SBITNationalSeminar

CapacitiveTouch 1

SBITNATIONALSEMINAREmbeddedSystemsanditsApplications

CAPACITIVETOUCH

SubmittedByAmitVerma(CSE.06.204)

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CapacitiveTouch 2

CAPACITIVETOUCH

Introduction

Acapacitororcondenserisapassiveelectroniccomponentconsistingofapairofconductorsseparatedbyadielectric.Whenavoltagepotentialdifferenceexistsbetween the conductors, an electric field is present in the dielectric. This fieldstoresenergyandproducesamechanicalforcebetweentheplates.Theeffectisgreatestbetweenwide,flat,parallel,narrowlyseparatedconductors.

Capacitivesensingisatechnologyfordetectingproximity,position,etc.,basedon capacitive coupling effects. Capacitive sensing as a human interface device(HID) technology, for example to replace the computer mouse, is becomingincreasingly popular. Capacitive sensors can be recognized in many popularconsumerproducts suchas laptop trackpads,MP3players, computermonitorsand cell phones, but it is certainly not limited to these applications.More andmore engineers choose capacitive sensors for their flexibility, unique human‐device interfaceandcost reductionovermechanical switches.Capacitive touchsensorshavebecomeapredominantfeatureinalargenumberofmobiledevicesandmp3playersandfeatureprominentlyinMIT'scyberfloraroboticsproject.

While capacitive sensing applications can replace mechanical buttons withcapacitive alternatives, other technologies such asmulti­touch and gesture­based touchscreens are also premised on capacitive sensing. The Apple iPodclick wheel and the iPhonemulti‐touch capable touch screen is a well‐knownimplementationofcapacitivesensing.

Design

Capacitive sensors can be constructed from many different media, such ascopper,Indiumtinoxide(ITO)andprintedink.Coppercapacitivesensorscanbeimplementedon standardFR4PCBsaswell ason flexiblematerial. ITOallowsthe capacitive sensor to be up to 90% transparent (for single layer solutions).The size and spacing of the capacitive sensor are both very important to thesensor's performance. In addition to the size of the sensor, and its spacingrelative to the groundplane, the type of groundplane used is very important.Since the parasitic capacitance of the sensor is related to the E‐Field's path toground,itisimportanttochooseagroundplanethatlimitstheconcentrationofE‐Fieldlineswithoutaconductiveobjectpresent.

Designingacapacitancesensingsystemrequiresfirstpickingthetypeofsensingmaterial(FR4,Flex,ITO,etc).Onealsoneedstounderstandtheenvironmentthedevicewilloperatein,suchasthefulloperatingtemperaturerange,whatradiofrequenciesarepresentandhowtheuserwillinteractwiththeinterface.

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CapacitiveTouch 3

Therearetwotypesofcapacitivesensingsystem.Transcapacitance,wheretheobject (finger, conductive stylus) alters the transcapacitance coupling betweenrow and column electrodes, which are scanned sequentially. The second isAbsolute Capacitance where the object (finger etc) senses the presence andabsolute position horizontally and vertically on the surface. Subtraction of apreceding absolute position from the present absolute position yields therelativemotionoftheobject/fingerduringthattime.

Theartofcapacitivetouchsensing

Typicalcapacitivesensordesignsspecifyanoverlayof3mmor less.Sensingafinger through an overlay becomes increasingly more difficult as the overlaythickness increases. In other words, as the overlay thickness increases, theprocessoftuningthesystemmovesfromsciencetoart.

FingerCapacitance

At the heart of any capacitive‐sensing system is a set of conductors, whichinteract with electric fields. The tissue of the human body is filled withconductive electrolytes covered by a layer of skin, a lossy dielectric. It is theconductivepropertyoffingersthatmakescapacitivetouchsensingpossible.

A simple parallel plate capacitor has two conductors separated by a dielectriclayer. Most of the energy in this system is concentrated directly between theplates. Someof the energy spills over into the areaoutside theplates, and theelectricfieldlinesassociatedwiththiseffectarecalledfringingfields.Partofthechallenge ofmaking a practical capacitive sensor is to design a set of printedcircuittraces,whichdirectfringingfieldsintoanactivesensingareaaccessibletoauser.Aparallelplatecapacitorisnotagoodchoiceforsuchasensorpattern.

Placingafingernearfringingelectricfieldsaddsconductivesurfaceareatothecapacitivesystem.Theadditionalchargestoragecapacityaddedbythefingerisknownasfingercapacitance,CF.ThecapacitanceofthesensorwithoutafingerpresentisdenotedasCPinthisarticle,whichstandsforparasiticcapacitance.

