WORK VEHICLE, CONTROLLER FOR WORK VEHICLE, AND SPEED CONTROL METHOD FOR WORK VEHICLE

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2023-118987, filed Jul. 21, 2023. The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION
Field of the Invention

The present invention relates to a work vehicle, a controller for the work vehicle, and a speed control method for the work vehicle.

Discussion of The Background

Japanese Patent Application Laid-Open No. 2017-053413 discloses a technique of measuring an input of a travel lever and a rotation speed of a travel motor and adjusting a pilot pressure of a travel pump so that the rotation speed of the travel motor matches a command based on the input of the travel lever. Japanese Patent Application Laid-Open No. 2020-038002 discloses a method of detecting a primary pressure of pilot oil supplied to a remote control valve and a rotational speed of a travel motor, and controlling the primary pressure so as to achieve a target vehicle speed based on the detected primary pressure and rotational speed.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present disclosure, a speed control method for a work vehicle includes receiving a setting stage set by a first operation member among a plurality of speed stages, the plurality of speed stages corresponding to a plurality of sets of target rotation speeds at which a first hydraulic motor and a second hydraulic motor respectively rotate to move the work vehicle straight, the first hydraulic motor being configured to drive a first traveling device provided on a left side of a vehicle body of the work vehicle, the second hydraulic motor being configured to drive a second traveling device provided on a right side of the vehicle body, obtaining a target moving direction of the work vehicle by receiving information corresponding to the target moving direction from a sensor to detect operation of a second operation member via which the target moving direction is set, the target moving direction being determined by a speed difference between a target rotation speed of the first hydraulic motor and a target rotation speed of the second hydraulic motor, determining an operating range of a first control parameter and an operating range of a second control parameter based on the setting stage when the second operation member is operated such that a deviation amount between the target moving direction and a straight direction is within a first range predetermined, a displacement volume of the first hydraulic pump configured to supply hydraulic fluid to the first hydraulic motor being determined by the first control parameter, a displacement volume of a second hydraulic pump configured to supply hydraulic fluid to the second hydraulic motor being determined by the second control parameter, determining a boost stage among the plurality of speed stages based on the deviation amount to determine at least one operating range based on the boost stage, when the second operation member is operated such that the deviation amount is out of the first range, each of the at least one operating range being an operating range of each of at least one control parameter of the first control parameter and the second control parameter, the boost stage corresponding to a set of target rotation speeds higher than the target rotation speeds of the setting stage, controlling the first control parameter within the operating range of the first control parameter determined based on the boost stage or the setting stage, controlling the second control parameter within the operating range of the second control parameter determined based on the boost stage or the setting stage, and setting a displacement volume of the first hydraulic motor and a displacement volume of the second hydraulic motor to respective constant volumes regardless of the plurality of speed stages to drive the first traveling device and the second traveling device by rotating the first hydraulic motor and the second hydraulic motor, respectively.

In accordance with a second aspect of the present disclosure, a work vehicle includes a vehicle body, a first traveling device and a second traveling device provided on a left side and a right side of the vehicle body, respectively, a first hydraulic motor configured to drive the first traveling device, a second hydraulic motor configured to drive the second traveling device, a first hydraulic pump configured to supply hydraulic fluid to the first hydraulic motor, a second hydraulic pump configured to supply the hydraulic fluid to the second hydraulic motor, a control mechanism configured to control a displacement volume of the first hydraulic pump and a displacement volume of the second hydraulic pump, a prime mover configured to rotate the first hydraulic pump and the second hydraulic pump, a storage configured to store a plurality of speed stages corresponding to a plurality of sets of target rotation speeds at which the first hydraulic motor and the second hydraulic motor respectively rotate to move the work vehicle straight, a first operation member configured to receive an input corresponding to a setting stage of the plurality of speed stages, a second operation member configured to receive an input corresponding to a target moving direction of the work vehicle determined by a speed difference between a target rotation speed of the first hydraulic motor and a target rotation speed of the second hydraulic motor, a sensor configured to detect an operation of the second operation member, and a hardware processor configured to control the motor and the control mechanism, the hardware processor being configured to receive the setting stage from the first operation member, receive information corresponding to the target moving direction from the sensor to obtain the target moving direction, determine an operating range of a first control parameter and an operating range of a second control parameter based on the setting stage, when the second operation member is operated such that a deviation amount between the target moving direction and a straight direction is within a first range predetermined, ac displacement volume of the first hydraulic pump being determined by the first control parameter, displacement volume of the second hydraulic pump being determined by the second control parameter, determine a boost stage among the plurality of speed stages based on the deviation amount to determine at least one operating range based on the boost stage, when the second operation member is operated such that the deviation amount is out of the first range, each of the at least one operating range being an operating range of each of at least one control parameter of the first control parameter and the second control parameter, the boost stage corresponding to a set of target rotation speeds higher than the target rotation speeds of the setting stage, control the control mechanism such that the first control parameter is within the operating range of the first control parameter and the second control parameter is within the operating range of the second control parameter, the operating ranges of the first control parameter and the second control parameter being determined based on a boost stage or the setting stage, and set a displacement volume of the first hydraulic motor and a displacement volume of the second hydraulic motor to respective constant volumes regardless of the plurality of speed stages to drive the first traveling device and the second traveling device by rotating the first hydraulic motor and the second hydraulic motor, respectively.

In accordance with a third aspect of the present disclosure, a controller for a work vehicle includes a hardware processor. The hardware processor is configured to receive a setting stage set by the first operation member among a plurality of speed stages, the plurality of speed stages corresponding to a plurality of sets of target rotation speeds at which a first hydraulic motor and a second hydraulic motor respectively rotate to move the work vehicle straight, the first hydraulic motor being configured to drive a first traveling device provided on a left side of a vehicle body of the work vehicle, the second hydraulic motor being configured to drive a second traveling device provided on a right side of the vehicle body, obtain a target moving direction of the work vehicle by receiving information corresponding to the target moving direction from a sensor to detect operation of a second operation member via which the target moving direction is set, the target moving direction being determined by a speed difference between a target rotation speed of the first hydraulic motor and a target rotation speed of the second hydraulic motor, determine an operating range of a first control parameter and an operating range of a second control parameter based on the setting stage when the second operation member is operated such that a deviation amount between the target moving direction and a straight direction is within a first range predetermined, a displacement volume of the first hydraulic pump configured to supply the hydraulic fluid to the first hydraulic motor being determined by the first control parameter, a displacement volume of the second hydraulic pump configured to supply the hydraulic fluid to the second hydraulic motor being determined by the second control parameter, determine a boost stage, which is among the plurality of speed stages based on the deviation amount to determine at least one operating range based on the boost stage, when the second operation member is operated such that the deviation amount is out of the first range, each of the at least one operating range being an operating range of each of at least one control parameter of the first control parameter and the second control parameter, the boost stage corresponding to a set of target rotation speeds higher than the target rotation speeds of the setting stage, based on the magnitude of the deviation amount, and determines an operation range of at least one control parameter of the first control parameter and the second control parameter based on the boost stage when the second operation member is operated such that the deviation amount is out of the first range, control the control mechanism such that the first control parameter is within the operating range of the first control parameter determined based on the boost stage or the setting stage, control the control mechanism such that the second control parameter is within the operating range of the second control parameter determined based on the boost stage or the setting stage, control a prime mover configured to rotate the first hydraulic pump and the second hydraulic pump to drive the first hydraulic pump and the second hydraulic pump such that the first hydraulic motor and the second hydraulic motor of which the displacement volumes are set to respective constant volumes regardless of the plurality of speed stages.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

FIG. 1 is a side view of a work vehicle.

FIG. 2 is a top view of the work vehicle.

FIG. 3 is a hydraulic circuit diagram of a travel system of the work vehicle.

FIG. 4 is a diagram showing a relationship between an engine rotation speed, a travel primary pressure, and a setting line.

FIG. 5 is a diagram showing a relationship between an operation position of an operation lever and a traveling secondary pressure.

FIG. 6 is a block diagram of the work vehicle.

FIG. 7 shows an example of a speed stage (first reference information) in the first embodiment.

FIG. 8 shows an example of second reference information in the first embodiment.

FIG. 9 is a flowchart showing the operation of the work vehicle according to the first embodiment.

FIG. 10 shows a hydraulic system (hydraulic circuit) of the work vehicle different from that of FIG. 6.

FIG. 11 is a hydraulic circuit diagram of the travel system of the work vehicle in the second embodiment.

FIG. 12 shows an example of the speed stage (first reference information) in the second embodiment.

FIG. 13 is a flowchart showing the operation of the work vehicle according to the second embodiment.

FIG. 14 is a hydraulic circuit diagram of a travel system of the work vehicle in a modified example of the second embodiment.

FIG. 15 is a hydraulic circuit diagram of the travel system of the work vehicle in the third embodiment.

FIG. 16 is a schematic view showing the tilting direction of an operation lever as viewed from above the operation lever in the downward direction.

FIG. 17 is a schematic view showing the tilting direction of the operation lever as viewed from the side of the operation lever in the lateral direction.

FIG. 18 is a schematic view showing the tilting direction of the operation lever as viewed from the rear of the operation lever toward the front.

FIG. 19 is a view showing a relationship between a tilt angle of the operation lever and a deemed operation position.

FIG. 20 is a view showing a relationship between a tilt angle of the operation lever and a deemed operation position.

FIG. 21 shows an example of the speed stage (first reference information) in the third embodiment.

FIG. 22 shows an example of second reference information in the third embodiment.

FIG. 23 is a flowchart showing the operation of the work vehicle according to the third embodiment.

DESCRIPTION OF THE EMBODIMENTS

The present invention will be described in detail below with reference to the drawings showing embodiments thereof. In the drawings, the same reference numerals denote corresponding or substantially the same components.

First Embodiment
<Overall Configuration>

Referring to FIGS. 1 and 2, a work vehicle 1, for example, a compact truck loader includes a vehicle body 2, a pair of traveling devices 3, and a work device 4. The vehicle body 2 supports the traveling device 3 and the work device 4. In the illustrated embodiment, the traveling device 3 is a crawler type traveling device. Therefore, each of the pair of traveling devices 3 includes a drive wheel 31, driven wheels 32 and 33, and rolling wheels 34, which are driven by the hydraulic motor device 30. However, each of the pair of traveling devices 3 is not limited to the crawler type traveling device. Each of the pair of traveling devices 3 may be, for example, a front wheel/rear wheel traveling device or a traveling device having a front wheel and a rear crawler. The work device 4 comprises work equipment (bucket) 41 at the distal end of the work device 4. A proximal end of the work device 4 is attached to a rear portion of the vehicle body 2. The work device 4 includes a pair of arm assemblies 42 for rotatably supporting the bucket 41 via a bucket pivot shaft 43. Each of the pair of arm assemblies 42 includes a link 44 and an arm 45.

The link 44 is rotatable relative to the vehicle body 2 about the fulcrum shaft 46. The arm 45 is rotatable relative to the link 44 about the axis L. The work device 4 further includes a plurality of arm cylinders 48 and at least one equipment cylinder 49. Each of the plurality of arm cylinders 48 is rotatably connected to the vehicle body 2 and the arm 45, and moves the link 44, the arm 45, and the like to raise and lower the bucket 41. The at least one implement cylinder 49 is configured to tilt the bucket 41. The vehicle body 2 includes a cabin 5. The cabin 5 includes a front window 51 that can be opened and closed, and the outer shape of the cabin 5 is defined by a cab frame 53. The front window 51 may be omitted. The work vehicle 1 includes a driver's seat 54 and an operation lever 55 in the cabin 5. As shown in FIG. 2, the cab frame 53 is rotatable around rotational shafts RSL and RSR on the vehicle body 2. FIGS. 1 and 2 illustrate the common pivot AXC defined by the rotational shafts RSL and RSR. That is, the cab frame 53 is attached to the vehicle body 2 so as to be rotatable around the pivot AXC.

In the embodiment according to the present application, a front-back direction D_FB(forward direction D_F/backward direction D_B) means a front-back direction (forward direction/backward direction) as seen from an operator seated on the driver's seat 54 of the cabin 5. A leftward direction D_L, a rightward direction D_R, a width direction D_Wmeans the left direction, the right direction, and the left-right direction as viewed from the operator, respectively. An upward direction D_U, a downward direction D_D, height direction DH means an upward direction, a downward direction, and a height direction as viewed from the operator. The front-back, left-right (width), and up-down (height) directions of the work vehicle 1 coincide with the front-back, left-right (width), and up-down (height) directions as viewed from the operator, respectively.

FIG. 1 shows the left side of the work vehicle 1. As shown in FIG. 2, the vehicle body 2 is substantially plane-symmetric with respect to the vehicle body center surface M, and is a first side surface 2L which is a left side surface and a second side surface 2R which is a right side face. Among the pair of traveling devices 3, the traveling device 3 provided on the first side surface 2L is shown as the first traveling device 3L, and the traveling device 3 provided on the second side surface 2R is shown as the second traveling device 3R. Among the pair of arm assemblies 42, the arm assembly 42 provided on the left side with respect to the vehicle body center surface M is shown as the first arm assembly 42L, and the arm assembly 42 provided on the right side with respect to the vehicle body center surface M is shown as the second arm assembly 42R. The link 44 provided on the left side of the vehicle body center surface M is shown as a first link 44L. An arm 45 provided on the left side of the vehicle body center surface M is shown as a first arm 45L, and an arm 45 provided on the right side of the vehicle body center surface M is shown as a second arm 45R. The fulcrum shaft 46 provided on the left side of the vehicle body center surface M is shown as the first fulcrum shaft 46L. The fulcrum shaft 46 provided on the right side with respect to the vehicle body center surface M is shown as a second fulcrum shaft 46R. The joint shaft 47 provided on the left side with respect to the vehicle body center surface M is shown as a first joint shaft 47L, and the joint shaft 47 provided on the right side with respect to the vehicle body center surface M is shown as a second joint shaft 47R. Among the hydraulic motor device 30, the hydraulic motor device 30 provided on the left side with respect to the vehicle body center surface M is shown as the first hydraulic motor device 30L. The hydraulic motor device 30 provided on the right side with respect to the vehicle body center surface M is shown as a second hydraulic motor device 30R.

Referring to FIGS. 1 and 2, the work vehicle 1 includes an engine 6 provided at a rear portion of the vehicle body 2, and a plurality of hydraulic pumps including the first hydraulic pump 7L and the second hydraulic pump 7R. The engine 6 drives a plurality of hydraulic pumps 7. The engine 6 is an example of a prime mover configured to rotate the first hydraulic pump 7L and the second hydraulic pump 7R. The first hydraulic pump 7L and the second hydraulic pump 7R are configured to discharge hydraulic fluid for driving a hydraulic motor device 30 for driving the drive wheel 31. The first hydraulic pump 7L and the second hydraulic pump 7R are collectively referred to as hydraulic pumps (7L, 7R). The plurality of hydraulic pumps 7 other than the first hydraulic pump 7L and the second hydraulic pump 7R is configured to discharge hydraulic fluid for driving a hydraulic actuator (a plurality of arm cylinders 48, at least one equipment cylinder 49, or the like) connected to the work device 4. The engine 6 is provided between a pair of arm assemblies 42 in the width direction D_Wof the work vehicle 1. The work vehicle 1 further includes a cover 8 for covering the engine 6. The work vehicle 1 further includes a bonnet cover 9 provided at the rear end of the vehicle body 2. The bonnet cover 9 is openable and closable such that a maintenance personnel can perform maintenance work on the engine 6 and the like.

FIG. 3 is a hydraulic circuit diagram of a travel system of the work vehicle 1. The work vehicle 1 includes a hydraulic circuit 20. The hydraulic circuit 20 includes a hydraulic fluid tank 70 and a pilot pump 71. The pilot pump 71 is a gear pump of a constant displacement type driven by the power of the engine 6. The engine 6 is configured to rotate the pilot pump 71. The pilot pump 71 is configured to discharge the hydraulic oil stored in the hydraulic fluid tank 70. In particular, the pilot pump 71 is configured to discharge the hydraulic oil mainly used for control. For convenience of description, the hydraulic oil used for control among the hydraulic oil discharged from the pilot pump 71 is referred to as pilot oil, and the pressure of the pilot oil is referred to as pilot pressure. In the hydraulic circuit 20, a mechanism that supplies the pilot oil and controls the pilot pressure is referred to as a control mechanism 25. In particular, the pilot pump 71 is configured to supply pilot oil to the first hydraulic pump 7L and the second hydraulic pump 7R.

