Work vehicle, work vehicle control device, and work vehicle control method

Abstract

To provide a work vehicle improving responsiveness when performing feedback control to a desired vehicle speed using fluctuations in oil pressure in an oil passage between a travel motor and a travel pump.SOLUTION: A control method of a work vehicle includes: preparing first reference information representing a first correspondence relation between an upper limit speed in a creep mode in which a work vehicle travels at a speed lower than the upper limit speed regardless of an operation amount of at least one operation device to which a user's speed change operation is input, an oil pressure in an oil passage between a hydraulic motor for traveling the work vehicle and a hydraulic pump for traveling the work vehicle, and a primary running pressure of a pilot oil input to an operation valve operated by a first operation device of at least one operation device corresponding to the upper limit speed and the oil pressure; obtaining the input upper limit speed; detecting the oil pressure; acquiring a primary running pressure corresponding to the obtained upper limit speed and the detected oil pressure from the first reference information; and controlling a control valve sending the pilot oil to the operation valve toward the acquired primary running pressure.SELECTED DRAWING: Figure 9

Description

[Technical Field]

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

[0002] Patent Document 1 discloses a technique for measuring the input of a travel lever and the rotational speed of a travel motor, and adjusting the pilot pressure of the travel pump so that the rotational speed of the travel motor matches the command based on the input of the travel lever.Patent Document 2 discloses a method for detecting the primary pressure of pilot oil supplied to a remote control valve and the rotational speed of the travel motor, and controlling the primary pressure to achieve a target vehicle speed based on the detected primary pressure and rotational speed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-053413 [Patent Document 2] Japanese Patent Publication No. 2020-038002 Summary of the Invention [Problem to be solved by the invention]

[0004] Both the methods of Patent Document 1 and Patent Document 2 use feedback that utilizes the rotational speed of the travel motor. The reason the actual rotational speed of the travel motor deviates from the target rotational speed is because the load related to the travel motor is transmitted to the travel pump via hydraulic oil, pushing back the swash plate of the travel pump. Therefore, fluctuations in the oil pressure in the oil passage between the travel motor and the travel pump occur faster than the deviation of the actual rotational speed of the travel motor from the target rotational speed. Therefore, feedback control that utilizes fluctuations in the oil pressure in the oil passage between the travel motor and the travel pump is expected to further improve responsiveness. [Means for solving the problem]

[0005] A control method for a work vehicle according to a first aspect of the present disclosure includes preparing first reference information that represents a first correspondence between an upper limit speed in a creep mode in which the work vehicle is caused to travel at or below the upper limit speed regardless of the amount of operation of at least one operating device to which a user's speed change operation is input, the oil pressure in an oil line between a hydraulic motor for traveling of the work vehicle and a hydraulic pump for traveling of the work vehicle, and a primary traveling pressure of pilot oil that is input to an operating valve operated by a first operating device of the at least one operating device corresponding to the upper limit speed and the oil pressure, acquiring the input upper limit speed, detecting the oil pressure, determining the primary traveling pressure that corresponds to the acquired upper limit speed and the detected oil pressure from the first reference information, and controlling a control valve that sends pilot oil to the operating valve so as to achieve the determined primary traveling pressure.

[0006] a hydraulic pump configured to discharge pilot oil; an operating valve configured to convert the pressure of the pilot oil from a primary traveling pressure to a secondary traveling pressure and output the pilot oil in accordance with a first operation amount of a first operating device of the at least one operating device; a control valve provided between the pilot pump and the operating valve and configured to convert the pressure of the pilot oil supplied to the operating valve into the primary traveling pressure; an input device for inputting the upper limit speed, which sets a creep mode in which the traveling device travels at or below an upper limit speed regardless of the operation amount of the at least one operating device; a memory that stores first reference information representing a first correspondence relationship between the upper limit speed, the hydraulic oil pressure, and the primary traveling pressure corresponding to the upper limit speed and the hydraulic oil pressure; and an electronic circuit configured to control the control valve. The electronic circuit is configured to acquire an upper limit speed from the input device, determine a primary traveling pressure corresponding to the oil pressure obtained from the oil pressure sensor and the acquired upper limit speed from the first reference information, and control the control valve to achieve the determined primary traveling pressure.

[0007] A control device for a work vehicle according to a third aspect of the present disclosure includes: a memory that stores first reference information that represents a first correspondence relationship between an upper limit speed in a creep mode that causes the work vehicle to travel at or below an upper limit speed regardless of the amount of operation of at least one operating device to which a user's speed change operation is input; a hydraulic pressure in an oil line between a hydraulic motor for traveling the work vehicle and a hydraulic pump for traveling the work vehicle; and a primary traveling pressure of pilot oil input to an operating valve operated by a first operating device of the at least one operating device, the primary traveling pressure corresponding to the upper limit speed and the hydraulic pressure; and an electronic circuit configured to control a control valve that sends pilot oil to the operating valve. The electronic circuit is configured to acquire the input upper limit speed, acquire the hydraulic pressure, determine the primary traveling pressure corresponding to the acquired upper limit speed and the acquired hydraulic pressure from the first reference information, and control the control valve to achieve the determined primary traveling pressure. [Effects of the Invention]

[0008] The technology disclosed in the present application makes it possible to provide a work vehicle that has improved responsiveness when feedback controlling to a desired vehicle speed by utilizing fluctuations in the oil pressure in the oil passage between the travel motor and the travel pump, for example. [Brief explanation of the drawings]

[0009] [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 block diagram of the work vehicle. [Figure 7] FIG. 7 shows an example of the first reference information in the first embodiment. [Figure 8]FIG. 8 shows an example of the second reference information in the first embodiment. [Figure 9] FIG. 9 shows a method of linear interpolation based on the first reference information and the second reference information. [Figure 10] FIG. 10 is a flowchart showing the operation of the work vehicle according to the first embodiment. [Figure 11] FIG. 11 shows an example of the first reference information in the second embodiment. [Figure 12] FIG. 12 shows an example of the second reference information in the second embodiment. [Figure 13] FIG. 13 is a flowchart showing the operation of the work vehicle according to the second embodiment. [Figure 14] FIG. 14 is a flowchart showing the operation of the work vehicle according to the third embodiment.

