Using polyhedral models to automatically sketch idealized geometry for structural analysis

Simplification of polyhedral models, which may incorporate large numbers of faces and nodes, is often required to reduce their amount of data, to allow their efficient manipulation, and to speed up computation. Such a simplification process must be adapted to the use of the resulting polyhedral model. Several applications require simplified shapes which have the same topology as the original model (e.g. reverse engineering, medical applications, etc.). Nevertheless, in the fields of structural analysis and computer visualization, for example, several adaptations and idealizations of the initial geometry are often necessary. To this end, within this paper a new approach is proposed to simplify an initial manifold or non-manifold polyhedral model with respect to bounded errors specified by the user, or set up, for example, from a preliminary F.E. analysis. The topological changes which may occur during a simplification because of the bounded error (or tolerance) values specified are performed using specific curvature and topological criteria and operators. Moreover, topological changes, whether or not they kept the manifold of the object, are managed simultaneously with the geometric operations of the simplification process

Discrete curvature approximations and segmentation of polyhedral surfaces

The segmentation of digitized data to divide a free form surface into patches is one of the key steps required to perform a reverse engineering process of an object. To this end, discrete curvature approximations are introduced as the basis of a segmentation process that lead to a decomposition of digitized data into areas that will help the construction of parametric surface patches. The approach proposed relies on the use of a polyhedral representation of the object built from the digitized data input. Then, it is shown how noise reduction, edge swapping techniques and adapted remeshing schemes can participate to different preparation phases to provide a geometry that highlights useful characteristics for the segmentation process. The segmentation process is performed with various approximations of discrete curvatures evaluated on the polyhedron produced during the preparation phases. The segmentation process proposed involves two phases: the identification of characteristic polygonal lines and the identification of polyhedral areas useful for a patch construction process. Discrete curvature criteria are adapted to each phase and the concept of invariant evaluation of curvatures is introduced to generate criteria that are constant over equivalent meshes. A description of the segmentation procedure is provided together with examples of results for free form object surfaces

Outils de base pour l'extraction de caractéristiques de surfaces numérisées

La construction d'une surface paramétrique à partir de données issues de la numérisation d'un objet réel est une étape longue et fastidieuse pour l'utilisateur. La difficulté principale de ce processus réside dans la décomposition de la surface de l'objet en carreaux comportant, de préférence, quatre côtés. La segmentation la plus naturelle pour l'utilisateur est une décomposition supportée par des lignes caractéristiques de la surface (arêtes vives, lignes de changement de courbure,...). En effet, l'utilisation de ces propriétés permet d'obtenir une décomposition représentative des caractéristiques géométriques de la surface. Les algorithmes développés utilisent un modèle polyédrique. La technique proposée repose sur des approximations de courbures faites sur les entités (sommets et arêtes) du polyèdre pour extraire, dans un premier temps, les "arêtes vives" et, par la suite, une première segmentation de la surface. Les "arêtes vives" sont identifiées par un algorithme basé sur des critères appelés invariants de courbures discrètes. Ces critères sont calculés pour chaque sommet et arête du polyèdre. Ils sont à rapprocher des approximations de courbures discrètes classiques, mais ils définissent la "forme" d'une surface au voisinage d'un sommet ou d'une arête. Le principe d'extraction consiste alors à sélectionner un ensemble ordonné d'arêtes vérifiant certaines propriétés géométriques. Chaque partition (ensemble connecté de faces) est une zone de la surface ayant une courbure locale plus ou moins constante. Les partitions sont identifiées grâce à une technique de propagation de fronts. Les faces adjacentes au front sont insérées dans celui-ci si les approximations de courbures calculées sur leurs sommets vérifient les critères de l'algorithme. Les "arêtes vives" préalablement extraites permettent de prendre en compte des discontinuités de courbures qui constituent des contraintes complémentaires pour la propagation de chaque front.In many areas of industry, it is desirable to create geometric models of existing objects for which no such model is available. Starting from a polyhedral representation on the digitized points measured on the object, this approach proposes a first phase of a segmentation process from a polyhedral surface prior to the generation of a NURBS model. Its main idea is to find a curve network, which divides the surfaces by means of a series of "feature polygonal lines". The advantage of this approach is that the patch structure will reflect the user's concept of the structure of the surface. Noise reduction and smoothing processes take place before the segmentation process to produce adequate input data for it. The approach is based on different approximations of curvature measurements of the surface to extract, at first, the sharp edges and secondly areas forming a first segmentation of the surface. This approach is interactive and allows the user to adapt threshold values to the various areas of the object. The sharp edges are found by an algorithm, which uses criteria based on discrete curvature invariant. These criteria are based on the approximation of curvatures (mean, gaussian, absolute), which strictly describe the local form of the surface around an edge or a vertex. According to a user threshold, the result of this algorithm is a set of list of edges. Each partition of the segmentation is an area of the surface with an almost constant curvature. Each of them is found with a frontal method. A front is initiated from a face, which satisfies a discrete curvature criterion. Faces adjacent to this front are admitted in it if the value of a curvature approximation for each of their vertices verifies the same discrete curvature criterion. Sharp edges defined beforehand express curvature or tangency discontinuities, which form complementary constraints for the front propagation algorithm

Using polyhedral models to automatically sketch idealized geometry for structural analysis

