A soil moisture measurement method by multi-frequency measurement of impedance

Abstract

The application relates to a soil moisture measurement method by measuring impedance at multiple frequencies, measuring impedance spectra of different water content soils, processing data, removing useless and related information by PCA, extracting principal components of the impedance spectra, obtaining principal components with higher variance in the data and average values of the data, retaining the first two principal components, then establishing a mathematical model, correlating the obtained principal component information with soil water content information, that is, obtaining soil water content by measuring impedance spectra obtained by a measuring system. Test proves that the method can well measure and show changes of soil water content. The mathematical model established by the method provided by the application overcomes the limitation that temperature, soil composition components and other interference factors must be error-corrected during traditional soil water content measurement, simplifies complexity during actual operation, and improves work efficiency.

Description

Technical Field

[0001] This invention belongs to the field of soil parameter detection, specifically relating to a method for measuring soil moisture by measuring impedance at multiple frequencies. Background Technology

[0002] As soil health deteriorates, there is an urgent need for methods to accurately monitor soil health. Therefore, monitoring soil information in a more substantial and quantifiable way is becoming increasingly important. In the past, soil monitoring meant going into the field and using physical methods to process and sample the soil. While this method of physically processing soil to obtain basic information still has high accuracy and reliability, today's technology makes remote soil monitoring and tracking parameters that cannot be easily or quickly measured manually possible. The development of soil sensors has made soil monitoring and measurement simpler and faster, allowing for real-time and precise observation of changes in various soil parameters. Soil moisture content, as an important indicator of soil quality, has a significant impact on crop growth, ecological environment improvement, and wildlife habitat; therefore, methods for measuring soil moisture content are receiving increasing attention.

[0003] Existing methods for measuring soil moisture mainly include the oven drying method, the neutron method, and the time domain reflectance method (TDR).

[0004] The drying method is the international standard method for determining soil moisture. However, the process requires disturbing the soil for sampling, and the drying time to constant weight is relatively long, resulting in slow results and making it unsuitable for continuous observation at fixed points.

[0005] The neutron method does not damage the soil, but it can lead to soil radiation contamination, and fast neutrons can easily escape from the soil, which may have some negative effects on human health.

[0006] Time-domain reflectometry (TDR) utilizes the difference in the speed of electromagnetic waves traveling in different media to detect soil moisture content. However, TDR is relatively expensive and its cost is too high, so it is not currently used on a large scale.

[0007] In addition, existing methods for measuring soil moisture content have limitations, requiring error correction for interfering factors such as temperature and soil composition. Summary of the Invention

[0008] In view of the above-mentioned problems in the prior art, the technical problem to be solved by the present invention is to provide a method for measuring soil moisture by measuring impedance at multiple frequencies.

[0009] This invention is implemented as follows:

[0010] A method for measuring soil moisture by measuring impedance at multiple frequencies, the method comprising:

[0011] Impedance spectrum information of soils with different moisture contents at different frequency points was collected.

[0012] By removing useless and relevant information through principal component analysis, the principal components of the impedance spectrum are extracted. The first two principal components are taken as the first principal component coeff1 and the second principal component coeff2.

[0013] The principal components of the soil for each water content are calculated as the first principal component score (score1) and the second principal component score (score2), respectively.

[0014] Using the first principal component score (score1) as the x-axis and the second principal component score (score2) as the y-axis, a scatter plot of the principal components was constructed for each soil sample. The angle between the coordinate vector formed by the two principal components and the positive x-axis changed regularly with decreasing water content. By fitting the relationship between the angle and the water content of different samples, a model of the relationship between principal component scores and soil water content was established.

[0015] ;

[0016] When measuring soil with unknown moisture content, the principal component score is obtained by substituting the data vector of the measured soil impedance spectrum into the following formula:

[0017] ,

[0018] In the formula, "score" is a matrix composed of principal component scores, including the scores of the first principal component. Second principal component scores A is composed of the first principal component coeff1 and the second principal component coeff2, and the D matrix is ​​a vector composed of the measured impedance spectrum data.

