Soft pneumatic actuators (SPAs) are widely studied and applied in the field of soft robotics. To expand their applications, the SPAs should be purpose-built to generate application-specific complex motions with multiple degrees of freedom. This article describes a new SPA consisting of a series of internal chambers with the same helix angle arranged in a row, which could generate bending and twisting motions simultaneously. The trajectory of the helical actuator was analyzed through the finite element method (FEM) by changing the angle of the chambers and the actuator length. We employed a three-dimensional printing method to directly fabricate the thin-walled and airtight helical actuators without applying any postfabrication process. The recorded trajectory of the actuator and the measured blocking force on the tipping point were compared with the corresponding simulation results from the FEM. The actuation behavior of the helical actuator has been compared with that of the actuator with zero chamber angle, but with the same size (i.e., a normal bending actuator generating a two-dimensional trajectory). It is found that the proposed helical actuator (with a maximum 2.10 N blocking force) had a higher mechanical output (or efficiency) than the normal bending actuator (with a maximum 1.19 N blocking force) under the same pressure input. We fabricated a soft helical actuator as the fingers of a four-finger gripper to grasp complex-shaped items. Furthermore, another four-finger gripper made of a hybrid actuator consisting of a half of the angled chambers and a half of the nonangled chambers was constructed to demonstrate that the proposed design and fabrication technique could be employed to establish application- and function-specific soft robotic systems.
Keywords: soft actuators; soft gripping; soft materials; soft robotics.