Failure modes, effects, and diagnostic analysis (FMEDA) is a systematic analysis technique to obtain subsystem / product level

failure rate Failure rate is the frequency with which an engineered system or component fails, expressed in failures per unit of time. It is usually denoted by the Greek letter λ (lambda) and is often used in reliability engineering. The failure rate of a ...

failure mode Failure causes are defects in design, process, quality, or part application, which are the underlying cause of a failure or which initiate a process which leads to failure. Where failure depends on the user of the product or process, then human er ...

s and

diagnostic Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine " cause and effect". In systems engine ...

capability. The FMEDA technique considers: * All components of a design, * The

functionality Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...

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 IEC 61508 is an international standard published by the International Electrotechnical Commission consisting of methods on how to apply, design, deploy and maintain automatic protection systems called safety-related systems. It is titled ''Functio ...

and

ISO 13849 ISO 13849 is a safety standard which applies to parts of machinery control systems that are assigned to providing safety functions (called safety-related parts of a control system). The standard is one of a group of sector-specific functional safe ...

. 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 In engineering, RAMS (reliability, availability, maintainability and safety)

Mechanical FMEDA Techniques

It became clear in the early 2000s that many products being used in safety critical applications had mechanical components. An FMEDA done without considering these mechanical components was incomplete, misleading, and potentially dangerous. The fundamental problem in using the FMEDA technique was the lack of a mechanical component database that included part failure rates and failure mode distributions. Using a number of published reference sources, exida began development of a mechanical component database in 2003. After a few years of research and refinement, the database has been published. This has allowed the FMEDA to be used on combination electrical / mechanical components and purely mechanical components.

Manual Proof Test Effectiveness

The FMEDA can predict the effectiveness of any defined manual proof test in the same way it can predict automatic diagnostic coverage. An additional column is added to the FMEDA and probability of detection for each component failure mode is estimated. The cumulative effectiveness of the proof test is calculated in the same way as automatic diagnostic coverage.

Product Useful Life

As each component within a product is reviewed, those with a relatively short useful life span are identified. One example of this is an electrolytic capacitor. Many designs have a useful life limitation of 10 years. Since constant failure rates are only valid during the useful life period, this metric is valuable for interpreting FMEDA result limitations.

The Future

Further refinement of the component database with selective calibration to different operation profiles is needed. In addition, comparisons of FMEDA results with field failure studies, have shown that

human factors Human factors and ergonomics (commonly referred to as human factors) is the application of psychological and physiological principles to the engineering and design of products, processes, and systems. Four primary goals of human factors learnin ...

, especially maintenance procedures, affect the failure rates and failure modes of products. As more data becomes available, the component database can be refined and updated. After a few years of research and refinement, the database has been published as required by new technology and new knowledge. The success of the FMEDA technique is supplying needed data in a relatively accurate way has allowed the probabilistic, performance approach to design to work.

References