基于小波包的点云数据平滑处理

点云数据的平滑处理是逆向工程曲面建模前的必要环节.提出了基于小渡包的点云数据平滑处理方法,通过小波包变换将点云数据分解成不同层次的小波系数,而点云数据中的噪声点属于高频信息范围,所以在小波系数域内通过去除高频信息重构点云数据,一方面可以实现点云数据的平滑处理,另一方面则可以针对不同的频谱信息进行噪点云除.同时,由于点云数据的离散间隔信息不均匀,采用离散小波进行处理时在采样方式上存在误差,故根据点云数据的获得原理,采用图像域的二维信息作为索引代替原三维点云数据的x,y坐标.小渡包重构后通过索引信息将原来的坐标信息还原,从而达到点云数据平滑的目的.整个算法通过图像域、小波系数空间和三维欧式空间的相互转换完成点云数据的平滑处理.最后以具体的实例验证所提方法的有效性.     

基于UG的点云数据滤波处理

目前UG还没有点云数据滤波处理的功能,为此本文以VC++作为开发平台,调用Matlab中平滑函数,在UG上开发了点云滤波模块,并给出了应用实例,证明了该模块的可行性。     

保持特征的散乱点云数据去噪

为保证在去除点云数据噪声的同时不损失模型的细节特征,提出了一种基于特征信息的加权模糊C均值聚类去噪算法。首先,构建点云K-D树拓扑结构,根据点的r半径球邻域点统计特性去除大尺度离群噪声点。然后,利用主元分析法估算点云的曲率和法向量,根据曲率特征标识点云数据的特征区域,并采用特征加权模糊C均值聚类算法对特征区域去噪,采用加权模糊C均值聚类算法对非特征区域去噪。最后,使用双边滤波器对点云模型进行平滑。对提出的算法进行了验证实验,结果显示:去噪后点云模型的最大偏差保持在模型尺寸的0.15%以内;标准偏差保持在模型尺寸的0.03%以内。本文算法能够在有效去除不同尺度和强度的噪声的同时不损失点云模型的细节特征,去噪精度高,且对不同的噪声模型具有较强的鲁棒性。     

基于特征信息分类的三维点数据去噪

为了有效去除获取三维点云数据时的噪声,同时又不损失模型的特征信息,提出了一种基于三维点云特征信息分类的去噪算法。首先采用主成分分析法和二次曲面拟合法估算三维点云的微分几何信息;然后根据点云平均曲率的局部特征权值,将点云数据划分为特征信息较少的平坦区域和特征信息丰富的区域,针对不同特征区域分别采用邻域距离平均滤波算法和自适应双边滤波算法进行去噪滤波。实验结果表明:滤波后点云数据的最大误差为0.144 7mm,标准偏差为0.021 0mm。在不同噪声强度下,该去噪算法均能够达到较好的去噪效果,并保留点云的高频特征信息。     

非接触式激光测量点云数据预处理

作为逆向工程中形状表面数字化应用最广泛的方式,非接触式激光测量具有接触式测量无可比拟的优势。然而激光测量所得的数据点云存在明显缺陷无法直接应用于三维模型重建。本文重点讨论了激光测量所得点云数据的特点,并针对其缺陷提出一些处理方法,使之可以应用于三维模型的重建。     

反求工程中一种数据平滑的方法

逆向工程中测量获得的点云数据中存在一定的噪声点，必须对其进行数据平滑处理。在比较弦高比阈值滤波、均值滤波和中值滤波的基础上，提出一种数据平滑的方法。该方法充分利用数据点间的相关性和位置信息，对数据点和噪声点采取不同的处理方式，避免了噪声的传播。实验研究表明，该方法既能较好地滤除噪声点又有较好的保边性。     

逆向工程中基于模糊聚类的点云数据分区

点云数据分区是逆向工程中重要而又难以解决的问题。首次将模糊聚类方法应用于逆向工程中的点云数据分区,用点的位置矢量、法矢量、高斯曲率和平均曲率8维向量作为特征向量,加权距离替代欧氏距离。在实现分区的同时,可以识别区域内部点和边界附近点,便于后续曲面特征参数精确提取。实验结果证明此算法具有较强的抗噪性,并具有较高的分区效率。     

逆向工程中分层点云数据的拼合及精简

在逆向工程多视图拼合过程中,由于数据测量误差以及计算等因素的影响,数据往往会产生分层的问题,这将对曲面重建工作造成很大困难。针对这一问题,文章提出了一种新的方法,对多个视图重合部分有分层现象处的点云进行二次拼合并精简,从而有效减小误差,为后续的数据分割和曲面拟合做好准备。     

