Leaving home while carrying a phone, an iPad and a laptop might also mean lugging along several tangled power cords. Now add radios and GPS devices. Now strap them to your person and wrap the cords around your body beneath your 30-pound armored vest. Oh, and you’re on patrol in Afghanistan, which means there’s no place to plug in when your phone’s batteries start to die. This explains why the Pentagon is keen on eliminating those cables with wireless chargers, and now wants to boost the range to more than 50 feet.
The plan involves spending $5-6 million using the branch’s research and development centers to “increase the efficiency of power transfer over longer distances,” according to an Army statement. If successful, it means that soldiers — instead of being limited to recharging their gadgets when returning to base, or by plugging into their vehicles — could go cordless. That means recharging by a wireless battery attached to their body — no plugs. The Army also hopes to build wireless transmitters on bases, allowing soldier gear to recharge passively, without having to plug anything in. And one day, it might be used on drones.
This isn’t the first time the Pentagon has explored wireless power. Last year, Darpa looked into the idea of building wireless power systems with ranges of up to two meters, roughly 6.6 feet; the project was not pursued. This was after MIT engineers built a system in 2007 capable of transferring 60 watts of power over two meters at 50 percent efficiency using “strongly-coupled magnetic resonators.” Fifty feet, though, is (currently) too far for any amount of energy to be useful. The range for practical consumer and military systems tap out at a few centimeters, at best.
The range is also limited by the size of the power coils and — of course — distance. MIT’s resonator coils were about the size of old refrigerator coils, says David Schatz, the director of business development and marketing for WiTricity, a Boston-area company founded by several former MIT researchers familiar with the project. MIT’s coils were large enough to demonstrate that the system worked, but were not large enough to go farther than two meters.
The greater the distance you want to send power, the bigger the coils have to be. But if the coils are too big, then they’re not practical enough for a soldier to carry around. The solution, said Schatz, are resonate “repeaters” that allow energy to “hop” from repeater to repeater.
“We’ve demonstrated room-scale energy transfer, hopping over a series of repeaters that are in turn, that are not wired together,” Schatz says. “The energy is able to hop to one to the next to the next and end up getting to the end source quite an efficient way.”
The resonators also allow for two different resonating devices — like a smartphone and a wireless battery — to focus on each other while ignoring (or interacting only weakly) with surrounding objects. Fortunately, non-metallic objects (like people) interact weakly, along with plastic and glass in vehicles. “You have to be a little more careful when there are metallic objects around,” Schatz says. “But you can manage that with careful design of resonators and shielding.”
It’s also necessary if the Army is serious about its desire to network soldiers — which comes with more gadgets, more power and more vulnerabilities. Any civilian dependent on a juice-sucking smartphone has an idea of what it’s like with a dead battery. The difference is that civilians are not out on patrol in isolated, hostile territory. But there are also other risks. If soldiers become dependent on smart phones linked to digital networks, then you’ve become only as effective as your battery power. If one soldier forgets to charge his or her battery before departing on an operation, then there’s a risk the soldier might drop out of the network when the battery dies.
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