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Hazel proof reading.

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Robin Clark 2010-11-30 13:09:19 +00:00
parent 0b81f9fbb7
commit f379067a9f
1 changed files with 7 additions and 5 deletions
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12
fmmd_design_aide/fmmd_design_aide.tex
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@ -41,7 +41,7 @@ The principle of FMMD analysis is a five stage process,
the collection of components into {\fg}s, the collection of components into {\fg}s,
which are analysed w.r.t. their failure mode behaviour, which are analysed w.r.t. their failure mode behaviour,
the failure mode behaviour is then viewed from the the failure mode behaviour is then viewed from the
{\fg} perspective (i.e. as a symptoms of the {\fg}), {\fg} perspective (i.e. as a symptoms of the {\fg}) and
common symptoms are then collected. The final stage common symptoms are then collected. The final stage
is to create a {\dc} which has the symptoms of the {\fg} is to create a {\dc} which has the symptoms of the {\fg}
it was sourced from, as its failure modes. it was sourced from, as its failure modes.
@ -71,7 +71,7 @@ From the analysis of the {\fg} we can create a {\dc}, where the failure modes
are the symptoms of the {\fg} we derived it from. are the symptoms of the {\fg} we derived it from.
% %
\paragraph{detectable and undetectable failure modes} \paragraph{detectable and undetectable failure modes}
The symptoms will be detectable (like a value of of range) The symptoms will be detectable (like a value out of range)
or undetectable (like a logic state or value being incorrect). or undetectable (like a logic state or value being incorrect).
The `undetectable' failure modes undertsandably, are the most worrying for the safety critical designer. The `undetectable' failure modes undertsandably, are the most worrying for the safety critical designer.
EN61058, the statistically based European Norm, using ratios EN61058, the statistically based European Norm, using ratios
@ -83,7 +83,7 @@ for detected and undetected failure modes \cite{EN61508}.
%failure modes. %failure modes.
For instance, out of range values, are easy to detect by For instance, out of range values, are easy to detect by
systems using the {\dc} supplying them. systems using the {\dc} supplying them.
An undetectable faults are ones that forward incorrect information Undetectable faults are ones that forward incorrect information
where we have no way of validating or testing it. where we have no way of validating or testing it.
% we know we can cope with; they % we know we can cope with; they
%are an obvious error condition that will be detected by any modules %are an obvious error condition that will be detected by any modules
@ -396,6 +396,7 @@ This test case looks at the transistor failure mode where TR1 is always OFF.
\paragraph{$\overline{TEST\_LINE}$ ON} \paragraph{$\overline{TEST\_LINE}$ ON}
Here TR1 should be OFF and R36 should be in series. Here TR1 should be OFF and R36 should be in series.
This is a dormant failure, we can only detect this failure This is a dormant failure, we can only detect this failure
when the $\overline{TEST\_LINE}$ is OFF.
\paragraph{$\overline{TEST\_LINE}$ OFF} \paragraph{$\overline{TEST\_LINE}$ OFF}
Here TR1 should be ON, but is OFF due to failure. Here TR1 should be ON, but is OFF due to failure.
The resistance R36 will always be in series. The resistance R36 will always be in series.
@ -497,8 +498,9 @@ We can surmise the symptoms in a list.
\section{conclusions} \section{conclusions}
With safety addition the undetectable failure mode of \textbf{low~reading} With the safety addition the undetectable failure mode of \textbf{low~reading}
disappears. The overall reliability though goes down ! disappears.
However, the overall reliability though goes down !
This is simply because we have more components that {\em can} fail. This is simply because we have more components that {\em can} fail.
%% Safety vs. reliability paradox. %% Safety vs. reliability paradox.