Intelligent skin multi-modal measurement system and method based on FBG taper cavity and bending composite microstructure

Abstract

The application relates to an intelligent skin multi-modal measurement system and method based on a FBG cone cavity and a bending composite microstructure. The system comprises a sensing module, an optical sensing module, a data acquisition and processing module, a force output module and a temperature control module; the sensing module comprises a flexible skin structure and a cone FBG and bending single-mode fiber composite structure embedded in the inside of the flexible skin structure; the cone FBG and bending single-mode fiber composite structure comprises a cone FBG section and a bending single-mode fiber section integrated in the same optical fiber; the optical sensing module and the data acquisition and processing module are optically connected with the cone FBG and bending single-mode fiber composite structure respectively; the force output module is used for applying a normal force to the flexible skin structure; and the temperature control module is used for providing a constant temperature field for the flexible skin structure. The application can realize accurate measurement of temperature and normal force and complete multi-modal sensing of the intelligent skin.

Description

Technical Field

[0001] This invention relates to the field of micro-nano measurement technology, specifically to an intelligent skin multimodal measurement system and method based on an FBG conical cavity and curved composite microstructure. Background Technology

[0002] As the core sensory organ for human interaction with the external environment, skin can accurately perceive various external physical stimuli such as temperature and normal force, providing crucial information for human action judgment and environmental perception. Intelligent skin (electronic skin), developed by mimicking the sensory characteristics of human skin, has demonstrated significant application value in fields such as robotic tactile sensing, medical prosthetics, smart wearables, and industrial micro-detection due to its flexibility and miniaturization, becoming a research hotspot in recent years. Developing intelligent skin with multimodal sensing capabilities for temperature and normal force can endow intelligent devices with a tactile perception level closer to that of the human body, further expanding their application scope in scenarios such as precision operation, environmental monitoring, and health detection, and has important practical application significance.

[0003] In the sensing technology system of smart skin, fiber Bragg grating (FBG) sensing technology has become the preferred alternative to traditional electrical sensing due to its advantages such as passive magnetoelectricity, resistance to electromagnetic interference, high resolution, small size, and embedding in flexible substrates. Although the fabrication process of electrical sensing structures is mature, they are susceptible to electrical noise and electromagnetic interference, resulting in a significant decrease in measurement accuracy under complex working conditions. Furthermore, the sensing site requires power, posing a potential electrical safety hazard, and its compatibility with flexible substrates is difficult to meet the large deformation application requirements of smart skin. In contrast, FBG sensors achieve sensing based on wavelength modulation, exhibiting strong anti-interference signal transmission and the ability to be directly embedded in flexible substrates such as silicone rubber. This makes them well-suited to the flexible and miniaturized design requirements of smart skin, and they are particularly suitable for the detection of normal force and temperature.

[0004] Currently, research on flexible sensing based on FBG (Fast-Film Group) largely revolves around the detection of single physical quantities. Specialized experimental research on multiple physical quantities, such as temperature and normal force, remains insufficient, and a complete multimodal sensing system has not yet been established. Existing technologies include some studies focusing on optimizing the normal force sensing performance of FBG sensors by adjusting the grating encapsulation structure to improve mechanical sensing sensitivity, but without conducting corresponding temperature experiments, thus failing to achieve comprehensive sensing of multiple physical quantities. While some studies involve temperature detection using FBG, they lack targeted experiments on normal force in conjunction with the actual application scenarios of smart skin, resulting in limited sensing functionality that cannot meet the multimodal sensing needs of smart skin.

[0005] Meanwhile, in experimental research on multi-physical quantity sensing using fiber optic cables (FBGs), multi-parameter detection experiments targeting FBGs with special structures are still relatively scarce. Existing studies mostly use traditional straight FBGs as the sensing core, and there are few applications of composite structures combining tapered FBGs with bent single-mode fibers in temperature and normal force detection experiments. There is a lack of systematic experimental analysis and data support for the sensing response characteristics of such special composite structures. Furthermore, existing experimental systems are mostly based on conventional packaging methods, simply embedding FBG sensors into flexible substrates without using rigid structures such as stainless steel needle tubes to shape and fix the bent fiber. This makes it difficult to accurately control key structural parameters such as the fiber bending radius, and also fails to guarantee the stability and consistency of the sensing structure after repeated stress and temperature changes. This results in poor repeatability of experimental results, making it difficult to provide reliable experimental basis for subsequent multi-physical quantity decoupling and sensing system optimization.

[0006] Chinese patent document CN 117268441 A discloses a multidimensional force vector sensing and measurement system and method based on FBG and flexible skin structure. The system includes: a sensing module, an optical sensing module, a demodulation optical path module, a signal conditioning module, a control module, a force vector output module, and a displacement driving module. The sensing module includes a flexible skin structure and a measuring FBG sensor. The optical sensing module includes an ASE broadband light source and a first coupler. The demodulation optical path module includes a second coupler, a demodulation grating, a photodetector, and a PZT piezoelectric ceramic stage. The force vector output module includes a three-dimensional precision micro-motion stage, a pressure plate, and a pressure sensor. This invention can achieve the purpose of simultaneously measuring and decoupling normal force and multi-directional shear force using a flexible skin structure, improving system stability. However, the FBG sensor in the sensing module adopts a traditional linear structure, which can only be used for the measurement and decoupling of normal force and multi-directional shear force. It cannot optimize signal differential processing and is easily affected by common-mode interference noise, resulting in low signal-to-noise ratio and measurement accuracy.

