Flexible differential sensor for non-destructive testing and method of making the same

Abstract

This invention provides a flexible differential sensor for nondestructive testing and its fabrication method. By introducing an island-bridge strain isolation structure and differential coil units, and integrating them using flexible PCB technology, the sensor achieves tight attachment on complex curved surfaces. Even under dynamic bending deformation, its differential output signal maintains high fidelity and stability, while also exhibiting excellent resistance to electromagnetic interference and temperature drift. The sensor includes a flexible insulating substrate, a coil layer, a flexible insulating protective layer, and a signal transmission interface. The coil layer is printed on the upper surface of the flexible insulating substrate using flexible PCB technology and is entirely encased under the flexible insulating protective layer. The signal transmission interface is located at one end of the sensor and electrically connected to the coil layer for connecting to external testing instruments. The coil layer includes a differential coil unit group and an island-bridge strain isolation structure for supporting the detection coil units.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology, and more specifically to a flexible differential sensor for nondestructive testing and its fabrication method. Background Technology

[0002] In fields such as energy and power, and rail transportation, numerous critical components endure long-term combined effects of load pressure, electrothermal stress, and chemical corrosion. This makes them highly susceptible to microscopic defects such as air gaps and cracks on their surfaces and interiors. The integrity and reliability of these critical component structures directly affect the safe operation of the entire system. Take the lead-sealed cable structure in a power system as an example. This structure is a critical sealing component at the termination and joint of high-voltage cables. It is typically hand-cast from lead or lead alloy materials to isolate external moisture and environmental corrosion, while also serving as an important connection point for electric field shielding and grounding. During actual service, the cable lead-sealed cable is subjected to mechanical fatigue caused by cyclic thermal expansion and contraction, electrochemical corrosion, and vibration loads in the laying environment. This makes it highly prone to developing microscopic defects such as micron-level air gaps, circumferential cracks, or localized peeling at the interface between the lead sheath and the cable's metal sheath. These initial defects are characterized by their high concealment and rapid propagation rate. Once they develop into penetrating damage, they will lead to leakage of insulating oil inside the cable termination, moisture intrusion, and ultimately, serious accidents such as discharge, flashover, or even insulation breakdown. Therefore, conducting highly sensitive and reliable non-destructive testing and imaging on key components in modern industry is of great significance for ensuring the safe operation of equipment and extending its service life.

[0003] To address the aforementioned needs, magnetic induction tomography, a non-destructive testing technique based on the principle of electromagnetic induction, offers advantages such as non-contact, non-destructive, visualization, high sensitivity, fast detection speed, and strong adaptability to conductive materials. Its basic principle involves introducing an alternating magnetic field into a conductor to generate an induced magnetic field. The distribution of the magnetic field changes with variations in conductivity, permeability, and structural integrity, thereby reflecting component defects and conductivity characteristics.

[0004] Existing technologies and equipment suffer from drawbacks when inspecting complex curved surfaces, including poor geometric fit and unstable electrical performance during attachment. Rigid probes are only suitable for inspecting fixed planes or simple curved surfaces. When used to inspect components with complex curvature, rigid probes cannot achieve good conformal attachment to the surface being measured, resulting in uneven and uncontrollable lift-off effects and reduced detection sensitivity. On the other hand, probes with flexible structures employ a continuous, integral coil pattern, which is prone to distortion during attachment, causing drift in key parameters such as resistance, inductance, and resonant frequency, leading to a decrease in the signal-to-noise ratio of the detection signal.

[0005] In summary, existing sensors for industrial non-destructive testing have significant shortcomings in terms of conformal attachment capability to curved surfaces and electrical stability under flexible conditions. Developing a new type of sensor capable of maintaining stable electrical performance under complex curved surface attachment conditions has become an urgent problem to be solved in the field of industrial non-destructive testing. Summary of the Invention

[0006] This invention aims to address the technical deficiencies of existing technologies by providing a flexible differential sensor for non-destructive testing and its fabrication method. By introducing an island-bridge strain isolation structure and a differential coil unit, and fabricating them in an integrated manner based on flexible PCB technology, the sensor can be tightly attached to complex curved surfaces. Furthermore, when subjected to dynamic bending deformation, its differential output signal can still maintain high fidelity and high stability, while also possessing excellent resistance to electromagnetic interference and temperature drift.

