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@ -2,21 +2,21 @@
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%% CHAPTER 4 : Failure Mode Modular Discrimination
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%% CHAPTER 4 : Failure Mode Modular Discrimination
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%%
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%%
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\ifthenelse {\boolean{paper}}
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% \ifthenelse {\boolean{paper}}
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{
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% {
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\abstract{
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% \abstract{
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This paper defines %what is meant by
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% This paper defines %what is meant by
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the terms
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% the terms
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components, derived~components, functional~groups, component fault modes and `unitary~state' component fault modes.
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% components, derived~components, functional~groups, component fault modes and `unitary~state' component fault modes.
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%The application of Bayes theorem in current methodologies, and
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% %The application of Bayes theorem in current methodologies, and
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%the suitability of the `null hypothesis' or `P' value statistical approach
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% %the suitability of the `null hypothesis' or `P' value statistical approach
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%are discussed.
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% %are discussed.
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The general concept of the cardinality constrained powerset is introduced
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% The general concept of the cardinality constrained powerset is introduced
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and calculations for it described, and then for
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% and calculations for it described, and then for
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calculations under `unitary state' fault mode conditions.
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% calculations under `unitary state' fault mode conditions.
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Data types and their relationships are described using UML.
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% Data types and their relationships are described using UML.
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Mathematical constraints and definitions are made using set theory.}
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% Mathematical constraints and definitions are made using set theory.}
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}
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% }
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{
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{
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\section{Overview}
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\section{Overview}
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This chapter defines the FMMD process and related concepts and calculations.
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This chapter defines the FMMD process and related concepts and calculations.
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@ -28,6 +28,14 @@ These {\dcs} are used to then build further {\fgs} until a hierarchy of {\fgs}
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and {\dcs} has been built, converging to a final {\dc}
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and {\dcs} has been built, converging to a final {\dc}
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at the top of the hierarchy.
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at the top of the hierarchy.
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Or in other words we take the traditional FMEA process, and modularise it.
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We break down each stage of reasoning
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into small manageable groups, and use the results of those groups, as {\dcs}
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to build higher level groups.
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%This has advantages of concentrating
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%effort in where modules interact,
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%% J. Howse 04MAY2012 REMOVEFirstly, %what is meant by
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%% J. Howse 04MAY2012 REMOVEFirstly, %what is meant by
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%% J. Howse 04MAY2012 REMOVEthe terms
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%% J. Howse 04MAY2012 REMOVEthe terms
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%% J. Howse 04MAY2012 REMOVEcomponents, failure~modes, derived~components, functional~groups, component fault modes and `unitary~state' component fault modes are defined.
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%% J. Howse 04MAY2012 REMOVEcomponents, failure~modes, derived~components, functional~groups, component fault modes and `unitary~state' component fault modes are defined.
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@ -51,12 +59,369 @@ paper
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{
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{
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chapter
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chapter
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}
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}
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starts with a worked example using the new methodology, Failure Mode Modular De-composition (FMMD), and then
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starts with a worked example using the new methodology, Failure Mode Modular De-composition (FMMD).
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develops an ontological structure for the methodology using UML class models.
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This is followed by a discussion on the design of the FMMD methodology and then
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A notation is then described to index and classify objects created in FMMD models.
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an ontological description is given using UML class models.
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A notation is then described to index and classify objects created in FMMD hierarchical models.
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We briefly analyse four current methodologies.
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Comprehensive overviews of these methodologies may be found
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in ~\cite{safeware,sccs}.
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\paragraph{Fault Tree Analysis (FTA).}
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FTA~\cite{nasafta,nucfta} is a top down methodology in which a hierarchical diagram is drawn for
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each undesirable top level failure/event, presenting the conditions that must arise to cause
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the event.
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%
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It is suitable for large complicated systems with few undesirable top
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level failures and focuses on those events considered most important or most catastrophic.
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%
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Effects of duplication/redundancy of safety systems can be readily assessed.
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It uses notations that are readily understood by engineers
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(logic symbols borrowed from digital electronics and a fault hierarchy).
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However, it cannot guarantee to model all base component failures
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or be used to determine system level errors other than those modelled.
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%
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Each FTA diagram models one top level event.
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This creates duplication of modelled elements,
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and it is difficult to cross check between diagrams. It has limited
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support for environmental and operational states.
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\paragraph{Fault Mode Effects Analysis (FMEA)} is used principally to determine system reliability.
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It is bottom-up and starts with component failure modes, which
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lead to top level failure/events.
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Each top level failure is assessed by its cost to repair (or perceived criticality) and its estimated frequency. %, using a
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%failure mode ratio.
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A list of failures according to their cost to repair~\cite{bfmea}, or effect on system reliability is then calculated.
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It is easy to identify single component failure to system failure mappings
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and an estimate of product reliability can be calculated.
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%This can be viewed as a prioritised `to~fix' list.
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%
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It cannot focus on complex
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component interactions that cause system failure modes or determine potential
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problems from simultaneous failures. It does not consider changing environmental
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or operational states in sub-systems or components. It cannot model
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self-checking safety elements or other in-built safety features or
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analyse how particular components may fail.
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\paragraph{Failure Mode Effects Criticality Analysis (FMECA)} is a refinement of FMEA, using
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extra variables: the probability of a component failure mode occurring,
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the probability that this will cause a given top level failure, and the perceived
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criticality. It gives better estimations of product reliability/safety and the
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occurrence of particular system failure modes than FMEA but has similar deficiencies.
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\paragraph{Failure Modes, Effects and Diagnostic Analysis (FMEDA)} is a refinement of
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FMEA and FMECA and in addition models self-checking safety elements. It assigns two
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attributes to component failure modes: detectable/undetectable and safe/dangerous.
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Statistical measures about the system can be made and used to classify a
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safety integrity level. It allows designs with in-built safety features to be assessed.
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Otherwise, it has similar deficiencies to FMEA.
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However, it has limited support
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for environmental and operational states in sub-systems or components,
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via self checking statistical mitigation. FMEDA is the methodology associated with
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the safety integrity standard EN61508~\cite{en61508}.
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\subsection{Summary of Deficiencies in Current Methods}
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\paragraph{Top Down approach: FTA} The top down technique FTA, introduces the possibility of missing base component
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level failure modes~\cite{faa}[Ch.9]. Since one FTA tree is drawn for each top level
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event, this leads to repeated work, with limited ability for cross checking/model validation.
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Also, the analysis process can miss top level events that bottom-up techniques
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can reveal.
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%\subsection{Bottom-up approach: }
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\paragraph{State Explosion problem for FMEA, FMECA, FMEDA.}
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The bottom-up techniques all suffer from % a problem of
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state explosion.
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To perform the analysis rigorously, we would need to consider the effect
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of a component failure against all other components. Adding environmental
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and operational states further increases the state explosion.
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Let $N$ be the number of components in our system, and $K$ be the average number of component failure modes
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(ways in which a component can fail). The approximate total number of base component failure modes
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is $N \times K$.
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%
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The total number of cases to examine, to determine the effect of all failure modes
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on all
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components
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will be approximately $(N-1) \times N \times K$. %, in effect a very large set cross product.
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%
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If $E$ is the number of environmental conditions to consider
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in a system, and $A$ the number of applied/operational states (or modes of the system),
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the bottom-up analyst is presented with two
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additional %cross product
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factors, yielding approximately
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$(N-1) \times N \times K \times E \times A$.
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%
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If we put some typical very small embedded system numbers\footnote{These figures would
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be typical of a very simple temperature controller, with a micro-controller, sensors, an RS485 interface,
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supporting circuitry and heater circuitry.}
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into this, say $N=100$, $K=2.5$, $A=2$, and $E=10$
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we have $99 \times 100 \times 2.5 \times 10 \times 2 = 495000 $ checks to perform.
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%
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To look in detail at half a million fault~scenarios is obviously impractical.
