Optical fiber tactile sensor based on friction enhancement, sensor array and preparation method

Abstract

The invention relates to the technical field of bionic touch, in particular to an optical fiber touch sensor based on friction enhancement, a sensor array and a preparation method. The tactile sensor comprises an optical fiber, the optical fiber comprises at least one fiber core and a cladding, and the cladding is arranged on the outer surface of the fiber core; at least one fiber bragg grating is arranged on the fiber core; the surface of the outer periphery of the cladding is provided with a microstructure, and the microstructure is arranged corresponding to the fiber bragg grating; a polymer layer is arranged on the outer surface of the microstructure; wherein the microstructure and the polymer layer are used for coupling sliding friction force into central wavelength dynamic modulation of the fiber bragg grating, and then spectral analysis is carried out according to a central wavelength time sequence to obtain a sliding spectrum. According to the tactile sensor structure, high-signal-noise output of the slip signal can be achieved, accurate slip detection can be achieved through the output slip spectrum, and meanwhile the friction state and material texture fingerprint output can be quantized.

Description

Technical Field

[0001] This invention relates to the field of biomimetic tactile technology, specifically to a friction-enhanced fiber optic tactile sensor, sensor array, and fabrication method. Background Technology

[0002] With the continuous development of wearable devices, wearable devices now possess flexible functions such as bending, folding, and stretching. These characteristics make flexible wearable devices highly adaptable, giving them great application potential in smart scenarios such as healthcare, sports, and daily life.

[0003] Existing tactile sensors mostly rely on structures such as resistors, capacitors, or piezoelectric films, which are susceptible to electromagnetic interference, difficult to wire over long distances, and have insufficient reliability in humid / hot / strong electromagnetic environments. Fiber optic FBG has advantages such as resistance to electromagnetic interference, long-distance distributed multiplexing, corrosion resistance, and miniaturization, and has been widely used in strain, pressure, or temperature monitoring.

[0004] In the process of realizing this invention, the inventors discovered that the current direct application of fiber Bragg gratings (FBGs) to tactile sensors still has the following problems in slip detection: Slip refers to the relative motion between an object and the sensor contact surface. When slip occurs in reality, the resulting slip friction force is mainly manifested as interfacial shear force and micro-vibration (friction spectrum). However, standard FBGs are relatively sensitive to axial strain, and the coupling of normal shear or tangential loads is weak, making it difficult to effectively transmit slip signals to the FBG. As a result, it is impossible to form distinguishable dynamic features. Therefore, standard FBGs cannot be used in tactile sensors to accurately detect slip. Summary of the Invention

[0005] One of the objectives of this invention is to provide a friction-enhanced fiber optic tactile sensor and pressure sensing method to address the shortcomings of existing FBG fiber optic sensor technology, such as inaccurate slip detection and the inability to form a slip spectrum array.

[0006] To solve the above-mentioned technical problems, the embodiments of the present invention are implemented as follows: In a first aspect, a friction-enhanced fiber optic tactile sensor is provided, comprising: an optical fiber, the optical fiber including at least one core and a cladding, the cladding being disposed on the outer surface of the core; At least one fiber Bragg grating is provided on the fiber core; The outer periphery of the cladding is provided with microstructures, and the microstructures are arranged corresponding to the fiber Bragg grating; The outer surface of the microstructure is provided with a polymer layer; The microstructure and the polymer layer are used to couple the slip friction force into dynamic modulation of the center wavelength of the fiber Bragg grating, and then perform spectral analysis based on the center wavelength time series to obtain the slip spectrum.

[0007] Furthermore, the microstructure is an anisotropic structure, used to obtain different spectral characteristics generated by sliding friction in different directions, so as to determine the sliding direction.

[0008] Furthermore, the microstructure includes one or more of periodic grooves, microridges, micropillars, spiral grooves, or serrated ridges.

[0009] Furthermore, the dynamic modulation of the center wavelength includes dynamic center wavelength drift, phase / intensity modulation, or short-time spectral characteristics. Furthermore, the polymer layer includes: an adhesive layer, which is adhered to the surface of the outer periphery of the cover layer and positioned relative to the microstructure; and a friction outer layer, disposed on the surface of the outer periphery of the adhesive layer.

[0010] Furthermore, the outer friction layer is PU / TPU or modified silicone rubber, and the shear modulus and loss factor of the outer friction layer are set within a preset range.

[0011] The second aspect also discloses a sensor array, including a flexible substrate for bonding to the device surface; At least two of the aforementioned friction-enhanced fiber optic tactile sensors described in the first aspect, wherein the sensors are disposed within the flexible substrate.

[0012] Furthermore, the sensor array also includes a reference fiber for data compensation as a reference to eliminate temperature drift and load drift.

[0013] The third aspect also discloses a method for obtaining a slip spectrum using the friction-enhanced fiber optic tactile sensor described in the first aspect, characterized in that... Obtain the center wavelength sequence of the FBG, which includes a time-varying sequence λB(t), an equivalent phase and / or intensity sequence; The FBG center wavelength sequence is preprocessed, including temperature compensation and detrending processing. The detection and acquisition of the slip event window is used to determine the window sequence, wherein the slip event window is obtained by detecting indicators such as short-time energy, spectral entropy or phase noise rise to obtain the slip start time or end time; The time-frequency analysis is performed on the window sequence to obtain the slip spectrum. The time-frequency analysis is to perform STFT or wavelet packet decomposition on the slip window to obtain the slip spectrum. The peak position, bandwidth, energy distribution, spectral centroid, or 1 / f noise slope change are extracted and output based on the slip spectrum to determine the slip intensity, direction, or material texture fingerprint.

[0014] The fourth aspect also discloses a method for fabricating a sensor, used to fabricate the friction-enhanced fiber optic tactile sensor described in the first aspect, comprising the following steps: Fix the optical fiber to the worktable, determine the region center of the fiber Bragg grating, and determine the start and end positions of the microstructure to be processed; Microstructures are fabricated on the cladding of the optical fiber, and the microstructures correspond to the center of the region of the fiber Bragg grating. The processing area of ​​the optical fiber is ultrasonically / solvent cleaned to remove molten redeposited debris, and the processing area is activated to improve polymer wetting and adhesion. A low-viscosity polymer precursor solution is dropped onto the microstructure, and the polymer precursor solution is completely filled or covered by the microstructure by capillary action or vacuum assistance. The polymer is cured after the microstructure of the optical fiber is covered.

