The Principle Optimization And Algorithm Research Of Digital Image Frequency Spectrum Method

In recent years,digital image technology(such as digital image correlation method)has been widely used in structural deformation detection.In order to improve the calculation efficiency and accuracy,the positioning time of the whole pixel displacement and the calculation precision of the sub-pixel displacement are the most popular technology for digital image displacement measurement technology.This paper proposes a new method for calculating displacement based on the frequency domain,and the correctness of this method is verified by experiments.The digital image frequency spectrum method(DIFS)proposed by this research group has the characteristics of fast and accurate positioning in whole pixel displacement without searching.This paper simplifies the number of equations derived from the principle and improves the computational efficiency.Since DIFS can only calculate the displacement magnitude of the whole pixel,and can not determine the direction of displacement,this propose three discriminant methods for the direction of displacement:Two-step translation method,one-step translation method and correlation coefficient comparison method.In this paper,the basic principles of three kinds of displacement direction discrimination methods are introduced in detail,and the advantages and disadvantages of these methods are compared.Comparing these methods with DIC in the efficiency of the whole pixel calculation,it is concluded that the spectrum method runs faster in the whole pixel calculation.The applicable scope of speckle particle radius and calculation window size is studied in this paper in the form of a simulated speckle.It is concluded that the whole pixel displacement calculation is completely correct when the speckle grain diameter is less than or equal to 7 pixels and the calculation window size is greater than or equal to 41×41 pixel.This paper combines the advantages of digital image spectrum frequency method and digital image correlation method,the displacement and direction of the whole pixel are calculated by the spectral method,and sub-pixel is calculated by using the surface fitting method and gradient method in DIC.This is called spectrum hybridization in this paper.The correctness of spectral hybridization method is verified by simulating speckle,and the accuracy of sub-pixel calculation is given.This method solves the sub-pixel computing problem of the spectrum method and advances the transformation from theoretical research to practical application.This paper designs a three-point bending experiment of low carbon steel,and use CCD camera to record the deformation of the specimen surface.Applying the method of frequency domain to measure the hybrid displacement field on the surface of the specimen,calculate the displacement field,on the surface of the specimen and the specimen cross the measured displacement and the load displacement pressure head of comparison and analysis,satisfactory results were obtained.The relative error of the experiment is within 5%.The accuracy and practicability of frequency domain hybridization are verified.