Hydraulic system of variable-amplitude oil cylinder and control method of hydraulic system

Abstract

The invention discloses a hydraulic system of a variable-amplitude oil cylinder and a control method of the hydraulic system in the field of hydraulic systems, and aims to solve the problem that in the prior art, the variable-amplitude oil cylinder shakes when descending under the variable-amplitude gravity, the hydraulic system comprises a hydraulic pump, the variable-amplitude oil cylinder and a three-position four-way reversing valve, and one end of the three-position four-way reversing valve is connected with the variable-amplitude oil cylinder; the balance valve is arranged on the variable-amplitude oil lifting path and is connected with the pilot oil path; the two-position two-way reversing valve is connected with the variable-amplitude oil lifting way and the variable-amplitude oil descending way, and the hydraulic half bridge is used for controlling and adjusting the pressure of a PIL pilot port on the pilot oil way. According to the system, the connection of the variable-amplitude ascending oil way and the variable-amplitude descending oil way is realized through the two-position two-way reversing valve, the differential compensation of the variable-amplitude oil cylinder during retraction is realized, pressure control can be accurately performed on the pilot oil way through the external pilot control port, the variable-amplitude oil cylinder is not influenced by variable-amplitude ascending and descending, meanwhile, the pressure fluctuation of the system is reduced, and the system reliability is improved. The problem that the amplitude variation gravity lowering of the amplitude variation oil cylinder is unstable is solved.

Description

Technical Field

[0001] The invention relates to the field of hydraulic systems, and in particular to a hydraulic system of a variable amplitude oil cylinder and a control method thereof. Background Art

[0002] A telescopic forklift is a device that can realize rapid loading, unloading and transshipment of goods. It is mostly used in construction sites, industrial enterprises, agriculture and animal husbandry and other occasions. A telescopic forklift consists of front and rear axles, suspension and retractable arms, and can forklift and stack goods by itself. A telescopic forklift is a multi-functional vehicle with off-road performance and its operating functions are relatively complex.

[0003] Therefore, in order to ensure the stability and safety of the vehicle during operation, the vehicle is required to have a reliable luffing and lifting function under load operation. When the vehicle is performing luffing movement, the system luffing cylinder should obtain stable pressure and flow to ensure that the boom luffing and lifting are smooth and stable, but the existing luffing balance valve will cause the luffing gravity to shake and the movement speed to be unstable, affecting normal use. Summary of the invention

[0004] The object of the present invention is to provide a hydraulic system of a luffing cylinder and a control system thereof, so as to solve the problem of jitter instability of the luffing cylinder during the luffing process.

[0005] In order to solve the above technical problems, the following technical solutions are adopted:

[0006] In a first aspect, the present invention provides a hydraulic system of a luffing cylinder, comprising a hydraulic pump and a luffing cylinder, and further comprising:

[0007] A variable amplitude raising oil circuit, a variable amplitude lowering oil circuit and a three-position four-way reversing valve, one end of the three-position four-way reversing valve is connected to the rodless chamber of the variable amplitude cylinder through the variable amplitude raising oil circuit, and is connected to the rod chamber of the variable amplitude cylinder through the variable amplitude lowering oil circuit; the other end of the three-position four-way reversing valve is connected to the second hydraulic oil tank, and is connected to the first hydraulic oil tank through the hydraulic pump;

[0008] A balancing valve is arranged on the variable amplitude lifting oil circuit and connected to the pilot oil circuit. When the pressure of the PIL pilot port on the pilot oil circuit is greater than a first preset value, the balancing valve opens;

[0009] A two-position two-way reversing valve is connected to the amplitude raising oil circuit and the amplitude lowering oil circuit, and is connected to the pilot oil circuit. A one-way valve is provided at one end of the two-position two-way reversing valve connected to the amplitude lowering oil circuit; when the pressure of the PIL pilot port on the pilot oil circuit is greater than a second preset value, the two-position two-way reversing valve is opened;

[0010] A hydraulic half-bridge is used to control the pressure of the PIL pilot port on the pilot oil circuit.

[0011] Optionally, the hydraulic half-bridge includes a fixed damping hole and a variable damping hole, the fixed damping hole is arranged on the overflow valve oil circuit and connected to the third hydraulic oil tank, and the variable damping hole is arranged on the pilot oil circuit.

