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53191mdash1432007mdash1849mdashVELUmdash246493mdash CRC ndash pp 1ndash15
copy 2007 by Taylor amp Francis Group LLC
Vibration MonitoringTesting and
Instrumentation
copy 2007 by Taylor amp Francis Group LLC
Mechanical Engineering SeriesFrank Kreith and Roop Mahajan - Series Editors
Published Titles
Computer Techniques in VibrationClarence W de SilvaDistributed Generation The Power Paradigm forthe New MillenniumAnne-Marie Borbely ampJan F KreiderElastoplasticity TheoryVlado A LubardaEnergy Audit of Building SystemsAn Engineering ApproachMoncefKrartiEngineering ExperimentationEuan SomerscalesEntropy Generation MinimizationAdrian BejanFinite Element Method Using MATLABreg2 n d EditionYoung W Kwon amp Hyochoong BangFluid Power Circuits and Controls Fundamentalsand ApplicationsJohn S CundiffFundamentals of Environmental DischargeModelingLorin R DavisHeat Transfer in Single and Multiphase SystemsGregF NatererIntroductory Finite Element MethodChandrakant S Desai amp Tribikram KunduIntelligent Transportation Systems New Principlesand ArchitecturesSumit Ghosh amp Tony LeeMathematical amp Physical Modeling of MaterialsProcessing OperationsOlusegun Johnson Ilegbusi Manabu Iguchi ampWalter E WahnsiedlerMechanics of Composite MaterialsAutarK Kaw
Mechanics of FatigueVladimir V BolotinMechanics of Solids and Shells Theories andApproximationsGerald Wempner amp Demosthenes TalaslidisMechanism Design Enumeration of KinematicStructures According to FunctionLung-Wen Tsai
The MEMS Handbook Second EditionMEMS Introduction and FundamentalsMEMS Design and FabricationMEMS Applications
Mohamed Gad-el-HakNanotechnology Understanding Small SystemsBen Rogers Jesse Adams amp Sumita PennathurNonlinear Analysis of StructuresM SathyamoorthyPractical Inverse Analysis in EngineeringDavid M Trujillo amp Henry R BusbyPressure Vessels Design and PracticeSomnath ChattopadhyayPrinciples of Solid MechanicsRowland Richards JrThermodynamics for EngineersKau-Fui WongVibration Damping Control and DesignClarence W de SilvaVibration Monitoring Testing and InstrumentationClarence W de SilvaVibration and Shock HandbookClarence W de SilvaViscoelastic SolidsRoderic S Lakes
copy 2007 by Taylor amp Francis Group LLC
Vibration MonitoringTesting and
Instrumentation
Edited by
Clarence W de SilvaThe University of British Columbia
Vancouver Canada
Ltfi) CRC PressVV^ J Taylor amp Francis Group
Boca Raton London New York
CRC Press is an imprint of theTaylor amp Francis Croup an informa business
copy 2007 by Taylor amp Francis Group LLC
This material was previously published in Vibration and Shock Handbook copy 2005 by CRC Press LLC
CRC PressTaylor amp Francis Group6000 Broken Sound Parkway NW Suite 300Boca Raton FL 33487-2742
copy 2007 by Taylor amp Francis Group LLCCRC Press is an imprint of Taylor amp Francis Group an Informa business
No claim to original US Government worksPrinted in the United States of America on acid-free paper10987654321
International Standard Book Number-101-4200-5319-1 (Hardcover)International Standard Book Number-13978-1-4200-5319-7 (Hardcover)
This book contains information obtained from authentic and highly regarded sources Reprinted material is quotedwith permission and sources are indicated A wide variety of references are listed Reasonable efforts have been made topublish reliable data and information but the author and the publisher cannot assume responsibility for the validity ofall materials or for the consequences of their use
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Library of Congress Cataloging-in-Publication Data
Vibration monitoring testing and instrumentation editor Clarence W de Silvap cm mdash (Mechanical engineering series)
Includes bibliographical references and indexISBN-13 978-1-4200-5319-7 (alk paper)ISBN-101-4200-5319-1 (alk paper)1 Vibration 2 Shock (Mechanics) I De Silva Clarence W II Title III Series
TA355V5228 200762030287-dc22 2006100139
Visit the Taylor amp Francis Web site athttpwwwtaylorandfranciscom
and the CRC Press Web site athttpwwwcrcpresscom
copy 2007 by Taylor amp Francis Group LLC
Preface
Individual chapters authored by distinguished leaders and experienced professionals in their
respective topics this book provides for engineers technicians designers researchers educators and
students a convenient thorough up-to-date and authoritative reference source on techniques tools
and data for monitoring testing and instrumentation of mechanical vibration and shock Included
are shock and vibration methodologies particularly for civil and mechanical engineering systems
instrumentation and testing methods including sensors exciters signal acquisition conditioning
and recording and LabVIEWw tools for virtual instrumentation testing and design for seismic
vibration and related regulatory issues and human response to vibration Important information
and results are summarized as windows tables graphs and lists throughout the chapters for easy
reference and information tracking References are given at the end of each chapter for further
information and study Cross-referencing is used throughout to indicate other places in the book
where further information on a particular topic is provided
In the book equal emphasis is given to theory and practical application Analytical
formulations design approaches monitoring and testing techniques and commercial software
tools are presented and illustrated Commercial equipment computer hardware and instrumenta-
tion are described analyzed and demonstrated for field application practical implementation and
experimentation Examples and case studies are given throughout the book to illustrate the use
and application of the included information The material is presented in a format that is
convenient for easy reference and recollection
Mechanical vibration is a manifestation of the oscillatory behavior in mechanical systems as a
result of either the repetitive interchange of kinetic and potential energies among components in
the system or a forcing excitation that is oscillatory Such oscillatory responses are not limited to
purely mechanical systems and are found in electrical and fluid systems as well In purely thermal
systems however free natural oscillations are not possible and an oscillatory excitation is needed
to obtain an oscillatory response Sock is vibration caused by brief abrupt and typically high-
intensity excitations Low levels of vibration mean reduced noise and improved work environment
Consequently a considerable effort is devoted today to monitoring studying and modifying the
vibration and shock generated by machinery components machine tools transit vehicles impact
processes civil engineering structures fluid flow systems and aircraft Before designing a system
for good vibratory or shock performance it is important to understand analyze and represent the
dynamic characteristics of the system This may be accomplished through monitoring testing and
analysis of test data which is the emphasis of this book
In recent years educators researchers and practitioners have devoted considerable effort towards
studying monitoring and testing vibration and shock in a range of applications in various branches
of engineering particularly civil mechanical aeronautical and aerospace and production and
v
53191mdash1432007mdash1850mdashVELUmdash246493mdash CRC ndash pp 1ndash15
copy 2007 by Taylor amp Francis Group LLC
manufacturing Specific applications are found in machine tools transit vehicles impact processes
civil engineering structures construction machinery industrial processes product qualification and
quality control fluid flow systems ships and aircraft This book is a contribution towards these
efforts
Clarence W de SilvaEditor-in-Chief
Vancouver Canada
Prefacevi
53191mdash1432007mdash1850mdashVELUmdash246493mdash CRC ndash pp 1ndash15
copy 2007 by Taylor amp Francis Group LLC
Acknowledgments
I wish to express my gratitude to the authors of the chapters for their valuable and highly professional
contributions I am very grateful to Michael Slaughter Acquisitions Editor-Engineering CRC Press for
his enthusiasm and support throughout the project Editorial and production staff at CRC Press have
done an excellent job in getting this volume out in print Finally I wish to lovingly acknowledge the
patience and understanding of my family
vii
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copy 2007 by Taylor amp Francis Group LLC
Editor-in-Chief
Dr Clarence W de Silva PEng Fellow ASME Fellow IEEE Fellow Canadian Academy of
Engineering is Professor of Mechanical Engineering at the University of British Columbia Vancouver
Canada and has occupied the NSERC-BC Packers Research Chair in Industrial Automation since 1988
He has earned PhD degrees from the Massachusetts Institute of Technology and the University of
Cambridge England De Silva has also occupied the Mobil Endowed Chair Professorship in the
Department of Electrical and Computer Engineering at the National University of Singapore He has
served as a consultant to several companies including IBM and Westinghouse in US and has led the
development of eight industrial machines and devices He is recipient of the Henry M Paynter
Outstanding Investigator Award from the Dynamic Systems and Control Division of the American
Society of Mechanical Engineers (ASME) Killam Research Prize Lifetime Achievement Award from the
World Automation Congress Outstanding Engineering Educator Award of IEEE Canada Yasundo
Takahashi Education Award of the Dynamic Systems and Control Division of ASME IEEE Third
Millennium Medal Meritorious Achievement Award of the Association of Professional Engineers of BC
and the Outstanding Contribution Award of the Systems Man and Cybernetics Society of the Institute
of Electrical and Electronics Engineers (IEEE)
