METHOD FOR CONTROLLING A STEERING DEVICE, AND VEHICLE

Abstract

A method of controlling a steering device including a pair of rack bars capable of steering right and left wheels in one and/or the other of opposite right and left directions, and a coupling mechanism which moves the rack bars in the right direction and/or the left direction by the same distance. The method includes issuing a command to couple or uncouple the coupling mechanism, and driving at least one of an engaging part and a receiving part of the coupling mechanism relative to the other if a coupling state sensor are unable to detect that the coupling mechanism has been coupled or uncoupled when a predetermined time has passed after issuing the command.

Claims

1. A method of controlling a steering device including a pair of rack bars capable of steering right and left wheels in one and/or the other of opposite right and left directions, respectively, and a coupling mechanism disposed between the pair of rack bars and including an engaging part provided on a side of one rack bar of the pair of rack bars, and a receiving part provided on a side of the other rack bar of the pair of rack bars, the steering device being configured such that when the engaging part and the receiving part are coupled to each other, the pair of rack bars are moved in one or the other of the opposite right and left directions by a same distance by the coupling mechanism, and when the engaging part and the receiving part are uncoupled from each other, the pair of rack bars are moved in the respective opposite right and left directions by a same distance, the method comprising: applying, to the coupling mechanism, a command to either couple the engaging part and the receiving part to each other, or uncouple the engaging part and the receiving part from each other; and performing a relative driving operation in which at least one of the engaging part and the receiving part are driven relative to the other of the engaging part and the receiving part with at least one steering drive arrangement, if a coupling state sensor provided at the coupling mechanism fails to sense that the engaging part has been coupled to or uncoupled from the receiving part when a predetermined time has passed after the command is applied.

2. The method of claim 1, wherein the pair of rack bars are connected to the respective right and left wheels via respective tie rods, wherein the steering device further comprises a synchronizing gear disposed between the pair of rack bars and configured to convert a movement of the one rack bar in either one of the opposite right and left directions to a movement of the other rack bar in the other of the opposite right and left directions, and wherein the steering device further comprises a rack bar moving arrangement including a first pinion gear in mesh with the one rack bar, a second pinion gear in mesh with the other rack bar, and the coupling mechanism, the coupling mechanism being disposed between the first pinion gear and the second pinion gear.

3. The method of claim 1, wherein a direction of the relative driving operation by the at least one steering drive arrangement is reversed, and every time the direction is reversed, a driving force for the relative driving operation is increased.

4. The method of claim 1, further comprising the step of terminating the relative driving operation if the coupling state sensor fails to sense that the engaging part has been coupled to or uncoupled from the receiving part as a result of the relative driving operation, based on determination that the coupling mechanism is malfunctioning.

5. The method of claim 1, wherein the at least one steering drive arrangement comprises one of a mode switching actuator, a normal steering actuator, and an in-wheel motor.

6. The method of claim 1, wherein the at least one steering drive arrangement comprises a plurality of steering drive arrangement selected from a mode switching actuator, a normal steering actuator, and an in-wheel motor, and configured to be driven either simultaneously or alternately with each other.

7. A vehicle in which travel modes are changed by the method of claim 1.

8. The method of claim 2, wherein a direction of the relative driving operation by the at least one steering drive arrangement is reversed, and every time the direction is reversed, a driving force for the relative driving operation is increased.

9. The method of claim 2, further comprising terminating the relative driving operation if the coupling state sensor fails to sense that the engaging part has been coupled to or uncoupled from the receiving part as a result of the relative driving operation, based on determination that the coupling mechanism is malfunctioning.

10. The method of claim 3, further comprising terminating the relative driving operation if the coupling state sensor fails to sense that the engaging part has been coupled to or uncoupled from the receiving part as a result of the relative driving operation, based on determination that the coupling mechanism is malfunctioning.

11. The method of claim 8, further comprising terminating the relative driving operation if the coupling state sensor fails to sense that the engaging part has been coupled to or uncoupled from the receiving part as a result of the relative driving operation, based on determination that the coupling mechanism is malfunctioning.

12. The method of claim 2, wherein the at least one steering drive arrangement comprises one of a mode switching actuator, a normal steering actuator, and an in-wheel motor.

