考虑定向能量沉积工艺特点的减材加工余量优化

针对定向能量沉积(DED)复杂曲面零件余量不均及台阶效应明显等问题,提出了考虑DED复杂曲面成形特点的中面动态配准技术,以优化后处理减材加工余量。首先根据扫描路径构建曲面零件最小外包络的截面线,用于在机检测毛坯点云;然后提取理论模型中面与测量毛坯中面点云,引入不同区域配准精度要求,利用动态加权迭代最近邻点算法实现加工余量优化,并通过两简单案例对算法可行性进行初步验证;最后,以离心叶轮叶片为复杂案例,分析所提余量优化方法的准确性,并与基于遗传算法的多配准精度要求的加工余量优化技术进行对比。结果表明,所提余量优化算法的配准精度及计算效率高,可用于DED制造复杂曲面类零件的余量快速优化。     

余量约束下的毛坯测量数据与CAD模型配准技术研究

为在加工前剔除变形的不合格毛坯或在加工时避免由于定位不够精确而出现余量不足,需在余量约束要求下将毛坯测量数据与CAD模型进行配准.针对此配准问题,提出“二步法”求解:第一步暂不考虑余量约束,用ICP算法进行配准,检查结果是否满足余量要求,满足则退出计算,否则,进行第二步,即考虑余量约束对ICP结果作进一步配准.实验结果表明,该方法能够有效实现毛坯测量数据与CAD模型在余量约束下的配准.     

基于点云与CAD模型配准对齐的毛坯加工定位技术

锻铸件毛坯在加工制造过程中,采用传统的划线定位方式存在废品率高、数控化率低等问题,为了免除划线工艺,实现毛坯加工余量的数字化检测和加工定位,提出了一套基于扫描点云与CAD模型配准对齐的毛坯加工定位技术,该技术首先建立毛坯加工余量优化分配的数学模型,然后采用遗传算法以双层迭代寻优的方式实现了模型的求解。最后,利用扫描点云和CAD模型配准对齐过程的位姿调整矩阵对CAD模型生成的刀位点进行后置变换处理,实现了毛坯的免划线加工。通过实际的加工实验,验证了所提出的加工定位技术。     

叶片零件毛坯余量分布优化问题研究

为了提高叶片零件的数控加工质量及效率,保证叶片毛坯各处加工余量的均匀性至关重要。从几何建模、曲面定位方法及相应的曲面匹配算法等方面,提出一种适用于叶片零件毛坯余量分布优化的一般方法。首先采用传统的六点定位法对工件粗定位,然后再通过对测量数据进行欧氏变换,实现测点与理论曲面的匹配,由此达到均化叶片零件加工余量的目的。     

序列图像约束的点云初始配准方法

针对现有算法难以精确配准重叠区域较小的点云,提出一种结合二维序列图像的点云初始配准方法。基于移动式三维测量方法获取实物表面灰度图像及三维数据,根据透视投影原理对相机在点云局部坐标系中的位置进行定位,获取点云到对应相机坐标系的变换矩阵,并以灰度图像的特征点及其匹配点作为匹配对,通过重建序列图像对相机外参数予以全局优化,根据初始配准公式实现点云初始配准。试验结果表明,该算法可正确配准重叠区域较小的点云,且能显著提高配准过程的稳健性及配准效率。     

基于点云几何信息改进的自动配准方法CSCD

针对工业流水线上激光扫描工件获得的点云数据的配准问题,提出了一种基于点云数据几何特征改进的点云自动配准新算法。新算法首先根据点云数据中法向量的变化规律选取特征点,作为初始的匹配点集;然后运用一种根据点对间距离约束优化的随机抽样一致(RANSAC)算法对数据初始匹配;并运用k-d tree加速改进的最近点迭代(ICP)算法进行精确匹配;并运用四元数法求得配准参数。分别对提出的新算法、PCA改进算法和经典ICP算法进行了实验,并对实验结果进行了对比。对比结果表明新算法能够实现配准,并显著提高了配准的速度和精度,表明了新算法的有效性,对实际应用具有一定的现实意义。     

基于点云几何信息改进的自动配准方法

针对工业流水线上激光扫描工件获得的点云数据的配准问题,提出了一种基于点云数据几何特征改进的点云自动配准新算法。新算法首先根据点云数据中法向量的变化规律选取特征点,作为初始的匹配点集;然后运用一种根据点对间距离约束优化的随机抽样一致(RANSAC)算法对数据初始匹配;并运用k-d tree加速改进的最近点迭代(ICP)算法进行精确匹配;并运用四元数法求得配准参数。分别对提出的新算法、PCA改进算法和经典ICP算法进行了实验,并对实验结果进行了对比。对比结果表明新算法能够实现配准,并显著提高了配准的速度和精度,表明了新算法的有效性,对实际应用具有一定的现实意义。     

