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

Abstract

To enable a work vehicle to quickly start running even if running resistance is large when starting to run.SOLUTION: A speed control method of a work vehicle includes: inputting a target rotational speed of a hydraulic motor constituted so as to drive a travel device through an operation member; determining a control parameter for determining displacement volume of a hydraulic pump constituted so as to supply hydraulic oil to the hydraulic motor according to operation of the operation member; rotating the hydraulic motor by rotating the hydraulic pump according to the control parameter; detecting a rotational speed of the hydraulic motor in sampling period; calculating speed difference obtained by subtracting the rotational speed from the target rotational speed in the sampling period; obtaining a reference value of the control parameter on the basis of the target rotational speed; calculating an offset value corresponding to the speed difference in the sampling period; and updating the control parameter in the sampling period by setting the control parameter as a value obtained by adding the offset value to the reference value.SELECTED DRAWING: Figure 10

Description

[Technical field]

[0001] Citation by reference This application claims the benefit of Japanese Patent Application No. 2022-131895, filed on August 22, 2022, each of which is incorporated by reference in its entirety herein.

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

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

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

[0005] Feedback is performed using the rotation speed of the travel motor in both the methods of Patent Document 1 and Patent Document 2. When the primary pressure is lowered as in Patent Document 2, with a general feedback method, if the running resistance is large when the work vehicle starts to move, it may take time to achieve a primary pressure that allows travel. [Means for solving the problem]

[0006] A speed control method for a work vehicle according to a first aspect of the present disclosure includes acquiring, via an operating member, a target rotational speed of a hydraulic motor configured to drive a traveling device. The method includes determining a control parameter for setting a displacement of a hydraulic pump configured to supply hydraulic oil to the hydraulic motor in response to operation of the operating member. The method includes controlling the displacement of the hydraulic pump in response to the control parameter and rotating the hydraulic pump, thereby rotating the hydraulic motor. The method includes detecting a rotational speed of the hydraulic motor at a sampling period. The method includes calculating, at a sampling period, a speed difference obtained by subtracting the target rotational speed from the rotational speed. The method includes determining a reference value of the control parameter based on the target rotational speed. The method includes calculating, at a sampling period, an offset value corresponding to the speed difference. The method includes updating the control parameter at a sampling period by setting the control parameter to a value obtained by adding the offset value to a reference value.

[0007] A work vehicle according to a second aspect of the present disclosure includes a traveling device, a hydraulic motor, a hydraulic pump, a prime mover, an operating member, a rotational speed sensor, a control mechanism, and a hardware processor. The hydraulic motor is configured to drive the traveling device. The hydraulic pump is configured to supply hydraulic oil to the hydraulic motor. The prime mover is configured to rotate the hydraulic pump. The operating member is configured to receive an input corresponding to a target rotational speed of the hydraulic motor. The rotational speed sensor is configured to detect the rotational speed of the hydraulic motor in a sampling period. The control mechanism is configured to control the hydraulic pressure of the pilot oil that changes the displacement of the hydraulic pump. The hardware processor is configured to control the prime mover and the control mechanism. The hardware processor is configured to obtain a reference value of a control parameter for controlling the control mechanism based on the target rotational speed, calculate a speed difference obtained by subtracting the target rotational speed from the rotational speed in a sampling period, calculate an offset value according to the speed difference in the sampling period, and update the control parameter in the sampling period by setting the control parameter to a value obtained by adding the offset value to the reference value.

[0008] A controller for a work vehicle according to a third aspect of the present disclosure includes a hardware processor. The hardware processor is configured to receive an input corresponding to a target rotational speed of a hydraulic motor configured to drive a traveling device from an operating member. The hardware processor is configured to determine a control parameter for determining a displacement volume of a hydraulic pump configured to supply hydraulic oil to the hydraulic motor in response to operation of the operating member. The hardware processor is configured to rotate the hydraulic motor via the hydraulic pump by sending a command to a prime mover configured to rotate the hydraulic pump in response to the control parameter to the prime mover. The hardware processor is configured to receive a rotational speed of the hydraulic motor detected in a sampling period from a rotational speed sensor. The hardware processor is configured to calculate a speed difference obtained by subtracting the rotational speed from the target rotational speed in a sampling period. The hardware processor is configured to determine a reference value of the control parameter based on the target rotational speed. The hardware processor is configured to calculate an offset value corresponding to the speed difference in a sampling period, and update the control parameter in a sampling period by setting the control parameter to a value obtained by adding the offset value to the reference value. Effect of the Invention

[0009] According to the techniques disclosed in the first to third aspects of the present application, for example, when a work vehicle starts moving, even if the moving resistance is large, the work vehicle can start moving quickly. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a side view of a work vehicle. [Diagram 2] FIG. 2 is a top view of the work vehicle. [Diagram 3] FIG. 3 is a hydraulic circuit diagram of a travel system of the work vehicle in the first embodiment. [Figure 4] FIG. 4 is a diagram showing the relationship between the engine speed, the primary pilot pressure, and the setting line. [Diagram 5] FIG. 5 is a diagram showing the relationship between the operating position of the operating lever and the secondary pilot pressure. [Figure 6] FIG. 6 is a block diagram of the work vehicle. [Figure 7] FIG. 7 shows a control block diagram in the first embodiment. [Figure 8] FIG. 8 is a diagram showing the relationship between the speed difference and the method of updating the offset value in the embodiment. [Figure 9] FIG. 9 is a flowchart showing the operation of the work vehicle according to the first embodiment. [Figure 10] FIG. 10 is a flowchart showing a method for determining the control parameters. [Figure 11] FIG. 11 is a hydraulic circuit diagram of a travel system of a work vehicle in the second embodiment. [Figure 12] FIG. 12 shows a control block diagram in the second embodiment. [Figure 13] FIG. 13 is a flowchart showing the operation of the work vehicle according to the second embodiment. [Figure 14] FIG. 14 is a hydraulic circuit diagram of a travel system of a work vehicle in a modified example of the second embodiment. [Figure 15] FIG. 15 shows a control block diagram of a modified example of the second embodiment. [Figure 16] FIG. 16 is a hydraulic circuit diagram of a travel system of a work vehicle in the third embodiment. [Figure 17] FIG. 17 shows a control block diagram in the third embodiment. [Figure 18] 18 is a flowchart showing the operation of the work vehicle according to the third embodiment.

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

[0012] 1 and 2, a work vehicle 1, for example, a compact track loader, includes a vehicle body 2, a pair of travel devices 3, and a working device 4. The vehicle body 2 supports the travel devices 3 and the working device 4. In the illustrated embodiment, the travel devices 3 are track-type travel devices provided on the vehicle body 2. Therefore, each of the pair of travel devices 3 includes a driving wheel 31, driven wheels 32 and 33, and rollers 34, which are driven by a hydraulic motor device 30. However, each of the pair of travel devices 3 is not limited to being a track-type travel device. Each of the pair of travel devices 3 may be, for example, a front-wheel / rear-wheel travel device, or a travel device having front wheels and rear crawlers. The working device 4 includes a work equipment (bucket) 41 at the distal end of the working device 4. A proximal end of the working device 4 is attached to the rear of the vehicle body 2. The working device 4 includes a pair of arm assemblies 42 for rotatably supporting a bucket 41 via a bucket pivot shaft 43. Each of the pair of arm assemblies 42 includes a link 44 and an arm 45.

[0013] The link 44 is rotatable relative to the vehicle body 2 around a fulcrum shaft 46. The arm 45 is rotatable relative to the link 44 around a joint shaft 47. The working device 4 further includes a plurality of arm cylinders 48 and at least one equipment cylinder 49. Each of the plurality of arm cylinders 48 is rotatably connected to the vehicle body 2 and the arm 45, and moves the link 44 and the arm 45, etc., to lift and lower the bucket 41. The at least one equipment cylinder 49 is configured to tilt the bucket 41. The vehicle body 2 includes a cabin 5. The cabin 5 includes a front window 51 that can be opened and closed, and its outer shape is defined by a cab frame 53. The front window 51 may be omitted. The work vehicle 1 includes a driver's seat 54 and an operation lever 55 in the cabin 5. The cab frame 53 is rotatable around rotational shafts RSL and RSR on the vehicle body 2, as shown in FIG. 2. In FIG. 1 and FIG. 2, a common pivot axis A defined by the rotation axes RSL and RSR is XC That is, the cab frame 53 is rotatable about a rotation axis A XC It is mounted so as to be rotatable around

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

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

[0016] 1 and 2, the work vehicle 1 further includes an engine 6 provided at the rear of the vehicle body 2, and multiple hydraulic pumps 7 including a left hydraulic pump 7L and a right hydraulic pump 7R. The engine 6 is configured to drive the multiple hydraulic pumps 7. The left hydraulic pump 7L and the right hydraulic pump 7R are configured to discharge hydraulic oil to drive a hydraulic motor device 30 that drives the drive wheels 31, etc. The left hydraulic pump 7L and the right hydraulic pump 7R are collectively referred to as hydraulic pumps (7L, 7R). The multiple hydraulic pumps 7 other than the left hydraulic pump 7L and the right hydraulic pump 7R are configured to discharge hydraulic oil to drive hydraulic actuators (multiple arm cylinders 48, at least one implement cylinder 49, etc.) connected to the working device 4. The engine 6 is configured to rotate in the width direction D of the work vehicle 1. W , and is provided between the pair of arm assemblies 42. The work vehicle 1 further includes a cover 8 for covering the engine 6. The work vehicle 1 further includes a bonnet cover 9 provided at the rear end of the vehicle body 2. The bonnet cover 9 can be opened and closed, allowing a maintenance technician to perform maintenance work on the engine 6, etc.

[0017] FIG. 3 is a hydraulic circuit diagram of the travel 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 fixed displacement gear pump driven by the power of the engine 6. The pilot pump 71 is configured to discharge hydraulic oil stored in the hydraulic oil tank 70. In particular, the pilot pump 71 is configured to discharge hydraulic oil mainly used for control. For convenience of explanation, the hydraulic oil used for control among the hydraulic oil discharged from the pilot pump 71 is referred to as pilot oil, and the pressure of the pilot oil is referred to as pilot pressure. In particular, the pilot pump 71 is configured to supply pilot oil to the left hydraulic pump 7L and the right hydraulic pump 7R.

[0018] The hydraulic circuit 1A includes a pilot supply oil passage PA1 connected to a discharge port of a pilot pump 71. Pilot oil flows through the pilot supply oil passage PA1. The hydraulic circuit 1A includes a plurality of switching valves (brake switching valve SV1, directional control valve SV2) connected to the pilot supply oil passage PA1, and a plurality of brake mechanisms 72. The brake switching valve SV1 is connected to the pilot supply oil passage PA1. The brake switching valve SV1 is a directional control valve (solenoid valve) for performing braking and releasing of braking by the plurality of brake mechanisms 72. The brake switching valve SV1 is a two-position switching valve configured to switch its valve body to a first position VP1a or a second position VP1b by excitation. The valve body of the brake switching valve SV1 is switched by a brake pedal 13 (see FIG. 6). The brake pedal 13 is provided with a sensor 14. An operation amount detected by the sensor 14 is input to a controller 10 consisting of an ECU (Electric Control Unit). The controller 10 may be referred to as a control device.

