Flexible capacitive sensor based on air gap electric field modulation

Abstract

The invention provides a flexible capacitive sensor based on air gap electric field modulation, which belongs to the field of capacitive sensor design and is formed by bonding a flexible interdigital electrode film and a high dielectric constant brittle film packaged in an elastomer. Wherein the interdigital electrode film is an interdigital conductive electrode on the flexible substrate, and the high-dielectric-constant brittle film is a high-dielectric-constant inorganic ceramic film with random cracks after pretreatment. When the sensor is bent forwards or backwards, air gaps among the crack high-dielectric-constant fragments are reversibly expanded or closed, electric field distribution and an effective dielectric constant in the dielectric layer are changed, and then a capacitance value is changed. And the bending curvature and direction of the measured object can be accurately represented by measuring the capacitance change. The device is simple in structure, has extremely high sensitivity, robustness and stability, can be directly attached to the surface of a soft robot, a wearable device and a bent structure, is used for scenes such as curvature monitoring, posture sensing and man-machine interaction, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of capacitive sensor design, and particularly relates to a flexible capacitive sensor based on air gap electric field modulation. Background Technology

[0002] In the field of soft robotics, flexible sensors, as sensing elements, play a crucial role in monitoring deformation, pressure, and contact states. They not only provide the system with tactile and proprioceptive capabilities but also lay the foundation for performance optimization and intelligent applications of flexible actuators. In recent years, with the development of flexible materials and microstructure manufacturing technologies, developing flexible sensors with high sensitivity, fast response, and high robustness has become an important research direction in this field.

[0003] Currently, various types of flexible sensors have received widespread attention and research. Among existing flexible sensing technologies, inductive sensors have lower resolution and response speed compared to bidirectional bending capacitive sensors. Furthermore, inductive sensors lack theoretical derivation formulas, making it impossible to deduce suitable inductor coil sizes from theoretical analysis. Therefore, capacitive sensors have gained widespread attention due to their advantages such as low power consumption, simple structure, and easy signal interpretation. However, traditional capacitive sensors often employ parallel plate structures. While this structure exhibits high linearity under small deformations, its sensitivity easily saturates under large deformations or complex stress conditions, and its structural flexibility is limited. In contrast, interdigitated electrode structures utilize edge field effects to detect capacitance changes, exhibiting better stretchability and mechanical stability. However, due to the weak edge field distribution, the signal change amplitude of interdigitated electrode structures is limited when detecting minute deformations, restricting their further application in high-sensitivity detection fields. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a flexible capacitive sensor based on air-gap electric field modulation, comprising a flexible interdigitated electrode film and a high-dielectric-constant brittle film encapsulated in an elastomer, bonded together. The interdigitated electrode film consists of interdigitated conductive electrodes on a flexible substrate, and the high-dielectric-constant brittle film is a pre-treated high-dielectric-constant inorganic ceramic film with random cracks. When the sensor bends in either the forward or reverse direction, the air gap between the cracked high-dielectric-constant fragments reversibly expands or closes, causing changes in the electric field distribution and equivalent dielectric constant within the dielectric layer, thereby altering the capacitance value. Measuring the capacitance change can accurately characterize the curvature and direction of the measured object. This invention offers advantages such as high sensitivity, simple manufacturing, good stability, a large measurement range, and long-term stable operation. It can be directly attached to soft robots, wearable devices, and curved structural surfaces for applications such as curvature monitoring, posture perception, and human-computer interaction, showing broad application prospects.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A flexible capacitive sensor based on air gap electric field modulation is composed of multiple thin films bonded together. The multiple thin films include interdigitated electrodes and a high dielectric constant brittle film. The interdigitated electrodes include conductive interdigitated electrode structures. The high dielectric constant brittle film is an inorganic ceramic film with cracks after pretreatment. The interdigitated electrodes and the high dielectric constant brittle film are bonded together by an adhesive layer formed by spin coating. There are air gaps between the crack fragments of the high dielectric constant brittle film. When the flexible capacitive sensor is strained as the measured object bends, the change in air gaps is amplified or reduced, thereby causing an increase or decrease in the overall dielectric constant of the high dielectric constant brittle film. The capacitance value measured by the interdigitated electrodes changes accordingly. The measured capacitance value is the coupling capacitance between the interdigitated electrodes, the high dielectric constant brittle film, and the air gaps, which depends on the change in the effective dielectric constant caused by the change in the proportion of air gaps between cracks.

