Pump control device and fluid pressure control system

Abstract

To realize various controls in a fluid pressure control system.SOLUTION: A discharge control unit 31 of a pump control device 30 has: a relief valve 40 that is provided in a discharge passage 15 and opens when a pressure in the discharge passage 15 reaches a predetermined valve opening pressure; a downstream throttle 32 that applies resistance to a flow of hydraulic oil that is guided from a supply passage 11 to the discharge passage 15 and passes through the relief valve 40; and a pressure sensor 35a that measures the pressure of the hydraulic oil guided from the relief valve 40 to a resistance unit. The controller 50 increases a discharge flow rate of a pump P as an operation input increases, pre-stores pressure-flow characteristics that are the relation between the pressure loss in the downstream throttle 32 and a passing flow rate, acquires a flow rate of the hydraulic oil passing through the downstream throttle 32 based on the pressure-flow characteristics of the downstream throttle 32 and the pressure measured by the pressure sensor 35a, and controls a discharge capacity of the pump P and an opening pressure of the relief valve 40 according to the flow rate of the hydraulic oil passing through the downstream throttle 32.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a pump control device and a fluid pressure control system. [Background technology]

[0002] A known fluid pressure control device for construction machinery uses a control valve to control the flow of working fluid guided from a pump to a fluid pressure actuator in response to operational input from an operator using an operating lever, etc., and acquires a pressure signal in a fluid pressure circuit and controls the pump's discharge capacity based on the pressure signal (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-303809 Summary of the Invention [Problem to be solved by the invention]

[0004] In conventional fluid pressure control systems, the flow rate of the working fluid supplied to the fluid pressure actuator and the discharge pressure of the pump are determined by the opening area of ​​the control valve and the discharge capacity of the pump, respectively. Therefore, when either the opening area of ​​the control valve or the discharge capacity of the pump is changed, the flow rate of the working fluid supplied to the fluid pressure actuator and the discharge pressure of the pump are changed accordingly. In contrast, in fluid pressure control systems, there is a need to realize various controls over the flow rate of the working fluid supplied to the fluid pressure actuator and the discharge pressure of the pump.

[0005] The present invention has been made in consideration of the above problems, and has an object to provide a pump control device and a fluid pressure control system that improve the versatility of control. [Means for solving the problem]

[0006] The present invention is a pump control device that controls the discharge capacity of a pump in a fluid pressure control system, and includes a discharge passage that guides working fluid in a supply passage to a tank, a discharge control unit that controls the flow of the working fluid discharged to the tank through the discharge passage, and a controller that controls the discharge capacity of the pump in response to an operation input by an operator. The discharge control unit has a relief valve that is provided in the discharge passage and opens when the pressure in the discharge passage reaches a predetermined valve opening pressure, and is configured so that the valve opening pressure is variable according to an input electrical signal, a resistance unit that applies resistance to the flow of working fluid that is guided from the supply passage to the discharge passage and passes through the relief valve, and a pressure measuring unit for obtaining a differential pressure before and after the resistance unit. The controller pre-stores pressure-flow characteristics that are the relationship between the pressure in the resistance unit and the passing flow rate, obtains the flow rate of the working fluid passing through the resistance unit based on the pressure-flow characteristics of the resistance unit and the pressure obtained by the pressure measuring unit, and changes at least one of the discharge capacity of the pump and the valve opening pressure of the relief valve in response to the flow rate of the working fluid passing through the resistance unit.

[0007] The present invention also provides a pump control device for controlling the discharge capacity of a pump in a fluid pressure control system, the pump control device comprising: a discharge passage for leading working fluid in a supply passage to a tank; a discharge control unit for controlling the flow of the working fluid discharged to the tank through the discharge passage; and a controller for controlling the discharge capacity of the pump in response to an operation input by an operator, the discharge control unit having: a relief valve provided in the discharge passage to open when the pressure in the discharge passage reaches a predetermined valve opening pressure and configured to vary the valve opening pressure in response to an input electrical signal; a resistance portion provided in the discharge passage for providing resistance to the flow of working fluid from the supply passage toward the relief valve; and a pressure measuring unit for obtaining a differential pressure before and after the resistance portion, the controller pre-stores pressure-flow characteristics which are the relationship between the pressure at the resistance portion and the passing flow rate, obtains the flow rate of the working fluid passing through the resistance portion based on the pressure-flow characteristics of the resistance portion and the differential pressure before and after obtained by the pressure measuring unit, and controls the discharge capacity of the pump and the valve opening pressure of the relief valve in response to the flow rate of the working fluid passing through the resistance portion.

[0008] In these inventions, at least one of the opening pressure of the relief valve and the discharge capacity of the pump is changed according to the flow rate of the working fluid passing through the resistance part of the discharge control part. The opening pressure of the relief valve and the discharge capacity of the pump can be adjusted independently or in cooperation with each other based on the pressure-flow rate characteristics of the resistance part. Therefore, even if the opening area of ​​the control valve and the operation input by the operator are the same, the pressure and flow rate of the working fluid led to the actuator can be increased by increasing the opening pressure of the relief valve according to an increase in the flow rate passing through the resistance part. In addition, it is also possible to reduce the discharge capacity of the pump by an amount corresponding to an increase in the opening pressure of the relief valve, thereby maintaining the pressure and flow rate of the working fluid led to the fluid pressure actuator and reducing only the flow rate passing through the resistance part. In this way, even if the operation input by the operator is the same, various controls over the operation of the fluid pressure actuator can be realized.

[0009] In addition, the present invention is characterized in that the discharge control portion has an additional resistance portion that is provided in the discharge passage and applies resistance to the flow of the working fluid from the supply passage toward the relief valve.

[0010] In this invention, the pressure-flow characteristics of the discharge control unit as a whole are affected by the pressure-flow characteristics of the additional resistor. By adjusting the pressure-flow characteristics of the additional resistor, the pressure-flow characteristics of the discharge control unit as a whole can be adjusted, improving the design freedom of the pressure-flow characteristics of the discharge control unit.

[0011] The present invention is also characterized in that the controller increases the valve opening pressure of the relief valve and reduces the discharge capacity of the pump in response to an increase in the flow rate of the working fluid passing through the resistance portion.

[0012] In this invention, the flow rate of the working fluid discharged to the tank through the discharge passage can be reduced by increasing the valve opening pressure of the relief valve. Also, by reducing the flow rate of the working fluid discharged to the tank through the discharge passage, the flow rate of the working fluid supplied from the pump to the fluid pressure actuator can be increased, so that the flow rate supplied to the fluid pressure actuator can be secured even if the discharge capacity of the pump is reduced. Therefore, the amount of work performed by the pump can be maintained while the energy consumed can be reduced.

[0013] The present invention is also characterized in that the pressure measuring unit further includes an upstream pressure measuring unit that measures the pressure of the working fluid guided from the relief valve to the resistance unit, and a tank pressure measuring unit that measures the pressure of the tank, and the controller obtains the flow rate of the working fluid passing through the resistance unit based on the differential pressure between the pressure measured by the upstream pressure measuring unit and the pressure measured by the tank pressure measuring unit.

[0014] In this invention, the flow rate of the working fluid passing through the resistance portion is obtained based on the actual measured value of the tank pressure, so that the flow rate can be obtained with high accuracy even if the tank pressure fluctuates.

[0015] The present invention also provides a fluid pressure control system for controlling the operation of a fluid pressure actuator, the system comprising: a variable displacement pump that discharges working fluid through a supply passage; a control valve that has a neutral position in which the working fluid discharged from the pump is guided to a tank when in a neutral position in which there is no operational input from an operator, and that controls the flow of working fluid supplied to and discharged from the fluid pressure actuator in response to the operational input; and a pump control device that controls the discharge capacity of the pump. The pump control device comprises a discharge passage that guides the working fluid in the supply passage to the tank, a discharge control unit that controls the flow of working fluid discharged to the tank through the discharge passage, and a controller that controls the capacity of the pump in response to operational input from an operator who operates the fluid pressure actuator. the discharge control unit has a relief valve that is provided in the discharge passage and opens when the pressure in the discharge passage reaches a predetermined valve opening pressure, and is configured so that the valve opening pressure can be varied by an input electrical signal; a resistance unit that applies resistance to the flow of working fluid that is guided from the supply passage to the discharge passage and passes through the relief valve; and a pressure measuring unit for obtaining a differential pressure before and after the resistance unit, the controller pre-stores pressure-flow characteristics that are the relationship between the pressure loss in the resistance unit and the passing flow rate, obtains the flow rate of the working fluid passing through the resistance unit based on the pressure-flow characteristics of the resistance unit and the differential pressure before and after obtained by the pressure measuring unit, and controls the discharge capacity of the pump and the valve opening pressure of the relief valve in accordance with the flow rate of the working fluid passing through the resistance unit.

[0016] Conventionally, negative control is known as a method for controlling the discharge capacity of a pump, in which the discharge capacity of the pump is controlled according to the pressure difference between before and after a throttle provided downstream of a control valve. In contrast, in this invention, the discharge capacity of the pump is controlled according to the operation input of the operator, so that the throttle provided for controlling the discharge capacity of the pump in negative control can be eliminated. Therefore, the working fluid discharged from the pump in neutral does not pass through the throttle downstream of the control valve as in negative control, so that energy loss can be suppressed. In addition, the valve opening pressure of the relief valve or the discharge capacity of the pump is controlled according to the flow rate of the working fluid passing through the resistance part of the discharge control part, so that the flow rate of the working fluid discharged from the pump and discharged to the tank through the discharge passage can be reduced.

