70811_27.pdf

CHAPTER 27

APPLICATION OF

DIGITAL COMPUTERS

Marcos A. Underwood

INTRODUCTION

This chapter introduces numerous applications and tools that are available on and

with digital computers for the solution of shock and vibration problems. First, the

types of computers that are used, the associated specialized processors, and their

input and output peripherals, are considered. This is followed by a discussion of com-

puter applications that fall into the following basic categories: (1) numerical analy-

ses of dynamic systems, (2) experimental applications that require the synthesis of

excitation (driving) signals for electrodynamic and electrohydraulic exciters (shak-

ers), and (3) the acquisition of the associated responses and the digital processing of

these responses to determine important structural characteristics.

The decision to employ a digital computer–based system for the solution of a

shock or vibration problem should be made with considerable care. Before particu-

lar computer software or hardware is selected, the following matters should be care-

fully considered.

1. The existing hardware and/or software that is or is not available to perform the

required task.

2. The extent to which the task or the existing software/hardware must be modified

in order to perform the task.

3. If no applicable software/hardware exists, the extent of the development effort

necessary to create the suitable software and/or hardware subsystems.

4. The detailed assumptions needed in the software/hardware in order to simplify

its development (e.g., linearity, proportional damping, frequency content, sam-

pling rates, etc.).

5. The ability of the software/hardware to measure and compute the output infor-

mation required (e.g., absolute vs. relative motion, phase relationships, rotational

information, etc.).

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CHAPTER TWENTY-SEVEN

6. The detailed input and output limitations of the needed system software and/or

hardware (types of excitation signals, voltage ranges, minimum detectable signal

amplitudes, calculation rates, control speed, graphic outputs, setup parameters,

etc.).

7. The processing power and time needed to perform the task.

8. The algorithms and hardware features that are needed to perform the task.

After these matters are resolved, the user must realize that the results obtained

from the output of a computer system can be no better than its available inputs. For

example, the quality of the natural frequencies and mode shapes obtained from a

structural analysis software system depends heavily on the degree to which the

mathematical model employed represents the actual mass, stiffness, and damping of

the physical structure being analyzed (see Chap. 21). Likewise, a spectral analysis of

a signal with poor signal-to-noise ratio will provide an accurate spectrum of the sig-

nal plus the measurement noise, but not of the signal amplitudes that fall below the

noise floor (see Chap. 22).

DIGITAL COMPUTER TYPES

The digital computer types that are used to solve shock and vibration problems are

varied. There are general-purpose or specialized digital computers. It is generally

better to use general-purpose computers whenever possible, since these types of dig-

ital computers are supported with the best graphics, applications development, sci-

entific and engineering tools, and the wider availability of preexisting applications

software. However, even within these general categories, there are various processor

or computer configurations available to help solve shock and vibration problems.

The following sections provide definitions, descriptions, and discussions of the appli-

cability of general-purpose computers and specialized processors that can help solve

shock and vibration problems.

GENERAL PURPOSE

General-purpose computers are computers designed to solve a wide range of prob-

lems. They are optimized to allow many individual users to access the particular

computer system’s resources. They range from large central systems like main-

frames, which can handle thousands of simultaneous users, to personal computers,

which are designed to serve one interactive user at a time and provide direct and

easy access to the computer system’s computational capability through thousands of

existing applications and its graphical user interface. These are personified by per-

sonal computers based on Wintel (i.e., Windows and Intel) or Power PC technolo-

gies. In the following, mainframes, workstations, personal computers, and palmtop

digital computers are discussed from the viewpoint of their applicability to solve

shock and vibration problems.

Mainframes. Mainframe computers are computer systems that are optimized to

serve many users simultaneously. They typically have large memories, many parallel

central processing units, large-capacity disk storage, and high-bandwidth local net-

work and Internet connections. These systems, when available, can be used to solve the

largest shock and vibration simulations, where very large finite element models or

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APPLICATION OF DIGITAL COMPUTERS

other discrete system models require large memories and the processing power that

mainframes provide. They are also used for web or disk server functions to networked

workstations and personal computers. Mainframe computers are increasingly being

replaced by either powerful workstation or personal computer–based systems.

