Hydraulic drive system and hydraulic drive method

Abstract

To provide a hydraulic drive system and a hydraulic drive method that have good fuel efficiency and operability while preventing an object from falling under its own weight.SOLUTION: A first direction switching valve 11 supplies hydraulic oil to a first port P1 on the basis of a first pilot signal. A second direction switching valve 12 discharges the hydraulic oil from a second port P2 on the basis of a second pilot signal. A descent control valve 13a is arranged between the second port P2 and the second direction switching valve 12. A controller 20 outputs the second pilot signal in accordance with descent operation of a working machine 3, outputs an instruction signal different from the second pilot signal on the basis of the second pilot signal, and opens the descent control valve 13a on the basis of the instruction signal.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a hydraulic drive system and a hydraulic drive method. [Background technology]

[0002] A hydraulic drive system that prevents an object from falling under its own weight in a work machine such as a hydraulic excavator is disclosed, for example, in Japanese Patent Application Laid-Open No. 2020-94644 (Patent Document 1). Patent Document 1 discloses a first boom directional control valve connected to each of the head-side port and rod-side port of the boom cylinder, and a lock valve disposed between the head-side port and the first boom directional control valve. The lock valve is a valve that maintains the position of the boom to prevent it from falling under its own weight when the operating lever is not operated, and corresponds to a so-called fall prevention valve. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2020-94644 Summary of the Invention [Problem to be solved by the invention]

[0004] In Patent Document 1, a lock valve is disposed between the rod-side port of the boom cylinder and the first boom direction control valve, which deteriorates fuel economy and operability due to the hardware characteristics of the lock valve.

[0005] An object of the present disclosure is to provide a hydraulic drive system and a hydraulic drive method that are fuel efficient and easy to operate while preventing an object from descending due to its own weight. [Means for solving the problem]

[0006] The hydraulic drive system disclosed herein is a hydraulic drive system that raises and lowers an object by supplying and discharging hydraulic oil to a first port and a second port of the hydraulic actuator to extend and retract the hydraulic actuator, and includes a first directional switching valve, a second directional switching valve, a descent control valve, and a controller. The first directional switching valve supplies hydraulic oil to the first port based on a first pilot signal. The second directional switching valve discharges hydraulic oil from the second port based on a second pilot signal. The descent control valve is disposed between the second port and the second directional switching valve. The controller outputs a second pilot signal in response to a lowering operation of the object, outputs a command signal different from the second pilot signal based on the second pilot signal, and opens the descent control valve based on the command signal.

[0007] The hydraulic drive method disclosed herein is a hydraulic drive method for raising and lowering an object by supplying and discharging hydraulic oil to a first port and a second port of the hydraulic actuator, thereby expanding and contracting the hydraulic actuator, and includes the following steps.

[0008] In response to a lowering operation of the object, a first pilot signal is output to control the first directional selector valve to supply hydraulic oil to the first port. In response to a lowering operation of the object, a second pilot signal is output to control the second directional selector valve to discharge hydraulic oil from the second port. Based on the second pilot signal, a command signal different from the second pilot signal is output to a descent control valve disposed between the second port and the second directional selector valve, thereby opening the descent control valve. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to realize a hydraulic drive system and a hydraulic drive method that are fuel efficient and easy to operate while preventing an object from descending due to its own weight. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a side view showing a configuration of a hydraulic excavator as an example of a work machine according to an embodiment of the present disclosure. [Figure 2]FIG. 1 is a diagram illustrating a configuration of a hydraulic drive system according to an embodiment of the present disclosure. [Figure 3] FIG. 3 is a diagram illustrating functional blocks of a controller illustrated in FIG. 2. [Figure 4] FIG. 1 is a flow diagram illustrating a hydraulic driving method according to an embodiment of the present disclosure. [Figure 5] FIG. 10 is a diagram showing the relationship between the pilot pressure (Pi pressure) supplied to the drop control valve and the opening area in each of an embodiment of the present disclosure and a conventional example. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.

[0012] In the specification and drawings, the same or corresponding components are denoted by the same reference numerals, and redundant explanations will not be repeated. In addition, in the drawings, configurations may be omitted or simplified for the sake of convenience.

[0013] In this disclosure, a hydraulic excavator will be used as an example of a work machine, but this disclosure can be applied to all devices that have a hydraulic drive system that raises and lowers objects such as work equipment 3 by extending and retracting hydraulic actuators, such as hydraulic excavators and cranes.

