A microwave non-destructive testing device and method for grain moisture content

Abstract

This invention discloses a microwave non-destructive testing device for grain moisture content. The device includes a shell, metamaterial lens antennas, a grain container, a 4.3-inch capacitive touchscreen, a coaxial cable, a near-infrared temperature probe, a microwave absorbing material plate, and a main control circuit board. The main control circuit board includes a main control chip, a 24GHz microwave transmitting circuit, and a 24GHz microwave receiving circuit. The main control circuit board is connected to symmetrical metamaterial lens antennas on both sides of the device via the coaxial cable. The device measures the change in scattering parameters between the two antennas using microwave signals, calculates the complex permittivity of the grain moisture content based on this change, and then uses a density-independent algorithm and a temperature calibration algorithm to invert the grain moisture content. The measurement results are displayed on the capacitive touchscreen. A microwave absorbing material plate is attached to the inner surface of the shell to reduce microwave reflection interference; a near-infrared temperature probe is used to quickly detect the grain temperature, reducing measurement errors caused by temperature.

Description

Technical Field

[0001] This invention relates to the field of grain testing technology, specifically to a microwave non-destructive testing device and method for grain moisture content. Background Technology

[0002] The moisture content of grains is a key factor in grain storage, processing, and quality assessment. On the one hand, excessive moisture content makes grains prone to mold and spoilage; on the other hand, insufficient moisture content makes grains dry, affecting their taste and nutritional value. Therefore, accurately detecting and controlling the moisture content of grains within a certain range is of great significance for ensuring grain quality, improving economic efficiency, and ensuring food safety.

[0003] With the development of grain testing technology, the requirements for testing methods are becoming increasingly stringent. Currently, common methods include high-frequency impedance method, capacitance method, and oven drying method. The capacitance method measures the moisture content of a sample by measuring the change in capacitance caused by the difference in moisture content of the dielectric material between the capacitor plates. This method is convenient, low-cost, and small in size, but its stability is poor and easily affected by factors such as grain type, density, and temperature. The high-frequency impedance method utilizes the different conductivity of grains with different moisture contents, measuring the moisture content by measuring their resistance. This method is inexpensive and simple in structure, but its accuracy is easily affected by factors such as grain bulk density and the contact state between the sensor and the grain. The oven drying method requires physically crushing and grinding the sampled grain into powder before testing. The powder is then placed in a drying box, and the temperature and time are set. The moisture content of the grain can be calculated by the difference in mass change of the sample before and after drying. This method has high accuracy and low cost, but the testing time is long and it can damage the grain itself.

[0004] Microwave methods for detecting moisture content offer advantages such as being non-destructive, rapid, and non-contact, and possess good penetration, making them particularly advantageous for measuring moisture content in stacked grains. This method detects moisture content by measuring the changes in microwave frequency, amplitude, and phase caused by microwave penetration through the grain. However, existing microwave detection equipment, which transmits and receives signals via circuits, is largely affected by antenna spherical wave multipath effects, diffraction effects, grain packing density, and temperature, resulting in low detection accuracy. To address the shortcomings of existing grain moisture detection devices, this invention proposes a non-destructive microwave detection device for grain moisture, incorporating a metamaterial lens antenna and density and temperature calibration algorithms to reduce the impact of spherical wave multipath effects, diffraction effects, density, and temperature on detection accuracy. Summary of the Invention

[0005] The purpose of this invention is to provide a microwave non-destructive testing device for grain moisture content, and the second objective of this invention is to provide a method for using the device to perform microwave non-destructive testing of grain moisture content.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0007] In the first aspect, the present invention discloses a microwave non-destructive testing device for grain moisture, comprising a rectangular shell body, a grain container (2), a 4.3-inch capacitive screen (3), a microwave absorbing material plate (4), a metamaterial lens antenna, a main control circuit board (7), and a near-infrared temperature probe (8); the shell body includes a top plate (1-1) and a bottom plate (1-2), the grain container (2) is inserted through the grain container opening set on the top plate (1-1) and placed centrally on the bottom plate (1-2), dividing the internal area of ​​the shell body into left and right chambers; the microwave absorbing material plate (4) is close to the side of the grain container (2) and is pasted on the inner surface of the shell body to reduce microwave reflection interference between the shell and the antenna, effectively reducing measurement errors caused by microwave signal reflection; the 4.3-inch capacitive screen (3) is horizontally fixed on the top plate (1-1) and outside the right chamber of the shell body;

