Ultra-low-frequency vertical vibration isolator based on a two-stage beam structure

Abstract

High-performance vertical vibration isolators are required in precision instruments and physical experiments to reduce the seismic noise, which limits the instrument performance and measurement results. For example, inertial references are needed in interferometric gravitational wave detectors and absolute gravimeters, in order to separate the useful signal from noise. Microseisms typically occur at around 0.07 Hz. The secondary microseisms occur at about 0.14 Hz. Buildings usually wobble at frequencies between 0.1 and 1 Hz. To reduce all these vibrations would require a spring-mass system with a resonance frequency lower than 0.05 Hz. The most commonly applied techniques use a passive vertical isolation system, which is easy to set up and cheap to build. However, to achieve low cut-off frequency, such as 0.05 Hz, there requires longer than 100 m static deflection for a simple passive isolator, which is impractical in most applications. An ultra-low-frequency active vertical vibration isolator, based on a two-stage beam structure, is proposed and demonstrated in this paper. Two beams are connected to a frame with flexural pivots. The upper beam is suspended from the frame with a normal hex spring. The lower beam is suspended from the upper one by a zero-length spring. The flexural pivots of the upper beam are not vertically placed above the pivots of the lower beam. With this special design, the attachment points of the zero-length spring to the beams can be moved to change the effective stiffness. A laser reflectometry is used to detect the angle between the two beams. A laser collimator, a mirror, a beam splitter and an optical detector are fixed to the upper beam, and another mirror is fixed to the lower beam. A laser beam from the collimator is directed to the detector via the mirrors and the beam splitter. The output of the detector is proportional to the angle between the two beams. The minimum detectable angle is 36 nrad. The angle signal is sent to a circuit to generate a control signal, which drives a voice coil mounted between the lower beam and the frame to maintain the angle between the two beams to a fixed value. The isolation system can achieve a natural period of 100 s by carefully adjusting the attachment points of the zero-length spring and the feedback parameters. This type of isolator has a simpler and more robust structure than the famous active vibration isolator-the super spring. The system is promising in applications such as precision instruments and experiments, especially in absolute gravimeters.