CN103207217B - Non-plug-in water content sensor of culture substrate - Google Patents

Abstract

The invention discloses a non-plug-in cultivation substrate water content sensor, which comprises a one-side sensitive capacitive probe, a circuit board provided with a capacitance measuring circuit, and an input and output interface cable. The one-side sensitive capacitive probe includes a driving plate, The induction plate, the shielding plate and the substrate, the driving plate and the sensing plate are located on the same surface of the substrate, the shielding plate is located on the other surface of the substrate, and the sensing plate and the shielding plate have equipotential. The invention also discloses a calibration model of the sensor. According to the output voltage signal of the sensor in the dry substrate and the known cultivation substrate water content at any point, the parameter β in the calibration model is determined, and the calibration equation is sequentially determined. The sensor of the present invention has the ability to obtain the water content of the surface layer of the cultivation substrate, has the advantages of low cost, stable performance, simple input and output interfaces, and a convenient and simple calibration process. Its calibration model and the two-point calibration model method based on this model are fast and simple. specialty.

Description

A sensor for moisture content of non-inserted cultivation substrate

technical field

The present invention relates to a non-inserted water content sensor and its calibration method, in particular to a single-side sensitive non-inserted sensor used for detecting the water content of a cultivation substrate and a two-point calibration device for its calibration model and its calibration method .

Background technique

Soil moisture measurement is the basis for implementing water-saving irrigation in precision agriculture and a key link in realizing agricultural irrigation automation. With the application of wireless distributed soil moisture monitoring network system becoming more and more mature, there is an urgent need for a water content sensor with high measurement accuracy, low power consumption, low price, small structure and simple calibration process. In addition, in protected gardening, especially in bag culture, potted planting or column planting, the soilless culture substrate is loose, making it difficult for the traditional probe-inserted moisture sensor probe to make good contact with the substrate, and it is easy to form gaps, which will affect the measurement accuracy ; And the inserted probe may damage the root system of the crop, therefore, a non-inserted cultivation substrate water content sensor is needed.

At present, the methods for measuring the water content of the cultivation substrate include drying method, ray method, dielectric method, nuclear magnetic resonance method, near-infrared spectroscopy and remote sensing method. Among them, the dielectric method is to indirectly measure the water content of the cultivation substrate by measuring the dielectric constant of the cultivation substrate, which has the advantages of rapidity and non-destructiveness. According to the measurement principle, it can be divided into time domain reflection method (TDR), frequency domain reflection method (FDR), Standing wave ratio method (SWR) and capacitance method. The capacitance method is to measure the equivalent capacitance of the electrode inserted into the cultivation substrate to measure the moisture content of the cultivation substrate. It has the advantages of relatively simple technology and low cost, and has been widely noticed by people very early. For example, Chinese Patent No. CN1504745A discloses that a capacitive probe is composed of two metal sheets coated with a layer of high-temperature sintered ceramic glaze film with insulation and non-absorbent properties; patents CN101694475B and CN201034964 similarly disclose a Two copper ring electrodes with insulating isolation rings are set on the hollow insulating rod and put into a PVC pipe, which can be used as a capacitive moisture sensitive device for soil profile moisture measurement; the probes of the above three patents are troublesome to manufacture or are not suitable for Insufficient insertion into the cultivation substrate and good contact with it. Both patents CN102023182A and 201210521545.3 disclose a soil moisture sensor probe processed based on printed circuit board (PCB) technology, which is a double-sided sensitive type. Both need to be inserted into the cultivation substrate to obtain water content information, and cannot obtain the moisture content information of the surface layer of the cultivation substrate. . In addition, sensor calibration models and methods are not proposed in the above five patents. Due to the large difference in physical and chemical properties of soil and soilless cultivation substrate, the scale transformation model established in one cultivation substrate has poor applicability. In actual use, it is necessary to use multi-point regression method to select a better model for re-calibration on different soils and soilless cultivation substrates. This method takes a long time, and there is no unified calibration model, which is not conducive to product promotion and use. Some scholars have studied the calibration method of capacitive soil moisture sensors. For example, Toshihiro Sakaki proposed a two-point α calibration model for dry soil and saturated wet soil for ECH ₂ O sensors, and achieved good results; Gao Yan, Bogena et al. proposed A two-step calibration method that first calibrates the relationship between the output voltage and the dielectric constant, and then calibrates the relationship between the dielectric constant and the water content. These two calibration methods have shortcomings: Toshihiro Sakak's saturated wet soil is difficult to quantitatively prepare, and it is inconvenient to operate; the better model established by Gao Yan and Bogena in the second step of calibration is also different due to the different properties of soil and soilless cultivation substrate. Not adaptable.

