Laboratory for Dynamics of Machines and Structures 
Tuned-Sinusoidal Method for the Operational Modal Analysis of Small and Light Structures
 D. Rovšček, J. Slavič and M. Boltežar
Journal of Mechanical Engineering, Volume 60, Issues 3, 2014, Pages 187–194

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Abstract
Small and light structures have some distinctive features that intensify the difficulties when measuring their modal parameters. The mass that is added to the structure by the sensors cannot be neglected and the resonant frequencies are usually relatively high. As a result, a wide frequency range of measurements is needed. There are also difficulties with ensuring the proper excitation, so that all the measured modes are excited well and that at the same time the excitation level is not too large, which would cause a larger response of the structure than the measuring ranges of the sensors can cover. In this study an innovative method for the operational modal analysis of small and light structures is presented. The method is non-contact; therefore, there is no added mass of the sensors to the structure. The structure is acoustically excited with a pure sine signal that is tuned to each resonant frequency. A single response measurement with the laser Doppler vibrometer in individual points is needed to determine the modal parameters. A mass-change strategy is used for the mass-normalisation of the measured mode shapes. The main contribution of the presented method compared to other similar methods is that the mode shapes are better accentuated (due to the sine excitation), which can improve the results of the modal analysis on small and light structures, where the response of the structure is weak. The method is also easy to perform, because only a single response measurement is needed for each point and the excitation force does not need to be measured. The presented method gives accurate results, and this was confirmed with a comparison of the experimental and the numerical results on a sample of simple geometry.
Authors

R&D

Domen Rovšček, PhD

  Hidria AET, d.d.
  domen.rovscek@hidria.com
  
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Associate Professor

Janko Slavič, PhD

  Ladisk, Faculty of Mechanical Engineering, University of Ljubljana
  janko.slavic@fs.uni-lj.si
  +386 1 4771 226
jankoslavic     jankoslavic    
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Professor

Miha Boltežar, PhD

  Ladisk, Faculty of Mechanical Engineering, University of Ljubljana
  miha.boltezar@fs.uni-lj.si
  +386 1 4771 608
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