面向RP的工业CT切片数据格式转换软件开发

针对原始的工业CT切片数据,应用矢量化软件提取工件封闭轮廓点数据,并通过轮廓配准、数据精简、三角网格划分、端面处理对轮廓点数据进行处理,实现了面向RP的工业CT切片数据格式转换.为使构成STL文件的三角网格更加优化,文中提出了平均点距值法数据精简与Delaunay三角化的网格精简相结合的数据精简算法,实例验证了该方法的正确性.     

基于MC算法的CT序列图像快速成型文件生成方法

快速成型技术在医学领域有着广泛而重要的应用,为了从医学图像中得到快速成型文件,详细论述了MC(marchingcubes)算法生成STL文件的步骤及需要注意的问题,并将其与轮廓连接生成STL文件的方法进行比较,说明了所提方法优势及可行性。方法先对CT序列图像进行阈值分割、区域增长等图像预处理,然后用MC算法得到用三角片组成的几何模型,最后根据文件格式标准,生成STL文件,其中着重讨论了STL文件中三角面片顶点顺序如何确定这一问题。     

基于医学图像序列面绘制的骨组织快速成型方法

针对人工骨组织快速成型中轮廓线轨迹生成复杂、分层效率低的问题,提出了一种简化三角片模型分层过程的方法。应用移动立方块（MC）算法对医学图像序列进行面绘制重建,根据重建过程的顺序对三角片集合分组,然后采用对边追踪的方法计算切平面与其对应三角片数组的交点轮廓线数据。简化后的分层效率相对于三角网格文件（STL）模型分层平均提高了4.65%。实验结果表明,所提方法可以直接从人体骨组织医学图像序列生成可供3D打印的轮廓线数据,从而实现骨组织的快速成型。     

基于二维轮廓序列的膝关节三维重建

提出了一个基于二维轮廓序列的四面体网格生成方法,用于医学图像三维几何模型重构.该方法首先对各选定的断层图像提取目标轮廓并做分支匹配等处理,然后生成各轮廓内部平面域的三角网格,最后在相邻断层之间根据三角网格连接四面体单元.该方法被应用于人体膝关节虚拟手术系统的三维几何建模,得到的膝部股骨模型包含494个节点和2 046个四面体单元,膝部脂肪模型包含2 854个节点和14011个四面体单元,这些模型被成功地应用于膝关节手术仿真,从而证明了该三维模型重建方法的可行性和有效性.     

基于NURBS曲线的STL模型截面轮廓平滑处理技术研究

由于STL文件格式的容易生成且不需要复杂的CAD系统支持，已经发展成为快速成型制造领域数据交换的准标准；但是STL模型的精度与其数据文件的尺寸之间存在严重的矛盾，为了在较小的STL文件下得到较高的轮廓精度，本文提出了一种基于三次NURBS曲线STL模型截面轮廓的重构技术，通过设置直线阈值和转角阈值的方法，分段拟合STL截面轮廓数据，使其截面轮廓具有更好的平滑度和更高的精度。     

Machine path generation for the SIS process

Because of the differences in the nature of rapid prototyping (RP) processes, there is no standard machine path code for them. Each RP process, based on its characteristics and requirements, uses the standard CAD file format to extract the required data for the process. Selective inhibition of sintering (SIS) is a new layered fabrication method based on powder sintering. Like other RP systems, SIS needs a specialized machine path generator to create an appropriate machine path file. Machine path (i.e., boundary path and hatch path) should produce the printing pattern that enables the SIS machine user to easily remove the fabricated part from the surrounding material. A new machine path algorithm, which generates appropriate boundary and hatch paths for the SIS process is proposed. The new machine path generator provides the ability to process CAD models of any size and complexity, the ability for machine path verification before sending the file to the SIS machine, and the ability to fix the possible STL files disconnection errors. In this system, very large STL files can be processed in a relatively small computer memory. The system has been successfully tested on STL files as large as 200 MB. The generated path files have been used in fabrication of several parts by the SIS process.     

Integrated reverse engineering and rapid prototyping

Reverse engineering is a methodology for constructing CAD models of physical parts by digitizing an existing part, creating a computer model and then using it to manufacture the component. When a digitized part is to be manufactured by means of rapid prototyping machines such as stereolithography apparatus (SLA) and selective laser sintering equipments (SLS), etc., it is not necessary to construct the CAD model of a digitized part. This will be described by the proposed novel method which can construct STL file (the de facto file format for rapid prototyping machines) directly from digitized part data. Further more, the STL file can even be constructed in a way that significant data reduction can be achieved at the users' discretion.     

