RU2638150C2 - Method and device for determining soil moisture content - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: spiral resonator is proposed as a moisture sensitive device mounted on an internal dielectric pipe which is fixed by discs inside an external dielectric pipe. The device also contains measuring and electronic units, two containers for soil inserted in housing at different distance from spiral resonator, and electric probes arranged diametrically opposite with optimal orientation.

EFFECT: increased sensitivity of the spiral resonator to a change of soil moisture content, expanded measurement range, as well as convenient operation with soil samples.

2 cl, 7 dwg, 2 ex

Description

The present invention relates to the field of technology for agriculture and can be used to quickly determine soil moisture with high sensitivity in any practical range of changes in moisture or salt concentration and taking into account effects that prevent accurate determination of moisture associated with runoff of moisture along the walls of the well or along the entered into the ground probe for known devices.

A known electric sensor [1] for measuring the distribution of soil moisture over depth, in which a measurement method is used, based on the dependence of the dielectric constant of the soil on its moisture. A change in the latter is reflected in the capacitance between the electrodes in the soil and the frequency of the generator, the element of which is said capacitance. However, the disadvantage of the sensor is that humidity information is obtained with low accuracy and uncontrolled error. This is explained by the dependence of the capacitance between the electrodes in the soil not only on moisture, but also on the soil structure and the processes of interaction of the soil with the electrodes, leading to polarization of the electrodes. The latter is used to monitor soil structure.

Known remote radiophysical method for determining moisture [2], which is based on sensing the soil surface in the microwave range and the use of previously obtained radiation-moisture dependencies in laboratory conditions. The disadvantage of this method is the need to use a fairly sophisticated technique and expensive equipment.

A known method and apparatus for measuring soil moisture [3], which uses two electrodes, the electrical capacitance between which is considered proportional to the moisture content in the soil between the electrodes. However, the disadvantage of this method also lies in the presence of an error due to the influence of the soil structure and polarization of the electrodes, which are not taken into account in any way.

A known method for determining soil moisture [4], taken as a prototype for the claimed method. The method consists in taking soil samples by horizons (depth) on the field where moisture monitoring is planned, weighing them, once determining the soil density at the beginning of the observation period, determining moisture by a standard thermostat-weight method, including once drying the samples, building a calibration curve of moisture dependence on soil density, with further observation, weighing soil samples directly in the field and determining moisture from the calibration curve. This method allows you to get rid of drying a soil sample with each determination of its moisture content, aimed at reducing the complexity of the measurements.

The disadvantage of this method is the lack of accuracy in determining moisture, because in the process of plant growth changes not only the soil moisture, but also its composition. The change in soil composition is reflected in the weight of soil samples regardless of moisture, which is not taken into account in the prototype.

The technical result of the claimed method is to increase the accuracy of determining soil moisture due to the use in the calibration and measurement of such an electrical parameter of the soil, which more adequately reflects a change in its moisture during plant growth. The dielectric constant of the soil was chosen as such a parameter, the value of which for free water, the most accessible to plant roots, is an order of magnitude greater than the values for other soil components.

The specified technical result for the method is achieved by the fact that when determining soil moisture, including taking soil samples horizontally in the field, drying the samples once with simultaneous calibration of soil moisture, according to the invention, the calibration uses the dependence of the dielectric constant of the soil on its moisture content in any practice the range of changes in humidity and determine the moisture content of the soil according to a calibration curve built on the basis of this dependence. Measurements show that soil moisture can vary from 0 to 45%. The upper limit is limited to dripping moisture down.

To implement the claimed method for determining soil moisture, you need a device that is capable of tracking with sufficient accuracy the change in the dielectric constant of the soil with a change in its moisture content.

The prior art device for measuring soil moisture [5], the main elements of which are a meter of amplitude-frequency characteristics and a sensitive element in the form of a circular waveguide resonator, where a controlled soil sample is placed. Soil moisture is calculated from the measured value of the resonant frequency and the known dependence of the dielectric constant of the soil on moisture, which was obtained in [6]. The disadvantage of the device is the impossibility of its use for soil of arbitrary composition, since these dependencies were obtained only for sandy and clay soils.

