30079_13a.pdf

CHAPTER 13

COMPUTER-AIDED DESIGN

Dr. Emory W. Zimmers, Jr., & Technical Staff

Enterprise Systems Center

Lehigh University

Bethlehem, PA

13.1 INTRODUCTION TO

COMPUTER-AIDED

DESIGN (CAD)

13.5.3 Mouse

291

13.5.4 Trackball

291

275

13.5.5 Light Pen

291

13.1.1 A Historical Perspective

of CAD

13.5.6 Digitizer

292

276

13.5.7 Scanner

293

13.1.2 The Design Process

276

13.1.3 Applying Computers to

Design

13.6

OUTPUT DEVICES

293

278

13.6.1 Electronic Displays

293

13.6.2 Hard Copy Devices

294

13.2 HARDWARE

282

13.2.1 Input/Output and Central

Processing Unit (CPU)

13.7

SOFTWARE

296

282

13.7.1 Operating Systems

296

13.7.2 Graphical User Interface

(GUI) and the X Window

System

13.3 THE COMPUTER

283

13.3.1 Computer Evolution

284

298

13.3.2 Categories of Computers

284

13.7.3 Computer Languages

299

13.3.3 Central Processing Unit

(CPU) 285

13.3.4 RISC and CISC Computers 285

13.3.5 Parallel Processing

13.8

CAD SOFTWARE

301

13.8.1 Graphics Software

301

287

13.8.2 Solid Modeling

302

13.4 MEMORYSYSTEMS

287

13.9

CAD STANDARDS AND

TRANSLATORS

13.4.1 Organizational Methods

287

309

13.4.2 Internal Memory and

Related Techniques

13.9.1 Analysis Software

311

288

13.4.3 External Memory

289

13.10 APPLICATIONSOFCAD

314

13.4.4 Magnetic Disks

289

13.10.1 Optimization

Applications

13.4.5 Magnetic Tape

290

314

13.4.6 Optical Data Storage

290

13.10.2 Virtual Prototyping

315

13.10.3 Rapid Prototyping

316

13.5 INPUTDEVICES

290

13.10.4 Computer-Aided

Manufacturing (CAM)

13.5.1 Keyboard

290

317

13.5.2 Touch Pad

291

13.1 INTRODUCTION TO CAD

Computer-aided design (CAD) uses the mathematical and graphic-processing power of the computer

to assist the engineer in the creation, modification, analysis, and display of designs. Many factors

have contributed to CAD technology becoming a necessary tool in the engineering world, such as

the computer's speed at processing complex equations and managing technical databases. CAD com-

bines the characteristics of designer and computer that are best applicable to the design process.

The combination of human creativity with computer technology provides the design efficiency

that has made CAD such a popular design tool. CAD is often thought of simply as computer-aided

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

drafting, and its use as an electronic drawing board is a powerful tool in itself. The functions of a

CAD system extend far beyond its ability to represent and manipulate graphics. Geometric mod-

eling, engineering analysis, simulation, and the communication of the design information can also

be performed using CAD.

13.1.1 A Historical Perspective of CAD

Graphical representation of data, in many ways, forms the basis of CAD. An early application of

computer graphics was used in the SAGE (Semi-Automatic Ground Environment) Air Defense Com-

mand and Control System in the 1950s. SAGE converted radar information into computer-generated

images on a cathode ray tube (CRT) display. It also used an input device, the light pen, to select

information directly from the CRT screen.

Another significant advancement in computer graphics technology occurred in 1963, when Ivan

Sutherland, in his doctoral thesis at MIT, described the SKETCHPAD system. The SKETCHPAD

system was driven by a Lincoln TX-2 computer. With SKETCHPAD, images could be created and

manipulated using the light pen. Graphical manipulations such as translation, rotation, and scaling

could all be accomplished on-screen using SKETCHPAD. Computer applications based on Suther-

land's approach have become known as interactive computer graphics (ICG). The graphical capabil-

ities of SKETCHPAD showed the potential for computerized drawing in design. The high cost of

computer hardware in the 1960s limited the use of ICG systems to large corporations, such as those

in the automotive and aerospace industries, which could justify the initial investment. With the rapid

development of computer technology, computers became more powerful, using faster processors and

greater data storage capabilities. Their physical size and cost decreased, and computers became

affordable to smaller companies and personal users. Today it is rare to find an engineering, design,

or architectural firm of any size without a working CAD system running on a personal computer or

a workstation.

13.1.2 The Design Process

Before any discussion of computer-aided design, it is necessary to understand the design process in

general. What is the series of events that leads to the beginning of a design project? How does the

engineer go about the process of designing something? How does one arrive at the conclusion that

the design has been completed? We address these questions by defining the process (Fig. 13.1) in

terms of six distinct stages:

1. Customer input and perception of need

2. Problem definition

3. Synthesis

4. Analysis and optimization

5. Evaluation

6. Final design and specification

A need is usually perceived in one of two ways. Someone must recognize either a problem in an

existing design or a customer-driven opportunity in the marketplace for a new product. In either case,

a need exists which can be addressed by modifying an existing design or developing an entirely new

design. Because the need for change may only be indicated by subtle circumstances, such as noise,

marginal performance characteristics, or deviations from quality standards, the design engineer who

identifies the need has taken a first step in correcting the problem. That step sets in motion processes

that may allow others to see the need more readily and possibly enroll them in the solution process.