The hydraulic circuit 20 includes a pilot oil supply passage PA1 connected to a discharge port of the pilot pump 71. The pilot oil is supplied in the pilot oil supply passage PA1. The hydraulic circuit 20 includes a plurality of switching valves (brake switching valves, direction switching valve SV2) connected to the pilot oil supply passage PA1, and a plurality of brake mechanisms 72. The brake switching valve SV1 is a direction switching valve (solenoid valve) for braking and releasing the braking by the plurality of brake mechanisms 72. The brake switching valve SV1 is a two-position switching valve configured to switch a valve element to the first position VP1a and the second position VP1b by exciting. Switching of the valve element of the brake control valve SV1 is performed by the brake pedal 13 (see FIG. 6). The brake pedal 13 is provided with a sensor 14. The operation amount detected by the sensor 14 is input to a controller 10 including an electric control unit (ECU). The controller 10 may be referred to as a control device.

The plurality of brake mechanisms 72 include a first brake mechanism 72L for braking the first traveling device 3L and a second brake mechanism 72R for braking the second traveling device 3R. The first brake mechanism 72L and the second brake mechanism 72R are connected to the brake switching valves via the oil passage PA2. The first brake mechanism 72L and the second brake mechanism 72R are configured to brake the traveling device 3 in accordance with pressures of pilot oil (hydraulic fluid). When the valve element of the brake switching valve SV1 is switched to the first position VP1a, the hydraulic fluid is discharged from the oil passage PA2 in a section between the brake switching valve SV1 and the brake mechanism 72, and the traveling device 3 is braked by the brake mechanism 72. When the valve element of the brake switching valve SV1 is switched to the second position VP1b, the braking by the brake mechanism 72 is released. Note that the braking by the brake mechanism 72 may be released when the valve element of the brake switching valves SV1 is switched to the first position VP1a, and the traveling device 3 may be braked by the brake mechanism 72 when the valve element of the brake switching valves SV1 is switched to the second position VP1b.

The direction switching valve SV2 is a solenoid valve for changing the rotation of the first hydraulic motor device 30L and the second hydraulic motor device 30R. The direction switching valve SV2 is a two-position switching valve configured to switch a valve clement to the first position VP2a or second position VP2b by excitation. The direction switching valve SV2 is switched by an operation member (not illustrated) or the like. The direction switching valve SV2 may be a proportional control valve capable of adjusting the flow rate of the hydraulic fluid to be discharged, instead of a two-position control valve.

The first hydraulic motor device 30L is a device that transmits power to the drive wheels 31 provided in the first traveling device 3L. The first hydraulic motor device 30L includes a first hydraulic motor 31L, a first swash plate switching cylinder 32L; and a first travel control valve (hydraulic switching valve) SV4. The first hydraulic motor 31L is a swash plate type variable capacity axial motor configured to drive the first traveling device 3L, and is a motor capable of changing the vehicle speed (rotation) to the first speed or the second speed. The first swash plate switching cylinder 32L is a cylinder configured to change the angle of the swash plate of the first hydraulic motor 31L by expansion and contraction. The first travel control valve SV4 is used to expand and contract the first swash plate switching cylinder 32L. The first travel control valve SV4 is a two-position switching valve configured to switch its valve element between a first position VP4a and a second position VP4b.

Switching of the first travel control valve SV4 is performed by a direction switching valve SV2 located on the upstream side and connected to the first travel control valve SV4. Specifically, the direction switching valve SV2 and the first travel control valve SV4 are connected by an oil passage PA3, and switching operation of the first travel control valve SV4 is performed by hydraulic fluid flowing through the oil passage PA3. For example, the valve element of the direction switching valve SV2 is switched to the first position VP2a, the pilot oil is released in the section between the direction switching valve SV2 and the first travel control valve SV4, and the valve element of the first travel control valve SV4 is switched to the first position VP4a. As a result, the first swash plate switching cylinder 32L contracts, and the speed of the first hydraulic motor 31L is changed to the first speed. When the valve element of the direction switching valve SV2 is switched to the second position VP2b by the operation of the operation member, the pilot oil is supplied to the first travel control valve SV4 through the direction switching valve SV2, and the valve element of the first travel control valve SV4 is switched to the second position VP4b. As a result, the first swash plate switching cylinder 32L is extended, and the speed of the first hydraulic motor 31L is changed to the second speed.

The second hydraulic motor device 30R is a device that transmits power to the drive wheels 31 provided in the second traveling device 3R. The second hydraulic motor device 30R includes a second hydraulic motor 31R, a second swash plate switching cylinder 32R, and second travel control valve (hydraulic switching valve) SV5. The second hydraulic motor 31R is a hydraulic motor configured to drive the second traveling device 3R, and operates in the same manner as the first hydraulic motor 31L. That is, the second hydraulic motor 31R operates in the same manner as the first hydraulic motor 31L. The first hydraulic motor 31L and the second hydraulic motor 31R are collectively referred to as hydraulic motors (31L, 31R). The second swash plate switching cylinder 32R operates in the same manner as the first swash plate switching cylinder 32L. The second travel control valve SV5 is a two-position switching valve configured to switch its valve element between the first position VP5a and the second position VP5b, and operates in the same manner as the first travel control valve SV4.

The hydraulic circuit 20 is connected to a drain oil passage DR1. The drain oil passage DR1 is an oil passage to make the pilot oil flow from a plurality of switching valves (brake switching valves SV1 and a direction switching valves SV2) to the hydraulic fluid tank 70. For example, the drain oil passage DR1 is connected to discharge ports of a plurality of switching valves (brake switching valves SV1 and direction switching valves SV2). That is, when the brake switching valve SV1 is at the first position VP1a, the hydraulic fluid is discharged from the oil passage PA2 to the drain oil passage DR1 in the interval between the brake switching valve SV1 and the brake mechanism 72. When the direction switching valve SV2 is at the first position VP1a, the pilot oil in the oil passage PA3 is discharged to the drain oil passage DR1.

The hydraulic circuit 1A further includes a first charge oil passage PA4 and a hydraulic drive device 75. The first charge oil passage PA4 is branched from the pilot oil supply passage PA1 and connected to the hydraulic drive device 75. The hydraulic drive device 75 is a device that drives the first hydraulic motor device 30L and the second hydraulic motor device 30R. The hydraulic drive device 75 includes a first drive circuit 76L for driving the first hydraulic motor device 30L and a second drive circuit 76R for driving the second hydraulic motor device 30R.

The first drive circuit 76L includes the first hydraulic pump 7L, a driving oil passage PA5L, PA6L, and a second charge oil passage PA7L. The driving oil passages PA5L and PA6L are oil passages that connect the first hydraulic pumps 7L and the first hydraulic motor 31L. The hydraulic circuit formed by the driving oil passages PA5L, PA6L is referred to as a first hydraulic circuit CL. The second charge oil passage PA7L, which is connected to the drive oil passages PA5L and PA6L is an oil passage for replenishing the drive oil passages PA5L and PA6L with the hydraulic fluid from the pilot pump 71. The first hydraulic motor 31L has a first connection port 31P1 connected to the drive oil passage PA5L and a second connection port 31P2 connected to the drive oil passage PA6L. The hydraulic fluid for rotating the first traveling device 3L in the backward direction is inputted to the first hydraulic motor 31L) via the first connection port 31P1, and the hydraulic fluid for rotating the first traveling device 3L in the forward direction is discharged from the first hydraulic motor 31L via the first connection port 31P1. The hydraulic fluid for rotating the first traveling device 3L in the backward direction is input to the first hydraulic motor 31L via the second connection port 31P2, and hydraulic fluid for rotating the first traveling device 3L in the forward direction is discharged from the first traveling device 3L.

Similarly, the second drive circuit 76R drives the second hydraulic pump 7R, driving oil passages PA5R and PA6R, and a third charge oil passage PAR. The driving oil passages PA5R and PA6R are oil passages that connect the second hydraulic pump 7R and the second hydraulic motor 31R. The hydraulic circuit formed by the driving oil passages PA5R and PA6R is referred to as a second hydraulic circuit CR. The third charge oil passage PA7R is an oil passage that is connected to the driving oil passages PA5R and PA6R and replenishes the driving oil passages PA5R and PA6R with the hydraulic fluid from the pilot pump 71. The second hydraulic motor 31R includes a third connection port 31P3 connected to the driving oil passage PA5R, and a fourth connection port 31P4 connected to the driving oil passage PA6R. The hydraulic fluid for rotating the second traveling device 3R in the forward direction is input to the second hydraulic motor 31R via the third connection port 31P3, and the hydraulic fluid for rotating the second traveling device 3R in the reverse direction is discharged from the second hydraulic motor 31R via the third connection port 31P3. The hydraulic fluid for rotating the second traveling device 3R in the reverse direction is input to the second hydraulic motor 31R via the fourth connection port 31P4, and the hydraulic fluid for rotating the second traveling device 3R in the forward direction is discharged from the second traveling device 3R. That is, the hydraulic motor (31L, 31R) is configured to drive the traveling device (3L, 3R). The hydraulic pumps (7L, 7R) are configured to discharge hydraulic fluid for driving the hydraulic motors (31L, 31R). The driving oil passages (PA5L, PA6L, PA5R, PA6R) are oil passages that connect the hydraulic pumps (7L, 7R) and the hydraulic motors (31L, 31R).

The first hydraulic pump 7L and a second hydraulic pump 7R are a swash plate type variable capacity axial pump which is driven by the power of the engine 6. The first hydraulic pump 7L which is connected to a first hydraulic motor 31L via a first hydraulic circuit CL includes a first port Pla and a second port PLb to which pilot pressure acts. The angle of the swash plate in the first hydraulic pump 7L is changed by the pilot pressure acting on the first port PLa and the second port PLb. Specifically, the first hydraulic pump 7L is configured to supply hydraulic fluid to a first hydraulic motor 31L via a first hydraulic circuit CL so as to drive a first traveling device 3L forward when the hydraulic pressure applied to a first port PLb is higher than the hydraulic pressure applied to a second port PLb, and to supply hydraulic fluid to the first hydraulic motor 31L via a first hydraulic circuit CL so as to drive the first traveling device 3L backward when the hydraulic pressure applied to a second port PLb is higher than the hydraulic pressure applied to a first port PLa. The first port PLa is a port via which the pilot pressure is applied to move the swash plate of the first hydraulic pump 7L so that the first hydraulic motor 31L drives the first traveling device 3L in the forward direction when the first hydraulic pump 7L is rotated. The second port PLb is a port via which the pilot pressure is applied to move the swash plate of the first hydraulic pump 7L so that the first hydraulic motor 31L drives the first traveling device 3L in the reverse direction when the first hydraulic pump 7L is rotated.

The second hydraulic pump 7R is connected to the second hydraulic motor 31R via the second hydraulic circuit CR, and has a third port PRa and a fourth port PRb to which the pilot pressure acts. The second hydraulic pump 7R is configured such that the angle of the swash plate is changed by the pilot pressure acting on the third port PRa and the fourth port PRb. To be more specific, the second hydraulic pump 7R is configured to supply the hydraulic fluid to the second hydraulic motor 31R via the second hydraulic circuit CR so as to drive the second traveling device 3R forward when the hydraulic pressure applied to the third port PRa is higher than the hydraulic pressure applied to the fourth port PRb, and to supply the hydraulic fluid to the second hydraulic motor 31R via the second hydraulic circuit CR so as to drive the second traveling device 3R backward when the hydraulic pressure applied to the fourth port PRb is higher than the hydraulic pressure applied to the third port PRa. The third port PRa is a port via which the pilot pressure is applied to move the swash plate of the second hydraulic pump 7R so that the second hydraulic motor 31R drives the second traveling device 3R in the forward direction when the second hydraulic pump 7R is rotated. The fourth port PRb is a port via which the pilot pressure is applied to move the swash plate of the second hydraulic pump 7R so that the second hydraulic motor 31R drives the second traveling device 3R in the reverse direction when the second hydraulic pump 7R is rotated. The first and second hydraulic pumps 7L and 7R can change outputs (discharge amounts of the hydraulic fluid) and discharge directions of the hydraulic fluid in accordance with the angle of the swash plate.

The outputs of the first and second hydraulic pumps 7L and 7R and the discharge direction of the hydraulic fluid are changed by the operation device 56 for operating the traveling direction of the work vehicle 1. To be specific, the outputs of the first and second hydraulic pumps 7L and 7R and the discharge directions of the hydraulic fluids are changed in accordance with the operation of the operation lever 55 included in the operation device 56. That is, the operation device 56 is a device configured to select at least one of the first traveling device 3L and the second traveling device 3R and instruct at least one of the traveling devices to move forward or backward, thereby operating the traveling direction of the work vehicle 1.

As shown in FIG. 3, the hydraulic circuit 20 is pilot supplying oil passage branched from the pilot oil supply passage PA1 and connected to the operation device 56, and a pilot pressure control valve CV1 provided on the pilot oil supply passage PA8. The pilot pressure control valve CV1 is a solenoid proportional valve and is configured to adjust the pilot pressure supplied to the operation device 56 by adjusting the opening degree thereof. The opening degree of the pilot pressure control valve CV1 is controlled by the controller 10. In the following embodiments, the pilot pressure control valve CV1 may be referred to as a hydraulic adjustment mechanism. The detailed operation of the pilot pressure control valve CV1 will be described later.

The operation device 56 includes an operation valve OVA for forward movement, an operation valve OVB for backward movement, an operation valve OVC for right turning, an operation valve OVD for left turning, and an operation lever 55. The operation device 56 includes first to fourth shuttle valves SVa, SVb, SVc, and SVd. The operation valves OVA, OVB, OVC, and OVD are operated by one operation lever 55. The operation valves OVA, OVB, OVC, and OVD change the pressure of the hydraulic oil in accordance with the operation of the operation lever 55, and supply the changed hydraulic fluid to the first port PLa and the second port PLb of the first hydraulic pump 7L and the third port PRa and the fourth port PRb of the second hydraulic pump 7R. In this embodiment, the operation valves OVA, OVB, OVC, and OVD are operated by one operation lever 55, but the number of operation levers 55 may be plural. In the following embodiments, one or more operation levers 55 may be referred to as a second operation member. The operation lever 55 is configured to receive an input corresponding to a target moving direction of the vehicle body 2 determined by a speed difference between the target rotation speed of the first hydraulic pump 7L and the target rotation speed of the second hydraulic pump 7R. That is, the target moving direction is set by the operation lever 55.

The operation valves OVA, OVB, OVC, and OVD each have an input port (primary-side port), a discharge port, and an output port (secondary-side port). As shown in FIG. 3, the input port is connected to the pilot oil supply passage PA8. The discharge port is connected to the drain oil passage DR2 leading to the hydraulic fluid tank 70. The operation lever can be tilted from a neutral position in the front-rear direction, a width direction orthogonal to the front-rear direction, and an oblique direction. That is, the operation lever 55 is rotatable in the front-rear and left-right directions. The operation valve of the operation device 56 is operated according to the tilting of the operation lever 55. Thus, the pilot pressure corresponding to the operation amount of the operation lever 55 from the neutral position is output from the secondary-side ports of the operation valves OVA, OVB, OVC, and OVD. The relationship between the pilot pressure applied to the primary-side port and the pilot pressure applied to the secondary-side port, which are output from the pilot pressure control valve CV1, will be described later.

The secondary-side port of the operation valve OVA and the secondary-side port of the operation valve OVC are connected to the input port of the first shuttle valves SVa, and the output port of the first shuttle valve is connected to the first port PLa of the first hydraulic pump 7L via the first pilot oil passage PA11. The first pilot oil passage PA11 connects the pilot pump 71 and the first port PLa via the pilot oil supply passage PA8. The secondary-side port of the operation valve OVA and the secondary-port of the operation valve OVD are connected to the input port of the second shuttle valves SVb, and an output port of the second shuttle valve SVb is connected to the third port PRa of the second hydraulic pump 7R via the third pilot oil passages PA13. The third pilot oil passage PA13 connects the pilot pump 71 and the third port PRa via the pilot oil supply passage PA8.

The secondary-side port of the operation valve OVB and the secondary-side port of the operation valve OVD are connected to the input port of the third shuttle valve SVc, and the output port of the third shuttle valve SVc are connected to the second port PLb of the first hydraulic pump 7L via the second pilot oil passage PA12. The second pilot oil passage PA12 connects the pilot pump 71 and the second port PLb via the pilot oil supply passage PA8. The secondary-side port of the operation valve OVB and the secondary-side port of the operation valve OVC are connected to the input ports of the fourth shuttle valve SVd, and the output port of the fourth shuttle valve SVd are connected to the second hydraulic pump 7R via the fourth pilot oil passage PA14. The fourth pilot oil passage PA14 connects the pilot pump 71 and the fourth port PRb via the pilot oil supply passage PA8.

