Numerical and experimental assessment of tilted-helical fiber orientation effects on deformation of pneumatic soft actuators

Pneumatic actuation of fiber-reinforced soft-material slender structures, and specifically inflatable cylinders, plays a key role in soft robotics, particularly for generating sought deformations in terms of, e.g. elongation, expansion, torsion, and bending. Fiber-reinforced tubes were widely investigated, in particular by considering regular helical patterns for the fibers, and multiple sets of fibers were introduced for achieving relatively richer deformations such as coiling (i.e. combined bending and torsion). However, coiling can be obtained even by introducing a single tilted-helical fiber, thus potentially simplifying actuator design and implementation. Therefore, in this study, we numerically investigated the effect of fiber orientation on the coiling behavior of tilted-helical fiber-reinforced cylindrical actuators. Specifically, we used finite element numerical simulations to determine the effect of fiber pitch and tilt angle on the curvature and twist angle of an inflated cylindrical actuator, and we assessed numerical results based on corresponding experiments. Finally, we addressed a preliminary design chart based on the inverse map linking coiling to the corresponding pitch and tilt angle. Our results take an initial step for the design of tilted-helical fiber-reinforced actuators, with potential application to a wider class of inflatable soft actuators to program a desired coiled configuration through the integration of a single tilted-helical fiber.