A dual pump source hydraulic system

Abstract

The application discloses a double-pump source hydraulic system, and belongs to the technical field of hydraulic control, and aims at solving the technical problem of poor system stability when a pump is switched from a flow control mode to a pressure control mode in a hydraulic system, and provides the following solution: the double-pump source hydraulic system comprises a driving module, a control module and an executing module, the executing module comprises two load driving devices, the driving module comprises an oil tank, a constant-displacement pump, an electro-hydraulic proportional variable pump, a servo motor and a three-phase frequency conversion synchronous motor; and the control module comprises a servo motor controller, a controller, a frequency converter and a reversing valve. The application can realize the independent operation of any one load driving device, that is, the double-pump source hydraulic system is in the pressure control mode or the flow control mode, and at the same time, the two load driving devices can be simultaneously operated, that is, the constant-displacement pump and the electro-hydraulic proportional pump are simultaneously operated, so that the control of the pump outlet flow and pressure under different load change conditions is realized under the premise of small energy loss, and the system stability is effectively enhanced.

Description

Technical Field

[0001] This invention belongs to the field of hydraulic control technology and relates to a dual-pump-source hydraulic system. Background Technology

[0002] In some applications, depending on the type of load, sometimes the load moves at a high speed, which requires a small change in the dynamic response speed of the system oil pressure, and in this case, the system needs a large oil flow rate; sometimes the individual load moves at a low speed, but the load changes quickly, which requires the system to have a fast dynamic response.

[0003] In current pump-valve coordinated control systems, when the load moves rapidly, i.e., the system requires a large flow rate, the flow resolution requirement is not high. At this time, it is in flow control mode, where the flow rate is mainly regulated by the pump and the throttle valve is fully open. When the load moves slightly, i.e., the system requires precise flow rate regulation, the flow rate is mainly regulated by the valve opening, and the pump is responsible for maintaining a constant pressure difference between the valve inlet and outlet. At this time, it is in pressure control mode. However, when the pump switches from flow control mode to pressure control mode, the system will vibrate. Therefore, a hydraulic control system that can enhance system stability when the system switches control modes is needed. Summary of the Invention

[0004] The main objective of this invention is to overcome the shortcomings of the prior art. This invention provides a dual-pump hydraulic system.

[0005] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0006] A dual-pump hydraulic system includes a drive module, a control module, and an execution module. The execution module includes two load drive devices, which are either two hydraulic cylinders, two hydraulic motors, or a combination of one hydraulic cylinder and one hydraulic motor.

[0007] The drive module includes an oil tank, a fixed displacement pump, an electro-hydraulic proportional variable displacement pump, a servo motor, and a three-phase variable frequency synchronous motor. The oil tank includes a first oil tank and a second oil tank. The inlet of the fixed displacement pump is connected to the first oil tank via a pipeline. The servo motor controls the flow rate at the outlet of the fixed displacement pump by controlling its rotational speed. The electro-hydraulic proportional variable displacement pump is the main pump source of the dual-pump-source hydraulic system. The inlet of the electro-hydraulic proportional variable displacement pump is connected to the second oil tank via a pipeline. The flow rate at the outlet of the electro-hydraulic proportional variable displacement pump is controlled by changing its swing angle, or by the three-phase variable frequency synchronous motor controlling its rotational speed. A check valve and a relief valve are respectively installed on the pipelines on the outlet side of the fixed displacement pump and the electro-hydraulic proportional variable displacement pump. The check valve is used to prevent the hydraulic oil in the system from flowing back to the pump outlet, and the relief valve is used as a safety valve to ensure that the system pressure is within a safe range.