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% Requirements for an improved methodology The deficiencies identified in the
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% current methodologies are used to establish criteria for an improved methodology.
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\paragraph{Reasoning distance - complexity and reach-ability.}
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Tracing a component level failure up to a top level event, without the rigour accompanying state explosion, involves
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working heuristically. A base component failure will typically
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be conceptually removed by several stages from a top level event.
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The `reasoning~distance' $R_D$ can be calculated by summing the failure modes in each component, for all components
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that must interact to reach the top level event.
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Where $C$ represents the set of components in a failure mode causation chain,
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$c_i$ represents a component in $C$ and
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the function $fm$ returns the failure modes for a given component, equation
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\ref{eqn:complexity}, returns the `reasoning~distance'.
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\begin{equation}
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R_D = \sum_{i=1}^{|C|} |{fm(c_i)}| %\; where \; c \in C
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\label{eqn:complexity}
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\end{equation}
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%
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The reasoning distance is a value representing the number of failure modes
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to consider to rigorously determine the causation chain
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from the base component failure to the system level event.
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%
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The reasoning distance serves to show that when the causes of a top level
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event are completely determined, a large amount of work not
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typical of heuristic or intuitive interpretation is required.
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% could have a chapter on this.
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% take a circuit or system and follow all the interactions
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% to the components that cause the system level event.
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\paragraph{Multiple Events from one base component failure mode.}
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A base component failure may potentially cause more than one
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system level failure mode.
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It could be possible to identify one top level event associated with
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a {\bc} {\fm} and not investigate other possibilities, using FMEA or FTA techniques.
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\paragraph{Software FMEA models are performed separately from Hardware FMEA.}
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%If software FMEA or Software FTA is performed it is always performed separately from
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%hardware FMEA.
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Work on SFMEA (where it is actually performed) does not seek to integrate
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hardware and software models, but to perform
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FMEA on the software in isolation~\cite{procsfmea}.
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Some work has been performed using databases
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to track the relationships between variables
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and system failure modes~\cite{procsfmeadb}, and work has been performed to
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introduce automation into the FMEA process~\cite{appswfmea} and code analysis
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automation~\cite{modelsfmea}. Performing these analyses separate breaks the reasoning chain for tracing
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failure causation through the software hardware interfaces.
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Although the SFMEA and hardware FMEAs are performed separately
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some schools of thought aim for FTA~\cite{nasafta}~\cite{nucfta} (top down - deductive) and FMEA (bottom-up inductive)
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to be performed on the same system to provide insight into the
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software hardware/interface~\cite{embedsfmea}.
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Although this
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would give a better picture of the failure mode behaviour, it
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is by no means a rigorous approach to tracing errors that may occur in hardware
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through to the top (and therefore ultimately controlling) layer of software.
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%\section{Requirements for a new static failure mode Analysis methodology}
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\section{Desirable Criteria.}
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From the deficiencies outlined above, we can form a set of desirable criteria for an enhanced failure mode methodology.
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{ %\small
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\label{criteria}
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\begin{enumerate}
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%\begin{itemize}
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\label{fmmdreq}
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\item Address the state explosion problem. % 1
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\item Ensure that all component failure modes are considered in the model. % 2
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\item Be easy to integrate mechanical, electronic and software models \cite{sccs}[p.287]. %3
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\item Be modular, in that commonly used {\fgs} can be re-used in other designs/projects. %4
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\item Have a formal basis, i.e. be able to produce mathematical traceability %5
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for its results, such as error causation trees.%, reliability and safety statistics.
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%\item It should be easy to use, ideally using a
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%graphical syntax (as opposed to a formal symbolic/mathematical text based language).
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%\item From the top down, the FMMD model of a system, should follow a logical de-composition of the functionality
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%to smaller and smaller functional groupings--or--modules. \cite{maikowski}.
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\item Be able to model multiple (simultaneous) failure modes.% 6 % from the base component level up.
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\end{enumerate}
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%\end{itemize}
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}
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%
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% The design process follows this
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%rationale, sub-systems are build t%o perform often basic functions from base components.
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%We can term these small groups {\fgs}.
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%
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% Components should be collected
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% into small functional groups to enable the examination of the effect of a
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% component failure mode on the other components in the group.
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% Once we have the failure modes, or symptoms of failure of a {\fg}
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% it can now be considered as `derived component' with a known set
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% of failure symptoms. We can use this `derived component' to build higher level
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% functional groups.
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%
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% This helps with the reasoning distance problem,
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% because we can trace failure modes back through complex interactions and have a structure to
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% base our reasoning on, at each stage.
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%
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%Development of the new methodology
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%
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% \section{An ontology of failure modes}
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% In order to address the state explosion problem, the process must be modular
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%and deal with small groups of components at a time. This approach should address the state explosion problem : criteria 1.
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% An ontology is now developed of
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% failure modes and their relationship to environmental factors,
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% applied/operational states and the hierarchical nature inherent in product design,
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% defining the relationships between the system as a whole, components,
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% failure modes, operational and environmental states.
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%
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%
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% Components have sets of failure modes associated with them.
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% Failure modes for common components may be found in
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% the literature~\cite{fmd91,mil1991}.
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% We can associate a component with its failure modes.
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% This is represented in UML in figure \ref{fig:component_concept}.
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%
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% \begin{figure}[h]
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% \centering
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% \includegraphics[width=200pt,keepaspectratio=true]{./component.png}
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% % component.:wq: 467x76 pixel, 72dpi, 16.47x2.68 cm, bb=0 0 467 76
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% \caption{Component with failure modes UML diagram}
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% \label{fig:component_concept}
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% \end{figure}
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%
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% \subsection{Modular Design}
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%
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% When designing a system from the bottom-up, small groups of components are selected to perform
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% simple functions. These can be termed {\fgs}.
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% When the failure mode behaviour, or symptoms of failure
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% of a {\fg} are determined, it can be treated as a component in its own right.
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%
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% % Functional groups
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% % are then brought together to form more complex and higher level {\fgs}.
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% Used in this way the {\fg} has become a {\dc}. The symptoms of failure
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% of the {\fg} can be considered the failure modes of its {\dc}.
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% Derived~Components can be used to create higher level {\fgs}.
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% Repeating this process will lead to identify-able higher level
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% groups, often referred to as sub-systems. We can call the entire collection/hierarchy
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% of sub-systems the system.
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\section{The proposed Methodology}
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\label{fmmdproc}
|
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% Any new static failure mode methodology must ensure that it
|
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% represents all component failure modes and it therefore should be bottom-up,
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% starting with individual component failure modes.
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To ensure all component failure modes are represented, the new methodology must be bottom-up.
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%
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This seems essential to satisfy criterion 2.
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The proposed methodology is therefore a bottom-up process
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starting with base~components.
|
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%
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Since we are only modelling failure modes, which could arise from
|
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mechanical, electronic or software components,
|
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criterion 3 is satisfied.
|
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%
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|
In order to address the state explosion problem, the process should be modular and hierarchical,
|
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|
dealing with small groups of components at a time; this should address criterion 1.
|
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%\paragraph{Outline of the Failure mode methodology.}
|
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%
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A {\em {\fg}}, is defined as a small collection of components
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that interact to provide
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a function or task within a system.
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%
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|
In the proposed methodology components are collected into functional groups
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|
and each component failure (and possibly multiple simultaneous component failures) are considered in the
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context of the {\fg}.
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|
%% GARK
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%
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|
The component failures are termed {\em{\fcs}}. %`test~cases'.
|
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|
For each {\fc}
|
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there will be a corresponding resultant failure, or `symptom', from the perspective of the {\fg}.
|
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%
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% MAYBE NEED TO DESCRIBE WHAT A SYMPTOM IS HERE
|
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%
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%From the perspective of the {\fg} failures of components will be symptoms.
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|
It is conjectured that many symptoms will be common. That is to say
|
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that component failures will often cause the same symptoms of failure
|
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from the perspective of a {\fg}.
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%
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|
A common symptom collection stage is now applied. Here common symptoms are collected
|
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|
from the results of the {\fcs}. Because it is possible to model combinations of failures,
|
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|
criterion 6 is satisfied.
|
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%
|
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|
With a collection of the {\fg} failure symptoms, we can create a {\em{\dc}}.
|
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The failure modes of this new {\dc} are the symptoms of the {\fg} it was derived from.
|
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This satisfies criterion 4, as we can now treat {\dcs} as pre-analysed
|
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modules available for re-use.
|
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By using {\dcs} in higher level functional groups, a hierarchy can be built representing
|
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|
the failure mode behaviour of a system. Because the hierarchy maintains information
|
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|
linking the symptoms to component failure modes (via {\fcs}), we have traceable
|
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reasoning connections from base component failures to top level failures.
|
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The traceability should satisfy criterion 5.
|
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|
% ONTOLOGY - NO ROOM IN 6 PAGES OF PAPER
|
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% \paragraph{Environmental Conditions, Operational States.}
|
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%
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% Any real world sub-system will exist in a variable environment
|
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% and may have several modes of operation.
|
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% In order to find all possible failures, a sub-system
|
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% must be analysed for each operational state
|
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% and environmental condition that could affect it.
|
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% %
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% A question is raised here: which objects should we
|
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% associate the environmental and the operational states with ?
|
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% There are three objects in our model to which these considerations could be applied.
|
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% We could apply these conditions
|
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% to {\fgs}, components, or {\dcs}.
|
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%
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|
% \paragraph {Environmental Conditions.}
|
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%
|
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% Environmental conditions are external to the
|
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% {\fg} and are often things over which the system has no direct control
|
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% ( e.g. ambient temperature, pressure or electrical interference levels).
|
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% %
|
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% Environmental conditions may affect different components in a {\fg}
|
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% in different ways.
|
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%
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% For instance, a system may be specified for
|
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% $0\oc$ to $85\oc$ operation, but some components
|
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% may show failure behaviour between $60\oc$ and $85\oc$
|
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% \footnote{Opto-isolators typically show marked performance decrease after
|
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% $60\oc$ \cite{tlp181}, whereas another common component, say a resistor, will be unaffected.}.
|
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% Other components may operate comfortably within that whole temperature range specified.
|
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% Environmental conditions will have an effect on the {\fg} and the {\dc},
|
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% but they will have specific effects on individual components.
|
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%
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% It seems obvious that
|
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|
% environmental conditions should apply to components.
|
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|
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% %A component will hold a set of environmental states that
|
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% %affect it.
|
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%
|
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|
% \paragraph {Operational States}
|
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%
|
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% Sub-systems may have specific operational states.
|
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|
% These could be a general health level, such as
|
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% normal operation, graceful degradation or lockout.
|
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|
% Alternatively they could be self~checking sub-systems that are either in a normal, alarm/lockout or self~check state.
|
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%
|
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|
% Operational states are conditions that apply to some functional groups, not individual components.
|
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|
%\section{The Non-Inverting Operational Amplifier}
|
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|
|
%When this constraint is complied with, we can use the FMMD method 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
|
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|
|
%This and the definition of a component are
|
|
|
@ -555,9 +920,11 @@ of component failures.
|
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|
|
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|
\section{Defining terms}
|
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|
|
\section{Defining terms}
|
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|
The worked example used terms such as {\fg}, component, and {\dc}.
|
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|
This chapter aims to define these terms.
|
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|
\subsection{Parts, components and base components}
|
|
|
|
A component is anything we use to build a product or system.
|
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|
|
A component is anything we use to build a product or system.
|
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|
|
It could be something quite complicated
|
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|
|
It could be something quite complicated
|
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|
|
like an integrated micro controller, or quite simple like the humble resistor.
|
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|
|
like an integrated micro controller, or quite simple like the humble resistor.
|
|
|
@ -581,7 +948,8 @@ We define {\bc} to mean a lowest level of assembly `part' or an atomic entity, w
|
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|
and component to mean either a part or a sub-assembly.
|
|
|
|
and component to mean either a part or a sub-assembly.
|
|
|
|
Definitions used in FMMD are listed in table~\ref{tbl:fmmd_defs} and discussed below.
|
|
|
|
Definitions used in FMMD are listed in table~\ref{tbl:fmmd_defs} and discussed below.
|
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|
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|
|
%%
|
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|
%% FUCKING STEREO SUB_SYSTEM EXAMPLE, THE FUCKING CHILDRENS SECTION
|
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|
\subsection{Systems, functional groups, sub-systems and failure modes: sound system example}
|
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|
\subsection{Systems, functional groups, sub-systems and failure modes: sound system example}
|
|
|
|
|
|
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|
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|
|
It is helpful here to define the terms, `system', `functional~group', `component', `base~component', `symptom' and `derived~component/sub-system'.
|
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|
|
It is helpful here to define the terms, `system', `functional~group', `component', `base~component', `symptom' and `derived~component/sub-system'.
|
|
|
@ -589,6 +957,8 @@ It is helpful here to define the terms, `system', `functional~group', `component
|
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|
A system, is any coherent entity that would be sold as a product. % safety critical product.
|
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|
A system, is any coherent entity that would be sold as a product. % safety critical product.
|
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|
|
A sub-system is a system that is part of some larger system.
|
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|
A sub-system is a system that is part of some larger system.
|
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|
|
A modular system common in most homes is the sound separates audio system or stereo hi-fi.
|
|
|
|
|
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|
|
This is now used as an example to describe terms used in FMMD.
|
|
|
|
For instance a stereo amplifier separate/slave is a sub-system. The
|
|
|
|
For instance a stereo amplifier separate/slave is a sub-system. The
|
|
|
|
whole sound system, consists perhaps of the following `sub-systems':
|
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|
|
whole sound system, consists perhaps of the following `sub-systems':
|
|
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CD-player, tuner, amplifier~separate, loudspeakers and ipod~interface.
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CD-player, tuner, amplifier~separate, loudspeakers and ipod~interface.
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@ -1047,8 +1417,8 @@ Operational states and environmental conditions must be factored into the UML mo
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levels of electrical interference, high voltage contamination on supply
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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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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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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
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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 \oc{60}. Most electrical components are far more robust to temperature.~\cite{tlp181}.}.
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which is typically affected at around \oc{60}. Most electrical components are far more robust to temperature.}.
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Environmental analysis is thus applicable to components.
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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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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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can be eliminated by down-rating of components as discussed in section~\ref{sec:determine_fms}.
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@ -1193,7 +1563,7 @@ in sections~\ref{sec:formalfmmd} and \ref{algotithmfmmd} respectively.
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\paragraph{Surjective constraint applied to symptom collection.}
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\paragraph{Surjective constraint applied to symptom collection.}
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We can stipulate that symptom collection process is surjective.
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We can stipulate that symptom collection process is surjective~\cite{dmfnt}.
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% i.e. $ \forall f in F $
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% i.e. $ \forall f in F $
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By stipulating surjection for symptom collection, we ensure
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By stipulating surjection for symptom collection, we ensure
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that each component failure mode maps to at least one symptom.
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that each component failure mode maps to at least one symptom.
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@ -1224,7 +1594,7 @@ are always orders of magnitude smaller than sum of {\fms} in its base components
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Integrated components such as OP-AMPS are already treated as {\dcs}
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Integrated components such as OP-AMPS are already treated as {\dcs}
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in literature.
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in literature.
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An Op-AMP is an integrated circuit comprising some hundred or so individual components
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An Op-AMP is an integrated circuit comprising some hundred or so individual components
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but in the literature ~\cite{fmd91} is is described as a simple component with a set of failure modes.
