Chipless rfid laminated double-frequency microstrip antenna strain sensor and simulation-experiment verification method thereof

Abstract

This invention discloses a chipless RFID stacked dual-frequency microstrip antenna strain sensor and its simulation-experimental verification method, belonging to the field of structural health monitoring technology. The sensor includes a metal ground plane, upper and lower dielectric substrates, upper and lower radiating patches, and a coaxial feed port. The lower patch is excited through a single port, and the upper patch is excited via interlayer coupling, achieving dual-frequency operation. When the sensor is attached to the surface of the structure under test, strain causes synchronous deformation of the two patches and the dielectric substrate, resulting in resonant frequency drift and enabling strain sensing. Key parameters are optimized through HFSS and COMSOL simulations to achieve impedance matching and stable resonance between low-frequency and high-frequency modes. Simulation and cantilever beam loading tests show that the low-frequency modal sensitivity is approximately 2.45 kHz / µε, the high-frequency modal sensitivity is approximately 3.41 kHz / µε, and the combined sensitivity is 5.71 kHz / µε. This invention features a compact design, independent dual frequencies, and high sensitivity, making it suitable for health monitoring of engineering structures and providing a technical means for strain measurement and structural condition assessment.

Description

Technical Field

[0001] This invention relates to the field of structural health monitoring and wireless passive sensing technology, specifically to a chipless RFID stacked dual-frequency microstrip antenna strain sensor and its simulation-experimental verification method. More specifically, this invention belongs to a composite sensor technology solution that combines chipless RFID technology, microstrip antenna resonant sensing technology, multi-layer stacked electromagnetic coupling technology, and structural surface strain detection technology. It can be used for long-term strain monitoring and structural health status assessment of bridges, buildings, steel structures, towers, large-span spatial structures, and other critical load-bearing components.

[0002] The sensor described in this invention employs a stacked dual-frequency microstrip antenna structure. It forms a multi-layered composite structure consisting of a metal ground plane, a lower dielectric substrate, a lower radiating patch, an upper dielectric substrate, and an upper radiating patch. A single-port feeding method excites the lower patch to achieve a low-frequency operating mode; simultaneously, electromagnetic coupling between the upper and lower patches excites the upper patch to achieve a high-frequency operating mode. This structure enables dual-resonant responses at both low and high frequencies within a single sensing unit, and structural strain can be detected by measuring the frequency shift of the resonant frequency as a function of strain.

[0003] This invention also relates to a simulation-experimental verification method for the strain sensor, including antenna geometry establishment, key parameter optimization, HFSS electromagnetic simulation, COMSOL multiphysics coupling simulation, dual-frequency modal impedance matching analysis, and S... 11 The process includes steps such as return loss curve extraction, strain-resonance frequency linear relationship fitting, and static loading experiment verification on a cantilever beam. Through these methods, the dual-frequency operational stability, strain sensitivity, impedance matching characteristics, and engineering applicability of the stacked dual-frequency microstrip antenna strain sensor can be systematically verified, providing a reliable basis for its application in structural health monitoring. Background Technology

[0004] As modern engineering structures become larger, more complex, and longer-term in service, bridges, buildings, tunnels, steel structure stadiums, offshore platforms, and energy infrastructure face increasingly complex external environments and loads during operation. Factors such as repeated vehicle loads, wind vibration, temperature gradient changes, material aging, fatigue accumulation, environmental corrosion, and accidental impact loads can all lead to micro-cracks, stress concentrations, loose connections, or fatigue damage in localized areas of the structure. When damage is in its early stages, its appearance is often not easily identifiable, but detectable abnormal changes in the strain state of the structural surface or interior often appear. Therefore, long-term, continuous, and accurate strain monitoring of critical structural components is a crucial foundation for achieving structural health monitoring, damage identification, risk warning, and life assessment.

[0005] Traditional strain measurement methods mainly include resistive strain gauges, vibrating wire sensors, fiber Bragg grating sensors, piezoelectric sensors, and other active electronic sensors. Among these, resistive strain gauges offer advantages such as mature measurement principles, low cost, and high sensitivity; however, they typically require extensive wiring and rely on a data acquisition device and external power supply. When monitoring large bridges or high-rise buildings, sensor deployment is dispersed, resulting in long on-site wiring distances, making the lines susceptible to environmental damage and electromagnetic interference, and increasing maintenance difficulty. Fiber Bragg grating sensors offer advantages such as strong resistance to electromagnetic interference and the ability to achieve distributed measurement; however, their demodulation equipment is expensive, and they require high installation quality and a suitable construction environment. While piezoelectric sensors and some smart sensors offer high sensitivity, they usually require internal power supplies or external excitation circuits, which is not conducive to long-term, low-maintenance operation.

[0006] In recent years, microstrip antenna strain sensors have attracted attention due to their thin and light structure, ease of attachment, wireless readability, and good compatibility with the measured structure. The basic mechanism of this type of sensor is as follows: when a microstrip antenna is attached to the surface of the measured structure, the deformation of the structure under stress causes synchronous deformation of the antenna patch and the dielectric substrate. This changes the effective electrical length, equivalent dielectric environment, and edge field distribution of the antenna, resulting in a drift in the antenna's resonant frequency. By detecting the change in the resonant frequency, the strain on the structural surface can be inferred. Compared with traditional wired strain sensors, microstrip antenna strain sensors reduce wiring requirements, are easier to deploy on complex structural surfaces, and have good engineering application potential.

[0007] However, existing microstrip antenna strain sensors still have several shortcomings. First, many existing solutions employ a single-layer, single-pattern, single-frequency operating mode, with only one main response peak at its resonant frequency, limiting sensitivity and dynamic measurement range. When the structural strain is small, the frequency shift at a single resonant point is limited and easily affected by the resolution of the measuring instrument, environmental noise, and local installation errors, resulting in insufficient strain identification accuracy. Second, single-frequency sensors struggle to simultaneously achieve the stability of low-frequency modes and the high sensitivity of high-frequency modes, failing to fully utilize the sensitivity enhancement effect brought about by the superposition of multi-mode frequency shifts. Third, some microstrip antenna sensors still require active circuitry, complex feedlines, or embedded chips to operate, which is not conducive to long-term passive monitoring in complex scenarios such as bridges, steel structures, and high-rise buildings.

