Differential microwave thickness sensor based on microstrip ring dual-mode resonator

Abstract

The application provides a differential microwave thickness sensor based on a microstrip ring dual-mode resonator, belongs to the technical field of microwave sensing, and comprises a dielectric substrate, a metal circuit layer and a metal ground layer; the metal circuit layer is integrated with a ring dual-mode resonator, a first T-shaped microstrip coupling structure, a second T-shaped microstrip coupling structure and a step impedance perturbation structure; a to-be-measured substance loading area is defined directly above the step impedance perturbation structure; the first and second T-shaped microstrip coupling structures are electromagnetically coupled with the ring dual-mode resonator through coupling gaps; the step impedance perturbation structure is integrally formed on the circumferential edge of the ring dual-mode resonator, and the step impedance perturbation structure is a microstrip line segment with an impedance mutation relative to the microstrip line constituting the ring dual-mode resonator. The differential microwave thickness sensor based on the microstrip ring dual-mode resonator is adopted, and the problems of complex structure and reduced sensing precision of the existing multi-resonator layout differential sensor are solved.

Description

Technical Field

[0001] This invention relates to the field of microwave sensing technology, and in particular to a differential microwave thickness sensor based on a microstrip ring dual-mode resonator. Background Technology

[0002] In modern industrial production, materials science, biomedicine, and electronics manufacturing, non-destructive, high-precision, real-time online measurement of the thickness and dielectric constant of transparent and non-transparent dielectric materials is a core technological requirement for product quality control and material performance characterization. Microwave resonant sensors, with their advantages of simple structure, low manufacturing cost, ease of planar integration, and good penetration capability into non-transparent media, have become one of the mainstream measurement methods in this field. Their core working principle is as follows: when the material under test is applied near a microwave resonant structure, it changes the local electromagnetic field distribution and equivalent dielectric parameters of the resonant region, thereby inducing a regular change in the resonant frequency or transmission characteristics. By detecting this change, parameters such as material thickness and dielectric constant can be retrieved.

[0003] However, existing microwave resonant thickness sensors still have many technical limitations, making it difficult to meet the high-precision measurement requirements under complex working conditions. Firstly, traditional single-resonator microwave sensors are highly sensitive to environmental factors. Even small fluctuations in ambient temperature and humidity can cause a drift in the dielectric constant of the sensor's substrate, leading to a non-targeted shift in the resonant frequency. This environment-induced frequency shift superimposed on the signal caused by changes in material thickness, introducing significant systematic measurement errors and drastically reducing the stability, repeatability, and accuracy of the measurement results. Secondly, to suppress environmental interference, existing technologies have proposed multi-resonator differential sensing improvements, using the frequency difference between two or more independent resonators to cancel out environmental common-mode interference. While these solutions offer advantages, they also require multiple physically independent resonators. This significantly increases the overall circuit footprint, reduces sensor integration, and makes them unsuitable for miniaturized planar applications. Furthermore, in miniaturized layouts, parasitic electromagnetic coupling easily occurs between independent resonators, leading to problems such as mode traction and frequency aliasing, resulting in sensor signal distortion. Even if the coupling effect is compensated by increasing the linewidth, it further increases the complexity of the structural design and manufacturing costs. Thirdly, multi-resonator differential solutions have stringent requirements for the physical dimensions and performance consistency of each resonator. Even small tolerances during the manufacturing process can lead to system performance degradation, resulting in low sensor yield and making it difficult to achieve large-scale mass production and engineering applications.

[0004] In summary, the industry urgently needs a microwave thickness sensing technology that is compact, highly integrated, environmentally robust, and can fundamentally avoid multi-resonator coupling problems and be compatible with standardized manufacturing processes, in order to solve many pain points of existing solutions in terms of measurement accuracy, environmental adaptability, structural design, and engineering implementation. Summary of the Invention

[0005] The purpose of this invention is to provide a differential microwave thickness sensor based on a microstrip ring dual-mode resonator, which solves the problem that existing multi-resonator differential sensors have complex structures and are prone to decreased sensing accuracy due to coupling between resonators.

[0006] To achieve the above objectives, the present invention provides a differential microwave thickness sensor based on a microstrip ring dual-mode resonator, comprising a dielectric substrate, a metal circuit layer disposed on the upper layer of the dielectric substrate, and a metal ground layer disposed on the lower surface of the dielectric substrate. The metal circuit layer integrates a ring dual-mode resonator, a first T-type microstrip coupling structure, a second T-type microstrip coupling structure, and a step impedance perturbation structure. The area of ​​the object under test is defined directly above the step impedance perturbation structure. The first T-type microstrip coupling structure and the second T-type microstrip coupling structure are electromagnetically coupled to the ring dual-mode resonator through coupling gaps. The step impedance perturbation structure is integrally formed on the circumferential edge of the ring dual-mode resonator, and the step impedance perturbation structure is a microstrip line segment that forms an impedance abrupt change relative to the microstrip line constituting the ring dual-mode resonator.

