Optical fiber shape sensor based on torsional stress release principle and manufacturing method

Abstract

The application provides a fiber shape sensor based on torsion stress release principle and a manufacturing method thereof. The fiber shape sensor comprises a fiber grating sensor for sensing the three-dimensional shape of the fiber in real time and a super-elastic sleeve for resisting torsion. The fiber grating sensor comprises a fiber sensing signal transmission part and a fiber shape sensing part, both of which are multicore optical fibers, and the fiber shape sensing part is engraved with a Bragg grating. The super-elastic sleeve is composed of a fixed head and a catheter which are fixedly connected. The fixed head is fixed at the connection between the fiber sensing signal transmission part and the fiber shape sensing part, and the catheter is sleeved outside the fiber shape sensing part. The application releases torsion stress by using the super-elastic sleeve, isolates torque, prevents the fiber grating sensor from being affected by external torque and being twisted, and makes the fiber grating sensor only produce bending deformation. The application improves the accuracy of three-dimensional shape reconstruction of the fiber and is beneficial to large-scale application of the fiber grating sensor.

Description

Technical Field

[0001] This invention relates to the field of fiber optic grating shape sensing, and more particularly to a fiber optic shape sensor based on the principle of torsional stress relief and its fabrication method. Background Technology

[0002] Fiber optic shape sensors can be used to measure the shape of slender components in areas inaccessible to vision, such as robots and flexible medical devices. Currently, the mainstream fiber optic shape sensing solutions include: weak grating distributed sensors based on optical frequency domain reflectometer (OFDR) technology and quasi-distributed sensors based on wavelength division multiplexing (WDM) Bragg gratings (FBG). OFDR technology offers high accuracy, but it has significant limitations in terms of cost and demodulation speed. WDM solutions are simple in structure and low in cost, potentially offering substantial cost savings, but their positioning accuracy needs improvement.

[0003] To improve the accuracy of wavelength division multiplexing (WDM) schemes, methods employed mainly include using multi-core fibers with more precise structures, increasing the number of FBG points, using twisted fibers, sensor encapsulation, and algorithm optimization. The main obstacle hindering further improvement in the accuracy of fiber optic shape sensors lies in the twisting of the multi-core fibers. While helical fibers can demodulate the twist amount, demodulation accuracy is limited, and achieving consistent fabrication of helical fibers is difficult. Therefore, preventing twisting of the multi-core fibers during use is crucial for further improving the accuracy of fiber optic shape sensors. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a fiber optic shape sensor based on the principle of torsional stress relief and its fabrication method. By installing a highly elastic sleeve on the fiber Bragg grating sensor, torsion during use is effectively prevented, improving the accuracy of fiber optic three-dimensional shape reconstruction and facilitating the large-scale commercial application of fiber Bragg grating sensors in the field of shape sensing.

[0005] The specific technical solution adopted in this invention is as follows:

[0006] A fiber optic shape sensor based on the principle of torsional stress relief comprises a fiber Bragg grating sensor for real-time sensing of the three-dimensional shape of the fiber and a superelastic sleeve for torsion resistance. The fiber Bragg grating sensor includes a fiber optic signal transmission section and a fiber optic shape sensing section, both of which are multi-core optical fibers. The fiber optic shape sensing section is etched with a Bragg grating. The superelastic sleeve consists of a fixed head and a guide tube, with the fixed head fixed at the connection between the fiber optic signal transmission section and the fiber optic shape sensing section, serving as the starting end of the fiber optic shape sensing section. The guide tube is sleeved outside the fiber optic shape sensing section.

[0007] The ultra-elastic sleeve is used to prevent the fiber shape sensing part in the fiber optic grating sensor from twisting, ensuring that the fiber shape sensing part only undergoes bending deformation, thereby improving the demodulation accuracy of the fiber optic grating sensor signal. The ultra-elastic sleeve consists of a fixing head and a guide tube. The fixing head is located at the starting end of the fiber shape sensing part and is used to securely fix the guide tube to the fiber sensing part. Simultaneously, the fixing head effectively prevents the twisting of the unsleeved fiber from causing the twisting of the sensing fiber. The guide tube is a slender, hollow circular tube with a diameter slightly larger than the diameter of the multi-core fiber, made of nickel-titanium alloy, but it can also be made of other alloy materials.

