Hydraulic system

Abstract

A hydraulic system of an embodiment of the present application includes a variable displacement hydraulic pump that discharges working oil and includes a swash plate; a swash plate angle sensor that measures an angle of the swash plate; a swash plate drive piston that moves the swash plate of the hydraulic pump according to a change in pressure applied to a large diameter portion; a swash plate control hydraulic line for supplying a portion of working oil discharged by the hydraulic pump to the large diameter portion; a control valve provided on the swash plate control hydraulic line and that controls a flow rate of working oil supplied to or discharged from the large diameter portion; an electronic proportional pressure reducing valve that generates a pilot pressure to be transmitted to one side of the control valve; an operating device that generates an operating signal; and a control device that controls the electronic proportional pressure reducing valve according to the operating signal of the operating device and angle information of the swash plate angle sensor.

Description

TECHNICAL FIELD

[0001] The present application relates to a hydraulic system, and more particularly, to a hydraulic system for improving the efficiency of a swash plate type variable displacement hydraulic pump. BACKGROUND

[0002] Generally, a hydraulic system transmits power by working oil discharged from a hydraulic pump to activate various driving devices. Such a hydraulic system is widely used in construction machines or industrial vehicles, etc. For example, a hydraulic system used in a construction machine drives a plurality of working devices such as a boom, a stick, a bucket, a traveling motor, a swing motor, etc. by working oil discharged from a hydraulic pump driven by an engine.

[0003] For a swash plate type variable displacement hydraulic pump as one of hydraulic pumps used in such a hydraulic system, an angle of a swash plate formed in the pump is adjusted by a flow control device such as a regulator, thereby controlling a discharge flow rate.

[0004] Such a hydraulic control device can be classified into a mechanical control method and an electronic control method. The mechanical control method has been mainly adopted in the past, but the electronic control method has been widely adopted in recent years. The electronic control method hydraulic control device controls a swash plate angle by applying an electric signal to a regulator. Such an electronic control method hydraulic control device controls a pressure control type electronic hydraulic pump. This pressure control type electronic hydraulic pump is controlled by a control device that receives an operation signal of an operation device provided in a cab of a construction machine and an angle value of a swash plate from an angle sensor installed in the electronic hydraulic pump as an electric signal, respectively, and outputs an electric signal for controlling pressure to a corresponding electronic hydraulic pump.

[0005] Further, a flow control device such as a regulator adjusts an angle of a swash plate of a hydraulic pump by controlling a valve to control an action of a swash plate driving piston. In addition, the valve controls the action of the swash plate driving piston by selectively supplying a part of working oil discharged from the hydraulic pump to the swash plate driving piston.

[0006] However, during the control of the action of the swash plate driving piston by the valve, a part of the working oil discharged from the hydraulic pump is discharged from the valve and wasted. Such discharge of the working oil from the valve is required to enable the valve to stably perform a control action.

[0007] However, since the working oil discharged from the hydraulic pump is wasted in an amount equivalent to the amount of the working oil discharged from the valve, there is a problem in that working oil of a flow rate less than a maximum flow rate of working oil that the hydraulic pump can actually discharge is used to drive various driving devices. Further, since a maximum displacement of the hydraulic pump is limited, such a problem can cause a performance of an equipment using the hydraulic system to be degraded. SUMMARY

[0008] Technical Problem

[0009] Embodiments of the present application provide a hydraulic system that minimizes unnecessary flow loss.

[0010] Technical Solution

[0011] According to embodiments of the present application, a hydraulic system includes: a variable displacement hydraulic pump that discharges working oil and includes a swash plate; a swash plate angle sensor that measures an angle of the swash plate; a swash plate drive piston that has a large diameter portion and a small diameter portion and moves the swash plate of the hydraulic pump according to a change in pressure applied to the large diameter portion; a swash plate control hydraulic line for supplying a portion of the working oil discharged by the hydraulic pump to the large diameter portion; a control valve provided on the swash plate control hydraulic line and controlling a flow rate of the working oil supplied to or discharged from the large diameter portion; an electronic proportional pressure reducing valve (EPPRV) that generates a pilot pressure to be transmitted to one side of the control valve; an operating device that generates an operating signal; and a control device that controls the EPPRV according to the operating signal of the operating device and the angle information of the swash plate angle sensor.