[0017] The present invention is also characterized in that the control valve has one or more operating positions that connect the supply passage and the fluid pressure actuator, and in any of the operating positions, the communication between the supply passage and the tank is blocked.

[0018] Some control valves that control the operation of a fluid pressure actuator control the flow rate of the working fluid supplied to the fluid pressure actuator by connecting a supply passage and a tank while providing resistance by throttling or the like in the operating position. Compared to such conventional control valves, the present invention is configured to block the supply passage and the tank in the operating position because the flow rate of the working fluid supplied to the fluid pressure actuator can be controlled by controlling the valve opening pressure of the relief valve in the discharge control section. Therefore, there is no need to provide the control valve with a throttle or the like that controls the flow rate from the supply passage to the tank in the operating position, and the configuration of the control valve can be simplified. Effect of the Invention

[0019] According to the present invention, a variety of controls can be realized by the pump control device and the fluid pressure control system. [Brief description of the drawings]

[0020] [Figure 1] 1 is a hydraulic circuit diagram of a fluid pressure control system according to a first embodiment of the present invention. [Diagram 2] 1 is a block diagram of a fluid pressure control system according to a first embodiment of the present invention. [Diagram 3] 1 is a hydraulic circuit diagram showing a configuration of a relief valve according to a first embodiment of the present invention. FIG. [Figure 4] FIG. 4 is a graph showing pressure-flow rate characteristics of a downstream throttle in the first embodiment of the present invention. [Diagram 5] FIG. 4 is a graph showing pressure-flow rate characteristics of a discharge control section in the first embodiment of the present invention. [Figure 6] FIG. 2 is a flowchart showing a control method according to the first embodiment of the present invention. [Figure 7]FIG. 5 is a hydraulic circuit diagram of a fluid pressure control system according to a second embodiment of the present invention. [Figure 8] FIG. 4 is a hydraulic circuit diagram of a fluid pressure system according to a comparative example of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] (First embodiment) A pump control device 30 according to an embodiment of the present invention and a fluid pressure control system 100 including the pump control device 30 will be described below with reference to the drawings. In the following, a pump control device 30 and a fluid pressure control system 100 used in a construction machine (e.g., a hydraulic excavator) will be described as an example.

[0022] The fluid pressure control system 100 includes an operating lever 1 as an operating part operated by an operator, a hydraulic cylinder 2 as a fluid pressure actuator that drives a driven object, a variable displacement pump P that discharges hydraulic oil as a working fluid, a tank T that stores the hydraulic oil, and a fluid pressure control device 10 that controls the operation of the hydraulic cylinder 2.

[0023] The operating lever 1 is operated by an operator to drive the hydraulic cylinder 2, and an operation input according to the operating direction and amount of the operation is input as an electric signal (operation signal) to the fluid pressure control device 10. Also, according to the operating direction and amount of the operation of the operating lever 1, a pilot pressure is supplied to a control valve 20 of the fluid pressure control device 10, which will be described later.

[0024] The pump P is driven by an engine (not shown) or a motor (not shown) to discharge hydraulic oil. The pump P is, for example, a swash plate type variable displacement piston pump, and the discharge capacity is changed by changing the inclination of the swash plate by a regulator R. The regulator R controls the inclination of the swash plate of the pump P based on a control signal transmitted from a controller 50 of a pump control device 30 described later. The pump P is configured to discharge a constant flow rate even when the tilt angle of the swash plate is at a minimum. In other words, the minimum discharge capacity of the pump P is a value greater than zero (hereinafter referred to as the "standby flow rate"). Since the configurations of the pump P and the regulator R can be well known, detailed explanations will be omitted.

[0025] The hydraulic cylinder 2 is a double-acting cylinder having a piston 4 that divides the inside of a cylinder tube 3 into a rod side chamber 6 and a bottom side chamber 7. A piston rod 5 is connected to the piston 4. Hydraulic oil is supplied to and discharged from the rod side chamber 6 of the hydraulic cylinder 2 through a rod side passage 13. Hydraulic oil is supplied to and discharged from the bottom side chamber 7 of the hydraulic cylinder 2 through a bottom side passage 14.

[0026] The hydraulic cylinder 2 extends when hydraulic oil is supplied to the bottom side chamber 7 and discharged from the rod side chamber 6. Conversely, the hydraulic cylinder 2 contracts when hydraulic oil is supplied to the rod side chamber 6 and discharged from the bottom side chamber 7.

[0027] The fluid pressure control device 10 controls the flow of hydraulic oil discharged from the pump P to control the expansion and contraction operation of the hydraulic cylinder 2. The hydraulic cylinder 2 expands and contracts when supplied with hydraulic oil discharged from the pump P.

[0028] The fluid pressure control device 10 includes a control valve 20 that is provided in a supply passage 11 that guides the hydraulic oil discharged from the pump P and controls the flow of the hydraulic oil supplied to and discharged from the hydraulic cylinder 2, a safety valve 27 that opens when the pressure in the supply passage 11 reaches a predetermined valve opening pressure (cracking pressure) and discharges the hydraulic oil in the supply passage 11 into a tank T, and a pump control device 30 that controls the flow of the hydraulic oil from the pump P to the control valve 20 through the supply passage 11 and also controls the discharge capacity of the pump P.

[0029] The control valve 20 is a spool valve that has a spool (not shown) and whose position is changed by the movement of the spool.

[0030] The control valve 20 has a pair of pilot chambers 21a, 21b and springs 22a, 22b as biasing members, and is actuated in response to the pressure difference between the pair of pilot chambers 21a, 21b.

[0031] Connected to the control valve 20 are the supply passage 11, the tank passage 12 communicating with the tank T, a branch supply passage 11a branching from the supply passage 11, a branch tank passage 12a branching from the tank passage 12, a rod side passage 13, and a bottom side passage 14. The supply passage 11 and the tank passage 12 form a center bypass passage that guides the hydraulic oil discharged from the pump P to the tank T. A check valve 25 is provided in the branch supply passage 11a to permit the flow of hydraulic oil from the pump P toward the control valve 20 and to restrict the flow in the opposite direction.

[0032] The control valve 20 has a neutral position 20A which communicates between the supply passage 11 and the tank passage 12 and opens the center bypass passage, an extended position 20B which is an operating position which communicates between the branch supply passage 11a and the bottom side passage 14 and also communicates between the branch tank passage 12a and the rod side chamber 6, and a contracted position 20C which is an operating position which communicates between the branch supply passage 11a and the rod side passage 13 and also communicates between the branch tank passage 12a and the bottom side passage 14.

[0033] When the operating lever 1 is not operated by an operator, pilot pressure is not introduced to the pair of pilot chambers 21a, 21b of the control valve 20, and the control valve 20 is held in the neutral position 20A by the pair of springs 22a, 22b. In the neutral position 20A, the rod side passage 13, the bottom side passage 14, the branch supply passage 11a, and the branch tank passage 12a are all blocked. Therefore, the hydraulic cylinder 2 does not expand or contract, and is maintained in a load holding state. In addition, the hydraulic oil discharged from the pump P is introduced from the supply passage 11 through the control valve 20 to the tank passage 12 and then to the tank T.

[0034] When the operator operates the operating lever 1 in one direction, pilot pressure is introduced into one pilot chamber 21a of the control valve 20 according to the amount of operation, and the control valve 20 switches to the extension position 20B. In the extension position 20B, communication between the supply passage 11 and the tank passage 12 is blocked, and the center bypass passage is closed. When the control valve 20 switches to the extension position 20B, hydraulic oil discharged from the pump P is supplied to the bottom side chamber 7 through the branch supply passage 11a and the bottom side passage 14. Hydraulic oil in the rod side chamber 6 is discharged to the tank T through the rod side passage 13 and the branch tank passage 12a. As a result, the hydraulic cylinder 2 is extended.

[0035] When the operator operates the operating lever 1 in the other direction, pilot pressure is introduced to the other pilot chamber 21b of the control valve 20 according to the amount of operation, and the control valve 20 switches to the contraction position 20C. In the contraction position 20C, communication between the supply passage 11 and the tank passage 12 is blocked, and the center bypass passage is closed. When the control valve 20 switches to the contraction position 20C, hydraulic oil discharged from the pump P is supplied to the rod side chamber 6 through the branch supply passage 11a and the rod side passage 13. Hydraulic oil in the bottom side chamber 7 is discharged to the tank T through the bottom side passage 14 and the branch tank passage 12a. As a result, the hydraulic cylinder 2 contracts.

[0036] The pump control device 30 includes a discharge passage 15 that guides the hydraulic oil in the supply passage 11 to the tank T, a discharge control unit 31 that controls the flow of the hydraulic fluid discharged to the tank T through the discharge passage 15, and a controller 50 that controls the discharge capacity of the pump P and the operation of the discharge control unit 31 in response to an operational input by an operator who operates the hydraulic cylinder 2.

[0037] The discharge control unit 31 includes a relief valve 40 provided in the discharge passage 15 which opens when the pressure in the discharge passage 15 reaches a predetermined valve opening pressure and has a variable valve opening pressure, a downstream orifice 32 as a resistance unit which provides resistance to the flow of hydraulic oil that is guided from the supply passage 11 to the discharge passage 15 and passes through the relief valve 40, an upstream orifice 33 provided in the discharge passage 15 as an additional resistance unit which provides resistance to the flow of hydraulic oil from the supply passage 11 toward the relief valve 40, and a pressure measuring unit 34 for obtaining the differential pressure before and after the downstream orifice 32.