A device is known having, as a moisture sensitive element, a resonator in the form of a long line segment and used in the implementation of the method for determining the moisture content of a substance [7]. In a long line, electromagnetic oscillations are excited, and one of the resonant frequencies is used as an informative parameter, in particular, the main resonant frequency, which is used to judge the value of moisture content. The use of such a sensitive element is focused only on some building materials with a known dielectric constant of dry material, which is a disadvantage of this method, as well as the device.

The closest in design to the claimed device is a radio wave sensor for determining the dielectric constant and air content in the fuel of an airplane or car, which is selected as a prototype [8]. The sensitive element of the sensor is a resonator in the form of a segment of a spiral waveguide shorted from the ends by metal plates. The measuring device of the sensor contains a resonator connected to the electronic unit. The latter in its composition has an analog or digital amplitude-frequency characteristic meter and a computer connected by a standard circuit to an electronic unit for observing the frequency response and measuring the resonant frequency. This frequency is a measured parameter and after calibration it is possible to determine the above fuel parameters.

A disadvantage of the known device is the low accuracy of the obtained value of the dielectric constant in the presence of significant energy losses of the electromagnetic field in the fuel.

This drawback is due to the inability to measure with sufficient accuracy the resonance frequency when the dielectric constant of the fuel, which has significant losses, changes, since the quality factor of the resonator becomes low.

The technical result of the claimed device is to increase the accuracy of determining soil moisture due to a more accurate measurement of its dielectric constant, including in the presence of significant energy losses for large values of moisture.

The technical result for the device is achieved in that in the known device comprising a housing, a spiral resonator and an electronic unit, according to the invention, the spiral resonator is made in the form of a hollow dielectric cylinder with a conductor located on it in the longitudinal direction, the ends of which are closed, the spiral resonator is placed on the inner a dielectric pipe, which is fixed with the help of disks inside the external dielectric pipe, two containers of symmetrical shape, external and internal, are inserted into the housing a different distance from the spiral resonator, which on the inner wall of the dielectric tube summed two coaxial cables, the ends of which are electrically probes, and the opposite ends of the cables are connected to the coaxial connectors to the cover inner dielectric tube for connection to the electronic unit.

A diagram of the inventive device for determining soil moisture is shown in FIG. 1 (a and b). In FIG. 1a shows the general construction, and FIG. 1b, the shape of the containers is shown more clearly. In the housing 1, the spiral resonator 2 is mounted on the inner dielectric tube 3, which is fixed with the help of the discs 4 inside the outer dielectric tube 5, two containers of a dielectric 6 and 7 of a symmetrical shape, external and internal, are inserted into the housing 1 at different distances from the spiral resonator, to which two coaxial cables 8 are connected along the wall of the internal dielectric pipe 3, the ends of which have electric probes, and the opposite ends of the cables are connected to the coaxial connectors 9 on the cover 10 of the external dielectric Uba 5 for connection to the electronic unit.

The containers have handles 11 and 12, the thickness of the dielectric walls affects the sensitivity and is chosen as low as possible to ensure strength. The difference between the inner container 6 and the outer container 7 is only in radial dimensions. The housing is equipped with handles 13 and is made of metal to eliminate the influence on the measurement results of external objects.

Electric probes (exciting and receiving) are located diametrically opposite. Their azimuthal orientation is changed by turning the cover 10 of the inner dielectric tube 3 and is fixed in the form of resonance curves on the screen of the electronic unit, which are generally two. By changing the orientation of the probes, it is possible to obtain one resonance curve with maximum quality factor. This eliminates the influence of the so-called polarization degeneracy effect in azimuthally symmetric vibrational systems.

The location of the conductor with closed ends is shown in FIG. 2 (a and b). Laying the conductor is performed on a segment of the hollow dielectric cylinder (Fig. 2A). The shape of the laid conductor is shown in FIG. 2b, for greater clarity, the dielectric cylinder is not shown, and the number of turns is selected small. The spiral resonator actually used in the device contains a larger number of turns. An example of a specific embodiment of such a resonator is shown in FIG. 3.

A device for determining soil moisture is used according to the proposed method as follows.

