Design of an Extensible Central Backbone for Stiffness-Shifting in Soft Robotic Arms

Abstract

Soft robotic arms, which are most commonly constructed from fluid-driven actuators, have demonstrated a high degree of compliance and the ability to move in simple and complex ways, from single bends to whole-arm gripping to drawer opening. However, their practical applications have been constrained by limited load-bearing capacities: soft arms rarely lift more than 3 N in cross workspace movements. Designing soft robotic arms to have a high load-bearing capacity is an unsolved challenge, as simply increasing actuation pressure dramatically reduces compliance at the arm’s surface. Antagonistic actuation (without a backbone) has shown promise, but is inherently limited to the available actuation pressures, while actuation against an inextensible backbone - similar to hyper-redundant manipulators - locks out axial deformation. In this article, we propose a third option: a passive, extensible backbone constructed from a tailorable architected geometry that adjusts stiffness, retains the ability to change length, and is compatible with unidirectional or antagonistic actuation. Our design consists of a multilayer stack of sinusoidal wave-shapes, inspired by recently developed trimmed helicoids, which shifts the center point of any soft arm’s stiffness range while otherwise preserving actuatable degrees of freedom. We characterize bending, axial, and shear stiffness through finite element analysis validated with prototype tests, and further experimentally show how these backbones can be used to improve load bearing capacity. Our results provide a general structural reinforcement strategy for soft robotic arms and demonstrate the potential for using our structure or similar structures to augment existing arm designs.