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 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.
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 smaller ones - sometimes USB DAQ systems can provide the ideal solutions. They can be scaled according to what is needed. In fact many different DAQ devices are avilable 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.
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.
Data acquisition definition:
Data acquisition is the process of sampling signals that measure real-world physical phenomena and the subsequent conversion of them into a digital form to be be processed by a computer with its associated software.
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 & data loggers
Data acquisition systems and data loggers are very similar in many ways, but they have soem 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.
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.
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..
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.
In essence a data logger is a simple form of data acquisition device, it cannot be totally be considered to be a full data acquisition system in view of the capabilities provided.
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 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.
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:
- Distance / displacement
- Angles, RPM, etc
- 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 or 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.
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 fort he 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.
Using something like a PXI rack for a DAQ 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.
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.
Read more about PXI rack system.
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 occasionally used and may be applicable in some instances, although thought should be given to the fact that it is now a very old scheme and more up to date technology should be considered.
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. That said, USB will tend to be used for smaller systems as well as for data logger applications.
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.
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