Hydraulic system and control device

Abstract

The present invention provides a hydraulic system (1) comprising: a hydraulic actuator (5); a control valve (4) connected to the hydraulic actuator (5); and a variable displacement hydraulic pump (21) connected to the control valve (4) via a pump line (31). The minimum discharge flow rate of the hydraulic pump (21) is set to be greater than zero. Further, the hydraulic system (1) comprises: a drain valve (34) provided on a drain line (33); actuator pressure detectors (81, 82) for detecting actuator pressure; and a control device (7). The control device (7) includes a processing circuit (71) that controls the drain valve (34) to have an opening area corresponding to the inflow flow command and the actuator pressure detected by the actuator pressure detectors (81, 82) when the inflow flow command to the hydraulic actuator (5) is greater than zero and less than the minimum discharge flow rate.

Description

Technical Field

[0001] This disclosure relates to a hydraulic system including a relief valve and a control device for the hydraulic circuit. Background Technology

[0002] Traditionally, hydraulic systems are used in construction machinery and industrial machinery, where hydraulic pumps supply working oil to hydraulic actuators via control valves. Some of these systems also include a relief valve that specifies the flow rate of working oil discharged from the hydraulic pump to the oil tank. This relief valve is also known as an unloading valve.

[0003] For example, Patent Document 1 discloses a hydraulic system including a variable displacement hydraulic pump. Specifically, in the hydraulic system of Patent Document 1, the hydraulic pump is connected to a control valve via a pump line, and the control valve is connected to a hydraulic actuator via a pair of supply and discharge lines. A drain line branches off from the pump line and connects to an oil tank, and a drain valve is provided on the drain line.

[0004] The hydraulic system of Patent Document 1 also includes an operating device for activating the hydraulic actuator. The operating device includes an operating section operated by an operator and outputs an operating signal corresponding to the amount of operation performed on the operating section. The relief valve is controlled such that its opening area decreases as the amount of operation on the operating section increases. That is, when the hydraulic actuator stops, the relief valve is fully open, and the discharge pressure of the hydraulic pump becomes low.

[0005] Furthermore, the amount of operation on the operating part of the operating device also serves as a speed command for the hydraulic actuator. That is, the greater the amount of operation on the operating part, the larger the discharge flow rate of the hydraulic pump and the opening area of ​​the control valve, thus increasing the supply flow rate of working oil to the hydraulic actuator. Existing technical documents Patent documents

[0006] Patent Document 1: Japanese Patent Publication No. 2019-2512 Summary of the Invention The problem that the invention aims to solve

[0007] However, in hydraulic systems, in order to quickly establish the discharge pressure of the hydraulic pump when the hydraulic actuator is initially energized, the minimum discharge flow rate of the hydraulic pump is sometimes set to be greater than zero. In such hydraulic systems, when the hydraulic actuator is energized at a very low speed (i.e., the required supply flow to the hydraulic actuator is less than the minimum discharge flow rate of the hydraulic pump), the relief valve will be almost fully open, which can sometimes lead to a delay in the establishment of the hydraulic pump's discharge pressure.

[0008] Therefore, the object of this disclosure is to provide a hydraulic system and a control device for the hydraulic circuit that can rapidly build up the discharge pressure of the hydraulic pump even when the hydraulic actuator is actuated at a very low speed. Means for solving the problem

[0009] From one perspective, this disclosure provides a hydraulic system comprising: a hydraulic actuator; a control valve connected to the hydraulic actuator; a variable displacement hydraulic pump connected to the control valve via a pump line and having a minimum discharge flow rate greater than zero; an oil tank for storing working oil; a drain valve provided on a drain line branching from the pump line and connected to the oil tank; an actuator pressure detector for detecting actuator pressure as the pressure of the working oil in the hydraulic actuator; and a control device comprising processing circuitry that controls the drain valve based on an inflow flow command to the hydraulic actuator, the processing circuitry causing the drain valve to be fully closed when the inflow flow command is greater than the minimum discharge flow rate, and controlling the drain valve to have an opening area corresponding to the inflow flow command and the actuator pressure detected by the actuator pressure detector when the inflow flow command is greater than zero and less than the minimum discharge flow rate.

