The shape memory effect is already thoroughly explored for many years and a technical adjustment to the respective use case is just a matter of rigor. Although the advantages of shape memory alloys (SMA), like a low weight, a high force to volume ratio, and eventually a low price, are perfectly apparent for all kinds of technical applications, an industrial breakthrough in astronautics did not happen to the current day.

Typical for SMA actuators in general are a linear or even rotatory output and a short travel range. Drives like that are commonly used in locking mechanisms, where a large force is necessary, and a short motion is sufficient. One of the most usual geometries of SMA elements is a wire that contracts under the influence of heat and moves an output component. The elongation back to its initial position is typically realized by a reset spring that is either pushed or pulled. Even though a setup like that is rather simple and not error-prone, it is a big challenge to establish a high-quality serial production for larger quantities. For a commercial product it is important that the properties as well as the behavior of the SMA is always identical. Beside a stable and reproducible manufacturing line, the shape memory technology offers the possibility to exploit the sensorial effect during assembly. During activation of the wire, the phase change from martensite to austenite causes a drop in electrical resistance. If this characteristic parameter is monitored, it is possible to deduce the status of the phase transformation and quantitative shortening of the wire, respectively. Since the graph is shifting after many activation cycles, further aspects about the condition of the SMA element can be recognized, like e.g. the lengthening of the wire or the structural fatigue of the material. Thus, it is expedient to link commonly used structural health monitoring (SHM) techniques with the actuator design.

In this paper the mentioned concepts are demonstrated using the example of an intelligent redundant spacecraft actuator. It was developed to provide a multifunctional drive for cubesats to either serve as a hold-down and release mechanism, a pointing system for antennas or solar arrays, or an adjustment of optical payloads. Right after assembly, the performance during the first activations can be surveilled and thereby an instant quality control is conducted by the evaluation of the electrical resistance.  Consequently, it is possible to assess the smoothness of the mechanical components and ensure frictionless translation or rotation without further investigations on the mounted components. Also, it is proved that the status of material fatigue can be recognized as part of a sophisticated data analysis.


Data Analytics Supported Quality Control of Serial-Produced SMA-Actuators for Space Applications

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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