Data Acquisition, or DAQ for short, is critical in today’s manufacturing environment. DAQ Systems are designed to gather, store, and analyze data. With this information, plant managers can make intelligent decisions concerning process improvement, quality, and efficiency. This white paper examines some basic DAQ terms and concepts. It is intended as an introduction to the world of Industrial Data Acquisition and Control systems.
Digital Input/Output (I/O)
Sometimes referred to as Discrete I/O, these types of inputs and outputs are much easier for a micro-processor based DAQ system to use since they are already a binary signal. The State of a digital signal is essentially the level of the signal (on or off, high or low). A common application is monitoring the condition of a switch (open or closed). The Rate of a digital signal is determined by measuring how often it changes state with respect to time. A common application is determining how fast a motor shaft spins. Unlike Analog Frequency, digital rate measures how often a signal occurs and an algorithm is not required.
There are several types of digital I/O:
Digital Inputs
- Sinking/NPN
- Sourcing/PNP
- Counter
Digital Outputs
- Sinking/NPN
- Sourcing/PNP
- Relay
Sinking and Sourcing
When choosing a digital I/O module, it is important to have an understanding of these terms. A sinking device provides a path for the current to ground and is not responsible for powering the device. Terms used to describe sinking devices include NPN and Open Collector. A sourcing device provides the power or a positive potential. Sourcing devices 'push' the current through the load. Terms used to describe sourcing devices include PNP and Open Emitter.
When selecting digital I/O, recognize that not all manufacturers define sinking and sourcing the same way. In general, Sinking/Sourcing describes a current signal flow relationship between field input/output devices in a control system and their power supply. Sourcing I/O modules supply (or source) current to sinking field devices. Sinking I/O modules receive (or sink) current from sourcing field devices. However, you should always consult your devices manual to determine the required I/O.
Counter
As the name implies, a counter is an input that increments a register with each successive pulse received. Using this information, a frequency or pulse rate can readily be determined. Counters are characterized by their number of bits. In an X bit counter, the maximum number of counts is equal to 2 raised to the X power. For example: a 16 bit counter can count up to 2 to the 16th power or 65.536.
Relay
A relay is a device in which power applied to a coil or input terminal causes the path between contacts open or close. These contacts are designed for interrupting and applying power to larger loads (i.e., integral horsepower motors) and significant resistance loads (i.e., lighting and heaters). Relay outputs come in several variations:
- Form A – Single Pole, Single Throw, Normally Open
- Form B – Single Pole, Single Throw, Normally Closed
- Form C – Single Pole, Double Throw, Break-before-make
Analog Input/Output (I/O)
Digital computers have replaced analog recording and display technologies in all but the simplest data acquisition applications. However, these computers speak only the binary language of ones and zeroes. Manufacturing processes and natural phenomena, however, are still by their very nature analog. To be manipulated by a computer, analog measurements such as pressure, temperature, or flow rate, must be translated into a digital equivalent. An algorithm is used to translate the signal based on several factors. The Analog Level gives important information, such as the intensity of a light source, pressure in a chamber, or temperature in a room. Analog signals can also be characterized by their Frequency. When the Analog Frequency is important, acquisition speed must be considered. The Shape of an Analog signal can be as important as the level. By measuring shape, the DC value, peak, and slope can be determined. This is of particular interest when the value changes rapidly. Sounds and vibration are some applications.
Analog I/O
- Current
- Voltage
- Resolution
- Temperature
Current
The 4-20 mA signal is the industry standard current signal for use with analog and digital controllers. A variation of the 4-20 mA signal is 0-20 mA.
Voltage
Common voltage signals used in the controls industry are 1-5 VDC, 2-10 VDC, 3-15 VDC, 0-5 VDC, 0-10 VDC and 0-15 VDC.
Resolution
ADC devices are defined by the resolution, ranging from 8-bit to 32-bit.The higher the resolution, the higher the accuracy and data integrity. But it also means that your costs will be higher. Because of this, it is essential to calculate exactly what your sampling rates will really be and balance that with budgeting needs.
Temperature
Temperature inputs come in two variations, Resistance Temperature Detector (RTD) or Thermocouple.
RTD
Simply put, an RTD is a sensor used to measure temperature by correlating the sensors electrical resistance with a specific temperature. RTD elements consist of a length of fine wire coiled around a ceramic or glass core. The wire is made of a pure material with a defined resistance. Platinum is the most popular and accurate. Copper is also used in some instances. A PT100 sensor has a resistance of 100 ohms at 0°C and is by far the most common type of RTD sensor. A PT1000 has 1000 ohms resistance at 0°C. Copper RTD’s are usually built at 10 ohms (CU10). Because of this low resistance, measured temperatures spans must be large to get an adequate signal. Therefore, CU10 RTD use is limited to devices such as motors or transformers.
2-Wire RTD - The lowest cost method is to keep the signal conditioner close enough to the sensor so the copper wire resistance change does not introduce an excessive error in the measurement.
3-Wire RTD - Lead compensation with three wires requires two conductors attached to one end of the RTD and a single conductor on the other end. This method requires the three lead wires to track each other in resistance change versus ambient temperature. It is effective for a total copper lead resistance equal to about 10% of the RTD value. The error introduced increases proportionally as the lead wire length increases.
4-Wire RTD - A four wire input provides the most accurate measurement. Current flows through two wires and the other two wires are used to measure the voltage across the RTD. The wire leads have no influence on the accuracy of the measurement.
Thermocouple
Based on the principle that when two dissimilar metals are joined, a predictable voltage will be generated that relates to the difference in temperature between the measuring junction and the reference junction or connection to the measuring device. The selection of the optimum thermocouple type (metals used in their construction) is based on application. Temperature Range, Atmosphere, Required length of service, Accuracy and Cost all play a factor in the decision.
| Thermocouple Type (IEC Color Code) |
Temperature Range (Short Term) |
Temperature Range (Continuous) |
|---|---|---|
| E - Purple | -400 to 900°C | 0 to 800°C |
| J - Black | -180 to 800°C | 0 to 700°C |
| K - Green | -180 to 1300°C | 0 to 100°C |
| T - Brown | -250 to 400°C | -185 to 300°C |
Data acquisition modules handle a broad range of analog and digital remote I/O and deliver that data over serial, Ethernet, USB or wireless connections. Compact, low cost modules fit a variety of applications to monitor sensors and control processes or equipment.
Courtesy of BB Electronics
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