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Wireless Power

March 16, 2017

Several years ago I designed an audio signal processing circuit for a hobby electronic kit company. I was asked by the owner of the company whether I would be interested in designing other, radio frequency (RF), circuits for them, since they were interested in such things as simple remote control devices and radar speed detectors. While I'm skilled in RF circuit design, I declined. RF circuits are my least favorite type of circuits because there's so much that can go wrong.

As Scrooge complained in Dickens', A Christmas Carol, it's "because... a little thing affects them."[1] Resistors have inductance because of the currents flowing through their leads, there's stray capacitance everywhere, coupling between parallel traces on printed circuit boards, and inductance in those same circuit board traces. Modern circuit boards mitigate most of these problems through use of surface mounted components with minimal lead length, and dedicated power and ground layers.

While the ultra high frequencies of cellphones and WiFi devices make for difficult circuit designs, working with low frequencies near the AM broadcast band, 530 - 1700 kHz in the US, is much easier. The recently popular idea of wirelessly charging batteries, including electric automobile batteries, is done by circuitry operating at even lower frequencies, some at 10 kHz in the audio frequency range. These inductive chargers make use of resonant inductive coupling between proximate coils (see figure).

Fig. 4 of US Patent No. 6,100,663, 'Inductively powered battery charger,' by John Talbot Boys and Andrew William Green, August 8, 2000.Fig. 4 of US Patent No. 6,100,663, "Inductively powered battery charger."

The resonant "tank circuit" of inductance 304 and capacitance 305 couples RF voltage into coil 402, which is rectified to charge capacitor 309.

(Via Google Patents.[2])

Quite a while ago I used inductive coupling to achieve something different than battery charging. I developed a method of remotely disabling a MEMS resonator by coupling voltage into a coil that discharged through a thin film. Although the film was acrylic, the idea was that it could be sodium azide, an explosive, and the voltage discharge would detonate the film.

The explosive force would be minuscule, but it would be enough to deposit mass onto the resonator to detune it. While I was doing this project, I was reminded of articles I has read about internal explosives designed to destroy hard disk drives in military systems lest they fall into enemy hands (look here for a current example). Now, we have lithium ion batteries to destroy our cellphone and tablet computer memory, instead.[3]

The present problem with wireless chargers is that the device to be charged needs to be very close to the charging station, since large air gaps diminish efficiency. The typical technique is to require laying the device onto a charging pad. How much more convenient would it be if your device could be charged, or powered, anywhere in a room? That's the problem that three researchers have tackled at Disney Research (Pittsburgh, Pennsylvania), and they've described their system in a recent open access paper in the journal, PLoS ONE.[4-5]
Qi wireless chargerA charging pad, based on the Qi international inductive power standard, a wireless charging solution by the Wireless Power Consortium, shown charging an LG smartphone.

(Via Wikimedia Commons.)

When pumping radio frequency power into an occupied volume, it's important to ensure that there are no harmful affects on people in that space, and on pets, as well. Government agencies have established safety guidelines for radio frequency exposure.[6] The research team was able to power multiple devices in a room by converting the room into an analog of a microwave oven, albeit safely with just the magnetic field component of the electromagnetic wave at the lower frequency of 1.32 MHz. The electric field was diminished by shifting the room's high-Q resonance to a deep sub-wavelength regime that effectively separated the electric field component from the magnetic field component in the electromagnetic wave.[4]

They built the room from aluminum sheets bolted to an aluminum frame to transform it into a cavity resonator. In this cavity, they were able to excite near-field standing waves that filled the room with uniform magnetic fields. Coupling these field to small receivers in the room enabled wireless power transfer to devices in the room. The system was able to safely deliver 1900 watts of power to devices nearly anywhere in the room with 40% to 95% efficiency.[4]

Radio frequency excitation components
Components for excitation of cavity resonance in the shielded room. A spiral coil (left), attached to a signal generator, excites the cavity modes. A vertical copper tube, broken at its center, shorts the electric current component through capacitors connecting the two halves.(Portion of fig. 3 of ref. 4. Licensed under the Creative Commons Attribution License.[4])

Power transfer efficiencycomputated efficiency of power transfer as a function of location in the room. Efficiency is highest near the central power source.

(Selected portions of fig. 4 of ref. 4. Licensed under the Creative Commons Attribution License.[4])

This was a reasonably large room of dimension 16' x 16' x 7.5' (4.9 m × 4.9 m × 2.3 m).[4] The central copper pole was 7.2 cm in diameter, and the two halves of pole, separated by a one inch gap, were electrically joined by fifteen high-Q discrete capacitors giving a total capacitance of 7.3 pF (see above figure).[4] While there was a four foot wide access panel in this otherwise shielded room, its presence had a negligible affect on system performance at the 1.32 MHz operating frequency.[4] The spiral drive coil was eight turns in a 28 cm diameter.[4]

Wirelessly powered devices in the cavity resonator room

Devices wirelessly powered in the cavity resonator room. (Portion of fig. 7 of ref. 4. Licensed under the Creative Commons Attribution License.[4])

Says Alanson Sample, leader of Disney Research's Wireless Systems Group and a co-author of the paper describing this research,

"In this work, we've demonstrated room-scale wireless power, but there's no reason we couldn't scale this down to the size of a toy chest or up to the size of a warehouse."[5]

While the demonstration room was specially constructed to act as a nearly ideal resonant cavity, Sample says that it may be possible to retrofit existing structures with modular panels or conductive paint, and larger spaces would function through use of using multiple copper poles.[5] As the graph shows, the input power to the demonstration room could be as high as 1.9 kilowatts, enough to simultaneously charge 320 smartphones, and still meet federal safety guidelines.[5]

Figure captionSafe operating region is shown in green. The red line is the absolute limit, and the black line is a cautionary 614 volts per meter level (see ref. 4 text).

(Fig. 6 of ref. 4. Licensed under the Creative Commons Attribution License.[4])

References:

  1. Charles Dickens, "A Christmas Carol," Chapman & Hall (London, 1843) on Project Gutenberg.
  2. John Talbot Boys and Andrew William Green, "Inductively powered battery charger," US Patent No. 6,100,663, August 8, 2000.
  3. Lithium battery dangers mean Samsung recall won't be last, Phys.org, October 26, 2016.
  4. Matthew J. Chabalko, Mohsen Shahmohammadi, and Alanson P. Sample, "Quasistatic Cavity Resonance for Ubiquitous Wireless Power Transfer," PLoS ONE, vol. 12, no. 2 (February 15, 2017), Article No. e0169045, http://dx.doi.org/10.1371/journal.pone.0169045. This is an open access article with a PDF version here.
  5. Wireless power transmission safely charges devices anywhere within a room, Disney Research Press Release, February 16, 2017.
  6. Radio Frequency Safety, Federal Communications Commission, March 2, 2011.
  7. Project Web Site (includes demonstration videos).

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