| Compared with traditional welding methods,friction stir welding has a relatively low heat input,the temperature in the welding process is lower than the melting point of the material,the material does not melt,which can avoid many defects in the fusion welding,and the residual stress and deformation of the welding seam are relatively small.The weld quality of friction stir welding is relatively high,and the strength of various materials after welding can reach more than 80% of the strength of the base metal.At present,friction stir welding is widely used in aerospace,automobile,shipbuilding,railway and other manufacturing fields,and in many fields such as weapons industry,construction,home appliances and other fields have shown great application potential.The robot has the advantages of high degree of freedom,strong flexibility and large working space in the industrial application,which has great potential in the application of friction stir welding.However,compared with parallel robots or gantry friction stir welders,the commonly used industrial robots have several disadvantages because of their open-chain series structure.The transmission errors of each joint of the robot lead to the accumulation of errors at the operation end,and the accuracy is reduced.The stiffness of the robot series structure is weak,and when the operating ends are subjected to welding force,torsion and bending deformation will occur,which will affect the accuracy of the end pose and cause structural vibration.In the process of applying friction stir welding combined with robot to industrial application,it is urgent to solve the deformation problem of robot in the welding process,and at the same time,it is necessary to consider the structural vibration caused by the process characteristics.In this paper,starting with the stiffness model of a 6-DOF industrial robot,the load deformation and machining vibration of the friction stir welding robot during welding are studied.Firstly,the research status of heavy-duty robot and robot friction stir welding at home and abroad was investigated,and the stiffness model and stiffness evaluation index of the robot were analyzed.Secondly,it introduces the equipment related to the project,including robot,positioner,motorized spindle and some sensing devices.The forward and inverse kinematics solutions of the robots used in the project were solved,the cooperative motion trajectory planning scheme of the robot-locator was analyzed,and the Jacobian matrix of the robot was calculated,which was used as the basis for establishing the stiffness model of the robot.Then,the stiffness matrix of the robot joint was established according to the stiffness conversion method and the finite element analysis method,and the Cartesian stiffness matrix of the robot was solved by establishing the mapping relationship.In the basic technological experiment of robot friction stir welding,obvious axial deformation and lateral deformation were found,which resulted in insufficient pressure and lateral deviation of welding seam.Use the stiffness model of the force-line displacement stiffness matrix was carried out on the deformation of the friction stir welding robot error prediction and compensation of six dimensional force sensor to measure different thickness of the aluminum alloy plate in the process of friction stir welding stress,the stress during welding and robot posture the stiffness matrix of the deformation forecast robot,Thus,the compensation process of robot deformation error in friction stir welding can be guided.Finally,the experimental data were collected and analyzed for the vibration of robot friction stir welding process.Based on the stiffness model of the robot,the stiffness ellipsoid of the operating end was established to analyze the directional stiffness of the robot in different directions,and then the stiffness evaluation index was determined.The six-dimensional force data collected by the force sensor were compared with different loading conditions on the robot machining path,and the relationship between the directional stiffness and vibration of the robot was analyzed.The acceleration sensor was used to measure the vibration data of the robot in each direction during friction stir welding,and the data processing software was compared.Combined with the analysis of the robot’s directional stiffness,the relationship between the vibration and the directional stiffness in the robot’s friction stir welding process was found out.Finally,based on the analysis and summary of the problems encountered in the friction stir welding experiment,a robot friction stir welding process guidance scheme based on directional stiffness was proposed. |
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