Fundamentals of Electrodynamic Vibration Testing …

1 Fundamentals ofElectrodynamic VibrationTesting 5/12/06 1:09 PM Page Electrodynamic ShakersSize & , Velocity, & & 5 Head Expanders & Vibration Data Acquisition & Playback (RDAP)..9 Sine with Resonant Search & ResponseSpectrum (SRS).. Amplifier Field Power Vibration Controllers & AccelerometersAdvantages & & & Universal Vibration 19G. References, Resources & Move-in & Installation Questions& 20I. Handy Equations & Engineering Handy Conversion Factors & Materials and published byThermotron Industries, Holland, MI Copyright Vibration HandbookTable Of 5/1/06 3:27 PM Page 1gVibration 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 tests are commonly usedto improve the reliability of mili-tary hardware, avionics instrumen-tation, consumer electronics, auto-motive components, and telecom-munications gear.

2 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 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 vibrationcontrol 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 speaker s woofer orvoice coil) moves up & down orback & forth in a magnetic 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 resource presents fundamen-tal concepts and the basic elementsthat comprise an electrodynamicvibration Vibration 5/1/06 3:27 PM Page 2gSize & Force (F=ma)When sizing an electrodynamicshaker for a specific applicationyou need to first take into accounttwo essential factors.

3 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. Don t 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 notuncommonfor the manufacturer of vibrationsystems to derate actual shakerforce capabilities to 80% of theirtrue value as a measure of conser-vative , 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.

4 Velocity lim-its shaker performance in a bandbetween the other two limits. Asan example, Thermotron s DS-2250 Vibration system has a dis-placement limit of 2 peak-peak, avelocitylimit of100 inchesper second,and anaccelera-tion limitof 100 g ofthose limitsappliesover a dif-ferent Vibration HandbookA. Electrodynamic ShakersAn example of a Force calculationF=maVerticalHorizontalProduct Mass25 lb25 lbFixture Mass40 lb40 lbCables Mass2 lb2 lbHead ExpanderMass60 lbNASlipplate MassNA65 lbArmature Mass23 lb23 lbTotal Mass150 lb155 lbAcceleration Level10 g10 gForce Required1500 lbf1550 5/1/06 3:27 PM Page 3 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 is standard practice to protectthe shaker from overtravel situa-tions by utilizing sensors that shutthe system down before themechanical limits of the shaker is the speed at which thearmature can move.

5 Velocity limitsfor Electrodynamic shakers canreach 100 inches per second ( ). The higher the velocitylimit, the greater the shaker s capa-bility of attaining a wider range ofshock is expressed in termsof gravitational units or g s. A sin-gle g is equal to the accelerationdue to gravity 32 ft/sec2( ), 2 g s is twice the acceler-ation due to gravity and so 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 areexpressed in terms of g pk, wherepk stands for peak. The accelera-tion component of a Vibration testis typically prescribed by the 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.

6 ArmaturesAmong the general rules of thumbfor armature design and construc-tion are:4gElectrodynamic Vibration Handbookabove: A 16 5/1/06 3:27 PM Page 4 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 & 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 systems possess theability to continuously center thearmature while a test is inprogress.

7 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 Expanders & PlatesIf 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) and5gElectrodynamic Vibration Handbookabove: Head expanders are used forlarge or multiple 5/1/06 3:27 PM Page 5should not be bolt-together struc-tures since bolted joints reduceenergy transmission.

8 Favoredmaterials of construction are mag-nesium or aluminum. FixturingIn addition to acting as a mountinginterface between the shaker andthe product to be tested, a vibra-tion fixture needs to be rigid andlightweight. The Vibration fixtureshould also transmit a uniform dis-tribution of energy from the arma-ture to the test item. In manycases, Vibration is applied in threeorthogonal axes. Specializedvibration fixture designs permit thefixture to be rotated for Testing inthe X, Y, and Z axis. It is commonto find the weight of a fixture to betwo to three times heavier than theproducts to be tested. Fixturesshould mount easily to the arma-ture and products should mounteasily to the fixture. Quickchanges from product to productand axis to axis help to maximizeequipment utilization and improvelab sliptable assembly is used whenvibration is required in the hori-zontal axis. For this purpose, theshaker is mounted in a trunnion which allows the shaker to berotated 90 . A single piece tableconstruction with a solid trunnion limits the relative bodymotion of the shaker and improveslow frequency/high displacementperformance of the overall systemby solidly linking all of the reac-tion mass of the shaker to the slipt-able base.

9 The solid trunnion andbase configuration also limits thepotential problems associated with6gElectrodynamic Vibration Handbookabove: Sliptable : A Thermotron horizontal sliptableavailable in various 5/1/06 3:27 PM Page 6misalignment which can plague atwo piece system . Beyond the standard oil film systems, sev-eral options such as guidelinetracking and hydrostatic bearings,are available to further control trueand consistent horizontal vibrationperformance. CoolingShakers consume significant elec-trical power which is converted toheat. Cooling of field coils andarmature coils is mandatory in anelectrodynamic Vibration up to 15,000 lbf are typi-cally air-cooled,while high forceshakers (20,000 lbfand above) are liq-uid-cooled. Air-cooled shakers canbe set up to exhaustwarm air outsidethe facility duringwarm can bemade to damper thewarm air back intothe facility duringcolder blower for anair-cooled shakercan be mountedoutdoors or it can beplaced in a sound deadening pack-age and remain InterfaceWhen integrated with a chamberto perform combined environmenttests, the shaker needs to beequipped with certain items.

10 Athermal barrier, using sheets ofG10 material and/or flexible sili-con rubber will protect the shakerfrom being exposed to the temper-ature extremes of the test designed, this flexibleseal will also provide a protectivebarrier against moisture. Castersand track are one way to roll the 7gElectrodynamic Vibration Handbookabove: Thermotron combined AGREE system allows for Electrodynamic andrepetitive shock 5/1/06 3:27 PM Page 7shaker under a chamber. Anotheroption is to utilize an air-glide sys-tem that floats the shaker on a cushion of compressed air. Applications where the shakerremains fixed in its position andthe chamber rolls back and forthpresent another viable ability to generate and runsophisticated combined environ-ment tests from a single user inter-face can be a huge simplifies data entry and syn-chronizes the individual pieces ofequipment so they work in concertto carry out a fully integrated IsolationElectrodynamic shakers are capa-ble of generating high forces overa wide range of frequencies.