[0008] As a preferred solution, to improve the fiber shape reconstruction performance of the fabricated fiber shape sensor, an anti-jitter coating can be added between the guide tube and the outer surface of the multi-core fiber. The anti-jitter coating is added by injecting an anti-jitter solution into the guide tube, then inserting the multi-core fiber into the guide tube for curing. After curing, an anti-jitter coating is obtained.

[0009] As a preferred embodiment, the anti-vibration coating is cured from polydimethylsiloxane (PDMS), wherein the anti-vibration solution consists of PDMS and a curing agent in a mass ratio of 10:1. Alternatively, the anti-vibration coating can also be cured from other substances with low Young's modulus and good flowability. To ensure good anti-vibration properties, the prepared anti-vibration solution should be allowed to stand or a vacuum dryer should be used to remove air bubbles, preventing hollow areas in the cured coating. The anti-vibration coating fills the space between the guide tube and the outer surface of the multi-core optical fiber, limiting the vibration range of the optical fiber within the guide tube. This effectively prevents vibration of the internal multi-core optical fiber within the guide tube during the use of the optical fiber shape sensor, resulting in good repeatability of the reconstructed three-dimensional shape of the optical fiber. The coefficient of friction between the anti-vibration coating and the outer surface of the multi-core optical fiber is very low, thus effectively reducing the torque exerted on the multi-core optical fiber by external forces and preventing significant twisting of the fiber that could affect the final reconstruction result.

[0010] As a preferred solution, to improve the torsion resistance of the cured optical fiber, a pre-stress can be applied to both ends of the optical fiber using an optical fiber straightener before curing. After the anti-vibration solution cures into an anti-vibration coating, the application of the pre-stress is stopped, so that the optical fiber inside the conduit is always in a taut state, further improving its torsion resistance.

[0011] Furthermore, the fiber Bragg grating sensor also includes a fan-in / fan-out module and several single-mode fibers. The fan-in / fan-out module is used to fan out the multi-core fiber to each single-mode fiber. When each single-mode fiber is connected to a multi-channel demodulator, the host can process the demodulated signal and reconstruct the three-dimensional shape of the fiber in real time, and display the reconstructed three-dimensional shape of the fiber on a monitor in real time.

[0012] Furthermore, the multi-core optical fiber is a major component of the optical fiber shape sensing unit used to sense the three-dimensional shape of the optical fiber and the optical fiber sensing signal transmission unit used to transmit demodulated signals. The multi-core optical fiber can be a conventional straight-core multi-core optical fiber or a demodulated twisted helical multi-core optical fiber.

[0013] Furthermore, each core of the multi-core optical fiber corresponding to the optical fiber shape sensing part is engraved with several Bragg gratings. The parameters of the Bragg gratings at the same position on the core are the same, and the center wavelength of the Bragg gratings at different positions on the core are different.

[0014] Furthermore, the fiber optic shape sensor needs to be used in conjunction with a multi-channel demodulator, a host computer, and a monitor. During use, the fiber optic shape sensing part of the sensor should be inserted into the internal clamp of the object being sensed or bound to its outer surface. The principle of the fiber optic shape sensing part sensing the three-dimensional shape of the fiber in real time is as follows: strain in the fiber causes a change in the center wavelength of the FBG at that location. Based on the change in the center wavelength of the FBG at that location, the three-dimensional curvature of the fiber at that location can be calculated. For the curvature at each location, interpolation or curve fitting methods are used to obtain the curvature function of the entire fiber optic shape sensing part. Finally, the three-dimensional shape of the fiber optic shape sensing part is reconstructed using a Flexner frame. During the use of the fiber optic shape sensor, the multi-channel demodulator sends optical signals to the fiber optic shape sensing part in real time and measures the change in the center wavelength of the FBG based on the received sensing signals. The host computer calculates the three-dimensional curvature at each location based on the change in the center wavelength of the FBG at each location, thus reconstructing the three-dimensional shape of the fiber.

[0015] Compared to traditional fiber Bragg grating sensors without anti-torsion packaging, the fiber shape sensor based on the torsional stress relief principle described in this invention does not undergo significant torsion during use, effectively improving the demodulation accuracy of the sensing signal and the accuracy of fiber optic three-dimensional shape reconstruction, which is beneficial for the large-scale application of fiber Bragg grating sensors in real life.

[0016] A method for fabricating an optical fiber shape sensor based on the principle of torsional stress relief includes the following steps:

[0017] Step 1: Inscribe Bragg gratings on each core of a multi-core optical fiber; wherein, the multi-core optical fiber with Bragg gratings is used as the optical fiber shape sensing part, and the multi-core optical fiber without Bragg gratings is used as the optical fiber sensing signal transmission part.

