Work vehicle and work vehicle control method

Abstract

To provide a work vehicle that performs anti-stall control with improved responsiveness while reducing waste of pilot oil.SOLUTION: A control method of a work vehicle includes performing control to make a pilot primary pressure become a first target value by a primary pressure control valve installed in a first pilot oil passage. The method further includes performing control to make a pilot secondary pressure become a second target value by a secondary pressure control valve installed in a second pilot oil passage. The method further includes detecting engine rotational speed. The method further includes controlling the primary pressure control valve and the secondary pressure control valve at the same time so that when the engine rotational speed detected by the rotational speed sensor falls below a target rotational speed by a predetermined threshold speed difference or more, the second target value is made higher than the first target value, and the first target value and the second target value are decreased.SELECTED DRAWING: Figure 6

Description

[Technical Field]

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

[0002] Patent Document 1 discloses a hydraulic circuit for a work vehicle that controls primary pilot pressure, which is the pilot pressure in a primary pressure circuit between a travel operation device and a pilot pump, when the engine rotation speed drops. Patent Document 2 discloses a hydraulic circuit for a work vehicle that controls secondary pilot pressure, which is the pilot pressure in a secondary pressure circuit between a travel operation device and a pilot port of a hydraulic pump, when the engine rotation speed drops. Patent Document 3 discloses a hydraulic circuit for a work vehicle that uses a relief valve as another method of controlling secondary pilot pressure. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5687971 [Patent Document 2] JP 2017-67100 A [Patent Document 3] Patent No. 6695791 Summary of the Invention [Problem to be solved by the invention]

[0004] Controlling pilot primary pressure as in Patent Document 1 results in little pilot oil waste but suffers from the problem of low responsiveness. On the other hand, controlling pilot secondary pressure using a relief valve with the method of Patent Document 2 as in Patent Document 3 results in high responsiveness but large pilot oil waste. Simultaneously executing the pilot primary pressure control of Patent Document 1 and the pilot secondary pressure control of Patent Documents 2 and 3 may be expected to reduce pilot oil waste while improving responsiveness. However, pilot oil flows out of the secondary pressure circuit and the operating valve between the primary and secondary pressure circuits, resulting in pressure loss due to the outflow of pilot oil. Therefore, simply controlling the pilot pressure in the same way poses the problem of the pilot pressure dropping below the desired control level due to the pressure loss. [Means for solving the problem]

[0005] A work vehicle according to a first aspect of the present disclosure includes a hydraulic motor, a hydraulic pump, an engine, a rotational speed sensor, a pilot pump, and a travel instruction input device. operation The hydraulic motor is configured to drive a traveling device. The hydraulic pump has a pilot port to which pilot oil pressure is applied, and is configured to supply hydraulic oil to the hydraulic motor in accordance with the pilot pressure. The engine is configured to drive the hydraulic pump. The rotational speed sensor is configured to detect the rotational speed of the engine. The pilot pump is configured to be driven by the engine and to discharge pilot oil. The travel instruction input device receives a travel direction instruction input from a user. operation The valve is connected to a first pilot oil passage and configured to control the pilot pressure by operation of the travel command input device. operation The first pilot oil passage is connected to the first valve, and pilot oil is fed through the first pilot oil passage. The primary pressure control valve is provided in the first pilot oil passage and is configured to control the pilot primary pressure, which is the pressure of the pilot oil in the first pilot oil passage. The second pilot oil passage is operationThe valve is connected to a pilot port, and pilot oil is supplied through the valve. The secondary pressure control valve is connected to a second pilot oil passage and is configured to control a pilot secondary pressure, which is the pressure of the pilot oil in the second pilot oil passage. The controller controls the engine rotation speed according to a target engine rotation speed. The controller is configured to control the primary pressure control valve and the secondary pressure control valve based on a first target value of the pilot primary pressure and a second target value of the pilot secondary pressure. When the engine rotation speed detected by the rotation speed sensor falls below the target rotation speed by more than a predetermined threshold speed difference, the controller controls both the primary pressure control valve and the secondary pressure control valve so as to make the second target value higher than the first target value and to lower the first target value and the second target value.

[0006] A control method for a work vehicle according to a second aspect of the present disclosure includes controlling an engine so that the rotational speed of the engine becomes a target rotational speed. The method further includes driving a hydraulic pump connected to the engine to discharge hydraulic oil to a hydraulic motor connected to a travel device. The method further includes driving a pilot pump connected to the engine to discharge pilot oil to a first pilot oil line. The method further includes driving a pilot pump connected to the engine to discharge pilot oil to a first pilot oil line in response to a travel direction command from a user. operation The method further includes converting a pilot primary pressure, which is the pressure of the input pilot oil, into a pilot secondary pressure by a valve and outputting the pilot secondary pressure to a second pilot oil line connected to a pilot port of the hydraulic pump. The method further includes controlling the pilot primary pressure to a first target value by a primary pressure control valve provided in the first pilot oil line. The method further includes controlling the pilot secondary pressure to a second target value by a secondary pressure control valve connected to the second pilot oil line. The method further includes detecting the engine rotation speed. The method further includes controlling both the primary pressure control valve and the secondary pressure control valve so that, when the engine rotation speed detected by the rotation speed sensor falls below the target rotation speed by more than a predetermined threshold speed difference, the second target value is made higher than the first target value and the first target value and the second target value are reduced. [Effects of the Invention]

[0007] According to the technology disclosed in the present application, by taking into account the pressure loss in the secondary pressure circuit and controlling the pilot pressure to match the target pilot primary pressure, it is possible to provide a work vehicle that performs anti-stall control with improved responsiveness while reducing waste of pilot oil. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a side view of a work vehicle. [Figure 2] FIG. 2 is a top view of the work vehicle. [Figure 3] FIG. 3 is a hydraulic circuit diagram of the travel system of the work vehicle. [Figure 4] FIG. 4 is a diagram showing the relationship between the engine rotation speed, the pilot primary pressure, and the setting line. [Figure 5] FIG. 5 is a diagram showing the relationship between the operating position of the operating lever and the traveling secondary source pressure. [Figure 6] FIG. 6 is a diagram showing the relationship between the engine rotation speed, the pilot secondary pressure, and the setting line. [Figure 7] FIG. 7 is a block diagram of a work vehicle. [Figure 8] FIG. 8 is a flowchart showing the operation of the controller of the work vehicle according to the embodiment. [Figure 9] FIG. 9 is a hydraulic circuit diagram showing a secondary pressure control valve according to a modified example.

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

[0010] 1 and 2, a work vehicle 1, for example, a compact track loader, includes a vehicle body 2, a pair of travelling devices 3, and a working device 4. The vehicle body 2 supports the travelling devices 3 and the working device 4. In the illustrated embodiment, the travelling devices 3 are track-type travelling devices. Therefore, each of the pair of travelling devices 3 includes a driving wheel 31, driven wheels 32 and 33, and rollers 34 driven by a hydraulic motor device 30. However, each of the pair of travelling devices 3 is not limited to being a track-type travelling device. Each of the pair of travelling devices 3 may be, for example, a front-wheel / rear-wheel travelling device or a travelling device having front wheels and rear crawlers. The working device 4 includes a work equipment (bucket) 41 at its distal end. A proximal end of the working device 4 is attached to the rear of the vehicle body 2. The working device 4 includes a pair of arm assemblies 42 for rotatably supporting the bucket 41 via a bucket pivot shaft 43. Each of the pair of arm assemblies 42 includes a link 44 and an arm 45 .