[0008] Chipless RFID technology offers a new approach to solving the aforementioned problems. Chipless RFID systems eliminate the need for integrated chips on the tags; instead, they rely on the electromagnetic scattering characteristics of the antenna or resonant structure itself to carry information. When an external reader transmits a radio frequency signal, the sensing structure generates a significant backscattering response near its resonant frequency. If the sensing structure is subjected to external strain, its resonant frequency drifts with changes in its geometry, allowing the reader to obtain structural status information by identifying these spectral changes. This technology features simple structure, requires no internal power supply, is easy to deploy in large quantities, and has low maintenance costs, making it particularly suitable for long-term structural health monitoring of large infrastructure projects.

[0009] While chipless RFID technology offers advantages in wireless passive sensing, existing sensor designs largely rely on single-layer microstrip antennas or single-frequency resonant units, making it difficult to simultaneously support both low-frequency and high-frequency operating modes within a single sensor unit. For dual-frequency sensors, the low-frequency mode typically exhibits better signal stability, while the high-frequency mode is more sensitive to strain changes. If both modes could be implemented within a single structure, ensuring relatively independent resonant responses and good impedance matching, the sensor's strain detection accuracy and reliability could be significantly improved. However, in practical designs, the electromagnetic coupling between upper and lower patches is complex. Parameters such as feed point location, patch size, dielectric thickness, and slotting type all affect the resonant frequency, input impedance, return loss, radiation pattern, and frequency shift sensitivity. Without a systematic parameter optimization method, mutual interference between the low-frequency and high-frequency modes can easily occur, leading to problems such as indistinct resonance valleys, insufficient impedance matching, and unstable frequency response.

[0010] The stacked microstrip antenna structure, through electromagnetic coupling between the upper and lower radiating patches and the dielectric layer, can form a dual-frequency operating mode within the same sensing element. Specifically, the lower patch acts as a directly fed radiating element to generate the low-frequency mode, while the upper patch acts as a parasitic radiating element to generate the high-frequency mode through interlayer coupling. This structure provides an effective way to achieve independent dual-frequency control and improve sensitivity. Compared with single-layer single-frequency sensors, the stacked dual-frequency microstrip antenna sensor has significant advantages in terms of structural compactness, rich frequency response, and multi-mode information extraction.

[0011] However, in the currently disclosed technologies, there is still a lack of a chip-free RFID laminated dual-frequency microstrip antenna strain sensor for structural strain monitoring and its systematic simulation-experiment verification method. The existing technologies generally have the following problems: First, the single-frequency operating mode limits the ability to detect微小应变 (tiny strains), making it difficult to achieve high-resolution monitoring; Second, the dual-frequency mode design lacks an effective parameter optimization process, and it is difficult to achieve relatively independent control between the low-frequency and high-frequency modes; Third, the impedance matching and resonance stability are insufficient, resulting in limited accuracy in extracting frequency drift; Fourth, there is a lack of a complete verification process that combines electromagnetic simulation, multi-physics simulation, and actual loading experiments, and it is impossible to fully prove the applicability of the sensor when working on the surface of a real structure.

[0012] Therefore, there is an urgent need to propose a laminated dual-frequency microstrip antenna strain sensor with reasonable structure, good dual-frequency independence, high sensitivity, stable impedance matching, passive wireless reading ability, and can be fully verified through simulation and experiments, to meet the actual needs of long-term health monitoring of large-scale engineering structures such as bridges and buildings. Summary of the Invention

[0013] Object of the Invention: The object of the present invention is to provide a chip-free RFID laminated dual-frequency microstrip antenna strain sensor and its simulation-experiment verification method, to solve the deficiencies of existing microstrip antenna strain sensors in terms of sensitivity, dual-frequency mode independent control, impedance matching, wireless passive reading, and engineering verification.

[0014] The present invention constructs a five-layer laminated microstrip antenna structure, enabling the lower radiation patch to form a low-frequency operating mode under the action of single-port feeding, and enabling the upper radiation patch to form a high-frequency operating mode through interlayer electromagnetic coupling, thereby achieving dual-mode strain response of low frequency and high frequency within the same sensor unit. After the sensor is attached to the surface of the structure, the strain effect will cause synchronous deformation of the patch and the dielectric substrate, change the effective electrical length of the antenna, and further cause resonance frequency drift. By detecting the frequency shift amounts of the two resonance points of low frequency and high frequency, high-sensitivity detection of strain can be achieved.

[0015] At the same time, the present invention proposes a simulation-experiment verification method配套 with the sensor structure. This method conducts electromagnetic characteristic simulation and parameter optimization through HFSS software, conducts force-electromagnetic multi-physics coupling simulation through COMSOL Multiphysics software, analyzes the drift law of dual-frequency resonance frequency under different strain actions, and conducts actual verification through a cantilever beam static loading experiment platform. This method can systematically evaluate the dual-frequency resonance characteristics, impedance matching characteristics, strain sensitivity, and engineering applicability of the sensor, providing a reliable basis for the subsequent application of the sensor.

[0016] To achieve the above object, the present invention adopts the following technical solutions.

[0017] A chipless RFID stacked dual-frequency microstrip antenna strain sensor includes a metal ground plane, a lower dielectric substrate, a lower radiating patch, an upper dielectric substrate, an upper radiating patch, and a coaxial feed port. The metal ground plane is located at the bottom of the sensor and serves to reflect electromagnetic waves, form a stable electromagnetic boundary, and reduce interference from the underlying structure on the antenna's radiation characteristics. The lower dielectric substrate is disposed above the metal ground plane, and the lower radiating patch is disposed above the lower dielectric substrate. The upper dielectric substrate is disposed above the lower radiating patch, and the upper radiating patch is disposed above the upper dielectric substrate. The coaxial feed port passes through the metal ground plane and the lower dielectric substrate and is electrically connected to the lower radiating patch to excite the lower patch to form a low-frequency mode.