[0012] The hydraulic system described above can further include: a drain line connected to the control valve; and a valve control hydraulic line branched from the swash plate control hydraulic line more upstream than the control valve and transmitting pressure to the other side of the control valve.

[0013] The generation and transmission of the pilot pressure to one side of the control valve by the EPPRV based on the control of the control device can include: a first pilot pressure that is smaller than the pressure applied to the other side of the control valve through the valve control hydraulic line; a second pilot pressure that is greater than the pressure applied to the other side of the control valve through the valve control hydraulic line; and a third pilot pressure that is greater than the second pilot pressure.

[0014] When the first pilot pressure is applied to one side of the control valve, the control valve can supply working oil to the large diameter portion of the swash plate drive piston through the swash plate control hydraulic line. When the second pilot pressure is applied to one side of the control valve, the control valve can drain the working oil of the large diameter portion of the swash plate drive piston to the drain line. In addition, when the third pilot pressure is applied to one side of the control valve, the draining of the working oil from the control valve to the drain line can be prevented.

[0015] At least a portion of the working oil discharged from the hydraulic pump and transmitted to the control valve through the swash plate control hydraulic line and the valve control hydraulic line can be discharged through the discharge line when the first pilot pressure and the second pilot pressure are applied to one side of the control valve.

[0016] The angle of the swash plate of the hydraulic pump can become smaller when the working oil is supplied to the large diameter portion of the swash plate drive piston, so that the discharge flow rate of the hydraulic pump decreases, and the angle of the swash plate of the hydraulic pump can become larger when the working oil is discharged from the large diameter portion of the swash plate drive piston, so that the discharge flow rate of the hydraulic pump increases.

[0017] The control device can control the electronic proportional pressure reducing valve to generate the third pilot pressure when the angle of the swash plate measured by the swash plate angle sensor becomes larger than a set angle after the second pilot pressure is applied to one side of the control valve.

[0018] The control device can control the electronic proportional pressure reducing valve to generate the third pilot pressure when the operation signal of the operation device requires the maximum discharge flow rate of the hydraulic pump after the working oil is maximally discharged from the large diameter portion of the swash plate drive piston while the second pilot pressure is applied to one side of the control valve.

[0019] The control device can calculate the pilot pressure required according to the flow rate control mode, the pilot pressure required according to the horsepower control mode, and the pilot pressure required according to the pressure control mode, respectively. Also, the control device can select the lowest pilot pressure from the calculated pilot pressures, and control the electronic proportional pressure reducing valve to generate the selected pilot pressure.

[0020] The control device can control the electronic proportional pressure reducing valve to generate the third pilot pressure when the pilot pressure required according to the flow rate control mode is lower than the pilot pressure required according to the horsepower control mode and the pilot pressure required according to the pressure control mode.

[0021] Further, according to an embodiment of the present application, a hydraulic system includes a variable displacement hydraulic pump that discharges working oil and includes a swash plate, a swash plate drive piston that has a large diameter portion and a small diameter portion and that moves the swash plate of the hydraulic pump according to a change in pressure applied to the large diameter portion, a swash plate control hydraulic line that supplies a portion of the working oil discharged by the hydraulic pump to the large diameter portion, a control valve that is provided on the swash plate control hydraulic line and that controls a flow rate of the working oil supplied to or discharged from the large diameter portion, an electronic proportional pressure reducing valve (EPPRV) that generates a pilot pressure to be transmitted to one side of the control valve, an operating device that generates an operating signal, and a control device that controls the EPPRV to generate a third pilot pressure that prevents the working oil from being discharged from the large diameter portion to a tank when the operating signal of the operating device requires a maximum discharge flow rate of the hydraulic pump.

[0022] The hydraulic system described above can further include a swash plate angle sensor that measures an angle of the swash plate, and the control device controls the EPPRV to generate the third pilot pressure when the angle of the swash plate measured by the swash plate angle sensor becomes larger than a set angle.

