Flexible large strain sensor, method of manufacture and use

Abstract

The present application relates to the technical field of flexible sensor, and discloses a flexible large-strain sensor, a preparation method and application, aiming to solve the problems of small range, poor adhesion of traditional sensors in large-strain monitoring of solid engine grain and other soft materials, and poor signal linearity and repeatability, complex preparation process and high cost of existing flexible sensors. A polymer film is used to make a flexible insulating substrate; a conductive film is prepared on the flexible insulating substrate to form a composite film layer structure; according to a preset pattern of the conductive film, laser etching and scribing are performed on the conductive film according to a first type of vector track, only penetrating the conductive film, to divide out strain resistance and other resistance, forming an off-axis serpentine structure; according to a preset pattern of the flexible insulating substrate, laser etching and scribing are performed on the composite film layer structure according to a second type of vector track, penetrating the composite film layer structure, to etch out the flexible insulating substrate of the sensor; lead-out wires are welded on the lead-out electrodes and encapsulated to obtain the flexible large-strain sensor.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible sensor, in particular, to a flexible large-strain sensor, a preparation method and application. BACKGROUND

[0002] The present application relates to the technical field of flexible sensor, in particular, to a flexible large-strain sensor, a preparation method and application.

[0003] Traditional metal strain gauges have the advantages of strong anti-interference ability, but their inherent rigidity is large, they cannot be well attached to soft materials such as solid propellants, and the typical measurement range is usually less than 2%, which is difficult to meet the large deformation measurement requirement. On the other hand, most existing flexible sensors achieve low modulus and large range through changes in the microstructure of the conductive network, but they generally have poor signal response repeatability and low linearity, resulting in insufficient reliability of measurement data. In addition, their preparation process is often complex, which is not conducive to mass production and stable application.

[0004] In the prior art, invention patent CN113551791A discloses a flexible sensor with a curved strain resistance, which adopts a rapid preparation scheme and is suitable for mass production. However, this sensor only relies on the bending part of a single wire to provide strain response, which limits its sensitivity in local small area strain measurement, and does not provide actual measurement data in specific application scenarios, so its practicality and effectiveness need to be verified. Another invention patent CN118776448A proposes a flexible sensor for full-site monitoring of solid propellants, which can achieve good attachment, but its manufacturing process includes multiple delicate steps, has high process complexity, is prone to errors, and is difficult to ensure consistency and cost control in mass production. SUMMARY

[0005] The present application provides a flexible large-strain sensor, a preparation method and application, which has high measurement accuracy and high linearity, is suitable for ultra-large strain range, has compact structure and is easy to integrate, has simple and reliable manufacturing method to support mass production, and can be effectively applied to harsh scenes such as solid propellant strain measurement, to solve the technical problems that existing flexible large-strain sensors are difficult to simultaneously realize mass production, high measurement accuracy and high linearity, and reliable measurement in actual application scenarios.

[0006] According to one aspect of the present application, a method for manufacturing a flexible large-strain sensor is provided, comprising the following steps: S100, manufacturing a flexible insulating substrate by using a polymer film; S200, manufacturing a conductive film on the flexible insulating substrate to form a composite film layer structure; S300, performing laser etching and scribing on the conductive film according to a preset pattern of the conductive film and a first type of vector trajectory, only penetrating the conductive film to separate a strain resistance and other resistances and form an off-axis serpentine structure; S400, performing laser etching and scribing on the composite film layer structure according to a preset pattern of the flexible insulating substrate and a second type of vector trajectory, penetrating the composite film layer structure to etch the flexible insulating substrate of the sensor; S500, welding lead wires on lead electrodes and packaging to obtain the flexible large-strain sensor.

[0007] Further, in the step S300 and the step S400, the distance by which the vector trajectory deviates from the contour and extends outward is equal to half of the diameter of the laser spot; the laser is an ultraviolet picosecond laser or an ultraviolet femtosecond laser, and the positioning accuracy of the laser is less than 2 microns; in the laser etching and scribing process, a single vector trajectory is selected for etching; or in the laser etching and scribing process, multiple parallel vector trajectories are selected for etching, and the gap between the multiple parallel vector trajectories is 1 / 4-1 times the width of the laser spot of the used laser.

[0008] Further, the gap between the multiple parallel vector trajectories is 1 / 3-1 / 2 times the width of the laser spot of the used laser.

[0009] Further, in the step S300, the etching and scribing parameters for forming the off-axis serpentine structure by laser etching and scribing are as follows: the ultraviolet laser power density is 0.05 W / μm²-0.5 W / μm², and the cutting rate is 50㎜ / s-200㎜ / s; in the step S400, the etching and scribing parameters for forming the flexible insulating substrate by laser etching and scribing are as follows: the ultraviolet laser power density is 0.1 W / μm²-1 W / μm², and the cutting rate is 50㎜ / s-200㎜ / s.

[0010] Further, in the step S200, the composite film layer structure is prepared by spin coating, and the spin coating speed is 100 r / min-500 r / min, and the spin coating time is 2 min-10 min.

[0011] Further, the film material of the flexible insulating substrate is polydimethylsiloxane; or the film material of the flexible insulating substrate is one of the following insulating polymer films: silicone rubber, polyimide, polyester, polyurethane, phenolic resin, and epoxy resin.

[0012] Further, the material of the conductive film is one of the following metals: constantan, new constantan, gold, aluminum, copper, nickel-chromium alloy, nickel-chromium-aluminum alloy, iron-chromium-aluminum alloy, platinum, and platinum-tungsten alloy; or the material of the conductive film is one of the following conductive materials: semiconductor single-crystal silicon and graphene.

[0013] Furthermore, step S500 is followed by S600, casting and encapsulation.

[0014] According to another aspect of the present invention, a flexible large strain sensor is also provided, which is fabricated using the above-described method for fabricating a flexible large strain sensor, comprising a flexible insulating substrate, a conductive film, and lead-out electrodes; the flexible insulating substrate has a planar structure, and the conductive film has a compact off-axis serpentine structure with multiple series-connected strain resistors, the compact off-axis serpentine structure comprising a curved section and a straight section, the curved section being a strain resistor and the straight section being other resistors; the conductive film and lead-out electrodes are located on the flexible insulating substrate, and the two ends of the conductive film are respectively connected to the lead-out electrodes.

[0015] Furthermore, the first lead-out electrode is connected to the straight segment, and the straight segment and the curved segment are connected end to end and arranged alternately; when the straight segment and the curved segment alternately approach the second lead-out electrode, the curved segment reverses direction and the straight segment and the curved segment are arranged alternately towards the first lead-out electrode, and so on until the predetermined number of reversals are completed and the second lead-out electrode is connected through the straight segment.

[0016] Furthermore, straight segments and curved segments are arranged alternately and repeatedly to form multiple straight segments arranged at equal intervals from the first lead-out electrode to the second lead-out electrode; and to form a group of concentric arc segments composed of multiple concentric arc segments arranged sequentially from the inside to the outside, with the multiple concentric arc segment groups arranged at equal intervals from the first lead-out electrode to the second lead-out electrode.

[0017] Furthermore, the curved segment is configured to be offset from the central axis in the direction of the center of the circle and connected to the corresponding straight segment at both ends.

