Data acquisition, or DAQ, is the measurement of a physical or electrical phenomenon such as current, voltage, pressure, sound, or temperature with a computer. A DAQ system is made up of different parts: sensors, DAQ hardware, and a computer with programmable software. Unlike a traditional measurement system, the PC-based DAQ system leverages the productivity, processing, connectivity and display capabilities of standard computers to provide a more economical, flexible and powerful measurement solution. Depending on the DAQ equipment used (Adam modules or Wise module), you can have access to different industrial protocols such as Profinet, Ethernet, Modbus, Profibus, etc. (Adam modules) or to wireless versions equipped with REST and MQTT protocols (modules WISE).
Elements of the DAQ System
To measure a physical phenomenon (intensity of a light source, temperature of a room, etc.), a sensor – also called a transducer – is necessary. The latter’s role is to convert the physical phenomenon into an electrical signal that can be measured. Among the most commonly used sensors, we find in particular:
- The optical sensor (for light)
- Thermocouple, thermistor, RTD or Resistance Temperature Detectors (for temperature)
- The microphone (for acoustics)
- The potentiometer, optical encoder, LVDT or Linear Variable Differential
- Transformer (for position and displacement)
- The piezoelectric transducer, the strain gauge (for force and pressure)
- The pH measuring electrode (for pH)
- The accelerometer (for acceleration)
Depending on the type of sensor, the electrical output is different. It could be :
- Of a tension
- Of a current
- With a different electrical attribute
A true interface between the computer and the various signals from the outside world, the main function of DAQ equipment is the digitization of the incoming analog signals so that a computer is able to interpret them. The 3 major elements of DAQ hardware that are essential for measuring a signal are as follows:
- The signal conditioning circuit: likely to be noisy or even too dangerous to be measured directly, the signal transmitted by the sensors must be transformed via a conditioning circuit. The latter then changes the signal to a form compatible with the input of a C A / D. This conditioning can involve attenuation, amplification, isolation and filtering.
- The analog-to-digital converter or C A / D: the conversion of analog signals from sensors into digital data is essential for manipulation via digital equipment such as a computer to be possible. A C A / D converter corresponds to an integrated circuit capable of providing a digital representation of analog signals at a given instant.
- The computer bus: DAQ hardware can connect to a computer through a port or slot. The computer bus acts as a communication interface between the computer and the DAQ hardware to allow the transfer of various instructions and measured data.
In a DAQ system, the computer with programmable software is responsible for controlling the operation of the acquisition hardware and for processing, viewing and storing the measurement data. Depending on the applications, different types of computers can be used:
- The desktop computer: ideal laboratory use to benefit from its processing power
- The laptop: use in the field
- The industrial computer: use in a production plant because of its great robustness
Concretely, a DAQ system consists of different software components: a driver and application software.
The latter makes the interaction between application software and DAQ hardware possible. It eliminates the programming of registers and low-level commands to the hardware and thereby simplifies communications with the DAQ.
The purpose of the application software is to facilitate the interaction between the computer and its user to obtain, analyze and display measurement data. It can be a prebuilt application with pre-established functionality or a programming environment designed to build applications with custom functionality. The use of custom applications is common for automating the functions of DAQ hardware, executing signal processing algorithms, and displaying custom user interfaces.
The different DAQ systems
DAQ Serial Communication Systems
They are ideal for use when measurements need to be performed at a location remote from the computer. While there are different communication standards, RS232 remains the most common although it supports transmission distances only up to 15.24m or 50ft. RS485, on the other hand, supports greater transmission distances, these being up to 1524 m or 5000 ft.
These acquisition systems are equipped with one or more wireless transmitters capable of sending data to a wireless receiver which is itself connected to a computer. Concretely, wireless transmitters can be available for:
- Relative humidity and ambient temperature
- RTD probes
- Sensors with pulse output
- Voltage transducers
- 4-20 mA transmitters
The universal serial bus
Also called Universal Serial Bus (USB), it corresponds to a new standard for connecting a PC to various peripherals such as printers, modems, monitors, etc. Among the advantages of USB, we can mention the possibility of powering power the device and the opportunity to take advantage of higher bandwidth (up to 12 Mbits / s).
Data acquisition cards
The acquisition cards are plugged directly into the computer bus. Their major advantages are cost (overload of power and packaging supplied by the computer) and speed (thanks to the direct connection to the bus). The functions of the cards differ depending on the speed, type and number of inputs (on / off, thermocouple, voltage), outputs, etc.