Hydraulic system with control strategies adapted to different load levels

Abstract

A hydraulic system includes a hydraulic actuator, a command input element that inputs an operation command of the hydraulic actuator by an operator, a main pump that supplies working hydraulic oil to the hydraulic actuator, a main valve that controls a direction and a pressure of the working hydraulic oil supplied from the main pump to the hydraulic actuator at a working valve position, a pilot valve that outputs a pilot pressure to the main valve to control an opening degree of the main valve, a load detecting element that detects a load received by the hydraulic actuator, and a controller that determines the pilot pressure output by the pilot valve based on the operation command input by the command input element and the load detected by the load detecting element, thereby controlling the opening degree of the main valve and a back pressure in the main valve so that: when the load is lower than or equal to a first load level, the main valve pilot pressure is controlled to be in a range equal to or lower than a first pressure; and when the load is higher than the first load level, the main valve pilot pressure is controlled to be in a range equal to or lower than a second pressure.

Description

Technical Field

[0001] This application relates to a hydraulic system that employs different control strategies for different load levels. Background Technology

[0002] Many mechanical devices have hydraulically driven working components. The workload on these components can vary significantly depending on the type of work the machine is performing. For example, an excavator's bucket is primarily used for leveling and digging. During leveling, the bucket moves quickly across the ground, bearing a relatively small load. During digging, the bucket digs into the ground, moving at a much slower speed than during leveling, and bearing a much larger load.

[0003] The movement of an excavator bucket is primarily achieved by driving the stick with the boom cylinder, although the boom cylinder also assists in some strokes. The main valve of the stick cylinder is typically designed to provide high back pressure to enable rapid bucket movement on level ground through hydraulic regeneration. However, during excavation operations, due to the heavy load, the high back pressure in the valve core not only fails to generate hydraulic regeneration but also creates an additional load on the hydraulic system, reducing the power efficiency of the hydraulic system's power source (such as the engine).

[0004] Similar situations exist in the hydraulic systems of other mechanical equipment. That is, the high back pressure design in the main valve spool or the valve spool with external regeneration function is beneficial for achieving rapid low-load operation, but it will lead to lower power efficiency of the power source during low-speed high-load operation. Summary of the Invention

[0005] One objective of this application is to employ different control strategies in a hydraulic system for different load levels, so as to take advantage of back pressure during fast, low-load operation and maximize the power efficiency of the power source during low-speed, high-load operation.

[0006] Therefore, this application provides a hydraulic system in one aspect, comprising: A hydraulic actuator configured to achieve at least first and second load levels of operation, wherein the second load level is higher than the first load level; A command input element configured to allow an operator to input action commands for the hydraulic actuator; The main pump is configured to supply working hydraulic oil to the hydraulic actuator; The main valve is configured to control the direction and pressure of the working hydraulic oil supplied by the main pump to the hydraulic actuator in the working valve position; and in the working valve position, an adjustable back pressure is established in the main valve, the back pressure in the main valve being related to the opening degree of the main valve and the return flow area. A pilot valve, configured to output pilot pressure to the main valve to control the opening degree of the main valve; A load sensing element configured to detect the load borne by the hydraulic actuator; The controller is configured to determine the pilot pressure output by the pilot valve based on the action command input by the command input element and the load detected by the load detection element, thereby controlling the opening degree of the main valve and the back pressure in the main valve, such that: When the load detected by the load detection element is lower than or equal to the first load level, the pilot pressure of the main control valve is in the range of equal to or lower than the first pressure; when the load detected by the load detection element is higher than the first load level, the pilot pressure of the main control valve is in the range of equal to or lower than the second pressure, and the second pressure is higher than the first pressure.

[0007] In one embodiment, the load detection element is a pressure sensor configured to detect the output pressure of the main pump or the input pressure of the hydraulic actuator.

[0008] In one embodiment, the main valve has a working port A, a working port B, an inlet port P, and a return port T, and includes a valve body and a valve core that is axially slidable in the valve body. The valve body has a first valve body oil groove communicating with the working port B and a second valve body oil groove communicating with the return port T. The valve core is configured to generate an adjustable back pressure in the first valve body oil groove when the valve is in the working position.

[0009] In one embodiment, the valve core is designed such that the first valve body oil groove and the second valve body oil groove have different return oil flow areas at different axial positions of the valve core relative to the valve body.

