Research On Accuracy Retention Evaluation And Maintenance Strategy Of CNC Machine Tools Considering Degradation Trajectory Fluctuation

CNC machine tools as equipment manufacturing industry,"industrial mother machine",its accuracy is to maintain the competitiveness of the important factors affecting the market.Accuracy retention evaluation is an important part of accuracy retention technology.Optimizing the maintenance strategy through the results of accuracy retention evaluation can slow down the trend of accuracy degradation under the premise of cost control,which is of great significance for promoting the development of domestic CNC machine tool industry.The whole machine accuracy maintainability of CNC machine tools is jointly determined by multiple accuracy degradation modes of each sub-system.At present,the accuracy degradation modes of each sub-system and their influence on the whole machine accuracy are still unclear,and it is difficult to extract the key degradation indexes that can effectively reflect the accuracy maintainability of the whole machine due to the complexity of the structure of the CNC machine tools.In addition,the current accuracy degradation model is limited by the number of sample points and ignores the influence of the degree of degradation trajectory fluctuation on the accuracy retention and the influence of the degree of degradation trajectory fluctuation on the transfer probability between different accuracy degradation states,which may result in over-maintenance of slight degradation states and under-maintenance of heavy degradation states in the production process.This paper firstly analyses the accuracy degradation mode and influence of CNC machine tools in service stage,extracts the key degradation indexes that can reflect the accuracy retention of the whole machine,and then reflects the accuracy retention of the machine tool by quantitatively evaluating the degree of fluctuation of the degradation trajectory.At the same time,the relationship between the degree of fluctuation of the degraded trajectory and multiple accuracy degradation states is established,and the maintenance strategy in the production process is optimized based on the accuracy degradation states to guarantee the accuracy retention of the CNC machine tool.The main contents of this paper are as follows:(1)Aiming at the problem that there are many indexes affecting the degradation of machine tool accuracy and they cannot reflect the accuracy retention of the whole machine,the key degradation indexes that can reflect the accuracy retention of the whole machine are extracted.By analyzing the accuracy degradation mode of each sub-system during the service stage of CNC machine tools,the causes of degradation and their influence on the whole machine,a 4-layer degradation index collection including key sub-systems,accuracy degradation indexes,accuracy degradation modes and detectable error items is established.Grey correlation analysis was used to calculate the correlation weights between each layer of metrics and the accuracy retention of the whole machine,and to determine the main error terms in the underlying set of metrics.Based on the multibody system theory fusion of multiple errors,the offset distance between the spatial error point and the error origin within the time series is calculated,and the spatial position accuracy is extracted as an effective key degradation index.(2)Aiming at the problem of insufficient data on accuracy degradation of CNC machine tools and the lack of evaluation methods for the degree of fluctuation of degradation trajectories,a method for evaluating the accuracy retention of CNC machine tools that takes into account the influence of the degree of fluctuation of degradation trajectories is proposed.The spatial position accuracy obtained based on multi-body system theory is used as the degradation modelling data,and the degradation data is expanded using radial basis function network to construct the degradation trajectory model.According to the number of radial basis function interpolation points,the degradation trajectory is divided into multiple tiny units,and the average degradation rate and the degree of accuracy fluctuation of CNC machine tools are quantified by calculating the mean value of the slope and the mean value of the area of the units in the consecutive time range,so as to realize quantitative assessment of accuracy retention.Finally,a sensitivity analysis of the two evaluated values was performed to quantitatively represent the degree of influence of the degradation of each axial accuracy index on the accuracy retention of the whole machine.(3)Aiming at the current accuracy degradation type faults only considering the degree of accuracy degradation,and ignoring the influence of the degree of degradation trajectory fluctuation on the transfer probability between different states in the assessment of accuracy degradation state,a method of evaluating the accuracy degradation state of CNC machine tools considering the influence of the degree of degradation trajectory fluctuation is proposed.The amount of accuracy degradation and the evaluation value of the degree of accuracy fluctuation are taken as the observed values of the degradation amount,and a Hidden Markov Model is established to correlate the observed values of the degradation amount with the degraded state of accuracy.Considering the influence of the average degradation rate of accuracy on the state transfer probability matrix,an improved Hidden Markov Model for multiple samples and multiple sequences of CNC machine tools is proposed to obtain the transfer probabilities of different accuracy degradation states of the machine tools and to calculate the trend of the reliability change of the accuracy based on the probability of the failure state,and to determine the complete service time under the natural degradation of the machine tools.(4)Aiming at the complete service process of CNC machine tools,considering the influence of different accuracy degradation states on the maintenance and inspection intervals of CNC machine tools and the probability of producing non-conforming products in the machining process respectively,a model of CNC machine tool maintenance strategy based on accuracy degradation states is proposed.Comprehensive cost constraint function and complete service time to determine the optimal maintenance inspection interval in the service process,and based on the Poisson process to determine the probability of nonconformity generation under the influence of different accuracy degradation states,so as to formulate the collection of maintenance methods considering the degree of maintenance,and based on the Markov decision-making process to establish the model of maintenance strategy within the complete service process,and to derive the value function under the influence of different accuracy degradation states.The optimal maintenance strategy in the service process of CNC machine tools is determined through the value iteration algorithm.