Digital Image Correlation Method Based On Refraction Error Correction In Automatic Measurement Of Skin Deformation

At present,stratospheric airship has been widely concerned because of its advantages such as flight height,coverage and time to stay in the air.In order to grasp the safety,operability and reliability of airship,it is necessary to study the mechanical ch aracteristics of airship skin.The digital image correlation method is one of the m ost widely used,full field,non-contact strain measurement methods.Based on the working environment of airship and the theory and operation steps of digital image correlation method,this paper makes an in-depth study on the effective use of this method in t he protection device at low temperature.Firstly,aiming at the problem that the classical digital image correlation methods commonly used in the industry can’t be carried out automatically and can’t effectively measure the deformation parameters under rotation and large strain,this paper proposes a digital image correlation method based on the initial value estimation of feature points.The essence of digital im age correlation method is to fi nd the optimal solution of nonlinear correlation function by initial value iteration.The selection of initial value is very important for iteration conver gence and runni ng time.Specifically,combining the stable and robust f eature points to estim ate the initial displacement value,this paper makes a detailed derivation,selects a pair of feature points to compl ete the featur e matching as the cent er of the sub region,selects four pairs of adjacen t feature points to represent the s ub region under the sub region constraint,and obtains the variable parameter through affine transformation,which is used as the seed point for subsequent iteration to obtain m ore accurate re sults.The results show that the relationship between affine transformation and deformation parameters in translation,rotation and strain is cons istent with the theoretical derivation,the initial value based on featu re point matching is stable and reliable,and the accuracy of the optimized measurement results is high.In addition,the method is applied to the measurement of skin strain of airship.The data show that the initial value of the me thod is reliable and accurate.This method can not only ef fectively deal with the proble ms of strain measurement under the conditions of translation,rotation and strain,but also has the characteristics of automation and high speed.In addition,the ef fective use of digital image correlation method in thermal insulation device at low tem perature is studied,and the specific form and correction method of displ acement deviation caused by optical window of therm al insulation box are give n.In this paper,firstly,the refraction optical path of glass is studied theoretically.The theoretical derivation shows that the displacement shift caused by the refraction of glass in the opt ical window of the heat preservation box cannot be ignored.Secondly,the first-order error and radial distortion error are studied step by step.Based on the shape function and correlation function of digital image correlation method,a correction method is proposed.Finally,the calibration plate test is used to analyze and correct the deviation error of the insulation box glass.The results verify t he correctness of the theoretical derivation.After the correction,the deformati on measurement accuracy is greatl y improved.At the same time,using this method to measure the skin strain of airship,the results show that the accu racy of displacement measurement is improved by about 5%,and the variance of displacement measurement is improved by about 60%,which will furthe r increase with the expansion of measurement area.This shows that the proposed m ethod can not only effectively improve the influence of thermal insulation device on th e accuracy of digital image correlation method,but also m ake the displacement measurement more stab le.To sum up,the digital image correlation correction model based on the initial value estimation of feature points proposed in this paper can au tomatically determine the reliable and stable initial value,improve the influence of optical window on the accuracy,and have important significance for the final realization of automatic,stable and high-precision displacement and strain measurement under the working condition of airship.