diff --git a/thesis_submission/CH5_Examples/copy.tex b/thesis_submission/CH5_Examples/copy.tex index c602702..10ef890 100644 --- a/thesis_submission/CH5_Examples/copy.tex +++ b/thesis_submission/CH5_Examples/copy.tex @@ -103,15 +103,19 @@ allowing re-use of modules and reducing the number of by-hand analysis checks to -\subsection{A detailed look at failure symptoms of two components: the op-amp and the resistor} +\subsection{Determining the failure modes of components} In order to apply any form of Failure Mode Effects Analysis (FMEA) we need to know the ways in which the components we are using can fail. +Typically when choosing components for a design, we look at manufacturers data sheets, +which describe the range and tolerances, and can indicate how a component may fail/behave +under certain conditions or environments. How base components could fail internally, its not of interest to an FMEA investigation. The FMEA investigator needs to know what failure behaviour a component may exhibit, or in other words, its modes of failure. A large body of literature exists which gives guidance for for determining component {\fms}. -For this study FMD-91~\cite{fmd91} and the gas burner standard EN298~\cite{en298}. +% +For this study FMD-91~\cite{fmd91} and the gas burner standard EN298~\cite{en298} are examined. %Some standards prescribe specific failure modes for generic component types. In EN298 failure modes for generic component types are prescribed, or determined by a procedure where failure scenarios of all pins OPEN and all adjacent pins shorted @@ -122,7 +126,7 @@ FMD-91 is a reference document released into the public domain by the United Sta and describes {\fms} of common electronic components. FMD-91 entries include descriptions of internal failures along with {\fms}. FMD-91 entries need, in some cases, some interpretation to be mapped to a clear set of -component failure modes. +component {\fms} suitable for use in FMEA. % One is from the US military document FMD-91, where internal failures @@ -141,8 +145,8 @@ component failure modes. % I hope to have chapter 5 finished by the end of March, chapter 5 being the % electronics examples for the FMMD methodology. -We look in detail at two common electrical components in this section and examine how -two sources of information on failure modes view their failure mode behaviour. +In this section we look in detail at two common electrical components and examine how +the two sources of information define their failure mode behaviour. We look at the reasons why some known failure modes are omitted, or presented in specific but unintuitive ways. %We compare the US. military published failure mode specifications wi @@ -150,16 +154,17 @@ specific but unintuitive ways. We then compare and contrast the failure modes determined for these components from the FMD-91 reference source and from the guidelines of the European burner standard EN298. +\subsection{Failure mode determination for generic resistor} %- Failure modes. Prescribed failure modes EN298 - FMD91 +\subsubsection{Resistor failure modes according to FMD-91} + -\subsection{resistor} - -The resistor is a ubiquitous component in electronics, and is there fore a good -example for examining it failure modes. +The resistor is a ubiquitous component in electronics, and is therefore a prime +example for examining its failure modes. FMD-91\cite{fmd91}[3-178] lists many types of resistor and lists many possible failure causes. -For instance for {\textbf Resistor,~Fixed,~Film} we are given the following failure causes: +For instance for {\textbf{Resistor,~Fixed,~Film}} we are given the following failure causes: \begin{itemize} \item Opened 52\% \item Drift 31.8\% @@ -172,7 +177,7 @@ This information may be of insterest to the manufacturer of resistors, but it do help a circuit designer. The circuit designer is not interested in the causes of resistor failure, but to build in contingecy against symptoms of failure that the resistor could exhibit. -We can determine these symptoms and map these failure causes to three symptoms, +We can determine these {\fms} and map these failure causes to three symptoms, drift (resistance value changing), open and short. \begin{itemize} @@ -189,6 +194,8 @@ modes do not include drift. If we can ensure that our resistors will not be exposed to overload conditions, drift or parameter