Automated Mode Shape Measurement By A Modulated-Fringe Technique Using Electronic Speckle Pattern Interferometry

Abstract

Electronic Speckle Pattern Interferometry (ESPI) is a full-field and non-contact technique for measuring surface displacements. However, one difficulty in the use of the technique is the ambiguity in fringe interpretation. This paper presents a modulated-fringe technique which removes the fringe ambiguity. By strobing the laser light, two speckle images of an object in steady state vibration can be digitally recorded. Shifting the illumination beam between exposures produces a linear phase variation which is added to the phase change induced by the vibrational displacements. The result is a modulated-fringe pattern which has monotonically increasing fringe orders. This permits the fringe orders to be determined in a staight forward manner and without ambiguity. The phase change due to vibrational displacement alone is then obtained by simply subtracting the known linear phase variation. Elimination of fringe ambiguity has paved a way for automatic data reduction of the vibration mode shape by digital image processing.