From ce4c551f226c90e85e93f40dc6b6cbae7bd41b12 Mon Sep 17 00:00:00 2001 From: Robin Date: Thu, 25 Mar 2010 22:11:15 +0000 Subject: [PATCH] FIT of 1 is 1 failure per 1.1 million years --- pt100/pt100.tex | 21 +++++++++++---------- 1 file changed, 11 insertions(+), 10 deletions(-) diff --git a/pt100/pt100.tex b/pt100/pt100.tex index 1afed32..c83ae7a 100644 --- a/pt100/pt100.tex +++ b/pt100/pt100.tex @@ -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} % typeset in {\Huge \LaTeX} \today