Projected Capacitive Touch: A Practical Guide for Engineers

198 Pages·2019·12.457 MB·English

by  Tony Gray

Tony Gray
Projected
Capacitive
Touch
A Practical Guide for Engineers
Projected Capacitive Touch
Tony Gray
Projected Capacitive Touch
A Practical Guide for Engineers
TonyGray
Plano,TX,USA
ISBN978-3-319-98391-2 ISBN978-3-319-98392-9 (eBook)
https://doi.org/10.1007/978-3-319-98392-9
LibraryofCongressControlNumber:2018954649
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Contents
1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 ProjectedCapacitiveTouchBasics. . . . . . . . . . . . . . . . . . . . . . . . . 5
3 SelfCapacitance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4 MeasuringCapacitance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5 EquivalentCircuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6 MeasuringOpticalQuality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7 TransparentConductors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
8 ElectrodePatterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
9 PerimeterTracesandTails. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
10 SensorStackUps. . .. . . .. . . . .. . . .. . . .. . . . .. . . .. . . . .. . . .. 91
Glass-Glass(GG). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Glass-Glass-Glass(GGG). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Double-SidedGlass(GG2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Glass-Film-Film(GFF). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Glass-Film(GF). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
OneGlassSolution(OGS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
11 DisplayBasics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
12 CoverLenses,Films,andCoatings. . . . . . . . . . . . . . . . . . . . . . . . . 117
13 TouchControllers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
14 CommunicationInterfaces,OperatingSystems,andDrivers. . . . . . 135
15 ElectricalNoise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
v
vi Contents
16 StylusandGlove. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
17 Calibration,Retransmission,andSuppression. . . . . . . . . . . . . . . . 159
18 WaterImmunity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
19 GUIDesignandGestureEngines. . . . . . . . . . . . . . . . . . . . . . . . . . 169
20 AutomatedTesting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
About the Author
TonyGray Tonyhasbeendesigning,tuning,andtestingprojectedcapacitivetouch
panelssince2009.InhisroleasDirectorofPCAPTouchTechnologyfirstatOcular
LCDandthenatDawarTechnologies,hehasbeenresponsibleforover100custom
projected capacitive touch panel design projects. Tony is highly regarded in the
marketplace for his knowledge of touch systems. He has presented at several
different technical conferences on projected capacitive touch technology and
publishednumerousarticlesandwhitepapers.Beforeworkinginthetouchindustry,
Tony spent 15 years writing embedded software for a variety of devices including
residential thermostats, smoke opacity meters, hospital food delivery systems,
robotic assay equipment, and a system used to capture and analyze 3D neuronal
structures. Tony has a Bachelor of Science in Computer Engineering from Lehigh
University.HelivesinPlano,Texas.
vii
Chapter 1
Introduction
OnJune29,2007,theworldchangedforever.Thatwasthedaythefirst-generation
®
iPhone was released to the public. There are several reasons that the iPhone had
suchahugeimpact.Itredefinedwhatacellphonecoulddo.Itlookedsleek,modern,
and(mostly)seamless.Ithadahigh-resolutionfullcolordisplay.Itincludedafully
functionalmusicplayer.Ithadaneco-systemofcustomapplications.Butthemost
important thing about the iPhone was that it could recognize two separate touches
simultaneously.Thatmaynotsoundlikeamajoraccomplishmenttoday,butin2007
itwasrevolutionary.
IsthatpictureofGrandmaandthekidstoosmall?Justusetwofingerstozoomin
and see the detail. Want to see how close Burkburnett Texas is to Dallas? Just use
twofingerstozoomoutuntilyoucanseeboth.Isthatmapofthezoonotorientedto
thedirectionyouarewalking?Justusetwofingerstorotateitsothatitlinesup.
Before the iPhone, the touch market was dominated by resistive touch sensors.
Resistivetouchsensorswerecheap,buttheyalsoscratchedeasily,hadtobereplaced
often, had to be calibrated on a regular basis, required a clunky bezel design, and
only supported one touch (there were some multi-touch resistive solutions on the
market, but they never worked well enough to capture significant market share).
Severalothertouchtechnologiesalsohadsignificantmarketshareincludingsurface
acousticwaveandinfrared,butresistivewasthemarketleaderformostapplications.
But once the iPhone was released, it established a new expectation for the touch
experience.
®
I do not mean to imply that Apple invented multi-touch. There were several
multi-touch technologies available going back to the 1980s. Apple did not even
