Kinematics of a Generalized Class of Pneumatic Artificial Muscles

Fluid filled fiber reinforced elastomeric enclosures (FREEs) have been a popular choice for actuators in prosthetics and soft robots owing to their high power density and cost effective manufacturing. While a narrow class of FREEs known as McKibben's actuators have been extensively studied, there is a wide unexplored class that could be potentially used as actuators and soft structural members. This paper analyzes the mobility of generalized FREEs based on simple geometric relationships that result from the inextensibility of fibers and fluidic actuation. The analysis conducted can be classified into instantaneous kinematics and global or large deformation kinematics. Instantaneous kinematics reveals that the most general deformation pattern of the FREE is a screw motion about the axis of its cylinder, whose pitch is a function of fiber orientations. Furthermore, a set of fiber angles, which do not deform under volumetric actuation were identified as the locked manifold (LM). Global kinematic analysis revealed that every FREE continued to deform until its fiber configuration approached the LM. These insights were corroborated with finite element analysis (FEA) and testing for a small sample of FREE actuators.