Steering device

Abstract

This steering device steers respective pairs of front and rear steered wheels (2, 2, 3, 3) of a front two-axle vehicle (1), and comprises: a first electric motor (12) that applies a steering assist force to a steering shaft (9); and a hydraulic device (56) that applies a steering assist force to a second shaft (5) on the rear side of the front two-axle vehicle (1) via a link mechanism (15). The hydraulic device (56) includes: a hydraulic cylinder (60) that provides the steering assist force to the rear-side second shaft (5); a hydraulic pump (57) that is capable of supplying hydraulic pressure to the hydraulic cylinder (60); a second electric motor (58) that drives the hydraulic pump (57); and a second ECU controller (59) that drives and controls the second electric motor (58) on the basis of steering torque detected by a torque sensor (27).

Description

Steering apparatus

[0001] The present invention relates to a steering apparatus.

[0002] As a steering apparatus used in a two-front-axle vehicle, the steering apparatus described in the following Patent Document 1 is known.

[0003] In the steering apparatus described in Patent Document 1, hydraulic pressure is generated by a pump driven by an engine or another device, and supplied into a steering housing including a steering shaft. Then, the hydraulic pressure is selectively used from the steering housing via an actuator switching valve to control the front steered wheels or the rear steered wheels of the two front axles.

[0004] In recent years, among common small automobiles driven by an engine with one front axle, so-called electric vehicles obtained by fully electrifying automobiles have tended to become widespread in order to suppress global warming caused by carbon dioxide in exhaust gas.

[0005] Furthermore, relatively large automobiles which are two-front-axle vehicles also tend to be electrified in order to suppress global warming. As one step toward this trend, there is a need to fully electrify the application of steering assist force to at least the first front axle among the two front axles, and to apply the steering assist force to the rear second axle with a simple configuration.

[0006] Japanese Patent Application Laid-Open No. 2011-201540

[0007] The present invention relates to a steering apparatus that steers a pair of front steered wheels and a pair of rear steered wheels of a two-front-axle vehicle, the steering apparatus comprising: a first electric motor that applies a steering assist force to a steering shaft; and a hydraulic device that applies a steering assist force to the rear axle of the two-front-axle vehicle via a link mechanism, the hydraulic device including a hydraulic cylinder for providing the steering assist force to the rear axle, a hydraulic pump capable of supplying hydraulic pressure to the hydraulic cylinder, a second electric motor that drives the hydraulic pump, and a second control device that drives and controls the second electric motor based on a steering torque detected by a torque sensor.

[0008] According to the present invention, the application of steering assist force to the first axle, which is the front of the two front axles, can be completely electrified, and the application of steering assist force to the second axle, which is the rear, can be done with a simple configuration.

[0009] This is a schematic top view of a vehicle including a steering device according to one embodiment. This is a longitudinal cross-sectional view of the steering device according to one embodiment. This is a schematic explanatory diagram of a hydraulic device used to provide steering assist force to the second axis. This is a cross-sectional view of the hydraulic device cut along the longitudinal direction of the hydraulic cylinder. This is a cross-sectional view of the hydraulic pump cut along line A-A in Figure 4.

[0010] Hereinafter, embodiments of the steering device of the present invention will be described with reference to the drawings.

[0011] Figure 1 is a schematic top view of a vehicle including a steering device according to one embodiment. Figure 2 is a longitudinal cross-sectional view of the steering device according to one embodiment. In Figure 1, a portion of the chassis of the two-axle front vehicle 1 is shown by a dashed line, but there are other parts of the chassis that are not shown in the figure. In Figure 2, the side of the steering shaft 9 that is linked to the steering wheel 8 (see Figure 1) (upper side in the figure) is referred to as "one end," and the side to which the ball screw mechanism 10 is attached (lower side in the figure) is referred to as "the other end."

[0012] The steering system is configured to steer the steering wheels 2, 2, 3, 3, which form a pair at the front and rear of the two-axle front vehicle 1. The two-axle front vehicle 1 has a first axle 4, which is located on the front side in the longitudinal direction of the vehicle and has steering wheels 2, 2, and a second axle 5, which is located on the rear side in the longitudinal direction of the vehicle and has steering wheels 3, 3, and a third axle 7, which is located on the rear side and has steering wheels 6, 6.

