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Researchers power a security camera with Wi-Fi signals alone

Thu 4 Jun 2015

A research department at the University of Washington has developed a new technique for drawing enough power from standard Wi-Fi signals to power a security camera and recharge a fitness tracker – and widespread adoption and refinement of it could have a pivotal influence on the emergence of the Internet of Things.

The paper Powering the Next Billion Devices with Wi-Fi [PDF], led by PhD candidate Vamsi Talla, documents the team’s experiments to extract a minimum operational threshold of 300 millivolts across the as-yet unlicensed ISM band which Wi-Fi uses, without disrupting the quality of conventional Wi-Fi data transmission or degrading the efficiency of existing small networks.

Eventually the group was able to extract the necessary voltage using Wi-Fi chipsets with ranges of 20 and 17 feet, and also to recharge nickel-metal hydride lithium-ion coin-cell batteries at a range of up to 28 feet.

vamsi-talla-uow-wireless-charged-devices[1]

Above are the prototype devices powered wirelessly by the system that the paper dubs ‘PoWiFi’ (Power Over Wi-Fi) – a camera (the OV7670 [PDF] low-consumption VGA image sensor by Omnivision), a generic temperature sensor, 2.4V nickel– metal hydride battery and 3.0 V lithium-ion coin-cell battery.

Asus-RT-AC68U[1]

The initial router used was the Asus RT-AC68U, which operates at 2.437 GHz outputting 23dBm from each of its three 4.04dBi gain antennas.

The chief problem that Valla and Co. needed to overcome was the irregular nature of Wi-Fi transmissions, which utilise a single channel in bursts that are not sufficient to constitute an adequate charging stream. They overcame this by jury-rigging a new router from three Atheros AR9580 (‘Peacock’) chipsets, programming it to broadcast noise in between data bursts to provide two parallel transmission channels to ensure that data rates were unaffected.

Using the hybridised Atheros rig the team were able to get usable recharging distances of 20-30 feet, and in certain circumstances to enable recharging to occur when the router was on the other side of a brick wall.

The security camera that the group was able to charge wirelessly is a humble one, for sure, producing 174 x 144 pixel black and white images at an energy cost of 10.4 milliJoules of energy per picture. An attached low-leakage capacitor activates when charged to 3.1V, cutting out again on a drop to 2.4V, with images stored in 64kb of non-volatile ferroelectric RAM. The camera succeeded in taking an image every 35 minutes initially, and the addition of a wirelessly-rechargeable battery increased the workable router distance from 16 feet to just under 23 feet.

powifi-general-purpose-USB_charger[1]

To demonstrate the potential of PoWiFi as a low-drain charging unit, Valla and his team created a 2dBi Wi-Fi antenna (pictured right) and used it to charge a Jawbone UP24 fitness band, increasing its charge status from zero to 41% in 2.5 hours, from Wi-Fi signals alone.

The later stage of the experiments involved the placement of these modified routers in a home deployment, and for this the team installed the devices in six homes in a metropolitan area, logging the router’s channel occupancy on each of the three Wi-Fi channels being used for data and power transmission. The results were inconclusive, but presented little or no determination of adverse affect of the high-occupancy setup in a standard domestic Wi-Fi network environment.

The paper concludes with broad visions for the possibility of PoWiFi not only in the new Internet of Things, but also in IoT as it emerges in the developing world. ‘PoWiFi can successfully deliver power via Wi-Fi with real-world Wi-Fi network conditions,’ it states. ‘We believe that our system is a general design for power delivery in the ISM bands. As Wi-Fi access and densities continue to grow in the ISM band, solutions that deteriorate Wi-Fi performance by jamming any specific frequency are not desirable. Our power delivery solution is integrated with the Wi-Fi protocol and hence can deliver power while having minimal impact on Wi-Fi traffic. Future designs would generalize our multi- channel approach to operate across multiple ISM bands (e.g., 900 MHz, 2.4 GHz and 5 GHz)’

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