an overview of vibration copyrighted material testing

1An Overview of Vibration

Testing

Rotating machinery diagnostics are being conducted using a portable vibrationfrequency analyzer specifically designed for such field in situ testing tasks. Suchmachinery monitoring is important to achieve optimum machine performance overlong periods of time with minimum unscheduled maintenance. (Photo courtesy ofSound and Vibration)

1

COPYRIG

HTED M

ATERIAL

2 AN OVERVIEW OF VIBRATION TESTING

1.1 INTRODUCTION

This book is dedicated to obtaining the maximum possible benefit whenapplying vibration testing techniques to a wide range of practical vibra-tion problems. Maximum benefits are obtained when appropriate instru-mentation and analysis techniques are used that are consistent with thedesired test objectives for a given machine and/or structure. The mostsophisticated and brilliant analysis methods are easily defeated by poor,inaccurate, or inappropriate data. Thus, one needs to understand the fun-damental principles that are involved in these tests so that an informedskepticism can be brought to bear on the test results. “Why is this glitchhere?” is often an important starting point in understanding that theremay be problems with a certain set of test data.

Vibration tests are run for a number of reasons. Among them are:

1. Engineering development testing2. Qualification testing3. Reliability qualification testing4. Production screening testing.5. Machinery condition monitoring.

The same types of instruments, frequency analyzers, and analysismethods are employed in nearly all types of tests. Different types ofvibration exciters are employed in other tests. In some cases, the testsare run in the laboratory. In other cases, the only way the test can beproperly conducted is in the field under actual operating conditions todetermine why a particular part of the structure is either quickly deterio-rating or failing to function correctly under service conditions. Becauseof this wide range of techniques and goals, the intent of this book isto cover a large range of fundamental concepts that are useful in mostvibration testing situations .

The point at which I became aware of vibration testing as an engi-neering activity isn’t important, but I do know that it was not during thelaboratory exercises during my first vibrations course. However, whenI worked at the Naval Ship Research Center at Carterrock, Maryland,I began to sense that getting good field vibration data was not a sim-ple matter. At any rate, over the years a number of experiences haveled me to attempt to identify what vibration testing is and how vibra-tion testing fits into the engineering function. One thing is evident: Inattempting to identify what vibration testing is, interesting facets cometo light.

1.1 INTRODUCTION 3

/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

Gun Turret

XA

Direction of acceleration measurement

Direction of major impulse

Figure 1.1.1 Schematic of a gun turret showing location of acceleration measure-ments.

I clearly remember an interesting event that occurred in the late 1960sat a Shock and Vibration Symposium. The essence of the story is thatthe U.S. Navy wanted an electronic device mounted on top of a largegun turret, as shown in Fig. 1.1.1. Apparently, navy personnel measuredthe vertical acceleration at point A, where the electronic device was tobe mounted, while several shells were fired from the gun. These accel-eration records were used to develop the device’s dynamic environment .The electronic device was designed, tested, and passed this dynamicenvironment. Yet when the device was installed, it failed to functionafter the first firing. Obviously, there was a problem in translating themeasured dynamic environment into an adequate test specification. Whathappened?

First, the vertical acceleration at point A is probably significantly lessthan the horizontal acceleration resulting from the reaction to firing thegun in a nearly horizontal direction. Second, the electronic device wasmounted so that it presented a significant frontal area to the gun’s airblast. It is clear that either of these two inputs could be the sourceof significant shock loading and large vibration levels. Obviously, theprocedures employed were inadequate and caused unnecessary grief toall parties concerned.

This story illustrates that significant questions need to be answered inplanning vibration tests, such as:

1. What field data should be taken?2. Where should the transducers be placed on the test item to achieve

the maximum useful information over a given frequency band-width?


3. How should the field data be stored for future reference and recall?4. What is the effect of changing boundary conditions between field

and test environments due to test fixtures?5. Which testing procedures are best suited in simulating a given field

environment?