[0008] The control module includes a servo motor controller, a controller, a frequency converter, and a reversing valve. The pipe connecting the outlet of the fixed displacement pump and the pipe connecting the outlet of the electro-hydraulic proportional variable pump are connected through an oil control main pipe. The oil control main pipe is connected to two three-position four-way electro-hydraulic proportional reversing valves. Overflow valves are installed on the pipes on the outlet side of the fixed displacement pump, the outlet side of the electro-hydraulic proportional variable pump, and the oil control main pipe. When the three-position four-way electro-hydraulic proportional reversing valve is in its initial state, it is in the neutral position. Port P is connected to the main oil control pipe. Port A of the three-position four-way electro-hydraulic proportional directional valve is connected to the rodless chamber of the load drive device. Port B of the three-position four-way electro-hydraulic proportional directional valve is connected to the rod chamber of the load drive device. Port T of the three-position four-way electro-hydraulic proportional directional valve is connected to the replenishing oil tank. Pressure sensors are installed at the P and A ports of the four-way electro-hydraulic proportional directional valve, and displacement sensors are installed on the piston rod of the load drive device to monitor the displacement of the hydraulic cylinder piston rod. Both the pressure sensors and the displacement sensors are electrically connected to the controller.

[0009] The servo motor controller is electrically connected to the servo motor, and the frequency converter is electrically connected to the three-phase variable frequency synchronous motor. The controller is electrically connected to both the servo motor controller and the frequency converter. The controller receives displacement feedback signals from the displacement sensor and then issues control signals. The controller controls the speed of the servo motor through the servo motor controller, controls the speed of the three-phase variable frequency synchronous motor through the frequency converter, and controls the valve opening of the three-position four-way electro-hydraulic proportional directional valve and the swing angle of the electro-hydraulic proportional variable pump.

[0010] Furthermore, the displacement of the metering pump and the speed of the servo motor are determined based on the leakage of the electro-hydraulic proportional variable pump at different pressures.

[0011] Furthermore, the speed range of the servo motor is determined based on the minimum speed and rated speed of the metering pump.

[0012] Furthermore, the individual operation state of any single load drive device:

[0013] 1) When the piston rod speed of the load drive device in operation is less than 0.2 m / s, the electro-hydraulic proportional variable pump is unloaded, the fixed displacement pump operates alone, and the dual-pump hydraulic system is in pressure control mode:

[0014] First, the target differential pressure value Δp is set for the three-position four-way electro-hydraulic proportional directional valve via the controller;

[0015] Then, calculate the flow rate required to reach the target differential pressure and replenish the leakage within the system:

[0016]

[0017] Q is the flow rate required to achieve the target differential pressure and replenish the system's internal leakage. C q Here, A0 is the flow coefficient, A0 is the flow area of ​​the three-position four-way electro-hydraulic proportional directional valve, Δp is the target pressure difference, and ρ is the liquid density.

[0018] The servo motor speed is calculated based on the required flow rate Q, and the servo motor speed is controlled by the servo motor controller to make the inlet and outlet pressure difference of the three-position four-way electro-hydraulic proportional directional valve reach the target value Δp.

[0019] Finally: Based on the real-time pressure difference between the valve inlet and outlet, adjust the valve opening of the three-position four-way electro-hydraulic proportional directional valve to stabilize the flow rate through the valve and keep the pressure difference between the valve inlet and outlet within the target range of ±0.05MPa.

[0020] 2) When the piston rod speed of the load drive device in operation is greater than or equal to 0.2 m / s, the dual-pump hydraulic system is in flow control mode:

[0021] The controller fully opens the valve of the three-position four-way electro-hydraulic proportional directional valve and controls the swing angle of the electro-hydraulic proportional variable pump to control its displacement. Simultaneously, the inverter controls the speed of the three-phase variable frequency synchronous motor, ultimately controlling the flow rate of the electro-hydraulic proportional variable pump. Furthermore, to ensure a rapid transition from pressure control to flow control mode in the dual-pump hydraulic system, the controller sets the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valve to ±0.5 MPa. It then calculates the flow rate required to reach the target pressure difference and partially compensate for system leakage. Based on the required flow rate, the servo motor speed is calculated, and the servo motor speed is adjusted by the servo motor controller to control the outlet pressure of the fixed displacement pump. Ultimately, the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valve is maintained within the target value of ±0.05 MPa.

