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Robin Clark 2010-11-24 17:28:21 +00:00
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42
fmmd_design_aide/fmmd_design_aide.tex
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@ -34,13 +34,31 @@ of its failure mode behaviour.
\section{How FMMD Analysis can reveal design flaws w.r.t. failure behaviour }
\ifthenelse {\boolean{paper}}
{
\paragraph{Overview of FMMD Methodology}
The principle of FMMD analysis is a four stage process,
The principle of FMMD analysis is a five stage process,
the collection of components into {\fg}s,
which are analysed w.r.t. their failure mode behaviour,
the failure mode behaviour is then viewed from the
{\fg} perspective (i.e. as a symptoms of the {\fg}),
common symptoms are then collected.
common symptoms are then collected. The final stage
is to create a {\dc} which has the symptoms of the {\fg}
it was sourced from, as its failure modes.
}
\paragraph{Overview of FMMD Methodology}
To re-cap from chapter \ref{symptomex},
the principle of FMMD analysis is a five stage process,
the collection of components into {\fg}s,
which are analysed w.r.t. their failure mode behaviour,
the failure mode behaviour is then viewed from the
{\fg} perspective (i.e. as a symptoms of the {\fg}),
common symptoms are then collected. The final stage
is to create a {\dc} which has the symptoms of the {\fg}
it was sourced from, as its failure modes.
{
%
%From the failure mode behaviour of the {\fg} common symptoms are collected.
@ -55,10 +73,11 @@ are the symptoms of the {\fg} we derived it from.
\paragraph{detectable and undetectable failure modes}
The symptoms will be detectable (like a value of of range)
or undetectable (like a logic state or value being incorrect).
The `undetectable' failure modes 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
of detected and undetected system failure modes to
classify the safety level \cite{EN61508}.
classify the sytems safety levels and describes sub-clasifications
for detected and undetected failure modes \cite{EN61508}.
%It is these that are, generally the ones that stand out as single
%failure modes.
@ -81,7 +100,7 @@ another failure mode becoming active, or an environmental
condition changing (for instance temperature). Some
component failure modes may lead to dormant failure modes.
By examining test cases from a functional group against all
input conditions and germane environmental conditions
operational states and germane environmental conditions
we can determine all the failure modes of the {\fg}.
\subsection{Iterative Design Example}
@ -100,13 +119,14 @@ paper
{
chapter
}
describes a milli-volt amplifier (see R18 in figure \ref{fig:mv1}), with an inbuilt safety\footnote{The `safety resistor' also acts
as a potential divider to provide a mill-volt offset. An offset is often required to allow for negative readings form the
describes a milli-volt amplifier (see figure \ref{fig:mv1}), with an inbuilt safety\footnote{The `safety resistor' also acts
as a potential divider to provide a mill-volt offset. An offset is often required to allow for negative readings from the
milli-volt source.}
resistor. The circuit is analysed and it is found that all but one component failure modes
resistor (R18). The circuit is analysed and it is found that all but one component failure modes
are detectable.
We then design a circuit to test for the `undetectable' failure mode
We then design a circuit to test for the `undetectable' failure modes
and analyse this with FMMD.
The test circuit addition can now be represented by a {\dc}.
With both {\dcs} we then use them to form a {\fg} which we can call our `self testing milli-volt amplifier'.
We then analsye the {\fg} and the resultant {\dc} failure modes/symptoms are discussed.
\section{An example: A Millivolt Amplifier}
@ -213,7 +233,7 @@ To take an example from a K type thermocouple, the offset of 1.86mV
%from the potential divider represents amplified to
would represent $\approx \; 46\,^{\circ}{\rm C}$ \cite{eurothermtables} \cite{aoe}.
\clearpage
%\clearpage
\subsection{Undetected Failure Mode: Incorrect Reading}
Although statistically, this failure is unlikely (get stats for R short FIT etc from pt100 doc)
@ -400,7 +420,7 @@ group w.r.t the failure modes in the two derived compoennts.
\begin{figure}[h]
\centering
\includegraphics[width=300pt,bb=0 0 698 631,keepaspectratio=true]{./testable_mvamp.jpg}
\includegraphics[width=300pt,bb=0 0 698 631,keepaspectratio=true]{./fmmd_design_aide/testable_mvamp.jpg}
% testable_mvamp.jpg: 698x631 pixel, 72dpi, 24.62x22.26 cm, bb=0 0 698 631
\caption{Testable milli-volt amplifier}
\label{fig:testable_mvamp}