[0015] The beneficial effects of the above-described technical solutions provided in the embodiments of the present invention include at least the following: This invention discloses a friction-enhanced fiber optic tactile sensor. By setting a microstructure at the position corresponding to the fiber optic cladding (FBG) and a polymer layer on the surface of the microstructure, a stable friction coupling interface and stress shaping layer are formed. Through this design, the embodiment can couple the normal load generated by slip to the fiber axis through the microstructure, thereby achieving efficient coupling of the shear friction / slip micro-vibration of the contact interface into FBG axial strain or equivalent grating periodic perturbation, and finally realizing the detection of slip using FBG.

[0016] The tactile sensor structure designed in this invention can achieve high signal-to-noise output of slip signals. The output slip spectrum can not only achieve accurate slip detection, but also quantify the friction state and material texture fingerprint output.

[0017] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1A partial schematic diagram of a friction-enhanced fiber optic tactile sensor provided for the first embodiment of the present invention; Figure 2 This is an embodiment of the present invention. Figure 1 A magnified view of part A; Figure 3 A partial schematic diagram of a friction-enhanced fiber optic tactile sensor according to a second embodiment of the present invention; Figure 4 This is an embodiment of the present invention. Figure 3 A magnified view of part B; Figure 5 A partial schematic diagram of a friction-enhanced fiber optic tactile sensor according to a third embodiment of the present invention; Figure 6 This is a calibration fitting curve of the normal force Fz and wavelength drift Δλ according to an embodiment of the present invention. Figure 7 This is a calibration fitting curve of tangential force Fx and wavelength drift Δλ according to an embodiment of the present invention. Figure 8 This is a graph showing the timing of the tangential force Fx(t) during the sliding process in an embodiment of the present invention. Figure 9 This is a time-series graph of the dynamic response Δλ(t) of the FBG during the sliding process in an embodiment of the present invention; Figure 10 This is a time-frequency analysis (STFT) amplitude spectrum of the slip spectrum according to an embodiment of the present invention. Figure 11 This is a frame of a thermal image of an array slip spectrum imaging system in the first state according to an embodiment of the present invention. Figure 12 This is a frame of a second-state array slip spectrum imaging thermogram from an embodiment of the present invention. Figure 13 This is a frame of the array slip spectrum imaging thermal image of the third state according to an embodiment of the present invention; Figure 14 This is a graph showing the change of the average glide spectrum energy of the array over time in an embodiment of the present invention.

[0020] The image shows: 10. Fiber core; 11. Fiber Bragg grating; 20. Cladding; 30. Microstructure; 40. Polymer layer; 400. Adhesive layer; 401. Friction outer layer. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] In this application, the term "exemplary" is used to mean "used as an example, illustration, or description." Any implementation described as "exemplary" in this application is not necessarily to be construed as being more preferred or advantageous than other implementations. The following description is provided to enable any person skilled in the art to implement and use this application. Details are set forth in the following description for illustrative purposes. It should be understood that those skilled in the art will recognize that this application can be implemented without using these specific details. In other instances, well-known structures and processes are not described in detail to avoid obscuring the description of this application with unnecessary detail. Therefore, this application is not intended to be limited to the embodiments shown, but rather to be consistent with the broadest scope of the principles and features disclosed in this application.

[0023] The fiber Bragg grating strain refers to the change in reflected wavelength λB when the physical properties of the grating region change, such as due to strain or temperature. This wavelength change is closely related to key measurement parameters such as strain and temperature and is an important indicator in structural health monitoring.

[0024] Before describing the embodiments of the present invention in detail, the design concept of the present invention will be summarized below.

[0025] This invention provides a friction-enhanced fiber optic tactile sensor, the specific implementation of which is as follows. Figure 1 As shown, the core process of this invention includes: setting microstructures at positions corresponding to the FBG in the fiber cladding 20, and then setting a polymer layer on the surface of the microstructures to form a stable friction coupling interface and stress shaping layer. Through this design embodiment, the normal load generated by slippage can be coupled to the fiber axis via the microstructures, thereby efficiently coupling the shear friction / slippage micro-vibrations at the contact interface into FBG axial strain or equivalent grating periodic perturbation, ultimately achieving slippage detection using the FBG. The tactile sensor structure designed in this invention can achieve high signal-to-noise output of the slippage signal. The output slippage spectrum not only enables accurate slippage detection but also quantifies the friction state and material texture fingerprint. Example 1:

[0026] Firstly, please refer to the appendix. Figure 1 and 2As shown, a friction-enhanced fiber optic tactile sensor is disclosed, comprising: an optical fiber, the optical fiber including at least one core 10 and a cladding 20, the cladding 20 being disposed on the outer surface of the core 10; At least one fiber Bragg grating 11 is provided on the fiber core 10; The cladding 20 has a microstructure 30 on its outer periphery, and the microstructure 30 is provided corresponding to the fiber Bragg grating 11. The outer surface of the microstructure 30 is provided with a polymer layer 40; The microstructure 30 and the polymer layer 40 are used to couple the slip friction force into dynamic modulation of the center wavelength of the fiber Bragg grating 11, and then perform spectral analysis based on the center wavelength time series to obtain the slip spectrum.

[0027] Understandably, directly using fiber Bragg gratings (FBGs) 11 in tactile sensors presents the following problems in slip detection: Slip refers to the relative motion between an object and the sensor's contact surface. In reality, when slip occurs, the resulting slip friction force mainly manifests as interfacial shear force and micro-vibration (friction spectrum). Standard FBGs are relatively sensitive to axial strain, but the coupling of normal shear or tangential loads is weak, making it difficult to effectively transmit slip signals to the FBG. As a result, distinguishable dynamic characteristics cannot be formed, and standard FBGs cannot be used in tactile sensors to accurately detect slip.

