Abstract
Currently, due to the rapid development and popularization of smartphones, the usage of ubiquitous smartphones has attracted growing interest in the field of structural health monitoring (SHM). The portable and rapid cable force measurement for cable-supported structures, such as a cable-stayed bridge and a suspension bridge, has an important and practical significance in the evaluation of initial damage and the recovery of transportation networks. The extraction of dynamic characteristics (natural frequencies) of cable is considered as an essential issue in the cable force estimation. Therefore, in this study, a vision-based approach is proposed for identifying the natural frequencies of cable using handheld shooting of smartphone camera. The boundary of cable is selected as a target to be tracked in the region of interest (ROI) of video image sequence captured by smartphone camera, and the dynamic characteristics of cable are identified according to its dynamic displacement responses in frequency domain. The moving average is adopted to eliminate the noise associated with the shaking of smartphone camera during measurement. A laboratory scale cable model test and a pedestrian cable-stayed bridge test are carried out to evaluate the proposed approach. The results demonstrate the feasibility of using smartphone camera for cable force estimation.
Highlights
Since cables are primary elements for ensuring the overall structural integrity and safety of cable-stayed bridges, the accurate cable force measurement has very important practical significance in the bridge health monitoring [1]
The boundary of cable for identifying the inclination angle of inclined stay cable and for selecting the region of interest (ROI) is detected by Hough transform (HT) on the image of the first scene captured by smartphone camera
The boundary of cable is detected by means of Hough transform (HT) on the image of the first scene captured by smartphone camera, from which the inclination angle of cable is determined, and the ROI is selected and rotated around its centroid on each of images converted from video file
Summary
Since cables are primary elements for ensuring the overall structural integrity and safety of cable-stayed bridges, the accurate cable force measurement has very important practical significance in the bridge health monitoring [1]. The authors investigated the feasibility and validity of vision-based cable force measuring method using smartphone camera [27]. The cable force estimation using smartphone is limited to the smartphone inner accelerometer, and in the case of using smartphone camera, the dynamic characteristics of cable are extracted from the actual displacement responses of a predefined target attached on the cable surface, by means of smartphone mounted on tripod. As the update of the target vision-based cable force measurement using a smartphone, a novel nontarget approach that is appropriate for handheld shooting of smartphone camera is proposed and its feasibility and practicality are preliminarily validated through a laboratory test and a field test. The comprehensive comparison tests between using smartphone builtin accelerometer and using smartphone camera, including target and nontarget method, fixed and handheld shooting, and smartphone application and MATLB postprocessing, demonstrate the feasibility and practicality of using smartphone camera for cable force estimation
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