Let's go through the process above one step at a time:
- Some physical phenomenon occurs. For example, a change in pressure
in process piping, a change in temperature in a heat exchanger, etc.
The phenomenon causes a sensor, flowmeter,
load
cell or transducer to electrically change
state.
And/or this industrial process system receives a
digital or analog signal from another industrial
process system. An example of this comes from the food industry: one
system may control (specializes in) recipe batching (mixing of ingredients); another process
system is dedicated to cooking or sterilization; yet another system
is dedicated to packaging, warehousing, and so on. The systems are
isolated because different, specialized vendors were called in to install systems
they specialized in. The different systems are linked via Ethernet data
highways.
- If the signal is high-voltage, "noisy" or "quiet" (low
level), it may need to be filtered, attenuated or amplified, respectively,
before reaching data acquisition (DAQ) hardware.
A signal conditioner is the tool for such a purpose. Often,
a signal conditioner will output digital data.
- After leaving the signal conditioner the signal finally
enters data
acquisition (DAQ) hardware. DAQ hardware may be located as an external
(stand-alone) rack-mounted unit or it may be installed inside a PC (much
like a sound
or video card).
- A PC terminal -- with data acquisition (DAQ) drivers
installed and running
special data acquisition
(DAQ) software -- is used to process,
record and analyze the information received by the DAQ hardware. The
information is used to control the process system; it may do this
by interfacing with a programmable logic controller (PLC). A data logger
-- such as chart recorder -- may also be used if a "hard copy" is necessary.
In the strictly-government-regulated food industry, the FDA may require
a chart recorder to record data live. Hence, one
(of the several) outputs from an input device -- pressure transducer,
RTD ,etc. -- may be
wired directly into the data logger (chart recorder). Many new data
loggers are hybrid models: they simultaneously write charts and save
data onto memory or an internal drive.
Types of Data Acquisition Systems
Three general usage types:
- Laboratory
- Distributed
- Portable
Laboratory and distributed DAQ systems are normally put in
a permanent location as these are comprised of relatively large or bulky
hardware and connect to desktop PCs in some way. These systems depend
upon the PC to access, process, and analyze input data and prepare it
for some type of presentation. Portable DAQ systems, on the other hand,
are small, lightweight units that are easily carried by hand and work
with laptops or, even, no computer at all when installed at a location
or site to only record data.
Laboratory and distributed data-acquisition systems
typically adhere to industry-based packaging standards. For instance,
some laboratory systems are mounted in standard 19-inch racks while distributed
systems often use track mountings. A subset of these systems includes
a host computer that accommodates data-acquisition plug-in boards. Portable systems,
in contrast, have no real standardized form -- they may come in various
sizes and shapes, but they are typically small and light. Further, portable
systems are additionally classified as either stand-alone units or those
which connect to a PC. Stand-alone units are self-contained data loggers
and do not need a PC connection to function.
Most modern data-acquisition systems, regardless of form factor, do
their intended tasks very well; i.e., they acquire and process data.
All systems have several factors in common; they need signal conditioners
to convert sensor signals and other electrical inputs to a form that
a processor can handle. They also have a wide variety of analog input
channels varying in number from two to several hundred -- or even thousands.
Further, inputs may or may not be isolated, and either single-ended or
differential, or both. Of the three types, portable systems are gaining
a larger market share as the other (legacy) systems age, and these portable
systems are increasingly outperforming them. Further, the new portable
systems can be easily configured for laboratories and distributed systems
as well as portable applications. When considering a data acquisition
system to purchase, however, the most important functional parameters
that differentiate them include accuracy, resolution, and sampling rate.
Above: Some data acquisition systems are: very compact,
available in in modular form, and can measure voltages and thermocouples
on every analog channel. An example of of such a system is IOTech’s Personal
Daq/3000. It features 16-bit, 1-MHz analog-to-digital conversion, with
16 inputs and counter.
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