Research On The Key Technology Of Microscopic Three-dimensional Digital Image Correlation

The microscopic three-dimensional digital image correlation method is a method that is based on three-dimensional digital image correlation method and combined with microscopic stereo vision principle to achieve the measurement of the three-dimensional shape,displacement and strain of micro-object,and has the advantages of non-contact,high precision and whole measurement.Considering the small field of vision and depth of the stereo microscope,and the complexity of the internal optical paths and components,researches on the methods of microscopic three-dimensional digital image was carried out from aspects of corner detection,calibration of microscopic stereoscopic vision and the image matching.In view of the error detection of the corner points of the chessboard calibration plate occurred in the process of the microscopic stereo vision calibration,which is caused by the distortion and blur produced during microscopic imaging.A corner detection method based on rotation invariance was proposed in this work,and studies were conducted on the geometrical characteristics of chessboard corner points,sorts of the feature corner points,and establishment of mathematical models through experiments.The confidence degrees of the selected feature corner were calculated,the position of the chessboard corner is extracted accurately,the accuracy of the method and the number of noise and the accuracy are validated by a series of experiments.The results shown that the accuracy of the chessboard corner detection method based on rotation invariance is 91.2%,and the mean variance of the reprojection error is about 0.1 pixels,suggesting that the accuracy and precision of the method is high enough.The small depth of the field of the microscope and the complex optical paths,making the macroscopic calibration method can not be applied to the problems of microscopic calibration,so a calibration method of fixed-point rotation is proposed.The relation between magnification and the depth of the field of the stereo light microscope was obtained,the maximum angle between the calibration plate and the XY plane was calculated,and the main point coordinates were optimized by the method of variation.The best angle to minimize the overall error of the calibration parameters was obtained as well.A large number of calibration experiments shown that the principal point coordinate error within 1.8 pixels,the maximum deviation of the relative translation vectors of Z component is not more than 0.012 mm,and the calibration result is relatively stable when the attitude number is 10 or above.A triangular template was put forward to replace the traditional square template due to the low fitting degree between the template and the edge of the hole in the object to be measured.The unknown quantity of the object deformation was determined by the three points of the triangle,adding to the point of the neighborhood to form a hexagon,and the overdetermined system of equation was solved by the weighting factor.The tensile test of the specimen with round hole was carried out,and the real-time strain around the round hole was obtained.The detections of the hole edge using triangular template and traditional square template were compared,the results shown that the triangular template is more fit to the hole edge,and the strain measurement of the object with the hole is more accurate.The following work was done using microscopic three-dimensional digital image correlation system: measurement the three-dimensional shape of the small area of the object;the vibration fatigue test of the specimen with precast crack to measure the strain distribution of the small area around the crack.