[0011] The link 44 is rotatable relative to the vehicle body 2 about a fulcrum shaft 46. The arm 45 is rotatable relative to the link 44 about a joint shaft 47. The work device 4 further includes a plurality of arm cylinders 48 and at least one equipment cylinder 49. Each of the plurality of arm cylinders 48 is rotatably connected to the vehicle body 2 and the arm 45, and moves the link 44, the arm 45, etc., to lift and lower the bucket 41. The at least one equipment cylinder 49 is configured to tilt the bucket 41. The vehicle body 2 includes a cabin 5. The cabin 5 is provided with a windshield 51 that can be opened and closed, and the outer shape of the cabin 5 is defined by a cab frame 53. The windshield 51 may be omitted. The work vehicle 1 includes a driver's seat 54 and a traveling instruction input device 55 within the cabin 5. The cab frame 53 is rotatable about rotational shafts RSL and RSR on the vehicle body 2, as shown in Fig. 2. In Figs. 1 and 2, a common pivot A defined by the rotational shafts RSL and RSR is XC That is, the cab frame 53 is rotatable about a rotation axis A XC It is mounted so as to be pivotable around

[0012] In the embodiment of the present application, FB (Forward direction D F / backward D B ) means the front-rear direction (forward direction / rear direction) as seen from the operator seated in the driver's seat 54 of the cabin 5. L , right direction D R , width direction D W The left, right, and left / right directions are respectively seen from the operator's perspective. U , downward D D , height direction D HThe terms "front-rear", "left-right (width)", and "up-down (height)" refer to the upward, downward, and height directions as seen from the operator. The front-rear, back-right (width), and up-down (height) directions of the work vehicle 1 respectively correspond to the front-rear, back-right (width), and up-down (height) directions as seen from the operator.

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

[0014] 1 and 2, the work vehicle 1 further includes an engine 6 provided at the rear of the vehicle body 2, and a plurality of hydraulic pumps 7 including a first hydraulic pump 7L and a second hydraulic pump 7R. The engine 6 drives the plurality of hydraulic pumps 7. The first hydraulic pump 7L and the second hydraulic pump 7R are configured to discharge hydraulic oil to drive a hydraulic motor unit 30 that drives drive wheels 31, etc. The plurality of hydraulic pumps 7 other than the first hydraulic pump 7L and the second hydraulic pump 7R are configured to discharge hydraulic oil to drive hydraulic actuators (a plurality of arm cylinders 48, at least one implement cylinder 49, etc.) connected to the work implement 4. The engine 6 is configured to drive the work vehicle 1 in the width direction D W and is provided between the pair of arm assemblies 42. The work vehicle 1 is further provided with a cover 8 for covering the engine 6. The work vehicle 1 is further provided with a bonnet cover 9 provided at the rear end of the vehicle body 2. The bonnet cover 9 can be opened and closed, allowing a maintenance technician to perform maintenance work on the engine 6, etc.

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

[0016] The hydraulic circuit 1A includes a pilot supply oil passage PA1 connected to a discharge port of a pilot pump 71. Pilot oil flows through the pilot supply oil passage PA1. The hydraulic circuit 1A includes a plurality of switching valves (brake switching valve SV1, directional control valve SV2) connected to the pilot supply oil passage PA1, and a plurality of brake mechanisms 72. The brake switching valve SV1 is connected to the pilot supply oil passage PA1. The brake switching valve SV1 is a directional control valve (solenoid valve) for applying and releasing the brakes by the plurality of brake mechanisms 72. The brake switching valve SV1 is a two-position switching valve configured to switch its valve element to a first position VP1a or a second position VP1b when energized. The valve element of the brake switching valve SV1 is switched by a brake pedal 13 (see FIG. 7). A sensor 14 is provided on the brake pedal 13. The operation amount detected by the sensor 14 is input to a controller 10 configured as an ECU (Electronic Control Unit).

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

[0018] The directional control valve SV2 is a solenoid valve that changes the rotation of the first hydraulic motor unit 30L and the second hydraulic motor unit 30R. The directional control valve SV2 is a two-position control valve that is configured to switch its valve element to a first position VP2a or a second position VP2b when excited. The directional control valve SV2 is switched by an operating member or the like (not shown). Note that the directional control valve SV2 may be a proportional valve that can adjust the flow rate of the hydraulic oil discharged, rather than a two-position control valve.

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

[0020] The first travel control valve SV4 is switched by the directional control valve SV2, which is located upstream and connected to the first travel control valve SV4. Specifically, the directional control valve SV2 and the first travel control valve SV4 are connected by an oil passage PA3, and the first travel control valve SV4 is switched by the hydraulic oil flowing through the oil passage PA3. For example, when the valve element of the directional control valve SV2 is switched to the first position VP2a by operating the operating member, pilot oil is released from the section between the directional control valve SV2 and the first travel control valve SV4, and the valve element of the first travel control valve SV4 is switched to the first position VP4a. As a result, the first swash plate switching cylinder 32L retracts, and the speed of the first hydraulic motor 31L changes to first gear. When the valve element of the directional control valve SV2 is switched to the second position VP2b by operating the operating member, pilot oil is supplied to the first travel control valve SV4 through the directional control valve SV2, and the valve element of the first travel control valve SV4 is switched to the second position VP4b. As a result, the first swash plate switching cylinder 32L extends, and the speed of the first hydraulic motor 31L changes to second gear.

[0021] The second hydraulic motor unit 30R transmits power to the drive wheels 31 provided on the second traveling unit 3R. The second hydraulic motor unit 30R includes a second hydraulic motor 31R, a second swash plate switching cylinder 32R, and a second traveling control valve (hydraulic switching valve) SV5. The second hydraulic motor unit 30R is a hydraulic motor for driving the second traveling unit 3R and operates in the same manner as the first hydraulic motor unit 30L. That is, the second hydraulic motor 31R operates in the same manner as the first hydraulic motor 31L. The second swash plate switching cylinder 32R operates in the same manner as the first swash plate switching cylinder 32L. The second traveling control valve SV5 is a two-position switching valve configured to switch its valve element between a first position VP5a and a second position VP5b, and operates in the same manner as the first traveling control valve SV4.

[0022] A drain oil passage DR1 is connected to the hydraulic circuit 1A. The drain oil passage DR1 is an oil passage that flows pilot oil from a plurality of switching valves (brake switching valve SV1, directional switching valve SV2) to the hydraulic oil tank 70. For example, the drain oil passage DR1 is connected to the discharge ports of the plurality of switching valves (brake switching valve SV1, directional switching valve SV2). That is, when the brake switching valve SV1 is in the first position VP1a, hydraulic oil is discharged from the oil passage PA2 to the drain oil passage DR1 in the section between the brake switching valve SV1 and the brake mechanism 72. When the directional switching valve SV2 is in the first position VP1a, pilot oil in the oil passage PA3 is discharged to the drain oil passage DR1.

