A non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency.

Abstract

This invention discloses a non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency (RFID). The method includes: designing and fabricating a flexible passive RFID sensor integrating a helical antenna and interdigitated electrodes to achieve a structured separation of electromagnetic coupling and sensing functions; and improving the response performance of ammonia detection by loading MXene / In₂O₃ composite material. The method performs non-line-of-sight passive detection of packaged agricultural products under non-line-of-sight conditions, studies the changes in reflection and absorption losses of electromagnetic waves caused by the medium and humidity, clarifies the frequency disturbance response of the detection signal to medium interference, achieves separation of signal amplitude-frequency changes, and establishes a relationship between medium interference and its induced S... 11 The correlation mechanism between amplitude changes enables self-compensation of interference signals.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, and in particular to a non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency. Background Technology

[0002] Food safety issues arising from the deterioration of agricultural product quality are becoming increasingly serious. Real-time monitoring and safety early warning during the storage and transportation of agricultural products have become urgent problems to be solved. Ammonia, as a marker gas in the deterioration process of fresh agricultural products, has a concentration change that is usually closely related to the quality status of the products. Currently, various detection methods have been applied to monitor and analyze ammonia during the spoilage process of agricultural products, including chromatographic analysis, spectral analysis, and traditional chemical gas sensing technology. However, these methods are mostly applicable to detection under line-of-sight conditions and usually rely on destructive treatment of packaging structures and the agricultural products themselves, thus making it difficult to achieve non-destructive detection of agricultural products under non-line-of-sight conditions during storage, transportation, or packaging.

[0003] Utilizing intelligent gas sensors to detect the quality of agricultural products can identify spoilage or deterioration signals at an early stage, thus possessing significant research value and practical implications. In recent years, flexible chipless RFID sensors have provided a new technical approach for non-contact, non-destructive testing of agricultural product quality. These sensors, characterized by being chipless and passive, can achieve non-contact detection of internal environmental parameters of agricultural products without damaging the packaging through electromagnetic coupling between the detection antenna and the sensing tag. Furthermore, they utilize a transmission antenna to wirelessly transmit radio frequency signals, thereby achieving passive sensing of the internal state of the packaging. However, existing work primarily focuses on ideal line-of-sight conditions, with insufficient attention paid to the widespread non-line-of-sight environments in actual agricultural product testing scenarios. In particular, research on the transmission loss, reflection interference, and coupling mechanisms between complex media such as packaging materials and gas detection signals introduced by these factors remains lacking. Therefore, providing a non-line-of-sight detection method for fresh agricultural product quality based on flexible passive radio frequency technology is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] This invention proposes a non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency. By using flexible passive radio frequency sensing and non-line-of-sight electromagnetic coupling, the method performs passive detection of fresh agricultural products inside the packaging. It also models and analyzes the reflection loss and absorption loss caused by the packaging medium and humidity, and uses amplitude-frequency separation and interference self-compensation methods to eliminate the influence of medium interference on the detection signal. This achieves stable sensing of the deterioration characteristics of fresh agricultural products and has the advantages of being non-contact, non-line-of-sight, and having high detection stability.

[0005] The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency according to an embodiment of the present invention includes the following steps: S1. Deploy a flexible passive radio frequency sensor inside the packaging of fresh agricultural products. The flexible passive radio frequency sensor integrates a signal transmission unit, an impedance matching unit, and a sensing unit. S2. An RF excitation and receiving device is set up outside the packaging to provide RF excitation to the flexible passive RF sensor through non-line-of-sight electromagnetic coupling and to collect the reflected RF signal of the flexible passive RF sensor. S3. The sensing unit is used to sense the change in ammonia concentration in the storage and transportation environment of fresh agricultural products inside the packaging. The ammonia causes the equivalent electrical parameters of the sensing unit to change, and triggers the change in the resonant characteristics of the flexible passive radio frequency sensor. S4. By adjusting the impedance relationship between the signal transmission unit and the sensing unit through the impedance matching unit, the change in equivalent electrical parameters caused by sensing is converted into the resonant frequency shift and amplitude change in the reflected radio frequency signal. S5. Perform passive wireless detection in non-line-of-sight scenarios, analyze the radio frequency signal reflection loss and absorption loss caused by packaging medium and ambient humidity, and distinguish the amplitude change caused by medium interference from the resonant frequency shift caused by ammonia sensing. S6. Based on the results of the dielectric interference analysis, establish the correspondence between dielectric interference and the amplitude change of the reflected radio frequency signal, perform dielectric interference self-compensation processing on the detection signal, and output the detection results characterizing the deterioration state of fresh agricultural products.

[0006] Optionally, S1 includes the following steps: Conductive patterns for a flexible passive radio frequency sensor are formed on a flexible thin polyimide substrate using a screen printing process. The conductive patterns correspond to the electrical connection paths between the signal transmission unit and the sensing unit. A near-field probe, an electromagnetic coupling area, and a spiral antenna are set in the signal transmission unit. The near-field probe is connected to a vector network analyzer and is used to output a sweep frequency excitation signal and generate an alternating magnetic field in the near-field region. During the frequency sweep process, an alternating magnetic field is applied to the helical antenna. The helical antenna obtains energy based on electromagnetic induction and generates a resonant response, which is then transmitted to the sensing unit connected to the helical antenna along the electrical connection path. Set the coupling distance between the near-field probe and the signal transmission unit, and perform multiple frequency sweep acquisitions within a preset distance set to obtain the reflection parameter curves corresponding to each distance; Extracting the resonant frequency based on the reflection parameter curve With minimum reflection amplitude and with Indicators characterizing coupling strength ,satisfy ; For distance set Compare and determine the working distance that meets the threshold constraint. The working distance is fixed in subsequent non-line-of-sight detection. Complete the excitation and signal acquisition of the flexible passive radio frequency sensor.