[0007] The present invention provides the following technical solution: On the one hand, it discloses a flexible differential sensor for non-destructive testing, including a flexible insulating substrate layer, a coil layer, a flexible insulating protective layer, and a signal transmission interface;

[0008] The coil layer is printed on the upper surface of the flexible insulating substrate using flexible PCB technology and is completely covered under the flexible insulating protective layer. The signal transmission interface is located at one end of the sensor and is electrically connected to the coil layer for connecting to external detection instruments.

[0009] The coil layer includes a differential coil unit group and an island-bridge type strain isolation structure for supporting the detection coil unit.

[0010] Furthermore,

[0011] The island-bridge strain isolation structure includes multiple rigid island regions and a flexible meandering connecting bridge. The differential coil unit group includes multiple differential coil units, which are fixedly disposed within the rigid island regions. The flexible meandering connecting bridge connects the differential coil units in each rigid island region to the signal transmission interface and undergoes elastic deformation when the sensor as a whole undergoes bending deformation, so as to absorb or isolate the mechanical strain applied to the rigid island regions.

[0012] Furthermore,

[0013] The rigid island region is a micro-region formed by local thickening printing, and its stiffness is greater than that of the flexible meandering connecting bridge.

[0014] Furthermore,

[0015] The differential coil unit is a planar circular coil formed within a rigid island region. This planar circular coil is made using high-conductivity ink and inkjet printing process. Its line width, line spacing, and number of turns are all controlled according to preset parameters.

[0016] Furthermore,

[0017] The flexible meandering connecting bridge has a serpentine routing pattern, with a line width smaller than that of the rigid island coil, and is printed using high-conductivity ink.

[0018] Furthermore,

[0019] The differential coil unit comprises three identical coils placed coaxially and perpendicularly, wherein:

[0020] The intermediate coil is the excitation coil, and its two ends are connected to the excitation signal source;

[0021] The two coils are receiving coils, symmetrically distributed on both sides of the middle coil within the rigid island area. The two receiving coils are connected in series with opposite polarities. The two ends of the series connection are respectively connected to the signal transmission interface to form a differential structure as a differential signal output terminal for connecting to external detection instruments.

[0022] The differential structure is configured such that when the sensor is in a defect-free or uniform conductivity detection environment, the signals induced by the receiving coils on both sides cancel each other out; when there are local defects or conductivity changes, the induced signals become unbalanced, thereby generating a differential signal characterizing the defect at the series output terminal of the coils on both sides.

[0023] Furthermore,

[0024] The differential coil unit group includes eight pairs of differential coil units.

[0025] Furthermore,

[0026] The rigid island region is designed as a square area, and a local thickening structure is formed by multiple overprinting processes.

[0027] Another aspect of the present invention discloses a method for fabricating a flexible differential sensor for nondestructive testing, comprising the following steps:

[0028] Select a flexible insulating substrate of appropriate thickness and size;

[0029] Rigid island regions, flexible meandering connecting bridges, and differential coil units are sequentially printed on the surface of a flexible insulating substrate using flexible PCB technology.

[0030] After printing, the coil is placed in a sintering device to cure the highly conductive ink. A flexible insulating protective layer is then attached to the coil layer, and the coil unit is connected to the signal transmission interface.

[0031] Furthermore,

[0032] The flexible insulating substrate layer is made of polyimide film.

[0033] This invention discloses a flexible differential sensor for nondestructive testing. This solution employs a dual mechanism of deformation isolation via an "island-bridge structure" and common-mode signal suppression via a "differential design," addressing the core bottlenecks of difficult surface adhesion and signal distortion under deformation in industrial nondestructive testing from both structural mechanics and circuit principle perspectives.

[0034] 1. Decoupling conformal attachment capability and electrical stability at the structural level:

[0035] By adopting an island-bridge strain isolation structure, the rigid differential coil unit is separated from the flexible interconnection bridge circuit. The sensor can be closely attached to the measured object with a complex curved surface, significantly reducing the lift-off effect. All tensile and bending deformations are mainly absorbed and borne by the flexible "bridge" structure. The flexible meandering connecting bridge absorbs mechanical strain, so that the coil maintains its geometric dimensions and electrical parameters unchanged when the sensor is bent significantly. This fundamentally solves the problem of electrical parameter drift caused by direct force on the functional layer during deformation in traditional flexible sensors, and achieves the effect of "the sensing unit remains stable no matter how the substrate is bent".

