In the previous study, a visually servoed paired structured light system (ViSP) which is composed of two sides facing each other, each with one or two lasers, a 2-DOF manipulator, a camera, and a screen has been proposed. The lasers project their parallel beams to the screen on the opposite side and 6-DOF relative displacement between two sides is estimated by calculating positions of the projected laser beams and rotation angles of the manipulators. To apply the system to massive civil structures such as long-span bridges or high-rise buildings, the whole area should be divided into multiple partitions and each ViSP module is placed in each partition in a cascaded manner. In other words, the movement of the entire structure can be monitored by multiplying the estimated displacements from multiple ViSP modules. In the multiplication, however, there is a major problem that the displacement estimation error is propagated throughout the multiple modules. To solve the problem, propagation error minimization method (PEMM) which uses Newton-Raphson formulation inspired by the error back-propagation algorithm is proposed. In this method, a propagation error at the last module is calculated and then the estimated displacement from ViSP at each partition is updated in reverse order by using the proposed PEMM that minimizes the propagation error. To verify the performance of the proposed method, various simulations and experimental tests have been performed. The results show that the propagation error is significantly reduced after applying PEMM.
To inspect structural conditions, structural displacement is needed to be monitored at any time. Therefore, our previous
study proposed a ViSP (Visually Servoed Paired structured light system) which is composed of two sides facing with
each other, each with a camera, a screen, and one or two lasers controlled by a 2-DOF manipulator. In this system, the
relative translational and rotational displacement between two sides can be estimated by calculating positions of the
projected laser beams on the screens and the rotation angles of the manipulators. To validate the performance of the
system, the various experimental tests with a two-story structural model were performed. The estimated results were
compared with the results from a laser displacement sensor which can be considered as a reference. The results show that
the presented system has potential of estimating the response of the structures with high accuracy in real time.
The displacement measurement in structural health monitoring (SHM) was not popular due to inaccessibility
and the huge size of the civil infrastructures. The frequently employed approaches such as accelerometer, strain
gauge, PZT, GPS, LVDT(Linear Variable Differential Transformer) require high cost and are difficult to install
and maintain. To develop an SHM system that directly measures the displacement of the structure using lowcost
sensor, we proposed a multiple paired structured light (SL) system. The proposed paired SL module which
uses two lasers and a camera in pair is inexpensive to implement and can directly measure the accurate relative
displacement between any two locations on the structure. The steepest descent and extended Kalman filter-based
displacement estimation methods was proposed by deriving a kinematic equation and its constraints. In this
paper, we theoretically justify the minimal configuration of the proposed paired structured light system. To
do so, another configurations are further investigated. The calibration method was proposed for this specific
configuration. After building a prototype of the paired SL module, some real experiments are performed to test the feasibility of the system for a structural displacement monitoring.