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

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

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

[0022] The first travel control valve SV4 is switched by the directional control valve SV2 located upstream and connected to the first travel control valve SV4. Specifically, the directional control valve SV2 and the first travel control valve SV4 are connected by an oil passage PA3, and the first travel control valve SV4 is switched by the hydraulic oil flowing through the oil passage PA3. For example, when the valve body of the directional control valve SV2 is switched to the first position VP2a by operating the input member, pilot oil is released from the section between the directional control valve SV2 and the first travel control valve SV4, and the valve body of the first travel control valve SV4 is switched to the first position VP4a. As a result, the first swash plate switching cylinder 32L contracts, and the speed of the left hydraulic motor 31L is changed to first gear. When the valve element of the directional control valve SV2 is switched to the second position VP2b by operating the input member, pilot oil is supplied to the first travel control valve SV4 through the directional control valve SV2, and the valve element of the first travel control valve SV4 is switched to the second position VP4b. As a result, the first swash plate switching cylinder 32L extends, and the speed of the left hydraulic motor 31L is changed to second gear.

[0023] The right hydraulic motor device 30R is a device that transmits power to the drive wheels 31 provided on the right traveling device 3R. The right hydraulic motor device 30R includes a right hydraulic motor 31R, a second swash plate switching cylinder 32R, and a second traveling control valve (hydraulic switching valve) SV5. The right hydraulic motor device 30R is a hydraulic motor for driving the right traveling device 3R, and operates in the same manner as the left hydraulic motor device 30L. That is, the right hydraulic motor 31R operates in the same manner as the left hydraulic motor 31L. The left hydraulic motor 31L and the right hydraulic motor 31R are collectively referred to as hydraulic motors (31L, 31R). The second swash plate switching cylinder 32R operates in the same manner as the first swash plate switching cylinder 32L. The second traveling control valve SV5 is a two-position switching valve configured to switch its valve body to a first position VP5a and a second position VP5b, and operates in the same manner as the first traveling control valve SV4.

[0024] A drain oil passage DR1 is connected to the hydraulic circuit 1A. The drain oil passage DR1 is an oil passage that flows pilot oil from a plurality of switching valves (brake switching valve SV1, directional control valve SV2) to the hydraulic oil tank 70. For example, the drain oil passage DR1 is connected to the discharge ports of the plurality of switching valves (brake switching valve SV1, directional control valve SV2). That is, when the brake switching valve SV1 is in the first position VP1a, hydraulic oil is discharged from the oil passage PA2 to the drain oil passage DR1 in the section between the brake switching valve SV1 and the brake mechanism 72. When the directional control valve SV2 is in the first position VP1a, pilot oil in the oil passage PA3 is discharged to the drain oil passage DR1.

[0025] The hydraulic circuit 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.

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

[0027] 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 an oil passage that is connected to the drive oil passages PA5R and PA6R and replenishes the hydraulic oil from the pilot pump 71 to the drive oil passages PA5R and PA6R. The 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. Hydraulic oil for rotating the right traveling device 3R in the forward direction is input to the right hydraulic motor 31R via the third connection port 31P3, and hydraulic oil for rotating the right traveling device 3R in the backward direction is discharged from the right hydraulic motor 31R via the third connection port 31P3. Hydraulic oil for rotating the right traveling device 3R in the backward direction is input to the right hydraulic motor 31R via the fourth connection port 31P4, and hydraulic oil for rotating the right traveling device 3R in the forward direction is discharged from the right traveling device 3R. That is, the hydraulic motors (31L, 31R) are configured to drive the traveling devices (3L, 3R). The hydraulic pumps (7L, 7R) are configured to discharge hydraulic oil for driving the hydraulic motors (31L, 31R). The drive oil passages (PA5L, PA6L, PA5R, PA6R) are oil passages that connect the hydraulic pumps (7L, 7R) and the hydraulic motors (31L, 31R).

[0028] The left hydraulic pump 7L and the right hydraulic pump 7R are swash plate 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 a pilot pressure acts. The left hydraulic pump 7L is configured to change the angle of the swash plate according to the pilot pressure acting on the first port PLa and the second port PLb, and to supply hydraulic oil to the left hydraulic motor 31L. Specifically, the left hydraulic pump 7L is configured to supply hydraulic oil to the left hydraulic motor 31L via the left hydraulic circuit CL so as to drive the left traveling device 3L forward when the hydraulic pressure applied to the first port PLa is higher than the hydraulic pressure applied to the second port PLb, and to supply hydraulic oil to the left hydraulic motor 31L via the left hydraulic circuit CL so as to drive the left traveling device 3L backward when the hydraulic pressure applied to the second port PLb is higher than the hydraulic pressure applied to the first port PLa.

[0029] The right hydraulic pump 7R is connected to the right hydraulic motor 31R via a right hydraulic circuit CR, and has a third port PRa and a fourth port PRb on which a pilot pressure acts. The right hydraulic pump 7R is configured to change the angle of the swash plate according to the pilot pressure acting on the third port PRa and the fourth port PRb, and to supply hydraulic oil to the right hydraulic motor 31R. Specifically, when the hydraulic pressure applied to the third port PRa is higher than the hydraulic pressure applied to the fourth port PRb, the right hydraulic pump 7R supplies hydraulic oil to the right hydraulic motor 31R via the right hydraulic circuit CR so as to drive the right traveling device 3R forward, and when the hydraulic pressure applied to the fourth port PRb is higher than the hydraulic pressure applied to the third port PRa, the right hydraulic pump 7R supplies hydraulic oil to the right hydraulic motor 31R via the right hydraulic circuit CR so as to drive the right traveling device 3R backward. The left hydraulic pump 7L and the right hydraulic pump 7R can change the output (discharge amount of hydraulic oil) and the discharge direction of hydraulic oil according to the angle of the swash plate.

[0030] The output of the left hydraulic pump 7L and the right hydraulic pump 7R and the discharge direction of the hydraulic oil are changed by an operation device 56 for operating the traveling direction of the work vehicle 1. Specifically, the output of the left hydraulic pump 7L and the right hydraulic pump 7R and the discharge direction of the hydraulic oil are changed in response to the operation of an operation lever 55 provided on the operation device 56. In other words, the operation device 56 is a device configured to operate the traveling direction of the work vehicle by selecting at least one of the left traveling device 3L and the right traveling device 3R and instructing at least one of the traveling devices to move forward or backward. A user's instruction for the traveling direction is input via the operation lever 55. The operation lever 55 may be referred to as an additional operation member.

[0031] As shown in FIG. 3, the hydraulic circuit 1A includes a pilot supply oil passage PA8 that branches off from a pilot supply oil passage PA1 and is connected to an operating device 56, and a primary pressure control valve CV1 that is provided on the pilot supply oil passage PA8. In the following embodiments, the pilot supply oil passage PA1 and the pilot supply oil passage PA8 are collectively referred to as a primary pilot oil passage. The primary pressure control valve CV1 is an electromagnetic proportional valve (control mechanism) that includes a solenoid, and is configured to adjust the pilot pressure supplied to the operating device 56 by adjusting the opening degree according to the current applied to the solenoid. The opening degree of the primary pressure control valve CV1 is controlled by the current sent from the controller 10. That is, the controller 10 is configured to control the primary pressure control valve CV1. Note that, as the magnitude of the current increases, the pilot pressure output from the primary pressure control valve CV1 may also increase, and, as the magnitude of the current increases, the pilot pressure output from the primary pressure control valve CV1 may also decrease. In the following embodiments, the primary pressure control valve CV1 may be referred to as a hydraulic adjustment mechanism. A detailed operation of the primary pressure control valve CV1 will be described later.

[0032] The operating device 56 includes a forward operating valve OVA, a reverse operating valve OVB, a right turning operating valve OVC, a left turning operating valve OVD, and an operating lever 55. The operating device 56 also includes first to fourth shuttle valves SVa, SVb, SVc, and SVd. The operating valves OVA, OVB, OVC, and OVD are operated by a single operating lever 55. The operating valves OVA, OVB, OVC, and OVD change the pressure of the hydraulic oil in response to the operation of the operating lever 55, and supply the changed hydraulic oil to the first port PLa and the second port PLb of the left hydraulic pump 7L and the third port PRa and the 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 the number of operating levers 55 may be multiple.

[0033] The operation valves OVA, OVB, OVC, and OVD have an input port (primary side port), a discharge port, and an output port (secondary side port). As shown in FIG. 3, the input port is connected to a pilot supply oil passage PA8. The discharge port is connected to a drain oil passage DR2 leading to a hydraulic oil tank 70. The operation lever 55 can be tilted from a neutral position in the front-rear direction, in the width direction perpendicular to the front-rear direction, and in an oblique direction. The neutral position is located when the operation lever 55 is not operated. The neutral position may be called a return position. The operation valves OVA, OVB, OVC, and OVD of the operation device 56 are operated in response to the tilting of the operation lever 55. Thereby, a pilot pressure according to the operation amount from the neutral position of the operation lever 55 is output from the secondary side port of the operation valves OVA, OVB, OVC, and OVD. The relationship between the pilot pressure applied to the primary side port output from the primary pressure control valve CV1 and the pilot pressure applied to the secondary side port will be described later.

[0034] The secondary side port of the control valve OVA and the secondary side port of the control valve OVC are connected to the input port of the first shuttle valve SVa, and the output port of the first shuttle valve SVa is connected to the first port PLa of the left hydraulic pump 7L via the first pilot oil passage PA11. The secondary side port of the control valve OVA and the secondary side port of the control valve OVD are connected to the input port of the second shuttle valve SVb, and the output port of the second shuttle valve SVb is connected to the third port PRa of the right hydraulic pump 7R via the third pilot oil passage PA13. The secondary side port of the control valve OVB and the secondary side port of the control valve OVD are connected to the input port of the third shuttle valve SVc, and the output port of the third shuttle valve SVc is connected to the second port PLb of the left hydraulic pump 7L via the second pilot oil passage PA12. The secondary side port of the control valve OVB and the secondary side port of the control valve OVC are connected to the input port of the fourth shuttle valve SVd, and the output port of the fourth shuttle valve SVd is connected to the fourth port PRb of the right hydraulic pump 7R via the fourth pilot oil passage PA14. That is, the pilot supply oil passage PA8, the first pilot oil passage PA11, and the fourth pilot oil passage PA14 connect the pilot pump 71 and the left hydraulic pump 7L. The pilot supply oil passage PA8, the second pilot oil passage PA12, and the third pilot oil passage PA13 connect the pilot pump 71 and the right hydraulic pump 7R.

[0035] When the control lever 55 is tilted forward, the control valve OVA for forward movement is operated and pilot pressure is output from the control valve OVA. This pilot pressure acts on the first port PLa from the first shuttle valve SVa via the first pilot oil passage PA11 that connects the control device 56 and the first port PLa of the left hydraulic pump 7L, and also acts on the third port PRa from the second shuttle valve SVb via the third pilot oil passage PA13 that connects the control device 56 and the third port PRa of the right hydraulic pump 7R. As a result, the output shaft of the left hydraulic pump 7L and the output shaft of the right hydraulic pump 7R rotate forward (forward rotation) at a speed corresponding to the tilt amount of the control lever 55, and the work vehicle 1 moves straight forward.

[0036] Furthermore, when the operating lever 55 is tilted rearward, the reverse operating valve OVB is operated and pilot pressure is output from the operating valve OVB. This pilot pressure acts on the second port PLb of the left hydraulic pump 7L from the third shuttle valve SVc via the second pilot oil passage PA12 connecting the operating device 56 and the second port, and also acts on the fourth port PRb from the fourth shuttle valve SVd via the fourth pilot oil passage PA14 connecting the operating device 56 and the fourth port PRb of the right hydraulic pump 7R. As a result, the output shaft of the left hydraulic pump 7L and the output shaft of the right hydraulic pump 7R rotate in the reverse direction (reverse rotation) at a speed corresponding to the tilt amount of the operating lever 55, and the work vehicle 1 moves straight backward.

