sensors without batteries

Eugen Leitl eugen at leitl.org
Sun May 21 03:14:43 PDT 2006


http://www.technologyreview.com/read_article.aspx?id=16864&ch=infotech

Sensors Without Batteries

In the future, the environment could be pervaded by sensors using the same
power-scavenging techniques as RFID tags.

By Kate Greene

Some technologists believe that in the future, seemingly invisible computers
will be embedded everywhere, collecting data about the environment and making
it useful to decision makers. One way to achieve this sort of ubiquitous
computing is to disperse tiny sensors that measure, for instance, light,
temperature, or motion.

But without a persistent power source, such sensors would need their batteries
replaced every few months. In other words, ubiquitous sensors could also mean
"ubiquitous dead batteries," says Josh Smith, a researcher at Intel Research
in Seattle.

Smith and his team are addressing this problem not by working on
longer-lasting batteries but by trying to eliminate the need for batteries
altogether. Instead, their prototype devices employ the same power-scavenging
technique used by battery-free radio frequency identification (RFID) tags.

The concept of throwing out the sensor battery is not new. Researchers have
proposed capturing energy from environmental vibrations or ambient light to
power a sensor (see "Free Electricity from Nano Generators"). But it is
unclear whether technology that captures ambient energy can be inexpensively
integrated into a sensing device.

By contrast, the technology used in RFID tags, which transmit a few bits of
information when scanned by an RFID reader, is cheap enough to integrate into
sensors and be mass produced; they're already widely used to track livestock
and cargo, as well as cars passing through "easy pass" lanes on highways.

Smith explains that Intel's sensor devices use off-the-shelf components: an
antenna to send and receive data and collect energy from a reader, and a
sensor-containing microcontroller -- a tiny computer that requires only a
couple hundred microwatts of power to collect and process data.

The antenna harvests this power directly from the radio waves emitted by an
RFID reader. When a tag comes within range of a reader, the reader's radio
signal passes through the antenna, generating a voltage that activates the
tag. The tag is then able to send information to the reader through a process
called backscattering, in which the antenna essentially reflects a
data-encoded variation of the received radio signal.

The microcontroller that Smith's team added to the RFID antenna includes a
16-bit microprocessor, 8 kilobytes of flash storage, and 256 bytes of
random-access memory.

One of the microcontroller's main jobs is to ensure that information is
transmitted to the reader error-free, which requires more computation than a
conventional RFID tag can handle. In a typical tag, the error-checking
information is precomputed and stored on the chip; but for a sensor, Smith
says, this information needs to be computed in real-time as data is gathered.

Just like RFID tags, the battery-free sensors turn on only when they encounter
a reader. As long as the RFID reader is within range of the device, Smith
says, it can collect data and send it to the reader.

Battery-free sensors could be useful in many areas, including medicine, says
Zeke Mejia, chief technology officer of St. Paul-based Digital Angel, an RFID
tag maker. They could "check the status and certain conditions in the body" at
any moment, Mejia says, from glucose levels in people with diabetes to the pH
of blood and other body fluids.

In their current form, Intel's sensors need to be within about a meter of a
reader to be activated. That's closer than would be ideal for some
applications, such as measuring the temperature of foods packed in large
crates or vibrations in thick walls. The problem is that while the
microcontroller needs only a milliwatt of power to run, it needs three volts
of electricity to turn on, and the sensor has to be within a meter of an
industry-standard RFID reader to generate that much energy. But with minor
changes to the way the microcontroller processes data, Smith says, the group
could reduce the voltage requirement to 1.8 volts, thus extending the range to
about five meters.

The team's latest prototype incorporates a light sensor, temperature sensor,
and even a tilt sensor into one battery-free device. The researchers are
working on ways to integrate the microcontroller and antenna into a single
chip that would be easier to install in the field. In the meantime, they have
developed a visual demonstration of just how much energy an RFID antenna can
garner from a reader: they've used it to power the second hand on a
wristwatch.

"It's surprising to people that this invisible form of energy b- radio waves
-b can actually make a watch hand move," Smith says. And a single tick of a
second hand, Smith says, takes about as much energy as sending one bit of data
from his sensor.


--
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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