[0022] Furthermore, when both load drive devices are running simultaneously, the quantitative pump and the electro-hydraulic proportional pump operate simultaneously:

[0023] First, determine the system pressure using the maximum load pressure. Keep the valve opening of the three-position four-way electro-hydraulic proportional directional valve on the same route constant at the maximum load pressure. Set the target pressure difference value between the inlet and outlet of this three-position four-way electro-hydraulic proportional directional valve.

[0024] Then, calculate the flow rate through the three-position four-way electro-hydraulic proportional directional valve with the maximum load pressure.

[0025] Finally, the swing angle of the electro-hydraulic proportional variable pump and the frequency of the frequency converter are calculated. By changing the swing angle of the electro-hydraulic proportional variable pump and changing the speed of the three-phase variable frequency synchronous motor through the frequency converter, the outlet flow rate of the electro-hydraulic proportional variable pump is changed. The outlet flow rate of the fixed displacement pump is calculated by the leakage of the electro-hydraulic proportional variable pump and the target pressure difference between the valve inlet and outlet, thereby calculating the speed of the servo motor. In addition, the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valve with the same pressure as the small load is calculated in real time by the controller. The target flow rate value is achieved by changing the valve opening of the three-position four-way electro-hydraulic proportional directional valve.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] The present invention provides a dual-pump hydraulic system that enables control of pump outlet flow and pressure under different load changes with minimal energy loss, effectively enhancing system stability. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the dual-pump hydraulic system structure of the present invention.

[0029] In the diagram, 1 is a fixed displacement pump, 2 is an electro-hydraulic proportional variable displacement pump, 3 is a servo motor, 4 is a three-phase variable frequency synchronous motor, 5 and 6 are check valves, 7 and 8 are three-position four-way electro-hydraulic proportional directional valves, 9 and 10 are load drive devices, 11 and 12 are oil tanks, 13 is a servo motor controller, 14 is a controller, 15 is a frequency converter, 16, 17, and 24 are overflow valves, 18, 19, 20, and 21 are pressure sensors, and 22 and 23 are displacement sensors. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0031] like Figure 1 The illustrated dual-pump hydraulic system includes a drive module, a control module, and an execution module. The execution module includes two load drive devices, which are either two hydraulic cylinders, two hydraulic motors, or a combination of one hydraulic cylinder and one hydraulic motor.

[0032] The drive module includes an oil tank, a fixed displacement pump 1, an electro-hydraulic proportional variable displacement pump 2, a servo motor 3, and a three-phase variable frequency synchronous motor 4. The oil tank includes a first oil tank 11 and a second oil tank 12. The oil inlet of the fixed displacement pump 1 is connected to the first oil tank 11 through a pipe. The servo motor 3 controls the oil flow rate at the outlet of the fixed displacement pump 1 by controlling its rotation speed. The electro-hydraulic proportional variable displacement pump 2 is the main pump source of the dual-pump-source hydraulic system. The oil inlet of the electro-hydraulic proportional variable displacement pump 2 is connected to the second oil tank 12 through a pipe. The oil flow rate at the outlet of the electro-hydraulic proportional variable displacement pump 2 is controlled by changing its swing angle, or the three-phase variable frequency synchronous motor 4 controls the oil flow rate at the outlet of the electro-hydraulic proportional variable displacement pump 2 by controlling its rotation speed. Check valves 5 and 6 and relief valves 16 and 17 are respectively installed on the pipes on the oil outlet side of the fixed displacement pump 1 and the electro-hydraulic proportional variable displacement pump 2.