[0007] In summary, conducting specialized experimental research on temperature and normal force as single physical quantities based on composite microstructure FBG, and clarifying the sensing response characteristics of FBG under the independent action of each physical quantity, is an important foundation for realizing multi-physical quantity sensing in smart skin. It has significant research value and engineering significance for making up for the shortcomings of existing FBG flexible sensing technology in multimodal physical quantity experimental research and promoting the practical application of smart skin. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a smart skin multimodal measurement system and method based on an FBG conical cavity and a curved composite microstructure. This system integrates a conical FBG with a curved single-mode optical fiber shaped by a stainless steel needle tube into the same optical fiber and embeds the entire structure into a flexible silicone rubber substrate. Simultaneously, a suitable experimental system is built to conduct precise measurements of temperature and normal force, thereby achieving multimodal sensing of the smart skin.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A smart skin multimodal measurement system based on FBG conical cavity and bending composite microstructure, the system includes: a sensing module, an optical sensing module, a data acquisition and processing module, a force output module and a temperature control module; The sensing module includes a flexible skin structure and a composite structure of a tapered FBG and a bent single-mode fiber embedded inside the flexible skin structure; the composite structure of the tapered FBG and the bent single-mode fiber includes a tapered FBG segment and a bent single-mode fiber segment integrated in the same fiber. The optical sensing module and the data acquisition and processing module are respectively connected to the composite structured light of the tapered FBG and the bent single-mode fiber; the force output module is used to apply a normal force to the flexible skin structure; and the temperature control module is used to provide a constant temperature field for the flexible skin structure.

[0010] Furthermore, the tapered FBG segment is formed by fused tapering of the grating region of a single-mode fiber; the bent single-mode fiber segment is formed by controlled bending and shaping of the pigtail extending from the tapered FBG segment.

[0011] Furthermore, the tapered FBG and bent single-mode fiber composite structure includes a first fiber segment, a second fiber segment, and a third fiber segment connected in sequence; wherein, the first fiber segment and the third fiber segment are arranged parallel and spaced apart, the second fiber segment is a bent single-mode fiber segment, which is connected between the same end of the first fiber segment and the third fiber segment to form a teardrop-shaped bent structure; the tapered FBG segment is disposed on the first fiber segment near one end of the second fiber segment, and the tapered FBG segment is formed by fused tapering of the grating region of the single-mode fiber. The tapered FBG and bent single-mode fiber composite structure is formed by tapering the FBG grating region of the single-mode fiber to form a tapered FBG segment, and bending its pigtail into a teardrop shape to form a bent single-mode fiber segment.

[0012] Furthermore, a stainless steel needle tube is sleeved on the outer side of the first and third optical fiber segments, and the bending radius of the second optical fiber segment is fixed by the stainless steel needle tube.

[0013] Furthermore, the optical sensing module includes an ASE broadband light source and a circulator; The input end of the circulator is connected to the output end of the ASE broadband light source, and the output end of the circulator is connected to the composite structured light of the tapered FBG and the bent single-mode fiber.

[0014] Furthermore, the data acquisition and processing module includes a spectrometer and a computer; The input end of the spectrometer is connected to the composite structured light of the tapered FBG and the bent single-mode fiber, and the output end of the spectrometer is connected to a computer. The computer is used to receive and process the transmission spectrum wavelength shift signal emitted by the composite structure of the tapered FBG and the bent single-mode fiber, and to determine the measurement results of temperature and normal force based on the transmission spectrum wavelength shift signal.

[0015] Furthermore, the force output module includes a pressure sensor, a pressure plate, and a three-dimensional precision micro-motion stage; the flexible skin structure is placed on the three-dimensional precision micro-motion stage, and the pressure plate is placed directly above the flexible skin structure and connected to the pressure sensor to apply a normal force to the sensing module.

[0016] Furthermore, the temperature control module includes a constant temperature chamber; The sensing module is placed in the constant temperature chamber, which provides a uniform and constant temperature field for the sensing module; the force output module is located inside the constant temperature chamber.

[0017] Furthermore, the flexible skin structure is made using Ecoflex00-30 silicone mold.

[0018] This invention also includes a measurement method for the above-mentioned intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure, the method comprising the following steps: S1. The optical sensing module transmits optical signals to the composite structure of the tapered FBG and bent single-mode fiber in the sensing module. S2. The tapered FBG and bent single-mode fiber composite structure transmits the received optical signal to the data acquisition and processing module. When the flexible skin structure is subjected to temperature and normal force to produce elastic deformation, strain or effective refractive index change occurs, causing the transmitted optical signal to shift in wavelength. S3. The force output module applies a normal force to the flexible skin structure in the sensing module; S4. The temperature control module provides a uniform temperature field with different gradients for the flexible skin structure in the sensing module. S5. The data acquisition and processing module establishes the relationship curves between wavelength, temperature, and normal force based on the received transmitted light signal and wavelength offset data, and determines the measurement results of temperature and normal force.

[0019] Compared with the prior art, the advantages of the present invention are: (1) Based on the multi-physical sensing characteristics of human skin, this invention embeds a composite sensing unit consisting of a tapered FBG and a teardrop-shaped curved fiber optic cable into a flexible silicone rubber substrate to form a biomimetic skin sensor. In the prior art, flexible sensing structures can generally only achieve single-function biomimicry, making it difficult to achieve a dual fit between structure and function. However, the silicone rubber substrate used in this invention has a flexibility and elastic modulus that are very close to human skin, which can accurately simulate the soft touch and deformation characteristics of human skin. The composite sensing unit is like a tactile receptor in the skin, which can respond synchronously to the action of normal force and temperature change, thereby achieving a dual unity of structural biomimicry and functional biomimicry, which is closer to the actual sensing logic of human skin.

