A Control Method and Control System for Hierarchical Utilization of Recovered Energy in a Forklift Hydraulic System

Abstract

The present invention discloses a control method for hierarchical utilization of recovered energy in a forklift hydraulic system. The control method includes the following steps: Step 1, detect the state of the forklift fork. If the fork is in the lifting state, then execute Step 2; Step 2, supply energy to the lifting cylinder 3 of the fork by releasing the high-pressure oil in different accumulators in the hierarchical energy storage unit; Step 3, determine the output flow rate of the variable displacement oil pump according to the lifting speed of the fork; Step 4, determine the lifting required power according to the load of the fork; Step 5, determine the output power of the energy storage unit according to the real-time output pressure value of the energy storage unit; Step 6, determine the minimum output power of the engine according to the lifting required power and the output power of the accumulator; Step 7, control the engine speed and the displacement of the variable displacement oil pump. The present invention proposes a method for hierarchical utilization of recovered energy by accumulators, and determines the accumulator oil supply mode according to the pressure in the rodless cavity and the oil supply pressure of each hierarchical accumulator, ensuring the maximization of the oil supply efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of energy recovery and reuse for forklifts, and particularly to a control method and control system for hierarchical utilization of recovered energy in a forklift hydraulic system. Background Art

[0002] As a commonly used cargo handling vehicle, counterbalanced forklifts are widely used in ports, freight yards, factory workshops, warehouses and other places, mainly for the circulation and distribution of goods. In the past two years, affected by national policies and the development of the logistics industry, the sales volume of forklifts has increased rapidly and has gradually become an indispensable part of industrial development. However, improving the energy utilization rate and working efficiency of forklifts is the driving force for people's research. At present, the main energy loss ways in the forklift lifting system include: overflow of the relief valve, throttling of the directional valve, pipeline bypass loss and power mismatch loss. At present, when energy conservation and emission reduction are the guiding principles, any form of energy loss is a waste.

[0003] However, the throttling loss during the lifting process of the forklift is mainly caused by the lack of power matching between the engine, hydraulic pump and load, resulting in the energy output by the engine and hydraulic pump exceeding the energy required for lifting the load. The engine often operates inefficiently. Therefore, by controlling the rotational speed of the hydraulic pump and reasonably outputting the energy required for lifting by the hydraulic pump, the throttling loss at the inlet and outlet of the directional valve can be effectively reduced. Summary of the Invention

[0004] The purpose of the present invention is to provide a control method and control system for hierarchical utilization of recovered energy in a forklift hydraulic system to solve the problems raised in the above background art.

[0005] To achieve the above purpose, the present invention provides the following technical solutions:

[0006] A control method for hierarchical utilization of recovered energy in a forklift hydraulic system includes an oil supply part and an energy storage part communicated with a lifting cylinder. The oil supply part includes a variable oil pump communicated with the lifting cylinder, and the energy storage part includes at least one accumulator. The control method includes the following steps:

[0007] Step 1, Detect the state of the forklift fork. If the fork is in the lifting state, then execute Step 2;

[0008] Step 2, Supply energy to the lifting cylinder 3 of the fork by releasing the high-pressure oil in different accumulators in the hierarchical energy storage part;

[0009] Step 3, Determine the output flow rate of the variable oil pump according to the lifting speed of the fork;

[0010] Step 4, Determine the lifting required power according to the load of the fork;

[0011] Step 5, Determine the output power of the energy storage part according to the real-time output pressure value of the energy storage part;

[0012] Step 6: Determine the minimum engine output power according to the lifting required power and the accumulator output power;

[0013] Step 7: Control the engine speed and the displacement of the variable oil pump.

[0014] As a further solution of the present invention: The accumulator is connected with a controller, the energy storage part includes a first accumulator and a second accumulator, and the control method for whether all the accumulators in the energy storage part are high-pressure oil in the above steps includes the following steps:

[0015] Step 2.1: The controller collects the oil pressures of the first accumulator, the second accumulator and the oil pressure at the outlet of the variable oil pump;

[0016] Step 2.2: Calculate the differences between the first accumulator and the second accumulator and the oil pressure at the outlet of the variable oil pump;

[0017] Step 2.3: Select the accumulator with the smallest pressure difference and satisfying the pressure difference greater than 0 to supply energy;

[0018] Step 2.4: The selected accumulator continuously supplies energy until the SOC state is 0, and then returns to Step 2.1.

