A multi-layer soil sensor based on temperature compensation

Abstract

The application discloses a kind of multilayer soil sensors based on temperature compensation, belong to multilayer soil sensor technical field, including PVC square tube, PVC square tube inside is vertically bonded with PVC sheet by epoxy resin AB glue, sensing assembly and acquisition mainboard are installed on PVC sheet, acquisition mainboard is electrically connected with sensing assembly, and acquisition mainboard includes microcontroller, storage module and communication module.The application adopts the above-mentioned multilayer soil sensor based on temperature compensation, high precision: by integrating temperature sensor in each measurement layer and applying temperature compensation algorithm, effectively eliminates the interference of temperature change on soil parameter measurement, significantly improves the accuracy and reliability of data;Strong profile monitoring capability: accurate data of temperature compensation can be obtained at different depths of soil at the same time, and the dynamic change rule of soil profile is truly reflected.

Description

Technical Field

[0001] This invention relates to the field of multilayer soil sensor technology, and in particular to a temperature-compensated multilayer soil sensor. Background Technology

[0002] Soil moisture is one of the basic conditions for the growth of ground plants. It is closely related to the growth and yield of agricultural and forestry crops, and also has an important relationship with the distribution and types of terrestrial plants. As an important soil parameter, soil moisture content is highly valued by workers in agriculture, forestry, urban planning, road transportation, and other fields.

[0003] Soil exhibits diverse vertical stratification, and the spatial distribution of moisture content correlates with its vertical dimension. Therefore, real-time monitoring of soil moisture at different depths is crucial. Exploring real-time measurement techniques for multi-layered soil moisture and developing multi-layered soil moisture measurement devices capable of real-time detection are highly helpful in understanding the vertical transport patterns of water in soil. Furthermore, they hold significant importance for agricultural yield assessment, soil moisture monitoring and forecasting, and other related parameter monitoring technologies and theoretical research.

[0004] Soil parameter measurements are significantly affected by ambient temperature. For example, the dielectric constant of soil moisture changes with temperature. Without considering the influence of temperature, measurement data can be significantly biased during periods of large diurnal temperature variations or seasonal transitions, leading to inaccurate monitoring results and impacting decisions regarding precision irrigation or scientific research. Therefore, developing a multilayer soil sensor capable of automatically compensating for temperature effects and providing high-precision measurement data has become a pressing technical challenge in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a temperature-compensated multilayer soil sensor with high accuracy: by integrating a temperature sensor in each measurement layer and applying a temperature compensation algorithm, the interference of temperature changes on soil parameter measurements is effectively eliminated, significantly improving the accuracy and reliability of the data; it also has strong profile monitoring capabilities: it can simultaneously acquire accurate temperature-compensated data from different soil depths, truly reflecting the dynamic changes in the soil profile.

[0006] To achieve the above objectives, the present invention provides a temperature-compensated multilayer soil sensor, comprising a PVC square tube, wherein a PVC sheet is vertically bonded inside the PVC square tube by epoxy resin AB glue, a sensing component and a data acquisition motherboard are mounted on the PVC sheet, the data acquisition motherboard is electrically connected to the sensing component, and the data acquisition motherboard includes a microcontroller, a storage module and a communication module. The microcontroller is used to receive the moisture content and temperature signals collected by each soil moisture and temperature monitoring unit; The storage module is used to store the temperature compensation algorithm model and calibration coefficients; The communication module is used for data exchange with external devices.

[0007] Preferably, the sensing component includes one or more soil moisture and temperature monitoring units, which are arrayed along the length of the PVC sheet. The soil moisture and temperature monitoring units are pluggable onto the PVC sheet, and each soil moisture and temperature monitoring unit includes a moisture sensor and a temperature sensor.

[0008] Preferably, the microcontroller corrects the received moisture content signal using a temperature compensation algorithm model based on the received temperature signal, and outputs the corrected moisture content value to eliminate the influence of ambient temperature changes on moisture content measurement.

[0009] Preferably, the temperature compensation algorithm model is a polynomial fitting model based on experimental calibration, and its expression is as follows: ; in, This is the corrected volumetric moisture content. For the original measurement of moisture content, The actual temperature value measured by the temperature sensor. The coefficients are the polynomial coefficients obtained by least-multiplication fitting. The value range is 1 to 3.

