Modeling And Control Of Robotic Drilling End-effector Based On Stewart Platform

Robotic drilling end-effector is the key component of a robotic drilling system.Its main functions are measuring and calculating the outward normal vector of the curved surface,and drilling holes after adjusting its own posture according with the normal vector.The robotic drilling system is more flexible,modular and low cost compared with the conventional large and customized automated drilling equipment.Therefore,the robotic drilling end-effector for curved surface has a promising application prospect in the field of aircraft fabrication and assembly.In this dissertation,a kind of parallel robotic drilling end-effector is designed based on Stewart platform under the support of National Natural Science Foundation of China in order to satisfy the mass demand of accurately drilling in aircraft manufacturing and assembly.The parallel end-effector has larger stiffness and dynamic position accuracy compared with series end-effector.Based on the structure design,a method of measuring and calculating the normal vector of the curved surface is proposed and kinematics analysis of normal measurement unit is carried out;And a control strategy is developed based on the dynamic analysis;The stiffness calculation method of the end-effector is proposed and calculated,which provides a theoretical basis for the vibration control of the end actuator.The main contents of this dissertation are as follows:(1)A kind of parallel robotic drilling end-effector is designed based on Stewart platform,which is comprised of drilling unit,normal measuring and calculating unit,normal adjusting unit and feed unit.And On this basis,the degree of freedom of the end-effector is calculated.(2)On the basis of structural design,the calculation method of normal vector outside the surface is proposed,and the kinematics analysis of normal adjustment unit is carried out.Simulation results show that the accuracy of this method is superior to the three-point plane method.(3)The dynamic equation of normal adjustment unit of automatic drilling end effector is established by combining Lagrange method with Newton-Euler method,and the dynamic characteristics of normal adjustment unit are analyzed.The Sim Mechanics models of normal adjustment unit and drilling unit are established to provide the basic model for its dynamic control.(4)Based on the dynamic analysis of the normal adjustment unit,a control strategy of sliding mode variable structure control is proposed.Simulation results show that the control performance of this control strategy is superior to that of the PID control.(5)The stiffness of the end-effector is calculated to provide the theorical basis for the vibration control.