[0020] (2) The composite structure of the tapered grating region, teardrop-shaped bent fiber optic cable, and stainless steel needle tube designed in this invention has significant advantages over existing single FBG sensors or simple bent fiber optic structures. The tapered FBG grating region greatly improves the response sensitivity to mechanical deformation; the teardrop-shaped bent structure increases the contact area with the bionic skin substrate, making the stress distribution more uniform and avoiding sensing deviation caused by local stress concentration; and the shaping effect of the stainless steel needle tube can ensure the precise constant bending radius (0.3cm), effectively avoiding the sensing performance drift caused by structural deformation during flexible packaging, and solving the defects of poor stability and easy parameter deviation of existing flexible sensing structures.

[0021] (3) This invention relies on the inherent passive magnetoelectric properties of FBG sensors and adopts optical transmission and detection methods, fundamentally avoiding the defects of traditional electrical sensing systems that are susceptible to electromagnetic interference and electrical noise, and require on-site power supply, which poses safety hazards. At the same time, the composite structure of tapered FBG and bent pigtail can enhance the response sensitivity to changes in physical quantities such as temperature and normal force. Combined with the high-precision spectral acquisition capability of the spectrometer, it can accurately capture the slight shift of the resonant valley wavelength, which can be adapted to complex experimental environments and high-precision measurement requirements such as robot tactile sensing and medical prostheses, making up for the shortcomings of weak sensitivity and anti-interference capability of existing FBG flexible sensing technology. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the measurement system in this invention; Figure 2 a is a schematic diagram of the sensing module in this invention; Figure 2 b is a schematic diagram of the composite structure of tapered FBG and bent single-mode fiber in this invention; Figure 3 This is a schematic diagram of the composite structure of the tapered FBG and the bent single-mode fiber and the stainless steel needle tube in this invention; Figure 4This is a schematic diagram of the transmission spectrum results of the bent optical fiber under different bending radii in this invention; Figure 5 This is a graph showing the relationship between the wavelength of the resonant valley of flexible skin and temperature in this invention; Figure 6 This is a graph showing the relationship between the wavelength of the resonant valley of flexible skin and the normal force in this invention; in: 1. Composite structure of tapered FBG and bent single-mode fiber; 2. Flexible skin structure; 3. ASE broadband light source; 4. Circulator; 5. Temperature chamber; 6. Spectrometer; 7. Computer; 8. Pressure sensor; 9. Three-dimensional precision micro-motion stage; 10. Pressure plate; 11. First fiber segment; 12. Second fiber segment; 13. Third fiber segment; 14. Tapered FBG segment; 15. Stainless steel needle tube. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings: Example 1 like Figure 1 As shown, the intelligent skin multimodal measurement system based on the FBG conical cavity and curved composite microstructure of the present invention is designed based on the sensing principle of conical FBG and consists of a sensing module, an optical sensing module, a data acquisition and processing module, a force output module, and a temperature control module. This system can achieve accurate detection of multiple parameters such as temperature and normal force, and has the characteristics of strong anti-interference, high sensitivity, and good stability, making it suitable for practical applications of intelligent skin multimodal sensing. Specifically, the sensing module is used to sense changes in temperature and normal force. The optical sensing module generates and transmits laser signals to the conical FBG and curved single-mode fiber composite structure in the flexible skin structure. The force output module applies normal force to the sensing module. The temperature control module provides a gradient temperature field for the sensing module. The data acquisition and processing module extracts the wavelength change of the resonance valley in the transmission spectrum, establishes a one-to-one correspondence between the wavelength change and the temperature and normal force changes, and establishes a corresponding linear relationship.

[0024] Specifically, such as Figure 2 As shown in (a), the sensing module includes a flexible skin structure 2 and a tapered FBG and bent single-mode fiber composite structure 1 embedded in the flexible skin structure 2. The flexible skin structure is in direct contact with the surface being measured and serves as the sensing carrier for physical quantities, transmitting external temperature and normal force signals to the built-in tapered FBG and bent single-mode fiber composite structure 1. After receiving the temperature and normal force signals, the transmission spectrum of the tapered FBG and bent single-mode fiber composite structure 1 will undergo a corresponding wavelength shift, thereby realizing the conversion of physical quantities into optical signals.

[0025] Furthermore, such as Figure 2As shown in (b), the tapered FBG and bent single-mode fiber composite structure 1 is fabricated from the same single-mode fiber, including a first fiber segment 11, a second fiber segment 12, and a third fiber segment 13 connected in sequence. These three are different functional sections of the same single-mode fiber. The first fiber segment 11 and the third fiber segment 13 are arranged parallel to each other and spaced apart. The second fiber segment 12 is a bent single-mode fiber, connected between the same end of the first fiber segment 11 and the third fiber segment 13, forming a teardrop-shaped bent structure. A tapered FBG segment 14 is provided on the first fiber segment 11 near the end of the second fiber segment 12. This tapered FBG segment 14 is formed by fused tapering of the grating region of the single-mode fiber. The bent single-mode fiber segment is formed by controllably bending and shaping the pigtail extending from the tapered FBG segment; preferably, the shaping is a bent structure with a radius of 0.3 cm.

[0026] To obtain a tapered FBG with a uniform transition in the tapered region and symmetrical ends, this embodiment employs the fused taper method. First, the voltage and current applied to the annular graphite element are controlled to generate heat, with an optimal power of 65-75 W, creating a stable and uniform temperature field around the fiber to be processed. Simultaneously, the fiber is stretched, with tension applied to both ends by a stepper motor driven by an FPGA, gradually forming a tapered cavity and dividing the original fiber grating region into two grating sections and a tapered cavity.

