M.SRINIVASSAN, ASST. PROF 1 RGCET YEAR/COMPUTER AIDED DESIG… · This might be the identification of some defect in a current machine design by an engineer or the perception of a

INTRODUCTION TO CAD

UNIT – I

MET71 COMPUTER AIDED DESIGN

Computer Aided Design (CAD) is assistance of computer in engineering processes such

as creation, optimization, analysis and modifications.

CAD involves creating computer models defined by geometrical parameters which can be

readily altered by changing relevant parameters. CAD systems enable designers to view objects

under a wide variety of representations and to test these objects by simulating real-world

conditions.

It is an integration of Mechanical and Computer technology to aid in the design process like

Modelling, Assembly, Drafting, Die Design, Tool Design, Sheet metal, analysis of products.

DESIGN PROCESS:

The process of designing something is characterized as an interactive procedure, which consists

of six identifiable steps or phases:

1. Recognition of need

2. Definition of problem

3. Synthesis

4. Analysis and optimization

5. Evaluation

6. Presentation

Recognition of need

Recognition of need involves the realization by someone that a problem exists for which

some corrective action should be taken

This might be the identification of some defect in a current machine design by an engineer

or the perception of a new product marketing opportunity by a salesperson

Definition of problem

Definition of the problem involves a thorough specification of the item to be designed

This specification includes physical and functional characteristics, cost, quality and

operating performance

Synthesis, Analysis and optimization

Synthesis and Analysis are closely related and highly iterative in the design process

A certain component or subsystem of the overall system is conceptualized by the

designer, subjected to analysis, improved through this analysis procedure and redesigned

This process is repeated until the design has been optimized within the constraints

imposed on the designer

The components and subsystems are synthesized into the final overall system in a similar

iterative manner

Evaluation and Presentation

M.SRINIVASSAN, ASST. PROF 1 RGCET


Evaluation is concerned with measuring the design against the specifications established

in the problem definition phase

This evaluation often requires the fabrication and testing of a prototype model to assess

operating performance, quality, reliability and other criteria

The final phase in the design process is the presentation of the design

This includes documentation of the design by means of drawings, material specifications,

assembly lists and so on

Essentially, the documentation requires that a design data base be created

Figure below illustrates the basic steps in the design process, indicating its iterative nature

The general design process by shiegly

Product Development and Manufacture:

Machines involved – Computers

Tasks – information processing

Use – assist in the definition and processing of information connected with design of products


Process involved in bringing the product to Market


In recent years there have been several attempts to provide a formal description of these

stages or elements of the design process. Some variation in these descriptions, both in

terminology and in detail, but in general the agree that design progress in a step – by – step

manner from some statement of need through identification of the problem , a search for solution

and development of the chosen solution to manufacture, test and use. These descriptions of

design are often called models of the design process.

MORPHOLOGY OF DESIGN

To illustrate these we will consider two models which give different but complementary

insights into the process.

I. Steps of the design process according to Pahl and Beitz(1984)

II. The design process according to Ohsuga



Steps of the design process according to Pahl and Beitz (1984)

In this model the design process is described by a flow diagram comprising four main phases

which may be summarized as:



Although it presents a straightforward sequence of stages through the process, in practice

the main phase are not always so clearly defined, and there is invariable feedback to previous

stages and often iteration between stages.

The design process according to Ohsuga

Ohsuga again describes design as a series of stages, in this case progressing from

requirements through conceptual design and preliminary design to detail design.

In this case however the various stages of design process are generalized into common

form in which models of design are developed through a process of analysis and

evaluation leading to modification and refinement of the model.

In the early stage the tentative solution is proposed by designer. In this stage the design

refined and evolution and modification repeated at a greater level of detail.


Application of design models:


Application of design model is to the receiver of the communication and considers the sort

of actions that are taken with the design information that is received.

This may be divided into two main classification

i. Evaluating actions

ii. Generative actions

In each case the actions involve the extraction of information from the design

representation and the combination of this with further information to form a new model.

This is shown diagrammatically in figure.

A design analyst might use this for the following assessments:

o A visual assessment

o An assessment of the mass of the components, by using the CAD model

o An evaluation of loads in the components, by considering them as parts of a

mechanism

o An evaluation of stresses, for example using the finite element model.

