Microwave resonant cavity sensing system and detection method for food metal impurities detection

Abstract

The invention discloses a microwave resonant cavity sensing system and a detection method for detecting metal impurities in food, comprising a microwave resonant cavity, a conveyor belt, a microwave network analyzer and a computer. The microwave resonant cavity is mainly formed by closing a waveguide through two end plates. The microwave resonant cavity adopts a radio frequency N-type adapter for signal input and output, and resonance is generated inside the cavity by extending an intermediate probe of the radio frequency N-type adapter. A pair of openings are provided in the middle of a narrow side at half the height of the microwave resonant cavity, and conduits are installed at both openings. The conveyor belt crosses the conduit. The operating frequency of the microwave resonant cavity needs to be lower than the cutoff frequency of the conduit. The non-invasive high-throughput online real-time non-destructive detection of metals in solid, liquid or powdered food can be performed. The volume of metal impurities can be quantitatively analyzed according to resonance change information. The operation is simple and convenient, and the microwave resonant cavity has low production cost and low detection power, and the detection system is easy to build.

Description

Technical Field

[0001] The present invention relates to the technical field of food detection, and in particular to a microwave resonant cavity sensing system and a detection method for detecting metal impurities in food. Background Art

[0002] Foreign matter may be introduced into food during harvesting, processing and final packaging; among all contaminants, metal objects (such as screws, bolts, nuts, small fragments of production line equipment or cleaning tools) pose a serious threat to consumer health if they are not detected and enter the final product. At the same time, food brands may be seriously affected by such incidents, resulting in great economic losses. Metals may also damage machinery and cause unnecessary line downtime; therefore, detecting metal objects in food is important from both health and economic perspectives.

[0003] At present, there are many non-invasive detection technologies for metal detection in food, but each technology has its own advantages, disadvantages and limitations, and only one technology cannot meet all the needs of the industry; for example, the instruments used in X-ray and nuclear magnetic resonance technology are relatively expensive, and require a large detection space, professional detection personnel and high power usage; the traditional ultrasonic method requires the food to be immersed in water, which is not convenient for actual industrial use. For dry food (common dry goods such as rice, peanuts, flour, fungus, mushrooms, tangerine peel, raisins, etc.), this setting is even more undesirable; for air-coupled ultrasound, the point scanning form is not conducive to rapid detection; terahertz detection has the disadvantages of high equipment price, cumbersome instrument setting and installation calibration, and low signal-to-noise ratio; optical technologies such as near-infrared spectroscopy and thermal imaging cannot well detect subsurface foreign matter; low-frequency electromagnetic detection systems involve complex signal processing, and the signal is easily affected by metal objects around the sensor. Therefore, a microwave resonant cavity sensing system and detection method for food metal impurity detection are urgently needed to solve the above problems. Summary of the invention

[0004] The present invention provides a microwave resonant cavity sensing system and a detection method for detecting metal impurities in food, which has a simple structure, is easy to operate, and can perform non-invasive high-throughput real-time non-destructive detection of metals in food, to solve the problems existing in the above-mentioned prior art.

[0005] To achieve the above-mentioned purpose, the present invention provides the following technical solution: a microwave resonant cavity for detecting metal impurities in food, which is mainly composed of a waveguide closed by two end plates, and the microwave resonant cavity adopts a radio frequency N-type adapter for signal input and output, and resonance is generated inside the cavity by extending the middle probe of the radio frequency N-type adapter, wherein a pair of openings are provided in the middle of the narrow side at half the height of the microwave resonant cavity, and conduits are installed at both of the openings, and the operating frequency of the microwave resonant cavity needs to be lower than the cutoff frequency of the conduit.

[0006] Preferably, the microwave resonant cavity adopts TE 102 The resonance mode of the microwave cavity TE 102 The resonant frequency of the mode is f r The calculation formula is: Where a and h are the wide side dimension and longitudinal height of the microwave resonant cavity cross section, respectively.

