A Pneumatically Driven Surgical Manipulator With a Flexible Distal Joint Capable of Force Sensing

Abstract

This paper presents a novel forceps manipulator for surgical robot systems. The forceps manipulator has a highly simplified flexible distal joint, which is actuated by push–pull motions of superelastic wires. Pneumatic cylinders are employed for its driving system to realize high backdrivability of the flexible mechanism, enabling external force estimation without using a force sensor. For the kinematic description, we newly introduce a three-degree-of-freedom (DOF) continuum model considering expansion and contraction of the flexible joint, which allows three-axis force sensing on the forceps tip. We also developed a practical dynamic model, including linear-approximated elastic forces and nonlinear friction forces dependent on the joint bending angle. Effectiveness of the dynamic model is validated by open-loop control performance of the joint angles. The position control system is designed using a PID-based cascade controller with a feedforward compensator based on the dynamic model. Resolution of the joint angle control is 1 $^\circ$ , satisfying the requirement for laparoscopic surgery. An external force estimation algorithm is developed, which realizes the three-axis sensing of translational forces acting on the forceps tip. The rigid-link approximation model is also employed to treat the calculation in singular attitude, the straight position of the flexible joint. Effectiveness of the force estimator is experimentally validated using a force sensor in two cases. Estimation error is 0.37 N at maximum with a force in a radial direction, and the estimation performance using the three-DOF force estimator is much better than the one using a conventional two-DOF force estimator.