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submission_thesis/CH2_FMEA/copy.tex
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@ -16,7 +16,7 @@ This chapter introduces Failure Mode Effect Analysis (FMEA).
%and then
It starts with a generic conceptual overview of the process.
It then looks at the stages of the FMEA process in greater detail, starting with
how we determine the failure modes associated with components.
how to determine the failure modes associated with components.
%
Two common electrical components, the resistor and the operational amplifier
are examined in the context of two sources of information that define failure modes.
@ -34,10 +34,8 @@ By using UML
the entities needed to implement FMEA
are defined.
%
The act
of defining relationships between the data objects
in FMEA raises questions about the nature of the process
and allows us to analytically discuss its strengths and weaknesses.
The act of defining relationships between the data objects in FMEA raises questions about the nature of the process
and allows analysis of its strengths and weaknesses.
@ -65,7 +63,7 @@ a brain-storming session
%in product design,
to formal submission as part of safety critical certification.
FMEA is a manual, % and therefore
time intensive process. To reduce the amount of manual work to perform,
time intensive process. To reduce the amount of manual work performed,
software packages~\cite{931423, 1778436820050601} and analysis strategies have
been developed~\cite{incrementalfmea, automatingFMEA1281774}.
%
@ -93,7 +91,7 @@ function that they perform.
\fmeagloss
\section{FMEA Process}
We begin FMEA with the basic, or starting components.
The initial stage of the FMEA process is with the basic, or starting components.
%
These components are the sort bought in or considered as pre-assembled modules.
These are termed {\bcs}; they are considered ``atomic'' i.e. they are not broken down further.
@ -126,7 +124,7 @@ In practise, each entry of an FMEA analysis of a {\bc} {\fm}
would typically be one line in a spreadsheet.
%
The analysis to symptom relationship is generally % considered
one-to-one, however here (see figure~\ref{fig:component_fm_rel_ana}), we allow for the possibility
one-to-one, however here (see figure~\ref{fig:component_fm_rel_ana}), allowance is made for the possibility
of more than one failure symptom.
%DIAGRAM of reasoning and Symptoms.
@ -152,7 +150,7 @@ In order to apply any form of FMEA the ways in which
the {\bcs}\footnote{A good introduction to hardware and software failure modes may be found in~\cite{sccs}[pp.114-124].} %used
can fail must be clearly defined.
%
In practise, this part of the process is guided by
In practice, this part of the process is guided by %%% PRACTICE NOUN Practice makes perfect.------------------- PRACTISE --- VERB I practise the piano.
the particular standard
which is being conformed to. %we are seeking to conform.% to.
%
@ -160,10 +158,22 @@ Standards may differ in their definitions for the {\fms} of {\bcs}.
The reasons for these differences are examined below using two example components.
%
%
Typically, when choosing components for a design, engineers will look at manufacturers' data sheets
%%%%%%%%%% DATA SHEETS and FAILURE MODES %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
Typically, when choosing components for a design, engineers will look at manufacturers' data~sheets
which describe functionality, physical dimensions,
environmental ranges, tolerances and by `reading~between~the~lines'
in some cases can indicate how a component may fail/misbehave.
environmental ranges, tolerances.
%
It is rare for a data~sheet to list failure modes.
%
Data~sheets after all are a sales tool as well as being a usage guide and technical description.
%
However, `reading~between~the~lines' or noting what is not~stated,
can in some cases indicate how a component could fail/misbehave.
%
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%under given conditions.
%
How %base
@ -242,12 +252,14 @@ and examined against two sources of {\fm} information. % define their failure mo
%
These definitions for a given generic component may not always agree.
%
The reasons why some {\fms}
can be found in one source but not in the others and vice versa, are discussed.
The reasons why, some {\fms}
can be found in one source, but not in the others and vice versa, are discussed.
%
Finally the failure modes determined %for these components
Finally, the failure modes determined %for these components
from the FMD-91~\cite{fmd91} reference source and from the guidelines of the
European burner standard EN298~\cite{en298} are compared and contrasted.
European burner standard EN298~\cite{en298}, are compared and contrasted.
\clearpage
\subsection{Failure mode determination for generic resistor.}
\label{sec:resistorfm}
@ -289,11 +301,11 @@ as listed below:
\item Lead damage 1.9\% $\mapsto$ OPEN.
\end{itemize}
%
Note that the main causes of resistor value drift are overloading. % of components.
Note, that the main cause of resistor value drift is overloading. % of components.
This is borne out in the FMD-91~\cite{fmd91}[232] entry for a resistor network where the failure
modes do not include drift.
