A review of technologies for sensing contact location on the surface of a display
`
`Geoff Walker (SID Member)
`
`Abstract — Touchscreen interactive devices have become increasingly important in both consumer and
`commercial applications. This paper provides a broad overview of all touchscreen technologies in use
`today, organized into 13 categories with 38 variations. The 13 categories are projected capacitive, analog
`resistive, surface capacitive, surface acoustic wave, infrared, camera-based optical, liquid crystal display
`in-cell, bending wave, force sensing, planar scatter detection, vision-based, electromagnetic resonance,
`and combinations of technologies. The information provided on each touchscreen technology includes a
`little history, some basic theory of operation, the most common applications, the key advantages and
`disadvantages, a few current issues or trends, and the author’s opinion of the future outlook for the
`technology. Because of its dominance, this paper begins with projected capacitive; more information is
`provided on this technology than on any of the other touch technologies that are discussed. This paper
`covers only technologies that operate by contact with a display screen; this excludes technologies such
`as 3D gesture recognition, touch on opaque devices such as interactive whiteboards, and proximity
`sensing. This is not a highly technical paper; it sacrifices depth of information on any one technology for
`breadth of information on multiple technologies.
`
`Keywords — touch technologies; touchscreen; touch panel; projected capacitive; in-cell; on-cell.
`
`DOI # 10.1002/jsid.100
`
`1
`
`Introduction
`
`1.1
`
`Context
`
`Touchscreen interactive devices have become increasingly
`important in both consumer and commercial applications,
`with over one billion touchscreens shipped in 2011.1 This
`paper provides a broad overview of all touchscreen technolo-
`gies in use today, organized into 13 categories with a total of
`38 variations. The information provided on each touch
`technology includes a little history, some basic theory of
`operation, the most common applications, the key advantages
`and disadvantages, a few current issues or trends, and the
`author’s opinion of the future outlook for the technology. This
`paper covers only technologies that operate by contact with a
`display screen; this excludes technologies such as three-
`dimensional
`(3D) gesture recognition,
`touch on opaque
`devices
`such as
`interactive whiteboards, and proximity
`sensing. This is not a highly technical paper; it sacrifices depth
`of technical information on any one technology for breadth of
`information on multiple technologies. In this paper (and
`throughout this issue of Journal of the Society for Information
`Display), the terms “touchscreen” and “touch panel” are
`synonymous; both refer to a module consisting of a touch
`sensor and a touch controller (the former term is more
`commonly used in the West, whereas the latter term is more
`commonly used in Asia). Also in this paper, projected capacitive
`touch technology is often abbreviated as “p-cap.” The touch
`industry has not yet settled on a single term for p-cap
`technology; it is also called “Pro-Cap,” “PCT” (p-cap touch
`[or] technology), and increasingly, just “capacitive,” as surface-
`capacitive technology becomes ever less relevant.
`
`shown in Fig. 1, analog resistive and p-cap touch
`As
`technologies dominate the touch landscape today. Together
`they accounted for more than 80% of revenue and 95% of units
`shipped in 2011. Resistive was historically always the largest
`technology in both revenue and units, but p-cap overtook
`resistive in revenue in 2010 and in units in 2011 1. Because of
`this dominance, this paper begins with p-cap; more information
`is provided on this technology than on any of the other touch
`technologies that are discussed.
`
`2
`
`Projected capacitive (p-cap)
`
`Worldwide sales of p-cap were less than $20m in 2006,
`growing to over $7b in 2011. More than 95% of the $7b was
`in the consumer electronics market, with more than 75% in
`smartphones and tablets. Contrary to popular belief, Apple
`did not invent p-cap (or multi-touch!). The history of p-cap
`is less clear than that of many other touch technologies. The
`basic concept of sensing touch by measuring a change in
`capacitance has been known since at least the 1960s. In fact,
`the first transparent touchscreen, invented in 1965 for use
`on air-traffic system-control terminals in the UK, used a form
`of capacitive sensing.2 Surface-capacitance touch technology
`(with an unpatterned sensor) was commercialized by Micro-
`Touch Systems around 1985. During the mid-1990s, several
`US companies developed transparent capacitive touchscreens
`
`Received 05/15/12; accepted 06/27/12.