At the later stage, detailed drawings will exist of the components of the design, and from

these, manufacturing engineers will extract information for tooling and for the control of

production machines.

Types of Design Models:

The design process model gives as hint of variety of representation needed in design.

There are phrases such as ‘develop preliminary layouts’ and complete detail drawings’.

In practice, the designer uses a host of different models depending on what property of

the design is to be modeled, and who or what is the target, or receiver, for any

communication.

The engineering designer has, at various times, to modeled the function of a design, its

structure, the form or shape of the components parts and the materials, surface

conditions and dimensions that are required.

He or she may also wish to form mathematical models, or computer based

representations, to assist in the evolution of design.

The potential targets for communication include, among others, fellow designers,

manufacturing and workshop staff, and user of the design.

For any particular combination of modeled property and receiver there will be type of

model and technique for its generation that will be most appropriate.

Of all the modeled properties, form and structure are of particular importance in

engineering, and the most appropriate method of representing these has traditionally

been graphical.

For many engineers a major part of their task is to define the shape and arrangement of

the component parts of the design. This is conventionally achieved by drawings of form.



Other engineers are more concentrated with the structure of the assembly of standard

elements to form a design, with the way these elements are connected together, and with

the flows between parts are often called a system engineering approach.

In the early stages of a design, the designer will often explore ideas by sketching, with

little or no detail. When information is being generated for manufacture, however, a more

diligent technique is required, and drawings and diagrams will be carefully produced to

show all necessary detail.

For any communication to be successful that must be understood by all those involved.

CONCURRENT ENGINEERING:

Concurrent engineering or Simultaneous Engineering is a methodology of restructuring

the product development activity in a manufacturing organization using a cross functional team

approach and is a technique adopted to improve the efficiency of product design and reduce the

product development cycle time.

This is also sometimes referred to as Parallel Engineering. Concurrent Engineering brings

together a wide spectrum of people from several functional areas in the design and manufacture

of a product. Representatives from R & D, engineering, manufacturing, materials management,

quality assurance, marketing etc. develop the product as a team.

Everyone interacts with each other from the start, and they perform their tasks in parallel.

The team reviews the design from the point of view of marketing, process, tool design and

procurement, operation, facility and capacity planning, design for manufacturability, assembly,

testing and maintenance, standardization, procurement of components and sub-assemblies,

quality assurance etc as the design is evolved.

Even the vendor development department is associated with the prototype development.

Any possible bottleneck in the development process is thoroughly studied and rectified. All the

departments get a chance to review the design and identify delays and difficulties.

The departments can start their own processes simultaneously. For example, the tool

design, procurement of material and machinery and recruitment and training of manpower which

contributes to considerable delay can be taken up simultaneously as the design development is

in progress. Issues are debated thoroughly and conflicts are resolved amicably.

Concurrent Engineering (CE) gives marketing and other groups the opportunity to review

the design during the modeling, prototyping and soft tooling phases of development. CAD

systems especially 3D modelers can play an important role in early product development phases.

In fact, they can become the core of the CE.

They offer a visual check when design changes cost the least. Intensive teamwork

between product development, production planning and manufacturing is essential for

satisfactory implementation of concurrent engineering.

The teamwork also brings additional advantages ; the co-operation between various

specialists and systematic application of special methods such as QFD (Quality Function

Deployment), DFMA (Design for Manufacture and Assembly) and FMEA (Failure Mode and



Effect Analysis) ensures quick optimization of design and early detection of possible faults in

product and production planning. This additionally leads to reduction in lead time which reduces

cost of production and guarantees better quality.

Comparison of Concurrent Engineering and Sequential Engineering

A comparison of concurrent and sequential engineering based on cost is attempted in this

section. The distribution of the product development cost during the product development cycle is

shown in Fig. This figure shows that though only about 15% of the budget is spent at the time of

design completion, whereas the remaining 85% is already committed.

The decisions taken during the design stage have an important bearing on the cost of the

development of the product. Therefore the development cost and product cost can be reduced by

proper and careful design. CE facilitates this. The significantly large number of nonconformities

detected in the later stages of product development cycle in sequential engineering results in

large time and cost overrun.