[0008] The metamaterial lens antenna consists of a nylon column (5-3), a metamaterial lens (5-4), and a feed antenna (5-5), and is fixedly mounted on the bottom plate (1-2) of the outer shell. The metamaterial lens antenna includes a transmitting antenna (5-1) and a receiving antenna (5-2). The transmitting antenna (5-1) and the receiving antenna (5-2) are located in the left and right chambers inside the main body of the outer shell, respectively, and are symmetrically arranged along the grain container (2). They are fixed on the absorbing material plate (4) pasted on the bottom plate (1-2) of the outer shell. The transmitting antenna (5-1) and the receiving antenna (5-2) are respectively connected to the coaxial transmitting line (6-1) and the coaxial receiving line (6-2).

[0009] The main control circuit board (7) is located in a groove on the bottom surface of the outer casing base plate (1-2). The coaxial transmitting line (6-1) and coaxial receiving line (6-2) are connected to the transmitting antenna (5-1) and receiving antenna (5-2) respectively, and extend from the groove on the bottom surface of the outer casing base plate (1-2) to the main control circuit board (7).

[0010] The near-infrared temperature probe (8) is fixed on the wall of the grain container, with the emission port facing the grain inside the container. The signal line and power line are connected and led out from the back, extending to the bottom of the outer shell and connected to the main control circuit board, for detecting the grain temperature.

[0011] As a further preferred embodiment of the present invention, the outer shell body is made of metal, and the outer shell dimensions (unit: mm) are: length × width × height = 350 × 150 × 160.

[0012] As a further preferred embodiment of the present invention, the outer shell body is made of aluminum alloy.

[0013] As a further preferred embodiment of the present invention, one connector of the coaxial transmitting line (6-1) and the coaxial receiving line (6-2) is a 3.5mm right-angle internal thread needle type, and the other connector is a 3.5mm straight internal thread needle type.

[0014] As a further preferred embodiment of the present invention, the main control circuit board (7) is a four-layer PCB board, the top layer is a 24GHz microwave transceiver circuit, the filling material of the middle dielectric layer is Rogers4350b with a relative permittivity of 3.48, the bottom layer is a data processing and control circuit, and the main control chip used is STM32F407ZGT6.

[0015] As a further preferred embodiment of the present invention, the dimensions (unit: mm) of the main control circuit board (7) are: length × width × height = 70 × 90 × 3.

[0016] As a further preferred embodiment of the present invention, the grain container (2) has a double-layer structure, and its material is acrylic sheet. The internal dimensions (unit: mm) are: length × width × height = 120 × 60 × 100, and the external dimensions (unit: mm) are: length × width × height = 130 × 70 × 110.

[0017] As a further preferred embodiment of the present invention, the metamaterial lens (5-4) is composed of 121 metamaterial phase compensation units, which are manufactured by 3D printing. The material is polylactic acid (PLA). The cross-section of the unit is square. Different sized grid-shaped medium grooves are removed from the center of the unit and 1 / 4 of the cylindrical medium material is cut off from the four corners of the unit to improve transmission efficiency.

[0018] Secondly, this invention discloses a method for non-destructive microwave detection of grain moisture using the aforementioned device. This device measures the change in scattering parameters between two antennas using microwave signals, calculates the complex permittivity of the grain moisture content based on this change, and then calculates the grain moisture content using a density-independent algorithm and a temperature calibration algorithm. The method includes:

[0019] S1. Combining the propagation characteristics of plane waves on the surface of multilayer media, straight-through and quarter-wavelength extension error models are established to reduce the influence of antenna errors. The straight-through model places the metasurface lens transmitting and receiving antennas 60mm apart, with no measuring object between them. The quarter-wavelength extension model increases the distance between the two antennas by 3.1mm based on the straight-through model. A system of equations is established based on the two error models to de-embedding the antenna error parameters.