Contents of the invention

In view of the above-mentioned deficiencies in the soil moisture sensor and soil moisture measurement technology in the prior art, and in view of the different physical and chemical properties of the cultivation substrate relative to the soil, the present invention provides a non-insert sensor with simple manufacture, low cost and unilateral sensitivity. Type cultivation substrate water content sensor, and a calibration model of this type of sensor is given, and a simple two-point calibration method is provided.

Technical scheme of the present invention is:

A non-inserted cultivation substrate water content sensor, comprising a one-sided sensitive capacitive probe, a main board provided with a capacitance measurement circuit, and an input-output interface cable, the one-sided sensitive capacitive probe including a driving plate and an inductive plate , a shielding pole plate and a substrate, the driving pole plate and the sensing pole plate are located on the same surface of the substrate, the shielding pole plate is located on the other surface of the substrate, the sensing pole plate and the shielding pole plate have etc. potential.

Further, the capacitance measuring circuit on the circuit board includes a power module, a resonance module, a two-stage frequency division module and a frequency-voltage converter; the power module is used to convert the power supplied by the dry battery into a stable, wireless power supply required by other electronic devices. Ripple voltage source; the resonance module is used to generate the frequency signal of the water content of the sensitive cultivation substrate; the secondary frequency division module includes a pre-frequency divider and a post-frequency divider, wherein the pre-frequency divider is used to realize Preset frequency division and level conversion function, the post frequency divider is a high-speed asynchronous counter, which is used to divide the frequency into the frequency that the frequency conversion circuit can handle; the frequency voltage converter is used to make the post frequency divider The output frequency-divided signal is output as an equivalent DC voltage signal and used as the output signal of the sensor.

Further, the surfaces of the driving plate, the sensing plate and the shielding plate are all coated with an insulating layer.

Further, the ratio of the width of the sensing plate and the shielding plate to the distance between them is between 0.8-1.

Further, the frequency of the frequency signal is determined by the equivalent capacitance of the probe with the cultivation substrate as the medium.

Further, the capacitance measurement circuit includes a printed circuit board and a probe printed board, and the printed circuit board and the probe printed board are integrally structured.

Further, the components in the capacitance measuring circuit are all located on the same side of the shielding plate.

Further, all the components are potted and sealed with electronic waterproof glue with good waterproof, moisture-proof, insulation and thermal conductivity.

A calibration model for the non-inserted cultivation substrate water content sensor, the calibration model is ; among them, U is the output voltage signal of the sensor, θ is the water content of the corresponding cultivation substrate, the parameters ε _w and ε _a are the relative dielectric constants of water and air, and the values of A and B can be determined by the output of the sensor in air and pure water Determine the voltage, U ₂ is the output voltage of the sensor in the dry cultivation medium, β is the empirical value of the mixed dielectric constant parameter, and its size is related to the geometric structure, composition, and direction of the electric field of the mixed medium.

A calibration method for a non-inserted cultivation substrate water content sensor based on the model. According to the output voltage signal of the sensor in the dry substrate and the known cultivation substrate moisture content at any point, the parameter β in the calibration model is determined, and the calibration equation is sequentially determined. .