[0007] Preferably, the first T-type microstrip coupling structure and the second T-type microstrip coupling structure are both arranged on adjacent sides of the ring dual-mode resonator, and the two are orthogonally arranged in terms of electrical characteristics, with an equivalent electrical angle difference of 90°.

[0008] Preferably, the first T-type microstrip coupling structure and the second T-type microstrip coupling structure are of the same size, and both include a main microstrip line and a branch microstrip line that are perpendicular to each other and integrally connected; the end of the main microstrip line away from the branch microstrip line is the input / output port of the microwave signal, the branch microstrip line is parallel to the edge of the ring dual-mode resonator, and the coupling gap between the two is an equal-width coupling gap.

[0009] Preferably, the step impedance perturbation structure is disposed on the circumference of the ring dual-mode resonator, and the step impedance perturbation structure is arranged at a 45° symmetrical position with respect to the first T-type microstrip coupling structure and the second T-type microstrip coupling structure; the step impedance perturbation structure is a low-impedance step microstrip line segment, which forms an impedance change with the ring dual-mode resonator by increasing the microstrip line width, and introduces a local capacitive perturbation inside the ring dual-mode resonator.

[0010] Preferably, the linewidth of the microstrip line in the step impedance perturbation structure is greater than the linewidth of the microstrip line constituting the main body of the ring dual-mode resonator. The loading region of the test object is the region directly above the step impedance perturbation structure. The step impedance perturbation structure couples with the local electromagnetic field at the test material, so that the change in the thickness of the test material can modulate the equivalent dielectric parameters and distributed capacitance of the loading region of the test object.

[0011] Preferably, when the step impedance perturbation structure is loaded onto the material under test, it causes the degenerate modes to split and forms two mutually separated transmission zeros in the frequency response of the transmission coefficient. The frequency difference between the two transmission zeros is used as the differential output of the thickness measurement. The sensor achieves differential sensing only through a single ring dual-mode resonator.

[0012] Preferably, the ring dual-mode resonator, the first T-type microstrip coupling structure, the second T-type microstrip coupling structure, and the step impedance perturbation structure are integrally formed on the metal circuit layer by a printed circuit board etching process, and both the metal circuit layer and the metal ground layer are copper layers.

[0013] Therefore, the present invention employs the aforementioned differential microwave thickness sensor based on a microstrip ring dual-mode resonator, and the technical effects are as follows:

[0014] 1. Achieve high-precision and stable measurement without compensation: The substrate dielectric constant drift caused by environmental temperature and humidity fluctuations is a common-mode interference, which will only cause the two transmission zeros of the sensor to shift in the same direction and in sync, and the frequency difference between the two can remain highly stable; this technology uses the frequency difference between the two transmission zeros as the differential output of thickness measurement, which can directly offset the systematic error caused by environmental factors, without the need to add an additional environmental compensation circuit.

[0015] 2. Improved integration and elimination of coupling errors at the source: Dual-mode differential sensing is achieved using a single microstrip ring dual-mode resonator, eliminating the need for an additional reference resonator, significantly reducing the circuit footprint, and greatly improving the sensor's integration. This is particularly suitable for miniaturized planar applications with strict size requirements. At the same time, the parasitic electromagnetic coupling effect caused by the close spacing of resonators in traditional multi-resonator schemes is completely eliminated from the physical structure, avoiding sensor signal distortion caused by mode pulling and frequency aliasing, ensuring the purity and independence of the sensor signal, and effectively improving the accuracy and repeatability of thickness measurement.

[0016] 3. Reduced costs and improved engineering feasibility: The sensor is designed based on the standard planar microwave transmission line theory, without the requirements of three-dimensional stacking or special irregular shape processing. Its manufacturing process is fully compatible with mature PCB etching process, resulting in low processing costs. At the same time, it abandons the stringent requirement of high consistency of multiple physical units in the multi-resonator scheme, which greatly reduces the impact of processing tolerance on system performance, greatly improves the yield of the sensor, and facilitates large-scale mass production. Attached Figure Description

[0017] Figure 1 This is a three-dimensional structural schematic diagram of an embodiment of the present invention;

[0018] Figure 2 This is a schematic diagram of the planar structure and parameters of an embodiment of the present invention;

[0019] Figure 3 This is a simulation diagram of S-parameters under different thicknesses of glass sheets in an embodiment of the present invention;

[0020] Figure 4 This is a fitting curve of the zero-point frequency difference as a function of thickness in an embodiment of the present invention.

