A dual-frequency excitation soil moisture detector and its testing method

Abstract

A dual-frequency excitation soil moisture detector and its testing method according to the present invention include a housing, a touch screen, a battery, a power switch, a probe and a circuit board unit. The touch screen and the power switch are installed on the top of the housing. The circuit board unit and the battery are installed inside the housing. The probe is installed at the lower part of the housing. A positive plate and a negative plate are arranged inside the probe. The positive plate, the negative plate and the circuit board unit are electrically connected. The touch screen, the battery and the power switch are also electrically connected to the circuit board unit. The circuit board unit includes a voltage regulator unit, a single-chip microcomputer, a square wave signal source unit, a frequency-selective analog switch, a precision resistor R, a peak detection unit and a temperature sensor. The capacitive probe is not easily worn, the circuit is simple, the capacitance value of the probe can be effectively measured, and the conductivity resistance performance of the instrument can be improved. The overall detector is designed in a portable manner, which is convenient for actual use.

Description

Technical Field

[0001] The present invention relates to the field of soil moisture detection, and particularly to a dual-frequency excitation soil moisture detector and a testing method thereof. Background Art

[0002] At present, there are more than a dozen soil moisture detection methods, and various types of soil moisture sensors have also emerged on the market. Among them, the sensing technologies based on the dielectric method principle include Time Domain Reflectometry (TDR), Time Domain Transmission (TDT), Standing Wave Ratio (SWR), and capacitance method. The capacitance method has the advantages of simple technology and low cost, and is the most widely used technology at present. From the perspective of capacitance detection technology, the capacitance method mostly adopts the resonance method, namely Frequency Domain Reflectometry (FDR), and the vector voltage technology to decompose the soil dielectric constant. Commercially available products include the SM300 sensor from the UK, the GS3, 10HS, EC-5, and Hydra Probe II sensors from the US, and the ML2x and PR2 sensors from the UK. Although there are commercial products for capacitive sensors, there are still some deficiencies - low accuracy and susceptibility to soil conductivity. Research on ECH2O EC-5, SM200, and ThetaProbe ML2x shows that the calibration model parameters of the sensors are related to soil texture and conductivity.

[0003] At present, a method to improve the anti-conductivity performance of sensors is to measure the soil conductivity simultaneously and implement it according to the conductivity compensation model established by experiments. For example, Patent CN109444227A, a low-frequency capacitance soil moisture detection method with conductivity compensation characteristics, and Patent CN202421116U, a sensor for detecting the water content and conductivity of soilless cultivation substrates, essentially measure the soil conductivity at direct current or low frequency and the soil moisture at high frequency. Another is to increase the excitation signal frequency. Research shows that the signal frequency must be greater than 500 MHz to obtain a soil moisture signal that is not affected by conductivity. The frequency of existing capacitance detection technology can reach 100 MHz. To further increase the frequency and ensure a high signal-to-noise ratio, the technical difficulty is high and the cost is expensive. On the other hand, starting from the principle of the polarization process of conductive material media, within an equivalent lumped parameter model, simply increasing the frequency cannot eliminate the ion migration polarization behavior.

[0004] Therefore, based on the principle of the series resistor-capacitor step response, this patent designs a capacitance detection technology, proposes to calculate the relative soil dielectric constant used to convert the soil moisture content from the dual-frequency response signal, and then measure the soil moisture. A dual-frequency excitation capacitive soil moisture detector with better anti-conductivity performance is provided. Summary of the Invention

[0005] The object of the present invention is to provide a dual-frequency excitation soil moisture detector and its testing method, so as to solve the problem that the soil moisture sensor has poor anti-conductivity performance, resulting in low testing accuracy of the soil moisture detector.

[0006] To achieve the above object, the technical solution provided by the present invention is: it includes a housing, a touch screen, a battery, a power switch, a probe and a circuit board unit. The touch screen and the power switch are installed on the top of the housing. The circuit board unit and the battery are installed inside the housing. The probe is installed at the lower part of the housing. A positive plate and a negative plate are arranged inside the probe. The positive plate, the negative plate and the circuit board unit are electrically connected. The touch screen, the battery and the power switch are also electrically connected to the circuit board unit. The circuit board unit includes a voltage regulator unit, a single-chip microcomputer, a square wave signal source unit, a frequency selection analog switch, a precision resistor R, a peak detection unit and a temperature sensor.

[0007] Further, the probe and the circuit board unit are an integrated structure, which is composed of a printed circuit board. The printed circuit board of the probe adopts a three-layer board structure, and the probe positive plate and negative plate are arranged on the middle layer.

