Antagonistic Pneumatic Actuators with Variable Stiffness for Soft Robotic Applications

Abstract

In this paper, we developed and demonstrated two variable stiffness soft elastomeric actuators, each of them composed of two symmetric pneumatic chambers (one chamber with segments and the other one with fiber reinforcement) and an inextensible layer embedded in the middle. Both actuators demonstrate enhanced properties by providing bi-directional motion and higher capacity of withstanding external loads. The symmetric assembly of two chambers in each actuator allows to demonstrate the antagonistic behavior. Thus, the proposed actuators are named Antagonistic Pneumatic Actuators (APA Pneunet and APA FRA - Fiber-Reinforced Actuator). The inextensible layer in the middle of the two chambers in each actuator limits their elongation; instead, an isometric expansion results when a pressure is applied to each chamber with a consequent bending of the structure. By applying equal pressure, these actuators increase their stiffening behavior. In such condition, each chamber supports the bending of the other one, and this generates a stiffer structure, where its stiffness is changed by varying the applied pressure on either chambers. We characterized the APA actuators in terms of actuation force displacement, both in axial and lateral load condition. The results showed a good performance of both the APA (Pneunet and FRA) actuators, providing stiffness at the same time of actuation. For instance, in axial position, the APA Pneunet and FRA actuators showed 19.93N and 29.6N in passive mode and 23.53N and 44.87N in active mode (maximum pressure 1.0 bar), respectively.