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Signal path description leading into XFMEA

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Robin Clark 2013-03-16 09:20:26 +00:00
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submission_thesis/CH2_FMEA/copy.tex
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@ -536,11 +536,71 @@ approach in looking for system failures.
In this section we examine some fundamental concepts and underlying philosophies of FMEA.
\paragraph{The unacceptability of a single component failure causing a catastrophe.}
\paragraph{The signal path.}
Most electronic systems are used to process a signal: with signal processing
there is usually a clear afferent to transform to efferent path.
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That is, there is an input, some processing and an output.
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Some circuits have feedback loops or even circular signal paths, but it
is normal for a signal path to exist.
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%can be identified.
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An FMEA investigation will often take the component {\fm} and examine its effect along this path,
in the direction of the signal,
echoing diagnostic/fault~finding methods~\cite{garrett, maikowski}. % loebowski}.
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The rationale and work-culture of those tasked to
perform FMEA are generally personnel who have performed fault finding.
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When fault finding we generally follow the signal path, checking for correct behaviour
along it: when we find something out of place we zoom in and measure
the circuit behaviour until we find a faulty component or module.
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With this style of fault finding, because it is based on experiment,
we can hop from module to module eliminating working modules, until we find the
failure.
FMEA is a theoretical discipline. It would be very unusual to build a circuit and then simulate
component failure modes. This would be very time consuming as it would involve building a circuit for each component {\fm} in the system.
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We cannot, as with fault finding, verify modules along the signal path for correct behaviour
and eliminate them from the investigation.
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With FMEA we therefore need to be more thorough.
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The question is by how much.
Too much and the task becomes impossible due to time/labour constraints.
Too little and the analysis could become meaningless because it misses
potential system failures.
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For a more complete analysis we should perhaps, examine each component {\fm} along the complete signal path,
forwards and backwards from the placement
of the component exhibiting the {\fm} under investigation.
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Also, whether following the effects through the signal path {\em only} is acceptable, and instead
looking at its effect on all other components in the system is necessary,
is a matter for debate, and is examined in section~\ref{sec:xfmea}.
\paragraph{Single component failure mode to system failure relation.}
% NEED SOME NICE HISTORICAL REFS HERE
FMEA, due to its inductive bottom-up approach, is good
at mapping potential single component failures to system level faults/events.
Used in the design phase of a project, FMEA is a useful tool
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The concept of the unacceptability of a single component failure causing a system failure, % catastrophe,
is an important and easily understood measurement of safety.
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It is easy to calculate
because we can usually find Mean Time to Failure (MTTF) statistics for commonly used components.
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Also, used in the design phase of a project, FMEA is a useful tool
for discovering potential failure scenarios~\cite{1778436820050601}.
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% Subject Object Wiki answers : Best Answer
@ -655,7 +715,7 @@ methodologies.
% cite for forward and backward search related to safety critical software
%{sfmeaforwardbackward}
\subsection{FMEA and the State Explosion Problem}
\label{sec:xfmea}
\paragraph{Exhaustive Single Failure FMEA.}
FMEA for a safety critical certification~\cite{en298,en61508} will have to be applied