[0012] Optionally, one end of the overflow valve oil circuit is connected to one end of the balancing valve close to the boom cylinder, and the other end is connected to the third hydraulic oil tank. The overflow valve oil circuit is connected to the pilot oil circuit, and an overflow valve and a filter are provided on one end of the overflow valve oil circuit close to the boom lifting oil circuit.

[0013] Optionally, one end of the pilot oil circuit is connected to the balancing valve, and the other end is connected to a pilot pump, and the pilot pump is connected to a pilot oil tank.

[0014] Optionally, the three-position four-way reversing valve includes a first working position and a second working position;

[0015] When the three-position four-way reversing valve is switched to the first working position, the hydraulic pump delivers the hydraulic oil to the rodless chamber of the luffing oil cylinder through the luffing oil path, and the hydraulic oil flows from the rodless chamber of the luffing oil cylinder to the rod chamber, pushing the cylinder rod of the luffing oil cylinder to extend, and the hydraulic oil then flows back from the rod chamber to the second hydraulic oil tank;

[0016] When the three-position four-way reversing valve is switched to the second working position, the hydraulic pump transports the hydraulic oil to the rod chamber of the luffing cylinder through the luffing oil circuit, the hydraulic oil flows from the rod chamber to the rodless chamber of the luffing cylinder, pushing the cylinder rod of the luffing cylinder to retract, and the hydraulic oil then flows back from the rodless chamber to the first hydraulic oil tank.

[0017] Optionally, the one-way valve is used to allow the hydraulic oil in the boom-raising oil circuit to flow into the boom-lowering oil circuit.

[0018] Optionally, a one-way valve is provided in the balancing valve, and the one-way valve is used to prevent the hydraulic oil from flowing back when the balancing valve is closed.

[0019] In a second aspect, the present invention provides a telescopic boom engineering vehicle, comprising a hydraulic system of a boom cylinder as described in any one of the first aspects.

[0020] In a third aspect, the present invention provides a control method for a hydraulic system of a luffing cylinder, wherein the control method adopts the hydraulic system of the luffing cylinder according to any one of the first aspects, and comprises:

[0021] In response to the extension of the luffing cylinder, the hydraulic oil in the first hydraulic oil tank is delivered to the three-position four-way reversing valve through the hydraulic pump, and then enters the rodless chamber of the luffing cylinder through the luffing raising oil path after passing through the three-position four-way reversing valve. The hydraulic oil enters the rod chamber from the rodless chamber to push the extension of the luffing cylinder. The hydraulic oil flows from the rod chamber to the luffing lowering oil path, reaches the three-position four-way reversing valve, and finally flows into the second hydraulic oil tank.

[0022] In response to the contraction of the luffing cylinder, the hydraulic oil in the first hydraulic oil tank is delivered to the three-position four-way reversing valve through the hydraulic pump, and the hydraulic oil is delivered to the luffing-down oil circuit; when the pressure of the PIL pilot port on the pilot oil circuit reaches a first preset value, the balancing valve opens, and the hydraulic oil enters the rod chamber of the luffing cylinder from the luffing-down oil circuit, and then flows into the rodless chamber, pushing the contraction of the luffing cylinder; the hydraulic oil flows from the luffing-up oil circuit to the three-position four-way reversing valve, and then flows into the second hydraulic oil tank.

[0023] Among them, in the process that the hydraulic oil flows from the amplitude raising oil circuit to the three-position four-way reversing valve, when the PIL pilot port pressure reaches a second preset value, the two-position two-way reversing valve opens, the amplitude raising oil circuit is connected with the amplitude lowering oil circuit, and the hydraulic oil in the amplitude raising oil circuit enters the amplitude lowering oil circuit through the two-position two-way reversing valve and the one-way valve, and the hydraulic oil then enters the rod chamber of the amplitude raising cylinder from the amplitude lowering oil circuit, pushing the amplitude raising cylinder to contract.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention realizes the connection between the luffing raising oil circuit and the luffing lowering oil circuit through a two-position two-way reversing valve, realizes differential compensation when the luffing cylinder retracts, and can accurately control the pressure in the pilot oil circuit through an external pilot control port. The luffing cylinder is not affected by the rise and fall of the luffing, and the fluctuation of the system pressure is also reduced, thus solving the problem of unstable luffing lowering of the luffing cylinder.