13. The method of claim 3, wherein the at least one steering drive arrangement comprises one of a mode switching actuator, a normal steering actuator, and an in-wheel motor.

14. The method of claim 2, wherein the at least one steering drive arrangement comprises a plurality of steering drive arrangement selected from a mode switching actuator, a normal steering actuator, and an in-wheel motor, and configured to be driven either simultaneously or alternately with each other.

15. The method of claim 3, wherein the at least one steering drive arrangement comprises a plurality of steering drive arrangement selected from a mode switching actuator, a normal steering actuator, and an in-wheel motor, and configured to be driven either simultaneously or alternately with each other.

16. A vehicle in which travel modes are changed by the method of claim 2.

17. A vehicle in which travel modes are changed by the method of claim 3.

18. A vehicle in which travel modes are changed by the method of claim 4.

19. A vehicle in which travel modes are changed by the method of claim 5.

20. A vehicle in which travel modes are changed by the method of claim 6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 is a plan view of a vehicle in which a method of controlling a steering device according to the present invention is used.

[0036] FIG. 2 is a plan view of the vehicle shown in FIG. 1, showing a normal travel mode (normal steering mode).

[0037] FIG. 3 is a plan view of the vehicle shown in FIG. 1, showing a pivot turn mode.

[0038] FIG. 4 is a plan view of the vehicle shown in FIG. 1, showing a lateral movement mode (parallel movement mode).

[0039] FIG. 5 is a back view of a steering device used in the vehicle shown in FIG. 1, showing its interior.

[0040] FIGS. 6(a) and 6(b) are back views of a coupling mechanism used in the steering device shown in FIG. 5, showing its coupled state (FIG. 6(a)) and its uncoupled state (FIG. 6(b)).

[0041] FIG. 7 is a block diagram showing how the vehicle is controlled.

[0042] FIG. 8 shows a flow of abnormality determination when switching travel modes, according to the present invention.

[0043] FIG. 9 shows a flow of abnormality determination when switching travel modes, in a conventional arrangement.

BEST MODE FOR EMBODYING THE INVENTION

[0044] The vehicle embodying the present invention is described with reference to the drawings. In particular, description is first made of the steering mechanism of the vehicle and various travel modes, and then a process flow is described in case an abnormal situation occurs when switching the travel mode.

[0045] Regarding the steering mechanism of the vehicle and travel modes

[0046] FIG. 1 shows the drive train and the control path of the vehicle 1. The vehicle 1 includes front left and front right wheels FL and FR which are coupled to a front steering device 10 through respective tie rods 12, and rear left and rear right wheels RL and RR coupled to a rear steering device 20 through respective tie rods 22.

[0047] Each steering device 10 (20) includes two rack bars for steering the right and left wheels w. Of the two rack bars, the rack bar coupled to the wheel on the left of the vehicle with respect to the forward direction of the vehicle is hereinafter referred to as the first rack bar 53, while the rack bar on the right of the vehicle is referred to as the second rack bar 54. The forward direction of the vehicle is indicated by the arrow on the left of each of FIGS. 1 and 2-4.

[0048] The rack bars 53 and 54 of each steering device 10 (20) have connecting members 11 and 21 connected to the right and left pair of front (rear) wheels w through the respective tie rods 12 (22). Members such as knuckle arms are disposed between the tie rods 12 and 22 and the respective wheels w.

[0049] As shown in FIG. 5, the first rack bar 53 and the second rack bar 54 of each steering device 10 (20) are received in a rack case (steering cylinder) 50 which extends in the right-and-left direction with respect to the direction in which the vehicle moves in a straight line (i.e., fore-and-aft direction of the vehicle). The rack case 50 is fixedly screwed, directly or indirectly through a flange, to the frame (chassis), not shown, of the vehicle 1.

[0050] The steering device 10 (20) includes two magnetic sensors 70 configured to sense the movements of the rack bars 53 and 54 so that it is possible to determine whether the right and left wheels w are being properly steered. The steering devices 10 and 20 each include a first rotary shaft 61 (pinion shaft). The first rotary shaft 61 of the steering device 10 is connected to a steering shat 3 through a steering joint, not shown.

[0051] The first rack bar 53 and the second rack bar 54 of each steering device 10 (20) can be moved together with each other to the right or left by the same distance, either directly by the operation of the steering wheel 2 by the driver, or by the actuation of a normal steering actuator 31 which is operatively associated with the operation of the steering wheel 2. Due to this movement, the right and left wheels w are simultaneously steered to the right or left during a normal travel mode (see FIG. 2).

[0052] As shown in FIG. 5, the steering device 10 (20) includes a rack bar moving means 60 which is capable of moving the first rack bar 53 and the second rack bar 54 in the opposite directions to each other along the right-and-left direction with respect to the direction in which the vehicle 1 moves in a straight line, namely, along the direction in which the rack bars moves relative to each other (i.e., the direction in which the teeth of the rack bars are arranged). The rack bar moving means 60 includes first synchronizing gears 55 which each mesh with a synchronizing rack gear 53a of the first rack bar 53 and a synchronizing rack gear 54a of the second rack bar 54, which is opposed to the synchronizing rack gear 53a of the first rack bar 53.

[0053] The first synchronizing gears 55 comprise three gears 55a, 55b and 55c which are arranged at regular intervals in the direction in which the teeth of each rack gear 53a (54a) are arranged. When the first rack bar 53 moves in either one direction along the direction in which the teeth of the synchronizing rack gear 53a are arranged under a driving force applied to the first rack bar 53 from the rack bar moving means 60, this movement is converted to the movement of the second rack bar 54 in the other direction by the distance equal the distance by which the first rack bar 53 moves in the one direction.

[0054] Gears 56a and 56b which constitute second synchronizing gears 56 are disposed, respectively, between the first synchronizing gears 55a and 55b, which are adjacent to each other, and between the first synchronizing gears 55b and 55c, which are adjacent to each other. The second synchronizing gears 56 are in mesh with neither of the synchronizing rack gear 53a of the first rack bar 53 and the synchronizing rack gear 54a of the second rack bar 54, and in mesh with only the first synchronizing gears 55, to allow the three first synchronizing gears 55a, 55b and 55c to be rotated in the same direction by the same angle. The second synchronizing gears 56 thus allow smooth relative movement between the first rack bar 53 and the second rack bar 54. The first rack bar 53 and the second rack bar 54 each include, besides the synchronizing rack gear 53a (54a), a steering rack gear 53b (54b).

[0055] As shown in FIG. 5, one of moving amount sensing gears 71 is in mesh with a second pinion gear 65, and one of the moving amount sensing gears 71 is provided with a pulse gear 72 opposed to one of the two magnetic sensors 70, and configured to rotate as the moving amount sensing gears 71 rotate. The magnetic sensor 70 includes a sensing element or a sensing coil for converting a change in magnetic field to an electric signal, and allows measurement of the amount of rotation of the pulse gear 72. The magnetic sensor 70, the moving amount sensing gear 71, and the pulse gear 72 constitute a moving amount sensing means 73 for sensing the amount of movement of the second rack bar 54 to the right and left. While not shown, another moving amount sensing means 73 is provided which is identical to the moving amount sensing means 73 shown, and which is capable of sensing the moving amount of the first rack bar 53 with one of its moving amount sensing gears 71 in mesh with a first pinion gear 62.

[0056] When the rack bars 53 and 54 of each steering device 10 (20) are moved to the right and/or left, and the first and second pinion gears 62 and 65 rotate correspondingly, the moving amount sensing gears 71 also rotate correspondingly, thus causing the pulse gears 72 to also rotate correspondingly. The rotation of the pulse gears 72 causes the magnetic sensors 70 to generate electric signals based on which a processing means of the steering device 10 (20) can calculate the numbers of revolutions of the respective pulse gears 72, and thus the steering angles of the right and left wheels w.

[0057] The pulse gears 72 used in this embodiment may be replaced with rotary magnetic encoders. Since the pulse gears 72, the rotary magnetic encoders, and the magnetic sensors 70 are less likely to be affected by dust and dirt, the moving amount sensing means 73, mounted on the vehicle 1, is capable of maintaining high sensing accuracy. However, if protective measures against dust and dirt are taken, optical rotation sensor arrangements may be used instead.

