Abstract

This experimental study demonstrates a horizontal seismic sensor based on an inverted vertical pendulum. The arm that keeps the proof mass inverted at the quiescent position is directly attached on an elastic metallic strip made from chrysocal (CuSn3Zn9) alloy that acts as a spring. This mass-suspension arrangement acts as pivot and restoring mechanism and reveals a relatively long natural period (3–4 sec), considering the overall dimensions of the pendulum. The application of multiple frequency-dependent feedback paths gave a flat to ground velocity response from 100 to 0.023 sec with a differential output 2 × 750 V/m/sec. The present study focused mainly on the pendulum arrangement with some emphasis on the suspension flexure and the feedback actuator design, whereas the electronic circuitry used was maintained according to the standard techniques used in the force-balanced sensors. Design and assembly simplicity was introduced with the use of a single uniform element as flexure without affecting the precision of the motion. Moreover, the planar coil adopted in the feedback actuator ensured a strong motor constant, without the limitations in the effective displacement usually met in the angular motion of the pendulum. Amplitude and spectral analyses of data sets obtained from local ground noise as well as from local, regional, and teleseismic earthquake activity, have been shown to be comparable with respective recordings of other standard broadband seismometers operated at the same vault. The experimental seismometer was found to be compatible with these reference seismometers in terms of amplitude and frequency response indicating the capability of the experimental sensor to resolve low-frequency and low-amplitude seismic signals.

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