| Sheet metal part,having the characteristics of light weight,high strength and strong bearing capacity,is widely used in aerospace,automobile,shipbuilding and other manufacturing industries.However,sheet metal part has poor stiffness and large deformation under the action of self-weight and processing load.Therefore,controlling the deformation of sheet metal part plays a key role in ensuring its processing accuracy.As one of the four major process equipment,fixture can directly affect the processing quality of sheet metal part.Excessive deformation of sheet metal part is often caused by unreasonable fixture positioning layout design.In order to control the deformation of sheet metal part after drilling and meet the requirement of machining accuracy,the multi-objective optimization design method of drilling fixture positioning layout for sheet metal part is studied in this paper,in the hope of providing reference and guidance for positioning layout planning of sheet metal part drilling fixture.The specific research content is as follows:(1)An intelligent reconfigurable drilling fixture for sheet metal part is designed.The drilling process,positioning and clamping mode and drilling fixture function of sheet metal part are analyzed.The intelligent reconfigurable drilling fixture is divided into three parts:perception control module,analysis and decision module and fixture mechanical body.Based on the analysis of functional requirement,the functional framework of intelligent reconfigurable drilling fixture for sheet metal part is designed,including perception-control function,analysis-decision-making function and reconfigurable positioning-clamping function.Based on the design of the fixture function frame,the overall scheme design,mechanical body design and key parts selection of intelligent reconfigurable drilling fixture for sheet metal part are completed.The designed drilling fixture can sense the actual position of the positioning element and the clamping force,adapt to different sizes and specifications of sheet metal part clamping within a certain range,and adjust to the optimal fixture positioning layout for different holes of the same sheet metal part.The designed fixture scheme lays the foundation for the subsequent simulation modeling and multi-objective optimization design of the positioning layout of the drilling fixture.(2)A simulation model of the positioning layout of the sheet metal part drilling fixture based on the fractal contact theory is established.The semi-elastic contact model is used to approximately replace the contact between the positioning element and the sheet metal part,and the contact stiffness and damping characteristics of contact surface are simulated by the normal spring damping unit.The functional relationships between the normal contact force and the contact stiffness and damping are obtained by using the fractal contact theory,the normal contact force at the location point is gained by finite element simulation,and the contact stiffness and damping at the location point are solved.The simulation model of the location layout of the drilling fixture for sheet metal part is established based on the fractal contact theory and the sheet metal part deformation after drilling is predicted.The feasibility and effectiveness of the modeling method of fixture positioning layout based on fractal contact theory are verified by two examples of flat plate and curved plate.The results of case analysis show that the accuracy of deformation prediction of sheet metal part is improved by the simulation model of positioning layout.(3)A multi-objective optimization method of drilling fixture locating layout based on back propagation neural network(BPNN)and particle swarm optimization(PSO)is proposed.The maximum deformation and the overall deformation of the sheet metal part after drilling under different fixture locating layouts are obtained by the positioning layout simulation model based on the fractal contact theory.The simulation results are taken as training samples to establish a multi-objective neural network prediction model between the locating layout and the maximum and overall deformation of the sheet metal part.On this basis,particle swarm optimization algorithm is used to obtain the Pareto optimal solution set of fixture positioning layout when drilling a single hole.The multi-objective optimization method of fixture positioning layout based on BPNN and PSO is used to analyze two examples of flat plate and curved plate.The optimization results show that the proposed method can effectively reduce the maximum deformation and overall deformation of flat and curved plates,the feasibility and effectiveness of the method is verified. |
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