Time Shift Frequency Domain (TSFD) modal analysis is a new identification technique giving the frequency, damping and mode shape from free-decay responses. It is assumed that the signal is composed of damped sinusoids and white noise. Since the main-lobe energy is associated with one sinusoid in the frequency domain, it is a function of the record length and time delay. If a delay is introduced between two Fast Fourier Transforms (FFTs) of the same signal, a relation is found to exist between the energy decay, the damping and the delay; this relation is used by the TSFD algorithm to evaluate modal damping. High cross-modal rejection (more than 90 dB) is then obtained with high side-lobe-rejection spectral windows. The TSFD algorithm, like the Ibrahim time domain analysis (ITD) and Z-plane modal analysis (ZMODAL) algorithms, does not require force inputs. Two FFTs are needed. The CPU time is less than for ZMODAL but the accuracy is lower. TSFD is recommended for summary signal evaluation and as a comparison algorithm. It has been tested for different experimental results and computer-generated signals, and shows good agreement with ITD and ZMODAL results.
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