Three-Degrees-of-Freedom Passive Gravity Compensation Mechanism Applicable to Robotic Arm with Remote Center of Motion for Minimally Invasive Surgery

For safety enhancement reasons, passive gravity compensation is widely applied in robotic systems used in minimally invasive surgery (MIS). MIS robotic systems have a remote center of motion (RCM) in which a surgical instrument conducts a fulcrum motion around a point of invasion. RCM mechanisms include three-degrees-of-freedom (3-DoF): roll, pitch, and translation. Existing studies to date have focused on multi-degrees-of-freedom gravity compensation mechanisms by installing springs and wires in a robot. However, a gravity compensation mechanism with 3-DoF that simultaneously uses all three-directional movements (roll, pitch, and translation) has not yet been researched. Here, we propose a novel gravity compensation mechanism applicable to a 3-DoF MIS robotic arm with an RCM mechanism. When a translational motion is exerted, the proposed gravity compensator can adjust the roll-pitch-directional compensating torque by utilizing a reduction gear box and wire cable. To verify the 3-DoF gravity compensation, a gravity-compensated robotic arm for MIS and customized torque sensors was manufactured and calibrated. The results showed that the proposed static balancing mechanism can compensate for the gravitational torque with respect to roll, pitch, and translation. The total torque error along the roll and pitch axis was less than 0.38 N·m. In particular, the torque variation due to the translational motion was less than 0.13 N·m.