Research On Modeling And Control Of Flexible Endoscopic Operation Robots For NOTES

Natural orifice transluminal endoscopic surgery(NOTES)is a typical minimally invasive surgery,which has been used in more and more surgical operations due to its inherent advantages.Flexible endoscopic operation technology is the basis for realizing NOTES.Although the flexible endoscope has the advantages of high safety,wide reachable range,and flexible operation,its promotion and application are limited due to the difficulty of manual operation,long learning cycle,high labor cost,requirements for extensive clinical experience,and harmful radiation damage during the procedure.It is expected to achieve much more safe,accurate,and intelligent NOTES procedure,by using flexible endoscopic robots to replace manual endoscopic operation.Up to now,for the research on flexible endoscopic operation robots,increasing works are focusing on the modeling and control methods of the tendon-sheath artificial muscle(TSAM)system in the flexible endoscope,and the path/trajectory planning strategy for the flexible endoscope’s tip-part.However,from the perspective of practical application,there are still some open problems unsolved: 1)The previous works do not fully consider the hysteresis,unknown dynamics,and other nonlinear characteristics of the tendon-driven with bending tip system,such as the flexible endoscope.These uncertainties pose great challenges to the modeling and control of the flexible endoscope bending;2)The modeling and control of the flexible endoscope twisting have barely been studied.Particularly,continuous unknown disturbances such as input saturation,time-varying parameters,discontinuous friction,unknown torsional strain,etc.,cause great difficulties for the precise control of the flexible endoscope.Besides,the traditional robust control method may lead to overshoot,chattering and even instability of the robotic system;3)There are complex coupling effects between the bending and twisting motions of the flexible endoscope’s tip-part,so the position control of the flexible endoscope in a 3-D workspace cannot be achieved by controlling the bending and twisting angle separately.Therefore,the trajectory planning strategy in the configuration space is expected for the multi-motions of the flexible endoscope.To conquer the above challenges,the modeling,control,and trajectory planning issues of the flexible endoscope’s motions are studied in depth.The main contributions of this thesis are listed as follows:1)Modelling of the bending and twisting motion of the flexible endoscope.In this thesis,a hysteresis phenomenon model is first established to describe the rela-tionship between the pulling wire distance and the bending angle.Then,an active modelling method that estimates the model error in the real-time is proposed to improve the model accuracy.Next,a dynamics model is built to describe the rela-tionship between the tendon force and the bending angle of the flexible endoscope by utilizing the Lagrange’s method.Furthermore,a novel three-element model is proposed to describe the dynamic characteristics of the flexible endoscope’s twisting motion based on the force analysis.This equivalent model can also ac-count for the hysteresis in the twisting motion.Next,by combining the rigid joint dynamics with the above-mentioned three-element equivalent dynamics model,the rigid-flexible hybrid dynamics model of the twisting motion is obtained.The mechanism relationship of the motor input torque and the distal twisting angle is also described by this hybrid dynamics model.In addition,a Koopman-based data-driven modelling strategy is suggested to investigate the data relationship between the input/output twisting angles.2)Controller design for the flexible endoscope bending.A novel active hysteresis observer is proposed based on the extended set membership filter to deal with the modeling deviation resulted from the asymmetric and non-uniform hysteresis loop in the bending motion.The active hysteresis observer can simultaneously estimate both the value and confidence interval of the hysteresis deviation,of-fering a dynamic boundary method to replace the constant boundary assumption in traditional robust control.Then,a practical sliding-mode-type tracking con-troller with pulling-wire speed input is proposed based on the active hysteresis observer.Moreover,the dynamics model of bending motion that contains the un-known parameters is used to design the controller with the flexible endoscope’s tendon force input.By reassembling these unknown parameters,a simplified nonlinear model is obtained and an adaptive controller is designed based on the backstepping method for flexible endoscope’s bending motion,while the reor-ganized parameters could be estimated online.Finally,the Lyapunov theory is used to prove the stability of the above two controllers,demonstrating both the convergence and boundness of the closed-loop system error are guaranteed the-oretically.The tracking performance and robustness of the proposed controllers are also validated by comparative experiments.3)Controller design for the flexible endoscope twisting.Firstly,based on the equiv-alent dynamics model,with consideration of the input angle saturation constraints,parameter uncertainties,and discontinuous friction disturbances of the TSAM twisting,a continuous nonlinear robust control strategy is proposed with a ‘shak-ing’ term.Then,based on the rigid-flexible hybrid model of ‘rigid link + flex-ible actuator’,considering the limitations of the overshoot and gain-adapting,a model-free gain-adapting control scheme is proposed by using the time-delay es-timation technique,and the barrier adaptive update law of the control gain is de-signed.Finally,focusing on the relationship between input and output data of the twisting motion,a data-driven-based control method is designed by estimating the finite-dimensional approximation error and its interval of the reference data-driven model in real time,and the safety-enhanced term is constructed from the barrier Lyapunov function.Rigorous mathematical analyses are carried out on the stability of the closed-loop system for the above-mentioned controllers.Be-sides,a series of hardware comparative experiments are conducted to verify the improved performance of the above control methods in the anti-disturbance and overshoot suppression.4)Planning-based position control for autonomous targeting.Considering the high nonlinearity,material creep,complex hysteresis and unknown coupling effects between motions,a modified hysteresis phenomenon model is proposed for the multi-motion of the flexible endoscope by combining the above-mentioned hys-teresis equivalent model with the coupling effects.Based on the proposed refer-ence model,a novel time-optimal trajectory planner with synchronous terminal is designed with the constraints of task requirements,hardware limitations and security boundaries,which can deal with the coupling effects in an intuitive separate control scheme with position input.The comparison results of hardware experiments verify that the proposed planning-based control scheme has good decoupling performance.