Evaluation of Bayesian Networks Used for Diagnostics[ 1] K. Wojtek Przytula: HRL Laboratories Denver Dash: University of Pittsburgh Don Thompson: Pepperdine University © 1998 HRL Laboratories, LLC. All Rights Reserved 1 DIAGNOSIS - MODEL BASED APPROACH DOMAIN MODEL APPLICATION DOMAIN EM 2000 ABCD EM 2000 Wiring 74 VDC Harness Power Supply 24 VDC Power Wiring Harness Sensor Component Wiring Annumciator Panel Pipe Wiring Harness Fuel System Crankshaft Spd SRS Fuel Tank Crankshaft Pos TRS Suction Strainer Performance Sensors Fuel Pump Fuel Press FPS Fuel Preheater Thermo Mod AMOT Valve 30 psi Bypass Primary Filter 120 PSI Relief Valve Secondary Fuel Filters Comm Interface & Fault Data Sensor Wiring Harness Protective System Oil P Pres OPS Oil Temp OTS Fuel Temp FTS Coolant P 1 Air Box P TBS Coolant P 2 Air Temp ATS Coolant P 3 Fuel Pressure Crankcase CCP Fuel Injectors 9 - 16 Left Injector Harness ECM Sender Fuel Injectors 1-8 Right Injector Harness ECM Receiver Cold Plate 40 psi Relief Valve To Fuel Tank TROUBLESHOOTER DIAGNOSIS SUPPORT TOOL QUERY DECISION © 1998 HRL Laboratories, LLC. All Rights Reserved 2 Bayesian Network Diagnostics •Bayesian Networks as models computerized diagnostic assistants for •Model evaluation has not been addressed •Model quality determines diagnosis quality •Evaluation provides a basis for model performance estimation © 1998 HRL Laboratories, LLC. All Rights Reserved 3 GRAPHICAL MODEL FOR DIAGNOSIS (GRAPH AND PROBABILITY THEORY) GRAPH (structure): • Two Fault Nodes: F1, F2. • Three Observation Nodes – e.g. symptoms and tests. • Causal Links PROBABILITIES (parameters) • Prior Probabilities of Faults • Conditional Probabilities of Observations given Faults • The Model constitutes a joint probability distribution over the nodes. • It is obtained from data or knowledge or both. © 1998 HRL Laboratories, LLC. All Rights Reserved 4 Bayesian Network for Example of Car Diagnostics © 1998 HRL Laboratories, LLC. All Rights Reserved 5 Bayesian Network Evaluation ] Using Inference, Monte Carlo Simulation, Visualization Techniques & –Step 1 –Set Defective Component –Execute Forward Inference –Step 2 –Sample Observation States –Execute Reverse Inference © 1998 HRL Laboratories, LLC. All Rights Reserved 6 Forward Inference © 1998 HRL Laboratories, LLC. All Rights Reserved 7 Reverse Inference © 1998 HRL Laboratories, LLC. All Rights Reserved 8 Evaluation Conclusions Identification of Critical Elements Responsible for Incorrect Diagnosis –Components with weak observations that cannot be diagnosed convincingly –Strongly coupled components that implicate each other, so they cannot be effectively separated in diagnosis –Components whose failures are misinterpreted as failures of other components © 1998 HRL Laboratories, LLC. All Rights Reserved 9 Sample Graph for Car Diagnosis Bayesian Network Model[ 1] Fuel Level Fuel Filter © 1998 HRL Laboratories, LLC. All Rights Reserved Battery Fuel Pump Starter Induction Coil Cable 10 2-D Matrix for Car Diagnosis Bayesian Network Model 1] Starter I M P L I C A T E D F A U L T Fuel Level Battery Cable Fuel Pump Fuel Filter Induction Coil © 1998 HRL Laboratories, LLC. All Rights Reserved True Defect Prior Probabilities 11 3-D Matrix for Car Diagnosis Bayesian Network Model 1] Prior Probabilities Starter Fuel Level Battery Cable Fuel Pump Starter Fuel Level Battery Cable Fuel Pump © 1998 HRL Laboratories, LLC. All Rights Reserved True Defect Fuel Filter Induction Coil Fuel FilterInduction Coil I M P L I C A T E D F A U L T 12 3-D Matrix for Bayesian Network Model for the Large Network [ 1] © 1998 HRL Laboratories, LLC. All Rights Reserved 13 Results and Conclusions [ 1] RESULTS • METHOD AND ALGORITHMS FOR ANALYSIS OF BAYESIAN NETWORKS FOR DIAGNOSTICS • SOFTWARE PACKAGE FOR COMPUTATION AND DISPLAY OF THE ANALYSIS RESULTS CONCLUSIONS • THE RESULTS CAN BE USED AS A GUIDE IN TESTING OF THE MODEL • THE METHOD CAN BE USED IN DESIGN OF SYSTEMS FOR DIAGNOSIBILITY • THE METHOD IS APPLICABLE NOT ONLY TO DIAGNOSTICS BUT TO GENERAL CLASS OF DECISION SUPPORT PROBLEMS © 1998 HRL Laboratories, LLC. All Rights Reserved 14
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