A novel compensation method for systematic effect in displacement measurement based on vision systems

Vision-based measurement techniques are often applied in experimental activities because of the possibility to obtain full-field dense measurement without any loading effect. Many different vision-based techniques are available in literature to measure a plurality of mechanical quantities. Digital image correlation (DIC) and pattern matching are very popular image processing techniques used in 2D and 3D measurement, in particular to estimate displacement and strain. Both these techniques show a systematic effect in displacement measurement. This paper proposed a novel method to compensate for the systematic effect in pattern matching and DIC. The basic idea is to shift the reference image backward and forward of an amount equal to 0.25 px using a Fourier-based technique, and then run the DIC analysis (or pattern matching analysis) two times, using each time one of the two new reference images. The final displacement is estimated by averaging the results of the two analyses. This approach allows significantly reducing the systematic effect in the results, as shown in the numerical and experimental validation. The numerical validation for different types of pattern and different noise levels is conducted for both DIC and pattern matching analysis. Moreover, the influence of the size of the pattern is discussed for pattern matching analysis. Finally the proposed technique is validated with an experimental test. To this purpose, a coordinate measuring machine (CMM) is used to impose subpixel displacement to a target and estimating the displacements with a pattern matching analysis on images acquired by a digital camera. The results of numerical validation and experimental validation demonstrate that the proposed compensation method allows reducing the systematic effect significantly. (paper)