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.
Southwest Research Institute
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