robin/Robin_PHD
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

waiting for my wheel to be trued by furtue cycles, fixed some typos etc

Browse Source 
after work......
This commit is contained in:
Robin P. Clark 2012-08-21 17:24:46 +01:00
parent 45929cb921
commit b91dade30c
1 changed files with 36 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

62
submission_thesis/CH2_FMEA/copy.tex
View File

@ -15,13 +15,14 @@ on the behaviour and safety of the system."
%\tableofcontents[currentsection]
FMEA is a broad term, and can mean anything from an informal check on how
FMEA is a broad term; it could mean anything from an informal check on how
how failures could affect some equipment in an initial brain-storming session
in product design to formal submissions as part of safety critical certification
procedures
This chapter describes the basic concepts, uses a simple example to
demonstrate an FMEA stage and then explores some concepts with which we can evaluate
the effectiveness of FMEA.
in product design, to formal submissions as part of safety critical certification.
%
This chapter describes basic concepts of FMEA, uses a simple example to
demonstrate a single FMEA analysis stage, describes the four main variants of FMEA in use today
and explores some concepts with which we can discuss and evaluate
the effectiveness of FMEA.
% \subsection{FMEA}
@ -46,8 +47,8 @@ demonstrate an FMEA stage and then explores some concepts with which we can eval
% % \item Analysis
% % \end{itemize}
\subsection{FMEA basic concept}
\clearpage
\paragraph{FMEA basic concept.}
\begin{itemize}
@ -62,7 +63,7 @@ demonstrate an FMEA stage and then explores some concepts with which we can eval
FMEA is a procedure based on the low level components of a system, and an example
analysis will serve to demonstrate it in practise.
\subsection{ FMEA Example: Milli-volt reader}
\paragraph{ FMEA Example: Milli-volt reader}
Example: Let us consider a system, in this case a milli-volt reader, consisting
of instrumentation amplifiers connected to a micro-processor
that reports its readings via RS-232.
@ -89,7 +90,7 @@ For the sake of example let us choose resistor R1 in the OP-AMP gain circuitry.
\subsection{FMEA Example: Milli-volt reader}
\paragraph{FMEA Example: Milli-volt reader}
% \begin{figure}
% \centering
% \includegraphics[width=80pt]{./mvamp.png}
@ -105,14 +106,21 @@ For the sake of example let us choose resistor R1 in the OP-AMP gain circuitry.
The analysis above has given us a result for one failure scenario i.e.
for one component failure mode.
A complete FMEA report would have to contain an entry
for each failure mode of all the components in the system under investigation.
%
Note here that we have had to look at the failure~mode
in relation to the entire circuit.
We have used intuition to determine the probable
effect of this failure mode.
We have not examined this failure mode
For instance we have assumed that the resistor R1 going SHORT
will not affect the ADC, the Microprocessor or the UART.
%
To put this in more general terms, have not examined this failure mode
against every other component in the system.
Perhaps we should.... this would be a more rigorous and complete
Perhaps we should: this would be a more rigorous and complete
approach in looking for system failures.
@ -135,20 +143,18 @@ are thus very reliable too. Reliable field data on failures will, therefore be s
Should we wish to prove a continuous demand system for say ${10}^{-7}$ failures\footnote{${10}^{-7}$ failures per hour of operation is the
threshold for S.I.L. 3 reliability~\cite{en61508}.}
per hour of operation, even with 1000 correctly monitored units in the field
we could only expect one failure per ten thousand hours (a little over one a year) to fail.
It would be impractical to get statistically significant data for equipment
we could only expect one failure per ten thousand hours (a little over one a year).
It would be utterly impractical to get statistically significant data for equipment
at these reliability levels.
However, we can use FMEA (more specifically the FMEDA variant, see section~\ref{sec:FMEDA}), working from known component failure rates, to obtain
statistical estimates of the equipment reliability.
\subsection{Rigorous FMEA --- State Explosion}
\subsection{Rigorous FMEA --- State Explosion Problem}
FMEA cannot consider---for practical reasons---a rigorous approach.
It must be applied by experts in the system under investigation
to be a meaningful analysis.
\paragraph{Rigorous Single Failure FMEA}
\paragraph{Rigorous Single Failure FMEA}
FMEA for a safety critical certification~\cite{en298,en61508} will have to be applied
to all known failure modes of all components within a system.
@ -191,7 +197,14 @@ For our theoretical 100 components with 3 failure modes each example, this is
$100*99*98*3=2,910,600$ failure mode scenarios.
\paragraph{Reliance of experts for meaningful FMEA Analysis.}
FMEA cannot consider---for practical reasons---a rigorous approach.
We define rigorous FMEA as examining the effect of every component failure mode
against the remaining components in the system under investigation.
%
Because we cannot perform rigorous FMEA,
we rely on experts in the system under investigation
to perform a meaningful FMEA analysis.
@ -214,7 +227,7 @@ $100*99*98*3=2,910,600$ failure mode scenarios.
\section{PFMEA - Production FMEA : 1940's to present}
\subsection{PFMEA}
Production FMEA (or PFMEA), is FMEA used to prioritise, in terms of
cost, problems to be addressed in product production.
@ -279,7 +292,6 @@ will return most cost benefit.
\subsection{PFMEA Example: Ford Pinto: 1975}
\begin{figure}[h]
\centering
@ -293,8 +305,6 @@ will return most cost benefit.
\subsection{PFMEA Example: Ford Pinto: 1975}
\begin{table}[ht]
\caption{FMEA Calculations} % title of Table
@ -315,7 +325,7 @@ will return most cost benefit.
http://www.youtube.com/watch?v=rcNeorjXMrE
% don't think this is relevant for the thesis: http://www.youtube.com/watch?v=rcNeorjXMrE