The Theory And Methodology For The Fault Tree Construction Of The Aeronautic Engineering System Based On Forward Reasoning

With the wide applications of fly-by-wire system and digital flight control computer to the aeronautic vehicle,the degree of modularization,information and automation is more and more advanced.Besides,the aircraft is fault tolerant.The system,however,is more fail prone with the subsequent increasing complexity and the amount of parts.Besides,for the enhanced coupling and relevance between the subsystems of the aircraft,the consequence will be rather serious when the accident occurs.Thus some method should be employed to evaluate the reliability of the complex engineering systems,so as to discover the failure and weakness,predict the possible fault modes and guide the people to improve the quality of the product.FTA is an effective approach for the reliability assessment of the complex system.It has an important academic significance and engineering value to reduce the latent failure and avoid the catastrophic accident.Fault tree construction is the basic technology and key step of FTA.Generally,the deductive method is employed to the manual construction of fault tree,which is a top-down way.This method,however,is laborious,error prone,full of repeated work and lack of global view.It is time-consuming,error prone and the fault tree constructed may vary with different engineers.In this thesis,several approaches such as the mixed entry decision tables are proposed to make up for the deficiency of fault tree construction of multi-state,non-coherent and multi-loop engineering systems,which is a bottom-up way.The main work of the thesis can be summarized as follows:(1)The AND-gate-subtree-union-set-method with the aid of decision tables is proposed.In the fault tree construction for subsystems,the logical relationships and output events are unknown.In this case,the fault tree construction will be difficult to be implemented.To solve this problem,the AND-gate-subtree-union-set-method with the aid of decision tables is proposed.The method has three steps: First,list all the possible state combinations of the input and the parts of some system by means of decision tables.At this moment,the output event is often unknown and to be determined.Second,analyze the fault propagation with a forward reasoning way to predict the state of the output.Then summarize the logical relationships between the state of the inputs,the parts and the outputs as a structure function of a subtree with an AND gate.Finally,find the union set of the subtree with the same output event so as to summarize the relationships between events.In this way,a multi-fault mode fault tree construction can be implemented,which is a bottom-up manner and the interactions between parts with different states in an engineering system are translated into a logic function.It’s global and the confusion of thoughts can be avoided.For there are several output events,the multi-tree can be constructed.It’s an extension and supplement of the mini-tree method.(2)A solution to the structure function determination of control loops in a complex system is proposed and verified.For the control loop system is non-coherent,it’s difficult to determine the structure function from the physical structure.To solve this problem,the controlled variable and the deviation are regarded as the input and the output event is regarded as the consequence that the input processed by the control loops.Thus the output event can be found in way of fault propagation analysis according to the state combinations of inputs and parts.In this process,the feature of control loops should be taken into account,such as self-healing etc..And then find the union set of the structure function of the subtree with the same output event to induce the structure function of the global fault tree.In this way,the interaction between parts with different states in the system is summarized as a logic expression and the fault tree construction of the control loops in the complex system is solved.(3)A solution to the fault tree construction of multi-state and non-coherent system by mixed entry decision tables is proposed.The non-coherent system is composed of multi-state,reversed action parts or control loops.The interaction between the parts is very complex and the fault tree construction should not be determined from the physical structures.To solve this problem,the states of each part in the system is classcified as normal and failure modes.The failure modes are classified as exclusive and inclusive.The output event is analyzed by fault paopagation on basis of the state combinations listed in the MEDT and the global structure function is found by synthesis of the structure function of the subtree.The superiority of MEDT is that the condition stub can be either event or part.If the state of the part is inclusive,the event can be taken as the condition stub.If the state of the part is exclusive,the part can be taken as the condition stub.The rules and work can be reduced a lot equivalently.(4)An equivalent reduction method for rules of decision tables is proposed.The number of rules of decision tables may increase dramatically with the number of condition stubs.But some of the state of the condition stubs are irrelevant to some action stub and the condition entry should be “don’t care”.Accordingly,some variables are irrelevant to the structure function of the subtree.The irrelevant variable makes the structure function very complex and should be ruled out or simplified equivalently.For this problem,the failure of the parts should be classified as two types: soft failure and hard failure.The irrelevant event can be ruled out by Boolean algebra and the failure types of the part,the relationships of upstream and downstream,fault propagation and the action of the monitoring system or with the aid of the decision tree.In this way,the irrelevant events can be ruled out effectively and the unnecessary work can be reduced a lot.(5)A method for variable order of BDD based on truth tables is proposed.The method for variable order is determined by the state and the position of the variables in the truth tables whose rules are equivalently combined.It can achieve the same result with the approach of traversing and easy to be read and processed by the program.The program for automatic generation of OBDD is compiled with Python.With this program,the OBDD with the optimized order can be achieved on basis of the truth tabes.The retraceable sets can be acquired,on basis of which the minimum cut set,prime implicant set are founded and the probability of the top event and importances of events can be calculated.In this thesis,a research is carried out about the theory and methodology about the fault tree construction in a reversed way for the complex engineering system of modern aeronautic equipment.And a detailed method and steps in a bottom-up way,based on decision tables for fault tree construction are developed.A program for OBDD automatic generation is compiled with Python.A multi-fault-mode fault tree construction and reliability assessment are performed for a hydraulic system and a transfer valve pressure system.The method for fault tree construction is proved to be feasible and the software compiled can be applied to the fault tree construction and reliability assessment of complex engineering system.