What is CAD?
CAD if often defined in a variety of ways and includes a large range of activities. Very broadly it can be said to be the integration of computer science (or software) techniques in engineering design. At one end when we talk of modeling, iIt encompasses the following:
  • Use of computers (hardware & software) for designing products
  • Numerical method, optimizations etc.
  • 2D/3D drafting
  • 3D modeling for visualization
  • Modeling curves, surfaces, solids, mechanism, assemblies, etc.
The models thus developed are first visualized on display monitors using a variety of techniques including wire frame display, shaded image display, and hidden surface removed display and so on. Once the designer is satisfied, these models are then used for various types of analysis / applications. Thus, at the other end it includes a number of analysis activities. These could be:
  • Stress (or deflection) analysis, i.e. numerical methods meant for estimating the behavior of an artifact with respect to these parameters. It includes tools like the Finite Element Method (FEM).
  • Simulation of actual use
  • Optimization
  • Other applications like
    • CAD/CAM integration
    • Process planning
These are activities which normally use models developed using one or more of the techniques mentioned above. These activities are often included in other umbrellas like CAM or CAE. A term often used is CAD to include this broad set of activities. They all use CAD models and often the kind of application they have to be determines the kind of a model to be developed. Hence, in this course I cover them under the umbrella of CAD. In this course we will strive to give an overview of modeling techniques followed by some applications, specifically CAM.
Thus there are three aspects to CAD.
  • Modeling
  • Display/ Visualization
  • Applications
Modeling typically includes a set of activities like
  • Defining objects
  • Defining relation between objects
  • Defining properties of objects
  • Defining the orientations of the objects in suitable co-ordinate systems
  • Modification of existing definition (editing)
The figure below explains what a typical CAD model would need to define, what kind of entities need to be defined and what relationships exist between them.

At the highest level we have the volume which is defined by (or "delimited by") a set of surfaces. These surfaces can be planar or curved / warped. A planar surface can be bounded by a set of curves. A curved surface can be seen as a net of curves. These curves are typically a succession of curve segments which define the complete the curve. The curve segment is defined using a set of end points / control points which govern the nature of the curve. Thus a relationship is defined between entities at each level.
Once such a relationship is defined, a geometric model of the artifact is available. In any design there might be many such artifacts. One then has to define properties of each of these artifacts and define a relationship between them. The properties and the relationships needed are dependent on the application the model is to be used for subsequently. But one common application that all models have to go through is visualization of the model (s).

Displaying the model requires the following:
  • Mapping objects onto screen coordinates: Models are typically made in a model coordinate system. This could be the world coordinate system, or a coordinate system local to the object. These coordinate systems are typically three dimensional in nature. To display the object on a 2D screen, the object coordinates need to be mapped on to the 2D coordinate system of the screen. This requires two steps:
    • Viewing transformations: The coordinates of the object are transformed in a manner as if one is looking at the object through the screen. This coordinate system is referred to as the viewing coordinate system.
    • Projections: The object in the viewing coordinate system is then projected onto the two dimensional plane of the screen.
  • Surface display or shading / rendering: In displaying the objects on the screen one often likes to get a shaded display of the object and get a good feel of the three dimensional shape of the object. This requires special techniques to render the surface based on its shape, lighting conditions and its texture.
  • Hidden line removal when multiple surfaces are displayed: In order to get a proper feel of the three dimensional shape of an object, one often desires that the lines / surfaces which are not visible should not be displayed. This is referred to as hidden line / surface removal.
Once a model is visualized on the screen and approved by the conceptual designer, it has to go through a number of analysis. Some of the kinds of usage this model might have to go through are the following:
  • Estimating stresses / strains / deflections in the objects under various static loading conditions
  • Estimating the same under dynamic loading conditions
  • Visualizing how a set of objects connected together would move when subject to external loading. This leads to a whole set of activities under simulation. These activities would vary depend upon the application the object is to be subject to.
  • Optimizing the objects for
  • Developing 2D engineering drawings of the object
  • Developing a process plan of the object
  • Manufacturing the object using NC / CNC machines and generating the programs for these machines so as to manufacture these objects.

Having given the overview of the kind of activities that can come under the umbrella of CAD the uses these CAD models can be put to, I know highlight what aspects of these would be covered in this course. Needless to say, all these activities would be well beyond the scope of one single course. Therefore this course, which is targeted to give an overview of CAD and its applications, would include the following:
  1. An overview of the hardware systems used in CAD
  2. 2D and 3D transformations used to shift between coordinate systems
  3. Projection transformation used to get the object in screen coordinate systems
  4. Modeling of curves and surfaces
  5. Modeling of solids

Typically, the primary output device in a graphics system is a video monitor (Fig. below). The operation of most video monitor is based on the standard cathode-ray tube (CRT) design.

Refresh Cathode-Ray Tubes
A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam towards specified position on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT.
The primary components of an electron gun in a CRT are the heated metal cathode and a control grid (fig. below). Heat is supplied to the cathode by directing a current through a coil of wire, called the filament, inside the cylindrical cathode structure. This causes electrons to be “boiled off” the hot cathode surface. In the vacuum inside the CRT envelope, negatively charged electrons are then accelerated toward the phosphor coating by a high positive voltage. The accelerating voltage can be generated with a positively charged metal coating on the inside of the CRT envelope near the phosphor screen, or an accelerating anode can be used, a in fig below . Sometimes the electron gun is built to contain the accelerating anode and focusing system within the same unit.