[0010] In another aspect, this disclosure provides a control device for a hydraulic circuit in which working oil is supplied from a hydraulic pump to a hydraulic actuator via a control valve, and a drain valve specifies the drain flow rate of the working oil discharged from the hydraulic pump to a tank. The control device includes a processing circuit that controls the drain valve based on an inflow flow command to the hydraulic actuator. The processing circuit fully closes the drain valve when the inflow flow command is greater than the minimum discharge flow rate of the hydraulic pump, and controls the drain valve to have an opening area corresponding to the inflow flow command and the actuator pressure, which is the working oil pressure within the hydraulic actuator, when the inflow flow command is greater than zero and less than the minimum discharge flow rate. Invention Effects

[0011] According to this disclosure, a hydraulic system and a control device for a hydraulic circuit are provided that can rapidly build up the discharge pressure of a hydraulic pump even when the hydraulic actuator is movable at a low speed. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of a hydraulic system according to one implementation method. Detailed Implementation

[0013] Figure 1 A hydraulic system 1 is shown, relating to one embodiment. The hydraulic system 1 is, for example, mounted on construction machinery such as hydraulic excavators or hydraulic cranes, industrial machinery such as press machinery, civil machinery such as tunnel excavators, or agricultural machinery such as tillers.

[0014] Specifically, the hydraulic system 1 includes a hydraulic circuit 10 and a control device 7 for the hydraulic circuit 10. The hydraulic circuit 10 includes a hydraulic actuator 5, a hydraulic pump 21 that supplies working oil to the hydraulic actuator 5 via a control valve 4, an oil tank 20 for storing the working oil, and a drain valve 34 that specifies the flow rate of working oil discharged from the hydraulic pump 21 to the oil tank 20. In the illustrated example, the combination of the hydraulic actuator 5 and the control valve 4 is one, but multiple combinations of hydraulic actuators 5 and control valves 4 can also be provided.

[0015] In this embodiment, the hydraulic actuator 5 is a double-acting cylinder or hydraulic motor that is movable in both directions by the supply of working oil. The hydraulic actuator 5 includes a first working chamber 51 movable in a first direction X and a second working chamber 52 movable in a second direction Y. However, the hydraulic actuator 5 may also be a single-acting cylinder movable in one direction by the supply of working oil.

[0016] The hydraulic pump 21 is connected to the oil tank 20 via the suction pipe 30, and to the control valve 4 via the pump pipe 31. The control valve 4 is connected to the oil tank 20 via the oil tank pipe 32, and to the first working chamber 51 and the second working chamber 52 of the hydraulic actuator 5 via a pair of supply and discharge pipes 41 and 42.

[0017] Hydraulic pump 21 is a variable-angle pump. For example, hydraulic pump 21 is an axial piston pump such as a swashplate pump or a swashplate pump. In this embodiment, the minimum discharge flow rate Qmin of hydraulic pump 21 is set to be greater than zero.

[0018] The tilt angle of the hydraulic pump 21 is changed by the adjuster 22. In this embodiment, the adjuster 22 is electrically connected to the control device 7. The control device 7 includes a processing circuit 71, which controls the adjuster 22. For example, if the hydraulic pump 21 is a swashplate pump, the adjuster 22 can be a device that electrically changes the oil pressure acting on a servo piston connected to the swashplate of the hydraulic pump 21, or it can be an electric actuator connected to the swashplate of the hydraulic pump 21.

[0019] However, the regulator 22 does not necessarily have to be controlled by the processing circuit 71; it can also be activated by the pressure of the working oil. For example, the regulator 22 can be a negative flow control method or a load sensing method.

[0020] In this embodiment, the hydraulic system 1 includes an operating device 6 for mobilizing the hydraulic actuator 5. The operating device 6 includes an operating part operated by an operator and outputs an operating signal corresponding to the operating direction and amount of the operating part. The operating part is, for example, an operating lever or a foot pedal.

[0021] In this embodiment, the operating device 6 is an electric joystick that outputs an electrical signal as an operating signal. However, the operating device 6 may also be a pilot-operated valve that outputs a pilot pressure as an operating signal.

