Fiducial marker-based decentralized computer vision for structural modal identification

Due to the advancement in optics and computer vision, the implementation of the vision-based technique is extensively being investigated for structural health monitoring. Compared with traditional contact sensing measurements, computer-vision technology offers contactless and remote measurements with high spatial density at low cost and instrumentation time. This study proposes an innovative contactless vision-based decentralized vibration measurement technique, where the fiducial marker is utilized as an inexpensive virtual sensor to extract structural vibration measurements using 3D pose estimation through camera calibration. Once the vibration measurements are extracted, covariance-driven stochastic subspace identification is employed due to its robustness for effective mode decomposition and noise reduction capabilities. This paper enables the extraction of 3D time series without deploying a stereo camera system and combines the multiple fields of view of different regions of interest from various cameras in a decentralized manner to capture high-density and high-resolution spatial data for full-field measurement. Two laboratory tests were conducted on a lab-scale building model and a lab-scale beam model to validate the robustness and effectiveness of the proposed methodology. Following the laboratory validation, field tests on a full-scale truss bridge were performed to demonstrate the efficacy of the proposed technique. The relative error in the estimation of the modal frequencies in lab-scale experimentation is <2 %, whereas, for the field study, it is <5.5 %, considering the working distance between the camera and field bridge is over 30 m. The modal assurance criterion (MAC) between the extracted mode shapes is also estimated, and the average MAC values for lab-scale building and lab-scale beam models are 98.61 % and 97.28 %, respectively. The proposed technique has proven to capture the minuscule vibration of the structure at a considerable distance between the structure and the vision-based system, including a detailed comparative study between the vision-based system and accelerometers.