[0007] Beneficial effects:

[0008] 1. This invention significantly enhances the dielectric polarization capability of a flexible substrate by introducing a cracked, high-dielectric-constant brittle thin film onto it, while improving the capacitive response while maintaining flexibility. Combining this high-dielectric-constant brittle thin film with an interdigitated electrode structure forms a parallel-enhanced capacitor structure, achieving a multiplication effect on the capacitive signal. This invention exhibits high sensitivity and fast response characteristics (response time <1 ms) in bending and micro-deformation detection, and maintains stable signal output even in the presence of localized damage or cracks, demonstrating excellent robustness.

[0009] 2. This invention significantly enhances the local concentration effect of the electric field and improves the capacitance response sensitivity by introducing a rigid-flexible coupling structure between a cracked, high-dielectric-constant brittle thin film and flexible interdigitated electrodes, thus achieving bidirectional high-resolution detection of bending direction and curvature. It has broad application prospects in fields such as bending angle detection in soft robots, texture recognition, and wearable motion monitoring. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the structure of a flexible capacitive sensor based on air gap electric field modulation according to the present invention;

[0011] Figure 2a A schematic diagram of the equivalent capacitance distribution model of interdigitated electrodes in a multilayer dielectric structure;

[0012] Figure 2b This is a schematic diagram of the coupling electric field relationship between a brittle thin film with high dielectric constant and a flexible interdigitated electrode with crack gaps.

[0013] Figure 3aThis is a schematic diagram showing the state of a sensor when it is bent forward, with the gap between the high dielectric constant brittle film cracks closing, the electric field concentrating, and the capacitance increasing.

[0014] Figure 3b This is a schematic diagram showing the state of the sensor when the crack gap expands, the electric field dissipates, and the capacitance decreases during reverse bending.

[0015] Figure 4a This is a schematic diagram illustrating the modulation mechanism of electric field distribution and equivalent capacitance of a brittle thin film with high dielectric constant; in the upper figure, d=10 micrometers and in the lower figure, d=100 micrometers.

[0016] Figure 4b The simulation results show the effect of brittle films with high dielectric constants on capacitance.

[0017] Figure 4c The diagram illustrates the effect of size; the top diagram is a cross-sectional view of the sensor, and the bottom diagram is a schematic diagram illustrating the effect of crack fragment size on sensor sensitivity.

[0018] Figure 4d The diagram illustrates the effect of sensitivity; the top diagram is a cross-sectional view of the sensor, and the bottom diagram is a schematic diagram illustrating the effect of the initial air gap on sensitivity.

[0019] Figure 5a This is a schematic diagram showing the change of sensor capacitance with bending angle.

[0020] Figure 5b A schematic diagram showing the capacitance change of the sensor under different loading rates;

[0021] Figure 5c Capacitance-time response curve of sensor under rapid loading / unloading conditions;

[0022] Figure 5d This is a schematic diagram of the capacitance response in 0.1° increments, with the inner diagram showing the minute capacitance changes in 0.01° increments.

[0023] Figure 5e This is a schematic diagram showing the change in capacitance of the sensor after 15,000 long-cycle cycles.

[0024] Figure 5f A schematic diagram showing that the sensor can still detect bidirectional bending even after localized damage;

[0025] Figure 6a This is a schematic diagram showing the capacitive response of fish fin fingers when scanning textured plates with different topologies.

[0026] Figure 6b Optical profile diagrams of different colored toners;

[0027] Figure 6cThis is a schematic diagram of the capacitive output response of a sensor used to detect traces of toner of different colors.

[0028] Figure 7 for Figure 5c A diagram of a device for rapid loading and unloading.

[0029] The reference numerals in the attached figures are: 1. Interdigitated electrode; 2. High dielectric constant brittle film; 3. Adhesive layer; 4. High dielectric constant brittle fragment; 5. Silicone encapsulation layer; 6. Air; 7. Polyethylene amine film. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0031] like Figure 1 As shown, the flexible capacitive sensor based on air gap electric field modulation of the present invention is formed by bonding multiple thin films, specifically including interdigitated electrodes 1 and a high dielectric constant brittle thin film 2. The interdigitated electrodes 1 include a flexible interdigitated electrode structure. The high dielectric constant brittle thin film 2 has cracks after pretreatment. The interdigitated electrodes 1 and the high dielectric constant brittle thin film 2 are bonded together by an extremely thin adhesive layer formed by spin coating.

