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Technology Brief 18Touchscreens and Active Digitizers

Touchscreen is the common name given to a wide varietyof technologies that allow computer displays to directlysense information from the user. In older systems, thisusually meant the display could detect and pinpoint wherea user touched the screen surface; newer systems candetect multiple touch locations as well as the associatedtouch pressures simultaneously, with very high resolution.This has led to a surge of applications in mobile comput-ing, cell phones, personal digital assistants (PDA), andconsumer appliances. Interactive touchscreens which de-tect multiple touches and interact with styli are now com-monly used in phones, tablet computers and e-readers.

Numerous technologies have been developed sincethe invention of the electronic touch interface in 1971 bySamuel C. Hurst. Some of the earlier technologies weresusceptible to dust, damage from repeat use, and poortransparency. These issues largely have been resolvedover the years (even for older technologies) as experienceand advanced material selection have led to improveddevices. With the explosion of consumer interest inportable, interactive electronics, newer technologies haveemerged that are more suitable for these applications.Figure TF18-1 summarizes the general categories oftouchscreens in use today. Historically, touchscreenswere manufactured separately from displays and addedas an extra layer of the display.More recently, display com-panies have begun to manufacture sensing technologydirectly into the displays; some of the newer technologiesreflect this.

Resistive

Resistive touchscreens are perhaps the simplest tounderstand. A thin, flexible membrane is separated froma plastic base by insulating spacers. Both the thinmembrane and the plastic base are coated on the insidewith a transparent conductive film (indium tin oxide (ITO)often is used). When the membrane is touched, the twoconductive surfaces come into contact. Detector circuitsat the edges of the screen can detect this change inresistance between the two membranes and pinpoint thelocation on the X–Y plane. Older designs of this type weresusceptible to membrane damage (from repeated flexing)and suffered from poor transparency.

Capacitive

Older capacitive touchscreens employ a single thin,transparent conductive film (usually indium tin oxide(ITO)) on a plastic or glass base. The conductive filmis coated with another thin, transparent insulator forprotection. Since the human body stores charge, a fingertip moved close to the surface of the film effectively formsa capacitor where the film acts as one of the plates andthe finger as the other. The protective coating and the airform the intervening dielectric insulator. This capacitivecoupling changes how a current flowing across the filmsurface is distributed; by placing electrodes at the screencorners and applying an ac electric signal, the location ofthe finger capacitance can be calculated precisely. Onevariant of this idea is to divide the sensing area into manysmaller squares (just like pixels on the display) and tosense the change in capacitance across each of themcontinuously and independently; this is commonly knownas self-capacitance sensing. A newer development,found in many modern portable devices, is the useof mutual capacitance sensing touchscreens, whichemploy two sets of conductive lines, each on a differentlayer. On one layer, the lines might run horizontally, whileon another layer below the first the lines run vertically. Ateach point of overlap between the lines on the two layers,a parallel plate capacitor is formed. If there are M lineson the top layer and N lines on the bottom, there will beM×N such nodes.Whenever a finger moves near a node,the capacitance of the node changes. By monitoring thecapacitance of each node continuously, the touchscreencan detect when touches occur and where. The principaladvantages of a touchscreen of this type are its abilityto detect many simultaneous touches and its ability todetect very light ones. Capacitive technologies are muchmore resistant to wear and tear (since they are not flexed)than resistive touchscreen and are somewhat more trans-parent (> 85 percent transparency) since they can havefewer films and avoid air gaps. These types of screenscan be used to detect metal objects as well, so pens withconductive tips can be used on writing interfaces.

Not all capacitive touch systems are integrated withscreens; a number of interactive media technologiesdeveloped over the last 15 years integrate the touchsensing technology into furniture, household objects, oreven countertops and overlay a display using nearbyprojection equipment. Some interactive tables operatethis way. A completely different way to detect touch relieson the measurement of acoustic energy on or near thetouchscreen. There are several ways to make use of

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394 TECHNOLOGY BRIEF 18: TOUCHSCREENS AND ACTIVE DIGITIZERS

(a) Resistive (b) Capacitive

(c) Pressure (d) Acoustic

(e) Infrared (f) Active digitizer

Spacer

MembraneConductive film

Plastic

Membrane

Cfinger

Conductive film

Plastic

Force

Stress Stress

Strainsensor

Strainsensor

Acousticsensor

Acousticemitter

5 MHz Acoustic wave

Acousticdampening

Screen

IR beam

Screen

DetectorLED

Electromagneticradiation

Pen

Wires

LCR

Figure TF18-1: Touchscreen technologies: (a) resistive, (b) capacitive, (c) pressure/strain sensor, (d) acoustic, (e) infrared,and (f) active digitizer.

acoustic energy to measure touch. One implementationrelies on transmission of high-frequency acoustic energyacross the surface of the display material.