Acommonmisconceptionaboutcapacitivesensorsisthatthefingerneedstobegrounded for thesystemtowork.A fingercanbesensedbecause itcanholdacharge,andthisoccursifthefingerisfloatingorgrounded.

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DiagramofaCapacitiveSensor

Figure1showsthetopviewofaprintedcircuit board (PCB) which implementsone of the capacitive sensor buttons inthisdesignexample.

The button diameter is 10 mm, theaverage size of an adult fingertip. ThePCB assembled for this demonstrationcircuit contains four buttons withcenters spaced 20 mm apart. Thegroundplaneisalsoonthetoplayer,asshowninthefigure.Thesensorpadisisolated from the ground plane by a uniform gap. The size of the gap is animportantdesignparameter.Ifthegapissettoosmall,toomuchfieldenergywillgo directly to ground. If set too large, control is lost over how the energy isdirected through the overlay. The selected gap size of 0.5mmworkswell fordirectingthefringingfieldsthrough10mmofglassoverlay.

Figure2showsacross‐sectionalviewofthesamesensorpattern.

Avia in thePCBconnectsthe sensor pad to thetrace on the bottom sideof theboard, as shown inthe figure. The dielectricconstant, εr, influenceshow tightly the electricfieldenergycanpackintothe material as the fieldtries to find the shortestpath to ground. Standardwindowglasshasanεrofaround8,whiletheFR4materialofthePCBhasanεrofaround4.Pyrexglass,whichiscommonlyusedinwhitegoods,hasanεrrofaround5. In thisdesignexample,standardwindowglass isused.Notethat theglasssheet ismountedon thePCBusing thenonconductiveadhesive film468‐MPfrom3M.

Tuningthesensor

Every time the "start scan" function is called in the application program, thecapacitanceofButton1ismeasured.Therawcountvaluesarestoredinanarray.Theusermodulealsotracksabaselinefortherawcounts.ThebaselinevalueofeachbuttonisanaveragerawcountlevelcomputedperiodicallybyanIIRfilterin software. The update rate for the IIR filter is programmable. The baselineenablesthesystemtoadapttodriftinthesystemduetotemperatureandotherenvironmental effects.A switch‐differencearray contains the rawcountvalues

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CapacitiveTouch 5

with the baseline offset removed. The current ON/OFF state of the buttons isdeterminedusing the switchdifference values.This allows theperformanceofthe system to remain constant even though the baselinemay be drifting overtime.Figure3showsthetransferfunctionbetweendifferencecountsandbuttonstatethatisimplementedinfirmware.

Thehysteresis in this transfer functionprovides clean transitionsbetweenONand OFF states even though the counts are noisy. This provides a debouncefunctionforthebuttons.ThelowerthresholdiscalledtheNoiseThreshold,andtheupperthresholdiscalledtheFingerThreshold.Settingofthethresholdlevelsdeterminestheperformanceofthesystem.Withverythickoverlays,thesignal‐to‐noise ratio is low. Setting the threshold levels in this kind of system ischallenging,andthisispartofartofcapacitivesensing.

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HowtheiPhoneCapacitiveTouchScreenWorks

ItisbeyonddoubtthattherealcredibilitytoCapacitiveSensingtechnologywasprovidedwhenApplefirstimplementeditinitsiPodfamilyofmusicplayers.TheCapacitive Sensing Clickwheel redefined the way people used MP3 players tobrowsetheirMusicCollection.TheiPhonegavetheCapacitivetouchtechnologya shot‐in‐the‐arm with its Multitouch display that made the device the besthardwareforbrowsingInternetonthego.

Toallowpeopletousetouchcommandsthatrequiremultiplefingers,theiPhoneuses a new arrangement of existing technology. Its touch‐sensitive screenincludes a layer of capacitive material, just like many other touch‐screens.However,theiPhone'scapacitorsarearrangedaccordingtoacoordinatesystem.Itscircuitrycansensechangesateachpointalongthegrid.Inotherwords,everypointonthegridgeneratesitsownsignalwhentouchedandrelaysthatsignaltothe iPhone's processor. This allows the phone to determine the location andmovementofsimultaneoustouchesinmultiplelocations.Becauseofitsrelianceon this capacitive material, the iPhone works only if you touch it with yourfingertip‐‐itwon'tworkifyouuseastylusorwearnon‐conductivegloves.

The iPhone's screen detects touch through one of two methods: MutualcapacitanceorSelf­capacitance.Inmutualcapacitance,thecapacitivecircuitryrequires two distinct layers ofmaterial. One houses driving lines,which carrycurrent, and theotherhousessensing lines,whichdetect thecurrentatnodes.Self‐capacitance uses one layer of individual electrodes connected withcapacitance‐sensing circuitry.Bothof thesepossible setups send touchdataaselectricalimpulses.

