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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 reg-
ister 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 resolu-
tion 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.
Today's prototypes are an opaque black, so
they're unsuitable as touch-screen inter-
faces 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 sev-
eral 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 indi-
vidual sensors to collect such data, but they
are too expensive to use over a large area.
The technology is also useful in multi-
touch 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 gad-
get, e ectively adding an ergonomic touch
input: users can control the game without
having their fingers block the screen. And
Rosenberg believes that by using a di erent
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 resis-
tance-sensing devices, says Andy Wilson,
a Microsoft researcher who developed Sur-
face, 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
di cult 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 tech-
nology and provide design assistance to com-
panies that want to build it into devices such
as mobile phones and e-readers. The compa-
ny's engineers are exploring additional appli-
cations---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 sur-
face, opening up a new dimension of multi-
4. The plastic sheets come in many shapes and sizes: one the size of a credit card could be used for small
handheld electronics, while circular pads are designed for electronic drum pads.
5. Rosenberg presses down on a completed pad with his fingers; software on the computer interprets the
range of pressures that he s exerting and displays them on the screen. The intense pressure near the center
of his fingertips shows up as red, while the lighter pressure around the edges is blue.
6. The team has developed several applications for the pressure-sensitive pad, including this demo pro-
gram, which allows users to manipulate a globe and alter its surface by pressing down on the pad.
To see these touch pads in
action take a snapshot of this
code with your smart phone (for
instructions, see p. 25) or visit
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