Work vehicle and work vehicle control method

Abstract

To improve safety while suppressing deceleration at the time of turning.SOLUTION: In a control method of a work vehicle, a first hydraulic pump and a second hydraulic pump are driven by an engine of the work vehicle respectively so that working fluid is supplied to a first hydraulic motor and a second hydraulic motor respectively, and a first travelling device and a second travelling device that are disposed opposed to each other are driven to run the work vehicle. An operation state of a direction input device for operating a running direction of the work vehicle is detected. It is determined whether the state of the movement of the work vehicle is a turning state on the basis of the detected operation state. If the state of the movement is determined to be a turning state, a rotation command is output to decrease a target rotation speed of the engine from a first rotation speed to a second rotation speed. A pilot pressure of pilot oil supplied to the first hydraulic pump and the second hydraulic pump respectively is controlled so that a displacement volume of the first hydraulic pump and the second hydraulic pump is maximized respectively when there is no load in the first hydraulic motor and the second hydraulic motor.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a work vehicle and a method for controlling a work vehicle. [Background technology]

[0002] Patent Document 1 discloses a hydraulic circuit that controls two traveling hydraulic motors arranged on the left and right sides of a work vehicle and two traveling hydraulic pumps that supply hydraulic oil to those hydraulic motors. Patent Document 2 discloses that the speed setting of the work vehicle is switched to a low speed setting when the load increases in order to improve safety when the work vehicle is turning. More specifically, Patent Document 2 discloses that the engine rotation speed is reduced when the low speed setting is used. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-8739 [Patent Document 2] Patent No. 2714821 Summary of the Invention [Problem to be solved by the invention]

[0004] In the method of Patent Document 2, a low speed is set when turning, which reduces the vehicle speed when turning and reduces workability. [Means for solving the problem]

[0005] A control method for a work vehicle according to a first aspect of the present disclosure includes: driving a first hydraulic pump and a second hydraulic pump, respectively, with an engine of the work vehicle, thereby supplying hydraulic oil to a first hydraulic motor and a second hydraulic motor, respectively, to drive a first traveling device and a second traveling device, respectively, which are arranged opposite to each other, and moving the work vehicle forward. An operation state of a direction input device for controlling the traveling direction of the work vehicle is detected. Based on the detected operation state, it is determined whether the movement state of the work vehicle is a turning state. If it is determined that the movement state is a turning state, a rotation command is output to reduce the target rotation speed of the engine from a first rotation speed to a second rotation speed. Pilot pressures of pilot oil supplied to the first hydraulic pump and the second hydraulic pump, respectively, are controlled so as to maximize the displacement of the first hydraulic pump and the second hydraulic pump, respectively, when there is no load on the first hydraulic motor and the second hydraulic motor.

[0006] 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 pilot pump, an engine, at least one pilot oil line, a hydraulic pressure adjustment mechanism, a directional input device, a sensor, and a controller. The vehicle body has a first side surface and a second side surface opposite the first side surface. The first traveling device is provided on the first side surface of the vehicle body. The second traveling device is provided on the second side surface of the vehicle body. 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 connected to the first hydraulic motor via a first hydraulic circuit and has a first port and a second port. The first hydraulic pump is configured to supply hydraulic oil to the first hydraulic motor via the first hydraulic circuit to forward drive the first traveling device when pressure applied to the first port is higher than pressure applied to the second port. The first hydraulic pump is configured to supply hydraulic oil to the first hydraulic motor via a first hydraulic circuit to drive the first traveling device in reverse when the pressure applied to the second port is higher than the pressure applied to the first port. The second hydraulic pump is connected to the second hydraulic motor via a second hydraulic circuit and has a third port and a fourth port. The second hydraulic pump is configured to supply hydraulic oil to the second hydraulic motor via the second hydraulic circuit to drive the second traveling device forward when the pressure applied to the third port is higher than the pressure applied to the fourth port. The second hydraulic pump is configured to supply hydraulic oil to the second hydraulic motor via the second hydraulic circuit to drive the second traveling device in reverse when the pressure applied to the fourth port is higher than the pressure applied to the third port. The pilot pump is configured to supply pilot oil to the first hydraulic pump and the second hydraulic pump. The engine is configured to drive the first hydraulic pump, the second hydraulic pump, and the pilot pump. At least one pilot oil passage connects the pilot pump to the first hydraulic pump and connects the pilot pump to the second hydraulic pump. The hydraulic pressure adjusting mechanism is provided in the at least one pilot oil passage and is configured to adjust the pilot pressure of each of the at least one pilot oil passage.The directional input device is configured to control the traveling direction of the work vehicle by instructing at least one of the first traveling device and the second traveling device to move forward or backward. The sensor is configured to detect operation of the directional input device. The controller is configured to determine whether the movement state of the work vehicle is a turning state based on the operation of the directional input device, and when it determines that the movement state is a turning state, to output a rotation command to decelerate the target rotation speed of the engine from a first rotation speed to a second rotation speed, and to control the hydraulic pressure adjustment mechanism to adjust the pilot pressure of each of the at least one pilot oil passage so as to maximize the displacement of the first hydraulic pump and the second hydraulic pump when there is no load on the first hydraulic motor and the second hydraulic motor. [Effects of the Invention]

[0007] According to the technology disclosed in the present application, for example, it is possible to provide a work vehicle that improves turning safety while suppressing deceleration during turning. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a side view of a work vehicle. [Figure 2] FIG. 2 is a top view of the work vehicle. [Figure 3] FIG. 3 is a hydraulic circuit diagram of the travel system of the 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. [Figure 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 hydraulic circuit diagram of a working system of a work vehicle. [Figure 7] FIG. 7 is a block diagram of a work vehicle. [Figure 8] FIG. 8 is a flowchart showing the operation of the controller of the work vehicle according to the first embodiment. [Figure 9A] FIG. 9A is a flowchart showing the operation of the controller of the work vehicle according to the second embodiment. [Figure 9B] FIG. 9B is a flowchart showing the operation of the controller of the work vehicle according to the second embodiment. [Figure 10A] FIG. 10A shows an example of a display on an operation panel of a work vehicle according to a modification of the first embodiment and the second embodiment. [Figure 10B] FIG. 10B shows an example of a display on the operation panel of the work vehicle according to the first embodiment and the modified example of the second embodiment. [Figure 10C] FIG. 10C shows an example of a display on the operation panel of the work vehicle according to the first embodiment and the modified example of the second embodiment. [Figure 10D] FIG. 10D shows an example of a display on the operation panel of the work vehicle according to the first embodiment and the modified example of the second embodiment.

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings showing embodiments thereof, in which the same reference numerals designate corresponding or substantially identical components. First Embodiment <Overall structure>

[0010] 1 and 2, a work vehicle 1, for example, a compact track loader, includes a vehicle body 2, a pair of travelling devices 3, and a working device 4. The vehicle body 2 supports the travelling devices 3 and the working device 4. In the illustrated embodiment, the travelling devices 3 are track-type travelling devices. Therefore, each of the pair of travelling 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 travelling devices 3 is not limited to being a track-type travelling device. Each of the pair of travelling devices 3 may be, for example, a front-wheel / rear-wheel travelling device or a travelling device having front wheels and rear crawlers. The working device 4 includes a work equipment (bucket) 41 at its distal end. 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 .

[0011] 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 work device 4 further includes a plurality of arm cylinders 48 and at least one equipment cylinder 49. Each of the plurality of arm cylinders 48 is rotatably connected to the vehicle body 2 and the arm 45 and moves the link 44, the arm 45, 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 is provided with a windshield 51 that can be opened and closed, and its outer shape is defined by a cab frame 53. The windshield 51 may be omitted. The work vehicle 1 includes a driver's seat 54 and an operating lever 55 within the cabin 5. The cab frame 53 is rotatable about rotational shafts RSL and RSR on the vehicle body 2, as shown in FIG. 2 . In Figures 1 and 2, a common pivot 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 pivotable around

[0012] In the embodiment of the present application, FB (Forward direction D F / backward D B ) means the front-rear direction (forward 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, and left / right directions are respectively seen from the operator's perspective. U , downward D D , height direction D H The terms "front-rear", "left-right (width)", and "up-down (height)" refer to the upward, downward, and height directions as seen from the operator. The front-rear, back-right (width), and up-down (height) directions of the work vehicle 1 respectively correspond to the front-rear, back-right (width), and up-down (height) directions as seen from the operator.

[0013] FIG. 1 shows the left side of a 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. 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 with respect to the vehicle body central plane M is shown as the first arm 45L, and the arm 45 provided on the right side with respect to the vehicle body central plane M is shown as the second arm 45R. The fulcrum shaft 46 provided on the left side of the vehicle body central plane M is shown as the first fulcrum shaft 46L, and the fulcrum shaft 46 provided on the right side of the vehicle body central plane M is shown as the second fulcrum shaft 46R. The joint shaft 47 provided on the left side of the vehicle body central plane M is shown as the first joint shaft 47L, and the joint shaft 47 provided on the right side of the vehicle body central plane M is shown as the second joint shaft 47R. Of the hydraulic motor units 30, the hydraulic motor unit 30 provided on the left side of the vehicle body central plane M is shown as the first hydraulic motor unit 30L, and the hydraulic motor unit 30 provided on the right side of the vehicle body central plane M is shown as the second hydraulic motor unit 30R.

[0014] 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 first hydraulic pump 7L and the second hydraulic pump 7R are configured to discharge hydraulic oil to drive hydraulic motor devices 30 that drive drive wheels 31, etc. 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 implement 4. The engine 6 is configured to drive the work vehicle 1 in the width direction D W and is provided between the pair of arm assemblies 42. The work vehicle 1 is further provided with a cover 8 for covering the engine 6. The work vehicle 1 is further provided with 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.

[0015] FIG. 3 is a hydraulic circuit diagram of the travel system of the work vehicle 1. The work vehicle 1 includes a hydraulic circuit 1A. The hydraulic circuit 1A includes a hydraulic oil tank 70 and a pilot pump 71. The pilot pump 71 is a fixed displacement gear pump driven by power from the engine 6. 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 that is mainly used for control. For ease of explanation, of the hydraulic oil discharged from the pilot pump 71, the hydraulic oil used for control is referred to as pilot oil, and the pressure of the pilot oil is referred to as pilot pressure. In particular, the pilot pump 71 is configured to supply pilot oil to the first hydraulic pump 7L and the second hydraulic pump 7R.