He has authored 16 technical books including Sensors and Actuators Control System Instrumentation
(Taylor amp Francis CRC Press 2007) MechatronicsmdashAn Integrated Approach (Taylor amp Francis CRC
Press Boca Raton FL 2005) Soft Computing and Intelligent Systems DesignmdashTheory Tools and
Applications (with F Karry Addison Wesley New York NY 2004) VIBRATION Fundamentals and
Practice (Taylor amp Francis CRC Press 2nd edition 2006) INTELLIGENT CONTROL Fuzzy Logic
Applications (Taylor amp Francis CRC Press 1995) Control Sensors and Actuators (Prentice Hall 1989) 14
edited volumes over 170 journal papers 200 conference papers and 12 book chapters He has served on
the editorial boards of 14 international journals in particular as the Editor-in-Chief of the International
Journal of Control and Intelligent Systems Editor-in-Chief of the International Journal of Knowledge-Based
Intelligent Engineering Systems Senior Technical Editor of Measurements and Control and Regional
Editor North America of Engineering Applications of Artificial Intelligence ndash the International Journal of
Intelligent Real-Time Automation He is a Lilly Fellow at Carnegie Mellon University NASA-ASEE Fellow
Senior Fulbright Fellow at Cambridge University ASI Fellow and a Killam Fellow Research and
development activities of Professor de Silva are primarily centered in the areas of process automation
robotics mechatronics intelligent control and sensors and actuators with cash funding of about $6
million as principal investigator
ix
53191mdash1432007mdash1850mdashVELUmdash246493mdash CRC ndash pp 1ndash15
copy 2007 by Taylor amp Francis Group LLC
Contributors
xi
Kourosh DanaiUniversity of Massachusetts
Amherst Massachusetts
Clarence W de SilvaThe University of British Columbia
Vancouver British Columbia Canada
P S HeynsUniversity of Pretoria
Pretoria South Africa
S HuangNational University of Singapore
Singapore
Hirokazu IemuraKyoto University
Kyoto Japan
Sarvesh Kumar JainMadhav Institute of Technology and
Science
Madhya Pradesh India
Christian LalanneEngineering Consultant
Jalles France
TH LeeNational University of Singapore
Singapore
YP LeowSingapore Institute of Manufacturing
Technology
Singapore
SY LimSingapore Institute of Manufacturing
Technology
Singapore
Jiaohao LinDalian University of Technology
Liaoning Peoplersquos Republic of China
W LinSingapore Institute of Manufacturing
Technology
Singapore
Chris K MechefskeQueenrsquos University
Kingston Ontario Canada
Priyan MendisUniversity of Melbourne
Melbourne Victoria Australia
Tuan NgoUniversity of Melbourne
Melbourne Victoria Australia
Mulyo Harris PradonoKyoto University
Kyoto Japan
C SchefferUniversity of Stellenbosch
Pretoria South Africa
KK TanNational University of Singapore
Singapore
KZ TangNational University of Singapore
Singapore
Yahui ZhangDalian University of Technology
Liaoning Peoplersquos Republic of China
53191mdash1432007mdash1850mdashVELUmdash246493mdash CRC ndash pp 1ndash15
copy 2007 by Taylor amp Francis Group LLC
Contents
1 Vibration Instrumentation Clarence W de Silva 1-1
11 Introduction 1-1
12 Vibration Exciters 1-3
13 Control System 1-15
14 Performance Specification 1-21
15 Motion Sensors and Transducers 1-27
16 Torque Force and Other Sensors 1-50
Appendix 1A Virtual Instrumentation for Data Acquisition Analysis
and Presentation 1-73
2 Signal Conditioning and Modification Clarence W de Silva 2-1
21 Introduction 2-2
22 Amplifiers 2-2
23 Analog Filters 2-15
24 Modulators and Demodulators 2-29
25 AnalogndashDigital Conversion 2-37
26 Bridge Circuits 2-43
27 Linearizing Devices 2-49
28 Miscellaneous Signal Modification Circuitry 2-56
29 Signal Analyzers and Display Devices 2-62
3 Vibration Testing Clarence W de Silva 3-1
31 Introduction 3-1
32 Representation of a Vibration Environment 3-3
33 Pretest Procedures 3-24
34 Testing Procedures 3-37
35 Some Practical Information 3-52
4 Experimental Modal Analysis Clarence W de Silva 4-1
41 Introduction 4-1
42 Frequency-Domain Formulation 4-2
43 Experimental Model Development 4-8
44 Curve Fitting of Transfer Functions 4-10
45 Laboratory Experiments 4-18
46 Commercial EMA Systems 4-24
xiii
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copy 2007 by Taylor amp Francis Group LLC
5 Mechanical Shock Christian Lalanne 5-1
51 Definitions 5-2
52 Description in the Time Domain 5-3
53 Shock Response Spectrum 5-4
54 Pyroshocks 5-17
55 Use of Shock Response Spectra 5-18
56 Standards 5-24
57 Damage Boundary Curve 5-26
58 Shock Machines 5-28
59 Generation of Shock Using Shakers 5-44
510 Control by a Shock Response Spectrum 5-52
511 Pyrotechnic Shock Simulation 5-58
6 Machine Condition Monitoring and Fault Diagnostics Chris K Mechefske 6-1
61 Introduction 6-2
62 Machinery Failure 6-2
63 Basic Maintenance Strategies 6-4
64 Factors Which Influence Maintenance Strategy 6-7
65 Machine Condition Monitoring 6-8
66 Transducer Selection 6-10
67 Transducer Location 6-14
68 Recording and Analysis Instrumentation 6-14
69 Display Formats and Analysis Tools 6-16
610 Fault Detection 6-21
611 Fault Diagnostics 6-25
7 Vibration-Based Tool Condition Monitoring Systems C Scheffer and PS Heyns 7-1
71 Introduction 7-1
72 Mechanics of Turning 7-2
73 Vibration Signal Recording 7-7
74 Signal Processing for Sensor-Based Tool Condition Monitoring 7-11
75 Wear ModelDecision-Making for Sensor-Based Tool Condition Monitoring 7-15
76 Conclusion 7-20
8 Fault Diagnosis of Helicopter Gearboxes Kourosh Danai 8-1
81 Introduction 8-1
82 Abnormality Scaling 8-5
83 The Structure-Based Connectionist Network 8-8
84 Sensor Location Selection 8-11
85 A Case Study 8-14
86 Conclusion 8-23
9 Vibration Suppression and Monitoring in Precision Motion Systems KK Tan
TH Lee KZ Tang S Huang SY Lim W Lin and YP Leow 9-1
91 Introduction 9-1
92 Mechanical Design to Minimize Vibration 9-2
93 Adaptive Notch Filter 9-10
94 Real-Time Vibration Analyzer 9-17
Contentsxiv
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copy 2007 by Taylor amp Francis Group LLC
95 Practical Insights and Case Study 9-29
96 Conclusions 9-35
10 Vibration and Shock Problems of Civil Engineering Structures Priyan Mendis
and Tuan Ngo 10-1
101 Introduction 10-2
102 Earthquake-Induced Vibration of Structures 10-3
103 Dynamic Effects of Wind Loading on Structures 10-22
104 Vibrations Due to FluidndashStructure Interaction 10-33
105 Blast Loading and Blast Effects on Structures 10-34
106 Impact Loading 10-47
107 Floor Vibration 10-51
11 Seismic Base Isolation and Vibration Control Hirokazu Iemura Sarvesh Kumar Jain
and Mulyo Harris Pradono 11-1
111 Introduction 11-1
112 Seismic Base Isolation 11-4
113 Seismic Vibration Control 11-33
12 Seismic Random Vibration of Long-Span Structures Jiahao Lin and Yahui Zhang 12-1
121 Introduction 12-2
122 Seismic Random-Excitation Fields 12-11
123 Pseudoexcitation Method for Structural Random Vibration Analysis 12-16
124 Long-Span Structures Subjected to Stationary Random Ground Excitations 12-27
125 Long-Span Structures Subjected to Nonstationary Random Ground Excitations 12-34
126 Conclusions 12-39
13 Seismic Qualification of Equipment Clarence W de Silva 13-1
131 Introduction 13-1
132 Distribution Qualification 13-1
133 Seismic Qualification 13-6
14 Human Response to Vibration Clarence W de Silva 14-1
141 Introduction 14-1
142 Vibration Excitations on Humans 14-2
143 Human Response to Vibration 14-3
144 Regulation of Human Vibration 14-6
Contents xv
53191mdash1432007mdash1850mdashVELUmdash246493mdash CRC ndash pp 1ndash15
copy 2007 by Taylor amp Francis Group LLC
1Vibration
Instrumentation
Clarence W de SilvaThe University of British Columbia
11 Introduction 1-1
12 Vibration Exciters 1-3Shaker Selection dagger Dynamics of Electromagnetic Shakers
13 Control System 1-15Components of a Shaker Controller dagger Signal-Generating
Equipment
14 Performance Specification 1-21Parameters for Performance Specification dagger Linearity dagger
Instrument Ratings dagger Accuracy and Precision
15 Motion Sensors and Transducers 1-27Potentiometer dagger Variable-Inductance Transducers dagger
Mutual-Induction Proximity Sensor dagger Selfinduction
Transducers dagger Permanent-Magnet Transducers dagger
Alternating Current Permanent-Magnet Tachometer dagger
Alternating Current Induction Tachometer dagger Eddy Current
Transducers dagger Variable-Capacitance Transducers dagger
Piezoelectric Transducers
16 Torque Force and Other Sensors 1-50Strain Gage Sensors dagger Miscellaneous Sensors
Appendix 1A Virtual Instrumentation for DataAcquisition Analysis and Presentation 1-73
Summary
Devices useful in instrumenting a mechanical vibrating system are presented in this chapter Shakers whichgenerate vibration excitations are discussed and compared A variety of sensors including motion sensorsproximity sensors forcetorque sensors and other miscellaneous sensors are considered Performance specificationin the time domain and the frequency domain is addressed Rating parameters of instruments are given
11 Introduction
Measurement and associated experimental techniques play a significant role in the practice of vibration
Academic exposure to vibration instrumentation usually arises in laboratories in the context of learning
training and research In vibration practice perhaps the most important task of instrumentation is the
measurement or sensing of vibration Vibration sensing is useful in the following applications
1 Design and development of a product
2 Testing (screening) of a finished product for quality assurance
3 Qualification of a good-quality product to determine its suitability for a specific application
1-1
53191mdash522007mdash1201mdashSENTHILKUMARmdash246494mdash CRC ndash pp 1ndash112
copy 2007 by Taylor amp Francis Group LLC
4 Mechanical aging of a product prior to carrying out a test program
5 Exploratory testing of a product to determine its dynamic characteristics such as resonances
