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A New Approach to RFID

University of Pittsburgh professor Marlin Mickle has developed a novel approach to RFID. His PENI tag "harvests" energy to transmit back a unique ID, which improves performance.
Jul 15, 2002—July 15, 2002 - Marlin Mickle wasn't interested in radio frequency identification, or any other form of auto-identification for that matter. As a professor at the University of Pittsburgh's School of Engineering, he was developing automatic remote sensing devices that could operate without batteries or wires and form ad hoc wireless networks. But he may have stumbled onto an approach that could transform radio frequency identification.

The PENI tag
Mickle's work was closely related to RFID. His sensors needed to gather power from the air and transmit information wirelessly, just as RFID tags do. But his sensors required more power than a typical RFID tag could muster, which led him to adopt a novel approach to RFID -- energy harvesting.

Most passive RFID tags – those without a battery to power their circuitry -- gather energy by coupling to the reader's communication field. That is, the coiled reader antenna sends out radio waves. When the waves reach the coiled antenna on the tag, they form an electromagnetic field. The tag draws power from this field.

The tag sends back data passively, usually by a method known as backscatter. Tags that use backscatter take the incoming waveform and reflect back a modulated wave. The modulated wave has to pass through all the energy coming from the reader, so the reader has to be very sensitive to pick it up. The system is inefficient. It would be like one boat communicating by flashing a light, and a second boat responding by using a mirror to reflect back what little light hits the mirror.

Backscatter has been used in RFID tags for decades. It traces its roots back to the dawn of radio frequency identification, when British radar installations sent signals that reflected off special paint on Royal Air Force planes. That enabled gunners on the ground to distinguish between friendly planes and German bombers.

The problem with this approach is that a lot of energy is wasted. So Mickle and his colleagues began exploring the possibility of having two antennas on their sensors. An antenna tuned to one frequency would harvest as much energy as it could from the electromagnetic field. The other would use the energy to actively transmit back at another frequency, much like a battery-powered tag. The result: a better signal-to-noise ratio and longer read distances.

"Our problem was we were measuring temperature, pressure, humidity, things like that," Mickle says. "We needed more energy than you could get with backscatter. So we had to evolve our own mechanism for converting energy. As a consequence, we have figured out how to gather more energy."

Mickle and his group at the university have also developed a way of building the antennas directly into standard complementary metallic oxide semiconductor (CMOS) chips. His tags can hold a unique serial number and be used for identification. And, he says, they can be made today for about nine and a half cents. Now, some of the largest retailers and consumer packaged goods companies in the world are knocking on his door.
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