| Wireless power technology offers the possibility of "cutting the last cord", thereby removing the last remaining wired connections required to power and recharge our electronic devices. A long standing goal has been to seamlessly deliver wireless power to large areas, thereby enabling users the freedom and mobility to use their devices in an unencumbered fashion. The realization of this paradigm would enable an expansive array of new consumer electronic devices and usage models. Additionally, it would create implanted medical devices capable of long term sensing and actuation and would enable embedded sensors and sensor networks capable of nearly unlimited operational life times.;However, existing technology falls short of this vision. Inductive charging solutions are limited in range/orientation and require a docking station or precise placement for effective operation. Far-field wireless power techniques have greater range, but have limits on the maximum allowable power transmitted for safety reasons. As a result, existing far-field wirelessly powered devices, such as RFID tags, are extremely limited in capability.;This dissertation explores several methods of wirelessly powering electronic devices and presents techniques for rectification, power management, and system design. Demonstrated transfer ranges vary from meters to kilometers. While delivered power levels vary from microwatts to tens of watts. The methods presented provide wireless power to large volumes of space, rather than surface or point charging, and insures safety for the general public.;The first topic uses magnetically coupled resonators for wireless power delivery. New analysis is presented that yields critical insight into key system concepts, such as frequency splitting, the maximum operating distance (critical coupling), and the behavior of the system as it becomes under-coupled. A new theoretical model has been developed and is validated against measured data, and shows an excellent agreement. An adaptive frequency tuning technique is demonstrated, which compensates for efficiency variations encountered when the transmitter to receiver distance and/or orientation are varied. This method allows the receiver to be moved to nearly any position and/or orientation within the range of the transmitter and still achieve a near constant efficiency of over 70% for a ranges of 0--70 cm. As an example application a wirelessly power laptop is demonstrated.;The second focus of this dissertation is on devices powered by far-field power transfer techniques. First, The Wireless Identification and Sensing Platform (WISP) has been developed as a programmable, battery-free sensing and computational platform, designed to explore sensor-enhanced radio frequency identification (RFID) applications. The WISP uses a 16-bit, ultra-low power microcontroller to perform sensing and computation, while exclusively operating from harvested RF energy at of range of 4.3 meters. Applications described in this document include photovoltaic enhanced RFID antennas for dual purpose energy harvesting and a capacitive touch interface for RFID tags. Our team has open sourced the WISP, and to date there are over 50 universities and research groups around the world actively working with it.;Additionally, the Wireless Ambient Energy Power (WARP) project has successfully demonstrated the ability to harvest ambient radio waves and use them to power both a commercially available home weather station and a custom built wireless sensor node. The node is capable of harvesting energy, sensing the environment, performing computation, and communicating wirelessly with a base station. The sensor node has been tested at a distance of 4.1 kilometers from a 916kW TV tower, resulting in an operational area of several tens of square kilometers, assuming line of sight to the tower. |