[0019] The water content of the soil to be tested was obtained by establishing a model relating principal component fractions to soil water content.

[0020] Furthermore, the acquisition of impedance spectrum information of soils with different moisture contents at different frequency points includes setting the acquired signals as n sets of observations, with each set of data consisting of impedance spectrum data from m frequency points, to obtain an n×m impedance spectrum matrix. Each row in the matrix represents the impedance spectrum information of a certain water content, and each column in the matrix represents the impedance spectrum information of different water contents at the same frequency.

[0021] Furthermore: Principal component analysis is used to remove useless and relevant information, and the principal components of the impedance spectrum are extracted, including:

[0022] Calculate the impedance spectrum covariance matrix:

[0023] For a soil sample, the impedance values ​​are observed at m frequency points. , ,… The impedance spectrum matrices of n groups of soils are:

[0024] X= =( , ,… ),

[0025] in: j=1,2,…m;

[0026] variance:

[0027] = i=1, 2…, n; j=1, 2,…, m;

[0028] and The covariance is:

[0029] ,

[0030] i=1, 2…, n; j, k=1, 2,…, m,

[0031] The matrix composed of variance and covariance is called the covariance matrix of the soil impedance spectrum:

[0032] C= ,

[0033] In the formula, That is, the elements on the diagonal of the covariance of the soil impedance spectrum are The variance, due to The covariance is a diagonal matrix;

[0034] Calculate the eigenvalues ​​of the soil impedance spectrum covariance matrix C and the corresponding orthogonalized unit eigenvectors ;if U and λ are the eigenvalues ​​and eigenvectors of the covariance matrix C, respectively. Then the following equation holds: CU = U, written as:

[0035] (C-) U=0;

[0036] The first a eigenvalues ​​of the soil impedance spectrum covariance matrix C It refers to the variance corresponding to the first a principal components. corresponding unit eigenvector This is called the load vector;

[0037] Principal components were selected based on the variance contribution rate, resulting in two principal components.

[0038] Compared with existing technologies, the advantages of this invention are as follows: By measuring the impedance spectra of soils with different moisture contents, and then using PCA (principal component analysis) to remove useless and irrelevant information, the principal components of the impedance spectra are extracted. Principal components with high variance and the average value of the data are obtained, and the first two principal components are retained (the cumulative contribution rate of these two principal components exceeds 85%; only when the cumulative contribution rate exceeds 85% can it be guaranteed that the selected principal components contain the vast majority of information from the original variables). Then, a mathematical model is established to correlate the obtained principal component information with soil moisture content information, that is, the soil water content is obtained through the impedance spectrum measured by the measurement system. Experiments have shown that this method can effectively measure and display changes in soil water content. The method provided by this invention overcomes the limitations of traditional soil water content measurements, which require error correction for interference factors such as temperature and soil composition. It simplifies the complexity of actual operation, improves work efficiency, and has the advantages of not polluting the environment and high accuracy. Attached Figure Description

[0039] Figure 1 This is a structural block diagram of the system used in the method of the present invention.

[0040] Figure 2 The sweep impedance diagrams are for different water contents used in the method of this invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0042] This invention employs, as follows Figure 1 The multi-frequency impedance soil moisture measurement system shown consists of 5 units. First, the STM32 controller sends a serial data control signal to the excitation unit (the excitation unit uses a DDS module AD9910) through ordinary wires to generate an electrical signal of a specific frequency. Then, the signal excitation unit simultaneously transmits an excitation signal of the set frequency to the measurement probe and the signal conversion unit.

[0043] The signal transmitted to the probe changes to varying degrees due to the different water content in the impedance system composed of the probe and the soil. This change is caused by the variation in soil water content affecting the polarization intensity of the soil under the influence of an electric field, thus directly impacting the soil's dielectric constant. Soil mainly consists of air, minerals, soil particles, and water. Water has a dielectric constant of approximately 80, while the relative dielectric constants of other substances range from 1 to 5. This directly results in the overall dielectric constant of the soil varying from 5 to 50 depending on the water content. Therefore, obtaining soil moisture content can be transformed into measuring the soil dielectric constant. The magnitude of the dielectric constant can be reflected by the signal changes in the impedance system.

