Control Evaluation of Antagonistic Series Elastic Actuation for a Robotic Endoscope Joint

Haptic feedback is typically missing during telemanipulation of surgical robots in minimally invasive surgeries, i.e., surgeons cannot feel the interaction forces between the instruments and tissues. Instead, surgeons have to solely rely on visual feedback, which increases complexity of guiding the instruments and poses the safety threat of unperceivable contacts outside the field of view. We propose a novel series elastic actuation design for articulated robotic endoscopes to overcome these limitations and evaluate an according device with one joint. Similar to the actuation of human fingers, the joint is driven by antagonistic tendons. Springs are integrated in the transmission between the tendons and the motors outside of the endoscope shaft. We estimated the joint angle and thereby the endoscope shape, measured spring deflection, estimated tendon forces from that deflection, and implemented force control for the endoscope joint. Zero torque control and impedance control were evaluated under application of both a continuous force and an impact force to the endoscope tip. The springs reduced impact forces at the tip of the endoscope through their inherent compliance. At the same time, feeding back the estimated force resulted in a stable tendon force control and a tunable endoscope joint control: Zero torque control effectively reduced the external forces, while the endoscope joint showed the expected stiffness in impedance control. These results show that antagonistic series elastic actuation is a promising concept for endoscope joint actuation and that it can lead towards safer robot–tissue interactions in surgical robotics.