[0018] Step 2: Place the ultra-elastic sleeve over the fiber shape sensing part and fix the fixing head to the starting end of the fiber shape sensing part of the multi-core fiber.

[0019] Furthermore, in step 1, Bragg gratings can be etched onto the multi-core optical fiber using methods such as phase masking or ultraviolet lithography. Optical fibers with etched Bragg gratings are prone to breakage; therefore, after grating etching, a protective coating layer should be applied to the grating area using an optical fiber coating machine to prevent breakage. In step 1, when the number of Bragg gratings is large and the center wavelength range is wide, making it impossible to etch the gratings all at once, the Bragg gratings can be etched in batches. After etching, the etched multi-core optical fibers are then fused together to form a complete multi-core optical fiber.

[0020] Furthermore, in step 2, to improve the fiber shape reconstruction performance of the fabricated fiber shape sensor, an anti-vibration coating can be added between the conduit and the outer surface of the multi-core fiber. Specifically, the fixing head is installed at the starting end of the fiber shape sensing part of the multi-core fiber; the anti-vibration solution is injected into the conduit, and then the multi-core fiber is inserted into the conduit until one end of the conduit contacts the fixing head, and then cured. After curing, an anti-vibration coating is obtained.

[0021] Furthermore, due to the extremely small inner diameter of the catheter, the liquid has difficulty flowing within it. Injecting the liquid from top to bottom will introduce air bubbles during the injection process, affecting the anti-vibration performance of the cured anti-vibration coating. Therefore, the anti-vibration coating solution should be slowly injected into the syringe from the bottom to the top of the catheter. After the anti-vibration solution is drawn into the syringe, it should be allowed to stand until all air bubbles in the solution in the syringe disappear before injection.

[0022] Furthermore, when injecting the anti-vibration coating solution from bottom to top using the syringe, a rubber sleeve should be placed over the syringe to prevent the solution from flowing out from the injection port edge. The inner diameter of the portion of the rubber sleeve connecting to the syringe is the same as the injection port, and the inner diameter of the portion connecting the rubber sleeve to the conduit is the same as the outer diameter of the conduit. The rubber sleeve effectively prevents the anti-vibration coating solution from flowing out from the injection port edge during injection, allowing it to flow upwards only through the lower end of the conduit. The anti-vibration solution can be cured by standing at room temperature or by using a dryer.

[0023] Furthermore, in step 2, to improve the anti-torsion performance of the cured optical fiber, a pre-set tensile stress can be applied to both ends of the multi-core optical fiber using an optical fiber straightener before curing, and this pre-set tensile stress is maintained until the anti-vibration coating is cured. By applying the pre-set tensile stress, the optical fiber inside the conduit remains taut and is not easily twisted.

[0024] The method for manufacturing an optical fiber shape sensor based on the torsional stress relief principle described in this invention has the advantages of being simple and quick to operate. Attached Figure Description

[0025] Figure 1The diagram shows a fiber optic shape sensor structure based on the torsional stress relief principle according to one embodiment of the present invention.

[0026] Figure 2 The diagram shows a block diagram of a fiber optic shape sensor based on the torsional stress relief principle according to one embodiment of the present invention.

[0027] Figure 3 The diagram shows a schematic of a fiber optic shape sensing part of a fiber optic shape sensor based on the torsional stress relief principle according to an embodiment of the present invention, which is encapsulated only by a superelastic sleeve.

[0028] Figure 4 The diagram shows a schematic of an encapsulation structure of an optical fiber shape sensor based on the torsional stress relief principle according to an embodiment of the present invention, which includes an optical fiber shape sensing part with a superelastic sleeve and a vibration-damping coating.

[0029] Figure 5 The diagram shows a fiber optic shape sensing part of a fiber optic shape sensor based on the torsional stress relief principle according to an embodiment of the present invention. The fiber optic shape sensing part has a super-elastic sleeve, a vibration-damping coating, and a pre-stressed encapsulation structure.

[0030] Figure 6 The diagram shows a straight multi-core optical fiber structure in an optical fiber shape sensing unit according to an embodiment of the present invention.

[0031] Figure 7 The diagram shows a multi-core spiral optical fiber structure in an optical fiber shape sensing unit according to an embodiment of the present invention.

[0032] Figure 8 The diagram illustrates the principle of one embodiment of the present invention.

[0033] Figure 9 The diagram shows an anti-vibration solution injection device used in the manufacturing process of one embodiment of the present invention.