工程装备逆向建模中的点云配准技术研究

通过应用一种基于区域划分的Super4PCS算法用于点云全局配准,根据源点云与目标点云的重叠比例对点云进行区域划分,计算出能够实现最大比例重叠的区域;最后求解出各区域配准的旋转矩阵和平移向量,并将其用于全局配准,同时采用了基于重启加速ADMM改进的稀疏ICP算法用于点云局部配准。实验结果表明,所用算法在两配准点云低重叠比例时,能够有效地提高配准精度与配准效率,同时避免了陷入局部最优解的问题。     

基于改进ICP算法的损伤零部件精确配准方法

针对损伤零部件点云模型与原始模型利用传统迭代最近点算法配准出现较大误差的问题,提出一种基于该算法的改进算法,以实现两模型间对应点的准确获取,从而实现损伤零部件点云与原始模型的准确配准。考虑到损伤零部件表面尺寸和形貌发生变化,该算法将对应点曲率约束与对应点间的距离约束结合,并设定曲率和距离阈值实现损伤点云的自动剔除,保证了配准点云对应点的准确性及配准的快速性。最后,运用MATLAB实现了算法编写,并通过损伤模具的配准验证了该算法的有效性。     

旋转台多视场扫描点云的自动配准方法

三维扫描需要将不同视角下获取的点云数据进行配准,并统一到全局坐标系。针对旋转台扫描得到的点云数据提出了一种自动配准方法,其基本思路是先用NDT算法作粗配准,将待配准的两片点云旋转平移到较好的初始位置,接着用ICP算法作精配准。经实验证明:该方法配准速度快、精度高。     

基于设计公差约束的螺旋桨配准问题

针对螺旋桨加工过程中叶面和叶背加工余量分布不均匀的问题,提出了一种公差约束条件下螺旋桨毛坯与设计曲面快速配准的方法,用以保证螺旋桨叶面和叶背加工余量均匀。在螺旋桨设计过程中,分析了半径、纵斜、轴向位置和桨叶夹角对设计曲面空间位置的影响,确定了基于设计公差变动范围的螺旋桨形貌变化约束条件。建立了约束条件下螺旋桨曲面配准数学模型,并基于模拟退火算法快速求解得到全局最优解。螺旋桨毛坯曲面与设计曲面的匹配验证结果表明,该算法能实现螺旋桨毛坯曲面和设计曲面的快速匹配。     

A Reverse Engineering Approach to Generating Interference-Free Tool Paths in Three-Axis Machining from Scanned Data of Physical Models

An NC tool path is usually generated by sweeping parametric surfaces of a CAD model. In modern design, freeform or sculptured surfaces are increasingly popular for representing complex geometry for aesthetic or functional purposes. Traditionally, a prototype is realised by machining the workpiece using the NC codes generated from a CAD model. The machined part can then be compared with the CAD model by measurement using a coordinate measuring machine. In this paper, a reverse engineering approach to generating interference-free tool paths in three-axis machining from the scanned data from physical models is presented. There are two steps in this procedure. First, a physical model is scanned by a 3D digitiser, and multiple data sets of the complex model are obtained. A surface registration algorithm is proposed to align and integrate those data to construct a complete 3D data set. A shortest-distance method is used to determine the connecting sequence of the neighbouring points between two adjacent scan lines, such that the scanned data are converted into triangular polygons. Tool paths are then generated from the tessellated surfaces. Using the Z-map method, interference-free cutter-location data are calculated, relative to the vertex, edges, and planes of the triangles. The algorithms for tool-path generation are usually different for cutters of different geometries. Some algorithms found in the literature require complex numerical calculations and are time consuming. In this paper, an efficient algorithm is developed to calculate interference-free cutter-location data by easy geometric reasoning without complex computation. This robust method is suitable for generally used cutters such as ball, flat, and filleted end mills, and the time taken to obtain full tool paths of compound surfaces is short. Some real applications are presented to validate the proposed approach.     

An unconstrained approach to blank localization with allowance assurance for machining complex parts

In machining large complex parts with critical stock allowance, even small deviations in the blank parts or slight inadequacy in the fixturing may result in local shortage of material (i.e., insufficient stock allowance). This paper presents an optimal localization algorithm that aligns the measured points from a blank part with the nominal model to assure the weakest allowance area with as much material to be cut as possible. Instead of exploiting extra constraints to force the allowance at each point larger than a specific value, which is a popular strategy for allowance assurance in the previous localization algorithms, we formulate the blank localization problem as an unconstrained max-min problem. To deal with the unsmoothness exhibiting in the max-min objective function, a method based on the entropy optimization principle is adopted to convert the non-differential objective function to an unconstrained differential one, which can be efficiently solved using the conventional Quasi-Newton algorithms. The unconstrained optimization result finally gives rise to localization with the maximum allowance margin. For the blank parts that the material shortage is inevitable, the method can still efficiently achieve reasonable localization results, which confine the material shortage to a least extent. The proposed method is easy to be implemented and works well for both sparse sample points and dense-scanned points. Case studies included justify the superiority of the proposed scheme over the existing non-linear constrained optimization solutions.     