[0020] S2. The spherical wave emitted by the feed antenna is converted into a plane wave after passing through the metasurface lens, which can reduce the error caused by the multipath effect and diffraction effect of the spherical wave. The microwave signal of samples with different moisture content and different grain types is collected by the metasurface lens transmitting antenna and the metasurface lens receiving antenna, and the signal is transmitted to the main control circuit board via the coaxial line. The change of scattering parameter S is obtained by using the microwave signal, and the complex permittivity ε of grain moisture content is calculated by the inversion algorithm formula 1: ε = ε' - jε".

[0021] S3. Change the density of the grain in the grain container and measure the scattering parameters of grain with different densities. Using the relationship between dielectric constant ε', loss factor ε", loss tangent tanδ and density ρ: Formula 2, Formula 3, Formula 4, Formula 5 is obtained to calculate the density calibration factor. The density calibration factor changes significantly with the moisture content and does not change with the density, which can reduce the influence of density on the accuracy of moisture detection.

[0022] S4. A near-infrared thermometer is used to collect grain temperature in real time and measure the moisture content of grain at different temperatures. Based on the relationship between grain moisture content and dielectric properties, a linear regression prediction model formula (Formula 6) for grain moisture content is established. Parameters a, b, and c can be calculated through the linear fitting equation of grain moisture content with temperature and density calibration factors, resulting in the moisture content calculation formula M = 94.607ξ + 0.097T - 11.016. This reduces the influence of density and temperature on detection, achieving accurate detection of grain moisture content. The measurement results are displayed in real time on a 4.3-inch capacitive touchscreen.

[0023]

[0024] M = aξ + bT + c (Formula 6)

[0025] In the formula, L is the width of the grain container (2), p is the phase parameter, λ0 is the free space wavelength at 24 GHz, and a f ε is the slope (1.23 at 24GHz), ε' is the dielectric constant, ε" is the loss factor, ξ is the density-independent factor, T is the grain temperature, and M is the grain moisture content.

[0026] This invention utilizes a metamaterial lens antenna to replace a traditional antenna, converting spherical waves into plane waves and reducing the effects of multipath and diffraction. Simultaneously, the metamaterial lens improves antenna gain. Combining the propagation characteristics of plane waves on multilayer dielectric surfaces, three error models—straight-through, quarter-wavelength extension, and reflection—are established. Based on the scattering parameters of these error models, a system of equations is established to de-embedding the antenna error parameters. A near-infrared temperature probe detects the grain temperature in real time and transmits the data to the main control circuit board. The 24GHz microwave transceiver circuit on the main control circuit board can transmit and receive signals via the metamaterial lens antenna. The microwave signal is used to measure the change in scattering parameters between the two antennas. Based on this change, the complex permittivity of the grain moisture content is calculated. Then, a density-independent algorithm and a temperature calibration algorithm are used to calculate the grain moisture content. The measurement results are displayed in real time on a capacitive screen, achieving rapid and non-destructive detection of grain moisture.

[0027] The measuring device of the present invention has the following beneficial effects:

[0028] 1. The device of the present invention has a simple and compact structure, can work in an environment of -20℃ to 40℃, has a measurement error of less than 0.5%, and a single measurement speed of less than 100ms. It has the characteristics of rapid and non-destructive testing and can be applied to grain production, transportation, storage and processing, etc., to meet the needs of rapid measurement of grain moisture in different scenarios.

[0029] 2. This invention uses a 24GHz microwave transceiver circuit on the main control circuit board to transmit and receive microwave signals via a metamaterial lens antenna. It employs 121 metamaterial phase compensation units as lenses. When the incident wave passes through the lens array, the spherical surface is transformed into a plane wave, reducing errors caused by multipath and diffraction effects of spherical waves, and increasing the antenna's peak gain by 15.97 dB compared to the feed antenna. The metamaterial lens antenna reduces the half-power beamwidth by 64° in the xoz plane and 70° in the yoz plane compared to the feed antenna. This reduces microwave scattering by granular grains and diffraction effects from container edges, increasing the stability of microwave measurements of grains.

