Design and Kinematics of a Robotic Instrument for Natural Orifice Transluminal Endoscopic Surgery

Natural orifice transluminal endoscopic surgery (NOTES) has attracted continuous attention in recent decades, but its confined workspace restricts the dexterity and manipulability of instruments. Instruments with multiple degrees of freedom (DOF) and variable stiffness are desired to achieve flexible operation with sufficient load capacity. This article presents a novel robotic instrument, which can assist NOTES. The instrument is 6.5 mm in diameter with 7 DOFs. The three continuum elastic joints are made of helical springs with high torsional rigidity, whose wires have a rectangular cross section. Patterned concentric tubes are designed to change the stiffness of the force transmission structure of the passive arm so that the stability and rigidity of the instrument can be improved. Analytical solutions of inverse kinematics are obtained based on the arrangement of coupled joints designed for creating triangulation. The disk-elastomer model is established to simplify the derivation of kinematics, and the relationship between the slack of the driving cable and the joint parameters is derived. The trajectory tracking accuracy, bending accuracy, and the ability of variable stiffness of the proposed instrument are verified with experiments.