[0023] Effects of the Invention

[0024] According to an embodiment of the present application, a hydraulic system can minimize unnecessary flow loss. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 FIG. 1 is a hydraulic circuit diagram of a hydraulic system according to an embodiment of the present application.

[0026] Figure 2 FIG. 2 is a sequence chart illustrating a control process of a control device of the hydraulic system of FIG. 1. Figure 1

[0027] Figure 3 FIG. 4 is a graph illustrating changes in a pilot pressure and a discharge flow rate corresponding to an operation of the hydraulic system according to an embodiment of the present application. DETAILED DESCRIPTION

[0028] Embodiments of the present application are described in detail below with reference to the accompanying drawings, so that those skilled in the art to which the present application pertains can easily practice the present application. The present application can be embodied in many different forms and is not limited to the embodiments described herein.

[0029] ​The accompanying drawings are diagrammatic and are not drawn to scale. The relative dimensions and proportions of the illustrated parts are shown exaggerated or reduced in size for the sake of clarity and convenience in the drawings and are not necessarily drawn to scale with respect to one another. Also, the same reference numerals are used to represent like structures, elements and components having similar features, functions or in similar groups throughout the drawings and the specification.

[0030] Embodiments of the present application specifically show the ideal embodiments of the present application. As a result, various modifications of the illustrations are expected. Therefore, the embodiments are not limited to the specific forms of the illustrated regions, for example, modifications of forms caused by manufacturing are also included.

[0031] Reference will now be made to Figures 1 to 3 A hydraulic system 101 of an embodiment of the present application will be described. The hydraulic system 101 of an embodiment of the present application can be used for construction machines or industrial vehicles, and can drive various driving devices such as a boom cylinder, a stick cylinder, a bucket cylinder, a swing motor, and a travel motor by working oil discharged from a hydraulic pump 310 driven by an engine.

[0032] As Figure 1 shown, the hydraulic system 101 of an embodiment of the present application includes the hydraulic pump 310, a swash plate angle sensor 730, a swash plate drive piston 200, a swash plate control hydraulic line 640, a control valve 300, an electronic proportional pressure reducing valve 500, an operating device 770, and a control device 700.

[0033] In addition, the hydraulic system 101 of an embodiment of the present application can further include a drain line 680, a valve control hydraulic line 630, a pilot pump 370, and a tank 800.

[0034] The hydraulic pump 310 is a swash plate type variable displacement type. That is, the hydraulic pump 310 includes a swash plate 314. In addition, the discharge flow rate of the hydraulic pump 310 can be adjusted by adjusting the angle of the swash plate 314.

[0035] The swash plate angle sensor 730 measures the angle of the swash plate 314. In addition, since the angle of the swash plate 314 is proportional to the working oil discharge flow rate of the hydraulic pump 310, the swash plate angle sensor 730 can measure the working oil discharge flow rate of the hydraulic pump 310.

[0036] The swash plate drive piston 200 adjusts the angle of the swash plate 314 of the hydraulic pump 310. The swash plate drive piston 200 has a large diameter portion 290 and a small diameter portion 210, and moves the swash plate 314 of the hydraulic pump 310 according to a change in the pressure applied to the large diameter portion 290.

[0037] The swash plate control hydraulic line 640 is provided so as to be able to supply a part of the working oil discharged from the hydraulic pump 310 to the large diameter portion 290 of the swash plate drive piston 200.

[0038] The control valve 300 is provided on the swash plate control hydraulic line 640 to control the flow rate of the working oil supplied to or discharged from the large diameter portion 290 of the swash plate drive piston 200. Specifically, the control valve 300 changes the internal flow path at the time of position switching of the spool, thereby supplying the working oil moved through the swash plate control hydraulic line 640 to the large diameter portion 290 of the swash plate drive piston 200 or discharging the working oil discharged from the large diameter portion 290 of the swash plate drive piston 200 through a drain line 680 to be described later.