[0008] Further, the voltage regulator unit converts the power of the battery into the working voltage of the single-chip microcomputer and the square wave signal source unit, and provides stable power for the single-chip microcomputer and the square wave signal source unit. The voltage regulator unit adopts a chip with the model of AMS1117-3.3V.

[0009] Further, the single-chip microcomputer is a single-chip microcomputer with built-in power-on reset, crystal oscillator and AD module. The single-chip microcomputer adopts a chip with the model of STC15W408AD.

[0010] Further, the frequency selection analog switch receives the control signal of the single-chip microcomputer, and then controls the square wave signal source unit to output square wave signals with different frequencies. The square wave signal source unit can generate two frequencies of square wave signals.

[0011] Further, one end of the precision resistor R is electrically connected to the output end of the square wave signal source unit, and the other end is electrically connected to the positive plate of the probe.

[0012] Further, the negative plate of the probe is electrically connected to the negative power supply.

[0013] Further, the peak detection unit includes a detection diode D, a capacitor and a resistor. The capacitor and the resistor are in parallel. One end of the parallel connection is electrically connected to the negative power supply, and the other end is electrically connected to the cathode of the detection diode D. The anode of the detection diode D is electrically connected to the positive plate. The detection diode D adopts a diode with the model of BAT15-03W.

[0014] Further, the temperature sensor adopts an integrated digital temperature sensor chip with the model of TMP275.

[0015] A test method for a soil moisture detector with dual-frequency excitation, comprising the following steps:

[0016] 1) Establish the test principle. Adopt the steady-state response principle of a first-order RC series circuit. Under the square-wave excitation, after the capacitor charging and discharging are stable, the relationship between the peak and valley voltages of the capacitor terminal voltage is

[0017] U T =U0+(U s -U0)(1-e -T / RC )(1)

[0018] In the formula, U T —— The peak voltage value on the capacitor, U0—— The valley voltage value on the capacitor, U s —— The square-wave high-level voltage value, T—— The square-wave period. The capacitor C includes the stray capacitance ΔC and the capacitance Cx caused by the change of soil moisture. Among them, Cx is proportional to the soil equivalent relative permittivity ε as

[0019] Cx=gε(2)

[0020] In the formula, g—— The proportionality coefficient

[0021] Formula (1) is sorted out as

[0022] U T =U s +(U s -U0)e -T / (R(ΔC+gε)) (3)

[0023] 2) Establish the test model. In the known relative permittivity ε solution, respectively calibrate the mathematical models of the relationships between the corresponding output signals U1, U2 and ε under the square-wave excitations of frequencies f1 and f2

[0024] ε=f(U1)|f1(4)

[0025] ε=f(U2)|f2(5)

[0026] The exponential model of formula (6) is selected for the said formula (4) and formula (5)

[0027] y=a+b·e (c / (x+d)) (6)

[0028] 3) Collect the test signals. The single-chip microcomputer controls the frequency-selective analog switch and then controls the square-wave signal source unit to generate two square-wave signals with frequencies f1 and f2, and collect the peak voltages of the two square-wave signals with frequencies f1 and f2;

[0029] 4) Process the test signals. Perform signal processing on the two peak voltages to calculate the soil moisture content. Processing the test signals includes the following three steps:

[0030] Step 1: Output signals U f1 and U f2 are obtained by exciting with square waves of frequencies f1 and f2, and the corresponding relative permittivities ε f1 and ε f2 are calculated respectively according to the calibration models of Formula (4) and Formula (5). Obviously, for the conductive medium soil, the ε f1 and ε f2 values calculated according to Formulas (4) and (5) contain ion migration polarization information, and large errors will occur when directly used to convert soil water content;

[0031] Step 2: Select a suitable model of the relative permittivity ε of soil and the test frequency F. Referring to the variation characteristics of soil permittivity with frequency in the literature and the mathematical characteristics of the exponential function, for the model of the relative permittivity ε of soil and the test frequency F, select the model of Formula (7), where a, b, c, and d are undetermined parameters. Here, c takes the value of f1 and d takes the value of f2. Then, from (f1, ε f1 ) and (f2, ε f2 ), a and b can be obtained. Formula (7) has the mathematical characteristic that when F - c is greater than 4 times d, ε(F) tends to the stable value a. Denote a as ε eff , which is the relative permittivity of soil used to convert soil water content

[0032]

[0033] Step 3: Refer to the Topp model of Formula (8) to calculate the corresponding soil volume water content θ of ε eff ,

[0034]

[0035] where θ is the soil volume water content;

[0036] 5) Display the measurement results, and the results in step 4) are displayed through the touch screen (2).

