SignalProcessinginSo.wareandElectricFieldSensingCSE466:So.wareforEmbeddedSystems

Winter2009

B.Mayton<bmayton@cs.washington.edu>UniversityofWashingtonCSE&IntelResearchSeaMle

CSE466Winter2009 1ElectricFieldSensing

Labs3and4:BuildingaSensor

•  You’llbuildanelectricfieldsensorwithbasiccomponentsandyourAVRmicrocontroller– Cansenseyourhandabovetheboardwithoutactuallytouchingit

– Transmitandreceivesignalsaregeneratedanddemodulatedinso.ware

– WillsendthesensorreadingstoaPCforfurtherprocessingandmovingaroundinacolorspace

CSE466Winter2009 2ElectricFieldSensing

E‐FieldSensinginNature

•  Generatesanelectricfieldaround1kHz

•  UsesE‐fieldtodetectprey,avoidobjects,andcommunicate

CSE466Winter2009 ElectricFieldSensing 3

BlackGhostKnifefish(Apteronotusalbifrons)

ElectricFieldSensors:Applicacons

•  CSE466Winter2008– added4‐channelE‐fieldsensor(the“AirScck”)totheiMote2toallowcontrolofasoccerplayerinamulcplayergame

CSE466Winter2009 ElectricFieldSensing 4

ElectricFieldSensors:Applicacons

•  PersonalRoboccsatIntelResearchSeaMle

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•  E‐Fieldaddssenseof“Pretouch”toarobothandandarm,allowingittodetectthatitisclosetoobjectswithouttouchingthem

TheoryofOperacon

•  CreateanACelectricfieldwithconstantamplitudewithtransmitelectrode

•  Measurecurrentinducedinreceiveelectrode•  Nearbygroundedobjectsshuntsomecurrenttoground,reducingthereceivedcurrent.

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TransmitReceive

Objectinrangeofsensor

PartsoftheSensor

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TransmitAntennaReceiveAntenna

AnalogFront‐EndTransmitResonator

WaveformGeneracon(Timer0OutputCompare)

Sampling(ADC)

Demodulacon(So.ware)

TransmiknganElectricField

•  UsecmerhardwareonAVRtogenerateasquarewave;frequencycanbetunedwithprescalervalueandoutputcomparevalue

•  Resonatorcircuitboosts5Vsquarewavetoasinewavewithanamplitudeofaround100V

•  TransmitinthetensofkHzrange;higherfrequenciesworkbeMerbutgivelesscmetoprocessinterrupts

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ResonantCircuits

•  Drivingaresonantcircuitwithsmallamountsofenergyattherightcmes(attheresonantfrequency)willcausehigh‐amplitudeoscillacons

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•  TheLCresonatoronthesensorboardallowstheAVRtocreatealargeelectricfieldfromasupplyofonly5V

•  TheresonantfrequencyofanLCcircuitis

€

f =1

2π LC

AmplifyingtheReceivedSignal

•  Transimpedanceamplifierconvertscurrentflowingintoreceivedelectrodeintovoltage

•  VoltagegainstageamplifiesthesignaltolevelsthatworkwellwiththeADCintheAVR

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OperaconalAmplifiers

•  Veryuseful,versacle,andubiquitousanalogcircuitdevices–  amplifyvoltages–  canturnhigh‐impedancesignalsintolow‐impedancesignals(weaksignalsintorobustsignals)

–  performmathemaccaloperaconsonsignalsintheanalogdomain(usedtobehowmostsignalprocessingwasdone)

•  Signalprocessinghasmovedintothedigitaldomain,butopampsaresclluseful,parccularlywhenitcomestointerfacingsensorswithmicrocontrollers

CSE466Winter2009 ElectricFieldSensing 11

OperaconalAmplifiers

•  Twoinputterminals:invercng(‐)andnon‐invercng(+)–  almostnocurrentflowsintotheinputs

(theyarehighimpedance)•  Voltageatoutputterminalisthe

differencebetweenthetwoinputsmulcpliedbysomegain

•  Outputchangestotrytokeepthevoltagesatbothinputsequaltoeachother

•  Outputislowimpedance:wecandrawsomecurrentfromitwithoutaffeccngitsvoltagesignificantly

CSE466Winter2009 ElectricFieldSensing 12

TheAnalogFront‐End

•  Nowthatweknowaboutopamps,we’lllookatthetwostagesoftheanalogfront‐end.

•  AGNDisat2.5V;thiswilladdaDCoffsettotheoutputvoltage,bringing“zero”intothemiddleofourusablerangefortheADC

CSE466Winter2009 ElectricFieldSensing 13

TheTransimpedanceAmplifier

•  Convertscurrententeringthereceiveelectrodeintoavoltagesignal– Whennocurrentisflowinginfromthele.,thereisnovoltagedropacrosstheresistor,andtheoutputvoltagewillbethesameasthetwoinputterminals.