[0019] As a further solution of the present invention: The output flow rate of the variable pump in the above step 3 is:

[0020] Q g =A g ·v g

[0021] In the formula, A g is the cross-sectional area of the rodless cavity of the lifting cylinder, m 2 ; v g is the speed of the lifting cylinder, m / s, where the cross-sectional area A g of the rodless cavity of the lifting cylinder is a fixed value of the system initial value, and the speed v g of the lifting cylinder is a preset value.

[0022] As a further solution of the present invention: The lifting required power in the above step 4 is:

[0023]

[0024] In the formula, F is the force for the lifting cylinder to drive the load, N; Q g is the flow rate for driving the lifting cylinder, ml / min; p g is the pressure of the rodless cavity for driving the lifting cylinder, Mpa.

[0025] As a further solution of the present invention: The output power of the energy storage part in the above step 5 is:

[0026] P a = p a ·Q g

[0027] Wherein, p a is the output pressure of the oil supply accumulator, Mpa.

[0028] As a further solution of the present invention: the engine output power in the step 6 is:[[]]

[0029] P e = P l - P a + P s

[0030] Wherein, P l is the load demand power, kW; P a is the output power of the energy storage unit, kW; P s is the pipeline loss power, kW, wherein, the load demand power P l and the output power P a of the energy storage unit are obtained from the above calculations, and P s is the system estimated value.

[0031] As a further solution of the present invention: the engine speed control includes the following steps:[[]]

[0032] Step 7.1.1, the controller takes the engine target speed n e determined according to the engine universal characteristics and the vehicle speed of the engine as input quantities, and takes the engine speed error e and the error change rate ec as input quantities, Δk p 、Δk i 、Δk d as the output quantities of the fuzzy controller, and then adds this output value to the initial value to obtain new parameters:[[]]

[0033]

[0034] Wherein, k p0 、k i0 、k d0 are the initial values of the PID controller, k p is the proportional gain of the controller, k i is the integral gain of the controller, k d is the derivative gain of the controller;[[]]

[0035] Step 7.1.2, design the language of the engine speed error e, the error change rate ec and the output throttle opening to as fuzzy subsets, [BS MS S M SB MB B], which respectively represent [big, medium, small, medium, small, zero, small, medium, big, big], the fuzzy domains of the engine speed error e and the error change rate ec are [-3, 3], and determine the output Δkp 、Δk i has a fuzzy universe of discourse of [-6, 6], and Δk d has a fuzzy universe of discourse of [-1, 5];

[0036] Step 7.1.3. The system needs to adjust the control system parameters according to the quantization factor, and determine the quantization factor as:

[0037] K e = n / e

[0038] K ec = m / ec

[0039] wherein, K e and K ec are respectively the quantization factors of the error and the error change rate; n and m are the levels of the universe of discourse;

[0040] Step 7.1.4. Determine the fuzzy rules as:

[0041] When the engine speed error e is large, in order to improve the speed response of the system, a large Δk p should be taken; and in order to prevent e from increasing excessively, a small Δk d should be taken; at the same time, in order to avoid response overshoot, take Δk i = 0;

[0042] When e is moderate, in order for the system to have a small overshoot, a smaller Δk p should be taken, and an appropriate Δk i , Δk d should be taken;

[0043] When e is small, in order to maintain the stability of the system and avoid oscillation, a larger Δk p , Δk i should be taken, and at the same time, the value of Δk d should be inversely proportional to |ec|;

[0044] Step 7.1.5. Obtain the fuzzy relation matrix according to the two-input single-output fuzzy controller as:

[0045]

[0046] wherein, E i is the i-th fuzzy state of e; EC j is the j-th fuzzy state of ec; Δkp ij , Δki ij , Δkd ij are the output quantity fuzzy states under the i-th e and the j-th ec fuzzy states;

[0047] Step 7.1.6: Use the maximum membership degree method to take the maximum value in the fuzzy set to defuzzify the output variable, obtain the throttle opening output value, and achieve engine speed control through throttle opening control.

[0048] As a further solution of the present invention: The variable oil pump displacement is determined as follows:

[0049]

[0050] Where n e is the engine target speed;

[0051] The variable oil pump displacement controller is constructed as follows:

[0052] Step 7.2.1: Determine the transfer function of the variable cylinder displacement of the variable oil pump to the solenoid valve opening of the variable oil pump according to the structural characteristics of the swash plate type variable oil pump as follows:

[0053]

[0054] In the formula, is the natural frequency of the hydraulic system; is the damping ratio of the hydraulic system; K q is the flow gain coefficient of the proportional valve, m 3 / s·pa; m t is the mass of the piston and the swash plate of the variable pump, kg; V t is the volume of the variable cylinder, m 3 ; A t is the force area of the variable oil cylinder, m 2 ; K c is the pipeline leakage coefficient, m 3 / s; β e is the volume compression modulus of the hydraulic oil, pa; s is the complex variable; x L (s) is the variable cylinder displacement; x t (s) is the solenoid valve opening.

