a beginners guide to data acquisition

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A Beginners Guide to Data Acquisition Terminology WP-30-REV0-4508-1/5 2008 by B&B Electronics.

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A Beginners Guide to Data AcquisitionTerminology

Data Acquisition, or DAQ for short, is critical in todaysmanufacturing environment. DAQ Systems are designed togather, store, and analyze data. With this information, plantmanagers can make intelligent decisions concerning processimprovement, quality, and efficiency. This white paper examinessome basic DAQ terms and concepts. It is intended as anintroduction to the world of Industrial Data Acquisition andControl systems.

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Digital Input / Output (I/O)

Sometimes referred to as Discrete I/O, these types of inputsand outputs are much easier for a micro-processor based DAQsystem to use since they are already a binary signal. The Stateof a digital signal is essentially the level of the signal (on or off,high or low). A common application is monitoring the conditionof a switch (open or closed). The Rate of a digital signal isdetermined by measuring how often it changes state withrespect to time. A common application is determining how fast amotor shaft spins. Unlike Analog Frequency, digital ratemeasures how often a signal occurs and an algorithm is notrequired.

Digital or Discrete I/O

concerns the digital

State (on or off, high or

low) and Rate, the

frequency at which the

state changes.


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There are several types of Digital I/O:

Digital Inputs Digital OutputsSinking / NPN Sinking / NPNSourcing / PNP Sourcing / PNPCounter Relay

Sinking & Sourcing

When choosing a Digital I/O module, it is important tohave an understanding of these terms. A sinking deviceprovides a path for the current to ground and is notresponsible for powering the device. Terms used todescribe sinking devices include NPN and OpenCollector.A sourcing device provides the power or apositive potential. Sourcing devices 'push' the currentthrough the load. Terms used to describe sourcingdevices include PNP and Open Emitter.

When selecting digital I/O, recognize that not allmanufacturers define sinking and sourcing the sameway. In general, Sinking / Sourcing describes acurrent signal flow relationship between field input /output devices in a control system and their powersupply. Sourcing I/O modules supply (or source)

current to sinking field devices. Sinking I/O modulesreceive (or sink) current from sourcing field devices.However, you should always consult your devicesmanual to determine the required I/O.

Counter

As the name implies, a counter is an input thatincrements a register with each successive pulsereceived. Using this information, a frequency or pulserate 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 theX power. For example: a 16 bit counter can count up to 2to the 16th power or 65,536.

When choosing I/O

modules, it is

important to

understand the concept

of Sinking and

Sourcing.


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WHTEPR Analog I/O

CurrentVoltageResolutionTemperature

Voltage- Common voltage signals used in the controlsindustry are 1-5 VDC, 2-10 VDC, 3-15 VDC, 0-5 VDC, 0-10 VDC and 0-15 VDC.

Current- The 4-20 mA signal is the industry standardcurrent signal for use with analog and digital controllers.A variation of the 4-20 mA signal is 0-20 mA.

ResolutionADC devices are defined by the resolution,ranging from 8-bit to 32-bit.The higher the resolution, thehigher the accuracy and data integrity. But it also meansthat your costs will be higher. Because of this, it is

essential to calculate exactly what your sampling rateswill really be and balance that with budgeting needs.Further information on data sampling, including how tocalculate your resolution needs, can be found in ourpaper, The Fine Art of Analog Signal Sampling.

Temperature Temperature inputs come in twovariations, Resistance Temperature Detector (RTD) orThermocouple.

RTD Simply put, an RTD is a sensor used to measure

temperature by correlating the sensors electricalresistance with a specific temperature. RTD elementsconsist of a length of fine wire coiled around a ceramic orglass core. The wire is made of a pure material with adefined resistance. Platinum is the most popular andaccurate. Copper is also used in some instances. APT100sensor has a resistance of 100 ohms at 0C andis by far the most common type of RTD sensor. APT1000has 1000 ohms resistance at 0C. Copper RTDsare usually built at 10 ohms (CU10). Because of this lowresistance, measured temperatures spans must be large

to get an adequate signal. Therefore, CU10 RTD use islimited to devices such as motors or transformers.

2-Wire RTD - The lowest cost method is to keep thesignal conditioner close enough to the sensor so thecopper wire resistance change does not introduce anexcessive error in the measurement.

Temperature is

measured using

Resistance

Temperature Detector

or Thermocouple

inputs.


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WHTEPR 3-Wire RTD - Lead compensation with three wires

requires two conductors attached to one end of the RTDand a single conductor on the other end. This methodrequires the three lead wires to track each other inresistance change versus ambient temperature. It iseffective for a total copper lead resistance equal to about10% of the RTD value. The error introduced increasesproportionally as the lead wire length increases.

4-Wire RTD - A four wire input provides the mostaccurate measurement. Current flows through two wiresand the other two wires are used to measure the voltageacross the RTD. The wire leads have no influence on theaccuracy of the measurement.

Thermocouple Based on the principle that when twodissimilar metals are joined, a predictable voltage will begenerated that relates to the difference in temperature betweenthe measuring junction and the reference junction or connectionto the measuring device. The selection of the optimumthermocouple type (metals used in their construction) is basedon application. Temperature Range, Atmosphere, Requiredlength of service, Accuracy and Cost all play a factor in thedecision.

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 from B&B Electronics handle a broadrange of analog and digital remote I/O and deliver that data overserial, Ethernet, USB or wireless connections. Compact, lowcost modules fit a variety of applications to monitor sensors andcontrol processes or equipment

B&B Electronics started in 1981 manufacturing a single product, an RS-232

tester. Since then B&B Electronics has had continuous growth in industrial

communications and automation. For a library of technical information and

one of industrys most popular industrial electronics and communications

catalo s visitwww.bb-elec.com

a beginners guide to data acquisition

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