IMPLEMENTATION OF CONCURRENT ENGINEERING

The cycle of engineering design and manufacturing planning involves interrelated

activities in different engineering disciplines simultaneously, than sequentially as shown in Fig.

(A). In addition, the activities necessary to complete a particular task within a specific engineering

discipline have to emerge wherever possible from their sequential flow into a concurrent workflow

with a high degree of parallelism as illustrated in Fig. (B).



Concurrency implies that members of the multidisciplinary project team work in parallel.

This also means that there is no strict demarcation of jobs among various departments. The

multi-disciplinary approach has the advantage of several inputs which can be focused effectively

early in the design process. Presently engineering departments are practicing this approach but

still with a high degree of manual involvement and redundancy.

CAD SYSTEM ARCHITECTURE:

Hardware: the computer and associated peripheral equipment

Software: the computer programs running on the hardware

Data: the data structure created and manipulated by the software:

Human Knowledge and activates

CAD systems are no more than computer programs, perhaps using specialized computing

hardware. The software normally comprises a number of different elements or functions that

process the data stored in the database in different ways. Those elements or functions are:

Model definition: for example, to add geometric elements to a model of the form of a

component;

Model manipulation: to move, copy, delete, edit or otherwise modify elements in design

models;

Picture generation: to generate images of the design model on a computer screen or on

some hard-copy device;



User interaction: to handle commands input by user and to present output to the user

about the operation of the system;

Database management: for the management of the files that make up the database;

Application: these elements of the software do not modify the design model, but use it to

generate information for evaluation, analysis or manufacture;

Utilities: a ‘catch-all’ term for parts of the software that do not directly affect the design

model, but modify the operation of the system in some way (e.g to set the color to be used

for display, or the units to be used for construction of a part model).

These features may be provided by multiple programs operating on a common database or

by a single program encompassing all of these elements.

CAD Hardware

Workstation – CPU

Mass storage – Magnetic tape storage, Magnetic Disc Storage, Magnetic drum storage

Input devices - (keyboard, light pen, thumb wheel, joy stick, mouse, digitizer, Touch Screen,

Track Ball) Output devices - (printers, plotters)

Display Devices- (storage tube – raster scan, vector refresh, plasma panel and LCD)

Central Processing Unit:

The CPU is the Heart of the digital Computer, since it coordinates and controls the

activities of all other units. The CPU consists of three separate subsections;

1. Control Unit

2. Arithmetic Logic unit

3. Memory


Control Unit:


The control unit is basically acts as an administrator in a computer. It coordinates the

operations of all other components. It controls the input and output of information between the

computer and the outside world through I/O devices, synchronizes the transfer of signals

between the various sections of the computer and regulates the other section to perform their

individual functions. The capability of the control unit to accomplish these operations is provided

by a set of instructions called executive program, which is stored in memory.

Arithmetic Logic unit

The ALU provides the circuitry required to perform the various calculations and

manipulations of data. Most ALU’s can add and subtract, but there are now some ALU’s that are

capable of performing multifunction and division and even other complex mathematical functions.

ALU’s with simplex circuits are capable of being programmed to perform these more complicated

operations, but more computing time is required. The more complex arithmetic logic units are

faster, but these units are more costly.

Memory

The memory section consists of binary storage units, which are organised into bytes. The

memory section stores all the instructions and data of a program. Therefore the CPU must

transfer these instructions and data. Two types of memory

Main memory (primary storage)

Auxiliary memory (Secondary storage)

Mass storage

The most common device used for computer storage technologies are

Magnetic tape storage

Magnetic Disc Storage

Magnetic drum storage

1) Magnetic tape storage

Magnetic storage is a good example of sequential access storage technology. Data are

stored on magnetic tape, similar to that used in audio systems. The major advantages of

magnetic tapes are that is relatively cheap when compared with other types of storage medium

and that it can easily hold large amount of data for its size. Magnetic tape unlike punched paper

tapes or cards can be used again by simply overwriting previously stored data.

