Vision-based 6-DOF displacement measurement of structures with a planar marker

The estimation of translational and rotational displacement of large structures is usually considered as major indicators for structural safety. Recently, several vision-based measurement methods have been developed. Most vision-based systems, however, estimate displacements in 1-D or 2-D space. There are six degree of freedom (6-DOF) measurement methods using combination of lasers and cameras. But, the system is complex to install and not easy to maintain. To mitigate this problem, this paper proposes a simple 6-DOF displacement measurement system using only one camera and a planar marker. Using the square shaped planar marker, whose world geometry is known a priori, the 6-DOF relation between the marker and the camera can be calculated. The camera with a built-in lens captures a marker image and detects corners of the marker. Using homography transformation, 6-DOF relative pose information to the structure is estimated. In order to verify the feasibility of the proposed system, experimental tests are performed. The system for experiments consists of a charge-coupled device (CCD) camera with a built-in 37x zoom lens for maker image processing. The square marker is installed about 20 meters distance away from the camera, and the displacement is estimated. The results show the applicability of the proposed 6-D measurement system to real structures.