| Efficient and high-quality plowing operations are the basis for high crop yields.The plowing operation has the complex soil environment and the wide variation range of the resistance.At present,it is a widely used technical means to improve the tillage efficiency relying on the automatic adjustment of the height of farm implements to change the plowing resistance to adapt to the traction adhesion performance,and then the slip rate is controlled in the permissible range.However,because of the frequent adjustment of the tillage depth,there is a common problem of uneven tillage depth in general,which leads to the unevenness of the furrow bottom and the ground surface after tillage,and affects the seed germination rate.This problem has not been well solved due to the lack of the resistance-adaptive operation control method under the condition of setting tillage depth.Therefore,in this paper an electric tractor with adjustable center of gravity was taken as the object,and two kinds of resistance adaptive operation control methods of the tractor were proposed according to the coupling regulation of the drive motor torque and sliding battery position,which can effectively adapt the tractor to the plowing resistance change without adjusting the tillage depth based on the automatic change of the traction adhesion performance,so as to improve the tillage efficiency and operation quality.The main research contents and conclusions are as follows:(1)The plowing dynamic modeling and analysis of the electric tractor with adjustable center of gravity.The structural characteristics and working principles of the electric tractor and its center of gravity adjustment mechanism were introduced.By ignoring the unevenness of the soil ground,the forces on the longitudinal plane of the tractor during plowing operation were analyzed,and the moment balance relationship in the plane was established according to the moment balance principle by considering the inter-axle load transfer effect.On this basis,the dynamic model of the tractor was developed.Based on the numerical solution of the model,the influences of plowing resistance changes on the traction force,slip rate and traction efficiency were studied,and the influence of the center of gravity adjustment on the traction adhesion performance of the electric tractor was investigated through simulation and experiment.The results showed that the variations of the operating speed and soil specific resistance can affect the magnitude of the plowing resistance,and the increase of the slip rate of the driving wheels can reduce the traction efficiency.By moving the sliding battery to adjust the center of gravity,the load distribution between the axles was changed,and the adhesion between the driving wheels and the ground was enhanced.(2)The research on the switching control method between the speed and slip rate of the plowing operation.The existing plowing operation control methods of tractors were analyzed,and the slip rate PID control based on plowing depth adjustment was established.Then,based on the established dynamic model,the switching control method of the tillage speed and slip rate was proposed under the assumption of constant tillage depth and according to the characteristics of adjustable motor torque and battery displacement.The slip rate was monitored in real time,when it was less than 20%,the speed control was executed to increase the speed to 6.0 km/h by using the sliding mode variable structure control algorithm.When the slip rate exceeded a threshold value of 20%,the RBF-PID controller was designed to control the slip rate to below 20%,where the initial values of the neural network parameters were optimized by the sparrow search algorithm to improve the learning rate.The control effects were compared and analyzed by simulation.The results showed that the proposed switching control method can effectively control the speed and slip rate when the tillage depth was kept at 20 cm with the changing plowing resistance.Compared with the traditional slip rate control method based on tillage depth adjustment,the average speed is higher by 8.79%and the traction efficiency is increased by 14.53% under constant tillage depth.(3)Research on the double-objective collaborative control method of the speed and slip rate considering the temperature rise of the motor.Due to the problem that the driving torque in the switching control is always high,resulting in significant temperature rise of the motor,according to the structural characteristics of the permanent magnet synchronous motor,the electromagnetic field simulation model was established,and the internal losses and their distribution rules in time and space during the motor operation were calculated and analyzed.Then the motor temperature field was established and the magneto-thermal coupling analysis was carried out,so that the stator winding resistance values and permanent magnet magnetic chain values corresponding to different temperatures were obtained.On this basis,the magnetic field orientation control model of the PMSM with temperature compensation was established,and the simulation results showed that it can effectively reduce the fluctuation of the output torque caused by the temperature rise and the error between it and the expected value.Then,considering that the system is prone to oscillation during the switching control,a dual-objective collaborative control method for the tractor considering the motor temperature rise was proposed.By using the fuzzy neural network algorithm to adjust the drive torque and battery displacement,the traction adhesion performance of the electric tractor was adapted to the resistance change,so as to realize the joint control of the speed and slip rate.The results showed that the average relative errors of the speed and slip rate were reduced by 59.52% and72.92%,respectively,compared with those before temperature compensation,and the traction efficiency was improved by 3.74%,which significantly improved the operational stability while ensuring the tillage efficiency.(4)Based on the self-developed electric tractor with adjustable center of gravity,a real vehicle test platform was built,the soil cone index of the test site was measured,and plowing tests were conducted under the switching control and the dual-objective collaborative control,respectively,and the operation effects were compared and analyzed with the uncontrolled operation.The results showed that the deviations of the tillage speed under the two control methods were reduced by 76.04% and 89.06%,the average slip rates were decreased by 38.26%and 43.28%,the traction efficiencies were increased by 22.84% and 25.04%,respectively,and the uniformity of the tillage depth was guaranteed.The temperature compensation control of the drive motor when its temperature reached to 52℃ led to reductions of 85.22% and 64.81%in the average relative errors of the tillage speed and slip rate relative to the target values,which greatly contributed to the improvement of the operating stability of the electric tractor while increasing the operating efficiency. |
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