Dynamic friction modeling and control for flexible tendon-sheath mechanism

Abstract

Natural Orifice Transluminal Endoscopic Sugery (NOTES) allows surgeons to perform efficient, higher fidelity, and complex tasks for surgical treatment while avoiding unexpected damages such as tissue trauma and reducing recovery time after the surgery. To actuate joints at the distal end of flexible endoscopic systems, tendon-sheath mechanism (TSM) is widely used. However, nonlinearities such as friction and backlash hysteresis degrade the accurate position control and system performances. In addition, force feedback information is challenging for online measurements since traditional sensors like torque gauges are difficult to integrate at the tool tips. Previous studies on the tendon-sheath systems only consider the tension transmission models of the TSM with pre-determined configurations and the information of sheath shapes must be known in advance; while discontinuity is still existing when the system operates at the vicinity of zero velocity. Efficient control strategies are still lacking since complex smooth inverses of the backlash hysteresis models are required to compensate the nonlinearities. In addition, disturbances and uncertainties have not considered yet in the literature. This thesis addresses the modeling, identification, and control problems for the TSM used in flexible endoscopic systems. New nonlinear dynamic friction models and compensation control schemes are developed to deal with discontinuous issues of the force feedback and significantly enhance the position tracking of the tendon-sheath systems. Two main aims are identified: (i) development of novel dynamic friction models consisting of the sliding and presliding regimes to provide continuous and accurate information of the distal force/torque under the absence of traditional sensors; and (ii) development of novel control schemes including direct inverse nonlinear models-based feedforward compensator under the presence of no position feedback during the compensation and nonlinear and adaptive control schemes with disturbances and uncertainties under the presence of output position feedback. The comparisons between the proposed model approaches and real-time experimental data demonstrate the accuracy and fidelity in predicting the distal force/torque and significantly enhancing the position tracking performances. The new structures of the proposed models allow for easy calculation, bringing more valuable practicability and providing accurate continuous force information to solve the haptic feedback problems for safe surgery and potentially analysis of a large set of possible surgical designs.