The pilot oil supply passage PA1, the pilot oil supply passage PA8, the first pilot oil passage PA11, and the second pilot oil passage PA12, which connect the pilot pump 71 and two pilot ports (PLa, PLb) of the first hydraulic pump 7L, are referred to as a first pilot hydraulic circuit PC1. The pilot oil supply passage PA1, the pilot oil supply passage PA8, the third pilot oil passage PA13, and the fourth pilot oil passage PA14, which connect the pilot pump 71 and the two pilot ports (PRa, PRb) of the second hydraulic pump 7R, are referred to as a second pilot hydraulic circuit PC2. The pilot pressure control valve CV1 is a primary pressure control valve provided so as to be connected to both the first pilot hydraulic circuit PCI and the second pilot hydraulic circuit PC2. At least one first pressure control valve PCI provided in the first pilot hydraulic circuit PCI and configured to control a first high pilot pressure, which is a higher one of two pilot pressures applied to the two pilot ports (PLa, PLb) of the first hydraulic pump 7L, includes operation valves OVA and OVB and a pilot pressure control valve CV1. At least one second pressure control valve PC2 provided in the second pilot hydraulic circuit PC2 and configured to control a second high pilot pressure, which is a higher one of the two pilot pressures applied to the two pilot ports (PRa, PRb) of the second hydraulic pump 7R, includes the operation valve OVC, the operation valve OVD, and the pilot pressure control valve CV1.

When the operation lever 55 is tilted forward, the operation valve OVA for forward movement is operated, and a pilot pressure is output from the operation valve OVA. The pilot pressure acts on the first port PLa from the first shuttle valve SVa via the first pilot oil passage PA11 connecting the operation device 56 and the first port PLa of the first hydraulic pump 7L, and acts on the third port PRa from the second shuttle valves SVb via the third pilot oil passage PA13 connecting the operation device 56 and the third port PRa of the second hydraulic pump 7R. As a result, the output shafts of the first and second hydraulic pumps 7L and 7R rotate in the normal direction (forward direction) at a speed corresponding to the amount of tilt of the operation lever 55, and the work vehicle 1 moves straight forward.

Further, the operation lever 55 is tilted backward, the operation valve for backward movement is operated, and the pilot pressure is output from the operation valve OVB. The pilot pressure acts on the second port PLb of the first hydraulic pump 7L from the third shuttle valve SVc via the second pilot oil passage PA12 connecting the operation device 56 and the second port, and acts on the fourth port PRb from the fourth shuttle SVd via the fourth pilot oil passage PA14 connecting the operation device 56 and the fourth port PRb of the second hydraulic pump 7R. As a result, the output shafts of the first and second hydraulic pumps 7L and 7R are reversed (backward rotation) at a speed corresponding to the amount of tilt of the operation lever 55, so that the work vehicle 1 travels straight backward.

When the operation lever 55 is tilted to the right side, the operation valve OVC for right turning is operated, and the pilot pressure is output from the operation valve OVC. This pilot pressure acts on the first port PLa of the first hydraulic pump 7L from the first shuttle valve SVa via the first pilot oil passage PA11, and acts on the fourth port PRb of the second hydraulic pump 7R from the fourth shuttle valve SBd via the fourth pilot oil passage PA14. Thereby the vehicle curves to the right with a degree of curvature corresponding to the operation position in the right direction of the operation lever 55.

When the operation lever 55 is tilted to the left side, the operation valve OVD for left turning is operated, and the pilot pressure is output from the operation valve OVD. This pilot pressure acts on the third port PRa of the second hydraulic pump 7R from the second shuttle valves SVb via the third pilot oil passage PA13, and also acts on the second port PLb of the first hydraulic pump 7L from the third shuttle valve SVc via the second pilot oil passage PA12. Thus, the vehicle turns leftward with a degree of bending corresponding to the leftward operation position of the operation lever 55.

That is, when the operation lever 55 is tilted obliquely forward to the left, the work vehicle 1 moves forward at a speed corresponding to the operation position of the operation lever 55 in the front-rear direction, and curves to the left with a degree of curve corresponding to the operation position of the operation lever 55 in the left direction. When the operation lever 55 is tilted obliquely forward to the right, the work vehicle 1 turns to the right while moving forward at a speed corresponding to the operation position of the operation lever 55. When the operation lever 55 is tilted obliquely rearward to the left, the work vehicle 1 turns to the left while moving rearward at a speed corresponding to the operation position of the operation lever 55. When the operation lever 55 is tilted obliquely rearward to the right, the work vehicle 1 turns to the right while moving backward at a speed corresponding to the operation position of the operation lever 55.

Next, the detailed operation of the pilot pressure control valve CV1 will be described. The work vehicle 1 includes a first setting member 11a and a second setting member 11b (see FIG. 6) for setting a target rotation speed of the engine 6. The first setting member 11a is an accelerator pedal which is a speed input device different from the operation device 56 described above or an accelerator lever which is supported so as to be swingable. The second setting member 11b is a rotatable indoor dial. The first setting member 11a is provided with a sensor 12a. The operation amount detected by the sensor 12a is input to the controller 10. The engine rotation speed corresponding to the second setting member 11b is the target rotation speed of the engine 6. In other words, the target rotation speed of the engine 6 is set based on the operation amount of the second setting member 11b. In a normal operating state (normal mode (described later)), the engine rotation speed is additionally increased or decreased based on the operation of the first setting member 11a. The controller 10 outputs a rotation command indicating, for example, a fuel injection amount, an injection timing, and a fuel injection rate to the injector so that the determined target rotation speed of the engine 6 is achieved. Alternatively, the controller 10 outputs a rotation command indicating a fuel injection pressure or the like to the supply pump or the common rail so that the determined target rotation speed of the engine 6 is achieved. In the following embodiments, the one or more operation levers 55 and the setting member 11 may be referred to as at least one operation device. A speed sensor 6a for detecting an actual engine rotation speed (referred to as an actual rotation speed of the engine 6) is connected to the controller 10, and the actual rotation speed of the engine 6 is input to the controller 10. The speed sensor 6a is, for example, a potentiometer configured to detect the rotation speed of a rotary member connected to the crankshaft of the engine 6. When a load is applied to the engine 6, the actual rotational speed of the engine 6 decreases from the target rotation speed of the engine 6. The amount of decrease in the actual rotational speed from the target rotation speed (the difference between the target rotation speed of the engine and the actual rotational speed of the engine) when a load is applied to the engine 29 is referred to as the amount of drop of the engine.

The pilot pressure control valve CV1 can set the a pilot pressure (primary pilot pressure) that acts on the input ports (primary-side ports) of the plurality of operation valves OVA, OVB, OVC, and OVD based on a decrease amount (drop amount) ΔE1 of the rotation speed of the engine 6 (engine rotation speed E1). That is, the pilot pressure control valve CV1 is a control valve that is provided between the pilot pump 71 and the operation valves OVA, OVB, OVC, and OVD, and is configured to send the pilot oil to the operation valves OVA, OVB, OVC, and OVD and convert the pressures of the pilot oil supplied to the operation valves OVA, OVB, OVC, and OVD into the primary pilot pressures. The rotation speed of the engine 6 can be detected by a speed sensor 6a of the engine rotation speed E1. The engine rotation speed E1 detected by the speed sensor 6a is input to the controller 10. The speed sensor 6a may be referred to as a speed sensor. FIG. 4 shows the relationship between the engine speed, the traveling primary pressure (primary pilot pressure), and the setting lines L1 and L2. The setting line L1 indicates the relationship between the engine rotation speed E1 and the traveling primary pressure when the amount of decrease ΔE1 is less than a predetermined value (less than the anti-stall determination value). The setting line L2 indicates the relationship between the engine rotation speed E1 and the travel primary pressure when the amount of decrease ΔE1 is equal to or greater than the anti-stall determination value. When the difference between the rotation speed RS determined based on the operation amount of the setting member 11 and the actual rotation speed of the engine 6 is smaller than a predetermined stall determination speed difference (anti-stall determination value), the primary pilot pressure corresponding to the rotation speed RS transitions in accordance with the third correspondence relationship indicated by the setting line L1. When the difference between the rotation speed RS and the actual rotation speed of the engine 6 is equal to or larger than a predetermined stall determination speed difference (anti-stall determination value), the primary pilot pressure corresponding to the rotation speed RS transitions in accordance with the fourth correspondence relationship indicated by the setting line L2.

When the amount of decrease ΔE1 is less than the anti-stall determination value, the controller 10 adjusts the opening degree of the pilot pressure control valve CV1 so that the relationship between the engine rotation speed E1 and the traveling primary pressure matches the reference pilot pressure indicated by the setting line L1. Further, when the amount of decrease ΔE1 is equal to or greater than the anti-stall determination value, the controller 10 adjusts the opening degree of the pilot pressure control valve CV1 so that the relationship between the engine rotation speed E1 and the traveling primary pressure matches the setting line L2 lower than the reference pilot pressure. In the setting line L2, the traveling primary pressure for a predetermined engine rotation speed E1 is lower than the traveling primary pressure of the setting line L1. That is, when focusing on the same engine rotation speed E1, the traveling primary pressure of the setting line L2 is set to be lower than the traveling primary pressure of the setting line L1. Therefore, the pressures (pilot pressures) of the hydraulic fluids entering the operation valves OVA, OVB, OVC, and OVD are suppressed to be low by the control based on the setting line L2. As a result, the angles of the swash plates of the first and second hydraulic pumps 7L and 7R are adjusted, and the load acting on the engine 6 is reduced, thereby preventing the engine 6 from stalling. Although one setting line L2 is shown in FIG. 4, a plurality of setting lines L2 may be provided. For example, the setting line L1 may be set for each engine rotation speed L2. Data or the control parameter such as the function indicating the setting line L1 and L2 is preferably included in the controller 10.

Next, the secondary pilot pressure output from the secondary port of the operation valves OVA, OVB, OVC, and OVD will be described. FIG. 5 is a diagram showing a relationship between the operation position of the operation lever and the traveling secondary pressure (secondary pilot pressure). Referring to FIG. 4, the origin of the lever operation position is an operation start position (neutral position, G0 position) which is a start position of the lever stroke, and the lever operation position approaches an operation end position (G5 position) which is an end position of the lever stroke as the lever operation position is away from the origin. The operation region of the operation lever 55 is divided into a neutral region RA1 (from the G0 position to the G1 position in the illustrated example) where the operation target does not move, a full operation vicinity region RA2 (from the G3 position to the G5 position in the illustrated example) near the operation end, and an intermediate region RA3 (from the G1 position to the G3 position in the illustrated example) between the neutral region RA1 and the full operation vicinity region RA2. Further, the intermediate region RA3 is divided into a very low speed region RA3A from the G1 position to the G2 position and an intermediate speed region RA3B from the G2 position to the G3 position.

In the neutral region RA1, the secondary pilot pressure is not supplied even if the operation lever 55 is operated. On the other hand, in the full operation vicinity region RA2, the speed of the operation target is not adjusted, and therefore, the operation lever 55 is operated to the operation terminal position (G5 position) without stopping in the middle. In the intermediate region RA3, the speed of the operation target is adjusted to a speed desired by the operator by stopping the operation lever 55 at an arbitrary position in the region or changing the position. For example, the ratios of each of the operation regions RA1, RA3A, RA3B, and RA2 to the lever stroke are as follows.

- Neutral region RA1: 0% or more and less than 15%
- Very low speed region RA3A: 15% or more and less than 45%
- Intermediate speed region RA3B: 45% or more and less than 75%
- Full operation vicinity region RA2: 75% to 100%

In the characteristic diagram shown in FIG. 5, when the operation lever 55 is operated from the G0 position to the G1 position, the secondary pilot pressure (Pa) is generated, and when the operation lever 55 is operated from the G1 position to the G4 position, the secondary pilot pressure increases from Pa to Pb in proportion to the operation amount of the operation lever 55. Further, at the G4 position, the primary pilot pressure is short-cut and flows to the secondary side, and the secondary pilot pressure rises from Pb to the maximum output pressure Pc at once. While the operation lever 55 is operated from the G4 position to the G5 position, the secondary pilot pressure is constant at the maximum output pressure (Pc) and is equal to the primary pilot pressure.

That is, the operation device 56 outputs the primary pilot pressure input to the operation device 56 to the first port PLa and the fourth port PRb when the shift of the operation lever 55 from the neutral position for instructing the movement in the left direction is equal to or larger than the first shift value (shift from G0 to G4). The operation device 56 outputs the primary pilot pressure input to the operation device 56 to the second port PLb and the third port PRa when the shift of the operation lever 55 from the neutral position for instructing the movement in the right direction is equal to or larger than a first shift value (shift from G0 to G4). The operation device 56 outputs the primary pilot pressure input to the operation device 56 to the first port PLa and the third port PRa when the shift of the operation lever 55 from the neutral position for instructing the movement in the forward direction is equal to or greater than a first shift value (shift from G0 to G4). The operation device 56 outputs the primary pilot pressure input to the operation device 56 to the second port PLb and the fourth port PRb when the shift of the operation lever 55 from the neutral position for instructing the movement in the rearward direction is equal to or larger than a first shift value (shift from G0 to G4).

The characteristic value of the secondary pilot pressure in the front-rear direction may be different from the characteristic value of the secondary pilot pressure in the lateral direction. When characteristic values of the secondary pilot pressure in the front-rear direction corresponding to G0 to G5 and Pa to Pc are G0′ to G5′ and Pa′ to Pc′, the operation device 56 may output the primary pilot pressure input to the operation device 56 to the first port PLa and the third port PRa when the shift of the operation lever 55 from the neutral position for instructing the movement in the front direction is equal to or larger than the second shift value (shift from G0′ to G4′). The operation device 56 may output the primary pilot pressure input to the operation device 56 to the second port PLb and the fourth port PRb when the shift of the operation lever 55 from the neutral position for instructing the movement in the backward direction is equal to or larger than a second shift value (shift from G0′ to G4′). Further, Pa and Pb (Pa′ and Pb′) are values that do not depend on the magnitude of the primary pilot pressure, but when the primary pilot pressure is lower than Pa or Pb (Pa′ or Pb′), the secondary pilot pressure reaches the maximum at the magnitude of the primary pilot pressure.

That is, the operation valve (OVA, OVB, OVC, OVD) is configured to convert the pressure of the pilot oil from the traveling primary pressure to the traveling secondary pressure in accordance with the first operation amount (operation lever position) of the operation device 56 and output the pilot oil. The pilot oil of the traveling secondary pressure is applied to the ports PLa, PRa, PLb, and PRb that provide the hydraulic pressure to the swash plates of the hydraulic pumps 7L and 7R. When the first operation amount is equal to or larger than a threshold amount (first displacement value), the operation valve (OVA, OVB, OVC, OVD) converts the primary traveling pressure into the traveling secondary pressure equal to the primary traveling pressure. The control mechanism 25 includes oil passages PA1 to PA14 extending from the pilot pump 71 to the ports (PLa, PRa, PLb, PRb), a pilot pressure control valve CV1, and operation valves OVA, OVB, OVC, OVD. The control mechanism 25 is configured to control the displacement volume of the first hydraulic pump 7L and the displacement volume of the second hydraulic pump 7R. The processor 10a is configured to control the control mechanism 25.

Based on the characteristics of the operation valves OVA, OVB, OVC, and OVD, the movement of the work vehicle 1 corresponding to the operation of the operation lever 55 will be described in more detail. When the operation amount of the operation lever 55 in the front-rear direction is larger than the operation amount in the right direction, the operation position in the right direction is operated from the G1 position to the G3 position, the first hydraulic pump 7L rotate in the same direction in a state where the magnitude of the rotation speed of the first hydraulic pump 7L is larger than the magnitude of the rotation speed of the second hydraulic pump 7R, whereby the work vehicle 1 turns to the right in a large circle. When the operation position of the operation lever 55 in the right direction becomes the same position as the operation position in the front-rear direction, the rotation speed of the second hydraulic pump 7R becomes 0, and only the first hydraulic pump 7L rotates, whereby the work vehicle 1 make a right pivot turn (right pivot turn). Further when the operation lever 55 is operated when the operation position in the right direction is between the G4 position and G5 position, the operation amount becomes larger than that of the operating position in the longitudinal direction, the output shaft of the first hydraulic pump 7L rotates in the normal direction and the output shaft of the second hydraulic pump 7R rotates in the reverse direction, so that the work vehicle 1 turns to the right side.