[0014] <Work machine configuration>

[0015] Fig. 1 is a side view schematically illustrating the configuration of a hydraulic excavator as an example of a work machine according to an embodiment of the present disclosure. As shown in Fig. 1, the hydraulic excavator 100 of this embodiment mainly includes a traveling body 1, a rotating body 2, and a work implement 3.

[0016] The work machine 100 is capable of traveling by means of a traveling body 1. A rotating body 2 is installed so as to be freely rotatable relative to the traveling body 1. The rotating body 2 mainly comprises an operator's room (cab) 2a, an operator's seat 2b, and an engine room 2c. Inside the operator's room 2a, there are arranged the operator's seat 2b for an operator to sit in, an operating lever 31 shown in FIG. 2, and the like. In the engine room 2c, there is arranged a hydraulic pump 15 shown in FIG. 2, and the like.

[0017] The work implement 3 is supported on the front side of the revolving structure 2, for example, on the right side of the operator's cab 2a. The work implement 3 has, for example, a boom 3a, an arm 3b, a bucket 3c, a boom cylinder 4a, an arm cylinder 4b, and a bucket cylinder 4c. The base end of the boom 3a is rotatably connected to the revolving structure 2 by a boom foot pin 5a. The base end of the arm 3b is rotatably connected to the tip of the boom 3a by a boom tip pin 5b. The bucket 3c is rotatably connected to the tip of the arm 3b by an arm tip pin 5c.

[0018] The boom 3a can be driven by a boom cylinder 4a. This drive allows the boom 3a to rotate vertically relative to the revolving body 2 around the boom foot pin 5a. The arm 3b can be driven by an arm cylinder 4b. This drive allows the arm 3b to rotate vertically relative to the boom 3a around the boom tip pin 5b. The bucket 3c can be driven by a bucket cylinder 4c. This drive allows the bucket 3c to rotate relative to the arm 3b around the arm tip pin 5c. In this way, the work machine 3 moves up and down by the extension and contraction of the cylinders 4a to 4c.

[0019] <Hydraulic drive system configuration>

[0020] Next, the configuration of the hydraulic drive system 50 applied to the above-described work machine 100 when a boom cylinder 4a is used as the hydraulic actuator AC will be described with reference to FIG.

[0021] Fig. 2 is a diagram showing the configuration of a hydraulic drive system according to one embodiment of the present disclosure. As shown in Fig. 2, the hydraulic drive system 50 supplies and discharges hydraulic oil to a first port P1 and a second port P2 of the hydraulic actuator AC. This causes the hydraulic drive system 50 to extend and retract the hydraulic actuator AC, thereby raising and lowering the work implement 3 (boom 3a).

[0022] The hydraulic drive system 50 includes a hydraulic actuator AC, a first directional control valve 11, a second directional control valve 12, a descent control device 13, a hydraulic pump 15, and a tank 16. The hydraulic drive system 50 further includes a controller 20, EPC (Electromagnetic Proportional Control) valves 21 to 24, an operating lever 31, an operation amount sensor 32, and a pressure sensor 41.

[0023] The hydraulic actuator AC is, for example, a boom cylinder 4a. The hydraulic actuator AC has a first oil chamber OR1 on the tube side of the piston and a second oil chamber OR2 on the rod side of the piston. The first oil chamber OR1 of the hydraulic actuator AC has a first port P1, and the second oil chamber OR2 has a second port P2. Hydraulic oil can be supplied to and discharged from a hydraulic pump 15 via the first port P1 and the second port P2, respectively, to the first oil chamber OR1 and the second oil chamber OR2 of the hydraulic actuator AC. The hydraulic pump 15 is, for example, a swash plate-type variable displacement pump, and the angle of the swash plate can be controlled by an EPC valve 24.

[0024] A first direction switching valve 11 is disposed between the first port P1 and the hydraulic pump 15. The first direction switching valve 11 has a rod-shaped spool 11a. The spool 11a of the first direction switching valve 11 is driven based on the first pilot pressure from the EPC valve 21. The first direction switching valve 11 controls the supply and discharge of hydraulic oil to and from the first oil chamber OR1 of the hydraulic actuator AC by driving the spool 11a.