Since data are stored sequentially access time is relatively slow. However, the low cost

per bit and high capacity of magnetic tape make it ideal for system backup. It is most suitable for

applications, which may be required in payroll, personnel management, inventory control and

customer invoicing where a large amount of data is to be processed sequentially.

2) Magnetic Disc Storage

Magnetic disk storage is also known as a random access storage device. The storage

medium is a magnetically coated disk. There are several types and sizes of disks each best

suited to a particular set of applications.


3) Floppy Disc


Floppy disks come in two standard sizes: the larger one is 8 inches in diameter and

smaller is 5 ¼ inches and is referred to as mini floppy.

4) Magnetic Drum Storage

The magnetic drum is direct access storage device with high capacity and high access

rates. The magnetic drum consists of a magnetically coated cylinder during operation. The drum

is rotated at a constant speed and data are recorded in the form of magnetized spots. The drum

can be read repeatedly without causing data loss.

Input devices

Keyboard

Mouse

Light pen

Thumb wheel

Joy stick

Digitizer

Touch Screen

Track Ball

Keyboard

The keyboard interacts with the computer on a hardware and software level. The

keyboard contains a keyboard controller (like 8042 or 8048) to check if any key is pressed or

released. If any key remains closed for more than half a second the controller sends a repeat

action at specific intervals. It has limited diagnostic and error checking capabilities. A buffer is

normally available to store a certain number of key actions if the computer is busy.

Mouse

Mouse is today one of the widely used input devices in graphics applications. Mouse can

be moved around by the operator on any flat surface to provide graphic input. Its ability to rapidly

position the cursor on the screen is its most important advantage. Mouse is available as a

mechanical or optical graphic input device. In the case of a mechanical mouse, the rolling ball at

the bottoms of the mouse causes two encoders to rotate. The movement of the mouse is thus

converted into pulses which move the cursor in the X and Y direction in proportion to the



movement of the mouse. Mouse can be operated in a limited space. Since the mouse can be

used without looking at it, the user can concentrate on the screen and hence design productivity

can be considerably increased.

Light pen

A light pen is a computer input device in the form of a light-sensitive wand used in

conjunction with a computer's CRT display.

It allows the user to point to displayed objects or draw on the screen in a similar way to a

touch screen but with greater positional accuracy. It was long thought that a light pen can

work with any CRT-based display, but not with LCDs and other display technologies. Thumb wheel

Thumb wheels are potentiometric devices. Two of them are provided for X and Y

movements of cursor. These also have the advantage that one can look at the screen and move

the cursor.

Joy stick

Joystick is a potentiometric device that contains sets of variable resistors which feed

signals that indicates the device position to the computer. These devices rely on the operator’s

sense of touch and hand-eye co-ordination to control the position of the cursor on the screen.

Joystick devices are normally set so that side-to-side movement produces change in X Co-

ordinates and front to back movements produce change in Y Co-ordinates.


http://en.wikipedia.org/wiki/Computer

http://en.wikipedia.org/wiki/Input_device

http://en.wikipedia.org/wiki/Cathode_ray_tube

http://en.wikipedia.org/wiki/Touchscreen

http://en.wikipedia.org/wiki/Touchscreen

http://en.wikipedia.org/wiki/Liquid_crystal_display

http://en.wikipedia.org/wiki/Display_technology

Digitizer


Digitizer boards or tablets are electro-mechanical vector graphic input devices that

resemble a drafting board. These are used together with a movable stylus or reticule called a

cursor or a puck. They are used to enter drawings into computer graphics systems by taping the

drawing to the surface of the digitizing board and placing the cursor over points whose co-

ordinates are to be entered. Figure shows a digitizer.

Touch Screen

Touch screens are direct devices. They are used by simply touching CRT display with

one’s finger or a pointing device. Two types of touch screens (mechanical and optical) are used

in CAD applications. Mechanical type is a transparent screen overlay which detects the location

of the touch.

Track Ball

Track ball has a ball and socket construction but the ball must be rolled with fingers or the

palm of the hand. The cursor moves in the direction of the roll at a rate corresponding to

rotational speed. The user must rely heavily on the tactile sense when using a trackball since

there is no correspondence between the position of the cursor and the ball. The ball momentum

provides a tactile feedback.