[0018] Furthermore, the thickness of the conductive film is less than or equal to 50 micrometers; the minimum width of the strain gauge is less than or equal to 20 micrometers; and the width of other resistors is three times or more the minimum width of the strain gauge.

[0019] Furthermore, the thickness of the conductive film is less than or equal to 20 micrometers; the minimum width of the strain gauge is less than or equal to 15 micrometers; and the width of other resistors is more than 5 times the minimum width of the strain gauge.

[0020] Furthermore, the overall thickness of the packaged sensor is ≤3mm.

[0021] According to another aspect of the present invention, an application of a flexible large strain sensor is also provided, wherein the aforementioned flexible large strain sensor is used for strain measurement of propellant grains in a solid rocket motor.

[0022] The present invention has the following beneficial effects:

[0023] 1. Achieved high-precision integrated manufacturing of function and structure: Step S300 selectively etches the conductive film through the first type of vector trajectory, and simultaneously completes the forming of the off-axis serpentine structure and the functional partitioning of the strain resistor and other resistors in one processing. This "one-time scribing, simultaneous realization" method avoids the errors that may be introduced by multiple patterning, alignment and etching processes in traditional photolithography and other processes, ensuring the high precision and consistency of the conductive pattern, and guaranteeing the uniformity and stability of the sensor's electrical performance from the source of manufacturing.

[0024] 2. Improved production efficiency and product yield: Step S400 cuts the entire composite film layer using the second type of vector trajectory, simultaneously completing the final shaping of the sensor's outer contour. Based on the same laser processing platform as step S300, it eliminates the need to change workpieces or perform secondary clamping and alignment, thus eliminating edge cutting errors and pattern-substrate misalignment caused by multiple positioning. This significantly simplifies the process flow, shortens processing time, facilitates large-scale mass production, and improves the overall product yield.

[0025] 3. Ensures the integrity and reliability of the final sensor structure: The method of this invention follows the processing sequence from the inside out (first etching the inner conductive pattern, then cutting the outer substrate contour), avoiding the impact of edge damage or stress concentration that may occur when cutting the outline first and then processing the fine pattern on the sensitive conductive structure. This ensures that the flexible substrate provides complete and uniform support and protection for the brittle conductive pattern, thereby enhancing the mechanical durability and signal stability of the sensor in subsequent tensile deformation.

[0026] 4. Balancing the demands of large-scale production and high performance: The core of the entire method relies on programmable laser etching technology. By simply adjusting the digital drawings of two types of vector trajectories, it is possible to quickly and flexibly design sensors of different sizes and sensitivities. It integrates complex micro-pattern processing and shape cutting into a set of efficient and digital processes, fundamentally solving the contradiction between "high process complexity" and "poor performance consistency" often faced in the fabrication of traditional flexible sensors. This provides a reliable path suitable for industrialization and promotion for the fabrication of high-performance flexible strain sensors.

[0027] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0028] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0029] Figure 1This is a two-dimensional annotated diagram of a preferred embodiment of the "compact" off-axis serpentine flexible large strain sensor of the present invention;

[0030] Figure 2 This is a three-dimensional annotated diagram of a preferred embodiment of the "compact" off-axis serpentine flexible large strain sensor of the present invention;

[0031] Figure 3 This is a physical image of a preferred embodiment of the "compact" off-axis serpentine flexible large strain sensor of the present invention, wherein... Figure 3 (a) is a comparison image of the actual object. Figure 3 (b) is a picture of tweezers picking up an object;

[0032] Figure 4 This is a diagram of a single laser etching vector trajectory cutting a conductive film according to a preferred embodiment of the present invention;

[0033] Figure 5 This is a diagram of multiple etching vector trajectories of the laser cutting the conductive film according to a preferred embodiment of the present invention;

[0034] Figure 6 This is a diagram of a preferred embodiment of the present invention, showing a "compact" off-axis serpentine flexible large strain sensor used to measure a uniaxial tensile test of a dumbbell-shaped solid propellant specimen.

[0035] Figure 7 This is a preferred embodiment of the present invention, showing the tensile-to-resistance ratio curve of a "compact" off-axis serpentine flexible large strain sensor for measuring the strain of a solid propellant specimen.

[0036] Legend:

[0037] 100. Flexible insulating substrate; 200. Conductive film; 300. Lead electrode; 400. Lead wire; 500. Strain gauge; 600. Other resistors; 700. Single etching vector track; 800. Multiple etching vector tracks. Detailed Implementation

[0038] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered below. Unless otherwise stated, the raw materials and reagents used in the following embodiments are commercially available or can be prepared by known methods.

[0039] The method for fabricating a flexible large strain sensor according to this embodiment includes the following steps: S100, fabricating a flexible insulating substrate 100 using a polymer thin film; S200, fabricating a conductive film 200 on the flexible insulating substrate to form a composite film structure; S300, according to the preset pattern of the conductive film 200, performing laser etching on the conductive film 200 according to a first type of vector trajectory, penetrating only the conductive film 200 to separate the strain resistor 500 and other resistors 600, forming an off-axis serpentine structure; S400, according to the preset pattern of the flexible insulating substrate 100, performing laser etching on the composite film structure according to a second type of vector trajectory, penetrating the composite film structure to etch out the flexible insulating substrate 100 of the sensor; S500, welding lead wires 400 on the lead electrode 300 and encapsulating it to obtain a flexible large strain sensor. The method for fabricating a flexible large strain sensor of the present invention includes step S300, which involves selectively etching the conductive film 200 using a first-type vector trajectory. This simultaneously completes the forming of the off-axis serpentine structure and the functional partitioning of the strain gauge 500 with other resistors 600 in a single processing step. This "one-time scribe, simultaneous realization" method avoids errors that may be introduced by multiple patterning, alignment, and etching processes in traditional photolithography, ensuring high precision and consistency of the conductive pattern and guaranteeing the uniformity and stability of the sensor's electrical performance from the manufacturing source. Step S400 involves cutting the entire composite film layer using a second-type vector trajectory, simultaneously completing the final shaping of the sensor's outer contour. Based on the same laser processing platform as step S300, this method eliminates the need to change workpieces or perform secondary clamping and alignment, thus eliminating edge cutting errors and pattern-substrate misalignment caused by multiple positioning steps. This significantly simplifies the process flow, shortens processing time, facilitates large-scale mass production, and improves the overall product yield. This invention follows a processing sequence from the inside out (etching the inner conductive pattern first, then cutting the outer substrate contour), avoiding the edge damage or stress concentration that may occur when cutting the outline first and then processing the fine pattern, thus ensuring that the flexible insulating substrate 100 provides complete and uniform support and protection for the brittle conductive pattern. This enhances the mechanical durability and signal stability of the sensor during subsequent tensile deformation. The core of the method relies on programmable laser etching technology. By simply adjusting the digital drawings of two types of vector trajectories, sensor designs of different sizes and sensitivities can be quickly and flexibly realized. It integrates complex micro-pattern processing and outline cutting into a single, efficient, and digital process, fundamentally solving the contradiction between "high process complexity" and "poor performance consistency" often faced in traditional flexible sensor fabrication. This provides a reliable path suitable for industrialization and promotion for the fabrication of high-performance flexible strain sensors.The fabrication method provided by this invention, through an innovative design and sequential execution of a two-step laser processing strategy of "functional etching first, then structural cutting," achieves integrated, high-precision, and high-efficiency manufacturing of the sensor's internal functional patterns and external overall structure. This not only simplifies the process flow and avoids multiple alignment errors, greatly improving the efficiency and product consistency of mass production, but also structurally ensures the mechanical reliability and electrical performance stability of the final sensor. It successfully solves the technical challenge of balancing high precision, high performance, low cost, and mass production in existing technologies. Other resistors 600 generally refer to the linear resistors other than the strain gauge 500, connecting the strain gauge 500 of each arc segment and the lead-out electrode 300. This conductive film provides relatively small strain deformation during structural tensile deformation, mainly undergoing translational motion along the tensile direction and rigid body displacement around the center of the arc segment, primarily providing larger elongation deformation and circuit connections for the structure.