[0044] The dielectric constant of the soil directly causes the impedance change of the impedance system. This impedance change caused by the change in soil moisture can be obtained by comparing the measured signal and the excitation signal. This is achieved by the signal conversion unit. The signal conversion unit can compare the excitation signal and the measured signal to obtain the change in amplitude and phase between the measured signal and the excitation signal, and directly convert this change into DC voltage output.

[0045] At this point, the voltage value obtained from the signal conversion block can be read using the STM32's built-in ADC and preliminarily processed. The data after this initial processing is still relatively raw and doesn't provide a clear indication of soil moisture content. Therefore, it's necessary to send the raw data to a host computer via serial port. The host computer can then obtain the impedance information corresponding to the current frequency point of the tested soil.

[0046] After acquiring impedance information data at one frequency point of the soil being tested, the controller will again control the signal excitation unit to generate excitation signals at other frequency points, continuously repeating the above measurement process until the impedance information corresponding to the preset frequency points has been acquired.

[0047] At this point, a multi-frequency impedance spectrum of the tested soil can be obtained, consisting of pre-set frequency points and the soil impedance information corresponding to each frequency point. Furthermore, the frequency of the signal excitation output can be controlled by the controller, allowing the output signal to vary within a specific frequency range using a user-defined frequency gradient.

[0048] Then, the impedance spectrum is processed by the algorithm in the host computer software, and the water content in the soil is displayed to the user more intuitively through images or numbers.

[0049] Specifically, the present invention provides a method for measuring soil moisture by measuring impedance at multiple frequencies, the method comprising:

[0050] Impedance spectrum information of soils with different moisture contents at different frequency points was collected. This involved setting the collected signals as n sets of observations, with each set consisting of impedance spectrum data from m frequency points, to obtain an n×m impedance spectrum matrix. Each row in the matrix represents the impedance spectrum information for a specific water content. See also... Figure 2 The sweep frequency impedance diagrams corresponding to different water contents used in the method of this invention are shown below.

[0051] Principal component analysis was used to remove useless and irrelevant information, and the principal components of the impedance spectrum were extracted. The first two principal components were selected as the first principal component (coeff1) and the second principal component (coeff2). Specifically, this included:

[0052] Calculate the impedance spectrum covariance matrix:

[0053] For a soil sample, the impedance values ​​are observed at m frequency points. , ,… The impedance spectrum matrices of n groups of soils are:

[0054] X= =( , ,… ),

[0055] in: j=1,2,…m,

[0056] variance:

[0057] = i=1, 2…, n; j=1, 2,…, m;

[0058] and The covariance is:

[0059] ,

[0060] i=1, 2…, n; j, k=1, 2,…, m,

[0061] The matrix composed of variance and covariance is called the covariance matrix of the soil impedance spectrum:

[0062] C= ,

[0063] In the formula, That is, the elements on the diagonal of the covariance of the soil impedance spectrum are The variance, due to The covariance is a diagonal matrix;

[0064] Calculate the eigenvalues ​​of the soil impedance spectrum covariance matrix C and the corresponding orthogonalized unit eigenvectors ;if U and λ are the eigenvalues ​​and eigenvectors of the covariance matrix C, respectively. Then the following equation holds: CU = U, written as:

[0065] (C-) U=0;

[0066] The first a eigenvalues ​​of the soil impedance spectrum covariance matrix C It refers to the variance corresponding to the first a principal components. corresponding unit eigenvector This is called the load vector;

[0067] Principal components were selected based on the variance contribution rate, resulting in two principal components with a cumulative contribution rate exceeding 85%.

[0068] Variance contribution rate of principal components A higher contribution rate indicates that the principal component contains more information about the original variables. Principal component analysis yields m principal components; however, their corresponding eigenvalues ​​decrease from largest to smallest, and the amount of information about the original variables they contain also decreases. Only when the cumulative contribution rate exceeds 85% can it be guaranteed that the selected principal components contain the vast majority of information about the original variables.