[0034] Figure 10 The diagram shows a comparison of the torsion resistance of an optical fiber shape sensor according to one embodiment of the present invention with that of an optical fiber shape sensor without a superelastic sleeve.

[0035] Figure 11 The diagram shows a reconstructed three-dimensional shape of a plurality of optical fiber shape sensors according to an embodiment of the present invention.

[0036] Symbol Explanation

[0037] 1-Multi-core optical fiber, 2-Fan-in / Fan-out module, 3-Single-mode optical fiber, 4-Fixed head, 5-Fiber optic sensing signal transmission unit, 6-Fiber optic shape sensing unit, 7-Ultra-elastic sleeve, 8-Bracket grating, 9-Fiber core, 10-Anti-vibration coating, 11-Preset tensile stress, 12-Rubber sleeve, 13-Injector. Detailed Implementation

[0038] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0039] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0040] The singular forms “a,” “the,” and “the” used in this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0041] It should be understood that although the terms first, second, third, etc., may be used in this application to describe various information, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, without departing from the scope of this application, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to determination."

[0042] This invention discloses an optical fiber shape sensor based on the principle of torsional stress relief and its manufacturing method. The optical fiber shape sensor needs to be used in conjunction with a demodulator, host and monitor. The device is simple, has excellent anti-torsion ability, and is suitable for high-precision sensing of the three-dimensional shape of optical fibers.

[0043] like Figure 1The diagram shown illustrates the structure of a fiber optic shape sensor based on the torsional stress relief principle of this embodiment. The fiber optic shape sensor comprises a fiber optic grating sensor for real-time sensing of the three-dimensional shape of the fiber and a super-elastic sleeve 7 for torsion resistance. The fiber optic grating sensor includes a fiber optic sensing signal transmission unit 5 and a fiber optic shape sensing unit 6, both of which are multi-core optical fibers 1. The fiber optic shape sensing unit 6 is etched with a Bragg grating 8. The super-elastic sleeve 7 consists of a fixed head 4 and a guide tube. The fixed head 4 is fixed at the connection between the fiber optic sensing signal transmission unit 5 and the fiber optic shape sensing unit 6. The fixed head of the super-elastic sleeve 7 serves as the starting end of the fiber optic shape sensing unit 6; that is, the portion of the multi-core optical fiber 1 encapsulated by the super-elastic sleeve 7 constitutes the fiber optic shape sensing unit 6, while the unencapsulated portion constitutes the fiber optic sensing signal transmission unit 5.

[0044] like Figure 2 The diagram shown is a block diagram illustrating the use of a fiber optic shape sensor based on the torsional stress relief principle described in this embodiment. The fiber optic shape sensor is used by inserting each single-mode fiber from a fiber Bragg grating sensor into a multi-channel demodulator. During three-dimensional shape sensing, the fiber optic shape sensing unit 6 of the fiber Bragg grating sensor converts its own shape into an optical signal in real time, which is transmitted to the multi-channel demodulator via the fiber optic sensing signal transmission unit 5 of the fiber Bragg grating sensor, the fan-in / fan-out module 2, and each single-mode fiber 3. The fan-in / fan-out module 2 is used to fan out each fiber core 9 of the multi-core fiber to each single-mode fiber 3. The multi-channel demodulator is used to send optical signals to the fiber optic shape sensing unit 6 in real time and demodulate the optical signals transmitted back from each single-mode fiber in the multi-core fiber in real time, sending a demodulated signal to the host computer. The host computer processes the demodulated signal in real time and reconstructs the three-dimensional shape of the fiber optic shape sensing unit of the anti-torsion fiber Bragg grating sensor according to the Flyner framework. The monitor displays the reconstructed three-dimensional shape of the fiber optic shape sensing unit in real time.

