Modeling, analysis and experiments of inter yarn compaction effects in braided composite actuators

Braided composite actuators are pressure-driven muscle-like actuators capable of large displacements as well as large blocking forces. Braided composite actuators can also exhibit a large change in effective stiffness through simple valve control when the working fluid has a high bulk modulus. This is due to the stiff fiber reinforcement of the braided sleeve and the high bulk modulus of the fluid resisting the volume change when a load is applied in the closed-valve condition. Several analytical models have been previously developed that capture the geometrical and material nonlinearities of the composite actuator, the compliance of the inner liner, and entrapped air in the fluid. This article focuses on inter yarn compaction in the fiber sleeve, which is shown to reduce the effective closed-valve stiffness. In this article, a new analytical model that considers inter fiber yarn compaction, fiber extension and entrapped air effect as well as the material and geometric nonlinearities is developed. Analysis and experimental results demonstrate that the new compaction model can improve the prediction of the response behavior of the actuator.