[0040] In this embodiment, in steps S300 and S400, the distance by which the vector trajectory deviates from the contour and extends outward is equal to half the diameter of the laser spot. The laser used is an ultraviolet picosecond laser or an ultraviolet femtosecond laser, with a repeatability accuracy of less than 2 micrometers. During laser etching, a single vector trajectory is selected for etching; or multiple parallel vector trajectories are selected for etching, with the gap between the parallel vector trajectories being 1 / 4 to 1 times the width of the laser spot. The distance by which the vector trajectory deviates from the contour and extends outward is equal to half the diameter of the laser spot, ensuring the integrity of the laser-processed contour and the accuracy of the conductive pattern dimensions. The laser spot has a certain energy distribution, with the highest energy at its center. By offsetting the trajectory outward by half the spot diameter, the center of the spot will be positioned on the edge line of the theoretical contour, thereby ensuring that the effective energy area of ​​the spot can completely cover and remove all material that needs to be removed outside the contour line, while avoiding thermal effects or accidental damage to the functional areas (such as conductive lines) that should be retained inside the contour line. Ultraviolet wavelengths have higher photon energy and a lower diffraction limit, which is beneficial for achieving fine processing. The ultrashort pulse characteristics of picosecond or femtosecond means that the interaction time between the laser and the material is extremely short, and the energy is deposited instantly in a cold processing manner, which greatly suppresses the generation of heat-affected zones and avoids the material melting, splashing, carbonization and other phenomena caused by thermal effects in traditional long pulse or continuous laser processing. Thus, when processing brittle conductive films 200 (such as metal films) and polymer substrates, clean, burr-free and thermally deformable cutting edges can be obtained. The repeatability positioning accuracy of less than 2 micrometers ensures that the above-mentioned biasing strategy and fine trajectory can be executed with high repeatability and precision from the perspective of equipment performance, which is the basis for achieving high consistency mass production. Choosing between single or multiple parallel vector trajectories for etching and setting the gap provides flexibility and optimization capabilities to meet different processing needs. Single-trajectory etching is suitable for finer lines or initial prototyping, offering high efficiency. Multiple parallel-trajectory etching is used for areas requiring the removal of larger areas of material (such as cutting an S400 shape or preparing wide lines). By decomposing wide slits into multiple closely spaced fine trajectories for scanning, heat accumulation can be effectively controlled, and cutting quality and efficiency can be improved. Setting the gap between parallel trajectories to 1 / 4 to 1 times the laser spot width ensures that the material is completely removed while achieving optimal processing efficiency and sidewall quality. A gap that is too small (e.g., less than 1 / 4) may lead to excessive overlap, causing heat accumulation and low efficiency. A gap that is too large (e.g., more than 1 times) may leave material that has not been completely removed.By synergistically limiting the laser trajectory offset, laser selection, positioning accuracy, and etching mode, significant improvements in processing accuracy, suppression of thermal damage, and optimization of processing quality and efficiency are achieved. This ensures that the two core steps of functional etching and structural cutting can achieve micron-level fine patterning and clean contour shaping with high repeatability and consistency through cold processing. This fundamentally guarantees the dimensional accuracy and electrical performance consistency of the final sensor, especially the off-axis serpentine conductive structure. Preferably, the gap between multiple parallel vector trajectories is 1 / 3 to 1 / 2 times the laser spot width of the laser used.

[0041] In this embodiment, the etching parameters for forming the off-axis serpentine structure by laser etching in step S300 are: ultraviolet laser power density 0.05W / μm²-0.5W / μm², and cutting rate 50mm / s-200mm / s. The short wavelength and high photon energy of the ultraviolet laser make it easily absorbed by the conductive film 200, facilitating fine processing and ensuring that the laser energy can accurately and effectively vaporize and remove the conductive film 200, penetrating only the conductive film 200 without damaging the underlying flexible insulating substrate 100. This clearly separates the strain gauge 500 from the other resistors 600, forming an off-axis serpentine structure with precise boundaries and consistent pattern. The matching of the power density and cutting rate constitutes an optimized energy input window, ensuring that the energy injected per unit time and per unit area is sufficient to effectively remove the conductive film 200 without being excessive, thus maximizing heat suppression. Effects such as melting, splashing, carbonization, or thermal damage to the substrate are mitigated, protecting the electrical integrity of the area containing the reserved 500mm strain gauge and other 600mm strain gauges. The power density and cutting rate parameters achieve a good balance between processing efficiency and quality. A scanning rate of 50mm / s-200mm / s ensures a reasonable processing cycle and production efficiency, avoiding inefficiency caused by excessively low speeds. Simultaneously, the power density ensures uniform and controllable etching trajectory width and depth, stable processing, and reduces the likelihood of quality defects due to parameter fluctuations, thus contributing to consistent performance in mass production. Precise control of energy input through laser parameters achieves high-precision, low-thermal-damage etching of the conductive film, ensuring accurate functional zoning and consistent electrical performance of the off-axis serpentine structure, while also balancing processing efficiency and process stability.

[0042] In this embodiment, the etching parameters for forming a flexible insulating substrate by laser etching in step S400 are: ultraviolet laser power density 0.1W / μm²-1W / μm², and cutting rate 50mm / s-200mm / s. Compared to step S300, which only requires removing the conductive film 200, step S400 requires simultaneously penetrating both the upper conductive film 200 and the lower flexible insulating polymer substrate, resulting in a larger material removal volume and greater difficulty. Increasing the power density to 0.1W / μm²–1W / μm² provides sufficient energy density for the laser, ensuring a clean and complete cut of the entire composite film structure in one operation, forming a neat and precisely sized sensor outline. This avoids problems such as incomplete cutting or edge burrs caused by insufficient energy. Maintaining a high cutting rate (50mm / s–200mm / s) similar to the inner layer etching, this parameter combination ensures efficient processing speed while maintaining cutting capability, thus ensuring production efficiency. Matching the rate and power prevents problems caused by excessively slow speeds. The extended laser dwell time effectively suppresses excessive heat accumulation, preventing the polymer substrate from melting, deforming, or carbonizing due to overheating, thus ensuring the quality and insulation performance of the cut edges. Since steps S400 and S300 are performed continuously on the same laser processing platform, secondary workpiece clamping and positioning are unnecessary. The outer layer cutting uses a scanning rate similar to the inner layer functional etching (50mm / s-200mm / s), which facilitates the entire system operating under similar dynamic performance. Combined with higher power density to accommodate thicker processing materials, this ensures extremely high relative positional accuracy between the second and first type of vector trajectories, fundamentally eliminating the pattern-substrate misalignment problem that may occur due to multiple alignments. This achieves integrated precision manufacturing of the sensor's internal functional structure and overall external contour. Step S400, by providing a higher energy density than step S300, ensures complete cutting of the composite layer structure. Simultaneously, through a scanning rate coordinated with the inner layer etching, it achieves efficient and high-quality forming of the sensor's external contour and ensures high-precision alignment between the inner and outer layer patterns, guaranteeing the integrity, dimensional accuracy, and mechanical reliability of the final sensor structure.

