Hydraulic system

Abstract

To provide a hydraulic device capable of improving the controllability in backpressure control.SOLUTION: The hydraulic device includes a first control part for controlling the rotating speed of a first hydraulic pump (P1) after receiving a pressure command signal, a flow amount command signal, and a signal showing the pressure of working oil detected by a first pressure sensor (PS1), and a second control part for controlling the rotating speeds of second hydraulic pumps (P2, P3). When performing the backpressure control of an actuator, the first control part outputs the signal showing the rotating speed of the first hydraulic pump (P1), and when performing the backpressure control of an actuator (10), the second control part controls the rotating speeds of the second hydraulic pumps (P2, P3) in accordance with an instruction by a signal (Rs) showing the rotating speed of the first hydraulic pump (P1) from the first control part.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a hydraulic device.

Background Art

[0002] Conventionally, as a hydraulic device, there is a cushioning device that controls the cushioning force of a press machine (see, for example, Japanese Patent Application Laid-Open No. 2006-315074 (Patent Document 1)). In the above cushioning device, the cushioning pressure is controlled while the hydraulic pump rotates in the reverse direction when the hydraulic cylinder descends (back pressure control).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the case of a hydraulic device that performs back pressure control in an industrial machine such as a press machine, the capacity of the hydraulic pump and the capacity of the motor that drives the hydraulic pump are determined according to the values of pressure and flow rate.

[0005] In particular, in a hydraulic device that uses a large-capacity hydraulic pump, since the inertia of the hydraulic pump and the motor is large, at the start of the operation of back pressure control, the pressure control by reverse rotation is delayed, and surge pressure is likely to occur, resulting in poor controllability.

[0006] The present disclosure proposes a hydraulic device capable of improving the controllability in back pressure control.

Means for Solving the Problems

[0007] The hydraulic device according to the first aspect of the present disclosure is a first hydraulic pump that supplies hydraulic oil from an oil tank to an actuator, A second hydraulic pump supplies hydraulic fluid from the oil tank to the actuator, A first pressure sensor for detecting the pressure of the hydraulic fluid discharged by the first hydraulic pump, A first control unit that receives a pressure command signal, a flow command signal, and a signal representing the pressure of the hydraulic fluid detected by the first pressure sensor, and controls the rotational speed of the first hydraulic pump, A second control unit that controls the rotation speed of the second hydraulic pump mentioned above, Equipped with, When the first control unit controls the back pressure of the actuator, it outputs a signal representing the rotational speed of the first hydraulic pump. When the second control unit controls the back pressure of the actuator, it controls the rotational speed of the second hydraulic pump based on a signal from the first control unit indicating the rotational speed of the first hydraulic pump.

[0008] According to this disclosure, by performing back pressure control using the first and second hydraulic pumps, the inertia of each of the first and second hydraulic pumps can be reduced compared to a single large-capacity hydraulic pump, thereby improving the controllability in back pressure control.

[0009] Furthermore, the hydraulic system according to the second aspect of this disclosure is In the hydraulic system of the first embodiment, When the second control unit controls the back pressure of the actuator, it controls the rotational speed of the second hydraulic pump to be less than or equal to the rotational speed of the first hydraulic pump.

[0010] According to this disclosure, when controlling the back pressure of an actuator, the rotational speed of the second hydraulic pump is controlled by the second control unit to be less than or equal to the rotational speed of the first hydraulic pump, thereby suppressing interference between the first and second hydraulic pumps that would otherwise reduce controllability.

[0011] Furthermore, the hydraulic system according to the third aspect of this disclosure is In the hydraulic system of the first or second embodiment, When the second control unit controls the back pressure of the actuator, it receives a signal representing a pressure limit value that is a predetermined pressure higher than the pressure command value represented by the pressure command signal, and controls the rotational speed of the second hydraulic pump so that the discharge pressure of the second hydraulic pump does not exceed the pressure limit value.

[0012] According to this disclosure, when controlling the back pressure of the actuator, the discharge pressure of the second hydraulic pump does not exceed the pressure limit value (= pressure command value + predetermined pressure), thereby suppressing interference between the first hydraulic pump and the second hydraulic pump that reduces controllability.