Further, when the operation amount of the operation lever 55 in the front-rear direction is larger than the operation amount of the operation lever 55 in the left direction and the operation position of the operation lever 55 in the left direction is operated from the G1 position to the G3 position, the second hydraulic pump 7R rotates in the same direction in a state where the magnitude of the rotation speed of the second hydraulic pump 7L is larger than the magnitude of the rotation speed of the first hydraulic pump 7L. When the operation position of the operation lever 55 in the left direction is the same as the operation position in the front-rear direction, the rotation speed of the first hydraulic pump 7L becomes 0, and only the second hydraulic pump 7R rotates. Further, when the operation lever 55 is operated to the left between the G4 position and the G5 position, the operation amount becomes larger than that of the operation position in the front-back direction, the second hydraulic pump 7R rotates in the normal direction, and the first hydraulic pump 7L rotates in the reverse direction. In the present embodiment, turning refers to the operation of the work vehicle 1 when the operation position in the right direction is operated between the G4 position to the G5 position, or when the operation position in the left direction is operated from the G4 position to the G5 position.

On the other hand, when the operation lever 55 is operated to the forward operation position between the G4 position and the G5 position, the operation amount becomes larger than that of the operation position in the lateral direction, and the first and second hydraulic pumps 7L and 7R rotate in the normal direction to move the work vehicle 1 forward at high speed. When the operation lever 55 is operated to the position between the G4 position and the G5 position, the operation amount in the rearward direction becomes larger than the operation amount in the lateral direction, and the drive shafts of the first and second hydraulic pumps 7L and 7R are reversed to move the work vehicle backward at a high speed. The other operations of the operation lever 55 in the front-rear direction are the same as those in the right-left direction.

The work vehicle 1 is provided with various switches and sensors connected to the controller 10 described above. FIG. 6 is a block diagram of the work vehicle 1. Referring to FIG. 6, the work vehicle 1 includes a creep setting member 16 provided around the driver's seat 54. The creep setting member 16 may be referred to as a first operation member. The creep setting member 16 is constituted by, for example, a touch panel, a slidable slide switch, or a dial. Creeping refers to control for causing the work vehicle 1 to travel at a speed equal to or lower than the upper limit speed regardless of the operation amount of at least one operation device (one or a plurality of operation levers 55) to which a speed change operation of the user is input.

The creep setting member 16 is configured to receive an input corresponding to the setting stage of the plurality of speed stages respectively corresponding to a plurality of sets of the target rotation speeds at which the first hydraulic motor 31L and the second hydraulic motor 31R respectively rotate to move the work vehicle 1 straight, the first hydraulic motor 31L and the second hydraulic motor 31R being configured to drive the first traveling device 3L and the second traveling device 3R, respectively. When a speed ratio between the target rotation speed of the first hydraulic motor 31L and a moving speed of the first traveling device 3L is equal to a speed ratio between the target rotation speed of the second hydraulic motor 31R and a moving speed of the second traveling device 3R, the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R in each speed stage may be equal. The target rotation speed of the speed stage corresponds to the traveling primary pressure at which the rotation speed of the first hydraulic motor 31L and the rotation speed of the second hydraulic motor 31R become the target rotation speed when the operation lever 55 is tilted in the forward direction (or the rearward direction) and the lever operation position is operated between G4 and G5.

In the first embodiment, a first high pilot pressure, which is a higher one of traveling secondary pressures which is applied to ports (PLa, PLb) and by which a displacement volume of the first hydraulic pump 7L is determined, is referred to as a first control parameter by which a displacement volume of the first hydraulic pump 7L is determined. The second high pilot pressure, which is the higher one of the traveling secondary pressures which is applied to the ports (PRa, PRb) and by which the displacement volume of the second hydraulic pump 7R is determined, is referred to as a second control parameter by which the displacement volume of the second hydraulic pump 7R is determined. The range of the first control parameter limited by the traveling primary pressure is referred to as an operating range of the first control parameter. The range of the second control parameter limited by the traveling primary pressure is referred to as an operating range of the second control parameter.

The speed stage may be represented by a number (stage number) represented by a natural number starting from 1, for example. The larger the stage number become, the larger the target rotation speed of the first hydraulic motor 31L (the maximum rotation speed of the first hydraulic motor 31L) and the target rotation speed of the second hydraulic motor 31R (the maximum rotation speed of the second hydraulic motor 31R) becomes. The difference of speed stages is referred to as a difference in stages. The difference in stages between the speed stage i and the speed stage k (I>k) is (i−k+1). FIG. 7 shows an example of the speed stage. In FIG. 7, when the maximum value of the speed stage is N and the speed stage is m, and the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R are the same Nm (m), the target rotation speed Nm (m) is set to monotonically increase as m increases.

The creep setting member 16 is configured to switch between a normal mode and a creep mode. A mode in which the setting stage is set by the creep setting member 16 is referred to as a creep mode. A state other than the creep mode is referred to as a normal mode. In the normal mode, the target rotation speed of the engine 6 is set by the operation of the first setting member 11a and the second setting member 11b, and the traveling primary pressure corresponding to the target rotation speed is obtained based on the setting line L1 or L2 of FIG. 4. The traveling secondary pressure is set based on the operation amount of one or more operation levers 55, and the hydraulic motors (31L, 31R) and the hydraulic pumps (7L, 7R) are controlled. That is, in the normal mode, the speed of the work vehicle 1 can be changed in accordance with the operation amount of at least one operation device, and the work vehicle 1 can be made to travel at a speed higher than the upper limit speed.

In the creep mode, the target rotation speed of the engine 6 is set by operating the second setting member 11b, and the maximum value of the angle of the swash plate of the hydraulic pumps (7L, 7R) required from the target rotation speed of the engine 6 is determined based on the target rotation speed of the first hydraulic motor 31L (maximum rotation speed of the first hydraulic motor 31L) and the target rotation speed of the second hydraulic motor 31R (maximum rotation speed of the second hydraulic motor 31R) predetermined in the speed stage, and the traveling primary pressure for outputting pilot pressure for realizing a maximum value of the swash plate is determined. That is, for example, in the example of FIG. 7, when the speed stage m is determined, the target rotation speeds of the first hydraulic motor 31L and the second hydraulic motor 31R are obtained as Nm (m), and thus the maximum values of the angles of the swash plates of the hydraulic pumps (7L and 7R) are obtained based on the target rotation speed of the engine 6 set by the second setting member 11b, and the first control parameter and the second control parameter are determined based on the obtained angles of the swash plates. Therefore, the setting line L1 or L2 of FIG. 4 is not used, and the traveling primary pressure is determined to be lower than the traveling primary pressure in the normal mode by using the speed stage. In the creep mode, the first setting member 11a is not used to change the target rotation speed of the engine 6, and the speed stage is switched based on the operation of the first setting member 11a. For example, the gear stage number of the speed stage temporarily increases only at the moment when the accelerator pedal is depressed. The setting of the traveling secondary pressure and thereafter in the creep mode is the same as in the normal mode, but the traveling secondary pressure is equal to or lower than the traveling primary pressure, and therefore, by limiting the traveling primary pressure, the speed of the work vehicle 1 is limited to be equal to or lower than the upper limit speed determined from the target rotation speed set by the speed stage, regardless of the operation amount of at least one operation device (one or more operation levers 55). In the creep mode, the first hydraulic motor 31L and the second hydraulic motor 31R are fixed at the first speed.

Referring to FIG. 3 and the work vehicle 1 includes a first hydraulic sensor SP11 for detecting the hydraulic pressure of the first pilot oil passage PA11, a second hydraulic sensor SP12 for detecting the hydraulic pressure of the second pilot oil passage PA12, a third hydraulic sensor SP13 for detecting the hydraulic pressure of the third pilot oil passage PA13, and a fourth hydraulic sensor SP14 for detecting the hydraulic pressure of the fourth pilot oil passage PA14. As described above, the secondary pilot pressure output from the secondary side port of each of the operation valves OVA, OVB, OVC, and OVD changes in accordance with the operation position of the operation lever 55. Therefore, the first hydraulic sensor SP11 to the fourth hydraulic pressure sensor SP14 are configured to detect the operation of the operation lever 55. The first hydraulic sensor SP11 to the fourth hydraulic sensor SP14 are sensors for detecting the traveling secondary pressure. The first hydraulic sensor SP11 to the fourth hydraulic sensor SP14 may be referred to as additional hydraulic sensors.

The work vehicle 1 includes a hydraulic sensor SP5L for detecting the hydraulic pressure of the driving oil passage PA5L, a hydraulic sensor SP6L for detecting the hydraulic pressure of the driving oil passage PA6L, a hydraulic sensor SP5R for detecting the hydraulic pressure of the driving oil passage PA5R, and a hydraulic sensor SP6R for detecting the hydraulic pressure of the driving oil passage PA6R. That is, the hydraulic sensors (SP5L, SP6L, SP5R, SP6R) are configured to detect the hydraulic pressure of the hydraulic fluid in the driving oil passages (PA5L, PA6L, PASR, PA6R). The states of the first hydraulic motor 31L and the second hydraulic motor 31R can be detected from the difference between the pressures of the hydraulic sensor SP5R and the hydraulic sensor SP6R and the difference between the pressures of the hydraulic sensor SP5R and the hydraulic sensor SP6R.

Referring to FIGS. 2, 3, and 6, the work vehicle 1 may further includes a rotation sensor SR31L that is connected to the rotation shaft of the first hydraulic motor 31L and detects a rotation speed of the first hydraulic motor 31L, and a rotation sensor SR31R that detects a rotation speed of the second hydraulic motor 31R. The states of the first hydraulic motor 31L and the second hydraulic motor 31R can be detected from the magnitude of the rotation direction and the rotation speed detected by the rotation sensor SR31L and the magnitude of the rotation direction and the rotation speed detected by the rotation sensor SR31R. The work vehicle 1 may include an operation detection sensor 18 for detecting the operation position of the operation lever 55. The operation detection sensor 18 is connected to the controller 10 described later. The operation detection sensor 18 is a position sensor or the like that detects the position of the operation lever 55.

In the present embodiment, considering that the travel primary pressure required for the work vehicle 1 to turn at a desired upper limit speed increases as the travel resistance received by the work vehicle 1 from the ground increases as the curvature of the turn of the work vehicle 1 increases (as the turning radius decreases), the controller 10 obtains the deviation amount between the target moving direction and the straight direction, and controls the travel primary pressure to be larger by setting a difference between the speed stage and the setting stage set by the creep setting member 16 larger as the deviation amount becomes larger. In the following description of the embodiment, the speed stage whose stage number is set is larger than the stage number of the setting stage is referred to as a boost stage for the above-described reason. The deviation amount is detected from the operation position of the operation lever 55, the traveling direction obtained from a change in position of a Global Positioning System device or the like mounted in the vehicle, the rotation speed difference between the rotation speed of the first hydraulic motor 31L detected by the rotation sensor SR31L and the rotation speed of the second hydraulic motor 31R detected by the rotation sensor SR31R, and the like. Hereinafter, a case where the deviation amount is detected from the secondary pilot pressure detected by the first hydraulic sensor SP11 to the fourth hydraulic sensor SP14 will be described in detail.

When the deviation amount is represented by using these secondary pilot pressures, the following method may be used. The controller 10 acquires the first forward pilot pressure lf (t) applied to the first port PLa from the first hydraulic sensor SP11. The controller 10 acquires the first backward pilot pressure lb (t) applied to the second port PLb from the second hydraulic sensor SP12. The controller 10 acquires the second forward pilot pressure rf (t) applied to the third port PRa from the third hydraulic sensor SP13. The controller 10 acquires the second reverse pilot pressure rb (t) applied to the fourth port PRb from the fourth hydraulic sensor SP14. The first forward pilot pressure lf (t), the first backward pilot pressure lb (t), the second forward pilot pressure rf (t), and the second lb (t) pilot pressure rb (t) correspond to information corresponding to the target moving direction. The controller 10 determines which of the following first to fourth instructions is input to the operation lever 55 from the first forward pilot pressure lf (t), the first backward pilot pressure lb (t), the second forward pilot pressure rf (t), and the second backward pilot pressure rb (t).

- (1) the first instruction: to rotate the first hydraulic motor 31L so as to drive the first traveling device 3L in the forward direction, and to rotate or stop the second hydraulic motor 31R so as to drive the second traveling device 3R in the backward direction with respect to the first traveling device 3L.
- (2) the second instruction: to rotate the first hydraulic motor 31L so as to drive the first traveling device 3L in the backward direction, and to rotate or stop the second hydraulic motor 31R so as to drive the second traveling device 3R in the forward direction with respect to the first traveling device 31R.
- (3) the third instruction: to rotate the second hydraulic motor 31R so as to drive the second traveling device 3R in the forward direction, and to rotate or stop the first hydraulic motor 31L so as to drive the first traveling device 3L in the backward direction with respect to the second traveling device 3R.
- (4) the fourth instruction: to rotate the second hydraulic motor 31R so as to drive the second traveling device 3R in the backward direction, and to rotate or stop the first hydraulic motor 31L so as to drive the first traveling device 3L in the forward direction with respect to the second traveling device 3R.

The determination as to which of the first to fourth instructions can be performed by the following method disclosed in Japanese Patent Publication No. 2022-033104. Japanese Patent Publication No. 2022-033104 is incorporated by reference. For example, when it is determined that the vehicle is turning right (step S11 in FIG. 16 of that patent publication) and is not moving backward (step S10 in FIG. 4 of that patent publication) from the four pilot pressures, the controller 10 can estimate that the first instruction has been issued. When it is determined from the four pilot pressures that the vehicle is turning to the right (step S11 in FIG. 16 of that patent publication) and is not moving forward (step S10 in FIG. 9 of that patent publication), the controller 10 can estimate that the second instruction has been issued.

When it is determined from the four pilot pressures that the vehicle is turning left (step S11 in FIG. 13 of that patent publication) and is not moving backward (step S10 in FIG. 4 of that patent publication, the controller 10 can estimate that the third instruction has been issued. When it is determined from the four pilot pressures that the vehicle is turning to the left (step S11 in FIG. 13 of that patent publication) and is not moving forward (step S10 in FIG. 9 of that patent publication), the controller 10 can estimate that the fourth instruction has been issued.

Thereafter, the controller 10 determines the operation degree corresponding to the target value of the speed difference between the rotation speed of the first hydraulic motor 31L and the rotation speed of the second hydraulic motor 31R in any of the above-described instructions from at least two pressures of the first forward pilot pressure lf (t), the first backward pilot pressure lb (t), the second forward pilot pressure rf (t), and the second backward pilot pressure rb (t). For example, in the case of the first instruction, the controller 10 can determine the operation degree such that the smaller rf(t)/lf(t)*lb(t)/rb(t) is, the larger the operation degree is. In the case of the second instruction, the controller 10 can determine the operation degree such that the smaller rb(t)/lb(t)*lf(t)/rf(t) is, the larger the operation degree is. In the case of the third instruction, the controller 10 can determine the operation degree such that the smaller the value of lf(t)/rf(t)*rb(t)/lb(t) is, the larger the operation degree is. In the case of the second instruction, the controller 10 can determine the operation degree such that the smaller lb(t)/rb(t)*rf(t)/lf(t) is, the larger the operation degree is. Recognizing any of the first instruction to the fourth instruction and obtaining the operation degree corresponds to obtaining the target moving direction. The controller 10 determines the deviation amount such that the deviation amount increases as the operation degree increases.

The deviation amount can be expressed in various ways, and the straightness can be used as one concept. The concept of the straightness will be described below. The state of high straightness is (1) a state where the pilot pressures at the ports (first port PLa, third port PRa) for the forward rotation of the outputs of the first and second hydraulic pumps 7L, 7R are sufficiently higher than the pilot pressures at the ports (second port PLb, fourth port PRb) for the backward rotation of the outputs of the first and second hydraulic pumps 7L, 7R. The state of high straightness is (1) a state where the pilot pressures at the ports (first port PLa, third port PRa) for the forward rotation of the outputs of the first and second hydraulic pumps 7L, 7R are sufficiently higher than the pilot pressures at the ports (second port PLb, fourth port PRb) for the backward rotation of the outputs of the first and second hydraulic pumps 7L, 7R, (2) The pilot pressures of the two ports determined to have high pilot pressures in (1) are substantially equal to each other (the value of the ratio of the two pilot pressures is within a predetermined range close to 1 (for example, between 0.9 and 1/0.9)).

Therefore, the straightness is obtained by the following algorithm. First, the larger value of lb (t) and rb (t) is substituted into a variable PV_Fstraight. The larger value of lf (t) and rf (t) is substituted into the variable PV_Bstraight.

Straightness in forward direction S_Fratio(t) is obtained by Equation (1). The backward straightness S_Bratio(t) is obtained by Equation (2).

S
_Fratio(t)={lf(t)+rf(t)}/{2×PV_Fstraight} (1)

S
_Bratio(t)={lb(t)+rb(t)}/{2×PV_Bstraight} (2)

Here, the processor 10a calculates S_Fratio(t) and S_Bratioand the larger value of the two is calculated as the degree of straightness.