`The author is with Walker Mobile, LLC, 799 Valencia Dr, Milpitas, CA 95035, USA; telephone 1-408-945-1221; e-mail: geoff@walkermobile.com.
`© Copyright 2012 Society for Information Display 1071-0922/12/2008-0100$1.00
`
`Journal of the SID 20/8, 2012
`
`413
`
`

`
`% of Units Shipped
`100%
`2%
`2%
`
`90%
`
`80%
`
`70%
`
`60%
`
`50%
`
`40%
`
`30%
`
`20%
`
`10%
`
`0%
`
`74%
`
`85%
`
`24%
`
`13%
`
`3%
`2%
`
`53%
`
`42%
`
`4%
`5%
`
`33%
`
`4%
`6%
`
`26%
`
`4%
`7%
`
`20%
`
`4%
`7%
`
`17%
`
`4%
`8%
`
`14%
`
`4%
`1%
`9%
`
`12%
`
`4%
`1%
`10%
`
`11%
`
`58%
`
`64%
`
`69%
`
`72%
`
`74%
`
`74%
`
`74%
`
`Others
`In-Cell
`On-Cell
`Resistive
`P-Cap
`
`2008A
`2009A
`2010A
`2011E
`2012E
`2013E
`2014E
`2015E
`2016E
`2017E
`FIGURE 1 — The touch world is already well into a transition from analog resistive (red) to
`projected capacitive (blue) as the dominant touch technology. The figure combines the
`opinions of an Asian investment bank (Guoxin Securities), the world’s largest touchscreen
`supplier (TPK) and world’s number one touch market research firm (DisplaySearch). Source:
`Guoxin Securities, TPK, and DisplaySearch.
`
`pair of electrodes, the capacitance of the human body to ground
`“steals” some of the charge between two electrodes, thus
`reducing the capacitance between the electrodes.4
`
`with patterned sensors by using indium tin oxide (ITO, the foun-
`dation of today’s p-cap). Two of these were Dynapro Thin Films
`and MicroTouch Systems; both of which were later acquired by
`3M (in 2000 and 2001, respectively) to form 3M Touch Systems.
`Dynapro Thin Films’ p-cap touchscreen technology, known as
`“Near-Field Imaging,” became 3M’s first p-cap product in
`2001. Also in 1994, an individual inventor in the UK named
`Ronald Peter Binstead developed a form of p-cap by using
`microfine (25 micron) wire as the sensing electrode.3 He
`licensed the technology to two UK companies: Zytronic in
`1998 and Visual Planet in 2003; both are still selling it today.