[0013] Furthermore, the hydraulic system according to the fourth aspect of this disclosure is In any one of the first to third embodiments of the hydraulic system, A relief valve that returns the hydraulic fluid discharged from the actuator to the oil tank, A third hydraulic pump, with its discharge side connected to the vent port of the above-mentioned relief valve, A second pressure sensor detects the vent pressure of the relief valve mentioned above, A third control unit controls the rotation speed of the third hydraulic pump in accordance with the vent pressure of the relief valve detected by the second pressure sensor. Equipped with, When the third control unit controls the back pressure of the actuator, it controls the set pressure of the relief valve by controlling the vent pressure of the relief valve with the pressure of the hydraulic fluid discharged from the third hydraulic pump.

[0014] According to this disclosure, when back pressure control occurs, if surge pressure is generated in the actuator and the back pressure of the actuator exceeds the set pressure of the relief valve, the relief valve operates and returns the hydraulic fluid discharged from the actuator to the oil tank, thereby suppressing the surge pressure generated from the actuator. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic block diagram of a hydraulic system according to the first embodiment of the present disclosure. [Figure 2] It is a control block diagram of the hydraulic device of the first embodiment. [Figure 3] It is a flowchart for explaining the back pressure control of the hydraulic device of the second embodiment. [Figure 4] It is a schematic block diagram of the hydraulic device of the second embodiment of the present disclosure. [Figure 5] It is a control block diagram of the hydraulic device of the second embodiment. [Figure 6] It is a diagram showing an example of the pressure change during the back pressure control of the hydraulic device of the second embodiment.

Mode for Carrying Out the Invention

[0016] Hereinafter, embodiments will be described. In the drawings, the same reference numerals represent the same or corresponding parts.

[0017] 〔First Embodiment〕 FIG. 1 is a schematic block diagram of the hydraulic device of the first embodiment of the present disclosure. The hydraulic device of this first embodiment supplies hydraulic oil to a hydraulic cylinder such as a press machine.

[0018] As shown in FIG. 1, the hydraulic device of this first embodiment includes a main hydraulic pump P1 that supplies hydraulic oil from an oil tank T to a hydraulic cylinder 10, a motor M1 that drives the hydraulic pump P1, a sub-hydraulic pump P2 that supplies hydraulic oil from the oil tank T to the hydraulic cylinder 10, a motor M2 that drives the sub-hydraulic pump P2, a sub-hydraulic pump P3 that supplies hydraulic oil from the oil tank T to the hydraulic cylinder 10, and a motor M3 that drives the sub-hydraulic pump P2. The hydraulic cylinder 10 is an example of an actuator.

[0019] Hydraulic pumps with the same performance (the discharge flow rate per revolution is the same) are used for the main hydraulic pump P1, the sub-hydraulic pump P2, and the sub-hydraulic pump P3, and motors with the same performance (for example, equivalent to a motor capacity of 37 kW) are used for the motor M1, the motor M2, and the motor M3. <​Main hydraulic pump P1 is an example of a first hydraulic pump. Sub-hydraulic pumps P2 and P3 are examples of second hydraulic pumps. Pressure sensor PS1 is an example of a first pressure sensor.

[0021] The hydraulic cylinder 10 includes a cylinder tube 11, a piston 12 that reciprocates within the cylinder tube 11, and a piston rod 13 with one end connected to the piston 12. Port 10a of the hydraulic cylinder 10 is connected to the discharge side of the main hydraulic pump P1.

[0022] The discharge side of sub-hydraulic pump P2 is connected to port 10a of hydraulic cylinder 10 via solenoid valve 20. The discharge side of sub-hydraulic pump P3 is connected to port 10a of hydraulic cylinder 10 via solenoid valve 30.

[0023] When the solenoid 23 is energized, the solenoid valve 20 is in the left switching position, and the first port 21 and the second port 22 are in communication. On the other hand, when the solenoid 23 is de-energized, the solenoid valve 20 is in the right switching position, and the first port 21 and the second port 22 are closed, respectively.

[0024] When the solenoid 33 is energized, the solenoid valve 30 is in the left switching position, and the first port 31 and the second port 32 are in communication. On the other hand, when the solenoid 33 is de-energized, the solenoid valve 30 is in the right switching position, and the first port 31 and the second port 32 are closed, respectively.

