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

Abstract

To provide a work vehicle that is improved in responsiveness when performing feed back control to a desired vehicle speed.SOLUTION: In a speed control method for a work vehicle, an engine is driven to rotate a first hydraulic pump, and hydraulic fluid is fed from the first hydraulic pump to a first hydraulic motor, which drives a first traveling device provided in a vehicle body, to rotate the first hydraulic motor. The method detects a rotation speed of the first hydraulic motor. The method performs feedback control, in which at least one of a control valve for a pilot pressure of the first hydraulic pump and an engine is a control target, so as to decrease the speed difference between a target rotation speed of the first hydraulic motor corresponding to a target vehicle speed and a detected rotation speed. The method sets a feedback gain of the feedback control on the basis of a first coefficient which becomes larger as the target rotation speed becomes lower.SELECTED DRAWING: Figure 8

Description

[Technical Field]

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

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

[0003] [Patent Document 1] Japanese Patent Publication No. 2017-053413 [Patent Document 2] Japanese Patent Publication No. 2020-038002 [Overview of the project] [Problems that the invention aims to solve]

[0004] Both Patent Document 1 and Patent Document 2 utilize the rotational speed of the drive motor for feedback. In normal feedback, the gain is the same regardless of the magnitude of the target vehicle speed, so the manipulated variable is determined by the difference between the target rotational speed of the drive motor corresponding to the target vehicle speed and the actual rotational speed of the drive motor. However, even with the same difference, when the target rotational speed of the drive motor is low, the ratio of this difference to the target rotational speed of the drive motor becomes higher, resulting in lower responsiveness compared to when the target rotational speed of the drive motor is high. Furthermore, in normal feedback, the gain is the same regardless of the magnitude of the load on the drive motor, so when the load is large, the swash plate of the drive pump is pushed back, making it difficult to reach the target rotational speed and resulting in lower responsiveness. [Means for solving the problem]

[0005] A speed control method for a work vehicle according to a first aspect of this disclosure involves driving an engine to rotate a first hydraulic pump, sending hydraulic fluid from the first hydraulic pump to a first hydraulic motor that drives a first running gear provided on the vehicle body, thereby rotating the first hydraulic motor. The method detects the rotational speed of the first hydraulic motor. The method performs feedback control on at least one of the pilot pressure control valve of the first hydraulic pump and the engine to reduce the speed difference between the target rotational speed of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed. The method determines the feedback gain of the feedback control based on a first coefficient that increases as the target rotational speed decreases.

[0006] A speed control method for a work vehicle according to a second aspect of this disclosure involves driving an engine to rotate a first hydraulic pump and supplying hydraulic fluid from the first hydraulic pump to a first hydraulic motor that drives a first running gear provided on the vehicle body. The method detects a first differential pressure of the first hydraulic motor. The method detects the rotational speed of the first hydraulic motor. The method performs feedback control on at least one of the pilot pressure control valve of the first hydraulic pump and the engine to reduce the speed difference between the target rotational speed of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed. The method determines the feedback gain of the feedback control based on a second coefficient that increases as the absolute value of the detected first differential pressure increases.

[0007] A work vehicle according to a third aspect of this disclosure comprises a vehicle body, a first hydraulic motor, a first hydraulic pump, an engine, a first rotational speed sensor, and a controller. The first hydraulic motor is configured to drive a first running gear provided on the vehicle body. The first hydraulic pump is configured to supply hydraulic fluid to the first hydraulic motor. The engine is configured to drive the first hydraulic pump. The first rotational speed sensor is configured to detect the rotational speed of the first hydraulic motor. The controller performs feedback control on at least one of the pilot pressure control valve of the first hydraulic pump and the engine to reduce the speed difference between the target rotational speed of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed. The controller determines the feedback gain of the feedback control based on a first coefficient that increases as the target rotational speed decreases.

[0008] A work vehicle according to a fourth aspect of this disclosure comprises a vehicle body, a first hydraulic motor, a first hydraulic pump, a first hydraulic sensor, an engine, a first rotational speed sensor, and a controller. The first hydraulic motor is configured to drive a first running gear provided on the vehicle body. The first hydraulic pump is configured to supply hydraulic fluid to the first hydraulic motor. The first hydraulic sensor detects a first differential pressure of the first hydraulic motor. The engine is configured to drive the first hydraulic pump. The first rotational speed sensor is configured to detect the rotational speed of the first hydraulic motor. The controller performs feedback control on at least one of the pilot pressure control valve of the first hydraulic pump and the engine to reduce the speed difference between the target rotational speed of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed. The controller determines the feedback gain of the feedback control based on a second coefficient that increases as the absolute value of the detected first differential pressure increases. [Effects of the Invention]

[0009] According to the technologies disclosed in the first and third aspects of this application, it is possible to provide a work vehicle in which the responsiveness of the feedback control changes little even when the magnitude of the target rotational speed of the travel motor is different. According to the technologies disclosed in the second and fourth aspects of this application, it is possible to provide a work vehicle in which the responsiveness of the feedback control changes little even when the magnitude of the hydraulic pressure in the oil passage between the travel motor and the travel pump is different. Both of these can improve the user experience. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a side view of the work vehicle. [Figure 2] Figure 2 is a top view of the work vehicle. [Figure 3] Figure 3 is a hydraulic circuit diagram of the travel system of the work vehicle in the first embodiment. [Figure 4] Figure 4 shows the relationship between engine rotational speed, primary pilot pressure, and the set line. [Figure 5] Figure 5 shows the relationship between the operating position of the control lever and the secondary pilot pressure. [Figure 6] Figure 6 is a block diagram of the work vehicles. [Figure 7] Figure 7 shows a control block diagram in the first embodiment. [Figure 8] Figure 8 shows the relationship between the target rotational speed of the first hydraulic motor and the first coefficient in the embodiment. [Figure 9] Figure 9 shows the relationship between the absolute value of the first differential pressure and the first coefficient in the embodiment. [Figure 10] Figure 10 is a flowchart showing the operation of the work vehicle according to the first embodiment. [Figure 11] Figure 11 is a flowchart showing the operation of the method for determining the first coefficient. [Figure 12] Figure 12 is a flowchart showing the operation of the method for determining the second coefficient. [Figure 13]Figure 13 is a hydraulic circuit diagram of the travel system of the work vehicle in the second embodiment. [Figure 14] Figure 14 shows a control block diagram in the second embodiment. [Figure 15] Figure 15 is a flowchart showing the operation of the work vehicle according to the second embodiment. [Figure 16] Figure 16 is a hydraulic circuit diagram of the travel system of a work vehicle in a modified example of the second embodiment. [Figure 17] Figure 17 shows a control block diagram in the third embodiment. [Figure 18A] Figure 18A is a flowchart showing the operation of the work vehicle according to the third embodiment. [Figure 18B] Figure 18B is a flowchart showing the operation of the work vehicle according to the third embodiment. [Figure 19] Figure 19 is a hydraulic circuit diagram of the travel system of the work vehicle in the fourth embodiment. [Figure 20] Figure 20 shows a control block diagram in the fourth embodiment. [Figure 21] Figure 21 is a flowchart showing the operation of the work vehicle according to the fourth embodiment. [Figure 22] Figure 22 shows a control block diagram in a modified example of the first embodiment. [Figure 23] Figure 23 shows a control block diagram in a modified example of the second embodiment. [Figure 24] Figure 24 shows a control block diagram in a modified example of the third embodiment. [Figure 25] Figure 25 shows a control block diagram in a modified example of the fourth embodiment.

[0011] The present invention will be described in detail below with reference to the drawings illustrating its embodiments. In the drawings, the same reference numerals indicate corresponding or substantially identical components. <First Embodiment> <Overall Structure> Referring to Figures 1 and 2, the work vehicle 1, for example, a compact track loader, comprises a vehicle body 2, a pair of running gears 3, and a work device 4. The vehicle body 2 supports the running gears 3 and the work device 4. In the illustrated embodiment, the running gears 3 are tracked running gears mounted on the vehicle body 2. Thus, each of the pair of running gears 3 includes a drive wheel 31, driven wheels 32, 33, and idler wheels 34, which are driven by a hydraulic motor device 30. However, each of the pair of running gears 3 is not limited to tracked running gear. Each of the pair of running gears 3 may be, for example, a front-wheel / rear-wheel running gear, or a running gear having front wheels and a rear crawler. The work device 4 includes work equipment (bucket) 41 at the distal end of the work device 4. The proximal end of the work device 4 is attached to the rear of the vehicle body 2. The working device 4 includes a pair of arm assemblies 42 for rotatably supporting the bucket 41 via a bucket pivot shaft 43. Each of the pair of arm assemblies 42 includes a link 44 and an arm 45.

[0012] Link 44 is rotatable with respect to the vehicle body 2 around a fulcrum shaft 46. Arm 45 is rotatable with respect to link 44 around a joint shaft 47. The working device 4 further includes a plurality of arm cylinders 48 and at least one equipment cylinder 49. Each of the plurality of arm cylinders 48 is rotatably connected to the vehicle body 2 and the arm 45, and moves links 44 and arm 45 etc. to raise and lower the bucket 41. At least one equipment cylinder 49 is configured to tilt the bucket 41. The vehicle body 2 includes a cabin 5. The cabin 5 has an openable and closable front window 51, and its outer shape is defined by a cab frame 53. The front window 51 may be omitted. The work vehicle 1 includes a driver's seat 54 and an operation lever 55 in the cabin 5. The cab frame 53 is rotatable around rotational shafts RSL and RSR on the vehicle body 2 as shown in FIG. 2. In FIGS. 1 and 2, a common pivot A defined by the rotational shafts RSL and RSR XC is illustrated. That is, the cab frame 53 is rotatably attached to the vehicle body 2 around the pivot A XC .

[0013] In the embodiment according to the present application, the front-rear direction D FB (front direction D (front direction D F / rear direction D B ) means the front-rear direction (front direction / rear direction) as viewed from an operator seated in the driver's seat 54 of the cabin 5. The left direction D L , the right direction D R , and the width direction D W mean the left direction, the right direction, and the left-right direction, respectively, as viewed from the operator; the upward direction D U , the downward direction D D , and the height direction D H mean the upward direction, the downward direction, and the height direction, respectively, as viewed from the operator. The front-rear / left-right (width) / up-down (height) directions of the work vehicle 1 shall coincide with the front-rear / left-right (width) / up-down (height) directions as viewed from the operator, respectively.

[0014] Figure 1 shows the left side of the work vehicle 1. As shown in Figure 2, the vehicle body 2 is generally symmetrical with respect to the central surface M of the vehicle body and includes a first side 2L, which is the left side, and a second side 2R, which is the right side. Of the pair of running gears 3, the running gear 3 provided on the first side 2L is shown as the left running gear 3L, and the running gear 3 provided on the second side 2R is shown as the right running gear 3R. Of the pair of arm assemblies 42, the arm assembly 42 provided on the left side with respect to the central surface M of the vehicle body is shown as the first arm assembly 42L, and the arm assembly 42 provided on the right side with respect to the central surface M of the vehicle body is shown as the second arm assembly 42R. The link 44 provided on the left side with respect to the central surface M of the vehicle body is shown as the first link 44L. The arm 45 provided on the left side with respect to the central surface M of the vehicle body is shown as the first arm 45L, and the arm 45 provided on the right side with respect to the central surface M of the vehicle body is shown as the second arm 45R. A pivot shaft 46 located on the left side of the vehicle body's central plane M is shown as the first pivot shaft 46L, and a pivot shaft 46 located on the right side of the vehicle body's central plane M is shown as the second pivot shaft 46R. A joint shaft 47 located on the left side of the vehicle body's central plane M is shown as the first joint shaft 47L, and a joint shaft 47 located on the right side of the vehicle body's central plane M is shown as the second joint shaft 47R. Of the hydraulic motor devices 30, the hydraulic motor device 30 located on the left side of the vehicle body's central plane M is shown as the left hydraulic motor device 30L, and the hydraulic motor device 30 located on the right side of the vehicle body's central plane M is shown as the right hydraulic motor device 30R.

[0015] Referring to Figures 1 and 2, the work vehicle 1 further comprises an engine 6 located at the rear of the vehicle body 2, and a plurality of hydraulic pumps 7, including a left hydraulic pump 7L and a right hydraulic pump 7R. The engine 6 is configured to drive the plurality of hydraulic pumps 7. The left hydraulic pump 7L and the right hydraulic pump 7R are configured to discharge hydraulic fluid to drive a hydraulic motor device 30 (etc.) that drives the drive wheels 31. The left hydraulic pump 7L and the right hydraulic pump 7R are collectively referred to as hydraulic pumps (7L, 7R). The plurality of hydraulic pumps 7 other than the left hydraulic pump 7L and the right hydraulic pump 7R are configured to discharge hydraulic fluid to drive hydraulic actuators (multiple arm cylinders 48, at least one tool cylinder 49, etc.) connected to the work device 4. The engine 6 is located in the width direction D of the work vehicle 1. W In this configuration, it is provided between a pair of arm assemblies 42. The work vehicle 1 further comprises a cover 8 for covering the engine 6. The work vehicle 1 further comprises a bonnet cover 9 provided at the rear end of the vehicle body 2. The bonnet cover 9 is openable and closable, allowing maintenance personnel to perform maintenance work on the engine 6 and other components.

[0016] Figure 3 is a hydraulic circuit diagram of the drive system of the work vehicle 1 in the first embodiment. The work vehicle 1 includes a hydraulic circuit 1A. The hydraulic circuit 1A includes a hydraulic oil tank 70 and a pilot pump 71. The pilot pump 71 is a constant-capacity gear pump driven by the power of the engine 6. The pilot pump 71 is configured to discharge the hydraulic oil stored in the hydraulic oil tank 70. In particular, the pilot pump 71 is configured to discharge hydraulic oil mainly used for control. For convenience of explanation, the hydraulic oil discharged from the pilot pump 71 that is used for control will be called pilot oil, and the pressure of the pilot oil will be called pilot pressure. In particular, the pilot pump 71 is configured to supply pilot oil to the left hydraulic pump 7L and the right hydraulic pump 7R.

[0017] The hydraulic circuit 1A includes a pilot supply oil passage PA1 connected to the discharge port of the pilot pump 71. Pilot oil flows through the pilot supply oil passage PA1. The hydraulic circuit 1A includes a plurality of switching valves (brake switching valve SV1, directional switching valve SV2) and a plurality of brake mechanisms 72 connected to the pilot supply oil passage PA1. The brake switching valve SV1 is connected to the pilot supply oil passage PA1. The brake switching valve SV1 is a directional switching valve (solenoid valve) for braking and releasing the brakes by the plurality of brake mechanisms 72. The brake switching valve SV1 is a two-position switching valve configured to switch its valve body to a first position VP1a or a second position VP1b by excitation. The switching of the valve body of the brake switching valve SV1 is performed by the brake pedal 13 (see Figure 6). A sensor 14 is provided on the brake pedal 13. The amount of operation detected by the sensor 14 is input to a controller 10 consisting of an ECU (Electric Control Unit). Controller 10 may also be referred to as a control device.