[0007] Preferably, the opening and the conduit are both rectangular, wherein the operating frequency of the microwave resonant cavity is less than the cut-off frequency of the rectangular conduit, that is, Among them, g and f are the wide side size of the rectangular duct cross section and the operating frequency respectively.

[0008] Preferably, the length of the rectangular conduit is determined based on a waveguide attenuation factor and a preset value of attenuation degree, wherein the waveguide attenuation factor α is calculated as:

[0009] Preferably, the opening is circular and the conduits are cylindrical, wherein the operating frequency of the microwave resonant cavity is less than the cutoff frequency of the rectangular conduit, that is, Where d and f are the inner diameter and operating frequency of the circular conduit, respectively.

[0010] Preferably, the length of the cylindrical conduit is determined based on the waveguide attenuation factor and the preset value of the attenuation degree, wherein the calculation formula of the waveguide attenuation factor α of the cylindrical waveguide is:

[0011] A microwave resonant cavity sensing system for detecting metal impurities in food comprises a microwave resonant cavity, a conveyor belt, a microwave network analyzer and a computer; the conveyor belt crosses the conduit and is used to transmit food into the microwave resonant cavity; the microwave network analyzer is connected to the microwave resonant cavity via a flexible coaxial radio frequency cable to collect resonant frequency change data in the microwave resonant cavity; the computer is connected to the microwave network analyzer via a network cable to receive and process the data collected by the microwave network analyzer.

[0012] Preferably, the shapes of the opening and the conduit of the microwave resonant cavity match the conveyor belt, and the heights of the opening and the conduit are much greater than the height of the food on the conveyor belt.

[0013] A detection method of a microwave resonant cavity sensing system for detecting metal impurities in food, comprising the following steps:

[0014] S1, start the conveyor belt to transport the food into the microwave resonant cavity;

[0015] S2. Read the transmission coefficient S from the microwave network analyzer through the programmable instrument standard command SCPI language code 21 Data, through programming to fit the resonance signal data, to obtain accurate resonance frequency value and quality factor;

[0016] S3. According to the distribution of the resonant electromagnetic field and the position of the food conveyor belt, the resonant frequency change caused by metal impurities is:

[0017]

[0018] Among them, f r1 and f r2 is the resonant frequency before and after the metal impurities disturb the resonant detection system; ε and μ are the dielectric constant and magnetic permeability distribution of the material filled in the resonant cavity respectively; and is the electric and magnetic field distribution in the resonant cavity before the disturbance, V c and V m are the volumes of the resonant cavity and metal impurities respectively;

[0019] S4. The volume of metal in the food is obtained by calculating and analyzing the change in resonant frequency.

[0020] Preferably, S 21 The value of and the frequency f satisfy the Lorentz equation:

[0021]

[0022] Among them, the transmission energy T=|S 21 | 2 , QL is the loaded quality factor.

[0023] Compared with the prior art, the present invention has the following beneficial effects: the microwave resonant cavity in the present invention can perform non-invasive high-throughput online real-time non-destructive detection of metals in solid, liquid or powdered foods, and can quantitatively analyze the volume of metal impurities based on the resonance change information. The operation is simple and convenient, and the microwave resonant cavity has low production cost, low detection power, and the detection system is easy to build. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0025] In the attached picture:

[0026] Figure 1 It is a schematic diagram of the structure of the microwave resonant cavity of the present invention;

[0027] Figure 2 is a cross-sectional view of the microwave resonant cavity of the present invention;

[0028] Figure 3 It is a structural schematic diagram of a radio frequency N-type switch in a microwave resonant cavity of the present invention;

[0029] Figure 4 It is a schematic structural diagram of a cylindrical conduit of a microwave resonant cavity of the present invention;

[0030] Figure 5 It is a schematic structural diagram of the microwave resonant cavity sensing system of the present invention;

[0031] Figure 6 It is a schematic diagram of the change of the resonance signal caused by metal impurities in the food of the present invention;

