BR102020024523A2 - SOIL MOISTURE SENSOR - Google Patents

Abstract

soil moisture sensor. the present invention is part of the field of electrochemistry with application in the field of soil science and describes a resistive sensor used to measure soil moisture. in this sense, the objectives of the invention are achieved by means of a soil moisture sensor characterized by comprising: a rod (1) with two conductive ends, the rod (1) being made from a sheet of electrical insulating material (3 ) with a layer of graphite (2) disposed thereon and a hole positioned at the top of the rod for connecting a wire of conductive material (4) to the layer of graphite (2) of the rod (1).

Description

SOIL MOISTURE SENSOR field of invention

[1] The present invention refers to a resistive sensor used to measure soil moisture with applications ranging from disaster prevention, such as breaking barriers, to applications in irrigation systems.

[2] The present invention is part of the field of electrochemistry with application in the field of soil science.

Fundamentals of the invention

[3] According to 2015 data from FAO 2015, water scarcity will lead to reduced agricultural production and income, and investments in technologies such as this sensor can mitigate the impacts of growing water scarcity. 2014 data from ANA show a reduction in rainfall in some regions since 2012, compromising the supply of water for human supply. Irrigation is the main water-consuming activity in Brazil, accounting for 68% of drinking water consumption (ANA, 2019). In addition, there is also an economic problem, which is the granting of the right to use water. Controlling the amount of water used in irrigation can also generate savings, especially for small producers who do not have the resources to purchase more sophisticated equipment.

[4] Currently, there is a wide variety of sensors used to measure soil moisture. The main types are resistive, capacitive and induction. In order for a humidity sensor to be considered of good quality, it must fulfill certain requirements, such as having a long service life, good sensitivity and fast response.

[5] One of the main problems encountered in the development of humidity sensors is related to the useful life of these sensors. This is because humidity sensors work based on an analog signal provided by a circuit connected to plates of a conductive material (usually metallic). Soil moisture is measured precisely through the resistance between the sheets, however, the electric current that runs through the sheets ends up resulting in an electrolysis process that culminates in the corrosion of the sheets and the consequent loss of material and area of contact with the ground. .

[6] Low-cost humidity sensors, due to the aforementioned wear process, end up working for periods of less than 20 days, depending on the application.

[7] With this in mind, the present invention presents a solution to increase the useful life of humidity sensors, through the development and evaluation of non-metallic sensors based on high performance conductive graphite, which allow the performance of the necessary measurements during long time intervals.

State of the Technique:

[8] A document that describes matter related to such technology is the document Li, Fu Sheng, et al. "Research of Sensing Probe Material in Wet and Dry Soil." Advanced Materials Research, vol. 807-809, Trans Tech Publications, Ltd., Sept. 2013, p. 1715-1718, Crowdsourced determination of movable device location, which presents an analysis of the conductive properties of a probe (conducting rod) of wet and dry soil in different structures under different humidity conditions. However, the document uses a continuous material of graphite in different concentrations, which leads to little ability to differentiate dry from wet soil.

[9] The Fiedler, Sabine et al. "Soil Redox Potential: Importance, Field Measurements, And Observations" Advances in Agronomy, Volume 94 Copyright 2007, Elsevier Inc. describes matter related to soil reduction potential. More specifically, how the measurements obtained are related to voltage records obtained over time from a system of electrodes inserted into the soil. However, this document uses a system with electrodes mounted to oxidize carbon in order to measure soil redox potential.

Brief description of the invention:

[10] The present invention refers to a resistive sensor used to measure soil moisture with applications from disaster prevention, such as breaking barriers, to applications in irrigation systems, and comprises: a rod with two conductive ends, the rod being made from a sheet of electrically insulating material with a layer of graphite disposed thereon and a hole positioned at the top of the rod for connecting a wire of conductive material to the graphite layer of the rod.

[11] The electrical insulating material of the sheet can be selected from: acrylic, phenolite, glass or ceramic.

