Intro Process Xmitters (Suresh)

8/14/2019 Intro Process Xmitters (Suresh)

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Introduction toIntroduction to

rocessrocess

Part IPart I


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transmittransmit::

effect conveyance of, pass on,effect conveyance of, pass on,communicate, serve as medium for thecommunicate, serve as medium for the

Transmit/Transmission/TransmitterTransmit/Transmission/Transmitter

TransmissionTransmission

involves ainvolves atransmittertransmitter, a transmission, a transmission

medium and a receivermedium and a receiver..


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The ProcessThe Process--variable Transmittervariable Transmitter a field device that operates in conjunctiona field device that operates in conjunction

with a sensor for processwith a sensor for process--variablevariablemeasurement, and communicatesmeasurement, and communicates

..

In almost all continuous process/batchIn almost all continuous process/batch

operation worldwide, these devices mustoperation worldwide, these devices must

transmittransmitaccurate signals regarding theaccurate signals regarding the

process.process.


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Process TransmitterProcess Transmitter

the general configuration of a measurement

transducer consists of a sensing element

combined with a driving element (transmitter).


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Transducers for process measurements convertTransducers for process measurements convert

the magnitude of a process variable (e.g., flow rate,the magnitude of a process variable (e.g., flow rate,

pressure, temperature, level, or concentration) intopressure, temperature, level, or concentration) intoa signal that can be sent directly to the controller.a signal that can be sent directly to the controller.

TheThe sensing element is required to convert thesensing element is required to convert the

measured quantity, that is, the process variable,measured quantity, that is, the process variable,

into some quantity more appropriate forinto some quantity more appropriate formechanical or electrical processing within themechanical or electrical processing within the

transducer.transducer.


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Standard Inst rumentat ion SignalStandard Inst rumentat ion Signal --LevelsLevels

Before 1960Before 1960--70, instrumentation in the process70, instrumentation in the process

industry utilized pneumatic (air pressure) signals toindustry utilized pneumatic (air pressure) signals to

transmit measurement and control information almosttransmit measurement and control information almost

exclusively.exclusively.

These devices make use of mechanical forceThese devices make use of mechanical force--

balance elements to generate signals in the range ofbalance elements to generate signals in the range of

3 to 15 psig (0.2 to 1.0 kg/cm2).3 to 15 psig (0.2 to 1.0 kg/cm2).

Since about 1960Since about 1960--70s, electronic instrumentation has70s, electronic instrumentation has

come into widespread use.come into widespread use.


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Pneumatic transmitterPneumatic transmitter

converts theconverts the physical variablephysical variable measured to ameasured to a

standard pressure signalstandard pressure signal such as in thesuch as in the

range 0.2 kg/cmrange 0.2 kg/cm22 to 1.0 kg/cmto 1.0 kg/cm22 (3(3--15 psig).15 psig).

The pressure signal is transmitted throughThe pressure signal is transmitted through

pipes / tubes to gauges and chart recorderspipes / tubes to gauges and chart recorders

and also used as feedback signals in controland also used as feedback signals in control

applicationsapplications


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PneumaticPneumatic

processprocess

transmitterstransmitters


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Pneumatic transmission:Pneumatic transmission: Problems?Problems?

internalinternal andand externalexternal leakages,leakages,

pressurepressure drops,drops,

availabilityavailability ofof clean,clean, drydry compressedcompressed airair (at(at properproperll r rr r

difficultydifficulty inin transmittingtransmitting pressurepressure signalssignals toto longerlongerdistances,distances,

noise,noise, mechanicalmechanical vibrations,vibrations,

stuckstuck--upup faults,faults, etcetc..


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In control applications:In control applications:

quite a number of moving parts andquite a number of moving parts and

assemblies,assemblies,

Pneumatic transmission: Problems?Pneumatic transmission: Problems?

hysteresis, nonhysteresis, non--linearity,linearity,

low response times,low response times,

difficulty in attaining repeatability of control,difficulty in attaining repeatability of control,

high frequency of service and associatedhigh frequency of service and associated

maintenance costs and inventory costs.maintenance costs and inventory costs.


