A hydraulic energy recovery system and control method for forklift forks

Abstract

The present invention discloses a hydraulic energy recovery system for a forklift fork, which includes a lifting cylinder. The lifting cylinder is connected to an oil supply part and an oil return part through a lifting solenoid valve. The oil supply part includes an oil supply pipeline and an oil pump arranged on the oil supply pipeline. The oil return part includes an oil return pipeline. The end of the oil return pipeline is communicated with the oil inlet of the oil pump. A energy storage part is connected to the oil return pipeline. The energy storage part includes at least one accumulator. The accumulator is communicated with the oil return pipeline through an accumulator valve. Based on the original conventional forklift hydraulic oil circuit, the present invention adds at least one accumulator for recovering the hydraulic energy of the lifting cylinder, which can greatly improve the energy utilization efficiency of the forklift and save costs. For the improvement of the forklift hydraulic system and control method, it can effectively improve the energy waste of the forklift lifting system, reduce the overflow loss, and can greatly improve the stability of the fork descent, making the operation safer. At the same time, it can significantly improve the working efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of forklifts, and specifically to a hydraulic energy recovery system and control method for forklift forks. Background Art

[0002] A forklift is a common cargo handling vehicle, widely used in ports, airports, factory workshops, warehouses, etc. With the development of the economy and the progress of technology, forklifts have replaced manual handling and become an indispensable part of modern industrial development. The energy conservation of forklifts has gradually become an urgent problem to be solved. During the working process, the lifting system accounts for most of the energy consumption, and the gravitational potential energy of the goods will be lost through overflow when the forks descend. Therefore, the recovery of this part of energy has great potential for energy recovery. Summary of the Invention

[0003] The purpose of the present invention is to provide a hydraulic energy recovery system and control method for forklift forks to solve the problems raised in the above background art.

[0004] To achieve the above purpose, the present invention provides the following technical solutions:

[0005] A hydraulic energy recovery system for forklift forks includes a lifting cylinder, and the lifting cylinder is connected to an oil supply part and an oil return part through a lifting solenoid valve.

[0006] The oil supply part includes an oil supply pipeline and an oil pump arranged on the oil supply pipeline.

[0007] The oil return part includes an oil return pipeline, the end of the oil return pipeline is communicated with the oil inlet of the oil pump, an energy storage part is connected to the oil return pipeline, the energy storage part includes at least one accumulator, and the accumulator is communicated with the oil return pipeline through an accumulator valve.

[0008] As a further solution of the present invention: the rodless cavity of the lifting cylinder is communicated with the lifting solenoid valve, the piston end of the lifting cylinder is connected with a fork, and a height sensor and a load sensor are arranged on the fork.

[0009] As a further solution of the present invention: the oil inlet of the oil pump is connected with a fuel tank, a check valve is arranged between the oil pump and the fuel tank, the connection part of the oil return pipeline and the oil pump is located between the check valve and the oil pump, and an oil supply pressure sensor and a safety valve are connected to the oil supply pipeline.

[0010] As a further solution of the present invention: the energy storage part includes a first accumulator and a second accumulator. The first accumulator is a high-pressure accumulator, and the second accumulator is a low-pressure accumulator. A first oil pressure sensor is connected to the first accumulator. The first accumulator is communicated with the oil return pipeline through a first accumulator valve. A second oil pressure sensor is connected to the second accumulator. The second accumulator is connected to the oil return pipeline through a second accumulator valve.

[0011] As a further solution of the present invention: an overflow valve and an oil return pressure sensor are connected to the oil return pipeline, and an oil supply valve is provided at one end of the oil return pipeline close to the oil pump.

[0012] A hydraulic energy recovery control method for a forklift fork includes a controller communicated with valves and sensors. The control method includes the following steps:

[0013] Step 1: When the fork is in the descending state, the high-pressure oil in the lifting cylinder is discharged through the oil return pipeline and reaches the energy storage part;

[0014] Step 2: All accumulators in the energy storage part perform energy recovery according to the oil return pressure gradient. During the recovery process, the controller selects the accumulator for recovering hydraulic energy to perform recovery energy storage according to the fork load and the pressure states of each accumulator;

[0015] Step 3: During the process of energy recovery through the accumulator in Step 2, the descending speed of the forklift fork during energy recovery is controlled by controlling the oil inlet speed of the accumulator.

