diff --git a/fmmd_design_aide/fmmd_design_aide.tex b/fmmd_design_aide/fmmd_design_aide.tex index 071e8b2..4a15f63 100644 --- a/fmmd_design_aide/fmmd_design_aide.tex +++ b/fmmd_design_aide/fmmd_design_aide.tex @@ -145,6 +145,20 @@ TC:8 $OP\_AMP$ LATCH DOWN & low amp output & Out of Range & \label{tab:fmmdaide1} \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 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 @@ -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 from the potential divider represents amplified to $\approx \, 342mV$ would represent $\approx \; 46\,^{\circ}{\rm C}$. +\clearpage \subsection{Undetected Failure Mode: Incorrect Reading} 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 allowance according to EN61508. + \section{Proposed Checking Method} Were we to switch in a a second resistor in parrallel with the diff --git a/fmmd_design_aide/mvamp_fmmd.dia b/fmmd_design_aide/mvamp_fmmd.dia new file mode 100644 index 0000000..e6c4a70 Binary files /dev/null and b/fmmd_design_aide/mvamp_fmmd.dia differ diff --git a/fmmd_design_aide/mvamp_fmmd.jpg b/fmmd_design_aide/mvamp_fmmd.jpg new file mode 100644 index 0000000..6d59048 Binary files /dev/null and b/fmmd_design_aide/mvamp_fmmd.jpg differ