A material moisture content detection device and detection method based on microwave cavity resonance

Abstract

The present invention discloses a material moisture content detection device and method based on microwave cavity resonance, belonging to the field of microwave application technology. The device comprises a sample to be tested, a microwave generator, a weighing unit, a sample tank, a temperature sensor, a microwave resonant cavity, an excitation probe, a detection probe, a detector, a control unit and a display output unit. In the present invention, the control unit controls the excitation voltage of the microwave generator and generates a microwave signal, which enters the microwave resonant cavity through the excitation probe and is received by the detection probe. The detection probe located inside the microwave resonant cavity detects the change in resonant frequency, and the temperature sensor located below the sample tank measures the temperature of the sample to be tested, establishes a functional relationship, and inverts the moisture content of the material to be tested. The present invention improves the universality and detection accuracy of the microwave moisture content detection device, and the result is not affected by the material temperature and stacking conditions, thereby realizing rapid, non-destructive and high-precision detection of the moisture content of granular, powdered or liquid materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave applications, and in particular relates to a material moisture content detection device and a detection method based on microwave cavity resonance. Background Art

[0002] In the processing and manufacturing industries of grain, soil, fertilizer, and petroleum, the moisture content of raw materials is a key indicator of product quality and is closely related to the product's price. Accurately, effectively, and conveniently detecting the moisture content of materials has become a hot topic in the field of testing technology. Determination of moisture content involves direct and indirect measurement. The direct measurement method primarily uses the wet-based method, calculating the moisture content by measuring the specific gravity of the difference in weight between the sample before and after drying. However, this method is slow and requires material destruction, and is generally used for moisture calibration in laboratories. Indirect methods for measuring moisture content primarily include resistance, capacitance, neutron, infrared, and microwave methods, each with varying characteristics, scope of application, and measurement accuracy. The microwave method utilizes the propagation characteristics of high-frequency electromagnetic waves in space at the speed of light. The measurement process does not destroy the sample structure, thus meeting the needs of rapid, non-destructive testing of material moisture content and possessing a wide range of applications.

[0003] At microwave frequencies, the complex dielectric constant of water is much higher than that of other dry substances. Therefore, by measuring physical quantities related to the dielectric constant, such as power attenuation and phase shift, after microwave interaction with the material, the moisture content of the material can be indirectly measured. Because microwaves are penetrating, the measurement results reflect the overall moisture content of the sample and are therefore more representative.

[0004] Material moisture content detection based on microwave technology is mainly divided into the space wave method, transmission line method and resonant cavity method. Among them, the space wave method is generally suitable for continuous online detection of large batches of samples. After the microwaves interact with the sample being measured, the space microwaves will produce reflection, transmission and scattering phenomena. The multiple reflections of electromagnetic waves will reduce the accuracy of moisture content measurement. The transmission line method sensor needs to be in direct contact with the material being measured, and the measurement process requires very high uniformity in the sampling of the material. At present, in the process of measuring the moisture content of materials based on the resonant cavity method, the sample needs to be evenly filled in the cavity and in direct contact with the microwave probe. It is impossible to quickly and accurately detect different types of samples in a short time. Based on the above situation, it is of great significance to research and develop economical, practical, universal and high-precision microwave moisture content detection methods and devices. Summary of the Invention

[0005] In response to the above-mentioned problems existing in the prior art, the present invention provides a material moisture content detection device and a detection method based on microwave cavity resonance. When the microwave frequency matches the natural frequency of the cavity, resonance will occur. The magnitude of the resonance frequency is related to the moisture content of the sample being tested in the cavity. The present invention establishes a functional relationship by detecting the change in the resonance frequency and inverts the moisture content of the material being tested. The detection device can realize rapid and non-destructive detection of the moisture content of granular, powder or liquid materials. The device has a simple structure and high measurement accuracy.