[0033] The control module includes a servo motor controller 13, a controller 14, a frequency converter 15, and reversing valves 7 and 8. The pipe connecting the outlet of the fixed displacement pump 1 and the pipe connecting the outlet of the electro-hydraulic proportional variable pump 2 are connected through an oil circuit control main pipe. The oil circuit control main pipe is connected to two three-position four-way electro-hydraulic proportional reversing valves 7 and 8 respectively. Overflow valves 16, 17, and 24 are respectively installed on the pipe on the outlet side of the fixed displacement pump 1, the pipe on the outlet side of the electro-hydraulic proportional variable pump 2, and the oil circuit control main pipe. When the three-position four-way electro-hydraulic proportional reversing valves 7 and 8 are in the initial state, they are in the neutral position. The P ports of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 are connected to the oil control main pipe, the A ports of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 are connected to the rodless chamber of the load drive device, the B ports of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 are connected to the rod chamber of the load drive device, and the T ports of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 are connected to the replenishment oil tank. Pressure sensors 18, 19, 20, and 21 are respectively installed at the P port and A port positions on the four-way electro-hydraulic proportional directional valves 7 and 8, and displacement sensors 22 and 23 are respectively installed on the piston rod of the load drive device. Pressure sensors 18, 19, 20, and 21 and displacement sensors 22 and 23 are all electrically connected to the controller 14.

[0034] The servo motor controller 13 is electrically connected to the servo motor 3, and the frequency converter 15 is electrically connected to the three-phase variable frequency synchronous motor 4. The controller 14 is electrically connected to both the servo motor controller 13 and the frequency converter 15. The controller 14 receives displacement feedback signals from displacement sensors 22 and 23, and then sends control signals. The controller 14 controls the speed of the servo motor 3 through the servo motor controller 13, controls the speed of the three-phase variable frequency synchronous motor 4 through the frequency converter 15, and controls the valve opening of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 and the swing angle of the electro-hydraulic proportional variable pump 2.

[0035] Furthermore, the displacement of the metering pump 1 and the rotational speed of the servo motor 3 are determined based on the leakage of the electro-hydraulic proportional variable pump 2 at different pressures.

[0036] Furthermore, the speed range of the servo motor 3 is determined based on the minimum speed and rated speed of the metering pump 1.

[0037] Example 1

[0038] In the dual-pump hydraulic system provided in Embodiment 1, when any one of the load drive devices is operating alone, and the piston rod speed of the operating load drive device is 0.1 m / s, the electro-hydraulic proportional variable pump 2 is unloaded, the fixed displacement pump 1 operates alone, and the dual-pump hydraulic system is in pressure control mode.

[0039] First, the target differential pressure value Δp is set for the three-position four-way electro-hydraulic proportional directional valves 7 and 8 through the controller 14. In this embodiment 1, the target differential pressure value Δp = 2MPa.

[0040] Then, calculate the flow rate required to reach the target differential pressure and replenish the leakage within the system:

[0041]

[0042] Q is the flow rate required to achieve the target differential pressure and replenish the system leakage, and the flow coefficient is C. q =0.7, A0 is the flow area of ​​the three-position four-way electro-hydraulic proportional directional valve orifice, the target pressure difference Δp is 2MPa, and the liquid density ρ is 850kg / m³. 3 ;

[0043] The rotational speed of servo motor 3 is calculated based on the required flow rate Q. The rotational speed of servo motor 3 is controlled by servo motor controller 13, thereby making the pressure difference between the inlet and outlet of three-position four-way electro-hydraulic proportional directional valves 7 and 8 reach the target value Δp = 2MPa.

[0044] Finally: Based on the real-time pressure difference between the valve inlet and outlet, adjust the valve opening of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 to maintain a stable flow rate through the three-position four-way electro-hydraulic proportional directional valves 7 and 8, and keep the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 within the target value ±0.05MPa range.

[0045] Example 2

[0046] In the dual-pump hydraulic system provided in Embodiment 2, when any one load drive device is operating independently and the piston rod speed of the operating load drive device is 0.25 m / s, the dual-pump hydraulic system is in flow control mode.