[0037] In addition, when the control lever 55 is tilted to the right, the control valve OVC for right turning is operated and pilot pressure is output from the control valve OVC. This pilot pressure acts on the first port PLa of the left hydraulic pump 7L through the first pilot oil passage PA11 from the first shuttle valve SVa, and also acts on the fourth port PRb of the right hydraulic pump 7R through the fourth pilot oil passage PA14 from the fourth shuttle valve SVd. As a result, the vehicle turns to the right with a degree of turning that corresponds to the rightward operating position of the control lever 55.

[0038] In addition, when the control lever 55 is tilted to the left, the control valve OVD for left turning is operated and pilot pressure is output from the control valve OVD. This pilot pressure acts on the third port PRa of the right hydraulic pump 7R via the second shuttle valve SVb and the third pilot oil passage PA13, and also acts on the second port PLb of the left hydraulic pump 7L via the third shuttle valve SVc and the second pilot oil passage PA12. As a result, the vehicle turns left with a degree of turning that corresponds to the leftward operating position of the control lever 55.

[0039] That is, when the operating lever 55 is tilted diagonally forward to the left, the work vehicle 1 moves forward at a speed corresponding to the operating position of the operating lever 55 in the forward / rearward direction, and turns left with a degree of turning corresponding to the operating position of the operating lever 55 in the left direction. When the operating lever 55 is tilted diagonally forward to the right, the work vehicle 1 turns right while moving forward at a speed corresponding to the operating position of the operating lever 55. When the operating lever 55 is tilted diagonally rearward to the left, the work vehicle 1 turns left while moving backward at a speed corresponding to the operating position of the operating lever 55. When the operating lever 55 is tilted diagonally rearward to the right, the work vehicle 1 turns right while moving backward at a speed corresponding to the operating position of the operating lever 55.

[0040] Next, detailed operation of the primary pressure control valve CV1 will be described. The work vehicle 1 includes a setting member 11 (see FIG. 6) that sets the target rotation speed of the engine 6. The setting member 11 is an accelerator pedal, an accelerator lever supported so as to be able to swing, or a rotatable indoor dial, which is a speed input device different from the above-mentioned operation device 56. The setting member 11 is provided with a sensor 12. The operation amount detected by the sensor 12 is input to the controller 10. The engine rotation speed corresponding to the operation amount detected by the sensor 12 is the target rotation speed of the engine 6. In other words, the target rotation speed of the engine 6 is set based on the operation amount of the setting member 11. The controller 10 outputs a rotation command indicating, for example, a fuel injection amount, an injection timing, and a fuel injection rate to the injector so as to achieve the determined target rotation speed of the engine 6. Alternatively, the controller 10 outputs a rotation command indicating a fuel injection pressure, etc., to a supply pump or a common rail so as to achieve the determined target rotation speed of the engine 6. In other words, the controller 10 is configured to control the engine 6. In the following embodiments, the one or more operating levers 55 and the setting member 11 described above may be referred to as at least one operating device. A speed sensor 6a for detecting an 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 referred to as the engine drop amount.

[0041] The primary pressure control valve CV1 can set a pilot pressure (primary pilot pressure) acting on the input ports (primary side ports) of the multiple control valves OVA, OVB, OVC, and OVD based on a drop amount (drop amount) ΔE1 of the rotation speed of the engine 6 (engine rotation speed E1). In other words, the primary pressure control valve CV1 is a control valve that is provided between the pilot pump 71 and the control valves OVA, OVB, OVC, and OVD, sends pilot oil to the control valves OVA, OVB, OVC, and OVD, and is configured to convert the pressure of the pilot oil supplied to the control valves OVA, OVB, OVC, and OVD into a primary pilot pressure. The rotation speed of the engine 6 can be detected by a speed sensor 6a for the engine rotation speed E1. The engine rotation speed E1 detected by the speed sensor 6a is input to the controller 10. The speed sensor 6a may be called a speed sensor. FIG. 4 shows the relationship between the engine rotation speed, the primary pilot pressure, and the setting lines L1 and L2. The setting line L1 shows the relationship between the engine speed E1 and the primary pilot pressure when the decrease amount ΔE1 is less than a predetermined value (less than the anti-stall determination value). The setting line L2 shows the relationship between the engine speed E1 and the primary pilot pressure when the decrease amount ΔE1 is equal to or greater than the anti-stall determination value. When the difference between the rotation speed RS1 determined based on the operation amount of the setting member 11 and the actual rotation speed of the engine 6 is smaller than a predetermined stall determination speed difference (anti-stall determination value), the primary pilot pressure corresponding to the rotation speed RS1 transitions according to the third correspondence relationship shown by the setting line L1. When the difference between the rotation speed RS1 and the actual rotation speed of the engine 6 is equal to or greater than the predetermined stall determination speed difference (anti-stall determination value), the primary pilot pressure corresponding to the rotation speed RS1 transitions according to the fourth correspondence relationship shown by the setting line L2.

[0042] When the amount of decrease ΔE1 is less than the anti-stall determination value, the controller 10 adjusts the opening of the primary pressure control valve CV1 so that the relationship between the engine speed E1 and the primary pilot pressure coincides with the reference pilot pressure indicated by the set line L1. When the amount of decrease ΔE1 is equal to or greater than the anti-stall determination value, the controller 10 adjusts the opening of the primary pressure control valve CV1 so that the relationship between the engine speed E1 and the primary pilot pressure coincides with the set line L2, which is lower than the reference pilot pressure. In the set line L2, the primary pilot pressure for a given engine speed E1 is lower than the primary pilot pressure of the set line L1. That is, when the same engine speed E1 is taken into consideration, the primary pilot pressure of the set line L2 is set lower than the primary pilot pressure of the set line L1. Therefore, the pressure (pilot pressure) of the hydraulic oil entering the operating valves OVA, OVB, OVC, and OVD is kept low by the control based on the set line L2. As a result, the swash plate angles of the left hydraulic pump 7L and the right hydraulic pump 7R are adjusted, the load acting on the engine 6 is reduced, and stalling of the engine 6 can be prevented. Although one setting line L2 is shown in FIG. 4, there may be multiple setting lines L2. For example, the setting line L2 may be set for each engine rotation speed E1. In addition, it is preferable that the controller 10 has data or functions indicating the setting lines L1 and L2.

[0043] Next, the pilot pressure (secondary pilot pressure) output from the secondary port of the operating valves OVA, OVB, OVC, and OVD will be described. Fig. 5 is a diagram showing the relationship between the operating position of the operating lever and the secondary pilot pressure. Referring to Fig. 4, the origin of the lever operating position is the operation start position (neutral position, G0 position) which is the start position of the lever stroke, and the further away from the origin, the closer to the operation end position (G5 position) which is the end position of the lever stroke. The operating region of the operating lever 55 is divided into a neutral region RA1 where the operating object does not move (in the illustrated example, from the G0 position to the G1 position), a full operation vicinity region RA2 near the operation end (in the illustrated example, from the G3 position to the G5 position), and an intermediate region RA3 between the neutral region RA1 and the full operation vicinity region RA2 (in the illustrated example, from the G1 position to the G3 position). Further, the intermediate region RA3 is divided into a slow-speed region RA3A from the G1 position to the G2 position, and an intermediate-speed region RA3B from the G2 position to the G3 position.

[0044] In the neutral region RA1, no secondary pilot pressure is supplied even if the control lever 55 is operated. On the other hand, in the full operation vicinity region RA2, the speed of the controlled object is not adjusted, and therefore the control lever 55 is operated to the operation end position (G5 position) without stopping midway. In the intermediate region RA3, the control lever 55 is stopped at any position within the region or its position is changed, and the speed of the controlled object is adjusted to the speed desired by the operator. For example, the ratio of each of the operation regions RA1, RA3A, RA3B, and RA2 to the lever stroke is as follows: Neutral region RA1: 0% to less than 15% Slow speed area RA3A: 15% to less than 45% Intermediate speed range RA3B: 45% to less than 75% Full operation area RA2: 75% to 100%

[0045] In the characteristic diagram shown in FIG. 5, when the operating lever 55 is operated from the G0 position to the G1 position, a secondary pilot pressure (Pa) is generated, and 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 of operation of the operating lever 55. Also, at the G4 position, the primary pilot pressure is shortcut and flows to the secondary side, and the secondary pilot pressure rises from Pb to the maximum output pressure Pc in one go. Then, while the operating lever 55 is operated from the G4 position to the G5 position, the secondary pilot pressure is constant at the maximum output pressure (Pc) and is equal to the primary pilot pressure. That is, when the displacement of the operating lever 55 from the neutral position for instructing a movement in the left direction is equal to or greater than the first displacement value (displacement from G0 to G4), the operating device 56 outputs the primary pilot pressure input to the operating device 56 to the 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 called operating the operating lever 55 at a full stroke. The operation device 56 outputs the primary pilot pressure input to the operation device 56 to the second port PLb and the third port PRa when the displacement from the neutral position of the operation lever 55 for instructing a movement in the right direction is equal to or greater than the first displacement value (displacement from G0 to G4). The operation device 56 outputs the primary pilot pressure input to the operation device 56 to the first port PLa and the third port PRa when the displacement from the neutral position of the operation lever 55 for instructing a movement in the forward direction is equal to or greater than the first displacement value (displacement from G0 to G4). The operation device 56 outputs the primary pilot pressure input to the operation device 56 to the second port PLb and the fourth port PRb when the displacement from the neutral position of the operation lever 55 for instructing a movement in the rearward direction is equal to or greater than the first displacement value (displacement from G0 to G4). Note that the characteristic value of the secondary pilot pressure in the forward / rearward direction may be different from the characteristic value of the secondary pilot pressure in the left / right direction.If the characteristic values ​​of the secondary pilot pressure in the forward / rearward direction corresponding to G0 to G5 and Pa to Pc are G0' to G5' and Pa' to Pc', the operation device 56 may output the primary pilot pressure input to the operation device 56 to the first port PLa and the third port PRa when the displacement of the operation lever 55 for instructing a movement in the forward direction from the neutral position is equal to or greater than the second displacement value (displacement from G0' to G4'). The operation device 56 may output the primary pilot pressure input to the operation device 56 to the second port PLb and the fourth port PRb when the displacement of the operation lever 55 for instructing a movement in the rearward direction from the neutral position is equal to or greater than the second displacement value (displacement from G0' to G4'). In addition, Pa and Pb (Pa' and Pb') are values ​​that do not depend on the magnitude of the primary pilot pressure, but when the primary pilot pressure is lower than Pa or Pb (Pa' or Pb'), the secondary pilot pressure is capped at the magnitude of the primary pilot pressure. That is, the operating valves (OVA, OVB, OVC, OVD) are configured to convert the pressure of the pilot oil from a primary pilot pressure to a secondary pilot pressure in accordance with a first operation amount (operation lever position) of the operating device 56, and output the pilot oil. The pilot oil at the secondary pilot pressure is applied to ports (PLa, PRa, PLb, PRb) that provide hydraulic pressure to the swash plates of the hydraulic pumps (7L, 7R). When the first operation amount is equal to or greater than a threshold amount (first displacement value), the operating valves (OVA, OVB, OVC, OVD) convert the primary pilot pressure to a secondary pilot pressure that is equal to the primary pilot pressure.

