Advanced frequency-domain modal analysis for dealing with measurement noise and parameter uncertainty

The poly-reference Least Squares Complex Frequency-domain (pLSCF) estimator -commercially known as the LMS PolyMAX method- has introduced an improvement in the field of modal analysis. The main advantages are its computational speed and the very clear stabilization diagrams it yields even in the case of highly damped systems and noisy FRF measurements. Moreover, the numerical stability of the algorithm allows for a large-bandwidth and high-model order analysis and makes it suitable both for lowly- and highly-damped structures. Regardless of the very clear stabilization diagrams, it has been observed that the damping estimates are not always reliable for highly damped modes when the FRFs are very noisy. In this paper, a two-step procedure is introduced to improve the damping estimates while maintaining the very clear stabilization diagrams. In the first step, a kind of smoothing is used to remove the noise from the data and in the second step; the pLSCF estimator is applied to the smoothed data resulting in improved (damping) estimates. In addition, the proposed procedure properly deals with uncertainty: the data variance due to measurement noise is taken into account to compute confidence bounds on the modal parameter estimates. The procedure is validated by means of simulated as well as experimental data. The presented procedure to process highly damped noisy vibration data leads to very accurate estimates in comparison to the traditional pLSCF approach. © The Society for Experimental Mechanics, Inc. 2012.