Active Vibration Isolation Issues in LIGO
Hong S. Bae
Dept. of Mechanical Engineering
Stanford University
August 1999
Abstract
This work has investigated the design of several controllers for a
prototype
vibration isolation and alignment system for LIGO. LIGO is designed to detect
gravitational waves generated by astrophysical activities with a ground-based
laser interferometer. The LIGO requirements call for very stringent isolation
and alignment of a test mass that will be supported by several passive and
active stages. To meet these requirements with a robust control design, we
use
colocated sensors and actuators on each leg of the platform. Geophones are
used at the top of the leg for feedback control, and at the bottom of each leg
for feedforward control. To avoid low frequency stability issues and to
provide alignment information, we also use strain gauge on each piezo
actuator. The primary objective of this thesis is to investigate
techniques to
design the feedforward and feedback controllers for this system. In the
process, we present an analysis of complementary filtering and illustrate an
“error signal correction process” that is required to account for the fact
that
the strain gauge also measures the ground motion.
The experimental results show a factor of 50 reduction in the transmissions
over 2-100 Hz. These results then provide a strong foundation for future LIGO
II designs.
Contact for a copy of
the thesis.
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