[0025] A second direction switching valve 12 and a descent control device 13 are arranged between the second port P2 and the hydraulic pump 15. The second direction switching valve 12 is separate from the first direction switching valve 11. The second direction switching valve 12 has a rod-shaped spool 12a. The spool 12a of the second direction switching valve 12 is driven based on the second pilot pressure from the EPC valve 22. The second direction switching valve 12 controls the supply and discharge of hydraulic oil to and from the second oil chamber OR2 of the hydraulic actuator AC by driving the spool 12a.

[0026] The descent control device 13 is disposed between the second port P2 and the second direction switching valve 12. The descent control device 13 has a descent control valve 13a and a check valve 13b. The descent control valve 13a and the check valve 13b are connected in parallel with each other between the second port P2 and the second direction switching valve 12. The descent control valve 13a prevents the work machine 3 from descending due to its own weight when the operation lever 31 is not operated. The descent control valve 13a has a rod-shaped spool 13aa. The spool 13aa of the descent control valve 13a is driven based on the third pilot pressure from the EPC valve 23. The descent control valve 13a switches between flowing and blocking of hydraulic oil between the second port P2 of the hydraulic actuator AC and the second direction switching valve 12 by driving the spool 13aa. The check valve 13b allows the flow of hydraulic oil from the second direction switching valve 12 toward the second port P2, and blocks the flow of hydraulic oil from the second port P2 toward the second direction switching valve 12.

[0027] Each of the EPC valves 21 to 24 is an electromagnetic proportional control valve, and operates based on a pilot signal from the controller 20. The EPC valve 21 supplies a first pilot pressure to the first direction switching valve 11 based on a first pilot signal from the controller 20. The EPC valve 21 drives the spool 11a of the first direction switching valve 11 based on, for example, the first pilot signal from the controller 20, so as to supply hydraulic oil from the hydraulic pump 15 to the first port P1. Note that an EPC valve is also provided that drives the spool 11a of the first direction switching valve 11 based on the pilot signal from the controller 20, so as to discharge hydraulic oil from the first port P1 to the tank 16, but this is not shown in the figure.

[0028] The EPC valve 22 supplies a second pilot pressure to the second direction switching valve 12 based on a second pilot signal from the controller 20. The EPC valve 22 drives the spool 12a of the second direction switching valve 12 so as to discharge hydraulic oil from the second port P2 to the tank 16, for example, based on the second pilot signal from the controller 20. Note that an EPC valve is also provided that drives the spool 12a of the second direction switching valve 12 based on the pilot signal from the controller 20 so as to supply hydraulic oil from the hydraulic pump 15 to the second port P2, but this is not shown in the figure.

[0029] The EPC valve 23 supplies a third pilot pressure to the descent control valve 13a based on a command signal from the controller 20. This causes the descent control valve 13a to switch between a closed state and an open state.

[0030] The EPC valve 24 can control the angle of the swash plate of the hydraulic pump 15 based on a control signal from the controller 20. This makes it possible to change the amount of hydraulic oil discharged from the hydraulic pump 15.

[0031] The control lever 31 is operated by an operator. The control lever 31 is used to control the operation of the work implement 3. The amount of operation of the work implement 3 is controlled according to the amount of operation of the control lever 31. Operation of the control lever 31 controls, for example, the rotational movement of the boom 3a about the boom foot pin 5a, the rotational movement of the arm 3b about the boom tip pin 5b, and the rotational movement of the bucket 3c about the arm tip pin 5c.

[0032] The operation amount sensor 32 detects the operation amount of the control lever 31. The operation amount sensor 32 is, for example, a potentiometer. The operation amount sensor 32 outputs the detected operation amount of the control lever 31 to the controller 20 as an operation signal (electrical signal). The controller 20 acquires the electrical signal indicating the operation amount of the control lever 31, and controls the first directional switching valve 11, the second directional switching valve 12, the descent control valve 13a, and the hydraulic pump 15 based on the electrical signal. Since the operation amount of the control lever 31 is output to the controller 20 as an electrical signal in this way, the control lever 31 is an electric lever. Note that the control lever 31 may also be a hydraulically driven lever that controls the directional switching valves 11, 12, etc. by changing the hydraulic pressure based on the operation amount of the control lever 31.

[0033] The pressure sensor 41 is disposed between the second directional switching valve 12 and the descent control device 13. The pressure sensor 41 detects the pressure of the hydraulic oil between the second directional switching valve 12 and the descent control device 13. The pressure sensor 41 outputs the detected pressure of the hydraulic oil to the controller 20.

