diff --git a/submission_thesis/CH2_FMEA/copy.tex b/submission_thesis/CH2_FMEA/copy.tex index 08cd09b..0494ab7 100644 --- a/submission_thesis/CH2_FMEA/copy.tex +++ b/submission_thesis/CH2_FMEA/copy.tex @@ -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}.