[0016] The hydraulic circuit 1A 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 1A 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 (solenoid valve) for applying and releasing the brakes by the plurality of brake mechanisms 72. The brake switching valve SV1 is a two-position switching valve configured to switch its valve element to a first position VP1a or a second position VP1b when energized. The valve element of the brake switching valve SV1 is switched by a brake pedal 13 (see FIG. 7). A sensor 14 is provided on the brake pedal 13. The operation amount detected by the sensor 14 is input to a controller 10 configured as an ECU (Electronic Control Unit).

[0017] 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 switch 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 in accordance with the pressure of pilot oil (hydraulic oil). When the valve body of the brake switch valve SV1 is switched to the first position VP1a, hydraulic oil is released from the oil passage PA2 in the section between the brake switch 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 switch 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.

[0018] 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 that is configured to switch its valve element to a first position VP2a or a second position VP2b when excited. The directional 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 that can adjust the flow rate of the hydraulic oil discharged, rather than a two-position control valve.

[0019] The first hydraulic motor unit 30L transmits power to the drive wheels 31 provided on the first traveling unit 3L. The first hydraulic motor unit 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 for driving the first traveling unit 3L and is a motor that can change the vehicle speed (rotation) between first and second speeds. 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 extending or retracting. The first traveling control valve SV4 is a valve for extending or retracting the first swash plate switching cylinder 32L. The first traveling control valve SV4 is a two-position switching valve configured to switch its valve element between a first position VP4a and a second position VP4b.

[0020] The first travel control valve SV4 is switched by the directional control valve SV2, which is 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 element of the directional control valve SV2 is switched to the first position VP2a by operating the operating member, pilot oil is released from the section between the directional control valve SV2 and the first travel control valve SV4, and the valve element of the first travel control valve SV4 is switched to the first position VP4a. As a result, the first swash plate switching cylinder 32L retracts, and the speed of the first hydraulic motor 31L changes 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 changes to second gear.

[0021] The second hydraulic motor unit 30R transmits power to the drive wheels 31 provided on the second traveling unit 3R. The second hydraulic motor unit 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 unit 30R is a hydraulic motor for driving the second traveling unit 3R and operates in the same manner as the first hydraulic motor unit 30L. That is, the second hydraulic motor 31R operates in the same manner as the first hydraulic motor 31L. 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 element between a first position VP5a and a second position VP5b, and operates in the same manner as the first traveling control valve SV4.

[0022] A drain oil passage DR1 is connected to the hydraulic circuit 1A. The drain oil passage DR1 is an oil passage that flows pilot oil from a plurality of switching valves (brake switching valve SV1, directional switching 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 switching 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 switching valve SV2 is in the first position VP1a, pilot oil in the oil passage PA3 is discharged to the drain oil passage DR1.

[0023] The hydraulic circuit 1A 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.

[0024] The first drive circuit 76L has a first hydraulic pump 7L, drive oil passages PA5L and PA6L, and a second charge oil passage PA7L. The drive oil passages PA5L and 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 and PA6L is called the first hydraulic circuit CL. The second charge oil passage PA7L is connected to the drive oil passages PA5L and PA6L and is an oil passage that replenishes hydraulic oil from the pilot pump 71 to the drive oil passages PA5L and 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 3L 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.

[0025] 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 connected to the drive oil passages PA5R and PA6R and is an oil passage that replenishes 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 connected to the drive oil passage PA5R and a fourth connection port 31P4 connected 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 reverse 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 reverse direction is input to the second hydraulic motor 3R 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.

[0026] The first hydraulic pump 7L and the second hydraulic pump 7R are swash plate-type variable displacement axial pumps driven by power from 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.

[0027] The second hydraulic pump 7R is connected to the second hydraulic motor 31R via a second hydraulic circuit CR and has a third port PRa and a fourth port PRb to 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, when the hydraulic pressure applied to the third port PRa is higher than the hydraulic pressure applied to the fourth port PRb, the second hydraulic pump 7R supplies hydraulic oil to the second hydraulic motor 31R via the second hydraulic circuit CR to drive the second traveling device 3R forward. When the hydraulic pressure applied to the fourth port PRb is higher than the hydraulic pressure applied to the third port PRa, the second hydraulic pump 7R supplies hydraulic oil to the second hydraulic motor 31R via the second hydraulic circuit CR to drive the second traveling device 3R backward. The first hydraulic pump 7L and the second hydraulic pump 7R can change their output (the amount of hydraulic oil discharged) and the direction of hydraulic oil discharge depending on the angle of their swash plates.

[0028] The output of the first hydraulic pump 7L and the second hydraulic pump 7R and the direction of hydraulic oil discharge are changed by an operating device 56 for operating the direction of travel of the work vehicle 1. Hereinafter, the operating device 56 may be referred to as a direction input device. Specifically, the output of the first hydraulic pump 7L and the second hydraulic pump 7R and the direction of hydraulic oil discharge are changed in accordance with 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 direction of travel 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 reverse.

[0029] As shown in Fig. 3, the hydraulic circuit 1A includes a pilot supply oil passage PA8 that branches off from a pilot supply oil passage PA1 and is connected to the operating device 56, and a pilot pressure control valve CV1 that is provided on the pilot supply oil passage PA8. The pilot pressure control valve CV1 is an electromagnetic proportional valve that 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. Hereinafter, the pilot pressure control valve CV1 may also be referred to as a hydraulic pressure adjustment mechanism. The detailed operation of the pilot pressure control valve CV1 will be described later.

[0030] The operating device 56 (directional input device) includes a forward operating valve OVA, a reverse operating valve OVB, a right-turn operating valve OVC, a left-turn 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 operation of the operating lever 55, and supply the changed hydraulic oil to a first port PLa and a second port PLb of the first hydraulic pump 7L and a third port PRa and a fourth port PRb of the second hydraulic pump 7R. In the embodiment according to the present application, the operating valves OVA, OVB, OVC, and OVD are operated by a single operating lever 55, but multiple operating levers 55 may be used.

[0031] The control valves OVA, OVB, OVC, and OVD each have an input port (primary port), a discharge port, and an output port (secondary 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 that leads to a hydraulic oil tank 70. The control lever 55 can be tilted from a neutral position in the front-to-rear direction, the width direction perpendicular to the front-to-rear direction, and diagonal directions. The control valves OVA, OVB, OVC, and OVD of the operating device 56 are operated in response to the tilt of the control lever 55. As a result, pilot pressures corresponding to the amount of operation of the control lever 55 from the neutral position are output from the secondary ports of the control valves OVA, OVB, OVC, and OVD. The relationship between the pilot pressure applied to the primary port output from the pilot pressure control valve CV1 and the pilot pressure applied to the secondary port will be described later.

[0032] The secondary side ports of the control valve OVA and 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 secondary side ports of the control valve OVA and the secondary side ports 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 secondary side ports of the control valve OVB and the secondary side ports of the control 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 secondary side ports of the control valve OVB and the secondary side ports of the control 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. That is, the pilot supply oil passage PA8, the first pilot oil passage PA11, and the fourth pilot oil passage PA14 connect the pilot pump 71 and the first hydraulic pump 7L. The pilot supply oil passage PA8, the second pilot oil passage PA12, and the third pilot oil passage PA13 connect the pilot pump 71 and the second hydraulic pump 7R.

[0033] When the control lever 55 is tilted forward, the forward control valve OVA is operated and pilot pressure is output from the control valve OVA. This pilot pressure acts from the first shuttle valve SVa to the first port PLa via a 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 from the second shuttle valve SVb to the third port PRa via a 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 that corresponds to the tilt amount of the control lever 55, and the work vehicle 1 moves straight forward.

[0034] 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 from the third shuttle valve SVc to the second port PLb of the first hydraulic pump 7L via a second pilot oil passage PA12 that connects the operating device 56 and the second port, and also acts from the fourth shuttle valve SVd to the fourth port PRb via a fourth pilot oil passage PA14 that connects 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 that corresponds to the tilt amount of the operating lever 55, causing the work vehicle 1 to move straight backward.

[0035] 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 this control valve OVC. This pilot pressure acts on the first port PLa of the first hydraulic pump 7L via the first shuttle valve SVa and the first pilot oil passage PA11, and also acts on the fourth port PRb of the second hydraulic pump 7R via the fourth shuttle valve SVd and the fourth pilot oil passage PA14. 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.

[0036] 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 at a degree of turning that corresponds to the leftward operating position of the control lever 55.

[0037] 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 / backward direction, and turns left at 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.

[0038] Next, the detailed operation of the pilot pressure control valve CV1 will be described. The work vehicle 1 includes a setting member 11 (see FIG. 7) that sets the target rotation speed of the engine 6. The setting member 11 is an accelerator pedal or a swingably supported accelerator lever, which is a speed input device separate from the direction input device described above. The setting member 11 is provided with a sensor 12. The operation amount detected by the sensor 12 is input to the controller 10. The engine rotation speed corresponding to the operation amount detected by the sensor 12 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 setting member 11. The controller 10 outputs a rotation command indicating, for example, the fuel injection amount, injection timing, and fuel injection rate to the injector so that the determined target rotation speed of the engine 6 is achieved. Alternatively, the controller 10 outputs a rotation command indicating the fuel injection pressure, etc. to the supply pump or common rail so that the determined target rotation speed of the engine 6 is achieved. A sensor 6a that detects the actual engine rotation speed (referred to as the 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 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 drops from the target rotation speed of the engine 6. The amount by which the actual rotation speed drops 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 referred to as the engine drop amount.

[0039] The pilot pressure control valve CV1 can set the pilot pressure (primary pilot pressure) acting on the input ports (primary ports) of the multiple control valves OVA, OVB, OVC, and OVD based on a drop ΔE1 in the rotational speed of the engine 6 (engine rotational speed E1). The rotational speed of the engine 6 can be detected by an engine rotational speed E1 detection sensor 6a. The engine rotational speed E1 detected by the detection sensor 6a is input to the controller 10. FIG. 4 shows the relationship between the engine rotational speed, the traveling primary pressure (primary pilot pressure), and setting lines L1 and L2. The setting line L1 shows the relationship between the engine rotational speed E1 and the traveling primary pressure when the drop ΔE1 is less than a predetermined value (less than the anti-stall determination value). The setting line L2 shows the relationship between the engine rotational speed E1 and the traveling primary pressure when the drop ΔE1 is equal to or greater than the anti-stall determination value. When the difference between the first rotation speed RS1 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 first rotation speed RS1 transitions in accordance with a first correspondence relationship shown by the setting line L1. When the difference between the first rotation speed RS1 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 first rotation speed RS1 transitions in accordance with a second correspondence relationship shown by the setting line L2.

