Experimental validation of different methods for controlling a flexible nozzle using embedded SMA wires as both positioning actuator and sensor

One of the main selling points of smart materials is the potential to exploit their multi-functional capabilities. For example, a shape memory alloy (SMA) wire can be used as a positioning actuator by heating the wire to induce contraction and as a positioning sensor by measuring the resistance across the length of the wire. While SMA's have found application in many on-off type applications, their ability to 'sense' their own change in length has not been fully exploited. This is because when coupled with a compliant structure, SMA wires exhibit non-linear, hysteretic behavior that depends not only on the phase transformation within the material, but also the thermal and force interactions between the wires and structure itself. If the resistance across an SMA can be reliably mapped to wire strain, a closed-loop controller can easily vary the length of the wire by changing the amount of electrical power put into the wire that causes Joule heating. This paper analyzes the fidelity of different mapping schemes when employed in a closed-loop controller. The schemes are tested in the context of a dual-joint flexible nozzle that is designed to control both the release position and trajectory of an emitted fluid flow. The mapping methods include consideration of the force coupling that results from opposing SMA actuators. The challenges of practical implementation issues are discussed alongside the results to develop the best mappingcontrol scheme for this application. Results show that simple linear-mapping solutions offer 2D nozzle tip position tracking with errors of 2 mm over a range of 12 mm on both axes, with minimal investment of calibration time, while more involved solutions that include force coupling and account for hysteresis can bring positioning errors to less than 500 um.