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外文翻译

ELECTRONICS’200422-24September,Sozopol,BULGARIA

MEASUREMENTOFTEMPERATUREANDHUMIDITYUSING

SHT11/71INTELLIGENTSENSOR

Assist.Prof.GrishaSpasov,PhD,BScNikolayKakanakov

DepartmentofComputerSystems,TechnicalUniversity–branchPlovdiv,25,“Tzanko

Djustabanov”Str.,4000Plovdiv,Bulgaria,+35932659576,e-mail:

gvs@tu-plovdiv.bg,

kakanak@tu-plovdiv.bg

Keywords:

temperatureandhumiditymeasurements,intelligentsensors,

distributedautomationandcontrol

Thepaperdiscussestheadvantagesofintelligentsensors.ItpresentsanSHT11/71temperature

andhumiditycombinedsensorfromSensirion.Thissensorisidealfordevelopingdistributed

embeddedsystemsformonitoringenvironmentalparameters.

AnexampleapplicationusingthissensorisaddedtopresentSHT11/71inrealworking

condition.Theapplicationisdistributedsystemformonitoringtemperatureandhumidity.Itusesa

microcontrollerwithintegratedwebservertoorganizethecommunicationandmanagementof

sensors.Thisapplicationisrealizedandtested.

1.INTRODUCTION

Measurementandcontroloftemperatureandrelativehumidityhassignificant

applianceinindustry,science,healthcareagricultureandcontrollingtechnological

processes.Thesetwoenvironmentalparametersstronglyinfluenceeachotherandit

iscriticalinsomeapplicationtomeasuretheminparallel.Usingmoderntechnologies

itispossibletocombinetemperaturemeasurementelement,humiditymeasurement

element,amplifier,ADC,digitalinterface,calibrationmemoryandCRCcalculation

logicinasinglechipwithverysmallsize[1,3].

Usingintelligentsensorsofthiskindcanshortenthedevelopmenttimeandcost.

IntegratingADCandamplifierintosensor’schipallowdeveloperstooptimizesensor

elementsforaccuracyandlong-termstability.Andthatisnotall–integratingdigital

interfacelogicsimplifiesconnectivityandmanagementofsensors.Theseadvantages

canreducewholetime-to-markettimeandevenprice[1,3].

InpresentedpaperweuseSHT11/71intelligentsensorfromSensirionasan

exampleandpresentitsadvantagesandmeasurementprocedures.Anexample

applicationisalsopresentedtodemonstrateitsworkinrealconditions.This

applicationisrealizedandtested.

2.INTELIGENTSENSORS–SHT11/71

SHT11/71isasinglechiprelativehumidityandtemperaturemultisensormodule

comprisingacalibrateddigitaloutput.Thedeviceincludesacapacitivepolymer

sensingelementforrelativehumidityandabandgaptemperaturesensor.Bothare

seamlesslycoupledtoa14bitADCandaserialinterfacecircuitonthesamechip.

Thisresultsinhighsignalquality,afastresponsetimeandinsensitivitytoexternal

disturbances（EMC）.EachSHT11/71isindividuallycalibratedandcalibration

coefficientsareprogrammedintotheOTPmemory.The2-wireserialinterfaceand

internalvoltageregulationalloweasyandfastsystemintegration[1].

ELECTRONICS’200422-24September,Sozopol,BULGARIA

TheSHT11/71isshownonfig.1[1].

Combiningtemperatureandhumiditysensingelementsinsingleunitenables

precisedeterminationofdewpoint,withoutincurringerrorsduetotemperature

gradientsbetweenelements.Thesignalamplificationnearthesensorallowsthe

polymerlayerstobeoptimizednotforsignalstrengths,butratherforlong-term

stability.Performinganalog-to-digitalconversion“inplace”makesthesignal

extremelyinsensitivetonoise.Achecksumgeneratedbythechipitselfisusedfor

additionalreliability.Thecalibrationdataloadedonthechipmemoryguaranteesthat

humiditysensorshaveidenticalspecificationsandthustheyare100%replaceable

[1].

SomeoftheadvancedfunctionsoftheSHT11/71areavailabletroughthestatus

register.Someofthemare:

internalheatingelement;regulationofmeasurement

resolutiontooptimizeforprecisionorforfastresponse;End-Of-Battery（EOB）

detection（lowvoltagedetection）.Statusregistersizeis8bitsbutonlyfourofthem

areused[1].

SHT11/71canbeconnecteddirectlytoanymicrocontrollerbymeansofthe

digital2-wireinterface.Thisinterfaceisoptimizedforsensorreadoutandpower

consumptionandisnotcompatiblewithI2Cinterfaces[1].（Seefig.2）

Twowiresareusedtotransferserialclock（SCK）anddata（DATA）.TheSCKis

usedtosynchronizethecommunicationbetweenamicrocontrollerandthe

SHT11/71.Sincetheinterfaceconsistsoffullystaticlogicthereisnominimum

frequency.TheDATAtristatepinisusedtotransferdatainandoutofthemodule.It

changesafterthefallingedgeandisvalidontherisingedgeoftheSCK.Duringthe

transmissiontheDATAlinemustremainstablewhileSCKishigh[1].

Signalsoverthetwolinesduringanexampletransmissionareshownonfig.3[1].