[0013] The steering system also mainly consists of a steering shaft 9 linked to the steering wheel 8, a sector gear 11 that provides steering to the steering wheels 2, 2, 3, 3 via a ball screw mechanism 10, a first electric motor 12 that provides steering assist force to the steering shaft 9, and a reduction gear 13 that reduces the rotation of the first electric motor 12. The sector gear 11 is connected to the steering wheels 2, 2, 3, 3 via a pitman arm 14 and a link mechanism 15 connected to the pitman arm 14.

[0014] The steering shaft 9 is arranged to be inserted into a bottomed cylindrical third housing 19, passing through a cylindrical first housing 16, a cylindrical second housing 17 to which the first housing 16 is attached, and an annular bearing retaining member 18 provided at the end of the second housing 17 opposite to the first housing 16. The steering shaft 9 includes an input shaft 20 to which rotational force from the steering wheel 8 is transmitted, and an output shaft 22 connected to the input shaft 20 via a torsion bar 21. The outer circumference of one end of the input shaft 20 is rotatably supported by a first ball bearing 23, which is a bearing provided on the inner circumference of the first housing 16. The other end of the input shaft 20 is located in a recess 22b that is recessed in the direction of the rotation axis P from the axial end 22a of one end of the output shaft 22. One end of the output shaft 22 is rotatably connected to the input shaft 20 via a torsion bar 21 and is supplied with steering assist torque input from a first electric motor 12 connected to its outer circumference via a reduction gear 13. The output shaft 22 is rotatably supported by a second ball bearing 24, which is a bearing provided on the inner circumference of the bearing holding member 18.

[0015] For the sake of convenience in the following explanation, we define the direction along the rotation axis P of the steering shaft 9 as the "axial direction," the direction perpendicular to the axial direction as the "radial direction," and the direction around the steering shaft 9 as the "circumferential direction."

[0016] The first housing 16 is formed in a cylindrical shape from a metal material, such as an aluminum alloy. The first housing 16 is located on one end of the steering shaft 9 and, as shown in Figure 2, is attached and fixed to the second housing 17 by fastening members, such as bolts 25. As shown in Figure 2, the first housing 16 has a small-diameter portion 16a located on one end of the steering shaft 9 and a large-diameter portion 16b formed integrally with the small-diameter portion 16a and having a larger diameter than the small-diameter portion 16a. In the space inside the small-diameter portion 16a, a dust seal 26 is located adjacent to one end of the first ball bearing 23 in the axial direction of the steering shaft 9, together with the first ball bearing 23, to suppress the intrusion of dust and other debris into the first housing 16 from the outside. In addition, a torque sensor 27 that detects steering torque corresponding to the rotation of the steering shaft 9 is located in the space inside the large-diameter portion 16b. The torque sensor 27 is housed in a sensor housing member 95 provided in the first housing 16.

[0017] The second housing 17 is made of a metallic material, such as an aluminum alloy. The second housing 17 houses a reduction gear 13 that reduces the rotation of the first electric motor 12, which is driven and controlled by a first control device, a first ECU controller (not shown).

[0018] The bearing retaining member 18 is made of a metal material, such as an aluminum alloy, and the above-mentioned second ball bearing 24 is provided on its inner circumference.

[0019] The third housing 19 is made of a metal material, such as an aluminum alloy. The third housing 19 has a ball screw mechanism housing portion 19a that houses the other end portion of the output shaft 22 and the ball screw mechanism 10, and a gear housing portion 19b that houses the sector gear 11 adjacent to the ball screw mechanism 10.

[0020] The torque sensor 27 mainly consists of a permanent magnet 28, a holder member 29, a pair of first and second yokes 30 and 31, a pair of first and second magnetic collecting rings 32 and 33, and a magnetic sensor (not shown). The permanent magnet 28, the holder member 29, the first and second yokes 30 and 31, and the magnetic collecting rings 32 and 33 are all arranged to be approximately concentric with the rotation axis P of the steering shaft 9.

[0021] The permanent magnet 28 is a magnetic member formed in a substantially cylindrical shape from a magnetic material and attached and fixed to the outer circumference of one end of the output shaft 22. The permanent magnet 28 is configured such that north poles and south poles are alternately arranged (magnetized) along the circumferential direction of the permanent magnet 28.

