Work vehicle and method for controlling work vehicle

An articulated work vehicle with linked front and rear frames includes a hydraulic actuator, a joystick lever operated by an operator, a control valve, a force imparting component, and a controller. The hydraulic actuator is driven hydraulically to change a steering angle of the front frame with respect to the rear frame. The control valve is linked to the joystick lever, to control flow of fluid supplied to the hydraulic actuator according to an operation amount of the joystick lever, and to restrict the operation amount of the joystick lever to a predetermined range. The force imparting component imparts an assist force or a counterforce to the operation of the joystick lever by the operator. The controller controls the force imparting component so as to decrease the assist force or increase the counterforce before the operation of the joystick lever is restricted by the control valve.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of International Application No. PCT/JP2016/081728, filed on Oct. 26, 2016. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2015-213788, filed in Japan on Oct. 30, 2015, the entire contents of which are hereby incorporated herein by reference.

BACKGROUND

Field of the Invention

The present invention relates to a work vehicle and a method for controlling a work vehicle.

Description of the Related Art

An articulated work vehicle has been disclosed with a configuration in which the steering angle is changed by controlling the flow of fluid supplied to a hydraulic actuator disposed from the front frame to the rear frame (see Japanese Laid-Open Patent Application H11-105723 and Japanese Patent Laid-Open Patent Application 11-321664, for example).

The work vehicles in Japanese Laid-Open Patent Application H11-105723 and Japanese Patent Laid-Open Patent Application 11-321664 are provided with a steering valve that adjusts the flow of fluid supplied to a hydraulic actuator according to an inputted pilot pressure, and a pilot valve that adjusts the pilot pressure supplied to the steering valve.

The pilot valve is provided with an operation input shaft and a feedback input shaft that are able to rotate relative to each other. The operation input shaft is linked to the joystick lever and rotates according to the rotational angle of the joystick lever. The feedback input shaft is linked to the front frame by a linking mechanism, and rotates according to the change in steering angle.

Such a pilot valve adjusts the pilot pressure inputted to the steering valve according to the difference between the rotational angle of the operation input shaft and the rotational angle of the feedback input shaft. The flow of fluid supplied from the steering valve to the hydraulic actuator is changed according to the adjusted pilot pressure, and the steering angle is changed.

Thus, the operator can change the steering angle by operating the joystick lever to rotate the operation input shaft of the pilot valve.

SUMMARY

With the pilot valve in the work vehicles of the above-mentioned Japanese Laid-Open Patent Application H11-105723 and Japanese Patent Laid-Open Patent Application 11-321664, however, since the amount of displacement of the operation input shaft with respect to the feedback input shaft is mechanically restricted to within a predetermined range, when the operator operates the joystick lever to rotate the operation input shaft and the amount of displacement reaches the restricted position (hereinafter also referred to as during catch-up), the operator's wrist is subjected to a sudden recoil.

Thus, a problem has been that a burden is placed on the wrist of the operator by the sudden recoil caused by displacement restriction of the valve.

In light of the above problems encountered with conventional work vehicles, it is an object of the present invention to provide a work vehicle and a work vehicle control method with which the sudden recoil caused by displacement restriction of the valve can be mitigated.

The work vehicle pertaining to the first aspect is an articulated work vehicle in which a front frame and a rear frame are linked, comprising a hydraulic actuator, a joystick lever, a control valve, a force imparting component, and a controller. The hydraulic actuator is driven hydraulically and changes the steering angle of the front frame with respect to the rear frame. The joystick lever is operated by an operator. The control valve is linked to the joystick lever, controls the flow of fluid supplied to the hydraulic actuator according to the operation amount of the joystick lever, and restricts the operation amount of the joystick lever to a predetermined range. The force imparting component imparts an assist force or a counterforce to the operation of the joystick lever by the operator. The controller controls the force imparting component so as to decrease the assist force or increase the counterforce before the operation of the joystick lever is restricted by the control valve.

Consequently, the operating force required to operate the joystick lever increases before the operation of the joystick lever is restricted. That is, since the tactile sensation gradually becomes heavier from before the joystick lever is restricted, the operation speed becomes slower as the joystick lever approaches the restricted position. Therefore, the sudden recoil caused by valve displacement restriction can be mitigated.

The work vehicle pertaining to the second aspect is the work vehicle pertaining to the first aspect, wherein the rotational angle of the joystick lever corresponds to the target steering angle of the front frame with respect to the rear frame. The work vehicle further comprises a target steering angle sensor and an actual steering angle sensor. The target steering angle sensor senses the target steering angle. The actual steering angle sensor senses the actual steering angle. The control valve restricts the movement of the joystick lever when the angular difference between the target steering angle and the actual steering angle reaches a first predetermined angle difference. The controller controls the force imparting component so that the assist force is gradually decreased or the counterforce is gradually increased toward the first predetermined angle difference, on the basis of the angular difference.

Thus, the approach to the restricted position can be detected by the angular difference between the target steering angle and the actual steering angle, and the operating force required to operate the joystick lever can be increased before the operation of the joystick lever is restricted.

The work vehicle pertaining to the third aspect is the work vehicle pertaining to the second aspect, wherein the control valve has a first input member, a second input member, and a restrictor. The first input member is linked to the joystick lever and is displaced according to the operation of the joystick lever. The second input member is displaced according to the actual steering angle. The restrictor restricts the displacement of the first input member to within a predetermined range and thereby restricts the operation amount of the joystick lever to within a predetermined range. The biasing component biases the first input member to a neutral position where the amount of displacement of the first input member matches the amount of displacement of the second input member. The difference between the amount of displacement of the first input member and the amount of displacement of the second input member corresponds to the angular difference. The joystick lever is operated against the biasing force of the biasing component.

Consequently, after the joystick lever is operated, the steering angle changes to follow the joystick lever, and the control valve goes into the neutral position when the operation amount of the joystick lever matches the steering angle.

Also, the control valve is thus provided with the biasing component, and the operator operates the joystick lever with an operating force that goes against the biasing force of the biasing component. The operating force required to operate the joystick lever can be increased by decreasing the assist force or increasing the counterforce for operation against this biasing force before there is restriction.

The work vehicle pertaining to the fourth aspect is the work vehicle pertaining to the third aspect, wherein the restrictor has a striking member and a struck member. The striking member is linked to the first input member and is displaced according to the displacement of the first input member. The struck member is formed on a member connected to the second input member and is struck by the striking member. When the difference in the amount of displacement of the first input member with respect to the second input member reaches a value corresponding to the first predetermined angle difference, the striking member strikes the struck member and displacement of the first input member with respect to the second input member is restricted.

Consequently, the difference in the amount of displacement of the first input member with respect to the second input member is restricted to within the range of the predetermined angular difference.

The work vehicle pertaining to the fifth aspect is the work vehicle pertaining to the second aspect, further comprising a torque sensor. The torque sensor senses the torque produced by operation of the joystick lever. The controller controls the force imparting component on the basis of the torque sensor.

Consequently, a force can be applied according to the torque applied by the operator to the joystick lever. For example, the assist force imparted by the force imparting component can be increased when the torque applied to the joystick lever by the operator is high, and the assist force can be decreased when the torque is low.

The work vehicle pertaining to the sixth aspect is the work vehicle pertaining to the fifth aspect, wherein the controller has a calculator and an operation controller. The calculator calculates the force to be imparted to the operation of the joystick lever by combining an imparted force preset for the sensed torque with a counterforce preset for the angular difference. The operation controller actuates the force imparting component so as to impart the calculated force.

Consequently, the operating force required to operate the joystick lever can be increased before the operation of the joystick lever is restricted, while an assist force or a counterforce is imparted by the force imparting component to the operation of the joystick lever.

The work vehicle pertaining to the seventh aspect is the work vehicle pertaining to the sixth aspect, further comprising a speed sensor. The speed sensor senses the speed of the work vehicle. The calculator calculates the force to be imparted to the operation of the joystick lever by changing the imparted force preset for the torque on the basis of the sensed speed, and combining this changed imparted force with a counterforce preset for the angular difference.

Consequently, the force imparted to the operation of the joystick lever by the force imparting component according to torque can also be changed according to the speed of the vehicle, and furthermore, the operating force required to operate the joystick lever can be increased before the operation of the joystick lever is restricted.

The work vehicle pertaining to the eighth aspect is the work vehicle pertaining to the first aspect, further comprising a link. The link links the joystick lever to the control valve. The force imparting component has an electric motor and a transmission mechanism. The electric motor generates the assist force or the counterforce. The transmission mechanism transmits the assist force or the counterforce produced by the electric motor to the link.

Consequently, the force of the electric motor can be transmitted to the link that links the joystick lever to the control valve, and the force required to operate the joystick lever can be changed.

The method for controlling a work vehicle pertaining to the ninth aspect is a method for controlling an articulated work vehicle in which a front frame and a rear frame are linked, said method comprising a load increasing step. This load increasing step involves decreasing the assist force or increasing the counterforce to be imparted to the operation of the joystick lever before the operation of the joystick lever is restricted, with a control valve that controls the flow of fluid supplied to a hydraulic actuator that changes the steering angle of the front frame with respect to the rear frame according to the operation amount of the joystick lever.

