Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (19 page)

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antee that the software will need a large amount of computational and network
power, and the Web does not always provide enough power. Of course, CAD us-
ers are generally only using one of these functions at a time, so with enough
“granularity” to the software, it could perhaps be served in small enough pieces
that the user is not affected by the poor performance of the network. Another dif-
ficulty is the large data files. Even meager 3-D models can produce rather large
data files. The size of these files can be another impediment to getting good re-
sponse time from the ASP approach.

An advantage of the ASP approach is outsourcing of the maintenance and
administration of the CAD software. In this case, a company can be relieved of
the very significant demands of managing and administering the complicated
CAD system. On the other hand, relying on an outside source for something as
vital and proprietary to a company as the information in the CAD system may be
a real problem. So the ASP approach may be the best solution for some compa-
nies, but not others (probably based on the overall size of the company).

3.6 DATA ACQUISITION SOFTWARE

Another type of software that may be relevant to CAD systems and users is data
acquisition. This is a class of software that is used in laboratories and shop floors.
They are meant to measure physical parameters and record the information on a
computer system. The physical parameters may just be geometric such as
lengths, diameters, and distances. The physical parameters may also be such
things as pressures, temperatures, accelerations, deflections, etc.

The two likely scenarios for interfacing a CAD system with data acquisi-
tion are analytical verification and manufacturing verification. For analytical ver-
ification, the product geometry contained in the CAD system may be used to
generate simulations of the product behavior (attempting to predict whether the
product will fail in service, how fast it can go, etc.). This computer-based analysis
often requires the information from the CAD system. Subsequently, when a phys-
ical prototype is built, the system then can be instrumented to acquire physical
data from the prototype. Finally, this data (created with the data acquisition sys-
tem) may need to be re-imported back into the CAD system. This new data or
geometry can then be compared with the original design geometry or the analyti-
cal models for design verification (as discussed by quality system standards such
as ISO 9001).

The other potential scenario for data acquisition systems being an issue for
a CAD system is manufacturing verification. In this case, parts or assemblies that
have been manufactured are checked against the actual design (as documented in
a drawing or as modeled in a 3-D model). Once again, there is a need to take
physical measurements (this time geometry-based measurements) and then “im-

Computer Software Basics 69

port” the data to the CAD system. These physical measurements are usually
made by a device called a Coordinate Measuring Machine (CMM). The CMM
has a heavy table or other grounding mechanism to make sure that the item to be
measured is not disturbed. Then there is usually a probe of some kind that is used
to touch the part at very specific locations. For instance, a CAD model may indi-
cate that at a point on a surface where X is 5 mm and Y is 20 mm, the Z value of
the point is supposed to be 10 mm. The probe is moved with respect to the
ground to the very precise X and Y value (such as 5 and 20). Then, the probe is
moved along the Z direction until it touches the part. This value of Z can then be
compared to the 10 mm theoretical value from the CAD model. Obviously, if the
values agree closely (say within a fraction of 1 mm), then the model can be con-
sidered verified.

Data acquisition software is rather unique. It runs on a computer system at
a low level. It needs to communicate directly with devices on a signal basis (as
opposed to using files or other traditional digital data), so at some point the sig-
nals (referred to as electrical analog information) need to be converted to digital
data (such as storing the physical characteristic as a real number in a real data
file). This conversion is performed by circuits referred to as Analog-to-Digital
Converters (or just “A-to-D”). The actual electrical signals connected to the pro-
totype or the devices that detect the location of the probe on a CMM are referred
to as transducers. As the analog signals from the transducers are sent to the com-
puter controlling the data acquisition, the computer often samples a number of
the wired connections from the transducers. These separate connections are often
referred to as channels. This may allow many transducers to be used on a single
test with a single computer. However, the amount of time that the computer soft-
ware analyzes the data from a particular transducer is reduced depending on the
number of transducers being sampled.

Data acquisition software is often closely tied to particular computer hard-
ware (CPU as well as transducers and data acquisition or DAC circuit boards).
The entire process of acquiring and formatting the data from the devices involves
knowing exactly how the devices function. Also, specific tests that a company
performs are often highly customized, so the combination of transducers and
software may only be applicable to a particular company.

When the data acquisition software and/or hardware is customized for a
particular company, special programming techniques may be used. In particular,
a Programmable Logic Controller (PLC) may be used to drive a specific se-
quence of steps or operations in the test (including sending data directly to a
CAD system). These PLCs often employ a type of programming called Relay
Ladder Logic (RLL). This programming would typically be done by specialists
in the data acquisition or test engineering fields (as opposed to a CAD user or
administrator).

