Highly Articulated Tube Mechanism With Variable Stiffness and Shape Restoration Using a Pneumatic Actuator

Recently, soft robotics research has focused on mechanisms to control the stiffness of robot structures. There are several types of variable stiffness mechanisms that can change the stiffness of such structures by altering friction force. Among these, the negative-pressurization method applies negative pressure to the particles or layers that are filled inside a bag so that they contact each other and increase stiffness. However, the maximum driving pressure that can be achieved using the negative-pressurization method is 0.1 MPa, which limits the stiffness. In this study, we devised a new fluid-driven mechanism that can change the stiffness through positive pressurization using pneumatic artificial muscles. The proposed structure constructed with hollow beads and inner muscles can hold arbitrary joint angles by the contraction force generated when a pressure is applied. We measured the torque in the pitch direction due to friction when the internal pressure and inner segment diameter were changed. We confirmed that the error between the measured and theoretical torque value was within a 20% at any inner diameter. Also, since a pneumatic artificial muscle is enclosed inside the structure, an actuation effect can occur when the pressure is applied. We measured the restoring angle and force of this effect when the valve opening ratio and inner segment diameter were changed. We confirmed the function that can change the shape restoration characteristics by changing the flow rate with a single rubber tube. Our approach will help design effective stiffening mechanisms using positive pressurization in soft robotics applications. In future, we will design a tensioner mechanism for arbitrarily changing the distance of segments in the axial direction, and we will investigate the holding torque, restoring angle and force when the distance of the segments is changed.