Modified ERA method for operational modal analysis in the presence of harmonic excitations

• Published in: Mechanical systems and signal processing 2006-01, Vol.20 (1), p.114-130
• Main Authors: Mohanty, Prasenjit, Rixen, Daniel J.
• Format: Article
• Language: English
• Subjects: Exact sciences and technology; Fundamental areas of phenomenology (including applications); Measurement and testing methods; Physics; Solid mechanics; Structural and continuum mechanics; Vibration, mechanical wave, dynamic stability (aeroelasticity, vibration control...)
• ISSN: 0888-3270; 1096-1216
• DOI: 10.1016/j.ymssp.2004.06.010

Operational modal analysis (OMA) is a procedure which allows to extract modal parameters of structures from measured responses to unknown excitation arising in operation. It is based on the assumption that the input to the structure is stationary white noise. In practice, however, structural vibration observed in operation cannot always be considered as pure white-noise excitation. In many practical cases, vibrations are induced by a combination of white-noise and harmonic excitations. Harmonic excitations in addition to random inputs can occur due to rotating components or fluctuating forces in electric actuators for instance. The usual way to compute modal parameters in the presence of harmonic excitations is to consider harmonically excited frequencies as being virtual eigenfrequencies of the structure. However, if the frequencies of the harmonic inputs are close to an eigenfrequency of the system, OMA procedures fail to identify the modal parameters properly. In this paper a modified ERA method is proposed, which can be applied as an identification procedure to include the effect of purely harmonic vibrations, assuming the harmonic frequencies are known a priori. The efficiency of the proposed approach is evaluated for an experimental example of a pinned–pinned beam structure excited by multi-harmonic loads superposed on random excitation.