[0040] When the decrease amount 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 setting line L1. Furthermore, when the decrease amount 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 setting line L2, which is lower than the reference pilot pressure. On the setting line L2, the traveling primary pressure for a given engine rotation speed E1 is lower than the traveling primary pressure of the setting line L1. That is, for the same engine rotation speed E1, the traveling primary pressure of the setting line L2 is set lower than the traveling primary pressure of the setting line L1. Therefore, the pressure (pilot pressure) of the hydraulic oil entering the operating valves OVA, OVB, OVC, and OVD is kept low by control based on the setting 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, there may be multiple setting lines L2. For example, a setting line L2 may be set for each engine rotation speed E1. Furthermore, it is preferable that the controller 10 has data indicating the setting lines L1 and L2, or control parameters such as functions.

[0041] Next, the secondary pilot pressure output from the secondary ports of the control valves OVA, OVB, OVC, and OVD will be described. Fig. 5 is a diagram showing the relationship between the operation position of the control lever and the traveling secondary pressure (secondary pilot pressure). Referring to Fig. 4, the origin of the lever operation position is the operation start position (neutral position, G0 position) which is the start position of the lever stroke, and as it moves away from the origin, it approaches the operation end position (G5 position) which is the end position of the lever stroke. The operation region of the control lever 55 is divided into a neutral region RA1 where the operation target does not move (in the illustrated example, from G0 position to G1 position), a near-full operation region RA2 near the operation end (in the illustrated example, from G3 position to G5 position), and an intermediate region RA3 between the neutral region RA1 and the near-full operation region RA2 (in the illustrated example, from G1 position to G3 position). Furthermore, 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.

[0042] In the neutral region RA1, no secondary pilot pressure is supplied even when the control lever 55 is operated. On the other hand, in the near-full operation 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 can be stopped or changed at any position within the region to adjust the speed of the controlled object to the speed desired by the operator. For example, the ratios of the lever strokes for each of the operation regions RA1, RA3A, RA3B, and RA2 are as follows: Neutral region RA1: 0% to less than 15% Slow-speed region RA3A: 15% or more and less than 45% Intermediate speed range RA3B: 45% or more and less than 75% Full operation area RA2: 75% to 100%

[0043] In the characteristic diagram shown in FIG. 5, when the control lever 55 is operated from the G0 position to the G1 position, a secondary pilot pressure (Pa) is generated. When the control lever 55 is operated from the G1 position to the G4 position, the secondary pilot pressure increases from Pa to Pb in proportion to the amount of operation of the control lever 55. At the G4 position, the primary pilot pressure is shortcut to the secondary side, and the secondary pilot pressure increases from Pb to the maximum output pressure Pc in one go. 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 becomes equal to the primary pilot pressure. In other words, when the displacement of the control lever 55 (directional input device) for instructing movement leftward from the neutral position is equal to or greater than the first displacement value (displacement from G0 to G4), the control device 56 outputs the primary pilot pressure input to the control device 56 to the first port PLa and the fourth port PRb. 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 (directional input device) for instructing movement in the right direction is equal to or greater than a 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 (directional input device) for instructing movement in the forward 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 fourth port PRb when the displacement from the neutral position of the operation lever 55 (directional input device) for instructing movement in the rearward direction is equal to or greater than the first displacement value (displacement from G0 to G4). Note that the characteristic value of the secondary pilot pressure in the forward / backward 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 / backward direction corresponding to G0 to G5 and Pa to Pc are G0' to G5' and Pa' to Pc', the operating device 56 may output the primary pilot pressure input to the operating device 56 to the first port PLa and the third port PRa when the displacement from the neutral position of the operating lever 55 (directional input device) for instructing forward movement 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 from the neutral position of the operation lever 55 (directional input device) for instructing movement in the rearward direction is equal to or greater than a second displacement value (displacement from G0' to G4'). Furthermore, Pa and Pb (Pa' and Pb') are values ​​that do not depend on the magnitude of the primary pilot pressure, but when the primary pilot pressure is lower than Pa or Pb (Pa' or Pb'), the secondary pilot pressure will reach a plateau at the magnitude of the primary pilot pressure.

[0044] Based on the above characteristics of the control valves OVA, OVB, OVC, and OVD, the movement of the work vehicle 1 corresponding to the operation of the control lever 55 will be described in more detail. When the amount of operation of the control lever 55 in the forward / backward direction is greater than the amount of operation in the rightward direction, and the rightward operation position of the control lever 55 is operated from the G1 position to the G3 position, the rotational speed of the first hydraulic pump 7L is greater than the rotational speed of the second hydraulic pump 7R, and they rotate in the same direction, causing the work vehicle 1 to make a wide turn to the right. When the rightward operation position of the control lever 55 is the same as the forward / backward operation position, the rotational speed of the second hydraulic pump 7R becomes zero, and only the first hydraulic pump 7L rotates, causing the work vehicle 1 to perform a right pivot turn. Furthermore, when the rightward operation position of the control lever 55 is operated between the G4 position and the G5 position, the rotational speed becomes greater than the forward / backward operation position, causing the output shaft of the first hydraulic pump 7L to rotate forward and the output shaft of the second hydraulic pump 7R to rotate reversely, causing the work vehicle 1 to turn right.

[0045] Furthermore, when the amount of operation of the control lever 55 in the forward / backward direction is greater than the amount of operation in the leftward direction, and the leftward operation position of the control lever 55 is operated from the G1 position to the G3 position, the rotational speed of the second hydraulic pump 7R is greater than the rotational speed of the first hydraulic pump 7L, and they rotate in the same direction, causing the work vehicle 1 to make a wide turn to the left. When the leftward operation position of the control lever 55 is the same as the forward / backward operation position, the rotational 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. Furthermore, when the leftward operation position of the control lever 55 is operated between the G4 position and the G5 position, it becomes greater than the forward / backward operation position, 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 the embodiment of the present application, turning refers to the movement of the work vehicle 1 when the operating position to the right is operated between the G4 position and the G5 position, or when the operating position to the left is operated between the G4 position and the G5 position.

[0046] 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. Note that operation of the other operation levers 55 in the forward / backward direction is the same as for the left / right direction.

[0047] FIG. 6 is a hydraulic circuit diagram of the work system of the work vehicle 1. As shown in FIG. 6, the hydraulic system of the work system is a system that operates the work implement 4 and the like. The hydraulic system of the work system is a system that operates the work implement 4 by using the engine 6 to drive hydraulic pumps other than the hydraulic pumps 7L and 7R of the travel system. The hydraulic system of the work system includes a plurality of control valves 81 and a main pump 80, which is a hydraulic pump that discharges hydraulic oil. The main pump 80 is a pump that is located in a different position from the above-mentioned pilot pump 71, and is configured by a low-capacity gear pump. The main pump 80 is configured to discharge hydraulic oil stored in the hydraulic oil tank 70. In particular, the main pump 80 is configured to discharge hydraulic oil that mainly operates the hydraulic actuators.

[0048] An oil passage 81f is provided on the discharge side of the pilot pump 71. A plurality of control valves 81 are connected to this oil passage 81f. The plurality of control valves 81 include an arm control valve 81a, a bucket control valve 81b, and a spare control valve 81c. The arm control valve 81a is a valve that controls the arm cylinder 48, the bucket control valve 81b is a valve that controls the implement cylinder 49, and the spare control valve 81c is a valve that controls the hydraulic actuator of the spare attachment.

[0049] The arm 45 and the implement (bucket) 41 can be operated by a work operation lever 57 provided on the operation device 56. The work operation lever 57 may also be called an arm operation device. The work operation lever 57 is a lever that is supported by a plurality of operation valves 59 and swings left and right (widthwise) or forward and backward. By tilting the work operation lever 57, the operation valves 59 provided below the work operation lever 57 can be operated.

[0050] The multiple operation valves 59 and the multiple control valves 81 are connected to one another by multiple work oil passages 82 (82a, 82b, 82c, 82d). Specifically, the operation valve 59a is connected to the arm control valve 81a via the work oil passage 82a. The operation valve 59b is connected to the arm control valve 81a via the work oil passage 82b. The operation valve 59c is connected to the bucket control valve 81b via the work oil passage 82c. The operation valve 59d is connected to the bucket control valve 81b via the work oil passage 82d. The multiple operation valves 59a to 59d can each set the pressure of the hydraulic oil to be output in response to the operation of the work operation lever 57.

[0051] When the work operation lever 57 is tilted forward, the operation valve 59a is operated and pilot pressure is output from the operation valve 59a. This pilot pressure acts on the pressure receiving portion of the arm control valve 81a, and the hydraulic oil that has entered the arm control valve 81a is supplied to the rod side of the arm cylinder 48 via oil passage PA21, causing the arm 45 to descend. When the work operation lever 57 is tilted rearward, the operation valve 59b is operated and pilot pressure is output from the operation valve 59b. This pilot pressure acts on the pressure receiving portion of the arm control valve 81a, and the hydraulic oil that has entered the arm control valve 81a is supplied to the bottom side of the arm cylinder 48 via oil passage PA22, causing the arm 45 to rise.

[0052] That is, arm control valve 81a can control the flow rate of hydraulic oil flowing to arm cylinder 48 according to the pressure of hydraulic oil set by operation of work operation lever 57 (pilot pressure set by operation valve 59a, pilot pressure set by operation valve 59b). When work operation lever 57 is tilted to the right, operation valve 59c is operated and pilot pressure acts on the pressure-receiving portion of bucket control valve 81b. As a result, bucket control valve 81b operates in a direction that extends tool cylinder 49, and tool (bucket) 41 performs a dump operation at a speed proportional to the amount of tilt of work operation lever 57.

[0053] When the work operation lever 57 is tilted to the left, the operation valve 59d is operated, and pilot oil acts on the pressure-receiving portion of the bucket control valve 81b. As a result, the bucket control valve 81b operates in a direction to retract the tool cylinder 49 by supplying oil to the rod side of the tool cylinder 49 via the oil passage PA23, and the tool (bucket) 41 performs a scooping operation at a speed proportional to the tilt amount of the work operation lever 57. In other words, the bucket control valve 81b can control the flow rate of hydraulic oil flowing to the tool cylinder 49 in accordance with the pressure of the hydraulic oil set by operation of the work operation lever 57 (the pilot pressure set by the operation valve 59c, the pilot pressure set by the operation valve 59d). In other words, the operation valves 59a, 59b, 59c, and 59d change the pressure of the hydraulic oil in accordance with the operation of the work operation lever 57 and supply the changed hydraulic oil to control valves such as the arm control valve 81a, the bucket control valve 81b, and the auxiliary control valve 81c.

