Soil moisture sensor calibration method

Abstract

The present invention relates to a soil moisture sensor calibration method. According to the present invention, the soil moisture sensor calibration method comprises the steps of: acquiring one or more pieces of soil moisture data at a wavelength in a preset range; converting the acquired one or more pieces of soil moisture data into a proxy permittivity of a preset single wavelength; and using, as input data, the converted permittivity proxy in a reverse permittivity model in which organic material was considered, so as to calculate soil moisture from which bias error due to the organic material has been removed. According to the present invention, a soil moisture sensor can be efficiently and accurately calibrated in organic soil without developing a separate calibration algorithm for each wavelength of soil moisture data acquired at one or more wavelengths, and thus soil moisture can be accurately measured even in soil with high organic material content.

Description

How to calibrate a soil moisture sensor

[0001] The present invention relates to a soil moisture sensor calibration method, and more particularly, to a soil moisture sensor calibration method for converting soil moisture data obtained at one or more wavelengths into a single proxy permittivity at a predefined wavelength and calibrating a soil moisture sensor in organic soil using an organic soil inverse permittivity model at the corresponding wavelength.

[0002] In smart farms, sensors that measure soil moisture are being used to build automatic irrigation systems and monitoring systems.

[0003] Methods for measuring soil moisture using these sensors include soil moisture tension method, gravimetric method, gypsum block method, neutron moisture measurement method, dielectric constant measurement method, and electrical resistance measurement method.

[0004] Among these, the dielectric constant measurement method is the most commonly used soil moisture measurement method. It is a method that transmits a signal from the main body of the measuring device, analyzes the characteristics of the echo wave to determine the dielectric constant of the soil, and then measures the soil moisture content using the correlation between the dielectric constant of the soil and the moisture content. It is based on the principle that the dielectric properties of the soil are affected by the amount of water in the soil.

[0005] However, existing dielectric constant measurement methods have limitations in that they are difficult to calibrate at more than one wavelength, especially at wavelengths more sensitive to organic matter, and the accuracy of the calibration is low, requiring the development of separate calibration algorithms for each wavelength of soil moisture data.

[0006] In addition, existing models determine the maximum permittivity of bound and free water by considering only the soil clay content, but the permittivity of actual soil is also greatly affected by the organic matter content.

[0007] Therefore, the existing model has a limitation in that the accuracy of soil moisture estimation may be low in soils with very high or low organic matter because it does not reflect organic matter in the setting of the maximum dielectric constant.

[0008] According to the present invention, there is provided a soil moisture sensor calibration method for converting soil moisture data obtained at one or more wavelengths into a single proxy permittivity at a predefined wavelength and calibrating a soil moisture sensor in organic soil using an organic soil inverse permittivity model at the wavelength.

[0009] According to one embodiment of the present invention for achieving such a technical task, a soil moisture sensor calibration method includes the steps of: acquiring one or more soil moisture data at a preset range of wavelengths; converting the acquired one or more soil moisture data into a proxy permittivity of a preset single wavelength; and using the converted permittivity proxy as input data to an inverse permittivity model considering organic matter to calculate soil moisture with bias error due to organic matter removed.

[0010] In this way, according to the present invention, soil moisture data obtained at one or more wavelengths can be efficiently and accurately calibrated in organic soil without having to develop a separate correction algorithm for each wavelength, thereby enabling accurate soil moisture measurement even in soil rich in organic matter.

[0011] Figure 1 is a drawing for explaining a method for estimating soil moisture.

[0012] FIG. 2 is a diagram illustrating a flow chart of a soil moisture sensor calibration method according to one embodiment of the present invention.

[0013] FIG. 3 is a diagram for explaining a step of obtaining soil moisture data according to one embodiment of the present invention.

[0014] FIG. 4 is a diagram illustrating a step of generating a dielectric proxy according to one embodiment of the present invention.

[0015] FIG. 5 is a drawing for explaining a step of correcting soil moisture according to one embodiment of the present invention.

[0016] FIG. 6 is a diagram illustrating an example comparing a dielectric proxy-based correction and a general correction at a wavelength of 100 MHz according to one embodiment of the present invention.

[0017] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. In this process, the thickness of lines and the sizes of components depicted in the drawings may be exaggerated for clarity and convenience of explanation.

