Mechanical reconfigurable spiral frequency selective surface with metal resonance unit cell on-off configuration

Abstract

The invention relates to the field of frequency selective surfaces, in particular to a metal resonance unit cell on-off configuration mechanical reconfigurable spiral frequency selective surface which comprises a substrate and metal unit cells adhered to the surface of the substrate, and the metal unit cells are periodically arranged on the substrate; the metal unit cell is a combined layer formed by stacking an upper metal layer and a lower insulating substrate; each metal unit cell is based on Archimedes double helix, two spiral lines are wound mutually and stretched outwards by a set width with the spiral lines as the center lines, the two spiral structures do not make contact with each other, and the two ends of each spiral structure are semicircular. The semicircle is adhered to the substrate; and then through mechanical uniaxial stretching, the Archimedes double-helix structure generates out-of-plane buckling deformation, the two groups of helix structures are in contact with each other, metal is in contact with each other to form a path, and the on-off effect is achieved. According to the three-dimensional spiral frequency selective surface, the on-off configuration of the metal resonance unit cells is changed through mechanical tensile deformation, and the electromagnetic wave transmission frequency is actively and greatly regulated and controlled.

Description

Technical Field

[0001] The present invention relates to the field of frequency selective surfaces, and specifically to a three-dimensional spiral frequency selective surface with a mechanically reconfigurable on-off configuration of a metal resonant unit cell. The on-off configuration of the metal resonant unit cell is changed by mechanical stretching and deformation, thereby actively and significantly regulating the transmission frequency of electromagnetic waves. Technical Background

[0002] Frequency selective surfaces (FSSs) are infinitely large, planar structures composed of periodically arranged metal patches or slots. They exhibit band-stop or band-pass filtering properties, selectively transmitting electromagnetic waves based on their frequency, polarization, or angle of incidence. Traditional FSSs, characterized by fixed electromagnetic properties and rigidity, have limited adaptability to complex electromagnetic environments and surfaces, making them difficult to meet the demands of diverse engineering applications. Consequently, research on FSSs that can modify the transmission characteristics of electromagnetic waves has attracted widespread attention.

[0003] The current common solution is the active frequency selective surface, which is usually electrically adjustable. The electromagnetic wave transmission characteristics are adjusted by loading active devices, and the resistance state of the structure is changed by controlling external excitation to achieve changes in the electromagnetic wave transmission characteristics. However, this method requires additional metal feeder design, which increases the design difficulty, and the impact of the metal feeder on the electromagnetic performance needs to be considered. In order to reduce or eliminate the design difficulties and adverse effects brought by the metal feeder, variable resistors such as photoresistors and thermistors can be introduced to effectively avoid the problems brought by the metal feeder. However, external excitation is still required, and internal light sources, heat sources and other structures need to be designed, which brings new problems to the design of the frequency selective surface. The frequency selective surface regulated by mechanical tensile deformation is different from the above-mentioned active frequency selective surface. It does not require external metal feeders and external active stimulation physical fields. It has a simple structure and convenient design, which has attracted the attention of researchers.

[0004] Frequency selective surfaces (FSSs) regulated by mechanical stretching are flexible and stretchable FSSs that can be conformally attached to non-developable surfaces. They actively control electromagnetic wave transmission performance through mechanical stretching. However, mechanical deformation generally alters the period, spacing, and other arrangements of the FSS without directly changing the unit cell configuration of the resonant metal units, limiting the tuning range and design margin. Summary of the Invention

[0005] In response to the problems of the prior art, the present invention realizes a three-dimensional spiral frequency selective surface with adjustable resonant frequency by mechanically stretching to change the on-off configuration of a metal resonant unit cell.

[0006] The present invention can make the structure produce out-of-plane buckling deformation through mechanical uniaxial stretching. Compared with the planar frequency selective surface structure, this three-dimensional frequency selective surface has an additional degree of design freedom, which can better construct the resonant cavity and achieve angular stability and filtering response with high selection performance.

