High sensitivity linearity-compensated thickness sensor based on interdigital slot structure

Abstract

The application discloses a high-sensitivity linearity compensation type thickness sensor based on an interdigital slot structure and relates to the field of microwave sensing.The thickness sensor comprises a dielectric substrate, a coplanar waveguide, an interdigital slot resonant structure and a stacked boundary structure.The surface of the dielectric substrate is provided with the coplanar waveguide, the interdigital slot resonant structure is etched on the ground belt of the coplanar waveguide, and the stacked boundary structure is located above the interdigital slot resonant structure.The stacked boundary structure comprises an insulating gasket and a metal reflection layer which are stacked from bottom to top.The high-sensitivity linearity retention type microwave resonant thickness sensor is adopted, the phenomenon that the sensitivity rapidly decays with the increase of the thickness of a sample to be measured is effectively inhibited while high sensitivity is achieved, the response linearity and uniformity of the sensor are significantly improved, the stability and reliability of measurement are maintained, and the structure is simple.

Description

Technical Field

[0001] This invention relates to the field of microwave sensing technology, and in particular to a high-sensitivity linearity-compensated thickness sensor based on an interdigitated groove structure. Background Technology

[0002] In radio frequency and microwave testing scenarios, high-precision real-time measurement of material thickness is urgently needed in industrial quality control, intelligent manufacturing, and non-destructive testing. Microwave resonant sensors, due to their high resolution, fast response, and ease of planar integration, have become an effective technological path to achieve these requirements. The basic working principle of this type of sensor is that when the material under test is placed in the sensor's resonant field, it changes its equivalent dielectric constant and electromagnetic field distribution, thereby causing changes in parameters such as resonant frequency, quality factor, or insertion loss. By detecting these changes in parameters, the material thickness can be deduced. Therefore, developing microwave sensors with high sensitivity and good response linearity has become a key requirement for current scientific research and engineering applications.

[0003] Existing microwave planar sensors mainly employ transmission line structures such as microstrip lines or coplanar waveguides. Traditional microstrip line sensors are widely used due to their simple fabrication process; however, because the signal line is located on the top layer of the dielectric substrate and the ground plane is on the bottom layer, the electromagnetic field is primarily confined within the dielectric between the signal line and the ground plane. This means that at the crucial dielectric interface (above the signal line), the electric field distribution available for coupling with the external loading medium is very weak, resulting in low sensitivity to changes in the thickness or dielectric constant of the external medium and minimal zero-point drift. To improve sensitivity, researchers have undertaken various efforts, proposing numerous sensors with high sensitivity characteristics. Aiza's team utilized a CPW structure, placing the ground plane on either side of the central conductor, increasing the exposed portion of the electromagnetic field and making it more sensitive to the influence of external loading materials. Morteza et al. introduced a defective ground structure (DGS) to concentrate the electromagnetic field energy in the slot region between the conductor and the ground strip, thereby increasing sensitivity.

[0004] Despite the significant advancements in sensitivity achieved by these structures, their sensitivity remains highly dependent on the electromagnetic field at the interface between the sensor and the measured medium. As material thickness increases, the sensing sensitivity exhibits a rapid, non-linear decline, directly limiting the sensing range of these thickness sensors. Summary of the Invention

[0005] The purpose of this invention is to provide a high-sensitivity linearity-compensated thickness sensor based on an interdigitated groove structure. While achieving high sensitivity, it effectively suppresses the phenomenon of rapid sensitivity decay as the thickness to be measured increases, significantly improving the response linearity and uniformity of the sensor, maintaining the stability and reliability of the measurement, and having a simple structure.

[0006] This invention provides a high-sensitivity linearity-compensated thickness sensor based on an interdigitated slot structure, comprising a dielectric substrate, a coplanar waveguide, an interdigitated slot resonant structure, and a stacked boundary structure. The surface of the dielectric substrate is provided with a coplanar waveguide, the interdigitated slot resonant structure is etched on the grounding band of the coplanar waveguide, and the stacked boundary structure is located above the interdigitated slot resonant structure. The stacked boundary structure includes an insulating pad and a metal reflective layer stacked from bottom to top.

