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Intel, University Researchers Develop Power-Storing Passive Tags
Intel and University of Washington scientists have created RFID tags that accumulate energy from ambient RF signals, allowing them to record sensor data when out of range of a reader.
Feb 18, 2009—It might sound like science fiction, but Joshua Smith, a principal engineer at Intel Research Seattle, and his team of scientists from Intel and the University of Washington are working to develop passive RFID-enabled sensors that operate on power pulled out of thin air. Stated more precisely, the tags would harvest ambient RF power—that is, already existing RF emissions transmitted by any number of sources, including television, radio and cell phone base stations.
In an experiment, the team mounted a prototype sensor tag on an outside balcony at the Seattle lab. The prototype included a power harvester consisting of a printed circuit board approximately 2 inches by 4 inches in size, wired to a set-top television antenna—the type a consumer would use to receive over-the-air television broadcasts. The researchers aimed the antenna at a Seattle TV transmission tower located slightly more than 4 kilometers (2.5 miles) away, and broadcasting 960kW of effective radiating power at 674 to 680 MHz.
According to Smith, the prototype Wireless Ambient Radio Power (WARP) device was able to harvest the ambient RF signals emitted by the tower and store up sufficient energy to power a commercially available thermometer/hygrometer with an LCD display. The prototype, he says, could be altered to harvest ambient RF power from other sources, such as FM radio, AM radio, or cell phone base station signals.
But WARP is only one part of the Wireless Identification and Sensing Platform (WISP), a project Smith and his group have undertaken to develop passive RFID tags capable of exploiting RF energy to support sensing applications. Other iterations of the tags are designed either to harvest ambient power from an RFID interrogator and store it in a capacitor to allow for data logging over a period of time when the tags are not being read, or to harvest enough power from a reader's signal to collect sensory information that is immediately transmitted back to the interrogator.
WISP tags have been built with sensors that measure light, temperature and strain (the amount of deformation an object experiences compared with its original size and shape). "The vision is to embed a lot of strain sensors throughout an airplane wing," Smith says. "They could be read either by a handheld RFID reader, or by one mounted somewhere in the airplane." WISP tags have also been used to test cryptographic computations for securing tag data, he adds.
Each WISP contains a microcontroller that not only communicates with an off-the-shelf EPC Gen 1 or 2 RFID interrogator but also controls the sensors linked to the tag. According to Smith, most RFID chips do not contain microcontrollers, which typically have at least 20,000 transistors. RFID chips have a finite-state machine, which usually has fewer transistors and carries out a fixed set of functions (implementing the EPC protocol, for instance). "The big difference," Smith says, "is that finite-state machines are not programmable—they don't run software."
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