Frequency-domain modal analysis in the OMAX framework

Abstract

Ambient noise is generally seen as an unwanted excitation that disturbs the estimation of vibration parameters. Averaging techniques are then used to decrease as much as possible the influence of the noise. However, this noise also excites the mechanical structure and thus increases the vibration response level. Moreover, it is possible that (broadband) noise excites vibration modes that are not well excited by the artificially applied forces. Those modes are missed by classical estimation methods. Recently, classical EMA and OMA were combined into the so-called OMAX framework. In this framework both the artificial force and the ambient excitation are considered useful in determining the modal parameters. In this paper it is shown that the classical frequency-domain modal parameter estimators (rational fraction polynomial based and state space based) can be used without changing them, if the correct non-parametric preprocessing is applied to calculate the frequency response function (FRF) and the power spectrum (PSD). Special attention is paid to the case of structure-exciter interaction, where a direct OMAX approach would result in erroneous results. Also the importance of scaling the FRF and PSD is discussed. The approach is demonstrated on a typical OMAX case: flight flutter test of an airplane wing. ©2010 Society for Experimental Mechanics Inc.