CN116626072A - A material moisture content detection device and detection method based on microwave cavity resonance - Google Patents

Abstract

The invention discloses a device and a method for detecting the moisture content of a material based on microwave cavity resonance, which belong to the technical field of microwave application and comprise a detected sample, a microwave generating device, 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; the invention controls the excitation voltage of the microwave generating device and generates a microwave signal, the microwave signal enters the microwave resonant cavity through the excitation probe and is received by the detection probe, the detection of the resonance frequency change is realized through the detection probe positioned in the microwave resonant cavity, the temperature of the detected sample is measured through the temperature sensor positioned below the sample groove, a functional relation is established, and the water content of the detected material is inverted; the invention improves the universality and the detection precision of the microwave water content detection device, and the result is not influenced by the material temperature and the accumulation condition, thereby realizing the rapid nondestructive high-precision detection of the water content of the particle, powder or liquid material.

Description

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

technical field

The invention belongs to the field of microwave application technology, and in particular relates to a material moisture content detection device and detection method based on microwave cavity resonance.

Background technique

In the processing and manufacturing industries of grain, soil, chemical fertilizer and petroleum, the moisture content of raw materials is an important indicator of product quality and is closely related to the price of the product. Accurate, effective and convenient detection of moisture content of materials has become a hot issue in the field of detection technology. The determination of moisture content includes two methods, direct measurement and indirect measurement. The direct measurement method mainly adopts the wet base method, and the moisture content is calculated by measuring the specific gravity of the difference between the weight of the sample before and after drying, but the measurement speed of this method is slow, and at the same time, the material needs to be tested. Destruction, generally used for moisture calibration in the laboratory. The indirect measurement methods of moisture content mainly include resistance method, capacitance method, neutron method, infrared method and microwave method, etc., and their characteristics, scope of application and measurement accuracy are different. Among them, the microwave method uses the light-speed propagation characteristics of high-frequency electromagnetic waves in space, and the measurement process does not destroy the sample structure, so it can meet the needs of rapid non-destructive testing of the moisture content of materials and has a wide range of application scenarios.

At microwave frequency, the complex permittivity of water is much higher than that of other dry substances. Therefore, by measuring the physical quantities related to permittivity, such as power attenuation and phase change after the microwave interacts with the material, the water content of the material can be measured indirectly. Rate. Microwaves are penetrating, and the measurement results reflect the overall water content of the sample, so they are more representative.

The moisture content detection of materials based on microwave technology is mainly divided into space wave method, transmission line method and resonant cavity method, among which the space wave method is generally suitable for continuous on-line detection of a large number of samples. After the microwave interacts with the tested sample, the space microwave will produce Reflection, transmission and scattering phenomena, multiple reflections of electromagnetic waves will reduce the accuracy of water content measurement. The transmission line method sensor needs to be in direct contact with the material to be measured, and the measurement process has high requirements on the uniformity of material sampling. 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 inside the cavity and directly contacted with the microwave probe, so it is impossible to perform fast and high-precision detection of different samples in a short time. Based on the above status quo, it is of great significance to research and develop economical, practical, universal, and high-precision microwave water content detection methods and devices.

Contents of the invention

Aiming at the above-mentioned problems existing in the prior art, the present invention provides a material moisture content detection device and detection method based on microwave cavity resonance. When the microwave frequency matches the natural frequency of the cavity, resonance will occur, and the resonance frequency The size is related to the water content of the sample to be tested in the cavity. The present invention establishes a functional relationship by detecting the change of the resonant frequency, and inverts the water content of the material to be tested; the detection device can realize the water content of the particle, powder or liquid material Fast non-destructive testing, simple device structure and high measurement accuracy.