[0016] As a further solution of the present invention: the pre-charge pressure of the accumulator is p0, and the initial volume of the accumulator is V0. The energy storage part includes a first accumulator and a second accumulator. The first accumulator is a high-pressure accumulator, and the second accumulator is a low-pressure accumulator. The pre-charge air pressure p 02 and volume V 02 of the first accumulator and the pre-charge air pressure p 01 and volume V 01 of the second accumulator are determined according to different load weight ranges.

[0017] As a further solution of the present invention: the calculation method of the pre-charge pressure p0 of the accumulator is as follows:

[0018] S1.1: When the fork descends, the load sensor transmits the collected fork load to the controller;

[0019] S1.2: Determine the pressure in the rodless cavity of the lifting cylinder according to the collected fork load. The specific calculation method is obtained by the following method:

[0020]

[0021] Wherein, p g is the pressure of the rodless cavity of the lifting cylinder, Mpa; F is the load on the piston rod, N; m0 is the total mass of the fork carriage, kg; m1 is the mass of the inner mast, kg; m2 is the mass of the piston rod, kg; A g is the area of the rodless cavity of the lifting cylinder, m 2 ;

[0022] S1.3. Divide the forklift load into load intervals with low load and high load, and determine the maximum working pressure p1 and minimum working pressure p2 of the accumulator according to the highest and lowest loads in each load interval respectively; the maximum working pressure p l1 and minimum working pressure p l2 of the second accumulator, and the maximum working pressure p h1 and minimum working pressure p h2 of the first accumulator;

[0023] S1.4. According to the working law of the accumulator, the precharge pressure p0 of the accumulator = 0.9p2, which is used to determine the precharge pressure of the second accumulator as p 01 and the precharge pressure of the first accumulator as p 02 ;

[0024] The method for determining the initial volume V0 of the accumulator is as follows:

[0025] S2.1. Determine the maximum and minimum volumes required for the accumulator to store energy. According to the volume of the oil generated by the single maximum stroke of the lifting cylinder, we can get:

[0026] V g = ΔV = A g ·L g

[0027] Wherein, V g is the maximum volume of the rodless cavity of the lifting cylinder, m 3 ; L g is the maximum stroke of the lifting cylinder, m; ΔV is the maximum and minimum volume required for the accumulator to store energy, m 3 ;

[0028] S2.2. Determine the initial volume V0 of the accumulator as:

[0029]

[0030] [[ID=, where n is the polytropic index of the gas. Due to the working characteristics, it can be known that the gas compression and release process in the airbag cavity is fast, so n = 1.4 is taken.

[0031] As a further solution of the present invention: The selection method of the accumulator in step 2 is carried out as follows:

[0032] Step 2.1: The controller collects the signals of the first oil pressure sensor, the second oil pressure sensor, and the return oil pressure sensor;

[0033] Step 2.2: Compare the pressure signal of the return oil pressure sensor with the pressure signals of the low-pressure p l and high-pressure p oil pressure sensors in sequence:

[0034] When the pressure signal of the return oil pressure sensor is greater than the pressure signal of the low-pressure p l oil pressure sensor, at this time, the second accumulator can be used for recovery, and the second accumulator valve corresponding to the second accumulator is selected to open;

[0035] When the pressure of the second accumulator gradually rises to the pressure value p of the first oil pressure sensor, control the second accumulator valve and the first accumulator valve to open simultaneously;

[0036] When the pressure value p of the second oil pressure sensor l rises to the maximum working pressure p of the second accumulator l1 , close the second accumulator valve and open the first accumulator valve;

[0037] When the pressures of the second accumulator and the first accumulator both rise to the maximum working pressures p h1 and p l1 , close the first accumulator valve and the second accumulator valve, and open the overflow valve for overflow.

[0038] As a further solution of the present invention: The control of the lowering speed of the forklift fork in step 3 includes the following steps:

[0039] Step 3.1: The controller predicts the lowering speed v of the fork according to the collected input quantities of the real-time pressure of the target accumulator, the return oil pressure sensor, and the load sensor value, and based on the force relationship of the lifting cylinder during the lowering stroke, in combination with the speed control strategy g ;

[0040] Step 3.2: Calculate and adjust the opening degree of the corresponding accumulator valve according to the lowering speed v of the fork g ;

[0041] The prediction of the lowering speed v of the fork g includes the following steps:

[0042] Step 3.1.1: The controller obtains the force on the lifting cylinder during the lowering stroke according to the collected input quantities of the real-time pressure of the target accumulator, the return oil pressure sensor, and the load sensor value:

[0043] F g -p g ·A g =m2·a g +b g ·vg -k g ·x g

[0044] In the formula, b g is the damping coefficient of hydraulic oil, N / m / s; x g is the displacement of the lifting cylinder, m; v g is the speed of the piston rod of the lifting cylinder, m / s; a g is the acceleration of the piston rod of the lifting cylinder, m / s 2 ; k g is the spring stiffness of the lifting cylinder, N / m;

[0045] Step 3.1.2, determine the state equation of the lifting cylinder system:

[0046]

[0047]

[0048] In the formula, z = [x g v g T is the system state vector; u = F g -p g ·A g is the system control input;

[0049] Step 3.1.3, discretize the system state equation in Step 3.1.2:

[0050] z(k + 1) = A k z(k) + B k u(k)

[0051]

[0052] In the formula, k is the sampling point; k + 1 is the next sampling point; T s is the sampling period;

[0053] The output equation is:

[0054] y(k) = Cz(k), C = [0 1]

[0055] In the formula, y(k) is the cylinder lowering speed, m / s;

[0056] Step 3.1.4, use the error between the output quantity and the reference value and the control increment as the evaluation index, and the objective function is:

[0057]

[0058] In the formula, y p (k + i|k) is the predicted value of the output speed; y​r (k + i|k) is the output speed reference value; (k + i|k) is the value at the (k + i)-th sampling point predicted at the k-th sampling point, where i = 1, 2,..., N p ; (k + i) is the value at the (k + i)-th sampling point, where i = 1, 2,..., N c -1; N p is the prediction time domain; N c is the control time domain;

[0059] Step 3.1.5. According to the system speed and acceleration constraints:

[0060] v min ≤ v(k + i) ≤ v max , i = 0, 1,..., N c -1

[0061] a min ≤ a(k + i) ≤ a max , i = 0, 1,..., N c -1

[0062] In the formula, v min and v max are the speed thresholds; a min and a max are the acceleration thresholds;

[0063] According to the initial state of the system, minimize the objective function under the constraints and solve to obtain the output speed prediction value y p (k + i|k) The opening degrees of the accumulator valve and the overflow valve are calculated as follows:

[0064]

[0065] In the formula, x f is the opening degree of the accumulator valve or the overflow valve, m; v g is the fork lowering speed, m / s; C d is the throttle flow coefficient, taking 0.61; ω is the wet perimeter of the solenoid valve, m; ρ is the density of the hydraulic oil, kg / m 3 ; V g is the volume of the rodless chamber of the lifting cylinder, m 3 ; β is the bulk modulus of compression; p g is the pressure in the rodless chamber of the lifting cylinder, pa; p b is the pressure acting on the airbag chamber from the oil chamber, pa.

[0066] Compared with the prior art, the beneficial effects of the present invention are:

[0067] 1. Based on the original conventional forklift hydraulic oil circuit, the present invention adds at least one accumulator for recovering the hydraulic energy of the lifting cylinder, which can greatly improve the energy utilization efficiency of the forklift and save costs;

[0068] 2. The present invention proposes a method for determining the pre-charge pressure and initial volume of each echelon accumulator, which can ensure the maximization of recovered energy for different load ranges;

[0069] 3. The present invention proposes a method for selecting an accumulator for recovering energy, which can recover the energy of the lifting cylinder to the greatest extent under different loads and different accumulator pressure states;

[0070] 4. The present invention proposes a descending speed control system, which can control the descending speed of the forklift forks to ensure operation safety while ensuring the maximum recovered energy. When the forklift forks carry goods and perform a descending operation, it can timely adjust the descending speed of the forklift forks to prevent the forklift forks from descending too fast and the goods will fall due to inertia and injure the staff;

[0071] 5. The improvement of the forklift hydraulic system and control method of the present invention can effectively improve the energy waste of the forklift lifting system, reduce the overflow loss, and can greatly improve the descending stability of the forklift forks, making the operation safer. At the same time, it can significantly improve the work efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 It is a schematic diagram of the energy recovery system of this embodiment;

[0073] In the figure: 1 - height sensor, 2 - load sensor, 3 - lifting cylinder, 31 - rodless cavity of the lifting cylinder, 41 - first accumulator, 42 - first oil pressure sensor, 43 - first accumulator valve, 51 - second accumulator, 52 - second oil pressure sensor, 53 - second accumulator valve, 6 - overflow valve, 7 - lifting solenoid valve, 8 - return oil pressure sensor, 9 - supply oil pressure sensor, 10 - oil pump, 11 - supply oil valve, 12v safety valve, 13 - fuel tank, 14 - controller. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0074] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0075] Please refer to Figure 1, in the embodiments of the present invention, a hydraulic energy recovery system for a forklift fork includes a lifting cylinder 3. The rodless cavity of the lifting cylinder 3 is communicated with a lifting solenoid valve 7. The piston end of the lifting cylinder 3 is connected with a fork. A height sensor 1 and a load sensor 2 are arranged on the fork. The lifting cylinder 3 is connected with an oil supply part and an oil return part through the lifting solenoid valve 7.

