Many of the
individual tasks within the overall design process can be performed using a
computer. As each of these tasks is made more efficient, the efficiency of the
overall process increases as well. The computer is especially well suited to
design in four areas, which correspond to the latter four stages of the general
design process (Fig. 1).
Figure 1 Application of computers to the design
process.
Computers function
in the design process through geometric modeling capabilities, engineering
analysis calculations, automated evaluative procedures, and automated drafting.
Geometric
Modeling
Geometricmodeling
is one of the keystones of CAD systems. It usesmathematical descriptions of
geometric elements to facilitate the representation and manipulation of
graphical images on a computer display screen. While the computer central
processing unit (CPU) and the graphics processing unit (GPU) provide the
ability to quickly make the calculations specific to the element, the software
provides the instructions necessary for efficient transfer of information between
the user and the CPU and the GPU.
Three types of
commands are used by the designer in computerized geometric modeling:
1. Input commands allow
the user to input the variables needed by the computer to represent basic
geometric elements such as points, lines, arcs, circles, splines, and ellipses.
2. Transformation commands are used to transform these elements.
Commonly performed transformations in CAD include scaling, rotation, and
translation.
3. Solid commands allow the various elements previously created
by the first two commands to be joined into a desired shape.
During the entire
geometric modeling process, mathematical operations are at work which can be
easily stored as computerized data and retrieved as needed for review,
analysis, andmodification. There are different ways of displaying the same data
on the computer monitor, depending on the needs or preferences of the designer.
One method is to
display the design as a two-dimensional (2D) representation of a flat object
formed by interconnecting lines.
Another method
displays the design as a three-dimensional (3D) representation of objects. In
3D representations, there are four types of modeling approaches:
Wire frame modeling
Surface modeling
Solid modeling
Hybrid solid modeling
Wire
Frame Model
A wire frame model
is a skeletal description of a 3D object. It consists only of points, lines, and
curves that describe the boundaries of the object. There are no surfaces in a
wire frame model. 3D wire frame representations can cause the viewer some
confusion because all of the lines defining the object appear on the 2D display
screen. This makes it hard for the viewer to tell whether the model is being
viewed from above or below, from inside the object or looking from outside.
Surface modeling
defines not only the edge of the 3D object but also its surface. Two different
types of surfaces can be generated: faceted surfaces using a polygon mesh and
true curve surfaces.
A polygonal mesh is
a surface approximated by polygons such as squares, rectangles, and hexagons.
The surface is created as if a mosaic of fine polygons. Depending on the detail
required by the designer, very fine surfaces cannot be created this way.
Instead, polygonal meshes allow for faster rendering of shapes, as opposed to
using curves.
The nonuniform
rational basis spline (NURBS) is a B-spline curve or surface defined by a series
of weighted control points and one or more knot vectors. It can exactly
represent a wide range of curves such as arcs and conics. The greater
flexibility for controlling continuity is one advantage of NURBS. NURBS can
precisely model nearly all kinds of surfaces more robustly than the
polynomial-based curves that were used in earlier surface models. Surface
modeling is more sophisticated than wire frame modeling. Here, the computer
still defines the object in terms of a wire frame but generates a surface
“skin” to cover the frame, thus giving the illusion of a “real” object.
However, because the computer has the image stored in its data as a wire frame
representation having no mass, physical properties cannot be calculated
directly from the image data. Surface models are very advantageous due to
point-to-point data collections usually required for numerical control (NC)
programs in computer-aided manufacturing (CAM) applications. Most surface
modeling systems also produce the stereolithographic data required for rapid
prototyping systems.
Solid
Modeling
Solid modeling
defines the surfaces of an object, with the added attributes of volume and
mass. This allows data to be used in calculating the physical properties of the
final product. Solid modeling software uses one of two methods to represent
solid objects in a computer: constructive solid geometry (CSG) or boundary
representation (B-rep).
The CSG method uses
Boolean operations such as union, subtraction, and intersection on two sets of
objects to define composite solid models. For example, to create a hole in a cube,
a small cylinder can be subtracted from a large cube. See Fig. 2.
