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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.
In order to determine the symptoms or failure modes of a {\fg}
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.
%
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.
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
%being derived from the functional~group.
%being derived from the {\fg}.
We can now create a new `{\dc}' which has
the failure symptoms of the functional~group as its set of failure modes.
This new derived~component is at a higher failure mode abstraction
the failure symptoms of the {\fg} as its set of failure modes.
This new {\dc} is at a higher failure mode abstraction
level than the {\bcs}.
%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,
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.
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}
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
%% with refs to nancy