mode shapes and even a complete dynamic model
6 Vibration monitoring for performance evaluation
7 Control and suppression of vibration
Figure 11 indicates a procedure typical of experimental vibration highlighting the essential
instrumentation Vibrations are generated in a device the test object in response to some excitation In
some experimental procedures primarily in vibration testing (see Figure 11) the excitation signal has
to be generated in a signal generator in accordance with some requirement (specification) and applied to
the object through an exciter after amplification and conditioning In some other situations primarily in
performance monitoring and vibration control the excitations are generated as an integral part of the
operating environment of the vibrating object and may originate either within the object (eg engine
excitations in an automobile) or in the environment with which the object interacts during operation
(eg road disturbances on an automobile) Sensors are needed to measure vibrations in the test object In
particular a control sensor is used to check whether the specified excitation is applied to the object
and one or more response sensors may be used to measure the resulting vibrations at key locations of
the object
The sensor signals have to be properly conditioned for example by filtering and amplification and
modified for example through modulation demodulation and analog-to-digital conversion prior to
recording analyzing and display The purpose of the controller is to guarantee that the excitation is
correctly applied to the test object If the signal from the control sensor deviates from the required
excitation the controller modifies the signal to the exciter so as to reduce this deviation Furthermore the
controller will stabilize or limit (compress) the vibrations in the object It follows that instruments in
experimental vibration may be generally classified into the following categories
1 Signal-generating devices
2 Vibration exciters
3 Sensors and transducers
4 Signal conditioningmodifying devices
5 Signal analysis devices
6 Control devices
7 Vibration recording and display devices
AnalogDigital
Interface
DigitalSignal
RecorderAnalyzerDisplay
FilterAmplifier
FilterAmplifier
SignalGenerator
and ExciterController
Reference (Required)Signal (Specification)
PowerAmplifier
MountingFixtures
TestObject
ResponseSensor
ControlSensor
Exciter
SwivelBase
FIGURE 11 Typical instrumentation in experimental vibration
Vibration Monitoring Testing and Instrumentation1-2
53191mdash522007mdash1201mdashSENTHILKUMARmdash246494mdash CRC ndash pp 1ndash112
copy 2007 by Taylor amp Francis Group LLC
Note that one instrument may perform the tasks
of more than one category listed here Also more
than one instrument may be needed to carry out
tasks in a single category In the following sections
we will provide some examples of the types of
vibration instrumentation giving characteristics
operating principles and important practical
considerations Also we will describe several
experiments which can be found in a typical
vibration laboratory
An experimental vibration system generally
consists of four main subsystems
1 Test object
2 Excitation system
3 Control system
4 Signal acquisition and modification system
These are schematically shown in Figure 12 Note that various components shown in Figure 11 may
be incorporated into one of these subsystems In particular component matching hardware and object
mounting fixtures may be considered interfacing devices that are introduced through the interaction
between the main subsystems as shown in Figure 12 Some important issues of vibration testing and
instrumentation are summarized in Box 11
12 Vibration Exciters
Vibration experimentation may require an external exciter to generate the necessary vibration This is the
case in controlled experiments such as product testing where a specified level of vibration is applied to the
test object and the resulting response is monitored A variety of vibration exciters are available with
different capabilities and principles of operation
Three basic types of vibration exciters (shakers) are widely used hydraulic shakers inertial shakers
and electromagnetic shakers The operation-capability ranges of typical exciters in these three categories
are summarized in Table 11 Stroke or maximum displacement is the largest displacement the exciter
is capable of imparting onto a test object whose weight is assumed to be within its design load limit
Maximum velocity and acceleration are similarly defined Maximum force is the largest force that could
be applied by the shaker to a test object of acceptable weight (one within the design load) The values
given in Table 11 should be interpreted with caution Maximum displacement is achieved only at very
low frequencies The achievement of maximum velocity corresponds to intermediate frequencies in the
operating frequency range of the shaker Maximum acceleration and force ratings are usually achieved at
high frequencies It is not feasible for example to operate a vibration exciter at its maximum
displacement and its maximum acceleration simultaneously
Consider a loaded exciter that is executing harmonic motion Its displacement is given by
x frac14 s sin vt eth11THORN
in which s is the displacement amplitude (or stroke) Corresponding velocity and acceleration are
_x frac14 sv cos vt eth12THORN
eurox frac14 2sv2sin vt eth13THORN
SignalModification
System
TestObject
ControlSystem
VibrationExciter (Shaker)
System
FIGURE 12 Interactions between major subsystems of
an experimental vibration system
Vibration Instrumentation 1-3
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copy 2007 by Taylor amp Francis Group LLC
If the velocity amplitude is denoted by v and the acceleration amplitude by a it follows from Equation
12 and Equation 13 thatv frac14 vs eth14THORN
and
a frac14 vv eth15THORN
Box 11
VIBRATION INSTRUMENTATION
Vibration Testing Applications for Products
Design and Development Production Screening and Quality Assessment Utilization and Qualification for Special Applications
Testing Instrumentation
Exciter (excites the test object) Controller (controls the exciter for accurate excitation) Sensors and Transducers (measure excitations and responses and provide excitation error
signals to controller) Signal Conditioning (converts signals to appropriate form) Recording and Display (perform processing storage and documentation)
Exciters
Shakers
1 Electrodynamic (high bandwidth moderate power complex and multifrequency
excitations)
2 Hydraulic (moderate to high bandwidth high power complex and multifrequency
excitations)
3 Inertial (low bandwidth low power single-frequency harmonic excitations)
TransientInitial Condition
1 Hammers (impulsive bump tests)
2 Cable Release (step excitations)
3 Drop (impulsive)
Signal Conditioning
Filters Amplifiers Amplifiers ModulatorsDemodulators ADCDAC
Sensors
Motion (displacement velocity acceleration) Force (strain torque)
Vibration Monitoring Testing and Instrumentation1-4
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copy 2007 by Taylor amp Francis Group LLC
TABLE 11 Typical Operation-Capability Ranges for Various Shaker Types
Shaker Type Typical Operational Capabilities
Frequency Maximum Displacement
(Stroke)
Maximum
Velocity
Maximum
Acceleration
Maximum
Force
Excitation
Waveform
Hydraulic
(electrohydraulic)
Low
(01ndash500 Hz)
High (20 in 50 cm) Intermediate
(50 insec
125 cmsec)
Intermediate
(20 g)
High (100000 lbf
450000 N)
Average flexibility (simple
to complex and random)
Inertial
(counter-rotating mass)
Intermediate
(2ndash50 Hz)
Low (1 in 25 cm) Intermediate
(50 insec
125 cmsec)
Intermediate
(20 g)
Intermediate
(1000 lbf 4500 N)
Sinusoidal only
Electromagnetic
(electrodynamic)
High
(2ndash10000 Hz)
Low (1 in 25 cm) Intermediate
(50 insec
125 cmsec)
High (100 g) Low to intermediate
(450 lbf 2000 N)
High flexibility and accuracy
(simple to complex and
random)
VibrationInstru
mentation
1-5
53191mdash522007mdash
1201mdashSENTHILKUMARmdash246494mdash
CRCndashpp1ndash112
copy 2007 by Taylor amp Francis Group LLC
An idealized performance curve of a shaker
has a constant displacementndashamplitude region a
constant velocityndashamplitude region and a con-
stant accelerationndashamplitude region for low
intermediate and high frequencies respectively
in the operating frequency range Such an ideal
performance curve is shown in Figure 13(a) on a
frequencyndashvelocity plane Logarithmic axes are
used In practice typical shaker performance
curves would be fairly smooth yet nonlinear
curves similar to those shown in Figure 13(b)
As the mass increases the performance curve
compresses Note that the acceleration limit of a
shaker depends on the mass of the test object
(load) Full load corresponds to the heaviest object
that could be tested The ldquono loadrdquo condition
corresponds to a shaker without a test object To
standardize the performance curves they are
usually defined at the rated load of the shaker A
performance curve in the frequencyndashvelocity
plane may be converted to a curve in the
frequencyndashacceleration plane simply by increasing
the slope of the curve by a unit magnitude (ie
20 dbdecade)
Several general observations can be made from
Equation 14 and Equation 15 In the constant-
peak displacement region of the performance
curve the peak velocity increases proportionally
with the excitation frequency and the peak
acceleration increases with the square of the excitation frequency In the constant-peak velocity region
the peak displacement varies inversely with the excitation frequency and the peak acceleration increases
proportionately In the constant-peak acceleration region the peak displacement varies inversely with the
square of the excitation frequency and the peak velocity varies inversely with the excitation frequency
