模态试验常见问题

1LMSModalAnalysisTechniquesX=SourceReceiver•Engine•Rotor•Road•Gearbox&transmission•Turbomachinery•Wheel&tire•Accessories•Environmentalsources•...Transferthroughthestructure•Noiseatdriver’s&Passengerears•Steeringwheelshake•Rearviewmirrorvibration•Cabincomfort•Structuralintegrity•Seatvibration•CockpitNVH•...模态试验范畴:SystemTransferSystemSystemTransferTransferModalModalAnalysisAnalysis2LMSModalAnalysisTechniques模态分析结构固有属性分析表现为共振频率阻尼比振型目的:验证产品振动与噪声问题提高产品耐久性提高数字模型准确性模态试验目的:3LMSModalAnalysisTechniques模态分析的简单描述:从属性到响应试验设置变频激励定频分析Asadesignengineer,needtoidentifythesefrequenciesandknowhowtheymightaffecttheresponseofmystructurewhenaforceexcitesthestructure.Understandingthemodeshapeandhowthestructurewillvibratewhenexcitedhelpsthedesignengineertodesignbetterstructures.4LMSModalAnalysisTechniques常见问题一:什么样的MAC值才能确定模态是可信的?模态振型验证只能给出振型间的相关性,不能提供响应预测的信息.振型可反映出输入力的分布变化和响应变化.如下公式:where[M],[C],[K]arethemass,dampingandstiffnessmatricesrespectively,转换成模态空间wherethediagonalmatricesarethemodalmass,modaldampingandmodalstiffness5LMSModalAnalysisTechniques常见问题一:什么样的MAC值才能确定模态是可信的?频响函数验证Figure2显示变频激励结果.输入信号的频率与结构共振频率不重合.如第二个激励频率,若二阶共振频率不准，偏移附近,则响应预测就会失真.应用实例1对于旋转机械,频响函数的准确要比振型准确更值得关注.这是因为输入激励与某一频率关联,而这对响应又有直接影响..2对于激励为宽带白噪声信号,则模态振型的准确会得到更多的关注6LMSModalAnalysisTechniques常见问题二:试验安装和刚体模态对柔性模态的影响?刚体模态与第一阶模态之比1:10or更多?三参考点计算:Thecompositeplatewassupportedonaverysoftelasticsystemandsubjecttoimpacttestingusingthemultiplereferenceimpacttechniqueusingthreereferenceaccelerometers.四种悬挂方式比较TheimpacttestwasconductedwiththefourdifferentsupportconfigurationsasshowninFigure1alongwithaphotoofoneofthetestsetups.7LMSModalAnalysisTechniques常见问题二:试验安装和刚体模态对柔性模态的影响?Sowhatshouldyoudo?1与数字模型相关性分析,振型与频响2检查各种边界限制与模态计算关系,如果影响很大,则试验安装时要小心.3RulesofThumbthatareusedareexactlythat–theymustbeevaluatedinmoredepthandshouldnotbeblindlyfollowed.Andrememberthatachangeinthestiffnessofthetestsupportmusthaveaneffectonallthefrequencies.Ifyouaddstiffnessthefrequenciesmustshiftupwards–thequestionishowmuchdothefrequenciesshiftandisitofimportanceorisitmeasurable.8LMSModalAnalysisTechniques常见问题三:简支梁激振器法–连杆的长短影响?理想的连杆只传递激振方向的力,且尽可能减少对测试结构侧向的输入.但实际使用中常忽略连杆长度对側向刚度的影响,或者说没考虑:thestingerwasrelativelyshortandtherewasapossibilitythattherotationalstiffnessofthestingermayaffectthebeamflexiblemodes.??9LMSModalAnalysisTechniques常见问题三:简支梁激振器法–连杆的长短影响?从上图可以看出真正原因:支撑刚度的影响短杆:相对高的旋转刚度–rotationalstiffness,causeTunedAbsorberEffect长杆:相对柔的刚度,对测试结构模态附加影响小很多.10LMSModalAnalysisTechniques常见问题四:抗混淆滤波器的影响?力锤敲击信号,震荡只是由于双击吗?