morning edit

fmmd_concept/System_safety_2011/submission.tex

@@ -83,9 +83,12 @@ These properties provide advantages in rigour and efficiency when compared to cu
 \section{Introduction}
 
 This paper describes and criticises the four current failure mode methodologies,
-presents a table of the deficiencies and advantages, and then presents a new proposed
-methodology. A worked example is then provided, which models the failure mode
+discusses their advantages and deficiencies and defines a desirable criteria list 
+for an ideal static failure mode methodology.
+A new proposed
+methodology is then described and discussed. A worked example is then provided, which models the failure mode
 behaviour of a non inverting op-amp.
+Using  the worked example the new methodology is evaluated.
 
  
 \paragraph{Current methodologies} 
@@ -318,15 +321,22 @@ mechanical, electronic or software components,
 criteria 3 is satisfied.
 %
 In order to address the state explosion problem, the process must be modular
-and deal with small groups of components at a time. This approach should address the state explosion problem : criteria 1.
+and deal with small groups of components at a time, should address criteria 1.
 In the proposed methodology components are collected into functional groups
 and each component failure mode (and optionally combinations) are considered in the 
 context of the {\fg}.
 %
 The component failure modes (and optional combinations) are termed `test~cases'. For each test~case
 there will be a corresponding resultant failure, from the perspective of the {\fg}.
+%
 % MAYBE NEED TO DESCRIBE WHAT A SYMPTOM IS HERE
-A symptom collection stage is then applied. Here common symptoms are collected
+%
+From the perspective of the {\fg} failures of components will be symptoms.
+It is proposed that many symptoms will be common. That is to say
+that component failure modes, will often cause the same symptoms of failure
+from the perspective of a {\fg}.
+%
+A common symptom collection stage is then applied. Here common symptoms are collected
 from the results of the test~cases. Because optional combinations of failures are possible,
 multiple failures can be modelled, satisfying criteria 6.
 %
@@ -547,10 +557,10 @@ we can assign a test case number (see table \ref{pdfmea}).
 Each test case is analysed to determine the `symptom'
 on the potential dividers' operation. For instance
 were the resistor $R_1$ to go open, the circuit would not be grounded and the 
-voltage output from it would be the +ve supply rail.
+voltage output from it would be high (+ve).
 This would mean the symptom of the failed potential divider, would be that it
-gives an output high voltage reading. We can now consider the {\fg}
-as a component in its own right, and its symptoms as its failure modes.
+gives an output high voltage reading.%We can now consider the {\fg}
+%as a component in its own right, and its symptoms as its failure modes.
 
 From table  \ref{pdfmea} we can see that resistor 
 failures modes lead to some common `symptoms'.
@@ -662,9 +672,9 @@ We can use the symbol $\bowtie$ to represent taking the analysed
 \ifthenelse {\boolean{dag}}
 {
 We can now represent the potential divider as a {\dc}.
-Because have its symptoms or failure mode behaviour,
+Because we have its symptoms or failure mode behaviour,
 we can treat these as the failure modes of a a new {\dc}.
-We can represent that as a DAG (see figure \ref{fig:dc1dag}).
+We can represent this as a DAG (see figure \ref{fig:dc1dag}).
 
 \begin{figure}[h+]
  \centering
@@ -692,7 +702,7 @@ We can represent that as a DAG (see figure \ref{fig:dc1dag}).
 
 Because the derived component is defined by its failure modes and 
 the functional group used to derive it, we can use it
-as a building block for other {\fgs} in the same way as we used the resistors $R1$ and $R2$.
+as a building block for other {\fgs} in the same way as we used the base components $R1$ and $R2$.
 
 %\clearpage
 
@@ -1064,10 +1074,13 @@ to assist in building models for FTA, FMEA, FMECA and FMEDA failure mode analysi
 \paragraph{Worked example. Effect on State explosion.}
 The potential divider {\dc} reduced the number of failures to consider from four to two.
 The op-amp and potential divider modelled together, reduced the number of 
-failure symptoms from eight to three. Because symptoms are collected, we can state
+base component failures from eight to three failure symptoms. 
+%
+In general,
+because symptoms are collected, we can state
 the the number of failure symptoms for a {\fg} will be less then or equal to the number
-of component failues. In practise the number of symptoms is usually around half the 
-number of component failure modes.
+of component failures. In practise the number of symptoms is usually around half the 
+number of component failure modes, at each stage of FMMD analysis.
 
 
 
@@ -1086,11 +1099,11 @@ the state explosion problem is effectively dealt with.
 
 \item{All component failure modes must be considered in the model.}
 The proposed methodology will be bottom-up. 
-This ensures that all component failure modes are handled.
+This means that we can ensure/check that all component failure modes are handled.
 
 
 \item{ It should be easy to integrate mechanical, electronic and software models.}
-Because component failure modes are considered, we have a generic entity to model.
+Because FMMD models in terms of  failure modes only, we have a generic failure mode entities to model.
 We can describe a mechanical, electrical or software component in terms of its failure modes. 
 %
 Because of this 
@@ -1099,21 +1112,24 @@ using a common notation.
 
 \item{ It should be re-usable, in that commonly used modules can be re-used in other designs/projects.}
 The hierarchical nature, taking {\fg}s and deriving components from them, means that 
-commonly used {\dcs} can be re-used in a design (for instance self checking digital inputs)
+commonly used {\dcs} can be re-used in a design% (for instance self checking digital inputs)
 or even in other projects where the same {\dc} is used.
 
 
 
 \item{ It should have a formal basis, data should be available to produce mathematical proofs
 for its results}
-Because the failure mode of a SYSTEM is a hierarchy of {\fg}s and derived components
+Because the failure mode model of a SYSTEM is a hierarchy of {\fg}s and {\dcs}
 SYSTEM level failure modes are traceable back down the fault tree to
-component level failure modes. This provides causation trees \cite{sccs} or, minimal cut sets~\cite{nasafta}[Ch.1pp3]
-for all SYSTEM failure modes.
+component level failure modes. 
+%
+This allows causation trees \cite{sccs} or, minimal cut sets~\cite{nasafta}[Ch.1pp3]
+to be determined by traversing the DAG from top level events down to their causes.
+
 
 \item{ It should be capable of producing reliability and danger evaluation statistics.}
 The minimal cuts sets for the SYSTEM level failures can have computed MTTF
-and danger evaluation statistics sourced from the component failure mode statistics \cite {mil1991}.
+and danger evaluation statistics sourced from the component failure mode statistics~\cite{fmd91,mil1991}.
 
 % \item{ It should be easy to use, ideally 
 % using a graphical syntax (as opposed to a formal mathematical one).}
@@ -1129,7 +1145,7 @@ The bottom-up approach fulfils the  logical de-composition requirement, because
 are built from components performing a given task. 
 
 
-\item{ Multiple failure modes may be modelled from the base component level up.}
+\item{ Multiple failure modes (conjunction) may be modelled from the base component level up.}
 By breaking the problem of failure mode analysis into small stages
 and building a hierarchy, the problems associated with the cross products of
 all failure modes within a system are reduced by an exponential order.
@@ -1150,7 +1166,7 @@ to be a %superset
 a more rigorous  and `data~complete' model than
 the current four approaches, that is to say, 
 from an FMMD model, we should be able to 
-derive models that the other four methodologies would have been 
+derive outline models that the other four methodologies would have been 
 able to create. As this approach is modular, many of the results of 
 analysed components may be re-used in other projects, so 
 test efficiency is improved.