Experimental Characterization of Actuators for Micro Air Vehicles

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Mini-UAVs and RC slow flyers require compact, lightweight and responsive actuators. Shape memory alloy wires can be used to design ultra-light micro-servos. This technology relies on the reversible change in crystalline structure that a SMA wire undergoes when electricity runs through it. The resulting contraction is used to deflect the aircraft control surfaces. This paper introduces SMA wires technology and its application to the design of a small-size and light-weight actuator for elevon type controls. A conventional servo is taken as a reference to compare static and dynamic performance of the realized wire configuration prototype. A wind tunnel experiment is set up to test the behavior at different airspeeds and the servos response to variable frequency input is recorded. Extensive data analysis is performed to estimate the system models and to predict their bandwidth. In particular, Prediction-Error Minimization method is applied and Akaike's Final Prediction-Error is used to evaluate the model fitting accuracy. Results show that the SMA servo, despite its excellent general characteristics (i.e. small size, light weight, high power to weight ratio, silent operation, long life) seems to be only partially suitable for small scale flying vehicles due to its low bandwidth. By contrast, the conventional low cost servo provides a faster response in terms of torque output but fails to be accurate and repeatable under dynamic load conditions.