These and certainly other questions need to be answered in orderto properly plan the field test as well as a corresponding laboratorysimulation.

In another instance, while I was preparing to conduct vibration testson a company’s product, I found the test specifications to be quite con-tradictory. If I followed one set of specifications, it appeared that nothingwould fail because the inputs were so low, whereas following another setof specifications could result in a premature failure because the inputswere so large. In struggling with these contradictory requirements, itbecame evident that the test specifications were quite arbitrary, sinceno one could provide me with actual field data. Thus, no clear meanswas available to determine what the test specifications should be, andwe were left to make our own judgments. Obviously, we chose to beconservative, which in turn added unnecessary weight to the product.

I had enlightening experiences during two summers where I wasinvolved in reviewing the shock and vibration testing procedures emplo-yed at Sandia National Laboratories. This review involved interviewswith more than a dozen of Sandia’s very experienced vibration testpersonnel. It became clearly evident that some underlying rules forconducting these tests were not being properly addressed by the teststandards they were required to use. Fundamental questions about howfield data is obtained, stored, and converted to useful test specificationswere often discussed during these interviews. Clearly, a better frame-work is required by which to judge what procedures should be used.While attempting to understand and explain what these rules should be,the basic thoughts contained in Chapter 8 of this book were developedas a framework to guide the processes used in establishing a vibrationtesting program for a given application. This area requires considerablymore thought and research to understand the implications of the manychoices we can make in writing a test specification from field vibrationdata.

A recent experience brought to light an interesting situation where acompany’s product was experiencing a high rate of field failures andyet passed the vibration test standard that is used in the industry. Theproblem was a lack of understanding the type of vibration they were

1.1 INTRODUCTION 5

trying to simulate in the laboratory. A different type of signal analysisgenerated a much more severe and realistic vibration test specification.Following the Sandia experience, I surveyed practitioners of vibrationtesting at more than a dozen sites in the United States in order to deter-mine a broader view of the current state of events in the vibration testingindustry. A striking result from this survey is that there is no general the-ory for vibration testing, so test specifications are often modified basedon the tester’s experience, much like I had to do when testing a com-pany’s product. The survey also revealed that if any model is referredto, it is most often that of a single degree of freedom (SDOF) system,a highly inappropriate model for most testing cases that involve rathercomplicated structural systems.

Typical comments that were returned are summarized as follows:

1. Writers of test specifications do not recognize test equipment lim-itations.

2. Required test spectra do not reflect actual vehicle (field) environ-ments.

3. The relation between test level and test item endurance seems tobe quite arbitrary.

4. Stress level and duration need to be sensibly related to reliabilityand endurance testing.

5. Test fixtures are usually much stiffer than the host vehicle.6. Unless a person oversees instrumentation installation and subse-

quent data reduction, there is usually insufficient data available toseparate sources of excitation.

7. When data is collected by others, sensor location is often poorlydefined.

8. “Many people operate in a particular way because that is what theylearned to do, without necessarily knowing why or how to predictsresults.”1

9. “It takes much more competence to do it correctly than it does todo it the ‘standard’ way!”1

It is hoped that this book will illuminate many of the required fun-damental concepts so that test personnel will become aware of oppor-tunities to conduct tests better—and hence obtain maximum possiblebenefits.

1Personal communication from E. A. Szymkowiak, July 1990.


1.2 PRELIMINARY CONSIDERATIONS

It is hard to decide where to start this adventure, but experience hasshown that an overview and precise definitions significantly improve thecommunication process. Figure 1.2.1 shows a general model of a struc-ture where it is assumed that a transducer (usually an accelerometer)measures the motion Xp at location p. Xp is used to represent displace-ment, velocity, or acceleration. A common notation used throughout thisbook is that lowercase letters represent a time domain variable, so thatx = x(t), and capital letters represent a frequency domain variable, sothat X = Xp(ω) is a frequency spectrum. This notation is most conve-nient, since we often have both time and frequency spectra informationavailable from our instrumentation.