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TheiPhone'sprocessorandsoftwarearecentraltocorrectlyinterpretinginputfromthetouch‐screen.Thecapacitivematerialsendsrawtouch‐locationdatatotheiPhone'sprocessor.TheprocessorusessoftwarelocatedintheiPhone'smemorytointerprettherawdataascommandsandgestures.Here'swhathappens:

1.Signalstravelfromthetouchscreentotheprocessoraselectricalimpulses.

2.Theprocessorusessoftwaretoanalyzethedataanddeterminethefeaturesofeachtouch.Thisincludessize,shapeandlocationoftheaffectedareaonthescreen.Ifnecessary,theprocessorarrangestoucheswithsimilarfeaturesintogroups.Ifyoumoveyourfinger,theprocessorcalculatesthedifferencebetweenthestartingpointandendingpointofyourtouch.

3.Theprocessorusesitsgesture‐interpretationsoftwaretodeterminewhichgestureyoumade.Itcombinesyourphysicalmovementwithinformationaboutwhichapplicationyouwereusingandwhattheapplicationwasdoingwhenyoutouchedthescreen.

4.Theprocessorrelaysyourinstructionstotheprograminuse.Ifnecessary,italsosendscommandstotheiPhone'sscreenandotherhardware.Iftherawdatadoesn'tmatchanyapplicablegesturesorcommands,theiPhonedisregardsitasanextraneoustouch.

Allthesestepshappeninaninstant‐‐youseechangesinthescreenbasedonyourinputalmostinstantly.ThisprocessallowsyoutoaccessandusealloftheiPhone'sapplicationswithyourfingers.

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ApplicationsofCapacitiveTouchSensing

Whilewidespreadadoptionisstilldistant,theCapacitiveTouchScreenSensingis enjoyinga rapid increase inutilization.Areasof implementation range fromConsumer Electronics to full‐fledged Robotics. Capacitive Touch Sensing hasbeen successful in replacing mechanical switches in a fairly large number ofdevices.

While its not possible to enumerate all the applications of Capacitive TouchSensingasitcoversalargedomainofdevices.LetustakealookatsomeoftheinnovationsrelatedtoCapacitiveTouchSensing.

TheiPhoneTo allow people to usetouch commands thatrequire multiple fingers,the iPhone uses a newarrangement of existingtechnology. Its touch‐sensitive screen includesa layer of capacitivematerial, just like manyothertouch‐screens.

The basic idea is prettysimple ‐‐ when you placeyour fingerora stylusonthe screen, it changes thestatethatthedeviceismonitoring.Inscreensthatrelyonsoundorlightwaves,your finger physically blocks or reflects some of the waves. Capacitive touch‐screensusea layerofcapacitivematerial toholdanelectricalcharge; touchingthe screen changes the amount of charge at a specific point of contact. Inresistivescreens,thepressurefromyourfingercausesconductiveandresistivelayersofcircuitrytotoucheachother,changingthecircuits'resistance.

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TheiPodTheClickWheel isa touch‐sensitivering that you use to navigatethrough all of iPod's menus andcontrolallofitsfeatures.Itprovidestwo ways to input commands: byslidingyourfingeraroundthewheeland by pressing buttons locatedunder and in the middle of thewheel.

UndertheplasticsurfaceoftheClickWheel, there are four mechanical buttons (Menu, back, forward, play/pause),andthere'sonebuttoninthecenter(select).

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TheMacbookApplereallyknowshow to use thecapacitive sensingtechnology to itsmaximumpotential.After thesuccessofiPodandiPhone, it was theturn of their line‐up of laptopscalled theMacbook, to get ataste of CapacitiveTouch Sensors. The trackpads of these notebook computersweremadeMulti‐touchcapabletoenabletheuserstocontrolmanyaspectsoftheirMacswiththehelpofgestures,whichtooktheHumanInterfacedimensiontothenextlevel.

TheMagicMouseItbeganwithiPhone.ThencameiPodtouch.ThenMacBookPro.Intuitive,smart,dynamic.Multi‐Touchtechnologyintroducedaremarkablybetterwaytointeractwithyourportabledevices—allusinggestures.NowApplehasreachedanothermilestonebybringinggesturestothedesktopwithamousethat’sunlikeanythingeverbefore.It'scalledMagicMouse.It'stheworld'sfirstMulti‐Touchmouse.

TheMulti‐TouchareacoversthetopsurfaceofMagicMouse,andthemouseitselfisthebutton.Scrollinanydirectionwithonefinger,swipethroughwebpagesandphotoswithtwo,andclickanddouble‐clickanywhere.InsideMagicMouseisachipthattellsitexactlywhatyouwanttodo.WhichmeansMagicMousewon’tconfuseascrollwithaswipe.Itevenknowswhenyou’rejustrestingyourhandonit.