[0054] The work vehicle 1 is provided with various switches and sensors connected to the controller 10 described above. FIG. 7 is a block diagram of the work vehicle 1. Referring to FIG. 7, the work vehicle 1 includes an operation panel 15 and a switch 16 for inputting operations for the auxiliary attachment described above. The operation panel 15 is, for example, a touch panel that displays various states of the work vehicle 1 and allows various settings according to the present embodiment to be made. The display and operation of the operation panel 15 will be described later. The switch 16 is provided around the driver's seat 54. The switch 16 is, for example, configured as a rocking seesaw switch, a sliding switch, or a push switch that can be pressed. The operation of the switch 16 is input to the controller 10. A first solenoid valve 83a and a second solenoid valve 83b, which are configured from solenoid valves and the like shown in FIG. 6, open according to the amount of operation of the switch 16. As a result, pilot oil is supplied to the auxiliary control valve 81c connected to the first solenoid valve 83a and the second solenoid valve 83b, and the auxiliary actuator of the auxiliary attachment is operated by the hydraulic oil supplied from the auxiliary control valve 81c.

[0055] 3 and 7, the work vehicle 1 includes a hydraulic sensor SP11 for detecting the hydraulic pressure in the first pilot oil passage PA11, a hydraulic sensor SP12 for detecting the hydraulic pressure in the second pilot oil passage PA12, a hydraulic sensor SP13 for detecting the hydraulic pressure in the third pilot oil passage PA13, and a hydraulic sensor SP14 for detecting the hydraulic pressure in the fourth pilot oil passage PA14. As described above, the secondary pilot pressures output from the secondary ports of the operation valves OVA, OVB, OVC, and OVD change in response to the operating position of the operation lever 55. Therefore, the hydraulic sensors SP11 to SP14 are sensors for detecting the operation of the operation device 56 (directional input device).

[0056] The work vehicle 1 includes a hydraulic sensor SP5L for detecting the hydraulic pressure in the drive oil passage PA5L, a hydraulic sensor SP6L for detecting the hydraulic pressure in the drive oil passage PA6L, a hydraulic sensor SP5R for detecting the hydraulic pressure in the drive oil passage PA5R, and a hydraulic sensor SP6R for detecting the hydraulic pressure in the drive oil passage 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 sensors SP5L and SP6L and the pressure difference between the hydraulic sensors SP5R and SP6R. Therefore, the hydraulic sensors SP5L to SP6R can be considered as sensors for detecting the operation of the operating device 56 (directional input device).

[0057] 2, 3, and 7, the work vehicle 1 may further include a rotation sensor SR31L connected to the rotary shaft of the first hydraulic motor 31L for detecting the rotation speed of the first hydraulic motor 31L and a rotation sensor SR31R for 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 magnitude of the rotation speed detected by the rotation sensor SR31L and the rotation direction and magnitude of the rotation speed detected by the rotation sensor SR31R. Therefore, the rotation sensors SR31L and SR31R can be considered as sensors for detecting the operation of the operating device 56 (directional input device).

[0058] 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, which will be described later. The operation detection sensor 18 is a position sensor or the like that detects the position of the operation lever 55. Therefore, the operation detection sensor 18 is a sensor that detects the operation of the operation device 56 (directional input device). Similarly, the work vehicle 1 may include a work operation detection sensor 19 for detecting the operation position of the work operation lever 57. The work operation detection sensor 19 is connected to the controller 10, which will be described later. The work operation detection sensor 19 is a position sensor or the like that detects the position of the work operation lever 57.

[0059] The work vehicle 1 may also include a gyroscope 20 that detects the angular acceleration of the vehicle body 2, an attitude detection sensor 21 that detects the attitude of the vehicle body 2, a temperature sensor ST70 that detects the temperature of hydraulic oil stored in a hydraulic oil tank 70, an oil pressure sensor SP21 that detects the oil pressure of oil passage PA21, an oil pressure sensor SP22 that detects the oil pressure of oil passage PA22, an oil pressure sensor SP23 that detects the oil pressure of oil passage PA23, an oil pressure sensor SP82a that detects the oil pressure of work oil passage 82a, an oil pressure sensor SP82b that detects the oil pressure of work oil passage 82b, an arm attitude sensor SS45 such as an angle sensor (such as a potentiometer) or a distance sensor that measures the length of the arm cylinder 48 that is provided on the joint shaft 47 to detect the attitude of the arm 45, and a load sensor SW21 that detects the load on the implement 41. The attitude detection sensor 21 can detect the roll angle (front-rear tilt angle) and pitch angle (left-right tilt angle) of the vehicle body 2 by detecting the tilt of the vehicle body 2 in the front-rear and left-right directions relative to the direction of gravitational acceleration. The attitude detection sensor 21 can detect the yaw angle (azimuth angle) of the vehicle body 2 by detecting the tilt of the vehicle body 2 in the longitudinal direction relative to the geomagnetism. The gyro 20 and the attitude detection sensor 21 may be realized by an inertial measurement unit. A method for utilizing the information from these sensors will be described later.

[0060] In the embodiment according to the present application, the arm 45 is raised and lowered by swinging one work operation lever 57 forward or rearward, and the bucket or other implement 41 is operated by swinging it left or right. However, instead of this, at least a pair of work operation levers 57 may be provided, and the arm 45 may be raised and lowered by swinging one work operation lever 57, and the implement 41 may be operated by swinging the other work operation lever 57. In this case, the operation valves 59a and 59b set the pilot pressure in response to the swing of one work operation lever 57, and the operation valves 59c and 59d set the pilot pressure in response to the swing of the other work operation lever 57. In other words, the arm 45 and the implement 41 may be capable of being operated in a combined manner. <Turning state>

[0061] Before describing the specific operation of the controller 10, the swing state in the embodiment of the present application will be described. As described above, the swing state refers to the operation of the work vehicle 1 when the operating lever 55 is operated rightward or leftward between positions G4 and G5. When the operating lever 55 is operated rightward or leftward between positions G4 and G5, primary pilot pressure Pc is applied to the first port PLa of the first hydraulic pump 7L and the fourth port PRb of the second hydraulic pump 7R, or to the second port PLb of the first hydraulic pump 7L and the third port PRa of the second hydraulic pump 7R. This primary pilot pressure Pc maximizes the displacement of each of the first hydraulic pump 7L and the second hydraulic pump 7R when there is no load on the first hydraulic motor 31L and the second hydraulic motor 31R. FIG. 4 shows the primary travel pressure for maximizing the displacement of each of the first hydraulic pump 7L and the second hydraulic pump 7R corresponding to the rotational speed of the engine 6. When the drop amount of the engine 6 is less than the anti-stall judgment value (setting line L1), if the engine rotation speed is equal to or higher than the idling speed RSi, the displacement volumes of the first hydraulic pump 7L and the second hydraulic pump 7R are maximized when there is no load on the first hydraulic motor 31L and the second hydraulic motor 31R.

[0062] In this turning state, the turning radius is very short (approximately half the width of the work vehicle 1), so the amount of crawler slippage per unit travel distance is large. This increases running resistance. When running resistance is high, the work vehicle 1 performs automatic deceleration control to switch the first hydraulic motor 31L and the second hydraulic motor 31R to first gear in order to reduce the running resistance. Specifically, when running resistance is high, the output of at least one of the hydraulic sensors SP5L, SP6L, SP5R, and SP6R exceeds a predetermined threshold, and based on this, the work vehicle 1 switches the first hydraulic motor 31L and the second hydraulic motor 31R to first gear. Details of this automatic deceleration control are disclosed in, for example, Japanese Patent Application Laid-Open No. 2021-067147, so please refer to that publication.

[0063] In this way, even if automatic deceleration control is performed when the vehicle is turning, the pressure in the drive oil passages PA5L, PA6L, PA5R, and PA6R due to the influence of running resistance can cause the swash plates of the first hydraulic pump 7L and the second hydraulic pump 7R to become unstable, resulting in variations in pump capacity and causing the work vehicle 1 to vibrate back and forth. This state of the work vehicle 1 is called flapping of the work vehicle 1. The controller 10 performs the following control to prevent this flapping of the work vehicle 1. <Detailed operation of controller 10>

[0064] The controller 10 determines whether the movement state of the work vehicle 1 is a turning state based on the operation of the operating device 56 (directional input device), and if it determines that the movement state is a turning state, outputs a rotation command to decelerate the target rotation speed of the engine 6 from a first rotation speed RS1 determined based on the amount of operation of the setting member 11 to a second rotation speed RS2. In other words, the controller 10 outputs a target rotation speed that is lower than the first rotation speed RS1 determined based on the amount of operation of the setting member 11. This reduces the amount of slippage of the crawler per unit time, thereby reducing running resistance and suppressing flapping of the work vehicle 1. However, because a significant deceleration impairs workability, the controller 10 is configured to control the pilot pressure control valve CV1 (hydraulic pressure adjustment mechanism) for each pilot pressure of at least one pilot oil line so as to maximize the displacement of the first hydraulic pump 7L and the second hydraulic pump 7R when there is no load on the first hydraulic motor 31L and the second hydraulic motor 31R. However, even if the pilot pressure is such that the displacement of the first hydraulic pump 7L and the second hydraulic pump 7R can be maximized when there is no load on the first hydraulic motor 31L and the second hydraulic motor 31R, it is still preferable to increase the pilot pressure as much as possible. This is because it makes it easier to fix the swash plates of the first hydraulic pump 7L and the second hydraulic pump 7R in positions that maximize the displacement of the first hydraulic pump 7L and the second hydraulic pump 7R against the pressure in the drive oil passages PA5L, PA6L, PA5R, and PA6R that is generated by travel resistance. To achieve the above-described processing, the controller 10 includes a processor 10a and a memory 10b as shown in FIG. 7. The memory 10b includes both volatile and nonvolatile memory. The memory 10b includes at least a travel control program 10c1 for implementing the above-described control and a work control program 10c2 for controlling the operation of the implement 41 (bucket) and the arm 45. The processor 10a executes the above-mentioned control by executing the travel control program 10c1, or by executing both the travel control program 10c1 and the work control program 10c2. The control method of the controller 10 will be described in detail below.