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E L E C T R O N I C T R A N S M I T T E R SE L E C T R O N I C T R A N S M I T T E R S

AnalogAnalog transmitterstransmitters

DigitalDigital transmitterstransmitters

AnalogAnalog Digital hybrid transmittersDigital hybrid transmitters


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Electronic TransmittersElectronic Transmitters

better accuracy,better accuracy,

linearity and repeatability,linearity and repeatability,

greatly simplified measurement & controlgreatly simplified measurement & control

re uce operat on an ma ntenance costsre uce operat on an ma ntenance costs improvedimproved transmittabilitytransmittabilityto greater distances.to greater distances.


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ElectronicsElectronics

housinghousing

SensorSensor

assemblyassembly


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Analog TransmittersAnalog Transmitters

AnalogAnalog transmitterstransmittersconvertconvert thethe processprocess variablevariable

measuredmeasured intointo anan equivalentequivalent electricalelectrical signalsignal

cable/wirescable/wires fromfrom measurementmeasurement sitesite toto thethe controlcontrol

roomroom forfor interfacinginterfacing toto displaydisplay oror processprocess

controllercontroller..


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Analog 2Analog 2--wire 4wire 4--20mA20mAtransmitterstransmitters


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voltagevoltage transmitterstransmitters

wirewire--resistance/impedanceresistance/impedance posesposes aa significantsignificant

problem,problem, especiallyespecially longlong transmissiontransmission lengthslengths

signalsignal attenuatedattenuated alongalong thethe lineline


TypicalTypical signalsignal rangesranges (corresponding(corresponding toto PVPV fromfrom 00%%toto 100100%%))

00 toto 1010V,V, 00 toto 55V,V, etcetc.. sometimes,sometimes, --1010VV toto ++1010VV oror --55VV toto ++55V,V, etcetc..


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currentcurrent transmitterstransmitters

transmittertransmitter isis inin essenceessence aa currentcurrent sourcesource (current(currentgenerator)generator) thatthat hashas veryvery highhigh outputoutput resistanceresistance..

wirewire--resistanceresistance hashas nono effecteffect onon thethe flowingflowing currentcurrentinin thethe currentcurrent looploop;;


signalsignal rangesranges 00--2020ma,ma, 44--2020mA,mA, 00--5050 mAmA andand 1010--5050mAmA

44--2020mAmA currentcurrent signalsignal isis thethe industryindustry standardstandard.. TheThe

44 toto 2020mAmA representsrepresents anan entireentire spanspan withwith zerozerostimulusstimulus ((00%%)) correspondingcorresponding toto 44mAmA andand maximummaximum((100100%%)) correspondingcorresponding toto 2020mAmA..


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Input Output

50 C 4 mA

150 C 20 mA

This instrument transducer has a lowerThis instrument transducer has a lower

( ) ( )

( )

20 mA 4 mA

mA 50 C 4 mA150 C 50 C

mA0.16 C 4 mA

C

mT T

T

= +

=

o

o o

o

o

limit orlimit or zerozeroof 50of 50 C and a range orC and a range or spanspanofof100100 C.C.

For the temperature transmitter discussedFor the temperature transmitter discussedabove, the relation between transducerabove, the relation between transduceroutput and input isoutput and input is


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Four wire transmittersFour wire transmitters

ToTo connectconnect sensorssensors ofof relativelyrelatively lowlow resistanceresistance suchsuch

asas piezopiezo--resistors,resistors, RTDs,RTDs, etcetc.. toto remotelyremotely locatedlocatedinterfaceinterface circuitscircuits

connectingconnecting wireswires resistanceresistance posepose aa seriousserious problemproblem

44--wirewire methodmethod allowsallows measurementmeasurement ofof resistanceresistance ofofconnectingconnecting conductorsconductors..

