title: A Self-Powered and Autonomous Fringing Field Capacitive Sensor Integrated into a Micro Sprinkler Spinner to Measure Soil Water Content.
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.
authors.affiliation: Department of Semiconductors, Instruments and Photonics, School of Electrical and Computer Engineering, University of Campinas, Campinas, SP 13083-820, Brazil. eduardoc@cpqd.com.br.
authors.forename: Eduardo Ferreira
authors.initials: EF
authors.lastname: da Costa
authors.affiliation: Department of Semiconductors, Instruments and Photonics, School of Electrical and Computer Engineering, University of Campinas, Campinas, SP 13083-820, Brazil. neo@fee.unicamp.br.
authors.forename: Nestor E
authors.initials: NE
authors.lastname: de Oliveira
authors.affiliation: Department of Semiconductors, Instruments and Photonics, School of Electrical and Computer Engineering, University of Campinas, Campinas, SP 13083-820, Brazil. flaviojom@tupa.unesp.br.|Faculty of Science and Engineering, São Paulo State University Júlio de Mesquita, Tupã, SP 17602-496, Brazil. flaviojom@tupa.unesp.br.
authors.forename: Flávio J O
authors.initials: FJ
authors.lastname: Morais
authors.affiliation: Department of Semiconductors, Instruments and Photonics, School of Electrical and Computer Engineering, University of Campinas, Campinas, SP 13083-820, Brazil. pcdias@utfpr.edu.br.|Department of Electrical Engineering, Paraná Federal University of Technology-UTFPR, Cornélio Procópio, PR 86300-000, Brazil. pcdias@utfpr.edu.br.|Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Barcelona 08930, Spain. pcdias@utfpr.edu.br.
authors.forename: Pedro
authors.initials: P
authors.lastname: Carvalhaes-Dias
authors.affiliation: Department of Semiconductors, Instruments and Photonics, School of Electrical and Computer Engineering, University of Campinas, Campinas, SP 13083-820, Brazil. lfduarte@utfpr.edu.br.|Department of Electrical Engineering, Paraná Federal University of Technology-UTFPR, Cornélio Procópio, PR 86300-000, Brazil. lfduarte@utfpr.edu.br.
authors.forename: Luis Fernando C
authors.initials: LF
authors.lastname: Duarte
authors.affiliation: Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Barcelona 08930, Spain. acabot@irec.cat.|Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, Barcelona 08010, Spain. acabot@irec.cat.
authors.forename: Andreu
authors.initials: A
authors.lastname: Cabot
authors.affiliation: Department of Semiconductors, Instruments and Photonics, School of Electrical and Computer Engineering, University of Campinas, Campinas, SP 13083-820, Brazil. siqueira@demic.fee.unicamp.br.|Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Barcelona 08930, Spain. siqueira@demic.fee.unicamp.br.
authors.forename: J A
authors.initials: JA
authors.lastname: Siqueira Dias
keywords: autonomous sensors; capacitive soil water content sensor; energy harvesting; micro sprinkler spinner generator; soil water content sensors; ultra-low-power circuits