Experimental testing of LIGO vibration isolation system
Author(s)Krull, Alexander G. (Alexander Gerhard)
Experimental testing of Laser Interferometer Gravitational-wave Observatory vibration isolation system
Massachusetts Institute of Technology. Dept. of Mechanical Engineering.
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The LIGO (Laser Interferometer Gravitational-wave Observatory) project is designed to detect gravitational waves using precision interferometry. The detection from astrophysical sources has the potential to test Einstein's Theory of General Relativity, and additionally open a new window into the universe and its origin. The Initial LIGO detectors are currently operating at a strain sensitivity of 10-21 Hz, or equivalently 1018 m/ [square root of] Hz, at 100 Hz. In order to attain improved sensitivity required for guaranteed detection of astrophysical sources, e.g. coalescing neutron star binaries and black holes, pulsars, and supernovae collapses, improvements of the strain sensitivity must be achieved. Next generation detectors such as Advanced LIGO are under development, which aims to improve the sensitivity by more than a factor of 10 at all frequencies, compared to initial LIGO. This improvement in sensitivity will be achieved in part by improved seismic isolation one component of which is an active vibration isolation platform. Currently, research and development is being conducted at MIT on a prototype of this vibration isolation system. The work described in this thesis focuses on the Internal Seismic Isolation (ISI) system under development for Advanced LIGO.(cont.) This system consists of a three-stage in-vacuum seismic isolation system which is supported by an external hydraulic actuation stage known as the Hydraulic External Pre-Isolation (HEPI) stages of the active vibration control system. HEPI uses forces generated by hydraulic pressure to cancel low frequency seismic noise, primarily due to forces from ground vibration. The ISI is an actively controlled platform, in which each stage is supported by three maraging steel blade springs. The vibration is sensed in six degrees of freedom and reduced by applying forces through a control feedback loop. In order for the feedback loop to function properly, it is important to know and be able to predict the position of the ISI stages to within a few thousandths of an inch. Since the load being applied to the spring blades is known, the compliance of each spring along with various shim thicknesses will determine the final position of the stages. Although compliance is a material and geometric property, and should remain constant from spring to spring, due to imperfections of the fabrication process and variation in the material properties, small variations in the long and short spring compliance value were detected using a Spring Tester.(cont.) The blades were designed based on their resonant frequencies and the load which they would be supporting - more specifically, their geometry (length, width, and thickness) were defined such that the load each supported brought them to a 1/3 of their failure stress. For my undergraduate thesis, I determined the compliance of multiple long and short springs was determined using a specially designed apparatus - the "Spring Tester." Ideally, three blade springs of identical compliance should be used to eliminate system imbalance, but to variation during fabrication may be difficult to achieve Using the Spring Tester the mean values for each set of long and short spring linear compliance data were found to be 0.729 +.008 mils/lb and 0.670 ±.027 mils/lb, respectively, while the means for the long and short angular compliance data were 0.078 + .001 mrad/lb and 0.089 ±.003 mrad/lb, respectively.
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (leaf 29).
DepartmentMassachusetts Institute of Technology. Dept. of Mechanical Engineering.
Massachusetts Institute of Technology