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UBC Theses and Dissertations

Interpolated ESPI using continuous measurements for moving surfaces and large deformations Pokharna, Harsh


Deformation measurement is a common experimental need when testing the behaviour of mechanical devices. Such measurements are the basis for the evaluation of mechanical and thermal stresses, also for health monitoring of biological samples, study of vibrating parts of a mechanical component, and for quality testing of MEMS products. Optical methods like Electronic Speckle Pattern Interferometry (ESPI) are particularly attractive for practical applications because they are non-contact, sensitive and because they provide full-field displacement map over an extended area of the specimen. Conventional phase-stepping ESPI requires that the test object remains in the state of rest during the image acquisition, both before and after the deformation. This limits the use of ESPI to static or quasi-static measurements. Another limitation of conventional static ESPI is the measurable displacement range. Over large displacements ranging more than the size of a pixel, the speckles decorrelate and correct displacement information is lost. This project aims at developing a robust measurement technique to calculate surface displacement map for moving objects and to extend the range of measurable displacement. A newly developed algorithm that includes a technique involving camera exposure-synced phase stepper and mathematical interpolation is described. This method enables the extraction of instantaneous phase information using single interferograms. The test object is no longer required to be quasi-static during image acquisition. Every pixel in this method behaves like an independent displacement sensor. In addition, this method is computationally inexpensive. Some of the post-processing techniques are also discussed. A set of experimental results are discussed to validate the proposed technique.

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