Wavelet coherence analysis applied to laser vibrometry measurements

Vibration measurement is of great interest in many fields (e.g. mechanical engineering). Laser Doppler Vibrometry (LDV) provides a non-contact method of measuring vibrations very exactly (due to zero mass loading) and greatly increases the investigation capabilities of experimental modal analysis. By applying a standard FFT-algorithm it is possible to analyze the frequency spectrum of the vibrating structure. To overcome many of the shortcomings of classical Fourier-based signal analysis, which are mainly the result of neglecting time resolution, the wavelet transform has been established as an important technique in time-frequency analysis combining high temporal resolution with good frequency resolution. By applying complex wavelets (e.g. the Morlet wavelet) amplitude and phase information can be extracted from the analyzed signal and time is kept as an additional parameter which allows measurement of signal coherence over time as well. The aim of this work is to achieve accurate and reliable quantitative measurements for the characterization of the vibration characteristics of different skis (even when measured under harsh industrial conditions). In this regard, coherence is very sensitive to fluctuations of linearity in phase, relatively less so to nonlinear fluctuations of amplitude and completely insensitive to linear fluctuations in amplitude. Thus, the application of wavelet coherence analysis can give additional detailed insight into the dynamics of vibrations. Experimental results of laser vibrometry measurements are presented and the wavelet-based coherence approach is discussed and compared to classical Fourier methods in order to show the advantages of the wavelet-based representation.