[0007] Optionally, S2 includes the following steps: An interdigitated electrode structure is formed in the sensing unit region of the flexible passive radio frequency sensor. The interdigitated electrodes are electrically connected to the helical antenna of the signal transmission unit, so that the interdigitated electrodes participate in the reflection parameter response as a capacitor component of the resonant circuit. A gas-sensitive material layer is loaded on the surface of the interdigital electrode. The gas-sensitive material layer is selected from MXene / In2O3 composite material, and the gas-sensitive material layer is limited to cover the effective interdigital area of ​​the interdigital electrode. Reflection parameter curves were acquired under non-line-of-sight frequency sweep excitation, and the resonant frequency was determined from the reflection parameter curves. and the corresponding reflection amplitude ; Under the influence of changes in ammonia concentration, the gas-sensitive material layer causes a change in the equivalent capacitance of the interdigitated electrodes, thus obtaining the equivalent capacitance. With ammonia concentration Related capacitance change and satisfy ,in This is the initial equivalent capacitance; Based on the equivalent inductance provided by the helical antenna With equivalent capacitance Calculate the resonant frequency, satisfying and the calculated Corresponding to the acquired resonant frequency; The change in resonant frequency As a characteristic of ammonia response, among which This is the reference resonant frequency.

[0008] Optionally, step S3 includes the above steps: In the design phase of the flexible passive radio frequency sensor, the set of structural parameters for the signal transmission unit and the sensing unit is determined. The structural parameters include the line width, line spacing, and number of turns of the helical antenna, as well as the finger width, finger spacing, and finger length of the interdigitated electrodes. The equivalent impedance model of the sensor is constructed based on the set of structural parameters. The equivalent inductance parameters of the signal transmission unit are combined with the equivalent resistance and capacitance parameters of the sensing unit to calculate the sensor input impedance. The matching state between the input impedance and the external excitation port impedance is used as the impedance matching criterion. During the design phase, the set of structural parameters is iteratively adjusted, and the corresponding reference resonant frequency and reflection parameters are recorded as the initial reference state. In actual operation, when the flexible passive radio frequency sensor is exposed to the storage and transportation environment of fresh agricultural products, when ammonia in the environment is adsorbed by the sensing unit, the local equivalent resistance and equivalent capacitance of the sensing unit change. Under the influence of changes in equivalent resistance and equivalent capacitance, the overall input impedance of the sensor deviates from the matching state in the design stage, resulting in impedance mismatch, and simultaneously causing changes in resonant frequency and reflection parameters. The reflection parameters and resonant frequency offset under impedance mismatch conditions are collected, and the resonant frequency offset is used as the output.

[0009] Optionally, step S4 includes the above steps: A radio frequency detection antenna is set on the outside of fresh agricultural products, and a flexible passive radio frequency sensor is set inside the packaging of fresh agricultural products. There is a non-line-of-sight spatial relationship between the radio frequency detection antenna and the flexible passive radio frequency sensor, which is isolated by the packaging medium. A radio frequency excitation signal within a preset frequency range is transmitted to the flexible passive radio frequency sensor through a radio frequency detection antenna, so that the radio frequency excitation signal penetrates the packaging medium during propagation and electromagnetically couples with the flexible passive radio frequency sensor. Under the action of electromagnetic coupling, the flexible passive radio frequency sensor can obtain energy from the radio frequency excitation signal and form a reflected radio frequency signal corresponding to its electrical parameter state; The radio frequency (RF) signal is received by the RF detection antenna and reflected by the flexible passive RF sensor, and the received RF signal is output. Under non-line-of-sight detection conditions, the coupling state between the radio frequency detection antenna and the flexible passive radio frequency sensor is controlled; Radio frequency features that characterize changes in internal environmental parameters of the packaging are extracted from the received reflected radio frequency signals, and these radio frequency features are used as passive detection inputs for the quality status of fresh agricultural products.

[0010] Optionally, step S5 includes the above steps: The reflected radio frequency signal of the flexible passive radio frequency sensor was acquired under non-line-of-sight detection conditions. The corresponding amplitude variation characteristics of the reflection parameters and the resonant frequency variation characteristics were obtained and used as the original detection signal. Based on the type of packaging material and the ambient humidity, we conduct a medium interference analysis on the electromagnetic propagation effect introduced by the packaging medium in the original detection signal, and identify the changes in reflection loss and absorption loss of electromagnetic waves under the action of the medium. In the process of dielectric interference analysis, the amplitude variation component and the resonant frequency variation component in the reflected radio frequency signal are distinguished to obtain the dielectric interference response characteristics dominated by amplitude variation and the sensing response characteristics dominated by frequency variation. Based on the characteristics of media interference response, a model of the correspondence between the degree of media interference and the amplitude change of reflection parameters is established, and this correspondence is used as the basis for characterizing the amount of media interference. The amount of medium interference is inferred from the change in the amplitude of the reflection parameters, and the resonant frequency disturbance component caused by the medium interference is calculated based on the amount of medium interference. The resonant frequency variation characteristics in the original detection signal are compensated to eliminate the frequency disturbance caused by medium interference, and the detection results of fresh agricultural product quality deterioration after medium interference self-compensation are output.

[0011] Optionally, the non-line-of-sight passive detection is performed while the fresh agricultural product packaging is intact. By setting a detection antenna on the outside of the packaging and transmitting a swept-frequency radio frequency signal to a flexible passive radio frequency sensor inside the packaging, the radio frequency signal penetrates the packaging medium and electromagnetically couples with the flexible passive radio frequency sensor. The flexible passive radio frequency sensor acquires electromagnetic energy and forms a reflected radio frequency signal corresponding to its electrical parameter state without the need for an external power supply. The detection antenna receives the reflected radio frequency signal and outputs it to the signal processing unit, thereby achieving passive sensing of the internal environmental parameters and quality status of the packaging without unpacking or damaging the agricultural product packaging structure.