[0007] Preferably, the coplanar waveguide includes a central conductor strip and grounding strips located on both sides of the central conductor strip.

[0008] Preferably, the interdigitated slot resonant structure includes interdigitated structures cascaded at equal intervals along the microwave signal propagation direction, each pair of interdigitated structures including finger-shaped metal arms extending inward from the grounding band and tortuous slots between the finger-shaped metal arms; the interdigitated structures form a defect ground structure (DGS) on the grounding band.

[0009] Preferably, the insulating pad directly covers the interdigitated slot resonant structure, and the projected area of ​​the metal reflective layer in the vertical direction covers both the interdigitated slot resonant structure and the insulating pad.

[0010] Preferably, a test area is formed between the stacked boundary structure and the dielectric substrate.

[0011] Preferably, the metal reflective layer is configured to reflect electromagnetic waves radiated from the coplanar waveguide back to the sensing area, and a quasi-closed electromagnetic coupling domain is formed between the dielectric substrate and the metal reflective layer.

[0012] Preferably, there is a signal transmission gap between the center conductor strip and the ground strip.

[0013] Preferably, the structure of the coplanar waveguide is mirror-symmetrical about the central axis of the central conductor strip.

[0014] Therefore, the present invention employs the high-sensitivity linearity-compensated thickness sensor based on the interdigitated groove structure described above, which effectively suppresses the phenomenon of rapid sensitivity decay as the thickness to be measured increases while achieving high sensitivity, significantly improving the response linearity and uniformity of the sensor, maintaining the stability and reliability of the measurement, and has a simple structure.

[0015] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the overall structure of the high-sensitivity linearity compensation thickness sensor based on the interdigitated groove structure of the present invention;

[0017] Figure 2This is a top view and parameter annotation diagram of the main body and unit interdigitated structure of the high-sensitivity linearity compensation thickness sensor based on the interdigitated groove structure of the present invention.

[0018] Figure 3 This is a schematic diagram of the assembly front view of the high-sensitivity linearity compensation thickness sensor based on the interdigitated groove structure of the present invention;

[0019] Figure 4 This is a schematic left view of the assembly of the high-sensitivity linearity compensation thickness sensor based on the interdigitated groove structure of the present invention.

[0020] Figure 5 This is a simulation diagram of the electromagnetic field distribution of the interdigitated groove resonant structure of the high-sensitivity linearity compensation thickness sensor based on the interdigitated groove structure of the present invention.

[0021] Figure 6 This is a simulation diagram comparing the S-parameters of the high-sensitivity linearity compensation thickness sensor based on the interdigitated groove structure of this invention, mounted on glass sheets of different thicknesses.

[0022] Figure Labels

[0023] 1. Dielectric substrate; 2. Coplanar waveguide; 3. Interdigitated slot resonant structure; 4. Insulating gasket; 5. Metal reflective layer; 21. Center conductor strip; 22. Grounding strip. 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] The terms "first," "second," and similar words used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0027] Example 1

[0028] like Figures 1-6As shown, this invention relates to a high-sensitivity linearity-compensated thickness sensor based on an interdigitated slot structure, comprising a dielectric substrate 1, a coplanar waveguide 2, an interdigitated slot resonant structure 3, and a stacked boundary structure. The surface of the dielectric substrate 1 is provided with the coplanar waveguide 2. The dielectric substrate 1 is made of FR4 glass fiber epoxy resin board with a relative permittivity of 4.4 and a loss tangent of 0.02. The coplanar waveguide 2 is formed by etching copper foil, and the input / output port impedance is designed to be 50Ω. The overall dimensions of the dielectric substrate 1 are: length... L =70mm, width W =30.4mm, thickness H =1mm.

[0029] The coplanar waveguide 2 includes a central conductor strip 21 and grounding strips 22 located on both sides of the central conductor strip 21, with a signal transmission gap between the central conductor strip 21 and the grounding strips 22; the structure of the coplanar waveguide 2 is mirror-symmetrical about the central axis of the central conductor strip 21. The width of the central conductor strip 21... cpww =4mm, length cpwl =30mm, the gap between the center conductor strip 21 and the two side grounding strips 22 cpwg =0.2mm. The two ends of the coplanar waveguide 2 are connected to the input port and the output port respectively, and the impedance of each is 50Ω, for use with external test instruments.

