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 X
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
