Work vehicle, work vehicle controller, and work vehicle speed control method

Abstract

SOLUTION: A work vehicle speed control method receives a set stage set by a first operation member among a plurality of speed stages; receives a target moving direction of a vehicle body defined by a speed difference between a target rotational speed of a first hydraulic motor and a target rotational speed of a second hydraulic motor from a second operation member for setting the target moving direction; determines operation ranges of a first control parameter for determining displacement volume of a first hydraulic pump constituted so as to supply hydraulic oil to the first hydraulic motor and a second control parameter for determining displacement volume of a second hydraulic pump on the basis of the set stage, when the second operation member is operated so that deviation amount between the target moving direction and a linear advancing direction falls in a predetermined first range; and determines a boost stage on the basis of magnitude of the deviation amount when the second operation member is operated so that the deviation amount falls outside the first range.EFFECT: To provide a work vehicle enabling improvement of responsiveness when feedback control is executed so as to be a desired vehicle speed when turning.SELECTED DRAWING: Figure 9

Description

[Technical field]

[0001] Citation by reference This application claims the benefit of Japanese Patent Application No. 2021-093045, filed on February 28, 2022, each of which is incorporated by reference in its entirety herein.

[0002] The present invention relates to a work vehicle, a controller for a work vehicle, and a speed control method for a work vehicle. [Background technology]

[0003] Patent Document 1 discloses a technique for measuring the input of a travel lever and the rotation speed of a travel motor, and adjusting the pilot pressure of a travel pump so that the rotation speed of the travel motor matches the command based on the travel lever input. Patent Document 2 discloses a method for detecting the primary pressure of pilot oil supplied to a remote control valve and the rotation speed of the travel motor, and controlling the primary pressure to achieve a target vehicle speed based on the detected primary pressure and rotation speed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-053413 A [Patent Document 2] JP 2020-038002 A Summary of the Invention [Problem to be solved by the invention]

[0005] In both of the methods of Patent Document 1 and Patent Document 2, feedback is performed using the rotation speed of the travel motor. However, since the feedback does not take into account the turning situation, there is a problem that it takes a long time to reach the desired speed when turning with a large curvature. [Means for solving the problem]

[0006] A speed control method for a work vehicle according to a first aspect of the present disclosure includes receiving a setting stage set by a first operating member out of a plurality of speed stages corresponding to a plurality of pairs of target rotational speeds of a first hydraulic motor and a target rotational speed of a second hydraulic motor configured to drive a first traveling device and a second traveling device, respectively, when driving the first traveling device and a second traveling device provided on the left and right sides of the vehicle body to move the vehicle body in a straight line, receiving information corresponding to a target movement direction of the vehicle body determined by a speed difference between the target rotational speed of the first hydraulic motor and the target rotational speed of the second hydraulic motor from a sensor that detects operation of a second operating member that sets the target movement direction, determining the target movement direction, and when the second operating member is operated so that a deviation amount between the target movement direction and the straight movement direction falls within a predetermined first range, determining an operating range of a first control parameter for determining a displacement volume of a first hydraulic pump configured to supply hydraulic oil to the first hydraulic motor and a second hydraulic motor and determining, based on the set stage, an operating range of a second control parameter for determining a displacement of a second hydraulic pump configured to supply hydraulic oil to the first hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the first hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the first hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the second hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the first hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the second hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the first hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the second hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the first hydraulic pump and a second hydraulic pump configured to supply hydraulic oil to the second hydraulic pump and a second hydraulic motor configured to supply hydraulic oil to the first hydraulic pump and a second hydraulic motor configured to supply hydraulic oil to the second ...

[0007] A work vehicle according to a second aspect of the present disclosure includes a vehicle body, a first traveling device, a second traveling device, a first hydraulic motor, a second hydraulic motor, a first hydraulic pump, a second hydraulic pump, a control mechanism, a prime mover, a storage device, a first operating member, a second operating member, a sensor, and a hardware processor. The first traveling device and the second traveling device are provided on the left and right sides of the vehicle body, respectively. The first hydraulic motor is configured to drive the first traveling device. The second hydraulic motor is configured to drive the second traveling device. The first hydraulic pump is configured to supply hydraulic oil to the first hydraulic motor. The second hydraulic pump is configured to supply hydraulic oil to the second hydraulic motor. The control mechanism is configured to control a displacement volume of the first hydraulic pump and a displacement volume of the second hydraulic pump. The prime mover is configured to rotate the first hydraulic pump and the second hydraulic pump. The storage device is configured to store a plurality of speed stages corresponding to a plurality of pairs of a target rotation speed of the first hydraulic motor and a target rotation speed of the second hydraulic motor when the first traveling device and the second traveling device are driven to move the vehicle body in a straight line. The first operating member is configured to receive an input corresponding to a set stage among the plurality of speed stages. The second operating member is configured to receive an input corresponding to a target movement direction of the vehicle body determined by a speed difference between the target rotation speed of the first hydraulic motor and the target rotation speed of the second hydraulic motor. The sensor is configured to detect the operation of the second operating member. The hardware processor is configured to control the prime mover and the control mechanism. The hardware processor is configured to receive the set stage from the first operating member, receive information corresponding to the target movement direction from the sensor, and determine the target movement direction. The hardware processor is configured to determine, based on a setting stage, an operating range of a first control parameter for controlling the displacement volume of the first hydraulic pump and an operating range of a second control parameter for controlling the displacement volume of the second hydraulic pump when the second operating member is operated so that the deviation amount between the target movement direction and the straight-ahead direction falls within a predetermined first range.The hardware processor is configured to determine a boost stage, which is a speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, based on the magnitude of the deviation when the second operating member is operated so that the deviation amount falls outside the first range, and to determine the operating range based on the boost stage. The hardware processor is configured to control the control mechanism to change the first control parameter and the second control parameter so as not to deviate from the operating range of the first control parameter and the operating range of the second control parameter determined based on the boost stage or the set stage. The hardware processor is configured to drive the first traveling device and the second traveling device by controlling the first hydraulic pump and the second hydraulic pump to rotate the first hydraulic motor and the second hydraulic motor, which are set to have a constant displacement regardless of the multiple speed stages.

[0008] A controller for a work vehicle according to a third aspect of the present disclosure includes a hardware processor. The hardware processor is configured to receive a setting stage set by a first operating member among a plurality of speed stages corresponding to a plurality of pairs of target rotational speeds of a first hydraulic motor and a second hydraulic motor configured to drive the first traveling device and the second traveling device, respectively, when driving the first traveling device and the second traveling device provided on the left and right sides of the vehicle body to move the vehicle body straight ahead. The hardware processor is configured to receive information corresponding to a target movement direction of the vehicle body determined by a speed difference between the target rotational speed of the first hydraulic motor and the target rotational speed of the second hydraulic motor from a sensor that detects operation of a second operating member that sets the target movement direction, and to determine the target movement direction. The hardware processor is configured to determine, based on the setting stage, an operating range of a first control parameter for determining a displacement volume of a first hydraulic pump configured to supply hydraulic oil to the first hydraulic motor and an operating range of a second control parameter for determining a displacement volume of a second hydraulic pump configured to supply hydraulic oil to the second hydraulic motor when the second operating member is operated so that the deviation amount between the target moving direction and the straight forward direction falls within a predetermined first range. The hardware processor is configured to determine, based on the magnitude of the deviation amount, a boost stage that is a speed stage corresponding to a target rotation speed higher than the target rotation speed of the setting stage when the second operating member is operated so that the deviation amount falls outside the first range, and to determine the operating range of at least one of the first control parameter and the second control parameter based on the boost stage. The hardware processor is configured to control a control mechanism that controls the first control parameter so as to change the first control parameter so as not to deviate from the operating range of the first control parameter determined based on the boost stage or the setting stage, and to control a control mechanism that controls the second control parameter so as not to deviate from the operating range of the second control parameter determined based on the boost stage or the setting stage.The hardware processor is configured to control a prime mover configured to rotate a first hydraulic pump and a second hydraulic pump to rotate a first hydraulic motor and a second hydraulic motor that are defined to have a constant displacement regardless of multiple speed stages. Effect of the Invention

[0009] According to the technology disclosed in the present application, it is possible to provide a work vehicle that has improved responsiveness when performing feedback control to achieve a desired vehicle speed, for example, when turning. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a side view of a work vehicle. [Diagram 2] FIG. 2 is a top view of the work vehicle. [Diagram 3] FIG. 3 is a hydraulic circuit diagram of a travel system of a work vehicle. [Figure 4] FIG. 4 is a diagram showing the relationship between the engine rotation speed, the primary traveling pressure, and the setting line. [Diagram 5] FIG. 5 is a diagram showing the relationship between the operating position of the operating lever and the secondary traveling pressure. [Figure 6] FIG. 6 is a block diagram of the work vehicle. [Figure 7] FIG. 7 shows an example of a speed stage (first reference information) in the first embodiment. [Figure 8] FIG. 8 shows an example of the second reference information in the first embodiment. [Figure 9] FIG. 9 is a flowchart showing the operation of the work vehicle according to the first embodiment. [Figure 10] FIG. 10 shows a hydraulic system (hydraulic circuit) of a work vehicle different from that shown in FIG. [Figure 11] FIG. 11 is a hydraulic circuit diagram of a travel system of a work vehicle in the second embodiment. [Figure 12] FIG. 12 shows an example of a speed stage (first reference information) in the second embodiment. [Figure 13]FIG. 13 is a flowchart showing the operation of the work vehicle according to the second embodiment. [Figure 14] FIG. 14 is a hydraulic circuit diagram of a travel system of a work vehicle in a modified example of the second embodiment. [Figure 15] FIG. 15 is a hydraulic circuit diagram of a travel system of a work vehicle in the third embodiment. [Figure 16] FIG. 16 is a schematic diagram showing the tilt direction of the operating lever as viewed downward from above. [Figure 17] FIG. 17 is a schematic diagram showing the tilt direction of the operating lever as viewed from the side of the operating lever. [Figure 18] FIG. 18 is a schematic diagram showing the tilt direction of the operating lever as viewed forward from behind the operating lever. [Figure 19] FIG. 19 is a diagram showing the relationship between the tilt angle of the operating lever and the deemed operating position. [Figure 20] FIG. 20 is a diagram showing the relationship between the tilt angle of the operating lever and the deemed operating position. [Figure 21] FIG. 21 shows an example of a speed stage (first reference information) in the third embodiment. [Figure 22] FIG. 22 shows an example of the second reference information in the third embodiment. [Diagram 23] FIG. 23 is a flowchart showing the operation of the work vehicle according to the third embodiment.

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the drawings showing embodiments thereof. In the drawings, the same reference numerals indicate corresponding or substantially identical components. First Embodiment <Overall composition>

[0012] 1 and 2, a work vehicle 1, for example, a compact track loader, includes a vehicle body 2, a pair of travel devices 3, and a working device 4. The vehicle body 2 supports the travel devices 3 and the working device 4. In the illustrated embodiment, the travel devices 3 are track-type travel devices. Therefore, each of the pair of travel devices 3 includes a driving wheel 31, driven wheels 32 and 33, and rollers 34 driven by a hydraulic motor device 30. However, each of the pair of travel devices 3 is not limited to being a track-type travel device. Each of the pair of travel devices 3 may be, for example, a front-wheel / rear-wheel travel device, or a travel device having front wheels and a rear crawler. The working device 4 includes a work equipment (bucket) 41 at a distal end of the working device 4. A proximal end of the working device 4 is attached to the rear of the vehicle body 2. The working 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 .

[0013] The link 44 is rotatable relative to the vehicle body 2 around a fulcrum shaft 46. The arm 45 is rotatable relative to the link 44 around a joint shaft 47. The working 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 and the arm 45, etc., to lift and lower the bucket 41. The at least one equipment 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 its outer shape 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. The cab frame 53 is rotatable around rotational shafts RSL and RSR on the vehicle body 2, as shown in FIG. 2. In FIG. 1 and FIG. 2, a common pivot axis A defined by the rotation axes RSL and RSR is XC That is, the cab frame 53 is rotatable about a rotation axis A XC It is mounted so as to be rotatable around

[0014] In the embodiment of the present application, FB (Forward direction D F / backward D B ) means the front-rear direction (front direction / rear direction) as seen from the operator seated in the driver's seat 54 of the cabin 5. L , right direction D R , width direction D W The left, right, left and right directions are respectively seen from the operator's point of view. U , downward D D , height direction D H The directions refer to the upward, downward, and height directions as seen from the operator. The forward / backward, left / right (width), and up / down (height) directions of the work vehicle 1 respectively correspond to the forward / backward, left / right (width), and up / down (height) directions as seen from the operator.

[0015] FIG. 1 shows the left side of the work vehicle 1. As shown in FIG. 2, the vehicle body 2 is generally symmetrical with respect to the vehicle body central plane M, and includes a first side surface 2L which is the left side surface, and a second side surface 2R which is the right side surface. Of 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. That is, the first traveling device 3L and the second traveling device 3R are provided on the left and right sides of the vehicle body 2, respectively. Of the pair of arm assemblies 42, the arm assembly 42 provided on the left side with respect to the vehicle body central plane 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 central plane M is shown as the second arm assembly 42R. The link 44 provided on the left side with respect to the vehicle body central plane M is shown as the first link 44L. The arm 45 provided on the left side of the vehicle body central plane M is shown as a first arm 45L, and the arm 45 provided on the right side of the vehicle body central plane M is shown as a second arm 45R. The fulcrum shaft 46 provided on the left side of the vehicle body central plane M is shown as a first fulcrum shaft 46L, and the fulcrum shaft 46 provided on the right side of the vehicle body central plane M is shown as a second fulcrum shaft 46R. The joint shaft 47 provided on the left side of the vehicle body central plane M is shown as a first joint shaft 47L, and the joint shaft 47 provided on the right side of the vehicle body central plane M is shown as a second joint shaft 47R. Of the hydraulic motor devices 30, the hydraulic motor device 30 provided on the left side of the vehicle body central plane M is shown as a first hydraulic motor device 30L, and the hydraulic motor device 30 provided on the right side of the vehicle body central plane M is shown as a second hydraulic motor device 30R.

[0016] 1 and 2, the work vehicle 1 further includes an engine 6 provided at the rear of the vehicle body 2, and a plurality of hydraulic pumps 7 including a first hydraulic pump 7L and a second hydraulic pump 7R. The engine 6 drives the 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 oil to the hydraulic motor device 30, etc., for driving the hydraulic motor device 30, etc., which drives the drive wheels 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 are configured to discharge hydraulic oil to drive hydraulic actuators (a plurality of arm cylinders 48, at least one implement cylinder 49, etc.) connected to the work device 4. The engine 6 is arranged in a width direction D of the work vehicle 1. W , and is provided between the pair of arm assemblies 42. 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 can be opened and closed, allowing a maintenance technician to perform maintenance work on the engine 6, etc.

[0017] FIG. 3 is a hydraulic circuit diagram of the traveling system of the work vehicle 1. The work vehicle 1 includes a hydraulic circuit 20. The hydraulic circuit 20 includes a hydraulic oil tank 70 and a pilot pump 71. The pilot pump 71 is a fixed displacement gear pump 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 hydraulic oil stored in the hydraulic oil tank 70. In particular, the pilot pump 71 is configured to discharge hydraulic oil mainly used for control. For convenience of explanation, the hydraulic oil used for control among the hydraulic oil discharged from the pilot pump 71 is called pilot oil, and the pressure of the pilot oil is called pilot pressure. In the hydraulic circuit 20, a mechanism that supplies pilot oil and controls the pilot pressure is called 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.

