FREEs – Bucknell Soft Robotics

In recent years, the safety considerations of robots working in human environments has taken on increased importance and heightened the need to study soft robotics. Soft robots exceed the capabilities of traditional rigid robots in several ways, such as compatibility, degrees of freedom, manufacturing costs, and safe interactions with the environment. As part of our research, we model Fiber Reinforced Elastomeric Enclosures (FREEs), as one special type of soft actuator using Finite Element Analysis (FEA). The developed finite element model is a powerful, valuable tool for rapidly investigating the capabilities of FREEs for use in a robotic arm. We have expanded upon existing research on similar types of pneumatic actuators by studying various cases and investigating the effect of fundamental material properties on overall response. In addition to proposing new modeling considerations and techniques, this research emphasizes the importance of material selection, geometry, and adhesives on the deformations of FREEs (extension, expansion, and rotation).

Finite Element Model

Our research assesses the behavior of a pressurized FREE using Finite Element Analysis (FEA), including a consideration of nonlinear geometric and material characteristics. In depth studies were conducted of the design space of FREEs as a function of fiber angle and the influence of constituent material properties on overall behavior. The FREE is modeled as an assemblage of a thin-walled latex tube wound with cotton fibers. The FEA model used tetrahedral elements with an Ogden hyperelastic model for the elastomer and truss elements with linear material properties for the fibers.

Experimental Results

The FEA model was validated by comparing data characterizing the rotation of a FREE as predicted with an Ogden material model (α = 1.2) with fiber winding angles of 20^o, 50^o, and 70^o and determined experimentally for three FREEs with the same winding angles and geometry. Rotation was measured based on images of a pattern on the endcap of the FREE at each pressure increment.

*Comparison of FEA and experimental data*

Design Space Study

Overall FREE behavior was studied using the FEA model for fiber angles ranging from 10^o to 80^o. A fiber winding angle greater than approximately 40^o led to a shortening of the FREE (at low pressures) and a fiber angle less than 40^oled to an elongation of the FREE. As fiber angle decreased, expansion decreased. Maximum rotation occurred at a fiber angle of approximately 30^o. To investigate the relative impact on response of the fiber and elastomer material properties, a parametric study was conducted using a 20^ofiber angle FREE by doubling and halving elastomer and fiber stiffnesses. Material properties of the elastomer were shown to have a much greater impact on deformation than the fiber material properties.

*FEA results predicting extension (λ_z), expansion (b/B), and twist per unit length (τ) as a function of applied pressure.*

Controlling a Single FREE

Open-loop control and dynamic response (LabVIEW)

Closed-loop PID control in simulation (Matlab)

Closed-loop PID control (LabVIEW)

Controlling Multiple FREEs

All FREEs in the following module have the same winding angle (40^o) and internal pressure (10 psi)

All following FREEs have the same winding angle (40^o); internal pressures are top-left/bottom-right pairs (1 psi), top-right/bottom-left pairs (10 psi)