Tube Mechanism With 3-Axis Rotary Joints Structure to Achieve Variable Stiffness Using Positive Pressure

Abstract

Studies on soft robotics have explored mechanisms of switching the stiffness of a robot structure. The hybrid soft-rigid approach, which combines soft materials and high-rigidity structures, is the most commonly-used method for the variable stiffness mechanism. The positive-pressurization method, in particular, has attracted significant attention in recent years as it can remove the limit on the driving pressure. Moreover, it can change the shape holding force according to the value of internal pressure. In this study, a variable stiffness mechanism, consisting of 3-axis rotary ball joints and a single chamber, is devised via frictional force using positive pressure. The prototype can change joint angles arbitrarily when not pressurized and can hold joint angles when a positive pressure is applied. By using a theoretical model of the torque required to hold the joint angle, we simulated the holding torque using finite element modeling analysis and measured the holding torque in the pitch and roll directions when internal pressure was applied. Based on the interaction of the theoretical model, measurement, and FEM analysis, it was confirmed that the value of the holding torque in the roll direction was approximately π/2 times larger than that in the pitch direction for each value of the internal pressure. Further, we evaluated the FEM value, theoretical value, and measured value of holding torque by numerical comparison with each other. We believe that our approach will aid the design of effective stiffening mechanisms for soft robotics applications.