The use case which underlies this work is a locking system based on electrically activated antagonistic NiTi-wires. Since this actuator system is designed to operate in aircraft interiors, high demands are placed on the system. Especially in the ambient temperature range, which is especially critical for NiTi-SMA based systems, a simple time-based control reaches its limits. If the system is undercooled due to low ambient temperatures, the provided activation energy is not sufficient for complete activation of the actuator wires. However, at higher ambient temperatures, the same amount of energy will cause the system to overheat, with negative effects on actuator performance and lifetime.

As external temperature monitoring is too complex, too expensive and, given the lightweight construction requirements in aircraft construction, too heavy, the change in the internal resistance of SMA is used to optimize actuator activation. The electrical resistence is dependent on the applied force, the ambient temperature and the wear of the wire. For this purpose, the applied current is measured and analyzed during activation at constant voltage. The changes in material resistance due to wear are based on changes in the microstructural state, elongation, mechanical stress and structural damage to the wire. They change over each activation cycle along phase transformation as well as over the entire service life of the actuator wire. This allows conclusions about the activation status, possible damage to the wire, occurring system faults (e.g. blockage) or the remaining lifetime. Since the ambient temperature has a direct influence on actuator activation, it can be monitored by changes in material resistance. Therefore, a resistance-based control of activation can compensate for the effects of ambient temperature on the actuator system.

The locking system uses a snapping mechanism. If the activation energy is not sufficient at low ambient temperatures, this mechanism cannot be overcome and excessive mechanical strain is placed on the wire. The same effect has a miscellaneous blocking of the locking actuator. Accordingly, both actuator states show the same characteristic resistance curves. In the undercooling state, the supplied activation energy must be increased to reach the necessary phase transformation temperature. If the activation energy would be increased in the case of a blockage of the system, this would lead to a mechanical overload of the wire and thus to serious damage to the actuator. Accordingly, constant temperature and condition monitoring is crucial in order to correctly distinguish the various actuator states and react appropriately.

In this paper, the authors use machine learning tools in order to classify electric resistance values. The classes contain temperature steps of 5°C between -20°C and 70°C. By this method a temperature difference of 5°C can be detected for a specific actuator setup without knowing the cycle count. Depending on the ambient temperature it is now possible to increase or decrease the actuation parameters to maintain actuator functionality. In long terms this is a base to optimize overall actuator performance and lifetime. This work is intended to ensure safe and low-wear operation of the SMA locking system. In addition, new potential applications for SMA-based actuator systems can be exploited.


Resistance-Based Temperature Monitoring Using Machine Learning and Adapted Activation for an SMA Locking System in Aircraft Interiors

Division / Sponsors: 
Aerospace Division

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

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