[0030] The interdigitated slot resonant structure 3 is etched onto the grounding band 22 of the coplanar waveguide 2. Each side of the structure consists of three pairs of cascaded interdigitated structures along the signal propagation direction (y-direction). Each pair of interdigitated structures consists of two sets of parallel metal finger arms and the slits between them. The length of the metal finger arm of each interdigitated structure is... fl= 7.4mm, width fw =0.2mm, the width of the interdigital groove between adjacent finger arms fg =0.2mm.

[0031] The stacked boundary structure is located above the interdigitated slot resonant structure 3, forming a test area between the stacked boundary structure and the dielectric substrate 1 to accommodate the sample under test. The stacked boundary structure includes an insulating pad 4 and a metal reflective layer 5 stacked from bottom to top to confine electromagnetic field energy and enhance penetration depth into the test medium. The insulating pad 4 directly covers the interdigitated slot resonant structure 3 for physical isolation; the projected area of ​​the metal reflective layer 5 in the vertical direction covers both the interdigitated slot resonant structure 3 and the insulating pad 4. The insulating pad 4 is made of PVC plastic sheet; the length of the insulating pad 4... materialx =40mm; Width materialy =20mm; thickness plh =0.5mm, the material of metal reflective layer 5 is aluminum block.

[0032] The metal reflective layer 5 is placed on top of the PVC insulating pad 4, with a length of materialx=40mm, width materialy =20mm, thickness alh The 10mm metal reflective layer 5 is configured to reflect electromagnetic waves radiated from the coplanar waveguide back to the sensing area, and a quasi-closed electromagnetic coupling domain is formed between the dielectric substrate 1 and the metal reflective layer 5.

[0033] The key dimensional parameters of the sensor shown in the figure are summarized in the table below:

[0034] Table 1 Summary of key dimensional parameters of the sensor

[0035]

[0036] A microwave signal is fed into the input port of the coplanar waveguide 2. Its energy excites the resonant mode of the interdigitated slot resonant structure 3 through electric field coupling, generating a distinct resonance valley in the transmission coefficient. When the sample under test (such as glass slides of different thicknesses) is placed below the metal reflective layer 5 and the PVC insulating gasket 4, the change in sample thickness modulates the local electric field in the interdigitated slot region, causing a shift in the resonant frequency. The thickness of the sample can be determined by monitoring this frequency shift.

[0037] A three-dimensional model was created using electromagnetic simulation software. Material parameters such as FR4 and the geometric dimensions listed in Table 1 were set, with a port impedance of 50Ω. The sensor was then simulated. Figure 5 As shown, the metal finger arms and slits work together, utilizing strong capacitive coupling to highly localize and concentrate microwave energy within the interdigitated slot gaps. This quasi-distributed capacitor array design is the core of achieving the high-sensitivity response of this invention. This structure significantly enhances the electric field coupling strength to changes in the thickness of the measured medium, thereby effectively improving the frequency shift amplitude caused by thickness variations.

[0038] In electromagnetic simulation software, using a glass slide as a sample, parametric scanning was performed from 0 to 2 mm in 0.5 mm intervals. The parameter curves for both cases with and without stacking boundaries were compared. Figure 6 As shown: (a) is the parameter diagram of the structure without stacked boundaries, and (b) is the parameter diagram after loading the stacked boundary structure; (a) without stacked boundaries: Due to the lack of upper boundary constraints, some electromagnetic energy is dissipated by radiation into free space, resulting in limited coupling efficiency between the electric field at the edge of the interdigitated groove and the sample under test, exhibiting a significant sensitivity saturation effect. Simulation results show that the resonant frequency shift range is narrow, with a total span of approximately 0.42 MHz, and the peak positions of the curves corresponding to each thickness are relatively close, resulting in low discrimination and making it difficult to achieve high-precision resolution.