Consequently, the operating force required to operate the joystick lever increases before the operation of the joystick lever is restricted. That is, since the tactile sensation gradually becomes heavier before the joystick lever is restricted, the operation speed slows down as the restricted position is approached. Accordingly, the sudden recoil caused by displacement restriction of the valve can be mitigated.

Effects of the Invention

The present invention provides a work vehicle and a method for controlling a work vehicle with which sudden recoil caused by displacement restriction of a valve can be mitigated.

DETAILED DESCRIPTION OF EMBODIMENT(S)

A wheel loader in an embodiment pertaining to the present invention will now be described through reference to the drawings.

1-1. Overview of Wheel Loader Configuration

FIG. 1is a simplified diagram of the configuration of a wheel loader1in this embodiment. The wheel loader1in this embodiment comprises a body frame2, a work implement3, a pair of front tires4, a cab5, an engine compartment6, a pair of rear tires7, and a steering operating device8(seeFIG. 2, discussed below).

The wheel loader1performs earth loading and other such work with the work implement3.

The body frame2is what is known as an articulated type, and has a front frame11, a rear frame12, and a linking shaft13. The front frame11is disposed in front of the rear frame12. The linking shaft13is provided in the center of the vehicle width direction, and pivotably links the front frame11to the rear frame12. The front tires4are attached on the left and right sides of the front frame11. The rear tires7are attached on the left and right sides of the rear frame12.

The work implement3is driven by hydraulic fluid from a work implement pump (not shown). The work implement3has a boom14, a bucket15, a lift cylinder16, and a bucket cylinder17. The boom14is mounted on the front frame11. The bucket15is attached to the distal end of the boom14.

The lift cylinder16and the bucket cylinder17are hydraulic cylinders. One end of the lift cylinder16is attached to the front frame11, and the other end of the lift cylinder16is attached to the boom14. The lift cylinder16telescopes in and out to pivot the boom14up and down. One end of the bucket cylinder17is attached to the front frame11, and the other end of the bucket cylinder17is attached to the bucket15via a bell crank18. The bucket cylinder17telescopes in and out to pivot the bucket15up and down.

The cab5is mounted on the rear frame12, inside of which are disposed a steering wheel or joystick lever24(discussed below; seeFIG. 2) for steering, a lever for controlling the work implement3, various display devices, and so forth. The engine compartment6is disposed on the rear frame12to the rear of the cab5, and houses an engine.

The steering operating device8will be discussed in detail below, but has steering cylinders21and22. The amount of fluid supplied to the steering cylinders21and22is varied to change the steering angle of the front frame11with respect to the rear frame12and to change the travel direction of the wheel loader1.

1-2. Steering Operation Device

FIG. 2is a hydraulic circuit diagram of the configuration of the steering operation device8. The steering operation device8in this embodiment mainly has a pair of steering cylinders21and22, a steering hydraulic circuit23, a joystick lever24, a link25, a linking mechanism26, a force imparting component27, and a controller28.

The steering cylinders21and22are driven hydraulically. The steering cylinders21and22are disposed side by side on the left and right sides in the vehicle width direction, flanking a linking shaft13. The steering cylinder21is disposed on the left side of the linking shaft13(seeFIG. 1). The steering cylinder22is disposed on the right side of the linking shaft13. The steering cylinders21and22are attached at one end to the front frame11, and at the other end to the rear frame12.

The steering cylinder21is provided with an extension port21aand a contraction port21b, and the steering cylinder22is provided with an extension port22aand a contraction port22b.

When fluid is supplied to the extension port21aof the steering cylinder21and the contraction port22bof the steering cylinder22and fluid is discharged from the contraction port21bof the steering cylinder21and the extension port22aof the steering cylinder22, the steering cylinder21extends and the steering cylinder22contracts. As a result, the steering angle θs changes and the vehicle turns to the right. When fluid is supplied to the contraction port21bof the steering cylinder21and the extension port22aof the steering cylinder22and fluid is discharged from the extension port21aof the steering cylinder21and the contraction port22bof the steering cylinder22, the steering cylinder21contracts and the steering cylinder22extends. As a result, the steering angle θs changes and the vehicle turns to the left.

A steering angle sensor104for detecting a steering angle θs is provided near the linking shaft13disposed arranged between the steering cylinders21and22. The steering angle sensor104is constituted by a potentiometer, for example, and the sensed steering angle θs is sent to the controller28as a sensing signal.

The steering cylinder21is provided with a cylinder stroke sensor106for detecting the stroke of the cylinder, and the steering cylinder22is provided with a cylinder stroke sensor107for detecting the stroke of the cylinder. Sensing values from these cylinder stroke sensors106and107may be sent to the controller28to find the steering angle θs.

1-2-2. Steering Hydraulic Circuit

The steering hydraulic circuit23is a hydraulic circuit for adjusting the flow of fluid supplied to the steering cylinders21and22. The steering hydraulic circuit23has a main hydraulic circuit30and a pilot hydraulic circuit40.

(a) Main Hydraulic Path

The main hydraulic circuit30is a circuit that supplies fluid from a main hydraulic pressure source31to the steering cylinders21and22, and has a steering valve32. The main hydraulic pressure source31is made up of a hydraulic pump, a relief valve, and the like.

The steering valve32is a flow control valve that adjusts the flow of the fluid supplied to the steering cylinders21and22according to the inputted pilot pressure. The steering valve32has a main pump port P1, a main drain port P2, a first steering port P3, and a second steering port P4. The main pump port P1is connected to the main hydraulic pressure source31via a main hydraulic line36. The main drain port P2is connected to a drain tank DT that collects fluid via the main drain line37. The first steering port P3is connected to the contraction port21bof the steering cylinder21and the extension port22aof the steering cylinder22via a first steering line38. The second steering port P4is connected to the extension port21aof the steering cylinder21and the contraction port22bof the steering cylinder22via a second steering line39.

Also, the steering valve32has a valve body33that can move between a neutral position Ns, a left steering position Ls, and a right steering position Rs. When the valve body33is in the neutral position Ns, the main pump port P1communicates with the main drain port P2. In this case, the first steering port P3and the second steering port P4are not in communication. When the valve body33is in the left steering position Ls, the main pump port P1communicates with the first steering port P3, and the main drain port P2communicates with the second steering port P4. When the valve body33is in the right steering position Rs, the main pump port P1communicates with the second steering port P4, and the main drain port P2communicates with the first steering port P3.

The steering valve32has a first pilot chamber34and a second pilot chamber35. In a state in which no pilot pressure is supplied to the first pilot chamber34or the second pilot chamber35, and the same pilot pressure is supplied to the first pilot chamber34and the second pilot chamber35, the valve body33is in the neutral position Ns. In a state in which the pilot pressure is supplied only to the first pilot chamber34, the valve body33is located in the left steering position Ls. In a state in which the pilot pressure is supplied only to the second pilot chamber35, the valve body33is located in the right steering position Rs. When the valve body33is located in the left steering position Ls and the right steering position Rs, the steering valve32changes the opening surface area through which the fluid from the main hydraulic pressure source31passes according to the supplied pilot pressure. Consequently, the steering valve32controls the flow of fluid supplied to the steering cylinder21or the steering cylinder22according to the pilot pressure.

(b) Pilot Hydraulic Circuit

The pilot hydraulic circuit40is a circuit for supplying the fluid from the pilot hydraulic pressure source43to the first pilot chamber34and the second pilot chamber35of the steering valve32.

The pilot hydraulic circuit40has a variable pressure reducer41and a pilot valve42.

(i) Variable Pressure Reducer

The variable pressure reducer41reduces and adjusts the hydraulic pressure sent from the pilot hydraulic pressure source43to the pilot valve42. The variable pressure reducer41incorporates an electromagnetic pressure reducing valve, and receives a command signal from the controller28to control the hydraulic pressure.

The pilot valve42is a rotary valve that adjusts the pilot pressure inputted from the pilot hydraulic pressure source43to the steering valve32.

Overview of Pilot Valve Configuration

The rotary pilot valve42has a pilot pump port P5, a pilot drain port P6, a first pilot port P7, and a second pilot port P8. The pilot pump port P5is connected to the variable pressure reducer41via a pilot hydraulic line44, and the variable pressure reducer41is connected to the pilot hydraulic pressure source43. The pilot drain port P6is connected to the drain tank DT for recovering fluid via a pilot drain line45. The first pilot port P7is connected to the first pilot chamber34of the steering valve32via a first pilot line46. The second pilot port P8is connected to the second pilot chamber35of the steering valve32via a second pilot line47.

The pilot valve42has a valve body component60that includes an operation spool71and an operation sleeve72. With the operation sleeve72as a reference, the operation spool71can move between a neutral position Np, a left pilot position Lp, and a right pilot position Rp.

When the operation spool71is in the neutral position Np with respect to the operation sleeve72, the pilot pump port P5, the pilot drain port P6, the first pilot port P7, and the second pilot port P8communicate with each other. When the operation spool71in the left pilot position Lp with respect to the operation sleeve72, the pilot pump port P5communicates with the first pilot port P7, and the pilot drain port P6communicates with the second pilot port P8. Also, when the operation spool71is in the right pilot position Rp with respect to the operation sleeve72, the pilot pump port P5communicates with the second pilot port P8, and the pilot drain port P6communicates with the first pilot port P7.

