A negative torque test and control method for a closed hydraulic system of a crane, and a crane

Abstract

The present invention discloses a negative torque test and control method for a closed hydraulic system of a crane and a crane. Under the set single-rope tension and engine speed, the crane hoist performs a lowering operation. The torque test method includes: detecting the engine speed change rate and the operating parameters of the engine and the closed hydraulic system at the current engine speed change rate; executing a set control strategy according to the detected engine speed change rate until engine flameout protection; recording the operating parameters of the engine and the closed hydraulic system when the crane hoist performs a lowering operation at different single-rope tensions and engine speeds; performing processing to obtain the engine's negative torque resistance ability in the full speed range under the set single-rope tension condition, and drawing a single-rope tension - engine negative torque resistance ability curve. The present invention can accurately obtain the negative torque resistance ability of the engine itself, and perform real-time negative torque control according to the negative torque resistance ability of the engine itself to avoid engine flameout.

Description

Technical Field

[0001] The present invention belongs to the technical field of cranes, and particularly relates to a method for testing and controlling negative torque of a closed hydraulic system of a crane, and a crane. Background Art

[0002] The control of negative torque in a closed hydraulic system mainly balances the negative torque by increasing the form of a braking or loading device. When the engine works under negative torque, the working load of the engine is increased through a hydraulic pump loading overflow valve, so as to relatively reduce or offset the negative torque on the engine and achieve the stability of the engine speed. This technology is currently widely applied in closed systems. When the loading pump absorbs the negative torque of the engine and overflows, a large amount of heat is generated, and this heat can be dissipated into the air through a hydraulic heat dissipation device, so as to achieve the heat balance of the hydraulic system.

[0003] Disadvantages of the prior art:

[0004] 1. Since the anti-negative torque ability of the engine itself cannot be accurately obtained, the control scheme is relatively conservative, reducing the working efficiency of the working mechanism;

[0005] 2. For small-power engines, due to their small anti-negative torque ability, by the method of real-time detecting the engine speed and adjusting, due to the delay in detection and control of the controller ECU and the like, the engine speed cannot be fully and real-time feedback, so the untimely control causes the engine to repeatedly stall during operation, greatly affecting the safety and operation experience of the crane;

[0006] 3. The continuous operation of the loading pump generates a large amount of energy loss, resulting in waste of energy. Summary of the Invention

[0007] To solve the deficiencies in the prior art, the present invention provides a method for testing and controlling negative torque of a closed hydraulic system of a crane, and a crane, which can accurately obtain the anti-negative torque ability of the engine itself, and perform real-time negative torque control according to the anti-negative torque ability of the engine itself to avoid engine stalling.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] In a first aspect, a method for testing the negative torque of a closed hydraulic system is provided. Under the set single-rope pulling force and engine speed, the crane hoist performs a lowering operation. The method includes: detecting the engine speed change rate and the operating parameters of the engine and the operating parameters of the closed hydraulic system at the current engine speed change rate; executing a set control strategy according to the detected engine speed change rate until engine flameout protection; repeating the above process, and recording the operating parameters of the engine and the operating parameters of the closed hydraulic system when the crane hoist performs a lowering operation at different single-rope pulling forces and engine speeds; processing the obtained several groups of operating parameters of the engine and the operating parameters of the closed hydraulic system to obtain the engine's negative torque resistance ability in the full speed range under the set single-rope pulling force condition, and drawing a single-rope pulling force - engine negative torque resistance ability curve.

[0010] Further, the executing the set control strategy according to the detected engine speed change rate includes: when the engine speed change rate satisfies: n1 ≤ n < n2, executing control strategy one; where n1 represents the first threshold of the engine speed change rate, n represents the currently detected engine speed change rate, and n2 represents the second threshold of the engine speed change rate; when the engine speed change rate satisfies: n2 ≤ n < n3, executing control strategy two; where n3 represents the third threshold of the engine speed change rate; when the engine speed change rate satisfies: n ≥ n3, engine flameout protection is performed.

[0011] Further, control strategy one includes that the loading pump starts to work at the working pressure Hi, and increases the positive torque load so that the engine speed change rate is stabilized within n2.

[0012] Further, control strategy two includes that the loading pump starts to work at the working pressure Hi, and the working pump reduces the control current value at a certain ratio and speed to reduce the output flow of the working pump, and reduces the output of the negative torque so that the engine speed change rate is stabilized within n3.