[0010] 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>

[0011] 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 .

[0012] 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

[0013] In the embodiment of the present application, FB (Forward 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.

[0014] 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.

[0015] 1 and 2, the work vehicle 1 further includes an engine 6 provided at the rear of the vehicle body 2, and multiple hydraulic pumps 7 including a first hydraulic pump 7L and a second hydraulic pump 7R. The engine 6 drives the multiple 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 first hydraulic pump 7L and the second hydraulic pump 7R are collectively referred to as hydraulic pumps (7L, 7R). The multiple 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 (multiple arm cylinders 48, at least one implement cylinder 49, etc.) connected to the work implement 4. The engine 6 is arranged in a width direction D of the work vehicle 1. W and is provided between the pair of arm assemblies 42. The work vehicle 1 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.

[0016] 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.

[0017] 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. 6). 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). The controller 10 may also be called a control device.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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. In other words, the second hydraulic motor 31R operates in the same manner as the first hydraulic motor 31L. The first hydraulic motor 31L and the second hydraulic motor 31R are collectively referred to as hydraulic motors (31L, 31R). The second swash plate switching cylinder 32R operates in the same manner as the first swash plate switching cylinder 32L. The second traveling control valve SV5 is a two-position switching valve configured to switch its valve element between a first position VP5a and a second position VP5b, and operates in the same manner as the first traveling control valve SV4.

[0023] 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.

[0024] 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.

[0025] 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 31L via the second connection port 31P2, and hydraulic oil for rotating the first traveling device 3L in the forward direction is discharged from the first traveling device 3L.

[0026] 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 31R via the fourth connection port 31P4, and hydraulic oil for rotating the second traveling device 3R in the forward direction is discharged from the second traveling device 3R. In other words, the hydraulic motors (31L, 31R) are configured to drive the traveling devices (3L, 3R). The hydraulic pumps (7L, 7R) are configured to discharge hydraulic oil for driving the hydraulic motors (31L, 31R). The drive oil passages (PA5L, PA6L, PA5R, PA6R) are oil passages that connect the hydraulic pumps (7L, 7R) and the hydraulic motors (31L, 31R).

[0027] 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.

[0028] 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.

[0029] 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. 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 response to 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.

[0030] 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. In the following embodiments, 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.

[0031] The operating device 56 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 the first port PLa and the second port PLb of the first hydraulic pump 7L and the third port PRa and the fourth port PRb of the second hydraulic pump 7R. In this embodiment, the operating valves OVA, OVB, OVC, and OVD are operated by a single operating lever 55, but multiple operating levers 55 may be used. In the following embodiments, one or more operating levers 55 may be referred to as a first operating device.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Next, detailed operation of the pilot pressure control valve CV1 will be described. The work vehicle 1 includes a setting member 11 (see FIG. 6) that sets the target rotation speed of the engine 6. The setting member 11 is a speed input device separate from the operating device 56, such as an accelerator pedal, a swingably supported accelerator lever, or a rotatable indoor dial. The setting member 11 is provided with a sensor 12. The amount of operation detected by the sensor 12 is input to the controller 10. The engine rotation speed corresponding to the amount of operation 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 amount of operation of the setting member 11. The controller 10 outputs a rotation command indicating, for example, a 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 a fuel injection pressure, etc. to the supply pump or common rail so that the determined target rotation speed of the engine 6 is achieved. In the following embodiments, the one or more operating levers 55 and the setting member 11 described above may be referred to as at least one operating device. A speed 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 speed sensor 6a is, for example, a potentiometer configured to detect the rotation speed of a rotating member connected to the crankshaft of the engine 6. When a load is applied to the engine 6, the actual rotation speed of the engine 6 decreases from the target rotation speed of the engine 6. The amount by which the actual rotation speed decreases 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.

[0040] 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 the amount of decrease (drop) ΔE1 in the rotational speed of the engine 6 (engine rotational speed E1). In other words, the pilot pressure control valve CV1 is a control valve provided between the pilot pump 71 and the control valves OVA, OVB, OVC, and OVD, configured to send pilot oil to the control valves OVA, OVB, OVC, and OVD and convert the pressure of the pilot oil supplied to the control valves OVA, OVB, OVC, and OVD into primary pilot pressure. The rotational speed of the engine 6 can be detected by a speed sensor 6a for the engine rotational speed E1. The engine rotational speed E1 detected by the speed sensor 6a is input to the controller 10. The speed sensor 6a may also be referred to as a speed sensor. Figure 4 shows the relationship between the engine rotational speed, the traveling primary pressure (primary pilot pressure), and the setting lines L1 and L2. The setting line L1 shows the relationship between the engine rotation speed E1 and the traveling primary pressure when the decrease amount ΔE1 is less than a predetermined value (less than the anti-stall determination value). The setting line L2 shows the relationship between the engine rotation speed E1 and the traveling primary pressure when the decrease amount ΔE1 is equal to or greater than the anti-stall determination value. When the difference between the rotation speed RS determined based on the operation amount of the setting member 11 and the actual rotation speed of the engine 6 is smaller than a predetermined stall determination speed difference (anti-stall determination value), the primary pilot pressure corresponding to the rotation speed RS transitions in accordance with the third correspondence relationship shown by the setting line L1. When the difference between the rotation speed RS and the actual rotation speed of the engine 6 is equal to or greater than the predetermined stall determination speed difference (anti-stall determination value), the primary pilot pressure corresponding to the rotation speed RS transitions in accordance with the fourth correspondence relationship shown by the setting line L2.

[0041] 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.

[0042] 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.

