Civil Engineering Association

The HAM-SAS Seismic Isolation System for the Advanced LIGO Gravitational Wave Interferometers

The three LIGO interferometers are full operative and under science run since November 2005. The acquired data are integrated with those obtained by th Virgo experiment within an international cooperation aimed to maximize the e.orts for the detection of gravitational waves.
From 2001 LIGO I is expected to be shut down and the construction and commissioning of Advanced LIGO to start. The objective of the new generation interferometers is a ten times greater sensibility with the purpose to extend of a factor of a thousand the space volume covered and to increase of the same order of magnitude the probability to detect events.
To increase the sensibility in the band below 40 Hertz, the main source of noise that Advanced LIGO have to face is the seismic noise. In this perspective, the SAS group (Seismic Attenuation Systems) of LIGO has developed a class of technologies on which the HAM-SAS system is based. Designed for the seismic isolation of the output mode cleaner optics bench and more in general for all the HAM vacuum chambers of LIGO, HAM-SAS, with little variations, can be extended to the BSC chambers as well. In HAM-SAS the legs of four inverted pendulums form the stage of attenuation of the horizontal degrees of freedom. Four GAS filters are included inside a rigid intermediate structure called Spring Box which is supported by the inverted pendulums and provide for isolation of the vertical degrees of freedom. The geometry is such that the horizontal degrees of freedom and the vertical ones are separate. Each GAS filter carries an LVDT position sensor and an electromagnetic actuator and so also each leg of the inverted pendulums. Eight stepper motors guarantee the DC control of the system.
A prototype of HAM-SAS has been constructed in Italy, at Galli & Morelli and then transferred to Massachusetts Institute of Technology in the US to be tested inside the Y-HAM vacuum chamber of the LIGO LASTI laboratory.
The test at LASTI showed that the vertical and horizontal degrees of freedom are actually uncoupled and can be treated as independent. It was possible to clearly identify the modes of the system and assume these as a basis by which to build a set of virtual position sensors and a set of virtual actuators from the real ones, respect with which the transfer function of the system was diagonal. Inside this modal space the control of the system was considerably simplified and moree.ective. We measured accurate physical plants responses for each degree of freedom and, based on these, designed specific control strategies. For the horizontal degrees of freedom we implemented simple control loops for the conservation of the static position and the damping of the resonances. For the vertical ones, be

direct link