[0010] Preferably, the polynomial coefficients are obtained by calibration as follows: under laboratory conditions, the sensor is placed in a constant temperature chamber, and multiple temperature points are set, such as 5℃, 15℃, 25℃, 35℃, and 45℃. At each temperature point, the original moisture content output of soil samples with different standard moisture contents is measured. The original moisture content output of soil samples with different standard moisture contents is measured with the measurement value at 25℃ as the reference.

[0011] Preferably, the moisture sensor uses the standing wave ratio method to measure soil moisture content. The high-frequency signal source inside the moisture sensor emits high-frequency electromagnetic waves into the soil. At the interface between the soil and the probe, the incident wave and the reflected wave are superimposed due to impedance mismatch to form a standing wave. The change in the amplitude of the standing wave voltage on the transmission line of the moisture sensor is detected to reflect the dielectric constant of the soil, and then the soil moisture content is calculated.

[0012] Preferably, the temperature sensor uses a constant current source method to measure soil temperature, and uses a platinum resistance thermometer PT100 as the temperature sensing element. A constant current is applied to the platinum resistance thermometer PT100, and its resistance value changes with temperature, causing the voltage across the platinum resistance thermometer PT100 to change. The voltage signal is amplified by a differential amplifier and converted into a current signal by a voltage-to-current converter to calculate the soil temperature.

[0013] Preferably, the acquisition motherboard also includes a power control module, which uses an electronic load switch chip to supply power to each soil moisture content and temperature monitoring unit in a time-sharing manner to reduce overall power consumption and achieve electrical isolation between each soil moisture content and temperature monitoring unit; the communication module includes two RS-485 modules, one for communication with an external host and the other for communication with each soil moisture content and temperature monitoring unit.

[0014] Therefore, the present invention employs a temperature-compensated multilayer soil sensor, which has high accuracy: by integrating a temperature sensor into each measurement layer and applying a temperature compensation algorithm, the interference of temperature changes on soil parameter measurement is effectively eliminated, significantly improving the accuracy and reliability of the data; it also has strong profile monitoring capabilities: it can simultaneously acquire accurate temperature-compensated data from different soil depths, truly reflecting the dynamic change patterns of the soil profile.

[0015] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of a temperature-compensated multilayer soil sensor according to the present invention; Figure 2 This is a schematic diagram of the structure of a temperature-compensated multilayer soil sensor acquisition motherboard according to the present invention.

[0017] Figure Labels 1. PVC square tube; 2. Epoxy resin AB glue; 3. PVC sheet; 4. Data acquisition mainboard; 5. Soil moisture content and temperature monitoring unit; 51. Moisture sensor; 52. Temperature sensor. Detailed Implementation

[0018] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0020] Example 1 like Figures 1 to 2 As shown, this invention provides a temperature-compensated multi-layer soil sensor, comprising a PVC square tube 1, with a PVC sheet 3 vertically bonded to the inside of the PVC square tube 1 using epoxy resin AB glue 2. Sensing components and a data acquisition mainboard 4 are mounted on the PVC sheet 3. The PVC square tube 1 serves as the external protective shell and main support frame of the multi-layer soil sensor, providing a mounting base for the sensing components and data acquisition mainboard 4, isolating it from soil sediment, moisture, acid and alkali corrosion, and external force compression and collision, making it suitable for long-term buried soil monitoring conditions and significantly extending the equipment's service life. The high bonding strength of the epoxy resin AB glue 2 ensures that the PVC sheet remains vertically fixed without shifting or shaking, guaranteeing the arrangement accuracy of the sensing components. Furthermore, the epoxy resin AB glue 2 possesses sealing, waterproofing, insulation, and moisture-proof properties, preventing groundwater and moisture from seeping in and damaging the circuit board and sensors, making the bonded structure less prone to delamination and failure. The PVC sheet 3 provides a mounting carrier for the sensing components and data acquisition mainboard 4, maintaining a vertical and flat mounting surface, ensuring that the soil moisture content and temperature detection units are neatly arrayed along the length direction, guaranteeing the spacing accuracy of multi-layer soil layer monitoring.