[0036] Integrated molding and structural reliability: The coil layer is directly printed on the substrate layer using flexible PCB technology, eliminating interface defects caused by traditional manual winding or adhesive bonding processes. This integrated design ensures no relative slippage between the coil layer and the substrate, avoiding signal abrupt changes caused by delamination or friction in dynamic bending or vibration environments. Furthermore, the coil drawings can be modified according to specific application scenarios, significantly shortening the R&D cycle and reducing manufacturing costs.

[0037] 2. At the functional level, common-mode suppression and anti-interference are achieved, improving detection accuracy and anti-interference capability:

[0038] The differential coil unit design effectively suppresses common-mode interference, highlights the difference between defects and conductivity signals, and significantly improves the detection signal-to-noise ratio. In industrial environments (such as near motors with electromagnetic interference) or due to background noise caused by the bending of the sensor itself (such as changes in the ambient magnetic field), the two ends of the differential coil will act simultaneously. After being subtracted by the differential circuit at the back end, they will be canceled out. The actual defect signal (such as the eddy current field distortion caused by cracks) is asymmetrical and will be significantly amplified by the differential structure, thereby greatly improving the detection signal-to-noise ratio and the detection rate of small defects.

[0039] Temperature drift compensation: The differential structure can also automatically compensate for the effect of temperature changes on coil impedance. When the ambient temperature changes, the impedance changes of the two differential coils have the same trend (common-mode signal), which automatically cancels each other out in the differential output, ensuring that the sensor can maintain zero-point stability under temperature fluctuations in industrial environments. Attached Figure Description

[0040] Figure 1A diagram showing the flexible differential sensor of the present invention attached to the differential wire structure of a cable lead seal junction;

[0041] Figure 2 This is a model diagram of eight pairs of flexible differential coils;

[0042] Figure 3 A mesh partitioning model of the COMSOL flexible differential sensor and the lead-sealed structure of the cable under test;

[0043] Figure 4 A comparison of the sensitivity of differential coil units and non-differential coil units. Detailed Implementation

[0044] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0045] like Figures 1-4 As shown, it illustrates a specific embodiment of the present invention:

[0046] like Figures 1-4 As shown, one aspect of the present invention discloses Embodiment 1:

[0047] A flexible differential sensor for non-destructive testing is characterized by comprising a flexible insulating substrate layer, a coil layer, a flexible insulating protective layer, and a signal transmission interface.

[0048] The coil layer is printed on the upper surface of the flexible insulating substrate using flexible PCB technology and is completely covered under the flexible insulating protective layer. The signal transmission interface is located at one end of the sensor and is electrically connected to the coil layer for connecting to external detection instruments.

[0049] The coil layer includes differential coil unit groups and an island-bridge strain isolation structure for supporting the detection coil units. The island-bridge strain isolation structure includes multiple rigid island regions and flexible, meandering connecting bridges. The differential coil unit groups include eight pairs of differential coil units, which are fixedly disposed within the rigid island regions. The flexible, meandering connecting bridges connect the differential coil units within each rigid island region to a signal transmission interface and undergo elastic deformation when the sensor as a whole bends, absorbing or isolating the mechanical strain applied to the rigid island regions. The rigid island regions are locally thickened micro-regions printed with stiffness greater than that of the flexible, meandering connecting bridges.

[0050] The differential coil unit is a planar circular coil formed within a rigid island region. This planar circular coil is made using high-conductivity ink and inkjet printing process. Its line width, line spacing, and number of turns are all controlled according to preset parameters.

[0051] The flexible meandering connecting bridge has a serpentine routing pattern, with a line width smaller than that of the rigid island coil, and is printed using high-conductivity ink.

[0052] The differential coil unit comprises three identical coils placed coaxially and perpendicularly, wherein:

[0053] The intermediate coil is the excitation coil, and its two ends are connected to the excitation signal source;

[0054] The two coils are receiving coils, symmetrically distributed on both sides of the middle coil within the rigid island area. The two receiving coils are connected in series with opposite polarities. The two ends of the series connection are respectively connected to the signal transmission interface to form a differential structure as a differential signal output terminal for connecting to external detection instruments.

[0055] The differential structure is configured such that when the sensor is in a defect-free or uniform conductivity detection environment, the signals induced by the receiving coils on both sides cancel each other out; when there are local defects or conductivity changes, the induced signals become unbalanced, thereby generating a differential signal characterizing the defect at the series output terminal of the coils on both sides.

