Speed regulation control of the dual-flow transmission system for a tractorusing slip rate-resistance interval division

Operation resistance of a tractor fluctuates frequently when working in the farmland, due to the complex field environment and terrain differences. As such, the resulting change of speed cannot guarantee the operation efficiency of the tractor. Taking a newly developed Hydraulic Mechanical Continuously Variable Transmission (HMCVT) as the research object, this study introduced the principle of transmission, mechanical structure and transmission efficiency in a Hydraulic Mechanical Transmission (HMT) transmission mode. The slip rate was divided into different intervals according to the traction efficiency curve of the tractor. In different intervals of slip rate, the first priority was to ensure the highest production efficiency and the traffic ability of a tractor. Specifically, if the slip rate of driving wheel was within the allowable range of slip rate, the first priority was to ensure the production efficiency of the tractor, where the strategy of variable speed control was implemented; if the slip rate of driving wheel exceeded the maximum allowable slip rate, the first priority was to ensure the traffic ability of a tractor, where the control strategy of slip rate was implemented, in order to avoid the serious consequences, particularly that the vehicle cannot move due to excessive slip of driving wheel. In speed control, the internal division of operation resistance was used to determine the transmission mode of HMCVT system suitable for the current resistance state. Taking the maximum production efficiency of tractor as the goal, the law of speed change was optimized to determine the adjustment surface of displacement ratio for the HMCVT system in HMT and hydrostatic transmission (HST) mode. A feedforward compensation was designed using the sliding mode control algorithm, in order to deal with the constant change of pump displacement. The reason was that the fluctuation of oil pressure caused the variation in the inclination angle of variable pump in the reverse direction. The working oil pressure inevitably fluctuated in the pump-controlled motor system, particularly when the working load of tractor fluctuated. According to the fluctuation of the working oil pressure, the feedforward controller generated a corresponding compensation signal, which was superimposed with the sliding mode control signal, and then output to the actuator, in order to reduce the influence of fluctuating oil pressure on the actuation accuracy of the servo mechanism. Simulink simulation results show that the proposed algorithm accurately followed the desired signal to change, indicating better tracking stability than the traditional PID control. A HMCVT drive system was constructed, where the strategy model of speed control was established. The simulation results show that the speed control strategy can constrain the slip rate of driving wheel within the allowable range of slip rate, as the conditions of load or road changed. The speed and acceleration of the tractor increased to 5.3 km/h and 0.15 m/s2, respectively, compared with the conventional change of dynamic speed. When the speed was lower than the lower limit of high-efficiency speed range, the speed of tractor increased greater than that in the traditional dynamic speed change, indicating a high production efficiency of tractor. The findings can offer great significance to improve the performance of tractors suitable in the complex and ever-changing operating environments for the higher agricultural productivity and efficiency.