[0032] Figure 7 It is a simulation curve diagram of the resonance response signal of the microwave sensing system of the present invention in the absence of metal;

[0033] Figure 8 It is a comparison diagram of the original data and the fitting results obtained by electromagnetic simulation in the absence of metal in the present invention;

[0034] Fig. 9 is a graph showing the relationship between the resonant frequency shift of the microwave sensor and the radius of the metal ball inside the sensor;

[0035] Numbers in the figure: 1. waveguide; 2. end plate; 3. RF N-type adapter; 4. intermediate probe; 5. rectangular opening; 6. rectangular conduit; 7. circular opening; 8. cylindrical conduit; 9. conveyor belt; 10. microwave network analyzer; 11. computer; 12. coaxial RF cable; 13. network cable. DETAILED DESCRIPTION

[0036] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0037] Example: Figure 1-Figure 2 As shown, a microwave resonant cavity for detecting metal impurities in food is mainly formed by closing a waveguide with two end plates, wherein the end plates and the waveguide can be connected by bolts, and the microwave resonant cavity uses a radio frequency N-type adapter for signal input and output;

[0038] refer to Figure 1 and Figure 3 As shown, the RF N-type adapter is installed at the position where the electric field strength is the largest (i.e., the intersection of the half-width side and the height 1 / 4), and resonance is generated inside the cavity by extending the middle probe of the RF N-type adapter;

[0039] In this embodiment, the microwave resonant cavity adopts TE 102 The resonant mode of the microwave cavity TE is used to avoid the influence of the parasitic mode on the detection signal. 102 The resonant frequency of the mode is f r The calculation formula is: Among them, a and h are the width and longitudinal height of the microwave resonant cavity cross section, respectively. To facilitate the production and application of detection equipment, the resonant frequency can be set within the ISM frequency band for industry, science and medicine specified by the International Telecommunication Union, such as 434MHz, 915MHz or 2.45GHz;

[0040] A pair of openings are provided in the middle of the narrow side at half the height of the microwave resonant cavity (i.e., the position where the surface current intensity is weak). Both openings are equipped with conduits to prevent signal leakage and also to support the conveyor belt. The flange of the conduit is connected to the resonant cavity by bolts. The operating frequency of the microwave resonant cavity must be lower than the cutoff frequency of the conduit.

[0041] Furthermore, the rectangular resonant cavity and the conduit of the sensor can be made of aluminum alloy or copper and processed by CNC machine tools; if a higher resonant cavity quality factor is required, silver or gold with higher conductivity can be used for inner surface plating.

[0042] In a specific embodiment, reference 1- Figure 2 As shown, the opening and the conduit are both rectangular, wherein the operating frequency of the microwave resonant cavity is less than the cutoff frequency of the rectangular conduit, that is, Wherein, g and f are the width dimension of the rectangular duct cross section (in m) and the operating frequency (in MHz), respectively;

[0043] The length of the rectangular duct is determined based on the waveguide attenuation factor and the preset value of the attenuation degree, wherein the waveguide attenuation factor α (in dB / m) is calculated as follows:

[0044] The attenuation degree of the detection signal in the catheter can be set to 40 dB.

[0045] In a specific embodiment, reference Figure 4 As shown, the opening is circular and the conduits are cylindrical, wherein the operating frequency of the microwave resonant cavity is less than the cutoff frequency of the rectangular conduit, that is, Where d and f are the inner diameter and operating frequency of the circular conduit, respectively;

[0046] The length of the cylindrical conduit is determined based on the waveguide attenuation factor and the preset value of the attenuation degree, wherein the calculation formula of the waveguide attenuation factor α of the cylindrical waveguide is:

[0047] refer to Figure 5As shown, a microwave resonant cavity sensing system for detecting metal impurities in food includes a microwave resonant cavity, a conveyor belt, a microwave network analyzer and a computer; the conveyor belt crosses the conduit, and the conveyor belt is used to transmit food to the microwave resonant cavity. The microwave network analyzer is connected to the microwave resonant cavity through a flexible coaxial radio frequency cable to collect resonant frequency change data in the microwave resonant cavity. The computer is connected to the microwave network analyzer through a network cable to receive and process the data collected by the microwave network analyzer.

