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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.

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.

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Credit: Porter Gifford
Video by Kate Greene

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

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