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

[0024] The first drive circuit 76L has a first hydraulic pump 7L, drive oil passages PA5L and PA6L, and a second charge oil passage PA7L. The drive oil passages PA5L and PA6L are oil passages that connect the first hydraulic pump 7L and the first hydraulic motor 31L. The hydraulic circuit formed by the drive oil passages PA5L and PA6L is called the first hydraulic circuit CL. The second charge oil passage PA7L is connected to the drive oil passages PA5L and PA6L and is an oil passage that replenishes hydraulic oil from the pilot pump 71 to the drive oil passages PA5L and PA6L. The first hydraulic motor 31L has a first connection port 31P1 that connects to the drive oil passage PA5L and a second connection port 31P2 that connects to the drive oil passage PA6L. Hydraulic oil for rotating the first traveling device 3L in the forward direction is input to the first hydraulic motor 31L via the first connection port 31P1, and hydraulic oil for rotating the first traveling device 3L in the reverse direction is discharged from the first hydraulic motor 31L via the first connection port 31P1. Hydraulic oil for rotating the first traveling device 3L in the reverse direction is input to the first hydraulic motor 31L via the second connection port 31P2, and hydraulic oil for rotating the first traveling device 3L in the forward direction is discharged from the first traveling device 3L.

[0025] Similarly, the second drive circuit 76R has a second hydraulic pump 7R, drive oil passages PA5R and PA6R, and a third charge oil passage PA7R. The drive oil passages PA5R and PA6R are oil passages that connect the second hydraulic pump 7R and the second hydraulic motor 31R. The hydraulic circuit formed by the drive oil passages PA5R and PA6R is called the second hydraulic circuit CR. The third charge oil passage PA7R is connected to the drive oil passages PA5R and PA6R and is an oil passage that replenishes hydraulic oil from the pilot pump 71 to the drive oil passages PA5R and PA6R. The second hydraulic motor 31R has a third connection port 31P3 connected to the drive oil passage PA5R and a fourth connection port 31P4 connected to the drive oil passage PA6R. Hydraulic oil for rotating the second traveling device 3R in the forward direction is input to the second hydraulic motor 31R via the third connection port 31P3, and hydraulic oil for rotating the second traveling device 3R in the reverse direction is discharged from the second hydraulic motor 31R via the third connection port 31P3. Hydraulic oil for rotating the second traveling device 3R in the reverse direction is input to the second hydraulic motor 31R via the fourth connection port 31P4, and hydraulic oil for rotating the second traveling device 3R in the forward direction is discharged from the second traveling device 3R.

[0026] The first hydraulic pump 7L and the second hydraulic pump 7R are swash plate-type variable displacement axial pumps driven by power from the engine 6. The first hydraulic pump 7L is connected to the first hydraulic motor 31L via a first hydraulic circuit CL and has a first pilot port PLa and a second pilot port PLb to which a pilot pressure is applied. The angle of the swash plate of the first hydraulic pump 7L is changed by the pilot pressure acting on the first pilot port PLa and the second pilot port PLb. Specifically, the first hydraulic pump 7L is configured to supply hydraulic oil to the first hydraulic motor 31L via the first hydraulic circuit CL so as to drive the first traveling device 3L forward when the hydraulic pressure applied to the first pilot port PLa is higher than the hydraulic pressure applied to the second pilot port PLb, and to supply hydraulic oil to the first hydraulic motor 31L via the first hydraulic circuit CL so as to drive the first traveling device 3L backward when the hydraulic pressure applied to the second pilot port PLb is higher than the hydraulic pressure applied to the first pilot port PLa. That is, the first hydraulic pump 7L is configured to supply hydraulic oil to the first hydraulic motor 31L in accordance with the pilot pressure.

[0027] The second hydraulic pump 7R is connected to the second hydraulic motor 31R via a second hydraulic circuit CR and has a third pilot port PRa and a fourth pilot port PRb to which a pilot pressure is applied. The second hydraulic pump 7R changes the angle of its swash plate according to the pilot pressure acting on the third pilot port PRa and the fourth pilot port PRb. Specifically, when the hydraulic pressure applied to the third pilot port PRa is higher than the hydraulic pressure applied to the fourth pilot port PRb, the second hydraulic pump 7R supplies hydraulic oil to the second hydraulic motor 31R via the second hydraulic circuit CR to drive the second traveling device 3R forward. When the hydraulic pressure applied to the fourth pilot port PRb is higher than the hydraulic pressure applied to the third pilot port PRa, the second hydraulic pump 7R supplies hydraulic oil to the second hydraulic motor 31R via the second hydraulic circuit CR to drive the second traveling device 3R backward. That is, the second hydraulic pump 7R supplies hydraulic oil to the second hydraulic motor 31R in accordance with the pilot pressure. The first hydraulic pump 7L and the second hydraulic pump 7R can change the output (discharge amount of hydraulic oil) and the discharge direction of hydraulic oil according to the angle of the swash plate.

[0028] The output of the first hydraulic pump 7L and the second hydraulic pump 7R and the discharge direction of the hydraulic oil are changed by an operating device 56 for operating the traveling direction of the work vehicle 1. Hereinafter, the operating device 56 may be referred to as a direction input device. Specifically, the output of the first hydraulic pump 7L and the second hydraulic pump 7R and the discharge direction of the hydraulic oil are changed in response to operation of a travel instruction input device 55 provided in the operating device 56. In other words, the operating device 56 is a device configured to operate the traveling direction of the work vehicle by selecting at least one of the first traveling device 3L and the second traveling device 3R and instructing at least one of the traveling devices to move forward or backward. In the following description, the travel instruction input device 55 is, for example, an operating lever. A user's instruction for the traveling direction is input via the travel instruction input device 55.

[0029] As shown in FIG. 3 , the hydraulic circuit 1A includes a pilot supply oil passage PA8 branched from a pilot supply oil passage PA1 and connected to the operating device 56, and a primary pressure control valve CV1 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 first pilot oil passage 11. Pilot oil is supplied via the first pilot oil passage 11. The primary pressure control valve CV1 is an electromagnetic proportional valve and is configured to adjust the pilot pressure supplied to the operating device 56 by adjusting its opening. The opening of the primary pressure control valve CV1 is controlled by a controller 10. The primary pressure control valve CV1 is provided on the first pilot oil passage 11 and is configured to control a pilot primary pressure, which is the pressure of the pilot oil in the first pilot oil passage 11. The operation of the primary pressure control valve CV1 will be described in detail later.

