Technology - April 2012
Turning the tables on vibration
As a method
for vibration isolation, the traditional air table is now
being challenged by the more compact effective Negative-Stiffness
vibration isolators, developed by Minus K Technology
For almost 40 years pneumatic vibration isolators - air
tables - have I been the mainstay fur stabilizing industry
and academia's most critical micro-engineering instrumentation.
But just as technology has been migrating steadily from
micro to nano, so has the need for more precise vibration
isolation in microelectronics fabrication, industrial
laser/ optical systems and biological research.
These so-called "passive system" air tables
are now being seriously challenged by the newer Negative-Stiffness
vibration isolators. Negative-Stiffness isolation is rapidly
gaining popularity in industrial and laboratory environments,
mainly because of its ability to isolate vibration effectively
in diverse and challenging environments.
Negative-Stiffness mechanism vibration isolation systems
were invented by Dr David Platus, who worked in the nuclear,
aerospace and defense industries conducting and directing
analysis and design projects in structural-mechanical
systems; he holds more than 20 patents related to shock
and vibration isolation.
In 1993 Platus founded Minus K Technology to develop,
manufacture and market vibration isolation products based
on the company's patented technology. The resulting products,
sold under the trade name Nano-K, are used in abroad spectrum
of applications, including nano technology, biological
sciences, semiconductors, materials research, zero-g simulation
of spacecraft and high-end audio.
Basically cans of air, air tables have been used for isolation
purposes since the 1960s and are still the most popular isolators
used But air tables with resonant frequencies at 2 to 2.5Hz
can typically handle vibrations down to only about 8 to l0
Hz, not quite low enough for optimum performance with modern
nano-equipment. Also, greater isolation efficiencies are needed
in the frequency ranges air isolators can handle.
Because of their very high isolation efficiencies, Negative-Stiffness
vibration isolation systems enable vibration-sensitive instruments,
such as scanning probe microscopes, micro-hardness testers,
profilers and scanning electron microscopes, to operate in
harsh conditions and severe vibration environments that would
not be practical with top-performance air tables and other
pneumatic isolation systems.
Negative-Stiffness isolators employ a different and completely
mechanical concept in low-frequency vibration isolation. Vertical-motion
isolation is provided by a stiff spring that supports a weight
load, combined with a Negative-Stiffness mechanism. The net
vertical stiffness is made very low without affecting the
static load-supporting capability of the spring.
Beam-columns connected in series with the vertical-motion
isolator provide horizontal-motion isolation. The horizontal
stiffness of the beam-columns is reduced by the "beam-column"
effect. (A beam-column behaves as a spring combined
with a Negative-Stiffness mechanism.)
The result is a compact passive isolator
capable of very low vertical and horizontal natural
frequencies and very high internal structural frequencies.
The isolators (adjusted to 0.5Hz) achieve 93% isolation
efficiency at 2Hz; 99% at 5Hz; and 99.7% at 10 Hz.
The following key points demonstrate the benefits of
Negative-Stiffness isolators compared with air isolation
Low Hertz perturbations: An air table will amplify
vibrations in a typical range of 2-7Hz; this is because
of the natural frequencies at which air tables resonate.
All isolators will amplify at their resonant frequency,
and then they will start isolating. So with an air table,
any vibration in that range could not only fail to be
mitigated, it could be amplified. The low cycle perturbations
would just come straight through to the instrument.
Negative-Stiffness isolators resonate at 0.5Hz, At this
frequency there is almost no energy present. It would
be very unusual to find a significant vibration at 0.5
Image clarity: Negative-Stiffness vibration isolation
can reduce vibration noise levels in Atomic Force Microscopes,
for example, by a factor of 2 to 3 when compared with top-performance
air tables. This is significant for noise levels in the sub-Angstrom
range. This results in clearer images and features not discernible
with pneumatic isolation systems.
Severe vibration environments: As nano-cquiprnent becomes
more prevalent, lab sites are being set up in much more severe
vibration-prone environments, such as upper floors of buildings
and cleanrooms. Such severe vibration locations are too extreme
for pneumatic isolators to do their job effectively. But Negative-Stiffness
isolators perform well in such environments, producing much
better images and data than can be obtained with even the
best high-performance air tables.
Harsh environments: Air tables are not particularly
compatible with operations in vacuums, extreme high and low
temperatures, and radiation. Yet these harsh environments
are often necessary when conducting research or testing, such
as with cryogenic chambers in semiconductor research.
