Calibrating a profile measurement system for dimensional inspection in rail rolling mills

Modern, high-speed, railway transportation requires rails to conform strictly to requirements specified in various standards. One key requirement is the conformance of the dimensions of the rail cross section to those of the corresponding rail model, within tight tolerances. This paper deals with a system for dimensional quality inspection during the manufacture of railway rails. Optical triangulation is used to build a profile from laser lines projected on the rails from four different locations. Then, the profile is compared to that of the corresponding rail model. The differences between certain numerical values (the dimensions) for the profile and the model are compared to standard tolerances for each dimension in order to detect dimensional defects. As a prerequisite for this, the cameras used to capture the laser lines must be calibrated. Standard calibration plates are unsuitable for sheet-of-light calibration in a production environment, as determining the location of the laser emitters relative to the plates would be an issue. For this reason, a cylinder-based calibration target is used instead. Different calibration algorithms are discussed and compared to said standard calibration. The results of accuracy and repeatability tests in the production environment are also shown. The accuracy of the system is found to be appropriate for the purpose of quality inspection under the requirements of applicable rail standards.