Design And Research Of The Stiffness-adjustable Cable-driven Snake-Arm Robot

Cable-driven snake-arm robot has been widely used in many fields such as the maintenance and detection in restricted environments,and minimally invasive surgery since its compact structure,multiple degrees of freedom(DOF)and flexible motion.However,for the common cable-driven snake-arm robot,the cables often are prone to loosen when the robot is subjected to external force,which will resulte in uncontrollable configuration.This is known as passive compliance.To avoid passive compliance,this paper firstly summarizes and analyzes the typical joint types and its structure characteristics.Secondly,this paper considers the typical joints applied to the cable-driven snake-arm robots as examples to analysis detailedly the characteristics of passive compliant mechanism,and puts forward a passive compliance index by studying the concrete influence between the joint structural parameters and the cable length changes.The requirements of joint parameters that can avoid passive compliance are obtained.Inspired by Jensen inequations,a method is proposed to avoid passive compliance by judging the property of cable length function as lower convex function.The requirements of structural parameters for avoiding passive compliance of typical joints are verified.For the fixed-pivot joints,the passive compliance can be avoided by adjusting the position of joint axis.In addition,by adjusting the axis position,the 2-DOF decoupling motion can be also realized to simplify robot kinematics mapping relation between the actuator space and joint space.The design difficulty of the robot’s driving system is decreased.For spherical/cylinder rolling joints,varying the relationship between the rolling radius and the initial height in a single joint can be implemented to avoid passive compliance.For flexible joints,the passive compliance can be avoided by adjusting the constraint length of the cable hole in the joint.Based on the contra-parelogram mechanism,this paper proposes a cable-driven adjustablestiffness snake-arm robot.On this basis,the kinematics of the cable-driven snake-arm robot is established according to the D-H method,and the configuration posture and motion space of the robot are obtained.Considering the security in the robotic work,the static model of the robotic distal segment is established,and the configuration change and displacement at the end are predicted based on the principle of minimum potential energy,when the end of the distal segment is subjected to external lateral forces.Based on virtual work principle,the adjustablestiffness model of the robotic proximal segment is verified by changing cables tension.Finally,the cable-driven snake-arm robot prototype is built to test its multiple-DOFs bending motion performance and verify the rationality of the static and stiffness models by load experiments.