[0034] Although the boom cylinder 4a has been described with reference to FIG. 2, the present disclosure can be similarly applied to the arm cylinder 4b.

[0035] <Hydraulic drive system operation>

[0036] Next, the operation of the hydraulic drive system shown in FIG. 2 will be described using an example in which the hydraulic actuator AC is the boom cylinder 4a.

[0037] 2, first, a description will be given of an operation of moving the tip of the boom 3a upward in the direction of gravity (boom-raising operation). In this case, an operation signal based on the boom-raising operation of the control lever 31 by the operator is output to the controller 20. The controller 20 controls the EPC valve 24 to drive the hydraulic pump 15 in accordance with the amount of operation of the control lever 31.

[0038] The controller 20 also controls the second direction switching valve 12 by outputting a second pilot signal to the EPC valve 22. As a result, the second direction switching valve 12 is controlled to supply hydraulic oil from the hydraulic pump 15 to the second port P2 of the hydraulic actuator AC. At this time, the hydraulic oil that has passed through the second direction switching valve 12 is supplied to the second oil chamber OR2 of the hydraulic actuator AC via the check valve 13b. The controller 20 also controls the first direction switching valve 11 by outputting a first pilot signal to the EPC valve 21. As a result, the first direction switching valve 11 is controlled to discharge hydraulic oil from the first port P1 of the hydraulic actuator AC to the tank 16. At this time, the controller 20 controls the descent control valve 13a to maintain it in a closed state.

[0039] As a result, hydraulic oil is supplied to the second oil chamber OR2 and the hydraulic oil is discharged from the first oil chamber OR1, causing the hydraulic actuator AC to extend. As the hydraulic actuator AC extends, the boom 3a rotates about the boom foot pin 5a, moving the tip of the boom 3a upward and performing a boom-raising operation.

[0040] Next, a description will be given of an operation for moving the tip of the boom 3a downward in the direction of gravity (boom lowering operation). In this case, an operation signal based on the boom lowering operation of the control lever 31 by the operator is output to the controller 20. The controller 20 controls the EPC valve 24 to drive the hydraulic pump 15 in accordance with the amount of operation of the control lever 31.

[0041] The controller 20 also controls the first direction switching valve 11 by outputting a first pilot signal to the EPC valve 21. As a result, the first direction switching valve 11 is controlled to supply hydraulic oil from the hydraulic pump 15 to the first port P1 of the hydraulic actuator AC. The controller 20 also controls the descent control valve 13a to be in an open state by outputting a third pilot signal to the EPC valve 23. As a result, the hydraulic oil in the second oil chamber OR2 of the hydraulic actuator AC can pass through the descent control valve 13a. The controller 20 also controls the second direction switching valve 12 by outputting a second pilot signal to the EPC valve 22. As a result, the second direction switching valve 12 is controlled to discharge the hydraulic oil that has passed through the descent control valve 13a to the tank 16.

[0042] As a result of the above, hydraulic oil is supplied to the first oil chamber OR1, and hydraulic oil in the second oil chamber OR2 is discharged to the tank 16, thereby retracting the hydraulic actuator AC. As the hydraulic actuator AC retracts, the boom 3a rotates about the boom foot pin 5a, moving the tip of the boom 3a downward and performing a boom-lowering operation.

[0043] Next, we will explain the state in which the control lever 31 is not operated and the tip of the work implement 3 is floating in the air and stopped. In this case, because the tip of the work implement 3 is floating in the air, the tip of the work implement 3 receives a force in the downward direction due to its own weight, and the tip of the boom 3a receives a force in the downward direction (boom lowering direction). At this time, the hydraulic actuator AC, which is the boom cylinder 4a, receives a force in the retracting direction.

[0044] However, when the operating lever 31 is not operated, the descent control valve 13a remains closed, preventing hydraulic oil from being discharged from the second oil chamber OR2 of the hydraulic actuator AC. As a result, the hydraulic actuator AC does not retract, preventing the tip of the work implement 3 from descending.

[0045] As described above, in the hydraulic drive system 50 of this embodiment, the descent of the tip of the work machine 3 is prevented, but the provision of the descent control valve 13a causes pressure loss of the hydraulic oil in the descent control valve 13a, deteriorating fuel economy and operability. Therefore, in this embodiment, the following functional blocks of the controller 20 and hydraulic drive method are adopted.