[0021] Figure 5 The following are simulation diagrams of S-parameters under different ambient humidity conditions according to an embodiment of the present invention.

[0022] Figure Labels

[0023] 1. Dielectric substrate; 2. Ring dual-mode resonator; 3. First T-type microstrip coupling structure; 4. Second T-type microstrip coupling structure; 5. Step impedance perturbation structure; 6. Loading region of the test object; 7. Metal grounding layer. Detailed Implementation

[0024] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0025] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0026] Example 1

[0027] like Figure 1 As shown, this invention provides a differential microwave thickness sensor based on a microstrip ring dual-mode resonator. This sensor integrates key components such as a dielectric substrate 1, a ring dual-mode resonator 2, a first T-type microstrip coupling structure 3, a second T-type microstrip coupling structure 4, a step impedance perturbation structure 5, a test object loading region 6, and a metal ground layer 7. The ring dual-mode resonator 2, the first T-type microstrip coupling structure 3, and the second T-type microstrip coupling structure 4 are all arranged on the upper metal circuit layer of the dielectric substrate 1 and precisely implemented using microstrip lines to ensure efficient and stable signal transmission.

[0028] The first T-shaped microstrip coupling structure 3 and the second T-shaped microstrip coupling structure 4 are symmetrically distributed along the adjacent sides of the ring dual-mode resonator. They are electrically strictly orthogonal, with an equivalent electrical angle difference of approximately 90°. This layout not only optimizes space utilization but also ensures that microwave signals can be efficiently electromagnetically coupled to the ring dual-mode resonator through the gap, thereby achieving precise input and output of microwave signals.

[0029] The step impedance perturbation structure 5 is positioned circumferentially on the ring dual-mode resonator, forming a 45° symmetrical angle with respect to the first T-type microstrip coupling structure 3 and the second T-type microstrip coupling structure 4. This structure, by increasing the microstrip line width, creates a step impedance microstrip segment with a lower characteristic impedance, thereby introducing a local capacitive perturbation within the ring dual-mode resonator. This perturbation design significantly improves the sensor's sensitivity to thickness variations, laying the foundation for subsequent high-precision measurements.

[0030] The material under test (DUT) is directly loaded above a low-impedance step-impedance microstrip line segment, enabling it to couple with the local electromagnetic field at that segment. When the thickness of the DUT changes, the equivalent dielectric constant and distributed capacitance of the region alter, introducing a controlled asymmetric perturbation into the originally symmetrical ring-type dual-mode resonator structure. This perturbation causes the originally degenerate modes to split, manifesting as a single transmission zero splitting into two separate transmission zeros in the frequency response of the transmission coefficient. Changes in the external environment simultaneously affect these two transmission zeros, causing their frequency positions to drift in the same direction and by approximately equal amounts. This common-mode effect can be effectively suppressed by differential measurement techniques. In contrast, the asymmetric perturbation introduced by changes in the DUT thickness primarily alters the relative splitting degree of the two modes; this differential effect becomes the key basis for our thickness measurement. By accurately extracting the frequency positions of these two transmission zeros and defining the frequency difference as the differential output, a mapping relationship between the DUT and the DUT thickness is established.

[0031] In such Figure 2 The specific implementation process shown is that the sensor uses FR4 glass fiber epoxy resin board as dielectric substrate 1, which has a relative permittivity of 4.4, a loss tangent of 0.02, a substrate thickness of 0.8mm, and dimensions of 70mm in both the x and y directions, ensuring good mechanical stability and electrical performance.

[0032] The metal circuit layer and the metal ground layer 7 are made of copper, which has good conductivity and processing performance. The ring dual-mode resonator, the first T-type microstrip coupling structure 3, the second T-type microstrip coupling structure 4, and the step impedance perturbation structure 5 are all integrated on this metal circuit layer and integrally formed by printed circuit board (PCB) etching process, which ensures the consistency of the structure and the processing accuracy.

[0033] The ring dual-mode resonator is designed as a square ring structure, with the outer side length L of the outer square ring. r The linewidth W of the toroidal microstrip line is 22mm. r The precision control is 0.3 mm. Without introducing any perturbations, this ring dual-mode resonator exhibits excellent geometric symmetry and can effectively excite degenerate modes under orthogonal coupling excitation, providing a stable foundation for subsequent differential sensing.