[0018] The upper radiating patch is not directly connected to the feed port, but instead acts as a parasitic radiating unit, forming a high-frequency operating mode through electromagnetic coupling with the lower radiating patch. By adjusting the size of the upper and lower patches, the thickness of the upper and lower dielectric substrates, and the position of the feed point, relatively independent control of the low-frequency and high-frequency modes can be achieved, ensuring that the two resonant frequencies are located near the target frequency band. The center frequency of the low-frequency mode is approximately 2.4 GHz, and the center frequency of the high-frequency mode is approximately 3.5 GHz. The low-frequency mode is mainly generated by direct feeding from the lower patch, and its frequency response is relatively stable; the high-frequency mode is mainly generated by the coupling excitation between the upper and lower patches, and it has a higher sensitivity to structural strain changes. Through the combined action of the two modes, the overall strain detection sensitivity of the sensor can be improved.

[0019] The sensor can be attached to the surface of bridges, buildings, or other structures being measured. When the measured structure undergoes tensile or bending deformation, the sensor deforms synchronously with the structure through the adhesive layer, and the upper and lower radiating patches and dielectric substrate change geometrically with the strain on the structural surface. Since the resonant frequency of a microstrip antenna is closely related to its effective electrical length, the resonant frequency typically drifts to lower frequencies as the effective electrical length increases. By detecting the change in resonant frequency, the strain on the structural surface can be inverted. Wireless passive reading is achieved using the electromagnetic scattering principle of chipless RFID. An external reader or vector network analysis device transmits radio frequency signals to the sensor. When the frequency of the incident signal approaches the sensor's resonant frequency, the sensor produces obvious backscattering characteristics or a return loss trough. After the resonant frequency drifts due to structural strain, the reading device can obtain strain information by identifying the change in the resonant frequency.

[0020] The dominant resonant frequency of a microstrip patch antenna can be approximated as:

[0021] (1)

[0022] in, The speed of light in a vacuum. The effective length of the patch, Let be the equivalent dielectric constant. The effective length, considering edge extension, can be expressed as:

[0023] (2)

[0024] Under strain, the change in patch length is as follows:

[0025] (3)

[0026] Therefore, the resonant frequency drift can be expressed as:

[0027] (4)

[0028] Within a small strain range, the above equation can be approximated as:

[0029] (5)

[0030] in, The strain sensitivity coefficient of the sensor. This is the initial resonant frequency without strain. The return loss of the microstrip antenna is related to its input impedance as follows:

[0031] (6)

[0032] (7)

[0033] The resonant frequency can be determined by S 11 The valley value of the curve is determined, and the structural strain is obtained through frequency drift inversion:

[0034] (8)

[0035] This invention employs a stacked dual-pattern structure in its sensor design. The upper patch acts as a parasitic unit, generating high-frequency modes through electromagnetic coupling, while the lower patch directly powers the sensor to generate low-frequency modes. The coupling between the two layers can be optimized by adjusting the feed point location, patch size, and dielectric thickness, ensuring that the low-frequency and high-frequency modes are independent and simultaneously meet impedance matching conditions. The optimization targets for the sensor structural parameters include input resistance approaching 50Ω for both low-frequency and high-frequency modes, input reactance approaching 0, and S...11 The minimum and peak values ​​are steep, thus ensuring a clear resonant frequency response and facilitating high-precision strain measurement.

[0036] The sensor design methodology includes establishing a five-layer stacked structure, HFSS and COMSOL multiphysics simulation parameter scanning, low-frequency and high-frequency mode modulation, impedance matching optimization, and verification of resonant frequency characteristics. Simulation results show that the low-frequency mode resonant frequency is approximately 2.4 GHz, and the high-frequency mode is approximately 3.5 GHz. Under small strain, the low-frequency and high-frequency modes exhibit sensitivities of approximately 2.45 kHz / µε and 3.41 kHz / µε, respectively, with the superposition effect significantly enhancing the overall sensitivity. Cantilever beam loading experiments verified that the sensor's strain sensing capability and frequency drift characteristics are consistent with the simulation, indicating that the design can operate stably under actual engineering conditions.

[0037] Regarding parameter optimization, the single-port feed position y n Patch length and width B1, B2, B 11 B 22 The thicknesses of the upper and lower media, h1 and h2, can both be used as adjustable parameters. Optimal dual-frequency performance can be obtained through scanning and optimization. The sensor can be compactly arranged on the surface of structures such as bridges and buildings, achieving high-precision measurement of minute strains while maintaining dual-frequency independence, and supporting remote passive wireless readout. Since wireless signals propagate in space according to radar equations, and received power attenuates to the fourth power with distance, the system should select a near-field or far-field starting position as the working area to balance signal stability and strength.

[0038] Based on the aforementioned theories and optimization methods, this invention provides a systematic design scheme for a stacked dual-frequency microstrip antenna. It combines microstrip antenna strain sensing theory with chipless RFID technology to achieve a strain sensing unit that is independent of both frequencies, has good impedance matching, and high sensitivity. This scheme not only enables high-precision frequency shift detection under minute strain conditions but also confirms its feasibility in structural health monitoring of bridges, buildings, and other structures through simulation verification and experimental testing.

