Vacuum Induced Tube Pinching Enables Reconfigurable Flexure Joints with Controllable Bend Axis and Stiffness

Soft continuum robots present novel advantages over their rigid, linkage-based counterparts, by expanding the range of achievable kinematic configurations through their flexibility and lack of discrete joints. However, the lack of discrete joints presents challenges for estimation and control of movement in continuum robots. In this paper we present an intermediate approach towards achieving the same versatility of continuum robots while maintaining the traditional control and estimation methods for rigid robots. Our design focuses around a soft tubular element which can be buckled through an internal negative pressure, with the buckling angle set by a confining sleeve. Once the tube is buckled it approximates a revolute joint with torsional stiffness. In this paper we present the design, fabrication, and performance of tube-pinching reconfigurable revolute joints. Through experiment and modeling we identify the appropriate sleeve shape that enables joint axis control to within an error of 5.4°. Force-displacement experiments demonstrate that internal vacuum pressure controls the torsional stiffness of the joint. Lastly, to demonstrate the applicability of soft joint reconfiguration we perform experiments with a flapping tail in water to observe how joint reconfiguration enables different swimming modes.