Brief description of figures:

[12] Figure 1A shows a schematic of the lead rod of the graphite-based moisture sensor, according to a preferred construction of the present invention.

[13] Figure 1B shows a photograph of the lead rod of the graphite-based moisture sensor, according to a preferred construction of the present invention.

[14] Figure 1C shows a connection region between the wire of conductive material and the graphite layer of the rod, according to a preferred construction of the present invention.

[15] Figure 2 presents a simplified circuit diagram assembled for monitoring the sensor, according to a configuration of the present invention.

[16] Figure 3 shows an electrical circuit diagram developed for the evaluation of sensors, according to an example of application of the present invention.

[17] Figure 4 shows a set of graphs that represent the data received from an experiment developed according to an example application of the present invention.

[18] Figure 5 presents a comparative graph between the theoretical humidity and the humidity read by the graphite sensors, according to an example of application of the present invention.

[19] Figure 6 presents a graph with the reading of data from graphite sensors, according to an example of application of the present invention.

Detailed description of the invention:

[20] The present invention falls within the field of electrochemistry and describes a resistive sensor used to measure soil moisture.

[21] The present invention describes an environmental sensor based on high performance non-metallic conductive material, comprising: a rod (1) with two conductive ends, the rod (1) being made from a sheet of electrical insulating material (3) with a layer of graphite (2) disposed thereon and a hole positioned at the top of the rod for connecting a wire of conductive material (4) to the layer of graphite (2) of the rod (1).

[22] The fabrication of the conductive rods that will be used to design soil moisture sensors can be carried out through the use of various metallic and non-metallic materials. In a specific example, preferably a high performance conductive graphite is used, and graphene, gold, platinum, stainless steel, among other conductive materials, can also be used. The process of applying this material for the manufacture of sensors can be done manually on sheets of non-conductive (or electrically insulating) material, such as acrylic, phenolite, glass, ceramics, etc., so that graphite ( 2) merges with the electrical insulator 3 leaving a thin layer of graphite (2) arranged over it.

[23] Next, the sheets are taken to a machining machine where they are preferably cut in the shape of a rod with two conductive ends, as shown in Figure 1B.

[24] As can be seen in Figure 1A, the rod with two conductive ends (1) of the sensor has a graphite layer (2) arranged on the acrylic sheet (3).

[25] In a preferred embodiment, a hole is drilled at the top of the rod to connect a copper wire (4) to the surface layer of graphite (2), preventing water from coming into contact with the conductive metal of the wire (4) and thus prevent corrosion. This preferred connection way is demonstrated in Figure 1C.

[26] In the constructive mode, the monitoring of the sensors is done through a monitoring circuit that comprises a micro controller (5), preferably an Arduino micro controller, which provides the creation of an environment for the development of interactive objects using the concept for data collection and tracking, a contact matrix ("breadboard"), a Real Time Clock (RTC) module and one or more resistors R up to 10 kΩ.

[27] As can be seen in Figure 2, an example of an electrical schematic of a monitoring circuit produced according to the above is presented. In this example, an analog port A of the microcontroller (5) is used to measure the voltage of the humidity sensor, and a resistor R, connects one of the conducting ends of the rod (1) to a point of the microcontroller 5 with voltage V1. A voltage of 5V is used by the Arduino, but strictly speaking there is no limitation on the voltage and theoretically either DC or AC voltage can be used, while the other end of the rod (1) is connected to ground.