[0018] Furthermore, the terms described below are defined based on their functions within the present invention, and may vary depending on the intent or custom of the user or operator. Therefore, the definitions of these terms should be based on the overall content of this specification.

[0019] Figure 1 is a drawing for explaining a method for estimating soil moisture.

[0020] First, referring to FIG. 1, a soil moisture estimation method for measuring soil moisture using a dielectric constant model in the present invention will be described.

[0021] First, the effective permittivity of completely dry organic soil is calculated using the following mathematical equation 1.

[0022]

[0023] At this time, is the effective permittivity of dry organic soil, is the clay permittivity, is the volume ratio of clay, is the permittivity of sand, is the volume ratio of sand.

[0024] also, is the silt permittivity, is the volume fraction of silt, is the organic dielectric constant, is the volume ratio of organic matter.

[0025] At this time, is calculated as shown in the following mathematical formula 2.

[0026]

[0027] At this time, SOM is the weight ratio of organic matter, is the volume density of organic matter, is the bulk density of mineral soil.

[0028] Also, the organic matter weight ratio can be converted into organic carbon weight ratio OC as shown in the following mathematical formula 3.

[0029]

[0030] At this time, is the conversion coefficient.

[0031] Also, a falsification point for estimating soil moisture (SM) is calculated as in the following mathematical expression 4, and the saturation point p is calculated as in the following mathematical expression 5.

[0032]

[0033]

[0034] When the soil moisture content is less than the withering point, the bound water acts dominantly due to the charge of the soil particles, attracting water molecules and attaching them to the surface of the soil particles, forming a thin moisture layer around the mineral particles and organic tissues.

[0035] As more water is added to the soil, this moisture layer becomes thicker, causing van der Waals forces to dominate over the charges on the particle surfaces.

[0036] When only these combinations exist, the soil moisture when the soil moisture is less than the withering point is calculated using the following mathematical equation 6. ) is estimated.

[0037]

[0038] In addition, when the soil moisture is greater than the withering point and less than the saturation point, the permittivity of the water in the soil is expressed as a complex of the permittivity of bound water and free water, and the soil moisture when the soil moisture is between the withering point and the saturation point is expressed by the following mathematical expression 7. ) is estimated.

[0039]

[0040] In addition, when the soil moisture is above the saturation point, the bound water no longer exists and all moisture becomes free water, and through the following mathematical equation 8, the soil moisture when the soil moisture is above the saturation point ( ) is estimated.

[0041]

[0042] Since soil moisture is the target of estimation, a model according to the soil moisture range cannot be applied, but soil moisture is estimated using all three models based on the observed permittivity, and then soil moisture is estimated in a discontinuous model by selecting the model with the second largest soil moisture value, as in Equation 9 below.

[0043]

[0044] Hereinafter, a soil moisture sensor calibration method according to one embodiment of the present invention will be specifically described with reference to FIGS. 2 to 5.

[0045] FIG. 2 is a diagram illustrating a flow of a soil moisture sensor calibration method according to an embodiment of the present invention, FIG. 3 is a diagram for explaining a step of acquiring soil moisture data according to an embodiment of the present invention, FIG. 4 is a diagram for explaining a step of generating a permittivity proxy according to an embodiment of the present invention, and FIG. 5 is a diagram for explaining a step of calibrating soil moisture according to an embodiment of the present invention.

[0046] As illustrated in FIGS. 2 and 3, a soil moisture sensor calibration method according to one embodiment of the present invention first acquires one or more soil moisture data at a wavelength within a preset range (e.g., 50 MHz to 18 GHz) (S110).

[0047] At this time, it is assumed that the bias error due to the applied organic matter uncertainty will be equally reflected within the error range set for soil moisture.

[0048] That is, soil moisture data is obtained without considering variables due to organic matter.

[0049] At this time, soil moisture data can be measured using sensors including TDR (Time Domain Reflectometry) or FDR (Frequency Domain Reflectometry).

[0050] Next, as illustrated in FIG. 4, one or more acquired soil moisture data are converted into a preset single wavelength (e.g., 50 MHz) proxy permittivity (S120).