[0048] The opening and conduit shape of the microwave resonant cavity match the conveyor belt, and the height of the opening and conduit are much greater than the height of the food on the conveyor belt. For example, the ratio of the two can be set to be greater than 10.

[0049] A detection method of a microwave resonant cavity sensing system for detecting metal impurities in food, comprising the following steps:

[0050] S1, start the conveyor belt to transport the food into the microwave resonant cavity;

[0051] S2. Set the signal power within 1mW and read the transmission coefficient S from the microwave network analyzer through the programmable instrument standard command SCPI language code. 21 Data, through programming to fit the resonance signal data, to obtain accurate resonance frequency value and quality factor, where S 21 The value of and the frequency f satisfy the Lorentz equation:

[0052]

[0053] Among them, the transmission energy T=|S 21 | 2 , QL is the loaded quality factor;

[0054] S3. When the food contains metal impurities, the volume of the resonant cavity decreases. According to the characteristics of the perturbation, it can be considered that the change in the electromagnetic field distribution before and after the disturbance is small. According to the resonant electromagnetic field distribution and the position of the food conveyor belt, the change in the resonant frequency caused by the metal impurities is:

[0055]

[0056] Among them, f r1 and f r2 is the resonant frequency before and after the metal impurities disturb the resonant detection system; ε and μ are the dielectric constant and magnetic permeability distribution of the material filled in the resonant cavity respectively; and is the electric and magnetic field distribution in the resonant cavity before the disturbance, V c and V m are the volumes of the resonant cavity and metal impurities respectively; Figure 6 The figure shows the change of resonance signal caused by metal impurities in food.

[0057] S4. According to TE 102 From the electromagnetic field distribution in the mode, it can be seen that the object at the center of the resonant cavity will cause the highest resonant frequency shift; therefore, when measuring, the maximum value of the resonant frequency shift is taken for analysis, and the TE 102 The electromagnetic field distribution in the mode is substituted into the above formula to calculate the volume of the metal to be measured. In addition, the metal object of standard volume can be measured, that is, the change in resonant frequency caused by unit metal volume can be obtained through calibration method.

[0058] In a specific embodiment, a microwave resonant cavity model for metal detection in food was established in CST electromagnetic simulation software. The inner dimensions of the resonant cavity were 363mm×327mm×2267mm and the wall thickness was 5mm. A rectangular conduit was used for electromagnetic shielding on the side of the microwave resonant cavity model. The inner dimensions were 261mm×235mm and the wall thickness was 5mm. The transmission coefficient S corresponding to the metal-free case was obtained by simulation. 21 The resonance curve is as follows Figure 7 As shown; the resonant frequency design value is 434MHz within the industrial, scientific and medical frequency band specified by the International Telecommunication Union; by fitting the CST simulation results, as shown Figure 8 As shown, the resonant frequency f r The value is 432.42MHz. The two are relatively close; and a higher quality factor value (32307) is calculated; therefore, the correctness of the design results and simulation model is verified.

[0059] At the same time, a metal sphere model was established at the center of the microwave resonant cavity to simulate impurities, and the effect of different sphere radii r on f was analyzed. r The impact of Fig. 9 As shown, the resonant frequency shift Δf r It increases with the increase of the sphere radius, which is consistent with the conclusion of theoretical analysis. r and r 3 (corresponding to the volume of the metal sphere) has a good linear relationship (R 2 The value is close to 1), which is also the same as the above-mentioned resonant frequency variation trend (i.e., for a given resonant cavity structure, the reference resonant frequency is fixed); the fitting results show that Δf generated by r = 0 r Although the value is not theoretically 0, it is small compared with the offset given by other radii and can be ignored; thus, it can be seen that the accuracy is confirmed by the simulation results of metal detection.