[0039] Beneficial effects: (1) The present invention adopts a stacked double patch microstrip antenna structure, forming two working modes, low frequency and high frequency, within a single sensor unit. The lower patch is directly excited by the coaxial feed port to form the low frequency mode, and the upper patch forms the high frequency mode through interlayer electromagnetic coupling, enabling the sensor to obtain two independent resonant responses simultaneously. Compared with a single-frequency sensor, this structure can use dual-frequency shift information to jointly characterize the strain state, which is beneficial to improving strain detection resolution and monitoring reliability. (2) The present invention achieves relatively independent control of the low frequency and high frequency modes by adjusting the feed point position, the size of the upper and lower patches, and the dielectric thickness. Both the low frequency mode and the high frequency mode can form clear resonance valley values ​​near the target frequency band, and the input resistance is close to 50Ω and the input reactance is close to zero through impedance matching optimization, thereby ensuring that the sensor has stable resonance characteristics in the working frequency band. (3) The present invention utilizes the principle of strain causing changes in the effective electrical length of the patch to convert the small deformation of the structural surface into resonant frequency drift, thereby achieving high-sensitivity detection of strain. Simulation results show that the low-frequency modal sensitivity is about 2.45 kHz / µε and the high-frequency modal sensitivity is about 3.41 kHz / µε; experimental results show that the low-frequency modal sensitivity is about 2.44 kHz / µε and the high-frequency modal sensitivity is about 3.27 kHz / µε, verifying that the sensor has a high response capability to strain changes. (4) This invention provides a complete simulation-experiment verification method. The input impedance, return loss, Smith chart and radiation pattern are analyzed by HFSS electromagnetic simulation, the frequency drift law under strain is studied by COMSOL multiphysics simulation, and the actual verification is carried out by cantilever beam loading experiment, forming a systematic technical route from structural design, parameter optimization to performance verification. (5) This invention has a compact structure and is easy to deploy. The five-layer stacked structure has a small thickness and can be attached to the surface of the structure under test by adhesive layer without obvious damage or complex modification to the structure under test. It is suitable for installation in bridge main beams, steel structure nodes, building components, cantilever beams or other key stress parts. (6) This invention can provide quantifiable data for structural health assessment. By using the linear fitting relationship between resonant frequency and strain, the frequency drift can be converted into structural strain values, providing data support for structural damage identification, fatigue state analysis, and long-term operational safety assessment. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the three-dimensional model of the upper and lower layers of the stacked dual-frequency microstrip antenna sensor of the present invention;

[0041] Figure 2 This is a schematic diagram of the overall structure of a stacked dual-frequency microstrip antenna strain sensor.

[0042] Figure 3 This is a graph showing the change of sensor input impedance with frequency in an HFSS simulation.

[0043] Figure 4 For sensor S 11 Return loss versus frequency curve;

[0044] Figure 5 This is a schematic diagram of the Smith Chart for the sensor.

[0045] Figure 6 (a) is the two-dimensional gain pattern of the sensor in the low-frequency mode, and (b) is the two-dimensional gain pattern of the sensor in the high-frequency mode.

[0046] Figure 7 (a) is a three-dimensional gain distribution diagram of the sensor's low-frequency mode, and (b) is a three-dimensional gain distribution diagram of the sensor's high-frequency mode.

[0047] Figure 8 Schematic diagram of simulation mesh generation and boundary conditions

[0048] Figure 9 (a) Si under different strains in the low-frequency mode of the sensor in the COMSOL simulation. 11 Curve (b) shows the S-values ​​of the sensor under different strains in the high-frequency mode during the Comsol simulation. 11 curve;

[0049] Figure 10 (a) shows the relationship between the resonant frequency and strain of the sensor in the low-frequency mode in the Comsol simulation; (b) shows the relationship between the resonant frequency and strain of the sensor in the high-frequency mode in the Comsol simulation.

[0050] Figure 11 The relationship between the integrated resonant frequency and strain of a stacked dual-frequency microstrip antenna strain sensor in a Comsol simulation;

[0051] Figure 12 This serves as a test platform for a stacked dual-frequency microstrip antenna cantilever beam strain monitoring system.

[0052] Figure 13 (a) is the curve showing the relationship between low-frequency modal resonant frequency and strain in the cantilever beam experiment, and (b) is the curve showing the relationship between high-frequency modal resonant frequency and strain in the cantilever beam experiment.

[0053] Figure 14 This relates the integrated resonant frequency of the sensor to the strain in a cantilever beam experiment.

[0054] In the figure: 1. Upper strain measurement patch antenna; 2. Lower strain measurement patch antenna; 3. Upper dielectric substrate; 4. Lower dielectric substrate; 5. Metal ground plane; 6. Coaxial feed point; 7. Curved slot; 8. Vector network analyzer; 9. Strain acquisition instrument; 10. RF port; 11. Cantilever beam with strain to be measured; 12. Vector network analyzer and strain acquisition terminal; 13. Resistive strain gauge; 14. Stacked dual-frequency microstrip antenna; 15. Hydraulic lever. Detailed Implementation

[0055] The embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0056] like Figure 1 As shown, this invention discloses a stacked dual-frequency microstrip antenna strain sensor based on chipless RFID technology. The three-dimensional model of the upper and lower layers of the sensor shows the relative positions of the patches and the dielectric substrate, as well as the feed point position y. n This model is used for simulation analysis and parameter optimization. By adjusting the patch size, feed point position, and dielectric thickness, impedance matching and stability of dual-frequency resonance can be achieved.

[0057] like Figure 2 The diagram illustrates the specific structure of a stacked dual-frequency microstrip antenna strain sensor, comprising a metal ground plane, a lower dielectric substrate, a lower radiating patch, an upper dielectric substrate, an upper radiating patch, and a coaxial feed port. The metal ground plane, located at the bottom, reflects electromagnetic waves and shields against interference from below. The lower dielectric substrate and the lower patch form a low-frequency mode radiation structure, excited via the coaxial feed port. The upper dielectric substrate and the upper patch form a high-frequency mode, achieving dual-frequency operation through interlayer coupling. This structure enables independent response and synchronous strain transfer between low-frequency and high-frequency modes, providing a foundation for high-sensitivity strain detection.

[0058] like Figure 3 As shown, the sensor input impedance versus frequency curve in the HFSS simulation illustrates the matching between low-frequency and high-frequency modes in the target design frequency band. This is achieved by adjusting the patch lengths B1, B2, and B... 11 B 22 and feed point bias y n It can achieve an ideal state with an input resistance close to 50Ω and an input reactance close to 0, thus ensuring accurate extraction of the resonant frequency.

[0059] like Figure 4 As shown, S 11 The return loss versus frequency curve shows clear dips in the low-frequency and high-frequency modes, with a center frequency of approximately 2.4 GHz for the low-frequency mode and approximately 3.5 GHz for the high-frequency mode. This curve is used to invert the strain signal, calculating the surface strain of the structure through frequency drift.

