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Model parameters of a horizontal seismometer with an optical diffraction vibration sensor

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Abstract

Subject of study. This study investigates the model of a horizontal seismometer composed of a physical pendulum and an optical diffraction sensor for angular vibrations that comprises a laser, an assembly of two relief phase diffraction gratings (DGs), a diaphragm isolating the radiation of the first diffraction order after the diffraction grating assembly, and a photodetector. The physical pendulum comprises a disk mounted on the rotation axis, pullback springs, and an additional weight on the disk edge. The assembly of DGs was positioned on the disk. The signal from the photodetector output was proportional to the small linear displacements of the additional weight owing to the inertial forces under horizontal vibrations of the seismometer base. Model parameters. The diameter and mass of the disk are 250 mm and 2 kg, respectively. The free swing period of the pendulum is 3 s. The mass of the additional weight is 0.1 kg. The laser power is 5 mW. Two relief phase DGs with a groove depth of 0.32 µm and a period of 0.1 mm are positioned on the opposite facets of a glass block of DGs with a thickness of 13 mm. The grooves of the first DG are parallel to the grooves of the second DG. An FD-24k photodiode, a load resistor, an amplifier, and a filter with a bandwidth of 1800 Hz are connected to the output. Test results. The model of the optical diffraction seismometer and SM-3 reference seismometer were mounted on a single platform. Periodic oscillations of the platform with a period of 0.475 s were applied in the first series of experiments. The dependence of the signals from the outputs of seismometers of both types were similar; however, a 90° phase shift of the output signal from the investigated model with respect to the SM-3 output signal was present. This is because the output signal of the sensor in the optical diffraction seismometer is proportional to the angular displacement of the physical pendulum, whereas the signal from the coil of the SM-3 sensor is proportional to the speed of pendulum movement. A slow weak linear force was applied to the platform with seismometers in the second experiment series. In this case, the SM-3 output signal did not exceed the noise level, whereas the output signal of the optical seismometer model was triple the noise level. The sensitivity of the seismometer model significantly exceeds the sensitivity of the SM-3 seismometer under slow perturbations. Practical significance. The output signal from the optical diffraction vibration sensor is proportional to the angular displacement of the pendulum instead of the speed of its movement. Thus, the sensitivity of the seismometer model does not decrease in the range of low frequency vibrations. A seismometer with negative feedback for investigation of weak low-frequency vibrations of the Earth’s surface can be designed based on this scheme.

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