- an overview or tutorial about data acquisition its applications, data
acquisition software and the test equipment used for data acquisition.
Data acquisition is widely used in many areas of
industry. Data acquisition is used to acquire data from sensors and other
sources under computer control and bring the data together and store and
manipulate it. In view of the wide variety of signals and parameters that
can be sampled and stored, data acquisition involves many techniques and
skills. There are many different components to a data acquisition system
including sensors, communication links, signal processors, computers,
databases, data acquisition software, etc. All these items have to operate
together to make a successful data acquisition system.
Data acquisition systems can take many forms from very
simple manual systems to high complicated computer controlled ones. The
simplest form may be a technician manually logging information such as the
temperature of an oven. However this form of data acquisition has its
limitations. Not only is it expensive because of the fact that someone has
to be available to take the measurements, but being manual it can be subject
to errors. Readings may not be taken at the prescribed times, and also there
can be errors resulting from the manual fashion in which the readings are
taken. As can be imagined the problems become worse if a large number of
readings need to be taken, as timing may become more of an issue, along with
the volume of work required.
To overcome this, the simple answer is to use computer
control to perform the data acquisition. As a result a definition of what is
normally taken to be data acquisition is gathering information in an
automated fashion from analogue and digital measurement sources, i.e.
sensors and devices under test. The sensors may range from thermocouples and
voltage and current sensors to strain gauges and displacement gauges and
much more.
Data acquisition measurements
Data acquisition systems may make any number of a huge variety of
measurements. These measurements typically measure analogue. Before they can
be transferred into any computer system they need to be in a digital format.
Although a huge number of data acquisition measurements
can be made, they basically boil down to a very few basic elements:
- Voltage
- Digital signals
- Frequency or time interval
The sensors that are used in data acquisition
measurements often return values of voltage in particular that can then be
converted to the values of displacement, temperature, or whatever is being
measured.
These data acquisition measurements that may be termed
primary measurements will be looked at in turn, and then their applications
in making other measurements in data acquisition systems will be covered.
other data acquisition measurements that can be taken
include: current (using an accurate resistor and a voltage measurement);
temperature sensing (using a thermocouple sensor that provides a voltage
according to temperature); resistance measurements (using an accurate
current source and a taking a voltage reading across the resistance under
test); strain gauges (using a Wheatstone bridge, an accurate voltage supply
and a then a measurement of the resultant voltage); and of course a host of
other measurements.
Further details of these and other data acquisition
measurements and measurement techniques can be found on this website - see
the related articles list under the main menu on the left hand side.
Typical data acquisition system
It is difficult to define a typical data acquisition system as the
requirements, and hence the implementations vary so considerably. Although
simple solutions may utilise a datalogger, these dataloggers may not be
suitable for data acquisition systems requiring data from a large variety of
different sensors to be made and collected and analysed.
The ideal solution for a data acquisition system is often
to use a central computer to control the system and to collect the data,
store and process it. In some instances the system may include a data
acquisition card. More comprehensive systems may include a larger amount of
test equipment and this may either be discrete stand-alone test instruments,
or a more likely scenario is that it may utilise VXI or PXI technologies. If
this is the case then VXI or PXI chassis will be used with the relevant
instruments contained on cards contained within a VXI or PXI rack
These days technologies such as PXI are widely used for
data acquisition as they are able to house and control the test instruments
within an electrical environment where fast data transfers can be made, and
a suitable level of control can be provided.
While PXI and VXI may be the most obvious candidates for
linking an instrument to a data acquisition system there are other methods
that can be used. The General Purpose Interface Bus, GPIB, is still widely
used and may be applicable in some instances. However for many, possibly
smaller, data acquisition systems it is worth considering using the
Universal Serial Bus, USB. A number of USB data acquisition modules are
available these days. USB provides a very cost effective form of
transferring data to a computer and these USB data acquisition modules can
form a very cost effective and compact solution for many applications. Where
more than one USB data acquisition mode is required, then it is possible to
use a USB expander to provide more connections.
Data acquisition software
In order to be able to acquire, store and process the data in a logical
format, specialised data acquisition software can be used. This data
acquisition software can be written in a variety of languages and can be
written for the particular application in mind. Alternatively there are a
number of proprietary data acquisition software packages that are available
and these can be utilised instead.
The advantage of the proprietary data acquisition
software packages is that all the development has been undertaken and the
problems encountered. Although a software maintenance charge will normally
be applicable, this will be considerably less than trying to maintain a
similar "home grown" data acquisition software package. Accordingly many
companies opt to buy their data acquisition software, and then use this to
develop the tests themselves for the particular system in use.
Summary
Data acquisition is a very important area of the test and measurement
industry. Data acquisition systems are required in many applications from
electronics manufacturing to chemical engineering, mechanical manufacture as
well as more diverse applications such as monitoring geographical data from
mountains and volcanoes as well as many other interesting and diverse uses.
In view of this there are many data acquisition products
available on the market that may use data acquisition cards directly
included in PXI or VXI chassis, or in computers. In some instances bus
systems may be incorporated into the data acquisition system - GPIB, USB,
and RS232 ware widely used. Wireless systems are also used in many
applications. With this level of flexibility it is possible to create
systems that can be tailored to a given applications to make the
measurements that are required.
