Textile overmolding for increased design space of strain limiting features in soft robotic structures

While soft robots demonstrate utility in applications requiring compliant and adaptive interactions, a current limitation of soft actuators is their inability to generate large forces and support heavy payloads. The primary way for pneumatic actuators to achieve higher force output and support heavier payloads is to operate at higher pressures, but soft actuators often operate at lower pressures to prevent actuators from bursting. One way to enable soft actuators to attain high burst pressures and sustain more cycles until failure is through strain limiting, where flexible but inextensible elements like fabric are incorporated into the actuators to limit local strain. Implementing strain limiting typically involves a highly manual process of either tying inextensible elements around a finished actuator or applying the elements between successive molding steps. In this paper, we propose two new fabrication techniques for overmolding fabric within soft actuators in a single monolithic molding step. We compare the performance of the actuators made with the two proposed methods to those made with a manually wrapped strain limiting feature external to the actuators, and a control without any strain limiting features. Our characterization includes fabrication time, cycle testing, and force output at both constant pressure and curvature across bellows-type (fast PneuNet) actuators for the different strain limiting strategies. We also look at the degradation of molds related to one of the fabrication strategies. The proposed fabrication methods increase average actuator force output over two-fold and expand the potential design space of strain limiting elements in soft robots, demonstrating a path toward scalable fabrication of strong and durable soft actuators.