ELECTRONICS’200422-24September,Sozopol,BULGARIA

3.MEASUREMENTOFTEMPERATUREANDRELATIVEHUMIDITY

AmeasurementusingSHT11/71isatwo-stepprocedure.First,itisnecessaryto

sendcommandformeasurementtothesensorandgetthedatathroughthedigital

interface.Second,itisnecessarytoconvertdatafromsensortorealphysicalvalues

andtocalculatethetemperaturecompensationofthehumiditydata[1].

3.1Sendingacommandandreceivingdata

Toinitiateatransmission,a“transmissionstart”sequencehastobeissued.It

consistsofaloweringoftheDATAlinewhileSCKishigh,followedbyalowpulse

onSCKandraisingDATAagainwhileSCKisstillhigh[1].

Thesubsequentcommandconsistsofthreeaddressbits（only.000.iscurrently

supported）andfivecommandbits.TheSHT11/71indicatestheproperreceptionofa

commandbypullingtheDATApinlow（ACKbit）afterthefallingedgeofthe8th

SCKclock.TheDATAlineisreleased（andgoeshigh）afterthefallingedgeofthe

9thSCKclock.TwobytesofmeasurementdataandonebyteofCRCchecksumare

transmitted.TheμCmustacknowledgeeachbytebypullingtheDATAlinelow.All

valuesareMSBfirst,rightjustified.Possiblecommandsare:

“gettemperature”,“get

humidity”,“changemeasurementresolution”,“getstatusregistervalue”,“setstatus

registervalue”[1].

CommunicationterminatesaftertheacknowledgebitoftheCRCdata.IfCRC-8

checksumisnotusedthecontrollermayterminatethecommunicationafterthe

measurementdataLSBbykeepingACKhigh.Thedeviceautomaticallyreturnsto

sleepmodeafterthemeasurementandcommunicationhaveended.[1]

3.2ConvertingOutputtoPhysicalValues

Tocompensatefornon-linearityofhumiditysensingelementandtoobtainthe

fullaccuracyitisrecommendedtoconvertthereadoutwiththefollowingformula

[1,3]:

RHlinear=c1+c2+c3.SORH

2.

Coefficientsc1,c2andc3dependonlyonmeasurementresolution.For12bit

resolution:

c1=–4,c2=0.0405andc3=–2.8*10-6.For8bitresolution:

c1=–4,c2=

0.648andc3=–7.2*10-4.

Fortemperaturessignificantlydifferentfrom25°Cthetemperaturecoefficientof

thehumiditysensorshouldbeconsidered[1,3]:

ELECTRONICS’200422-24September,Sozopol,BULGARIA

RHtrue=（T°C–25）.（t1+t2+SORH）+RHlinear,

wheret1=0.01andt2=0.00008for14bitandt2=0.00128for8bitresolution[1].

ThebandgapPTAT（ProportionalToAbsoluteTemperature）temperaturesensor

isverylinearbydesign.Usethefollowingformulatoconvertfromdigitalreadoutto

temperature[1,3]:

Temperature=d1+d2.SOT.

Sincehumidityandtemperaturearebothmeasuredonthesamemonolithicchip,

theSHT11/71allowssuperbdewpointmeasurement.

4.DISTRIBUTEDSYSTEMFORMONITORINGTEMPERATUREAND

HUMIDITY

Onthefigure4belowafunctionalschemeofdistributedsystemformonitoring

temperatureandhumidity,basedonIPC@Chipisshown[2,4].

Thefunctionsofthesystemareseparatedbetweenthefourmainblocks,as

follows[2,5]:

−Theinternetbrowserassuresthatthesystemhasafamiliaruserinterface.

Thevisualizationofthemonitoredparametersisplacedhere.

ELECTRONICS’200422-24September,Sozopol,BULGARIA

−TheintegratedWebserverinconjunctionwiththeCGIdealswiththe

controloftheremoteprocessesandthetransferofdynamicHTMLpagesto

theclient.Thereal-timeoperatingsystem（RTOS）oftheIPC@Chipmanages

withtheWebserver’stasks,TCP/IPcommunication,localperipherals,user

tasksandinterconnectionbetweenthem.

−Users’applicationsandtasks,runningonthecontroller,areusedfor

automationpurposes.Inourcase,thisisthesensordrivingandmanagement

oftheSMScommunication.

Thesoftwareofthissystemconsistsofseveralblocks.Oneblockisfor

temperatureandhumiditymeasurement,one–forcommunicationwiththeGSM

gatewayandsendingSMSandoneisforgeneratingdynamicHTMLdocumentswith

thedatafromthesensor.

Beforeeverymeasurementasoftwarerestartingofthesensor’sinterfacetake

place.Afterthataninitializationcommandissentandcontrollerwaits11ms.After

thisprocedurearealmeasurementcanstart.Themeasurementblockconsistsoftwo

identicalparts,one–fortemperaturemeasurementandone–forhumidity

measurement.Themicrocontrollersendsacommandtothesensortogetdata.After

gettingdataμCnormalizesandstoresitintheoperatingmemory.Thetwoidentical

partsareexecutedsequentiallyandarerepeatedtogether.Betweeneverytwo

iterationsofthiscyclethetask“sleeps”foraboutoneminutetofreetheprocessorfor

taskswithlowpriority（likeFTP,Web,Telnet）.

Thecommunicationbetweenthecontrollerandthesensoristrough2-line

interface（dataandsynchronization）.AC-librarybasedonthesoftwareinterruptsof

theRTOSismadeforthiscommunication.Thislibraryhasthefollowingfunctions:

−shttransstart（）–startsatransmission;

−shtreset（）–forrestartingofthesensor;

−shtini