[0065] FIG. 8 is a flowchart showing the operation of the controller 10 of the work vehicle 1. In this flowchart, the processes from step S1 to step S12 are executed at predetermined sampling intervals (e.g., 20 μs). In step S1, the temperature sensor ST70 detects the temperature of the hydraulic oil, and the controller 10 determines whether the temperature of the hydraulic oil detected by the temperature sensor ST70 is below a threshold temperature. This threshold temperature is set to a temperature at which the viscosity of the hydraulic oil becomes higher than a predetermined value. High viscosity reduces pressure fluctuations in the drive oil passages PA5L, PA6L, PA5R, and PA6R caused by running resistance, reducing flapping of the work vehicle 1. Therefore, if the temperature of the hydraulic oil detected by the temperature sensor ST70 is below the threshold temperature (Yes in step S1), the engine rotation speed is not reduced. That is, the controller 10 outputs a rotation command (step S11) that sets the first rotation speed RS1, which is determined based on the amount of operation of the setting member 11, as the target rotation speed. If the temperature of the hydraulic oil detected by the temperature sensor ST70 is equal to or higher than the threshold temperature (No in step S1), the process proceeds to step S2.

[0066] In step S2, one of the following three processes is performed. (1) The arm posture sensor SS45 detects the posture of the arm 45 attached to the vehicle body 2 of the work vehicle 1 and supporting the implement 41 (bucket) of the work vehicle 1. When the height of the arm 45 relative to the vehicle body 2 is equal to or greater than a predetermined threshold height (No in step S2), the controller 10 proceeds to step S3. For example, whether the height of the arm 45 relative to the vehicle body 2 is equal to or greater than the predetermined threshold height can be determined by thresholding the output value of an angle sensor provided on the joint shaft 47, or thresholding the output value of a distance sensor that measures the length of the arm cylinder 48.

[0067] (2) The hydraulic sensors SP21 and SP22 detect that the arm 45 is being operated based on fluctuations in their respective pressure values. Alternatively, the arm posture sensor SS45 may detect that the arm 45 is being operated. For example, it is also possible to detect that the arm 45 is being operated based on changes over time in the output value of an angle sensor provided on the joint shaft 47, or changes over time in the output value of a distance sensor that measures the length of the arm cylinder 48. When the arm 45 is being operated (No in step S2), the controller 10 proceeds to step S3.

[0068] (3) The hydraulic pressure sensors SP82b and SP82a detect input of operation of the work operation lever 57 based on a change in the difference between their respective pressure values. Input of operation of the work operation lever 57 may be detected based on the operation position of the work operation lever 57 detected by the work operation detection sensor 19. When input of operation of the work operation lever 57 is detected (No in step S2), the controller 10 proceeds to step S3. When the height of the arm 45 relative to the vehicle body 2 is less than a predetermined threshold height, the arm 45 is not being operated, and operation of the work operation lever 57 is not being input (Yes in step S2), the controller 10 does not perform processing to reduce the engine rotation speed. In other words, the controller 10 outputs a rotation command to set the target rotation speed to the first rotation speed RS1 determined based on the operation amount of the setting member 11 (step S11). When the arm is being operated, for which the result in step S2 is No, the center of gravity of the work vehicle 1 fluctuates, making it prone to fluttering. By limiting the engine rotation speed when fluttering is likely to occur, high workability can be ensured. Note that step S2 may be omitted.

[0069] In step S3, the sensor 14 of the brake pedal 13 detects that the hydraulic brake is being applied. Alternatively, it is determined whether the pressures of the hydraulic sensors SP11 to SP14 are all equal to or lower than the secondary travel pressure Pd corresponding to the lever operation position G2, or are equal to or lower than the pressure between Pa and Pd taking hysteresis into consideration, i.e., whether all forward, backward, left, and right operations are in the slow-speed range. When all forward, backward, left, and right operations are in the slow-speed range or the hydraulic brake is not being applied (Yes in step S3), the controller 10 outputs a rotation command to set the target rotation speed to a first rotation speed RS1 determined based on the operation amount of the setting member 11 (step S11). When all forward, backward, left, and right operations are in the slow-speed range or the hydraulic brake is being applied, fluttering is unlikely to occur because the movement speed is low. High workability can be ensured by limiting the engine rotation speed when fluttering is likely to occur. If the result in step S3 is No, the process proceeds to step S4.

[0070] In step S4, the controller 10 determines whether the first rotation speed RS1 described above is less than a predetermined first threshold speed. When the first rotation speed RS1 is lower than the first threshold speed RSth1 (Yes in step S4), the controller 10 does not perform a process to decelerate the engine rotation speed. That is, the controller 10 outputs a rotation command to set the target rotation speed of the engine 6 to the first rotation speed RS1 (step S11). If the target rotation speed is reduced to or below the first threshold speed RSth1, the risk of the engine 6 stalling increases. The first threshold speed RSth1 is, for example, 2208 rpm. Therefore, when the target rotation speed of the engine 6 is a rotation speed at which the risk of the engine 6 stalling is high, the stall can be prevented by not performing a process to decelerate the engine rotation speed. If the result in step S4 is No, the process proceeds to step S5.

[0071] In step S5, the sensor 6a detects the rotation speed of the engine 6 (the actual rotation speed of the engine 6). When the detected actual rotation speed is lower than a predetermined second threshold speed RSth2 that is lower than the first threshold speed RSth1, the controller 10 outputs a rotation command to set the target rotation speed of the engine 6 to the first rotation speed RS1 (step S11). If the actual rotation speed drops to or below the second threshold speed RSth2, there is a high risk of the engine 6 stalling. The second threshold speed RSth2 is, for example, 1750 rpm. Therefore, when the actual rotation speed of the engine 6 is a rotation speed at which there is a high risk of the engine 6 stalling, stalling can be prevented by not performing the engine rotation speed deceleration process. If the answer to step S5 is No, the process proceeds to step S6.

[0072] In step S6, at least one of the hydraulic sensors SP11 to SP14, operation detection sensor 18, gyro 20, hydraulic sensors SP5L, SP6L, SP5R, SP6R, and rotation sensors SR31L, SR31R detects the operation state of the operation device 56 (directional input device) for operating the direction of travel of the work vehicle 1. Based on the detected operation state, the controller 10 determines whether the movement state of the work vehicle 1 is turning. For example, (1) the controller 10 determines that the movement state of the work vehicle 1 is turning when the ratio of the pressure value of the hydraulic sensor SP11 to the pressure value of the hydraulic sensor SP14 is within a predetermined range close to 1 (for example, between 0.9 and 1 / 0.9), and the value obtained by multiplying the average value of the pressure values ​​of the hydraulic sensors SP11 and SP14 by a predetermined coefficient (for example, 0.5) is greater than the larger of the pressure value of the hydraulic sensor SP12 and the pressure value of the hydraulic sensor SP13. Alternatively, the controller 10 determines that the movement state of the work vehicle 1 is a turning state when the ratio between the pressure value of the hydraulic sensor SP12 and the pressure value of the hydraulic sensor SP13 is within a predetermined range close to 1, and the value obtained by multiplying the average value of the pressure values ​​of the hydraulic sensors SP12 and SP13 by a predetermined coefficient (for example, 0.5) is greater than the larger of the pressure value of the hydraulic sensor SP11 and the pressure value of the hydraulic sensor SP14.

[0073] In other words, the controller 10 determines that the traveling state is a turning state when the first pilot pressure (pressure value of hydraulic sensor SP11) applied to the first port PLa for driving the first hydraulic pump 7L forward and the fourth pilot pressure (pressure value of hydraulic sensor SP14) applied to the fourth port PRb for driving the second hydraulic pump 7R reverse are substantially equal, and the average of the first pilot pressure and the fourth pilot pressure is greater than the larger of the second pilot pressure (pressure value of hydraulic sensor SP12) applied to the second port PLb for driving the first hydraulic pump 7L reverse and the third pilot pressure (pressure value of hydraulic sensor SP12) applied to the third port PRa for driving the second hydraulic pump 7R forward, or when the second pilot pressure and the third pilot pressure are substantially equal and the average of the second pilot pressure and the third pilot pressure is greater than the larger of the first pilot pressure and the fourth pilot pressure. When the control lever 55 is operated in the left-right direction in the near-full operation region RA2, the primary pilot pressure is applied to the first port PLa and the fourth port PRb (the second port PLb and the third port PRa), so the first pilot pressure and the fourth pilot pressure (the second pilot pressure and the third pilot pressure) become substantially equal. Furthermore, since the control lever 55 is not operated in the near-full operation region RA2 in the forward-backward direction, a pressure equal to or less than Pb, which is smaller than the primary pilot pressure, should be output to the remaining ports, and the average of the first pilot pressure and the fourth pilot pressure (the average of the second pilot pressure and the third pilot pressure) should be greater than the larger of the second pilot pressure and the third pilot pressure (the first pilot pressure and the fourth pilot pressure). The controller 10 detects the above-described states of the control levers based on the output values ​​of the hydraulic sensors SP11 to SP14.

[0074] (2) When the left-right operation position of the operation detection sensor 18 detected by the operation detection sensor 18 is between the G4 position and the G5 position, the controller 10 determines that the movement state of the work vehicle 1 is a turning state. In other words, the controller 10 determines that the traveling state is a turning state when the displacement from the neutral position of the operation lever 55 (directional input device) for instructing movement in the left-right direction is equal to or greater than the first displacement value described above, and the displacement from the neutral position of the operation lever 55 (directional input device) for instructing movement in the forward-backward direction is less than the second displacement value.

[0075] (3) When the angular acceleration detected by the gyroscope 20 exceeds a predetermined threshold value that indicates a turning state, the controller 10 determines that the movement state of the work vehicle 1 is turning. This threshold value increases as the first rotational speed RS1 increases. This threshold value may be set, for example, for each first rotational speed RS1, to a value greater than the angular acceleration that is affected by the turning speed when pilot pressure Pb is applied to the first port PLa and the fourth port PRb (the second port PLb and the third port PRa).

