OVERVIEW OF CAD/CAM
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
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).
DISPLAY / VISUALIZATION
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:
- An overview of
the hardware systems used in CAD
- 2D and 3D
transformations used to shift between coordinate systems
- Projection
transformation used to get the object in screen coordinate systems
- Modeling of
curves and surfaces
- Modeling of
solids
VIDEO DISPLAY DEVICES
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.
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/
Comments
Post a Comment