[0010] In one embodiment, a valve core oil groove is formed on the outer periphery of the valve core. In the original position of the main valve, the first valve body oil groove faces the valve core oil groove. The second valve body oil groove is closed by the grooveless valve core portion. There is a return oil groove between the grooveless valve core portion and the valve core oil groove. Along the direction from the grooveless valve core portion toward the valve core oil groove, the diameter of the return oil groove gradually decreases.

[0011] In one embodiment, the controller is configured to: determine the input command-related demand pilot pressure based on the relationship between the input command of the command input element and the main valve demand pilot pressure, and determine the load-related maximum demand pilot pressure based on the relationship between the load of the hydraulic actuator and the maximum value of the main valve demand pilot pressure, and determine the actual main valve pilot pressure based on the input command-related demand pilot pressure and the load-related maximum demand pilot pressure.

[0012] In one implementation, the controller is configured to determine the actual main valve pilot pressure as the minimum of the maximum value of the main valve pilot pressure associated with the input command and the maximum value of the load-related pilot pressure.

[0013] In one embodiment, in the expression of the relationship between the input command of the command input element and the pilot pressure required by the main valve, the pilot pressure required by the main valve increases as the input command of the command input element increases.

[0014] In one embodiment, in the expression of the relationship between the load of the hydraulic actuator and the maximum required pilot pressure of the main valve, the maximum required pilot pressure is the first pressure corresponding to the first load level, and the maximum required pilot pressure is the second pressure corresponding to the second load level.

[0015] In one embodiment, the hydraulic system is an excavator hydraulic system, and the hydraulic actuator is an excavator stick cylinder.

[0016] The hydraulic system of this application employs different control strategies for different load levels. With a back pressure design in place for the main valve spool, under high load operation, the back pressure of the spool decreases, the pump flow increases, and the power efficiency is improved. Attached Figure Description

[0017] The foregoing and other aspects of this application will be more fully understood and appreciated through the following detailed description with reference to the accompanying drawings, in which: Figure 1 This is a hydraulic circuit diagram of a hydraulic system according to one embodiment of this application; Figure 2 This is a schematic curve showing the relationship between the back pressure in the main valve spool of the hydraulic system of this application and the return oil flow area and the valve spool position. Figures 3 to 5 This is a partial schematic diagram of an exemplary internal structure of a main valve capable of implementing the control strategy; Figure 6 This is a partial schematic diagram of another exemplary internal structure of the main valve; Figure 7 This is a graph showing the relationship between a working command and the pilot pressure required by the main valve, which is used in the control strategy of this application. Figure 8 This is a graph showing the relationship between the pump output pressure and the maximum value of the main valve's required pilot pressure, which is used in the control strategy of this application. Detailed Implementation

[0018] This application generally relates to a hydraulic system for driving a primary actuating element (e.g., the stick of an excavator) in mechanical equipment, where the spatial movement of the end effector (e.g., the bucket of an excavator) is mainly achieved by this actuating element during operation of the mechanical equipment under different levels of load.

[0019] Figure 1The image illustrates a hydraulic system according to one embodiment of this application. The hydraulic system includes a command input element 1, such as an operating lever, for generating a motion command for a primary actuator of a mechanical device, operated by an operator (such as a driver). Based on the position of the command input element 1, command signals such as the desired direction of movement and speed of movement of the primary actuator can be generated. Furthermore, the hydraulic system may also include other command input elements, such as parallel operating levers or foot pedals, for inputting motion commands for other components of the mechanical device.

[0020] The hydraulic system also includes a controller 2, which is connected (via cable, wireless, etc.) to a command input element 1 (and other possible command input elements) and receives action commands from the operator via the command input element 1 (and other possible command input elements).

[0021] The hydraulic system also includes a main pump 3, a main valve 4, and a hydraulic actuator 5 for hydraulically driving the main actuating element. The main pump 3 is controlled by a controller 2. The main pump 3 is preferably a variable displacement pump, whose displacement can be adjusted by the controller 2. The main pump 3 supplies high-pressure hydraulic oil to the actuator 5. The actuator 5 in the figure is a hydraulic cylinder, but it can also be other types of actuators (such as hydraulic motors). The main pump 3 can also be used to supply hydraulic oil to actuators of other actuating elements of the mechanical equipment.

