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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.
sma actuator; flexible actuator; rapid control prototyping; hammerstein–wiener model; position and velocity control