Error-based segmentation of cloud data for direct rapid prototyping

This paper proposes an error-based segmentation approach for direct rapid prototyping (RP) of random cloud data. The objective is to fully integrate reverse engineering and RP for rapid product development. By constructing an intermediate point-based curve model (IPCM), a layer-based RP model is directly generated from the cloud data and served as the input to the RP machine for fabrication. In this process, neither a surface model nor an STL file is generated. This is accomplished via three steps. First, the cloud data is adaptively subdivided into a set of regions according to a given subdivision error, and the data in each region is compressed by keeping the feature points (FPs) within the user-defined shape tolerance using a digital image based reduction method. Second, based on the FPs of each region, an IPCM is constructed, and RP layer contours are then directly extracted from the models. Finally, the RP layer contours are faired with a discrete curvature based fairing method and subsequently closed to generate the final layer-based RP model. This RP model can be directly submitted to the RP machine for prototype manufacturing. Two case studies are presented to illustrate the efficacy of the approach.     

基于工业CT切片数据的CAD模型重构

为了使工业CT（computed tomography）扫描获得的切片数据能够被CAD软件直接处理并进行模型重构，在得到切片轮廓点坐标数据的基础上，提出了用IGES（Initial Graphics Exchange Specification）格式作为边缘轮廓坐标数据和CAD软件交换标准的方法，并采用VC^＋＋设计了IGES格式转换软件。实现了IGES格式文件的生成，并用逆向软件Imageware重构出三维CAD模型，取得了较好的效果。     

基于2维流形的STL曲面网格重建算法

STL（stereo lithography）作为3D扫描数据和快速原型制造事实上的标准,其广泛应用于娱乐、制造业和Internet等领域.但随着3D模型越来越复杂,数据量越来越庞大,从STL文件难以快速获得完整拓扑关系以及其存在大量冗余信息的缺点,制约着STL网格模型的进一步优化处理与应用.为此,需要针对STL网格模型进行网格重建.本文针对2维流形的STL三角形曲面网格模型,提出了一种快速的网格重建方法.主要利用删除在重建过程中达到饱和的顶点,以便减少需要比对的顶点数,并结合STL文件数据的相关性来提高顶点搜索与比较的效率.对于非封闭的曲面网格,本文算法在提高曲面网格重建效率的同时,还能有效地提取曲面网格模型的边界信息.另外,重建的曲面网格数据文件大大地减少了存储空间,有效地去除了冗余数据.实验结果表明本文算法的高效性及鲁棒性.     

A grid generator for 3-D explosion simulations using the staircase boundary approach in Cartesian coordinates based on STL models

In this paper, an automatic grid generator based on STL models is proposed. The staircase boundary treatment is implemented to handle irregular geometries and the computation domain is discretized using a regular Cartesian grid. Using the grid generator, staircase grids that are suitable for fast and accurate finite difference analysis could be generated. Employing the slicing algorithm in RP technologies [1], the STL models are sliced with a set of parallel planes to generate 2D slices after the STL files obtained from a CAD system undergo topology reconstruction. To decrease the staircase error (increase accuracy) and enhance working efficiency, the cross-section at the middle of the layer is taken to represent the cross-section of whole layer. The scan line filling technique of computer graphics [2] is used to achieve grid generation after slicing. Finally, we demonstrate an application of the introduced method to generate staircase grids, which allows successful FDM simulation in the field of explosion. The example shows that the automatic grid generator based on STL models is fast and gives simulation results that are in agreement with practical observations.     

基于压缩体素模型的快速成型直接切片算法*

基于STL文件模型的间接分层切片算法存在缺乏拓扑信息, 面误差诊断修复困难等问题,结合压缩表示形式的体素模型特点,提出了一种新的基于压缩体素模型直接切片分层优化算法.该算法利用Dexel模型记录的Dexel射线和物体交点所表达的物体表面点位置,通过设定分层平面厚度等参数获得截面轮廓数据,生成可直接适用于快速成型系统的共用层接口CLI文件.该算法在虚拟油泥造型系统中得到了应用,实验结果表明算法稳定、分层效果良好.     

基于CAD模型的直接快速成型软件

目前国内普遍使用的基于STL模型的快速成型软件在精确度和灵活性上都存在着很多缺陷，在一定程度上影响了快速成型技术在生产中所起的作用，以三维几何造型系统GEMS6．0为平台，实现了一个基于CAD模型的直接快速成型软件，可进行分层、填充、构造支撑体以及生成CLI文件等，针对其中的若干问题给出了相应的算法和应用实例。     

Adaptive tool-path generation of rapid prototyping for complex product models

Rapid prototyping (RP) provides an effective method for model verification and product development collaboration. A challenging research issue in RP is how to shorten the build time and improve the surface accuracy especially for complex product models. In this paper, systematic adaptive algorithms and strategies have been developed to address the challenge. A slicing algorithm has been first developed for directly slicing a Computer-Aided Design (CAD) model as a number of RP layers. Closed Non-Uniform Rational B-Spline (NURBS) curves have been introduced to represent the contours of the layers to maintain the surface accuracy of the CAD model. Based on it, a mixed and adaptive tool-path generation algorithm, which is aimed to optimize both the surface quality and fabrication efficiency in RP, has been then developed. The algorithm can generate contour tool-paths for the boundary of each RP sliced layer to reduce the surface errors of the model, and zigzag tool-paths for the internal area of the layer to speed up fabrication. In addition, based on developed build time analysis mathematical models, adaptive strategies have been devised to generate variable speeds for contour tool-paths to address the geometric characteristics in each layer to reduce build time, and to identify the best slope degree of zigzag tool-paths to further minimize the build time. In the end, case studies of complex product models have been used to validate and showcase the performance of the developed algorithms in terms of processing effectiveness and surface accuracy.     

