By R. Colin Johnson
Minus K Technology Inc. claims that the "negative
stiffness" mechanism it has developed isolates objects
from vibrations better than traditional solutions. Such
techniques provide the stable platform
and angstrom-level accuracy needed to test microelectro-mechanical
systems, nanoscale metrology and semiconductor fabrication
tools, for example.
"Our negative stiffness mechanism exerts an opposing
force that cancels out the stiffness in a spring,"
said David Platus, president and CEO of Minus K (Inglewood,
Calif.). "That gives us isolation that is twice as
good as other active systems but for half the price of air
table-style passive vibration isolation systems."
Even the subsonic background resonance of waves pounding
the shores worldwide can be too noisy for some testing environments.
"The U.S. Air Force couldn't find a place quiet enough
to test their next-generation accelerometers and gyros,"
said Platus, "since they wanted isolation even from
the .07-Hz background of nano-g-scale vibrations from waves
crashing the shores worldwide. That got me thinking about
a negative stiffness mechanism to cancel out vibrations."
To isolate atomic-force and scanning-tunneling microscopes
from vibrations, researchers have traditionally used passive
air tables that support weight on a cushion of air, or they
use active electronic feedback to send cancelling forces
that damp out oscillations in springs. Now Paltus claims
his patented negative-stiffness mechanism outperforms active
systems while underpricing passive ones.
Platus, who founded Minus K, now has a patent portfolio
protecting its negative stiffness mechanism. The company
offers vibration isolation payload capacities ranging from
a 10-pound tabletop to 10,000-pound floor panels. When adjusted
to a .5-Hz natural frequency, the Minus K vibration isolators
achieve 93 percent isolation efficiency at 2 Hz, 99 percent
at 5 Hz and 99.7 percent at 10 Hz.
Blocking the vibrations
The problem is that any platform have a certain positive
stiffness coefficient that determines their natural resonant
frequency--usually 1 Hz and up. But by subtracting negative
stiffness from the positive stiffness of the spring, Minus
K's negative stiffness mechanism can block nearly all vibrations
higher than .5 Hz.
An example of a commonly known negative stiffness mechanism
is the bottom on an oil can, fabricated so that it is puckers
out. When the user pushes the bottom, the surface flips
into a concave shape, thus sharply forcing out the oil.
Since stiffness is the force needed to move something, the
bottom of the oil can is said to have negative stiffness.
The slightest push makes the bottom jump forward from under
the finger to squirt out an oil stream--mathematically exhibiting
a negative stiffness coefficient.
"The bottom of the oil can is buckled, but in the
center of its travel it is unstable, flipping to one side
or the other," said Platus. "If you were to monitor
the force deflection behavior at the center of that travel,
its stiffness would be negative."
Minus K exploits this passive materials phenomenon by supporting
its payload with a spring that is at the center of travel
of a negative stiffness flexure. Any positive force pushing
on the springs of its vibration isolation platform is opposed
by the negative stiffness of the flexure.
"At the top of the negative stiffness mechanism there
are horizontally oriented metal flexures coming from the
center of the spring out, and they are compressed so that
the flexure wants to pop up
or down," said Platus. "By carefully tuning the
magnitude of this negative stiffness to be a little less
than the stiffness of the supporting springs, we can reduce
the vibration isolation platform's natural frequency of
oscillation to be less than half a hertz, so that higher-frequency
vibrations can't get through."
For the vertical direction, the isolation platform loads
a beam column in the same way it does for the horizontal
flexures. By keeping the beam column
near buckling, its horizontal stiffness gets less and less
with increased load.
"Theoretically, if you load the vertical beam columns
at their critical buckling load, no force will move it back
and forth," said Platus.
By balancing a vertical spring against the negative stiffness
of the vertical beam columns, the vibration isolation system
achieves vertical .5-Hz natural resonance. The combined
effect of the negative stiffness of the horizontal flexures
and the vertical beam columns isolates all vibrations greater
than .5 Hz in both vertically and horizontally.
"Transistors have critical dimensions down around
25 nanometers," said nanotechnology
researcher David Ferry, who is an EE professor at Arizona
State University. "And the most critical dimension
is the oxide thickness, which is 1 nanometer." Consider,
he said, that "you have to control 1-nm vertical thickness
over 300 millimeters of lateral dimension. That defines
modern manufacturing technology's need for effective vibration
isolation, which has never been greater than today, and
will continue to become more demanding as the nano-industry
progresses."
Arizona State University isolates its atomic-force and
scanning-tunneling microscopes with Minus-K vibration isolation
platforms.
"At the top of the negative stiffness mechanism there
are horizontally oriented metal flexures coming from the
center of the spring out, and they are compressed so that
the flexure wants to pop up
or down," said Platus. "By carefully tuning the
magnitude of this negative stiffness to be a little less
than the stiffness of the supporting springs, we can reduce
the vibration isolation platform's natural frequency of
oscillation to be less than half a hertz, so that higher-frequency
vibrations can't get through."
For the vertical direction, the isolation platform loads
a beam column in the same way it does for the horizontal
flexures. By keeping the beam column
near buckling, its horizontal stiffness gets less and less
with increased load.
"Theoretically, if you load the vertical beam columns
at their critical buckling load, no force will move it back
and forth," said Platus.
By balancing a vertical spring against the negative stiffness
of the vertical beam columns, the vibration isolation system
achieves vertical .5-Hz natural resonance. The combined
effect of the negative stiffness of the horizontal flexures
and the vertical beam columns isolates all vibrations greater
than .5 Hz in both vertically and horizontally.
"Transistors have critical dimensions down around
25 nanometers," said nanotechnology
researcher David Ferry, who is an EE professor at Arizona
State University. "And the most critical dimension
is the oxide thickness, which is 1 nanometer." Consider,
he said, that "you have to control 1-nm vertical thickness
over 300 millimeters of lateral dimension. That defines
modern manufacturing technology's need for effective vibration
isolation, which has never been greater than today, and
will continue to become more demanding as the nano-industry
progresses."
Arizona State University isolates its atomic-force and
scanning-tunneling microscopes with Minus-K vibration isolation
platforms.
