| When soybean seeding monomer works,there is always the contact between mechanical components(i.e.,opener,seeding device,as well as the covering and compacting parts)and particles(soil particles and soybean seed particles),and between particles.At the same time,it also involves the movement process of mechanical components and particle materials.The final spatial position of soybean seed particles is the result of the combined action of mechanical components and particle materials.At present,for the design,optimization,performance and working process analysis of seeding monomer,experiments or traditional continuum mechanics methods are often used in China and abroad,but the experimental method has the disadvantages of large amount of experiments and long cycle.The continuous medium analysis method can only consider the particle group as a whole,and cannot analyze the movement of a single seed particle and its contact with mechanical components and soil particles.The Discrete Element Method(DEM)can not only solve the above problems,but also analyze the movement of seeding monomer,the interaction between mechanical components and particles,and the movement of particles and the interaction between particles.When the DEM is used to analyze the working process of seeding monomer,the discrete element model of mechanical components and particles should be established first.In this paper,the DEM modeling of soybean seed particles is taken as the starting point.Firstly,after analyzing the shape characteristics of soybean seed particles in detail,a general geometric modeling method of soybean seed particles is proposed and established.The applicability of the particle model established in this paper is verified by comparing simulation and experiment.Then,for soil particles,the geometric model is established after analyzing their geometric shape characteristics.According to their texture classification,the appropriate contact model between particles is selected,and the relevant parameters are calibrated.Finally,the DEM and Multi-rigid-body dynamics(MBD)coupling method are used to simulate and analyze the working process of soybean seeding monomer.The main work and conclusions of this paper are as follows.(1)The selection of soybean varieties throughout the country,the shape and size of soybean seeds were systematically analyzed,and the multi-sphere method and ellipsoid model were proposed.The main contents and conclusions are as follows.(1)Three soybean varieties planted in recent years were applied to each of the four research institutes in the main soybean planting areas in China.Three seeds with good appearance were selected for each variety,and the length,width and thickness of soybean seeds were measured to generate the corresponding ellipsoid.At the same time,the three-dimensional scanner was used to scan it to obtain the spatial point cloud data of soybean seeds.The point cloud data were compared with the ellipsoid of the corresponding size,which proved that the soybean seeds of all varieties could be approximated as ellipsoid.(2)The size distribution and three-axis size relationship of 12 varieties were analyzed systematically,and the three-axis size conformed to normal distribution.The width is defined as the main size,and the particle group model is established according to the relationship between the primary and secondary sizes.(3)The variation range of sphericity of all varieties was calculated to be(80.6 % –95.42%).In this paper,three varieties with high,medium and low sphericity(SN42,sphericity of 94.78 %),(JD17,sphericity of 86.86 %)and(ZD39,sphericity of80.6 %)were selected as the research objects,and the 5S,9S and 13 S models suitable for different sphericities were established,and the filling accuracy was analyzed.(2)The physical and mechanical properties of three varieties of soybean seed particles were measured,mainly involving moisture content,particle density,stiffness coefficient,elastic modulus,friction coefficient and restitution coefficient.(3)The multi-sphere model,ellipsoid model and the multi-sphere model proposed by different scholars are verified by self-flow screening test and angle of repose test.The influence of multi-contact phenomenon on particle group simulation is analyzed.The main contents and conclusions are as follows.(1)The aperture and inclination angle of the self-flow screening device were determined by simulation.The length and width of the angle of repose device and the number of simulated particles were determined.(2)Using the soybean seed particle model established in this paper,the multi-sphere model established by different scholars and the ellipsoid model established by ellipsoid equation method,the simulation of self-flow screening and angle of repose was carried out,and the simulation and experimental results were compared and analyzed.The results show that the multi-sphere particle model proposed in this paper is more applicable.(3)By adjusting the restitution coefficient and comparing different contact models,the multi-contact point problem in the multi-sphere particle model was analyzed and explored through self-flow screening and angle of repose tests and simulation.The results show that the multi-contact problem has little effect on the simulation results of the multi-sphere particle model group.The reason why multi-contact points have little effect on particle group simulation is further analyzed.(4)The soil used in this experiment was systematically analyzed,and the geometric model of soil was established.At the same time,the Edinburgh model(EEPA)was selected for the simulation analysis of soil particles,and the simulation parameters of soil particles were calibrated by direct shear test and simulation.The main contents and conclusions are as follows.(1)The soil particles with particle size less than 3 mm were screened by screening,and the particle size distribution was counted.The soil was determined as loam in the experiment.The geometric shapes of soil particles in different particle size ranges were observed by two-dimensional scanner.After classifying and counting the shapes,the soil particles were divided into spherical and triangular-like particles,and their geometric models were established.(2)The natural moisture content,natural density and particle density of soil were measured.(3)The EEPA model was selected as the contact model of soil particle simulation.The corresponding parameters were calibrated by direct shear test.Firstly,the sensitivity of parameters was analyzed by PB test.Then the BBD test was designed to calibrate the sensitivity parameters by response surface method.(5)The parameters between soybean seed particles and soil particles were calibrated by response surface method,and the seed throwing experiment and simulation analysis of soybean seed particles were carried out.The main contents and conclusions are as follows.(1)The interaction parameters between soybean seed particles and soil were calibrated by angle of repose test,and the static friction coefficient and rolling friction coefficient were determined as sensitive factors by PB experiment.The optimized parameters were determined by CCD test.(2)The high-speed camera was used to conduct the seed throwing experiment,and the effects of throwing height,soil plane inclination angle,collision orientation,soil moisture content and relative throwing speed on the bounce and rolling distance of soybean seeds were analyzed in detail.(3)The soybean seed particle model,soil particle model established in this paper and calibration parameters were used to analyze the seed throwing process by simulation.By comparing the simulation and experimental results,the trend of simulation and experimental results is basically the same.(6)The DEM and MBD coupling method were used to simulate the working process of seeding monomer,including opening,seed throwing,soil covering and compacting,and the results were compared with the experimental results,the conclusions are as follows.(1)Using EDEM and Recur Dyn coupling,the working process of seeding monomer is simulated and analyzed.With the increase of seeding monomer working speed,the opening angle of soil decreases gradually.(2)The simulation results of horizontal and vertical position changes of soybean seed particles after covering and pressing were compared with the experimental results.The results showed that the trend of simulation and experimental results was basically the same,and there was little difference between them.From the simulation results,it can be seen that the depth of soybean seed particles does not change significantly after soil covering.(3)Under different seeding speed,the simulation average grain spacing is basically consistent with the theoretical grain spacing.The smaller the working speed of seeding monomer,the better the uniformity of soybean seed particle spacing. |
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