[0046] <Controller function block>

[0047] The functional blocks of the controller in this embodiment will be described with reference to FIG.

[0048] Fig. 3 is a diagram showing functional blocks of the controller shown in Fig. 2. As shown in Fig. 3, the controller 20 has an operation amount acquisition unit 20A, an operation amount determination unit 20B, a work implement speed determination unit 20C, an opening determination unit 20D, an EPC valve control unit 20E, and a memory 20F.

[0049] The controller 20 includes a processor, a main memory, and storage (memory 20F). The processor is, for example, a CPU (Central Processing Unit). The main memory includes, for example, a non-volatile memory such as a ROM (Read Only Memory) and a volatile memory such as a RAM (Random Access Memory).

[0050] The controller 20 reads out the program stored in the storage, loads it into the main memory, and executes predetermined processing in accordance with the program. The program may be distributed to the controller 20 via a network.

[0051] The controller 20 and the operation lever 31 may each be mounted on the work machine 100, or may be located remotely outside the work machine 100. When the controller 20 and the operation lever 31 are each located remotely outside the work machine 100, the controller 20 and the operation lever 31 may each be wirelessly connected to the EPC valves 21-24, the pressure sensor 41, etc. The controller 20 may be stored in a server remote from the work machine 100. Furthermore, by having the operation lever 31 remote from the work machine 100, the operator may operate the work machine 100 remotely without being inside the cab 2a of the work machine 100.

[0052] <Hydraulic drive method>

[0053] Next, the hydraulic driving method in this embodiment will be described with reference to FIGS.

[0054] 4 is a flow diagram showing a hydraulic drive method according to an embodiment of the present disclosure. As shown in FIGS. 3 and 4, in step S1, the amount of operation of the control lever 31 by the operator is acquired by the operation amount acquisition unit 20A of the controller 20. Specifically, the operation amount acquisition unit 20A acquires an operation signal (electrical signal) output from the operation amount sensor 32.

[0055] In step S2, the operation amount determination unit 20B of the controller 20 determines whether the operation amount acquired by the operation amount acquisition unit 20A is equal to or greater than a predetermined value. At this time, the operation amount determination unit 20B refers to the predetermined value of the operation amount stored in the memory 20F.

[0056] If the operation amount determination unit 20B determines that the operation amount is less than the predetermined value, step S1 is repeated. On the other hand, if the operation amount determination unit 20B determines that the operation amount is equal to or greater than the predetermined value, the operating speed (work implement speed) of the work implement 3 is calculated from the operation amount in step S3. In calculating the work implement speed, a table showing the relationship between the operation amount and the work implement speed, which is stored in memory 20F, is referenced.

[0057] In step S4, the work implement speed determination unit 20C of the controller 20 determines whether the calculated work implement speed is equal to or less than a predetermined value. At this time, the work implement speed determination unit 20C refers to the predetermined value of the work implement speed stored in the memory 20F.

[0058] If the work implement speed determination unit 20C determines that the work implement speed is greater than the predetermined value, the steps from step S1 onwards are repeated. On the other hand, if the work implement speed determination unit 20C determines that the work implement speed is equal to or less than the predetermined value, the stroke position of the spool 12a of the second direction switching valve 12 is calculated in step S5.

[0059] In step S6, the opening area of ​​the spool 12a is calculated from the stroke position of the spool 12a. In calculating the opening area of ​​the spool 12a, a table showing the relationship between the stroke position of the spool 12a and the opening area, which is stored in the memory 20F, is referenced.

[0060] In step S7, the opening determination unit 20D of the controller 20 determines whether the calculated opening area of ​​the spool 12a is equal to or smaller than a predetermined value. At this time, the opening determination unit 20D refers to the predetermined value of the opening area of ​​the spool 12a stored in the memory 20F.

[0061] The opening determination unit 20D outputs a signal indicating the determination result of the opening area of ​​the spool 12a and a signal indicating the opening area of ​​the spool 12a of the second direction switching valve 12 to the EPC valve control unit 20E.

[0062] The EPC valve control unit 20E calculates the opening area of ​​the spool 13aa in the descent control valve 13a from the opening area of ​​the spool 12a of the second direction switching valve 12. When calculating the opening area of ​​the spool 13aa, a table showing the relationship between the opening area of ​​the spool 12a and the opening area of ​​the spool 13aa, which is stored in the memory 20F, is referenced. The descent speed of the boom 3a is determined according to the opening area of ​​the spool 13aa.

