The silicone strip acts as a flexible cantilever that bends back and forth with body movements. The scientists attached the cantilever to a conducting metal coil with a strong neodymium, NdFeB, magnet inside, all enclosed in a polymer casing. When a conductor moves through a magnetic field a current is induced in the conductor.
In order to extract the electricity generated, there is a need to include special circuitry that takes only the positive voltage and passes it along to a rechargeable battery. In previous work, this circuitry includes a rectifying diode that allows current to flow in one positive direction only and blocks the reverse, negative, current. Unfortunately, the development of kinetic chargers has been stymied by current diode technology that requires a voltage of around 200 millivolts to drive a current.
University of Auckland researchers, Jiayang Song and Kean Aw side-stepped the voltage obstacle by using a tiny electrical transformer and a capacitor, which acts like a microelectronic battery. The charger, which weighs a few grams, oscillates and moves the coil back and forth through the neodymium magnetic field and produces 40 millivolts. The transformer captures this voltage and stores up the charge in the capacitor in fractions of a second. Once the capacitor is full it discharges sending a positive pulse to the rechargeable battery, thus acting as its own rectifier.
The development is first step towards a viable trickle charger that could be used to keep medical devices, monitors and sensors trickle charged while a person goes about their normal lives without the need for access to a power supply.
The system might be even more useful if it were embedded in an implanted medical device to prolong battery life without the need for repeated surgical intervention to replace a discharged battery.
‘An energy harvester from human vibrational kinetic energy for wearable biomedical devices’ in Int. J. Biomechatronics and Biomedical Robotics, 2014, 3, 54-61
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