[0030] The operating device 56 (directional input device) includes a forward operating valve OVA, a reverse operating valve OVB, a right-turn operating valve OVC, a left-turn operating valve OVD, and a travel command input device 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 the travel command input device 55 (one operating lever). The operating valves OVA, OVB, OVC, and OVD change the pressure of the pilot oil in response to operation of the travel command input device 55, and supply the changed hydraulic oil to the first pilot port PLa and the second pilot port PLb of the first hydraulic pump 7L and the third pilot port PRa and the fourth pilot port PRb of the second hydraulic pump 7R. In other words, the operating valves OVA, OVB, OVC, and OVD are connected to the first pilot oil passage 11 and are configured to control the pilot pressure in response to operation of the travel command input device 55. In the embodiment according to the present application, the control valves OVA, OVB, OVC, and OVD are operated by one control lever, but a plurality of control levers may be used.

[0031] The operating valves OVA, OVB, OVC, and OVD each have an input port (primary port), a discharge port, and an output port (secondary port). As shown in FIG. 3, the input port is connected to a pilot supply oil passage PA8. The discharge port is connected to a drain oil passage DR2 leading to the hydraulic oil tank 70. The travel command input device 55 can be tilted from a neutral position in the front-to-rear direction, the width direction perpendicular to the front-to-rear direction, and an oblique direction. The operating valves OVA, OVB, OVC, and OVD of the operating device 56 are operated in response to the tilting of the travel command input device 55. As a result, pilot pressures corresponding to the amount of operation of the travel command input device 55 from the neutral position are output from the secondary ports of the operating valves OVA, OVB, OVC, and OVD. The relationship between the pilot pressure applied to the primary port output from the primary pressure control valve CV1 and the pilot pressure applied to the secondary port will be described later.

[0032] The secondary 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 pilot port PLa of the first hydraulic pump 7L via the first secondary 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 pilot port PRa of the second hydraulic pump 7R via the third secondary 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 pilot port PLb of the first hydraulic pump 7L via the second secondary 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 pilot port PRb of the second hydraulic pump 7R via the fourth secondary pilot oil passage PA14.

[0033] That is, the pilot supply oil passage PA8, the first secondary pilot oil passage PA11, and the fourth secondary pilot oil passage PA14 connect the pilot pump 71 and the first hydraulic pump 7L. The pilot supply oil passage PA8, the second secondary pilot oil passage PA12, and the third secondary pilot oil passage PA13 connect the pilot pump 71 and the second hydraulic pump 7R. In the following embodiments, the first secondary pilot oil passage PA11, the fourth secondary pilot oil passage PA14, the second secondary pilot oil passage PA12, and the third secondary pilot oil passage PA13 are collectively referred to as a plurality of second pilot oil passages 12. Each of the plurality of second pilot oil passages 12 connects one of the operation valves OVA, OVB, OVC, and OVD to one of the pilot ports PLa, PLb, PRa, and PRb. Pilot oil is supplied via the second pilot oil passage 12.

[0034] Hydraulic circuit 1A further includes fifth shuttle valve SVe, sixth shuttle valve SVf, secondary pressure control valves CV11 and CV12, and discharge oil passages DR3 and DR4. The first secondary pilot oil passage PA11 and the second secondary pilot oil passage PA12 are connected to an input port of fifth shuttle valve SVe, and the output port of fifth shuttle valve SVe is connected to the input port of secondary pressure control valve CV11 via discharge oil passage DR3. The higher of the hydraulic pressure in first secondary pilot oil passage PA11 and the hydraulic pressure in second secondary pilot oil passage PA12 is applied to the input port of fifth shuttle valve SVe.

[0035] The third secondary pilot oil passage PA13 and the fourth secondary pilot oil passage PA14 are connected to the input port of the sixth shuttle valve SVf, and the output port of the sixth shuttle valve SVf is connected to the input port of the secondary pressure control valve CV12 via a discharge oil passage DR4. The higher of the oil pressure in the third secondary pilot oil passage PA13 and the oil pressure in the fourth secondary pilot oil passage PA14 is applied to the input port of the sixth shuttle valve SVf.

[0036] The secondary pressure control valve CV11 is a proportional electromagnetic relief valve that is configured to open when the higher of the hydraulic pressures applied to its input port in the first secondary pilot oil passage PA11 and the second secondary pilot oil passage PA12 is greater than the hydraulic pressure corresponding to the current applied to the solenoid. When the secondary pressure control valve CV12 is opened, the pilot oil in the pilot oil passage with the higher hydraulic pressure, either the first secondary pilot oil passage PA11 or the second secondary pilot oil passage PA12, is discharged to the hydraulic oil tank 70. In this way, the pilot secondary pressure, which is the pressure of the pilot oil in the first secondary pilot oil passage PA11 and the second secondary pilot oil passage PA12, is controlled by adjusting the current applied to the solenoid.

[0037] The secondary pressure control valve CV12 is a proportional electromagnetic relief valve that opens when the higher of the hydraulic pressures in the third secondary pilot oil passage PA13 and the fourth secondary pilot oil passage PA14 applied to its input port is greater than the hydraulic pressure corresponding to the current applied to the solenoid. In this manner, the secondary pressure control valves CV11 and CV12 are collectively referred to as the secondary pressure control valve CV2. Discharge oil passages DR3 and DR4 branch off from the second pilot oil passage and are connected to the secondary pressure control valve CV2. The secondary pressure control valve CV2 is connected to the second pilot oil passage 12 and is configured to control the pilot secondary pressure, which is the pressure of the pilot oil in the second pilot oil passage 12. The detailed operation of the secondary pressure control valve CV2 will be described later.

[0038] The hydraulic circuit 1A further includes throttles TH1 to TH4. The throttle TH1 is provided in a first secondary pilot oil passage PA11 between the first shuttle valve SVa and the fifth shuttle valve SVe and is configured to reduce the flow rate of pilot oil in the first secondary pilot oil passage PA11. The throttle TH2 is provided in a second secondary pilot oil passage PA12 between the second shuttle valve SVb and the fifth shuttle valve SVe and is configured to reduce the flow rate of pilot oil in the second secondary pilot oil passage PA12. The throttle TH3 is provided in a third secondary pilot oil passage PA13 between the third shuttle valve SVc and the fifth shuttle valve SVe and is configured to reduce the flow rate of pilot oil in the third secondary pilot oil passage PA13. The throttle TH4 is provided in a fourth secondary pilot oil passage PA14 between the fourth shuttle valve SVd and the fifth shuttle valve SVe and is configured to reduce the flow rate of pilot oil in the fourth secondary pilot oil passage PA14. In the following embodiments, the throttles TH1 to TH4 will be collectively referred to as throttles TH. In other words, the throttle TH is connected to the operating valves OVA, OVB, OVC, OVD and the discharge oil passage. DR3, DR4 The throttle TH is provided in the second pilot oil passage 12 between the first and second pilot oil passages 12 and 12. The throttle TH may be omitted.

