When a person carries a loaded backpack, the pack too moves up and down the same distance at the same time. By detaching the frame of the pack from the load it carries, the scientists were able to capture the energy of this up-and-down movement.
The backpack frame sits on the carrier's back while the load is suspended from the frame by vertical springs. The springs allow the load to slide up and down with the same motion of the hip, but lagging by a fraction of a second.
"You get a differential movement between the frame of the backpack and the load, and that's where we generate the electricity," Rome said.
The pogo-stick like movement of the load turns a gear connected to a generator at the top of the pack. This gear rotates coils of wire within a magnetic field inside the generator, creating electricity.
The amount of power generated depends on how much weight is in the pack and how fast the user walks. Researchers measured a maximum output of 7.4 watts. Gadgets like cell phones require less than one watt.
While the separate movement of the cargo compartment and the frame sounds awkward and uncomfortable, Rome said "it's actually more comfortable than a normal backpack."
In a commentary accompanying the Science report, Arthur Kuo, a mechanical engineer at the University of Michigan in Ann Arbor, writes that the backpack is easy to wear and works surprisingly well.
"Why it works so well is unclear," he writes. "Perhaps the device reduces the mechanical work required of muscles to walk while carrying a load."
Rome speculated that the added comfort may stem from the springs, which reduce the need to jerk the load up and down.
"Imagine that you step up on a rock with a rigid backpack. That weight has to go up instantlyit can't hang on the ground. [I]n a suspended backpack, when you step up, the load lags behind you," he said. The effect is like that of a shock absorber, he explained.
In addition, Rome and colleagues found the backpack altered the gait of test wearers, causing their up-and-down hip movement to become smaller.
The researchers also learned that the self-powering backpack weighs only slightly more than a conventional packequivalent to carrying around an extra candy bar.
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