[0012] Optionally, the media interference analysis includes the following steps: The reflection parameters of the flexible passive radio frequency sensor were collected under non-line-of-sight detection conditions, and the corresponding reflection amplitude variation characteristics were obtained within a preset frequency sweep range to characterize the radio frequency propagation loss introduced by the packaging medium. Based on transmission line theory, an input impedance model for electromagnetic waves in the packaging medium of fresh agricultural products is established to address the propagation process of electromagnetic waves. The input impedance of electromagnetic waves in the product packaging medium is defined as follows: ; in, The relative permittivity of the packaging medium. The relative permeability of the packaging medium. For the thickness of the medium, For radio frequency signal frequency, The speed at which electromagnetic waves propagate in free space; Based on free space input impedance Input impedance of packaging medium The difference between them is used to calculate the reflection loss of electromagnetic waves at the interface of the medium. The reflection loss is defined as: ; in, The input impedance of electromagnetic waves in free space. The input impedance of electromagnetic waves in the product packaging medium; and There are inherent differences between them, and the reflection loss value will change accordingly; Based on electromagnetic wave propagation theory, an absorption loss model is established for electromagnetic waves propagating in packaging media. The propagation constant of electromagnetic waves in the media is defined as follows: ,in, It is the attenuation constant. It is the phase constant and the attenuation constant. satisfy: ; in, , , The magnetic permeability in air; Based on attenuation constant The propagation distance of electromagnetic waves in a medium Calculate the absorption loss of electromagnetic waves in the packaging medium. The absorption loss is defined as: ; in, For conversion factors, This refers to the propagation distance of electromagnetic waves in the packaging medium. Reflection loss and absorption loss are used as outputs in the dielectric interference analysis.

[0013] Optionally, step S6 includes the above steps: Under non-line-of-sight detection conditions, the reflected radio frequency signals corresponding to different packaging media are acquired, and the amplitude characteristics and resonant frequency characteristics of the reflection parameters corresponding to each packaging medium are extracted under the same frequency sweep conditions. To address the difference in electromagnetic parameters between the packaging medium and air, the correspondence between the incident signal and the reflected signal was recorded when electromagnetic waves were incident on the interface of the packaging medium, and the change in reflection loss caused by interface reflection was used as a characterization parameter of the medium's influence. After electromagnetic waves are transmitted into the packaging medium, the amplitude attenuation characteristics of the transmitted radio frequency signal are collected based on the propagation process inside the medium, and the energy attenuation caused by absorption inside the medium is used as another characterization parameter of the medium's influence. The non-line-of-sight detection process was repeated under different packaging medium thickness conditions. The corresponding changes in reflection loss and absorption loss were recorded, and the relationship between medium thickness and transmission loss was used as the input for medium influence analysis. In the process of humidity effect analysis, the humidity level of the detection environment is adjusted, and reflected radio frequency signals are collected under different humidity conditions to record the changes in the amplitude of reflection parameters caused by humidity changes. The changes in ambient humidity are mapped to changes in the equivalent dielectric parameters and dielectric loss parameters of the packaging medium, and the characteristics of the influence of humidity are obtained based on the changes in reflection loss and absorption loss under humidity change conditions. By jointly analyzing the characteristics of the influence of the medium and the characteristics of the influence of humidity, the enhancement of reflection loss and the enhancement of absorption loss under the combined influence of the medium and humidity are obtained as the output of the medium interference analysis results.

[0014] Optionally, the medium interference self-compensation includes signal amplitude-frequency separation and interference signal self-compensation, wherein signal amplitude-frequency separation includes the following steps: The reflected radio frequency signal of the flexible passive radio frequency sensor is acquired under non-line-of-sight detection conditions, and the amplitude variation characteristics of the reflection parameters and the resonant frequency variation characteristics are extracted within a preset frequency sweep range as the original amplitude-frequency joint signal. Based on the results of the medium interference analysis, the amplitude variation component and the resonant frequency variation component in the reflection parameters are independently characterized to obtain the amplitude characteristic quantity used to describe the medium interference response and the frequency characteristic quantity used to describe the gas response. In the process of dielectric interference analysis, the corresponding relationship between the reflection loss and absorption loss caused by the dielectric and the impact on the amplitude of the radio frequency signal is established, and the amplitude changes caused by the reflection loss and absorption loss are collected as dielectric interference amplitude components. In the gas response analysis process, the resonant frequency shift caused by the change of the equivalent electrical parameters of the sensing unit is collected as the gas response frequency component, and this frequency component is defined as the main characterization feature of the ammonia response. Amplitude-frequency separation processing is performed on the original amplitude-frequency combined signal to decouple the amplitude component of the medium interference from the frequency component of the gas response, forming independent amplitude signal channels and frequency signal channels; Based on the amplitude-frequency separation results, the frequency disturbance response range of the detection signal to the medium interference is determined, and the separated frequency signal is used as the input for subsequent interference signal self-compensation processing.