[0028] The optical fiber, short for optical fiber conduit fiber, is a fiber made of glass or plastic that serves as a light transmission tool. The optical fiber includes a core 10 and a cladding 20, with the cladding 20 wrapping around the outer periphery of the core 10. In embodiments, the number of cores 10 is varied depending on the specific situation, and they are located inside the cladding 20. When the optical fiber is applied to the field of tactile sensors, FBG11 is etched onto the fiber. Tactile detection is achieved by detecting changes in the center wavelength of the FBG under axial stress. Furthermore, FBG inherently supports WDM / TDM multiplexing, enabling the formation of long-distance, large-scale arrays, and is resistant to electromagnetic interference, making it suitable for strong electromagnetic / humid / high-pressure environments.

[0029] The microstructure 30 is a microstructure disposed on the fiber cladding 20, a micrometer- or millimeter-scale structure with a specific geometric morphology. Specifically, it is a non-flat micrometer-scale structure disposed on the outer or inner sidewall of the cladding 20. When a small slip occurs on the surface, these microstructures 30 generate a stick-slip frictional vibration phenomenon, which manifests as a periodic alternating motion of "stickiness" and "sliding" between contact surfaces, commonly seen in scenarios with low relative motion speeds or high system elasticity. When the microstructure 30 slips on the surface, it can produce at least the following effects: 1. Friction enhancement effect, specifically by increasing the effective friction coefficient of the interface and the number of micro-contacts, making the slip generate stable and repeatable stick-slip micro-events; 2. Stress shaping and conversion effect, the microstructure 30 converts tangential shear loads into axial tension or compression or periodic disturbances that can be sensed by the FBG, and increases the dynamic coupling bandwidth. The microstructure 30 enables interpretable and repeatable measurement of the slip spectrum, that is, it enables high signal-to-noise output of the slip signal.

[0030] The polymer layer 40 is a material layer with corresponding requirements for equivalent shear modulus and loss factor. The equivalent shear modulus is selected according to the target frequency band and load. In this embodiment, the equivalent shear modulus G is controlled between 0.2 MPa and 20 MPa; the loss factor tanδ is controlled between 0.01 and 0.2, and the lower the tanδ, the better it is for retaining the high-frequency slip spectrum. The materials used to make the polymer layer 40 include polyurethane (PU / TPU), modified silicone rubber, or elastomer composite materials. SiO2, carbon black, or microparticles can be added to the material to improve friction and wear resistance. The polymer layer 40 is disposed on the surface of the microstructure 30 and plays the following roles: 1. Improve frictional coupling; 2. Provide controllable viscoelasticity to adjust the spectral energy distribution; 3. Protect the microstructure 30 and improve wear resistance. In this embodiment, by filling / covering the surface of the microstructure 30 with a polymer material with adjustable elastic modulus and loss characteristics, a stable frictional coupling interface and stress shaping layer are formed on the surface of the microstructure 30, achieving an adjustable spectral energy distribution and enabling interpretable slip spectrum measurement.

[0031] In this process, the center wavelength of the fiber Bragg grating 11 is dynamically modulated, and the slip spectrum is obtained by spectral analysis based on the center wavelength time series. The dynamic modulation of the center wavelength refers to the process of actively adjusting the reflected wavelength of the FBG, i.e., the Bragg wavelength, through real-time changes in external physical quantities such as strain or temperature. The dynamic modulation of the center wavelength includes dynamic center wavelength drift, phase / intensity modulation, or short-time spectral characteristics. Specifically, the micro-vibration and friction spectrum during the slip process is output as a dynamically modulated FBG center wavelength, where the output characteristics include dynamic center wavelength drift, phase / intensity modulation, or short-time spectral characteristics, and the slip spectrum is obtained by spectral analysis based on the obtained center wavelength time series.

[0032] According to the above embodiments, by employing an optical fiber with at least one fiber core 10 and setting at least one FBG on the fiber core 10, and by setting a microstructure 30 at the position corresponding to the FBG in the cladding 20 of the optical fiber, and then setting a polymer layer 40 on the surface of the microstructure 30, a stable friction coupling interface and stress shaping layer are formed. With this structure, the normal load generated by slip can be coupled to the optical fiber axis through the microstructure 30, thereby efficiently coupling the shear friction / slip micro-vibration of the contact interface into FBG axial strain or equivalent grating periodic perturbation, ultimately enabling the detection of slip using the FBG. Furthermore, the slip spectrum can not only detect "whether slip has occurred," but also quantify the friction state and material texture fingerprint, supporting robot dexterous grasping closed-loop and quantifiable palpation.

[0033] For a specific embodiment, please refer to the appendix. Figure 3 and 5 As shown, the optical fiber includes a core 10 and a cladding 20. The core 10 is provided with eight staggered fiber girders (FBGs). Microridge regions are respectively formed on the outer peripheral surface of the cladding 20 corresponding to the eight FBGs. The microridges in each microridge region are arranged in the same direction, but the microridges in the eight microridge regions are not completely arranged in the same direction. A modified silicone rubber layer is covered on the eight microridge regions. In another specific embodiment, the optical fiber includes three cores 10 and a cladding 20. The three cores 10 are respectively provided with six staggered FBGs. Microridge regions are respectively formed on the outer peripheral surface of the cladding 20. A spiral region is set for each of the six FBGs, and the spiral directions of these six spiral regions are not exactly the same. A polyurethane layer is covered on the six spiral regions. In another specific embodiment, the optical fiber includes a fiber core 10 and a cladding 20. The fiber core 10 is provided with four staggered FBGs. Micro-groove regions are set on the outer peripheral surface of the cladding 20 corresponding to the four FBGs. Each groove in each groove region is arranged in the same direction, but the micro-ridges in these four groove regions are not exactly the same. A modified silicone rubber layer is covered on the four groove regions.

[0034] In a further embodiment, the microstructure 30 is an anisotropic structure, used to obtain different spectral characteristics generated by sliding friction in different directions, so as to determine the sliding direction.

[0035] Understandably, in order to solve the problem that current fiber optic touch sensors cannot accurately detect the sliding direction, the embodiment is to set the microstructure 30 as an anisotropic structure.

[0036] The anisotropic structure refers to a material exhibiting different physical properties in different directions. In an embodiment, the microstructure 30 is designed as a directional microstructure, so that when it slides along different directions, the microstructure 30 will receive completely different feedback, thereby realizing the detection of direction.

