This paper describes a flexible Shape Memory Alloy (SMA) actuator
designed to increase the limited displacement that these alloys can
induce. The SMA actuator has been designed so that it can be bent up to
about 180°, providing more freedom of movements and a better integration
in wearable robots, specially in soft wearable robots, than standard
rigid solutions. Although the actuator length is relatively short, this
original design allows a great linear displacement, because it can have
one or more loops of the same SMA wire inside the actuator. This implies
that the length of the SMA wire is at least two times greater than the
length of the actuator. The adopted strategy for both position and speed
control that overcomes the hysteresis and prevents overheating the
actuator is also described. The control algorithm has been implemented
in a rapid control prototyping (RCP) system based on a low cost hardware
platform. Finally, the application of this novel actuator in a wrist
exoskeleton prototype is shown to demonstrate the feasibility of using
the flexible SMA actuator in a real soft wearable robot.
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