Research On Key Performance Of An Industrial Robot Based On A Manipulator With Hybrid Open-and Closed-Loop Kinematic Chains

With the rapid development of the robot technology,industrial robots have been gradually used in the fields of drilling,milling,grinding and friction stir welding(FSW)because of their advantages of large workspace,compact structure,high flexibility and low cost,instead of material handling,spot welding,arc welding,coating,sealing,stamping,die casting and other simple operations.Industrial robots can partially replace manual operations which greatly improve the machining accuracy and production efficiency.In the fields of aviation,aerospace,shipbuilding and high-speed rail,a large number of drilling,milling and boring processes need to be carried out during assembly and the assemble parts usually have large size,complex structure and equipment.However,traditional multi-axis NC machining centers cover larger area;have low flexibility and accessibility which makes them unable to process such large complex structures.However,there still have some problems to be solve for traditional industrial robots in practical applications:(1)The driving parts of traditional industrial robots are composed of servo motors and reducers at the rotary joint,and the compliance of reducers makes the overall stiffness of the open-kinematic chain manipulator worse,which results in the deviation of the end-effector from the desired trajectory in heavy load cutting,and affects the machining accuracy of the robot directly;(2)The backlash of gear reducers affects the positioning accuracy of the robot,and it also causes vibration due to rapid change of external loads.Eliminating the backlash is an important way to improve the accuracy of robots;(3)The traditional industrial robot is a series structure with a big arm and a small,and the equivalent external moment of the robot is fully shared by servo motors which increases the driving power of the robotic arm.In order to solve the main problems of traditional industrial robots in machining process,a novel five degree of freedom(DOF)non-backlash industrial robot based on the manipulator with hybrid open-and closed-loop kinematic chains is designed according to the actual need of robotic machining.And the key performance of the robot are deeply studied,including energy consumption characteristics of the robotic arms,the global static stiffness of the robot,the stiffness weight ratio of the manipulator and anti-backlash characteristics of the dual-motor drive robotic joint,etc.The analytical and semi-analytical stiffness modeling methods for the manipulator with hybrid open-and closed-loop kinematic chains,and the anti-backlash control method based on switching bias torque for the dual-motor driving system are proposed.The structure optimization of the manipulator based on the best stiffness mass ratio is also presented.The proposed theory and methods are verified by the simulation,the experimental and the comprehensive analysis results.The research results provide theoretical basis for establishing new machining system and extending the means and the ways of robotic machining.The main contents of the dissertation are shown as follows:(1)The background and significance of the research are discussed,the important role and the development status of the robots that used for industrial applications are introduced;in addition,the methods of improving machining accuracy,stiffness modeling of the robot,energy consumption of robotic arms,robotic structural parameters optimal design and backlash elimination methods are summarized.(2)In view of the weak stiffness of traditional industrial robots,and the effect of joint backlash on robot precision,a five-DOF non-backlash industrial robot based on the manipulator with hybrid open-and closed-loop chains is designed.Different from the open-chain manipulator of traditional industrial robots,a parallelogram frame structure with diagonal driven using an electric cylinder is adopted in both big and small arms,that is,the closed chain is introduced in the part of the open chain manipulator to increase the robot stiffness.Double screw nuts with preload are used in ball screws of electric cylinders in the robotic big and small arms to eliminate the reverse backlash.In addition,dual-motor drive is used for each DOF of the waist and the wrist joint in order to implement anti-backlash control method to eliminate the transmission backlash.(3)In order to compare the driving power and energy consumption of the robotic arm with hybrid open-and closed-loop kinematic chains and the traditional industrial robotic arm,the driving power and energy consumption models of the two kinds robotic arms are established,respectively.Through the theoretical calculation results,the simulation results and the experimental results,it is shown that compared with the traditional open-chain industrial manipulator,the hybrid open-and closed-chain manipulator based on the parallelogram framework that driven in the diagonal direction can reduce the power and energy consumption of the external load of the robot effectively.(4)In order to investigate the static stiffness performance of the proposed robot in global workspace,analytical and semi-analytical methods for calculating the global static stiffness according to strain energy method and Castigliano's theorem are proposed,combining with the structural parameters and equivalent stiffness of robotic arms,based on the manipulator with hybrid open-and closed-loop chains,.The calculation accuracy and efficiency are both taken into account.The effectiveness of the methods is verified through the simulation results of the finite element analysis(FEA)and the experimental results of the robot principle prototype,and the global stiffness performance is also evaluated.(5)Aiming at the two important performance indexes of robot stiffness and mass,the robot structural parameters are optimized using the nonlinear programming genetic algorithm(NPGA),which takes the stiffness and mass ratio of the manipulator as the objective function,and takes the size parameters of the manipulator,the stiffness matching conditions and the end stiffness as the constraint conditions.By changing the constraint conditions,the structural parameters of the manipulator with different stiffness and mass can be obtained,so that the manipulator can meet the operating performance in the specified application.The results provide theoretical basis for the parameter optimization of the robotic structure.(6)In view of the anti-backlash performance of robotic joints driven by dual-motor in this paper,a switching bias torque anti-backlash control method according to the motor currents is proposed based on the analysis of the shortcomings of typical anti-backlash control methods.Depending on the motor currents and the switching function,a bias torque is output dynamically to realize the transformation from anti-backlash control to torque synchronization control.Meanwhile,the method of selecting the current setting point in the switching function is also presented which provides a theoretical basis for the practical engineering application.The anti-backlash performance of the robotic joint driven by dual-motor is verified by the simulation and the experimental results of the dual-motor anti-backlash driving system.(7)In order to investigate the machining performance of the proposed manipulator with hybrid open-and closed-loop kinematic chains,the feasibility of the proposed robot for machining is preliminary verified by the drilling experimental results using the robot principle prototype and the prediction results of the drilling performance using the full-scale industrial robot.Finally,the contents of the dissertation are summarized,and the contents of further research are analyzed and pointed out.