[0022] The holder member 29 is formed in a substantially cylindrical shape and is attached and fixed to the outer circumference of the axial center of the input shaft 20.

[0023] The pair of first and second yokes 30 and 31 are both formed in a substantially cylindrical shape from a soft magnetic material and are connected to the input shaft 20 via a holder member 29, with the other end facing the permanent magnet 28 in the radial direction. The first and second yokes 30 and 31 are welded and fixed to the holder member 29 by an annular welding plate (not shown).

[0024] The pair of magnetic collecting rings 32 and 33 are substantially annular rings that concentrate the magnetic flux from the permanent magnets that leaks to one end of both yokes 30 and 31 into a predetermined range. The magnetic collecting ring 32 is positioned on the outer circumference of the torque sensor 27, while the magnetic collecting ring 33 is positioned on the inner circumference of the torque sensor 27, facing the magnetic collecting ring 32. A magnetic collecting portion 32a having a flat surface (not shown) is provided at a predetermined position in the circumferential direction of the magnetic collecting ring 32, while a magnetic collecting portion 33a having a flat surface (not shown) parallel to the flat surface of the magnetic collecting portion 32a is provided at a position in the circumferential direction of the magnetic collecting ring 33 facing the magnetic collecting portion 32a.

[0025] The magnetic sensor consists of a Hall element (not shown) housed in the radial gap between the flat surface of the magnetic collecting section 32a and the flat surface of the magnetic collecting section 33a, and a connection terminal (not shown) for connecting the Hall element to a substrate (not shown). The magnetic sensor utilizes the Hall effect of the Hall element to detect the magnetic flux passing between the opposing flat surfaces of the magnetic collecting sections 32a and 33a, and outputs a signal corresponding to this magnetic flux to a substrate (not shown). This allows for the calculation of the relative rotation angle between the input shaft 20 and the output shaft 22 on the substrate, and the calculation of the steering torque based on this relative rotation angle. In the calculation of the relative rotation angle, the calculation is based on the steering direction, which is the rotation direction of the input shaft 20, so the steering direction of the steering shaft 9 can be determined. Therefore, the steering direction of the steering shaft 9 can be detected using the torque sensor 27.

[0026] The ball screw mechanism 10 is composed of an output shaft side ball screw groove 22b, which is a helical groove provided on the outer circumference of the output shaft 22; a nut side ball screw groove 34a, which is a helical groove provided on the inner circumference of the nut 34; and a plurality of balls 35 arranged between the ball screw grooves 22b and 34a. The balls 35 support the nut 34 so that it can rotate relative to the output shaft 22. The outer surface of the nut 34 is in slidable contact with the inner surface of the ball screw mechanism housing 19a. As a result, the nut 34 can slide along the rotation axis P of the output shaft 22 within the ball screw mechanism housing 19a.

[0027] Furthermore, multiple rack teeth 34b are formed on the outer circumference of the nut 34 on the sector gear 11 side. These rack teeth 34b are designed to mesh with the teeth 11a of the sector gear 11.

[0028] The sector gear 11 is pivotably mounted within the gear housing 19b. One axial end of the sector gear 11 is connected to the nut 34 via the teeth 11a, while the other end is linked to the steering wheels 2, 2, 3, 3 via the pitman arm 14 and the linkage mechanism 15.

[0029] The reduction gear 13 reduces the rotation from the first electric motor 12 and consists of a worm gear formed by meshing a worm shaft 36 fixed to the motor shaft of the first electric motor 12 with a worm wheel 37.

[0030] The worm wheel 37 has a generally cylindrical metal core portion 37a and a synthetic resin gear forming portion 37b provided on the outer circumference of the core portion 37a. The gear forming portion 37b meshes with the worm formed on the worm shaft 36.

[0031] The first electric motor 12 is connected to the output shaft 22 via a reduction gear 13 and is configured as a three-phase AC brushless motor that applies steering assist torque to the output shaft 22 according to the amount of twist of the torsion bar 21. As shown in Figure 2, the first electric motor 12 is integrated with a first EPS controller (not shown), which is a first control device that controls the first electric motor 12 based on the steering torque from the torque sensor 27.

