Wireless charging taps quantum mechanics for moving objects
Wireless charging for moving vehicles is just emerging, with Qualcomm HALO demonstrating a test system in France to charge vehicles moving at speed. MIT demonstrated the transfer of 60W over 2m back in 2007, and resonant wireless charging company WiTricity is a resulting spin-off.
Instead the team at Stanford has used a principle called Parity-Time (PT) Symmetry that is effective in photonics for a wireless charging system that couples together identical systems.
The researchers believe this overcomes a major hurdle to wireless power transfer to a nearby moving object. Their results are published in a letter in the June 15 edition of Nature and they have patented the technique.
“In addition to advancing the wireless charging of vehicles and personal devices like cellphones, our new technology may untether robotics in manufacturing, which also are on the move,” said Shanhui Fan, a professor of electrical engineering. “We still need to significantly increase the amount of electricity being transferred to charge electric cars, but we may not need to push the distance too much more.”
The researchers eliminated the radio-frequency source in the transmitter and replaced it with a commercially available voltage amplifier and feedback resistor that adjusts the power for different distances using a non-linear saturation gain element based on PT symmetry calculations.
“Adding the amplifier and resistor allows power to be very efficiently transferred across most of the three-foot range and despite the changing orientation of the receiving coil,” said graduate student Sid Assawaworrarit. “This eliminates the need for automatic and continuous tuning of any aspect of the circuits.”
Assawaworrarit tested the approach by placing a 1mW LED bulb on the receiving coil. In a conventional setup without active tuning, LED brightness would diminish with distance. In the new setup, the brightness remained constant as the receiver moved away from the source to around 1m.
The group used an off-the-shelf, general-purpose amplifier with a relatively low efficiency of about 10 percent. They say custom-made amplifiers can improve that efficiency to more than 90 percent.
The researchers suggest that this is independent of the coil orientation, which would be a significant step forward. However, if it is relying on PT symmetry, it requires identical systems. This could be a limitation for small receivers in medical devices or consumer electronics, and for moving vehicle systems with a long charging path.
The letter is at www.nature.com/nature/journal/v546/n7658/full/nature22404.html
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