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应用指南.docx

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应用指南

SharpGP2D12

Applicationnoteforaninfrared,triangultation-baseddistancesensorwithananalog,non-linearoutput

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∙Overview

ThisapplicationnoteisintendedtoprovideagoodoverviewoftheSharpGP2D12sensor（Fig.1）anditsuse,especiallyfordynamicsapplication.Ihopeitwillhelpsomerobotiststomakethissensorworkquiteeasilyandtoavoidcertainpitfalls.Thisrangefinderisoneofthemostcommonlyusedinautonomousroboticsapplicationsforhobbyistsandalsoinacademicresearch.Thereasonsforthisareessentiallyitslowprice（about14$atAcroname,forexample）,itscompact（~40x14x13mm）andlightweightpackage（acomparisonbetweenseveralsensorsmaybefoundhere）.TheGP2D12isbasedonthetriangulationprinciplewithacollimatedinfraredLEDfortheemittingelementandaPSD（PositionSensingDevice）whichconstitutesthereceiver.

Fig.1-SharpGP2D12distancesensor

Fig.2-SharpGP2D12outputpattern

Althoughitexistsaverysimilarsensorwithadigitaloutput（theGP2D02）,IwillfocusonlyontheGP2D12whichhasananalogoutput（seeFig.2）.ThemainmotivationforthischoiceisthatthedigitalversionisalmosttwiceslowerthantheGP2D12（updateperiodabout75msagainst40msfortheanalogversion）.Table1givesabriefoverviewoftheGP2D12specifications（Hereisthefullandlatestofficialdatasheet）.

Range:

10to80cm

Updatefrequency/period:

25Hz/40ms

Directionofthemeasureddistance:

Verydirectional,duetotheIRLED

Maxadmissibleangleonflatsurface:

>40°

Powersupplyvoltage:

4.5to5.5V

Noiseontheanalogoutput:

<200mV

Meanconsumption:

35mA

Peakconsumption:

about200mA

Table1-OverviewoftheSharpGP2D12specifications

Note:

itisveryimportanttoputagoodcapacitor（somethinglike22uF）betweenGNDandVcc,directlyonthesensoritself,inordertoreducethenoiseonthe5VpowersupplyduetothecurrentrequiredbytheemittingLED.Intheotherhand,avoidtheuseofacapacitorbetweenthesignaloutputandGNDorVcc:

itmaydramaticallyreducethesensordynamics（lowpassfilter）.Below,wewillseehowtonumericallyfilterthisoutputinordertoimprovetheprecisionwithoutdecreasingthedynamics.

∙ADconversionofthesensoranalogoutput

Whenpoweredwitha5Vpotential,theSharpsensorhasamaximumoutputvoltageofabout2.45V,forclosedistances.Thehighestuseabledistancegivesapproximately0.45V（seeFig.2）.ItisthereforewisetouseanADconverterwithtwoexternaladjustablevoltagereferences:

VrefandCOM（theoneIemployedistheMAX148BCAP）.Inordertoobtainthebestprecision,thehighestlimitoftheconverter（255on8bits）shouldmoreorlesscorrespondtothehighestoutputvoltageofthesensor,thatisthelowestmeasurabledistance,andviceversa.Indoingso,thefullscaleoftheconverterisused.Fig.3showsthemeasuredresult.

Fig.3-GP2D12outputpatternafterADconversion（0->~0.45V&250->~2.45V）

Basedonthesemeasurements,alookuptablecanbeimplementedinamicrocontroller.Formyownapplication,IusedaUbicomSX52microcontrollerwitha256entrieslookuptable.

∙Statisticalanalysisofthesensoroutput

Inordertomeasurethenoisepresentonthesensoroutput,ImadesometestsandstatisticswithaGP2D12sensorplacedatabout25cminfrontofaflatwhitewall.Thedistributionof10'000successivevaluesacquiredbythemicrocontroller（at1kHz）isshownonFig.4.

Fig.4-StatisticalanalysisoftheSharpGP2D12output

ThevaluespresentedherearecharacteristicoftheGP2D12output.Inparticular,thestandarddeviationdoesnotdependonthemeasureddistance.However,theprecisionofthedistancecannotbedirectlydeducedbecausetheoutputisnonlinear.Aconstantstandarddeviationoverthefullrangeofthesensoroutputdoesnotleadtoaconstantprecisionofthecomputeddistance.

Thisgraph（Fig.4）showsthatthenoiseoftheSharpsensoroutputfollowsanormal-likedistributionlaw.Therefore,itwouldbewisetoaverageseveralsuccessiveoutputvaluestoimprovetheprecision.

Notethatthisquiteimportantnoiseisessentiallyduetothesensoritself（electronicnoise）.Thenoiseintroducedbytheconverterismarginal.SeeFig.8bellowtoconvinceyourself.

∙Lookuptableforlinearization&Precisionevaluation

Astheemployed8-bitmicrocontrollerisnotabletoefficientlyhandlemultiplicationsanddivisions,alookuptablepreferabletoconverttheoutputvaluesofthesensorsintodistances（anotherapproach,usingapowerregression,isexplainedhere）.AstheconvertedvaluesfromtheADconverterare8-bitcoded,thebestwecandoistousealookuptablewith256entries.Themicrocontrolleroffersthepossibilitytostore12-bitdataintheprogrammemory.Itisthereforeeasytostorethedistancesinmillimeters（from100to800）thatwouldrequire10bits.