[0045] like Figure 3 The diagram shows a schematic of the encapsulation structure of an optical fiber shape sensor based on the torsional stress relief principle according to an embodiment of the present invention, where the optical fiber shape sensing part consists only of a superelastic sleeve. The superelastic sleeve 7 comprises a fixing head 4 and a guide tube. The fixing head 4 is located at the starting end of the optical fiber shape sensing part 6. The fixing head 4 firmly fixes the end face of the starting end of the optical fiber shape sensing part 6, preventing the optical fiber of the optical fiber shape sensing part 6 from twisting due to the torsion of the optical fiber sensing signal transmission part 5, effectively isolating the torque of the optical fiber sensing signal transmission part 5. The guide tube is a slender hollow cylindrical tube with a certain rigidity, encapsulated on the outer surface of the optical fiber of the optical fiber shape sensing part 6. The diameter of the guide tube is slightly larger than the diameter of the multi-core optical fiber. The guide tube is made of an alloy material. Figure 3-5 The conduits used in this process are all made of nickel-titanium alloy. Only one highly elastic sheath is encapsulated. Figure 3Compared to traditional fiber optic shape sensors without anti-torsion components, this reduces the torque applied to the fiber optic shape sensing unit 6, resulting in more accurate demodulation of the sensing signal. However, due to limitations in the material and manufacturing process of the ultra-elastic sleeve 7, Figure 3 The torsional resistance is limited, the inner wall of the conduit is not smooth enough, and the friction coefficient between the inner wall of the conduit and the outer surface of the optical fiber is relatively large. During use, the friction force on the outer surface of the optical fiber will cause the optical fiber shape sensing part to still produce a certain degree of torsion. Figure 3 A gap exists between the conduit and the multi-core optical fiber. During use, the multi-core optical fiber vibrates within the conduit, resulting in a lack of repeatability in the shape reconstruction results. Using a smaller inner diameter ultra-elastic sleeve 7 can limit the vibration of the multi-core optical fiber, but it increases the probability of friction between the outer surface of the fiber and the inner wall of the conduit. Conversely, using a larger inner diameter ultra-elastic sleeve 7 can reduce the probability of friction between the outer surface of the fiber and the inner wall of the conduit, but it will result in a larger vibration range for the multi-core optical fiber and may also cause axial slippage of the fiber within the conduit. To solve these problems, an anti-vibration coating 10 can be added between the conduit and the outer surface of the multi-core optical fiber. This coating should have a low Young's modulus and good flowability.

[0046] like Figure 4 The diagram shown is a schematic representation of the packaging structure of an optical fiber shape sensor based on the torsional stress relief principle according to an embodiment of the present invention, which includes an optical fiber shape sensing part with a superelastic sleeve and an anti-vibration coating. Figure 4 A layer of anti-vibration coating is filled between the middle guide tube and the outer surface of the multi-core optical fiber 10. Figure 4-5 The selected anti-vibration coating 10 is made of PDMS, with a PDMS solution to curing agent mass ratio of 10:1, which can be adjusted according to actual usage requirements. In this embodiment, the PDMS mixture is cured by drying at 80°C for 2 hours using a dryer. The anti-vibration coating 10 can also be cured at room temperature. The presence of the cured anti-vibration coating 10 limits the vibration range of the multi-core optical fiber, effectively preventing the internal multi-core optical fiber from vibrating within the guide tube during use of the anti-torsion fiber Bragg grating sensor, thus ensuring good repeatability of the reconstructed three-dimensional shape. The coefficient of friction between the anti-vibration coating 10 and the outer surface of the multi-core optical fiber is very small. Compared to encapsulation without the anti-vibration coating 10, the frictional force on the outer surface of the optical fiber is greatly reduced, effectively reducing the torque exerted on the multi-core optical fiber by external forces and preventing large twisting of the optical fiber under external influence. To improve the anti-torsion performance of the optical fiber shape sensor, a pre-set tensile stress can be applied to both ends of the multi-core optical fiber before curing until the anti-vibration coating 10 is fully cured.

[0047] like Figure 5The diagram shows a fiber shape sensing section of a fiber shape sensor based on the torsional stress relief principle according to an embodiment of the present invention, comprising a super-elastic sleeve, an anti-vibration coating, and a pre-set tensile stress encapsulation structure. Because a pre-set tensile stress 11 is applied to both ends of the fiber using a fiber straightener before and during the curing process of the anti-vibration coating 10, the fiber inside the fiber shape sensing section 6 remains taut after curing when the application of the pre-set tensile stress 11 is stopped. Twisting a taut fiber requires a larger torque, thus further reducing the possibility of twisting in the fiber shape sensing section. The taut fiber may experience tensile strain, which can also cause a change in the center wavelength of the Bragg grating 8. Failure to calibrate these tensile strains will affect the demodulation accuracy of the demodulated signal. Therefore, for... Figure 5 The pre-stressed encapsulation method shown requires individual calibration of the center wavelength of each grating of the fiber optic shape sensor after encapsulation is completed.

