Fault Tree Analysis Approach For NC Turret Considering Uncertainty And Fault Polymorphism

NC turret is the most critical and influential functional component in determining the reliability of CNC lathes and turning centers.Employing a fault tree model for fault analysis and tracing is the primary technical approach to enhance product reliability and maintainability.However,the complexity of NC turrets’ structure and the diversity of failure modes,particularly the multi-state and dynamic characteristics of some major failure modes in addition to binary states(referred to as polymorphism),coupled with uncertainties such as cognitive biases in quantitative fault tree analysis or inadequate maintenance information and failure data,pose significant challenges.Consequently,the conventional two-state fault tree method falls short in accurately depicting the actual system degradation levels and temporal relationships between failure modes.This limitation hinders the provision of objective and precise failure analysis and reliability assessments for NC turrets.It can even lead to erroneous outcomes,preventing the accurate guidance required for improving NC turret design and making maintenance decisions.To address these issues,this paper introduces the research theme of fault tree analysis methods for NC turrets,which consider uncertainty and fault polymorphism.The primary research achievements within this theme are outlined below:(1)A two-state fault tree analysis method for NC turrets based on the weakest triangular paradigm fuzzy theory is proposed.In view of the problem that it is difficult to accurately determine the failure probability of the basic events of the binary fault tree of NC turret caused by insufficient failure data of parts and incomplete maintenance information,expert judgment and fuzzy mathematical theory are used to quantify the failure probability of the basic events.Aiming at the problem that the existing fuzzy algorithms are prone to excessive accumulation of fuzziness in the quantization process,an improved fuzzy algorithm based on the weakest triangular paradigm is proposed and extended to the quantitative analysis of the binary fault tree and the evaluation of the importance of the process.Taking the case that the NC turret’s cutter cannot be rotated,the proposed algorithm is verified to be effective in solving the problem of uncertainty of two-state faults,and at the same time,the fuzzy fault probability and fuzzy importance degree of each event are obtained,and the reliability analysis and the identification of the weak links are completed.(2)A multi-state fault tree analysis method for NC turret based on gray fuzzy TS model is proposed.Aiming at the problem of multi-state characteristics of the fault states of certain fault modes of NC turret,the T-S model is introduced to extend the two-state fault tree into the T-S multi-state fault tree.Considering the problem of experts’ hesitation degree in the process of fault state judgment,the expert decisionmaking hesitation factor is introduced in the fuzzy operation,and a method of quantifying the probability of basic event failure based on triangular intuitionistic fuzzy set is proposed.On this basis,for the problem that the existing T-S static logic gate cannot quantify the uncertainty of the top-event fault state caused by the multistate characteristic of the basic event fault state,the gray system theory is introduced into the T-S model,and the intuitionistic fuzzy T-S polymorphic algorithm based on the gray number of intervals is put forward,which realizes the quantitative analysis of the polymorphic fault tree of the numerical control tool holder under the uncertainty,and further puts forward the gray T-S polymorphic Importance analysis method.The effectiveness of the proposed algorithm in solving the problem of uncertainty of polymorphic faults is verified by taking the abnormal noise of NC turret as an example,and the fuzzy fault probability intervals and fuzzy importance intervals of each event under different fault states,as well as the support and hesitation of the results,are obtained,and the reliability analysis and the identification of the weak links are accomplished.(3)A dynamic fault tree analysis method for NC turrets based on discrete-time evidence T-S model is proposed.Aiming at the problem of the dynamic characteristics of the fault states of certain fault modes of the NC turret,the discretetime model is further combined with the T-S model to construct a discrete-time T-S dynamic fault tree model.Aiming at the problem that the existing T-S dynamic logic gate cannot quantify the uncertainty of the top event fault state caused by the dynamic characteristics of the basic event fault state,combined with the evidence theory,the discrete-time evidence T-S dynamic algorithm based on the bursty and degraded faults is proposed,which realizes the quantitative analysis of the dynamic fault tree of the NC turret under uncertainty.Taking the case of the NC turret’s blade not being able to lock,it is verified that the proposed algorithm can effectively solve the dynamic fault uncertainty problem,and at the same time,the fuzzy fault probability trust intervals of each event in discrete time corresponding to different fault states and the uncertainty of the results are obtained,and the reasonableness of the results is analyzed in a comparative manner.(4)The reliability assessment and analysis method of NC turret under the influence of maintenance based on parameter probability box theory and dynamic fault tree is proposed.Aiming at the problem that the reliability of NC turret is affected by maintenance,the fault tree model of repairable system under static logic and dynamic logic is constructed on the basis of "normal-failure(maintenance)-normal" state update process.Considering the influence of different fault distributions of basic events on the quantitative analysis of the fault tree,the uncertainty of fault distribution parameter intervals is quantified based on the method of coefficient of variation and extended parameter probability box.On this basis,a Monte Carlo simulation method based on equivalent minimum cut set is proposed.Still taking the case of the NC turret’s blade not being able to lock,the average fault-free working time interval of the top event is obtained through simulation,and the influence of the replacement strategy of the parts under the same time node on the reliability of the system is compared,so as to realize the reliability assessment of the dynamic fault tree of the NC turret under the influence of maintenance.