[0032] As shown in Figure 1, the link mechanism 15 connects the sector gear 11 to the first axle 4 and the second axle 5 of the front two-axle vehicle 1. The link mechanism 15 includes a first axle side link component 15a that transmits the steering assist force, which is the output from the sector gear 11, to the first axle 4 via the pitman arm 14, and a second axle side link component 15b that transmits the steering assist force, which is the output from the sector gear 11, to the second axle 5 via the pitman arm 14.

[0033] The first shaft-side linkage component 15a includes a first drag link 38 and a first knuckle arm 39. One end of the first drag link 38 is rotatably supported on the pitman arm 14 via a first joint 40, while the other end of the first drag link 38 is rotatably supported on one end of the first knuckle arm 39 via a second joint 41. The middle portion 39a of the first knuckle arm 39 is attached to one of the pair of steering wheels 2, 2. The other end of the first knuckle arm 39 is rotatably supported on one end of the first shaft 4 via a third joint 42. The other end of the first shaft 4 is rotatably supported on one end of the second knuckle arm 44 via a fourth joint 43. The other end of the second knuckle arm 44 is attached to the other of the pair of steering wheels 2, 2.

[0034] The second shaft-side link configuration 15b includes a second drag link 45, an intermediate link 46, a third knuckle arm 47, and a branch link 48. One end of the second drag link 45 is rotatably supported on the pitman arm 14 via a fifth joint 49, while the other end of the second drag link 45 is rotatably supported on one end of the intermediate link 46 and one end of the branch link 48 via a sixth joint 50. The other end of the intermediate link 46 is rotatably supported on one end of the third knuckle arm 47 via a seventh joint 51. The other end of the branch link 48 is rotatably supported on a part of the chassis not shown via an eighth joint 52. A link mounting member 87, located at one end of the hydraulic cylinder 60 of the hydraulic system 56 (described later), is attached to the central part of the branch link 48. The connecting portion 60c at the other end of the hydraulic cylinder 60 is rotatably supported on a part of the chassis not shown. The middle portion 47a of the third knuckle arm 47 is attached to one of the pair of steering wheels 3, 3. The other end of the third knuckle arm 47 is rotatably supported on one end of the second shaft 5 via the ninth joint 53. The other end of the second shaft 5 is rotatably supported on one end of the fourth knuckle arm 55 via the tenth joint 54. The other end of the fourth knuckle arm 55 is attached to the other of the pair of steering wheels 3, 3.

[0035] Figure 3 is a schematic diagram illustrating the hydraulic system 56 used to provide steering assist force to the second shaft 5. Figure 4 is a cross-sectional view of the hydraulic system 56 cut along the longitudinal direction of the hydraulic cylinder 60. Note that in Figure 4, for the sake of simplicity, the cross-sections of the second electric motor 58, the second ECU controller 59, the first check valve 65 and the second check valve 67, as well as the first tank 64 and the second tank 66, are omitted. Figure 5 is a cross-sectional view of the hydraulic pump 57 cut along line A-A in Figure 4.

[0036] The hydraulic system 56 applies steering assist force to the rear second axle 5 of the front two-axle vehicle 1 via the second axle side link component 15b of the link mechanism 15. The hydraulic system 56 includes a hydraulic pump 57, a second electric motor 58, a second ECU controller 59, and a hydraulic cylinder 60.

[0037] The hydraulic pump 57 is capable of supplying hydraulic pressure to the hydraulic cylinder 60 and is configured as an internal gear pump capable of switching hydraulic pressure in two directions according to the rotation direction of the second electric motor 58. The hydraulic pump 57 may be configured as an external gear pump instead of an internal gear pump. Furthermore, the hydraulic pump 57 is not limited to an internal or external gear pump, and may be other types of pumps capable of switching in two directions. The hydraulic pump 57 comprises a housing member 61, a drive gear 62, a driven gear 63, a first tank 64, a first check valve 65, a second tank 66, a second check valve 67, a gear ring member 68, and a cover member 69.