[0038] The relief valve 40 is an electromagnetic relief valve having a set spring 40a as a biasing member for setting a valve opening pressure, and a solenoid 40b that generates an electromagnetic force when energized. The solenoid 40b generates an electromagnetic force in response to an electric signal (valve opening pressure signal) input from the controller 50, which moves a plunger (not shown) and expands or contracts the set spring 40a, thereby adjusting the valve opening pressure of the relief valve 40. The valve opening pressure of the relief valve 40 is set to be at least higher than the tank pressure. The hydraulic oil that has passed through the upstream throttle 33 and the hydraulic oil that has passed through the downstream throttle 32 (tank pressure) are respectively introduced to the relief valve 40 as pilot pressures. The relief valve 40 opens when the pressure between the upstream throttle 33 and the relief valve 40 (the downstream pressure of the upstream throttle 33, i.e., the upstream pressure of the relief valve 40) and the tank pressure (hereinafter referred to as the "pressure difference between the upstream and downstream sides" of the relief valve 40) reaches the valve opening pressure, thereby opening the discharge passage 15. In addition, when the relief valve 40 is in a closed state, the upstream pressure of the relief valve 40 is equal to the upstream pressure of the upstream throttle 33 (and thus the pressure in the supply passage 11), so the relief valve 40 opens when the differential pressure between the upstream pressure of the upstream throttle 33 and the tank pressure substantially reaches the valve opening pressure. When the relief valve 40 is in an open state, it operates using the differential pressure between the pressure downstream of the upstream throttle 33 and upstream of the relief valve 40 and the tank pressure as a pilot pressure.

[0039] The configuration of the relief valve 40 will be described in detail with reference to Fig. 3. As shown in Fig. 3, the relief valve 40 has a housing 41, a main poppet 42 and a pilot poppet 46 provided in the housing 41, a main spring 44a as a biasing member that biases the main poppet 42 in the valve closing direction, a set spring 40a that biases the pilot poppet 46 in the valve closing direction, and a solenoid 40b that expands and contracts the set spring 40a.

[0040] The main poppet 42 divides the housing 41 into a first upstream chamber 43a, a first downstream chamber 43b, and a first pilot chamber 43c. The first upstream chamber 43a communicates with the discharge passage 15 on the downstream side of the upstream throttle 33, and hydraulic oil in the discharge passage 15 that has passed through the upstream throttle 33 is guided thereto. The first downstream chamber 43b communicates with the discharge passage 15 on the upstream side of the downstream throttle 32, and hydraulic oil in the first downstream chamber 43b is guided to the downstream throttle 32. When the main poppet 42 is in a closed state, communication between the first upstream chamber 43a and the first downstream chamber 43b is blocked by the main poppet 42, and when the main poppet 42 is opened, the first upstream chamber 43a and the first downstream chamber 43b communicate with each other. The first pilot chamber 43c always communicates with the first upstream chamber 43a through the poppet throttle 44b. The pressure receiving area for the first upstream chamber 43a when the main poppet 42 is closed is set to be equal to or smaller than the pressure receiving area for the first pilot chamber 43c.

[0041] The pilot poppet 46 divides the housing 41 into a second upstream chamber 47a, a second downstream chamber 47b, and a second pilot chamber 47c. The second upstream chamber 47a is always in communication with the first pilot chamber 43c divided by the main poppet 42. The second downstream chamber 47b is in communication with the exhaust passage 15 downstream of the downstream throttle 32, and is in communication with the tank T through the exhaust passage 15. When the pilot poppet 46 is in a closed state, the communication between the second upstream chamber 47a and the second downstream chamber 47b is blocked by the pilot poppet 46, and when the pilot poppet 46 is opened, the second upstream chamber 47a and the second downstream chamber 47b are in communication. The second pilot chamber 47c is always in communication with the second downstream chamber 47b. The pressure-receiving area of ​​the pilot poppet 46 with respect to the second upstream chamber 47a is smaller than the pressure-receiving area of ​​the pilot poppet 46 with respect to the second pilot chamber 47c. The second pilot chamber 47c accommodates a set spring 40a and a solenoid 40b. The pilot poppet 46 is biased in the valve closing direction by the biasing force of the set spring 40a and the thrust of the solenoid 40b.

[0042] The hydraulic oil that has passed through the upstream throttle 33 of the discharge passage 15 is led to the second upstream chamber 47a through the poppet throttle 44b. The second downstream chamber 47b is connected to the tank T through the discharge passage 15. When the thrust exerted on the pilot poppet 46 by the pressure led to the second upstream chamber 47a is equal to or less than the resultant force of the thrust exerted by the pressure (tank pressure) of the second pilot chamber 47c and the thrust exerted by the solenoid 40b and the set spring 40a, the pilot poppet 46 is closed. When the pilot poppet 46 is closed, the hydraulic oil in the first pilot chamber 43c is not discharged to the tank T, and therefore the main poppet 42 is also closed.

[0043] When the thrust exerted on the pilot poppet 46 by the pressure of the second upstream chamber 47a exceeds the resultant force of the thrust exerted by the pressure of the second pilot chamber 47c (tank pressure) and the thrust exerted by the solenoid 40b and the set spring 40a, the pilot poppet 46 opens. As a result, the hydraulic oil in the first pilot chamber 43c is guided from the second upstream chamber 47a to the second downstream chamber 47b and discharged from the discharge passage 15 to the tank T. Accordingly, the main poppet 42 (and thus the relief valve 40) is opened by the pressure of the first upstream chamber 43a, and the hydraulic oil in the discharge passage 15 is discharged to the tank T through the relief valve 40. In this way, the relief valve 40 opens when the differential pressure (front-rear differential pressure) between the pressure on the downstream side of the upstream throttle 33 in the discharge passage 15 and the pressure on the downstream side of the downstream throttle 32 (tank pressure) reaches a predetermined valve opening pressure, and the hydraulic oil in the discharge passage 15 is discharged to the tank T.

[0044] As described above, the opening pressure of the relief valve 40 is determined according to the thrust of the set spring 40a and the solenoid 40b, as well as the pressure-receiving area difference between the first upstream chamber 43a and the first pilot chamber 43c relative to the main poppet 42, the pressure-receiving area difference between the second upstream chamber 47a and the second pilot chamber 47c relative to the pilot poppet 46, the poppet throttle 44b, the main spring 44a, etc.

[0045] The configuration of the relief valve 40 is not limited to that shown in Fig. 3. Any configuration that can realize the function of the relief valve 40 described in this specification and can be represented by a circuit symbol such as that shown in Fig. 1 can be adopted as the relief valve 40.

[0046] The upstream throttle 33 and the downstream throttle 32 are fixed throttles constituted by, for example, orifices. The upstream throttle 33 and the downstream throttle 32 may be flow path resistances (piping resistances) formed by the flow paths (piping) constituting the discharge passage 15 as fixed throttles.

[0047] The pressure measurement unit 34 has a pressure sensor 35a serving as an upstream pressure measurement unit that measures the pressure of the hydraulic oil guided from the relief valve 40 to the downstream orifice 32, and a tank pressure sensor 35b serving as a tank pressure measurement unit that measures the pressure in the discharge passage 15 downstream of the downstream orifice 32. The pressure sensor 35a measures the pressure between the relief valve 40 and the downstream orifice 32 in the discharge passage 15 (in other words, the pressure upstream of the downstream orifice 32). The measurement result of the pressure sensor 35a is input to the controller 50.

[0048] The tank pressure sensor 35b measures the pressure downstream of the downstream orifice 32, which is led to the relief valve 40 as the pilot pressure. The measurement result of the tank pressure sensor 35b is input to the controller 50. The tank pressure sensor 35b may measure the pressure inside the tank passage 12 or the tank T. The difference between the pressure measured by the pressure sensor 35a and the pressure measured by the tank pressure sensor 35b is the differential pressure before and after the downstream orifice 32.

[0049] Further, a second pressure sensor 26 that acquires the pressure of the supply passage 11 is provided in the supply passage 11. The measurement result of the second pressure sensor 26 is input to the controller 50.

[0050] The controller 50 is composed of a computer equipped with a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and an I / O interface (Input / Output Interface). The RAM stores data for CPU processing, the ROM stores the CPU control program and the like in advance, and the I / O interface is used for inputting and outputting information to and from connected devices. The controller 50 is programmed to be capable of executing at least the processes required for executing the control according to this embodiment and the modified examples. The controller 50 may be configured as a single device, or may be divided into multiple devices and configured to perform distributed processing of each control among the multiple devices. The controller 50 may also be used in common with other controllers provided in the construction machine.

[0051] As shown in FIG. 2, the controller 50 includes a storage unit 51 that stores information required for controlling the discharge capacity of the pump P in advance, and a processing unit 52 that executes a process for controlling the discharge capacity of the pump P.

[0052] The memory unit 51 stores in advance the relationship between the operation amount of the operating lever 1 and the discharge capacity of the pump P (not shown), the pressure-flow characteristic (FIG. 4) showing the relationship between the pressure and flow rate of the hydraulic oil passing through the downstream throttle 32 alone, and the pressure-flow characteristic (FIG. 5) of the discharge control unit 31 as a whole. Normally, the pressure-flow characteristic of the relief valve 40 is such that the passing flow rate increases rapidly when the valve opening pressure is reached. In contrast, the pressure-flow characteristic of the discharge control unit 31 is such that the passing flow rate increases gradually with increasing pressure in a pressure range equal to or higher than the valve opening pressure of the relief valve 40, as shown in FIG. 5, by providing throttles (upstream throttle 33, downstream throttle 32) upstream and downstream of the relief valve 40. In other words, the pressure-flow characteristic of the discharge control unit 31 has a smaller slope of the flow rate change with respect to the pressure change than the pressure-flow characteristic of a general relief valve. In this way, the pressure-flow characteristic of the discharge control unit 31 is determined according to the pressure-flow characteristic of each of the relief valve 40, the downstream throttle 32, and the upstream throttle 33. The relationship between the amount of operation of the operating lever 1 and the discharge capacity of the pump P, and the pressure-flow rate characteristics of the discharge control section 31 and the downstream throttle 32 are stored in the memory section 51 in the form of, for example, a map.