B-rep is a
representation of a solid model that defines an object in terms of its surface boundaries:
faces, edges, and vertices. In the case of the cube with a hole, a square
surface could be created with a hole (as two mirrored surfaces) and then
extruded to create the model. See Fig. 3.
Hybrid
Solid Modeling
Hybrid solid
modeling allows the user to represent a part with a mixture of wire frame,
surface modeling, and solid geometry. The Siemens product lifecycle management
(PLM) program offers this representation feature.
Figure 2 Solid subtraction.
Figure 3 Surface solid subtraction.
Figure 4 Wire frame model.
Figure 4 Wire frame model.
Figure 5 Wire frame model with
hidden lines removed.
In CAD software,
certain features have been developed to minimize the ambiguity of wire frame representations
(Fig. 4). These features include using dashed lines to represent the
background of a view or removing those background lines altogether (Fig. 5).
The latter method is appropriately referred to as “hidden-line removal.” The
hidden-line removal feature makes it easier to visualize the model because the
back faces are not displayed. Shading removes hidden lines and assigns flat
colors to visible surfaces. Rendering is the process by which light is added
and adjusted and textures are applied to the surfaces in order to produce
realistic effects. Shading and rendering can greatly enhance the realism of the
3D image.
Engineering
analysis can be performed using one of two approaches: analytical or experimental.
Using the analytical method, the design is subjected to simulated conditions
using any number of analytical formulas. By contrast, the experimental approach
to analysis requires that a prototype be constructed and subsequently subjected
to various experiments to yield data that might not be available through purely
analytical methods.
There are various
analytical methods available to the designer using a CAD system, such as
finite-element analysis (FEA), static and dynamic analysis, and kinematic
analysis.
Finite-Element
Analysis
Finite-element
analysis is a computer numerical analysis program used to solve complex
problems in many engineering and scientific fields, such as structural analysis
as it relates to stress, deflection, vibration, thermal analysis (steady state
and transient), and fluid dynamics analysis (laminar and turbulent flow).
The finite-element
method (FEM) divides a given physical or mathematical model into smaller and
simpler elements, performs analysis on each individual element using required mathematics,
and then assembles the individual solutions of the elements to reach a global solution
for the model. FEA software programs usually consist of three parts: the
preprocessor, the solver, and the postprocessor.
The program inputs
are prepared in the preprocessor. Model geometry can be defined or imported
from CAD software. Meshes are generated on a surface or solid model to form the
elements. Element properties and material descriptions can be assigned to the
model. Finally, the boundary conditions and loads are applied to the elements and
their nodes. Certain checks must be completed before analysis solving is
executed. These include checking for duplication of nodes and elements and
verifying the element connectivity of the surface elements so that the surface
normals are all in the same direction. In order to optimize disk space and
running time, the nodes and elements should usually be renumbered and
sequenced.
Many analysis
options are available in the analysis solver to execute the model. The element stiffness
matrices can be formulated and solved to form a global stiffness value for the model
solution. The results of the analysis data are then interpreted by the
postprocessor. The postprocessor in most FEA applications offers graphical
output and animation displays. Vendors of CAD software are developing pre- and
postprocessors that allow the user to graphically visualize their input and
output. FEA is a powerful tool in effectively developing a design to
achieve a superior
product.
Kinematic
Analysis and Synthesis
Kinematic analysis
and synthesis allow for the study of the motion or position of a set of rigid bodies
in a system without reference to the forces causing that motion or the mass of
the bodies. It allows engineers to see how the mechanisms they design will function
and interact in motion. This kinematic modeler enables the designer to avoid a
faulty design and to apply a variety of scenarios to the model without
constructing a physical prototype. A superior design may be developed after analyzing
the data extracted from kinematic analysis after numerous motion iterations.
The behavior of the resulting model mechanism may be understood prior to production.
