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Added more FMEDA

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Need to put mill-volt amp in as a block diagram
Then need to put the SYSTEM SAFETY presentation
in at the end
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Robin Clark 2011-09-30 08:52:00 +01:00
parent 3b6f18d149
commit b6a7d4893b
1 changed files with 58 additions and 2 deletions
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presentations/fmea/fmea_pres.tex
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@ -333,8 +333,63 @@ FMEDA is the methodology behind statistical (safety integrity level)
type standards (EN61508/IOC5108).
It provides a statistical overall level of safety
and allows diagnostic mitigation for self checking etc.
It provides guidelines for the design and architecture
of computer/software systems for the four levels of
safety Integrity.
For Hardware
FMEDA does force the user to consider all components in a system
by requiring that a MTTF value is assigned.
This MTTF may be statistically mitigated (improved)
if it can be shown that selfchecking will detect failure modes.
\end{frame}
\begin{frame}
Failure modes are classified as Safe or Dangerous according
to the putative system level failure they will cause.
The Failure modes are also classified as Detected or
Undetected.
This gives us four level failure mode classifications:
Safe-Detected (SD), Safe-Undetected (SU), Dangerous-Detected (DD) or Dangerous-Undetected (DU),
and the probablistic failure rate of each classification
is represented by lambda variables
(i.e. $\lambda_{SD}$, $\lambda_{SU}$, $\lambda_{DD}$, $\lambda_{DU}$).
\end{frame}
\begin{frame}
\textbf{Diagnostic Coverage.}
The diagnostic coverage is simply the ratio
of the dangerous detected probabilities
against the probability of all dangerous failures,
and is normally expressed as a percentage. $\Sigma\lambda_{DD}$ represents
the percentage of dangerous detected base component failure modes, and
$\Sigma\lambda_D$ the total number of dangerous base component failure modes.
$$ DiagnosticCoverage = \Sigma\lambda_{DD} / \Sigma\lambda_D $$
\end{frame}
\begin{frame}
The diagnostic coverage for safe failures, where $\Sigma\lambda_{SD}$ represents the percentage of
safe detected base component failure modes,
and $\Sigma\lambda_S$ the total number of safe base component failure modes,
is given as
$$ SF = \frac{\Sigma\lambda_{SD}}{\Sigma\lambda_S} $$
\textbf{Safe Failure Fraction.}
A key concept in FMEDA is Safe Failure Fraction (SFF).
This is the ratio of safe and dangerous detected failures
against all safe and dangerous failure probabilities.
Again this is usually expressed as a percentage.
$$ SFF = \big( \Sigma\lambda_S + \Sigma\lambda_{DD} \big) / \big( \Sigma\lambda_S + \Sigma\lambda_D \big) $$
\end{frame}
\begin{frame}
SIL Levels are how they are calculated
\end{frame}
\section{FMEA - General Criticism}
\begin{frame}
@ -347,8 +402,9 @@ and allows diagnostic mitigation for self checking etc.
\pause \item FMEA type methodologies were designed for simple electro-mechanical systems of the 1940's to 1960's.
\end{itemize}
FMEDA is an extension of FMEA, in that it will give higher ratings
for self checking. It
FMEDA is a modern extension of FMEA, in that it will allow for
self checking features, and provides detailed recommendations for computer/software architecture,
but
\end{frame}