Design of the walking driving system for a blueberry harvester based on contact mechanical behavior of wheel-soil

Abstract: A contact mechanics model of the wheel-soil was established to improve the working road surface quality of the self-propelled blueberry harvester in China. After analyzing the mechanical behaviors of the wheel driving torque, wheel load, wheel subsidence and drawbar pull, the influence factors on the contact mechanics model were obtained, including the structure parameters of wheels, the property of ground soils and the walking speed of the harvester. A simulation model of the wheel-soil contact mechanics for the blueberry harvester was established by using the discrete element method. The variation ranges of the wheel resistance torque and the soil fluctuation velocity are obtained when taking the wheel parameters and walking speed as test factors. The simulation results show that the compressive force between the wheel and soil decreases from the center to the periphery in the wheel-soil contact model. The compressive force gradually returns to zero after the traffic passing, and then begins to rise on the approaching road. The resistance torque of the rear wheels is generally higher than that of the front wheels. In the same walking speed of the harvester, the resistance torque of the wheels increases as the increase of the wheel radius and wheel width. The maximum increasing amount of the peak value is 271.6 N·m. When the structure parameters of the wheels are fixed, the walking speed rises from 3.1 km/h to 11 km/h, as well the resistance torque increases. The growth maximum of the peak value is 452.3 N·m. The fluctuation velocity of soil increases as the wheel structure parameters and walking speed increase. The closed hydrostatic four-wheel walking driving system was designed for the self-propelled blueberry harvester based on the extreme values of the resistance torque. The AMESim model of the walking driving system was also established to analyze the condition adaptability in the application. The simulation results show that the motor output torque of the four wheels is up to 86.8 N·m, and the output revolving speed is 83.6 r/min after overcoming the resistance torque of 300 N·m. This makes the harvester walking stable at the speed of 10 km/h and the output characteristics of the four wheels. When the rear wheels sink into the soil and their resistance torque was up to 700 N·m, the motor output torque of the front wheels increased by 25.5 N·m, and that of the rear wheels increased by 44.1 N·m. The revolving speed of the motor output for the front wheels increased from 75.6 r /min to 120.3 r /min and 103.4 r /min, respectively, whereas that for the rear wheels decreased to 44.3 r/min and 24.8 r/min, respectively. After the subsidence condition remains for 3 s, the output torque and revolving speed of each wheel return to the initial value, when the rear wheels running cross the subsidence area. The field test of the prototype verified the consistency output of the four wheels for the walking driving system, and the maximum deviation of the center line of the tire is 180 mm. The duration for running across the subsidence is 3.3 s, while the whole machine has no purposeful steering deviation. The two systems of driving and harvesting are well matched, with the harvesting efficiency of 7.01 kg/min, the collected rate of fruits of 92%, and the tree damage rate of 11.5%.