FIG. 3is a cross section of the configuration of the pilot valve42.

The pilot valve42mainly has the valve body component60, an operation input shaft61, a feedback input shaft62, a housing63, a first spring64, a second spring65, and a feedback component66.

Operation Input Shaft

The operation input shaft61is provided so as to be rotatable around its center axis O, and is inserted into the housing63. The operation input shaft61is linked to the joystick lever24(discussed below) via the link25. The operation input shaft61rotates at the same rotational angle as the rotational angle θin to the left and right of the joystick lever24.

Feedback Input Shaft

The feedback input shaft62is disposed coaxially with the operation input shaft61, and is provided so as to be rotatable around the center axis O. The feedback input shaft62is inserted into the housing63so as to be opposite the operation input shaft61. The feedback input shaft62is linked to the front frame11via a linking mechanism26(discussed below) and rotates at the same rotational angle as the steering angle θs of the front frame11with respect to the rear frame12.

Housing

A substantially cylindrical space is formed in the housing63, and the operation input shaft61and the feedback input shaft62are inserted as mentioned above. The housing63accommodates the valve body component60and the feedback component66, and the pilot pump port P5, the pilot drain port P6, the first pilot port P7, and the second pilot port P8are formed.

Valve Body Component

The valve body component60has the operation spool71and the operation sleeve72, and moves between the neutral position Np, the left pilot position Lp, and the right pilot position Rp when the operation spool71rotates with respect to the operation sleeve72.

The operation spool71is substantially cylindrical in shape and disposed coaxially with the operation input shaft61, and is connected to the operation input shaft61. The joystick lever24is connected to the operation input shaft61via the link25(discussed below). When the operator operates the joystick lever24to the right side by the rotational angle θin, the operation input shaft61and the operation spool71also rotate to the right around the center axis O by the rotational angle θin. Slits71aand71bare formed in the operation spool71near the operation input shaft61along the peripheral direction at two positions opposite each other so as to sandwich the center axis O in between.

The operation sleeve72has a substantially cylindrical shape and is disposed on the outside of the operation spool71and inside the housing63so as to be rotatable with respect to the operation spool71and the housing63.

In this Specification, the terms right rotation and left rotation indicate the rotation direction when viewed from above.

First Spring

The first spring64is inserted between the operation spool71and the operation sleeve72, which are rotatable with respect to each other, and generates a counterforce corresponding to the difference in rotational angle between the two.

FIG. 4Ais a cross section along the AA′ line perpendicular to the center axis O. As shown inFIG. 4A, rectangular holes71cand71dare provided to the operation spool71on diametrically opposed walls. Rectangular grooves72cand72dare formed in the diametrically opposed walls at the end of the operation sleeve72on the operation input shaft61side. The first spring64is formed by two leaf spring units64ain which a plurality of convex leaf springs are stacked. The two leaf spring units64aare disposed so that their convex parts are opposite each other so as to form an X shape as inFIG. 4A. The two leaf spring units64ago through the holes71cand71din the operation spool71, and both ends thereof go into the grooves72cand72dof the operation sleeve72. The operation spool71and the operation sleeve72are thus linked by the first spring64.

As shown inFIG. 4A, a state in which the positions of the hole71cand the groove72cin the peripheral direction substantially coincide, and the positions of the hole71dand the groove72din the peripheral direction substantially coincide, is a state in which the valve body component60is in the neutral position Np.

Also, when the joystick lever24is operated, the operation spool71rotates with respect to the operation sleeve72as shown inFIG. 4B, and the operation spool71moves with respect to the operation sleeve72to the left pilot position Lp or the right pilot position Rp. When the joystick lever24is rotated to the right, the operation spool71rotates to the right with respect to the operation sleeve72and moves to the right pilot position Rp. When the joystick lever24is rotated to the left, the operation spool71rotates to the left with respect to the operation sleeve72and moves to the left pilot position Lp.

In this movement, since the operator moves the joystick lever24against the spring force of the first spring64, a lever counterforce is generated in the joystick lever24. In other words, the first spring64biases the operation spool71to the neutral position Np with respect to the operation sleeve72.

Feedback Component

Meanwhile, the feedback component66feeds back the steering angle θs of the front frame11with respect to the rear frame12to the valve body component60. The feedback component66mainly has a feedback spool73, a feedback sleeve74, a drive shaft75, a first center pin76, and a restrictor78.

The drive shaft75is disposed between the operation input shaft61and the feedback input shaft62, coaxially with the operation input shaft61and the feedback input shaft62(center axis O). The drive shaft75is disposed inside the operation spool71. The first center pin76is disposed perpendicular to the center axis O at the end of the drive shaft75on the operation input shaft61side. Both ends of the first center pin76go through the slits71aand71band are fixed to the operation sleeve72. As will be described in detail below, the first center pin76and the slits71aand71brestrict the rotational angle of the operation spool71with respect to the operation sleeve72to an angle within a predetermined range. Since the first center pin76is fixed to the operation sleeve72and the drive shaft75, the operation sleeve72that is integrated with the drive shaft75also rotates when the drive shaft75is rotated.

The feedback spool73has a substantially cylindrical shape and is disposed coaxially with the feedback input shaft62, and is linked to the feedback input shaft62. Slits73aand73bare formed near the feedback input shaft62of the feedback spool73along the peripheral direction at two locations that are opposite each other and sandwich the central axis O in between. The drive shaft75is disposed inside the feedback spool73. The feedback input shaft62is linked to the front frame11via the linking mechanism26(discussed below), and when the front frame11rotates to right by the steering angle θs with respect to the rear frame12, the feedback input shaft62and the feedback spool73also rotate to the right by the same rotational angle θs as the steering angle θs.

The feedback sleeve74is substantially cylindrical in shape, and is disposed outside of the feedback spool73and inside the housing63, rotatably with respect to the feedback spool73and the housing63.

The restrictor78restricts the rotation of the feedback sleeve74with respect to the feedback spool73to an angle within a predetermined range. The restrictor78is made up of a second center pin77and walls73aeand73be(discussed below; seeFIG. 7) at both ends in the peripheral direction of the slits73aand73b.

The second center pin77is disposed perpendicular to the center axis O, at the end of the drive shaft75on the feedback input shaft62side. Both ends of the second center pin77are fixed to the feedback sleeve74through the slits73aand73b. The second center pin77and the slits73aand73brestrict the rotation of the feedback sleeve74with respect to the feedback spool73to an angle within a predetermined range. Also, since the second center pin77is fixed to the feedback sleeve74and the drive shaft75, when the feedback sleeve74rotates, the drive shaft75that is integrated with the feedback sleeve74also rotates. The rotation of the drive shaft75causes the operation sleeve72that is fixed to the drive shaft75by the first center pin76to rotate.

Second Spring

The second spring65is inserted between the feedback spool73and the feedback sleeve74, which are able to rotate relative to each other, and generates a counterforce corresponding to the rotational difference between the two.FIG. 4Cis cross section along the BB′ line inFIG. 3.

As shown inFIG. 4C, square holes73cand73dare provided to the diametrically opposed walls of the feedback spool73.

Also, rectangular grooves74cand74dare formed in the diametrically opposed walls at the end of the feedback sleeve74on the feedback input shaft62side. The second spring65is formed from two leaf spring units65ain which a plurality of convex leaf springs are stacked. The two leaf spring units65aare disposed so that their convex parts are opposite each other so as to form an X shape as inFIG. 4C. The two leaf spring units65ago through the holes73cand73din the feedback spool73, and both ends thereof go into the grooves74cand74dof the feedback sleeve74. The feedback spool73and the feedback sleeve74are thus linked by the second spring65. In the state inFIG. 4C, the hole73cand the groove74ccoincide in the peripheral direction, and the hole73dand the groove74dcoincide in the peripheral direction. The feedback sleeve74is biased by the second spring65so that the positions of the grooves74cand74din the peripheral direction match the positions of the holes73cand73dof the feedback spool73in the peripheral direction.

The first spring64bends until the operation spool71is restricted with respect to the operation sleeve72, but the second spring65is set so that it begins to bend when subjected to a force that is greater than the counterforce produced by the first spring64until the operation spool71is restricted.

As described later in below through reference toFIG. 7, when the operation spool71rotates with respect to the operation sleeve72up to the angle at which the operation spool71is restricted, and the joystick lever24is then operated, as shown inFIG. 4D, the second spring65bends and the feedback sleeve74rotates with respect to the feedback spool73.FIG. 4Dis a cross section along the BB′ line inFIG. 3, and since the view is from below, the arrow indicating the rotational direction is reversed from that inFIG. 4B.

That is, when the joystick lever24is operated past the angle at which the operation spool71is restricted with respect to the operation sleeve72, the operator must operate the joystick lever24against the biasing force of the second spring65.

With the above configuration of the feedback unit66, when the feedback input shaft62rotates in accordance with a change in the steering angle, the feedback spool73rotates, and the feedback sleeve74that is linked to the feedback spool73via the second spring65also rotates. Then, the operation sleeve72, which is fixed to the feedback sleeve74via the second center pin77, the drive shaft75, and the first center pin76, also rotates, which produces a change in the difference in rotational angle between the operation spool71and the operation sleeve72and changes the pilot pressure.

