Research On Fault Diagnosis Approach Based On Digraph Model And Its Application In Aerospace

Fault diagnosis technique is very important to ensure the safety and reliability of spacecraft. This dissertation studies the spacecraft fault diagnosis based on the digraph model. The approach is a qualitative model-based approach, the model qualitatively describes the characteristic of system structure, and can express cause-effect relation among process variables and propagation relation among the faults, and the model is a deep knowledge-based model, and can solve the bottleneck problem in KA and identify some unknown fault, so the approach provide a new solutions for fault diagnosis of spacecraft. The main purposes of this dissertation are to make a systematic study on digraph-based fault diagnosis technique to realize the on-board autonomous fault diagnosis. For the purpose, the main work is as follows:According to the characteristic in fault diagnosis domain of spacecraft, this dissertation proposes a novel qualitative diagnosis approach based on hierarchical digraph integrating the goodness of the fault propagation graph and SDG(signed directed graph), the model has the better completeness and explanation facility of SDG, and can express the connection relation among all components. The hierarchical strategy was adopted for digraph so that it reduces search space for failure sources; the fault propagation consistent branches was backward searched by qualitative relations between measured nodes ,and set of failure source candidates were found, the furthermore, the failure source candidates are ranked according to the fault possibility rate. The diagnosis system for primary power system of some satellite was modeled with the approach, and diagnosis simulation result show the approach is valid.In the aerospace field,because the fault experience and fault data is insufficient, this has brought a lot of difficulties for the fault diagnosis modeling, so the idea that modeling by the data in all normal work modes of system is adapted. In addition, due to the work threshold and relation between processes variables vary with the work state, the tradition SDG model is insufficient expression of the qualitative relation, and has lower resolution of diagnosis. so the fuzzy theory introduced into SDG model, the node variable was expressed as fuzzy variable , each fuzzy subset of which describes the state in each working mode , the cause-effect relationship between the nodes was described by fuzzy relation matrix. The algorithm is presented to identify fuzzy consistent branch by fuzzy reasoning and to search the set of failure source candidates. The validity of approach was identified by the application example.Under the condition of the unmeasured node, the approach based on the fuzzy SDG model will be not available, In order to solve the problem, the fuzzy probabilistic SDG is proposed based on model of fuzzy SDG and Bayesian inference, the cause-effect relationship between the nodes was described by conditional probabilities table (CPT), the set of failure source candidates is found out by Bayesian inference and backtracking algorithm under the condition of the unmeasured node. The primary electrical power supply system in certain a satellite was modeled with the proposed approach, the diagnosis simulation result show the approach is valid.The accuracy of diagnosis depends on precision of the system model. To improve the precision of system model, on the basis of the previous method for modeling, the novel modeling method was proposed that the technology of QTA (qualitative trend analysis) can aid in modeling. The qualitative trend information by QTA can transform the valid information for SDG model, the thresholds and dynamic qualitative relation of node variable are determined by analyzing the information, furthermore, the synthetical modeling method combined with other modeling methods is summarized. At last the primary electrical power supply system in certain a satellite was modeled, the diagnosis simulation result show the modeling method improve the precision of the model and the accuracy of the diagnosis .The core problem of fault detection is the sensor location. According to the digraph model, the design scheme of sensor location based on observability, reliability was respectively given. The applications and analysises based on the schemes were presented. Through the above research, the new design idea of sensor location based on reliability was proposed, and the reliability problem was described again based on the idea, furthermore, the optimal design scheme for location of detection sensors was presented. this scheme takes into account both the fault observability, the reliability of fault detection, and sensor placement costs. The greedy heuristics algorithm was designed for the scheme. Furthermore, the proposed approach was applied to primary electrical power supply system in certain a satellite, and the diagnosis simulation results show that the design scheme has met the design requirements of the observability and the reliability, and can rapidly enhancement reliability and is more suitable for system needs.