[0076] (4) The controller 10 determines that the movement state of the work vehicle 1 is in a turning state when the absolute value of the pressure difference between the drive oil passages PA5L, PA6L (the difference between the hydraulic pressure at the first connection port 31P1 and the hydraulic pressure at the second connection port 31P2) is equal to or greater than a predetermined turning determination threshold pressure, the absolute value of the pressure difference between the drive oil passages PA5R, PA6R (the difference between the hydraulic pressure at the third connection port 31P3 and the hydraulic pressure at the fourth connection port 31P4) is equal to or greater than a predetermined turning determination threshold pressure, and the hydraulic pressure at the first connection port 31P1 is higher than the hydraulic pressure at the second connection port 31P2 and the hydraulic pressure at the fourth connection port 31P4 is higher than the hydraulic pressure at the third connection port 31P3, or the hydraulic pressure at the second connection port 31P2 is higher than the hydraulic pressure at the first connection port 31P1 and the hydraulic pressure at the third connection port 31P3 is higher than the hydraulic pressure at the fourth connection port 31P4. This turning threshold pressure increases as the first rotation speed RS1 increases. The turning threshold pressure can be determined by measuring the oil pressure received by each connection port when pilot pressure Pb is applied to the first port PLa and the fourth port PRb (second port PLb and third port PRa) for each first rotational speed RS1 in an ideal environment with low running resistance, and setting a value greater than the differential pressure obtained from the measurement results.

[0077] (5) The controller 10 determines that the movement state of the work vehicle 1 is turning when the rotation direction is reversed based on the outputs of the rotation sensors SR31L and SR31R and the rotation speed is within a range estimated from the engine target rotation speed (first rotation speed RS1). This is because in the embodiment described above, the reduction ratio between the hydraulic motor and the hydraulic pump generally falls within a predetermined range. The higher the first rotation speed RS1, the larger the upper and lower limits of this rotation speed range. Specifically, for example, the lower limit of the range may be set to a value obtained by adding a predetermined margin to the value output from the rotation sensors SR31L and SR31R when pilot pressure Pb is applied to the first port PLa and the fourth port PRb (second port PLb and third port PRa) for each first rotation speed RS1 in an ideal environment with low running resistance. The upper limit of this range may be set to the value output from the rotation sensors SR31L, SR31R when the pilot pressure Pc is applied to the first port PLa and the fourth port PRb (the second port PLb and the third port PRa) for each first rotation speed RS1 in an ideal environment with low running resistance. Note that the controller 10 may determine that the movement state of the work vehicle 1 is a turning state based on a combination of the conditions listed in (1) to (5).

[0078] In step S7, if the controller 10 determines that the moving state is a turning state (Yes in step S6), it outputs a rotation command to decrease the target rotation speed of the engine 6 from the first rotation speed RS1 to a second rotation speed RS2. The second rotation speed RS2 is a value obtained by multiplying the first rotation speed RS1 by a predetermined ratio. The predetermined ratio is, for example, 0.92.

[0079] Because the speed difference between the second rotational speed RS2 and the first rotational speed RS1 is greater than the stall determination speed difference, the actual rotational speed decreases rapidly, and the primary pilot pressure is normally controlled according to the setting line L2. However, in step S8, when the traveling state is determined to be a turning state, the controller 10 controls the primary pilot pressure to a pressure corresponding to the second rotational speed RS2 based on the first correspondence relationship according to the setting line L1. In FIG. 4, line L3 indicates the primary pilot pressure required to maximize the displacement of the first hydraulic pump 7L and the second hydraulic pump 7R when there is no load on the first hydraulic motor 31L and the second hydraulic motor 31R. This primary pilot pressure is referred to as the maximum volume threshold pressure. The maximum volume threshold pressure varies depending on the engine rotational speed. When the first rotational speed RS1 is set in a range higher than the first threshold speed RSth1, the primary pilot pressure according to the setting line L1 corresponding to the second rotational speed RS2 (indicated by a circle in the figure) is greater than the maximum volume threshold pressure (indicated by a triangle in the figure).

[0080] Therefore, when it is determined that the traveling state is a turning state, the controller 10 controls the primary pilot pressure to be greater than the maximum volume threshold pressure that maximizes the displacement of each of the first hydraulic pump 7L and the second hydraulic pump 7R when there is no load on the first hydraulic motor 31L and the second hydraulic motor 31R. In other words, it can be said that the controller 10 controls the pilot pressure of the pilot oil supplied to each of the first hydraulic pump 7L and the second hydraulic pump 7R to maximize the displacement of each of the first hydraulic pump 7L and the second hydraulic pump 7R when there is no load on the first hydraulic motor 31L and the second hydraulic motor 31R.

[0081] When the first rotation speed RS1 is relatively high, as is often the case with the control according to this embodiment, the primary pilot pressure (indicated by a square in the figure) may be greater than the maximum volume threshold pressure (indicated by a triangle in the figure) even if anti-stall control is performed. However, as in this embodiment, by controlling the primary pilot pressure to a pressure corresponding to the second rotation speed RS2 based on the first correspondence relationship following the setting line L1, it is possible to set the primary pilot pressure to a value sufficiently greater than the maximum volume threshold pressure. This makes it easier to fix the swash plates of the first hydraulic pump 7L and the second hydraulic pump 7R in positions that maximize the displacement of each of the first hydraulic pump 7L and the second hydraulic pump 7R against the pressure in the drive oil passages PA5L, PA6L, PA5R, and PA6R caused by running resistance. Therefore, fluctuations in the displacement of each of the first hydraulic pump 7L and the second hydraulic pump 7R are less likely to occur, further suppressing fluttering.

[0082] When the controller 10 determines that the moving state is not a turning state (No in step S6), the controller 10 determines in step S9 whether the moving state is a straight moving state. This straight-ahead state is a state that satisfies two conditions: (1) the pilot pressure related to the ports (first port PLa, third port PRa) through which the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate forward is sufficiently greater than the pilot pressure related to the ports (second port PLb, fourth port PRb) through which the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate reversely, or the pilot pressure related to the ports (second port PLb, fourth port PRb) through which the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate reversely is sufficiently greater than the pilot pressure related to the ports (first port PLa, third port PRa) through which the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate forwardly; 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)).

[0083] The state variable for defining such a state is called the degree of straightness. The controller 10 calculates the degree of straightness using the following algorithm. The first pilot pressure (pressure value of hydraulic sensor SP11) applied to the first port PLa is defined as lf(t), the second pilot pressure (pressure value of hydraulic sensor SP12) applied to the second port PLb is defined as lb(t), the third pilot pressure (pressure value of hydraulic sensor SP12) applied to the third port PRa is defined as rf(t), and the fourth pilot pressure (pressure value of hydraulic sensor SP14) applied to the fourth port PRb is defined as rb(t). First, it is determined whether lf(t) / rf(t) or lb(t) / rb(t) is within a predetermined range close to 1 (for example, between 0.9 and 1 / 0.9). If it is determined that lf(t) / rf(t) is within the predetermined range, the larger of lb(t) and rb(t) is set as the variable PV Fstraight If it is determined that lb(t) / rb(t) is within the predetermined range, the larger value of lf(t) and rf(t) is assigned to the variable PV Bstraight Substitute into

[0084] PV Fstraight When a value is assigned to , the forward straightness S Fratio (t) is calculated using equation (1). PV Bstraight When a value is assigned to , the straightness of the backward movement S 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) S Fratio (t) or S Bratio If (t) exceeds a predetermined threshold (for example, 300 or more), it is determined that the above two conditions are satisfied, and the controller 10 determines that the movement state is a straight-ahead state.

[0085] If the controller 10 determines that the traveling state is neither a straight traveling state nor a turning state (No in step S9), it outputs a rotation command to gradually increase the target rotation speed of the engine 6 from the second rotation speed RS2 to the first rotation speed RS1 (step S10). This increase may be rapid or gradual. In the case of a gradual increase, if the controller 10 determines No at each sampling interval, the target rotation speed of the engine 6 is determined by multiplying the first rotation speed RS1 by a value obtained by gradually approaching 1, which is a predetermined ratio for determining the second rotation speed RS2. When the controller 10 determines No in step S9, the traveling state is an intermediate state between a straight traveling state and a turning state. Therefore, gradually increasing the target rotation speed can smooth the deceleration state between the straight traveling state and the turning state, thereby improving the user's operational feel. After step S10 is completed, the process proceeds to step S8.

[0086] Once step S10 or S11 is complete, the process proceeds to step S12. In step S12, the controller 10 moves the work vehicle 1 based on the pilot pressure and target rotational speed set as described above. Specifically, the controller 10 executes a process of driving the first hydraulic pump 7L and the second hydraulic pump 7R using the engine 6 of the work vehicle 1 based on the pilot pressure and target rotational speed set as described above, thereby supplying hydraulic oil to the first hydraulic motor 31L and the second hydraulic motor 31R, and driving the first traveling device 3L and the second traveling device 3R arranged opposite each other, thereby moving the work vehicle 1 forward. <Actions and Effects of the First Embodiment>

[0087] The control method for the work vehicle 1 according to the first embodiment or the processing by the controller 10 of the work vehicle 1 determines whether the movement state of the work vehicle 1 is turning, and if it is determined that the movement state is turning, outputs a rotation command to reduce the target rotation speed of the engine 6 from the first rotation speed RS1 to the second rotation speed RS2, and controls the pilot pressure of the pilot oil supplied to each of the first hydraulic pump 7L and the second hydraulic pump 7R so as to maximize the displacement of each of the first hydraulic pump 7L and the second hydraulic pump 7R when there is no load on the first hydraulic motor 31L and the second hydraulic motor 31R. This makes it possible to provide a work vehicle 1 that improves turning safety while suppressing deceleration during turning. <Modification of the first embodiment>

[0088] In the first embodiment, some or all of the processes in steps S1 to S2 and the various sensors corresponding to those processes may be omitted. The sensors for determining the turning state and some of the determination processes may also be omitted.

[0089] The straight-ahead state may also be determined by a factor other than the pilot pressure. For example, the determination may be made using an algorithm that determines whether the rotations are in the same direction and the absolute values ​​of the rotation speeds are substantially equal, based on the output of the rotation sensor SR31L of the first hydraulic motor 31L and the output of the rotation sensor SR31R of the second hydraulic motor 31R. The degree of straight-ahead movement may also be determined by the operation detection sensor 18, the gyro 20, and the hydraulic pressure of the drive oil passages PA5L, PA6L, PA5R, and PA6R.

[0090] The control of the first pilot pressure applied to the first port PLa, the second pilot pressure applied to the second port PLb, the third pilot pressure applied to the third port PRa, and the fourth pilot pressure applied to the fourth port PRb may not only be by controlling the primary pilot pressure input to the operating device 56, but may also be in a form that controls the secondary pilot pressure output from the operating device 56 according to the movement state.

