Page 221 - Mechanical Engineers' Handbook (Volume 2)

210 Data Acquisition and Display Systems

mind, this chapter has been updated to focus more on the latter and less on the former. Other
chapters in this book cover details of the electronics, transducers, sampling, and calibration.
To control any process or understand what occurs during the life cycle of a process, the
system (a human or machine) must have information about what is occurring. In the simplest
of control loops, the measured variable must be converted to usable units, comparison in
some form to a target occurs, and a response is determined. At the plant level, improvement
of plant operation relies upon understanding the relationships between processes within the
plant (not only current, but historical), which in turn requires collecting data throughout the
plant, characterizing the relationship of the data with other data, storing the data in such a
way as to be retrievable in a useful, timely way, and manipulating the data for presentation
and hopefully providing an aid to understanding the relationships between processes. In
today’s competitive environment, focusing on local control and ignoring the interaction be-
tween processes, both internal to the plant and external, can be disastrous. If one is not
focused on improvement, one can bet the competitor is. Larger corporations, especially, can
bring analytical tools to bear to improve local processes, plantwide processes, and their
relationships to external influences, such as the supply chain. On the other hand, today’s
computing tools bring very powerful data acquisition and analysis capability within the reach
of the average technician with a little bit of motivation.
Data acquisition and display systems have changed dramatically. Twenty years ago,
terms referring to specialized systems such as SCADA (supervisory control and data acqui-
sition) and data loggers were common terms. Now, with the proliferation and broadening
role of computer systems and their intrusion into every aspect of manufacturing, many of
the features that used to be in specialized instruments and systems are now part of the
everyday tools available to anyone with a computer. This chapter attempts to cover aspects
of data acquisition and manipulation that may help the engineer better understand issues and
give a foundation for using and even constructing tools. The organization is as follows:

• The initial sections cover the nature of data and the acquisition and conversion of data
to usable units and includes some discussion of useful display techniques. The dis-
cussion attempts to identify issues of which the engineer should be aware and give
guidelines on how to manage data.
• The latter sections cover the coordination, storage, access, and manipulation of data.
A discussion of pros and cons of different strategies should help the reader understand
the trade-offs in system selection and construction. It is difficult to do this without
describing specific technologies and brands, but the author has endeavored to level
the discussion in such a way that changes in technology will not change the value of
the discussion. Time will tell if the approach is effective.

2 DATA ACQUISITION

Data acquisition includes the following: (1) acquiring raw data from the process being mea-
sured and (2) converting data to usable units. Included in this section are also some topics
of data display closely related to the nature of the data being acquired. Other aspects of data
display will be covered in later sections.
In process industries, much of the data are analog in nature, such as pressure, temper-
ature, and flow rate. The values acquired are sampled representations of process data that
have a scale and a range, with various issues around effective range and whether values
over a range are linear or more complex. When acquired in a data acquisition system there
are a number of issues that must be addressed related to how data is sampled; how it is