Effect of broad-band phase-based motion magnification on modal parameter estimation

In this paper, broad-band phase-based motion magnification (BPMM) is used to improve the modal parameter estimation from high-speed video of a structure undergoing low amplitude vibration. The authors use a novel application of the phase-based motion magnification technique to obtain improved results in the presence of image noise. A numerical simulation is performed to demonstrate and quantify the effect of broad-band motion magnification. A clear correlation is found between the magnification factor and the error in the motion obtained from 2-dimensional point tracking (2DPT). In a laboratory experiment, operational modal analysis is performed on a metallic nozzle with a 5.3 in exit diameter and a constant wall thickness of 0.03 inches. Fluorescent markers are painted on the nozzle lip. The nozzle is attached to a test stand and excited by pressurized air. The induced vibrations are captured by a single high-speed camera, which takes images of the nozzle lip. Using a 2DPT algorithm, the displacement history of the markers in the nozzle exit plane is extracted. As is typical for small and stiff structures, the resonant frequencies are quite high and the resulting vibration amplitudes are fairly low. This leads to low signal-to-noise ratio in the higher frequencies, which makes the following operational modal analysis much harder. By using BPMM as a preprocessor, the authors demonstrate that the outcome of modal analysis is greatly improved. Comparison with Finite Element analysis shows that the mode shapes agree much better with the experimental results when motion magnification is used. The modal parameters of the first six modes are found in the frequency range 0-1400 Hz using the new methodology, whereas without motion magnification only the first five modes are found. (C) 2020 Elsevier Ltd. All rights reserved.