| Industrial robot,due to its intelligent,high flexibility and low-cost,is considered as a promising option to accomplish the drilling operation in aircraft assembly.However,limited by its serial structure,the stiffness of industry robot is less than 1/50~1/20 of the stiffness compared to CNC machine tools.In the field of precise machining application,the properties of robot stiffness will lead to machining trajectory error and quality defects.In aircraft assembly,component manufacture is high value added and time-consuming.Robot drilling quality and stability is significantly important to component performance and productive efficiency.In this paper,robot drilling system is taken as the main research object.The mechanism of robot stiffness properties to drilling process is analysied intensively,which will provide theortical foundation to the method of improving robot machining performance.The specific research content has:(1)The stiffness model of robotic drilling system is established,and the prodiction of robot deformation under drilling force is realized.By analyzing the force balance of drilling end effector and robot joints,the mapping relationship of joint stiffness matrix and Cartensian stiffness matrix is established.Throuth static loading test,robot joint stiffness matrix is identified experimentally under the drilling working state.By calculating the Cartensian stiffness matrix,robot displacement is predicted.(2)A drilling task-oriented robot stiffness model is proposed.And by analyzing the distribution character of machining force,the the robot stiffness in the certain direction is evaluated quanititatively.By analyzing the difference between robotself properties and the needed machining performance,a stiffness model in the certain direction is proposed.Robot machining performance in the drilling force directions is evaluated quantitatively.The stiffness changing and developing tendency is studied in different robot postures,and the principle of robot Cartensian stiffness is achieved.(3)It is found that drilling quality changes with robot machining posture regularly.By analyzing the developing trend of cutting force and robot loading state,the mechanism and influences of robot stiffness properties on drilling process is revealed.By analyzing the three directional coupled character of robot displacement,the springback of robot is found when the drilling force is unload.It reveales that the mechanism of robot stiffness properties to drilling process is leading to the path deviation of feeding drill,and hole diameter accuracy and quality is influenced.It also reveals that the substantial founction of pressing force is to enhance robot stiffness.It is found that the capacity of robot to drilling thrust force is various under different robot postures,and hence countersink depth error is caused.By optimizing drilling posture and improving axial stiffness,the accuracy of countersink depth will be highly improved.(4)The equivalent stiffness model under pressing force is established.By analyzing the robot loading state character under pressing force,the effectiveness of pressing force is evaluated quantitatively.It will provide theoretical foundation to pressing force optimization,robot performance evaluation and processing parameter optimization.Through analyzing the applied force on the robot,the equivalent stiffness model under pressing force is established and the enhancement of robot stiffness could be calculated quantitatively.By defining stiffness promotion factor,the effectiveness of pressing force is evaluated quantitatively.The friction between pressure foot and workpiece will suppress the deformation of robot,and matching criterion between robot drilling posture and processing parameters is proposed by further analysis of the friction.Robot stable machining condition is calculated,which will lead to realize the optimization of pressing force,evaluating the limitation of robot process performance and processing parameters optimization.(5)Coupled with robot stiffnes and kinematics properties,an integrated robot drilling posture optimization method is proposed.The stiffness,manipulability and joint limit are all taken into consideration.Based on the function redundancy of robot,a robotic posture optimized method is proposed.By choosing the optimized drilling posture robot machining performance is improved.The method and conclusion proposed in this paper has significant importance in robotic drilling application.By using the robot stiffness promotion method,drilling hole diameter accuracy and machining stablility are highly improved.The drilling quality for aircraft assembly application could be fulfilled. |
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