[0053] The controller 50 receives an operation signal transmitted from the operating lever 1 in response to an operation input of the operating lever 1 by an operator, and pressure signals indicating respective measured values ​​input from the pressure sensor 35a, the tank pressure sensor 35b, and the second pressure sensor 26. The processing unit 52 calculates a differential pressure before and after the relief valve 40 from the pressure signals of the pressure sensor 35a and the tank pressure sensor 35b. The processing unit 52 also controls the discharge capacity of the pump P and the valve opening pressure of the relief valve 40, based on the operation signal of the operating lever 1 and the pressure signals of the pressure sensor 35a, the tank pressure sensor 35b, and the second pressure sensor 26.

[0054] A method for controlling the displacement of the pump P in this embodiment will now be described.

[0055] A control method of this embodiment will be described below with reference to the flowchart shown in Fig. 6. The controller 50 executes the process shown in Fig. 6 at predetermined execution intervals.

[0056] In step S10, an operation signal of the operating lever 1 by the operator is acquired.

[0057] In step S11, pressure signals that are the measurement results of the pressure sensor 35a and the tank pressure sensor 35b of the discharge control unit 31 are acquired.

[0058] Here, when the operator has not operated the operating lever 1 and the control valve 20 is in the neutral position 20A, the hydraulic oil discharged from the pump P is guided to the tank T through the center bypass passage (the supply passage 11 and the tank passage 12). Therefore, the pressure in the discharge passage 15 does not reach the valve opening pressure of the relief valve 40, so that the relief valve 40 is closed and the hydraulic oil discharged from the pump P does not pass through the downstream throttle 32.

[0059] The pressure in the supply passage 11 is determined according to the pressure (load pressure) of hydraulic oil required for the hydraulic cylinder 2. Even if the operator operates the operation lever 1 and the control valve 20 switches to the operating position (extension position 20B, contraction position 20C), if the load pressure of the hydraulic cylinder 2 is relatively low, the pressure in the supply passage 11 does not reach the valve opening pressure of the relief valve 40 of the discharge control unit 31. For this reason, the relief valve 40 is in a closed state, and the hydraulic oil discharged from the pump P does not pass through the downstream throttle 32.

[0060] Therefore, when the operating lever 1 is not operated and when the required pressure of the hydraulic cylinder 2 is low (when the required pressure is lower than the opening pressure of the relief valve 40), in step S11, a pressure signal indicating the tank pressure is input from the pressure sensor 35a to the controller 50.

[0061] When the operator operates the operating lever 1 to supply hydraulic oil from the pump P to the hydraulic cylinder 2, the pressure in the supply passage 11 rises with a drive pressure corresponding to the load acting on the hydraulic cylinder 2 from the driven object. When the pressure in the supply passage 11 reaches the valve opening pressure of the relief valve 40, the relief valve 40 opens, and a portion of the hydraulic oil discharged from the pump P passes through the relief valve 40 and the downstream throttle 32 (producing an excess flow rate) and is discharged into the tank T. In this way, the excess flow rate to the hydraulic cylinder 2 is discharged into the tank T. Therefore, in this case, in step S11, a pressure signal indicating the pressure of the hydraulic oil corresponding to the excess flow rate is input from the pressure sensor 35a to the controller 50.

[0062] In step S12, the flow rate (excess flow rate) passing through the downstream orifice 32 is calculated based on the pressure signal acquired in step S11. The excess flow rate can be calculated by calculating the differential pressure P1 across the relief valve 40 from the pressure signals of the pressure sensor 35a and the tank pressure sensor 35b acquired in step S11, and reading the flow rate L1 passing through the downstream orifice 32 at the differential pressure P1 from the pressure flow rate characteristics shown in FIG.

[0063] The pressure inside the tank T may fluctuate to a certain degree because the hydraulic oil discharged from the hydraulic cylinder 2 is led to the tank T. In contrast, in this embodiment, the pressure in the discharge passage 15 downstream of the downstream orifice 32 is measured as the tank pressure, and the flow rate through the downstream orifice 32 is calculated by calculating the differential pressure before and after the relief valve 40 from the measured tank pressure. Since the flow rate through the downstream orifice 32 is calculated based on the actually measured tank pressure, the calculation accuracy can be improved. Note that the tank pressure sensor 35b is not an essential component, and the flow rate through the downstream orifice 32 may be calculated based on a tank pressure that is previously determined, without providing the tank pressure sensor 35b.

[0064] When the operating lever 1 is not operated or when the required pressure of the hydraulic cylinder 2 is low, as described above, the relief valve 40 does not open and a pressure signal corresponding to the tank pressure is input to the controller 50. Therefore, in these cases, in step S12, the surplus flow rate is calculated as zero.

[0065] When the relief valve 40 opens, a pressure signal indicating a pressure higher than the tank pressure is input from the pressure sensor 35a. Therefore, in step S12, the flow rate of the hydraulic oil that has passed through the downstream throttle 32 (i.e., the surplus flow rate discharged to the tank T) is calculated based on the pressure signal and the pressure-flow rate characteristics of the downstream throttle 32 (see FIG. 4).

[0066] In step S13, the valve opening pressure signal input to the relief valve 40 is controlled in accordance with the surplus flow rate determined in step S12.

[0067] If the surplus flow rate is calculated to be zero in step S12, control is performed in step S13 to maintain (not change) the valve opening pressure of the relief valve 40.

[0068] If an excess flow rate greater than zero is calculated in step S12, then in step S13, as shown in FIG. 5, for example, the valve opening pressure of the relief valve 40 is increased from Pc1 to Pc2 by an amount corresponding to the excess flow rate.

[0069] In step S14, the discharge capacity of the pump P is controlled in accordance with the operation signal acquired in step S10 and the surplus flow rate calculated in step S12.

[0070] When the operating lever 1 is not operated and the surplus flow rate is zero, in step S14, the discharge capacity of the pump P is controlled to be minimum. In this manner, when the control valve 20 is in the neutral position 20A, hydraulic oil is discharged from the pump P at the standby flow rate and led to the tank T through the center bypass passage.

[0071] If the operating lever 1 is operated but the surplus flow rate is calculated to be zero in step S12, the discharge capacity of the pump P is determined in step S14 based only on the amount of operation of the operating lever 1. In other words, when the relief valve 40 is closed, the controller 50 controls the discharge capacity of the pump P to increase as the operation input of the operating lever 1 increases.

[0072] When the operating lever 1 is operated and an excess flow rate greater than zero is discharged to the tank T through the discharge passage 15, in step S14, the discharge capacity of the pump P calculated from the operation amount of the operating lever 1 is corrected in accordance with the excess flow rate. Then, the pump P is controlled so that the discharge capacity becomes the corrected discharge capacity in accordance with the excess flow rate. Specifically, the pump P is controlled so that the discharge capacity of the pump P corresponding to the current operation amount of the operating lever 1 is reduced by an amount corresponding to the excess flow rate.

[0073] In step S13, the valve opening pressure of the relief valve 40 is increased, and in step S14, the discharge flow rate of the pump P is reduced, thereby making it possible to reduce the surplus flow rate discharged through the discharge passage 15 while maintaining the flow rate of hydraulic oil supplied to the hydraulic cylinder 2 (while preventing a change in the supplied flow rate). In other words, it is possible to maintain the supply flow rate to the hydraulic cylinder 2 while reducing the surplus flow rate of the pump P, and prevent a change in the operating state of the hydraulic cylinder 2 that would be caused by an increase in the valve opening pressure of the relief valve 40.

[0074] More specifically, for example, as shown in FIG. 5, when the control of this embodiment is not yet performed, if the valve opening pressure of the relief valve 40 is Pc1 and the discharge pressure of the pump P (pressure of the supply passage 11) is P2, the relief valve 40 opens and hydraulic oil is discharged to the tank T through the discharge passage 15 at a flow rate L2. When the control of this embodiment is performed and the valve opening pressure of the relief valve 40 is increased from Pc1 to Pc2 (Pc2>Pc1) in step S13, the pressure flow rate characteristic of the relief valve 40 changes from the broken line state to the solid line state. At this time, if the load on the hydraulic cylinder 2 does not change, and the discharge capacity of the pump P is maintained, the flow rate of the hydraulic oil supplied to the hydraulic cylinder 2 increases by the amount of the surplus flow rate reduced from L2 to L3. As a result, the speed of the hydraulic cylinder 2 increases despite the absence of operation of the operating lever 1 or change in the load on the hydraulic cylinder 2.

[0075] Therefore, in this embodiment, by reducing the discharge capacity of the pump P as the valve opening pressure of the relief valve 40 increases, it is possible to suppress an increase in the supply flow rate of hydraulic oil to the hydraulic cylinder 2 that accompanies an increase in the valve opening pressure of the relief valve 40. Specifically, when the discharge pressure of the pump P is P2, the discharge capacity of the pump P is reduced by the difference between the flow rate L2 before control is performed and the flow rate L3 after control is performed (L2>L3), thereby making it possible to maintain the supply flow rate to the hydraulic cylinder 2. In this way, according to the control of this embodiment, it is possible to reduce the excess flow rate discharged through the discharge passage 15 while maintaining the amount of work performed by the pump P, thereby reducing the energy consumed by the pump P.

