161 Single Motion Actuated Shape Memory Alloy Coupling 

A. O. Perez*, J. H. Newman** and M. Romano* 

Abstract 

The objective of the single-motion-actuated-shape-memory-alloy coupling (SMA2C) is to produce a small,  single motion actuator used to secure and then release the solar panels on a CubeSat, or other very small satellite, upon command. The SMA 2C consists of a nickel titanium (NiTi) cylindrical shape memory  alloy (SMA) press-fit into a stainless steel bushing, surrounded by a Kapton foil heater, a spring, a holding bolt, and a polyethermide isolation washer all in a structural housing. Heating the SMA above its activation temperature causes it to contract, permitting the spring to push the SMA, and therefore the holding bolt, out of the stainless steel bushing. This releases whatever the holding bolt is attached to from the CubeSat structure.   Introduction 

In the past, CubeSats, very small satellites of about one to four kilograms, have used various ways to secure their solar panels while in their CubeSat launchers. Some have been simply held closed by the walls of the launcher itself. The CubeSat’s solar panels would slide along the walls of the launcher and, when fully released, the solar panels would deploy. Sometimes the solar panel is secured by tying fishing wire around the solar panel and having a mechanism burn the wire for a controlled release. When this method is successful, it is quite simple and effective, but there have been premature releases occurring during vibration tests. In addition, there is a limit to the amount of force that the fishing wire can withstand, restricting its application to other types of deployments. SMA 2C is a release mechanism that addresses  both control of deployment and capacity of holding force, which can be scaled for many applications.   SMA 2C Background 

NiTi is a shape memory alloy with particular properties for our application. Shape memory alloys are a class of materials that exhibit a mechanical change in property with a non-mechanical input such as a temperature change. SMAs undergo three types of transformations; Austenite, Twinned Martensite, and Detwined Martensite. In its natural state, SMA begins in its memory state also known as the Austenite phase. Once cooled it enters into the Twinned Martensite phase in which the crystal structures realign and produce a minimal change in overall expansion. If stress is not introduced during the Twinned Martensite phase and heated back up, the SMA’s crystal structure returns back to its Austenite phase and  a minimal amount of overall contraction occurs. On the other hand, when stress ( ) is added in the cooled  Twinned Martensite phase, an increase in strain ( expansion) of the Shape memory alloy brings it to  the Detwinned Martensite phase wherein its crystal structure has been further modified. In order to go  from the Detwinned Martensite phase back to Austenite phase, all that’s needed is an increase in temperature. This will cause the crystal structure to return to its original memory state (contraction). The amount of overall expansion is closely related to the amount of stress added during the Twinned Martensite phase. Figure 1 is a visual representation of this process.                                                              * Mechanical and Astronautical Engineering Department, Naval Postgraduate School, Monterey, CA  ** Space Systems Academic Group, Naval Postgraduate School, Monterey, CA 

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012