Work vehicle and method for controlling the speed of a work vehicle

Abstract

To provide a work vehicle that improves usability of a user by controlling the vehicle speed to a desirable vehicle speed even if loads on a traveling device are increased.SOLUTION: A speed control method of a work vehicle includes sending hydraulic fluid from a first hydraulic pump to a first hydraulic motor that drives a first traveling device provided in the vehicle body. The method includes detecting a first differential pressure across the first hydraulic motor. The method includes controlling at least one of the first pump pilot pressure applied to the first pump pilot port of the first hydraulic pump and the rotation speed of the engine for driving the first hydraulic pump according to the absolute value of the first differential pressure to maintain the vehicle speed at a predetermined target speed.SELECTED DRAWING: Figure 7

Description

[Technical Field]

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

[0002] Patent Document 1 discloses a technique for limiting the primary pilot pressure in the oil passage between a pilot pump and an operating valve operated by a travel lever when limiting the travel speed of a work vehicle. Patent Document 2 discloses a technique for limiting the secondary pilot pressure in the oil passage between an operating valve operated by a travel lever further down the line and a hydraulic pump for travel when limiting the travel speed of a work vehicle. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2017-053413 [Patent Document 2] Patent No. 6695791 [Overview of the project] [Problems that the invention aims to solve]

[0004] In both Patent Document 1 and Patent Document 2, the swash plate angle of the hydraulic pump for driving is controlled by a pilot pressure set by a separate setting member. However, when the load on the driving device increases, the load pushes back the swash plate angle, and even if the accelerator lever, indoor dial, or operating lever is operated to its full stroke, the speed set by the setting member cannot be reached. This creates a problem that reduces the user experience. [Means for solving the problem]

[0005] A method for controlling the speed of a work vehicle according to a first aspect of the present disclosure includes supplying hydraulic fluid from a first hydraulic pump to a first hydraulic motor that drives a first running gear provided on the vehicle body. The method includes detecting a first differential pressure of the first hydraulic motor. The method includes controlling at least one of the following, according to the absolute value of the first differential pressure: a first pump pilot pressure applied to a first pump pilot port of the first hydraulic pump, and the rotational speed of an engine for driving the first hydraulic pump, so that the vehicle speed maintains a predetermined target speed.

[0006] A work vehicle according to a second aspect of this disclosure comprises a vehicle body, a first running gear, a first hydraulic motor, a first oil passage and a second oil passage, a first hydraulic sensor, a second hydraulic sensor, a pilot pump, an engine, and a controller. The first running gear is provided on the vehicle body. The first hydraulic motor is configured to drive the first running gear. The first hydraulic pump has a first pump pilot port and is configured to supply hydraulic fluid to the first hydraulic motor in accordance with the first pump pilot pressure applied to the first pump pilot port. The first oil passage and the second oil passage connect the first hydraulic pump and the first hydraulic motor, and hydraulic fluid is delivered through the first oil passage and the second oil passage. The first hydraulic sensor is configured to detect the first hydraulic pressure in the first oil passage. The second hydraulic sensor is configured to detect the second hydraulic pressure in the second oil passage. The pilot pump is configured to supply pilot oil to the first pump pilot port. The engine is configured to drive the first hydraulic pump and the pilot pump. The controller is configured to determine the absolute value of the first differential pressure, which is the difference between the first hydraulic pressure and the second hydraulic pressure, and to control at least one of the first pump pilot pressure and the engine rotational speed according to the absolute value of the first differential pressure, so that the vehicle speed maintains a predetermined target speed. [Effects of the Invention]

[0007] According to the technology disclosed herein, the first pump pilot pressure or the engine speed is determined according to the absolute value of the first differential pressure by utilizing the fact that fluctuations in the hydraulic pressure in the oil passage between the hydraulic motor and the hydraulic pump occur faster than the deviation of the actual rotational speed of the hydraulic motor for driving from the target rotational speed. Therefore, even if the load on the driving system increases, it is possible to provide a work vehicle that improves the user experience by allowing quick control to the desired vehicle speed. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a side view of the work vehicle. [Figure 2] Figure 2 is a top view of the work vehicle. [Figure 3] Figure 3 is a hydraulic circuit diagram of the travel system of the work vehicle in the first embodiment. [Figure 4] Figure 4 shows the relationship between engine rotational speed, primary pilot pressure, and the set line. [Figure 5] Figure 5 shows the relationship between the operating position of the control lever and the secondary pilot pressure. [Figure 6] Figure 6 is a block diagram of the work vehicles. [Figure 7] Figure 7 shows an example of the third reference information in the first and second embodiments. [Figure 8] Figure 8 is a flowchart showing the operation of the work vehicle according to the first embodiment. [Figure 9] Figure 9 is a hydraulic circuit diagram of the travel system of the work vehicle in the second embodiment. [Figure 10] Figure 10 is a flowchart showing the operation of the work vehicle according to the second embodiment. [Figure 11] Figure 11 is a hydraulic circuit diagram of the travel system of a work vehicle in a modified example of the second embodiment. [Figure 12] Figure 12 shows an example of the third reference information in the third embodiment. [Figure 13] Figure 13 shows an example of the fourth reference information in the third embodiment. [Figure 14A]FIG. 14A is a flowchart showing the operation of the work vehicle according to the third embodiment. [Figure 14B] FIG. 14B is a flowchart showing the operation of the work vehicle according to the third embodiment. [Figure 15] FIG. 15 is a hydraulic circuit diagram of the traveling system of the work vehicle according to the fourth embodiment. [Figure 16] FIG. 16 shows an example of the third reference information in the fourth embodiment. [Figure 17] FIG. 17 is a flowchart showing the operation of the work vehicle according to the fourth embodiment.

[0009] Hereinafter, the present invention will be specifically described based on the drawings showing its embodiments. In the drawings, the same reference numerals indicate corresponding or substantially identical configurations. <First Embodiment> <Overall Configuration>

[0010] Referring to FIGS. 1 and 2, a work vehicle 1, for example, a compact track loader, includes a vehicle body 2, a pair of traveling devices 3, and a working device 4. The vehicle body 2 supports the traveling device 3 and the working device 4. In the illustrated embodiment, the traveling device 3 is a crawler-type traveling device provided on the vehicle body 2. For this reason, each of the pair of traveling devices 3 includes a drive wheel 31 driven by a hydraulic motor device 30, driven wheels 32 and 33, and idler wheels 34. However, each of the pair of traveling devices 3 is not limited to a crawler-type traveling device. Each of the pair of traveling devices 3 may be, for example, a front-wheel / rear-wheel traveling device or a traveling device having a front wheel and a rear crawler. The working device 4 includes a tool (work equipment) (bucket) 41 at the distal end of the working device 4. The proximal end of the working device 4 is attached to the rear part of the vehicle body 2. The working device 4 includes a pair of arm assemblies 42 for rotatably supporting the bucket 41 via a bucket pivot shaft 43. Each of the pair of arm assemblies 42 includes a link 44 and an arm 45.

[0011] Link 44 is rotatable with respect to the vehicle body 2 around a fulcrum shaft 46. Arm 45 is rotatable with respect to link 44 around a joint shaft 47. The working device 4 further includes a plurality of arm cylinders 48 and at least one equipment cylinder 49. Each of the plurality of arm cylinders 48 is rotatably connected to the vehicle body 2 and the arm 45, and moves the link 44, the arm 45, etc. to raise and lower the bucket 41. At least one equipment cylinder 49 is configured to tilt the bucket 41. The vehicle body 2 includes a cabin 5. The cabin 5 has an openable and closable front window 51, and its outer shape is defined by a cab frame 53. The front window 51 may be omitted. The work vehicle 1 includes a driver's seat 54 and an operation lever 55 in the cabin 5. The cab frame 53 is rotatable around rotational shafts RSL and RSR on the vehicle body 2 as shown in FIG. 2. In FIGS. 1 and 2, a common pivot A defined by the rotational shafts RSL and RSR XC is illustrated. That is, the cab frame 53 is rotatably attached to the vehicle body 2 around the pivot A XC around.

[0012] In the embodiment according to the present application, the front-rear direction D FB (front direction D F / rear direction D B ) means the front-rear direction (front direction / rear direction) as seen from an operator seated in the driver's seat 54 of the cabin 5. The left direction D L , the right direction D R , and the width direction D W mean the left direction, the right direction, and the left-right direction, respectively, as seen from the operator. The upward direction D U , the downward direction D D , and the height direction D H mean the upward direction, the downward direction, and the height direction, respectively, as seen from the operator. The front-rear / left-right (width) / up-down (height) direction of the work vehicle 1 shall coincide with the front-rear / left-right (width) / up-down (height) direction as seen from the operator, respectively.

[0013] Figure 1 shows the left side of the work vehicle 1. As shown in Figure 2, the vehicle body 2 is generally symmetrical with respect to the central surface M of the vehicle body and includes a first side 2L, which is the left side, and a second side 2R, which is the right side. Of the pair of running gears 3, the running gear 3 provided on the first side 2L is shown as the left running gear 3L, and the running gear 3 provided on the second side 2R is shown as the right running gear 3R. Of the pair of arm assemblies 42, the arm assembly 42 provided on the left side with respect to the central surface M of the vehicle body is shown as the first arm assembly 42L, and the arm assembly 42 provided on the right side with respect to the central surface M of the vehicle body is shown as the second arm assembly 42R. The link 44 provided on the left side with respect to the central surface M of the vehicle body is shown as the first link 44L. The arm 45 provided on the left side with respect to the central surface M of the vehicle body is shown as the first arm 45L, and the arm 45 provided on the right side with respect to the central surface M of the vehicle body is shown as the second arm 45R. A pivot shaft 46 located on the left side of the vehicle body's central plane M is shown as the first pivot shaft 46L, and a pivot shaft 46 located on the right side of the vehicle body's central plane M is shown as the second pivot shaft 46R. A joint shaft 47 located on the left side of the vehicle body's central plane M is shown as the first joint shaft 47L, and a joint shaft 47 located on the right side of the vehicle body's central plane M is shown as the second joint shaft 47R. Of the hydraulic motor devices 30, the hydraulic motor device 30 located on the left side of the vehicle body's central plane M is shown as the left hydraulic motor device 30L, and the hydraulic motor device 30 located on the right side of the vehicle body's central plane M is shown as the right hydraulic motor device 30R.

[0014] Referring to Figures 1 and 2, the work vehicle 1 further comprises an engine 6 located at the rear of the vehicle body 2, and a plurality of hydraulic pumps 7, including a left hydraulic pump 7L and a right hydraulic pump 7R. The engine 6 drives the plurality of hydraulic pumps 7. The left hydraulic pump 7L and the right hydraulic pump 7R are configured to discharge hydraulic fluid to drive a hydraulic motor device 30 (etc.) that drives the drive wheels 31. The left hydraulic pump 7L and the right hydraulic pump 7R are collectively referred to as hydraulic pumps (7L, 7R). The plurality of hydraulic pumps 7 other than the left hydraulic pump 7L and the right hydraulic pump 7R are configured to discharge hydraulic fluid to drive hydraulic actuators (a plurality of arm cylinders 48, at least one tool cylinder 49, etc.) connected to the work device 4. The engine 6 is located in the width direction D of the work vehicle 1. W In this configuration, it is provided between a pair of arm assemblies 42. The work vehicle 1 further comprises a cover 8 for covering the engine 6. The work vehicle 1 further comprises a bonnet cover 9 provided at the rear end of the vehicle body 2. The bonnet cover 9 is openable and closable, allowing maintenance personnel to perform maintenance work on the engine 6 and other components.

