fundamentals of electrodynamic vibration testing .dynamic range ... vibration testing is performed

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  • Fundamentals ofElectrodynamic Vibration

    Testing Handbook

    VibeCover.hls 5/12/06 1:09 PM Page 3

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    Introduction................................2

    A. Electrodynamic ShakersSize & Force...........................3Displacement, Velocity, & Acceleration........................3Frequency Range....................4Armatures................................4Centering & Support.............. 5Head Expanders & Plates.......5Fixturing..................................6Sliptables.................................6Cooling....................................7Chamber Interface.................. 7Vibration Isolation..................8Noise Levels...........................8

    B. Vibration ModesRandom...................................9Real Data Acquisition & Playback (RDAP)...............9Sine with Resonant Search & Dwell................................ 10Classical Shock.....................10Shock ResponseSpectrum (SRS)....................11Sine-on-Random...................11Random-on-Random.............11

    C. Amplifier Console.............. 12Inverters................................12Magnetic Field Power Supply...................................12Console Design.....................12

    D. Vibration Controllers & Instrumentation..................13Software................................13Dynamic Range.................... 14Spectral Resolution...............14Data Acquisition...................15

    E. AccelerometersAdvantages & Size............... 16Mounting..............................16Types & Conditioning..........17Sensitivity & Environments.18

    F. Universal Vibration Calculator........................... 19

    G. References, Resources & Websites.......................... 19

    H. Move-in & Installation Questions& Considerations.............. 20

    I. Handy Equations & Engineering Reference.......23

    J. Handy Conversion Factors & Materials Properties......24

    Written and published byThermotron Industries, Holland, MI Copyright 2006.

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    Table Of Contents

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    Vibration testing is performed fora variety of reasons: to determineif a product can withstand the rig-ors of its intended use environ-ment, to insure the final designwill not fall apart during shipping,for Environmental StressScreening to weed out productiondefects, or even as a form ofAccelerated Stress Testing.Vibration tests are commonly usedto improve the reliability of mili-tary hardware, avionics instrumen-tation, consumer electronics, auto-motive components, and telecom-munications gear.

    Electrodynamic vibration systemsare capable of performing manydifferent tests that specify sine,random, shock, sine-on-random,random-on-random and other com-plex waveforms as well as repli-cating data that is collected fromreal world conditions.

    Breaking the electrody-namic vibration sys-tem down into its dis-crete components, weare quite simply leftwith something anal-ogous to a stereo sys-tem a big and pow-erful, industrialstrength stereo sys-tem. Using a vibration

    control system (synonymous withthe CD player), a signal is sentthrough an amplifier (similar to theamplifier used for a home stereo),to the shaker (something like aspeaker, but made mostly out ofsteel and weighing several tons),where the armature (comparable tothe stereo speakers woofer orvoice coil) moves up & down orback & forth in a magnetic field.An added element to the vibrationsystem is an accelerometer thatsenses the output of the shaker andsends this signal back to the con-troller for fine tuning. The con-troller in turn sends a drive signalback to the amplifier which pro-vides accurate, closed-loop controland spectral shaping of the testbeing performed.

    This resource presents fundamen-tal concepts and the basic elementsthat comprise an electrodynamicvibration system.

    2g Electrodynamic Vibration Handbook

    Introduction

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    Size & Force (F=ma)When sizing an electrodynamicshaker for a specific applicationyou need to first take into accounttwo essential factors. What is themoving mass(armature, fixture and product)and what acceleration level needsto be achieved? Multiplying thesetwo factors together provides theforce required to perform the testfunction. In the event that theshaker is attached to a sliptable,the mass of the slip plate and driv-er bar attachment must beaccounted for. When a shakerinterfaces with an environmentaltest chamber, the mass of the ther-mal barrier must be added to thetotal moving mass. Dont forgetto account for miscellaneous massthat mightotherwise beoverlookedsuch as: headexpanders orplates, bolts &nuts, cables,etc. Force canbe expressedin the Englishunits, lbf orthe metricequivalent,kN. It is notuncommon

    for the manufacturer of vibrationsystems to derate actual shakerforce capabilities to 80% of theirtrue value as a measure of conser-vative safety.