[0018] The multiple brake mechanisms 72 include a first brake mechanism 72L for braking the left running gear 3L and a second brake mechanism 72R for braking the right running gear 3R. The first brake mechanism 72L and the second brake mechanism 72R are connected to the brake switching valve SV1 via an oil passage PA2. The first brake mechanism 72L and the second brake mechanism 72R are configured to brake the running gear 3 in accordance with the pressure of the pilot oil (hydraulic fluid). When the valve body of the brake switching valve SV1 is switched to the first position VP1a, hydraulic fluid is released from the oil passage PA2 in the section between the brake switching valve SV1 and the brake mechanism 72, and the running gear 3 is braked by the brake mechanism 72. When the valve body of the brake switching valve SV1 is switched to the second position VP1b, the braking by the brake mechanism 72 is released. Furthermore, when the valve body of the brake switching valve SV1 is switched to the first position VP1a, the braking by the brake mechanism 72 is released, and when the valve body of the brake switching valve SV1 is switched to the second position VP1b, the running gear 3 may be braked by the brake mechanism 72.

[0019] The directional control valve SV2 is a solenoid valve that changes the rotation of the left hydraulic motor unit 30L and the right hydraulic motor unit 30R. The directional control valve SV2 is a two-position directional control valve configured to switch its valve body to a first position VP2a or a second position VP2b by excitation. The switching of the directional control valve SV2 is performed by an operating member or the like (not shown). Note that the directional control valve SV2 may be a proportional valve capable of adjusting the flow rate of the discharged hydraulic fluid instead of a two-position directional control valve.

[0020] The left hydraulic motor unit 30L is a device that transmits power to the drive wheel 31 provided on the left travel unit 3L. The left hydraulic motor unit 30L includes a left hydraulic motor 31L, a first swash plate switching cylinder 32L, and a first travel control valve (hydraulic switching valve) SV4. The left hydraulic motor 31L is a swash plate type variable displacement axial motor for driving the left travel unit 3L, and is a motor that can change the vehicle speed (rotation) to first or second speed. The first swash plate switching cylinder 32L is a cylinder configured to change the angle of the swash plate of the left hydraulic motor 31L by extending and retracting. The first travel control valve SV4 is a valve for extending and retracting the first swash plate switching cylinder 32L. The first travel control valve SV4 is a two-position switching valve configured to switch its valve body between a first position VP4a and a second position VP4b.

[0021] The first travel control valve SV4 is switched by the directional control valve SV2, which is located upstream and connected to the first travel control valve SV4. Specifically, the directional control valve SV2 and the first travel control valve SV4 are connected by an oil passage PA3, and the first travel control valve SV4 is switched by the hydraulic fluid flowing through the oil passage PA3. For example, when the valve body of the directional control valve SV2 is switched to the first position VP2a by operating the operating member, pilot oil is released in the section between the directional control valve SV2 and the first travel control valve SV4, and the valve body of the first travel control valve SV4 is switched to the first position VP4a. As a result, the first swash plate switching cylinder 32L retracts, and the speed of the left hydraulic motor 31L is changed to first gear. Furthermore, when the valve body of the directional control valve SV2 is switched to the second position VP2b by the operation of the operating member, pilot oil is supplied to the first travel control valve SV4 through the directional control valve SV2, and the valve body of the first travel control valve SV4 is switched to the second position VP4b. As a result, the first swash plate switching cylinder 32L extends, and the speed of the left hydraulic motor 31L is changed to second speed.

[0022] The right hydraulic motor unit 30R is a device that transmits power to the drive wheel 31 provided on the right travel unit 3R. The right hydraulic motor unit 30R includes the right hydraulic motor 31R, the second swash plate switching cylinder 32R, and the second travel control valve (hydraulic switching valve) SV5. The right hydraulic motor unit 30R is a hydraulic motor for driving the right travel unit 3R and operates in the same way as the left hydraulic motor unit 30L. In other words, the right hydraulic motor 31R operates in the same way as the left hydraulic motor 31L. The left hydraulic motor 31L and the right hydraulic motor 31R are collectively referred to as the hydraulic motor (31L, 31R). The second swash plate switching cylinder 32R operates in the same way as the first swash plate switching cylinder 32L. The second travel control valve SV5 is a two-position switching valve configured to switch its valve body between the first position VP5a and the second position VP5b, and operates in the same way as the first travel control valve SV4.

[0023] The hydraulic circuit 1A is connected to a drain oil passage DR1. The drain oil passage DR1 is an oil passage that carries pilot oil from multiple switching valves (brake switching valve SV1, directional switching valve SV2) to the hydraulic oil tank 70. For example, the drain oil passage DR1 is connected to the discharge ports of multiple switching valves (brake switching valve SV1, directional switching valve SV2). In other words, when the brake switching valve SV1 is in the first position VP1a, hydraulic oil is discharged from the oil passage PA2 to the drain oil passage DR1 in the section between the brake switching valve SV1 and the brake mechanism 72. When the directional switching valve SV2 is in the first position VP1a, the pilot oil in the oil passage PA3 is discharged to the drain oil passage DR1.

[0024] The hydraulic circuit 1A further includes a first charge oil passage PA4 and a hydraulic drive unit 75. The first charge oil passage PA4 branches off from the pilot supply oil passage PA1 and is connected to the hydraulic drive unit 75. The hydraulic drive unit 75 is a device that drives the 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.

[0025] The first drive circuit 76L includes a left hydraulic pump 7L, drive oil passages PA5L and PA6L, and a second charge oil passage PA7L. The drive oil passages PA5L and PA6L are oil passages that connect the left hydraulic pump 7L and the left hydraulic motor 31L. The hydraulic circuit formed by the drive oil passages PA5L and PA6L is called the left hydraulic circuit CL. The second charge oil passage PA7L is connected to the drive oil passages PA5L and PA6L and is an oil passage that replenishes the drive oil passages PA5L and PA6L with hydraulic fluid from the pilot pump 71. The left hydraulic motor 31L has a first connection port 31P1 that connects to the drive oil passage PA5L and a second connection port 31P2 that connects to the drive oil passage PA6L. Through the first connection port 31P1, hydraulic fluid that rotates the left travel device 3L in the forward direction is input to the left hydraulic motor 31L, and hydraulic fluid that rotates the left travel device 3L in the reverse direction is discharged from the left hydraulic motor 31L through the first connection port 31P1. Through the second connection port 31P2, hydraulic fluid that rotates the left travel device 3L in the reverse direction is input to the left hydraulic motor 31L, and hydraulic fluid that rotates the left travel device 3L in the forward direction is discharged from the left travel device 3L.

[0026] Similarly, the second drive circuit 76R has a right hydraulic pump 7R, drive oil passages PA5R and PA6R, and a third charge oil passage PA7R. The drive oil passages PA5R and PA6R are oil passages that connect the right hydraulic pump 7R and the right hydraulic motor 31R. The hydraulic circuit formed by the drive oil passages PA5R and PA6R is called the right hydraulic circuit CR. The third charge oil passage PA7R is connected to the drive oil passages PA5R and PA6R and is an oil passage that replenishes the drive oil passages PA5R and PA6R with hydraulic fluid from the pilot pump 71. The right hydraulic motor 31R has a third connection port 31P3 that connects to the drive oil passage PA5R and a fourth connection port 31P4 that connects to the drive oil passage PA6R. Through the third connection port 31P3, hydraulic fluid that rotates the right travel unit 3R in the forward direction is input to the right hydraulic motor 31R, and hydraulic fluid that rotates the right travel unit 3R in the reverse direction is discharged from the right hydraulic motor 31R through the third connection port 31P3. Through the fourth connection port 31P4, hydraulic fluid that rotates the right travel unit 3R in the reverse direction is input to the right hydraulic motor 31R, and hydraulic fluid that rotates the right travel unit 3R in the forward direction is discharged from the right travel unit 3R. In other words, the hydraulic motors (31L, 31R) are configured to drive the travel units (3L, 3R). The hydraulic pumps (7L, 7R) are configured to discharge the hydraulic fluid that drives the hydraulic motors (31L, 31R). The drive oil passages (PA5L, PA6L, PA5R, PA6R) are oil passages that connect the hydraulic pumps (7L, 7R) and the hydraulic motors (31L, 31R).

[0027] The left hydraulic pump 7L and the right hydraulic pump 7R are swashplate type variable displacement axial pumps driven by the power of the engine 6. The left hydraulic pump 7L is connected to the left hydraulic motor 31L via the left hydraulic circuit CL and has a first port PLa and a second port PLb on which pilot pressure acts. The left hydraulic pump 7L is configured to change the angle of the swashplate in accordance with the pilot pressure acting on the first port PLa and the second port PLb, and to supply hydraulic fluid to the left hydraulic motor 31L. Specifically, the left hydraulic pump 7L is configured to supply hydraulic fluid to the left hydraulic motor 31L via the left hydraulic circuit CL to drive the left travel device 3L forward when the hydraulic pressure on the first port PLa is higher than the hydraulic pressure on the second port PLb, and to supply hydraulic fluid to the left hydraulic motor 31L via the left hydraulic circuit CL to drive the left travel device 3L backward when the hydraulic pressure on the second port PLb is higher than the hydraulic pressure on the first port PLa.

[0028] The right hydraulic pump 7R is connected to the right hydraulic motor 31R via the right hydraulic circuit CR and has a third port PRa and a fourth port PRb on which pilot pressure acts. The right hydraulic pump 7R is configured to change the angle of its swash plate in accordance with the pilot pressure acting on the third port PRa and the fourth port PRb, and to supply hydraulic fluid to the right hydraulic motor 31R. Specifically, the right hydraulic pump 7R is configured to supply hydraulic fluid to the right hydraulic motor 31R via the right hydraulic circuit CR to drive the right travel device 3R forward when the hydraulic pressure applied to the third port PRa is higher than the hydraulic pressure applied to the fourth port PRb, and to supply hydraulic fluid to the right hydraulic motor 31R via the right hydraulic circuit CR to drive the right travel device 3R backward when the hydraulic pressure applied to the fourth port PRb is higher than the hydraulic pressure applied to the third port PRa. The left hydraulic pump 7L and the right hydraulic pump 7R can change their output (amount of hydraulic fluid discharged) and the direction of hydraulic fluid discharge in accordance with the angle of their swash plates.

[0029] The output and hydraulic fluid discharge direction of the left hydraulic pump 7L and the right hydraulic pump 7R are changed by an operating device 56 for controlling the direction of travel of the work vehicle 1. Specifically, the output and hydraulic fluid discharge direction of the left hydraulic pump 7L and the right hydraulic pump 7R are changed in response to the operation of the operating lever 55 provided on the operating device 56. In other words, the operating device 56 is configured to control the direction of travel of the work vehicle by selecting at least one of the left travel device 3L and the right travel device 3R and instructing at least one of the travel devices to move forward or backward. The user inputs the direction of travel via the operating lever 55. The operating lever 55 may also be called a travel instruction input device.

[0030] As shown in Figure 3, the hydraulic circuit 1A includes a pilot supply oil passage PA8 that branches off from the pilot supply oil passage PA1 and is connected to the operating device 56, and a primary pressure control valve CV1 provided on the pilot supply oil passage PA8. In subsequent embodiments, the pilot supply oil passage PA1 and the pilot supply oil passage PA8 are collectively referred to as the primary pilot oil passage. The primary pressure control valve CV1 is an electromagnetic proportional valve including a solenoid and is configured to adjust the pilot pressure supplied to the operating device 56 by adjusting its opening degree according to the current applied to the solenoid. The opening degree of the primary pressure control valve CV1 is controlled by a current sent from the controller 10. Note that the pilot pressure output from the primary pressure control valve CV1 may increase as the magnitude of the current increases, or it may decrease as the magnitude of the current increases. In subsequent embodiments, the primary pressure control valve CV1 may also be referred to as the hydraulic adjustment mechanism. The detailed operation of the primary pressure control valve CV1 will be described later.

[0031] The operating device 56 includes an operating valve OVA for forward movement, an operating valve OVB for reverse movement, an operating valve OVC for right turn, an operating valve OVD for left turn, and an operating lever 55. The operating device 56 also has first to fourth shuttle valves SCa, SVb, SVc, and SVd. The operating valves OVA, OVB, OVC, and OVD are operated by a single operating lever 55. The operating valves OVA, OVB, OVC, and OVD change the pressure of the hydraulic fluid in response to the operation of the operating lever 55, and supply the changed hydraulic fluid to the first port PLa and second port PLb of the left hydraulic pump 7L and the third port PRa and fourth port PRb of the right hydraulic pump 7R. In this embodiment, the operating valves OVA, OVB, OVC, and OVD are operated by a single operating lever 55, but there may be multiple operating levers 55. In subsequent embodiments, one or more operating levers 55 may be referred to as the first operating device.

[0032] The control valves OVA, OVB, OVC, and OVD have an input port (primary port), a discharge port, and an output port (secondary port). As shown in Figure 3, the input port is connected to the pilot supply oil passage PA8. The discharge port is connected to the drain oil passage DR2 leading to the hydraulic oil tank 70. The operating lever 55 can be tilted from the neutral position in the forward / backward direction, in the width direction perpendicular to the forward / backward direction, and in the diagonal direction. The control valves OVA, OVB, OVC, and OVD of the operating device 56 are operated according to the tilt of the operating lever 55. As a result, a pilot pressure corresponding to the amount of operation of the operating lever 55 from the neutral position is output from the secondary port of the control valves OVA, OVB, OVC, and OVD. The relationship between the pilot pressure applied to the primary port output from the primary pressure control valve CV1 and the pilot pressure applied to the secondary port will be described later.

[0033] The secondary port of control valve OVA and the secondary port of control valve OVC are connected to the input port of the first shuttle valve SVa, and the output port of the first shuttle valve SVa is connected to the first port PLa of the left hydraulic pump 7L via the first pilot oil passage PA11. The secondary port of control valve OVA and the secondary port of control valve OVD are connected to the input port of the second shuttle valve SVb, and the output port of the second shuttle valve SVb is connected to the third port PRa of the right hydraulic pump 7R via the third pilot oil passage PA13. The secondary port of control valve OVB and the secondary port of control valve OVD are connected to the input port of the third shuttle valve SVc, and the output port of the third shuttle valve SVc is connected to the second port PLb of the left hydraulic pump 7L via the second pilot oil passage PA12. The secondary port of control valve OVB and the secondary port of control valve OVC are connected to the input port of the fourth shuttle valve SVd, and the output port of the fourth shuttle valve SVd is connected to the fourth port PRb of the right hydraulic pump 7R via the fourth pilot oil passage PA14. In other words, pilot supply oil passage PA8, first pilot oil passage PA11, and fourth pilot oil passage PA14 connect pilot pump 71 to left hydraulic pump 7L. Pilot supply oil passage PA8, second pilot oil passage PA12, and third pilot oil passage PA13 connect pilot pump 71 to right hydraulic pump 7R.

[0034] When the operating lever 55 is tilted forward, the forward operating valve OVA is operated and pilot pressure is output from the operating valve OVA. This pilot pressure acts from the first shuttle valve SVa to the first port PLa via the first pilot oil passage PA11 connecting the operating device 56 and the first port PLa of the left hydraulic pump 7L, and also acts from the second shuttle valve SVb to the third port PRa via the third pilot oil passage PA13 connecting the operating device 56 and the third port PRa of the right hydraulic pump 7R. As a result, the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R rotate forward at a speed corresponding to the amount of tilt of the operating lever 55, causing the work vehicle 1 to move straight forward.