[0047] The controller 14 controls the valve ports of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 to be fully opened. The controller 14 controls the swing angle of the electro-hydraulic proportional variable pump 2 to achieve the control of the displacement of the electro-hydraulic proportional variable pump 2. At the same time, the frequency converter 15 controls the speed of the three-phase variable frequency synchronous motor 4 to finally achieve the flow control of the electro-hydraulic proportional variable pump 2.

[0048] In addition, to ensure that the dual-pump hydraulic system can quickly switch from pressure control mode to flow control mode, the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valves 7 and 8 is set to ±0.5MPa by the controller 14, so that the fixed displacement pump 1 can maintain a low speed of about 600r / min.

[0049] Then, calculate the flow rate required to reach the target differential pressure and replenish the leakage within the system:

[0050]

[0051] Q is the flow rate required to achieve the target differential pressure and replenish the system leakage, and the flow coefficient is C. q =0.7, A0 is the flow area of ​​the three-position four-way electro-hydraulic proportional directional valve orifice, the target pressure difference Δp is 0.5MPa, and the liquid density ρ is 850kg / m³. 3 ;

[0052] Finally, based on the required flow rate, the rotation speed of servo motor 3 is calculated to be 600 r / min. The rotation speed of servo motor 3 is adjusted by servo motor controller 13, thereby controlling the outlet pressure of quantitative pump 1 to be 0.5 MPa higher than the maximum load pressure, so that the pressure difference between the inlet and outlet of three-position four-way electro-hydraulic proportional directional valves 7 and 8 is kept within the target value of ±0.05 MPa.

[0053] Example 3

[0054] This embodiment 3 provides a dual-pump hydraulic system in which, when two load drive devices are running simultaneously, the fixed displacement pump 1 and the electro-hydraulic proportional pump 2 are operating simultaneously:

[0055] First, the system pressure is determined to be 25MPa by the maximum load pressure. The valve opening of the three-position four-way electro-hydraulic proportional directional valve with the same maximum load pressure remains unchanged. The target pressure difference value Δp between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valve is set.

[0056] Then, calculate the flow rate through the three-position four-way electro-hydraulic proportional directional valve in the same path as the maximum load pressure:

[0057]

[0058] Q is the flow rate required to achieve the target differential pressure and replenish the system leakage, and the flow coefficient is C. q=0.7, A0 is the flow area of ​​the three-position four-way electro-hydraulic proportional directional valve orifice, the target pressure difference Δp is 2MPa, and the liquid density ρ is 850kg / m³. 3 ;

[0059] Finally, the swing angle of the electro-hydraulic proportional variable pump 3 and the frequency of the frequency converter 15 are calculated. The swing angle of the electro-hydraulic proportional variable pump 2 is changed, and the speed of the three-phase variable frequency synchronous motor 4 is changed by the frequency converter 15, thereby changing the outlet flow rate of the electro-hydraulic proportional variable pump 2. The outlet flow rate of the fixed displacement pump 3 is calculated by the leakage of the electro-hydraulic proportional variable pump 2 and the target pressure difference between the valve inlet and outlet, thereby calculating the speed of the servo motor 3. In addition, the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valve with the same pressure under low load is calculated in real time by the controller 14, and the target flow rate value is achieved by changing the valve opening of the three-position four-way electro-hydraulic proportional directional valve.

