Coaxial fibrosis TDR (time domain reflectometry) sensor, preparation method thereof and distributed monitoring system

Abstract

The invention discloses a coaxial fibrosis TDR sensor and a preparation method thereof, and a distributed monitoring system, and relates to the technical field of flexible distributed sensor design, the sensor is realized by adopting a flexible coaxial structure comprising a central conductor, a sensitive medium layer, an outer braided shielding layer and a protective layer which are sequentially arranged from inside to outside; the central conductor and the sensitive dielectric layer serve as signal transmission conductors and are used for bearing and transmitting excitation pulse signals; the sensitive dielectric layer adopts any one of a temperature, pressure and strain single sensitive dielectric layer or a temperature-strain and pressure-strain coupling sensitive dielectric layer, and is used for correspondingly responding to changes of external temperature, pressure and strain respectively, changing the dielectric constant and the geometric dimension of the sensitive dielectric layer, causing local impedance disturbance, and realizing the temperature, pressure and strain sensing. The TDR reflection signal generates time delay and amplitude change; and the outer braided shielding layer and the protective layer are respectively used for shielding external electromagnetic interference and protecting machinery and environment. According to the invention, distributed positioning and quantitative analysis of multiple physical quantities can be realized.

Description

Technical Field

[0001] This invention relates to the field of flexible distributed sensor design technology, and in particular to a coaxial fiberized TDR sensor, its fabrication method, and a distributed monitoring system. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] With the increasing demand for structural health monitoring, wearable devices, and implantable electronics, the design and application of flexible sensors are rapidly expanding. As an information acquisition port, sensors can convert external physical forces such as temperature, pressure, and strain into electrical signals, thereby enabling real-time monitoring of complex environments and structural states. Distributed flexible sensors, as a new generation of key components, place higher demands on sensitivity, accuracy, scalability, and environmental adaptability.

[0004] However, on the one hand, existing distributed sensing technologies still have significant limitations: while fiber optic distributed sensing can provide high spatial resolution and long-distance monitoring capabilities, it is expensive, complex to lay and maintain, and fiber optic materials are insufficient in terms of weavability and flexibility, making it difficult to meet the application needs of textiles, composite materials, and biological environments; traditional coaxial cable-based TDR (Time-Domain Reflectometry) technology is mostly used for cable fault detection, and its targets are usually rigid or semi-rigid cables, making it difficult to be compatible with flexible woven materials, and also unable to achieve comprehensive monitoring of multiple physical quantities. On the other hand, existing flexible sensors have structural limitations, namely, most of them are "point-type" or "array-type" structures, requiring independent electrodes and complex wiring to achieve multi-point acquisition, lacking continuous distributed sensing capabilities. When facing complex structures or dynamic environments, single-point sensing data cannot fully characterize the overall state, limiting its practical application. In other words, existing sensing technologies cannot meet the design requirements of distributed flexible sensors.

[0005] In addition, existing sensing systems have limited resistance to external noise and electromagnetic interference, making it difficult to obtain stable and reliable monitoring results in real-world environments. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, this invention provides a coaxial fiberized TDR sensor and its fabrication method, as well as a distributed monitoring system, which solves the problems that existing sensing technologies cannot meet the requirements for sensing multiple physical quantities (such as temperature, pressure, strain, etc.), continuous distributed positioning and quantification, flexible weavability, and anti-interference stability.

[0007] In a first aspect, the present invention provides a coaxial fiberized TDR sensor.

[0008] A coaxial fiber TDR sensor is realized by adopting a flexible coaxial structure consisting of a central conductor, a sensitive dielectric layer, an outer braided shielding layer, and a protective layer arranged sequentially from the inside to the outside. The central conductor and the sensitive dielectric layer serve as signal transmission conductors, used to carry and transmit excitation pulse signals; The sensitive medium layer can be a single sensitive medium layer of temperature, pressure, and strain or a coupled sensitive medium layer of temperature-strain and pressure-strain, respectively, to respond to changes in external temperature, pressure, and strain, change the dielectric constant and geometric dimensions of the sensitive medium layer, cause local impedance disturbance, and cause time delay and amplitude change of the TDR reflected signal. The outer braided shielding layer is used to shield against external electromagnetic interference and provide mechanical protection. The protective layer is used to provide environmental and mechanical protection for the inner layer.

[0009] A further technical solution involves using a temperature-sensitive dielectric layer. The composite medium consists of a pressure-sensitive medium layer made of foamed TPU or silicone microporous structure layer and a strain-sensitive medium layer made of solid TPU or silicone film layer. The temperature-strain coupling sensitive medium layer is made of a stretchable temperature-sensitive composite material, which is composed of an elastic matrix and a temperature-sensitive filler. The pressure-strain coupling sensitive medium layer is made of a stretchable pressure-sensitive composite material, which is made of a highly elastic compressible polymer matrix and high dielectric filler particles.

[0010] Secondly, the present invention provides a method for fabricating a coaxial fiberized TDR sensor.

[0011] A method for fabricating a coaxial fiber-reinforced TDR sensor, comprising: A central conductor is formed by selecting copper wire, tungsten wire, silver wire, or carbon fiber wire of a set diameter and subjecting it to ultrasonic surface cleaning and low-temperature plasma activation treatment. Based on the monitoring target, select a temperature, pressure or strain sensitive material system, and coat it onto the outside of the central conductor through melt extrusion or spin coating process to form a sensitive dielectric layer of a set thickness; Copper wire, tungsten wire, silver wire or carbon fiber wire is woven into a mesh structure with a set coverage by a braiding machine to form a braided shielding layer and cover the outside of the sensitive medium layer. An elastic silicone rubber layer is wrapped around the outer woven shielding layer and cured at room temperature to form a protective layer, thus preparing a coaxial fiberized TDR sensor.