In the present embodiment, for example, 300 that can be generally regarded as straight traveling is set as a reference straightness, and it can be regarded that the deviation amount between the target moving direction and the straight traveling direction increases as the straightness decreases.

The controller 10 includes a processor 10a and a memory 10b as shown in FIG. 6 in order to realize the control of the vehicle speed in the creep mode. The processor 10a may be referred to as an electronic circuit. The memory 10b includes volatile memory and non-volatile memory. The memory 10b may be referred to as a storage unit. The memory 10b includes at least a travel control program 10c1 for realizing the above-described control and first reference information 10r1, second reference information 10r2, third reference information 10r3 and fourth reference information 10r4.

As shown in FIG. 7, the first reference information 10r1 represents a first correspondence relationship between the above-described speed stages in the creep mode and sets of the target rotation speeds of the first hydraulic motor 31L and the target rotation speeds of the second hydraulic motor 31R each corresponding to the speed stages. That is, the memory 10b is configured to store a plurality of speed stages corresponding to a plurality of sets of target rotation speeds at which the first hydraulic motor 31L and the second hydraulic motor 31R respectively rotate when the first traveling device 3L and the second traveling device 3R are driven to move the vehicle body 2 straight. In principle, in the same speed stage, the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R are the same value. However, when the reduction gear ratio of the reduction gear between the first hydraulic motor 31L and the first traveling device 3L is different from the reduction gear ratio of the reduction gear between the first hydraulic motor 31L and the first traveling device 3L, the target rotation speed of the first hydraulic motor 31L may be different from the target rotation speed of the second hydraulic motor 31R. Further, the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R may be changed in accordance with the operation degree in each of the first to fourth instructions. Although the primary pressure Pp (m), which is the first control parameter and the second control parameter, is also displayed in FIG. 7, since these are values calculated from the target rotation speed of the engine 6 and the target rotation speed Nm (m), the first reference information 10r1 may not include the primary pressure Pp (m). The second reference information 10r2 represents the relationship between the setting stage and the boost stage with respect to the deviation amount between the target moving direction and the straight direction. The second reference information 10r2 will be described in detail later.

The third reference information 10r3 represents a third correspondence relationship between the rotation speed RS of the engine 6 detected by the speed sensor 6a and the traveling primary pressure in the normal mode. That is, the third reference information 10r3 represents the third correspondence relationship represented by the setting line L1 of FIG. 4. The fourth reference information 10r4 represents a fourth correspondence relationship between the rotation speed RS of the engine 6 detected by the speed sensor 6a and the traveling primary pressure, which is used for the control of the traveling primary pressure when the drop amount of the engine 6 is large in the normal mode. That is, the fourth reference information 10r4 represents the fourth correspondence relationship represented by the setting line L2 of FIG. 4.

The processor 10a executes the following control while executing the travel control program 10c1 with reference to the first reference information 10r1, the second reference information 10r2, the third reference information 10r3, and the fourth reference information 10r41. First, the processor 10a is configured to acquire the rotation speed RS of the engine 6 from the speed sensor 6a when the normal mode is selected by the creep setting member 16, to obtain the traveling primary pressure corresponding to the detected rotation speed RS of the engine 6 from the third reference information, and to control the pilot pressure control valve CV1 so as to achieve the obtained traveling primary pressure. When the drop amount of the engine 6 is large in the normal mode, the processor 10a is configured to obtain the traveling primary pressure corresponding to the rotation speed RS of the engine 6 detected by the speed sensor 6a from the fourth reference information, and control the pilot pressure control valve CV1 so as to achieve the obtained traveling primary pressure.

The processor 10a receives the setting stage from the creep setting member 16, and receives information on the target moving direction from the first hydraulic sensor SP11 to the fourth hydraulic sensor SP14 that detect the operation of the operation lever 55. The processor 10a calculates a degree of straightness representing the target moving direction. And the processor 10a calculates the target moving direction and the straight direction obtained from the straightness. When it is determined that the creep setting member 16 is operated so that the deviation amount is within the predetermined first range Rdev1, the operating range (range having the travel primary pressure based on the setting stage as the upper limit) of the first control parameter (first high pilot pressure) for controlling the displacement volume of the first hydraulic pump 7L and the operating range (range having the travel primary pressure based on the setting stage as the upper limit) of the second control parameter (second high pilot pressure) for controlling the displacement volume of the second hydraulic pump 7R are determined based on the setting stage. More specifically, the processor 10a determines the operating range of the first control parameter and the operating range of the second control parameter based on the target rotation speed of the engine 6 set by the first setting member 11a and the target rotation speed Nm (m) obtained from the setting stage. The first range Rdev1 is a range of the deviation amount based on which rotation of at least one of the first hydraulic motor 31L and the second hydraulic motor 31R is operable when the setting stage is set to the minimum speed stage among the plurality of speed stages (the speed stage whose stage number is 1) which is a speed stage corresponding to a set of target rotation speeds lowest among target rotation speeds of the plurality of speeds. When it is determined that the creep setting member 16 is operated so that the deviation amount is out of the first range Rdev1, the processor 10a determines a boost stage which is a speed stage corresponding to a set of target rotation speeds higher than the target rotation speeds of the setting stage based on the magnitude of the deviation amount, and determines an operating range of at least one control parameter of the first control parameter and the second control parameter based on the boost stage. In the present embodiment, the at least one control parameter includes a first control parameter and a second control parameter.

FIG. 8 shows an example of the second reference information 10r2 according to the first embodiment. In FIG. 8, the horizontal axis represents the straightness, and the vertical axis represents the difference in stages (boost stage—setting stage). Since a high straightness indicates that the deviation amount between the target moving direction and the straight direction is small, the first range Rdev1 described above is set as a range of higher straightness than the threshold value Th1 of the straightness degree. When the straightness is equal to or greater than the threshold value Th1, the operating range of the first control parameter and the operating range of the second control parameter are determined based on the setting stage, and thus the difference in stages between the boost stage and the setting stage is 0. The threshold value Th1 is a value empirically obtained as a minimum degree of straightness at which rotation of at least one of the first hydraulic motor 31L and the second hydraulic motor 31R is operable when the minimum speed stage (the speed stage whose stage number is 1) is set.

In the present embodiment, when the number of speed stages representing a set of a target rotation speed of the first hydraulic motor 31L between the target rotation speed of the first hydraulic motor 31L of the boost stage and the target rotation speed of the first hydraulic motor 31L of the setting stage and a target rotation speed of the second hydraulic motor 31R between the target rotation speed of the second hydraulic motor 31R of the boost stage and the target rotation speed of the second hydraulic motor 31R of the setting stage is m (m is an integer of 0 or more), the boost stage is determined such that the difference in stages increases as the deviation amount increases when the deviation amount is out of the first range Rdev1. That is, the difference in stages between the boost stage and the setting stage is controlled to increase as the straightness decreases from the threshold value Th1.

However, when the boost stage is set to the minimum turnable stage in which the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R are the minimum among at least one of the speed stages in which rotation of at least one of the first hydraulic motor 31L and the second hydraulic motor 31R is operable regardless of any operation executed by the operation lever, the difference in stages between the boost stage and the setting stage reaches a ceiling at a predetermined upper limit because it is not necessary to further increase the stage number of the boost stage. That is, among the plurality of speed stages, when a number of the speed stages corresponding to a set of the target rotation speed of the first hydraulic motor 31L between the target rotation speed of the first hydraulic motor 31L of the minimum turnable speed stage and the target rotation speed of the first hydraulic motor 31L of the minimum speed stage (speed stage whose the stage number is 1), and the target rotation speed of the second hydraulic motor 31R between the target rotation speed of the second hydraulic motor 31R of the minimum turnable speed stage and the target rotation speed of the second hydraulic motor 31R of the minimum speed stage (the speed stage with the stage number 1) is n (n is an integer of 0 or more), n+1 is set as the maximum value of the difference in stages. The stage number of the minimum turnable speed stage is n+2.

In the example of FIG. 8, the difference in stages between the boost stage and the setting stage is linearly changed to increase as the straightness decreases between the threshold value Th0 (Th0<Th1) and the threshold value Th1, and the difference in stages between the boost stage and the setting stage is set to the upper limit DMAX (=n+1) when the straightness is less than the threshold value Th0. When the threshold value of the straightness corresponding to the arbitrary difference in stages j and j+1 are SD (j) and SD (j+1), respectively, the threshold values SD (j) and SD (j+1) are empirically determined so that at least one of the first hydraulic motor 31L and the second hydraulic motor 31R is rotatable at the speed stage (j+1) if the straightness is between SD (j) and SD (j+1). That is, the difference in stages is determined such that at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates in the range of the deviation amount (SD (j) and SD (j+1)) based on which the boost stage is determined. In other words, the difference in stages is determined such that at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates in the range (SD (j) to SD (j+1)) of the deviation amount based on which the boost stage is determined.

Note that, when the straightness is between threshold value Th0 (Th0<Th1) and threshold value Th1, the relationship between the straightness and the difference in stages may be a monotonous decrease, and does not necessarily have to be a linear decrease. That is, when the threshold value of the straightness corresponding to the arbitrary difference in stages j and j+1 are SD (j) and SD (j+1), respectively, the relationship between the straightness and the difference in stages is determined so that SD (j+1)<SD (j) is satisfied.

The processor 10a is configured to control the pilot pressure control valve CV1 (primary pressure control valve) so that the set pressure controlled as the traveling primary pressure determined based on the boost stage or the setting stage is set to the upper limit value of the operating range of the first control parameter (first high pilot pressure) and the operating range of the second control parameter (second high pilot pressure). The processor 10a is configured to control the control mechanism 25 that controls the first control parameter not to deviate from the operating range of the first control parameter determined based on the boost stage or the setting stage, and control the control mechanism 25 that controls the second control parameter not to deviate from the operating range of the second control parameter determined based on the boost stage or the setting stage. The processor 10a is configured to control the engine 6 configured to rotate the first and second hydraulic pumps 7L and 7R so as to rotate the first and second hydraulic motors 31L and 31R displacement volumes of which are set to their respective constant displacement volumes (set to the first speed) irrespective of the plurality of speed stages.

FIG. 9 is a flowchart showing the operation of the work vehicle 1 according to the first embodiment. In the work vehicle 1 according to the present embodiment, in the present flowchart, the processing from step S1 to step S14 is executed at predetermined sampling intervals (for example, 20 μs). In step S1, the processor 10a acquires the rotation speed RS of the engine 6 detected by the speed sensor 6a. That is, the method of controlling the work vehicle 1 according to the present embodiment includes acquiring the rotation speed RS of the engine 6 detected by the speed sensor 6a. In step S2, the processor 10a determines whether or not the creep mode is selected by the creep setting member 16. That is, the control method according to the present embodiment includes determining whether or not the creep mode is selected by the creep setting member 16. When the creep mode is set, that is, when the setting stage is set (Yes in step S2), the process proceeds from step S3 to S8. When the normal mode is set, that is the upper limit speed is not set or the setting stage is not set (No in step S2), the process proceeds from step S9 to S11.

When the creep mode is set (Yes in step S3), the processor 10a receives the setting stage set by the creep setting member 16 (first operation member). Thereafter, in step S4, the processor 10a acquires the first forward pilot pressure lf (t) from the first hydraulic sensor SP11. The processor 10a acquires the first backward pilot pressure lb (t) from the second hydraulic sensor SP12. The processor 10a acquires the second forward pilot pressure rf (t) from the third hydraulic sensor SP13. The processor 10a acquires the second backward pilot pressure rb (t) from the fourth hydraulic sensor SP14. The processor 10a determines the straightness from the above-mentioned values based on the above-mentioned algorithm. That is, in the control method according to the present embodiment, in step S4, the processor 10a receives information (lf (t), lb (t), rf (t), rb (t)) indicating the target moving direction of the vehicle body 2 determined by the speed difference between the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R from the sensors (SP11, SP12, SP13, SP14) detecting the operation of the operation lever 55 (second operation member) for setting the target moving direction, and obtains the straightness corresponding to the target moving direction.

In step S5, the processor 10a determines whether or not the operation lever 55 (second operation member) is operated such that the deviation amount between the target moving direction and the straight direction is within the predetermined first range Rdev1. When the operation lever 55 (second operation member) is operated so that the deviation amount is within the predetermined first range Rdev1 (Yes in step S5), the processor 10a determines the traveling primary pressure based on the setting stage in step S6. That is, the processor 10a determines, based on the setting stage, an operating range (a range having the traveling primary pressure based on the setting stage as an upper limit) of a first control parameter (first high pilot pressure) by which a displacement volume of the first hydraulic pumps 7L configured to supply the hydraulic fluid to the first hydraulic motor 31L is determined, and an operating range (a range having the traveling primary pressure based on the setting stage as an upper limit) of a second control parameter (second high pilot pressure) by which a displacement volume of the second hydraulic pump 7R configured to supply the hydraulic fluid to the second hydraulic motor 31R is determined. To be more specific, the processor 10a obtains the maximum value of the angle of the swash plate of the hydraulic pumps (7L, 7R) based on the target rotation speed of the engine 6 set by the first setting member 11a and the target rotation speed of the first hydraulic motor 31L and the second hydraulic motor 31R known from the setting stage, and determines the traveling primary pressure so that the swash plate has the angle when the operation lever 55 is tilted to the maximum. Strictly speaking, the rotation speed RS of the engine 6 detected by the speed sensor 6a and obtained in step SI is different from the target rotation speed of the engine 6 set by the first setting member 11a, but the processor 10a performs feedback control for bringing the rotation speed RS of the engine 6 close to the target rotation speed of the engine 6, separately from the speed control method according to the present embodiment.

When the operation lever 55 (second operation member) is operated so that the deviation amount is out of the predetermined first range Rdev1 (No in step S5), in step S7, the processor 10a refers to the second reference information 10r2 and determines the boost stage which is the speed stage corresponding to the set of the target rotation speeds higher than the target rotation speeds of the setting stage based on the magnitude of the deviation amount (threshold value Th—degree of straightness obtained in the step S4). In step S8, the processor 10a is configured to, based on the boost stage, determine an operating range of (a range having the traveling primary pressure based on the setting stage as an upper limit) at least one of a first control parameter and a second control parameter. In particular, the processor 10a determines the operating range of the first control parameter and the operating range of the second control parameter based on the boost stage. To be more specific, the processor 10a obtains the maximum value of the angle of the swash plate of the hydraulic pumps (7L, 7R) based on the target rotation speed of the engine 6 set by the first setting member 11a and the target rotation speed of the first hydraulic motor 31L and the second hydraulic motor 31R known from the boost stage, and determines the traveling primary pressure so that the swash plate has the angle when the operation lever 55 is tilted to the maximum.

In step S12, the processor 10a controls the pilot pressure control valve CV1 that feeds the pilot oil to the operation valves OVA, OVB, OVC, and OVD so that the traveling primary pressure obtained in step S6 or S8 is achieved. In step S13, the operation valves OVA, OVB, OVC, and OVD convert the traveling primary pressure into the traveling secondary pressure based on the lever position (first operation amount) of the operation lever 55 (second operation member). That is, the control method according to the present embodiment includes converting the traveling primary pressure into the traveling secondary pressure based on the lever position (first operation amount) of the operation lever 55 (first operation device) by the operation valves OVA, OVB, OVC, and OVD. In step S12 and step S13, the control method according to the present embodiment controls the first control parameter (first high pilot pressure) determined based on the boost stage or the setting stage not to deviate from the operating range (range having the traveling primary pressure based on the setting stage as the upper limit) of the first control parameter, and controls the second control parameter (second high pilot pressure) not to deviate from the operating range (range having the traveling primary pressure based on the setting stage as the upper limit) of the second control parameter.

In a step S14, the processor 10a controls the engine 6 configured to rotate the first and second hydraulic pumps 7L, 7R. Accordingly, the traveling secondary pressure of the pilot oil is applied to the ports Pla, Pra, PLb, and PRb through which the hydraulic pressure is applied to the swash plates of the hydraulic pumps 7L and 7R. Thus, the processor 10a causes rotation of the first and second hydraulic pumps 7L and 7R whose displacement volumes are set to their respective constant displacement volumes (set to the first speed) regardless of the plurality of speed stages. The control method according to the present embodiment drives the first traveling device 3L and the second traveling device 3R by rotating the first hydraulic pump 7L and the second hydraulic pump 7R whose displacement volumes are set to their constant displacement volumes regardless of the plurality of speed stages.