`P-cap remained a little-known niche technology until
`Apple used it in the first iPhone in 2007. Apple’s engaging
`and immersive user interface was an instant hit, causing most
`other smartphone manufacturers to immediately adopt the
`technology. Over the next 5 years, p-cap sets a new standard
`for the desirable characteristics of touch in the minds of more
`than one billion consumers, as follows:
`(cid:129) Multiple simultaneous touches (“multi-touch” for zoom)
`(cid:129) Extremely
`light
`touch with flick/swipe gestures
`(no pressure required)
`(cid:129) Flush touch surface (“zero bezel”)
`(cid:129) Excellent optical performance
`(cid:129) Extremely smooth and fast scrolling
`(cid:129) Reliable and durable
`(cid:129) Fully integrated into the device user experience so that
`using it is effortless and fun
`
`P-cap fundamentals
`2.1
`There are two basic kinds of p-cap: self-capacitance and
`mutual capacitance. Both are illustrated in Fig. 2. Self-
`capacitance is based on measuring the capacitance of a “single”
`electrode with respect to ground. When a finger is near the
`electrode, the capacitance of the human body increases the
`self-capacitance of the electrode with respect to ground. In
`contrast, mutual capacitance is based on measuring the capaci-
`tance between a “pair” of electrodes. When a finger is near the
`
`FIGURE 2 — Self-capacitance (a) is the capacitance of a single electrode
`to ground. When a finger is near the electrode, human body capacitance to
`ground “increases” the total self-capacitance of
`the electrode. Mutual
`capacitance (b) is the capacitance between two electrodes. When a finger
`is near the electrodes, it “steals” some charge from the drive electrode,
`“reducing” the mutual capacitance between the two electrodes. In an X–Y
`self-capacitance grid (c,
`left), each row and column electrode is
`scanned individually. If the sensor is touched with two fingers that are
`diagonally separated, the controller sees two maximums on each axis, but
`cannot tell which pair of maximums is the real touch points. In an X–Y
`mutual-capacitance grid (c, right), each electrode intersection is scanned
`individually, allowing multiple touch points to be unambiguously identified.
`Source: 3M and Touch International; redrawn by the author.
`
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`
`Walker / Review of touch technologies
`
`

`
`Although it seems that the difference between self and mutual
`capacitance could be determined by the number of electrodes,
`the key difference is actually in how the electrodes are measured.
`Regardless of how they are configured, the electrodes in a self-
`capacitance touchscreen are measured individually, one at a time.
`For example, even if the electrodes are configured in a two-layer
`X–Y matrix, all the X-electrodes are measured, and then all the Y-
`electrodes are measured in sequence. If a single finger is touch-
`ing the screen, the result is that the nearest X-electrode and the
`nearest Y-electrode will both be detected as having maximum ca-
`pacitance. However, as shown in Fig. 2(c), if the screen is touched
`with two or more fingers that are diagonally separated, there will
`be multiple maximums on each axis, and “ghost” touch points will
`be detected as well as “real” touch points (ghost points are false
`touches positionally related to real touches). Note that this disad-
`vantage does not eliminate the possibility of using two-finger ges-
`tures on a self-capacitive touchscreen. Rather than using the
`ambiguous “location” of the reported points, software can use
`the “direction of movement” of the points. In this situation, it does
`not matter that four points resulted from two touches; as long as
`pairs are moving toward or away from each other (for example), a
`zoom gesture can be recognized. For this reason and because
`self-capacitance can be of lower cost than mutual capacitance,
`the former is often used on lower-capability mobile phones.
`In contrast,
`in a mutual-capacitive touchscreen, each
`electrode “intersection” is measured individually. Generally, this
`is accomplished by driving a single X-electrode, measuring each
`Y (intersecting) electrode, and then repeating the process until
`all the X-electrodes have been driven. This measurement
`methodology allows the controller to unambiguously identify
`every touch point on the touchscreen. Because of its ability to
`correctly process multiple touch points (moving or not), mutual
`capacitance is used in preference to self-capacitance in most
`smartphones and tablets today.
`
`P-cap controllers
`2.2
`In every case, the measurement of electrode capacitance
`is accomplished by a touch controller. Figure 3 illustrates the
`basic structure of a controller for a mutual-capacitance
`touchscreen. A sensor driver excites each X-electrode one at
`a time. An analog front-end measures the capacitance at the
`intersection of each Y-electrode and the excited X-electrode;
`the analog values are converted to digital by an analog-to-
`
`Cmutual
`
`Analog-
`to-Digital
`Converter
`(ADC)
`
`Digital
`Signal
`Processor
`(DSP)
`
`Analog Front-
`End (AFE)
`
`Sensor Driver
`
`Touch Controller
`Touch Sensor
`FIGURE 3 — A projected capacitive touch controller consists of only four
`main elements: a sensor driver to excite the drive electrodes, an analog front-
`end (AFE) to read the sense electrodes, an analog-to-digital converter (ADC),
`and a digital signal processor (DSP). Source: Maxim Integrated Products.