[0015] Figure 3 is a hydraulic circuit diagram of the drive system of the work vehicle 1 in the first embodiment. 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 constant-capacity gear pump driven by the power of the engine 6. The pilot pump 71 is configured to discharge the hydraulic oil stored in the hydraulic oil tank 70. In particular, the pilot pump 71 is configured to discharge hydraulic oil mainly used for control. For convenience of explanation, the hydraulic oil discharged from the pilot pump 71 that is used for control will be called pilot oil, and the pressure of the pilot oil will be called pilot pressure. In particular, the pilot pump 71 is configured to supply pilot oil to the left hydraulic pump 7L and the right hydraulic pump 7R.

[0016] The hydraulic circuit 1A includes a pilot supply oil passage PA1 connected to the discharge port of the 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 switching valve SV2) and a plurality of brake mechanisms 72 connected to the pilot supply oil passage PA1. The brake switching valve SV1 is connected to the pilot supply oil passage PA1. The brake switching valve SV1 is a directional switching valve (solenoid valve) for braking 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 body to a first position VP1a or a second position VP1b by excitation. The switching of the valve body of the brake switching valve SV1 is performed by the brake pedal 13 (see Figure 6). A sensor 14 is provided on the brake pedal 13. The amount of operation detected by the sensor 14 is input to a controller 10 consisting of an ECU (Electric Control Unit). Controller 10 may also be referred to as a control device.

[0017] The multiple brake mechanisms 72 include a first brake mechanism 72L for braking the left running gear 3L and a second brake mechanism 72R for braking the right running gear 3R. The first brake mechanism 72L and the second brake mechanism 72R are connected to the brake switching valve SV1 via an oil passage PA2. The first brake mechanism 72L and the second brake mechanism 72R are configured to brake the running gear 3 in accordance with the pressure of the pilot oil (hydraulic fluid). When the valve body of the brake switching valve SV1 is switched to the first position VP1a, hydraulic fluid is released from the oil passage PA2 in the section between the brake switching valve SV1 and the brake mechanism 72, and the running gear 3 is braked by the brake mechanism 72. When the valve body of the brake switching valve SV1 is switched to the second position VP1b, the braking by the brake mechanism 72 is released. Furthermore, when the valve body of the brake switching valve SV1 is switched to the first position VP1a, the braking by the brake mechanism 72 is released, and when the valve body of the brake switching valve SV1 is switched to the second position VP1b, the running gear 3 may be braked by the brake mechanism 72.

[0018] The directional control valve SV2 is a solenoid valve that changes the rotation of the left hydraulic motor unit 30L and the right hydraulic motor unit 30R. The directional control valve SV2 is a two-position directional control valve configured to switch its valve body to a first position VP2a or a second position VP2b by excitation. The switching of the directional control valve SV2 is performed by an operating member or the like (not shown). Note that the directional control valve SV2 may be a proportional valve capable of adjusting the flow rate of the discharged hydraulic fluid instead of a two-position directional control valve.

[0019] The left hydraulic motor unit 30L is a device that transmits power to the drive wheel 31 provided on the left travel unit 3L. The left hydraulic motor unit 30L includes a left hydraulic motor 31L, a first swash plate switching cylinder 32L, and a first travel control valve (hydraulic switching valve) SV4. The left hydraulic motor 31L is a swash plate type variable displacement axial motor for driving the left travel unit 3L, and is a motor that can change the vehicle speed (rotation) to first or second speed. The first swash plate switching cylinder 32L is a cylinder configured to change the angle of the swash plate of the left hydraulic motor 31L by extending and retracting. The first travel control valve SV4 is a valve for extending and retracting the first swash plate switching cylinder 32L. The first travel control valve SV4 is a two-position switching valve configured to switch its valve body between a first position VP4a and a second position VP4b.

[0020] The first travel control valve SV4 is switched by the directional control valve SV2, which is located upstream and connected to the first travel control valve SV4. Specifically, the directional control valve SV2 and the first travel control valve SV4 are connected by an oil passage PA3, and the first travel control valve SV4 is switched by the hydraulic fluid flowing through the oil passage PA3. For example, when the valve body of the directional control valve SV2 is switched to the first position VP2a by operating the operating member, pilot oil is released in the section between the directional control valve SV2 and the first travel control valve SV4, and the valve body of the first travel control valve SV4 is switched to the first position VP4a. As a result, the first swash plate switching cylinder 32L retracts, and the speed of the left hydraulic motor 31L is changed to first gear. Furthermore, when the valve body of the directional control valve SV2 is switched to the second position VP2b by the operation of the operating member, pilot oil is supplied to the first travel control valve SV4 through the directional control valve SV2, and the valve body 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 left hydraulic motor 31L is changed to second speed.

[0021] The right hydraulic motor unit 30R is a device that transmits power to the drive wheel 31 provided on the right travel unit 3R. The right hydraulic motor unit 30R includes the right hydraulic motor 31R, the second swash plate switching cylinder 32R, and the second travel control valve (hydraulic switching valve) SV5. The right hydraulic motor unit 30R is a hydraulic motor for driving the right travel unit 3R and operates in the same way as the left hydraulic motor unit 30L. In other words, the right hydraulic motor 31R operates in the same way as the left hydraulic motor 31L. The left hydraulic motor 31L and the right hydraulic motor 31R are collectively referred to as the hydraulic motor (31L, 31R). The second swash plate switching cylinder 32R operates in the same way as the first swash plate switching cylinder 32L. The second travel control valve SV5 is a two-position switching valve configured to switch its valve body between the first position VP5a and the second position VP5b, and operates in the same way as the first travel control valve SV4.

[0022] The hydraulic circuit 1A is connected to a drain oil passage DR1. The drain oil passage DR1 is an oil passage that carries pilot oil from multiple 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 multiple switching valves (brake switching valve SV1, directional switching valve SV2). In other words, 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, the pilot oil in the oil passage PA3 is discharged to the drain oil passage DR1.

[0023] The hydraulic circuit 1A further includes a first charge oil passage PA4 and a hydraulic drive unit 75. The first charge oil passage PA4 branches off from the pilot supply oil passage PA1 and is connected to the hydraulic drive unit 75. The hydraulic drive unit 75 is a device that drives the left hydraulic motor unit 30L and the right hydraulic motor unit 30R. The hydraulic drive unit 75 has a first drive circuit 76L for driving the left hydraulic motor unit 30L and a second drive circuit 76R for driving the right hydraulic motor unit 30R.

[0024] The first drive circuit 76L includes a left 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 left hydraulic pump 7L and the left hydraulic motor 31L. The hydraulic circuit formed by the drive oil passages PA5L and PA6L is called the left 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 the drive oil passages PA5L and PA6L with hydraulic fluid from the pilot pump 71. The left 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. Through the first connection port 31P1, hydraulic fluid that rotates the left travel device 3L in the forward direction is input to the left hydraulic motor 31L, and hydraulic fluid that rotates the left travel device 3L in the reverse direction is discharged from the left hydraulic motor 31L through the first connection port 31P1. Through the second connection port 31P2, hydraulic fluid that rotates the left travel device 3L in the reverse direction is input to the left hydraulic motor 31L, and hydraulic fluid that rotates the left travel device 3L in the forward direction is discharged from the left travel device 3L.

[0025] Similarly, the second drive circuit 76R has a right 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 right hydraulic pump 7R and the right hydraulic motor 31R. The hydraulic circuit formed by the drive oil passages PA5R and PA6R is called the right 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 the drive oil passages PA5R and PA6R with hydraulic fluid from the pilot pump 71. The right hydraulic motor 31R has a third connection port 31P3 that connects to the drive oil passage PA5R and a fourth connection port 31P4 that connects to the drive oil passage PA6R. Through the third connection port 31P3, hydraulic fluid that rotates the right travel unit 3R in the forward direction is input to the right hydraulic motor 31R, and hydraulic fluid that rotates the right travel unit 3R in the reverse direction is discharged from the right hydraulic motor 31R through the third connection port 31P3. Through the fourth connection port 31P4, hydraulic fluid that rotates the right travel unit 3R in the reverse direction is input to the right hydraulic motor 31R, and hydraulic fluid that rotates the right travel unit 3R in the forward direction is discharged from the right travel unit 3R. In other words, the hydraulic motors (31L, 31R) are configured to drive the travel units (3L, 3R). The hydraulic pumps (7L, 7R) are configured to discharge the hydraulic fluid that drives 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).

[0026] The left hydraulic pump 7L and the right hydraulic pump 7R are swashplate type variable displacement axial pumps driven by the power of the engine 6. The left hydraulic pump 7L is connected to the left hydraulic motor 31L via the left hydraulic circuit CL and has a first port PLa and a second port PLb on which pilot pressure acts. The left hydraulic pump 7L is configured to change the angle of the swashplate in accordance with the pilot pressure acting on the first port PLa and the second port PLb, and to supply hydraulic fluid to the left hydraulic motor 31L. Specifically, the left hydraulic pump 7L is configured to supply hydraulic fluid to the left hydraulic motor 31L via the left hydraulic circuit CL to drive the left travel device 3L forward when the hydraulic pressure on the first port PLa is higher than the hydraulic pressure on the second port PLb, and to supply hydraulic fluid to the left hydraulic motor 31L via the left hydraulic circuit CL to drive the left travel device 3L backward when the hydraulic pressure on the second port PLb is higher than the hydraulic pressure on the first port PLa.

[0027] The right hydraulic pump 7R is connected to the right hydraulic motor 31R via the right hydraulic circuit CR and has a third port PRa and a fourth port PRb on which pilot pressure acts. The right hydraulic pump 7R is configured to change the angle of its swash plate in accordance with the pilot pressure acting on the third port PRa and the fourth port PRb, and to supply hydraulic fluid to the right hydraulic motor 31R. Specifically, the right hydraulic pump 7R is configured to supply hydraulic fluid to the right hydraulic motor 31R via the right hydraulic circuit CR to drive the right travel device 3R forward when the hydraulic pressure applied to the third port PRa is higher than the hydraulic pressure applied to the fourth port PRb, and to supply hydraulic fluid to the right hydraulic motor 31R via the right hydraulic circuit CR to drive the right travel device 3R backward when the hydraulic pressure applied to the fourth port PRb is higher than the hydraulic pressure applied to the third port PRa. The left hydraulic pump 7L and the right hydraulic pump 7R can change their output (amount of hydraulic fluid discharged) and the direction of hydraulic fluid discharge in accordance with the angle of their swash plates.

[0028] The output and hydraulic fluid discharge direction of the left hydraulic pump 7L and the right hydraulic pump 7R are changed by an operating device 56 for controlling the direction of travel of the work vehicle 1. Specifically, the output and hydraulic fluid discharge direction of the left hydraulic pump 7L and the right hydraulic pump 7R are changed in response to the operation of the operating lever 55 provided on the operating device 56. In other words, the operating device 56 is configured to control the direction of travel of the work vehicle by selecting at least one of the left travel device 3L and the right travel device 3R and instructing at least one of the travel devices to move forward or backward. The user inputs the direction of travel via the operating lever 55. The operating lever 55 may also be called a travel instruction input device.

[0029] As shown in Figure 3, the hydraulic circuit 1A includes a pilot supply oil passage PA8 that branches off from the pilot supply oil passage PA1 and is connected to the operating device 56, and a primary pressure control valve CV1 provided on the pilot supply oil passage PA8. In subsequent embodiments, the pilot supply oil passage PA1 and the pilot supply oil passage PA8 are collectively referred to as the primary pilot oil passage. The primary pressure control valve CV1 is an electromagnetic proportional valve including a solenoid and is configured to adjust the pilot pressure supplied to the operating device 56 by adjusting its opening degree according to the current applied to the solenoid. The opening degree of the primary pressure control valve CV1 is controlled by a current sent from the controller 10. Note that the pilot pressure output from the primary pressure control valve CV1 may increase as the magnitude of the current increases, or it may decrease as the magnitude of the current increases. In subsequent embodiments, the primary pressure control valve CV1 may also be referred to as the hydraulic adjustment mechanism. The detailed operation of the primary pressure control valve CV1 will be described later.