%
If it is ensured that our resistors will not be exposed to overload conditions, the
If it is ensured that resistors will not be exposed to overload conditions, the
probability of drift (sometimes called parameter change) %occurring
is significantly reduced, enough for some standards to exclude it~\cite{en298,en230}.
@ -301,7 +313,7 @@ is significantly reduced, enough for some standards to exclude it~\cite{en298,en
\paragraph{Resistor failure modes according to EN298.}
EN298, the European gas burner safety standard,
tends to be give failure modes more directly
tends to give failure modes that are more directly
usable for performing FMEA than FMD-91.
%
The certification process for EN298 requires that a full FMEA be undertaken, examining all failure modes
@ -345,8 +357,10 @@ limit of resolution in any failure analysis methodology.
\subsubsection{Resistor Failure Modes}
\label{sec:res_fms}
The differences in resistor failure modes between FMD-91 and EN298 are that FMD-91 would
include the failure mode DRIFT. EN298 does not include this, mainly because it imposes circuit design constraints
The difference in resistor failure modes between FMD-91 and EN298 is that FMD-91 would
include the failure mode DRIFT.
%
EN298 does not include this, mainly because it imposes circuit design constraints
that effectively side step that problem.
%
For this study the conservative view from EN298 is taken, and the failure
@ -355,17 +369,15 @@ to return a set of failure modes,
i.e.
\label{ros}
$$ fm(R) = \{ OPEN, SHORT \} . $$
%
%
% Mention tolerance here
%
% hmmmmmm
%
\subsection{Failure modes determination for generic operational amplifier}
%
\subsection{Failure modes determination for a generic operational amplifier}
%
The operational amplifier (op-amp) %is a differential amplifier and
is very widely used in nearly all fields of modern analogue electronics.
%
@ -380,14 +392,12 @@ components types not specifically listed in it.
Operational amplifiers are typically packaged in dual or quad configurations---meaning
that a chip will typically contain two or four amplifiers.
%
For the purpose of example for EN298, %we look at
a typical op-amp designed for instrumentation and measurement, the dual packaged version of the LM358~\cite{lm358}
(see figure~\ref{fig:lm258}) is examined.
The failure modes determined from the FMD-91 entries are presented and then
the failure mode determination procedure of EN298
is applied to a typical op-amp designed for instrumentation and measurement, the dual packaged version of the LM358~\cite{lm358}
(see figure~\ref{fig:lm258}).
%
With the results from both sources of {\fm} definition %
%we compare
the failure mode definitions for FMD-91 and EN298
relating to operational amplifiers are compared.
The results from both sources of {\fm} definition are then compared.
\paragraph{Failure Modes of an Op-Amp according to FMD-91.}
\fmodegloss
@ -400,7 +410,7 @@ For Op-Amp failures modes, FMD-91\cite{fmd91}{3-116] states,
\item Opened $V_+$ open 6.3\%
\end{itemize}
Again these are mostly internal causes of failure, more of interest to the component manufacturer
These are mostly internal causes of failure, more of interest to the component manufacturer
than a test engineer % designer
looking for the symptoms of failure.
%
@ -437,29 +447,30 @@ of the Op-Amp applied to it, and the output will be forced HIGH or LOW.
This failure cause maps to $HIGH$ or $LOW$.
\paragraph{Collecting Op-Amp failure modes from FMD-91.}
An Op-Amps' failure mode behaviour, under FMD-91 definitions will have the following {\fms}.
An Op-Amp's failure mode behaviour, under FMD-91 definitions will have the following {\fms}:
\begin{equation}
\label{eqn:opampfms}
fm(OpAmp) = \{ HIGH, LOW, NOOP, LOW_{slew} \}
fm(OpAmp) = \{ HIGH, LOW, NOOP, LOW_{slew} \} .
\end{equation}
\paragraph{Failure Modes of an Op-Amp according to EN298.}
EN298 does not specifically define OP\_AMPS failure modes; these can be determined
EN298 does not specifically define op-amp failure modes; these can be determined
by following a procedure for `integrated~circuits' outlined in
annex~A~\cite{en298}[A.1 note e].
%
This demands that all open connections, and shorts between adjacent pins be considered as failure scenarios.
We examine these failure scenarios on the dual packaged $LM358$~\cite{lm358} %\mu741$
and determine its {\fms} in table ~\ref{tbl:lm358}.
%
In table ~\ref{tbl:lm358} these failure scenarios on the dual packaged $LM358$~\cite{lm358} %\mu741$
are examined and from this its {\fms} are determined.
%
% Collecting the op-amp failure modes from table ~\ref{tbl:lm358} we obtain the same {\fms}
% that we got from FMD-91, listed in equation~\ref{eqn:opampfms}, except for
% $LOW_{slew}$.
