Joint rotation angle sensing of flexible endoscopic surgical robots

Abstract

Accurate motion control of surgical robots is critical for the efficiency and safety of both state-of-the-art teleoperated robotic surgery and the ultimate autonomous robotic surgery. However, fine motion control for a flexible endoscopic surgical robot is highly challenging because of the shape-dependent and speed-dependent motion hysteresis of tendon-sheath mechanisms (TSMs) in the long, tortuous, and dynamically shape-changing robot body. Aiming to achieve precise closed-loop motion control, we propose a small and flexible sensor to directly sense the large and sharp rotations of the articulated joints of a flexible endoscopic surgical robot. The sensor - a Fiber Bragg Grating (FBG) eccentrically embedded in a thin and flexible epoxy substrate - can be significantly bent with a large bending angle range of [-62.9°, 75.5°] and small bending radius of 6.9 mm. Mounted in-between the two pivot-connected links of a joint, the sensor will bend once the joint is actuated, resulting in the wavelength shift of the FBG. In this study, the relationship between the wavelength shift and the rotation angle of the joint was theoretically modeled and then experimentally verified before and after the installation of the sensor in a robotic endoscopic grasper. The sensor, with the calibrated model, can track the rotation of the robotic joint with an RMSE of 3.34°. This small and flexible sensor has good repeatability, high sensitivity (around 147.5 pm/degree), and low hysteresis (7.72%). It is suitable for surgical robots and manipulators whose articulated joints have a large rotation angle and small bending radius.