Measurement of objects with low vibration frequencies using modified Michelsoninterferometry

Abstract

A simple, low-cost, and non-contact optical method using modified Michelsoninterferometry is proposed for measuring the vibration frequencies of objects. Two beamswhich are split from a laser beam project onto two rough surfaces (compared withthe laser-beam wavelength) of a fixed reference object and a vibrating testedobject respectively, and two quasi-spherical waves scattered or diffused from thetwo rough surfaces individually are combined to form interference fringes on anobservation plane. The vibration frequencies of the tested object can be derived byanalyzing the time-dependent gray-value signal (caused by the fringe-shiftingphenomena) for a specified position on the observation plane. The signal is transformedfrom the time domain to the frequency domain to form a spectrum by first usingthe fast Fourier transformation. Then, all the spectrum values correspondingto different frequencies in the spectrum are compared with a threshold value.Finally, all frequencies in the spectrum with local peaks higher than the thresholdvalue are considered to be the vibration frequencies of the tested object. Theexperiment is a non-contact type with no lenses and is able to be conducted easily withhigh accuracy and low cost for testing objects vibrating with one, two, or threefrequencies. The results and features confirm the utility of the proposed method.