[0018] The hydraulic circuit 20 includes a pilot supply oil passage PA1 connected to a discharge port of a pilot pump 71. Pilot oil flows through the pilot supply oil passage PA1. The hydraulic circuit 20 includes a plurality of switching valves (brake switching valve SV1, directional control valve SV2) connected to the pilot supply oil passage PA1, and a plurality of brake mechanisms 72. The brake switching valve SV1 is connected to the pilot supply oil passage PA1. The brake switching valve SV1 is a directional control valve (electromagnetic valve) for performing braking and releasing of braking by the plurality of brake mechanisms 72. The brake switching valve SV1 is a two-position switching valve configured to switch its valve body to a first position VP1a or a second position VP1b by excitation. The valve body of the brake switching valve SV1 is switched by a brake pedal 13 (see FIG. 6). The brake pedal 13 is provided with a sensor 14. An operation amount detected by the sensor 14 is input to a controller 10 consisting of an ECU (Electric Control Unit). The controller 10 may be referred to as a control device.

[0019] The multiple 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 changeover valve SV1 via an oil passage PA2. The first brake mechanism 72L and the second brake mechanism 72R are configured to brake the traveling device 3 according to the pressure of pilot oil (hydraulic oil). When the valve body of the brake changeover valve SV1 is switched to the first position VP1a, hydraulic oil is released from the oil passage PA2 in the section between the brake changeover valve SV1 and the brake mechanism 72, and the traveling device 3 is braked by the brake mechanism 72. When the valve body of the brake changeover valve SV1 is switched to the second position VP1b, braking by the brake mechanism 72 is released. In addition, when the valve body of the brake switching valve SV1 is switched to the first position VP1a, braking by the brake mechanism 72 may be released, and when the valve body of the brake switching valve SV1 is switched to the second position VP1b, the brake mechanism 72 may brake the traveling device 3.

[0020] The directional control valve SV2 is a solenoid valve that changes the rotation of the first hydraulic motor unit 30L and the second hydraulic motor unit 30R. The directional control valve SV2 is a two-position control valve configured to switch its valve element to a first position VP2a or a second position VP2b when excited. The direction control valve SV2 is switched by an operating member or the like (not shown). Note that the directional control valve SV2 may be a proportional valve capable of adjusting the flow rate of the discharged hydraulic oil, rather than a two-position control valve.

[0021] The first hydraulic motor device 30L is a device that transmits power to the drive wheels 31 provided on 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 traveling control valve (hydraulic switching valve) SV4. The first hydraulic motor 31L is a swash plate type variable displacement axial motor configured to drive the first traveling device 3L, and is a motor that can change the vehicle speed (rotation) to first speed or 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 expanding and contracting. The first traveling control valve SV4 is a valve for expanding and contracting the first swash plate switching cylinder 32L. The first traveling control valve SV4 is a two-position switching valve configured to switch its valve body to a first position VP4a and a second position VP4b.

[0022] The first travel control valve SV4 is switched by the directional control valve SV2 located upstream and connected to the first travel control valve SV4. Specifically, the directional control valve SV2 and the first travel control valve SV4 are connected by an oil passage PA3, and the first travel control valve SV4 is switched by the hydraulic oil flowing through the oil passage PA3. For example, when the valve body of the directional control valve SV2 is switched to the first position VP2a by operating the operating member, pilot oil is discharged from the section between the directional control valve SV2 and the first travel control valve SV4, and the valve body 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 first gear. When the valve element of the directional control valve SV2 is switched to the second position VP2b by operating the operating member, pilot oil is supplied to the first travel control valve SV4 through the directional control 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 extends, and the speed of the first hydraulic motor 31L is changed to second gear.

[0023] The second hydraulic motor device 30R is a device that transmits power to the drive wheels 31 provided on 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 a second traveling 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. In other words, 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 traveling control valve SV5 is a two-position switching valve configured to switch its valve body to a first position VP5a and a second position VP5b, and operates in the same manner as the first traveling control valve SV4.

[0024] A drain oil passage DR1 is connected to the hydraulic circuit 20. The drain oil passage DR1 is an oil passage that flows pilot oil from a plurality of switching valves (brake switching valve SV1, directional control valve SV2) to the hydraulic oil tank 70. For example, the drain oil passage DR1 is connected to the discharge ports of the plurality of switching valves (brake switching valve SV1, directional control valve SV2). That is, when the brake switching valve SV1 is in the first position VP1a, hydraulic oil is discharged from the oil passage PA2 to the drain oil passage DR1 in the section between the brake switching valve SV1 and the brake mechanism 72. When the directional control valve SV2 is in the first position VP1a, pilot oil in the oil passage PA3 is discharged to the drain oil passage DR1.

[0025] The hydraulic circuit 20 further includes a first charge oil passage PA4 and a hydraulic drive unit 75. The first charge oil passage PA4 branches off from the pilot supply oil passage PA1 and is connected to the hydraulic drive unit 75. The hydraulic drive unit 75 is a device that drives the first hydraulic motor unit 30L and the second hydraulic motor unit 30R. The hydraulic drive unit 75 has a first drive circuit 76L for driving the first hydraulic motor unit 30L and a second drive circuit 76R for driving the second hydraulic motor unit 30R.

[0026] The first drive circuit 76L has a first hydraulic pump 7L, drive oil passages PA5L, PA6L, and a second charge oil passage PA7L. The drive oil passages PA5L, PA6L are oil passages that connect the first hydraulic pump 7L and the first hydraulic motor 31L. The hydraulic circuit formed by the drive oil passages PA5L, PA6L is called the first hydraulic circuit CL. The second charge oil passage PA7L is an oil passage that is connected to the drive oil passages PA5L, PA6L and replenishes the hydraulic oil from the pilot pump 71 to the drive oil passages PA5L, PA6L. The first hydraulic motor 31L has a first connection port 31P1 that connects to the drive oil passage PA5L and a second connection port 31P2 that connects to the drive oil passage PA6L. Hydraulic oil for rotating the first traveling device 3L in the forward direction is input to the first hydraulic motor 31L via the first connection port 31P1, and hydraulic oil for rotating the first traveling device 3L in the reverse direction is discharged from the first hydraulic motor 31L via the first connection port 31P1. Hydraulic oil for rotating the first traveling device 3L in the reverse direction is input to the first hydraulic motor 31L via the second connection port 31P2, and hydraulic oil for rotating the first traveling device 3L in the forward direction is discharged from the first traveling device 3L.

[0027] Similarly, the second drive circuit 76R has a second hydraulic pump 7R, drive oil passages PA5R and PA6R, and a third charge oil passage PA7R. The drive 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 drive oil passages PA5R and PA6R is called the second hydraulic circuit CR. The third charge oil passage PA7R is an oil passage that is connected to the drive oil passages PA5R and PA6R and replenishes the hydraulic oil from the pilot pump 71 to the drive oil passages PA5R and PA6R. The second hydraulic motor 31R has a third connection port 31P3 that connects to the drive oil passage PA5R and a fourth connection port 31P4 that connects to the drive oil passage PA6R. Hydraulic oil 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 hydraulic oil for rotating the second traveling device 3R in the backward direction is discharged from the second hydraulic motor 31R via the third connection port 31P3. Hydraulic oil for rotating the second traveling device 3R in the backward direction is input to the second hydraulic motor 31R via the fourth connection port 31P4, and hydraulic oil for rotating the second traveling device 3R in the forward direction is discharged from the second traveling device 3R. In other words, the hydraulic motors (31L, 31R) are configured to drive the traveling devices (3L, 3R). The hydraulic pumps (7L, 7R) are configured to discharge hydraulic oil for driving the hydraulic motors (31L, 31R). The drive oil passages (PA5L, PA6L, PA5R, PA6R) are oil passages that connect the hydraulic pumps (7L, 7R) and the hydraulic motors (31L, 31R).

[0028] The first hydraulic pump 7L and the second hydraulic pump 7R are swash plate type variable displacement axial pumps driven by the power of the engine 6. The first hydraulic pump 7L is connected to the first hydraulic motor 31L via a first hydraulic circuit CL, and has a first port PLa and a second port PLb to which a pilot pressure acts. The angle of the swash plate of 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 oil to the first hydraulic motor 31L via the first hydraulic circuit CL so as to drive the first traveling device 3L forward when the hydraulic pressure applied to the first port PLa is higher than the hydraulic pressure applied to the second port PLb, and to supply hydraulic oil to the first hydraulic motor 31L via the first hydraulic circuit CL so as to drive the first traveling device 3L backward when the hydraulic pressure applied to the second port PLb is higher than the hydraulic pressure applied to the first port PLa. The first port PLa is a port for moving the swash plate of the first hydraulic motor 31L so that the first hydraulic motor 31L drives the first hydraulic motor 31L in the forward direction when the first hydraulic pump 7L is rotated. The second port PLb is a port for moving the swash plate of the first hydraulic motor 31L 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.

[0029] 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 on which a pilot pressure acts. The angle of the swash plate of the second hydraulic pump 7R is changed by the pilot pressure acting on the third port PRa and the fourth port PRb. Specifically, the second hydraulic pump 7R is configured to supply hydraulic oil 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 hydraulic oil 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 for moving 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 for moving 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 hydraulic pump 7L and the second hydraulic pump 7R can change the output (discharge amount of hydraulic oil) and the discharge direction of the hydraulic oil according to the angle of the swash plate.

[0030] The output of the first hydraulic pump 7L and the second hydraulic pump 7R and the discharge direction of the hydraulic oil are changed by an operating device 56 for operating the traveling direction of the work vehicle 1. Specifically, the output of the first hydraulic pump 7L and the second hydraulic pump 7R and the discharge direction of the hydraulic oil are changed in response to the operation of an operating lever 55 provided on the operating device 56. In other words, the operating device 56 is a device configured to operate the traveling direction of the work vehicle by selecting at least one of the first traveling device 3L and the second traveling device 3R and instructing at least one of the traveling devices to move forward or backward.

[0031] As shown in Fig. 3, the hydraulic circuit 20 includes a pilot supply oil passage PA8 branched from a pilot supply oil passage PA1 and connected to the operating device 56, and a pilot pressure control valve CV1 provided on the pilot supply oil passage PA8. The pilot pressure control valve CV1 is an electromagnetic proportional valve and is configured to adjust the pilot pressure supplied to the operating device 56 by adjusting its opening. The opening of the pilot pressure control valve CV1 is controlled by a controller 10. In the following embodiments, the pilot pressure control valve CV1 may be referred to as a hydraulic adjustment mechanism. A detailed operation of the pilot pressure control valve CV1 will be described later.

[0032] The operating device 56 includes a forward operating valve OVA, a reverse operating valve OVB, a right turning operating valve OVC, a left turning operating valve OVD, and an operating lever 55. The operating device 56 also includes first to fourth shuttle valves SVa, SVb, SVc, and SVd. The operating valves OVA, OVB, OVC, and OVD are operated by a single operating lever 55. The operating valves OVA, OVB, OVC, and OVD change the pressure of the hydraulic oil in response to the operation of the operating lever 55, and supply the changed hydraulic oil 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 operating valves OVA, OVB, OVC, and OVD are operated by a single operating lever 55, but there may be a plurality of operating levers 55. In the following embodiments, the single or multiple operating levers 55 may be referred to as a second operating member. The operating lever 55 is configured to receive an input corresponding to a target movement direction of the vehicle body 2, which is determined by the speed difference between the target rotation speed of the first hydraulic pump 7L and the target rotation speed of the second hydraulic pump 7R. In other words, the target movement direction is set by the operating lever 55.

[0033] The operation valves OVA, OVB, OVC, and OVD 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 a pilot supply oil passage PA8. The discharge port is connected to a drain oil passage DR2 leading to a hydraulic oil tank 70. The operation lever 55 can be tilted from a neutral position in the front-rear direction, in the width direction perpendicular to the front-rear direction, and in the diagonal direction. That is, the operation lever 55 can be rotated in the front-rear direction and the left-right direction. The operation valves OVA, OVB, OVC, and OVD of the operation device 56 are operated in response to the tilting of the operation lever 55. Thereby, a pilot pressure according to the operation amount from the neutral position of the operation lever 55 is output from the secondary side port of the operation valves OVA, OVB, OVC, and OVD. The relationship between the pilot pressure applied to the primary side port output from the pilot pressure control valve CV1 and the pilot pressure applied to the secondary side port will be described later.

[0034] The secondary side port of the control valve OVA and the secondary side port of the control valve OVC are connected to the input port of the first shuttle valve SVa, and the output port of the first shuttle valve SVa 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 to the first port PLa via the pilot supply oil passage PA8. The secondary side port of the control valve OVA and the secondary side port of the control valve OVD are connected to the input port of the second shuttle valve SVb, and the 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 passage PA13. The third pilot oil passage PA13 connects the pilot pump 71 to the third port PRa via the pilot supply oil passage PA8.

[0035] 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 is 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 to the second port PLb via the pilot supply oil 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 port of the fourth shuttle valve SVd, and the output port of the fourth shuttle valve SVd is connected to the fourth port PRb of the second hydraulic pump 7R via the fourth pilot oil passage PA14. The fourth pilot oil passage PA14 connects the pilot pump 71 to the fourth port PRb via the pilot supply oil passage PA8.

[0036] The pilot supply oil passage PA1, the pilot supply oil passage PA8, the first pilot oil passage PA11, and the second pilot oil passage PA12, which connect the pilot pump 71 and the two pilot ports (PLa, PLb) of the first hydraulic pump 7L, are referred to as a first pilot hydraulic circuit PC1. The pilot supply oil passage PA1, the pilot supply oil 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 PC1 and the second pilot hydraulic circuit PC2. At least one first pressure control valve PCV1 is provided in the first pilot hydraulic circuit PC1 and controls a first high pilot pressure, which is the higher of two pilot pressures applied to the two pilot ports (PLa, PLb) of the first hydraulic pump 7L, and includes an operating valve OVA, an operating valve OVB, and a pilot pressure control valve CV1. At least one second pressure control valve PCV2 is provided in the second pilot hydraulic circuit PC2 and controls a second high pilot pressure, which is the higher of two pilot pressures applied to the two pilot ports (PRa, PRb) of the second hydraulic pump 7R, and includes an operating valve OVC, an operating valve OVD, and a pilot pressure control valve CV1.

[0037] When the control lever 55 is tilted forward, the control valve OVA for forward movement is operated and pilot pressure is output from the control valve OVA. This pilot pressure acts on the first port PLa from the first shuttle valve SVa via the first pilot oil passage PA11 that connects the control device 56 and the first port PLa of the first hydraulic pump 7L, and also acts on the third port PRa from the second shuttle valve SVb via the third pilot oil passage PA13 that connects the control device 56 and the third port PRa of the second hydraulic pump 7R. As a result, the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate forward (forward rotation) at a speed corresponding to the tilt amount of the control lever 55, and the work vehicle 1 moves straight forward.

[0038] Furthermore, when the operating lever 55 is tilted rearward, the reverse operating valve OVB is operated and pilot pressure is output from the operating valve OVB. This 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 operating device 56 and the second port, and also acts on the fourth port PRb from the fourth shuttle valve SVd via the fourth pilot oil passage PA14 connecting the operating device 56 and the fourth port PRb of the second hydraulic pump 7R. As a result, the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate in the reverse direction (reverse rotation) at a speed corresponding to the tilt amount of the operating lever 55, and the work vehicle 1 moves straight backward.

[0039] Furthermore, when the control lever 55 is tilted to the right, the control valve OVC for right turning is operated and pilot pressure is output from the control valve OVC. This pilot pressure acts on the first port PLa of the first hydraulic pump 7L through the first pilot oil passage PA11 from the first shuttle valve SVa, and also acts on the fourth port PRb of the second hydraulic pump 7R through the fourth pilot oil passage PA14 from the fourth shuttle valve SVd. This causes the vehicle to turn to the right with a degree of turning that corresponds to the rightward operating position of the control lever 55.

