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 θv 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 μA, 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.

Autonomous Soil Water Content Sensors Based on Bipolar Transistors Encapsulated in Porous Ceramic Blocks

“…In this work we present a signal conditioning system that operates from a single 5-V power supply, obtained from an energy-harvesting system with a supercapacitor, based on a DC microgenerator attached to the top of an irrigation micro-sprinkler [12]. The simplified block diagram of the developed power management circuit is presented in Figure 2.…”

“…To avoid the presence of entrapped air in the pores, the sensor was soaked for twelve hours in water (at atmospheric pressure), followed by an additional one hour under a pressure of approximately 70 kPa, to guarantee a complete saturation of the ceramic [12]. Based on the observed weight change from dry to saturated, the saturated volumetric water content of the porous ceramic was calculated as…”

“…By measuring the thermal conductivity of the porous ceramic filled with water, accurate estimations of the water content in the soil surrounding the sensor can be obtained. The use of a porous ceramic block is so efficient in overcoming the soil contact problems that even capacitive sensors have been fabricated using porous blocks [12].…”

Fringing Electric Field Sensors for Anti-Attack at System-Level Protection

“…Non-contact measurements with FEF sensors provide reliable information about material’s characteristic and property [12]. Due to possessing these features, the FEF sensor is used in many fields, for example, for soil moisture measurement, making changes in a material’s dielectrics, in motion sensors, to improve a material’s properties and to measure the moisture content of agricultural commodities [13,14,15]. The FEF sensors’ performance is largely impacted by their penetration depth, signal strength, measurement sensitivity and linearity, which are determined by factors such as sensor’s pattern, area and geometrical parameters [16].…”

Towards Improved Field Application of Using Distributed Temperature Sensing for Soil Moisture Estimation: A Laboratory Experiment

“…Another problem might be that both single probe heat pulse and dual probe heat pulse DTS measurements suffer from poor soil-cable contact. For some sensors, the usage of porous blocks represents one possibility to eliminate this problem [21]. These problems are normally overcome by introducing an apparent spacing L app to take Sensors 2020, 20, 29 3 of 16 into account the probe geometry, materials, non-zero radius, and contact resistance [22].…”