1) During calibration, a sample of moist soil is placed in one of the containers removed from the housing. In the process of drying a wet soil sample, its moisture is determined by the thermostatic-weight method, and at the same time, the resonant frequency of the electronic unit is noted when the container with the sample is placed in the device. At high initial humidity, a larger container is used. At low initial humidity - smaller. The latter is due to the need to have a higher sensitivity to changes in moisture of drier sandy soil. Sensitivity is understood as the modulus of the ratio of changes in the resonant frequency to a small change in humidity in MHz. A calibration curve is constructed in the form of a dependence of the resonant frequency on soil moisture. This dependence is due to a decrease in permittivity during the drying process.

2) To determine the moisture during the growth of plants, a soil sample is placed in the appropriate container that was used for calibration, the container is inserted into the housing of the proposed device, the value of the resonance frequency is indicated by the electronic unit and the soil moisture is determined by the calibration curve.

The difference is that instead of weighing the soil sample, the resonance frequency is counted over the electronic unit, i.e. they use the dependence of the dielectric constant of the soil on its moisture, on the basis of this dependence they build a calibration curve, which determines the moisture, which gives a significant increase in accuracy to determine the soil moisture, and this is of great importance in solving many problems in agriculture, in particular for determining the time of sowing , agricultural chemistry and in solving many other pressing problems in solving the problems of import-substituting products and technologies.

An example of the implementation of the claimed device, on which the following measurements were performed, is presented in FIG. 4, which shows: 1 - housing, 2 - spiral resonator, 14 - measuring unit, 15 - electronic unit, 16 - container with soil sample, 17 - exciting probe, 18 - receiving probe, 19 - synthesizer, 21 - amplifier, 21 — synchronous detector, 22 — analog-to-digital converter, 23 — data converter, 24 — data transmission channel, 25 — computer.

The claimed invention was tested in laboratory conditions of St. Petersburg State University and Techpribor Public Joint Stock Company in St. Petersburg. The device has a measuring unit 14 and an electronic unit 15, which is standard for measuring the resonant frequency and quality factor of the resonator in laboratory conditions. One of the containers with the soil sample 16 is located in the housing 1 next to the spiral resonator 2, so that the soil sample is in the cavity field when the latter is excited. An empty container is not used for measurements, and in FIG. 4 is not shown. The signal from the synthesizer 19 is fed to the exciting probe 17 through the amplifier 20. From the receiving probe 18, the signal is fed to the synchronous detector 21 and, after detection, to the analog-to-digital converter 22. The blocks are connected through the connectors 9 (Fig. 1). Data preparation for the software module, which is available in computer 25, is implemented in the data converter 23 and transmitted via Ethernet 24 to computer 25. The amplitude-frequency characteristic observed on the computer screen allows you to adjust the gain to eliminate non-linear distortions and determine the optimal probe orientation, about which It was said above. Starting the software module allows you to get the value of the resonant frequency of the spiral resonator. In this case, the signal transmitted through the resonator is amplified and used to accumulate digital information about the amplitude-frequency characteristic, which is reflected on the computer screen. According to the accumulated information for one clock cycle, the resonant frequency is determined using the appropriate software.

When designing the claimed device, an approximate theoretical assessment of the dependence of the resonant frequency of the spiral resonator on the relative permittivity of the external environment is possible by analogy with the calculation performed in [8]. In FIG. 5 shows an example of such a relationship using the resonator shown in FIG. 3, having an average diameter of a hollow dielectric cylinder on which a copper wire is laid, 40 mm, a dielectric thickness of 3 mm, a wire diameter of 0.5 mm and a height of 20 mm. The indicated range of changes in the relative permittivity of the environment corresponds to the calculated change in soil moisture from 0 to 10%. The methodology for this calculation was taken from [9].

In the manufacture of the proposed device in the factory for work on determining soil moisture in the field, the electronic unit is based on known electronic components in the form of a compact device with a small monitor without a computer.

Measurements were taken that showed the operability of the proposed device when it is used to implement the proposed method. The diameter of the external dielectric tube is 50 mm, the height of the casing is 100 mm, and its diameter is 200 mm.

