Research On Accelerated Life Test Technology Of Grating Ruler Based On Step Stress Loading

Grating ruler is a high-precision linear displacement measurement sensor used in the closed-loop feedback of CNC machine control system,and it is one of the key components that affect the reliability of CNC machine.However,compared with foreign well-known brands,domestic grating ruler has frequent failures and low reliability level and has become the bottleneck of industry development.Therefore,it is of great significance to carry out accelerated life test,quickly stimulate,expose the weak reliability parts and implement reliability improvement design of grating ruler for the reliability improvement of domestic grating ruler and even domestic high-grade CNC machine tools to.In this paper,the domestic high-precision closed absolute grating ruler is taken as the research object,and the accelerated life test technology of grating ruler is systematically studied from the aspects of fault analysis,test system construction,test scheme formulation and test data statistical analysis.The main contents of this paper are as follows:1.Fault analysis of the grating ruler.Based on the detailed study of the structure and working principle,the grating ruler is divided into three subsystems: mechanical system,optical system and electronic system.To solve the problem of uncertainty in the process of expert evaluation,an improved failure mode and effect analysis(FMEA)method based on interval 2-tuple linguistic variable(ITLV)and technique for order preference by similarity to ideal solution(TOPSIS)is proposed.This method is used to analyze the risk degree of each potential failure of grating scale and to determine the main stress load that affects the reliability of grating scale.2.A grating scale accelerated life test system is built.Based on the failure analysis,the sensitive stress of grating ruler is selected and the accelerated life test system of grating ruler is built.The stress loading part includes the servo driving platform for velocity stress loading and the temperature and humidity comprehensive test chamber for temperature and humidity stress loading.Based on this system,an on-line measurement method based on laser interferometer is proposed to measure the degradation of grating ruler caused by the decrease of precision.On this basis,a stress loading scheme for reliability accelerated life test of grating ruler is developed,which takes temperature and humidity as accelerated stress and loads step by step,and speed as conventional stress whose load spectrum simulating the actual working condition of grating ruler.3.The Weibull life distribution model and the double stress acceleration model are established.The Weibull distribution model is used to describe the life distribution of the grating ruler after the failure data obtained from the test are converted.The unknown parameters in the Weibull model and the double stress acceleration model of temperature and humidity are solved by the maximum likelihood estimation method.On this basis,the reliability level of the tested grating ruler is evaluated,and the acceleration factors under each acceleration stress are given.4.The competitive failure life distribution model of grating ruler is established.Based on the comprehensive analysis of the reliability characteristics of the three subsystems of the tested grating ruler,a more accurate competitive failure model is established to describe the life distribution of the grating ruler.When the test data contains incomplete data with unknown failure location,the expected maximization algorithm is used to solve the unknown parameters in the competitive failure model.