基于离散元法的旋耕刀受力分析

为分析旋耕刀在有秸秆覆盖和无秸秆覆盖土壤中工作时的受力情况,该文采用离散元法构建土壤-旋耕刀相互作用和秸秆-土壤-旋耕刀相互作用的三维离散元模型。在土槽试验中采用扭矩传感器测量旋耕刀所受扭矩,在仿真中导出每个时刻旋耕刀受到的扭矩和力数据,试验和仿真都在恒定前进速度0.222 m/s和耕作深度100 mm及4种转速77、100、123、146 r/min下进行。结果显示旋耕刀最大扭矩值随着转速增加呈现增加的趋势;在无秸秆覆盖土壤中,试验值与仿真值的相对误差为16.3%;在有秸秆覆盖土壤中二者相对误差为19.1%。耕作过程中旋耕刀所受合力、水平力及侧向力都呈现先升后降的趋势;合力、水平力和垂直力的最大值,都随着转速的增加而增加。旋耕刀刚开始切土时,在有秸秆覆盖的土壤中受到的阻力总比无秸秆覆盖的土壤中所受阻力要大;之后旋耕刀在有秸秆覆盖的土壤中受到的最大力要大于在无秸秆覆盖的土壤中受到的最大力。仿真的水平力和垂直力与旋耕刀理论模型计算得到的旋耕刀水平力和垂直力变化一致。利用离散元法研究旋耕刀切土过程,对探讨刀具与土壤的相互作用机理及设计和优化高秸秆含量土壤中工作的耕作机械具有重要意义。

Force analysis of rotary blade based on distinct element method

Abstract: Tillage practice is one of the energy-consumed links in agricultural production, while the energy can be saved through proper management of tillage practices. In order to manage the tillage process reasonably, a better understanding of soil-tool interaction should be developed. In fact, the soil-tool interaction is a complex process because of the spatial variability of soil, tool dynamics and soil movement. The process becomes more complex due to the addition of straw, as straw-soil-tool interaction involves the incorporation, displacement and movement of straw. Basically, soil-tool interactions are usually characterized by forces arising at the soil-tool interface. In case of rotary tillage, the force variation of rotary blade plays a pivotal role in the field of blade geometry optimization and energy consumption. However, it is very difficult to acquire this information in field tests due to the presence of many uncontrollable variables such as weather and soil moisture content. The simulation of tillage process by a rotary blade using distinct element method (DEM) is hypothesized to provide a better understanding of the force variation in complex field conditions. Therefore, the present study was carried out with the DEM simulation on the basis of the experiments performed in an indoor soil bin. Based on these situations, the DEM models for the interaction of soil and rotary blade and the interaction of straw, soil, and rotary blade were established. Secondly, the indoor soil bin experiments were conducted. Blade torques at every moment were used to study torque requirement in simulation, and a torque sensor (LKN207) was installed between the output shaft of motor and the rotor of blade through chain transmission to measure the torque force at 0.025 s interval. Both simulation and experiment were performed with 4 rotational speeds of blade (77, 100, 123, and 146 r/min), the constant forward speed of 0.222 m/s and the depth of 100 mm under 2 soil conditions (with and without straw covered). It was observed that the torque requirement in both soil bin experiment and simulation increased with increasing rotational speed of blade. In the simulation, the torque first increased with the increasing of rotational angle and then decreased gradually. This phenomenon was also observed for the soil bin experiment, but the torque forces obtained were slightly higher than those obtained by simulation. The average error of torque between simulation results and experimental results was about 16.3% for the soil without straw covered while 19.1% for the soil with straw covered. The resultant force, forward and side force under both soil conditions increased from 0 to a maximum value and then decreased to 0. The vertical force increased from 0 to a maximum and then decreased to 0 in upward direction, and later another rising and falling period occurred in the opposite direction. The maximum resultant force, horizontal force and vertical force during the tillage increased with the increasing of rotational speeds, whereas there was no clear trend of variance for the side force. The variations of 3-dimensional force with different rotational speeds under 2 soil conditions were also compared. The results revealed that all the forces acting on the blade under the soil with straw covered were higher than those observed under the soil without straw covered in the initial period of the tillage, and later the maximum forces of blade working in the soil with straw covered were also higher. A high correlation between the simulation results and theoretical results was obtained in horizontal and vertical directions. It can be inferred that the simulation of soil-blade and straw-soil-blade interaction using the DEM provides a better understanding of force and torque requirement during the tillage, which is helpful for the design and optimization of rotary blade.