A knowledge-based approach to the generation of IDEF0 models

Though a powerful tool for modelling manufacturing systems, IDEF0 is not without weaknesses. It is generally recognised that the process of IDEF0 modelling can be time-consuming and inconsistent. However, the process may be automated to improve time-effectiveness and consistency. This paper proposes a knowledgebased system for automating the process. The system, which is meant for the discrete manufacturing industry, is based on the concept of reference models. It is intended for the modellers of manufacturing systems, as an aid rather than as their replacement. A prototype of the proposed system has been developed. The paper explains the knowledge-based approach to the generation of IDEF0 models and describes the work that has been done at Gintic on the development of the prototype system.     

An apporach to automatic three-dimensional finite element mesh generation

Recently developed solid modelling systems for the design of complex physical solids using interactive computer graphics offer the exciting possibility of an integrated design/analysis system. Called geometric modellers, these systems build complex solids from primitive solids (cubes, cylinders, spheres, solid patches, etc.) and macro solids (combination of primitives)^{3, 4, 8, 16, 18, 25, 38}. To provide an effective structural analysis capability for these systems, methods must be devised to ease the burden of discretizing the solid geometry into a user controlled (usually locally graded) finite element mesh. The purpose of this paper is to describe an interactive solid mesh generation system capable of generating valid meshes of well-proportional tetrahedral finite elements for the decomposition of multiply connected solid structures. The system uses a semi-automatic node insertion procedure to locate element node points within and on the surface of a structure. An independent automatic three-dimensional triangulator then accepts these nodes as input and connects them to form a valid finite element mesh oftetrahedral elements. Although this report makes use of a modeller based on a constructive solid geometry representation (a so-called CSG modeller), the mesh generation strategy elaborated herein is completely general and makes no particular use of the CSG representation.     

A framework for CAD-based geometric reasoning for robot assembly language†

For automatic robot programming, world modelling of the robot's environment is one of the most important phases of the task planning. World modelling requires that the robot know the environment in which it operates, including the spatial configuration of all objects in the task environment. In robotic assembly planning at the task level, representation of these objects requires symbolic feature and shape identification of the objects to be assembled by the robot. In this paper, we present a framework for reasoning about objects based on their shapes and features and the representation of such objects for robotic assembly planning when the modelling is done on a CAD system. We show the importance of AI languages in the communication of constructive solid geometry (CSG) based information from modellers. Finally, we present the schematic for a formalism, based on Prolog, for expressing object properties and assembly situations.     

Finite elements and related areas of computer-aided engineering

The past fifteen years have seen an explosion in the use of numerical methods, such as finite elements and turnkey CAD systems, in engineering design centres. Rapid development is currently taking place in second generation CAD systems utilising geometric modellers and sculptured surface packages which can be linked to finite element models and other previously separate areas of computer application such a N.C. machining. The strategy of this paper is to discuss some such systems which are just emerging from the development phase at Lucas Research Centre and to show some typical applications.     

Virtual reality prototyping of bio-molecules

Modelling, sharing and transmission of three-dimensional (3D) graphics data of biomolecules are essential in many bio visualization tasks from collaborative research and education to molecular simulation and drug discovery. In the current paper, modelling and representing of bio-molecular structure for virtual and physical rapid prototyping is presented. Our aim is to devise a uniform solution for visualizing, browsing, interacting and prototyping of bio-molecules in various environments including internet, immersive virtual reality (VR), and rapid manufacturing. To do so, we use non uniform rational B-spline surfaces (NURBS) to represent protein secondary structure and surface structure. NURBS protein structures are then tessellated to form bio-molecular graphics models. Their triangular mesh representation is next extracted from their scene graph. A geometric optimization process is followed to make data compatible for their formatting in compact and consistent VR standard to support protein internet browsing, protein VR visualization, protein 3D rapid prototyping and crystal sub-surface laser engraving.     

Virtual reality prototyping of bio-molecules

Modelling, sharing and transmission of three-dimensional (3D) graphics data of biomolecules are essential in many bio visualization tasks from collaborative research and education to molecular simulation and drug discovery. In the current paper, modelling and representing of bio-molecular structure for virtual and physical rapid prototyping is presented. Our aim is to devise a uniform solution for visualizing, browsing, interacting and prototyping of bio-molecules in various environments including internet, immersive virtual reality (VR), and rapid manufacturing. To do so, we use non uniform rational B-spline surfaces (NURBS) to represent protein secondary structure and surface structure. NURBS protein structures are then tessellated to form bio-molecular graphics models. Their triangular mesh representation is next extracted from their scene graph. A geometric optimization process is followed to make data compatible for their formatting in compact and consistent VR standard to support protein internet browsing, protein VR visualization, protein 3D rapid prototyping and crystal sub-surface laser engraving.     