[0043] 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%

[0044] 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 and flows 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 from the neutral position for instructing movement to the left 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 for instructing movement in the rightward 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 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 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 for instructing forward movement is greater than or equal to the second displacement value (displacement from G0' to G4').The operation device 56 may output the primary pilot pressure input to the operation device 56 to the second port PLb and the fourth port PRb when the displacement of the operation lever 55, which commands rearward movement, from the neutral position is equal to or greater than a second displacement value (a displacement from G0' to G4'). Furthermore, Pa and Pb (Pa' and Pb') are values ​​independent of the magnitude of the primary pilot pressure. However, when the primary pilot pressure is lower than Pa or Pb (Pa' or Pb'), the secondary pilot pressure peaks at the magnitude of the primary pilot pressure. In other words, the operation valves (OVA, OVB, OVC, OVD) are configured to convert the pressure of the pilot oil from the traveling primary pressure to the traveling secondary pressure in accordance with the first operation amount (operation lever position) of the operation device 56 and output the pilot oil. The pilot oil of the traveling secondary pressure is applied to the ports (PLa, PRa, PLb, PRb) that provide hydraulic pressure to the swash plates of the hydraulic pumps (7L, 7R). When the first manipulated variable is equal to or larger than a threshold variable (first displacement value), the operating valves (OVA, OVB, OVC, OVD) convert the primary traveling pressure into a secondary traveling pressure that is equal to the primary traveling pressure.

[0045] 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.

[0046] 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 this embodiment, 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.

[0047] 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.

[0048] The work vehicle 1 is provided with various switches and sensors connected to the controller 10 described above. FIG. 6 is a block diagram of the work vehicle 1. Referring to FIG. 6, the work vehicle 1 includes a creep setting member 16 provided around the driver's seat 54. The creep setting member 16 may also be referred to as an input device. The creep setting member 16 is configured, for example, as a touch panel, a freely slidable slide switch, or a dial. Creep refers to control that causes the work vehicle 1 to travel at or below an upper limit speed, regardless of the amount of operation of at least one operation device (setting member 11, one or more operation levers 55) through which a user's speed change operation is input. The upper limit speed is input by the creep setting member 16. The creep setting member 16 is configured to switch between a normal mode and a creep mode. The state in which the upper limit speed is set by the creep setting member 16 is called creep mode. A state other than creep mode is called normal mode.

[0049] In normal mode, a target rotation speed of the engine 6 is set by operating the setting member 11, and a primary traveling pressure corresponding to the target rotation speed is determined based on the setting line L1 or L2 in FIG. 4. The secondary traveling pressure is then set based on the amount of operation of one or more operating levers 55, and the hydraulic motors (31L, 31R) and the hydraulic pumps (7L, 7R) are controlled. That is, in normal mode, the speed of the work vehicle 1 is changed according to the amount of operation of at least one operating device, and the work vehicle 1 can travel at a speed higher than the upper limit speed. On the other hand, in creep mode, the setting line L1 or L2 in FIG. 4 is not used to determine the primary traveling pressure, and the primary traveling pressure is determined to be lower than the primary traveling pressure in normal mode using first reference information 10r1, etc., which will be described later. The settings from the secondary traveling pressure onwards in creep mode are the same as in normal mode, but because the secondary traveling pressure is lower than the primary traveling pressure, limiting the primary traveling pressure limits the speed of the work vehicle 1 to be lower than the upper limit speed, regardless of the amount of operation of at least one operating device (the setting member 11, one or more operating levers 55).

[0050] 3 and 6, 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 accordance with the operating position of the operation lever 55. Therefore, the hydraulic sensors SP11 to SP14 are sensors for detecting the traveling secondary pressure. The hydraulic sensors SP11 to SP14 may also be referred to as additional hydraulic sensors.

[0051] 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. That is, the hydraulic sensors (SP5L, SP6L, SP5R, SP6R) are configured to detect the hydraulic pressure of the working oil in the drive oil passages (PA5L, PA6L, PA5R, PA6R). The states of the first hydraulic motor 31L and the second hydraulic motor 31R can be detected from the pressure difference between the hydraulic sensor SP5L and the hydraulic sensor SP6L and the pressure difference between the hydraulic sensor SP5R and the hydraulic sensor SP6R.

[0052] 2, 3, and 6, 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. The work vehicle 1 may also 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. <Configuration of Controller 10>

[0053] The controller 10 has a processor 10a and a memory 10b as shown in FIG. 7 to control the vehicle speed in the creep mode described above. The processor 10a may also be referred to as an electronic circuit. The memory 10b includes a volatile memory and a non-volatile memory. The memory 10b includes at least a travel control program 10c1, first reference information 10r1, second reference information 10r2, third reference information 10r3, and fourth reference information 10r4 to realize the control described above. The first reference information 10r1 represents a first correspondence relationship between the upper limit speed described above in the creep mode, the oil pressure in the oil passages (CL, CR) between the hydraulic motors (31L, 31R) for travel of the work vehicle 1 and the hydraulic pumps (7L, 7R) for travel of the work vehicle 1, and the primary travel pressure of the pilot oil input to the operation valves OVA, OVB, OVC, and OVD corresponding to the upper limit speed and the oil pressure. The hydraulic pressure in the oil passages (CL, CR) between the hydraulic motors (31L, 31R) and the hydraulic pumps (7L, 7R) is the average value of the highest and second highest hydraulic pressures among the four hydraulic pressures measured by the hydraulic pressure sensors SP5L, SP6L, SP5R, and SP6R. First reference information 10r1 represents a first correspondence relationship when the rotational speed of the engine 6 of the work vehicle 1 is a first rotational speed RS1. The first rotational speed RS1 is a standard engine rotational speed when the anti-stall control described above is performed.

[0054] FIG. 7 shows an example of the first reference information 10r1. To clearly explain the first correspondence relationship, FIG. 7 illustrates the upper limit speed set by the creep setting member 16 on the horizontal axis and the control pressure output representing the primary traveling pressure on the vertical axis. The hydraulic pressure in the oil passages (CL, CR) between the hydraulic motors (31L, 31R) and the hydraulic pumps (7L, 7R) is shown as the main effective pressure. As described above, the main effective pressure is the average value of the highest and second highest hydraulic pressures among the four hydraulic pressures from the hydraulic sensors SP5L, SP6L, SP5R, and SP6R. While FIG. 7 uses a line graph to show the relationship between the upper limit speed and the primary traveling pressure when the main effective pressure is 10 MPa, 20 MPa, and 30 MPa, the first correspondence relationship may also include relationships between the upper limit speed and the primary traveling pressure for other main effective pressures. The range not set as the upper limit speed is a speed range that cannot be set by the creep setting member 16.