[0027] The use of a single-sided tapered FBG in this embodiment is mainly due to two considerations: first, a double-sided tapered FBG would significantly increase the cost of the optical fiber itself and the tapering process; second, a double-sided structure would make the transmission spectrum more complex, which would be detrimental to subsequent data acquisition and signal processing.

[0028] Furthermore, such as Figure 3 As shown, a stainless steel needle tube 15 is sleeved on the outer side of the first optical fiber segment 11 and the second optical fiber segment 12; the tapered FBG segment and the bent single-mode optical fiber segment are continuously and seamlessly connected along the axial direction of the single-mode optical fiber; the first optical fiber segment 11 and the third optical fiber segment 13 with tapered FBG segment are placed inside the stainless steel needle tube and fixed axially; the second optical fiber segment 12 with bent single-mode optical fiber structure forms a bent structure with a fixed bending radius on the outer side of the stainless steel needle tube, and the three together constitute an integrated composite sensing structure.

[0029] The tapered FBG and bent single-mode fiber composite structure uses a single single-mode fiber with an inscribed apodized FBG as the substrate, and is integrated into a single structure through segmented processing and shaping. The tapered FBG segment is located in the middle of the single-mode fiber, and is formed into a tapered microstructure with a gradually decreasing diameter by fused tapering of the fiber grating region. The bent single-mode fiber segment is directly formed by the pigtail extending from the tapered FBG segment, without tapering, and only through a controlled bending process to achieve a stable bending shape. During assembly, the tapered FBG segment is placed entirely inside a stainless steel needle tube, which is used for axial fixation and protection. The bent single-mode fiber segment located outside the stainless steel needle tube is bent into a curved structure with a preset radius, and the bending radius is precisely shaped and fixed by the stainless steel needle tube. When external temperature and normal force are applied to the flexible skin structure, they are transmitted to the composite structure through the silicone matrix, causing wavelength shifts in the tapered FBG segment and changes in the transmission spectrum of the bent single-mode fiber segment, thereby achieving synchronous sensing of temperature and normal force.

[0030] The core sensing mechanism of the intelligent skin multimodal measurement system based on the FBG conical cavity and curved optical fiber composite microstructure described in this invention relies on the optical response characteristics of the conical FBG and the curved optical fiber. The sensing principle and detection mechanism of the two in the flexible skin structure are as follows: Tapered fiber Bragg gratings (FBGs) are passive optical devices formed by tapering a standard FBG with periodically modulated refractive index through a fused taper process. The sensing principle of a tapered FBG is based on the dual effects of Bragg reflection and mode coupling. Specifically, when a broadband beam of light is incident on a tapered FBG, on the one hand, light of a specific wavelength satisfying the Bragg condition (λ = 2nΛ, where λ is the reflected wavelength, n is the effective refractive index of the tapered fiber core, and Λ is the grating period) undergoes Bragg reflection; on the other hand, the tapered structure causes the effective refractive index of the fiber core to gradually change along the axial direction, inducing coupling between the fundamental mode and the cladding mode. This allows some light satisfying the coupling condition to couple from the core to the cladding and interfere. When a tapered FBG is integrated into a flexible skin structure, changes in ambient temperature or slight deformation caused by normal forces will synchronously alter the effective refractive index and grating period of the tapered FBG, leading to a shift in the reflected wavelength and changes in coupling loss. By detecting changes in the aforementioned optical parameters, information related to temperature and normal force can be obtained, enabling accurate detection of multiple parameters.

[0031] In this embodiment, the tapered FBG segment adopts a tapered FBG structure. Compared with traditional FBGs, the tapered treatment of the grating region significantly improves the response sensitivity to mechanical deformation and temperature changes, enabling precise capture of minute deformations and temperature fluctuations in flexible skin, thus greatly improving measurement accuracy. Simultaneously, the tapered FBG and bent single-mode fiber composite structure are integrally embedded in a silicone rubber matrix. The flexible buffering properties of silicone rubber effectively isolate external mechanical impacts and environmental interference, providing comprehensive protection for the fiber structure. This prevents fiber breakage and performance drift during stress, encapsulation, and long-term use, significantly improving the structural stability and lifespan of the sensing system.

[0032] In conjunction with the tapered FBG, the bent fiber, as another core sensing unit, is a passive optical device with a specific bending shape formed by the controlled bending of ordinary single-mode fiber. Its core characteristics are determined by the bending state of the fiber. The bending fiber sensing principle is based on the fiber mode transmission loss mechanism. Specifically, when a broadband light beam is incident on a bent fiber, the bending of the fiber disrupts the total internal reflection condition, causing leakage of the fundamental mode light transmitted in the fiber core. Some light energy is coupled from the core to the cladding. At the bend end of the fiber, the light that has leaked into the cladding will couple again with the light transmitted in the core, forming an interference output. When the bent fiber is integrated into a flexible skin structure, when the structure is subjected to temperature and normal force, causing a slight deformation, the bending radius of the bent fiber will change, thereby changing the transmission spectrum. By detecting the change in the transmission spectrum, relevant information on temperature and normal force can be obtained, enabling multi-parameter detection.

