Study On Characteristics, Control And Fault Diagnosis Of Tractor Hydro-Mechanical CVT

The tractor has complex working condition, such as plowing, planting, transport and others. As a consequence, more gearshifts are required to adapt to the different load and running speed, making the gearbox with complex structure, inconvenient operation, and uneconomical fuel consumption. In order to solve this problem, the most effective method is to realize the stepless speed regulation. The hydro-mechanical CVT successfully was applied to tractor as the first tractor continuously variable transmission in history, which combines the advantages of both mechanical and hydraulic transmission with high transmission efficiency, hign power output, low dynamic load impact, and large speed ratio range. While, this study in China started late and coupled with technical blockade, causing that the country has failed to use this technology in the domestic tractor.To this end, a new kind of tractor hydro-mechanical CVT with rated power of 132.5 kW has been designed under the financial support of "948" project "The introduction and development of automatic shift transmission of large tractor" funded by Ministry of Agriculture (fund No.:2010-Z18), which becomes the first hydro-mechanical CVT designed for wheeled tractor in China. For the first experimental transmission prototype, extensive theoretical and experimental studies have been made in this paper. The main research work and obtained results are as follows:(1) The test bench for hydro-mechanical CVT was build, and the equipments were calibrated. Furthermore, a remote control system based on SCM and Matlab GUI has been developed to control the transmission and the dymo.(2) The transmission characteristics of the hydro-mechanical CVT were studied theoretically and experimentally. On the one hand, the characteristics of CVT speed, torque, oil pressure and flow were calculated and tested. On the other hand, the characteristic of the power split was analyzed and the simulation model of CVT efficiency was built. According to the calibration test and repeating parameters correction, the simulation model was well fitted to the real CVT system. And then, the efficiency characteristics of the CVT were studied under this simulation platform. The results prove that:The speed ratio of the hydro-mechanical CVT can be changed steplessly under the synergic control of variable displacement pump and wet clutches, which verifies the correctness of the design theory. According to the results of calculation, there is hydraulic cycle power in range HM1/HM3 with positive pump displacement and range HM2/HM4 with negative pump displacement, because of the cycle power, leading to the transmission efficiency reduced in these working conditions. The efficiency of the CVT is proportional to the level of engine torque, and is inversely proportional to the level of engine speed. And, the full load efficiency under rated engine speed is about 81.5%. Confirmed by further studies, by increasing the gear ratio of the gear pair before the pump shaft, the transmission efficiency would be increased because of the increase of working pressure and the decrease of viscous damping loss in hydraulic system, although the method can not change the theoretical value of the hydraulic split power level. By calculation, if the ratio of the transmission gear and the parameters are optimized, the full load efficiency of the CVT can be basically limited to the range form 80 to 90%.(3) The dynamic characteristics of the hydro-mechanical CVT were studied theoretically and experimentally. On the one hand, according to theoretical analysis and the step excitation test on hydraulic system identification, a 3-order dynamic mathematical model was built; on the other hand, according to a series of tests including oil charging test, power shift test with single and multiple factors considered revealed that the significance of the factors affecting the shift quality from highest to lowest are clutch charging flow, load torque, oil pressure and engine speed. And, the best parameter combinations are flow 5 L/min and oil pressure 4 MPa.(4) The control algorithm of the hydro-mechanical CVT was studied theoretically and experimentally. On the one hand, the algorithm of point-to-point control, neural PID control with relay logic and fuzzy control were studied. The results proved all algorithms to be feasible:there is a certain speed regulation time during the open loop control based on the point-to-point algorithm, and the process would not be interfered by the external factors such as the governor control characteristics of the engine; the closed loop control algorithm based on neural PID controller with relay logic combines the two control modes of open loop and closed loop, both of the speed regulation and the power shift process were stable, and there was no static error during the control process; the load adaptive control based on fuzzy reasoning can be applied to the power control of engine, not only to prevent overload, but also for realizing the fuel economy of the tractor. On the other hand, the findings of the oil pressure drop during power shift process revealed the safty of overlapping sequence. And the further tests showed that this strategy can improve the shift quality significantly.(5) The speed ratio matching between the hydro-mechanical CVT and the engine have been studied. On the one hand, a model of this hydro-mechanical CVT and tractor has been built under the platform of simulationX;on the other hand, the matching strategies with one or two parameters are considered have been analyzed under the simulation. In the single-parameter matching, the engine can be controlled to work along the characteristic curve with lower fuel consumption by adjusting the speed ratio of CVT. Futhermore, in order to increase the transmission efficiency of CVT and ensure the economy of the whole tractor, the actual displacement of the pump must be limited in a narrow range. In the two-parameter matching, the economical engine speed have been calculated, the results show that the displacement of pump corresponding to the economical engine speed is always closing to zero. For each matching strategy, both of the work condition plowing and sowing were simulated under SimulationX so as to test and verify their feasibility.(6) The fault diagnosis algorithm of the hydro-mechanical CVT was studied theoretically and experimentally. Given that there have been abundant of studies on fault diagnosis of mechanical transmission system in the past, this study focuses the discussion on fault diagnosis of hydraulic system, especially the clutch control system:on the one hand, the training and testing samples were obtained first through a series of bench test, and then, the useful characteristic quantities were selected and the identification models were built for each fault pattern based on Fisher criterion and kernel method. The results prove the method to be effective. For the 20 groups of test samples, the diagnostic accuracy of the algorithm can exceed 95%. And in most cases the value can reach 100%. On the other hand, based on the algorithm above and the platform of VB, a system of condition monitoring and fault diagnosis for hydro-mechanical CVT have been developed finally.In summary, the study on the characteristics, control and fault diagnostics of the designed hydro-mechanical CVT, not only paves the way for the next field experiment, but also provides a theoretical reference and support for the optimization of the transmission and the controller.