[0035] Furthermore, when the operating lever 55 is tilted to the rear, the reverse operating valve OVB is operated and pilot pressure is output from the operating valve OVB. This pilot pressure acts from the third shuttle valve SVc to the second port PLb of the left hydraulic pump 7L via the second pilot oil passage PA12 connecting the operating device 56 and the second port, and also acts from the fourth shuttle valve SVd to the fourth port PRb of the right hydraulic pump 7R via the fourth pilot oil passage PA14 connecting the operating device 56 and the fourth port PRb of the right hydraulic pump 7R. As a result, the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R reverse (rotate in reverse) at a speed corresponding to the amount of tilt of the operating lever 55, causing the work vehicle 1 to move straight backward.

[0036] Furthermore, when the operating lever 55 is tilted to the right, the operating valve OVC for right rotation is operated and pilot pressure is output from the operating valve OVC. This pilot pressure acts on the first port PLa of the left hydraulic pump 7L via the first pilot oil passage PA11 from the first shuttle valve SVa, and also acts on the fourth port PRb of the right hydraulic pump 7R via the fourth pilot oil passage PA14 from the fourth shuttle valve SVd. As a result, the system curves to the right with a degree of curvature corresponding to the rightward operating position of the operating lever 55.

[0037] Furthermore, when the operating lever 55 is tilted to the left, the operating valve OVD for left rotation is operated, and pilot pressure is output from the operating valve OVD. This pilot pressure acts on the third port PRa of the right hydraulic pump 7R via the third pilot oil passage PA13 from the second shuttle valve SVb, and also acts on the second port PLb of the left hydraulic pump 7L via the second pilot oil passage PA12 from the third shuttle valve SVc. As a result, the system curves to the left with a degree of curvature corresponding to the leftward operating position of the operating lever 55.

[0038] In other words, when the operating lever 55 is tilted diagonally forward to the left, the work vehicle 1 moves forward at a speed corresponding to the operating position of the operating lever 55 in the forward and backward direction, and turns to the left with a curve corresponding to the operating position of the operating lever 55 to the left. When the operating lever 55 is tilted diagonally forward to the right, the work vehicle 1 moves forward and turns to the right at a speed corresponding to the operating position of the operating lever 55. When the operating lever 55 is tilted diagonally rear to the left, the work vehicle 1 moves backward and turns to the left at a speed corresponding to the operating position of the operating lever 55. When the operating lever 55 is tilted diagonally rear to the right, the work vehicle 1 moves backward and turns to the right at a speed corresponding to the operating position of the operating lever 55.

[0039] Next, the detailed operation of the primary pressure control valve CV1 will be described. The work vehicle 1 includes a setting member 11 (see Figure 6) for setting the target rotational speed of the engine 6. The setting member 11 is a speed input device separate from the operating device 56 described above, which is an accelerator pedal, a pivotably supported accelerator lever, or a rotatable indoor dial. A sensor 12 is provided on the setting member 11. The manipulated amount detected by the sensor 12 is input to the controller 10. The engine rotational speed corresponding to the manipulated amount detected by the sensor 12 is the target rotational speed of the engine 6. In other words, the target rotational speed of the engine 6 is set based on the manipulated amount of the setting member 11. The controller 10 outputs a rotation command to the injector indicating, for example, the fuel injection amount, injection timing, and fuel injection rate, so that the engine 6 reaches this determined target rotational speed. Alternatively, the controller 10 outputs a rotation command indicating the fuel injection pressure, etc., to the supply pump or common rail so that the engine 6 reaches this determined target rotational speed. In subsequent embodiments, the one or more operating levers 55 and setting member 11 described above may be referred to as at least one operating device. A speed sensor 6a that detects the actual engine rotation speed (referred to as the actual rotation speed of the engine 6) is connected to the controller 10, and the actual rotation speed of the engine 6 is input. The speed sensor 6a is, for example, a potentiometer configured to detect the rotation speed of a rotating member connected to the crankshaft of the engine 6. When a load is applied to the engine 6, the actual rotation speed of the engine 6 decreases from the target rotation speed of the engine 6. The amount of decrease in the actual rotation speed from the target rotation speed when a load is applied to the engine 29 (the difference between the target rotation speed of the engine 6 and the actual rotation speed of the engine 6) is called the engine drop.

[0040] The primary pressure control valve CV1 can set the pilot pressure (primary pilot pressure) acting on the input ports (primary ports) of multiple operating valves OVA, OVB, OVC, and OVD based on the amount of drop ΔE1 of the engine speed (engine speed E1). In other words, the primary pressure control valve CV1 is installed between the pilot pump 71 and the operating valves OVA, OVB, OVC, and OVD, and is configured to supply pilot oil to the operating valves OVA, OVB, OVC, and OVD, and to convert the pressure of the pilot oil supplied to the operating valves OVA, OVB, OVC, and OVD into primary pilot pressure. The engine speed E1 can be detected by the speed sensor 6a. The engine speed E1 detected by the speed sensor 6a is input to the controller 10. The speed sensor 6a may also be called a speed sensor. Figure 4 shows the relationship between engine speed, primary pilot pressure, and set lines L1 and L2. The setting line L1 shows the relationship between engine speed E1 and primary pilot pressure when the decrease amount ΔE1 is less than a predetermined value (less than the anti-stall judgment value). The setting line L2 shows the relationship between engine speed E1 and primary pilot pressure when the decrease amount ΔE1 is equal to or greater than the anti-stall judgment value. When the difference between the rotational speed RS1 determined based on the operating amount of the setting member 11 and the actual rotational speed of the engine 6 is less than a predetermined stall judgment speed difference (anti-stall judgment value), the primary pilot pressure corresponding to rotational speed RS1 transitions according to the third correspondence shown in the setting line L1. When the difference between rotational speed RS1 and the actual rotational speed of the engine 6 is equal to or greater than a predetermined stall judgment speed difference (anti-stall judgment value), the primary pilot pressure corresponding to rotational speed RS1 transitions according to the fourth correspondence shown in the setting line L2.

[0041] If the decrease amount ΔE1 is less than the anti-stall judgment value, the controller 10 adjusts the opening degree of the primary pressure control valve CV1 so that the relationship between the engine speed E1 and the primary pilot pressure matches the reference pilot pressure indicated by the set line L1. If the decrease amount ΔE1 is greater than or equal to the anti-stall judgment value, the controller 10 adjusts the opening degree of the primary pressure control valve CV1 so that the relationship between the engine speed E1 and the primary pilot pressure matches the set line L2, which is lower than the reference pilot pressure. At the set line L2, the primary pilot pressure for a given engine speed E1 is lower than the primary pilot pressure at the set line L1. That is, when considering the same engine speed E1, the primary pilot pressure at the set line L2 is set lower than the primary pilot pressure at the set line L1. Therefore, control based on the set line L2 keeps the pressure (pilot pressure) of the hydraulic fluid entering the operating valves OVA, OVB, OVC, and OVD low. As a result, the swash plate angles of the left hydraulic pump 7L and the right hydraulic pump 7R are adjusted, reducing the load acting on the engine 6 and preventing the engine 6 from stalling. Although Figure 4 shows one setting line L2, there may be multiple setting lines L2. For example, a setting line L2 may be set for each engine rotational speed E1. Furthermore, it is preferable that the controller 10 has the data or control parameters such as functions that indicate the setting lines L1 and L2.

[0042] Next, we will explain the pilot pressure (secondary pilot pressure) output from the secondary ports of the control valves OVA, OVB, OVC, and OVD. Figure 5 shows the relationship between the operating position of the control lever and the secondary pilot pressure. Referring to Figure 4, the lever operating position starts at the neutral position (G0 position), where the origin is the starting position of the lever stroke, and approaches the end position (G5 position), which is the end position of the lever stroke, as it moves away from the origin. The operating range of the control lever 55 is divided into a neutral region RA1 where the object being operated does not move (in the example, from the G0 position to the G1 position), a near-full-operation region RA2 near the end of operation (in the example, from the G3 position to the G5 position), and an intermediate region RA3 between the neutral region RA1 and the near-full-operation region RA2 (in the example, from the G1 position to the G3 position). Furthermore, the intermediate region RA3 can be divided into the slow-velocity region RA3A from position G1 to position G2, and the intermediate-velocity region RA3B from position G2 to position G3.

[0043] In the neutral region RA1, operating the control lever 55 does not supply secondary pilot pressure. On the other hand, in the near-full-operation region RA2, the speed of the object being operated is not adjusted, and therefore the control lever 55 is operated all the way to the end position (G5 position) without stopping midway. In the intermediate region RA3, the control lever 55 can be stopped or its position changed at any point within the region to adjust the speed of the object being operated to the operator's desired speed. For example, the ratio of each operating region RA1, RA3A, RA3B, and RA2 to the lever stroke is as follows.

[0044] Neutral zone RA1: 0% to less than 15% Low-velocity region RA3A: 15% to less than 45% Intermediate speed range RA3B: 45% or more and less than 75% Full operation range RA2: 75% to 100% In the characteristic diagram shown in Figure 5, when the operating lever 55 is operated from the G0 position to the G1 position, a secondary pilot pressure (Pa) is generated. When the operating lever 55 is operated from the G1 position to the G4 position, the secondary pilot pressure rises from Pa to Pb in proportion to the amount the operating lever 55 is operated. At the G4 position, the primary pilot pressure is bypassed and flows to the secondary side, causing the secondary pilot pressure to rise rapidly from Pb to the maximum output pressure Pc. While the operating lever 55 is operated from the G4 position to the G5 position, the secondary pilot pressure remains constant at the maximum output pressure (Pc) and is equal to the primary pilot pressure. In other words, when the displacement of the operating lever 55 from the neutral position to instruct movement to the left is greater than or equal to the first displacement value (displacement from G0 to G4), the operating device 56 outputs the primary pilot pressure input to the operating device 56 to the first port PLa and the fourth port PRb. In the following embodiments, operating the operating lever 55 between the G4 position and the G5 position is referred to as operating the operating lever 55 with its full stroke. The operating device 56 outputs primary pilot pressure input to the operating device 56 to the second port PLb and the third port PRa when the displacement of the operating lever 55 for instructing movement to the right from the neutral position is greater than or equal to a first displacement value (displacement from G0 to G4). The operating device 56 outputs primary pilot pressure input to the operating device 56 to the first port PLa and the third port PRa when the displacement of the operating lever 55 for instructing movement forward from the neutral position is greater than or equal to a first displacement value (displacement from G0 to G4). The operating device 56 outputs primary pilot pressure input to the operating device 56 to the second port PLb and the fourth port PRb when the displacement of the operating lever 55 for instructing movement backward from the neutral position is greater than or equal to a first displacement value (displacement from G0 to G4). Note that the characteristic value of the secondary pilot pressure in the forward / backward direction may differ from the characteristic value of the secondary pilot pressure in the left / right direction.If the characteristic values ​​of the secondary pilot pressure in the forward and backward directions corresponding to G0 to G5 and Pa to Pc are G0' to G5' and Pa' to Pc', then the operating device 56 may output the primary pilot pressure input to the operating device 56 to the first port PLa and the third port PRa when the displacement of the operating lever 55 for instructing forward movement from the neutral position is greater than or equal to the second displacement value (displacement from G0' to G4'). The operating device 56 may also output the primary pilot pressure input to the operating device 56 to the second port PLb and the fourth port PRb when the displacement of the operating lever 55 for instructing backward movement from the neutral position is greater than or equal to the second displacement value (displacement from G0' to G4'). Furthermore, Pa and Pb (Pa' and Pb') are values ​​that do not depend on the magnitude of the primary pilot pressure, but if the primary pilot pressure is lower than Pa or Pb (Pa' or Pb'), the secondary pilot pressure will cap out at the magnitude of the primary pilot pressure. In other words, the control valves (OVA, OVB, OVC, OVD) are configured to convert the pilot oil pressure from primary pilot pressure to secondary pilot pressure and output pilot oil according to the first operating amount (operating lever position) of the operating device 56. The pilot oil at secondary pilot pressure is applied to the ports (PLa, PRa, PLb, PRb) that supply hydraulic pressure to the swash plate of the hydraulic pump (7L, 7R). When the first operating amount is greater than or equal to a threshold amount (first displacement value), the control valves (OVA, OVB, OVC, OVD) convert the pressure to a secondary pilot pressure equal to the primary pilot pressure.

[0045] Based on the characteristics of the control valves OVA, OVB, OVC, and OVD described above, the movement of the work vehicle 1 corresponding to the operation of the control lever 55 will be explained in more detail. When the amount of operation of the control lever 55 in the forward / backward direction is greater than the amount of operation in the rightward direction, and the position of the control lever 55 in the rightward direction is operated from position G1 to position G3, the left hydraulic pump 7L rotates in the same direction as the right hydraulic pump 7R, with the magnitude of the rotational speed being greater than the magnitude of the rotational speed of the right hydraulic pump 7R, causing the work vehicle 1 to make a wide turn to the right. When the position of the control lever 55 in the rightward direction becomes the same as the position of operation in the forward / backward direction, the rotational speed of the right hydraulic pump 7R becomes 0, and only the left hydraulic pump 7L rotates, causing the work vehicle 1 to perform a right pivot turn. Furthermore, when the position of the control lever 55 in the rightward direction is operated between position G4 and position G5, it becomes greater than the position of operation in the forward / backward direction, causing the output shaft of the left hydraulic pump 7L to rotate forward and the output shaft of the right hydraulic pump 7R to rotate backward, causing the work vehicle 1 to turn to the right.

[0046] Furthermore, when the amount of operation of the operating lever 55 in the forward / backward direction is greater than the amount of operation in the leftward direction, and the operating position of the operating lever 55 in the leftward direction is operated from position G1 to position G3, the rotational speed of the right hydraulic pump 7R is greater than that of the left hydraulic pump 7L, and they rotate in the same direction, causing the work vehicle 1 to make a wide turn to the left. When the operating position of the operating lever 55 in the leftward direction becomes the same as the operating position in the forward / backward direction, the rotational speed of the left hydraulic pump 7L becomes 0, and only the right hydraulic pump 7R rotates, causing the work vehicle 1 to perform a left pivot turn. Moreover, when the operating position of the operating lever 55 in the leftward direction is operated between position G4 and position G5, it becomes greater than the operating position in the forward / backward direction, causing the output shaft of the right hydraulic pump 7R to rotate forward and the output shaft of the left hydraulic pump 7L to rotate backward, causing the work vehicle 1 to turn to the left. In this embodiment, turning refers to the operation of the work vehicle 1 when the operating position to the right is operated between position G4 and position G5, or when the operating position to the left is operated between position G4 and position G5.

[0047] On the other hand, when the operating lever 55 is operated to a forward position between the G4 and G5 positions, it becomes larger than the operating position in the left-right direction, causing the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R to rotate forward and the work vehicle 1 to move forward at high speed. When the operating lever 55 is operated to a backward position between the G4 and G5 positions, it becomes larger than the operating position in the left-right direction, causing the output shafts of the left hydraulic pump 7L and the right hydraulic pump 7R to reverse and the work vehicle 1 to move backward at high speed. The operation of the other operating levers 55 in the forward and backward directions is the same as in the left-right direction.