[0025] When controlling the upward movement of the piston 12 of the hydraulic cylinder 10, the above hydraulic system de-energizes the solenoids 23 and 33 of the solenoid valves 20 and 30 to the right-side switching position, closing the first port 21 and the second port 22 of solenoid valve 20, and closing the first port 31 and the second port 32 of solenoid valve 30. On the other hand, when controlling the back pressure to lower the piston 12 of the hydraulic cylinder 10, the solenoids 23 and 33 of the solenoid valves 20 and 30 are energized to the left-side switching position, connecting the first port 21 and the second port 22 of solenoid valve 20, and connecting the first port 31 and the second port 32 of solenoid valve 30.

[0026] The pressure sensor PS1 detects the pressure of the hydraulic fluid in the flow path between the port 10a of the hydraulic cylinder 10 and the main hydraulic pump P1.

[0027] Figure 2 is a control block diagram of the hydraulic system.

[0028] As shown in Figure 2, the hydraulic system includes a main control unit 101 that controls the rotational speed of the main hydraulic pump P1 in response to a pressure command signal Pi, a flow rate command signal Qi, a back pressure command signal BP, and a signal representing the hydraulic fluid pressure detected by a pressure sensor PS1 from the main controller 200; a sub-control unit 102 that controls the rotational speed of the sub-hydraulic pump P2 in response to a pressure limit signal Pis from the main controller 200; and a sub-control unit 103 that controls the rotational speed of the sub-hydraulic pump P3 in response to a pressure limit signal Pis from the main controller 200. The main control unit 101 is an example of a first control unit. The sub-control units 102 and 103 are examples of second control units.

[0029] The pressure limit value pis, represented by the pressure limit signal Pi from the main controller 200, is the pressure obtained by adding a predetermined pressure α to the pressure command value pi, represented by the pressure command signal Pi.

[0030] The back pressure command signal BP is output from the main controller 200 when back pressure control is initiated. When the back pressure command signal BP is not input to the main control unit 101, and the control is to raise the piston 12 of the hydraulic cylinder 10, the main control unit 101 de-energizes the solenoids 23 and 33 of the solenoid valves 20 and 30 and sets them to the right switching position. Then, the main control unit 101 controls the motor M1 in accordance with the pressure command signal Pi and flow command signal Qi from the main controller 200 to rotate the main hydraulic pump P1 in the forward direction. At this time, the solenoid valves 20 and 30 are closed and the sub-hydraulic pumps P2 and P3 are stopped.

[0031] On the other hand, when a back pressure command signal BP is input to the main control unit 101 and back pressure control is performed to lower the piston 12 of the hydraulic cylinder 10, the main control unit 101 energizes the solenoids 23 and 33 of the solenoid valves 20 and 30 to the left switching position. Then, the main control unit 101 controls the motor M1 in accordance with the pressure command signal Pi and flow command signal Qi from the main unit controller 200 to reverse rotation of the main hydraulic pump P1 and outputs a rotation speed signal Rs representing the rotation speed of the main hydraulic pump P1.

[0032] At this time, the sub-control unit 102 receives a rotation speed signal Rs from the main control unit 101 and controls the motor M2 to rotate the sub-hydraulic pump P2 in the opposite direction at the same rotation speed as the main hydraulic pump P1. At this time, the sub-control unit 102 controls the sub-hydraulic pump P2 so that the pressure is less than the pressure indicated by the pressure limit signal Pis from the main controller 200.

[0033] Similarly, the sub-control unit 103 receives a rotational speed signal Rs from the main control unit 101 and controls the motor M3 to rotate the sub-hydraulic pump P3 in the opposite direction at the same rotational speed as the main hydraulic pump P1. At this time, the sub-control unit 103 controls the sub-hydraulic pump P3 so that the pressure is less than the pressure indicated by the pressure limit signal Pis from the main controller 200.

[0034] Here, the main control unit 101 controls the rotation speed of the main hydraulic pump P1 so that the hydraulic fluid pressure detected by the pressure sensor PS1 becomes equal to the pressure represented by the pressure command signal Pi.

[0035] In this case, the sub-hydraulic pumps P2 and P3 were rotated in the opposite direction at the same rotational speed as the main hydraulic pump P1 based on the rotational speed signal Rs from the main control unit 101. However, the rotational speed signal Rs from the main control unit 101 may also be used as a rotational speed limit, and the sub-hydraulic pumps P2 and P3 may be rotated in the opposite direction at a rotational speed less than that of the main hydraulic pump P1. In other words, when the sub-control units 102 and 103 perform back pressure control of the hydraulic cylinder 10, they only need to control the rotational speed of the second hydraulic pumps P2 and P3 based on instructions from the rotational speed signal Rs representing the rotational speed of the first hydraulic pump P1 from the main control unit 101.

