Cambridge, MA--If you've spent much time in a helicopter, you know what a noisy, bone-rattling experience it can be. Now SatCon Technology Corp. claims to have developed a simple, cost-effective solution. The company's Reaction Mass Actuator (RMA) senses and suppresses noise and vibration to make a helicopter's passengers and pilot more comfortable.
Designed by SatCon as part of a Small Business Innovative Research contract awarded by the Army Aviation Applied Technology Directorate, the RMA is mounted on the rotor transmission housing of a helicopter. At the heart of the RMA is a cylinder of Terfenol-D, a proprietary version of a crystalline alloy of terbium, iron, and dysprosium originally developed by the former Naval Ordinance Laboratory. A magnetostrictive material, Terfenol exhibits a high force/volume ratio when subjected to a magnetic field.
"Compared with other materials used for this purpose--piezoelectrics and electrostrictives, for example-- Terfenol-D has a very large strain capability. In fact, its strain capability is three to four times that of piezos," says SatCon senior engineer Shankar Jagannathan. The RMA also requires a smaller on-board power supply than piezos--the prototype consumed just 3.3A peak at 33V zero-to-peak--thus cutting overall system weight by 90%. Other advantages include an operating temperature range of -50 to 150C and the virtual absence of any known fatigue mechanism.
SatCon has supplied RMA units to several helicopter manufacturers for evaluation. When designed for use on a helicopter transmission, an RMA measures 1.7 inches in diameter and 2.9 inches in height, and weighs 1.3 lbs. Each unit consists of a cylinder of Terfenol-D approximately 1/8 inch in diameter, concentric with a coil, a permanent magnet, and an outer steel casing that forms a reaction mass.
During operation, a 0.5-lb control electronics package senses fuselage vi-bration frequency and transmits a drive voltage to the RMA's coil. The magnetic field induced by the coil causes the Terfenol-D to rapidly expand and relax at the frequency of the drive signal. SatCon won't specify the RMA's operating frequency range, but notes that other, non-Terfenol devices generally resonate at 500 to 2,000 Hz. The regulated movement of the reaction mass against the rotor housing partially cancels vibration produced by the transmission. SatCon claims a noise reduction as great as 40 dB can be achieved by using the RMA.
At high frequencies, inductance dominates coil impedance. Producing forces required by the system would exceed the capacity of a 28 V dc aircraft bus if not for two design approaches used by SatCon. First, the actuator is designed to exhibit a mechanical resonance at the required frequency band. Second, designers increase the diameter of the coil wire and reduce the number of turns to reduce coil inductance. The RMA employs both these techniques, and is capable of producing an output of plus or minus 210 lbf.
Because the RMA does not operate in the aircraft's load path, Jagannathan says that SatCon expects a relatively short certification process. Tunable and scalable, the device may prove useful to engineers trying to reduce noise and vibration in other types of equipment.
Additional details...Contact John Berry, Manager, Energy Systems Integrated Product Team, SatCon Technology Corp., 161 First St., Cambridge, MA 02142, (617) 661-0540.
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