[0055] FIG. 8 shows an example of the second reference information 10r2. The second reference information 10r2 represents a second correspondence relationship between the upper limit speed, the main effective pressure, and the traveling primary pressure when the rotational speed of the engine 6 of the work vehicle 1 is a second rotational speed RS2 different from the first rotational speed RS1. The second rotational speed RS2 is lower than the first rotational speed RS1. The second rotational speed RS2 is, for example, the idling speed of the engine 6. The vertical axis, the horizontal axis, and the main effective pressure in FIG. 8 have the same meanings as those in FIG. 7. Because the second rotational speed RS2 is significantly lower than the first rotational speed RS1, even if the capacity of the hydraulic pumps (7L, 7R) is maximized, the maximum value of the rotational speed of the hydraulic motors (31L, 31R) is limited, and the upper limit speed is also limited. For this reason, the range of the upper limit speed is narrower than that in FIG. 7. When the upper limit speed set by the creep setting member 16 falls outside the range represented by the second correspondence relationship, speed control in normal mode is performed.

[0056] The third reference information 10r3 represents a third correspondence relationship between the rotation speed RS of the engine 6 detected by the speed sensor 6a and the primary traveling pressure in the normal mode. That is, the third reference information 10r3 represents the third correspondence relationship represented by the setting line L1 in FIG. 4. The fourth reference information 10r4 represents a fourth correspondence relationship between the rotation speed RS of the engine 6 detected by the speed sensor 6a and the primary traveling pressure, which is used for controlling the primary traveling pressure when the drop amount of the engine 6 is large in the normal mode. That is, the fourth reference information 10r4 represents the fourth correspondence relationship represented by the setting line L2 in FIG. 4.

[0057] The processor 10a executes the following control while executing the cruise control program 10c1 with reference to the first reference information 10r1, the second reference information 10r2, the third reference information 10r3, and the fourth reference information 10r4. First, when the normal mode is selected by the creep setting member 16, the processor 10a is configured to acquire the rotation speed RS of the engine 6 from the speed sensor 6a, determine the primary traveling pressure corresponding to the detected rotation speed RS of the engine 6 from the third reference information, and control the pilot pressure control valve CV1 to achieve the determined primary traveling pressure. When the drop amount of the engine 6 is large while the normal mode is selected, the processor 10a is configured to determine the primary traveling pressure corresponding to the rotation speed RS of the engine 6 detected by the speed sensor 6a from the fourth reference information, and control the pilot pressure control valve CV1 to achieve the determined primary traveling pressure.

[0058] When the creep mode is selected by the creep setting member 16, the processor 10a acquires the upper limit speed input by the creep setting member 16, acquires the main effective pressure, and acquires the rotational speed RS of the engine 6 detected by the speed sensor 6a. Based on the acquired rotational speed RS, the processor 10a extracts information for calculating the primary traveling pressure from the first reference information 10r1 and the second reference information 10r2. The processor 10a then determines the primary traveling pressure based on the extracted information. For example, when the absolute value of the difference between the acquired rotational speed RS and the first rotational speed RS1 is less than a threshold, the processor 10a calculates the primary traveling pressure corresponding to the acquired upper limit speed and the acquired main effective pressure from the first reference information. When the absolute value of the difference between the acquired rotational speed RS and the second rotational speed RS2 is less than a threshold, the processor 10a calculates the primary traveling pressure corresponding to the acquired upper limit speed and the acquired main effective pressure from the second reference information 10r2. Alternatively, linear interpolation may be used, as shown in FIG. 10 . The first reference information 10r1 and the second reference information 10r2 have a first correspondence relationship and a second correspondence relationship corresponding to the lever position of the operating lever 55 (for example, the lever operating positions G1 to G5 in FIG. 5). When the engine rotation speed RS is between the rotation speed RS1 and the rotation speed RS2, an interior division point in FIG. 10 where a:b=(RS-RS2):(RS1-RS) is found, and a correspondence relationship between the traveling primary pressure and the upper limit speed can be found by connecting these points. Furthermore, when the acquired main effective pressure is not shown in the correspondence relationships shown in FIG. 7 or FIG. 8, a correspondence relationship for finding the effective primary pressure may be extracted by linear interpolation or the like based on the difference between the acquired main effective pressure and one of the main effective pressures in the first reference information 10r1 and the second reference information 10r2, or the difference between the acquired main effective pressure and another main effective pressure in the first reference information 10r1 and the second reference information 10r2. Then, the processor 10a controls the pilot pressure control valve CV1 so as to achieve the determined primary traveling pressure. <Work vehicle operation>

[0059] FIG. 10 is a flowchart showing the operation of the work vehicle 1 according to the first embodiment. 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 processor 10a acquires the rotation speed RS of the engine 6 detected by the speed sensor 6a. In other words, the control method for the work vehicle 1 according to this embodiment includes acquiring the rotation speed RS of the engine 6 detected by the speed sensor 6a. In step S2, the processor 10a determines whether or not the creep mode has been selected by the creep setting member 16. In other words, the control method according to this embodiment includes determining whether or not the creep mode has been selected by the creep setting member 16. When the creep mode is set, that is, when an upper limit speed has been set (Yes in step S2), the process proceeds from step S3 to S6. When the normal mode is set, that is, when an upper limit speed has not been set or an invalid upper limit speed that does not have the first or second correspondence relationship has been set (No in step S2), the process proceeds from step S7 to S9.