[0048] Traditional fiber optic shape sensors lack anti-torsion components, inevitably leading to fiber twisting during use. This twisting affects the center wavelength of the fiber Bragg grating, impacting the demodulated signal of the multi-channel demodulator and reducing the accuracy of shape sensor demodulation. In this embodiment... Figure 3-5 The packaging methods shown can all prevent the fiber optic shape sensor from experiencing a decrease in shape reconstruction accuracy due to twisting.

[0049] like Figure 6 The diagram shown illustrates the multi-core fiber structure in the fiber shape sensing section of the fiber shape sensor described in this embodiment. For ease of illustration, the coating layer of the multi-core fiber is not shown. In actual use, the multi-core fiber 1 is sheathed with a super-elastic sleeve 7. The number of cores in the multi-core fiber 1 can be 4, 5, 7, etc. In this embodiment, the multi-core fiber has 7 cores. The cores 9 are distributed as follows: one core 9 at the center of the end face, and the remaining six cores 9 are evenly distributed around the central core, exhibiting a centrosymmetric shape. Each core 9 of the multi-core fiber has six Bragg gratings 8 engraved on its sensing portion. The parameters of the Bragg gratings 8 at the same position on the core 9 are identical, while the center wavelengths of the Bragg gratings 8 at different positions on the core 9 are different. When the shape of the fiber shape sensing section changes, the center wavelength of the Bragg gratings 8 at each deformation position also changes in real time. The deformation at the corresponding position is calculated based on the change in center wavelength, and combined with the Flyner frame, the three-dimensional shape of the fiber shape sensing section can be reconstructed with high precision in real time. Figure 7The diagram shows a schematic representation of the fiber shape sensing section of an embodiment of a fiber shape sensor made of multi-core helical fiber. The central core of the multi-core helical fiber remains straight, while the outer cores are helically arranged around the central core. The fiber in the fiber shape sensing section 6 is uniformly etched with Bragg gratings 8, and the grating parameters are identical at the same locations on the fiber core. The biggest advantage of using a multi-core helical fiber shape sensor compared to a fiber shape sensor using straight multi-core fiber is that the latter cannot demodulate fiber torsion, only bending, while the former can demodulate fiber torsion as well, thereby greatly improving the accuracy of fiber 3D shape reconstruction.

[0050] like Figure 8 The figure shown is a cross-sectional view of the straight multi-core optical fiber used in this embodiment, used to illustrate the shape sensing principle. The fiber coating is not shown in the figure, but this does not affect the explanation of the principle. Since the bending radius of the optical fiber is much larger than the cross-sectional radius, the optical fiber can be considered as a Kirchhoff elastic rod, and therefore its three-dimensional shape can be reconstructed using a Flyner frame. Specifically, when deformation occurs at the grating location on the optical fiber, the center wavelength λ of the Bragg grating 8 at that location... B The change will occur. The multi-channel demodulator will send optical signals to the fiber shape sensor 6 in real time and demodulate the change in center wavelength Δλ based on the signal returned by the fiber sensor signal transmission unit 5. B The change in center wavelength Δλ can be observed. B The fiber deformation ε at this location is calculated using the following formula:

[0051]

[0052] Where P e It is a constant, determined by the optical fiber material. Figure 6 Establishing a yoz coordinate system with the central core of a multi-core optical fiber as the origin, the relationship between the bending strain of a single core and the bending curvature of the optical fiber can be expressed as:

[0053]

[0054] Where R i Let be the bending radius of fiber core i. Let d be the bending curvature of the optical fiber. i Let θ be the distance from fiber core i to the bending neutral plane, r be the distance from fiber core to the origin, and θ be the distance from fiber core i to the origin. b For the bending angle, θ i Let be the angle between fiber core i and the y-axis.

[0055] Apparent curvature κ of multi-core straight optical fiber app It can be represented as:

[0056]

[0057] The actual bending curvature of this multi-core straight optical fiber can be expressed as:

[0058]

[0059] Where N is the total number of fiber optic sensing points. Let be the unit vector in the y-direction. This is the unit vector in the z-direction. The multi-core fiber used in this embodiment is a seven-core fiber. When reconstructing the three-dimensional shape using other multi-core fibers, the reconstruction formula should be modified according to the number of fiber cores.

[0060] The bending direction of the optical fiber at this location is characterized by the deflection angle θ, and can be expressed as:

[0061] θ = angle(κ) app )

[0062] angle(κ app ) represents κ app The angle.