[0026] 2. The present invention arranges an overflow valve oil circuit to connect to the second hydraulic oil tank, which can play a role in safety protection. The pressure control of the PIL pilot control port on the pilot oil circuit realizes the locking of the oil cylinder. The present invention integrates speed adjustment, differential compensation, locking cylinder and pilot control functions. The differential compensation solves the problem of the rod cavity being sucked empty when the boom cylinder is lowered by gravity, reduces the loss of hydraulic oil energy, realizes the adjustment of the boom speed, and improves work efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a structural schematic diagram of the hydraulic system of the luffing cylinder in an embodiment of the present invention.

[0028] Description of reference numerals:

[0029] 1. Hydraulic pump; 2. Fixed damping hole; 3. Two-position two-way reversing valve; 4. Balance valve; 5. Overflow valve; 6. Filter; 7. Variable-length cylinder; 8. First hydraulic oil tank; 9. One-way valve; 10. Three-position four-way reversing valve; 11. Variable damping hole; 12. Second hydraulic oil tank; 13. Third hydraulic oil tank; 14. Pilot pump; 15. Pilot oil tank. DETAILED DESCRIPTION

[0030] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use.

[0031] Example 1

[0032] like Figure 1 As shown, this embodiment provides a hydraulic system of a luffing cylinder, comprising a hydraulic pump 1 and a luffing cylinder 7, wherein the hydraulic pump 1 is connected to a first hydraulic oil tank 8, and further comprising:

[0033] A variable amplitude raising oil circuit, a variable amplitude lowering oil circuit and a three-position four-way reversing valve 10, one end of the three-position four-way reversing valve 10 is connected to the rodless chamber of the variable amplitude cylinder 7 through the variable amplitude raising oil circuit, and is connected to the rod chamber of the variable amplitude cylinder 7 through the variable amplitude lowering oil circuit, and the other end of the three-position four-way reversing valve 10 is connected to the second hydraulic oil tank, and is connected to the first hydraulic oil tank 8 through the hydraulic pump 1;

[0034] A balancing valve 4 is arranged on the variable amplitude lifting oil circuit and connected to the pilot oil circuit. When the pressure of the PIL pilot port on the pilot oil circuit is greater than a first preset value, the balancing valve 4 is opened;

[0035] A two-position two-way reversing valve 3 is connected to the amplitude raising oil circuit and the amplitude lowering oil circuit, and is connected to the pilot oil circuit. A one-way valve 9 is provided at one end of the two-position two-way reversing valve 3 connected to the amplitude lowering oil circuit; when the pressure on the PIL pilot port on the pilot oil circuit is greater than a second preset value, the two-position two-way reversing valve 3 is opened;

[0036] A hydraulic half-bridge is used to control the pressure of the PIL pilot port on the pilot oil circuit.

[0037] When in use, the pressure of the PIL pilot port is controlled by the hydraulic half-bridge to prevent pressure fluctuations in the system from affecting the pressure of the PIL pilot port and affecting the on-off of the balancing valve 4 and the two-position two-way reversing valve 3. The connection control between the amplitude raising oil circuit and the amplitude lowering oil circuit is achieved through the two-position two-way balancing valve 4, so that differential compensation is achieved when the amplitude cylinder 7 retracts, thereby improving the utilization effect of hydraulic energy and increasing system stability.

[0038] Example 2

[0039] This embodiment provides a hydraulic system of a variable-luff cylinder based on the embodiment 1, and the difference from the embodiment 1 is that:

[0040] like Figure 1 As shown, the three-position four-way reversing valve 10 includes four oil ports A, B, P, and T. The A oil port is connected to the rodless chamber of the amplitude-changing cylinder 7 through the amplitude-changing raising oil circuit, the B oil port is connected to the rod chamber of the amplitude-changing cylinder 7 through the amplitude-changing lowering oil circuit, the P oil port is connected to the hydraulic pump 1, the hydraulic pump 1 is connected to the first hydraulic oil tank 8, and the T oil port is connected to the second hydraulic oil tank 12.

