Compact vibration isolation and suspension for Australian International Gravitational Observatory: local control system

High performance vibration isolators are required for ground based gravitational wave detectors. To attain very high performance at low frequencies we have developed multistage isolators for the proposed Australian International Gravitational Observatory detector in Australia. New concepts in vibration isolation including self-damping, Euler springs, LaCoste springs, Roberts linkages, and double preisolation require novel sensors and actuators. Double preisolation enables internal feedback to be used to suppress low frequency seismic noise. Multidegree of freedom control systems are required to attain high performance. Here we describe the control components and control systems used to control all degrees of freedom. Feedback forces are injected at the preisolation stages and at the penultimate suspension stage. There is no direct actuation on test masses. A digital local control system hosted on a digital signal processor maintains alignment and position, corrects drifts, and damps the low frequency linear and torsional modes without exciting the very high Q-factor test mass suspension. The control system maintains an optical cavity locked to a laser with a high duty cycle even in the absence of an autoalignment system. An accompanying paper presents the mechanics of the system, and the optical cavity used to determine isolation performance. A feedback method is presented, which is expected to improve the residual motion at 1 Hz by more than one order of magnitude.