The beneficial effects of the present invention are:

Using simple PCB processing technology and circuit principles, a non-inserted cultivation substrate water content sensor is provided, which has the ability to obtain the surface moisture content of the cultivation substrate; a unified calibration model for this type of sensor is given, and a fast , A simple two-point calibration model method; this sensor has the advantages of low cost, stable performance, simple input and output interfaces, and convenient and simple calibration process.

Description of drawings

Figure 1 is a schematic front view of the overall structure of a non-inserted cultivation substrate water content sensor;

Figure 2 is a schematic diagram of the reverse side of the overall structure of the non-inserted cultivation substrate water content sensor;

Fig. 3 is a schematic block diagram of the capacitance measurement circuit;

Fig. 4 is the schematic diagram of the resonant module circuit;

Figure 5 is a schematic diagram of the two-stage frequency division module;

Fig. 6 is a schematic diagram of a frequency-to-voltage converter circuit;

Figure 7 is a scatter diagram of calibration test data.

In the figure: 1. One-sided sensitive capacitive probe; 2. Circuit board; 3. Input and output interface cables; 4. Drive plate; 5. Induction plate; 6. Shield plate; 7. Substrate.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

The present invention aims at the above-mentioned deficiencies of the existing soil water content sensors, and aims at the different physical and chemical properties of the cultivation substrate relative to the soil, and proposes a non-insertable cultivation substrate moisture sensor with simple production, low cost, and one-sided sensitivity, and provides A calibration model of this type of sensor is proposed, and a simple two-point calibration method is provided.

In order to achieve the above object, the present invention discloses a non-insertable cultivation substrate water content sensor and its calibration method, including a single-side sensitive capacitive probe made by printed circuit board (PCB) technology for sensitive cultivation substrate moisture content; Based on the resonance principle, a capacitance measurement circuit with a frequency-to-voltage conversion module; a calibration model ( β model) derived based on the empirical formula of the mixed model of permittivity, and its two-point calibration method.

The one-sided sensitive capacitive probe is made by printed circuit board technology, and it is composed of a driving plate, a sensing plate, a shielding plate and a substrate; the driving plate and the sensing plate are located on the same surface of the substrate; the shielding plate is located on the base The other surface of the plate; the surface of the driving plate, the sensing plate, and the shielding plate are all coated with an insulating layer, which can effectively avoid the leakage current between the plates; the sensing plate and the shielding plate have equipotential; in order to take into account the sensitivity of the probe and the sensitive depth, the ratio of the width of the sensing plate and the shielding plate to the distance between them is between 0.8-1.

Capacitance measurement circuit consists of four parts: power module, resonant unit, two-stage frequency division module and frequency-to-voltage converter (F/V); The voltage source; the resonance unit is used to generate the frequency signal sensitive to the water content of the cultivation substrate, and its core device is an integrated voltage-controlled oscillator (VCO) chip MC12148; the frequency of the frequency signal is determined by the probe with the cultivation substrate as the medium, etc. The effective capacitance is determined by the size of the effective capacitance; the equivalent capacitance of the probe is used as the capacitance required in the external resonant tank circuit of the chip MC12148; the two-stage frequency division module includes a pre-frequency divider and a post-frequency divider; the pre-frequency divider has a preset Frequency division and level conversion function; the post frequency divider is a high-speed asynchronous counter, which can divide the frequency into the frequency that the frequency conversion circuit can handle; The signal is output as an equivalent DC voltage signal and used as the output signal of the sensor;

The printed circuit board of the capacitance measurement circuit and the printed circuit board of the probe are integrally structured, and the components of the capacitance measurement circuit are located on the side of the shielding plate.

The components of the circuit are potted and sealed with electronic waterproof glue with good waterproof, moisture-proof, insulation and thermal conductivity to ensure long-term stable and reliable operation of the sensor.