[0032] The flexible capacitive sensor based on air gap electric field modulation is disposed on the surface of the object being measured or embedded in the object. When the object bends and strains, the change in the air gap of the high-dielectric-constant brittle thin film 2 is amplified / reduced. This change causes an increase / decrease in the overall dielectric constant, and the capacitance value measured by the interdigital electrode 1 changes accordingly. The measured capacitance is the coupling capacitance between the interdigital electrode and the equivalent dielectric layer formed by the high-dielectric-constant brittle thin film 2, and its value depends on the change in the effective dielectric constant caused by the change in the proportion of air gaps between cracks. The air gaps between cracks have an initial gap width; bidirectional bending causes the entire gap width to shrink / expand, resulting in a change in the effective dielectric constant and affecting the capacitive response. The high-dielectric-constant brittle thin film 2 forms fragments, and air gaps are formed between the fragments, filled with low-dielectric-constant air.

[0033] Preferably, the high dielectric constant brittle film 2 is pre-encapsulated to induce cracks, thereby creating a penetrating air gap.

[0034] Preferably, the interdigital electrode 1 is an interdigital electrode made of solid conductive material, a deformable interdigital electrode made of liquid metal, or a flexible interdigital electrode structure formed of flexible conductive material.

[0035] Preferably, the shape of the interdigitated electrode 1 is rectangular, square, elliptical, or other two-dimensional shape.

[0036] Preferably, the interdigitated electrode 1 is one or more layers.

[0037] Preferably, the substrate of the interdigitated electrode 1 is a flexible but non-stretchable polymer film, or a stretchable hyperelastic film.

[0038] Preferably, the high dielectric constant brittle film 2 is a brittle film composed of lead zirconate titanate (PZT), barium titanate (BaTiO3), strontium titanate (SrTiO3) or its doped and modified forms, which has a high dielectric constant (> 50) and can generate a crack structure through pretreatment.

[0039] Preferably, after the high dielectric constant brittle film 2 cracks, the length change caused by the strain on the sensor is entirely provided by the air gaps between the brittle fragments in the high dielectric constant brittle film, and the change in the air gaps will cause a change in the overall dielectric constant of the material.

[0040] Preferably, the interdigitated electrode 1 and the high dielectric constant brittle film 2 are bonded together by an extremely thin adhesive layer 3 formed by spin coating. The adhesive layer 3 is formed by high-speed spin coating of adhesive with a spin coater and has a thickness of less than 10 μm. This adhesive layer 3 ensures a strong bond between the two layers while maintaining the continuity of electric field coupling, thereby improving the sensitivity and signal stability of the sensor.

[0041] Preferably, during the bending deformation of the object being measured, the crack gap in the high dielectric constant brittle film 2 changes with the bending direction. The change in crack gap causes a readjustment of the electric field distribution inside the dielectric layer, which in turn changes the equivalent dielectric constant of the dielectric layer, thereby causing the overall capacitance value to change with the bending state.

[0042] Preferably, the sensor is suitable for non-conductive and non-magnetic objects being measured.

[0043] Preferably, the capacitance value of the sensor exhibits a monotonic functional relationship with the curvature of the object being measured within a certain range. This function determines the curvature of the object, thereby realizing the measurement of the object's curvature. The specific angle calculation formula is: y = 0.3223x 2 - 13.859x + 1.1685. Where y represents the angle and x represents the change in capacitance.

[0044] The principle of this invention is as follows: due to the large stiffness difference between the air gap and the high dielectric constant fragment, the strain generated by bending deformation is entirely borne by the air gap. At this time, the dielectric constant of the equivalent dielectric layer formed by the high dielectric constant brittle film 2 and the air gap changes, and the coupling with the edge field of the interdigitated electrode 1 causes the capacitance to change. Thus, the curvature of the object can be measured by measuring the change in capacitance of the bidirectional bending capacitance sensor.