[0039] When the travel command input device 55 is tilted forward, the forward operation valve OVA is operated and pilot pressure is output from the operation valve OVA. This pilot pressure acts from the first shuttle valve SVa to the first pilot port PLa via a first secondary pilot oil passage PA11 that connects the operation device 56 and the first pilot port PLa of the first hydraulic pump 7L, and also acts from the second shuttle valve SVb to the third pilot port PRa via a third secondary pilot oil passage PA13 that connects the operation device 56 and the third pilot port PRa of the second hydraulic pump 7R. As a result, the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate forward (forward rotation) at a speed that corresponds to the amount of tilt of the travel command input device 55, and the work vehicle 1 moves straight forward.

[0040] Furthermore, when the travel command input device 55 is tilted rearward, the reverse operation valve OVB is operated and pilot pressure is output from the operation valve OVB. This pilot pressure acts from the third shuttle valve SVc to the second pilot port PLb of the first hydraulic pump 7L via a second secondary pilot oil passage PA12 that connects the operation device 56 and the second pilot port PLb, and also acts from the fourth shuttle valve SVd to the fourth pilot port PRb via a fourth secondary pilot oil passage PA14 that connects the operation device 56 and the fourth pilot port PRb of the second hydraulic pump 7R. As a result, the output shaft of the first hydraulic pump 7L and the output shaft of the second hydraulic pump 7R rotate in the reverse direction (reverse rotation) at a speed that corresponds to the tilt amount of the travel command input device 55, causing the work vehicle 1 to travel straight backward.

[0041] Furthermore, when the travel command input device 55 is tilted to the right, the right turn operation valve OVC is operated and pilot pressure is output from the operation valve OVC. This pilot pressure acts on the first pilot port PLa of the first hydraulic pump 7L via the first shuttle valve SVa and the first secondary pilot oil passage PA11, and also acts on the fourth pilot port PRb of the second hydraulic pump 7R via the fourth shuttle valve SVd and the fourth secondary pilot oil passage PA14. As a result, the vehicle turns right at a degree of turning that corresponds to the rightward operating position of the travel command input device 55.

[0042] Furthermore, when the travel command input device 55 is tilted to the left, the left turn operation valve OVD is operated and pilot pressure is output from the operation valve OVD. This pilot pressure acts on the third pilot port PRa of the second hydraulic pump 7R via the second shuttle valve SVb and the third secondary pilot oil passage PA13, and also acts on the second pilot port PLb of the first hydraulic pump 7L via the third shuttle valve SVc and the second secondary pilot oil passage PA12. As a result, the vehicle turns left at a degree of turning that corresponds to the leftward operating position of the travel command input device 55.

[0043] That is, when the driving instruction input device 55 is tilted diagonally forward to the left, the work vehicle 1 moves forward at a speed corresponding to the operating position of the driving instruction input device 55 in the forward / backward direction, and turns left at a degree of turning corresponding to the operating position of the driving instruction input device 55 in the left direction. When the driving instruction input device 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 driving instruction input device 55. When the driving instruction input device 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 driving instruction input device 55. When the driving instruction input device 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 driving instruction input device 55.

[0044] Next, the detailed operation of the primary pressure control valve CV1 will be described. The work vehicle 1 includes a setting member 16 (see FIG. 7) that sets the target rotation speed of the engine 6. The setting member 16 is an accelerator pedal or a swingably supported accelerator lever, which is a speed input device separate from the direction input device described above. A sensor 17 is provided on the setting member 16. The amount of operation detected by the sensor 17 is input to the controller 10. The engine rotation speed corresponding to the amount of operation detected by the sensor 17 is the target rotation speed of the engine 6. In other words, the target rotation speed of the engine 6 is set based on the amount of operation of the setting member 16. The controller 10 controls the rotation speed of the engine 6 by outputting, for example, a rotation command indicating a fuel injection amount, injection timing, and fuel injection rate to the injector so that the rotation speed of the engine 6 becomes the determined target rotation speed of the engine 6. Alternatively, the controller 10 controls the rotation speed of the engine 6 by outputting a rotation command indicating a fuel injection pressure, etc. to a supply pump or a common rail so that the rotation speed of the engine 6 becomes the determined target rotation speed of the engine 6.

[0045] A rotation speed sensor 6a configured to detect the actual engine rotation speed (referred to as the actual rotation speed of the engine 6) is connected to the controller 10, and the actual rotation speed of the engine 6 is input. The rotation 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 drops from the target rotation speed of the engine 6. The amount by which the actual rotation speed drops from the target rotation speed when a load is applied to the engine 29 (the difference between the target rotation speed of the engine and the actual rotation speed of the engine) is referred to as the engine drop amount.

[0046] The primary pressure control valve CV1 can set the pilot primary pressure acting on the input ports (primary ports) of the multiple control valves OVA, OVB, OVC, and OVD based on a drop amount ΔE1 in the rotational speed of the engine 6 (engine rotational speed E1). The rotational speed of the engine 6 can be detected by a rotational speed sensor 6a for the engine rotational speed E1. The engine rotational speed E1 detected by the rotational speed sensor 6a is input to the controller 10. FIG. 4 shows the relationship between the engine rotational speed, the pilot primary pressure, and setting lines L1 and L2. The setting line L1 shows the relationship between the engine rotational speed E1 and the pilot primary pressure when the drop 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 rotational speed E1 and the pilot primary pressure when the drop amount ΔE1 is equal to or greater than the anti-stall determination value. When the difference between the first rotation speed RS1 determined based on the operation amount of the setting member 16 and the actual rotation speed of the engine 6 is smaller than a predetermined stall determination speed difference (anti-stall determination value), the pilot primary pressure corresponding to the first rotation speed RS1 transitions in accordance with a first correspondence relationship shown by a setting line L1. When the difference between the first rotation speed RS1 and the actual rotation speed of the engine 6 is equal to or greater than the predetermined stall determination speed difference (anti-stall determination value), the pilot primary pressure corresponding to the first rotation speed RS1 transitions in accordance with a second correspondence relationship shown by a setting line L2.

[0047] When the decrease amount 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 pilot primary pressure coincides with the reference pilot pressure indicated by the setting line L1. Furthermore, when the decrease amount ΔE1 is equal to or greater than the anti-stall determination value, the controller 10 adjusts the opening of the primary pressure control valve CV1 so that the relationship between the engine speed E1 and the pilot primary pressure coincides with the setting line L2, which is lower than the reference pilot pressure. On the setting line L2, the pilot primary pressure for a given engine speed E1 is lower than the pilot primary pressure of the setting line L1. That is, for the same engine speed E1, the traveling primary pressure of the setting line L2 is set lower than the traveling primary pressure of the setting line L1. Therefore, the pressure (pilot pressure) of the hydraulic oil entering the operating valves OVA, OVB, OVC, and OVD is kept low by control based on the setting line L2. As a result, the swash plate angles of the first hydraulic pump 7L and the second hydraulic pump 7R are adjusted, the load acting on the engine 6 is reduced, and stalling of the engine 6 can be prevented. Although one setting line L2 is shown in FIG. 4, there may be multiple setting lines L2. For example, a setting line L2 may be set for each engine rotation speed E1. Furthermore, it is preferable that the controller 10 has data indicating the setting lines L1 and L2, or control parameters such as functions.