[0021] The sensing component includes one or more soil moisture and temperature monitoring units 5. The number of soil moisture and temperature monitoring units 5 ranges from 1 to 8, and the position and number of the units can be adjusted according to actual measurement needs. The soil moisture and temperature monitoring units 5 are arrayed along the length of the PVC sheet 3, evenly installed at 10cm intervals. This allows for layered acquisition of raw data on soil moisture and temperature at different depths, achieving layered and multi-layered synchronous monitoring without the need for separate sensor deployment at multiple points, saving construction costs and installation space. The soil moisture and temperature monitoring units 5 are pluggable onto the PVC sheet 3. Each unit includes a moisture sensor 51 and a temperature sensor 52. The moisture sensor 51 detects the soil moisture content, and the temperature sensor 52 detects the soil temperature. The pluggable connection allows for independent module assembly, disassembly, and maintenance, facilitating repair.

[0022] The moisture sensor 51 uses the standing wave ratio method to measure soil moisture content. The high-frequency signal source inside the moisture sensor 51 emits high-frequency electromagnetic waves into the soil. At the interface between the soil and the probe, the incident wave and the reflected wave are superimposed due to impedance mismatch to form a standing wave. The change in the amplitude of the standing wave voltage on the transmission line of the moisture sensor is detected to reflect the dielectric constant of the soil, and then the soil moisture content is calculated.

[0023] According to transmission line theory, specifically: ; in, The potential difference between the two ends of the transmission line, This represents the oscillation amplitude of the high-frequency oscillator. The detection impedance of the ring probe is denoted as . For transmission line impedance, The reflection index of the transmission line.

[0024] when No standing waves are generated on the transmission line, and the voltage at both ends of the transmission line is zero.

[0025] The impedance characteristics of a ring probe are related to the dielectric constant of its internal filling material, that is: ; in, For the ring probe capacitor, A constant related to shape and size. The dielectric constant of the medium surrounding the probe. is the dielectric constant in a vacuum.

[0026] The expression for the capacitance of the probe is as follows: ; ; ; in, The characteristic capacitance of the ring probe is... The stray capacitance generated by the electric field. The capacitance is represented by the PVC mounting pipe surrounding the sensor probe. To measure the characterizing capacitance of soil, These are soil-related constants. The dielectric constant of the soil surrounding the probe. These are constants related to the PVC pipe body. The dielectric constant of the PVC pipe surrounding the probe is denoted as .

[0027] Detection impedance of the ring probe With admittance The relationship can be represented as: ; in, The imaginary unit, This is the test angular frequency of the sensor.

[0028] During the detection process, the soil ionic conductivity affects the impedance characteristics of the probe, as specifically expressed by: ; in, The impedance is caused by the influence of soil ionic conductivity.

[0029] Since the sensor uses fixed-point measurement, the influence of the PVC pipe and stray capacitance of the circuit can be considered a fixed value. Therefore, its impedance is mainly determined by the sensor probe size, the dielectric constant of the soil being detected, and the operating frequency.

[0030] Soil moisture content refers to the percentage of water in the total mass / volume of a soil sample, denoted as . The specific formula for soil volumetric water content is: ; in, Soil volumetric water content, This represents the volume of water in the measured soil. This represents the total volume of the soil being measured.

[0031] Soil volumetric water content With soil dielectric constant There is a single-value relationship between them, and the specific formula is: ; Based on the above theory, soil moisture content can be measured by measuring the change in the impedance of the sensor probe.

[0032] Temperature sensor 52 measures soil temperature using a constant current source method. It uses a platinum resistance thermometer PT100 as the temperature sensing element. A constant current is applied to the platinum resistance thermometer PT100, and its resistance value changes with temperature, causing the voltage across the platinum resistance thermometer PT100 to change. The voltage signal is amplified by a differential amplifier and converted into a current signal by a voltage-to-current converter to calculate the soil temperature.

[0033] The chip generates a reference voltage, which is converted into a constant current source (1mA) by an operational amplifier. The current flows through a platinum resistor, generating a voltage drop. A differential amplifier then picks up the voltage signal across the platinum resistor, and finally, a voltage-to-current converter converts the output voltage signal into a 4–20mA current signal. The specific formula is as follows: ; ; in, The voltage output by the sensor. For reference voltage, we take 2.49V. For the ideal resistance value, we take 2.49kΩ. This refers to the resistance value of a PT100 platinum resistance thermometer. This is the gain of the differential amplifier. This represents the voltage-to-current conversion coefficient of the voltage-to-current converter.

[0034] The acquisition motherboard 4 is electrically connected to the sensing components. The acquisition motherboard 4 includes a microcontroller, a storage module, and a communication module. The acquisition motherboard 4 is the core of the whole machine for data processing and control. It can accept raw data of moisture content and temperature, realize the temperature compensation function, eliminate the influence of soil temperature changes on the detection accuracy of moisture sensor, and improve the accuracy of soil moisture content measurement.

