| A wheeled tractor equipped with a new type of hydraulic mechanical continuously variable transmission(HMCVT)is studied in this paper.The following contents are studied,including transmission characteristics of the new type HMCVT,tractor speed control strategy based on slip rate-resistance classification,the speed regulation of HMCVT aiming at achieving the maximum productivity under three different transmission modes and the control of speed control mechanism of HMCVT system,which are of great significance to improve the tractor to adapt to the complex and changeable working environment and ensure the production efficiency of tractor-implement combination.The main works are as follows:(1)The mechanical construction of the new type HMCVT system is analyzed.In order to point out its innovation and advantages,the structure of its front-end module is compared with the traditional dual flow transmission.The transmission principle and characteristics of pure mechanical transmission mode,pure hydraulic transmission mode and double flow transmission mode of HMCVT system are introduced.(2)The characteristics of speed ratio,hydraulic power distribution,transmission efficiency fluctuation and power cycle in HMT transmission mode are analyzed theoretically.In AMESim,the model of HMCVT transmission system is established.The starting characteristics of HMCVT system under different transmission modes,starting characteristics under different displacement ratio and acceleration shifting process on flat ground are simulated.(3)Based on the analysis of the recognition of the wheel slip rate,the influence of the slip rate on the tractor production efficiency and the tractor working resistance,a tractor speed control strategy based on the slip rate-resistance classification is proposed.The priority of the tractor speed control strategy is determined by classifying the wheel slip rate.When the tractor production efficiency is the first priority,according to the classification of working resistance,the law of tractor's maximum productivity speed change based on the classification of resistance is formulated.The ergodic optimization algorithm is designed.Taking the maximum driving power of the driving wheel as the optimization objective,the relationship between the displacement ratio of the hydraulic pump-motor and the tractor speed and the load oil pressure under the constant throttle opening is obtained.(4)The main speed control mechanism of HMCVT system—diesel engine and hydraulic pump are studied.The speed characteristics and universal characteristics of YC4DK-58.8 are analyzed.The speed control algorithm of diesel engine is proposed based on the global fast terminal sliding mode,and the control simulation model is built in Simulink to compare the speed control effect with the traditional PID algorithm.Considering that the fluctuation of the load oil pressure will affect the angle of the swashplate of the hydraulic pump,a control algorithm of the displacement adjustment of the hydraulic pump based on sliding mode control with feedforward compensator is proposed,and the servo mechanism model is built in Simulink to verify the control algorithm.(5)Based on AMESim and Simulink joint simulation,the effectiveness of speed control strategy of HMCVT system in MT and HMT transmission modes are verified by simulation. |
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