[0022] The main pump 3 is connected to the input and output ports of the actuator 5 via a hydraulic circuit. The main valve 4, located in the hydraulic circuit, controls the direction and speed of the actuator 5. The main valve 4 is a hydraulically controlled three-position five-way proportional directional valve, with each control port connected to a pilot valve in the form of a proportional valve 6. These two proportional valves 6 are controlled by the controller 2. The controller 2 determines which proportional valve 6 is open and its output pilot pressure based on the input signal from the command input element 1. The corresponding proportional valve 6 outputs pilot pressure to the main valve 4 to determine the valve position and opening degree of the main valve 4.

[0023] The main valve 4 has five ports: an inlet port P, two return ports T, and working ports A and B. The inlet port P is connected to the output port of the main pump 3, the two return ports T are connected to the oil tank, and the working ports A and B are connected to the input and output ports of the actuator 5, respectively.

[0024] The intermediate valve position (home position) of main valve 4 corresponds to the stopped state of actuator 5. In the intermediate valve position, the ports of main valve 4 are not connected to each other.

[0025] The first valve position of main valve 4 (the left valve position in the diagram) corresponds to the forward position of actuator 5. In the first valve position, working port A is connected to inlet port P, and working port B is connected to a return port T. The hydraulic oil output from main pump 3 will drive actuator 5 to perform a forward movement.

[0026] It should be noted that in order to increase the forward speed of actuator 5, in the first valve position of main valve 4, working port B has a check valve leading to working port A. This check valve opens when the pressure of working port B (and the corresponding oil chamber of actuator 5) is higher than the pressure of working port A (and the corresponding oil chamber of actuator 5), and a portion of the working oil will directly enter working port A from working port B to achieve hydraulic regeneration.

[0027] The second valve position of main valve 4 (the right valve position in the diagram) corresponds to the retracted state of actuator 5. In the second valve position, working port B is connected to inlet port P, and working port A is connected to a return port T. The hydraulic oil output from main pump 3 will push actuator 5 to perform a retracted action.

[0028] It is understandable that the main valve 4 can also be a three-position four-way directional valve with an oil inlet P, an oil return port T, and working oil ports A and B.

[0029] The first and second valve positions can be referred to as the working valve positions of the main valve 4. Each working valve position is achieved by a corresponding proportional valve 6 and a corresponding control port on the main valve 4. In each working valve position, the pilot pressure from the corresponding proportional valve 6 determines the spool position of the main valve 4, which in turn determines the opening degree of the main valve 4. The opening degree of the main valve 4 determines the flow rate of hydraulic oil output from the main pump 3 to the actuator 5. In this way, the controller 2 can control the action (direction, speed, etc.) of the actuator 5 based on the input commands from the command input element 1.

[0030] In addition, a pressure sensor 7 is provided on the output side of the main pump 3 to detect the output pressure of the main pump 3. The controller 2 is connected to the pressure sensor 7, and thus can acquire the detection result of the pressure sensor 7. In the illustrated example, the pressure sensor 7 is located upstream of the main valve 4. However, the pressure sensor 7 can also be located downstream of the main valve 4, for example, at the input port of the actuator 5. Through the pressure sensor 7, the controller 2 can detect the output pressure of the main pump 3 in real time, which is essentially the high-pressure chamber pressure when the actuator 5 moves forward.

[0031] When actuator 5 moves forward, the oil chamber connected to working port A is supplied with high pressure, while the oil chamber connected to working port B also experiences a certain back pressure. This facilitates the rapid forward movement of actuator 5 and hydraulic regeneration. Correspondingly, back pressure also exists in the oil groove of the main valve 4 in the first valve position, which communicates with working port B. The back pressure in the main valve 4 is related to the return oil flow area in the oil groove of the main valve 4 that communicates with working port B.

[0032] Figure 2The diagram schematically illustrates the relationship between the return flow area Sout and the back pressure Pback in the oil groove of the main valve 4, which communicates with the working port B, when the main valve 4 is in the first valve position, and the pilot pressure Pst received by the main valve 4 (corresponding to the valve spool position). It can be seen that when the pilot pressure Pst is less than the first pressure P1 (e.g., 20 bar), the back pressure Pback is at a relatively high value (essentially constant, or decreases as the pilot pressure Pst increases). As the pilot pressure Pst increases above the first pressure P1, the back pressure Pback drops sharply. When the pilot pressure Pst is higher than the first pressure P1 but less than the second pressure P2 (e.g., 30 bar), the back pressure Pback is at a relatively low value (essentially constant).