[0040] Furthermore, when the control lever 55 is tilted to the left, the control valve OVD for left turning is operated and pilot pressure is output from the control valve OVD. This pilot pressure acts on the third port PRa of the second hydraulic pump 7R via the second shuttle valve SVb and the third pilot oil passage PA13, and also acts on the second port PLb of the first hydraulic pump 7L via the third shuttle valve SVc and the second pilot oil passage PA12. This causes the vehicle to turn left with a degree of turning that corresponds to the leftward operating position of the control lever 55.

[0041] That is, when the operating lever 55 is tilted diagonally forward to the left, the work vehicle 1 moves forward at a speed corresponding to the operating position of the operating lever 55 in the forward / rearward direction, and turns left with a degree of turning corresponding to the operating position of the operating lever 55 in the left direction. When the operating lever 55 is tilted diagonally forward to the right, the work vehicle 1 turns right while moving forward at a speed corresponding to the operating position of the operating lever 55. When the operating lever 55 is tilted diagonally rearward to the left, the work vehicle 1 turns left while moving backward at a speed corresponding to the operating position of the operating lever 55. When the operating lever 55 is tilted diagonally rearward to the right, the work vehicle 1 turns right while moving backward at a speed corresponding to the operating position of the operating lever 55.

[0042] 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 separate from the above-described operation device 56, or an accelerator lever supported so as to be freely 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 reached. Alternatively, the controller 10 outputs a rotation command indicating a fuel injection pressure, etc. to a supply pump or a common rail so that the determined target rotation speed of the engine 6 is reached. 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 that detects 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. The speed sensor 6a is, for example, a potentiometer configured to detect the rotation speed of a rotating member connected to the crankshaft of the engine 6. When a load is applied to the engine 6, the actual rotation speed of the engine 6 decreases from the target rotation speed of the engine 6. The amount of decrease in the actual rotation speed from the target rotation speed when a load is applied to the engine 29 (the difference between the target rotation speed of the engine and the actual rotation speed of the engine) is called the amount of engine drop.

[0043] The pilot pressure control valve CV1 can set a pilot pressure (primary pilot pressure) acting on the input ports (primary side ports) of the multiple operating valves OVA, OVB, OVC, and OVD based on a drop amount (drop amount) ΔE1 of the rotation speed of the engine 6 (engine rotation speed E1). In other words, the pilot pressure control valve CV1 is a control valve that is provided between the pilot pump 71 and the operating valves OVA, OVB, OVC, and OVD, sends pilot oil to the operating valves OVA, OVB, OVC, and OVD, and is configured to convert the pressure of the pilot oil supplied to the operating valves OVA, OVB, OVC, and OVD into a primary pilot pressure. The rotation speed of the engine 6 can be detected by a speed sensor 6a for 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 called a speed sensor. FIG. 4 shows the relationship between the engine rotation speed, the traveling primary pressure (primary pilot pressure), and the setting lines L1 and L2. The setting line L1 shows the relationship between the engine rotation speed E1 and the traveling primary pressure when the decrease amount ΔE1 is less than a predetermined value (less than the anti-stall determination value). The setting line L2 shows the relationship between the engine rotation speed E1 and the traveling primary pressure when the decrease amount 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 according to the third correspondence relationship shown 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 greater than the predetermined stall determination speed difference (anti-stall determination value), the primary pilot pressure corresponding to the rotation speed RS transitions according to the fourth correspondence relationship shown by the setting line L2.

[0044] When the amount of decrease ΔE1 is less than the anti-stall determination value, the controller 10 adjusts the opening of the pilot pressure control valve CV1 so that the relationship between the engine rotation speed E1 and the traveling primary pressure coincides with the reference pilot pressure indicated by the set line L1. When the amount of decrease ΔE1 is equal to or greater than the anti-stall determination value, the controller 10 adjusts the opening of the pilot pressure control valve CV1 so that the relationship between the engine rotation speed E1 and the traveling primary pressure coincides with the set line L2 which is lower than the reference pilot pressure. In the set line L2, the traveling primary pressure for a given engine rotation speed E1 is lower than the traveling primary pressure of the set line L1. That is, when the same engine rotation speed E1 is taken into consideration, the traveling primary pressure of the set line L2 is set lower than the traveling primary pressure of the set line L1. Therefore, the pressure (pilot pressure) of the hydraulic oil entering the operating valves OVA, OVB, OVC, and OVD is kept low by the control based on the set line L2. As a result, the swash plate angles of the first hydraulic pump 7L and the second hydraulic pump 7R are adjusted, the load acting on the engine 6 is reduced, and stalling of the engine 6 can be prevented. Although one setting line L2 is shown in FIG. 4, a plurality of setting lines L2 may be provided. For example, the setting line L2 may be set for each engine rotation speed E1. In addition, it is preferable that the controller 10 has data indicating the setting lines L1 and L2, or control parameters such as functions.

[0045] Next, the secondary pilot pressure output from the secondary side port of the operating valves OVA, OVB, OVC, and OVD will be described. FIG. 5 is a diagram showing the relationship between the operating position of the operating lever and the traveling secondary pressure (secondary pilot pressure). Referring to FIG. 4, the lever operating position has an origin at the operation start position (neutral position, G0 position) which is the start position of the lever stroke, and approaches the operation end position (G5 position) which is the end position of the lever stroke as it moves away from the origin. The operating region of the operating lever 55 is divided into a neutral region RA1 where the operating object does not move (in the illustrated example, from the G0 position to the G1 position), a full operation vicinity region RA2 near the operation end (in the illustrated example, from the G3 position to the G5 position), and an intermediate region RA3 between the neutral region RA1 and the full operation vicinity region RA2 (in the illustrated example, from the G1 position to the G3 position). Further, the intermediate region RA3 is divided into a slow-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.

[0046] In the neutral region RA1, no secondary pilot pressure is supplied even if the control lever 55 is operated. On the other hand, in the full operation vicinity region RA2, the speed of the controlled object is not adjusted, and therefore the control lever 55 is operated to the operation end position (G5 position) without stopping midway. In the intermediate region RA3, the control lever 55 is stopped at any position within the region or its position is changed, and the speed of the controlled object is adjusted to the speed desired by the operator. For example, the ratio of each of the operation regions RA1, RA3A, RA3B, and RA2 to the lever stroke is as follows: Neutral region RA1: 0% to less than 15% Slow speed area RA3A: 15% to less than 45% Intermediate speed range RA3B: 45% to less than 75% Full operation area RA2: 75% to 100%

[0047] In the characteristic diagram shown in Figure 5, when the control lever 55 is operated from the G0 position to the G1 position, a secondary pilot pressure (Pa) is generated, and when the control lever 55 is operated from the G1 position to the G4 position, the secondary pilot pressure rises from Pa to Pb in proportion to the amount of operation of the control lever 55. Also, at the G4 position, the primary pilot pressure is shortcut and flows to the secondary side, and the secondary pilot pressure rises from Pb to the maximum output pressure Pc in one go. Then, while the control lever 55 is operated from the G4 position to the G5 position, the secondary pilot pressure remains constant at the maximum output pressure (Pc) and is equal to the primary pilot pressure.

[0048] 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 displacement from the neutral position of the operation lever 55 for instructing movement in the left direction is equal to or greater than the first displacement value (displacement 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 displacement from the neutral position of the operation lever 55 for instructing movement in the right direction is equal to or greater than the first displacement value (displacement 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 displacement from the neutral position of the operation lever 55 for instructing movement in the forward direction is equal to or greater than the first displacement value (displacement from G0 to G4). The operating device 56 outputs the primary pilot pressure input to the operating device 56 to the second port PLb and the fourth port PRb when the displacement from the neutral position of the operating lever 55 for instructing movement in the rearward direction is equal to or greater than a first displacement value (displacement from G0 to G4).

[0049] The characteristic value of the secondary pilot pressure in the forward / rearward direction may be different from the characteristic value of the secondary pilot pressure in the left / right direction. If the characteristic values ​​of the secondary pilot pressure in the forward / rearward direction corresponding to G0-G5 and Pa-Pc are G0'-G5' and Pa'-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 displacement of the operation lever 55 for instructing a movement in the forward direction from the neutral position is equal to or greater than the second displacement value (displacement 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 displacement of the operation lever 55 for instructing a movement in the rearward direction from the neutral position is equal to or greater than the second displacement value (displacement from G0' to G4'). In addition, Pa and Pb (Pa' and Pb') are values ​​that are independent of the magnitude of the primary pilot pressure, but if the primary pilot pressure is lower than Pa or Pb (Pa' or Pb'), the secondary pilot pressure will plateau at the magnitude of the primary pilot pressure.

[0050] That is, the operating valves (OVA, OVB, OVC, OVD) are configured to convert the pressure of the pilot oil from the primary traveling pressure to the secondary traveling pressure according to the first operation amount (operation lever position) of the operating device 56, and output the pilot oil. The pilot oil of the secondary traveling pressure is applied to the ports (PLa, PRa, PLb, PRb) that provide hydraulic pressure to the swash plates of the hydraulic pumps (7L, 7R). When the first operation amount is equal to or greater than a threshold amount (first displacement value), the operating valves (OVA, OVB, OVC, OVD) convert the primary traveling pressure to the secondary traveling pressure. The control mechanism 25 includes oil passages PA1 to PA14 leading from the pilot pump 71 to the ports (PLa, PRa, PLb, PRb), the pilot pressure control valve CV1, and the operating 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 10 a is configured to control the control mechanism 25 .

[0051] Based on the above characteristics of the operating valves OVA, OVB, OVC, and OVD, the movement of the work vehicle 1 corresponding to the operation of the operating lever 55 will be described in more detail. When the operation amount of the operating lever 55 in the forward / rearward direction is larger than the operation amount in the rightward direction, and the operation position of the operating lever 55 in the rightward direction is operated from the G1 position to the G3 position, the first hydraulic pump 7L rotates in the same direction with the rotation speed of the first hydraulic pump 7L being larger than the rotation speed of the second hydraulic pump 7R, and the work vehicle 1 turns to the right in a wide turn. When the operation position of the operating lever 55 in the rightward direction is the same as the operation position in the forward / rearward direction, the rotation speed of the second hydraulic pump 7R becomes 0, and only the first hydraulic pump 7L rotates, and the work vehicle 1 performs a right pivot turn (right pivot turn). Furthermore, when the operation position of the operating lever 55 in the rightward direction is operated between the G4 position and the G5 position, it becomes larger than the operation position in the forward / rearward direction, and the output shaft of the first hydraulic pump 7L rotates forward and the output shaft of the second hydraulic pump 7R rotates reversely, and the work vehicle 1 turns to the right.

[0052] In addition, when the operation amount of the operation lever 55 in the forward / rearward direction is greater than the operation amount in the leftward direction, and the operation position of the operation lever 55 in the leftward direction is operated from the G1 position to the G3 position, the second hydraulic pump 7R rotates in the same direction with the rotation speed of the first hydraulic pump 7L being greater than that of the second hydraulic pump 7R, causing the work vehicle 1 to make a wide turn to the left. When the operation position of the operation lever 55 in the leftward direction is the same as the operation position in the forward / rearward direction, the rotation speed of the first hydraulic pump 7L becomes 0, and only the second hydraulic pump 7R rotates, causing the work vehicle 1 to make a left pivot turn (left pivot turn). Furthermore, when the operation position of the operation lever 55 in the leftward direction is operated between the G4 position and the G5 position, it becomes greater than the operation position in the forward / rearward direction, causing the output shaft of the second hydraulic pump 7R to rotate forward and the output shaft of the first hydraulic pump 7L to rotate reversely, causing the work vehicle 1 to turn left. In this embodiment, turning refers to the movement of the work vehicle 1 when the operating position in the right direction is operated between the G4 position and the G5 position, or when the operating position in the left direction is operated between the G4 position and the G5 position.

[0053] On the other hand, when the operation lever 55 is operated between positions G4 and G5 in the forward direction, the operation position becomes larger than the operation position in the left / right direction, the output shafts of the first hydraulic pump 7L and the second hydraulic pump 7R rotate forward, and the work vehicle 1 moves forward at high speed. When the operation lever 55 is operated between positions G4 and G5 in the rearward direction, the operation position becomes larger than the operation position in the left / right direction, the output shafts of the first hydraulic pump 7L and the second hydraulic pump 7R rotate in the reverse direction, and the work vehicle 1 moves backward at high speed. The operation of the other operation levers 55 in the forward / rearward direction is the same as that in the left / right direction.

[0054] 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 operating member. The creep setting member 16 is configured, for example, with a touch panel, a freely slidable slide switch, or a dial. Creep refers to control that causes the work vehicle 1 to travel at or below an upper limit speed, regardless of the amount of operation of at least one operating device (one or more operating levers 55) to which a user's speed change operation is input.

[0055] The creep setting member 16 is configured to receive an input corresponding to a set stage among a plurality of speed stages corresponding to a plurality of pairs of the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R configured to drive the first traveling device 3L and the second traveling device 3R, respectively, when driving the first traveling device 3L and the second traveling device 3R to linearly move the vehicle body 2. When the speed ratio between the target rotation speed of the first hydraulic motor 31L and the movement speed of the first traveling device 3L is equal to the speed ratio between the target rotation speed of the second hydraulic motor 31R and the movement 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 at each speed stage may be equal. The target rotational speed for this speed stage corresponds to the primary traveling pressure such that the rotational speed of the first hydraulic motor 31L and the rotational speed of the second hydraulic motor 31R become the above-mentioned target rotational speed when the operating lever 55 is tilted forward (or backward) and the lever operating position is operated between G4 and G5.

[0056] In the first embodiment, the higher of the traveling secondary pressures applied to the ports (PLa, PLb) for determining the displacement of the first hydraulic pump 7L is called the first control parameter for determining the displacement of the first hydraulic pump 7L. The higher of the traveling secondary pressures applied to the ports (PRa, PRb) for determining the displacement of the second hydraulic pump 7R is called the second control parameter for determining the displacement of the second hydraulic pump 7R. The range of the first control parameter limited by the traveling primary pressure is called the operating range of the first control parameter. The range of the second control parameter limited by the traveling primary pressure is called the operating range of the second control parameter.

[0057] The speed stages may be expressed by a number (stage number) expressed by a natural number starting from 1, for example. The larger the stage number, 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). The difference between the number of speed stages is called the number of stages. The number of stages between speed stages i and speed stage k (i>k) is (i-k+1). FIG. 7 shows an example of the speed stages. In FIG. 7, the maximum value of the speed stage is N, and if the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R at speed stage m are the same, Nm(m), the target rotation speed Nm(m) is set to monotonically increase as m increases.

[0058] The creep setting member 16 is configured to switch between a normal mode and a creep mode. A state in which the setting stage is set by the creep setting member 16 is called a creep mode. A state other than the creep mode is called a normal mode. In the normal mode, a target rotation speed of the engine 6 is set by operating the first setting member 11a and the second setting member 11b, and a primary traveling pressure corresponding to the target rotation speed is obtained based on the setting line L1 or L2 in FIG. 4. Then, a secondary traveling pressure is set based on the operation amount of one or more operating 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 is changed according to the operation amount of at least one operating device, and the work vehicle 1 can be made to travel at a speed higher than the upper limit speed.

[0059] On the other hand, 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 swash plate angle of the hydraulic pump (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 (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) that are predetermined in the speed stage, and the primary traveling pressure for outputting the pilot pressure for realizing the maximum value of the swash plate angle 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 therefore the maximum value of the swash plate angle of the hydraulic pump (7L, 7R) is 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 swash plate angle. For this reason, the setting line L1 or L2 in FIG. 4 is not used, and the speed stage is used to determine a primary traveling pressure that is smaller than the primary traveling pressure in the normal mode. 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 number of speed stages increases temporarily only at the moment the accelerator pedal is depressed. The settings after the secondary traveling pressure in the creep mode are the same as those in the normal mode, but since the secondary traveling pressure is equal to or lower than the primary traveling 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 operating device (one or more operating levers 55). In the creep mode, the first hydraulic motor 31L and the second hydraulic motor 31R are fixed at the first speed.

