Cable vibration measurement based on broad-band phase-based motion magnification and line tracking algorithm

Cables are important components of cable-supported bridges and the measurement of cable vibration is very useful for the condition assessment of bridges. The recently developed computer vision (CV) based vibration measurement method has many merits over the traditional contact method. However, most existing CV-based methods require installing the artificial targets on the structure and are only suitable for situations with large vibration amplitude. Therefore, this study proposes a method based on the broad-band phase-based video motion magnification (BPVMM) and the line tracking algorithms for the vibration measurement of cables with small-amplitude vibration. The BPVMM algorithm is used to magnify the small vibrations and the line tracking algorithm is used to extract the vibration displacement from the magnified video for the final identification of cable frequencies. The proposed method is then verified by the laboratory cable test and the field test of a cable-stayed bridge. The results indicate that using the BPVMM algorithm to magnify the small vibration can significantly improve the identification accuracy of displacements and frequencies. The proposed method performs motion magnification over a wide frequency band including all the frequencies of interest, without prior knowledge of the structure frequencies, which greatly improves the computational efficiency and facilitate the measurement of cable vibration under environmental excitation in practical applications.