Page 298 - Sensors and Control Systems in Manufacturing

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Cha p te r

box being detailed and those boxes under it. The same relationship
holds for the parent diagram as the diagram directly preceding the
child diagrams.
The interfaces linking boxes through their neighbors at the same
level are the input, output, and control actions. In graphical terms,
these interfaces are designated by arrows. Methodically proceeding
through a given network, the entire system can be modeled in terms
of boxes and arrows.
While SADT provides a realistic approach to modeling any system,
it cannot provide the solution to any problem. The integrated computer-
aided manufacturing definition method comes one step closer to the
realization of a functional CIM system. It utilizes teamwork and
demands that all correspondence and analysis be in written form so
others can obtain a grasp of the situation and errors can be more read-
ily located. Because the written word is required during the implemen-
tation phases, the documentation that usually is done at the end of
most major projects can be nearly eliminated. Keeping accurate records
also plays an important role in debugging the system in the future.

5.6.2 A Multiobjective Approach for Selection of Sensors in
Manufacturing
Sensors are evaluated using six criteria:

• Cost is simply the price for the sensor and its integrated cir-
cuitry if it should be purchased.
• Integrability is the degree to which the sensor can be used in
conjunction with the manufacturing system it serves. This
can usually be measured in terms of compatibility with exist-
ing hardware control circuits and software.
• Reliability is the quality of the sensors as indicated by the mean
time between failures (MTBF), and can be measured by perform-
ing a simple stress test on the sensor under severe limits of
operation. If the sensor operates under a certain high temper-
ature for a certain period of time, it will assure the user that
the system will perform satisfactorily under normal operating
conditions. It will also indicate that the electronic control cir-
cuits are reliable, according to the burn-in philosophy.
• Maintenance involves the total cost to update and maintain
the sensor and how often the sensor needs to be serviced.
• Expandability is how readily the sensor can be modified or
expanded as new needs arise because of a changing environment.
• User friendliness indicates the ease of using and understand-
ing the unit. It may include the quality of documentation in
terms of simplicity, completeness, and step-by-step descrip-
tions of procedures.