آناليز مودال عملياتي در حضور تحريك هاي هارمونيك

چكيده انگليسي :

Abstract Nowadays experimental modal analysis has found a wide range of applications in dynamicimprovement of structures enhancing analytical models optimal dynamic design vibration control faultdetection and structural health monitoring in numerous fields of mechanical aerospace and civilengineering The modal test is normally performed at laboratory where the boundary conditions arereplicated and the system is excited by artificial forces By processing the data acquired from system sresponse and input force measurements frequency response functions are computed through whichdifferent modal analysis methods enable to obtain modal parameters Experimental vibration analyseswhich concerns large industrial or masonry structures encounter technical difficulties and limitations Forinstance transferring a bridge for this purpose to the laboratory site is neither practical nor plausible infinancial terms Furthermore measuring the system s frequency response in its location will necessitateshutting it off and also eliminating mechanical and electrical disturbance noises not to mention thedifficulty of applying excitation forces in such conditions On the other side the system operationalconditions are not identical to those present in the test and hence the results are not appropriate to ourpurpose In order to alleviate these limitations the operational modal analysis OMA has been developed Thismethod is purposed to obtain modal parameters by measuring vibrations under the influence of ambientforces in normal work conditions of the system One of the obvious advantages of this method comparing tothe conventional modal analysis resides in the dispensability of using specific excitation sources recreatingactual boundary conditions or stopping the system s normal operation hence enhancing the speed andreducing the cost In this method the model linearized about the actual force and boundaries of the wholesystem and not a separated part of it is obtained Modal analysis of rotary machines and bearing structuresfinds many applications in investigating induced vibrations complications but is not applicable viaperforming conventional experimental modal tests because of above mentioned limitations OMA can besuitable for the purpose of obtaining the modal parameters of rotary machines The vibration response ofrotary machines contains a multitude of powerful harmonics which cause problem to the method as theexcitation force is essentially assumed as white noise In order to rectify the OMA method to deal with theseharmonics present in the excitation source these latter are identified in a first attempt and their influence onthe resulting modal parameters are investigated Another method may consist of splitting the responsesignals into stochastic and periodic parts then using the former for further analysis In this thesis different indexes for identifying harmonic components are investigated in order to reacha more sensitive and accurate method Methods based on frequency domain decomposition FDD consisting of EFDD and CFDD are studied and implemented for obtaining the modal parameters In thesemethods the harmonics parts identified via the harmonic index are eliminated from the frequency responseby performing linear interpolation For enhancing the performance a novel MCFDD method has beenproposed Another method presented in this project for getting rid of the harmonic excitation forces consistsof separating the response signals before performing the operational modal analysis The stochastic part ofthe signal is used for obtaining the modal parameters via the FDD method in the frequency domain and SSImethod in the time domain Accuracy and effectiveness of existing and proposed methods are investigatedand compared through computer simulations Then their effectiveness has been evaluated in practical testsperformed on a steel beam and a large industrial fan in Mobarakeh Steel Complex Both simulations andtests show enhancements in the results in comparison

مسجديان، محمد حسن

آناليز مودال عملياتي در حضور تحريك هاي هارمونيك

ماشين دوار , جداسازي سيگنال