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Über dieses Buch

This book is concerned with problems and solutions associated with the exchange of data between different computer aided design, engineering and manufacturing (CAx) systems. After an analysis of the current problems a new strategy consisting of a test methodology, check software and tools for the improvement of the data exchange process are discussed. The particular problems associated with the transfer of curve and surface data are expanded upon and new methods to overcome them presented. With all these tools a system-specific adaption of neutral files is made possible. Thus the integration of several incompatible CAx systems within devel- opment and production processes can be effectively improved. In order to exclude incorrect data a new methodology for neutral file processor tests has been worked out. Finally, the benefits resulting from this new strategy are shown by the example of data transfer not only between CAx systems but also between consecutive production processes.



CAD*I Project Overview

During the past 25 years computers have been introduced in industry to perform technical tasks such as drafting, design, process planning, data acquisition, process control and quality assurance. Computer-based solutions, however, are still in most cases single isolated devices within a manufacturing plant
Raymond J. Goult, Peter A. Sherar

1. CAD/CAM Data Exchange in the Industrial Environment — Methodology and Tools

In the last years the entire process of industrial development has become more and more computer-aided. In almost all areas and stages of product development, process planning and product manufacturing itself, CAx systems are in constant use to support the work in terms of productivity, quality and reliability. Due to the diversity of CAx applications, the systems used are generally well specified and appropriate to the relevant application. This results in the implementation of special sets of entities and structures in the systems databases.
H. Scheder, D. Trippner

2. Exchange of Curve and Surface Data

In the previous chapter the industrial and commercial necessity to exchange product definition data has been described, as well as some of the problems encountered with using current neutral file specifications for this purpose. Many of the least tractable problems are encountered when attempting to communicate data concerned with the description of complex curves and surfaces in 3D space. The basic source of all these problems is the wide variety of definition formats used for curve and surface data by different CAD/CAM systems. In the past analytic, parametric and procedural forms have all been used for the definitions of curves and surfaces in CAD/CAM systems but parametric forms are by far the most popular. In some cases it is possible to perform, theoretically at least, an exact mathematical conversion between two different forms of representation, algorithms for this purpose are described in Chapter 4. In far too many cases no exact conversion is possible and the best that can be achieved is to produce some form of ‘good’ approximation to the original data which is acceptable to the receiving system. Some useful approximation algorithms which preserve as much as possible of the geometric intent of the original design are described in Chapters 5 and 6.
R. J. Goult

3. Neutral File Interface Requirements

Ideally a neutral file interface for CAD data exchange should make it possible to communicate 100% of the data between any two systems without loss of accuracy or information. The effectiveness of this transfer depends not only upon the neutral file specification but also upon the quality of the pre- and post-processors provided. In the context of sculptured surface data fundamental incompatibilities between the forms of surface representation used by different systems mean that no matter how good the neutral file specification and the processors the objective of 100% interchange between all systems will never be attainable. The practical limit to the amount of data which can be fully exchanged is the intersection of the capabilities of the two systems, the neutral file specification and of the processors concerned. The design of the neutral file is important in that it defines the ultimate scope of the exchange and strongly influences the quality of pre- and post-processors. This chapter will describe some of the important considerations in defining a neutral file and describe how these have influenced the design of the CAD*I neutral file and the ISO draft proposal STEP. The emphasis is on those entities in the neutral file concerned with the communication of parametric curve and surface data.
R. J. Goult, M. A. Lachance

4. Conversions Between Representations

A wide variety of CAD systems are in use today. The mathematical forms of the curve and surface entities within such systems show a similar diversity. This can lead to two types of problem when considering the transfer of such data between two dissimilar systems. First, there may be a basic incompatibilty in the curve and surface description employed, e.g the sending system might use rational polynomials while the receiving system may only support non-rational forms of a specified degree. Second, the two systems may employ curve and surface entities based on the same underlying building blocks, in particular parametric polynomials of a given degree, but differ in the actual representation used. The first of these problems requires effective approximation of the sending system form to suit that required by the receiving one and is expanded on in chapters 5 and 6. The second problem can in principle be solved by an exact conversion between the two representations involved. We shall be concerned with the solution to the second problem here.
P. A. Sherar

5. Degree Reduction Approximations

The majority of CAD systems provide some form of parametric curve and surface representation but the precise form of this representation varies considerably. Some systems use simple parametric polynomial curves and surfaces, others provide some form of rational parametric representation. Even amongst those systems using parametric polynomials they differ in the maximum degree allowed and the form of representation, which may be explicit polynomial base, B-spline or Bézier. Provided the degrees are the same, exact conversions between these different representations are possible as detailed in Chapter 4 but approximation problems arise when transferring data from a rational or other non-polynomial based system to a polynomial system. The generalised Chebyshev technique described in this chapter addresses the degree reduction problem, the orthogonal polynomial methods described in Chapter 6 can be used for all other types of approximation problem.
M. A. Lachance, P. A. Sherar, R. J. Goult

6. More General Curve and Surface Approximations

The Chebyshev economisation technique described in chapter 5 is restricted in its application to degree reduction of parametric polynomial curves and surfaces. In practice rational, trigonometric or procedurally defined parametric surfaces are frequently used in CAD systems, and some more general technique of polynomial approximation is needed for data exchange applications. In general the problem is to find a polynomial approximation of specified degree, according to the capabilities of the receiving system, which gives an acceptable fit to the original data. One well known criterion for ‘goodness of fit’ is the least squares criterion. This can be combined with appropriate orthogonal polynomials, in order to avoid the numerical instability problems commonly found in solving the normal equations associated with the simple least squares method.
R. J. Goult


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