[0058] Next, the operation of the rack bar moving means 60 of each steering device 10 (20) is described.

[0059] The rack bar moving means 60 includes a second rotary shaft 64 arranged coaxially with the first rotary shaft 61, and a second pinion gear 65 mounted to the second rotary shaft 64 so as to be rotatable in unison with the second rotary shaft 64. As shown in FIG. 5, the first pinion gear 62 meshes with the steering rack gear 53b of the first rack bar 53, while the second pinion gear 65 meshes with the steering rack gear 54b of the second rack bar 54.

[0060] A coupling mechanism 63 is mounted between the first pinion gear 62 and the second pinion gear 65, and is capable of selectively coupling together the first rotary shaft 61 and the second rotary shaft 64 so as to be not rotatable relative to each other (coupled state, shown in FIG. 6(a)), and uncoupling the first rotary shaft 61 and the second rotary shaft 64 from each other so as to be rotatable relative to each other (uncoupled state, shown in FIG. 6(b)).

[0061] As shown in FIGS. 6(a) and 6(b), the coupling mechanism 63 includes a moving part 63a (engaging part) provided on the side of the first rotary shaft 61, and a fixed part 63b (receiving part) provided on the side of the second rotary shaft 64. The moving part 63a is biased toward the fixed part 63b by an elastic member, not shown, such as a spring. When the moving part 63a is pushed toward the fixed part 63b by the elastic member until protrusions 63c of the moving part 63a are engaged in recesses 63d of the fixed part 63b (coupled state, shown in FIG. 6(a)), the first and second rotary shafts 61 and 64 are coupled together so as not to be rotatable relative to each other. The protrusions 63c may be formed, not on the moving part 63a, but on the fixed part 63b, with the recesses 63d formed in the moving part 63a.

[0062] The moving part 63a of the coupling mechanism 63 can be moved axially upward (in FIG. 6(a)) and away from the fixed part 63b with a force from an external driving source, not shown, such as a push solenoid, until the fixed part 63b is uncoupled from the moving part 63a, and thus first rotary shaft 61 and the second rotary shaft 64 become rotatable independently of each other (uncoupled state, shown in FIG. 6(b)). In this state, the first pinion gear 62 and the second pinion gear 65 are also rotatable independently of each other.

[0063] The first pinion gear 62 and the second pinion gear 65 are in mesh with the first rack bar 53 and the second rack bar 54, respectively, and the first rack bar 53 and the second rack bar 54 are both in mesh with the first synchronizing gears 55. Thus, when the first pinion gear 62 is rotated while, as shown in FIG. 6(b), being uncoupled from the second pinion gear 65, the first rack bar 53 is moved in one of the right and left directions of the vehicle 1, i.e. one of the opposite directions along which the rack teeth of the first rack bars 53 are arranged, and the first synchronizing gears 55 rotate, thereby moving the second rack bar 54 in the other of the opposite directions by the same distance the first rack bar 53 is moved in the one of the opposite directions. Corresponding to the movement of the second rack bar 54, the second pinion gear 65 rotates.

[0064] Thus, by selectively coupling and uncoupling the first pinion gear 62 and the second pinion gear 65 by means of the coupling mechanism 63, it is possible to easily change between the state in which pair of rack bars 53 and 54 are moved in the same direction by the same distance, and the state in which the pair of rack bars 53 and 54 are moved in opposite directions to each other by the same amount.

[0065] As shown in FIGS. 6(a) and 6(b), the moving part 63a is formed with a protruding portion 76 (sensed portion) protruding outwardly from the outer periphery of the moving part 63a such that when the coupling mechanism 63 is coupled and uncoupled, the protruding portion 76 moves together with the moving part 63a in the axial direction of the moving part 63a. The coupling mechanism 63 further includes an induction type proximity sensor 77a (77) located so as to face the moving part 63a when the coupling mechanism 63 is coupled, thereby sensing the coupled state of the coupling mechanism 63, and an induction type proximity sensor 77b (77) located so as to face the moving part 63a when the coupling mechanism 63 is uncoupled, thereby sensing the uncoupled state of the coupling mechanism 63. The protruding portion 76a and the induction type proximity sensors 77a and 77b constitute a coupling state sensing means 78. The induction type proximity sensors 77 sense the approach of the protruding portion 76 by sensing the magnetic loss corresponding to the approach of the protruding portion 76.

