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126 CHAPTER FOUR
N components are necessary for the successful operation of the system, the designers
add one more system, hence the terminology N 1. If one of the N components fails,
the spare component will take up the slack and the overall system will keep running. If,
however, a second component fails before the broken one is repaired, then the system
fails. The calculations for MTTF and availability are somewhat complex in a case such
as this. For further information, see the following sites:
www.mapleapps.com/categories/engineering/manufacturing/html/reliability.html
www.mathpages.com/home/kmath326.htm
www.mathpages.com/home/kmath498.htm

Done properly, availability goes up quite high, but at the price of a single extra com-
ponent. Repair is basically instantaneous, except for the case where two or more com-
ponents fail at the same time. It’s therefore wise to make the repair to broken units as
rapidly as possible. This, too, increases the availability.

COMPONENTS

Another way to increase the MTTF of a system is to avoid unreliable components. The
Bellcore component tables list the failure rates of many different types of components.
Without the tables in front of me, I’ll list the components I remember as having high
failure rates. These are components we should avoid in the design and construction of
the robot.

Connectors
Connectors of all sorts are lower-reliability parts. Every single pin on every connector
must be counted. Connectors generally work by spring pressure. A bent piece of metal
pushes on another piece of metal and thus excludes gas and dirt. Often, a wiping action
is made as the connection is made. The following problems can make connectors fail.

Contaminants If we have too many contaminants or gases are not excluded,
corrosion can creep into the connection and ruin it.
Currents In addition, if high currents move over the contacts, they can corrode
from too much heat.
Vibration Movement can cause contacts to break open, interrupting signals.
Operator error Service technicians can fail to seat connectors properly. Stating
a cynical view, the function of a switch or connector is to be installed wrong or
set to the wrong position. If connectors are not used, they cannot present such
problems.