Use of fault tree analysis for evaluation of system-reliabilityimprovements in design phase

摘要

Traditional failure mode and effects analysis is applied as abottom-up analytical technique to identify component failure modes andtheir causes and effects on the system performance, estimate theirlikelihood, severity and criticality or priority for mitigation. Failuremodes and their causes, other than those associated with hardware,primarily electronic, remained poorly addressed or not addressed at all.Likelihood of occurrence was determined on the basis of componentfailure rates or by applying engineering judgement in their estimation.Resultant prioritization is consequently difficult so that only theapparent safety-related or highly critical issues were addressed. Whenthoroughly done, traditional FMEA or FMECA were too involved to be usedas a effective tool for reliability improvement of the product design.Fault tree analysis applied to the product as a top down in view of itsfunctionality, failure definition, architecture and stress andoperational profiles provides a methodical way of following productsfunctional flow down to the low level assemblies, components, failuremodes and respective causes and their combination. Flexibility ofmodeling of various functional conditions and interaction such asenabling events, events with specific priority of occurrence, etc.,using FTA, provides for accurate representation of their functionalityinterdependence. In addition to being capable of accounting for mixedreliability attributes (failure rates mixed with failure probabilities),fault trees are easy to construct and change for quick tradeoffs as rollup of unreliability values is automatic for instant evaluation of thefinal quantitative reliability results. Failure mode analysis usingfault tree technique that is described in this paper allows for real,in-depth engineering evaluation of each individual cause of a failuremode regarding software and hardware components, their functions,stresses, operability and interactions