[0037] For a specific embodiment, please refer to the appendix. Figure 3 and 4 As shown, the microstructure 30 is an anisotropic microridge structure with ridges arranged in one direction. When sliding along the direction of the ridge of the microridge structure, the coefficient of friction is very low and the sliding is smooth. When sliding against the direction of the ridge, the coefficient of friction increases sharply, producing a stopping effect. Thus, sliding in different directions causes the microstructure 30 to produce different spectral characteristics in different directions. The embodiment utilizes this directional difference to achieve accurate detection of the direction of micro-motion.

[0038] Please refer to the attached document. Figure 1 and 2 As shown in the embodiment, the microstructure 30 further includes one or more of periodic grooves, microridges, micropillars, spiral grooves, or serrated ridges.

[0039] Understandably, in order to solve the problem that standard FBG cannot detect slip, in this embodiment, a different form of microstructure 30 is used to couple slip to the FBG axis for detection, thereby realizing slip detection.

[0040] Please refer to the appendix. Figure 2 As shown, the periodic grooves are a micro-groove array arranged according to a certain pattern, specifically a micro-groove array in which the grooves are arranged at a certain interval and in the same direction.

[0041] Please refer to the attached document. Figure 3 As shown, the microridge is an array of ridges arranged along one direction, which can cause the slip to produce different spectral characteristics in different directions, thereby realizing the slip direction determination.

[0042] The spiral groove is a spiral groove structure. In this embodiment, the tangential force can be decomposed into an axial component by the helix angle of the spiral groove, thereby enhancing the axial strain response of the FBG.

[0043] The serrated ridges are arranged in an array along one direction, and a serrated structure is provided on the upper part of the ridges. The serrated structure of each ridge can also be staggered. In this embodiment, the serrated structure makes the spectral peaks and energy distributions of forward / reverse sliding different, thereby improving directional resolution.

[0044] In this embodiment, different microstructures 30 can be selected according to different usage scenarios. The microstructure 30 can consist of one, two, or three microstructures, thereby improving the detection effect of slippage. When slippage occurs on the surface, the microstructure 30 can produce at least the following effects: 1. Friction enhancement effect: specifically, by increasing the effective friction coefficient of the interface and the number of micro-contacts, the slippage generates stable and repeatable stick-slip effect micro-events; 2. Stress shaping and conversion effect: the microstructure 30 converts tangential shear loads into axial tension / compression or periodic disturbances that can be sensed by the FBG, and improves the dynamic coupling bandwidth. The microstructure 30 enables interpretable and repeatable measurement of the slip spectrum, that is, it enables high signal-to-noise output of the slip signal.

[0045] In a further embodiment, the center wavelength dynamic modulation includes center wavelength dynamic drift, phase / intensity modulation, or short-time spectral characteristics.

[0046] In this embodiment, the dynamic modulation of the center wavelength refers to the process of actively adjusting the reflected wavelength of the FBG, i.e., the Bragg wavelength, through real-time changes in external physical quantities, such as strain or temperature. The dynamic modulation of the center wavelength includes dynamic center wavelength drift, phase / intensity modulation, or short-time spectral characteristics. Specifically, the micro-vibration and friction spectrum during the slip process is output as a dynamically modulated FBG center wavelength, wherein the output characteristics include dynamic center wavelength drift, phase / intensity modulation, or short-time spectral characteristics, and the slip spectrum is obtained by spectral analysis based on the obtained center wavelength time series.

[0047] Please refer to the attached document. Figure 4 As shown in the embodiment, the polymer layer 40 further includes: an adhesive layer 400, which is adhered to the outer periphery of the cover layer 20 and positioned relative to the microstructure 30; and a friction outer layer 401, which is disposed on the outer periphery of the adhesive layer 400.

[0048] Understandably, the bonding layer 400 is used to bond with the microstructure 30 of the fiber cladding 20 to ensure that external forces can be accurately transmitted to the FBG, thus solving the problems of interface bonding and stress transmission.

[0049] The outer friction layer 401 directly contacts the object being measured, such as a finger, fabric, or metal surface. It can directly interact with the complex external world, generating friction signals that determine what textures and forces the sensor can detect, thus addressing the issues of signal generation and durability.

[0050] In one embodiment, the polymer layer 40 can also be made into a replaceable patch. The bonding layer 400 of the polymer layer 40 has a pre-formed microstructure 30, which is aligned with the fiber optic microstructure 30 region via a positioning groove, ensuring the accuracy of polymer layer 40 replacement. When the polymer layer 40 wears out, it can be quickly replaced, maintaining long-term consistency of the slip spectrum and facilitating engineering applications and batch maintenance.

[0051] In a further embodiment, the outer friction layer 401 is PU / TPU or modified silicone rubber, and the shear modulus and loss factor of the outer friction layer 401 are set within a preset range.

[0052] In this embodiment, by filling / covering the surface of the microstructure 30 with a polymeric material having adjustable elastic modulus and loss characteristics, a stable frictional coupling interface and stress shaping layer are formed on the surface of the microstructure 30, thereby achieving an adjustable spectral energy distribution and enabling interpretable slip spectrum measurement.

[0053] The shear modulus, as mentioned here, is a measure of a material's resistance to shear deformation when subjected to shear force or a force parallel to its surface. It is directly related to the material's hardness or stiffness. For viscoelastic materials, the higher the shear modulus, the harder the material; the lower the shear modulus, the softer the material. Here, the equivalent shear modulus is controlled between 0.2 MPa and 20 MPa, and selected according to the target frequency band and load.

[0054] The loss factor refers to a measure of the internal friction of a viscoelastic material, reflecting the material's ability to convert mechanical energy into heat energy during deformation. The loss factor tanδ is controlled between 0.01 and 0.2, where a lower tanδ is more conducive to preserving the high-frequency slip spectrum.

[0055] In this embodiment, by setting the preset range of these two parameters, the purpose is essentially to customize the tactile conduction characteristics of the friction outer layer 401, making it like the skin of a human hand: by setting the shear modulus appropriately, the friction outer layer 401 has sufficient softness to conform to the object, and by setting the loss factor appropriately, the friction outer layer 401 can sensitively sense the subtle vibrations of the texture, thereby obtaining a high-quality slip spectrum.

