Vibration measurement in non-rigid test environment with speckle interferometry

We propose a new method for vibration measurement in non-rigid test environment with electronic speckle pattern interferometry (ESPI). The ESPI is useful for non-contact, real-time analysis of vibration. This method needs rigid test environment, however. When the interferometer and a vibration surface are on a non-rigid table or their environments are separated, especially at manufacturing areas, high-amplitude, low-frequency noise fluctuation overwhelms a vibration signal and the amplitude fringes disappear. We use electronic shutter function of a TV camera and reduce exposure time of an image sensor. With the time reduction, we may extract an image from many input images, during whose acquisition time noise fluctuation turns back and its magnitude is so small that the vibration signal goes to be included in the image. We accumulate the images and increase the contrast of the amplitude fringe map. We evaluated usefulness of this method with circular saw vibration. The interferometer and the saw are fixed on a rigid board and the noise fluctuation is electronically superposed on the vibration signal with sine wave. This method is successful for a fluctuation amplitude of 60 mu m.