Multiplexing Embedded Nitinol Actuators to Obtain Increased Bandwidth in Structural Control

A wide variety of applications of shape memory alloys is seen in devices where the response required of them is essentially quasi-static. At the same time, the high damping intrinsic to the alloys known as "quiet metals," such as Nitinol, have led to their use as passive distributed dampers. However, it is generally felt that the frequency response of SMA actuators is too slow to permit their use for structural control where they need to supply a time-dependent force at the natural frequency of a structure. In many applications the response time of SMA actuators is ultimately limited by heat transfer. Typically, very rapid heating can be achieved but the time needed for cooling is long compared to the vibratory period of typical mechanical or civil structures. A unique approach has been found suitable to overcome this inherent limitation. The approach is based on using several Nitinol wires as actuators in parallel and energizing subsets of these during successive cycles of structural motion, effectively trading reduced control authority for increased frequency response. Thus, with an array of actuators an effective bandwidth can be achieved that is demonstrated to be greater than the bandwidth possible with a single actuator. This technique of multiplexing several actuators was established in principle by the senior author of this paper in 1990. Most recently, upon completion of the effort reported here, the authors learned that Wilson et al. (1990) had employed a similar approach to obtain a higher frequency bandwidth. The approach has now been extended to demonstrate its validity by applying it to a robust box beam made of steel. Furthermore, successful performance of the multiplexing scheme has led to the analysis, design, fabrication and testing of a composite beam in which NiTiNOL fibers were embedded. A series of vibration tests were conducted on the composite beam along with temperature measurements using an infrared camera. With the multiplexing approach, the first two modes of the composite beam, at 23.5 Hz and 144 Hz respectively, were excited. This unique approach can now be developed further to design structural systems of interest to industry and build smart composite structures. This serves the eventual goal of controlling vibration at frequencies higher than was thought possible with this material.