[0056] Let's take a small ridge as an example to illustrate: In this embodiment, 4-32 FBGs with different center wavelengths are written onto the core 10 of a single-mode optical fiber, and the spacing between adjacent FBGs is controlled at 5-30 mm. Microstructures 30 are fabricated on the cladding 20 of the optical fiber corresponding to each FBG location. Multiple stripe microridge arrays can be formed on the cladding 20 using processes such as laser micromachining, etching, or molding. The microridge parameters are set as follows: period p controlled at 20-80 µm, height h controlled at 10-60 µm, and width w controlled at 0.3 p-0.6 p. Then, a layered polymer is coated onto the microstructure 30 region. The bonding layer 400 is a low-shrinkage adhesive layer with a thickness controlled at <10 µm, and the friction outer layer 401 is a PU friction skin with a thickness controlled at 20-150 µm. The optical fiber is embedded into a flexible substrate and fitted to the curved surface of a fingertip. High-speed demodulation at ≥5 kS / s / channel is used to acquire λB(t), and the slip spectrum is obtained through short-time spectral analysis, outputting the slip initiation and intensity.

[0057] Example 2: Based on the same inventive concept, a second aspect discloses a sensor array including a flexible base layer for attaching to the surface of a device. At least two of the aforementioned friction-enhanced fiber optic tactile sensors described in the first aspect, wherein the sensors are disposed within the flexible substrate.

[0058] In this embodiment, a single multi-FBG optical fiber is embedded in a flexible substrate, or multiple optical fibers are arranged in parallel to form a two-dimensional array. By using wavelength division multiplexing (WDM) or time division multiplexing (TDM) demodulation, a pixel coordinate and center wavelength mapping table can be established, and temperature compensation and calibration can be completed to output tactile images, such as Fz, Fx / Fy, and glide spectrum.

[0059] Since each FBG in the friction-enhanced fiber optic tactile sensor is paired with the microstructure 30, during detection, the slip is coupled to the axial strain of the FBG through the microstructure 30, i.e., an independent sensing point, which can be regarded as a tactile pixel. By arranging multiple sensors horizontally and vertically or in a uniform orientation within the flexible substrate, a two-dimensional tactile image composed of multiple tactile pixels is achieved. In this embodiment, the slip vector field, such as direction and local shear distribution, can be estimated jointly by multiple pixels, and tactile images, such as Fz, Fx / Fy, and slip spectrum, can be output.

[0060] In a further embodiment, the sensor array also includes a reference fiber for data compensation as a reference to eliminate temperature drift and load drift.

[0061] Understandably, in order to reduce cross-contamination of temperature, the embodiment can set a stress-free reference optical fiber in the flexible substrate and perform temperature compensation through differential processing, thereby eliminating the effects of temperature drift and load drift and obtaining more accurate data.

[0062] In some embodiments, multiple friction-enhanced fiber optic tactile sensors are disposed within the flexible substrate, and the sensors are arranged in an alternating manner. For example, the sensors are divided into two layers: a first layer is arranged along a first direction, and a second layer is arranged along a second direction, with the first and second directions intersecting. Each sensor's fiber core 10 has a plurality of alternating FBGs, and the cladding 20 has a plurality of microstructures 30 corresponding to the FBGs. This design enables two-dimensional tactile imaging.

[0063] A concrete example: M=4-32 optical fibers are laid in parallel on a flexible substrate, with each fiber containing N=4-32 FBGs, thus forming an M×N array of sensors for tactile imaging. Each pixel region employs microstructures 30 arranged in mutually orthogonal directions, or in an interleaved structure in adjacent pixel directions, to achieve decoupling and estimate the tangential direction during slip detection. The array output of this embodiment can reconstruct the normal distribution and slip vector field, enabling slip closed-loop control and material recognition in robot grasping. Figure 14 This is a graph showing the change of the average glide spectrum energy of the array over time in an embodiment of the present invention, which can reflect the energy change of the glide in real time.

[0064] Example 3: Based on the same inventive concept, a third aspect discloses a method for acquiring a slip spectrum using the friction-enhanced fiber optic tactile sensor described in the first aspect. Step 001: Obtain the center wavelength sequence of the FBG, which includes a time-varying sequence λB(t), an equivalent phase and / or intensity sequence.

[0065] The acquisition of the FBG center wavelength sequence includes using a high-speed FBG demodulator, scanning method-tunable filtering, sideband modulation demodulation, or a method based on narrow linewidth light source and interference / phase demodulation to read the λB(t), equivalent phase, and / or intensity sequence of the FBG center wavelength of the sensor over time.

[0066] The The sequence is the most fundamental physical quantity, reflecting the overall strain change of the microstructure 30. For slip signals, it is usually manifested as a high-frequency AC signal superimposed on the DC component.

[0067] The equivalent phase sequence is obtained through phase demodulation techniques (such as interferometric demodulation or PGC demodulation). Phase information is more sensitive to minute vibrations (nanometer-level displacements) than wavelength and can capture micro-tremors in the initial stage of slip.

[0068] The intensity sequence directly corresponds to the vibration signal in the rapid fluctuation of light intensity. Although the accuracy is low, the sampling rate can be made very high, which is suitable for capturing transient slip.

[0069] Please refer to the attached document. Figure 8 The figure shows the time series curve of the tangential force Fx(t) during the sliding process. By observing the time series change of the tangential force, the dynamic process of static friction accumulation and sliding to micro-vibration during the sliding process is intuitively displayed. Please refer to the attached document. Figure 9 As shown, the curve of the dynamic response Δλ(t) of the FBG during the slip process is a graph. It can be seen that the dynamic change of the FBG wavelength drift quantifies the mechanical behavior of "stick-slip-micro-vibration" during the slip process: periodic fluctuations correspond to the macroscopic cycle of stick-slip, and high-frequency micro-vibration corresponds to the dynamic response of the micro-texture.

[0070] Step 002: Preprocess the center wavelength sequence of the FBG, including temperature compensation and detrending processing.

[0071] In this embodiment, the preprocessing is for the purpose of purifying the dynamic signal, that is, before extracting the slip spectrum, environmental interference must be removed to ensure the accuracy of the data.

