Display and Touch Technology Choices Put a Touchscreen on That! Design & Manufacturing Midwest 2011...

Display and Touch Technology Choices

Put a Touchscreen on That!

Design & Manufacturing Midwest 2011Conference Session D301

D S S Robert Phares

Display Sourcing & Service LLC

Elements of a Touchscreen System

• Hardware– Touchscreen– Controller– Cables

• Software– Drivers for standard OSs, or – Touch processing code in the application

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Take the Easy Way Out!

• External display/separate enclosure?– Finished displays, open frame chassis with touch

• Often available “unbranded”

– Controller usually in display housing– Some choice of touch technology usually available– Standard OS drivers provided by most manufacturers– Avoids many display obsolescence issues

• Usually doesn’t apply in a stand-alone product…

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Display Technologies

• “Standard” display panels --Most common is transmissive AMLCD

• Limited availability of reflective/transflective panels—usually smaller sizes

– Emissive panels• Plasma• VF• AMOLED becoming available in small sizes

– System processor may dictate display type and size/resolution

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Display Technologies, cont’d

• Display/touchscreen interactions can affect display AND touchscreen choice. Things to consider will largely be in the areas of: • Optics• EMI• Environment

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Touchscreen Technologies

• Commercially available touchscreens:– Analog resistive—four wire (4W) and five wire (5W)– Surface (SCAP) and projected capacitive (PCAP)– Surface acoustic wave (SAW)– Dispersive signal technology (DST) and acoustic

pulse recognition (APR) – Scanning infrared– Camera-based optical– Pen-based electronic digitizers

• Developing technologies– Multitouch resistive—analog and digital– “In cell” touch (several variations)

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Touchscreen Technology Descriptions

• Resistive– Concept: a two axis voltage divider– Two major types—four and five wire

• 4W—a dedicated conductive sheet for each axis

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• Resistive– DC Bias is applied sequentially to each sheet, with

the opposing sheet serving as a voltage probe– Position is proportional to voltage

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• Resistive– Important assumptions include:

• Uniformity of the resistivity of the transparent conductive film• High conductivity of the bass bars vs the film, to insure

reasonably uniform drive potential/current on the buss bars

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• Resistive important details– “Separator dots” between the two active sheets:

• Hold the sheets apart except when touched• ~100-200um (0.004-0.008 “) diameter• ~10-40um height• Screen or foil printed

– Construction can be “film-film” laminated to rigid backplate, film-glass, or glass-glass

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• Resistive important details– “Construction can be “film-film” laminated to rigid

backplate, film-glass, or glass-glass– Typical cross section:

Glass

Plastic (Coversheet)

ITO CoatingAdhesive Spacer

Separator Dots

Touchscreen Technology Descriptions• Resistive important details

– Connecting cable attached to glass or film with Anisotropic Conductive Film (ACF) and essentially any length or shape can be had, for a price.

• Durability– 4W resistive is NOT durable.

• ITO damage due to repeated flexure of upper film layer affects linearity

• Controllers– 4W controllers are cheap, widely available in board or chip

solutions, and most importantly, may be included for “FREE” in your system microprocessor.

– Usually have your choice of communications interface—USB, RS232 for PNP, I2C, SPI, etc. for embedded

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Touchscreen Technology Descriptions• Why consider 4W resistive in a new design?

– Cheap and widely available. • Small off the shelf touchscreen is a $5-10 solution depending

on the controller• Usually available up to ~19” diagonal

– Least restrictive touch input--finger, glove, any stylus– Very low current requirements– Reasonable environmental performance

• Water resistant with proper seal • Why avoid 4W resistive in a new design?

– Poor long term durability– Poor visible light transmission (VLT) —typ 80%--and

very reflective• OK for many indoor apps, poor in direct sunlight.

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Touchscreen Technology Descriptions• Five wire (5W) touchscreen

– Uses single drive layer to develop orthogonal position sensing planes

– 5W is somewhat higher current than 4W due to resistive network

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Resistive Network

1 2 4 5X H H L LY H L H L

1

2

4

5 Touchscreen Glass


• 5W touchscreen– Now the upper film is always the voltage

probe• Uniformity of the conductive coating is less critical• Results in significantly better field life than

equivalent 4W touchscreen– Better candidate for optical and environmental

enhancements

– Basic construction very similar to 4W– Many of the design features similar to 4W

also

Touchscreen Technology Descriptions• Why consider 5W resistive in a new design?

– Relatively inexpensive and widely available. • 15-30% premium over 4W• Controller circuit seldom included in system microprocessor,

but many inexpensive chip/board level solutions available– Least restrictive touch input

• Finger, with any type of glove, or any stylus– Reasonable environmental performance

• Water resistant with proper seal • Why avoid 5W resistive in a new design?

– Fair long term durability– Poor light transmission—typ 80%--and very reflective

• OK for many indoor apps, poor in direct sunlight.– Standard 5W does not support multi-touch

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• Surface Capacitive– Concept: much like a 5W resistive design

with no coversheet– Features

• Always glass-based• Touch surface (usually ITO, ATO or TO) protected

by SiO2 or sol-gel coating– More durable than resistive (no plastic coversheet)

• Better VLT than resistive unless EMI shield is needed, and always less reflective

• Excellent finger sensitivity

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• Surface Capacitive– Controllers

• Touch detection method by ratios of AC current through the corners of the resistive network

• Some third party board level solutions available—usually USB or RS232

• Few chip solutions available• Integration more difficult than resistive. Issues with:

– Grounding – Cable routing– EMI

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Touchscreen Technology Descriptions• Why consider Surface Capacitive in a new

design?– Established technology in gaming, kiosks and some

ATMs—customer familiarity– Easy environmental/contaminant sealing– Good durability in public areas

• Why avoid Surface Capacitive in a new design?– Cost, may be ~50% or more higher than resistive– Poor calibration stability in many designs– Limited gloved hand/stylus operation– Portable/mobile operation poor– Safety issues in some markets unless touchscreen is

bonded to display

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Touchscreen Technology Descriptions• Projected Capacitive

– Despite similarity of name, completely different from Surface Capacitive

– Concept: Finger contact or proximity to the touchscreen surface changes the capacitance between one or more electrodes of an array and its neighbors. Two types:

• Self capacitance—measured to a common electrode for both axes• Mutual capacitance—measured between a given electrode and

nearby electrodes in the 2nd axis array. Supports multi-touch.– Measurement not a trivial issue—capacitance is usually in the

1picofarad range!– Apple and others have improved on very old technology from

3M/Microtouch (ThroughGlass) and 3M/Dynapro (Near Field Imaging) to make this technology the darling of the touch industry today.

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• Projected Capacitive– Features

• Many different glass and/or film constructions possible—can address many different applications

• Cable and EMI issues have led to “chip on flex” controller solutions from many vendors

– Nice complete solution if it fits your design form factor

• Many vendors concentrating on smaller sizes—for cell phones and tablets

– Availability of larger sizes as component limited– Few third party controllers available in any form

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• Projected Capacitive– Features, cont’d

• Excellent finger sensitivity– But poor gloved hand/stylus sensitivity

• Good portable/mobile performance—none of the SCAP issues

• Multi-touch performance with mutual capacitance types• Excellent VLT, and virtually invisible arrays in later generation

products, especially if optically bonded to display• Reasonable power requirements—less than 5W, more than

4W resistive• Relatively narrow borders; EMI is the major integration issue• Not amenable to top surface optical enhancements other

than non-conductive coatings or AG finish—no CP filters

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Touchscreen Technology Descriptions• Why consider Projected Capacitive in a new design?

– Multi-touch support– Excellent VLT without enhancements– Durability– Reasonably low power for portable/mobile

• Why avoid Projected Capacitive in a new design?– Limited component solutions in larger sizes– 3-4X the cost of analog resistive, may have high design-in cost– Glove/stylus limitations– Possible EMI issues– Not compatible with some optical enhancements for outdoor use

• CP filters, etc.

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• Surface Acoustic Wave (SAW)– Concept: Shear waves propagating in the surface

of a piece of glass are attenuated by finger touch

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YT

YR

XR

XT

Y Axis

X Axis

Piezo-electric transducers (4)

5.53MHz“burst” signal from XT, YT

Return signal from XR, YR

Time in usec ~5 100+


– SAW Features• Always glass based• Excellent optics--no conductive coatings• Variety of medium-large sizes available

– Not space efficient in smaller sizes (border)– But, chip on flex improves controller solution– Flat front surface on current product from Elo

• Multi-touch (2) capability from several suppliers now

• Scalable to at least 46”diagonal• Desktop+ sizes cost competitive with other

technologies


– SAW Features, cont’d• Good EMI performance• Vandal resistant--adapts to surface scratches• Finger or soft stylus touch only

– “Point and shoot” touch excellent– Dragging may produce skips—glass finish, finger

moisture, stylus type, etc.

• Some issues with environmental sealing and contamination sensitivity

• Similar to PCAP re optical enhancements—no add on filters, etc., to top surface

Touchscreen Technology Descriptions• Why consider SAW in a new design?

– Mature technology, many suppliers, cost competitive with other high performance touch

– Multi-touch support– Excellent optics, good un-attended survivability– Overall excellent choice for interactive digital signage

and other large format touch displays • Why avoid SAW in a new design?

– Size, integration in small enclosures– Higher relative cost in smaller sizes– Contamination/sealing– Few optical enhancements possible (outdoor)– Possible safety issues without bonding (exposed

glass)

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• APR (Elo) and DST (3M)– Concept: Touch position determined by

analyzing time of flight (DST) or acoustic signature (APR) of bending waves at several piezo transducers attached to the back side of a piece of glass

– APR = “Acoustic Pulse Recognition”– DST=“Dispersive Signal Technology”– These rather similar touchscreens offered as

monitors only, at present, by Elo and 3M.

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• APR (Elo) and DST (3M)– Features

• Plain glass with backside piezo-electric pickups• Excellent optics—no conductive coatings• Very up scalable—large touch monitors available• Works with fingers and most stylii—may be some

issues with rubber gloves• Easy environmental sealing• No touch and hold capability (no signal after initial

touch)

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• Why consider APR or DST in a new design?– Competitive solution to SAW or optical for a

large finished monitor– Should be available as a component solution

in the future– Good optics, good survivability in public

situations • Why avoid APR or DST in a new design?

– Sole source– Touch hold/sensitivity/gloves potential issues

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• Optical Touchscreens—Scanning Infrared– One of the earliest commercial touchscreens– Roots in the PLATO project at Univ of Illinois– Concept: Finger or stylus breaks the path of IR

light between LED and photo transistor

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IR Diode Photo transistor

Touchscreen Technology Descriptions• Optical Touchscreens—Scanning Infrared,

cont’d– Features

• Scalable, but cost increases with size/component count• Few standard designs—most custom• Requires IR transparent bezel over optics• Relatively slow• Stylus independent above minimum size threshold• Resolution improved with better interpolation algorithms,

but still coarse• Near perfect environmental seal when using display lens• Contamination susceptibility (beam blocking)• Now supports multi-touch from some manufacturers• Some new designs decrease component count (RPO)

and improve overall performance/reliability

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• Why consider Scanning IR in a new design?– Excellent optics– Good environmental performance– Stylus independence– Supports multi-touch

• Why avoid Scanning IR in a new design?– Cost—needs large quantity to overcome NRE– Speed, resolution– Some sunlight de-sense issues

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• Optical Touchscreens-Camera-based Infrared– Features

• Many variations on this concept• Fast• Scales easily • Low component count• Potential for multi-touch • Some issues with false touch• Display has to be really flat!• New suppliers showing up, so costs should come down

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• Why consider Camera-based IR in a new design?– Cost vs other large format touch—SAW, DST, APR,

Scanning IR– Performance/speed – Stylus independence– Excellent optics

• Why avoid Camera-based IR in a new design?– False touches– Display flatness– Some contamination issues– Will your supplier be there tomorrow?

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• Pen-based Electromagnetic Digitizers– Concept: Tuned circuit in the pen locally couples energy out of

grid of active wires in the sensor.– Features

• If you must have data entry and/or annotation as well as some “traditional” touch function, this might be the one

• High resolution, good writing fidelity• Pen “hover” function• Easy to seal, so good for harsh environments• Small-medium sizes available• Historically, lots of manufacturers, but most have not survived• High cost• Pens are almost always proprietary and unique, so a lost or failed

pen is at least a minor disaster in the field• Some EMI issues

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• Why consider an electronic digitizer in a new design?– Because you need it for writing, annotating, drawing

or some other special purpose that the other technologies can’t handle

• Why avoid an electronic digitizer in a new design?– Because you don’t need any of this– Will your supplier be there tomorrow?

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Touchscreen Technology Descriptions• And all the other not quite ready for prime time

technologies:– Best of the rest, near term, is probably “digital multi-touch

resistive” if some materials issues with top film durability can be solved.

• There are existing devices, but uncertain reliability• Much cheaper than PCAP• Stylus independent

– Longer term, one of the several “in cell” technologies may be sufficiently matured to consider

• The manufacturers would really like to eliminate add-on touchscreens, allowing them to capture the additional revenue from an in-cell approach

• Will be largely confined to rather small displays at first• Considerable danger that some approaches will fail—don’t be one

of those customers!

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• Conclusions:– Display and touch selection is an interactive process– Still no perfect touchscreen

• Concentrate of the features that really matter– Do sanity checks on your design early

• An app written for mouse control can be ported to a touch computer/display for this purpose

• Carefully consider the application and the user when selecting the touchscreen

– Many more detailed F&B analyses can be found in technical literature– The classic “this touchscreen for this app” tables may be biased, but

usually aren’t too far off– Fixing failed touchscreens in the field is a potential economic disaster– Be wary of the opinions of one technology touchscreen vendors—they

only have one thing to sell you.

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• Conclusions:– For further reading:

• “Information Display” from SID– http://www.informationdisplay.org/pastissue.cfm – Numerous issues featuring touch:

» December 2006» December 2007» March 2010» March 2011

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Display and Touch Technology Choices Put a Touchscreen on That! Design & Manufacturing Midwest 2011...

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