Page 140 - Anatomy of a Robot

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RELIABILITY, SAFETY, AND COMPLIANCE 125
If we combine two systems with MTTF1 and MTTF2, we can find l1 and l2 by
inversion, add them together, invert the result, and get the MTTF for the combined
system. In this way, the reliability of a large system can be predicted.
To predict the MTTF of a large PCB, for instance, perform the following steps:
1. Obtain the Bill of Material (BOM) for the board, the list of parts, and the quan-
tity of each.
2. Make a spreadsheet of the part types, grouping similar parts together. All the
resistors of the same wattage, for instance, can be thrown into the same group.
3. Don’t forget to count components like feed-throughs on the PCB. They have
failure rates, too! Count all connections and wires.
4. List the quantities in each group.
5. In the Bellcore tables, look up the l failure rates for each component type.
6. Multiply the failure rate by the quantity for each component-type failure rate,
and add up the component-type failure rates to get a failure rate for the PCB.
Invert it to obtain the PCB’s MTTF.
MTTR is the Mean Time to Repair a Failure. Basically, it’s the average time it takes
to repair a failed component and bring it back to functioning status.
MTBF is the Mean Time Between Failures. It’s related to the MTTF and MTTR in
that it is the sum of the two (MTBF MTTF MTTR). Basically, once a component
is repaired after a failure, it will take an average of MTTF MTTR for it to fail again
and be repaired. (For more info, go to www.ab.com/harry/mtbf.html or www.its
.bldrdoc.gov/fs-1037/dir-022/_3254.htm.)

AVAILABILITY

The reliability of many systems is measured as availability. People who use computer
systems get very impatient if the computer goes down and cannot be used. They are
specifically interested in the percentage of time that the computer system will be usable.
Thus, availability is defined as follows:
Availability MTTF>1MTTF MTTR2 MTTF>MTBF

From the middle term in the equation, we can see that if MTTR is very short, or if
the MTTF is very long, then availability approaches 100 percent. Engineers who need
high availability can work on both the MTTF and MTTR to achieve their goals. The
MTTR can be lowered in several ways.
If a failed unit requires a trip to the repair depot, it can take quite a while. But if a
spare component is right on site, repair can take just a few minutes.
If a hot backup component can switch over automatically, it will take a few seconds.
Hot backups can be accomplished with what’s called an “N 1” backup. If a total of