[0060] 3 and 6, 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 the operation valve OVA, OVB, OVC, OVD changes in response to the operation position of the operation lever 55. Therefore, the first hydraulic sensor SP11 to the fourth hydraulic 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.

[0061] The work vehicle 1 includes a hydraulic sensor SP5L for detecting the hydraulic pressure of the drive oil passage PA5L, a hydraulic sensor SP6L for detecting the hydraulic pressure of the drive oil passage PA6L, a hydraulic sensor SP5R for detecting the hydraulic pressure of the drive oil passage PA5R, and a hydraulic sensor SP6R for detecting the hydraulic pressure of the drive oil passage PA6R. That is, the hydraulic sensors (SP5L, SP6L, SP5R, SP6R) are configured to detect the hydraulic pressure of the working oil in the drive oil passages (PA5L, PA6L, PA5R, PA6R). The states of the first hydraulic motor 31L and the second hydraulic motor 31R can be detected from the pressure difference between the hydraulic sensor SP5L and the hydraulic sensor SP6L and the pressure difference between the hydraulic sensor SP5R and the hydraulic sensor SP6R.

[0062] 2, 3, and 6, the work vehicle 1 may further include a rotation sensor SR31L connected to a rotating shaft of the first hydraulic motor 31L and detecting the rotation speed of the first hydraulic motor 31L and a rotation sensor SR31R detecting the 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 rotation direction and the magnitude of the rotation speed detected by the rotation sensor SR31L and the rotation direction and the magnitude of 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.

[0063] In this embodiment, the greater the curvature at which the work vehicle 1 turns (the smaller the turning radius), the greater the running resistance that the work vehicle 1 receives from the ground, and therefore taking into consideration the fact that the primary traveling pressure required to turn at a desired upper limit speed increases, the controller 10 determines the amount of deviation between the target moving direction and the straight-ahead direction, and controls the primary traveling pressure to be increased by increasing the number of speed steps from the set step set by the creep setting member 16 as the deviation increases. In the following explanation of the embodiment, the speed step that is controlled to be a step greater than the set step for the reasons described above will be referred to as a boost step. This deviation amount can also be detected from the operating position of the operating lever 55, the direction of travel determined from position changes of a GPS (Global Positioning System) installed in the vehicle, and the rotational speed difference between the rotational speed of the first hydraulic motor 31L detected by the rotational sensor SR31L and the rotational speed of the second hydraulic motor 31R detected by the rotational sensor SR31R, but below we will provide a detailed description of the 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.

[0064] When using these secondary pilot pressures to represent the amount of deviation, 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 reverse 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 reverse pilot pressure lb(t), the second forward pilot pressure rf(t), and the second reverse pilot pressure rb(t) correspond to information corresponding to the target movement direction. The controller 10 determines which of the following first to fourth instructions has been input to the operating lever 55 based on the first forward pilot pressure lf(t), the first reverse pilot pressure lb(t), the second forward pilot pressure rf(t), and the second reverse pilot pressure rb(t). (1) First command: rotate the first hydraulic motor 31L so as to drive the first traveling device 3L in the forward direction, and rotate or stop the second hydraulic motor 31R so as to drive the second traveling device 3R in the reverse direction relative to the first traveling device 3L. (2) Second command: rotate the first hydraulic motor 31L so as to drive the first traveling device 3L in the backward direction, and rotate or stop the second hydraulic motor 31R so as to drive the second traveling device 3R in the forward direction relative to the first traveling device 3L. (3) A third command: 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 reverse direction relative to the second traveling device 3R. (4) Fourth command: rotate the second hydraulic motor 31R so as to drive the second traveling device 3R in the reverse direction, and rotate or stop the first hydraulic motor 31L so as to drive the first traveling device 3L in the forward direction relative to the second traveling device 3R.

[0065] The determination of which of the first to fourth instructions corresponds can be performed by the following method shown in Japanese Patent Application No. 2021-093045. For example, when the above four pilot pressures are determined to be a right turn (step S11 in FIG. 16 of the application) and other than reverse (step S10 in FIG. 4 of the application), the controller 10 can estimate that the first instruction has been given. When the above four pilot pressures are determined to be a right turn (step S11 in FIG. 16 of the application) and other than forward (step S10 in FIG. 9 of the application), the controller 10 can estimate that the second instruction has been given.

[0066] The controller 10 can deduce that a third command has been issued when it is determined from the above four pilot pressures that the command is to turn left (step S11 in FIG. 13 of the application) and is other than reverse (step S10 in FIG. 4 of the application). The controller 10 can deduce that a fourth command has been issued when it is determined from the above four pilot pressures that the command is to turn left (step S11 in FIG. 13 of the application) and is other than forward (step S10 in FIG. 9 of the application).

[0067] Thereafter, the controller 10 determines the degree of operation 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-mentioned instructions from at least two pressures among the first forward pilot pressure lf(t), the first reverse pilot pressure lb(t), the second forward pilot pressure rf(t), and the second reverse pilot pressure rb(t). For example, in the case of the first instruction, the controller 10 can determine the degree of operation so that the smaller rf(t) / lf(t)*lb(t) / rb(t) is, the greater the degree of operation. In the case of the second instruction, the controller 10 can determine the degree of operation so that the smaller rb(t) / lb(t)*lf(t) / rf(t) is, the greater the degree of operation. In the case of the third instruction, the controller 10 can determine the degree of operation so that the smaller lf(t) / rf(t)*rb(t) / lb(t) is, the greater the degree of operation. In the case of the second instruction, the controller 10 can determine the degree of operation so that the smaller lb(t) / rb(t)*rf(t) / lf(t) is, the larger the degree of operation becomes. Recognizing any one of the first instruction to the fourth instruction and determining the degree of operation corresponds to determining the target movement direction. The controller 10 determines the amount of deviation so that the greater the degree of operation is, the larger the deviation becomes.

[0068] The amount of deviation can be expressed in various ways, but one concept that can be used is straightness. The concept of straightness will be explained below. This state of high straightness is a state that satisfies two conditions: (1) the pilot pressure related to the ports (first port PLa, third port PRa) for the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R to rotate in the forward direction is sufficiently greater than the pilot pressure related to the ports (second port PLb, fourth port PRb) for the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R to rotate in the reverse direction, or the pilot pressure related to the ports (second port PLb, fourth port PRb) for the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R to rotate in the reverse direction is sufficiently greater than the pilot pressure related to the ports (first port PLa, third port PRa) for the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R to rotate in the forward direction; and (2) the pilot pressures of the two ports determined to have high pilot pressure in (1) are substantially equal (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)).

[0069] Therefore, the straightness is calculated by the following algorithm: First, the larger value of lb(t) and rb(t) is set as the variable PV Fstraight The larger value of lf(t) and rf(t) is assigned to the variable PV Bstraight Assign to .

[0070] Straightness in forward direction S Fratio (t) is calculated by the formula (1). Bratio (t) is calculated using 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 is S Fratio (t) and S Bratio The larger value of (t) and (t) is calculated as the straightness.

[0071] In this embodiment, for example, 300, which is generally regarded as straight, is set as the reference straightness, and it can be regarded that the smaller the straightness, the greater the deviation between the target movement direction and the straight-line direction. <Controller 10 Configuration>

[0072] The controller 10 has a processor 10a and a memory 10b as shown in FIG. 6 in order to realize the above-mentioned control of the vehicle speed in the creep mode. The processor 10a may be called an electronic circuit. The memory 10b includes a volatile memory and a non-volatile memory. The memory 10b may be called a storage device. The memory 10b includes at least a driving control program 10c1, a first reference information 10r1, a second reference information 10r2, a third reference information 10r3, and a fourth reference information 10r4 in order to realize the above-mentioned control.

[0073] As shown in Fig. 7, the first reference information 10r1 represents a first correspondence relationship between the above-mentioned speed stages in the creep mode and sets of the target rotation speeds of the first hydraulic motor 31L and the second hydraulic motor 31R corresponding to each speed stage. That is, the memory 10b is configured to store a plurality of speed stages corresponding to a plurality of sets of the target rotation speeds of the first hydraulic motor 31L and the second hydraulic motor 31R when the first traveling device 3L and the second traveling device 3R are driven to move the vehicle body 2 straight ahead. In principle, the target rotation speeds of the first hydraulic motor 31L and the second hydraulic motor 31R are the same value at the same speed stage. However, when the reduction ratio of the reducer between the first hydraulic motor 31L and the first traveling device 3L is different from the reduction ratio of the reducer between the first hydraulic motor 31L and the first traveling device 3L, the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R may be different. In addition, the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R may be changed according to the degree of operation in each of the first instruction to the fourth instruction. Although FIG. 7 also shows the primary pressure Pp(m), which is the first control parameter and the second control parameter, these are values ​​calculated from the target rotation speed of the engine 6 and the target rotation speed Nm(m), so the first reference information 10r1 does not have to include the primary pressure Pp(m). The second reference information 10r2 represents the relationship between the set 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.

[0074] 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 primary traveling pressure in the normal mode. That is, the third reference information 10r3 represents the third correspondence relationship represented by the set line L1 in 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 primary traveling pressure, which is used for controlling the primary traveling 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 set line L2 in FIG. 4.

[0075] The processor 10a executes the following control while executing the driving 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 10r4. First, when the normal mode is selected by the creep setting member 16, the processor 10a is configured to obtain the rotation speed RS of the engine 6 from the speed sensor 6a, obtain the primary traveling pressure corresponding to the detected rotation speed RS of the engine 6 from the third reference information, and control the pilot pressure control valve CV1 so as to achieve the obtained primary traveling pressure. When the drop amount of the engine 6 is large when the normal mode is selected, the processor 10a is configured to obtain the primary traveling 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 primary traveling pressure.

[0076] 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 the straightness representing the target moving direction. Then, when the processor 10a determines that the creep setting member 16 has been operated so that the deviation amount between the target moving direction calculated from the straightness and the straight moving direction is within a predetermined first range Rdev1, the processor 10a determines the operating range (the range whose upper limit is the primary traveling pressure based on the setting stage) of the first control parameter (first high pilot pressure) for controlling the displacement of the first hydraulic pump 7L and the operating range (the range whose upper limit is the primary traveling pressure based on the setting stage) of the second control parameter (second high pilot pressure) for controlling the displacement of the second hydraulic pump 7R 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) that can be known from the setting stage. The first range Rdev1 is a range of deviation amounts in which at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates when the setting stage is set to the minimum speed stage (speed stage with the number of stages 1) that is the speed stage corresponding to the smallest set of target rotation speeds among the multiple speed stages. When it is determined that the creep setting member 16 has been operated so that the deviation amount falls outside the first range Rdev1, the processor 10a determines the boost stage that is the speed stage corresponding to the target rotation speed higher than the target rotation speed of the setting stage based on the magnitude of the deviation amount, and determines the operating range of at least one of the first control parameter and the second control parameter based on the boost stage. In this embodiment, the at least one control parameter includes the first control parameter and the second control parameter.

[0077] FIG. 8 shows an example of the second reference information 10r2 according to the first embodiment. In FIG. 8, the horizontal axis indicates the straightness, and the vertical axis indicates the number of steps (boost stage-set stage). A high straightness means that the deviation between the target moving direction and the straight direction is small, so the above-mentioned first range Rdev1 is set as a range of straightness higher than the straightness threshold Th1. When the straightness is equal to or greater than the threshold Th1, the operating range of the first control parameter and the operating range of the second control parameter are determined based on the set stage, so the number of steps between the boost stage and the set stage is 0. The threshold Th1 is a value empirically determined as the minimum straightness at which at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates when set to the minimum speed stage (speed stage with the number of stages being 1).

[0078] In this embodiment, when the number of speed stages representing a set of the target rotation speed of the first hydraulic motor 31L between the target rotation speed of the first hydraulic motor 31L in the boost stage and the target rotation speed of the first hydraulic motor 31L in the set stage, and the target rotation speed of the second hydraulic motor 31R between the target rotation speed of the second hydraulic motor 31R in the boost stage and the target rotation speed of the second hydraulic motor 31R in the set stage is m (m is an integer of 0 or more), if the number of steps is m+1, when the deviation amount is outside the first range Rdev1, the boost stage is determined so that the number of steps increases as the deviation amount increases. In other words, the straightness is controlled so that the number of steps between the boost stage and the set stage increases as the straightness decreases from the threshold value Th1.

[0079] However, if this control sets the boost stage to the minimum speed stage allowing rotation, which is the speed stage where the target rotation speed of the first hydraulic motor 31L and the target rotation speed of the second hydraulic motor 31R are the smallest, among at least one speed stage at which at least one of the first hydraulic motor 31L and the second hydraulic motor 31R can rotate regardless of any operation of the operating lever 55, then there is no need to increase the number of boost stages any further, and the number of steps between the boost stage and the set stage will be capped at a predetermined upper limit. That is, when the number of speed stages representing a pair 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 speed stage capable of swing and the target rotation speed of the first hydraulic motor 31L of the minimum speed stage (speed stage with 1 stage) 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 speed stage capable of swing and the target rotation speed of the second hydraulic motor 31R of the minimum speed stage (speed stage with 1 stage) among the multiple speed stages is n (n is an integer of 0 or more), n+1 is set as the maximum number of stages. The number of stages of the minimum speed stage capable of swing is n+2.

[0080] In the example of FIG. 8, when the straight-ahead degree is between the threshold Th0 (Th0 < Th1) and the threshold Th1, as the straight-ahead degree decreases, the number of steps between the boost stage and the set stage linearly changes so as to increase. When the straight-ahead degree is less than the threshold Th0, an example is shown in which the number of steps between the boost stage and the set stage is set to the upper limit DMAX (= n + 1). When the threshold values of the straight-ahead degree corresponding to arbitrary step numbers j and j + 1 are SD(j) and SD(j + 1) respectively, if the straight-ahead degree is between SD(j) and SD(j + 1), and if it is the speed stage (j + 1), 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. That is, in the range of the deviation amount (SD(j) and SD(j + 1)) in which the boost stage is determined, the number of steps is determined so that at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates. In other words, in the range of the deviation amount (SD(j) to SD(j + 1)) in which the boost stage is determined, the number of steps is determined so that at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates.

[0081] Note that when the straight-ahead degree is between the threshold Th0 (Th0 < Th1) and the threshold Th1, the relationship between the number of steps and the straight-ahead degree only needs to be monotonically decreasing, and does not necessarily have to decrease linearly. That is, when the threshold values of the straight-ahead degree corresponding to arbitrary step numbers j and j + 1 are SD(j) and SD(j + 1) respectively, the relationship between the straight-ahead degree and the number of steps is determined so that SD(j + 1) < SD(j) holds.

[0082] 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 set stage is 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 so as not to deviate from the operating range of the first control parameter determined based on the boost stage or the set stage, and to control the control mechanism 25 that controls the second control parameter so as not to deviate from the operating range of the second control parameter determined based on the boost stage or the set stage. The processor 10a is configured to control the engine 6 that is configured to rotate the first hydraulic pump 7L and the second hydraulic pump 7R so as to rotate the first hydraulic motor 31L and the second hydraulic motor 31R that are set to a constant displacement volume (set to 1st gear) regardless of a plurality of speed stages. <Work vehicle operation>

[0083] 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 this embodiment, in this flowchart, the processes from step S1 to step S14 are executed at a predetermined sampling interval (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 control method of the work vehicle 1 according to this 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 has been selected by the creep setting member 16. That is, the control method according to this embodiment includes determining whether or not the creep mode has been selected by the creep setting member 16. When the creep mode is set, that is, 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.

