In-Plane Displacement Measurement Using Moire Fringes And Pattern Matching
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Date
2009-05
Authors
Yen, Kin Sam
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Abstract
A system for crack displacement sensing and measurement in concrete
structures using circular grating moire fringes and pattern matching was reported in a
recent research. However, the imperfection of the moire fringe extraction operation
and the angular displacement detection caused error in pattern matching for
displacement prediction. In this study, an improved in-plane (2D) displacement
measurement system using the moire method integrated with image processing
techniques has been developed. A set of simulated moire patterns was generated
using two circular gratings of different pitch for the development of the moire fringes
extraction algorithm. Circular gratings were first transformed into horizontal line
gratings using Cartesian-to-Polar spatial transformation technique. The transformed
line gratings were removed using a I-D vertical average filter and the unfiltered
fringe pattern was inversely transformed to recover the complete circular moire
fringes without the residual gratings. The proposed moire extraction method proved
to have the highest gratings removal rate (41.4%) as compared to the iterative 2-D
average filtering operation proposed in the past. The extracted moire fringes were
compared to the stored fringe patterns in the database using the area-based pattern
matching approach to estimate the displacement represented by the moire fringes. A
total of 151 fringes patterns were captured for every 0.02 mm displacement
increment from 0 to 3.00 mm in the x-direction and stored in the database for the
pattern matching purpose. The matching of the moire fringes in y- and the x- and
directions was done after angular compensation using a rotational angle detection
algorithm. The average difference and its standard deviation of the pattern matching
approach compared to the actual displacement were found to be 0.00 I mm and 0.022
mm respectively. The small difference between the actual displacement and the
estimated displacement shows the high accuracy of the proposed pattern matching
approach. To assess the applicability and practicality, the in-plane displacement
measuring system was applied to measure the crack growth in a wall crack model
and to measure the beam deflection in a three-point bending test to determine the
elastic moduli of foam material. In crack growth measurement, the proposed
measuring system produced output displacements with the average error of 0.003
mm. In determination of the elastic moduli of foam material, the proposed measuring
system produces an elastic modulus of 27.27 MPa compared to the elastic modulus
of 17.65MPa determined using conventional three-point bending test.
Description
Keywords
An improved in-plane (2D) displacement measurement system using the moire method , integrated with image processing techniques has been developed.