Development of SMA Based Actuator Mechanisms for Deployment of Control Surfaces

The research and development of SMA based actuator mechanisms are being pursued at many places. In this paper the on going research and development efforts at ACD, NAL to realize a SMA based actuator mechanism for deployment of a control surface model is discussed. The mechanism can be modified to either give a rotary or a linear output. The deployment of this control surface model was successfully demonstrated using the SMA based rotary actuator mechanism along with computerised powering devices and controls. This control surface model is deployed only during landing in order to improve the landing characteristics of the aircraft and remains retracted at all other times. The real challenge is the design and development of the SMA based mechanism, which will satisfy the weight and volume budget and more importantly consume minimum power. An innovative 3-gear relay arrangement was chosen primarily to save the power. In this type of arrangement 3 different input gears mounted on 3 independent input shafts are used to drive the same output shaft on which the control surface model is mounted. Independent banks of SMA wires drive each of the 3 input shafts. The actuation of the input shafts is done in a sequential manner and each them rotates the control surface model by different angles. At any given time only one bank of SMA is energized and therefore the power consumption reduces significantly in contrast to a case where the complete rotation of the control surface model has to be effected by a single input shaft power by a single bank of SMA. The state of art electronics and controls in order to achieve the above sequential actuation include miniaturized power devices, data acquisition, and controller of SMA actuation and health monitoring of SMA. The actuators have both hysterisis and non-linearity especially during the phase transformation when the electrical resistance changes abruptly. SMAs can be actuated by heating using external heaters or by resistive heating. For better control, high efficiency, compactness and silent operation, resistive heating is preferred. Individual miniaturized power devices (DC-DC converters) are developed to electrically isolate the SMA actuators. Adequate redundancy in terms of both mechanical actuators and powering devices is built in