Compact four-degree-of-freedom seismometer with capacitive readout

Seismic noise in both translational and angular degrees of freedom poses significant challenges to highprecision measurements. To mitigate low-frequency seismic noise coupling, active vibration-isolation platforms have been developed, in which a combination of low-noise and single- or multiple-degreeof-freedom seismometers play a critical role. This paper presents the design and performance evaluation of a four-degree-of-freedom (4D) seismometer based on a single flexure inverted pendulum within a 15cm cube. This seismometer employs a differential capacitive sensing scheme to measure the motion of a test mass, similar to that used in the Laser Interferometer Space Antenna gravitational-wave detector. Two with an optimal translational noise floor of 2 x 10-10 m/s2/root Hz around 1 Hz. In the 4D mode, a single sensing modes are employed: the 2D and 4D modes. In the 2D mode, translational motions are measured, seismometer can measure both translational and angular motion, whereas two spatially separated xyz seismometers would typically be employed; however, the angular sensitivity is limited by the compactness of the seismometer and the intrinsic stiffness of the flexure, which is left for future optimization.