Optimizing Specific Actuation Force of Soft Composite Pneumatic Artificial Muscles Using Additively Manufactured Components

Abstract

Pneumatic artificial muscles (PAMs) are composite actuators comprised of a Kevlar braid, latex bladder, and two aluminum end-fittings through which inflation, deflation, and transfer of external loads can be achieved. PAMs can exhibit extensile or contractile forces based on the choice of braid angle. PAMs show exceptional specific actuation force when normalized by their weight. However, the end fittings are typically machined from aluminum. Lighter materials, such as Acrylonitrile Butadiene Styrene (ABS-R), could be used to reduce the weight of the end-fittings and increase the specific actuation force. This research focuses on the design, fabrication, and testing of contractile PAMs for which aluminum end fittings are replaced with lightweight 3D-printed end fittings. 3D-printed PAMs were tested under quasi-static axial loads for a range of inflation pressures. PAMs were tested in blocked force and free contraction using a Materials Testing System (MTS) servo-hydraulic testing machine. Burst and actuation testing was conducted to determine the PAM’s maximum failure load, and fully characterize their output force with respect to percent contraction. A 120-psi burst pressure was chosen to provide a safety factor of 1.2 when operating the PAM between 0 and 100 psi. PAMs tested in actuation were cycled quasi-statically between normal operating pressures from blocked force to free contraction. After additive manufacturing and design alterations, the PAMs burst pressure was improved from 363 lbf at 43 psi to 1160 lbf at 123 psi, showing their ability to withstand large axial forces within the specified pressure limits. 3D-printed PAM actuation characteristics show comparable performance to PAMs built using aluminum end fittings and show no loss in force per unit contraction when switching from aluminum to 3D-printed end fittings.