[0034] The first T-shaped microstrip coupling structure 3 and the second T-shaped microstrip coupling structure 4 are cleverly arranged on two adjacent sides of the ring dual-mode resonator. They are electrically strictly orthogonal, with an equivalent electrical angle difference of approximately 90°, ensuring efficient signal transmission and coupling. Each T-shaped microstrip coupling structure includes a main microstrip line and branch microstrip lines, where the length of the main microstrip line is L. t It is 23.5mm, and the line width is W. t The length of the branched microstrip line is 3.6 mm; L c 10mm, line width W c The width is 0.25mm. The branched microstrip line is connected to the ring dual-mode resonator via a distance W. w A tight electromagnetic coupling is achieved with a coupling gap of 0.25mm, which serves as the input and output ports for microwave signals, respectively. The characteristic impedance of each port is carefully designed to reach 50Ω to meet the system matching requirements.

[0035] The step impedance perturbation structure 5, one of the key innovations of this embodiment, is positioned circumferentially on the ring dual-mode resonator, at a 45° symmetrical angle with respect to the first T-type microstrip coupling structure 3 and the second T-type microstrip coupling structure 4. This structure is implemented using a low-impedance step microstrip line segment with a linewidth W. l It is 0.5mm, and the length is L. l The thickness is 6 mm. Since the linewidth of this microstrip segment is significantly larger than that of the microstrip line constituting the ring dual-mode resonator, its characteristic impedance is relatively low. This allows for the successful introduction of local capacitive perturbations within the ring dual-mode resonator, forming an electromagnetically sensitive region and enabling high-precision thickness measurement.

[0036] The test object loading region 6 is set above the low impedance step microstrip line segment, and the test material is placed directly in this region so that it couples with the local electromagnetic field at the step impedance perturbation structure 5.

[0037] like Figure 3 As shown, with changes in the thickness of the material under test, the equivalent dielectric constant and distributed capacitance of the sensitive region change, thereby introducing controlled asymmetric perturbations into the originally symmetrical ring-type dual-mode resonator structure, causing the originally degenerate modes to split. In the frequency response of the transmission coefficient, this split manifests as a single transmission zero splitting into two separate transmission zeros.

[0038] This invention uses the frequency difference between two transmission zeros as the differential output to extract the corresponding frequencies of the two transmission zeros. and And define the differential output as:

[0039] .

[0040] Differential output It is insensitive to disturbances such as test link drift and changes in the equivalent dielectric parameters of the substrate, which significantly reduces the dependence on absolute drift at a single frequency point during the thickness inversion process.

[0041] At the same time, both transmission zeros are stable and identifiable feature points in the response curve, which facilitates high-resolution frequency point extraction, thereby reducing the thickness inversion error introduced by reading error and improving measurement accuracy and repeatability.

[0042] like Figure 3 As shown, the transmission coefficient varies under conditions of loading glass sheets of different thicknesses. Two transmission zeros in the response and The positions separate significantly with increasing thickness. For example... Figure 4 As shown, the frequency difference data extracted above... Linear fitting analysis was performed on the thickness H of the glass sheet.

[0043] The experimental data are shown in Table 1 below:

[0044] As shown in Table 1, the zero-point frequency difference is within the thickness range of 0.1 mm to 0.7 mm for the tested material. The frequency difference increases monotonically with the thickness of the material under test, from 0.92 GHz to 1.3 GHz, and exhibits good approximate linear characteristics, thus allowing the establishment of a zero-point frequency difference. The mapping relationship between the thickness of the material and the thickness of the material under test enables differential inversion measurement of the thickness.

[0045] Table 1 Frequency Difference Data Linear fitting data analysis of glass sheet thickness H

[0046]

[0047] To verify the robustness of the differential output of this invention to changes in ambient humidity, this embodiment refers to Paul's findings on the influence of humidity on the dielectric parameters of FR4, and equates humidity changes to the relative permittivity of the substrate. The drift, while maintaining the thickness of the material being measured. With mm constant, electromagnetic simulations were performed for five different operating conditions to obtain the corresponding transmission coefficients. A family of frequency response curves, among which As a standard value. Figure 5 As shown, the two transmission zero frequencies change when the substrate dielectric constant varies. and Drift will occur, but the frequency difference will be... The changes are relatively small, extracted based on simulation results. The data is shown in Table 2.