[0038] The housing member 61 is generally formed in the shape of a cylindrical block and has circular recesses 61b formed on the end faces 61a of the housing member that are opposite the lid member 69. The driving gear 62, the driven gear 63, the first check valve 65, the second check valve 67, and the gear ring member 68 are mainly housed in these circular recesses 61b.

[0039] The drive gear 62 is fixed to the outer circumference of the motor shaft 58a of the second electric motor 58. As shown in Figure 5, the outer circumference of the drive gear 62 has a plurality of teeth 62a (six in this embodiment) that are arranged at equal intervals along the circumferential direction of the drive gear 62.

[0040] The driven gear 63 is annular in shape and is provided around the drive gear 62. As shown in Figure 5, the inner circumference of the driven gear 63 has multiple (seven in this embodiment) teeth 63a arranged at equal intervals along the circumferential direction of the driven gear 63. When the drive gear 62 rotates, the driven gear 63 rotates in the same direction as the drive gear 62 through the meshing of the teeth 62a and teeth 63a. As shown in Figure 5, a space is provided between the four teeth 63a of the driven gear 63 arranged in the circumferential direction and the four teeth 62a of the drive gear 62 arranged in the circumferential direction. As shown in Figure 5, when the second electric motor 58 rotates to the right (clockwise rotation in Figure 5), oil in the first tank 64, which is the low-pressure side, is supplied to the space between the teeth 62a, 63a via the first check valve 65, flows along the oil flow direction X, which is the right rotation direction, and flows into the second flow path 72, which will be described later and is the high-pressure side. On the other hand, when the second electric motor 58 rotates to the left (counterclockwise in Figure 5), the low-pressure section and the high-pressure section are swapped, and the oil in the second tank 66, which is the low-pressure section, is supplied to the space between the teeth 62a and 63a via the second check valve 67, flows in the opposite direction to the oil flow direction X, and flows into the first flow path 71, which will be described later and is the high-pressure section.

[0041] The gear ring member 68 is annular in shape and houses a drive gear 62 fixed to the motor shaft 58a of the second electric motor 58, and a driven gear 63 surrounding the drive gear 62. The end faces 68a, 62b, and 63b of the gear ring member 68, the drive gear 62, and the driven gear 63 on the cover member 69 side are flush with the end face 61a of the housing member.

[0042] The cover member 69 is formed in a disc shape, and is provided so as to close the opening surface of the recessed portion 61b of the housing member 61 while being adjacent to each of the end surfaces 68a, 62b, 63b and the housing member end surface 61a. In a portion of the interior of the cover member 69 on the first check valve 65 side that faces the boundary between the drive gear 62 and the driven gear 63, a first flow path inlet portion 73 that forms a part of the first flow path 71 and has an L-shaped cross section is formed. The first flow path inlet portion 73 communicates with a first relay portion 74 that also forms a part of the first flow path 71 and has an internal passage 74a communicating with the first flow path inlet portion 73. The first relay portion 74 communicates with a first pipe portion 75 that forms a part of the first flow path 71. The first pipe portion 75 communicates with a second relay portion 76 that also forms a part of the first flow path 71 and has an internal passage 76a communicating with the first pipe portion 75. The second relay portion 76 communicates with a first cylinder chamber 60a of the hydraulic cylinder 60, which will be described later.

[0043] Further, in a portion of the interior of the cover member 69 on the second check valve 67 side that faces the boundary between the drive gear 62 and the driven gear 63, a second flow path inlet portion 77 that forms a part of the second flow path 72 and has an L-shaped cross section is formed. The second flow path inlet portion 77 communicates with a third relay portion 78 that also forms a part of the second flow path 72 and has an internal passage 78a communicating with the second flow path inlet portion 77. The third relay portion 78 communicates with a second pipe portion 79 that forms a part of the second flow path 72. The second pipe portion 79 communicates with a fourth relay portion 80 that also forms a part of the second flow path 72 and has an internal passage 80a communicating with the second pipe portion 79. The fourth relay portion 80 communicates with a second cylinder chamber 60b of the hydraulic cylinder 60, which will be described later.

[0044] The second electric motor 58 is configured as a three-phase brushless motor similarly to the first electric motor 12, and drives the hydraulic pump 57. The driving of the second electric motor 58 is controlled by a second ECU controller 59.