Simplification of polyhedral models, which may incorporate large numbers of faces and nodes, is often required to reduce their amount of data, to allow their efficient manipulation, and to speed up computation. Such a simplification process must be adapted to the use of the resulting polyhedral model. Several applications require simplified shapes which have the same topology as the original model (e.g. reverse engineering, medical applications, etc.). Nevertheless, in the fields of structural analysis and computer visualization, for example, several adaptations and idealizations of the initial geometry are often necessary. To this end, within this paper a new approach is proposed to simplify an initial manifold or non-manifold polyhedral model with respect to bounded errors specified by the user, or set up, for example, from a preliminary F.E. analysis. The topological changes which may occur during a simplification because of the bounded error (or tolerance) values specified are performed using specific curvature and topological criteria and operators. Moreover, topological changes, whether or not they kept the manifold of the object, are managed simultaneously with the geometric operations of the simplification process

REVERSE ENGINEERING IN MODELING OF AIRCRAFT PROPELLER BLADE - FIRST STEP TO PRODUCT OPTIMIZATION

ABSTRACT: Propeller aircrafts have had many ups and downs throughout their use in the aviation history. Due to the current economic recession and price hikes in fuels, propeller aircrafts may yet again be a choice for aerial transport and has thus re-emerged as an active area for research. On modern propeller aircrafts old aluminum propellers are being replaced with fiber reinforced composite propellers. However, owing to their reliability, strength, and integrity, aluminum propellers are still used in military aircrafts. One of the challenges that engineers of these aircraft-type have had to deal with is the non-availability of engineering drawings of these propellers. It is practically impossible to carry out any study, research or modification on such propellers in the absence of correct CAD data. This article proposes a methodology wherein a CAD model of a C-130 aircraft propeller blade can be constructed using reverse engineering techniques. Such a model would help in future aerodynamic as well as structural analyses which includes investigation on structural integrity and the fluid dynamics characteristics of propeller blades. Different steps involved in this process are discussed; starting from laser scanning to obtain the cloud of points data and subsequently generating a CAD model in a commercial CAD software. The model is then imported into an analysis software where quality surface meshes are generated using tetrahedral elements. The purpose is to prepare a meshed model for future computational analysis including CFD (Computational Fluid Dynamics) and FE (Finite Element) analysis. ABSTRAK: Pesawat bebaling mempunyai tempoh pasang surutnya sepanjang penggunaanya dalam sejarah penerbangan. Kini disebabkan oleh kemelesetan ekonomi dan kenaikan harga minyak, pesawat bebaling mungkin akan merupakan pengangkutan udara pilihan dan seterusnya muncul semula sebagai ruangan aktif penyelidikan. Pada pesawat bebaling moden, bebaling aluminium yang lama digantikan dengan bebaling komposit yang diperkuatkan gentian. Namun, disebabkan oleh keupayaan, kekuatan dan integritinya, bebaling aluminium masih diguna pakai bagi pesawat tentera. Salah satu cabaran jurutera bagi pesawat jenis ini, adalah apabila berhadapan dengan tidak adanya lukisan kejuruteraan bagi bebaling ini. Agak mustahil untuk menjalankan sebarang ujian, kajian atau modifkasi terhadap bebaling jenis ini kerana tidak adanya data yang tepat daripada Reka Bentuk Berbantukan Komputer (CAD). Artikel ini mencadangkan metodologi di mana model Reka Bentuk Berbantukan Komputer (CAD) bagi kipas bebaling pesawat C-130 boleh dibina menggunakan teknik kejuruteraan balikan. Model sebegini akan bakal dapat membantu bidang aerodinamik di masa hadapan. Begitu juga dengan analisis struktur yang melibatkan penyelidikan terhadap integriti struktur dan ciri-ciri dinamik bendalir kipas bebaling.  Beberapa proses berbeza diperbincangkan; bermula dengan imbasan laser untuk memperolehi poin kumpulan data dan seterusnya menjanakan model Reka Bentuk Berbantukan Komputer (CAD) dengan menggunakan pengaturcara komersil. Model kemudiannya diimport ke dalam pengaturcara analisis di mana jejaring permukaan dijanakan dengan menggunakan unsur tetrahedron. Ini bertujuan bagi menyediakan model jejaring untuk analisis berkomputer di masa hadapan, termasuklah analisis CFD (Gambar Rajah Aliran Kawalan) dan FE (Unsur Terhingga). KEYWORDS: reverse engineering; cloud data; laser scanning; CAD modeling; propeller blad

Isotropic Surface Remeshing

International audienceThis paper proposes a new method for isotropic remeshing of tri- angulated surface meshes. Given a triangulated surface mesh to be resampled and a user-specified density function defined over it, we first distribute the desired number of samples by generalizing error diffusion, commonly used in image halftoning, to work directly on mesh triangles and feature edges. We then use the resulting sam- pling as an initial configuration for building a weighted centroidal Voronoi tessellation in a conformal parameter space, where the specified density function is used for weighting. We finally create the mesh by lifting the corresponding constrained Delaunay trian- gulation from parameter space. A precise control over the sampling is obtained through a flexible design of the density function, the latter being possibly low-pass filtered to obtain a smoother grada- tion. We demonstrate the versatility of our approach through vari- ous remeshing examples

Automatic Mesh Repair and Optimization for Quality Mesh Generation

Ph.DDOCTOR OF PHILOSOPH

Aplicabilidade da digitalização 3D em ambientes de engenharia simultânea /

Dissertação (Mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico