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Robin Clark 2010-08-24 21:27:04 +01:00
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component_failure_modes_definition/component_failure_modes_definition.tex
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@ -4,22 +4,26 @@
\ifthenelse {\boolean{paper}} \ifthenelse {\boolean{paper}}
{ {
\abstract{ \abstract{
This paper defines what is meant by the terms This paper defines %what is meant by
the terms
components, derived~components, functional~groups, component fault modes and `unitary~state' component fault modes. components, derived~components, functional~groups, component fault modes and `unitary~state' component fault modes.
%The application of Bayes theorem in current methodologies, and %The application of Bayes theorem in current methodologies, and
%the suitability of the `null hypothesis' or `P' value statistical approach %the suitability of the `null hypothesis' or `P' value statistical approach
%are discussed. %are discussed.
The general concept of the cardinality constrained powerset is introduced The general concept of the cardinality constrained powerset is introduced
and calculations for it corrected for the `unitary state' fault mode conditions. and calculations for it described, and then for
calculations under `unitary state' fault mode conditions.
Data types and their relationships are described using UML. Data types and their relationships are described using UML.
Mathematical constraints and definitions are made using set theory.} Mathematical constraints and definitions are made using set theory.}
} }
{ {
\section{Overview} \section{Overview}
This chapter defines what is meant by the terms This chapter defines %what is meant by
the terms
components, derived~components, functional~groups, component fault modes and `unitary~state' component fault modes. components, derived~components, functional~groups, component fault modes and `unitary~state' component fault modes.
The general concept of the cardinality constrained powerset is introduced The general concept of the cardinality constrained powerset is introduced
and calculations for it corrected for the `unitary state' fault mode conditions. and calculations for it described, and then for
calculations under `unitary state' fault mode conditions.
Data types and their relationships are described using UML. Data types and their relationships are described using UML.
Mathematical constraints and definitions are made using set theory. Mathematical constraints and definitions are made using set theory.
} }
@ -30,9 +34,9 @@ This chapter describes the data types and concepts for the Failure Mode Modular
When analysing a safety critical system using When analysing a safety critical system using
this technique, we need clearly defined failure modes for this technique, we need clearly defined failure modes for
all the components that are used to model the system. all the components that are used to model the system.
These failure modes have a constraint such that In our model we have a constraint that
the component failure modes must be mutually exclusive. the component failure modes must be mutually exclusive.
When this constraint is complied with we can use the FMMD process to When this constraint is complied with we can use the FMMD method to
build hierarchical bottom-up models of failure mode behaviour. build hierarchical bottom-up models of failure mode behaviour.
%This and the definition of a component are %This and the definition of a component are
%described in this chapter. %described in this chapter.
@ -47,7 +51,7 @@ build hierarchical bottom-up models of failure mode behaviour.
%% Paragraph component and its relationship to its failure modes %% Paragraph component and its relationship to its failure modes
%% %%
\section{ Defining the term `Component' } \section{ Defining the term Component }
\begin{figure}[h] \begin{figure}[h]
@ -59,7 +63,8 @@ build hierarchical bottom-up models of failure mode behaviour.
\end{figure} \end{figure}
Let us first define a component. This is anything which we use to build a Let us first define a component. This is anything which we use to build a
product or system with. This could be something quite complicated product or system with.
It could be something quite complicated
like an integrated microcontroller, or quite simple like the humble resistor. like an integrated microcontroller, or quite simple like the humble resistor.
We can define a We can define a
component by its name, a manufacturers' part number and perhaps component by its name, a manufacturers' part number and perhaps
@ -75,27 +80,26 @@ The UML diagram in figure
structure with its associated failure modes. structure with its associated failure modes.
From this diagram we see that each component must have at least one failure mode. From this diagram we see that each component must have at least one failure mode.
Also to clearly show that the failure modes are unique events associated with one component, To clearly show that the failure modes are unique events associated with one component,
each failure mode is referenced back to only one component. each failure mode is referenced back to only one component.
This modelling constraint is due to the fact that even generic components with the same
failure mode types, may have different statistical MTTF properties within the same
circuitry\footnote{For example, consider resistors one of high resistance and one low.
The generic failure modes for a resistor will be the same for both.
The lower resistance part will draw more current and therefore have a statistically higher chance of failure.}.
%% sharing failure modes arrrgghh so irrelevant
%% wrong as well perhaps, as each component will have environmental constraints
%% that determine its statistical behaviour. A 1 Meg ohm resistor
%% is less stressed than a 100 ohm in the same circuit etc
% Perhaps talk here about the failure modes being shared, but by being referenced
% by the component ?
%%-%% MTTF STATS CHAPTER MAYBE ??
%%-%%
%%-%% This modelling constraint is due to the fact that even generic components with the same
%%-%% failure mode types, may have different statistical MTTF properties within the same
%%-%% circuitry\footnote{For example, consider resistors one of high resistance and one low.
%%-%% The generic failure modes for a resistor will be the same for both.
%%-%% The lower resistance part will draw more current and therefore have a statistically higher chance of failure.}.
A product naturally consists of many components and these are traditionally A product naturally consists of many components and these are traditionally
kept in a `parts list'. For a safety critical product this is usually a formal document kept in a `parts list'. For a safety critical product this is usually a formal document
and is used by quality inspectors to ensure the correct parts are being fitted. and is used by quality inspectors to ensure the correct parts are being fitted.
For our UML diagram the parts list is simply a collection of components For our UML diagram the parts list is simply a collection of components
as shown in figure \ref{fig:componentpl}. as shown in figure \ref{fig:componentpl}. The parts list is shown for
completeness here, as people involved with PCB and electronics production, verification
and testing would want to know where it lies in the model.
The parts list is not actively used in the FMMD method.
\begin{figure}[h] \begin{figure}[h]
\centering \centering
\includegraphics[width=400pt,bb=0 0 712 68,keepaspectratio=true]{component_failure_modes_definition/componentpl.jpg} \includegraphics[width=400pt,bb=0 0 712 68,keepaspectratio=true]{component_failure_modes_definition/componentpl.jpg}