In-plane Vibration Measurement of an Aluminum Plate Using a Three-Dimensional Continuously Scanning Laser Doppler Vibrometer System

Abstract

AbstractA three-dimensional (3D) continuously scanning laser Doppler vibrometer (CSLDV) system that contains three CSLDVs and an external controller is developed to conduct full-field scanning to measure 3D vibrations of an aluminum plate with free boundary conditions under sinusoidal excitation. A reference object parallel to the plane of the plate is used as the measurement coordinate system to obtain in-plane vibration components. Calibration among three CSLDVs in the 3D CSLDV system based on the geometrical model of its scan mirrors is conducted to adjust their rotational angles to ensure that three laser spots can continuously and synchronously move along the same 2D scan trajectory on the plate. The demodulation method is used to process the measured response to obtain operating deflection shapes (ODSs) of the plate. By using frequencies that are close to damped natural frequencies of the plate as sinusoidal excitation frequencies, four in-plane ODSs, including shear and longitudinal ones, are obtained in the frequency range between 0 and 5000 Hz. These ODSs are in good agreement with those obtained by traditional stepwise scanning and theoretical undamped mode shapes of the plate calculated from its finite element (FE) model. Modal assurance criterion (MAC) values between the first four in-plane ODSs from 3D CSLDV measurements and those from stepwise scanning measurements are larger than 95%. MAC values between ODSs from 3D CSLDV measurements and corresponding mode shapes from the FE model are larger than 90%. However, the 3D CSLDV system can scan more measurement points in much less time than 3D stepwise scanning.KeywordsPlateIn-plane vibrationOperating deflection shapeThree-dimensional continuously scanning laser Doppler vibrometer systemDemodulation method