基于HC-CFCN模型的肝脏CT图像分割

在计算机断层扫描(CT)图像中肝脏与相邻器官灰度值近似,且不同患者的肝脏轮廓存在差异性,导致肝脏CT图像的精确分割成为医学图像处理中的难题之一。为实现肝脏CT图像的自动分割,构建一种层间上下文级联式的全卷积神经网络模型HC-CFCN。利用第1级网络实现肝脏轮廓的粗略分割,并将其分割结果与原始CT图像、肝脏能量图共同作为第2级网络的输入,优化分割结果。在LiTS数据集上的实验结果表明,与U-Net、FCN+3DCRF和V-Net模型相比,HC-CFCN模型的分割精度较高。     

激光直接制造和再制造中的三维CAD模型直接分层技术

在分析SolidWorks软件平台下CAD模型数据的内部表达方法以及拓扑信息和几何信息提取方法的基础上,研究三维CAD模型直接分层技术.对SolidWorks进行二次开发,调用SolidWorks应用程序接口函数中的曲面一曲面求交函数对CAD模型曲面与分层平面求交,得到的交线首尾相连形成轮廓轨迹;同时研究了光栅填充扫描算法及程序实现.为实现切片数据的通用化,设计了记录切片数据的文件格式,用直线、圆弧或圆描述分层轮廓.对上述的直接分层不仅进行了软件模拟,还用于直接制造.制作的试件与STL间接分层试件比较结果表明,采用直接分层的试件的精度和表面质量优于STL间接分层.     

以STL为接口的CAD/CAE集成应用

提出了以STL为接口实现CAD/CAE系统的多对多集成方案,通过对原始STL的分析,检查,纠正以及整合处理,实现了基于STL的三维实体网格自动剖分,生成了可被CAE系统接受的有限元网格数据,实例表明,文中方法简单实用,效果良好。     

Automatic recognition of features from freeform surface CAD models

This paper reports the design and implementation of a system for automatic recognition of features from freeform surface CAD models of sheet metal parts represented in STL format. The developed methodology has three major steps viz. STL model preprocessing, Region segmentation and automated Feature recognition. The input CAD model is preprocessed to get a healed and topology enriched STL model. A new hybrid region segmentation algorithm based on both edge- and region-based approaches has been developed to segment the preprocessed STL model into meaningful regions. Geometrical properties of facets, edges and vertices such as gauss and mean curvature at vertices, orientations of facet normals, shape structure of triangles, dihedral edge angle (angle between facets), etc. have been computed to identify and classify the regions. Feature on a freeform surface is defined as a set of connected meaningful regions having a particular geometry and topology which has some significance in design and manufacturing. Feature recognition rules have been formulated for recognizing a variety of protrusion and depression features such as holes, bends, darts, beads, louvres, dimples, dents, ridges/channels (blind and through) etc. occurring on automotive sheet metal panels. The developed system has been extensively tested with various industrial sheet metal parts and is found to be robust and consistent. The features data can be post processed and linked to various downstream CAD/CAM applications like automated process planning, sheet metal tool design, refinement of FEM meshes and product redesign.     

STL实体模型的拓扑重建及其缺陷修复

STL文件是CAD系统与快速成型制造系统之间进行数据交换的标准格式文件，由于CAD系统在生成STL文件时使用的转换方法不当，使得由STL文件描述的实体模型有许多缺陷，要修复缺陷建立实体模型的拓扑关系是必要的。该文首先对STL实体模型进行了拓扑关系重建，然后修复了一些实体模型中常见的缺陷。     

Extension of surface reconstruction algorithm to the global stitching and repairing of STL models

In the design of complex parts involving free-form or sculptured surfaces, the design is usually represented by a B-rep model. But in production involving rapid prototyping (RP) or solid machining, the B-rep model is often converted to the popular STL model. Due to defects such as topological and geometric errors in the B-rep model, the resulting STL model may contain gaps, overlaps, and inconsistent orientations. This paper presents the extension of a surface reconstruction algorithm to the global stitching of STL models for RP and solid machining applications. The model to be stitched may come from the digitization of physical objects by 3D laser scanners, or the triangulation of trimmed surfaces of a B-rep model. Systematic procedures have been developed for each of these two different but equally important cases. The result shows that the proposed method can robustly and effectively solve the global stitching problem for very complex STL models.