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working on design aid chapter.

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Robin Clark 2010-10-14 19:31:10 +01:00
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fmmd_design_aide/fmmd_design_aide.tex
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@ -145,6 +145,20 @@ TC:8 $OP\_AMP$ LATCH DOWN & low amp output & Out of Range &
\label{tab:fmmdaide1} \label{tab:fmmdaide1}
\end{table} \end{table}
This analysis process, which given the components R18,R22,R26,R30,IC1, has derived
the component "milli-volt amplifier" with two failure modes, `Out of Range' and
`Low reading'.
we can represent this in an FMMD hierarchy diagram, see figure \ref{fig:mvamp_fmmd}.
\begin{figure}[h]
\centering
\includegraphics[width=200pt,keepaspectratio=true]{./fmmd_design_aide/mvamp_fmmd.jpg}
% mvamp_fmmd.jpg: 281x344 pixel, 72dpi, 9.91x12.14 cm, bb=0 0 281 344
\caption{FMMD analysis Hierarchy for Milli-Volt Amplifier}
\label{fig:mvamp_fmmd}
\end{figure}
The table \ref{tab:fmmdaide1} shows two possible causes for an undetectable The table \ref{tab:fmmdaide1} shows two possible causes for an undetectable
error, that of a low reading due to the loss of the offset millivolt signal. error, that of a low reading due to the loss of the offset millivolt signal.
Typically this type of circuit would be used to read a thermocouple Typically this type of circuit would be used to read a thermocouple
@ -153,6 +167,7 @@ beleive that the temperature reading is lower than it actually is.
To take an example from a K type thermocouple, the offset of 1.86mV To take an example from a K type thermocouple, the offset of 1.86mV
from the potential divider represents amplified to $\approx \, 342mV$ would represent $\approx \; 46\,^{\circ}{\rm C}$. from the potential divider represents amplified to $\approx \, 342mV$ would represent $\approx \; 46\,^{\circ}{\rm C}$.
\clearpage
\subsection{Undetected Failure Mode: Incorrect Reading} \subsection{Undetected Failure Mode: Incorrect Reading}
Although statistically, this failure is unlikely (get stats for R short FIT etc from pt100 doc) Although statistically, this failure is unlikely (get stats for R short FIT etc from pt100 doc)
@ -163,6 +178,7 @@ test $R_{off}$ periodically.
For instance were we to check $R_off$ every $\tau = 20mS$ work out detection For instance were we to check $R_off$ every $\tau = 20mS$ work out detection
allowance according to EN61508. allowance according to EN61508.
\section{Proposed Checking Method} \section{Proposed Checking Method}
Were we to switch in a a second resistor in parrallel with the Were we to switch in a a second resistor in parrallel with the

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