| As an important engineering construction equipment,hydraulic excavators are widely used in national defense,energy resource exploitation,mineral exploitation and other major infrastructure field.By the end of 2017,there are more than 1.7 million units of hydraulic excavators in China,and the annual output in 2018 has reached 200,000.Thus,it is one of the most important pillar industries in China.In the excavator system,a displacement variable pump driven by an internal-combustion engine is normally applied as power source to supply high-pressure oil,and the four-sides spool valves are used to distribute the flow and to control the actuators.However,in this scheme,the very large throttling loss of control valve,the long-term operation of power source in low efficiency area and the dissipation of large capacity potential and kinetic energy are the root causes of the large installed power,energy dissipation and serious heat of the whole machine.These three key links are also the international problems that restrict the high energy efficiency operation of engineering equipment for a long time.It is reported that the throttling loss and the waste potential and kinetic energy account for 35%~39% and 20%~25% of the output energy of the hydraulic pump respectively,resulting in the energy efficiency of the whole excavator only about 40%.If the engine efficiency(up to 40%)and the hydraulic pump efficiency(80%)are considered,the overall efficiency of the hydraulic excavator will be lower.Take a 20 t hydraulic excavator as an example,during the operation,the average output of the diesel engine is about 80 kW,2400 hours per year,0.3 L of diesel fuel consumption per kWh,and the fuel consumption caused by throttling loss and wasted potential energy is as high as 34560 L,and the carbon dioxide emitted is as much as 93600 kg.It can be seen that the reducing of throttling loss and the reusing of potential energy have great significance for energy saving and emission reduction of hydraulic excavators.Focusing on the goal of green,low-carbon and efficient operation of hydraulic excavation equipment,this dissertation carries out research on energy-saving technology of hydraulic excavator under the support key project of NSFC-Shanxi Low-carbon Based Coal Joint Fund(U1510206)and the project of National Natural Science Foundation of China(51575374).Considering the development trend of electric engineering equipment and all aspects of energy transfer,this paper tries to explore the real-time and efficient matching mechanism of electro-hydraulic power source and load,high-efficiency conversion and utilization large mount of potential and kinetic energy,energy efficient conversion mechanism of multi-load system,and thus to realize high-speed,stable and low-cost operation of hydraulic excavation equipment under low energy loss.The topic has important theoretical and academic value.Based on the above objectives,a new distributed scheme combined speed variable and displacement variable open volume control and asymmetric pump controlled asymmetric cylinder closed volume control is proposed.The decoupling of compound action of hydraulic excavator is realized from structure,so that each actuator can be controlled according to volume control mode,and the throttling loass can be eliminated,especially the throttling loss caused by load difference of each actuator.In the innovative closed volume drive circuit of asymmetric pump,high-pressure accumulator is introduced,and based on the same hydraulic pump,the large potential and kinetic energy can be quickly and efficiently converted and reused.And the whole machine is driven and controlled by high-efficiency speed variable electro-hydraulic power source to completely eliminate pollution emissions.The designed scheme only needs three pumps to reduce the loss of energy transmission in each link of hydraulic excavator.The open volume drive circuit can also be used to drive the whole excavator,which has strong redundancy.It is the best scheme for small and medium-sized electric-driven hydraulic excavators at present.The proposed hydraulic excavator scheme is driven by speed variable electro-hydraulic power source with common DC bus,which is the basic control unit of the whole machine.How to achieve the real-time dynamic matching of the electric motor,hydraulic pump and load power by coordinating the variables such as speed,displacement,torque and pressure under the premise of low cost,for obtaining the best output performance of electro-hydraulic power source is the main difficulty.In order to improve the dynamic response performance of the electro-hydraulic power source,an innovative configuration of pressure-flow compound power source based on inverter motor and constant pressure variable pump was proposed,and a hydraulic accumulator was set at the suction port of the pump for assisting the power source to start.A flow control strategy based on piecewise speed control and continuous displacement control was designed to realize high dynamic response and energy efficiency operation of the power source.Furthermore,the configuration of speed variable and fixed displacement pump based on direct torque control is proposed.The pressure control of electro-hydraulic power source is realized by direct torque control of the electric motor.When the load pressure changes,there is no need to control the motor speed,which has the advantages of faster dynamic response and simple control structure than the conventional constant pressure pump.The research shows that the energy efficiency of the power source in the whole working range is improved by using these two innovative configurations.The average efficiency of the power source can reach more than 50%.The driving and control of the power source which coordinates the energy efficiency and dynamic performance are realized.The innovative solution can be used for the performance optimization of the electro-mechanical-hydraulic hybrid transmission system.The speed variable and displacement variable electro-hydraulic power source designed in the above is applied to the open volume