FMEA
EXAMPLE
The above block diagram shows a simple
redundant system in which two computers are connected to an internally
redundant Interface Unit (IU). Only one of the two computers
is required
to output a control signal for some mission critical operation. Also,
one of
the IU buses is required to complete the mission (i.e.: the
outputs from
MUX_A are on the A-Bus and the outputs from MUX_B
are on
the B-Bus). The IU receives the two control signals CONTROL1
and CONTROL2 into dual redundant multiplex devices labeled MUX_A
and MUX_B. Each mux chip has a bypass capacitor connected
across the
power supply input to ground.
This looks to be a robust redundant
system.
However, review of the FMEA shows that there are single point
failures
lurking in the design…
The FMEA worksheet for this
circuit
is at WORKSHEET. The process is to
hypothesize
a failure and determine that failure’s effect on the rest of the
system.
The failure mode of each device may be determined by the data in
FMD-97. FMD-97
is a database compiled by Reliability Analysis Center (RAC). This
database
contains many electronic devices and the failure modes of the devices.
For
example, the capacitor used in the circuit shown is a Ceramic 1 uF
device.
The failure modes of this generic
device are
as follows:
DRIFT 62%
SHORTED 30%
OPEN 6%
OTHER 2%
The failure mode of the IC in the
circuit
can also be found from the database. The failure modes Including
single point
failures (SPFs) are shown in the FMEA worksheet. Go to
the WORKSHEET.
There are single point failures in this
design. See this link for a fix to the design to remove the Single
point
failures…FMEA_NO_SPF
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