已阅读5页,还剩10页未读, 继续免费阅读
版权说明:本文档由用户提供并上传,收益归属内容提供方,若内容存在侵权,请进行举报或认领
文档简介
APracticalApproachtoVibrationDetectionandMeasurementPhysicalPrinciplesandDetectionTechniquesBy: JohnWilson, theDynam ic Consultant, LLCThistutorial addressesthephysicsofvibration; dynam ics ofaspringm asssystem ; dam ping; displacem ent, velocity,andacceleration; andtheoperatingprinciplesofthesensorsthat detect andm easure theseproperties.Vibrationisoscillatorym otion resultingfrom theapplicationofoscillatoryorvaryingforcestoastructure. Oscillatorym otion reversesdirection. Asweshall see,theoscillationm ay becontinuousduringsom e tim e periodofinterest orit m ay beinterm ittent. It m ay beperiodicornonperiodic, i.e., it m ay orm ay not exhibitaregularperiodofrepetition. Thenatureoftheoscillationdependsonthenatureoftheforcedrivingit andonthestructurebeingdriven.Motionisavectorquantity, exhibitingadirectionaswell asam agnitude. Thedirectionofvibrationisusuallydescribedinterm s ofsom e arbitrarycoordinatesystem (typicallyCartesianororthogonal)whosedirectionsarecalledaxes. Theoriginfortheorthogonal coordinatesystemofaxesisarbitrarilydefinedat som econvenient location.Most vibratoryresponsesofstructurescanbem odeled assingle-degree-of-freedom springm asssystem s, andm any vibrationsensorsuseaspringm ass system asthem echanical partoftheirtransductionm echanism . Inadditiontophysical dim ensions, aspringm ass system canbecharacterizedbythestiffnessofthespring, K, andthem ass, M,orweight, W, ofthem ass. Thesecharacteristicsdeterm ine not onlythestaticbehavior(staticdeflection, d)ofthestructure, but alsoitsdynam iccharacteristics. Ifgistheaccelerationofgravity:F=MAW=MgK=F/d=W/dd=F/K=W/K=Mg/KDynamicsofaSpringMassSystemThedynam ics ofaspringm ass system canbeexpressedbythesystem s behaviorinfreevibrationand/orinforcedvibration.FreeVibration. Freevibrationisthecasewherethespringisdeflectedandthenreleasedandallowedtovibratefreely.Exam ples includeadivingboard, abungeejum per, andapendulum orswingdeflectedandleft tofreelyoscillate.Twocharacteristicbehaviorsshouldbenoted. First, dam ping inthesystemcausestheam plitude oftheoscillationstodecreaseovertim e. Thegreaterthedam ping, thefastertheam plitudedecreases. Second, thefrequencyorperiodoftheoscillationisindependent ofthem agnitude oftheoriginal deflection(aslongaselasticlim its arenot exceeded).Thenaturallyoccurringfrequencyofthefreeoscillationsiscalledthenaturalfrequency, fn:ForcedVibration. Forcedvibrationisthecasewhenenergyiscontinuouslyaddedtothespringm ass system byapplyingoscillatoryforceat som e forcingfrequency, ff. Twoexam ples arecontinuouslypushingachildonaswingandanunbalancedrotatingm achineelem ent. Ifenoughenergytoovercom e thedam ping isapplid, them otion willcontinueaslongastheexcitationcontinues.Forcedvibrationm ay taketheform ofself-excitedorexternallyexcitedvibration.Self-excitedvibrationoccurswhentheexcitationforceisgeneratedinoronthesuspendedm ass; externallyexcitedvibrationoccurswhentheexcitationforceisappliedtothespring. Thisisthecase, forexam ple, whenthefoundationtowhichthespringisattachedism oving.Transmissibility. Whenthefoundationisoscillating, andforceistransm ittedthroughthespringtothesuspendedm ass,them otion ofthem ass will bedifferentfrom them otion ofthefoundation. Wewillcall them otion ofthefoundationtheinput,I, andthem otion ofthem ass theresponse,R. TheratioR/Iisdefinedasthetransm issibility, Tr:Tr=R/IResonance. At forcingfrequencieswell belowthesystem s natural frequency,RI, andTr 1. Astheforcingfrequencyapproachesthenatural frequency,transm issibility increasesduetoresonance.Resonanceisthestorageofenergyinthem