Example 1. The outer container was filled with dried earth with the further addition of water to change the humidity. The measurement results are shown in FIG. 6 in the form of a calibration curve. With a diameter d of an external container of 120 mm, clear resonance curves are observed with a Q factor exceeding 85. The dependence of the Q factor on humidity is shown in FIG. 7. With increasing humidity in the region of its large values, a slight increase in the quality factor is observed. The analysis showed that this region corresponds to the regime of soil supersaturation with moisture, in which moisture flows down, and does not apply to the calibration curve. Therefore, in the process of calibration, it is useful to monitor the quality factor and provide such an opportunity when creating an electronic unit. The dependence of the resonant frequency on humidity should be monotonic. The given numerical values can be considered indicative when designing the proposed device.

Example 2. Measurements were taken with soil containing sand. The conductivity of such soil is lower than in example 1. Samples of sandy soil is best placed in an internal container. In this case, the sensitivity increases for small values of moisture, which is essential for sandy soils, but the Q factor of the resonance curve does not significantly decrease.

The claimed device is free from the disadvantages of the device adopted as a prototype. Technical and economic efficiency of the invention is determined by the following characteristics:

- sufficient sensitivity to a change in soil moisture during plant growth and fertilizing, due to the large value of the relative dielectric constant of free water, the most accessible to plant roots, compared with values for other soil components,

- the need to dry the soil sample for the selected field and horizon only during calibration,

- obtaining information about the moisture content of the soil sample directly in the field,

- a weak dependence of the measurement results on the concentration of salt in water, due to the weak dependence of the dielectric constant of water on salt concentration,

- the convenience of working with soil samples placed during measurements in a separate container,

- the ability to take measurements in any practical range of changes in humidity or salt concentration,

- the absence of interfering effects associated with runoff of moisture along the walls of the well or along the probe introduced into the ground for known devices.

Literature

1. Dolgopyatov P.M., Katz R.I., Belnkoev V.V. An electrical sensor for measuring the distribution of soil moisture over depth. AS of the USSR No. 573743, 1977.

2. Komarov S.A., Mironov V.L., Romanov A.N. and others. Remote radiophysical method for determining soil moisture. RF patent No. 2010219, 1994.

3. WILLS ROBERT H. Method and apparatus for measuring soil salinity. US Application No. 4646000, 1984.

4. Yasonidi O.E., Putrina M.V., Serezhnikov D.I., Yasonidi A.O. A method for determining soil moisture. RF patent No. 2439559, 2010 (prototype for the np method).

5. Akhobadze G.N. A device for measuring soil moisture. RF patent No. 2433393, 2010.

6. Leshchinsky Yu.I., Lebedev G.N., Shumilin V.D. Electrical parameters of sand and clay soil in the range of centimeter, decimeter and meter waves. - University proceedings, Radiophysics, 1971, T. XIV, No. 4, p. 562-569.

7. Sovlukov A.S. A method for determining the moisture content of a substance. RF patent No. 2468358, 2012.

8. Stolyarov OI, Novikov A.YU., Latyshev E.E., Naumov D.I. Study of a spiral resonator as a sensitive element of a radio wave sensor. Regional XIX Conference on the Propagation of Radio Waves. Conference proceedings. St. Petersburg, 2013, p. 123-126 (prototype for np device).

9. Lichtenecker K., Rother K. Die herleitung des logarithmischen mischungsgesetzes des allgemeinen prinzipien der stationaren stromung // Physikalishe zeitschrift. - 1931. - Bd 32, No. 6. - S. 255-260.

Claims (2)

1. A device for determining soil moisture, comprising a housing, a spiral resonator and an electronic unit, characterized in that the spiral resonator is made in the form of a hollow dielectric cylinder with a conductor located on it in the longitudinal direction, the ends of which are closed, the spiral resonator is placed on the inner dielectric pipe, which, with the help of disks, is fixed inside the external dielectric pipe, two containers of symmetrical shape, external and internal, are inserted into the housing at different distances from the spiral cut an onator, to which two coaxial cables are connected along the wall of the internal dielectric pipe, the ends of which have electric probes, and the opposite ends of the cables are connected to the coaxial connectors on the cover of the internal dielectric pipe for connection with the electronic unit. 2. The method of determining soil moisture using the device according to claim 1, including taking soil samples horizontally in the field, drying samples once with simultaneous calibration of soil moisture, characterized in that the calibration uses the dependence of the dielectric constant of the soil on its moisture and determines the soil moisture according to the calibration curve constructed on the basis of this dependence.

Images