[0084] 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 operating member). Then, 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 reverse 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 reverse pilot pressure rb(t) from the fourth hydraulic sensor SP14. The processor 10a calculates the straightness from the above values ​​based on the above algorithm. In other words, in the control method of this embodiment, in step S4, the processor 10a receives information (lf(t), lb(t), rf(t), rb(t)) representing the target movement direction of the vehicle body 2, which is determined by the speed difference between the target rotational speed of the first hydraulic motor 31L and the target rotational speed of the second hydraulic motor 31R, from sensors (SP11, SP12, SP13, SP14) that detect the operation of the operating lever 55 (second operating member) that sets the target movement direction, and calculates the straightness corresponding to the target movement direction.

[0085] In step S5, the processor 10a determines whether the operation lever 55 (second operation member) is operated so that the deviation amount between the target moving direction and the straight traveling direction is within a 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), in step S6, the processor 10a determines the traveling primary pressure based on the setting stage. That is, based on the setting stage, the processor 10a determines the operating range (a range whose upper limit is the traveling primary pressure based on the setting stage) of the first control parameter (first high pilot pressure) for determining the displacement volume of the first hydraulic pump 7L configured to supply hydraulic oil to the first hydraulic motor 31L, and the operating range (a range whose upper limit is the traveling primary pressure based on the setting stage) of the second control parameter (second high pilot pressure) for determining the displacement volume of the second hydraulic pump 7R configured to supply hydraulic oil to the second hydraulic motor 31R. Specifically, the processor 10a obtains the maximum value of the swash plate angle of the hydraulic pump (7L, 7R) based on the target rotation speed of the engine 6 set by the first setting member 11a and the target rotation speeds of the first hydraulic motor 31L and the second hydraulic motor 31R known from the setting stage, and determines the primary traveling pressure so that the swash plate angle is obtained 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 obtained in step S1 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 to bring the rotation speed RS of the engine 6 closer to the target rotation speed of the engine 6, separately from the speed control method according to this embodiment.

[0086] When the operating lever 55 (second operating member) is operated so that the deviation amount falls outside 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 a speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, based on the magnitude of the deviation amount (threshold Th--the straightness obtained in step S4). In step S8, the processor 10a determines the operating range (a range with the primary traveling pressure based on the set stage as an upper limit) of at least one of the first control parameter and the second control parameter based on the boost stage. 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 boost stage. Specifically, the processor 10a determines the maximum value of the swash plate angle 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 speeds of the first hydraulic motor 31L and the second hydraulic motor 31R determined from the boost stage, and determines the primary traveling pressure so that this swash plate angle is achieved when the operating lever 55 is tilted to its maximum.

[0087] In step S12, the processor 10a controls the pilot pressure control valve CV1 that sends pilot oil to the control valves OVA, OVB, OVC, and OVD so as to achieve the primary traveling pressure calculated in step S6 or step S8. In step S13, the control valves OVA, OVB, OVC, and OVD convert the primary traveling pressure to the secondary traveling pressure based on the lever position (first operation amount) of the control lever 55 (second operation member). In other words, the control method according to this embodiment includes converting the primary traveling pressure to the secondary traveling pressure by the control valves OVA, OVB, OVC, and OVD based on the lever position (first operation amount) of the control lever 55 (first operation device). In steps S12 and S13, the control method of this embodiment includes controlling the first control parameter (first high pilot pressure) determined based on the boost stage or the setting stage so that it does not deviate from its operating range (a range having an upper limit of the primary traveling pressure based on the setting stage), and controlling the second control parameter (second high pilot pressure) so that it does not deviate from its operating range (a range having an upper limit of the primary traveling pressure based on the setting stage).

[0088] In step S14, the processor 10a controls the engine 6 configured to rotate the first hydraulic pump 7L and the second hydraulic pump 7R. As a result, the secondary traveling pressure of the pilot oil is applied to the ports (PLa, PRa, PLb, PRb) that provide hydraulic pressure to the swash plates of the hydraulic pumps (7L, 7R). As a result, the processor 10a rotates the first hydraulic pump 7L and the second hydraulic pump 7R that are set to a constant displacement volume (set to 1st speed) regardless of multiple speed stages. The control method according to this 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 that are set to a constant displacement volume regardless of multiple speed stages.

[0089] 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 greater than the anti-stall determination value. When there is no engine drop (No in step S9), in step S10, the processor 10a obtains the primary running pressure from the third reference information 10r3 based on the rotation speed RS of the engine 6. That is, in the control method according to the present embodiment, the third reference information 10r3 is prepared, and when the normal mode is selected from the creep mode and the normal mode, the primary running 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 there is an engine drop (Yes in step S9), in step S11, the processor 10a obtains the primary running pressure from the fourth reference information 10r4 based on the rotation speed RS of the engine 6. That is, in the control method according to the present embodiment, the fourth reference information 10r4 is prepared, and when the normal mode is selected, when an engine drop occurs, the primary traveling 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. After the process of step S10 or step S11 ends, the processes of steps S12 to S14 are executed. <Actions and Effects of the First Embodiment>

[0090] In the control method for the work vehicle 1 or the work vehicle 1 according to the first embodiment, when the operation lever 55 (second operation member) is operated so that the deviation amount between the target moving direction and the straight-ahead direction falls outside the first range Rdev1, the processor 10a determines the boost stage, which is a speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, based on the magnitude of 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 so that the control parameter does not 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, so that the time required to reach the desired speed can be shortened even in a turn with a large curvature. <Modification of the first embodiment>

[0091] In this embodiment, the first control parameter is a first high pilot pressure, which is the higher of the two pilot pressures applied to the ports (PLa, PLb) for determining the displacement of the first hydraulic pump 7L, but the first control parameter may be a first pilot effective pressure obtained by subtracting a lower first low pilot pressure from a higher first high pilot pressure of two pilot pressures for determining the displacement of the first hydraulic pump 7L. In this embodiment, the second control parameter is a second high pilot pressure, which is the higher of the two pilot pressures applied to the ports (PRa, PRb) for determining the displacement of the second hydraulic pump 7R, but the second control parameter may be a second pilot effective pressure obtained by subtracting a lower second low pilot pressure from a higher second high pilot pressure of two pilot pressures for determining the displacement of the second hydraulic pump 7R. At this time, at least one third pressure control valve PCV3 provided in the first pilot hydraulic circuit PC1 and controlling a first pilot effective pressure which is a difference between a higher pilot pressure and a lower pilot pressure of two pilot pressures applied to two pilot ports (PLa, PLb) of the first hydraulic pump 7L includes an operating valve OVA (first pressure reducing valve), an operating valve OVB (second pressure reducing valve), and a pilot pressure control valve CV1. At least one fourth pressure control valve PCV4 provided in the second pilot hydraulic circuit PC2 and controlling a second pilot effective pressure which is a difference between a higher pilot pressure and a lower pilot pressure of two pilot pressures applied to two pilot ports (PRa, PRb) of the second hydraulic pump 7R includes an operating valve OVC (third pressure reducing valve), an operating valve OVD (fourth pressure reducing valve), and a pilot pressure control valve CV1.

[0092] FIG. 10 shows a hydraulic system (hydraulic circuit 20A) of a work machine different from that shown in FIG. 3. The control mechanism according to this modification may be called a control mechanism 25A. The hydraulic system of the work machine in FIG. 10 differs in the connection between the operating device 56 and the first hydraulic pump 7L and the second hydraulic pump 7R. The other configuration in FIG. 10 is the same as that in FIG. 3. In FIG. 10, the first to fourth shuttle valves SVa, SVb, SVc, and SVd are not provided. The forward operation valve OVA is connected to the first pilot oil passage PA11, the reverse operation valve OVB is connected to the second pilot oil passage PA12, the right turn operation valve OVC is connected to the third pilot oil passage PA13, and the left turn operation valve OVD is connected to the fourth pilot oil passage PA14.

[0093] The operating 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 a secondary pressure according to the rotation direction and amount of rotation of the operating lever 55. The operating valve OVB is a second pressure reducing valve provided in the second pilot oil passage PA12 and configured to convert the primary pressure supplied by the pilot pump 71 into a secondary pressure according to the rotation direction and amount of rotation of the operating lever 55. The operating 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 a secondary pressure according to the rotation direction and amount of rotation of the operating lever 55. The operating 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 a secondary pressure according to the rotation direction and amount of rotation of the operating lever 55.

[0094] In this modified example, the second control parameter may be a first high pilot pressure, which is the higher of the two pilot pressures applied to the ports (PLa, PLb) for determining the displacement of the first hydraulic pump 7L, or a first pilot effective pressure obtained by subtracting a lower first low pilot pressure from a higher first high pilot pressure of two pilot pressures for determining the displacement of the first hydraulic pump 7L. The second control parameter may be a second high pilot pressure, which is the higher of the two pilot pressures applied to the ports (PRa, PRb) for determining the displacement of the second hydraulic pump 7R, or a second pilot effective pressure obtained by subtracting a lower second low pilot pressure from a higher second high pilot pressure of two pilot pressures for determining the displacement of the second hydraulic pump 7R.

[0095] In this case, at least one first pressure control valve PCV1 includes an operating valve OVA (first pressure reducing valve), an operating valve OVB (second pressure reducing valve), and a pilot pressure control valve CV1. At least one second pressure control valve PCV2 includes an operating valve OVC (third pressure reducing valve), an operating valve OVD (fourth pressure reducing valve), and a pilot pressure control valve CV1. At least one third pressure control valve PCV3 includes an operating valve OVA (first pressure reducing valve), an operating valve OVB (second pressure reducing valve), and a pilot pressure control valve CV1. At least one fourth pressure control valve PCV4 includes an operating valve OVC (third pressure reducing valve), an operating valve OVD (fourth pressure reducing valve), and a pilot pressure control valve CV1.

[0096] In this hydraulic circuit, the determination of which of the first to fourth instructions corresponds can be performed by the following method shown in Japanese Patent Application No. 2021-93045. For example, when the above four pilot pressures are determined to be a right turn (step S27 in FIG. 17 of the application) and other than reverse (step S28 in FIG. 7 of the application), the controller 10 can estimate that the first instruction has been given. When the above four pilot pressures are determined to be a right turn (step S27 in FIG. 17 of the application) and other than forward (step S28 in FIG. 11 of the application), the controller 10 can estimate that the second instruction has been given.

[0097] The controller 10 can estimate that a third command has been issued when it is determined from the above four pilot pressures that the command is to turn left (step S27 in FIG. 15 of the application) and is other than reverse (step S28 in FIG. 7 of the application). The controller 10 can estimate that a fourth command has been issued when it is determined from the above four pilot pressures that the command is to turn left (step S27 in FIG. 15 of the application) and is other than forward (step S28 in FIG. 11 of the application).

[0098] Thereafter, the controller 10 determines the degree of operation 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-mentioned instructions from at least two pressures among the first forward pilot pressure lf(t), the first reverse pilot pressure lb(t), the second forward pilot pressure rf(t), and the second reverse pilot pressure rb(t). For example, in the case of the first instruction, the controller 10 can determine the degree of operation so that the greater the rf(t) / lf(t)*lb(t) / rb(t) is, the greater the degree of operation. In the case of the second instruction, the controller 10 can determine the degree of operation so that the greater the rb(t) / lb(t)*lf(t) / rf(t) is, depending on the hydraulic circuit. In the case of the third instruction, the controller 10 can determine the degree of operation so that the greater the lf(t) / rf(t)*rb(t) / lb(t) is, depending on the hydraulic circuit. In the case of the fourth command, the controller 10 can determine the degree of operation so that the greater the value of lb(t) / rb(t)*rf(t) / lf(t) is, the greater the degree of operation becomes, depending on the hydraulic circuit.

[0099] In this modification, the parameter corresponding to the degree of straightness based on the degree of operation described above may also be regarded as the amount of deviation between the target movement direction and the straight-line direction. <Second embodiment>

[0100] In the first embodiment, an example in which the primary pilot pressure is controlled to realize the creep mode has been given, but the secondary pilot pressure may be controlled. FIG. 11 is a hydraulic circuit diagram of a traveling system of a work vehicle in a second embodiment. FIG. 11 shows a configuration added to FIG. 3. In FIG. 11, the same configuration as in FIG. 3 is given the same reference numerals, and detailed description is omitted. In the second embodiment, the work vehicle 1 includes a hydraulic circuit 21. The hydraulic circuit 21 differs from the hydraulic circuit 20 in that it further includes a first relief valve CV23, a 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 control mechanism 25 further includes a first relief valve CV23, a 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.

[0101] The first relief valve CV23 and the second relief valve CV24 are balanced relief valves whose set pressure for opening is variable based on the pressure of the pilot oil, and have control ports 23a and 24a that receive the pilot oil. The first relief valve CV23 is configured to open when the pressure related to the input port is greater than a first set pressure applied to the control port 23a. The second relief valve CV24 is configured to open when the pressure related to the input port is greater than a second set pressure applied to the control port 24a. At this time, the pilot oil is discharged to the hydraulic oil tank 70. The proportional valves CV21 and CV22 are connected to hydraulic oil passages 211 and 212 connected to the control ports 23a and 24a, and pilot oil is supplied from the pilot pump 71. The proportional valves CV21 and CV22 are electromagnetic proportional valves whose opening degree can be changed by exciting a solenoid, and are controlled by the controller 10.

[0102] The proportional valves CV21 and CV22 are connected to the pilot supply oil passage PA1, and in the creep mode, the pressure is controlled to be the pressure obtained by adding an offset α to the pilot pressure control valve CV1 (primary pressure control valve) in the first embodiment, taking into consideration the outflow of pilot oil from the first relief valve CV23, the second relief valve CV24, etc., and in the normal mode, the proportional valves CV21 and CV22 operate to be a value obtained by adding the offset α to the set line L1 when anti-stall control is not performed, and to be a value obtained by adding the offset α to the set line L2 when anti-stall control is performed. Of the proportional valves CV21 and CV22, the proportional valve that controls the hydraulic pressure of the pilot oil in the first pilot hydraulic circuit PC1 may be referred to as the secondary pressure control valve CV2, and the proportional valve that controls the pilot oil in the second pilot hydraulic circuit PC2 may be referred to as the additional secondary pressure control valve 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 the additional secondary pilot pressure, which is the hydraulic pressure of the pilot oil in the second pilot hydraulic circuit PC2.

[0103] The first relief valve CV23 is connected to a first pilot oil passage PA11 between the operating valve OVA (first pressure reducing valve) and the first port PLa, and to a second pilot oil passage PA12 between the operating 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 operating valve OVA (first pressure reducing valve) and the secondary pressure input to the operating valve OVB (second pressure reducing valve) exceeds the first set pressure. The second relief valve CV24 is connected to a third pilot oil passage PA13 between the operating valve OVC (third pressure reducing valve) and the third port PRa, and to a fourth pilot oil passage PA14 between the operating 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 operating valve OVC (third pressure reducing valve) and the secondary pressure input to the operating valve OVD (fourth pressure reducing valve) exceeds the second set pressure.

[0104] The discharge oil passage DR3 is connected to a first pilot oil passage PA11. The discharge oil passage DR4 is connected to a second pilot oil passage PA12. The discharge oil passage DR5 is connected to a third pilot oil passage PA13. The discharge oil passage DR6 is connected to a fourth pilot oil passage PA14. The check valves CK1-CK4 close the discharge oil passages DR3-DR6 unless the pressure on the side where the throttles TH1-TH4 are located becomes higher than the pressure on the side where the relief valves CV23, CV24 are located by a predetermined value or more.

