73 lines
3.0 KiB
TeX
73 lines
3.0 KiB
TeX
\label{sec:chap3}
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\section{Historical Origins of FMEA}
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\subsection{FMEA designed for simple electro-mechanical systems}
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FMEA traces it roots to the 1940s when it was used to identify the most costly
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failures arising from car mass-production~\cite{pfmea}.
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It was later modified slightly to include severity of the top level failure (FMECA~\cite{fmeca}).
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In the 1980s FMEA was extended again (FMEDA~\cite{fmeda}) to provide statistics
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for predicting failure rates.
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However a typical entry in each of the above methodologies, starts with a
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particular component failure mode and associates it with a system---or top level---failure symptom.
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This analysis philosophy has not changed since FMEA was first used.
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\subsection{FMEA does not support modularity.}
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It is a common practise in the process control industry to buy in sub-systems, typically sensors and actuators connected to an industrially hardened computer bus, i.e. CANbus~\cite{can,canspec}, modbus~\cite{modbus} etc.
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Most sensor systems now are `smart', that is to say, they contain programmatic elements
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even if their outputs are %they supply
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analogue signals. For instance a liquid level sensor that
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supplies a {\ft} output, would have been typically have been implemented
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in analogue electronics before the 1980s. After that time, it would be common to use a micro-processor
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based system to perform the functions of reading the sensor and converting it to a current (\ft) output.
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For the non-safety critical systems integrator this brings with it the advantages
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that come with using a digital system (increased accuracy, self checking and ease of
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calibration etc. ). For a safety critical systems integrator this can be very problematic when it
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comes to approvals. Even if the sensor manufacturer will let you see the internal workings and software
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we have a problem with tracing the FMEA reasoning through the sensor, through the sensors software
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and then though the system being integrated.
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This problem is compounded by the fact that traditional FMEA cannot integrate software into FMEA models~\cite{sfmea,safeware}.
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\section{Reasoning Distance used to measure Comparison Complexity}
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\section{FMEA - General Criticism}
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\subsection{FMEA - General Criticism}
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\begin{itemize}
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\item FMEA type methodologies were designed for simple electro-mechanical systems of the 1940's to 1960's.
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\item Reasoning Distance - component failure to system level symptom
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\item State explosion - impossible to perform rigorously
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\item Difficult to re-use previous analysis work
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\item Very Difficult to model simultaneous failures.
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\end{itemize}
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%
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\subsection{FMEA - Better Methodology - Wish List}
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\subsection{FMEA - Better Methodology - Wish List}
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\begin{itemize}
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\item State explosion
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\item Rigorous (total coverage)
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\item Reasoning Traceable
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\item Re-useable
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\item Simultaneous failures
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% \item
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\end{itemize}
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%FMEDA is a modern extension of FMEA, in that it will allow for
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%self checking features, and provides detailed recommendations for computer/software architecture,
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%but
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