Actuators typically have large footprints and mass to meet the power output needed for operation, leading to design hurdles for aircraft and space applications.
Researchers at NASA Glenn Research Center pioneered and matured shape memory alloys (SMAs) to enable future aircraft with morphing airfoil surfaces and adaptive wings that fold to respond to different flight condition demands. SMAs are functional metals with unique properties that can go through solid-state phase transformations — stretched, bent, heated, and cooled — and recover their original shape.
The team created two novel mechanical actuators with SMA transmission elements to move parts with rotational angular twisting, delivering the same power as traditional actuators but in a lightweight, smaller-footprint form factor. The new rotary actuator and ring drive actuator move in response to external stimuli such as heating.
The rotary actuator uses nested tubes of SMAs to provide torque output or angular displacement; the ring drive actuator uses SMA tubes in a drive gear element to provide continual rotary output in either clockwise or counterclockwise directions. The SMA tubes are deformed in their martensitic condition and when exposed to a thermal stimulus, the tubes will revert to their original state while providing rotary motion.
One variation of the innovation nests the SMA tubes within a rotary actuator imparting several technical benefits. Nested SMA tubes can decrease the length of the actuator while achieving the same twist angle. For the same actuator length, a nested configuration of SMA tubes can multiply the twist angle and improve the power output.
A second variation utilizes SMA components as transmission elements in a ring drive gear to enable continuous rotation in one direction. Previous similar SMA actuators rotate in one direction while heating and the other while cooling, which can limit the output of the rotary actuator. The innovation developed by NASA allows for continuous rotation in any direction, thereby allowing the rotational output capability to be independent from the amount of cyclic angular twist provided by the SMA tubes.
Despite the smaller footprint, the SMA tubes achieve higher power density and similar power output to larger actuators, enabling new designs based on compact actuators.