[0030] The operating device 56 includes an operating valve OVA for forward movement, an operating valve OVB for reverse movement, an operating valve OVC for right turn, an operating valve OVD for left turn, and an operating lever 55. The operating device 56 also has first to fourth shuttle valves SCa, 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 fluid in response to the operation of the operating lever 55, and supply the changed hydraulic fluid to the first port PLa and second port PLb of the left hydraulic pump 7L and the third port PRa and fourth port PRb of the right hydraulic pump 7R. In this embodiment, the operating valves OVA, OVB, OVC, and OVD are operated by a single operating lever 55, but there may be multiple operating levers 55. In subsequent embodiments, one or more operating levers 55 may be referred to as the first operating device.

[0031] The control valves OVA, OVB, OVC, and OVD have an input port (primary port), a discharge port, and an output port (secondary port). As shown in Figure 3, the input port is connected to the pilot supply oil passage PA8. The discharge port is connected to the drain oil passage DR2 leading to the hydraulic oil tank 70. The operating lever 55 can be tilted from the neutral position in the forward / backward direction, in the width direction perpendicular to the forward / backward direction, and in the diagonal direction. The control valves OVA, OVB, OVC, and OVD of the operating device 56 are operated according to the tilt of the operating lever 55. As a result, a pilot pressure corresponding to the amount of operation of the operating lever 55 from the neutral position is output from the secondary port of the control valves OVA, OVB, OVC, and OVD. The relationship between the pilot pressure applied to the primary port output from the primary pressure control valve CV1 and the pilot pressure applied to the secondary port will be described later.

[0032] The secondary port of control valve OVA and the secondary port of 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 left hydraulic pump 7L via the first pilot oil passage PA11. The secondary port of control valve OVA and the secondary port of 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 right hydraulic pump 7R via the third pilot oil passage PA13. The secondary port of control valve OVB and the secondary port of 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 left hydraulic pump 7L via the second pilot oil passage PA12. The secondary port of control valve OVB and the secondary port of 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 right hydraulic pump 7R via the fourth pilot oil passage PA14. In other words, pilot supply oil passage PA8, first pilot oil passage PA11, and fourth pilot oil passage PA14 connect pilot pump 71 to left hydraulic pump 7L. Pilot supply oil passage PA8, second pilot oil passage PA12, and third pilot oil passage PA13 connect pilot pump 71 to right hydraulic pump 7R.

[0033] When the operating lever 55 is tilted forward, the forward operating valve OVA is operated and pilot pressure is output from the operating valve OVA. This pilot pressure acts from the first shuttle valve SVa to the first port PLa via the first pilot oil passage PA11 connecting the operating device 56 and the first port PLa of the left hydraulic pump 7L, and also acts from the second shuttle valve SVb to the third port PRa via the third pilot oil passage PA13 connecting the operating device 56 and the third port PRa of the right hydraulic pump 7R. As a result, the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R rotate forward at a speed corresponding to the amount of tilt of the operating lever 55, causing the work vehicle 1 to move straight forward.

[0034] Furthermore, when the operating lever 55 is tilted to the rear, 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 left hydraulic pump 7L via the second pilot oil passage PA12 connecting the operating device 56 and the second port, and also acts from the fourth shuttle valve SVd to the fourth port PRb of the right hydraulic pump 7R via the fourth pilot oil passage PA14 connecting the operating device 56 and the fourth port PRb of the right hydraulic pump 7R. As a result, the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R reverse (rotate in reverse) at a speed corresponding to the amount of tilt of the operating lever 55, causing the work vehicle 1 to move straight backward.

[0035] Furthermore, when the operating lever 55 is tilted to the right, the operating valve OVC for right rotation is operated and pilot pressure is output from the operating valve OVC. This pilot pressure acts on the first port PLa of the left hydraulic pump 7L via the first pilot oil passage PA11 from the first shuttle valve SVa, and also acts on the fourth port PRb of the right hydraulic pump 7R via the fourth pilot oil passage PA14 from the fourth shuttle valve SVd. As a result, the system curves to the right with a degree of curvature corresponding to the rightward operating position of the operating lever 55.

[0036] Furthermore, when the operating lever 55 is tilted to the left, the operating valve OVD for left rotation is operated, and pilot pressure is output from the operating valve OVD. This pilot pressure acts on the third port PRa of the right hydraulic pump 7R via the third pilot oil passage PA13 from the second shuttle valve SVb, and also acts on the second port PLb of the left hydraulic pump 7L via the second pilot oil passage PA12 from the third shuttle valve SVc. As a result, the system curves to the left with a degree of curvature corresponding to the leftward operating position of the operating lever 55.

[0037] In other words, 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 and backward direction, and turns to the left with a curve corresponding to the operating position of the operating lever 55 to the left. When the operating lever 55 is tilted diagonally forward to the right, the work vehicle 1 moves forward and turns to the right at a speed corresponding to the operating position of the operating lever 55. When the operating lever 55 is tilted diagonally rear to the left, the work vehicle 1 moves backward and turns to the left at a speed corresponding to the operating position of the operating lever 55. When the operating lever 55 is tilted diagonally rear to the right, the work vehicle 1 moves backward and turns to the right at a speed corresponding to the operating position of the operating lever 55.

[0038] Next, the detailed operation of the primary pressure control valve CV1 will be described. The work vehicle 1 includes a setting member 11 (see Figure 6) for setting the target rotational speed of the engine 6. The setting member 11 is a speed input device separate from the operating device 56 described above, which is an accelerator pedal, a pivotably supported accelerator lever, or a rotatable indoor dial. A sensor 12 is provided on the setting member 11. The manipulated amount detected by the sensor 12 is input to the controller 10. The engine rotational speed corresponding to the manipulated amount detected by the sensor 12 is the target rotational speed of the engine 6. In other words, the target rotational speed of the engine 6 is set based on the manipulated amount of the setting member 11. The controller 10 outputs a rotation command to the injector indicating, for example, the fuel injection amount, injection timing, and fuel injection rate, so that the engine 6 reaches this determined target rotational speed. Alternatively, the controller 10 outputs a rotation command indicating the fuel injection pressure, etc., to the supply pump or common rail so that the engine 6 reaches this determined target rotational speed. In subsequent embodiments, the one or more operating levers 55 and 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 of decrease in the actual rotation speed from the target rotation speed when a load is applied to the engine 29 (the difference between the target rotation speed of the engine 6 and the actual rotation speed of the engine 6) is called the engine drop.

[0039] The primary pressure control valve CV1 can set the pilot pressure (primary pilot pressure) acting on the input ports (primary ports) of multiple operating valves OVA, OVB, OVC, and OVD based on the amount of drop ΔE1 of the engine speed (engine speed E1). In other words, the primary pressure control valve CV1 is installed between the pilot pump 71 and the operating valves OVA, OVB, OVC, and OVD, and is configured to supply pilot oil to the operating valves OVA, OVB, OVC, and OVD, and to convert the pressure of the pilot oil supplied to the operating valves OVA, OVB, OVC, and OVD into primary pilot pressure. The engine speed E1 can be detected by the speed sensor 6a. The engine speed E1 detected by the speed sensor 6a is input to the controller 10. The speed sensor 6a may also be called a speed sensor. Figure 4 shows the relationship between engine speed, primary pilot pressure, and set lines L1 and L2. The setting line L1 shows the relationship between engine speed E1 and primary pilot pressure when the decrease amount ΔE1 is less than a predetermined value (less than the anti-stall judgment value). The setting line L2 shows the relationship between engine speed E1 and primary pilot pressure when the decrease amount ΔE1 is equal to or greater than the anti-stall judgment value. When the difference between the rotational speed RS1 determined based on the operating amount of the setting member 11 and the actual rotational speed of the engine 6 is less than a predetermined stall judgment speed difference (anti-stall judgment value), the primary pilot pressure corresponding to rotational speed RS1 transitions according to the third correspondence shown in the setting line L1. When the difference between rotational speed RS1 and the actual rotational speed of the engine 6 is equal to or greater than a predetermined stall judgment speed difference (anti-stall judgment value), the primary pilot pressure corresponding to rotational speed RS1 transitions according to the fourth correspondence shown in the setting line L2.

[0040] If the decrease amount ΔE1 is less than the anti-stall judgment value, the controller 10 adjusts the opening degree of the primary pressure control valve CV1 so that the relationship between the engine speed E1 and the primary pilot pressure matches the reference pilot pressure indicated by the set line L1. If the decrease amount ΔE1 is greater than or equal to the anti-stall judgment value, the controller 10 adjusts the opening degree of the primary pressure control valve CV1 so that the relationship between the engine speed E1 and the primary pilot pressure matches the set line L2, which is lower than the reference pilot pressure. At the set line L2, the primary pilot pressure for a given engine speed E1 is lower than the primary pilot pressure at the set line L1. That is, when considering the same engine speed E1, the primary pilot pressure at the set line L2 is set lower than the primary pilot pressure at the set line L1. Therefore, control based on the set line L2 keeps the pressure (pilot pressure) of the hydraulic fluid entering the operating valves OVA, OVB, OVC, and OVD low. As a result, the swash plate angles of the left hydraulic pump 7L and the right hydraulic pump 7R are adjusted, reducing the load acting on the engine 6 and preventing the engine 6 from stalling. Although Figure 4 shows one setting line L2, there may be multiple setting lines L2. For example, a setting line L2 may be set for each engine rotational speed E1. Furthermore, it is preferable that the controller 10 has the data or control parameters such as functions that indicate the setting lines L1 and L2.

[0041] Next, we will explain the pilot pressure (secondary pilot pressure) output from the secondary ports of the control valves OVA, OVB, OVC, and OVD. Figure 5 shows the relationship between the operating position of the control lever and the secondary pilot pressure. Referring to Figure 4, the lever operating position starts at the neutral position (G0 position), where the origin is the starting position of the lever stroke, and approaches the end position (G5 position), which is the end position of the lever stroke, as it moves away from the origin. The operating range of the control lever 55 is divided into a neutral region RA1 where the object being operated does not move (in the example, from the G0 position to the G1 position), a near-full-operation region RA2 near the end of operation (in the example, from the G3 position to the G5 position), and an intermediate region RA3 between the neutral region RA1 and the near-full-operation region RA2 (in the example, from the G1 position to the G3 position). Furthermore, the intermediate region RA3 can be divided into the slow-velocity region RA3A from position G1 to position G2, and the intermediate-velocity region RA3B from position G2 to position G3.

