Non-contact measurement of the dynamic displacement of railway bridges using an advanced video-based system

Abstract This article describes the development of a non-contact dynamic displacement measurement system for railway bridges based on video technology. The system, consisting of a high speed video camera, an optical lens, lighting lamps and a precision target, can perform measurements with acquisition frame rates ranging from 64 fps to 500 fps, and be articulated with other measurement systems, which promotes its integration in structural health monitoring (SHM) systems. Preliminary tests of the system have shown that the measurements’ precision can be affected by: (i) movements of the camera stand and, therefore, rigid supports should be used and the camera should be protected from air flows; (ii) the distortion of the field of view, caused by the flow of heat waves generated by IR incandescent lighting and, therefore, the operating time of the lamps should be limited. The system was used to measure the displacement of a railway bridge’s deck, induced by the passage of trains at speeds between 120 km/h and 180 km/h, yielding a very good agreement between the results of displacement measurement obtained with the video system and with a LVDT. The achieved precision was below 0.1 mm for distances from the camera to the target up to 15 m, and in the order of 0.25 mm for a distance of 25 m. The application of an image processing technique at subpixel level resulted in real precisions generally inferior to the theoretical precisions.