Yao-Chen Tsai

VIRTUAL 3D PLANNING OF PELVIC FRACTURE REDUCTION AND IMPLANT PLACEMENT

In pelvic surgery, CT images are commonly used for diagnosis and surgical planning. The images are often displayed and manipulated in a 2D environment, which is insufficient owing to the complex geometry of a pelvic structure. Moreover, a pre-bend of the reconstruction plates is necessary, but difficult to complete, because of the lack of appropriate templates. In this study, an integrated preoperative planning system is developed to provide 3D models and physical templates so that the surgeon can simulate and observe his planning and prepare better-fitted curved plates before surgery. The proposed method can be divided into the following four stages: 3D display, bone segmentation, bone reduction, and implant placement. As for the designed curved plates, they can be fabricated as templates by means of rapid prototyping technology. Several examples, including artificial bones, real CT images and real pelvic surgery, have been presented to demonstrate the feasibility of the proposed method.

Automatic reconstruction of B-spline surfaces with constrained boundaries

The aim of this study is to present an automatic surface reconstruction method that can take practical restrictions on scanned points into consideration and efficiently and reliably output a group of G^1 surfaces. The proposed method is mainly composed of three phases: quadrangle frame generation, point and curve networks planning, and surface patches reconstruction. In the first phase, the original triangle mesh is reduced and converted into a quadrangle mesh, the edges of which serve as the frame of the surfaces. In the second phase, the boundary data of the surfaces are prepared. These include a network of serial points, frame curves and surface normals which are also expressed as curves. In the final phase, surface initialization, harmonization mapping and surface warping are presented to yield the desired surfaces. The main advantage of the proposed method is that it can relax the pre-processing of a scanned triangle mesh, and hence, increase the efficiency and quality of the surface reconstruction. Several examples of various types of air bags are presented to demonstrate the feasibility of the proposed method.

Recognition of virtual loops on 3D CAD models based on the B-rep model

Loops are critical elements in boundary representation (B-rep) models because they link all edges corresponding to a face. Loops can be used in feature recognition for identifying depressions or protrusions. In real 3D CAD models, however, features typically lie across multiple faces, which is beyond the data structure of current B-rep models. This study presents a virtual loop concept to account for all loop types used in CAD models, and develops algorithms for recognizing them. In accordance with the complexity of the recognition algorithm, this study defines three types of loop: single, virtual, and multivirtual. A single loop is the current loop recorded in the B-rep model. A virtual loop lies across faces that are at least G1 continuous. Finally, a multivirtual loop lies across faces that are either G0 or G1 continuous. The proposed loop structure provides a more complete data structure for recognizing various types of features in feature-recognition modules. Several realistic CAD models are presented to confirm the feasibility of the proposed loop-recognition and feature-recognition methods.

Recognition of depression and protrusion features on B-rep models based on virtual loops

ABSTRACTLoops are vital elements in B-rep models and are used to describe the boundary contours of faces. A loop is only defined on a single face, which does not reflect real situations in which features mostly lie across multiple faces. The objective of this study is to detect virtual loops across multiple faces and subsequently use them for recognizing depression and protrusion features in computer-aided design models. Three loop types are defined: single, virtual, and multivirtual loops; virtual and multivirtual loops lie across multiple faces with different boundary conditions across faces. The data of the detected loops are then used to develop a feature recognition algorithm for identifying various depression and protrusion types, ranging from simple circular holes on a face to complex irregular pockets on multiple faces with fillets. This paper provides a detailed description of the proposed algorithm and presents several examples to illustrate its feasibility.

Recognition and decomposition of rib features in thin-shell plastic parts for finite element analysis

ABSTRACTFeature recognition has been extensively studied for applications in computer-aided design (CAD), computer-aided manufacturing, and computer-aided process planning. However, its application in finite element analysis (FEA) is limited. The primary motivation for applying feature recognition in FEA is to facilitate high-quality mesh generation, which requires not only identifying the geometric entities of a feature but also decomposing and computing the region data. This study proposes an approach based on feature recognition to recognize ribs from a CAD model and decompose them into regions that can be meshed with hexahedral or prismatic meshes. Ribs in a CAD model are generally connected to form a complex structure. A rib recognition algorithm to recognize all faces belonging to a rib structure is proposed, and a rib decomposition algorithm was also developed to decompose the rib structure into rib segment and transition regions. After both types of region were defined, the data to decompose each ...