[0013] Further, the processing the obtained several groups of operating parameters of the engine and the operating parameters of the closed hydraulic system to obtain the engine's negative torque resistance ability in the full speed range under the set single-rope pulling force condition includes: obtaining the rated power of the engine at different speeds according to the engine performance curve; calculating the engine's negative load torque resistance under the set single-rope pulling force and each group of engine speed conditions according to the operating parameters of the engine and the operating parameters of the closed hydraulic system; calculating the engine's negative torque resistance ability in the full speed range under the set single-rope pulling force condition according to the rated power of the engine at different speeds, the set single-rope pulling force, and the engine's negative load torque resistance under each group of engine speed conditions.

[0014] Further, according to the operating parameters of the engine and the operating parameters of the closed hydraulic system, calculate the anti-negative load torque of the engine under the set single-rope tension and the engine speed conditions of each group, including:

[0015] (1) The pump displacement Di under the pump current Ci corresponding to the engine speed increase point is:

[0016] Di = (Ci - 200) / (600 - 200) * D

[0017] In the formula, D is the maximum displacement corresponding to the maximum control current;

[0018] (2) The single-pump torque Wi and power Gi corresponding to the engine speed increase point are:

[0019] Wi = Di × Ei / (2 × π × η mh1 )

[0020] In the formula, η mh1 is the hydraulic-mechanical efficiency of the pump, and Ei is the output pressure of the pump at the Di displacement when the engine is at the speed increase point;

[0021] Gi = Di × (Ni / 1000) × Ei / (600 × η t1 )

[0022] In the formula, η t1 is the total efficiency of the pump, and Ni is the set engine speed;

[0023] (3) The torque Ki and power Mi of the loading pump corresponding to the engine speed increase point are:

[0024] Ki = Ji × Hi / (2 × π × η mh2 )

[0025] In the formula, η mh2 is the hydraulic-mechanical efficiency of the loading pump, Ji is the working displacement of the positive torque loading pump, and Hi is the output pressure of the positive torque loading pump;

[0026] Mi = Ji × (Ni / 1000) × Hi / (600 × η t2 )

[0027] In the formula, η t2 is the total efficiency of the pump;

[0028] (4) The torque T and power X of the other load pumps corresponding to the engine speed increase point are:

[0029] T = Q × R / (2 × π × η mh3 )

[0030] In the formula, η mh3η is the hydraulic-mechanical efficiency of other load pumps, Q is the output displacement of other positive torque load pumps, and R is the output pressure of other positive torque load pumps;

[0031] X = Q × (Ni / 1000) × R / (600 × η t3 )

[0032] In the formula, η t3 is the total efficiency of other load pumps;

[0033] Because, the system torque balance is maintained in the stable state before the engine speed increase, that is, the system positive torque T Z and the system negative torque T F are balanced, where, T Z = torque of the make-up oil pump + torque of the loading pump + engine anti-negative load torque, T F = the corresponding torque of the main pump when the double winch descends; the engine anti-negative load torque can be obtained under the conditions of each set of set single-rope pulling force and each set of engine speeds.

[0034] Furthermore, the engine anti-negative torque ability in the full speed range under the condition of the set single-rope pulling force is obtained by using the method of taking the average. First, the engine anti-negative torque ability Zi in the full speed range under the condition of the set single-rope pulling force Fi is obtained, and the average value is obtained after multiple tests.

[0035] In the second aspect, a method for controlling negative torque of a closed hydraulic system is provided, including: when the crane winch executes the falling operation, obtaining the engine speed corresponding to the throttle and the single-rope pulling force of the suspended heavy object; calculating the torque value borne by the current engine according to the engine speed corresponding to the throttle and the single-rope pulling force of the suspended heavy object; comparing the torque value borne by the engine with the engine anti-negative torque ability in the full speed range under the condition of the set single-rope pulling force obtained by the closed hydraulic system negative torque test method described in the first aspect, and performing negative torque control according to the comparison result.

[0036] Furthermore, the performing negative torque control according to the comparison result includes: when the torque value borne by the engine is less than the curve set value, no negative torque control is performed, otherwise, hierarchical loading control is performed according to the set loading program until the engine flameout protection.

[0037] In the third aspect, a crane is provided, and the crane performs negative torque control when the crane winch executes the falling operation according to the closed hydraulic system negative torque control method described in the second aspect.