Feature vector: a graph-based feature recognition methodology

Recognition of machining features is a vital link for the effective integration of various modules of computer integrated manufacturing systems (CIMS). Graph-based recognition is the most researched method due to the sound mathematical background of graph theory and a graph's structural similarity with B-Rep computer-aided design modellers’ database. The method, however, is criticized for its high computational requirement of graph matching, its difficulty in building a feature template library, its ability to handle only polyhedral parts and its inability to handle interacting features. The paper reports a new edge classification scheme to extend the graph-based algorithms to handle test parts with curved faces. A unique method of representing a feature, called a feature vector, is developed. The feature vector generation heuristic results in a recognition system with polynomial time complexity for any arbitrary attributed adjacency graph. The feature vector can be generated automatically from B-Rep modellers. This helps in building incrementally a feature library as per the requirements of the specific domain. The proposed system is implemented in VC++ using an ACIS® 3D solid modelling toolkit.     

Transport networks and their use: how real can modelling get?

The context for modelling transport systems and their use is set out. Such modelling is based on representing the transport system by a network of nodes and links, and the characteristics of this representation needed to reflect the principal realities of the system are outlined. The characteristics of use of the system that need to be reflected are described. Purposes of the modelling are set out and its evolution is described, starting from the basic traffic assignment model and discussing its generalizations and extensions in the search for greater realism-first in steady-state modelling for fixed demand, and then considering variable demand and time dependence. Further progress towards appropriate realism is seen as requiring communication and cooperation between the modellers and the users of models, helped perhaps by combining the advantages of analytical modelling and microsimulation.     

A new class of transitional blended finite elements for the analysis of solid structures

Recently developed computer aided design systems for the design and modification of complex physical solids using interactive computer graphics offer the exciting possibility of an integrated design/analysis system. Called geometric modellers, these systems build complex solids from primitive solids (cubes, cylinders, spheres, etc.) and macro solids (combinations of primitives). To provide an effective finite element analysis capability for these systems, methods must be devised to ease the burden of discretizing the solid geometry into a user controlled finite element mesh. In this paper we describe a new class of transitional blended finite elements which make substantially simpler the task of finite element mesh generation and local mesh refinement. Computational experience indicates that numerical accuracy is not compromised by use of these flexible elements.     

Modelling of food transportation systems – a review

In 2002, over a million refrigerated road vehicles, 400,000 refrigerated containers and many thousands of other forms of refrigerated transport systems are used to distribute chilled and frozen foods throughout the world. All these transportation systems are expected to maintain the temperature of the food within close limits to ensure its optimum safety and high quality shelf life.Increasingly, modelling is being used to aid the design and optimisation of food refrigeration systems. Much of this effort has concentrated on the modelling of refrigeration processes that change the temperature of the food such as chilling, freezing and thawing. The purpose of a refrigerated transport system is to maintain the temperature of the food and appears to have attracted less attention from modellers. This paper reviews the work that has been carried out specifically on the modelling of food temperature, microbial growth and other parameters in the transportation of food.     

Voxelization and fabrication of freeform models

We present a volume graphics system for the generation of freeform models and synthesis of complex objects. A unique non-uniform rational B-spline (NURBS) volume representation and its voxelization algorithm are proposed. The forward difference technique is employed to speed up the voxelization process while the desired topological connectivity is preserved. For synthesis of our models and other three-dimensional objects, such as computed tomography images, voxel-based cutting tools are developed for the AutoCAD user interface. Volume rendering is applied to visualize the intermediate and final models. An isosurface is then extracted over the synthesized model and exported to a rapid prototyping machine as an STL file for fabrication. The approach taken in this system has been proven robust and efficient in modelling and fabrication of complex freeform models.     

Status and future directions for turbulence modelling

Progress in the development of a hierarchy of turbulence models for Reynolds-averaged Navier-Stokes codes used in aerodynamic applications is discussed. Although steady progress is demonstrated, transfer of the modelling technology has not kept pace with the development and demands of the CFD tools. An examination of the process of model development leads to recommendations for a better process involving close coordination between modellers, CFD developers and application engineers. In instances where the process is changed and cooperation enhanced, timely transfer is realized. A turbulence modelling information database is proposed to refine the process and open it to greater participation among modelling and CFD practitioners. The citations in this paper are not in our usual format     

Efficient explicit formulation for practical fuzzy structural analysis

This paper presents a practical approach based on High Dimensional Model Representation (HDMR) for analysing the response of structures with fuzzy parameters. The proposed methodology involves integrated finite element modelling, HDMR based response surface generation, and explicit fuzzy analysis procedures. The uncertainties in the material, geometric, loading and structural parameters are represented using fuzzy sets. To facilitate efficient computation, a HDMR based response surface generation is employed for the approximation of the fuzzy finite element response quantity.     