[0063] The EPC valve control unit 20E calculates the pilot pressure of the descent control valve 13a from the calculated opening area of ​​the spool 13aa of the descent control valve 13a. When calculating the pilot pressure of the descent control valve 13a, a table showing the relationship between the opening area of ​​the spool 13aa and the pilot pressure of the descent control valve 13a, which is stored in the memory 20F, is referenced.

[0064] The EPC valve control unit 20E calculates, from the calculated pilot pressure of the descent control valve 13a, a command current value to be supplied to the EPC valve 23. When calculating this command current value, a table showing the relationship between the pilot pressure of the descent control valve 13a and the command current value to be supplied to the EPC valve 23, which is stored in the memory 20F, is referenced.

[0065] In step S8a, when EPC valve control unit 20E obtains from opening determination unit 20D a determination result that the opening area of ​​spool 12a is equal to or smaller than a predetermined value, it outputs a command signal to EPC valve 23 so that the opening area of ​​spool 13aa in descent control valve 13a becomes equal to or smaller than the predetermined area, based on the output of the second pilot signal to second direction switching valve 12. This makes it possible to keep the opening area of ​​descent control valve 13a equal to or smaller than the predetermined area in region R (FIG. 5) where the pilot pressure (Pi) supplied to second direction switching valve 12 is low, thereby suppressing descent of work machine 3 even if the cylinder hose bursts.

[0066] In step S8b, when EPC valve control unit 20E obtains from opening determination unit 20D a determination result that the opening area of ​​spool 12a is larger than a predetermined value, it outputs a command signal to EPC valve 23 to maximize the opening area of ​​spool 13aa in descent control valve 13a, based on the output of the second pilot signal to second direction switching valve 12. In this way, when it is desired to lower work machine 3 by making the opening area of ​​second direction switching valve 12 larger than a predetermined value, maximizing the opening area of ​​descent control valve 13a makes it easier for hydraulic oil to pass through the opening of descent control valve 13a than when the opening of descent control valve 13a is narrowed down, thereby further suppressing pressure loss of hydraulic oil and further improving fuel economy and operability.

[0067] In the hydraulic drive method of this embodiment, the boom 3a is lowered at a predetermined speed in response to an operation to lower the object, for example, an operation to lower the boom 3a.

[0068] <Effects>

[0069] Next, the effects of this embodiment will be described with reference to FIG.

[0070] FIG. 5 is a diagram showing the relationship between the pilot pressure (Pi pressure) supplied to the descent control valve 13a and the opening area of ​​the control valve in each of an embodiment of the present disclosure and a conventional example. As shown in FIG. 5, the thick solid line in the diagram shows the relationship between the pilot pressure and the opening area of ​​the spool 13aa in the descent control valve 13a of this embodiment. The thin solid line in the diagram shows the relationship between the pilot pressure and the opening area of ​​the spool 13aa in the descent control valve 13a of the conventional example. The dashed line in the diagram shows the ideal relationship between the pilot pressure and the opening area of ​​the spool 13aa in the descent control valve 13a. The dashed line in the diagram shows the relationship between the pilot pressure and the opening area of ​​the spool 12a in the second directional control valve 12 of each of this embodiment and the conventional example.

[0071] The hydraulic drive system of this embodiment has the configuration shown in Fig. 2. On the other hand, in the hydraulic drive system of the conventional example, the first directional switching valve 11 and the second directional switching valve 12 are configured as a single valve in the configuration shown in Fig. 2, and the opening and closing operations of both the first directional switching valve 11 and the second directional switching valve 12 are controlled by the operation of a single spool. Other than this, the configuration of the conventional example is substantially the same as the configuration of this embodiment shown in Fig. 2, so description thereof will not be repeated.

[0072] When the opening and closing operations of both the first direction switching valve 11 and the second direction switching valve 12 are controlled by the operation of a single spool, as in the conventional example, the second direction switching valve 12 is subject to the operation constraints of the first direction switching valve 11. In the conventional example, it is necessary to control the opening area of ​​the descent control valve 13a under such constraints. For this reason, as shown by the thin solid line in Figure 5, it is difficult to rapidly increase the opening area of ​​the descent control valve 13a. As a result, pressure loss of the hydraulic oil occurs at the opening of the descent control valve 13a, deteriorating fuel economy and operability.

