Touch Screen Principle2

8/12/2019 Touch Screen Principle2

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Primary applications of LCD monitors with touch panels. These devices are used in manywidespread spheres.

While a touch panel requires a wide range of characteristics, including display visibility above all,along with precision in position sensing, rapid response to input, durability, and installation costs,their characteristics differ greatly depending on the methods used to sense touch input. Some typicaltouch-panel sensing methods are discussed below.

Resistive film touch panels

As of 2010, resistive film represented the most widely used sensing method in the touch panelmarket. Touch panels based on this method are called pressure-sensitive or analog-resistive filmtouch panels. In addition to standalone LCD monitors, this technology is used in a wide range ofsmall to mid-sized devices, including smartphones, mobile phones, PDAs, car navigation systems,and the Nintendo DS.

With this method, the position on screen contacted by a finger, stylus, or other object is detectedusing changes in pressure. The monitor features a simple internal structure: a glass screen and afilm screen separated by a narrow gap, each with a transparent electrode film (electrode layer)attached. Pressing the surface of the screen presses the electrodes in the film and the glass to comeinto contact, resulting in the flow of electrical current. The point of contact is identified by detectingthis change in voltage.


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The advantages of this system include the low-cost manufacture, thanks to its simple structure. Thesystem also uses less electricity than other methods, and the resulting configurations are stronglyresistant to dust and water since the surface is covered in film. Since input involves pressure applied

to the film, it can be used for input not just with bare fingers, but even when wearing gloves or usinga stylus. These screens can also be used to input handwritten text.

Drawbacks include lower light transmittance (reduced display quality) due to the film and twoelectrode layers; relatively lower durability and shock resistance; and reduced precision of detectionwith larger screen sizes. (Precision can be maintained in other waysfor example, splitting thescreen into multiple areas for detection.)

Capacitive touch panels

Capacitive touch panels represent the second most widely used sensing method after resistive film

touch panels. Corresponding to the terms used for the above analog resistive touch panels, thesealso are called analog capacitive touch panels. Aside from standalone LCD monitors, these are oftenused in the same devices with resistive film touch panels, such as smartphones and mobile phones.

With this method, the point at which the touch occurs is identified using sensors to sense minorchanges in electrical current generated by contact with a finger or changes in electrostatic capacity(load). Since the sensors react to the static electrical capacity of the human body when a fingerapproaches the screen, they also can be operated in a manner similar to moving a pointer within anarea touched on screen.


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Two types of touch panels use this method: surface capacitive touch panels and projectivecapacitive touch panels. The internal structures differ between the two types.

Surface capacitive touch panels

Surface capacitive touch panels are often used in relatively large panels. Inside these panels, atransparent electrode film (electrode layer) is placed atop a glass substrate, covered by a protectivecover. Electric voltage is applied to electrodes positioned in the four corners of the glass substrate,generating a uniform low-voltage electrical field across the entire panel. The coordinates of theposition at which the finger touches the screen are identified by measuring the resulting changes inelectrostatic capacity at the four corners of the panel.

While this type of capacitive touch panel has a simpler structure than a projected capacitive touchpanel and for this reason offers lower cost, it is structurally difficult to detect contact at two or morepoints at the same time (multi-touch).

Projected capacitive touch panelsProjected capacitive touch panels are often used for smaller screen sizes than surface capacitivetouch panels. They've attracted significant attention in mobile devices. The iPhone, iPod Touch, andiPad use this method to achieve high-precision multi-touch functionality and high response speed.

The internal structure of these touch panels consists of a substrate incorporating an IC chip forprocessing computations, over which is a layer of numerous transparent electrodes is positioned inspecific patterns. The surface is covered with an insulating glass or plastic cover. When a finger


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approaches the surface, electrostatic capacity among multiple electrodes changes simultaneously,and the position were contact occurs can be identified precisely by measuring the ratios betweenthese electrical currents.

A unique characteristic of a projected capacitive touch panel is the fact that the large number ofelectrodes enables accurate detection of contact at multiple points (multi-touch). However, theprojected capacitive touch panels featuring indium-tin-oxide (ITO) found in smartphones and similardevices are poorly suited for use in large screens, since increased screen size results in increasedresistance (i.e., slower transmission of electrical current), increasing the amount of error and noise indetecting the points touched.

Larger touch panels use center-wire projected capacitive touch panels in which very thin electricalwires are laid out in a grid as a transparent electrode layer. While lower resistance makes center-wire projected capacitive touch panels highly sensitive, they are less suited to mass production thanITO etching.

Above, we've summarized the differences between the two types of capacitive touch panels. Theoverall characteristics of such panels include the fact that unlike resistive film touch panels, they donot respond to touch by clothing or standard styli. They feature strong resistance to dust and waterdrops and high durability and scratch resistance. In addition, their light transmittance is higher, ascompared to resistive film touch panels.

