Experiments on Structural Displacement Monitoring Using ViSP (Visually Servoed Paired Structured Light System)

Abstract

As structural displacement is considered as one of important indicators to assess structural conditions, various displacement monitoring systems or methods have been researched. To estimate 6-DOF relative displacement regardless of environmental changes, a visually servoed paired structured light system (ViSP) was proposed in the previous study. The system is composed of two sides facing each other, each with one or two lasers, a camera, a screen, and a 2-DOF manipulator. A total of three lasers project their parallel beams to the screen on the opposite side and the camera near the screen captures an image of the screen. The translational and rotational displacements each in three axes are estimated by calculating positions of the projected laser beams and rotation angles of the manipulators. Since the distance between the camera and the screen is short, typically less than 20 cm, the system is robust to environmental changes such as weather or illumination changes. By controlling the visually servoed manipulator which holds the lasers, the 6-DOF displacement can be estimated regardless of a magnitude of the displacement despite the limited screen size. To validate the performance of ViSP, a field test using a five-story steel frame building structure was carried out. The prototype of the system was built and installed on the building structure in vertical direction. The movements of the structure under external loads were monitored by using ViSP and then the results were compared with reconstructed displacements from a 3-axes accelerometer. The estimated translational displacements from ViSP agree with those from the accelerometer and it is found that the system has a strong advantage that it can estimate not only the translational displacements but also the rotational displacement with low noise level. Through the experimental results, the performance of the system and its applicability to real civil structures were verified.