190J.T.Devarajuetal./Measurement76(2015)189–200

Micro-ElectromechanicalSystem(MEMS)[7,8].Interfacingthesemeteorologicalsensorstomicrocontrolleriseasyandinexpensiveusingsimpleelectroniccircuitstoacquireaccurateweathermeasurementsautomaticallywithoutanyinterruptionandmaintenance[9–11].Thus,theoveralllifecyclecostofWeatherMonitoringStationcanbeminimized.

Inthispaper,anattempthasbeenmadetodesignandimplementtheWirelessPortableWeatherMonitoringSta-tiontomeasurerelativehumidity,atmosphericpressure,rainfall,solarradiation,windspeed,winddirection,surfaceandambienttemperaturebyinterfacingmeteorologicalgradesensorstoPIC16F887microcontroller.Further,industrialstandardModbuscommunicationprotocolshasbeenimplementedtoacquiredatafromWeatherMonitor-ingStationandcommunicatedtobasestation(PCnLaptop)overZigbeewireless(XBee-Proradiomodules)andserialinterfaces(RS232nRS485)seamlessly.Atthebasestation,theacquireddatafromWeatherMonitoringStationisloggedanduploadedtoonlineMYSQLdataservertopro-videubiquitoussharingoftheacquiredweatherdata.

2.Relatedwork

Authorof[9]haspresentsaimplementationofweatherstationwhichconsistsofTEMT600luminositysensor,SCP1000pressuresensorandSHT15temperaturecumhumiditysensorinterfacedtoATmega328microcontrollerusingSEN-08311USBWeatherBoard,alongwithGSMmodule.ThestationcanbecontrolledthroughtheSMSser-viceofmobilephones.Authorsof[10]hasimplementedlowcostdistributedmonitoringsystemforcollectingenvi-ronmentalparametersliketemperature,humidityandwinddirectionusingTINImicrocontrollercard.TCP/IPpro-tocolisusedfordatatransmissionandGUIhasbeendevel-opedtoobserveenvironmentalparameters.Authorsof[11]havedesignedaprototypeoflow-costmicrocontrollerbasedsystemforcontinuousandautomatedmonitoringofcropconditionsusinginexpensiveelectroniccomponentsandsolid-statesensors.Thissystemdesignedtomonitorsoilmoisture,soiltemperature,airtemperature,andcanopy-temperaturelevelsincropped elds.Authorsof[12]haveimplementedanIntelligentTransportationSys-tem(ITS)employingTxDOTEnvironmentalSystem.TxDOTEnvironmentalSystemincludessensorssuchasroadwaywaterdepth,rainfallgauge,windspeed,winddirection,temperature,streamvelocityalongwithpavementtem-peratureandmoisturesensors.Thevaluesfromthesesen-sorsaresenttolocal oodcontrolagencyusinglowfrequencyradiostoalertthemotoristduringsevereweatherconditions.Authorsof[13]haveimplementedaMulti-TieredPortableWirelessSystemtomonitorforest res.Thelocalareaweatherconditionssuchasrelativehumidity,winddirection,windspeedandtemperatureareacquiredusingMica2sensornodes.Awebcamispro-videdtomonitorlivevideooftheregionsaffectedbyforest res.Theacquireddataiscommunicatedtolocalbasesta-tionthroughmulti-hoptechnique.Authorsof[14]haveimplementedOn-sitedynamicwirelesssensormonitoringsystemtooptimizeenergyconsumptionandtoimprove

usercomfortswithinthebuilding.ThesystemincludesZigbeedevicestoacquireparameterssuchaslightinten-sity,temperature,relativehumidityandairqualitywithinthebuilding.Further,Zigbeedevicesmeasuresbatteryandbuildingelectricalpowerconsumptionthroughacurrent/ammetersensorinterfacedtothesensornodes.AsoftwaretoolcalledBuildingMonitoringSystemreceivessensorval-uesandupdatesitsdatabasethroughwellestablishedwirelesssensornetworkandperformscontrolling,moni-toringandreportingoperations.

TherestofthepaperisorganizedasfollowsSection3andSection4describestheblockdiagramoftheimple-mentedsystemandsoftwarearchitecture.Section5outli-nesthetestingprocedureanditsresults.Section6concludesthepaper.

3.Hardwareimplementation

TheblockdiagramofWirelessPortableMicrocontrollerbasedWeatherMonitoringStationisshownintheFig.1.Theimplementedstationconsistsoftemperatureandhumiditysensor,windspeedandwinddirectionsensor,raingaugesensor,solarradiationsensor,pressuresensor,surfaceandambienttemperaturesensorsalongwithTLV2543serialADC,PIC16F887microcontrollerandcon-trolswitches.TheXBee-Promoduleprovideswirelesscommunication,MAX-232andMAX-485modulesarepro-videdforserialcommunication.

3.1.Relativehumidityandtemperaturesensor(SHT11)TheRelativeHumidity(RH)isde nedastheratiooftheamountofwatervaporintheairatanygiventemperaturetothemaximumamountofwatervaporthattheaircanhold.IngeneralRHisexpressedintermsofpercentage(%).Similarly,theatmospherictemperatureisde nedasthemeasureoftemperatureatdifferentlevelsofEarth’satmospherewhichisexpressedindegreeCelsius(°C).TheSHT11sensorisemployedtomeasurebothrelativehumidityandatmospherictemperature.SHT11incorpo-ratesacapacitivesensorelementtomeasurerelativehumidityandbandgapsensortomeasuretemperature.Thesesensorsareconnectedinternallytoserialinterfacethroughhighprecision14bitAnalogtoDigitalConverter(ADC).Ontherequestofthehostmicrocontroller,theSHT11communicatestherelativehumidity(SORH)andtemperature(SOT)readoutsthroughserialinterfaceusingI2Cprotocols[15].TheinterfacingschematicforSHT11sensortoPIC16F887isshowninFig.2.

ThereadoutsSORHisconvertedtotruerelativehumid-ityandSOTisconvertedtotruetemperatureusingEqs.(1)and(2)respectively.TheEqs.(1)and(2)arereferredfromtheSHT1xdatasheet[15].Thecoef cientsC1,C2,C3inEq.(1)andd1,d2inEq.(2)areselectedfromTables1and2respectivelybasedupontheconversionbitlengthofSORHandvoltage(VDD)appliedtosensor.

Relativehumidity¼C1þC2ÂSORHþC3ÂSO2RH%ð1ÞTemperature¼d1þd2ÂSOT C

ð2Þ