[0012] A further technical solution, the preparation of the central conductor, is as follows: Highly conductive metal or carbon-based filaments with a diameter of 0.2-1.0 mm are selected and subjected to ultrasonic chemical cleaning with isopropanol and deionized water, followed by low-temperature plasma activation treatment for 20-60 seconds to reduce interfacial contact resistance. To enhance the interfacial bonding with the sensitive medium layer; The highly conductive metal or carbon-based filaments can be any one of copper wire, silver wire, tungsten wire, nickel-chromium wire, or 12K carbon fiber bundle.

[0013] A further technical solution, the preparation of the temperature-sensitive dielectric layer, is as follows: Will powder and Nanoparticles are mixed and blended by twin-screw extrusion at 180-210℃, and the melt is uniformly coated on the outside of the central conductor to form a temperature-sensitive dielectric layer with a thickness of 0.3-0.8 mm. in, and The molar ratio is 75:25. The nanoparticles have a particle size of 80-150 nm and a mass fraction of 20-30%.

[0014] A further technical solution, the preparation of the pressure-sensitive medium layer, is as follows: TPU is dissolved in DMF solution, and then n-octane is added. The mixture is magnetically stirred at 130-140℃ for 60-90 min to form a thermally induced phase separation microporous structure. After the microporous structure is spin-coated or extruded into a film, it is quenched in a water bath to complete the non-solvent exchange, thus obtaining a porous foamed TPU thin film with a pore size of 20-200 μm, which is then coated on the outside of the central conductor. The mass ratio of TPU to n-octane is 1:(1.5–2.5).

[0015] A further technical solution for the fabrication of the strain-sensitive medium layer is as follows: Solid TPU or silicone is selected and extruded to form a continuous elastic dielectric layer with a uniform thickness of 0.5-1.0 mm, which is then wrapped around the central conductor to allow for reversible radial shrinkage under tensile strain.

[0016] A further technical solution, the preparation of the outer braided shielding layer, is as follows: Copper wire, tungsten wire, silver wire, or carbon fiber wire is used to form a mesh structure with 24–64 spindles of copper wire, silver-clad copper wire, or high-modulus carbon fiber wire in a braiding angle of 25–60° and a coverage of 70–95%, which is uniformly wrapped on the outside of the sensitive medium layer.

[0017] A further technical solution involves setting up termination and protection layers, as follows: The excitation end of the coaxial fiber TDR sensor is connected in series / parallel with matching resistors and SMA / MCX RF coaxial connectors. The remote end is selected as either an absorption terminal or an open / short circuit reference depending on the application to realize signal input and output. The outer woven shielding layer is covered with liquid silicone rubber and cured at room temperature for 2 hours to form a sealed protective layer.

[0018] Thirdly, the present invention also provides a distributed monitoring system based on the TDR mechanism.

[0019] A distributed monitoring system based on the TDR mechanism includes the aforementioned coaxial fiber TDR sensor, signal excitation circuit, reflected wave acquisition module, and host computer processing unit, wherein: Signal excitation circuit, used to output excitation pulse signal; Coaxial fiber TDR sensor is used to transmit excitation pulse signal and respond to external temperature, pressure and strain, change the dielectric constant and geometry of the sensitive medium layer, cause local impedance disturbance, and cause time delay and amplitude change of TDR reflected signal; The reflected wave acquisition module is used to acquire TDR reflected signals; The host computer processing unit is used to analyze the time delay and amplitude changes of the TDR reflected signal, and realize the spatial distributed positioning and quantitative measurement of temperature, pressure and strain.

[0020] The above one or more technical solutions have the following beneficial effects: This invention proposes a coaxial fibrous TDR sensor and its fabrication method, as well as a distributed monitoring system, addressing the limitations of existing sensing technologies in meeting the requirements for multi-physical quantity (temperature, pressure, strain, etc.) sensing, continuous distributed positioning and quantification, flexible weavability, and anti-interference stability. The fibrous TDR sensor designed in this invention features a flexible structure that can be woven into fabrics or composite materials, possessing the ability to sense multiple physical quantities such as temperature, pressure, and strain. Through optimized design of the coaxial weaving and dielectric layer, superior sensitivity, recoverability, and anti-interference performance are achieved. Distributed detection is achieved using the TDR principle, with millimeter-level positioning accuracy and a monitoring length that can be extended to several meters or even tens of meters. The use of elastic silicone rubber encapsulation enhances environmental adaptability and biocompatibility, preventing instability factors such as moisture or bodily fluid intrusion. Combined with a distributed monitoring system based on reflected wave analysis, it can acquire spatial distribution information in real time, making it widely applicable in civil engineering, energy equipment, wearable devices, and medical devices.

[0021] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0022] 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 improper limitation of the invention.

[0023] Figure 1 This is a schematic diagram of the distributed fiber TDR sensor proposed in an embodiment of the present invention; Figure 2 This is a schematic diagram illustrating the principle of TDR coaxial connection in an embodiment of the present invention; Figure 3 This is a waveform diagram of the reflection response of a temperature-sensitive fiber TDR sensor at different temperatures in an embodiment of the present invention; Figure 4 This is a schematic diagram of the reflection response of a temperature-sensitive fiber TDR sensor at different locations when temperature excitation is applied in an embodiment of the present invention; Figure 5 This is a schematic diagram of the reflection response of a pressure-sensitive fiber TDR sensor at the same location when different pressure excitations are applied in an embodiment of the present invention. Figure 6 This is a partial schematic diagram of the reflection response of a stress-sensitive fiber TDR sensor at the same location under different stress excitations in an embodiment of the present invention. Figure 7 The image shown is a SEM micrograph of the fibrous tissue in an embodiment of the present invention; wherein, ( a () is a side view, ( b ( ) is a cross-sectional view. Detailed Implementation