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

[0049] In normal mode, the target rotational speed of the engine 6 is set by operating the setting member 11, and the primary pilot pressure corresponding to the target rotational speed is determined based on the setting line L1 or L2 in Figure 4. Then, the secondary pilot pressure is set based on the amount of operation of one or more operating levers 55, and the hydraulic motors (31L, 31R) and hydraulic pumps (7L, 7R) are controlled. In other words, in normal mode, the speed of the work vehicle 1 can be changed according to the amount of operation of at least one operating device, making it possible to run the work vehicle 1 at a speed greater than the upper limit speed. On the other hand, in creep mode, the setting line L1 or L2 in Figure 4 is not used to determine the primary pilot pressure, and instead, the first reference information 10r1, etc., described later is used to determine a value that is lower than the primary pilot pressure in normal mode. The settings from the secondary pilot pressure onwards in creep mode are the same as in normal mode, but since the secondary pilot pressure is less than or equal to the primary pilot pressure, the primary pilot pressure is limited, and the speed of the work vehicle 1 is limited to less than or equal to the upper limit speed regardless of the amount of operation of at least one operating device (setting member 11, one or more operating levers 55).

[0050] Referring to Figures 3 and 6, the work vehicle 1 includes a hydraulic sensor SP11 for detecting the hydraulic pressure of the first pilot oil passage PA11, a hydraulic sensor SP12 for detecting the hydraulic pressure of the second pilot oil passage PA12, a hydraulic sensor SP13 for detecting the hydraulic pressure of the third pilot oil passage PA13, and a hydraulic sensor SP14 for detecting the hydraulic pressure of the fourth pilot oil passage PA14. As described above, the secondary pilot pressure output from the secondary ports of the control valves OVA, OVB, OVC, and OVD changes in accordance with the operating position of the control lever 55. Therefore, hydraulic sensors SP11 to SP14 are sensors for detecting the secondary pilot pressure. Hydraulic sensors SP11 to SP14 may also be called additional hydraulic sensors.

[0051] The work vehicle 1 includes a hydraulic sensor SP5L for detecting the hydraulic pressure of the drive oil passage PA5L, a hydraulic sensor SP6L for detecting the hydraulic pressure of the drive oil passage PA6L, a hydraulic sensor SP5R for detecting the hydraulic pressure of the drive oil passage PA5R, and a hydraulic sensor SP6R for detecting the hydraulic pressure of the drive oil passage PA6R. In other words, the hydraulic sensors (SP5L, SP6L, SP5R, SP6R) are configured to detect the hydraulic pressure of the hydraulic fluid in the drive oil passages (PA5L, PA6L, PA5R, PA6R). The state of the left hydraulic motor 31L and the right hydraulic motor 31R can be detected from the pressure difference between hydraulic sensor SP5L and hydraulic sensor SP6L, and from the pressure difference between hydraulic sensor SP5R and hydraulic sensor SP6R.

[0052] Referring to Figures 2, 3, and 6, the work vehicle 1 may further include a rotational speed sensor SR31L connected to the rotation axis of the left hydraulic motor 31L and configured to detect the rotational speed of the left hydraulic motor 31L, and a rotational speed sensor SR31R configured to detect the rotational speed of the right hydraulic motor 31R. The state of the left hydraulic motor 31L and the right hydraulic motor 31R can be detected from the rotational direction and magnitude of rotational speed detected from the rotational speed sensor SR31L and the rotational direction and magnitude of rotational speed detected from the rotational speed sensor SR31R. The work vehicle 1 may also include an operation detection sensor 18 configured to detect the operating position of the operating lever 55. The operation detection sensor 18 is connected to a controller 10, which will be described later. The operation detection sensor 18 is a position sensor or the like that detects the position of the operating lever 55. <Controller 10 Configuration> The controller 10 has a processor 10a and a memory 10b as shown in Figure 6 in order to realize the control of the vehicle speed in the creep mode described above. The processor 10a may also be called an electronic circuit. The memory 10b includes volatile memory and non-volatile memory. The memory 10b includes a driving control program 10c1, first reference information 10r1, and second reference information 10r2 for realizing the control described above.

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

[0054] The processor 10a executes the following control while running the driving control program 10c1, referring 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 obtains the rotational speed RS of the engine 6 from the speed sensor 6a, determines the primary pilot pressure corresponding to the detected rotational speed RS of the engine 6 from the first reference information 10r1, and controls the primary pressure control valve CV1 to achieve the determined primary pilot pressure. When the engine drop is large while the normal mode is selected, the processor 10a determines the primary pilot pressure corresponding to the rotational speed RS of the engine 6 detected by the speed sensor 6a from the second reference information 10r2, and controls the primary pressure control valve CV1 to achieve the determined primary pilot pressure.

[0055] 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 determines the target rotational speed RSi of the hydraulic motors (31L, 31R) by acquiring the upper limit speed input by the creep setting member 16. From the information obtained from the hydraulic sensors SP5L, SP6L, SP5R, and SP6R, the processor 10a determines the first differential pressure DP1, which is the differential pressure with the larger absolute value among the differential pressures of each of the hydraulic motors (31L, 31R). The processor 10a acquires the rotational speed RSd of the first hydraulic motor among the hydraulic motors (31L, 31R) whose differential pressure is the first differential pressure DP1 from the first rotational speed sensor among the rotational speed sensors (SR31L, SR31R) which is configured to detect the rotational speed of the first hydraulic motor. The processor 10a performs feedback control on the pilot pressure control valve (primary pressure control valve CV1) of the first hydraulic motor in order to reduce the speed difference e between the target rotational speed RSi of the first hydraulic motor corresponding to the target vehicle speed and the rotational speed RSd of the first hydraulic motor detected by the first rotational speed sensor. The processor 10a extracts information for determining the primary pilot pressure from the third reference information 10r3 and the fourth reference information 10r4, and determines the gain of the feedback control based on the extracted information, the absolute value |DP1| of the first differential pressure DP1, and the target rotational speed RSi.

[0056] In the following embodiments, of the left travel device 3L and the right travel device 3R, the travel device driven by the first hydraulic motor will be referred to as the first travel device. Of the first swash plate switching cylinder 32L and the second swash plate switching cylinder 32R, the cylinder provided on the first hydraulic motor will be called the first motor pilot port. The pilot pressure applied to the first motor pilot port will be called the first motor pilot pressure. Of the hydraulic pumps (7L, 7R), the hydraulic pump that supplies hydraulic fluid to the first hydraulic motor will be called the first hydraulic pump. Of the ports of the first hydraulic pump (any of PLa, PRa, PLb, PRb), the port on which the pilot pressure input by the primary pilot pressure is limited will be called the first pump pilot port. The pilot pressure applied to the first pump pilot port will be called the first pump pilot pressure. Of the drive oil passages (PA5L, PA6L, PA5R, PA6R), of the two oil passages connecting the first hydraulic motor and the first hydraulic pump, one will be called the first oil passage and the other will be called the second oil passage. The pilot pressure of the first oil passage is called the first oil pressure, and the pilot pressure of the second oil passage is called the second oil pressure. Of the oil pressure sensors (SP5L, SP6L, SP5R, SP6R), the oil pressure sensor configured to detect the first oil pressure is called the first oil pressure sensor, and the oil pressure sensor configured to detect the second oil pressure is called the second oil pressure sensor. Of the first to fourth pilot oil passages PA11 to PA14, the oil passages connecting the control valves OVA, OVB, OVC, OVD and the first pump pilot port are called secondary pilot oil passages.

[0057] Of the left running gear 3L and the right running gear 3R, the running gear located on the opposite side of the vehicle body 2 from the first hydraulic motor is called the second running gear. Of the hydraulic motors (31L, 31R), the hydraulic motor configured to drive the second running gear is called the second hydraulic motor. The differential pressure of the second hydraulic motor is called the second differential pressure. Of the first swash plate switching cylinder 32L and the second swash plate switching cylinder 32R, the cylinder located on the second hydraulic motor is called the second motor pilot port. The pilot pressure applied to the second motor pilot port is called the second motor pilot pressure. Of the hydraulic pumps (7L, 7R), the hydraulic pump that supplies hydraulic fluid to the second hydraulic motor is called the second hydraulic pump. Of the ports of the second hydraulic pump (any of PLa, PRa, PLb, PRb), the port on which the pilot pressure input by the primary pilot pressure is limited is called the second pump pilot port. The pilot pressure applied to the second pump pilot port is called the second pump pilot pressure. Of the drive oil passages (PA5L, PA6L, PA5R, PA6R), one of the two oil passages connecting the second hydraulic motor and the second hydraulic pump is called the third oil passage, and the other is called the fourth oil passage. The pilot pressure of the third oil passage is called the third hydraulic pressure, and the pilot pressure of the fourth oil passage is called the fourth hydraulic pressure. Of the hydraulic sensors (SP5L, SP6L, SP5R, SP6R), the hydraulic sensor configured to detect the third hydraulic pressure is called the third hydraulic pressure sensor, and the hydraulic sensor configured to detect the fourth hydraulic pressure is called the fourth hydraulic pressure sensor. Of the rotational speed sensors (SR31L, SR31R), the rotational speed sensor configured to detect the rotational speed of the second hydraulic motor is called the second rotational speed sensor. Of the first to fourth pilot oil passages PA11 to PA14, the oil passage connecting the operating valves OVA, OVB, OVC, OVD and the second pump pilot port is called the additional secondary pilot oil passage.

[0058] Figure 7 shows a control block diagram in the first embodiment. The control system 20A according to the first embodiment includes a feedforward controller Cf v1 And, the feedback controller Cb v1 This includes the feedforward controller Cf v1 This corresponds to the primary pilot pressure u0 of the target rotational speed RSi of the first hydraulic motor.V1 It outputs the primary pilot pressure u0 corresponding to the target rotational speed RSi. V1 It stores a list or conversion formula. The processor 10a calculates the target rotational speed RSi of the first hydraulic motor from the upper limit speed input by the creep setting member 16. Feedforward controller Cf v1 The processor 10a, which performs this operation, uses a stored list or conversion formula to calculate the primary pilot pressure u0 from the target rotational speed RSi. V1 Derive the following.

[0059] Feedback controller Cb v1 The gain K1 is applied to the speed difference e between the actual rotational speed RSd of the first hydraulic motor detected by the first rotational speed sensor and the target rotational speed RSi. v1 and gain K2 v1 Multiply by and to adjust the primary pilot pressure u1 V1 The following is derived. In the subsequent first embodiment, the gain K1 v1 Gain K2 v1 These are also referred to as the first coefficient and the second coefficient, respectively. In other words, the controller 10 controls the feedback gain of the feedback control using the first coefficient (gain K1 v1 ) and the second coefficient (gain K2 v1 ) is determined based on the above. Then, the processor 10a sets the primary pilot pressure u0 V1 and adjust primary pilot pressure u1 V1 The sum of and the manipulated variable u V1 This is input to the primary pressure control valve CV1. In other words, the processor 10a determines that the primary pilot pressure output from the primary pressure control valve CV1 is u V1 Current is input to the solenoid of the primary pressure control valve CV1 so that this occurs.

[0060] Gain K1 v1 This value increases as the target rotational speed RSi decreases, and the third reference information 10r3 is the first coefficient (gain K1 v1 The correspondence between the target rotational speed RSi of the first hydraulic motor and the first coefficient (gain K1) is stored. Figure 8 shows the target rotational speed RSi of the first hydraulic motor and the first coefficient (gain K1) in the embodiment. v1This figure shows the relationship between the target rotational speed RSi of the first hydraulic motor and the first coefficient (gain K1). As shown in Figure 8, v1 The correspondence with ) is represented by either setting line L3 or L4. Setting line L3 represents the correspondence in the normal case where at least one of the conditions 1 to 3 described below is not met. Setting line L4 represents the correspondence in the special case where all of the conditions 1 to 3 described below are met.

[0061] Referring to the setting line L3, the first coefficient (gain K1 v1 ) is the first value K1 when the target rotational speed RSi is less than or equal to the first target rotational speed RSi1. v1 Let _1. When the target rotational speed RSi is greater than or equal to the second target rotational speed RSi2, which is greater than the first target rotational speed RSi1, the first coefficient (gain K1 v1 ) is the first value K1 v1 Second value K1 that is less than _1 v1 Let it be _2. Preferably, the first value K1 v1 _1 is the second value K1 v1 It is 1.2 times _2. Preferably, the first target rotational speed RSi1 is 1 rpm and the second target rotational speed RSi2 is 50 rpm. When the target rotational speed RSi is greater than the first target rotational speed RSi1 and less than the second target rotational speed RSi2, the first coefficient (gain K1 v1 ) is the first value K1 v1 _1 and the second value K1 v1 It takes a value between _2. Preferably, when the target rotational speed RSi is greater than the first target rotational speed RSi1 and less than the second target rotational speed RSi2, the first coefficient (gain K1 v1 ) decreases monotonically as the target rotational speed RSi increases.

[0062] Target rotational speed RSi and first coefficient (gain K1 v1 The relationship between ) and the setting line L4 is expressed when all of the following conditions 1 to 3 are met. [Condition 1] The absolute value of the change per unit time of the actual rotational speed RSd of the first hydraulic motor is less than the first threshold, or less than the second threshold, which is less than -1 times the first threshold. For this reason, the controller 10 determines the change per unit time of the rotational speed RSd of the first hydraulic motor detected by the first rotational speed sensor. The first threshold is a positive value, and the second threshold is a negative value. If the absolute value of the change is less than the first threshold, it means that the work vehicle 1 is effectively stopped, and if the change is less than the second threshold, it means that the work vehicle 1 is effectively decelerating significantly. When the work vehicle 1 effectively stops or decelerates significantly despite the user's intention to operate it, it is highly likely to be due to special circumstances such as decelerating on an uphill slope or getting stuck on an obstacle (such as a rock). In such cases, the aim is to improve the user experience by increasing the gain to provide control closer to the user's intention. [Condition 2] The absolute value of the speed difference e described above is greater than the third threshold. When this condition is met, it means that the actual rotational speed RSd is not equal to the target rotational speed RSi, taking noise into consideration. The third threshold is determined by taking into account the noise of the rotational speed sensors (SR31L, SR31R). This condition is intended to prevent the gain from increasing when the speed difference e is negligibly small. [Condition 3] The target rotational speed RSi is less than or equal to the second target rotational speed RSi2 and less than or equal to the third target rotational speed RSth. Gain K1 is lower when the target rotational speed is low. v1When the user or manufacturer wants to increase the speed further, a third target rotational speed RSth is set. By default, the third target rotational speed RSth is set to be equal to the second target rotational speed RSi2. The third target rotational speed RSth can be either less than or equal to the first target rotational speed RSi1 (represented as RSth(1) in Figure 8) or greater than the first target rotational speed RSi1 and less than or equal to the second target rotational speed RSi2 (represented as RSth(2) in Figure 8). In either case, if the above conditions [Condition 1] and [Condition 2] are met, the relationship will shift from setting line L3 to setting line L4 when RSth becomes less than or equal to RSi2.