[0066] In the coupled state (FIG. 6(a)), the induction type proximity sensor 77a for sensing the coupled state senses the approach of the protruding portion 76, while the induction type proximity sensor 77b for sensing the uncoupled state does not sense the approach of the protruding portion 76. In the uncoupled state (FIG. 6(b)), the induction type proximity sensor 77a for sensing the coupled state does not sense the approach of the protruding portion 76, while the induction type proximity sensor 77b for sensing the uncoupled state senses the approach of the protruding portion 76.

[0067] In this embodiment, the sensed portion is the protruding portion 76, and the sensors 77 are induction type proximity sensors 77a and 77b. Instead, a magnet may be used as the sensed portion, and magnetic sensing elements may be used as the sensors 77. Magnetic sensing elements maintain stable sensing capability and are less likely to malfunction in an environment where there exists a large amount of dust and dirt.

[0068] FIG. 7 shows a block diagram of a control arrangement of the embodiment, which includes an electronic control unit (ECU) 40 configured to actuate an external driving source such as a push solenoid to drive the moving part 63a of the coupling mechanism 63 of each steering device 10 (20). Then, the ECU 40 determines whether or not the coupling mechanism 63 has been actually coupled or uncoupled as commanded by the ECU 40, based on the feedback from the coupling state sensing means 78.

[0069] When the moving part 63a is separated from the fixed part 63b, the pair of rack bars 53 and 54 become movable in opposite directions to each other. In this state, the state of the steering device 10 (20) is changeable between a normal travel mode and various special travel modes. When the moving part 63a is joined to the fixed part 63b, the pair of rack bars 53 and 54 become movable in the same direction, so that the right and left wheels w can be steered in the same direction.

[0070] Various travel modes of the vehicle 1, on which are mounted the steering devices 10 and 20, are now described.

[0071] (Normal Travel Mode)

[0072] With the wheels positioned as shown in FIG. 1, in which the vehicle is movable in a straight line, the coupling mechanism 63 of the first steering device 10 for the front wheels is coupled (as shown in FIG. 6(a)) so that the first rack bar 53 and the second rack bar 54 of the first steering device 10 are movable together with each other in one of the right and left directions, in a rack case 50 of the steering device 10 which is fixed to the frame of the vehicle 1.

[0073] When the first rack bar 53 and the second rack bar 54 of the steering device 10 are moved together with each other by the same distance in one of the right and left directions relative to the direction in which the vehicle moves in a straight line, under the driving force from the normal steering actuator 31 or by the operation of the steering wheel 2, the front right and front left wheels w are steered by predetermined angles. FIG. 2 shows the state when the front wheels are steered to the right. Thus, when the two rack bars 53 and 54 are completely fixed to each other, the vehicle 1 can travel in the same manner as ordinary vehicles. During the normal travel mode, the operation of the steering wheel 2 by a driver is transmitted to the front wheels through the steering device 10 for the front wheels, so that the vehicle 1 can be moved in a straight line, or turned right or left by operating the steering wheel 2.

[0074] (Pivot Turn Mode)

[0075] FIG. 3 shows a pivot turn mode. By uncoupling the coupling mechanism 63 (as shown in FIG. 6(b)) of each steering device 10 (20), the first rack bar 53 and the second rack bar 54 of the steering device 10 (20) become separately movable. Thus, when a driving force is applied to the first pinion gear 62 from a mode switching actuator 32, the first rack bar 53 and the second rack bar 54 are moved in opposite directions to each other by the same distance. That is, due to the first synchronizing gears 55 being disposed between the first rack bar 53 and the second rack bar 54, when the first rack bar 53 moves in one of the right and left directions, the second rack bar 54 moves in the other or the right and left directions.

[0076] When the first rack bar 53 and the second rack bar 54 of each steering device 10 (20) are moved in opposite directions to each other until all of the four wheels w are positioned such that, as shown in FIG. 3, their respective axes approximately pass through the center of the vehicle, the respective coupling mechanisms 63 are coupled (as shown in FIG. 6(a)). In this state, since all of the four wheels w are positioned such that their axes approximately pass through the center of the vehicle, by actuating the in-wheel motors M mounted in the respective wheels w, the vehicle can be turned with its center stationary (or substantially stationary).