The present invention also provides a calibration model for this type of non-inserted cultivation substrate water content sensor, the calibration model is:

(1)

Among them, U is the output voltage signal of the sensor, θ is the water content of the corresponding cultivation substrate, the parameters ε _w and ε _a are the relative permittivity of water and air, and the values of A and B can be determined by the output voltage of the sensor in air and pure water Determined, U2 is the sensor output voltage of the sensor in the dry cultivation medium, β is the empirical value of the mixed dielectric constant parameter, and its size is related to the geometric structure, composition, and direction of the electric field of _the mixed medium;

The method of the above two-point calibration model refers to determining the parameter β in the calibration model according to the output voltage signal of the sensor in the dry matrix and the water content of the known cultivation medium at any point, so the calibration equation is determined.

The measurement principle of this embodiment: the capacitance method is a kind of capacitance detection technology by measuring the cultivation substrate composed of substrate particles, water and air as the capacitor medium; As the moisture content changes, the dielectric constant of the soil changes, causing a change in the capacitance of the capacitor.

As shown in Figure 1 and Figure 2, it is a front and back schematic diagram of the overall structure of the non-inserted cultivation substrate water content sensor of the present invention. From the structural diagram, the present invention includes a single-side sensitive capacitive probe 1 for sensitive cultivation substrate moisture, a circuit board 2 for measuring the capacitance of the probe, and an input and output interface cable 3 .

The capacitive probe 1 is made by printed circuit board technology, and it is composed of a driving plate 4, a sensing plate 5, a shielding plate 6 and a substrate 7; the driving plate 4 and the sensing plate 5 are located on the same surface of the substrate 7; the shielding The pole plate 6 is located on the other surface of the substrate 7; the surfaces of the driving pole plate 4, the sensing pole plate 5, and the shielding pole plate 6 are all coated with an insulating layer to effectively avoid leakage current between the pole plates; the sensing pole plate 5, the shielding pole plate 6 is equipotential.

When designing the capacitive probe 1, the sensitivity and sensitive depth of the probe should be taken into consideration. Since the driving plate 4 and the sensing plate 5 of the capacitive probe 1 are on the same plane, and the shielding plate 6 exists, it is very difficult to analyze the accurate analytical solution of the probe space electric field. With the help of ANSYS finite element analysis software, assuming that the plate L is infinitely long and the thickness of the plate is infinitely thin, the numerical solution is used to simulate the electric field of the probe in two dimensions, and the sensitivity of the probe, the sensitive depth and the ratio of the pole distance a to the pole width b are analyzed ( a /b ) relationship, according to the simulation results: it is considered that the probe structure is better when a/b takes a value between 0.8-1.

The single-side sensitive capacitive probe 1 is integrated with the circuit board 2 for measuring the capacitance of the probe, so that the overall structure of the sensor is compact.

As shown in Figure 3, it is a functional block diagram of the medium capacitance measurement circuit of the present invention; it includes four parts: a power supply module, a resonance module, a two-stage frequency division module and a frequency-to-voltage converter (F/V).

The power module is used to convert the power supply powered by the dry battery into a stable and ripple-free 5V voltage source required by other electronic devices; the circuit can use the AMS1117-5.0 voltage regulator chip, which integrates overheating protection and current limiting circuit inside. A good choice for sensors.

The resonance module is used to generate frequency signals sensitive to the water content of the cultivation substrate. Its core device is an integrated voltage-controlled oscillator (VCO) chip MC12148, as shown in Figure 4, which is the schematic diagram of the resonance unit circuit. The frequency of the frequency signal is determined by the equivalent capacitance Cx of the probe with the cultivation substrate as the medium. The equivalent capacitance Cx of the probe is used as the capacitance required in the external resonant tank circuit of the chip MC12148, and the output signal of the MC12148 is MECL level, the typical maximum frequency can reach 225MHz, and the spectrum purity is high. Under the 5.0V DC power supply voltage, the maximum current consumption is 19mA, and the frequency calculation formula of the output signal is:

(2)

Among them, C is the intrinsic capacitance inside the chip MC12148, and L is the size of the inductance L1 in Fig. 3 .