[0030] 3. This invention provides a microwave non-destructive testing device for grain moisture content, calibrating antenna error, grain pile density, and temperature error. The invention rapidly measures grain surface temperature using infrared compensation and reduces the influence of grain temperature on the measurement results through a temperature calibration algorithm, enabling the instrument to operate in environments ranging from -20℃ to +40℃. Combining the propagation characteristics of plane waves on multilayer media surfaces, three error models—through, quarter-wavelength extension, and reflection—are established. Based on the scattering parameters of these error models, a system of equations is established to de-embedding the antenna error parameters. The change in scattering parameters between the two antennas is measured using microwave signals, and the complex permittivity of the grain moisture content is calculated based on this change. Finally, the moisture content of the grain is calculated using a density-independent algorithm and a temperature calibration algorithm. Experiments were conducted on various grains with different moisture contents, temperatures, and densities. The experimental data were compared with the moisture content obtained by the standard drying method, and the results show that the detection error of this device is ≤0.5%. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of a microwave non-destructive testing device for grain moisture according to the present invention;

[0032] Figure 2 for Figure 1 Side view;

[0033] Figure 3 for Figure 1 Top view;

[0034] Figure 4 This is a schematic diagram of the structure of the metamaterial lens antenna and the metamaterial lens unit of the present invention;

[0035] Figure 5 This is a reflection coefficient diagram of the metamaterial lens antenna of the present invention;

[0036] Figure 6 This is a comparison diagram of the radiation direction of the metamaterial lens antenna of the present invention;

[0037] Figure 7 The simulated electric field distribution of the metamaterial lens antenna of the present invention in the xoz and yoz planes is shown.

[0038] Figure 8 The transmission characteristic curve of the metamaterial lens unit of this invention;

[0039] Figure 9 This is a graph showing the results of detecting the real part of the complex permittivity of grain moisture using a microwave non-destructive testing device for grain moisture according to the present invention.

[0040] Figure 10 This is a diagram showing the results of detecting the imaginary part of the complex permittivity of grain moisture using a microwave non-destructive testing device of the present invention.

[0041] Figure 11 This is a comparison chart of the predicted moisture content and the actual moisture content of grain using a microwave non-destructive testing device of the present invention.

[0042] Figure 12 This is a diagram of the capacitive display screen operation interface of a microwave non-destructive testing device for grain moisture according to the present invention; the markings in the diagram are: 1-1, outer shell body, 1-2, outer shell bottom plate, 2, grain container, 3, 4.3-inch capacitive screen, 4, absorbing material plate, 5-1, metamaterial lens antenna transmitting antenna, 5-2, metamaterial lens antenna receiving antenna, 5-3, nylon column, 5-4, metamaterial lens, 5-5, feed antenna, 6-1, coaxial transmitting line, 6-2, coaxial receiving line, 7, main control circuit board, 8, near-infrared temperature probe. Detailed Implementation

[0043] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0044] like Figure 1 As shown: A microwave non-destructive testing device for grain moisture includes a rectangular shell, a grain container 2, a 4.3-inch capacitive screen 3, a microwave absorbing material plate 4, a metamaterial lens antenna, a coaxial transmitting line 6-1, a coaxial transmitting and receiving line 6-2, a main control circuit board 7, and a near-infrared temperature probe 8.

[0045] The main body of the outer shell includes a top plate 1-1 and a bottom plate 1-2. The dimensions of the outer shell (unit: mm) are: length × width × height = 350 × 150 × 160, and the material is aluminum alloy.

[0046] The metamaterial lens antenna includes a transmitting antenna 5-1 and a receiving antenna 5-2. The transmitting antenna 5-1 and the receiving antenna 5-2 are located in the left and right chambers of the outer shell body, respectively, and are symmetrically arranged along the grain container 2. They are fixed on the wave-absorbing material plate 4 pasted on the bottom plate 1-2 of the outer shell. The transmitting antenna 5-1 and the receiving antenna 5-2 are respectively connected to the coaxial transmitting line 6-1 and the coaxial receiving line 6-2.

