Finite Element Modeling of Soft Fluidic Actuators: Overview and Recent Developments

Abstract

Soft robotics has experienced an exponential growth in publications in the last two decades. [1] Unlike rigid conventional manipulators, [2,3] soft robots based on hydrogels, [4,5] electroactive polymers, [6,7] and elastomers [7-9] are physically resilient and can adapt to delicate objects and environments due to their conformal deformation. [10,11] They also show increased safety and dexterity can be lightweight and used within constrained environments with restricted access. [12,13] Many soft robots have a biologically inspired design coming from snakes, [14-17] worms, [18-20] fishes, [21-24] manta rays, [25,26] and tentacles. [27-29] The scope of applications includes minimally invasive surgery, [30,31] rehabilitation, [32,33] elderly assistance, [34] safe human-robot interaction, [35,36] and handling of fragile materials. [37,38] Important features of soft robotics design, fabrication, modeling, and control are covered in the soft robotics toolkit. [39,40] The building blocks of soft robots are the soft actuators. The most popular category of soft actuator is the soft fluidic actuator (SFA), where actuation is achieved using hydraulics or pneumatics. [8,41] These actuators are usually fabricated with silicone rubbers following a 3D molding process, [42] although directly 3D printing the soft actuators is also possible. [43,44] Silicone rubber is a highly flexible/extensible elastomer with high-temperature resistance, lowtemperature flexibility, and good biocompatibility. [45] Elastomers can withstand very large strains over 500% with no permanent deformation or fracture. [46] For relatively small strains, simple linear stress-strain relationships can be used, and two of the following parameters can be used to describe the elastic properties: bulk compressibility, shear modulus, tensile modulus (Young's modulus of elasticity), or Poisson's ratio. [45] For large deformations, nonlinear solid mechanic models using hyperelasticity should be considered. [8,32,47-50] Due to the strong nonlinearities in SFAs and their complex geometries, analytical modeling is challenging. [51] A brief review of the analytical methods for modeling of soft robotic structures is provided in the following. 1.1. Analytical Modeling of Soft Actuators The majority of soft/continuum robots with bending motion can be approximated as a series of mutually tangent constant curvature sections, i.e., piecewise constant curvature. [52] This is a result of the fact that the internal potential energy is uniformly distributed along each section for pressure-driven robots. [53] This approach has also been validated using Hamilton's principle by Gravagne et al. [54] As discussed by Webster and Jones, [52] the kinematics of continuum robots can be separated into robotspecific and robot-independent components in this approach. The robot specific mapping transforms the input pressures P or actuator space q to the configuration space κ, ϕ, l, and the robot-independent mapping goes from the configuration space to the task space x. The actuator space contai...

A Soft Pneumatic Two-Degree-of-Freedom Actuator for Endoscopy

“…For example, varying the reinforcement pattern and the fiber angle on a fully extruded catheter could be promising, to limit locally the bending ( Connolly et al, 2017 ). Finally, developing a finite element model could allow for a finer prediction of the performance and optimization of the design for specific applications ( Xavier et al, 2021 ).…”

“…Euler–Bernoulli beam theory can provide a first interpretation of the working principle of the actuator ( Xavier et al, 2021 ; Gorissen et al, 2017 ; Decroly et al, 2020 ). It can be considered that the pressure p generates a moment M ∼ p , constant along the length L of the actuator, which causes the bending of the actuator with a constant curvature k and an angle α : .…”

“…The results can be further analyzed to generalize the actuators’ characterization, to compare it to that in the literature, and to study further the material influence. This analysis is based on several assumptions and, in particular, on the assumption of a constant curvature, which is a strong assumption, although common ( Xavier et al, 2021 ). The extruded material is not included in this study, since no estimated Young’s modulus is provided.…”

MOOSE-Based Finite Element Hyperelastic Modeling for Soft Robot Simulations

“…ABAQUS has also been used to model the compelx electromechanical behavior of hyperelastic soft sensors [16], to analyze multi-material fiber-reinforced pneumatic actuators [17] and to assist in the automatic design and modeling of soft actuators [18]. In addition to these works, a comprehensive review of the current state of soft robotic modeling is given in [19]. COMSOL, ANSYS, and ABAQUS are all powerful simulation platforms that are capable of accurately modeling soft robots.…”

Video1.MP4

“…Euler-Bernoulli beam theory can provide a first interpretation of the working principle of the actuator (Xavier et al, 2021;Gorissen et al, 2017;Decroly et al, 2020). It can be considered that the pressure p generates a moment M ∼ p, constant along the length L of the actuator, which causes the bending of the actuator with a constant curvature k and an angle α: α…”