Search for: All records

Abstract Soft robots join body and actuation, forming their structure from the same elements that induce motion. Soft actuators are commonly modeled or characterized as primary movers, but their second role as support structure introduces strain–pressure combinations outside of normal actuation. This article examines a more complete set of possible strain–pressure combinations for McKibben actuators, including passive or unpressurized, deformation, pressurized extension and compression of a pressurized actuator beyond the maximum actuation strain. Each region is investigated experimentally, and empirical force–displacement–pressure relationships are identified. Particular focus is placed on ensuring that empirical relationships are consistent at boundaries between an actuator’s strain–pressure regions. The presented methodology is applied to seven McKibben actuator designs, which span variations in wall thickness, enclosure material, and actuator diameter. Empirical results demonstrate a trade-off between maximum contraction strain and force required to passively extend. The results also show that stiffer elastomers require an extreme increase in pressure to contract without a compensatory increase in maximum achieved force. Empirical force–displacement–pressure models were developed for each variant across all the studied strain–pressure regions, enabling future design variation studies for soft robots that use actuators as structures. 

Current models of bending in soft arms are formulated in terms of experimentally determined, arm-specific parameters, which cannot evaluate fundamental differences in soft robot arm design. Existing models are successful at improving control of individual arms but do not give insight into how the structure of the arm affects the arm’s capabilities. For example, omnidirectional soft robot arms most frequently have three parallel actuators, but may have four or more, while common biological arms, including octopuses, have tens of distinct longitudinal muscle bundles. This article presents a quasi-static analytical model of soft arms bent with longitudinal actuators, based on equilibrium principles and assuming an unknown neutral axis location. The model is presented as a generalizable framework and specifically implemented for an arm with [Formula: see text] fluid-driven actuators, a subset of which are pressurized to induce a bend with a certain curvature and direction. The presented implementation is validated experimentally using planar (2D) and spatial (3D) bends. The planar model is used to initially estimate pressure for a closed-loop curvature control system and to bound the accessible configurations for a rapidly-exploring random trees (RRT) motion planner. A three-segment planar arm is demonstrated to navigate along a planned trajectory through a gap in a wall. Finally, the model is used to explore how the arm morphology affects maximum curvature and directional resolution. This research analytically connects soft arm structure and actuator behavior to unloaded arm performance, and the results may be used to methodically design soft robot arms. 

Bending permits soft arms to access a workspace that is not colinear with the initial arm axis; the size and shape of this space depends on the characteristics of the soft arm. Soft bending actuators and arms have developed for specific applications, but not characterized for the general relationship between design variables and performance. This paper defines a class of soft bending arms based on its design, considering the arm as a system constructed from many contracting actuators organized into segments. A modular segment design is presented, and seven variants of this design were constructed and tested for bend radius, bend direction, lateral stiffness and contraction. The variants isolate system parameters, in this case, arm radius and number of actuators within a given segment, to quantify how these parameters affect performance. A trade-off was found between lateral stiffness and bend radius, which can be controlled by altering the arm radius or the number of actuators. Bend direction was found to be coupled to both bend radius and arm load. Finally, a three-segment arm following a bio-inspired design is presented to demonstrate how the experimental results apply to soft robot system design. 

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 ν = 0.5 mitigate this loss. Finally, the feasibility of the concept is demonstrated with McKibben actuators wound around foam.