Simulation and Isolation Design for the TorPeDO Gravitational Sensor

Abstract

The Torsion Pendulum Dual Oscillator (TorPeDO) is a dual torsion pendulum gravitational sensor under development by The Australian National University (ANU), Centre for Gravitational Astrophysics (CGA). This sensor uses measurements of differential rotation between two low resonant frequency (~26 mHz) torsion pendulums to infer transients in the curl of the gravitational potential field. Target sensitivity for the TorPeDO is set at 1.5 x 10^-14 rad/sqrt[Hz] at 0.1 Hz differential rotation between the two pendulums. This target sensitivity, combined with torsion pendulum resonant frequencies of ~ 26 mHz, aims to create an observation frequency band for gravitational sources between 30 mHz and 10 Hz. If this target sensitivity is achieved the proposed applications of TorPeDO sensor are wide ranging, with both scientific and practical applications. This thesis focuses upon the development of the TorPeDO for two primary applications: the detection of Newtonian noise sources, and detection of earthquakes by transients in the local gravitational potential field.

A controls prototype for the TorPeDO sensor having already been fabricated, this thesis explores several design challenges that have arisen during its commissioning and operation. The initial chapters of this thesis focus on the simulation of the current prototype, and necessary steps required to mitigate environmentally induced mechanical disturbances to the system. These environmental factors, combined with technical noises source, limit on the sensitivity of the TorPeDO controls prototype. By examining a predictive Lagrangian mechanics based model of the TorPeDO, calculated in a full three-dimensional Cartesian space, this thesis explores how cross-coupling facilitates the propagation of noise from several sources within the detector's design. Methods for mitigating the propagation of each potential noise source, both in the mechanics of the system and in the design of the controls architecture, are then proposed. As part of this examination, how the design of the TorPeDO's pendulums contributes to the range and antenna pattern of the TorPeDO detector is considered.

Later chapters of this thesis focus upon the suppression of residual motion of the TorPeDO transducer suspension point to below 1.9 x 10^-10 m/sqrt[Hz] linear motion and 3.9 x 10^-12 rad/sqrt[Hz] rotational motion at 0.1 Hz using a novel seismic isolation chain design. This suspension point requirement is determined through the simulation as being required to ensure the sensor is not limited by seismic noise. The design and control of the proposed seismic isolation chain design is a wholly new system being fabricated and commissioned at the ANU because of the work presented here. Where possible, practical results arising from initial testing of the mechanics and control systems of this new isolation chain are presented alongside simulations to demonstrate the potential performance of the system.

In the concluding chapters of this thesis proposals will be made for improvement of the TorPeDO sensor based upon the current state of the system and predictive modelling of the new seismic isolation chain design. This will provide the reader with a more complete picture of the current state of the TorPeDO sensor, the state of the system upon the completion of the seismic chain commissioning, and current ongoing avenues of development. Show more