8-Temperature sensors


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Sensors& Actuators
Dr. Hassan SHARABATY
BY
Aleppo –May 2012
Department ofMechatronicsEngineering
EEM304 Mechatronics
8-Temperaturesensors
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
2/15
Introduction
Temperature can have a significant effect on materials and processes at the molecular level
It is the most widely sensed of all variables.
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
3/15
Contact temperature sensing:
Requires the sensor to be in direct physical contact with the media or object being sensed
Used to monitor temperature of solids, liquids or gases over an extremely wide temperature range.
Non-contact measurement:
Interprets the radiant energy of a heat.
Used to monitor temperature of non-reflective solids and liquids but is not effective with gases due to their natural transparency.
Two basic types of temperature sensing:
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
4/15
Three families Sensor Types and Technologies
Electro-mechanical sensors
EX:Bi-metalthermostats
Twodifferentmetalsbondedtogetherunderheatandpressuretoformasinglestripofmaterial.Byemployingthedifferentexpansionratesofthetwomaterials,thermalenergycanbeconvertedintoelectro-mechanicalmotion.
Electronic sensors
Resistive sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
5/15
Electro-mechanical sensors
Electronic sensors
Resistive sensors
Silicon sensors
Infrared (IR) pyrometry
Thermocouples
Three families Sensor Types and Technologies
Dr. Hassan SHARABATY Sensors & Actuators – Temperature sensors 6/15
Electronic sensors (1)
 Silicon sensors : use of the electrical resistance properties of
semiconductor materials.
• IC-type devices can provide a direct, digital temperature reading,
so there’s no need for an A/D converter.
Ex. PN Junction
   