[0047] The grain container 2 is made of acrylic sheet, with dimensions (unit: mm): length × width × height = 140 × 60 × 150 mm and a thickness of 5 mm. The grain container is inserted through the container opening on the top surface of the outer shell top plate 1-1 and placed centrally on the outer shell bottom plate 1-2, dividing the internal area of ​​the outer shell body into left and right chambers. The wave-absorbing material plate on the inner surface of the outer shell bottom plate 1-2 is close to the side of the grain container 2, and together with the container opening on the top surface of the outer shell top plate 1-1, it fixes the grain container 2 in place.

[0048] When the transmitting antenna of the metamaterial lens antenna 5-1 is excited, the metamaterial lens will convert the spherical wave into a plane wave, which will reduce the spherical wave error and improve the peak gain of the antenna. It will also reduce the scattering of microwaves by granular grains and the diffraction effect of transmission at the edge of the container, and increase the stability of microwave measurement of grains.

[0049] The main control circuit board 7 is a four-layer PCB with dimensions (in mm): length × width × height = 70 × 90 × 3. The top layer is a 24GHz microwave transceiver circuit, the middle dielectric layer is filled with Rogers 4350b with a relative permittivity of 3.48, and the bottom layer is the data processing and control circuit. The main control chip used is STM32F407ZGT6, which adopts a modular design to improve the convenience of debugging. It mainly includes power supply circuits, filter circuits, microwave oscillation circuits, and power amplifier circuits.

[0050] The absorbing material plate 4 is pasted on the inner surface of the housing, which can reduce the reflection interference between the housing and the antenna and increase the accuracy of the measurement.

[0051] The 4.3-inch capacitive screen 3 is fixed to the upper right corner of the top plate 1-1 of the outer casing, 2cm from the top edge and 2cm from the right edge. It is connected to the main control circuit board 7 via an FPC cable and can display the type of grain being measured and the moisture detection results.

[0052] The main control circuit board 7 is fixed in the groove on the bottom surface of the bottom plate 1-2 of the outer casing, and the coaxial line extends from the groove at the bottom of the outer casing to the connector on the back of the antenna, forming a microwave signal circuit with the antenna.

[0053] The near-infrared temperature probe 8 is fixed on the side wall of the grain container 2, with its emission port aimed at the grain inside the grain container 2. The signal line and power line are connected and led out from the back, extending to the bottom 1-2 of the outer shell and connected to the main control circuit board 7, thus realizing the temperature measurement function.

[0054] One connector of the coaxial transmitting line 6-1 and the coaxial receiving line 6-2 is a 3.5mm right-angle internal thread needle type, and the other connector is a 3.5mm straight internal thread needle type.

[0055] The device's metasurface lens converts the spherical wave emitted by the feed antenna into a plane wave, reducing errors caused by multipath and diffraction effects of the spherical wave and improving detection accuracy. An absorbing material plate is attached inside the device's outer shell to absorb microwaves reflected between the shell and the grain container, reducing antenna reflection interference and improving measurement accuracy. Based on the propagation characteristics of plane waves on the surface of a multilayer medium, a straight-through and quarter-wavelength extension error model is established. An equation system is then established based on the error model to de-embedding antenna error parameters, reducing the impact of antenna errors on detection accuracy. The main control circuit board controls the metasurface lens transmitting and receiving antennas via a coaxial transmission line, collecting microwave signals from samples with different moisture content, temperature, density, and grain types. The microwave signal... The changes in scattering parameters S were analyzed, and the complex dielectric constant ε of grain moisture was calculated using inversion algorithm formula 1: ε = ε' - jε". The density of grain in the container was varied, and the scattering parameters of grains with different densities were measured. Formulas 2, 3, and 4, relating the dielectric constant ε', loss factor ε", loss tangent tanδ, and grain density ρ and moisture content, were used to derive formula 5 for calculating the density calibration factor, thus reducing the influence of density on the accuracy of moisture content detection. A near-infrared thermometer was used to collect grain temperature in real time, and the dielectric properties of the grain at different temperatures were measured. Based on the relationship between grain moisture content and dielectric properties, a linear regression prediction model for grain moisture content was established: M = 94.607ξ + 0.097T - 11.016. Where M is the grain moisture content, ξ is the density-independent factor, and T is the grain temperature. This prediction model reduces the influence of temperature and density on detection, achieving accurate detection of grain moisture content. The measurement results, including temperature and grain type, are displayed in real time on a 4.3-inch capacitive touchscreen.