[0039] (b) Adding Stacked Boundaries: After adding insulating pads and a metal reflective layer, the reflective block effectively reflects the upward-propagating electromagnetic waves back to the sensing area, forming a quasi-resonant cavity with concentrated energy on the sensor surface. This mechanism significantly enhances the interaction strength between the electric field and the sample under test. Simulation results show that the resonant frequency shift range is significantly expanded, with a total span of 0.7 MHz, an improvement of approximately 67% compared to the case without boundaries. The resonant curves corresponding to different thicknesses are highly differentiated and clearly spaced. Data comparison shows that the stacked boundary structure not only suppresses radiation loss by forming a closed domain, but more importantly, it significantly improves the sensor's response sensitivity to changes in medium thickness, suppresses the trend of rapid sensitivity decay with increasing thickness, effectively improves the sensor's response linearity, and enhances measurement resolution.

[0040] Therefore, this invention employs the aforementioned high-sensitivity linearity-compensated thickness sensor based on an interdigitated groove structure to achieve localized enhancement of electric field energy and high coupling strength. It can sensitively respond to minute thickness changes in the medium, meeting high-precision measurement requirements. The metal reflective layer and insulating pad constitute the upper conductor boundary, suppressing upward radiation and forming a quasi-closed coupling domain, improving the equivalent penetration depth and effective coupling energy. This alleviates the inherent problem of rapid sensitivity decay with increasing thickness in traditional planar sensors, enhancing the response capability to samples of different thicknesses. The symmetrical layout of the interdigitated grooves suppresses lateral unbalanced excitation, the stacked boundary constrains field energy and reduces environmental interference, and the insulating pad alleviates impedance abrupt changes and bonding errors, comprehensively improving sensor stability, reliability, measurement consistency, and long-term drift performance. The interdigitated groove structure optimizes sensor size and reduces manufacturing complexity. Based on standard PCB processes and FR4 material, it combines low cost and high batch consistency advantages.

[0041] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended 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 high-sensitivity linearity-compensated thickness sensor based on an interdigitated groove structure, characterized in that, It includes a dielectric substrate, a coplanar waveguide, an interdigitated slot resonant structure, and a stacked boundary structure. The surface of the dielectric substrate is provided with a coplanar waveguide. The interdigitated slot resonant structure is etched on the grounding strip of the coplanar waveguide. The stacked boundary structure is located above the interdigitated slot resonant structure. The stacked boundary structure includes an insulating pad and a metal reflective layer stacked from bottom to top.

2. The high-sensitivity linearity compensated thickness sensor based on an interdigitated groove structure according to claim 1, characterized in that, The coplanar waveguide includes a central conductor strip and ground strips located on both sides of the central conductor strip, with a signal transmission gap between the central conductor strip and the ground strip; the structure of the coplanar waveguide is mirror-symmetrical about the central axis of the central conductor strip.

3. The high-sensitivity linearity-compensated thickness sensor based on an interdigitated groove structure according to claim 1, characterized in that, The interdigitated slot resonant structure includes interdigitated structures cascaded at equal intervals along the microwave signal propagation direction. Each pair of interdigitated structures includes finger-shaped metal arms extending inward from the grounding band and tortuous slots between the finger-shaped metal arms. The interdigitated structures form a defect ground structure (DGS) on the grounding band.

4. The high-sensitivity linearity compensated thickness sensor based on an interdigitated groove structure according to claim 1, characterized in that, The insulating pad is directly covered on the interdigitated slot resonant structure, and the projected area of ​​the metal reflective layer in the vertical direction covers both the interdigitated slot resonant structure and the insulating pad.

5. The high-sensitivity linearity-compensated thickness sensor based on an interdigitated groove structure according to claim 1, characterized in that, A test area is formed between the stacked boundary structure and the dielectric substrate.

6. The high-sensitivity linearity-compensated thickness sensor based on an interdigitated groove structure according to claim 1, characterized in that, The metal reflective layer is configured to reflect electromagnetic waves radiated from the coplanar waveguide back to the sensing area, and a quasi-closed electromagnetic coupling domain is formed between the dielectric substrate and the metal reflective layer.

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