[0051] More specifically, one or more acquired soil moisture data are subjected to a permittivity proxy calculation using the following mathematical expression 10 for additional correction by organic matter.

[0052]

[0053] At this time, refers to the soil moisture value observed from a soil moisture sensor in a wavelength range of a preset range (e.g., 50 MHz to 18 GHz), is a proxy for dielectric constant.

[0054] At this time, the soil moisture value observed by the soil moisture sensor is used as a parameter of the regression model between the soil moisture and dielectric constant observations so that the error becomes 0 using a second-order polynomial-based regression model (e.g., Seyfried model). , and Decide.

[0055] For example, when simulating the permittivity by inputting the soil moisture gravimetrically observed in SMAPVEX12 into the SM of the second-order polynomial, the parameters of the regression model with the minimum error from the actual observation of the 50MHz permittivity are , and am.

[0056] The parameters of the regression model determined in this way will be used in the future to provide a permittivity proxy ( ) is converted to .

[0057] Next, the converted permittivity proxy is used as input data for an inverse permittivity model that takes organic matter into account to calculate soil moisture with the bias error due to organic matter removed (S130).

[0058] More specifically, first, as in the following mathematical expression 11, the dielectric constant proxy converted to the inverse dielectric constant model, which is a maximum dielectric constant model of bound water and free water considering the soil clay content and organic matter content, is used as input data to obtain the dielectric constant information.

[0059]

[0060]

[0061]

[0062]

[0063] At this time, is the maximum permittivity of the bonding number, is the maximum permittivity of free water, OM is the organic matter content, , , , , and is an empirical constant.

[0064] The empirical constant is determined by the observational data on which it is based, and in the case of SMAPVEX12 observational data, , , , , and It is decided by .

[0065] Next, in Equations 10 and 11, , , After calculating, the calculated , , By applying to mathematical equations 6 to 8, , , and produce, and the produced , , The second value is determined as the final soil moisture.

[0066] FIG. 6 is a diagram illustrating an example comparing a dielectric proxy-based correction and a general correction at a wavelength of 100 MHz according to one embodiment of the present invention.

[0067] As illustrated in Fig. 6, the soil moisture sensor can be calibrated with a 50MHz-based dielectric model without developing a dielectric model including organic matter at a 100MHz wavelength.

[0068] In this way, according to the present invention, soil moisture data obtained at one or more wavelengths can be efficiently and accurately calibrated in organic soil without having to develop a separate correction algorithm for each wavelength, thereby enabling accurate soil moisture measurement even in soil rich in organic matter.

[0069] While the present invention has been described with reference to the embodiments illustrated in the drawings, these are merely exemplary, and those skilled in the art will understand that various modifications and equivalent embodiments are possible. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the following claims.

Claims

1. A step of acquiring one or more soil moisture data at a wavelength within a preset range; A step of converting one or more of the above-obtained soil moisture data into a proxy permittivity of a preset single wavelength; and A soil moisture sensor calibration method comprising a step of calculating soil moisture by removing bias errors due to organic matter by using the above-mentioned converted permittivity proxy as input data to an inverse permittivity model that takes organic matter into account.

2. In paragraph 1, The step of acquiring one or more soil moisture data comprises: A soil moisture sensor calibration method for obtaining soil moisture data without considering variables caused by organic matter.

3. In paragraph 1, The above soil moisture data is, A method for calibrating a soil moisture sensor measured using a sensor including a TDR (Time Domain Reflectometry) or FDR (Frequency Domain Reflectometry).

4. In paragraph 1, The step of converting the above single wavelength proxy permittivity is: A soil moisture sensor calibration method for performing a permittivity proxy calculation using the following mathematical formula for additional correction of one or more of the above-obtained soil moisture data by organic matter: At this time, is the soil moisture value observed from the soil moisture sensor in the preset wavelength range, is a proxy for dielectric constant, , and is a parameter of the regression model between soil moisture and dielectric constant observations.

5. In paragraph 1, The above inverse permittivity model is, Soil moisture sensor calibration method using the maximum bound and free water permittivity model considering soil clay content and organic matter content as shown in the following mathematical formula: At this time, is the maximum permittivity of the bonding number, is the maximum permittivity of free water, OM is the organic matter content, , , , , , is a preset empirical constant.

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