[0063] The first coefficient (gain K1) is represented by the setting line L4. v1 The value of ) is the first coefficient (gain K1) represented by the setting line L3. v1 The value is greater than ). In setting line L4, when the target rotational speed RSi is less than or equal to the first target rotational speed RSi1, the first coefficient (gain K1 v1 ) is the third value K1 v1 Let _3. When the target rotational speed RSi is greater than the first target rotational speed RSi1 and less than the second target rotational speed RSi2, the first coefficient (gain K1 v1 ) is the third value K1 v1 _3 and the second value K1 v1 It takes a value between _2. Preferably, when the target rotational speed RSi is greater than the first target rotational speed RSi1 and less than the second target rotational speed RSi2, the first coefficient (gain K1 v1 ) decreases monotonically as the target rotational speed RSi increases. In this way, the controller 10 determines the first to third conditions, and when all of the first to third conditions are true, the first coefficient (gain K1 v1 ) is the first coefficient (gain K1) when any of the first to third conditions are false. v1 Increase the value from ).

[0064] Gain K2 v1 This value increases as the absolute value of the first differential pressure DP1 |DP1| increases, and the fourth reference information 10r4 is the second coefficient (gain K2 v1Figure 9 stores the correspondence between the absolute value of the first differential pressure DP1 and the second coefficient (gain K2). v1 This figure shows the relationship between the absolute value of the first differential pressure DP1 |DP1| and the second coefficient (gain K2). As shown in Figure 9, the absolute value of the first differential pressure DP1 |DP1| and the second coefficient (gain K2 v1 The correspondence with ) is represented by either setting line L5 or L6. Setting line L5 represents the correspondence in the normal case where at least one of the aforementioned conditions 1 and 2 and the later condition 4 is not met. Setting line L6 represents the correspondence in the special case where all of the aforementioned conditions 1 and 2 and the later condition 4 are met.

[0065] Referring to setting line L5, the second coefficient (gain K1 v2 ) is the fourth value K1 when the absolute value of the first differential pressure DP1 |DP1| is less than or equal to the first threshold pressure Pth1. v2 Let _4. When the absolute value of the first differential pressure DP1| is greater than or equal to the second threshold pressure Pth2, which is greater than the first threshold pressure Pth1, the second coefficient (gain K1 v2 ) is the fourth value K1 v2 The fifth value K1 is greater than 4. v2 Let it be _5. Preferably, the 5th value K1 v2 _5 is the fourth value K1 v2 It is 1.4 times _4. Preferably, the first threshold pressure Pth1 is 10 MPa and the second threshold pressure Pth2 is 35 MPa. When the absolute value |DP1| is greater than the first threshold pressure Pth1 and less than the second threshold pressure Pth2, the second coefficient (gain K1 v2 ) is the fourth value K1 v2 _4 and the 5th value K1 v2 It takes a value between _5. Preferably, when the absolute value |DP1| is greater than the first threshold pressure Pth1 and less than the second threshold pressure Pth2, the second coefficient (gain K1 v2 ) increases monotonically as its absolute value |DP1| becomes larger.

[0066] Target rotational speed RSi and second coefficient (gain K2) v1 The relationship between ( ) and the setting line L6 is expressed when all of the aforementioned conditions 1 to 3 and the following condition 4 are met. [Condition 4] The absolute value of the first differential pressure DP1 |DP1| is greater than or equal to the third threshold pressure Pth, which is greater than or equal to the first threshold pressure Pth1. When the absolute value of the first differential pressure DP1 |DP1| is large, the gain K2 v1 When the user or manufacturer wants to increase the threshold pressure further, they can set a third threshold pressure Pth. By default, the third threshold pressure Pth is set to be equal to the first threshold pressure Pth1. The third threshold pressure Pth can be either greater than or equal to the second threshold pressure Pth2 (represented as Pth(1) in Figure 9) or greater than or equal to the first threshold pressure Pth1 and less than the second threshold pressure Pth2 (represented as Pth(2) in Figure 9). In either case, if the above conditions [Condition 1] and [Condition 2] are met, the relationship will shift from setting line L5 to setting line L6 when it becomes greater than or equal to Pth.

[0067] The second coefficient (gain K2) is represented by the setting line L6. v1 The value of ) is the second coefficient (gain K2) represented by the setting line L5. v1 The value is greater than ). At setting line L6, when the absolute value of the first differential pressure DP1 |DP1| is greater than or equal to the second threshold pressure Pth2, the second coefficient (gain K2 v1 ) is the sixth value K2 v1 Let's assume _6. When the absolute value of the first differential pressure DP1| is greater than the first threshold pressure Pth1 and less than the second threshold pressure Pth2, the second coefficient (gain K2 v1 ) is the fourth value K1 v2 _4 and the 6th value K2 v1 It takes a value between _6. Preferably, when the absolute value of the first differential pressure DP1| is greater than the first threshold pressure Pth1 and less than the second threshold pressure Pth2, the second coefficient (gain K2 v1 ) increases monotonically as the absolute value of the first differential pressure DP1|| becomes larger. In this way, the controller 10 determines the first to fourth conditions, and when all of the first to fourth conditions are true, the second coefficient (gain K2 v1 ) is the second coefficient (gain K2) when any of the first to fourth conditions are false. v1 Increase the value from ). <Operation of the work vehicle according to the first embodiment> Figure 10 is a flowchart showing the operation of the work vehicle 1 according to the first embodiment. In this flowchart, the processes from step S1 to step S14 are executed at predetermined sampling intervals (for example, 20 μs). In step S1, the processor 10a drives the engine 6 to rotate the first hydraulic pump. The processor 10a sends hydraulic fluid from the first hydraulic pump to the first hydraulic motor that drives the first running gear provided on the vehicle body 2, thereby rotating the first hydraulic motor. The processor 10a also sends hydraulic fluid from the second hydraulic pump to the second hydraulic motor that drives the second running gear provided on the opposite side of the vehicle body 2 from the first running gear, thereby rotating the second hydraulic motor. In step S2, the processor 10a determines whether or not the creep mode has been selected by the creep setting member 16. In other words, the control method according to this embodiment includes determining whether or not the creep mode has been selected by the creep setting member 16. When the creep mode is set, that is, when the upper limit speed is set (Yes in step S2), the process proceeds from step S3 to S8. If normal mode is set, meaning no upper speed limit is set, or if an invalid upper speed limit is set that does not have the first or second correspondence (No in step S2), proceed from step S9 to S11.

[0068] In creep mode (Yes in step S2), in step S3, the processor 10a acquires the upper limit speed input by the creep setting member 16, i.e., the target rotational speed RSi of the first hydraulic motor. In other words, the control method according to this embodiment acquires the upper limit speed input by the creep setting member 16, i.e., the target rotational speed RSi of the first hydraulic motor. In step S4, the processor 10a controls the feedforward controller Cf v1 The process is executed, and the primary pilot pressure u0 V1 The output is generated. In step S5, the first rotational speed sensor detects the rotational speed RSd of the first hydraulic motor, and the processor 10a acquires the detected rotational speed RSd.

[0069] In step S6, the processor 10a refers to the third reference information 10r3 to determine the first coefficient (gain K1 v1 ). FIG. 11 is a flowchart showing the operation of the method for determining the first coefficient (gain K1 v1 ). In step S61, the processor 10a obtains the amount of change per unit time of the rotational speed RSd. In step S62, the processor 10a determines a first condition that is true when the absolute value of the amount of change is smaller than the first threshold value or the amount of change is smaller than a second threshold value that is -1 times the first threshold value, and false otherwise. When the first condition is true (Yes in step S62), the process proceeds to step S63. When the first condition is false (No in step S62), the process proceeds to step S65. In step S63, the processor 10a determines a second condition that is true when the absolute value of the speed difference e is greater than the third threshold value, and false otherwise. When the second condition is true (Yes in step S63), the process proceeds to step S64. When the second condition is false (No in step S63), the process proceeds to step S65. In step S64, the processor 10a determines a third condition that is true when the target rotational speed RSi is less than or equal to a third target rotational speed RSth that is less than the second target rotational speed RSi2, and false otherwise. When the third condition is true (Yes in step S64), the process proceeds to step S66. When the third condition is false (No in step S64), the process proceeds to step S65. When any one of the first condition, the second condition, and the third condition is false (No in any of steps S62 to S64), in step S65, the processor 10a determines the first coefficient (gain K1 v1 ) corresponding to the target rotational speed RSi based on the setting line L3. When all of the first condition, the second condition, and the third condition are true (Yes in all of steps S62 to S64), in step S65, the processor 10a determines the first coefficient (gain K1 v1 ) corresponding to the target rotational speed RSi based on the setting line L4. Since the setting line L4 is above the setting line L3 when the target rotational speed RSi is less than or equal to the third target rotational speed RSth, in step S66, the processor 10a determines the first coefficient (gain K1 v1) is increased from the value determined in step S65.

[0070] In step S7, the processor 10a refers to the fourth reference information 10r4 and determines the second coefficient (gain K2 v1 ). FIG. 12 is a flowchart showing the operation of the method for determining the second coefficient (gain K2 v1 ). In step S71, the hydraulic pressure sensors SP5L, SP6L, SP5R, and SP6R detect the first differential pressure DP1 and the second differential pressure, and the processor 10a obtains the first differential pressure DP1 and the second differential pressure. In step S72, the processor 10a determines the above-described first condition. When the first condition is true (Yes in step S72), the process proceeds to step S73, and when the first condition is false (No in step S72), the process proceeds to step S76. In step S73, the processor 10a determines the above-described second condition. When the second condition is true (Yes in step S73), the process proceeds to step S74, and when the second condition is false (No in step S73), the process proceeds to step S76. In step S74, the processor 10a determines a fourth condition that is true when the absolute value |DP1| of the first differential pressure DP1 is greater than or equal to a third threshold pressure Pth that is greater than the first threshold pressure Pth1, and false otherwise. When the fourth condition is true (Yes in step S74), the process proceeds to step S77, and when the third condition is false (No in step S74), the process proceeds to step S76. When any one of the first condition, the second condition, and the fourth condition is false (when No in any of steps S72 to S74), in step S76, the processor 10a determines the second coefficient (gain K2 v1 ) corresponding to the absolute value |DP1| of the first differential pressure DP1 based on the setting line L5. The absolute value of the second differential pressure is smaller than the absolute value of the first differential pressure. According to FIG. 9, the larger the absolute value |DP1| of the first differential pressure DP1, the larger the second coefficient (gain K2 v1 ) is set. Therefore, when the absolute value |DP1| of the first differential pressure DP1 is greater than the absolute value of the second differential pressure, the processor 10a makes the second coefficient (gain K2 v1It can be said that this increases the gain K2. When all of the first, second, and fourth conditions are true (when all of steps S72 to S74 are Yes), in step S77, the processor 10a calculates the second coefficient (gain K2) corresponding to the absolute value |DP1| of the first differential pressure DP1 based on the set line L6. v1 ) is determined. When the absolute value of the first differential pressure DP1| is less than or equal to the third threshold pressure Pth, the set line L6 is above the set line L5, so in step S77, the processor 10a determines the second coefficient (gain K2 v1 ) is increased from the value determined in step S76.

[0071] In step S8, the processor 10a calculates the primary pilot pressure u0 V1 , First coefficient (gain K1 v1 ), and the second coefficient (gain K2 v1 Based on this, the manipulated variable u V1 =u0 V1 +K1 v1 *K2 v1 *determines e. In other words, the manipulated variable u in the feedback control. V1 This is used as the input to the primary pressure control valve CV1, which adjusts the primary pilot pressure, which is the hydraulic pressure of the primary pilot oil passage connecting a pilot pump 71 for discharging pilot oil toward the pilot ports (PLa, PRa, PLb, PRb) of at least one hydraulic pump (first hydraulic pump, second hydraulic pump) and an operating valve (OVA, OVB, OVC, OVD) operated by a travel instruction input device (operating lever 55) into which the user inputs an instruction for the direction of travel.

[0072] In step S12, the processor 10a controls the primary pressure control valve CV1 so that the calculated manipulated amount becomes the primary pilot pressure output from the primary pressure control valve CV1. In this way, the processor 10a performs feedback control on the pilot pressure control valve (primary pressure control valve CV1) of the first hydraulic pump to reduce the speed difference e between the target rotational speed RSi of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed RSd. This feedback gain is a first coefficient (gain K1) that increases as the target rotational speed RSi decreases. v1 It is determined based on the second coefficient (gain K2), which increases as the absolute value of the detected first differential pressure DP1|| DP1| increases, and increases as the value increases. v1 It is determined based on the following.

[0073] In normal mode (No in step S2), in step S9, the processor 10a obtains the rotational speed RS of the engine 6 detected by the speed sensor 6a. Then, the processor 10a determines whether or not there is engine drop. That is, in step S9, the processor 10a determines whether or not the drop amount ΔE1 of the engine 6 is greater than or equal to the anti-stall judgment value. If there is no engine drop (No in step S9), in step S10, the processor 10a obtains the primary pilot pressure from the first reference information 10r1 based on the rotational speed RS of the engine 6. If there is engine drop (Yes in step S9), in step S11, the processor 10a obtains the primary pilot pressure from the second reference information 10r2 based on the rotational speed RS of the engine 6. After the completion of the processing in step S10 or step S11, the processing in step S12 is executed.

[0074] In step S12, the processor 10a controls the primary pressure control valve CV1, which supplies pilot oil to the operating valves OVA, OVB, OVC, OVD, so that the primary pilot pressure is the primary pilot pressure determined in step S10 or step S11. In step S13, the operating valves OVA, OVB, OVC, OVD convert the primary pilot pressure to secondary pilot pressure based on the lever position (first operating amount) of the operating lever 55 (first operating device). In step S14, the secondary pilot pressure of the pilot oil is applied to the ports (PLa, PRa, PLb, PRb) that supply hydraulic pressure to the swash plates of the hydraulic pumps (first hydraulic pump, second hydraulic pump), thereby controlling the hydraulic pumps (first hydraulic pump, second hydraulic pump) and hydraulic motors (31L, 31R). <Operation and Effects of the First Embodiment> When feeding back the vehicle speed of the work vehicle 1, even with the same speed difference e, a smaller target vehicle speed results in a larger deviation from the target, while a larger target vehicle speed results in a smaller deviation from the target. In the control method for the work vehicle 1 or the work vehicle 1 according to the first embodiment, the feedback gain is increased as the travel speed decreases, so even with the effects of such deviations, the vehicle speed can be quickly returned to the target speed. Also, when feeding back the vehicle speed of the work vehicle 1, the higher the absolute value |DP1| of the first differential pressure DP1, the more difficult it is to control the swashplate of the hydraulic pumps (first hydraulic pump, second hydraulic pump). In the control method for the work vehicle 1 or the work vehicle 1 according to the first embodiment, the feedback gain is increased as the absolute value |DP1| of the first differential pressure DP1 increases, so regardless of the magnitude of the first differential pressure DP1, the primary pilot pressure can be controlled to quickly return the vehicle speed to the target speed. <Second Embodiment> In the first embodiment, an example was given in which the primary pilot pressure is controlled to achieve creep mode, but the secondary pilot pressure may also be controlled. Figure 13 is a hydraulic circuit diagram of the running system of the work vehicle in the second embodiment. Figure 13 shows the configuration added to Figure 3. In Figure 13, the same components as in Figure 3 are denoted by 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 configuration of the hydraulic circuit 1A in that it further includes relief valves CV23, CV24, proportional valves CV21, CV22, discharge oil passages DR3 to DR6, check valves CK1 to CK4, and throttles TH1 to TH4.