[0036] Furthermore, the main hydraulic pump P1 and the sub-hydraulic pumps P2 and P3 may be hydraulic pumps with different performance characteristics, and the motors M1, M2, and M3 may be motors with different performance characteristics. In this case, the main control unit 101 and the sub-control units 102 and 103 will respectively appropriately control the rotational speed of the hydraulic pumps according to the performance characteristics of the hydraulic pumps and motors.

[0037] Next, we will explain back pressure control of the hydraulic system according to the flowchart in Figure 3.

[0038] First, when the back pressure control of the hydraulic system starts, the process proceeds to step S1 shown in Figure 3, and a back pressure command signal BP is input to the main control unit 101.

[0039] Next, the process proceeds to step S2, where the main control unit 101 reverses the rotation of the main hydraulic pump P1 to perform pressure feedback control (pressure control). More specifically, in pressure feedback control, the rotation speed of the main hydraulic pump P1 is feedback-controlled so that the hydraulic fluid pressure detected by the pressure sensor PS1 becomes equal to the pressure represented by the pressure command signal Pi.

[0040] Next, the process proceeds to step S3, where the rotation speed signal Rs is input from the main control unit 101 to the sub-control units 102 and 103.

[0041] Next, the process proceeds to step S4, where the sub-control units 102 and 103 control the rotation speed of the sub-hydraulic pumps P2 and P3 by rotating them in opposite directions. Specifically, sub-control unit 102 controls sub-hydraulic pump P2 so that it reaches the rotation speed indicated by the rotation speed signal Rs, while sub-control unit 103 controls sub-hydraulic pump P3 so that it reaches the rotation speed indicated by the rotation speed signal Rs. Here, sub-control units 102 and 103 control only the rotation speed of sub-hydraulic pumps P2 and P3 and do not perform pressure control; only the main control unit 101 performs pressure control.

[0042] With the hydraulic system configured as described above, back pressure control is performed by the main hydraulic pump P1 (first hydraulic pump) and sub-hydraulic pumps P2 and P3 (second hydraulic pumps). Compared to a single large-capacity hydraulic pump, the inertia of the main hydraulic pump P1 and the sub-hydraulic pumps P2 and P3 can be reduced, thereby improving the controllability of back pressure control and enabling the hydraulic cylinder 10 to be lowered at high speed. In addition, reducing the inertia of the main hydraulic pump P1 and the sub-hydraulic pumps P2 and P3 can suppress surge generation.

[0043] In the case of high-flow hydraulic systems, it is necessary to select hydraulic pumps and motors that match the high flow rate, which results in very high prices for the hydraulic pumps and motors. Generally, motor prices increase sharply above 37 kW, and hydraulic pumps are generally limited to a flow rate of around 300 L / min. For example, using a single hydraulic pump in the 110 kW class to handle a flow rate of 900 L / min would be extremely expensive and difficult to obtain, leading to problems such as difficulty in dealing with malfunctions. In contrast, the hydraulic system disclosed herein uses multiple general-purpose, low-cost hydraulic pumps and motors, improving the availability of hydraulic pumps and motors and enabling quick response in the event of a malfunction, thereby significantly improving maintainability.

[0044] Furthermore, when controlling the back pressure of the hydraulic cylinder 10 (actuator), the rotational speeds of the sub-hydraulic pumps P2 and P3 are controlled by the sub-control units 102 and 103 (second control units) to be less than or equal to the rotational speed of the main hydraulic pump P1. This suppresses interference between the main hydraulic pump P1 and the sub-hydraulic pumps P2 and P3, which would otherwise reduce controllability.

[0045] Furthermore, when controlling the back pressure of the hydraulic cylinder 10, by ensuring that the discharge pressure of the sub-hydraulic pumps P2 and P3 does not exceed the pressure limit value pis (= pressure command value pi + predetermined pressure α) represented by the pressure limit signal Pis, it is possible to suppress interference between the main hydraulic pump P1 and the sub-hydraulic pumps P2 and P3, which would otherwise reduce controllability.

[0046] In the hydraulic system of the first embodiment described above, when controlling the raising of the piston 12 of the hydraulic cylinder 10, the solenoid valves 20 and 30 were closed and the hydraulic cylinder 10 was driven using only the main hydraulic pump P1. However, the piston 12 of the hydraulic cylinder 10 may be raised using the main hydraulic pump P1 and sub-hydraulic pumps P2 and P3 without using the solenoid valves 20 and 30.