[0022] The operation signal output from the operating device 6 is input to the control device 7. The processing circuit 71 of the control device 7 controls the regulator 22 so that the greater the operation amount of the operating part of the operating device 6, the greater the discharge flow of the hydraulic pump 21. That is, when the operating part of the operating device 6 is not operated, in other words, when the hydraulic actuator 5 is not movable and is stopped, the discharge flow of the hydraulic pump 21 is maintained at the minimum discharge flow Qmin.

[0023] Control valve 4 switches between a neutral position and either a first or second operating position. In the neutral position, control valve 4 disconnects pump line 31, oil tank line 32, and a pair of supply and discharge lines 41 and 42. In the first operating position, control valve 4 connects pump line 31 to supply and discharge line 41 and connects supply and discharge line 42 to oil tank line 32. This supplies working oil from hydraulic pump 21 to the first working chamber 51 of hydraulic actuator 5, making hydraulic actuator 5 movable in the first direction X. In the second operating position, control valve 4 connects pump line 31 to supply and discharge line 42 and connects supply and discharge line 41 to oil tank line 32. This supplies working oil from hydraulic pump 21 to the second working chamber 52 of hydraulic actuator 5, making hydraulic actuator 5 movable in the second direction Y.

[0024] In this embodiment, the control valve 4 is controlled by the processing circuit 71 of the control device 7. It should be noted that... Figure 1 To simplify the accompanying drawings, some signal lines have been omitted. Control valve 4 is a spool valve type valve that includes a spool valve and a drive unit that receives command current and drives the spool valve. Command current is supplied to the drive unit from the processing circuit 71. For example, the drive unit may include a pair of electromagnetic proportional valves that output secondary pressure acting on the spool valve in opposite directions, or it may be a direct-acting mechanism connected to the spool valve, including a motor and a ball screw.

[0025] When the operating part of the operating device 6 is operated in the first direction, the processing circuit 71 switches the control valve 4 from the neutral position to the first operating position, and increases the opening area of ​​the inlet and return oil throttling of the control valve 4 as the amount of operation on the operating part increases. Conversely, when the operating part of the operating device 6 is operated in the second direction, the processing circuit 71 switches the control valve 4 from the neutral position to the second operating position, and increases the opening area of ​​the inlet and return oil throttling of the control valve 4 as the amount of operation on the operating part increases.

[0026] However, control valve 4 does not necessarily have to be controlled by processing circuit 71. For example, as described above, when operating device 6 is a pilot-operated valve, control valve 4 can replace the drive unit that receives command current and include a pair of pilot ports, on which a pilot-operated valve is connected.

[0027] The drain line 33 branches off from the pump line 31. The drain line 33 is connected to the oil tank 20. The drain valve 34 described above is installed on the drain line 33.

[0028] In this embodiment, the drain valve 34 has a pilot port. The higher the pilot pressure, the smaller the opening area of ​​the drain valve 34 becomes as it moves from the fully open state to the fully closed state. However, the opening area of ​​the drain valve 34 can also increase as the pilot pressure increases, moving from the fully closed state to the fully open state. Alternatively, the drain valve 34 may not necessarily be actuated by a pilot pressure; it can also be actuated by an electrical signal.

[0029] The drain valve 34 is controlled by the processing circuit 71 of the control device 7 via the solenoid proportional valve 36. Specifically, the pilot port of the drain valve 34 is connected to the secondary pressure port of the solenoid proportional valve 36 via the secondary pressure line 35. The primary pressure port of the solenoid proportional valve 36 is connected to the fixed-displacement hydraulic pump 23 via the primary pressure line 37. The discharge pressure of the hydraulic pump 23 is maintained at the set pressure by the relief valve.

[0030] In this embodiment, the electromagnetic proportional valve 36 is a positively proportional type, where the command current supplied to the electromagnetic proportional valve 36 and the secondary pressure output by the electromagnetic proportional valve 36 are positively correlated. However, the electromagnetic proportional valve 36 can also be an inversely proportional type, where the command current supplied to the electromagnetic proportional valve 36 and the secondary pressure output by the electromagnetic proportional valve 36 are negatively correlated.