This further explains why rated stroke maximum velocity and maximum acceleration values are not
simultaneously realized
121 Shaker Selection
Vibration testing is accomplished by applying a specified excitation to the test package using a shaker
apparatus and monitoring the response of the test object Test excitation may be represented by its
response spectrum The test requires that the response spectrum of the actual excitation known as the
test response spectrum (TRS) envelops the response spectrum specified for the particular test known as
the required response spectrum (RRS)
A major step in the planning of any vibration testing program is the selection of a proper shaker
(exciter) system for a given test package The three specifications that are of primary importance in
selecting a shaker are the force rating the power rating and the stroke (maximum displacement) rating
Force and power ratings are particularly useful in moderate to high frequency excitations and the stroke
rating is the determining factor for low frequency excitations In this section a procedure is given to
determine conservative estimates for these parameters in a specified test for a given test package
Frequency domain considerations are used here
Stroke
Limit M
ax
Acceleration
MaxVelocity
Peak
Vel
ocity
(cm
s)
01 1 10 100
FullLoad
NoLoad
Frequency (Hz)(a)
Peak
Vel
ocity
(cm
s)
100
10
1
100
10
1
01 1 10 100
FullLoad
NoLoad
Frequency (Hz)(b)
FIGURE 13 Performance curve of a vibration exciter
in the frequencyndashvelocity plane (log) (a) ideal (b)
typical
Vibration Monitoring Testing and Instrumentation1-6
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copy 2007 by Taylor amp Francis Group LLC
1211 Force Rating
In the frequency domain the (complex) force at the exciter (shaker) head is given by
F frac14 mHethvTHORNasethvTHORN eth16THORNin which v is the excitation frequency variable m is the total mass of the test package including
mounting fixture and attachments asethvTHORN is the Fourier spectrum of the support-location (exciter head)
acceleration and H(v) is frequency response function that takes into account the flexibility and
damping effects (dynamics) of the test package apart from its inertia In the simplified case where the
test package can be represented by a simple oscillator of natural frequency vn and damping ratio by zt
this function becomes
HethvTHORN frac14 1thorn 2jztv=vn=12 ethv=vnTHORN2 thorn 2jztv=vn eth17THORNin which j frac14 ffiffiffiffi
21p
This approximation is adequate for most practical purposes The static weight of the
test object is not included in Equation 16 Most heavy-duty shakers which are typically hydraulic
have static load support systems such as pneumatic cushion arrangements that can exactly balance the
dead load The exciter provides only the dynamic force In cases where shaker directly supports the
gravity load in the vertical test configuration Equation 16 should be modified by adding a term to
represent this weight
A common practice in vibration test applications is to specify the excitation signal by its response
spectrum This is simply the peak response of a simple oscillator expressed as a function of its natural
frequency when its support location is excited by the specified signal Clearly the damping of the simple
oscillator is an added parameter in a response spectrum specification Typical damping ratios ethzrTHORN used inresponse spectra specifications are less than 01 (or 10) It follows that an approximate relationship
between the Fourier spectrum of the support acceleration and its response spectrum is
as frac14 2jzrarethvTHORN eth18THORNThe magnitude larethvTHORNl is the response spectrumEquation 18 substituted into Equation 16 gives
F frac14 mHethvTHORN2jzrarethvTHORN eth19THORNIn view of Equation 17 for test packages having low damping the peak value of H(v) is
approximately 1=eth2jztTHORN this should be used in computing the force rating if the test package has a
resonance within the frequency range of testing On the other hand if the test package is assumed to be
rigid then HethvTHORN oslash 1 A conservative estimate for the force rating is
Fmax frac14 methzr=ztTHORNlarethvTHORNlmax eth110THORNIt should be noted that larethvTHORNlmax is the peak value of the specified (required) response spectrum (RRS)
for acceleration
1212 Power Rating
The exciter head does not develop its maximum force when driven at maximum velocity Output power
is determined by using
p frac14 Refrac12FvsethvTHORN eth111THORNin which vsethvTHORN is the Fourier spectrum of the exciter velocity and Re [ ] denotes the real part of a complex
function Note that as frac14 jvvs Substituting Equation 16 and Equation 18 into Equation 111 gives
p frac14 eth4mz2r =vTHORNRefrac12jHethvTHORNa2r ethvTHORN eth112THORNIt follows that a conservative estimate for the power rating is
pmax frac14 2methz2r =ztTHORNfrac12larethvTHORNl2=v max eth113THORN
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53191mdash522007mdash1201mdashSENTHILKUMARmdash246494mdash CRC ndash pp 1ndash112
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Representative segments of typical acceleration RRS curves have slope n as given by
a frac14 k1vn eth114THORN
It should be clear from Equation 113 that the maximum output power is given by
pmax frac14 k2v2n21 eth115THORN
This is an increasing function for n 1=2 and a decreasing function for n 1=2 It follows that the power
rating corresponds to the highest point of contact between the acceleration RRS curve and a line of slope
equal to 12 A similar relationship may be derived if velocity RRS curves (having slopes n2 1) are used
1213 Stroke Rating
From Equation 18 it should be clear that the Fourier spectrum xs of the exciter displacement time
history can be expressed as
xs frac14 2zrarethvTHORN=jv2 eth116THORNAn estimate for stroke rating is
xmax frac14 2zrfrac12larethvTHORNl=v2 max eth117THORNThis is of the form
xmax frac14 kvn22 eth118THORNIt follows that the stroke rating corresponds to the highest point of contact between the acceleration
RRS curve and a line of slope equal to two
Example 11
A test package of overall mass 100 kg is to be
subjected to dynamic excitation represented by the
acceleration RRS (at 5 damping) as shown in
Figure 14 The estimated damping of the test
package is 7 The test package is known to have a
resonance within the frequency range of the
specified test Determine the exciter specifications
for the test
Solution
From the development presented in the previous
section it is clear that the point F (or P) in
Figure 14 corresponds to the force and output
power ratings and the point S corresponds to
the stroke rating The co-ordinates of these
critical points are F P frac14 eth42 Hz 40 gTHORN and S frac14 eth08 Hz 075 gTHORN Equation 110 gives the force
rating as
Fmax frac14 100 pound eth005=007THORN pound 40 pound 981 N frac14 2803 N
Equation 113 gives the power rating as
pmax frac14 2 pound 100 pound eth0052=007THORN pound frac12eth40 pound 981THORN2=42 pound 2p watts frac14 417 W
Equation 117 gives the stroke rating as
xmax frac14 2 pound 005 pound frac12eth075 pound 98THORN=eth08 pound 2pTHORN2 m frac14 3 cm
Acc
eler
atio
n(g
)
Con
stan
t Dis
plac
emen
t
Constant Velo
city
Frequency (Hz)
S
FP
0101
10 10 100
10
10
FIGURE 14 Test excitation specified by an accelera-
tion RRS (5 damping)
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copy 2007 by Taylor amp Francis Group LLC
1214 Hydraulic Shakers
A typical hydraulic shaker consists of a piston-cylinder arrangement (also called a ram) a servo-valve a
fluid pump and a driving electric motor Hydraulic fluid (oil) is pressurized (typical operating pressure
4000 psi) and pumped into the cylinder through a servo-valve by means of a pump that is driven by an
electric motor (typical power 150 hp) The flow (typical rate 100 galmin) that enters the cylinder is
controlled (modulated) by the servo-valve which in effect controls the resulting piston (ram) motion
A typical servo-valve consists of a two-stage spool valve which provides a pressure difference and a
controlled (modulated) flow to the piston which sets it in motion
The servo-valve itself is moved by means of a linear torque motor which is driven by the excitation-
input signal (electrical) A primary function of the servo-valve is to provide a stabilizing feedback to the
ram In this respect the servo-valve complements the main control system of the test setup The ram is
coupled to the shaker table by means of a link with some flexibility The cylinder frame is mounted on the
support foundation with swivel joints This allows for some angular and lateral misalignment which
might be caused primarily by test-object dynamics as the table moves
Two-degree-of-freedom (Two-DoF) testing requires two independent sets of actuators and three-DoF
testing requires three independent actuator sets Each independent actuator set can consist of several
actuators operated in parallel using the same pump and the same excitation-input signal to the torque
motors
If the test table is directly supported on the vertical actuators they must withstand the total dead
weight (ie the weight of the test table the test object the mounting fixtures and the instrumentation)
This is requirement is usually prevented by providing a pressurized air cushion in the gap between the
test table and the foundation walls Air should be pressurized so as to balance the total dead weight
exactly (typical required gage pressure 3 psi)
Figure 15(a) shows the basic components of a typical hydraulic shaker The corresponding
operational block diagram is shown in Figure 15(b) It is desirable to locate the actuators in a pit in the
test laboratory so that the test tabletop is flushed with the test laboratory floor under no-load conditions
