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Energy harvesting and sensing for backscatter tags

Daskalakis Spyridon-Nektarios

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Year 2014
Type of Item Diploma Work
Bibliographic Citation Spyridon-Nektarios Daskalakis, "Energy harvesting and sensing for backscatter tags", Diploma Work, School of Electronic and Computer Engineering, Technical University of Crete, Chania, Greece, 2014
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Nowadays, RF energy harvesting has become an increasingly attractive research topic due to the proliferation of radio frequency emitters. The goal is to collect unused ambient RF energy and supply small sensors with power. In the first part, an efficient, low-cost and low-complexity rectenna-grid is analyzed, fabricated and measured, for low-power RF input and RF density. Despite the fact that a lossy and low-cost FR-4 substrate was used, the RF-to-DC rectification efficiency of 20.5% and 35.3% is achieved at -20 and -10dBm power input, respectively. The rectenna was connected to a custom DC-to-DC converter and the open voltage was increased from 298mV to 1.4V, charging a 1mF capacitor at 37min. Monitoring, environmental variables such as soil moisture, relative humidity, etc. with wireless sensor networks (WSNs) is invaluable for precision agriculture applications. However, the effectiveness of existing conventional (e.g. ZigBee-type) radios in large-scale deployments is limited by power consumption, cost and complexity constraints. While the existing WSN solutions employing non-conventional, scatter-radio principles have been restricted to communication ranges of up to a few meters, a novel joint analog design of wireless transmitter (tag) with scatter radio and extended communication ranges is presented in this thesis. Two sensing elements that can be connected with the tag were implemented, a soil moisture sensor and a plant signal sensor. The network tag with the soil moisture sensor was selected to be evaluated in terms of accuracy and communication range. The design is based on a custom microstrip capacitor, exploits bistatic analog scatter radio principles and is able to wirelessly convey soil moisture percentage by mass (% MP) with RMS error of 1.9%. Power consumption and communication range is on the order of 100 uW and 100 meters, respectively. It is tailored for ultra-low cost (5 Euro per sensor) agricultural sensor network applications for soil moisture.

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