Self-Sensing Actuation and Control with Shape Memory Alloys

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Cate Brinson, Michael Peshkin, Bruce Wilson Northwestern University Shape memory alloys (SMAs) have been used in a variety of actuation, energy-absorbing, and sensing applications. The key feature of this material is its ability to undergo large seemingly-plastic strains and subsequently recover these strains when a load is removed or the material is heated. This unique ability occurs due to a reversible thermoelastic phase transformation between austenite and martensite. The key feature allows SMAs to serve as very compact actuators. As SMAs can be used for both material-stiffening and energy-absorption, they have generated much interest in the smart structures field. Further, no other material or device can generate significant tensile forces over a large displacement while occupying such a small volume. A second useful feature of many SMAs is a change in resistivity with a change in strain. The change in resistivity as an SMA undergoes strain has enabled investigators to use them as coarse position sensors. The coarseness is due to a complex relationship between resistivity and the material state and its coupling with mechanical load and temperature. Given the material characteristics above, economical, power-dense self-sensing-actuation (SSA) can be achieved with shape memory alloys. However, the nonlinear nature of SMA actuation and sensing, incomplete understanding of SMA thermomechanical response and the lack of suitable models for control result in an under-utilization of this useful material as an actuator, sensor, and SSA. In this research, a focused effort will be targeted at (1) improving the characterization of SMAs for a sensing and control context, (2) refining material modeling, (3) developing model-based control algorithms, and (4) demonstrating these in hardware to advance the understanding and range of applications of SMAs as actuators, sensors, and SSAs.