This minimizes the effort required to place the test object on the test table Otherwise the test object has
to be lifted onto the test table with a forklift Also installation of an aircushion to support the system
dead weight is difficult under these circumstances of elevated mounting
Hydraulic actuators are most suitable for heavy load testing and are widely used in industrial and civil
engineering applications They canbeoperated at very low frequencies (almost direct current [DC]) aswell
as at intermediate frequencies (see Table 11) Large displacements (stroke) are possible at low frequencies
Hydraulic shakers have the advantage of providing high flexibility of operation during the test their
capabilities include variable-force and constant-force testing and wide-band random-input testing The
velocity and acceleration capabilities of hydraulic shakers are moderate Although any general excitation-
input motion (for example sine wave sine beat wide-band random) can be used in hydraulic shakers
faithful reproduction of these signals is virtually impossible at high frequencies because of distortion and
higher-order harmonics introduced by the high noise levels that are common in hydraulic systems This
is only a minor drawback in heavy-duty intermediate-frequency applications Dynamic interactions are
reduced through feedback control
1215 Inertial Shakers
In inertial shakers or ldquomechanical excitersrdquo the force that causes the shaker-table motion is generated by
inertia forces (accelerating masses) Counter-rotating-mass inertial shakers are typical in this category
To understand their principle of operation consider two equal masses rotating in opposite directions at
the same angular speed v and in the same circle of radius r (see Figure 16) This produces a resultant
force equal to 2mv2r cos vt in a fixed direction (the direction of symmetry of the two rotating arms)
Consequently a sinusoidal force with a frequency of v and an amplitude proportional to v2 is generated
This reaction force is applied to the shaker table
Figure 17 shows a sketch of a typical counter-rotating-mass inertial shaker It consists of two identical
rods rotating at the same speed in opposite directions Each rod has a series of slots in which to
Vibration Instrumentation 1-9
53191mdash522007mdash1201mdashSENTHILKUMARmdash246494mdash CRC ndash pp 1ndash112
copy 2007 by Taylor amp Francis Group LLC
place weights In this manner the magnitude of
the eccentric mass can be varied to achieve various
force capabilities The rods are driven by a
variable-speed electric motor through a gear
mechanism that usually provides several speed
ratios A speed ratio is selected depending on the
required test-frequency range The whole system is
symmetrically supported on a carriage that is
directly connected to the test table The test object
is mounted on the test table The preferred
mounting configuration is horizontal so that the
excitation force is applied to the test object in a
horizontal direction In this configuration there
are no variable gravity moments (weight pounddistance to center of gravity) acting on the drive
mechanism Figure 17 shows the vertical con-
figuration In dynamic testing of large structures
the carriage can be mounted directly on the
structure at a location where the excitation force
should be applied By incorporating two pairs of
counter-rotating masses it is possible to generate
test moments as well as test forces
FIGURE 15 A typical hydraulic shaker arrangement (a) schematic diagram (b) operational block diagram
m m
2mw2r cos wt
wtw w
wt
FIGURE 16 Principle of operation of a counter-
rotating-mass inertial shaker
Vibration Monitoring Testing and Instrumentation1-10
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copy 2007 by Taylor amp Francis Group LLC
Reaction-type shakers driven by inertia are widely used for the prototype testing of civil engineering
structures Their first application dates back to 1935 Inertial shakers are capable of producing
intermediate excitation forces The force generated is limited by the strength of the carriage frame The
frequency range of operation and the maximum velocity and acceleration capabilities are also
intermediate for inertial shakers whereas the maximum displacement capability is typically low A major
limitation of inertial shakers is that their excitation force is exclusively sinusoidal and that the force
amplitude is directly proportional to the square of the excitation frequency As a result complex and
random excitation testing constant-force testing (for example transmissibility tests and constant-force
sine-sweep tests) and flexibility to vary the force amplitude or the displacement amplitude during a test
are not generally feasible with this type of shakers Excitation frequency and amplitude can be varied
during testing however by incorporating a variable-speed drive for the motor The sinusoidal excitation
generated by inertial shakers is virtually undistorted which gives them an advantage over the other types
of shakers when used in sine-dwell and sine-sweep tests Small portable shakers with low-force capability
are available for use in on-site testing
1216 Electromagnetic Shakers
In electromagnetic shakers or ldquoelectrodynamic excitersrdquo the motion is generated using the principle of
operation of an electric motor Specifically the excitation force is produced when a variable excitation
signal (electrical) is passed through a moving coil placed in a magnetic field
The components of a commercial electromagnetic shaker are shown in Figure 18 A steady magnetic
field is generated by a stationary electromagnet that consists of field coils wound on a ferromagnetic base
that is rigidly attached to a protective shell structure The shaker head has a coil wound around it When
the excitation electrical signal is passed through this drive coil the shaker head which is supported on
flexure mounts will be set in motion The shaker head consists of the test table on which the test object
is mounted Shakers with interchangeable heads are available The choice of appropriate shaker head is
based on the geometry and mounting features of the test object The shaker head can be turned to
different angles by means of a swivel joint In this manner different directions of excitation (in biaxial
and triaxial testing) can be obtained
FIGURE 17 Sketch of a counter-rotating-mass inertial shaker
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copy 2007 by Taylor amp Francis Group LLC
122 Dynamics of Electromagnetic Shakers
Consider a single axis electromagnetic shaker (Figure 18) with a test object having a single natural
frequency of importance within the test frequency range The dynamic interactions between the shaker
and the test object give rise to two significant natural frequencies (and correspondingly two significant
resonances) These appear as peaks in the frequency response curve of the test setup Furthermore the
natural frequency (resonance) of the test package alone causes a ldquotroughrdquo or depression (antiresonance)
in the frequency response curve of the overall test setup To explain this characteristic consider the
dynamic model shown in Figure 19 The following mechanical parameters are defined for
Figure 19(a) m k and b are the mass stiffness and equivalent viscous damping constant respectively
of the test package and me ke and be are the corresponding parameters of the exciter (shaker) Also in
the equivalent electrical circuit of the shaker head as shown in Figure 19(b) the following electrical
parameters are defined Re and Le are the resistance and (leakage) inductance and kb is the back
electromotive force (back emf) of the linear motor Assuming that the gravitational forces are supported
FIGURE 18 Schematic sectional view of a typical electromagnetic shaker manufactured by Bruel and Kjaer
Denmark
Vibration Monitoring Testing and Instrumentation1-12
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copy 2007 by Taylor amp Francis Group LLC
by the static deflections of the flexible elements and that the displacements are measured from the static
equilibrium position we have the following system equations
Test object meuroy frac14 2kethy2 yeTHORN2 beth_y2 _yeTHORN eth119THORNShaker head me euroye frac14 fe thorn kethy2 yeTHORN thorn beth_y2 _yeTHORN2 key 2 be _ye eth120THORN
Electrical Lediedt
thorn Reie thorn kb _ye frac14 vethtTHORN eth121THORN
The electromagnetic force fe generated in the shaker head is a result of the interaction of the magnetic
field generated by the current ie with coil of the moving shaker head and the constant magnetic field
(stator) in which the head coil is located Here we have
fe frac14 kbie eth122THORNNote that v(t) is the voltage signal that is applied by the amplifier to the shaker coil ye is the displacement
of the shaker head and y is the displacement response of the test package
It is assumed that kb has consistent electrical and mechanical units (Vmsec and NA) Usually
the electrical time constant of the shaker is quite small compared with the primarily mechanical
time constants of the shaker and the test package In such cases the Ledie=dt term in Equation 121 may
be neglected Consequently the equations from Equation 119 through Equation 122 may be
expressed in the Laplace (frequency) domain with the Laplace variable s taking the place of the derivative
d=dt as
ethms2 thorn bsthorn kTHORNy frac14 ethbsthorn kTHORNye eth123THORN
FIGURE 19 Dynamic models of an electromagnetic shaker and a flexible test package (a) mechanical model
(b) electrical model
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copy 2007 by Taylor amp Francis Group LLC
frac12mes2 thorn ethbthorn beTHORNsthorn ethkthorn keTHORN ye frac14 ethbsthorn kTHORNy thorn kb
Rev2
k2bs
Reye eth124THORN
It follows that the transfer function of the shaker head motion with respect to the excitation voltage is