[0038] Compared with the prior art, the beneficial effects achieved by the present invention:

[0039] (1) The present invention obtains the accurate ability of the engine to resist negative torque through testing, so as to be targeted and more precise in the negative torque control of the engine, thereby making full use of the engine's ability and improving the efficiency of the working mechanism;

[0040] (2) According to the curve of the engine's ability to resist negative torque, the present invention enables the control curve to work below this curve, avoiding the impact problem caused by repeated loading of the loading pump;

[0041] (3) According to the engine negative torque curve data, the present invention always ensures that the system negative torque does not exceed the engine's ability during the control process, eliminating the control lag caused by the delay of real-time detection and control, and thus avoiding the problem of easy engine stalling;

[0042] (4) The present invention can inversely calculate the actual ability that the pump can exert through the engine negative torque curve data, and the pump is controlled within this working range in advance by the controller, so that the loading pump only loads when needed, thereby reducing energy loss and waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 FIG. is a schematic diagram of the main flow of a method for testing negative torque of a closed hydraulic system of a crane provided by an embodiment of the present invention;

[0044] Figure 2 FIG. is the engine speed-torque curve in an embodiment of the present invention;

[0045] Figure 3 FIG. is the engine speed-power curve in an embodiment of the present invention;

[0046] Figure 4 FIG. is a single-rope tension-engine negative torque resistance percentage ability curve diagram drawn according to a method for testing negative torque of a closed hydraulic system of a crane provided by an embodiment of the present invention;

[0047] Figure 5 FIG. is a schematic diagram of the main flow of a method for controlling negative torque of a closed hydraulic system of a crane provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0048] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and should not be used to limit the protection scope of the present invention.

[0049] Embodiment 1:

[0050] As Figures 1 to 4As shown in the figure, a method for testing the negative torque of a closed hydraulic system. Under the set single-rope tension and engine speed, the crane hoist executes the lowering operation. The method includes: detecting the engine speed change rate and the operating parameters of the engine and the operating parameters of the closed hydraulic system at the current engine speed change rate; executing the set control strategy according to the detected engine speed change rate until the engine shuts down for protection; repeating the above process, and recording the operating parameters of the engine and the operating parameters of the closed hydraulic system when the crane hoist executes the lowering operation at different single-rope tensions and engine speeds; processing the obtained several groups of operating parameters of the engine and the operating parameters of the closed hydraulic system to obtain the engine's negative torque resistance ability in the full speed range under the set single-rope tension condition, and drawing the single-rope tension - engine negative torque resistance ability curve.

[0051] This embodiment is based on the lowering conditions of the large-tonnage crane with a closed system for the hoisting mechanism 1 and the hoisting mechanism 2. In this embodiment, the hoist pump is an electro-hydraulic proportional variable control pump, and the hoist motor is an electro-hydraulic proportional variable motor. To ensure the negative torque resistance ability of the system, the hydraulic system is provided with multiple loadings and controls. When the hoist executes the lowering operation, at different single-rope tensions (different load rates) and different engine speeds set, the parameters of the hydraulic system are measured, and the engine's negative torque resistance ability is obtained after calculation. To ensure the negative torque resistance ability of the system, the hydraulic system is provided with multiple loadings and controls.

[0052] Executing the set control strategy according to the detected engine speed change rate includes:

[0053] When the engine speed change rate satisfies: n1 ≤ n < n2, execute Control Strategy 1: The loading pump starts to work at the working pressure Hi, and increases the positive torque load to make the engine speed change rate stable within n2; where, n1 represents the first threshold of the engine speed change rate, n represents the currently detected engine speed change rate, and n2 represents the second threshold of the engine speed change rate;

[0054] When the engine speed change rate satisfies: n2 ≤ n < n3, execute Control Strategy 2: The loading pump starts to work at the working pressure Hi, and the working pump reduces the control current value at a certain ratio and speed to reduce the output flow of the working pump and reduce the output of negative torque, making the engine speed change rate stable within n3; where, n3 represents the third threshold of the engine speed change rate;

[0055] When the engine speed change rate satisfies: n ≥ n3, the engine shuts down for protection.

[0056] Processing the obtained several groups of operating parameters of the engine and the operating parameters of the closed hydraulic system, the specific calculation method is as follows:

[0057] After obtaining the test data, calculations are performed for the points of the rotational speed change rate n3 to obtain the curve of the engine's anti-negative torque ability within the range of n3 for the rotational speed change rate. The anti-negative torque ability curve is the percentage of negative torque corresponding to different single-rope tensions (the percentage of the torque ability to resist negative loads in the output torque ability).