[0055] Step 7.2.2: Describe the transfer function in step 1 with a mathematical model as follows:

[0056]

[0057] In the formula, y(k) is the output at time k; u(k) is the input at time k.

[0058] Step 7.2.3: Write the non-minimum realization form of the system transfer function according to the Diophantine equation. Since the order of the controlled system is n = 3 and m = 0, the relative order n * = 3, and we can get:

[0059]

[0060] Wherein, Q(s) = s 2 + g1s + g0; D(s) = s 3 + d2s 2 + d1s + d0; R(s) = r1s + r0;

[0061] H(s) = h2s 2 + h1s + h0.

[0062] Step 7.2.4. Simplify the non-minimal realization form of the system as follows:

[0063] y(k) = Θ T (k)ζ(k)

[0064] Wherein, Θ T (k) = (b0 r1 r0 h2 h1 h0) is the parameter to be adjusted; b0, r1, h2, h1, h0 are functions of time k;

[0065] Step 7.2.5. Determine the system output estimation:

[0066]

[0067] Wherein, is the adaptive adjustment parameter at time k - 1; is the system output estimation.

[0068] Step 7.2.6. The adaptive rate can be obtained as:

[0069]

[0070]

[0071] Wherein, Γ is a positive definite symmetric matrix, and e(k) is the error between the output quantity and the target quantity at time k.

[0072] A control system for hierarchical utilization of recovered energy of a forklift hydraulic system, including a lifting cylinder, the piston end of the lifting cylinder is power-connected to a forklift fork, a height sensor and a load sensor are installed on the forklift fork, and the lifting cylinder is connected to an oil supply part and an oil return part through a lifting solenoid valve;

[0073] The oil supply part includes an oil supply pipeline and a variable oil pump arranged on the oil supply pipeline, an overflow valve and an oil return pressure sensor are connected to the oil supply pipeline, and an oil supply valve is arranged at one end of the oil supply pipeline close to the variable oil pump;

[0074] The oil return part includes an oil return pipeline, the end of the oil return pipeline is communicated with the oil inlet of the variable 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;

[0075] The oil inlet of the variable oil pump is connected with a fuel tank, a check valve is arranged between the variable oil pump and the fuel tank, the connection part of the oil return pipeline and the variable oil pump is located between the check valve and the variable oil pump, and an oil supply pressure sensor and a safety valve are connected to the oil supply pipeline;

[0076] The rodless cavity of the lifting cylinder is connected with a lifting solenoid valve, the lifting solenoid valve is communicated with the energy storage part and the variable oil pump, the variable oil pump is connected with a fuel tank, the oil outlet end of the variable oil pump is connected with an oil supply pressure sensor and a safety valve, the variable oil pump is power-connected to the engine, a speed sensor is arranged on the engine, the swash plate of the variable oil pump of the variable oil pump is connected with a variable cylinder, the variable cylinder is connected with a variable oil pump solenoid valve, one end of the variable cylinder is connected with a displacement sensor, and an amplifier is arranged between the position sensor and the variable oil pump solenoid valve.

[0077] As a further scheme of the present invention: the energy storage part includes a first accumulator and a second accumulator, the first accumulator is a high-pressure accumulator, 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 supply pipeline through a first accumulator valve, a second oil pressure sensor is connected to the second accumulator, and the second accumulator is connected to the oil supply pipeline through a second accumulator valve.

[0078] Compared with the prior art, the beneficial effects of the present invention are:

[0079] 1. The present invention proposes a method for recycling and cascading utilization of energy by accumulators, determines the oil supply mode of the accumulator according to the pressure of the rodless cavity and the oil supply pressure of each stage of accumulators, and ensures the maximization of oil supply efficiency; at the same time, a method for power matching between the engine-variable pump-accumulator and the load is proposed, and the engine power output is determined according to the power required by the load and the power supplied by the energy storage part, greatly reducing the redundant power loss of the engine and improving the energy utilization efficiency;

[0080] 2. The present invention proposes a method for determining the flow rate of the variable pump, and according to the requirement of the fork lift speed, the forklift ensures the operation safety on the basis of meeting the working efficiency, and avoids the goods falling and injuring the staff during the working process;

