Control Strategy Development For Multi-Range Hydro-Mechanical Continuously Variable Transmission In Tractors

Hydro-mechanical continuously variable transmission (HMCVT) combines the hydrostatic power flow and mechanical power flow to realize stepless speed change with high efficiency and large power, so it has many advantages in the power train system of heavy vehicles. To develop and utilize the HMCVT in large power tractors, This thesis studies its principle and characteristics, modeling and simulation, stepless speed change and range shift law, control strategy and implementation method, as well as electronic control technology.Based on the analysis of compositive principle, fundamental types and kinematic characteristics, according to the work feature of tractors and the need of power train system, a novel principle of multi-range HMCVT is developed, which has a wide stepless speed change range and high efficiency in the main work speed field of tractors. The analysis methods of transmission ratio, torque, power split ratio and efficiency characteristic are derived. For the tractive characteristic of tractors equipped multi-range HMCVT, an analysis and drawing method is represented. These research will establish the theory foundation of HMCVT on the design, characteristic analysis, match method and control technology.For the development of control system, with the modular method, the model of multi-range HMCVT and tractor's powertrain system are established. The mechanical components are simplified as lumped mass-dashpot system to meet the need of control simulation and reduce the complexity of model. Based on the principle of finite state machine(FSM) and software of MATLAB, a multimode transfer simulation system of multi-range HMCVT is developed, which can simulate the control process of continuously speed change and range shift. By control simulation and analysis, it is presented that a variable parameters PID control principle which can increase the adjusting speed of transmission ratio, and that a throttle change rate forward control principle which can avoid the fluctuation of vehicle speed. The validity of modeling method is proved by bench test. A simulation test condition is created for the development of control strategy.According to the main purpose of obtaining highest productivity for tractors, based on getting largest tractive power, a speed change and range shift law for the multi-range HMCVT is represented. A control strategy of engineering implementation is established, which uses throttle position, engine speed, range number and real transmission ratio as the control parameters. It is expounded that the calculating method of engine work speed and range shift speed in the speed change and range shift law. The control strategy of engineering implementation is proved to be validity by simulation test, which is the theory support to develop the control system of multi-range HMCVT.By analyzing the main influencing factors of economical efficiency for tractors, it is represented that the rate of fuel consumption and the multi-range HMCVT's efficiency (ge/ηb) must be keep least to realize the highest economical efficiency, because the efficiency of multi-range HMCVT changes in a wide field with the transmission ratio. Based on getting the highest economical efficiency of tractors, a speed change and range shift law for the multi-range HMCVT is presented, which can ensure ge/ηb to be the least and the vehicle speed to be correct on the condition of coordinate control of transmission ratio and engine speed. A control strategy of engineering implementation is established, which uses object vehicle speed, engine speed, injection pump gear rack position, optimal transmission ratio and slip rate as the control parameters. The real vehicle speed is held to be equal to the object vehicle speed on any work conditions by real time adjusting the slip rate that is adjusted by slip rate correction factor on different condition of soil and vertical load. The calculating method of optimal transmission ratio is derived. A range shift law is made which can avoid the recirculation range shift. The control strategy of engineering implementation is proved to be validity by simulation test.According to the established work modes of tractors equipped with HMCVT, the hydrostatic and electronic control system of multi-range HMCVT is developed. Because the control precision of transmission ratio is decided by the displacement ratio, a weight dynamic change fuzzy-PID integrated control principle for the displacement ratio control is presented. The control of transmission ratio is done by this principle in the bench test. The result showed that the rapidity and accuracy is good and the dynamic follow error is small.