[0048] Next, the pilot pressure output from the secondary ports of the control valves OVA, OVB, OVC, and OVD will be described. Hereinafter, this pilot pressure will be referred to as the secondary pilot source pressure. FIG. 5 is a diagram showing the relationship between the operation position of the control lever and the secondary pilot secondary source pressure. Referring to FIG. 4, the origin of the lever operation position is the operation start position (neutral position, G0 position) which is the start position of the lever stroke, and as it moves away from the origin, it approaches the operation end position (G5 position) which is the end position of the lever stroke. The operation region of the travel instruction input device 55 is divided into a neutral region RA1 where the operation target does not move (in the illustrated example, from G0 position to G1 position), a near-full operation region RA2 near the operation end (in the illustrated example, from G3 position to G5 position), and an intermediate region RA3 between the neutral region RA1 and the near-full operation region RA2 (in the illustrated example, from G1 position to G3 position). Furthermore, the intermediate region RA3 is divided into a slow speed region RA3A from the G1 position to the G2 position, and an intermediate speed region RA3B from the G2 position to the G3 position.

[0049] In the neutral region RA1, pilot secondary source pressure is not supplied even when the travel command input device 55 is operated. On the other hand, in the near-full operation region RA2, the speed of the controlled object is not adjusted, and therefore the travel command input device 55 is operated to the operation end position (G5 position) without stopping midway. In the intermediate region RA3, the travel command input device 55 is stopped or moved at any position within the region, and the speed of the controlled object is adjusted to the speed desired by the operator. For example, the ratios of the lever strokes for each of the operation regions RA1, RA3A, RA3B, and RA2 are as follows: Neutral region RA1: 0% to less than 15% Slow-speed region RA3A: 15% or more and less than 45% Intermediate speed range RA3B: 45% or more and less than 75% Full operation area RA2: 75% to 100%

[0050] In the characteristic diagram shown in FIG. 5, when the travel instruction input device 55 is operated from the G0 position to the G1 position, a pilot secondary source pressure (Pa) is generated. When the travel instruction input device 55 is operated from the G1 position to the G4 position, the pilot secondary source pressure increases from Pa to Pb in proportion to the amount of operation of the travel instruction input device 55. At the G4 position, the pilot primary source pressure is bypassed and flows to the secondary side, and the pilot secondary source pressure increases from Pb to the maximum output pressure Pc in one go. While the travel instruction input device 55 is operated from the G4 position to the G5 position, the pilot secondary source pressure remains constant at the maximum output pressure (Pc) and becomes equal to the pilot primary pressure. In other words, when the displacement of the travel instruction input device 55 from the neutral position for issuing an instruction to move left is equal to or greater than the first displacement value (displacement from G0 to G4), the operating device 56 outputs the pilot primary pressure input to the operating device 56 to the first pilot port PLa and the fourth pilot port PRb. The operating device 56 outputs the pilot primary pressure input to the operating device 56 to the second pilot port PLb and the third pilot port PRa when the displacement from the neutral position of the travel instruction input device 55 for instructing movement in the rightward direction is equal to or greater than a first displacement value (displacement from G0 to G4). The operating device 56 outputs the pilot primary pressure input to the operating device 56 to the first pilot port PLa and the third pilot port PRa when the displacement from the neutral position of the travel instruction input device 55 for instructing movement in the forward direction is equal to or greater than the first displacement value (displacement from G0 to G4). The operating device 56 outputs the pilot primary pressure input to the operating device 56 to the second pilot port PLb and the fourth pilot port PRb when the displacement from the neutral position of the travel instruction input device 55 for instructing movement in the backward direction is equal to or greater than the first displacement value (displacement from G0 to G4).

[0051] The characteristic value of the pilot secondary source pressure in the forward / rearward direction may be different from the characteristic value of the pilot secondary source pressure in the left / right direction. If the characteristic values ​​of the pilot secondary source pressure in the forward / rearward direction corresponding to G0-G5 and Pa-Pc are G0'-G5' and Pa'-Pc', respectively, the operation device 56 may output the pilot primary pressure input to the operation device 56 to the first pilot port PLa and the third pilot port PRa when the displacement from the neutral position of the travel instruction input device 55 for instructing movement in the forward direction is equal to or greater than a second displacement value (a displacement from G0' to G4'). The operation device 56 may output the pilot primary pressure input to the operation device 56 to the second pilot port PLb and the fourth pilot port PRb when the displacement from the neutral position of the travel instruction input device 55 for instructing movement in the rearward direction is equal to or greater than the second displacement value (a displacement from G0' to G4'). Furthermore, Pa and Pb (Pa' and Pb') are values ​​that do not depend on the magnitude of the pilot primary pressure, but if the pilot primary pressure is lower than Pa or Pb (Pa' or Pb'), the pilot secondary source pressure will reach a plateau at the magnitude of the pilot primary pressure.

[0052] Next, the detailed operation of the secondary pressure control valve CV2 will be explained. Similar to FIG. 4, FIG. 6 shows the relationship between the engine rotation speed, the pilot secondary pressure controlled by the secondary pressure control valve CV2, and the setting lines L11 and L12 described later. The controller 10 sets the value of the secondary pressure control valve CV2 based on the first target value of the primary pressure control valve CV1 and the second target value of the pilot secondary pressure. ,one Secondary pressure control valve CV1 and secondary pressure control valve CV2. Specifically, when the primary pressure control valve CV1 is controlled based on the setting line L2 in Fig. 4 and the operation valves OVA, OVB, OVC, and OVD output a pilot secondary source pressure equal to the pilot primary pressure, the secondary pressure control valve CV2 is controlled by the controller 10 so that the second target value of the pilot secondary pressure is higher than the first target value of the pilot primary pressure controlled by the primary pressure control valve CV1.

[0053] Immediately after the anti-stall control is started (immediately after the reduction amount ΔE1 of the rotation speed of the engine 6 becomes equal to or greater than the anti-stall judgment value), even if the pilot primary pressure is reduced by the primary pressure control valve CV1, the pilot pressure applied to the pilot ports PLa, PLb, PRa, and PRb will not be reduced immediately if there is no secondary pressure control valve CV2. On the other hand, if the secondary pressure control valve CV2 is operated in the same way as the primary pressure control valve CV1, the secondary pressure control valve CV2 will not operate in the discharge oil path D R3, DR4 Because of pressure loss that occurs when pilot oil flows through the primary pressure control valve CV1, the pilot secondary pressure is controlled to be lower than the pressure value based on the setting line L2 in Figure 4, which is the first target value that is ultimately desired to be the pilot pressure. For this reason, as shown by setting line L12 in Figure 6, the second target value of the pilot secondary pressure is determined according to the engine rotation speed so that it is higher than the setting line L2 by an offset ΔV that takes pressure loss into account. By controlling in this way, the pilot secondary pressure can be smoothly reduced to a target pressure that is close to the pressure set by the primary pressure control valve CV1.