[0060] In creep mode (Yes in step S2), in step S3, the processor 10a acquires the upper limit speed input by the creep setting member 16. That is, the control method according to this embodiment acquires the upper limit speed input by the creep setting member 16. In step S3, the processor 10a acquires the oil pressures detected by the oil pressure sensors SP5L, SP6L, SP5R, and SP6R, and determines the main effective pressure, which is the average value of the highest and second highest oil pressures among these oil pressures. That is, the control method according to this embodiment detects the main effective pressure, which is the oil pressure in the oil passages (CL, CR) between the hydraulic motors (31L, 31R) for traveling of the work vehicle 1 and the hydraulic pumps (7L, 7R) for traveling of the work vehicle 1.

[0061] In step S5, the processor 10a extracts information for calculating the primary traveling pressure from the first reference information 10r1 and the second reference information 10r2 based on the rotation speed RS of the engine 6. The extraction method is as described above. That is, the control method according to this embodiment includes preparing the first reference information 10r1 and the second reference information 10r2. In step S6, the processor 10a calculates the primary traveling pressure corresponding to the upper limit speed and the main effective pressure from the extracted information. That is, the control method according to this embodiment includes calculating the primary traveling pressure from the first reference information 10r1 and the second reference information 10r2 based on the detected rotation speed RS, the first correspondence relationship, and the second correspondence relationship. When the absolute value of the difference between the acquired rotation speed RS and the first rotation speed RS1 is less than a threshold, the control method according to this embodiment includes calculating the primary traveling pressure corresponding to the acquired upper limit speed and the acquired main effective pressure from the first reference information. After the process of step S6 is completed, the process of step S10 is executed.

[0062] In step S10, the processor 10a controls the pilot pressure control valve CV1 that sends pilot oil to the operation valves OVA, OVB, OVC, and OVD so as to achieve the primary traveling pressure calculated in step S6. That is, the control method according to this embodiment includes controlling the pilot pressure control valve CV1 that sends pilot oil to the operation valves OVA, OVB, OVC, and OVD so as to achieve the calculated primary traveling pressure.

[0063] In the normal mode (Yes in step S2), the processor 10a determines in step S7 whether or not an engine drop has occurred. That is, in step S7, the processor 10a determines whether or not the drop amount ΔE1 of the engine 6 is equal to or greater than the anti-stall determination value. When there is no engine drop (No in step S7), the processor 10a calculates the primary traveling pressure from the third reference information 10r3 based on the rotation speed RS of the engine 6 in step S8. That is, in the control method according to this embodiment, the third reference information 10r3 is prepared, and when the normal mode is selected from the creep mode and the normal mode, the primary traveling pressure corresponding to the rotation speed RS of the engine 6 detected by the speed sensor 6a is calculated from the third reference information 10r3. When there is an engine drop (Yes in step S7), the processor 10a calculates the primary traveling pressure from the fourth reference information 10r4 based on the rotation speed RS of the engine 6 in step S9. That is, in the control method according to this embodiment, the fourth reference information 10r4 is prepared, and when the normal mode is selected, if an engine drop occurs, the primary traveling pressure corresponding to the rotation speed RS of the engine 6 detected by the speed sensor 6a is calculated from the fourth reference information 10r4. After the processing of step S8 or step S9 is completed, the processing of step S10 is executed.

[0064] In step S10, the processor 10a controls the pilot pressure control valve CV1 that sends pilot oil to the control valves OVA, OVB, OVC, and OVD so that the traveling primary pressure becomes the one calculated in step S8 or step S9. That is, the control method according to this embodiment includes controlling the pilot pressure control valve CV1 that sends pilot oil to the control valves OVA, OVB, OVC, and OVD so that the traveling primary pressure becomes the one calculated. In step S11, the control valves OVA, OVB, OVC, and OVD convert the traveling primary pressure into the traveling secondary pressure based on the lever position (first operation amount) of the control lever 55 (first operation device). That is, the control method according to this embodiment includes converting the traveling primary pressure into the traveling secondary pressure by the control valves OVA, OVB, OVC, and OVD based on the lever position (first operation amount) of the control lever 55 (first operation device).

[0065] In step S12, the secondary travel pressure of the pilot oil is applied to the ports (PLa, PRa, PLb, PRb) that provide hydraulic pressure to the swash plates of the hydraulic pumps (7L, 7R). The control method according to this embodiment includes applying the secondary travel pressure of the pilot oil to the ports (PLa, PRa, PLb, PRb) that provide hydraulic pressure to the swash plates of the hydraulic pumps (7L, 7R). <Actions and Effects of the First Embodiment>

[0066] In the work vehicle 1 or control method for the first embodiment, the processor 10a acquires the upper limit speed input by the creep setting member 16, acquires the main effective pressure, calculates the traveling primary pressure corresponding to the acquired upper limit speed and main effective pressure from the first reference information 10r1, and controls the pilot pressure control valve CV1 that sends pilot oil to the operation valves OVA, OVB, OVC, and OVD so as to achieve the calculated traveling primary pressure. Feedback control using fluctuations in the main effective pressure can improve responsiveness in creep mode. Second Embodiment

[0067] In the first embodiment, the same primary traveling pressure is calculated for creep mode regardless of whether the work vehicle 1 is turning or not. However, the greater the curvature at which the work vehicle 1 turns (the smaller the turning radius), the greater the running resistance that the work vehicle 1 experiences from the ground, and therefore the greater the primary traveling pressure required to turn at a desired upper limit speed. The processor 10a according to the second embodiment calculates the straightness corresponding to the turning radius of the work vehicle 1, and changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same main effective pressure in the first correspondence relationship based on the deviation between the calculated straightness and a reference straightness.

[0068] First, the concept of the straightness described above will be explained. This state of high straightness is a state that satisfies two conditions: (1) the pilot pressure related to the ports (first port PLa, third port PRa) through which the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate in the forward direction 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 in the reverse direction, 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 in the reverse direction 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 in the forward direction, and (2) the pilot pressures of the two ports determined to have high pilot pressures in (1) are substantially equal (the value of the ratio of the two pilot pressures is within a predetermined range close to 1 (for example, between 0.9 and 1 / 0.9)).

