| The pump control systems instead of valve control systems and the recovery and reuse of boom potential energy are effective methods and approaches for improving efficiency and energy conservation in excavator hydraulic systems.Digital pumps are the best choice for hydraulic power components in future hydraulic pump control systems,because they have the feature of independent control for each plunger chamber,which solve the problem of rapid efficiency decline of traditional v ariable displacement pumps when working with small displacement.However,the current digital pump can only work under one operating condition of pump or motor.When they are used for boom pump control and potential energy recovery,the system structure is complex,there are many energy conversion processes,and the cost is high.Therefore,this article proposes a digital pump with direct energy recovery.By adding a dedicated energy recovery valve and energy recovery port,the high-pressure oil that needs to be recovered is introduced into the plunger chamber during the suction stroke.When the digital pump operates,some of the plunger chambers are in pump mode and some are in motor mode,and the torque is output by the p lunger working in motor condition,which drives the digital pump to work together with the prime mover,which achieves direct energy recovery and utilization.When it is applied to pump control and potential energy recovery of excavator arm,the system structure is simplified and the efficiency of excavator hydraulic system is improved.The main work of the paper is as follows:(1)Research on the energy direct recovery flow distribution and variable displacement control strategies of digital pump.A digital flow distribution method for direct energy recovery of the plunger chamber is proposed.A two-position three-way valve is used as the high-pressure digital flow distribution valve(energy recovery valve).By coding and controlling it and the low-pressure flow distribution valve,direct recycling of external high-pressure oil is realized during the oil suction stroke of the plunger chamber.The flow distribution method is applied to a five-piston radial piston pump,and the configuration of digital pump with direct energy recovery is obtained.The mathematical model of single plunger chamber with digital flow distribution is established,and simulation calculations are performed to obtain the changing rules of plunger displacement and speed with the angle of the pump shaft,as well as the theoretical output flow curves of single plunger chamber with digital variable displacement are acquired.Two control strategies,"full plunger variable effective stroke control" and "plunger enable and variable effective stroke collaborative control",are used to conduct simulation analysis of variable displacement control for digital pump.The theoretical output flow pulsation of digital pumps at 20% ～ 100% displacement are compared.It is concluded that the output flow stability is better under the strategy of "full plunger variable effective stroke control".(2)Study on digital valve modeling and dynamic respo nse characteristics.The structure and working principle of the digital valve are analyzed,and the mathematical models of the electromagnet,the mech anical movement of the valve core and the fluid power of the valve port are established.With the fastest dynamic response speed in the opening and closing process of the flow distribution valve,The AMESim simulation model of digital valve is built,analyze the influence laws of different pre-loaded voltage duty cycle ratio and holding voltage duty cycle ratio on valve response speed when a hybrid voltage control strategy is adopted to control valve,and the best values of the two voltage duty cycle ratios are obtained.(3)Research on adaptive flow distribution control and working characteristics of direct energy recovery digital pump.Aiming at the problem that the originally set opening lag time and closing advance time of the flow distribution valve can not be adaptively matched when the motor speed fluctuates and the valve dynamic characteristics change,an adaptive flow distribution control strategy for a digital pump is proposed.The proposed strategy is based on the influence of mismatched flow distribution parameters on the pressure change of the plunger chamber,which uses the detected pump shaft rotation angle and the inlet and outlet pressure difference signals of the low-pressure flow distribution valve to correct the opening lag time and closing advance time of the flow distribution parameters.The adaptive flow control is realized in response to motor speed fluctuations and valve dynamics change.The AMESim simulation model of adaptive flow distribution control and working characteristics of direct energy recovery digital pump are established,and the effectiveness and feasibility of the proposed adaptive flow distribution control strategy and the pump mode,motor mode and direct energy recovery mode of the new digital pump are verified.(4)Investigations on the application of energy direct recovery digital pump in the drive system of excavator boom.The direct energy recovery type digital pump is applied to the boom drive system,and the working principle of its hydraulic system is designed.The working principle and energy conversion path of the new boom drive system under three operating modes of pure electric drive,energy recovery and electro-hydraulic combined drive are analyzed.The AMESim simulation model of the boom digital hydraulic drive system is established.Under no-load and full-load conditions,the energy recovery and electro-hydraulic combined drive working modes of the new drive system are simulated and analyzed.The simulation results show that the new boom digital hydraulic drive system works in the energy recovery mode when the boom is down,and hydraulic energy converted from the gravitational potential energy of the boom is realized and stored in the hydraulic accumulator;When the boom rises,the system works in electro-hydraulic compound drive mode.The hydraulic energy stored in the accumulator and the electric energy input by the motor jointly provide energy for the boom to rise,and realize the direct reuse of the stored energy in the accumulator.(5)Experimental study on working characteristics of digital pump with direct energy recovery.The five-plunger radial piston pump is reformed,the oil discharge check valve of the original plunger chamber is cancelled,and the oil outlet of each plunger chamber is drawn out separately.The experimental platform for the working characteristics of the digital pump is built,which is mainly composed of three parts:The main hydraulic experimental circuit of the digital pump is to connect the single plunger pair of the new digital pump to the hydraulic circuit to test its various performance;Energy recovery auxiliary experimental circuit is mainly to provide high pressure oil for energy recovery;The real-time simulation measurement and control system mainly measures and controls the p iston chamber pressure,relief valve working pressure,recovered hydraulic energy pressure,pump shaft rotation Angle,digital pump input torque and other parameters in real time.Baseed on the experimental platform,the direct energy recovery and utilizat ion mode of the digital pump is experimentally tested,and the change curves of plunger chamber pressure and pump shaft torque before and after energy recovery are obtained.The feasibility of the direct energy recovery digital flow distribution method was preliminarily verified. |
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