OUTPUT DEVICES

A CAD system is not complete unless it can make hard copies of designs or analysis

created on the computer. Determining the best output device for a typical CIM application is a

three-step process: specifying how hard copies will be used, identifying quality and cost criteria

and selecting equipment most suitable for the application. Hard copies are used for a variety of

purposes, including shop use, file storage, reports and presentations. Design iterations can be

reduced by making hard copies at crucial stages and distributing them to key personnel for

review. Documents and drawings are required for archival purposes, to be used in proposals,



reports, as well as illustrations. Quality of the hard copy depends on the resolution of the hard

copy unit. Speed and frequency of operation of hard copy equipment are also of importance.

Printers

Plotters

PLOTTERS

Plotters are classified based several factors. Depending on the maximum size of the

drawing plotters are designated as A0, A1, A2, A3 and A4. There are plotters capable of creating

drawings larger than A0 size. Generally plotters plot drawings on cut sheets. Some special

plotters are capable of creating drawings on rolls also. Drawings are created through a series of

short vectors which requires movement to the pen in X and Y direction. Plotters can be classified

on the basis of their construction. A flat bed plotter has the pen moving on a flat surface on which

the drawing paper is fixed. The linear movements in the X and Y direction generate the required

drawing. In the case of a drum plotter, the paper is wound around on a cylindrical drum. The pen

holder is attached to a moving slide.

The co-ordinate motion generated by the rotation of the drum and linear movement draws

the pictures on the paper. In the third type, i.e. the pinch roller plotter, the paper is tightly held

between two sets of rollers. One roller in each pair has a rough surface and the linear motion to

the paper in one direction is imparted by the rotation of the roller. The movement in the other

direction is through a linear motion imparted to the pen holder. Plotters can also classify as pen

plotters and electrostatic plotters. Pen plotters use 1, 4, 8 or more different color pens. The

drawings thus can be made in several colors. Pencil plotters are also available. Electrostatic

plotters are faster but there is no color variety. They are also cheaper.

PRINTERS

Several types of printers are available:

(i) Impact printers: They use small hammers or print heads containing small pins to strike a

ribbon to form dot matrix images. Colors are introduced through the use of multiple ribbons or

single ribbons with different color bands. Color intensity is fixed and creating shades is almost

impossible. Because of the low resolution, copy quality is poor. Impact printers are suitable for

high speed, low cost, high volume hard copies.



(ii) Inkjet printer: Inkjet printers produce images by propelling fine droplets of ink on to the

medium to be printed. Droplets can be generated in continuous streams or pulses. Some of the

droplets get charged and are returned to the reservoir, while uncharged droplets attach to the

printing surface to form graphics. The laser jet printers are capable of giving good quality color

prints with shading at reasonable cost.

(iii) Laser printer: Laser printer is one of the most widely used output devices. This type

combines high speed with high resolution and the quality of output is very fine.

DISPLAY DEVICES:

The graphics display of a workstation is considered its most important component

because the quality of the displayed image influences the perception of generated design

on the CAD/CAM system.

In addition to viewing images, the graphics display enables the user to communicate with

the displayed image by adding, deleting, blanking, and moving graphics entities on the

display screen.

As a matter of fact, this communication process is what gives interactive graphics its

name to differentiate it from passive graphics, as in the case of a home television set,

that the user cannot change.

Variable display technologies are now available to the user to choose from. They are all

based on the concept of converting computer’s electrical signals, controlled by the

corresponding digital information, into visible images at high speed.

Technologies

Cathode Ray Tube(CRT)

Laser Display

Flat Panel Display or Plasma panel Display

1. In the first a laser beam instead of an electron beam is used to trace an image in a film.

2. In the second a liquid crystal display (LCD) and light emitting diodes (LEDs) are used to

generate images

3. The plasma display uses small neon bulbs arranged in a panel which provides a medium

resolution display.

Thus far, none of these display technologies has been able to displace the CRT as the dominant

graphics display device.

CATHODE RAYS TUBE:

The operation of CRT is based on the concept of energizing an electron beam that strikes

the phosphor coating at very high speed. The energy transfer from the electron to the phosphor

due to the impact causes it to illuminate and glow.



