How it works: The technology of touch screens

Here's a question: What is a technology that you can't see, but is essential to smartphones, tablets and other mobile devices -- and is estimated to generate $16 billion in revenues this year (according to DisplaySearch)? The answer is multitouch touch screens -- which have sparked the explosive growth of the mobile device market.

Will touchscreens kill the keyboard?

It was not so long ago that we would tap away on a PalmPilot with a tiny stylus, or exercise our thumbs on a BlackBerry micro-keyboard. Then, in January 2007, along came the Apple iPhone, and everything changed. Suddenly, people were wiping their fingers across screens, pinching images and performing other maneuvers that had not previously been part of the smartphone interface.

Now we not only take touch input for granted, we expect to be able to use multitouch (using more than one finger on the screen at a time) and gestures as well. What made this touch screen revolution possible, and where is it likely to take us?

Many paths to touch

To begin with, not all touch is created equal. There are many different touch technologies available to design engineers.

According to touch industry expert Geoff Walker of Walker Mobile, there are 18 distinctly different touch technologies available. Some rely on visible or infrared light; some use sound waves and some use force sensors. They all have individual combinations of advantages and disadvantages, including size, accuracy, reliability, durability, number of touches sensed and -- of course -- cost.

As it turns out, two of these technologies dominate the market for transparent touch technology applied to display screens in mobile devices. And the two approaches have very distinct differences. One requires moving parts, while the other is solid state. One relies on electrical resistance to sense touches, while the other relies on electrical capacitance. One is analog and the other is digital. (Analog approaches measure a change in the value of a signal, such as the voltage, while digital technologies rely on the binary choice between the presence and absence of a signal.) Their respective advantages and disadvantages present clearly different experiences to end users.

Resistive touch

The traditional touch screen technology is analog resistive. Electrical resistance refers to how easily electricity can pass through a material. These panels work by detecting how much the resistance to current changes when a point is touched.

This process is accomplished by having two separate layers. Typically, the bottom layer is made of glass and the top layer is a plastic film. When you push down on the film, it makes contact with the glass and completes a circuit.

The glass and plastic film are each covered with a grid of electrical conductors. These can be fine metal wires, but more often they are made of a thin film of transparent conductor material. In most cases, this material is indium tin oxide (ITO). The electrodes on the two layers run at right angles to each other: parallel conductors run in one direction on the glass sheet and at right angles to those on the plastic film.

When you press down on the touch screen, contact is made between the grid on the glass and the grid on the film. The voltage of the circuit is measured, and the X and Y coordinates of the touch position is calculated based on the amount of resistance at the point of contact.

This analog voltage is processed by analog-to-digital converters (ADC) to create a digital signal that the device's controller can use as an input signal from the user.

What's so special about Gorilla Glass?

Many vendors are quick to trumpet the use of Corning's Gorilla Glass in their products. The glass is used as a protective outer layer for many devices, from smartphones to large flat panel televisions. But what makes Gorilla Glass different?

The answer lies in the composition of the glass itself. Most display glass is an alumina silicate formulation, which is made up of aluminum, silicon, and oxygen. The glass also contains sodium ions spread throughout the material. And this is where the difference starts.

The glass is put in a bath of molten potassium at about 400 degrees. The sodium ions are replaced by potassium ions in a process that's a bit like soaking a pickle in salty brine. It's a diminishing process: More of the sodium ions are replaced by potassium at the surface of the glass, and then fewer and fewer are exchanged as you go further into the glass.

Why change from sodium to potassium? Sodium (Na) has an atomic number of 11, while potassium (K) has an atomic number of 19. If you remember your high school chemistry, this indicates that the potassium atoms are significantly larger than the sodium atoms. (The atomic radius of a neutral sodium atom measures out as 180 picometers and potassium at 220 picometers, so potassium measures out as more than 20% larger.)

Imagine that you have a box packed tightly with tennis balls. What would happen if you took out the top layer of tennis balls and replaced them -- one for one -- with larger softballs? The softball layer would be squeezed together much more tightly and it would be harder to get one out.

That's what happens with glass when the potassium ions take the place of the sodium ions. The potassium ions take up more space and create compression in the glass. This makes it more difficult for a crack to start, and even if one does start, it is much less likely to grow through the glass.

The concept of strengthening glass through ion exchange is not new; it has been known since at least the 1960s. And other companies offer glass that has been strengthened by this type of process. Corning's Gorilla brand of strengthened glass has gained considerable market share, however, and has a very visible presence in the marketplace.

One of the big advantages of resistive touch panels is that they are relatively inexpensive to make. Another is that you can use almost anything to create an input signal: finger tip, fingernail, stylus -- just about anything with a smooth tip. (Sharp tips would damage the film layer.)

This technology has a lot of disadvantages, however. First, the analog system is susceptible to drift, so the user may have to recalibrate the touch panel from time to time. (If you owned a PalmPilot or other PDA, you may remember having to occasionally go through the recalibration process on their PalmPilot.) Next, the ITO material used for the conductors is brittle and not well suited for bending. Over time, repeated use can cause the ITO to crack, which disrupts the flow of electricity and can result in a dead spot on the touch screen.