Motion Xp is caused by numerous excitation sources. These sourcescan be internal forces (such as rotating unbalances) that are representedby Sq(= Sq(ω)) as well as external forces . The external forces are bro-ken into two subgroups: those that are due to external sources and arerepresented by Fq(= Fq(ω)) and those that are due to the presence of aboundary and are represented by Bq(= Bq(ω)). Then, if we use the lin-ear input-output frequency domain representation Hpq(= Hpq(ω)), wecan express the frequency domain output motion as

Xp =n1∑

q=1

Hpq Sq

internal

+n2∑

n1+1

Hpq Fq

external

+n3∑

n2+1

HpqBq

boundary

(1.2.1)

External Fq

Boundary Bq

o

Sn1

S1

Sqo o

pHpq

Xp

Internal

i

j

k

Figure 1.2.1 General structure showing internal, external, and boundary types ofexcitation forces.

1.2 PRELIMINARY CONSIDERATIONS 7

where Hpq is the frequency response function (abbreviated asFRF) that is a function of frequency ω representsthe output motion at p due the excitation forces atlocation q,

n1 is the number of internal sources,(n2 − n1 − 1) is the number of external forces.(n3 − n2 − 1) is the number of boundary forces.

FRF Hpq can represent receptance, mobility , oraccelerance dependent on Xp being displacement,velocity, or acceleration, respectively.

The situation shown in Fig. 1.2.1 is characteristic of the general vibra-tion testing situation where the structure (or machine) vibrations aremonitored in order to determine either its dynamic characteristics orits mechanical health. The purpose of the test depends on the end useof the test data. In order to determine the structure’s dynamic charac-teristics, we might do a modal analysis. On the other hand, we maymeasure the machine’s dynamic characteristics in order to determine itsoperating condition, a process that is often called machine monitoring .The test structure could be either an aircraft engine or a steam turbine.Obviously, the boundary conditions, test conditions, and instrumentationrequirements are strikingly different for each of these two machines.

Now suppose that a critical circuit board is used in controlling thejet engine, and this circuit board cannot fail during operation. We needto ensure that the circuit board will not only survive but continue tofunction properly when in this dynamic environment. The question is,how do we go about achieving this goal?

We begin by breaking the problem down as illustrated in Fig. 1.2.2,where we identify several major components that are involved in the pro-cess of gathering data, interpreting this data, communicating the resultsto the major participants. The components are the aircraft, which weshall call the vehicle, the circuit board, which we shall call the test item ,the vibration laboratory, which we shall call the laboratory , and thefinite element computer program or other design tool, which we shallcall simply design , and the process . In the process part we need to:

1. Obtain experimental field data, which may involve installation ofa prototype test item, a different test item, or no test item

2. Analyze the field data, taking field test conditions into account3. Communicate the field data in the proper form to the designer for

use during the design stage


Vehicle Laboratory

Test Item

Design

Process

Figure 1.2.2 Defintion of major elements and data flow under various test configu-rations.

4. Provide vibration test personnel with realistic data so that ade-quate laboratory testing is performed to achieve a realistic dynamicenvironment

This process often requires communications between persons fromdifferent government and/or industrial organizations, and these peoplemay have their own vested interests that are not completely compatiblewith each other. Consequently, interesting personal relationship situ-ations often occur in large projects, and the importance of skills forworking together becomes evident. However, if everyone can keep theireye on the target and cooperate for the common good of the project,the result will be a successful product that is reliable, cost effective, andperforms as expected. The full implications of Fig. 1.2.2 will be evidentas the reader proceeds through the book.