given by
yevfrac14 kb
Re
DethsTHORNDdethsTHORN eth125THORN
where DethsTHORN is the characteristic function of the primary dynamics of the test object
DethsTHORN frac14 ms2 thorn bsthorn k eth126THORN
and DdethsTHORN is the characteristic function of the primary dynamic interactions between the shaker and thetest object
DcethsTHORN frac14 mmes4 thorn frac12methbe thorn bthorn boTHORN thornmeb s
3 thorn frac12methke thorn kTHORN thornmekthorn bethbe thorn boTHORN s2thorn frac12bke thorn ethbe thorn boTHORNk sthorn kke
eth127THORN
where
bo frac14 k2bRe
eth128THORN
It is clear that under low damping conditions DdethsTHORN will produce two resonances as it is fourth order in sand similarly DethsTHORN will produce one antiresonance (trough) corresponding to the resonance of the testobject Note that in the frequency domain s frac14 jv and hence the frequency response function given by
Equation 125 is in fact
yevfrac14 kb
Rb
DethjvTHORNDdethjvTHORN eth129THORN
The magnitude of this frequency response function for a typical test system is sketched in Figure 110
Note that this curve is for the ldquoopen-looprdquo case where there is no feedback from the shaker controller
In practice the shaker controller will be able
to compensate for the resonances and anti-
resonances to some degree depending on its
effectiveness
The main advantages of electromagnetic
shakers are their high frequency range of
operation their high degree of operating flexi-
bility and the high level of accuracy of the
generated shaker motion Faithful reproduction
of complex excitations is possible because of the
advanced electronics and control systems used in
this type of shakers Electromagnetic shakers are
not suitable for heavy-duty applications (large test
objects) however High test-input accelerations
are possible at high frequencies when electromag-
netic shakers are used but their displacement and
velocity capabilities are limited to low or
intermediate values (see Table 11)
100
10
1 10 100Excitation Frequency (Hz)
Antiresonance
Resonance
Shak
erD
ispl
acem
entM
agni
tude
Resonance
1000
FIGURE 110 Frequency response curve of a typical
electromagnetic shaker with a test object
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copy 2007 by Taylor amp Francis Group LLC
1221 Transient Exciters
Other varieties of exciters are commonly used in
transient-type vibration testing In these tests
either an impulsive force or an initial excitation is
applied to the test object and the resulting
response is monitored The excitations and the
responses are ldquotransientrdquo in this case Hammer
test drop tests and pluck tests fall into this
category For example a hammer test may be
conducted by hitting the object with an instru-
mented hammer and then measuring the response
of the object The hammer has a force sensor at its
tip as sketched in Figure 111 A piezoelectric or
strain-gage type force sensor may be used More
sophisticated hammers have impedance heads in
place of force sensors An impedance head
measures force and acceleration simultaneously
The results of a hammer test will depend on many
factors for example the dynamics of the hammer
body how firmly the hammer is held during the
impact how quickly the impact is applied and
whether there are multiple impacts
13 Control System
The two primary functions of the shaker control system in vibration testing are (1) to guarantee that the
specified excitation is applied to the test object and (2) to ensure that the dynamic stability (motion
constraints) of the test setup is preserved An operational block diagram illustrating these control
functions is given in Figure 112 The reference input to the control system represents the desired
excitation force that should be applied to the test object In the absence of any control however the force
reaching the test object will be distorted primarily because of (1) dynamic interactions and
nonlinearities of the shaker the test table the mounting fixtures the auxiliary instruments and the test
object itself (2) noise and errors in the signal generator amplifiers filters and other equipment and (3)
external loads and disturbances acting on the test object and other components (for example external
restraints aerodynamic forces friction) To compensate for these distorting factors response
measurements (displacements velocities acceleration and so on) are made at various locations in the
test setup and are used to control the system dynamics In particular the responses of the shaker the test
table and the test object are measured These responses are used to compare the actual excitation felt by
FIGURE 111 An instrumented hammer used in bump
tests or hammer tests
ExcitationInput
(Reference)
Feedback Paths
DriveSignal
ShakerResponse
TestTable
TestObject
Test ObjectResponse
Test TableResponseExciter
(Shaker)(Ram)
Controllerand
Amplifier
FIGURE 112 Operational block diagram illustrating a general shaker control system
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copy 2007 by Taylor amp Francis Group LLC
the test object at the shaker interface with the desired (specified) input The drive signal to the shaker is
modified depending on the error that is present
Two types of control are commonly employed in shaker apparatus simple manual control and
complex automatic control Manual control normally consists of simple open-loop trial-and-error
methods of manual adjustments (or calibration) of the control equipment to obtain a desired dynamic
response The actual response is usually monitored (on an oscilloscope or frequency analyzer screen for
example) during manual-control operations The pretest adjustments in manual control can be very
time-consuming as a result the test object might be subjected to overtesting which could produce
cumulative damage is undesirable and could defeat the test purpose Furthermore the calibration
procedure for the experimental setup must be repeated for each new test object
The disadvantages ofmanual control suggest that automatic control is desirable in complex test schemes
in which high accuracy of testing is desired The first step of automatic control involves automatic
measurement of the system response using control sensors and transducers The measurement is then fed
back into the control system which instantaneously determines the best drive signal to actuate the shaker
in order to get the desired excitation This may be done by either analog or digital methods
Primitive control systems require an accurate mathematical description of the test object This
dependency of the control system on the knowledge of test-object dynamics is clearly undesirable
Performance of a good control system should not be considerably affected by the dynamic interactions
and nonlinearities of the test object or by the nature of the excitation Proper selection of feedback signals
and control-system components can reduce such effects and will make the system robust
In the response-spectrum method of vibration testing it is customary to use displacement control at
low frequencies velocity control at intermediate frequency and acceleration control at high frequencies
This necessitates feedback of displacement velocity and acceleration responses Generally however the
most important feedback is the velocity feedback In sine-sweep tests the shaker velocity must change
steadily over the frequency band of interest In particular the velocity control must be precise near the
resonances of the test object Velocity (speed) feedback has a stabilizing effect on the dynamics which is
desirable This effect is particularly useful in ensuring stability in motion when testing is done near
resonances of lightly damped test objects On the contrary displacement (position) feedback can have a
destabilizing effect on some systems particularly when high feedback gains are used
The controller usually consists of various instruments equipment and computation hardware and
software Often the functions of the data-acquisition and processing system overlap with those of the
controller to some extent As an example consider the digital-controller of vibration testing apparatus
First the responses are measured through sensors (and transducers) filtered and amplified
(conditioned) These data channels may be passed through a multiplexer whose purpose is to select
one data channel at a time for processing Most modern data acquisition hardware does not need a
separate multiplexer to handle multiple signals The analog data are converted into digital data using
analog-to-digital converters (ADCs) The resulting sampled data are stored on a disk or as a block data in
the computer memory The reference input signal (typically a signal recorded on an FM tape) is also
sampled (if it is not already in the digital form) using an ADC and fed into the computer Digital
processing is done on the reference signal and the response data with the objective of computing the
command signal to drive the shaker The digital command signal is converted into an analog signal using
a digital-to-analog converter (DAC) and amplified (conditioned) before it is used to drive the exciter
The nature of the control components depends to a large extent on the nature and objectives of the
particular test to be conducted Some of the basic components in a shaker controller are described in
the following subsections
131 Components of a Shaker Controller
1311 Compressor
A compressor circuit is incorporated in automatic excitation control devices to control the excitation-
input level automatically The level of control depends on the feedback signal from a control sensor and
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copy 2007 by Taylor amp Francis Group LLC
the specified (reference) excitation signal Usually the compressor circuit is included in the excitation-
