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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