Flatness Measurement of Platform Screen System Welding Assembly Using Stereo Vision and Grid Pattern Projector

The welding assemblies are the main components of the platform screen door system, which can offer the subway with safety, economy, and silence. The deformed welding assemblies can reduce the carrying capacity of the platform screen door system, so the flatness needs to be measured before installation. In this study, a flatness measuring robot based on stereo vision and grid pattern projector measuring system is developed to online measure the welding assembly flatness in workshop. The developed robot is capable of measuring the flatness by incorporating the following solutions: 1) measuring system consisting of stereo vision and grid pattern projector is designed; 2) grid intersection location method using grid intersection probability calculation, candidate preselection, grid intersection reexamination and sub-pixel refinement is proposed, which can consider both false positives and false negatives by using preselection-and-reexamination; 3) 3D coordinates of sampling points (grid intersection) are calculated based on stereo camera calibration and robot hand-eye calibration; 4) flatness measuring method based on the least squares plan is employed to calculate the flatness of welding assembly. The developed flatness measuring robot is robust, accurate, and efficient, as demonstrated by the following performances: an average measuring bias of 0.10 mm in actual working conditions, and an average measurement time of 4.1 s for single welding assembly, meeting the engineering requirements. It is foreseeable that the developed flatness measuring robot has great application market in the online measurement with a middling accuracy and high efficiency.