Production integrated laser-based measurement system for railway sleepers

Deutsche Bahn (German Railways) requires its suppliers to deliver concrete sleepers with high precision dimensions for critical sections. To meet these demands suppliers have to continuously monitor the production process. Critical dimensions are the track width and the widths of both rail supporting planes. Also their orientation must not exceed given tolerances. That demands measurements with a precision of 0.2 mm or better. The contactless measurement system developed uses the triangulation approach based on laser plane projection. It consists of two CCD-cameras and four lasers projecting planes under 45degrees each. A third camera serves for the recognition of the mould numbers. Software detects if a sleeper is in a measuring position, the camera images are frozen and the measurements are done using line approximation and subpixel accuracy. Additionally the measurement of the positions of peg holes is integrated, these data serve for the control of a screw-in robot. The system is added to the running production line in a harsh industrial environment. It works fully automatically. All measured data are transferred to the production monitoring for evaluation and archiving. The system has been working now for two years, more than half a million sleepers have been reliably monitored.