[28] The operating principle of the resistive humidity sensors of the present invention is based on the application of a potential difference (voltage) between the two conductive ends (1) that were embedded in the ground. In this way, the portion of wet soil existing between the two conductive ends (1) provides an electrical resistance value that varies according to the volume of water contained therein.

onde:
Sa = sinal analógico
Rs = resistência do solo, em kΩ
R = resistência do circuito, em kΩ[29] In this sense, in a preferred configuration of the present invention, by means of an analog-to-digital converter (ADC) included in the monitoring circuit, the ground resistance is converted into an analog signal, where 0 represents an open circuit and 1023 an analog signal. short circuit. The formula used for the conversion is:

Where:
Sa = analog signal
Rs = ground resistance, in kΩ
R = circuit resistance, in kΩ

[30] Preferably, the maximum analog signal values obtained during a given time interval can be compared to experimental or theoretical data in order to define a threshold humidity value.

onde:
P = massa total do solo úmido, em gramas
Ps = massa do solo seco, em gramas
Pa = massa de água, em gramas
h = umidade do solo[31] In a preferred embodiment, the percentage of soil moisture can be calculated using the following formula:

Where:
P = total mass of wet soil, in grams
Ps = dry soil mass, in grams
Pa = mass of water, in grams
h = soil moisture

[32] In this way, in the example configuration above, a basis for comparing the data read from the graphite-based moisture sensor with the moisture data provided from the mass of the set is established. Using the maximum values of analog signal detected over time Vs, that is, the maximum values of conductivity, it is calculated at these points, from Equation 2, a value of minimum soil moisture Um to be considered as a parameter for soil saturated with water.

onde:
Us = Umidade do solo, em porcentagem
Vs = valor máximo de sinal analógico detectado, em bits
Um = valor de umidade do solo mínima para solo saturado de água, em porcentagem[33] Finally, with the data obtained according to the configuration above, the analog signal read from the graphite sensor monitoring circuit (5) can be converted into soil moisture percentage values by the following equation:

Where:
Us = Soil moisture, in percentage
Vs = maximum detected analog signal value, in bits
Um = minimum soil moisture value for water saturated soil, in percentage

[34] In another preferred embodiment of the present invention, the measured soil strength can be used to provide a soil water potential.

Example 1: Experimental test

[35] Experimental tests to simulate the use of graphite sensors in real wet and dry soil situations were performed according to the procedure below.

[36] The sensor used has a layer of graphite deposited on an acrylic sheet and, on top, a hole that connects a copper wire to the surface layer of graphite.

[37] The circuit used to monitor the sensors can be seen in Figure 3.

[38] This circuit has an Arduino microcontroller, a breadboard, a set of relays and resistors.

[39] In the tests, the relays were used to avoid the generation of an electric field that could influence the measurement of sensors placed next to each other, so that when one sensor sent the data, the others would be turned off.

[40] In addition, a digital balance was also used for the experimental tests to measure the mass of the water + soil set and a dehumidifier to accelerate the process of evaporation of water from the soil.

[41] The experiment was isolated from the external environment through a glass box sealed with plastic and tape.

[42] Over 50 days, 3 graphite-based sensors and 1 commercial sensor, which has two copper tracks with a thin layer of tin on top and a layer of phenolite on the bottom, were exposed to a mixture of 268 grams. of soil and 430 ml of tap water. After 29 days the set was replenished with 300 ml of water.

[43] The data received over 50 days can be seen in Figure 4.

[44] The digital balance was tared with the mass of the aforementioned experimental set, so that, as water evaporates from the system, the mass over time is given in a negative way.

[45] The analog signal shown in the graphs of Figure 4 and received by the sensors in the experiment directly represents the variation of the wet soil resistance over time, and the analog signal relating the circuit resistance to the soil resistance can be calculated according to Equation 1 described above.

[46] Next, the percentage of soil moisture is calculated with the mass of water and the mass of dry soil through Equation 2, in order to use the mass graph of the set in Figure 4 as a reference for real soil moisture. Thus, a comparative basis of the data read from the graphite sensors is obtained with the real moisture data provided by the mass of the set.

[47] From the analog signal data collected in the graphs of Figure 4 and using Equation 2, the maximum values of conductivity Vs for each sensor can be obtained and related to a minimum soil moisture Um so that this maximum value of conductivity, which represents water-saturated soil, is achieved.

