Pilots experience spatial disorientation when they have an erroneous sense of their position and motion relative to the Earth's surface. It costs the Department of Defense hundreds of millions of dollars each year in aircraft crashes and, prior to the war on terrorism, spatial disorientation accounted for one fatal crash every eleven days. Captain Angus Rupert, a NASA flight surgeon at the Naval Aerospace Medical Research Laboratory (NAMRL), is about to make the statistics on this aerospace problem do a nosedive using flight vests with small, fast-acting pneumatic valves that help pilots orient themselves and their aircraft.
Tactors in the TSAS vest stimulate in accordance with changes in the aircraft's orientation. For example, if the aircraft dives, the pilot feels pressure in the lower abdominal region.
The vest uses the human body's orientation senses, which are made up of vision, the vestibular system of fluid-filled channels in the inner ear, and proprioceptors—nerves in muscles, skin, and other tissues. Unlike vision, which becomes ineffective at night and during some bad weather conditions, and the vestibular system, which may transmit false sensations of motion to the brain after 10 to 20 seconds of constant angular acceleration, proprioceptors provide tactile feedback.
"Your visual and vestibular systems can give you false information when flying," says Rupert. "But with a tactile indication, you can rely on your sense of touch, your muscles, your joints—and the seat of your pants—to orient you and tell you where down is."
Providing tactile feedback to pilots is the idea behind a new flight vest, The Tactile Situation Awareness System (TSAS) developed by Rupert. It helps helicopter pilots hold a stationary hover and fixed-winged aircraft pilots maintain horizontal flight—all without total dependence on the vestibular system, the instrument panel, and other visual inputs.
Rupert explains that if a pilot banks a fixed-wing aircraft to the right, the pilot's sense of down is perpendicular to the wings. "Down is thought to be in the direction of your seat, even if the seat is upside-down," says Rupert.
A cable from the valve-driver circuit board plugs into the avionics bus of the aircraft to pick up inertial signals from the aircraft's systems. Using attitude and altitude information from the aircraft's inertial reference systems, the TSAS flight vest actuates specific combinations of tactors to stimulate the skin and indicate the direction of deviation from a straight and level path. "Determining range is also possible using frequency or amplitude," adds Rupert.
Each tactor has a rubber diaphragm stretched over a small ring that is inflated by small pneumatic valves. The diaphragms' displacement imparts small impact forces upon the skin, which provides the tactile stimulation.
The vest uses dozens of small pneumatic X-Valves from Pneutronics (Hollis, NH) to actuate each of the nickel-sized tactors. Up to 96 tactors are included in a single vest.
The X-Valves operate between 80 and 200 Hz and their response times are in the 1-10 ms range. Running at these frequencies translates into higher tactile simulation. "Humans are more sensitive to stimulus at certain frequencies," explains Paul Browne, a mechanical engineering consultant to NAMRL who designed the pneumatically powered tactor used in the flight vest. "This valve gets us to an optimal frequency that feels stronger."
Each of the custom-made X-Valves has a 12V coil inside. "Normally, this size coil would overheat in an application like ours, but our duty cycle is very low, so overheating wasn't an issue for us," adds Browne.
Each valve measures approximately 8 × 9 × 23 mm and weighs about 5 grams. In this 40-psi application, each X-Valve operates at more than six times the 6-psi rated pressure.
"Large airflow rates are crucial in developing pressure waves for increased impulse strength," says Bob Irwin, a project engineer at Pneutronics. Browne agrees that ample flow from the X-Valve's large orifice is important. "We need a rapid pressure rise," says Browne. "The last thing we wanted was for a small orifice to restrict flow in the tactor."
Future applications for the TSAS will improve the ability to spatially track targets, helping fighter pilots differentiate between friendly and enemy troops while increasing the efficiency and precision with which the military carries out missions. In the future, portable units may provide ground forces threat and target information. For now, Rupert is keeping details on these applications close to the vest, so to speak.
For more information about pneumatic valves from Pneutronics: Enter 541