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Power wireless sensor networks by harvesting energy

29 Jun 2012  | Dan Strassberg

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At a distance of 2.4m (eight feet) between the primary and the secondary, the efficiency of power transfer is almost 40%, according to the company. Unlike higher-frequency wireless-energy-transfer schemes, which carry energy in both electric and magnetic fields, WiTricity's energy transfer depends almost entirely on the magnetic field. The technology achieves best results if the excitation signal is sinusoidal, but the system suffers only a slight reduction in efficiency if the excitation waveform is square or trapezoidal.

The bugaboo, however, is shorted turns. Suppose that the transformer's primary and secondary coils center one above the other in parallel horizontal planes. When you apply an excitation voltage to the primary coil, a toroidal, or donut-shaped, RF-magnetic field with a vertical axis surrounds the two coils.

Perpetuum vibration-energy harvester

Figure 6: The Perpetuum vibration-energy harvester can power wireless-sensor networks built from National Instruments´┐Ż wireless-sensor-network components.

If you mount the coils on metal plates, you must slit each plate along a coil radius so that no low-resistance current path—that is, no shorted turn—surrounds the toroidal field's axis. If you were to create such a path, a large current would circulate in it and would counteract the field from the coils, thereby reducing nearly to zero the efficiency of the coil-to-coil power transfer. The circulating current could also cause dangerous heating of the metal plates.

A similar shorted-turn problem potentially exists with a more common power-transfer approach: ac-line-frequency inductive coupling, such as is used to charge electric-toothbrush batteries. In that case, however, the primary and secondary coils are in close proximity, and the coil supports are made of nonconductive plastic. Unlike the metal plates in the earlier example, there are no legitimate concerns about their structural integrity.

Acknowledgment
I much appreciate the assistance of industry sources who helped me to research this article. In particular, I wish to thank Harry Ostaffe, vice president of marketing and sales at Powercast Corp, and Bharat Rawal, manager of BestCap technology at AVX Corp.

References
Akyildiz, IF; W Su; Y Sankarasubramaniam; and E Cayirci, "Wireless sensornetworks: a survey," Georgia Institute of Technology, 2002.

Conner, Margery, "Advances in energy-storage technology power wirelessdevices," EDN, Feb 3, 2011.

Gummeson, Jeremy; Shane S Clark; Kevin Fu; and Deepak Ganesan, "Onthe Limits of Effective Hybrid Micro-Energy Harvesting on Mobile CRFIDSensors," University of Massachusetts—Amherst.

Ostaffe, Harry, "Wireless power energizeswireless sensor networks," Powercast, March 19, 2009.

"Boostcap ultracapacitors," Maxwell Technologies, 2009.

Seah, Winston KG; Zhi Ang Eu; and Hwee-Pink Tan, "Wireless Sensor NetworksPowered by Ambient EnergyHarvesting (WSN-HEAP)—Survey andChallenges," Institute for Infocomm Research, 2009.


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