1.3 GENERAL INPUT-OUTPUT RELATIONSHIPSIN THE FREQUENCY DOMAIN

The input-output frequency response function (FRF) plays an impor-tant role in experimental vibration testing in general and modal analysisin particular. Generally, standard testing arrangements employ eitherone or more electrodynamic vibration exciters or an impulse hammeras excitation sources. These excitation sources usually have minimumbending moments if properly implemented. However, in general, bound-ary forces actually consist of both a resultant force (a vector) and aresultant moment (a vector). Each of these vectors has three orthogonalcomponents in unit vector directions of i, j, and k as shown in Fig. 1.2.1.Similarly, the resultant motion at any point can have three orthogonal

1.3 GENERAL INPUT-OUTPUT RELATIONSHIPS IN THE FREQUENCY DOMAIN 9

linear motions: and three orthogonal angular motions; one motion foreach (i, j ,k) direction.

The frequency-domain input-output relationship between the mea-sured output motion at location p due to an input excitation appliedat location q as shown in Fig. 1.2.1, can be stated as

{U }p = [H ]pq {P }q (1.3.1)

where {U }p = {U(ω)}p is the frequency-domain motion outputvector composed of six entries, three linear motionsand three angular motions, that are associated withlocation p.

{P }q = {P(ω)}q is the frequency-domain input vector that isapplied at location q and is composed of six loadcomponents, three forces, and three moments.

[H ]pq = [H(ω)]pq is the 6 × 6 input-output FRF matrixrelating these input and output quantities, and thuscontains 36 FRFs for a single pair of measurementpoints on the SUT! An expanded form of Eq. (1.3.1)is given by

{{X}{�}

}

p

=[

[H ]xF [H ]xM

[H ]θF [H ]θM

]

pq

{{F }{M}

}

q

(1.3.2)

In Eq. (1.3.2), {X} = {X(ω)} and {�} = {�(ω)} are each 1 × 3 vec-tors containing the linear and angular output motions, respectively, and{F } = {F(ω)} and {M} = {M(ω)} are each 1 × 3 vectors containing theforce and moment input components. As can be seen from Eq. (1.3.2), thefull FRF matrix [H(ω)] is partitioned into four submatrices where three ofthese submatrices contain information about rotational DOFs. The 3 × 3[H ]xF matrix relates linear motions to the forces and corresponds to thedata obtained in most standard testing procedures. The [H ]xM HxM

= [H ]θFmatrices contain the FRFs relating either output linear motions due tomoment inputs or angular motions due to force inputs. The [H ]θMHθM

matrix relates the angular output motions due to moment inputs.The importance of rotational degrees of freedom becomes clear when

an element of the linear motion vector is obtained from Eq. (1.3.2)such as

X1p = H11pqF1q + H12pq F2q + H13pq F3q + H̃11pqM1q

+ H̃12pq M2q + H̃13pq M3q (1.3.3)


where subscripts 1, 2, and 3 refer to directions parallel to orthogonalCartesian x, y , and z directions and H̃rcpq are the linear output/momentinput-output FRFs. Thus, Eq. (1.3.3) shows that six terms are requiredto fully characterize the relationship between structure’s linear motionX1p and its force/moment inputs.

If a single axis accelerometer is attached at the pth location with itsprimary sensitivity axis oriented in the 1-direction, and only a singleinput force F1q is measured, then Eq. (1.3.3) reduces to

X1p∼= H11pq F1q (1.3.4)

which is only approximately true unless care is taken to reduce theunmeasured inputs to being negligible, since the remaining five termsfrom Eq. (1.3.3) are not accounted for in the measurements describedby Eq. (1.3.3). This situation becomes more troublesome when the teststructure is attached to another structure at several points. Generally,the test fixture will not simulate the actual field structure, so that thetest item’s dynamic performance may be significantly different in thelaboratory compared to the field conditions. These issues are discussedin Chapter 8.

1.4 OVERVIEW OF EQUIPMENT EMPLOYED

There are many different types of vibration tests. Some involve fieldmeasurements while the structure is in its normal operational state; othersinvolve situations where the structure is excited by some external means,either in place in the field or in a laboratory setting. The purpose of thesetests can cover a wide range of reasons, such as vibration monitoringin order to determine a machine’s suitability for operation, a generalvibration survey to find out what is happening, a complete modal analysisto determine the structure’s dynamic characteristics, and so on. However,in each instance, commonly used concepts and equipment are involved.