[0041] A balancing valve 4 is provided on the variable amplitude lifting oil circuit, and the balancing valve 4 is connected to the pilot oil circuit. The on-off of the balancing valve 4 is controlled by the pressure of the pilot port PIL on the pilot oil circuit. When the pressure of the pilot port PIL is greater than the first preset value, the balancing valve 4 is opened, and the oil port C2 on the variable amplitude lifting oil circuit is directly connected to the oil port V2. A one-way valve is also provided inside the balancing valve 4. When the balancing valve 4 is closed, the one-way valve only allows the fluid to flow from the oil port V2 to the oil port C2. When the balancing valve 4 is opened, the oil port C2 is directly connected to the oil port V2, and the one-way valve does not work.

[0042] The amplitude raising oil circuit and the amplitude lowering oil circuit are connected by an intermediate oil circuit. The oil port connected to the amplitude raising oil circuit is oil port a, and the oil port connected to the amplitude lowering oil circuit is oil port b. A two-position two-way reversing valve 3 is provided on the intermediate oil circuit. The two ends of the two-position two-way reversing valve 3 are respectively connected to the amplitude raising oil circuit and the amplitude lowering oil circuit through oil circuits. A one-way valve is provided at one end of the two-position two-way reversing valve 3 close to the oil port b. The direction of the one-way valve is to only allow the fluid to go from the oil port a to the oil port b. The two-position two-way reversing valve 3 is connected to the pilot oil circuit at the same time. The on-off of the two-position two-way reversing valve 3 is affected by the pressure of the PIL pilot port. When the pressure of the PIL pilot port is greater than the second preset value, the two-position two-way reversing valve 3 is opened, and the hydraulic oil of the oil port a passes through the two-position two-way reversing valve 3 and the one-way valve 9 to the oil port b.

[0043] The three-position four-way reversing valve 10 includes two working positions, namely the first working position and the second working position. When the three-position four-way reversing valve 10 is placed in the first working position, the oil port A is connected to the oil port P, and the oil port B is connected to the oil port T. The hydraulic oil flows from the variable amplitude lifting oil path into the rodless chamber of the variable amplitude cylinder 7, and flows from the rodless chamber to the rod chamber. The cylinder rod of the variable amplitude cylinder 7 extends, and then flows from the rod chamber to the three-position four-way reversing valve 10, and flows into the second hydraulic oil tank through the three-position four-way reversing valve 10. In this process, the balance valve 4 is closed, and the one-way valve in the balance valve 4 works, and the hydraulic oil can only flow from the oil port V2 to the oil port C2 to prevent the hydraulic oil from flowing back.

[0044] When the three-position four-way reversing valve 10 is placed in the second working position, the A oil port is connected to the T oil port, the B oil port is connected to the P oil port, and the hydraulic pump 1 delivers the hydraulic oil to the rod chamber of the luffing oil cylinder 7. The hydraulic oil flows from the rod chamber of the luffing oil cylinder 7 to the rodless chamber, pushing the luffing oil cylinder 7 to retract, and the hydraulic oil then flows back from the rodless chamber to the second hydraulic oil tank 12. In this process, the pressure of the PIL pilot port is greater than the first preset value, the balance valve 4 is opened, and the oil port C2 is directly connected to the oil port V2. When the pressure at the PIL pilot port is greater than the second preset value, the two-position two-way reversing valve 3 opens. Due to the suction in the amplitude-dropping oil circuit, the pressure in the amplitude-dropping oil circuit is greater than the pressure in the amplitude-raising oil circuit. The hydraulic oil in the amplitude-raising oil circuit flows from oil port a through the two-position two-way reversing valve 3 and the one-way valve 9 to oil port b, forming a differential connection. The hydraulic oil flowing to oil port b flows into the rod chamber of the amplitude-changing cylinder 7, and then flows into the rodless chamber, pushing the amplitude-changing cylinder 7 to retract.

