Study On Control Strategy Of Tractor Hydro-mechanical Continuously Variable Transmission

Hydro-mechanical Continuously Variable Transmission(HMCVT)typically includes power split and power confluent mechanism.Engine power transmitted to the mechanical part and the hydraulic circuit.The mechanical part transfers certain power,and the motor output speeds continuously adjustable by changing pump displacement.With the help of power confluent mechanism,two power flow combined and output,then drive the vehicle to travel and the load to rotate.A continuously variable output speed can be obtained with a certain input speed.Our university self-developed HMCVT by adding the clutch to the output side,expand the gear ratio range,so that it can satisfy the need of high-power tractors.For the application of HMCVT loaded on tractor,it needs to design an appropriate control strategy to ensure automatically adjust the transmission ratio according to the tractor requirements.To complete HMCVT control strategy design,this paper carried out the following main tasks:1.According to the principle of HMCVT structure,the simulation model is established.It includes gear transmission simulation model,the pump motor closed system simulation model and the wet clutch simulation model.Meanwhile,in order to facilitate control strategy simulation,it defined the interface module(control rules),created a pump adjustment rule and clutch switching rules.The module input value is named Ratio;the output is control signal of pump and clutch.There is a correspondence between input and output.The characteristic of HMCVT was simulated by using HMCVT simulation model.By analyzing the oil pressure and speed changes of the hydraulic pump motor during transmission ratio adjusting process,it proved the existence of power cycle.And this will lead to abrupt speed variation of motor;HMCVT output speed decreases.So we need to optimize the shifting rule to avoid this kind discontinuity.2.Design the control strategy which based on the fuel-economy curve,its core is to determine the economic curve and design the fuzzy controller.The input to the controller is deviation and its change rate.The deviation is the difference between fuel economy speed and actual speed.The output is the Ratio which accorded with the requirement of the interface modules.In addition,the fuzzy controller was established and carried on the simulation.From the results,the control strategy can adjust the speed ratio of HMCVT,and ensures that engine can work on fuel economy speed.3.To improve fuel-economic control strategy,the economic control strategy based on the driver's intention was present.The control strategy can recognize the driving intention by using the fuzzy controller,and calculate target HMCVT ratio by using the PID controller.Through the co-simulation,the results verified feasibility and correctness of the composite controller's design.4.After selecting S7-200 PLC as the main controller a corresponding control system was established.In order to reduce the impact of switching clutch engagement,the output of control rules was optimized to avoid the unstable gear shifting when controller output stabilized in shifting point.Compiling the control program of clutch switching and pump displacement adjustment according to the rules of output control,and the core control algorithm according to the control strategy laid a foundation for the bench test later.5.Verify this two control strategy by carrying out bench test.This result shows that two kinds of control strategies can adjust HMCVT ratio,switch clutch engagement and ensure a stable work in engine fuel economy curve very well.For the composite control strategy,it can adjust the change rate of output value Ratio according to pedal change.And it is also a feedback of the driving intention.