In the normal mode (Yes in step S2), in step S9, the processor 10a determines whether or not there is an engine drop. That is, in step S9, the processor 10a determines whether or not the drop amount ΔE1 of the engine 6 is equal to or larger than the anti-stall determination value. When the engine drop does not occur (No in step S9), the processor S10 obtains the traveling primary pressure from the third reference information 10r3 based on the rotation speed RS of the engine 6 in step S10. That is, the control method according to the present embodiment includes preparing the third reference information 10r3, and when the normal mode is selected from the creep mode and the normal mode, the traveling primary pressure corresponding to the rotation speed RS of the engine 6 detected by the speed sensor 6a is obtained from the third reference information 10r3. When the engine drop is present (Yes in step S9), the processor S11 obtains the traveling primary pressure from the fourth reference information 10r4 based on the rotation speed RS of the engine 6 in step 10r4. That is, the control method according to the present embodiment includes preparing the fourth reference information 10r4, and when the normal mode is selected, the traveling primary pressure corresponding to the rotation speed RS of the engine 6 detected by the speed sensor 6a is obtained from the fourth reference information 10r4 when the engine drop occurs. After the process of step S10 or step S11 is finished, the processes of steps S12 to S14 are executed.

In the work vehicle 1 and the control method of the work vehicle 1 according to the first embodiment, when the operation lever 55 (second operation member) is operated such that the deviation amount between the target moving direction and the straight direction is out of the first range Rdev1, the processor 10a determines the boost stage as the speed stage corresponding to the set of the target rotation speeds higher than the target rotation speeds of the setting stage based on the deviation amount, determines the operating range of at least one of the first control parameter (first high pilot pressure) and the second control parameter (second high pilot pressure) based on the boost stage, and controls the control mechanism 25 not to deviate from the determined operating range. As a result, the first control parameter (first high pilot pressure) and the second control parameter (second high pilot pressure) are controlled in consideration of the turning situation, and thus the time required for the vehicle to reach a desired speed can be shortened even when the vehicle is turning with a large curvature.

In the present embodiment, the first control parameter is the first high pilot pressure, which is a higher one of the traveling secondary pressures which is applied to the ports (PLa, PLb) and by which the displacement volume of the first hydraulic pump 7L is determined, but the first control parameter may be the first pilot effective pressure obtained by subtracting the first low pilot pressure, which is the lower one of the two pilot pressures by which the displacement volume of the first hydraulic pump 7L is determined, from the first high pilot pressure, which is the higher one of the two pilot pressures. In the present embodiment, the second control parameter is the second high pilot pressure of the traveling secondary pressures which is applied to the ports (PRa, PRb) and by which the displacement volume of the second hydraulic pump 7R, is determined but the second control parameter may be the second pilot effective pressure obtained by subtracting the lower second low pilot pressure from the second high pilot pressure of the two pilot pressures by which the displacement volume of the second hydraulic pump 7R is determined. At this time, the at least one of the third pressure control valve PCV3 provided in the first pilot hydraulic circuit PCI to control the first pilot effective pressure, which is the difference between the two pilot pressures applied to the two pilot ports (PLa, PLb) of the first hydraulic pump 7L, includes the operation valve OVA (first pressure reducing valves), the operation valve OVB (second pressure reducing valves), and the pilot pressure control valve CV1. The at least one fourth pressure control valve PCV4 provided in the second pilot hydraulic circuit PC2 to control the second pilot effective pressure, which is the difference between the two pilot pressures applied to the two pilot ports (PRa, PRb) of the second hydraulic pump 7R, includes the operation valve OVC (third pressure reducing valve), the operation valve OVD (fourth pressure reducing valve), and the pilot pressure control valve CV1.

FIG. 10 shows a hydraulic system (hydraulic circuit 20A) of a work machine different from that of FIG. 3. Note that the control mechanism according to the present modification may be referred to as a control mechanism 25A. The hydraulic system of the work machine of FIG. 10 is different in the connection between the operation device 56 and the first and second hydraulic pumps 7L and 7R. In FIG. 10, the other configuration is the same as that of FIG. 3. In FIG. 10, the first to fourth shuttle valves SVa, SVb, SVc, and SVd are not provided. The operation valve OVA for forward movement is connected to the first pilot oil passage PA11, the operation valve OVB for backward movement is connected to the second pilot oil passage PA12, the operation valve OVC for right turning is connected to the third pilot oil passage PA13, and the operation valve OVD for left turning is connected to the fourth pilot oil passage PA14.

The operation valve OVA is a first pressure reducing valve provided in the first pilot oil passage PA11 and configured to convert the primary pressure supplied by the pilot pump 71 into the secondary pressure according to the rotation direction and the rotation amount of the operation lever 55. The operation valve OVB is a second pressure reducing valve that is provided in the second pilot oil passage PA12 and is configured to convert the primary pressure supplied by the pilot pump 71 into a secondary pressure according to the rotation direction and the rotation amount of the operation lever 55. The operation valve OVC is a third pressure reducing valve provided in the third pilot oil passage PA13 and configured to convert the primary pressure supplied by the pilot pump 71 into the secondary pressure according to the rotation direction and the rotation amount of the operation lever 55. The operation valve OVD is a fourth pressure reducing valve provided in the fourth pilot oil passage PA14 and configured to convert the primary pressure supplied by the pilot pump 71 into the secondary pressure according to the rotation direction and the rotation amount of the operation lever 55.

In this modification, the first control parameter may be a high pilot pressure which is a higher one of the traveling secondary pressures applied to the ports (PLa, PLb) by which the displacement volume of the first hydraulic pump 7L is determined, or may be the first pilot effective pressure obtained by subtracting the lower first low pilot pressure from the higher first high pilot pressure of the two pilot pressures by which the displacement volume of the first hydraulic pump 7L is determined. The second control parameter may be a second high pilot pressure which is a higher one of the traveling secondary pressures applied to the ports (PRa, PRb) by which the displacement volume of the second hydraulic pump 7R is determined, or may be a second pilot effective pressure obtained by subtracting a second low pilot pressure which is a lower one of the two pilot pressures by which the displacement volume of the second hydraulic pump 7R is determined from the second high pilot pressure which is a higher one of the two pilot pressures.

In this case, the at least one first pressure control valve PCV1 includes the operation valve OVA (first pressure reducing valve), the operation valve OVB (second pressure reducing valve), and the pilot pressure control valve CV1. The at least one second pressure control valve PCV2 includes the operation valve OVC (third pressure reducing valve), the operation valve OVD (fourth pressure reducing valve), and the pilot pressure control valve CV1. At least one third pressure control valve PCV3 includes the operation valve OVA (first pressure reducing valve), the operation valves OVB (second pressure reducing valve), and the pilot pressure control valve CV1. The at least one fourth pressure control valve PCV4 includes the operation valve OVC (third pressure reducing valve), the operation valve OVD (fourth pressure reducing valve), and the pilot pressure control valve CV1.

The determination of which of the first to fourth instructions is applied in the hydraulic circuit can be performed by the following method disclosed in Japanese Patent Publication No. 2022-033104 which is incorporated by reference. For example, when it is determined that the vehicle is turning to the right (step S27 in FIG. 17 of that patent publication) and is not moving backward (step S28 in FIG. 7 of that patent publication) from the four pilot pressures, the controller 10 can estimate that the first instruction has been issued. When it is determined from the four pilot pressures that the vehicle is turning to the right (step S27 in FIG. 17 of that patent publication) and is not moving forward (step S28 in FIG. 11 of that patent publication), the controller 10 can estimate that the second instruction has been issued.

When it is determined from the four pilot pressures that the vehicle is turning left (step S27 in FIG. 15 of that patent publication) and is not moving backward (step S28 in FIG. 7 of that patent publication), the controller 10 can estimate that the third instruction has been issued. When it is determined from the four pilot pressures that the vehicle is turning to the left (step S27 in FIG. 15 of that patent publication) and is not moving forward (step S28 in FIG. 11 of that patent publication), the controller 10 can estimate that the fourth instruction has been issued.

Thereafter, the controller 10 determines the operation degree corresponding to the target value of the speed difference between the rotation speed of the first hydraulic motor 31L and the rotation speed of the second hydraulic motor 31R in any of the above-described instructions from at least two pressures of the first forward pilot pressure lf (t), the first backward pilot pressure lb (t), the second forward pilot pressure rf (t), and the second backward pilot pressure rb (t). For example, in the case of the first instruction, the controller 10 can determine the operation degree such that the operation degree increases as rf (t)/lf(t)*lb(t)/rb(t) increases. In the case of the second instruction, the controller 10 can determine the operation degree so that the operation degree is increased as rb(t)/lb(t)*lf(t)/rf(t) is larger depending on the hydraulic circuit. In the case of the third instruction, the controller 10 can determine the operation degree so that the operation degree is increased as lf(t)/rf(t)*rb(t)/lb(t) is larger, depending on the hydraulic circuit. In the case of the fourth instruction, the controller 10 can determine the operation degree so that the operation degree is increased as lb(t)/rb(t)*rf(t)/lf(t) is larger, depending on the hydraulic circuit.

In this modification, a parameter corresponding to the straightness based on the operation degree may be regarded as the deviation amount between the target moving direction and the straight direction.

Second Embodiment

In the first embodiment, the primary pilot pressure is controlled to realize the creep mode, but the secondary pilot pressure may be controlled. FIG. 11 is a hydraulic circuit diagram of the travel system of the work vehicle 1 in the second embodiment. FIG. 11 shows a configuration added to FIG. 3. In FIG. 11, the same components as those in FIG. 3 are denoted by the same reference numerals, and detailed description thereof will be omitted. In the second embodiment, the work vehicle 1 includes a hydraulic circuit 21. The hydraulic circuit 21 includes a first relief value CV23, a second relief value CV24, proportional valves CV21 and CV22, discharge oil passages DR3 to DR6, check valves CK1 to CK4, and throttles TH1 To TH4. The control mechanism 25 further includes first relief valve CV23, second relief valve CV24, proportional valves CV21 and CV22, discharge oil passages DR3 to DR6, check valves CK1 to CK4, and throttles TH1 to TH4 in addition to the configuration of the first embodiment.

The first and second relief valves CV23 and CV24 are balance type relief valves in which a set pressure for opening the valves is variable, and have control ports 23a and 24a for receiving the pilot oil. The first relief valve CV23 is configured to open when the pressure applied to input port is at a greater than a first set pressure applied to the control port 23a. The second relief valve CV24 is configured to open when the pressure applied to the input port is at a greater than a second set pressure applied to the control port 24a. At this time, the pilot oil is discharged to the hydraulic fluid tank 70. The proportional valves CV21 and CV22 are connected to the hydraulic fluid passages 211 and 212 connected to the control ports 23a and 24a, and are supplied with the pilot fluid from the pilot pump 71. The proportional valves CV21 and CV22 are electromagnetic proportional valves whose opening degrees can be changed by exciting solenoids, and are controlled by the controller 10.

The proportional valves CV21, CV22 are connected to the pilot oil supply passage PA1, and is configured to control to be a pressure of which an offset a in consideration of the outflow of the pilot oil is added from the first and second relief valves CV23 and CV24 and the like to the pilot pressure control valve CV1 (primary pressure control valve) according to the first embodiment in the creep mode, and the operation is performed in the normal mode so that the value of the setting line L1 plus the offset a is obtained when the anti-stall control is not performed, and the value of the setting line L2 plus the offset a is obtained when the anti-stall control is performed. Among the proportional valves CV21 and CV22, the proportional valves that control the hydraulic pressure of the pilot oil in the first pilot hydraulic circuit PCI may be referred to as secondary pressure control valves CV2, and the proportional valves that control the hydraulic pressure of the pilot oil in the second pilot hydraulic circuit PC2 may be referred to as additional secondary pressure control valves ACV2. That is, the secondary pressure control valve CV2 controls the secondary pilot pressure which is the hydraulic pressure of the pilot oil in the first pilot hydraulic circuit PC1. The additional secondary pressure control valve ACV2 controls an additional secondary pilot pressure which is the hydraulic pressure of the pilot oil in the second pilot hydraulic circuit PC2.

The first relief valve CV23 are connected to first pilot oil passages PA11 between the operation valve OVA (first pressure reducing valve) and the first port PLa and second pilot oil passage PA12 between the operation valve OVB (second pressure reducing valve) and the second port PLb, and is configured to reduce the secondary pressure exceeding the first set pressure to the first set pressure when at least one of the secondary pressure input to the operation valve OVA (first pressure reducing valve) and the secondary pressure input to the operation valve OVB (second pressure reducing valve) exceeds the first set pressure. The second relief valve CV24 is connected to the third pilot oil passages PA13 between the operation valve OVC (third pressure reducing valve) and the third port PRa and the fourth pilot oil passage PA14 between the operation valve OVD (fourth pressure reducing valve) and the fourth port PRb, and is configured to reduce the secondary pressure exceeding the second set pressure to the second set pressure when at least one of the secondary pressure input to the operation valve OVC (third pressure reducing valve) and the secondary pressure input to the operation valve OVD (fourth pressure reducing valve) exceeds the second set pressure.

The discharge oil passage DR3 is connected to the first pilot oil passage PA11. The discharge oil passage DR4 is connected to the second pilot oil passage PA12. The discharge oil passage DR5 is connected to the third pilot oil passage PA13. The discharge oil passage DR6 is connected to the fourth pilot oil passage PA14. The check valves CK1 to CK4 block the discharge oil passages DR3 to DR6 unless the pressures on the sides of the throttle TH1 to TH4 become higher than the pressures on the sides of the relief valves CV23 and CV24 by a predetermined value or more.

Since the pilot pressure of the discharge oil passage DR3 and the discharge oil passage DR4 becomes high when the first hydraulic pump 7L rotates in the normal direction and the reverse direction, respectively, when the pilot pressure of one of the discharge oil passages DR3 and DR4 becomes equal to the primary pilot pressure, the other becomes significantly smaller than the primary pilot pressure. Since the pilot pressure of the discharge oil passage DR5 and the discharge oil passage DR6 becomes high when the second hydraulic pumps 7R are rotated in the normal direction and in the reverse direction, respectively, the pilot pressure of one of the discharge oil passages DR5 and DR6 becomes much smaller than the primary pilot pressure when the pilot pressure of the other one of the discharge oil passages DR5 and DR6 becomes equal to the primary pilot pressure. Therefore, only one of the check valves CK1 and CK2 is normally opened. Therefore, the above-described control can be executed by controlling the pressures of the proportional valves CV21 and CV22 so that the pressures of the proportional valves CV21 and CV22 are pressures obtained by adding a loss of pressures due to the pilot oil flowing out from the relief valves CV23 and CV24 to the pressures by the control of the pilot pressure control valve CV1 (primary pressure control valve) according to the first embodiment.

The throttle TH1 is provided in the first pilot oil passage PA11 between the first shuttle valve SVa and the discharge oil passage DR3, and is configured to reduce the flow rate of the pilot oil in the first pilot oil passage PA11. The throttle TH2 is provided in the second pilot oil passage PA12 between the second shuttle valve SVb and the discharge oil passage DR4, and is configured to reduce the flow rate of the pilot oil in the second pilot oil passage PA12. The throttle TH3 is provided in the third pilot oil passage PA13 between the third shuttle valves SVc and the discharge oil passage DR5, and is configured to reduce the flow rate of the pilot oil in the third pilot oil passage PA13. The throttle TH4 is provided in the fourth pilot oil passage PA14 between the fourth shuttle valves SVd and the discharge oil passage DR6, and is configured to reduce the flow rate of the pilot oil in the fourth pilot oil passage PA14.

In the present embodiment, the first control parameter may be the first high pilot pressure or the first pilot effective pressure, and the second control parameter may be the second high pilot pressure or the second pilot effective pressure. In the present embodiment, the at least one first pressure control valve PCV1 includes the operation valve OVA, the operation valve OVB, the first relief valve CV23, and the secondary pressure control valve CV2. The at least one second pressure control valve PCV2 includes an operation valve OVC, an operation valve OVD, a second relief value CV24, and an additional secondary pressure control valve ACV2. The at least one third pressure control valve PCV3 includes the operation valve OVA and the operation valve OVB, the first relief valve CV23, and the secondary pressure control valve CV2. The at least one fourth pressure control valve PCV4 includes the operation valve OVC, the operation valve OVD, the second relief valve CV24, and the additional secondary pressure control valve ACV2.