[0031] The control device 7 is electrically connected to the first actuator pressure detector 81, the second actuator pressure detector 82, and the oil tank pressure detector 83. The first actuator pressure detector 81 detects the pressure in the first working chamber 51 of the hydraulic actuator 5 as actuator pressure Pa1, and the second actuator pressure detector 82 detects the pressure in the second working chamber 52 of the hydraulic actuator 5 as actuator pressure Pa2. Both actuator pressures Pa1 and Pa2 are the pressures of the working oil inside the hydraulic actuator 5. The oil tank pressure detector 83 detects the oil tank pressure Pt, which is the pressure inside the oil tank 20.

[0032] In this embodiment, the first actuator pressure detector 81 and the second actuator pressure detector 82 are provided on the supply and discharge pipelines 41 and 42, but the first actuator pressure detector 81 and the second actuator pressure detector 82 can also be provided on the hydraulic actuator 5.

[0033] Regarding control device 7, the functions of the elements disclosed in this specification can be performed using a circuit or processing circuit that includes a general-purpose processor, a special-purpose processor, an integrated circuit, an ASIC (Application Specific Integrated Circuit), conventional circuits, and / or combinations thereof configured or programmed to perform the disclosed functions. A processor, because it contains transistors and other circuitry, is considered a processing circuit or circuit. In this disclosure, a circuit, unit, or component is hardware that performs the listed functions, or hardware programmed to perform the listed functions. The hardware can be the hardware disclosed in this specification, or other known hardware programmed or configured to perform the listed functions. Where the hardware is considered a type of circuit, i.e., a processor, the circuit, unit, or component is a combination of hardware and software, with the software used to constitute the hardware and / or processor.

[0034] The processing circuit 71 of the control device 7 determines the inflow flow command Qi to the hydraulic actuator 5 based on the operation signal from the operating device 6. The inflow flow command Qi is the supply flow that the hydraulic pump 21 should supply to the hydraulic actuator 5. That is, the greater the operation amount of the operating part of the operating device 6, the greater the inflow flow command Qi.

[0035] The processing circuit 71 controls the drain valve 34 based on the inflow flow command Qi. When the hydraulic actuator 5 is movable in the first direction X and the second direction Y, if the inflow flow command Qi is greater than the minimum discharge flow rate Qmin, the processing circuit 71 fully closes the drain valve 34. Furthermore, when the operating part of the operating device 6 is not operated and the hydraulic actuator 5 is stopped, i.e., when the inflow flow command Qi is zero, the processing circuit 71 fully opens the drain valve 34.

[0036] On the other hand, when the inflow flow command Qi is greater than zero and less than the minimum discharge flow Qmin, the processing circuit 71 uses the actuator pressure Pa1 detected by the first actuator pressure detector 81 to control the drain valve 34 when the hydraulic actuator 5 is movable in the first direction X, and uses the actuator pressure Pa2 detected by the second actuator pressure detector 82 to control the drain valve 34 when the hydraulic actuator 5 is movable in the second direction Y.

[0037] More specifically, when the hydraulic actuator 5 is movable in the first direction X, the processing circuit 71 controls the drain valve 34 to have an opening area corresponding to the inflow flow command Qi and the actuator pressure Pa1 detected by the first actuator pressure detector 81. In this embodiment, the processing circuit 71 calculates the opening area A of the drain valve 34 based on the inflow flow command Qi and the differential pressure ΔP between the actuator pressure Pa1 detected by the first actuator pressure detector 81 and the tank pressure Pt detected by the tank pressure detector 83, and controls the drain valve 34 to have the calculated opening area A.

[0038] For example, processing circuit 71 calculates the opening area A of drain valve 34 using the following orifice calculation formula with differential pressure ΔP as the target pressure. In this case, before calculating the opening area A, processing circuit 71 calculates the drain flow rate Qb through drain valve 34 by subtracting the inflow flow command Qi from the minimum discharge flow rate Qmin. C in the formula is a coefficient.

