Design of a discrete bending joint using multiple unit PREF joints for isotropic 2-DOF motion

Miniature steerable robots are required for various medical applications. Although a typical continuum joint mechanism has a great advantage of miniaturization, it is difficult to have a small bending radius of curvature. With a discrete joint, it is easy to secure definite bending with strength; however, a coarsely structured joint cannot provide a stable distal rolling motion to the end-effector. This paper proposes a method to construct a 2-DOF discrete bending joint using multiple pulleyless rolling joints with an example of a 4-segmental joint. The effects of the stacking sequence on its performance are analyzed. Then, three evaluation criteria are established, and the best stacking sequences are determined. The proposed design method is valid for various numbers of unit joints, and it can be easily applied to the structural design of soft robots resembling snakes or elephant trunks.