Study On Control System Of Power-Shift Transmission For High-Powered Tractor

Under the premise of satisfying the performance of high-powered tractor,it’s transmission module adopts a combined transmission scheme based on the consideration of cost-saving and composition-simplifying of control system,in which the power-shift transmission is made up of 3 wet double clutch packages.If the transmission must shift automatically,two problems must be solved in the control system.One is shift strategy and the other is power-shift control strategy.The traditional two parameter shift strategy often meets the problem of cycle shift for high-powered tractor under load operation.The cycle shift will greatly reduce the clutch life,increase the transmission failure rate,and cause the vehicle speed to be unstable and affect the quality of operation.Then this paper analyses the causes of circle shift under load operations for tractor based on the change characteristics of engine’s working point under shifting,and furthermore proposes a shift strategy to avoid circle shift.Simulation results show that the strategy can avoid the circle shift effectively and make tractor operate continuously under different working resistance,and the fuel economy is also improved.For the power shift control problem,based on the integrated inertia of two degree freedom model,a constant-torque power shift control strategy is proposed with the constant transmission output torque as the control target.In the shift process,only one clutch is in the sliding state,which is beneficial to reduce the friction loss.However,during the shifting process,the power of the friction loss is required to be supplemented by engine for constant output power and the output torque of the transmission.Based on the constant-torque power shift control strategy,considering the speed regulation characteristics of the diesel engine,the practical power shift control strategy can rely on the inherent speed characteristics of the engine to increase the output torque adaptively in the process of power transfer.The simulation results show that: compared with mainstream power-shift strategy,the practical power-shift strategy proposed by this paper can achieve the same transmission output torque while reducing sliding friction loss greatly.In addition,this paper has studied the influence of oncoming clutch pressure limit and gear overlap time on transmission output torque and sliding friction loss.The results show that higher pressure limit is conducive to reduce shift time,but will produce larger mutation of output torque at the end of shift process,which results in larger jerk,and gear overlap time will have a great influence on transmission output torque,if the time is too short,the transmission output torque will appear larger trough.In the end,this paper discusses basic function modules of transmission hydraulic control system,and then establishes a digital simulation model of clutch pressure control system and next simulates open-loop and closed-loop pressure control based on the model.The simulation results show that open-loop control and closed-loop control can both realize clutch pressure control,but closed-loop control is more stable and can control pressure accurately under different supply oil pressure,and furthermore it proves that I control law can achieve better effect between I,PI and PID control law,whose overshoot is smaller and setting time is shorter.