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FIT of 1 is 1 failure per 1.1 million years

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Robin 2010-03-25 22:11:15 +00:00
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pt100/pt100.tex
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@ -59,7 +59,7 @@ Note that the low reading goes down as temperature increases, and the higher rea
For this reason the low reading will be reffered to as {\em sense-}
and the higher as {\em sense+}.
\subsection{Accuracy despite variable resistance in cables}
\subsection{Accuracy despite variable \\ resistance in cables}
For electronic and accuracy reasons a four wire circuit is preffered
because of resistance in the cables. Resistance from the supply
@ -69,7 +69,7 @@ is carried by the two `sense' lines the resistance back to the ADC
causes only a negligible voltage drop, and thus the four wire
configuration is more accurate.
\subsection{Calculating Temperature from the sense line voltages}
\subsection{Calculating Temperature from \\ the sense line voltages}
The current flowing though the
whole circuit can be measured on the PCB by reading a third
@ -100,7 +100,7 @@ Where this occurs a circuit re-design is probably the only sensible course of ac
\subsection{Single Fault FMEA Analysis of PT100 Four wire circuit}
\subsection{Single Fault FMEA Analysis \\ of PT100 Four wire circuit}
\label{fmea}
This circuit simply consists of three resistors.
@ -208,7 +208,7 @@ for any single error (short or opening of any resistor) this bounds check
will detect it.
\section{Single Fault FMEA Analysis of PT100 Four wire circuit}
\section{Single Fault FMEA Analysis \\ of PT100 Four wire circuit}
\subsection{Single Fault Modes as PLD}
@ -250,7 +250,7 @@ for the circuit shown in figure \ref{fig:vd}.
\subsection{Proof of Out of Range Values for Failures}
\subsection{Proof of Out of Range \\ Values for Failures}
\label{pt110range}
Using the temperature ranges defined above we can compare the voltages
we would get from the resistor failures to prove that they are
@ -287,7 +287,7 @@ With pt100 at the high end of the temperature range 300\oc.
$$ highreading = 5V $$
$$ lowreading = 5V.\frac{212.02\Omega}{2k2+212.02\Omega} = 0.44V$$
Thus with $R_2$ shorted both readingare outside the
Thus with $R_2$ shorted both readings are outside the
proscribed range in table \ref{ptbounds}.
\subsubsection{ TC : 4 Voltages $R_2$ OPEN }
@ -368,7 +368,7 @@ in the other 10\%, so we can use this.
A standard fixed film resistor, for use in a benign environment, non military spec at
temperatures up to 60\oc is given a probability of 13.8 failures per billion ($10^9$)
hours of operation. This figure is referred to as a FIT\footnote{FIT values are measured as failures per billion hours of operation, roughly 114,000 years}, Failure in time.
hours of operation. This figure is referred to as a FIT\footnote{FIT values are measured as the number of failures per billion hours of operation, (roughly 1.1 Million years). The smaller the FIT number the more reliable the fault~mode}, Failure in time.
A thermistor, bead type, non military spec is given a FIT of 3150.
@ -378,7 +378,7 @@ showing the FIT values for all faults considered.
\begin{table}[h+]
\caption{PT100 FMEA Single Fault Statistics} % title of Table
\caption{PT100 FMEA Single // Fault Statistics} % title of Table
\centering % used for centering table
\begin{tabular}{||l|c|c|l|l||}
\hline \hline
@ -401,7 +401,8 @@ TC:6 $R_2$ OPEN & High Fault & High Fault & 1.38 \\ \hline
\end{table}
The FIT for the circuit as a whole is the sum of MTTF values for all the
test cases. The PT100 circuit here has a FIT of 3177.6.
test cases. The PT100 circuit here has a FIT of 3177.6. This is a MTTF of
about 360 years per circuit.
A Probablistic tree can now be drawn, with a FIT value for the PT100
circuit and FIT values for all the component fault modes that it was calculated from.
@ -425,7 +426,7 @@ The next analysis phase looks at how the circuit will behave under double simult
conditions.
\clearpage
\section{ PT100 Double Simultaneous Fault Analysis}
\section{ PT100 Double Simultaneous \\ Fault Analysis}
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