invent projected capacitive multi-touch. That had been around since at least 1984
whenitwasusedtocreatea16keycapacitivetouchpad,althoughitwasnotcalled
“projectedcapacitive”until1999.
Apple’strueinnovationwasnotthetechnology,buttheuseofit.TheiPhonegave
youareasontoneedmultipletouches.Onceusershadthiscapabilityandunderstood
howusefulandintuitiveitwas,theybegantoexpectitonotherdevicesaswellfrom
©SpringerInternationalPublishingAG,partofSpringerNature2019 1
T.Gray,ProjectedCapacitiveTouch,https://doi.org/10.1007/978-3-319-98392-9_1
2 1 Introduction
thermostats to exercise bikes to infusion pumps to slot machines to pretty much
everything.
While the iPhone deservedly gets a lot of credit for introducing projected
capacitive (PCAP) touch technology to the world, there were several other factors
that also contributed to the rise of PCAP. Full color displays started getting less
expensive,allowingdesignerstoreplacemonochrome,segmenteddisplayswithfull
color bitmap displays. Processors continued to get less expensive, which was
important because it is just about impossible to drive a 320 (cid:1) 480 256 color TFT
withan8051embeddedprocessorandnooperatingsystem. Speakingofoperating
systems,anotherimportantfactorwasthecontinuedgrowthofLinuxintheembed-
ded systems market. All of these changes fed into one another allowing product
designerstoputtogetherinexpensivefullcolordisplays,afreeoperatingsystem,and
ahigh-endprocessorcapableofdisplayingamulticolorGUI.
When all this upheaval started happening in 2007, I was working for Invensys
Controls writing software for residential thermostats using a segmented LCD, a
16-bit processor, and no operating system. In 2007, our design team finally con-
vinced marketing that we should develop a high-end thermostat with a 5.7in TFT
and a resistive touch sensor. This was my first glimpse of the changes coming in
productdesign.
In2008,IleftInvensysControlstoworkforOcularLCD.Ocularhadbeenvery
successful supplying custom segmented LCDs to various embedded markets. But
fairlysoonafterIjoinedthecompany,itbecameobviousthatsegmentedLCDswere
goingtobereplacedbyfullcolorTFTsinjustafewyears.MybossLarryMozdzyn,
the CTO and one of the co-founders of the company, realized that the same
equipmentweusedtobuildLCDscouldalsobeusedtomakeprojectedcapacitive
touch sensors. Over the next few years Ocular transformed itself from an LCD
suppliertoamajor supplierofPCAPtouchsensors.And fortunatelyforme, Larry
pulledmeintothePCAPsideofthebusiness.
AtthetimeIdidnotknowanythingaboutPCAP,soIdidwhatanynerdwould
do:IGoogled“howdoesprojectedcapacitivetouchwork.”Ifoundafewwebsites
®
that did not go into a lot of detail. Then I tried searching Amazon for a book on
PCAP. I was a bit surprised that I could not find one, although this was not too
shocking since PCAP was just starting to become popular. I attended trainings
® ®
offered by the touch controller vendors we used at the time, Cirque and Atmel .
Iaskedalotofquestions(nodoubttothepointofannoyingtheengineersleadingthe
trainings).IreadeverybitofdocumentationIcouldget.Iprintedoutthedocuments,
wrote notes on each page, and then e-mailed dozens of questions to the support
teams at Cirque and Atmel. I wrote a lot of utilities for the controllers including
configuration apps, test apps, programming apps, and even tuning wizards that
automated the tuning process. Eventually I developed an understanding of PCAP
including the fundamental physics of the technology, the operating principles of a
touchcontroller,communicationprotocolsanddriversupport,accuracyandlinearity
testing,physicalconstruction,andmanufacturing.
Occasionally, I would do another search on Amazon looking for the definitive
bookonprojectedcapacitivetouchtechnology,butIneverfoundone.Ihadwrittena
1 Introduction 3
fewarticlesovertheyears,someonembeddedsoftwaredevelopmentandsomeon
PCAP.Eventually,itoccurredtomethatifnobodyelsewasgoingtowriteabookon
PCAP,Ishoulddoit.
This book is essentially the book I wish I had found on Amazon in 2008. It
explainsjustabouteverythingthereistoknowaboutprojectedcapacitivetouchthat
isnotproprietary(ifyouarelookingforabooktoexplainthesecretsoftheinsert-
name-heretouchcontroller,sorrybutNDAspreventmefromgettingintothatkind
of detail). This book starts with a basic explanation of how a projected capacitive
touch sensor works. It explains mutual vs. self capacitance, how capacitance is
measured,howatouchsensorcanbemodeledasanelectroniccircuit,opticalquality
metrics,transparentconductors,andsomuchmore(checktheTableofContentsfor
a complete list of topics). In short, this book includes just about everything I have
learned about PCAP over the last 10 years that I am allowed to share. While it is
targetedatmechanical,electrical,andsoftwareengineerswhoareconsideringusing
PCAP in a project, I am hopeful that it will also prove useful to a wide variety of
peoplefromsalesandmarketingtoPCAPdesignerstophysicsstudentstonerdswho
justwanttolearnmore(mypeople).IhopeyouallenjoyreadingitasmuchasIdid
writingit.