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

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°, 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.