Independent Tendons Increase Stiffness of Continuum Robots without Actuator Coupling

Tendon-driven continuum robots have drawn interest for a wide variety of applications. Prior work in this area has elucidated the coupled kinematics and statics models that describe the motion and coupling of the robot’s elastic backbone with the driving tendons that are tensioned to change the shape of the robot. However, the full design freedom associated with the routing of the tendon through the supporting “eyelets” in the structure has not been explored. This article describes designs that have multiple tendon paths designed to influence the shape of only one continuously deformable section. It is known that this type of solution generally results in highly coupled tendon kinematics, but we show experimentally that there exist paths for which the tendons are so weakly coupled (kinematically) that they can be locked off to provide configuration-independent stiffening. They could also be displaced independently from one another to control independent deformation modes. The approach reveals a strategy for reducing the uncontrolled compliance of the robot’s body, including the torsional compliance, while retaining simplicity in design and control. In particular, we show that tendons that are routed sinusoidally and helically do not strongly couple to constant-curvature actuating tendons as long as they meet an orthogonality constraint. The added tendons increase the stiffness at the cantilevered end by 4.85x over straight tendons alone without impacting the range of motion in the stiffened condition.