[0042] In the neutral region RA1, operating the control lever 55 does not supply secondary pilot pressure. On the other hand, in the near-full-operation region RA2, the speed of the object being operated is not adjusted, and therefore the control lever 55 is operated all the way to the end position (G5 position) without stopping midway. In the intermediate region RA3, the control lever 55 can be stopped or its position changed at any point within the region to adjust the speed of the object being operated to the operator's desired speed. For example, the ratio of each operating region RA1, RA3A, RA3B, and RA2 to the lever stroke is as follows. Neutral zone RA1: 0% to less than 15% Low-velocity region RA3A: 15% to less than 45% Intermediate speed range RA3B: 45% or more and less than 75% Full operation range RA2: 75% to 100%

[0043] In the characteristic diagram shown in Figure 5, when the operating lever 55 is operated from the G0 position to the G1 position, a secondary pilot pressure (Pa) is generated. When the operating lever 55 is operated from the G1 position to the G4 position, the secondary pilot pressure rises from Pa to Pb in proportion to the amount the operating lever 55 is operated. At the G4 position, the primary pilot pressure is bypassed and flows to the secondary side, causing the secondary pilot pressure to rise rapidly from Pb to the maximum output pressure Pc. While the operating lever 55 is operated from the G4 position to the G5 position, the secondary pilot pressure remains constant at the maximum output pressure (Pc) and is equal to the primary pilot pressure. In other words, when the displacement of the operating lever 55 from the neutral position to instruct movement to the left is greater than or equal to the first displacement value (displacement from G0 to G4), the operating device 56 outputs the primary pilot pressure input to the operating device 56 to the first port PLa and the fourth port PRb. In the following embodiments, operating the operating lever 55 between the G4 position and the G5 position is referred to as operating the operating lever 55 with its full stroke. The operating device 56 outputs primary pilot pressure input to the operating device 56 to the second port PLb and the third port PRa when the displacement of the operating lever 55 for instructing movement to the right from the neutral position is greater than or equal to a first displacement value (displacement from G0 to G4). The operating device 56 outputs primary pilot pressure input to the operating device 56 to the first port PLa and the third port PRa when the displacement of the operating lever 55 for instructing movement forward from the neutral position is greater than or equal to a first displacement value (displacement from G0 to G4). The operating device 56 outputs primary pilot pressure input to the operating device 56 to the second port PLb and the fourth port PRb when the displacement of the operating lever 55 for instructing movement backward from the neutral position is greater than or equal to a first displacement value (displacement from G0 to G4). Note that the characteristic value of the secondary pilot pressure in the forward / backward direction may differ 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 and backward directions corresponding to G0 to G5 and Pa to Pc are G0' to G5' and Pa' to Pc', then 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 of the operating lever 55 for instructing forward movement from the neutral position is greater than or equal to the second displacement value (displacement from G0' to G4'). The operating device 56 may also output the primary pilot pressure input to the operating device 56 to the second port PLb and the fourth port PRb when the displacement of the operating lever 55 for instructing backward movement from the neutral position is greater than or equal to the second displacement value (displacement from G0' to G4'). Furthermore, Pa and Pb (Pa' and Pb') are values ​​that do not depend on the magnitude of the primary pilot pressure, but if the primary pilot pressure is lower than Pa or Pb (Pa' or Pb'), the secondary pilot pressure will cap out at the magnitude of the primary pilot pressure. In other words, the control valves (OVA, OVB, OVC, OVD) are configured to convert the pilot oil pressure from primary pilot pressure to secondary pilot pressure and output pilot oil according to the first operating amount (operating lever position) of the operating device 56. The pilot oil at secondary pilot pressure is applied to the ports (PLa, PRa, PLb, PRb) that supply hydraulic pressure to the swash plate of the hydraulic pump (7L, 7R). When the first operating amount is greater than or equal to a threshold amount (first displacement value), the control valves (OVA, OVB, OVC, OVD) convert the pressure to a secondary pilot pressure equal to the primary pilot pressure.

[0044] Based on the characteristics of the control valves OVA, OVB, OVC, and OVD described above, the movement of the work vehicle 1 corresponding to the operation of the control lever 55 will be explained 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 position of the control lever 55 in the rightward direction is operated from position G1 to position G3, the left hydraulic pump 7L rotates in the same direction as the right hydraulic pump 7R, with the magnitude of the rotational speed being greater than the magnitude of the rotational speed of the right hydraulic pump 7R, causing the work vehicle 1 to make a wide turn to the right. When the position of the control lever 55 in the rightward direction becomes the same as the position of operation in the forward / backward direction, the rotational speed of the right hydraulic pump 7R becomes 0, and only the left hydraulic pump 7L rotates, causing the work vehicle 1 to perform a right pivot turn. Furthermore, when the position of the control lever 55 in the rightward direction is operated between position G4 and position G5, it becomes greater than the position of operation in the forward / backward direction, causing the output shaft of the left hydraulic pump 7L to rotate forward and the output shaft of the right hydraulic pump 7R to rotate backward, causing the work vehicle 1 to turn to the right.

[0045] Furthermore, when the amount of operation of the operating lever 55 in the forward / backward direction is greater than the amount of operation in the leftward direction, and the operating position of the operating lever 55 in the leftward direction is operated from position G1 to position G3, the rotational speed of the right hydraulic pump 7R is greater than that of the left 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 operating position of the operating lever 55 in the leftward direction becomes the same as the operating position in the forward / backward direction, the rotational speed of the left hydraulic pump 7L becomes 0, and only the right hydraulic pump 7R rotates, causing the work vehicle 1 to perform a left pivot turn. Moreover, when the operating position of the operating lever 55 in the leftward direction is operated between position G4 and position G5, it becomes greater than the operating position in the forward / backward direction, causing the output shaft of the right hydraulic pump 7R to rotate forward and the output shaft of the left hydraulic pump 7L to rotate backward, causing the work vehicle 1 to turn to the left. In this embodiment, turning refers to the operation of the work vehicle 1 when the operating position to the right is operated between position G4 and position G5, or when the operating position to the left is operated between position G4 and position G5.

[0046] On the other hand, when the operating lever 55 is operated to a forward position between the G4 and G5 positions, it becomes larger than the operating position in the left-right direction, causing the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R to rotate forward and the work vehicle 1 to move forward at high speed. When the operating lever 55 is operated to a backward position between the G4 and G5 positions, it becomes larger than the operating position in the left-right direction, causing the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R to reverse and the work vehicle 1 to move backward at high speed. The operation of the other operating levers 55 in the forward and backward directions is the same as in the left-right direction.

[0047] The work vehicle 1 is equipped with various switches and sensors connected to the controller 10 described above. Figure 6 is a block diagram of the work vehicle 1. Referring to Figure 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 called an input device. The creep setting member 16 is composed of, for example, a touch panel, a sliding switch, or a dial. Creep refers to a control that drives the work vehicle 1 at or below the upper limit speed, regardless of the amount of operation of at least one operating device (setting member 11, one or more operating levers 55) to which the 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 the creep mode. The state other than the creep mode is called the normal mode.

[0048] In normal mode, the target rotational speed of the engine 6 is set by operating the setting member 11, and the primary pilot pressure corresponding to the target rotational speed is determined based on the setting line L1 or L2 in Figure 4. Then, the secondary pilot pressure is set based on the amount of operation of one or more operating levers 55, and the hydraulic motors (31L, 31R) and hydraulic pumps (7L, 7R) are controlled. In other words, in normal mode, the speed of the work vehicle 1 can be changed according to the amount of operation of at least one operating device, making it possible to run the work vehicle 1 at a speed greater than the upper limit speed. On the other hand, in creep mode, the setting line L1 or L2 in Figure 4 is not used to determine the primary pilot pressure, and instead, the first reference information 10r1, etc., described later is used to determine a value that is lower than the primary pilot pressure in normal mode. The settings from the secondary pilot pressure onwards in creep mode are the same as in normal mode, but since the secondary pilot pressure is less than or equal to the primary pilot pressure, the primary pilot pressure is limited, and the speed of the work vehicle 1 is limited to less than or equal to the upper limit speed regardless of the amount of operation of at least one operating device (setting member 11, one or more operating levers 55).

[0049] Referring to Figures 3 and 6, the work vehicle 1 includes a hydraulic sensor SP11 for detecting the hydraulic pressure of the first pilot oil passage PA11, a hydraulic sensor SP12 for detecting the hydraulic pressure of the second pilot oil passage PA12, a hydraulic sensor SP13 for detecting the hydraulic pressure of the third pilot oil passage PA13, and a hydraulic sensor SP14 for detecting the hydraulic pressure of the fourth pilot oil passage PA14. As described above, the secondary pilot pressure output from the secondary ports of the control valves OVA, OVB, OVC, and OVD changes in accordance with the operating position of the control lever 55. Therefore, hydraulic sensors SP11 to SP14 are sensors for detecting the secondary pilot pressure. Hydraulic sensors SP11 to SP14 may also be called additional hydraulic sensors.

[0050] The work vehicle 1 includes a hydraulic sensor SP5L for detecting the hydraulic pressure of the drive oil passage PA5L, a hydraulic sensor SP6L for detecting the hydraulic pressure of the drive oil passage PA6L, a hydraulic sensor SP5R for detecting the hydraulic pressure of the drive oil passage PA5R, and a hydraulic sensor SP6R for detecting the hydraulic pressure of the drive oil passage PA6R. In other words, the hydraulic sensors (SP5L, SP6L, SP5R, SP6R) are configured to detect the hydraulic pressure of the hydraulic fluid in the drive oil passages (PA5L, PA6L, PA5R, PA6R). The state of the left hydraulic motor 31L and the right hydraulic motor 31R can be detected from the pressure difference between hydraulic sensor SP5L and hydraulic sensor SP6L, and from the pressure difference between hydraulic sensor SP5R and hydraulic sensor SP6R.

[0051] Referring to Figures 2, 3, and 6, the work vehicle 1 may further include a rotational speed sensor SR31L connected to the rotation shaft of the left hydraulic motor 31L to detect the rotational speed of the left hydraulic motor 31L, and a rotational speed sensor SR31R to detect the rotational speed of the right hydraulic motor 31R. The state of the left hydraulic motor 31L and the right hydraulic motor 31R can be detected from the rotational direction and magnitude of rotational speed detected by the rotational speed sensor SR31L and the rotational direction and magnitude of rotational speed detected by the rotational speed sensor SR31R. The work vehicle 1 may also include an operation detection sensor 18 configured to detect the operating position of the operating lever 55. The operation detection sensor 18 is connected to a 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 operating lever 55. <Configuration of Controller 10>

[0052] The controller 10 has a processor 10a and a memory 10b as shown in Figure 6 in order to realize the control of the vehicle speed in the creep mode described above. The processor 10a may also be called an electronic circuit. The memory 10b includes volatile memory and non-volatile memory. The memory 10b includes at least a driving control program 10c1 for realizing the control described above, first reference information 10r1, second reference information 10r2, and third reference information 10r3.

[0053] The first reference information 10r1 represents the first correspondence between the rotational speed RS of the engine 6 detected by the speed sensor 6a and the primary pilot pressure in normal mode. In other words, the first reference information 10r1 represents the first correspondence represented by the setting line L1 in Figure 4. The second reference information 10r2 represents the second correspondence between the rotational speed RS of the engine 6 detected by the speed sensor 6a and the primary pilot pressure, which is used for controlling the primary pilot pressure when the drop amount of the engine 6 is large in normal mode. In other words, the second reference information 10r2 represents the second correspondence represented by the setting line L2 in Figure 4.

[0054] The third reference information 10r3 represents a third correspondence between the upper limit speed in creep mode, the absolute value of the first differential pressure, and the pressure output from the primary pressure control valve CV1, which determines the primary pilot pressure of the pilot oil input to the operating valves OVA, OVB, OVC, and OVD, corresponding to the upper limit speed and the absolute value of the first differential pressure. The first differential pressure is the differential pressure with the larger absolute value between the differential pressure between hydraulic sensors SP5L and SP6L, and the differential pressure between hydraulic sensors SP5R and SP6R. The third correspondence does not depend on the rotational speed of the engine 6 of the work vehicle 1.