[0024] It should be noted that the following detailed descriptions are exemplary and are intended only to describe specific embodiments and to provide further explanation of the invention, and are not intended to limit the scope of exemplary embodiments of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0025] To improve the sensor's sensitivity and multi-parameter sensing capability, this invention employs a coaxial three-layer structure consisting of a central conductor, a sensitive dielectric layer, and an outer braided shielding conductor. Distributed detection of temperature, pressure, and strain is achieved through different material systems. Specifically, considering the sensor's susceptibility to electromagnetic noise and external disturbances in complex environments, different functional designs are incorporated into the dielectric layer: the temperature-sensitive layer uses... The composite material enhances the dielectric temperature response; the pressure-sensitive layer uses foamed TPU (thermoplastic polyurethane) or silicone to form a microporous structure, improving compression sensitivity; the strain-sensitive layer uses solid TPU or silicone to ensure stability and recoverability under tensile conditions; an outer braided shielding conductor is introduced to provide good electromagnetic shielding performance and improve anti-interference capability. Finally, using this fibrous TDR sensor, combined with time delay and amplitude analysis of the reflected waveform, a distributed monitoring system is constructed, enabling spatially distributed real-time monitoring of temperature, pressure, and strain outside the body or on the exterior of the structure. This system is suitable for engineering structure health management and flexible wearable applications.

[0026] Example 1 This embodiment provides a coaxial fiber TDR sensor, employing a continuous flexible coaxial three-layer configuration. While its appearance is similar to conventional braided wires, it features a systematic design in the dielectric layer material and outer braiding, termination matching, and time-domain reflectometry (TDR) link. Figure 1 As shown, the physical structure of the fiber sensor is a flexible coaxial structure consisting of a central conductor, a sensitive medium layer, an outer braided shielding layer, and a protective layer arranged sequentially from the inside out.

[0027] Specifically, the central conductor and the sensitive dielectric layer serve as signal transmission conductors, carrying and transmitting excitation pulse signals. Furthermore, the sensitive dielectric layer, in addition to being a covering layer for the central conductor, also functions as an effective component of the TDR transmission line, participating in the transmission of the excitation pulse signals. As a sensing medium for external physical stimuli (such as temperature, pressure, and strain), it can adaptively change its local impedance according to the sensing conditions, which is reflected in the TDR reflected signal, laying the foundation for distributed monitoring. In this embodiment, the sensitive dielectric layer can be either a single temperature, pressure, or strain sensitive dielectric layer, or a temperature-strain or pressure-strain coupled sensitive dielectric layer, used to respond to changes in external temperature, pressure, and strain, respectively, changing the dielectric constant and geometric dimensions of the sensitive dielectric layer, causing local impedance disturbances, and resulting in time delays and amplitude changes in the TDR reflected signal.

[0028] In addition, in this embodiment, an outer braided shielding layer and a protective layer are sequentially wrapped around the sensitive medium layer. The outer braided shielding layer is used to shield against external electromagnetic interference and provide mechanical protection. The central conductor, the sensitive medium layer, the electromagnetic field, and the outer braided shielding layer together determine the signal propagation characteristics. The protective layer is used to provide environmental and mechanical protection for the inner layer and prevent external water vapor and other bodily fluids from penetrating and affecting the monitoring results.

[0029] Furthermore, in this embodiment, the temperature-sensitive dielectric layer (also referred to as a temperature-sensitive type dielectric layer) adopts... A composite dielectric layer is used to enhance the dielectric temperature response; the pressure-sensitive dielectric layer (also known as a pressure-sensitive type dielectric layer) employs a microporous structure formed from foamed TPU or silicone to improve compression sensitivity; the strain-sensitive dielectric layer (also known as a strain-sensitive dielectric layer) uses solid TPU or silicone to ensure stability and recoverability under tensile conditions. These temperature, pressure, and strain-sensitive dielectric layers respond to changes in external temperature, pressure, and strain, respectively, altering their local equivalent dielectric constant and geometry, resulting in local characteristic impedance. Disturbances are reflected in the amplitude changes and time delays of the TDR reflected signal, thereby enabling spatial distributed positioning and quantitative monitoring.

[0030] Preferably, compared to the design of a single sensitive medium layer, in order to further simplify the measurement process, the sensitive medium layer can also be realized by a single-layer coupled sensitive medium layer. Through the coordinated control of material system and structural parameters, the medium layer can generate distinguishable TDR response characteristics under the action of temperature, normal pressure and axial tension, thereby realizing the spatial distributed identification and quantitative inversion of multiple parameters.

[0031] Specifically, when using temperature-strain dual-mode measurement, it is preferable to use a stretchable temperature-sensitive composite material in the coaxial sensitive medium layer. This composite material consists of an elastic matrix (such as TPU) and a temperature-sensitive filler (such as... The composite material is composed of nanoparticles, and a small amount of oriented short fibers are locally introduced within the dielectric layer through radial volume fraction gradients or axial stripes to enhance strain sensitivity. By using this composite material to create a coupled sensitive dielectric layer, the synergistic sensing of both temperature and strain parameters can be achieved.

[0032] Similarly, when using pressure-strain dual-mode measurement, it is preferable to use a stretchable pressure-sensitive composite material in the coaxial sensitive medium layer. This composite material uses a highly elastic compressible polymer matrix (such as foamed TPU, PDMS, or silicone rubber) and incorporates high-dielectric filler particles (such as... , A composite medium made of nanoparticles, possessing both compressibility and dielectric response properties. By employing this composite material to create a coupling-sensitive medium layer, the coordinated sensing of both pressure and strain parameters can be achieved.

[0033] Example 2 This embodiment provides a method for fabricating a coaxial fiber TDR sensor, including the processing of the central conductor, the forming of the dielectric layer, the construction of the outer braided shielding layer, and the encapsulation of the protective layer, specifically including the following steps: Step S1: Select copper wire, tungsten wire, silver wire or carbon fiber wire of a set diameter, and perform ultrasonic surface cleaning and low-temperature plasma activation treatment to form a central conductor.