[0105] The pilot pressures of the discharge oil passages DR3 and DR4 become high when the first hydraulic pump 7L rotates forward and backward, respectively, so that when the pilot pressure of one of the passages becomes equal to the primary pilot pressure, the other passage becomes significantly smaller than the primary pilot pressure. The pilot pressures of the discharge oil passages DR5 and DR6 become high when the second hydraulic pump 7R rotates forward and backward, respectively, so that when the pilot pressure of one of the passages becomes equal to the primary pilot pressure, the other passage becomes significantly smaller than the primary pilot pressure. For this reason, usually, only one of the check valves CK1 and CK2 is opened. For this reason, the proportional valves CV21 and CV22 can execute the above-mentioned control by controlling the pressure of the proportional valves CV21 and CV22 so that the pressure is equal to the pressure caused by the control of the pilot pressure control valve CV1 (primary pressure control valve) according to the first embodiment plus the pressure loss caused by the pilot oil flowing out from the relief valves CV23 and CV24.

[0106] 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 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 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 valve SVc and the discharge oil passage DR5, and is configured to reduce the flow rate of 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 valve SVd and the discharge oil passage DR6, and is configured to reduce the flow rate of pilot oil in the fourth pilot oil passage PA14.

[0107] 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. In this embodiment, at least one first pressure control valve PCV1 includes an operating valve OVA, an operating valve OVB, a first relief valve CV23, and a secondary pressure control valve CV2. At least one second pressure control valve PCV2 includes an operating valve OVC, an operating valve OVD, a second relief valve CV24, and an additional secondary pressure control valve ACV2. At least one third pressure control valve PCV3 includes an operating valve OVA, an operating valve OVB, a first relief valve CV23, and a secondary pressure control valve CV2. At least one fourth pressure control valve PCV4 includes an operating valve OVC, an operating valve OVD, a second relief valve CV24, and an additional secondary pressure control valve ACV2.

[0108] In this 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 to correspond to the traveling primary pressure according to the speed stage shown in FIG. 12. FIG. 12 shows an example of the first reference information 10r1 representing the speed stage according to the second embodiment. The first reference information 10r1 in 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), so that the maximum value of the swash plate angle of the hydraulic pump (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) is determined based on the obtained swash plate angle. The traveling secondary pressure Ps(m) corresponds to the first set pressure and the second set pressure described above. In Fig. 12, the secondary traveling pressure Ps(m), which is both the first and second control parameters, is also displayed, but 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 does not need to include the secondary traveling pressure Ps(m). If the secondary traveling pressure at the speed stage m is Ps(m), the secondary traveling pressure Ps(m) is set to monotonically increase as m increases. The processor 10a receives the set stage from the creep setting member 16 and calculates the straightness representing the target moving direction.

[0109] When the processor 10a determines that the creep setting member 16 has been operated so that the deviation amount between the target moving direction calculated from the straightness and the straight traveling direction falls within a predetermined first range Rdev1, the processor 10a determines the operating range (the range with the traveling secondary pressure based on the set stage as the upper limit) of the first control parameter (the first high pilot pressure or the first pilot effective pressure) for controlling the displacement of the first hydraulic pump 7L and the operating range (the range with the traveling secondary pressure based on the set stage as the upper limit) of the second control parameter (the second high pilot pressure or the second pilot effective pressure) for controlling the displacement of the second hydraulic pump 7R based on the set stage. The processor 10a determines the boost stage, which is a speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, based on the magnitude of the deviation amount, and determines the operating range (the range with the traveling secondary pressure based on the set stage as the upper limit) of at least one 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. 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, of the first control parameter and the second control parameter, a control parameter for a hydraulic pump that drives the traveling device that moves on the outside of the turning, of the first traveling device 3L and the second traveling device 3R. In addition, the control parameter for the hydraulic pump that does not correspond to the at least one control parameter is determined based on the setting stage.

[0110] The processor 10a is configured to control the control mechanism 25 (secondary pressure control valve CV2) that controls the first control parameter so as not to deviate from the operating range of the first control parameter determined based on the boost stage or the setting stage (the range with the traveling secondary pressure based on the setting stage or the boost stage as the upper limit), and to control the control mechanism 25 (additional secondary pressure control valve ACV2) that controls the second control parameter so as not to deviate from the operating range of the second control parameter determined based on the boost stage or the setting stage (the range with the traveling secondary pressure based on the setting stage or the boost stage as the upper limit). The processor 10a is configured to control the engine 6 that is configured to rotate the first hydraulic pump 7L and the second hydraulic pump 7R so as to rotate the first hydraulic motor 31L and the second hydraulic motor 31R that are set to a constant displacement volume (set to 1st gear) regardless of a plurality of speed stages. <Operation of the work vehicle according to the second embodiment>

[0111] 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 will be omitted. In the work vehicle 1 according to this embodiment, when it is determined that the deviation amount between the target moving direction and the straight traveling direction is within the first range Rdev1 (Yes in step S5), in step S6A, the processor 10a determines the traveling secondary pressure based on the setting stage. That is, based on the setting stage, the processor 10a determines the operating range (a range with the traveling secondary pressure based on the setting stage as the upper limit) of the first control parameter (first high pilot pressure or first pilot effective pressure) for determining the displacement of the first hydraulic pump 7L configured to supply hydraulic oil to the first hydraulic motor 31L, and the operating range (a range with the traveling secondary pressure based on the setting stage as the upper limit) of the second control parameter (second high pilot pressure or second pilot effective pressure) for determining the displacement of the second hydraulic pump 7R configured to supply hydraulic oil to the second hydraulic motor 31R. More specifically, the processor 10a determines the maximum value of the swash plate angle of the hydraulic pumps (7L, 7R) based on the target rotational speed of the engine 6 set by the first setting member 11a and the target rotational speeds of the first hydraulic motor 31L and the second hydraulic motor 31R determined from the setting stage, and determines the secondary traveling pressure Ps(m) so that this swash plate angle is obtained when the operating lever 55 is tilted to the maximum.

[0112] After step S7, in step S8A, the processor 10a determines the operating range (the range with the secondary traveling pressure based on the setting stage as the upper limit) of at least one of the first and second control parameters based on the boost stage. Specifically, the processor 10a obtains the maximum value of the swash plate angle of the hydraulic pump (7L, 7R) based on the target rotation speed of the engine 6 set by the first setting member 11a and the target rotation speeds of the first hydraulic motor 31L and the second hydraulic motor 31R known from the boost stage, and determines the secondary traveling pressure Ps(m) so that the swash plate angle is obtained when the operation lever 55 is tilted to the maximum. In step S12A, the processor 10a controls the pilot pressure control valve CV1 that sends pilot oil to the operation valves OVA, OVB, OVC, and OVD so that the primary traveling pressure is obtained in step S10 or step S11. In steps S6A and S8A, the primary traveling 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 at least one control parameter so 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 valve CV23 so that the first set pressure becomes the upper limit value of the operating range (the range whose upper limit is the traveling secondary pressure Ps(m) based on the setting stage). When the at least one control parameter includes the second control parameter, the processor 10a controls the second relief valve CV24 so that the second set pressure becomes the upper limit value of the operating range (the range whose upper limit is the traveling secondary pressure Ps(m) based on the setting stage). Steps S13 and S14 are the same as those in the first embodiment. <Actions and Effects of the Second Embodiment>

[0113] In the control method of the work vehicle 1 or 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 traveling direction falls outside the first range Rdev1, the processor 10a determines the boost stage, which is a speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, based on the magnitude of 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) so that the determined operating range (a range with the traveling secondary pressure based on the set stage or the boost stage as an upper limit) is not exceeded. As a result, 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) are controlled in consideration of the turning situation, so that the time required to reach the desired speed can be shortened even in a turn with a large curvature. <Modification of the second embodiment>

[0114] FIG. 14 is a hydraulic circuit diagram according to a modified example 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 in the example of FIG. 11. The shuttle valve SV12 connects the oil passage having the higher hydraulic pressure out of the discharge oil passage DR3 and the discharge oil passage DR4 to the relief valve CV23. The shuttle valve SV34 connects the oil passage having the higher hydraulic pressure out of the discharge oil passage DR5 and the discharge oil passage DR6 to the relief valve CV24. Even with this hydraulic circuit configuration, the above-mentioned control can be executed. In addition, in the circuit of FIG. 11 or FIG. 14, the pilot pressure control valve CV1 (primary pressure control valve) may be omitted. In addition, at least one of the combination of the secondary pressure control valve CV2 and the balanced relief valve and the combination of the additional secondary pressure control valve ACV2 and the balanced relief valve may be realized by an electromagnetic proportional relief valve. <Third embodiment>

[0115] Although the operation lever 55 according to the above 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 the control valves that control 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 in the second embodiment can be realized by adjusting the operation amount detected by the sensor. FIG. 15 is a hydraulic circuit diagram of the traveling system of the work vehicle 1 in the third embodiment. In FIG. 15, the same components as those in FIG. 3 are given the same reference numerals, and detailed description is omitted. In the third embodiment, the work vehicle 1 includes a hydraulic circuit 22. The hydraulic circuit 22 includes pilot control valves CV31 to CV34 that control the pilot pressures applied to the ports (PLa, PRa, PLb, PRb), respectively, instead of the operation valves OVA, OVB, OVC, and OVD and the first to fourth shuttle valves SVa, SVb, SVc, and SVd. The pilot control valves CV31 to CV34 are electromagnetic proportional valves including 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 the pilot pressure.

[0116] In this embodiment, the pilot supply oil passage PA8 connects the pilot control valves CV31-CV34 and the pilot supply oil passage PA8, and the first to fourth pilot oil passages PA11-PA14 are connected to the pilot control valves CV31-CV34, respectively. In this embodiment, the pilot supply oil passages PA1, PA8, the first pilot oil passage PA11, and the second pilot oil passage PA12 correspond to the first pilot hydraulic circuit PC1. The pilot supply oil passages PA1, PA8, the third pilot oil passage PA13, and the fourth pilot oil passage PA14 correspond to the second pilot hydraulic circuit PC2. In this embodiment, since there are no operation valves OVA, OVB, OVC, and OVD, there is no difference between a primary pilot pressure and a secondary pilot pressure. Therefore, in this embodiment, these are simply called pilot pressure without distinction.

[0117] The pilot control valve CV31 is a first electromagnetic pressure control valve provided in the first pilot oil passage PA11 and configured to convert the pilot pressure supplied by the pilot pump 71 in response to 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 convert 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 convert 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 convert the pilot pressure supplied by the pilot pump 71 in response to a fourth signal from the controller 10.

[0118] 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 this embodiment, the at least one first pressure control valve PCV1 includes 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 pilot control valves CV33 to CV34.

[0119] In the normal mode, the controller 10 controls the pilot control valves CV31 to CV34 so that a pilot pressure corresponding to FIG.

[0120] FIG. 16 is a schematic diagram showing the tilt direction of the operating lever 55 as viewed from above. FIG. 17 is a schematic diagram showing the tilt direction of the operating lever 55 as viewed from the side. FIG. 18 is a schematic diagram showing the tilt direction of the operating lever 55 as viewed from behind. FIGS. 16 to 18 show directions D1, D2, and D3 to clarify the correspondence between the directions. Direction D3 is the direction in which the operating lever 55 faces when in the neutral position. FIG. 16 is a view of the operating lever 55 as viewed in the opposite direction to direction D3. When the operation detection sensor 18 is a potentiometer configured to detect the tilt angle of the operating lever 55, the operation detection sensor 18 detects the tilt angle θ shown in FIG. 17. T and the tilt angle θ shown in FIG. U The tilt angle θ T The direction in which the operating lever 55 faces when in the neutral position is set to 0 degrees, the angle at which it is tilted when moving forward is set to positive, and the angle at which it is tilted when moving backward is set to negative. U The direction in which the operating lever 55 faces when in the neutral position is taken as 0 degrees, the angle at which it is tilted to the right is taken as positive, and the angle at which it is tilted to the left is taken as negative.

[0121] Tilt angle θ T corresponds to the lever operation positions of the operating valves OVA and OVB in FIG. 5. U corresponds to the lever operation positions of the operating valves OVC and OVD in FIG. 5. In the normal mode, the tilt angle θ T is the angle 0~θ 1TH This range corresponds to the lever operation positions G0 to G1 in FIG. 5. The maximum tilt angle of the operation lever 55 is θ MAX Then, the tilt angle θ in FIG. T (θ MAX -θ 2TH )~θ MAX This range corresponds to the lever operation positions G4 to G5 in FIG. 5. U is the angle 0~θ 3TH This range corresponds to the lever operation positions G0 to G1 in FIG. 5. The maximum tilt angle of the operation lever 55 is θ MAX Then, the tilt angle θ in FIG. U (θ MAX -θ4TH )~θ MAX corresponds to the lever operation positions G4 to G5 in FIG.

[0122] FIG. 19 shows the tilt angle θ T 20 is a diagram showing the relationship between the tilt angle θ of the operating lever 55 and the assumed operating position r. U and the deemed operation position r. In the creep mode, angles 0 to θ corresponding to G0 to G1 are 1TH , 0~θ 3TH corresponds to the play when not operated, so the deemed operation position r corresponds to G0 regardless of the speed stage. T (θ MAX -θ 2TH )~θ MAX When the operation is performed between U (θ MAX -θ 4TH )~θ MAX If the tilt angle θ is between 0 and 1, it can be considered that the operation is fully performed by taking into account the play, and therefore it can be considered that the operation is performed to the maximum deemed operation position Ga(m). T is θ 1TH ~(θ MAX -θ 2TH ) when the tilt angle θ T The assumed operation position r should be changed linearly from G1 to Ga(m) according to the magnitude of the tilt angle θ U is θ 3TH ~(θ MAX -θ 4TH ) when the tilt angle θ U It is advisable to change the assumed operation position r linearly from G1 to Ga(m) according to the magnitude of

[0123] Tilt angle θ F The tilt angle θ is defined as 0 degrees when the operating lever 55 is tilted in the forward tilt direction, and is positive in the clockwise direction, ranging from -90 degrees to 90 degrees. F is the tilt angle θ T is positive. F is the tilt angle θ Tand tilt angle θ U Using these, it is expressed as follows (Equation 1). θ F =arctan(sinθ U / sinθ T ) (Formula 1)

[0124] Tilt angle θ B The tilt angle θ is defined as 0 degrees when the operating lever 55 is tilted in the forward tilt direction, and is positive in the clockwise direction, ranging from -90 degrees to 90 degrees. B is the tilt angle θ T is negative. B is the tilt angle θ T and tilt angle θ U Using these, it is expressed as follows (Equation 2). θ B =arctan(sinθ U / sinθ T ) (Formula 2)

[0125] Tilt angle θ F (θ B The absolute value of the tilt angle θ ) corresponds to the deviation between the target moving direction and the straight moving direction. This is different from the straight moving direction shown in the first and second embodiments, and is F (θ B ) is larger, the amount of deviation is larger. FIG. 22 shows an example of the second reference information 10r2a according to the third embodiment. FIG. 22 shows the relationship between the tilt angle θ F (θ B The absolute value of the tilt angle θ is plotted on the horizontal axis, and the number of steps (boost step - set step) is plotted on the vertical axis. F (θ B ) is large means that the deviation between the target moving direction and the straight moving direction is large. Therefore, the above-mentioned first range Rdev1 is a tilt angle θ smaller than the tilt angle threshold value θTh1. F (θ B The range of the absolute value of the tilt angle θ F (θ B) is equal to or less 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 set stage, and therefore the number of steps between the boost stage and the set stage is 0. The threshold value θTh1 is a value empirically determined as the minimum straightness at which at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates when set to the minimum speed stage (speed stage with step number 1).

