Automatic segmentation of digitized data for reverse engineering applications

Reverse engineering is the process of developing a Computer Aided Design (CAD)model and a manufacturing database for an existing part. This process is used in CAD modeling of part prototypes, in designing molds, and in automated inspection of parts with complex surfaces. The work reported in this paper is on the automatic segmentation of 3-Dimensional (3-D) digitized data captured by a laser scanner or a Coordinate Measuring Machine (CMM) for reverse engineering applications. Automatic surface segmentation of digitized data is achieved using a combination of region and edge based approaches. It is assumed that the part surface contains planar as well as curved surfaces that are embedded in a base surface. The part surface should be visible to a single scanning probe (21/2D object). Neural network algorithms are developed for surface segmentation and edge detection. A back propagation network is used to segment part surfaces into surface primitives which are homogenous in their intrinsic differential geometric properties. The method is based on the computation of Gaussian and mean curvatures of the surface. They are obtained by locally approximating the object surface using quadratic polynomials. The Gaussian and mean curvatures are used as input to the neural network which outputs an initial region-based segmentation in the form of a curvature sign map. An edge based segmentation is also performed using the partial derivatives of depth values. Here, the output of the Laplacian operator and the unit surface normal are computed and used as input to a Self-Organized Mapping (SOM) network. This network is used to find the edge points on the digitized data. The combination of the region based and the edge based approaches, segment the data into primitive surface regions. The uniqueness of our approach is in automatic calculation of the threshold level for segmentation, and on the adaptability of the method to various noise levels in the digitized data. The developed algorithms and sample results are described in the paper     

Automatic segmentation and approximation of digitized data for reverse engineering

Reverse engineering is the process of developing a computer-aided design (CAD) model and a manufacturing database for an existing object. This process is used in CAD modelling of part prototypes, in designing moulds and in automated inspection of parts with a complex surface. This paper reports on the automatic segmentation and approximation of three-dimensional digitized points for reverse engineering. Based on an innovation that uses the properties of a non-uniform rational B-spline (NURBS) or B-spline and makes ordered digitized points be control points directly to construct a NURBS or B-spline surface, which takes less computation time than traditional algorithms in calculating surface normals and curvatures at digitized points, an algorithm was developed for automatic segmentation and NURBS surfaces fitting for digitized points.     

Sculptured surface reconstruction from CMM measurement data by a software iterative approach

The reverse engineering approach is being widely applied in designing and manufacturing of sculptured surfaces. The first step in the reverse engineering process is collection of data lying on the surface. When using a coordinate measuring machine (CMM) with a touch trigger probe for digitizing data points, the probe radius must be compensated according to normal vectors. Previous works were addressed on the iterative measurement procedure to reduce the error caused by the compensation. The major problem of iterative methods is time consumption in digitization. This paper proposes a new architecture based on look-up table that keeps the estimated normal vectors of the measurement data to refine the data points digitized by CMM. The digitized data are first fitted into several NURBS curves by interpolation. The skinning method is then applied to interpolate these curves. Thus a smooth NURBS surface can be constructed and more accurate normal vectors of the measurement points can be obtained to compensate for the probe radius. By using this software iterative technique, we need only one digitization procedure. Therefore, much time can be saved and the efficiency of reverse engineering can be improved.     

三坐标扫描数据的规律与快速图形化

扫描测量功能是三坐标测量机一项重要功能，扫描测量方法效率高，可以反映出零件表面自由形状的细部特征，因此，该方法多用于具有复杂型面零件的测量和逆向工程。由于不同型号三坐标测量机的扫描数据格式都不相同，如何识别不同的扫描数据格式，并将其转换为CAD曲线或曲面，以便进一步进行数据分析或建立三维模型，是亟待解决的问题。针对此问题自主开发的CAD接口，可自动识别三种以上数据文件格式，且效率很高。     

基于三视图的实体重建技术研究

基于三视图的三维实体重建技术是根据已有二维视图中的几何信息和拓扑信息,生成相应的三维实体模型。以AutoCAD绘制的三视图及DXF格式的文件信息为基础,提出并实现了一种由三视图重建三维实体的算法。通过对三视图的规则处理,由计算机自动实现实体重建,对三视图分别进行平移、旋转、拉伸操作,然后作相应的布尔运算即可反求出该视图所对应的三维实体。     

基于3D数模的三坐标测量机曲面检测

计算机辅助设计／制造技术的发展，对曲面工件的检测提出了更高的要求，这一任务的完成通常依靠三坐标测量机。本文结合ZCRMDT三坐标测量机测量软件的研发，从数据转换、对齐、测尖补偿、理论值捕获等四个关键方面，对基于3D数模的三坐标测量机曲面检测技术进行了深入剖析。同时对三坐标测量机测量软件现状和发展趋势进行了探讨。     

