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COB91 ANÁLISE TEÓRICA DO USO DE JANELA EXPONENCIAL NA AQUISIÇÃO DE DADOS EM TESTE DE IMPACTO/ THEORETICAL ANALYSIS OF EXPONENTIAL WINDOWING IN IMPACT TEST
Newton S. Soeiro1, Gustavo D. P. Silva2 & Samir N.Y. Gerges2
1 Departamento de Engenharia Mecânica, UFPA - Belém, CEP 66075-900 Pará, Brasil - E-mail: soeiro@mbox1.ufsc.br
2 Departamento de Engenharia Mecânica, UFSC - Florianópolis, CEP 88040-900 Santa Catarina, Brasil
E-mail: gustavo@mbox1.ufsc.br e gerges@mbox1.ufsc.br
Impact excitation is one of the most common methods for experimental modal analysis. It is versatile, mobile and produces reliable results. In the other hand, when a structure is lightly damped the response to the hammer impact may be sufficiently long that it is impractical to capture the entire signal. To suppress the truncation of response signal a common practice is to artificially force it to decay within the data capture window by multiplying it by an exponential function. However, the aplication of the exponential window must be considered carefully since it may adversely affect the estimated spectra. The purpose of this paper is to examine the efects of the exponential window over the modal parameters and estimated spectra from the experimental modal analysis developed by impulse excitation technical. Thus, the characteristics of impact test procedure are examined with analitical functions developed for an idealized test and to examine the influence of the exponential window over the closely spaced modes.
Keywords: Análise Modal, Excitação por Impacto, Janela Exponencial, Modal Analysis, Impact Excitation, Exponential windowing
COB203 COMPARAÇÃO ENTRE RESULTADOS NUMÉRICOS E EXPERIMENTAIS DA ANÁLISE MODAL DE CONES DE ALTO-FALANTES / COMPARISON BETWEEN NUMERICAL AND EXPERIMENTAL RESULTS FROM MODAL ANALYSIS OF LOUDSPEAKER CONES
André L. Goldstein & Samir N. Y. Gerges
Laboratório de Vibrações e Acústica, Depto.de Engenharia Mecânica - UFSC -CP: 476,CEP 88040-900
Florianópolis, SC, Brasil-E-mail: andre@gva.ufsc.br , gerges@mbox1.ufsc.br
Numerical techniques are a powerful tool for predicting the behavior of structures that can't be modeled in a closed analytical form. However, if the computational model is to be used with confidence, it must be validated by comparison, for example, with experimental measurements. This paper presents the modeling of the vibrational behavior of a loudspeaker cone with modal analysis by the finite element method (FEM) and the comparison with results of experimental modal analysis. The experimental results show the existence of non-axisymmetric modes that can't be calculated by the ideal axissymmetric loudspeaker model and that are well predicted by the tridimensional FEM model. The existence of complex mode shapes in the experimental tests, that implies in non-proportional damping,, suggests the limitation of the proportional damping model used by the FEM.
Keywords: Alto-falantes, Método de Elementos Finitos, Análise Modal Experimental Loudspeakers, Finite Element Method, Experimental Modal Analysis
COB413 PROBLEMAS EXPERIMENTAIS PARA A ANÁLISE MODAL EM PLACAS DE MATERIAL COMPOSTO
Alessandro B. de S. Oliveira & Fernando J. R. Neves
Departamento de Engenharia Mecânica, Faculdade de Tecnologia - UnB,
CEP 70910-900 Brasília - Brasil - E-mail : fernando@orion.enm.unb.br
This work is centered on the experimental problems related to modal testing of composite material plates. The results presented show the effects of non uniformity of properties of the plate along its surface. A carbon fiber sandwich plate with aluminium honeycomb core was used in the tests.
Keywords: Análise Modal, Materiais Compostos, Elementos Finitos, Caracterização Dinâmica, Resposta em Frequência.
COB445 IDENTIFICAÇÃO DE PARÂMETROS MODAIS DE UMA VIGA NO DOMÍNIO DO TEMPO : TÉCNICA EXPERIMENTAL / MODAL PARAMETERS IDENTIFICATION IN A CANTILEVER BEAM IN TIME DOMAIN : EXPERIMENTAL TECHNIQUE
Luiza Yoko Taneguti & Mario Francisco Mucheroni
Departamento de Engenharia Mecânica, Escola de Engenharia de São Carlos – USP – São Carlos
CEP 13560-250 São Carlos, SP Brasil – E-mail : taneguti@sc.usp.br
In the last decade several methods in time domain have been developed to identify modal parameters of a mechanical structure. Usually these methods require data from the impulse response function or free decay responses. In this paper the Ibrahim Time Domain Method – ITD was used to obtain a unique set of modal parameter of natural frequencies, damping factors and mode shapes from a set of free vibration measurements in a single analysis. The applicability of this method was verified by one experiment using a cantilever beam. A technique for acquisition of the responses at measurement points was used for minimization of the amount of instrumentation required.
Keywords: Identificação, parâmetros modais, domínio do tempo, método ITD, viga engastada / identification, modal parameters, time domain, ITD method.
COB452 INVESTIGATION ON THE USE OF THE WAVELET TRANSFORM IN THE SOLUTION OF THE MODEL UPDATING PROBLEM
Mauro J. Atalla, Ph.D.
State University of Campinas, FEM - Department of Mechanical Design, 15083-970 Campinas, SP Brazil, E-mail: mjatalla@vt.edu
Existing model updating techniques, based on modal data or frequency responses use data experimentally obtained to update the model of the structure with the implicit assumption that the experimental data is correct. However, noise present in the experimental data is known to effect the computation of the updated parameters. In particular, the Fourier transform used to compute the frequency response functions averages the noise over the duration of the signal, smearing its effect over the frequency axis. An alternative to the Fourier transform is the wavelet transform, which has been successfully used in the electrical engineering and computer science fields to filter noise and compress data. This paper investigates the use of the wavelet transform in the solution of the model updating problem, and presents a formulation analogous to the Fourier-based formulation. Numerical results show the feasibility of this approach in updating models based on noisy measurements.
Keywords: Model updating, Wavelet transform, Ajuste de modelos, Transformada wavelet