[0006] The present invention is achieved through the following technical solutions:

[0007] A material moisture content detection device based on microwave cavity resonance, comprising a sample to be tested 1, a microwave generator 2, a weighing unit 3, a sample tank 4, a temperature sensor 5, a microwave resonant cavity 6, an excitation probe 7, a detection probe 8, a detector 9, a control unit 10 and a display output unit 11; wherein the microwave generator 2 is used to generate a swept frequency microwave signal, which is connected to the excitation probe 7 through a coaxial line, and transmits the microwave excitation signal into the microwave resonant cavity 6. After the microwave signal propagates in the cavity, it is received by the detection probe 8 and then transmitted to the detector 9 outside the microwave resonant cavity 6 through the coaxial line for power detection; the weighing unit 3 is used to generate a swept frequency microwave signal, which is connected to the excitation probe 7 through a coaxial line, and transmits the microwave excitation signal into the microwave resonant cavity 6. The weighing unit 3 is located at the bottom of the microwave resonant cavity 6 and is used to record the weight when empty and the weight when fully loaded with materials; the sample slot 4 is located at the top of the microwave resonant cavity 6, and a temperature sensor 5 is provided below the sample slot 4 for real-time measurement of the temperature of the sample 1 under test; the control unit 10 is connected to the detector 9, the temperature sensor 5 and the weighing unit 3 through signal lines, and controls the excitation voltage of the microwave generating device 2. The control unit 10 calculates the moisture content information of the sample 1 under test through an internal function based on the read detection voltage, temperature and weight information, and outputs the measurement result through the display output unit 11.

[0008] Furthermore, the sample 1 to be tested is a non-metallic water-containing material with uniform density distribution, such as food, soil, fertilizer, etc.

[0009] Furthermore, the microwave generating device 2 is a voltage-controlled swept frequency signal source, the voltage control range is 0-5V, and the frequency output is in the range of 3GHz-12GHz.

[0010] Furthermore, the sample tank 4 is a cylindrical structure with a cover, made of metal, with a height range of 30-70 mm, located at the top of the resonant cavity, and the bottom is a non-metallic partition, which can be a low-attenuation material for microwaves such as ceramic, plastic or glass. It is connected to the resonant cavity 6 and constitutes a part of the resonant cavity.

[0011] Furthermore, the effective temperature measurement range of the temperature sensor 5 is not less than 0-100°C, and the accuracy is not less than ±0.5°C.

[0012] Furthermore, the microwave resonant cavity 6 is a cylindrical structure, made of metal material, with an inner radius of the cavity ranging from 10 to 40 mm and a cavity length ranging from 20 to 80 mm.

[0013] Furthermore, the excitation probe 7 and the detection probe 8 are both made of metal materials and are vertically inserted into the cavity wall of the microwave resonant cavity 6. The two probes are at 90° in the cavity of the microwave resonant cavity 6, and the length range of the vertical cavity coupling is 5-20 mm.

[0014] Furthermore, the control unit 10 includes a power supply, an A / D converter, a D / A converter and a single-chip microcomputer operation control unit, wherein the A / D converter is used to convert the detection voltage signal output by the detector 9 into a digital quantity for internal operation of the single-chip microcomputer, and the effective bit number of the A / D converter is not less than 12 bits; the D / A converter is used to convert the single-chip microcomputer control signal into a voltage value to control the microwave generating device 2 to generate a swept-frequency microwave signal, and the effective bit number of the D / A converter is also not less than 12 bits.

[0015] The measurement principle of the material moisture content detection device based on microwave cavity resonance of the present invention is described as follows:

[0016] For a resonant cavity, resonance occurs when the microwave frequency matches the cavity's natural frequency. If the sample to be tested contains water, the water molecules will absorb part of the microwave energy, resulting in energy loss in the electromagnetic field. These losses ultimately manifest as a decrease in the quality factor (Q) of the microwave resonant cavity, and the cavity's natural frequency peak becomes wider and moves toward lower frequencies.

[0017] For a cylindrical resonant cavity, the resonant frequency satisfies the following calculation relationship:

[0018]

[0019] Where c is the speed of light, ε m is the dielectric constant, l is the length of the resonant cavity, and R is the radius.

[0020] For a resonant cavity of fixed size, the radius R and length l are constant, and the above formula is transformed into:

[0021]

[0022] in, is a constant.