[0045] Figure 2a A model of the equivalent capacitance distribution of interdigitated electrodes under a multilayer dielectric structure is presented. Figure 2b The coupling electric field relationship between a brittle thin film with high dielectric constant and crack gaps and a flexible interdigitated electrode is shown. The formula for calculating the capacitance of a multilayer interdigitated electrode can be written as:

[0046] ;

[0047] in, Let n represent the capacitance of the interdigitated electrode, n represent the number of different dielectric layers on the interdigitated electrode, and i represent which layer. This represents the dielectric constant value of the i-th layer. This represents the height of the i-th dielectric layer. This represents the internal capacitance of the i-th layer. This represents the capacitance of the air dielectric layer. Figure 2a In this diagram, i = 1-4, V represents voltage, +V is positive voltage, and -V is negative voltage. Air 6 is at the top, followed by the silicone encapsulation layer 5, the high-dielectric-constant brittle fragment 4, the adhesive layer 3, and the interdigitated electrode 1. The high-dielectric-constant brittle fragment 4 is formed from the high-dielectric-constant brittle film 2. Capacitance is directly proportional to the dielectric constant, which is affected by the air gaps in the cracks. This is the fundamental reason why strain causes changes in capacitance.

[0048] like Figure 3a , Figure 3b The figure shows the simulation diagram of the electric field distribution of the sensor under forward and reverse bending conditions. Since the entire sensor has a certain thickness, according to elasticity, no strain occurs in the neutral layer, while the two sides are stretched / compressed, and the corresponding crack gaps will also expand / compress accordingly. When the sensor is bent forward, the crack gaps decrease as the bending angle increases, thus reducing the amount of electric field dissipation and concentrating more in the PZT portion. Therefore, the capacitance of the sensor will further increase. Conversely, the crack gaps increase as the reverse bending angle increases, and the capacitance of the sensor will further decrease. The simulation results clearly illustrate the mechanism of crack gap modulation of electric field distribution and capacitance. The polyethylene amine film 7 is positioned below the interdigitated electrode 1.

[0049] The sensor fabrication process is as follows: First, E41 epoxy resin diluted with cyclohexane is spin-coated onto a glass substrate at a speed of 500 rpm for 1 min to obtain a uniform encapsulation layer. Then, a high-dielectric-constant brittle film (such as PZT or other high-dielectric-constant materials) is gently placed on top and compacted. After curing at room temperature for about 12 h, the cured encapsulation layer is peeled off from the glass substrate with a spatula. Due to the difference in shrinkage stress and substrate constraint, random cracks naturally form on the surface of the high-dielectric film, penetrating the thickness direction, while the encapsulation layer provides constraint. At the same time, interdigitated electrodes are attached to the spin-coated sheet, and 1 mL of E41 dilution solution (E41:cyclohexane = 1:2) is dropped onto it. Then, a spin coater (KW-4L, SETCAS, China) is used to spin-coat the film at 2500 rpm for 1 min. The interdigitated electrode was then removed and bonded to the previously prepared cracked PZT. It was then wrapped and fixed to a 25mm acrylic rod with tape to form a pre-stretched crack structure, which was left to stand for 12 hours. Once the PZT and interdigitated electrode were bonded, it was removed from under the glass rod, at which point the sample preparation was complete.

[0050] like Figure 4a As shown, d represents the distance between the high dielectric constant brittle film and the interdigitated electrode, and 10 and 100 represent distances of 10 micrometers, respectively. Figure 4a (as shown in the image above) and 100 micrometers ( Figure 4a The figure below shows the simulation results of the effect of the relative distance between the flexible interdigitated electrode and the high dielectric constant brittle film on the capacitance. Figure 4a In this case, the closer the two are, the stronger the influence of the high dielectric constant brittle film on the electric field. Figure 4b Simulations show that the smaller the distance between the interdigitated electrodes and the high dielectric layer, the stronger the electric field coupling and the larger the capacitance. Figure 4c This indicates that the smaller the crack fragment size, the more crack elements are present in the same area, and the more significant the change in the equivalent dielectric constant caused by strain, thus resulting in higher sensor sensitivity. Figure 4c The image above is a cross-sectional view of the sensor. Figure 4c The figure below is a schematic diagram illustrating the effect of crack fragment size on sensor sensitivity; Figure 4d This indicates that the smaller the initial air gap, the more sensitive the equivalent dielectric constant is to changes in strain, resulting in higher overall sensitivity; among them, Figure 4d The image above is a cross-sectional view of the sensor. Figure 4d The figure below illustrates the effect of the initial air gap on sensitivity. Therefore, the electrode spacing, high dielectric layer thickness, and crack feature size can be optimized based on simulation results to obtain the desired sensitivity and range.