[0072] The temperature compensation involves using a reference grating or differential processing method to eliminate wavelength drift caused by changes in ambient temperature, ensuring that the remaining signal is purely caused by mechanical contact. For example, in this embodiment, by adding a stress-free reference fiber, when slippage occurs, since the reference fiber is stress-free, only the temperature-induced change in the FBG center wavelength occurs under the same environment. Therefore, this serves as reference data for temperature compensation of the slipped FBG center wavelength change.

[0073] The detrending process removes the influence of slowly varying normal loads / attitudes to obtain a pure signal with zero mean, containing only high-frequency vibration components. Specifically, the detrending includes: removing the influence of normal force: gripping force (normal load) is usually a slowly varying low-frequency signal that can drown out weak slip vibrations; high-pass filtering or adaptive moving average is used to remove low-frequency trends; removing attitude interference: the movement of the robotic arm can cause baseline drift, which needs to be eliminated through dynamic baseline estimation.

[0074] Step 003: Detect and acquire the slip event window to determine the window sequence, wherein the slip event window is acquired by detecting indicators such as short-time energy, spectral entropy, or phase noise rise to obtain the slip start time or end time.

[0075] In this embodiment, to avoid wasting computing resources, the system needs to know when a slip occurs in order to initiate subsequent spectrum analysis. This embodiment uses the window at which the slip event occurs as a trigger mechanism. Specifically, when a slip occurs, the detection device can obtain changes in indicators such as short-time energy, spectral entropy, or phase noise rise, which are used as the start and end times of the slip event, thus obtaining the window sequence of slip events.

[0076] The short-term energy refers to the frictional vibration energy that rises sharply when slippage occurs, and is determined to be the start of slippage if it exceeds a set threshold.

[0077] The spectral entropy measures the complexity and randomness of a signal. Specifically, when the signal is stationary or in a stable grip, it is usually smooth with a low spectral entropy; when the slip begins, the signal becomes chaotic and rich, and the spectral entropy suddenly increases.

[0078] The phase noise rise occurs in the very early stages of slip, before vibration even appears. The phase noise begins to increase due to the interaction of surface micro-roughness. This indicator can be used to achieve early slip warning.

[0079] The window sequence refers to the sequence of the center wavelength of the FBG when the slip occurs, based on the start and end times of the slip event. In the embodiment, the sensor converts the normal load of the slip to the FBG axis, thereby detecting the λB(t) of the center wavelength of the FBG over time or the equivalent phase / intensity sequence.

[0080] Step 004: Perform time-frequency analysis based on the window sequence to obtain the slip spectrum. The time-frequency analysis involves performing STFT or wavelet packet decomposition on the slip window to obtain the slip spectrum.

[0081] In this example, after capturing the sliding window sequence, the acquired data needs to be analyzed. Here, time-frequency analysis is used to transform the one-dimensional time signal into a two-dimensional time-frequency graph. .

[0082] Window capture: Capture valid slip signal segments based on the detected start and end times.

[0083] STFT (Short-Time Fourier Transform): Applicable to smooth sliding processes. By applying a windowed Fourier transform, the changes in frequency components over time during the sliding process can be observed.

[0084] Wavelet packet decomposition is suitable for non-stationary, transient slip signals, such as stick-slip impacts. Wavelet packets can provide high frequency resolution in both low and high frequency bands. They can accurately capture the high-frequency impact at the initial stage of slip and the low-frequency components during the viscous phase.

[0085] In one example, after time-frequency processing, a sliding spectrum is output with time as the horizontal axis, frequency as the vertical axis, and energy (or amplitude) as the color value. This spectrum is similar to the fingerprint of an object.

[0086] refer to Figure 10 The STFT amplitude spectrum of the slip spectrum in this embodiment of the invention intuitively shows the dynamic characteristics of the time and frequency domain during the slip process. It converts the time domain signal (FBG wavelength drift) of the slip process into the time and frequency domain, and quantifies the periodicity, frequency components and energy distribution of the slip: the periodic frequency strips correspond to the stick-slip cycle, the main frequency corresponds to the microstructure 30 response, and the amplitude distribution reflects the signal energy.

[0087] Figure 11 , Figure 12 and Figure 13 The sliding process can be dynamically represented by array sliding spectrum imaging thermal image frames.

[0088] Step 005: Extract the output spectral peak position, bandwidth, energy distribution, spectral centroid, or 1 / f noise slope change based on the slip spectrum to determine the slip intensity, direction, or material texture fingerprint.

[0089] In this embodiment, the required physical features are extracted from the obtained slip spectrum. The extracted physical features include the position of the spectral peak, bandwidth, energy distribution, spectral centroid, or 1 / f noise slope change. The slip intensity, direction, or material texture fingerprint is determined by these extracted material features.

[0090] Wherein, the spectral peak position, the spectral peak position, and the spacing of the surface texture of the object during slippage. With slip velocity Together they determine the resonant frequency The position of the spectral peak corresponds to the dominant frequency during the slip process, directly reflecting the interaction between velocity and texture.

[0091] For example, in determining direction, if the sliding speed increases, such as when the finger slides faster... If the rate increases, the spectral peak position shifts to higher frequencies; if the rate decreases, it's like decelerating. A decrease in spectral peak position indicates a shift towards lower frequencies. The dynamic trend of the spectral peak position, such as rising or falling, can indicate the direction of slippage, i.e., acceleration or deceleration. For example, when a robot grasps an object, if the spectral peak position rises rapidly, it indicates accelerated slippage, meaning the object is sliding away from the robot, and the gripping force needs to be increased immediately.

[0092] The natural frequency of the microstructure 30 or the resonant frequency of the contact system is used to identify the contact stiffness and determine whether the object is soft or hard.

[0093] The bandwidth is the frequency range of the vibration energy distribution. For example, a wide bandwidth typically implies a rough surface or severe slippage; a narrow bandwidth may imply a smooth surface or slight vibration.

[0094] The spectral centroid is the average position of energy in the frequency domain. It is used for texture recognition. Rough textures typically have high friction and strong impact, so their centroid is biased towards high frequencies; smooth textures have their centroid biased towards low frequencies. It is also used to determine slip velocity; the faster the velocity, the more high-frequency components are typically excited.

