Geometric Anti Spring (GAS) Filter Used for vertical seismic

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Geometric Anti Spring (GAS) Filter
Used for vertical seismic isolation
Demonstration model,
designed and built at NIKHEF
• A large mass (payload 4,8kg) is suspended from 3 blade springs.
Initial resonance frequency f0 ≈ 5 Hz.
• The blades are mounted on the filter plate.
• The filter plate is set into oscillation with frequency f ≈ 10 Hz, using coil
springs. This mimics seismic motion.
• As f is larger than f0, the mass oscillates with smaller amplitude than the plate.
• By horizontal compression of the blades the stiffness strongly decreases,
resulting into smaller f0 (≈ 0. 3 Hz) and a much larger attenuation.
• The motion of plate and mass are measured with LVDT displacement sensors.
keystone
Undeformed maraging steel blade
filter plate
Horizontal compression of blades
suspension wire
Fx
Top view
200 mm
Mathematical analysis
Finite element analysis
prestressed blade

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