[0033] Furthermore, when the pilot pressure Pst is less than the first pressure P1, the flow area Sout is at a low value (basically constant, or increases with increasing pilot pressure Pst). As the pilot pressure Pst increases above the first pressure P1, the flow area Sout increases sharply. When the pilot pressure Pst is greater than the first pressure P1 and less than the second pressure P2, the flow area Sout is at a large value (basically constant).

[0034] The aforementioned changes in flow area Sout and back pressure Pback are achieved through the valve core design of main valve 4.

[0035] For example, according to Figures 3 to 5 In one exemplary embodiment shown, the valve core 9 is arranged in an axially slidable manner in the valve body 8 of the main valve 4.

[0036] The valve body 8 of the main valve 4 has an oil groove (cutting groove) Bslot communicating with the working oil port B and an oil groove (cutting groove) Tslot communicating with a return oil port T. An oil groove (cutting groove) 9a is formed on the outer periphery of the valve core 9. Figure 3 In the intermediate valve position of the main valve 4 (where no pilot pressure Pst is applied to the main valve 4), oil groove 9a and oil groove B slot face each other and are therefore connected. Oil groove T slot faces the valve core portion 9b, which does not form an oil groove, and is closed by the valve core portion 9b.

[0037] A return oil groove 9c is provided in the portion of the valve core portion 9b adjacent to the oil groove 9a. The number of return oil grooves 9c is at least one, preferably multiple. The radial depth of the return oil grooves 9c gradually increases from the valve core portion 9b toward the oil groove 9a. Figure 3 The diagram shows an increase in the form of multiple steps, and the circumferential width of the oil return groove 9c also gradually increases. Figure 3 The intermediate valve position of the main valve 4 shown has the return oil groove 9c generally facing the grooveless part 8a in the valve body 8 located between the oil groove Tslot and the oil groove Bslot, and is closed by the grooveless part 8a.

[0038] exist Figure 3In the intermediate valve position of the main valve 4 shown, the oil sump Tslot is completely closed by the valve core part 9b. Therefore, the oil sump Tslot and the oil sump Bslot are not connected, and the flow area Sout between them is zero.

[0039] like Figure 4 As shown, when the controller 2 controls the opening of the proportional valve 6 corresponding to the first valve position, causing the proportional valve 6 to output a pilot pressure Pst less than the first pressure P1 to the corresponding control port of the main valve 4, the valve core 9 moves axially in the direction of achieving the first valve position. A certain flow area Sout is generated between the return oil groove 9c and the grooveless part 8a, and a connection is established between the oil groove Tslot and the oil groove Bslot. The working oil port B is connected to the corresponding return oil port T. At this time, the flow area Sout is limited by the larger diameter part of the return oil groove 9c. The flow area Sout is small, so the back pressure Pback in the oil groove B is high, and the flow rate from the oil groove Bslot into the oil groove Tslot through the flow area Sout is small.

[0040] like Figure 5 As shown, when the controller 2 increases the opening of the proportional valve 6 corresponding to the first valve position, causing the proportional valve 6 to output a pilot pressure Pst to the corresponding control port of the main valve 4 that is greater than the first pressure P1 and less than the second pressure P2, the valve core 9 moves further axially in the direction of achieving the first valve position, and the flow area Sout between the return oil groove 9c and the grooveless part 8a increases. At this time, the flow area Sout is limited by the minimum diameter portion of the return oil groove 9c. Therefore, the flow rate from oil groove B slot into oil groove T slot through the flow area Sout increases, and the back pressure Pback in oil groove B decreases.

[0041] Through the structural design of the valve core 9 described above, the following can be achieved: Figure 2 The diagram shows the switching of flow area Sout and back pressure Pback under different pilot pressures Pst.

[0042] Alternatively, the return groove 9c of the valve core 9 can be designed in other forms, such as circumferential slots, to achieve a similar switching between flow area Sout and back pressure Pback.

[0043] For example, in Figure 6 In the example shown, the return oil groove 9c is a smoothly transitioned conical shape.

[0044] Alternatively, the 9C section can be designed with circumferential slots. With this design, the switching of the flow area (Sout) and the switching of the back pressure (Pback) are eliminated. Figure 2 The abrupt change shown is not a smooth change, but it can still produce a state with a small flow area Sout and a high back pressure Pback, as well as a state with a large flow area Sout and a low back pressure Pback.

[0045] Other forms of oil return grooves 9c can also be conceived, as long as the above switching can be achieved.

[0046] Controller 2 is configured to determine the opening degree of main valve 4 in the first valve position based on the load during actuator 5's forward movement. The load during actuator 5's forward movement is represented by the detection value of pressure sensor 7. Controller 2 sets the load level during actuator 5's forward movement, namely a lower first load level (e.g., the normal load level of the stick cylinder during excavator leveling operations) and a second load level that jumps relative to the first load level (e.g., the normal load level of the stick cylinder during excavator digging operations). Controller 2 determines the output pilot pressure of proportional valve 6 based on the detection value of pressure sensor 7 and the action command of command input element 1.

[0047] In general, the control strategy of this application is as follows: when the load level of the actuator 5 during its forward movement is equal to or lower than the first load level, the proportional valve 6 corresponding to the first valve position is controlled to output a pilot pressure of the maximum value of the first pressure P1 (e.g., 20 bar), thereby controlling the main valve 4 to have a high back pressure. At this time, the check valve in the first valve position of the main valve 4 opens, and a portion of the working oil will directly enter the working oil port A from the working oil port B to achieve hydraulic regeneration, which is beneficial to the rapid action of the actuator 5.

[0048] When the load level of actuator 5 during its forward movement is higher than the first load level, for example, reaching a second load level significantly higher than the first load level, the proportional valve 6 corresponding to the first valve position is controlled to output a maximum pilot pressure of the second pressure P2 (e.g., 30 bar), thereby controlling the main valve 4 to have a low back pressure. This low back pressure is beneficial to the power efficiency of the hydraulic system under high load conditions.

[0049] The following describes a feasible implementation method for the controller 2 to execute the control strategy.

[0050] First, controller 2 is configured to establish a relationship between the input command of command input element 1 and the required pilot pressure of the main valve, and a relationship between the output pressure of main pump 3 and the maximum value of the required pilot pressure of the main valve. Then, using these two relationships, the corresponding required pilot pressure of the main valve is determined based on the input command of command input element 1 and the output pressure of main pump 3.

[0051] Figure 7 The diagram illustrates an exemplary relationship between the input command Cmd of command input element 1 and the required pilot pressure Pst of the main valve. Generally, as the input command Cmd of command input element 1 increases (e.g., as the operating lever displacement increases), the required pilot pressure Pst of the main valve 4 increases. This relationship can be direct proportional or expressed as a curve.

[0052] Figure 8The diagram illustrates an exemplary relationship between the main pump output pressure Ppump (represented by the reading from pressure sensor 7) and the maximum required pilot pressure Pst_max of the main valve. Generally, when the main pump output pressure Ppump indicates that the load level during actuator 5's advance is equal to or lower than the first load level, the maximum required pilot pressure Pst_max of the main valve is the first pressure P1. When the main pump output pressure Ppump indicates that the load level during actuator 5's advance is higher than the second load level, the maximum required pilot pressure Pst_max of the main valve is the second pressure P2. As the main pump output pressure Ppump increases from the first load level to the second load level, the maximum required pilot pressure Pst_max of the main valve increases from the first pressure P1 to the second pressure P2.

[0053] After the controller 2 determines the main valve demand pilot pressure based on the input command of the command input element 1 and the maximum value of the main valve demand pilot pressure determined by the output pressure of the main pump 3, it takes the smaller of the two as the actual pilot pressure Pst, and controls the corresponding proportional valve 6 to output the actual pilot pressure Pst to the main valve 4.

[0054] For example, in level ground operation, assuming the input command of command input element 1 indicates a desired forward speed of the main actuator of the mechanical equipment at full speed, controller 2 determines, based on the relationship between the input command of command input element 1 and the main valve's required pilot pressure, that the current main valve's required pilot pressure Pst is higher than the first pressure P1 (e.g., 30 bar). Simultaneously, due to the smaller load in level ground operation, controller 2 determines, based on the relationship between the output pressure of the main pump 3 and the maximum value of the main valve's required pilot pressure, that the current maximum value of the main valve's required pilot pressure Pst_max is equal to the first pressure P1 (e.g., 20 bar). In this case, controller 2 determines the actual pilot pressure Pst to be equal to the lower of the two determined values, i.e., the first pressure P1 (i.e., 20 bar). At this time, the main valve core 4 has high back pressure, hydraulic regeneration opens, which is beneficial for rapid action.

[0055] For example, during excavation operations, assuming that controller 2 determines, based on the relationship between the input command from command input element 1 and the main valve's required pilot pressure, that the current main valve's required pilot pressure Pst is higher than the first pressure P1 (e.g., 30 bar). Simultaneously, due to the large load of the excavation operation, controller 2 determines, based on the relationship between the output pressure of the main pump 3 and the maximum value of the main valve's required pilot pressure, that the current maximum value of the main valve's required pilot pressure Pst_max is the second pressure P2 (e.g., 30 bar). In this case, controller 2 determines the actual pilot pressure Pst to be equal to the lower of the two determined values, i.e., 30 bar. At this point, the main valve core 4 has low back pressure, avoiding additional energy consumption and improving power efficiency.

[0056] Based on the principles of this application, those skilled in the art can design other specific implementations of the controller 2 controlling the pilot pressure of the main valve 4 based on the input commands of the command input element 1 and the output pressure of the main pump 3.

[0057] Furthermore, those skilled in the art can make various modifications to the details of the hydraulic system disclosed in this application.

[0058] For example, pressure sensor 7 is actually a load sensing element that detects the load on actuator 5. It is understood that pressure sensor 7 can be replaced with other load sensing elements capable of detecting the load on actuator 5, such as a force sensor. The controller determines the load level of the actuator based on the detection value of the load sensing element, and thus, in conjunction with the input command from the command input element, determines the maximum pilot pressure of the main valve.

[0059] For example, the valve core 9 of the main valve 4 described above with reference to the attached drawings includes a return oil groove 9c disposed between a valve core portion 9b corresponding to a return oil port T and an oil groove 9a corresponding to a working oil port B. This valve core structure can generate different levels of back pressure in the valve body oil groove Bslot corresponding to the working oil port B in the first valve position of the main valve 4. These different back pressures are beneficial for the forward movement of the actuator 5 at different load levels. It is conceivable that a similar return oil groove can also be formed between the grooveless valve core portion corresponding to the other return oil port T of the valve core 9 and the oil groove corresponding to the working oil port A. This valve core structure can generate different levels of back pressure in the valve body oil groove corresponding to the working oil port A in the second valve position of the main valve 4. These different back pressures are powerful for the backward movement of the actuator 5 at different load levels.

[0060] For example, controller 2 can also be configured to execute the aforementioned control strategies for different load levels on hydraulic actuators of other moving parts of mechanical equipment (such as the boom cylinder of an excavator). Of course, in this case, the main valve spool of the hydraulic actuators of other moving parts also needs to be designed with the aforementioned back pressure regulation structure based on pilot pressure.

[0061] In summary, this application discloses a hydraulic system for driving a primary actuator in a conventional operating mode of mechanical equipment (such as an excavator). The controller of the hydraulic system controls the back pressure in the main valve based on the input commands from the primary actuator command input element and the load control pressure of the primary actuator. Thus, the benefits of different back pressures in the main valve are utilized at different load levels.

[0062] While this application has been described herein with reference to specific exemplary embodiments, the scope of this application is not limited to the details shown. Various modifications may be made to these details without departing from the basic principles of this application.