[0081] 3. The present invention proposes a control method for the engine speed and the displacement of the variable pump, which can effectively improve the energy waste of the forklift lifting system, reduce the throttling loss of the valve, realize the adjustment of the engine speed and the displacement of the variable pump, and improve the working efficiency. Description of the Drawings

[0082] Figure 1 This is a schematic diagram of the energy recovery system for this embodiment;

[0083] Figure 2 This is the cascaded utilization process of the accumulator;

[0084] Figure 3 This is the block diagram of the power matching control system;

[0085] Figure 4 This is the structural diagram of the displacement adjustment mechanism of the variable oil pump;

[0086] 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, 12 - safety valve, 13 - fuel tank, 14 - controller, 15 - rotational speed sensor, 16 - engine, 17 - variable oil pump solenoid valve, 18 - variable oil pump swash plate, 19 - variable cylinder, 20 - displacement sensor, 21 - amplifier. Specific embodiments

[0087] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying 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 of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0088] Please refer to Figures 1-4 , a power matching and energy cascaded utilization system for a forklift hydraulic system, including a lifting cylinder 3, and the lifting cylinder 3 is connected to a supply oil part and a return oil part through a lifting solenoid valve 7;

[0089] The supply oil part includes a supply oil pipeline and a variable oil pump 10 arranged on the supply oil pipeline;

[0090] The return oil part includes a return oil pipeline, the end of the return oil pipeline is communicated with the oil inlet of the variable oil pump 10, a energy storage part is connected to the return oil pipeline, the energy storage part includes at least one accumulator, and the accumulator is communicated with the return oil pipeline through an accumulator valve;

[0091] The rodless cavity 31 of the lifting oil cylinder 3 is communicated with the lifting solenoid valve 7. The piston end of the lifting oil cylinder 3 is connected with a forklift fork, and a height sensor 1 and a load sensor 2 are arranged on the forklift fork. The height sensor is used to detect the height of the forklift fork, and the load sensor is used to detect the load signal.

[0092] The inlet of the variable oil pump 10 is connected with an oil tank 13. A one-way valve is arranged between the variable oil pump 10 and the oil tank 13. The connection part of the return oil pipeline and the variable oil pump 10 is located between the one-way valve and the variable oil pump 10. An oil supply pressure sensor 9 and a safety valve 12 are connected to the oil supply pipeline.

[0093] 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 supply 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 supply pipeline through a second accumulator valve 53.

[0094] An overflow valve 6 and a return oil pressure sensor 8 are connected to the oil supply pipeline. An oil supply valve 11 is arranged at one end of the oil supply pipeline close to the variable oil pump 10.

[0095] The variable oil pump 10 is connected to the engine 16 through a transmission shaft. A speed sensor 15 is connected to the transmission shaft. The variable swash plate 18 of the variable oil pump 10 is connected to a variable cylinder 19. The variable cylinder 19 is connected to a variable oil pump solenoid valve 17. One end of the variable cylinder 19 is connected to a displacement sensor 20. An amplifier 21 is arranged between the displacement sensor 20 and the variable oil pump solenoid valve 17.

[0096] A control method for hierarchical utilization of recovered energy of a forklift hydraulic system includes the following steps:

[0097] Step 1: Detect the state of the forklift fork. If the forklift fork is in the lifting state, execute Step 2.

[0098] Step 2: Supply energy to the lifting oil cylinder 3 of the forklift fork by releasing high-pressure oil in different accumulators in the hierarchical energy storage part. During the utilization process, the controller 14 selects the accumulator for energy supply according to the pressure state of the rodless cavity of the lifting oil cylinder and the pressures of each accumulator to supply energy to the inlet of the variable oil pump.

[0099] The control method for high-pressure oil in all accumulators in the hierarchical energy storage part includes the following steps:

[0100] Step 2.1: The controller 14 collects the oil pressures of the first accumulator 41 and the second accumulator 51, as well as the oil pressure at the outlet of the variable oil pump 10. In this embodiment, the controller 14 collects the signals of the first oil pressure sensor 42, the second oil pressure sensor 52, and the oil supply pressure sensor 9;

[0101] Step 2.2: Calculate the differences between the oil pressures of the first accumulator 41 and the second accumulator 51 and the oil pressure at the outlet of the variable oil pump 10;

[0102] Step 2.3: Select the accumulator with the smallest pressure difference and satisfying the condition that the pressure difference is greater than 0 to supply energy;

[0103] Step 2.4: The selected accumulator continuously supplies energy until the SOC state reaches 0, and then returns to Step 2.1

[0104] Step 3: Determine the output flow rate of the variable oil pump according to the fork lifting speed;

[0105] The method for determining the variable pump flow rate is as follows:

[0106] Q g =A g ·v g

[0107] In the formula, A g is the cross-sectional area of the rodless cavity of the lifting cylinder, m 2 ; v g is the speed of the lifting cylinder, m / s. Among them, the cross-sectional area A g of the rodless cavity of the lifting cylinder is a fixed initial value of the system, and the speed v g of the lifting cylinder is a preset value

[0108] Step 4: Determine the lifting required power according to the load of the fork;

[0109] The method for determining the load lifting required power is as follows:

[0110]

[0111] In the formula, F is the force for the lifting cylinder to drive the load, N; Q g is the flow rate for driving the lifting cylinder, ml / min; p g is the pressure of the rodless cavity of the driving lifting cylinder, Mpa. Among them, the load force F is collected by the load sensor (2), and the pressure p g of the rodless cavity of the lifting cylinder is collected by the oil supply pressure sensor (9)

[0112] Step 5: Determine the output power of the energy storage unit according to the real-time output pressure value of the energy storage unit;

[0113] The method for determining the output power of the energy storage unit is as follows:

[0114] Pa = p a ·Q g

[0115] In the formula, p a is the output pressure of the oil supply accumulator, in Mpa. Its value is collected by the oil return pressure sensor (8).

[0116] Step 6: Determine the minimum engine output power according to the lifting demand power and the accumulator output power;

[0117] The method for determining the engine output power is as follows:

[0118] P e = P l - P a + P s

[0119] In the formula, P l is the load demand power, in kW; P a is the output power of the energy storage unit, in kW; P s is the pipeline loss power, in kW. Among them, the load demand power P l and the output power P a of the energy storage unit are obtained from the above calculations, and P s is the system estimated value

[0120] Step 7: Control the engine speed and the displacement of the variable oil pump. That is, the speed sensor 15 collects the real-time engine speed signal and transmits it to the controller 14. An engine speed controller is constructed through adaptive fuzzy PID control, and at the same time, a variable oil pump displacement controller is constructed through model reference adaptive control.

[0121] The construction of the engine speed controller is carried out as follows:

[0122] Step 7.1.1: The controller takes the engine target speed determined according to the engine universal characteristics and the real-time engine speed signal of the speed sensor, uses the engine speed error e and the error change rate ec as input quantities, and Δk p 、Δk i 、Δk d as the output quantities of the fuzzy controller, and then adds this output value to the initial value to obtain new parameters:

[0123]

[0124] In the formula, k p0 、k i0 、k d0 are the initial values of the PID controller, k p is the proportional gain of the controller, k i is the integral gain of the controller, k dis the derivative gain of the controller.

[0125] Step 7.1.2: Design the linguistic fuzzy subsets of the engine speed error e, error change rate ec, and output throttle opening to as [BS MS S M SB MB B], which represent [big, medium, small, medium, small, zero, small, medium, big, big] respectively. The fuzzy domains of the speed error e and error change rate ec are [-3, 3], and determine the output Δk p and Δk i The fuzzy domain of is [-6, 6], and Δk d The fuzzy domain of is [-1, 5]

[0126] Step 7.1.3: The system needs to adjust the control system parameters according to the quantization factor, and determine the quantization factor as:

[0127] K e = n / e

[0128] K ec = m / ec

[0129] In the formula, K e and K ec are the quantization factors of the error and error change rate respectively; n and m are the domain levels.

[0130] Step 7.1.4: Determine the fuzzy rule as

[0131] When the engine speed error e is large, in order to improve the speed response of the system, a large Δk should be taken p ; and in order to prevent e from increasing excessively, a small Δk should be taken d ; at the same time, in order to avoid response overshoot, take Δk i = 0;

[0132] When e is moderate, in order for the system to have a small overshoot, a smaller Δk should be taken p , take an appropriate Δk i and Δk d ;

[0133] When e is small, to maintain the stability of the system and to avoid oscillation, a larger Δk p and Δk i should be taken, and at the same time the value of Δk d should be inversely proportional to |ec|.

[0134] Step 7.1.5: Obtain the fuzzy relation matrix according to the two-input single-output fuzzy controller as:

[0135]

[0136] In the formula, E i is the i-th fuzzy state of e; ECj is the j-th fuzzy state of ec; Δkp ij , Δki ij , Δkd ij is the output quantity fuzzy state at the i-th e and the j-th ec fuzzy state.

[0137] Step 7.1.6: Use the maximum membership degree method to take the maximum value in the fuzzy set to defuzzify the output variable, and obtain the throttle opening output value.