The present invention realizes through following technical scheme:

A material moisture content detection device based on microwave cavity resonance, including 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 Needle 8, wave detector 9, control unit 10 and display output unit 11; wherein, the microwave generating device 2 is used to generate frequency-sweeping microwave signals, connected to the excitation probe 7 through a coaxial line, and transmits the microwave excitation signal into the microwave In the resonant cavity 6, after the microwave signal propagated in the cavity is received by the detection probe 8, it is transmitted to the detector 9 outside the microwave resonant cavity 6 through the coaxial line for power detection; the weighing unit 3 is located in the microwave resonant cavity The bottom of 6 is used to record the weight when it is empty and the weight when it is fully loaded with materials; the sample tank 4 is located on the top of the microwave resonator 6, and a temperature sensor 5 is provided below the sample tank 4 for real-time measurement of the sample to be tested 1 temperature; the control unit 10 is respectively connected with the wave detector 9, the temperature sensor 5 and the weighing unit 3 through signal wires, and controls the excitation voltage of the microwave generator 2, and the control unit 10 is based on the detection voltage read, For the temperature and weight information, the water content information of the sample 1 to be tested is calculated through internal functions, and the measurement results are output through the display and output unit 11 .

Furthermore, the tested sample 1 is a non-metallic water-containing material with uniform density distribution such as grain, soil, and fertilizer.

Further, the microwave generating device 2 is a voltage-controlled sweep signal source, the voltage control range is 0-5V, and the frequency output is in the range of 3GHz-12GHz.

Further, the sample tank 4 is a cylindrical structure with a cover, made of metal, with a height range of 30-70 mm, located on the top of the resonant cavity, and a non-metallic partition at the bottom, which can be made of ceramics, plastics or glass, etc., which are low in microwave resistance. The attenuation material is connected with the resonant cavity 6 to form a part of the resonant cavity.

Further, the temperature measurement effective range of the temperature sensor 5 is not lower than 0-100°C, and the accuracy is not lower than ±0.5°C.

Further, the microwave resonant cavity 6 is a cylindrical structure made of metal material, the inner radius of the cavity is 10-40 mm, and the cavity length is 20-80 mm.

Further, the excitation probe 7 and the detection probe 8 are all metal materials, and are vertically inserted into the cavity wall of the microwave resonator 6, and the two probes are 90° in the cavity of the microwave resonator 6, and the vertical cavity coupling The length range is 5-20mm.

Further, the control unit 10 includes a power supply, an A/D converter, a D/A converter and a single-chip computer 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 supply The internal operation of the single-chip microcomputer, the effective number of digits of the A/D converter is not less than 12; the D/A converter is used to convert the control signal of the single-chip microcomputer into a voltage value, and controls the microwave generator 2 to generate a frequency-sweeping microwave signal. The D/A converter The effective number of digits is also not less than 12.

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

For a resonant cavity, when the microwave frequency matches the natural frequency of the cavity, resonance will occur. If the sample to be tested contains water, the water molecules will absorb a part of the microwave energy, resulting in energy loss in the electromagnetic field; these losses will eventually appear as microwave resonant cavity The Q value of the quality factor decreases, and the peak of the natural frequency of the cavity becomes wider and moves to the lower frequency.

For a cylindrical resonator, the resonant frequency satisfies the following calculation relation:

Among them, 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.

For a resonant cavity with a fixed size, the radius R and the length l are fixed, and the above formula is transformed into:

in, is a constant.

The above formula 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 much higher dielectric constant of water than other dry substances. After placing different samples in the cavity, the change of resonance frequency can directly reflect the change of sample moisture.

In addition, the dielectric properties of materials are affected by both temperature and sample stacking conditions. Temperature affects the energy state of water molecules in the material, and packing density affects the spatial distribution of water molecules in the material. For a sample tank with a fixed volume, the change in the mass of the sample inside it directly reflects the change in the bulk density of the sample. The device of the invention simultaneously measures the resonant frequency, temperature, and mass of the sample to be tested, and establishes a functional relational expression with temperature and bulk density compensation to invert the water content of the sample.