[0007] On the other hand, by rationally designing the unit cell structure of a three-dimensional frequency selective surface, buckling deformation can be used to achieve contact control at specific points within the unit cell, altering the path of the surface current excited by the metal resonant unit cell. Mechanical stretching achieves the effect of a diode controlling the on / off state of a circuit, actively and significantly regulating the resonant frequency of electromagnetic wave transmission on the frequency selective surface. Using mechanical stretching, a simple and easy-to-use active control method, to adjust the electromagnetic wave transmission characteristics of a three-dimensional spiral frequency selective surface has potential applications in electromagnetic shielding, electromagnetic stealth, and other fields.

[0008] The technical solution adopted by the present invention is as follows:

[0009] A spiral frequency selective surface with a mechanically reconfigurable on-off configuration of a metal resonant unit cell includes a substrate made of flexible silicone Ecoflex material and metal unit cells adhered to the surface of the substrate. The metal unit cells are periodically arranged on the substrate. The metal unit cells are divided into an upper metal layer and a lower insulating layer according to the shape of the components. Specifically, the metal unit cells are formed by stacking a Cu layer and a PI layer to form a Cu / PI composite layer, with the Cu layer above the PI layer.

[0010] The shape of the metal unit cell is inspired by the traditional Archimedean double helix and three-dimensional buckled dipole frequency selective surfaces. Each metal unit cell is a partially bonded Archimedean double helix structure. Specifically, based on the Archimedean double helix, two intertwined helices are drawn and adjusted to a suitable shape. These helices are then stretched outward a certain width from the centerline to ensure they do not touch each other, forming a two-dimensional double helix structure with semicircular ends.

[0011] Only the two semicircular ends of each spiral structure are bonded to the substrate. The Cu layer and the PI layer have the same planar shape and completely overlap when bonded.

[0012] The spiral frequency selective surface can be subjected to mechanical uniaxial stretching to produce out-of-plane buckling deformation, which increases the x-direction length and z-direction deflection of the Cu layer (adding a degree of design freedom in the z-direction, which is more conducive to constructing a resonant cavity, achieving high-performance filtering effects and structural miniaturization). This changes the structural shape and size of the unit cell. This change allows the resonant frequency and other electromagnetic wave transmission properties to be actively and continuously controlled when exposed to electromagnetic waves.

[0013] As the applied strain increases, the unit cell period length of the spiral frequency selective surface gradually increases, the Cu layer undergoes three-dimensional buckling, and the height continues to increase. When the strain is greater than a certain value, the two sets of spiral structures contact each other, and the metal contact forms a pathway, achieving a diode-like on-off effect through mechanical stretching.

[0014] Mechanical simulations were performed using the commercial software ABAQUS. The upper and lower layers of the metal unit cell are both thin metal sheets, and the substrate thickness is much greater than that of the metal unit cell. Therefore, the metal unit cell can be approximated as a shell in computer modeling and simulation. Finite element models were constructed using shell elements (S4R) and three-dimensional solid elements (C3D8R) for the Cu / PI composite layer and substrate, respectively, to improve computational efficiency and accuracy. When the spiral frequency selective surface was subjected to uniaxial tensile strain, the maximum principal strain distribution and three-dimensional buckling configuration of the Cu / PI composite layer were determined by applying the corresponding strain to the substrate.

[0015] The commercial electromagnetic simulation software CST was used to model the spiral frequency selective surface and simulate its projection performance. For each uniaxial tensile strain, the commercial mechanical simulation software ABAQUS was first used to simulate and obtain the deformed Cu / PI composite layer and substrate shape. The deformed mesh units were sequentially imported into HyperMesh (finite element software for pre-processing and post-processing) to obtain the corresponding geometric configuration. The geometric configurations under different uniaxial tensile strains were used to import into CST for further simulation. Secondly, the frequency domain analysis model was used for electromagnetic simulation, and the mesh units of the spiral frequency selective surface were divided using hexahedral units. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic diagram of the geometrical dimensions of a unit cell of a spiral frequency selective surface according to the present invention;