[0060] Finally, it should be noted that the above description is only a preferred example of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A microwave resonant cavity for detecting metal impurities in food, which is mainly formed by closing the waveguide with two end plates, and is characterized by: The microwave resonant cavity uses a radio frequency N-type adapter for signal input and output, and generates resonance inside the cavity by extending the middle probe of the radio frequency N-type adapter, wherein a pair of openings are provided in the middle of the narrow side at half the height of the microwave resonant cavity, and conduits are installed at both of the openings, and the operating frequency of the microwave resonant cavity needs to be lower than the cutoff frequency of the conduit; The opening and the conduit are both rectangular, or the opening is circular and the conduit is cylindrical; When the opening and the conduit are both rectangular, the operating frequency of the microwave resonant cavity is less than the cutoff frequency of the rectangular conduit, that is, Wherein, g and f are the width dimension of the rectangular conduit cross section and the operating frequency, respectively; the length of the rectangular conduit is determined based on the waveguide attenuation factor and the preset value of the attenuation degree, wherein the waveguide attenuation factor α is calculated as follows: When the opening is circular and the conduits are cylindrical, the operating frequency of the microwave resonant cavity is less than the cutoff frequency of the rectangular conduit, that is, Wherein, d and f are the inner diameter and operating frequency of the circular conduit, respectively; the length of the cylindrical conduit is determined based on the waveguide attenuation factor and the preset value of the attenuation degree, wherein the calculation formula of the waveguide attenuation factor α of the cylindrical waveguide is:

2. The microwave resonant cavity for detecting metal impurities in food according to claim 1, characterized in that: The microwave resonant cavity adopts TE 102 The resonance mode of the microwave cavity TE 102 The resonant frequency of the mode is f r The calculation formula is: Where a and h are the wide side dimension and longitudinal height of the microwave resonant cavity cross section, respectively.

3. A microwave resonant cavity sensing system for detecting metal impurities in food, comprising the microwave resonant cavity according to any one of claims 1 to 2, as well as a conveyor belt, a microwave network analyzer and a computer; characterized in that: The conveyor belt crosses the conduit and is used to transport food into the microwave resonant cavity. The microwave network analyzer is connected to the microwave resonant cavity through a flexible coaxial radio frequency cable to collect data on changes in the resonant frequency in the microwave resonant cavity. The computer is connected to the microwave network analyzer through a network cable to receive and process the data collected by the microwave network analyzer.

4. The microwave resonant cavity sensing system for detecting metal impurities in food according to claim 3, characterized in that: The shapes of the opening and the conduit of the microwave resonant cavity match the conveyor belt, and the heights of the opening and the conduit are much greater than the height of the food on the conveyor belt.

5. A detection method based on the microwave resonant cavity sensing system for detecting metal impurities in food according to claim 4, characterized in that: The steps include: S1, start the conveyor belt to transport the food into the microwave resonant cavity; S2. Read the transmission coefficient S from the microwave network analyzer through the programmable instrument standard command SCPI language code 21 Data, through programming to fit the resonance signal data, to obtain accurate resonance frequency value and quality factor; S3. According to the distribution of the resonant electromagnetic field and the position of the food conveyor belt, the resonant frequency change caused by metal impurities is: Among them, f r1 and f r2 is the resonant frequency before and after the metal impurities disturb the resonant detection system; ε and μ are the dielectric constant and magnetic permeability distribution of the material filled in the resonant cavity respectively; and is the electric and magnetic field distribution in the resonant cavity before the disturbance, V c and V m are the volumes of the resonant cavity and metal impurities respectively; S4. The volume of metal in the food is obtained by calculating and analyzing the change in resonant frequency.

6. The detection method of the microwave resonant cavity sensing system for detecting metal impurities in food according to claim 5, characterized in that: S 21 The value of and the frequency f satisfy the Lorentz equation: Among them, the transmission energy T=|S 21 | 2 , QL is the loaded quality factor.

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