[0060] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A dual-pump hydraulic system, comprising a drive module, a control module, and an execution module, wherein the execution module includes two load drive devices, wherein the two load drive devices are two hydraulic cylinders, or two hydraulic motors, or a combination of one hydraulic cylinder and one hydraulic motor, characterized in that: The drive module includes an oil tank, a fixed displacement pump (1), an electro-hydraulic proportional variable displacement pump (2), a servo motor (3), and a three-phase variable frequency synchronous motor (4). The oil tank includes a first oil tank (11) and a second oil tank (12). The oil inlet of the fixed displacement pump (1) is connected to the first oil tank (11) through a pipeline. The servo motor (3) controls the oil flow rate of the fixed displacement pump (1) by controlling the rotation speed. The electro-hydraulic proportional variable displacement pump (2) is the main pump source of the dual-pump hydraulic system. The oil inlet of the electro-hydraulic proportional variable displacement pump (2) is connected to the first oil tank (11) through a pipeline. Connected to the second oil tank (12), the electro-hydraulic proportional variable pump (2) controls the flow rate at the outlet of the electro-hydraulic proportional variable pump (2) by changing the swing angle, or the three-phase variable frequency synchronous motor (4) controls the flow rate at the outlet of the electro-hydraulic proportional variable pump (2) by controlling the speed. The displacement of the metering pump (1) and the speed of the servo motor (3) are determined according to the leakage of the electro-hydraulic proportional variable pump (2) at different pressures. Check valves (5, 6) are respectively installed on the pipelines on the outlet side of the metering pump (1) and the electro-hydraulic proportional variable pump (2). The control module includes a servo motor controller (13), a controller (14), a frequency converter (15), and reversing valves (7, 8). The pipe connecting the outlet of the fixed displacement pump (1) and the pipe connecting the outlet of the electro-hydraulic proportional variable pump (2) are connected through the oil circuit control main pipe. The oil circuit control main pipe is connected to two three-position four-way electro-hydraulic proportional reversing valves (7, 8). Overflow valves (16, 17, 24) are respectively installed on the pipe on the outlet side of the fixed displacement pump (1), the pipe on the outlet side of the electro-hydraulic proportional variable pump (2), and the oil circuit control main pipe. When the three-position four-way electro-hydraulic proportional reversing valves (7, 8) are in the initial state, the three-position four-way electro-hydraulic proportional reversing valves (7, 8) are in the middle position, and the three-position four-way electro-hydraulic proportional reversing valves (7, 8) are in the middle position. The P ports of the three-position four-way electro-hydraulic proportional directional valves (7, 8) are connected to the oil control main pipe respectively. The A ports of the three-position four-way electro-hydraulic proportional directional valves (7, 8) are connected to the rodless chamber of the load drive device respectively. The B ports of the three-position four-way electro-hydraulic proportional directional valves (7, 8) are connected to the rod chamber of the load drive device respectively. The T port of the three-position four-way electro-hydraulic proportional directional valves (7, 8) is connected to the oil replenishment tank. Pressure sensors (18, 19, 20, 21) are respectively installed at the P port and A port on the four-way electro-hydraulic proportional directional valves (7, 8). Displacement sensors (22, 23) are respectively installed on the piston rod of the load drive device. The pressure sensors (18, 19, 20, 21) and displacement sensors (22, 23) are all electrically connected to the controller (14). The servo motor controller (13) is electrically connected to the servo motor (3), the frequency converter (15) is electrically connected to the three-phase variable frequency synchronous motor (4), and the controller (14) is electrically connected to the servo motor controller (13) and the frequency converter (15) respectively. The controller (14) receives the displacement feedback signal from the displacement sensors (22, 23), and then the controller (14) sends a control signal. The controller (14) controls the speed of the servo motor (3) through the servo motor controller (13), and controls the speed of the three-phase variable frequency synchronous motor (4) through the frequency converter (15). The controller (14) controls the valve opening of the three-position four-way electro-hydraulic proportional directional valve (7, 8) and the swing angle of the electro-hydraulic proportional variable pump (2).

2. The dual-pump hydraulic system according to claim 1, characterized in that: The speed range of the servo motor (3) is determined based on the minimum speed and rated speed of the metering pump (1).