%
Collecting the op-amp failure modes from table ~\ref{tbl:lm358} the same {\fms}
that we got from FMD-91 are obtained---listed in equation~\ref{eqn:opampfms}---except for
Collating the op-amp failure modes from table ~\ref{tbl:lm358} the same {\fms}
from FMD-91 are obtained---listed in equation~\ref{eqn:opampfms}---except for
$LOW_{slew}$.
@ -539,13 +550,13 @@ $LOW_{slew}$.
\subsubsection{Failure modes of an Op-Amp}
\label{sec:opamp_fms}
For the purpose of the examples to follow, the op-amp will
have the following failure modes:-
For the purpose of the examples to follow in this document, op-amp's
are assigned the following failure modes:
%
$$ fm(OPAMP) = \{ LOW, HIGH, NOOP, LOW_{slew} \} . $$
%
$$ fm(OPAMP) = \{ LOW, HIGH, NOOP, LOW_{slew} \} $$
\subsection{Comparing the component failure mode sources}
\subsection{Comparing the component failure mode sources: EN298 vs FMD-91}
The EN298 pinouts failure mode technique cannot reveal failure modes due to internal failures,
@ -625,11 +636,16 @@ be used throughout the FMEA and FMMD process.
\section{FMEA worked example: milli-volt reader.}
FMEA is a bottom-up procedure which starts with the failure modes of the low level components of a system, an example
analysis will serve to demonstrate it in practise.
Example: Let us consider a system, in this case a simple milli-volt reader, consisting
%
FMEA is a bottom-up procedure which starts with the failure modes of the low level components of a system.
%
An example analysis will serve to demonstrate it in practice.
%
%
Consider a system of a simple milli-volt reader, consisting
of instrumentation amplifiers connected to a micro-processor
that reports its readings via RS-232.
%
\begin{figure}
\centering
\includegraphics[width=175pt]{./CH2_FMEA/mvamp.png}
@ -642,11 +658,10 @@ that reports its readings via RS-232.
\subsection{FMEA Example: Milli-volt reader}
Let us perform an FMEA and consider how one of its resistors failing could affect
it.
%For the sake of example
Let us choose resistor R1 in the OP-AMP gain circuitry.
\subsection{FMEA Example: Milli-volt reader}
%
Undertaking an FMEA on the milli-volt reader to consider how one of its resistors failing could affect
it and choosing the resistor R1 in the OP-AMP gain circuitry:
% \begin{figure}
% \centering
% \includegraphics[width=175pt]{./mvamp.png}
@ -662,31 +677,33 @@ Let us choose resistor R1 in the OP-AMP gain circuitry.
% % mvamp.png: 561x403 pixel, 72dpi, 19.79x14.22 cm, bb=0 0 561 403
% \end{figure}
\begin{itemize}
\item \textbf{F - Failures of given component} The resistor (R1) could fail by going OPEN or SHORT (EN298 definition).
\item \textbf{M - Failure Mode} Consider the component failure mode SHORT
\item \textbf{E - Effects} This will drive the minus input LOW causing a HIGH OUTPUT/READING
\item \textbf{A - Analysis} The reading will be out of the normal range, i.e. will have an erroneous milli-volt reading
\item \textbf{F - Failures of given component} The resistor (R1) could fail by going OPEN or SHORT (EN298 definition),
\item \textbf{M - Failure Mode} Consider the component failure mode SHORT,
\item \textbf{E - Effects} This will drive the minus input LOW causing a HIGH OUTPUT/READING,
\item \textbf{A - Analysis} The reading will be out of the normal range, i.e. will have an erroneous milli-volt reading.
\end{itemize}
\fmeagloss
The analysis above has given a result for % one failure %scenario i.e.
one single 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.
%
In theory it would be necessary to look at the failure~mode
in relation to the entire circuit.
%
Intuition has been used to determine the probable
effect of this failure mode.
%
For instance it has been assumed that the resistor R1 going SHORT
will not affect the ADC, the Microprocessor or the UART.
%
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%% WE removal project ends here today 08SEP2013 %%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
The analysis above has given us a result for % one failure %scenario i.e.
one single 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.
%
In theory we have had to look at the failure~mode
in relation to the entire circuit.
%
We have used intuition to determine the probable
effect of this failure mode.
%
For instance we have assumed that the resistor R1 going SHORT
will not affect the ADC, the Microprocessor or the UART.
%
We have taken the {\bc} {\fm} R1 SHORT and then followed the failure reasoning path through to a putative system level symptom.
We have not looked in detail at any side effects of this {\fm}.