[0095] The change in the 1 / f noise slope reflects the attenuation law of low-frequency noise and is positively correlated with the friction coefficient of the material. High-friction materials, such as rubber, have high friction and more random vibrations, resulting in a steeper slope; low-friction materials, such as PTFE, have low friction and a gentler slope.

[0096] In this embodiment, slip intensity, direction, or material texture fingerprint is determined: slip intensity is determined by combining spectral energy and spectral centroid; slip direction is determined by the fact that, if based on a multi-directional microstructure 30 or a triaxial FBG, slip in different directions will excite different modes of vibration, and the direction can be inferred from the spectral features; material texture fingerprint is determined by establishing a spectral feature database and comparing the real-time slip spectrum with the database to achieve blind-like tactile object recognition. These methods can be referenced from existing ones and will not be elaborated further here.

[0097] For example: When a robot grasps an object, it needs to determine the sliding state (whether sliding has occurred), the sliding direction, and the sliding intensity (the magnitude of the force) to adjust its grip strength strategy. Slip detection: Determines whether an object is sliding by observing changes in the position of spectral peaks or the centroid of the spectrum; Direction determination: Determine the sliding direction by the trend of the spectral peak position / spectral centroid, such as the object sliding away from the robot; Intensity quantification: The magnitude of the slip force is quantified by the total energy or peak amplitude. For example, "strong slip" requires an increase of 20% in grip strength. Material compatibility: Identify materials by bandwidth, energy distribution, and 1 / f noise slope, such as rubber requiring flexible grip and metal requiring rigid grip.

[0098] For example, when a robot grasps a rubber ball, it detects a drop in the spectral peak position, indicating deceleration and slippage; a large bandwidth, indicating rough texture; and a steep 1 / f noise slope, indicating high friction. This indicates "strong slippage and high friction material," requiring an immediate increase in gripping force and adjustment of the grasping posture.

[0099] Example 4: Based on the same inventive concept, a fourth aspect discloses a method for fabricating a sensor, used to fabricate the friction-enhanced fiber optic tactile sensor described in any one of the first aspects above, comprising the following steps: Step 100: Fix the optical fiber to the worktable, determine the regional center of the fiber Bragg grating 11, and determine the start and end positions of the microstructure 30 to be processed.

[0100] Understandably, before fabricating the microstructure 30 on the cladding 20 of the optical fiber, it is necessary to locate the FBG region so that the fabricated microstructure 30 can correspond to the FBG region.

[0101] Before processing, the cladding 20 of the optical fiber needs to be cleaned. Specifically, standard single-mode fiber or micro / nano fiber is selected, and FBGs are written onto the fiber using ultraviolet phase masking, femtosecond dot writing, or tapered post-gradient grating writing. If there is an outer coating layer in the corresponding FBG area of ​​the fiber, the outer coating layer needs to be removed. Then, the exposed cladding 20 is cleaned with isopropanol / deionized water, and finally dried to complete the cleaning.

[0102] In one specific embodiment, the cleaned optical fiber is fixed on a high-precision three-axis or five-axis displacement stage, so that the FBG region is within the working distance of the femtosecond laser processing focus. Then, the center of the FBG region and the start and end positions of the processing segment of the microstructure 30 to be processed are located by microscopic imaging to ensure the accuracy of processing.

[0103] Step 200: A microstructure 30 is fabricated on the cladding 20 of the optical fiber, the microstructure 30 corresponding to the region center of the fiber Bragg grating 11.

[0104] In this embodiment, after locating the FBG region of the optical fiber and determining the starting position for processing, processing can be performed on the determined region so that the required microstructure 30 is processed on the cladding 20 of the optical fiber. The processed microstructure 30 corresponds to the FBG region, so that the FBG of the fiber core 10 inside the optical fiber and the microstructure 30 processed in the cladding 20 form multiple individual tactile sensing points.

[0105] In one specific embodiment, after locating the FBG region of the optical fiber, annular grooves are directly fabricated using a femtosecond laser. Specifically, during femtosecond laser processing, the center wavelength is controlled at 1030 nm / 515 nm, and the pulse width is controlled between 200–800 fs. Annular scanning processing is performed on the outer surface of the cladding 20 to form a groove array. The grooves can be complete closed loops or segmented closed loops. Preferably, the grooves are distributed circumferentially around the fiber to generate stable frictional micro-events in any slip direction.

[0106] The resulting grooves are designed to ensure parameter control and batch consistency. Specifically, the groove depth d, width w, and pitch p are controlled by adjusting the single pulse energy, repetition frequency, scanning speed, and number of ring scans. Recommended range: d = 1–20 µm, w = 2–30 µm, p = 5–200 µm. Single-scale or multi-scale p can be used within the same FBG pixel area, such as p1+p2 superimposed, to form a "slip spectral fingerprint".

[0107] Step 300 involves ultrasonic / solvent cleaning of the processed area of ​​the optical fiber to remove molten redeposited debris, and activation of the processed area to improve polymer wetting and adhesion.

[0108] Understandable. The processing area is ultrasonically / solvent cleaned to remove molten redeposited debris; plasma treatment or silanization may be performed to improve polymer wetting and adhesion.

[0109] Step 400: A low-viscosity polymer precursor solution is dropped onto the microstructure 30, and the polymer precursor solution is completely filled or covered by the microstructure 30 by capillary action or vacuum assistance.

[0110] Understandably, this is to improve the sensor's frictional coupling; provide controllable viscoelasticity to adjust the spectral energy distribution; protect the microstructure 30; and enhance its wear resistance. In one embodiment, by filling / covering the surface of the microstructure 30 with a polymeric material having adjustable elastic modulus and loss characteristics, a stable frictional coupling interface and stress-shaping layer are formed on the surface of the microstructure 30, achieving an adjustable spectral energy distribution and enabling interpretable slip spectrum measurements.

[0111] In one embodiment, a polymer precursor solution is dropped onto the processed microstructure 30, thereby filling or covering the microstructure 30 with a layer of polymer solution. It is also possible to cover the microstructure 30 with a thin film made of polymer, thereby forming a polymer layer 40.

[0112] In one specific embodiment, a polymer layer 40 is obtained by polymer filling and molding. The specific steps involve preparing a low-viscosity polymer precursor from materials such as PU / TPU / modified silicone rubber, and then dropping the low-viscosity polymer precursor into the groove-shaped microstructure 30 region. During this process, capillary action or vacuum assistance can be used to completely fill the annular groove.