`
`digital converter. A digital signal processor runs highly sophis-
`ticated algorithms to process the array of digital capacitance
`data and convert it into touch locations and areas, along with
`a variety of related processing such as “grip suppression” (the
`elimination of undesired touches near the edge of the screen
`resulting from holding a device) and “palm rejection” (the
`elimination of unintended touches resulting from the edge or
`base of your palm contacting the screen in the process of
`touching with a finger). A p-cap touch controller is an
`example of an application-specific integrated circuit (ASIC).5
`Controllers are where most of the innovation is happening in
`p-cap today, although the geometry of the sensor pattern is also
`an ongoing contributor to performance improvement. The top
`three controller suppliers (Atmel, Cypress, and Synaptics, who
`together accounted for more than half of the p-cap controller
`unit shipments in 2011) are all US-based companies.6 This could
`be taken as a sign of the relative youth of the p-cap controller
`industry because most system-level ASICs eventually become
`commoditized with suppliers based in Asia. An example of recent
`p-cap controller innovation is the significant increase in touch
`system signal-to-noise ratio (SNR) that has occurred during the
`last 18 months. The value of this innovation is that is allows
`p-cap touchscreens to support an active or passive stylus with a
`1-mm tip, rather than just a human finger. Multiple p-cap
`controller suppliers have demonstrated or talked about this
`capability with regard to their latest controllers, although there
`has not been enough time for it to show up in consumer
`electronic products on the shelf yet.4,7
`A fine-tipped stylus adds a large amount of value to a
`smartphone or tablet. It allows the user to “create” data
`(drawings, notes, etc.) rather than just “consume” media. In
`Asia,
`it is highly desirable to write Kanji characters on a
`smartphone, and finger writing is impractical because the tip
`of your finger obscures what you are writing. A fine-tipped
`stylus is also excellent as a pointing device for use with software
`that was not designed for touch (e.g., legacy Windows applica-
`tions running on a Windows 8 tablet in “desktop” mode).
`
`P-cap sensors
`2.3
`A p-cap sensor is at heart a set of transparent conductive electro-
`des used by the controller to determine touch locations. In self-
`capacitance touchscreens, transparent conductors are patterned
`into spatially separated electrodes in either a single layer or two
`layers. When the electrodes are in a single layer, each electrode
`represents a different touch coordinate pair and is connected in-
`dividually to a controller. When the electrodes are in two layers,
`they are usually arranged in a layer of rows and a layer of col-
`umns. The intersection of each row and column represents
`unique touch coordinate pairs; however, as noted in the previous
`section, in self-capacitance, each electrode is measured individu-
`ally rather than measuring each intersection with other electro-
`des, so the multi-touch capability of this configuration is limited.
`In a mutual-capacitance touchscreen, there are almost
`always two sets of spatially separated electrodes. In higher-
`performance touchscreens (such as that in the iPhone), the
`
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`

`
`electrodes are usually arranged in a rectilinear grid of rows
`and columns, spatially separated by an insulating layer or a
`film or glass substrate. In contrast, the most commonly used
`electrode pattern is an interlocking diamond consisting of
`
`angle, connected at two corners via a small
`squares on a 45
`bridge. When this pattern is used on two spatially separated
`layers, the processing of each layer is straightforward. How-
`ever, this pattern is often applied in a single “coplanar” layer
`to achieve the thinnest possible touchscreen. In this case, the
`bridges require additional processing steps to (1) insulate the
`first ITO bridge before depositing the second (intersecting)
`ITO bridge or (2) omit
`the second ITO bridge during
`deposition and replace it with a metal “microcrossover” bridge.
