Research On A Variable-stiffness Continuum Manipulator For Minimally Invasive Surgery

With the rapid development and wide application of robotic technology,the traditional minimally invasive surgery is developing towards a more minimally invasive,automated and intelligent direction.The robot-assisted minimally invasive surgery emerges as the times require.The traditional minimally invasive surgical robot can show good flexibility in structure.However,it may also bring negative effects such as poor stiffness and low strength.If a minimally invasive surgery robot has the ability of variable stiffness,it means that it can more safely navigate and intervene in the human body,naturally reducing the difficulty of minimally invasive surgery.In order to solve the problem of lack of variable stiffness in robots for minimally invasive surgery,a variable-stiffness continuous robot for minimally invasive surgery is designed in this paper.Its bending element can realize continuous adjustment of stiffness,and it also has the basic motion ability of translation,rotation and bending.In this paper,the implementation process of traditional minimally invasive surgery and the natural environment characteristics of human aortic vessels are analyzed.On the basis of this,the basic design requirements of minimally invasive surgery are put forward.The basic structure of the bending element(CDBM-0)and the mechanical structure of the driving system of the robot are designed.At the same time,the overall layout and planning of the robot system are studied.Then,through studying the transformation mechanism of shape memory alloy,the variable-stiffness method of unidirectional bending element is explored.Structural design and theoretical modeling of the variable-stiffness device are studied,including SMA elastic modulus model,thermoelectric model and mechanical model derivation,and its variable-stiffness capability is preliminarily verified by finite element simulation.After completing the memory shape setting process of SMA wire,the unidirectional variable-stiffness bending element(CDVSBM-1)is assembled and its variable-stiffness capability was verified via experiments.After that,considering the limitation of variable-stiffness method in unidirectional variable-stiffness bending element(CDVSBM-1),the variable-stiffness method of omnidirectional bending element is further studied.A variable-stiffness shape memory alloy sheath is designed,and the thermo-electrical model of the sheath is simulated and analyzed.The variable-stiffness ability of the sheath is preliminarily verified by finite element simulation.Then the omnidirectional variable-stiffness bending element(CDVSBM-2)is fabricated and its variable-stiffness capability is verified by experiments.Finally,the kinematics model of the robot with several bending elements(CDVSBM-2)in series is established,and the mapping relations among driving space,joint space and operation space are derived.At the same time,the reachable workspace under the cooperation of single bending element,two bending elements in series,three bending elements in series and linear push element is simulated and analyzed.The master-slave control method of the robot is studied,and the master-slave mapping relationship under joint control motion is deduced.In addition,the overall structure of the robot motion system is completed,including the establishment of prototype system and the development of software interface.The kinematics modeling and master-slave control method are verified by the joint control motion experiment and the point control motion experiment of the robot bending element.