Smart home sensors use electrical wiring as an antenna
By Darren Quick
September 16, 2010
Smart homes of the future will automatically adapt to their surroundings using an array of sensors to record everything from the building’s temperature and humidity to the light level and air quality. One hurdle impeding the development of such intelligent homes is the fact that existing technology is still power hungry and today’s wireless devices either transmit a signal only several feet, or consume so much energy they need frequent battery replacements. Researchers have now developed sensors that run on extremely low power thanks to using a home’s electrical wiring as a giant antenna to transmit information.
The technology devised by researchers at the University of Washington and the Georgia Institute of Technology uses a home’s copper electrical wiring as a giant antenna to receive wireless signals at a set frequency, allowing for wireless sensors that run for decades on a single watch battery. A low-power sensor placed within 10 to 15 feet of electrical wiring can use the antenna to send data to a single base station plugged in anywhere in the home.
The device is called Sensor Nodes Utilizing Powerline Infrastructure, or SNUPI. It originated when Shwetak Patel, a UW assistant professor of computer science and of electrical engineering, and co-author Erich Stuntebeck were doctoral students at Georgia Tech and worked with thesis adviser Gregory Abowd to develop a method using electrical wiring to receive wireless signals in a home. They discovered that home wiring is a remarkably efficient antenna at 27 megahertz. Since then, Patel's team at the UW has built the actual sensors and refined this method.
"Here, we can imagine this having an out-of-the-box experience where the device already has a battery in it, and it's ready to go and run for many years," Patel said. Users could easily sprinkle dozens of sensors throughout the home, even behind walls or in hard-to-reach places like attics or crawl spaces.
In testing the system sensors were placed in five locations in each room of a 3,000-square-foot house. The team found that only five percent of the house was out of range, compared to 23 percent when using over-the-air communication at the same power level. They also discovered that the sensors can transmit near bathtubs because the electrical grounding wire is typically tied to the copper plumbing pipes, that a lamp cord plugged into an outlet acts as part of the antenna, and that outdoor wiring can extend the sensors' range outside the home.
While traditional wireless systems can have trouble sending signals through walls, this system actually does better around walls that contain electrical wiring. Even more impressive is the fact that, according to the researchers, SNUPI uses less than one percent of the power for data transmission compared to the next most efficient model.
"Existing nodes consumed the vast majority of their power, more than 90 percent, in wireless communication," said lead student researcher, Gabe Cohn. "We've flipped that. Most of our power is consumed in the computation, because we made the power for wireless communication almost negligible."
The existing prototype uses UW-built custom electronics and consumes less than 1 milliwatt of power when transmitting, with less than 10 percent of that devoted to communication. Depending on the attached sensor, the device could run continuously for 50 years, much longer than the decade-long shelf life of its battery.
"Basically, the battery will start to decompose before it runs out of power," Patel said.
The team suggests longer-term applications might consider using more costly medical-grade batteries, which have a longer shelf life. The team is also looking to reduce the power consumption even further so no battery would be needed. They say they're already near the point where solar energy or body motion could provide enough energy. The researchers are commercializing the base technology, which they believe could be used as a platform for a variety of sensing systems.
The technology, which could be used in home automation or medical monitoring, doesn’t interfere with electricity flow or with other emerging systems that use electrical wiring to transmit Ethernet signal between devices plugged into two outlets.
It will be presented this month at the Ubiquitous Computing conference in Copenhagen, Denmark.
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