`Figure 4 illustrates the stack-up of a typical mutual-
`capacitance touchscreen. To keep this and all similar drawings
`in this paper as easy to understand as possible, several
`simplifications have been made, as follows. (1) The electrode
`pattern shown (rows 3 and 5) is a rectilinear grid rather than
`the more common interlocking diamond; row 3 shows the end
`views of the Y-electrodes, whereas row 5 shows a side view of
`one X-electrode. (2) The common use of optically clear
`adhesive has been omitted; for example, the space between
`rows 2 and 3 is typically filled with optically clear adhesive. (3)
`The touchscreen is shown using a glass substrate; in lower-end
`mobile phones, the substrate is often two layers of polyethylene
`terephthalate (PET) film, one for each set of electrodes. (4) All
`the layers below the thin film transistor (TFT)-array glass in the
`liquid crystal display (LCD) (e.g., bottom polarizer, brightness
`enhancement films, and backlight) have been omitted.
`One of the key points made in Fig. 4 is that the touchscreen
`adds a fourth sheet of glass to the stack-up. All LCDs use two
`sheets of glass, and essentially, every mobile device adds a third
`sheet of glass (or Poly(methyl methacrylate) (PMMA)) as a pro-
`tective and decorative covering over the LCD. Adding a fourth
`sheet of glass is generally considered to be undesirable because
`it adds weight, thickness, and cost to the mobile device. There
`are two basic methods of eliminating the fourth sheet of glass:
`(1) the method used by the touchscreen industry, called “one-
`glass solution,” “sensor on lens,” or a variety of company-specific
`
`names, and (2) the method used by the LCD industry, called
`“on-cell touch.” These methods are in direct competition.
`Figure 5 illustrates the one-glass solution,
`in which the
`touchscreen electrodes are moved to the underside of the dec-
`orated cover glass (“lens”).8 In this solution, the touchscreen
`manufacturer either purchases the decorated cover glass from
`an appropriate supplier or vertically integrates and acquires
`the equipment and skills necessary to manufacture the cover
`glass. The touchscreen manufacturer then builds the touch
`module (sensor plus controller) by using the decorated cover
`glass as a substrate and sells the entire assembly to a mobile de-
`vice Original Equipment Manufacturer/Original Design Manu-
`facturer (OEM/ODM) (as is often the case, the touchscreen
`manufacturer may also obtain the LCD on consignment from
`the device OEM/ODM and integrate the touchscreen module
`with the LCD). The advantage of the one-glass solution to the
`end user is that the mobile device is lighter and thinner because
`of the elimination of the fourth piece of glass. The advantage of
`the one-glass solution to the touchscreen manufacturer is that
`they continue to derive revenue from the production of
`touchscreens instead of forfeiting revenue to the LCD industry.
`Figure 6 illustrates the on-cell touch solution, in which the
`fourth piece of glass is eliminated by moving the touchscreen
`
`FIGURE 5 — This figure depicts the p-cap “one-glass solution” (also called
`“sensor on lens”) configuration used by the touchscreen industry. To
`eliminate the fourth piece of glass, the p-cap electrodes are moved to the
`bottom surface of the decorated cover glass (rows 3–5). ITO, indium tin
`oxide; TFT, thin film transistor. Source: the author.
`
`FIGURE 4 — All smartphones and tablets use some form of “decorated
`covering” (rows 1 and 2) to protect the LCD (rows 6–11) from damage.
`When a projected capacitive touchscreen is added, most commonly, the
`electrodes are located on a fourth piece of glass (rows 3–5). ITO, indium
`tin oxide; TFT, thin film transistor. Source: the author.
`
`FIGURE 6 — In the on-cell touch sensor configuration used by the liquid
`crystal display industry, the fourth piece of glass is eliminated by moving
`the p-cap electrodes to the top of the color filter glass, underneath the
`top polarizer (rows 4–6). The touch functionality is exactly the same as in
`Figure 5. ITO, indium tin oxide; TFT, thin film transistor. Source: the author.