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but in the literature ~\cite{fmd91}[3-116] is is described as a simple component with a set of failure modes.
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% Idea stage on this section, integrated circuits and some compond parts (like digital resistors)
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% Idea stage on this section, integrated circuits and some compond parts (like digital resistors)
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% are treated like base components. i.e. this sets a precedent for {\dcs}.
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% are treated like base components. i.e. this sets a precedent for {\dcs}.
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@ -1408,10 +1778,166 @@ failure modes combinations have been analysed as failure scenarios.
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%This AUTOMATIC check can reveal WHEN double checking no longer necessary
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%This AUTOMATIC check can reveal WHEN double checking no longer necessary
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%in the hierarchy to cover dub sum !!!!! YESSSS
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%in the hierarchy to cover dub sum !!!!! YESSSS
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\section{Conclusion}
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%\subsection{Evaluation of FMMD}
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%By applying the methodology in section \ref{fmmdproc}, the wishlist can
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%now be evaluated for the proposed FMMD methodology.
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We evaluate the FMMD method using the criteria in section \ref{fmmdreq}.
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Table \ref{tbl:comparison} compares the current methodologies and FMMD using these criteria.
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{ %\small
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\begin{itemize}
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\item{State explosion is reduced,}
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%State Explosion is reduced,
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because small collections of components are dealt within functional groups
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which are used to create derived components which are then used in an hierarchical manner.
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\item{All component failure modes must be considered in the model.}
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%All component failure modes must be considered in the model.
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Since the proposed methodology is bottom-up,
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this means that we can ensure/check that all component failure modes are handled.
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\item{ It should be straightforward to integrate mechanical, electronic and software models,}
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%It should be straight forward to integrate mechanical, electronic and software models,
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because FMMD models in terms of failure modes only. % we have a generic failure mode entities to model.
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%We can describe a mechanical, electrical or software component in terms of its failure modes.
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%
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Because of this
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we can model and analyse integrated electromechanical systems, controlled by computers,
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using a common notation. An electronic/software FMMD example is given in~\ref{sec:elecsw}.
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\item{ It should be re-usable, in that commonly used modules can be re-used in other designs/projects.}
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%It should be re-usable, in that commonly used modules can be re-used in other designs/projects.
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The hierarchical nature, taking {\fg}s and deriving components from them, means that
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commonly used {\dcs} can be re-used in a design % (for instance self checking digital inputs)
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or even in other projects where the same {\dc} is used. Several examples of re-used {\dcs}
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may be found in chapter~\ref{sec:chap5}.
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\item{ Formal basis: data should be available to produce mathematical proofs and traceability.}
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%It should have a formal basis, data should be available to produce mathematical proofs
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%for its results
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Because the failure mode model of a system is a hierarchy of {\fg}s and {\dcs},
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system level failure modes are traceable back down the fault tree to
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component level failure modes.
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%
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This allows cut sets~\cite{nasafta}[Ch.1p3]
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to be determined by traversing the DAG from top level events down to their causes.
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% \item{ It should be capable of producing reliability and danger evaluation statistics.}
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% The minimal cuts sets for the system level failures can have computed MTTF
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% and danger evaluation statistics sourced from the component failure mode statistics~\cite{fmd91,mil1991}.
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% \item{ It should be easy to use, ideally
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% using a graphical syntax (as opposed to a formal mathematical one).}
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% A modified form of constraint diagram (an extension of Euler diagrams) has
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% been developed to support the FMMD methodology.
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% This uses Euler circles to represent failure modes, and spiders to collect symptoms, to
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% advance a {\fg} to a {\dc}.
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% \item{ From the top down the failure mode model should follow a logical de-composition of the functionality
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% to smaller and smaller functional modules \cite{maikowski}.}
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% The bottom-up approach fulfils the logical de-composition requirement, because the {\fg}s
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% are built from components performing a given task.
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%
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\item{ Multiple failure modes (conjunction - where more that one failure mode is active)
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may be modelled from the base component level up.}
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%Multiple failure modes (conjunction) may be modelled from the base component level up.