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3.7 CHAPTER EXERCISES

1. Determine the machine code instructions for the CPU chip that was
found in the computer in the Chapter 2 Exercises. Record how many instructions
are used by the chip.

2. Determine how many different operating systems are in use at your
campus or company. Record the version numbers, maintenance levels, service
packs, etc. for these operating systems.

3. Create and edit an ASCII file with the computer system’s file editor
program.

4. Using computer system documentation as needed, create a shell script,
command shortcut, or batch file that starts the CAD software loaded on the
system.

5. Record what disk drive or storage resources available for the computer
system is actually accessed via the network.

3.8 CHAPTER REVIEW

1. List each of the software layers that are found on a typical computer

system that runs CAD. Describe each of the layers.

2. Why can’t a binary file be reliably transferred between different oper-

ating systems?

3. In an ASCII file, how does the operating system “know” when each

line in the file ends?

4. What operating system is likely involved if a file is identified as the

following: /u/sharon/config.sjs?

5. What operating system is likely involved if a file is identified as the

following: C:\CADMAN\CONFIG.SJS?

4

Drawings and 2-D Design

4.1 INTRODUCTION

This chapter presents information on the standards of “drafting” or mechanical
drawings. This is certainly an essential background for understanding 2-D CAD
and CAD systems in general. The production of these drawings is generally still
the end “result” of the CAD system for many companies.

Generally, if one is interested in fully creating a design (and not just creat-
ing drawings), then 3-D CAD is certainly the more appropriate choice. Although
3-D modeling is discussed in later chapters, keep in mind that drawings and 2-D
design still has a role to play in 3-D CAD systems. First of all, 3-D models may
be used as the “source” of geometric information in drawings (created automati-
cally by the software). Secondly, the first step in the creation of 3-D models is
often to create 2-D “sketch” geometry on a plane. So, the information in this
chapter can provide a foundation for 3-D CAD use, as well.

4.2 BACKGROUND

For perhaps a century or more, the creation and use of paper drawings was the
best and perhaps only means of truly documenting a design. These drawings took

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various forms ranging from small handmade sketches to large sheets of paper
many meters long using drafting machines. Eventually, the overall format as well
as the smaller details were standardized across entire nations, and to some degree
internationally.

Although the author is convinced that paper drawings and 2-D electronic
data eventually will be completely eclipsed by the sophistication of 3-D models
(particularly when portable 3-D model viewing devices are cheaply available), it
remains an necessity that those dealing with design work be familiar with draw-
ings. Indeed, once 3-D models are the only medium, many of the concepts in
drawings will continue to be found in annotated 3-D models.

There are a number of types of drawings. A basic list is shown in Table 4.1.
These drawings can generally be categorized based on the engineering discipline
they are related to. For instance, electrical schematics would be typical for elec-
trical engineering, A/E/C and survey drawings for civil engineering, plant draw-
ings for industrial engineering, and, of course, mechanical drawings for
mechanical engineering. There are specialized CAD software packages that are
geared toward specific disciplines, but many CAD packages can be used for more
than one of these disciplines. It is worth mentioning that for electrical engineer-
ing, in particular, that there is a class of software that is referred to as E-CAD.
These packages are usually meant for electronic, printed circuit board (PCB), or
integrated circuit (IC) development activities. They also may have the ability to
simulate the electronic behavior, and create schematic drawings based on the
component layout and design. However, E-CAD is beyond the scope of this
work, although there are occasional references to it.

Mechanical drawings are one of the most important types of documentation
used by the manufacturing sector. These drawings have been standardized and
refined to serve the needs of design, manufacturing, purchasing, and legal depart-

TABLE
4.1

Some Basic Types and Uses of Drawings

Type Use

Mechanical drawings Document the design and to some degree the manufacturing

processes of individual mechanical parts (often called “de-

tails”) as well as the assembly or arrangements of parts.
Schematics Used by the electrical, hydraulic, and pneumatic disciplines.

They document the wiring or piping of a system as well as

the constituent componentry of the circuit.

A/E/C Used in various types of construction projects (architectural,

civil engineering, and job sites).

Plant engineering Used for building and/or facilities management.

Surveys Document various types of property.

Drawings and 2-D Design 73

ments of countless firms involved with manufacturing products worldwide. Fur-
thermore, they are “living” documents to a great extent since they are not just
created for a particular project (such as with many architectural or civil engineer-
ing projects). The mechanical product may be manufactured for many decades
and mass produced into millions of copies that must be revised and “change con-
trolled” throughout its life cycle.

Due to the complexity of the mechanical drawings, their tendency to domi-
nate the attention of manufacturers, and since the author is a mechanical engi-
neer, mechanical drawings will form the foundation of the information presented
by this work.