Accuracy of the Sampling Moire Method and its Application to Deflection Measurements of Large-Scale Structures

Measuring accurate displacement distributions for large-scale structures is an important issue and a very challenging task. Recently, a simple and accurate phase measurement technique called sampling moire method [Exp Mech 50-4: 501-508, (2010)] has been developed for small-displacement distribution measurements. In this method, the phase distribution of moire fringes can be analyzed from a single grating image by simultaneously performing down-sampling image processing and intensity-interpolation to generate multiple phase-shifted moire fringe images. In addition, the phase of the original grating can also be obtained from the phase of the moire fringe by adding the phase of the sampling grating. In this study, the measurement accuracy of the sampling moire method was analyzed through computer simulations and a displacement measurement experiment. Four factors of the sampling moire method were investigated, including the sampling pitch, the order of the intensity-interpolation, random noise, and the form of grating. The results show that determining the optimal sampling pitch is an important factor for obtaining better results but it is not critical. In addition, a practical application of the sampling moire method is presented that involves a deflection measurement on a 10-meter-long crane. The experimental results demonstrate that submillimeter deflections of the crane can be successfully detected.