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By breaking the problem of failure mode analysis into small stages
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and building a hierarchy, the problems associated with needing to
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analyse all possible combinations of base level components
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within a system are reduced.
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% by an exponential order.
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This is because the multiple failure modes considered
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within {\fgs} have fewer failure modes to consider
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at each FMMD stage.
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Where appropriate, multiple simultaneous failures can be modelled by
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introducing {\fcs} %test~cases
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where the conjunction of failure modes is considered.
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An example demonstrating multiple failure mode analysis may be found in section~\ref{sec:Pt100}.
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\end{itemize}
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}
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{ %\tiny
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\begin{table}[ht]
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\caption{Features of static Failure Mode analysis methodologies} % title of Table
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%\centering % used for centering table
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\begin{tabular}{||l|c|c|c|c|c||}
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\hline \hline
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% \textbf{Des.} & \textbf{FTA} & \textbf{FMEA} & \textbf{FMECA} & \textbf{FDEMA} & \textbf{FMMD} \\
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\textbf{\tiny Des.} & \textbf{\tiny FTA} & \textbf{\tiny FMEA} & \textbf{\tiny FMECA} & \textbf{\tiny FDEMA} & \textbf{\tiny FMMD} \\
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\textbf{\tiny Crit.} & \textbf{} & \textbf{} & \textbf{} & \textbf{} & \textbf{} \\
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% R & wire & res + & res - & description
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\hline
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\hline
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C1: % state exp
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& partial & & & & $\tickYES$ \\ \hline
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C2: % $\forall$ failures
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& &$\tickYES$ & $\tickYES$ & $\tickYES$ & $\tickYES$ \\ \hline
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C3: %mech,elec,s/w & $\tickYES$
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& & & & & $\tickYES$ \\ \hline
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C4: %modular
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& & & & partial & $\tickYES$ \\ \hline
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C5: %formal
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& partial & partial & partial & partial & $\tickYES$ \\ \hline
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C6: %multiple fm
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& $\tickYES$ & & & partial & $\tickYES$ \\ \hline
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\hline
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\hline
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\end{tabular}
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\label{tbl:comparison}
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\end{table}
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}
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%\clearpage
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%\subsection{}
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%This new approach is called
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Failure Mode Modular De-Composition (FMMD) is designed
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to be a more rigorous and `data~complete' model than
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the current four approaches.
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%
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That is,
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from an FMMD model, we should be able to
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derive outline models that the other four methodologies would have been
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able to create. As this approach is modular, many of the results of
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analysed components may be re-used in other projects, so
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test efficiency is improved.
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%Clearly the more complex the original system is the more benefit,
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%i.e. less components and derived components, will be produced from decomposing the
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%system into functional groups.
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FMMD is based on generic failure modes, so it is not constrained to a
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particular field. It can be applied to mechanical, electrical or software domains.
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It can therefore be used to analyse systems comprised of electrical,
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mechanical and software elements in one integrated model.
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Furthermore the reasoning path is traceable. By being able to trace a
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top level event down through derived components, to base component
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failure modes, with each step annotated as {\fcs}, the model is easier to maintain.
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The example used here is deliberately small for the purpose
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of being presenting the methodology showing more than only one stage of hierarchy,
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worked examples for common electronic circuits, digital analogue hybrids and software electronic
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hybrid systems are given in chapter~\ref{sec:chap5}.
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%FMMD has been applied
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%to larger systems encompassing mechanical, electrical and software
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%elements.
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FMMD represents a new technique in that it
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can address all the criteria in table 3, whereas the other methodologies
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can only cover some.
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FMMD not only has advantages of efficiency (reduction of state explosion), it also
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provides the capability to model entire electro-mechanical-software systems using a common notation
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and process.
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%\ifthenelse {\boolean{paper}}
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%\ifthenelse {\boolean{paper}}
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%{
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%{
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%\input{abstract}
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%\input{abstract}
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