What is Data Acquisition

Data Acquisition Includes:
Understanding data acquisition     Data logging systems     DAQ systems     DAQ cards & modules     DAQ measurements    

Data acquisition sensors: Thermocouple temperature sensor     Flow meters    

Data acquisition, often abbreviated to DAQ and sometimes DAS, is widely used in many areas of industry. Data acquisition is used to acquire data from sensors and other sources under computer control to bring the data together, store it and then 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.

Data acquisition systems can take many forms from very simple DAQ systems to high complicated computer controlled ones. They can also take many forms from large rack DAQ systems to compact DAQ systems - sometimes USB DAQ systems can provide the ideal solutions.

In fact data acquisition can be scaled according to what is needed. In fact many different DAQ devices are available to accommodate all the various requirements that may appear.

Many parameters can be measured - everything from temperature to voltage, strain to flow, and many more items. Different DAQ sensors can be used to measure different types of parameters and several different types of sensor can be used within one overall system as required for the particular situation.

Often data acquisition is used to measure the different parameters that need to be monitored in manufacturing processes, or they may be used for monitoring a large number of sensors used for geological monitoring, or fr any application where very many sensors need to be monitored in a central location or computer.

Unlike traditional bench testing systems where oscilloscopes, spectrum analysers, voltage meters and the like are used to make ongoing measurements, data acquisition systems take measurements of various parameters, returning these samples at regular intervals from a variety of sources to look at trends and also ensure that processes such as those in manufacturing plants, etc are running as they should.

Data acquisition definition

When talking about data acquisition systems, it can help to have a basic understanding of what one is, and a definition can be helpful.

There are many different components to a data acquisition system. The different DAQ devices can include: sensors, communication links, DAQ modules and cards, computers, databases, data acquisition software, etc. All these different DAQ devices have to operate together to make a successful data acquisition system.

Data acquisition systems vs data loggers

Data acquisition systems and data loggers are very similar in many ways, but they have some subtle differences.

When looking at data acquisition systems it is helpful to be able to understand the differences in terminology between them and different applications that suit one rather than the other better.

There are no actual hard and fast definitions for the different types of data acquisition systems, but some of the ones that have been seen are described here. It is worth mentioning that different people and companies may have different views. However it helps to have a view of the different types of DAQ system and the different descriptions used.

• Data acquisition systems:   DAQ systems tend to be used for more demanding situations. Both dynamic and static measurements can be made and therefore DAQ systems are capable of both high speed and low-speed sampling.

  As a result of the demands placed on them, DAQ systems usually have a high-powered computer associated within them, either built in or connected to them. In this way they offer the maximum performance and flexibility, but at additional cost. These systems may be in larger racks although a number of compact DAQ systems are available.

• Data acquisition device:   This description has been seen being used by more than one manufacturer, although it is not particularly widespread. It has been included here for completeness.

  Essentially this is a DAQ system that is more complex than a data logging system, but does not have the complexity of a full rack based data acquisition system. It is likely to use a single device to which all the sensors are connected - hence the name. This type of data acquisition system is less costly than the full rack based systems, but provides more functionality than a data logger. Many of these items are USB data acquisition systems.

• Data loggers:   Data Loggers are often just that - they log data. In reality the ease with which small processors and even computers can be used means that the lines are probably a bit blurred, but data loggers tend to be smaller and they are often intended to measure relatively slow signals. Many are intended to collect data over a long period of time. Data may be collected on a smaller computer and sent of over a data link, or it may even be stored in a removable memory device such as a flash memory card.

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  A data logging system by definition will have a more limited set of inputs, and these tend to be more basic in their format: voltage current, temperature, although geological data may also be collected for long term monitoring of a number of items.

  Read more about . . . . Data Logging & Data Loggers.
  
  

Although the boundaries and definitions are a little arbitrary, it gives a guide to the different types of data logging and data acquisition systems available.

When setting out to acquire data in one form or another, it is worth looking at what will best meet the needs of the requirement. Both data acquisition systems and devices as well as data loggers have their place. Selecting the right format will enable the best return on investment as well as making it easier to use and gain the required data. From a large rack system to a smaller or compact DAQ system, a DAQ device or a data logging system there is a wide choice.

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.

It is possible measure a huge number of different parameters using a DAQ system - the most common ones are listed below:

• Voltage
• Current
• Temperature
• Strain
• Pressure
• Vibration
• Distance / displacement
• Angles, RPM, etc
• Weight
• Digital signals
• Frequency or time interval

Separate modules or sensors may be used for the different measurements, although there are many multi-input general purpose DAQ devices that can interface to a variety of sensors of different types..

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.

Often a DAQ module and sensor will use a transducer of some form and a measurement of a parameter such as voltage is made. This is called the primary measurement, and it is then converted into the actual parameter that is required. In this way it is possible to make or obtain DAQ modules that can measure almost any parameter required.

As a result it is possible to obtain a vast array of data acquisition modules that are able to measure almost anything. It is also possible to obtain DAQ modules that can be used for special sensor designs. In this way these DAQ modules can be customised for specific measurements with specific sensors. They are able to meet the needs of very specific requirements.

Data acquisition systems

Data acquisition systems are made up from multiple elements, and these can be tailored to meet the individual requirements for any given situation.

Although simple systems with low sense and data rates may be best suited to a data logger, larger applications requiring fast data acquisition and more data being collected will need a data acquisition system.

Data acquisition systems tend to utilise a form of rack system for the basic hardware. This makes them much easier to customise for a particular application and also reconfigure if required.

To this end there are several manufacturers who make suitable rack systems for data acquisition, DAQ.

Data acquisition cards & modules

Most data acquisition systems consist of a rack module that is filled with cards to provide the various measurement functions that are needed.

These cards obviously need to conform to the overall system that is used in both mechanical and electrical interfaces.

Fortunately the rack systems used are often standardised and modules are often available from a number of manufacturers making selection of the required module easy.

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. Some companies have their own data acquisition platforms like their NI Compact DAQ system which offers good flexibility, but within a compact format. 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.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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