Vehicle and steering apparatus

1. A vehicle comprising:

a front wheel drive device for driving front wheels; a rear wheel drive device for driving rear wheels independently of the front wheel drive device;

a drive mode control unit for controlling the front wheel drive device and the rear wheel drive device to control drive modes of the front wheels and the rear wheels; and

a steering apparatus for steering steerable wheels, which are at least the front wheels or the rear wheels;

wherein the drive mode control unit performs at least one of:

a first switching operation of switching between a front-wheel-only-drive mode in which only the front wheels are driven, and a rear-wheel-only-drive mode in which only the rear wheels are driven;

a second switching operation of switching between the rear-wheel-only-drive mode, and a composite drive mode in which both the front wheels and the rear wheels are driven; and

a third switching operation of switching between the composite drive mode and the front-wheel-only-drive mode; and

the steering apparatus further comprises:

manual steering means that manually steers the steerable wheels;

assistive force generating means, which generates a steering assistive force acting in a same direction as or in an opposite direction to a steering force applied to the manual steering means, for assisting in steering the steerable wheels with the manual steering means; and

assistive force control means that controls the steering assistive force; and

when the drive mode control unit performs any one of the first switching operation, the second switching operation, and the third switching operation, the assistive force control means changes a control process thereof for controlling the steering assistive force, and

wherein the drive mode control unit performs the first switching operation such that when the drive mode control unit changes from one of the front-wheel-only-drive mode and the rear-wheel-only-drive mode to the other mode, the drive mode control unit interposes the composite drive mode therebetween.

2. The vehicle according to claim 1 , wherein the steering apparatus includes manually steered state acquiring means, which acquires an operated state of the manual steering means, and vehicle speed acquiring means, which acquires a vehicle speed of the vehicle;

wherein the assistive force control means:

calculates a target steering assistive quantity using a target base steering assistive quantity based on the operated state of the manual steering means and the vehicle speed, and a corrective quantity based on one of the drive modes; and

when the drive mode control unit performs any one of the first switching operation, the second switching operation, and the third switching operation, the assistive force control means changes the corrective quantity, and calculates the target steering assistive quantity using the target base steering assistive quantity and the changed corrective quantity.

3. The vehicle according to claim 2 , wherein the assistive force control means:

calculates a target base current depending on the target base steering assistive quantity and a corrective current depending on the corrective quantity;

adds the corrective current to the target base current thereby to calculate a target current depending on the target steering assistive quantity; and

controls the assistive force generating means based on the target current.

4. The vehicle according to claim 2 , wherein the assistive force control means further comprises an amplifier for amplifying the target base steering assistive quantity with a predetermined coefficient; and

the amplifier uses different values for the predetermined coefficient before and after the drive mode control unit performs any one of the first switching operation, the second switching operation, and the third switching operation.

5. The vehicle according to claim 1 , wherein the assistive force control means:

controls the assistive force generating means using control maps, which represent control quantities for an operated state of the manual steering means; and

uses different ones of the maps before and after the drive mode control unit performs any one of the first switching operation, the second switching operation, and the third switching operation.

6. The vehicle according to claim 1 , wherein the front wheels comprise the steerable wheels, and the rear wheels comprise non-steerable wheels.

7. The vehicle according to claim 6 , wherein, when the drive mode control unit performs the first switching operation, the assistive force control means changes the control process for controlling the steering assistive force in order to make the steering assistive force greater in the front-wheel-only-drive mode than in the rear-wheel-only-drive mode, when the drive mode control unit performs the second switching operation, the assistive force control means changes the control process for controlling the steering assistive force in order to make the steering assistive force greater in the composite drive mode than in the rear-wheel-only-drive mode, and when the drive mode control unit performs the third switching operation, the assistive force control means changes the control process for controlling the steering assistive force in order to make the steering assistive force greater in the front-wheel-only-drive mode than in the composite drive mode.

8. The vehicle according to claim 1 , wherein when the drive mode control unit changes from one of the front-wheel-only-drive mode and the rear-wheel-only-drive mode to the other mode, a drive force applied to one of the front wheels and the rear wheels, which are to stop being driven, is progressively reduced, and a drive force applied to one of the front wheels and the rear wheels, which start being driven, is progressively increased.

9. The vehicle according to claim 8 , wherein in the composite drive mode while changing from one of the front-wheel-only-drive mode and the rear-wheel-only-drive mode to the other mode, a total of the drive force applied to one of the front wheels and the rear wheels, which are to stop being driven, and the drive force applied to one of the front wheels and the rear wheels, which start being driven, is kept constant.