[48] Table 1 correlates these values for graphite sensors as follows:

[49] Therefore, it is possible to calculate the percentage value of soil moisture Us for each analog signal reading point over time by the graphite sensors with Equation 3 above.

[50] Figure 5 shows a graph that compares the theoretical humidity and the humidity read by the graphite sensor (2) over a period of 29 days, discarding the period where there was data loss.

[51] The initial section of the graph in Figure 5 shows a section where the theoretical curve (theoretical humidity) and the graphite sensor curve (sensory humidity) do not match, which is due to the fact that the graphite sensor reaches its maximum value of conductivity, that is, from a certain point the amount of water in the soil no longer interferes with the sensor reading. It is also demonstrated that, after the theoretical humidity reaches the point of 116% humidity (value calculated in table 1 for sensor 2) and starts to decrease, the curves start to present a behavior pattern with a high degree of correspondence.

[52] Therefore, the results obtained by graphite sensors demonstrate that such sensors represent a technological advance and can be great substitutes for commercial sensors, since they provide longer duration to the equipment and resistance to the corrosion process that affects the usual sensors, as well as as a high accuracy in moisture measurement as seen in the graph of Figure 5.

Example 2: Experimental test

[53] In this example, two types of sensors were tested and compared: the graphite sensor and a commercial type 2 sensor.

[54] The commercial type 2 sensor is a Watermark 200 SS, which has two rings of concentric electrodes and a layer of plaster for stabilization against changes in salinity arranged between the rings.

[55] The experiment was conducted using 15 samples of both types of sensors deposited in potted plants in a controlled environment, where the soil was irrigated every 2 days.

[56] The graphite sensors in this experiment do not have a manual reader like the commercial type 2 sensor and, therefore, their data are treated from a circuit as shown in Figure 2, with an Arduino micro controller, a breadboard, an RTC module and 10 kΩ resistors. In other tests, these sensors are compatible with Watermark readers, that is, they work on equipment that is already on the market and, depending on the application, they would not need their own equipment.

[57] Data from the graphite sensors were collected at 1 minute intervals and stored on a micro SD card, while the commercial type 2 sensor readings were manually collected at a two-day time interval with the scanner's own reader. equipment, before and after soil irrigation.

[58] Both sensors have the same working principle, differing only in relation to the information provided by them through the resistance between the electrodes. For the graphite sensor, an analog-to-digital converter (ADC) present in the designed circuit is responsible for converting the resistance into analog signal bits according to equation 1 shown above. In the commercial type 2 sensor, the resistance is used to provide the soil water potential measured in kPa.

[59] Figure 6 shows data collected from graphite sensors over 6 days with water replenishment intervals.

[60] The reading peaks verified in Figure 6 occur on the days of water replacement, being the moments when the soil presents the highest humidity and, consequently, the greatest propensity to conduct electric current. Other conductivity peaks outside of water replenishment periods may be associated with rising humidity at the site at these times.

[61] Data from commercial sensors was collected by the product's own equipment. By way of comparison, the same equipment was used to collect data from the graphite sensors, since they have the same working principle. The comparative results for the soil water potential values, in kPa, measured with the commercial type 2 sensor reader can be seen in table 2 below:

[62] As shown in Table 2, the commercial type 2 sensor does not capture variations in the period of 2 days without soil irrigation, while the graphite sensor is capable of being sensitive to this irrigation interval, having an average variation of 5 kPa in the period without water replacement.

Claims (2)

Soil moisture sensor characterized by comprising:
a rod (1) with two conductive ends, the rod (1) being made from a sheet of electrically insulating material (3) with a layer of graphite (2) disposed thereon and a hole positioned at the top of the rod for connect a wire of conductive material (4) to the graphite layer (2) of the rod (1). Sensor, according to claim 1, characterized in that the electrical insulating material (3) of the sheet is one of: acrylic, phenolite, glass or ceramic.

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