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Sensors 2017, 17(3), 575; doi:10.3390/s17030575

A Self-Powered and Autonomous Fringing Field Capacitive Sensor Integrated into a Micro Sprinkler Spinner to Measure Soil Water Content

1
Department of Semiconductors, Instruments and Photonics, School of Electrical and Computer Engineering, University of Campinas, Campinas, SP 13083-820, Brazil
2
Department of Electrical Engineering, Paraná Federal University of Technology—UTFPR, Cornélio Procópio, PR 86300-000, Brazil
3
Faculty of Science and Engineering, São Paulo State University Júlio de Mesquita, Tupã, SP 17602-496, Brazil
4
Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Barcelona 08930, Spain
5
Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, Barcelona 08010, Spain
These authors contributed equally to this work.
*
Author to whom correspondence should be addressed.
Academic Editors: Manuel Gasulla and Ferran Reverter
Received: 29 November 2016 / Revised: 4 March 2017 / Accepted: 8 March 2017 / Published: 12 March 2017
(This article belongs to the Special Issue Autonomous Sensors)
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Abstract

We present here the design and fabrication of a self-powered and autonomous fringing field capacitive sensor to measure soil water content. The sensor is manufactured using a conventional printed circuit board and includes a porous ceramic. To read the sensor, we use a circuit that includes a 10 kHz triangle wave generator, an AC amplifier, a precision rectifier and a microcontroller. In terms of performance, the sensor’s capacitance (measured in a laboratory prototype) increases up to 5% when the volumetric water content of the porous ceramic changed from 3% to 36%, resulting in a sensitivity of S = 15.5 pF per unity change. Repeatability tests for capacitance measurement showed that the θ v sensor’s root mean square error is 0.13%. The average current consumption of the system (sensor and signal conditioning circuit) is less than 1.5 μ A, which demonstrates its suitability for being powered by energy harvesting systems. We developed a complete irrigation control system that integrates the sensor, an energy harvesting module composed of a microgenerator installed on the top of a micro sprinkler spinner, and a DC/DC converter circuit that charges a 1 F supercapacitor. The energy harvesting module operates only when the micro sprinkler spinner is irrigating the soil, and the supercapacitor is fully charged to 5 V in about 3 h during the first irrigation. After the first irrigation, with the supercap fully charged, the system can operate powered only by the supercapacitor for approximately 23 days, without any energy being harvested. View Full-Text
Keywords: autonomous sensors; soil water content sensors; capacitive soil water content sensor; energy harvesting; micro sprinkler spinner generator; ultra-low-power circuits autonomous sensors; soil water content sensors; capacitive soil water content sensor; energy harvesting; micro sprinkler spinner generator; ultra-low-power circuits
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

da Costa, E.F.; de Oliveira, N.E.; Morais, F.J.O.; Carvalhaes-Dias, P.; Duarte, L.F.C.; Cabot, A.; Siqueira Dias, J.A. A Self-Powered and Autonomous Fringing Field Capacitive Sensor Integrated into a Micro Sprinkler Spinner to Measure Soil Water Content. Sensors 2017, 17, 575.

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