[0138] The variable oil pump displacement is determined as follows:

[0139]

[0140] n e is the engine target speed;

[0141] The variable pump displacement controller is constructed as follows:

[0142] Step 7.2.1: According to the structural characteristics of the swash plate type variable oil pump, determine the transfer function of the displacement of the variable cylinder 19 of the variable oil pump to the opening of the variable oil pump solenoid valve 17 as follows:

[0143]

[0144] In the formula, is the natural frequency of the hydraulic system; is the damping ratio of the hydraulic system; K q is the proportional valve flow gain coefficient, m 3 / s·pa; m t is the mass of the piston and the variable pump swash plate, kg; V t is the variable cylinder volume, m 3 ; A t is the force area of the variable oil cylinder, m 2 ; K c is the pipeline leakage coefficient, m 3 / s; β e is the bulk modulus of compression of the hydraulic oil, pa; s is the complex variable; x L (s) is the variable cylinder displacement; x t (s) is the opening of the variable oil pump solenoid valve.

[0145] Step 7.2.2: Describe the transfer function in step 1 with a mathematical model as follows:

[0146]

[0147] In the formula, y(k) is the output quantity at time k; u(k) is the input quantity at time k.

[0148] Step 7.2.3. Write the non - minimum realization form of the system transfer function according to the Diophantine equation. Since the order of the controlled system \(n = 3\) and \(m = 0\), the relative order \(n\) * = 3, and we can get:

[0149]

[0150] In the formula, \(Q(s)=s\) 2 +g1s + g0; \(D(s)=s\) 3 +d2s 2 +d1s + d0; \(R(s)=r1s + r0\);

[0151] \(H(s)=h2s\) 2 +h1s + h0.

[0152] Step 7.2.4. Simplify the non - minimum realization form of the system as follows:

[0153] y(k)=Θ T (k)ζ(k)

[0154] In the formula, Θ T (k)=(b0r1 r0 h2 h1 h0) is the parameter to be adjusted; b0, r1, h2, h1, h0 are functions of time k;

[0155] Step 7.2.5. Determine the system output estimation:

[0156]

[0157] In the formula, is the adaptive adjustment parameter at time k - 1; is the system output estimation.

[0158] Step 7.2.6. The adaptive rate can be obtained as:

[0159]

[0160]

[0161] In the formula, Γ is a positive definite symmetric matrix, and e(k) is the error between the output at time k and the target value.

[0162] Through the above control, the error between the output and the target value can be obtained. Furthermore, according to the requirements of the actual target value, the output of the variable pump can be controlled, and then the variable pump can be controlled to reduce the loss of the variable pump.

[0163] Embodiment

[0164] Taking a common 3-ton forklift as an example;

[0165] Apply the energy cascade utilization system of the accumulator in this embodiment to a common 3-ton forklift. The measured parameters of the forklift are 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, and the cross-sectional area of the rodless cavity of the lifting cylinder is 0.0024m 2 , the transmission method is hydraulic transmission. The parameters of the first accumulator selected for cascade recovery are 8L and 16 Mpa, and the parameters of the second accumulator are 10L and 8 Mpa.

[0166] Taking the load of 2.5t on the forklift forks as an example, when the forklift forks are lifted, it is measured that the pressure in the rodless cavity of the lifting cylinder is 24 Mpa, and the pressures of the high-pressure and low-pressure bladder accumulators are 20 Mpa and 15 Mpa respectively. According to the high-pressure and low-pressure sequence, the high-pressure accumulator can be first selected for hydraulic energy recovery. The required speed of the lifting cylinder is 0.25 m / s, and the output flow of the variable pump is determined to be 60 L / min. Under the pressure supply of the accumulator, the initial pressure difference between the inlet and outlet of the variable pump is 4 Mpa, the load demand power is 25 kW, the output power of the accumulator is 20 kW, and the power loss is 1 kW. At this time, the minimum output power of the engine is 6 kW, reducing the power output by 20 kW.

[0167] During the ascending process, the output pressure of the high-pressure accumulator continuously decreases, and the pressure difference between the inlet and outlet of the variable oil pump continuously increases until the pressure of the high-pressure accumulator drops to 16 Mpa, then switch to the low-pressure accumulator for energy supply. When the pressure of the low-pressure accumulator drops to 8 Mpa, close the oil supply valve, and the variable oil pump directly sucks oil from the fuel tank.

[0168] 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-limiting. 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 included in the present invention. Any reference signs in the claims should not be regarded as limiting the claimed rights.

