87 lines
3.7 KiB
TeX
87 lines
3.7 KiB
TeX
\label{sec:chap8}
|
|
\section{Further Work}
|
|
|
|
\subsection{Environment, operational states and inhibit gates: additions to the UML model.}
|
|
|
|
FTA~\cite{nasafta,nucfta} models environmental, operational state and inhibit gates, and these can be incorporated into
|
|
the FMMD model.
|
|
|
|
A system will be expected to perform in a given environment.
|
|
%
|
|
Environment in the context of this study
|
|
means external influences under which the System could be expected to work. % under.
|
|
%
|
|
A typical data sheet for an electrical component will give
|
|
a working temperature range, for instance.
|
|
Mechanical components could be specified for stress and loading limits.
|
|
|
|
|
|
Systems may have distinct operational states. For instance, a safety critical controller
|
|
may have a LOCKOUT state where it has detected a serious problem and will not continue to operate until
|
|
authorised human intervention takes place.
|
|
A safety critical circuit may have a self test mode which could be operated externally.
|
|
%
|
|
Operational states and environmental conditions must be factored into the UML model.
|
|
|
|
\paragraph{Environmental Modelling.} The external influences/environment could typically be temperature ranges,
|
|
levels of electrical interference, high voltage contamination on supply
|
|
lines, radiation levels etc.
|
|
Environmental influences will affect specific components in specific ways.\footnote{A good example of a part
|
|
affected by environmental conditions, in this case temperature, is the opto-isolator~\cite{tlp181}
|
|
which is typically affected at around {60 \oc}. Most electrical components are more robust to temperature variations.}.
|
|
Environmental analysis is thus applicable to components.
|
|
Environmental influences, such as over stress due to voltage
|
|
can be eliminated by down-rating of components as discussed in section~\ref{sec:determine_fms}.
|
|
With given environmental constraints, we can therefore eliminate some failure modes from the model.
|
|
|
|
|
|
\paragraph{Operational states.}
|
|
Within the field of safety critical engineering, we often encounter
|
|
elements that include test or self-test facilities.
|
|
%
|
|
We also encounter degraded performance
|
|
(such as only performing functions in an emergency) and lockout/emergency conditions.
|
|
These can be broadly termed operational states. %, and apply to the
|
|
%functional groups.
|
|
%
|
|
We need to determine which UML class is most appropriate to hold a relationship
|
|
to operational states.
|
|
%
|
|
Consider for instance an electrical circuit that has a TEST line.
|
|
When the TEST line is activated, it supplies a test signal
|
|
which will validate the circuit. This circuit will have two operational states,
|
|
NORMAL and TEST mode.
|
|
%
|
|
It seems better to apply the operational states to {\fgs}.
|
|
%
|
|
Functional groupings by definition implement functionality, or purpose, and therefore are the best objects to model
|
|
operational states.% with.
|
|
|
|
\paragraph{Inhibit Conditions.}
|
|
A third data class may be required if modelling of inhibit conditions~\cite{nasafta}[p.40] is desired.
|
|
Some failure modes may only be active given specific environmental conditions
|
|
or when other failures are already active.
|
|
To model this, an `inhibit' class has been added.
|
|
This is an optional attribute of
|
|
a failure mode. This inhibit class can be triggered
|
|
on a combination of environmental or failure modes.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
\paragraph{UML Diagram Additional Objects.}
|
|
The additional objects System, Environment and Operational States
|
|
are added to UML diagram in figure \ref{fig:cfg} are represented in figure \ref{fig:cfg2}.
|
|
|
|
\label{completeumlfurtherwork}
|
|
|
|
\begin{figure}[h]
|
|
\centering
|
|
\includegraphics[width=400pt,keepaspectratio=true]{./CH7_Conclusion/master_uml_further_work.png}
|
|
% cfg2.png: 702x464 pixel, 72dpi, 24.76x16.37 cm, bb=0 0 702 464
|
|
\caption{FMMD UML diagram, incorporating Environmental, Operational State and Inhibit gates}
|
|
\label{fig:cfg2}
|
|
\end{figure}
|