Claims

1. A hydraulic system, comprising: Hydraulic actuator (5) configured to achieve at least first and second load levels of operation, the second load level being higher than the first load level; The instruction input element (1) is configured to allow the operator to input the action command of the hydraulic actuator (5); The main pump (3) is configured to supply working hydraulic oil to the hydraulic actuator (5); The main valve (4) is configured to control the direction and pressure of the working hydraulic oil supplied by the main pump (3) to the hydraulic actuator (5) in the working position; and in the working position, an adjustable back pressure is established in the main valve (4), the back pressure in the main valve (4) being related to the opening of the main valve (4) and the return flow area. A pilot valve (6) is configured to output pilot pressure to the main valve (4) to control the opening degree of the main valve (4); A load detection element configured to detect the load borne by the hydraulic actuator (5); The controller (2) is configured to determine the pilot pressure output by the pilot valve (6) based on the action command input by the command input element (1) and the load detected by the load detection element, thereby controlling the opening of the main valve (4) and the back pressure in the main valve (4) such that: When the load detected by the load detection element is lower than or equal to the first load level, the pilot pressure of the control main valve (4) is in the range of equal to or lower than the first pressure (P1); when the load detected by the load detection element is higher than the first load level, the pilot pressure of the control main valve (4) is in the range of equal to or lower than the second pressure (P2) and higher than the first pressure (P1), and the second pressure (P2) is higher than the first pressure (P1). The main valve (4) has a working port A, a working port B, an inlet port P, and a return port T, and includes a valve body (8) and a valve core (9) that can slide axially in the valve body (8). The valve body (8) has a first valve body oil groove (Bslot) communicating with the working port B and a second valve body oil groove (Tslot) communicating with the return port T. The valve core (9) is configured to generate an adjustable back pressure in the first valve body oil groove (Bslot) in the working valve position.

2. The hydraulic system as described in claim 1, wherein, The load detection element is a pressure sensor (7), which is configured to detect the output pressure of the main pump (3) or the input pressure of the hydraulic actuator (5).

3. The hydraulic system as described in claim 1, wherein, The valve core (9) is designed such that at different axial positions of the valve core (9) relative to the valve body (8), the first valve body oil groove (Bslot) and the second valve body oil groove (Tslot) have different return oil flow areas (Sout).

4. The hydraulic system as claimed in claim 1, wherein, The outer periphery of the valve core (9) is formed with a valve core oil groove (9a). In the original position of the main valve (4), the first valve body oil groove (Bslot) faces the valve core oil groove (9a), and the second valve body oil groove (Tslot) is closed by the grooveless valve core part (9b). There is a return oil groove (9c) between the grooveless valve core part (9b) and the valve core oil groove (9a). Along the direction from the grooveless valve core part (9b) towards the valve core oil groove (9a), the diameter of the return oil groove (9c) gradually decreases.

5. The hydraulic system as described in claim 3, wherein, The outer periphery of the valve core (9) is formed with a valve core oil groove (9a). In the original position of the main valve (4), the first valve body oil groove (Bslot) faces the valve core oil groove (9a), and the second valve body oil groove (Tslot) is closed by the grooveless valve core part (9b). There is a return oil groove (9c) between the grooveless valve core part (9b) and the valve core oil groove (9a). Along the direction from the grooveless valve core part (9b) towards the valve core oil groove (9a), the diameter of the return oil groove (9c) gradually decreases.

6. The hydraulic system as described in any one of claims 1 to 5, wherein, The controller (2) is configured to: The relationship between the input command of the command input element (1) and the pilot pressure required by the main valve is expressed to determine the pilot pressure required by the input command, and the relationship between the load of the hydraulic actuator (5) and the maximum value of the pilot pressure required by the main valve is expressed to determine the maximum value of the pilot pressure required by the load. The actual pilot pressure of the main valve is determined based on the pilot pressure required by the input command and the maximum value of the pilot pressure required by the load.

7. The hydraulic system as claimed in claim 6, wherein, The controller (2) is configured to determine the actual main valve pilot pressure as the minimum of the maximum value of the main valve pilot pressure related to the input command and the maximum value of the load-related pilot pressure.

8. The hydraulic system as claimed in claim 6, wherein, In the expression of the relationship between the input command of the command input element (1) and the pilot pressure required by the main valve, as the input command of the command input element (1) increases, the pilot pressure required by the main valve increases.

9. The hydraulic system as claimed in claim 6, wherein, In the expression of the relationship between the load of the hydraulic actuator (5) and the maximum value of the pilot pressure required by the main valve, the maximum value of the pilot pressure required by the main valve is the first pressure (P1) corresponding to the first load level, and the maximum value of the pilot pressure required by the main valve is the second pressure (P2) corresponding to the second load level.

10. The hydraulic system as claimed in any one of claims 1 to 5, wherein, The hydraulic system is the excavator hydraulic system, and the hydraulic actuator (5) is the excavator stick cylinder.

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