Phase-Based Motion Magnification for Structural Vibration Monitoring at a Video Streaming Rate

Abstract

Here we propose a novel approach to image magnification that enables frame-by-frame motion magnification application at a video streaming rate of around 30 fps. This novel approach can be used instead of batch processing an image file in conventional phase-based magnification (PBM). This new PBM method can instantly show a magnified video streaming on display at 30 fps, which is helpful for vibration measurement and for monitoring tasks where magnified images need to be viewed simultaneously. To accomplish video streaming-rate magnification, the proposed PBM uses time-domain convolution in temporal bandpass filtering for frame-by-frame operation whereas conventional PBM employs a frequency-domain filter that is applied to the entire image file at once. An experiment was conducted to monitor the vibration of a cantilever using a webcam streaming at the same frame rate, and data were collected simultaneously using a laser Doppler vibrometer for comparison. The experiment confirmed that the proposed PBM approach is more effective than the conventional magnification method and it also analyzed the system performance for vibration measurement. Additionally, the proper orthogonal mode could be found through the singular value decomposition from vibration displacement data of the cantilever that was collected instantly from the magnified image frames. Furthermore, the dominant mode could be effectively extracted from excitation at the resonance frequency. Because of magnification factors, the vibration displacements from the proposed method were estimated using linear regression and the accuracy of the estimated displacements was within the permissible error bound. Since the proposed PBM is a frame-by-frame operation, instantaneous adjustment of magnification parameters is available, even while the magnification is being processed. In addition, the proposed PBM is independently adjustable to the number of image frames for temporal FIR filter order.