[0039] The electronic proportional pressure reducing valve 500 (EPPRV) generates a pilot pressure to be transmitted to one side of the control valve 300. Such an electronic proportional pressure reducing valve 500 is an electronically controlled valve, and generates a pilot pressure in accordance with a current signal supplied from a control device 700 to be described later. That is, the electronic proportional pressure reducing valve 500 can adjust the magnitude of the pilot pressure generated in proportion to the magnitude of the current signal supplied from the control device 700.

[0040] The pilot pump 370 is used to generate a pilot pressure. That is, the pressure of the working oil discharged from the pilot pump 370 is processed by the electronic proportional pressure reducing valve 500 as a pilot pressure to be transmitted to the control valve 300.

[0041] The valve control hydraulic line 630 can be branched from the swash plate control hydraulic line 640 more upstream than the control valve 300 and transmit a pressure to the other side of the control valve 300. Here, upstream refers to the direction from the control valve 300 to the hydraulic pump 310 on the basis of the flow direction of the working oil. That is, the other side of the control valve 300 is applied with the pressure possessed by the working oil discharged from the hydraulic pump 310. At this time, the pressure of the working oil transmitted to the other side of the control valve 300 can also be processed according to the situation.

[0042] The drain line 680 is connected to the control valve 300. As the spool of the control valve 300 is switched, the working oil transmitted to the control valve 300 and the working oil discharged from the large diameter portion 290 of the swash plate drive piston 200 can be discharged through the drain line 680.

[0043] The oil tank 800 is connected to the drain line 680, and can store the working oil discharged through the drain line 680. In addition, the hydraulic pump 310 can discharge the working oil stored in the oil tank 800.

[0044] The operation device 770 generates an operation signal. For example, the operation device 770 can include a control lever, an operation lever, a pedal, a touch screen, a button, and the like, which are provided in the cab in such a manner that the operator can operate various driving devices. The operation device 770 is operated by the user, and the control device 700 to be described later generates a current signal in accordance with the operation signal of the operation device 770 and transmits it to the electronic proportional pressure reducing valve 500.

[0045] The control device 700 controls the electronic proportional pressure reducing valve 500 in accordance with the operation signal of the operation device 770 and the angle information of the swash plate angle sensor 730. That is, the electronic proportional pressure reducing valve 500 generates a pilot pressure in accordance with the current signal transmitted from the control device 700.

[0046] Thus, based on the control of the control device 700, the pilot pressure generated by the electronic proportional pressure reducing valve 500 is transmitted to the control valve 300, and the control valve 300 acts in accordance with the received pilot pressure to control the action of the swash plate drive piston 200. In addition, the angle of the swash plate 314 of the hydraulic pump 310 is adjusted in accordance with the action of the swash plate drive piston 200 to control the discharge flow rate of the hydraulic pump 310.

[0047] For example, the pilot pressure generated by the electronic proportional pressure reducing valve 500 based on the control of the control device 700 and transmitted to one side of the control valve 300 can be classified into a first pilot pressure, a second pilot pressure, and a third pilot pressure.

[0048] The first pilot pressure is a pressure smaller than the pressure applied to the other side of the control valve 300 through the valve control hydraulic line 640. Therefore, when the first pilot pressure is applied to one side of the control valve 300, the control valve 300 supplies working oil to the large diameter portion 290 of the swash plate drive piston 200 through the swash plate control hydraulic line 640. Then, when working oil is supplied to the large diameter portion 290 of the swash plate drive piston 200, the angle of the swash plate 314 of the hydraulic pump 310 becomes small, so that the discharge flow rate of the hydraulic pump 310 decreases.

[0049] The second pilot pressure is a pressure greater than the pressure applied to the other side of the control valve 300 through the valve control hydraulic line 640. Therefore, when the second pilot pressure is applied to one side of the control valve 300, as the position of the spool of the control valve 300 is switched, the internal flow path changes, so that the control valve 300 discharges working oil of the large diameter portion 290 of the swash plate drive piston 200 to the discharge line 680. Then, when working oil is discharged from the large diameter portion 290 of the swash plate drive piston 200, the angle of the swash plate 314 of the hydraulic pump 310 becomes large, so that the discharge flow rate of the hydraulic pump 310 increases.