control system of the whole machine scheme,and the principle of pressure-flow matching independent metering control loop based on electro-hydraulic power source and control valve is designed.By matching the output flow and pressure of the electro-hydraulic power source with the demand of the actuator,the system throttling loss are greatly reduced.And the dynamic response and energy efficiency of the electro-hydraulic power source under large-scale and fast-changing load conditions are significantly improved.In addition,according to the movement direction of the actuator,the flow of the inlet and outlet of the actuator is controlled to realize the flow matching of the system and to ensure the smooth movement of the actuator in all directions.The results show,that compared with the load sensing system driving by a fuel engine,the operation cost can be reduced by more than 50% in one typical excavation cycle.In the proposed scheme,the boom and arm cylinder are controlled by an innovative asymmetric pump.The asymmetric pump has three oil stream windows,two of which are connected with the rodless chamber and rod chamber of the hydraulic cylinder.The displacement ratio of the two stream windows is consistent with the area ratio of the two chambers of the hydraulic cylinder,which solves the problem of complex flow compensation system and poor control characteristics of the conventional closed pump-controlled asymmetric hydraulic cylinder system.The third stream window of the asymmetric pump is connected with a high-pressure accumulator,which is used to recover and to reuse the gravitational potential energy of the working device,thus realizing the efficient recovery and utilization of the gravitational potential energy of the working device and greatly reducing the installed power of the driving system.The results show that the potential energy recovery rate of the working device can reach 83%,and the energy-saving ratio of the boom and arm system can reach more than 70%,which is the highest energy efficiency electro-hydraulic control system so far.The basic structure of this dissertation is as follows:In the first chapter,the energy loss patterns of each link of energy flow of hydraulic excavator are analyzed.Then the current research status of power source optimization,throttling loss reduction,and potential and kinetic energy recovery of hydraulic excavator are summarized,and the existing problems are summarized.Finally,the corresponding solutions are put forward,and the main research contents of the subject are given.In chapter 2,on the basis of literature review,the structure and power supply type of electric hydraulic excavator are introduced.Combining with the team's innovative speed variable asymmetric pump controlled asymmetric hydraulic cylinder technology,the system scheme structure of speed variable volume controlled hydraulic excavator are discussed,and the overall scheme and research content framework of hydraulic excavator are further determined.In the third chapter,the principle of anti-flow saturated load sensing system commonly used in hydraulic excavators is introduced,and the operation characteristics and energy consumption of the whole hydraulic excavator under various working conditions are experimentally studied.On this basis,a fuel engine model based on universal characteristic curve is constructed,and the test data are imported into the engine model.The fuel consumption characteristics of hydraulic excavator under various working conditions are obtained,which can provide reference and comparison for the follow-up work.In chapter 4,the control principle and dynamic characteristics of converter motor and servo motor,displacement variable hydraulic pump and constant pressure pump are studied firstly with theoretical analysis and experimental data.On this basis,the effects of rotational speed,torque,flow rate and pressure on the dynamic performance of electro-hydraulic power source are studied through experiments.The principle of electro-hydraulic power source driving by converter motor are designed and a hydraulic accumulator is used to assist starting the power source.And the principle of constant pressure electro-hydraulic power source driving by servo motor based on direct torque control are designed.Finally,the energy efficiency of converter motor,servo motor,hydraulic pump and speed variable displacement variable electro-hydraulic power source under various load conditions is studied,and the validity of the relevant conclusions is verified by experiments.In the fifth chapter,on the basis of the previous research work,a high dynamic and energy-efficient control strategy of electro-hydraulic power source based on piecewise speed variable and continuous displacement is designed to drive the hydraulic excavator with independent metering control principle.On this basis,the compound flow matching control strategy of pump and valve is designed.Finally,the feasibility of the designed control strategy is studied through experiments.In chapter 6,the principle of asymmetric pump-controlled asymmetric hydraulic cylinder is introduced firstly,and the working principle of asymmetric axial piston pump is expounded.Secondly,the operation characteristics of asymmetric pump controlled boom cylinder are studied.The influence of hydraulic accumulator on the recovery rate of gravitational potential energy of the working device is analyzed theoretically.A test system of asymmetric pump controlled boom cylinder is constructed.The feasibility of the integrated efficient driving and recovery and utilization of potential energy is verified.Finally,the working characteristics of asymmetric pump controlled asymmetric hydraulic cylinder which under four-quadrant are simulated,and the feasibility and energy consumption characteristics of asymmetric pump controlled four-quadrant hydraulic cylinder are studied experimentally.In chapter 7,the main research structure of this dissertation,the innovative points of the research,and looks forward to the future research work are summarized. |
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