[0034] Specifically, highly conductive metal or carbon-based filaments (such as copper wire, silver wire, tungsten wire, nickel-chromium wire, or 12K carbon fiber bundles) with a diameter of 0.2-1.0 mm are selected. Their resistivity ρ and surface roughness Ra can be reduced through ultrasonic chemical cleaning with isopropanol and deionized water, followed by low-temperature plasma activation treatment for 20-60 seconds to improve their interfacial contact resistance. To enhance the bonding with the interface of the sensitive medium layer.

[0035] Step S2: Based on the monitoring target, select a temperature, pressure or strain sensitive material system, and coat it onto the outside of the central conductor through melt extrusion or spin coating process to form a sensitive dielectric layer of a set thickness.

[0036] In this embodiment, the following is selected: Composite, foamed TPU, or solid TPU are used as the functional medium. The thickness t of the sensitive medium layer is adjustable from 0.30 to 1.00 mm according to the targeted physical quantity (i.e., the physical quantity monitored by the target, such as temperature, pressure, and strain). Its equivalent dielectric constant is... The compressibility / scalability determines the sensitivity and linearity of the TDR.

[0037] Step S2.1: Prepare a temperature-sensitive dielectric layer.

[0038] In this embodiment, the temperature-sensitive dielectric layer adopts The composite material was prepared by taking particles with a diameter of 80-150 nm and a mass fraction of 20-30%. Nanoparticles, after surface modification of the powder with a coupling agent (such as KH550 0.5-1.0wt%), are combined with... The powders (75 / 25 molar ratio) are mixed and melt-blended and extruded by a twin-screw extruder at 180–210°C. Vacuum degassing is used to eliminate microbubbles. The melt is extruded around the central conductor to form a uniform layer, i.e., a temperature-sensitive medium layer with a thickness of 0.3–0.8 mm.

[0039] like Figure 3 and Figure 4 The temperature-sensitive working mechanism shown is as follows: The equivalent dielectric constant of the composite medium increases monotonically or quasi-monotonically with temperature T, which can be expressed as: ; in, Let T be the relative permittivity at temperature T, which characterizes the dielectric response of the material at temperature T. Reference temperature The dielectric constant of the following is usually taken as or ; It is a first-order temperature coefficient, which characterizes the slope of the linear change of dielectric constant with temperature; It is a second-order temperature coefficient used to correct nonlinear trends at high temperatures; typically, it exists in the range of 25–90 °C. , Obtained through calibration; This is the temperature offset, representing the amount of change in the current temperature relative to the reference temperature.

[0040] Furthermore, changes in the dielectric constant affect the local impedance characteristics, which are expressed as: ; With dielectric constant An increase in the reflection coefficient leads to a decrease in local impedance, thereby affecting the reflection coefficient. Changes; and the propagation speed of TDR. As the temperature T decreases, the reflected signal shifts slightly to the right on the time axis.

[0041] Therefore, in response to changes in external temperature, the dielectric constant of the sensitive medium layer can be altered, causing local disturbances in the characteristic impedance, resulting in time delay and amplitude changes in the TDR reflected signal, which facilitates distributed positioning and quantification of the sensor.

[0042] Preferably, to improve temperature-sensitive stability, The mass fraction is 20–30 wt%, and the particle size is 80–150 nm. Excessive content will increase the dielectric loss (tanδ) and introduce scattering noise. The extrusion temperature is divided into three zones (inlet / middle / outlet: 185 / 195 / 205 ℃), and the traction line speed is 0.5–1.0 m·min. -1 Dual-zone cooling (air + deionized water mist) suppresses anisotropy caused by excessive crystal orientation.

[0043] As one implementation method, the temperature calibration process is as follows: using a segmented constant temperature cavity (±0.1 ℃), along the fiber at a known temperature... The micro-film heating element is attached to the position or placed in a gradient groove for recording. and Curves, constructing local sensitivity With group velocity function Then, spline fitting and Tikhonov regularization are used to invert the temperature field. In the presence of noise (equivalent input noise density) In this scenario, the absolute temperature error can be controlled within ±0.8–1.2 ℃, thereby achieving accurate temperature calibration.

[0044] Step S2.2: Prepare the pressure-sensitive medium layer.

[0045] In this embodiment, the pressure-sensitive dielectric layer adopts a microporous structure formed by foamed TPU or silicone. The preparation process is as follows: TPU is dissolved in a DMF (N,N-Dimethylformamide, N,N-dimethylformamide, with a solid content of 15–20 wt%) solution. Then, n-octane is added at a TPU:n-octane mass ratio of 1:(1.5–2.5). The mixture is magnetically stirred at 130–140°C for 60–90 min to form a thermally induced phase separation microporous structure. After spin-coating or extrusion to form a film, non-solvent exchange is completed by quenching in a water bath, resulting in a closed / open-pore mixed microporous structure, i.e., a porous foamed TPU thin layer with a pore size of 20–200 μm, which is then coated around the central conductor. Preferably, the pore size can be controlled by adjusting the n-octane ratio and cooling rate.

[0046] like Figure 5 The pressure-sensitive working mechanism shown is that the foamed TPU microporous medium is subjected to normal pressure. P Thickness shrinkage occurs under the action of With porosity decay ,in Calibrated by mercury porosimetry or stress-strain testing, The initial porosity, The attenuation coefficient is used; the equivalent dielectric constant can be approximated using the Lichtenecker logarithmic mixture approximation, which increases with increasing P, and is expressed as: ; in, is the equivalent dielectric constant of the composite medium, representing the overall effective dielectric response; The dielectric constant of the matrix material (such as PVDF-TrFE or TPU) characterizes the low dielectric component; For filler materials (such as The dielectric constant of particles characterizes the high dielectric component; The filler volume fraction represents the proportion of the filler in the composite system, and its value ranges from 0 to 1.