[0045] The second ECU controller 59 receives, via CAN communication, information on the required motor torque (the motor torque required for driving the hydraulic cylinder 60) calculated by the first ECU controller associated with the first electric motor 12 based on the steering torque and steering direction. Then, the second ECU controller 59 drives and controls the second electric motor 58 in accordance with the motor torque based on the steering torque and steering direction, so that the hydraulic pressure required for the steering assist of the second shaft 5 by the hydraulic cylinder 60 is supplied to the first cylinder chamber 60a or the second cylinder chamber 60b of the hydraulic cylinder 60.

[0046] Furthermore, instead of receiving information from the first ECU controller on the first electric motor 12 side, the second ECU controller 59 may directly receive the steering torque and steering direction from the torque sensor 27, independently detect the driver's intention, and calculate the motor torque required for driving the hydraulic cylinder 60.

[0047] The hydraulic cylinder 60 is used for applying a steering assist force to the rear second shaft 5. The hydraulic cylinder 60 includes a cylinder tube 81, a piston 82, a rod 83, a second closing member 84, a third closing member 85, a rubber boot 86, and a link attachment member 87.

[0048] The cylinder tube 81 has a cylindrical shape. A first insertion hole 81b, into which the second relay portion 76 of the first flow path 71 is inserted, is formed penetrating along the radial direction of the cylinder tube 81 on the outer peripheral portion near one end 81a of the cylinder tube 81. Furthermore, a second insertion hole 81d, into which the fourth relay portion 80 of the second flow path 72 is inserted, is formed penetrating along the radial direction of the cylinder tube 81 on the outer peripheral portion near the other end 81c of the cylinder tube 81.

[0049] The piston 82 is disc-shaped with an outer diameter slightly smaller than the inner diameter of the cylinder tube 81 and is slidably mounted inside the cylinder tube 81. A hole 82a is formed through the center of the piston 82, extending along the thickness direction of the piston 82, into which a bolt 89 for attaching the other end 83b of the rod 83 is inserted. The piston 82 divides the inside of the cylinder tube 81 into a first cylinder chamber 60a located on the second relay section 76 side and a second cylinder chamber 60b located on the fourth relay section 80 side. An annular groove is formed on the outer circumferential surface of the piston 82, which accommodates an annular first seal member 88. The first seal member 88 provides a liquid-tight seal between the outer circumferential surface of the piston 82 and the inner circumferential surface of the cylinder tube 81.

[0050] The rod 83 is slender and cylindrical. One end 83a of the rod 83 is provided with a link mounting member 87 to which a branch link 48 (see Figure 1) is attached. The other end 83b of the rod 83 is attached and fixed to the piston 82 via a fixing member, such as a bolt 89.

[0051] The second closure member 84 is annular in shape, and a through hole 84a is formed in its center, extending along the thickness direction of the second closure member 84, into which the rod 83 is slidably inserted. A first annular projection 84b is formed on the outer circumferential surface of the second closure member 84, projecting radially outward from the outer circumferential surface. One end 81a of the cylinder tube 81 abuts against the piston 82 side surface of the first annular projection 84b. The first annular projection 84b is attached and fixed to a first flange portion 81e provided on one end 81a of the cylinder tube 81 via a fixing member, such as a bolt 90. An annular groove is formed on the inner circumferential surface of the second closure member 84 (the inner circumferential surface of the through hole 84a) near the rubber boot 86, into which an annular second seal member 91 is fitted. The second seal member 91 provides a liquid-tight seal between the inner circumferential surface of the second closure member 84 and the outer circumferential surface of the rod 83. Furthermore, an annular groove is formed on the outer surface of the second closure member 84 at a position closer to the first cylinder chamber 60a, into which an annular third sealing member 92 is fitted. The third sealing member 92 provides a liquid-tight seal between the outer surface of the second closure member 84 and the inner surface of the cylinder tube 81.