基于逆向工程的点云数据预处理技术研究

激光测量机获得的行扫描数据,含有部分噪声点和大量的冗余数据,不利于曲面的重构,针对LSH600激光测量机的测点数据,提出利用孤立点统计排异法剔除粗大误差,采用最大允许偏差法进行点云数据的精简。实例表明,该数据预处理算法简单实用,对行扫描数据有较好的消噪和简化效果。     

应用改进迭代最近点方法的点云数据配准

提出了基于点云边界特征点的改进迭代最近点(ICP)方法来提高逆向工程中点云数据配准的效率和精度.首先,提出了基于点云边界特征点的初始配准方法.对点云最小包围盒进行三维空间划分,建立空间网格模型;运用边界种子网格识别及生长算法,从点云边界提取特征点,运用奇异值矩阵分解法(SVD)求出点云的变换矩阵,得到初始配准结果.然后,提出了改进的ICP精确配准方法.对点云对应点赋予权重,剔除权重大于阈值的点,通过对目标函数引入M-估计(M-estimation),剔除异常点.最后,在初始配准的基础上,运用改进的ICP方法精确配准.对经典ICP方法和改进ICF方法做对比实验,结果显示,改进方法的配准效率提高了70％以上,误差减小到0.02％.实验表明,本文方法大幅提高了点云配准的效率和精度.     

基于扩展高斯球的点云数据与CAD模型配准

针对点云数据和三维CAD模型坐标配准过程中初始对应位置无法定量确定及最临近点计算效率问题,提出基于扩展高斯球的快速模板匹配算法,通过点云数据扩展高斯球空间与笛卡尔空间的相互转换,使外特征配准算法中测量点云与三维CAD模型初始位置的确定得到定量解决.为提高配准算法中点云数据与三维CAD模型最临近点的计算效率,采用三维CAD模型引导点云对测量数据与三维CAD模型进行桥接,从而将测量点与自由曲面的数值迭代计算问题转化为三维空间中最临近点的空间搜索,使算法的效率得到提升,实例验证了算法的有效性.     

基于照片的实体建模方法的现状及展望

基于照片的实体建模方法指通过摄像、图像处理获得物体三维模型的方法。该方法源于计算机视觉,是一种方便快捷的三维建模方法。对基于照片的实体建模基础理论与方法进行了论述,并对目前国内外的研究动态进行了分析,提出了进一步研究的方向。     

基于热平衡分析法的喷射器引射系数计算与分析

采用以喷射器出口混合气体中工作蒸气处于过热状态为基础的热平衡分析法计算引射系数,由于热平衡分析时,既按照喷嘴前后工作蒸气的膨胀与扩压器前后混合气体的压缩之热平衡关系,又考虑到喷射器吸气口状态与喷嘴出口状态之间的温度差影响,以及气体混合过程热值的变化情况,包含喷射器内各状态间的全部热力变化关系都集中反映在计算式的推导内。故这种方法不仅可获得接近实际运行状态的喷射器出口混合气体温度,还提高了计算结果与实际运行情况的相符程度。     

基于反求工程的个性化残肢重建方法的研究

为克服传统石膏复型方法在假肢接受腔制作中存在的缺点,提出了反求残肢骨骼和皮肤软组织数字化模型。通过CT扫描获得残肢内外组织的二维图像,利用图像处理技术及反求工程技术实现了残肢骨骼和皮肤软组织三维模型的重建。三维重建的模型与原实物高度相似并再现了残肢的内外组织结构,此外,该方法为将来残肢接受腔的设计提供了科学的依据。     

基于主动轮廓模型的序列图像分割与重建

基于断层测量的表面重建是逆向工程中的重要研究方向,而图像分割是其中的关键性技术。针对这一问题,提出了一种基于Greedy算法的B样条主动轮廓模型,可快速稳定地实现目标图像的分割,适用于基于断层图像的分割与表面重建处理。通过对磁共振序列断层图像的分割与表面重建验证了该方法的有效性。     

ANALYSIS AND RESEARCH OF COMPUTER-AIDED MODEL OF HIP JOINT BASED ON REVERSE ENGINEERING

Former research work about the modeling of hip joint focus on the upper segment of femoral, and assumes the acetabulum cup is sphere concave, and the acetabulum prostheses is semisphere. A method of acquiring the point data on the surface of the hipbone using the reverse engineering technology is presented. After analyzing the acetabulum surface fitting error, a rotation ellipsoid CAD model is applied to fit the acetabulum surface, and then optimization technique is used to find the geometric parameters of the model. The fitting error between the sphere and rotation ellipsoid is compared and gets the result that the fitting error of rotation ellipsoid is smaller than sphere, and the rotation ellipsoid can describe the shape of the acetabulum better.     