[0047] Meanwhile, the geometric term in the local impedance In this context, changes in the thickness of the effective medium will also cause... The decrease, combined with the effect of both, leads to a positive increase in the reflection amplitude.

[0048] Therefore, in response to changes in external pressure, the dielectric constant of the sensitive medium layer can be altered, causing local disturbances in the characteristic impedance and resulting in a change in the amplitude of the TDR reflected signal, which facilitates the distributed positioning and quantification of the sensor.

[0049] Preferably, the phase separation process described above uses a three-parameter coupling of solvent mass fraction, phase separation temperature, and cooling rate to control pore size and pore size distribution: TPU / DMF solid content 18 wt%, n-octane mass ratio 2.0, holding at 135 ℃ for 60 min, and water bath temperature difference ≥80 ℃, can obtain a continuous microporous layer with an average pore size of approximately 60–120 μm and a porosity of 45–55%. After extrusion, it is rapidly placed in a water bath for 10–30 s, followed by curing at room temperature for 24 h to release internal stress. Furthermore, to improve anti-interference and return-line stability, high coverage weaving (≥90%) is preferentially used on the outer side of the foam layer, and sparse anti-collapse ribs (micron-level low-medium fiber short chops) are added to the coating layer to avoid permanent deformation caused by high pressure in small areas.

[0050] As one implementation method, the pressure calibration process involves using a miniature loading stage and an equidistant positioning fixture, in... Apply a step load of 10–200 kPa, holding each step for 10 seconds, and collect data. And by taking a steady-state average, we obtain With linearity Considering the spatial point spread function (PSF) introduced by boundary diffusion and finite pulse width, the host computer uses Richardson–Lucy or Wiener deconvolution to reconstruct the spatial distribution in the time domain. The typical spatial resolution can be better than 5 mm, the pressure reading repeatability (1σ) is better than ±3 kPa, and the drift after 1000 cycles is <±2%.

[0051] Step S2.3: Prepare the strain-sensitive medium layer.

[0052] In this embodiment, the strain-sensitive dielectric layer is made of solid TPU or medical-grade silicone rubber (Shore A 25–45), which is extruded with constant thickness to form a continuous elastic dielectric layer with a uniform thickness of 0.5-1.0 mm, low strain drift, and low creep, and is wrapped around the central conductor so that it can undergo reversible radial shrinkage under tensile strain.

[0053] like Figure 6 The strain-sensitive working mechanism shown is that the strain-sensitive medium layer uses a solid TPU / silicone medium under axial strain. Radial Poisson contraction occurs, which can be represented as: In the formula And thus An increase in intensity causes localized reflection enhancement and subtle phase changes.

[0054] Therefore, in response to changes in external stress, the geometry of the sensitive medium layer can be altered, causing local disturbances in the characteristic impedance. This results in a time delay (opposite to temperature changes) and amplitude change in the TDR reflected signal, facilitating distributed positioning and quantification of the sensor.

[0055] Preferably, in order to suppress viscoelastic hysteresis and creep, this embodiment controls the ratio of soft segments to hard segments in the formulation and adds a small amount of anti-creep agent (such as 0.5 wt% siloxane lubricating modifier). At the same time, high modulus carbon fiber filaments are selected in the braiding layer to achieve circumferential restraint and avoid nonlinearity caused by excessive radial shrinkage. In terms of process, a dense medium layer with a thickness of 0.50–0.80 mm is obtained by using a die of equal thickness (tolerance ±20 μm) and constant tension traction.

[0056] As one implementation method, the strain calibration process is as follows: A tension jig is held at both ends, and a load is applied in increments of 0–10% in 0.5% steps, recording the results. and ,get Under dynamic frequency sweep of 1–5 Hz, the amplitude and phase response amplitude flatness is better than ±1.5 dB and the hysteresis area is <5%FS, indicating that it is suitable for monitoring low-frequency human motion or slow structural deformation. In high strain situations (ε>15%), a higher outer layer coverage and a thicker protective layer are used to delay the buckling of the braided yarn.

[0057] Step S2.4: Prepare a temperature-strain coupling sensitive medium layer.

[0058] In this embodiment, when using temperature-strain dual-mode measurement, it is preferable to use a stretchable temperature-sensitive composite material in the coaxial sensitive medium layer. This composite material consists of an elastic matrix (such as TPU) and a temperature-sensitive filler (such as... The medium is composed of nanoparticles, and a small amount of oriented short fibers are locally introduced into the medium layer through radial volume fraction gradients or axial strips to enhance strain sensitivity.

[0059] The working mechanism of the above-mentioned coupling dielectric layer is as follows: when the temperature changes, the relative permittivity... The changes in temperature primarily cause changes in propagation speed, which manifest as time delay in the TDR reflected signal. Under axial strain, changes in local geometry and orientation of the filler network mainly cause disturbances in local characteristic impedance, which manifest as changes in amplitude / phase and spectral characteristics in the TDR reflected signal. Based on this, the host computer can extract multi-dimensional features such as time delay, amplitude, spectrum, and phase, and combine them with a pre-calibrated sensitivity matrix or regression model to achieve joint decoupling and quantitative inversion of temperature and strain.

[0060] Step S2.5: Prepare the pressure-strain coupled sensitive medium layer.

[0061] In this embodiment, in order to achieve the coordinated sensing of both pressure and strain parameters, the sensitive medium layer of the coaxial fiber TDR sensor adopts a single-layer composite medium coordinated response design. This design, through the coordinated control of material system and structural parameters, enables the medium layer to generate distinguishable TDR response characteristics under normal pressure and axial tension, thereby realizing the spatial distributed identification and quantitative inversion of multiple parameters.

