| The transmission system of loader mainly includes hydrodynamic transmission,mechanical transmission and hydrostatic transmission.Although hydrodynamic transmission has good adaptability,its high efficiency zone is narrow.Although the mechanical transmission has simple structure and high transmission efficiency,the shift operation is complicated,and there will be power flow interruption in the process of shifting.This has a negative impact on the productivity of construction machinery with frequent shift and reversing.Hydrostatic transmission employs the way of volume speed regulation,the liquid from the hydraulic pump is input into the hydraulic motor,and there is no overflow loss.It has higher efficiency,more flexible structure layout and relatively simple operation than hydrodynamic transmission.In this paper,a single pump-double motor hydrostatic transmission system scheme was determined for a 7-ton loader.A speed control strategy was developed based on the hydrostatic transmission system,and the whole machine system simulation based on this speed control strategy was carried out on the AMESim software platform.The correctness of the simulation and control strategy of the hydrostatic transmission system was verified by experiments.The main research contents and conclusions of this paper are as follows.(1)The single pump-double motor hydrostatic transmission scheme of a 7-ton loader was determined.The engine selection was realized by calculating the driving resistance and shoveling resistance of the loader.The external characteristic and universal characteristic curve of QSL9.3 engine were analyzed,and the mathematical model of engine was built on this basis.Then,the engine speed characteristic equation was fitted,and the dynamic speed curve and economic speed curve of the engine were obtained.The working characteristics of the single pump-double motor equal displacement loop system and the single pump-double motor unequal displacement loop system were compared,and the speed characteristics and torque characteristics of the pump-controlled double motor closed hydraulic circuit were analyzed.Working pressure of the hydrostatic transmission system was determined,and the main components of the hydrostatic transmission system were calculated and selected.On the AMESim software platform,the overall simulation model was built for the hydrostatic transmission system circuit of a 7-ton loader.(2)A speed regulation control strategy was proposed for V-type operation of loader.The speed control characteristics of the hydrostatic transmission system of the loader were analyzed.Combined with the characteristics of the V-type working condition of the loader,the whole operation was divided into no-load forward stage,shovel excavation stage,full load reversing stage,full load forward stage,and no-load reversing stage.A speed control strategy for the V-type working condition of the loader was formulated.The relationship between the speed of the loader and the control signal of variable pump displacement and variable motor displacement was analyzed.The control algorithm was compiled by AMESim-Statechart tool to realize the control of the loader under V-type working condition.(3)The overall model of hydrostatic transmission system built on AMESim platform was used to analyze the V-type working conditions of loader.Under V-type operating conditions,the speed control strategy of single pump-double motor hydrostatic transmission system was employed,and the whole model was simulated.The changes of system parameters under this condition were obtained,including vehicle speed,engine speed,traction,system flow rate,pump displacement ratio,motor displacement ratio and system pressure,etc.(4)The cycle test of the loader based on the speed regulation control strategy was carried out under V-type operating condition.The control strategy and algorithm were written into the controller,and the V-type operating condition of the hydrostatic loader was tested,and the wheel speed,engine speed,system flow rate,pump displacement ratio,motor displacement ratio and system pressure of the loader were collected.The simulation results of various parameters under the V-type operating condition of the loader were compared with the experimental data of the whole machine.It was found that the simulation results of the engine speed,the vehicle speed,the pressure and flow rate of the hydraulic system,the displacement ratio of the pump and motor and the traction force are basically consistent with the experimental results,which proved the rationality of the speed control strategy. |
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