Boss recognition algorithm for application to finite element analysis

ABSTRACTIn finite element analysis (FEA), computer-aided design (CAD) models must be converted into solid meshes so that the solver can perform the desired analysis and simulation. Hexahedral and prism meshes are better than tetrahedral meshes, but are inherently more complex and difficult to generate. The purpose of this study was to propose an approach based on feature recognition for generating better quality solid meshes for FEA applications. Particularly, this study focused on the development of a boss recognition algorithm, the output of boss data for meshing, and the development of a process for automatic boss meshing. The proposed boss recognition method contains three parts: the preliminary functions, the data base, and the boss recognition. The first two parts provide a framework that can be used for other types of feature recognition. The core of the boss recognition is the tube recognition, which involves five main steps of validation. The output data of a boss includes the rib, tube, and hole...

Recognition and decomposition of bosses on CAD models for hexahedral meshes generation in CAE analysis

Solid meshes are used in finite element analysis to represent the object geometry for simulation and analysis. Hexahedral and prism meshes are inherently more preferable to tetrahedral meshes, but are difficult to generate automatically. The purpose of this study was to develop an approach for meshing using feature recognition to decompose the target feature. Particularly, a boss recognition algorithm was proposed and the data for automatic meshing was established. A detailed description of the proposed algorithm was described, and several examples were presented to illustrate its feasibility.

Small blend suppression from B-rep models in computer-aided engineering analysis

Abstract In mechanical design, small blending is a common operation used to improve the strength and aesthetics of the workpiece. In blending operations, certain smooth faces are added to the boundary representation (B-rep) model to smooth sharp edges and vertices. These faces are termed ‘blend faces.’ However, they may affect the quality of analysis; these faces should be meshed with tiny meshes, which may result in an increase in computational time and reduced accuracy of simulation results. Hence, small blend suppression is an approach for improving the quality of generated meshes. Blend suppression is a technique for reverting a blended computer-aided design model to its original unblended status. This study proposes a blend suppression algorithm for removing blend faces in computer-aided engineering (CAE) analysis. The proposed algorithm operates as follows: (1) edge blend faces and vertex blend faces are grouped individually, (2) new geometric data are computed, (3) new elements are added to the data structure of the B-rep model, (4) all data related to blend faces are deleted, and (5) all topological data in the B-rep model are updated. Several examples are presented in this paper to demonstrate the feasibility of the proposed method for blend suppression as well as its advantage in improving the quality of meshes in CAE analysis.

Automatic recognition and decomposition of rib features in thin-shell parts for mold flow analysis

In mold flow analysis, for greater accuracy, convergence, and application specificity, it is beneficial to apply a hybrid combination of pyramidal, prismatic, and hexahedral meshes to replace traditional tetrahedral meshes. This, however, requires the recognition of specific features from the computer-aided design (CAD) model and decomposition into regions, so that high-quality meshes can be applied. The purpose of this study is to propose a rib recognition and decomposition algorithm for detecting all the types of ribs on thin-shell plastic parts, and decomposing them into rib regions. Conventional rib recognition methods emphasizing only the recognition of the constructing faces are not suitable for high-quality mesh generation, because they cannot provide sufficient information for region decomposition. We employ a concept of parallel pairs of edges to identify regions of regular shape on a rib structure. This not only recognizes the constructing faces, but also evaluates the constructing edges on each regular region. With the edge data on regular regions, the transition regions are recognized and the associated face and edge data are evaluated. This study focuses on the development of a feasible algorithm that can be used to deal with industrial CAD models with various kinds of geometric conditions. Several CAD examples are presented to demonstrate the feasibility of the proposed algorithm.

Automatic recognition and suppression of holes on mold bases for finite element applications

As the required accuracy of mold flow analysis has recently increased, it has become necessary to consider all components of a mold base and to generate solid meshes for all of them. If holes on all mold components are kept in mesh generation, the mesh size near each hole must be adapted to maintain the shape accurately. This, however, not only increases the effort required to edit the meshes, but also increases the total amount of meshes tremendously. The objective of this study is to present a method for the recognition and suppression of holes on mold bases. A preliminary computation of the data required for hole recognition is described, which includes entity grouping, fillet recognition, and loop recognition. An algorithm for recognizing holes on all mold components can thus be implemented, which includes the recognition of all face compositions on various kinds of “ladder” structures that involve several holes connected to each other. All hole structures can then be suppressed automatically. The recognition and suppression of holes on two mold bases are presented to demonstrate the feasibility of the proposed technique.