[0069] The principal components of the soil for each water content are calculated as the first principal component score (score1) and the second principal component score (score2), respectively.

[0070] Using the first principal component score (score1) as the x-axis and the second principal component score (score2) as the y-axis, a scatter plot of the principal components was constructed for each soil sample. The angle between the coordinate vector formed by the two principal components and the positive x-axis changed regularly with decreasing water content. By fitting the relationship between the angle and the water content of different samples, a model of the relationship between principal component scores and soil water content was established.

[0071] ;

[0072] When measuring soil with unknown moisture content, the principal component score is obtained by substituting the data vector of the measured soil impedance spectrum into the following formula:

[0073] ,

[0074] In the formula, "score" is a matrix composed of principal component scores, including the scores of the first principal component. Second principal component scores A is composed of the first principal component coeff1 and the second principal component coeff2, and the D matrix is ​​a vector composed of the measured impedance spectrum data.

[0075] The water content of the soil to be tested was obtained by establishing a model relating principal component fractions to soil water content.

[0076] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A soil moisture measurement method by multi-frequency measurement of impedance, characterized in that: The method comprises: Collecting impedance information corresponding to different frequency points of different water content soils; Removing useless and related information through principal component analysis, extracting principal components of the impedance spectrum, and taking the first two principal components as a first principal component coeff1 and a second principal component coeff2; The principal components of the soil for each water content are calculated to be a first principal component score and a second principal component score ; Based on the first principal component score The x-axis represents the scores of the second principal components. Using the ordinate as the vertical axis, a scatter plot of the principal components is constructed for each soil sample. The angle between the coordinate vector formed by the two principal components and the positive x-axis changes regularly with decreasing water content. By fitting the relationship between the angle and the water content of different samples, a model is established to show the relationship between principal component scores and soil water content. ； When measuring the soil with unknown water content, the data vector of the measured soil impedance spectrum is brought into the following formula to obtain the principal component score: ， where score is a matrix of principal component scores, including a first principal component score and a second principal component score , A is composed of the first principal component coeffl and the second principal component coeff2, and D is a vector of measured impedance spectrum data. Through the established model of the relationship between the principal component score and the soil water content, the water content of the soil to be measured is obtained.

2. A method of soil moisture measurement by measuring impedance at multiple frequencies as claimed in claim 1, wherein: The impedance information of the soil with different water contents at different frequency points is collected, including setting the collected signals as n groups of observation values, each group of data being impedance spectrum data of m frequency points, and obtaining an n*m impedance spectrum matrix , each row of the matrix representing impedance spectrum information of one water content.

3. A method of soil moisture measurement by measuring impedance at multiple frequencies as claimed in claim 2, wherein: Removing useless and related information through principal component analysis, extracting principal components of the impedance spectrum, including: Calculating the impedance spectrum covariance matrix: For one soil sample, observe impedance values at m frequency points , ,... , impedance spectrum matrix of n groups of soil is: X= =（ ， ，… ）； wherein: , j = 1, 2,... m; variance: = i = 1, 2,..., n; j = 1, 2,..., m; and The covariance of X and Y is: ， i=1, 2…, n; j, k=1, 2, …, m; The matrix composed of the variance and the covariance becomes the covariance matrix of the soil impedance spectrum: C= ， wherein i.e. the elements on the diagonal of the covariance of the soil impedance spectrum are the variance of the covariance is diagonal. Eigenvalues of the soil impedance spectrum covariance matrix C are computed and the corresponding orthonormal unit eigenvectors ; if and U are the eigenvalues and eigenvectors of the covariance matrix C, respectively, then the following holds: CU= U, written as: (C- ) U = 0; the first a eigenvalues of the covariance matrix C of the soil impedance spectrum are the variances corresponding to the first a principal components, the corresponding unit eigenvectors are called loading vectors; Selecting the principal component according to the variance contribution rate.

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