Vegard Joa Moseng – Student meeting
RELIABILITY ANALYSIS USING ISOGRAPH
A LITTLE BIT ABOUT SYSTEM RELIABILITY:
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Reliability: The ability of an item to perform a required function, under given
environmental and operational conditions and for a stated period of time
[ISO 8402].
The resistance to failure of an item over time.
System reliability: Systems functioning only if all the components are
functioning. The reliability of a system is equal to the products of the
reliabilities of the individual components which make up the system. A large
number of components in a system may therefore cause the system
reliability to be rather low even if the individual components have high
reliabilities.
FAILURES:
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Normally, we are interested in the failures that stands in the way of a
reliable system. This is measured using failure rates.
Failure rates: is the frequency with which a system or component fails,
expressed, for example, in failures per hour. This number is denoted by
lambda, λ.
Because λ often (hopefully) ends up being a high negative exponent it is
common to instead use 1/λ (Mean Time To Failure) to more easily
understand the numbers.
MTTF is however only focused on the «linear» part of the «useful life period»
in an expected bathtub curve.
MTTF FROM FIT:
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The failure rate can be defined as the following: The total number of failures
within an item population, divided by the total time expended by that
population, during a particular measurement interval under stated
conditions.
Example from SI4162DY N-channel MOSFET reliability calculations.
HOW I CALCULATE THE FAILURE RATE:
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The number extracted from the data sheet is put into Isograph. FIT stands
for Failures In Time and denotes the number of failures per 10^9 device
hours. E.g. 1000 devices for 1 million hours, or 1 million devices for 1000
hours each, or some other combination.
RESULT OF PREDICTION:
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The result of the parameters affect on the component is calculated into a
final failure rate after the estimation of the MIL-HDBK-217F.
RESULT OF PREDICTION - TEMPERATURE:
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Also the failure rates relative to the surroundings temperature can be
plotted to better see the effects of temperature variations.
FMECA
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After predictions have been made about the reliability of the single
components this information can be implemented in a Failure Mode, Effects
and Criticality Analysis. The purpose of this is to identify what can go wrong
depending on the failure modes. To help indentify what can go wrong there
are military handbooks and standards which contains for example expected
failure modes for electronic components. From MIL-HDBK-338B:
FMECA - EXAMPLE
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Implementing this standard on, for example, a capacitor, means we can
understand what the expected failure modes are and what effect the
failures will have. The FMECA is constructed with respect to outputs.
FMECA – ADDITIONAL NOTES
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After a FMECA is constructed we can also see the new failure rates
calculated based on the parameters such as probability of failure mode,
criticality level, danger level and so on.
FAULT TREE
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The information gathered thus far can then be put into a Fault Tree, where
we can use boolean logic to better see and understand what failure modes
can affect the critical points in our system and what failure rate each of the
failure modes have.