All metal Negative-Stiffness systems can be configured to
be compatible with high vacuums and other adverse environments,
such as extreme high and low temperatures, and radiation.
With vacuums, for example, Negative-Stiffness isolators can
be used right inside the vacuum chambers. This offers other
advantages, such as much lower payload weights and more compact
systems, and eliminates problems associated with vacuum chamber
|Compressed air: Air tables require a constant
supply of compressed air, via either a dedicated compressed
air line plumbed into the lab, a tank of pressurized gas
or a small compressor. Even with a dedicated compressed
air line, the table's location is still limited by the
length of the air line.
Furthermore, large tanks of compressed gas have to be
mounted very securely to minimize their danger. Changing
the tanks can be quite difficult and inconvenient as well.
Compressors are sources of both mechanical and acoustic
noise and are very
poor choices from a vibration standpoint
It is advantageous to isolate the nano-environment mechanically
without recourse to compressed air. Negative-Stiffness
isolators do not require compressed air and operate purely
in a mechanical mode.
Location selection: Air tables are big, bulky structures
that take up a lot of lab space. The high-performance
air tables are even bigger. This can be a limiting factor
when laying out equipment in the lab. Negative-Stiffness
isolators are available in high-performance benchtop configurations,
considerably more compact than air tables and easy to
They are also available as workstations, tables and floor
platforms where these configurations are required.
Load adjustment: Low-frequency passive vibration isolators
are somewhat sensitive to small changes in weight loads, as
well as to large displacements. Pneumatic systems utilize
leveling valves to mitigate the problem.
Negative-Stiffness isolators provide a very simple manual
adjustment to accommodate variations in weight loads. For
applications where manual load adjustment is not practical
they provide an auto-adjust system that maintains the isolator
in a precise vertical equilibrium position.
Scanning probe microscopes: Scanning probe microscopes
(SPMs) have vibration isolation requirements that are unparalleled
in the metrology world. The vertical axis is the most sensitive
for most SPMs. They can also be quite sensitive to vibrations
in the horizontal axes. To achieve the lowest possible noise
floor, i.e. on the order of an Angstrom, isolation is always
Benchtop air systems provide limited isolation vertically
and very little isolation horizontally Negative-Stiffness
isolators provide increased isolation performance for SPMs
over air tables, while offering better ease-of-use and no
facility requirements. Negative-Stiffness isolators have the
flexibility of custom tailoring resonant frequencies vertically
Laser and optical equipment: Laser and optical systems,
whether used in an academic lab or in an industrial environment,
are very susceptible to vibrations from the environment. These
instruments almost always need vibration isolation. Traditionally
large air tables have been the isolators preferred for optical
systems, but Negative-Stiffness isolators are becoming a popular
choice because they provide 10 to 100 times the performance
(depending on vibration frequency). Laser-based interferometers
are extremely sensitive devices capable of resolving nanometre-scale
motions and features. They often have very long mechanical
paths, which makes them even more sensitive to vibrations.
The sophisticated modern ellipsometry techniques that allow
this high performance rely on low noise to be able to detect
fringe movement. Properly isolating an interferometer will
allow it to provide the highest possible resolution.
Optical profilers have similar sensitivity to vibrations.
Optical component systems are often quite complex. The long
optical paths can lead to angular magnification of vibrations.
Optical air tables can make the problems worse since they
have a resonant frequency that often matches that of floor
vibrations. Negative-Stiffness 0.5Hz isolators provide isolation
in these environments when air tables simply cannot.
Maintenance and expense: Because Negative-Stiffness
isolators utilize simple elastic structures and viscoelastic
materials that deform, their isolation performance does not
degrade with micromotions typical of lab floors and fabrication
rooms, as do conventional pneumatic isolators. Negative-Stiffness
isolators are also comparable in price to air isolators or
lower priced for many applications.
As industrial researchers and universities continue to broaden
their nanotech work, necessitating more sensitive equipment
and expanded lab facilities, vibration-handicapped environments
will become more prevalent, and a better vibration isolation
solution will be required than that provided by air tables
for the past half-century. It appears Negative-Stiffness vibration
isolation will fill that void. CT
Minus K Technology
460 Hindry Ave. Unit C
Inglewood, CA 90301, USA
+1 310 348 9656
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