[0076] It should be noted that the degree to which the opening pressure of the relief valve 40 is increased in response to the generated surplus flow rate can be set arbitrarily. For example, the opening pressure of the relief valve 40 may be set to be equal to or higher than the pressure at the start of control (P2 in the example of FIG. 5) so that the relief valve 40 is closed, or, as explained in FIG. 5, the opening pressure may be set to be lower than the pressure P2 at the start of control so that a predetermined surplus flow rate (flow rate L3) occurs even after control.

[0077] As described above, in this embodiment, the valve opening pressure of the relief valve 40 and the discharge capacity of the pump P are controlled based on the flow rate (excess flow rate) passing through the discharge control unit 31 in addition to the amount of operation of the operating lever 1.

[0078] When the pressure in the supply passage 11 reaches the opening pressure of the safety valve 27, which is higher than the opening pressure of the relief valve 40, the safety valve 27 opens to discharge a portion of the hydraulic oil in the supply passage 11 into the tank passage 12. This makes it possible to protect the entire system regardless of the opening pressure of the relief valve 40.

[0079] In addition to the above controls, the controller 50 also executes horsepower control to control the discharge capacity of the pump P based on the pressure signal from the second pressure sensor 26 so that the load on the engine or the like that drives the pump P does not exceed an allowable limit. Since the horsepower control is a known configuration, a detailed description thereof will be omitted.

[0080] Here, in order to facilitate understanding of the present invention, a comparative example of the present invention will be described with reference to Fig. 8. In the comparative example, the same components as those in the above embodiment are denoted by the same reference numerals as those in the above embodiment, and the description thereof will be omitted.

[0081] The fluid pressure control system 300 according to the comparative example controls the discharge capacity of the pump P by so-called negative control. The fluid pressure control system 300 has a negative control control section 210 provided downstream of the control valve 220, instead of the discharge control section 31 of the present embodiment.

[0082] The negative control control unit 210 has a negative control relief valve 211 provided in the tank passage 12 downstream of the control valve 220, a negative control throttle 212 provided in parallel to the negative control relief valve 211, and a pressure sensor 213 that measures the upstream pressure of the negative control throttle 212 (the pressure between the control valve 220 and the negative control throttle 212).

[0083] Furthermore, in the fluid pressure control system 300 of the comparative example, when the control valve 220 is in the contraction position 220C, the center bypass passage is not blocked by the control valve 220 but is opened through the control orifice 214. In other words, when the control valve 220 is in the contraction position 220C, a portion of the hydraulic oil discharged from the pump P is guided from the supply passage 11 through the control orifice 214 of the control valve 220 to the tank passage 12 and discharged into the tank T. Although not shown in the figures, the control orifice 214 is formed, for example, by machining a groove-shaped notch on the outer periphery of the land portion of the spool of the control valve 220.

[0084] When the hydraulic cylinder 2 is operated with the control valve 220 in the contraction position 220C, unlike the extension position 20B in which the center bypass passage is blocked, part of the discharge flow rate of the pump P passes through the control orifice 214 of the control valve 220 and is led to the negative control control unit 210. For this reason, the upstream pressure of the negative control orifice 212 becomes high. Here, when the load on the hydraulic cylinder 2 changes, even if the throttling amount of the control orifice 214 is constant, the flow rate supplied to the hydraulic cylinder 2 and the flow rate passing through the control orifice 214 change, and the upstream pressure of the negative control orifice 212 also changes. In this way, the flow rate of hydraulic oil passing through the control orifice 214 is a flow rate that is not used for the operation of the hydraulic cylinder 2 and is part of the energy loss.

[0085] The controller 50 controls the discharge capacity of the pump P based on the upstream pressure of the negative control throttle 212 input from the pressure sensor 213 of the negative control control unit 210 (negative control). The controller 50 stores the pressure-flow rate characteristics of the negative control throttle 212 in advance, and can calculate the flow rate passing through the negative control throttle 212 from the pressure of the pressure sensor 213 and the pressure-flow rate characteristics of the negative control throttle 212.

[0086] When the control valve 220 is in the neutral position 20A, the hydraulic oil discharged from the pump P is discharged into the tank T through the negative control throttle 212 in the center bypass passage.

[0087] When the control valve 220 is switched to the contraction position 220C, a portion of the hydraulic oil discharged from the pump P is supplied to the hydraulic cylinder 2 through the control valve 220. This reduces the flow rate passing through the negative control throttle 212. Therefore, the controller 50 increases the discharge capacity of the pump P based on the upstream pressure of the negative control throttle 212, which decreases in accordance with the decrease in the flow rate passing through the negative control throttle 212.

[0088] In addition, in the negative control, by providing the negative control control section 210 in the tank passage 12, the hydraulic oil discharged from the pump P is discharged to the tank T through the negative control throttle 212 even when the control valve 220 is in the neutral position 20A. Therefore, even when the control valve 220 is in the neutral position 20A and the pump P is in a state where it discharges hydraulic oil of the standby flow rate, energy is consumed to pass the hydraulic oil of the standby flow rate through the negative control throttle 212.

[0089] In contrast to this, in this embodiment, the valve opening pressure of the relief valve 40 of the pump control device 30 can be adjusted, and the discharge capacity of the pump P can be controlled according to the flow rate passing through the downstream throttle 32. Therefore, even if the required pressure of the hydraulic cylinder 2 and the operation amount of the operating lever 1 are the same, by adjusting the valve opening pressure of the relief valve 40, it is possible to adjust the pressure of the supply passage 11 and the flow rate of hydraulic oil discharged to the tank T through the discharge passage 15, and ultimately the flow rate supplied to the hydraulic cylinder 2. Therefore, various controls over the hydraulic cylinder 2 can be realized.

[0090] Furthermore, in this embodiment, negative control is not performed, and the surplus flow rate of the pump P is discharged to the tank T through the discharge passage 15 branching off from the supply passage 11 upstream of the control valve 20, and the surplus flow rate is controlled by the discharge control unit 31. When the control valve 20 is in the neutral position 20A, the hydraulic oil discharged from the pump P is discharged to the tank T through the center bypass passage that is not provided with the negative control throttle 212. Therefore, according to this embodiment, energy loss caused by the hydraulic oil passing through the negative control throttle 212 can be suppressed.

[0091] Furthermore, in this embodiment, the control valve 20 is configured to block the center bypass passage in the operating positions (extension position 20B, contraction position 20C), and is not configured to open the center bypass passage through the control throttle 214 in the operating positions as in the comparative example. In this embodiment, instead of the control throttle 214 of the control valve 220 as in the comparative example, the valve opening pressure of the relief valve 40 is controlled, thereby making it possible to mitigate a sudden pressure change in the hydraulic oil supplied from the pump P to the hydraulic cylinder 2. Therefore, in this embodiment, the control throttle 214 can be eliminated from the control valve 20, and therefore the structure of the control valve 20 can be simplified, the number of steps for forming the control throttle 214 can be reduced, and costs can be reduced.

[0092] From the viewpoint of reducing energy loss, in the process of switching from the neutral position 20A to the operating position, it is desirable that the timing at which the supply passage 11 and the tank passage 12 are shut off (the center bypass passage is shut off) is earlier (earlier) than or the same as the timing at which the branch supply passage 11a communicates with the rod side passage 13 or the bottom side passage 14. This makes it possible to control (reduce) the amount of leakage of hydraulic oil discharged from the pump P to the tank T due to reasons other than the operation of the hydraulic cylinder 2 via the control valve 20 below the valve opening pressure of the safety valve 27, thereby reducing energy loss to the maximum extent. In addition, in such a case, it is not necessary to form a configuration such as a notch that forms the control throttle 214 of the comparative example in the land portion of the spool of the control valve 20 that shuts off the communication between the supply passage 11 and the tank passage 12, so that the structure of the control valve 20 can be simplified.

[0093] On the other hand, if the supply passage 11 and the tank passage 12 are blocked before or at the same time as the branch supply passage 11a communicates with the rod side passage 13 or the bottom side passage 14, a momentary pressure rise in the supply passage 11 may be unavoidable. If it is desired to avoid such a pressure rise in the supply passage 11, the timing at which the supply passage 11 and the tank passage 12 are blocked may be after the timing at which the branch supply passage 11a communicates with the rod side passage 13 or the bottom side passage 14. In other words, as a transition state between a state in which the control valve 20 is in the neutral position 20A and the supply passage 11 and the tank passage 12 are connected to each other and a state in which the supply passage 11 and the tank passage 12 are blocked by switching to the operating position, a state in which the supply passage 11 and the tank passage 12 are connected to each other and the branch supply passage 11a and the rod side passage 13 or the bottom side passage 14 are connected to each other may temporarily occur. According to this, even when the hydraulic cylinder 2 is in an operating state, a sudden pressure rise in the supply passage 11 is suppressed regardless of the valve opening pressure of the relief valve 40, and the pressure rises gradually. In the transition state, the hydraulic cylinder 2 is in a stopped state without operating, or in a slightly operated state. Then, when the supply passage 11 and the tank passage 12 are completely blocked, the pressure of the hydraulic oil guided to the supply passage 11 and thus to the hydraulic cylinder 2 rises, and the hydraulic cylinder 2 operates.