[0060] like Figure 5 As shown in the Smith Chart, the impedance characteristics of the sensor in dual-frequency operation are illustrated. The input impedances of the low-frequency and high-frequency modes fall near the target impedance circle, indicating that the two modes achieve good matching, ensuring stable response and high-sensitivity measurement of the sensor under small strains.

[0061] like Figure 6 As shown, the two-dimensional gain pattern of the sensor further illustrates the energy distribution of low-frequency and high-frequency modes in the main radiation direction. Figure 6 (a) is a two-dimensional gain pattern of the low-frequency mode, showing that its radiation is mainly concentrated in the specified direction, forming a clear main lobe and a small side lobe, which ensures the signal stability of the low-frequency mode under small strain. Figure 6 (b) shows the two-dimensional gain pattern of the high-frequency mode. Its radiation directionality is similar to that of the low-frequency mode, but the energy distribution and amplitude are different, ensuring that the high-frequency mode has independent radiation characteristics during wireless readout. The two-dimensional pattern is used to evaluate the radiation uniformity and directional selectivity of the sensor in the plane, providing a basis for subsequent remote wireless measurements.

[0062] like Figure 7 As shown, the three-dimensional gain distribution diagram of the sensor illustrates the radiation intensity distribution of the dual-frequency modes in space. Figure 7 (a) The three-dimensional gain distribution of the low-frequency mode is shown. Its radiation intensity is the largest along the main beam direction and the lateral attenuation is obvious, which is beneficial to reduce interference. Figure 7 (b) The three-dimensional gain distribution of the high-frequency mode is displayed, and its radiation characteristics do not interfere with the low-frequency mode in the spatial direction, providing conditions for the extraction of dual-frequency independent signals. Through three-dimensional gain distribution analysis, antenna layout, antenna position reading, and signal acquisition strategies can be effectively guided, ensuring the reliability of wireless passive reading.

[0063] like Figure 8 As shown, the simulation mesh generation and boundary condition diagram illustrates the modeling method of the COMSOL multiphysics simulation model. In the simulation, the upper and lower patches of the sensor and the dielectric substrate are divided into fine meshes to ensure the accuracy of electromagnetic field calculations. Simultaneously, reasonable boundary conditions are set to simulate the surface adhesion effect of the structure, ensuring that the simulation results are highly consistent with actual experimental conditions. Mesh generation and boundary condition optimization are key steps in achieving accurate frequency response and sensitivity prediction, providing a theoretical basis for parameter optimization.

[0064] like Figure 9 As shown, S under different strain conditions in COMSOL simulation 11 The return loss curve illustrates the strain response characteristics of the dual-frequency mode. Figure 9 (a) represents the low-frequency mode S 11 As strain increases, the return loss curve shifts significantly to the left, and the low-frequency resonant frequency changes linearly with strain. Figure 9 (b) represents the high-frequency mode S 11 The curve also exhibits the characteristic of drifting with strain. By comparing the S11 curves under different strains, the sensor frequency shift can be accurately extracted, providing a reliable basis for micro-strain measurement.

[0065] like Figure 10 As shown, the relationship between the resonant frequencies and strain of the low-frequency and high-frequency modes in the COMSOL simulation further quantifies the sensitivity of the sensor. Figure 10 (a) shows the linear fitting relationship between the low-frequency modal resonant frequency and strain, with the fitting formula being: The fitting results show that R² is approximately 0.9903, indicating that the low-frequency mode exhibits a highly linear response to small strains. Figure 10 (b) shows the relationship between the high-frequency modal resonant frequency and strain, with the fitting formula as follows: The fitted R² is approximately 0.9962, indicating that the high-frequency mode has a relatively stable linear response to strain.

[0066] like Figure 11 As shown in the figure, the COMSOL simulation presents the relationship between the combined resonant frequency and strain after the sensor is superimposed. The fitting formula for the combined resonant frequency and strain using the dual-frequency modal superposition response is: The sensor has a total sensitivity of 5.86 kHz / µε and a fitting R² of approximately 0.994, enabling high-precision measurement of minute strains.

[0067] like Figure 12 As shown, this embodiment demonstrates the actual strain test of a stacked dual-frequency microstrip antenna sensor on an experimental platform. The experimental platform consists of a cantilever beam, a hydraulic rod, strain gauges, a virtual network analyzer (VNA), a strain acquisition instrument, and a data terminal. The sensor is attached to the surface of the cantilever beam, and precise strain is applied through the hydraulic rod. Simultaneously, the strain gauge acquires a reference strain signal, which is compared with the change in the sensor's resonant frequency.

[0068] like Figure 13 As shown, the frequency shift and strain relationships of the low-frequency and high-frequency modes are fitted respectively. Figure 13 (a) shows the linear fitting relationship between the low-frequency modal resonant frequency and strain. The linear fitting formula for the low-frequency operating mode is: The low-frequency mode antenna sensitivity is 2.44 kHz / µε, and the fitting coefficient R0 is [missing value]. 2 It is 0.989; Figure 13 (b) shows the linear fitting relationship between the high-frequency modal resonant frequency and strain. The linear fitting formula for the high-frequency operating mode is: The high-frequency mode antenna sensitivity is 3.27 kHz / µε, and the fitting coefficient R0 is [missing value]. 2 It is 0.9954.

[0069] like Figure 14 As shown, the relationship between the superimposed resonant frequency and strain is fitted, and a linear fitting relationship between the combined resonant frequency and strain is given. The linear fitting formula for the superimposed modal resonant frequency is: The antenna's overall sensitivity is 5.71 kHz / µε, and the fitting coefficient R0 is [missing value]. 2 It is 0.998.