[0035] The microcontroller is an STM-32 microcontroller, used to receive the moisture content and temperature signals collected by each soil moisture and temperature monitoring unit; The microcontroller uses a temperature compensation algorithm model to correct the received moisture content signal based on the received temperature signal, and outputs the corrected moisture content value to eliminate the influence of ambient temperature changes on moisture content measurement.

[0036] The temperature compensation algorithm model is a polynomial fitting model based on experimental calibration, and its expression is shown below: ; in, This is the corrected volumetric moisture content. For the original measurement of moisture content, The actual temperature value measured by the temperature sensor. The coefficients are the polynomial coefficients obtained by least-multiplication fitting. The value range is 1 to 3.

[0037] The polynomial coefficients were obtained by calibration as follows: under laboratory conditions, the sensor was placed in a constant temperature chamber, and multiple temperature points were set, such as 5℃, 15℃, 25℃, 35℃, and 45℃. At each temperature point, the original moisture content output of soil samples with different standard moisture contents was measured. The original moisture content output of soil samples with different standard moisture contents was measured with the measurement value at 25℃ as the reference.

[0038] The storage module is an SD card storage unit used to store the temperature compensation algorithm model and calibration coefficients; The communication module is used for data exchange with external devices. It includes two RS-485 modules: one for communication with the external host and the other for communication with each soil moisture and temperature monitoring unit.

[0039] The data acquisition motherboard also includes a power control module, which uses an electronic load switch chip to supply power to each soil moisture content and temperature monitoring unit in a time-sharing manner to reduce overall power consumption and achieve electrical isolation between each soil moisture content and temperature monitoring unit.

[0040] Before using the multi-layer soil sensor, a PVC square tube needs to be buried at the test point using the specified tools. During testing, adjust the position and number of soil moisture and temperature monitoring units according to the actual measurement needs, insert the moisture sensor and temperature sensor into the PVC tube, connect the power supply and data cable, and tighten the sealing cap to perform online real-time measurement of soil moisture and temperature at different depths.

[0041] Example 2 Soil volumetric water content With soil dielectric constant There is a single-valued relationship between the dielectric constant and the soil moisture content, but the dielectric constant is significantly affected by temperature. Studies have shown that at the same moisture content, the dielectric constant can change by 5% to 10% for every 10°C change in temperature, leading to a large error in moisture content measurement. Therefore, it is necessary to monitor soil temperature in real time using a temperature sensor and correct the moisture content measurement values.

[0042] A polynomial fitting model was used for temperature compensation. This model expresses the moisture content measurement deviation as a function of temperature and corrects it to a standard temperature of 25℃. The model expression is as follows: ; in, The temperature deviation term can be expanded into a polynomial: ; In practical applications, using the first two or three terms is sufficient to achieve high accuracy. (Coefficients) Obtained through experimental calibration.

[0043] The experimental equipment required for the calibration method is as follows: Temperature control chamber (temperature control accuracy ±0.5℃), standard soil samples (known volumetric moisture content, such as 5%, 10%, 15%, 20%, 25%, 30%, 35%), multi-layer soil sensor to be calibrated, and data acquisition instrument.

[0044] The calibration method steps are as follows: The sensor probe was placed in a constant temperature chamber, and the temperature was set to 5℃, 15℃, 25℃, 35℃ and 45℃ respectively. After the temperature stabilized for 2 hours at each temperature point, the measurement was performed. For each temperature point, the raw output values ​​of standard samples with different moisture contents were measured sequentially. ; Using the measurement at 25℃ as a baseline, calculate the deviation at other temperatures. ; For each temperature point, the deviation will be... With temperature difference Perform polynomial fitting to obtain the coefficients. For example, the least squares method can be used to solve overdetermined systems of equations.

[0045] Example 3 The compensation formula for calibrating a certain type of probe is as follows: ; The formula has a correction error of less than ±1% (volume moisture content) within the temperature range of 5~45℃.

[0046] When the sensor is actually working, the microcontroller performs compensation according to the following steps: Step 1: Obtain the raw moisture content value using a moisture sensor. ; Step 2: Obtain the current soil temperature using a temperature sensor. ; Step 3: Read the pre-stored polynomial coefficients from memory ; Step 4: Calculate the compensation value ; Step 5: Output the corrected moisture content .