3. The dual-pump hydraulic system according to claim 1, characterized in that: Individual operation of any load drive device: 1) When the piston rod of the load drive device in operation moves at a speed less than 0.2 m / s, the electro-hydraulic proportional variable pump (2) is unloaded, the fixed displacement pump (1) operates alone, and the dual-pump hydraulic system is in pressure control mode: First, the target differential pressure value Δp is set for the three-position four-way electro-hydraulic proportional directional valves (7, 8) through the controller (14); Then, calculate the flow rate required to reach the target differential pressure and replenish the leakage within the system: ； In the formula, Q is the flow rate required to achieve the target differential pressure value and replenish the leakage in the system, and C is the flow rate required to achieve the target differential pressure value and replenish the leakage in the system. q Here, A0 is the flow coefficient, A0 is the flow area of ​​the three-position four-way electro-hydraulic proportional directional valve, Δp is the target pressure difference, and ρ is the liquid density. The speed of the servo motor (3) is calculated based on the required flow rate Q. The speed of the servo motor (3) is controlled by the servo motor controller (13) so that the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valve (7, 8) reaches the target value Δp. Finally: Based on the real-time pressure difference between the valve inlet and outlet, change the valve opening of the three-position four-way electro-hydraulic proportional directional valve (7, 8) to stabilize the flow through the three-position four-way electro-hydraulic proportional directional valve (7, 8) and keep the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valve (7, 8) within the target value ±0.05MPa range. 2) When the piston rod speed of the load drive device in operation is greater than or equal to 0.2 m / s, the dual-pump hydraulic system is in flow control mode: The controller (14) controls the valve ports of the three-position four-way electro-hydraulic proportional directional valves (7, 8) to be fully opened. The controller (14) controls the swing angle of the electro-hydraulic proportional variable pump (2) to achieve the control of the displacement of the electro-hydraulic proportional variable pump (2). At the same time, the frequency converter (15) controls the speed of the three-phase variable frequency synchronous motor (4) to finally achieve the flow control of the electro-hydraulic proportional variable pump (2). In addition, in order to ensure that the dual-pump source hydraulic system can quickly switch from pressure control mode to flow control mode, the controller (14) controls the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valves (7, 8) to be ±0.5MPa. Then, the flow rate required to reach the target pressure difference value and partially supplement the leakage in the system is calculated. The speed of the servo motor (3) is calculated according to the required flow rate. The speed of the servo motor (3) is adjusted by the servo motor controller (13) to control the outlet pressure of the metering pump (1). Finally, the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valves (7, 8) is kept within the target value of ±0.05MPa.

4. A dual-pump hydraulic system according to claim 1, characterized in that: When both load drive devices are running simultaneously, the quantitative pump (1) and the electro-hydraulic proportional pump (2) are operating simultaneously: First, determine the system pressure using the maximum load pressure. Keep the valve opening of the three-position four-way electro-hydraulic proportional directional valve on the same route constant at the maximum load pressure. Set the target pressure difference value between the inlet and outlet of this three-position four-way electro-hydraulic proportional directional valve. Then, calculate the flow rate through the three-position four-way electro-hydraulic proportional directional valve with the maximum load pressure. Finally, the swing angle of the electro-hydraulic proportional variable pump (3) and the frequency of the inverter (15) are calculated. The swing angle of the electro-hydraulic proportional variable pump ((2)) is changed. The speed of the three-phase variable frequency synchronous motor (4) is changed by the inverter (15), thereby changing the outlet flow of the electro-hydraulic proportional variable pump (2). The outlet flow of the fixed pump (3) is calculated by the leakage of the electro-hydraulic proportional variable pump (2) and the target pressure difference between the valve inlet and outlet, thereby calculating the speed of the servo motor (3). In addition, the pressure difference between the inlet and outlet of the three-position four-way electro-hydraulic proportional directional valve with the same small load pressure is calculated in real time by the controller (14). The target flow value is achieved by changing the valve opening of the three-position four-way electro-hydraulic proportional directional valve.

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