In the present embodiment, the secondary pressure controlled by the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 or the traveling secondary pressure controlled by the additional secondary pressure control valve ACV2 is controlled as a pressure corresponding to the traveling primary pressure corresponding to the speed stage shown in FIG. 12. FIG. 12 illustrates an example of the first reference information 10r1 indicating the speed stage according to the second embodiment. The first reference information 10r1 of FIG. 12 also includes the first correspondence relationship of the first embodiment. In the example of FIG. 12, when the speed stage m is determined, the target rotation speeds of the first hydraulic motor 31L and the second hydraulic motor 31R are obtained as Nm (m), and thus the maximum value of the angle of the swash plate of the hydraulic pumps (7L, 7R) is obtained based on the target rotation speed of the engine 6 set by the second setting member 11b, and the traveling secondary pressure Ps (m) The angle θ is determined based on the obtained angle of the swash plate. The traveling secondary pressure Ps (m) corresponds to the first set pressure and the second set pressure described above. In FIG. 12, the traveling secondary pressure Ps (m) which is the first control parameter and the second control parameter is also displayed, but these are values calculated from the target rotation speed of the engine 6 and the target rotation speed Nm (m), and therefore the first reference information 10r1 may not include the traveling secondary pressure Ps (m). When the traveling secondary pressure at the time of the speed stage m is set to Ps (m), the traveling secondary pressure Ps (m) is set to monotonously increase as m increases. The processor 10a receives the setting stage from the creep setting member 16 and calculates the straightness representing the target moving direction.

When determining that the creep setting member 16 is operated so that the deviation amount between the target moving direction obtained from the straightness and the straight direction is within the predetermined first range Rdev1, the processor 10a sets the operating range (range having the traveling secondary pressure based on the setting stage as an upper limit) of the first control parameter (first high pilot pressure or first pilot effective pressure) for controlling the displacement volume of the first hydraulic pump 7L, and the operating range (range having the traveling secondary pressure based on the setting stage as an upper limit) of the second control parameter (second high pilot pressure or second pilot effective pressure) for controlling the displacement volume of the second hydraulic pump 7R. The processor 10a determines a boost stage which is a speed stage corresponding to a set of target rotation speeds higher than the target rotation speeds of the setting stage based on the magnitude of the deviation amount, and determines an operating range (a range having the traveling secondary pressure based on the setting stage as an upper limit) of at least one control parameter of the first control parameter and the second control parameter based on the boost stage. The method of determining the boost stage is the same as that of the first embodiment. Note that the at least one control parameter may be one of the first control parameter and the second control parameter. In this case, the at least one control parameter is a control parameter of the hydraulic pumps that drive the traveling devices that move on the outer side of the turn, of the first traveling device 3L and the second traveling device 3R, among the first control parameter and the second control parameter. Also, the control parameter of the hydraulic pump which does not correspond to the at least one control parameter is determined on the basis of the setting stage.

The processor 10a is configured to control the control mechanism 25 (secondary pressure control valve CV2) for controlling the first control parameter not to deviate from an operating range of the first control parameter determined based on the boost stage or the setting stage (a range having the traveling secondary pressure based on the setting stage or the boost stage as an upper limit), and to control the control mechanism 25 (additional secondary pressure control valve ACV2) for controlling the second control parameter not to deviate from an operating range of the second control parameter determined based on the boost stage or the setting stage (a range having the traveling secondary pressure based on the setting stage or the boost stage as an upper limit). The processor 10a is configured to control the engine 6 configured to rotate the first and second hydraulic pumps 7L and 7R so as to rotate the first and second hydraulic motors 31L and 31R whose displacement volumes are set to constant displacement volumes (set to the first speed) irrespective of the plurality of speed stages.

FIG. 13 is a flowchart showing the operation of the work vehicle 1 according to the second embodiment. Here, the same operations as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. In the work vehicle 1 according to the present embodiment, when it is determined that the deviation amount between the target moving direction and the straight ahead direction is within the first range Rdev1 (Yes in step S5), the processor S6A determines the traveling secondary pressure based on the setting stage in step 10a. That is, the processor 10a determines, based on the setting stage, an operating range (a range having the travel secondary pressure based on the setting stage as an upper limit) of a first control parameter (first high pilot pressure or first pilot effective pressure) by which the displacement volume of the first hydraulic pumps 7L configured to supply the hydraulic fluid to the first hydraulic motor 31L is determined, and an operating range (a range having the travel secondary pressure based on the setting stage as an upper limit) of a second control parameter (second high pilot pressure or second pilot effective pressure) by which the displacement volume of the second hydraulic pump 7R configured to supply the hydraulic fluid to the second hydraulic motors 31R is determined. More specifically, the processor 10a obtains the maximum value of the angle of the swash plate of the hydraulic pumps (7L, 7R) based on the target rotation speed of the engine 6 set by the first setting member 11a and the target rotation speed of the first hydraulic motor 31L and the second hydraulic motor 31R known from the setting stage, and determines the traveling secondary pressure Ps (m) so that the swash plate has the angle when the operation lever 55 is tilted to the maximum.

After step S7, in step S8A, the processor 10a determines the operating range (the range having the traveling secondary pressure based on the setting stage as the upper limit) of at least one control parameter of the first control parameter and the second control parameter based on the boost stage. To be more specific, the processor 10a obtains the maximum value of the angle of the swash plate of the hydraulic pumps (7L, 7R) based on the target rotation speed of the engine 6 set by the first setting member 11a and the target rotation speed of the first hydraulic motor 31L and the second hydraulic motor 31R known from the boost stage, and determines the traveling secondary pressure Ps (m) so that the swash plate has the angle when the operation lever 55 is tilted to the maximum. In step S12A, the processor 10a controls the pilot pressure control valve CV1 that feeds the pilot oil to the operation valves OVA, OVB, OVC, and OVD so that the traveling primary pressure obtained in step S10 or step S11 is obtained. In the step S6A and the step S8A, the traveling primary pressure is determined in the same manner as in the normal mode. Further, in step S12A, the processor 10a controls at least one of the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 corresponding to the at least one control parameter such that the traveling secondary pressure Ps (m) obtained in step S6A or step S8A becomes the upper limit. When the at least one control parameter includes the first control parameter, the processor 10a controls the first relief value CV23 such that the first set pressure is set to the upper limit value of the operating range (the range having the traveling secondary pressure Ps (m) based on the setting stage as the upper limit). When the at least one control parameter includes the second control parameter, the processor 10a controls the second relief value CV24 such that the second set pressure becomes the upper limit value of the operating range (the range having the traveling secondary pressure Ps (m) based on the setting stage as the upper limit). The steps S13 and S14 are the same as those of the first embodiment.

In the work vehicle 1 and the control method of the work vehicle 1 according to the second embodiment, when the operation lever 55 (second operation member) is operated so that the deviation amount between the target moving direction and the straight direction is out of the first range Rdev1, the processor 10a determines the boost stage as the speed stage corresponding to the set of the target rotation speeds higher than the target rotation speeds of the setting stage based on the deviation amount, determines the operating range of at least one of the first control parameter (first high pilot pressure or first pilot effective pressure) and the second control parameter (second high pilot pressure or second pilot effective pressure) based on the boost stage, and controls the control mechanism 25 (at least one of the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2). As a result, the first control parameter (the first high pilot pressure or the first pilot effective pressure) and the second control parameter (the second high pilot pressure or the second pilot effective pressure) are controlled in consideration of the turning situation, and thus it is possible to shorten the time required to achieve a desired speed even when the vehicle turns with a large curvature.

FIG. 14 is a hydraulic circuit diagram according to a modification of the second embodiment. In the example of FIG. 14, shuttle valves SV12 and SV34 are provided instead of the check valves CK1 to CK4 of the example of FIG. 11. The shuttle valve SV12 connects the discharge oil passage with the higher hydraulic pressure among the discharge oil passage DR3 and the discharge oil passage DR4 to the relief CV23. The shuttle valve SV34 connects the discharge passage with the higher hydraulic pressure among the discharge oil passage DR5 and the discharge oil passage DR6 to the relief CV24. Even with this configuration of the hydraulic circuit, the above-described control can be executed. In the circuit of FIG. 11 or 14, the pilot pressure control valve CV1 (primary pressure control valve) may be omitted. Further, at least one of the combination of the secondary pressure control valve CV2 and the balanced relief valves and the combination of the additional secondary pressure control valve ACV2 and the balanced relief valves may be realized by proportional solenoid relief valves.

Third Embodiment

Although the operation lever 55 according to the above-described embodiment directly controls the operation valves OVA, OVB, OVC, and OVD, the work vehicle 1 may detect the operation amount of the operation lever 55 by a separate sensor such as a potentiometer and control a control valve that controls the first high pilot pressure (first pilot effective pressure) and the second high pilot pressure (second pilot effective pressure) based on the operation amount detected by the sensor. In this case, the same control as that of the second embodiment can be realized by adjusting the operation amount detected by the sensor. FIG. 15 is a hydraulic circuit diagram of the travel system of the work vehicle 1 in the third embodiment. In FIG. 15, the same components as those in FIG. 3 are denoted by the same reference numerals, and detailed description thereof will be omitted. In the third embodiment, the work vehicle 1 includes a hydraulic circuit 22. The hydraulic circuit 22 may include pilot control valves CV 31 to CV34 for controlling the pilot pressure applied to each of the ports (PLa, PRa, PLb, PRb), instead of the operation valves OVA, OVB, OVC, OVD and the first to fourth shuttle valves SVa, SVb, SVc, SVd. The pilot control valves CV31 to CV34 is a proportional valve includes a solenoid that receives a signal from the controller 10. The hydraulic circuit 22 includes a control mechanism 27 that supplies pilot oil and controls pilot pressure.

In the present embodiment, the pilot oil supply passage PA8 connects the pilot control valves CV31 to CV34 and the pilot oil supply passage PA8, and the first to fourth pilot oil passages PA1l to PA14 are connected to the pilot control valves CV31 to CV34, respectively. In the present embodiment, the pilot oil supply passages PA1 and PA8, the first pilot oil passage PA11, and the second pilot oil passage PA12 correspond to a first pilot hydraulic circuit PC1. The pilot oil supply passages PA1 and PA8, the third pilot oil passage PA13, and the fourth pilot oil passage PA14 correspond to the second pilot hydraulic circuit PC2. In the present embodiment, since the operation valves OVA, OVB, OVC, and OVD are not provided, there is no difference between the primary pilot pressure and the secondary pilot pressure. Therefore, in the present embodiment, these pressures are simply referred to as pilot pressures without being distinguished from each other.

The pilot control valve CV31 is a first electromagnetic pressure control valve that is provided in the first pilot oil passage PA11 and is configured to change the pilot pressure supplied by the pilot pump 71 in accordance with a first signal from the controller 10. The pilot control valve CV32 is a second electromagnetic pressure control valve provided in the second pilot oil passage PA12 and configured to change the pilot pressure supplied by the pilot pump 71 in response to a second signal from the controller 10. The pilot control valve CV33 is a third electromagnetic pressure control valve provided in the third pilot oil passage PA13 and configured to change the pilot pressure supplied by the pilot pump 71 in response to a third signal from the controller 10. The pilot control valve CV34 is a fourth electromagnetic pressure control valve provided in the fourth pilot oil passage PA14 and configured to change the pilot pressure supplied by the pilot pump 71 in response to a fourth signal from the controller 10.

In this embodiment, the first control parameter may be the first high pilot pressure or the first pilot effective pressure, and the second control parameter may be the second high pilot pressure or the second pilot effective pressure. The control mechanism 27 includes a pilot pump 71, a first pilot oil passage PA11, a second pilot oil passage PA12, at least one first pressure control valve PCV1 (or at least one third pressure control valve PCV3), and at least one second pressure control valve PCV2 (or at least one fourth pressure control valve PCV4). In the present embodiment, the at least one first pressure control valve PCV1 includes the pilot control valves CV31 to CV32. The at least one second pressure control valve PCV2 includes pilot control valves CV33 to CV34. The at least one third pressure control valve PCV3 includes pilot control valves CV31 to CV32. The at least one fourth pressure control valve PCV4 includes the pilot control valves CV33 to CV34.

In the normal mode, the controller 10 is configured to control the pilot control valves CV31 to CV34 to output the pilot pressure corresponding to FIG. 5 in accordance with the operation position detected by the operation detection sensor 18.

FIG. 16 is a schematic view showing the tilting direction of the operation lever 55 as viewed from above. FIG. 17 is a schematic view showing the tilting direction of the operation lever 55 as viewed from the side. FIG. 18 is a schematic view showing the tilting direction of the operation lever 55 as viewed from the rear. FIGS. 16 to 18 illustrate the directions D1, D2, and D3 in order to clarify the correspondence relationship between the orientations. The direction D3 is a direction in which the operation lever 55 is oriented when the operation lever 55 is at the neutral position. FIG. 16 is a view of the operation lever 55 as viewed in the direction opposite to the direction D3. When the operation detection sensor 18 is a potentiometer configured to detect the tilt angle of the operation lever 55, the operation detection sensor 18 can detect the tilt angle θ_Tas shown in FIG. 17 and the tilt angle θ_Uas shown in FIG. 18. The tilt angle θ_Tis set such that the direction in which the operation lever 55 is directed when the operation lever 55 is at the neutral position is 0 degrees, the tilt angle when the vehicle moves forward is positive, and the tilt angle when the vehicle moves backward is negative. The tilt angle θ_Tis set such that the direction in which the operation lever 55 is oriented when the operation lever 55 is at the neutral position is 0 degrees, the angle of tilt in the forward direction is positive, and the angle of tilt in the backward direction is negative. The tilt angle θ_Uis set such that the direction in which the operation lever 55 is oriented when the operation lever 55 is at the neutral position is 0 degrees, the angle of tilt in the right direction is positive, and the angle of tilt in the left direction is negative.

The tilt angle θ_Tcorresponds to the lever operation position of the operation valve OVA and the operation valve OVB in FIG. 5. The tilt angle θ_Ucorresponds to the lever operation position of the operation valve OVC and the operation valve OVD in FIG. 5. In the normal mode, the range of the of the tilt angle θ_Tfrom θ_T0 to θ_1THcorresponds to the lever operation positions G0 to G1 in FIG. 5. When the maximum angle of the tilting of the operation lever 55 is set to θ_MAX, the range of the tilting angle θ_Tfrom (θ_MAX−θ_2TH) to θ_MAXin FIG. 17 corresponds to the lever operation positions G4 to G5 in FIG. 5. The range of the tilt angle θ_Ufrom 0 to θ_3THin FIG. 18 corresponds to the lever operation positions G0 to G1 in FIG. 5. When the maximum angle of the tilting of the operation lever 55 is set to θ_MAX, the tilting angle θ_Uin FIG. 18 from (θ_MAX−θ_4TH) to θ_MAXcorresponds to the lever operation positions G4 to G5 in FIG. 5.

FIG. 19 is a diagram showing the relationship between the tilt angle θ_Tof the operation lever 55 and the deemed operation position r. FIG. 20 is a diagram showing a relationship between the tilt angle θ_Uof the operation lever 55 and the deemed operation position r. In the creep mode, the angles 0 to θ_1TH, 0 to θ_3THG0 and 0 to θ corresponding to G0 to G1 corresponds to play when not operated, and thus the last operation position r related to the speed stage corresponds to G0. When the tilt angle θ_Tis operated between (θ_MAX−θ_2TH) to θ_MAX, and when the tilt angle θ_Uis operated between (θ_MAX−θ_4TH) to θ_MAX, it can be considered that the full operation is performed in consideration of play, and thus it can be considered that the operation is performed at the deemed maximum operation position Ga (m). When the tilt angle θ_Tchanges in the section from θ_1THto (θ_MAX−θ_2TH), the deemed operation position r may be linearly changed from G1 to Ga (m) in accordance with the magnitude of the tilt angle θ_T. When the tilt angle θ_Uchanges in the section from θ_3THto (θ_MAX−θ_4TH), the deemed operation position r may be linearly changed from G1 to Ga (m) in accordance with the magnitude of the tilt angle θ_U.

The tilt angle θ_Bis defined between −90 degrees and 90 degrees, with 0 degrees being the tilt angle when the operation lever 55 is oriented in the tilt direction for forward travel, and the clockwise direction being positive. The tilt angle θ_Fis determined when the tilt angle θ_Tis positive. Tilt angle θ_FT is expressed by the following equation (1) by using the tilt angle θ_Tand the tilt angle θ_U.

$\begin{matrix} θ_{F} = \arctan (\sin θ_{U} / \sin θ_{T}) & (Equation 1) \end{matrix}$

The tilt angle θ_Bis defined between −90 degrees to 90 degrees, with 0 degrees being the tilt angle when the operation lever 55 is oriented in the tilt direction for forward travel, and the clockwise direction being positive. The tilt angle θ_Bis determined when the tilt angle θ_Tis negative. The tilt angle θ_Bis expressed by the following (Equation 2) using the tilt angle θ_Tand the tilt angle θ_U.

