Finite Element Analysis of Fiber Reinforced Elastomeric Enclosures:
Keith W. Buffinton, Benjamin B. Wheatley, Soheil Habibian, Joon Shin, Aditi Vijayvergia, and Amanda E. Christy
Abstract— Increasing attention has been focused in recent years on the development and analysis of “soft” robots. Such tools can perform a variety of simple tasks in and around humans with minimal risk of injury to people and the environments in which people typically live and work. Some studies have developed computational models, such as finite element models, to compare to experimental analysis. While these models generally show strong agreement with experiments, there has been little use of models to investigate modeling assumptions, buckling behavior, or design spaces. This paper seeks to address these opportunities and challenges through finite element analysis (FEA) of a particular type of soft actuators known as Fiber Reinforced Elastomeric Enclosures (FREEs), both individually and in modules. The FREEs studied in this work are fabricated from thin-walled latex tubes with helically wound cotton fibers at particular angles relative to the tube. Our results suggest that elastomer material behavior can be better described with an Ogden rather than neo-Hookean material model at large deformations and by modeling fibers with 1D truss elements. Additionally, the material properties of the elastomer were found to greatly influence FREE extension, expansion, and rotation (with strains in excess of 25%), while changes to fiber stiffness resulted in negligible differences in deformation. The implications of these results are that in the design and manufacturing of FREEs, substantial attention must be given to accurately measuring, modeling, and understanding elastomer and adhesive properties, and that these may be used for design tuning. Additional results showed that modules made up of multiple FREEs can be effectively studied using FEA to determine range of motion, buckling, and workspace.
Force Generation by Parallel Combinations of Fiber-Reinforced Fluid-Driven Actuators
Daniel Bruder, Audrey Sedal, Ram Vasudevan, and C. David Remy, Member, IEEE
Abstract— The compliant structure of soft robotic systems enables a variety of novel capabilities in comparison to traditional rigid-bodied robots. A subclass of soft fluid-driven actuators known as fiber reinforced elastomeric enclosures (FREEs) is particularly well suited as actuators for these types of systems. FREEs are inherently soft and can impart spatial forces without imposing a rigid structure. Furthermore, they can be configured to produce a large variety of force and moment combinations. In this paper we explore the potential of combining multiple differently configured FREEs in parallel to achieve fully controllable multi-dimensional soft actuation. To this end, we propose a novel methodology to represent and calculate the generalized forces generated by soft actuators as a function of their internal pressure. This methodology relies on the notion of a state dependent fluid Jacobian that yields a linear expression for force. We employ this concept to construct the set of all possible forces that can be generated by a soft system in a given state. This force zonotope can be used to inform the design and control of parallel combinations of soft actuators. The approach is verified experimentally with the parallel combination of three carefully designed actuators constrained to a 2DOF testing rig. The force predictions matched measured values with a root-mean-square error of less than 1.5 N force and 6.5 × 10−3 Nm moment, demonstrating the utility of the presented methodology
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Model Based Control of Fiber Reinforced Elastofluidic Enclosures
Daniel Bruder1∗ , Audrey Sedal1∗ , Joshua Bishop-Moser1 , Sridhar Kota1 , and Ram Vasudevan
Abstract— Fiber-Reinforced Elastomeric Enclosures (FREEs), are a subset of pneumatic soft robots with an asymmetric continuously deformable skin that are able to generate a wide range of deformations and forces, including rotation and screw motions. Though these soft robots are able to generate a variety of motions, simultaneously controlling their end effector rotation and position has remained challenging due to the lack of a simple model. This paper presents a model that establishes a relationship between the pressure, torque due to axial loading, and axial rotation to enable a model-driven open-loop control for FREEs. The modeling technique relies on describing force equilibrium between the fiber, fluid, and an elastomer model which is computed via system identification. The model is experimentally tested as these variables are changed, and the results are compared to the analytical model, illustrating that the model provides good agreement. To further illustrate the potential of the model, a precision open-loop control experiment of opening a rotational combination lock is presented.