[0094] In addition, in such a transition state, the supply passage 11 and the tank passage 12 may be communicated with each other via a restriction such as a notch (hereinafter referred to as a "communication restriction"). Such a communication restriction is formed, for example, in a land portion of a spool of the control valve 20 that blocks communication between the supply passage 11 and the tank passage 12 in the actuated position. This makes it possible to suppress a sudden pressure rise in the supply passage 11 and to suppress leakage of hydraulic oil to be supplied to the hydraulic cylinder 2 into the tank T.

[0095] The communication orifice is formed in a position on the spool of the control valve 20 similar to that of the control orifice 214 of the comparative example, but does not communicate between the supply passage 11 (branch supply passage 11a) and the tank passage 12 in the operating position, and does not control the flow rate of hydraulic oil discharged from the pump P to the tank T during operation of the hydraulic cylinder 2. For this reason, the machining range of the spool (length, width, depth, etc. of the notch) is smaller and the amount of machining is less, compared to the control orifice 214 of the comparative example. Therefore, even if a communication orifice is provided, it is possible to simplify the configuration of the control valve 20 and reduce the number of machining steps, compared to the comparative example.

[0096] Next, modified examples of this embodiment will be described. The following modified examples are also within the scope of the present invention, and it is possible to combine the configurations shown in the modified examples with the configurations described in the above embodiment, or to combine the configurations described in the different modified examples below.

[0097] In the above embodiment, the controller 50 stores the pressure-flow characteristics of the entire discharge control unit 31 and the downstream throttle 32. Alternatively, the controller 50 may store the pressure-flow characteristics of the discharge control unit 31 and the downstream throttle 32 for each predetermined temperature range, and use the pressure-flow characteristics of the discharge control unit 31 and the downstream throttle 32 according to the temperature of the hydraulic oil. In this case, the temperature of the hydraulic oil may be measured by a temperature sensor and input to the controller 50. By using the pressure-flow characteristics according to the temperature of the hydraulic oil, the flow rate passing through the downstream throttle 32 can be calculated with high accuracy.

[0098] In the above embodiment, when the excess flow rate passing through the discharge control section 31 increases, the controller 50 increases the valve opening pressure of the relief valve 40 and reduces the discharge capacity of the pump P. This makes it possible to reduce the excess flow rate while maintaining the pressure required for the hydraulic cylinder 2, thereby saving energy. Meanwhile, the control of the valve opening pressure of the relief valve 40 and the discharge capacity of the pump P by the controller 50 is not limited to the configuration of the above embodiment. The controller 50 is not limited to a configuration in which the valve opening pressure of the relief valve 40 and the discharge capacity of the pump P are changed simultaneously in response to the excess flow rate, and may be configured to change at least one of them.

[0099] In other words, in this specification, controlling the valve opening pressure of the relief valve 40 (operation of the discharge control unit 31) and the discharge capacity of the pump P means not only changing the valve opening pressure of the relief valve 40 and the discharge capacity of the pump P, but also maintaining them unchanged. Furthermore, controlling the discharge capacity of the pump P and the valve opening pressure of the relief valve 40 in accordance with the flow rate of the hydraulic oil passing through the downstream throttle 32 means maintaining the discharge capacity of the pump P or the valve opening pressure of the relief valve 40 in accordance with the flow rate of the hydraulic oil passing through the downstream throttle 32 without changing them.

[0100] For example, the controller 50 may change one of the valve opening pressure of the relief valve 40 or the discharge capacity of the pump P in accordance with the excess flow rate passing through the discharge control section 31, and control the other to be maintained.

[0101] The controller 50 may be configured to be able to execute a plurality of switchable control modes. For example, the controller 50 may be configured to set the control of the above embodiment as an "energy saving mode" and to execute a "high response mode" in which the opening pressure of the relief valve 40 and the discharge capacity of the pump P are not controlled according to the surplus flow rate in addition to the energy saving mode. In the high response mode, the discharge capacity of the pump P is controlled according to the operation amount of the operation lever 1, not depending on the surplus flow rate. In the energy saving mode as in the above embodiment, the opening pressure of the relief valve 40 and the discharge capacity of the pump P are controlled so as to reduce the surplus flow rate when the surplus flow rate increases, so that energy can be saved. In contrast, the high response mode does not reduce the surplus flow rate, so that when the load state of the hydraulic cylinder 2 fluctuates and the required pressure (or required flow rate) increases, the surplus flow rate can be immediately supplied to the hydraulic cylinder 2, and is excellent in responsiveness to load fluctuations. By configuring the controller 50 to be able to switch between the energy saving mode and the high response mode by the operation of a button or the like by an operator, appropriate control can be performed according to the situation.

[0102] In the above embodiment, the upstream throttle 33, which is the additional resistance portion, is provided in the discharge passage 15 as a separate body from the relief valve 40. In contrast to this, the additional resistance portion may be configured, for example, by a slit or the like formed in the main poppet 42 of the relief valve 40 or a seat portion (not shown) on which the main poppet 42 sits. In other words, the additional resistance portion may not be separate from the relief valve 40, but may be provided as part of the configuration of the relief valve 40 and may be built into the relief valve 40. Also, if the discharge control portion 31 can achieve the desired pressure-flow rate characteristics, the additional resistance portion may be eliminated.

[0103] In the above embodiment, the fluid pressure control system 100 is a so-called open center type system in which the control valve 20 is in the neutral position 20A and the center bypass passage is opened when the operating lever 1 is not operated, and a positive control is performed to control the discharge capacity of the pump P according to the operation amount of the operating lever 1. In contrast, the pump control device 30 may be applied to a closed center type system in which the supply passage 11 and the tank passage 12 are not communicated by the control valve 20 when the operating lever 1 is not operated. Even in this case, various controls can be realized in the control of the hydraulic oil supplied to the hydraulic cylinder 2 by controlling at least one of the valve opening pressure of the relief valve 40 and the discharge capacity of the pump P based on the flow rate of the hydraulic oil passing through the discharge passage 15. In addition, in the case of the closed center type, when the operating lever 1 is not operated (the lever angle of the operating lever 1 is within a predetermined angle range from the neutral position), the supply passage 11 may be opened to the tank T via the relief valve 40 by cutting off the power supply to the relief valve 40 to minimize the valve opening pressure of the relief valve 40.

[0104] The pump control device 30 may be incorporated inside the fluid pressure control device 10, or may be incorporated inside the pump P. The pump control device 30 may be provided as a separate unit for each of the fluid pressure control device 10 and the pump P.

[0105] Next, a fluid pressure control system 200 and a pump control device 130 according to a second embodiment of the present invention will be described with reference to FIG.

[0106] In the first embodiment, the resistance portion is the downstream throttle 32 provided downstream of the relief valve 40. The pressure measurement portion 34 has a pressure sensor 35a and a tank pressure sensor 35b, and measures the differential pressure before and after the downstream throttle 32. The controller 50 obtains the flow rate of the hydraulic oil passing through the discharge passage 15 from the differential pressure before and after the downstream throttle 32, and controls the discharge capacity of the pump P and the valve opening pressure of the relief valve 40 based on the flow rate.

[0107] In contrast to this, in the second embodiment, the resistance portion is an upstream throttle 33 provided upstream of the relief valve 40. In the second embodiment, the function of the upstream throttle 33 which is the additional resistance portion in the first embodiment and the function of the downstream throttle 32 which is the resistance portion are interchanged. Accordingly, the pressure measuring portion 134 is configured to measure the differential pressure before and after the upstream throttle 33. The specific configuration of the second embodiment will be described below.

[0108] In the second embodiment, the downstream throttle 32 functions as an additional resistance portion for adjusting the pressure-flow characteristics of the relief valve 40. In the second embodiment, the downstream throttle 32 is not an essential component, and may not be provided.

[0109] The pressure measurement unit 134 of the second embodiment has, instead of the pressure sensor 35a and the tank pressure sensor 35b of the first embodiment, a second pressure sensor 26 as an upstream pressure measurement unit that measures the pressure upstream of the upstream throttle 33, and a third pressure sensor 35c as a downstream pressure measurement unit that measures the pressure downstream of the upstream throttle 33 (the pressure between the upstream throttle 33 and the relief valve 40). The measurement results of the second pressure sensor 26 and the third pressure sensor 35c are input to the controller 50.

[0110] The second pressure sensor 26 has the same configuration as the second pressure sensor in the first embodiment, and is provided as a part of the pressure measurement unit 134 in this embodiment. Since the pressure in the discharge passage 15 upstream of the upstream throttle 33 is substantially the same as the pressure in the supply passage 11, the second pressure sensor 26 can measure the pressure upstream of the upstream throttle 33. Note that the second pressure sensor 26 may be provided to directly measure the pressure in the discharge passage 15 upstream of the upstream throttle 33.

[0111] The controller 50 preliminarily stores the pressure-flow characteristic of the upstream throttle 33. The controller 50 obtains the differential pressure across the upstream throttle 33 from the measurement results of the second pressure sensor 26 and the third pressure sensor 35c, and obtains the passing flow rate (excess flow rate) of the discharge passage 15 based on the differential pressure across the upstream throttle 33 and the pressure-flow characteristic of the upstream throttle 33.

[0112] In the second embodiment, the surplus flow rate discharged to the tank T through the discharge passage 15 can be obtained from the differential pressure across the upstream throttle 33 and the pressure-flow rate characteristics, and therefore the same effects as in the first embodiment can be achieved.