[0017] Figure 2 This is a schematic structural diagram of a cross-sectional view of a spiral frequency selective surface of the present invention;

[0018] Figure 3 This is the frequency-selective surface optical image before uniaxial stretching of the present invention;

[0019] Figure 4 This is an optical diagram of a spiral frequency selective surface that undergoes three-dimensional buckling after uniaxial stretching according to the present invention;

[0020] Figure 5 The three-dimensional buckling morphology and maximum principal strain of the Cu / PI composite layer under uniaxial tensile strain obtained by simulation in the present invention;

[0021] Figure 6 The variation trend of the maximum deflection of Cu in the z-axis direction with uniaxial tensile strain in the present invention is shown in FIG.

[0022] Figure 7 is the electromagnetic wave transmittance obtained through simulation when the present invention is subjected to uniaxial tensile strain;

[0023] Figure 8 The change trend of the resonant frequency of the present invention with the increase of uniaxial tensile strain;

[0024] Figure 9 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the substrate when the substrate thickness t3 is 1 mm.

[0025] Figure 10 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the substrate when the substrate thickness t3 is 2 mm in the present invention;

[0026] Figure 11 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the substrate when the substrate thickness t3 is 3 mm in the present invention;

[0027] Figure 12 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the substrate when the substrate thickness t3 is 4 mm in the present invention;

[0028] Figure 13 The changing trend of the influence of 60% uniaxial tensile strain on the resonant frequency regulation of the spiral frequency selective surface of the present invention;

[0029] Figure 14 This is the effect of 60% uniaxial tensile strain on its electromagnetic wave transmittance when the length of R1 is 0.8 mm;

[0030] Figure 15 This is the effect of 60% uniaxial tensile strain on its electromagnetic wave transmittance when the length of R1 is 1.12 mm;

[0031] Figure 16 This is the effect of 60% uniaxial tensile strain on its electromagnetic wave transmittance when the length of R1 is 1.44 mm;

[0032] Figure 17 This is the effect of 60% uniaxial tensile strain on its electromagnetic wave transmittance when the length of R1 is 1.76 mm;

[0033] Figure 18 is the electromagnetic wave transmittance obtained by electromagnetic simulation when the present invention is subjected to uniaxial tensile strain;

[0034] Figure 19 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the present invention when the x-direction gap d1 is 2 mm;

[0035] Figure 20The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the present invention when the x-direction gap d1 is 4 mm;

[0036] Figure 21 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the present invention when the x-direction gap d1 is 6 mm;

[0037] Figure 22 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the present invention when the x-direction gap d1 is 8 mm;

[0038] Figure 23 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the present invention when the y-direction gap d2 is 2 mm;

[0039] Figure 24 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the present invention when the y-direction gap d2 is 3 mm;

[0040] Figure 25 The effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance of the present invention when the y-direction gap d2 is 4 mm;

[0041] Figure 26 This is the effect of 60% uniaxial tensile strain on the electromagnetic wave transmittance when the y-direction gap d2 is 5 mm. DETAILED DESCRIPTION

[0042] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0043] like Figure 1 The spiral frequency selective surface is composed of a flexible silicone Ecoflex substrate and periodically arranged metal units attached to its surface. The metal units are divided into two layers according to the shape of the components. The geometric shape is as follows Figure 1 As shown, the upper Archimedean double helix structure is made of Cu layer and PI layer, which is partially attached to the lower substrate Ecoflex. Figure 1 Shown in the red area.

[0044] The geometric dimensions are described below. This geometric dimension sample is only an example. The present invention protects frequency selective surfaces with different geometric dimensions. The unit cell radius R1 = 0.8 mm, R2 = 1.6 mm, width w = 0.344 mm, gap d1 = 2 mm, d2 = 2 mm. The cross-sectional view of the spiral frequency selective surface is shown in FIG. Figure 2 As shown, the Cu layer thickness t1 = 0.018 mm, the PI thickness t2 = 0.0225 mm, and the Ecoflex thickness t3 = 3 mm.