[0051] Figures 5a-5fThe relationship between the bending angle and capacitance of the sensor is demonstrated. The experimental curves show that the simple interdigitated electrode structure is insensitive to changes in the bending angle, while the sensitivity is significantly improved after adding a high-dielectric-constant cracked film. This invention focuses on some basic characteristics of a bidirectional bending capacitive sensor, showcasing its advantages such as high sensitivity, good dynamic characteristics, high resolution, and good repeatability. Figure 5a As shown, the angle and capacitance exhibit a monotonic function relationship within a certain range. Therefore, after obtaining their calibration curves, the bending angle of the object being measured can be determined by measuring the capacitance of the sensor. Figure 5b As shown, after being loaded at different rates, the angle-capacitance curves of this sensor basically overlap, indicating that the hysteresis of this sensor is extremely small. Figure 5c The sensor demonstrated extremely fast response speed, achieving a response time of 0.9 ms during loading and unloading. Figure 5d The diagram shows the capacitance change of the sensor when it bends forward / reverse by 0.1°. A further smaller diagram shows the capacitance change when the sensor bends forward by 0.01°, demonstrating the high resolution of the bidirectional bending capacitive sensor. The blue line represents the capacitance response in 0.1° increments, and the orange line shows the minute capacitance change in 0.01° increments. Figure 5e The experiment demonstrates the change in relative capacitance of the flexible interdigitated electrode during 15,000 cycles as the bending angle of the sensor varies from 60° to 90°, reflecting that the bidirectional bending capacitive sensor can operate stably over a long period of time. Figure 5f This demonstrates that the sensor maintains a stable output response for bidirectional bending after three tests (Test 1, Test 2, Test 3), reflecting the good robustness of the bidirectional bending capacitive sensor.

[0052] Example:

[0053] One embodiment of the present invention integrates the bidirectional bending capacitive sensor of the present invention into the fingertip of a fin-like soft finger for scanning surface textures of different heights and periods. Specifically, when the sensor slides along the sample surface at a constant speed, the undulations of the surface texture cause changes in the local bending degree of the sensor surface, modulating the internal electric field distribution and equivalent dielectric constant, resulting in capacitance fluctuations in the output signal consistent with the surface period. Thus, the waveform of the sensor output signal shows a good correspondence with the surface texture period, indicating that the sensor can perform high-resolution detection and recognition of surface texture features at the micrometer scale. Figure 6a As shown, the height ranges of surface micro-undulations measured by microscope were 10 μm, 50 μm, and 100 μm, respectively. Figure 6aIn this context, 50μm and 500μm represent the scale size. Correspondingly, the sensor output signal exhibits periodic capacitance fluctuations consistent with the surface period, indicating its ability to accurately perceive microscale texture features. For example... Figure 6b As shown, #181717, #767171 and #D0CECE are hexadecimal color codes. Using different color codes will result in different toner thicknesses when printing. Figure 6b The six digits represent the heights of the lightest and darkest colors in the optical profilometer's color chart. To further verify the sensor's texture discrimination capability, this invention tested toners of different printing colors. At a constant scanning speed, the capacitance signal waveforms generated by different samples showed significant differences; the amplitude was correlated with the surface height, while the signal frequency corresponded to the texture period. The three-dimensional reconstruction results of the measured surface morphology were highly consistent with the sensor's output waveform, further demonstrating the sensor's reliability in texture recognition. SEM (Scanning Electron Microscopy) analysis revealed a minimum detectable thickness of approximately 1 μm, further showcasing the sensor's excellent sensitivity. Figure 6c As shown, the sensor scanned the printed coated paper at a constant scanning speed. The sensor output response was different for different colors of printing marks, and the width of the marks was also different (corresponding to the image scanned by the optical profilometer; the lighter the color, the smaller the width of the toner, and the darker the color, the larger the width of the toner). 3.42 fF (Flyfar) corresponds to the sensor output of the first mark, 2.57 fF corresponds to the second mark, and 2.1 fF corresponds to the last mark. Two consecutive marks were tested for each sample, so the capacitance curve has two waves.