[0015] Optionally, the interference signal self-compensation includes the following steps: After completing the signal amplitude-frequency separation, the amplitude change of the reflection parameter used to characterize the medium interference and the resonant frequency change used to characterize the gas response are obtained, and the amplitude change of the reflection parameter is used as the input for characterizing the medium interference. Based on the correspondence model between media interference and the amplitude change of reflection parameters established in the media interference analysis stage, the mapping relationship between the amplitude change of reflection parameters and the properties of packaging media is determined. After measuring the change in the amplitude of the reflection parameters, the corresponding medium property parameters are deduced based on the mapping relationship, and the resonant frequency shift caused by the medium interference is calculated by combining the transmission loss characteristics corresponding to the medium property. The original resonant frequency measured under non-line-of-sight detection conditions is compared with the calculated resonant frequency offset, and frequency compensation processing is performed to subtract the resonant frequency offset from the original resonant frequency to obtain the compensated resonant frequency. The compensated resonant frequency is used as the gas response characteristic after eliminating the influence of medium interference, which is used to characterize the quality deterioration state of fresh agricultural products inside the packaging. The interference signal self-compensation process is repeatedly executed under different packaging media and humidity conditions to keep the compensated resonant frequency decoupled from the changes in the medium and only reflect the frequency response caused by the change in ammonia concentration.

[0016] The beneficial effects of this invention are: This invention introduces a flexible passive radio frequency sensor and a non-line-of-sight electromagnetic coupling detection method to achieve passive detection of the internal environment of fresh agricultural products without damaging the packaging structure. This effectively overcomes the technical limitations of traditional detection methods that rely on line-of-sight conditions and destructive sampling, and improves the applicability and continuity of quality monitoring during the storage and transportation of fresh agricultural products.

[0017] This invention systematically models and analyzes the reflection loss and absorption loss caused by packaging media and environmental humidity, and treats media interference as an independent interference source. This enables a clear characterization of the media interference mechanism in non-line-of-sight detection scenarios, and enhances the adaptability of the detection process to different packaging materials and environmental conditions.

[0018] This invention constructs an amplitude-frequency separation and media interference self-compensation method to distinguish between the amplitude changes of radio frequency signals caused by media interference and the resonant frequency shift caused by gas sensing. This effectively eliminates the influence of media interference on the detection results, enabling the detection signal to stably reflect the deterioration characteristics of fresh agricultural products and improving the accuracy and reliability of the detection results. Attached Figure Description

[0019] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This invention relates to a non-line-of-sight detection method for quality deterioration of fresh agricultural products based on a flexible passive radio frequency sensor. Figure 2 This is a structural diagram of a flexible passive radio frequency sensor; Figure 3This shows the changes in reflection loss of different media under different humidity conditions. From left to right, the media are plastic, cardboard, foam, and glass. Figure 4 This shows the changes in absorption loss of different media under different humidity conditions. From left to right, the media are plastic, cardboard, foam, and glass. Figure 5 It is the equivalent circuit model of line-of-sight scene and the equivalent circuit model of non-line-of-sight medium interference; Figure 6 The correlation mechanism between non-line-of-sight media transmission loss and the resulting change in S11 amplitude is shown. The media from left to right are plastic, cardboard, foam, and glass. Figure 7 This is a flowchart of the self-compensation process for media interference. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, illustrating only the basic structure of the invention, and therefore only show the components relevant to the invention.

[0021] refer to Figures 1-7 A non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency includes the following steps: This embodiment includes a fresh agricultural product quality deterioration sensing terminal U1, a non-line-of-sight passive detection U2, and a media interference analysis and self-compensation U3.

[0022] In this embodiment, the non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency is as follows: Figure 1 As shown.

[0023] In this embodiment, the fresh agricultural product quality deterioration sensing terminal U1 includes a flexible passive radio frequency sensor U11, which comprises a signal transmission unit U111, an impedance matching unit U112, and a sensing unit U113. The signal transmission unit U111 achieves efficient coupling of electromagnetic energy, the sensing unit U113 senses changes in ammonia concentration in the storage and transportation environment of fresh agricultural products, and the impedance matching unit U112 studies the influence of the structural dimensions of the signal transmission unit U111 and the sensing unit U113 on the sensor's impedance matching. This embodiment improves the problem of uneven electromagnetic coupling energy distribution in traditional sensors by structurally separating the electromagnetic coupling function and the environmental sensing function.

[0024] In this embodiment, the flexible passive radio frequency sensor U11 uses a 0.3mm thick flexible polyimide substrate. Based on screen printing technology, copper ink is uniformly coated onto the substrate surface along the structural pattern, and a layer of gold is attached to the copper surface to prevent oxidation.

[0025] In this embodiment, the flexible passive radio frequency sensor U11 has the following structure: Figure 2 As shown. The sensor's center resonant frequency is 13.56MHz, and it is loaded with MXene / In2O3 composite gas-sensitive material.

[0026] In this embodiment, the signal transmission unit U111 includes a near-field probe U1111, an electromagnetic coupler U1112, and a helical antenna U1113. The near-field probe U1111 emits a sweep frequency signal through a vector network analyzer, generating an alternating magnetic field in the near-field region, which excites the helical antenna on the sensor to resonate. In this embodiment, the helical antenna U1113 adopts a helical antenna structure, and obtains energy through electromagnetic induction near its resonant frequency and transmits it to the sensing unit connected thereto. Electromagnetic coupling U1112 studies the electromagnetic coupling between the near-field probe U1111 and the helical antenna U1113. During the sensing process, the signal is emitted through the near-field probe, coupled by the helical antenna, and transmitted to the sensing unit. The electromagnetic coupling strength directly determines the sensing performance of the sensor, and the electromagnetic coupling strength is related to the distance between the near-field probe and the signal transmission unit.

[0027] As the distance between the two increases, the electromagnetic coupling strength decreases significantly due to the attenuation effect of near-field coupling energy and the reduction in signal transmission efficiency. Experimental verification in this embodiment shows that a distance of 20mm is chosen, achieving a balance between electromagnetic coupling strength and practicality.