[0126] In the normal mode, the pilot pressure controlled by the pilot control valve CV31 is T The tilt angle θ is determined according to the operation position of FIG. T When the tilt angle θ is positive, the pilot pressure controlled by the pilot control valve CV32 is 0. The pilot pressure controlled by the pilot control valve CV32 is T The tilt angle θ is determined according to the operation position of FIG. T When the tilt angle θ is negative, the pilot pressure controlled by the pilot control valve CV31 is 0. The pilot pressure controlled by the pilot control valve CV33 is 0 when the tilt angle θ is positive. U The tilt angle θ is determined according to the operation position of FIG. U When the tilt angle θ is positive, the pilot pressure controlled by the pilot control valve CV34 is 0. The pilot pressure controlled by the pilot control valve CV34 is U The tilt angle θ is determined according to the operation position of FIG. U When is negative, the pilot pressure controlled by the pilot control valve CV33 is 0.

[0127] In the creep mode, the pilot pressure controlled by the pilot control valve CV31 is set at a positive tilt angle θ T 5 corresponding to the assumed operating position r in FIG. 19 estimated for the tilt angle θ T When the tilt angle θ is positive, the pilot pressure controlled by the pilot control valve CV32 is 0. The pilot pressure controlled by the pilot control valve CV32 is T5 corresponding to the assumed operating position r in FIG. 19 estimated for the tilt angle θ T When the tilt angle θ is negative, the pilot pressure controlled by the pilot control valve CV31 is 0. The pilot pressure controlled by the pilot control valve CV33 is 0 when the tilt angle θ is positive. U 5 corresponding to the assumed operating position r in FIG. 20 estimated for the tilt angle θ U When the tilt angle θ is positive, the pilot pressure controlled by the pilot control valve CV34 is 0. The pilot pressure controlled by the pilot control valve CV34 is U 5 corresponding to the assumed operating position r in FIG. 20 estimated for the tilt angle θ U When is negative, the pilot pressure controlled by the pilot control valve CV33 is 0.

[0128] In this embodiment, a maximum deemed operation position Ga or a pilot pressure Pt corresponding to the maximum deemed operation position Ga is set for each speed stage. FIG. 21 shows an example of the first reference information 10r1 representing the speed stage according to the third embodiment. The first reference information 10r1 in 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 value of the swash plate angle of the hydraulic pump (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) corresponding to the deemed operation position Ga(m) is determined based on the obtained swash plate angle. The pilot pressure Pt(m) is set as the maximum value of the pilot pressure output by the pilot control valves CV31 to CV34.

[0129] FIG. 21 shows an example of the first reference information 10r1 representing the speed stage according to the third embodiment. The first reference information 10r1 in FIG. 21 also includes the first correspondence relationship in 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 pump (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 maximum deemed operating position Ga(m) is determined from the pilot pressure Pt(m) based on the correspondence relationship in FIG. 5. If the maximum deemed operating position at the speed stage m is Ga(m) and the pilot pressure at that time is Pt(m), the pilot pressure Pt(m) is set so as to monotonically increase as m increases. For most of the speed stages m, Ga(m) is set between the operating positions G1 to G4 in FIG. 5. However, at the maximum speed stage or the like, Ga(m) may be set between the operating positions G4 and G5 in FIG.

[0130] In this embodiment as well, the boost stage is determined so that the number of steps increases as the deviation between the target moving direction and the straight-line direction increases. However, the number of steps between the boost stage and the set stage is controlled so that the number of steps increases as the straightness increases from the threshold value θTh1. In this embodiment as well, when the number of speed stages representing 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 speed stage capable of turning and the target rotation speed of the first hydraulic motor 31L of the minimum speed stage (speed stage with 1 stage) 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 speed stage capable of turning and the target rotation speed of the second hydraulic motor 31R of the minimum speed stage (speed stage with 1 stage) is n (n is an integer of 0 or more), the maximum number of steps is n+1. The number of stages of the minimum speed stage capable of turning is n+2.

[0131] In the example of FIG. 22, the tilt angle θ F (θ B When the absolute value of the tilt angle θ is between the threshold value θTh0 and the threshold value θTh1 (θTh1<θTh0), F (θB The larger the absolute value of θ, the larger the number of steps between the boost stage and the set stage. F (θ B ) is greater than the threshold value θTh0, the number of steps between the boost stage and the set stage is set to the upper limit DMAX (=n+1). F (θ B ) are SD(j) and SD(j+1), respectively, the thresholds SD(j) and SD(j+1) are empirically determined so that if the straightness is between SD(j) and SD(j+1) and at the speed stage (j+1), at least one of the first hydraulic motor 31L and the second hydraulic motor 31R can rotate. In other words, the number of steps is determined so that at least one of the first hydraulic motor 31L and the second hydraulic motor 31R rotates within the deviation amount range (SD(j) to SD(j+1)) for which the boost stage is determined.

[0132] However, the tilt angle θ F (θ B When the absolute value of the tilt angle θ F (θ B The relationship between the absolute value of j and the number of steps does not necessarily have to be linear, but only needs to be monotonically increasing. That is, the tilt angle θ corresponding to any step number j or j+1 F (θ B ) are the thresholds of the absolute values ​​of j and j+1, respectively. The tilt angle θ is set so that SD(j+1)>SD(j). F (θ B The relationship between the absolute value of (a) and the number of steps is determined.

[0133] The processor 10a is configured to generate a first signal or a second signal corresponding to a first high pilot pressure and a first signal or a second signal corresponding to a first low pilot pressure based on the deviation amount between the target moving direction and the straight forward direction and the upper limit value of the operating range of the first parameter (the pilot pressure corresponding to the maximum deemed operating position Ga(m) based on the set stage or the boost stage). The processor 10a is configured to generate a third signal or a fourth signal corresponding to a second high pilot pressure and a third signal or a fourth signal corresponding to a second low pilot pressure based on the deviation amount and the upper limit value of the operating range (the pilot pressure corresponding to the maximum deemed operating position Ga(m) based on the set stage or the boost stage).

[0134] That is, the processor 10a is configured to control the control mechanism 25 (pilot control valves CV31-CV32) that controls the first control parameter so as not to deviate from the operating range of the first control parameter determined based on the boost stage or the set stage (the range with the upper limit being the pilot pressure corresponding to the maximum deemed operating position Ga(m) based on the set stage or the boost stage), and to control the control mechanism 25 (pilot control valves CV31-CV32) that controls the second control parameter so as not to deviate from the operating range of the second control parameter determined based on the boost stage or the set stage (the range with the upper limit being the pilot pressure corresponding to the maximum deemed operating position Ga(m) based on the set stage or the boost stage). The processor 10a is configured to control the engine 6 that is configured to rotate the first hydraulic pump 7L and the second hydraulic pump 7R so as to rotate the first hydraulic motor 31L and the second hydraulic motor 31R that are set to a constant displacement volume (set to 1st gear) regardless of a plurality of speed stages. <Operation of the work vehicle according to the third embodiment>

[0135] 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 given the same reference numerals, and detailed explanations will be omitted. In the work vehicle 1 according to this embodiment, after step S3, in step S4B, the processor 10a calculates a tilt angle θ corresponding to the deviation amount between the target movement direction and the straight-ahead direction.F (θ B ) is calculated. When the deviation amount is within the first range Rdev1 (Yes in step S5), the processor 10a determines in step S6B the maximum deemed operation position Ga(m) or the pilot pressure Pt(m) corresponding to the maximum deemed operation position Ga(m) based on the setting stage. That is, the processor 10a determines, based on the setting stage, the operating range (a range having an upper limit of the pilot pressure Pt(m) corresponding to the maximum deemed operation position Ga(m) based on the setting stage) of the first control parameter (first high pilot pressure or first pilot effective pressure) for determining the displacement of the first hydraulic pump 7L configured to supply hydraulic oil to the first hydraulic motor 31L, and the operating range (a range having an upper limit of the pilot pressure Pt(m) corresponding to the maximum deemed operation position Ga(m) based on the setting stage) of the second control parameter (second high pilot pressure or second pilot effective pressure) for determining the displacement of the second hydraulic pump 7R configured to supply hydraulic oil to the second hydraulic motor 31R.

[0136] When the deviation amount is outside the first range Rdev1 (No in step S5), in step S7B, the processor 10a refers to the second reference information 10r2a shown in FIG. 19 and determines the magnitude of the deviation amount (the tilt angle θ F (θ B)-threshold value θTh1). In step S8B, the processor 10a determines the operating range of at least one of the first and second control parameters based on the boost stage (a range with an upper limit of the pilot pressure Pt(m) corresponding to the maximum deemed operation position Ga(m) based on the setting stage). 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 boost stage. Note that the at least one control parameter may be one of the first and second control parameters. In that case, the at least one control parameter is a control parameter of a hydraulic pump that drives the traveling device that moves on the outside of the turning, of the first traveling device 3L and the second traveling device 3R, of the first and second control parameters. In addition, the control parameter of the hydraulic pump that does not correspond to the at least one control parameter is determined based on the setting stage.

[0137] When there is no engine drop (No in step S9), in step S10B, 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. When there is an engine drop (Yes in step S9), in step S11B, 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. After the processing of step S6B, S8B, S10B, or S11B is completed, in step S13B, the processor 10a controls each of the pilot control valves CV31 to CV34 so as to obtain a pilot pressure based on the lever position (first operation amount) of the operating lever 55 (second operating member).

[0138] That is, the control method according to this embodiment includes controlling the pilot control valves CV31 to CV34 so as to output pilot pressures based on the lever position (first operation amount) of the operation lever 55 (first operation device) by the pilot control valves CV31 to CV34. In step S13B, the control method according to this 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 so as not to deviate from the operating range (a range with an upper limit of the pilot pressure Pt corresponding to the maximum deemed operating position Ga based on the setting stage) and controlling the second control parameter (second high pilot pressure or second pilot effective pressure) so as not to deviate from the operating range (a range with an upper limit of the pilot pressure Pt corresponding to the maximum deemed operating position Ga based on the setting stage). Step S14 is the same as in the first embodiment. <Actions and Effects of the Third Embodiment>

[0139] In the control method for the work vehicle 1 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-ahead direction falls outside the first range Rdev1, the processor 10a determines the boost stage, which is a speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, based on the magnitude of 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 (pilot control valves CV31 to CV34) so ​​that the determined operating range (a range with an upper limit of the pilot pressure Pt(m) corresponding to the maximum deemed operating position Ga(m) based on the set stage or the boost stage) is not exceeded. As a result, 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) are controlled in consideration of the turning situation, so that the time required to reach the desired speed can be shortened even in a turn with a large curvature. <Modification of the third embodiment>

[0140] As an alternative to the third embodiment, each port (PLa, PRa, PLb, PRb) of the hydraulic pump (7L, 7R) may receive electromagnetic input, such as a solenoid. In this case, the pilot control valves CV31-34 may be omitted. In this case, instead of the pilot control valves CV31-34, a device that receives electromagnetic input and is provided in the hydraulic pump (7L, 7R) may be regarded as the control mechanism 27. <Modifications of all embodiments>

[0141] In a work vehicle 1 that does not include the technology of the first or 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. Also, in the second embodiment, the straightness may be calculated from the output values ​​of the hydraulic sensors SP5L, SP6L, SP5R, and SP6R.

[0142] The values ​​of the various threshold values ​​may be changed depending on the characteristics of the first hydraulic pump 7L, the second hydraulic pump 7R, the first hydraulic motor 31L, and the second hydraulic motor 31R, the characteristics of the reducer connected to the first hydraulic motor 31L, and the characteristics of the reducer connected to the second hydraulic motor 31R, and the characteristics of the various control valves.

[0143] In the above-described embodiment, the first hydraulic pump 7L and the second hydraulic pump 7R are rotated by the engine 6. However, the first hydraulic pump 7L and the second hydraulic pump 7R may be configured to be rotated by another prime mover, such as an electric motor.

[0144] In this application, the term "comprises" and its derivatives are open-ended terms that describe the presence of elements and do not exclude the presence of other elements not listed. This also applies to the terms "have", "include" and their derivatives.

[0145] The terms "member," "part," "element," "body," and "structure" may have multiple meanings, such as a single part or multiple parts.

[0146] Ordinal numbers such as "first" and "second" are merely terms used to identify components and do not have any other meaning (such as a particular 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."

[0147] Words expressing degree, such as "substantially," "about," and "approximately," may mean a reasonable deviation that does not significantly change the final result, unless otherwise specified in the embodiment. All numerical values ​​described in this application may be interpreted to include words such as "substantially," "about," and "approximately."

[0148] In this application, the phrase "at least one of A and B" should be interpreted as including A only, B only, and both A and B.

[0149] It is apparent that various changes and modifications of the present invention are possible in light of the above disclosure, and therefore, the present invention may be practiced otherwise than as specifically disclosed herein without departing from the spirit and scope of the present invention.

Claims

1. a first operating member that receives a set speed setting from among a plurality of speed settings corresponding to a plurality of pairs of a target rotation speed of a first hydraulic motor and a target rotation speed of a second hydraulic motor that are configured to drive a first traveling device and a second traveling device that are provided on the left and right sides of a vehicle body, respectively, when driving the first traveling device and the second traveling device to move the vehicle body in a straight line; receiving information corresponding to a target movement direction of the vehicle body, the target movement direction being determined by a speed difference between a target rotational speed of the first hydraulic motor and a target rotational speed of the second hydraulic motor, from a sensor that detects operation of a second operating member that sets the target movement direction; and determining the target movement direction; When the second operating member is operated so that a deviation amount between the target movement direction and a straight forward direction falls within a predetermined first range, an operating range of a first control parameter for determining a displacement volume of a first hydraulic pump configured to supply hydraulic oil to the first hydraulic motor and an operating range of a second control parameter for determining a displacement volume of a second hydraulic pump configured to supply hydraulic oil to the second hydraulic motor are determined based on the setting stage; When the second operating member is operated so that the deviation amount falls outside the first range, a boost stage, which is the speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, is determined based on the magnitude of the deviation amount, and an operating range of at least one of the first control parameter and the second control parameter is determined based on the boost stage; controlling the first control parameter so as not to deviate from an operating range of the first control parameter determined based on the boost stage or the setting stage; controlling the second control parameter so as not to deviate from an operating range of the second control parameter determined based on the boost stage or the setting stage; The first traveling device and the second traveling device are driven by rotating the first hydraulic motor and the second hydraulic motor, which have a constant displacement volume regardless of the multiple speed stages. Including, A method for controlling the speed of a work vehicle.

2. The first range is a range of the deviation amount such that at least one of the first hydraulic motor and the second hydraulic motor rotates when the set stage is set to a minimum speed stage that is a speed stage corresponding to the smallest set of target rotation speeds among the multiple speed stages. The speed control method according to claim 1 .

3. Among the multiple speed stages, the number of speed stages representing a set of the target rotational speed of the first hydraulic motor between the target rotational speed of the first hydraulic motor in the boost stage and the target rotational speed of the first hydraulic motor in the set stage, and the target rotational speed of the second hydraulic motor between the target rotational speed of the second hydraulic motor in the boost stage and the target rotational speed of the second hydraulic motor in the set stage is m (m is an integer equal to or greater than 0), and the number of steps between the boost stage and the set stage is m+1. When the deviation amount is outside the first range, the boost stage is determined such that the number of steps increases as the deviation amount increases. The speed control method according to claim 2.

4. The number of steps is determined so that at least one of the first hydraulic motor and the second hydraulic motor rotates within the range of the deviation amount in which the boost stage is determined. The speed control method according to claim 3.

5. Among at least one speed stage at which at least one of the first hydraulic motor and the second hydraulic motor can rotate regardless of any operation of the second operating member, the speed stage at which the target rotation speed of the first hydraulic motor and the target rotation speed of the second hydraulic motor are the smallest is defined as a minimum speed stage capable of turning. Among the multiple speed stages, when the number of speed stages representing a pair of the target rotation speed of the first hydraulic motor between the target rotation speed of the first hydraulic motor at the minimum speed stage for turning and the target rotation speed of the first hydraulic motor at the minimum speed stage, and the target rotation speed of the second hydraulic motor between the target rotation speed of the second hydraulic motor at the minimum speed stage for turning and the target rotation speed of the second hydraulic motor at the minimum speed stage is n (n is an integer equal to or greater than 0), n+1 is set as the maximum number of steps. The speed control method according to claim 3.