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

S1. No-load signal detection;

Keep the sample tank 4 unloaded, the control unit 10 controls the change of the frequency sweep voltage of the microwave generator 2, synchronously detects and records the value of the detection voltage of the detector 9, records the value of the frequency sweep voltage corresponding to the extreme point of the detection voltage, and converts it accordingly is the resonant frequency f ₀ ; synchronously measure the output weight m ₀ of the weighing unit 3 at no-load;

S2, full load signal detection;

The sample fills the sample slot 4, the control unit 10 controls the frequency 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 frequency sweep voltage corresponding to the extreme point of the detection voltage, and converts it to Resonant frequency f ₁ ; synchronous measurement of weighing unit 3 output weight m ₁ and temperature sensor 5 output sample temperature value T when fully loaded;

S3, material moisture content calculation;

The single-chip microcomputer inside the control unit 10 adopts the following formula to calculate the moisture content:

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

Among them, W represents the water content of the material to be 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 obtained by The coefficient obtained by fitting the moisture content data measurement, T is the temperature value of the measured sample;

S4, real-time display output of moisture content;

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

Compared with prior art, advantage of the present invention is as follows:

(1) The present invention designs a cylindrical resonator structure based on the working principle of the microwave resonator; while realizing the integrity of the cavity, it has good sealing performance, anti-electromagnetic interference, light structure, economical and practical;

(2) The detection device of the present invention has a simple circuit, easy parameter acquisition, analysis and processing, and can compensate the sample temperature and quality at the same time. The moisture content detection accuracy is high and the speed is fast, and it is suitable for the detection of various processing and manufacturing materials.

Description of drawings

In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings that need to be used in the description of the specific embodiments or the prior art. Throughout the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, elements or parts are not necessarily drawn in actual scale.

Fig. 1 is a structural schematic diagram of a material moisture content detection device based on microwave cavity resonance of the present invention;

Fig. 2 is the top view of microwave cavity of the present invention;

Fig. 3 is the structural representation of sample tank of the present invention;

Fig. 4 is the measurement signal control flowchart of detection device of the present invention;

Fig. 5 is a schematic diagram of the change in resonance frequency before and after adding samples to the detection device of the present invention;

In the figure: the sample to be tested 1, the microwave generator 2, the weighing unit 3, the sample tank 4 and the temperature sensor 5 located below the sample tank 4, the microwave resonant cavity 6, the excitation probe 7, the detection probe 8, and the detector 9 , a control unit 10 , and a display output unit 11 .

Detailed ways

In order to clearly and completely describe the technical solution of the present invention and its specific working process, in conjunction with the accompanying drawings, the specific implementation of the present invention is as follows:

Example 1

As shown in Figure 1, it is a schematic structural diagram of a material moisture content detection device based on microwave cavity resonance in this embodiment, and the device includes a sample to be tested 1, a microwave generating device 2, a weighing unit 3, and a sample tank 4 , temperature sensor 5, microwave resonant cavity 6, excitation probe 7, detection probe 8, wave detector 9, control unit 10 and display output unit 11; The wire is connected with the excitation probe 7, and the microwave excitation signal is transmitted 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 resonator 6, and a temperature sensor 5 is arranged below the sample tank 4; the microwave generating device 2 and the detector 9 are connected to the microwave resonator 6 through a coaxial line; The line is connected to the detection probe 8; 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;

In this embodiment, the microwave generator 2 is a voltage-controlled frequency sweep signal source, the voltage control range is 0-4.5V, and the frequency sweep output range is 8-10GHz.

In this embodiment, the weighing sensor of the weighing unit 3 is a bridge structure with a maximum capacity of 3KG and an accuracy of 0.1g.

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

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

In this embodiment, the control unit 10 includes a power supply, an A/D converter, a D/A converter, and a single-chip computer operation control unit. The single-chip computer operation control unit adopts a 32-bit ARM core STM32F103 series processor, and adopts a 3.3-volt power supply. The working frequency of the chip is set to 72MHz, and the A/D converter is selected to integrate the analog-to-digital converter inside the STM32 processor, and the conversion precision of 12 bits and the acquisition time of single conversion of 1μs are set.