[0046] Based on the above characteristics of the operation valves OVA, OVB, OVC, and OVD, the movement of the work vehicle 1 corresponding to the operation of the operation lever 55 will be described in more detail. When the operation amount of the operation lever 55 in the forward / rearward direction is larger than the operation amount in the rightward direction, and the operation position of the operation lever 55 in the rightward direction is operated from the G1 position to the G3 position, the left hydraulic pump 7L rotates in the same direction with the rotation speed of the left hydraulic pump 7R being larger than that of the right hydraulic pump 7R, and the work vehicle 1 turns to the right in a wide turn. When the operation position of the operation lever 55 in the rightward direction is the same as the operation position in the forward / rearward direction, the rotation speed of the right hydraulic pump 7R becomes 0, and only the left hydraulic pump 7L rotates, and the work vehicle 1 performs a right pivot turn (right pivot turn). Furthermore, when the operation position of the operation lever 55 in the rightward direction is operated between the G4 position and the G5 position, it becomes larger than the operation position in the forward / rearward direction, and the output shaft of the left hydraulic pump 7L rotates forward and the output shaft of the right hydraulic pump 7R rotates reversely, and the work vehicle 1 turns to the right.

[0047] In addition, when the operation amount of the operation lever 55 in the forward / rearward direction is greater than the operation amount in the leftward direction, and the operation position of the operation lever 55 in the leftward direction is operated from the G1 position to the G3 position, the right hydraulic pump 7R rotates in the same direction with the rotation speed of the right hydraulic pump 7R being greater than the rotation speed of the left hydraulic pump 7L, causing the work vehicle 1 to make a wide turn to the left. When the operation position of the operation lever 55 in the leftward direction is the same as the operation position in the forward / rearward direction, the rotation speed of the left hydraulic pump 7L becomes 0, and only the right hydraulic pump 7R rotates, causing the work vehicle 1 to make a left pivot turn (left pivot turn). Furthermore, when the operation position of the operation lever 55 in the leftward direction is operated between the G4 position and the G5 position, it becomes greater than the operation position in the forward / rearward direction, causing the output shaft of the right hydraulic pump 7R to rotate forward and the output shaft of the left hydraulic pump 7L to rotate reversely, causing the work vehicle 1 to turn left. In this embodiment, turning refers to the movement of the work vehicle 1 when the operating position in the right direction is operated between the G4 position and the G5 position, or when the operating position in the left direction is operated between the G4 position and the G5 position.

[0048] On the other hand, when the operation lever 55 is operated between positions G4 and G5 in the forward direction, the operation position becomes larger than the operation position in the left and right direction, the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R rotate forward, and the work vehicle 1 moves forward at high speed. When the operation lever 55 is operated between positions G4 and G5 in the backward direction, the operation position becomes larger than the operation position in the left and right direction, the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R rotate in the opposite direction, and the work vehicle 1 moves backward at high speed. Note that the operation of the other operation levers 55 in the forward and backward directions is similar to that in the left and right directions.

[0049] The work vehicle 1 is provided with various switches and sensors connected to the controller 10 described above. FIG. 6 is a block diagram of the work vehicle 1. Referring to FIG. 6, the work vehicle 1 includes a creep setting member 16 provided around the driver's seat 54. The creep setting member 16 may be called an operation member. The creep setting member 16 is configured, for example, as a touch panel, a freely slidable slide switch, or a dial. Creep refers to a control that causes the work vehicle 1 to travel at an upper limit speed or less, regardless of the amount of operation of at least one operation device (the setting member 11, one or more operation levers 55) to which a user's speed change operation is input. The upper limit speed is input by the creep setting member 16. The creep setting member 16 is configured to switch between a normal mode and a creep mode. A state in which the upper limit speed is set by the creep setting member 16 is called a creep mode. A state other than the creep mode is called a normal mode.

[0050] In the normal mode, the target rotation speed of the engine 6 is set by operating the setting member 11, and the primary pilot pressure corresponding to the target rotation speed is obtained based on the setting line L1 or L2 in FIG. 4. Then, the secondary pilot pressure is set based on the operation amount of one or more operation levers 55, and the hydraulic motors (31L, 31R) and the hydraulic pumps (7L, 7R) are controlled. That is, in the normal mode, the speed of the work vehicle 1 is changed according to the operation amount of at least one operation device, and the work vehicle 1 can be driven at a speed higher than the upper limit speed. On the other hand, in the creep mode, the setting line L1 or L2 in FIG. 4 is not used to determine the primary pilot pressure, and the primary pilot pressure is determined to be lower than the primary pilot pressure in the normal mode using the first reference information 10r1 described later. The settings after the secondary pilot pressure in the creep mode are the same as those in the normal mode, but since the secondary pilot pressure is equal to or lower than the primary pilot pressure, the primary pilot pressure is limited, and the speed of the work vehicle 1 is limited to be equal to or lower than the upper limit speed, regardless of the operation amount of at least one operation device (the setting member 11, one or more operation levers 55).

[0051] 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 pressures output from the secondary ports of the operation valves OVA, OVB, OVC, and OVD change in response to the operation position of the operation lever 55. Therefore, the hydraulic sensors SP11 to SP14 are sensors for detecting the secondary pilot pressures. The hydraulic sensors SP11 to SP14 may be referred to as additional hydraulic sensors.

[0052] The work vehicle 1 includes a hydraulic sensor SP5L for detecting the hydraulic pressure of the drive oil passage PA5L, a hydraulic sensor SP6L for detecting the hydraulic pressure of the drive oil passage PA6L, a hydraulic sensor SP5R for detecting the hydraulic pressure of the drive oil passage PA5R, and a hydraulic sensor SP6R for detecting the hydraulic pressure of the drive oil passage PA6R. That is, the hydraulic sensors (SP5L, SP6L, SP5R, SP6R) are configured to detect the hydraulic pressure of the working oil in the drive oil passages (PA5L, PA6L, PA5R, PA6R). The states of the left hydraulic motor 31L and the right hydraulic motor 31R can be detected from the pressure difference between the hydraulic sensor SP5L and the hydraulic sensor SP6L and the pressure difference between the hydraulic sensor SP5R and the hydraulic sensor SP6R.

[0053] 2, 3, and 6, the work vehicle 1 further includes a rotation speed sensor SR31L connected to a rotating shaft of the left hydraulic motor 31L and configured to detect the rotation speed of the left hydraulic motor 31L at a sampling period Ts (e.g., 20 μs) and a rotation speed sensor SR31R configured to detect the rotation speed of the right hydraulic motor 31R at a sampling period Ts. The states of the left hydraulic motor 31L and the right hydraulic motor 31R can be detected from the rotation direction and the magnitude of the rotation speed detected by the rotation speed sensor SR31L and the rotation direction and the magnitude of the rotation speed detected by the rotation speed sensor SR31R. The work vehicle 1 may include an operation detection sensor 18 configured to detect the operation position of the operation lever 55. The operation detection sensor 18 is connected to the controller 10 described later. The operation detection sensor 18 is a position sensor that detects the position of the operation lever 55, or the like. <Controller 10 Configuration>

[0054] The controller 10 has a processor 10a and a memory 10b as shown in FIG. 6 in order to realize the above-mentioned control of the vehicle speed in the creep mode. The processor 10a may be called a hardware processor or an electronic circuit. The memory 10b includes a volatile memory and a non-volatile memory. The memory 10b includes a driving control program 10c1, a first reference information 10r1, and a second reference information 10r2 in order to realize the above-mentioned control.

[0055] The first reference information 10r1 represents a first correspondence relationship between the rotation speed RS of the engine 6 detected by the speed sensor 6a and the primary pilot pressure in the normal mode. That is, the first reference information 10r1 represents the first correspondence relationship represented by the set line L1 in FIG. 4. The second reference information 10r2 represents a second correspondence relationship between the rotation 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 the normal mode. That is, the second reference information 10r2 represents the second correspondence relationship represented by the set line L2 in FIG. 4.

[0056] The processor 10a executes the following control while executing the travel control program 10c1 with reference to the first reference information 10r1 and the second reference information 10r2. First, when the normal mode is selected by the creep setting member 16, the processor 10a is configured to obtain the rotation speed RS of the engine 6 from the speed sensor 6a, obtain a primary pilot pressure corresponding to the detected rotation speed RS of the engine 6 from the first reference information 10r1, and control the primary pressure control valve CV1 to achieve the obtained primary pilot pressure. When the drop amount of the engine 6 is large while the normal mode is selected, the processor 10a is configured to obtain a primary pilot pressure corresponding to the rotation speed RS of the engine 6 detected by the speed sensor 6a from the second reference information 10r2, and control the primary pressure control valve CV1 to achieve the obtained primary pilot pressure.

[0057] The memory 10b further includes third reference information 10r3 and fourth reference information 10r4. When the creep mode is selected by the creep setting member 16, the processor 10a receives the upper limit speed input by the creep setting member 16 to determine the target rotation speed RSi of the hydraulic motors (31L, 31R). This target rotation speed RSi is the target rotation speed of the hydraulic motors (31L, 31R) when it is assumed that the work vehicle 1 travels straight ahead. When the operation lever 55 is operated in this state, the operation device 56 sets the secondary pilot pressure based on the characteristics shown in FIG. 5. However, as shown in FIG. 5, the secondary pilot pressure is a pressure that maximizes the primary pilot pressure, and considering that the operation lever 55 is normally operated to be pushed down to its maximum in the creep mode, the primary pilot pressure is an example of a control parameter for setting the displacement of the hydraulic pumps (7L, 7R) configured to supply hydraulic oil to the hydraulic motors (31L, 31R). In this embodiment, the primary pressure control valve CV1 is an example of a control mechanism configured to control the hydraulic pressure of pilot oil that changes the displacement volume of the hydraulic pump (31L, 31R). Since the primary pilot pressure is determined based on a current applied to a solenoid of the primary pressure control valve CV1 (control mechanism), the control parameter may be a current value (control current value) that controls the primary pressure control valve CV1 (control mechanism). Hereinafter, this embodiment will be described assuming that the control parameter is a current value that controls the primary pressure control valve CV1 (control mechanism).

[0058] In the creep mode, the processor 10a sets the target rotation speed of the engine 6 based on the amount of operation of the creep setting member 16. The processor 10a outputs a rotation command indicating, for example, a fuel injection amount, an injection timing, and a fuel injection rate to the injector so that the determined target rotation speed of the engine 6 is reached. Alternatively, the processor 10a outputs a rotation command indicating a fuel injection pressure, etc., to the supply pump or the common rail so that the determined target rotation speed of the engine 6 is reached. For example, the processor 10a may rotate the engine 6 at the same predetermined rotation speed for all target rotation speeds RSi. When the setting member 11 is operated in the creep mode, the target rotation speed RSi or the target rotation speed of the engine 6 may be changed in accordance with the operation, as in the normal mode, instead of changing the target rotation speed of the engine 6 in accordance with the operation.

[0059] FIG. 7 shows a control block diagram in the creep mode in the first embodiment. The control system 20A according to the first embodiment includes a feedforward controller Cf v1 and the feedback controller Cbb v1 The feedforward controller Cf v1 is the control current value u0 of the primary pressure control valve CV1 corresponding to the target rotation speed RSi of the hydraulic motor (31L, 31R). V1 The memory 10b outputs the control current value u0 corresponding to the target rotation speed RSi. V1 The processor 10a calculates the target rotation speed RSi of the hydraulic motors (31L, 31R) from the upper limit speed input by the creep setting member 16. v1 The processor 10a executes the above calculation to obtain the control current value u0 from the calculated target rotation speed RSi by using a stored list or a conversion formula. V1 The determined control current value u0 V1 is called the reference value of the control parameter.