[0033] The flexible skin structure 2 in this invention is prepared using Ecoflex™ 00-30 silicone rubber through a demolding process. The mold is made from imported photosensitive resin Imagine Black. The molding process must be carried out in a well-ventilated environment. Operators must wear safety glasses, long-sleeved protective clothing, and rubber gloves to avoid contact between the silicone material and auxiliary reagents and the skin and mucous membranes. To improve demolding convenience and prevent the silicone rubber from sticking to the mold surface, Ease Release™ 200 is evenly sprayed onto the inner wall of the mold. After spraying, it is gently applied with a soft brush to ensure complete coverage of the mold surface. After the release agent dries, components A and B of the Ecoflex 00-30 silicone rubber are thoroughly mixed at a weight or volume ratio of 1:1 until the mixture is homogeneous and free of air bubbles. The mixed silicone material is then poured into the mold and cured at room temperature for 4 hours. After the silicone rubber is fully formed, demolding is performed to obtain the flexible skin structure.

[0034] Before tapering the original FBG grating region, the optical fiber is pre-processed. The optical fiber with a dovetail FBG is selected as the tapered base fiber. Its grating period is 1550±2nm, the fiber length is 15±1mm, the 3dB bandwidth is not less than 0.4nm, and the reflectivity is ≥90%.

[0035] This invention employs a melt-heating and tapered stretching technique based on a ring-shaped graphite element to tapere the original FBG grating region. This process includes two steps: heating and stretching. First, heating is achieved by controlling the voltage and current applied to the ring-shaped graphite element, with a preferred heating power of 65-75 W, thereby creating a stable and uniform temperature field around the fiber to be processed. Second, a stepper motor driven by an FPGA is used to stretch both ends of the fiber, gradually thinning the FBG region and forming a tapered cavity. This divides the original FBG region into three parts: the two grating sections on either side and the tapered cavity in the middle. The diameter of the tapered cavity is determined by the preheating time, the stretching speed V, and the stretching displacement X; different sizes of tapered FBGs can be obtained by changing these parameters.

[0036] After tapering, the optical fiber undergoes further bending and shaping to form a composite structure of a tapered FBG and a bent single-mode fiber. During shaping, the tapered FBG segment is placed entirely within a stainless steel needle tube for axial fixation and protection. The bent single-mode fiber segment located outside the stainless steel needle tube is bent into a pre-defined radius, and the bending radius is precisely fixed using the stainless steel needle tube to ensure structural stability and consistent optical parameters during subsequent encapsulation and use. The shaped tapered FBG and bent single-mode fiber composite structure is then embedded entirely into a flexible skin structure. The composite structure is completely embedded within the flexible skin structure, with the stainless steel needle tube tightly fitted to the flexible skin structure, and no fiber exposed. External temperature and normal force can be effectively transmitted to the tapered FBG segment and the bent single-mode fiber segment through the flexible skin structure, causing the composite structure to produce a corresponding optical response, thereby achieving simultaneous multimodal measurement of temperature and normal force.

[0037] In some embodiments, the optical sensing module includes an ASE broadband light source 3 and a circulator 4. The input of the circulator 4 is connected to the output of the ASE broadband light source 3, and the output of the circulator 4 is optically connected to the tapered FBG and bent single-mode fiber composite structure 2. The ASE broadband light source 3 provides a broadband light source and outputs a broadband optical signal. The circulator 4 enables directional transmission and guidance of the optical signal.

[0038] In some embodiments, the data acquisition and processing module includes a spectrometer 6 and a computer 7. The input end of the spectrometer 6 is connected to the tapered FBG and bent single-mode fiber composite structure 1, and the output end of the spectrometer 6 is connected to the computer 7. The spectrometer 6 is used to capture the transmission spectrum changes of the overall structure under changes in temperature and normal force. The computer 7 is used to process the wavelength data of the resonance valleys in the transmission spectrum and establish the relationship curve between wavelength and temperature and normal force based on the wavelength shift signal of the transmission spectrum of the tapered FBG and bent single-mode fiber composite structure.

[0039] In some embodiments, the force output module includes a pressure sensor 8, a pressure plate 10 corresponding to the pressure sensor 8, and a three-dimensional precision micro-motion stage 9. The three-dimensional precision micro-motion stage 9 is mounted on the base of the pressure sensor 8, and the pressure plate 10 is mounted at the probe of the pressure sensor 8 and located above the three-dimensional precision micro-motion stage 9. The three-dimensional precision micro-motion stage 9 is used to apply a normal force to the flexible skin structure 2. The pressure sensor is used to sense the magnitude of the normal force applied to the flexible skin structure 2. The pressure plate 10 is in direct contact with the flexible skin structure 2 and is used to apply normal forces of different magnitudes.

[0040] In some embodiments, the temperature control module includes a constant temperature chamber 5. The flexible skin structure 2 is placed in the constant temperature chamber 5, which provides a uniform and constant temperature field for the flexible skin structure 2.

[0041] The aforementioned measurement system uses a composite structure of tapered FBG and bent single-mode fiber as the sensing unit, a flexible skin structure as the encapsulation carrier, and is equipped with an optical sensing module, a data acquisition and processing module, a force output module, and a temperature control module. It can realize the synchronous measurement of temperature and normal force, providing a feasible solution for the development of intelligent bionic skin using fiber optic sensors to achieve multimodal detection.

[0042] Example 2 The aforementioned intelligent skin multimodal measurement system based on FBG conical cavity and bending composite microstructure is used to measure three-dimensional force vectors. This invention also discloses a measurement method for this system, which includes the following steps: S1. The optical sensing module transmits optical signals to the composite structure of the tapered FBG and the bent single-mode fiber in the sensing module.