[0039] Conversely, when the hydraulic actuator 5 is movable in the second direction Y, the processing circuit 71 controls the drain valve 34 to have an opening area corresponding to the inflow flow command Qi and the actuator pressure Pa2 detected by the second actuator pressure detector 82. In this embodiment, the processing circuit 71 calculates the opening area A of the drain valve 34 based on the inflow flow command Qi and the differential pressure ΔP between the actuator pressure Pa2 detected by the second actuator pressure detector 82 and the tank pressure Pt detected by the tank pressure detector 83, and controls the drain valve 34 to have the calculated opening area A.

[0040] For example, the processing circuit 71 calculates the opening area A of the drain valve 34 using the above-described orifice calculation formula with the differential pressure ΔP as the target pressure. In this case, similarly to the above, before calculating the opening area A, the processing circuit 71 calculates the drain flow rate Qb through the drain valve 34 by subtracting the inflow flow command Qi from the minimum discharge flow rate Qmin.

[0041] As explained above, in the hydraulic system 1 of this embodiment, when the hydraulic actuator 5 is movable at a very low speed with the inflow flow command Qi to the hydraulic actuator 5 being less than the minimum discharge flow Qmin of the hydraulic pump 21, the discharge pressure of the hydraulic pump 21 can be quickly built up to the actuator pressure Pa1 or Pa2 because the drain valve 34 has an opening area corresponding to the inflow flow command Qi and the actuator pressure Pa1 or Pa2. Furthermore, the drain flow Qb through the drain valve 34 can be controlled to be the difference between the minimum discharge flow Qmin of the hydraulic pump 21 and the inflow flow command Qi to the hydraulic actuator 5, i.e., the remaining flow. Therefore, the discharge of excess working oil to the oil tank 20 through the drain line 33 is suppressed, resulting in good hydraulic efficiency.

[0042] <Variation Example> This disclosure is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of this disclosure.

[0043] For example, one of the first actuator pressure detector 81 and the second actuator pressure detector 82 can be omitted, and the processing circuit 71 can perform the above-mentioned control only when the hydraulic actuator 5 is movable in either the first direction X or the second direction Y. However, if configured as described in the above embodiment, the discharge pressure of the hydraulic pump 21 can be quickly built up to the actuator pressure Pa1 or Pa2 regardless of whether the hydraulic actuator 5 is movable in either the first direction X or the second direction Y.

[0044] Alternatively, the tank pressure detector 83 can be omitted, and it can be assumed that the tank pressure Pt, which is the downstream pressure of the drain valve 34, is constant at atmospheric pressure, i.e., zero Pascal. In the calculation formula for the throttle orifice, the actuator pressure Pa1 detected by the first actuator pressure detector 81 or the actuator pressure Pa2 detected by the second actuator pressure detector 82 can be used instead of ΔP. However, if configured as described in the above embodiment, the drain flow rate Qb can be controlled more accurately compared to the case where the tank pressure Pt, which is the downstream pressure of the drain valve 34, is assumed to be constant at atmospheric pressure.

[0045] When the machine equipped with the hydraulic system 1 is an unmanned machine, the operating device 6 can be omitted, and the processing circuit 71 determines the inflow command Qi to the hydraulic actuator 5 based on the image captured by the camera.

[0046] Furthermore, when multiple hydraulic actuators 5 and control valves 4 are combined, and the drain line 33 also serves as a central bypass line passing through multiple control valves 4, the drain valve 34 can also be located downstream of all control valves 4.

[0047] When multiple hydraulic actuators 5 and control valves 4 are configured, the processing circuit 71 can control the drain valve 34 based on the maximum actuator pressure among the actuator pressures of the multiple hydraulic actuators 5 when multiple hydraulic actuators 5 are simultaneously actuated.

[0048] Summary As a first aspect, this disclosure provides a hydraulic system comprising: a hydraulic actuator; a control valve connected to the hydraulic actuator; a variable displacement hydraulic pump connected to the control valve via a pump line and having a minimum discharge flow rate greater than zero; an oil tank for storing working oil; a drain valve provided on a drain line branching from the pump line and connected to the oil tank; an actuator pressure detector for detecting actuator pressure, which is the pressure of the working oil in the hydraulic actuator; and a control device comprising a processing circuit that controls the drain valve based on an inflow flow command to the hydraulic actuator, the processing circuit closing the drain valve fully when the inflow flow command is greater than the minimum discharge flow rate, and controlling the drain valve to have an opening area corresponding to the inflow flow command and the actuator pressure detected by the actuator pressure detector when the inflow flow command is greater than zero and less than the minimum discharge flow rate.