That is, with the pilot valve42, the position of the operating spool71with respect to the operation sleeve72moves to the neutral position Np, the left pilot position Lp, or the right pilot positions Rp, according to the difference α between the rotational angle θin of the operation input shaft61and the rotational angle θfb (matches the steering angle θs) of the feedback input shaft62. When the rotational angle difference α is zero, the operation spool71is in the neutral position Np with respect to the operation sleeve72. Also, when the operation spool71is in the left pilot position Lp or the right pilot positions Rp with respect to the operation sleeve72, the pilot valve42changes the opening surface area through which fluid from the pilot hydraulic source43passes, according to the rotational angle difference α. Consequently, the pilot pressure sent from the pilot valve42to the steering valve32is adjusted according to the rotational angle difference α.

A first rotational angle sensor101, constituted by a rotary sensor, for example, is provided to the input shaft61. The first rotational angle sensor101senses the rotational angle θin of the operation input shaft61. A second rotational angle sensor102, constituted by a rotary sensor, for example, is provided to the feedback input shaft62. The second rotational angle sensor102senses the rotational angle θfb (=θs) of the feedback input shaft62. The rotational angles θin and θfb sensed by the first rotational angle sensor101and the second rotational angle sensor102are sent as sensing signals to the controller28.

As discussed above, the steering angle θs at the linking shaft13is also sensed by a steering angle sensor104, but since the rotational angle θfb of the feedback input shaft62matches the steering angle θs, the steering angle sensor104may be omitted.

FIG. 5is a side view of the configuration inside the cab5. An operator's seat5ain which the operator sits is provided inside the cab5. A steering box80is disposed on the left side in the vehicle width direction of the operator's seat5a.

The joystick lever24is disposed protruding obliquely upward toward the front from the steering box80.

The link25links the joystick lever24and the pilot valve42. The link25mainly has a steering operation shaft81, a linking bar82, and a universal joint83.

The steering operation shaft81is disposed vertically, and is supported rotatably around its center axis E by the steering box80. The linking bar82is disposed inside the steering box80, and links the joystick lever24to the steering operation shaft81.

More precisely, the steering operation shaft81is made up of a lever-side shaft81a, an input shaft81b, and a valve-side shaft81cthat are connected in that order (seeFIG. 8discussed below). That is, one end of the lever-side shaft81ais linked to the linking bar82, and the other end of the lever-side shaft81ais linked to one end of the input shaft81b. The other end of the input shaft81bis connected to one end of the valve-side shaft81c, and the other end of the valve-side shaft81cis connected to the universal joint83. An assist force or a counterforce from the force imparting component27(discussed below) is inputted to the input shaft81b.

The universal joint83links the steering operation shaft81to the operation input shaft61of the pilot valve42disposed near the operator's seat5a. The universal joint83has a telescoping center portion83aand joint portions83band83cdisposed at both ends of the center portion83a. The joint portion83bis linked to the steering operation shaft81. The joint portion83cis linked to the operation input shaft61.

FIG. 6is a plan view of the area near the joystick lever24as seen from above. As shown inFIG. 6, the joystick lever24is formed protruding obliquely upward from an arc-shaped hole84formed in the upper face of the steering box80. The joystick lever24is capable of turning horizontally around the steering operation shaft81(more precisely, the center axis E). Also, the edge of the right end of the hole84of the steering box80is marked with an R, and the edge of the left end is marked with an L.

For example, as shown inFIG. 6, when the operator rotates joystick lever24by the rotational angle θin to the right from the center position, the steering operation shaft81also rotates to the right by the rotational angle θin. This rotation of the steering operation shaft81by the rotational angle θin is transmitted through the universal joint83to the operation input shaft61, and the operation input shaft61also rotates to the right by the rotational angle θin. The same applies when the joystick lever24is rotated to the left.

The linking mechanism26has a follow-up lever91, a follow-up link92, and a bracket93. The follow-up link92is fixed to the feedback input shaft62of the pilot valve42. The bracket93is fixed to the front frame11. The follow-up link92is linked to the follow-up lever91and the bracket93.

This linking mechanism26links the front frame11to the pilot valve42disposed on the rear frame12.

The linking mechanism26makes the steering angle θs of the front frame11with respect to the rear frame12be the same as the rotational angle θfb of the feedback input shaft62.

That is, when the front frame11rotates to the right side around the linking shaft13with respect to the rear frame12by the steering angle θs, the feedback input shaft62also rotates right by the rotational angle θs via the linking mechanism26, and when the front frame11rotates to the left side by the steering angle θs, the feedback input shaft62also rotates left by the rotational angle θs via the linking mechanism26.

The lever counterforce produced by the first spring64and the second spring65when the joystick lever24is operated will now be described.

FIG. 7Ais a simplified diagram of the pilot valve42.FIG. 7Bis a graph of the relation between lever counterforce and the body-lever angular deviation. The body-lever angular deviation α is the difference (θin −θfb) between the rotational angle θin of the joystick lever24and the steering angle θs of the front frame11with respect to the rear frame12.FIG. 7Cis a cross section along the CC′, DD′, EE′, and FF′ lines inFIG. 7Awhen the angular deviation α is zero.FIG. 7Dis a cross section along the CC′, DD′, EE′, and FF′ lines inFIG. 7Awhen the angular deviation α is θ2, andFIG. 7Eis a cross section along the CC′, DD′, EE′, and FF lines inFIG. 7Awhen the angular deviation α is θ3. As shown inFIG. 7A, the cross sections along the CC′, DD′, EE′, and FF′ lines are all as seen from above. InFIG. 7B, play in the joystick lever24is not taken into account in order to make the illustration easier to understand.

When the operator rotates the joystick lever24by the rotational angle θin from the center position, the operation input shaft61also rotates by the rotational angle θin. Meanwhile, since the response of the steering cylinders21and22is delayed, the steering angle θs increases gradually in accordance with the rotational angle θin. The rotational angle θin of the joystick lever24represents the target steering angle, while the steering angle θs indicates the actual steering angle. The feedback input shaft62also rotates by the same rotational angle θs in response to a change in the steering angle θs. The feedback spool73also rotates together with the feedback input shaft62, and this rotation causes the feedback sleeve74linked via the second spring65to rotate as well.

Since the feedback sleeve74and the operation sleeve72are integrated with the first center pin76, the second center pin77, and the drive shaft75, rotation of the feedback sleeve74causes the operation sleeve72to rotate as well.

Specifically, the difference between the rotational angle of the operation spool71and the rotational angle of the operation sleeve72corresponds to the angular deviation α (seeFIG. 4B).

Since the first spring64biases the operation spool71to the neutral position Np with respect to the operation sleeve72, the joystick lever24must be operated against the biasing force of the first spring64in order to increase the angular deviation α.

The first spring64has the spring property S1shown inFIG. 7B. With the spring property S1of the first spring64, the joystick lever24must be operated with a force at or above an initial counterforce F1(the force required to begin to bend the first spring64) in order to rotate the operation input shaft61. Also, with the spring property S1of the first spring64, the lever counterforce increases in proportion to the angular deviation α. That is, as the angular deviation α increases, the force required to operate the joystick lever24increases.

As shown inFIG. 7C, in the neutral position Np where the angular deviation α is zero, the first center pin76is disposed in the center of the slits71aand71bof the operation the spool71. The second center pin77is disposed in the center of the slits73aand73bof the feedback spool73.

The joystick lever24is then rotated to the right side, for example, to increase the angular deviation α, and when the angular deviation α reaches the angle θ2, as shown inFIG. 7D, the first center pin76hits the wall71aeformed in the peripheral direction of the slit71a, and the wall71beformed in the peripheral direction of the slit71b. At this point the second center pin77is disposed in the center of the slits73aand73bof the feedback spool73. This is because if we let F2be the counterforce produced by the first spring64when the angular deviation α is the angle θ2, the initial counterforce (the force needed to start bending the second spring65) is set to F2as indicated by the spring property S2of the second spring65. The initial counterforce of the second spring65may be set higher than F2, or may be greater than or equal to F2.

Furthermore, the operator must operate the joystick lever24against the counterforce of the second spring65to rotate it to the right side. That is, when the joystick lever24is further rotated to the right side, since the first center pin76is hitting the walls71aeand71be, it is necessary to rotate the operation sleeve72if an attempt is made to rotate the operation spool71. Also, as discussed above, the operation sleeve72is integrated with the feedback sleeve74, and the feedback spool73is connected to the feedback input shaft62. Therefore, when the joystick lever24is further rotated to the right side, the operator operates against the counterforce of the second spring65, as shown inFIG. 4D.

When the angular deviation α reaches θ3, as shown inFIG. 7E, the second center pin77hits the wall73aeformed in the peripheral direction of the slit73aand the wall73beformed in the peripheral direction of the slit73b. Thus, the second center pin77is able to rotate by an angle (θ3−θ2). That is, the pilot valve42is configured so that the angular deviation α will not exceed the angle θ3. Therefore, as shown inFIG. 7B, the lever counterforce goes straight up at the angle θ3. If the second center pin77strikes the walls73aeand73bewith sufficient energy, a sharp rebound will be generated to put a burden on the operator's wrist. This angle θ3is also referred to as the catch-up angle.

