Analysis of Pneumatic Artificial Muscles and the Inelastic Braid Assumption

Abstract

Pneumatic artificial muscles (PAMs) are becoming an increasingly popular form of soft actuator due to their unique actuation characteristics. The creation of accurate PAM actuation models is important for their successful implementation. However, PAM studies often employ actuation models that use simplifying assumptions which make the models easier to formulate and use, but at the cost of reduced accuracy. One of the most commonly used assumptions, the inelastic braid assumption, suggests that the braid does not stretch, and therefore would not affect its geometry or actuation force. Although this assumption has often been cited as a likely source of model error, its use has persevered for decades due to researchers’ inability to directly measure the effects of braid elasticity. The recent development of a photogrammetric method to accurately measure PAM geometry now enables this analysis. This study seeks to assess the current default adoption of the inelastic braid assumption in PAM models by attempting to quantify the braid elasticity effects. This research finds that current models that use the inelastic braid assumption can underestimate PAM diameter by as much as 30%, and overestimate actuation force by as much as 70%. These results show that braid elasticity can have a substantial effect on the geometry and actuation force of PAMs, and demonstrates that the inelastic braid assumption may not be a suitable universal assumption for PAM modeling and analyses, especially when low-stiffness braid materials are used.