A novel RCM mechanism using pneumatically driven flexible joint for laparoscopic forceps holder

Surgical robot systems for minimally invasive surgery (MIS), such as laparoscopic surgery, typically employ remote center of motion (RCM) mechanisms consisting of rigid links. Although the rigid body mechanisms have high reliability based on their stiffness, their large volume interferes the workspace of surgeons and the rigidity of pivot point requires precise positioning during the setup of robots. In this paper, a space-saving and light-weight manipulator to hold a forceps is developed, which has a compliant RCM mechanism by using a flexible joint. The manipulator consists of elastic metal springs and a wire to inflect the joint. Since the proposed flexible link has 2-DOF motion of protrusion and bending, the location of the pivot point can be variable, allowing easier adjustment of pivot point before or during the operation. We proposed the method to vary the radius of curvature of the flexible joint and pitch angle of forceps by controlling the position of the joint and the tension of the wire with two pneumatic cylinders. For more accurate manipulation, we modeled the holder in terms of material mechanics and experimentally identified static characteristics of the flexible joints. Experimental results show that the proposed model and control law achieve the rotational motion of the forceps with a remote and variable pivot point.