22 wireswires connectconnect toto currentcurrent source,source, andand anotheranother 22

wireswires toto highhigh inputinput impedanceimpedance voltagevoltagemeasurementmeasurement circuitrycircuitry..


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Digital transmittersDigital transmitters

PulsePulse transmissiontransmission

Switch/contactSwitch/contact--closureclosure--digitaldigital

transmissiontransmission

SerialSerial datadata transmissiontransmission


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Pulse transmitterPulse transmitter

InstrumentsInstruments /devices/devices areare incorporatedincorporated withwith aa

sensingsensing mechanismmechanism thatthat producesproduces pulsepulse outputoutput

inin proportionproportion toto thethe variablevariable beingbeing measuredmeasured

(generally,(generally, motion)motion)..

TheThe frequencyfrequency ofof pulsespulses isis proportionproportion toto

measurementmeasurement..


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capacitive proximity sensors,capacitive proximity sensors,

inductive transducers,inductive transducers,

magnetic reedmagnetic reed--switches,switches,

Pulse output mechanismsPulse output mechanisms

Magnetic proximity sensors,Magnetic proximity sensors,

optical encoders, photooptical encoders, photo--

sensors,sensors,


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ExamplesExamples

FlowmetersFlowmeters::

TurbineTurbine flowmetersflowmeters, positive, positive--displacementdisplacementflowmetersflowmeters, vortex, vortex flowmetersflowmeters, etc., etc.

Displacement Encoders:Displacement Encoders:

tachometers, rotary position encoders,tachometers, rotary position encoders, quadraturequadrature

sensors, etcsensors, etc


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signals are normally weak, of low amplitude,signals are normally weak, of low amplitude,improper / nonimproper / non--uniform shapes, unsuitable foruniform shapes, unsuitable fortransmissiontransmission

Pulse transmitters..Pulse transmitters..

Signal Conditioning

on on ng: eve s , pu seon on ng: eve s , pu se--s ap ng,s ap ng,amplification, etc.amplification, etc.

typical output peaktypical output peak--toto--peak amplitudes 5V, 12V,peak amplitudes 5V, 12V,

24V or bipolar24V or bipolar

5V,5V,

12V12V Usually output waveUsually output wave--shape is square, or of fixedshape is square, or of fixed

pulsepulse--width, or sinusoidal;width, or sinusoidal;


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frequency of pulses is proportional tofrequency of pulses is proportional toinstantaneous value of process variableinstantaneous value of process variable --s eed flow etc.s eed flow etc.

Pulse transmitters..Pulse transmitters..

Pulse Outputs are Popular

integration of process variable over a periodintegration of process variable over a periodof time is simpleof time is simple totalisingtotalising/counting of the/counting of thepulsespulses


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Switch/contact closure transmittersSwitch/contact closure transmitters

SimilarSimilar toto pulsepulse transmitterstransmitters;; butbut frequencyfrequency

ofof pulsepulse oror switch/contactswitch/contact closureclosure isis veryverysmallsmall..

,, ,,

statusstatus etcetc.. toto remoteremote indicators,indicators,anunciatorsanunciators andand displaydisplay systemssystems..

ControlControl applications,applications, ON/OFF,ON/OFF, etcetc..


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Serial data transmissionSerial data transmission

communicates serially between Computers,communicates serially between Computers,peripheral devices, Instruments, etc.peripheral devices, Instruments, etc.

RSRS--232232

introduced in 1962 ,introduced in 1962 , widely used throughout the industry.widely used throughout the industry.

singlesingle--ended data transmission at relativelyended data transmission at relatively

slow data rates (20slow data rates (20 kBaudkBaud))

short distances (short distances (uptoupto 50 ft.)50 ft.)

Intro Process Xmitters (Suresh)

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Transcript of Intro Process Xmitters (Suresh)