[0023] The above equation shows that the resonant frequency is directly related to the dielectric constant. At microwave frequencies, water molecules have a strong dipole moment, resulting in a dielectric constant much higher than that of other dry substances. After placing different samples in the cavity, changes in the resonant frequency directly reflect changes in the sample's moisture content.

[0024] Furthermore, the dielectric properties of materials are affected by both temperature and sample packing conditions. Temperature influences the energy state of water molecules within the material, while packing density influences the spatial distribution of water molecules within the material. For a sample tank with a fixed volume, changes in the sample mass within it directly reflect changes in the sample's packing density. The device of the present invention simultaneously measures the resonant frequency, temperature, and mass of the sample being tested, establishing a functional relationship that compensates for temperature and packing density to invert the sample's moisture content.

[0025] On the other hand, the present invention also provides a detection method for the above-mentioned material moisture content detection device based on microwave cavity resonance, which specifically includes the following steps:

[0026] S1, no-load signal detection;

[0027] Keeping the sample tank 4 empty, the control unit 10 controls the sweeping voltage change of the microwave generator 2, synchronously detects and records the value of the detection voltage of the detector 9, records the value of the sweeping voltage corresponding to the extreme value of the detection voltage, and converts it into the resonant frequency f0; synchronously measures the output weight m0 of the weighing unit 3 when it is empty;

[0028] S2, full load signal detection;

[0029] When the sample tank 4 is filled with the sample, the control unit 10 controls the sweep voltage change of the microwave generator 2, synchronously detects and records the value of the detection voltage of the detector 9, records the value of the sweep voltage corresponding to the extreme value of the detection voltage, and converts it into the resonant frequency f1; synchronously measures the weight m1 output by the weighing unit 3 and the sample temperature value T output by the temperature sensor 5 when fully loaded;

[0030] S3. Calculation of material moisture content;

[0031] The microcontroller inside the control unit 10 uses the following formula to calculate the moisture content:

[0032] W=a(f1-f0) / (m1-m0)+bT+c

[0033] Where W represents the moisture content of the material being tested, f0 and f1 are the resonant frequencies measured at no load and full load, respectively; m0 and m1 are the weights measured at no load and full load, respectively; a, b, and c are coefficients obtained by fitting the known moisture content data; and T is the temperature of the sample being tested.

[0034] S4, real-time display and output of moisture content;

[0035] After the single chip microcomputer in the control unit 10 completes the moisture content calculation, the display output unit 11 outputs and displays the moisture content information of the tested sample 1 .

[0036] Compared with the prior art, the advantages of the present invention are as follows:

[0037] (1) Based on the working principle of microwave resonant cavity, the present invention designs a cylindrical resonant cavity structure; while achieving cavity integrity, it has good sealing and electromagnetic interference resistance, a light structure, and is economical and practical;

[0038] (2) The detection device of the present invention has a simple circuit, and is easy to obtain, analyze and process parameters. It can also compensate for sample temperature and mass. The moisture content detection is accurate and fast, and is suitable for the detection of materials in various processing and manufacturing industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly describes the drawings required for the specific embodiments or the description of the prior art. Similar elements or parts are generally identified by similar reference numerals throughout the drawings. Elements or parts in the drawings are not necessarily drawn to scale.

[0040] Figure 1 This is a schematic structural diagram of a material moisture content detection device based on microwave cavity resonance according to the present invention;

[0041] Figure 2 A top view of the microwave resonant cavity of the present invention;

[0042] Figure 3 It is a structural schematic diagram of the sample tank of the present invention;

[0043] Figure 4 This is a flow chart of measurement signal control of the detection device of the present invention;

[0044] Figure 5 This is a schematic diagram of the change in resonant frequency of the detection device of the present invention before and after adding a sample;

[0045] In the figure: the sample to be tested 1, the microwave generating device 2, the weighing unit 3, the sample slot 4 and the temperature sensor 5 located below the sample slot 4, the microwave resonant cavity 6, the excitation probe 7, the detection probe 8, the detector 9, the control unit 10, and the display output unit 11. DETAILED DESCRIPTION

