Not only are quadrotors fun, they're useful for applications like surveillance and are even showing promise in building construction. Here's a practical use we hadn't thought of though - remote wireless charging. The folks from NIMBUS lab at the University of Nebraska-Lincoln are developing a quadrotor equipped with a system that uses strongly-coupled magnetic resonances to transmit power from its batteries to the receiving device without ever needing to make physical contact. The roboticists see this as a solution for powering devices that are otherwise inaccessible to conventional electrical sources.
Not surprisingly, none of this came to pass because the original idea was to basically broadcast power as high-voltage radio waves. This was not only incredibly inefficient, involving blasting out millions of watts all over the place with the power available dropping off alarmingly within a very short distance, but it was also extremely dangerous. Just about anything made of metal would act as a receiving antenna, so it was probably a good thing that it never got past the speculation stage or many people would have suffered the unpleasant fate of being electrocuted by their bridgework.
The alternative plan was to forego broadcast power in favor of beaming it in the form of microwaves or lasers. This was certainly a more efficient alternative, though still hazardous, since a laser or microwave powerful enough to run a device is also liable to be dangerous to anything that got in the way.
Perhaps the only practical form of broadcast power at the moment is induction - a mode of power transmission that's most commonly used in cordless electric toothbrushes and is being developed for electric cars. Induction works on the same principle as an electrical transformer - two electrical coils are placed in proximity to one another, an alternating current is passed through one coil and the continually collapsing electromagnetic field caused by the alternation sets up a current in the second coil. Used in power transmission, one coil is the transmitter and the other is installed in the device being charged. It’s a system that works, but only over very short ranges.
This is seen in the party trick of having a trained opera singer hitting the exactly right note that makes a wine glass vibrate at its resonance frequency. The glass vibrates, the sound energy is stored in the form of greater and greater vibrations until the glass shatters. But it’s more than a party trick. In 1940, the power of resonance frequencies was demonstrated to terrifying effect when the Tacoma Narrows Bridge in Washington State was buffeted by winds at exactly the right (wrong) frequency and the bridge literally shook itself to pieces.
The important thing about resonance, however, isn’t that it can shatter a wine glass. What is important is that other wine glasses with a different resonance frequency don't shatter. In other words, only that particular glass is affected.
Now here’s the clever bit. Until these two fields come together, no power is transmitted. When the quadrotor and the target are apart, their fields become simple, inert magnetic fields like that around a compass needle. This is very different from, for example, a Tesla coil, which is always pumping out energy when it’s on and which will charge anything within range that can act like an antenna. With a strongly-coupled magnetic resonance, the fields will only transmit or receive power when they’re in contact. What’s more, only the target will receive any power. Anything else in the vicinity remains largely unaffected.
Because the resonance fields interact very poorly with anything they don’t resonate with, other devices, people, animals, walls or other obstacles are more or less transparent as far as the field is concerned. What all this adds up to is that a strongly-coupled magnetic resonance field is more efficient for transmitting power, operates over a longer range and can’t electrocute someone, for example, who’s carrying a steel pole in the vicinity.
Experiments with strongly-coupled magnetic resonance have already been carried out at places like MIT, but these have been with stationary equipment while NIMBUS is working with a mobile system. The idea is to bring the power source to the device instead of the device to the power source. So far, there's been a measure of success, with 5.5 watts of power transmitted over 20 cm (8 inches) with 35 percent efficiency.
The purpose of the quadrotor is to recharge devices such as remote sensors, buried equipment or devices that are inaccessible, like those installed on bridges or atop radio masts or ones that can’t use solar panels for aesthetic reasons. A variation on the quadrotor could periodically visit the device and feed power to it without ever needing to come into contact.
The current design is still very much in the experimental stage and NIMBUS plans to make improvements to handle greater power as well as making the quadrotor operate autonomously so it can seek out and hover near its target without flitting about as the current version does.
There’s still a ways to go, but it may be that someday you may see someone being followed down the road by a quadrotor and you’ll have to decide whether he’s being spied on or the ‘rotor is just charging his tablet for him.
The video below shows NIMBUS Lab's quadrotor remotely charging an experimental target.
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