Experimental investigation of hybrid hydraulic fiber-reinforced actuators incorporating shape memory alloy

Abstract

The emerging field of soft robotics aims to offer solutions to many of the limiting factors present in traditional, “hard” robotics, such as complexity, material rigidity, and a lack of compliance. Fiber-reinforced elastomeric enclosure (FREE) actuators, including the well-researched McKibben pneumatic artificial muscle, are some of the most prevalent applications of this field due to their high power density and ability to actuate an entire structure with many degrees of freedom from a single pressure source. The kinematic behavior of these actuators is governed by the wrap architecture of the inextensible reinforcing fibers, and tailored motions can be achieved by simply adjusting the number of fiber families and their respective wrap angles. In this experimental investigation, eight hybrid hydraulic FREE prototypes are developed to explore the kinematic effects of incorporating a single wire of shape memory alloy (SMA) into various wrap geometries, including asymmetric configurations, which are previously unexplored. Changes in actuator length and rotation for each prototype are analyzed using discrete marker tracking as functions of the control variables of volumetric input and applied current, and kinematic behaviors are compared over various parameters of wrap geometry. Trends are established for the effects of wrap angles and their relationships to a generalized kinematic design space for FREES on the actuation potential of an incorporated fiber of SMA. Promising results are described for two additional proof of concept prototypes in an initial exploration of incorporating more than one SMA fiber into a hybrid actuator and expanding to more complex wrap configurations. This research demonstrates the capability of SMA to both augment passive actuator behavior and produce new forms of multimodal motion, providing a stepping stone towards more tailored applications of hybrid soft actuators.