Static
Analysis
Static analysis
determines reaction forces at the joint positions of resting mechanisms when a constant
load is applied. As long as zero or constant velocity of the entire system
under study is assumed, static analysis can also be performed on mechanisms at
different points of their range of motion. Static analysis allows the designer
to determine the reaction forces on mechanical systems as well as
interconnection forces transmitted to individual joints. Data extracted from
static analysis can be useful in determining compatibility with the various
criteria set out in the problem definition. These criteria may include reliability,
fatigue, and performance considerations to be analyzed through stress analysis
methods.
Dynamic
Analysis
Dynamic analysis
combines motion with forces in a mechanical system to calculate positions, velocities,
accelerations, and reaction forces on parts in the system. The analysis is
performed stepwise within a given interval of time. Each degree of freedom is
associated with a specific coordinate for which initial position and velocity
must be supplied. Defining the system in various ways creates the computer
model from which the design is analyzed. The user must supply joints, forces,
and overall system coordination either directly or through a manipulation of
data within the software.
Experimental
Analysis
Experimental
analysis involves fabricating a prototype and subjecting it to various
experimental methods.Although this usually takes place in the later stages of
design,CADsystems enable the designer to make more effective use of
experimental data, especially where analytical methods are thought to be
unreliable for the given model. CAD also provides the platform for
incorporating experimental results into the design process.
Design review can
be easily accomplished using CAD. The accuracy of the design can be checked
using automated routines for tolerancing and dimensioning to reduce the
possibility of error. Layering is a technique that allows the designer to
superimpose images on one another. This can be quite useful during the
evaluative stage of the design process by allowing the designer to visually
check the dimensions of a final design against the dimensions of stages of the
design’s proposed manufacture, ensuring that sufficient material is present in
preliminary stages for correct manufacture. Interference checking can also be
performed using CAD. This procedure checks the models and identifies when two
parts of a design occupy the same space at the same time.
Automated
Drafting
Automated drafting
capabilities in CAD systems facilitate presentation, which is the final stage of
the design process. CAD data, stored in computer memory, can be sent to a
plotter or other hard-copy device to produce a detailed drawing printout. In
the early days of CAD, this feature was the primary rationale for investing in
a CAD system. Drafting conventions, including but not limited to dimensioning,
crosshatching, scaling of the design, and enlarged views of parts or other
design areas, can be included automatically in nearly all CAD systems. Detail
and assembly drawings, bills of materials (BOM), and cross-sectioned views of
design parts are also automated and simplified through CAD parts databases. In
addition, most systems are capable of presenting as many as six views of the
design automatically (front, side left, side right, top, bottom, rear).
Drafting standards defined by a company can be programmed into the system such
that all final drafts will comply with the company standards.
Product
Data Management
Product data
management (PDM) is an important application associated with CAD. PDM allows
companies to make CAD data available across the enterprise on computer
networks. For example, PDMsoftware may operate in conjunction with CAD software
and word processing software. This approach holds significant advantages over
conventional data management. PDM is not simply a database holding CAD data as
a library for interested users. PDM systems offer increased data management
efficiency, for example, through a client-server environment. Benefits of
implementing a PDM system include faster retrieval of CAD files through keyword
searches and other search features such as model parameters like color or
serial number, automated distribution of designs to management,manufacturing engineers,
and shop-floor workers for design review, record-keeping functions that provide
a history of design changes, and data security functions limiting access levels
to design files. PDM facilitates the exchange of information characteristic of
the agile workplace. As companies face increased pressure to provide clients
with customized solutions to their individual needs, PDM systems allow an
augmented level of teamwork among personnel at all levels of product design and
manufacturing, cutting down on costs often associated with information lag and
rework.
Although CAD has
made the design process less tedious andmore efficient than traditional methods,
the fundamental design process remains unchanged. It still requires the human
input and ingenuity to initiate and proceed through the many iterations of the
design process. CAD is a powerful, time-saving design tool that competing in
the engineering world without it is difficult if not impossible. The CAD system
will now be examined in terms of its components: the hardware and software of a
computer.
Mechanical Engineers’
Handbook, Volume 2: Design, Instrumentation, and Controls
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