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Added alpha factor to FMECA

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Robin Clark 2011-02-14 18:36:24 +00:00
parent d1ae9c67eb
commit 8673ec8318
1 changed files with 10 additions and 8 deletions
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fmmd_concept/fmmd_concept.tex
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@ -356,15 +356,16 @@ electronic components was published by the DOD
in 1991 (MIL HDK 1991 \cite{mil1991}) and is a typical in 1991 (MIL HDK 1991 \cite{mil1991}) and is a typical
source for MTFF data. source for MTFF data.
% %
FMECA has a probability factor for a component error becoming % causing FMECA has a probability factor for a component error occuring % causing
a SYSTEM level error. termed the $\alpha$ factor.
This is termed the $\beta$ factor.
%\footnote{for a given component failure mode there will be a $\beta$ value, the %\footnote{for a given component failure mode there will be a $\beta$ value, the
%probability that the component failure mode will cause a given SYSTEM failure}. %probability that the component failure mode will cause a given SYSTEM failure}.
% %
This lacks precision, or in other words, determinability prediction accuracy \cite{fafmea}, A second probability factor, the liklihood that the component failure
as often the component failure mode cannot be proven to cause a SYSTEM level failure, but is mode will cause a given SYSTEM failure, is termed the $\beta$ factor.
assigned a probability $\beta$ factor by the design engineer. The use of a $\beta$ factor As often the component failure mode cannot be proven to cause a SYSTEM level failure, but is
assigned a heuristic probability $\beta$ factor by the design engineer.
The use of a $\beta$ factor
is often justified using Bayes theorem \cite{probstat}. is often justified using Bayes theorem \cite{probstat}.
%Also, it can miss combinations of failure modes that will cause SYSTEM level errors. %Also, it can miss combinations of failure modes that will cause SYSTEM level errors.
% %
@ -387,8 +388,9 @@ Again this essentially produces a prioritised `to~do' list.
\begin{itemize} \begin{itemize}
\item Possibility to miss the effects of failure modes at SYSTEM level. \item Possibility to miss the effects of failure modes at SYSTEM level.
\item Possibility to miss environmental affects. \item Possibility to miss environmental affects.
\item The $\beta$ factor is based on heuristics and does not reflect any rigorous calculations. Applying failure rates of individual components rather than individual failure modes \item The $\beta$ factor is based on heuristics and typically does not reflect any rigorous calculations.
makes the factor less statistically reliable. \item The $\alpha$ factor is generally taken from lierature for generic component such as MIL~HDBK~\cite{mil1992}
and may not take into account operational states and environemtnal factors.
\item Complex component interaction effects can be missed. \item Complex component interaction effects can be missed.
\item No possibility to model base component level double failure modes. \item No possibility to model base component level double failure modes.
\end{itemize} \end{itemize}