[0037] The beneficial effects of the present invention are as follows: A soil moisture detector with dual - frequency excitation and its testing method. The capacitive probe has good insulation and is not easily worn. The peak detection technology circuit given is simple and can effectively measure the probe capacitance value. Based on the processing of dual - frequency excitation response signals, the anti - conductivity performance of the instrument can be improved. The overall tester is designed to be portable, which is convenient for actual use. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a three - dimensional structural schematic diagram of an embodiment of the present invention;

[0039] Figure 2 is the working principle diagram of the present invention;

[0040] Figure 3 This is the structural block diagram of the circuit of the circuit board unit of the present invention;

[0041] Figure 4 This is the schematic circuit diagram of the circuit board unit of an embodiment of the present invention. Specific implementation manner

[0042] Embodiment 1

[0043] As Figures 1-4 shown, it includes a housing 1, a touch screen 2, a battery 3, a power switch 4, a probe 5 and a circuit board unit 6. The touch screen 2 and the power switch 4 are installed on the top of the housing 1. The circuit board unit 6 and the battery 3 are installed inside the housing 1. The probe 5 is installed at the lower part of the housing 1. A positive plate 7 and a negative plate 8 are arranged inside the probe 5. The positive plate 7 and the negative plate 8 are electrically connected to the circuit board unit 6. The touch screen 2, the battery 3 and the power switch 4 are also electrically connected to the circuit board unit 6. The circuit board unit 6 includes a voltage regulator unit 11, a single-chip microcomputer 12, a square wave signal source unit 13, a frequency selection analog switch 14, a precision resistor R15, a peak detection unit 16 and a temperature sensor 17.

[0044] The probe 5 and the circuit board unit 6 are of an integrated structure and are composed of a printed circuit board. The printed circuit board of the probe 5 adopts a three-layer board structure, and a probe positive plate and a negative plate are arranged on the middle layer.

[0045] The voltage regulator unit 11 converts the power of the battery 3 into the working voltage of the single-chip microcomputer 12 and the square wave signal source unit 13, and provides stable power for the single-chip microcomputer 12 and the square wave signal source unit 13. The voltage regulator unit 11 adopts a chip with the model AMS1117-3.3V.

[0046] The single-chip microcomputer 12 is a single-chip microcomputer with built-in power-on reset, crystal oscillator and AD module. The single-chip microcomputer 12 adopts a chip with the model STC15W408AD.

[0047] The frequency selection analog switch 14 receives the control signal of the single-chip microcomputer 12, and then controls the square wave signal source unit 13 to output square wave signals with different frequencies. The square wave signal source unit 13 can generate square wave signals with two frequencies.

[0048] One end of the precision resistor R15 is electrically connected to the output end of the square wave signal source unit 13, and the other end is electrically connected to the positive plate 7 of the probe 5.

[0049] The negative plate 8 of the probe 5 is electrically connected to the negative power supply.

[0050] As Figure 4As shown, the peak detection unit 16 includes a detection diode D, a capacitor, and a resistor. The capacitor and the resistor are in parallel. One end of the parallel combination is electrically connected to the negative pole of the power supply, and the other end is electrically connected to the cathode of the detection diode D. The anode of the detection diode D is electrically connected to the positive plate 7. The detection diode D uses a BAT15-03W diode, the capacitor is C6, and the resistor is R5.

[0051] The temperature sensor 17 uses a TMP275 integrated digital temperature sensor chip.

[0052] A testing method for a soil moisture detector with dual-frequency excitation includes the following steps:

[0053] 1) Establish a testing principle. Adopt the steady-state response principle of a first-order RC series circuit. Under the square-wave excitation, after the capacitor charging and discharging are stable, the relationship between the peak and valley voltages of the capacitor terminal voltage is

[0054] U T = U0 + (U s - U0)(1 - e -T / RC )(1)

[0055] In the formula, U T —— Peak voltage value on the capacitor, U0—— Valley voltage value on the capacitor, U s —— Square-wave high-level voltage value, T—— Square-wave period, R—— Resistance value. The capacitor C includes the stray capacitance ΔC and the capacitance Cx caused by the change of soil moisture. Among them, Cx is proportional to the soil equivalent relative permittivity ε as

[0056] Cx = gε(2)

[0057] In the formula, g—— Proportionality coefficient

[0058] Formula (1) is sorted out as

[0059] U T = U s + (U s - U0)e -T / (R(ΔC+gε)) (3)

[0060] 2) Establish a testing model. In a solution with a known relative permittivity ε, respectively calibrate the mathematical models of the relationships between the corresponding output signals U1, U2 and ε under the square-wave excitations of frequencies f1 and f2

[0061] ε = f(U1)|f1(4)

[0062] ε = f(U2)|f2(5)

[0063] The exponential model of formula (6) is selected for formula (4) and formula (5)

[0064] y = a + b·e(c / (x+d)) (6)

[0065] The known relative dielectric constant ε of the solution is preferably shown in Table 1.

