Non-target image technique for spatial-temporal structural responses measurement

Image sequences recorded by high-resolution digital video cameras contain vast amount of spatial-temporal information of targeted objects. With the rapid increase of image resolution, these digital cameras can now be used to measure three-dimensional complex motion of structures with sufficient accuracy for the purpose of modal analysis or health monitoring applications. In this paper, a measurement technique based on image sequence analysis is proposed for extracting spatial-temporal responses of continuous structures. Two digital cameras are used to record two-dimensional (2D) image sequences of a three-dimensional (3D) structural vibration response. This structural vibration response is reconstructed using the two 2D image sequences through the epipolar geometry theory without the use of any pre-installed targets. The obtained displacement response provides a more direct way to quantify the modal properties of the structure. To demonstrate, a laboratory test was conducted to measure the free vibration of a cantilever steel rod. Results show that the proposed technique can obtain excellent results as compared to the analytical solution. The proposed technique provides a low-cost alternative to measure 3D vibration of low-frequency flexible structures such as bridge cables in a non-contact and non-target fashion.