`
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`Walker / Review of touch technologies
`
`

`
`electrodes to the top of the color filter glass, underneath the
`LCD’s top polarizer. Note that an on-cell configuration is stan-
`dard p-cap with exactly the same functionality as in Figs. 3 and
`4; only the location of the electrodes is different. The advantage
`of the on-cell solution to the end user is exactly the same as the
`one-glass solution—the mobile device is lighter and thinner be-
`cause of the elimination of the fourth piece of glass. The advan-
`tage of the on-cell solution to the LCD manufacturer is that it
`increases their revenue because of the added value of touch
`functionality (but the touchscreen manufacturer loses revenue).
`One other factor in on-cell’s favor is that with the touch
`sensor integrated into the LCD, it makes sense to consider
`integrating the touch controller and the display driver
`together into a single ASIC or at least establishing a direct
`connection between the two chips to enable cooperation.
`Manufacturing yield can be more of an issue with on-cell
`because depositing the electrodes on the top surface of the
`color filter glass substantially increases the value of that one
`piece of glass; if either the color filter or the touch electrode
`deposition is defective, both must be discarded. Product-line
`management is also an issue for the LCD manufacturer—for
`example, should every LCD be designed with on-cell touch
`included or only some models? Should there be two versions
`of a high-volume LCD, one with on-cell and one without?
`It should be clear from the aforementioned that on-cell
`touch is not necessarily an automatically better solution than
`one-glass. There are factors to be considered on both sides,
`and some of those factors are more business-related and
`operational-related than technical. Competition between
`touch module manufacturers and LCD manufacturers will
`remain a major factor in the progression of on-cell. The author
`believes that on-cell will achieve only limited success in the next
`5 years, accounting for no more than 10%–15% of all p-cap
`touch in consumer electronics applications and much less
`(if any) in commercial applications.
`
`ITO-replacement materials for p-cap
`2.4
`sensors
`ITO-replacement materials eliminate the need for vacuum
`sputtering; patterning of ITO-replacement materials can be
`carried out at room temperature in a normal atmosphere
`without the need for an expensive fab. This is potentially a
`highly disruptive technology.
`Because of the fine resolution required in creating the
`pattern (e.g., 20-micron-wide ITO conductors) and the
`relatively large number of electrode connections that must fit
`in a very narrow space at the edge of the touchscreen, most
`glass-based sensors are patterned using photolithography on a
`fabrication plant (“fab”). There are three basic sources of fabs:
`(1) converted from LCD color filter fabs, (2) converted from
`passive LCD fabs, and (3) purpose built. Existing p-cap fabs
`were expanded at a very rapid rate in 2011; the author estimates
`that the total capital expenditures (“capex”) spent by the p-cap
`touch industry in 2011 was around $2b. The necessity of
`
`creating the sensor on a fab contributes substantially to the high
`cost of a p-cap touchscreen today. For example, a glass touch-
`screen module for a 10-in Android tablet in high volume
`currently costs the device OEM/ODM around $25 for the
`sensor, whereas the controller is typically under $5 (this does
`not include the cost of the cover glass and lamination).
`There are at least five different materials competing to
`become the dominant ITO-replacement material, including
`copper metal mesh, silver nanowires, carbon nanotubes,
`conductive polymers, and ITO inks. In the author’s opinion,
`the material with the most market traction so far is metal
`mesh. Two examples of companies working with metal mesh
`include Atmel and Unipixel. Atmel recently announced their
`XSenseTM sensor film; it uses a metal mesh printed roll-to-roll
`on film.9 Atmel’s partner
`for
`the mesh and printing
`equipment
`is Conductive Inks Technology in the UK.
`Because the transparent conductor is metal, the material’s
`sheet resistance is very low (less than 10 ohms/square and in
`some cases, as low as 0.6 ohms/square). This provides increased
`noise immunity and helps support both active and passive styli.