[0054] In the hydraulic circuit 1A shown in Fig. 3, a similar pressure loss occurs even when anti-stall control is not performed, and therefore, when the pilot primary pressure is controlled based on the setting line L1 in Fig. 4, the pilot secondary pressure is controlled in accordance with the engine speed so that it is higher than the setting line L1 by an offset ΔV that takes the pressure loss into account, as shown by the setting line L11 in Fig. 6. By controlling in this way, even when anti-stall control is not performed, if the viscosity of the pilot oil is high and the pilot secondary pressure increases, the secondary pressure control valve CV2 can be opened to forcibly flow pilot oil, which has the advantage of allowing the engine to warm up smoothly.

[0055] Based on the characteristics of the above-mentioned operating valves OVA, OVB, OVC, and OVD, the movement of the work vehicle 1 in response to operation of the travel instruction input device 55 will be described in more detail. When the amount of operation of the travel instruction input device 55 in the forward / backward direction is greater than the amount of operation in the rightward direction, and the rightward operation position of the travel instruction input device 55 is operated from position G1 to position G3, the rotational speed of the first hydraulic pump 7L is greater than the rotational speed of the second hydraulic pump 7R, and they rotate in the same direction, causing the work vehicle 1 to make a wide turn to the right. When the rightward operation position of the travel instruction input device 55 becomes the same as the forward / backward operation position, the rotational speed of the second hydraulic pump 7R becomes 0, and only the first hydraulic pump 7L rotates, causing the work vehicle 1 to perform a right pivot turn. Furthermore, when the travel instruction input device 55 is operated to a rightward operating position between the G4 position and the G5 position, the operating position becomes larger than the forward / backward operating position, the output shaft of the first hydraulic pump 7L rotates forward and the output shaft of the second hydraulic pump 7R rotates reversely, and the work vehicle 1 turns to the right.

[0056] Furthermore, when the amount of operation of the travel instruction input device 55 in the forward / backward direction is greater than the amount of operation in the leftward direction, and the leftward operation position of the travel instruction input device 55 is operated from the G1 position to the G3 position, the rotational speed of the second hydraulic pump 7R is greater than the rotational speed of the first hydraulic pump 7L, and they rotate in the same direction, causing the work vehicle 1 to make a wide turn to the left. When the leftward operation position of the travel instruction input device 55 is the same as the forward / backward operation position, the rotational speed of the first hydraulic pump 7L becomes 0, and only the second hydraulic pump 7R rotates, causing the work vehicle 1 to make a left pivot turn. Furthermore, when the leftward operation position of the travel instruction input device 55 is operated between the G4 position and the G5 position, it becomes greater than the forward / backward operation position, causing the output shaft of the second hydraulic pump 7R to rotate forward and the output shaft of the first hydraulic pump 7L to rotate reversely, causing the work vehicle 1 to turn left. In the embodiment of the present application, turning refers to the movement of the work vehicle 1 when the operating position to the right is operated between the G4 position and the G5 position, or when the operating position to the left is operated between the G4 position and the G5 position.

[0057] On the other hand, when the operation position of the travel instruction input device 55 in the forward direction is operated between the G4 position and the G5 position, it becomes larger than the operation position in the left / right direction, the output shafts of the first hydraulic pump 7L and the second hydraulic pump 7R rotate forward, and the work vehicle 1 moves forward at high speed. When the operation position of the travel instruction input device 55 in the backward direction is operated between the G4 position and the G5 position, it becomes larger than the operation position in the left / right direction, the output shafts of the first hydraulic pump 7L and the second hydraulic pump 7R rotate in the reverse direction, and the work vehicle 1 moves backward at high speed. Note that other operations of the travel instruction input device 55 in the forward / backward direction are the same as those in the left / right direction.

[0058] The work vehicle 1 is provided with various switches and sensors connected to the controller 10 described above. FIG. 7 is a block diagram of the work vehicle 1. Referring to FIG. 7, the work vehicle 1 includes an operation panel 15. The operation panel 15 is, for example, a touch panel that displays various states of the work vehicle 1 and allows various settings according to the present embodiment to be configured. As described above, the controller 10 is configured to control both the primary pressure control valve CV1 and the secondary pressure control valve CV2 so as to increase the second target value above the first target value and to reduce the first target value and the second target value when the rotational speed of the engine 6 detected by the rotational speed sensor 6a falls below the target rotational speed by more than a predetermined threshold speed difference (anti-stall determination value). Specifically, the controller 10 is configured to simultaneously control the primary pressure control valve CV1 and the secondary pressure control valve CV2 when the rotational speed of the engine 6 detected by the rotational speed sensor 6a falls below the target rotational speed by more than a predetermined threshold speed difference (anti-stall determination value).

[0059] To achieve the above-described processing, the controller 10 has a processor 10a and a memory 10b as shown in FIG. 7. The memory 10b includes a volatile memory and a nonvolatile memory. The memory 10b includes a cruise control program 10c1 for achieving the above-described control. The processor 10a executes the cruise control program 10c1 to perform the above-described control. A control method using the controller 10 and the operation device 56 will be described in detail below.

[0060] FIG. 8 is a flowchart showing the operation of controller 10 of work vehicle 1. In step S1, controller 10 controls engine 6 so that the rotational speed of engine 6 becomes a target rotational speed. In step S2, controller 10 drives hydraulic pump 7 connected to engine 6, causing hydraulic pump 7 to discharge hydraulic oil to hydraulic motor device 30 connected to traveling device 3. In step S3, controller 10 drives pilot pump 71 connected to engine 6, causing pilot pump 71 to discharge pilot oil to first pilot oil line 11. In step S4, in response to a travel direction command from the user, operation valves OVA, OVB, OVC, and OVD connected to first pilot oil line 11 convert pilot primary pressure, which is the pressure of pilot oil input to operation valves OVA, OVB, OVC, and OVD, into pilot secondary pressure, and outputs the pilot secondary pressure to second pilot oil line 12 connected to the pilot port of hydraulic pump 7.

[0061] In step S5, the controller 10 controls the primary pressure control valve CV1 provided in the first pilot oil line 11 so that the pilot primary pressure becomes a first target value. In step S6, the controller 10 controls the secondary pressure control valve CV2 connected to the second pilot oil line 12 so that the pilot secondary pressure becomes a second target value. In step S7, the rotational speed sensor 6a detects the rotational speed of the engine 6, and the controller 10 receives the detected rotational speed of the engine 6. In step S8, the controller 10 determines whether the rotational speed of the engine 6 detected by the rotational speed sensor 6a is lower than the target rotational speed by a predetermined threshold speed difference (anti-stall determination value) or more.

