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