Nontarget image-based technique for small cable vibration measurement

In this paper, a proof-of-concept image-based technique is proposed for measuring small cable vibration. The technique analyzes an image sequence of a vibrating cable segment captured by a camera. An optical flow method is used to calculate variation of optical intensity of an arbitrary selected region of interest (ROI) on the cable image sequence. The obtained optical flow vector provides the direction of vibration for the ROI on the cable segment, which then can be used to estimate displacement of the ROI on the image plane. Furthermore, actual displacement of the ROI can be extracted when some conditions are met. The proposed technique is validated both in the laboratory using a rigid pipe and in the field on a small pedestrian bridge cable. Results show that the technique is able to measure the pipe motion and the cable vibration accurately. The proposed technique requires only one commercial camera, and no prior camera calibration is needed. In addition, the use of an optical flow method eliminates the need to attach any target to the cable and makes the technique very easy to implement. Despite these advantages, the technique still needs further development before it can be applied to long-span bridge cables.