[0049] Based on the above configuration, when a hydraulic actuator is actuated at a very low speed, with the inflow command to the hydraulic actuator being less than the minimum discharge flow of the hydraulic pump, the discharge pressure of the hydraulic pump can be rapidly built up to the actuator pressure because the drain valve has an opening area corresponding to the inflow command and the actuator pressure. Furthermore, the drain flow through the drain valve can be controlled to be the difference between the minimum discharge flow of the hydraulic pump and the inflow command to the hydraulic actuator, i.e., the remaining flow. Therefore, the discharge of excess working oil to the oil tank through the drain line is suppressed, resulting in good hydraulic efficiency.

[0050] As a second embodiment, the first embodiment may further include a tank pressure detector that detects the tank pressure, which is the pressure inside the tank. The processing circuit can calculate the opening area of ​​the drain valve based on the inflow flow command and the pressure difference between the actuator pressure detected by the actuator pressure detector and the tank pressure detected by the tank pressure detector. With this configuration, the drain flow rate can be controlled more accurately than if the tank pressure, which is the pressure downstream of the drain valve, is assumed to be atmospheric pressure and remains constant.

[0051] As a third form, in the first or second form, the hydraulic actuator includes a first working chamber movable in a first direction and a second working chamber movable in a second direction. The actuator pressure detector includes a first actuator pressure detector that detects the pressure of the first working chamber as the actuator pressure, and a second actuator pressure detector that detects the pressure of the second working chamber as the actuator pressure. The processing circuit can use the actuator pressure detected by the first actuator pressure detector to control the drain valve when the hydraulic actuator is movable in the first direction, and can use the actuator pressure detected by the second actuator pressure detector to control the drain valve when the hydraulic actuator is movable in the second direction. According to this configuration, regardless of whether the hydraulic actuator is movable in the first or second direction, the discharge pressure of the hydraulic pump can be rapidly built up to the actuator pressure.

[0052] As a fourth form, in any of the first to third forms, for example, the hydraulic system described above may also include an operating device that outputs an operating signal corresponding to the amount of operation on the operating unit, and the processing circuit may determine the inflow flow command based on the operating signal.

[0053] As a fifth aspect, this disclosure provides a control device for a hydraulic circuit in which working oil is supplied from a hydraulic pump to a hydraulic actuator via a control valve, and a drain flow rate for the working oil discharged from the hydraulic pump to a tank is specified by a drain valve. The control device includes a processing circuit that controls the drain valve based on an inflow flow command to the hydraulic actuator. The processing circuit fully closes the drain valve when the inflow flow command is greater than the minimum discharge flow rate of the hydraulic pump, and controls the drain valve to have an opening area corresponding to the inflow flow command and the actuator pressure, which is the working oil pressure within the hydraulic actuator, when the inflow flow command is greater than zero and less than the minimum discharge flow rate.

[0054] Based on the above configuration, when a hydraulic actuator is actuated at a very low speed, with the inflow command to the hydraulic actuator being less than the minimum discharge flow of the hydraulic pump, the discharge pressure of the hydraulic pump can be rapidly built up to the actuator pressure because the drain valve has an opening area corresponding to the inflow command and the actuator pressure. Furthermore, the drain flow through the drain valve can be controlled to be the difference between the minimum discharge flow of the hydraulic pump and the inflow command to the hydraulic actuator, i.e., the remaining flow. Therefore, the discharge of excess working oil to the oil tank through the drain line is suppressed, resulting in good hydraulic efficiency.

[0055] As a sixth form, in the fifth form, the processing circuit can calculate the opening area of ​​the drain valve based on the inflow flow command and the pressure difference between the actuator pressure and the tank pressure (which is the pressure inside the tank). According to this configuration, the drain flow rate can be controlled more accurately compared to the case where the tank pressure (which is the pressure downstream of the drain valve) is assumed to be atmospheric pressure and remains constant.

