Adaptive control of shape memory alloy actuators for underwater biomimeticapplications

In actuator technology active or smart materials have opened up new horizon s in terms of actuation simplicity, compactness, and miniaturization potent ial. One such material is the nickel-titanium shape memory alloy (NiTi SMA) , which is gaining widespread use in a variety of applications. The numerou s advantages of SMA over traditional actuators are of particular interest i n the area of underwater vehicle design, particularly the development of hi ghly maneuverable vehicles of a design based on the swimming techniques and anatomic structure of fish. An SMA actuation cycle consists of heating/coo ling 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 excelle nt heat-transfer properties of water, thus enabling much higher actuation f requencies. A controller for SMA actuators must account not only for large hysteretic nonlinearities between actuator output (strain or displacement) and input (temperature), but also the thermal control for resistive 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 S MA is very difficult in water, We overcome this problem by using a simplifi ed thermal model to estimate the temperature of the wire in conjunction wit h an adaptive hysteresis model, which relates the actuator output to the es timated temperature. Experimental results are provided, showing that this m ethod for control of an SMA wire works equally well both in air and in wate r, with only rough estimates (easily obtained) of the thermal parameters. S uccessful tracking of reference displacement signals with frequencies up to 2 Hz and relatively large amplitudes have been demonstrated experimentally .