Modal analysis it is a kind of measure and signal processing executed in order to identify the vibration modes of a structure.
This tecnique can put in evidence the mechanical properties of the structure, it can confirm experimentally its design or reveal differences that will be subject of further investigation.
With the modal analysis the dynamic properties of the structure are revealed. It is related to the seismic response a key factor to evaluate the capability to resist to earthquakes.
Using very sensitive instruments natural vibration induced by human activities or natural excitation phenomena like wind, also called background noise, can be used as source of excitation signal for the structure. In case of very stiff structures and/or where large amplitudes are desirable, then external specific forces with vibrodines, percussions, or even controlled explosions in free field can be used to generate the appropriate signal burst to inject in the structure.
For modal analysis in the strict sensr you absolutely need to use more than one sensor and they have to be all perfectly synchronized to each other (AceBox, VelBox o Digital Array). Synchronicity must be guaranteed "to the sample"; every single data of the various sensors involved in the measurement must be sampled in phase with the others. Use of radio links could be not sufficient for this purpose. Cable synchronization, if possible, is preferred; else GPS synchronization can perform very well.
Data are subjected to pre-processing (deconvolved where necessary) and examined for the modal analysis itself.
Immagine di Ajay B.Harish (https://commons.wikimedia.org/wiki/File:ModalAnalysis_TrussBridge.gif)
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