Data acquisition tutorial
- 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:
- 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.
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.
By Ian Poole
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