[0047] [Second Embodiment] Figure 4 is a schematic block diagram of a hydraulic system according to a second embodiment of the present disclosure. This hydraulic system of the second embodiment has the same configuration as the hydraulic system of the first embodiment, except for the relief valve 40, motor M4, hydraulic pump P4, pressure sensor PS2, and relief valve control unit 104.

[0048] The hydraulic system of the second embodiment includes a relief valve 40 that returns the hydraulic fluid discharged from the hydraulic cylinder 10 to the oil tank T, a pressure sensor PS2 that detects the vent pressure of the relief valve 40, a hydraulic pump P4 that controls the set pressure Pset of the relief valve 40, and a motor M4 that drives the hydraulic pump P4. The hydraulic pump P4 is an example of a third hydraulic pump. The pressure sensor PS2 is an example of a second pressure sensor.

[0049] Port 10a of the hydraulic cylinder 10 is connected to the inlet port 41 of the relief valve 40, and the outlet port 42 of the relief valve 40 is connected to the oil tank T. The relief valve 40 is a pilot-operated relief valve, and the discharge side of the hydraulic pump P4 is connected to the vent port 43 of the relief valve 40. As a result, the discharge pressure of the hydraulic pump P4 is supplied to the vent port 43 of the relief valve 40, and the set pressure Pset of the relief valve 40 is controlled.

[0050] Figure 5 is a control block diagram of the hydraulic system of the second embodiment. The control block of this hydraulic system of the second embodiment has the same configuration as the control block of the hydraulic system of the first embodiment, except for the relief valve control unit 104. The relief valve control unit 104 is an example of a third control unit.

[0051] The control for raising the piston 12 of the hydraulic cylinder 10 and the back pressure control for lowering the piston 12 of the hydraulic cylinder 10 are the same as those of the hydraulic system in the first embodiment, except for the operation of the relief valve 40.

[0052] The relief valve control unit 104 receives a pressure command signal Pi, a flow rate command signal Qi, a back pressure command signal BP, and a signal representing the hydraulic fluid pressure detected by the pressure sensor PS2 from the main unit controller 200, and controls the rotation speed of the hydraulic pump P4.

[0053] Figure 6 shows an example of pressure change during back pressure control of the hydraulic system of the second embodiment. In Figure 6, the vertical axis represents pressure [MPa] and the horizontal axis represents time [arbitrary scale].

[0054] In the example shown in Figure 6, the target pressure for back pressure control of the hydraulic cylinder 10 is set to 20 MPa. Furthermore, the relief valve control unit 104 controls the hydraulic pump P4 to set the relief valve 40's set pressure Pset to 20 MPa.

[0055] When back pressure control of the hydraulic cylinder 10 is initiated, the main hydraulic pump P1 and the sub-hydraulic pumps P2 and P3 rotate in opposite directions, similar to the first embodiment.

[0056] For example, in back pressure control of a press machine, surge pressure is generated in the hydraulic cylinder 10. When the pressure at port 10a of the hydraulic cylinder 10 exceeds the set pressure Pset of the relief valve 40, the relief valve 40 operates and returns the hydraulic fluid discharged from the hydraulic cylinder 10 to the oil tank T. This suppresses the surge pressure generated from the hydraulic cylinder 10.

[0057] Subsequently, the relief valve control unit 104 controls the hydraulic pump P4 to change the set pressure Pset of the relief valve 40 to 22 MPa, thereby closing the relief valve 40. After the relief valve 40 is closed, the main hydraulic pump P1 and sub-hydraulic pumps P2 and P3 return the hydraulic fluid discharged from the hydraulic cylinder 10 to the oil tank T.

[0058] With the hydraulic system configured as described above, when back pressure control is performed, surge pressure is generated in the hydraulic cylinder 10 (actuator). When the back pressure of the hydraulic cylinder 10 exceeds the set pressure Pset of the relief valve 40, the relief valve 40 operates and returns the hydraulic fluid discharged from the hydraulic cylinder 10 to the oil tank T, thereby suppressing the surge pressure generated from the hydraulic cylinder 10.

[0059] The hydraulic system of the second embodiment described above has the same effects as the hydraulic system of the first embodiment.