InFIG. 7B, an example was illustrated in which the joystick lever24was rotated to the right side, but the same applies when the rotation is to the left side, in which case the angular deviation α becomes a negative value, in left and right symmetry as indicated by the two-dot chain line L7shown inFIG. 10B(discussed below). That is, the first center pin76hits the walls71aeand71beat an angle of −θ2, and the second center pin77hits the walls73aeand73beat −θ3. Thus, the pilot valve42is configured so that the absolute value of the angular deviation α will not exceed the angle θ3.

Until the angular deviation α reaches θ2, there will be a difference between the rotational angle of the operation spool71and the rotational angle of the operation sleeve72, but once the angle θ2 is exceeded, there is no longer any difference between the rotational angles of the operation spool71and the operation sleeve72, so the aperture of the pilot valve42stays constant. Also, while the aperture of the pilot valve42remains constant when the angular deviation α is between the angles θ2 and θ3, the pilot pressure should be varied according to the angular deviation by controlling the variable pressure reducer41.

1-2-6. Force Imparting Component

FIG. 8is an oblique view of the force imparting component27. The force imparting component27imparts an assist force or counterforce to the operation of the joystick lever24. The force imparting component27has an electric motor111and a worm gear112. The worm gear112has a cylindrical worm112aand a worm wheel112b. The worm wheel112bis provided around the above-mentioned input shaft81b, and meshes with the cylindrical worm112a. The output shaft of the electric motor111is connected to the cylindrical worm112a, and rotates the cylindrical worm112aaround its center axis. The electric motor111is driven on the basis of a command from a drive circuit204provided to the controller28.

The first end81b1of the input shaft81bis connected to the lever-side shaft81a, and the second end81b2is connected to the valve-side shaft81c.

When the electric motor111is driven, the cylindrical worm112arotates, this rotation causes the worm wheel112bto rotate, and rotational force is also produced at the input shaft81bthat is fixed to the worm wheel112b. Rotational force can be applied for left rotation or right rotation to the input shaft81bby changing the direction of rotation of the cylindrical worm112a.

For example, when the joystick lever24is rotated to the right, an assist force is imparted to the operation of the joystick lever24by applying a force in the right rotation direction to the input shaft81b. Also, when the joystick lever24is rotated to the right, a counterforce is imparted to the operation of the joystick lever24by applying a force in the left rotation direction to the input shaft81b.

A torque sensor103is provided to the input shaft81b. The torque sensor103senses the torque generated at the input shaft81bexerted on the joystick lever24by the operator. The torque sensor103in this embodiment, for example, senses the torque generated at the input shaft81band the rotation direction of the input shaft81bby sensing the twisting of a torsion bar with a coil, for example. The sensed rotation direction and torque T are outputted to the controller28as a steering torque signal.

FIG. 9is a block diagram of the configuration of the controller28. As shown inFIG. 9, the controller28includes a storage unit200, a first assist torque deciding component201, a second assist torque deciding component202, a calculator203, and a drive circuit204. The first assist torque deciding component201, the second assist torque deciding component202, and the calculator203are executed by a CPU or another such computing device.

The storage unit200stores the relation of the imparted assist torque with respect to lever input torque (first assist torque information) for each speed. The storage unit200also stores the relation of imparted assist torque with respect to the deviation angle α (second assist torque information). The first assist torque information and the second assist torque information are preset. The first assist torque information and the second assist torque information will be discussed in detail below. The storage unit200may be provided inside the controller28, or may be provided outside the controller28. Also, the storage unit200is made up of a RAM, a ROM, an HDD, or the like.

The first assist torque deciding component201decides the first assist torque from the steering torque signal from the torque sensor103and the speed signal from the vehicle speed sensor105on the basis of the first assist torque information stored in the storage unit200.

The second assist torque deciding component202calculates the difference between the rotational angle θin sensed by the first rotational angle sensor101and the rotational angle θfb (=θs) sensed by the second rotational angle sensor102, and calculates the deviation angle α (θin−θfb). The second assist torque deciding component202decides the second assist torque from the deviation angle α on the basis of the second assist torque information stored in the storage unit200.

The calculator203calculates the sum of the first assist torque decided by the first assist torque deciding component201and the second assist torque decided by the second assist torque deciding component202, and calculates the target assist torque to be imparted to the input shaft81b.

The drive circuit204drives the electric motor111on the basis of the calculated target assist torque.

Thus, the controller28can impart an assist force or counterforce to the operation of the joystick lever24by the operator on the basis of the torque T, the deviation angle α, and the speed V. The controller28also controls the variable pressure reducer41as shown inFIG. 2on the basis of the rotational angle θin, the rotational angle θfb (=θs), and the vehicle speed V. This allows the source pressure of the pilot pressure sent to the pilot valve42to be controlled so that the flow of fluid to the left and right steering cylinders21and22does not change abruptly.

Also, control of the electric motor111and the variable pressure reducer41by the controller28may be performed by wire or wirelessly.

The steering operation with the wheel loader1in this embodiment will now be described.

2-1. Steering Operation

If the joystick lever24is in the center position, the operation input shaft61is located in a predetermined initial position, and the rotational angle θin produced by the operation input shaft61is zero. Also, since the steering angle θs is zero, the feedback input shaft62is also located in a predetermined initial position. In this embodiment, as shown inFIG. 7A, the steering angle θs indicates the angle from a state in which the angle along the longitudinal direction with respect to the rear frame12is zero. As shown inFIG. 6, the rotational angle θin indicates the rotational angle from the center position of the joystick lever24. Also, in finding the angular deviation, computation may be performed using a positive angle for rotation to the right and a negative angle for rotation to the left, for example.

At this point, the operation spool71is located in the neutral position Np shown inFIG. 4Awith respect to the operating sleeve72. In this case, the pilot pressure in the first pilot chamber34and in the second pilot chamber35of the steering valve32is the same, and the valve body33of the steering valve32is also in the neutral position Ns. Therefore, no fluid is supplied or discharged to or from the left and right steering cylinders21and22, the steering angle θs is maintained at zero, and the rotational angle θft) (=θs) of the feedback input shaft62is also maintained at zero.

Next, the operator exerts an operation force Fin to rotate the joystick lever24to the right side from the center position as shown inFIG. 6. When the operating force Fin exceeds F1of the first spring64, the operation input shaft61rotates to the right the same as the joystick lever24, and the rotational angle θin of the first operation input shaft61is increased. At this point, because of the delay in the response of the left and right steering cylinders21and22, the steering angle θs is still at zero, and the rotational angle θfb (=θs) of the feedback input shaft62is also zero. Therefore, the angular deviation (α=θin−θs) between the rotational angle θin and the steering angle θs increases.

The operation spool71rotates to the right with respect to the operation sleeve72together with the rotation of the operation input shaft61. Here, the operation sleeve72is integrated with the feedback sleeve74, and the feedback sleeve74is linked to the feedback spool73by the second spring65. The initial counterforce F2of the second spring65is at or above the counterforce of the spring property S1of the first spring64shown inFIG. 7B. Therefore, the operation sleeve72does not rotate along with the operating spool71, and operating the spool71rotates to the right with respect to the operation sleeve72.

Thus, the operating spool71rotates to the right with respect to the operation sleeve72and moves to the right pilot positions Rp, pilot pressure is supplied to the second pilot port P8, and the pilot pressure is supplied to the second pilot chamber35.

Thus, the valve body33of the steering valve32moves to the right steering position Rs, fluid is supplied to the extension port21aof the steering cylinder21and the contraction port22bof the steering cylinder22, and fluid is discharged from the contraction port21bof the steering cylinder21and the extension port22aof the steering cylinder22. This gradually increases the steering angle θs, and the front frame11is oriented in the right direction with respect to the rear frame12(see R inFIG. 2). This change in the steering angle θs is transmitted by the linking mechanism26to the feedback input shaft62, and the feedback input shaft62rotates at the rotational angle θs.

When the operator stops the joystick lever24at a predetermined rotational angle θ1, the operation input shaft61also stops at the rotational angle θ1. On the other hand, since the steering angle θs is gradually increasing, the rotational angle θs of the feedback input shaft62also increases. The feedback spool73also rotates along with the feedback input shaft62, and the feedback sleeve74linked via the second spring65to the feedback spool73also rotates. Since the feedback sleeve74is integrated with the operation sleeve72via the first center pin76, the second center pin77, and the drive shaft75, the operation sleeve72also rotates along with the rotation of the feedback sleeve74. Rotation of the operation sleeve72reduces the difference in the rotational angle (deflection angle α) between the operation sleeve72and the operation spool71. When the steering angle θs (the rotational angle θs of the feedback input shaft62) catches up with the rotational angle θ1 (the rotational angle θin of the operation input shaft61), the angular deviation α drops to zero. At this point, the operation spool71of the pilot valve42is located in the neutral position Np with respect to the operation sleeve72. In this case, the pilot pressure in the first pilot chamber34and the second pilot chamber35of the steering valve32is the same, and the steering valve32is also in the neutral position Ns. Therefore, no fluid is supplied or discharged to or from the left and right steering cylinders21and22, and the steering angle θs is maintained at the rotational angle θ1.

When the joystick lever24is thus rotated to the right side and stopped at a predetermined rotational angle θ1, the steering angle θs is also maintained at the same rotational angle θ1. This keeps the front frame11oriented in the direction of the rotational angle θ1, to the right with respect to the rear frame12.