[0077] (Lateral Movement Mode)

[0078] FIG. 4 shows a lateral movement mode, in which with the coupling mechanism 63 uncoupled (as shown in FIG. 6(b) as in the pivot turn mode, the mode switching actuator 32 of each steering device 10 (20) rotates the first pinion gear 62 to move the first rack bar 53 and the second rack bar 54 in each steering device 10 (20) in opposite directions to each other until all of the four wheels w are positioned such that their respective axes extend perpendicular to (i.e. in the right-and-left direction relative to) the direction in which the vehicle moves in a straight line, and in this state, the coupling mechanism 63 is coupled (as shown in FIG. 6(a)) to fix the pair of rack bars 53 and 54 in position.

[0079] At that time, it is possible to finely adjust the directions (angles) of the wheels w by moving the first rack bar 53 and the second rack bar 54 in one of the right and left directions relative to the direction in which the vehicle moves in a straight line, under the driving force of the normal steering actuator 31 or by operating the steering wheel 2.

[0080] FIG. 4 shows the positional relationship between the steering devices 10 and 20 for the front and rear wheels w, respectively, and the directions of the wheels w, during the lateral movement mode. Compared to during the pivot turn mode, the rack bars 53 and 54 protrude laterally outwardly to a larger degree. Thus, in this mode, the connecting points between the tie rods 12 and 22 and the respective wheels w are located at the outermost positions in the width direction of the vehicle. During the lateral movement mode too, it is possible to finely adjust the directions (angles) of the wheels w under the driving force of the normal steering actuator 31 or by operating the steering wheel 2.

[0081] Besides the above-mentioned travel modes, the steering arrangement shown is capable of performing various other travel modes.

[0082] FIG. 8 shows a processing flow according to the present invention which is carried out if abnormality occurs in the steering device 10 or 20 while the steering device is being switched from one travel mode to another.

[0083] In this flow, the ECU 40 first generates a driving signal for coupling or uncoupling the coupling mechanism 63 (Step S10). If the ECU 40 fails to receive, within a predetermined time period after generating the driving signal, a signal indicative of completion of coupling or uncoupling of the coupling mechanism 63 from the coupling state sensing means 78 (Yes in Step S11), the ECU 40 proceeds to Step S12 in which the ECU 40 determines whether or not it has proceeded to Step S12 more than a predetermined number of times (e.g. three times). If this is not the case (No in Step 12), the ECU 40 proceeds to steps for driving steering drive means such as the mode switching actuator. If the number of times the ECU has proceed to Step S12 exceeds the above predetermined number of times (Yes in Step S12), the ECU 40 proceeds to step S13 in which it determines that it has failed to couple or uncouple the coupling mechanism 63, terminates the processing flow, and notifies the driver of this fact.

[0084] If the ECU 40 determines that abnormality exists during this processing flow, the reason why the coupling mechanism cannot be coupled or uncoupled is most probably not simply due to the moving part and the fixed part getting stuck on each other but due e.g. to wear of the coupling mechanism 63 itself or inclusion of foreign matter in the coupling mechanism 63. Thus, in such a case, the ECU 40 terminates the processing flow (in Step S13) instead of proceeding to the steps for coupling or uncoupling the coupling mechanism 63, thereby preventing the abnormality of the coupling mechanism 63 from becoming more serious.

[0085] On the other hand, while the number of times the ECU 40 has proceeded to Step S12 does not exceed the above predetermined number of times (No in Step S12), the ECU 40 actuates one of the mode switching actuator 32, the normal steering actuator 31, and each of the in-wheel motors M (as the steering drive means) to rotate the moving part 63a (engaging part) and the fixed part 63b (receiving part) relative to each other about their axes (Step S14).

