From f379067a9f14aba04762c67ee937c6e4cd30e09f Mon Sep 17 00:00:00 2001 From: Robin Clark Date: Tue, 30 Nov 2010 13:09:19 +0000 Subject: [PATCH] Hazel proof reading. --- fmmd_design_aide/fmmd_design_aide.tex | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/fmmd_design_aide/fmmd_design_aide.tex b/fmmd_design_aide/fmmd_design_aide.tex index 38afbd0..277bf55 100644 --- a/fmmd_design_aide/fmmd_design_aide.tex +++ b/fmmd_design_aide/fmmd_design_aide.tex @@ -41,7 +41,7 @@ The principle of FMMD analysis is a five stage process, the collection of components into {\fg}s, which are analysed w.r.t. their failure mode behaviour, the failure mode behaviour is then viewed from the -{\fg} perspective (i.e. as a symptoms of the {\fg}), +{\fg} perspective (i.e. as a symptoms of the {\fg}) and common symptoms are then collected. The final stage is to create a {\dc} which has the symptoms of the {\fg} it was sourced from, as its failure modes. @@ -71,7 +71,7 @@ From the analysis of the {\fg} we can create a {\dc}, where the failure modes are the symptoms of the {\fg} we derived it from. % \paragraph{detectable and undetectable failure modes} -The symptoms will be detectable (like a value of of range) +The symptoms will be detectable (like a value out of range) or undetectable (like a logic state or value being incorrect). The `undetectable' failure modes undertsandably, are the most worrying for the safety critical designer. EN61058, the statistically based European Norm, using ratios @@ -83,7 +83,7 @@ for detected and undetected failure modes \cite{EN61508}. %failure modes. For instance, out of range values, are easy to detect by systems using the {\dc} supplying them. -An undetectable faults are ones that forward incorrect information +Undetectable faults are ones that forward incorrect information where we have no way of validating or testing it. % we know we can cope with; they %are an obvious error condition that will be detected by any modules @@ -396,6 +396,7 @@ This test case looks at the transistor failure mode where TR1 is always OFF. \paragraph{$\overline{TEST\_LINE}$ ON} Here TR1 should be OFF and R36 should be in series. This is a dormant failure, we can only detect this failure +when the $\overline{TEST\_LINE}$ is OFF. \paragraph{$\overline{TEST\_LINE}$ OFF} Here TR1 should be ON, but is OFF due to failure. The resistance R36 will always be in series. @@ -497,8 +498,9 @@ We can surmise the symptoms in a list. \section{conclusions} -With safety addition the undetectable failure mode of \textbf{low~reading} -disappears. The overall reliability though goes down ! +With the safety addition the undetectable failure mode of \textbf{low~reading} +disappears. +However, the overall reliability though goes down ! This is simply because we have more components that {\em can} fail. %% Safety vs. reliability paradox.