Failure modes, effects, and diagnostic analysis (FMEDA) is a systematic analysis technique to obtain subsystem / product level failure rates, failure modes and diagnostic capability. The FMEDA technique considers: All components of a design, The functionality of each component, The failure modes of each component, The effect of each component failure mode on the product functionality, The ability of any automatic diagnostics to detect the failure, The design strength (de-rating, safety factors) and The operational profile (environmental stress factors).
Given a component database calibrated with field failure data that is reasonably accurate, the method can predict product level failure rate and failure mode data for a given application. The predictions have been shown to be more accurate than field warranty return analysis or even typical field failure analysis given that these methods depend on reports that typically do not have sufficient detail information in failure records.
The abstract of an FMEDA report typically mentions the Safe Failure Fraction (rate of failures that are neither dangerous nor undetected over the total rate) and the Diagnostic Coverage (rate of detected dangerous failures over the rate of all dangerous failures). Each term is defined equivalently in both standards, IEC 61508 and ISO 13849.
The name was given by Dr. William M. Goble in 1994 to the technique that had been in development since 1988 by Dr. Goble and other engineers now at exida.
Antecedents
A failure modes and effects analysis, FMEA, is a structured qualitative analysis of a system, subsystem, process, design or function to identify potential failure modes, their causes and their effects on (system) operation. The concept and practice of performing a FMEA, has been around in some form since the 1960s. The practice was first formalized in 1970s with the development of US MIL-STD-1629/1629A. In early practice its use was limited to select applications and industries where cost of failure was particularly high. The primary benefits were to qualitatively evaluate the safety and reliability of a system, determine unacceptable failure modes, identify potential design improvements, plan maintenance activities and help understand system operation in the presence of potential faults. The failure modes, effects and criticality analysis (FMECA) was introduced to address a primary barrier to effective use of the detailed FMEA results by the addition of a criticality metric. This allowed users of the analysis to quickly focus on the most important failure modes/effects in terms of risk. This allowed prioritization to drive improvements based on cost / benefit comparisons.
Development
The FMEDA technique was developed in the late 1980s by exida engineers based in part on a paper in the 1984 RAMS Symposium. The initial FMEDA added two additional pieces of information to the FMEA analysis process. The first piece of information added in an FMEDA is the quantitative failure data (failure rates and the distribution of failure modes) for all components being analyzed. The second piece of information added to an FMEDA is the probability of the system or subsystem to detect internal failures via automatic on-line diagnostics. This is crucial to achieving and maintaining reliability in increasingly complex systems and for systems that may not be fully exercising all functionality under normal circumstances such as a low demand Emergency Shutdown System, ESD System. There is a clear need for a measurement of automatic diagnostic capability. This was recognized in the late 1980s In that context the principles and basic methods for the modern FMEDA were first documented in the book Evaluating Control System Reliability. The actual term FMEDA was first used in 1994 and after further refinement the methods were published in the late 1990s. The method was explained to members of the IEC 61508 committee in the late 90s and included in the standard as a method of determining failure rate, failure mode and diagnostic coverage for products. FMEDA techniques have been further refined during the 2000s primarily during IEC 61508 preparation work. The key changes have been: 1. Use of Functional Failure Modes; 2. Mechanical Component Usage; 3. Prediction of manual proof test effectiveness; and 4. Prediction of product useful life. With these changes, the FMEDA technique has matured to become more complete and useful.