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robin 2012-04-07 18:01:04 +01:00
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15
papers/software_fmea/software_fmea.tex
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@ -373,13 +373,16 @@ Let us define any value outside the 4mA to 20mA range as an error condition.
As a voltage, we use ohms law~\cite{aoe} to determine the voltage ranges: $V=IR$, $0.004A * \ohms{220} = 0.88V$ As a voltage, we use ohms law~\cite{aoe} to determine the voltage ranges: $V=IR$, $0.004A * \ohms{220} = 0.88V$
and $0.020A * \ohms{220} = 4.4V$. and $0.020A * \ohms{220} = 4.4V$.
% %
Our acceptable voltage range is therefore $$V >= 0.88 \wedge V<= 4.4 \; .$$ Our acceptable voltage range is therefore
$$(V \ge 0.88) \wedge (V \le 4.4) \; .$$
This voltage range forms our input requirement. This voltage range forms our input requirement.
% %
We can now examine a software function that performs a conversion from the voltage read to We can now examine a software function that performs a conversion from the voltage read to
a per~mil representation of the {\ft} input current. a per~mil representation of the {\ft} input current.
% %
or the purpose of example the `C' programming language is used. For the purpose of example the `C' programming language is used.
We initially assume a function \textbf{read\_ADC} which returns a floating point %double precision We initially assume a function \textbf{read\_ADC} which returns a floating point %double precision
value which holds the voltage read (see code sample in figure~\ref{fig:code_read_4_20_input}). value which holds the voltage read (see code sample in figure~\ref{fig:code_read_4_20_input}).
@ -684,8 +687,8 @@ We can now form a functional group with the {\dc} $RADC$ and the software compon
\textbf{Failure} & \textbf{failure} & \textbf{Symptom} \\ \textbf{Failure} & \textbf{failure} & \textbf{Symptom} \\
\textbf{Scenario} & \textbf{effect} & \textbf{RADC } \\ \hline \textbf{Scenario} & \textbf{effect} & \textbf{RADC } \\ \hline
\hline \hline
1: $RI_{VRGE}$ & voltage & $OUT\_OF\_RANGE$ \\ 1: $RI_{VRGE}$ & voltage & $OUT\_OF\_$ \\
& outside range & \\ \hline & outside range & $RANGE$ \\ \hline
2: $RADC_{VV_ERR}$ & voltage & $VAL\_ERR$ \\ 2: $RADC_{VV_ERR}$ & voltage & $VAL\_ERR$ \\
& incorrect & \\ \hline \hline & incorrect & \\ \hline \hline
@ -697,8 +700,8 @@ We can now form a functional group with the {\dc} $RADC$ and the software compon
4: $RADC_{LOW}$ & ADC may read & $OUT\_OF\_RANGE$ \\ 4: $RADC_{LOW}$ & ADC may read & $OUT\_OF\_$ \\
& wrong channel & \\ \hline & wrong channel & $RANGE$ \\ \hline
\hline \hline