PressureTouch also can be detected mechanically. Pressuresensors can be placed at the corners of the display screen

or even the entire display assembly, so whenever thescreen is depressed, the four corners will experiencedifferent stresses depending on the (X,Y) position ofthe pressure point. Pressure screens benefit from highresistance to wear and tear and no losses in transparency(since there is no need to add layers over the displayscreen).

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AcousticA completely different way to detect touch relies on thetransmission of high-frequency acoustic energy acrossthe surface of the display material. Bursts of 5 MHz tonesare launched by acoustic actuators from two corners ofthe screen. Acoustic reflectors all along the edges of thescreen re-direct the incoming waves to the sensors. Anytime an object comes into contact with the screen, itdampens or absorbs some fraction of the energy travelingacross the material. The exact (X,Y) position can becalculated from the energy hitting the acoustic sensors.The contact force can be calculated as well, because theacoustic energy is dampened more or less depending onhow hard the screen is pressed.

Another approach is to listen, with very sensitiveacoustic transducers (i.e., microphones) to the char-acteristic pressure signal (e.g., sound) made in thetouchscreen material when it is touched. By placingseveral transducers around the edge of the screen, thesystem can determine if a touch occurred and where.Onedrawback is that motionless fingers cannot be detected.However, this does provide an advantage in that restingobjects (i.e., your cheek) do not trigger the screen.This method is sometimes known as acoustic pulserecognition.

InfraredOne of the oldest and least used technologies is theinfrared touchscreen. This technology relies on infraredemitters (usually infrared diodes) aligned along twoadjoining edges of the screen and infrared detectorsaligned across from the emitters at the other two edges.The position of a touch event can be determined througha process based on which light paths are interrupted.The detection of multiple simultaneous touch events ispossible. Infrared screens are somewhat bulky, prone todamage or interference from dust and debris, and needspecial modifications to work in daylight.They largely havebeen displaced by newer technologies.

Electromagnetic ResonanceAnother technology in widespread use is the electromag-netic resonance detection scheme used by many tabletPCs. Strictly speaking, many tablet PC screens are nottouchscreens; they are called active digitizers becausethey can detect the presence and location of the tabletpen as it approaches the screen (even without contact).In this scheme, a very thin wire grid is integrated within

the display screen (which usually is a flat-profile LCDdisplay). The pen itself contains a simple RLC resonator(see Section 6-1) with no power supply. The wire gridalternates between two modes (transmit and receive)every ∼ 20 milliseconds. The grid essentially acts as anantenna.During the transmit mode, an ac signal is appliedto the grid and part of that signal is emitted into the airaround the display. As the pen approaches the grid, someenergy from the grid travels across to the pen’s resonatorwhich begins to oscillate. In receive mode, the grid is usedto “listen” for ac signals at the resonator frequency; if thosesignals are present, the grid can pinpoint where they areacross the screen. A tuning fork provides a good analogy.Imagine a surface vibrating at a musical note; if a tuningfork designed to vibrate at that note comes very close tothat surface, it will begin to oscillate at the same frequency.Even if we were to stop the surface vibrations, the tuningfork will continue to make a sound for a little while longer(as the resonance dies down). In a similar way, the laptopscreen continuously transmits a signal and listens forthe pen’s electromagnetic resonance. Functions (such asbuttons and pressure information) can be added to thepen by having the buttons change the capacitance valueof the LCR when pressed; in this way, the resonancefrequency will shift (see Section 6-2), and the shift canbe detected by the grid and interpreted as a button press.

Increased Integration

Mobile devices have largely driven the development ofadvanced touch technologies in the last few years. Giventhe constant pressure to miniaturize and integrate, anumber of companies have or are developing integratedtouch and display systems. Unlike the earlier-generationtechnologies, the display and the touch sensor arenot manufactured separately and then integrated duringassembly. Rather, the touch sensor conductors (in thecase of capacitive sensing) are designed into the verydisplay itself, either into the conductive traces in/on thepixels of the display or immediately over them. In otherdesigns, light-sensing pixels are manufactured into eachdisplay pixel of a display, giving the display not only theability to produce images but also to sense nearby objectsthat occlude light landing on the sensing pixels. Eventhe integrated circuits are increasingly being integrated;earlier-generation systems relied on stand-alone touchcontroller IC chips that managed the sensor informationand communicated it to the application processor inthe mobile devices. There is a push to integrate thisfunctionality into some phone processors directly.