[0045] An overflow valve oil circuit is provided on the luffing oil circuit, one end of the overflow valve oil circuit is connected to the luffing oil circuit, and the other end is connected to the third hydraulic oil tank 13 through the oil port T. One end of the overflow valve oil circuit connected to the luffing oil circuit is between the luffing cylinder 7 and the balance valve 4. An overflow valve 5 and a filter 6 are provided on the overflow valve oil circuit, and the filter 6 is used to filter impurities in the hydraulic oil. When the pressure in the luffing oil circuit is too high, the hydraulic oil flows from the overflow valve oil circuit to the third hydraulic oil tank 13 to reduce the pressure in the luffing oil circuit.

[0046] A fixed damping hole 2 is provided on the overflow valve oil circuit, and a variable damping hole 11 is provided on the pilot oil circuit. The fixed damping hole 2 and the variable damping hole 11 form a hydraulic half-bridge. By limiting the flow rate of the hydraulic oil and increasing the resistance, a local pressure drop is formed to control the pressure of the PIL pilot port, avoid pressure fluctuations in the system, and stabilize the pressure and reduce the impact.

[0047] One end of the pilot oil circuit is connected to the balance valve 4 , and the other end is connected to the pilot pump 14 , and the pilot pump 14 is connected to the pilot oil tank 15 .

[0048] When in use, the control switch of the boom cylinder 7 provides an extension / retraction signal to the three-position four-way reversing valve 10, and the three-position four-way reversing valve 10 is placed in the first working position or the second working position according to the signal. When the extension signal is received, the three-position four-way reversing valve 10 is placed in the first working position, and the hydraulic pump 1 pumps the hydraulic oil in the first hydraulic oil tank 8 to the boom raising oil circuit, and enters the rodless cavity of the boom cylinder 7 after passing through the one-way valve in the balance valve 4. The hydraulic oil flows from the rodless cavity to the rod cavity, pushing the boom cylinder 7 to extend, and the hydraulic oil in the rodless cavity then flows from the boom lowering oil circuit to the oil port B of the three-position four-way reversing valve 10, and then flows from the oil port B to the oil port T, and finally flows into the second hydraulic oil tank 12, completing a cycle of the hydraulic oil and realizing the extension of the boom cylinder 7. When the three-position four-way valve receives the retraction signal, it is placed in the second working position, and the hydraulic oil in the first hydraulic oil tank 8 is pumped to the variable amplitude lowering oil circuit through the hydraulic pump 1, enters the rod chamber of the variable amplitude cylinder 7, and flows into the rodless chamber to push the retraction of the variable amplitude cylinder 7. In this process, the pilot hydraulic pump 1 pumps the hydraulic oil in the pilot oil tank 15 to the pilot oil circuit. When the pressure of the PIL pilot port is greater than the first preset value, the balance valve 4 is opened, and the oil port C2 is directly connected with the oil port V2. The hydraulic oil flows from the rodless chamber through the balance valve 4 to the oil port A of the three-position four-way reversing valve 10, and then flows into the oil port T to flow into the second hydraulic oil tank 12. In this process, when the pressure of the PIL pilot port is greater than the second preset value, the two-position two-way reversing valve 3 connecting the variable amplitude raising oil circuit and the variable amplitude lowering oil circuit is opened, and the hydraulic oil in the oil port a flows to the oil port b, and continues to flow into the rod chamber of the variable amplitude cylinder 7 to push the retraction of the variable amplitude cylinder 7. The pressure of the PIL pilot port can be controlled by the fixed damping hole 2 and the variable damping hole 11, so as to avoid the pressure fluctuation in the oil circuit affecting the pressure of the PIL pilot port, and can play the role of stabilizing the pressure and reducing the impact.

[0049] Example 3

[0050] This embodiment provides a telescopic arm engineering vehicle based on Embodiment 1 or Embodiment 2, including the hydraulic system of the boom cylinder provided in Embodiment 1 or Embodiment 2, and the telescopic arm on the engineering vehicle is driven to extend and retract through the hydraulic system, thereby avoiding the shaking of the telescopic arm caused by gravity lowering and unstable movement speed, and increasing the stability of the telescopic arm's boom lowering work.