Automated feature-based registration for reverse engineering of human models

In order to reconstruct a full 3D human model in reverse engineering (RE), a 3D scanner needs to be placed arbitrarily around the target model to capture all part of the scanned surface. Then, acquired multiple scans must be registered and merged since each scanned data set taken from different position is just given in its own local co-ordinate system. The goal of the registration is to create a single model by aligning all individual scans. It usually consists of two sub-steps: rough and fine registration. The fine registration process can only be performed after an initial position is approximated through the rough registration. Hence an automated rough registration process is crucial to realize a completely automatic RE system. In this paper an automated rough registration method for aligning multiple scans of complex human face is presented. The proposed method automatically aligns the meshes of different scans with the information of features that are extracted from the estimated principal curvatures of triangular meshes of the human face. Then the roughly aligned scanned data sets are further precisely enhanced with a fine registration step with the recently popular Iterative Closest Point (ICP) algorithm. Some typical examples are presented and discussed to validate the proposed system.     

损伤零件点云模型配准的ICP算法

针对损伤零件的传统点云模型配准过程中存在着运算效率低与损伤部位损伤量确定误差较大等问题,提出一种基于原始ICP算法的改进算法。考虑模型因为损伤而引起的特征与表面形貌的改变,利用法矢夹角进行点云数据的精简,保留模型主要特征,再利用对应点的曲率约束与距离约束设定阈值剔除损伤区域点云,保证对应点之间快速准确地配准。最后,运用Matlab实现改进算法,并利用损伤的轮机叶片点云数据的配准验证该算法的有效性。     

Generating NC tool paths from random scanned data using point-based models

This paper presents a new approach for the generation of NC tool paths from random scanned data. Instead of using smooth or triangulated surfaces reconstructed from raw data, which is usually a time-consuming reverse engineering approach, the point-based surfel models computed by a GPU (graphics processing unit) are used to generate NC tool paths. The tool-path generation is highly efficient and still maintains the advantage of having accurate and smooth machining result. The word “surfel” itself is the combination of the two words “surface” and “element”. It is originally applied to the rendering of scanned data. In this paper, the point-based model is created using an elliptical Gaussian re-sampling filter that is based on a signal re-sampling algorithm. Since the input scanned data is of discrete and random nature, the warping process is utilized to transform the input data into a continuous surface and then re-sample the continuous surface by using GPU. Because the re-sampled data can accurately represent the original surface, tool paths can be generated based on the point data set. For cutting tools with various sizes, adaptive re-sampling schemes are employed to generate sufficient sampled points for the generation of accurate and smooth tool-paths.     

A unified localization approach for machining allowance optimization of complex curved surfaces

The goal of workpiece localization is of interest to find the optimal Euclidean transformation that aligns the sampled points to the nominal CAD model to ensure sufficient stock allowance during the machining process. In this paper, a unified localization technique is developed for sculptured surface machining. This technique concerns an alignment process to satisfy a user-defined set of constraints for some specific surfaces where the machining allowance is preferentially guaranteed. The mathematical model of the constrained optimization alignment is firstly established, and is efficiently solved by a combination of the multipliers method and the BFGS algorithm to handle the large number of constraints in allowance optimization. To efficiently calculate the Euclidean oriented distance, a novel approach, which combines the robust arithmetic for multivariate Bernstein-form polynomials and Bezier surface segmentation algorithm, is presented based on recursive quadtree decomposition. Two typical sculptured surfaces are used to test the developed algorithm and comparisons between the proposed algorithm and the existing algorithms are given. Experiment results show that the proposed method is appropriate and feasible to distribute the stock allowance for proper sculptured surface machining.     

基于轮廓约束点的B样条曲面拟合算法

提出了一种面向截面测量数据的B样条曲面拟合算法。首先对原始数据点列进行降噪处理,然后遴选出曲率优势点,并将其作为初始的轮廓约束点,得到插值于约束点的初始曲线。再在需改善拟合精度的区域增加约束点,直至获得满足精度要求的B样条曲线。最后以约束点数目最多的曲线为准,在其余的曲线上增加差额数目的约束点,并进行平均弦长参数化,构造出B样条曲线簇,最终获得B样条拟合曲面。仿真实验结果表明,该方法可显著压缩曲面模型的控制顶点数目,具有较高的曲面重构效率。