[0075] Relief valves CV23 and CV24 are balanced relief valves whose set pressure for opening is variable based on the pressure of the pilot oil, and have control ports 23a and 24a that receive pressure from the pilot oil. Relief valves CV23 and CV24 are configured to open when the pressure applied to the input port is greater than the pressure applied to the control ports 23a and 24a. At this time, the pilot oil is discharged into the hydraulic oil tank 70. Proportional valves CV21 and CV22 are connected to hydraulic oil passages 21 and 22 which are connected to the control ports 23a and 24a, and pilot oil is supplied from the pilot pump 71. Proportional valves CV21 and CV22 are electromagnetic proportional valves whose opening degree can be changed by exciting a solenoid, and are controlled by the controller 10.

[0076] The proportional valves CV21 and CV22 are connected to the pilot supply oil passage PA1. In creep mode, they are controlled to have a pressure equal to the primary pressure control valve CV1 in the first embodiment plus an offset α that takes into account the outflow of pilot oil from the relief valves CV23 and CV24. In normal mode, if anti-stall control is not in place, they operate to have a pressure equal to the set line L1 plus an offset α, and if anti-stall control is in place, they operate to have a pressure equal to the set line L2 plus an offset α. Of the proportional valves CV21 and CV22, the proportional valve that controls the hydraulic pressure of the pilot oil in the secondary pilot oil passage may be called the secondary pressure control valve CV2, and the proportional valve that controls the pilot oil in the additional secondary pilot oil passage may be called the additional secondary pressure control valve ACV2. That is, the secondary pressure control valve CV2 controls the secondary pilot pressure, which is the hydraulic pressure of the pilot oil in the secondary pilot oil passage. The additional secondary pressure control valve ACV2 controls the additional secondary pilot pressure, which is the hydraulic pressure of the pilot oil in the additional secondary pilot oil passage. The controller 10 performs feedback control on the pilot pressure control valves (secondary pressure control valve CV2 and additional secondary pressure control valve ACV2) of the first hydraulic pump to reduce the speed difference e between the target rotational speed RSi of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed RSd. Furthermore, the inputs for the feedback control are set to the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2, which adjust the secondary pilot pressure, which is the hydraulic pressure of the secondary pilot oil passage connecting the operating valves OVA, OVB, OVC, OVD, which are controlled by the driving instruction input device (operating lever 55) into which the user inputs the direction of travel, and the pilot ports (PLa, PRa, PLb, PRb) of at least one hydraulic pump (first hydraulic pump, second hydraulic pump).

[0077] Discharge oil passage DR3 is connected to the first pilot oil passage PA11. Discharge oil passage DR4 is connected to the second pilot oil passage PA12. Discharge oil passage DR5 is connected to the third pilot oil passage PA13. Discharge oil passage DR6 is connected to the fourth pilot oil passage PA14. Check valves CK1 to CK4 shut off discharge oil passages DR3 to DR6 if the pressure on the side with throttles TH1 to TH4 does not exceed a predetermined value above the pressure on the side with relief valves CV23 and CV24.

[0078] In discharge oil passages DR3 and DR4, the pilot pressure increases when the left hydraulic pump 7L rotates forward and reverses, respectively. Therefore, when the pilot pressure on one side of either passage equals the primary pilot pressure, the other side becomes significantly smaller than the primary pilot pressure. In discharge oil passages DR5 and DR6, the pilot pressure increases when the right hydraulic pump 7R rotates forward and reverses, respectively. Therefore, when the pilot pressure on one side of either passage equals the primary pilot pressure, the other side becomes significantly smaller than the primary pilot pressure. For this reason, normally only one of check valves CK1 and CK2 will open. For this reason, the above control can be performed by controlling the pressure of proportional valves CV21 and CV22 so that the pressure is the sum of the pressure controlled by the primary pressure control valve CV1 according to the first embodiment and the pressure loss due to the outflow of pilot oil from relief valves CV23 and CV24.

[0079] Throttle TH1 is provided in the first pilot oil passage PA11 between the first shuttle valve SVa and the discharge oil passage DR3, and is configured to reduce the flow rate of pilot oil in the first pilot oil passage PA11. Throttle TH2 is provided in the second pilot oil passage PA12 between the second shuttle valve SVb and the discharge oil passage DR4, and is configured to reduce the flow rate of pilot oil in the second pilot oil passage PA12. Throttle TH3 is provided in the third pilot oil passage PA13 between the third shuttle valve SVc and the discharge oil passage DR5, and is configured to reduce the flow rate of pilot oil in the third pilot oil passage PA13. Throttle TH4 is provided in the fourth pilot oil passage PA14 between the fourth shuttle valve SVd and the discharge oil passage DR6, and is configured to reduce the flow rate of pilot oil in the fourth pilot oil passage PA14.

[0080] Figure 14 shows a control block diagram in the second embodiment. The main difference between the control system 20B in the second embodiment and the control system 20A in the first embodiment is that the controlled object is the secondary pressure control valve CV2, but since it is largely the same as the control system 20A, the differences will be explained mainly. The control system 20B includes a feedforward controller Cf v2 And, the feedback controller Cb v2 This includes the feedforward controller Cf v2 This corresponds to the secondary pilot pressure u0 of the target rotational speed RSi of the first hydraulic motor. V2 It outputs the secondary pilot pressure u0 corresponding to the target rotational speed RSi. V2 It stores a list or conversion formula. Feedforward controller Cf v2 The processor 10a, which performs this operation, uses a stored list or conversion formula to calculate the secondary pilot pressure u0 from the target rotational speed RSi. V2 Derive the following.

[0081] Feedback controller Cb v2 The gain K1 is applied to the speed difference e between the actual rotational speed RSd of the first hydraulic motor detected by the first rotational speed sensor and the target rotational speed RSi. v2 and gain K2 v2Multiply by and to adjust secondary pilot pressure u1 V2 The following is derived. In the subsequent first embodiment, the gain K1 v2 Gain K2 v2 These are also referred to as the first coefficient and the second coefficient, respectively. In other words, the controller 10 controls the feedback gain of the feedback control using the first coefficient (gain K1 v2 ) and the second coefficient (gain K2 v2 ) is determined based on the above. Then, the processor 10a sets the secondary pilot pressure u0 V2 and adjustment secondary pilot pressure u1 V2 The sum of and the manipulated variable u V2 This is input to the secondary pressure control valve CV2. In other words, the processor 10a determines that the secondary pilot pressure output from the secondary pressure control valve CV2 is u V2 Current is input to the solenoid of the secondary pressure control valve CV2 so that this occurs.

[0082] Gain K1 v2 This value increases as the target rotational speed RSi decreases, and the third reference information 10r3 is the first coefficient (gain K1 v2 The correspondence between this value and the target rotational speed RSi of the first hydraulic motor is stored. This correspondence is shown in Figure 8, first value K1 v1 _1 gain K1 v2 The first value K1 corresponding to v2 Replacing it with _1, the second value K1 in Figure 8 v1 _2 gain K1 v2 The corresponding second value K1 v2 Reinterpreting as _2, the third value K1 in Figure 8 v1 _3 gain K1 v2 The third value K1 corresponding to v2 It can be expressed by substituting _3. Note that the first value K1 v2 _1 and the second value K1 v2 _2 and the third value K1 v2 If the relative size relationship with _3 does not change, K1 v1 _1 and K1 v2 _1 may be the same value or a different value, K1 v1 _2 and K1 v2 _2 may be the same value or a different value, K1 v1 _3 and K1v2 _3 may be the same value or different values. The characteristics of setting line L3 and setting line L4, and the conditions for transitioning between setting line L3 and setting line L4, are the same as in the first embodiment.

[0083] Gain K2 v2 This value increases as the absolute value of the first differential pressure DP1 |DP1| increases, and the fourth reference information 10r4 is the second coefficient (gain K2 v2 The correspondence between the absolute value of the first differential pressure DP1 and |DP1| is stored. This correspondence is the fourth value K2 in Figure 9. v1 _4 Gain K2 v2 The corresponding fourth value K2 v2 Reinterpreting it as _4, the fifth value K2 in Figure 9 v1 _5 Gain K2 v2 The fifth value K2 corresponds to this. v2 Reinterpreting it as _5, the sixth value K3 in Figure 9 v1 _6 gain K2 v2 The corresponding sixth value K2 v2 It can be expressed by substituting _6. Note that the fourth value K2 v2 _4 and the 5th value K2 v2 _5 and the sixth value K2 v2 If the relative size relationship with _6 remains unchanged, then K2 v1 _4 and K2 v2 _4 may be the same value or a different value, K2 v1 _5 and K2 v2 _5 may be the same value or a different value, K2 v1 _6 and K3 v2 _6 may have the same value or different values. The characteristics of setting line L5 and setting line L6, and the conditions for transitioning between setting line L5 and setting line L6, are the same as in the first embodiment.

[0084] Figure 15 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 S14 are executed at predetermined sampling intervals (e.g., 20 μs). In Figure 15, the same processes as in Figure 10 are given the same step numbers and are therefore omitted from explanation. In normal mode (Yes at step S2), in step S21, the processor 10a controls the proportional valves CV21 and CV22 so that the pressure applied to the relief valves CV23 and CV24 becomes higher than the pressure output by the primary pressure control valve CV1 as described above. As a result, the relief valves CV23 and CV24 are closed.

[0085] In creep mode (Yes in step S2), after step S3, in step S4A, the processor 10a controls the feedforward controller Cf v2 The process is executed, and the secondary pilot pressure u0 V2 The output is given by step S5, and then in step S6A, the processor 10a outputs the first coefficient (gain K1 v2 ) and the third reference information 10r3 which stores the correspondence between the first hydraulic motor and the target rotational speed RSi, the first coefficient (gain K1 v2 The determination method is the same as in the first embodiment. Then, in step S7A, the processor 10a determines the second coefficient (gain K2 v2 Referencing the fourth reference information 10r4 which stores the correspondence between ) and the absolute value |DP1| of the first differential pressure DP1, the second coefficient (gain K2 v2 The determination method is the same as in the first embodiment.

[0086] In step S8A, the processor 10a calculates the secondary pilot pressure u0 V2 , First coefficient (gain K1 v2 ), and the second coefficient (gain K2 v2 Based on this, the manipulated variable u V2 =u0 V2 +K1 v2 *K2 v2 *determines e. In other words, the manipulated variable u in the feedback control. V2This is used as input to the secondary pressure control valve CV2 and additional secondary pressure control valve ACV2, which adjust the secondary pilot pressure, which is the hydraulic pressure of the secondary pilot oil passage connecting the control valves (OVA, OVB, OVC, OVD), which are operated by a travel instruction input device (operation lever 55) that receives a direction of travel instruction from the user, to the pilot ports (PLa, PRa, PLb, PRb) of at least one hydraulic pump (first hydraulic pump, second hydraulic pump).

[0087] Subsequently, in step S22, the processor 10a controls the pressure applied to the relief valves CV23 and CV24 to control the amount u V2 The secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 (proportional valves CV21, CV22) are controlled so that the secondary pilot pressure is equal to the target speed RSi of the first hydraulic motor corresponding to the target vehicle speed, and the detected rotational speed RSd. In other words, the processor 10a performs feedback control on the pilot pressure control valves (secondary pressure control valve CV2, additional secondary pressure control valve ACV2) of the first hydraulic pump to reduce the speed difference e between the target rotational speed RSi of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed RSd. After step S22, the process proceeds to step S9. <Operation and Effects of the Second Embodiment> In the control method or the work vehicle 1 according to the second embodiment, the secondary pilot pressure can be controlled to quickly return the vehicle to the target speed, similar to the first embodiment. <Modified form of the second embodiment> Figure 16 is a hydraulic circuit diagram relating to a modified example in the second embodiment. In the example of Figure 16, shuttle valves SV12 and SV34 are provided instead of the check valves CK1 to CK4 in the example of Figure 13. Shuttle valve SV12 connects the oil passage with the higher oil pressure among the discharge oil passages DR3 and DR4 to the relief valve CV23. Shuttle valve SV34 connects the oil passage with the higher oil pressure among the discharge oil passages DR5 and DR6 to the relief valve CV24. The above control can be performed even with this hydraulic circuit configuration. Also, the primary pressure control valve CV1 may be omitted in the circuit of Figure 9 or Figure 11. Furthermore, at least one of the combinations of a secondary pressure control valve CV2 and a balanced relief valve, and an additional secondary pressure control valve ACV2 and a balanced relief valve, may be implemented using electromagnetic proportional relief valves.

[0088] In the second embodiment, the processor 10a controls the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump according to the absolute value of the first differential pressure when the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure. However, when the difference between the absolute value of the first differential pressure and the absolute value of the second differential pressure is within a predetermined range, the processor 10a may control the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump according to the absolute value of the second differential pressure. In that case, the absolute value of the first differential pressure DP1|| in Figure 9 is read as the absolute value of the second differential pressure, and the processor 10a calculates the product of the second coefficient on the vertical axis and the second differential pressure, and the secondary pilot pressure u0 V2 The system should be controlled so that the sum of the two values ​​is output from the additional secondary pressure control valve ACV. In this way, when the absolute value of the first differential pressure is not significantly different from the absolute value of the second differential pressure, the left and right travel devices can be controlled separately to achieve maneuvers such as turning that are close to the user's desired state. <Third Embodiment> In the first and second embodiments, examples were shown in which the controller 10 controls the first pump pilot pressure and the second pump pilot pressure. However, it is also possible to control not only these pilot pressures but also the engine rotational speed. Figure 17 shows a control block diagram in the third embodiment. The main difference between the control system 20C in the third embodiment and the control system 20A in the first embodiment is that the controlled object is the target rotational speed of the engine 6. However, since it is largely the same as the control system 20A, the differences will be explained mainly. The control system 20C includes a feedforward controller Cf E And, the feedback controller Cb E This includes the feedforward controller Cf E This corresponds to the target engine rotational speed u0 of the operating amount rE of the setting member 11. E It outputs the target engine rotational speed u0 corresponding to the manipulated variable rE. E It stores a list or conversion formula. Feedforward controller Cf E The processor 10a, which performs this operation, uses a stored list or conversion formula to calculate the target engine rotational speed u0 from the manipulated variable rE. E Derive the following.

[0089] Feedback controller Cb E The gain K1 is applied to the speed difference e between the actual rotational speed RSd of the first hydraulic motor detected by the first rotational speed sensor and the target rotational speed RSi. E and gain K2 E Multiply by and to adjust target engine rotation speed u1 E The following is derived. In the third embodiment, the gain K1 E Gain K2 E These are also referred to as the first coefficient and the second coefficient, respectively. In other words, the controller 10 controls the feedback gain of the feedback control using the first coefficient (gain K1 E ) and the second coefficient (gain K2 E ) is determined based on the following. Then, the processor 10a determines the target engine rotation speed u0 E and adjustment target engine rotation speed u1 E The sum of and the manipulated variable u EThe manipulated quantity u E The rotational command is output to the injector, supply pump, or common rail.

