Wireless sensors scattered throughout a building can monitor everything from humidity and temperature to air quality and light levels. This seems like a good idea–until you consider the hassle and cost of replacing the sensors’ batteries every couple of years. The problem is that most wireless sensors transmit data in a way that drains battery power.
Researchers at the University of Washington have come up with a way to reduce the amount of power a sensor uses to transmit data by leveraging the electrical wiring in a building’s walls as an antenna that propagates the signal. The approach extends a wireless sensor’s range, and it means that its battery can last up to five times longer than existing sensors, say the researchers.
The technology, called Sensor Nodes Utilizing Powerline Infrastructure (SNUPI), sends a small trickle of data wirelessly at a frequency that resonates with the copper wiring in a building’s walls, says Shwetak Patel, professor of computer science and electrical engineering at the University of Washington. The copper wiring, which can be up to 15 feet away from the sensors, picks up the signal and acts as a giant receiving antenna, transmitting the data at 27 megahertz to a base station plugged into an electrical outlet somewhere in the building.
“The powerline has an amplification effect,” says Patel. While many low-power sensors only have a range of a few feet, he says, his prototype sensors can cover most of a 3,000-square-foot home. In most wireless sensor schemes, Patel says, walls impede transmission of sensor data, but with SNUPI, “the more walls in the home, the better our system works.” A paper describing the work will be presented at the Ubiquitous Computing conference in Copenhagen, Denmark, in September.
“Most academic research on in-building sensor nodes has looked at building infrastructure as a problem,” says Matt Reynolds, professor of electrical and computer engineering at Duke University. Patel’s work is interesting because it “turns the problem on its head,” he says. “The building’s wiring is part of the solution rather than part of the problem.”
Using powerlines to transmit data is not a new idea. Broadband over powerlines, or BPL, uses the power grid to provide Internet connectivity. But using powerlines to extend the range of ambient sensors, and reduce their power consumption, is novel.
The researchers’ prototype uses less than one milliwatt of power when transmitting data to the powerline antenna, and less than 10 percent of that power is used for communication. Future versions, says Patel, will reduce the amount of power the sensor uses for computation, and will also include a receiving antenna for two-way communication between the sensors and the base station. This could enable the sensor to accept confirmation that all of the data has been received properly.
Patel, who founded an in-home energy-monitoring startup called Zenzi that was sold to Belkin earlier this year, has launched another company to commercialize SNUPI. He suspects that the approach can be used for more than monitoring air quality in homes–it could also be used to collect data from wearable sensors or implanted medical devices. In fact, Patel says, preliminary studies have shown that the popular pedometer called FitBit, which sends data to a base station wirelessly, could last for a year on a single charge, rather than its current duration of 14 days, using the SNUPI scheme.