[0076] The oil supply part includes an oil supply pipeline and an oil pump 10 arranged on the oil supply pipeline. The oil return part includes an oil return pipeline. The end of the oil return pipeline is communicated with the oil inlet of the oil pump 10. The oil inlet of the oil pump 10 is connected with an oil tank 13. A check valve is arranged between the oil pump 10 and the oil tank 13. The connection part of the oil return pipeline and the oil pump 10 is located between the check valve and the oil pump 10. An oil supply pressure sensor 9 and a safety valve 12 are connected to the oil supply pipeline. An energy storage part is connected to the oil return pipeline. The energy storage part includes at least one accumulator. The accumulator is communicated with the oil return pipeline through an accumulator valve. In this embodiment, the energy storage part includes a first accumulator 41 and a second accumulator 51. The first accumulator 41 is a high-pressure accumulator and the second accumulator 51 is a low-pressure accumulator. A first oil pressure sensor 42 is connected to the first accumulator 41. The first accumulator 41 is communicated with the oil return pipeline through a first accumulator valve 43. A second oil pressure sensor 52 is connected to the second accumulator 51. The second accumulator 51 is connected to the oil return pipeline through a second accumulator valve 53. An overflow valve 6 and an oil return pressure sensor 8 are connected to the oil return pipeline. An oil supply valve 11 is arranged at one end of the oil return pipeline close to the oil pump 10.

[0077] In addition, it further includes a controller 14. The controller 14 is in signal communication with the height sensor 1, the load sensor 2, the lifting solenoid valve 7, the oil supply pressure sensor 9, the first oil pressure sensor 42, the second oil pressure sensor 52, the first accumulator valve 43, the second accumulator valve 53, the overflow valve 6, and the oil return pressure sensor 8. And the controller 14 internally sets the following energy recovery control method, and then can control all the sensors and valves through the controller to control the energy recovery system of this embodiment.

[0078] A hydraulic energy recovery control method for a forklift fork. The pre-charge pressure of the accumulator is p0, and the initial volume of the accumulator is V0. In this embodiment, the energy storage part includes a first accumulator 41 and a second accumulator 51. The first accumulator 41 is a high-pressure accumulator and the second accumulator 51 is a low-pressure accumulator. The pre-charge air pressure p 02 and volume V 02 of the first accumulator 41 and the pre-charge air pressure p 01 and volume V 01 of the second accumulator 51 are determined according to different load weight ranges.

[0079] The calculation method of the accumulator pre-charge pressure p0 is as follows:

[0080] S1.1. When the forklift forks descend, the load sensor 2 transmits the detected load weight of the forklift forks to the controller 14;

[0081] S1.2. Determine the pressure in the rodless chamber of the lifting cylinder based on the detected load weight of the forklift forks. The specific calculation method is obtained as follows:

[0082]

[0083] In the formula, p g is the pressure in the rodless chamber of the lifting cylinder, in Mpa; F is the load on the piston rod, in N; m0 is the total mass of the forklift fork carriage, in kg; m1 is the mass of the inner mast, in kg; m2 is the mass of the piston rod, in kg; A g is the area of the rodless chamber of the lifting cylinder, in m 2 ;

[0084] S1.3. Divide the load weight of the forklift forks into load intervals of low load and high load, and determine the maximum working pressure p1 and the minimum working pressure p2 of the accumulator based on the highest and lowest load weights in each load interval respectively; the maximum working pressure p l1 and the minimum working pressure p l2 of the second accumulator 51, the maximum working pressure p h1 and the minimum working pressure p h2 of the first accumulator 41;

[0085] S1.4. According to the working law of the accumulator, the pre-charge pressure p0 of the accumulator = 0.9p2, and use it to determine the pre-charge pressure of the second accumulator 51 as p 01 and the pre-charge pressure of the first accumulator 41 as p 02 ;

[0086] The calculation method of the initial volume V0 of the accumulator includes the following steps:

[0087] S2.1. Determine the minimum value of the maximum volume required for the accumulator to store energy. According to the volume of the oil generated by the single maximum stroke of the lifting cylinder, we can get:

[0088] V g = ΔV = A g ·L g

[0089] In the formula, V g is the maximum volume of the rodless chamber of the lifting cylinder, in m 3 ; Lg is the maximum stroke of the lifting cylinder, in m; ΔV is the minimum value of the maximum volume required for the accumulator to store energy, in m 3 ;

[0090] S2.2. Determine the initial volume V0 of the accumulator as:

[0091]

[0092] In the formula, n is the polytropic index of the gas. Due to the working characteristics, it can be known that the gas compression and release process in the airbag cavity is fast, so n = 1.4 is taken.

