Adaptive Control of Shape Memory Alloy Actuators for Underwater Biomimetic Applications

In actuator technology active or smart materials have opened up new horizons in terms of actuation simplicity, compactness, and miniaturization potential. One such material is the nickel-titanium shape memory alloy (NiTi SMA), which is gaining widespread use in a variety of applications. The numerous advantages of SMA over traditional actuators are of particular interest in the area of underwater vehicle design, particularly the development of highly maneuverable vehicles of a design based on the swimming techniques and anatomic structure of e sh. An SMA actuation cycle consists of heating/cooling half-cycles, currently imposing a limit on the frequency of actuation to well below 1 Hz in air because of slow cooling. The aquatic environment of underwater vehicles lends itself to cooling schemes that use the excellent heat-transfer properties of water, thus enabling much higher actuation frequencies. A controller for SMA actuators must account not only for large hysteretic nonlinearities betweenactuatoroutput (strainordisplacement )andinput(temperature ),butalso thethermalcontrolforresistive heating via an applied current. The control of SMA in water presents a problem not encountered when actuating in air: accurate temperature feedback for the SMA is very dife cult in water. We overcome this problem by using a simplie ed thermal model to estimate the temperatureof the wire in conjunction with an adaptivehysteresismodel, which relates the actuator output to the estimated temperature. Experimental results are provided, showing that this method for control of an SMA wire works equally well both in air and in water, with only rough estimates (easily obtained )ofthethermal parameters.Successful tracking of referencedisplacementsignals with frequencies up to 2 Hz and relatively large amplitudes have been demonstrated experimentally. I. Introduction I N aerodynamicsand hydrodynamics birds and e sh have inspired and guided the development of aircraft and underwater vehicles. These manmademachinesseemsoprimitive compared to their natural counterparts in terms of intelligence, efe ciency, agility, adaptability, and functionalcomplexity. These and other similar observationsandissuesthathavebeenaddressedbythescientie ccommunity havetriggered theformulation of thescience ofbiomimetics and have inspired new approaches to old problems. In the area of underwater vehicle design, the development of highly maneuverable vehicles is presently of interest, with their design being based on the swimming techniques and anatomic structure of e sh; primarily the undulatory body motions, the highly controllable e ns, and the large aspect ratio lunate tail. The tailoring and implementation of the accumulated knowledge into biomimetic vehicles is a task of multidisciplinary nature with two of the dominant e elds being