[0048] Table 2 Extracted from simulation results data

[0049]

[0050] Based on the formula for calculating relative error:

[0051] ;

[0052] In the formula For the zero-point frequency difference under simulated conditions, This represents the zero-point frequency difference under standard environmental conditions. Calculations show that even when the substrate dielectric constant varies within the range of 4.2–4.6, simulating different environmental humidity conditions, The maximum relative error compared to the standard value is only about 1.24%. Within the normal fluctuation range of 4.4 to 4.5, the error is even lower, as low as 0.93%. These data strongly demonstrate that although changes in ambient humidity cause drastic drift in the sensor's resonant frequency, this invention effectively cancels out this interference using a dual-mode differential structure, resulting in a stable output. It maintained extremely high stability. This indicates that the sensor has excellent environmental robustness, enabling high-precision and high-reliability measurement of the thickness of the object under test without the need for complex environmental compensation circuitry.

[0053] Therefore, the present invention adopts the above-mentioned differential microwave thickness sensor based on a microstrip ring dual-mode resonator. The dielectric substrate is made of FR4 glass fiber epoxy resin, and the metal circuit layer and metal ground layer are copper layers. Through the precisely designed ring dual-mode resonator, the orthogonally arranged first and second T-type microstrip coupling structures, and the step impedance perturbation structure, high-precision differential measurement of material thickness and resistance to environmental interference are achieved. It has the advantages of compact structure, high integration, low manufacturing cost and easy mass production.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A differential microwave thickness sensor based on a microstrip ring dual-mode resonator, characterized in that, It includes a dielectric substrate, a metal circuit layer disposed on the upper layer of the dielectric substrate, and a metal ground layer disposed on the lower surface of the dielectric substrate. The metal circuit layer integrates a ring dual-mode resonator, a first T-type microstrip coupling structure, a second T-type microstrip coupling structure, and a step impedance perturbation structure. The loading area of ​​the test object is defined directly above the step impedance perturbation structure. Both the first T-type microstrip coupling structure and the second T-type microstrip coupling structure form electromagnetic coupling with the ring dual-mode resonator through coupling gaps; the step impedance perturbation structure is integrally formed on the circumferential edge of the ring dual-mode resonator, and the step impedance perturbation structure is a microstrip line segment that forms an impedance abrupt change relative to the microstrip line constituting the ring dual-mode resonator.

2. The differential microwave thickness sensor based on a microstrip ring dual-mode resonator according to claim 1, characterized in that, The first T-type microstrip coupling structure and the second T-type microstrip coupling structure are both arranged on adjacent sides of the ring dual-mode resonator. They are orthogonal in electrical characteristics and their equivalent electrical angles differ by 90°.

3. The differential microwave thickness sensor based on a microstrip ring dual-mode resonator according to claim 1, characterized in that, The first T-type microstrip coupling structure and the second T-type microstrip coupling structure are of the same size. Both include a main microstrip line and a branch microstrip line that are perpendicular to each other and integrally connected. The end of the main microstrip line away from the branch microstrip line is the input / output port of the microwave signal. The branch microstrip line is parallel to the edge of the ring dual-mode resonator. The coupling gap between the two is an equal-width coupling gap.

4. The differential microwave thickness sensor based on a microstrip ring dual-mode resonator according to claim 1, characterized in that, The step impedance perturbation structure is set on the circumference of the ring dual-mode resonator, and the step impedance perturbation structure is arranged at a 45° symmetrical position with respect to the first T-type microstrip coupling structure and the second T-type microstrip coupling structure. The step impedance perturbation structure is a low impedance step microstrip line segment, which forms an impedance change with the ring dual-mode resonator by increasing the microstrip line width, and introduces a local capacitive perturbation inside the ring dual-mode resonator.

5. The differential microwave thickness sensor based on a microstrip ring dual-mode resonator according to claim 1, characterized in that, The microstrip linewidth of the step impedance perturbation structure is larger than that of the microstrip linewidth constituting the main body of the ring dual-mode resonator. The loading region of the test object is the region directly above the step impedance perturbation structure. The step impedance perturbation structure couples with the local electromagnetic field at the test material, so that the thickness change of the test material can modulate the equivalent dielectric parameter and distributed capacitance of the loading region of the test object.

6. The differential microwave thickness sensor based on a microstrip ring dual-mode resonator according to claim 5, characterized in that, When the step impedance perturbation structure is loaded onto the material under test, it causes the degenerate modes to split and forms two mutually separated transmission zeros in the frequency response of the transmission coefficient. The frequency difference between the two transmission zeros is used as the differential output for thickness measurement. The sensor achieves differential sensing only through a single ring dual-mode resonator.

7. The differential microwave thickness sensor based on a microstrip ring dual-mode resonator according to claim 1, characterized in that, The ring dual-mode resonator, the first T-type microstrip coupling structure, the second T-type microstrip coupling structure, and the step impedance perturbation structure are integrally formed on the metal circuit layer through a printed circuit board etching process. Both the metal circuit layer and the metal ground layer are copper layers.

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