[0169] In addition, it should be understood that although this specification is described according to 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 control method for hierarchical utilization of recovered energy in a forklift hydraulic system, characterized in that, It includes an oil supply section and an energy storage section that are connected to the lifting cylinder (3). The oil supply section includes a variable oil pump (10) that is connected to the lifting cylinder (3). The energy storage section includes at least one accumulator, and the accumulator is connected to a controller (14). The energy storage section includes a first accumulator (41) and a second accumulator (51). The control method includes the following steps: Step 1: Detect the state of the forklift forks. If the forks are in the lifting state, then execute Step 2; Step 2: Supply energy to the lifting cylinder (3) of the forks by releasing the high-pressure oil in different accumulators in the hierarchical energy storage section; The control method for the hierarchical release of high-pressure oil in all accumulators in the energy storage section in Step 2 includes the following steps: Step 2.1: The controller (14) collects the oil pressures of the first accumulator (41) and the second accumulator (51) and the oil pressure at the outlet of the variable oil pump (10); Step 2.2: Calculate the difference between the oil pressures of the first accumulator (41) and the second accumulator (51) and the oil pressure at the outlet of the variable oil pump (10); Step 2.3: Select the accumulator with the smallest pressure difference and satisfying the pressure difference greater than 0 to supply energy; Step 2.

4. The selected accumulator continuously supplies energy until SOC the state is 0, and then return to Step 2.1 Step 3: Determine the output flow rate of the variable oil pump according to the lifting speed of the forks; Step 4: Determine the lifting required power according to the load of the forks; Step 5: Determine the output power of the energy storage section according to the real-time output pressure value of the energy storage section; Step 6: Determine the minimum output power of the engine according to the lifting required power and the output power of the accumulator; Step 7: Control the engine speed and the displacement of the variable oil pump.

2. The control method for hierarchical utilization of recovered energy in a forklift hydraulic system according to claim 1, characterized in that, The output flow rate of the variable oil pump in Step 3 is: In the formula, is the cross-sectional area of the rodless cavity of the hoisting cylinder, ; is the speed of the hoisting cylinder, , where the cross-sectional area of the rodless cavity of the hoisting cylinder is the initial fixed value of the system, and the speed of the hoisting cylinder is the preset value.

3. A control method for cascaded utilization of recovered energy in a forklift hydraulic system according to claim 2, characterized in that, The lifting required power in Step 4 is: Wherein, is the force for the hoisting cylinder to drive the load, ; is the flow rate for driving the hoisting cylinder, ; is the pressure in the rodless cavity for driving the hoisting cylinder, .

4. A control method for hierarchical utilization of recovered energy in a forklift hydraulic system according to claim 3, characterized in that, The output power of the energy storage section in Step 5 is: In the formula, is the output pressure of the oil supply accumulator, .

5. A control method for hierarchical utilization of recovered energy in a forklift hydraulic system according to claim 4, characterized in that, The output power of the engine in Step 6 is: Wherein, is the load demand power, ; is the output power of the energy storage unit, ; is the pipeline loss power, , wherein, the load demand power and the output power of the energy storage unit are obtained from the above calculations, is the system estimated value.

6. A control method for hierarchical utilization of recovered energy in a forklift hydraulic system according to claim 1, characterized in that, The engine speed control includes the following steps: Step 7.1.1, the target engine speed determined by the controller according to the engine universal characteristic , the speed of the engine at the current speed, and the engine speed error e and the rate of change of the error ec are used as input quantities, , , are used as the output quantities of the fuzzy controller, and then this output value is added to the initial value to obtain a new parameter: In the formula, , , are the initial values of the PID controller, is the proportional gain of the controller, is the integral gain of the controller, is the derivative gain of the controller; Step 7.1.2, design the engine speed error e , the error change rate ec and the output throttle opening to are fuzzy subsets of the language, , which respectively represent [large, medium-large, medium, small, small-large, medium-small, small], the speed error e and the error change rate ec have a fuzzy universe of discourse of , determine the output , has a fuzzy universe of discourse of , has a fuzzy universe of discourse of ; Step 7.1.3: The system needs to adjust the control system parameters according to the quantization factor, and determine the quantization factor as: In the formula, and are the quantization factors of the error and the error change rate respectively; n, m is the universe of discourse level; Step 7.1.4: Determine the fuzzy rules as: When the engine speed error e is large, in order to improve the speed response of the system, a large should be taken; and in order to prevent e from increasing excessively, a small should be taken; at the same time, in order to avoid response overshoot, take = 0; When e is moderate, to make the system have a small overshoot, a smaller should be taken, and appropriate , ; When e is small, to maintain the stability of the system and to avoid oscillations, a larger , should be taken. At the same time, the value of should be inversely proportional to ; Step 7.1.5: According to the two-input single-output fuzzy controller, obtain the fuzzy relation matrix as: In the formula, is e the i nth fuzzy state; is ec the j mth fuzzy state; , , are the i kth e , the j lth ec output quantity fuzzy states under the fuzzy state; Step 7.1.6: Use the maximum membership degree method to take the maximum value in the fuzzy set to defuzzify the output variable, obtain the throttle opening output value, and realize the engine speed control through the throttle opening control.