A methodology for smoothing of point cloud data based on anisotropic heat conduction theory

It is necessary to smooth point cloud data in reverse engineering or the inspection of free-form surfaces because noisy points will have a negative influence on the post-processing of this data. The big problem in smoothing point cloud data is how to solve the dilemma between removing noisy points and keeping feature boundary information, whilst controlling the diffusiveness of noisy points. In this paper, the theory of anisotropic heat conduction is adopted to establish a mathematical model of point cloud data smoothing. The point cloud data can be considered as a temperature field with an adiabatic boundary. So the heat is only conducted inside the temperature filed and has no effect on the outer side. For point cloud data, it means that the smoothing is only on the local area, which makes a good balance between deleting noisy points and keeping feature boundary information. The method has been implemented by using two cases for practical application, and the result proves its efficiency.     

基于表面法矢的散乱数据分割与几何特征提取

使用区域增长法进行散乱数据的区域分割以及二次曲面几何特征的提取.算法首先自动地布置种子区域,然后通过迭代/拟合这一循环过程来实现区域的增长.为了提高算法的可靠性,提出基于散乱数据表面法矢的二次曲面直接拟合方法.该方法根据特定类型二次曲面的几何性质,应用线性最小二乘技术求解其几何参数,从根本上解决以往二次曲面拟合算法因几何参数初值的设置精度不高而导致的求解效率低,进而失败的问题.试验结果表明提出的算法实现简单,且稳定可靠.     

CUTTING TEMPERATURE MODELING BASED ON NON-UNIFORM HEAT INTENSITY AND PARTITION RATIO

ABSTRACT

The understanding of temperature distribution along the tool-chip interface is important for machining process planning and tool design. Among many temperature modeling studies, uniform heat partition ratio and/or uniform heat intensity along the interface are frequently assumed. This assumption is not true in actual machining and can lead to ill-estimated results at the presence of sticking and sliding. This paper presents a new analytical cutting temperature modeling approach that considers the combined effect of the primary and the secondary heat sources and solves the temperature rise along the tool-chip interface based on the non-uniform heat partition ratio and non-uniform heat intensity along the interface. For the chip side, the effect of the primary shear zone is modeled as a uniform moving oblique band heat source, while that of the secondary shear zone is modeled as a non-uniform moving band heat source within a semi-infinite medium. For the tool side, the effect of the secondary heat source is modeled as a non-uniform static rectangular heat source within a semi-infinite medium; and the primary heat source affects the temperature distribution on the tool side indirectly by affecting the heat partition ratio along the interface. Imaginary heat sources are considered as a result of the adiabatic boundary condition involved along the tool-chip interface and of the insulated boundary conditions along both the chip back side and the tool flank face. The temperature matching condition along the tool-chip interface leads to the solution of distributed heat partition ratio by solving a set of linear equations. The proposed model is verified based on the published experimental data of the conventional turning process and it shows both satisfactory accuracy and improved match.     

CUTTING TEMPERATURE MODELING BASED ON NON-UNIFORM HEAT INTENSITY AND PARTITION RATIO

The understanding of temperature distribution along the tool-chip interface is important for machining process planning and tool design. Among many temperature modeling studies, uniform heat partition ratio and/or uniform heat intensity along the interface are frequently assumed. This assumption is not true in actual machining and can lead to ill-estimated results at the presence of sticking and sliding. This paper presents a new analytical cutting temperature modeling approach that considers the combined effect of the primary and the secondary heat sources and solves the temperature rise along the tool-chip interface based on the non-uniform heat partition ratio and non-uniform heat intensity along the interface. For the chip side, the effect of the primary shear zone is modeled as a uniform moving oblique band heat source, while that of the secondary shear zone is modeled as a non-uniform moving band heat source within a semi-infinite medium. For the tool side, the effect of the secondary heat source is modeled as a non-uniform static rectangular heat source within a semi-infinite medium; and the primary heat source affects the temperature distribution on the tool side indirectly by affecting the heat partition ratio along the interface. Imaginary heat sources are considered as a result of the adiabatic boundary condition involved along the tool-chip interface and of the insulated boundary conditions along both the chip back side and the tool flank face. The temperature matching condition along the tool-chip interface leads to the solution of distributed heat partition ratio by solving a set of linear equations. The proposed model is verified based on the published experimental data of the conventional turning process and it shows both satisfactory accuracy and improved match.