Commack, NY Researchers at Intel Research Seattle Lab used last week's Rawcon Conference in San Diego to disclose details of an ambient RF energy-harvesting scheme that scavenged 60 microwatts, or enough energy to drive a thermometer/hygrometer and its associated LCD, from a TV tower at a distance of 4.1 km.
The demonstration of RF scavenging, called Wireless Ambient Radio Power (WARP) by the researchers, broadens the range of potential ambient energy sources, which already includes vibration, solar and heat. It was shown as another application of the Wireless Identification and Sensing Platform (WISP), a platform for sensing and computation that is powered and read by a commercial off-the-shelf UHF RFID reader. Each WISP consumes 2 microwatts to 2 milliwatts and can be operated at distances of up to several meters from the reader.
According to Joshua Smith, the presenter and co-author of the Rawcon paper, "Experimental Results with Two Wireless Power Transfer Systems," a WISP is essentially an RFID tag with a microcontroller on it. In this case, a Texas Instruments MSP430.
As advanced as it may sound, the key enabler behind the WISP is not so much nifty circuit design or an extraordinary breakthrough in component physics. Instead, said Smith, it's directly attributable to Moore's Law, whereby the increased integration and falling power consumption of digital circuitry has enabled improved functionality per microwatt of scavenged energy.
The improvements are advancing at such a rate that, "The range at which you can power a device [with a given amount of ambient RF energy] should double every four years," said Smith, who may well have just stated his own new Law, in the context of RF energy harvesting.
Each WISP comprises an antenna, impedance-matching components, RF power harvester, demodulator to extract reader-to-WISP data, backscatter modulator for WISP-to-reader data, voltage regulator, programmable microcontroller (the MSP430), and optional external sensors. The harvester itself comprises a four-stage charge pump. For more details, see the paper itself, with associated diagrams.
To date, the WISP has been used for a variety of sensing and other applications, including accelerometers, temperature, strain gage, capacitance and a custom neural amplifier. However, the Rawcon discussion was the first to focus on TV broadcast RF energy.
From a balcony at the Intel Research Seattle lab the researchers harvested RF power from the KING-TV tower 4.1 km away, which broadcasts 960kW ERP on channel 48, at 674 - 680 MHz. Energy was collected using a manually oriented broadband log periodic antenna (5 dBi) designed for TV applications and a 4 stage power harvesting circuit of the same design as WISP, but with a front end tuned to the desired channel. Across an 8KOhm load the team measured 0.7V, corresponding to 60 microwatts of power harvested. According to Smith, that's sufficient to drive many of the WISP's sensing applications. In this case, it drove a thermometer/hygrometer and its LCD display.
Smith envisions many applications of the two power-harvesting techniques (RFID + TV), including RF-powered sensor devices that log their data until interrogated by an RFID reader, thereby enabling a perpetual sensing platform with no need for batteries.
However, that is but one application of the WISP. To see what other applications can bubble up, Intel Research has launched what it's calling the WISP Challenge whereby it will send a full WISP kit to potential collaborators in academia upon receipt of a suitable proposal.
RF will be but one of the many harvesting technologies to be discussed during a panel on Energy Harvesting at the up-coming Embedded Systems Conference, Silicon Valley, where the application and potential of energy harvesting in general will be the focus.