Shape-Memory Alloy Actuators for Small Satellites
Session
Pre-Workshop Session 2: Communication
Abstract
A frequently‐used hold‐and‐release mechanism for spacecraft deployables is the “meltwire” or “burnwire”. Here an electrically resistive heating element melts through a fusible restraint to free some sliding, pivoting, or flexing element such as a hinged solar array, an instrument boom, or an antenna. The heating element is often a length of nichrome wire, but discrete electrical resistors, both surfacemount and through‐hole have also been used. Meltwire release mechanisms present a unique challenge for testing in that they cannot be reused; once melted, the fusible restraint must be replaced. Since this is often a laborious and time‐consuming task, the testing regime is often limited, particularly for small, low‐budget spacecraft. In addition, since the fusible restraint must be replaced after each use, the component that ultimately flies will be one that has not, itself, been tested. We have found that simple mechanisms actuated by Shape Memory Alloy (SMA) devices are an effective alternative to meltwires for small spacecraft applications. In contrast to meltwires, these mechanisms are easily resettable, enabling a low‐cost but robust test campaign to ensure operational reliability. In each case the heart of the device is a length of nitinol wire, nitinol being a nickel titanium alloy that can be configured to contract 3 to 4% on heating. Heating is accomplished by passing an electrical current through the wire. The resulting contraction exerts a force that can be used to move a structure attached to the wire, so implementation of SMA‐actuated release mechanisms revolves around design of mechanisms that can be operated through a simple pull force. When the nitinol returns to its original temperature, it can be stretched mechanically to its original dimension with essentially no degradation of its properties (provided it has not been overheated), allowing the mechanism to be reset and ready for the next actuation. Many of the features of SMA wire actuators are particularly suited (but not limited to) to the kinds of one‐time‐use mechanisms found on satellites. They are light, simple, and easily tested and reset. They present no magnetic signature when not energized. They can also be built into more complex devices that have multiple discrete positions and can even be used to produce or control a motor. In addition, they are clean; they can often be designed without greases or other lubricants that could pose a contamination risk, and they produce no debris when actuated. While there are many reasons to consider SMA wire actuators, prudence demands an understanding of their limitations, particularly with respect to actuation speed, electrical efficiency, longevity, and force limitations. Another key consideration is that the performance of SMA actuators can be severely degraded by overheating. As such, the thermal design must take into account operations in both air and vacuum, and particular care must be taken to ensure that vacuum test procedures do not result in unacceptable thermal stresses. SMA actuators have flown and operated successfully on six spacecraft in the AeroCube series, and these actuators are designed into a number of upcoming missions. This paper will review design, assembly, and testing techniques and present several case studies from recent AeroCube flights.
Presentation
Shape-Memory Alloy Actuators for Small Satellites
A frequently‐used hold‐and‐release mechanism for spacecraft deployables is the “meltwire” or “burnwire”. Here an electrically resistive heating element melts through a fusible restraint to free some sliding, pivoting, or flexing element such as a hinged solar array, an instrument boom, or an antenna. The heating element is often a length of nichrome wire, but discrete electrical resistors, both surfacemount and through‐hole have also been used. Meltwire release mechanisms present a unique challenge for testing in that they cannot be reused; once melted, the fusible restraint must be replaced. Since this is often a laborious and time‐consuming task, the testing regime is often limited, particularly for small, low‐budget spacecraft. In addition, since the fusible restraint must be replaced after each use, the component that ultimately flies will be one that has not, itself, been tested. We have found that simple mechanisms actuated by Shape Memory Alloy (SMA) devices are an effective alternative to meltwires for small spacecraft applications. In contrast to meltwires, these mechanisms are easily resettable, enabling a low‐cost but robust test campaign to ensure operational reliability. In each case the heart of the device is a length of nitinol wire, nitinol being a nickel titanium alloy that can be configured to contract 3 to 4% on heating. Heating is accomplished by passing an electrical current through the wire. The resulting contraction exerts a force that can be used to move a structure attached to the wire, so implementation of SMA‐actuated release mechanisms revolves around design of mechanisms that can be operated through a simple pull force. When the nitinol returns to its original temperature, it can be stretched mechanically to its original dimension with essentially no degradation of its properties (provided it has not been overheated), allowing the mechanism to be reset and ready for the next actuation. Many of the features of SMA wire actuators are particularly suited (but not limited to) to the kinds of one‐time‐use mechanisms found on satellites. They are light, simple, and easily tested and reset. They present no magnetic signature when not energized. They can also be built into more complex devices that have multiple discrete positions and can even be used to produce or control a motor. In addition, they are clean; they can often be designed without greases or other lubricants that could pose a contamination risk, and they produce no debris when actuated. While there are many reasons to consider SMA wire actuators, prudence demands an understanding of their limitations, particularly with respect to actuation speed, electrical efficiency, longevity, and force limitations. Another key consideration is that the performance of SMA actuators can be severely degraded by overheating. As such, the thermal design must take into account operations in both air and vacuum, and particular care must be taken to ensure that vacuum test procedures do not result in unacceptable thermal stresses. SMA actuators have flown and operated successfully on six spacecraft in the AeroCube series, and these actuators are designed into a number of upcoming missions. This paper will review design, assembly, and testing techniques and present several case studies from recent AeroCube flights.