[0060] Control current value u0 from target rotation speed RSi V1It is preferable to derive based on the following: As described above, when the rotation speed of the engine 6 in the creep mode is known from the target rotation speed RSi, and the primary pilot pressure corresponding to the target rotation speed RSi is applied to the hydraulic pumps (7L, 7R), the processor 10a derives the reference value u0 of the control current value of the primary pressure control valve CV1 so that the hydraulic motors (31L, 31R) rotate at the target rotation speed RSi in a state in which the left and right traveling devices (3L, 3R) are not subjected to resistance from the road surface. V1 is determined.

[0061] Feedback controller Cbb v1 is an offset value uA corresponding to the speed difference e with a larger absolute value among the speed differences (eL, eR) obtained by subtracting the target rotation speed RSi from the rotation speed (RSdL, RSdR) of the hydraulic motor (31L, 31R) detected at the sampling period Ts by the rotation speed sensor (SR31L, SR31R). V1 The processor 10a is configured to receive the rotation speeds (RSdL, RSdR) of the hydraulic motors (31L, 31R) detected in a sampling period Ts from the rotation speed sensors (SR31L, SR31R), and calculate a speed difference (eL, eR) obtained by subtracting a target rotation speed RSi from the rotation speeds (RSdL, RSdR) in the sampling period Ts. The processor 10a calculates an offset value uA according to the speed difference e, the speed difference having a larger absolute value among the speed differences (eL, eR) in the sampling period Ts. V1 The processor 10a is configured to calculate the reference value u0 V1 and offset value uA V1 The sum of and the control parameter u V1 That is, the processor 10a inputs the control current value u V1 The processor 10a applies a current of u to the solenoid of the primary pressure control valve CV1. V1 The processor 10a sends a command corresponding to the above-mentioned control parameter u to the primary pressure control valve CV1 (control mechanism) that controls the displacement of the hydraulic pump (7L, 7R). V1The processor 10a controls the displacement of the hydraulic pumps (7L, 7R) in response to the rotation of the hydraulic motors (31L, 31R) by controlling the engine 6 to rotate the hydraulic pumps (7L, 7R).

[0062] The processor 10a calculates the control parameter u at the sampling period Ts. V1 More specifically, the processor 10a updates the offset value uA V1 FIG. 8 shows the speed difference e and the offset value uA V1 8, the processor 10a (1) updates the offset value uA to reduce the displacement when the speed difference e is greater than a predetermined positive first threshold af. V1 to the first feedback control value d1 v1 (2) subtracting an offset value uA to increase the displacement when the speed difference e is less than a predetermined negative second threshold value -ae V1 to the second feedback control value d2 v1 (3) When the speed difference e is between the second threshold value -ae and the first threshold value af, the offset value uA v1 (4) When the speed difference e is greater than the third threshold ag, which is greater than the first threshold af, the offset value uA V1 is set to 0, and the offset value uA V1 The system is configured to update the

[0063] The processor 10a calculates the offset value uA as described above while referring to the speed difference e at the sampling period Ts. V1 Increase / decrease / maintain / reset the offset value uA V1 is obtained by accumulating the above-mentioned process in the sampling period Ts. In other words, if the speed difference e is smaller than the predetermined negative second threshold value -ae for a period of five times the sampling period Ts, the offset value uA V1 is the second feedback control value d2 v1 The offset value uA V1 The initial value of is 0. The reference value of the control current value u0V1 Since it is assumed that there is no external load, the reference value u0 V1 If the speed difference e is controlled only by the third feedback control value d3, the speed difference e tends to be negative and generally falls below the second threshold value −ae. v1 This also adds to the offset value uA V1 is positive. Also, the reference value u0 V1 offset value uA V1 The value (u V1 ) is the predetermined upper limit u maxv1 When the control parameter u V1 The upper limit value u maxv1 The device is configured to:

[0064] When the speed difference e is smaller than a predetermined negative second threshold value −ae, the second feedback control value d2 v1 By increasing the offset value uA V1 The absolute value d1 of the first feedback control value can be increased to shorten the time until the vehicle starts to run. v1 is the absolute value of the second feedback control value d2 v1 This prevents the work vehicle 1 from accelerating too much due to a large decrease in running resistance at the start of movement, by using the offset value uA V1 This can be prevented by rapidly decreasing

[0065] Furthermore, when the operating lever 55 is shifted from the neutral position, the processor 10a calculates the offset value uA V1 to the second feedback control value d2 v1 A third feedback control value d3 that is greater than v1 Since the operation lever 55 is often set to the neutral position when the work vehicle 1 is stationary, when the torque of the hydraulic motors (31L, 31R) is required from when the vehicle is stationary to when the vehicle starts moving, the offset value uA V1 Since the vehicle speed can be increased rapidly, the time until the vehicle starts running can be shortened. <Operation of the work vehicle according to the first embodiment>

[0066] FIG. 9 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 S25 are executed at a predetermined sampling period (for example, 20 μs). In step S1, the processor 10a drives the engine 6. In step S2, the processor 10a determines whether or not the creep mode has been selected by the creep setting member 16. That is, the control method according to this embodiment includes determining whether or not the creep mode has been selected by the creep setting member 16. When the creep mode is set, that is, when an upper limit speed is set (Yes in step S2), the process proceeds from step S9 to S10. When the normal mode is set, that is, when an upper limit speed is not set or an invalid upper limit speed that does not have the first correspondence relationship or the second correspondence relationship is set (No in step S2), the process proceeds to step S3.

[0067] In the normal mode (No in step S2), in step S3, the processor 10a acquires the rotation speed RS of the engine 6 detected by the speed sensor 6a. Then, the processor 10a judges whether or not there is an engine drop. That is, in step S9, the processor 10a judges whether or not the drop amount ΔE1 of the engine 6 is equal to or larger than the anti-stall judgment value. When there is no engine drop (No in step S3), in step S4, the processor 10a obtains a control parameter (control current value of the primary pressure control valve CV1) corresponding to the primary pilot pressure from the first reference information 10r1 based on the rotation speed RS of the engine 6. When there is an engine drop (Yes in step S3), in step S5, the processor 10a obtains a control parameter (control current value of the primary pressure control valve CV1) corresponding to the primary pilot pressure from the second reference information 10r2 based on the rotation speed RS of the engine 6. After the process of step S4 or step S5 is completed, the process of step S6 is executed.

[0068] In step S6, the processor 10a applies a current corresponding to the control parameter (control current value of the primary pressure control valve CV1) obtained in step S4 or step S5 to the solenoid of the primary pressure control valve CV1 that sends the current to the operation valves OVA, OVB, OVC, and OVD. That is, the control method according to this embodiment includes controlling the displacement of the hydraulic pump (7L, 7R) according to the control parameter (control current value of the primary pressure control valve CV1). In step S7, the operation valves OVA, OVB, OVC, and OVD convert the primary pilot pressure into a secondary pilot pressure based on the operation lever position (first operation amount) of the operation lever 55 (first operation device). In step S8, the secondary pilot pressure of the pilot oil is applied to the ports (PLa, PRa, PLb, PRb) that provide hydraulic pressure to the swash plates of the hydraulic pumps (7L, 7R), and the hydraulic pumps (7L, 7R) and the hydraulic motors (31L, 31R) are controlled. Through steps S1 and S8, the processor 10a is configured to rotate the hydraulic motors (31L, 31R) via the hydraulic pumps (7L, 7R) by sending a command to the engine 6, which is configured to rotate the hydraulic pumps (7L, 7R), to drive the engine 6 in accordance with the control parameters. The control method according to this embodiment includes rotating the hydraulic motors (31L, 31R) by rotating the hydraulic pumps (7L, 7R).

[0069] In the creep mode (Yes in step S2), in step S9, the processor 10a receives an input corresponding to the target rotation speed RSi of the hydraulic motors (31L, 31R) configured to drive the traveling gears (3L, 3R) from the creep setting member 16. That is, in the control method according to this embodiment, the target rotation speed RSi of the hydraulic motors (31L, 31R) configured to drive the traveling gears (3L, 3R) is obtained via the creep setting member 16. In step S10, the processor 10a controls the feedforward controller Cf v1 and the feedback controller Cbb v1 and the control parameter u V1That is, the control method according to the present embodiment outputs a control parameter u for setting the displacement of the hydraulic pump (7L, 7R) configured to supply hydraulic oil to the hydraulic motor (31L, 31R) in response to the operation of the creep setting member 16. V1 After that, in steps S6 to S8, the same control as in the normal mode is carried out.

[0070] FIG. 10 shows the control parameter u V1 5. In step S11, the processor 10a determines whether or not the operating lever 55 is in the neutral position. Specifically, the processor 10a determines whether or not the operating lever 55 is in the neutral region RA1 based on the outputs of the hydraulic sensors (SP5L, SP6L, SP5R, SP6R). ​​For example, the processor 10a can determine whether or not the secondary pilot pressure obtained from the outputs of the hydraulic sensors (SP5L, SP6L, SP5R, SP6R) based on the characteristics of FIG. 5 by using threshold processing to determine whether or not it can be considered that the secondary pilot pressure is equal to or lower than Pa in FIG. 5, thereby determining whether or not the operating lever 55 is in the neutral region RA1.

[0071] When the operating lever 55 is in the neutral position (Yes in step S11), in step S12, the processor 10a V1 is set to 0, and the offset value uA V1 Reset to 0.

[0072] When the operating lever 55 is not in the neutral position (No in step S11), in step S13, the processor 10a calculates the control parameter u based on the target rotation speed RSi. V1 The reference value u0 V1In step S13, the processor 10a receives the rotational speeds (RSdL, RSdR) of the hydraulic motors (31L, 31R) detected in the sampling period Ts from the rotational speed sensors (SR31L, SR31R). In step S15, the processor 10a calculates the speed difference (eL, eR) obtained by subtracting the rotational speeds (RSdL, RSdR) from the target rotational speeds RSi in the sampling period Ts.

[0073] Next, in step S16, the processor 10a determines whether or not the operating lever 55 has been shifted from the neutral position. This may be done by determining that the operating lever 55 is in the neutral position at the immediately preceding sampling time, and determining whether or not the operating lever 55 is determined to be not in the neutral position at the current sampling time in step S11, but in consideration of noise and the like, the processor 10a may determine that the operating lever 55 has been shifted from the neutral position when it is determined in step S11 that the operating lever 55 is not in the neutral position continuously for a predetermined multiple of the sampling period Ts, and immediately before that, the operating lever 55 is determined to be not in the neutral position.

[0074] When it is determined that the operating lever 55 has been shifted from the neutral position (Yes in step S16), in step S17, the processor 10a V1 to the third feedback control value d3 v1 When it is determined that the operating lever 55 has not been shifted from the neutral position (No in step S16) or when step S17 ends, the process proceeds to step S18. In step S18, the processor 10a determines whether the speed difference e, which has a larger absolute value of the speed differences (eL, eR), is within the following range.

[0075] When the speed difference e is greater than the third threshold ag, in step S19, the processor 10a calculates the offset value uA V1When the speed difference e is greater than the first threshold af and equal to or smaller than the third threshold ag, in step S20, the processor 10a resets the offset value uA V1 to the first feedback control value d1 v1 When the speed difference e is equal to or smaller than the first threshold af and equal to or larger than the second threshold −ae, in step S21, the processor 10a subtracts the offset value uA V1 When the speed difference e is smaller than the second threshold value −ae, in step S22, the processor 10a does not change the offset value uA V1 to the second feedback control value d2 v1 By the processes from step S19 to S22, the processor 10a calculates an offset value uA according to the speed difference e at the sampling period Ts. V1 Calculate.

