Page 248 - Mechanical Engineers' Handbook (Volume 2)

9 Other Data Acquisition and Display Topics 237

question–answer or menu-type systems. When the system must be very flexible or custom-
ized, it may be more appropriate to write a custom program. When considering this approach,
be cautious, for the cost of implementing a program is often much higher than expected.
For instance, if one wanted to perform simple data acquisition and storage from a sensor,
the cost to write a program would probably be higher than buying a small off-the-shelf
system and entering the parameters for data collection. Writing a program involves analysis,
design, development, debugging the program, and testing of results. The cost of documenting
a program is often a large unplanned cost. If the results are intended to be used to make
economic or process-related decisions, then the program must be tested carefully. Addition-
ally, maintenance of the program can be quite expensive. Someone must be trained in the
technologies used to build the program, the logic of the program, and the installation of the
program. Another factor to consider is that costs of improvement of third-party software are
borne by many customers and driven by many customers. The net result to the user of this
software is that it is normally constantly improving, constantly tested, and maintained by a
group of developers whose primary job is software development. One reason to build and
maintain software internally is that a company can keep special knowledge within the com-
pany and thus maintain competitive advantage.

9.4 Store and Forward
When data acquisition and data storage are on two separate machines, it is important to
provide methods to retain data in case the link between systems is broken. Message buses
provide automated methods of maintaining a link between data acquisition systems. The
developer inserts data into the message bus. If the link between the two systems is broken,
the message bus queues up the data messages on the collection machine. When the data
storage machine connection is reestablished, the message bus passes on the data to the data
storage machine.
When a message bus is not available or feasible, a simplified mechanism can be created
where a file representing each sample of data is created. If the data collection and data
storage system are linked, then the data storage system monitors the directory of the collector
for a new data file. If the data storage system detects one or more files on the data collection
computer, then it will process them into storage. If the link is broken, then the files build
up until the link is reestablished. A related technique is to store data in a database or similar
mechanism on the data collection computer and scan it periodically for missing data from
the storage computer. This is particularly useful when connection to the data collection
computer is unreliable.

9.5 Additional Communications Topics
When considering transmission media, some points may provide value to the engineer. Fiber-
optic cabling is less sensitive to noise than other transmission media. Wireless access points
provide increased flexibility in positioning of sensors and greatly reduce wiring costs. Par-
ticularly, if one wishes to collect data from sites that may move, such as environmental
sampling sites, the costs of wiring and rewiring can be quite significant. Using wireless
transmission technology eliminates much of the wiring costs and facilitates moving the sen-
sors from one location to another. Wireless transmission has a set of concerns that must be
taken into account by the engineer, including security, since other units can monitor signals
(still evolving) and ability to be jammed.