MULTIPLEXING EMBEDDED NITINOL ACTUATORS TO OBTAIN INCREASED BANDWIDTHIN STRUCTURAL CONTROL

A wide variety of applications of shape memory alloys is seen in devic es where the response required of them is essentially quasi-static. At the same time, the high damping intrinsic to the alloys known as ''qu iet metals,'' such as NiTiNOL, have led to their use as passive distri buted dampers. However, it is generally felt that the frequency respon se of SMA actuators is too slow to permit their use for structural con trol where they need to supply a time-dependent force at the natural f requency of a structure. In many applications the response time of SMA actuators is ultimately limited by heat transfer. Typically, very rap id heating can be achieved but the time needed for cooling is long com pared to the vibratory period of typical mechanical or civil structure s. A unique approach has been found suitable to overcome this inherent limitation. The approach is based on using several NiTiNOL wires as a ctuators in parallel and energizing subsets of these during successive cycles of structural motion, effectively trading reduced control auth ority for increased frequency response. Thus, with an array of actuato rs an effective bandwidth can be achieved that is demonstrated to be g reater than the bandwidth possible with a single actuator. This techni que of multiplexing several actuators was established in principle by the senior author of this paper in 1990. Most recently. upon completio n of the effort reported here, the authors learned that Wilson ct al. (1990) had employed a similar approach to obtain a higher frequency ba ndwidth. The approach has now been extended to demonstrate its validit y by applying it to a robust box beam made of steel. Furthermore, succ essful 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 th e composite beam along with temperature measurements using an infrared camera. With the multiplexing approach, the first two modes of the co mposite beam, at 23.5 Hz and 144 Hz respectively, were excited. This u nique approach can now be developed further to design structural syste ms of interest to industry and build smart composite structures. This serves the eventual goal of controlling vibration at frequencies highe r than was thought possible with this material.