When the operator then returns the joystick lever24from the right side position to the center position, the operation input shaft61similarly rotates, which reduces the rotational angle θin of the operation input shaft61. At this point, because of the delay in the response of the left and right steering cylinders21and22, the steering angle θs is still the rotational angle θ1. Therefore, the rotational angle difference α (=θin−θs) decreases from zero and becomes negative. Then, the operation spool71rotates to the left with respect to the operation sleeve72and moves to the left pilot position Lp, and pilot pressure is supplied to the first pilot port P7. Consequently, the valve body33of the steering valve32moves to the left steering position Ls, fluid is supplied to the contraction port21bof the steering cylinder21and the extension port22aof the steering cylinder22, and fluid is discharged from the extension port21aof the steering cylinder21and the contraction port22bof the steering cylinder22. This gradually reduces the steering angle θs from the rotational angle θ1. This change in the steering angle θs is transmitted by the linking mechanism26to the feedback input shaft62, and the feedback input shaft62rotates at the same change in rotational angle as the change in the steering angle θs.

When the operator stops the joystick lever24in its center position, the operation input shaft61also stops at its initial position, that is, at a position where the rotational angle θin is zero. Meanwhile, since the steering angle θs is gradually decreasing from the rotational angle θ1, the difference in rotational angle (angular deviation) a decreases gradually. When the steering angle θs reaches zero, the rotational angle θfb (=θs) of the feedback input shaft62also reaches zero, and the rotational angle difference α becomes zero. At this point, the operation spool71is disposed in the neutral position Np with respect to the operation sleeve72. In this case, the pilot pressure in the first pilot chamber34and the second pilot chamber35of the steering valve32is the same, and the steering valve32is also in the neutral position Ns. Therefore, no fluid is supplied or discharged to or from the left and right steering cylinders21and22, and the steering angle θs goes back to zero and is maintained there. Consequently, the front frame11is returned to an orientation along the longitudinal direction with respect to the rear frame12.

The situation is the same when the joystick lever24is rotated to the left side, and will therefore not be described here.

2-2. Control of Force Imparting Component

Next, control of the force imparting component27when the joystick lever24is operated as discussed above will be described.

The wheel loader1in this embodiment changes the assist torque to be imparted to the operation of the joystick lever24according to torque and speed on the basis of the first assist torque information.

Furthermore, with the wheel loader1in this embodiment, the assist torque is changed so a larger force will gradually be required to operate the joystick lever24before the operation of the joystick lever is restricted by the pilot valve42on the basis of the second assist torque information.

First, the first assist torque information and the second assist torque information will be described.

2-2-1. First Assist Torque Information

FIG. 10Ais a graph of imparted assist torque (first assist torque information) at various vehicle speeds versus input torque. InFIG. 10A, the solid line L1indicates the assist torque information at a vehicle speed of 0 km/h, the dotted line L2indicates the assist torque information at a vehicle speed of 25 km/h, and the one-dot chain line L3indicates the assist torque information at a vehicle speed of 40 km/h.

In the graph shown inFIG. 10A, a positive lever input torque indicates the torque produced by rotation of the joystick lever24to the right side, and a negative lever input torque indicates the torque produced by rotation of the joystick lever24to the left side. Also, a positive assist torque indicates when a force is applied to the input shaft81bin the right rotation direction, and a negative assist torque indicates when a force is applied to the input shaft81bin the left rotation direction.

Specifically, L1aindicates the assist torque when the joystick lever24is rotated to the right side at a vehicle speed of 0 km/h, and L1bindicates the assist torque when the joystick lever24is rotated to the left side at a vehicle speed of 0 km/h. L2aindicates the assist torque when the joystick lever24is rotated to the right side at a vehicle speed of 25 km/h, and L2bindicates the assist torque when the joystick lever24is rotated to the left side at a vehicle speed of 25 km/h. L3aindicates the assist torque when the joystick lever24is rotated to the right side at a vehicle speed of 40 km/h, and L3bindicates the assist torque when the joystick lever24is rotated to the left side at a vehicle speed of 40 km/h.

L1a, L2a, and L3ashow the case when the joystick lever24is rotated to the right side, and since the assist torque here is a positive value, a force is applied to the input shaft81bfor the right rotation. L1b, L2b, and L3bshow the case when the joystick lever24is rotated to the left side, and since the assist torque is a negative value, a force is applied to the input shaft81bfor the left rotation. An assist force is thus imparted to the operation of the joystick lever24.

Also, L1aand L1bare symmetrical with respect to the origin, L2aand L2bare symmetrical with respect to the origin, and L3aand L3bare symmetrical with respect to the origin. Therefore, the assist force with respect to the absolute value of the input torque is in left and right symmetry.

FIG. 10Bis a graph of lever counterforce versus body-lever deviation angle when the assist torque shown inFIG. 10Ais and is not imparted. InFIG. 10B, a positive deviation angle α indicates when the joystick lever24is moved to the right side, and a negative deviation angle α indicates when the joystick lever24is moved to the left side. That is, as shown inFIG. 7E, the angle θ3 indicates the angle at which the operation is restricted when the joystick lever24is rotated in the right rotation direction, and the angle −θ3 indicates the angle at which the operation is restricted when the joystick lever24is rotated in the left rotation direction. As shown inFIG. 7D, the angle θ2 indicates the angle at which the first center pin76strikes the walls71aeand71bewhen the joystick lever24is rotated in the right rotation direction, and the angle −θ2 indicates the angle at which the first center pin76strikes the walls71aeand71bewhen the joystick lever24is rotated in the left rotation direction.

The solid line L4indicates lever counterforce versus deviation angle at a vehicle speed of 0 km/h, the dotted line L5indicates lever counterforce versus deviation angle at a vehicle speed of 25 km/h, and the one-dot chain line L6indicates lever counterforce versus deviation angle at a vehicle speed of 40 km/h. Also, inFIG. 10B, the two-dot chain line L7indicates when the assist torque is not imparted. The two-dot chain line L7inFIG. 10Bshows the same state as inFIG. 7B.

As shown inFIG. 10B, L4to L7are in line symmetry with respect to the vertical axis, and in L4to L6, an assist force is imparted symmetrically to left and right operations, and the lever counterforce is smaller than when no assist torque is imparted (L7).

Also, the lever counterforce is set to increase as the speed goes up. This makes it possible to achieve both good operability at low speed and good running stability at high speed.

2-2-2. Second Assist Torque Information

The second assist torque information indicates the assist torque to be imparted to mitigate the recoil that suddenly occurs at the joystick lever24due to the restriction of the pilot valve42.FIG. 1IA is a graph of the assist torque (second assist torque information) with respect to the body-lever deviation angle (α). Again inFIG. 11A, a positive body-lever deviation angle α (=θin −θs) indicates when the joystick lever24is operated to the right side, and a negative body-lever deviation angle α indicates when the joystick lever24is operated to the left side. Also, a positive assist torque indicates when a force is applied to the input shaft81bfor the right rotation, and a negative assist torque indicates when a force is applied to the input shaft81bfor the left rotation.

With the second assist torque information shown inFIG. 11A, a counterforce is generated when the deviation angle α reaches an angle of ±θ4, and the assist torque is set so that the counterforce increases exponentially as the absolute value of the deviation angle increases.

More precisely, with the second assist torque information, when the joystick lever24is rotated to the right side and the deviation angle α reaches an angle of +θ4, the assist torque is set so as to apply a force to the input shaft81bin the left rotation direction. When the joystick lever24is rotated to the left side and the deviation angle α reaches an angle of −θ4, the assist torque is set so as to apply a force to the input shaft81bin the right rotation direction. The angle θ4 is set between the angles θ2 and θ3 shown inFIG. 11A. The angle −θ4 is set between −θ2 and −θ3.

FIG. 11Bis a graph of lever counterforce versus body-lever deviation angle when assist torque is and is not imparted on the basis of the second assist torque information shown inFIG. 11A. The solid line L8indicates when the assist torque is imparted, and the dotted line L9indicates when the assist torque is not imparted.

As shown inFIG. 11B, when the deviation angle α reaches the angle ±θ4 and the absolute value of the angle becomes large, the lever counterforce increases exponentially.

Because the counterforce in thus exponentially increased, the operation of the joystick lever24becomes heavier as the second center pin77approaches the walls73aeand73be, so the second center pin77does not strike the walls73aeand73bewith as much momentum.

2-2-3. Control Operation

FIG. 12is a flowchart of the control operation of the force application unit27.

When the joystick lever24is operated, in step S110the second assist torque deciding component202of the controller28acquires the rotational angle θin of the operation input shaft61from the first rotational angle sensor101, and acquires the rotational angle θfb (=θs) of the feedback input shaft62from the second rotational angle sensor102. The second assist torque deciding component202then calculates the deviation angle α (=θin−θs).

Next, in step S120the first assist torque deciding component201of the controller28receives a steering torque signal from the torque sensor103. The steering torque signal includes information related to the amount of torque and the rotation direction. For example, information related to the amount of torque and the rotation direction can be included in the torque value, so that if the torque has a positive value, it means the torque is produced by the right rotation of the input shaft81b, and if the torque has a negative value, it means the torque is produced by the left rotation of the input shaft81b.