[0056] The rigid island region is designed as a square area, and a local thickening structure is formed by multiple overprinting processes.

[0057] Figure 1 This diagram illustrates the flexible differential sensor attached to the lead-sealed cable structure in this embodiment. Specifically, the flexible insulating substrate is a polyimide film with a thickness of 50 μm, a length of 315 mm, and a width of 100 mm. This material possesses excellent high-temperature resistance, low dielectric loss, and good chemical stability, enabling it to adapt to complex industrial environments. Before use, the polyimide film is sequentially cleaned in anhydrous ethanol and deionized water to remove surface oil and particulate contaminants. After cleaning, it is placed in an indoor environment to dry for several hours.

[0058] The coil layer is printed on the upper surface of the flexible insulating substrate using flexible PCB technology. In this embodiment, the coil layer includes eight pairs of differential coil units and an island-bridge strain isolation structure for supporting the detection coil units. The overall sensor size is 315mm × 100mm, with the eight pairs of coil units arranged in a 1×8 array, where one row corresponds to the circumferential direction of the pipe, eight columns correspond to the axial direction of the pipe, and the center-to-center distance between adjacent units is 39.5mm.

[0059] Each coil unit in the coil layer adopts a differential structure design, comprising three identical planar circular coils placed coaxially and perpendicularly. The specific parameters of the planar circular coils are as follows:

[0060] Inner diameter: 26.5mm, outer diameter: 35mm, number of turns: 8, wire width: 0.25mm, wire spacing: 0.25mm.

[0061] like Figure 2 As shown, in the diagram, the middle coil serves as the excitation coil, while the two side coils act as receiving coils. The positive terminal of the middle coil is connected to the positive terminal of a 10Vpp, 10MHz excitation signal, and the negative terminal is connected to the negative terminal of the excitation signal. The negative terminal of one side coil is connected to the positive terminal of the other side coil. The positive and negative terminals of the two side coils are respectively connected to the signal transmission interface for connecting to external testing instruments. The two side receiving coils are symmetrically distributed with respect to the middle excitation coil within the rigid island region, with the center of each side coil being 0.25mm from the center of the middle coil. This differential structure layout ensures that when the sensor detects a defect-free component or a fluid with uniform conductivity, the electromagnetic fields sensed by the two side receiving coils are completely identical. When a local defect or a change in internal conductivity is detected, the internal operating conditions affect the two side coils differently, thus generating a differential output signal.

[0062] To visually demonstrate the improvement in coil detection sensitivity brought about by the differential structure, COMSOL software was used to conduct a comparative analysis of the sensitivity of single-layer coils and differential coils. For example... Figures 3-4 As shown, the specific steps of its sensitivity comparison analysis are as follows:

[0063] (1) Derive the real and imaginary parts of the induced voltage of the excitation coil in the three-layer differential coil, and extract the phase signal as single-layer coil data;

[0064] (2) The phase amplitude of the differential coil as a whole is derived by using the real and imaginary parts of the induced voltage obtained by the difference between the upper and lower receiving coils, and is used as the differential coil data;

[0065] (3) Plot the curves of the phase of the two sensors as a function of the conductivity of the salt water, and compare the trends of change. It can be concluded that the differential coil unit has greater sensitivity.

[0066] During the process of the salt water conductivity increasing from 0 S / m to 10 S / m, for example... Figure 4 As shown in the middle left figure, the blue line represents the phase change corresponding to the real and imaginary parts of the overall induced voltage of the differential coil, decreasing from 89.2° to 80.4°; the yellow line represents the phase change corresponding to the real and imaginary parts of the induced voltage when the lower receiving coil is used as a single-layer coil, which shows almost no change; after magnification, as shown... Figure 4As shown in the middle right figure, the blue and red lines represent the phase changes of the real and imaginary parts of the induced voltage when the upper and lower receiving coils are used as single-layer coils, respectively, decreasing from 89.428° to 89.262° and 89.280°.

[0067] The magnitude of the phase change of the induced voltage signal received by the differential coil and the single-layer coil with the change of salt water conductivity indicates that the differential coil has greater sensitivity to changes in the conductivity of the measured object.