[0043] Specifically, the ASE broadband light source in the optical sensing module emits laser light, which enters the tapered FBG and bent single-mode fiber composite structure in a specific sequence through a circulator. The tapered FBG pigtail is bent into a 3 mm radius, and the bending radius is fixed using a stainless steel needle tube to ensure the stability of the bending shape. The fiber position is then adjusted so that the tapered FBG is completely placed inside the stainless steel needle tube. These steps complete the fabrication of the tapered FBG and bent single-mode fiber composite structure.

[0044] S2. The tapered FBG and bent single-mode fiber composite structure transmits the received optical signal through the circulator output port to the spectrometer in the data acquisition and processing module. When the flexible skin structure is subjected to temperature and normal force to produce elastic deformation, the tapered FBG and bent fiber undergo strain or change in effective refractive index, causing the transmitted optical signal to shift in wavelength and be recorded by the spectrometer.

[0045] S3, the force output module applies a normal force to the flexible skin structure in the sensing module.

[0046] Specifically, the pressure sensor is kept stationary, and the Z-axis of the three-dimensional precision micro-motion stage is adjusted while the pressure sensor readings are observed. When the pressure reading reaches the desired threshold, the triggering condition is considered met, and the adjustment of the three-dimensional precision micro-motion stage is stopped. Simultaneously, the transmission spectrum at this point is recorded. Under different normal force loading conditions, the tapered FBG and the bent single-mode fiber experience varying degrees of axial strain, causing a shift in the transmission spectrum of the entire structure.

[0047] S4, the temperature control module provides a uniform temperature field with different gradients for the flexible skin structure in the sensing module.

[0048] Specifically, the biomimetic skin structure is placed in a temperature-controlled chamber, placing it in a sealed environment. The chamber's temperature control function provides the biomimetic skin structure with uniform, constant temperature fields of varying gradients. Different temperatures alter the effective refractive index of the tapered FBG and the bent single-mode fiber, causing a shift in the transmission spectrum of the entire structure.

[0049] S5. The computer in the data acquisition and processing module establishes the relationship curve between wavelength and temperature and normal force based on the received transmitted light signal and wavelength offset data, and determines the measurement results of temperature and normal force.

[0050] Specifically, by using a linear relationship, the corresponding temperature value and normal force magnitude can be deduced from the detected wavelength change, thereby achieving synchronous measurement of temperature and normal force.

[0051] The principle governing the relationship between wavelength change and temperature and normal force change is as follows: For a bent fiber structure, if the fundamental mode in the fiber couples with the m-th cladding mode to produce an interference effect, and the optical path difference between the two paths at the fiber output end satisfies (2k+1)π, a resonance valley will appear. The wavelength of this resonance valley can be expressed as: ; in, The wavelength of the resonance valley. There are some refractive index differences, L is the effective interference length, and k is a positive integer.

[0052] Temperature changes alter the refractive index difference and fiber length through thermo-optical effects and thermal expansion, leading to resonance valley drift. The formula for resonance valley drift caused by temperature changes is: ; in, This represents the change in wavelength at the resonance valley. The wavelength of the m-th order mode. This is the coefficient of thermal expansion of the optical fiber. This refers to the thermo-optic coefficient of the optical fiber. This represents the change in temperature.

[0053] For a conical FBG, temperature changes alter the grating period and effective refractive index parameters, ultimately causing a shift in the interference valleys of the transmission spectrum. The mathematical relationship can be expressed as follows: ; in, It is the initial wavelength. It is the temperature sensitivity coefficient.

[0054] axial stress The change in the center wavelength of the interference valley under the action of (the deformation of the flexible skin normal force) It can be represented as .

[0055] in, It is the effective photoelastic coefficient. It is axial stress.

[0056] The working process of the measurement system described in this invention is as follows: (1) Place the flexible skin structure 2 on the upper surface of the three-dimensional precision micro-motion stage 9 and align its center with the center of the pressure plate 10 located above the flexible skin structure; adjust the Z-axis of the three-dimensional precision micro-motion stage 9 so that the flexible skin structure 2 rises to the set gap with the pressure plate 10 above and then stop adjusting.

[0057] (2) Keep the pressure sensor 8 fixed and continue to adjust the Z-axis of the three-dimensional precision micro-motion stage 9 while observing the value of the pressure sensor 8 in real time; set a threshold value for the pressure value in the pressure sensor 8. When the pressure value detected by the pressure sensor 8 reaches the threshold value, it is determined that the flexible skin structure 2 and the pressure plate 10 meet the triggering conditions. At this time, stop adjusting the three-dimensional precision micro-motion stage 9. Under different normal force loading conditions, the tapered FBG segment and the bent single-mode fiber segment produce different degrees of axial strain, which in turn causes the transmission spectrum of the entire composite structure to shift.

[0058] (3) The flexible skin structure 2 is placed in a constant temperature chamber 5 to keep it in a sealed environment. The temperature control function of the constant temperature chamber 5 provides a uniform and constant temperature field with different gradients for the flexible skin structure 2. Different temperatures will change the effective refractive index of the tapered FBG and the bent single-mode fiber, thereby causing a shift in the transmission spectrum of the entire composite structure.

[0059] (4) Turn on the ASE broadband light source 3. The light signal emitted by the ASE broadband light source 3 is transmitted through the circulator 4 to the tapered FBG and bent single-mode fiber composite structure 1 in the flexible skin structure 2. The light signal transmitted from the other end of the composite structure 1 is received by the spectrometer 6. The spectrometer 6 transmits the collected spectral data to the computer 7.

[0060] (5) The computer 7 processes the received spectral data related to temperature and normal force: by extracting the wavelength change of the resonance valley in the transmission spectrum, linear relationships between wavelength and temperature and wavelength and normal force are established respectively. Using the linear relationships, the corresponding temperature value and normal force are deduced from the detected wavelength change, thereby realizing the synchronous measurement of temperature and normal force.