[0045] like Figure 3 and Figure 4 , showing optical images of the spiral frequency selective surface before and after uniaxial stretching. The area where the metal unit cell of the spiral frequency selective surface is locally bonded is a semicircle with a diameter of only 0.344mm, which requires a very high level of process precision. The specific preparation process of the spiral frequency selective surface is described as follows:

[0046] (1) Preparation of metal unit cells. Use a programmable UV laser cutting machine to cut the commercial PI copper film into the desired spiral shape;

[0047] (2) Pick up the metal unit cells. In order to maintain the periodicity of the unit cells, use water-soluble tape (WST) (AQUASOL) to pick up the periodically arranged metal unit cells at one time;

[0048] (3) Prepare Ecoflex substrate. Mix Ecoflex 00-30 (Smooth-On) A / B components at a ratio of 1:1, use a deaerator to mix evenly, remove bubbles, and pour into an acrylic mold for curing.

[0049] (4) Bonding the metal unit cells. To apply the commercial glue (Dinglifeng) only to the connection area, the commercial PI copper film was cut into a mask again and attached to the surface of the periodically arranged metal unit cells fixed with water-soluble tape. After applying a layer of glue, the mask was removed. The metal unit cells were partially attached to the Ecoflex substrate, and then the water-soluble tape was rinsed with deionized water to remove it.

[0050] like Figure 5 , showing the finite element analysis results when the uniaxial tensile strain applied by the spiral frequency selective surface is 0%, 9.8%, 21.3%, 30%, 40%, 50%, 60%, and 66.8%, respectively. The commercial software ABAQUS was used for mechanical simulation to simulate the tensile deformation process of the sample, and the configuration of the sample after stretching was obtained in turn. Due to the huge difference in thickness between the Cu / PI composite layer and the Ecoflex layer, in order to improve the calculation efficiency and accuracy, the Cu / PI composite layer and the Ecoflex substrate were respectively established with shell elements (S4R) and three-dimensional solid elements (C3D8R) to establish finite element models. Ecoflex, PI and Cu all use linear elastic constitutive relations, and the elastic modulus E and Poisson's ratio ν are described as follows: E Ecoflex =0.06MPa,ν Ecoflex =0.49; E PI =2.5GPa,ν PI =0.34; E Cu =119GPa,ν Cu = 0.34. In order to ensure the accuracy of the calculation, the convergence of the grid size was tested.

[0051] As the applied strain increases, the unit cell period length of the spiral FSS gradually increases, and the Cu undergoes three-dimensional buckling, increasing in height. Finite element analysis results show that the maximum principal strain of Cu is always less than 0.9%, providing a theoretical guarantee that the FSS can withstand repeated stretching without compromising its service life. When the uniaxial tensile strain reaches 66.8%, the two Cu Archimedean spirals symmetrical along the y-axis of the unit cell of the spiral FSS are about to come into contact.

[0052] like Figure 6 To quantitatively demonstrate the effect of uniaxial tensile strain on the deformation morphology of Cu, the figure shows the variation of the maximum deflection of Cu along the z-axis of the unit cell as the tensile strain increases from 0% to 66.8%. The maximum deflection of Cu along the z-axis increases monotonically with uniaxial tensile strain, but the increase is not linear; it gradually levels off with increasing uniaxial tensile strain.