[0043] In this embodiment, step S200 uses spin coating to prepare the composite film structure, with a spin coating speed of 100 r / min-500 r / min and a spin coating time of 2 min-10 min. Preferably, step S200 uses spin coating to prepare the composite film structure, with a spin coating speed of 200 r / min and a spin coating time of 10 min. Setting the spin coating speed to 100 r / min-500 r / min and maintaining it for 2 min-10 min, this combination of low-speed and long-time process parameters adopts a coating strategy close to static leveling, ensuring that the polymer precursor solution or conductive paste has sufficient flow and spreading time on the surface of the flexible insulating substrate 100. Thus, under the combined action of surface tension and centrifugal force, thickness unevenness, swirl marks, or edge effects are effectively eliminated, ultimately forming an extremely thin and highly uniform conductive film. The high controllability of film thickness and uniformity lays the foundation for subsequent laser etching to prepare strain gauges 500 and other resistors 600 with consistent electrical properties. Low-speed spin coating reduces defects such as bubbles that may be generated inside the solution due to rapid shearing. Simultaneously, the gentle film-forming process facilitates the full wetting and orderly arrangement of the conductive material on the polymer substrate surface, thereby enhancing the interfacial bonding between the conductive film 200 and the flexible insulating substrate 100. The 2-10 minute spin coating time also ensures that the solvent can evaporate fully and slowly, avoiding intrinsic defects such as pinholes and cracks caused by excessively rapid evaporation, thus guaranteeing the compactness and integrity of the final composite film structure. A uniform, defect-free conductive film 200 with good bonding to the flexible insulating substrate 100 is a prerequisite for the high-precision laser etching described in S300 and S400. The prepared conductive film 200 exhibits high uniformity in its physical and chemical properties at both the macroscopic and microscopic scales, making the energy absorption and material removal processes during laser etching more stable and controllable. This significantly reduces the risk of uneven etching line widths, depths, or even breakdown failures due to unevenness in the film layer itself, thereby improving the reliability and yield of the entire fabrication process. One of the key prerequisites for mass production.

[0044] In this embodiment, the film material of the flexible insulating substrate 100 is polydimethylsiloxane; or the film material of the flexible insulating substrate 100 is one of silicone rubber, polyimide, polyester, polyurethane, phenolic resin, or epoxy resin insulating polymer film. The materials selected for the flexible insulating substrate 100 (such as PDMS, silicone rubber, and polyurethane) all have the common characteristics of low elastic modulus and high elongation at break, which enables the prepared flexible insulating substrate 100 to withstand large bending, stretching, or torsional deformation without breaking, thereby providing a reliable mechanical support platform for the conductive sensing structure attached thereto. The mechanical properties of this type of flexible insulating substrate 100 can be well matched with the measured object such as solid propellant, ensuring that the sensor can deform accordingly without debonding or slipping when large strain occurs, thereby transmitting strain truthfully and accurately. The flexible insulating substrate 100 is made of high-performance polymer insulating materials with extremely high volume resistivity and breakdown field strength. These materials effectively isolate the sensing circuit from the external environment, preventing short circuits or signal leakage and ensuring the stability and signal-to-noise ratio of the sensor's electrical signal acquisition. In addition, materials such as polyimide (PI) have excellent chemical stability and resistance to high and low temperatures, and polydimethylsiloxane (PDMS) has hydrophobicity, enabling the sensor to adapt to complex working environments and improving the long-term environmental durability and reliability of the device. The flexible insulating substrate 100 is made from a variety of mainstream flexible insulating materials, which are highly compatible with the preparation processes such as spin coating (S200) and laser etching (S300, S400) in the method steps of this invention. For example, PDMS is easy to mold and adheres well to a variety of conductive materials; PI has a smooth surface, which is conducive to uniformly coating the conductive film 200. This selectivity provides flexibility for material selection based on different performance priorities (such as stability at extreme temperatures, biocompatibility, or cost control), while ensuring that the selected materials can be smoothly integrated into the overall preparation process.

[0045] In this embodiment, the conductive film 200 is made of one of the following metals: constantan, neoconstantan, gold, aluminum, copper, nickel-chromium alloy, nickel-chromium-aluminum alloy, iron-chromium-aluminum alloy, platinum, or platinum-tungsten alloy. The conductive film 200 is also made of semiconductor single-crystal silicon or graphene-based conductive materials. The selected metal or alloy materials (such as constantan, nickel-chromium alloy, platinum-tungsten alloy, etc.) and semiconductor single-crystal silicon all exhibit a clear piezoresistive effect, meaning their resistivity changes regularly with mechanical strain. This provides a reliable and predictable strain-resistance signal conversion mechanism for the sensor, forming the physical basis for achieving high-precision, high-linearity measurements. Metal alloy materials (such as constantan) are known for their excellent linearity and temperature stability, while semiconductor materials (such as single-crystal silicon) provide extremely high sensitivity (high strain coefficient). Graphene combines excellent conductivity and mechanical strength; its resistance change originates from the change in carrier mobility caused by lattice deformation. The diversity of material selection provides a wide and reliable range of choices for designs with different sensitivity, linearity, and temperature stability requirements. The conductive film 200 is made of a metal or alloy material with excellent conductivity and mechanical stability, maintaining stable electrical connections and a low temperature coefficient of resistance during repeated stretching and deformation, ensuring the reliability and repeatability of the sensor output signal. New materials such as graphene also possess high conductivity and fatigue durability. High-quality, dense films can be formed on the flexible insulating substrate 100 through sputtering, evaporation, and other methods, and are highly compatible with laser etching processes. This ensures that the strain gauges 500 and other resistors 600 formed by laser segmentation have consistent and stable initial electrical properties, laying the foundation for the overall performance consistency of the sensor. Although most of the listed bulk materials have limited ductility, their brittleness is overcome by fabricating them into thin films and integrating them into an "off-axis serpentine" flexible structure design. Supported by the flexible insulating substrate 100, the thin-film conductive material can adapt to large strains through structural deformation rather than material extension, thus achieving a perfect combination of rigid functional materials and the flexible insulating substrate 100, resolving the contradiction that traditional metal strain gauges cannot be used for large strain measurements.

