Research On The Method And Technique Of CNC System Hardware Reliability Growth

Computerized Numerical Control (CNC) system is the brain of CNC machine tools. Improving the reliability of CNC system is very important to improve the reliability level of CNC machine tools. Endeavoring to develop numerical control technology, which is the core of advanced manufacturing technology, has become an important way to accelerate economic development and enhance comprehensive national strength. Thus, increasing reliability, analyzing the reliability status of CNC system, as well as developing and applying advanced technologies for improving reliability have become key for the development of CNC system. The aim of this dissertation is to study the reliability of CNC system hardware, which is one of the most important parts of a CNC system. Improving CNC system hardware reliability is an important way to improve the reliability of the full CNC system. In this dissertation;the analysis data is obtained by tracking the performance of30CNC Machining Center units. Based on the tracked data, strict fault data collection procedures and methods, the preparation of a detailed specification of the failure positions, failure causes, failure modes, and failure codes, CNC system hardware database forms are proposed in order to ensure the quality and quantity of fault data information. RAM (Reliability, Availability and Maintainability), FMEA (Failure Mode and Effects Analysis), FMECA (Failure Mode and Effects Critical Analysis) and FTA (Fault Tree Analysis) are used to analyze the running CNC system hardware. Fault data, the criteria of the fault locations, fault mode, and cause are obtained from these analyses. By analyzing fault data, the frequent fault of CNC system hardware parts (electrical systems, PLC unit and pre-processing module), the fault modes (damaged components, line or bad cable connection), and frequent fault causes are thereby found. Based on the critical analysis of CNC system hardware subsystems, the weaknesses of CNC system hardware are found and methods for improving design reliability are proposed. The FTA framework of CNC system hardware is established based on the careful analysis of fault data. In traditional fault tree analysis (FTA), which is an established technique in hazard identification, ambiguous and imprecise events, such as human errors, can not be handled efficiently. Fuzzy method can generate failure probability values with only a little available quantitative information. The probabilities of basic events are treated as fuzzy numbers. Therefore fuzzy methods are effective tools to deal with subjective judgment. The method to handle the fuzzy problems in the FTA is proposed in this dissertation by combining expert elicitation with fuzzy set theory. In this dissertation we propose9analysis steps of the46most important fault events that affect the fault rate of top events to improve CNC system hardware reliability. Central Processing Unit (CPU), Programmable Logic Controller (PLC) Unit, and Detection Unit failures are common in CNC system hardware. Thus, improving these units’ reliability will be the most significant contribution to enhance CNC system hardware. By analyzing the RAM system data, the corresponding probability distribution model, statistical characteristic parameters are obtained. The result of this analysis which determines the reliability characteristics, the nature of failures, the frequency of failures, the failure positions, the failure modes, and the maintainability requirements of CNC system hardware components, can be used as the reference for CNC system hardware operation, maintaining and repairing activities, as well as improving the reliability of CNC system hardware. Based on the failure data, the most well-known qualitative reliability methods, FMEA and FMECA, are used to analyze the reliability of CNC system hardware. Index evaluation of CNC system hardware such as failure rates, MTBF (Mean Time Between Failures), MTTR (Mean Time To Repairs), and are calculated to propose reliability evaluation methods for domestic CNC system hardware. After obtaining the FMEA result, further preventive/detection actions are defined in order to minimize the risk of the potential failure causes. The optimized risk assessment of CNC system hardware and the reduction in RPN (Risk Priority Number) are shown in this dissertation. Some problems of reliability evaluation are pointed out and some suggestions for system hardware reliability improvement are given.