[0113] Here, the first embodiment and the second embodiment will be compared and described. In the first embodiment, the resistance portion is the downstream throttle 32 provided downstream of the relief valve 40. Therefore, even if the flow rate passing through the discharge passage 15 is the same, the pressure measured by the pressure sensor 35a is lower than the pressure measured by the third pressure sensor 35c of the second embodiment due to the pressure loss caused by the upstream throttle 33 and the relief valve 40. For this reason, the pressure sensor 35a can use a sensor having a lower measurement range than the third pressure sensor 35c. In general, even if the same pressure value is measured, the lower the measurement range, the finer the measurement resolution can be. Therefore, the first embodiment is easier to improve the measurement accuracy of the differential pressure before and after the resistance portion, and therefore the calculation accuracy of the excess flow rate passing through the discharge passage 15, compared to the second embodiment. Also, in the first embodiment, the second downstream chamber 47b of the relief valve 40 is connected to the discharge passage 15 downstream of the downstream throttle 32 and communicates with the tank T. Therefore, for example, even if a high surge pressure is input to the relief valve 40, the pressure in the second downstream chamber 47b is discharged to the tank T through the discharge passage 15, and the second downstream chamber 47b and therefore the second pilot chamber 47c are prevented from becoming high pressure. Therefore, the durability of the solenoid 40b housed in the second pilot chamber 47c can be ensured, and the solenoid 40b is not required to have high pressure resistance.

[0114] On the other hand, in the first embodiment, as shown in Fig. 3, the pressure sensor 35a measures the pressure of the hydraulic oil guided from the first upstream chamber 43a to the downstream throttle 32 by opening the main poppet 42 of the relief valve 40. In contrast, when the relief valve 40 opens, a flow of hydraulic oil is also generated that is guided to the second upstream chamber 47a and guided to the discharge passage 15 by opening the pilot poppet 46 without passing through the downstream throttle 32. That is, in the first embodiment, the flow rate of the hydraulic oil passing through the downstream throttle 32 does not match the total flow rate of the hydraulic oil passing through the discharge passage 15, and is slightly less by the amount that passes through the pilot poppet 46 (second upstream chamber 47a). Normally, such an error in measurement of the surplus flow rate is not so large as to affect the control of the discharge capacity of the pump P and the opening pressure of the relief valve 40, and therefore does not cause a problem in control.

[0115] In the second embodiment, the upstream throttle 33 on the upstream side of the relief valve 40 serves as the resistance portion, and therefore no measurement error in the excess flow rate occurs due to a portion of the hydraulic oil passing through the pilot poppet 46, as in the first embodiment in which the downstream throttle 32 serves as the resistance portion. Therefore, the second embodiment is suitable for cases in which high-precision control is performed in which the discharge capacity of the pump P and the opening pressure of the relief valve 40 are affected by measurement error in the excess flow rate.

[0116] The configuration, operation, and effects of the embodiment of the present invention will be described below.

[0117] The pump control device 30 of this embodiment includes a discharge passage 15 that introduces hydraulic oil in the supply passage 11 to a tank T, a discharge control unit 31 that controls the flow of hydraulic oil discharged to the tank T through the discharge passage 15, and a controller 50 that controls the capacity of the pump P in response to an operation input by an operator who operates the hydraulic cylinder 2. The discharge control unit 31 includes a relief valve 40 that is provided in the discharge passage 15 and opens when the pressure in the discharge passage 15 reaches a predetermined valve opening pressure, and is configured so that the valve opening pressure is variable according to an input electric signal, and a pressure regulator 40 that is led from the supply passage 11 to the discharge passage 15 and controls the pressure regulator 40 through the relief valve 40. The controller 50 increases the discharge flow rate of the pump P as the operational input increases, pre-stores pressure-flow characteristics which are the relationship between the pressure at the downstream orifice 32 and the passing flow rate, obtains the flow rate of the hydraulic oil passing through the downstream orifice 32 based on the pressure-flow characteristics of the downstream orifice 32 and the differential pressure before and after measured by the pressure measuring unit 34, and changes at least one of the discharge capacity of the pump P and the opening pressure of the relief valve 40 in accordance with the flow rate of the hydraulic oil passing through the downstream orifice 32.

[0118] The pump control device 30 of this embodiment includes a discharge passage 15 that guides the hydraulic oil in the supply passage 11 to the tank T, a discharge control unit 31 that controls the flow of the hydraulic oil discharged to the tank T through the discharge passage 15, and a controller 50 that controls the capacity of the pump P in response to an operation input by an operator who operates the hydraulic cylinder 2. The discharge control unit 31 includes a relief valve 40 that is provided in the discharge passage 15 and opens when the pressure in the discharge passage 15 reaches a predetermined valve opening pressure, and is configured so that the valve opening pressure is variable according to an input electrical signal, and a controller 50 that controls the flow of the hydraulic oil from the relief valve 40 to the tank T. The controller 50 increases the discharge flow rate of the pump P as the operational input increases, pre-stores pressure-flow characteristics which are the relationship between the pressure at the upstream throttling 33 and the flow rate passing through it, obtains the flow rate of hydraulic oil passing through the upstream throttling 33 based on the pressure-flow characteristics of the upstream throttling 33 and the differential pressure before and after measured by the pressure measuring unit 134, and changes at least one of the discharge capacity of the pump P and the opening pressure of the relief valve 40 in accordance with the flow rate of hydraulic oil passing through the upstream throttling 33.

[0119] In these configurations, at least one of the valve opening pressure of the relief valve 40 and the discharge capacity of the pump P is changed according to the flow rate of the hydraulic fluid passing through the downstream throttle 32 or the upstream throttle 33 of the discharge control unit 31. The valve opening pressure of the relief valve 40 and the discharge capacity of the pump P can be adjusted independently or in cooperation with each other based on the pressure-flow rate characteristics of the downstream throttle 32 or the upstream throttle 33. Therefore, even if the opening area of ​​the control valve 20 and the operation input by the operator are the same, the pressure and flow rate of the hydraulic fluid guided to the hydraulic cylinder 2 can be increased by increasing the valve opening pressure of the relief valve 40 according to an increase in the flow rate passing through the downstream throttle 32 or the upstream throttle 33. In addition, it is also possible to reduce the discharge capacity of the pump P by an amount corresponding to an increase in the valve opening pressure of the relief valve 40, thereby maintaining the pressure and flow rate of the hydraulic fluid guided to the hydraulic cylinder 2 and reducing only the flow rate passing through the downstream throttle 32 or the upstream throttle 33. In this way, even if the operation input by the operator is the same, various controls can be realized for the operation of the hydraulic cylinder 2 actuator.

[0120] In these configurations, the controller 50 controls the discharge capacity of the pump P in response to the operator's operation input rather than negative control, so that the throttle provided for controlling the discharge capacity of the pump P in negative control can be eliminated. Therefore, the hydraulic oil discharged from the pump P in neutral does not pass through the throttle downstream of the control valve 20 as in negative control, so that energy loss can be suppressed. In addition, the valve opening pressure of the relief valve 40 or the discharge capacity of the pump P is controlled in response to the flow rate of the hydraulic oil passing through the downstream throttle 32 of the discharge control unit 31, so that the flow rate of the hydraulic oil discharged from the pump P and discharged to the tank T through the discharge passage 15 can be reduced.

[0121] Furthermore, the pump control device 30 is characterized in that the discharge control section 31 has an upstream throttle 33 that is provided in the discharge passage 15 and applies resistance to the flow of hydraulic oil from the supply passage 11 toward the relief valve 40 .

[0122] In this configuration, the pressure-flow characteristics of the discharge control unit 31 as a whole are affected by the pressure-flow characteristics of the upstream throttle 33 and the downstream throttle 32. Since the pressure-flow characteristics of the discharge control unit 31 as a whole can be adjusted by adjusting the pressure-flow characteristics of the upstream throttle 33 and the downstream throttle 32, the design freedom of the pressure-flow characteristics of the discharge control unit 31 is improved.

[0123] Furthermore, in the pump control device 30, the controller 50 increases the valve opening pressure of the relief valve 40 and reduces the discharge capacity of the pump P as the flow rate of the hydraulic oil passing through the resistance portion increases.

[0124] In this configuration, the flow rate of hydraulic oil discharged to the tank T through the discharge passage 15 can be reduced by increasing the valve opening pressure of the relief valve 40. Furthermore, the flow rate of hydraulic oil supplied from the pump P to the hydraulic cylinder 2 is increased by reducing the flow rate of hydraulic oil discharged to the tank T through the discharge passage 15, so that the flow rate supplied to the hydraulic cylinder 2 can be ensured even if the discharge capacity of the pump P is reduced. Therefore, the amount of work performed by the pump P can be maintained while the energy consumed can be reduced.

[0125] In addition, in the pump control device 30, the pressure measurement unit 34 further includes a pressure sensor 35a that measures the pressure of the hydraulic oil guided from the relief valve 40 to the downstream orifice 32, and a tank pressure sensor 35b that measures the pressure of the tank T, and the controller 50 obtains the flow rate of the hydraulic oil passing through the downstream orifice 32 based on the differential pressure between the pressure measured by the pressure sensor 35a and the pressure measured by the tank pressure sensor 35b.

[0126] In this configuration, the flow rate of the hydraulic oil passing through the downstream throttle 32 is obtained based on the actual measured value of the tank pressure, so that the flow rate can be obtained with high accuracy even if the tank pressure fluctuates.

[0127] In addition, in the fluid pressure control system 100, the control valve 20 has two operating positions (extension position 20B and contraction position 20C) that connect the supply passage 11 to the hydraulic cylinder 2, and in either operating position, the communication between the supply passage 11 and the tank T is blocked.