[0091] 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 reducer connected to the second hydraulic motor 31R, and the characteristics of the various control valves. Second Embodiment

[0092] The operation of the controller 10 of the work vehicle 1 is not limited to the operation of the flowchart in Figure 8 of the first embodiment. In the second embodiment, an operation of the controller 10 different from that of the first embodiment will be described. The hardware configuration of the work vehicle 1 of the second embodiment is the same as that of the first embodiment, so a description thereof will be omitted. Figures 9A and 9B show a flowchart showing the operation of the controller 10 of the work vehicle 1 according to the second embodiment. In Figures 9A and 9B, the same operations as those in Figure 8 are assigned the same reference numerals, and a description thereof will be omitted.

[0093] In the second embodiment, instead of step S7 in Fig. 8, the controller 10 executes steps S71 and S72. In step S71, the work vehicle 1 measures at least one of the forward / backward tilt angle (pitch angle) and the left / right tilt angle (roll angle) of the work vehicle 1 using the attitude detection sensor 21. In step S72, the controller 10 determines the second rotation speed described above in accordance with at least one of the tilt angles.

[0094] More specifically, when the at least one tilt angle is less than a predetermined first threshold angle, the controller 10 sets the second rotation speed equal to the first rotation speed. This first threshold angle is empirically determined. Note that the first threshold angle when the at least one tilt angle is a pitch angle may be different from the first threshold angle when the at least one tilt angle is a roll angle. When the at least one tilt angle includes both a pitch angle and a roll angle, the controller 10 sets the second rotation speed equal to the first rotation speed when the pitch angle is less than the first threshold angle corresponding to the pitch angle and the roll angle is less than the first threshold angle corresponding to the roll angle.

[0095] When at least one tilt angle is equal to or greater than the first threshold angle and less than a second threshold angle greater than the first threshold angle, the controller 10 determines the second rotation speed to be a value obtained by multiplying the first rotation speed by a predetermined first ratio. This second threshold angle is empirically determined. Note that the second threshold angle when at least one tilt angle is a pitch angle may be different from the second threshold angle when at least one tilt angle is a roll angle. When at least one tilt angle includes both a pitch angle and a roll angle, when the pitch angle is equal to or greater than the first threshold corresponding to the pitch angle and less than the second threshold corresponding to the pitch angle, and the roll angle is equal to or greater than the first threshold corresponding to the roll angle and less than the second threshold corresponding to the roll angle, the controller 10 determines the second rotation speed to be a value obtained by multiplying the first rotation speed by a predetermined first ratio. This first ratio may be equal to the predetermined ratio in step S7 according to the first embodiment.

[0096] When at least one tilt angle is equal to or greater than the second threshold angle described above, the controller 10 sets the second rotational speed to a value obtained by multiplying the first rotational speed by a predetermined second ratio that is smaller than the first ratio. Note that when at least one tilt angle includes both a pitch angle and a roll angle, when the pitch angle is equal to or greater than the second threshold angle corresponding to the pitch angle, or when the roll angle is equal to or greater than the second threshold angle corresponding to the roll angle, the controller 10 sets the second rotational speed to a value obtained by multiplying the first rotational speed by the second ratio. Through the above processing, fluttering can be further suppressed by reducing the engine rotational speed only when fluttering of the work vehicle 1 is occurring (at least one tilt angle is equal to or greater than the first threshold angle), or by further reducing the engine rotational speed when fluttering is particularly severe (at least one tilt angle is equal to or greater than the second threshold angle).

[0097] Furthermore, in the second embodiment, in step S81 after step S8, the controller 10 acquires the hydraulic pressure detected by the hydraulic sensors SP5L, SP6L, SP5R, and SP6R and determines whether the traveling pressure, which is at least one of the hydraulic pressure in the hydraulic circuit CL connecting the first hydraulic pump 7L and the first hydraulic motor 31L (the pressure difference between the hydraulic sensors SP5L and SP6L) and the hydraulic pressure in the hydraulic circuit CR connecting the second hydraulic pump 7R and the second hydraulic motor 31R (the pressure difference between the hydraulic sensors SP5R and SP6R), is equal to or greater than a threshold pressure. This threshold pressure is an empirically determined value. When the traveling pressure is equal to or greater than the threshold pressure (YES in step S82), if the timer is not activated (NO in step S82), the controller 50 resets and activates the timer (step S83). When the timer is activated (NO in step S82), the controller 50 executes step S84. When the running pressure is less than the threshold pressure (NO in step S82), or when the process proceeds to step S11 described above, the controller 50 turns off the timer (step S111).

[0098] In step S84, the controller 50 determines whether the time during which the traveling pressure remains equal to or higher than the threshold pressure exceeds a threshold time. Specifically, the controller 50 determines whether the time measured by the timer exceeds the threshold time. If the time during which the traveling pressure remains equal to or higher than the threshold pressure exceeds the threshold time (YES in step S84), the controller 50 outputs a rotation command to further increase the target rotation speed from the second rotation speed to the first rotation speed (step S85). If the rotation speed is increased in step S10, the target rotation speed is set to a rotation speed that is closer to the first rotation speed than the rotation speed determined in step S10. When step S85 or S111 ends, or if step S84 is NO, the controller 50 executes the process of step S12. The processes of steps S81 to S85 can reduce the risk of engine stall when the engine load is high. <Modifications of the First and Second Embodiments>

[0099] The processes of the first and second embodiments described above may be changed by changing the settings on the operation panel 15. FIGS. 10A to 10D show examples of the display on the operation panel 15 of a work vehicle according to modifications of the first and second embodiments. FIG. 10A shows a display screen 15A of the operation panel 15 when the processes of FIGS. 8, 9A, and 9B are not executed. Not executing the processes of FIGS. 8, 9A, and 9B means that steps S1 to S10, S71, S72, S81 to 85, and S111 are omitted, and the target rotation speed is not changed to the second rotation speed. The display screen 15A includes a setting icon 150 and a status display gauge 151. Touching the setting icon 150 displays a pop-up window 15P shown in FIG. 10B. The Quick mode in the pump-up window 15P is a mode in which the target rotation speed is not changed to the second rotation speed. The Quick mode may also be referred to as the first mode. In both Normal mode and Mild mode, the processes of steps S1 to S10, S71, S72, S81 to S85, and S111 are executed, but the predetermined rate in step S7, the first rate in S72, and the second rate in Normal mode are greater than the predetermined rate in step S7, the first rate in S72, and the second rate in Mild mode, respectively. That is, the speed decelerates more significantly in Mild mode. When the desired mode is selected using the radio button on the right edge of the pump-up window 15P and the enter button 15R is tapped, the mode selected by the radio button is set. FIG. 10A shows a status display gauge 151 in Quick mode, FIG. 10C shows a status display gauge 152 in Normal mode, and FIG. 10D shows a status display gauge 153 in Mild mode.

[0100] In this way, by providing an interface for the operation panel 15, the work vehicle control method can further include selecting one mode from a plurality of modes. In these modes, the ratio of the second rotational speed to the first rotational speed differs, and different predetermined ratios in step S7 or different combinations of the first ratio and the second ratio in step S72 are set. The second rotational speed, the predetermined ratio in step S7, and the first ratio and the second ratio in step S72 are determined according to the selected mode. Furthermore, when the first mode (Quick mode), which is one of the plurality of modes, is selected, the controller 50 outputs a rotation command that sets the target rotational speed to the first rotational speed, even if the moving state is determined to be a turning state. By providing such modes, control that suits the user's preferences can be performed.

[0101] The above-mentioned operation panel 15 does not have to be a touch panel, and the mode may be set by an input device such as a dial or key provided on the operation panel 15. The number of modes may also be two, or four or more. When there are two modes, the above-mentioned first mode and other modes are selected. When there are four or more modes, different values ​​may be adopted for the predetermined ratio in step S7 and the first ratio and second ratio in step S72 depending on the mode.

[0102] 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.

[0103] The terms "member," "part," "element," "body," and "structure" may have multiple meanings, such as a single part or multiple parts.

[0104] Ordinal numbers such as "first" and "second" are merely terms used to identify components and do not have any other meaning (e.g., 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."

[0105] Words expressing degrees, such as "substantially," "about," and "approximately," can 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 can be interpreted to include words such as "substantially," "about," and "approximately."

[0106] In this application, the phrase "at least one of A and B" should be interpreted to include A only, B only, and both A and B.

[0107] 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 of the present invention.

Claims

1. The first hydraulic pump and the second hydraulic pump are driven by the engine of the work vehicle, thereby supplying hydraulic oil to the first hydraulic motor and the second hydraulic motor, respectively, to drive the first traveling device and the second traveling device arranged opposite to each other, thereby moving the work vehicle forward; detecting an operation state of a direction input device for operating the traveling direction of the work vehicle; determining whether the movement state of the work vehicle is a turning state based on the detected operation state; When it is determined that the moving state is the turning state, a rotation command is output to decrease the target rotation speed of the engine from a first rotation speed to a second rotation speed; controlling a pilot pressure of pilot oil supplied to each of the first hydraulic pump and the second hydraulic pump so as to maximize a displacement volume of each of the first hydraulic pump and the second hydraulic pump when there is no load on the first hydraulic motor and the second hydraulic motor; A method for controlling a work vehicle.

2. the first rotation speed is determined based on an operation amount of a speed input device separate from the direction input device of the work vehicle, and further including outputting the rotation command for setting a target rotation speed of the engine of the work vehicle to the first rotation speed even when the movement state is determined to be the turning state when the first rotation speed is lower than a predetermined first threshold speed. The control method according to claim 1 .

3. The second rotation speed is a value obtained by multiplying the first rotation speed by a predetermined ratio. The control method according to claim 2 .

4. When it is determined that the movement state is the turning state, a tilt angle (pitch angle) of the work vehicle in the front and rear directions is measured, and the second rotation speed is determined in accordance with the tilt angle. The control method according to claim 2 .

5. When it is determined that the movement state is the turning state, a left / right tilt angle (roll angle) of the work vehicle is measured, and the second rotation speed is determined in accordance with the tilt angle. The control method according to claim 2 .

6. The control method according to claim 4 or 5, wherein the second rotation speed is made equal to the first rotation speed when the tilt angle is less than a predetermined first threshold angle.

7. when the tilt angle is equal to or greater than the first threshold angle and less than a second threshold angle that is greater than the first threshold angle, the second rotation speed is set to a value obtained by multiplying the first rotation speed by a predetermined first ratio, When the tilt angle is equal to or greater than the second threshold angle, the second rotation speed is set to a value obtained by multiplying the first rotation speed by a predetermined second ratio that is smaller than the first ratio. The control method according to claim 6.

8. Detecting the rotational speed of the engine; When the detected rotation speed is lower than a predetermined second threshold speed that is lower than the first threshold speed, the rotation command is output to set the target rotation speed to the first rotation speed. The control method according to any one of claims 3 to 5.

9. Detecting the temperature of the hydraulic oil; When the temperature is lower than a predetermined threshold temperature, the rotation command is output to set the target rotation speed to the first rotation speed. A control method according to any one of claims 1 to 5.

10. outputting the rotation command to set the target rotation speed to the first rotation speed when neither the height of the arm of the work vehicle relative to the vehicle body is equal to or greater than a predetermined threshold height nor the arm is being operated; A control method according to any one of claims 1 to 5.

11. determining whether or not the movement state has changed to a straight-ahead state after the movement state has changed to the turning state; When it is determined that the moving state has become the straight traveling state, the rotation command is output so as to return the target rotation speed of the engine to the first rotation speed. A control method according to any one of claims 1 to 5.

12. when it is determined that the moving state is neither the straight traveling state nor the turning state, the rotation command is output to increase the target rotation speed of the engine from the second rotation speed to the first rotation speed. The control method according to claim 11.

13. when a time during which at least one of the hydraulic pressure in a hydraulic circuit connecting the first hydraulic pump and the first hydraulic motor and the hydraulic pressure in a hydraulic circuit connecting the second hydraulic pump and the second hydraulic motor continues to be equal to or higher than a threshold pressure exceeds a threshold time, the rotation command is output to further increase the target rotation speed from the second rotation speed to the first rotation speed. The control method according to claim 12.

14. when a time during which at least one of the hydraulic pressure in a hydraulic circuit connecting the first hydraulic pump and the first hydraulic motor and the hydraulic pressure in a hydraulic circuit connecting the second hydraulic pump and the second hydraulic motor continues to be equal to or higher than a threshold pressure exceeds a threshold time, the rotation command is output to increase the target rotation speed from the second rotation speed to the first rotation speed. A control method according to any one of claims 1 to 5.

15. 6. The control method according to claim 1, wherein the movement state is determined to be the turning state when a displacement from a neutral position of the directional input device for instructing movement in a left-right direction is equal to or greater than a first displacement value and a displacement from a neutral position of the directional input device for instructing movement in a forward-backward direction is less than a second displacement value.

16. 16. The control method according to claim 15, wherein the movement state is determined to be the turning state when a first pilot pressure applied to a first port for driving the first hydraulic pump forward and a fourth pilot pressure applied to a fourth port for driving the second hydraulic pump reverse are substantially equal, and an average of the first pilot pressure and the fourth pilot pressure is greater than a larger value of a second pilot pressure applied to a second port for driving the first hydraulic pump reverse and a third pilot pressure applied to a third port for driving the second hydraulic pump forward, or when the second pilot pressure and the third pilot pressure are substantially equal, and an average of the second pilot pressure and the third pilot pressure is greater than a larger value of the first pilot pressure and the fourth pilot pressure.

17. The turning state is determined when an absolute value of the difference between the hydraulic pressure at a first connection port of the first hydraulic motor, through which hydraulic oil for rotating the first traveling device in a forward direction is input to the first hydraulic motor, and the hydraulic pressure at a second connection port of the first hydraulic motor, through which hydraulic oil for rotating the first traveling device in a reverse direction is input to the first hydraulic motor, is equal to or greater than a turning determination threshold pressure corresponding to the first rotation speed; an absolute value of the difference between the hydraulic pressure at a third connection port of the second hydraulic motor, through which hydraulic oil for rotating the second traveling device in a forward direction is input to the second hydraulic motor, and the hydraulic pressure at a fourth connection port of the second hydraulic motor, through which hydraulic oil for rotating the second traveling device in a reverse direction is input to the second hydraulic motor, is equal to or greater than the turning determination threshold pressure; and the hydraulic pressure at the first port is higher than the hydraulic pressure at the second port, and the hydraulic pressure at the fourth port is higher than the hydraulic pressure at the third port, or the hydraulic pressure at the second port is higher than the hydraulic pressure at the first port, and the hydraulic pressure at the third port is higher than the hydraulic pressure at the fourth port.

17. The control method of claim 16.

18. When the rotation directions of the first hydraulic motor and the second hydraulic motor are opposite to each other and the magnitudes of the rotation speeds of the first hydraulic motor and the second hydraulic motor are within a range of rotation speeds estimated from the first rotation speed, it is determined that the vehicle is in the turning state. A control method according to any one of claims 1 to 5.

19. the directional input device outputs a primary pilot pressure to be input to the directional input device to the first port and the fourth port when a displacement of the directional input device from a neutral position for instructing movement in a left direction is equal to or greater than the first displacement value; the direction input device outputs a primary pilot pressure to be input to the direction input device to the second port and the third port when a displacement of the direction input device from a neutral position for instructing movement in a right direction is equal to or greater than the first displacement value; the directional input device outputs a primary pilot pressure to be input to the directional input device to the first port and the third port when a displacement of the directional input device from a neutral position for instructing movement in a forward direction is equal to or greater than the second displacement value; the direction input device outputs a primary pilot pressure to be input to the direction input device to the second port and the fourth port when a displacement of the direction input device from a neutral position for instructing movement in a rearward direction is equal to or greater than the second displacement value; When the movement state is determined to be the turning state, the primary pilot pressure is made larger than a maximum volume threshold pressure that maximizes the displacement volumes of the first hydraulic pump and the second hydraulic pump when there is no load on the first hydraulic motor and the second hydraulic motor.

17. The control method of claim 16.

20. Detecting the rotational speed of the engine; when a difference between the first rotational speed and the detected rotational speed of the engine is smaller than a predetermined stall determination speed difference, the primary pilot pressure corresponding to the first rotational speed transitions in accordance with a first correspondence relationship; when a difference between the first rotational speed and the detected rotational speed of the engine is equal to or greater than a predetermined stall determination speed difference, the primary pilot pressure corresponding to the first rotational speed transitions in accordance with a second correspondence relationship; the primary pilot pressure based on the second correspondence relationship when the first rotational speed is the same is lower than the primary pilot pressure based on the first correspondence relationship when the first rotational speed is the same; a speed difference between the second rotation speed and the first rotation speed is greater than the stall determination speed difference, When the moving state is determined to be the turning state, the primary pilot pressure is controlled to a pressure corresponding to the second rotation speed based on the first correspondence relationship.

20. The control method of claim 19.

21. 6. The control method according to claim 1, further comprising selecting one mode from a plurality of modes in which a ratio of the second rotation speed to the first rotation speed is different, and the second rotation speed is determined according to the selected one mode.

22. 22. The control method according to claim 21, wherein when a first mode that is one of the plurality of modes is selected, the rotation command for setting the target rotation speed to the first rotation speed is output even if the moving state is determined to be the turning state.

23. 4. The control method according to claim 3, further comprising selecting one mode from a plurality of modes in which different predetermined ratios are set, wherein the predetermined ratio is determined according to the selected one mode.

24. 8. The control method according to claim 7, further comprising selecting one mode from a plurality of modes in which different combinations of the first ratio and the second ratio are set, and the first ratio and the second ratio are determined according to the selected one mode.

25. a vehicle body having a first side surface and a second side surface opposite to the first side surface; a first traveling device provided on the first side surface of the vehicle body; a second traveling device provided on the second side surface of the vehicle body; a first hydraulic motor for driving the first traveling device; a second hydraulic motor for driving the second traveling device; a first hydraulic pump connected to the first hydraulic motor via a first hydraulic circuit, having a first port and a second port, configured to supply hydraulic oil to the first hydraulic motor via the first hydraulic circuit when pressure applied to the first port is higher than pressure applied to the second port so as to drive the first traveling device forward, and to supply hydraulic oil to the first hydraulic motor via the first hydraulic circuit when pressure applied to the second port is higher than pressure applied to the first port so as to drive the first traveling device backward; a second hydraulic pump connected to the second hydraulic motor via a second hydraulic circuit, having a third port and a fourth port, configured to supply hydraulic oil to the second hydraulic motor via the second hydraulic circuit when the pressure applied to the third port is higher than the pressure applied to the fourth port so as to drive the second traveling device forward, and to supply hydraulic oil to the second hydraulic motor via the second hydraulic circuit when the pressure applied to the fourth port is higher than the pressure applied to the third port so as to drive the second traveling device backward; a pilot pump configured to supply pilot oil to the first hydraulic pump and the second hydraulic pump; an engine configured to drive the first hydraulic pump, the second hydraulic pump, and the pilot pump; At least one pilot oil passage connecting the pilot pump and the first hydraulic pump and connecting the pilot pump and the second hydraulic pump; a hydraulic pressure adjusting mechanism provided in the at least one pilot oil passage and configured to adjust the pilot pressure of each of the at least one pilot oil passages; a direction input device configured to operate the traveling direction of the work vehicle by instructing at least one of the first traveling device and the second traveling device to move forward or backward; a sensor for detecting an operation of the directional input device; a controller configured to determine whether the movement state of the work vehicle is a turning state based on operation of the directional input device, and when it is determined that the movement state is the turning state, output a rotation command to decelerate a target rotation speed of the engine from a first rotation speed to a second rotation speed, and control the hydraulic pressure adjustment mechanism to adjust the pilot pressure of each of the at least one pilot oil passage to maximize the displacement of each of the first hydraulic pump and the second hydraulic pump when there is no load on the first hydraulic motor and the second hydraulic motor; A work vehicle equipped with:

26. The at least one pilot oil passage a first pilot oil passage connecting the direction input device and the first port; a second pilot oil passage connecting the direction input device and the second port; a third pilot oil passage connecting the direction input device and the third port; a fourth pilot oil passage connecting the direction input device and the fourth port; a pilot supply oil passage connecting the pilot pump and the direction input device; Including, The hydraulic pressure adjustment mechanism is a pilot pressure control valve provided in the pilot supply oil passage and configured to adjust the pilot pressure of the pilot supply oil passage.

26. A work vehicle according to claim 25.

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