[0070] In the sensor design process described in this invention, the feed point location, upper and lower patch sizes, dielectric thickness, and slotting structure can all be used as optimization parameters. Specifically, the feed point location primarily affects the input impedance and resonant valley depth; the length and width of the lower patch primarily affect the low-frequency mode resonant frequency; the length and width of the upper patch primarily affect the high-frequency mode resonant frequency; and the thickness of the upper and lower dielectric substrates affects the coupling strength between patches and the resonant bandwidth. Parameter scanning is performed using HFSS. First, the dielectric material and initial patch size are fixed, and the lower patch size is adjusted to bring the low-frequency mode close to 2.4 GHz; then, the upper patch size is adjusted to bring the high-frequency mode close to 3.5 GHz; subsequently, the feed point offset position is adjusted to ensure good impedance matching for both modes; finally, the dielectric thickness and local slotting parameters are fine-tuned to achieve the desired S... 11 The curve has clear valleys, steep resonance peaks, and minimal mutual interference between the two frequency responses.

[0071] In practical engineering applications, this sensor can be attached to the surface of bridge main beams, steel structure nodes, building beam-column connection areas, key parts of towers, or other components requiring strain monitoring. Since the sensor does not require internal power and can be wirelessly and passively read via chipless RFID, it is suitable for long-term monitoring and scenarios where wiring is difficult. During deployment, good adhesion between the sensor and the structural surface should be ensured, and installation in areas with severely rough, damp, oily, or obviously damaged surfaces should be avoided. The sensor can be deployed at a single point for local strain monitoring of key areas, or in a multi-point array for regional structural condition sensing. When multiple sensors are used on the same structure, they can be distinguished by operating frequency band, spatial location, or encoding method to achieve multi-point strain information acquisition. Due to its compact structure and relatively low cost, the sensor is suitable for widespread application in large-scale structural health monitoring systems.

[0072] The frequency drift characteristics under strain are analyzed below using COMSOL multiphysics simulation.

[0073] A stacked dual-frequency microstrip antenna was modeled and analyzed using the COMSOL Multiphysics simulation platform. By establishing a three-dimensional simulation model and utilizing the solid mechanics and electromagnetic wave frequency domain modules, the Ss of the antenna under different strain conditions was investigated. 11The curve variation and resonant frequency drift law are analyzed, along with the sensitivity improvement effect combined with the dual-frequency superposition mechanism. The simulation model mainly consists of a metal ground plane, a double-layer dielectric substrate, upper and lower radiating patches, and a coaxial feed port. The metal part uses copper, and the dielectric substrate is FR4 with a relative permittivity of 4.4 and a loss factor of 0.02. The antenna uses a single coaxial feed port to excite the lower patch, and the upper patch forms a parasitic resonance through interlayer electromagnetic coupling, thus achieving dual-frequency operation. To simulate the electromagnetic propagation environment in free space, a spherical air domain is set outside the antenna, and a perfectly matched layer is introduced on its outer layer. After setting the material properties, physical fields, and boundary conditions, the model is meshed.

[0074] The simulation utilizes deformation geometry to map the geometric deformation of the structure under different strains to the electromagnetic module in real time, ensuring that the electromagnetic analysis is performed on the true geometry after strain. In the solid mechanics module, one end of the antenna is fixed, and a target direction displacement is applied to the other end to generate strain. In the electromagnetic wave module, a feed port is used to excite the antenna, and scattering boundary conditions are applied outside the air box to simulate an open space environment. During the simulation, a linear strain from 0 με to 2700 με is applied to the antenna, with an increment step of 450 με. SLS ... 11 Parameter Calculation and Analysis. Based on the above steps, the return loss curves of the microstrip antenna strain sensor under different strains are calculated. As the strain increases, S... 11 The curve gradually shifts to the left. Simultaneously, the S frequency within the sweep range... 11 The values ​​are all less than -10dB, meaning the antenna is less affected by impedance mismatch within this deformation range. 11 The lowest point of the curve is the resonant frequency of the sensor.

[0075] Based on the strain sensor with a stacked dual-frequency microstrip antenna, the resonant frequencies of both operating frequencies show a linear relationship with the deformation. The fitting formula for the linear relationship of the low-frequency operating mode is as follows: The antenna sensitivity is 2.45 kHz / µε, and the fitting coefficient R0 is 2.45 kHz / µε. 2 The value is 0.9903; the linear relationship fitting formula for the high-frequency operating mode is... The antenna sensitivity is 3.41 kHz / µε, and the fitting coefficient R0 is 3.41 kHz / µε. 2 The value is 0.9962. COMSOL multiphysics simulation results show that structural strain can be indirectly measured through the resonant frequency.

[0076] To verify the strain sensing capability and measurement performance of the stacked dual-frequency microstrip antenna strain sensor described in this invention under actual mechanical conditions, an experimental platform based on static loading of a cantilever beam was built, and the resonant frequency response law of the sensor under different strain conditions was systematically tested.

[0077] Equipped with a stacked dual-frequency microstrip antenna strain sensor test platform, the entire test system mainly consists of a vector network analyzer, a VNA data acquisition terminal, RF connection lines, a stacked dual-frequency microstrip antenna strain sensor, a resistive strain gauge, a strain acquisition instrument, a cantilever beam loading structure, hydraulic rods, and an experimental support frame, which together complete the synchronous measurement of the strain state of the cantilever beam and the antenna resonance characteristics.

[0078] The test plan is as follows:

[0079] (1) Installation of the stacked dual-frequency microstrip antenna strain sensor and strain gauges. Before installation, the surface of the cantilever beam was first ground, degreased, and cleaned to reduce the impact of surface roughness and contaminants on the consistency of strain transmission. Then, the stacked dual-frequency microstrip antenna strain sensor was attached to the upper surface of the cantilever beam near the fixed end using a stable structural adhesive, and resistive strain gauges were attached to both sides of the center of the sensor as reference measuring elements. After attachment, the sample was left to stand at room temperature for a sufficient time to allow the adhesive layer to fully cure.

[0080] (2) Connect the network analyzer and strain gauge. During high-frequency testing, the oscillation of the RF cable may affect the impedance state of the microstrip antenna. Necessary segmentation and stress relief treatment was performed on the RF coaxial cable near the sensor. After proper wiring, the vector network analyzer was calibrated to eliminate systematic errors introduced by mismatches in test cables, connectors, and ports. After calibration, the RF connection status was kept constant throughout the experiment to ensure consistency of test conditions across different loading conditions.