On the other hand, these touch panels require either a finger or a special stylus. They cannot beoperated while wearing gloves, and they are susceptible to the effects of nearby metal structures.


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Surface acoustic wave (SAW) touch panels

Surface acoustic wave (SAW) touch panels were developed mainly to address the drawbacks of lowlight transmittance in resistive film touch panelsthat is, to achieve bright touch panels with highlevels of visibility. These are also called surface wave or acoustic wave touch panels. Aside from

standalone LCD monitors, these are widely used in public spaces, in devices like point-of-saleterminals, ATMs, and electronic kiosks.

These panels detect the screen position where contact occurs with a finger or other object using theattenuation in ultrasound elastic waves on the surface. The internal structure of these panels isdesigned so that multiple piezoelectric transducers arranged in the corners of a glass substratetransmit ultrasound surface elastic waves as vibrations in the panel surface, which are received bytransducers installed opposite the transmitting ones. When the screen is touched, ultrasound wavesare absorbed and attenuated by the finger or other object. The location is identified by detectingthese changes. Naturally, the user does not feel these vibrations when touching the screen. Thesepanels offer high ease of use.

The strengths of this type of touch panel include high light transmittance and superior visibility, sincethe structure requires no film or transparent electrodes on the screen. Additionally, the surface glassprovides better durability and scratch resistance than a capacitive touch panel. Another advantage isthat even if the surface does somehow become scratched, the panel remains sensitive to touch. (Ona capacitive touch panel, surface scratches can sometimes interrupt signals.) Structurally, this typeof panel ensures high stability and long service life, free of changes over time or deviations inposition.


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Weak points include compatibility with only fingers and soft objects (such as gloves) that absorbultrasound surface elastic waves. These panels require special-purpose styluses and may react tosubstances like water drops or small insects on the panel.

All in all, however, these touch panels offer relatively few drawbacks. Recent developments such as

improvements in manufacturing technology are also improving their cost-performance.

Optical touch panels (infrared optical imaging touch panels)

The category of optical touch panels includes multiple sensing methods. The number of productsemploying infrared optical imaging touch panels based on infrared image sensors to sense positionthrough triangulation has grown in recent years, chiefly among larger panels.

A touch panel in this category features one infrared LED each at the left and right ends of the top ofthe panel, along with an image sensor (camera). Retroreflective tape that reflects incident light alongthe axis of incidence is affixed along the remaining left, right, and bottom sides. When a finger or

other object touches the screen, the image sensor captures the shadows formed when the infraredlight is blocked. The coordinates of the location of contact are derived by triangulation.

Electromagnetic induction touch panels


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While this type differs somewhat from the above touch panels, let's touch on the subject ofelectromagnetic induction touch panels. This method is used in devices like LCD graphics tablets,tablet PCs, and purikura photo sticker booths.

This input method for graphics tablets, which originally did not feature monitors, achieves high-precision touch panels by combining a sensor with the LCD panel. When the user touches the

screen with a special-purpose stylus that generates a magnetic field, sensors on the panel receivethe electromagnetic energy and use it to sense the position of the pen.

Since a special-purpose stylus is used for input, input using a finger or a general-purpose stylus isnot possible, and the method has limited applications. Still, this has both good and bad points. Iteliminates input errors due to the surrounding environment or unintended screen manipulation.Since the technology was intended for use in graphics tablets, it offers superior sensor precisionmaking it possible, for example, to change line width smoothly by precisely sensing the pressurewith which the stylus is pressed against the screen (electrostatic capacity). This design approachalso gives the screen high light transmittance and durability.

Summary of trends in touch-panel sensing methods

The table below summarizes the characteristics of the touch panels we've looked at. Keep in mindthat even in devices based on the same sensing method, performance and functions can vary widelyin the actual products. Use this information only as an introduction to general product characteristics.

Additionally, given daily advances in touch-panel technological innovations and cost reductions, theinformation below is only a snapshot of current trends as of September 2010.

Differences in and characteristics of main touch-panel sensing methods

Sensingmethod

Resistivefilm

Capacitive SAW Infraredopticalimaging

Electromagneticinduction

Lighttransmittance

Not sogood

Good Good Excellent Excellent

Finger touch Excellent Excellent Excellent Excellent No

Gloved touch Excellent No Good Excellent No

Stylus touch Excellent Not so good(special-purposestylus)

Good(depends onmaterial)

Good(dependson material)

Excellent (special-purpose stylus)

Durability Not sogood

Excellent Excellent Excellent Excellent

Resistance towater drops

Excellent Excellent Not so good Good Excellent

Cost Reasonable Not soreasonable

Reasonable Not soreasonable

Not so reasonable


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Each touch-panel type offers its own strengths and weaknesses. No single sensing method currentlyoffers overwhelming superiority in all aspects. Choose a product after considering the intended useand environmental factors.

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Touch Screen Principle2