Toward a solution to the snapping problem in a concentric-tube continuum robot: Grooved tubes with anisotropy

The concentric-tube continuum robot generates distal end motions by translating and rotating the proximal ends of pre-curved tubes that overlap concentrically. This robot does not require additional actuators along the tubes because the overall curvature and distal end position are determined solely by interactions between the inner and outer tubes. However, under certain conditions, the rotation of the distal end is hindered as the actuation energy accumulates into torsional energy of the tubes. As the distal ends are rotated further, the accumulated energy from the twisting is suddenly released, which makes the tubes snap to a remote position. This is called the snapping problem, and it considerably limits the performance of the robot. In this paper, we propose a novel design for the concentric tubes to eliminate the snapping problem. The new design creates groove patterns on superelastic nitinol tubes to make the tubes more flexible to bending than twisting. Simulations and experiments were performed to verify that the tubes with our groove patterns had anisotropic structural characteristics, and video image analysis verified that this structural property can eliminate the snapping problem. A concentric-tube robot with this new tube design can have a larger workspace area because tubes with greater curvatures can be used without the snapping problem.