[0063] To reconstruct the three-dimensional shape of the entire optical fiber, it is necessary to determine the actual bending curvature κ and deflection angle θ at each sensing point on the fiber. The actual bending curvature κ and deflection angle θ at all sensing points can be represented as two discrete sequences κ(n) and θ(n), where n is the sensing point index. Smooth functions κ(s) and θ(s) with respect to the sensing fiber length s are fitted using κ(n) and θ(n), where 0 ≤ s ≤ L, and L is the length of the fiber shape sensing section.

[0064] The torsion function τ(s) is:

[0065] τ(s)=θ′(s)

[0066] θ'(s) is the derivative of θ(s);

[0067] Substituting the obtained κ(s) and θ(s) into the Fryner framework, we get:

[0068]

[0069] T(s), N(s), and B(s) represent the tangent vector, principal normal vector, and secondary normal vector at a point, respectively. When the initial position of the fiber is fixed, κ(0) and θ(0) are both 0. T(s), N(s), and B(s) are obtained using numerical methods, and the three-dimensional shape of the fiber shape sensor is reconstructed using the integral of T(s).

[0070]

[0071] in It is a three-dimensional vector form of the fiber optic shape sensor. The deformation at each location is calculated based on the change in the center wavelength of each Bragg grating 8 on the fiber shape sensor, and the three-dimensional shape of the fiber shape sensor is finally reconstructed. All of the above calculations are performed by the host computer.

[0072] The method for fabricating the fiber optic shape sensor in this embodiment is as follows:

[0073] Step 1: Bragg gratings 8 are etched on multi-core optical fibers using ultraviolet lithography. After etching, optical fibers at each grating location are coated using an optical fiber coating machine. The multi-core optical fiber with Bragg gratings 8 etched serves as the optical fiber shape sensing unit 6, while the multi-core optical fiber without Bragg gratings 8 etched serves as the optical fiber sensing signal transmission unit 5.

[0074] Step 2: Install the fixing head at the starting end of the fiber optic shape sensing unit 6.

[0075] Step 3: Fill the conduit with PDMS mixture, then insert the optical fiber into the conduit and assemble it with the fixing head to form a super-elastic sleeve 7. Place the multi-core optical fiber with the super-elastic sleeve 7 into a dryer to dry and cure.

[0076] Step 4: Using a fiber optic fusion splicer and a fan-in / fan-out module, each core 9 of the multi-core fiber is fanned out into each single-mode fiber. After fanning out, the fiber shape sensor is complete, and each single-mode fiber can be directly connected to a multi-channel demodulator for use.

[0077] In step 3, the PDMS mixture used is prepared by mixing PDMS solution and curing agent at a mass ratio of 10:1. After preparation, the PDMS mixture is allowed to stand to remove air bubbles, preventing hollow areas in the cured coating from affecting its anti-vibration and anti-torsion effects. The mass ratio of PDMS solution to curing agent can be adjusted according to actual needs, and air bubbles in the mixture can also be removed using a vacuum dryer.

[0078] In step 3, the PDMS mixture injection device used in this embodiment is as follows: Figure 9As shown. Due to the small inner diameter of the catheter, which is comparable to the diameter of a conventional syringe needle, the PDMS mixture cannot be directly injected into the catheter through the syringe needle. Therefore, injection is performed by inserting the catheter into a syringe filled with the PDMS mixture. The injection device consists of a syringe 13 with the syringe needle removed and a rubber sleeve 12. The inner diameter of the portion of the rubber sleeve 12 used to connect the syringe is the same as the injection port of the syringe 13, and the inner diameter of the portion of the rubber sleeve 12 used to connect the catheter is the same as the outer diameter of the catheter. During injection, the syringe is first used to draw the PDMS mixture. After all the air bubbles in the PDMS mixture in the syringe have disappeared, the syringe injection port is inserted into the rubber sleeve 12, and the catheter is inserted into the rubber sleeve 12 and connected to the injection port of the syringe 13. Because the inner diameter of the catheter is extremely small, the liquid is difficult to flow inside the catheter. When injecting from top to bottom, air bubbles are very likely to be introduced during the injection process, reducing the anti-vibration performance of the cured anti-vibration coating 10. Therefore, during injection, the injection device should slowly inject from the bottom of the catheter upwards.

[0079] In step 3, curing can also be carried out at room temperature, but the curing time will be significantly increased. In this embodiment, a vacuum dryer is used for curing, with the dryer set to a temperature of 80°C and a time of 2.5 hours.