[0046] In this embodiment, step S500 is followed by step S600: casting and encapsulation. Optionally, the encapsulation film material is polydimethylsiloxane (PDMS) or Ecoflex series silicone rubber. Optionally, the thickness of the encapsulation film is 0.2 mm to 0.25 mm. By using flexible polymers such as polydimethylsiloxane (PDMS) or Ecoflex series silicone rubber for casting and encapsulation, a continuous, dense, and robust encapsulation layer can be formed outside the sensitive conductive pattern structure and solder joints. This effectively isolates direct contact with moisture, oxygen, dust, and chemicals in the external environment, preventing oxidation, corrosion, or other physical and chemical property degradation of the conductive material, thereby significantly improving the long-term environmental stability and service life of the sensor under complex operating conditions. The encapsulation layer physically isolates the delicate conductive structure from the external environment, preventing damage during installation and use due to direct friction, scratches, or accidental impacts. Simultaneously, the selected encapsulation material (such as PDMS or Ecoflex) possesses excellent flexibility and high elasticity, allowing it to deform in tandem with the flexible insulating substrate 100 and the sensing structure. This avoids excessive rigidity of the encapsulation layer constraining the overall deformation of the sensor or introducing additional stress, ensuring that the encapsulated sensor can still freely sense large strains without distortion. Limiting the thickness of the encapsulation film to a relatively thin range of 0.2 mm to 0.25 mm provides ample protection while minimizing the impact of the encapsulation material itself on the sensor's flexibility. The encapsulation layer completely covers the circuitry and solder joints without significantly increasing the overall stiffness of the sensor, ensuring that strain energy is efficiently transferred from the measured object through the encapsulation layer to the internal sensitive structure. Furthermore, this thickness effectively protects mechanically weak points such as solder joints, preventing failure due to stress concentration during repeated stretching.

[0047] like Figure 1 and Figure 2As shown, the flexible large strain sensor of this embodiment is fabricated using the aforementioned method and includes a flexible insulating substrate 100, a conductive film 200, and lead-out electrodes 300. The flexible insulating substrate 100 has a planar structure, and the conductive film 200 has a compact off-axis serpentine structure with multiple series-connected strain gauges 500. The compact off-axis serpentine structure includes a curved section and a straight section. The curved section is a strain gauge 500, and the straight section is another resistor 600. The conductive film 200 and the lead-out electrodes 300 are located on the flexible insulating substrate 100, and the two ends of the conductive film 200 are respectively connected to the lead-out electrodes 300. The compact off-axis serpentine structure is a mechanical structure that achieves extensibility through geometric design. When the flexible insulating substrate 100 is under tension, it adapts to deformation through the stretching of its curved section and the deflection of its straight arm section, thereby significantly reducing the actual strain applied to the conductive material itself. This makes it possible to achieve large strain measurement using high-performance conductive materials with limited intrinsic extensibility (such as metal alloys), fundamentally solving the drawback of the small strain range of traditional metal strain gauges. Designing the conductive film 200 as a multi-strand serpentine structure significantly extends the total effective inductive conductive path length. According to the resistance law (R=ρL / A), a longer initial length (L) means that the same absolute elongation (ΔL) can produce a larger relative resistance change rate (ΔR / R), thus significantly improving the sensor's sensitivity (i.e., strain coefficient GF). Simultaneously, the multi-strand series design increases the resistance value of the sensitive area (strain resistor 500), helping to reduce the influence of lead resistance and contact resistance, thereby improving the signal-to-noise ratio and anti-interference capability of the measurement signal. By concentrating the sensing function in the bending section (defined as strain resistor 500) and assigning the interconnection function to the straight section (defined as other resistors 600), functional zoning is achieved. During stretching, the bending section undergoes more significant and uniform deformation due to its geometric characteristics, and its resistance change dominates the overall signal output. The resistance change in the straight section is relatively small, ensuring that the sensor's resistance change mainly originates from the structurally defined and predictably behaving sensitive area, rather than the disordered changes of the entire network. This contributes to better response linearity, repeatability, and consistency. The compact off-axis design maximizes the path length within a limited substrate area, resulting in high space utilization. This allows the sensor to achieve large strain measurement capabilities while miniaturizing its overall size, making it easier to deploy and integrate in space-constrained environments (such as inside solid rocket motor propellant grains). This invention's flexible large strain sensor, with its compact off-axis serpentine structure featuring multiple 500Ω series strain gauges, cleverly unifies characteristics that are difficult to achieve simultaneously in traditional technologies: large strain capacity, high sensitivity, good linearity, and compact size. This enables the sensor to reliably convert mechanical strain into significant, stable, and linear electrical signal changes, thereby meeting the demands of demanding applications (such as health monitoring of solid rocket motor propellant grains) and high-performance requirements.

[0048] like Figure 1 As shown, in this embodiment, the first lead-out electrode is connected to the straight segment, and the straight segment and the curved segment are connected end to end and arranged alternately. When the straight segment and the curved segment alternately approach the second lead-out electrode, the curved segment reverses direction and the straight segment and curved segment are arranged alternately towards the first lead-out electrode. This process is repeated until the predetermined number of reversals, after which the second lead-out electrode is connected through the straight segment. The reversal arrangement of the compact off-axis serpentine structure means that the conductive line is not directly connected when it approaches the lead-out electrode 300, but is rotated 180 degrees through the curved segment and then arranged alternately in the opposite direction. This process can be repeated many times (a predetermined number of times), so that the effective sensing area of ​​the sensor (i.e., all the series-connected strain resistors 500) is compactly limited to the limited area between the two lead-out electrodes 300. This achieves the integration of the longest sensitive line with the smallest footprint, greatly improving the space utilization and structural compactness of the sensor, so as to facilitate its arrangement in space-constrained situations (such as inside the propellant grain of a solid rocket motor). All the serpentine loops formed by reversal (compact off-axis serpentine structure) are arranged in the same area on the flexible insulating substrate 100. When strain occurs in this area, all the series-connected sensitive units (bending sections) will be in a basically identical large strain environment, undergoing predictable and directional deformation together. This avoids signal cancellation or nonlinear superposition caused by the distribution of sensing units in different strain regions, allowing the resistance changes of all sensitive units to accumulate linearly and contribute to the overall signal output, thus ensuring high output signal amplitude (high sensitivity) and excellent response consistency and linearity. The wiring structure of the compact off-axis serpentine structure forms a uniform mechanical system as a whole. Through multiple reversals, the distribution of conductive lines on the flexible insulating substrate 100 is more balanced, avoiding excessive stress concentration in a certain local area. This reduces the risk of fatigue failure of the structure under repeated large strain cycles, improving the mechanical durability and long-term measurement reliability of the sensor. Optionally, the predetermined number of cycles is 2-6. Preferably, the predetermined number of cycles is 3.

[0049] like Figure 1As shown, in this embodiment, straight segments and curved segments are alternately and repeatedly arranged to form multiple straight segments arranged at equal intervals from the first lead-out electrode to the second lead-out electrode; and to form a group of concentric arc segments composed of multiple concentric arc segments arranged sequentially from the inside to the outside, with the multiple concentric arc segment groups arranged at equal intervals from the first lead-out electrode to the second lead-out electrode. The compact off-axis serpentine structure combines the equally spaced arrangement of straight segments with the equally spaced arrangement of concentric arc segment groups, so that the core sensitive unit (curved segment) and the interconnecting unit (straight segment) of the sensor present a highly regular and periodic arrangement in space. This orderly arrangement allows strain energy to be uniformly transferred to each sensitive unit, avoiding stress concentration or uneven strain sensing caused by structural disorder, so as to enable the sensor to obtain a highly consistent and highly repeatable response signal. The concentric arc segment group, composed of concentric arc segments, maintains a standard and consistent geometry at each reversal point, except for the structures at both ends. This consistency ensures that each bending segment deforms (e.g., unfolds) in a highly predictable and uniform manner during stretching. This allows all the series-connected strain gauges 500 to contribute synchronously and equally to the overall strain. This high degree of synergy in response directly translates into excellent linearity in the sensor output signal, meaning a good proportional relationship is maintained between resistance change and mechanical strain. The equidistant arrangement and concentric arc segment layout greatly simplify the programming of vector trajectories during laser etching, making the fabrication process easy to control and highly repeatable. Therefore, different sensors manufactured based on this design exhibit extremely high structural and performance consistency, ensuring high yield and reliability for mass-produced products.

