Development of a Pneumatically-Driven Robotic Forceps with a Flexible Wrist Joint

The present paper describes a pneumatically-driven forceps manipulator for minimally-invasive robot surgery that has a simplified flexible wrist joint for easy fabrication, cost reduction and future miniaturization. The joint structure consists only of a machined spring, and four NiTi super-elastic wires fixed to the joint-end are actuated by pneumatic cylinders in push-pull motions. This mechanism realizes two degrees-of-freedom bending motions of the wrist joint and provides higher manipulation stiffness than other flexible joint mechanisms. Theoretical models of the proposed mechanism are developed; kinematic relation between joint positions and actuator displacements is described as a simple ideal continuum model, and dynamic model of the manipulator is described considering frictional and elastic forces of the joint mechanism. The joint position control system employs a cascade structure: the outer loop of the position control with dynamics compensation and the inner loop of the pneumatic force control. Performances of the joint position control are evaluated through elementary experiments, and effectiveness of the proposed joint mechanism is demonstrated.