put proof for comparison equations in

PLT
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Robin Clark 2011-11-11 18:35:43 +00:00
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@ -1,6 +1,6 @@
PNG_DIA = circuit1_dag.png mvampcircuit.png pd.png invamp.png shared_component.png tree_abstraction_levels.png PNG_DIA = circuit1_dag.png mvampcircuit.png pd.png invamp.png shared_component.png tree_abstraction_levels.png three_tree.png

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@ -813,7 +813,7 @@ This can be simplified if we can determine the total number of failure modes in
equation~\ref{eqn:rd} becomes $$ RD(fg) = fT.(|fg|-1).$$ equation~\ref{eqn:rd} becomes $$ RD(fg) = fT.(|fg|-1).$$
\pagebreak[4] \pagebreak[4]
\subsection{Reasoning Distance Examples} \subsection{Reasoning Distance Examples}(c-1)
The potential divider discussed in section~\ref{potdivfmmd} has a four failure modes and two components and therefore has an $RD$ of 4. The potential divider discussed in section~\ref{potdivfmmd} has a four failure modes and two components and therefore has an $RD$ of 4.
$$RD(potdiv) = \sum_{n=1}^{2} |2|.(|1|) = 4 $$ $$RD(potdiv) = \sum_{n=1}^{2} |2|.(|1|) = 4 $$
@ -823,5 +823,96 @@ having 3 failure modes each, we would have an $RD$ of
$$RD(fictitious) = \sum_{n=1}^{81} |3|.(|80|) = 19440 .$$ $$RD(fictitious) = \sum_{n=1}^{81} |3|.(|80|) = 19440 .$$
This would be the polynomial ($O(N^2)$) result of applying FMEA rigorously (we could term this
Rigorous FMEA (RFMEA).
\pagebreak[4]
\subsection{Using the concept of Reasoning Distance to compare RFMEA with FMMD}
\begin{figure}
\centering
\includegraphics[width=400pt,keepaspectratio=true]{./three_tree.png}
% three_tree.png: 851x385 pixel, 72dpi, 30.02x13.58 cm, bb=0 0 851 385
\caption{FMMD Hierarchy with $(|fg| = 3) \wedge (|fm(c)| = 3)$}
\label{fig:three_tree}
\end{figure}
Because components have variable numbers of failure modes,
and {\fgs} have variable numbers of components it is difficult to
come up with a general formula for comparing the number of checks to make for
RFMEA and FMMMD.
If we were to create an example by fixing the number of components in a {\fg}
and the number of failure modes per component, we can derive formulae
to represent the number of checks to make.
Consider $k$ to be the number of components in a {\fg} (i.e. $k=|fg|$),
$f$ is the number of failure modes per component (i.e. $f=|fm(c)|$), and
$L$ to be the number of levels in the hierarchy of an FMMD analysis.
We can represent the number of failure scenarios to check in an FMMD
with equation~\ref{eqn:anscen}.
\begin{equation}
\label{eqn:anscen}
\sum_{n=0}^{L} {k}^{n}.k.f.(k-1)
\end{equation}
The thinking behind equation~\ref{eqn:anscen}, is that for each level of analysis -- counting down from the top --
there are ${k}^{n}$ {\fgs} within each level; we need to apply RFMEA to each {\fg} on the level.
The number of checks to make for RFMEA is number of components $k$ multiplied by the number of failure modes $f$
checked against the remaining components in the {\fg} $(k-1)$.
If, for the sake of example we fix the number of components in a {\fg} to three and
the number of failure modes per component to three, an FMMD hierarchy
would look like figure~\ref{fig:three_tree}.
\subsection{Worked Example}
Using the diagram in figure~\ref{fig:three_tree}, we have three levels of analysis.
Starting at the top, we have a {\fg} with three derived components, each of which has
three failure modes.
Thus the number of checks to make in the top level is $3^0.3.2.3=18$.
On the level below that, we have three {\fgs} each with a
an identical number of checks, $3^1.3.2.3=56$.{\fg}
On the level below that we have nine {\fgs}, $3^2.3.2.3=168$.
Adding these together gives $242$ checks to make to perform RFMEA \textbf{within}
{\fgs}.
If we were to take the system represented in figure~\ref{fig:three_tree}, and
apply RFMEA on it as a whole system, we can use equation~\ref{eqn:rd},
$ RD(fg) = \sum_{n=1}^{|fg|} |fm(c_n)|.(|fg|-1)$, where $|fg|$ is 27, $fm(c_n)$ is 3
and $(|fg|-1)$ is 26.
This gives:
$ RD(fg) = \sum_{n=1}^{27} |3|.(|27|-1) = 2106$
In order to get general equations with which to compare RFMEA with FMMD
we can re-write equation~\ref{eqn:rd} in terms of the number of levels
in an FMMD hierarchy. The number of components in is number of components
in a {\fg} raised to the power of the level plus one.
Thus we re-write equation~\ref{eqn:rd} as:
\begin{equation}
\label{eqn:fmea_state_exp21}
\sum_{n=1}^{k^{L+1}}.(k^{L+1}-1).f \; , % \\
%(N^2 - N).f
\end{equation}
or
\begin{equation}
\label{eqn:fmea_state_exp22}
k^{L+1}.(k^{L+1}-1).f \;. % \\
%(N^2 - N).f
\end{equation}
We can now use equation~\ref{eqn:anscen} and \ref{eqn:fmea_state_exp22} to compare (for fixed sizes of $|fg|$ and $|fm(c)|$)
the two approaches, for the work required to perform rigorous checking.
\subsection{Exponential squared to Exponential}
can I say that ?
\end{document} \end{document}

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