Coil springs with constrained-layer viscoelastic damping for passive isolation

摘要

Abstract: This paper summarizes the design, optimization, development, fabrication, and testing of a vacuum compatible coil spring with embedded constrained layer visco-elastic damping. The spring is developed as part of the NSF funded LIGO (Laser Interferometer Gravity Wave Observatory) project. Large numbers of those springs are the primary components of multi- stage, in-vacuum, passive seismic isolation stacks that provide high attenuation ($MIN@160 dB/decade above 15 Hz) of floor vibrations for ultra-sensitive (better than 10$+$MIN@18$/ m/$ROOT@Hz noise floor between 40 and 1000 Hz) laser interferometers. The spring design addresses both requirements for passive isolation within a single, self- contained, vacuum tight envelope: low stiffness for maximum attenuation and non-viscous damping to limit resonant amplitudes in the stack. This is achieved with a tubular coil spring design with an internal torsional constrained layer damping structure. The paper presents the analysis of this spring using closed-form analytical expressions, trend studies showing the strong dependence of spring performance on key design parameters, and explicit numerical design optimization. Manufacturing issues are briefly discussed. Finally, experimental results from static and dynamic tests performed on prototype units are presented. Results show loss factors of the order of 1.5% in the transverse direction to 3% in the axial direction, at frequencies from 1 to 2 Hz. !10