[0128] Some control valves for controlling the operation of hydraulic cylinders communicate with a supply passage and a tank while applying resistance to the flow of hydraulic oil by throttling or the like in the actuated position to control the flow rate of hydraulic oil supplied to the hydraulic cylinder. Compared to such conventional control valves, the above-mentioned configuration is configured so that the supply passage and the tank T are blocked in the actuated position because the flow rate of hydraulic oil supplied to the hydraulic cylinder 2 can be controlled by controlling the valve opening pressure of the relief valve 40 of the discharge control unit 31. Therefore, there is no need to provide the control valve 20 with a throttle or the like for controlling the flow rate from the supply passage 11 to the tank T in the actuated position, and the configuration of the control valve 20 can be simplified.

[0129] Moreover, the fluid pressure control system 100 of this embodiment includes a variable displacement pump P that discharges hydraulic oil through a supply passage 11, a hydraulic cylinder 2 that is operated by the hydraulic oil discharged from the pump P, a control valve 20 that controls the flow of hydraulic oil supplied to and discharged from the fluid pressure actuator in response to an operation input by an operator, and a pump control device 30 that controls the discharge capacity of the pump P. The pump control device 30 has a discharge passage 15 that introduces the hydraulic oil in the supply passage 11 to a tank T, a discharge control unit 31 that controls the flow of the hydraulic oil discharged to the tank T through the discharge passage 15, and a controller 50 that controls the capacity of the pump P in response to an operation input by an operator who operates the fluid pressure actuator. The discharge control unit 31 is provided in the discharge passage 15 and controls the capacity of the pump P in response to an operation input by an operator who operates the fluid pressure actuator. the downstream orifice 32 that applies resistance to the flow of hydraulic oil that has been guided from the supply passage 11 to the discharge passage 15 and passed through the relief valve 40; and a pressure measuring unit 34 for obtaining a differential pressure before and after the downstream orifice 32. The controller 50 increases the discharge flow rate of the pump P as the operation input increases, stores in advance a pressure-flow characteristic that is a relationship between the pressure loss in the downstream orifice 32 and the passing flow rate, obtains the flow rate of hydraulic oil passing through the downstream orifice 32 based on the pressure-flow characteristic of the downstream orifice 32 and the pressure measured by the pressure measuring unit 34, and controls at least one of the discharge capacity of the pump P and the opening pressure of the relief valve 40 in accordance with the flow rate of hydraulic oil passing through the downstream orifice 32.

[0130] In this configuration, the discharge capacity of the pump P is controlled according to the operation input by the operator, so that the throttle provided for controlling the discharge capacity of the pump P in negative control can be eliminated. Therefore, the hydraulic oil discharged from the pump P when the control valve 20 is in neutral does not pass through the throttle downstream of the control valve as in negative control, so that energy loss can be suppressed. In addition, the valve opening pressure of the relief valve 40 or the discharge capacity of the pump P is changed according to the flow rate of the hydraulic oil passing through the downstream throttle 32, so that the flow rate of the hydraulic oil discharged from the pump P and discharged to the tank T through the discharge passage 15 can be reduced.

[0131] Although the embodiments of the present invention have been described above, the above-mentioned embodiments merely show some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above-mentioned embodiments. [Explanation of symbols]

[0132] 30,130...pump control device, 100,200...fluid pressure control system, 2...hydraulic cylinder (fluid pressure actuator), 11...supply passage, 15...discharge passage, 20...control valve, 20A...neutral position, 20B...extension position (operating position), 20C...contraction position (operating position), 31...discharge control section, 32...downstream restriction (resistance section, additional resistance section), 33...upstream restriction (additional resistance section, resistance section), 34,134...pressure measurement section, 35a...pressure sensor (upstream pressure measurement section), 35b...tank pressure sensor (tank pressure measurement section), 40...relief valve, 50...controller, P...pump, T...tank

Claims

1. A pump control device for controlling a discharge capacity of a pump in a fluid pressure control system including a variable displacement pump that discharges a working fluid to a fluid pressure actuator through a supply passage, and a control valve that controls a flow of the working fluid supplied to or discharged from the fluid pressure actuator in response to an operation input by an operator who drives the fluid pressure actuator, a discharge passage for leading the working fluid in the supply passage to a tank; a discharge control unit for controlling a flow of the working fluid discharged to the tank through the discharge passage; a controller that controls the discharge capacity of the pump and the operation of the discharge control unit in response to the operation input by an operator, The discharge control unit is a relief valve that is provided in the discharge passage and opens when the pressure in the discharge passage reaches a predetermined valve opening pressure, and the valve opening pressure is variable in response to an input electric signal; a resistance portion that applies resistance to a flow of the working fluid that is guided from the supply passage to the discharge passage and passes through the relief valve; A pressure measuring unit for acquiring a differential pressure before and after the resistor, The controller: A pressure-flow rate characteristic, which is a relationship between the pressure at the resistance portion and the flow rate passing through the resistance portion, is stored in advance; acquiring a flow rate of the working fluid passing through the resistance portion based on the pressure-flow rate characteristic of the resistance portion and the differential pressure before and after the pressure measurement portion; A pump control device comprising: a pump control section for changing at least one of the discharge capacity of the pump and the valve opening pressure of the relief valve in response to a flow rate of the working fluid passing through the resistance portion.

2. 2. The pump control device according to claim 1, The pump control device according to claim 1, wherein the discharge control section has an additional resistance section that is provided in the discharge passage and that applies resistance to a flow of the working fluid from the supply passage toward the relief valve.

3. The pump control device according to claim 1 or 2, The pump control device according to claim 1, wherein the controller increases the valve opening pressure of the relief valve and reduces the discharge capacity of the pump in response to an increase in a flow rate of the working fluid passing through the resistance portion.

4. The pump control device according to claim 1 or 2, The pressure measuring unit is an upstream pressure measuring unit that measures a pressure of the working fluid guided from the relief valve to the resistance unit; A tank pressure measuring unit for measuring the pressure of the tank is further provided, A pump control device characterized in that the controller obtains the flow rate of the working fluid passing through the resistance unit based on the differential pressure between the pressure measured by the upstream pressure measuring unit and the tank pressure measuring unit and the pressure measured by the tank pressure measuring unit.

5. A fluid pressure control system for controlling an operation of a fluid pressure actuator, comprising: a variable displacement pump that discharges the working fluid through a supply passage; a control valve having a neutral position for directing the working fluid discharged from the pump to a tank when in a neutral position where there is no operational input from an operator, and controlling a flow of the working fluid supplied to and discharged from the fluid pressure actuator in response to the operational input; A pump control device that controls the discharge capacity of the pump, The pump control device includes: a discharge passage for guiding the working fluid in the supply passage to the tank; a discharge control unit for controlling a flow of the working fluid discharged to the tank through the discharge passage; a controller that controls the discharge capacity of the pump and the operation of the discharge control unit in response to the operation input by an operator who operates the fluid pressure actuator, The discharge control unit is a relief valve that is provided in the discharge passage and opens when the pressure in the discharge passage reaches a predetermined valve opening pressure, and the valve opening pressure is variable in response to an input electric signal; a resistance portion that applies resistance to a flow of the working fluid that is guided from the supply passage to the discharge passage and passes through the relief valve; A pressure measuring unit for acquiring a differential pressure before and after the resistor, The controller: A pressure-flow characteristic, which is a relationship between a pressure loss in the resistance portion and a flow rate passing through the resistance portion, is stored in advance; acquiring a flow rate of the working fluid passing through the resistance portion based on the pressure-flow rate characteristic of the resistance portion and the differential pressure before and after the pressure measurement portion; A fluid pressure control system comprising: a pressure adjusting section for adjusting at least one of the discharge capacity of the pump and the valve opening pressure of the relief valve in response to a flow rate of the working fluid passing through the resistance section.

6. 6. The fluid pressure control system according to claim 5, the control valve has an operating position that communicates the supply passage with the fluid pressure actuator, 4. A fluid pressure control system comprising: a supply passage for supplying a fluid to the tank;

7. A pump control device for controlling a discharge capacity of a pump in a fluid pressure control system including a variable displacement pump that discharges a working fluid to a fluid pressure actuator through a supply passage, and a control valve that controls a flow of the working fluid supplied to or discharged from the fluid pressure actuator in response to an operation input by an operator who drives the fluid pressure actuator, a discharge passage for leading the working fluid in the supply passage to a tank; a discharge control unit for controlling a flow of the working fluid discharged to the tank through the discharge passage; a controller that controls the discharge capacity of the pump and the operation of the discharge control unit in response to the operation input by an operator, The discharge control unit is a relief valve that is provided in the discharge passage and opens when the pressure in the discharge passage reaches a predetermined valve opening pressure, and the valve opening pressure is variable in response to an input electric signal; a resistance portion provided in the discharge passage and configured to provide resistance to a flow of the working fluid from the supply passage toward the relief valve; A pressure measuring unit for acquiring a differential pressure before and after the resistor, The controller: A pressure-flow rate characteristic, which is a relationship between the pressure at the resistance portion and the flow rate passing through the resistance portion, is stored in advance; acquiring a flow rate of the working fluid passing through the resistance portion based on the pressure-flow rate characteristic of the resistance portion and the differential pressure before and after the pressure measurement portion; A pump control device comprising: a pump control section for changing at least one of the discharge capacity of the pump and the valve opening pressure of the relief valve in response to a flow rate of the working fluid passing through the resistance portion.

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