What is Data Acquisition, Anyway?
Do you really know what a data acquisition system is, or do you
just have sort of a fuzzy picture that you think of when you hear
the words? Even if somebody gave you a good, workable definition
of a data acquisition system sometime in the past, does it still
fit the data acquisition technology of today?
We need to start this book off by drawing a Venn diagram and identify
what belongs in and what belongs out. We have to start out with
a sharp, clear borderline , even if we intend to be a little flexible
about applying it. We need a clear starting pointsomething
to refer back to when you have to make those tough decisions. Otherwise,
if you start out fuzzy, you can get lost really fast.
So, what is a data acquisition system and, more importantly, what
DAQ Is as DAQ Does
I think we can identify a data acquisition system by what it does
more readily (or at least more reliably) than by what it is. What
a data acquisition system is can change dramatically with technological
advances. What it does is likely to be more stable.
What a data acquisition system does is capture information about
a real-world physical (as opposed to, say, financial) system and
archive that data in a form that is readily available for scientific
or engineering analysis. I think we should also require the data
acquisition system to do that job automaticallywithout real-time
The next question, if you buy my definition of what a data acquisition
system does, is: "What elements do we expect to find in a data
Figure 1.1 shows a conceptual block diagram that includes all the
main functions of a data acquisition system.
Figure 1.1: A data acquisition system includes elements to perform
seven essential functions.
Obviously, if youre going to capture information about a
real-world physical system, youd better have some sensors
out there that measure things about the physical system and convert
them to data signals. These signals can come off the sensors as
either analog or digital signals, but theyd sure better be
in some form that can travel from the sensors to a central location
where they can be collected, sorted, cataloged and stored.
That travelling, collecting, sorting, cataloging and storing doesnt
have to happen in real time, though. You arent going to look
at that data in real time. What youre going to do with it
is analyze it after the fact, anyway. If you tried to analyze it
in real time, youd end up with an incomplete picture of the
experiment and youd probably interfere with the more time-critical
job of capturing the next piece of data. Although microprocessors
think fast enough that they seem able to do more than one thing
at a time, thats really an illusion. One processor can do
only one thing at one time. If your processor is off doing a trend
analysis of the data-set-so-far when the time comes to grab the
next data point, it will miss that next data point.
The travelling has to happen along some kind of data-transmission
link. This link can be as simple as a couple of wires leading from
a thermocouple, or as complex as the Internet.
The third element is a means of collecting the information into
a data stream. That is usually accomplished by a data-acquisition
board, but there are other possibilities as well. The point is that
the "collector" takes multiple analog and/or digital signals
from sensors and puts them out as a digital data stream (usually)
on a computer bus.
The sorting, cataloging and storing happens in a general-purpose
computer. By the way, an embedded controller is just a general-purpose
computer with blinders on so that it has fewer distractions than,
say, a desktop PC.
In order to do its sorting and cataloging, however, the computer
needs a little added information about the data. Just knowing that
a thermocouple reported a temperature of 625.7° C doesnt
do the job. When that measurement was made is probably the most
useful thing to know about it, so youd expect to see a reliable
real-time clock providing time tags for the data points.
What else is going on in the physical system at the time the reading
was taken is also important. So, the data acquisition system has
to have some kind of event triggering. In many systems, the computer
actually controls the physical system, so control outputs are a
Finally, what we need to have in the end is a permanent record
of all this information that the data acquisition system collected.
In the end, it goes into data files stored on some kind of (usually)
magnetic media, such as floppy disks or magnetic tape.
The Seven Elements
So, as Figure 1.2 shows, the elements we expect to see in a data
acquisition systemwhat the data acquisition system isinclude
sensors, data transmission links, a collecting means (such as a
data acquisition board), a processor, a clock, triggering and/or
control hardware and a means of laying down a permanent electronic
Figure 1.2: Venn diagram showing what constitutes a data acquisition
system, and what doesnt.
If you run across a thing that has these seven elements, its
a data acquisition system. If it has some, but not all, of them,
it could be a component of a data acquisition system. If it has
these elements and more, then it is a bigger system of which a data
acquisition system is a part.
Chapter 1 | Chapter 2
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