[0086] At that time, a constant driving force may be applied to the moving part 63a and the fixed part 63b in one direction to rotate them relative to each other. Preferably, however, the direction of such a driving force is reversed, and also, the driving force is gradually increased. With this arrangement, at some point while the driving force is gradually increasing, the driving force necessarily cancels out external forces, thus minimizing the frictional force between the moving part 63a and the fixed part 63b, so that the moving part 63a can be smoothly coupled to or uncoupled from the fixed part 63b at that moment. With this arrangement, compared to an arrangement in which a large driving force is applied to the coupling mechanism 63 from the beginning, the moving part 63a can be coupled to or uncoupled from the fixed part with a minimum necessary driving force, so that it is possible to prevent damage to the coupling mechanism 63.

[0087] Instead of applying the driving force of only one of the above-mentioned steering drive means (such as the driving force of only the mode switching actuator 32) to the coupling mechanism 63, the driving forces of more than one of the steering drive means (such as the driving forces of the mode switching actuator 32 and the in-wheel motors M) may be applied, simultaneously or alternately, to the coupling mechanism 63.

[0088] After applying the driving force of at least one of the steering drive means (such as the mode switching actuator 32) to the coupling mechanism 63, the ECU 40 determines whether or not the coupling mechanism 63 has been coupled or uncoupled based on the feedback from the coupling state sensing means 78. That is, if the feedback from the coupling state sensing means 78 indicates that the moving part 63a has moved to the predetermined separated position or coupled position (Yes in Step S15), the ECU 40 determines that the coupling mechanism 63 has been successfully coupled or uncoupled (Step S16), and terminates the steps for coupling or uncoupling the coupling mechanism 63.

[0089] As long as the feedback from the coupling state sensing means 78 does not indicate that the coupling mechanism 63 has been successfully coupled or uncoupled (No in Step S15), the ECU 40 continuously checks the feedback from the coupling state sensing means 78 (in Step S11) until a predetermined time period has passed. If the feedback from the coupling state sensing means 78 confirms, before the predetermined time period has passed (No in Step S11), that the coupling mechanism 63 has been coupled or uncoupled (Yes in Step S15), the ECU 40 determines (in Step S16) that the coupling mechanism 63 has been successfully coupled or uncoupled, and terminates the steps for coupling or uncoupling the coupling mechanism 63.

[0090] By applying a driving force to the coupling mechanism 63 from e.g. the mode switching actuator 32 according to the above-described processing flow, it is possible to disengage the moving part 63a and the fixed part 63b of the coupling mechanism 63 from each other, thereby allowing them to be coupled together or uncoupled from each other, and allowing a smooth change in travel mode.

[0091] According to this invention, if the moving part 63a and the fixed part 63b of the coupling mechanism 63 get stuck on each other, at least one of them is rotated relative to the other by at least one of the steering drive means to cancel out external forces that cause the moving part to get stuck on the fixed part, thereby disengaging the moving part from the fixed part. Once the moving part disengages from the fixed part, the coupling mechanism 63 can be driven smoothly with a small driving force. Thus, compared to an arrangement in which the moving part 63a is moved against the force with which the moving part get stuck on the fixed part, it is possible to reduce the size of the steering drive means such as an actuator, as well as its energy consumption.

[0092] The above described structures of the steering devices 10 and 20, and the vehicle 1, as well as the above-described processing flow for determining abnormality while a travel mode is being switched to another travel mode, are mere examples, and these structures and processing flow may be modified, provided they can achieve the object of the present invention, namely, they allow smooth steering of the wheels of the vehicle, of which the four wheels are steerable, if it becomes impossible or difficult to couple or uncouple one of the coupling mechanisms 63 for driving the rack bars 53 and 54, by allowing smooth coupling or uncoupling of the coupling mechanism 63.

DESCRIPTION OF THE NUMERALS

[0093] 3. Steering shaft [0094] 12, 22, Tie rod [0095] 31. Normal steering actuator [0096] 32. Mode switching actuator [0097] 53, 54 Rack bar [0098] 55. Synchronizing gear [0099] 60. Rack bar moving means [0100] 62. First pinion gear [0101] 63. Coupling mechanism [0102] 63a. Engaging part (moving part) [0103] 63b. Receiving part (fixed part) [0104] 65. Second pinion gear [0105] 78. Coupling state sensing means [0106] M. In-wheel motor [0107] w. Wheel