[0073] In contrast, in this embodiment, as shown in FIG. 2, the second direction switching valve 12 is controlled to open and close based on a pilot signal different from the pilot signal that operates the first direction switching valve 11. This makes it possible to control the second direction switching valve 12 independently of the first direction switching valve 11. In this manner, in this embodiment, the opening area of ​​the descent control valve 13a can be controlled without the constraints imposed by the conventional example. Therefore, as shown by the thick solid line in FIG. 5, the opening area of ​​the spool 13aa in the descent control valve 13a can be increased more rapidly than that of the conventional descent control valve 13a (thin solid line). This allows the relationship in this embodiment to approach the ideal relationship between the pilot pressure and the opening area of ​​the spool 13aa in the descent control valve 13a, shown by the dashed-dotted line. In the ideal relationship shown by the dashed-dotted line, the opening area of ​​the descent control valve 13a instantly increases to nearly its maximum value. This reduces the resistance the hydraulic oil experiences when passing through the opening of the descent control valve 13a, thereby reducing pressure loss of the hydraulic oil. In this embodiment, the opening area of ​​the descent control valve 13a is closer to the ideal relationship than in the conventional example, and therefore the pressure loss of the hydraulic oil at the opening of the descent control valve 13a is suppressed, resulting in improved fuel economy and operability.

[0074] Furthermore, as shown in Figure 5, if the opening area of ​​the descent control valve 13a is increased in region R where the pilot pressure (Pi) supplied to the second direction switching valve 12 is small, the descent speed of the work implement 3 increases when the cylinder hose bursts.

[0075] Therefore, in this embodiment, as shown in step S8a in Fig. 4, when the opening area of ​​the spool 12a in the second direction switching valve 12 is equal to or less than a predetermined value, the opening area of ​​the descent control valve 13a is controlled to be equal to or less than a predetermined area. For example, the opening area of ​​the spool 13aa is controlled to be equal to or less than a predetermined value so that the predetermined opening area is large enough to prevent the working implement 3 from descending even when the cylinder hose bursts. This prevents the working implement 3 from descending even when the cylinder hose bursts.

[0076] Furthermore, in this embodiment, as shown in step S8b in Fig. 4, when the opening area of ​​second direction switching valve 12 is larger than a predetermined value, the opening area of ​​descent control valve 13a is controlled to be maximized. In this way, when it is desired to lower work implement 3 by making the opening area of ​​second direction switching valve 12 larger than a predetermined value, by maximizing the opening area of ​​descent control valve 13a, the resistance when hydraulic oil passes through the opening of descent control valve 13a is smaller than when the opening of descent control valve 13a is narrowed, thereby further suppressing pressure loss of hydraulic oil and further improving fuel economy and operability.

[0077] In this embodiment, as shown in Fig. 2, the controller 20 acquires an electric signal indicating the amount of operation of the control lever 31 by the operator, and controls the descent control valve 13a based on the electric signal. Because the control can be performed by an electric lever in this way, the rise in the opening area of ​​the descent control valve 13a, indicated by the thick solid line in Fig. 5, can be made steeper than when a hydraulically driven lever is used. This makes it possible to further improve fuel economy and operability.

[0078] 2, in this embodiment, the first direction switching valve 11 and the second direction switching valve 12 are separate from each other. This allows the first direction switching valve 11 to supply and discharge hydraulic oil to and from the first port, and the second direction switching valve 12 to supply and discharge hydraulic oil to and from the second port.

[0079] <Additional Notes>

[0080] The above-described embodiment includes the following technical idea.

[0081] (Appendix 1) A hydraulic drive system that raises and lowers an object by supplying and discharging hydraulic oil to a first port and a second port of a hydraulic actuator to extend and contract the hydraulic actuator, a first directional control valve that supplies hydraulic oil to the first port based on a first pilot signal; a second directional control valve that discharges hydraulic oil from the second port based on a second pilot signal; a drop control valve disposed between the second port and the second directional control valve; a controller that outputs the second pilot signal in response to a lowering operation of the object, outputs a command signal different from the second pilot signal based on the second pilot signal, and opens the descent control valve based on the command signal.

[0082] (Appendix 2) 2. The hydraulic drive system of claim 1, wherein the controller controls the opening area of ​​the descent control valve to be equal to or less than a predetermined area when the opening area of ​​the second directional switching valve based on the second pilot signal is equal to or less than a predetermined value.