[0055] Figure 7 shows an example of the first reference information 10r1. To clearly explain the third correspondence, Figure 7 shows the absolute value of the first differential pressure on the horizontal axis and the pressure output from the primary pressure control valve CV1 (output pressure) on the vertical axis. This output pressure corresponds to the primary pilot pressure to be controlled. In Figure 7, the relationship between the absolute value of the first differential pressure and the output pressure is shown as a line graph for the target rotational speeds x[rpm], y[rpm], and z[rpm] of the hydraulic motor corresponding to upper speed limits of 1 km / h, 8 km / h, and 15 km / h. However, the third correspondence may also include the relationship between the absolute value of the first differential pressure and the output pressure at other target rotational speeds of the motor. Here, when the first differential pressure is up to P0, the setting member 11 (for example, the accelerator pedal) is operated to be equal to or greater than the predetermined target rotational speed, and the output pressure determined to reach the upper speed limit of the creep mode when the secondary pilot pressure becomes equal to the primary pilot pressure and the running device 3 is unloaded is stored as the first reference information 10r1. When the first differential pressure is greater than P0, the output pressure increases as the target rotational speed increases. Specifically, the output pressure changes linearly with respect to the absolute value of the first differential pressure, such that the slope becomes steeper as the target rotational speed (upper speed limit) increases. Although the example in Figure 7 shows a linear change, other changes are also acceptable as long as they are monotonically increasing. The range below the lower limit and above the upper limit of the upper speed limit specified in Figure 7 is a speed range that cannot be set by the creep setting member 16. The 1 km / h specified as the lower limit and 15 km / h specified as the upper limit in Figure 7 are just examples, and other values ​​may be set. Between the upper and lower limits, output pressures corresponding to multiple speeds may be set. Output pressures corresponding to upper speed limits that are not set may be estimated by linear interpolation or other methods. If the upper speed limit set by the creep setting member 16 falls outside the range represented by the third correspondence, speed control in normal mode is performed. Furthermore, since the third correspondence is below the primary pilot pressure shown by the setting line L2 in Figure 4, it also has an anti-stall effect.

[0056] The processor 10a executes the following control while running the driving control program 10c1, referring to the first reference information 10r1, the second reference information 10r2, and the third reference information 10r3. First, when the normal mode is selected by the creep setting member 16, the processor 10a obtains the rotational speed RS of the engine 6 from the speed sensor 6a, determines the primary pilot pressure corresponding to the detected rotational speed RS of the engine 6 from the first reference information 10r1, and controls the primary pressure control valve CV1 to achieve the determined primary pilot pressure. When the engine drop is large while the normal mode is selected, the processor 10a determines the primary pilot pressure corresponding to the rotational speed RS of the engine 6 detected by the speed sensor 6a from the second reference information 10r2, and controls the primary pressure control valve CV1 to achieve the determined primary pilot pressure.

[0057] When the creep mode is selected by the creep setting member 16, the processor 10a determines the target rotational speed by obtaining the upper limit speed input by the creep setting member 16, determines the absolute value of the first differential pressure from the information obtained from the hydraulic sensors SP5L, SP6L, SP5R, and SP6R, extracts information for determining the primary pilot pressure from the third reference information 10r3, and determines the primary pilot pressure based on the extracted information. Then, the processor 10a controls the primary pressure control valve CV1 to achieve the determined primary pilot pressure. Once the primary pilot pressure is controlled, the upper limits of the ports (PLa, PRa, PLb, PRb) that supply hydraulic pressure to the swash plate of the hydraulic pumps (7L, 7R) are controlled.

[0058] In the following embodiments, the hydraulic motor with the larger differential pressure among the hydraulic motors (31L, 31R) will be referred to as the first hydraulic motor. Of the left travel device 3L and the right travel device 3R, the travel device driven by the first hydraulic motor will be referred to as the first travel device. Of the first swash plate switching cylinder 32L and the second swash plate switching cylinder 32R, the cylinder provided on the first hydraulic motor will be referred to as the first motor pilot port. The pilot pressure applied to the first motor pilot port will be referred to as the first motor pilot pressure. Of the hydraulic pumps (7L, 7R), the hydraulic pump that supplies hydraulic fluid to the first hydraulic motor will be referred to as the first hydraulic pump. Of the ports (PLa, PRa, PLb, PRb) of the first hydraulic pump, the port on which the pilot pressure input by the primary pilot pressure is limited will be referred to as the first pump pilot port. The pilot pressure applied to the first pump pilot port will be referred to as the first pump pilot pressure. Of the drive oil passages (PA5L, PA6L, PA5R, PA6R), one of the two oil passages connecting the first hydraulic motor and the first hydraulic pump is called the first oil passage, and the other is called the second oil passage. Hydraulic to the first hydraulic and second oil passages Hydraulic This is called the second hydraulic pressure. Of the hydraulic pressure sensors (SP5L, SP6L, SP5R, SP6R), the hydraulic pressure sensor configured to detect the first hydraulic pressure is called the first hydraulic pressure sensor, and the hydraulic pressure sensor configured to detect the second hydraulic pressure is called the second hydraulic pressure sensor. Of the rotational speed sensors (SR31L, SR31R), the rotational speed sensor configured to detect the rotational speed of the first hydraulic motor is called the first rotational speed sensor. Of the first to fourth pilot oil passages PA11 to PA14, the oil passage connecting the operating valves OVA, OVB, OVC, OVD to the first pump pilot port is called the secondary pilot oil passage.

[0059] Of the left running gear 3L and the right running gear 3R, the running gear located on the opposite side of the vehicle body 2 from the first hydraulic motor is called the second running gear. Of the hydraulic motors (31L, 31R), the hydraulic motor configured to drive the second running gear is called the second hydraulic motor. Of the first swash plate switching cylinder 32L and the second swash plate switching cylinder 32R, the cylinder located on the second hydraulic motor is called the second motor pilot port. The pilot pressure applied to the second motor pilot port is called the second motor pilot pressure. Of the hydraulic pumps (7L, 7R), the hydraulic pump that supplies hydraulic fluid to the second hydraulic motor is called the second hydraulic pump. Of the ports (PLa, PRa, PLb, PRb) of the second hydraulic pump, the port whose input pilot pressure is limited by the primary pilot pressure is called the second pump pilot port. The pilot pressure applied to the second pump pilot port is called the second pump pilot pressure. Of the drive oil passages (PA5L, PA6L, PA5R, PA6R), one of the two oil passages connecting the second hydraulic motor and the second hydraulic pump is called the third oil passage, and the other is called the fourth oil passage. Hydraulic to the third hydraulic and fourth oil passages Hydraulic This is called the fourth hydraulic pressure. Of the hydraulic pressure sensors (SP5L, SP6L, SP5R, SP6R), the hydraulic pressure sensor configured to detect the third hydraulic pressure is called the third hydraulic pressure sensor, and the hydraulic pressure sensor configured to detect the fourth hydraulic pressure is called the fourth hydraulic pressure sensor. Of the rotational speed sensors (SR31L, SR31R), the rotational speed sensor configured to detect the rotational speed of the second hydraulic motor is called the second rotational speed sensor. Of the first to fourth pilot oil passages PA11 to PA14, the oil passage connecting the operating valves OVA, OVB, OVC, OVD to the second pump pilot port is called the additional secondary pilot oil passage.

[0060] In the first embodiment, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 is configured to determine the absolute value of the first differential pressure, which is the difference between the first hydraulic pressure and the second hydraulic pressure, and to control the first pump pilot pressure and the second pump pilot pressure according to the absolute value of the first differential pressure so that the vehicle speed maintains a predetermined target speed (upper limit speed). Specifically, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 is configured to control the primary pilot pressure by controlling the primary pressure control valve CV1 so that the vehicle speed maintains a target speed. As shown in the correspondence in Figure 7, the controller 10 controls the output pressure output from the primary pressure control valve CV1, i.e., the primary pilot pressure, to increase as the absolute value of the first differential pressure increases. <Operation of the work vehicle according to the first embodiment>

[0061] Figure 8 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 S11 are executed at predetermined sampling intervals (for example, 20 μs). In step S1, the processor 10a rotates the engine 6 and sends hydraulic fluid from the first hydraulic pump to the first hydraulic motor that drives the first running device provided on the vehicle body 2. The processor 10a then acquires the rotational 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 rotational 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 the upper speed limit is set (Yes in step S2), the process proceeds from step S3 to S5. If normal mode is set, meaning no upper speed limit is set, or if an invalid upper speed limit is set that does not have the first or second correspondence (No in step S2), proceed from step S6 to S8.

[0062] 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, i.e., the target rotational speed of the first hydraulic motor. In other words, the control method according to this embodiment acquires the upper limit speed input by the creep setting member 16, i.e., the target rotational speed of the first hydraulic motor. In step S3, the processor 10a acquires the hydraulic pressure detected by hydraulic sensors SP5L, SP6L, SP5R, and SP6R, and calculates the first differential pressure from these. In other words, the control method according to this embodiment detects the first differential pressure, which is the larger of the differential pressures of the hydraulic motors (31L, 31R) for driving the work vehicle 1. It also detects the second differential pressure, which is the smaller of the differential pressures of the hydraulic motors (31L, 31R) for driving the work vehicle 1.

[0063] In step S5, the processor 10a refers to the third reference information 10r3 to determine the output pressure, i.e., the primary pilot pressure, that is, the output pressure from the primary pressure control valve CV1 corresponding to the target rotational speed and the absolute value of the first differential pressure. After the processing of step S5 is completed, the processing of step S9 is executed. In step S9, the processor 10a controls the primary pressure control valve CV1 that sends pilot oil to the operating valves OVA, OVB, OVC, and OVD so that the primary pilot pressure is the primary pilot pressure determined in step S6. In other words, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the processor 10a controls the first pump pilot pressure applied to the first pump pilot port and the second pump pilot port of the first hydraulic pump according to the absolute value of the first differential pressure so that the vehicle speed maintains a predetermined target speed. Controlling the first pump pilot pressure and the second pump pilot port involves controlling the primary pilot pressure, which is the hydraulic pressure of the primary pilot oil passage connecting a pilot pump for discharging pilot oil toward the first and second pump pilot ports, and an operating valve controlled in response to input to a travel instruction input device into which user instructions for the direction of travel are input. Specifically, the processor 10a controls the first pump pilot pressure and the second pump pilot port to increase as the absolute value of the first differential pressure increases.

[0064] In normal mode (No in step S2), in step S6, the processor 10a determines whether or not there is engine drop. That is, in step S6, the processor 10a determines whether or not the engine drop amount ΔE1 is greater than or equal to the anti-stall judgment value. If there is no engine drop (No in step S6), in step S7, the processor 10a determines the primary pilot pressure from the first reference information 10r1 based on the rotational speed RS of the engine 6. If there is engine drop (Yes in step S6), in step S8, the processor 10a determines the primary pilot pressure from the second reference information 10r2 based on the rotational speed RS of the engine 6. After the completion of the processing in step S7 or step S8, the processing in step S9 is executed.

