Abstract: This paper describes the real-time measurement of in-plane vibration using an improved double-aperture laser speckle interferometer. To detect the in-plane vibration in real- time, a new signal processing system which can detect the phase angle of the frequency component of the interference fringes in real-time by calculating Fourier coefficients is proposed. The proposed real-time signal processing system is composed of a pulse generator, a sin/cos generator and an A/D converter controlled by a personal computer, and of an analog electronic circuit made of analog IC's. The linearity of the output signal of the prototype due to a number of the detected fringes and noises is experimentally examined. In- plane static displacement and vibration displacements of a specimen driven by a piezostack are measured with a new laser speckle interferometer equipped with the proposed signal processing system and are calibrated with a gap sensor. The measured static in-plane displacements varied in proportion to the given displacement with reproductive error variation with the maximum value of 0.5 $mu@m. The vibration displacements measured with the laser speckle interferometer were detected with the maximum sampling frequency of 470Hz. This new laser speckle interferometer thus achieved the measurement of in-plane vibration in real-time. !4
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