6. The first control parameter is a first high pilot pressure that is a higher one of two pilot pressures for determining a displacement volume of the first hydraulic pump, The second control parameter is a second high pilot pressure, which is a higher of two pilot pressures for determining a displacement volume of the second hydraulic pump. A speed control method according to any one of claims 1 to 5.

7. The first control parameter is a first pilot effective pressure obtained by subtracting a first low pilot pressure, which is a lower one of two pilot pressures, from a first high pilot pressure, which is a higher one of two pilot pressures for determining a displacement volume of the first hydraulic pump; The second control parameter is a second pilot effective pressure obtained by subtracting a second low pilot pressure, which is lower than a second high pilot pressure, from a second high pilot pressure, which is higher than a second low pilot pressure, for determining a displacement volume of the second hydraulic pump. A speed control method according to any one of claims 1 to 5.

8. A vehicle body, a first traveling device and a second traveling device provided on the left and right sides 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; and a first hydraulic pump configured to supply hydraulic fluid to the first hydraulic motor; a second hydraulic pump configured to supply hydraulic fluid to the second hydraulic motor; a control mechanism configured to control a displacement of the first hydraulic pump and a displacement of the second hydraulic pump; a prime mover configured to rotate the first hydraulic pump and the second hydraulic pump; a storage device configured to store a plurality of speed stages respectively corresponding to a plurality of pairs of a target rotation speed of the first hydraulic motor and a target rotation speed of the second hydraulic motor when the first traveling device and the second traveling device are driven to move the vehicle body in a straight line; A first operating member configured to receive an input corresponding to a setting stage among the plurality of speed stages; a second operating member configured to receive an input corresponding to a target movement direction of the vehicle body defined 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 operation of the second operating member; a hardware processor configured to control the prime mover and the control mechanism; Equipped with The hardware processor includes: receiving the setting stage from the first operating member; receiving information corresponding to the target movement direction from the sensor, and determining the target movement direction; when the second operating member is operated so that a deviation amount between the target movement direction and a straight forward direction falls within a predetermined first range, an operating range of a first control parameter for controlling the displacement volume of the first hydraulic pump and an operating range of a second control parameter for controlling the displacement volume of the second hydraulic pump are determined based on the setting stage; When the second operating member is operated so that the deviation amount falls outside the first range, a boost stage, which is the speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, is determined based on the magnitude of the deviation amount, and an operating range of at least one of the first control parameter and the second control parameter is determined based on the boost stage; controlling the control mechanism to vary the first and second control parameters so as not to deviate from the operating ranges of the first and second control parameters determined based on the boost stage or the setting stage; The first traveling device and the second traveling device are driven by controlling the first hydraulic pump and the second hydraulic pump to rotate the first hydraulic motor and the second hydraulic motor, which are set to have a constant displacement volume regardless of the multiple speed stages. Work vehicle.

9. The first range is a range of the deviation amount such that at least one of the first hydraulic motor and the second hydraulic motor rotates when the set stage is set to a minimum speed stage that is a speed stage corresponding to the smallest set of target rotation speeds among the multiple speed stages. A work vehicle according to claim 8.

10. Among the multiple speed stages, when the number of speed stages representing a pair of the target rotational speed of the first hydraulic motor between the target rotational speed of the first hydraulic motor in the boost stage and the target rotational speed of the first hydraulic motor in the set stage, and the target rotational speed of the second hydraulic motor between the target rotational speed of the second hydraulic motor in the boost stage and the target rotational speed of the second hydraulic motor in the set stage is m (m is an integer equal to or greater than 0), the number of steps is m+1. When the amount of deviation is outside the first range, the greater the amount of deviation, the greater the number of steps. A work vehicle according to claim 9.

11. The number of steps is determined so that at least one of the first hydraulic motor and the second hydraulic motor rotates within the range of the deviation amount in which the boost stage is determined. A work vehicle according to claim 10.

12. Among at least one speed stage at which at least one of the first hydraulic motor and the second hydraulic motor can rotate regardless of any operation of the second operating member, the speed stage at which the target rotation speed of the first hydraulic motor and the target rotation speed of the second hydraulic motor are the smallest is defined as a minimum speed stage capable of turning. Among the multiple speed stages, when the number of speed stages representing a pair of the target rotation speed of the first hydraulic motor between the target rotation speed of the first hydraulic motor at the minimum speed stage for turning and the target rotation speed of the first hydraulic motor at the minimum speed stage, and the target rotation speed of the second hydraulic motor between the target rotation speed of the second hydraulic motor at the minimum speed stage for turning and the target rotation speed of the second hydraulic motor at the minimum speed stage is n (n is an integer equal to or greater than 0), n+1 is set as the maximum number of steps. A work vehicle according to claim 10.

13. The control mechanism includes: A pilot pump configured to supply pilot oil to the first hydraulic pump and the second hydraulic pump; a first pilot hydraulic circuit connecting the pilot pump and two pilot ports of the first hydraulic pump; a second pilot hydraulic circuit connecting the pilot pump and two pilot ports of the second hydraulic pump; at least one first pressure control valve provided in the first pilot hydraulic circuit and configured to control a first high pilot pressure, which is a higher one of two pilot pressures applied to the two pilot ports of the first hydraulic pump; at least one second pressure control valve provided in the second pilot hydraulic circuit and configured to control a second high pilot pressure, which is higher of two pilot pressures applied to the two pilot ports of the second hydraulic pump; Including, the prime mover is configured to rotate the pilot pump; The first control parameter is the first high pilot pressure, The second control parameter is the second high pilot pressure. A work vehicle according to any one of claims 8 to 12.

14. The control mechanism includes: a pilot pump that supplies pilot oil to the first hydraulic pump and the second hydraulic pump; a first pilot hydraulic circuit connecting the pilot pump and two pilot ports of the first hydraulic pump; a second pilot hydraulic circuit connecting the pilot pump and two pilot ports of the second hydraulic pump; at least one third pressure control valve provided in the first pilot hydraulic circuit and configured to control a first pilot effective pressure which is a difference between a higher pilot pressure and a lower pilot pressure of two pilot pressures applied to the two pilot ports of the first hydraulic pump; at least one fourth pressure control valve provided in the second pilot hydraulic circuit and configured to control a second pilot effective pressure which is a difference between a higher pilot pressure and a lower pilot pressure of two pilot pressures applied to the two pilot ports of the second hydraulic pump; Including, the prime mover is configured to rotate the pilot pump; The first control parameter is the first pilot effective pressure, The second control parameter is the second pilot effective pressure. A work vehicle according to any one of claims 8 to 12.

15. The at least one control parameter includes the first control parameter and the second control parameter. A work vehicle according to claim 13.

16. The second operating member is an operating lever that can be rotated in the front-rear and left-right directions, the two pilot ports of the first hydraulic pump include a first port for moving a swash plate of the first hydraulic pump so that the first hydraulic motor drives the first traveling device in a forward direction when the first hydraulic pump is rotated, and a second port for moving a swash plate of the first hydraulic pump so that the first hydraulic motor drives the first traveling device in a reverse direction when the first hydraulic pump is rotated, the two pilot ports of the second hydraulic pump include a third port for moving a swash plate of the second hydraulic pump so that the second hydraulic motor drives the second traveling device in a forward direction when the second hydraulic pump is rotated, and a fourth port for moving a swash plate of the second hydraulic pump so that the second hydraulic motor drives the second traveling device in a reverse direction when the second hydraulic pump is rotated, The first pilot hydraulic circuit includes: a first pilot oil passage connecting the pilot pump and the first port; a second pilot oil passage connecting the pilot pump and the second port; Including, The second pilot hydraulic circuit includes: a third pilot oil passage connecting the pilot pump and the third port; a fourth pilot oil passage connecting the pilot pump and the fourth port; Including, The work vehicle is a first hydraulic pressure sensor that detects a first forward pilot pressure that is a pilot pressure applied to the first port; a second hydraulic pressure sensor that detects a first reverse pilot pressure that is a pilot pressure applied to the second port; a third hydraulic sensor that detects a second forward pilot pressure that is a pilot pressure applied to the third port; a fourth hydraulic pressure sensor that detects a second reverse pilot pressure that is the pilot pressure applied to the fourth port; Further equipped with The hardware processor includes: The first forward pilot pressure is obtained from the first hydraulic pressure sensor; The first reverse pilot pressure is obtained from the second hydraulic pressure sensor; The second forward pilot pressure is obtained from the third hydraulic pressure sensor; The second reverse pilot pressure is obtained from the fourth hydraulic pressure sensor; a first instruction to rotate the first hydraulic motor to drive the first traveling device in the forward direction and to rotate or stop the second hydraulic motor to drive the second traveling device in the reverse direction relative to the first traveling device; a second instruction to rotate the first hydraulic motor to drive the first traveling device in the reverse direction and to rotate or stop the second hydraulic motor to drive the second traveling device in the forward direction relative to the first traveling device; a third instruction to rotate the second hydraulic motor to drive the second traveling device in the forward direction and to rotate or stop the first hydraulic motor to drive the first traveling device in the reverse direction relative to the second traveling device; a fourth instruction to rotate the second hydraulic motor to drive the second traveling device in the reverse direction and to rotate or stop the first hydraulic motor to drive the first traveling device in the forward direction relative to the second traveling device; which of the commands is input to the operation lever is determined based on the first forward pilot pressure, the first reverse pilot pressure, the second forward pilot pressure, and the second reverse pilot pressure; determining an operation degree corresponding to a target value of a speed difference between the rotational speed of the first hydraulic motor and the rotational speed of the second hydraulic motor in any of the instructions from at least two pressures of the first forward pilot pressure, the first reverse pilot pressure, the second forward pilot pressure, and the second reverse pilot pressure; The greater the degree of the operation, the greater the amount of deviation is determined. A work vehicle according to claim 13.

17. The at least one first pressure control valve includes: a first pressure reducing valve provided in the first pilot oil passage and configured to convert a primary pressure supplied by the pilot pump into a secondary pressure in accordance with a rotation direction and a rotation amount of the operation lever; a second pressure reducing valve provided in the second pilot oil passage and configured to convert a primary pressure supplied by the pilot pump into a secondary pressure in accordance with a rotation direction and a rotation amount of the operation lever; a first relief valve connected to the first pilot oil passage between the first pressure reducing valve and the first port and to the second pilot oil passage between the second pressure reducing valve and the second port, and 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 first pressure reducing valve and the secondary pressure input to the second pressure reducing valve exceeds a first set pressure; Including, The at least one second pressure control valve a third pressure reducing valve provided in the third pilot oil passage and configured to convert a primary pressure supplied by the pilot pump into a secondary pressure in accordance with a rotation direction and a rotation amount of the operation lever; a fourth pressure reducing valve provided in the fourth pilot oil passage and configured to convert a primary pressure supplied by the pilot pump into a secondary pressure in accordance with a rotation direction and a rotation amount of the operating lever; a second relief valve connected to the third pilot oil passage between the third pressure reducing valve and the third port and to the fourth pilot oil passage between the fourth pressure reducing valve and the fourth port, and 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 third pressure reducing valve and the secondary pressure input to the fourth pressure reducing valve exceeds a second set pressure; Including, The hardware processor includes: When the at least one control parameter includes the first control parameter, the first relief valve is controlled so that the first set pressure is set to an upper limit value of the operating range; When the at least one control parameter includes the second control parameter, the second relief valve is controlled so that the second set pressure is set to an upper limit value of the operating range. A work vehicle according to claim 13.

18. The at least one first pressure control valve includes: a first electromagnetic pressure control valve provided in the first pilot oil passage and configured to convert a pilot pressure supplied by the pilot pump in response to a first signal from the hardware processor; a second electromagnetic pressure control valve provided in the second pilot oil passage and configured to convert the pilot pressure supplied by the pilot pump in response to a second signal from the hardware processor; Including, The at least one second pressure control valve a third electromagnetic pressure control valve provided in the third pilot oil passage and configured to convert the pilot pressure supplied by the pilot pump in response to a third signal from the hardware processor; a fourth electromagnetic pressure control valve provided in the fourth pilot oil passage and configured to convert the pilot pressure supplied by the pilot pump in response to a fourth signal from the hardware processor; Including, The hardware processor includes: generating the first signal or the second signal corresponding to the first high pilot pressure based on the deviation amount and an upper limit value of the operating range; generating the third signal or the fourth signal corresponding to the second high pilot pressure based on the deviation amount and an upper limit value of the operating range; A work vehicle according to claim 13.

19. The at least one first pressure control valve includes: a first pressure reducing valve provided in the first pilot oil passage and configured to convert a primary pressure supplied by the pilot pump into a secondary pressure in accordance with a rotation direction and a rotation amount of the operation lever; a second pressure reducing valve provided in the second pilot oil passage and configured to convert a primary pressure supplied by the pilot pump into a secondary pressure in accordance with a rotation direction and a rotation amount of the operation lever; Including, The at least one second pressure control valve a third pressure reducing valve provided in the third pilot oil passage and configured to convert a primary pressure supplied by the pilot pump into a secondary pressure in accordance with a rotation direction and a rotation amount of the operation lever; a fourth pressure reducing valve provided in the fourth pilot oil passage and configured to convert a primary pressure supplied by the pilot pump into a secondary pressure in accordance with a rotation direction and a rotation amount of the operating lever; Including, The at least one first pressure control valve and the at least one second pressure control valve are both provided to be connected to the first pilot hydraulic circuit and the second pilot hydraulic circuit, and are configured to set the primary pressure to a set pressure. Further comprising: The hardware processor includes: controlling the primary pressure control valve so that the set pressure is the upper limit value of the operating range; A work vehicle according to claim 13.

20. a first operating member that receives a set speed setting from among a plurality of speed settings corresponding to a plurality of pairs of target rotation speeds of a first hydraulic motor and a second hydraulic motor that are configured to drive the first traveling device and the second traveling device, respectively, when driving the first traveling device and the second traveling device that are provided on the left and right sides of the vehicle body to move the vehicle body in a straight line; receiving information corresponding to a target movement direction of the vehicle body, the target movement direction being determined by a speed difference between a target rotational speed of the first hydraulic motor and a target rotational speed of the second hydraulic motor, from a sensor that detects operation of a second operating member that sets the target movement direction; and determining the target movement direction; When the second operating member is operated so that a deviation amount between the target movement direction and a straight forward direction falls within a predetermined first range, an operating range of a first control parameter for determining a displacement volume of a first hydraulic pump configured to supply hydraulic oil to the first hydraulic motor and an operating range of a second control parameter for determining a displacement volume of a second hydraulic pump configured to supply hydraulic oil to the second hydraulic motor are determined based on the setting stage; When the second operating member is operated so that the deviation amount falls outside the first range, a boost stage, which is the speed stage corresponding to a target rotation speed higher than the target rotation speed of the set stage, is determined based on the magnitude of the deviation amount, and an operating range of at least one of the first control parameter and the second control parameter is determined based on the boost stage; controlling a control mechanism for controlling the first control parameter so as not to deviate from an operating range of the first control parameter determined based on the boost stage or the setting stage; controlling the control mechanism for controlling the second control parameter so as not to go outside an operating range of the second control parameter determined based on the boost stage or the setting stage; A prime mover configured to rotate the first hydraulic pump and the second hydraulic pump is controlled so as to rotate the first hydraulic motor and the second hydraulic motor, each of which has a constant displacement volume regardless of the multiple speed stages. a hardware processor configured to: Work vehicle controller.

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