$\begin{matrix} θ_{B} = \arctan (\sin 0_{U} / \sin θ_{T}) & (Equation 2) \end{matrix}$

The absolute value of the tilt angle θ_F(θ_B) corresponds to the deviation amount between the target moving direction and the straight direction. This is different from the straightness shown in the first embodiment and the second embodiment, and the deviation amount increases as the absolute value of the tilt angle θ_F(θ_B) increases. FIG. 22 shows an example of the second reference information 10r2a according to the third embodiment. FIG. 22 shows the absolute value of the tilt angle θ_F(θ_B) as the horizontal axis and the difference in stages of (boost stage—setting stage) as the vertical axis. Since the large absolute values of the tilt angles θ_F(θ_B) mean that the deviation amount between the target moving direction and the straight direction is large, the first range Rdev1 is set as a range of the absolute values of the tilt angles θ_F(θ_B) smaller than the threshold value θTh1 of the tilt angles. When the tilt angle θF (θ_B) is equal to or smaller than the threshold value θTh1, the operating range of the first control parameter and the operating range of the second control parameter are determined based on the setting stage, and thus the difference in stages between the boost stage and the setting stage is 0. The threshold value θTh1 is a value empirically obtained as the minimum degree of straight movement at which rotation of at least one of the first hydraulic motor 31L and the second hydraulic motor 31R is operable when the minimum speed stage (the speed stage whose stage number is 1) is set.

In the normal mode, the pilot pressure controlled by the pilot control valve CV31 is determined in accordance with the operation position in FIG. 5 estimated for the positive tilt angle θ_T. When the tilt angle θ_Tis positive, the pilot pressure controlled by the pilot control valve CV32 is 0. The pilot pressure controlled by the pilot control valve CV32 is determined in correspondence with the operation position of FIG. 5 estimated for the negative tilt angle θ_T. When the tilt angle θ_Tis negative, the pilot pressure controlled by the pilot control valve CV31 is 0. The pilot pressure controlled by the pilot control valve CV33 is determined in correspondence with the operation position of FIG. 5 estimated for the positive tilt angle θ_U. When the tilt angle θ_Uis positive, the pilot pressure controlled by the pilot control valve CV34 is 0. The pilot pressure controlled by the pilot control valve CV34 is determined in correspondence with the operation position of FIG. 5 estimated for the negative tilt angle θ_U. When the tilt angle θ_Uis negative, the pilot pressure controlled by the pilot control valve CV33 is 0.

In the creep mode, the pilot pressure controlled by the pilot control valve CV31 is determined to be the pilot pressure in FIG. 5 corresponding to the deemed operation position r in FIG. 19 estimated for the positive tilt angle θ_T. When the tilt angle θ_Tis positive, the pilot pressure controlled by the pilot control valve CV32 is 0. The pilot pressure controlled by the pilot control valve CV32 is determined to be the pilot pressure of FIG. 5 corresponding to the deemed operation position r of FIG. 19 estimated for the negative tilt angle θ_T. When the tilt angle θ_Tis negative, the pilot pressure controlled by the pilot control valve CV31 is 0. The pilot pressure controlled by the pilot control valve CV33 is determined in correspondence with the pilot pressure of FIG. 5 corresponding to the deemed operation position r of FIG. 20 estimated for the positive tilt angle θ_U. When the tilt angle θ_Uis positive, the pilot pressure controlled by the pilot control valve CV34 is 0. The pilot pressure controlled by the pilot control valve CV34 is determined in correspondence with the pilot pressure of FIG. 5 corresponding to the deemed operation position r of FIG. 20 estimated for the negative tilt angle θ_U. When the tilt angle θ_Uis negative, the pilot pressure controlled by the pilot control valve CV33 is 0.

In the present embodiment, the deemed maximum operation position Ga or the pilot pressure Pt corresponding to the deemed maximum operation position Ga is set for each speed stage. FIG. 21 illustrates an example of the first reference information 10r1 indicating the speed stage according to the third embodiment. The first reference information 10r1 of FIG. 21 also includes the first correspondence relationship of the first embodiment. In the example of FIG. 21, when the speed stage m is determined, the target rotation speeds of the first hydraulic motor 31L and the second hydraulic motor 31R are obtained as Nm (m), and therefore the maximum values of the angles of the swash plates of the hydraulic pumps (7L, 7R) are obtained based on the target rotation speed of the engine 6 set by the second setting member 11b, and the pilot pressure Pt (m) corresponding to the deemed operation position Ga (m) is determined based on the obtained angles of the swash plates. The pilot pressure Pt (m) is set as the maximum value of the pilot pressure output by the pilot control valves CV31 to CV34.

FIG. 21 illustrates an example of the first reference information 10r1 indicating the speed stage according to the third embodiment. The first reference information 10r1 of FIG. 21 also includes the first correspondence relationship of the first embodiment. In the example of FIG. 21, when the speed stage m is determined, the target rotation speed of the first hydraulic motor 31L and the second hydraulic motor 31R is obtained as Nm (m), and therefore the maximum value of the swash plate angle of the hydraulic pumps (7L, 7R) is obtained based on the target rotation speed of the engine 6 set by the second setting member 11b, and the pilot pressure Pt (m) is determined based on the obtained swash plate angle. The deemed maximum operating position Ga (m) is determined from the pilot pressure Pt (m) based on the correspondence relationship of FIG. 5. When the deemed maximum operation position at the time of the speed stage m is set as Ga (m) and the pilot pressure at that time is set as Pt (m), the pilot pressure Pt (m) is set to monotonically increase as m increases. Note that Ga (m) is set between the operation positions G1 to G4 in FIG. 5 for most of the speed stage m. However, in the maximum speed stage or the like, Ga (m) may be set between the operation positions G4 to G5 in FIG. 5.

In the present embodiment, the boost stage is determined such that the difference in stages increases as the deviation amount between the target moving direction and the straight direction increases. However, the difference in stages between the boost stage and the setting stage is controlled to increase as the straightness increases from the threshold value θTh1. In this embodiment, when the number of speed stages corresponding to a set of a target rotation speed of the first hydraulic motor 31L between the target rotation speed of the first hydraulic motor 31L of the minimum turnable speed stage and the target rotation speed of the first hydraulic motor 31L of the minimum speed stage (speed stage whose stage number is 1) and a target rotation speed of the second hydraulic motor 31R between the target rotation speed of the second hydraulic motor 31R of the minimum turnable speed stage and the target rotation speed of the second hydraulic motor 31R of the minimum speed stage (speed stage whose stage number is 1) is n (n is an integer of 0 or more), n+1 is set as the maximum value of the difference in stages. The stage number of the minimum turnable speed stage is n+2.

In the example of FIG. 22, the difference in stages between the boost stage and the setting stage is linearly changed to increase as the absolute values of the tilt angles θ_F(θ_B) increase in which the tilt angles θ_F(θ_B) is between the threshold value θTh0 and θTh1 (θTh1<θTh0), and the difference in stages between the boost stage and the setting stage is set to the upper limit DMAX (=n+1) when the absolute value of the tilt angle θ_F(θ_B) is larger than the threshold value θTh0. When the threshold value of the absolute values of the tilt angles θ_F(θ_B) corresponding to the arbitrary step numbers j and j+1 are SD (j) and SD (j+1), respectively, the threshold values SD (j) and SD (j+1) are empirically determined so that at least one of the first hydraulic motor 31L and the second hydraulic motor 31R can rotate at the speed stage (j+1) if the straightness is between SD (j) and SD (j+1). That is, the difference in stages is determined such that at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates in the range (SD (j) to SD (j+1)) of the deviation amount based on which the boost stage is determined.

However, when the absolute value of the tilt angle θ_F(θ_B) is between the thresholds θTh0 and θTh1 (θTh1<θTh0), the relationship between the absolute value of the tilt angle θ_F(θ_B) and the step numbers may be a monotonous increase, and does not necessarily have to be a linear increase. That is, when the threshold values of the absolute values of the tilt angles θ_F(θ_B) corresponding to the arbitrary step numbers j and j+1 are SD (j) and SD (j+1), respectively, the relationship between the absolute values of the tilt angles θ_F(θ_B) and the difference in stages is determined so that SD (j+1)>SD (j) is satisfied.

The processor 10a is configured to generate the first signal or the second signal corresponding to the first high pilot pressure and the first signal or the second signal corresponding to the first low pilot pressure based on the deviation amount between the target moving direction and the straight advancing direction and the upper limit value of the operating range of the first parameter (pilot pressure corresponding to the deemed maximum operation position Ga (m) based on the setting stage or the boost stage). The processor 10a is configured to generate the third signal or the fourth signal corresponding to the second high pilot pressure and to generate the third signal or the fourth signal corresponding to the second low pilot pressure based on the deviation amount and the upper limit value of the operating range (the pilot pressure corresponding to the deemed maximum operation position Ga (m) based on the setting stage or the boost stage).

That is, the processor 10a is configured to control the control mechanism 25 (pilot control valves CV31 to CV32) that controls the first control parameter not to deviate from the operating range of the first control parameter determined based on the boost stage r the setting stage (range having the pilot pressure corresponding to the deemed maximum operation position Ga (m) based on the setting stage or the boost stage as an upper limit), and control the control mechanism 25 (pilot control valves CV31 to CV32) that controls the second control parameter not to deviate from the operating range of the second control parameter determined based on the boost stage or the setting stage (range having the pilot pressure corresponding to the deemed maximum operation position Ga (m) based on the setting stage or the boost stage as an upper limit). The processor 10a is configured to control the engine 6 configured to rotate the first and second hydraulic pumps 7L and 7R so as to rotate the first and second hydraulic motors 31L and 31R whose displacement volumes are set to their constant displacement volumes (set to the first speed) irrespective of the plurality of speed stages.

FIG. 23 is a flowchart showing the operation of the work vehicle 1 according to the third embodiment. Here, the same operations as those in the second embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. In the work vehicle 1 according to the present embodiment, after step S3, the processor 10a obtains the absolute value of the tilt angles θ_F(θ_B) corresponding to the deviation amount between the target movement direction and the straight ahead direction in the step SB4. When the deviation amount is within the first range Rdev1 (Yes in step S5), in the step S6B, the processor 10a determines the deemed maximum operation position Ga (m) or the pilot pressure Pt (m) corresponding to the deemed maximum operation position Ga (m) based on the setting stage. That is, the processor 10a determines, based on the setting stage, the operating range (range having the pilot pressure Pt (m) corresponding to the deemed maximum operation position Ga (m) based on the setting stage as an upper limit) of the first control parameter (first high pilot pressure or first pilot effective pressure) by which the displacement volume of the first hydraulic pump 7L configured to supply the hydraulic fluid to the first hydraulic motor 31L is determined and the operating range (range having the pilot pressure Pt (m) corresponding to the deemed maximum operation position Ga (m) based on the setting stage as an upper limit) of the second control parameter (second high pilot pressure or second pilot effective pressure) by which the displacement volume of the second hydraulic pump 7R configured to supply the hydraulic fluid to the second hydraulic motor 31R is determined.

When the deviation amount is out of the first range Rdev1 (No in step S5), the processor 10a determines the boost stage based on the magnitude of the deviation amount (absolute value of the tilt angle θ_F(θ_B)—threshold value θTh1) with reference to the second reference information 10r2a illustrated in FIG. 19 in step S7B. In step S8B, the processor 10a determines the operating range (a range having the pilot pressure Pt (m) corresponding to the deemed maximum operation position Ga (m) based on the setting stage as an upper limit) of at least one control parameter of the first control parameter and the second control parameter based on the boost stage. In particular, the processor 10a determines the operating range of the first control parameter and the operating range of the second control parameter based on the boost stage. Note that the at least one control parameter may be one of the first control parameter and the second control parameter. In this case, the at least one control parameter is a control parameter among the first control parameter and the second control parameter of the hydraulic pump to drive a traveling device of the first traveling device 3L and the second traveling device 3R that moves on the outer side of the turn, of the first traveling device 3L and the second traveling device 3R. Also, the control parameter of the hydraulic pump which does not correspond to the at least one control parameter is determined on the basis of the setting stage.

When the engine drop does not occur (No in step S9), the processor 10a obtains the maximum output pressure Pc of FIG. 5 from the third reference information 10r3 based on the rotation speed RS of the engine 6 in step S10B. When the engine drop is present (Yes in step S9), the processor 10a obtains the maximum output pressure Pc of FIG. 5 from the fourth reference information 10r4 based on the rotation speed RS of the engine 6 in step S11B. After the process of step S6B, S8B, S10B, or S11B is finished, in step S13B, the processor 10a controls each of the pilot control valves CV31 to CV34 so as to obtain the pilot pressure based on the lever position (first operation amount) of the operation lever 55 (second operation member).

That is, the control method according to the present embodiment includes controlling the pilot control valves CV31 to CV34 so that the pilot control valves CV31 to CV34 output the pilot pressure based on the lever position (first operation amount) of the operation lever 55 (first operation device). In step S13B, the control method according to the present embodiment includes controlling the first control parameter (first high pilot pressure or first pilot effective pressure) determined based on the boost stage or the setting stage not to deviate from the operating range (range having the pilot pressure Pt corresponding to the deemed maximum operation position Ga based on the setting stage as an upper limit), and controlling the second control parameter (second high pilot pressure or second pilot effective pressure) not to deviate from the operating range (range having the pilot pressure Pt corresponding to the deemed maximum operation position Ga based on the setting stage as an upper limit). Step S14 is the same as that of the first embodiment.

In the control method or the work vehicle 1 according to the third embodiment, when the operation lever 55 (second operation member) is operated so that the deviation amount between the target moving direction and the straight direction is out of the first range Rdev1, the processor 10a determines the boost stage as the speed stage corresponding to a set of the target rotation speeds higher than the target rotation speeds of the setting stage based on the deviation amount, and determines the operating range of at least one control parameter of the first control parameter (the first high pilot pressure or the first pilot effective pressure) and the second control parameter (the second high pilot pressure or the second pilot effective pressure) based on the boost stage, and controls the control mechanism 25 (pilot control valves CV34 to SL) such that the at least one control parameter does not deviate from the determined operating range (range having the pilot pressure Pt (m) corresponding to the deemed maximum operation position Ga (m) based on the setting stage or the boost stage +as an upper limit). As a result, the first control parameter (the first high pilot pressure or the first pilot effective pressure) and the second control parameter (the second high pilot pressure or the second pilot effective pressure) are controlled in consideration of the turning situation, and thus it is possible to shorten the time required to achieve a desired speed even when the vehicle turns with a large curvature.

Instead of the third embodiment, each of the ports (PLa, PRa, PLb, PRb) of the hydraulic pumps (7L, 7R) may be configured to receive electromagnetic input, such as solenoids. In this case, the pilot control valves CV31 to 34 may be omitted. In this case, devices that are provided in the hydraulic pumps (7L, 7R) and electromagnetically input may be regarded as the control mechanism 27 instead of the pilot control valves CV31 to 34.

In the work vehicles 1 that do not include the technology of the first embodiment or the second embodiment, the first hydraulic sensor SP11, the second hydraulic sensor SP12, the third hydraulic sensor SP13, and the fourth hydraulic sensor SP14 may be omitted. In the second embodiment, the straightness may be calculated from the outputs of the hydraulic sensors SP5L, SP6L, SP5R, and SP6R.

The values of the various thresholds may be changed according to the characteristics of the first hydraulic pumps 7L, the second hydraulic pumps 7R, the first hydraulic motor 31L, and the second hydraulic motor 31R, the characteristics of the reduction gears connected to the first hydraulic motor 31L and the reduction gears connected to the second hydraulic motor 31R, and the characteristics of the various control valves.

In the above-described embodiment, the case where the first hydraulic pump 7L and the second hydraulic pump 7R are rotated by the engine 6 has been described, but the first hydraulic pump 7L and the second hydraulic pump 7R may be rotated by another prime mover such as an electric motor.

In this application, the word “comprise” and its derivatives are used as open-ended terms to describe the presence of elements but not to exclude the presence of other elements not listed. This applies to “having”, “including” and derivatives thereof.

The terms “member,” “part,” “element,” “body,” and “structure” may have a plurality of meanings, such as a single portion or a plurality of portions.

The ordinal numbers such as “first” and “second” are merely terms for identifying the configuration, and do not have other meanings (for example, a specific order). For example, the presence of a “first element” does not imply the presence of a “second element,” and the presence of a “second element” does not imply the presence of a “first element.”

Terms of degree such as “substantially”, “about”, and “approximately” may mean a reasonable amount of deviations such that the end result is not significantly changed, unless the embodiment is specifically described otherwise. All numerical values recited herein may be construed to include terms such as “substantially,” “about,” and “approximately.”

The phrase “at least one of A and B” as used herein should be interpreted to include A alone, B alone, and both A and B.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that the invention may be practiced otherwise than as specifically described herein without departing from the scope of the invention.

WORK VEHICLE, CONTROLLER FOR WORK VEHICLE, AND SPEED CONTROL METHOD FOR WORK VEHICLE

Information

Publication Number

Date Filed

Date Published

Inventors

Original Assignees

CPC

International Classifications

Abstract

Description

Claims

Priority Claims (1)