[0069] Therefore, the straightness is calculated using the following algorithm. Let lf(t) be the first pilot pressure (pressure value of hydraulic sensor SP11) applied to the first port PLa, lb(t) be the second pilot pressure (pressure value of hydraulic sensor SP12) applied to the second port PLb, rf(t) be the third pilot pressure (pressure value of hydraulic sensor SP12) applied to the third port PRa, and rb(t) be the fourth pilot pressure (pressure value of hydraulic sensor SP14) applied to the fourth port PRb. 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

[0070] 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) Here, the processor 10a Fratio (t) and S Bratio The larger value of (t) and (t) is calculated as the straightness.

[0071] In this embodiment, for example, 300, which is generally considered to be straight traveling, is set as the reference straightness, and when the straightness is equal to or greater than the reference straightness, the primary traveling pressure is calculated using the correspondence relationships marked with circles in Figures 11 and 12. The primary traveling pressure is calculated using a correspondence relationship (marked with a diamond) in which the smaller the straightness becomes from the reference straightness, the greater the primary traveling pressure becomes compared to the correspondence relationships marked with a circle.

[0072] Alternatively, as another method, an intermediate value 150 may be set as the reference straightness, and when the straightness is the reference straightness (150), the primary running pressure may be calculated using the correspondence relationships marked with circles in Figures 11 and 12. The primary running pressure may be calculated using a correspondence relationship (marked with a diamond) in which the smaller the straightness obtained in (1) or (2) above is compared to the reference straightness, the greater the primary running pressure becomes compared to the correspondence relationship marked with a circle. Furthermore, the primary running pressure may be calculated using a correspondence relationship (marked with a triangle) in which the larger the straightness obtained in (1) or (2) above is compared to the reference straightness, the smaller the primary running pressure becomes compared to the correspondence relationship marked with a circle.

[0073] FIG. 13 is a flowchart showing the operation of the work vehicle 1 according to the second embodiment. Here, the same operations as in the first embodiment are assigned the same reference numerals, and detailed description will be omitted. In the work vehicle 1 according to this embodiment, after step S4, in step S41, the hydraulic sensors SP11 to SP14 detect the secondary traveling pressure. The processor 10a acquires the pressure values ​​of the hydraulic sensors SP11 to SP14 and determines the straightness based on the above-mentioned algorithm. In other words, the control method according to this embodiment includes detecting the secondary traveling pressure using the hydraulic sensors SP11 to SP14 and determining the straightness in step S41.

[0074] Next, in S51, which replaces step S5 in the first embodiment, the processor 10a changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same main effective pressure in the first correspondence relationship based on the difference between the determined straightness and the reference straightness. The processor 10a changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same main effective pressure in the second correspondence relationship based on the difference between the determined straightness and the reference straightness. That is, the processor 10a corrects the first correspondence relationship and the second correspondence relationship read from the first reference information 10r1 and the second reference information 10r2 according to the straightness, as in the above-described algorithm. Then, the processor 10a calculates the primary traveling pressure based on the corrected first correspondence relationship and the second correspondence relationship in the same manner as in the first embodiment.

[0075] That is, the control method according to this embodiment changes the value of the primary pressure corresponding to the same upper limit speed and the same main effective pressure in the first correspondence relationship based on the difference between the determined straightness and the reference straightness, thereby changing the value of the primary pressure corresponding to the same upper limit speed and the same main effective pressure in the first correspondence relationship. The control method according to this embodiment changes the value of the primary pressure corresponding to the same upper limit speed and the same main effective pressure in the second correspondence relationship based on the difference between the determined straightness and the reference straightness. That is, the control method according to this embodiment corrects the first and second correspondence relationships read from the first reference information 10r1 and the second reference information 10r2 in accordance with the straightness, as in the above-described algorithm. Then, the primary pressure is calculated based on the corrected first and second correspondence relationships in the same manner as in the first embodiment. <Actions and Effects of the Second Embodiment>

[0076] In the work vehicle 1 or control method according to the second embodiment, hydraulic sensors SP11 to SP14 detect the secondary traveling pressure, and processor 10a calculates the straightness from the secondary traveling pressure, and changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same main effective pressure in the first and second correspondence relationships based on the difference between the calculated straightness and the reference straightness. Therefore, when the straightness is small, at which point the work vehicle 1 experiences greater running resistance from the ground, the primary traveling pressure can be increased, allowing the work vehicle 1 to turn at an upper limit speed close to the speed desired by the operator. <Third embodiment>

[0077] In the first and second embodiments, the travel speed of the work vehicle 1 is controlled without using the rotational speed of the output of the hydraulic motors (31L, 31R), but by feeding back the rotational speed of the output of the hydraulic motors (31L, 31R), it is possible to make the work vehicle 1 travel at an upper limit speed that is closer to the speed desired by the operator. Creep mode is used to enable the user to operate the lever at an easy-to-operate position (G4 to G5 in FIG. 5) when moving the work vehicle 1 in a speed range where delicate operation is required in normal mode (for example, lever operation positions G1 to G4 in FIG. 5). Therefore, the following control is performed on the assumption that the secondary traveling pressure is equal to the primary traveling pressure.

[0078] When the rotation speeds of the outputs of the hydraulic motors (31L, 31R) are detected by the rotation sensors SR31L and SR31R, the actual vehicle speed of the work vehicle 1 can be calculated from the reduction ratio of the reducer connected to the first hydraulic motor 31L, the reduction ratio of the reducer connected to the second hydraulic motor 31R, and the shape of the traveling device 3. When this actual vehicle speed is equal to the upper speed limit set by the creep setting member 16, the primary traveling pressure is determined using the correspondence relationship marked with a circle in FIGS. 11 and 12. The lower the actual vehicle speed is compared to the upper speed limit, the higher the primary traveling pressure is determined using the correspondence relationship (marked with a diamond) in FIGS. 11 and 12. Conversely, the higher the actual vehicle speed is compared to the upper speed limit, the lower the primary traveling pressure is determined using the correspondence relationship (marked with a triangle) in FIGS. 11 and 12.