[0062] Specifically, when using pressure-strain dual-mode measurement, it is preferable to use a stretchable pressure-sensitive composite material in the coaxial sensitive medium layer. This composite material uses a highly elastic compressible polymer matrix (such as foamed TPU, PDMS, or silicone rubber) and incorporates high-dielectric filler particles (such as... , Composite media made of nanoparticles that combine compressibility and dielectric response properties.

[0063] The working mechanism of the above-mentioned coupling dielectric layer is as follows: when a normal pressure is applied from the outside, the dielectric layer is compressed, the thickness t decreases, and the local dielectric constant decreases. The increase in the fiber length leads to an increase in the amplitude of the TDR signal reflection and a decrease in the characteristic impedance. When an axial tensile strain is applied, the overall fiber length L changes, the signal propagation path becomes longer, and the time position of the TDR reflection event is delayed along the time axis, while the change in the dielectric constant is not significant.

[0064] Therefore, by analyzing the amplitude variation and time drift characteristics of the reflected waveform, it is possible to distinguish and quantitatively invert pressure and strain signals. Pressure primarily affects the reflection amplitude ΔV, while strain primarily affects the reflection time delay Δt. A two-parameter response function is established as follows: ; By combining multi-point calibration or fitting models, the host computer can achieve joint decoupling and spatial distribution imaging of pressure-strain signals.

[0065] Preferably, the aforementioned composite medium can be directly coated onto the surface of the central conductor via a one-step co-extrusion process to form a continuous coaxial dielectric layer with a thickness of 0.5–0.8 mm. A typical material ratio is: 70–80 wt% TPU matrix, 15–25 wt% BaTiO3 filler, and 2–5 wt% foamed microspheres. By adjusting the particle content and pore structure, pressure sensitivity and strain linear response performance can be optimized.

[0066] In summary, this embodiment, through a single-layer dual-parameter collaborative response mechanism, can achieve the differentiation and quantitative monitoring of temperature-strain and pressure-strain in a coaxial structure, avoiding the interface reflection problem of multi-layer stacked structures. It has high integration, high robustness and weavable characteristics, and is suitable for scenarios such as flexible smart fabrics and structural health monitoring.

[0067] Step S3: Using a braiding machine, copper wire, tungsten wire, silver wire or carbon fiber wire are braided into a mesh structure with a set coverage rate to form a braided shielding layer and cover the outside of the sensitive medium layer.

[0068] In this embodiment, the outer braided shielding layer uses copper wire, tungsten wire, silver wire, or carbon fiber wire. 24–64 spindles of copper wire, silver-clad copper wire, or high-modulus carbon fiber wire are used to form a mesh structure with a braiding angle of 25–60° and a coverage rate of 70–95%, uniformly covering the outside of the sensitive dielectric layer. It serves as both an outer conductor and provides electromagnetic shielding and resistance to mechanical damage. The overall structure is a continuous, flexible fiber that can be woven into fabrics or composite materials. Preferably, the braiding tension and pitch are controlled by a closed-loop tension control (±3%) to suppress outer diameter runout (OD runout <±3%), ensuring... Axial uniformity.

[0069] Step S4: Wrap elastic silicone rubber around the outer layer of the outer braided shielding layer, and cure it at room temperature to form a protective layer, thus preparing a coaxial fiberized TDR sensor.

[0070] In this embodiment, termination and protection layers are also provided. Specifically, to reduce multiple reflections and port echoes, the excitation end of the coaxial fiber TDR sensor is connected in series / parallel with matching resistors (24.9–50.0 Ω, 0.1%) and SMA / MCX RF coaxial connectors. The remote end is selected with an absorption terminal (matching load) or an open / short circuit reference depending on the application, so as to perform reference curve differential and realize signal input and output.

[0071] Meanwhile, the outermost layer is coated with liquid silicone rubber, such as Ecoflex or a thin FEP / PFA coating (0.10–0.30 mm thick), which cures at room temperature for 2 hours to form a sealed protective layer. This achieves biocompatibility, water vapor resistance, and resistance to sweat / salt spray, with a water vapor transmission rate (WVTR) <15 g·m -2 ·day -1 Dielectric drift <±1.0% after 48 h of body fluid osmosis.

[0072] Furthermore, to verify structural consistency and fabrication controllability, such as Figure 7 As shown, the fibers were characterized using SEM (scanning electron microscope). Figure 7 The cross-sectional view shown in (b) clearly distinguishes the sandwich structure (i.e., the three-layer structure) of the central conductor, dielectric layer, and braided layer, with dielectric thickness uniformity (3σ) < ±5%; Figure 7 The side view shown in (a) indicates that the braided filaments are evenly interwoven and have a stable pitch, with a mesh size of approximately 80–120 μm. The foamed TPU sample shows equiaxed pores with a mixture of closed and open pores, and the pore walls are continuous without obvious cracks. The ceramic nanoparticles in the sample were well dispersed, with no agglomerations larger than 500 nm, indicating that the twin-screw shearing and coupling modification was effective. Metallographic observation of the interface after polishing showed that there were no inclusions or delamination at the conductor-dielectric-braided layer interface, and the interfacial shear strength was >18 MPa as determined by micro-pull-out testing.

[0073] Figure 3 , Figure 5 and Figure 6 These correspond to typical TDR curve families for temperature, pressure, and strain responses, respectively: the horizontal axis represents time delay (which can be converted to spatial location), and the vertical axis represents the reflection coefficient amplitude. Temperature rise causes the reflection peak at the same location to shift slightly to the right and increase in amplitude; pressure loading shows a positive amplitude enhancement and peak broadening trend at the corresponding location; strain leads to an increase in peak amplitude accompanied by a finite phase shift. Combined with... Figure 7 The SEM side and cross-sectional images shown confirm the uniformity of the three-layer structure and the controllable distribution of micropores. To ensure reproducibility of the results, the host computer software records the pulse morphology, front-end gain, and temperature baseline for each test. All inversions are performed based on the same batch of calibration curves. When the external temperature field changes significantly, the system first executes a "temperature self-calibration sequence" (collecting baselines in two known temperature ranges) before performing formal monitoring to avoid v(T) mismatch.