[0047] In a field environment, this sensor was compared with a traditional uncompensated sensor. Under a daily temperature variation of 15°C, the moisture content measurement value of the uncompensated sensor fluctuated by ±3.5%, while the fluctuation of this sensor, after temperature compensation, was less than ±0.8%, significantly improving measurement stability.

[0048] Therefore, the present invention employs a temperature-compensated multilayer soil sensor, which has high accuracy: by integrating a temperature sensor into each measurement layer and applying a temperature compensation algorithm, the interference of temperature changes on soil parameter measurement is effectively eliminated, significantly improving the accuracy and reliability of the data; it also has strong profile monitoring capabilities: it can simultaneously acquire accurate temperature-compensated data from different soil depths, truly reflecting the dynamic change patterns of the soil profile.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A temperature-compensated multilayer soil sensor, characterized in that: It includes a PVC square tube, inside which a PVC sheet is vertically bonded with epoxy resin AB glue. A sensing component and a data acquisition motherboard are mounted on the PVC sheet. The data acquisition motherboard is electrically connected to the sensing component. The data acquisition motherboard includes a microcontroller, a storage module and a communication module. The microcontroller is used to receive the moisture content and temperature signals collected by each soil moisture and temperature monitoring unit; The storage module is used to store the temperature compensation algorithm model and calibration coefficients; The communication module is used for data exchange with external devices.

2. The multi-layer soil sensor based on temperature compensation according to claim 1, characterized in that: The sensing component includes one or more soil moisture and temperature monitoring units, which are arrayed along the length of the PVC sheet. The soil moisture and temperature monitoring units are pluggable onto the PVC sheet, and each soil moisture and temperature monitoring unit includes a moisture sensor and a temperature sensor.

3. A temperature-compensated multilayer soil sensor according to claim 2, characterized in that: The microcontroller uses a temperature compensation algorithm model to correct the received moisture content signal based on the received temperature signal, and outputs the corrected moisture content value to eliminate the influence of ambient temperature changes on moisture content measurement.

4. A temperature-compensated multilayer soil sensor according to claim 3, characterized in that: The temperature compensation algorithm model is a polynomial fitting model based on experimental calibration, and its expression is shown below: ； in, This is the corrected volumetric moisture content. For the original measurement of moisture content, The actual temperature value measured by the temperature sensor. The coefficients are the polynomial coefficients obtained by least-multiplication fitting. The value range is 1 to 3.

5. A temperature-compensated multilayer soil sensor according to claim 4, characterized in that: The polynomial coefficients were obtained by calibration as follows: under laboratory conditions, the sensor was placed in a constant temperature chamber, and multiple temperature points were set, such as 5℃, 15℃, 25℃, 35℃, and 45℃. At each temperature point, the original moisture content output of soil samples with different standard moisture contents was measured. The original moisture content output of soil samples with different standard moisture contents was measured with the measurement value at 25℃ as the reference.

6. A temperature-compensated multilayer soil sensor according to claim 5, characterized in that: The moisture sensor uses the standing wave ratio method to measure soil moisture content. The high-frequency signal source inside the moisture sensor emits high-frequency electromagnetic waves into the soil. At the interface between the soil and the probe, the incident wave and the reflected wave are superimposed due to impedance mismatch to form a standing wave. The change in the amplitude of the standing wave voltage on the transmission line of the moisture sensor reflects the dielectric constant of the soil, and then the soil moisture content is calculated.

7. A temperature-compensated multilayer soil sensor according to claim 6, characterized in that: The temperature sensor uses the constant current source method to measure soil temperature. It uses a platinum resistance thermometer (PT100) as the temperature sensing element. A constant current is applied to the PT100, and its resistance value changes with temperature, causing the voltage across the PT100 to change. The voltage signal is amplified by a differential amplifier and converted into a current signal by a voltage-to-current converter to calculate the soil temperature.

8. A temperature-compensated multilayer soil sensor according to claim 7, characterized in that: The data acquisition motherboard also includes a power control module, which uses an electronic load switch chip to supply power to each soil moisture content and temperature monitoring unit in a time-sharing manner to reduce overall power consumption and achieve electrical isolation between each soil moisture content and temperature monitoring unit; the communication module includes two RS-485 modules, one for communication with an external host and the other for communication with each soil moisture content and temperature monitoring unit.

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