From 7a78aa264679c818505bf50301a0694f03a04857 Mon Sep 17 00:00:00 2001 From: "Robin P. Clark" Date: Fri, 13 Sep 2013 15:39:06 +0100 Subject: [PATCH] C Garret CH2 comments 13SEP2013 --- submission_thesis/CH2_FMEA/copy.tex | 66 +++++++++++++++-------------- 1 file changed, 34 insertions(+), 32 deletions(-) diff --git a/submission_thesis/CH2_FMEA/copy.tex b/submission_thesis/CH2_FMEA/copy.tex index ffb36c8..52dfff2 100644 --- a/submission_thesis/CH2_FMEA/copy.tex +++ b/submission_thesis/CH2_FMEA/copy.tex @@ -94,7 +94,7 @@ function that they perform. 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. +These are termed `{\bcs}'; they are considered ``atomic'' i.e. they are not broken down further. % The first requirement for a {\bc} is to define the ways in which it can fail, this relationship %between a {\bc} and its failure modes, @@ -177,7 +177,7 @@ can in some cases indicate how a component could fail/misbehave. %under given conditions. % How %base -components could fail internally, is not of interest to an FMEA investigation. +components could fail internally is not of interest to an FMEA investigation. The FMEA investigator needs to know what failure behaviour a component could exhibit. %, or in other words, its modes of failure. % A large body of literature exists giving guidance for the determination of component {\fms}. @@ -219,7 +219,7 @@ FIT claims for modern integrated micro-controllers are typically less than five~ % The FMEA variant\footnote{EN61508 (and related standards) are based on the FMEA variant Failure Mode Effects and Diagnostic Analysis (FMEDA)} used for European standard EN61508~\cite{en61508} -requires statistics for Meantime to Failure (MTTF) for all {\bc} failure modes. +requires statistics for Mean Time to Failure (MTTF) for all {\bc} failure modes. % One is from the US military document FMD-91, where internal failures @@ -268,11 +268,11 @@ European burner standard EN298~\cite{en298}, are compared and contrasted. \paragraph{Resistor failure modes according to FMD-91.} \fmodegloss -The resistor is a ubiquitous component in electronics, and is therefore a good candidate for detailed examination of its failure modes. +%The resistor is a ubiquitous component in electronics, and is therefore a good candidate for detailed examination of 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}} the following failure causes are given: +and lists many possible failure causes, +for instance for {\textbf{Resistor,~Fixed,~Film}} the following failure causes are given: \begin{itemize} \item Opened 52\% , \item Drift 31.8\% , @@ -302,6 +302,8 @@ as listed below: \item Lead damage 1.9\% $\mapsto$ OPEN. \end{itemize} % +The symptomatic descriptor chosen is based on experience and are not unique. +% 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. @@ -364,7 +366,7 @@ 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 +For this study the conservative view from EN298, but restrictive view from FMD-91 (i.e. no DRIFT) is taken, and the failure modes for a generic resistor taken to be both OPEN and SHORT. The function $fm$ is used to return a set of failure modes, i.e. @@ -407,8 +409,8 @@ For Op-Amp failures modes, FMD-91\cite{fmd91}{3-116] states, \begin{itemize} \item Degraded Output 50\% Low Slew rate - poor die attach \item No Operation - overstress 31.3\% - \item Shorted $V_+$ to $V_-$, overstress, resistive short in amplifier 12.5\% - \item Opened $V_+$ open 6.3\% + \item Shorted inputs (labelled $V_+$ to $V_-$), overstress, resistive short in amplifier 12.5\% + \item Opened input (labelled $V_+$) open 6.3\% \end{itemize} These are mostly internal causes of failure, more of interest to the component manufacturer @@ -428,7 +430,7 @@ This means that the op-amp will not react quickly to changes on its input termin 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 entirely. -This failure cause can be mapped to a symptomatic {\fm} called $LOW_{slew}$. +This failure cause can be mapped to a symptomatic {\fm} called $LOW\_SLEW$. \paragraph{No Operation - over stress.} Here the OP-Amp has been damaged, and the output may be held HIGH or LOW, or may be @@ -445,13 +447,13 @@ This failure cause maps to $HIGH$ or $LOW$. \paragraph{Open input: $V_+$.} 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. -This failure cause maps to $HIGH$ or $LOW$. +This failure cause maps to $HIGH$ or $LOW$.\footnote{No failure mode for open input $V_-$ was listed in this FMD-91 entry~\cite{fmd91}{3-116].} \paragraph{Collecting Op-Amp failure modes from FMD-91.