Study On The Control Strategy Of Hydro-Mechanical Continuously Variable Transmission Of Tractor

Tractor is a kind of versatile agricultural machine, which can complete various jobs equipped with different implements. Integrating the merits of hydrostatic transmission and mechanical transmission, Hydro-Mechanical Continuously Variable Transmission (HMCVT) can realize the transmission ratio varying continuously in a large scale, improve the efficiency of whole powertrain, and reduce the dynamic load. HMCVT has a good application prospect in the large horsepower tractor. In the study of HMCVT, the control for automatic adjustment of transmission ratio and shifting quality is a key issue to realize the advantages of HMCVT. In this paper, the control strategy of the two aspects was studied based on the HMCVT equipped on DFH1302R tractor.The principle of HMCVT was analyzed, and the transmitting processes of power and speed characteristics were studied. The optimal economy and performance operating curves of engine were worked out based on the fundamentals of matching a CVT with an engine and the test data of engine. According to the characteristics of HMCVT ratio adjustment, the match between HMCVT and tractor was carried out, the target shift and the target displacement ratio of pump and motor for the optimal economy and performance operating were worked out. To improve the harmony among human, machine and environment, and to enhance the combination property, an intelligent control strategy based on fuzzy inference of driver's intention and tractor resistance characteristic was proposed, whose aim is to improve tractor dynamic performance under transient operating conditions and fuel economy under steady operating conditions. A model of tractor was established based on the bond graph theory, a simulation model was built up on the platforms of Matlab/Simulink and Matlab/Stateflow. The simulation results showed that the intelligent control strategy enable tractors to reach the optimal combination property.According to the working character of tractor and shifting process of HMCVT, an evaluation criterion of shifting quality was established. The shifting process was analyzed and the shifting states were partitioned. A hybrid modeling method, based on the continuous system under bond graph theory and on the discrete system under Finite State Machine, was proposed. A simulation model was established on the platforms of Matlab/Simulink and Matlab/Stateflow. The characteristics of speed and torque of input shaft, middle shaft and output shaft were analyzed. The effects of clutch oil pressure, tractor resistance and clutches overlap timing on the shifting quality were studied. A clutch control strategy of oil pressure and clutches overlap timing was proposed. The simulation was carried out to verify the validity of control strategy.On the HMCVT performance test bench, according to the testing program, the correctness of whole tractor model and hybrid model of shifting process was checked; the validity of transmission ratio control strategy and shifting control strategy was verified.