[0060] In the first and second embodiments described above, a single-rod hydraulic cylinder was used as the actuator for the hydraulic cylinder 10, but a double-rod hydraulic cylinder or a hydraulic motor may also be used.

[0061] In the first and second embodiments described above, sub-hydraulic pumps P2 and P3 were used as the second hydraulic pump, but the number of second hydraulic pumps may be 1 or 3 or more.

[0062] In the first and second embodiments described above, a hydraulic system was described in which the main hydraulic pump P1 (first hydraulic pump), sub-hydraulic pump P2 (second hydraulic pump), and sub-hydraulic pump P3 (second hydraulic pump) all have the same performance, and the motors M1, M2, and M3 all have the same performance. However, the performance of the first and second hydraulic pumps and the motors may differ. In that case, it is preferable that the first control unit that controls the first hydraulic pump and the second control unit that controls the second hydraulic pump appropriately control the rotational speed of the hydraulic pumps according to the performance of the hydraulic pumps and motors being controlled.

[0063] While specific embodiments of this disclosure have been described, this disclosure is not limited to the first and second embodiments described above, and can be implemented with various modifications within the scope of this disclosure. [Explanation of Symbols]

[0064] 10… Hydraulic cylinder (actuator) 10a...port 11…Cylinder tube 12... Piston 13…Piston rod 20, 30… Solenoid valves 40…Relief valve 101...Main Control Unit (First Control Unit) 102, 103... Sub-control units (2nd control unit) 104…Relief valve control unit (3rd control unit) M1, M2, M3, M4… Motors P1…Main hydraulic pump (first hydraulic pump) P2, P3... Sub-hydraulic pumps (second hydraulic pumps) P4…Hydraulic pump (3rd hydraulic pump) PS1... Pressure sensor (first pressure sensor) PS2... Pressure sensor (second pressure sensor) T... Oil tank

Claims

1. A first hydraulic pump (P1) supplies hydraulic fluid from an oil tank (T) to the actuator (10), A second hydraulic pump (P2, P3) supplies hydraulic fluid from the oil tank (T) to the actuator (10), A first pressure sensor (PS1) detects the pressure of the hydraulic fluid discharged by the first hydraulic pump (P1) described above, A first control unit (101) controls the rotational speed of the first hydraulic pump (P1) in response to a pressure command signal (Pi), a flow rate command signal (Qi), and a signal representing the pressure of the hydraulic fluid detected by the first pressure sensor (PS1). The second control unit (102, 103) controls the rotational speed of the second hydraulic pump (P2, P3) mentioned above. Equipped with, When the first control unit (101) controls the back pressure of the actuator, it outputs a signal (Rs) representing the rotational speed of the first hydraulic pump (P1). The second control unit (102, 103) controls the rotational speed of the second hydraulic pump (P2, P3) based on a signal (Rs) from the first control unit (101) indicating the rotational speed of the first hydraulic pump (P1) when controlling the back pressure of the actuator (10).

2. In the hydraulic system according to claim 1, The second control unit (102, 103) controls the rotational speed of the second hydraulic pump (P2, P3) to be less than or equal to the rotational speed of the first hydraulic pump (P1) when controlling the back pressure of the actuator (10).

3. In the hydraulic system according to claim 1 or 2, The second control unit (102, 103) described above controls the back pressure of the actuator (10) and, upon receiving a signal (Pis) representing a pressure limit value that is a predetermined pressure higher than the pressure command value represented by the pressure command signal (Pi), controls the rotational speed of the second hydraulic pumps (P2, P3) so that the discharge pressure of the second hydraulic pumps (P2, P3) does not exceed the pressure limit value.

4. In the hydraulic system according to claim 1 or 2, A relief valve (40) returns the hydraulic fluid discharged from the actuator (10) to the oil tank (T), A third hydraulic pump (P4) is connected to the vent port of the above-mentioned relief valve (40) on its discharge side, A second pressure sensor (PS2) detects the vent pressure of the above-mentioned relief valve (40), A third control unit (104) controls the rotation speed of the third hydraulic pump (P4) in accordance with the vent pressure of the relief valve (40) detected by the second pressure sensor (PS2) and Equipped with, The third control unit (104) controls the set pressure of the relief valve (40) by controlling the vent pressure of the relief valve (40) with the pressure of the hydraulic fluid discharged from the third hydraulic pump (P4) when controlling the back pressure of the actuator (10).

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