Transcript Diversity - ntpu.edu.tw

Introduction to FMECA

What is a FMECA?
– An Analysis technique which facilitates the
identification of potential design problems by
examining the effects of lower level failures on system
operation.
“Obviously, a major
malfunction.”
- Stephen A Nesbitt
NASA Public Affairs Officer
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Why is it Important?




Provides a basis for identifying root failure
causes and developing effective corrective
actions
Identifies reliability/safety critical components
Facilitates investigation of design alternatives at
all stages of the design
Provides a foundation for other maintainability,
safety, testability, and logistics analyses
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Definitions

FMECA - Failure Mode, Effects, and Criticality
Analysis.

FMEA - Failure Mode and Effects Analysis.

COMPENSATING PROVISIONS - Actions
available or that can be taken to lessen or
eliminate the effect of a failure on a system.

NEXT HIGHER EFFECT - The consequence a
failure mode has upon the operation, function,
or status at the next higher level of assembly.

END EFFECT - The consequence a failure mode
has upon the operation, function, or status at
the highest level of indenture.
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How is it Done?
What are the effects
of box failures on
the system?
What are the effects
of board failures on
the box?
What are the effects
of part failures on
the board?
Note: This is a bottoms up example.
Top down examples are possible.
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Simple Example: Flashlight
This flashlight is for use by special operations forces involved in
close combat missions (especially hostage rescue) during low
visibility conditions in urban areas. The light is to mounted coaxially
with the individual's personal weapon to momentarily illuminate
and positively identify targets before they are engaged. The exterior
casing including the transparent light aperture are from an existing
ruggidized design and can be considered immune to failure.
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Simple Example: Flashlight (cont.)
How can it fail?
What is the effect? Note
Part
that Next Higher Effect =
End Effect in this case.
Item
Failure Mode
End Effect
bulb
dim light
no light
flashlight output dim
no flashlight output
switch
stuck closed
stuck open
intermittent
constant flashlight output
no flashlight output
flashlight sometimes will not turn on
contact
poor contact
no contact
intermittent
flashlight output dim
no flashlight output
flashlight sometimes will not turn on
battery
low power
no power
flashlight output dim
no flashlight output
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Severity

SEVERITY classifies the degree of injury,
property damage, system damage, and mission
loss that could occur as the worst possible
consequence of a failure. For a FMECA these
are typically graded from I to IV in decreasing
severity.

The standard severities defined in MIL-STD1682
may be used or equipment specific severities
may be defined with customer concurrence
(recommended).
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Simple Example: Flashlight (cont.)

Severity
– Severity I
Light stuck in the “on” condition
– Severity II
Light will not turn on
– Severity III
Degraded operation
– Severity IV
No effect
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Simple Example: Flashlight (cont.)
Item
Failure Mode
End Effect
bulb
dim light
no light
flashlight output dim
no flashlight output
III
II
switch
stuck closed
stuck open
intermittent
constant flashlight output
no flashlight output
flashlight sometimes will not turn on
I
II
III
contact
poor contact
no contact
intermittent
flashlight output dim
no flashlight output
flashlight sometimes will not turn on
III
II
III
battery
low power
no power
flashlight output dim
no flashlight output
III
II
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Severity
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Criticality

CRITICALITY is a measure of the frequency of
occurrence of an effect.
– May be based on qualitative judgement or
– May be based on failure rate data
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Simple Example: Flashlight (cont.)
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Simple Example: Flashlight (cont.)
Can circled items be designed out or mitigated?
(There may be others that need to addressed also.)
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Integrated FMECA

FMECAs are often used by other functions such
as Maintainability, Safety, Testability, and
Logistics.
– Coordinate your effort with other functions up front
– Integrate as many other tasks into the FMECA as
possible and as make sense (Testability, Safety,
Maintainability, etc.)

Integrating in this way can save considerable cost over
doing the efforts separately and will usually produce a
better product.

If possible, use the same analyst to accomplish these
tasks for the same piece of hardware. This can be a huge
cost saver.
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FMECA Facts and Tips

FMECAs should begin as early as possible
– This allows the analyst to affect the design before it is
set in stone.
– If you start early (as you should) expect to have to
redo portions as the design is modified.

FMECAs take a lot of time to complete.

FMECAs require considerable knowledge of
system operation necessitating extensive
discussions with software/hardware Design
Engineering and System Engineering.

Spend time developing groundrules with your
customer up front.
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