[0083] (Appendix 3) 3. The hydraulic drive system according to claim 1, wherein the controller controls the opening area of ​​the descent control valve to a maximum when the opening area of ​​the second directional switching valve based on the second pilot signal is greater than a predetermined value.

[0084] (Appendix 4) Further provided with an operating lever operated by an operator, 4. The hydraulic drive system according to claim 1, wherein the controller acquires an electrical signal indicating an amount of operation of the operating lever, and controls the descent control valve based on the electrical signal.

[0085] (Appendix 5) 5. The hydraulic drive system according to claim 1, wherein the first direction switching valve and the second direction switching valve are separate from each other.

[0086] (Appendix 6) A hydraulic drive method for lifting and lowering an object by supplying and discharging hydraulic oil to a first port and a second port of a hydraulic actuator to extend and contract the hydraulic actuator, comprising: outputting a first pilot signal to control a first directional switching valve to supply hydraulic oil to the first port in response to a lowering operation of the object; outputting a second pilot signal to control a second directional control valve to discharge hydraulic oil from the second port in response to a lowering operation of the object; and outputting a command signal different from the second pilot signal to a descent control valve disposed between the second port and the second directional switching valve based on the second pilot signal to open the descent control valve.

[0087] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0088] 1 Traveling body, 2 Swinging body, 2a Operator's cab, 2b Operator's seat, 2c Engine room, 3 Work equipment, 3a Boom, 3b Arm, 3c Bucket, 4a Boom cylinder, 4b Arm cylinder, 4c Bucket cylinder, 5a Boom foot pin, 5b Boom tip pin, 5c Arm tip pin, 11 First direction switching valve, 12 Second direction switching valve, 11a, 12a, 13aa Spool, 13 Descent control device, 13a Descent control valve, 13b Check valve, 15 Hydraulic pump, 16 Tank, 20 Controller, 20A Operation amount acquisition unit, 20B Operation amount determination unit, 20C Work equipment speed determination unit, 20D Opening determination unit, 20E Valve control unit, 20F Memory, 21, 22, 23, 24 EPC valve, 31 Operation lever, 32 Operation amount sensor, 41 Pressure sensor, 50 Hydraulic drive system, 100 work machine, AC hydraulic actuator, OR1 first oil chamber, OR2 second oil chamber, P1 first port, P2 second port, R area.

Claims

1. A hydraulic drive system that raises and lowers an object by supplying and discharging hydraulic oil to a first port and a second port of a hydraulic actuator to extend and contract the hydraulic actuator, a first directional control valve that supplies hydraulic oil to the first port based on a first pilot signal; a second directional control valve that discharges hydraulic oil from the second port based on a second pilot signal; a drop control valve disposed between the second port and the second directional control valve; a controller that outputs the second pilot signal in response to a lowering operation of the object, outputs a command signal different from the second pilot signal based on the second pilot signal, and opens the descent control valve based on the command signal.

2. 2. The hydraulic drive system according to claim 1, wherein the controller controls the opening area of ​​the descent control valve to be equal to or less than a predetermined area when the opening area of ​​the second directional switching valve based on the second pilot signal is equal to or less than a predetermined value.

3. The hydraulic drive system according to claim 1 , wherein the controller controls the opening area of ​​the descent control valve to a maximum when the opening area of ​​the second directional control valve based on the second pilot signal is larger than a predetermined value.

4. Further provided with an operating lever operated by an operator, The hydraulic drive system according to claim 1 , wherein the controller acquires an electric signal indicating an amount of operation of the control lever, and controls the descent control valve based on the electric signal.

5. The hydraulic drive system according to claim 1 , wherein the first directional control valve and the second directional control valve are separate from each other.

6. A hydraulic drive method for lifting and lowering an object by supplying and discharging hydraulic oil to a first port and a second port of a hydraulic actuator to extend and contract the hydraulic actuator, comprising: outputting a first pilot signal to control a first directional control valve to supply hydraulic oil to the first port in response to a lowering operation of the object; outputting a second pilot signal to control a second directional control valve to discharge hydraulic oil from the second port in response to a lowering operation of the object; and outputting a command signal different from the second pilot signal to a descent control valve disposed between the second port and the second directional switching valve based on the second pilot signal to open the descent control valve.

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