Straight-Fiber-Type Artificial Muscle Deformation Under Pressurization

Currently, researchers are actively conducting studies and experiments on soft robots due to their increasing abilities to cooperate with humans. Particular attention is being paid to pneumatic artificial muscles because of their flexibility and relatively light weight as actuators. Among the multiple types of pneumatic artificial muscles, the straight-fiber-type has a structure, which is typically configured in a cylindrical rubber tube reinforced by fibers in the axial direction. The application of an internal pressure expands the tube in the radial direction, while it contracts in the axial direction. In this study, the deformation of such tubes was theoretically analyzed. First, the differential equation for the cylinder profile was derived, and the solution then obtained in the closed form, when the rubber is relatively soft enough compared to the fiber. Finally, for a large enough inner pressure, the profile of the reinforced rubber cylinder was derived from the incomplete elliptic integral. Moreover, the spring constant, the contraction ratio, and the maximum stress were obtained regarding the cylinder length. The spring constant linearly increased with the initial length of the cylinder. The contraction ratio also increased with the initial cylinder length up to 54.3%. The maximum stress was found to be related to the elastic modulus of the rubber but not to the inner pressure; when the cylinder length was three to five times larger than the initial diameter of the cylinder, the rubber exhibited small a maximum stress and large contraction ratio.