Examining the Coiling Motion of Soft Actuators Reinforced With Tilted Helix Fibers

This study aims to examine the coiling and uncoiling motion of a soft pneumatic actuator reinforced with tilted helix fibers. Coiling motion can be quite useful for robotic manipulation and locomotion purposes. This research proposes and investigates a novel actuator that is inspired and derived from the unique cell wall architecture in the seed appendage of Stork’s Bill plant (Erodium Gruinum). These plant cells are reinforced by cellulose fibers distributed in a tilted helix pattern — helixes that are tilted at a certain angle with respect to the longitudinal axis of the cell. As a result, the seed appendage can coil and uncoil via a combination of twisting and bending. This paper discusses the design, fabrication, and testing of a soft actuator that can mimic this sophisticated motion. This actuator consists of Kevlar fiber thread wrapped around a silicon rubber body that has the shape of a tube. The tube will be capped at both ends so that it can be pressurized internally to induce motion. Once the design parameter has been chosen, the soft actuator are fabricated by 1) designing and 3D printing molds, 2) tube casting and fiber wrapping, and 3) creating the end caps for pressure sealing. Carefully executing these fabrication steps is essential because any errors could give undesired deformation. Several soft actuators prototypes are fabricated based on different design choices regarding the actuator radius, tube wall thickness, and the number of tilted helix fibers (aka. fiber coverage). Proof-of-concept tests show that these actuator prototypes can indeed exhibit a combined twisting and bending under internal pressurization: all are the necessary receipts to achieve the coiling and uncoiling motion. Result of this paper can pave the way for a new family of soft actuators capable of unprecedented and sophisticated actuation motions, which are particularly appealing for soft robot application.