    Displacement, Velocity &AccelerationThe three functional limits to elec-trodynamic shaker performanceare displacement, velocity andacceleration. Displacement limitsshaker operation at the lowest fre-quencies, and acceleration limitsthe shaker performance at thehighest frequencies. Velocity lim-its shaker performance in a bandbetween the other two limits. Asan example, Thermotrons DS-2250 vibration system has a dis-placement limit of 2 peak-peak, a

    velocitylimit of100 inchesper second,and anaccelera-tion limitof 100 gs.Each ofthose limitsappliesover a dif-ferent fre-quencyrange.

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    A. Electrodynamic Shakers

    An example of a Force calculationF=ma

    Vertical Horizontal

    Product Mass 25 lb 25 lb

    Fixture Mass 40 lb 40 lb

    Cables Mass 2 lb 2 lb

    Head ExpanderMass

    60 lb NA

    Slipplate Mass NA 65 lb

    Armature Mass 23 lb 23 lb

    Total Mass 150 lb 155 lb

    Acceleration Level 10 g 10 g

    Force Required 1500 lbf 1550 lbf

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  • Displacement of an electrodynam-ic shaker is a function of how farup and down the armature is capa-ble of traveling. Most shaker sys-tems are limited to 2 (50 mm)peak-to-peak travel. This meansthat an armature can travel up oneinch (25 mm) and down one inch(25 mm) from its center position.It is standard practice to protectthe shaker from overtravel situa-tions by utilizing sensors that shutthe system down before themechanical limits of the shaker areexceeded.

    Velocity is the speed at which thearmature can move. Velocity limitsfor electrodynamic shakers canreach 100 inches per second (2.5m/sec). The higher the velocitylimit, the greater the shakers capa-bility of attaining a wider range ofshock pulses.

    Acceleration is expressed in termsof gravitational units or gs. A sin-gle g is equal to the accelerationdue to gravity 32 ft/sec2 (9.8m/sec2), 2 gs is twice the acceler-ation due to gravity and so on.When the term grms is encoun-tered, it is used to specify the rms(root mean square) g level of arandom acceleration profile. Sineand shock acceleration levels are

    expressed in terms of g pk, wherepk stands for peak. The accelera-tion component of a vibration testis typically prescribed by the testspecification.

    Frequency RangeElectrodynamic shakers operatethrough a wide frequency rangethat is typically from 5 Hz to 3,000Hz. Most test specifications in theautomotive and transportationindustry emphasize low frequen-cies (ie: below 1,000 Hz) whilemilitary vibration specificationsnormally call for testing out to2,000Hz and electronics industryspecs can go as high as 3,000 Hz.

    Armatures

    Among the general rules of thumbfor armature design and construc-tion are:

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    above: A 16 armature

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  • A lightweight armature is favor-able and will permit testing at higher g levels.

    The armature structure should provide a significant amount of stiffness.

    Material of construction is oftenmagnesium or aluminum.

    Magnesium has a very high strength-to-weight ratio and provides superior damping, making it a favored material.

    Smaller lighter armatures may be appropriate for testing small-er products, while larger arma-tures can eliminate the need to use a head expander, reducing system mass and improving transmissibility.

    Centering & SupportThe armature needs to remain cen-tered in its travel. Using an opti-cal sensor to locate the armatureand an automated pneumatic fill &drain system, the armature, baretable or loaded, will stay true to itscourse. Merely centering thearmature at the beginning of a testis neither adequate nor safe.Advanced systems possess theability to continuously center thearmature while a test is inprogress. Another feature of mostshakers is a centerpole and bearingshaft that helps to keep the arma-

    ture properly aligned during oper-ation. It is good practice to loadfixture and product weight overthe center of the armature to avoidoverturning moments. In theevent the payload center of gravitycannot be located over the centerof the armature, additional guid-ance may need to be added to thesystem to prevent shaker damage.

    Head Expanders & Plates

    If large or multiple productsextend too far beyond the edge ofthe armature, the product could bedamaged or overtested. Using ahead expander or head plate toincrease the armature mountingarea and properly tie the fixturingand products should alleviate thispotential problem.

    Head expanders and plates shouldbe designed for proper stiffness(ie: gusseted and welded) and

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