An innovative Hilbert-Huang transform (HHT) based frequency identification approach designated as adaptive magnitude spectrum algorithm (AMSA) is proposed in this paper. Characterized by the a posteriori property, the AMSA does not need the a priori information about the modal frequencies to be identified, and the situations that a modal frequency may be contained along specific segments of the whole time duration of one or more intrinsic mode functions (IMFs) are allowed for automatically in the resulting adaptive magnitude spectrum (AMS). The algorithm introduces a banded frequency sweep procedure, during which a series of digital filters are designed to process the original signal. Then upon applying HHT to the filtered signals, the forward weighted averages and the backward weighted averages are computed to construct the AMS, based on which the frequencies can be clearly identified. Two numerically simulated examples, i.e. the free vibration signal from a concrete slab subjected to impact loading and the random vibration signal generated by the Phase Ⅰ IASC-ASCE structural health monitoring analytical benchmark problem, and one experimental example, i.e. the free vibration signal based on the Phase Ⅱ IASC-ASCE structural heath monitoring experimental benchmark problem, are used to demonstrate the efficacy of the algorithm. The results indicate that the AMSA is an effective frequency identification technique.
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