[0068] The island-bridge strain isolation structure consists of multiple rigid island regions and flexible meandering connecting bridges connecting the rigid island regions. Specifically, the rigid island regions are designed as square areas and a local thickening structure is formed by multiple overprinting processes. That is, three layers of conductive ink are repeatedly printed in the rigid island regions so that the thickness of the region after drying reaches about 3μm. The flexible meandering connecting bridge regions are printed with only one layer, with a thickness of about 1μm, thereby ensuring that the stiffness of the rigid island regions is significantly greater than that of the flexible meandering connecting bridges.

[0069] The flexible, meandering connecting bridge is designed with a serpentine wiring structure, which allows it to fully absorb tensile or compressive strain through geometric deformation when the sensor bends. The connecting bridge serves not only as a mechanical connection but also as an electrical interconnect, connecting the differential coil units within each rigid island to the signal transmission interface.

[0070] Based on the differential coil unit design drawings, a piezoelectric inkjet printing system was used to first print a flexible, meandering connecting bridge. Then, three rigid island regions were formed by multiple overlapping printings within the same area. Finally, the differential coil units were printed within these rigid island regions. After printing, the printed differential coil units were rapidly sintered using a sintering device. A polyimide precursor solution was coated onto the sintered coil layer to form a flexible insulating protective layer approximately 10 μm thick. This protective layer possesses excellent insulation properties and mechanical flexibility, effectively protecting the coil layer from physical damage and environmental corrosion. After the flexible insulating protective layer cured, it was cut along the sensor contour to expose the signal lead-out pads. The positive and negative terminals of the receiving coils in each coil unit were then thermo-pressed to the pads using anisotropic conductive adhesive. Finally, epoxy resin was applied to the connection points for reinforcement to improve connection reliability.

[0071] Another aspect of the present invention discloses Embodiment 2:

[0072] The method for fabricating a flexible differential sensor for nondestructive testing includes the following steps:

[0073] A flexible insulating substrate of suitable thickness and size is selected, and the material of the flexible insulating substrate is a polyimide film. Specifically, a polyimide (PI) film is selected as the flexible insulating substrate, with a thickness of 50 μm and a size of 315 mm × 100 mm. The film is immersed and washed in anhydrous ethanol and deionized water for 5 minutes respectively to remove surface dust, etc. After washing, it is placed in a ventilated place to air dry for 2 hours.

[0074] Rigid island regions, flexible meandering connecting bridges, and differential coil units are sequentially printed on the surface of a flexible insulating substrate using flexible PCB technology.

[0075] The rigid island area is set in 1 row and 8 columns. The 1st row corresponds to the circumferential direction of the cable lead sealing structure, and the 8th column corresponds to the axial direction of the cable lead sealing structure. The center of the rigid island area in the 1st column is 20mm away from the left edge of the flexible insulation substrate, and the center-to-center distance between adjacent rigid island areas is 39mm. The center of the rigid island area in the 8th column is 20mm away from the right edge of the flexible insulation substrate, and the center of each of the 8 rigid island areas is 50mm away from the top and bottom edges of the substrate. A locally thickened structure is formed by 3 layers of overprinting, and the final thickness is about 3μm, forming a square with a side length of 35mm.

[0076] A flexible, meandering connecting bridge is located between adjacent rigid island regions for mechanical connection and stress dispersion. Designed as a serpentine trace, it extends from the midpoint of the right edge of the first rigid island region to the midpoint of the left edge of the second, and so on. The flexible, meandering connecting bridge has a line width of 0.15 mm and a printing thickness of approximately 1 μm. Three layers of coaxial circular coils are printed within each rigid island region. The geometric center of the circular coils should coincide with the center point of the rigid island region. The coil parameters are: inner diameter 26.5 mm, outer diameter 35 mm, 8 turns, line width 0.25 mm, and line spacing 0.25 mm. The spacing between each coil layer is approximately 0.4 mm. The three coil layers are connected in a differential structure to form a coil unit.

[0077] After printing, the ink with high conductivity is placed in a sintering device to cure, and then naturally cooled after sintering.

[0078] A flexible insulating protective layer is attached to the coil layer. In a specific embodiment, a polyimide precursor solution is spin-coated onto the surface of the sintered coil layer to form a uniformly covered protective layer to ensure electrical insulation.

