Echinoderm inspired variable stiffness soft actuator with connected ossicle structure

Abstract

Soft robots have a potential to interact with the human counterparts safely and handle fragile objects without breaking due to their adaptation ability in shape and stiffness to the environment. This capability makes soft robots to be considered as a breakthrough in many robotics applications including grasping hands, rehabilitation robots and physical human-robot interactions. However, their inherent softness from the material also provides reduced structural behaviors and it limits the application and payloads of the soft robot. Therefore, variable stiffness soft actuator is highly expected in soft robotics. In this research, the stiffness modulation method that is inspired by the connected ossicle structures of an echinoderm is proposed. Echinoderm’s body wall actively modulates their stiffness more than 10 times through the material and structural features of the calcite ossicles, connective tissue and inter-ossicular muscle. Based on the analyzation result of echinoderm’s body wall, suggested method is designed and stiffening layer was fabricated with combination of three design parameters which gives large stiffness factor. To figure out that the actuator is fabricated and actuate as intended, micro-CT was used to observe and analyze the change of the internal ossicle position and porous structure according to the vacuum level. As an application to demonstrate the feasibility of the proposed design, the robotic gripper is selected. Bending motion generating parts, needed for use as a finger, was selected based on the ability to robustly realize the performance, such as bending curvature and blocking force, even with vacuum level change. Lastly, grasping ability change by applying suggesting variable stiffness method was tested with three different types of robotic gripper design for different task purpose. This variable stiffness mechanism can be applied not only to robotic gripper but also to various applications where the transition in between compliant and rigid state are needed like active orthosis or pneumatic artificial muscles. Consequentially, this study aims to expand the application range of soft robot by suggesting variable stiffness mechanism to control the rigidity of soft robot according to the given task.