In addition, there needs to be a gap between the two sensor planes that must be bridged in order to make contact between the two. Just about the only material suitable for this gap is air, but this presents some problems of its own.

First, the gap adds to the combined thickness of the display and touch module. As the consumers demand thinner and thinner devices, a single millimeter can be a big deal.

Another problem has to do with the optical properties of the different layers. If you look at a drinking straw in a glass of water, it will look as though it is slightly bent where it enters the water, even though it is straight. This is because light can bend, or "refract," when it makes the transition from one material to another. If the materials have the same index of refraction, the light won't change its path, but if the index of refraction is different, the light will bend.

The space between the plastic and glass layers of a resistive touch panel is filled with air, and the air has a different index of refraction than the other layers, which makes the light bend as it passes from one layer to another. This can create visible artifacts that can impact the display quality.

The air gap is especially a problem when you view the display under high ambient light conditions, such as outdoors in bright sunlight. The outside light passes through the top layer, then bends when it hits the air gap, and can then reflect between the glass and plastic layers before exiting out the front of the display again. This bouncing light can reduce the image's contrast, making the display look washed out and impossible to see.

But probably the biggest problem with resistive panels in consumers' eyes is that they can sense only one touch at a time. If you touch the panel in two places at once, the combined effect will produce one coordinate for the touch point, and that will be different from either of the two actual points. There are ways to create resistive panels that can sense multiple touches at one time, but these can be expensive and complex, such as creating a matrix of separate contact pads on one of the layers.

Projected capacitance

Fortunately, there's a better way. Many mobile devices now rely on a technology known as "projected capacitance," often referred to in the industry as "p-cap" or "pro-cap." According to various sources, resistive touch has rapidly lost market share to pro-cap and is forecast to continue to decline.

Pro-cap is a solid-state technology, which means that it has no moving parts (unlike the resistive touch technology). Instead of being based on electrical resistance, it relies on electrical capacitance.

When you apply an electrical charge to an object, the charge can build up if there is no place for the electrons to flow. This "holding" of electrons is known as "capacitance." You have probably experienced this effect first-hand. When you walk across a carpet in rubber-soled shoes in the winter time, electrons can build up in your body. If you should then reach for a light switch or some other conductive object that does not have a similar built-up charge, those electrons can flow from your body to the object, producing a spark of electricity.

If you apply a charge to a conductor, and then bring another conductor near it, the second conductor will "steal" some of the charge from the first one, just as the light switch did when your finger approached it. If you know what the charge was to start with, you can tell when the amount of the charge has changed. This is the principle behind pro-cap touch screens.

Early capacitance touch technologies required that you actually touch a conductive layer. This approach left the conductor exposed to wear and damage. Today's projective capacitance technology relies on the fact that an electromagnetic field "projects" above the plane of the conductive sensor layer. You can cover the touch module with a sheet of thin glass, for example, and it will still sense when a conductor comes near.

Pro-cap touch screens use two layers of conductors, separated by an insulator (such as a thin sheet of glass, though other insulating layers can be used). The conductors typically are made of transparent ITO, just as with the resistive designs. The conductor layers never have to bend, however, so its brittle nature is not a problem with pro-cap screens.

The conductors in each layer are separate, so that the capacitance of each one can be measured separately. As with a resistive panel, the conductors run at right angles to each other, so that the device can sense an X and a Y position when touched. The difference is that the separate conductors are scanned in rapid sequence, so that all the possible intersections are measured many times per second.

When you touch the screen with your finger, it steals a little of the charge from each layer of conductors at that point. The electrical charge involved is tiny, which is why you don't feel any shock when you touch the screen, but this little change is enough to be measured. Because each conductor is checked separately, it is possible to identify multiple simultaneous touch points.

Pro-cap technology is not without its challenges. The system of conductors is susceptible to electrical noise from electromagnetic interference (EMI). This can be a problem for display devices such as LCD and OLED panels that rely on an active matrix backplane of transistors to rapidly switch the individual subpixels on and off. The touch screen controller must be able to filter out this background noise and figure out which signals are from actual touch points.

The controller is often asked to make other decisions as well. Comparing results from adjacent coordinates can help determine if the touch is hard or soft, or if it is the result of the palm of the hand resting on the screen and thus should be ignored. Some smartphones rely on the touch screen to signal when the phone is being held next to the user's face, so that the screen can be turned off to save power.

All these tasks require significant processing power, which makes the controller more expensive. In addition, the touch screen only works when you apply a conductor; the ball of your finger works, but not your fingernail. Some pro-cap screens will work even if you're wearing thin surgical gloves, but they won't work if you have thick winter gloves on. (The exception is if the gloves themselves are conductive; you can buy gloves with conductors woven into the fingertips so that they can conduct the charge from the screen to your finger.)

In spite of these shortcomings, pro-cap technology has become the dominant choice for mobile devices. And there are improvements on the way that could make them even better.

Can't be too thin or too light

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