Quasi-Articulation of a Continuous Robotic Manipulator Enabled by Stiffness-Switching Origami Joints

Soft robots possess a nearly infinite number of kinematic degrees of freedom due to the compliance of their underlying materials which enables them to accomplish incredible feats of movement and adaptation. However, their severely underactuated structures limit their controllability and the degree of precision that can be achieved. As demonstrated by the octopus when fetching prey, it is possible to achieve precise movement in an otherwise “soft” arm by stiffening select sections of the arm while keeping other sections flexible, in effect generating a quasi-articulated structure and reducing the degrees of freedom from practically infinite to a finite number of angles.\nIn this study, we use the bistable generalized Kresling origami to emulate this strategy. Both experimental and computational modeling procedures are conducted to evaluate the bending mechanics of the structure at each of its two stable states (extended and contracted). As the model accurately predicts the major trends observed in experiments, it is used to perform a parametric study on the bending stiffness ratio, defined as the ratio of bending stiffness at the extended state to the bending stiffness at the contracted state. Using the results of the parametric study, we discover that the Kresling design which maximizes the bending stiffness ratio is that possessing the greatest angle ratio λ, the lowest contracted height Lc, and the largest number of sides of the base polygon n, enabling the transformation of the structure from rigid to flexible.\nTo complete the study, we use the optimal Kresling design in the fabrication of a tendon-driven reconfigurable manipulator composed of three Kresling modules. We find that by reconfiguring the Kresling module states (rigid or flexible), the manipulator can effectively transform into 2m different configurations where m corresponds to the number of modules. Through this reconfiguration, the manipulator can generate a quasi-articulated structure which reduces its effective degrees of freedom and enables linkage-like motion.\nUnlike other methods of stiffness modulation, this solution reduces system complexity by using a bistable structure as both the body of the robot and as a mechanism of stiffness-switching. The structure’s primary reliance on geometry for its properties makes it a scalable solution, which is appealing for minimally invasive surgical applications where both precision and adaptability are vital. The manipulator may also be used as an inspection or exploration robot to access areas that may be inaccessible to humans or rigid robots.