[0080] In step 3, to improve the torsional resistance of the multi-core optical fiber, pre-tension stress can be applied to both ends of the multi-core optical fiber before curing until the curing is complete. In this embodiment, an optical fiber straightener is used to apply pre-tension stress to both ends of the optical fiber by straightening them. To eliminate the influence of tensile strain on the demodulation signal accuracy of the taut optical fiber, the center wavelength of each grating of the optical fiber shape sensor needs to be individually calibrated after curing.

[0081] like Figure 10 The diagram shows a comparison between the anti-torsion performance of the fiber optic shape sensor according to one embodiment of the present invention and that of a fiber optic shape sensor without a super-elastic sheath. In practical use, the torsion angle of the multi-core fiber in this embodiment remains almost constant, while the torsion angle of the multi-core fiber without a super-elastic sheath can reach a maximum of 43°. Therefore, the fiber optic shape sensor described in this embodiment exhibits excellent anti-torsion performance.

[0082] like Figure 11The image shows the reconstruction results of various three-dimensional shapes of the fiber optic shape sensor according to one embodiment of the present invention, where c1 to c8 represent eight different three-dimensional shapes. The fiber optic shape sensing part of the fiber optic shape sensor can be inserted into the internal clamping channel of the object being sensed or bound to the outer surface of the object. When the shape of the object being sensed changes, the shape of the fiber optic shape sensing part changes accordingly. When the fiber optic shape sensing part 6 of the fiber optic shape sensor is bent into different three-dimensional shapes, the monitor displays in real time the three-dimensional shape of the fiber optic shape sensing part reconstructed by the host based on the demodulation signal of the multi-channel demodulator and the Flyner frame. In this embodiment, the fiber optic shape sensor is simply encapsulated with a super-elastic sleeve, without adding an anti-vibration coating or pre-set tensile stress. Bending the fiber optic shape sensing part into different three-dimensional shapes, the reconstructed three-dimensional shapes of the fiber optic all conform well to the actual three-dimensional shape of the fiber, demonstrating the feasibility of the present invention.

[0083] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A fiber optic shape sensor based on the principle of torsional stress relief, characterized in that, The device comprises a fiber Bragg grating sensor for real-time sensing of the three-dimensional shape of an optical fiber and a super-elastic sleeve for torsion resistance. The fiber Bragg grating sensor includes an optical fiber sensing signal transmission section and an optical fiber shape sensing section, both of which are multi-core optical fibers. The optical fiber shape sensing section is etched with a Bragg grating. The super-elastic sleeve consists of a fixed head and a guide tube. The fixed head is fixed at the connection between the optical fiber sensing signal transmission section and the optical fiber shape sensing section, serving as the starting end of the optical fiber shape sensing section. The guide tube is fitted over the optical fiber shape sensing section. An anti-vibration coating is also provided between the guide tube and the optical fiber shape sensing section.

2. The fiber optic shape sensor according to claim 1, characterized in that, The catheter is made of nickel-titanium alloy.

3. The fiber optic shape sensor according to claim 1, characterized in that, The anti-vibration coating is formed by curing an anti-vibration solution.

4. The fiber optic shape sensor according to claim 3, characterized in that, The anti-vibration solution is composed of polydimethylsiloxane and a curing agent in a mass ratio of 10:

1.

5. The fiber optic shape sensor according to claim 1, characterized in that, The fiber optic grating sensor has a preset tensile stress.

6. A method for manufacturing an optical fiber shape sensor according to any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Bragg gratings are etched on each core of a multi-core optical fiber. The multi-core optical fiber with Bragg gratings is used as the optical fiber shape sensing part, and the multi-core optical fiber without Bragg gratings is used as the optical fiber sensing signal transmission part. Step 2: Place the ultra-elastic sleeve over the fiber optic shape sensing element and fix the fixing head to the starting end of the fiber optic shape sensing element of the multi-core fiber; specifically: Install the fixing head at the starting end of the fiber shape sensing part of the multi-core optical fiber; inject the anti-vibration solution into the conduit, then insert the fiber shape sensing part into the conduit until one end of the conduit contacts the fixing head, and then cure it to complete the process. Sleeve the ultra-elastic conduit over the fiber shape sensing part and fix the fixing head at the starting end of the fiber shape sensing part of the multi-core optical fiber.

7. The method according to claim 6, characterized in that, The anti-vibration solution is injected into the catheter using a syringe from bottom to top.

8. The method according to claim 7, characterized in that, During the curing process, a fiber straightener is used to apply a pre-set tensile stress to both ends of the multi-core fiber and maintain the pre-set tensile stress until the anti-vibration coating is cured.

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