Study Of Operational Modality Parameter Identification Method Based On Transmissibility

Dynamic characteristics of large-scale engineering equipments, civil structures, locomotives and astrovehicles, etc, needs to be considered in the design and these structures also suffer by accident from the effect of extreme load, wind load, pulsation of the earth, earthquake and aerodynamic force. Unreasonable dynamic characteristics and these random ambient excitation cause directly structural vibration and mechanical fatigue and subsequent noise and destruction. Structure optimization designing, fault diagnosis, damage identification, health detection, have great engineering significance, and the modal analysis and modal parameter identification is a best mean to solve these problems. Experimental modality analysis theory is mature but restricted in practical application; Operational modality analysis is flexible in practical application but it still needs an assumption that excitation must be pure white noise.In order to precisely extract modal parameters with operational modality analysis method under no pure white noise excitation condition, this paper studied transmissibility-based operational modality parameter identification method based on power spectrum-based operational modality parameter identification method.Under pure white noise excitation, power spectrum method achieved clear stabilization diagrams. But the stabilization diagrams of transmissibility method were not clear:more poles and worse stabilization. Under white noise with harmonics excitation, power spectrum method had pseudo poles and their stabilization of these pseudo poles was much better than physical pole of system, which made the physical pole not obvious, even submerged by these pseudo poles. In contrary, transmissibility method had no pseudo poles because of harmonics interference and its stabilization diagrams were clear relatively--retained "single peak" characteristics as under pure white noise excitation.Power spectrum method and transmissibility method both brought about pseudo poles. The former brought about pseudo poles because of noise interference and the pseudo poles were stable under one given excitation. The latter brought about pseudo poles because of formation characteristics of transmissibility function itself and the pseudo poles were not stable under two given excitations. Physical poles are irrelevant to excitation and never change as polynomial fitting order changes. Pseudo poles are relevant to excitation and always change as polynomial fitting order changes. The stabilization diagram changed successively the order of polynomial fitting to reflect the stable poles and achieved the purpose of identifying physical poles. Study shows that, the combination of transmissibility-based operational modality parameter identification method with stabilization diagram can extract correctly modal parameters of the system under harmonics interference and overcome the shortage of power spectrum-based operational modality parameter identification method in this aspect, finally achieves the purpose of this research.