Design Con 2015

Far-field RF energy transport can charge “smart building” wireless sensors

Hubregt J. Visser -December 06, 2012

Smart-building integration requires hundreds of wireless sensors per building for sensing light, temperature, occupants’ presence, and other characteristics. Cabling all those sensors is too costly, as are the continual replacement and disposal of primary batteries.

An alternative solution is to recharge sensor batteries remotely using far-field RF energy transport. Such technology uses a dedicated RF energy source, transmitting RF radiation in the 2446- to 2454-MHz ISM band. Restrictions imposed on the effective isotropic radiated power, however, make it challenging to realize dc power levels on the order of 100 µW over distances of multiple meters.

Imec and Holst Centre (Eindhoven, Netherlands) have proposed measures to improve the energy efficiency of the RF energy transport system and thereby allow an increase of the operating distance between the energy source and battery. Researchers from Imec and Holst Centre presented their results at the 2nd International Workshop on Wireless Energy Transport and Harvesting (IWWETH), jointly organized by the two organizations.

The components of a far-field RF energy transmission and reception system are shown.

The researchers showed that by characterizing the indoor environment and adapting the transmitting radiation pattern to those characteristics, more power may be obtained at larger distances using less transmit power. A geometrical-optics-based model of the environment can optimize the transmitting-radiation pattern. Simulations reveal that more power can be received by using a multibeam antenna that points at the receiving antenna and at the first-order reflection points of a corridor, for example, than by using an antenna that is directed to the receiver antenna only.

Since the transmit-antenna beam shape depends on the environment, a transmit antenna with beam-shaping capabilities is required. Imec and Holst Centre propose the use of a cost-effective switched array antenna, in which one dipole/monopole element will be driven and parasitic dipoles/monopoles will be switched open or short-circuited/grounded. Modeling can determine the best switching scheme to illuminate the receiver, as well as the wall positions for in-phase reflected wave contributions.

Besides maximizing the power incident on the receiver, the design must optimize the subcomponents of the receiving rectifying antenna, or rectenna. The functional blocks of the rectenna are the antenna, the rectifier, the dc-to-dc voltage boost converter, and the load. Optimization lies in identifying the impedance-matching efficiencies between the blocks and the conversion efficiencies over the blocks.

Consider a packaged, remote 2.45-GHz RF battery charger and a commercially available 433-MHz wireless temperature and humidity sensor, with an operating distance currently limited to 5 meters. By optimizing all the subsystems and interconnects, it is estimated that this distance can be doubled.

Approximate and full-wave normalized radiation pattern simulation results are shown for a
switched array antenna comprising seven Yagi-Uda arrays (beam steered to 30°).

Hubregt J. Visser began his affiliation with Holst Centre in 2006 and has been with Imec as well since 2009, working on wireless energy transfer. He earned a PhD from Eindhoven University of Technology (Netherlands) and Katholieke Universiteit (Leuven, Belgium) in 2009 and is an associate professor at the Eindhoven university, where he teaches antenna theory. Hubregt is author of Array and Phased Array Antenna Basics (Wiley, 2005), Approximate Antenna Analysis for CAD (Wiley, 2009), and Antenna Theory and Applications (Wiley, 2012).

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