Modeling and experimental characterization of the stress, strain, and resistance of shape memory alloy actuator wires with controlled power input

Recently, a novel power controller has been presented that simultaneously controls electric power input and measures resistance of a commercially available Flexinol shape memory alloy wire. This work exploits the new power controller by plotting shape memory alloy stress, strain, and resistance versus Joule heating power instead of input voltage or current. Heating power is directly related to shape memory alloy temperature, whereas standard constant voltage or constant current inputs cause more or less heating as the resistance of the wire changes due to phase transformation. A simple experimental setup consisting of a 50-µm-diameter Flexinol wire mounted in series with the tip of a compliant cantilever beam is used to systematically study the shape memory alloy behavior. Actuator performance is reported for a range of prestress values and actuation frequencies. All the experimental data are compared with simulated behavior that is derived from a free energy–based numerical model for shape memory alloy material. Additionally, a new resistance model based on the simulated wire temperature and phase fractions is introduced and compared to experimental data. Exploiting a multifunctional power controller and an improved understanding of the resistance change during phase transformation will help enable the use of shape memory alloy wires in simultaneous sensing and actuating applications.