[0053] like Figure 7 This study demonstrates the simulated transmittance of a spiral FSS for 9-17 GHz electromagnetic waves under uniaxial tensile strains of 0%, 9.8%, 21.3%, 30%, 40%, 50%, and 60%. First, the geometric configurations of the spiral FSS under different uniaxial tensile strains were obtained. For each uniaxial tensile strain, simulations were performed using the commercial mechanical simulation software ABAQUS to obtain the deformed shapes of Cu, PI, and Ecoflex. The deformed mesh elements were then sequentially imported into HyperMesh (a finite element software used for pre- and post-processing) to obtain the corresponding geometric configurations. The geometric configurations under different uniaxial tensile strains were then imported into the commercial electromagnetic simulation software CST for further simulation. Electromagnetic simulations were then performed using a frequency domain analysis model. The spiral FSS mesh was meshed using hexahedral elements. The boundary conditions were set as "unit cell" in the x and y directions and "open (add space)" in the z direction. The relative dielectric constants (εr) of Ecoflex and PI are 3.2 and 3.4, respectively, and Cu was simplified to a perfect electrical conductor (PEC). It can be seen that the spiral frequency selective surface exhibits stable band-stop frequency selection characteristics and the reliability of mechanically controlling electromagnetic wave transmission performance.

[0054] like Figure 8 In order to further study in detail the influence of uniaxial tensile strain on the frequency selection characteristics of the spiral frequency selective surface, the changing trend of the resonant frequency of the spiral frequency selective surface with uniaxial tensile strain is demonstrated. The simulation results show that when the spiral frequency selective surface is uniaxially stretched, the resonant frequency decreases from 14.7GHz to 10.8GHz, the offset is 3.9GHz, and the offset rate is 26.7%, achieving a wide range of electromagnetic tuning effect.

[0055] In practical electromagnetic applications, different requirements exist for electromagnetic wave modulation. Therefore, structural parameterized analysis can be used to study the tuning effects of uniaxially stretched three-dimensional spiral frequency selective surfaces of varying sizes. This approach will help expand the application of these structures in a wider range of fields. Therefore, this paper investigates the effects of substrate thickness, period size, and gap length on the electromagnetic wave tuning characteristics of three-dimensional spiral frequency selective surfaces.

[0056] like Figures 9 to 12 , showing the effect of 60% uniaxial tensile strain on its electromagnetic wave transmittance when other parameters remain unchanged and t3 is 1mm, 2mm, 3mm, and 4mm respectively, all of which show good frequency selection characteristics. As the thickness of the substrate increases, the resonant frequency under each strain state shifts to low frequency as a whole.

[0057] like Figure 13 , showing the changing pattern of the effect of 60% uniaxial tensile strain on the resonant frequency regulation under different substrate thicknesses. It can be seen that as the substrate thickness increases, the active tuning performance of the spiral frequency selective surface continues to improve when subjected to uniaxial tensile strain, and the resonant frequency offset and offset rate both increase monotonically. However, the resonant frequency offset and offset rate do not increase linearly with the increase in substrate thickness. As the substrate thickness gradually increases, both gradually tend to a fixed value, which is consistent with the fact that the resonant frequency of the unilaterally loaded frequency selective surface can be reduced to a maximum of: This is consistent with the theoretical results. It can be concluded that by varying the substrate thickness, both the resonant frequency band and the resonant frequency shift rate can be adjusted: increasing the substrate thickness shifts the resonant frequency toward lower frequencies, and the resonant frequency shift rate increases. This analysis facilitates the parameterized customization of spiral frequency selective surfaces of specific thicknesses in various practical applications, tailored to the specific electromagnetic wave control requirements.

[0058] like Figures 14 to 17 , it shows that when the ratios of w, d1, d2, R1, and R2 remain unchanged and the structural dimensions are enlarged proportionally: R1 is 0.8mm, 1.12mm, 1.44mm, and 1.76mm respectively (for convenience, R1 is used here instead of P1 and P2, that is, when the period length is 1 times, 1.4 times, 1.8 times, and 2.2 times), the effect of 60% uniaxial tensile strain on its electromagnetic wave transmittance shows good frequency selection characteristics. With the increase of the period length, the resonant frequency under each strain state shifts to low frequency as a whole.