[0046] In order to clearly and completely describe the technical solution and specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings:

[0047] Example 1

[0048] like Figure 1: As shown in the figure, it is a structural schematic diagram of a material moisture content detection device based on microwave cavity resonance of this embodiment, the device includes a sample to be tested 1, a microwave generating device 2, a weighing unit 3, a sample tank 4, a temperature sensor 5, a microwave resonant cavity 6, an excitation probe 7, a detection probe 8, a detector 9, a control unit 10 and a display output unit 11; wherein, the microwave generating device 2 generates a swept frequency microwave signal, is connected to the excitation probe 7 through a coaxial line, transmits the microwave excitation signal into the microwave resonant cavity 6, and the microwave signal propagated in the cavity is received by the detection probe 8; the weighing unit 3 is located at the bottom of the microwave resonant cavity 6; the sample tank 4 is located at the top of the microwave resonant cavity 6, and a temperature sensor 5 is provided below the sample tank 4; the microwave generating device 2 and the detector 9 are connected to the microwave resonant cavity 6 through a coaxial line; the detector 9 is connected to the detection probe 8 through a coaxial line; the control unit 10 is connected to the detector 9, the temperature sensor 5 and the weighing unit 3 respectively through signal lines; the output display unit 11 is connected to the control unit 10;

[0049] In this embodiment, the microwave generating device 2 is a voltage-controlled swept-frequency signal source with a voltage control range of 0-4.5V and a swept-frequency output range of 8-10GHz.

[0050] In this embodiment, the weighing sensor of the weighing unit 3 is a bridge structure with a maximum measuring range of 3 kg and an accuracy of 0.1 g.

[0051] In this embodiment, the microwave resonant cavity 6 is a cylindrical structure made of stainless steel, with an inner radius of 11 mm and a cavity length of 40 mm.

[0052] In this embodiment, the temperature sensor 5 is a DS18B20 temperature sensor with a measuring temperature range of -55-+125°C and an accuracy of ±0.5°C.

[0053] In this embodiment, the control unit 10 includes a power supply, an A / D converter, a D / A converter, and a single-chip microcomputer operation control unit. The single-chip microcomputer operation control unit adopts a 32-bit ARM core STM32F103 series processor, is powered by 3.3 volts, and the chip operating frequency is set to 72 MHz. The A / D converter uses the analog-to-digital converter integrated inside the STM32 processor, and sets a 12-bit conversion accuracy and a single conversion acquisition time of 1 μs.

[0054] like Figure 2 As shown, in this embodiment, the excitation probe 7 and the detection probe 8 are both made of metal materials, with a length of 8 mm. They are installed in the middle position of the resonant cavity 6 and are vertically inserted into the cavity wall of the microwave resonant cavity 6. The length of the coupling into the cavity is 5 mm, and the two probes are at 90° in the cavity.

[0055] like Figure 3As shown, in this embodiment, the sample tank 4 is a cylindrical structure with a cover, made of metal, 10 mm in height, located at the top of the resonant cavity, with a ceramic partition at the bottom, 2 mm in thickness, and an inner radius of the sample tank of 11 mm.

[0056] like Figure 4 As shown, the signal control process of the material moisture content detection device of this embodiment is as follows:

[0057] The control unit 10 controls the excitation voltage of the microwave generating device 2 to gradually change from small to large through the internal D / A digital-to-analog conversion. The generated swept-frequency microwave signal enters the microwave resonant cavity 6 through the excitation probe 7, and the microwave signal after propagating through the sample slot 4 is received by the detection probe 8; the detector 9 performs power detection on the microwave signal voltage inside the cavity through the detection probe 8; the weighing unit 3 collects the weight information of the empty and full loads; the temperature sensor 5 collects the temperature information of the sample 1 under test; the control unit 10 calculates the moisture content information of the sample 1 under test through the internal function according to the read detection voltage, temperature and weight information, and outputs the measurement result through the display output unit 11.

