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A Touch of Ingenuity

An inexpensive pressure-sensitive pad could make surfaces smarter.
August 18, 2009

Now that more and more smart phones and MP3 players have touch-screen interfaces, people have grown accustomed to interacting with gadgets using only taps and swipes of their fingers. But on the 11th floor of a downtown Manhattan building, New York University researchers Ilya Rosenberg and Ken Perlin are developing an interface that goes even further. It’s a thin pad that responds precisely to pressure from not only a finger but a range of objects, such as a foot, a stylus, or a drumstick. And it can sense multiple inputs at once.

Ken Perlin (left), a professor of computer science at NYU, and Ilya Rosenberg, an NYU graduate student, show off the plastic sheets that are the starting point for their pressure-sensitive touch pads.

The idea for the pad occurred to Rosenber­g, a graduate student at NYU, a few years ago when he was working with a conductive polymer called force-­sensing resistor ink, which is often used in electronic music keyboards. When pressure is applied to the ink, its molecules reorient themselves in a way that alters its electrical resistance, which is easy to measure. Rosenber­g originally used the ink to create sensors that could be embedded under tennis­-court boundaries to automate line calls, but he wondered if it might be the basis of a good multi­touch interface for computers. He began collaborating with Perlin, a professor in NYU’s Media Research Laboratory, to make a pressure-sensitive touch pad to replace a computer mouse.

Pressure-sensitive pads have existed for years, but most have been limited to ­simple applications, such as sensing when a car seat is occupied. Devices like the ­Palm­ Pilot, which use a stylus to input data, typically detect touch by measuring changes in electrical resistance when an object taps the screen. But these screens can register only a single touch at a time. Touch screens on smart phones, meanwhile, use a sensor that detects changes in capacitance, or the material’s ability to hold an electric charge; capacitance changes when objects containing water–including fingers–move across the screen. Such screens can sense multiple touches, but they can’t detect pressure.

Rosenberg and Perlin’s touch pad, by contrast, combines some advantages of all these technologies. It can simultaneously register the pressure and location of several touches, and it can be simply and inexpensively shrunk to the size of a pendant or scaled up to cover a tabletop.

Painted Plastic
To build a pressure-sensitive touch pad, Rosenberg starts with sheets of plastic slightly thicker than a piece of paper. He uses a special program to design a pattern of lines that will be printed on each sheet, tailoring the pattern to the device’s intended use. The lines are laid down on the plastic in metal to make them electrically conductive; the sheet is then covered with an even coat of the black pressure-sensitive ink. In bulk, the printed sensors would cost about $100 per square meter, but since these letter-sized prototypes are one-offs, each one is about $100.

Rosenberg places two of the prepared sheets against each other with the polymer ink side facing in, orienting them so that the conductive lines create a grid. Then he sticks the sheets together with double-sided tape. Every sixth metal line terminates at one edge of the plastic sheets in a short, flexible tail that is connected to a rigid circuit board by a clamp. Though the rest of the wires are not connected to electronics, they influence the electrical characteristics of the active lines, which helps software infer where a touch is coming from.

The circuit board itself contains a microchip programmed to scan the sensor pad, supplying power to each active wire in quick succession. The chip also converts the pressure data from a continuous analog signal into a digital format that a computer can interpret. Finally, it compresses the data and sends it to a computer via a USB connection or (for musical applications) a MIDI port.

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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