[0090] Gain K1 E This value increases as the target rotational speed RSi decreases, and the third reference information 10r3 is the first coefficient (gain K1 E The correspondence between this value and the target rotational speed RSi of the first hydraulic motor is stored. This correspondence is shown in Figure 8, first value K1 v1 _1 gain K1 E The first value K1 corresponding to E Replacing it with _1, the second value K1 in Figure 8 v1 _2 gain K1 E The corresponding second value K1 E Reinterpreting as _2, the third value K1 in Figure 8 v1 _3 gain K1 E The third value K1 corresponding to E It can be expressed by substituting _3. Note that the first value K1 E _1 and the second value K1 E _2 and the third value K1 E The relationship between _3 and other values ​​is the same as in the first embodiment. The characteristics of setting line L3 and setting line L4, and the conditions for transitioning between setting line L3 and setting line L4 are the same as in the first embodiment.

[0091] Gain K2 E This value increases as the absolute value of the first differential pressure DP1 |DP1| increases, and the fourth reference information 10r4 is the second coefficient (gain K2 E The correspondence between the absolute value of the first differential pressure DP1 and |DP1| is stored. This correspondence is the fourth value K2 in Figure 9. v1 _4 Gain K2 E The corresponding fourth value K2 E Reinterpreting it as _4, the fifth value K2 in Figure 9 v1 _5 Gain K2 E The fifth value K2 corresponds to this. E Reinterpreting it as _5, the sixth value K3 in Figure 9 v1 _6 gain K2 E The corresponding sixth value K2 EIt can be expressed by substituting _6. Note that the fourth value K2 E _4 and the 5th value K2 E _5 and the sixth value K2 E The relationship between _6 and other values ​​is the same as in the first embodiment. The characteristics of setting line L5 and setting line L6, and the conditions for transitioning between setting line L5 and setting line L6 are the same as in the first embodiment.

[0092] Figures 18A and 18B are flowcharts illustrating the operation of the work vehicle 1 according to the third embodiment. Figure 18A is a flowchart illustrating the operation of the work vehicle 1 according to the third embodiment, which has the hydraulic circuit 1A of the first embodiment. Figure 18B is a flowchart illustrating the operation of the work vehicle 1 according to the third embodiment, which has the hydraulic circuit 1B of the second embodiment. In Figure 18A, the same reference numerals are used for the same operations as in the first embodiment, and detailed explanations are omitted. In Figure 18B, the same reference numerals are used for the same operations as in the second embodiment, and detailed explanations are omitted.

[0093] Referring to Figure 18A, in step S4B following step S4 of the first embodiment, the processor 10a controls the feedforward controller Cf E The process is executed, and the target engine rotation speed u0 corresponds to the manipulated amount rE of the setting member 11. E The output is given by step S5, and then in step S6B, the processor 10a outputs the first coefficient (gain K1 E ) and the third reference information 10r3 which stores the correspondence between the first hydraulic motor and the target rotational speed RSi, the first coefficient (gain K1 E The method for determining the second coefficient (gain K2) is the same as in the first embodiment. Then, in step S7B, the processor 10a determines the second coefficient (gain K2). E Referencing the fourth reference information 10r4 which stores the correspondence between ) and the absolute value |DP1| of the first differential pressure DP1, the second coefficient (gain K2 E The determination method is the same as in the first embodiment.

[0094] In step S8B, the processor 10a determines the target engine rotation speed u0 E, First coefficient (gain K1 E ), and the second coefficient (gain K2 E Based on this, the manipulated variable u E =u0 E +K1 E *K2 E *e is determined. Processor 10a controls the feedback control variable u E This is the target rotational speed of the engine 6 that rotates the hydraulic pumps (first hydraulic pump, second hydraulic pump).

[0095] In step S31, the processor 10a determines the operation amount u E A rotation command with a target rotational speed of is output to the injector, supply pump, or common rail. As a result, the processor 10a performs feedback control on the engine 6 to reduce the speed difference e between the target rotational speed RSi of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed RSd. This feedback gain is a first coefficient (gain K1) that increases as the target rotational speed RSi decreases. E It is determined based on the second coefficient (gain K2), which increases as the absolute value of the detected first differential pressure DP1|| DP1| increases, and increases as the value increases. E It is determined based on the primary pilot pressure u0 obtained in step S4. In step S12, the processor 10a determines the primary pilot pressure u0 V1 The primary pressure control valve CV1 is controlled so that the output pressure from the primary pressure control valve CV1 becomes the primary pilot pressure.

[0096] In Figure 18B, the same reference numerals are used for the same processes as in Figures 15 and 18A. Referring to Figure 18B, after the processing of step S5, steps S6B to S8B are executed. In step S22B, which replaces step S22 in the second embodiment, the processor 10a determines the secondary pilot pressure by referring to the third reference information 10r3 as shown in Figure 12. The secondary pilot pressure u0 obtained in step S4A V2The secondary pressure control valve CV2 is controlled so that the output from the secondary pressure control valve CV2 becomes the secondary pilot pressure. After step S22B, step S31 described above is executed. After step S31, the process proceeds to step S9. <Operation and Effects of the Third Embodiment> In the control method or work vehicle 1 according to the third embodiment, the rotational speed of the engine 6 can be controlled to quickly return to the target vehicle speed, similar to the first embodiment. <Fourth Embodiment> In the above-described embodiment, the operating lever 55 directly controlled the operating valves OVA, OVB, OVC, and OVD. However, the work vehicle 1 may also control control valves that control the first pump pilot pressure and the second pump pilot pressure based on the amount of operation of the operating lever 55 detected by a separate sensor such as a potentiometer. In this case, the same control as in the second embodiment can be achieved by adjusting the amount of operation detected by the sensor. Figure 19 is a hydraulic circuit diagram of the travel system of the work vehicle 1 in the fourth embodiment. In Figure 19, the same components as in Figure 3 are denoted by the same reference numerals, and detailed explanations are omitted. In the fourth embodiment, the work vehicle 1 includes a hydraulic circuit 1C. The hydraulic circuit 1C includes pilot control valves CV31 to CV34 that control the pilot pressure applied to each of the ports (PLa, PRa, PLb, PRb) instead of the operating valves OVA, OVB, OVC, and OVD and the first to fourth shuttle valves SVA, SVb, SVc, and SVd. The pilot control valves CV31 to CV34 are electromagnetic proportional valves including solenoids.

[0097] In this embodiment, the pilot supply oil passage PA8 connects the pilot control valves CV31 to CV34 to the pilot supply oil passage PA8, and the first to fourth pilot oil passages PA11 to PA14 are each connected to the pilot control valves CV31 to CV34. In this embodiment, the pilot supply oil passages PA1 and PA8 and the first to fourth pilot oil passages PA11 to PA14 correspond to the pilot oil supply circuit that connects the pilot pump to the first pump pilot port or the second pump pilot port. In this embodiment, since there are no operating valves OVA, OVB, OVC, and OVD, there is no difference between primary pilot pressure and secondary pilot pressure. Therefore, in this embodiment, these are simply referred to as pilot pressure without distinction.

[0098] In normal mode, the controller 10 controls the pilot control valves CV31 to CV34 so that it can output the pilot pressure corresponding to Figure 5, corresponding to the operating position detected by the operation detection sensor 18. In creep mode, in order to limit the vehicle speed, even if the operating lever 55 is actually operated to its full stroke, it is considered to have been operated to the deemed operating position Ga. Specifically, when the operating position is between position G0 and position Ga, the pilot pressure is determined from the correspondence relationship in Figure 5, corresponding to the operating position detected by the operation detection sensor 18. When the operating position is greater than or equal to position Ga, it is considered to have been operated to the deemed operating position Ga. In this embodiment, the amount of operation from position G0 to position Ga is called the deemed operating amount r.

[0099] Figure 20 shows a control block diagram in the fourth embodiment. The control system 20D according to the fourth embodiment differs from the control system 20A according to the first embodiment mainly in that the controlled object is a deemed manipulated variable r, but since it is largely the same as the control system 20A, the differences will be explained mainly. The control system 20D is a feedforward controller Cf R And, the feedback controller Cb R and the converter Cb v This includes the feedforward controller Cf RThis outputs a deemed manipulated variable r0 corresponding to the target rotational speed RSi of the first hydraulic motor. Memory 10b stores a list or conversion formula for the deemed manipulated variable r0 corresponding to the target rotational speed RSi. Feedforward controller Cf E The processor 10a, which performs this operation, derives the assumed manipulated variable r0 from the target rotational speed RSi using a stored list or conversion formula.

[0100] Feedback controller Cb R The gain K1 is applied to the speed difference e between the actual rotational speed RSd of the first hydraulic motor detected by the first rotational speed sensor and the target rotational speed RSi. R and gain K2 R Multiplying by the two, the adjusted assumed control amount r1 is derived. In the following fourth embodiment, gain K1 R Gain K2 R These are also referred to as the first coefficient and the second coefficient, respectively. In other words, the controller 10 controls the feedback gain of the feedback control using the first coefficient (gain K1 R ) and the second coefficient (gain K2 R ) is determined based on the above. Then, the processor 10a takes the sum of the deemed manipulated variable r0 and the adjusted deemed manipulated variable r1 as the manipulated variable r. Converter Cb v This involves the manipulated variable r and the pilot pressure u, as shown in Figure 5. V It maintains a relationship with the pilot pressure u corresponding to the input manipulated variable r. V It outputs the pilot pressure u. V Control pilot control valves CV31 to CV34 to output the specified value.

[0101] Gain K1 R This value increases as the target rotational speed RSi decreases, and the third reference information 10r3 is the first coefficient (gain K1 R The correspondence between this value and the target rotational speed RSi of the first hydraulic motor is stored. This correspondence is shown in Figure 8, first value K1 v1 _1 gain K1 R The first value K1 corresponding to R Replacing it with _1, the second value K1 in Figure 8 v1 _2 gain K1R The corresponding second value K1 R Reinterpreting as _2, the third value K1 in Figure 8 v1 _3 gain K1 R The third value K1 corresponding to R It can be expressed by substituting _3. Note that the first value K1 R _1 and the second value K1 R _2 and the third value K1 R The relationship between _3 and other values ​​is the same as in the first embodiment. The characteristics of setting line L3 and setting line L4, and the conditions for transitioning between setting line L3 and setting line L4 are the same as in the first embodiment.

[0102] Gain K2 R This value increases as the absolute value of the first differential pressure DP1 |DP1| increases, and the fourth reference information 10r4 is the second coefficient (gain K2 R The correspondence between the absolute value of the first differential pressure DP1 and |DP1| is stored. This correspondence is the fourth value K2 in Figure 9. v1 _4 Gain K2 R The corresponding fourth value K2 R Reinterpreting it as _4, the fifth value K2 in Figure 9 v1 _5 Gain K2 R The fifth value K2 corresponds to this. R Reinterpreting it as _5, the sixth value K3 in Figure 9 v1 _6 gain K2 R The corresponding sixth value K2 R It can be expressed by substituting _6. Note that the fourth value K2 R _4 and the 5th value K2 R _5 and the sixth value K2 R The relationship between _6 and other values ​​is the same as in the first embodiment. The characteristics of setting line L5 and setting line L6, and the conditions for transitioning between setting line L5 and setting line L6 are the same as in the first embodiment.

[0103] Figure 21 is a flowchart showing the operation of the work vehicle 1 according to the fourth embodiment. In Figure 21, the same reference numerals are used for the same operations as in the first embodiment, and detailed explanations are omitted. In this flowchart, the processes from step S1 to step S14 are executed at predetermined sampling intervals (e.g., 20 μs). After the completion of step S1, in step S41, the processor 10a acquires the first manipulated variable from the operation detection sensor 18. When there is no engine drop (No in step S9), in step S10C, the processor 10a determines the maximum output pressure (Pc) in Figure 5 from the first reference information 10r1 based on the rotational speed RS of the engine 6. When there is engine drop (Yes in step S9), in step S11C, the processor 10a determines the maximum output pressure (Pc) in Figure 5 from the second reference information 10r2 based on the rotational speed RS of the engine 6. After the completion of step S10D or step S11C, in step S43, the processor 10a determines the pilot pressure according to the first manipulated variable while the maximum output pressure (Pc) is limited, and controls the pilot control valves CV31 to CV34 so that the determined pilot pressure is applied.

[0104] In step S4C, which replaces step S4, the processor 10a controls the feedforward controller Cf R The process is executed and the assumed manipulated variable r0 is output. From the processing of step S5 onward, in step S6C, the processor 10a calculates the first coefficient (gain K1 R ) and the third reference information 10r3 which stores the correspondence between the first hydraulic motor and the target rotational speed RSi, the first coefficient (gain K1 R The determination method is the same as in the first embodiment. Then, in step S7C, the processor 10a determines the second coefficient (gain K2 R Referencing the fourth reference information 10r4 which stores the correspondence between ) and the absolute value |DP1| of the first differential pressure DP1, the second coefficient (gain K2 R The determination method is the same as in the first embodiment.

[0105] In step S8C, the processor 10a calculates the assumed manipulated variable r0 and the first coefficient (gain K1 R ), and the second coefficient (gain K2 R Based on this, the manipulated variable r = r0 + K1 R *K2 R *e is determined. In other words, the manipulated amount r for the feedback control is set to the manipulated amount of the travel instruction input device (operating lever 55) into which the user inputs an instruction for the direction of travel. Then, in step S42, the processor 10a determines whether the first manipulated amount is greater than or equal to the manipulated amount r. In creep mode, the first manipulated amount is usually operated so that it is greater than or equal to the manipulated amount r. If the first manipulated amount is less than the manipulated amount r (No in step S42), the process proceeds to step S43. If the first manipulated amount is greater than or equal to the manipulated amount r (Yes in step S42), in step S44, the processor 10a determines the pilot pressure according to the deemed manipulated amount and controls the pilot control valves CV31 to CV34 so that the determined pilot pressure is applied. In other words, the processor 10a adjusts the hydraulic pressure of the pilot circuit between the pilot pump and the pilot port for discharging pilot oil toward the pilot port of at least one hydraulic pump (first hydraulic pump, second hydraulic pump) according to the manipulated amount r of the travel instruction input device (operating lever 55). In other words, the processor 10a performs feedback control on the pilot pressure control valves (CV31~CV34) of the first hydraulic pump to reduce the speed difference e between the target rotational speed RSi of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed RSd. <Operation and Effects of the Fourth Embodiment> In the control method or work vehicle 1 according to the fourth embodiment, it is possible to control the deemed operation amount to quickly return to the target vehicle speed, similar to the first embodiment. <Modified form of the fourth embodiment> In the fourth embodiment, when the absolute value of the first differential pressure DP1||DP1| is greater than the absolute value of the second differential pressure, the processor 10a determines a second coefficient (gain K2) for controlling the second pump pilot pressure applied to the second pump pilot port of the second hydraulic pump according to the absolute value of the first differential pressure DP1|DP1|. RThis shows an example of adjusting (gain K2). However, when the difference between the absolute value of the first differential pressure and the absolute value of the second differential pressure is within a predetermined range, the processor 10a adjusts (gain K2) according to the absolute value of the second differential pressure. R ) may be adjusted. In that case, the absolute value of the first differential pressure in Figure 9 should be read as the absolute value of the second differential pressure, and the processor 10a should be configured to control the additional secondary pressure control valve ACV based on the deemed operation amount on the vertical axis. In this way, when the absolute value of the first differential pressure is not significantly different from the absolute value of the second differential pressure, the left and right travel devices can be controlled separately to achieve operations such as turning that are close to the user's desire. In addition, in the second embodiment, the travel instruction input device (operation lever) 55 is equipped with an operation detection sensor 18, and the deemed operation amount described above is calculated based on the detection result of the operation detection sensor 18, and the secondary pressure control valve CV2 and the additional secondary pressure control valve ACV2 may be controlled accordingly. In this case, the relief valves CV23 and CV24 are controlled using the deemed operation amount, so the work vehicle 1 can be controlled to reach the desired vehicle speed more quickly than when controlled by the pilot control valves CV31 to CV34, which are electromagnetic proportional valves, as shown in Figure 19. <Variations relating to all embodiments> The values ​​of the various thresholds may be changed depending on the characteristics of the left hydraulic pump 7L, the right hydraulic pump 7R, the left hydraulic motor 31L, and the right hydraulic motor 31R, the characteristics of the reduction gear connected to the left hydraulic motor 31L and the reduction gear connected to the right hydraulic motor 31R, and the characteristics of the various control valves.