具有复杂内腔和外形的斗齿快速逆向设计

为了获得某型挖掘机斗齿点云逆向造型的实体化模型和CAD工程图样,研究了斗齿点云快速实体化的方法.采用基于“曲面重构-实体化-工程图”的逆向设计途径,提出把对精度要求高的内腔和销孔部位及对精度要求一般的斗齿外形分别用Imageware及Geomagic Studio软件完成曲面重构和基于Pro/E软件完成曲面合并和实体化的逆向策略,介绍了斗齿点云的数据分割、曲面重构与合并、实体化以及其工程图获取过程中的主要过程和方法.研究结果表明：对于型面数量多而复杂的斗齿,曲面逆向工具的选取及点云的数据分割方法是否得当是决定曲面和实体模型重建效率和成败的关键,通过恰当选择逆向策略和工具,合理简化点云的分割、曲面重构与合并以及实体模型的建立等,可显著提高逆向设计效率.     

基于锥束扫描三维CT的逆求技术研究

对于内外结构复杂的实物原型，难以采用传统的测量方法如三坐标测量机(CMM)、激光扫描等手段获取内外表面完整的点云数据以逆求实物原型的CAD模型。基于此种问题，介绍了ICT(Industrial Computed Tomography)辅助逆求工程技术。通过对三维ICT图像进行对相应的图像、图形处理，获取实物内外表面轮廓矢量数据，并将其输入Pro/E软件，逆求出实物原型的CAD模型。     

Flexible inspection system based on a vision guided coordinate measuring machine

The objective of this paper is to develop a flexible three-dimensional inspection system for sculptured surfaces by employing 'Coordinate Measuring Machine (CMM), CAD database and vision system technology. New surface identification and rough localization methods are suggested using a concept called ‘Z-layers’ and a vision guided system. The Z-layers are generated from a CAD database which classifies the three-dimensional models. The three-dimensional models are reconstructed using a vision system that identifies the model and roughly identifies the orientation and location of the object implementing Z-layers. Then inspection planning and fine localization methods are mapped out which are based on the CAD database. The experimental results indicate that the developed inspection system, with the proposed algorithm, can be implemented in modern industry to reduce measurement errors and increase productivity.     

贵州少数民族文化遗产的逆向工程应用研究

以少数民族文化遗产的保护和开发方法研究为目的,从实物测量、三维CAD模型、曲面模型评价体系、CAD模型重建等方面详细阐述了逆向工程技术在文化遗产保护方面的应用.选用有代表性的贵州少数民族文物--布依族酒壶、水壶以及木制傩戏面具作为研究对象,采用Hiscanner 激光扫描测头对其进行数据采集,扫描得到的点云数据可刻录到光盘上作为文物数据永久保存;更进一步地,采用目前流行的逆向工程软件Geomagic studio对点云数据进行数据精简、数据滤波、数据修补、提取特征线等处理后得到NURBS曲面;为保证曲面模型重建的精度,在曲面重建模型后对其误差评价,通过控制误差的方式指导曲面模型重建,使重建的文物曲面模型能真实完整地反映原状;最后,在功能强大的三维造型软件Solidworks中导入重构的NURBS曲面进行曲面缝合、放样等,将其生成实体CAD模型,并对模型的局部特征进行适当的再设计以实现文物实体模型的优化.对该模型进行快速成型制造可得到文物复制品或成品模具,即实现逆向工程技术在文化遗产保护中的应用研究.逆向工程技术应用于少数民族文化遗产文物保护是一个全新的课题,该项技术的应用能进一步推动文化遗产保护措施的发展,也能为类似课题的研究提供借鉴.     

特征识别中实体虚实性的判别方法

 提出了一种根据环之间的相互关系从机械工程图二维视图中自动判断实体虚实性的方法。     

Improved neural network model for reverse engineering

While conventional engineering transforms engineering concepts into real parts, in reverse engineering real parts are transformed into engineering models. The construction of a surface from three-dimensional (3D) measuring data points is an important problem in reverse engineering. This paper presents a reconstruction method for the sculptured surfaces from the 3D measuring data points. The surface reconstruction scheme is presented based on a neural network. The reconstruction of the existing surfaces is realized by training the network. A series of measuring points from existing sculptured surfaces is used as a training set. Once the neural network has been trained, it serves as a geometric model to generate all the points that are needed. However, the learning rate for the neural network is relatively slow, and the learning accuracy is often unacceptably low. In this paper, to improve the performance of the neural network, a pre-processor is proposed before the input layer. The pre-processor maps the input into the larger space by generating a set of linearly independent values. The effect of the pre-processor is to increase modelling accuracy, and reduce learning time. Based on this method, experimental results are given to show that the reconstructed surfaces are faithful to the original data points. The proposed scheme is useful for regular or irregular digitized data.     