The two-stage frequency division module includes a pre-frequency divider and a post-frequency divider. Figure 5 is a circuit schematic diagram of the two-stage frequency division module. The prescaler adopts chip MC12017, which has 64/63 preset frequency division function and level conversion function prescaler, the maximum operating frequency is 225MHz, and the output level can be compatible with COMS and TTL levels; The post frequency divider uses a 12-bit high-speed COMS asynchronous counter chip 74HC4040, which can achieve a maximum frequency division of 2 ^{to 12} , which is enough to divide the frequency after the preset frequency division to the frequency that the frequency-voltage conversion circuit can handle.

The frequency-to-voltage converter is to make the frequency signal output by the post divider output as an equivalent DC voltage signal. Its core device is the chip LM331, which is a very cost-effective V/F and F/V converter produced by NS company in the United States. chip. It uses a new temperature-compensated bandgap reference circuit, which has extremely high conversion accuracy over the entire operating temperature range and as low as 4.0V supply voltage. When used as F/V and connected as shown in the circuit in Figure 6, the average current flowing out of pin 1 of the LM331 chip is filtered by the 100kΩ resistor R2 and the 1uf capacitor C9, and the peak value of the ripple is 10mV, but it is relatively slow , the time constant is 0.1s, and the accuracy of 0.1% is achieved after a stabilization time of 0.7s. The relationship between the output voltage signal and the input signal frequency is:

U=k f _s (3)

Among them, U is the sensor output voltage, and k is the frequency-to-voltage conversion coefficient.

The present invention also provides a calibration model for this type of non-inserted cultivation substrate water content sensor. The specific derivation process of the model is as follows: According to the empirical formula of the Jihe Tenneko two-phase dielectric constant mixed model, the cultivation substrate can be seen The relative dielectric constant of the cultivation substrate can be expressed as:

(4)

Among them, ε _b is the relative permittivity of soil, ε _a , ε _s , ε _w are the relative permittivity of air, soil particles and water respectively, f _a , f _s , θ are the volumes of air, soil particles and water respectively β is the empirical value of the mixed permittivity parameter, and its size is related to the geometric structure, composition, and direction of the electric field of the mixed medium; in addition, the relationship between f _a , f _s , and θ is:

(5)

(6)

Among them, φ is the soil porosity, according to formula (4)-(6):

(7)

When the soil volume moisture content changes, the equivalent capacitance of the probe changes accordingly, and the equivalent capacitance of the probe can be expressed as:

C _x = C ₀ + ξ ε _b ε ₀ (8)

Among them, C _x is the equivalent capacitance of the probe, C ₀ is the equivalent capacitance formed by the driving plate and the shielding plate, ξ is the sensitivity of the probe, and ε ₀ is the vacuum dielectric constant.

According to the calculation formula (2) of the resonant signal frequency of the resonant unit, the F/V conversion circuit chip frequency conversion relationship formula (3), and the formula (8), it can be obtained:

(9)

make , , assuming that the output voltages of the sensor probes are respectively U ₀ and U ₁ when the sensor probes are in contact with the surface layer of air and deionized water, then:

(10)

(11)

(12)

A and B can be determined by formulas (11) and (12). In addition, assuming that the sensor is in a dry matrix ( θ is 0), the output voltage is U ₂ . Then from equations (7) and (10), we can get:

(13)

According to formula (7), (10), (13) can get:

(14)

Equation (14) is an empirical calibration model based on β parameters ( β model). When the β model is used to calibrate, the parameters ε _w , ε _a are constants, and the values of A and B for a certain sensor are the same, so it is only necessary to determine the parameters U ₂ and β for different cultivation substrates. Among them, U ₂ is the output voltage of the sensor in the dry cultivation medium. According to the calibration model formula (14), a two-point calibration model method can be proposed.

