| Stay cables are the key force-transmitting components of cable-based bridge structures.The accurate measurement of the cable force is the prerequisite for evaluating the mechanical performance and safety status of the structure.The existing cable force measurement methods are mostly based on contact sensors,the results are reliable,but there are disadvantages such as difficulty in installation of sensors,time-consuming,labor-consuming,introducing additional mass and so on.Therefore,based on the National Natural Science Foundation of China project "Image contour overlap analysis to obtain bridge holographic deformation and structure state deduction method exploration(51778094)",micro-vibration video and static image of cable were analyzed to identify its force.By writing an operable GUI program,the non-contact,long distance,high efficiency,and low-cost cable force measurement is realized.The influence of shooting angle,shooting distance and other parameters on the field of view size and image sampling accuracy is studied,and the calculation formula of the special scaling factor of any pixel in the image frame is derived.Two noise reduction methods for cable image data are proposed,which highlight the cable structure information in the image and weaken the background environment.A target location algorithm is proposed based on the matrix correlation theory to accurately locate the target in the image.Furthermore,a hierarchical optimization algorithm is proposed based on reducing the matching step size and compressing the matching region to improve the computational efficiency and accuracy,which can save more than95% of the time in processing high-resolution images.By fitting the quadric surface of the correlation coefficient in the neighborhood of the best matching point in pixel level,and tracking the coordinates of the peak point of the surface,the sub-pixel level positioning calculation is realized.Based on the characteristics of cable micro vibration video,the prior knowledge of cable vibration amplitude is used to reduce the search area of positioning calculation,which can effectively save the calculation time.A multi-target tracking and positioning algorithm based on natural texture is proposed,which can simultaneously obtain the displacement time history vectors of multiple points on the cable.Then the cable force is calculated according to its frequencies.The experiment of cable force identification based on video was carried out.The results show that the algorithm can accurately measure the first four frequencies of the cable,and the results of each measuring point are basically consistent.The accuracy of the cable force calculated by the first and third frequency is higher,and the error is less than 2.0%.The GUI program is used to analyze the cable vibration video,which can obtain the vibration displacement time history,frequency,and cable force values of multiple points on the cable in real time.The theoretical relationship between cable sag and cable force is derived when only the dead weight and axial tension of the cable are considered.The length coefficient of cable segment and position deviation coefficient are defined,and their influence on the calculation accuracy of cable force is studied.The static image of the cable is evenly divided into several computing units along the span direction,and the multi view holographic shape curve splicing of the cable is realized by using the uniqueness of the tangent angle at any position of the cable.The experiment of cable force identification based on holographic shape of cable was carried out.The results show that the larger the segment length coefficient is,the higher the measurement accuracy is.And when the cable length coefficient is greater than 0.85,the maximum error of cable force measurement is 5.53%.When the single view cannot capture the long cable image with high accuracy,the cable force measurement method of multi view splicing is recommended,which can effectively improve the cable force measurement accuracy. |
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