Next, in step S130the controller28determines the steering direction of the joystick lever24on the basis of the steering torque signal. The rotation direction of the electric motor111when a force is imparted can be determined by this steering direction.

Then, in step S140the first assist torque deciding component201of the controller28acquires a signal for the vehicle speed V from the vehicle speed sensor105.

Next, in step S150the first assist torque deciding component201decides the first assist torque on the basis of the first assist torque information stored in the storage unit200.

The controller28stores the three sets of first assist torque information shown inFIG. 13(at vehicle speeds of 0 km/h, 25 km/h, and 40 km/h). If the sensed value from the vehicle speed sensor105is among the three speeds (for example, 12 km/h), the controller28calculates the assist torque at that speed by interpolation. The controller28thus decides the first assist torque by interpolation. The assist torque can be varied continuously according to the speed change by calculating the first assist torque by the interpolation.

Next, in step S160, the second assist torque deciding component202decides the second assist torque on the basis of the second assist torque information shown inFIG. 11Afrom the deviation angle α calculated in step S110.

Next, in step S170the calculator203combines the first assist torque and the second assist torque to calculate the target assist torque. Here, the target assist torque is a positive or negative value, and also includes information about the rotation direction. For instance, when the rotation is to the right, an assist force is imparted with the first assist torque information shown inFIG. 10A, but with the second assist torque information shown inFIG. 11A, a counterforce is imparted once the deviation angle α exceeds the angle θ4. The calculator203combines these values, and if the absolute value of the counterforce is greater than the absolute value of the assist force, the target assist torque becomes a negative value, and a force obtained by subtracting the absolute value of the assist force from the absolute value of the counterforce is imparted in the left rotation direction. On the other hand, if the absolute value of the assist force is greater than the absolute value of the counterforce, the target assist torque becomes a positive value, and a force obtained by subtracting the absolute value of the counterforce from the absolute value of the assist force is imparted in the right rotation direction.

Next, in step S180the controller28outputs a command torque signal to the drive circuit204on the basis of the decided target assist torque. Consequently, the electric motor111is driven, and a force is imparted to the operation of the joystick lever24via the link25.

The wheel loader1in this embodiment (an example of a work vehicle) is an articulated type in which the front frame11and the rear frame12are linked. The wheel loader1comprises the steering cylinders21and22(an example of a hydraulic actuator), the joystick lever24, the pilot valve42(an example of a control valve), the force imparting component27, and the controller28. The steering cylinders21and22are driven hydraulically and change the steering angle θs of the front frame11with respect to the rear frame12. The joystick lever24is operated by the operator. The pilot valve42is linked to the joystick lever24and controls the flow of fluid supplied to the steering cylinders21and22according to the deviation angle α (an example of an operation amount) produced by operation of the joystick lever, and restricts the deviation angle α produced by the joystick lever24to between −θ3 and +θ3 (an example of a predetermined range). The force imparting component27imparts an assist force or a counterforce to operation of the joystick lever by the operator. The controller28controls the force imparting component27so as to decrease the assist force or increase the counterforce before the operation of the joystick lever24is restricted by the pilot valve42.

Consequently, as shown inFIGS. 11A and 11B, the operating force required to operate the joystick lever24increases before the operation of the joystick lever24is restricted. That is, since the tactile sensation gradually becomes heavier before the joystick lever24is restricted, the operation slows down as the restricted position is approached. Therefore, the sudden recoil produced by displacement restriction of the pilot valve42can be mitigated.

With the wheel loader1in this embodiment (an example of a work vehicle), the rotational angle θin of the joystick lever24corresponds to the target steering angle θs of the front frame11with respect to the rear frame12. The wheel loader1further comprises the first rotational angle sensor101(an example of a target steering angle sensor) and the second rotational angle sensor102(an example of an actual steering angle sensor). The first rotational angle sensor101senses the rotational angle θin (an example of a target steering angle). The second rotational angle sensor102senses the rotational angle θfb (=θs) (an example of the actual steering angle). The pilot valve42restricts movement of the joystick lever24when the absolute value of the deviation angle α (an example of an angular difference), which is the difference between the rotational angle θin (an example of a target steering angle) and the steering angle θs (an example of the actual steering angle) becomes the angle θ3 (an example of a first predetermined angular difference). The controller28controls the force imparting component27so as to gradually decrease the assist force or gradually increase the counterforce toward the angle +θ3 or −θ3, on the basis of the deviation angle α.

Thus, the approach to the restricted position is detected from the deviation angle α of the rotational angle θin and the rotational angle θfb (=θs), and the operating force required to operate the joystick lever24can be increased before the operation of the joystick lever24is restricted.

With the wheel loader1in this embodiment (an example of a work vehicle), the pilot valve42(an example of a control valve) has the operation input shaft61(an example of a first input member), the feedback input shaft62(an example of a second input member), the restrictor78, the first spring64(an example of a biasing component), and the second spring65(an example of a biasing component). The operation input shaft61is linked to the joystick lever24and is displaced according to the operation of the joystick lever24. The feedback input shaft62is displaced according to the steering angle θs (an example of the actual steering angle). The restrictor78restricts the deviation angle α (an example of an operation amount) produced by the operation of the joystick lever24to an angle between −θ3 and +θ3 by restricting displacement of the operation input shaft61to an angle between −θ3 and +θ3 (an example of a predetermined range). The first spring64and the second spring65bias the operation input shaft61to the neutral position Np at which the rotational angle θin (an example of an amount of displacement) of the operation input shaft61matches the rotational angle θfb (=θs) (an example of an amount of displacement) of the feedback input shaft62. The difference between the rotational angle of the rotational angle θin of the operation input shaft61and the rotational angle θfb (=θs) of the feedback input shaft62corresponds to the deviation angle α. The joystick lever24is operated against the biasing force of the first spring64and the second spring65.

Consequently, after the joystick lever24has been operated, the actual steering angle θs changes to follow the joystick lever24, and the pilot valve42goes into the neutral position when the rotational angle θin of the joystick lever24matches the steering angle θs.

Also, the first spring64and the second spring65are thus provided to the pilot valve42, and the operator operates the joystick lever24with an operating force that goes against the biasing force produced by the first spring64and the second spring65. The operating force required to operate the joystick lever24can be increased by decreasing the assist force or increasing the counterforce before restriction, to the operation against this biasing force.

With the wheel loader1in this embodiment (an example of a work vehicle), the restrictor78has the second center pin77(an example of a striking member) and the walls73aeand73be(an example of a struck member). The second center pin77is linked to the operation input shaft61and is displaced according to displacement of the operation input shaft61. More precisely, the second center pin77is displaced according to displacement of the operation input shaft61after the deviation angle α of the operation input shaft61and the feedback input shaft62goes past the angle θ2 shown inFIG. 7B. The walls73aeand73beare formed on the feedback spool73(an example of a connected member) connected to the feedback input shaft62, and are struck by the second center pin77. The second center pin77strikes the walls73aeand73bewhen the difference α in the rotational angle of the operation input shaft61with respect to the feedback input shaft62reaches a value corresponding to the angle θ3 (first predetermined angle difference), and displacement of the input shaft61with respect to the feedback input shaft62is restricted.

Consequently, the difference in the amount of displacement of the operation input shaft61with respect to the feedback input shaft62is restricted to an angle between −θ3 and +θ3 (an example of within a predetermined angle difference range).

The wheel loader1in this embodiment (an example of a work vehicle) further comprises the torque sensor103. The torque sensor103senses the torque produced by operation of the joystick lever24. The controller28controls the force applying unit27on the basis of the torque sensor103.

Consequently, a force can be imparted according to the torque applied by the operator to the joystick lever24. For example, the amount of force that is imparted can be controlled so that the assist force imparted by the force imparting component27is increased when the torque applied to the joystick lever24by the operator is high, and the assist force is decreased when the torque is low.

With the wheel loader1in this embodiment (an example of a work vehicle), the controller28has the calculator203and the drive circuit204(an example of an operation controller). The calculator203, as shown inFIG. 9, calculates the force to be imparted to the operation of the joystick lever24by combining the imparted force preset for the sensed torque with the counterforce preset for the angular difference.

Consequently, the operating force required to operate the joystick lever24can be increased before the operation of the joystick lever24is restricted, while an assist force or a counterforce is imparted by the force imparting component27to operation of the joystick lever24.

The wheel loader1in this embodiment (an example of a work vehicle) further comprises the vehicle speed sensor105(an example of a speed sensor). The speed sensor105(an example of a speed sensor) senses the speed of the wheel loader1. The calculator203, as shown inFIG. 9, calculates the force to be imparted to the operation of the joystick lever24by changing the imparted force preset for the torque on the basis of the sensed speed, and combining this changed imparted force with a counterforce preset for the angular difference.

Consequently, the force imparted to the operation of the joystick lever24by the force imparting component27according to torque can also be changed according to the speed of the vehicle, and furthermore, the operating force required to operate the joystick lever24can be increased before the operation of the joystick lever24is restricted.

The work vehicle1in this embodiment further comprises the link25. The link25links the joystick lever24to the pilot valve42. The force imparting component27has the electric motor111and the worm gear112(an example of a transmission mechanism). The electric motor111generates the assist force or the counterforce. The worm gear112transmits the assist force or the counterforce produced by the electric motor111to the link25.