[0056]

[0057] M = aξ + bT + c (Formula 6)

[0058] In the formula, L is the width of the grain container (2), p is the phase parameter, λ0 is the free space wavelength at 24 GHz, and a f ε is the slope (1.23 at 24GHz), ε' is the dielectric constant, ε" is the loss factor, ξ is the density-independent factor, T is the grain temperature, and M is the grain moisture content.

[0059] like Figure 2 As shown, the outer shell of this invention is 350mm long, 150mm wide, and 160mm high. The two metamaterial planar lens antennas have a vertical length of 78mm, and their nylon pillars are equal in length (48mm). The internal dimensions (in mm) of the grain container are: length × width × height = 120 × 60 × 100, and the external dimensions (in mm) are: length × width × height = 130 × 70 × 110.

[0060] like Figure 3 As shown, the dimensions of the main control circuit board 7 are: 70mm wide and 90mm long. The coaxial transmitting line 6-1 and the coaxial receiving line 6-2 are of equal length and equal to 210mm, with a coaxial cable diameter of 3mm. The boundary dimensions of the capacitive screen 3 are 62.0mm wide, 104.7mm long, and 1mm thick.

[0061] like Figure 4 The diagram shows the structure of the metamaterial lens antennas 5-1 and 5-2 and the metamaterial lens unit of this invention. The metamaterial lens antenna consists of a nylon column 5-3, a metamaterial lens, and a feed antenna 5-5 (5-4). The feed antenna is a rectangular microstrip antenna, and the dielectric layer has a thickness of 0.508 mm and a relative permittivity ε. r Rogers RO4350 sheet metal with a loss tangent of 3.66 and a loss angle tangent of tanδ = 0.004, and metamaterial lenses fabricated by 3D printing, are made of PLA with a relative permittivity ε. r =2.55, loss tangent tanδ = 0.01. The metamaterial lens array consists of 121 metamaterial units, and the metamaterial phase compensation unit satisfies the following: when the incident electromagnetic wave passes through the plane wave lens array, the phase corresponding to each phase compensation unit is:

[0062]

[0063] In the formula, Let λ be the initial phase of the central unit of the metamaterial lens, x and y be the horizontal and vertical distances between the centers of any unit on the array surface and the origin, λ0 be the free-space wavelength, and f be the focal distance. The thickness of the metamaterial lens unit is h, and its size is between 1 mm and 21 mm. Other dimensions are: unit side length d = 8 mm, lattice slot width w = 0.6 mm, lattice slot length a = 5.5 - 0.13 h, and radius r = 2.2 - 0.1 h.

[0064] like Figure 5 The figure shows the reflection coefficient of the metamaterial lens antenna of this invention. It can be seen from the figure that the reflection coefficient S of the feed antenna is [value missing] in the frequency range of 23.40 GHz to 24.26 GHz. 11 <dB, the device's reflection coefficient S in the frequency range of 23.23GHz to 24.26GHz 11 < dB.

[0065] like Figure 6 The diagram shows a comparison of the radiation directions of a microwave non-destructive testing device for grain moisture according to the present invention. The peak gain of the antenna reaches 22.75 dBi. Compared with the feed antenna, the half-power beamwidth of the metamaterial lens antenna is reduced by 64° in the xoz plane and by 70° in the yoz plane.

[0066] like Figure 7 The figure shows the simulated electric field distribution of the metamaterial lens antenna of the present invention in the xoz and yoz planes. It can be seen from the figure that the metamaterial lens antenna of the device can convert the spherical wave emitted by the feed antenna into a plane wave, thereby reducing the measurement error caused by the sample edge effect.

