How it works: The technology of touch screens

From single-touch to multitouch and why all displays are not equal.

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.

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.

touchscreen

Resistive touch screen technology. © All rights reserved by IntelFreePress

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.

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