Data Acquisition Systems, DAQ Systems

Data acquisition systems, DAQ systems consist of many items assembled as a system that is used to collect and analyse data.

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 system, or DAQ system is the overall term used to describe the collection of items used for data acquisition.

Where any data acquisition is to be undertaken, it will be necessary to assemble a number of items together to provide the functionality that is required. The overall collection can then be referred to as the data acquisition system or DAQ system.

These data acquisition systems of DAQ systems can take many forms and can be built around a number of different platforms. Although these platforms will work well, understanding how the overall system will fit together and operate will provide the best insight into which platform to use and how to set the system up.

Typical data acquisition system

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.

Within any data acquisition system there are several elements:

Sensor: The sensor is the item that actually measures the parameter of the physical phenomenon. It could be a simple voltage sensor, or current probe. IT could also be a temperature thermocouple or strain gauge, etc. This will return an electrical impulse, most often in the form of a voltage.
Signal conditioning: Some signal conditioning is normally required to present the input from the sensor into a form that is suitable for digital conversion. It may require passing through a differential amplifier. It may require some filtering to remove noise. There may be other processing required.
Analogue to digital conversion: This one of the key processes in the system as it allows the analogue value to be converted into a digital format for processing within the computer.
Computer for processing data and controlling the measurements: The computer is key to the operation of the DAQ system. It takes in the measurements in the form of digital data and processes them. It may need to convert what are termed primary measurements into the measurement that is needed - i.e. many sensors for items such as temperature, strain and the like return measurements in terms of voltage that need to be converted into the required parameters.

The DAQ computer also organises the different measurements, often triggering the different measurements to be made, and it often will process the data into a form for use or further processing elsewhere. It could even be linked via a special data communications link or the Internet to another remote computer, etc. Having a computer enables the system to be set up as required.
Software: Having the right software is essential for the overall system. Although software could be written for the computer from scratch, it is far more usual to utilise an existing package and configure it as needed for the given situation.

Although the signal conditioning and digital to analogue conversion have been listed as two separate functions, they are often contained within a single DAQ card or DAQ module. This will enable the sensor data to be converted into the right format using a single card.

DAQ cards and DAQ modules are often available off the shelf as most measurements are relatively standard. By using a standard DAQ card it is possible to get a reliable item very quickly and for a much lower cost than if it had been specially designed.

Many DAQ modules have multiple inputs which can be configured for a variety of different types of sensor. This makes it very easy to collect as much data as required.

DAQ system technologies

When setting up a DAQ system, it is usually advantageous to use a proprietary system standard. Rack systems like PXI are ideal. Even the older VXI systems can be used.

Other companies have their own systems as well. One such system id the CompactDAQ from NI. As the name indicates this provides a compact DAQ system. However there are many other manufacturers offering their own systems which are able to provide excellent performance.

Using something like a PXI rack, or another proprietary rack system, it is possible for the rack to have its own controller, or use a linked computer - which ever is more convenient. The DAQ modules or DAQ cards can them be slotted into the relevant slots in the rack. This provides a really effective way of setting up the system.

The other advantage of this approach is that it is possible to fully customise the system for what is required. As there are many different types of card and module available, it is possible to configure the DAQ system to provide exactly what is needed.

A typical PXI chassis with modules - NI 1085 PXI chassis — A typical PXI chassis loaded with modules & cards

Using something like a PXI DAQ system, it is very easy to set up and run a new data acquisition system as different DAQ cards and DAQ modules can be selected, slotted into the rack and the system configured using all standard cards, software and the like.

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.

Note on PXI Systems:

The PXI system is based on the PCI rack standard but with some additional features for instrumentation. A variety of rack sizes are available and it can contain its own controller or link to an external computer. It is ideal for a variety of test, measurement, control and data acquisition activities.

Data Acquisition Systems, DAQ Systems