% % Make the revision and doc number macro's then they are defined in one place \begin{abstract} The PT100, or platinum wire \ohms{100} sensor is a wisely used industrial temperature sensor that is are slowly replacing the use of thermocouples in many industrial applications below 600\oc, due to high accuracy\cite{aoe}. This chapter looks at the most common configuration, the four wire circuit, and analyses it from an FMEA perspective twice. Once considering single faults (cardinality constrained powerset of 1) and then again, considering the possibility of double simultaneous faults (cardinality constrained powerset of 2). The analysis is performed using Propositional Logic diagrasms to aid in the reasoning process, which takes the failure modes of the components, and produces a failure mode model for the circuit as a whole. Thus after the analysis the PT100 temperature sensing circuit, may be veiwed from an FMEA persepective as a component itsself, with a set of know failure modes. \end{abstract} \begin{figure}[h] \centering \includegraphics[width=400pt,bb=0 0 714 180,keepaspectratio=true]{./pt100/pt100.jpg} % pt100.jpg: 714x180 pixel, 72dpi, 25.19x6.35 cm, bb=0 0 714 180 \caption{PT100 four wire circuit} \label{fig:pt100} \end{figure} \section{Overview of PT100 four wire circuit} The PT100 four wire circuit consists of teo resistors supplying a current to a third, the thermistor or PT100. By measuring volatges from sections of this circuit forming potential dividers, we can determine the current resistance of the platinum wire sensor. The resistance of this is directly related to temperature, and may be determined by look-up tables or a suitable polynomial expression. \begin{figure}[h] \centering \includegraphics[width=150pt,bb=0 0 273 483,keepaspectratio=true]{./pt100/vrange.jpg} % pt100.jpg: 714x180 pixel, 72dpi, 25.19x6.35 cm, bb=0 0 714 180 \caption{PT100 expected voltage range} \label{fig:pt100vrange} \end{figure} The voltage ranges we expect from from this three stage potential divider are shown in figure \ref{fig:pt100vrange}. Note that there is an expected range for each reading for a given temperature span. \subsection{Accuracy despite variable resistance in cables} For electronic and accuracy reasons the four wire circiut is used because of resistance in the cables. Resitance from the supply causes a slight voltage drop in the supply to the PT100. As no significant current is carried by the two `sense' lines the resistance back to the ADC causes only a negligible voltage drop. The current flowing though the whole circuit can be measured on the PCB by reading a third sense voltage from one of the load resistors. Knowing the current flowing through the circuit and knowing the voltage drop over the PT100, we can calculate its resistance by ohms law $V=I.R$, $R=\frac{I}{V}$. Thus a little loss of supply current due to resistance in the cables does not impinge on accuracy. The resistance to temperature conversion is achieved through the published PT100 tables\cite{eurothermtables}. \section{Safety case for 4 wire circuit} This sub-section looks at the behaviour of the PT100 four wire circuit for the effects of component failures. All components have a set of known `failure modes'. In other words we know that a given component can fail in several distict ways. Studies have been published which list common component types and their sets of failure modes, often with MTTF statistics \cite{mil1991}. Thus for each component, an analysis is made for each of it failure modes, with respect to its effect on the circuit. Each one of these scenarios is termed a `test case'. The resultant circuit behaviour for each of these test cases is noted. The worst case for this type of analysis would be a fault that we cannot detect. Where this occurs a circuit re-design is probably the only sensible course of action. \subsection{Single Fault FMEA Analysis of PT100 Four wire circuit} \label{fmea} Looking at this circuit, it simply consists of three resistors. Resistors according to the DOD Electronic component fault handbook 1991, fail by either going OPEN or SHORT circuit \cite{mil1991}. %Should wires become disconnected these will have the same effect as %given resistors going open. For the purpose of his analyis; $R_{1}$ is the \ohms{2k2} from 5V to the thermistor, $R_p$ is the PT100 thermistor and $R_{2}$ connects the thermistor to ground. \begin{table}[ht] \caption{PT100 FMEA Single Faults} % title of Table \centering % used for centering table \begin{tabular}{||l|c|c|l|l||} \hline \hline \textbf{Test} & \textbf{Result} & \textbf{Result } & \textbf{General} \\ \textbf{Case} & \textbf{sense +} & \textbf{sense -} & \textbf{Symtom Description} \\ % R & wire & res + & res - & description \hline \hline $R_1$ SHORT & High & - & Value Out of Range Value \\ \hline $R_1$ OPEN & Low & Low & Both values out of range \\ \hline \hline $R_p$ SHORT & Low & High & Both values out of range \\ \hline $R_p$ OPEN & High & Low & Both values out of range \\ \hline \hline $R_2$ SHORT & - & Low & Value Out of Range Value \\ $R_2$ OPEN & High & High & Both values out of range \\ \hline \hline \end{tabular} \label{ptfmea} \end{table} From table \ref{ptfmea} it can be seen that any component failure in the circuit will cause a common symptom, that of one or more of the values being out of range. Temperature range calculations and detailed calculations on the effects of each test case are found in section \ref{pt100range} and \ref{pt100temp}. So by defining an acceptable measurement/temperature range, and ensuring the values are always within these bounds we can be