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MicrosoftSurfaceMicrosoft Surface is a revolutionarymulti‐touchcomputerthatrespondsto natural hand gestures and real‐world objects, helping peopleinteract with digital content in asimple and intuitive way. With alarge, horizontal user interface,Surface offers a unique gatheringplace where multiple users cancollaboratively and simultaneouslyinteractwithdataandeachother.

ForUsers:Microsoft Surface represents a fundamental change in thewayweinteractwithdigitalcontent.Leavethemouseandkeyboardbehind.Surfaceletsyou grab digital content with your hands and move information with simplegesturesandtouches.Surfacealsoseesandinteractswithobjectsplacedonthescreen,allowingyoutomoveinformationbetweendeviceslikemobilephonesorcameras.Theresultisafun,socialandexcitingcomputingexperiencelikeyou’veneverhadbefore.

For Businesses: Microsoft Surface provides tremendous potential forbusinessestoimprovecommunication,andbemoreefficientinhowtheydeliverinformation and services to their customers. The intuitive and approachableinterface is easy to learn and the familiar software platformmakes it easy tomanage, too. The multi‐touch and multi‐user capabilities create an incrediblycollaborativeexperience,wheresharingandexploringinformationiseasierthanever.

Microsoft Surface has four key capabilities that make it such a uniqueexperience:

*Directinteraction.Userscangrabdigitalinformationwiththeirhandsandinteractwithcontenton‐screenbytouchandgesture–withoutusingamouseorkeyboard.

*Multi­userexperience.Thelarge,horizontal,30‐inchdisplaymakesiteasyfor several people to gather and interact together with Microsoft Surface ‐providingacollaborative,face‐to‐facecomputingexperience.

* Multi­touch. Microsoft Surface responds to many points of contactsimultaneously‐notjustfromonefinger,aswithatypicaltouchscreen,butfromdozensofcontactpointsatonce.

*Objectrecognition.Userscanplacephysicalobjectsonthescreentotriggerdifferent types of digital responses – providing for amultitude of applicationsandthetransferofdigitalcontenttomobiledevices.

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ThePCAfter all theMacintosh devices utilizing the power of Capacitive Screens, howcanPC be far behind?ManyPCmanufacturers such asHewlett‐Packard, Sony,Lenovoetc.have launchedcapacitivedisplayPCsthattakeshumaninterfacetoanaltogetherdifferentlevel.

Thehi‐definitionlarge‐iconinterfacemakesUIsoamazingtolookandoperate.DifferentPCmanufacturersuse their ownproprietaryUser Interface skinsbuttheideologyremainssame:TomakeHuman‐Interfacewiththemachinesomuchmoreenjoyable.

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SonyEricssonMotionSensingEarphonesThismight look like an odd‐entry in the category of Capacitive Touch Sensing

applications, but a little research uponthese wonderful earplugs leads us tofindoutthat,they,infact,useCapacitiveTouch technology to Stop or Resumemusicplaybackwhentheyaretakenoutoftheuser’sears.

Sony Ericsson's MH907s are button‐free, minimalistic headphones thatactivateuponthemagicaltouchofyour

skin.Well,yourearstobeprecise.

Unfortunately,SonyEricssonisbeingverytight‐lippedaboutitssecrettechnologythatisbeinAmit Verma Page 13 12/11/09g used in these pair of earplugs. Initially weassumeditwasskincontactonbothshinypadsoneachbud,butsoonwerealizedtheactualculpritwasthecanalextension‐‐thepartcoveredbytheremovablerubbercap.Whatwehavehereiseffectivelyacapacitivetouchswitch:thecanalextensionactsasa lone electrode insulated by the rubber cap, and when in contact with thehumanskin itscapacitance is increased,thusactivatingtheswitch.Fascinating,eventhoughthisisn'tnewtechnology.

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CONCLUSION

CapacitiveTouchSensinghascomealongwaysincetheAppleClickWheel.Itisnowpoweringdevices,whichwereveryfarfromitsdomainofapplication.EngineershavedevisedwaystoimproviseuponthisEmbeddedSystemapproachtofindnewwaysHumansinterfacewithmachinesordevices.EngineeringwaystomakeCapacitiveInputtechnologymuchmorereliableandaccurateisalwaysanobviouscall.Fasterembeddedsystemscontrollersarebeingdevelopedthatmaketheprocessfarmoreaccurateandefficient.CompanieslikeSynapticsaredevelopingCapacitivedisplayswithembeddedHapticFeedbacktechnologythatreverbsafeedbacktotheuserupontouch.ManysuchinnovationswouldcontinuetoembarktheEmbeddedSystemsapproachtocomputing.