[0052] The third closure member 85 is formed in a generally cylindrical shape. On the outer circumferential surface of the third closure member 85, a second annular projection 85a is formed, projecting radially outward from the outer circumferential surface of the third closure member 85, in the portion adjacent to the second cylinder chamber 60b. The other end 81c of the cylinder tube 81 abuts against the piston 82 side surface of the second annular projection 85a. The second annular projection 85a is attached and fixed to a second flange portion 81f provided on the other end 81c of the cylinder tube 81 via a fixing member, such as a bolt 93. Furthermore, an annular groove is formed on the third closure member 85 near the second cylinder chamber 60b, into which an annular fourth seal member 94 is fitted. The fourth seal member 94 provides a liquid-tight seal between the outer circumferential surface of the third closure member 85 and the inner circumferential surface of the cylinder tube 81.

[0053] The rubber boot 86 is formed in a bellows shape from rubber material and is provided between the second closing member 84 and the link mounting member 87, at a position adjacent to the second closing member 84. The rubber boot 86 prevents foreign matter from entering the gap between the outer circumferential surface of the rod 83 and the inner circumferential surface of the second closing member 84 from outside the hydraulic cylinder 60.

[0054] In the hydraulic system 56 configured in this way, the hydraulic cylinder 60 applies steering assist force to the second shaft 5 to compensate for the deficiency of the steering assist force to the second shaft 5 provided by the first electric motor 12. More specifically, when the driver steers the steering wheel 8, the steering assist force from the first electric motor 12 is transmitted to both the first shaft 4 and the second shaft 5 via the output shaft 22 of the steering shaft 9, the ball screw mechanism 10, the sector gear 11, the pitman arm 14, the first shaft side link component 15a, and the second shaft side link component 15b. At this time, the steering assist force transmitted to the first shaft 4 is greater than the steering assist force transmitted to the second shaft 5. Therefore, the deficiency of the steering assist force to the second shaft 5 relative to the steering assist force to the first shaft 4 is compensated for by the hydraulic cylinder 60.

[0055] In this hydraulic system, when the driver steers the steering wheel 8 to the left, the second electric motor 58 rotates to the left (counterclockwise rotation in Figure 5), causing oil from the second tank 66 to flow into the first cylinder chamber 60a via the second check valve 67, the space between the teeth 62a and 63a, and the first flow path 71. The increase in hydraulic pressure in the first cylinder chamber 60a causes the piston 82 to be pressed towards the other end 81c of the cylinder tube 81, and consequently, the rod 83 also moves towards the other end 81c of the cylinder tube 81. As a result, the branch link 48 shown in Figure 1 is pulled in a direction toward the hydraulic cylinder 60, causing one of the steering wheels 3 (the right steering wheel 3 in Figure 1) to be steered to the left via the intermediate link 46 and the third knuckle arm 47. Simultaneously, the other steering wheel 3 (the left steering wheel 3 in Figure 1) is also steered to the left via the second shaft 5 and the fourth knuckle arm 55.

[0056] Furthermore, when the driver steers the steering wheel 8 to the right, the second electric motor 58 rotates to the right (clockwise rotation direction in Figure 5), causing oil from the first tank 64 to flow into the second cylinder chamber 60b via the first check valve 65, the space between teeth 62a and 63a, and the second flow path 72. As the hydraulic pressure in the second cylinder chamber 60b increases, the piston 82 is pressed towards one end 81a of the cylinder tube 81, and consequently, the rod 83 also moves towards one end 81a of the cylinder tube 81. This pushes the branch link 48 shown in Figure 1 away from the hydraulic cylinder 60, causing one of the steering wheels 3 (the right steering wheel 3 in Figure 1) to be steered to the right via the intermediate link 46 and the third knuckle arm 47. Simultaneously, the other steering wheel 3 (the left steering wheel 3 in Figure 1) is also steered to the right via the second shaft 5 and the fourth knuckle arm 55.

[0057] Furthermore, the second ECU controller 59 continues to control the drive of the second electric motor 58 and activates the hydraulic cylinder 60 when the first electric motor 12 fails. Here, the second ECU controller 59 drives the second electric motor 58 to stably steer the steering wheels 3, 3 on the second axle 5 side via the second axle side link component 15b of the link mechanism 15, at least until the vehicle is brought to an emergency stop. At this time, along with the steering of the steering wheels 3, 3, the steering wheels 2, 2 on the first axle 4 side are also steered until the vehicle is brought to an emergency stop.

