面向切割过程仿真的小麦茎秆模型参数标定与试验验证

Parameter Calibration and Experimental Validation of Wheat Straw Model for Cutting Process Simulation

摘要:

基于离散元法的小麦植株切割过程仿真，对于小麦收获机械切割部件分析与优化具有重要的意义，而准确测量及标定小麦植株力学参数是仿真重要前提。本文基于Hertz-Mindlin with Bonding模型在自主研发的AgriDEM软件中建立小麦茎秆模型。搭建单摆切割试验装置并进行了茎秆动态切割试验，探究了茎秆相关物理特性及切割参数对茎秆切断力的影响，同时，基于AgriDEM软件进行了茎秆单摆动态切割仿真并与实际试验进行对比，结果表明利用准静态粘接力学参数进行动态切割仿真时，茎秆切断力仿真误差随着切割速度增大而增大。为降低不同切割速度下仿真误差，提出了2种茎秆粘接力学参数标定方法。①以刚度和临界应力为试验因素的多变量中心组合优化标定方法，得到摆切线速度为1m/s的情况下，法向刚度、切向刚度、临界法向应力和临界切向应力优化值分别为2.82×1011N/m3、1.32×1011N/m3、9.50×106Pa和6.46×106Pa。②以粘接半径放大因子为单变量的标定方法，得到摆切线速度为0.5、1.0、1.5、2.0、2.5、3.0m/s时，粘接半径放大因子优化值分别为0.997、1.106、1.163、1.213、1.323、1.439，并得到切割速度与粘接半径放大因子标定值间的拟合方程。通过最大切割速度为1m/s的往复式切割试验与仿真对比可知，采用刚度和临界应力组合优化标定方法，茎秆切断力仿真误差由22.55%降低到9.13%；采用粘接半径放大因子的方法，当粘接半径放大因子为1.068时，茎秆切断力仿真误差由22.55%降低至6.58%。证明了本文提出的两种参数标定方法可有效降低茎秆动态切割仿真误差，且利用有效切割速度中间值对应的粘接半径放大因子标定值开展往复式切割作业仿真是准确且可行的方法，研究结果为小麦切割作业过程仿真和关键切割部件优化设计提供参考。

Abstract:

Simulation analysis of wheat plant cutting process based on discrete element method (DEM) is of great significance for the optimization of cutting parts of wheat harvesting machinery, while accurate measurement and calibration of mechanical parameters of wheat plant is an important prerequisite for simulation. Firstly, a wheat straw model based on the Hertz-Mindlin with Bonding model was established in the self-developed AgriDEM software. Then a single pendulum cutting test device was built and dynamic cutting test was carried out to investigate the influence of straw-related physical properties and cutting parameters on the straw-cutting force. At the same time, a dynamic straw-cutting simulation was carried out based on the AgriDEM software. Comparing the simulation results with the actual test, the results showed that the simulation error of straw-cutting forces was increased with the increase of cutting speed when using quasi-static bonding parameters for dynamic cutting simulation. To reduce the simulation error at different cutting speeds, two calibration methods for the bonding parameters of wheat straws were proposed. The first method was a multi-variable central combination optimization calibration method with stiffness coefficient and critical stress as test factors, which obtained the optimized values of the normal stiffness coefficient, tangential stiffness coefficient, critical normal stress, and critical tangential stress for a cutting speed of 1m/s, respectively, as 2.82×1011N/m3, 1.32×1011N/m3, 9.50×106Pa and 6.46×106Pa, respectively. The second method was a single-variable calibration method with the bonding radius scaling factor, and the optimized values of the bonding radius scaling factor were obtained as 0.997, 1.106, 1.163, 1.213, 1.323 and 1.439 for the pendulum-cutting speeds of 0.5m/s, 1m/s, 1.5m/s, 2m/s, 2.5m/s, 3m/s, and a fitting equation between the cutting speed and the calibration value of the bonding radius scaling factor was obtained. Finally, by comparing the reciprocating cutting test with simulation at cutting speed of 1m/s, it was found that using the first calibration method with stiffness coefficient and critical stress, the simulation error of the straw cut-off force was reduced from 22.55% to 9.13%. Using the second calibration method, when the bonding radius scaling factor were 1.068, the simulation error of the straw cut-off force was reduced from 22.55% to 6.58%. This preliminary demonstrated that the two bonding parameter calibration methods proposed can effectively reduce the simulation error of straw-cut-off force, and the use of calibration value of the bonding radius scaling factor corresponded to the middle value of the effective cutting speed to simulate the reciprocating cutting operation was the most accurate and feasible method. The research result can provide a reference for the simulation of wheat cutting operation process and the optimized design of key cutting components.