MET71 COMPUTER AIDED DESIGN

Transcription

1 UNIT I INTRODUCTION TO CAD 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

2 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 M.SRINIVASSAN, ASST. PROF 2 RGCET

3 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 M.SRINIVASSAN, ASST. PROF 3 RGCET

4 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: M.SRINIVASSAN, ASST. PROF 4 RGCET

5 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. M.SRINIVASSAN, ASST. PROF 5 RGCET

6 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. M.SRINIVASSAN, ASST. PROF 6 RGCET

7 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 M.SRINIVASSAN, ASST. PROF 7 RGCET

8 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). M.SRINIVASSAN, ASST. PROF 8 RGCET

9 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; M.SRINIVASSAN, ASST. PROF 9 RGCET

10 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 M.SRINIVASSAN, ASST. PROF 10 RGCET

11 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. M.SRINIVASSAN, ASST. PROF 11 RGCET

12 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 M.SRINIVASSAN, ASST. PROF 12 RGCET

13 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 Coordinates and front to back movements produce change in Y Co-ordinates. M.SRINIVASSAN, ASST. PROF 13 RGCET

14 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 coordinates 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, M.SRINIVASSAN, ASST. PROF 14 RGCET

15 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. M.SRINIVASSAN, ASST. PROF 15 RGCET

16 (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. M.SRINIVASSAN, ASST. PROF 16 RGCET

17 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. M.SRINIVASSAN, ASST. PROF 17 RGCET

18 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. M.SRINIVASSAN, ASST. PROF 18 RGCET

19 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. M.SRINIVASSAN, ASST. PROF 19 RGCET

20 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. M.SRINIVASSAN, ASST. PROF 20 RGCET

21 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. M.SRINIVASSAN, ASST. PROF 21 RGCET

22