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Vision‐based displacement measurement for structural health monitoring has gained popularity in recent years but still has several limitations in practical applications. Unmanned aerial vehicles (UAVs) provide opportunities to address the bottleneck problems of camera resolution insufficiency and mounting inconvenience due to their high maneuverability. However, existing methods using UAVs for structural displacement measurement are often complicated to operate due to the use of multiple stationary markers or multiple UAVs. This paper describes a novel vision‐based displacement measurement approach, using only one UAV, along with a motionless laser spot projected from a distance away as a reference. The positions of the marker and the laser spot are precisely calculated using a two‐step strategy, in which a designed black and white marker of known size is applied to the structure for scale definition and precise positioning. The adaptive region of interest (ROI) and adaptive binarization methods are utilized to improve the automatic applicability of the proposed approach with various background and brightness values. In this way, the motion of the UAV parallel and perpendicular to the plane of the structure can be eliminated by the stationary reference laser spot and the constantly updated scaling factors, respectively. The performance of the proposed method is validated on a two‐story frame and a suspension bridge. The results show that the displacement measured using the UAV agrees with the reference data obtained using the laser displacement sensor and the stationary camera, thereby demonstrating the accuracy and feasibility of the proposed method for displacement measurement for small‐ and large‐scale infrastructure. [ABSTRACT FROM AUTHOR]

• Propose a laser-and-camera (LAC) displacement measurement system that is portable, simple to assemble, and capable of measuring multipoint displacements in the field at a low manual and economic cost. • Provide an efficient video processing flow with an improved binarization strategy for pinpointing the laser spot's center to extract the displacement from the video collected by LAC devices. • Implement the laser-assisted vision-based measurement technique for structural modal identification for the first time. Vision-based techniques for modal analysis of full-scale civil infrastructure have been constrained by the challenge of collecting measurements of all points on a large structure using limited-resolution video devices. This paper proposes a portable, simple-to-assemble laser-and-camera (LAC) system capable of measuring multipoint displacements at a low cost. An efficient video processing flow with an improved binarization strategy for pinpointing the laser spot's center is provided to extract the displacement from the video collected by LAC devices. In addition, the laser-assisted vision-based measurement technique is implemented for structural modal identification for the first time. To evaluate the performance of LAC devices in displacement measurement and structural modal identification, an error analysis test, a measurement accuracy test, and a scaled bridge model experiment are conducted in succession. The results with different types of laser receiving panels and under variable light environments demonstrate the feasibility and application potential of the proposed LAC system and approach for structural health monitoring. [ABSTRACT FROM AUTHOR]

• Energy distribution characteristics of displacement and acceleration are revealed. • The estimated covariances on modal flexibility are used for comparison. • Lower uncertainty is obtained for modal parameters when using displacement. • Displacement response is more suitable for structural flexibility identification. For civil engineering structures, the dynamics-based structural health monitoring is commonly based on inverse analysis of structural acceleration response data to identify the structural parameters. As the measurement of structural displacement response becomes more feasible and cost-effective thanks to advances in computer vision techniques, a comparison of the displacement and acceleration response data for structural identification has become of considerable interest among researchers. This paper presents a comprehensive analysis and comparison of structural flexibility identification based on displacement versus acceleration response data through uncertainty quantification technique. Firstly, theoretical derivation reveals that the energy spectral density of acceleration response is equal to the fourth power of the frequency of the energy spectral density of displacement response, which affects the distribution of measurement noise across different frequency bands. Secondly, the uncertainty quantification method for modal flexibility from subspace-based system identification is utilized to compare the derivation process of covariance estimation on modal flexibility. Finally, numerical analysis and experimental example are carried out to compare the structural flexibility identification results when using the displacement versus acceleration. It shows that more precise identification results of lower-order modal parameters are obtained when using displacement, which further improves the precision of modal flexibility identification. The displacement is more suitable for identifying the modal flexibility when the measurement noises are at the same level. [ABSTRACT FROM AUTHOR]