[0050] The third pilot pressure is a pressure greater than the second pilot pressure. When the third pilot pressure is applied to one side of the control valve 300, the spool of the control valve 300 is switched again, and the discharge of the working oil from the control valve 300 to the drain line 680 is prevented. Thus, when the discharge of the working oil from the control valve 300 is prevented, a portion of the working oil discharged from the hydraulic pump 310 is no longer moved along the swash plate control hydraulic line 640, and thus the working oil discharged from the hydraulic pump 310 can be used entirely to drive the drive device. That is, it is possible to prevent the working oil discharged from the hydraulic pump 310 from being wasted by being unnecessarily discharged through the control valve 300.

[0051] On the other hand, when the first pilot pressure and the second pilot pressure are applied to one side of the control valve 300, at least a portion of the working oil discharged from the hydraulic pump 310 and passed through the swash plate control hydraulic line 640 and the valve control hydraulic line 630 to the control valve 300 is discharged through the drain line 680. Thus, the working oil is discharged from the control valve 300 in order to cause the control valve 300 to stably perform a control action. The control valve 300 changes a flow path with the pressure of the working oil as the spool inside is switched.

[0052] Therefore, the hydraulic pump 310 does not need to discharge the working oil at the maximum flow rate, and when the discharge flow rate continues to change as the operation device 770 is operated, the electronic proportional pressure reducing valve 500 generates a pilot pressure lower in magnitude than the third pilot pressure in accordance with the current signal passed by the control device 700, and the control valve 300 activates the swash plate drive piston 200 to increase or decrease the discharge flow rate of the working oil of the hydraulic pump 310 in accordance with the magnitude of the applied pilot pressure.

[0053] Further, as shown in FIG. 1, the control device 700 can control the control valve 300 in a control mode selected from a flow control mode, a horsepower control mode, and a pressure control mode. Figure 3

[0054] In the flow control mode, the control device 700 receives feedback of information from the swash plate angle sensor 730 and calculates a pilot pressure Pi to be generated by the electronic proportional pressure reducing valve 500 so that the hydraulic pump 310 discharges the working oil in compliance with a target discharge flow rate command of the hydraulic pump 310 determined by an operation signal of the operation device 770.

[0055] In the horsepower control mode, the control device 300 receives feedback of information from the swash plate angle sensor 730 and the rotational speed of the engine and calculates a pilot pressure Pd to be generated by the electronic proportional pressure reducing valve 500 so that the rotational speed of the engine is restored when the rotational speed of the engine supplied to the hydraulic pump 310 drops to a set rotational speed while limiting the horsepower required by the hydraulic pump 310 to not exceed a set horsepower.

[0056] ​In the pressure control mode, the control device 700 limits the discharge pressure of the hydraulic pump 310 to not exceed the maximum pressure set for each operation, and calculates the pilot pressure Pc to be generated by the electronic proportional pressure reducing valve 500 using the limited value.

[0057] Then, the control device 700 selects the lowest pilot pressure among the three pilot pressures Pi, Pd, Pc calculated as described above, and controls so that the electronic proportional pressure reducing valve 500 generates the selected pilot pressure.

[0058] Further, in an embodiment of the present application, the control device 700 can control the electronic proportional pressure reducing valve 500 to generate the third pilot pressure which is the selected pilot pressure plus an additional pilot pressure under certain conditions. In this way, the third pilot pressure is the selected pilot pressure plus the additional pilot pressure.

[0059] Specifically, for stability of control, the control device 700 controls so that the electronic proportional pressure reducing valve 500 generates the third pilot pressure only when certain conditions are satisfied.

[0060] After the second pilot pressure is applied to one side of the control valve 300, when the angle of the swash plate 314 measured by the swash plate angle sensor 730 becomes larger than a set angle, the control device 700 can control so that the electronic proportional pressure reducing valve 500 generates the third pilot pressure. Here, the set angle is an angle close to the maximum angle of the swash plate 314 of the hydraulic pump 310. The set angle is set to be slightly lower than the maximum angle in consideration of stability and allowable threshold of the hydraulic pump 310. For example, the set angle can have a size greater than 95% of the maximum angle.