如下图,不同硬度锤头的不同信号,及带宽影响这是一典型抗混滤波器截止频率对分析信号的影响.Typically,ifyouaremeasuringlowerfrequencyranges,thelowerfrequencyfilterisemployed.Ifasoftimpacttipisusedthenthiswillnotsignificantlycauseanyfilterring.Butifaslightlyhardertipisused,thentheupperfrequencyrangeofthehammerexcitationmayexcitethelowfrequencyanaloganti-aliasingfilter.Thefiltergetsexcitedandhasadynamicresponsecharacteristicwhichmanifestsitselfontheforcepulseasthisfilterring.Noticethatthe400Hzbandwidthhassignificantlymorefilterringasthehammertipsgofromsoftertoharder.Thisisbecausethehardertipexcitesawiderfrequencyrangeandhasagreatpossibilityofexcitingthelowfrequencyanaloganti-aliasingfilter.Generally,asoftertipisabetterselectiontoassurethatthefilterringdoesnotoccur.Ifthereisfilterringthenitmakessensetoselectahigherfrequencyrangesothatthefilterringisminimized.Thenitisnotaseriousissueandtheproblemisresolved.11LMSModalAnalysisTechniques常见问题五:激振器激励信号选择对模态试验的影响?随机信号:每次采样数据不同优点:适用噪声干扰下数据平滑,对弱非线性结构,平均计算则可以最优描述线性化的结果缺点:泄露及加窗对原始数据的影响结论:不是最佳激励信号伪随机信号:相同频谱数据多次循环播放优点:无泄露及加窗对原始数据的影响缺点:只对线性结构合适结论:不是最佳激励信号猝发随机信号:随机+伪随机优点:两者结合缺点:需注意时域窗内数据的收敛程度结论:最佳激励信号之一12LMSModalAnalysisTechniques常见问题五:激振器激励信号选择对模态试验的影响?鸟鸣正弦信号:扫频正弦信号的一种优点:与伪随机相同,且力可控制,从而可研究结构非线性缺点:结论:最佳激励信号之一步进正弦信号:单一频谱数据多次循环播放优点:无泄露及加窗对原始数据的影响,无量化误差缺点:相对长的试验时间结论:最佳激励信号之一信号选取建议1.Ingeneral,burstrandomusuallyworksbestoverall.ButIhavealsousedsinechirponmanyoccasionswhenthestructureswerefairlylinearinnature.2.WhenIhaveneededextremelyhighresolutionFRFs,thenIhaveuseddigitalsteppedsine.Onceortwice,Ihaveusedeitherpseudo-randomandrandomtoverylightlydampedlinearstructure3.Whenastructuredoeshavesomenonlinearcharacter,thensinechirpprovestobeaverygoodexcitationforthistypeoftest.13LMSModalAnalysisTechniques常见问题六:时域/频域/模态域的基本概念?简支梁不同域的物理描述:likemoney☺解耦解析式模态域Thereisreallynodifferencebetweenthetimedomain,frequencydomain,modalspaceandphysicalspace.Eachdomainisjustaconvenientwayforpresentingorviewingdata.However,sometimesonedomainismucheasiertoseethingsthananotherdomain.Forinstance,thetotaltimeresponsedoesnotclearlyidentifyhowManymodestherearecontributingtotheresponseofthebeam.ButthetotalFRFinthefrequencydomainismuchclearerinshowinghowmanymodesareactivatedandthefrequencyofeachofthemodes.Sooftenwetransformfromonedomaintoanotherdomainsimplybecausethedataismucheasiertointerpret.IRFRF14LMSModalAnalysisTechniques常见问题七:锤击法与激振器法有区别吗?互易性:h13=h31理论上二者是一致的!但实际操作中,很多因素会造成两种方法的不同1.悬挂系统2.传感器附加质量3.顶杆附加刚度4.…15LMSModalAnalysisTechniques常见问题八:两种锤击法有区别吗?实际操作中,两种做法可以使两种锤击法测得的矩阵一致1.Setup1,敲击三向,用单向传感器,就可得到一完整列向量2.Setup1,敲击三向,Setup2,敲击三向,就可得到三完整列向量移动