10. A steering apparatus for use in a vehicle including a front wheel drive device for driving front wheels, a rear wheel drive device for driving rear wheels independently of the front wheel drive device, and a drive mode control unit for controlling the front wheel drive device and the rear wheel drive device to control drive modes of the front wheels and the rear wheels, wherein the drive mode control unit performs at least one of:

a first switching operation of switching between a front-wheel-only-drive mode in which only the front wheels are driven and a rear-wheel-only-drive mode in which only the rear wheels are driven;

a second switching operation of switching between the rear-wheel-only-drive mode, and a composite drive mode in which both the front wheels and the rear wheels are driven; and

a third switching operation of switching between the composite drive mode and the front-wheel-only-drive mode; and

the steering apparatus further comprises:

manual steering means that manually steers steerable wheels, which are at least the front wheels or the rear wheels;

assistive force generating means, which generates a steering assistive force acting in a same direction as or in an opposite direction to a steering force applied to the manual steering means, for assisting in steering the steerable wheels with the manual steering means; and

assistive force control means that controls the steering assistive force; and

when the drive mode control unit performs any one of the first switching operation, the second switching operation, and the third switching operation, the assistive force control means changes a control process thereof for controlling the steering assistive force, and

wherein the drive mode control unit performs the first switching operation such that when the drive mode control unit changes from one of the front-wheel-only-drive mode and the rear-wheel-only-drive mode to the other mode, the drive mode control unit interposes the composite drive mode therebetween.

11. A steering apparatus for use in a vehicle including a front wheel drive device for driving front wheels, a rear wheel drive device for driving rear wheels independently of the front wheel drive device, and a drive mode control unit for controlling the front wheel drive device and the rear wheel drive device to control drive modes of the front wheels and the rear wheels, wherein the drive mode control unit performs at least one of:

a first switching operation of switching between a front-wheel-only-drive mode in which only the front wheels are driven and a rear-wheel-only-drive mode in which only the rear wheels are driven;

a second switching operation of switching between the rear-wheel-only-drive mode, and a composite drive mode in which both the front wheels and the rear wheels are driven; and

a third switching operation of switching between the composite drive mode and the front-wheel-only-drive mode; and

the steering apparatus further comprises:

manual steering device that manually steers steerable wheels, which are at least the front wheels or the rear wheels;

assistive force generator, which generates a steering assistive force acting in a same direction as or in an opposite direction to a steering force applied to the manual steering device, for assisting in steering the steerable wheels with the manual steering device; and

assistive force controller that controls the steering assistive force; and

when the drive mode control unit performs any one of the first switching operation, the second switching operation, and the third switching operation, the assistive force controller changes a control process thereof for controlling the steering assistive force, and

wherein the drive mode control unit performs the first switching operation such that when the drive mode control unit changes from one of the front-wheel-only-drive mode and the rear-wheel-only-drive mode to the other mode, the drive mode control unit interposes the composite drive mode therebetween.

12. A method for controlling a vehicle including a front wheel drive device for driving front wheels, a rear wheel drive device for driving rear wheels independently of the front wheel drive device, a drive mode control computer for controlling the front wheel drive device and the rear wheel drive device to control drive modes of the front wheels and the rear wheels, and a steering apparatus including a manual steering device that manually steers steerable wheels which are at least the front wheels or the rear wheels,

wherein the method comprising:

(i) performing, by the drive mode control computer, at least one of:

a first switching operation of switching between a front-wheel-only-drive mode in which only the front wheels are driven and a rear-wheel-only-drive mode in which only the rear wheels are driven;

a second switching operation of switching between the rear-wheel-only-drive mode, and a composite drive mode in which both the front wheels and the rear wheels are driven; and

a third switching operation of switching between the composite drive mode and the front-wheel-only-drive mode; and

(ii) generating a steering assistive force acting in a same direction as or in an opposite direction to a steering force applied to the manual steering device for assisting in steering the steerable wheels with the manual steering device, and controlling, by a computer, the steering assistive force wherein a control process in controlling the steering assistive force is changed when performing any one of the first switching operation, the second switching operation, and the third switching operation,

wherein when the first switching operation changes from one of the front-wheel-only-drive mode and the rear-wheel-only-drive mode to the other mode, the drive mode control computer interposes the composite drive mode therebetween.