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.

The quest for wireless charging

The quadrotor as flying charge point is the latest in the over a century-old dream of doing away with wires and batteries for powering electrical devices. That denizen of school physics lab, the Tesla coil is an early example of this as it lights fluorescent tubes across the room by what seems like magic, but is actually basic physics. Its inventor, Nikola Tesla spent decades in the late 19th and early 20th Centuries searching for practical ways to send electric power through the air, but without much success. In the 1920s and ‘30s, one of Tesla’s greatest admirers, the American publisher Hugo Gernsback, filled his science fiction and popular science magazines with all manner of visions of a future time in when power lines have given way to huge antennae beaming power to everything from typewriters to ocean liners.

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.

NIMBUS Lab quadrotor

The NIMBUS quadrotor works on a very different principle called “strongly-coupled magnetic resonances.” That sounds intimidating, but coupled resonances are actually a universal phenomenon found in not only electromagnetism, but also mechanics, civil engineering, acoustics and many others. It’s based on the idea that many things have what is called a natural resonance frequency. That’s to say, when they are made vibrate at a particular rate, they store the energy by making the amplitude of the vibrations stronger and stronger. If energy keeps being fed into an object at that resonance frequency long enough, the object will vibrate so violently that it destroys itself.

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.

Strongly-coupled magnetic resonance

The same holds true of magnetic resonators. This is what the NIMBUS quadrotor uses, but instead of singers and wine glasses, it uses a pair of magnetic fields that are “coupled” or designed so that one resonates in response to the other. How it works is that the quadrotor has a copper wire wrapped around it. This is configured to act as the power transmitter. The target has a similar wire ring, which is configured to receive. Both of these, when switched on, generate magnetic fields. When these two fields come into contact with one another, they couple and resonate strongly (hence the name “strongly-coupled magnetic resonances”) and power is transmitted from the quadrotor to the target.

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.

Sources: NIMBUS Lab, IEEE Spectrum