A novel 6-DOF displacement measurement system based on vision and laser sensors

Abstract

The main objective of this thesis is development of a novel structural 6-DOF displacement measurement system in order to monitor the structural safety. The displacement measurement system, a visually servoed paired structured light system (ViSP), is composed of two sides facing each other, each with one or two lasers, a 2-DOF manipulator, a camera, and a screen. The lasers on each side project parallel beams to the screen on the opposite side and the camera near the screen captures the image of the screen. A 2-DOF manipulator on each side controls the pose of lasers to prevent the projected laser beams be on the screen all the time. Since the distance between the screen and the camera is short, the system is robust to environmental changes. By calculating positions of the laser beams projected on the screens and rotation angles of the manipulators, 6-DOF displacement between two sides can be estimated. To reduce computation time, an incremental displacement estimation (IDE) method which updates the previously estimated displacement based on the difference between the previous and the current observed data is proposed. To apply the proposed system to massive structures, the whole area should be partitioned and each ViSP module is placed in each partition in a cascaded manner. The estimated displacement between adjoining ViSPs is combined with the next partition so that the entire movement of the structure can be estimated. The multiple ViSPs, however, have a major problem that the error is propagated through the partitions. Therefore, a displacement estimation error back-propagation (DEEP) method which uses Newton-Raphson or gradient descent formulation inspired by the error back-propagation algorithm; and a pose-graph optimized displacement estimation (PODE) method based on a pose-graph optimization technique recently used in mobile robotics are proposed. To validate the performance of the proposed system and methods, various simulations and experimental tests...