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Helically wound soft actuators for torsion control

Individual soft actuators have been developed for elongation, contraction, bending and twist, but these actuators and their combinations have yet to demonstrate the range and flexibility of motion seen in common sources of biological inspiration, such as cephalopods. This paper presents a method for torsion control via sets of opposing contracting actuators wound helically around a cylindrical structure. By shortening one set of actuators, twist is developed, similar to the oblique muscles within octopus arms. The addition of helical actuators to systems with longitudinal and transverse actuators will enable control over orientation of the arm and antagonistic stiffening. A geometric model is used to quantify best-case developed twist, representing application to a constant dimension cylinder. This model is validated experimentally using a cable-driven prototype on a rigid cylinder with no torsional stiffness. To evaluate the interaction with a system of actuators, a mechanics model of the torsion actuators wrapped around a deformable center is proposed. This model is used to extend the solution given by W.M. Kier [Zoological Journal of the Linnean Society, Vol. 83, No. 4, 307-324, 1985], and shows that while significant twist can be lost to deformations of the internal structure, those with a Poisson's ratio approaching v = 0.5 mitigate this loss. Finally, the feasibility of the concept is demonstrated with McKibben actuators wound around foam.