echanical system . At forcingfrequenciesnearthenatural frequency, energyisstoredandbuildsup, resultinginincreasingresponseam plitude. Dam ping alsoincreaseswithincreasingresponseam plitude, however, andeventuallytheenergyabsorbedbydam ping, percycle,equalstheenergyaddedbytheexcitingforce, andequilibrium isreached. Wefindthepeaktransm issibility occurringwhenff fn. Thisconditioniscalledresonance.Isolation. Iftheforcingfrequencyisincreasedabovefn, Rdecreases. Whenff =1.414fn, R=IandTr=1; at higherfrequenciesR0.1in., tom ake them practical.Thechangeinintensityorangleofalight beam directedontoareflectivesurfacecanbeusedasanindicationofitsdistancefrom thesource. Ifthedetectionapparatusisfast enough, changesofdistancecanbedetectedaswell.Them ost sensitive, accurate, andpreciseoptical deviceform easuringdistanceordisplacem ent isthelaserinterferom eter. Withthisapparatus, areflectedlaserbeam ism ixed withtheoriginal incident beam . Theinterferencepatternsform ed bythephasedifferencescanm easure displacem ent downto1MHzinsom e PRshockaccelerom eters.Most contem porary PRsensorsarem anufactured from asinglepieceofsilicon.Ingeneral, theadvantagesofsculptingthewholesensorfrom onehom ogeneous blockofm aterial arebetterstability, lesstherm alm ism atch betweenparts, andhigherreliability. Underdam ped PRaccelerom eters tendtobelessruggedthanPEdevices. Single-crystal siliconcanhaveextraordinaryyieldstrength, particularlywithhighstrainrates, but it isabrittlem aterial nonetheless. Internal frictioninsiliconisverylow, soresonanceam plification canbehigherthanforPEtransducers. Boththesefeaturescontributetoitscom parative fragility, althoughifproperlydesignedandinstalledtheyareusedwithregularitytom easure shockswell above100,000g. TheygenerallyhavewiderbandwidthsthanPEtransducers(com paring m odels ofsim ilar full-scalerange), aswell assm aller nonlinearities,zeroshifting, andhysteresischaracteristics.BecausetheyhaveDCresponse, theyareusedwhenlong-durationm easurem ents aretobem ade.Inatypical m onolithic siliconsensingelem ent ofaPRaccelerom eter, the1m msquaresiliconchipincorporatestheentirespring, m ass, andfour-arm PRstraingaugebridgeassem bly. Thesensorism ade fromasingle-crystal siliconbym eans ofanisotropicetchingandm icrom achiningtechniques. Straingaugesareform ed byapatternofdopant intheoriginallyflatsilicon. Subsequent etchingofchannelsfreesthegaugesandsim ultaneouslydefinesthem asses assim ply regionsofsiliconoforiginal thickness.Thebridgecircuit canbebalancedbyplacingcom pensation resistor(s)inparallelorserieswithanyofthelegs, correctingforthem atching ofeithertheresistancevaluesand/orthechangeofthevalueswithtem perature. Com pensation isanart;becausethePRtransducercanhavenonlinearcharacteristics, it isinadvisabletooperateit withexcitationdifferent fromtheconditionsunderwhichit wasm anufactured orcalibrated. Forexam ple,PRsensitivityisonlyapproxim atelyproportional toexcitation, whichisusuallyaconstant voltageor, insom e cases,constant current, whichhassom eperform ance advantages. Becausetherm alperform ance will ingeneral changewithexcitationvoltage, thereisnot apreciseproportionalitybetweensensitivityandexcitation. Anotherprecautionindealingwithvoltage-drivenbridges, particularlythosewithlowresistance, istoverifythatthebridgegetstheproperexcitation. Theseriesresistanceoftheinput leadwiresactsasavoltagedivider. Takecarethat theinput leadwireshavelowresistance, orthat asix-wirem easurem ent bem ade (withsenselinesat thebridgetoallowtheexcitationtobeadjusted)sothebridgegetstheproperexcitation.Constant current excitationdoesnothavethisproblem