[0081] (3) Measurement. This experiment employed a static graded loading method to apply external deformation to the cantilever beam. During loading, the displacement of the hydraulic rods was controlled to induce progressively increasing deflection responses at the free end of the cantilever beam, resulting in different levels of bending strain on the beam surface. After each load level was applied, the load was maintained for a period of time to allow the beam deformation and adhesive layer transfer state to stabilize. Then, strain gauge readings and S-strain readings from the microstrip antenna sensor were collected. 11 Curve. The specific testing steps are as follows:

[0082] Record the initial readings of the strain gauges and the initial S-values ​​of the stacked dual-frequency microstrip antenna sensor under no-load conditions. 11 The curve is used to extract the initial frequencies of the two resonant points, the low-frequency and the high-frequency points, which are denoted as f. L0 and f H0This state serves as a reference for subsequent calculations of frequency drift.

[0083] External loads are applied in stages, and the following two measurements are performed simultaneously under each loading stage: First, the output values ​​of the resistance strain gauges are recorded by a strain acquisition instrument, and the average value is taken as the actual strain under that load stage; Second, the return loss curve of the stacked dual-frequency microstrip antenna sensor within the set frequency sweep range is obtained by a vector network analyzer, and the return loss curve is determined by identifying S... 11 The positions of the two resonance valleys in the curve are used to extract the low-frequency mode resonant frequency f. L With high-frequency modal resonant frequency f H .

[0084] To reduce the impact of random noise and instrument fluctuations on the test results, the measurements were repeated several times under each load level, and the average value of the multiple measurements was calculated.

[0085] Test results show that as the surface strain of the cantilever beam gradually increases, the low-frequency and high-frequency resonant frequencies of the stacked dual-frequency microstrip antenna strain sensor both show a gradual decreasing trend, and the frequency shift amplitude of the high-frequency operating mode is generally greater than that of the low-frequency operating mode. The frequency variation law measured experimentally is generally consistent with the simulation results. To quantitatively evaluate the measurement performance of the stacked dual-frequency microstrip antenna strain sensor, the frequency shift and strain relationship of the low-frequency and high-frequency modes were fitted separately. The linear relationship fitting formula for the low-frequency operating mode is as follows: The low-frequency mode antenna sensitivity is 2.44 kHz / µε, and the fitting coefficient R0 is [missing value]. 2 The value is 0.989; the linear relationship fitting formula for the high-frequency operating mode is... The high-frequency mode antenna sensitivity is 3.27 kHz / µε, and the fitting coefficient R0 is [missing value]. 2 The value is 0.9954; the linear fitting formula for the superimposed modal resonant frequency is... The antenna's overall sensitivity is 5.71 kHz / µε, and the fitting coefficient R0 is [missing value]. 2 The final sensitivity of the entire stacked dual-frequency microstrip antenna strain sensor reached 5.71 kHz / µε under the superposition effect. Comparing the simulation results, there is a certain deviation between the experimental and simulation results; the antenna sensitivity decreased by approximately 150 Hz / µε compared to the simulation. This may be because the thickness of the adhesive layer between the sensor and the cantilever beam, the curing uniformity, and the interface strain transfer efficiency are difficult to achieve perfectly in practice, causing attenuation or lag in the transfer of the actual strain from the structural surface to the antenna patch. Furthermore, the actual electromagnetic coupling and edge effects between the two layers of patches in the stacked structure are likely more complex than in the ideal simulation model, which is also a significant reason for the difference between the experimental and theoretical sensitivities.

[0086] In summary, this invention discloses a chipless RFID stacked dual-frequency microstrip antenna strain sensor and its systematic simulation and experimental verification method. Addressing the technical bottlenecks of existing traditional strain measurement methods, such as complex wiring and high maintenance costs, and the limited sensitivity and difficulty in handling multi-mode applications of single-frequency microstrip antenna strain sensors, this invention employs a five-layer stacked microstrip antenna structure design. Through direct single-port feeding excitation of the lower layer patch and interlayer electromagnetic coupling of the upper parasitic patch, relatively independent control and stable response of low-frequency and high-frequency dual operating modes are achieved within a single sensing unit. This invention provides a complete technical route from theoretical simulation to practical engineering verification. Impedance matching and resonance characteristic optimization of the dual-frequency modes are achieved through HFSS electromagnetic simulation, ensuring that the return loss valley meets the operating conditions. COMSOL multiphysics coupling simulation is used to simulate the resonance variation law of the antenna under applied strain. Finally, a static loading experiment with a cantilever beam realistically verifies the sensitivity and reliability of the sensor in a real mechanical environment. Experimental results verify that, under the superposition effect of dual-frequency modes, the overall sensitivity of the sensor can reach 5.71 kHz / µε, which can meet the high-precision measurement requirements of small strain on structural surfaces.

Claims

1. A coreless RFID laminated dual-band microstrip antenna strain sensor and its simulation-experimental verification method, characterized in that, The device comprises a metal ground plane, a lower dielectric substrate, a lower radiating patch, an upper dielectric substrate, an upper radiating patch, and a coaxial feed port. The stacked dual-frequency microstrip antenna strain sensor consists of a metal ground plane, a lower dielectric substrate, a lower radiating patch, an upper dielectric substrate, and an upper radiating patch stacked sequentially from bottom to top, forming a five-layer stacked structure. The coaxial feed port is used to directly excite the lower radiating patch. The upper radiating patch forms another resonant mode through interlayer electromagnetic coupling with the lower radiating patch, enabling the sensor to achieve a dual-frequency resonant response. When the sensor is attached to the surface of the measured structure and the measured structure experiences strain, the upper and lower radiating patches and the dielectric substrate deform synchronously, causing the two resonant frequencies to drift. The strain is sensed and measured by detecting the resonant frequency drift.

