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Researchers create wireless sensor chip the size of glitter

sensor processing chip
UC Berkeley researchers have developed a new wireless mote that integrates radio frequency communication onto a sensor processing chip, shown above. The sensor chip measures a mere 5 square millimeters. (Jason Hill)

– Engineers at the University of California, Berkeley, have successfully tested a wireless sensor chip so small that if someone were to sneeze, it just might blow away. The new "smart dust" chip integrates sensors and transmitters onto a platform that measures a mere 5 square millimeters, or slightly bigger than a fleck of glitter.

The chip will be the brains behind the new generation mote, dubbed "Spec" by its creators. It's a significant milestone in the Smart Dust and TinyOS projects, which are part of UC Berkeley's Center for Information Technology Research in the Interest of Society (CITRIS). The projects seek to create low-powered, low-cost wireless sensor devices, or motes, roughly the size of a grain of sand.

Massive numbers of these millimeter-scaled motes could be used in self-organizing wireless sensor networks for such innovative applications as monitoring seabird nests in remote habitats, pinpointing structural weaknesses in a building after an earthquake, or warning of the presence of biochemical toxins.

"Spec is our first mote to integrate radio frequency communication and custom circuits onto a chip that runs the TinyOS operating system," said Kris Pister, professor of electrical engineering and computer sciences and the pioneer behind the smart dust project. "It's a major step towards sensors that cost less than a dollar a piece and that are integrated into the products that we own, the buildings that we live and work in, and the freeways we drive on. The potential for such sensor networks is enormous."

The Spec mote brings together years of Smart Dust and TinyOS research led by Pister and David Culler, professor of computer sciences. While Pister led research into miniaturizing the hardware components for Smart Dust, Culler and his research team developed the TinyOS operating system that allows the mini-motes to communicate with one another.

Their work led to generations of wireless sensor motes slightly bigger than pocket change. But the breakthrough came when Jason Hill, a former graduate student who worked with Culler on TinyOS and Pister, successfully integrated the hardware and software components onto a silicon chip small enough to fit under the wings of the eagle on the back of a quarter.

What is remarkable about Spec is the range of components the UC Berkeley researchers were able to fit onto a single chip and its system-driven architecture. Spec combines a micro-radio, an analog-to-digital converter, a temperature sensor, and the TinyOS operating system onto a piece of silicon 2 by 2.5 millimeters square.

"This is the first fully integrated and fully operational mote on an individual chip, and its design is driven by the networking and system requirements," said Culler. "Single chip integration makes the mote very cheap because it reduces post-assembly requirements. This opens the path to very low cost deployments of a large number of motes."

Researchers tested the new chip at the Intel Research Laboratory in Berkeley. Spec was able to transmit radio signals at 902 megahertz over 40 feet at 19200 kilobytes per second.

"We were in a lab environment with a lot of high-power equipment that generates interference," said Hill, who received his PhD in electrical engineering and computer sciences from UC Berkeley in May. "If we went outdoors and had direct line of sight between the two motes, we could realistically transmit about twice the distance we did indoors."

Hill said a drastic reduction in power consumption of components allowed the sensor mote to be shrunk down. Spec includes special hardware accelerators to help the core pieces of TinyOS run more efficiently. For example, the accelerators allowed data encryption to be performed by the hardware, which is thousands of times more efficient than performing the same function in software. In addition, the radio transmitter uses 1,000 times less power than a cell phone.

Another key development was the reduction in power of the micro-radio. Al Molnar, a doctoral student in the Department of Electrical Engineering and Computer Sciences, developed the ultra-low powered radio by distributing the energy - in the form of electrons - more efficiently.

Compared to traditional radio transmitters that can transmit dozens of watts over thousands of feet, this micro-radio only transmits a few hundred microwatts over dozens of feet, far enough to communicate with the nearest sensor mote in the system. The transmitter requires so little power that excess energy is left over. Instead of dissipating, however, the extra energy is diverted to power other components of the radio, reducing overall power consumption.

The fundamental principle behind smart dust is to enable wireless sensor motes to talk to other nearby motes rather than pump up the power of a single mote so that it can transmit through hundreds of feet of building space.

Why the focus on power? Simply put, lower power means smaller batteries. For instance, the two AA batteries used to power the Mica mote account for the bulk of its size.

"As the volume of battery goes down, the energy it contains decreases," explained Hill. "The major hurdle in miniaturizing the motes was the power. One of the huge advantages of this chip over prior generations of motes is a 30-fold reduction in total power consumption."

In its commercial form, Spec will be hundreds of times smaller than its predecessor, Mica, a mote available commercially through San Jose-based Crossbow Technology, Inc. In addition to the chip, Spec still requires an inductor, an antenna, a 32 kilohertz watch crystal and a power source. The researchers said that these components will add little to the size of the mote. Companies are already developing millimeter-scale batteries that could power a commercial version of Spec, they said.

"We're not that far off from getting a mote that is 2.5 cubic millimeters for the complete device," said Hill. "The technological time required to accomplish that would be anywhere from months to a year away."

In addition to its potential for academic and consumer use, smart dust has generated interest from the military for its potential uses on the battlefield or to monitor troop movements. Two years ago, dozens of Rene motes, the precursor to Mica, were dropped from a plane alongside a road in a test at Twentynine Palms, Calif. The matchbox-sized motes successfully tracked the speed and direction of passing vehicles based upon the vibrations in the ground.

Pister, who has taken an industrial leave of absence this year to launch a startup based upon Smart Dust, said he plans to develop Spec into a commercial product within the next year. He said the finished product, which will include the battery and casing, will likely be the size of an aspirin.

The research was funded by a Network Embedded Systems Technology grant from the Defense Advanced Research Projects Agency (DARPA).

Other researchers on this project include UC Berkeley graduate students Ben Cook, Mike Scott and Brett Warneke.

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