[0065] In step S9, the processor 10a controls the primary pressure control valve CV1, which supplies pilot oil to the operating valves OVA, OVB, OVC, OVD, so that the primary pilot pressure is the primary pilot pressure determined in step S8 or step S9. In step S10, the operating valves OVA, OVB, OVC, OVD convert the primary pilot pressure to secondary pilot pressure based on the lever position (first operating amount) of the operating lever 55 (first operating device). In step S11, the secondary pilot pressure of the pilot oil is applied to the ports (PLa, PRa, PLb, PRb) that supply hydraulic pressure to the swash plate of the hydraulic pumps (7L, 7R), thereby controlling the hydraulic pumps (7L, 7R) and hydraulic motors (31L, 31R). <Operation and Effects of the First Embodiment>

[0066] In the control method or work vehicle 1 according to the first embodiment, the processor 10a acquires the upper limit speed (target rotational speed of the first motor) input by the creep setting member 16, acquires the absolute value of the first differential pressure, determines the output pressure (primary pilot pressure) output from the primary pressure control valve CV1 corresponding to the acquired target rotational speed and the absolute value of the first differential pressure from the third reference information 10r3, and controls the primary pressure control valve CV1 to send pilot oil to the operating valves OVA, OVB, OVC, and OVD so that the primary pilot pressure becomes the determined primary pilot pressure. By controlling the primary pilot pressure using fluctuations in the first differential pressure, it is possible to improve the user experience in creep mode. <Second Embodiment>

[0067] In the first embodiment, an example was given in which the primary pilot pressure is controlled to achieve creep mode, but the secondary pilot pressure may also be controlled. Figure 9 is a hydraulic circuit diagram of the running system of the work vehicle in the second embodiment. Figure 9 shows the configuration added to Figure 3. In Figure 9, the same reference numerals are used for the same components as in Figure 3, and detailed explanations are omitted. In the second embodiment, the work vehicle 1 includes a hydraulic circuit 1B. The hydraulic circuit 1B differs from the configuration of the hydraulic circuit 1A in that it further includes relief valves CV23, CV24, proportional valves CV21, CV22, discharge oil passages DR3 to DR6, check valves CK1 to CK4, and throttles TH1 to TH4.

[0068] Relief valves CV23 and CV24 are balanced relief valves whose set pressure for opening is variable based on the pressure of the pilot oil, and have control ports 23a and 24a that receive pressure from the pilot oil. Relief valves CV23 and CV24 are configured to open when the pressure applied to the input port is greater than the pressure applied to the control ports 23a and 24a. At this time, the pilot oil is discharged into the hydraulic oil tank 70. Proportional valves CV21 and CV22 are connected to hydraulic oil passages 21 and 22 which are connected to the control ports 23a and 24a, and pilot oil is supplied from the pilot pump 71. Proportional valves CV21 and CV22 are electromagnetic proportional valves whose opening degree can be changed by exciting a solenoid, and are controlled by the controller 10.

[0069] The proportional valves CV21 and CV22 are connected to the pilot supply oil passage PA1. In creep mode, the secondary pressure control valve CV2 is controlled so that the pressure is the primary pressure control valve CV1 in the first embodiment plus an offset α that takes into account the outflow of pilot oil from the relief valves CV23, CV24, etc. In normal mode, if anti-stall control is not in place, the secondary pressure control valve CV2 is operated so that the set line L1 is the value obtained by adding an offset α. Of the proportional valves CV21 and CV22, the proportional valve that controls the hydraulic pressure of the pilot oil in the secondary pilot oil passage may be called the secondary pressure control valve CV2, and the proportional valve that controls the pilot oil in the additional secondary pilot oil passage may be called the additional secondary pressure control valve ACV2. That is, the secondary pressure control valve CV2 controls the secondary pilot pressure, which is the hydraulic pressure of the pilot oil in the secondary pilot oil passage. The additional secondary pressure control valve ACV2 controls the additional secondary pilot pressure, which is the hydraulic pressure of the pilot oil in the additional secondary pilot oil passage. When the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 is configured to control the first pump pilot pressure corresponding to the secondary pilot pressure by controlling the secondary pressure control valve CV2, thereby controlling the vehicle speed to maintain the target speed. In other words, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 is configured to determine the absolute value of the first differential pressure, which is the difference between the first hydraulic pressure and the second hydraulic pressure, and to control the first pump pilot pressure according to the absolute value of the first differential pressure, so that the vehicle speed maintains a predetermined target speed (upper limit speed). The controller 10 controls the output pressure, i.e., the secondary pilot pressure, output from the secondary pressure control valve CV2, as the absolute value of the first differential pressure increases.When the target rotational speed of the engine 6 set by the setting member 11 is at a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 calculates the absolute value of the second differential pressure, which is the difference between the third hydraulic pressure and the fourth hydraulic pressure. If the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure, the controller 10 is configured to control the second pump pilot pressure according to the absolute value of the first differential pressure so that the vehicle speed maintains a predetermined target speed. Specifically, the controller 10 controls the second pump pilot pressure to increase as the absolute value of the first differential pressure increases.

[0070] Discharge oil passage DR3 is connected to the first pilot oil passage PA11. Discharge oil passage DR4 is connected to the second pilot oil passage PA12. Discharge oil passage DR5 is connected to the third pilot oil passage PA13. Discharge oil passage DR6 is connected to the fourth pilot oil passage PA14. Check valves CK1 to CK4 shut off discharge oil passages DR3 to DR6 if the pressure on the side with throttles TH1 to TH4 does not exceed the pressure on the side with relief valves CV23 and CV24.

[0071] In discharge oil passages DR3 and DR4, the pilot pressure increases when the left hydraulic pump 7L rotates forward and reverses, respectively. Therefore, when the pilot pressure on one side of either passage equals the primary pilot pressure, the other side becomes significantly smaller than the primary pilot pressure. In discharge oil passages DR5 and DR6, the pilot pressure increases when the right hydraulic pump 7R rotates forward and reverses, respectively. Therefore, when the pilot pressure on one side of either passage equals the primary pilot pressure, the other side becomes significantly smaller than the primary pilot pressure. When proportional valves CV21 and CV22 are controlled as shown in Figure 7, only one of check valves CK1 and CK2 will open. Therefore, the above control can be performed by controlling the pressure of proportional valves CV21 and CV22 so that the pressure is the sum of the pressure obtained by controlling the primary pressure control valve CV1 according to the first embodiment and the pressure loss due to the outflow of pilot oil from relief valves CV23 and CV24.

[0072] Throttle TH1 is provided in the first pilot oil passage PA11 between the first shuttle valve SVa and the discharge oil passage DR3, and is configured to reduce the flow rate of pilot oil in the first pilot oil passage PA11. Throttle TH2 is provided in the second pilot oil passage PA12 between the second shuttle valve SVb and the discharge oil passage DR4, and is configured to reduce the flow rate of pilot oil in the second pilot oil passage PA12. Throttle TH3 is provided in the third pilot oil passage PA13 between the third shuttle valve SVc and the discharge oil passage DR5, and is configured to reduce the flow rate of pilot oil in the third pilot oil passage PA13. Throttle TH4 is provided in the fourth pilot oil passage PA14 between the fourth shuttle valve SVd and the discharge oil passage DR6, and is configured to reduce the flow rate of pilot oil in the fourth pilot oil passage PA14.

[0073] Figure 10 is a flowchart showing the operation of the work vehicle 1 according to the second embodiment. In this flowchart, the processes from step S1 to step S11 are executed at predetermined sampling intervals (e.g., 20 μs). In Figure 10, the same processes as in Figure 8 are given the same step numbers and are therefore omitted from explanation. In normal mode (Yes at step S2), the processor 10a controls the proportional valves CV21 and CV22 so that the pressure applied to the relief valves CV23 and CV24 is higher than the pressure output by the primary pressure control valve CV1 as described above. As a result, the relief valves CV23 and CV24 are closed.

[0074] In creep mode (Yes in step S2), after step S4, in step S22, the processor 10a refers to the third reference information 10r3 to determine the output pressure, i.e., the secondary pilot pressure, that is, the output pressure from the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 (proportional valves CV21, CV22) corresponding to the target rotational speed and the absolute value of the first differential pressure. In step S23, the processor 10a controls the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 (proportional valves CV21, CV22) so that the pressure applied to the relief valves CV23, CV24 becomes the differential pressure of the secondary pilot pressure + check valves CK1 to CK4. In other words, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the processor 10a controls the first pump pilot pressure applied to the first pump pilot port of the first hydraulic pump in accordance with the absolute value of the first differential pressure, so that the vehicle speed maintains a predetermined target speed. Controlling the first pump pilot pressure includes controlling the secondary pilot pressure, which is the hydraulic pressure of the secondary pilot oil passage connecting the operating valve and the first pump pilot port. Specifically, the processor 10a controls the first pump pilot pressure to increase as the absolute value of the first differential pressure increases.

[0075] When the target rotational speed of the engine 6 set by the setting member 11 is at a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the processor 10a controls the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump according to the absolute value of the first differential pressure, so that the vehicle speed maintains a predetermined target speed, if the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure. Controlling the second pump pilot pressure includes controlling the additional secondary pilot pressure, which is the hydraulic pressure of the additional secondary pilot oil passage connecting the operating valve and the second pump pilot port. Specifically, the processor 10a controls the second pump pilot pressure to increase as the absolute value of the first differential pressure increases. After step S23, the process proceeds to step S6. <Operation and Effects of the Second Embodiment>

[0076] In the control method or work vehicle 1 according to the second embodiment, the processor 10a acquires the upper limit speed (target rotational speed of the first motor) input by the creep setting member 16, acquires the absolute value of the first differential pressure, and obtains the output pressure (secondary pilot pressure) output from the secondary pressure control valve CV2 and additional secondary pressure control valve ACV2 (proportional valves CV21, CV22) corresponding to the acquired target rotational speed and the absolute value of the first differential pressure from the third reference information 10r3, and controls the secondary pressure control valve CV2 and additional secondary pressure control valve ACV2 (proportional valves CV21, CV22) so that the secondary pilot pressure becomes the obtained secondary pilot pressure. By controlling the secondary pilot pressure using fluctuations in the first differential pressure, it is possible to improve the user experience in creep mode. <Modified form of the second embodiment>

[0077] Figure 11 is a hydraulic circuit diagram relating to a modified example in the second embodiment. In the example of Figure 11, shuttle valves SV12 and SV34 are provided instead of the check valves CK1 to CK4 in the example of Figure 9. Shuttle valve SV12 connects the oil passage with the higher oil pressure among the discharge oil passages DR3 and DR4 to the relief valve CV23. Shuttle valve SV34 connects the oil passage with the higher oil pressure among the discharge oil passages DR5 and DR6 to the relief valve CV24. The above control can be performed even with this hydraulic circuit configuration. In addition, the primary pressure control valve CV1 may be omitted in the circuit of Figure 9 or Figure 11. Furthermore, at least one of the combinations of a secondary pressure control valve CV2 and a balanced relief valve, and an additional secondary pressure control valve ACV2 and a balanced relief valve, may be implemented using electromagnetic proportional relief valves.

[0078] In the second embodiment, the processor 10a controls the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump according to the absolute value of the first differential pressure when the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure. However, when the difference between the absolute value of the first differential pressure and the absolute value of the second differential pressure is within a predetermined range, the processor 10a may control the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump according to the absolute value of the second differential pressure. In that case, the absolute value of the first differential pressure in Figure 7 should be read as the absolute value of the second differential pressure, and the processor 10a should be controlled so that the output pressure on the vertical axis is output from the additional secondary pressure control valve ACV2. In this way, when the absolute value of the first differential pressure is not significantly different from the absolute value of the second differential pressure, the left and right travel devices can be controlled separately to achieve operations such as turning that are close to the user's desire. <Third Embodiment>

[0079] In the first and second embodiments, examples were shown in which the controller 10 controls the first pump pilot pressure and the second pump pilot pressure according to the absolute value of the first differential pressure, but it is also possible to control not only these pilot pressures but also the engine rotational speed. In the third embodiment, the controller 10 obtains the target rotational speed set by the setting member 11, the first differential pressure, and the upper limit speed in creep mode described above. The controller 10 calculates the amount of speed increase to be increased from the target rotational speed based on the first differential pressure and the upper limit speed in creep mode described above. The controller 10 outputs a rotation command to the injector, supply pump, or common rail based on a modified target rotational speed obtained by adding the calculated rotational speed increase amount to the target rotational speed. In the third embodiment, the memory 10b further includes fourth reference information 10r4 representing a fourth correspondence between the target rotational speed of the first hydraulic motor corresponding to the upper limit speed in creep mode, the absolute value of the first differential pressure, and the rotational speed increase amount. When the work vehicle 1 of the third embodiment has the hydraulic circuit 1A of the first embodiment, the controller 10 limits the output pressure output from the primary pressure control valve CV1 in order to achieve the upper limit speed in creep mode. When the work vehicle 1 of the third embodiment has the hydraulic circuit 1B of the second embodiment, the controller 10 limits the output pressure output from the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 of the second embodiment in order to achieve the upper limit speed in creep mode.