[0061] To investigate the spectral response characteristics of the FBG conical cavity and bending composite microstructure under different bending radii, this invention utilizes RSoft optical simulation software to perform transmission spectrum simulation analysis on the bending structure of a single-mode fiber, providing a theoretical basis for the optimized design of device bending parameters and subsequent experimental fabrication.

[0062] Figure 3 The figure shows the simulation results of the transmission spectrum of single-mode fiber under different bending radii. The horizontal axis represents the free space wavelength, and the vertical axis represents the normalized transmission intensity (Mon. val.). The simulation curves correspond to the bending radii R=2000μm, R=3000μm, R=4000μm, and R=5000μm, respectively.

[0063] Depend on Figure 3It can be seen that when a single-mode fiber is at different bending radii, its transmission spectrum shape, resonance valley depth, and spectral undulation characteristics all change significantly, indicating that bending significantly alters the internal mode coupling state and optical field transmission characteristics of the fiber. As the bending radius decreases, the bending effect of the fiber intensifies, and the coupling between the core mode and the cladding mode becomes more pronounced, making the resonance characteristics in the transmission spectrum more prominent. Conversely, when the bending radius increases, the bending disturbance of the fiber weakens, the modulation depth of the transmission spectrum relatively decreases, and the resolution of characteristic peaks and valleys also declines.

[0064] Further comparison Figure 3 The simulation curves show that when the bending radius is 3000 μm (i.e., 0.3 cm), the transmission spectrum of the single-mode fiber exhibits relatively obvious and stable characteristic changes, with clear spectral undulations, high resonant valley identification, and good spectral response regularity. Compared to the excessive loss or spectral instability that may occur under smaller bending radii, and the problems of insufficient bending modulation and low sensing response sensitivity under larger bending radii, a bending radius of 0.3 cm achieves a good balance between the obviousness of spectral characteristics, the sensitivity of physical quantity response, and the feasibility of device structure. Therefore, this invention determines 0.3 cm as the optimal bending radius for the bending section in the composite structure of tapered FBG and bent single-mode fiber.

[0065] Figure 4 This is a graph showing the relationship between the wavelength of the resonant valley of the flexible skin in this invention and temperature. The horizontal axis represents temperature, and the vertical axis represents the wavelength of the resonant valley. The resonant valley that drifts with temperature is named DIP1, and the sharp peak in the middle of the transmission spectrum is the characteristic transmission peak of the cone-shaped FBG.

[0066] Depend on Figure 4 It can be seen that the transmission spectrum of the structure of this invention undergoes a regular shift with temperature changes. Specifically, the position of the resonance valley DIP1 exhibits a blue shift with temperature, while the position of the cone-shaped FBG peak exhibits a red shift, indicating that the structure has significant differences in its temperature response characteristics. Through extraction and fitting analysis of the characteristic wavelength positions at different temperatures, it is found that DIP1 shows a good linear relationship with temperature, with a linear correlation coefficient of 0.9985. In contrast, the temperature linear correlation coefficient corresponding to the cone-shaped FBG peak is 0.9944, which, although also showing a good linear response, is slightly lower than that of DIP1.

[0067] Therefore, temperature experiments show that multiple spectral feature points, i.e. multiple feature wavelengths, in the flexible skin structure of the present invention all have temperature response capabilities. Among them, DIP 1 exhibits better linear characteristics, indicating that the structure can achieve stable and accurate temperature sensing, providing a reliable foundation for subsequent multi-parameter sensing.

[0068] Figure 5 This is a graph showing the relationship between the wavelength of the resonant valley of the flexible skin and the normal force in this invention. The horizontal axis represents the normal force, and the vertical axis represents the wavelength of the resonant valley. The resonant valley that drifts with the normal force is named DIP2, and the peak of the conical FBG is used as another characteristic wavelength for comparative analysis.

[0069] Depend on Figure 5 It can be seen that the spectral curves of the structure of this invention undergo certain changes under different normal force loading conditions. Among them, the wavelength position of DIP2 shows a relatively obvious drift pattern with the change of normal force, indicating that this feature point has a good response capability to external normal force. Linear fitting of the relationship between the characteristic wavelength of DIP2 and the change of normal force shows a linear correlation coefficient of 0.9916, indicating that there is a good linear correspondence between DIP2 and normal force, and it can be used as an important characteristic parameter for normal force detection. Meanwhile, the peak of the conical FBG hardly changes significantly during the change of normal force, and its center wavelength remains basically stable, indicating that this characteristic peak is insensitive or has low sensitivity to normal force. Therefore, it is evident that different spectral features in the structure of this invention exhibit significant differences in their response to external pressure.

[0070] Therefore, the normal force experiment results show that DIP2 in the flexible skin structure described in this invention has good response characteristics and high linearity, while the cone-shaped FBG peak is basically insensitive to changes in normal force. By utilizing the difference in their response characteristics, the normal force parameter can be effectively identified.

[0071] In summary, this invention utilizes a composite microstructure of FBG cone cavity and bending to construct an intelligent skin multimodal measurement system, achieving the following beneficial effects: First, addressing the issues of traditional electrical sensors being susceptible to electromagnetic interference, having electrical noise, requiring power supply which poses safety hazards, and having poor compatibility with flexible substrates, this invention leverages the passive magnetoelectric properties of FBG to achieve significant advantages such as resistance to electromagnetic radiation, no electrical crosstalk, and low power consumption. It can operate stably for a long time under complex working conditions, adapting to the long-term stable measurement needs in complex scenarios such as robotic tactile sensing and medical prostheses.

