A NEW OPTICAL METHOD FOR DETECTING SEISMIC VIBRATIONS

In this paper we have described a new method to detecting characteristics of an oscillation system based on the Moiré technique. We can determine amplitude, resonance frequency and damping coefficient of an oscillation system both in vertical and horizontal direction. To do this approach, the displacements of oscillatory mass must be determined as possible as accurate. These displacements are recorded by Moiré detecting procedure. A spring-suspended mass (f0=10 Hz), whose position is monitored by Moiré technique, is used to testing this idea. Our detecting system consists of a pair of similar gratings which are installed near together without physical contact. The planes of the gratings are parallel and the lines of gratings have small angle respect together. Also, a laser diode (1 mW), a silicon photo-diode and a narrow slit have been used and fixed to the frame to illumination fringes movements due to the suspended mass movement. Due to moving the oscillatory mass and the fringes movements, the light intensity on the detector varies and is recorded as voltage. The output signal can be used to measure the oscillator characteristics. This method can detect displacements of the order of micron. Also, the experimental result and theoretical simulation are compared.