withseriesresistance.However, PRtransducersaregenerallycom pensated assum ing constant voltageexcitationandm ight not givethedesiredperform ance withconstant current. ThebalanceofthePRbridgeisitsm ostsensitivem easure ofhealth, andisusuallythedom inant featureinthetotaluncertaintyofthetransducer. Thebalance,som etim es calledbias, zerooffset, orZMO(zerom easurand output, theoutput with0g), canbechangedbyseveral effectsthatareusuallytherm al characteristicsorinternallyorexternallyinducedshiftsinstrainsinthesensors. Transducercasedesignsattem pt toisolatethesensorsfromexternal strainssuchastherm al transients,basestrain, orm ounting torque. Internalstrainchanges, e.g., epoxycreep, tendtocontributetolong-term instabilities. Allthesegenerallylow-frequencyeffectsarem ore im portant forDCtransducersthanforAC-coupleddevicesbecausetheyoccurm ore ofteninthewiderfrequencybandoftheDC-coupledtransducer.Som e PRdesigns, particularlyhigh-sensitivitytransducers, aredesignedwithdam ping toextendfrequencyrangeandoverrangecapability. Dam pingcoefficientsof0.7areconsideredideal.Suchdesignsoftenuseoil orsom e otherviscousfluid. Twocharacteristicsdictatethat thetechniqueisuseful onlyatrelativelylowfrequencies: dam ping forcesareproportional toflowvelocity, andadequateflowvelocityisattainedbypum ping thefluidwithlargedisplacem ents.Thisisahappycoincidenceforsensitivetransducersinthat theyoperateat thelowaccelerationfrequencieswheredisplacem ents areadequatelylarge.Viscousdam ping caneffectivelyelim inateresonanceam plification, extendtheoverrangecapability, andm ore thandoubletheuseful bandwidth. However, becausetheviscosityofthedam ping fluidisastrongfunctionoftem perature, theusefultem perature rangeofthetransducerissubstantiallylim ited.VariableCapacitance. VCtransducersareusuallydesignedasparallel-plateairgapcapacitorsinwhichm otion isperpendiculartotheplates. Insom e designstheplateiscantileveredfromoneedge, som otion isactuallyrotation;otherplatesaresupportedaroundtheperiphery, asinatram poline. ChangesincapacitanceoftheVCelem ents duetoaccelerationaresensedbyapairofcurrentdetectorsthat convert thechangesintovoltageoutput. ManyVCsensorsarem icrom achined asasandwichofanisotropicallyetchedsiliconwaferswithagaponlyafewm icrons thicktoallowairdam ping. Thefact that airviscositychangesbyjust afewpercent overawideoperatingtem perature rangeprovidesafrequencyresponsem ore stablethanisachievablewithoil-dam ped PRdesigns.InaVCaccelerom eter, ahigh-frequencyoscillatorprovidesthenecessaryexcitationfortheVCelem ents.Changesincapacitancearesensedbythecurrent detector. Output voltageisproportional tocapacitancechanges, and,therefore, toacceleration. Theincorporationofovertravel stopsinthegapcanenhanceruggednessinthesensitivedirection, althoughresistancetooverrangeintransversedirectionsm ust relysolelyonthestrengthofthesuspension, asistrueofall othertransducerdesignswithoutovertravel stops. Som e designscansurviveextrem ely highaccelerationoverrangeconditions-asm uch as1000full-scalerange.Thesensorofatypicalm icrom achined VCaccelerom eter isconstructedofthreesiliconelem entsbondedtogethertoform aherm eticallysealedassem bly. Twooftheelem ents aretheelectrodesofanairdielectric,parallel-platecapacitor. Them iddleelem ent ischem ically etchedtoform arigidcentral m ass suspendedbythin,flexiblefingers. Dam ping characteristicsarecontrolledbygasflowintheorificeslocatedonthem ass.VCsensorscanprovidem any ofthebest featuresofthetransducertypesdiscussedearlier: largeoverrange, DCresponse, low-im pedance output, andsim ple external signal conditioning.Disadvantagesarethecost andsizeassociatedwiththeincreasedcom