ref16 - E N E R G Y H A RV E S T I N G C O N S E RVAT I O N...

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28 PERVASIVE computing Published by the IEEE CS and IEEE ComSoc 1536-1268/05/$20.00 © 2005 IEEE ENERGY HARVESTING & CONSERVATION Improving Power Output for Vibration-Based Energy Scavengers P ervasive networks of wireless sensor and communication nodes have the potential to significantly impact soci- ety and create large market opportuni- ties. For such networks to achieve their full potential, however, we must develop practi- cal solutions for self-powering these autonomous electronic devices. Fixed-energy alternatives, such as batteries and fuel cells, are impractical for wireless devices with an expected lifetime of more than 10 years because the applications and environments in which these devices are deployed usually preclude changing or re-charging of batteries. There are several power-generating options for scavenging ambient environment energy, including solar energy, thermal gradients, and vibration-based devices. However, it’s unlikely that any single solution will satisfy all application spaces, as each method has its own constraints: solar methods require sufficient light energy, thermal gradients need sufficient temperature vari- ation, and vibration-based systems need sufficient vibration sources. Vibration sources are generally more ubiquitous, however, and can be readily found in inaccessible locations such as air ducts and building structures. We’ve modeled, designed, and built small can- tilever-based devices using piezoelectric materials that can scavenge power from low-level ambient vibration sources. Given appropriate power con- ditioning and capacitive storage, the resulting power source is sufficient to support networks of ultra-low-power, peer-to-peer wireless nodes. These devices have a fixed geometry and—to max- imize power output—we’ve individually designed them to operate as close as possible to the fre- quency of the driving surface on which they’re mounted. Here, we describe these devices and pre- sent some new designs that can be tuned to the fre- quency of the host surface, thereby expanding the method’s flexibility. We also discuss piezoelectric designs that use new geometries, some of which are microscale (approximately hundreds of microns). Problem overview We first analyze the wireless sensor nodes’ power requirements, and then investigate the var- ious sources that can fill those demands. Power demand Assuming an average distance between wireless sensor nodes of approximately 10 meters—which means that the radio transmitter should operate at approximately 0 dBm (decibels above or below 1 milliwatt)—the radio transmitter’s peak power consumption will be around 2 to 3 mW, depend- ing on its efficiency. Using ultra-low-power tech- niques, 1 the receiver should consume less than 1 mW. Including the dissipation of the sensors and Given appropriate power conditioning and capacitive storage, devices made from piezoelectric materials can scavenge power from low-level ambient sources to effectively support networks of ultra-low-power, peer- to-peer wireless nodes.
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ref16 - E N E R G Y H A RV E S T I N G C O N S E RVAT I O N...

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