Research On Thermal Error Modeling And Compensating For Heavy-duty CNC Machine Tools

The heavy-duty CNC machine tool is the key equipment of manufacturing,which is widely used to machine large parts in many fields,like chemical industry,ship,aerospace and military.With the development and progress of global industrialization,the manufacturing industry has an increasing demand for the machining accuracy of the heavy-duty CNC machine tool.Many researches show that thermal error accounts for 40%-70% of all error sources that affect machining accuracy of the heavy-duty CNC machine tool,so reducing thermal error can significantly improve machining accuracy of the heavy-duty CNC machine tool,and then improve the manufacturing industry level.Thermal error compensation has become one of important ways to reduce thermal error of the heavy-duty CNC machine tool because of its low cost and effectiveness.Many scholars at home and abroad have carried out a lot of research work on it and achieved rich results,but the relevant research work has not considered characteristics of the heavy-duty CNC machine tool,such as huge volume,wide distribution of heat sources and using for long-term continuous machining,to form a complete theoretical system and implementation scheme of thermal error compensation.Taking the ZK5540 A heavy-duty CNC machine tool as the research object and combining its structure and application features,this thesis carries out theoretical research and propose relevant solutions about thermal error modeling and compensating for the heavy-duty CNC machine tool through focusing on key technologies,such as thermal error mechanism,temperature field monitoring,key temperature measurement points selection,thermal error prediction modeling,thermal error compensation method and strategy.The main research contents and achievements include:(1)Analysis on thermal characteristics of the heavy-duty CNC machine tool and new method of temperature filed monitoring based on fiber Bragg grating(FBG).Cosindering the mechanical structure features of the ZK5540 A heavy-duty CNC machine tool,distribution and influence of its internal and external heat sources are analyzed in detail.Mechanism analysis and finite element simulation of temperature field and thermal deformation are conducted for the machine tool.Aiming at the problem that electrical temperature sensors are not proper to be applied to monitor temperature field of the heavy-duty CNC machine tool,a novel temperature field monitoring method based on FBG sensoring is put forward and a kind of substrate FBG temperature sensor is developed.128 FBG temperature sensors are installed on the main parts of the heavy-duty CNC machine tool to monitor its temperature field comprehensively according to thermal analysis results.Through thermal error experiments under the simulated machining state,temperature field,thermal error and working state parameters of the heavy-duty CNC machine tool are monitored synchronously and sufficient experimental data are obtained.(2)Research on temperature measurement points optimization of the heavy-duty CNC machine tool.Aiming at the problem of large-scale temperature measurement points(TMPs)optimization for the heavy-duty CNC machine tool,comman used TMPs optimization methods are classified and analyzed.A similarity calculation method for TMPs based on dynamic time swapping algorithm is put forward to avoid the phenomenon that the similarity calculation is inaccurate due to the phase difference between TMPs caused by the time-consuming of heat conduction.The density peak clustering algorithm is used to classify 128 TMPs fast and clustering centers which can best represent the temperature field information of the machine tool are selected as the key TMPs.(3)Research on thermal error modeling of the heavy-duty CNC machine tool.Initial weights and thresholds of back propagation(BP)neural network algorithm are optimized through cuckoo search(CS)optimization algorithm,and then a CSBP thermal error prediction model is established to prodicet current thermal error using current temperature data from key TMPs of the heavy-duty CNC machine tool.In order to resolve the problem that the predicted thermal error lags behind the actual one when thermal error is compensated during machining process of the heavy-duty CNC machine tool,a forward thermal error prediction model based on long-short term memory neural network algorithm is proposed,which could predict future thermal error with historical and current temperature data from key TMPs.This model effectively improves the accuracy of thermal error compensation for the heavy-duty CNC machine tool.(4)Study on thermal error compensation methods and strategies for the heavy-duty CNC machine tool.In view of the generality of the thermal error compensation method for the CNC machine tool,the interaction between thermal error compensation information and CNC system during the thermal error compensation implement for CNC machine tools,various access ways of thermal error information are analyzed and compared.A kind of thermal error compensation method through embedding parameters into numerical control(NC)code is put forward considering actual machining conditions of the heavy-duty CNC machine tool.Typical machining process of the heavy-duty CNC machine tool are studied,and thermal error compensation strategy guided by machining process knowledge is given to determine proper compensation time for realizing smooth compensation and reducing the secondary error caused by thermal error real-time compensation in the machining process.(5)Design and development of thermal error compensation system based on cyber physical system.Aiming at the limitation of the thermal error compensation controller based on embedded system,a new realization method of the thermal error compensation system for the heavy-duty CNC machine tool based on the cyber physical system(CPS)is proposed.The cyber physical machine tool system(CPMTS)is proposed.Its functions and hierarchical structure are defined.The thermal error compensation system based on CPMTS is designed and developed through being divided into compensation controller and service cloud platform.Its feasibility and effectiveness are verified via thermal error compensation comparative experiments during plane machining processes of the ZK5540 A heavy-duty CNC machine tool.