Once the decision has been made to take corrective action to the need at hand, the problem must

be defined as a particular problem to be solved such that all significant parameters in the problem

are defined. These parameters often include cost limits, quality standards, size and weight character-

istics, and functional characteristics. Often, specifications may be defined by the capabilities of the

manufacturing process. Anything that will influence the engineer in choosing design features must

be included in the definition of the problem. Careful planning in this stage can lead to fewer iterations

in subsequent design stages.

Once the problem has been fully defined in this way, the designer moves on to the synthesis

stage, where knowledge and creativity can be applied to conceptualize an initial design. Teamwork

can make the design more successful and effective at this stage. That design is then subjected to

various forms of analysis, which may reveal specific problems in the initial design. The designer

then takes the analytical results and applies them in an iteration of the synthesis stage. These iterations

may continue through several cycles of synthesis and analysis until the design is optimized.

The design is then evaluated according to the parameters set forth in the problem definition. A

scale prototype is often fabricated to perform further analysis and to assess operating performance,

quality, reliability, and other criteria. If a design flaw is revealed during this stage, the design moves

back to the synthesis/analysis stages for reoptimization, and the process moves in this circular manner

until the design clears the evaluative stage and is ready for presentation.

CORRECT EXISTING DESIGN

PROBLEMS OR CUSTOMER

INPUT AND PERCEPTION

OF NEED - OPPORTUNITY

PROBLEM

DEFINITION

* SYNTHESIS

ANALYSIS AND

OPTIMIZATION

EVALUATION

FINAL DESIGN

AND

SPECIFICATION

Fig. 13.1 The general design process.

Final design and specification represents the last stage of the design process. Communicating the

design to others in such a way that its manufacture and marketing are seen as vital to the organization

is essential. When the design has been fully approved, detailed engineering drawings are produced,

complete with specifications for components, subassemblies, and the tools and fixtures required to

manufacture the product and the associated costs of production. These can then be transferred man-

ually or digitally, using CAD data, to the various departments responsible for manufacture.

In every branch of engineering, prior to the implementation of CAD, design has traditionally been

accomplished manually on the drawing board. The resulting drawing, complete with significant de-

tails, was then subjected to analysis using complex mathematical formulae and then sent back to the

drawing board with suggestions for improving the design. The same iterative procedure was followed

and, because of the manual nature of the drawing and the subsequent analysis, the whole procedure

was time-consuming and labor-intensive. CAD has allowed the designer to bypass much of the manual

drafting and analysis that was previously required, making the design process flow more smoothly

and much more efficiently.

It is helpful to understand the general product development process as a step-wise process. How-

ever, in today's engineering environment, the steps outlined above have become consolidated into a

more streamlined approach called concurrent engineering. This approach enables teams to work

concurrently by providing common ground for interrelated product development tasks. Product in-

formation can be easily communicated among all development processes: design, manufacturing,

marketing, management, and supplier networks. Concurrent engineering recognizes that fewer itera-

tions result in less time and money spent in moving from design concept to manufacture and from

manufacturing to market. The related processes of Design for Manufacturing (DFM) and Design for

Assembly (DFA) have become integral parts of the concurrent engineering approach.

Design for Manufacturing and Design for Assembly methods use cross-disciplinary input from a

variety of sources (e.g., design engineers, manufacturing engineers, suppliers, and shop-floor repre-

sentatives) to facilitate the efficient design of a product that can be manufactured, assembled, and

marketed in the shortest possible period of time. Products designed using DFM and DFA are often

simpler, cost less, and reach the marketplace in far less time than traditionally designed products.

DFM focuses on determining what materials and manufacturing techniques will result in the most

efficient use of available resources in order to integrate this information early in the design process.

The DFA methodology strives to consolidate the number of parts wherever possible, uses gravity-

assisted assembly techniques, and calls for careful review and consensus approval of designs early

in the process. By facilitating the free exchange of information, DFM and DFA methods allow

engineering companies to avoid the costly rework often associated with repeated iterations of the

design process.

13.1.3 Applying Computers to Design

Many of the individual tasks within the overall design process can be performed using a computer.

As each of these tasks is made more efficient, the efficiency of the overall process increases as well.

The computer is especially well suited to design in four areas, which correspond to the latter four

stages of the general design process. Computers function in the design process through geometric

modeling capabilities, engineering analysis calculations, automated testing procedures, and automated

drafting. Figure 13.2 illustrates the relationship between CAD technology and the final four stages

of the design process.

Geometric modeling is one of the keystones of CAD systems. It uses mathematical descriptions

of geometric elements to facilitate the representation and manipulation of graphical images on a

computer display screen. While the central processing unit (CPU) provides the ability to quickly

make the calculations specific to the element, the software provides the instructions necessary for

efficient transfer of information between user and the CPU.