[0051] Example 4

[0052] This embodiment provides a control method for the hydraulic system of a luffing cylinder, which is implemented using the hydraulic system of the luffing cylinder 7 in Embodiment 1 or Embodiment 2, and specifically includes:

[0053] In response to the extension of the luffing cylinder 7, the hydraulic oil in the first hydraulic oil tank 8 is delivered to the three-position four-way reversing valve 10 through the hydraulic pump 1, and then enters the rodless chamber of the luffing cylinder 7 through the luffing raising oil path after passing through the three-position four-way reversing valve 10, and the hydraulic oil enters the rod chamber from the rodless chamber, and flows from the rod chamber to the luffing lowering oil path, reaches the three-position four-way reversing valve 10, and flows into the second hydraulic oil tank 12;

[0054] In response to the contraction of the luffing cylinder 7, the hydraulic oil in the first hydraulic oil tank 8 is delivered to the three-position four-way reversing valve 10 through the hydraulic pump 1, and the hydraulic oil is delivered to the luffing-down oil circuit. When the pressure of the PIL pilot port on the pilot oil circuit reaches a first preset value, the balance valve 4 opens, and the hydraulic oil enters the rod chamber of the luffing cylinder 7 from the luffing-down oil circuit, and then flows into the rodless chamber, pushing the contraction of the luffing cylinder 7. The hydraulic oil flows from the luffing-up oil circuit to the three-position four-way reversing valve 10, and then flows into the hydraulic oil tank.

[0055] Among them, in the process that the hydraulic oil flows from the amplitude raising oil circuit to the three-position four-way reversing valve 10, when the PIL pilot port pressure reaches a second preset value, the two-position two-way reversing valve 3 opens, the amplitude raising oil circuit is connected with the amplitude lowering oil circuit, and the hydraulic oil in the amplitude raising oil circuit passes through the two-position two-way reversing valve 3 and the one-way valve into the amplitude lowering oil circuit, and the hydraulic oil then enters the rod chamber of the amplitude raising cylinder 7 from the amplitude lowering oil circuit, pushing the amplitude raising cylinder 7 to contract.

[0056] By setting a two-position two-way reversing valve 3 to control the on-off between the amplitude raising oil circuit and the amplitude lowering oil circuit, differential compensation is achieved when the amplitude raising oil cylinder 7 retracts to avoid energy waste. The setting of the one-way valve in the balancing valve 4 avoids the backflow of hydraulic oil and increases the stability of the system. The pressure fluctuation in the system is regulated by the hydraulic half-bridge to avoid the pressure fluctuation affecting the pressure value of the PIL pilot port, thereby avoiding the on-off errors of the balancing valve 4 and the two-position two-way reversing valve 3 and increasing the stability of the entire system.

[0057] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention / invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention / invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention / invention, unless otherwise specified, "multiple" means two or more. In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood by specific circumstances.

[0058] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A hydraulic system for a luffing cylinder, comprising a hydraulic pump and a luffing cylinder, characterized in that: Also includes: A variable amplitude raising oil circuit, a variable amplitude lowering oil circuit and a three-position four-way reversing valve, one end of the three-position four-way reversing valve is connected to the rodless chamber of the variable amplitude cylinder through the variable amplitude raising oil circuit, and is connected to the rod chamber of the variable amplitude cylinder through the variable amplitude lowering oil circuit; the other end of the three-position four-way reversing valve is connected to the second hydraulic oil tank, and is connected to the first hydraulic oil tank through the hydraulic pump; A balancing valve is arranged on the variable amplitude lifting oil circuit and connected to the pilot oil circuit. When the pressure of the PIL pilot port on the pilot oil circuit is greater than a first preset value, the balancing valve opens; A two-position two-way reversing valve is connected to the amplitude raising oil circuit and the amplitude lowering oil circuit, and is connected to the pilot oil circuit. A one-way valve is provided at one end of the two-position two-way reversing valve connected to the amplitude lowering oil circuit; when the pressure of the PIL pilot port on the pilot oil circuit is greater than a second preset value, the two-position two-way reversing valve is opened; A hydraulic half-bridge is used to control the pressure of the PIL pilot port on the pilot oil circuit.

2. The hydraulic system of the luffing cylinder according to claim 1, characterized in that: The hydraulic half-bridge comprises a fixed damping hole and a variable damping hole. The fixed damping hole is arranged on the overflow valve oil circuit and connected to the third hydraulic oil tank. The variable damping hole is arranged on the pilot oil circuit.