[0076] In step S23, the processor 10a calculates the reference value u0 V1 and offset value uA V1 The sum of these is a predetermined upper limit u maxv1 It is judged whether it exceeds the reference value u0 V1 and offset value uA V1 The sum of these is the upper limit u maxv1 If it exceeds the control parameter u (Yes in step S23), in step S24, the processor 10a V1 The upper limit value u maxv1 The reference value u0 V1 and offset value uA V1 The sum of these is the upper limit u maxv1 If it is equal to or less than (No in step S23), in step S25, the processor 10a adjusts the control parameter u V1 The reference value u0 V1 offset value uA V1 By adding the control parameter u V1 Update. <Actions and Effects of the First Embodiment>

[0077] In the work vehicle 1 according to this embodiment, when the speed difference e is smaller than the second threshold value −ae, the offset value uAV1 to the second feedback control value d2 v1 Therefore, even if the running resistance is large when the work vehicle 1 starts running, the work vehicle 1 can start running quickly. Furthermore, when the speed difference e is greater than the first threshold value af and equal to or less than the third threshold value ag, the work vehicle 1 adds the offset value uA V1 to the second feedback control value d2 v1 The first feedback control value d1v1 having an absolute value greater than the absolute value of the speed difference e is subtracted from the first feedback control value d1v1. Furthermore, when the speed difference e is greater than the third threshold value ag, in step S19, the processor 10a resets the offset value uAV1 to 0. This makes it possible to prevent the work vehicle 1 from accelerating too much due to a large decrease in running resistance at the start of movement. <Modification of the first embodiment>

[0078] As a modification of the first embodiment, the control system 20A uses, as a feedback controller, the feedback controller Cb v1 Alternatively, the control system 20A may be a combination of the feedback controller Cb v11 and Cb v12 It may also be a combination of the above. <Second embodiment>

[0079] In the first embodiment, an example was given in which the primary pilot pressure was controlled to realize the creep mode, but the secondary pilot pressure may also be controlled. Fig. 11 is a hydraulic circuit diagram of a traveling system of a work vehicle in a second embodiment. Fig. 11 shows a configuration added to Fig. 3. In Fig. 11, the same components as those in Fig. 3 are given the same reference numerals, 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 hydraulic circuit 1A in that it further includes relief valves CV23, CV24, proportional valves CV21, CV22, drain oil passages DR3-DR6, check valves CK1-CK4, and throttles TH1-TH4.

[0080] The relief valves CV23 and CV24 are balanced relief valves whose set pressure for opening varies based on the pressure of the pilot oil, and have control ports 23a and 24a that receive the pilot oil. The relief valves CV23 and CV24 are configured to open when the pressure on the input port is greater than the pressure on the control ports 23a and 24a. At this time, the pilot oil is discharged to the hydraulic oil tank 70. The proportional valves CV21 and CV22 are connected to hydraulic oil passages 21 and 22 that are connected to the control ports 23a and 24a, and pilot oil is supplied from a pilot pump 71. The proportional valves CV21 and CV22 are electromagnetic proportional valves whose opening can be changed by exciting a solenoid, and are controlled by the controller 10.

[0081] Proportional valves CV21 and CV22 are connected to a pilot supply oil passage PA1, and in creep mode, are controlled to a pressure equal to the primary pressure control valve CV1 in the first embodiment plus an offset α that takes into consideration the outflow of pilot oil from relief valves CV23, CV24, etc., and in normal mode, operate to a value equal to a set line L1 plus the offset α when anti-stall control is not performed, and to a value equal to a set line L2 plus the offset α when anti-stall control is performed. Proportional valves CV21 and CV22 may also be referred to as secondary pressure control valves CV2.

[0082] The controller 10 performs feedback control of the control valves (proportional valves CV21, CV22) for the pilot pressure of the hydraulic pumps (7L, 7R) based on the speed difference (eL, eR) between the target rotation speed RSi of the hydraulic motors (31L, 31R) corresponding to the target vehicle speed and the detected rotation speed RSd. In addition, the control parameters are input to the secondary pressure control valve CV2 that adjusts the secondary pilot pressure, which is the hydraulic pressure of the pilot oil passage connecting the operation valves OVA, OVB, OVC, OVD controlled by the travel command input device (operation lever 55) to which the user inputs a command for the travel direction and the pilot ports (PLa, PRa, PLb, PRb) of the hydraulic pumps (7L, 7R).

[0083] The discharge oil passage DR3 is connected to a first pilot oil passage PA11. The discharge oil passage DR4 is connected to a second pilot oil passage PA12. The discharge oil passage DR5 is connected to a third pilot oil passage PA13. The discharge oil passage DR6 is connected to a fourth pilot oil passage PA14. The check valves CK1-CK4 close the discharge oil passages DR3-DR6 unless the pressure on the side where the throttles TH1-TH4 are located becomes higher than the pressure on the side where the relief valves CV23, CV24 are located by a predetermined value or more.

[0084] The pilot pressures of the discharge oil passages DR3 and DR4 become high when the left hydraulic pump 7L rotates forward and backward, respectively, so that when the pilot pressure of one of the drain oil passages DR3 and DR4 becomes equal to the primary pilot pressure, the pilot pressure of the other drain oil passage DR4 becomes significantly smaller than the primary pilot pressure. The pilot pressures of the discharge oil passages DR5 and DR6 become high when the right hydraulic pump 7R rotates forward and backward, respectively, so that when the pilot pressure of one of the drain oil passages DR5 and DR6 becomes equal to the primary pilot pressure, the pilot pressure of the other drain oil passage DR4 becomes significantly smaller than the primary pilot pressure. For this reason, usually, only one of the check valves CK1 and CK2 opens. For this reason, the proportional valves CV21 and CV22 can execute the above-mentioned control by controlling the pressure of the proportional valves CV21 and CV22 so that the pressure is equal to the pressure caused by the control of the primary pressure control valve CV1 according to the first embodiment plus the pressure loss caused by the pilot oil flowing out from the relief valves CV23 and CV24.

[0085] The throttle TH1 is provided in the first pilot oil passage PA11 between the first shuttle valve SVa and the discharge oil passage DR3, and is configured to reduce the flow rate of pilot oil in the first pilot oil passage PA11. The throttle TH2 is provided in the second pilot oil passage PA12 between the second shuttle valve SVb and the discharge oil passage DR4, and is configured to reduce the flow rate of pilot oil in the second pilot oil passage PA12. The throttle TH3 is provided in the third pilot oil passage PA13 between the third shuttle valve SVc and the discharge oil passage DR5, and is configured to reduce the flow rate of pilot oil in the third pilot oil passage PA13. The throttle TH4 is provided in the fourth pilot oil passage PA14 between the fourth shuttle valve SVd and the discharge oil passage DR6, and is configured to reduce the flow rate of pilot oil in the fourth pilot oil passage PA14.

[0086] The target rotation speed RSi is a target rotation speed of the hydraulic motor (31L, 31R) when it is assumed that the work vehicle 1 travels straight ahead. In this embodiment, the secondary pilot pressure is an example of a control parameter for setting the displacement of the hydraulic pump (7L, 7R) configured to supply hydraulic oil to the hydraulic motor (31L, 31R). In this embodiment, the secondary pressure control valve CV2 is an example of a control mechanism configured to control the hydraulic pressure of the pilot oil that changes the displacement of the hydraulic pump (31L, 31R). Since the secondary pilot pressure is determined based on the current applied to the solenoid of the secondary pressure control valve CV2 (control mechanism), the control parameter may be a current value (control current value) that controls the secondary pressure control valve CV2 (control mechanism). In this embodiment, the control parameter will be described as a current value that controls the secondary pressure control valve CV2 (control mechanism).

[0087] FIG. 12 shows a control block diagram in the second embodiment. The control system 20B according to the second embodiment is mainly different from the control system 20A according to the first embodiment in that the controlled object is the secondary pressure control valve CV2. However, since the control system 20B is the same as the control system 20A in most respects, the differences will be mainly described. The control system 20B includes a feedforward controller Cf v2 and the feedback controller Cbbv2 The feedforward controller Cf v2 is the control current value u0 of the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 corresponding to the target rotation speed RSi of the hydraulic motor (31L, 31R). V2 When the work vehicle 1 starts moving, it usually moves straight ahead, so the control current value u0 of the proportional valves CV21 and CV22 (secondary pressure control valve CV2) is V2 The memory 10b stores the control current value u0 corresponding to the target rotation speed RSi. V2 The feedforward controller Cf v2 The processor 10a executes the above calculation to obtain the control current value u0 from the calculated target rotation speed RSi by using a stored list or a conversion formula. V2 The determined control current value u0 V2 are the reference values ​​of the control parameters according to the second embodiment.

[0088] Control current value u0 from target rotation speed RSi V2 It is preferable to derive based on the following: As described above, when the rotational speed of the engine 6 in the creep mode is known from the target rotational speed RSi and a secondary pilot pressure corresponding to the target rotational speed RSi is applied to the hydraulic pumps (7L, 7R), the processor 10a derives the reference value u0 of the control current value of the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 so that the hydraulic motors (31L, 31R) rotate at the target rotational speed RSi in a state in which the left and right traveling devices (3L, 3R) are not subjected to resistance from the road surface. V2 is determined.

[0089] Feedback controller Cbb v2 is an offset value uA corresponding to the speed difference e with a larger absolute value among the speed differences (eL, eR) obtained by subtracting the target rotation speed RSi from the rotation speed (RSdL, RSdR) of the hydraulic motor (31L, 31R) detected at the sampling period Ts by the rotation speed sensor (SR31L, SR31R). V2The processor 10a is configured to receive the rotational speeds (RSdL, RSdR) of the hydraulic motors (31L, 31R) detected in a sampling period Ts from the rotational speed sensors (SR31L, SR31R), and calculate the speed differences (eL, eR) obtained by subtracting the target rotational speed RSi from the rotational speeds (RSdL, RSdR) in the sampling period Ts.

[0090] The processor 10a calculates an offset value uA according to the speed difference e, which has a larger absolute value among the speed differences (eL, eR) in the sampling period Ts. V2 The processor 10a is configured to calculate the reference value u0 V2 and offset value uA V2 The sum of and the control parameter u V2 The processor 10a inputs the control current value u V2 The processor 10a applies a current of u to the solenoid of the secondary pressure control valve CV2. V2 The processor 10a sends a command corresponding to the above-mentioned control parameter u to the secondary pressure control valve CV2 (control mechanism) that controls the displacement of the hydraulic pump (7L, 7R). V2 The processor 10a controls the displacement of the hydraulic pumps (7L, 7R) in response to the rotation of the hydraulic motors (31L, 31R) by controlling the engine 6 to rotate the hydraulic pumps (7L, 7R).

[0091] The processor 10a calculates the control parameter u at the sampling period Ts. V2 More specifically, the processor 10a updates the offset value uA V2 The updating method and the speed difference thresholds af, -ae, and ag (see FIG. 8) may be the same as those in the first embodiment. In addition, the first feedback control value -d1 is updated based on the difference in the valve characteristics between the primary pressure control valve CV1 and the secondary pressure control valve CV2. v2 , second feedback control value +d2 v2 , the third feedback control value +d3 v2 , upper limit u maxv2The value of the first feedback control value -d1 of the first embodiment v1 , second feedback control value +d2 v1 , the third feedback control value +d3 v1 , upper limit u maxv1 may differ from.

