Monolithic Folded Pendulum Accelerometers for Seismic Monitoring and Active Isolation Systems

Abstract—A new class of very low noise low-frequency force-balance accelerometers is presented. The device has been designed for advanced mirror isolation systems of interferometric gravita-tional wave detectors. The accelerometer consists of a small mono-lithic folded pendulum with 2 s of natural period and an in-vacuum mechanical quality factor of 3000. The folded pendulum geom-etry, combined with the monolithic design, allows a unique 0.01% cross-axis residual coupling. Equipped with a high-resolution ca-pacitance position sensor, it is capable of a noise-equivalent iner-tial displacement of 1-nm root mean square integrated over all the frequencies above 0.01 Hz. The main features of this new ac-celerometer are here reviewed. New possible applications of mono-lithic folded pendula in geophysical instrumentation are discussed. Index Terms—Accelerometer, pendulum, tiltmeter. I