An improved shape memory alloy actuator for rotor blade tracking

The design, analysis, and testing of an improved Shape Memory Alloy (SMA)-based tracking tab actuator is described in this paper. The goal of the actuator is to provide in-flight tracking capability for a helicopter rotor in order to minimize 1/rev vibrations due to rotor dissimilarities. Previous SMA-based actuator designs demonstrated the potential for in-flight rotor tracking but admitted drawbacks that led to inconsistent operation under air-loads. The current research builds upon the existing knowledge base and addresses the challenges encountered in previous designs. The objective is to achieve a deflection of +/-5degrees with an accuracy of 0.1degrees under realistic aerodynamic loading conditions. The present actuation concept is based on the bidirectional motion of a pair of antagonistic SMA wires, with a passive friction brake to lock the tab position. A theoretical model of the actuator was developed based on Brinson's thermomechanical model. The model was used to predict the behavior of the actuator under external loading and applied as a design tool to identify optimal actuator parameters. The actuator was integrated into a NACA 0012 12 in. chord blade section and tested in an open-jet wind tunnel at speeds of up to 120 ft/s (0.107 M) and at angles of attack up to 15degrees. Closed-loop tracking was implemented using a PID controller with gains selected by Ziegler-Nichols tuning. The improved SMA actuator meets the project goals by achieving repeatable,tab deflection of up to +/-5degrees with an average accuracy of 0.05degrees. Position hold under power-off conditions and a duty cycle of 20 cycles/h were also demonstrated.