[0058] (1) Given the engine performance curves, namely the rotational speed n, power PD, and torque TD curves, obtain the performance parameters of the engine at different rotational speeds;

[0059] (2) Obtain data through testing:

[0060] a. Output pressure of the main pump: The main pump is set as P m1 、P m2 , in this embodiment, the two main pumps are electronically controlled closed-loop variable pumps with the same variable characteristics, and the working pressures are basically the same. Take the pressure E when a single pump is working i for calculation.

[0061] b. Through the test curve, obtain the engine speed increase inflection point and the corresponding handle current (i.e., the current C that controls the pump displacement), and calculate the corresponding output displacement Di of the pump according to the pump current control curve. i

[0062] c. Output pump pressures of other positive loads during system operation: Set as P 3 、P 4 etc. In this embodiment, it is a make-up oil pump with a comprehensive displacement of Q, and the working pressure is taken as R for calculation.

[0063] d. Loading pump pressure: Set as P j , in the negative torque test scheme of this embodiment, the displacement of the loading pump during operation is J i , and the working pressure is taken as H i for calculation.

[0064] List of relevant calculation parameters:

[0065]

[0066] Table 1 Test parameter table

[0067] Instructions:

[0068] a. Single-rope tension setting: In this case, tests are carried out according to 60%, 80%, 100%, 110%, and 125% of the rated load.

[0069] b. Test group setting: Group tests are carried out according to the engine speed gears. Taking 800 r / m at idle speed as the gear starting point, and increasing the speed at intervals according to the test requirements, and measuring i groups of data.

[0070] ​c. Test data: The test can obtain the pump control current, pump output pressure, loading pump pressure, and output pressure of other load pumps at the engine speed increasing point.

[0071] d. Calculation data: Through the test data, the output displacement of the hoist pump, the power torque of the hoist pump, the power torque of the loading pump, and the power torque of other positive load pumps can be calculated at the engine speed increasing point.

[0072] The specific calculations are as follows:

[0073] (1) The pump displacement Di under the condition of the pump current Ci corresponding to the engine speed increasing point is:

[0074] Di = (Ci - 200) / (600 - 200) * D (ml / r)

[0075] In the formula, D is the maximum displacement corresponding to the maximum control current; the initial pump variable current is 200 mA, and the maximum control current is 600 mA.

[0076] (2) The single-pump torque Wi and power Gi corresponding to the engine speed increasing point are:

[0077] Wi = Di × Ei / (2 × π × η mh1 )(Nm)

[0078] In the formula, Ei is the output pressure of the pump at the displacement Di when the engine speed is increasing;

[0079] η mh1 is the hydro-mechanical efficiency of the pump. In this case, η mh1 = 0.92.

[0080] Gi = Di × (Ni / 1000) × Ei / (600 × η t1 )(kW)

[0081] In the formula, Ni is the set engine speed; η t1 is the total efficiency of the pump.

[0082] (3) The torque Ki and power Mi of the loading pump corresponding to the engine speed increasing point are:

[0083] Ki = Ji × Hi / (2 × π × η mh2 )(Nm)

[0084] In the formula, Ji is the working displacement of the positive torque loading pump, and Hi is the output pressure of the positive torque loading pump;

[0085] η mh2 is the hydro-mechanical efficiency of the loading pump. In this case, η mh2 = 0.95

[0086] Mi = Ji × (Ni / 1000) × Hi / (600 × η t2 )(kW)

[0087] Where η t2 is the total efficiency of the pump.

[0088] (4) The torque T and power X of other load pumps corresponding to the engine speed increase point are as follows:

[0089] T = Q × R / (2 × π × η mh3 )(Nm)

[0090] Where Q is the output displacement of other positive torque load pumps, and R is the output pressure of other positive torque load pumps; η mh3 is the hydro-mechanical efficiency of other load pumps.

[0091] X = Q × (Ni / 1000) × R / (600 × η t3 )(kW)

[0092] Where η t3 is the total efficiency of other load pumps.

[0093] The system torque balance is maintained in a stable state before the engine speed increase, that is, the system positive torque T Z and the system negative torque T F are balanced:

[0094] T Z = make-up oil pump torque + loading pump torque (confirm whether loaded) + engine anti-negative load torque (Li);

[0095] T F = the corresponding torque of the main pump when the double winches lower;

[0096] It can be obtained that under the conditions of each set of set single rope tensions and each set of engine speeds, the engine anti-negative load torque / engine rated torque, such as Y1 in Table 2 ～ Yi.