Spots of light are produced on the screen by the transfer of the CRT beam energy to the phosphor. When the electrons in the beam collide with the phosphor coating, they are stopped and there are stopped and their kinetic energy is absorbed by the phosphor. Part of the beam energy s converted by friction into heat energy, and the remainder causes electron in the phosphor atoms to move up to higher quantum-energy levels. After a short time, the “excited” phosphor electrons begin dropping back to their stable ground state, giving up their extra energy as small quantum’s of light energy. What we see on the screen is the combined effect of all the electrons light emissions: a glowing spot that quickly fades after all the excited phosphor electrons have returned to their ground energy level. The frequency (or color) of the light emitted by the phosphor is proportional to the energy difference between the excited quantum state and the ground state.

Different kinds of phosphor are available for use in a CRT. Besides color, a major difference between phosphors is their persistence: how long they continue to emit light (that is, have excited electrons returning to the ground state) after the CRT beam is removed. Persistence is defined as the time it takes the emitted light from the screen to decay to one-tenth of its original intensity. Lower-persistence phosphors require higher refresh rates to maintain a picture on the screen without flicker. A phosphor with low persistence is useful for animation; a high-persistence phosphor is useful for displaying highly complex, static pictures. Although some phosphor have persistence greater than 1 second, graphics monitor are usually constructed with persistence in the range from 10 to 60 microseconds.

·       Raster-Scan Displays

In a raster- scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in memory area called the refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and “painted” on the screen one row (scan line) at a time (fig. below). Each screen point is referred to as a pixel or pel (shortened forms of picture element).

Refreshing on raster-scan displays is carried out at the rate of 60 to 80 frames per second, although some systems are designed for higher refresh rates. Sometimes, refresh rates are described in units of cycles per second, or Hertz (Hz), where a cycle corresponds to one frame. At the end of each scan line, the electron beam returns to the left side of the screen to begin displaying the next scan line. The return to the left of the screen, after refreshing each scan line, is called the horizontal retrace of the electron beam. And at the end of each frame (displayed in 1/80th to 1/60th of a second), the electron beam returns (vertical retrace) to the top left corner of the screen to begin the next frame.

On some raster-scan systems (and in TV sets), each frame is displayed in two passes using an interlaced refresh procedure. In the first pass, the beam sweeps across every other scan line from top to bottom. Then after the vertical retrace, the beam sweeps out the remaining scan lines (fig. below). Interlacing of the scan lines in this way allows us to see the entire screen displayed in one-half the time it would have taken to sweep across all the lines at once from top to bottom.

Random-Scan Displays

Random scan monitors draw a picture one line at a time and for this reason are also referred to as vector displays (or stroke-writing or calligraphic displays).The component lines of a picture can be drawn and refreshed by a random-scan system in any specified order.

Refresh rate on a random-scan system depends on the number of lines to be displayed. Picture definition is now stored as a set of line-drawing commands in an area of memory referred to as the refresh display file. Sometimes the refresh display file is called the display list, display program, or simply the refresh buffer. To display a specified picture, the system cycles through the set of commands in the display file, drawing each component line in turn. After all line- drawing commands have been processed, the system cycles back to the first line command in the list. Random-scan displays are designed to draw al the component lines of a picture 30 to 60times each second.

Color CRT Monitors

The beam penetration method for displaying color pictures has been used with random-scan monitors. Two layers of phosphor, usually red and green, are coated on to the inside of the CRT screen, and the displayed color depends on how far the electron beam penetrates into the phosphor layers.
Shadow-mask methods are commonly used in raster-scan systems (including color TV) because they produce a much wider range of color than the beam penetration method. A shadow-mask CRT has three phosphor color dots at each pixel position. One phosphor dot emits a red light, another emits a green light, and the third emits a blue light. This type of CRT has three electron guns, one for each color dot, and a shadow- mask grid just behind the phosphor –coated screen. Fig.below illustrates the delta-delta shadow-mask method, commonly used in color CRT systems. The three electron beam are deflected and focused as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns. When the three beams pass through a hole in the shadow mask, they activate a dot triangle, which appears as a small color spot the screen the phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. 
Flat-Panel Displays

The term flat–panel displays refers to a class of video devices that have reduced volume, weight, and power requirements compared to a CRT. A significant feature of flat-panel displayed is that they are thinner than CRTs, and we can hang them on walls or wear them on our wrists.

We can separate flat-panel displays into two categories: emissive displays and non-emissive displays. The emissive displays (or emitters) are devices that displays and light - emitting diodes are examples of emissive displays. Non emissive displays (or non-emitters) use optical effects to convert sunlight or light from some other source into graphics patterns. The most important example of a non-emissive flat-panel display is a liquid- crystal device.

Plasma panels, also called gas discharge displays, are constructed by filling the region between two glass plates with a mixture of gases that usually include neon. A series of vertical conducting ribbons is placed on one glass panel, and a set of horizontal ribbons is built into the other glass panel (fig. below). Firing voltages applied to a pair of horizontal and vertical conductors cause the gas at the intersection of the two conductors to break down into a glowing plasma of electrons and ions. Picture definition is stored in a refresh buffer, and the firing voltages are applied to refresh the pixel positions (at the intersections of the conductors) 60 times per second.
Another type of emissive device is the light-emitting diode (LED). A matrix of diodes is arranged to form the pixel positions in the display, and picture definition is stored in refresh buffer. As in scan- line refreshing of a CRT, information is read from the refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display. Liquid- crystal displays (LCDs) are commonly used in systems, such as calculators (fig. below) and portable, laptop computers (fig. below). These non-emissive devices produce a picture by passing polarized light from the surrounding or from an internal light source through a liquid- crystal material that can be aligned to either block or transmit the light.

References: - www.nptel.iitm.ac.in/


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