The electrons are generated via the electron gun that contains the cathode and focused

into a beam via the focusing unit shown in figure. By controlling the beam direction and intensity

in a way related to the graphics information generated in the computer, meaningful and desired

graphics can be displayed on the screen.

The graphics display can be divided into two types based on the scan technology used to

control the electron beam.

Random Scan

Raster Scan

In Random scan graphics can be generated by drawing vectors or line segments on the screen

in a random order which is controlled by the user input and the software. The word “random”

indicates that the screen is not scanned in a particular order.



Raster Scan system, the screen is scanned from top to bottom, left to right all the time to

generate graphics. This is similar to home television scan system, thus suggesting the name

digital scan.

The three existing CRT display that are based on these techniques are

i. Refresh display (calligraphic)

ii. Direct view storage tube

iii. Raster display

Refresh Display

The refresh buffer stores the display file or program, which contains points, lines,

characters and other attributes of picture to drawn. These commands are interpreted and

processed by the display processor.

The electron beam accordingly excites the phosphor, which glows for a short period. To

maintain a steady flicker – free image, the screen must be refreshed or redrawn at least 30 to 60

times per second, that is, at a rate of 30 to 60 Hz.



Changes made to the display file by the software must be synchronized with the display

refresh cycle to prevent the display of an incomplete picture. If the software updates the file fast

enough, then it is possible to use the dynamic techniques such as animation to simulate

movements as well as developing responsive user interfaces.

The principal advantage to refresh displays is its high resolution (4096 x 4096) and thus

its generation of high quality pictures. However, the need to refresh the picture places a limit on

the number of vectors that can be displayed without flicker

In addition, being a binary display, the refresh display is able to generate only two level of

color intensity. In some displays, the intensity of the electron beam can vary to provide better

color capabilities.

Direct View Storage Tube (DVST):

Refresh display were very expensive in the 1960s due to the required refresh buffer

memory and fast display processor, and could only display a few hundred vectors on the screen

without flicker. At the end of 1960s the DVST was introduced by Tektronix as an alternative and

inexpensive solution.

The DVST eliminates refresh processors completely and consequently the refresh buffer

used with refreshes display.



It also uses a special type of phosphor that has a long – lasting glowing effect. The

phosphor is embedded in a storage tube. In addition, the speed of the electron beam in the DVST

is slower than in the refresh display due to elimination of refresh cycle.

In the DVST the picture is stored as a charge in the phosphor mesh located behind the

screen’s surface. Therefore, complex pictures could be drawn without flicker at high resolution.

Once displayed, the picture remains on the screen until it is explicitly erased. This

is why the name “storage tube” was suggested.

In addition to the lack of selective erasure, the DVST cannot provide colors, animation

and use of light pen as an input device.

Raster Display:

The inability of the DVST to meet the increasing demands by various CAD/CAM

applications for colors, shaded images and animation motivated hardware designer to continue

searching for a solution.

During the late 1970s raster display based on the standard television technology began to

emerge as a viable alternative. The drop in memory price due to advances in solid states made

large enough refresh buffers available support high resolution display.

A typical resolution of raster display is 1280 x 1204 with a possibility to reach 4096 x 4096

as the DVST. Raster displays are very popular and nearly al recent display research and

development focus on them.



In raster display, the display screen area is divided horizontally and vertically into matrix of small

elements called picture element or pixel. A pixel is a small addressable area on the screen. An N

x M resolution defines on a screen with N rows and M Columns. Each row defines a scan line. A

rasterization process is needed in order to display either a shaded area or graphics entities.

In this process the area or entities are converted into their corresponding pixels whose

intensity and color are controlled by the image processing system.

Working:

Images are displayed by converting geometric information into pixel values which then

converted into electron beam deflection through display processor and deflection system. If the

display is monochrome, the pixel value is used to control the intensity level or the gray level on

the screen. For color displays, the value is used to control the color mapping into a color map.

The creation of transfer format data from geometric information is known as scan

conversion or rasterization. A rasterizer that forms the mage-creation system is mainly a set of

scan conversion algorithms. Due to the universal need for these algorithms, the scan conversion

or rasterization process is implemented.