[0097]

[0098] Table 2 Calculation parameter table of single rope tension - engine negative torque capacity

[0099] The calculation and processing of the test data under each set of single rope tension settings mainly involve: capturing the inflection point of the engine speed increase change, screening out extreme data, calculating the average value, etc. In this case, an algorithm for calculating the average value is used to obtain the engine anti-negative torque capacity Zi in the full speed range under the condition of the set single rope tension Fi. The above operation tests need to be carried out multiple times. Taking the single rope tension F1 as an example, calculate the . Z1 is the average value of the measured multiple times.

[0100] Furthermore, the engine's negative torque percentage resistance ability can be obtained, as shown in Table 3:

[0101] Table 3 Single-rope Tension - Engine's Negative Torque Percentage Resistance Ability

[0102]

[0103] Finally, through curve fitting, a curve graph of the single-rope tension F corresponding to the engine's negative torque percentage Z can be obtained by fitting.

[0104] Through the designed test method of the present invention, the negative torque performance of the engine is obtained, and the engine's negative torque curve is used as the reference curve for engine negative torque control. A negative torque control strategy is designed to keep the working torque of the hydraulic system always lower than the corresponding negative torque performance curve, maintaining the stability and safety of the hydraulic system during operation.

[0105] Embodiment 2:

[0106] Based on the closed-loop hydraulic system negative torque test method described in Embodiment 1, this embodiment provides a closed-loop hydraulic system negative torque control method, as Figure 5 shown, including: when the crane hoist executes the lowering operation, obtaining the engine speed corresponding to the throttle and the single-rope tension of the suspended heavy object; calculating the torque value borne by the current engine based on the engine speed corresponding to the throttle and the single-rope tension of the suspended heavy object; comparing the torque value borne by the engine with the engine's negative torque resistance ability in the full speed range under the set single-rope tension condition obtained by the closed-loop hydraulic system negative torque test method described in the first aspect, and performing negative torque control according to the comparison result.

[0107] Performing negative torque control according to the comparison result includes: when the torque value borne by the engine is less than the curve set value, no negative torque control is executed; otherwise, hierarchical loading control is performed according to the set loading program until engine flameout protection.

[0108] The present invention is mainly applicable to the test and control of the negative torque generated by the heavy object doing work on the engine during the descent of a crane controlled by a closed-loop hydraulic system. By accurately testing the negative torque that the engine can withstand at different speeds and different loads, and taking this negative torque curve as the boundary, the torque generated by the hydraulic pump is controlled below this boundary by a certain proportion through PLC control, avoiding the danger of the engine speed soaring out of control due to the negative torque generated by the hydraulic system exceeding the negative torque that the engine can withstand, and ensuring the stability and safety of the crane during the descent process.

[0109] Embodiment 3:

[0110] Based on the negative torque test method for a closed hydraulic system described in Embodiment 1 and the negative torque control method for a closed hydraulic system described in Embodiment 2, this embodiment provides a crane. The crane performs negative torque control when the hoist of the crane executes the lowering operation according to the negative torque control method for the closed hydraulic system described in Embodiment 2.

[0111] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and deformations can be made, and these improvements and deformations should also be regarded as the protection scope of the present invention.

Claims

1. A method for testing the negative torque of a closed hydraulic system, characterized in that, under the set single-rope tension and engine speed, the crane hoist performs a lowering operation, and the method includes: detecting the engine speed change rate and the operating parameters of the engine and the operating parameters of the closed hydraulic system at the current engine speed change rate; executing the set control strategy according to the detected engine speed change rate until the engine shuts down for protection; repeating the above process, and recording the operating parameters of the engine and the operating parameters of the closed hydraulic system when the crane hoist performs a lowering operation at different single-rope tensions and engine speeds; processing the obtained several groups of operating parameters of the engine and the operating parameters of the closed hydraulic system to obtain the engine's negative torque resistance ability in the full speed range under the set single-rope tension condition, and drawing a single-rope tension - engine negative torque resistance ability curve; wherein, the executing the set control strategy according to the detected engine speed change rate includes: when the engine speed change rate satisfies: n1 ≤ n < n2, execute control strategy one; where, n1 represents the first threshold of the engine speed change rate, n represents the currently detected engine speed change rate, and n2 represents the second threshold of the engine speed change rate; when the engine speed change rate satisfies: n2 ≤ n < n3, execute control strategy two; where, n3 represents the third threshold of the engine speed change rate; when the engine speed change rate satisfies: n ≥ n3, the engine shuts down for protection; Control strategy one includes that the loading pump starts to work at the working pressure Hi, and increases the positive torque load so that the engine speed change rate is stabilized within n2; Control strategy two includes that the loading pump starts to work at the working pressure Hi, and the working pump reduces the control current value at a set ratio and speed to reduce the output flow of the working pump, reduces the output of negative torque, and makes the engine speed change rate stabilized within n3.

