Designing systems of fiber reinforced pneumatic actuators using a pseudo-rigid body model

Abstract

Fiber Reinforced Elastomeric Enclosures (FREEs) are fundamental building blocks of pneumatic soft robots, with McKibben muscles as common examples. When a system of FREEs are arranged in a planar configuration, they undergo large bending deformations due to internal loads and moments. This paper models the bending behavior of FREEs using a modified three-spring Pseudo-Rigid Body (PRB) model, which is commonly used to capture large bending behavior of flexures. The springs of the PRB model are shown to effectively capture the axial and bending stiffness of contracting FREEs, while an internal force accounts for actuation due to internal pressurization. An optimization process is carried out to fit the PRB model parameters with the experimental response curve of different FREEs with varying length to diameter ratio and fiber angles. Unlike conventional models applied to compliant mechanisms, the model parameters are shown to depend on fluid pressure. The PRB model is used to demonstrate and analyze a pennate arrangement of contracting FREEs that exhibit variable gear ratio behavior. The pennate FREEs can serve as building blocks for the design of soft robots.