[0058] As described above, in this embodiment, the steering device includes a first electric motor 12 capable of transmitting steering assist force to both the first axle 4 and the second axle 5 via the output shaft 22 of the steering shaft 9, the ball screw mechanism 10, the sector gear 11, the pitman arm 14, the first axle-side link component 15a, and the second axle-side link component 15b by applying steering force to the steering shaft 9, and a hydraulic device 56 that applies steering assist force to the rear second axle 5 of the front two-axle vehicle 1 via the second axle-side link component 15b of the link mechanism 15. The hydraulic device 56 includes a hydraulic cylinder 60 used to apply steering assist force to the rear second axle 5, a hydraulic pump 57 capable of supplying hydraulic pressure to the hydraulic cylinder 60, a second electric motor 58 that drives the hydraulic pump 57, and a second control device that drives and controls the second electric motor 58 based on the steering torque detected by the torque sensor 27. With the steering device configured in this way, the generation of steering assist force within the housing (including the first housing 16, etc.) containing the steering shaft 9 is completely electrified by the first electric motor 12, without the use of a hydraulic pump driven by the engine. Therefore, the application of steering assist force to the front first shaft 4 of the two front shafts is completely electrified, and the application of steering assist force to the rear second shaft 5 can be done with a simple configuration. In other words, the application of steering assist force to the first shaft 4 is completely electrified, while the application of steering assist force to the rear second shaft 5 is provided by the first electric motor 12 via the second shaft side link component 15b of the link mechanism 15, and any deficiency in steering assist force can be compensated for using a relatively simple hydraulic cylinder 60.

[0059] Furthermore, in this embodiment, the first ECU controller associated with the first electric motor 12 calculates the steering direction of the steering wheel 8 and the motor torque of the second electric motor 58 corresponding to this steering direction based on the steering torque detected by the torque sensor 27. The hydraulic cylinder 60 can switch hydraulic pressure bidirectionally by moving the piston 82 in accordance with the motor torque. Therefore, since the steering torque used to control the first electric motor 12 is also used to control the hydraulic cylinder 60 via the second electric motor 58, steering assist force can be applied to the second shaft 5 more efficiently compared to when the motor torque is calculated using other parameters.

[0060] Furthermore, in this embodiment, when the first electric motor 12 fails, the second ECU controller 59 continues to control the drive of the second electric motor 58 and activates the hydraulic cylinder 60 to stably steer the steering wheels 3, 3 on the second axle 5 side. As a result, even when the first electric motor 12 fails, the steering wheels 3, 3 on the second axle 5 side are steered, and consequently the steering wheels 2, 2 on the first axle 4 side are also steered, allowing the front two-axle vehicle 1 to be safely brought to an emergency stop.

Claims

1. A steering device for steering a pair of steering wheels, one at the front and one at the rear, of a two-axle front vehicle, comprising: a steering shaft to which rotation from a steering wheel is input; a sector gear to which the rotation of the steering shaft is transmitted; a link mechanism connecting the sector gear to the front axle and the rear axle of the two-axle front vehicle; a torque sensor for detecting steering torque corresponding to the rotation of the steering shaft; a first electric motor for applying steering assist force to the steering shaft and for transmitting steering assist force to both the front axle and the rear axle; and a first control device for driving and controlling the first electric motor based on the steering torque detected by the torque sensor. A hydraulic system for applying steering assist force to the rear axle of the two front axle vehicle via the link mechanism, comprising: a hydraulic cylinder for applying steering assist force to the rear axle; a hydraulic pump capable of supplying hydraulic pressure to the hydraulic cylinder; a second electric motor for driving the hydraulic pump; and a second control device for controlling the driving of the second electric motor based on the steering torque detected by the torque sensor.

2. A steering device according to claim 1, wherein the first control device or the second control device calculates the steering direction of the steering wheel and the motor torque of the second electric motor corresponding to the steering direction based on the steering torque detected by the torque sensor, and the hydraulic cylinder is capable of switching hydraulic pressure bidirectionally by the movement of a piston corresponding to the steering direction of the steering wheel and the motor torque of the second electric motor.

3. A steering device according to claim 1, wherein the second control device, in the event of failure of the first electric motor, continues to control the drive of the second electric motor and acts on the hydraulic cylinder to steer the steering wheel attached to the rear axle of the front two axle vehicle via the link mechanism.

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