change can be reasonably excluded. +\subsubsection{Resistor failure modes according to EN298} + EN298 ,the European gas burner safety standard,tends to be give more symptom centric failure modes than FMD-91, and requires that a full FMEA be undertaken, examining all characterisic failure modes of all components~\cite{en298}[11.2 5]. @@ -226,15 +233,24 @@ i.e. $$ fm(R) = \{ OPEN, SHORT \} . $$ -\subsection{op-amp} +\subsection{Failure modes determination for generic OP-AMP} -The op-amp is a differential amplifier and is very widely used. +\begin{figure}[h+] + \centering + \includegraphics[width=200pt]{./lm258pinout.jpg} + % lm258pinout.jpg: 478x348 pixel, 96dpi, 12.65x9.21 cm, bb=0 0 359 261 + \caption{Pinout for an LM358 dual OP-AMP} + \label{fig:lm258} +\end{figure} + +The op-amp is a differential amplifier and is very widely used in nearly all fields of modern electronics. They are typically packaged in dual or quad configurations---meaning that a chip will typically contain two or four amplifiers. For the purpose of example, we look at -a typical op-amp designed for instrumentation and measurement, the dual packaged version of the LM358~\cite{lm358}. +a typical op-amp designed for instrumentation and measurement, the dual packaged version of the LM358~\cite{lm358} +(see figure~\ref{fig:lm258}). -\subsubsection{FMD-91 Op-AMP Failure Modes} +\subsubsection{ Failure Modes of an OP-AMP according to FMD-91 } %Literature suggests, latch up, latch down and oscillation. For OP-AMP failures modes, FMD-91\cite{fmd91}{3-116] states, @@ -244,10 +260,10 @@ For OP-AMP failures modes, FMD-91\cite{fmd91}{3-116] states, \item Opened $V_+$ open\% \end{itemize} -These are internal causes of failure, more of interest to the component manufacter +Again these are mostly internal causes of failure, more of interest to the component manufacturer than a designer looking for the symptoms of failure. -We need to translate these failure causes within the OP-AMP into symptoms. -We can look at each failure cause in turn. +We need to translate these failure causes within the OP-AMP into {\fms}. +We can look at each failure cause in turn, and map it to potential {\fms}. \paragraph{OP-AMP failure cause: Poor Die attach} The symptom for this is given as a low slew rate. This means that the op-amp @@ -255,12 +271,12 @@ will not react quickly to changes on its input terminals. This is a failure symptom that may not be of concern in a slow responding system like an instrumentation amplifier. However, where higher frequencies are being processed a signal may be lost. -We can map this failure cause to a failure symptom, and we can call it $LOW_{slew}$. +We can map this failure cause to a {\fm}, and we can call it $LOW_{slew}$. \paragraph{No Operation - over stress} Here the OP\_AMP has been damaged, and the output may be held HIGH LOW, or may be effectively tri-stated -, i.e. not able to drive circuitry in along the next stages of te signal path: we can call theis state NOOP (no Operation). - +, i.e. not able to drive circuitry in along the next stages of the signal path: we can call this state NOOP (no Operation). +% We can map this failure cause to three symptoms, $LOW$, $HIGH$, $NOOP$. \paragraph{Shorted $V_+$ to $V_-$} @@ -273,10 +289,11 @@ This failure cause will mean that the minus input will have the very high gain of the OP-AMP applied to it, and the output will be forced HIGH or LOW. We map this failure cause to $HIGH$ or $LOW$. -\paragraph{Collecting failure symptoms from FMD-91} -We can define an OP-AMP, under FMD-91 definitions to have the following failure mode symptoms. +\paragraph{Collecting OP-AMP failure modes from FMD-91} +We can define an OP-AMP, under FMD-91 definitions to have the following {\fms}. $$fm(OP-AMP) = \{ HIGH, LOW, NOOP, LOW_{slew} \} $$ +\subsubsection{Failure Modes of an OP-AMP according to EN298} EN298 does not specifically define OP\_AMPS failure modes; these can be determined by following a procedure for `integrated~circuits' outlined in @@ -286,13 +303,6 @@ We can examine these failure modes by taking a typical instrumentation op-amp, s and examining these conditions. -\begin{figure} - \centering - \includegraphics[width=200pt]{./lm258pinout.jpg} - % lm258pinout.jpg: 478x348 pixel, 96dpi, 12.65x9.21 cm, bb=0 0 359 261 - \caption{Pinout for an LM358 dual OP-AMP} - \label{fig:lm258} -\end{figure}