Target-free vision method for planar displacement measurement of structures subjected to out-of-plane movement by UAV

In the field of structural health monitoring, one of the essential tasks is to measure dynamic responses such as vibration displacement. With the recent advancement in computer vision, cameras are developed as alternative tools to traditional displacement sensors. Unmanned aerial vehicles (UAVs) offer mobility and can handle complex real-world situations. However, limitations of using UAVs, particularly self-motions, have hindered the accuracy in vibration displacement measurements. This paper proposes a target-free vision-based approach for measuring dynamic displacement responses using UAV. A series of videos of a beam subjected to planar motions are captured using a UAV. Stationary features on the background and target features on the structure are detected using features from accelerated segment test with adaptive threshold strategy and tracked using the Kanade-Lucas-Tomasi. UAV self-motions are estimated using motion-only bundle adjustment. Dynamic displacement responses of the structure are computed based on the displacements of target features and UAV self-motions. Results of the displacement responses and identified natural frequencies with different planar motion types and shooting angles are obtained. They are compared with those obtained by linear variable differential transducers, which demonstrates the accuracy of the proposed method for vibration displacement response measurement.