Workstations. Workstations are computer systems that provide dedicated com-

puter processing for individual users that typically are involved in technically spe-

cialized and complex computing activities. These computer systems usually run a

version of the UNIX operating system using a graphical user interface that is based

on X-windows; X-windows is a set of libraries of graphical software routines, devel-

oped by an industry consortium that provide a standard access to the workstation’s

graphics hardware through a graphical user interface. Workstations often are based

on reduced instruction set computer systems, to be discussed in a later section, with

significant floating point processing power, sophisticated graphic hardware systems,

and access to large disk and random access memory systems. This suits them for

computer-assisted engineering activities like large-scale simulations, mechanical and

electrical system design and drafting, significant applications in the experimental

area that involve many channels of data acquisition and analysis, and the control of

multiexciter vibration test systems. They are designed to efficiently serve one user,

but are inherently multiuser, multitasking, and multiprocessor in nature, and can

serve as a suitable replacement for mainframes in the server arena. These systems

are now mature, with capability still expanding, but merging in the future with high-

powered personal computers. However, due to their maturity, they have an inherent

reliability advantage over personal computers, and thus have a higher suitability for

mission-critical applications. Newer versions of UNIX, like LINUX, allow personal

computer hardware to be used as a workstation, affording the power and reliability

of workstations with the convenience of personal computer hardware.

Personal Computers. Personal computers (PCs) are computer systems that are

intended to be used by casual users and are designed for simplicity of use. PCs orig-

inally were targeted to be used as home- and hobby-oriented computers. Over the

years, PCs have evolved into systems that have central processing units that rival

those of workstations and some older mainframes. PC operating systems have also

evolved to provide access to large disk and random access memories, and a sophisti-

cated graphical user interface. They have many applications in the shock and vibra-

tion arena that are available commercially. These applications include sophisticated

word processors, spreadsheet processors, graphics processors, system modeling tools

like Matlab

, design applications, and countless other computer-aided engineering

applications.

There are also many experimental applications like modal analysis, signal analy-

sis, and vibration control systems that are implemented using PCs. These types of

systems are typically less expensive when they are built using PCs rather than work-

stations. At this time, however, workstations still provide greater performance and

reliability than PCs. PC operating systems are not as robust as those that run on

workstations, although this may change in the future. PCs, however, are ubiquitous

and the hardware and software used to make them continues to expand in capabil-

ity and reliability. It is likely that the PC and workstation categories will ultimately

merge, hopefully preserving the best of both worlds. Currently, most PCs are based

on Wintel technologies, with a smaller percentage based on Power PC technologies.

Palmtops. Palmtop computers (also called hand-held computers ) are computer

systems that are designed for extreme portability and moderate computing applica-

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CHAPTER TWENTY-SEVEN

tions. This type of digital computer system is an outgrowth of electronic organizers.

They are small enough to fit in a shirt pocket, are battery-powered, have small

screens, and thus are useful for note-taking, simple calculations, simple word pro-

cessing, and Internet access. They support simplified versions of popular personal

computer applications with many also supporting handwriting and voice recogni-

tion. They can be employed in the shock and vibration field as remote data gather-

ers that can connect to a host computer to transfer the acquired data to it for further

processing. The host computer is typically a personal computer or workstation.

SPECIALIZED PROCESSORS

Specialized processors are designed for a particular activity or type of calculation

that is being performed. They consist of embedded, distributed, digital signal proces-

sors, and reduced instruction set computer processor architectures. These systems

typically afford the most performance for shock and vibration applications, but at a

higher level of complexity than that associated with the general purpose computers

that were previously discussed. Included in this category are specialized peripherals

such as analog-to-digital (A/D) converters and digital-to-analog (D/A) converters

that provide the fundamental interfaces between computer systems and physical

systems like transducers and exciters, which are used for many shock and vibration

testing and analysis applications. Specialized processor architectures are used exten-

sively in shock and vibration experimental applications, since they provide the nec-

essary power and structure to be able to accomplish some of the more demanding

applications like the control of single or multiple vibration test exciters, or applica-

tions that involve the measurement and analysis of many response channels from a

shock and vibration test.