[0050] like Figure 1As shown, in this embodiment, the curved segment is positioned off-center from the central axis and connected to corresponding straight segments at both ends. Traditionally, the connection point between a U-shaped curved segment and a straight segment is located on the central axis, which easily becomes a stress concentration point during tensile deformation, increasing the risk of fatigue fracture of the conductive material. This invention, however, shifts the entire curved segment towards the center (i.e., inside the folded structure), moving the connection point away from the central axis region where stress is greatest. This offset design optimizes the force transmission path and effectively disperses the stress at the connection point, significantly improving the sensor's mechanical durability and structural reliability under repeated large strain cycles. The deviation of the curved segment from the central axis makes its motion trajectory and deformation pattern more stable and controllable during tensile unfolding. This suppresses potential lateral swaying or torsional deformation during unfolding, concentrating its deformation within the expected plane. This ensures more consistent and predictable resistance change behavior for each curved segment (strain gauge 500), contributing to improved repeatability and linearity of the sensor's output signal. By shifting the curved section inward, the space between the two straight sections can be utilized more effectively, allowing adjacent serpentine loops to be arranged with a smaller spacing. Without sacrificing mechanical performance, the space utilization of the sensor is further improved, making the overall structure more compact and highly consistent with the "compact" design goal.

[0051] In this embodiment, the thickness of the conductive film 200 is less than or equal to 50 micrometers; the minimum width of the strain gauge 500 is less than or equal to 20 micrometers; and the width of the other resistors 600 is three times or more the minimum width of the strain gauge 500. The minimum width of the strain gauge 500 is limited to less than or equal to 20 micrometers, forming an extremely fine conductive linewidth. According to the resistance law (R=ρL / A), when the length (L) and material resistivity (ρ) are constant, the smaller the cross-sectional area (A) of the conductor, the greater its resistance value (R). Therefore, the initial resistance value of the sensing element is significantly increased, resulting in a larger absolute resistance change (ΔR) under the same strain (ΔL / L), thereby greatly improving the sensor's sensitivity (strain coefficient). Simultaneously, the higher initial resistance helps reduce the operating current of the measurement circuit, meeting the application requirements for low power consumption. By setting the width of the other resistors 600 to three times or more the minimum width of the strain gauge 500, a clear functional partitioning is achieved. The wide area where the other resistors 600 are located (the interconnection area) significantly reduces the resistance value of this part and its rate of change during stretching. This ensures that the overall resistance change of the sensor mainly comes from the area where the narrow strain gauge 500 is located. This not only optimizes the signal-to-noise ratio, making the signal more purely reflective of strain information, but also enhances the mechanical strength and current carrying capacity of the wires in the wide interconnection area, ensuring the stability and durability of the electrical connection and avoiding failure at the interconnection due to excessive current density or insufficient mechanical strength. Limiting the thickness of the conductive film 200 to less than or equal to 50 micrometers ensures that the entire functional layer is very thin. This thin-film treatment greatly reduces the contribution of the conductive material itself to the overall stiffness of the sensor, making its mechanical properties mainly dominated by the flexible substrate. This ensures the sensor's excellent flexibility, enabling it to fit well with the measured object (such as solid propellant) and achieve true strain transmission. At the same time, the thin material has less impact from neutral layer displacement during bending deformation, which is beneficial to improving the accuracy of strain measurement. Preferably, the thickness of the conductive film 200 is less than or equal to 20 micrometers; the minimum width of the strain resistor 500 is less than or equal to 15 micrometers; and the width of the other resistors 600 is more than 5 times the minimum width of the strain resistor 500.

[0052] In this embodiment, the overall thickness of the packaged sensor is ≤3 mm.

[0053] The application of the flexible large strain sensor in this embodiment is to use the aforementioned flexible large strain sensor for the strain measurement of solid rocket motor propellant grains.

[0054] In practice, a "compact" off-axis serpentine flexible large strain sensor and its fabrication method are provided. The sensor includes: a conductive film 200, a flexible insulating substrate 100, lead wires 400, and lead electrodes 300; the flexible insulating substrate 100 is made of a polymer film and has a planar structure, and the conductive film 200 and lead electrodes 300 are respectively fixed on the flexible insulating substrate 100; the conductive film 200 is designed as a "compact" off-axis serpentine structure with multiple series-connected strain resistors 500, divided into strain resistors 500 in the curved portion and other resistors 600 in the straight portion; the bending resistors (strain resistors 500) are distributed in the inner portion off the central axis. During the deformation process of the flexible large strain sensor, the real-time resistance change of the strain resistors 500 is large, while the real-time resistance change of the other resistors 600 is small. During the production process, a conductive film 200 is first deposited on a flexible insulating substrate 100. Then, an ultraviolet laser is used to etch the strain resistor 500 and the flexible insulating substrate 100 according to the preset patterns. After two etching processes, the main body of the product can be produced. Finally, the lead wire 400 and the upper substrate are welded on the lead electrode 300 to complete the sensor fabrication.

[0055] A flexible, high-strain sensor with a "compact" off-axis serpentine structure comprises: a sensitive layer: constantan foil (0.1mm-0.2mm thick), formed into an off-axis serpentine structure by ultraviolet laser cutting; and an encapsulation layer: PDMS material (0.2mm-0.25mm thick) encapsulating the sensitive layer. The cutting parameters for the off-axis serpentine structure are: ultraviolet laser power density of 5W and cutting rate of 50mm / s.

[0056] A method for fabricating a flexible, large-strain sensor with a "compact" off-axis serpentine structure includes: spin-coating a PDMS-constantan composite film (200 r / min, 10 min); ultraviolet laser cutting of the constantan layer to form a serpentine pattern; secondary laser removal of edge residues; welding of 300mm lead-out electrodes; and PDMS casting and encapsulation. The ultraviolet laser is a picosecond / femtosecond laser. The overall thickness of the encapsulated sensor is ≤3mm.

[0057] The method for fabricating the flexible large strain sensor of the present invention comprises the following steps:

[0058] a. Prepare a polymer film for fabricating the flexible insulating substrate 100;

[0059] b. Prepare a conductive film 200 on an insulating film to form a composite film structure;

[0060] c. Using a laser, according to the preset pattern of the conductive film 200, laser etching is performed on the conductive film 200 according to the first type of vector trajectory, penetrating only the conductive film 200 to separate the strain resistor 500 and other resistors 600.

[0061] d. Using a laser, laser etching is performed on the composite film structure according to the preset pattern of the flexible insulating substrate 100 and the second type of vector trajectory, penetrating the composite film structure and etching out the flexible insulating substrate 100 of the sensor.

