Design - January/February 2008
Negative-stiffness vibration holds experiments
The air tables many research labs use to isolate
delicate experiments from vibration can be insufficient for
measuring at the angstrom and micron levels. Yale University's
Professor Lawrence Cohen found a better solution for his brain-imaging
experiments in Negative Stiffness vibration isolators from
Minus K Technology, Inglewood, Calif., (minusk.com). "The
device reduces vibrations better than air tables because it
works in all three axes," says Cohen.
Negative-stiffness isolators use a completely
mechanical concept to isolate vibrations. The firm says
its tests show that a properly adjusted unit can cut
vibration 93% at 2 Hz, 97% at 5 Hz and 99.7% at l0 Hz.
The University' design uses three isolators stacked
in series: a tilt-motion isolator sits on a horizontal-motion
isolator and these mount on a vertical-motion isolator.
To view micron-level changes, Cohen's
lab uses a camera capable of 2,000 frames/sec. "Measuring
brief changes requires vibration isolation because they
are so small," says Cohen. "Any movement in
the lab has a big effect. If you are viewing at ten
microns, and the apparatus vibrates by ten microns,
you are in big trouble."
The passive vibration isolation machine
from Minus K Technology is at work in Lawrence Cohen's
lab at Yale
The vertical-motion isolator uses a
conventional spring connected to the company's negative-stiffness
mechanism which consists of two bars hinged at the center,
supported at their outer ends on pivots, and loaded
in compression by a force P. The spring is compressed
by a weight W to an operating position. The stiffness
of the isolator is K= Ks - Ka where Ks = spring stiffness,
and Kn = magnitude of a negative stiffness which is
a function of the bar lengths and load P. The developer
says isolator stiffness can approach zero.
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