[0056] As a seventh form, in the fifth or sixth form, the hydraulic actuator includes a first working chamber movable in a first direction and a second working chamber movable in a second direction. The processing circuit, when the hydraulic actuator is movable in the first direction, can use the pressure of the first working chamber as the actuator pressure to control the drain valve; and when the hydraulic actuator is movable in the second direction, it can use the pressure of the second working chamber as the actuator pressure to control the drain valve. According to this configuration, regardless of whether the hydraulic actuator is movable in the first or second direction, the discharge pressure of the hydraulic pump can be rapidly built up to the actuator pressure.

[0057] As an eighth form, in any of the fifth to seventh forms, for example, the processing circuit can determine the inflow flow command based on the operation signal output from the operating device.

Claims

1. A hydraulic system, characterized in that, include: Hydraulic actuators; The control valve connected to the hydraulic actuator; A variable displacement hydraulic pump connected to the control valve via a pump pipeline, with a minimum discharge flow rate greater than zero; An oil tank for storing working oil; An oil drain valve is provided on the oil drain line that branches off from the pump line and connects to the oil tank; An actuator pressure detector that detects the actuator pressure, which is the working oil pressure within the hydraulic actuator; and The control device includes processing circuitry that controls the relief valve based on inflow command to the hydraulic actuator. The processing circuit fully closes the drain valve when the inflow flow command is greater than the minimum discharge flow, and controls the drain valve to have an opening area corresponding to the inflow flow command and the actuator pressure detected by the actuator pressure detector when the inflow flow command is greater than zero and less than the minimum discharge flow.

2. The hydraulic system according to claim 1, characterized in that, It also includes a tank pressure detector that detects the tank pressure, which is the pressure inside the tank. The processing circuit calculates the opening area of ​​the drain valve based on the inflow flow command and the pressure difference between the actuator pressure detected by the actuator pressure detector and the oil tank pressure detected by the oil tank pressure detector.

3. The hydraulic system according to claim 1 or 2, characterized in that, The hydraulic actuator includes a first working chamber movable in a first direction and a second working chamber movable in a second direction. The actuator pressure detector includes a first actuator pressure detector that detects the pressure in the first chamber as the actuator pressure, and a second actuator pressure detector that detects the pressure in the second chamber as the actuator pressure. The processing circuit uses the actuator pressure detected by the first actuator pressure detector to control the drain valve when the hydraulic actuator is movable in the first direction, and uses the actuator pressure detected by the second actuator pressure detector to control the drain valve when the hydraulic actuator is movable in the second direction.

4. The hydraulic system according to claim 1 or 2, characterized in that, It also includes an operating device that outputs an operating signal corresponding to the operating amount of the operating unit. The processing circuit determines the inflow flow command based on the operation signal.

5. A control device for a hydraulic circuit, in which working oil is supplied from a hydraulic pump to a hydraulic actuator via a control valve, and a drain valve specifies the drain flow rate of the working oil discharged from the hydraulic pump to a tank, wherein... The control device includes: The processing circuit controls the drain valve based on the inflow command to the hydraulic actuator. The processing circuit fully closes the drain valve when the inflow flow command is greater than the minimum discharge flow of the hydraulic pump, and controls the drain valve to have an opening area corresponding to the inflow flow command and the actuator pressure, which is the working oil pressure in the hydraulic actuator, when the inflow flow command is greater than zero and less than the minimum discharge flow.

6. The control device according to claim 5, characterized in that, The processing circuit calculates the opening area of ​​the drain valve based on the inflow flow command and the pressure difference between the actuator pressure and the oil tank pressure, which is the pressure inside the oil tank.

7. The control device according to claim 5 or 6, characterized in that, The hydraulic actuator includes a first working chamber movable in a first direction and a second working chamber movable in a second direction. The processing circuit uses the pressure of the first working chamber as the actuator pressure to control the drain valve when the hydraulic actuator is movable in the first direction, and uses the pressure of the second working chamber as the actuator pressure to control the drain valve when the hydraulic actuator is movable in the second direction.

8. The control device according to claim 5 or 6, characterized in that, The processing circuit determines the inflow flow command based on the operation signal output from the operating device.

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