[0066] Table 1

[0067]

[0068] 3) Collect the test signal. The single-chip microcomputer 12 controls the frequency-selective analog switch 14 and then controls the square-wave signal source unit 13 to generate two square-wave signals with frequencies f1 and f2, and collect the peak voltages of the two square-wave signals with frequencies f1 and f2.

[0069] 4) Process the test signal. Perform signal processing on the two peak voltages to calculate the soil moisture content. Processing the test signal includes the following three steps:

[0070] Step 1: Output signals U f1 and U f2 under the excitation of the square waves with frequencies f1 and f2, and respectively calculate the corresponding relative dielectric constants ε f1 and ε f2 according to the calibration models of equations (4) and (5). Obviously, as the conductive soil medium, the values of ε f1 and ε f2 calculated by equations (4) and (5) contain ion migration polarization information, and there will be a large error when directly used to convert the soil water content.

[0071] Step 2: Select a suitable model of the soil relative dielectric constant ε and the test frequency F. Refer to the variation characteristics of the soil dielectric constant with frequency in the literature and the mathematical characteristics of the exponential function. For the model of the soil relative dielectric constant ε and the test frequency F, select the model of equation (7), where a, b, c, and d are undetermined parameters. Here, c takes the value of f1 and d takes the value of f2. Then, from (f1, ε f1 ), (f2, ε f2 ), a and b can be obtained. Equation (7) has the mathematical characteristic that when F - c is greater than 4 times d, ε(F) approaches the stable value a. Denote a as ε eff , which is the soil relative dielectric constant used to convert the soil water content.

[0072]

[0073] Step 3: Refer to the Topp model of equation (8) to calculate the soil volume water content θ corresponding to ε eff .

[0074]

[0075] In the formula, θ——soil volume water content;

[0076] 5) Display the measurement result, and the result in 4) is displayed through the touch screen (2).

[0077] A dual-frequency excitation soil moisture detector and its testing method according to this embodiment. The capacitive probe has good insulation and is not easily worn. The peak detection technology circuit is simple and can effectively measure the capacitance value of the probe. Based on the processing of the dual-frequency excitation response signal, the anti-conductivity performance of the instrument can be improved. The overall tester is designed to be portable, which is convenient for actual use.