[0061] Further, after the second pilot pressure is applied to one side of the control valve 300 so that the working oil is discharged from the large diameter portion 290 of the swash plate drive piston 200 to the maximum extent, when the operation signal of the operation device 770 requires the hydraulic pump 310 to discharge the maximum flow rate, the control device 700 can control so that the electronic proportional pressure reducing valve 500 generates the third pilot pressure.

[0062] Further, when the pilot pressure Pi required according to the flow control mode is lower than the pilot pressure Pd required according to the horsepower control mode and the pilot pressure Pc required according to the pressure control mode, the control device 700 can control so that the electronic proportional pressure reducing valve 500 generates the third pilot pressure.

[0063] In addition, when all of the three conditions are satisfied, or when one or more of the conditions are satisfied depending on the situation, the control device 700 can control the electronic proportional pressure reducing valve 500 to generate the third pilot pressure which is the selected pilot pressure plus an additional pilot pressure.

[0064] On the other hand, when none of the three conditions is satisfied, or when any of the profiles is not satisfied, the control device 700 can control the electronic proportional pressure reducing valve 500 to generate the selected pilot pressure therefrom.

[0065] Figure 3 As shown in the hydraulic system 101 of an embodiment of the present application, the pilot pressure generated by the control device 700 controlling the electronic proportional pressure reducing valve 500 varies the flow rate of the working oil discharged from the hydraulic pump 310 and supplied to the various driving devices.

[0066] As shown in the hydraulic system 101 of an embodiment of the present application, the pilot pressure generated by the control device 700 controlling the electronic proportional pressure reducing valve 500 varies the flow rate of the working oil discharged from the hydraulic pump 310 and supplied to the various driving devices. Figure 3 As shown, as the pilot pressure transmitted to the control valve 300 rises, the swash plate driving piston 200 increases the angle of the swash plate 314 of the hydraulic pump 310, so that the discharge flow rate of the hydraulic pump 310 gradually increases.

[0067] After the angle of the swash plate 314 becomes maximum, at the time point A at which the discharge flow rate of the hydraulic pump 310 cannot be further increased, when the hydraulic pump 310 is continuously required to discharge the maximum flow rate, as the highest third pilot pressure is transmitted to the control valve 500, the position of the spool of the control valve 300 is switched, so that the working oil is prevented from being discharged from the control valve 300.

[0068] Thus, a part of the working oil discharged from the hydraulic pump 310 is no longer directed toward the control valve 300, so that the working oil discharged from the hydraulic pump 310 is all used for driving the driving devices.

[0069] As such, when the flow rate of the working oil discharged from the hydraulic pump 310 and supplied to the driving devices increases (ΔQ), an effect equivalent to improving the performance of the hydraulic pump 310 can be substantially obtained.

[0070] According to this configuration, the hydraulic system 101 of an embodiment of the present application can minimize unnecessary flow rate loss.

[0071] That is, when the hydraulic pump 310 is required to discharge the maximum flow rate, the working oil is prevented from being discharged from the control valve 300, so that the working oil discharged from the hydraulic pump 310 is all used for driving the driving devices, thereby enabling the performance of the hydraulic pump 310 to be maximally utilized.

[0072] Although embodiments of the present application have been described above with reference to the accompanying drawings, it will be understood by those skilled in the art that the present application can be embodied in other specific forms without changing the technical idea or essential characteristics thereof.

[0073] Therefore, the embodiments described above are to be understood in all respects as illustrative only, and not restrictive, the scope of the application being indicated by the appended claims rather than by the foregoing description, all changes and modifications that come within the meaning of the claims, the scope of the claims and any equivalents thereof are to be embraced by the claims.

[0074] Reference Signs

[0075] 101: hydraulic system, 200: swash plate drive piston, 210: small diameter portion, 290: large diameter portion, 300: control valve, 310: hydraulic pump, 314: swash plate, 370: pilot pump, 500: electronic proportional pressure reducing valve, 630: valve control hydraulic line, 640: swash plate control hydraulic line, 700: control device, 730: swash plate angle sensor, 770: operating device, 800: oil tank.