The method of the two-point calibration model refers to determining the parameter β in the calibration model according to the output voltage signal of the sensor in the dry matrix and the water content of the known cultivation medium at any point, so that the calibration equation is determined.

In order to illustrate the intended effect of the present invention, both the practicability and reliability of the sensor; and the adaptability of the β model and the practicability of the two-point calibration method; with soil, and commonly used soilless culture substrate peat and vinegar grains as test objects , to carry out the calibration test. The calibration method is as follows: remove impurities from the air-dried soil, peat and vinegar grains, put them in a drying oven at 105°C for 3 hours, and cool to room temperature for later use; take a plastic bucket with an inner diameter of 13cm and a depth of 25cm, and weigh the empty bucket with a precision electronic scale , determine the volume of soil and matrix to be filled, and mark the corresponding height in the bucket; calculate the required water and dry soil, peat and vinegar grains according to the calculated results Take dry soil, peat, vinegar residue and mix with water, stir well, seal it in a plastic bag and let it stand for 24 hours, then put it into a plastic bucket by layered compaction method and compact it to the mark; When measuring, the room temperature is controlled at (25±3)°C, and the sensitive side of the non-inserted cultivation substrate water content sensor is in close contact with the top of the prepared sample, and the measurement is made once every 120° rotation, and the average value of the output voltage of 3 times is taken as the measurement value. The samples are numbered according to the water content from small to large, and the water content is 0, and the number is 1. In addition, the experimental test obtained: U ₀ =1.354V, U ₁ =0.405V. Figure 7 is the calibration data, and the data is carried out to the least squares method to fit the β parameter model, and the correlation coefficient R ^and the root mean square error (RMSE) of the fitting are as follows:

It can be seen from the table that the β calibration model can better describe the relationship between the output voltage signal and the water content of the matrix, and the minimum correlation coefficient R ² =0.9911.

According to the two-point calibration method proposed by the present invention, we bring the test data of any point with a large number into formula (16), solve the parameter β , and determine the calibration model; calculate the calibration value and true value of the other points outside itself according to the calibration model The maximum absolute value of the error is M _|△| . The calculated β and M _|△| data are as follows:

It can be seen from the table that the β calibration model determined by the two-point calibration method is better for the measurement of soil, peat and vinegar grains, and their maximum error is less than 0.025, which meets the measurement accuracy requirements for the moisture content of the cultivation substrate in agricultural production.

Finally, it should be noted that the above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that the present invention can still be modified Or an equivalent replacement, any modification or partial replacement without departing from the spirit and scope of the present invention shall fall within the scope of the claims of the present invention.

Claims (2)

1. A non-insertable cultivation substrate water content sensor, the non-insertion cultivation substrate moisture content sensor includes a single-side sensitive capacitive probe (1), a circuit board (2) with a capacitance measurement circuit, and an input and output interface A cable (3), the single-side sensitive capacitive probe (1) includes a driving plate (4), a sensing plate (5), a shielding plate (6) and a substrate (7), the driving plate (4) and the sensing plate (5) are located on the same surface of the substrate (7), the shielding plate (6) is located on the other surface of the substrate (7), and the sensing plate (5) and The shielding plate (6) has an equipotential, and the non-insertable cultivation substrate water content sensor is calibrated according to a calibration model, which is characterized in that: the calibration model is ; among them, U is the output voltage signal of the sensor, θ is the water content of the corresponding cultivation substrate, the parameters ε _w and ε _a are the relative dielectric constants of water and air, and the values of A and B can be determined by the output of the sensor in air and pure water Determine the voltage, U ₂ is the output voltage of the sensor in the dry cultivation medium, β is the empirical value of the mixed dielectric constant parameter, and its size is related to the geometric structure, composition, and direction of the electric field of the mixed medium. 2. The non-insertable cultivation substrate water content sensor according to claim 1, characterized in that: according to the output voltage signal of the known cultivation substrate moisture content in the dry substrate and any point of the sensor, the parameter β in the calibration model is determined , and then determine the calibration equation.