[0106] The above example shows an embodiment with only P control, but I control and D control may also be included. In the above embodiment, the feedback gain of the feedback controller is determined based on a first coefficient and a second coefficient, but either the first coefficient or the second coefficient may be omitted. If the second coefficient is omitted, the feedback gain is set to the first coefficient, and if the first coefficient is omitted, the feedback gain is set to the second coefficient. In that case, the manipulated variable can be determined by setting the omitted coefficient to 1 in step S6 or S7 in Figure 10, S6A or S7A in Figure 15, S6B or S7B in Figure 18A, S6B or S7B in Figure 18B, and S6C or S7C in Figure 21.

[0107] In the above example, both the first and second coefficients are multiplied and fed back, but they may also be added together and fed back. Figure 22 shows a control block diagram in a modified example of the first embodiment. Figure 23 shows a control block diagram in a modified example of the second embodiment. Figure 24 shows a control block diagram in a modified example of the third embodiment. Figure 25 shows a control block diagram in a modified example of the fourth embodiment. The control system 20AM according to the modified example of the first embodiment is a feedback controller Cb of the control system 20A. v1 Instead, the feedback controller Cb v11 and feedback controller Cb v12 This includes.

[0108] Feedback controller Cb v11 The speed difference e is given by gain K1 v1 Multiply by to adjust primary pilot pressure u1 V11 Derive the following: Feedback controller Cb v12 This is the speed difference e with gain K2 v1 Multiply by to adjust primary pilot pressure u1 V12 The following is derived. The processor 10a has a primary pilot pressure u0 V1 and adjust primary pilot pressure u1 V11 and adjust primary pilot pressure u1 V12 The sum of and the manipulated variable u V1 This is input to the primary pressure control valve CV1. In other words, the manipulated amount u V1=u0 V1 +K1 v1 *e+K2 v1 *e. In other respects, it is the same as the first embodiment. Note that the gain K1 v1 , K2 v1 The size of is the gain K1 in the first embodiment. v1 , K2 v1 The size may differ, but the properties shown in Figures 8 and 9 are the same.

[0109] The control system 20BM according to a modified example of the second embodiment is a feedback controller Cb of the control system 20B. v2 Instead, the feedback controller Cb v21 and feedback controller Cb v22 Includes the feedback controller Cb. v21 The speed difference e is given by gain K1 v2 Multiply by to adjust secondary pilot pressure u1 V21 Derive the following: Feedback controller Cb v22 This is the speed difference e with gain K2 v2 Multiply by to adjust secondary pilot pressure u1 V22 The following is derived. The processor 10a has a secondary pilot pressure u0 V2 and adjustment secondary pilot pressure u1 V21 and adjustment secondary pilot pressure u1 V22 The sum of and the manipulated variable u V2 This is input to the secondary pressure control valve CV2. In other words, the manipulated variable u V2 =u0 V2 +K1 v2 *e+K2 v2 *e. In other respects, it is the same as the second embodiment. Note that the gain K1 v2 , K2 v2 The size of is the gain K1 in the second embodiment. v2 , K2 v2 The size may differ, but the properties shown in Figures 8 and 9 are the same.

[0110] The control system 20CM according to a modified example of the third embodiment includes the feedback controller Cb of the control system 20C. E Instead, the feedback controller Cb E1and feedback controller Cb E2 Includes the feedback controller Cb. E1 The speed difference e is given by gain K1 E Multiply by u1 to adjust target engine rotation speed E1 Derive the following: Feedback controller Cb E2 This is the speed difference e with gain K2 E Multiply by u1 to adjust target engine rotation speed E2 The following is derived. Processor 10a determines the target engine rotation speed u0 E and adjustment target engine rotation speed u1 E1 and adjustment target engine rotation speed u1 E2 The sum of and the manipulated variable u E The manipulated quantity u E The rotation command applied to it is output to the injector, supply pump, or common rail. In other words, the manipulated amount u E =u0 E +K1 E *e+K2 E *e. In other respects, it is the same as the third embodiment. Note that the gain K1 E , K2 E The size of is the gain K1 in the third embodiment. E , K2 E The size may differ, but the properties shown in Figures 8 and 9 are the same.

[0111] The control system 20DM according to a modified example of the fourth embodiment is a feedback controller Cb of the control system 20D. R Instead, the feedback controller Cb R1 and feedback controller Cb R2 Includes the feedback controller Cb. R1 The speed difference e is given by gain K1 R Multiply by this to derive the adjusted assumed control variable r11. Feedback controller Cb R2 This is the speed difference e with gain K2 R The adjusted deemed manipulated variable r12 is derived by multiplying by . The processor 10a takes the sum of the deemed manipulated variable r0, the adjusted deemed manipulated variable r11, and the adjusted deemed manipulated variable r12 as the manipulated variable r and converts this manipulated variable r to the converter Cb v Output to: That is, the manipulated variable r = r0 + K1R *e+K2 R *e. In other respects, it is the same as the fourth embodiment. Note that the gain K1 R , K2 R The size of is the gain K1 in the fourth embodiment. R , K2 R The size may differ, but the properties shown in Figures 8 and 9 are the same.

[0112] In the above embodiment, each feedback controller obtains the output by multiplying the speed difference e by a gain, but it is also possible to have an output for the speed difference e in map format and obtain the output by reading out that map.

[0113] In this application, “equipped with” and its derivatives are non-restrictive terms that describe the existence of a component and do not exclude the existence of other components not described. This also applies to “having,” “including,” and their derivatives.

[0114] The terms "~member," "~part," "~element," "~body," and "~structure" can have multiple meanings, such as a single part or multiple parts.

[0115] Ordinal numbers such as "1st" and "2nd" are simply terms used to identify components and do not carry any other meaning (such as a specific order). For example, the existence of a "1st element" does not implicitly mean the existence of a "2nd element," nor does the existence of a "2nd element" implicitly mean the existence of a "1st element."

[0116] Unless otherwise specifically stated in the embodiments, terms such as "substantially," "about," and "approximately" can mean a reasonable deviation that does not significantly alter the final result. All numerical values ​​described in this application may be interpreted as including terms such as "substantially," "about," and "approximately."

[0117] 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.

[0118] Based on the above disclosure, it is clear that various changes and modifications to the present invention are possible. Therefore, the present invention may be implemented in a manner different from the specific disclosures of this application, without departing from the spirit of the invention.

Claims

1. The engine is driven to rotate the first hydraulic pump, The first hydraulic pump supplies hydraulic fluid to the first hydraulic motor that drives the first running gear installed on the vehicle body, thereby rotating the first hydraulic motor. The rotational speed of the first hydraulic motor is detected, Determine the rate of change per unit time of the aforementioned rotational speed, Feedback control is performed on the pilot pressure control valve of the first hydraulic pump and the engine to reduce the speed difference between the target rotational speed of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed. The feedback gain of the aforementioned feedback control is determined based on a first coefficient that increases as the target rotational speed decreases. This includes, The first coefficient is, When the aforementioned target rotational speed is less than or equal to the first target rotational speed, it is taken as the first value. When the aforementioned target rotational speed is greater than or equal to a second target rotational speed which is greater than the first target rotational speed, the second value is set to be smaller than the first value. A first condition is determined to be true if the absolute value of the amount of change is less than a first threshold, or if the amount of change is less than a second threshold which is less than the value obtained by multiplying the first threshold by -1, and false otherwise. The second condition is determined to be true when the absolute value of the speed difference is greater than the third threshold, and false otherwise. A third condition is determined, which is true if the aforementioned target rotational speed is less than or equal to the third target rotational speed which is less than or equal to the second target rotational speed, and false otherwise. When all three conditions are true, increase the first coefficient from the value of the first coefficient when any one of the three conditions is false. A method for controlling the speed of a work vehicle.

2. The method according to claim 1, wherein the feedback gain is the first coefficient.

3. The first differential pressure of the first hydraulic motor is detected, The feedback gain of the feedback control is the product of the first coefficient and a second coefficient, which increases as the absolute value of the detected first differential pressure increases. The method according to claim 1.

4. A second hydraulic pump supplies hydraulic fluid to a second hydraulic motor that drives a second running device, which is located on the opposite side of the vehicle body from the first running device, causing the second hydraulic motor to rotate. The second differential pressure of the second hydraulic motor is detected, The method according to claim 3, wherein when the absolute value of the first differential pressure is greater than the absolute value of the second differential pressure, the second coefficient is increased as the absolute value of the first differential pressure increases.

5. The second coefficient is, When the absolute value of the first differential pressure is less than or equal to the first threshold pressure, it is taken as the fourth value. When the absolute value of the first differential pressure is greater than or equal to the second threshold pressure which is greater than the first threshold pressure, the fifth value is set to be greater than the fourth value. The method according to claim 3.

6. The second coefficient is, When the absolute value of the first differential pressure is less than or equal to the first threshold pressure, it is taken as the fourth value. When the absolute value of the first differential pressure is greater than or equal to the second threshold pressure which is greater than the first threshold pressure, the fifth value is set to be greater than the fourth value. The method according to claim 4.

7. A fourth condition is determined, which is true when the absolute value of the first differential pressure is equal to or greater than the third threshold pressure (which is equal to or greater than the first threshold pressure), and false otherwise. When all of the first, second, and fourth conditions are true, increase the second coefficient from the value of the second coefficient when any of the first to fourth conditions are false. The method according to claim 5.

8. A fourth condition is determined, which is true when the absolute value of the first differential pressure is equal to or greater than the third threshold pressure (which is equal to or greater than the first threshold pressure), and false otherwise. When all of the first, second, and fourth conditions are true, increase the second coefficient from the value of the second coefficient when any of the first to fourth conditions are false. The method according to claim 6.

9. The first differential pressure of the first hydraulic motor is detected, The feedback gain of the feedback control is the sum of the first coefficient and a second coefficient, which increases as the absolute value of the detected first differential pressure increases. The method according to claim 1.

10. The amount of the feedback control is used as an input to a primary pressure control valve that adjusts the primary pilot pressure, which is the hydraulic pressure in the primary pilot oil passage connecting a pilot pump for discharging pilot oil toward the pilot port of at least one hydraulic pump and an operating valve operated by a travel instruction input device that receives a travel direction instruction from the user. The method according to any one of claims 1 to 8.

11. The amount of the feedback control is input to a secondary pressure control valve that adjusts the secondary pilot pressure, which is the hydraulic pressure of the secondary pilot oil passage connecting an operating valve controlled by a travel instruction input device that receives a travel direction instruction from the user, and the pilot port of the at least one hydraulic pump. The method according to any one of claims 1 to 8.

12. The amount of the feedback control is set to the amount of the operation of the driving instruction input device into which the user inputs an instruction for the direction of travel. In accordance with the amount of operation of the drive instruction input device, the hydraulic pressure of the pilot circuit between the pilot pump and the pilot port for discharging pilot oil toward the pilot port of the at least one hydraulic pump is adjusted. The method according to any one of claims 1 to 8.

13. The amount of the feedback control is set to the target rotational speed of the engine that rotates the hydraulic pump. The method according to any one of claims 1 to 8.

14. The vehicle body and A first hydraulic motor configured to drive a first running device provided on the vehicle body, A first hydraulic pump configured to supply hydraulic fluid to the first hydraulic motor, An engine configured to drive the first hydraulic pump, A first rotational speed sensor configured to detect the rotational speed of the first hydraulic motor, A controller that performs feedback control on at least one of the engines, and controls the pilot pressure control valve of the first hydraulic pump, and reduces the speed difference between the target rotational speed of the first hydraulic motor corresponding to the target vehicle speed and the detected rotational speed, Equipped with, The controller determines the feedback gain of the feedback control based on a first coefficient that increases as the target rotational speed decreases. The aforementioned controller, Determine the rate of change per unit time of the aforementioned rotational speed, A first condition is determined to be true if the absolute value of the amount of change is less than a first threshold, or if the amount of change is less than a second threshold which is less than the value obtained by multiplying the first threshold by -1, and false otherwise. The second condition is determined to be true when the absolute value of the speed difference is greater than the third threshold, and false otherwise. A third condition is determined, which is true if the aforementioned target rotational speed is less than or equal to the third target rotational speed which is less than or equal to the second target rotational speed, and false otherwise. When any of the first, second, and third conditions is false, the first coefficient is set to a first value when the target rotational speed is less than or equal to the first target rotational speed which is less than the second target rotational speed, and to a second value which is less than the first value when the target rotational speed is equal to or greater than the second target rotational speed. When all three conditions are true, increase the first coefficient from the value of the first coefficient when any one of the three conditions is false. Work vehicle.

15. The system further includes a first hydraulic sensor for detecting a first differential pressure of the first hydraulic motor, The controller sets the feedback gain of the feedback control to the product of a second coefficient, which increases as the absolute value of the detected first differential pressure increases, and the first coefficient. The work vehicle according to claim 14.

16. The aforementioned controller, A fourth condition is determined, which is true when the absolute value of the first differential pressure is equal to or greater than the third threshold pressure (which is equal to or greater than the first threshold pressure), and false otherwise. If any of the first, second, and fourth conditions is false, the second coefficient is set to the fourth value when the absolute value of the first differential pressure is less than or equal to the first threshold pressure, and to the fifth value which is greater than the fourth value when the absolute value of the first differential pressure is greater than or equal to the second threshold pressure which is greater than the first threshold pressure. The work vehicle according to claim 15, wherein when all of the first to fourth conditions are true, the second coefficient is increased from the value of the second coefficient when any of the first to fourth conditions are false.

17. Further comprising a first hydraulic sensor for detecting a first differential pressure of the first hydraulic motor, The controller sets the feedback gain of the feedback control to the sum of a second coefficient, which increases as the absolute value of the detected first differential pressure increases, and the first coefficient. The work vehicle according to claim 14.

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