Modal Identification of a Turbine Blade with a Curved Surface Under Random Excitation by a 3D CSLDV System and the Extended Demodulation Method

This study develops a novel general-purpose 3D continuously scanning laser Doppler vibrometer (CSLDV) system to measure 3D full-field vibration of a turbine blade with a curved surface under random excitation and proposes an operational modal analysis (OMA) method to identify its modal parameters. The 3D CSLDV system developed in this study contains three CSLDVs, an external controller, and a profile scanner. A 3D zig-zag scan trajectory is designed on the blade surface based on profile scanning, and scan angles of mirrors in CSLDVs are adjusted based on relations among their laser beams to ensure that three laser spots can continuously and synchronously move along the same scan trajectory. The OMA method referred to as the extended demodulation method is used to process the measured response of the blade under random excitation to obtain its damped natural frequencies and 3D full-field undamped mode shapes. Comparison between the first six modal parameters from the proposed 3D CSLDV system and those from a commercial 3D scanning laser Doppler vibrometer (SLDV) system is made in this study. Errors between the first six damped natural frequencies from 3D CSLDV measurement and those from 3D SLDV measurement are less than 1.5%, and modal assurance criterion values between the first six undamped mode shapes from 3D CSLDV measurement and corresponding damped mode shapes from 3D SLDV measurement are larger than 95%. It took the 3D SLDV system about 900 seconds to scan 85 measurement points in the experiment, and the 3D CSLDV system 115.5 seconds to scan 132,000 points, which means that the 3D CSLDV system can measure much more points in much less time than the 3D SLDV system in 3D full-field vibration measurement.