[0058] Example 2

[0059] In this embodiment, corn is used as the measurement object to illustrate the specific measurement method of the detection device. The initial moisture content of the naturally air-dried corn is 9.8%. By adding water to the sample and continuously stirring it evenly, six corn samples with different moisture contents are finally obtained. The moisture content varies from 9.8% to 24.2%. To ensure the uniformity of sample stacking, the sample is ground into powder using a grinder before measurement.

[0060] This embodiment provides a detection method for a material moisture content detection device based on microwave cavity resonance, and the specific steps are as follows:

[0061] S1, no-load signal detection;

[0062] Keeping the sample tank 4 empty, the control unit 10 controls the sweep voltage of the microwave generator 2 and simultaneously detects and records the detection voltage value of the detector 9. The sweep voltage value corresponding to the extreme value of the detection voltage is recorded and converted to the resonant frequency f0. The weight m0 output by the weighing unit 3 is also measured when the sample tank 4 is empty.

[0063] S2, full load signal detection;

[0064] When the sample tank 4 is filled with sample, control unit 10 controls the sweep voltage of microwave generator 2 and simultaneously detects and records the detection voltage value of detector 9. The sweep voltage value corresponding to the extreme value of the detection voltage is recorded and converted to the resonant frequency f1. The fully loaded weighing unit 3 outputs weight m1, and the temperature sensor 5 outputs sample temperature T.

[0065] Figure 5 The figure shows the change of resonance frequency in the measurement of corn with no load and 9.8% moisture content. The resonance frequency is 9.425 GHz when no load is added, and the resonance frequency is 9.270 GHz when the sample is added.

[0066] Table 1 lists the measurement results of the moisture content W, no-load resonant frequency f0, full-load resonant frequency f1, no-load weight m0, full-load weight m1 and sample temperature T of the six corn samples with different moisture contents in the experiment.

[0067] S3. Calculation of material moisture content

[0068] The microcontroller inside the control unit 10 uses the following formula to calculate the moisture content:

[0069] W=a(f1-f0) / (m1-m0)+bT+c

[0070] Where W represents the moisture content of the material being measured, f0 and f1 are the resonant frequencies measured at no load and full load, respectively, and m0 and m1 are the weights measured at no load and full load, respectively. a, b, and c are coefficients derived from fitting data with known moisture content, and T is the temperature of the sample being measured. For a given sample, the fitting coefficient is a constant. Substituting the different moisture contents W, no-load resonant frequency f0, full-load resonant frequency f1, no-load weight m0, full-load weight m1, and sample temperature T from Table 1 into the above formula, and performing a linear fit using Origin data processing software, we obtain fitting coefficients a of 632.52, b of 1.43, and c of 5.47.

[0071] Table 1: Corn sample sensor measurement data

[0072]

[0073] S4, real-time display output of moisture content

[0074] After the single chip microcomputer in the control unit 10 completes the moisture content calculation, the display output unit 11 outputs and displays the moisture content information of the tested sample 1 .