signal generator (for example in a sine generator) The control by this means may be done on the basis of
a single-frequency component (eg the fundamental frequency)
1312 Equalizer (Spectrum Shaper)
Random-signal equalizers are used to shape the spectrum of a random signal in a desired manner In
essence and equalizer consists of a bank of narrow-band filters (for example 80 filters) in parallel over
the operating frequency range By passing the signal through each filter the spectral density (or the mean
square value) of the signal in that narrow frequency band (for example each one-third-octave band) is
determined This is compared with the desired spectral level and automatic adjustment is made in that
filter in case there is an error In some systems response-spectrum analysis is made in place of power
spectral density analysis In that case the equalizer consists of a bank of simple oscillators whose
resonant frequencies are distributed over the operating frequency range of the equalizer The feedback
signal is passed through each oscillator and the peak value of its output is determined This value is
compared with the desired response spectrum value at that frequency If there is an error automatic gain
adjustment is made in the appropriate excitation signal components
Random-noise equalizers are used in conjunction with random signal generators They receive
feedback signals from the control sensors In some digital control systems there are algorithms (software)
that are used to iteratively converge the spectrum of the excitation signal felt by the test object into the
desired spectrum
1313 Tracking Filter
Many vibration tests are based on single-frequency excitations In such cases the control functions
should be performed on the basis of the amplitudes of the fundamental-frequency component of the
signal A tracking filter is simply a frequency-tuned band-pass filter It automatically tunes the center
frequency of its very narrow-band-pass filter to the frequency of a carrier signal Then when a noisy
signal is passed through the tuned filter the output of the filter will be the required fundamental
frequency component in the signal Tracking filters also are useful in obtaining amplitudendashfrequency
plots using an XndashY plotter In such cases the frequency value comes from the signal generator (sweep
oscillator) which produces the carrier signal to the tracking filter The tracking filter then determines the
corresponding amplitude of a signal that is fed into it Most tracking filters have dual channels so that two
signals can be handled (tracked) simultaneously
1314 Excitation Controller (Amplitude Servo-Monitor)
An excitation controller is typically an integral part of the signal generator It can be set so that automatic
sweep between two frequency limits can be performed at a selected sweep rate (linear or logarithmic)
More advanced excitation controllers have the capability of an automatic switch-over between constant-
displacement constant-velocity and constant-acceleration excitation-input control at specified
frequencies over the sweep frequency interval Consequently integrator circuits should be present
within the excitation controller unit to determine velocities and displacements from acceleration signals
Sometimes integration is performed by a separate unit called a vibration meter This unit also offers
the operator the capability of selecting the desired level of each signal (acceleration velocity or
displacement) There is an automatic cut-off level for large displacement values that could result from
noise in acceleration signals A compressor is also a subcomponent of the excitation controller The
complete unit is sometimes known as an amplitude servo-monitor
132 Signal-Generating Equipment
Shakers are force-generating devices that are operated using drive (excitation) signals generated from a
source The excitation-signal source is known as the signal generator Three major types of signal
generators are used in vibration testing applications (1) oscillators (2) random-signal generators and
(3) storage devices In some units oscillators and random-signal generators are combined We shall
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copy 2007 by Taylor amp Francis Group LLC
discuss these two generators separately however because of their difference in function It also should be
noted that almost any digital signal (deterministic or random) can be generated by a digital computer
using a suitable computer program the signal eventually can be passed through a DAC to obtain
the corresponding analog signal These lsquodigitalrsquo signal generators along with analog sources such as
magnetic tape players (FM) are classified into the category of storage devices
The dynamic range of equipment is the ratio of the maximum and minimum output levels (expressed
in decibels) at which it is capable of operating without significant error This is an important specification
for many types of equipment particularly for signal-generating devices The output level of the signal
generator should be set to a value within its dynamic range
1321 Oscillators
Oscillators are essentially single-frequency generators Typically sine signals are generated but other
waveforms (such as rectangular and triangular pulses) are also available in most oscillators Normally
an oscillator has two modes of operation (1) sweeping up and down between two frequency limits and
(2) dwelling at a specified frequency In the sweep operation the sweep rate should be specified This can
be done either on a linear scale (Hzmin) or on a logarithmic scale (octavesmin) In the dwell operation
the frequency points (or intervals) should be specified In either case a desired signal level can be chosen
using the gain-control knob An oscillator that is operated exclusively in the sweep mode is called a sweep
oscillator
The early generation of oscillators employed variable inductor-capacitor types of electronic circuits to
generate signals oscillating at a desired frequency The oscillator is tuned to the required frequency by
varying the capacitance or inductance parameters A DC voltage is applied to energize the capacitor
and to obtain the desired oscillating voltage signal which subsequently is amplified and conditioned
Modern oscillators use operational amplifier circuits along with resistor capacitor and semiconductor
(SC) elements Also common are crystal (quartz) parallel-resonance oscillators used to generate voltage
signals accurately at a fixed frequency The circuit is activated using a DC-voltage source Other
frequencies of interest are obtained by passing this high-frequency signal through a frequency converter
The signal is then conditioned (amplified and filtered) Required shaping (for example rectangular
pulse) is obtained using a shape circuit Finally the required signal level is obtained by passing the
resulting signal through a variable-gain amplifier A block diagram of an oscillator illustrating various
stages in the generation of a periodic signal is given in Figure 113
A typical oscillator offers a choice of several (typically six) linear and logarithmic frequency ranges and
a sizable level of control capability (for example 80 dB) Upper and lower frequency limits in a sweep can
be preset on the front panel to any of the available frequency ranges Sweep-rate settings are continuously
DC Voltage
Oscillator
FrequencySpecification
FrequencyConverter
FilterAmplifier
ShaperPeriodicSignal
FrequencyCounter
Fixed-FrequencySignal
Variable-GainOutput
Amplifier
SignalSpecification
LevelSpecification
FIGURE 113 Block diagram of an oscillator-type signal generator
Vibration Monitoring Testing and Instrumentation1-18
53191mdash522007mdash1202mdashSENTHILKUMARmdash246494mdash CRC ndash pp 1ndash112
copy 2007 by Taylor amp Francis Group LLC
- 53191fm
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- Vibration Monitoring Testing and Instrumentation
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- Preface
- Acknowledgments
- Editor-in-Chief
- Contributors
- Contents
- Related Titles
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- 53191ch1
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- Table of Contents
- Chapter 1 Vibration Instrumentation
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- Summary
- 11 Introduction
- 12 Vibration Exciters
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- 121 Shaker Selection
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- 1211 Force Rating
- 1212 Power Rating
- 1213 Stroke Rating
- 1214 Hydraulic Shakers
- 1215 Inertial Shakers
- 1216 Electromagnetic Shakers
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- 122 Dynamics of Electromagnetic Shakers
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- 1221 Transient Exciters
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- 13 Control System
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- 131 Components of a Shaker Controller
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- 1311 Compressor
- 1312 Equalizer (Spectrum Shaper)
- 1313 Tracking Filter
- 1314 Excitation Controller (Amplitude Servo-Monitor)
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- 132 Signal-Generating Equipment
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- 1321 Oscillators
- 1322 Random Signal Generators
- 1323 Tape Players
- 1324 Data Processing
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- 14 Performance Specification
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- 141 Parameters for Performance Specification
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- 1411 Time-Domain Specifications
- 1412 Frequency-Domain Specifications
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- 142 Linearity
- 143 Instrument Ratings
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- 1431 Rating Parameters