[0079] The coil unit is connected to the signal transmission interface. Specifically, die-cutting is performed in an area 10mm from the bottom of each differential coil unit to leave a 20mm×10mm signal lead-out pad area for each coil unit. The positive and negative terminals of the internal coil unit are connected to the solder joints, and conductive adhesive is used to connect the solder joints to the connecting wires. Epoxy resin is applied to the solder joint area to improve tensile strength and durability.

[0080] This preparation method first selects a 50μm thick polyimide (PI) film as a flexible insulating substrate, which is the optimal choice to ensure curved surface adhesion and process support. The 50μm thickness ensures that the sensor can closely adhere to structures with small curvature radii, such as cable lead sealing, and also provides a flat and stable substrate for subsequent multilayer printing. The substrate material is then cleaned using a two-step cleaning process of "anhydrous ethanol + deionized water" followed by natural drying. This not only removes oil and particles but also avoids secondary adhesion of electrostatic dust that might be introduced during drying. Interface cleanliness is a prerequisite for the subsequent strong bonding between the high-conductivity ink and the substrate, preventing microcracks, and directly affects the stability of the electrical pathways in the flexible state.

[0081] The thickness of the rigid island region is increased to 3μm through three-layer overprinting (far exceeding the 1μm of the connecting bridge), which is key to achieving mechanical decoupling. This localized thickening significantly improves the stiffness of the "island" region. When the sensor as a whole bends, the deformation is confined to the flexible "bridge" section, while the "island" carrying the core sensing element is almost unaffected by mechanical strain, thus ensuring that the inductance / impedance of the differential coil does not drift under curved surface conditions. The design of the rigid island size (35mm×35mm) and spacing (39mm), along with the serpentine routing of the meandering bridge, together constitute a precise strain guidance system. The presence of the serpentine bridge transforms tensile or bending stress into minute torsion of the connecting bridge, avoiding stress concentration at solder joints or coils, and greatly improving the sensor's fatigue life.

[0082] The design employs a vertically stacked three-layer coaxial coil, which, compared to a single-layer coil, increases the equivalent cross-sectional area and number of turns within the same planar dimensions, significantly enhancing the excitation magnetic field strength and signal reception sensitivity. The 0.4mm spacing between each layer precisely balances interlayer parasitic capacitance and magnetic coupling efficiency.

[0083] Printed using flexible PCB technology, the geometric parameters (26.5mm inner diameter, 35mm outer diameter, 8 turns) of the eight coil units were highly consistent. This high symmetry is the foundation for the effective operation of differential detection technology. In defect-free areas, the signals from the two coils cancel each other out; when a defect is encountered, the differential signal becomes prominent, thereby effectively suppressing common-mode noise (such as temperature drift and external electromagnetic interference) and extracting weak defect responses. The design of the coil's 26.5mm inner diameter and 35mm outer diameter is an optimization result for the curvature and defect feature size of specific inspected objects such as cable lead sealing, ensuring that the magnetic field lines can effectively penetrate the measured surface and cover the key inspection area.

[0084] The 1x8 array is configured to meet the circumferential and axial inspection requirements of the cable sealing structure. The layout of the first column, 20mm from the edge and 39mm from the center, ensures that the sensor coverage area (315mm × 100mm) matches the standard sealing size, enabling one-time coverage of a large inspection area and improving inspection efficiency. The 0.15mm linewidth of the connecting bridge falls within the scope of high-precision printing. While maintaining conductivity, the narrow-linewidth meandering structure has lower bending stiffness, further reducing stress transmission to the rigid islands and ensuring electrical stability.

[0085] The specific preparation process uses spin-coating of polyimide precursor solution instead of simple film coating, which has two major advantages:

[0086] First, it is a void-free filling: the solution can penetrate and fill the tiny grooves in the coil layer, and after curing, it forms a completely dense, bubble-free encapsulation layer, eliminating interlayer air, improving the stability of high-frequency signals, and preventing moisture corrosion.

[0087] Second, interface fusion: The precursor and the substrate PI material have similar chemical properties, and the interface bonding strength is extremely high after curing, and delamination will not occur when repeatedly bent.

[0088] Its signal output terminals have a mechanical enhancement effect. The pad area adopts a triple process of "die-cutting for pads + conductive adhesive connection + epoxy resin dotting": die-cutting ensures the consistency of the output position and the convenience of welding; conductive adhesive avoids thermal damage to the flexible substrate caused by high-temperature welding; epoxy resin dotting plays a stress buffering and anchoring role, enabling the fragile solder joint connection to withstand the cable pulling force, making it more reliable.

