A Hybrid Active/Passive Wrist Approach for Increasing Virtual Fixture Stiffness in Comanipulated Robotic Minimally Invasive Surgery

In minimally invasive surgery (MIS), the incision point acts as a fulcrum about which the surgical instrument pivots. Most robotic MIS systems foresee procedures to carefully align the robot with the incision in the patient. Such procedures disrupt the normal workflow. This hampers the clinical translation of such assistive technology. In contrast, backdrivable, wristed (BW) robots, especially when operated as comanipulation devices, have a minimal impact on the traditional surgical workflow in MIS, as they are readily equipped with algorithms that automate fulcrum point estimation. To improve safety, accuracy and/or task completion time, virtual fixtures can be implemented at the distal instrument tip, using an assistive BW system. This letter formalizes the problems related to such distal virtual fixtures. It then develops a hybrid active/passive wrist control strategy to improve the performance of virtual fixtures with BW systems while minimizing stability issues. Aside from an analytical example, experimental validation is added to show that the new algorithm increases the achievable stiffness of distal virtual fixtures.