Multi-scale optimal control for Einstein Telescope active seismic isolation

TL;DR

This paper introduces a multi-scale optimal control framework for active seismic isolation in the Einstein Telescope, improving low-frequency noise reduction and sensor optimization for gravitational-wave detection.

Contribution

It presents a unified control approach that optimizes feedback and blending filters for seismic isolation, adaptable to different sensors and environmental conditions.

Findings

OmniSens achieves up to 100x reduction in platform motion near microseism.

Framework enables efficient re-optimization under varying conditions.

Sensor noise can be projected to instrument performance metrics.

Abstract

We present a multi-scale optimal control framework for active seismic isolation in the Einstein Telescope, a third-generation gravitational-wave observatory. Our approach jointly optimizes feedback and blending filters in a cross-coupled opto-mechanical system using a unified cost function based on the "acausal optimum," which quantifies sensor signal-to-noise ratios across frequencies. This method enables efficient re-optimization under varying sensor configurations and environmental conditions. We apply the framework to two candidate sensing systems using their modeled sensitivity: OmniSens-a six-degree-of-freedom inertial isolation system-and BRS-T360, which combines Beam Rotation Sensor (BRS) as an inertial tilt sensor with T360 as a horizontal seismometer. We demonstrate superior low-frequency isolation with OmniSens, reducing platform motion by up to two orders of magnitude near the microseism. The framework allows for ready optimization and projection of sensor noise to metrics relevant to the performance of the instrument, aiding the design of the Einstein Telescope.