[0080] Figure 12 shows an example of the third reference information 10r3 according to the third embodiment. As shown in Figure 12, in the third embodiment, the output pressure output from the control valve remains constant regardless of the magnitude of the first differential pressure, and is set to increase as the target rotational speed of the first hydraulic motor corresponding to the upper limit speed increases. This is the target secondary pilot pressure to be applied to the first pilot port of the first hydraulic motor according to the target rotational speed. This may be the output of the primary pressure control valve CV1 according to the first embodiment, or it may be the output of the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 according to the second embodiment. Figure 13 shows an example of the fourth reference information 10r4 according to the third embodiment. Referring to Figure 13, when the first differential pressure is up to P0, the target rotational speed of the engine 6 is the target rotational speed r0 of the engine 6 set by the setting member 11. When the first differential pressure is greater than P0, the target rotational speed of the engine 6 increases as the target rotational speed of the first hydraulic motor increases. Specifically, the target rotational speed of engine 6 changes linearly with respect to the absolute value of the first differential pressure, such that the slope becomes steeper as the target rotational speed (upper limit speed) of the first hydraulic motor increases. While the example in Figure 13 shows a linear change, other changes are acceptable as long as they are monotonically increasing. The range below the lower limit and above the upper limit of the upper speed specified in Figure 13 is a speed range that cannot be set by the creep setting member 16. The third reference information 10r3 is information such as a map set for each target rotational speed r0, or an algorithm for determining the target rotational speed of engine 6 for each target rotational speed r0 relative to the target rotational speed of the first hydraulic motor. The 1 km / h specified as the lower limit and 15 km / h specified as the upper limit in Figure 13 are examples, and other values ​​may be set. Multiple speeds may be set between the upper and lower limits. Target rotational speeds corresponding to upper speeds that are not set may be estimated by linear interpolation or other methods.

[0081] In this embodiment, the controller 10 is configured to increase the target rotational speed of the engine 6 to compensate for the speed reduction due to load, from the vehicle speed that can be reached under no load when the engine 6 rotates at the engine rotational speed set by the setting member 11. In other words, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 is configured to control the target rotational speed of the engine 6 by referring to the fourth reference information 10r4, thereby controlling the vehicle speed to maintain the target speed (upper limit speed). In other words, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 is configured to determine the absolute value of the first differential pressure, which is the difference between the first hydraulic pressure and the second hydraulic pressure, and to control the rotational speed of the engine 6 according to the absolute value of the first differential pressure, so that the vehicle speed maintains a predetermined target speed (upper limit speed). The controller 10 controls the rotational speed of the engine 6 to increase as the absolute value of the first differential pressure increases. Specifically, the controller 10 controls the rotational speed of the engine 6 to increase as the absolute value of the first differential pressure increases.

[0082] Figures 14A and 14B are flowcharts showing the operation of the work vehicle 1 according to the third embodiment. Figure 14A is a flowchart showing the operation of the work vehicle 1 according to the third embodiment, which has the hydraulic circuit 1A of the first embodiment. Figure 14B is a flowchart showing the operation of the work vehicle 1 according to the third embodiment, which has the hydraulic circuit 1B of the second embodiment. In Figure 14A, the same reference numerals are used for the same operations as in the first embodiment, and detailed explanations are omitted. In Figure 14B, the same reference numerals are used for the same operations as in the second embodiment, and detailed explanations are omitted.

[0083] Referring to Figure 14A, in step S5A, which replaces step S5 of the first embodiment, the processor 10a determines the primary pilot pressure by referring to the third reference information 10r3 as shown in Figure 12. Following step S5A, in step S31, the processor 10a determines the target rotational speed of the engine 6 corresponding to the target rotational speed of the first hydraulic motor and the absolute value of the first differential pressure by referring to the fourth reference information 10r4. However, step S5A may be omitted. Next, in step S32, the processor 10a controls the injector, supply pump, and common rail to increase the rotational speed of the engine 6 based on the determined target rotational speed.

[0084] In other words, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the processor 10a controls the rotational speed of the engine 6 for driving the first hydraulic pump in accordance with the absolute value of the first differential pressure, so that the vehicle speed maintains a predetermined target speed. Specifically, the processor 10a controls the rotational speed of the engine 6 to increase as the absolute value of the first differential pressure increases. Steps S32 onwards proceed to step S9.

[0085] Referring to Figure 14B, in step S22A, which replaces step S22 in the second embodiment, the processor 10a determines the secondary pilot pressure by referring to the third reference information 10r3 as shown in Figure 12. After step S23, steps S31 and S32 described above are executed. From step S32 onward, the process proceeds to step S9. However, steps S22A and S23 may be omitted. <Operation and Effects of the Third Embodiment>

[0086] In the control method or work vehicle 1 according to the third embodiment, the processor 10a acquires the upper limit speed (target rotational speed of the first motor) input by the creep setting member 16, acquires the absolute value of the first differential pressure, and controls the injector, supply pump, and common rail to achieve the target rotational speed of the engine 6, which is determined from the acquired absolute value of the first differential pressure. By controlling the target rotational speed of the engine 6 using fluctuations in the first differential pressure, it is possible to improve the user experience in creep mode. <Fourth Embodiment>

[0087] In the above-described embodiment, the operating lever 55 directly controlled the operating valves OVA, OVB, OVC, and OVD. However, the work vehicle 1 may also control control valves that control the first pump pilot pressure and the second pump pilot pressure based on the amount of operation of the operating lever 55 detected by a separate sensor such as a potentiometer. In this case, the same control as in the second embodiment can be achieved by adjusting the amount of operation detected by the sensor. Figure 15 is a hydraulic circuit diagram of the travel system of the work vehicle 1 in the fourth embodiment. In Figure 15, the same components as in Figure 3 are denoted by the same reference numerals, and detailed explanations are omitted. In the fourth embodiment, the work vehicle 1 includes a hydraulic circuit 1C. The hydraulic circuit 1C includes pilot control valves CV31 to CV34 that control the pilot pressure applied to each of the ports (PLa, PRa, PLb, PRb) instead of the operating valves OVA, OVB, OVC, and OVD and the first to fourth shuttle valves SVA, SVb, SVc, and SVd. The pilot control valves CV31 to CV34 are electromagnetic proportional valves including solenoids.

[0088] In this embodiment, the pilot supply oil passage PA8 connects the pilot control valves CV31 to CV34 to the pilot supply oil passage PA8, and the first to fourth pilot oil passages PA11 to PA14 are each connected to the pilot control valves CV31 to CV34. In this embodiment, the pilot supply oil passages PA1 and PA8 and the first to fourth pilot oil passages PA11 to PA14 correspond to the pilot oil supply circuit that connects the pilot pump to the first pump pilot port or the second pump pilot port. In this embodiment, since there are no operating valves OVA, OVB, OVC, and OVD, there is no difference between primary pilot pressure and secondary pilot pressure. Therefore, in this embodiment, these are simply referred to as pilot pressure without distinction.

[0089] In normal mode, the controller 10 controls the pilot control valves CV31 to CV34 so that it can output the pilot pressure corresponding to Figure 5, corresponding to the operating position detected by the operation detection sensor 18. In creep mode, in order to limit the vehicle speed, even if the operating lever 55 is actually operated to its full stroke, it is considered to have been operated to the deemed operating position Ga. Specifically, when the operating position is between position G0 and position Ga, the pilot pressure is determined from the correspondence relationship in Figure 5, corresponding to the operating position detected by the operation detection sensor 18. When the operating position is greater than or equal to position Ga, it is considered to have been operated to the deemed operating position Ga. In this embodiment, the amount of operation from position G0 to position Ga is called the deemed operating amount OA. In this embodiment, the memory 10b includes third reference information 10r3a instead of third reference information 10r3 according to the first and second embodiments.

[0090] Figure 16 shows an example of the third reference information 10r3a in the fourth embodiment. The third reference information 10r3a differs from the third reference information 10r3 only in that the vertical axis represents the deemed operating amount. When the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 refers to the third reference information 10r3a, converts the operating amount detected by the operation detection sensor 18 into a deemed operating amount based on the absolute value of the first differential pressure, and controls the first pump pilot pressure by controlling at least one pilot pressure control valve (pilot control valves CV31 to CV34) according to the deemed operating amount, thereby controlling the vehicle speed to maintain the target speed. In other words, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 is configured to determine the absolute value of the first differential pressure, which is the difference between the first hydraulic pressure and the second hydraulic pressure, and to control the first pump pilot pressure according to the absolute value of the first differential pressure so that the vehicle speed maintains a predetermined target speed. When the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the controller 10 is configured to determine the absolute value of the second differential pressure, which is the difference between the third hydraulic pressure and the fourth hydraulic pressure, and when the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure, the controller 10 is configured to control the second pump pilot pressure according to the absolute value of the first differential pressure so that the vehicle speed maintains a predetermined target speed. As shown in Figure 16, the controller 10 controls the amount of operation so that the deemed operation increases as the absolute value of the first differential pressure increases.

[0091] Figure 17 is a flowchart showing the operation of the work vehicle 1 according to the fourth embodiment. In Figure 17, the same reference numerals are used for the same operations as in the first embodiment, and detailed explanations are omitted. In this flowchart, the processes from step S1 to step S11 are executed at predetermined sampling intervals (e.g., 20 μs). After the completion of step S1, the processor 10a acquires the first manipulated variable from the operation detection sensor 18. When there is no engine drop (No in step S6), in step S7A, the processor 10a determines the maximum output pressure (Pc) in Figure 5 from the first reference information 10r1 based on the rotational speed RS of the engine 6. When there is engine drop (Yes in step S6), in step S8A, the processor 10a determines the maximum output pressure (Pc) in Figure 5 from the second reference information 10r2 based on the rotational speed RS of the engine 6. After the completion of step S7A or step S8A, in step 10A, the processor 10a determines the pilot pressure according to the first manipulated variable while the maximum output pressure (Pc) is limited, and controls the pilot control valves CV31 to CV34 so that the determined pilot pressure is applied.

