Feb. 8, 2008 -- Talk about power walking. A new knee-mounted device converts the kinetic energy of strolling, sauntering and striding into usable electricity.
The energy harvested could work to power portable electronics, such as GPS locators or cell phones or supply electricity to motorized prosthetic joints and implanted neurotransmitters.
"There is power to be harvested from various places in the body, and you can use that to generate electricity. The knee is probably the best place," said Arthur Kuo, an associate professor of mechanical engineering at the University of Michigan in Ann Arbor.
Kuo and colleagues from Simon Fraser University in Canada and the University of Pittsburgh describe the technology in this week's issue of Science.
At first glance, the knee brace looks like the future of orthopedic support. But this brace goes beyond buttressing to not only making walking easier but to also generate electricity along the way.
A small oblong generator and an aluminum chassis -- complete with transmission, clutch, sensor and computer controls -- are mounted to the outside of the brace.
As the person walks, each leg swings naturally forward and back, with a stance in the middle, when the foot is firmly on the ground. The sensor, called a potentiometer, monitors the knee's angle during the swing and stance phases and feeds that information to the computer.
Only when the leg is moving from the back of the swing (when it's behind the walker) to the point where it steps on the ground, does the computer engage the generator.
It's during this phase of walking that the leg muscles are working to slow the leg motion. Without the brace, this braking energy would be wasted. But with the brace, the kinetic energy is captured and converted into electricity.
In experiments, Kuo and his team found that a person used up less body energy to produce a watt of electricity.
"You're letting the generator be the break for you and because of that…you can get this electricity for free," said Larry Rome, professor of biology at the University of Pennsylvania in Philadelphia and inventor of Lightning Packs, backpacks that generate electricity while their wearers hike.
Expending as little metabolic energy as possible can be critical for people, such as soldiers, working in remote places. They typically use, for example, hand-crank generators to produce electricity, but such a device can use up more than 6 watts of muscle energy just to generate one watt of electricity.
If Kuo and his team want to improve upon their device, said Rome, they have three main challenges: Make the thing comfortable and lighter (right now it weighs 3.5 pounds). Use a more efficient generator and gear system. Design a component that can use the generated electricity to actually charge a battery.
"You can't hook the battery up to the device without some other intervening electronics that control the charging."
Kuo and his team are already looking forward to improvements, including making the brace more form-fitting and constructing it out of lightweight carbon-fiber material.
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