 
2 10
0 2 0 1
T2 T1
I T I T

  C
mV

 2.5
dt
dV
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
7/15
Infrared(IR)pyrometer:
Allobjectsemitinfraredenergy(>0Kelvin).
directcorrelationbetweentheinfraredenergyanobjectemitsanditstemperature.
IRsensorsmeasuretheinfraredenergyemittedfromanobjectandconvertthereadingtoavoltage.
TypicalIRtechnologyusesalenstoconcentrateradiatedenergyontoathermopile.Theresultingvoltageoutputisamplifiedandconditionedtoprovideatemperaturereading.
Electronic sensors (2)
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
8/15
Thermocouples:Twoelectricalconductorsofdissimilarmetalsoralloysarejoinedatoneendofacircuit.
Allthermocoupleshave:
a“hot”(ormeasurement)junction
a“cold”(orreference)junction.
Electronic sensors (3)
Whentheendsaresubjectedtodifferenttemperatures,acurrentwillflowinthewiresproportionaltotheirtemperaturedifference.
Temperatureatthemeasurementjunctionisdeterminedbyknowing:
typeofthermocoupleused,
magnitudeofthemillivoltpotential,
temperatureofthereferencejunction.
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
9/15
Electro-mechanical sensors
Electronic sensors
Resistive sensors
Thermistors
RTD
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
10/15
Thermistors(orthermallysensitiveresistors)aredevicesthatchangetheirelectricalresistanceinrelationtotheirtemperature.
Theytypicallyconsistofacombinationoftwoorthreemetaloxidesthataresinteredinaceramicbasematerialandhaveleadwiressolderedtoasemiconductorwaferorchip,whicharecoveredwithepoxyorglass.
TwodifferenttypesThermistors:
positivetemperaturecoefficient(PTC):PTCdevicesexhibitapositivechangeorincreaseinresistanceastemperaturerises.
negativetemperaturecoefficient(NTC):NTCdevicesexhibitanegativechangeordecreaseinresistancewhentemperatureincreases.
ThechangeinresistanceofNTCdevicesistypicallyquitelarge,providingahighdegreeofsensitivity.
Theyalsohavetheadvantageofbeingavailableinextremelysmallconfigurationsforextremelyrapidthermalresponse.
Resistive Devices (1)
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
11/15
RTDs(resistivetemperaturedevices),likethermistors,employachangeinelectricalresistancetomeasureorcontroltemperature.
RTDsconsistofasensingelement,connectionwiresbetweentheelementandmeasurementinstrument,andasupportforpositioningtheelementintheprocess.
Resistive Devices (2)
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
12/15
Themetalsensingelementisanelectricalresistorthatchangesresistancewithtemperature.Theelementusuallycontainsacoilofwireorconductivefilmwithconductorsetchedorcutintoit.Itisusuallyhousedinceramicandsealedwithceramiccementorglass.
Thesensingelementshouldbepositionedwhereitcanreachprocesstemperaturequickly.Wirewounddevicesshouldbeadequatelysecuredinhighvibrationandshockapplications.Extensionwiresbetweentheelementandinstrumentallowresistancetobemeasuredfromgreatdistances.
Resistive Devices (2)
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
13/15
References:
For further reading students are referred to:
SensorTechnologyHandbook,copyright©2005,Elsevierinc.
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
14/15
Infrared(IR)pyrometer:
FactorsthataffecttheaccuracyofIRsensing:
reflectivity(themeasureofamaterial’sabilitytoreflectinfraredenergy),
transmissivity(themeasureofamaterial’sabilitytotransmitorpassinfraredenergy),
emissivity(theratiooftheenergyradiatedbyanobjecttotheenergyradiatedbyaperfectradiatorofthesurfacebeingmeasured).
Remark:
Anobjectthathas:
-transmissivityof0isaperfectreflector
-transmissivityof1absorbs100%oftheinfraredenergyappliedtoit.
Antransmissivityof1iscalleda“blackbody”anddoesnotexistintherealworld.
Electronic sensors (2)
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
15/15
How to select the best temperature sensor?
GeneralConsiderations
Eachsensortypehasitsadvantagesanddisadvantages.
Forexample:
■Thermistors:highresolution,mostsensitiveandlowcost
havethewidestrangeofapplications
butarenonlinearandhavelimitedtemperaturerange.
■Thermocoupleshavethehighesttemperatureregionandaredurableforhighvibrationandhigh-shockapplications,butrequirespecialextensionwire.
■RTDsarenearlylinearandarehighlyaccurateandstable,buttheyarelargeandexpensive.
■Silicontypesarelowcostandnearlylinear,
buthavealimitedtemperaturerange.
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
16/15
Animportantconsiderationinselectingthermalsensorsarethematerialsused,whichhavetemperaturelimitations.
Toleranceandaccuracyarealsoimportant.
Regardlessofthesensortechnologyselected,usersafetyshouldbetheprimaryconcern.
Neverselectadevicesolelybecauseithasthelowestcost.Choosethedevicethatoffersthebestperformanceforitspriceandalwaysadheretothemanufacturer’sguidelinesandrecommendations.
Eachtemperaturesensingapplicationcanpresentitsownuniquesetofrequirementsandproblemsandneedstobeevaluatedonanindividualbasis.
Herearesomequestionstoconsider.
How to select the best temperature sensor?
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
17/15
Does the application require contact or non-contact sensing?
Iftheapplicationismovingorifphysicalcontactisnotpracticalduetocontaminationorhazardousmaterialissues,infraredisthetechnologyofchoice.
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
18/15
Thermocouples have the broadest temperature range, –200°C to 2315°C.
(But Depending on design and material)
Thermsistors have a usable range of –100 to 500°C.
Bi-metal thermostats can handle temperatures from –85 to 371°C.
Forcryogenictemperatures,RTDsandsomesilicon-baseddevicesarecapableofapproachingabsolutezero(0K).
Withallofthesedevices,itispossibletoexceedtheserangesthroughtheuseofthermowellsorbyplacingthedeviceinalocationrelativetotheheatsource.However,thistypeofapproachcanaffecttheaccuracyandresponseofthesystem.
What temperature range is the sensor required to control or monitor?
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
19/15
Whatistherateoftemperaturechangeoftheapplication?
Forapplicationswheretherateoftemperaturechangeisrapid(>1.0°C/minute),themassofthesensormaybecomeanissue.Thethermalinertiaofthesensorisbasedonitsmass.
Forapplicationswherethesensorwillberemotelylocatedduetoenvironmentalorotherissues,designverificationtestingshouldbeperformed.Thisinvolvesusingtwoormoresensorstomonitorthetemperatureoftheapplication,whileanothersensormonitorsthetemperatureattheproposedsensorlocation.Inthisway,sensorlocationcanbeoptimized.
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
20/15
Other Questions
Application.
Cost.
.
.
.
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
21/15
AdvantagesDisadvantagesEx. Thermistor
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
22/15
Advantages
■Low component cost
■Fast thermal response
■Large change in resistance vs. temperature for more resolution
■Extremely small size means faster reaction to change in temperature and ability to use in variety of assemblies
■High resistance values so no lead wire compensation necessary
■Limited temperature range
■Lower temperature exposures than RTDs or thermocouples
■No established resistance standards
■Self heating can affect accuracy
■Non-linear resistance change
■Additional components/circuitry required to control application loads
Disadvantages
Dr. Hassan SHARABATY
Sensors& Actuators -Temperature sensors
23/15
END

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