The figure shows a prototype of a magnetostrictively actuated mechanism that would rotate an optical filter wheel for a high-performance infrared camera or telescope. Typically, a high-performance infrared instrument and its filter wheel are operated inside a cryogenic system. In conventional practice, the filter wheel is turned by use of a stepping motor in a warmer location. The mechanical connection between the stepping motor and the filter wheel is designed to be as thermally isolating as possible, but it still leaks appreciable heat into the cryogenic system. In contrast, the magnetostrictive drive can be operated with minimal heat leakage because it can be mounted inside the cryogenic system along with the filter wheel and infrared instrument. Moreover, in comparison with a stepping-motor drive, the magnetostrictive drive is simpler, less expensive, and more reliable.
The magnetostrictive drive is an inertial-reaction motor; that is, a motor that exploits a combination of stick/slip and inertial effects. A linear actuator in frictional contact with an armature (in this case, the wheel) is excited by a sawtooth signal, causing one end of the actuator to repeatedly accelerate slowly in one direction, then rapidly in the reverse direction back to the starting point. Because of the frictional contact, the armature moves with the end of the actuator during the initial slow acceleration. However, the force of the reverse acceleration is greater than the frictional force between the actuator and the armature, so that the armature does not snap back along with the actuator. The cycle then repeats, producing another increment of armature motion (in this case, an increment of rotation of the wheel). By reversing the polarity of the sawtooth waveform, one can reverse the direction of motion.
In this case, the linear actuator is a magnetostrictive rod connected to a friction pawl that makes contact with the wheel. When no power is applied to the actuator, friction holds the wheel in position. The magnetostrictive material is a terbium/dysprosium alloy, which exhibits a large magnetostriction at cryogenic temperatures. To minimize the heat load of the cryogenic system, the magnetostrictive rod can be driven by use of a superconducting solenoid. For an operating temperature of about 77 K (liquid-nitrogen temperature), one would have to use a solenoid made of a high-temperature superconductor.
In a test at a temperature of 77 K, the magnetostrictive drive was found to produce rotation with angular increments as small as an arc second and with a slew rate as high as 1.5 r/min. Inasmuch as the magnetostrictive rod generates a longer stroke at lower temperature, it should be possible to achieve a greater slew rate at the typical liquid-helium temperature of 4 K.
The Magnetostrictive Filter-Wheel Drive is a robust, reliable inertial-reaction motor that contains only a few moving parts. When operated in a cryogenic system, it contributes minimally to the heat load.
This work was done by Robert Chave and Christian Lindensmith of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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