Localization, Sensing, Energy Delivery and Communication in Wirelessly Powered Systems
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Wirelessly powered sensors are proven as an effective solution for sensing and monitoring in undeveloped, inaccessible environments or where the difficulty of maintaining devices dramatically increases as the network increases in scale. In addition, small low power consumer electronics, such as wearable sensors, are constrained by the properties of their batteries. The use of a battery limits the device’s lifetime, size, hardware cost and maintenance effort (charging or replacing the battery). Among the latest battery-free solutions, radio frequency (RF) wireless power is one of the most promising technologies for supplying power, because it not only wirelessly charges passive or semi-passive devices, but also has intrinsic compatibility with existing radio communication systems. This dissertation will discuss a type of RF wirelessly powered system with extremely constrained available power, that leverages existing infrastructure, such as an RFID reader or a cellphone, for its power source. In general, the power available on these wireless devices is around a few uW to a few mW. However, some applications using these devices require power-hungry computing, sensing or communications, which require much more power than is available from RF harvesting. Therefore, the performance attainable for computation, sensing, and communication is significantly affected by energy harvesting efficiency as well as the duty cycle strategy. This dissertation will use a few application examples to demonstrate how RF energy harvesting efficiency affects the performance of wirelessly powered platforms using existing infrastructure with regard to localization, sensing and communication. This dissertation categorizes wireless power systems into two types based on the target operation range and available wireless power level. The first type comprises far-field systems whose operating ranges are beyond a few meters and which support harvested power on the order of a few to hundreds of uW. The second type comprises near field systems whose operating ranges are closer than 10cm, where the available power is greater than a few mW. The first portion of this thesis addresses the challenges and trade-offs in a far field wirelessly powered sensing system. This dissertation presents a working prototype of an RFID-based system that localizes a custom, battery-free, EPC Gen2-compatible UHF tag, and uses it as an example to discuss the typical features of a far-field system. The presented system detects the 3D position and motion of a battery-free RFID tag incorporating an ultrasound detector and an accelerometer. Combining the tag’s acceleration with absolute 3D location permits adaptive power management and supports activity recognition. It uses the RFID communication channel for synchronization and inventory, and uses acoustic propagation delays for distance measurement. We characterize the system's localization performance in open space as well as implement it in a smart wet lab application. In the wet lab setting, the system is used to track the real-time location and motion of objects with wireless powered sensing tags, as well as recognize pouring actions performed on the objects to which the tag is attached. This thesis discusses the trade-offs between sensing accuracy, latency, tag power consumption and position. The second portion of this dissertation presents the NFC-WISP as an example of a typical near-field wireless power system. The NFC-WISP is a programmable, computationally enhanced platform designed to explore new near field battery-free sensing and user interface applications. Not only is the NFC-WISP be wirelessly powered and read by commercially available RFID readers (including NFC-enabled smart phones), but the harvested wireless power is sufficient to power external power hungry sensors and peripherals such as an active bistable matrix E-ink display. Firstly, we illustrate possible applications which use the NFC-WISP as a passive display tag, such as a battery-assisted perishable goods temperature display, and a motion monitor with wireless charging features. Secondly, this dissertation discusses how to improve the communication and power harvesting efficiencies by varying the number and the quality factor (Q) of receiver coils. A revision of NFC-WISP is demonstrated with improved communication and energy harvesting performance. Finally, this dissertation discusses how to choose antenna designs to meet the requirements of different systems.
- Electrical engineering