Flapping actuation using temperature feedback control of coated shape memory alloy actuators

Abstract

Shape memory alloys (SMAs) have been considered as actuators for micro application due to its high strength to weight ratio. Although it is mostly considered for linear or rotary actuation, in this study a technique using a temperature feedback control system has been proposed to produce oscillation at steady state suitable for flapping wing actuation in micro air vehicles (MAVs). A proportional-integral-derivative (PID) controller was used in the temperature feedback system to tune the frequency of oscillation of the shape memory alloy actuator. This system is compact as the oscillation can be produced without using components to translate from linear to rotary motion. Another challenge to be addressed is the slow cooling rate of the SMA actuator that occurs through natural convection resulting in slower flapping actuation. Thus, a viable approach to improve the actuation response is by coating the SMA to reduce hysteresis. An experimental study was conducted to evaluate the effects of heat convection on the SMA actuator's control performance by using SMA wire coated with silicone rubber. Its actuation was compared to the uncoated SMA wire to establish the effects of heat convection on SMA actuation behaviour. The response of the temperature feedback system of the SMA actuator for the flapping actuation under wind loading was evaluated using wind tunnel testing. The experimental results show that coated SMA wire resulted in increased actuation frequency. However, this effect became negligible as the airflow provided sufficient cooling and resulted in higher flapping frequency at Reynolds number greater than 7.5 × 103. Wind tunnel testing demonstrated that shape memory alloy actuators using a PID based temperature feedback control system may be used to produce flapping actuation at low Reynolds numbers.