Embedded Processors. Embedded processors are computer systems that do not

interact directly with the user and are used to accomplish a specialized application.

This type of system is part of a larger system where the embedded portion serves as

an intelligent peripheral for a general purpose computer host like a workstation or

personal computer–based system. The embedded subsystem is used to perform

time-critical functions that are not suitable for a general purpose system due to lim-

itations in its operating systems. The operating system used for embedded proces-

sors is optimized for real-time response and dedicated, for example, to the signal

synthesis, signal acquisition, and processing tasks. The embedded system typically

communicates with the host processor through a high-speed interface like Ethernet,

small computer system interconnect (SCSI), or a direct communication between the

memory busses of the embedded and host computer systems. An embedded com-

puter system does not interface directly with the computer system user, but uses the

host computer system for this purpose. An example of an embedded system, which

uses distributed processors, is shown in Fig. 27.1. Here the host computer is used to

set the parameters for the particular activity, for example, shock and vibration con-

trol and analysis, and uses the embedded computer subsystem to accomplish the

control and analysis task directly. This frees the host processor to simply receive the

results of the shock and vibration task, and to create associated graphic displays for

the system user.

Distributed Computer Systems. Distributed computer systems are digital com-

puters that accomplish their task by using several computer processor systems in

tandem to solve a problem that cannot be suitably solved by an individual computer

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APPLICATION OF DIGITAL COMPUTERS

FIGURE 27.1

Diagram of distributed and embedded system.

or processor system. This type of computer system typically partitions its task in such

a way that each part can be executed in parallel by its respective processor. This

enables the use of several specialized processors to separately accomplish a

demanding subtask, and thus the overall shock and vibration task, in a way that may

not be possible with the use of a single general purpose computer system.

An example of this type of system, as shown in Fig. 27.1, is a distributed and

embedded computer system that uses digital signal processors to process data being

received from an A/D converter by filtering it and extracting the pertinent signal

characteristics needed as part of a shock and vibration test. This filtered data, and its

extracted characteristics, are subsequently sent to a more general processor to per-

form additional analysis on the data. The results of this more general analysis may

yield a time-series data stream that is sent to another digital signal processor for fil-

tering, and then sent to an output D/A converter to produce signals that are used to

excite a system under test. Figure 27.1 also shows, in the form of a block diagram, a

typical form and application of a distributed and embedded subsystem as it would be

used in a shock and vibration test. A specialized embedded operating system is typ-

ically used by the distributed system’s central processing unit (CPU) to coordinate

the communications between and with the two digital signal processor subsystems.

The host processing system is used to interface with the overall system’s user.

Digital Signal Processors. Digital signal processors (DSPs) are specialized

processors that are optimized for the multiply-accumulate operations that are used

in digital filtering and linear algebra–related processing. They are used extensively

in shock and vibration signal analysis and vibration control systems. These proces-

sors are ideal to implement digital filters, for sample-rate reduction and aliasing pro-

tection 1 (see Chap. 14), fast Fourier transform (FFT)–based algorithms (see Chap.

22), and digital control systems. Linear algebra problems, like those encountered in

signal estimation, filtering, and prediction, are also performed efficiently by this

architecture. 2,3 The previous example of an embedded and distributed system in Fig.

27.1 also shows a typical application of DSP technology. The development of this

digital computer architecture has empowered much of the audio and video signal

processing systems in current use. It has also enabled many of the shock and vibra-

tion experimental applications now in use.

Reduced Instruction Set Computer. Reduced instruction set computer (RISC)

systems are computer systems based on specialized processors that are optimized to

execute their computer instructions in a single CPU cycle. In order to execute

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