[0076] Industrial Applicability

[0077] According to the embodiment of the present application, the hydraulic system can be used to minimize unnecessary flow loss.

Claims

1. A hydraulic system characterized by, Comprising: a variable displacement hydraulic pump that discharges working oil and includes a swash plate; a swash plate angle sensor that measures an angle of the swash plate; a swash plate drive piston that has a large diameter portion and a small diameter portion and moves the swash plate of the hydraulic pump according to a change in pressure applied to the large diameter portion; a swash plate control hydraulic line for supplying a portion of the working oil discharged from the hydraulic pump to the large diameter portion; a control valve provided on the swash plate control hydraulic line and controls a flow rate of the working oil supplied to or discharged from the large diameter portion; an electronic proportional pressure reducing valve that generates a pilot pressure to be transmitted to one side of the control valve; an operating device that generates an operating signal; a control device that controls the electronic proportional pressure reducing valve according to the operating signal of the operating device and the angle information of the swash plate angle sensor; a discharge line connected to the control valve; and a valve control hydraulic line that branches from the swash plate control hydraulic line more upstream than the control valve and transmits pressure to the other side of the control valve, the electronic proportional pressure reducing valve generating and transmitting the pilot pressure to one side of the control valve based on the control of the control device includes: a first pilot pressure that is smaller than the pressure applied to the other side of the control valve through the valve control hydraulic line; a second pilot pressure that is greater than the pressure applied to the other side of the control valve through the valve control hydraulic line; and a third pilot pressure that is greater than the second pilot pressure.

2. The hydraulic system according to claim 1, wherein when the first pilot pressure is applied to one side of the control valve, the control valve supplies working oil to the large diameter portion of the swash plate drive piston through the swash plate control hydraulic line; when the second pilot pressure is applied to one side of the control valve, the control valve discharges working oil of the large diameter portion of the swash plate drive piston to the discharge line; when the third pilot pressure is applied to one side of the control valve, the discharge of working oil from the control valve to the discharge line is prevented.

3. The hydraulic system according to claim 2, wherein when the first pilot pressure and the second pilot pressure are applied to one side of the control valve, at least a portion of the working oil discharged from the hydraulic pump and transmitted to the control valve through the swash plate control hydraulic line and the valve control hydraulic line is discharged through the discharge line.

4. The hydraulic system according to claim 2, wherein when working oil is supplied to the large diameter portion of the swash plate drive piston, the angle of the swash plate of the hydraulic pump becomes smaller so that the discharge flow rate of the hydraulic pump decreases; when working oil is discharged from the large diameter portion of the swash plate drive piston, the angle of the swash plate of the hydraulic pump becomes larger so that the discharge flow rate of the hydraulic pump increases.

5. The hydraulic system according to claim 2, wherein After the second pilot pressure is applied to one side of the control valve, when the swash plate angle measured by the swash plate angle sensor becomes larger than a set angle, the control device controls the electronic proportional pressure reducing valve to generate the third pilot pressure.

6. The hydraulic system according to claim 2, wherein After the second pilot pressure is applied to one side of the control valve to discharge the working oil from the large diameter portion of the swash plate drive piston to the maximum extent, when the operation signal of the operation device requires the hydraulic pump to discharge the maximum flow rate, the control device controls the electronic proportional pressure reducing valve to generate the third pilot pressure.

7. The hydraulic system according to claim 5 or 6, wherein The control device calculates the pilot pressure required according to the flow rate control mode, the pilot pressure required according to the horsepower control mode, and the pilot pressure required according to the pressure control mode, respectively; and The control device selects the lowest pilot pressure from the calculated pilot pressures, and controls the electronic proportional pressure reducing valve to generate the selected pilot pressure.

8. The hydraulic system according to claim 7, wherein When the pilot pressure required according to the flow rate control mode is lower than the pilot pressure required according to the horsepower control mode and the pilot pressure required according to the pressure control mode, the control device controls the electronic proportional pressure reducing valve to generate the third pilot pressure.

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