[0067] like Figure 8 The figure shows the transmission characteristic curve of the metamaterial lens unit of the present invention. As can be seen from the figure, the thickness h of the antenna patch is in the range of 1 mm to 21 mm, the transmission phase can change from -180° to 180°, the total transmission phase can cover 360°, and the transmission amplitude of the transmission unit is above 0.9.

[0068] like Figure 9 The figure shows the curve of the real part of the complex dielectric constant of grain as a function of frequency and moisture content. It can be seen from the figure that the real part of the complex dielectric constant increases with the increase of moisture content, showing a linear trend, and can be fitted with the corresponding linear regression equation.

[0069] like Figure 10 The figure shows the curve of the imaginary part of the complex dielectric constant as a function of frequency and moisture when detecting wheat moisture according to the present invention. It can be seen from the figure that the real part of the complex dielectric constant increases with the increase of moisture, showing a linear trend, and can be fitted with the corresponding linear regression equation.

[0070] like Figure 11 The figure shown is a comparison between the predicted grain moisture content and the actual grain moisture content of the microwave non-destructive testing device of the present invention. As can be seen from the figure, the predicted result and the actual result are basically on the same line, indicating that the prediction result is good.

[0071] like Figure 12 The diagram shown is an operation display interface of the capacitive display screen of the microwave non-destructive testing device for grain moisture according to the present invention. The interface display includes: sample number display, grain type display, moisture content display, temperature display, bulk density display, moisture regain display, sample library entry button, device software setting entry button, and statistics interface entry button.

[0072] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A microwave non-destructive testing device for grain moisture content, characterized in that: The device includes a rectangular shell, a grain container (2), a 4.3-inch capacitive screen (3), a wave-absorbing material plate (4), a metamaterial lens antenna, a main control circuit board (7), and a near-infrared temperature probe (8). The shell includes a top plate (1-1) and a bottom plate (1-2). The grain container (2) is inserted through the grain container opening on the top plate (1-1) and placed centrally on the bottom plate (1-2), dividing the internal area of ​​the shell into two chambers. The wave-absorbing material plate (4) is placed close to the side of the grain container (2) on the inner surface of the shell. The 4.3-inch capacitive screen (3) is horizontally fixed to the top plate (1-1) and the outside of the right chamber of the shell. The metamaterial lens antenna consists of a nylon column (5-3), a metamaterial lens (5-4), and a feed antenna (5-5), and is fixedly mounted on the bottom plate (1-2) of the outer shell; The metamaterial lens antenna includes a transmitting antenna (5-1) and a receiving antenna (5-2). The transmitting antenna (5-1) and the receiving antenna (5-2) are located in the left and right chambers inside the main body of the outer shell, respectively, and are symmetrically arranged along the grain container (2). They are fixed on the wave-absorbing material plate (4) pasted on the bottom plate (1-2) of the outer shell. The transmitting antenna (5-1) and the receiving antenna (5-2) are respectively connected to the coaxial transmitting line (6-1) and the coaxial receiving line (6-2). The main control circuit board (7) is located in a groove on the bottom surface of the outer casing base plate (1-2). The coaxial transmitting line (6-1) and coaxial receiving line (6-2) are connected to the transmitting antenna (5-1) and receiving antenna (5-2) respectively, and extend from the groove on the bottom surface of the outer casing base plate (1-2) to the main control circuit board (7). The near-infrared temperature probe (8) is fixed on the wall of the grain container, with the emission port facing the grain inside the container. The signal line and power line are connected and led out from the back, extending to the bottom of the outer shell and connected to the main control circuit board, for detecting the grain temperature.

2. The microwave non-destructive testing device for grain moisture according to claim 1, characterized in that: The outer shell is made of metal, and its dimensions (in mm) are: length × width × height = 350 × 150 × 160.

3. The microwave non-destructive testing device for grain moisture according to claim 2, characterized in that: The outer shell is made of aluminum alloy.

4. The microwave non-destructive testing device for grain moisture according to claim 1, characterized in that: One connector of the coaxial transmitting line (6-1) and the coaxial receiving line (6-2) is a 3.5mm right-angle internal thread needle type, and the other connector is a 3.5mm straight internal thread needle type.