[0113] In one embodiment, the polymer layer 40 may be configured as a layered structure, including an adhesive layer 400, which is attached to the outer periphery of the cover layer 20 and positioned relative to the microstructure 30, with a thickness controlled to be <10 µm, and a friction outer layer 401 disposed on the adhesive layer 400, with a thickness of 10-500 µm.

[0114] Step 500: After covering the microstructure 30 of the optical fiber with polymer, a curing process is performed.

[0115] After the polymer material is filled onto the microstructure 30, curing is required. In this embodiment, depending on the material system, thermosetting or UV curing can be selected. After curing, secondary surface microtexturing can be performed, and a protective coating of Parylene / thin PU film can be applied to improve the wear resistance and long-term consistency of the polymer layer 40.

[0116] In practical applications, array encapsulation and multiplexing are also required. Specifically, a single multi-FBG optical fiber is embedded in a flexible substrate, or multiple optical fibers are arranged in parallel to form a two-dimensional array. Wavelength division multiplexing (WDM) / time division multiplexing (TDM) demodulation is used to establish a mapping table between pixel coordinates and center wavelength, and temperature compensation and calibration are completed.

[0117] Based on the same inventive concept, the fifth aspect also discloses an application method of a friction-enhanced fiber optic tactile sensor, which is applied in wearable health monitoring, robot tactile sensing and human-computer interaction and other application scenarios.

[0118] Specific applications include: the present invention uses optical fiber and polymer packaging, and the process is compatible with batch writing of gates and templated microstructures to form biomimetic skin; the array can achieve fewer cables and more pixels through WDM / TDM, which is suitable for large-scale deployment.

[0119] In the field of robotic tactile sensing, it enables robots to achieve dexterous grasping by providing closed-loop control and anti-falling in wet / dusty / oil film environments. In the field of wearables / prosthetics, it is used for fingertip tactile skin to achieve sliding feedback and material sensing; In the field of medical palpation, slip spectrum and shear distribution are used to assist in the assessment of tissue stiffness / fibrosis. It is used in the field of industrial inspection to monitor surface roughness, coating condition, and friction degradation online; Applications in strong electromagnetic environments, such as power equipment and magnetic resonance environments, require tactile sensing that is resistant to electromagnetic interference.

[0120] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. This disclosure is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this disclosure is limited only by the appended claims. Thus, if these modifications and variations of the invention fall within the scope of the claims of the invention and their equivalents, the invention is also intended to include these modifications and variations.

Claims

1. A fiber optic tactile sensor based on tribological enhancement, characterized in that, include: An optical fiber, the optical fiber comprising at least one core and a cladding, the cladding being disposed on the outer surface of the core; At least one fiber Bragg grating is provided on the fiber core; The outer periphery of the cladding is provided with microstructures, and the microstructures are arranged corresponding to the fiber Bragg grating; The outer surface of the microstructure is provided with a polymer layer; The microstructure and the polymer layer are used to couple the slip friction force into dynamic modulation of the center wavelength of the fiber Bragg grating, and then perform spectral analysis based on the center wavelength time series to obtain the slip spectrum.

2. The sensor according to claim 1, characterized in that, The microstructure is an anisotropic structure, used to obtain different spectral characteristics generated by sliding friction in different directions, so as to determine the sliding direction.

3. The sensor according to claim 2, characterized in that, The microstructure includes one or more of the following: periodic grooves, microridges, microcolumns, spiral grooves, or serrated ridges.

4. The sensor according to claim 2, characterized in that, The center wavelength dynamic modulation includes center wavelength dynamic drift, phase / intensity modulation, or short-time spectral characteristics.

5. The sensor according to claim 2, characterized in that, The polymer layer includes: an adhesive layer, which is adhered to the surface of the outer periphery of the cover layer and positioned relative to the microstructure; and a friction outer layer, disposed on the surface of the outer periphery of the adhesive layer.

6. The sensor according to claim 5, characterized in that, The outer friction layer is made of PU / TPU or modified silicone rubber, and the shear modulus and loss factor of the outer friction layer are set within a preset range.

7. A sensor array, characterized in that, Includes flexible substrates for bonding to device surfaces; At least two of the friction-enhanced fiber optic tactile sensors according to any one of claims 1-6, wherein the sensor is disposed within the flexible substrate.

8. The sensor according to claim 7, characterized in that, The sensor array also includes a reference fiber for data compensation to eliminate temperature drift and load drift.

9. A method for acquiring a slip spectrum using the sensor array described in claim 7 or 8, characterized in that, Obtain the center wavelength sequence of the FBG, which includes a time-varying sequence λB(t), an equivalent phase and / or intensity sequence; The FBG center wavelength sequence is preprocessed, including temperature compensation and detrending processing. The detection and acquisition of the slip event window is used to determine the window sequence, wherein the slip event window is obtained by detecting indicators such as short-time energy, spectral entropy or phase noise rise to obtain the slip start time or end time; The time-frequency analysis is performed on the window sequence to obtain the slip spectrum. The time-frequency analysis is to perform STFT or wavelet packet decomposition on the slip window to obtain the slip spectrum. The peak position, bandwidth, energy distribution, spectral centroid, or 1 / f noise slope change are extracted and output based on the slip spectrum to determine the slip intensity, direction, or material texture fingerprint.

10. A method for manufacturing a sensor, characterized in that, The steps for preparing the friction-enhanced fiber optic tactile sensor according to any one of claims 1-6 include: Fix the optical fiber to the worktable, determine the region center of the fiber Bragg grating, and determine the start and end positions of the microstructure to be processed; Microstructures are fabricated on the cladding of the optical fiber, and the microstructures correspond to the center of the region of the fiber Bragg grating. The processing area of ​​the optical fiber is ultrasonically / solvent cleaned to remove molten redeposited debris, and the processing area is activated to improve polymer wetting and adhesion. A low-viscosity polymer precursor solution is dropped onto the microstructure, and the polymer precursor solution is completely filled or covered by the microstructure by capillary action or vacuum assistance. The polymer is cured after the microstructure of the optical fiber is covered.

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