[0092] However, the first feedback control value d1 v2 , second feedback control value d2 v2 , third feedback control value d3 v2 , upper limit u maxv2 The magnitude relationship of the absolute values ​​of the offset values ​​uA and uB is the same as that of the first embodiment. V1 When the torque of the hydraulic motors (31L, 31R) from the standstill to the start of movement is required, the offset value uA V1 Since the vehicle speed can be increased rapidly, the time until the vehicle starts running can be shortened.

[0093] FIG 13 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 S31 are executed at predetermined sampling intervals (e.g., 20 μs). In FIG 13, the same processes as in FIG 9 are given the same step numbers, and their explanations will be omitted. In the normal mode (Yes in step S2), in step S31, the processor 10a controls the proportional valves CV21, CV22 so that the pressure applied to the relief valves CV23, CV24 is higher than the pressure output by the primary pressure control valve CV1 as described above. This causes the relief valves CV23, CV24 to be closed.

[0094] In the creep mode (Yes in step S2), in step S10A after step S9, the processor 10a controls the feedforward controller Cf v2 and the feedback controller Cbb v2 and the control parameter u V2That is, the control method according to the present embodiment outputs a control parameter u for setting the displacement of the hydraulic pump (7L, 7R) configured to supply hydraulic oil to the hydraulic motor (31L, 31R) in response to the operation of the creep setting member 16. V2 The method of determination is the same as in the first embodiment. <Actions and Effects of the Second Embodiment>

[0095] In the control method for the work vehicle 1 or the work vehicle 1 according to the second embodiment, similarly to the first embodiment, it is possible to control the secondary pilot pressure so that the work vehicle 1 can start traveling quickly even when the running resistance is large. Also, in the control method or the work vehicle 1, it is possible to control the secondary pilot pressure so that the work vehicle 1 can be prevented from accelerating too much due to a large reduction in running resistance at the start of movement. <Modification of the second embodiment>

[0096] FIG. 14 is a hydraulic circuit diagram according to a modified example of the second embodiment. In the example of FIG. 14, shuttle valves SV12 and SV34 are provided instead of the check valves CK1 to CK4 in the example of FIG. 11. The shuttle valve SV12 connects the oil passage having the higher hydraulic pressure out of the discharge oil passage DR3 and the discharge oil passage DR4 to the relief valve CV23. The shuttle valve SV34 connects the oil passage having the higher hydraulic pressure out of the discharge oil passage DR5 and the discharge oil passage DR6 to the relief valve CV24. Even with this hydraulic circuit configuration, the above-mentioned control can be performed. Also, in the circuit of FIG. 11 or FIG. 14, the primary pressure control valve CV1 may be omitted. Also, at least one of the combination of the proportional valve CV21 and the balanced relief valve and the combination of the proportional valve CV21 and the balanced relief valve may be realized by an electromagnetic proportional relief valve.

[0097] As a modified example of the second embodiment, the control system 20B may be combined with the feedback controller Cbv2 described in Japanese Patent Application No. 2022-131895 as a feedback controller. Alternatively, the control system 20A may be combined with the feedback controller Cbv2 described in Japanese Patent Application No. 2022-131895 as a feedback controller. v21 and Cb v22 In the second embodiment described above, the hydraulic pumps (7L, 7R) and the hydraulic motors (31L, 31R) are controlled by the same feedback loop, but the left hydraulic pump 7L and the left hydraulic motor 31L and the right hydraulic pump 7R and the right hydraulic motor 31R may be controlled by separate feedback loops. FIG. 15 shows a control block diagram in a modification of the second embodiment.

[0098] Referring to FIG. 15, the control system 20BA according to this modification includes a feedforward controller Cf v2L and the feedback controller Cbb v2L and a feedforward controller Cf for controlling the right hydraulic pump 7R and the right hydraulic motor 31R. v2R and the feedback controller Cbb v2R The feedforward controller Cf v2L is the control current value u0 of the proportional valve CV21 corresponding to the target rotation speed RSiL of the hydraulic motor 31L. V2L The feedforward controller Cf v2R is the control current value u0 of the proportional valve CV22 corresponding to the target rotation speed RSiR of the hydraulic motor 31R. V2R Since the work vehicle 1 normally moves straight ahead when it starts to move, the target rotation speed RSiL of the hydraulic motor 31L and the target rotation speed RSiR of the hydraulic motor 31R may be the same.

[0099] Feedback controller Cbb v2Lis an offset value uA corresponding to a speed difference eL obtained by subtracting a target rotation speed RSiL from the rotation speed RSdL of the hydraulic motor 31L detected by the rotation speed sensor SR31L at a sampling period Ts. V2L The feedback controller Cbb is derived. v2R is an offset value uA corresponding to a speed difference eR obtained by subtracting a target rotation speed RSiR from the rotation speed RSdR of the hydraulic motor 31R detected by the rotation speed sensor SR31R at a sampling period Ts. V2R Derive.

[0100] The processor 10a sets the reference value u0 V2L and offset value uA V2L The sum of and the control parameter u V2L That is, the processor 10a inputs the control current value u V2L The processor 10a applies a current of u to the solenoid of the proportional valve CV21. V2L The processor 10a controls the displacement of the hydraulic pump 7L according to the reference value u0 V2R and offset value uA V2R The sum of and the control parameter u V2R That is, the processor 10a inputs the control current value u V2R The processor 10a applies a current of u to the solenoid of the proportional valve CV22. V2R In accordance with this, the displacement volume of the hydraulic pump 7R is controlled. As described above, the left hydraulic pump 7L and the left hydraulic motor 31L and the right hydraulic pump 7R and the right hydraulic motor 31R can be controlled separately. <Third embodiment>

[0101] Although the operation lever 55 according to the above embodiment directly controls the operation valves OVA, OVB, OVC, and OVD, the work vehicle 1 may detect the operation amount of the operation lever 55 by a separate sensor such as a potentiometer, and control the control valves that control the first pump pilot pressure and the second pump pilot pressure based on the operation amount detected by the sensor. In this case, the same control as in the second embodiment can be realized by adjusting the operation amount detected by the sensor. FIG. 16 is a hydraulic circuit diagram of the traveling system of the work vehicle 1 in the third embodiment. In FIG. 16, the same components as those in FIG. 3 are given the same reference numerals, and detailed description is omitted. In the third embodiment, the work vehicle 1 includes a hydraulic circuit 1C. The hydraulic circuit 1C includes pilot control valves CV31 to CV34 that control the pilot pressures applied to the ports (PLa, PRa, PLb, PRb), respectively, instead of the operation valves OVA, OVB, OVC, and OVD and the first to fourth shuttle valves SVa, SVb, SVc, and SVd. The pilot control valves CV31 to CV34 are electromagnetic proportional valves including solenoids.

[0102] In this embodiment, the pilot supply oil passage PA8 connects the pilot control valves CV31-CV34 and the pilot supply oil passage PA8, and the first to fourth pilot oil passages PA11-PA14 are connected to the pilot control valves CV31-CV34, respectively. In this embodiment, the pilot supply oil passages PA1, PA8 and the first to fourth pilot oil passages PA11-PA14 correspond to a pilot oil supply circuit that connects the pilot pump to the first pump pilot port or the second pump pilot port. In this embodiment, there is no difference between the primary pilot pressure and the secondary pilot pressure because there are no operation valves OVA, OVB, OVC, OVD. Therefore, in this embodiment, these are simply called pilot pressure without distinction.

[0103] In the normal mode, the controller 10 controls the pilot control valves CV31 to CV34 so that the pilot pressure corresponding to FIG. 5 can be output in accordance with the operation position detected by the operation detection sensor 18. In the creep mode, in order to limit the vehicle speed, even if the operation lever 55 is actually operated at a full stroke, it is deemed to have been operated to the deemed operation position Ga. Specifically, when the operation position is between the G0 position and the Ga position, the pilot pressure is obtained from the correspondence relationship in FIG. 5 in accordance with the operation position detected by the operation detection sensor 18. When the operation position is the Ga position or higher, it is deemed to have been operated to the deemed operation position Ga. In this embodiment, the operation amount from the G0 position to the Ga position is called the deemed operation amount r.

[0104] The target rotation speed RSi is the target rotation speed of the hydraulic motors (31L, 31R) when it is assumed that the work vehicle 1 travels straight ahead. In this embodiment, the deemed operation amount r is an example of a control parameter for setting the displacement of the hydraulic pumps (7L, 7R) configured to supply hydraulic oil to the hydraulic motors (31L, 31R). Note that the control parameter may be the output pressure of the pilot control valves CV31 to CV34, but since this is substantially the same as in the second embodiment, a description of this case will be omitted. In this embodiment, the pilot control valves CV31 to CV34 are an example of a control mechanism configured to control the hydraulic pressure of the pilot oil that changes the displacement of the hydraulic pumps (31L, 31R).

[0105] FIG. 17 shows a control block diagram in the third embodiment. The control system 20C according to the third embodiment is mainly different from the control system 20A according to the first embodiment in that the controlled object is the deemed manipulated variable r. However, since the control system 20C is the same as the control system 20A in most respects, the differences will be mainly described. The control system 20C includes a feedforward controller Cf R and the feedback controller Cbb R and the converter Cbb v The feedforward controller Cf Routputs a reference deemed manipulated variable r0 corresponding to the target rotation speed RSi of the hydraulic motor (SR31L, SR31R). The reference deemed manipulated variable r0 is called the reference value of the control parameter r. The memory 10b stores a list or a conversion formula of the deemed manipulated variable r0 corresponding to the target rotation speed RSi. R The processor 10a executing the above derives the deemed manipulated variable r0 from the target rotation speed RSi by using the stored list or a conversion formula.

[0106] Feedback controller Cbb R derives an adjustment deemed manipulated variable r1 according to the speed difference e with the larger absolute value of the speed differences (eL, eR) obtained by subtracting the target rotational speed RSi from the rotational speeds (RSdL, RSdR) of the hydraulic motors (31L, 31R) detected at the sampling period Ts by the rotational speed sensors (SR31L, SR31R). The adjustment deemed manipulated variable r1 is called the offset value of the control parameter. The processor 10a then determines the sum of the reference deemed manipulated variable r0 and the adjustment deemed manipulated variable r1 as the deemed manipulated variable (control parameter) r. Converter Cbb v holds the relationship between the deemed operation amount r and the pilot pressure Pt as shown in FIG. 5, and the control current value u of the pilot control valves CV31 to CV34 corresponding to the pilot pressure Pt V Output.

[0107] The processor 10a updates the control parameter r at the sampling period Ts. More specifically, the processor 10a updates the offset value r1 at the sampling period Ts. This updating method and the speed difference thresholds af, -ae, and ag (see FIG. 8) may be the same as those in the first embodiment. However, in this embodiment, since the operation lever position of the operation lever 55 is detected by the operation detection sensor 18, the determination as to whether the operation lever 55 is in the neutral position or has been shifted from the neutral position is made from the output of the operation detection sensor 18 as to whether the operation lever has deviated from the operation lever position corresponding to the neutral region RA1 in FIG. 5. In addition, the first feedback control value -d1 is updated based on the difference in the characteristics of the control system. r , second feedback control value +d2r , the third feedback control value +d3 r , upper limit u maxr The value of the first feedback control value -d1 of the first embodiment v1 , second feedback control value +d2 v1 , the third feedback control value +d3 v1 , upper limit u maxv1 may differ from.