plexityoftheonboardconditioning. Also,high-frequencycapacitancedetectioncircuitsareused, andsom e ofthehigh-frequencycarrierusuallyappearsontheoutput signal. It isgenerallynot evennoticed, beinguptothreeordersofm agnitude (i.e., 1000)higherinfrequencythantheoutput signals.Servo(ForceBalance). Althoughservoaccelerom eters areusedpredom inantly ininertial guidancesystem s,som e oftheirperform ance characteristicsm ake them desirableincertainvibrationapplications. All theaccelerom eter typesdescribedpreviouslyareopen-loopdevicesinwhichtheoutput duetodeflectionofthesensingelem ent isreaddirectly. Inservo-controlled, orclosed-loop,accelerom eters, thedeflectionsignal isusedasfeedbackinacircuit that physicallydrivesorrebalancesthem ass backtotheequilibrium position. Servoaccelerom eterm anufacturers suggest that open-loopinstrum ents that relyondisplacem ent (i.e.,strainingofcrystalsandpiezoresistiveelem ents) toproduceanoutput signal oftencausenonlinearityerrors. Inclosed-loopdesigns, internal displacem ents arekeptextrem ely sm all byelectrical rebalancingoftheproofm ass, m inim izing nonlinearity.Inaddition, closed-loopdesignsaresaidtohavehigheraccuracythanopen-looptypes.However, definitionoftheterm accuracyvaries. Checkwiththesensorm anufacturer.Servoaccelerom eters cantakeeitheroftwobasicgeom etries: linear(e.g.,loudspeaker)andpendulous(m eterm ovem ent).Pendulousgeom etry ism ost widelyusedincom m ercial designs. Until recently,theservom echanism wasprim arily basedonelectrom agnetic principles. Forceisusuallyprovidedbydrivingcurrentthroughcoilsonthem ass inthepresenceofam agnetic field. Inthependulousservoaccelerom eter withanelectrom agneticrebalancingm echanism , thependulousm ass developsatorqueproportional totheproduct oftheproofm ass andtheappliedacceleration. Motionofthem ass isdetectedbythepositionsensors(typicallycapacitivesensors), whichsendanerrorsignal totheservosystem . Theerrorsignaltriggerstheservoam plifier tooutput afeedbackcurrent tothetorquem otor,whichdevelopsanopposingtorqueequalinm agnitude totheacceleration-generatedtorquefrom thependulousm ass. Output istheapplieddrivecurrent itself(oracrossanoutput resistor), which, analogoustothedeflectionintheopen-looptransducers, isproportional totheappliedforceandthereforetotheacceleration.Incontrast totheruggedspringelem ents oftheopen-looptransducers, therebalancingforceinthecaseoftheclosed-loopaccelerom eter isprim arilyelectrical andexistsonlywhenpowerisprovided. Thespringsareasflim sy inthesensitivedirectionasfeasibleandm ostdam ping isprovidedthroughtheelectronics. UnlikeotherDC-responseaccelerom eters whosebiasstabilitydependssolelyonthecharacteristicsofthesensingelem ent
温馨提示
- 1. 本站所有资源如无特殊说明,都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
- 2. 本站的文档不包含任何第三方提供的附件图纸等,如果需要附件,请联系上传者。文件的所有权益归上传用户所有。
- 3. 本站RAR压缩包中若带图纸,网页内容里面会有图纸预览,若没有图纸预览就没有图纸。
- 4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
- 5. 人人文库网仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对用户上传分享的文档内容本身不做任何修改或编辑,并不能对任何下载内容负责。
- 6. 下载文件中如有侵权或不适当内容,请与我们联系,我们立即纠正。
- 7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
最新文档
- 第一单元素养评价卷(单元练习)-2023-2024学年语文一年级下册(统编版)
- 2024-旋挖钻孔桩合同范本模板
- 汽车专业:汽车电气设备考试试题(三)
- 如何正确协调产品生产与质量检验的关系
- 幼儿园大班春游方案:小脚丫踏青
- 小学语文教学交流发言稿
- 咖啡店营销方案策划
- 餐饮会所经营管理方案
- 儿童癫痫的长期管理策略
- 儿童慢性肾病的透析与移植
- 学生选课系统数据流图
- 实验室异常情况调查处理相关规范
- 《登岳阳楼》优质公开课获奖教学设计 (人教版高二选修) -1
- 红色喜庆元宵节文化介绍PPT模板
- 山区铁路隧道施工安全风险管理毕业论文
- DB51∕T 1464-2012 杉木二元立木材积表、单木出材率表
- 中西方绘画的特点与差异讲课教案课件
- 【运营】2020年万达某轻资产项目上线计划模块节点
- 小学综合实践活动_诗意汤圆教学设计学情分析教材分析课后反思
- DB13(J)∕T 8053-2019 市政基础设施工程施工质量验收统一标准
- 盾构同步注浆技术及常见问题处理措施72页(附二次注浆讲解)_ppt课件
评论
0/150
提交评论