} 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} @@ -468,11 +470,11 @@ 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 -% $LOW_{slew}$. +% $LOW\_SLEW$. % 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 -$LOW_{slew}$. +$LOW\_SLEW$. %\paragraph{EN298: Open and shorted pin failure symptom determination technique} @@ -507,23 +509,23 @@ $LOW_{slew}$. & & infinite gain on B-input & & $LOW_B$ or $HIGH_B$ \\ \hline FS6: PIN 6 OPEN & & B-input disconnected, & & \\ - FS6: PIN 6 OPEN & & infinite gain on B+input & & $LOW_B$ or $HIGH_B$ \\ \hline + FS6: & & infinite gain on B+input & & $LOW_B$ or $HIGH_B$ \\ \hline FS7: PIN 7 OPEN & & B output open & & $NOOP_B$ \\ \hline FS8: PIN 8 OPEN & & power to chip & & \\ - FS8: PIN 8 OPEN & & (Vcc) disconnected & & $NOOP_A$ and $NOOP_B$ \\ \hline + FS8: & & (V+ supply) disconnected & & $NOOP_A$ and $NOOP_B$ \\ \hline & & & & \\ % & & & & \\ % & & & & \\ \hline - FS9: PIN 1 $\stackrel{short}{\longrightarrow}$ PIN 2 & & A -ve 100\% Feed back, low gain & & $LOW_A$ \\ \hline + FS9: PIN 1 $\stackrel{short}{\longrightarrow}$ PIN 2 & & A -ve 100\% Feed back, unity gain & & $LOW_A$ \\ \hline FS10: PIN 2 $\stackrel{short}{\longrightarrow}$ PIN 3 & & A inputs shorted, & & \\ & & output controlled by internal offset & & $LOW_A$ or $HIGH_A$ \\ \hline - FS11: PIN 3 $\stackrel{short}{\longrightarrow}$ PIN 4 & & A + input held to ground & & $LOW_A$ \\ \hline + FS11: PIN 3 $\stackrel{short}{\longrightarrow}$ PIN 4 & & A + input held to ground & & $LOW_A$ or $HIGH_A$ \\ \hline FS12: PIN 5 $\stackrel{short}{\longrightarrow}$ PIN 6 & & B inputs shorted, & & \\ & & output controlled by internal offset & & $LOW_B$ or $HIGH_B$ \\ \hline @@ -554,14 +556,14 @@ $LOW_{slew}$. 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: EN298 vs FMD-91} The EN298 pinouts failure mode technique cannot reveal failure modes due to internal failures, -and that is why it misses the $LOW_{slew}$. +and that is why it misses the $LOW\_SLEW$. % The FMD-91 entries for op-amps are not directly usable as component {\fms} in FMEA and require interpretation. @@ -783,13 +785,13 @@ of the electronic circuit for each analysis. \paragraph{Single component failure mode to system failure relation.} - - +% +% % NEED SOME NICE HISTORICAL REFS HERE FMEA, due to its inductive bottom-up approach, is good at mapping potential single component failures to system level faults/events. % -The concept of the unacceptability of a single component failure causing a system failure, % catastrophe, +The concept of the unacceptability of a single component failure causing a system failure % catastrophe, is an important and easily understood measurement of safety. % They are easy to calculate @@ -1327,7 +1329,7 @@ or realistic levels of risk. % SIL levels are intended to classify the statistical safety of installed plant: -salesmen’s terms such as a `SIL~3~sensor' or other `device' given a SIL level, are meaningless. +sales terms such as a `SIL~3~sensor' or other `device' given a SIL level, are meaningless. % SIL analysis is concerned with `safety~loops', not individual modules, sensors, computing devices or actuators. % @@ -1449,13 +1451,13 @@ by statistically determining how frequently it can fail dangerously. \section{FMEA used for Safety Critical Approvals} \fmmdglossDFMEA \subsection{DESIGN FMEA: Safety Critical Approvals FMEA} -\begin{figure}[h] - \centering - \includegraphics[width=300pt,keepaspectratio=true]{./CH2_FMEA/tech_meeting.png} - % tech_meeting.png: 350x299 pixel, 300dpi, 2.97x2.53 cm, bb=0 0 84 72 - \caption{FMEA Meeting} - \label{fig:tech_meeting} -\end{figure} +% \begin{figure}[h] +% \centering +% \includegraphics[width=300pt,keepaspectratio=true]{./CH2_FMEA/tech_meeting.png} +% % tech_meeting.png: 350x299 pixel, 300dpi, 2.97x2.53 cm, bb=0 0 84 72 +% \caption{FMEA Meeting} +% \label{fig:tech_meeting} +% \end{figure} %Static FMEA, Design FMEA, Approvals FMEA % Experts from Approval House and Equipment Manufacturer @@ -1517,7 +1519,7 @@ potential strategies are listed below: % \begin{itemize} \item Look at all components electronically adjacent (i.e. connected to the affected component), - \item Look at all components connected (as above) and those one removed (those connected to those connected to the affected component), + \item Look at all components connected (as above) and those once removed (those connected to those connected to the affected component), \item Look at components forward of the {\fm} in the signal path, \item Look at all components in the signal path, \item Look at all components in the signal path including those one connection removed,