When technicians position solar arrays on orbiting satellites, they don’t want to worry about electrical noise and intermittent signals from the positioning equipment.
That’s why engineers at Moog Space and Defense Group, East Aurora, N. Y., developed a noise-free potentiometer to gauge the position of actuators on satellites. Called QuieSense, the device provides essentially noise-free feedback on the position of solar array drives and antenna-pointing mechanisms.
Traditional potentiometers usually employ a conductive wiper element that slides over resistive material, thus providing variable resistance. Over time, the wiper contact generates particle debris that prevents the wiper from making solid contact with the resistive material. The result is electrical noise and signal drop-out.
Moog says it has solved this problem by engineering its potentiometer with membrane technology, not unlike the kind used with iPod controls. The wiper touches a Kapton flex material patterned with a copper trace on its bottom surface. The trace, in turn, touches the resistive material where the wiper pushes down. This construction eliminates the sliding resistive contact that causes problems in ordinary potentiometers. Moog also had to qualify the device’s materials to meet spaceflight requirements. For example, the new potentiometer operates in a vacuum and is expected to handle more than 100,000 revolutions without creating signal noise.
Moog settled on this design after studying numerous position encoding technologies, including capacitive and optical sensors, inductive sensors such as resolvers and Inductosyns, and Hall-effect devices. Moog concluded all these alternatives would force the satellite to carry more onboard electronics, an expensive proposition, says Moog. Most satellite makers rely on heritage electronic systems that would require redesign and requalification.
Some membrane potentiometers Moog tested also used nonmetallic parts prone to permanent set when left at a given position for several months. In satellites, it is not uncommon for actuators to sit for years without operating. So any type of permanent set is a showstopper. Moog eventually used technology based on Hoffman & Krippner’s Sensofoil Hybrid, which is vacuum compatible and operates over a wide temperature range. So the wiper rides on an upper membrane protected by a highly flexible metal bond. This keeps dents from forming during inactive periods.
The commercial version of the potentiometer used a polyoxymethylene wiper, a material with limited high-temperature performance. (Moog engineers determined the potentiometer would have to operate at temperatures as high as 105°C.) So a Vespel polyimide wiper went into the spaceflight version.
Engineers concluded that as long as resistance remained below 300 Ω, the potentiometer would not have drop-out noise. Even after 800,000 cycle tests, Moog found that contact resistance remained below 163 Ω.
Standard Sensofoil potentiometers can hit an absolute linearity of around 1%. Moog’s QuieSense noisefree potentiometers are trimmed to produce linearities of better than 0.25%.
QuieSense devices could find use providing position feedback in a variety of spaceflight applications such as thrust-vector control systems, which direct a rocket’s position. A commercial satellite manufacturer has already opted to use the new potentiometer and actuators on a communications satellite scheduled for launch in early 2014. Moog is also developing QuieSense potentiometers sized to let satellite makers replace traditional potentiometers used on various spaceflight actuators.