CSE466Winter2009 ElectricFieldSensing 14

0µA

0V+ ‐

2.5V2.5V

TheTransimpedanceAmplifier

•  Convertscurrententeringthereceiveelectrodeintoavoltagesignal–  Currententeringfromthe

le.can’tgointotheinvercnginput,soitgoesthroughR3andcreatesavoltagedrop

–  Inordertokeepthevoltageattheinvercnginputequaltothenon‐invercnginput,theoutputvoltagemustbedecreased

CSE466Winter2009 ElectricFieldSensing 15

1µA

1V+ ‐

2.5V1.5V

TheVoltageAmplifier

•  Amplifies(andinverts)theinputvoltage.

•  Basicinvercngop‐ampconfiguracon

•  Outputvoltageforthecircuitattherightis

CSE466Winter2009 ElectricFieldSensing 16

€

vout = −vinR5R4

Wherearewenow?

•  We’vecreatedanelectricfieldandit’sinducedacurrentinourreceiveelectrode.

•  We’veamplifiedthereceivedsignalandbroughtitintoausablerangeforourAVR’sADCtosample.

•  Now,weneedtomakesenseofthesignal

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Demodulacon

•  We’llbereceivingoursignal,buttherewillalsobealotofnoise.

•  Needtorecovertheamplitudeofoursignal,butignorethenoise.

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Demodulacon:BasicIdea•  Insteadofonesample,we’ll

accumulatemulcplesamplesinanintelligentmanner.

•  Sampleatmulcplepointsonthereceivedwaveform

•  Ifweaddsampleswhenwe’retransmikngaposicvesignalandsubtractsignalswhenwe’retransmikngnegacvesignal,wecanceloutalotofunwantednoise(andtheDCoffset)

•  Accumulateabout20to255samplesforameasurement

•  Resultofaccumulaconrepresentsamplitude

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+ ‐ + ‐ ...

Whataboutphaseshi.s?

•  Ifthesamplesaren’tperfectlyinphasewiththereceivedwaveform,we’remissingoutonsignal‐to‐noise

•  Gekngperfectlylinedupisdifficult—athightransmitfrequencies,evenoneinstrucconcycleshi.sthesamplebyquiteabit

•  Solucon:alsosampleat90˚and270˚inaddiconto0˚and180˚,inaseparateaccumulator

•  Theaccumulatorfor0˚and180˚isthein‐phasecomponent,andthe90˚and270˚accumulatoristhequadraturecomponent

•  Wecannowrecoverthemagnitudeofthereceivedsignalregardlessofitsphase:

CSE466Winter2009 ElectricFieldSensing 20

€

magnitude = inphase2 + quad2

TimingIssues•  TheADCisn’tfastenoughtomakeallthesesamplesonevery

periodofthereceivedwaveform.•  Liningupthesampleswiththerightpartsofthewaveformisa

challenge.

CSE466Winter2009 ElectricFieldSensing 21

•  TheADCtakessomecmetoperformconversions,buttheactualsample‐and‐holdwindowisshortandcanbepreciselyplaced.

•  WecanmaketheADC’ssamplessynchronouswiththetransmiMedwaveformbyusingfree‐runningmodeandseknguptheprescalerssothattheADCsampleseveryn+¼periodsofthetransmiMedwaveform.

but...

Timing•  Theresultisthatwe’reonly

samplingeveryfewperiodsofthewaveform,butwestaylinedupbecausewe’reusingthesameclockforthetransmiMedsignalandtheADC.

•  SincewesetuptheADCtosampleeveryn+¼periodsofthetransmiMedwaveform,eachsamplewillbeoffsetby90˚.

•  Eachsetoffoursamplesgivesustheposicveandnegacvevaluestoaddtoeachofourtwoaccumulators.

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ImplementaconinLabs3and4•  Forlabs3and4,youwill:

–  calculateprescalerandoutputcomparevaluestoenablesynchronousundersamplinganddemodulacon

–  calculatethecapacitorvalueneededtomakethetransmiMerresonantatyourtransmitfrequency

–  buildthee‐fieldsensorhardware–  useSPIandUSBtosendthevaluesofthein‐phaseandquadrature

accumulatorstoaPC–  usethePCtocomputethemagnitudeofthereceivedwaveformfrom

itscomponents–  usethevaluefromthesensorasavirtualknobtomovearoundina

colorspace–  displayacolorwheelindicacngthecurrentcoloronthePCscreen,and

senditbacktoyourAVRtobedisplayedonyourtri‐colorLED

CSE466Winter2009 ElectricFieldSensing 23