[0062] The outward width of the vector trajectory relative to the pattern outline is half the width of the laser spot. The repeatability of the laser is less than or equal to 2 micrometers. The laser is an ultraviolet picosecond laser or an ultraviolet femtosecond laser. During the laser etching process, a single vector trajectory can be used for etching, or multiple parallel vector trajectories can be used. The gap between multiple parallel vector trajectories is 1 / 4 to 1 times the width of the laser spot.

[0063] The thickness of the conductive film 200 is less than or equal to 20 micrometers. The minimum width of the strain gauge 500 is less than or equal to 15 micrometers. The width of other resistors 600 is more than 5 times the minimum width of the strain gauge 500.

[0064] The "compact" off-axis serpentine structure with multiple 500 series strain gauges adopted in this invention not only greatly reduces the sensor size and improves the sensitivity coefficient and strain transfer efficiency of strain measurement, but also allows the main body of the product to be manufactured by only two consecutive laser etching processes. Compared with the manufacturing process of traditional metal strain gauges and existing large-range flexible strain sensors, this greatly simplifies the processing flow, shortens the production time, and reduces the manufacturing cost, making it very suitable for the mass production and widespread use of this type of flexible strain sensor.

[0065] The preparation process provided by this invention can complete the entire process in one go and continuously, and also greatly improves the processing accuracy. Currently, the minimum linewidth of the strain gauge 500 prepared by the laser etching method provided by this invention can reach 10 micrometers, and the accuracy is less than or equal to 2 micrometers.

[0066] The flexible large strain sensor provided by this invention can be more widely used in the measurement of flexible large-variable structure strain of solid propellants.

[0067] like Figure 1 and Figure 2 As shown, a flexible large strain sensor with a "compact" off-axis serpentine structure provided by the present invention includes: a conductive film 200, a flexible insulating substrate 100, lead-out electrodes 300, and lead-out wires 400. The flexible insulating substrate 100 is made of an insulating thin film material and has a planar structure; the conductive film 200 is fixed on the flexible insulating substrate 100. The curved structure of the conductive film 200 can be one or a combination of several of the following: circular arc, elliptical arc, and sine curve. This invention... Figure 1The conductive film 200 provided has an arc-shaped structure. On the conductive film 200, the central axis of the strain resistor 500 on the portion with greater bending deformation is located to one side of this central axis. Figure 1 It can be seen that the integrated conductive film 200 includes a strain resistor 500 in an arc segment and other resistors 600 in a straight segment. During the stretching deformation process of the flexible large strain sensor, the strain resistor 500 in the arc segment generates a larger part of the bending deformation, while the other resistors 600 in the straight segment generate a smaller part of the bending deformation, and the difference in bending deformation between the two is relatively large.

[0068] The flexible large strain sensor provided by this invention has a conductive film 200 with a thickness of 50 micrometers or less, such as 40 micrometers, 30 micrometers, 20 micrometers, 15 micrometers, 10 micrometers, 8 micrometers, or 5 micrometers. The strain gauge 500 has a minimum width of 20 micrometers, such as 15 micrometers, 10 micrometers, 8 micrometers, 5 micrometers, or 3 micrometers. The width of other resistors 600 is three times or more the minimum width of the strain gauge 500.

[0069] The insulating film material of the flexible insulating substrate 100 can be selected from polydimethylsiloxane (PDMS) or one of the following insulating polymer films: Ecoflex series silicone rubber, polyimide, polyester, polyurethane, phenolic resin, and epoxy resin. The conductive film 200 can be made of any of the following metals: constantan, neoconstantan, gold, aluminum, copper, nickel-chromium alloy, nickel-chromium-aluminum alloy, iron-chromium-aluminum alloy, platinum, platinum-tungsten alloy, or semiconductor single-crystal silicon or graphene conductive materials.

[0070] Fabrication of the flexible large strain sensor provided by this invention:

[0071] The preset pattern outline of the conductive film 200 is set as the etching vector trajectory. According to the process requirements, it can be set as a single etching vector trajectory 700 or multiple etching vector trajectories 800.

[0072] An insulating film is prepared for fabricating the flexible insulating substrate 100. A conductive film 200 is then fabricated on the insulating film to form a composite film structure. Using a laser, lines are etched onto the conductive film 200 according to a preset pattern, using a single etching vector trajectory 700 or multiple etching vector trajectories 800, to separate the strain gauge 500 and other resistors 600. The laser-etched flexible insulating substrate 100 is then removed, completing the main manufacturing process. Afterwards, depending on the specific application environment, an encapsulation film can be used for encapsulation. Encapsulation is achieved by wrapping the encapsulation film around the flexible large strain sensor. The encapsulation film can be made of polydimethylsiloxane or Ecoflex series silicone rubber.

[0073] This invention, by employing laser etching technology, not only significantly shortens the production process but also controls the average manufacturing cost of a single flexible large strain sensor to the level of a few yuan. In contrast, the plasma etching process used in existing technologies is expensive, with the cost per unit reaching several thousand yuan; while etching methods, although reducing costs to some extent and lowering the price per unit to tens of yuan, still suffer from insufficient processing precision.

[0074] like Figure 4 and Figure 5 As shown, during the laser etching process, the etching vector trajectory can be selected as a single vector trajectory or multiple parallel vector trajectories. For example... Figure 4 A local structural diagram of a single vector trajectory etched, such as... Figure 5 This is a local structural diagram of the etching process using multiple vector trajectories. The gap between the multiple parallel vector trajectories is 1 / 3 to 1 / 2 times the width of the laser spot used.

[0075] The flexible large-strain sensor proposed in this invention features high precision, large range, and high linearity, with a significantly shortened manufacturing time; the entire process can be completed in just 3-5 minutes. Compared to existing plasma etching or chemical etching techniques, the processing efficiency is improved by up to 120 times. This substantial reduction in production cycle effectively lowers manufacturing costs, facilitating the large-scale production and widespread application of this sensor. Furthermore, this method supports continuous processing throughout the entire process in a single operation, further enhancing the overall precision level of the process.

[0076] The following specific examples of the sensor fabrication and its strain testing on solid propellants illustrate the characteristics of the flexible large strain sensor in this invention.

[0077] Example 1:

[0078] for Figure 1 , Figure 2 , Figure 3 An embodiment of a flexible large strain sensor with the structure shown.

[0079] The flexible insulating substrate 100 is made of PDMS film, and the conductive film 200 is made of constantan foil.

[0080] The PDMS film is 0.2 mm thick, with a rectangular length of 15 mm and a width of 8 mm; the conductive film 200 constantan foil is 5 μm thick; the constantan foil linewidth of the 500 strain gauge in the arc segment is 10 μm, with the edge 35 μm from the centerline of the arc segment; and the constantan foil linewidth of the other 600 resistors in the straight segment is 90 μm.

[0081] Preparation method:

[0082] Purchase or spin-coat a polymer film as a flexible insulating substrate 100;

[0083] A metal film is deposited on the flexible insulating substrate 100 to form a composite film layer;

[0084] Using a UV femtosecond laser, a "compact" off-axis serpentine structure of strain gauges 500 and other resistors 600 is etched along the preset pattern of the conductive film 200 using three parallel etching vector tracks. This separates the strain gauges 500, the other resistors 600, and the remaining area of ​​the conductive film 200, ensuring they are not interconnected. The spacing between the three parallel etching vector tracks is 3 micrometers, with the middle vector track extending 5 micrometers beyond the preset pattern outline of the strain gauge 500.