[0074] Example 3 This embodiment provides a distributed monitoring system based on the TDR mechanism, including the coaxial fiber TDR sensor proposed in Embodiment 1, a signal excitation circuit, a reflected wave acquisition module, and a host computer processing unit, wherein: Signal excitation circuit, used to output excitation pulse signal; Coaxial fiber TDR sensor is used to transmit excitation pulse signal and respond to external temperature, pressure and strain, change the dielectric constant and geometry of the sensitive medium layer, cause local impedance disturbance, and cause time delay and amplitude change of TDR reflected signal; The reflected wave acquisition module is used to acquire TDR reflected signals; The host computer processing unit is used to analyze the time delay and amplitude changes of the TDR reflected signal, and realize the spatial distributed positioning and quantitative measurement of temperature, pressure and strain.

[0075] like Figure 2 As shown, the entire system consists of three parts: sensing fiber, excitation acquisition hardware, and host computer algorithm. The host computer injects excitation into one end of the fiber through a nanosecond pulse with a rising edge ≤1 ns or a fast-edge step signal. The pulse travels along the fiber at a speed of Propagation, encountering any local characteristic impedance Points that do not match the reference impedance will generate reflection. The acquisition end samples and records the reflected voltage at an equivalent or real-time rate of ≥1GS / s. Then, the host computer completes noise reduction, baseline correction, dispersion compensation, time delay extraction and amplitude inversion, thereby mapping the "time axis" to the "spatial coordinate axis" and reconstructing the spatial distribution of different physical quantities such as temperature, pressure or strain into a visual profile. This enables the spatial positioning and quantitative inversion of the points of action of multiple physical quantities.

[0076] Specifically, when the external environment causes changes in temperature, pressure, and strain, the sensitive dielectric layer of the sensing fiber will respond to the external stimulus, manifested as changes in local dielectric constant and geometric parameters, resulting in changes in characteristic impedance along the line. When a disturbance occurs, the reflected wave acquisition module acquires the reflected signal obtained by the time-domain reflectometry (TDR) method in real time. The host computer processing unit analyzes the time delay and amplitude changes of the reflected waveform to distinguish the type of external disturbance and achieve quantitative inversion.

[0077] Based on the above analysis, temperature changes mainly cause the dielectric constant of the sensitive dielectric layer to increase monotonically with temperature, and the propagation speed... This decreases accordingly, causing a significant rightward shift of the reflection event on the time axis, although the amplitude change is relatively small; pressure loading mainly causes compression of the dielectric or insulating layer thickness, i.e., the conductor spacing ratio. Decrease leads to characteristic impedance The amplitude of the reflected wave decreases significantly, but its time drift is relatively small; strain causes axial tensile deformation of the fiber, increasing the conductor spacing and accompanied by a slight decrease in the dielectric constant, thus... As the temperature rises, the amplitude of the reflected wave increases slightly and a time drift occurs, which is opposite to the direction of the temperature change.

[0078] In one implementation method, the host computer processing unit performs noise reduction, baseline correction, and dispersion compensation on the acquired reflected signals, and then calculates the time drift. With the change in reflection coefficient The joint analysis enables the identification of multiple parameters, and the analytical relationship can be expressed as follows: = A ; Among them, matrix The sensitivity matrix obtained through calibration experiments reflects the response coefficients of temperature, pressure, and strain to the time delay and amplitude of the reflected wave.

[0079] The temperature change can be obtained by solving the problem using least squares or regularized inversion. Pressure change and strain change The independent components enable decoupling and quantitative inversion of multiple physical quantities. Based on this, the host computer converts the time coordinates of the reflected signal using a relational formula. Mapped to spatial coordinate axes, a distributed physical quantity profile along the direction of the sensing fiber is constructed to realize the spatial positioning and quantitative reconstruction of temperature, pressure, and strain in multiple points or continuous regions.

[0080] In other words, this embodiment combines the analysis of the time drift characteristics, amplitude variation characteristics and waveform morphology differences of the reflected signal, and combines them with the temperature, pressure and strain response models of the medium material to form a complete multi-parameter identification and inversion calculation mechanism. This allows for accurate determination of the source type of external changes and the precise measurement and visualization of spatially distributed multi-physical quantities.

[0081] In summary, this embodiment achieves a distributed fiber sensing platform that is woven, can be laid over long distances, and is sensitive to temperature, pressure, and strain through integrated collaboration of "coaxial structure design, functional medium control, braided shielding forming, termination matching, and TDR inversion algorithm". While ensuring millimeter-level spatial positioning resolution and engineering robustness, it also achieves comprehensive capabilities such as low cost, scalable manufacturing, and decoupling of multiple parameters.

[0082] The steps involved in Examples 2 and 3 above correspond to those in Example 1. For specific implementation details, please refer to the relevant description section of Example 1.

[0083] The above description is only a preferred embodiment of the present invention. Although the specific implementation of the present invention has been described in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that, based on the technical solution of the present invention, various modifications or variations that can be made by those skilled in the art without creative effort are still within the scope of protection of the present invention.