3. The hydraulic system of the luffing cylinder according to claim 2, characterized in that: One end of the overflow valve oil circuit is connected to one end of the balance valve close to the boom cylinder, and the other end is connected to the third hydraulic oil tank. The overflow valve oil circuit is connected to the pilot oil circuit. An overflow valve and a filter are provided on one end of the overflow valve oil circuit close to the boom lifting oil circuit.

4. The hydraulic system of the luffing cylinder according to claim 2, characterized in that: One end of the pilot oil circuit is connected to the balance valve, and the other end is connected to the pilot pump, and the pilot pump is connected to the pilot oil tank.

5. The hydraulic system of the luffing cylinder according to claim 1, characterized in that: The three-position four-way reversing valve comprises a first working position and a second working position; When the three-position four-way reversing valve is switched to the first working position, the hydraulic pump delivers the hydraulic oil to the rodless chamber of the luffing oil cylinder through the luffing oil path, and the hydraulic oil flows from the rodless chamber of the luffing oil cylinder to the rod chamber, pushing the cylinder rod of the luffing oil cylinder to extend, and the hydraulic oil then flows back from the rod chamber to the second hydraulic oil tank; When the three-position four-way reversing valve is switched to the second working position, the hydraulic pump transports the hydraulic oil to the rod chamber of the luffing cylinder through the luffing oil circuit, the hydraulic oil flows from the rod chamber to the rodless chamber of the luffing cylinder, pushing the cylinder rod of the luffing cylinder to retract, and the hydraulic oil then flows back from the rodless chamber to the first hydraulic oil tank.

6. The hydraulic system of the luffing cylinder according to claim 1, characterized in that: The one-way valve is used to allow the hydraulic oil in the boom-raising oil circuit to flow into the boom-lowering oil circuit.

7. The hydraulic system of the luffing cylinder according to claim 1, characterized in that: A one-way valve is arranged inside the balancing valve, and the one-way valve is used to prevent the hydraulic oil from flowing back when the balancing valve is closed.

8. A telescopic boom engineering vehicle, characterized in that: A hydraulic system comprising the luffing cylinder according to any one of claims 1 to 7.

9. A control method for a hydraulic system of a variable-luff cylinder, characterized in that: The control method adopts the hydraulic system of the luffing cylinder according to any one of claims 1 to 7, comprising: In response to the extension of the luffing cylinder, the hydraulic oil in the first hydraulic oil tank is delivered to the three-position four-way reversing valve through the hydraulic pump, and then enters the rodless chamber of the luffing cylinder through the luffing raising oil path after passing through the three-position four-way reversing valve. The hydraulic oil enters the rod chamber from the rodless chamber to push the extension of the luffing cylinder. The hydraulic oil flows from the rod chamber to the luffing lowering oil path, reaches the three-position four-way reversing valve, and finally flows into the second hydraulic oil tank. In response to the contraction of the luffing cylinder, the hydraulic oil in the first hydraulic oil tank is delivered to the three-position four-way reversing valve through the hydraulic pump, and the hydraulic oil is delivered to the luffing-down oil circuit. When the pressure of the PIL pilot port on the pilot oil circuit reaches a first preset value, the balance valve opens, and the hydraulic oil enters the rod chamber of the luffing cylinder from the luffing-down oil circuit, and then flows into the rodless chamber, pushing the contraction of the luffing cylinder. The hydraulic oil flows from the luffing-up oil circuit to the three-position four-way reversing valve, and flows into the second hydraulic oil tank. Among them, in the process that the hydraulic oil flows from the amplitude raising oil circuit to the three-position four-way reversing valve, when the PIL pilot port pressure reaches a second preset value, the two-position two-way reversing valve opens, the amplitude raising oil circuit is connected with the amplitude lowering oil circuit, and the hydraulic oil in the amplitude raising oil circuit enters the amplitude lowering oil circuit through the two-position two-way reversing valve and the one-way valve, and the hydraulic oil then enters the rod chamber of the amplitude raising cylinder from the amplitude lowering oil circuit, pushing the amplitude raising cylinder to contract.

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