism is a hydraulic steering servomechanism including a hydraulic steering servomechanism including a hydraulic steering valve that generates a pilot hydraulic signal in relation to the rotational angle of the steering column and at least one hydraulic steering cylinder connected to adjust the steering angle of the one or more ground-engaging members, the pilot hydraulic signal being piloted to control the hydraulic steering cylinder.

4. A steerable self-propelled vehicle according to claim 1 , wherein the second steering servomechanism is an electric power steering servomechanism including a motor for generating a torque that acts on the steering column, and a signal generator for generating control signals determining the magnitude and direction of action of the torque.

5. A steerable self-propelled vehicle according to claim 1 , wherein the other locations include fields and roads.

6. A steerable self-propelled vehicle according to claim 1 , including two or more selectively operable steering input members connected to the steering column that include a steering wheel and a joystick.

7. A steerable self-propelled vehicle according to claim 6 , wherein the steering wheel is mechanically coupled to the steering column.

8. A steerable self-propelled vehicle according to claim 6 , wherein the joystick is electronically coupled to the second steering servomechanism.

9. A steerable self-propelled vehicle according to claim 1 , including at least one receiver of transmitted vehicle guidance signals and wherein the controller converts received guidance signals to steering input commands that are input to at least one of the first steering servomechanism and the second steering servomechanism.

10. A steerable self-propelled vehicle according to claim 1 , wherein the steering of the tractor is adjustable depending on at least one of the following signals:

vehicle speed or wheel speed;

steering wheel angle;

electric motor position;

joystick position;

steerable wheel alignment angle;

torque on the steering column;

hydraulic pressure in one or more hydraulic steering cylinder(s);

driver input torque on the steering wheel; and

electric motor supply current or voltage.

11. A steerable, self-propelled vehicle including an agricultural tractor, the steerable self-propelled vehicle comprising:

a steering mechanism having a rotatable steering column that is connected to control a steering angle of one or more ground-engaging members of the vehicle, the vehicle including

a hydraulic steering servomechanism acting on the steering mechanism in a first location, the hydraulic steering servomechanism having at least a first steering assistance characteristic;

an electric power steering servomechanism acting on the steering mechanism in a second location, the electric power servo mechanism having a second steering assistance characteristic; and

a programmable controller for causing the first and second steering assistance characteristics to influence the steering of the vehicle in dependence on one or more control commands generated in the controller; and

a human-machine interface (HMI) configured to receive driver input commands selecting two or more programmed modes of operation of the steering servomechanisms,

wherein the HMI includes respective switches for selecting between programmed modes that control steering during movement of the tractor respectively in a field headland and in other locations, and sequentially selecting sub-modes following selection of a said programmed mode, and

wherein the sub-modes available following selection of a headland operation include a damped steering sub-mode, a maximal assistance sub-mode, an automatic steering centring sub-mode, and a lock-to-lock assistance sub-mode, and the sub-modes available following selection of other location operation include an on-road sub-mode, a damped steering sub-mode, and a maximal assistance sub-mode.

12. A steerable self-propelled vehicle according to claim 11 that is driver-operated and includes one or more steering input members operatively connected to the steering column, wherein the controller generates control commands in dependence on one or more driver inputs via at least one steering input member.

13. A steerable self-propelled vehicle according to claim 11 , wherein the hydraulic steering servomechanism includes a hydraulic steering valve that generates a pilot hydraulic signal in relation to the rotational angle of the steering column and at least one hydraulic steering cylinder connected to adjust the steering angle of the one or more ground-engaging members, the pilot hydraulic signal being piloted to control the hydraulic steering cylinder.

14. A steerable self-propelled vehicle according to claim 11 , wherein the electric power steering servomechanism includes a motor for generating a torque that acts on the steering column, and a signal generator for generating control signals determining the magnitude and direction of action of the torque.

15. A steerable self-propelled vehicle according to claim 11 , wherein the other locations include fields and roads.

16. A steerable self-propelled vehicle according to claim 11 , including two or more selectively operable steering input members connected to the steering column that include a steering wheel mechanically coupled to the steering column and a joystick.

17. A steerable self-propelled vehicle according to claim 16 , wherein the steering wheel is mechanically coupled to the steering column.

18. A steerable self-propelled vehicle according to claim 16 , wherein the joystick is electronically coupled to the electric power steering servomechanism.

19. A steerable self-propelled vehicle according to claim 11 , including at least one receiver of transmitted vehicle guidance signals and wherein the controller converts received guidance signals to steering input commands that are input to at least one of the hydraulic steering servomechanism and the electric power steering servomechanism.