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Software on the computer calculates both the position of objects that contact the pad and the amount of pressure they exert. If an object touches at the inter­section of two conductive lines, the electronics register a strong current there; but the farther away from the intersection it touches, the weaker the current, owing to the resistivity of the ink. Prototypes already have resolution high enough to accurately sense finger and stylus input for tablet PCs. For a single touch, it can record forces from five grams to five kilograms with a 2.5 percent margin of error–enough range to interpret the light tap of a stylus or a strike on a digital drum. Perlin says that because so few of the wires need to be powered, larger versions of the pad can achieve similar sensitivity without much more complexity or cost.

Market Pressure
Today’s prototypes are an opaque black, so they’re unsuitable as touch-screen interfaces for cell phones and other electronic gadgets. But such a precise and inexpensive pressure-sensitive interface still has many potential uses, Perlin says.

For instance, Rosenberg and Perlin have collaborated with other researchers on several medical and scientific applications. ­Perlin says the pad could be added to shoes to monitor gait and to hospital beds to alert nurses when a patient has been still for too long, increasing the risk of pressure sores. The pad is even sensitive enough to measure pressure waves in water and air; this could lead to better fluid-dynamics models that might help with designing airplanes and boats. Today, researchers use arrays of individual sensors to collect such data, but they are too expensive to use over a large area.

The technology is also useful in multi­touch interfaces for electronic devices. Patric­k Baudisch, a researcher at the Hasso Plattner Institute in Germany, has integrated the pad onto the back of a small gaming gadget, effectively adding an ergonomic touch input: users can control the game without having their fingers block the screen. And Rosenberg believes that by using a different type of pressure-sensitive ink and making the lines thinner, he and his colleagues can build a transparent sensor usable in touch screens on mobile phones and tablet PCs.

Rosenberg and Perlin’s touch pad is much more sensitive than other resistance-sensing devices, says Andy Wilson, a Microsoft researcher who developed Surface, a commercially available multitouch table. “Many of the applications focus on using the pressure sensor in interesting ways,” he says. He adds, however, that the technology is still in its early stages, and it’s difficult to say how much cheaper it will be than today’s touch interfaces.

In April, Rosenberg and Perlin launched Touchco, a startup that will license the technology and provide design assistance to companies that want to build it into devices such as mobile phones and e-reader­s. The company’s engineers are exploring additional applications–such as the first electronic hand drum, which would be impossible without a sensor capable of such fine resolution.

Eventually, these thin, unobtrusive touch pads could be built into virtually any surface, opening up a new dimension of multi­touch interaction.

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Credit: Porter Gifford
Video by Kate Greene

Tagged: Computing, touch screen, plastic, plastic films, pressure-sensitive

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