2. The method for testing the negative torque of a closed hydraulic system according to claim 1, characterized in that, the processing the obtained several groups of operating parameters of the engine and the operating parameters of the closed hydraulic system to obtain the engine's negative torque resistance ability in the full speed range under the set single-rope tension condition includes: obtaining the rated power of the engine at different speeds according to the engine performance curve; calculating the engine's negative load torque resistance under the set single-rope tension and each group of engine speed conditions according to the operating parameters of the engine and the operating parameters of the closed hydraulic system; calculating the engine's negative torque resistance ability in the full speed range under the set single-rope tension condition according to the rated power of the engine at different speeds, the set single-rope tension and the engine's negative load torque resistance under each group of engine speed conditions.

3. The method for testing the negative torque of a closed hydraulic system according to claim 2, characterized in that, calculating the engine's negative load torque resistance under the set single-rope tension and each group of engine speed conditions according to the operating parameters of the engine and the operating parameters of the closed hydraulic system includes: (1) The pump displacement Di under the pump current Ci corresponding to the engine speed increase point is: Di = (Ci - 200) / (600 - 200) D where, D is the maximum displacement corresponding to the maximum control current; (2) The single-pump torque Wi and power Gi corresponding to the engine speed increase point are: Wi = Di × Ei / (2 × π × η mh1 ) where η mh1 is the hydro-mechanical efficiency of the pump, and Ei is the output pressure of the pump at displacement Di at the engine speed-up point; Gi = Di×(Ni / 1000)×Ei / (600×η t1 ) where η t1 is the total efficiency of the pump, and Ni is the set engine speed; (3)The torque Ki and power Mi of the loading pump corresponding to the engine speed increasing point are as follows: Ki = Ji×Hi / (2×π×η mh2 ) where η mh2 is the hydro-mechanical efficiency of the loading pump, Ji is the displacement of the positive torque loading pump, and Hi is the output pressure of the positive torque loading pump; Mi = Ji×(Ni / 1000)×Hi / (600×η t2 ) where η t2 is the overall efficiency of the pump; (4)The torque T and power X of other load pumps corresponding to the engine speed increasing point are as follows: T = Q×R / (2×π×η mh3 ) where η mh3 is the hydro-mechanical efficiency of other load pumps, Q is the output displacement of other positive torque load pumps, and R is the output pressure of other positive torque load pumps; X = Q×(Ni / 1000)×R / (600×η t3 ) where η t3 is the total efficiency of other load pumps; Because, before the engine speed increases, the system torque balance is maintained in a stable state, that is, the system positive torque T Z is balanced with the system negative torque T F , where T Z = torque of the make-up oil pump + torque of the loading pump + engine anti-negative load torque, and T F = corresponding torque of the main pump during double-winch lowering; it can be concluded that the engine anti-negative load torque under the conditions of each set of set single-rope pulling force and each set of engine speeds.

4. The method for testing negative torque of a closed hydraulic system according to claim 3, characterized in that the anti-negative torque ability of the engine in the full speed range under the condition of a set single-rope tension is obtained by the method of taking the average. First, the anti-negative torque ability Zi of the engine in the full speed range under the condition of a set single-rope tension Fi is obtained, and the average value is obtained after multiple tests.

5. A method for controlling negative torque of a closed hydraulic system, characterized in that it includes: when the crane hoist executes the lowering operation, obtaining the engine speed corresponding to the throttle and the single-rope tension of the suspended heavy object; calculating the torque value borne by the current engine according to the engine speed corresponding to the throttle and the single-rope tension of the suspended heavy object; comparing the torque value borne by the engine with the anti-negative torque ability of the engine in the full speed range under the condition of a set single-rope tension obtained by the method for testing negative torque of a closed hydraulic system according to any one of claims 1 to 4, and performing negative torque control according to the comparison result.

6. The method for controlling negative torque of a closed hydraulic system according to claim 5, characterized in that the performing of negative torque control according to the comparison result includes: when the torque value borne by the engine is less than the curve set value, no negative torque control is performed; otherwise, hierarchical loading control is performed according to the set loading program until engine flameout protection.

7. A crane, characterized in that the crane performs negative torque control when the crane hoist executes the lowering operation according to the method for controlling negative torque of a closed hydraulic system according to claim 6.

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