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

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

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

The control unit 10 controls the excitation voltage of the microwave generator 2 to gradually change from small to large through the internal D/A digital-to-analog conversion, and the generated frequency-sweeping microwave signal enters the interior of the microwave resonator 6 through the excitation probe 7 and propagates through the sample slot 4. The microwave signal is received by the detection probe 8; the wave detector 9 detects the power of the microwave signal voltage inside the cavity through the detection probe 8; the weighing unit 3 collects the weight information of no-load and full-load; the temperature sensor 5 collects the sample to be tested 1 temperature information; the control unit 10 calculates the moisture content information of the tested sample 1 through internal functions according to the read detection voltage, temperature and weight information, and outputs the measurement results through the display and output unit 11.

Example 2

In this example, corn is used as the measurement object to illustrate the specific measurement method of the detection device. The initial moisture content of the naturally dried corn is 9.8%, and 6 corn samples with different moisture content are finally obtained by adding water to the sample and stirring continuously , the moisture content ranges from 9.8% to 24.2%. In order to ensure the uniformity of sample accumulation, the sample is ground into powder by a pulverizer and then measured.

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

S1. No-load signal detection;

Keep the sample tank 4 unloaded, the control unit 10 controls the change of the frequency sweep voltage of the microwave generator 2, synchronously detects and records the value of the detection voltage of the detector 9, records the value of the frequency sweep voltage corresponding to the extreme point of the detection voltage, and converts it accordingly is the resonant frequency f ₀ . The weight m ₀ output by the weighing unit 3 is measured synchronously without load.

S2, full load signal detection;

The sample fills the sample slot 4, the control unit 10 controls the frequency 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 frequency sweep voltage corresponding to the extreme point of the detection voltage, and converts it to Resonant frequency f ₁ . The weighing unit 3 outputs the weight m ₁ and the temperature sensor 5 outputs the sample temperature value T when the load is measured synchronously.

Figure 5 shows the comparison of the change of resonance frequency in the measurement of corn with no load and 9.8% moisture content. The no-load resonance frequency is 9.425GHz, and the resonance frequency after adding samples is 9.270GHz.

Table 1 lists the measurement results of moisture content W, no-load resonant frequency f ₀ , full-load resonant frequency f ₁ , no-load weight m ₀ , full-load weight m ₁ and sample temperature T of 6 corn samples with different moisture content in the test .

S3, material moisture content calculation

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

Among them, W represents the moisture content of the material to be tested, 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 the coefficients obtained from the fitting of known moisture content data, and T is the temperature value of the measured sample. For a determined measurement sample, the fitting coefficient is constant, and the different water content W, no-load resonant frequency f ₀ , full-load resonant frequency f ₁ , no-load weight m ₀ , full-load weight m ₁ , and sample temperature T in Table 1 are brought into After the above formula was linearly fitted by origin data processing software, the fitting coefficients a was 632.52, b was 1.43, and c was 5.47.

Table 1: Corn sample sensor measurement data

S4, real-time display output of moisture content

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

The preferred embodiment of the present invention has been described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the specific details of the above embodiment, within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, These simple modifications all belong to the protection scope of the present invention.

In addition, it should be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable way if there is no contradiction. The combination method will not be described separately.

In addition, various combinations of different embodiments of the present invention can also be combined arbitrarily, as long as they do not violate the idea of the present invention, they should also be regarded as the disclosed content of the present invention.

Claims (9)