`Unipixel has been working for several years on its UniBoss
`copper metal mesh with a conductor size of 5 microns
`(invisible). The mesh can be printed roll-to-roll in a single pass
`at
`room temperature; Unipixel appears
`to be nearing
`production readiness.10 In fact, scuttlebutt within the touch in-
`dustry in June 2012 indicates that Unipixel is already providing
`(under NDA) small quantities of metal mesh for production of
`32-in p-cap touchscreens.
`Silver nanowires are a close second behind metal mesh.
`The leading supplier of
`this material
`is Cambrios;
`the
`optical and electrical properties (transmissivity and sheet
`resistance) of Cambrios’ material are highly competitive
`with ITO. The material has been used by Synaptics in the
`first non-ITO p-cap touchscreen used in a smartphone
`(Samsung’s CricKetTM brand, sold only in Asia).11 This is
`much more important than it may seem; it is the beginning
`of direct competition for capital-intensive, high-cost p-cap
`sensor manufacturing.
`3M is an example of a company working with both silver
`nanowires and metal mesh. 3M is planning to combine
`their well-known microreplication process with a solution-
`processable metal mesh or silver-nanowire material to create
`roll-to-roll printed p-cap sensors on film that can be laminated
`to glass. A joint venture between 3M and Quanta has been
`launched in Singapore to market 3M’s p-cap sensors to the
`consumer electronics OEM/ODM manufacturing tablet and
`larger products (but not smartphones).12
`The author believes that within 5 years, metal mesh and/or
`silver nanowires will be used in up to half of all tablet-sized
`and larger p-cap sensors because it will substantially reduce
`the cost of sensor production. This will put intense pressure
`on the owners of p-cap fabs, particularly those who specialize
`in larger touchscreens. If they cannot compete, many of those
`p-cap fabs will either become idle or be converted to some
`other use—similar to what happened to passive LCD fabs
`when TFT LCDs became dominant.
`
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`

`
`P-cap made with 10-micron wire instead
`2.5
`of ITO
`There are currently two forms of p-cap that are made with
`10-micron wire instead of ITO. These are (1) self-capacitive,
`supporting one or two touches, and (2) mutual capacitive,
`supporting 10+ touches with palm rejection. Both of these
`forms are available on glass or plastic.
`The self-capacitive form of wire-based p-cap has been on the
`market more than 10 years; it works by measuring a change in
`radio frequency (RF) signal frequency caused by the addition
`of human body capacity to an electrode (Binstead’s IP) rather
`than directly measuring a change in the capacitance of the
`electrode. The best known supplier of this form of p-cap is
`Zytronic in the UK; their products have typically been glass
`based in 5-in to 15-in sizes and used in commercial applications
`such as Automatic Teller Machine (ATM) machines and point-
`of-sale terminals. In large-format applications, the best known
`supplier is Visual Planet (also in the UK); their products have
`typically been film based in 40-in to 100-in sizes and used in
`“through store-window” applications, where closed retailers en-
`gage potential customers outside of business hours by letting
`them interact with (for example) a product selection application
`through the store’s windows. The significant visibility of the
`rather widely spaced wire pattern has always been somewhat
`of an impediment to this technology, although in applications
`where the viewing time is very short (such as in ATM machines),
`it is less of a problem.
`The mutual-capacitive form of wire-based p-cap was
`introduced to the market in June 2012 by Zytronic.13 It uses
`the more common technique of directly measuring the change
`in capacitance between electrodes rather than the RF-based
`technique used in the older self-capacitive products. The
`mutual-capacitive wire pattern is much denser than the self-
`capacitive version, consisting of 6 6 mm cells containing wires
`spaced about 1.5 mm apart. The wires in each cell cross (as
`expected in mutual capacitive) without problems because the
`wires are insulated. Because of its much higher density, the
`mutual-capacitive wire pattern is much harder to see (less
`visible)
`than the self-capacitive wire pattern. This lower
`visibility, along with the highly flexible automation that Zytronic
`has applied to the process of creating these touchscreens in any
`size up to 72 in (and larger later), portends a longer life than
`expected for the technology.