[0093] The energy recovery control method of this embodiment includes the following steps:

[0094] Step 1: When the forklift forks are in the descending state, the high-pressure oil in the lifting cylinder 3 is discharged through the oil return pipeline and reaches the energy storage part;

[0095] Step 2: All the accumulators in the energy storage part perform energy recovery according to the oil return pressure gradient. During the recovery process, the controller 14 selects the accumulator for recovering hydraulic energy for recovery energy storage according to the forklift load and the pressure state of each accumulator; the accumulator selection method is as follows:

[0096] Step 2.1: The controller 14 collects the signals of the first oil pressure sensor 42, the second oil pressure sensor 52 and the oil return oil pressure sensor 8;

[0097] Step 2.2: Compare the pressure signal of the oil return oil pressure sensor 8 with the pressure signals of the low-pressure p l and the high-pressure p oil pressure sensors in sequence:

[0098] When the pressure signal of the oil return oil pressure sensor 8 is greater than the pressure signal of the low-pressure p l oil pressure sensor, at this time, the second accumulator 51 can be used for recovery, and the second accumulator valve 53 corresponding to the second accumulator 51 is opened;

[0099] When the pressure of the second accumulator 51 gradually rises to the pressure value p of the first oil pressure sensor 42, control the second accumulator valve 53 and the first accumulator valve 43 to open simultaneously;

[0100] When the pressure value p l of the second oil pressure sensor 52 rises to the maximum working pressure p l1 of the second accumulator 51, close the second accumulator valve 53 and open the first accumulator valve 43;

[0101] When the pressures of the second accumulator 51 and the first accumulator 41 both rise to the maximum working pressures p h1 and p l1 , close the first accumulator valve 43 and the second accumulator valve 53, and open the overflow valve 6 for overflow;

[0102] Step 3: During the process of energy recovery through the accumulator in Step 2, control the descending speed of the forklift forks during energy recovery by controlling the oil inlet speed of the accumulator;

[0103] The control of the descending speed of the forklift forks includes the following steps:

[0104] Step 3.1. The controller 14 predicts the fork lowering speed v according to the collected input values of the target accumulator real-time pressure, the return oil pressure sensor 8, and the load sensor 2, based on the force relationship during the lowering stroke of the lifting cylinder and in combination with the speed control strategy. g ;

[0105] Step 3.2. Calculate and adjust the corresponding accumulator valve opening according to the fork lowering speed v g .

[0106] The prediction of the fork lowering speed v g includes the following steps:

[0107] Step 3.1.1. The controller 14 obtains the force during the lowering stroke of the lifting cylinder based on the collected input values of the target accumulator real-time pressure, the return oil pressure sensor 8, and the load sensor 2:

[0108] F g -p g ·A g =m2·a g +b g ·v g -k g ·x g

[0109] In the formula, b g is the hydraulic oil damping coefficient, N / m / s; x g is the displacement of the lifting cylinder, m; v g is the speed of the piston rod of the lifting cylinder, m / s; a g is the acceleration of the piston rod of the lifting cylinder, m / s 2 ; k g is the spring stiffness of the lifting cylinder, N / m;

[0110] Step 3.1.2. Determine the state equation of the lifting cylinder system:

[0111]

[0112]

[0113] In the formula, z = [x g v g T is the system state vector; u = F g -p g ·A g is the system control input;

[0114] Step 3.1.3. Discretize the system state equation in Step 3.1.2:

[0115] z(k + 1) = Ak z(k) + B k u(k)

[0116]

[0117] where k is the sampling point; k + 1 is the next sampling point; T s is the sampling period;

[0118] The output equation is:

[0119] y(k) = Cz(k), C = [0 1]

[0120] where y(k) is the cylinder lowering speed, m / s;

[0121] Step 3.1.4: Taking the error between the output and the reference value and the control increment as the evaluation index, the objective function is:

[0122]

[0123] where y p (k + i|k) is the predicted value of the output speed; y r (k + i|k) is the reference value of the output speed; (k + i|k) is the value predicted at the k sampling point for the k + i sampling point, i = 1, 2,..., N p ; (k + i) is the value at the k + i sampling point, i = 1, 2,..., N c - 1; N p is the prediction horizon; N c is the control horizon;

[0124] Step 3.1.5: According to the system speed and acceleration constraints:

[0125] v min ≤ v(k + i) ≤ v max , i = 0, 1,..., N c - 1

[0126] a min ≤ a(k + i) ≤ a max , i = 0, 1,..., N c - 1

[0127] where v min and v max are the speed thresholds; a min and a max are the acceleration thresholds;

[0128] According to the initial state of the system, minimize the objective function under the constraints to solve for the predicted value of the output speed y p(k + i|k) The opening degrees of the accumulator valve and the overflow valve are calculated by the following formula:

[0129]

[0130] In the formula, x f is the opening degree of the accumulator valve or the overflow valve, m; v g is the descending speed of the forklift forks, m / s; C d is the flow coefficient of the throttle port, taking 0.61; ω is the wetted perimeter of the solenoid valve, m; ρ is the density of the hydraulic oil, kg / m 3 ; V g is the volume of the rodless cavity of the lifting cylinder, m 3 ; β is the bulk modulus of compression; p g is the pressure of the rodless cavity of the lifting cylinder, Pa; p b is the pressure acting on the airbag cavity from the oil chamber, Pa.

[0131] Example 1

[0132] Take a forklift with a common 3-ton manually controlled valve operating the forklift forks as an example;

[0133] Apply the safety control system of this embodiment to a forklift with a common 3-ton manually controlled valve operating the forklift forks, and measure the parameters of the forklift as follows: the rated load of the forklift is 3 tons, the maximum lifting height is 3 meters, the maximum stroke of the lifting cylinder is 1.5 meters, the cross-sectional area of the rodless cavity of the lifting cylinder is 0.0024 m 2 , and the transmission method is hydraulic transmission.

[0134] Calculation of the accumulator precharge pressure

[0135] Divide the forklift load into low load and high load according to the full load weight, that is, 0 - 1.5t is the low load, and 1.5t - 3t is the high load. Determine that the pressures of the rodless cavity of the lifting cylinder at load weights of 3t and 1.5t are 27 Mpa and 17 Mpa respectively, corresponding to the highest and lowest working pressures of the high-pressure airbag type accumulator. Determine that the precharge pressure of the high-pressure airbag type accumulator is 17 * 0.9 = 15.3 Mpa. Similarly, the precharge pressure of the low-pressure airbag type accumulator can be determined to be 6 Mpa.

[0136] Calculation of the initial volume of the accumulator

[0137] According to the known volume of the oil generated by a single stroke of the lifting cylinder, it can be known that the minimum effective volume of the accumulator is 4L. The highest and lowest working pressures of the accumulator are 27 Mpa and 17 Mpa. According to the formula in step 2.2 of claim 3, the volumes of the high-pressure and low-pressure airbag type accumulators are calculated to be 10L and 8L.

[0138] Taking the 2.5t load capacity of the forklift forks as an example, when the forklift forks are lowered, it is measured that the pressure in the rodless chamber of the lifting cylinder is 24 Mpa, and the pressures of the high-pressure and low-pressure bladder accumulators are 16 Mpa and 8 Mpa respectively. According to the low-pressure and high-pressure sequence, the low-pressure accumulator can be selected first for hydraulic energy recovery. During this process, the controller collects the real-time pressure of the accumulator at a certain moment as 24 Mpa and the oil return pressure value as 16 Mpa, predicts the lowering speed of the forklift forks in combination with the speed control strategy, and then calculates the valve opening value as 50% according to Claim 7 and controls the valve opening adjustment. As the recovery progresses, when the pressure of the low-pressure accumulator gradually rises to 16 Mpa, both the high-pressure and low-pressure accumulators are opened for recovery. When the pressure of the low-pressure accumulator rises to the maximum working pressure value of the accumulator, which is 17 Mpa, the valve of the low-pressure accumulator is closed, and the valve of the high-pressure accumulator remains open until the pressure rises to the maximum working pressure value of the high-pressure accumulator, which is 27 Mpa. If the lifting cylinder has not reached the bottom yet, calculate the opening and open the overflow valve for overflow.

[0139] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and without departing from the spirit or basic characteristics of the present invention, the present invention can be implemented in other specific forms. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be encompassed within the present invention. Any reference signs in the claims should not be regarded as limiting the claims involved.

[0140] In addition, it should be understood that although this specification is described according to the embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A hydraulic energy recovery system for a forklift fork, comprising a lifting cylinder (3), characterized in that, The lifting cylinder (3) is connected to an oil supply part and an oil return part through a lifting solenoid valve (7); The oil supply part includes an oil supply pipeline and an oil pump (10) arranged on the oil supply pipeline; The oil return part includes an oil return pipeline. The end of the oil return pipeline is communicated with the oil inlet of the oil pump (10). An energy storage part is connected to the oil return pipeline. The energy storage part includes at least one accumulator. The accumulator is communicated with the oil return pipeline through an accumulator valve; The rodless cavity of the lifting cylinder (3) is communicated with the lifting solenoid valve (7). The piston end of the lifting cylinder (3) is connected with a forklift fork. A height sensor (1) and a load sensor (2) are arranged on the forklift fork; The oil inlet of the oil pump (10) is connected with an oil tank (13). A check valve is arranged between the oil pump (10) and the oil tank (13). The connection part of the oil return pipeline and the oil pump (10) is located between the check valve and the oil pump (10). An oil supply pressure sensor (9) and a safety valve (12) are connected to the oil supply pipeline; The energy storage part includes a first accumulator (41) and a second accumulator (51). The first accumulator (41) is a high-pressure accumulator and the second accumulator (51) is a low-pressure accumulator. A first oil pressure sensor (42) is connected to the first accumulator (41). The first accumulator (41) is communicated with the oil return pipeline through a first accumulator valve (43). A second oil pressure sensor (52) is connected to the second accumulator (51). The second accumulator (51) is connected to the oil return pipeline through a second accumulator valve (53).

2. The hydraulic energy recovery system for a forklift fork according to claim 1, wherein, An overflow valve (6) and an oil return pressure sensor (8) are connected to the oil return pipeline. An oil supply valve (11) is arranged at one end of the oil return pipeline close to the oil pump (10).

3. A hydraulic energy recovery control method for a forklift fork using the energy recovery system of claim 1, characterized in that, It includes a controller (14) communicated with valves and sensors. The control method includes the following steps: Step 1. When the forklift fork is in the descending state, the high-pressure oil in the lifting cylinder (3) is discharged through the oil return pipeline and reaches the energy storage part; Step 2. All accumulators in the energy storage part perform energy recovery according to the oil return pressure gradient. During the recovery process, the controller (14) selects the accumulator for recovering hydraulic energy for recovery energy storage according to the load weight of the forklift fork and the pressure states of each accumulator; Step 3. During the process of energy recovery through the accumulator in Step 2, the descending speed of the forklift fork during energy recovery is controlled by controlling the oil inlet speed of the accumulator.

4. A hydraulic energy recovery control method for a forklift fork according to claim 3, characterized in that, The pre-charge pressure of the accumulator is , the initial volume of the accumulator is , the energy storage part includes a first accumulator (41) and a second accumulator (51). The first accumulator (41) is a high-pressure accumulator and the second accumulator (51) is a low-pressure accumulator. The pre-charge pressure of the first accumulator (41) is determined according to different load weight ranges and volume , the pre-charge pressure of the second accumulator (51) and volume .

5. A hydraulic energy recovery control method for a forklift fork according to claim 4, characterized in that The accumulator pre-charge pressure The calculation method is as follows: S1.

1. When the forklift fork descends, the load sensor (2) transmits the collected load weight of the forklift fork to the controller (14); S1.

2. Determine the pressure in the rodless cavity of the lifting cylinder according to the collected load weight of the forklift fork. The specific calculation method is obtained as follows: In the formula, is the pressure in the rodless chamber of the lifting cylinder, ; is the load on the piston rod, ; is the total mass of the fork carriage, ; is the mass of the inner gantry, ; is the mass of the piston rod, ; is the area of the rodless chamber of the lifting cylinder, ; S1.

3. Divide the forklift load capacity into load intervals of low load and high load, and determine the maximum working pressure and minimum working pressure of the accumulator according to the highest and lowest load weights in each load interval respectively. and the minimum working pressure ; The maximum working pressure of the second accumulator (51) and the minimum working pressure , the maximum working pressure of the first accumulator (41) and the minimum working pressure ; S1.

4. According to the working law of the accumulator, the pre-charge pressure of the accumulator is respectively used to determine that the pre-charge pressure of the second accumulator (51) is , and the pre-charge pressure of the first accumulator (41) is ; The initial volume of the accumulator The determination method is as follows: S2.

1. Determine the minimum value of the maximum volume required for the accumulator to store energy. According to the volume of the oil generated by the single maximum stroke of the lifting cylinder, it can be obtained: Wherein, is the maximum volume of the rodless cavity of the hoisting cylinder, ; is the maximum stroke of the hoisting cylinder, ; is the minimum of the maximum volume required for energy storage of the accumulator, ; S2.

2. Determine the initial volume of the accumulator It is as follows: In the formula, is the polytropic index of the gas. Due to the working characteristics, it can be known that the gas compression and release process in the airbag cavity is fast. Therefore, is taken.

6. A hydraulic energy recovery control method for a forklift fork according to claim 4, characterized in that, The accumulator selection method in Step 2 is carried out as follows: Step 2.

1. The controller (14) collects the signals of the first oil pressure sensor (42), the second oil pressure sensor (52) and the oil return pressure sensor (8); Step 2.

2. Compare the pressure signals of the oil return pressure sensor (8) in sequence according to the pressure signals of the low-pressure and high-pressure oil pressure sensors: When the pressure signal of the oil return pressure sensor (8) is greater than the pressure signal of the low-pressure oil pressure sensor, the second accumulator (51) can be used for recovery at this time, and the second accumulator valve (53) corresponding to the second accumulator (51) is selected to open; When the pressure of the second energy accumulator (51) gradually rises to the pressure value of the first oil pressure sensor (42) control the second energy accumulator valve (53) and the first energy accumulator valve (43) to open simultaneously; When the pressure value of the second oil pressure sensor (52) rises to the maximum working pressure of the second accumulator (51) close the second accumulator valve (53) and open the first accumulator valve (43); When the pressures of the second energy accumulator (51) and the first energy accumulator (41) both rise to the maximum working pressure , close the first energy accumulator valve (43) and the second energy accumulator valve (53), and open the overflow valve (6) for overflow.

7. A hydraulic energy recovery control method for a forklift fork according to claim 4, characterized in that, The control of the lowering speed of the forklift fork in step 3 includes the following steps: Step 3.

1. The controller (14) predicts the fork lowering speed based on the real-time pressure of the input amount target accumulator, the return oil pressure sensor (8), and the load sensor (2) values collected, in combination with the force relationship during the lowering stroke of the lifting cylinder and the speed control strategy ; Step 3.

2. Calculate and adjust the opening degree of the corresponding accumulator valve according to the fork lowering speed ​ Fork lowering speed The prediction includes the following steps: Step 3.1.1: The controller (14) obtains the force on the lowering stroke of the lifting cylinder based on the real-time pressure of the input quantity target accumulator, the oil return pressure sensor (8), and the load sensor (2) values collected: In the formula, is the damping coefficient of the hydraulic oil, ; is the displacement of the lifting cylinder, ; is the speed of the piston rod of the lifting cylinder, ; is the acceleration of the piston rod of the lifting cylinder, ; is the spring stiffness of the lifting cylinder, ; Step 3.1.2: Determine the system state equation of the lifting cylinder system: wherein, is the system state vector; is the system control input; Step 3.1.3: Discretize the system state equation in step 3.1.2: In the formula, is the sampling point; is the next sampling point; is the sampling period; The output equation is: In the formula, is the descending speed of the oil cylinder, ; Step 3.1.4: Use the error between the output quantity and the reference value and the control increment as evaluation indicators, and the objective function is: In the formula, is the predicted output speed value; is the reference value of the output speed; is at the predicted sampling point value at the sampling point, ; is at the value of the sampling point, ; is the prediction time domain; is the control time domain; Step 3.1.5: According to the system speed and acceleration constraints: In the formula, and are speed thresholds; and are acceleration thresholds; According to the initial state of the system, minimize the objective function under constraints, and solve to obtain the predicted value of the output speed The calculation formula for the opening degrees of the accumulator valve and the overflow valve is: In the formula, is the opening of the accumulator valve or the overflow valve, ; is the lowering speed of the forklift forks, ; is the flow coefficient of the throttle orifice, taking 0.61; is the wetted perimeter of the solenoid valve, ; is the density of the hydraulic oil, ; is the volume of the rodless chamber of the lifting cylinder, ; is the bulk modulus of compression; is the pressure in the rodless chamber of the lifting cylinder, ; is the pressure exerted by the oil chamber on the airbag chamber, .

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