[0075] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0076] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0077] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A detection method for a material moisture content detection device based on microwave cavity resonance, characterized in that: The detection method is implemented by a material moisture content detection device, which includes a sample to be tested (1), a microwave generator (2), a weighing unit (3), a sample tank (4), a temperature sensor (5), a microwave resonant cavity (6), an excitation probe (7), a detection probe (8), a detector (9), a control unit (10) and a display output unit (11); wherein the microwave generator (2) is used to generate a swept frequency microwave signal, is connected to the excitation probe (7) via a coaxial line, transmits the microwave excitation signal into the microwave resonant cavity (6), and the microwave signal propagated in the cavity is received by the detection probe (8) and then transmitted to the detector (9) outside the microwave resonant cavity (6) via the coaxial line for power detection. The weighing unit (3) is located at the bottom of the microwave resonant cavity (6) and is used to record the weight when empty and the weight when fully loaded with materials; the sample tank (4) is located at the top of the microwave resonant cavity (6), and a temperature sensor (5) is provided below the sample tank (4) for real-time measurement of the temperature of the sample (1) under test; the control unit (10) is connected to the detector (9), the temperature sensor (5) and the weighing unit (3) through signal lines, and controls the excitation voltage of the microwave generating device (2); the control unit (10) calculates the moisture content information of the sample (1) under test through an internal function based on the read detection voltage, temperature and weight information, and outputs the measurement result through the display output unit (11); The detection method specifically comprises the following steps: S1, no-load signal detection; The sample tank (4) is kept empty, and the control unit (10) controls the sweeping voltage change of the microwave generating device (2), synchronously detects and records the value of the detection voltage of the detector (9), records the value of the sweeping voltage corresponding to the extreme value of the detection voltage, and converts it into the resonant frequency f0; synchronously measures the output weight m0 of the weighing unit (3) when empty; S2, full load signal detection; When the sample tank (4) is filled with the sample, the control unit (10) controls the change of the sweeping frequency voltage of the microwave generating device (2), synchronously detects and records the value of the detection voltage of the detector (9), records the value of the sweeping frequency voltage corresponding to the extreme value of the detection voltage, and converts it into the resonant frequency f1; synchronously measures the weight m1 output by the weighing unit (3) and the sample temperature value T output by the temperature sensor (5) when the sample is fully loaded; S3. Calculation of material moisture content; The single chip microcomputer inside the control unit (10) uses the following formula to calculate the moisture content: W=a(f1-f0) / (m1-m0)+bT+c Where W represents the moisture content of the material being tested, f0 and f1 are the resonant frequencies measured at no load and full load, respectively; m0 and m1 are the weights measured at no load and full load, respectively; a, b, and c are coefficients obtained by fitting the known moisture content data; and T is the temperature of the sample being tested. S4, real-time display and output of moisture content; After the single chip microcomputer inside the control unit (10) completes the moisture content calculation, the display output unit (11) outputs and displays the moisture content information of the tested sample (1).

2. The method for detecting moisture content of a material based on microwave cavity resonance according to claim 1, characterized in that: The sample (1) to be tested is a non-metallic water-containing material such as grain, soil or fertilizer with uniform density distribution.

3. The method for detecting moisture content of a material based on microwave cavity resonance according to claim 1, characterized in that: The microwave generating device (2) is a voltage-controlled frequency sweep signal source, with a voltage control range of 0-5V and a frequency output within the range of 3GHz-12GHz.

4. The method for detecting moisture content of a material based on microwave cavity resonance according to claim 1, wherein: The sample tank (4) is a cylindrical structure with a cover and is made of metal. The height of the sample tank (4) ranges from 30 to 70 mm. The sample tank (4) is located at the top of the resonant cavity. The bottom of the sample tank (4) is a non-metallic partition. The non-metallic partition is made of ceramic, plastic or glass with low microwave attenuation. The bottom is connected to the resonant cavity (6) and constitutes a part of the resonant cavity.

5. The detection method of a material moisture content detection device based on microwave cavity resonance according to claim 1, characterized in that: The temperature sensor (5) has a temperature measurement range of 0-100°C and an accuracy of not less than ±0.5°C.

6. The detection method of a material moisture content detection device based on microwave cavity resonance according to claim 1, characterized in that: The microwave resonant cavity (6) is a cylindrical structure and is made of metal material. The inner radius of the cavity ranges from 10 to 40 mm, and the cavity length ranges from 20 to 80 mm.

7. The method for detecting moisture content of a material based on microwave cavity resonance according to claim 1, characterized in that: The excitation probe (7) and the detection probe (8) are both made of metal materials and are vertically inserted into the cavity wall of the microwave resonant cavity (6). The two probes are at 90 degrees in the cavity of the microwave resonant cavity (6).

8. The method for detecting moisture content of a material based on microwave cavity resonance according to claim 1, wherein: The control unit (10) includes a power supply, an A / D converter, a D / A converter, and a single-chip microcomputer operation control unit, wherein the A / D converter is used to convert the detection voltage signal output by the detector (9) into a digital value for internal operation of the single-chip microcomputer, and the effective number of bits of the A / D converter is not less than 12 bits; the D / A converter is used to convert the single-chip microcomputer control signal into a voltage value to control the microwave generating device (2) to generate a swept-frequency microwave signal, and the effective number of bits of the D / A converter is also not less than 12 bits.

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