[0085] Using an ultraviolet picosecond laser, the rectangular flexible insulating substrate 100 is etched through the composite film layer according to the required rectangular size of the flexible insulating substrate 100. The etched rectangular flexible insulating substrate 100 is then removed to obtain the main structure of the flexible large strain sensor covered with a conductive film 200 with a "compact" off-axis serpentine structure.

[0086] Example 2:

[0087] for Figure 6 , Figure 7 An embodiment of the flexible large strain sensor is shown.

[0088] Propellant specimens were prepared according to the standard procedure specified in GJB 770B—2005 "Test Methods for Gunpowder", and uniaxial tensile tests were conducted using a WDW-10 electronic universal tensile testing machine. The response curve was plotted with the actual strain of the propellant specimen obtained by optical measurement as the X-axis and the relative change in resistance of the flexible strain gauge (ΔR / R0) as the Y-axis. Figure 7 As shown, the sensor exhibits high linear response characteristics within a strain range of 0% to 50%, with a fitting coefficient of determination R0. 2 =0.9959, indicating that it has good strain sensing performance and reliability.

[0089] Matters not covered in this invention are common knowledge.

[0090] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0091] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

[0092] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for fabricating a flexible large strain sensor, characterized in that, Includes the following steps: S100, A flexible insulating substrate (100) is made of polymer film. S200, a conductive film (200) is prepared on a flexible insulating substrate to form a composite film structure; S300. According to the preset pattern of the conductive film (200), laser etching is performed on the conductive film (200) according to the first type of vector trajectory, penetrating only the conductive film (200) to separate the strain resistor (500) and other resistors (600) to form an off-axis serpentine structure. The off-axis serpentine structure has multiple series strain gauges (500), including a curved section and a straight section. The curved section is a strain gauge (500), and the straight section is other resistors (600). Step S300 selectively etches the conductive film using the first type of vector trajectory, simultaneously completing the forming of the off-axis serpentine structure and the functional partitioning of the strain resistor and other resistors in one processing step. S400. According to the preset pattern of the flexible insulating substrate (100), laser etching is performed on the composite film structure according to the second type of vector trajectory to penetrate the composite film structure and etch out the flexible insulating substrate (100) of the sensor. Step S400 cuts the entire composite film layer using the second type of vector trajectory, simultaneously completing the final shaping of the sensor's outer contour. It is based on the same laser processing platform as step S300, eliminating the need to change the workpiece or perform secondary clamping and alignment, thus eliminating edge cutting errors and pattern-substrate misalignment caused by multiple positioning. S500, a flexible large strain sensor is fabricated by welding lead wires (400) onto the lead electrode (300) and then encapsulating it. The process follows a sequence from the inside out, first etching the inner conductive pattern and then cutting the outer substrate outline.

2. The method for fabricating a flexible large strain sensor according to claim 1, characterized in that, In steps S300 and S400, the distance by which the vector trajectory deviates from the contour and extends outward is equal to half the diameter of the laser spot. The laser used is an ultraviolet picosecond laser or an ultraviolet femtosecond laser, and the laser's repeatability is less than 2 micrometers. During laser etching, a single vector trajectory can be selected for etching; or multiple parallel vector trajectories can be selected for etching, with the gap between the multiple parallel vector trajectories being 1 / 4 to 1 times the width of the laser spot used.

3. The method for fabricating a flexible large strain sensor according to claim 1, characterized in that, The etching parameters for forming the off-axis serpentine structure by laser etching in step S300 are: ultraviolet laser power density 0.05W / μm²-0.5W / μm², cutting rate 50mm / s-200mm / s; The etching parameters for forming a flexible insulating substrate by laser etching in step S400 are: ultraviolet laser power density 0.1W / μm²-1W / μm², cutting rate 50mm / s-200mm / s.

4. The method for fabricating a flexible large strain sensor according to any one of claims 1 to 3, characterized in that, Step S200 involves spin coating to prepare the composite film structure. The spin coating speed is 100 r / min-500 r / min, and the spin coating time is 2 min-10 min.

5. The method for fabricating a flexible large strain sensor according to any one of claims 1 to 3, characterized in that, The film material of the flexible insulating substrate (100) is one of polydimethylsiloxane, silicone rubber, polyimide, polyester, polyurethane, phenolic resin or epoxy resin insulating polymer film.

6. The method for fabricating a flexible large strain sensor according to any one of claims 1 to 3, characterized in that, The conductive film (200) is made of one of the following metals: constantan, new constantan, gold, aluminum, copper, nickel-chromium alloy, nickel-chromium-aluminum alloy, iron-chromium-aluminum alloy, platinum, or platinum-tungsten alloy. The conductive film (200) is made of semiconductor single-crystal silicon or graphene-based conductive materials.

7. The method for fabricating a flexible large strain sensor according to any one of claims 1 to 3, characterized in that, Step S500 is followed by: S600, casting encapsulation.

8. A flexible large strain sensor, characterized in that, The flexible large strain sensor is prepared by any one of claims 1 to 7, comprising a flexible insulating substrate (100), a conductive film (200), and an lead-out electrode (300). The flexible insulating substrate (100) has a planar structure, and the conductive film (200) has a compact off-axis serpentine structure with multiple series strain resistors (500). The compact off-axis serpentine structure includes a curved section and a straight section. The curved section is a strain resistor (500), and the straight section is other resistors (600). The conductive film (200) and the lead-out electrode (300) are located on the flexible insulating substrate (100) and the two ends of the conductive film (200) are respectively connected to the lead-out electrode (300).

9. The flexible large strain sensor according to claim 8, characterized in that, The lead-out electrode (300) includes a first lead-out electrode and a second lead-out electrode; The first lead-out electrode is connected to the straight segment, and the straight segment and the curved segment are connected end to end and arranged alternately. When the straight segments and curved segments alternate to approach the second lead electrode, the curved segments are reversed and the straight segments and curved segments are alternately arranged towards the first lead electrode. This process continues until the predetermined number of reversals is completed, after which the second lead electrode is connected by a straight segment.

10. The flexible large strain sensor according to claim 8, characterized in that, Straight segments and curved segments are arranged alternately and repeatedly to form multiple straight segments arranged at equal intervals from the first lead-out electrode to the second lead-out electrode; And forming a group of concentric arc segments arranged sequentially from the inside out, with the group of concentric arc segments arranged at equal intervals from the first lead-out electrode to the second lead-out electrode.

11. The flexible large strain sensor according to any one of claims 8 to 10, characterized in that, The curved section is set off from the central axis in the direction of the center and connected to the corresponding straight section at both ends.

12. The flexible large strain sensor according to claim 11, characterized in that, The thickness of the conductive film (200) is less than or equal to 50 micrometers; The minimum width of the strain gauge (500) is less than or equal to 20 micrometers; The width of the other resistors (600) is three times or more the minimum width of the strain gauge resistor (500).

13. The flexible large strain sensor according to claim 11, characterized in that, The overall thickness of the packaged sensor is ≤3mm.

14. An application of a flexible large strain sensor, characterized in that, The flexible large strain sensor according to any one of claims 8 to 13 is used for strain measurement of solid rocket motor propellant grains.

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