From 80df6f9548171ce49a0086af917235f908d32acf Mon Sep 17 00:00:00 2001 From: Robin Clark Date: Wed, 24 Nov 2010 17:28:21 +0000 Subject: [PATCH] Wednesday edit --- fmmd_design_aide/fmmd_design_aide.tex | 42 ++++++++++++++++++++------- 1 file changed, 31 insertions(+), 11 deletions(-) diff --git a/fmmd_design_aide/fmmd_design_aide.tex b/fmmd_design_aide/fmmd_design_aide.tex index 489c252..38afbd0 100644 --- a/fmmd_design_aide/fmmd_design_aide.tex +++ b/fmmd_design_aide/fmmd_design_aide.tex @@ -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}