| Hydrostatic transmission has the advantages of silence,high power density and simple control.It is more and more widely used in the mobile machinery.At present,the hydrostatic transmission system of forklift basically adopts the closed loop of variable displacement pump driven by the internal combustion engine,but the transmission system has the disadvantages of narrow efficient zone,polluting the environment,poor heat dissipation and high cost.In order to solve the above problems and improve the efficiency of forklift system,this paper takes 3.5t paper roll forklift as the research object.A set of open hydrostatic drive system using AC motor to drive gear pump is designed by combining frequency conversion drive technology and electro-hydraulic proportional technology,and the flow matching of the hydraulic system is studied.The main research contents are as follows:1)Taking a 3.5t paper roll forklift as an object,the original motor is changed from internal combustion engine to three-phase AC asynchronous motor,and the motor and synchronous belt are designed.Then the installation and debugging of the electric forklift are completed,and the battery endurance,stability and noise are tested.The test results show that the AC variable frequency drive scheme can meet the requirements of normal operation of forklift.2)On the basis of AC variable frequency drive scheme,a set of hydraulic system of electric hydrostatic forklift is designed.The hydraulic system adopts single motor and single pump open loop and low speed motor scheme,which can realize the function of forklift to move,steer and load and unload cargo,and the braking problem in open hydrostatic transmission is solved.The main components of the hydraulic system are selected.Then the hydraulic valve block is designed to integrate the hydrostatic transmission system.Finally,the layout and key structure of forklift are designed,and the system of forklift is integrated.3)The control strategy of the hydrostatic transmission system is put forward to realize the walking speed control of forklift.By controlling motor,solenoid valve and proportional valve,the functions of micromotion,continuously variable speed,automatic shift and smooth braking are realized.The model of hydraulic system and control system is built by AMESim and simulated.The simulation results show that the hydrostatic transmission system can meet the technical requirements of forklift.The performances of the system are as follows: the maximum speed of forklift running on flat ground with full load is 15.89 km/h,and the maximum speed of forklift running on large slope with full load is 7.88 km/h.It can realize micro-motion driving with speed as low as 0.085 m/s,and the maximum climbing gradient is 15% at full load.When the power is insufficient,the motor displacement can be automatically switched.The braking distance can be effectively controlled by the brake-pedal travel and the system has the function of emergency braking.4)A flow matching method is proposed for the single pump multi-actuator system of hydrostatic transmission forklift,which includes flow distribution control method and overflow control method to realize pressure matching and flow matching of the system.The model of flow control system is built by AMESim and the effect of flow control is analyzed by simulation.The simulation results show that the method of flow distribution control distributes the flow to the walking system and the gantry system on demand and eliminates the throttle overflow.Moreover,the constant pressure system is changed into a variable pressure system and the throttling loss is reduced.The overflow control method realizes that the motor speed can be adjusted according to the overflow.Except for the overflow caused by idle speed limitation of the motor,almost no other overflow occurs,and the flow matching of the system is realized. |
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