Southwest Research Institute (SwRI)
has
built a unique facility for developing and testing
microelectromechanical
systems (MEMS) in vacuum conditions. Recent tests using the system
yielded
significant findings about how MEMS devices work in vacuums, and offer
important information about how MEMS can be used in space applications.
“If MEMS can be made to operate well in vacuum, they hold the promise
of
revolutionizing many space instruments and systems,” says Dr. David J.
McComas, head of the facility and executive director of the SwRI Space
Science and Engineering Division.
Researchers found that MEMS operate in vacuum, the environment found in
space, differently than they operate in atmosphere in two ways: the
voltages
required for resonant operation are much lower and the energetic
amplifications are much larger. The team found during testing that
oscillators needed only a tenth of the voltage normally required in
air.
“This is incredibly significant for space applications because instead
of
hundred volt supplies, which are heavier and more expensive to launch,
we
might be able to run space MEMS on standard low voltages of only 10 to
15
volts,” he says.
Testing also showed that the oscillators had an amplification that was
hundreds of times greater. “If you whack a tuning fork, it has a high
resonance, or amplification, which causes it to ring a long time,” he
continues. “For MEMS that are driven at resonance, this means they will
have
much larger amplification while operating on less power in vacuum.”
Researchers had also worried that “stiction,” a combination of
stickiness
and friction, and vacuum welding, the tendency for metal parts to bond
together in vacuum conditions, could be major factors in space MEMS —
yet
that has not been the case thus far. Water vapor and air act as
lubricants
for MEMS surfaces that slide on or touch each other. In vacuum,
however,
parts that touch lack that layer of gas between the surfaces, leading
to the
possibility that surfaces could exchange atoms and eventually bond.
This
effect most likely led to an antenna on the Galileo spacecraft being
unable
to open.
The cost of launching payloads into space — tens of thousands of
dollars
per pound, depending on the launch vehicle — makes
microelectromechanical
systems highly desirable for space applications. Researchers have used
novel
methods for years to miniaturize electronics, power supplies, and
overall
structures, but the miniaturization of science instruments can be
particularly challenging because many require large aperture sizes to
collect samples. Though possible, miniaturizing many space instruments
overall isn’t practical because the smaller size gives the sensor less
signal, such that it receives fewer particles or photons and can’t
measure
the highly tenuous particle distributions or dim emissions it was
intended
to measure.
MEMS will enable space instruments to have large aperture sizes in a
flat
panel shape that will be much thinner than current sensors, resulting
in
tremendous mass savings. MEMS devices are also highly reliable, and
space
instruments will use arrays of many thousands of identical MEMS. This
redundancy enables an instrument that suffers failure of a small number
of
its devices to continue to operate at nearly full sensitivity.
“With MEMS, the laws of physics are of course the same, but how they
work on
that scale is quite different,” says McComas. “Effects that you’re used
to
seeing in normal life — gravity and inertia — mean very little, while
small electrical forces and the damping of motions in air are
incredibly
important.”
In addition to space applications, MEMS could be vacuum packaged for
Earth-bound applications if the lower voltages or higher amplifications
are
of benefit. Currently, the most widely used MEMS are the accelerometers
used
in automobile airbags, but more and more are being seen in a variety of
sensors, gyros, smart munitions, pacemakers, valve pumps, printers, and
projectors.
McComas says his team is continuing tests in the SwRI facility and
beginning
the development of a space science instrument that uses MEMS.
EDITORS: Photos of the vacuum microprobe system are available at
www.swri.org/press/mems.htm.
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SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 2,700
employees and
an annual research volume of more than $339 million.