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- 144 Accuracy and Precision
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- 15 Motion Sensors and Transducers
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- 151 Potentiometer
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- 1511 Potentiometer Resolution
- 1512 Optical Potentiometer
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- 152 Variable-Inductance Transducers
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- 1521 Mutual-Induction Transducers
- 1522 Linear-Variable Differential Transformer
- 1523 Signal Conditioning
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- 153 Mutual-Induction Proximity Sensor
- 154 Selfinduction Transducers
- 155 Permanent-Magnet Transducers
- 156 Alternating Current Permanent-Magnet Tachometer
- 157 Alternating Current Induction Tachometer
- 158 Eddy Current Transducers
- 159 Variable-Capacitance Transducers
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- 1591 Capacitive Displacement Sensors
- 1592 Capacitive Angular Velocity Sensor
- 1593 Capacitance Bridge Circuit
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- 1510 Piezoelectric Transducers
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- 15101 Sensitivity
- 15102 Piezoelectric Accelerometer
- 15103 Charge Amplifier
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- 16 Torque Force and Other Sensors
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- 161 Strain Gage Sensors
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- 1611 Equations for Strain Gage Measurements
- 1612 Bridge Sensitivity
- 1613 The Bridge Constant
- 1614 The Calibration Constant
- 1615 Data Acquisition
- 1616 Accuracy Considerations
- 1617 Semiconductor Strain Gages
- 1618 Force and Torque Sensors
- 1619 Strain Gage Torque Sensors
- 16110 Deflection Torque Sensors
- 16111 Variable-Reluctance Torque Sensor
- 16112 Reaction Torque Sensors
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- 162 Miscellaneous Sensors
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- 1621 Stroboscope
- 1622 Fiber Optic Sensors and Lasers
- 1623 Fiber-Optic Gyroscope
- 1624 Laser Doppler Interferometer
- 1625 Ultrasonic Sensors
- 1626 Gyroscopic Sensors
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- References
- Appendix 1A Virtual Instrumentation for Data Acquisition Analysis and Presentation
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- Summary
- List of Abbreviations
- 1A1 Dynamic Signals
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- 1A11 Acquiring and Simulating Dynamic Signals
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- 1A111 Aliasing
- 1A112 Time Continuity
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- 1A2 Measurement Configuration Considerations
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- 1A21 Input Signal Considerations
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- 1A211 Input Pseudodifferential and Differential Configuration
- 1A212 Gain
- 1A213 Input Coupling
- 1A214 Integrated Electronic Piezoelectric Excitation
- 1A215 Nyquist Frequency and Bandwidth
- 1A216 Analog-to-Digital Conversion
- 1A217 Antialias Filters
- 1A218 Input Filter Delay
- 1A219 Overload Detection
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- 1A22 Output Signal Considerations
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- 1A221 Output Pseudodifferential and Differential Configuration
- 1A222 Attenuation
- 1A223 Digital-to-Analog Conversion
- 1A224 Anti-imaging and Interpolation Filters
- 1A225 Output Filter Delay
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- 1A3 Scaling and Calibration
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- 1A31 Scaling to Engineering Units
- 1A32 Performing System Calibration
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- 1A321 Propagation Delay Calibration
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- 1A4 Limit Testing Analysis
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- 1A41 Limit Testing Overview
- 1A42 Using the SVT Limit Testing VI
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- 1A5 Integration
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- 1A51 Introduction to Integration
- 1A52 Implementing Integration
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- 1A521 Challenges when Integrating Vibration Data
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- 1A53 Time-Domain Integration
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- 1A531 Single-Shot Acquisition and Integration
- 1A532 Continuous Acquisition and Integration
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- 1A54 Frequency-Domain Integration
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- 1A6 Vibration-Level Measurements
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- 1A61 Measuring the Root Mean Square Level
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- 1A611 Single-Shot Buffered Acquisition
- 1A612 Continuous Signal Acquisition
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- 1A62 Performing a Running RMS Level Measurement
- 1A63 Computing the Peak Level
- 1A64 Computing the Crest Factor
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- 1A7 Frequency Analysis
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- 1A71 FFT Fundamentals
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- 1A711 Number of Samples
- 1A712 Frequency Resolution
- 1A713 Maximum Resolvable Frequency
- 1A714 Minimum Resolvable Frequency
- 1A715 Number of Spectral Lines
- 1A716 Relationship between Time-Domain and Frequency-Domain Specifications and Parameters
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- 1A72 Increasing Frequency Resolution
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- 1A721 Zoom FFT Analysis
- 1A722 Frequency Resolution of the Zoom FFT VIs
- 1A723 Zoom Measurement
- 1A724 Zoom Settings
- 1A725 Subset Analysis
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- 1A8 Transient Analysis
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- 1A81 Transient Analysis with the Sound and Vibration Toolkit
- 1A82 Performing an STFT vs Time
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- 1A821 Selecting the FFT Block Size
- 1A822 Overlapping
- 1A823 Using the SVT STFT vs Time VI
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- 1A83 Performing an STFT vs Rotational Speed
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- 1A831 Converting the Pulse Train to Rotational Speed
- 1A832 STFT vs RPM
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- 1A84 Measuring a Shock Response Spectrum
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- 1A9 Waterfall Display
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- 1A91 Using the Display VIs
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- 1A911 Initializing the Display
- 1A912 Sending Data to the Display
- 1A913 Waterfall Display for Frequency Analysis
- 1A914 Waterfall Display for Transient Analysis
- 1A915 Waterfall Display for Octave Spectra
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- 1A10 Swept-Sine Measurements
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- 1A101 Swept-Sine Overview
- 1A102 Choosing Swept-Sine vs FFT Measurements
- 1A103 Taking a Swept-Sine Measurement
- 1A104 Swept-Sine Measurement Example
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- Bibliography
- Related Titles
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- 53191ch2
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- Table of Contents
- Chapter 2 Signal Conditioning and Modification
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- Summary
- 21 Introduction
- 22 Amplifiers
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- 221 Operational Amplifier
- 222 Use of Feedback in Opamp
- 223 Voltage Current and Power Amplifiers
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- 2231 Charge Amplifiers
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- 224 Instrumentation Amplifiers
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- 2241 Differential Amplifier
- 2242 Common Mode
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- 225 Amplifier Performance Ratings
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- 2251 Common-Mode Rejection Ratio
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- 226 Component Interconnection
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- 2261 Impedance Characteristics
- 2262 Cascade Connection of Devices
- 2263 AC-Coupled Ampliers
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- 23 Analog Filters
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- 231 Passive Filters and Active Filters
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- 2311 Number of Poles
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- 232 Low-Pass Filters
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- 2321 Low-Pass Butterworth Filter
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- 233 High-Pass Filters
- 234 Band-Pass Filters
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- 2341 Resonance-Type Band-Pass Filters
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- 235 Band-Reject Filters
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- 24 Modulators and Demodulators
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- 241 Amplitude Modulation
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- 2411 Modulation Theorem
- 2412 Side Frequencies and Side Bands
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- 242 Application of Amplitude Modulation
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- 2421 Fault Detection and Diagnosis
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- 243 Demodulation
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- 25 Analog ndashDigital Conversion