[0092] After step S4 is completed, in step S5A, the processor 10a refers to the third reference information 10r3a and determines the deemed manipulated amount based on the absolute value of the first differential pressure. Then, in step S42, the processor 10a determines whether the first manipulated amount is greater than or equal to the deemed manipulated amount. In creep mode, the operation is usually performed so that the first manipulated amount is greater than or equal to the deemed manipulated amount. In steps S41, S5A, and S42, the processor 10a detects the manipulated amount of the travel instruction input device (operating lever) 55 and converts the detected manipulated amount into a deemed manipulated amount based on the absolute value of the first differential pressure. When the first manipulated amount is greater than or equal to the deemed manipulated amount (Yes in step S42), in step 10B, the processor 10a determines the pilot pressure according to the deemed manipulated amount and controls the pilot control valves CV31 to CV34 so that the determined pilot pressure is applied. In other words, the processor 10a controls the first pump pilot pressure and the second pump pilot pressure based on the deemed manipulated amount. As described above, when the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the processor 10a controls the first pump pilot pressure applied to the first pump pilot port of the first hydraulic pump in accordance with the absolute value of the first differential pressure so that the vehicle speed maintains a predetermined target speed. When the target rotational speed of the engine 6 set by the setting member 11 is a rotational speed that can achieve the upper limit speed set by the creep setting member 16, and the operating lever 55 is operated to its full stroke, the processor 10a controls the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump in accordance with the absolute value of the first differential pressure so that the vehicle speed maintains a predetermined target speed, when the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure. <Operation and Effects of the Fourth Embodiment>

[0093] In the control method or work vehicle 1 of the fourth embodiment, the processor 10a acquires the upper limit speed (target rotational speed of the first motor) input by the creep setting member 16, acquires the absolute value of the first differential pressure, and applies a pilot pressure to the first pump pilot port based on a deemed operation amount corresponding to the acquired target rotational speed and the absolute value of the first differential pressure. By controlling the pilot pressure using fluctuations in the first differential pressure, it is possible to improve the user experience in creep mode. <Modified form of the fourth embodiment>

[0094] In the fourth embodiment, the processor 10a controls the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump according to the absolute value of the first differential pressure when the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure. However, when the difference between the absolute value of the first differential pressure and the absolute value of the second differential pressure is within a predetermined range, the processor 10a may control the second pump pilot pressure according to the absolute value of the second differential pressure. In that case, the absolute value of the first differential pressure in Figure 16 should be read as the absolute value of the second differential pressure, and the processor 10a should be configured to control the additional secondary pressure control valve ACV based on the deemed operation amount on the vertical axis. In this way, when the absolute value of the first differential pressure does not differ significantly from the absolute value of the second differential pressure, the left and right travel devices can be controlled separately to achieve maneuvers such as turning that are close to the user's desire. In the second embodiment, the driving instruction input device (operating lever) 55 is equipped with an operation detection sensor 18, and the deemed operation amount is calculated based on the detection result of the operation detection sensor 18, and the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 are controlled accordingly. In this case, the relief valves CV23 and CV24 are controlled using the deemed operation amount, so the work vehicle 1 can be controlled to reach the desired speed more quickly than when controlled by the pilot control valves CV31 to CV34, which are electromagnetic proportional valves, as shown in Figure 19. <Variations relating to all embodiments>

[0095] The values ​​of the various thresholds may be changed depending on the characteristics of the left hydraulic pump 7L, the right hydraulic pump 7R, the left hydraulic motor 31L, and the right hydraulic motor 31R, the characteristics of the reduction gear connected to the left hydraulic motor 31L and the reduction gear connected to the right hydraulic motor 31R, and the characteristics of the various control valves.

[0096] In this application, “equipped with” and its derivatives are non-restrictive terms that describe the existence of a component and do not exclude the existence of other components not described. This also applies to “having,” “including,” and their derivatives.

[0097] The terms "~member," "~part," "~element," "~body," and "~structure" can have multiple meanings, such as a single part or multiple parts.

[0098] Ordinal numbers such as "1st" and "2nd" are simply terms used to identify components and do not carry any other meaning (such as a specific order). For example, the existence of a "1st element" does not implicitly mean the existence of a "2nd element," nor does the existence of a "2nd element" implicitly mean the existence of a "1st element."

[0099] Unless otherwise specifically stated in the embodiments, terms such as "substantially," "about," and "approximately" can mean a reasonable deviation that does not significantly alter the final result. All numerical values ​​described in this application may be interpreted as including terms such as "substantially," "about," and "approximately."

[0100] 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.

[0101] Based on the above disclosure, it is clear that various changes and modifications to the present invention are possible. Therefore, the present invention may be implemented in a manner different from the specific disclosures of this application, without departing from the spirit of the invention.

Claims

1. A work vehicle that can switch between a normal mode in which the speed of the work vehicle is changed according to the amount of operation of at least one control device into which a user's speed change operation is input, and a creep mode in which the work vehicle is driven at an upper limit speed lower than the speed of the normal mode when the amount of operation becomes greater than a predetermined amount, wherein the upper limit speed input in the creep mode is acquired, A hydraulic fluid is supplied from a first hydraulic pump to a first hydraulic motor that drives a first running gear installed on the vehicle body. The first differential pressure, which is the difference between the hydraulic pressure at the inlet port and the hydraulic pressure at the outlet port of the first hydraulic motor, is detected. In the creep mode, when the amount of operation is greater than the predetermined amount, at least one of the first pump pilot pressure applied to the first pump pilot port of the first hydraulic pump and the rotational speed of the engine for driving the first hydraulic pump is controlled in accordance with the absolute value of the first differential pressure so that the vehicle speed maintains the upper limit speed. A method for controlling the speed of a work vehicle.

2. Controlling the first pump pilot pressure means Controlling the primary pilot pressure, which is the hydraulic pressure of the primary pilot oil passage connecting a pilot pump for discharging pilot oil toward the first pump pilot port and an operating valve controlled in response to an input to at least one operating device, Controlling the secondary pilot pressure, which is the hydraulic pressure of the secondary pilot oil passage connecting the control valve and the first pump pilot port, The operation amount is detected, the detected operation amount is converted into a deemed operation amount based on the absolute value of the first differential pressure, and the first pump pilot pressure is controlled based on the deemed operation amount. Including at least one of the following: The method according to claim 1.

3. The higher the absolute value of the first differential pressure, the more the system controls to increase at least one of the first pump pilot pressure and the engine rotational speed. The method according to claim 1 or 2.

4. A second hydraulic pump supplies hydraulic fluid to a second hydraulic motor that drives a second running device, which is located on the opposite side of the vehicle body from the first running device. The second differential pressure, which is the difference between the hydraulic pressure at the inlet port and the hydraulic pressure at the outlet port of the second hydraulic motor, is detected. In the creep mode, when the operating amount is greater than the predetermined size and the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure, at least one of the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump and the rotational speed of the engine for driving the second hydraulic pump is controlled in accordance with the absolute value of the first differential pressure so that the vehicle speed maintains the upper limit speed. The method according to claim 1 or 2.

5. Controlling the pilot pressure of the second pump means Controlling the primary pilot pressure, which is the hydraulic pressure of the primary pilot oil passage connecting a pilot pump for discharging pilot oil toward the second pump pilot port and an operating valve controlled in response to an input to at least one operating device, Controlling the additional secondary pilot pressure, which is the hydraulic pressure of the additional secondary pilot oil passage connecting the control valve and the second pump pilot port, The operation amount is detected, the detected operation amount is converted into a deemed operation amount based on the absolute value of the first differential pressure, and the second pump pilot pressure is controlled based on the deemed operation amount. Including at least one of the following: The method according to claim 4.

6. The higher the absolute value of the first differential pressure, the more the system controls the system to increase at least one of the second pump pilot pressure and the engine rotational speed. The method according to claim 4.

7. The vehicle body and The first running device provided on the vehicle body, A first hydraulic motor configured to drive the first traveling device, A first hydraulic pump having a first pump pilot port and configured to supply hydraulic fluid to a first hydraulic motor in accordance with the first pump pilot pressure applied to the first pump pilot port, The first hydraulic pump and the first hydraulic motor are connected, and the first and second oil passages through which the hydraulic fluid is supplied are provided. A first hydraulic pressure sensor configured to detect the first hydraulic pressure of the first oil passage, A second hydraulic pressure sensor configured to detect the second hydraulic pressure of the second oil passage, A pilot pump configured to supply pilot oil to the first pump pilot port, The first hydraulic pump and the engine configured to drive the pilot pump, At least one control device into which the user's speed change operation is input, Regardless of the amount of operation of the at least one operating device, a creep setting member receives the upper limit speed in a creep mode that causes the work vehicle to travel at or below the upper limit speed, In the creep mode, when the operating amount is greater than a predetermined amount, the absolute value of the first differential pressure, which is the difference between the first hydraulic pressure and the second hydraulic pressure, is determined, and a controller is configured to control at least one of the first pump pilot pressure and the engine rotational speed according to the absolute value of the first differential pressure, so that the vehicle speed maintains the upper limit speed. A work vehicle equipped with the following features.

8. The work vehicle according to claim 7, wherein the engine's rotational speed is controlled to increase as the absolute value of the first differential pressure increases.

9. An operating valve configured to control the first pump pilot pressure by at least one operating device, A primary pilot oil passage connecting the pilot pump and the control valve, A primary pressure control valve is provided on the primary pilot oil passage and controls the primary pilot pressure, which is the hydraulic pressure of the pilot oil in the primary pilot oil passage and is the upper limit of the first pump pilot pressure. It further includes, The controller is configured to control the primary pilot pressure by controlling the primary pressure control valve, thereby controlling the vehicle speed to maintain the upper limit speed. The work vehicle according to claim 7.

10. The work vehicle according to claim 9, wherein the primary pilot pressure is controlled to increase as the absolute value of the first differential pressure increases.

11. An operating valve configured to control the first pump pilot pressure by at least one operating device, A secondary pilot oil passage connecting the control valve and the first pump pilot port, A secondary pressure control valve that controls the secondary pilot pressure, which is the hydraulic pressure of the pilot oil in the secondary pilot oil passage, It further includes, The controller is configured to control the first pump pilot pressure corresponding to the secondary pilot pressure by controlling the secondary pressure control valve, thereby controlling the vehicle speed to maintain the upper limit speed. The work vehicle according to claim 7.

12. The work vehicle according to claim 11, wherein the secondary pilot pressure is controlled to increase as the absolute value of the first differential pressure increases.

13. An operation detection sensor configured to detect the operation amount, A pilot oil supply circuit connecting the pilot pump and the first pump pilot port, A pilot pressure control valve is provided on the pilot oil supply circuit and controls the hydraulic pressure of the pilot oil, It further includes, The controller converts the operation amount detected by the operation detection sensor into a deemed operation amount based on the absolute value of the first differential pressure, and controls the first pump pilot pressure by controlling at least one pilot pressure control valve according to the deemed operation amount, thereby controlling the vehicle speed to maintain the upper limit speed. The work vehicle according to claim 7.

14. The work vehicle according to claim 13, wherein the deemed operating amount is controlled to increase as the absolute value of the first differential pressure increases.

15. A second running device is provided on the opposite side of the first running device of the vehicle body, A second hydraulic motor configured to drive the second travel device, A second hydraulic pump having a second pump pilot port and configured to supply hydraulic fluid to the second hydraulic motor in accordance with the second pump pilot pressure applied to the second pump pilot port, The second hydraulic pump and the second hydraulic motor are connected, and the third and fourth oil passages through which the hydraulic fluid is supplied are provided. A third hydraulic pressure sensor configured to detect the third hydraulic pressure of the third oil passage, A fourth hydraulic pressure sensor configured to detect the fourth hydraulic pressure of the fourth oil passage, Furthermore, The engine is configured to drive the second hydraulic pump, The pilot pump is configured to supply the pilot oil to the second pump pilot port, The controller is configured to determine the absolute value of a second differential pressure, which is the difference between the third hydraulic pressure and the fourth hydraulic pressure, and when the operating amount is greater than a predetermined amount in the creep mode and the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure, it controls at least one of the second pump pilot pressure and the rotational speed of the engine according to the absolute value of the first differential pressure, so that the vehicle speed maintains the upper limit speed, as described in any one of claims 7 to 14.

16. The work vehicle according to claim 15, wherein the second pump pilot pressure is controlled to increase as the absolute value of the first differential pressure increases.

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