2. The stacked dual-frequency microstrip antenna strain sensor of claim 1, wherein, The length and width of the lower radiating patch are B1 and B2, respectively, and the length and width of the upper radiating patch are B... 11 and B 22 The position parameter of the coaxial feed point is y. n The lower dielectric substrate has a thickness of h1, the upper dielectric substrate has a thickness of h2, and the length and width of the dielectric substrate are both a. The dielectric substrate is made of FR4 epoxy resin material, which has a relative permittivity of approximately 4.

4. The lower radiating patch serves as the main radiating unit, which is directly excited through the coaxial feed port to form a low-frequency resonant mode. The upper radiating patch serves as the parasitic radiating unit, which forms a high-frequency resonant mode through interlayer coupling.

3. The stacked dual-frequency microstrip antenna strain sensor according to claim 1, characterized in that, The resonant frequencies of the low-frequency mode and the high-frequency mode respectively satisfy: (1) (2) in, It is the low-frequency modal resonant frequency. It is the high-frequency modal resonant frequency. The effective resonant length of the lower radiating patch. The effective resonant length of the upper radiating patch. The equivalent dielectric constant is the value of the lower radiating patch. The equivalent dielectric constant corresponding to the upper radiating patch is... The speed of light in a vacuum.

4. The stacked dual-frequency microstrip antenna strain sensor according to claim 4, characterized in that, Under strain, the resonant frequencies of the lower and upper radiating patches exhibit an approximately linear relationship with structural deformation, satisfying the following conditions respectively: (3) (4) in, and These are the initial resonant frequencies of the low-frequency mode and the high-frequency mode under strain-free conditions, respectively. and These are the frequency sensitivities corresponding to the low-frequency mode and the high-frequency mode, respectively. For structural strain.

5. The chipless RFID laminated dual-band microstrip antenna strain sensor according to claim 5, wherein, The frequency shifts generated by the low-frequency and high-frequency modes under the same strain jointly constitute the total frequency response of the sensor, and the total frequency shift satisfies: (5) wherein , is the total frequency offset; the integrated sensitivity of the strain sensor satisfies: (6) wherein, is the low frequency modal sensitivity, is the high frequency modal sensitivity, is the overall sensitivity of the stacked dual-band microstrip antenna strain sensor.

6. The stacked dual-frequency microstrip antenna strain sensor according to any one of claims 1 to 6, characterized in that, By adjusting the coaxial feed point position parameter y n The lower layer radiating patch has a length B1 and a width B2, and the upper layer radiating patch has a length B. 11 and width B 22 The input resistance, input reactance, and resonant point location in both low-frequency and high-frequency modes are optimized to achieve impedance matching within the two target frequency bands. (7) in, For input resistance, For input reactance, when Approaching 50Ω and When the voltage is close to zero, the sensor meets the power supply matching requirements.

7. The stacked dual-frequency microstrip antenna strain sensor of claim 6, wherein, The optimized structural parameters are: a=40mm, B1=34mm, B2=25.66mm, B 11 =27mm, B 22 =19.7mm, h1=1.6mm, h2=1mm, y n =7.1mm; the sensor achieves dual-frequency resonance in two operating frequency bands: 2.4GHz and 3.5GHz.

8. A design method of a stacked dual-band microstrip antenna strain sensor based on chipless RFID technology, characterized by, The process includes the following steps: establishing a model of a stacked microstrip antenna including a metal ground plane, a double-layer dielectric substrate, double-layer radiating patches, and a coaxial feed port; parametrically scanning the feed point location and the size parameters of the upper and lower radiating patches in an electromagnetic simulation platform; optimizing the structural parameters based on the input impedance, return loss, and the location of the dual-frequency resonant point; and obtaining a stacked dual-frequency microstrip antenna structure that meets the requirements for dual-frequency operation and can be used for strain sensing.

9. The method of claim 9, wherein, The electromagnetic simulation platform is HFSS. By parametrically scanning the coaxial feed point position parameter yn, the lower radiating patch length B1, the lower radiating patch width B2, the upper radiating patch length B11, and the upper radiating patch width B22, the variation patterns of input resistance, input reactance, and S11 parameters under low-frequency and high-frequency modes are obtained, and the location of the dual-frequency resonant point and impedance matching state are determined accordingly. After completing the HFSS electromagnetic simulation optimization, the chipless RFID stacked dual-frequency microstrip antenna is established using the COMSOL multiphysics simulation platform. A three-dimensional model of a linear strain sensor was constructed, and the changes in the S11 curve and the resonant frequency drift of the sensor under different strain conditions were analyzed using the solid mechanics module and the electromagnetic wave frequency domain module. In the simulation, the geometric deformation of the structure under different strains was mapped to the electromagnetic module through the deformation geometry function. A linear strain from 0 με to 2700 με was applied to the sensor with an increment of 450 με. The return loss and resonant frequency of the low-frequency mode and the high-frequency mode were calculated to verify the linear relationship between the resonant frequency and the strain.

10. A test verification method of the coreless RFID laminated dual-band microstrip antenna strain sensor according to any one of claims 1 to 9, characterized in that, The strain sensor was attached to the surface of the cantilever beam near the fixed end, and resistive strain gauges were placed near the sensor as reference measuring elements. An external load was applied to the cantilever beam using a static graded loading method, causing different levels of bending strain on the beam surface. The S11 curve of the strain sensor within a set frequency sweep range was acquired using a vector network analyzer, and the corresponding resonant frequencies were extracted by identifying the positions of the low-frequency and high-frequency modal resonance valleys in the S11 curve. The strain gauge output values ​​were recorded using a strain acquisition instrument, and the correspondence between the low-frequency modal resonance frequency, the high-frequency modal resonance frequency, and the actual strain was established to complete the experimental calibration and performance verification of the strain sensor. Specifically, COMSOL multiphysics simulation yielded a low-frequency modal sensitivity of 2.45 kHz / µε, a high-frequency modal sensitivity of 3.41 kHz / µε, and a combined sensitivity of 5.86 kHz / µε. The cantilever beam loading experiment yielded a low-frequency modal sensitivity of 2.44 kHz / µε, a high-frequency modal sensitivity of 3.27 kHz / µε, and a combined sensitivity of 5.71 kHz / µε.

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