Output-only System Identification using Wavelet Transform

System identification of ambient vibration structures using output-only identification techniques has become a key issue in the structural health monitoring and the assessment of engineering structures. Modal parameters of the ambient vibration structures consist of natural frequencies, mode shapes and modal damping ratios. So far, a number of mathematical models on the output-only identification techniques have been developed and roughly classified by either parametric methods in the time domain or nonparametric ones in the frequency domain. Each identification method in either the time domain or the frequency one has its own advantage and limitation. Parametric methods in the time domain are preferable for estimating modal damping but difficulty in natural frequencies, mode shapes extraction, whereas the nonparametric ones in the frequency domain advantage on natural frequencies, mode shapes extraction, but uncertainty in damping estimation. Most recently, new approach in both time and frequency domains based on Wavelet Transform (WT) has been developed for output-only identification techniques with a new concept of timefrequency identification in the time-frequency plane. This paper will present theoretical bases of the wavelet transform for output-only system identification of the ambient vibration data. Measured data will be applied for identifying natural frequencies and damping ratios. Modified complex Morlet wavelet function will be used for more adaptation on the time and frequency resolutions. Frequency resolution adjustment techniques have been proposed to estimate modal parameters of high-order modes.