Subdivision-based digital geometry processing as a fundamental building block of digital manufacturing

Digital manufacturing relies on geometric modelling techniques, virtual environments, and a solid common platform that can support multiple disciplines and applications. Subdivision methods offer powerful computational representations and algorithms for digital geometry processing (DGP) and can create a common model and unified representation for geometric design, numerical analysis, and numerical control (NC) machining. Subdivision-based digital geometry processing can be a fundamental building block of digital manufacturing. In this paper, models for geometric design, finite/boundary element analysis (FEA/BEA), and NC machining are created using subdivision surfaces. Automatic mesh generation for the FEA of sheet metal structures and for the BEA of general three-dimensional structures is fulfilled through subdivision-based digital geometry processing. Models with progressive refinement for progressive transmission are created. Subdivision-based digital geometry processing for NC machining is discussed.     

A generative process planning system for parts produced by rapid prototyping

This paper presents a generative process planning system for parts produced by the rapid prototyping process (i.e. fused deposition modelling–FDM). The proposed process planning involves optimal selection of orientating the model with a proper support structure and then provides an intelligent slicing methodology, such as direct or adaptive, to minimise the built up time, keeping the geometry and cusp height errors in control. Pre- and post-slicing processes have been used to minimise the sliced data error. The Computer Aided Process Planning (CAPP) model has been arranged into five modules: orientation, support structure generation, slicing, path planning and Numerical Control (NC) program generation, and model build up. The CAPP model has been implemented in C language having a unique methodology consisting of 42 simplified steps. The CAPP model has been tested for several examples and shows satisfactory results.     

Fracture mechanics modelling of constant and variable amplitude fatigue behaviour of field corroded 7075‐T6511 aluminium

For ageing airframe structures, a critical challenge for next generation linear elastic fracture mechanics (LEFM) modelling is to predict the effect of corrosion damage on the remaining fatigue life and structural integrity of components. This effort aims to extend a previously developed LEFM modelling approach to field corroded specimens and variable amplitude loading. Iterations of LEFM modelling were performed with different initial flaw sizes and crack growth rate laws and compared to detailed experimental measurements of crack formation and small crack growth. Conservative LEFM‐based lifetime predictions of corroded components were achieved using a corrosion modified‐equivalent initial flaw size along with crack growth rates from a constant K_max‐decreasing ΔK protocol. The source of the error in each of the LEFM iterations is critiqued to identify the bounds for engineering application.     

Subdivision surface-based finish machining

Subdivision surfaces combine smooth spline surfaces and polygonal meshes together, therefore, a smooth design model and discrete machining models may be unified and subdivision surfaces may be used as a common representation for geometric design and machining. Motivated by the idea, this paper presents the study of finish machining of objects represented by subdivision surfaces with emphasis on geometric error control involved in tool-path generation. First, given a design model, chordal error is controlled during finishing model building. A chordal error-driven adaptive subdivision method is used to build finishing models with less data. Second, a surface decomposition machining strategy is used to control the cusp height error. A simple iso-slope curve tracing and surface decomposition algorithm is presented to partition the model into flat and steep regions. Contour-map tool-paths are generated in the steep regions while iso-planar tool-paths are generated in the flat regions. The gouge problem is easily handled through two-dimensional (2D) tool-path correction algorithms. The implementation results demonstrate that subdivision is capable of serving as a unified representation for both geometric modelling and machining.     

Short messages

This paper has three purposes. The first one is to explain to a general audience what is involved in retrieving a web page or performing some other complex network transaction, and what can make it slow, and why the problem of slowness is likely to get worse as networked applications become more complex. The second is to describe, to those who program networked applications, certain facts that we have learnt from modelling communication networks, notably the fact of heavy-tailed distributions in traffic, which may allow more efficient applications to be written. The third is to describe to network modellers an interesting class of problems relating to algorithm design for communication networks.     

Fractal raster tool paths for layered manufacturing of porous objects

The present work describes an approach for layered manufacturing (LM) of porous objects using an appropriate modelling scheme, a pre-processing algorithm for slicing and a raster tool path generation based on the porosity information. Initially an overall framework of modelling and data transfer that includes controlled porosity information apart from the external geometry of porous objects and its transfer for LM is presented. A novel raster path generation methodology using space-filling fractal curves for LM of porous models is presented later. Specifically, the geometry and space-filling characteristics of fractal curves are studied for application to raster tool path generation in LM. Finally, boundary-constrained raster patterns are generated based on the surface geometry. The resulting data can be translated into a machine language file that can be imported by an LM system. Case studies are presented to illustrate the efficacy of this approach.