[0079] FIG. 14 is a flowchart showing the operation of the work vehicle 1 according to the third embodiment. Here, the same operations as in the first embodiment are assigned the same reference numerals, and detailed description will be omitted. In the work vehicle 1 according to this embodiment, after step S4, in step S42, the rotation speed of the output of the hydraulic motors (31L, 31R) is detected from the rotation sensors SR31L and SR31R. The processor 10a obtains the rotation speed of the output of the hydraulic motors (31L, 31R) from the rotation sensors SR31L and SR31R, and calculates the actual vehicle speed. In other words, the control method according to this embodiment includes calculating the actual vehicle speed from the rotation speed in step S42.

[0080] Next, in S52, which replaces step S5 in the first embodiment, the processor 10a changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same main effective pressure in the first correspondence relationship based on the difference between the calculated actual vehicle speed and the upper limit speed. The processor 10a changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same main effective pressure in the second correspondence relationship based on the difference between the calculated actual vehicle speed and the upper limit speed. That is, the processor 10a corrects the first and second correspondence relationships read from the first reference information 10r1 and the second reference information 10r2 according to the difference between the actual vehicle speed and the upper limit speed, as in the above-described algorithm. Then, the processor 10a calculates the primary traveling pressure based on the corrected first and second correspondence relationships in the same manner as in the first embodiment.

[0081] That is, the control method according to this embodiment changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same main effective pressure in the first correspondence relationship based on the difference between the calculated actual vehicle speed and the upper limit speed. The control method according to this embodiment changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same main effective pressure in the second correspondence relationship based on the difference between the calculated actual vehicle speed and the upper limit speed. That is, the control method according to this embodiment corrects the first correspondence relationship and the second correspondence relationship read from the first reference information 10r1 and the second reference information 10r2 according to the difference between the actual vehicle speed and the upper limit speed, as in the above-described algorithm. Then, the primary traveling pressure is calculated based on the corrected first correspondence relationship and the second correspondence relationship in the same manner as in the first embodiment. <Actions and Effects of the Third Embodiment>

[0082] In the work vehicle 1 or control method for the work vehicle 1 according to the third embodiment, the rotation sensors SR31L and SR31R detect the rotational speed of the output of the hydraulic motors (31L, 31R), and the processor 10a calculates the actual vehicle speed from the rotational speed, and changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same hydraulic pressure in the first correspondence relationship and the second correspondence relationship based on the deviation between the calculated actual vehicle speed and the upper limit speed set by the creep setting member 16. Therefore, by feeding back the rotational speed of the output of the hydraulic motors (31L, 31R), the work vehicle 1 can turn at an upper limit speed close to the speed desired by the operator. <Modification>

[0083] In a work vehicle 1 that does not include the second embodiment, the hydraulic sensors SP11, SP12, SP13, and SP14 may be omitted. Furthermore, in the second embodiment, the straightness may be calculated from the output values ​​of the hydraulic sensors SP5L, SP6L, SP5R, and SP6R. In a work vehicle 1 that does not include the third embodiment, the rotation sensors SR31L and SR31R may be omitted.

[0084] Although the above embodiment shows a case where two pieces of reference information, the first reference information 10r1 and the second reference information 10r2, the control may be performed as described above by including a third reference information corresponding to a different engine rotation speed. Also, the second reference information 10r2 may be omitted.

[0085] 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 controlled by controlling the primary pilot pressure input to the operating device 56, but may also be controlled directly by controlling the secondary pilot pressure output from the operating device 56 according to the movement state.

[0086] 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.

[0087] 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.

[0088] The terms "member," "part," "element," "body," and "structure" may have multiple meanings, such as a single part or multiple parts.

[0089] 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."

[0090] 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."

[0091] 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.

[0092] 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. In a work vehicle that can be switched between a normal mode in which the speed of the work vehicle is changed in accordance with the amount of operation of at least one operating device to which a user's speed change operation is input, and a creep mode in which the work vehicle is caused to travel at or below an upper limit speed that is lower than the speed in the normal mode when the amount of operation becomes greater than a predetermined magnitude, first reference information is prepared that represents a first correspondence relationship between the upper limit speed, a hydraulic pressure in an oil line between a hydraulic motor for traveling the work vehicle and a hydraulic pump for traveling the work vehicle, and a primary traveling pressure of pilot oil that is input to an operating valve operated by a first operating device of the at least one operating device, corresponding to the upper limit speed and the hydraulic pressure; The upper limit speed input in the creep mode is acquired, Detecting the oil pressure; A primary traveling pressure corresponding to the acquired upper limit speed and the detected hydraulic pressure is obtained from the first reference information; Controlling a control valve that sends the pilot oil to the operation valve so as to achieve the determined primary traveling pressure. A method for controlling a work vehicle.

2. Further preparing second reference information representing a second correspondence relationship between the upper limit speed, the hydraulic pressure, and the primary traveling pressure, the second correspondence relationship corresponding to a second rotation speed lower than a first rotation speed of the engine of the work vehicle to which the first correspondence relationship corresponds; Detecting the rotational speed of the engine; determining the primary traveling pressure from the first reference information and the second reference information based on the detected rotation speed, the first correspondence relationship, and the second correspondence relationship; The control method according to claim 1 .

3. Further, third reference information is provided that indicates a third correspondence relationship between the rotation speed of the engine in the normal mode and the primary traveling pressure corresponding to the rotation speed of the engine; Detecting the rotational speed of the engine; When the normal mode is selected from the creep mode and the normal mode, the primary pressure corresponding to the detected rotation speed of the engine is obtained from the third reference information. The control method according to claim 1 .

4. In the normal mode, the work vehicle can be caused to travel at a speed greater than the upper limit speed. The control method according to claim 3 .

5. converting the primary traveling pressure into a secondary traveling pressure based on a first operation amount of the first operation device by the operation valve; 5. The control method according to claim 1, further comprising applying the secondary travel pressure of the pilot oil to a port that provides hydraulic pressure to a swash plate of the hydraulic pump.

6. The control method according to claim 5 , wherein when the first manipulated variable is equal to or larger than a threshold variable, the operating valve converts the secondary traveling pressure to be equal to the primary traveling pressure.

7. Detecting the secondary pressure while traveling, A straightness corresponding to a turning radius of the work vehicle is calculated based on the secondary traveling pressure; changing a value of the primary traveling pressure corresponding to the same upper limit speed and the same hydraulic pressure in the first correspondence relationship based on a deviation between the straightness and a reference straightness; The control method according to claim 5.

8. detecting a rotation speed of the hydraulic motor by a rotation sensor connected to a rotary shaft of the hydraulic motor; An actual vehicle speed of the work vehicle is calculated from the rotational speed; changing a value of the primary traveling pressure corresponding to the same upper limit speed and the same hydraulic pressure in the first correspondence relationship based on a difference between the actual vehicle speed and the upper limit speed; A control method according to any one of claims 1 to 4.

9. a hydraulic motor configured to drive the travel gear; a hydraulic pump configured to discharge hydraulic fluid to drive the hydraulic motor; an oil passage connecting the hydraulic pump and the hydraulic motor; a hydraulic pressure sensor configured to detect the hydraulic pressure of the hydraulic oil in the oil passage; at least one operation device for inputting a speed change operation by a user; a pilot pump configured to discharge pilot oil; an operating valve configured to convert the pressure of the pilot oil from a traveling primary pressure to a traveling secondary pressure in accordance with a first operation amount of a first operating device among the at least one operating device, and output the pilot oil; a control valve provided between the pilot pump and the operation valve and configured to convert the pressure of the pilot oil supplied to the operation valve into the traveling primary pressure; an input device that is switchable between a normal mode in which the speed of the work vehicle is changed in accordance with the amount of operation of the at least one operating device, and a creep mode in which the work vehicle is caused to travel at an upper limit speed that is lower than the speed in the normal mode when the amount of operation becomes greater than a predetermined magnitude, and that is configured to receive the upper limit speed; a memory that stores first reference information that indicates a first correspondence relationship between the upper limit speed, the hydraulic pressure of the hydraulic oil, and the primary traveling pressure corresponding to the upper limit speed and the hydraulic pressure; an electronic circuit configured to control the control valve; Equipped with The electronic circuit acquiring the upper limit speed from the input device in the creep mode; determining, from the first reference information, the primary traveling pressure corresponding to the hydraulic pressure obtained from the hydraulic pressure sensor and the acquired upper limit speed; controlling the control valve so as to achieve the determined primary traveling pressure; A work vehicle configured as follows.

10. a speed sensor configured to detect a rotational speed of the engine; the memory further stores second reference information representing a second correspondence relationship between the upper limit speed, the hydraulic pressure, and the primary traveling pressure, the second correspondence relationship corresponding to a second rotation speed lower than the first rotation speed of the engine to which the first correspondence relationship corresponds; 10. The work vehicle according to claim 9, wherein the electronic circuit is configured to determine the primary traveling pressure from the first reference information and the second reference information based on the detected rotational speed, the first correspondence relationship, and the second correspondence relationship.

11. a speed sensor configured to detect a rotational speed of the engine; the memory further stores third reference information representing a third correspondence relationship between the detected rotation speed and the primary traveling pressure in the normal mode; 10. The work vehicle according to claim 9, wherein the electronic circuit is configured to determine the primary traveling pressure corresponding to the detected rotational speed from the third reference information when the normal mode is selected by the input device.

12. The operation valve converts the primary traveling pressure into a secondary traveling pressure based on the first operation amount, The work vehicle according to any one of claims 9 to 11, wherein the pilot oil of the secondary travel pressure is applied to a port that provides hydraulic pressure to a swash plate of the hydraulic pump.

13. The work vehicle according to claim 12 , wherein when the first operation amount is equal to or greater than a threshold amount, the operation valve converts the secondary traveling pressure to be equal to the primary traveling pressure.

14. Further provided is an additional pressure sensor for detecting the secondary pressure while traveling, the electronic circuit determines a straightness degree corresponding to a turning radius of the work vehicle based on the secondary traveling pressure detected by the additional pressure sensor, and changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same hydraulic pressure in the first correspondence relationship based on a deviation between the straightness degree and a reference straightness degree. The work vehicle according to claim 12.

15. a rotation sensor connected to a rotation shaft of the hydraulic motor and configured to detect a rotation speed of the hydraulic motor; the electronic circuit determines an actual vehicle speed of the work vehicle from the rotational speed, and changes the value of the primary traveling pressure corresponding to the same upper limit speed and the same hydraulic pressure in the first correspondence relationship based on a difference between the actual vehicle speed and the upper limit speed. A work vehicle according to any one of claims 9 to 11.

16. In the work vehicle, which is switchable between a normal mode in which the speed of the work vehicle is changed in accordance with the amount of operation of at least one operating device to which a user's speed change operation is input, and a creep mode in which the work vehicle travels at or below an upper limit speed that is lower than the speed in the normal mode when the amount of operation becomes greater than a predetermined magnitude, a memory that stores first reference information that represents a first correspondence relationship between the upper limit speed, a hydraulic pressure in an oil line between a hydraulic motor for traveling the work vehicle and a hydraulic pump for traveling the work vehicle, and a primary traveling pressure of pilot oil that is input to an operating valve operated by a first operating device of the at least one operating device, corresponding to the upper limit speed and the hydraulic pressure; an electronic circuit configured to control a control valve that sends the pilot oil to the operating valve; Equipped with The electronic circuit The upper limit speed input in the creep mode is acquired, Obtaining the hydraulic pressure; determining a primary traveling pressure corresponding to the acquired upper limit speed and the acquired hydraulic pressure from the first reference information; controlling the control valve so as to achieve the determined primary traveling pressure; It is configured as follows: Control device for work vehicle.

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