Claims

1. A coaxial fiber-reinforced TDR sensor, characterized in that, It is achieved by using a flexible coaxial structure consisting of a central conductor, a sensitive dielectric layer, an outer braided shielding layer, and a protective layer arranged sequentially from the inside out. The central conductor and the sensitive dielectric layer serve as signal transmission conductors, used to carry and transmit excitation pulse signals; The sensitive medium layer can be a single sensitive medium layer of temperature, pressure, and strain or a coupled sensitive medium layer of temperature-strain and pressure-strain, respectively, to respond to changes in external temperature, pressure, and strain, change the dielectric constant and geometric dimensions of the sensitive medium layer, cause local impedance disturbance, and cause time delay and amplitude change of the TDR reflected signal. The outer braided shielding layer is used to shield against external electromagnetic interference and provide mechanical protection. The protective layer is used to provide environmental and mechanical protection for the inner layer.

2. The coaxial fiber TDR sensor as described in claim 1, characterized in that, Temperature-sensitive dielectric layer uses The composite medium consists of a pressure-sensitive medium layer made of foamed TPU or silicone microporous structure layer and a strain-sensitive medium layer made of solid TPU or silicone film layer. The temperature-strain coupling sensitive medium layer is made of a stretchable temperature-sensitive composite material, which is composed of an elastic matrix and a temperature-sensitive filler. The pressure-strain coupling sensitive medium layer is made of a stretchable pressure-sensitive composite material, which is made of a highly elastic compressible polymer matrix and high dielectric filler particles.

3. A method for fabricating a coaxial fiberized TDR sensor as described in any one of claims 1-2, characterized in that, include: A central conductor is formed by selecting copper wire, tungsten wire, silver wire, or carbon fiber wire of a set diameter and subjecting it to ultrasonic surface cleaning and low-temperature plasma activation treatment. Based on the monitoring target, select a temperature, pressure or strain sensitive material system, and coat it onto the outside of the central conductor through melt extrusion or spin coating process to form a sensitive dielectric layer of a set thickness; Copper wire, tungsten wire, silver wire or carbon fiber wire is woven into a mesh structure with a set coverage by a braiding machine to form a braided shielding layer and cover the outside of the sensitive medium layer. An elastic silicone rubber layer is wrapped around the outer woven shielding layer and cured at room temperature to form a protective layer, thus preparing a coaxial fiberized TDR sensor.

4. The method for fabricating the coaxial fiberized TDR sensor as described in claim 3, characterized in that, The preparation of the central conductor is as follows: Highly conductive metal or carbon-based filaments with a diameter of 0.2-1.0 mm are selected and subjected to ultrasonic chemical cleaning with isopropanol and deionized water, followed by low-temperature plasma activation treatment for 20-60 seconds to reduce interfacial contact resistance. To enhance the interfacial bonding with the sensitive medium layer; The highly conductive metal or carbon-based filaments can be any one of copper wire, silver wire, tungsten wire, nickel-chromium wire, or 12K carbon fiber bundle.

5. The method for fabricating the coaxial fiberized TDR sensor as described in claim 3, characterized in that, The temperature-sensitive dielectric layer is prepared as follows: Will powder and Nanoparticles are mixed and blended by twin-screw extrusion at 180-210℃, and the melt is uniformly coated on the outside of the central conductor to form a temperature-sensitive dielectric layer with a thickness of 0.3-0.8 mm. in, and The molar ratio is 75:

25. The nanoparticles have a particle size of 80-150 nm and a mass fraction of 20-30%.

6. The method for fabricating the coaxial fiberized TDR sensor as described in claim 3, characterized in that, The pressure-sensitive medium layer is prepared as follows: TPU is dissolved in DMF solution, and then n-octane is added. The mixture is magnetically stirred at 130-140℃ for 60-90 min to form a thermally induced phase separation microporous structure. After the microporous structure is spin-coated or extruded into a film, it is quenched in a water bath to complete the non-solvent exchange, thus obtaining a porous foamed TPU thin film with a pore size of 20-200 μm, which is then coated on the outside of the central conductor. The mass ratio of TPU to n-octane is 1:(1.5–2.5).

7. The method for fabricating the coaxial fiberized TDR sensor as described in claim 3, characterized in that, The strain-sensitive medium layer is prepared as follows: Solid TPU or silicone is selected and extruded to form a continuous elastic dielectric layer with a uniform thickness of 0.5-1.0 mm, which is then wrapped around the central conductor to allow for reversible radial shrinkage under tensile strain.

8. The method for fabricating the coaxial fiberized TDR sensor as described in claim 3, characterized in that, The outer braided shielding layer is prepared as follows: Copper wire, tungsten wire, silver wire, or carbon fiber wire is used to form a mesh structure with 24–64 spindles of copper wire, silver-clad copper wire, or high-modulus carbon fiber wire in a braiding angle of 25–60° and a coverage of 70–95%, which is uniformly wrapped on the outside of the sensitive medium layer.

9. The method for fabricating the coaxial fiberized TDR sensor as described in claim 3, characterized in that, Configure the termination and protection layer as follows: The excitation end of the coaxial fiber TDR sensor is connected in series / parallel with matching resistors and SMA / MCX RF coaxial connectors. The remote end is selected as either an absorption terminal or an open / short circuit reference depending on the application to realize signal input and output. The outer woven shielding layer is covered with liquid silicone rubber and cured at room temperature for 2 hours to form a sealed protective layer.

10. A distributed monitoring system based on the TDR mechanism, characterized in that, Includes the coaxial fiber TDR sensor, signal excitation circuit, reflected wave acquisition module, and host computer processing unit as described in any one of claims 1-2, wherein: Signal excitation circuit, used to output excitation pulse signal; Coaxial fiber TDR sensor is used to transmit excitation pulse signal and respond to external temperature, pressure and strain, change the dielectric constant and geometry of the sensitive medium layer, cause local impedance disturbance, and cause time delay and amplitude change of TDR reflected signal; The reflected wave acquisition module is used to acquire TDR reflected signals; The host computer processing unit is used to analyze the time delay and amplitude changes of the TDR reflected signal, and realize the spatial distributed positioning and quantitative measurement of temperature, pressure and strain.

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