[0054] like Figure 7 The diagram shows the experimental setup for testing the sensor's response speed. A linear displacement stage is used to move the sensor at a relatively high speed with a small displacement. Loading and unloading curves are then measured, and the response time is calculated from these curves. An acrylic plate is fixed to the displacement stage, and another acrylic plate is mounted on the base. The sensor is then fixed to the acrylic plates using polyethylene tape. Both ends of the sensor are fixed to the acrylic plates. When one end of the sensor begins to move, the sensor bends. Moving the linear displacement stage back and forth causes a change in the bending angle of the sensor.

[0055] As can be seen from the above technical solution, the present invention has the advantages of simple manufacturing, low cost, stable and reliable performance, strong robustness, wide measurement range, high sensitivity, and high resolution. It can be directly attached to the surface of the object being measured or embedded inside it. The curvature of the object can be measured by the change in capacitance of the sensor caused by the deformation of the object when it bends.

[0056] The parts of this invention not disclosed in detail are well-known technologies in the field.

[0057] Although the illustrative specific embodiments of the present invention have been described above to enable those skilled in the art to understand the invention, it should be understood that the invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of the present invention are protected.

Claims

1. A flexible capacitive sensor based on air gap electric field modulation, characterized in that: Composed of multiple thin films, the multilayer thin films include interdigitated electrodes and a high-dielectric-constant brittle film. The interdigitated electrodes include conductive interdigitated electrode structures. The high-dielectric-constant brittle film is an inorganic ceramic film with cracks after pretreatment. The interdigitated electrodes and the high-dielectric-constant brittle film are bonded together by an adhesive layer formed by spin coating. There are air gaps between the crack fragments of the high-dielectric-constant brittle film. When the flexible capacitive sensor is strained as the measured object bends, the change in air gaps is amplified or reduced, thereby causing an increase or decrease in the overall dielectric constant of the high-dielectric-constant brittle film. The capacitance value measured by the interdigitated electrodes changes accordingly. The measured capacitance value is the coupling capacitance between the interdigitated electrodes, the high-dielectric-constant brittle film, and their air gaps, which depends on the change in the effective dielectric constant caused by the change in the proportion of air gaps between cracks.

2. The flexible capacitive sensor based on air gap electric field modulation according to claim 1, characterized in that: The conductive interdigitated electrode structure is an interdigitated electrode structure made of solid conductive material, a deformable interdigitated electrode structure made of liquid metal, or a flexible interdigitated electrode structure formed of flexible conductive material.

3. A flexible capacitive sensor based on air gap electric field modulation according to claim 1 or 2, characterized in that: The interdigitated electrodes are rectangular, square, or elliptical in shape, and are single-layered or double-layered.

4. A flexible capacitive sensor based on air gap electric field modulation according to claim 1, characterized in that: The substrate of the interdigitated electrode is a flexible but non-stretchable polymer film or a stretchable hyperelastic film.

5. A flexible capacitive sensor based on air gap electric field modulation according to claim 1, characterized in that: The inorganic ceramic film is a brittle film composed of lead zirconate titanate, barium titanate, strontium titanate, or doped and modified materials.

6. A flexible capacitive sensor based on air gap electric field modulation according to claim 5, characterized in that, The inorganic ceramic thin film is bonded to silicone with an adhesive before cracking to form a brittle film with a high dielectric constant. Even after cracks are generated during pretreatment, the film still maintains its adhesive relationship, thus forming a flexible sensing unit.

7. A flexible capacitive sensor based on air gap electric field modulation according to claim 5, characterized in that: The inorganic ceramic thin film has a dielectric constant >50 and can generate a crack structure through pretreatment.

8. A flexible capacitive sensor based on air gap electric field modulation according to claim 1, characterized in that: The thickness of the adhesive layer is less than 10 μm.

9. A flexible capacitive sensor based on air gap electric field modulation according to claim 1, characterized in that: It can be directly attached to the surface of the object being tested or embedded in the object being tested.

10. A flexible capacitive sensor based on air gap electric field modulation according to claim 1, characterized in that: The capacitance value measured by the flexible capacitive sensor has a monotonic functional relationship with the curvature of the object being measured within the range of -90° to 90°. This monotonic functional relationship is used to determine the curvature of the object being measured.

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