[0062] When the rotation speed of the engine 6 detected by the rotation speed sensor 6a falls below the target rotation speed by more than a predetermined threshold speed difference (anti-stall determination value) (YES in step S8), the controller 10 controls both the primary pressure control valve CV1 and the secondary pressure control valve CV2 so as to make the second target value higher than the first target value and to lower the first target value and the second target value. More specifically, when the rotation speed of the engine 6 detected by the rotation speed sensor 6a falls below the target rotation speed by more than a predetermined threshold speed difference (anti-stall determination value) (YES in step S8), the controller 10 simultaneously controls the primary pressure control valve CV1 and the secondary pressure control valve CV2. <Actions and Effects of the Embodiment>

[0063] The control method of the work vehicle 1 according to the embodiment or the processing by the controller 10 of the work vehicle 1 is configured to control both the primary pressure control valve CV1 and the secondary pressure control valve CV2 so that, when the rotational speed of the engine 6 detected by the rotational speed sensor 6a falls below the target rotational speed by a predetermined threshold speed difference (anti-stall determination value) or more, the second target value is made higher than the first target value and the first target value and the second target value are reduced. DR3, DR4 By controlling the pilot pressure in accordance with the target pilot primary pressure while taking into account the pressure loss, it is possible to provide a work vehicle 1 that performs anti-stall control with improved responsiveness while reducing the waste of pilot oil. <Modification>

[0064] The configuration of the secondary pressure control valve CV2 is not limited to the configuration shown in Fig. 3. Fig. 9 is a hydraulic circuit diagram showing a modified secondary pressure control valve CV2. In the example of Fig. 9, check valves CK1 to CK4 are provided in the discharge oil passages DR3 to DR6. These check valves CK1 to CK4 are collectively called check valves CK. The check valves CK close the discharge oil passages DR3 to DR6 unless the pressure on the side with the throttle TH is greater than the pressure on the side with the secondary pressure control valve CV2. DR3~DR6 In this modification, the secondary pressure control valve CV2 is made up of proportional valves CV21 and CV22 and relief valves CV23 and CV24.

[0065] 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 at the input port is greater than the pressure at the control ports 23a and 24a. At this time, the pilot oil is discharged into 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 energizing the solenoids, and are controlled by the controller 10.

[0066] The pilot pressure of the discharge oil passages DR3 and DR4 increases when the first hydraulic pump 7L rotates forward and reverse, respectively, so one side of either one becomes high and the other side becomes low. The pilot pressure of the discharge oil passages DR5 and DR6 increases when the second hydraulic pump 7R rotates forward and reverse, respectively, so one side of either one becomes high and the other side becomes low. For this reason, only one of the check valves CK1 and CK2 opens, and the check valve CK 3 and C.K. 4 Therefore, the proportional valves CV21 and CV22 are controlled in the same way as the secondary pressure control valves CV11 and CV12, so that the above-mentioned control can be carried out.

[0067] The secondary pressure control valves CV11 and CV12 in Fig. 3 may be replaced with the proportional valves CV21 and CV22 and the relief valves CV23 and CV24 in Fig. 9. The proportional valves CV21 and CV22 and the relief valves CV23 and CV24 in Fig. 9 may be replaced with the secondary pressure control valves CV11 and CV12 in Fig. 3.

[0068] 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.

[0069] The terms "member," "part," "element," "body," and "structure" may have multiple meanings, such as a single part or multiple parts.

[0070] Ordinal numbers such as "first" and "second" are merely terms used to identify components and do not have any other meaning (e.g., a particular order). For example, the presence of a "first element" does not imply the presence of a "second element," and the presence of a "second element" does not imply the presence of a "first element."

[0071] Words expressing degrees, such as "substantially," "about," and "approximately," can mean a reasonable deviation that does not significantly change the final result, unless otherwise specified in the embodiment. All numerical values ​​described in this application can be interpreted to include words such as "substantially," "about," and "approximately."

[0072] In this application, the phrase "at least one of A and B" should be interpreted to include A only, B only, and both A and B.

[0073] It is apparent that various changes and modifications of the present invention are possible in light of the above disclosure, and therefore, the present invention may be practiced otherwise than as specifically disclosed herein without departing from the spirit of the present invention.

Claims

1. a hydraulic motor configured to drive the travel gear; a hydraulic pump having a pilot port to which a pilot oil pressure is applied, and configured to supply hydraulic oil to the hydraulic motor in response to a pilot pressure that is the pressure of the pilot oil; an engine configured to drive the hydraulic pump; a rotational speed sensor configured to detect a rotational speed of the engine; a pilot pump driven by the engine and configured to discharge the pilot oil; a driving instruction input device into which a user inputs a driving direction instruction; an operating valve configured to control the pilot pressure by operation of a travel instruction input device; a first pilot oil passage connecting the pilot pump and the operation valve and through which the pilot oil is sent; a primary pressure control valve provided in the first pilot oil passage and configured to control a pilot primary pressure which is the pressure of the pilot oil in the first pilot oil passage; a second pilot oil passage connecting the operation valve and the pilot port and through which the pilot oil is sent; a secondary pressure control valve connected to the second pilot oil passage and configured to control a pilot secondary pressure that is the pressure of the pilot oil in the second pilot oil passage; a controller configured to control a rotational speed of the engine in accordance with a target rotational speed of the engine, and to control the primary pressure control valve and the secondary pressure control valve based on a first target value of the pilot primary pressure and a second target value of the pilot secondary pressure; Equipped with the controller is configured to control both the primary pressure control valve and the secondary pressure control valve so as to make the second target value higher than the first target value and to reduce the first target value and the second target value when the rotational speed of the engine detected by the rotational speed sensor falls below the target rotational speed by a predetermined threshold speed difference or more. Work vehicle.

2. 2. The work vehicle according to claim 1, wherein the controller is configured to simultaneously control the primary pressure control valve and the secondary pressure control valve when the engine rotational speed detected by the rotational speed sensor falls below the target rotational speed by a predetermined threshold speed difference or more.

3. a discharge oil passage branching from the second pilot oil passage and connected to the secondary pressure control valve; a throttle provided in the second pilot oil passage between the operation valve and the discharge oil passage; The work vehicle according to claim 1 or 2, further comprising:

4. Controlling the engine so that the engine rotation speed reaches a target rotation speed; Driving a hydraulic pump connected to the engine to cause the hydraulic pump to discharge hydraulic oil to a hydraulic motor connected to a traveling device; a pilot pump connected to the engine is driven to cause the pilot pump to discharge pilot oil into a first pilot oil passage; A pilot primary pressure, which is the pressure of the pilot oil input, is converted into a pilot secondary pressure by an operation valve connected to the first pilot oil passage in response to a travel direction instruction by a user, and the pilot secondary pressure is output to a second pilot oil passage connected to a pilot port of the hydraulic pump, controlling a primary pressure control valve provided in the first pilot oil passage so that the pilot primary pressure becomes a first target value; controlling a secondary pressure control valve connected to the second pilot oil line so that the pilot secondary pressure becomes a second target value; Detecting the rotational speed of the engine; when the detected engine rotation speed falls below the target rotation speed by a predetermined threshold speed difference or more, the second target value is made higher than the first target value, and both the primary pressure control valve and the secondary pressure control valve are controlled so as to reduce the first target value and the second target value. A method for controlling a work vehicle.

5. A control method as described in claim 4, further comprising simultaneously controlling the primary pressure control valve and the secondary pressure control valve when the engine rotational speed detected by a rotational speed sensor falls below the target rotational speed by more than a predetermined threshold speed difference.

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