The Method Of Representing Boundaries Of 3D DEM Based On 2D CAD Model

When the farm machinery works, massively exists the flow process of granular materials. Such as when the seeder sows seeds,there is always exist the flowing of the seeds, the fertilizer and the soil. In addition to the flow of granular materials is related with mechanical properties of itself,but also with the structural style and the size parameter of correlated working part of the farm machinery.A good farm machinery design,should cause granular materials to flow according to the expectated formula, reduces the nonessential damage in the flow process, saves the mechanical power consumption.So the question of the interaction between granular materials and related mechanical parts must be concerned.Cundall proposed the discrete element method(DEM) in 70's of the 20th century.Its basic thought is turn the discrete elements that have the certain shape and the quality into a set ,endow with the mechanics model and parameters so as to consider about the contaction among elements and the contaction between elements and the boundaries,also about the physical,mechanical property of the element and the machine.Since DEM proposed in 70's of the 20th century, the main boundary modelling method on roughly have been divided into two directions: One method is compiles the boundary through the computer program; Another method is use general modelling software,establishes the boundary model, then through the computer secondary development, obtain the boundary information which is needed and join into the DEM software. The merit of modeling through the computer program is through this method, the boundary model is closely related to the DEM.In the realization process,the operation should quite be strong regarding the boundary. But this method has very big drawbacks. The main drawback is that the workload of establishing complex boundary model is huge, boundary complex becomes the index relations with the workload. Therefore although the mentality of this method is quite clear, regarding the actual application,the significance is not very big. Use general modelling software can easily establish complex boundary model, and there is also quite splendid modelling software in the market that already has successfully demonstrated in the direction of 2D boundary modeling. But in the 3D direction, there is not a very unified method. At present,there are two main solutions. One is use 3D drawing software draws up the boundary,then read the information through the computer;The other is use 2D drawing software draws up 2D graph, then through the matching arithmetic,obtain the 3D geometric information and establish the 3D DEM model.The method this paper used is obtain the 3D geometric information based on three views 2D CAD model. This method uses the bottom-up thinking.Obtain 3D points through 2D points,then connect 3D points to determine 3D lines.Through these 3D lines,we can confirm 3D surface,and finally identified the 3D entity.But this method still has some drawbacks.One drawback is that in the process of 3D entity projection,due to the accumulation or some other reasons ,part of the geometric information will be lost.So totally use the computer for 3D reconstruction will encounter many problems with no solution or multi-solutions.Another drawback is for some mechanical components with comlex forms, totally use the computer for matching will become the exponential increase in workload.For the purpose of rebuilding 3D DEM model from 2D CAD model,this paper presents an idea which is split the complex 3D entity into a number of basic 3D entities.By rebuilding the basic 3D entities to achieve the purpose of building the complex 3D entity.For the problem of non-solution caused by the accumulation of projection,the method of automaticly drawing the intelligent supporting line is used to regain the lost information of points.For the problem of multi-solution,the thinking of adding human factor into design is proposed.Ensure the accuracy through interacting with user and also minimize user's workload.To increase the intelligence,the method of getting geometric attributes of basic 3D entities with same shape by calculation is used..Set the tractarian property for each basic 3D entity by interacting with user after getting the geometrical property,then save the information into database for the calculation of 3D DEM afterward.Based AutoCAD software as an example,this paper detailedly analyse the the memory structure of 2D AutoCAD model, and the methods of identify and map the geometric to obtain information.I achieved the design of building the 3D DEM boundary model from 2D CAD model by the secondary development of AutoCAD.I also achieved the integation of AutoCAD software and 3D DEM software .Some examples of building 3D DEM boundary model in the fourth chapter testify the accuracy and the validity of the method this paper proposed .This method provide basis for the in-depth study on 3D DEM software.