| With the advantages of less trauma,short recovery time and without cutaneous incision,natural orifice transluminal endoscopic surgery has become the main development trend of minimal invasive endoscopic surgery.During the operation,the tools delivered into the body should be controlled from outside to perform the necessary operations,which limits the application of traditional tools with slender,rigid and straight structure of traditional machinery and demands higher requirements for the flexibility of instruments.Tendon-sheath mechanism with the advantages of small size,simple structure,adaptable to complex path and transmission for long distance of force and displacement is commonly applied in flexible endoscopic system.However,the tension attenuation and motion hysteresis caused by nonlinear friction between tendon and sheath seriously affect the motion speed and operation accuracy of end effectors which place challenges in the development of surgical robots.What’s more,due to the strong requirement of small size and biological compatibility,there is great limitations for integration of high-precision sensors which can provide information for feedback controller.As a result,more difficulties are added for precise control.According to the existing researches,the feedforward controller based on model can compensate hysteresis without providing any feedback information,which is a suitable control strategy for natural orifice transluminal endoscopic surgery robot.Moreover,feedforward strategy combined with feedback strategy can improve robustness of the system to a new level if the feedback information is available in some conditions.This paper aims to study the tension and motion transmission of tendon-sheath mechanism and realize the motion hysteresis compensation without any feedback information.The study toward above targets is carried out as follows:Firstly,the motion phases of the tendon-sheath mechanism are analyzed.Based on Coulomb friction law,the theoretical formulas of tension transmission,elongation and displacement transmission of the tendon-sheath mechanism with arbitrary and constant curvature configuration in sliding stage are analyzed,which provides guidance for hysteresis modeling.Secondly,a hysteresis model to capture the motion transmission of tendon-sheath mechanism is established based on classical Bouc-Wen model.And the genetic algorithm is applied to identified the parameters of classical Bouc-Wen model.Because of the limitation when Bouc-Wen model is directly used to capture the motion transmission of tendon-sheath mechanism,a modified Bouc-Wen model is proposed based on the analysis of motion characteristics of tendon-sheath mechanism,and the parameter identification method is designed for the modified model.The simulation results show that compared with the classical Bouc-Wen model,the modified model with higher accuracy and parameter identification efficiency,performs better in modeling the motion hysteresis of tendon-sheath mechanism.Then,the feedforward compensation method based on the direct inverse model is designed for the classical Bouc-Wen model and the modified model.Simulation is carried out in Simulink to validate the effect of feedforward compensation.Finally,an experimental platform of a single tendon-sheath system is established.And experiments are designed from following two aspects.(1)The theoretical formula of force transmission and elongation based on Coulomb friction law are validated under constant curvature configuration in sliding stage with low speed applied;(2)Modeling accuracy of two hysteresis models and the effect of direct inverse model-based feedforward compensator for the motion transmission hysteresis are validated under arbitrary fixed configuration. |
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