`
`3
`
`Analog resistive
`
`The invention of analog resistive touchscreens is generally
`attributed to Elographics (now Elo TouchSystems) in 1971.14
`The original resistive technology was used in an opaque pen dig-
`itizer; it was not until 1977 that a transparent version (curved to
`fit the face of a CRT monitor) was developed. There is some
`possibility that Sierracin/Intrex’s four-wire analog resistive
`touchscreen may actually predate Elo’s transparent version
`
`418
`
`Walker / Review of touch technologies
`
`because Sierracin/Intrex first started selling ITO-coated PET
`film in 1973.15 In any case, at 35 years, resistive is the oldest
`touch technology currently in mass production.
`An analog resistive touchscreen is simply a mechanical
`switch mechanism used to locate a touch. The construction
`of a typical resistive touchscreen is shown in Fig. 7. A glass
`substrate and a flexible film (usually PET) are both coated on
`one side with the transparent conductor ITO. With the two
`coated sides facing each other, the two conductive surfaces
`are separated by very small, transparent, insulating spacer dots.
`A voltage is applied across one or both of the sheets (depending
`on the type of resistive touchscreen). When a finger presses on
`the flexible film, the two conductive surfaces make electrical
`contact. The resistance of the ITO creates a voltage divider at
`the contact point; the ratio of the voltages is used to calculate
`the touch position.
`
`Analog resistive variations
`3.1
`Resistive touch technology has three key variations: (1) the
`number of “wires,” (2) the layer construction, and (3) the
`options. The number of wires refers to the number of
`connections to the sensor; the three common types are four-
`wire, five-wire, and eight-wire.
`In a four-wire touchscreen (shown in Fig. 8), connections
`are made to bus bars on the left and right (X) edges of
`one conductive sheet, and bus bars on the top and bottom (Y)
`edges of the other. To determine the X position of the touch,
`the controller applies a voltage across the X connections and
`measures the voltage at one of the Y connections. The
`controller then reverses the process, applying voltage across
`
`FIGURE 7 — An analog resistive touchscreen is simply a mechanical switch
`mechanism used to locate a touch. Two conductive layers are separated by
`tiny insulating spacer dots; when the two layers are pressed together, an
`electrical contact is made. The touch location is calculated from the ratio of
`voltages on the conductive layers. Source: Elo TouchSystems.
`
`

`
`FIGURE 8 — In a four-wire touchscreen, a voltage gradient is applied between the two X-axis
`bus bars on the glass, and the resulting voltage is measured on the coversheet. Then, the
`voltage gradient is applied between the two Y-axis bus bars on the coversheet, and the
`resulting voltage is measured on the glass. Source: the author.
`
`the Y connections and measuring the voltage at one of the X
`connections to determine the Y location.16
`In a five-wire touchscreen (shown in Fig. 9), the X and Y
`voltages are applied to the four corners of
`the lower
`conductive sheet, and the upper sheet is used only as a
`contact point (wiper). To determine the X position, the
`controller applies a voltage to the two right-hand X-axis
`corners and grounds the two left-hand X-axis corners. The
`coversheet (the fifth wire) is used as a voltage probe to
`measure the X position. The controller then reverses the
`process, applying a voltage to the top two Y-axis contacts and
`grounding the bottom two Y-axis connections. Again, the
`coversheet is used as a voltage probe to measure the Y position.
`A five-wire touchscreen is always ready for a touch; when
`waiting for a touch, the four corners are driven with the same
`voltage, and the coversheet
`is grounded through a high
`resistance. When there is no touch on the screen, the voltage
`on the coversheet is zero. When the screen is touched