Vision-based displacement measurement using a camera mounted on a structure with stationary background targets outside the structure

While structural displacements are essential information for structural health monitoring, they are not being widely used in practice due to the inconvenience. Recently, vision-based displacement measurement methods have been introduced, which are more convenient and cost effective. However, vision-based methods have generally not been used primarily for the continuous monitoring of structures, due to spatial constraints on obtaining an appropriate location to secure the field of view. A vision device shows not only the changes of objects in the scene but also the relative changes of view according to changes in the position to which it is mounted. Accordingly, this study proposes a methodology for measuring the structural displacement of a location where a camera is mounted, based on a camera motion-induced relative view change. The method is organized into three steps. First, camera calibration is conducted with background targets to derive the camera parameters and coordinates of the target feature points. Second, by tracking the relative changes in the feature points according to the camera motion, the changed 2D-image coordinates of the points are derived. Third, the displacement is calculated through the relationship between the changed 2D-image coordinates and fixed 3D-world coordinates of the target feature points using the camera parameters. The changes in view according to the camera motion are analyzed with simulation tests, and the applicability of the proposed method is verified through experimental tests. The results show that the proposed method can be used to rationally measure structural displacements.