confident that none of the resistors in this circuit has failed. \subsection{Single Fault Modes as PLD} % Place in PLD diagram \subsection{Range and PT100 Calculations} \label{pt100temp} PT100 resistors are designed to have a resistance of ohms{100} at 0 \oc \cite{eurothermtables}. A suitable `wider than to be expected range' was considered to be {-100\oc} to {300\oc}. According to the Eurotherm PT100 tables \cite{eurothermtables}, this corresponded to the resistances \ohms{60.28} and \ohms{212.02} respectively. From this the potential divider circuit can be analysed and the maximum and minimum acceptable voltages determined. These can be used as bounds results to apply the findings from the PT100 FMEA analysis in section \ref{fmea}. As the PT100 forms a potential divider with the \ohms{2k2} load resistors, the upper and lower readings can be calculated thus: $$ highreading = 5V.\frac{2k2+pt100}{2k2+2k2+pt100} $$ $$ lowreading = 5V.\frac{2k2}{2k2+2k2+pt100} $$ To convert these to twelve bit ADC (\adctw) counts: $$ highreading = 2^{12}.\frac{2k2+pt100}{2k2+2k2+pt100} $$ $$ lowreading = 2^{12}.\frac{2k2}{2k2+2k2+pt100} $$ \begin{table}[ht] \caption{PT100 Maximum and Minimum Values} % title of Table \centering % used for centering table \begin{tabular}{||c|c|c|l|l||} \hline \hline \textbf{Temperature} & \textbf{PT100 resistance} & \textbf{Lower} & \textbf{Higher} & \textbf{Description} \\ \hline {-100 \oc} & {\ohms{68.28}} & 2.46V & 2.53V & Boundary of \\ & & 2017\adctw & 2079\adctw & out of range LOW \\ \hline {0 \oc} & {\ohms{100}} & 2.44V & 2.56V & Mid Range \\ & & 2002\adctw & 2094\adctw & \\ \hline {+300 \oc} & {\ohms{212.02}} & 2.38V & 2.62V & Boundary of \\ & & 1954\adctw & 2142\adctw & out of range HIGH \\ \hline \hline \end{tabular} \label{ptbounds} \end{table} Table \ref{ptbounds} gives ranges that determine correct operation. In fact it can be shown that for any single error (short or opening of any resistor) this bounds check will detect it. \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 `out of range'. There are six cases and each will be examined in turn. \subsubsection{ Voltages $R_1$ SHORT } With pt100 at -100\oc $$ highreading = 5V $$ Since the highreading or sense+ is directly connected to the 5V rail, both temperature readings will be 5V.. $$ lowreading = 5V.\frac{2k2}{2k2+68\Omega} = 4.85V$$ With pt100 at the high end of the temperature range 300\oc. $$ highreading = 5V $$ $$ lowreading = 5V.\frac{2k2}{2k2+212.02\Omega} = 4.56V$$ Thus with $R_1$ shorted both readingare outside the proscribed range in table \ref{ptbounds}. \subsubsection{ Voltages $R_1$ OPEN } In this case the 5V rail is disconnected. All voltages read are 0V, and therefore both readings are outside the proscribed range in table \ref{ptbounds}. \subsubsection{ Voltages $R_p$ SHORT } Here the potential divider is simply between the two 2k2 load resistors. Thus it will read a nominal; 2.5V. Assuming the load resistors are precision components, and then taking an absolute worst case of 1\% either way. $$ 5V.\frac{2k2*0.99}{2k2*1.01+2k2*0.99} = 2.475V $$ $$ 5V.\frac{2k2*1.01}{2k2*1.01+2k2*0.99} = 2.525V $$ These readings both lie outside the proscribed range. Also the sense+ and sense- readings would have the same value. \subsubsection{ Voltages $R_p$ OPEN } Here the potential divider is broken. The sense- will read 0V and the sense+ will read 5V. Both readings are outside the proscribed range. \subsubsection{ Voltages $R_2$ SHORT } With pt100 at -100\oc $$ lowreading = 0V $$ Since the lowreading or sense- is directly connected to the 0V rail, both temperature readings will be 0V. $$ lowreading = 5V.\frac{68\Omega}{2k2+68\Omega} = 0.15V$$ 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 proscribed range in table \ref{ptbounds}. \subsubsection{ Voltages $R_2$ OPEN } Here there is no potential divider operating and both sense lines will read 5V, outside of the proscibed range. %\vbox{ %\subsubsection{Calculating Bounds: High Value : HP48 RPL} % % %HP RPL calculator program to take pt100 resistance %and convert to voltage and {\adctw} values. % %\begin{verbatim} %<< -> p % << % p 2200 + 2200 2200 + p + / 5 * DUP 5 % / 4096 * % >> %>> %\end{verbatim} %} % %\vbox{ %\subsubsection{Calculating Bounds: LOW Value : HP48 RPL} % % %HP RPL calculator program to take pt100 resistance %and convert to voltage and {\adctw} values. % %\begin{verbatim} %<< -> p % << % p 2200 2200 p 2200 + + / 5 * DUP 5 % / 4096 * % >> %>> %\end{verbatim} %} % %\subsection{Implementation of Four Wire Circuit} % %A standard 4 wire PT100\cite[pp 992]{aoe} circuit is read by %ports on the 12 bit ADC of the PIC18F2523\cite{pic18f2523}. %Three readings are taken. A reading to confirm the voltage level %over $R_2$ is taken, %from which the current can be determined. %The two sense lines then give the voltage over the PT100 thermistor. %As we know the current flowing through it we can determine the %resistance. % %After verification (PT100 voltages/readings in range etc) the temperature %value is determined by interpolation via the PT100 tables \cite{eurothermtables}. %First order low pass filtering is then applied to smooth the value. %\section{Water Level Readings - \ft Inputs} %\label{wl} %After h/w revision 0.4, water level sensor \ft connections are wired to the TDS daughterboard, %but are passed to the main unit via a multiplexer, and connect to the %14 pin harwin (to PIN 13 of JP1 \cite{pcbAI222562}). % %The safety critical \ft water~level readings are thus handled in the \wlc. % \subsection{Single Fault FMEA Analysis of PT100 Four wire circuit} typeset in {\Huge \LaTeX} \today