Figure 1.4.1 shows a generic test item with its motion is measured byone transducer system (usually an accelerometer) and the input force ismeasured by a second transducer system (usually a force transducer). Ineither case, these transducers often employ either the piezoelectric or thestrain gage type of sensing element. The electronic signals from thesetransducers are amplified electronically and analyzed. The frequencyanalyzer is commonly employed for analysis purposes. It then becomesthe engineers’ job to interpret the resulting frequency spectra and to store

1.4 OVERVIEW OF EQUIPMENT EMPLOYED 11

Exciter o

Frequencyanalyzer

XOutput fromstructure

oTestItem

Amplifier

Amplifier

Force transducer measuresinput to structure

o

oComputer fordata storage and analysis

Figure 1.4.1 Generic test items showing force and motion transducers, amplifiers,frequency analyzer, and computer data storage and analysis.

the data in a suitable form. A computer is often used for this purpose.As one thinks about the processes shown in Fig. 1.4.1, it becomes

evident that certain common concepts and physical laws are employed.The task is then to organize these concepts and laws in an orderly fashionso that we can explore their importance. This information is organizedinto seven additional chapters as follows.

Chapter 2, “Dynamic Signal Analysis,” is devoted to the basic conceptsinvolved in dynamic signal analysis, since these concepts underliemuch of what we do in later chapters. Periodic, transient, and randomsignals are considered on a theoretical basis.

Chapter 3, “Vibration Concepts,” is a review of basic vibration concepts.While we could cite this material from other texts, we have includedit here because a certain set of fundamental ideas and view points arerequired in order to understand what is happening in vibration testingand why procedures should be done in a certain way.

Chapter 4, “Transducer Measurement Considerations,” is concerned withhow transducers behave. Models are dveloped for the mechanical andelectrical characteristics of common transducers. Experience indicatesthat transducers can do interesting and unpredictable things that arecaused by the environment that we put them into. Often, the user candramatically change the transducer’s behavior, so an informed user isrequired to obtain reliable data.

Chapter 5, “The Digital Frequency Analyzer,” carefully examines thecharacteristics of the digital analyzer. The rules by which a digitalanalyzer works are important to understanding what the measuredfrequency spectrum means. If the analyzer is improperly set, the cal-culated results may contain grossly distorted information.


Chapter 6, “Vibration Excitation Mechanisms,” concerned with the dy-namic characteristics of vibration exciters. These excitations includestatic release, mechanical, and electromagnetic-type devices. The inter-action of these exciters with their test environment is also explored.

Chapter 7, “The Application of Basic Concepts to Vibration Testing,”looks at what happens when we put everything together while attempt-ing to conduct an actual test. Several simple examples are used toillustrate significant factors that must be considered and how thesefactors can influence the experimental results.

Chapter 8, “General Vibration Testing Model: From the Field to theLaboratory,” explores the framework for conducting field tests andconverting the results into a meaningful laboratory test specification.

1.5 SUMMARY

This introduction suggests that a broad range of physical laws and math-ematical concepts are involved in the practical execution of a successfulvibration test. This book is dedicated to exploring these laws and con-cepts so that practicing test engineers as well as graduate students willbe able to understand some of the subtleties that can occur. No resultscan be better than the instruments employed to derive them. As willbe seen, recent research shows that instruments as well as frequencyanalyzer use can be the Achilles’ heel to the entire process.

In closing this first chapter, we offer the following definition:

Vibration testing. The art and science of measuring and understanding astructure’s response while exposed to a specific dynamic environment; andif necessary, simulating this environment in a satisfactory manner to ensurethat the structure will either survive or function properly when exposed tothis dynamic environment under field conditions.

As with all definitions, this one has its limitations. If any reader canfind a more inclusive statement, we would appreciate your sharing itwith us.

an overview of vibration copyrighted material testing

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