[0108] However, the first feedback control value d1 r , second feedback control value d2 r , third feedback control value d3 r , upper limit u maxr The magnitude relationship between the absolute values ​​of these is the same as in the first embodiment. As a result, by suddenly decreasing the offset value r1, it is possible to prevent the work vehicle 1 from accelerating too much due to a large decrease in running resistance when the vehicle starts to move, and since the offset value r1 can be suddenly increased when torque of the hydraulic motors (31L, 31R) is required from when the vehicle is stationary to when the vehicle starts to move, the time until the vehicle starts to move can be shortened.

[0109] FIG. 18 is a flowchart showing the operation of the work vehicle 1 according to the third embodiment. In FIG. 18, the same operations as in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. In this flowchart, the processes from step S1 to step S44 are executed at a predetermined sampling interval (e.g., 20 μs). After completion of step S1, in step S41, the processor 10a acquires a first operation amount from the operation detection sensor 18. That is, the operation amount r of the feedback control is set to the operation amount of the travel instruction input device (operation lever 55) to which an instruction for the travel direction is input by the user.

[0110] When there is no engine drop (No in step S3), in step S4B, the processor 10a determines the maximum output pressure (Pc) in Fig. 5 from the first reference information 10r1 based on the rotation speed RS of the engine 6. When there is an engine drop (Yes in step S3), in step S5B, the processor 10a determines the maximum output pressure (Pc) in Fig. 5 from the second reference information 10r2 based on the rotation speed RS of the engine 6. After the completion of step S4B or step S5B, in step S42, the processor 10a determines a pilot pressure according to the first manipulated variable with the maximum output pressure (Pc) limited, and controls the pilot control valves CV31 to CV34 so that the determined pilot pressure is applied.

[0111] In step S10B instead of step S10, the processor 10a controls the feedforward controller Cf R and the feedback controller Cbb R and outputs a control parameter r. That is, the control method according to this embodiment includes determining a control parameter r for setting a displacement volume of the hydraulic pump (7L, 7R) configured to supply hydraulic oil to the hydraulic motor (31L, 31R) in response to the operation of the creep setting member 16. The determination method is the same as that of the first embodiment, except that since the operation lever position of the operation lever 55 is detected by the operation detection sensor 18, it is determined whether the operation lever 55 is in the neutral position or has been shifted from the neutral position by determining whether the operation lever has deviated from the operation lever position corresponding to the neutral region RA1 in FIG. 5 from the output of the operation detection sensor 18.

[0112] Thereafter, in step S43, the processor 10a determines whether the first manipulated variable is equal to or larger than the deemed manipulated variable r. In the creep mode, the first manipulated variable is usually operated so that it is equal to or larger than the deemed manipulated variable r. When the first manipulated variable is smaller than the deemed manipulated variable (control parameter) r (No in step S43), the process proceeds to step S42. When the first manipulated variable is equal to or larger than the control parameter r (Yes in step S43), in step S44, the processor 10a determines a pilot pressure according to the control parameter r, and controls the pilot control valves CV31 to CV34 so that the determined pilot pressure is applied. <Actions and Effects of the Third Embodiment>

[0113] In the control method for the work vehicle 1 or the work vehicle 1 according to the third embodiment, as in the first embodiment, for example, even if the running resistance is large when the work vehicle starts to move, the work vehicle can start moving quickly. Also, in the control method or the work vehicle 1, the running resistance is significantly reduced when the work vehicle starts to move, so that the work vehicle 1 can be prevented from accelerating too much. <Modification of the third embodiment>

[0114] Instead of the third embodiment, each port (PLa, PRa, PLb, PRb) of the hydraulic pump (7L, 7R) may be electromagnetically inputted, such as a solenoid. In that case, the pilot control valves CV31 to CV34 may be omitted. In this case, a control current value u is supplied to the solenoid of the hydraulic pump (7L, 7R). V Instead of the pilot control valves CV31 to CV34, an electromagnetically input device provided in the hydraulic pumps (7L, 7R) may be regarded as the control mechanism.

[0115] In addition to the above-described embodiment, the control system 20A includes, as a feedback controller, the feedback controller Cb R Alternatively, the control system 20A may be a combination of the feedback controller CbR1 and Cb R2 In the third embodiment, similarly to the second embodiment, the left hydraulic pump 7L and the left hydraulic motor 31L and the right hydraulic pump 7R and the right hydraulic motor 31R may be controlled by separate feedback loops. <Modifications of all embodiments>

[0116] In the above embodiment, the hydraulic pumps (7L, 7R) are rotated by the engine 6. However, the hydraulic pumps (7L, 7R) may be configured to be rotated by other prime movers such as electric motors.

[0117] The values ​​of the various threshold values ​​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 reducer connected to the left hydraulic motor 31L and the reducer connected to the right hydraulic motor 31R, and the characteristics of the various control valves.

[0118] In addition to the above examples, the control systems 20A to 20C may further include other controls such as D control.

[0119] In this application, the term "comprises" and its derivatives are open-ended terms that describe the presence of elements and do not exclude the presence of other elements not listed. This also applies to the terms "have", "include" and their derivatives.

[0120] The terms "member," "part," "element," "body," and "structure" may have multiple meanings, such as a single part or multiple parts.

[0121] Ordinal numbers such as "first" and "second" are merely terms used to identify components and do not have any other meaning (such as a particular order). For example, the presence of a "first element" does not imply the presence of a "second element," and the presence of a "second element" does not imply the presence of a "first element."

[0122] Words expressing degree, such as "substantially," "about," and "approximately," may mean a reasonable deviation that does not significantly change the final result, unless otherwise specified in the embodiment. All numerical values ​​described in this application may be interpreted to include words such as "substantially," "about," and "approximately."

[0123] In this application, the phrase "at least one of A and B" should be interpreted as including A only, B only, and both A and B.

[0124] It is apparent that various changes and modifications of the present invention are possible in light of the above disclosure, and therefore, the present invention may be practiced otherwise than as specifically disclosed herein without departing from the spirit and scope of the present invention.

Claims

1. Obtaining a target rotation speed of a hydraulic motor configured to drive the traveling device via an operation member; determining a control parameter for setting a displacement volume of a hydraulic pump configured to supply hydraulic oil to the hydraulic motor in response to operation of the operating member; Controlling the displacement of the hydraulic pump in accordance with the control parameters to rotate the hydraulic pump, thereby rotating the hydraulic motor; Detecting the rotation speed of the hydraulic motor at a sampling period; Calculating a speed difference obtained by subtracting the target rotation speed from the rotation speed at the sampling period; determining a reference value of the control parameter based on the target rotation speed; Calculating an offset value according to the speed difference at the sampling period; updating the control parameter at the sampling period by adding the offset value to the reference value; Including, A method for controlling the speed of a work vehicle.

2. Calculating the offset value at the sampling period includes: subtracting a first feedback control value from the offset value to reduce the displacement when the speed difference is greater than a first predetermined positive threshold; adding a second feedback control value to the offset value to decrease the displacement when the speed difference is less than a second predetermined negative threshold; when the speed difference is between the second threshold and the first threshold, the offset value is not changed; 2. The speed control method of claim 1, comprising:

3. when the additional operating member is shifted from a return position in which the additional operating member is located when not being operated, a third feedback control value greater than the second feedback control value is added to the offset value; The speed control method according to claim 2.

4. The additional operating member includes an operating lever, The return position is a neutral position. The speed control method according to claim 3.

5. When the speed difference is greater than a third threshold value that is greater than the first threshold value, the offset value is set to 0. The speed control method according to claim 1 .

6. 2. The speed control method according to claim 1, wherein when a value obtained by adding said offset value to said reference value exceeds a predetermined upper limit value, said control parameter is set to said upper limit value.

7. The speed control method of claim 2 , wherein an absolute value of the first feedback control value is greater than an absolute value of the second feedback control value.

8. The control parameter is a current value that controls a control mechanism configured to control the hydraulic pressure of the pilot oil to change the displacement volume of the first hydraulic pump. A speed control method according to any one of claims 1 to 6.

9. The running gear, A hydraulic motor configured to drive the travelling device; a hydraulic pump configured to supply hydraulic fluid to the hydraulic motor; a prime mover configured to rotate the hydraulic pump; an operating member for receiving an input corresponding to a target rotational speed of the hydraulic motor; a rotation speed sensor configured to detect a rotation speed of the hydraulic motor at a sampling period; A control mechanism configured to control the hydraulic pressure of the pilot oil to change the displacement volume of the hydraulic pump; a hardware processor configured to control the prime mover and the control mechanism; Equipped with The hardware processor includes: determining a reference value of a control parameter for controlling the control mechanism based on the target rotation speed; Calculating a speed difference obtained by subtracting the target rotation speed from the rotation speed at the sampling period; Calculating an offset value according to the speed difference at the sampling period; The control parameter is updated at the sampling period by setting the control parameter to a value obtained by adding the offset value to the reference value. It is configured as follows: Work vehicle.

10. The hardware processor includes: subtracting a first feedback control value from the offset value to reduce the displacement when the speed difference is greater than a first predetermined positive threshold; adding a second feedback control value to the offset value to increase the displacement when the speed difference is less than a second predetermined negative threshold; By not changing the offset value when the speed difference is between the second threshold and the first threshold. configured to update the offset value at the sampling period; A work vehicle according to claim 9.

11. When the operating member is shifted from a return position in which the operating member is located when not operated, the hardware processor is configured to add a third feedback control value to the offset value, the third feedback control value being greater than the second feedback control value. A work vehicle according to claim 10.

12. The operating member includes an operating lever, The return position is a neutral position. A work vehicle according to claim 11.

13. the hardware processor is configured to set the offset value to zero when the speed difference is greater than a third threshold value that is greater than the first threshold value; A work vehicle according to claim 9.

14. when a value obtained by adding the offset value to the reference value exceeds a predetermined upper limit value, the hardware processor sets the control parameter to the upper limit value. A work vehicle according to claim 9.

15. The absolute value of the first feedback control value is greater than the absolute value of the second feedback control value. A work vehicle according to claim 10.

16. The control parameter is a current value for controlling the control mechanism. A work vehicle according to any one of claims 9 to 15.

17. receiving an input from an operating member corresponding to a target rotational speed of a hydraulic motor configured to drive the travel device; determining a control parameter for setting a displacement volume of a hydraulic pump configured to supply hydraulic oil to the hydraulic motor in response to operation of the operating member; sending a command corresponding to the control parameter to a control mechanism that controls the displacement of the hydraulic pump; Rotating the hydraulic motor via the hydraulic pump by sending a command to a prime mover configured to rotate the hydraulic pump, the prime mover driving the prime mover; receiving a rotation speed of the hydraulic motor detected at a sampling period from a rotation speed sensor; Calculating a speed difference by subtracting the rotation speed from the rotation speed at the sampling period; determining a reference value of the control parameter based on the target rotation speed; Calculating an offset value according to the speed difference at the sampling period; The control parameter is updated at the sampling period by setting the control parameter to a value obtained by adding the offset value to the reference value. a hardware processor configured to: Work vehicle controller.

18. Calculating the offset value at the sampling period includes: subtracting a first feedback control value from the offset value to reduce the displacement when the speed difference is greater than a first predetermined positive threshold; adding a second feedback control value to the offset value to increase the displacement when the speed difference is less than a second predetermined negative threshold; leaving the offset value unchanged when the speed difference is between the second threshold and the first threshold.

20. The controller of claim 17.

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