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Time Domain Modal Estimation



S Fahey, Jon R. Pratt


In a previous technical feature,2 we described some motives for performing structural dynamic tests and subsequently reviewed some frequency domain techniques for uncovering modal parameters. This time, we take up some applicable time domain algorithms by reviewing the complex exponential algorithm, Pisarenko?s harmonic de-composition, Ibrahim?s time domain method, and the eigen-system realization algorithm. A numerical example is developed to compare and contrast these techniques. One significant difference between time and frequency do-main techniques is the quality of fit as a function of system damping. Time domain techniques generally out-perform frequency domain techniques when the system damping is less than 0.5 percent of critical as a rule of thumb. Frequency domain techniques, on the other hand, generally provide more reasonable results when systems exhibit greater than 4.0 percent damping. This trend can be attributed to how the modal response appears in each domain. For example, a lightly damped response may distribute its effects over the entire duration of a time domain record but produce only a few spectral lines when transformed to the frequency domain.
Experimental Techniques


modal analysis


Fahey, S. and Pratt, J. (1998), Time Domain Modal Estimation, Experimental Techniques (Accessed June 24, 2024)


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Created December 31, 1997, Updated October 12, 2021