[0028] In this embodiment, the sensing unit U113 includes interdigitated electrodes U1131, a sensing mechanism U1132, and a gas-sensitive material U1133. The interdigitated electrodes U1131 are interdigitated electrodes, providing a suitable carrier for the gas-sensitive material while ensuring good electromagnetic response; the gas-sensitive material U1133 is an MXene / In2O3 composite material, which has p... The n-heterojunction structure increases surface active sites and enhances electronic modulation effects, which is beneficial for ammonia molecule adsorption and improves the response performance of ammonia detection. The sensing mechanism U1132 study investigates the passive detection mechanism of environmental ammonia concentration by the sensor under the combined action of the helical antenna U1113, interdigitated electrodes U1131, and gas-sensitive material U1133. The helical antenna and interdigitated electrodes together form an LC resonant circuit, where the helical antenna provides the equivalent inductance L, and the interdigitated electrodes and their surface-loaded MXene / In2O3 composite material provide the equivalent capacitance CIDE. The equivalent capacitance and the circuit resonant frequency can be defined as follows: ; ; in, The resonant frequency, The initial capacitance of the interdigitated electrode loaded with the gas-sensitive material is denoted as . The capacitance increment after ammonia adsorption is the increase in capacitance CIDE after the MXene / In2O3 composite material adsorbs NH3. This increases the capacitance CIDE and equivalent resistance of the interdigitated electrode, disrupting the original impedance matching and altering the resonant characteristics of the LC circuit.

[0029] In this embodiment, impedance matching U112 studies the influence of the structural dimensions of the signal transmission unit U111 and the sensing unit U113 on the impedance matching of the sensor. During the design phase, impedance matching between the signal transmission unit and the sensing unit is achieved by optimizing relevant dimensional parameters, thereby improving the energy coupling efficiency and signal transmission performance of the sensor. In actual operation, the helical coil antenna receives the radio frequency energy emitted by the near-field probe and completes electromagnetic coupling. The interdigitated electrodes and sensitive materials, acting as sensing units, experience local resistance and capacitance changes due to ammonia adsorption, leading to impedance mismatch in the sensor. This, in turn, causes a detectable shift in the resonant frequency and reflection parameters, enabling passive sensing of the ambient ammonia concentration.

[0030] This study investigates passive wireless detection of agricultural products within packaging in non-line-of-sight scenarios. Traditional detection methods are mostly applicable to line-of-sight conditions and typically rely on destructive treatment of the packaging structure and the agricultural products themselves. In this example, through electromagnetic coupling between the detection antenna and the flexible passive radio frequency sensor U11, non-contact detection of environmental parameters inside the packaging is achieved without damaging the packaging. The radio frequency signal is then wirelessly transmitted using a transmission antenna, thus enabling passive sensing of the internal state of the packaging.

[0031] The Medium Interference Analysis and Self-Compensation U3 comprises Medium Interference Analysis U31 and Medium Interference Self-Compensation U32. Medium Interference Analysis U31 studies the changes in reflection and absorption losses of electromagnetic waves caused by the influence of media and humidity. Medium Interference Self-Compensation U32 separates the amplitude-frequency changes of the signal, clarifies the frequency disturbance response of the detection signal to medium interference, and establishes the correlation mechanism between medium interference and the amplitude change of S11 it causes, thus achieving self-compensation of the interference signal.

[0032] In this embodiment, the dielectric interference analysis U31 includes loss characteristics U311, dielectric influence U312, and humidity influence U313. Loss characteristics U311 mainly include reflection loss and absorption loss. The received RF signal strength under unobstructed line-of-sight conditions is significantly higher than under non-line-of-sight conditions with obstacles, primarily because the obstructing medium has absorption and reflection effects on electromagnetic waves. Based on transmission line theory, the input impedance of electromagnetic waves in the product packaging medium can be defined as: ; in, The relative permittivity of different packaging media, Changes in the parameters of the formula and The change in impedance alters the input impedance of the electromagnetic wave in the medium. .

[0033] Based on the above definitions, reflection loss can be defined as: ; in, The input impedance of electromagnetic waves in free space. This refers to the input impedance of electromagnetic waves in the product packaging medium. and There are inherent differences between them, and the reflection loss value will change accordingly.

[0034] According to the theory of electromagnetic wave propagation, when an electromagnetic wave propagates in a medium, energy absorption occurs due to the complex permittivity ε' of the medium. The propagation constant of electromagnetic waves in a medium is... The absorption can be determined by the attenuation constant α: ; in, , , ρ is the magnetic permeability in air.

[0035] The electromagnetic parameters, such as the relative permittivity, differ among different packaging media, which in turn affect the attenuation constant α, ultimately leading to changes in the absorption loss value. The absorption loss can be defined as: ; in, For conversion factors, For distance.

[0036] In this embodiment, the influence of the medium (U312) studies the effect of the medium on electromagnetic signal transmission. When an electromagnetic wave encounters a non-line-of-sight medium during propagation, due to the significant difference in dielectric constant between the medium and air, the incident wave undergoes partial reflection at the interface, and the transmitted wave is further attenuated by absorption effects as it propagates within the medium. Simultaneously, as the thickness of the packaging medium increases, its transmission loss gradually increases. The influence of humidity (U313) studies the effect of humidity on electromagnetic signal transmission. During the measurement process, as the ambient humidity increases, water molecules are adsorbed on the surfaces of different packaging media, leading to an increase in the equivalent dielectric constant and the dielectric loss tangent.

[0037] Figure 3 The image shows the variation of reflection loss of different media under different humidity conditions. From left to right, the media are plastic, cardboard, foam, and glass. Figure 4The absorption loss of different media under different humidity conditions is shown. From left to right, the media are plastic, cardboard, foam, and glass.

