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Robin Clark 2010-08-16 19:40:54 +01:00
parent 393adae570
commit 3a66d2b7b4
2 changed files with 14 additions and 6 deletions
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10
fmmdset/fmmdset.tex
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@ -96,7 +96,7 @@ From the point of view of fault analysis, we are not interested in the component
For this study a {\fg} will mean a collection of components. For this study a {\fg} will mean a collection of components.
In order to determine the symptoms or failure modes of a {\fg} In order to determine the symptoms or failure modes of a {\fg}
we need to consider all failure modes of its components. we need to consider all failure modes of its components.
By analysing the fault behaviour of a `functional group' with respect these failure modes By analysing the fault behaviour of a `{\fg}' with respect these failure modes
we can derive a new set of possible failure modes. we can derive a new set of possible failure modes.
% %
This new set of faults is the set of derived faults from the perspective of the {\fg}, and is thus at a higher level of This new set of faults is the set of derived faults from the perspective of the {\fg}, and is thus at a higher level of
@ -105,10 +105,10 @@ fault~mode abstraction. Thus we can say that the {\fg} as an entity, can fail in
In other words we have taken a {\fg}, and analysed how it can fail according to the failure modes of its parts. In other words we have taken a {\fg}, and analysed how it can fail according to the failure modes of its parts.
These new failure~modes are derived failure modes. These new failure~modes are derived failure modes.
%The ways in which the module can fail now becomes a new set of fault modes, the fault~modes %The ways in which the module can fail now becomes a new set of fault modes, the fault~modes
%being derived from the functional~group. %being derived from the {\fg}.
We can now create a new `{\dc}' which has We can now create a new `{\dc}' which has
the failure symptoms of the functional~group as its set of failure modes. the failure symptoms of the {\fg} as its set of failure modes.
This new derived~component is at a higher failure mode abstraction This new {\dc} is at a higher failure mode abstraction
level than the {\bcs}. level than the {\bcs}.
%What this means is the `fault~symptoms' of the module have been derived. %What this means is the `fault~symptoms' of the module have been derived.
% %
@ -390,7 +390,7 @@ via the RS232 serial line.
This has a number of obvious {\fgs}, the PCB power supply, the milli-volt amplifiers, This has a number of obvious {\fgs}, the PCB power supply, the milli-volt amplifiers,
the analog to digital conversion circuitry, the micro processor and the UART (serial link - RS232 transceiver). the analog to digital conversion circuitry, the micro processor and the UART (serial link - RS232 transceiver).
It would make sense when analysing this system to take each one of these functional~groups in turn and examine them closely. It would make sense when analysing this system to take each one of these {\fgs} in turn and examine them closely.
It would be sensible if the system could detect the most obvious fault~modes by self testing. It would be sensible if the system could detect the most obvious fault~modes by self testing.
When these have been examined and diagnostic safeguard strategies have been thought up, When these have been examined and diagnostic safeguard strategies have been thought up,

10
introduction/introduction.tex
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@ -491,8 +491,16 @@ This element of FTA is discussed in \ref{surveysc}
\section{Therac 25} \section{Therac 25}
The therac-25 was a computer controlled radiation therapy machine, which
overdosed 6 people between 1985 and 1987.
An earlier computerised version of the therac-25 (the therac-20) used the same software but kept the
hardware interlocks from the previous manual operation machines. The hardware interlocks
on the therac-20 functioned correctly and the faulty software in it caused no accidents.
A safety study for the device, using Fault Tree Analysis% \cite{nucfta}
carried out in 1983
excluded the software \cite{safeware}[App. A].
\cite{safeware}[App. A]
%% Here need more detail of what therac 25 was and roughly how it failed %% Here need more detail of what therac 25 was and roughly how it failed
%% with refs to nancy %% with refs to nancy