Asthemicrocontrollerhasnotalotofprogrammemory,inthecaseoftheuseofseveralGP2D12,itseemswisetohaveasingletableforallthesensors.Thefollowinggraph（Fig.5）showsthemeasuredoutputpatternsof4differentsensorsredsimultaneouslyandseveraltimesatthesamedistance（onlytheaverageforeachdistancesandeachsensorsareshown）.

Fig.5-CharacteristicsoffourdifferentSharpGP2D12sensors

40referencedistancesforeachsensorhavebeendefinedinordertoplotthesegraphs.Alinearapproximationbetweeneachpairofsuccessivepointsismade.Onthefollowinggraph（Fig.6）,onlytheactuallymeasuredpoints（foroneofthefoursensors）andtheircorrespondingstandarddeviationareshown.

Fig.6-Setof40measuredvaluesandtheircorrespondingstandarddeviation

Basedonthisgraph,arapidevaluationoftheprecisioncanbedone.Itiseasytounderstandthatthevaluesabove700mmarealmostunusable.At600mm,theprecisionisonly20mmandbecomesbetterwithcloserdistance（about2mmat200mm）.Ofcourse,usingauniquelookuptableforseveralsensorsslightlydecreasestheprecision,buthowevermustrepresentagoodcompromiseincertaincases.

∙Dynamicdistancemeasurement

TheanalogoutputoftheSharpGP2D12sensorsmovesinvoltagestepswhenthemeasureddistancechanges.Thesestepslastabout40ms（seeFig.7）.

Fig.7-SharpGP2D12operationovertime

Thisbehavioriscertainlyduetothefactthatitprocessesanoutputsignalbyinternallyaveragingsuccessivevalues.Let'sassumethatthebestwaytointerpretthissignalistotakethesteplevelasthevaluemeasuredatitsmiddle.Becausethereissomenoiseonthesensoroutput,wecangetanimprovedvalueofthedistancebyaveragingtheoutputalongthestep.

Fig.8showsthetypicalsensoroutputduringafastmovementofthetargetobject.Asyoucansee,thenoiseonthestepscanreachabout±100mV.

Fig.8-Sharpsensoroutputduringdynamicmeasurement（200mVand20msperdivision）

Whatismoreinterestingforusisthesamesignalseenbythemicrocontroller（Fig.9）.

Fig.9-Sharpsensoroutputseenbythemicrocontrollerduringdynamicmeasurement

Thereadingfrequencyoftheconverteris1kHz.Thereforetheprocessorcanhaveanupdatedvalueevery1msandthusabout40valuesperstep.These40valueswillbeaveragedovertimeinordertoobtainagoodapproximationofthesteplevelwithoutnoise（thisprocedureactslikeadigitallow-passfilter）.Toachievethis,thepointsbelongingtothesamestepsmustberecognizedandclustered.

Thereareessentiallytwocriteriatoachievethisclustering.Thefirstiscertainlythedifferencebetweensuccessivedata.Ifthedifferenceisgreaterthanaspecifiedthreshold,theprocessorcanknowthatitisthenextstep.Thesecondoneisthemaximumtimeallowableforastep.Iftwoofthemaremerged,wecanendthesetofsuccessivevaluesbelongingtothesamestepbyknowingthemaximumlengthofthisset.Themaximumnumberofconversionduringastepis43.Soifthereisnosignificanttransitionafter43readings,wecanassumethatfuturedataarepartofthenextstep.Ifsomedataarenotwellclassified,inthiscase,thisisnotsoimportantbecauseitwillnotsignificantlychangethemeanvalueofthesteps.

Ifallthestepsareatthesameheight（staticcase）,thisclusteringprocesswilllosethesynchronization.But,assoonasatransitionappears,thealgorithmwillautomaticallysynchronizeagain.

Thisalgorithmhasbeenimplementedinthemicrocontroller.ThecomputedmeansofthestepsarerepresentedinblackonFig.10.Pleasenotethattheclusteringisappliedtotheroughsensorvaluesandnottothedistancesafterconversionthroughthelookuptable.Thisalgorithmisintendedtofilterthenoiseofthesensoroutput,whichisconstantoverthefullscaleofdistances.Ifthesuccessivevaluesfromthesensorwerefirstconvertedintodistances,thenoisewouldhavenotalwaysthesamemagnitude.

Fig.10-Resultoftheclusteringprocess

Thethresholdforthedifferenceissetto15andthemaximumnumberofpointsbelongingtothesamestepwas43.

Wecanseethattherewasnotenoughdifferencebetweenthefourthandthefifthsteps.Thereforethe43firstpointshavebeenallocatedtothefourthstep.Andthelastpoints（actually38）beforethenextrecognizedtransitionarepartofthefifthstep.

Notethatthecomputationofthisaveragedoesnotrequire40or43registersinmemory.Asumvariableon16-bitisupdatedateachreadingoftheconverter.Whentheendofaclusterisdetected,thesumisdividedbythenumberofdataintheset.Eachcomputedaveragewillbethentransformedintoadistanceviathelookuptable.Thisdistanceistime