RELIABILITY ANALYSIS USING ISOGRAPH

Vegard Joa Moseng – Student meeting

A LITTLE BIT ABOUT SYSTEM RELIABILITY:

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:

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:

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:

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:

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:

Also the failure rates relative to the surroundings temperature can be plotted to better see the effects of temperature variations.

FMECA

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

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

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

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.