[0038] Combination Figure 3 and Figure 4 As can be seen, under the combined effects of the medium influence U312 and the humidity influence U313, the propagation loss of electromagnetic waves in the medium increases significantly, specifically manifested as the simultaneous enhancement of reflection loss and absorption loss. Figure 5 For the equivalent circuit model of line-of-sight scene and the equivalent circuit model of non-line-of-sight medium interference, combined Figure 5 From this perspective, compared to the original line-of-sight LC resonant circuit, the introduction of a non-line-of-sight medium is equivalent to adding a resistor and a capacitor in parallel to the original line-of-sight LC resonant circuit. Their synergistic effect is manifested as a decrease in the effective resistance RMedium and an increase in the equivalent capacitance CMedium, which in turn changes the resonance conditions and impedance matching characteristics, resulting in a shift in the sensor response value.

[0039] In this embodiment, the dielectric interference self-compensation U32 includes signal amplitude-frequency separation U321 and interference signal self-compensation U322. Signal amplitude-frequency separation U321 effectively decouples the dielectric interference signal from the gas response signal. Based on the reflection loss and absorption loss mechanism, the interference of dielectric materials to electromagnetic waves mainly manifests as disturbances to the amplitude and resonant frequency of the radio frequency signal. The core of dielectric interference loss is the change in the amplitude of the radio frequency signal, while the ammonia gas sensing process mainly causes a shift in the resonant frequency by adjusting the equivalent parameters of the dielectric, with a relatively weak impact on the amplitude. Therefore, this embodiment separates the amplitude-frequency signal to decouple the dielectric interference signal from the gas response signal, clarifying the frequency disturbance response of the dielectric interference to the ammonia gas detection signal. Interference signal self-compensation U322 establishes a correlation mechanism between the dielectric interference and the amplitude change of S11 it causes, achieving interference signal self-compensation. The pre-established linear relationship between the two is as follows: Figure 6 As shown in the figure, the media from left to right are plastic, cardboard, foam, and glass. By combining the transmission loss characteristics corresponding to different media properties, after measuring S11, the media properties are inferred from its amplitude change, thus obtaining the offset of the resonant frequency. The offset is then subtracted from the measured resonant frequency to obtain the compensated resonant frequency. This achieves self-compensation for interference signals caused by frequency disturbances from different media, providing a new technical approach for non-line-of-sight detection of quality deterioration in fresh agricultural products.

[0040] To more clearly illustrate the self-compensation part of the medium interference in this embodiment, the self-compensation process of the medium interference is introduced. Figure 7 Here is a flowchart of the self-compensation process for media interference, such as Figure 7 As shown, after obtaining the initial non-line-of-sight detection signal, based on the S caused by media interference revealed in previous studies... 11To address the response difference between signal amplitude attenuation and resonant frequency shift caused by ammonia adsorption, an amplitude-frequency separation method for the S11 signal is proposed, effectively decoupling the dielectric interference response and the gas response at the radio frequency signal level. Based on the dielectric interference response, combined with the known S11 amplitude and a pre-established linear relationship between dielectric interference and S11 amplitude changes, the transmission loss of different media is calculated, and the corresponding media characteristics are inferred, thus yielding the S11 signal frequency shift under dielectric interference. Subsequently, the frequency shift is subtracted from the frequency measured by non-line-of-sight passive detection to obtain the self-compensated resonant frequency. Based on this frequency, the actual concentration of ammonia in the environment is determined, achieving self-compensation for dielectric interference.

[0041] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency, characterized in that, include: S1. Deploy a flexible passive radio frequency sensor inside the packaging of fresh agricultural products. The flexible passive radio frequency sensor integrates a signal transmission unit, an impedance matching unit, and a sensing unit. S2. An RF excitation and receiving device is set up outside the packaging to provide RF excitation to the flexible passive RF sensor through non-line-of-sight electromagnetic coupling and to collect the reflected RF signal of the flexible passive RF sensor. S3. The sensing unit is used to sense the change in ammonia concentration in the storage and transportation environment of fresh agricultural products inside the packaging. The ammonia causes the equivalent electrical parameters of the sensing unit to change, and triggers the change in the resonant characteristics of the flexible passive radio frequency sensor. S4. By adjusting the impedance relationship between the signal transmission unit and the sensing unit through the impedance matching unit, the change in equivalent electrical parameters caused by sensing is converted into the resonant frequency shift and amplitude change in the reflected radio frequency signal. S5. Perform passive wireless detection in non-line-of-sight scenarios, analyze the radio frequency signal reflection loss and absorption loss caused by packaging medium and ambient humidity, and distinguish the amplitude change caused by medium interference from the resonant frequency shift caused by ammonia sensing. S6. Based on the results of the dielectric interference analysis, establish the correspondence between dielectric interference and the amplitude change of the reflected radio frequency signal, perform dielectric interference self-compensation processing on the detection signal, and output the detection results characterizing the deterioration state of fresh agricultural products.

2. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 1, characterized in that, S1 includes the following steps: Conductive patterns for a flexible passive radio frequency sensor are formed on a flexible thin polyimide substrate using a screen printing process. The conductive patterns correspond to the electrical connection paths between the signal transmission unit and the sensing unit. A near-field probe, an electromagnetic coupling region, and a spiral antenna are set in the signal transmission unit. The near-field probe is connected to a vector network analyzer, outputs a sweep frequency excitation signal, and generates an alternating magnetic field in the near-field region. During the frequency sweep process, an alternating magnetic field is applied to the helical antenna. The helical antenna obtains energy based on electromagnetic induction and generates a resonant response, which is then transmitted to the sensing unit connected to the helical antenna along the electrical connection path. Set the coupling distance between the near-field probe and the signal transmission unit, and perform multiple frequency sweep acquisitions within a preset distance set to obtain the reflection parameter curves corresponding to each distance; Extracting the resonant frequency based on the reflection parameter curve With minimum reflection amplitude and with Indicators characterizing coupling strength ,satisfy ; For distance set Compare and determine the working distance that meets the threshold constraint. The working distance is fixed in subsequent non-line-of-sight detection. Complete the excitation and signal acquisition of the flexible passive radio frequency sensor.

3. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 1, characterized in that, S2 includes the following steps: An interdigitated electrode structure is formed in the sensing unit region of the flexible passive radio frequency sensor. The interdigitated electrodes are electrically connected to the helical antenna of the signal transmission unit, so that the interdigitated electrodes participate in the reflection parameter response as a capacitor component of the resonant circuit. A gas-sensitive material layer is loaded on the surface of the interdigital electrode. The gas-sensitive material layer is selected from MXene / In2O3 composite material, and the gas-sensitive material layer is limited to cover the effective interdigital area of ​​the interdigital electrode. Reflection parameter curves were acquired under non-line-of-sight frequency sweep excitation, and the resonant frequency was determined from the reflection parameter curves. and the corresponding reflection amplitude ; Under the influence of changes in ammonia concentration, the gas-sensitive material layer causes a change in the equivalent capacitance of the interdigitated electrodes, thus obtaining the equivalent capacitance. With ammonia concentration Related capacitance change and satisfy ,in This is the initial equivalent capacitance; Based on the equivalent inductance provided by the helical antenna With equivalent capacitance Calculate the resonant frequency, satisfying and the calculated Corresponding to the acquired resonant frequency; The change in resonant frequency As a characteristic of ammonia response, among which This is the reference resonant frequency.

4. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 1, characterized in that, S3 includes the following steps: In the design phase of the flexible passive radio frequency sensor, the set of structural parameters for the signal transmission unit and the sensing unit is determined. The structural parameters include the line width, line spacing, and number of turns of the helical antenna, as well as the finger width, finger spacing, and finger length of the interdigitated electrodes. The equivalent impedance model of the sensor is constructed based on the set of structural parameters. The equivalent inductance parameters of the signal transmission unit are combined with the equivalent resistance and capacitance parameters of the sensing unit to calculate the sensor input impedance. The matching state between the input impedance and the external excitation port impedance is used as the impedance matching criterion. During the design phase, the set of structural parameters is iteratively adjusted, and the corresponding reference resonant frequency and reflection parameters are recorded as the initial reference state. In actual operation, when the flexible passive radio frequency sensor is exposed to the storage and transportation environment of fresh agricultural products, when ammonia in the environment is adsorbed by the sensing unit, the local equivalent resistance and equivalent capacitance of the sensing unit change. Under the influence of changes in equivalent resistance and equivalent capacitance, the overall input impedance of the sensor deviates from the matching state in the design stage, resulting in impedance mismatch, and simultaneously causing changes in resonant frequency and reflection parameters. The reflection parameters and resonant frequency offset under impedance mismatch conditions are collected, and the resonant frequency offset is used as the output.

5. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 1, characterized in that, S4 includes the following steps: A radio frequency detection antenna is set on the outside of fresh agricultural products, and a flexible passive radio frequency sensor is set inside the packaging of fresh agricultural products. There is a non-line-of-sight spatial relationship between the radio frequency detection antenna and the flexible passive radio frequency sensor, which is isolated by the packaging medium. A radio frequency excitation signal within a preset frequency range is transmitted to a flexible passive radio frequency sensor via a radio frequency detection antenna; Under the action of electromagnetic coupling, the flexible passive radio frequency sensor can obtain energy from the radio frequency excitation signal and form a reflected radio frequency signal corresponding to its electrical parameter state; The radio frequency (RF) signal is received by the RF detection antenna and reflected by the flexible passive RF sensor, and the received RF signal is output. Under non-line-of-sight detection conditions, the coupling state between the radio frequency detection antenna and the flexible passive radio frequency sensor is controlled; Radio frequency features that characterize changes in internal environmental parameters of the packaging are extracted based on the received reflected radio frequency signals.

6. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 1, characterized in that, S5 includes the following steps: The reflected radio frequency signal of the flexible passive radio frequency sensor was acquired under non-line-of-sight detection conditions. The corresponding amplitude variation characteristics of the reflection parameters and the resonant frequency variation characteristics were obtained and used as the original detection signal. Based on the type of packaging material and the ambient humidity, we conduct a medium interference analysis on the electromagnetic propagation effect introduced by the packaging medium in the original detection signal, and identify the changes in reflection loss and absorption loss of electromagnetic waves under the action of the medium. In the process of dielectric interference analysis, the amplitude variation component and the resonant frequency variation component in the reflected radio frequency signal are distinguished to obtain the dielectric interference response characteristics dominated by amplitude variation and the sensing response characteristics dominated by frequency variation. Based on the characteristics of media interference response, a model of the correspondence between the degree of media interference and the amplitude change of reflection parameters is established, and the correspondence is used as the basis for characterizing the amount of media interference. The amount of medium interference is inferred from the change in the amplitude of the reflection parameters, and the resonant frequency disturbance component caused by the medium interference is calculated based on the amount of medium interference. The resonant frequency variation characteristics in the original detection signal are compensated to eliminate the frequency disturbance caused by medium interference, and the detection results of fresh agricultural product quality deterioration after medium interference self-compensation are output.

7. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 6, characterized in that, The media interference analysis includes the following steps: The reflection parameters of the flexible passive radio frequency sensor were collected under non-line-of-sight detection conditions, and the corresponding reflection amplitude variation characteristics were obtained within a preset frequency sweep range. Based on transmission line theory, an input impedance model for electromagnetic waves in the packaging medium of fresh agricultural products is established to address the propagation process of electromagnetic waves. The input impedance of electromagnetic waves in the product packaging medium is defined as follows: ； in, The relative permittivity of the packaging medium. The relative permeability of the packaging medium. For the thickness of the medium, For radio frequency signal frequency, The speed at which electromagnetic waves propagate in free space; Based on free space input impedance Input impedance of packaging medium The difference between them is used to calculate the reflection loss of electromagnetic waves at the interface of the medium. The reflection loss is defined as: ； in, The input impedance of electromagnetic waves in free space. The input impedance of electromagnetic waves in the product packaging medium; and There are inherent differences between them, and the reflection loss value will change accordingly; Based on electromagnetic wave propagation theory, an absorption loss model is established for electromagnetic waves propagating in packaging media. The propagation constant of electromagnetic waves in the media is defined as follows: ,in, It is the attenuation constant. It is the phase constant and the attenuation constant. satisfy: ； in, , , The magnetic permeability in air; Based on attenuation constant The propagation distance of electromagnetic waves in a medium Calculate the absorption loss of electromagnetic waves in the packaging medium. The absorption loss is defined as: ； in, For conversion factors, This refers to the propagation distance of electromagnetic waves in the packaging medium. Reflection loss and absorption loss are used as outputs in the dielectric interference analysis.

8. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 1, characterized in that, S6 includes the following steps: Under non-line-of-sight detection conditions, the reflected radio frequency signals corresponding to different packaging media are acquired, and the amplitude characteristics and resonant frequency characteristics of the reflection parameters corresponding to each packaging medium are extracted under the same frequency sweep conditions. To address the difference in electromagnetic parameters between the packaging medium and air, the correspondence between the incident signal and the reflected signal was recorded when electromagnetic waves were incident on the interface of the packaging medium, and the change in reflection loss caused by interface reflection was used as a characterization parameter of the medium's influence. After electromagnetic waves are transmitted into the packaging medium, the amplitude attenuation characteristics of the transmitted radio frequency signal are collected based on the propagation process inside the medium, and the energy attenuation caused by absorption inside the medium is used as another characterization parameter of the medium's influence. The non-line-of-sight detection process was repeated under different packaging medium thickness conditions. The corresponding changes in reflection loss and absorption loss were recorded, and the relationship between medium thickness and transmission loss was used as the input for medium influence analysis. In the process of humidity effect analysis, the humidity level of the detection environment is adjusted, and reflected radio frequency signals are collected under different humidity conditions to record the changes in the amplitude of reflection parameters caused by humidity changes. The changes in ambient humidity are mapped to changes in the equivalent dielectric parameters and dielectric loss parameters of the packaging medium, and the characteristics of the influence of humidity are obtained based on the changes in reflection loss and absorption loss under humidity change conditions. By jointly analyzing the characteristics of the influence of the medium and the characteristics of the influence of humidity, the enhancement of reflection loss and the enhancement of absorption loss under the combined influence of the medium and humidity are obtained as the output of the medium interference analysis results.

9. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 6, characterized in that, The media interference self-compensation includes signal amplitude-frequency separation and interference signal self-compensation, wherein signal amplitude-frequency separation includes the following steps: The reflected radio frequency signal of the flexible passive radio frequency sensor is acquired under non-line-of-sight detection conditions, and the amplitude variation characteristics of the reflection parameters and the resonant frequency variation characteristics are extracted within a preset frequency sweep range as the original amplitude-frequency joint signal. Based on the results of the medium interference analysis, the amplitude variation component and the resonant frequency variation component in the reflection parameters are independently characterized to obtain the amplitude characteristic quantity describing the medium interference response and the frequency characteristic quantity describing the gas response. In the process of dielectric interference analysis, the corresponding relationship between the reflection loss and absorption loss caused by the dielectric and the impact on the amplitude of the radio frequency signal is established, and the amplitude changes caused by the reflection loss and absorption loss are collected as dielectric interference amplitude components. In the gas response analysis process, the resonant frequency shift caused by the change of the equivalent electrical parameters of the sensing unit is collected as the gas response frequency component, and the frequency component is defined as the main characterization feature of the ammonia response. Amplitude-frequency separation processing is performed on the original amplitude-frequency combined signal to decouple the amplitude component of the medium interference from the frequency component of the gas response, forming independent amplitude signal channels and frequency signal channels; Based on the amplitude-frequency separation results, the response range of the medium interference to the frequency disturbance of the detection signal is determined, and the separated frequency signal is output.

10. The non-line-of-sight detection method for the quality of fresh agricultural products based on flexible passive radio frequency as described in claim 9, characterized in that, The interference signal self-compensation includes the following steps: After completing the signal amplitude-frequency separation, the amplitude change of the reflection parameter characterizing the medium interference and the resonant frequency change characterizing the gas response are obtained, and the amplitude change of the reflection parameter is used as the input for characterizing the medium interference. Based on the correspondence model between media interference and the amplitude change of reflection parameters established in the media interference analysis stage, the mapping relationship between the amplitude change of reflection parameters and the properties of packaging media is determined. After measuring the change in the amplitude of the reflection parameters, the corresponding medium property parameters are deduced based on the mapping relationship, and the resonant frequency shift caused by the medium interference is calculated by combining the transmission loss characteristics corresponding to the medium properties. The original resonant frequency measured under non-line-of-sight detection conditions is compared with the calculated resonant frequency offset, and frequency compensation processing is performed to subtract the resonant frequency offset from the original resonant frequency to obtain the compensated resonant frequency. The compensated resonant frequency is used as the gas response characteristic after eliminating the influence of medium interference, which is used to characterize the quality deterioration state of fresh agricultural products inside the packaging. The interference signal self-compensation process is repeatedly executed under different packaging media and humidity conditions to keep the compensated resonant frequency decoupled from the changes in the medium and only reflect the frequency response caused by the change in ammonia concentration.

Images