[0072] Secondly, the tapered FBG after tapering and the teardrop-shaped bent single-mode fiber composite structure are integrated into a single fiber and embedded in a flexible silicone rubber substrate. With the addition of a stainless steel needle tube for shaping and fixation, this not only simplifies the wiring and installation process of the measurement system and ensures the mechanical stability and optical parameter controllability of the composite structure, but also avoids the disadvantages of traditional electrical sensors, such as complicated wiring and inconvenient maintenance.

[0073] Finally, by optimizing the parameters of the composite microstructure, such as determining 0.3 cm as the optimal bending radius and performing taper treatment on the grating area, the sensing performance can be further improved, resulting in higher temperature and normal force response sensitivity and excellent linear response characteristics. These unique qualities effectively compensate for the shortcomings of existing FBG flexible sensing technology in multi-physical quantity experimental research, realizing the dual unity of biomimetic structure and function of intelligent skin. This plays an indelible role in promoting breakthroughs in multimodal sensing technology of intelligent skin, improving the fiber optic sensing and measurement system, and promoting industrial applications in related fields.

[0074] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A smart skin multimodal measurement system based on FBG conical cavity and curved composite microstructure, characterized in that, The system includes: a sensing module, an optical sensing module, a data acquisition and processing module, a force output module, and a temperature control module; The sensing module includes a flexible skin structure and a composite structure of a tapered FBG and a bent single-mode fiber embedded inside the flexible skin structure; the composite structure of the tapered FBG and the bent single-mode fiber includes a tapered FBG segment and a bent single-mode fiber segment integrated in the same fiber. The optical sensing module and the data acquisition and processing module are respectively connected to the composite structured light of the tapered FBG and the bent single-mode fiber; the force output module is used to apply a normal force to the flexible skin structure; and the temperature control module is used to provide a constant temperature field for the flexible skin structure.

2. The intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure according to claim 1, characterized in that, The tapered FBG and bent single-mode fiber composite structure includes a first fiber segment, a second fiber segment, and a third fiber segment connected in sequence. The first fiber segment and the third fiber segment are arranged parallel to each other and spaced apart. The second fiber segment is a bent single-mode fiber segment connected between the same end of the first and third fiber segments, forming a teardrop-shaped bent structure. The tapered FBG segment is disposed on the first fiber segment near the end of the second fiber segment. This tapered FBG segment is formed by fused tapering of the grating region of the single-mode fiber. The bent single-mode fiber segment is formed by controlled bending of the pigtail extending from the tapered FBG segment.

3. The intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure according to claim 1, characterized in that, The tapered FBG and bent single-mode fiber composite structure is formed by tapering the FBG grating region of the single-mode fiber to form a tapered FBG segment, and bending its pigtail into a teardrop shape to form a bent single-mode fiber segment.

4. The intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure according to claim 2, characterized in that, A stainless steel needle tube is fitted on the outer side of the first and third optical fiber segments, and the bending radius of the second optical fiber segment is fixed by the stainless steel needle tube.

5. The intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure according to claim 1, characterized in that, The optical sensing module includes an ASE broadband light source and a circulator; The input end of the circulator is connected to the output end of the ASE broadband light source, and the output end of the circulator is connected to the composite structured light of the tapered FBG and the bent single-mode fiber.

6. The intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure according to claim 1, characterized in that, The data acquisition and processing module includes a spectrometer and a computer; The input end of the spectrometer is connected to the composite structured light of the tapered FBG and the bent single-mode fiber, and the output end of the spectrometer is connected to a computer. The computer is used to receive and process the transmission spectrum wavelength shift signal emitted by the composite structure of the tapered FBG and the bent single-mode fiber, and to determine the measurement results of temperature and normal force based on the transmission spectrum wavelength shift signal.

7. The intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure according to claim 1, characterized in that, The force output module includes a pressure sensor, a pressure plate, and a three-dimensional precision micro-motion stage; the flexible skin structure is placed on the three-dimensional precision micro-motion stage, and the pressure plate is placed directly above the flexible skin structure and connected to the pressure sensor to apply a normal force to the sensing module.

8. The intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure according to claim 1, characterized in that, The temperature control module includes a constant temperature chamber; The sensing module is placed in the constant temperature chamber, which provides a uniform and constant temperature field for the sensing module; the force output module is located inside the constant temperature chamber.

9. The intelligent skin multimodal measurement system based on FBG conical cavity and curved composite microstructure according to claim 1, characterized in that, The flexible skin structure is made using Ecoflex 00-30 silicone.

10. The measurement method of the intelligent skin multimodal measurement system based on the FBG conical cavity and bending composite microstructure according to any one of claims 1 to 9, characterized in that, The method includes the following steps: S1. The optical sensing module transmits optical signals to the composite structure of the tapered FBG and bent single-mode fiber in the sensing module. S2. The tapered FBG and bent single-mode fiber composite structure transmits the received optical signal to the data acquisition and processing module. When the flexible skin structure is subjected to temperature and normal force to produce elastic deformation, strain or effective refractive index change occurs, causing the transmitted optical signal to shift in wavelength. S3. The force output module applies a normal force to the flexible skin structure in the sensing module; S4. The temperature control module provides a uniform temperature field with different gradients for the flexible skin structure in the sensing module. S5. The data acquisition and processing module establishes the relationship curves between wavelength, temperature, and normal force based on the received transmitted light signal and wavelength offset data, and determines the measurement results of temperature and normal force.

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