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Devices might harvest energy from the air

Monday, September 04, 2000

By Byron Spice, Science Editor, Post-Gazette

We can't see them, we can't feel them, and, except for those of us with CIA-implanted chips in our heads, we can't hear them.

Yet radio waves are all around us, emitted by a growing menagerie of devices, from navigation radars and TV broadcast transmitters to garage-door openers and wireless Internet devices.

But this is more than just a sea of static, say researchers at the University of Pittsburgh. These ambient radio waves could be a source of electricity.

By harvesting the power that is available all around us, Pitt engineers hope to develop electronic sensors, keys, tags and perhaps even simple computers that are not only wireless, but battery-free.

The amount of power that can be skimmed off the radio wave sea might be relatively small, acknowledged Marlin Mickle, a professor of electrical engineering. But as computer chips and other electronic devices have shrunk, so have their power demands. Harvesting just a volt or maybe two out of thin air might be sufficient for many applications.

"It's a very cool technology," said Phil Weilerstein, director of the National Collegiate Inventors and Innovators Alliance, which has funded some of the early Pitt research. It could become a key piece of what's known as the smart environment -- small sensors and processors that communicate with each other to autonomously perform tasks.

For instance, Mickle and his Pitt colleagues are working on a "keyless key," that might be worn on a watchband. The device would automatically open locked doors for people with proper authorization -- no keys, no swiping of magnetic-strip cards, no waving an electronic smart card near a sensor. It might eventually replace the key chain fobs used for remote-controlled door locks on cars; a car's owner might simply walk up to her locked car, open the door and push an ignition button.

Sensors embedded in concrete decks and pilings might stand sentry over the life of a bridge, providing civil engineers with information about stress. Thermometers or other sensors that must operate in corrosive environments could be embedded in epoxy resin, with no need for protruding wires. Home smoke alarms might charge themselves and never fail because of an old battery.

"It's a wide open area," Mickle said. "I'm having a ball."

It all began in 1996 as a search for lost hearing aids.

Mickle was teaching a project course, which challenged a class of junior and senior students to develop innovative and sometimes off-the-wall technological solutions to problems. By speaking with the operator of a Mt. Lebanon nursing home, Mickle learned that misplaced or dropped hearing aids represent a common nuisance and a major expense. So he suggested that his students concoct a way to find hearing aids that might be lost in the laundry or on a dinner tray.

The students obviously couldn't use a radioactive tag on something that must be worn by patients, and they didn't want to use a battery because of fears of interference with the hearing aid electronics. So they opted for a signaling device that normally would be without power but could be charged remotely when the hearing aid was lost.

What the students had in mind was something akin to the charging system used on some electric toothbrushes -- the toothbrush handle is placed in a holder, which uses electric coils to induce a current inside the handle to recharge its battery. But they needed to figure out how to induce those currents over a greater distance.

Mickle, 64, suggested a slightly different approach, one based on an invention that predated them all: the crystal radio set.

Crystal radios don't use a power supply. They get all their power from the radio signals themselves, which are captured with a very long antenna wire and passed through the set as an electrical current.

They were popular during radio's infancy and are so simple to make that they remained a favorite of young hobbyists through the 1960s. Mickle was 11 years old when he made his own crystal radio, using a cigar box, a cylindrical snuff can wrapped with wire and a hunk of galena crystal.

Some enthusiasts claim they have trickle-charged car batteries by hooking them up to their crystal sets.

Mickle's students came up with three or four versions of the hearing-aid locator. At the end of the course, several students expressed an interest in continuing the work and finding other purposes for radio waves besides listening to Howard Stern or Jim Krenn.

Mickle recruited some reinforcements for the project, including Kevin Wells, a 35-year-old electrical engineer at Marconi Communications who has been working on his bachelor's degree for the past 10 years. After Mickle picked his brain a bit, Wells did some experiments at home.

Wells would eventually build the first Active Remote Sensor, a palm-size circuit board that held a computer chip, an antenna, a transmitter and a pair of switches. The device gathered ambient radio frequency energy and converted it to direct current to operate the computer chip. The device could communicate the position of the switches to a remote computer, proving that ambient RF energy could be used to power off-the-shelf electronic components.

The sensor was later displayed at the Smithsonian Institute in 1999 as part of a March Madness for the Mind program sponsored by the National Collegiate Inventors and Innovators Alliance.

Later devices that used the Active Remote Sensor, or ARS, technology replaced the original's whip antenna with four flat wire coils, and its switches with a temperature-sensing device called a thermistor. Mickle and his cohorts tried embedding the devices inside epoxy resin, building a network of devices for detecting temperature and eventually a relay network to extend the range of transmissions.

In addition to funding from the inventors alliance, Mickle has obtained sponsors at the Pittsburgh Digital Greenhouse, the Defense Advanced Research Projects Agency and the Defense Logistics Agency. Pitt attorneys have filed for several patents on ARS technology.

"Now we're beginning to play more seriously," Mickle said.

One goal is to shrink the devices so they can fit on a computer chip no bigger than two or three cubic millimeters. But first the Pitt faculty and student researchers must determine how small they can make the antennas and figure out how much energy can practically be harvested. They are now working on an energy farm with four square antennas, each measuring 1.2 millimeters on each side.

"We don't know whether it's going to work," Mickle said, explaining that the antennas are far smaller than those used in cell phones or for almost any other application. "The antenna experts say this is a whole new ballgame."

The amount of power that can be harvested will determine how useful the ARS technology will be. In addition to powering electronic devices, Mickle said the RF energy farms might play a role in the infant field of nanotechnology, powering microscopic machines.

Wells, listed as a co-inventor with Mickle on the ARS patent application, said the technology could improve upon existing radio-frequency identification tags, which have been replacing bar codes for some applications. Passive radio-frequency ID tags emit identification numbers only when stimulated by radio waves and have limited range and information capacity. Active versions are more capable, but require a power source.

The ARS technology would provide greater range and would allow more on-board storage and processing of information.

ARS-enabled tags might record not only the serial number and manufacture date of a piece of equipment, but also its maintenance and ownership history.

Wells, still a few hours short of his bachelor's degree in electrical engineering, sees his future tied to ARS technology.

"I'm really hoping the patents come through soon," he said, "so I can go to work for a company pumping these things out."

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