Design and Characterisation of a 3D-Printed Pneumatic Rotary Actuator Exploiting Enhanced Elastic Properties of Auxetic Metamaterials

This paper describes the design and characterisation of a novel hybrid pneumatic rotational actuator that aims to overcome the limitations of both rigid and soft actuators while combining their advantages; indeed, the designed actuator consists of a soft air chamber having an auxetic structure constrained between two rigid frames connected by a soft hinge joint inspired by the musculoskeletal structure of a lobster leg. The main goal is to integrate the advantages of soft actuation, such as inherent compliance and safe human–robot interaction, with those of rigid components, i.e., the robustness and structural stability limiting the ineffective expansion of the soft counterpart of the actuator. The air chamber and its auxetic structure are capable of leveraging the hyper-elastic properties of the soft fabrication material, thereby optimising the response and extending the operational range of the rotational actuator. Each component of the hybrid actuator is fabricated using a 3D-printing method based on Fused Deposition Modeling technology; the soft components are made of thermoplastic polyurethane, and the rigid components are made of polylactic acid. The design phases were followed by some experimental tests to characterise the hybrid actuation by reproducing the typical operating conditions of the actuator itself. In particular, the actuator response in unconstrained expansion and isometric and isobaric conditions has been evaluated. The experimental results show linearity, good repeatability, and sensitivity of the actuator response vs. pneumatic pressure input, other than a small percentage hysteresis, which is ten times less than that observed in commercial soft pneumatic actuators.