5. The microwave non-destructive testing device for grain moisture according to claim 1, characterized in that: The main control circuit board (7) is a four-layer PCB board. The top layer is a 24GHz microwave transceiver circuit, the filling material of the middle dielectric layer is Rogers4350b with a relative permittivity of 3.48, and the bottom layer is a data processing and control circuit. The main control chip used is STM32F407ZGT6.

6. The microwave non-destructive testing device for grain moisture according to claim 1, characterized in that: The dimensions (unit: mm) of the main control circuit board (7) are: length × width × height = 70 × 90 × 3.

7. The microwave non-destructive testing device for grain moisture according to claim 1, characterized in that: The grain container (2) has a double-layer structure, and its material is acrylic sheet. The internal dimensions (unit: mm) are: length × width × height = 120 × 60 × 100, and the external dimensions (unit: mm) are: length × width × height = 130 × 70 × 110.

8. The microwave non-destructive testing device for grain moisture according to claim 1, characterized in that: The metamaterial lens (5-4) consists of 121 metamaterial phase compensation units, which are manufactured by 3D printing. The material is polylactic acid (PLA). The cross-section of the unit is square. Different sized grid-shaped medium grooves are removed from the center of the unit, and 1 / 4 of the cylindrical medium material is cut off from the four corners of the unit to improve transmission efficiency.

9. A method for non-destructive microwave detection of grain moisture using the apparatus according to any one of claims 1-8, characterized in that: This device uses microwave signals to measure the change in scattering parameters between two antennas, calculates the complex permittivity of the grain moisture content based on this change, and then calculates the moisture content of the grain using a density-independent algorithm and a temperature calibration algorithm. S1. Based on the propagation characteristics of plane waves on the surface of multilayer media, establish straight-through and quarter-wavelength extension error models. In the straight-through model, the metasurface lens transmitting antenna and receiving antenna are placed 60mm apart, with no measuring object between the two antennas. The quarter-wavelength extension model is based on the straight-through model, with the distance between the two antennas increased by 3.1mm. Based on the two error models, establish a system of equations to complete the de-embedding of antenna error parameters. S2. The spherical wave emitted by the feed antenna is converted into a plane wave after passing through the metasurface lens, which can reduce the error caused by the multipath effect and diffraction effect of the spherical wave. The microwave signal of samples with different moisture content and different grain types is collected by the metasurface lens transmitting antenna and the metasurface lens receiving antenna, and the signal is transmitted to the main control circuit board via the coaxial line. The change of scattering parameter S is obtained by using the microwave signal, and the complex permittivity ε of grain moisture content is calculated by the inversion algorithm formula 1: ε = ε' - jε". S3. Change the density of the grain in the grain container and measure the scattering parameters of grain with different densities. Using the relationship between dielectric constant ε', loss factor ε", loss tangent tanδ and density ρ: Formula 2, Formula 3, Formula 4, Formula 5 is obtained to calculate the density calibration factor. The density calibration factor changes significantly with the moisture content and does not change with the density, which can reduce the influence of density on the accuracy of moisture detection. S4. Grain temperature is collected in real time using a near-infrared temperature probe, and the moisture content of grain at different temperatures is measured. Based on the relationship between grain moisture content and dielectric properties, a linear regression prediction model formula for grain moisture content is established (Formula 6). Parameters a, b, and c can be calculated through the linear fitting equation of grain moisture content with temperature and density calibration factor, resulting in the moisture content calculation formula M = 94.607ξ + 0.097T - 11.

016. This reduces the influence of density and temperature on detection, achieving accurate detection of grain moisture content. The measurement results are displayed in real time on a 4.3-inch capacitive screen. M = aξ + bT + c (Formula 6) In the formula, L is the width of the grain container (2), p is the phase parameter, λ0 is the free space wavelength at 24 GHz, and a f ε is the slope (1.23 at 24GHz), ε' is the dielectric constant, ε" is the loss factor, ξ is the density-independent factor, T is the grain temperature, and M is the grain moisture content.

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