Mirror-aided registration-free geometric quality inspection of planar-type prefabricated elements using terrestrial laser scanning

This study presents a new technique that enables entire surface geometric quality inspection (GQI) of planar-type prefabricated construction elements without registration of point cloud data. Laser scanning technology has gained much attention in the area of GQI of manufactured planar-type construction elements such as precast concrete (PC) elements during the fabrication or manufacturing stage, due to its speed and accuracy. However, the scope of existing GQI techniques using laser scanning are limited to single surface GQI of planar-type construction elements. Because of this, a registration process is inevitably necessary to merge different sets of point cloud data acquired at multiple scanning locations. The current multiple scans and registration approach for the GQI of planar-type construction elements are, however, time consuming and error prone due to registration errors. This study aims to develop a registration-free GQI technique that is capable of scanning invisible side surfaces of planar-type elements from the terrestrial laser scanner (TLS) using flat mirrors in order to tackle the limitations of the prior GQI studies. Firstly, the geometrical relationship between the flat mirror and the invisible surfaces of the planar-type element is identified based on the mirror reflection principle of laser beams. The virtual scan points of the invisible side surfaces generated by the flat mirror are then transformed to the location of the actual side surface based on the identified geometrical relationship and the mirror plane estimated using rectangular patches attached to the mirror. To investigate the feasibility of the proposed registration-free GQI technique, a series of tests on a laboratory-scale specimen with varying scan parameters are conducted. In addition, comparison tests with traditional registration methods are performed to further analyze the performance of the proposed technique in terms of GQI accuracy and efficiency. The results show that the proposed technique provides more accurate GQI results while reducing scanning time compared to traditional registration methods, demonstrating great potential for application in the GQI of planar-type prefabricated construction elements during the fabrication stage.