In the previous study, a paired structured light system which incorporates lasers, cameras, and screens was proposed, and experimental tests validated the potential of a displacement measurement system for large structures. However, the estimation of relative translational and rotational displacements between two sides was based on an assumption that there is zero initial displacement and that three laser beams are always on the screens. In this paper, a calibration method is proposed to offset the initial displacement using the first captured image. The calibration matrix derived from the initial offset is used for subsequent displacement estimation. A newly designed 2-DOF manipulator for each side is visually controlled to prevent the laser beams from leaving the screen. As the manipulator actively controls the laser beams to target the center of the screen, it contains all three laser points within the bounds of the screen. To verify the feasibility of the proposed system, various simulations and experimental tests were performed. The results show that the proposed visually servoed paired structured light system solves the main problem with the former system and that it can be utilized to enlarge the estimation range of the displacement.

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Over the past several decades, there has been rapid growth in research on the displacement measurement system for large structures. However, widely used sensors such as accelerometers, strain gauges, PZT, and GPS have disadvantages that they indirectly measure the displacement, are difficult to install, or costly to maintain. To solve the aforementioned problem, a paired structured light (SL) system was introduced in the previous study. Each module is composed of two screens facing with each other each with one or two lasers and a camera. Though the system can successfully estimate 6-DOF displacement, the measurable displacement range is limited due to the limited screen size. In this paper, therefore, a visually servoed paired SL system is proposed. The newly proposed system uses a visually servoed 2-DOF manipulator in order to make the projected laser beams are always on the screen. In other words, the laser pointer is controlled by a manipulator before it gets off. Various experiments were performed to validate the proposed system. The results show that the proposed system estimates 6-DOF displacement with high accuracy and with largely expanded measurement range. By cascading multiple modules, the proposed system can be applied to the massive structures such as long-span bridges or high-rise buildings.

As civil structures are exposed to various external loads such as traffic, wind, and wave loads, it is essential to inspect structural conditions in every moment. Especially the structural displacement, which is one of the important categories of structural health monitoring (SHM), must be estimated to inspect the structural safety. In the previous study, therefore, a structural inspection robot was proposed to directly measure the structural displacement with high accuracy and low cost. The proposed robot is composed of two screens facing with each other, each with one or two lasers, and a camera. By cascading multiple modules, the inspection robot system can estimate the displacement of the entire structure. Although the proposed robot can estimate translational and rotational displacements in 6-DOF, the measurable range is limited due to the limited screen size. To solve this problem, structural inspection robot with visually servoed manipulator is introduced. In this paper, the architecture of structural inspection robot is described in detail and the actuated structural inspection robot is also introduced briefly.