[0078] Obviously, the above embodiments are only examples for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A testing method for a soil moisture detector with dual - frequency excitation. The soil moisture detector includes a housing (1), a touch screen (2), a battery (3), a power switch (4), a probe (5) and a circuit board unit (6). The touch screen (2) and the power switch (4) are installed on the top of the housing (1). The circuit board unit (6) and the battery (3) are installed inside the housing (1). The probe (5) is installed at the lower part of the housing (1). A positive plate (7) and a negative plate (8) are arranged inside the probe (5). The positive plate (7), the negative plate (8) and the circuit board unit (6) are electrically connected. The touch screen (2), the battery (3) and the power switch (4) are also electrically connected to the circuit board unit (6). The circuit board unit (6) includes a voltage regulator unit (11), a single - chip microcomputer (12), a square - wave signal source unit (13), a frequency - selective analog switch (14), a precision resistor R (15), a peak - detection unit (16) and a temperature sensor (17). It is characterized in that, It includes the following steps: 1) Set up the test principle. Adopt the steady-state response principle of a first-order RC series circuit. Under the square-wave excitation, after the capacitor charges and discharges stably, the relationship between the peak and valley voltages of the capacitor terminal voltage is U T = U0 + (U s - U0)(1 - e -T / RC )(1) Where U T —— the peak voltage value on the capacitor, U0—— the valley voltage value on the capacitor, U s —— the high-level voltage value of the square wave, T—— the square wave period, R—— the resistance value, the capacitor C includes the stray capacitance ΔC and the capacitance Cx caused by the change of soil moisture, where Cx is proportional to the equivalent relative dielectric constant ε of the soil as Cx = gε (2) where g is the proportionality coefficient Equation (1) is arranged as U T = U s + (U s - U0) e -T / (R(ΔC+gε)) (3) 2) Establish a test model. In a solution with a known relative permittivity ε, respectively calibrate the mathematical models of the relationships between the output signals U1, U2 and ε under the square-wave excitations of frequencies f1 and f2 ε = f(U1)|f1 (4) ε = f(U2)|f2 (5) The exponential model of Equation (6) is selected for the said Equation (4) and Equation (5) y = a + b·e (c / (x+d)) (6) 3) Collect test signals. The single-chip microcomputer (12) controls the frequency-selective analog switch (14) and then controls the square-wave signal source unit (13) to generate two square-wave signals with frequencies f1 and f2, and collect the peak voltages of the two square-wave signals with frequencies f1 and f2 4) Process the test signals. Perform signal processing on the two peak voltages to calculate the soil moisture content. Processing the test signals includes the following three steps: Step 1: Output signals U f1 and U f2 are respectively calculated according to the calibration models of formula (4) and formula (5) to obtain the corresponding relative permittivities ε f1 and ε f2 . Obviously, for the conductive soil medium, the values of ε f1 and ε f2 calculated according to formula (4) and formula (5) contain ion migration polarization information, and large errors will occur if they are directly used to convert soil water content; Step 2: Select a suitable model for the soil relative permittivity ε and the test frequency F. Referring to the variation characteristics of the soil permittivity with frequency in the literature and the mathematical characteristics of the exponential function, for the model of the soil relative permittivity ε and the test frequency F, select the model of Equation (7), where a, b, c, and d are undetermined parameters. Here, c is taken as f1 and d is taken as f2, and then a and b are obtained from (f1, ε f1 ), (f2, ε f2 ). Equation (7) has the mathematical characteristic that when F - c is greater than 4 times d, ε(F) approaches the stable value a. Denote a as ε eff , the soil relative permittivity used to convert the soil water content Step 3: Refer to Topp model formula (8) to calculate ε eff The corresponding soil volumetric water content θ where θ is the soil volumetric water content 5) Display the measurement result. The result in 4) is displayed through the touch screen (2).

2. The testing method of a soil moisture detector with dual-frequency excitation according to claim 1, characterized in that: The said probe (5) and the circuit board unit (6) are of an integrated structure and are composed of a printed circuit board. The printed circuit board of the probe (5) adopts a three-layer board structure, and the middle layer is provided with the positive electrode plate (7) and the negative electrode plate (8) of the probe 3. The testing method of a soil moisture detector with dual-frequency excitation according to claim 1, characterized in that: The voltage regulator unit (11) converts the power supply of the battery (3) into the working voltages of the single-chip microcomputer (12) and the square-wave signal source unit (13), and provides a stable power supply for the single-chip microcomputer (12) and the square-wave signal source unit (13). The voltage regulator unit (11) adopts a chip of model AMS1117-3.3V 4. The testing method of a soil moisture detector with dual-frequency excitation according to claim 1, characterized in that: The said single-chip microcomputer (12) is a single-chip microcomputer with built-in power-on reset, crystal oscillator and AD module. The single-chip microcomputer (12) adopts a chip of model STC15W408AD 5. The test method of a soil moisture detector with dual-frequency excitation according to claim 1, characterized in that: The said frequency-selective analog switch (14) receives the control signal of the single-chip microcomputer (12), and then controls the square-wave signal source unit (13) to output square-wave signals of different frequencies. The square-wave signal source unit (13) can generate square-wave signals of two frequencies 6. The testing method of a soil moisture detector with dual-frequency excitation according to claim 1, characterized in that: One end of the precision resistor R (15) is electrically connected to the output end of the square-wave signal source unit (13), and the other end is electrically connected to the positive electrode plate (7) of the probe (5).

7. The test method of a soil moisture detector with dual-frequency excitation according to claim 1, characterized in that: The negative electrode plate (8) of the said probe (5) is electrically connected to the negative power supply 8. The testing method of a soil moisture detector with dual-frequency excitation according to claim 1, characterized in that: The said peak detection unit (16) includes a detection diode D, a capacitor and a resistor. The capacitor and the resistor are in parallel. One end of the parallel connection is electrically connected to the negative power supply, and the other end is electrically connected to the cathode of the said detection diode D. The anode of the said detection diode D is electrically connected to the positive electrode plate (7). The said detection diode D adopts a diode of model BAT15-03W 9. The test method of a soil moisture detector with dual-frequency excitation according to claim 1, characterized in that: The said temperature sensor (17) adopts a chip of model TMP275 integrated digital temperature sensor

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