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waiting for my wheel to be trued by furtue cycles, fixed some typos etc

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after work......
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Robin P. Clark 2012-08-21 17:24:46 +01:00
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submission_thesis/CH2_FMEA/copy.tex
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@ -15,12 +15,13 @@ on the behaviour and safety of the system."
%\tableofcontents[currentsection] %\tableofcontents[currentsection]
FMEA is a broad term, and can mean anything from an informal check on how FMEA is a broad term; it could mean anything from an informal check on how
how failures could affect some equipment in an initial brain-storming session how failures could affect some equipment in an initial brain-storming session
in product design to formal submissions as part of safety critical certification in product design, to formal submissions as part of safety critical certification.
procedures %
This chapter describes the basic concepts, uses a simple example to This chapter describes basic concepts of FMEA, uses a simple example to
demonstrate an FMEA stage and then explores some concepts with which we can evaluate demonstrate a single FMEA analysis stage, describes the four main variants of FMEA in use today
and explores some concepts with which we can discuss and evaluate
the effectiveness of FMEA. the effectiveness of FMEA.
@ -46,8 +47,8 @@ demonstrate an FMEA stage and then explores some concepts with which we can eval
% % \item Analysis % % \item Analysis
% % \end{itemize} % % \end{itemize}
\clearpage
\subsection{FMEA basic concept} \paragraph{FMEA basic concept.}
\begin{itemize} \begin{itemize}
@ -62,7 +63,7 @@ demonstrate an FMEA stage and then explores some concepts with which we can eval
FMEA is a procedure based on the low level components of a system, and an example FMEA is a procedure based on the low level components of a system, and an example
analysis will serve to demonstrate it in practise. analysis will serve to demonstrate it in practise.
\subsection{ FMEA Example: Milli-volt reader} \paragraph{ FMEA Example: Milli-volt reader}
Example: Let us consider a system, in this case a milli-volt reader, consisting Example: Let us consider a system, in this case a milli-volt reader, consisting
of instrumentation amplifiers connected to a micro-processor of instrumentation amplifiers connected to a micro-processor
that reports its readings via RS-232. that reports its readings via RS-232.
@ -89,7 +90,7 @@ For the sake of example let us choose resistor R1 in the OP-AMP gain circuitry.
\subsection{FMEA Example: Milli-volt reader} \paragraph{FMEA Example: Milli-volt reader}
% \begin{figure} % \begin{figure}
% \centering % \centering
% \includegraphics[width=80pt]{./mvamp.png} % \includegraphics[width=80pt]{./mvamp.png}
@ -105,14 +106,21 @@ For the sake of example let us choose resistor R1 in the OP-AMP gain circuitry.
The analysis above has given us a result for one failure scenario i.e.
for one component failure mode.
A complete FMEA report would have to contain an entry
for each failure mode of all the components in the system under investigation.
%
Note here that we have had to look at the failure~mode Note here that we have had to look at the failure~mode
in relation to the entire circuit. in relation to the entire circuit.
We have used intuition to determine the probable We have used intuition to determine the probable
effect of this failure mode. effect of this failure mode.
We have not examined this failure mode For instance we have assumed that the resistor R1 going SHORT
will not affect the ADC, the Microprocessor or the UART.
%
To put this in more general terms, have not examined this failure mode
against every other component in the system. against every other component in the system.
Perhaps we should.... this would be a more rigorous and complete Perhaps we should: this would be a more rigorous and complete
approach in looking for system failures. approach in looking for system failures.
@ -135,18 +143,16 @@ are thus very reliable too. Reliable field data on failures will, therefore be s
Should we wish to prove a continuous demand system for say ${10}^{-7}$ failures\footnote{${10}^{-7}$ failures per hour of operation is the Should we wish to prove a continuous demand system for say ${10}^{-7}$ failures\footnote{${10}^{-7}$ failures per hour of operation is the
threshold for S.I.L. 3 reliability~\cite{en61508}.} threshold for S.I.L. 3 reliability~\cite{en61508}.}
per hour of operation, even with 1000 correctly monitored units in the field per hour of operation, even with 1000 correctly monitored units in the field
we could only expect one failure per ten thousand hours (a little over one a year) to fail. we could only expect one failure per ten thousand hours (a little over one a year).
It would be impractical to get statistically significant data for equipment It would be utterly impractical to get statistically significant data for equipment
at these reliability levels. at these reliability levels.
However, we can use FMEA (more specifically the FMEDA variant, see section~\ref{sec:FMEDA}), working from known component failure rates, to obtain However, we can use FMEA (more specifically the FMEDA variant, see section~\ref{sec:FMEDA}), working from known component failure rates, to obtain
statistical estimates of the equipment reliability. statistical estimates of the equipment reliability.
\subsection{Rigorous FMEA --- State Explosion} \subsection{Rigorous FMEA --- State Explosion Problem}
FMEA cannot consider---for practical reasons---a rigorous approach.
It must be applied by experts in the system under investigation
to be a meaningful analysis.
\paragraph{Rigorous Single Failure FMEA} \paragraph{Rigorous Single Failure FMEA}
@ -191,7 +197,14 @@ For our theoretical 100 components with 3 failure modes each example, this is
$100*99*98*3=2,910,600$ failure mode scenarios. $100*99*98*3=2,910,600$ failure mode scenarios.
\paragraph{Reliance of experts for meaningful FMEA Analysis.}
FMEA cannot consider---for practical reasons---a rigorous approach.
We define rigorous FMEA as examining the effect of every component failure mode
against the remaining components in the system under investigation.
%
Because we cannot perform rigorous FMEA,
we rely on experts in the system under investigation
to perform a meaningful FMEA analysis.
@ -214,7 +227,7 @@ $100*99*98*3=2,910,600$ failure mode scenarios.
\section{PFMEA - Production FMEA : 1940's to present} \section{PFMEA - Production FMEA : 1940's to present}
\subsection{PFMEA}
Production FMEA (or PFMEA), is FMEA used to prioritise, in terms of Production FMEA (or PFMEA), is FMEA used to prioritise, in terms of
cost, problems to be addressed in product production. cost, problems to be addressed in product production.
@ -279,7 +292,6 @@ will return most cost benefit.
\subsection{PFMEA Example: Ford Pinto: 1975}
\begin{figure}[h] \begin{figure}[h]
\centering \centering
@ -294,8 +306,6 @@ will return most cost benefit.
\subsection{PFMEA Example: Ford Pinto: 1975}
\begin{table}[ht] \begin{table}[ht]
\caption{FMEA Calculations} % title of Table \caption{FMEA Calculations} % title of Table
%\centering % used for centering table %\centering % used for centering table
@ -315,7 +325,7 @@ will return most cost benefit.
http://www.youtube.com/watch?v=rcNeorjXMrE % don't think this is relevant for the thesis: http://www.youtube.com/watch?v=rcNeorjXMrE