Yunfeng Ji

Identification of structural dynamic behavior for continuous system based on videogrammetric technique

The availability of inexpensive and high-resolution commercial digital video cameras has brought forth a new area of application that based on the processing of high quality of digital images. Videogrammetry is a three-dimensional measurement technique that combines the traditional photogrammetry and the computer vision technique. This technique has been previously demonstrated to provide reliable accuracy comparable to that of the traditional sensors for dynamic measurement. Potentially, the technique can measure three-dimensional deformation time history of either a few selected targets or a continuous spatial profile on a structure. In this paper, a novel technique based on videogrammetry is proposed for investigation of structural dynamic behavior, which can measure deformation with sub-pixel accuracy is first established to extract the temporal-spatial deformation of a structure. A simple modal identification method in frequency domain is then applied to extract structural vibration parameters using dynamic responses reconstructed from the image sequence. For demonstration of proof-of-concept, a lab test of identifying the free vibration of a steel cantilever beam is performed. Results have indicated that the proposed technique can achieve a good agreement with the analytical analysis, and show its significant potential for vibration-based real applications.

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.

High precision videogrammetric technique for structural dynamic response measurement

The past few years have seen unprecedented technological advancement in commercial digital cameras. The image resolution of these cameras has increased from below 1 million pixels a few years ago to over 10 million pixels today, with little increase in cost. These low cost high-resolution digital cameras have opened up new areas of application for various engineering disciplines, including civil engineering. The objective of this study is to investigate the application of videogrammetric principle for measuring structural response. A general videogrammetric framework for high precision measurement of three-dimensional structural response is proposed using two commercial digital cameras. Some important issues such as camera calibration, feature point detection and 3D point reconstruction are discussed. In order to evaluate the performance of the technique, three experiments involving capturing the trajectories of different types of motion are performed. The test results indicate that the videogrammetric technique can provide sub-pixel measurement accuracy and can be used to measure both static and dynamic responses of structures in laboratory.