[0059] like Figure 18, showing the changing pattern of the influence of 60% uniaxial tensile strain on the resonant frequency regulation under different period lengths. It can be seen that with the increase of period length, the active tuning performance of the spiral frequency selective surface remains basically unchanged when subjected to uniaxial tensile strain, and the resonant frequency offset decreases monotonically (gradually tends to a fixed value), but its offset rate remains basically stable. From this, it can be concluded that by changing the period length, the frequency band in which the resonant frequency is located can be adjusted while keeping the resonant frequency offset rate basically unchanged: as the period length increases, the resonant frequency shifts to low frequencies. This analysis result is helpful to parameterize and customize spiral frequency selective surfaces of specific period lengths according to different electromagnetic wave control requirements in different practical application scenarios.

[0060] like Figures 19 to 22 , showing the effect of 60% uniaxial tensile strain on its electromagnetic wave transmittance when d1 is 2mm, 4mm, 6mm, and 8mm, respectively, when other parameters remain unchanged. All of them show good frequency selectivity, and the resonant frequency shows a consistent monotonic decrease with increasing uniaxial tensile strain. However, as the x-direction gap length increases, the overall resonant frequency of the three-dimensional spiral frequency selective surface under various strain states shows no obvious shift pattern, and the same is true for the resonant frequency shift and rate of change. Notably, as the x-direction gap length gradually increases, the distance between the second-order resonant frequency and the first-order resonant frequency under each uniaxial tensile strain state gradually decreases, so that when the x-direction gap length is greater than 6mm, the second-order resonant frequency under 60% uniaxial tensile strain appears near the first-order resonant frequency under 0% initial strain. This will obviously affect the effectiveness of mechanically controlling the frequency selectivity. Therefore, in practical applications, it is necessary to choose a smaller x-direction gap length as much as possible to achieve a more compact periodic arrangement and achieve better frequency selectivity.

[0061] like Figures 23 to 26 , showing the effect of 60% uniaxial tensile strain on its electromagnetic wave transmittance when d2 is 2mm, 3mm, 4mm, and 5mm respectively when other parameters remain unchanged, all of which show good frequency selection characteristics, and the resonant frequency decreases monotonically with the increase of uniaxial tensile strain. However, with the increase of the y-direction gap length, there is no obvious overall shift pattern in the resonant frequency of the three-dimensional spiral frequency selective surface under various strain states, and the same is true for the resonant frequency offset and change rate. Therefore, in practical applications, it is best to choose a smaller y-direction gap to achieve a more compact periodic arrangement and achieve better frequency selection effects.

Claims

1. A mechanically reconfigurable spiral frequency selective surface with a metal resonant unit cell on-off configuration, characterized by: The invention comprises a substrate and metal unit cells attached to the surface of the substrate, wherein the metal unit cells are periodically arranged on the substrate; the metal unit cells are composed of an upper metal layer and a lower insulating layer stacked into a composite layer; The shape of the metal unit cell is an Archimedean double helix structure. Based on the Archimedean double helix, the two helices are intertwined and stretched outward to a set width with the helix as the center line. The two helices do not touch each other, forming a two-dimensional double helix structure. The two ends of the helical structure are semicircles; the semicircles are adhered to the substrate. Then, through mechanical uniaxial stretching, the Archimedean double helix structure produces out-of-plane buckling deformation, which increases the x-direction length and z-direction deflection of the combined layer, causing the combined layer to undergo three-dimensional buckling. When the strain is greater than a certain value, the two sets of spiral structures contact each other, and the metal contact forms a pathway to achieve the on-off effect.

2. The mechanically reconfigurable spiral frequency selective surface with an on-off configuration of a metal resonant unit cell according to claim 1, characterized in that: The substrate is made of silicone Ecoflex material.

3. The mechanically reconfigurable spiral frequency selective surface with an on-off configuration of a metal resonant unit cell according to claim 1, characterized in that: The composite layer includes a Cu layer and a PI layer, wherein the Cu layer is above the PI layer.

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