Holographic Shape Monitoring And Damage Identification Of Bridge Structure Based On Fixed Axis Rotation Photography

Compared with the traditional monitoring system based on the measuring point sensor,the application of the image-based monitoring system in the service state of bridge structures has advantages such as holography,convenience and economy.In addition,this technology highly conforms with the future trend of the development in digital information.With the support of NSFC projects “Research on the method of obtaining the bridge deformation and deducing the structural state based on the differential analysis of image contour(51778094)” and “Evolution and failure process of multiscale analysis method of bridge structure based on the surface shape of spatio-temporal data(51708068)”,systematic studies are carried out in this paper on the holographic shape monitoring and damage identification method of bridge structure,based on fixed axis rotation photography.The main research work is as follows:The digital image acquisition method of bridge structure under the condition of single camera fixed axis rotation is explored.The relationship between the maximum rotation angle of camera and image resolution is investigated,as well as the relationship between the adjacent image rotation angle and image overlap.The data-collecting strategy has been proposed for the image sequence in the bridge structure monitoring;A bridge image acquisition hardware system with low cost,non-fixed field of view and adjustable sight distance is constructed.The influence of the rotating camera micro pose offset on the image point coordinate is analyzed.The corresponding error model is established,and the mechanism of image point coordinate deviation caused by projection center deviation is revealed.Aiming at the inconsistency of image sequence projection center caused by the relaxation of hardware system axis,the theoretical model has been established for the calibration from offset projection center to set baseline projection center,and the analytical equation of conjugate points deviation is derived.The iterative adjustment method to correct the inconsistent projection center of image sequence is proposed with the equal coordinates constraint of conjugate points in the overlapping area of continuous image sequence.Using the mathematical representation method of image "feature points" as "homonymous points",the scale invariance and rotation invariance of SIFT feature descriptors are proved in the study by experiments,leading to a correction method of bridge structure image sequence projection center deviation.The experiment of bridge structure image sequence acquisition with single camera fixed axis rotation is carried out,and the equivalent orthophoto elevation image of test beam structure is obtained.Based on the theory of fuzzy set,the approach of structure fuzzy image enhancement is discussed;The "generalized cross validation criterion" is introduced to distinguish image noise,and the fuzzy filtering of image is completed by constructing image filtering window.The continuity of the edge distribution of the bridge structure is improved by using the gray morphology opening and closing operation to deal with the discontinuous edge;The resolution of image monitoring was calibrated,and the main shape size of the test beam is obtained;By comparing the three-dimensional laser scanning shape data with the image monitoring shape data,the accuracy of the structure shape in the test beam equivalent orthophoto elevation image is verified;Analyzed the leading factors of the oscillation effect in different regions of the structural edge line,and constructed an improved wavelet threshold denoising function,which effectively filters the regular highfrequency noise of the structural edge line;The holographic deformation data of the structural edge are obtained by stacking difference analysis.Based on the strong characteristics and strict error standards of the fixed calibration plate,a constraint construction method of conjugate points pair in the measurement plane of the structure image is proposed;Aiming at the problem of partial displacement vector mismatching under the natural texture condition of the structure surface,according to the deformation continuity characteristics of the structure and the deflection and angle information contained in the edge holographic deformation,the constraint condition of displacement vector field consistency is proposed,and the mismatch vector with obvious deviation in length and direction of the vector is eliminated,the whole field displacement vector of the structure surface under the natural texture condition is extracted;Compared with the experimental beam shape data obtained by three-dimensional laser scanning,it is verified that the full field displacement vector can accurately reflect the deformation characteristics of the structure.The method of structural damage identification based on surface displacement vector is explored.The singularity and linear superposition of Dirac function are used to describe the sudden change of flexibility caused by multiple damage of beam;and the deflection equation of beam with explicit response to the location and degree of damage is derived.Through theoretical analysis and numerical simulation,the mechanism of adjacent vector angle sudden change caused by the structural cracks is studied;By using the same position displacement vector rotation angles superposition of the structure,the change rate of the rotation angle before and after the structure damage is calculated.Through the significance test of the displacement vector rotation angle change rate,the judgment of the damage position of the structure component is realized;The numerical simulation of the test beam is carried out.Based on the formula of angle difference-damage degree obtained by theoretical analysis,the fitting function of member damage numerical analysis is established,and the relationship equation between angle difference and damage degree of different components is obtained.By extracting the angle difference of the test beam damage position,the identification of the damage degree is realized;The validity of the damage identification method is verified by experiment.