Consequently, the force of the electric motor111can be transmitted to the link25that links the joystick lever24to the pilot valve42, and the force required to operate the joystick lever24can be changed.

The method for controlling the wheel loader1in this embodiment (an example of a work vehicle) is a method for controlling an articulated wheel loader in which the front frame11and the rear frame12are linked, said method comprising step S170(an example of a load increasing step). This step S170(an example of a load increasing step) involves decreasing the assist force or increasing the counterforce imparted to the operation of the joystick lever24before the operation of the joystick lever24is restricted by the pilot valve42(an example of a control valve) that controls the flow of fluid supplied to the steering cylinders21and22(an example of a hydraulic actuator) that change the steering angle θs of the front frame11with respect to the rear frame12, according to the rotational angle θin of the joystick lever24(an example of an operation amount).

Consequently, the operating force required to operate the joystick lever24is increased before the operation of the joystick lever24is restricted. Specifically, since the tactile sensation gradually becomes heavier from before the joystick lever24is restricted, the operation speed slows down as the joystick lever24approaches the restricted position. Accordingly, the sudden recoil produced by displacement restriction of the pilot valve42can be mitigated.

Other Embodiments

An embodiment of the present invention was described above, but the invention is not limited to or by the above embodiment, and various modifications are possible without departing from the gist of the present invention.

With the wheel loader1in the above embodiment, when the absolute value of the torque with respect to left and right operations of the joystick lever24is the same, the same assist force is imparted, but the assist force may be different for left and right operations of the joystick lever24.

FIG. 13is a graph of first assist torque information when the assist force applied to the left rotation operation of the joystick lever24is greater than the assist force applied to the right rotation operation of the joystick lever24. InFIG. 13, the solid line L11indicates the assist torque information at a vehicle speed of 0 km/h, the dotted line L12indicates the assist torque information at a vehicle speed of 25 km/h, and the one-dot chain line L13indicates the assist torque information at a vehicle speed of 40 km/h. Also, L11a, L12a, and L13aindicate the assist torque when the joystick lever24is rotated to the right side at 0 km/h, 25 km/h, and 40 km/h, and L11b, L12b, and L13bindicate the assist torque when the joystick lever24is rotated to the left side at 0 km/h, 25 km/h, and 40 km/h.

The assist torque information (L11), the assist torque information (L12), and the assist torque information (L13) shown inFIG. 13are set so that rotating the joystick lever24to the right side will require more operating force than rotating it to the left side. For instance, as indicated by L11aand L11bfor the assist torque information (L11) at a vehicle speed of 0 km/h, when the absolute value of the lever input torque is the same, the absolute value of the assist torque is set to be higher in the left rotation (L11b) than in the right rotation (L11a).

In operating the joystick lever24, it is generally easier for an operator to bend his wrist to the inside than to the outside. In this embodiment, as shown inFIG. 5, since the joystick lever24is disposed on the left side of the operator's seat5a, the joystick lever24is easier to rotate to the right side than to the left side. Therefore, as shown inFIG. 13, rotation to the right side is set to require more operating force than rotation to the left side, which allows the tactile sensation to be equal to the right and left.

Also, with the assist torque information (L12) at a vehicle speed of 25 km/h, as indicated by L12a, the counterforce is set to be imparted to the right rotation operation of the joystick lever24. That is, the force imparting component27imparts force in the left rotation direction to the input shaft81bfor the right rotation operation of the joystick lever24. On the other hand, as indicated by L12b, an assist force is set to be imparted to the left rotation operation of the joystick lever24. That is, for the left rotation operation of the joystick lever24, the force imparting component27imparts a force in the left rotation direction to the input shaft81b.

Also, with the assist torque information (L13) at a vehicle speed of 40 km/h, as indicated by L13a, the counterforce is set to be imparted to clockwise operation of the joystick lever24. That is, the force imparting component27imparts force in the left rotation direction to the input shaft81bfor the right rotation operation of the joystick lever24. As indicated by L13b, a counterforce is also set to be imparted to counterclockwise operation of the joystick lever24. That is, for the left rotation operation of the joystick lever24, the force imparting component27imparts a force in the right rotation direction to the input shaft81b. The absolute value of the counterforce is set to be greater in the right rotation of the joystick lever24than in the left rotation.

The target assist torque may be calculated by combining the above-mentioned first assist torque information and the second assist torque information shown inFIG. 11A.

In the above embodiment, the controller28stored first assist torque information for three speeds (0 km/h, 25 km/h, and 40 km/h), the speeds are not limited to these. The first assist torque information is not limited to three sets, and there may be only two, or four or more. When the assist torque is smoothly varied according to speed, it is preferable for three or more sets of information to be provided.

In the above embodiment, the controller28stored three sets of first assist torque information, and the assist torque was continuously varied according to the speed by interpolation, but it may instead varied in steps.

For instance, let the first assist torque information at low speed be the solid line L11inFIG. 13, the first assist torque information at medium speed be the dotted line L12inFIG. 13, and the first assist torque information at high speed be the one-dot chain line L13inFIG. 13. Then, for example, let the low speed be less than 15 km/h, the medium speed be at least 15 km/h and less than 25 km/h, and the high speed be at least 25 km/h and no more than 40 km/hour. Also, for example, 15 km/h can be set as a first threshold, and 25 km/h as a second threshold.

In such a case, when the joystick lever24is operated, the controller28compares the speed sensed by the vehicle speed sensor105to the first threshold and the second threshold, and determines whether or not the vehicle speed corresponds to low, medium, or high speed. The first assist torque information at the determined speed is then used to decide a first assist torque from the steering torque signal. The number of stages is not limited to three, may be divided into only two stages, and may also be divided up more finely into more than three stages.

With the wheel loader1in the above embodiment, first assist torque information was provided for each speed, but it need not be provided for each speed. That is, the first assist torque may be decided on the basis of only the sensed value from the torque sensor103.

Also, in deciding the first assist torque, with the above embodiment, the operation direction of the joystick lever24was sensed by the torque sensor103, but the operation direction may be sensed using the lever-body deviation angle α in step S110.

Also, the body-lever deflection angle α may be calculated from the rotational angle θin sensed by the first rotational angle sensor101and the steering angle θs sensed by the steering angle sensor104, without using the value sensed by the second rotational angle sensor102.

Furthermore, the body-lever deflection angle α may be calculated from the rotational angle θin sensed by the first rotational angle sensor101and the steering angle θs calculated from the values sensed by the cylinder stroke sensors106and107.

In the above embodiment, the target assist torque for driving the electric motor111was found by combining the second assist torque information (FIG. 11A) with the first assist torque information (FIG. 10A), but assist torque may be imparted using just the second assist torque information, without using the first assist torque information. In this case, no force is imparted from the force imparting component27to the operation of the joystick lever24until the deviation angle α reaches an angle of ±θ4, and assist torque corresponding to the second assist torque inFIG. 11Ais imparted while the deviation angle α is between angles of −θ3 and −θ4 and between angles of +θ3 and +θ4.

Also, in this case, as mentioned in the (A) above, the assist force may be changed for left and right operations of the joystick lever24. For instance, the counterforce for operation toward the inside with respect to the operator's seat5a(operation to right side in the above embodiment) may be set higher than the counterforce for operation toward the outside with respect to the operator's seat5a(operation to left side in the above embodiment).

In the above embodiment, the assist torque was increased exponentially from the angle +θ3 toward +θ4, and the assist torque was decreased exponentially from the angle −θ4 toward −θ3, but this is not the only option, and the assist torque may be increased or decreased linearly.

In the above embodiment, two springs (the first spring64and the second spring65) were provided, but the second spring65need not be provided. In this case, for example, the part between the feedback spool73and the feedback sleeve74may be fixed.

Also, if the second spring65is not provided, when the deviation angle α reaches an angle of ±θ2 (seeFIG. 7B), the first center pin76will strike the walls71aeand71beand rotation of the operation input shaft61with respect to the feedback input shaft62will be restricted, in which case the first center pin76and the walls71aeand71beconstitute an example of the restrictor. That is, the second assist torque information may be set so that the angle +θ2 becomes the catch-up angle and the counterforce increases toward the catch-up angle.

In the above embodiment, the amount of fluid supplied from the steering valve32to the steering cylinders21and22was controlled according to the pilot pressure inputted from the pilot valve42(an example of a control valve), but the configuration may be such that the fluid from the pilot valve42is supplied directly to the steering cylinders21and22.

In the above embodiment, a force was generated by the electric motor111, but instead of an electric motor, a hydraulic motor or the like may be used. In other words, it should be an actuator or the like with which the force to be imparted can be generated.

In the above embodiment, the drive circuit204was included in the controller28, but it need not be included in the controller28, and only the drive circuit204may be mounted by itself. Furthermore, the drive circuit204may be mounted to an electric motor.

In the above embodiment, the wheel loader1was given as an example of a work vehicle, but a wheel loader is not the only option, and may instead be an articulated dump truck, motor grader, or the like, so long as it is an articulated work vehicle.

INDUSTRIAL APPLICABILITY

The work vehicle and method for controlling a work vehicle of the present invention have the effect of mitigating sudden recoil produced by displacement restriction of the valve, and are useful in a wheel loader or the like.