1. A material moisture content detection device based on microwave cavity resonance, characterized in that it includes a sample to be tested (1), a microwave generating device (2), a weighing unit (3), a sample tank (4), a temperature sensor (5), microwave resonant cavity (6), excitation probe (7), detection probe (8), wave detector (9), control unit (10) and display output unit (11); wherein, the microwave generation The device (2) is used to generate a frequency-sweeping microwave signal, which is connected to the excitation probe (7) through a coaxial line, and transmits the microwave excitation signal into the microwave resonator cavity (6), and the microwave signal propagated in the cavity passes through the detection probe (8) After receiving, it is transmitted to the detector (9) outside the microwave resonator (6) through the coaxial line for power detection; the weighing unit (3) is located at the bottom of the microwave resonator (6) for Record the weight when unloaded and the weight when fully loaded with materials; the sample tank (4) is positioned at the top of the microwave resonator (6), and a temperature sensor (5) is provided below the sample tank (4) for real-time measurement Measure the temperature of the sample (1); the control unit (10) is respectively connected with the detector (9), the temperature sensor (5) and the weighing unit (3) through the signal line, and the excitation of the microwave generating device (2) The voltage is controlled, and the control unit (10) calculates the moisture content information of the tested sample (1) through an internal function according to the read detection voltage, temperature and weight information, and outputs the measurement result through the display output unit (11). 2. a kind of material moisture content detection device based on microwave cavity resonance as claimed in claim 1, is characterized in that, described tested sample (1) is the non-metallic water-containing material with uniform density distribution such as grain, soil, fertilizer . 3. A kind of material water content detection device based on microwave cavity resonance as claimed in claim 1, is characterized in that, described microwave generation device (2) is voltage-controlled frequency sweep signal source, and voltage control range is 0-5V , the frequency output is in the range of 3GHz-12GHz. 4. A kind of material moisture content detection device based on microwave cavity resonance as claimed in claim 1, is characterized in that, described sample tank (4) is the cylindrical structure with cover, is metal material, and height range is 30- 70mm, located on the top of the resonant cavity, and the bottom is a non-metallic partition, which is made of ceramics, plastics or glass with low microwave attenuation, connected with the resonant cavity (6) to form a part of the resonant cavity. 5. A material moisture content detection device based on microwave cavity resonance according to claim 1, characterized in that the temperature measurement effective range of the temperature sensor (5) is not lower than 0-100°C, and the accuracy is not low at ±0.5°C. 6. a kind of material water content detection device based on microwave cavity resonance as claimed in claim 1, is characterized in that, described microwave resonant cavity (6) is a cylindrical structure, is a metal material, and the inner radius range of the cavity is 10-40mm, the cavity length range is 20-80mm. 7. A kind of material water content detection device based on microwave cavity resonance as claimed in claim 1, is characterized in that, described excitation probe (7), detection probe (8) are all metal materials, all vertically inserted The cavity wall of the microwave resonant cavity (6), the two probes in the cavity of the microwave resonant cavity (6) form a 90° angle, and the length range of vertical cavity coupling is 5-20mm. 8. A kind of material water content detection device based on microwave cavity resonance as claimed in claim 1, is characterized in that, described control unit (10) comprises power supply, A/D converter, D/A converter and 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 the internal calculation of the single-chip microcomputer, and the effective number of bits of the A/D converter is not less than 12; D/A The converter is used to convert the single-chip microcomputer control signal into a voltage value, and controls the microwave generating device (2) to generate frequency-sweeping microwave signals, and the effective number of bits of the D/A converter is also not less than 12 bits. 9. The detection method of a material moisture content detection device based on microwave cavity resonance as claimed in claim 1, characterized in that, it specifically comprises the following steps: S1. No-load signal detection; Keep the sample tank (4) as no-load, the control unit (10) controls the frequency sweep voltage change of the microwave generator (2), synchronously detects and records the value of the detection voltage of the detector (9), and records the value corresponding to the extreme point of the detection voltage The numerical value of the frequency sweep voltage is correspondingly converted into the resonant frequency f ₀ ; the output weight m ₀ of the weighing unit (3) is measured synchronously without load; S2, full load signal detection; The sample fills the sample slot (4), the control unit (10) controls the frequency sweep voltage change of the microwave generator (2), simultaneously detects and records the value of the detection voltage of the detector (9), and records the sweep corresponding to the extreme point of the detection voltage. The value of the frequency voltage is correspondingly converted into the resonant frequency f ₁ ; the weighing unit (3) outputs the weight m ₁ and the temperature sensor (5) outputs the sample temperature value T when it is fully loaded; S3, material moisture content calculation; The single-chip microcomputer inside the control unit (10) adopts the following formula to calculate the moisture content: W=a(f1-f0)/(m1-m0)+bT+c Among them, W represents the water content of the material to be 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 obtained by The coefficient obtained by fitting the moisture content data measurement, T is the temperature value of the measured sample; S4, real-time display output of moisture content; After the single-chip microcomputer in the control unit (10) completes the calculation of the moisture content, the display output unit (11) outputs and displays the moisture content information of the sample (1) to be tested.