Three types of commands are used by the designer in computerized geometric modeling. The

first type of command allows the user to input the variables needed by the computer to represent

CUSTOMER INPUT

AND PERCEPTION

OF NEED

PROBLEM

DEFINITION

SYNTHFSIS

«.

GEOMETRIC

SYNTHESIS

MODELING

ANALYSISAND

ENGINEERING

OPTIMIZATION *

ANALYSIS

CWAiIiATiOM

DESIGNREVIEW

EVALUATION

ANDEVALUATION

FINAL DESIGN AND

AUTOMATED

SPECIFICATION *

DRAFTING

Fig. 13.2 Application of computers to the design process.

basic geometric elements such as points, lines, arcs, circles, splines, and ellipses. The second type

of command is used to transform these elements. Commonly performed transformations in CAD

include scaling, rotation, and translation. The third type of command allows the various elements

previously created by the first two commands to be joined into a desired shape.

During the whole geometric modeling process, mathematical operations are at work that can be

easily stored as computerized data and retrieved as needed for review, analysis, and modification.

There are different ways of displaying the same data on the CRT screen, depending on the needs or

preferences of the designer. One method is to display the design as a two-dimensional representation

of a flat object formed by interconnecting lines. Another method displays the design as a three-

dimensional representation of objects. In three-dimensional representations, there are four types of

modeling approaches:

• Wireframe modeling

• Surface modeling

• Solid modeling

• Hybrid solid modeling

A "wireframe model is a skeletal description of a three-dimensional object. It consists only of

points, lines, and curves that describe the boundaries of the object. There are no surfaces in a

wireframe model. Three-dimensional wireframe representations can cause the viewer some confusion

because all of the lines defining the object appear on the two-dimensional display screen. This makes

it hard for the viewer to tell whether the model is being viewed from above or below, inside or

outside.

Surface modeling defines not only the edge of the three-dimensional object, but also its surface.

In surface modeling, two different types of surfaces can be generated: faceted surfaces using a

polygon mesh and true curve surfaces. NURBS (Non-Uniform Rational B-Spline) is a B-spline curve

or surface defined by a series of weighted control points and one or more knot vectors. It can exactly

represent a wide range of curves such as arcs and conies. The greater flexibility for controlling

continuity is one advantage of NURBS. NURBS can precisely model nearly all kinds of surfaces

more robustly than the polynomial-based curves that were used in earlier surface models. The surface

modeling is more sophisticated than wireframe modeling. Here, the computer still defines the object

in terms of a wireframe but can generate a surface "skin" to cover the frame, thus giving the illusion

of a "real" object. However, because the computer has the image stored in its data as a wireframe

representation having no mass, physical properties cannot be calculated directly from the image data.

Surface models are very advantageous due to point-to-point data collections usually required for

Numerical Control (NC) programs in computer-aided manufacturing (CAM) applications. Most sur-

face modeling systems also produce the stereolithographic data required for rapid prototyping

systems.

Solid modeling defines the surfaces of an object, with the added attributes of volume and mass.

This allows image data to be used in calculating the physical properties of the final product. Solid

modeling software uses one of two methods: constructive solid geometry (CSG) or boundary rep-

resentation (B-rep). The CSG method uses Boolean operations (union, subtraction, intersection) on

two sets of objects to define composite models. For example, a cylinder can be subtracted from a

cube. B-rep is a representation of a solid model that defines an object in terms of its surface bound-

aries: faces, edges, and vertices.

Hybrid solid modeling allows the user to represent a part with a mixture of wireframe, surface

modeling, and solid geometry. The I-DEAS Master Modeler offers this representation feature.

In CAD software, the hidden-line command can remove the background lines of the object in a

model. Certain features have been developed to minimize the ambiguity of wireframe representations.

These features include using dashed lines to represent the background of a view, or removing those

background lines altogether. The latter method is appropriately referred to as hidden-line removal.

The hidden-line removal feature makes it easier to visualize the model because the back faces are

not displayed. Shading removes hidden lines and assigns flat colors to visible surfaces. Rendering

adds and adjusts lights and materials to surfaces to produce realistic effects. Shading and rendering

can greatly enhance the realism of the 3D image. Figures 13.3(a) and (b) show the same object,

represented as a pure wireframe and a wireframe with hidden-line removal.

Engineering analysis can be performed using one of two approaches: analytical or experimental.

Using the analytical method, the design is subjected to simulated conditions, using any number of

analytical formulae. By contrast, the experimental approach to analysis requires that a prototype be

constructed and subsequently subjected to various experiments to yield data that might not be avail-

able through purely analytical methods.

There are various analytical methods available to the designer using a CAD system. Finite element

analysis and static and dynamic analysis are all commonly performed analytical methods available

in CAD.

Finite element analysis (FEA) is a computer numerical analysis program (Fig. 13.4) used to solve

the complex problems in many engineering and scientific fields, such as structural analysis (stress,

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