7. A control method for hierarchical utilization of recovered energy in a forklift hydraulic system according to claim 2, characterized in that The method for determining the displacement of the variable oil pump is as follows: Among them, is the engine target speed; The construction of the variable oil pump displacement controller is carried out as follows: Step 7.2.1: According to the structural characteristics of the swash plate type variable oil pump, determine the transfer function of the displacement of the variable cylinder (19) of the variable oil pump to the opening of the variable oil pump solenoid valve (17) as follows: In the formula, is the natural frequency of the hydraulic system; is the damping ratio of the hydraulic system; is the flow gain coefficient of the proportional valve, ; m is the mass of the piston and the swash plate of the variable pump, ; is the volume of the variable cylinder, ; is the force area of the variable oil cylinder, ; is the pipeline leakage coefficient, ; is the bulk modulus of compression of the hydraulic oil, ; is a complex variable; is the displacement of the variable cylinder; is the opening of the solenoid valve; Step 7.2.2: Describe the transfer function in Step 7.2.1 with a mathematical model as follows: In the formula, is the output at time is the input at time Step 7.2.

3. Write the non-minimal realization form of the system transfer function according to the Diophantine equation. Since the order of the controlled system , , the relative order , and we can obtain: Wherein, ; ; ; ； Step 7.2.4: Simplify the non-minimum realization form of the system as follows: In the formula, ; is the parameter to be adjusted; , , , , are functions of time ; ; Step 7.2.5: Determine the system output estimation: In the formula, is the adaptive adjustment parameter at the moment; is the system output estimate; Step 7.2.6: The adaptive rate can be obtained as: In the formula, is a positive definite symmetric matrix, is the error between the output quantity and the target quantity at the moment.

8. A control system for a method of recycling and cascaded utilization of energy in a forklift hydraulic system according to any one of claims 1-7, characterized in that, It includes a lifting cylinder (3). The piston end of the lifting cylinder (3) is power-connected to the forks. A height sensor (1) and a load sensor (2) are installed on the forks. The lifting cylinder (3) is connected to an oil supply section and an oil return section through a lifting solenoid valve (7); The fuel supply section includes a fuel supply pipeline and a variable oil pump (10) provided on the fuel supply pipeline. An overflow valve (6) and an oil return pressure sensor (8) are connected to the fuel supply pipeline. A fuel supply valve (11) is provided at one end of the fuel supply pipeline close to the variable oil pump (10). The oil return section includes an oil return pipeline. The end of the oil return pipeline is communicated with the oil inlet of the variable oil pump (10). An energy storage section is connected to the oil return pipeline. The energy storage section includes at least one accumulator. The accumulator is communicated with the oil return pipeline through an accumulator valve. The oil inlet of the variable oil pump (10) is connected to a fuel tank (13). A one-way valve is provided between the variable oil pump (10) and the fuel tank (13). The connection between the oil return pipeline and the variable oil pump (10) is located between the one-way valve and the variable oil pump (10). A supply oil pressure sensor (9) and a safety valve (12) are connected to the fuel supply pipeline. The rodless cavity (31) of the lifting cylinder (3) is connected to a lifting solenoid valve (7). The lifting solenoid valve (7) is communicated with the energy storage section and the variable oil pump (10). The variable oil pump (10) is connected to a fuel tank (13). The oil outlet end of the variable oil pump (10) is connected to a supply oil pressure sensor (9) and a safety valve (12). The variable oil pump (10) is power-connected to an engine (16). A speed sensor (15) is provided on the engine (16). The variable oil pump swash plate (18) of the variable oil pump (10) is connected to a variable cylinder (19). The variable cylinder (19) is connected to a variable oil pump solenoid valve (17). One end of the variable cylinder (19) is connected to a displacement sensor (20). An amplifier (21) is provided between the displacement sensor (20) and the variable oil pump solenoid valve (17).

9. The control system for hierarchical utilization of recovered energy of a forklift hydraulic system according to claim 8, characterized in that, The energy storage section 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 fuel supply 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 fuel supply pipeline through a second accumulator valve (53).

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