Experimental study of the effect of digitizing parameters on digitizing uncertainty with a CMM

Planning of an automated digitization or inspection process takes time and requires experience. The accuracy of digitization and inspection depends directly on the planning of digitization strategies. This paper uses the fractional factorial experimentation approach to investigate the relationship between digitizing (inspection) uncertainty and digitizing (inspection) parameters with a coordinate-measuring machine (CMM). The parameters include travel speeds, pitch, probe angles (part orientations), probe sizes and feature sizes. Robust digitization (inspection) strategies are identified given the part geometry, dimensions and accuracy. Finally, future research directions are highlighted.     

An Experimental Data Acquisition System for Ultrasound Imaging

An experimental digital data acquisition system for computerized ultrasound imaging studies is developed. The system requirements for the digital reconstruction of tomographic images from the envelope of ultrasound signals of several megahertz are described. To effectively digitize ultrasound signals, a new digital averager is implemented. Several consecutive ultrasound signals are averaged on a real-time base to enhance the signal-to-noise ratio, to reduce the volume of the data, and to achieve a match of data flow rates between the ultrasound signals and the usual computer input devices. The positions of the ultrasound transducers and the target are controlled by a microprocessor controller. These positions are sensed and also digitized by the averager. The averager transfers the digitized data, both the ultrasound signals and position signals, to a general-purpose computer for further data processing. Experimental data on reconstructing the cross section of a simple target from the ultrasound signals acquired by this system will be presented as an illustration.     

基于顺向工程与逆向工程结合的创新产品设计流程规划

产品逆向工程是在产品设计中广泛应用的一种非常重要的设计方法,文中立足于在传统正向设计思路中引入逆向工程概念,合理有效的应用目前的Cax(计算机辅助设计系统包括CAD,CAM,CAID等)系统.该方法可加快产品的造型设计速度,大大缩短新产品的开发周期.以一个汽车设计实例验证了该方法.     

三坐标测量机智能技术

介绍了智能坐标测量机的关键技术,包括CAD数据的识别和转化、测量策略的制定、面向测量工件的误差补偿和精度优化和防碰撞算法等。并指出智能化必将是坐标测量机发展的重要方向。     

三目视觉测头在自由曲面测量中的应用

自由曲面的逆向工程是先进制造技术中的一个关键技术．开发了一种线结构光三目视觉测头，这种包括3个CCD摄像机在内的测量系统具有许多优点，避免了双目视觉中存在的匹配现象．它不但具有高效率、高精度和高可靠性的特点，而且适用于复杂自由曲面的在线测量．针对工作原理以及相关问题，对其进行了详细的讨论，其中包括系统优化、摄像机标定、采样策略、特征提取、图像匹配和数据重构．实验证明三目视觉测头在精度和可靠性方面都优于双目测头．首次研制测头的精度达到了0．02mm．     

面向三维重建的三视图数据结构

提出了一种面向三维重建的三视图数据结构,这是一种采用分层的双向链表结构,它可以完整地表达三视图。     

逆向工程中自由曲面的自适应数字化算法研究

分析比较了对自由曲面自适应测量的几种方法。在有界曲率的平面曲线的有界性的基础上，提出圆弧外延法，即后续测量点根据已测前三点求得的圆弧曲率沿切线方向或等曲率外延来进行预测，并根据测点矢量来估测保证数字化几何信息的充分性。这样就可利用待测曲面的几何(曲率)特性，使得测点的分布与曲面的曲率保持一致。最后给出了对这一算法的仿真结果。     

Automatic coarse registration of three-dimensional surfaces by information theoretic selection of salient points

We describe a new method to register surface data measured by optical three-dimensional (3-D) sensors from various views of an object. With our method, complete 3-D models of objects can be generated without user interaction. Circumferential acquisition of 3-D objects is done by taking several views from different directions. To generate a complete 3-D-model, the views must be aligned with each other. This process is called registration and is commonly done interactively by searching for so-called corresponding points in the different views and by use of these points to calculate the appropriate rotation and translation. Our approach is based on automatically finding points that are eye catching or salient compared with other surface points. We derive a quantitative measure of point salience and a feature definition for free-form surfaces by introducing a concept to measure pragmatic information. Experiments confirm that our salient points can be robustly located on general free-form surfaces, even if there are no corners or edges. Furthermore, the neighborhoods of the salient points are highly distinguishable from each other. This results in a large reduction of the complexity of the subsequent geometric matching. The computing time is only a few seconds. We present results from various fields of application.