[0089] This fabrication method achieves strain isolation and surface conformal design by forming a rigid island-borne differential coil through localized thickening printing, combined with a serpentine bridge to disperse stress. The use of flexible PCB technology ensures the high precision and consistency of the multi-layer coaxial coils. Spin-coating for seamless encapsulation and epoxy anchoring enhances environmental protection and interface strength. This method solves the problem of electrical drift in flexible sensors attached to complex curved surfaces, combining high sensitivity, anti-interference capabilities, and engineering durability.

[0090] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. These changes involve related technologies well known to those skilled in the art, and all of them fall within the protection scope of the present invention.

[0091] Many other changes and modifications can be made without departing from the concept and scope of this invention. It should be understood that this invention is not limited to the specific embodiments, and the scope of this invention is defined by the appended claims.

Claims

1. A flexible differential sensor for non-destructive testing, characterized in that, It includes a flexible insulating substrate, a coil layer, a flexible insulating protective layer, and a signal transmission interface; The coil layer is printed on the upper surface of the flexible insulating substrate using flexible PCB technology and is completely covered under the flexible insulating protective layer. The signal transmission interface is located at one end of the sensor and is electrically connected to the coil layer for connecting to external detection instruments. The coil layer includes a differential coil unit group and an island-bridge type strain isolation structure for supporting the detection coil unit.

2. The flexible differential sensor for non-destructive testing according to claim 1, characterized in that, The island-bridge strain isolation structure includes multiple rigid island regions and a flexible meandering connecting bridge. The differential coil unit group includes multiple differential coil units, which are fixedly disposed within the rigid island regions. The flexible meandering connecting bridge connects the differential coil units in each rigid island region to the signal transmission interface and undergoes elastic deformation when the sensor as a whole undergoes bending deformation, so as to absorb or isolate the mechanical strain applied to the rigid island regions.

3. The flexible differential sensor for non-destructive testing according to claim 2, characterized in that, The rigid island region is a micro-region formed by local thickening printing, and its stiffness is greater than that of the flexible meandering connecting bridge.

4. The flexible differential sensor for non-destructive testing according to claim 3, characterized in that, The differential coil unit is a planar circular coil formed within a rigid island region. This planar circular coil is made using high-conductivity ink and inkjet printing process. Its line width, line spacing, and number of turns are all controlled according to preset parameters.

5. The flexible differential sensor for nondestructive testing according to any one of claims 2-4, characterized in that, The flexible meandering connecting bridge has a serpentine routing pattern, with a line width smaller than that of the rigid island coil, and is printed using high-conductivity ink.

6. The flexible differential sensor for non-destructive testing according to claim 5, characterized in that, The differential coil unit comprises three identical coils placed coaxially and perpendicularly, wherein: The intermediate coil is the excitation coil, and its two ends are connected to the excitation signal source; The two coils are receiving coils, symmetrically distributed on both sides of the middle coil within the rigid island area. The two receiving coils are connected in series with opposite polarities. The two ends of the series connection are respectively connected to the signal transmission interface to form a differential structure as a differential signal output terminal for connecting to external detection instruments. The differential structure is configured such that when the sensor is in a defect-free or uniform conductivity detection environment, the signals induced by the receiving coils on both sides cancel each other out; when there are local defects or conductivity changes, the induced signals become unbalanced, thereby generating a differential signal characterizing the defect at the series output terminal of the coils on both sides.

7. The flexible differential sensor for non-destructive testing according to claim 1, characterized in that, The differential coil unit group includes eight pairs of differential coil units.

8. The flexible differential sensor for non-destructive testing according to claim 3, characterized in that, The rigid island region is designed as a square area, and a local thickening structure is formed by multiple overprinting processes.

9. A method for fabricating a flexible differential sensor for nondestructive testing, characterized in that, Includes the following steps: Select a flexible insulating substrate of appropriate thickness and size; Rigid island regions, flexible meandering connecting bridges, and differential coil units are sequentially printed on the surface of a flexible insulating substrate using flexible PCB technology. After printing, the coil is placed in a sintering device to cure the highly conductive ink. A flexible insulating protective layer is then attached to the coil layer, and the coil unit is connected to the signal transmission interface.

10. The method for fabricating a flexible differential sensor for nondestructive testing according to claim 9, characterized in that, The flexible insulating substrate is made of polyimide film.

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