Three-dimensional shape reconstruction system and sensor preparation method combined with grating inversion technology

Abstract

This invention discloses a three-dimensional shape reconstruction system and sensor fabrication method combining grating inversion technology, belonging to the field of fiber optic sensing technology. The system includes a shape sensor, a grating demodulation device, and a shape reconstruction device; a fiber bundle or multi-core fiber composed of several fiber grating strings is bonded to the outside of a substrate, but not completely bonded, only point-constrained, forming the shape sensor; the demodulation device acquires spectral information in real time; and the reconstruction device combines spectral inversion technology to accurately reconstruct and display the three-dimensional shape. This invention also includes a method for fabricating the shape sensor: first, fabricating the fiber bundle or multi-core fiber; second, placing the fiber bundle in a hollow flexible tube bonded to the side of the substrate, fixing only one end. This invention solves the problem of low spatial resolution in fiber grating shape sensing technology, and the shape sensor structure can eliminate strain on the outside of the substrate when the substrate is bent, protecting the optical fiber.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, and particularly relates to a high-precision three-dimensional shape sensing and reconstruction system combining fiber optic grating inversion technology and a method for preparing the sensor used therein. Background Technology

[0002] In recent years, surgical robots in minimally invasive surgery have attracted much attention, providing doctors with more precise and safer operating methods and achieving remarkable results. Shape sensing reconstruction technology is particularly crucial for achieving precise operation of surgical robots. This technology can monitor and track the shape and position of surgical tools or robots in real time, providing accurate navigation and control for surgery. Among various shape sensors, fiber optic shape sensors have garnered significant attention due to their unique working principle and technological advantages. Fiber optic shape sensors utilize optical principles to transmit light signals to an optical fiber on the object being measured. By monitoring the reflection or transmission of the light signal, they capture real-time changes in the object's shape. Compared to other shape sensors, fiber optic shape sensors offer advantages such as high precision, real-time monitoring, and interference resistance.

[0003] In fiber optic shape sensing technology, FBG (fiber optic girders) sensing is widely used in high-precision 3D shape sensing and reconstruction techniques due to its high sensitivity to strain and strong anti-interference capabilities. However, due to its quasi-distributed sensing characteristics, it can only obtain a limited number of strain points over a certain length. Therefore, FBG shape sensors using center wavelength demodulation typically require densely distributed gratings to acquire sufficient information points. However, in long-distance shape sensing, when some sections have complex bending conditions, this method is prone to losing important strain information, leading to significant errors in the reconstruction results.

[0004] Therefore, there is an urgent need for a high-precision three-dimensional shape sensing and reconstruction system that can acquire more information points without increasing the number of gratings to solve the above measurement problems.

[0005] During bending, the strain experienced by a FBG (Factor-Free Group) is typically not uniform, but rather a continuous, smooth, non-uniform strain, and its reflection spectrum can contain information about the non-uniform strain in the grating region. FBG spectral inversion technology can utilize the reflection spectrum signal of an FBG under non-uniform strain to invert the non-uniform strain field applied to a single grating. Inversion methods include particle swarm optimization, simulated annealing, deep learning, and others. Then, smooth non-uniform strain information across the entire shape sensor is obtained through data processing methods such as interpolation, including spline interpolation and Hermite interpolation. Finally, a reconstruction algorithm is used to achieve high-precision 3D shape reconstruction from continuous non-uniform strain.

[0006] Secondly, the core sensing element in a shape sensor is typically a bundle of multiple bonded optical fibers or a multi-core optical fiber, usually tightly bonded to the substrate of the sensor, or the core sensing element itself is the object to be reconstructed. In surgical robots, a central space needs to be preserved to integrate surgical equipment; therefore, shape sensors are best placed on the outside of the instruments. When the optical fiber is located outside a large-diameter substrate and the substrate undergoes significant bending, the outer side of the substrate itself will experience strain relative to the neutral axis of the substrate, and the larger the diameter, the greater the strain. For substrates with large diameters and complex bending shapes, a sensing structure where the optical fiber is tightly bonded to the substrate will cause the optical fiber to endure extremely high strain, thereby reducing the device's lifespan or even making it impossible to accurately measure the bending. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention employs a hollow flexible tube tightly bonded to a substrate to control the position of the optical fiber, while allowing for relative movement between the optical fiber and the hollow flexible tube. This structure can counteract the additional external strain caused by bending of the large-diameter substrate, allowing the optical fiber to withstand only the strain generated when its fiber bundle bends, thereby significantly improving its service life and expanding its measurement range.

[0008] In order to achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0009] A high-precision three-dimensional shape sensing and reconstruction system combining fiber optic grating inversion technology includes a shape sensor 1, a real-time fiber optic spectral demodulation device 2, and a three-dimensional shape reconstruction device 3 combining spectral inversion technology. The shape sensor comprises a hollow substrate 1, a hollow flexible tube 7, and an optical fiber bundle 8. The hollow flexible tube 7 is fixed to the outer wall of the hollow substrate 1, the optical fiber bundle 8 is located inside the hollow flexible tube 7, the front end of the optical fiber bundle 8 is fixed to the front end of the hollow flexible tube 7, and the rear end of the optical fiber bundle 8 is a free end connected to the real-time fiber optic spectral demodulation device 2.

[0010] The fiber bundle 8 is made of multiple fiber optic grating strings 6 bonded together in parallel.

[0011] Each fiber Bragg grating string 6 contains multiple fiber Bragg gratings 10, with the fiber Bragg gratings 10 near the front end having a smaller spacing and the fiber Bragg gratings 10 in the remaining parts having a larger spacing.

[0012] Three fiber Bragg grating strings 6 are bonded together with acrylic adhesive 9. In cross-section, the three fiber Bragg grating strings 6 are at an angle of 60° to each other. Each fiber Bragg grating string 6 contains ten fiber Bragg gratings 10. The four fiber Bragg gratings 10 closest to the front end are spaced 1-3 mm apart, while the remaining fiber Bragg gratings 10 are spaced 8-12 mm apart.

[0013] Another design: The fiber bundle 8 is a multi-core fiber with a uniform and stable arrangement, and multiple gratings are engraved inside the fiber.

[0014] The reconstruction algorithm used in the reconstruction device incorporates spectral inversion technology. The spectrum generated after the grating is bent contains non-uniform strain information of all grating regions. Spectral inversion can be used to obtain the non-uniform strain on each grating from the spectral information. The spectral inversion method can be particle swarm optimization algorithm, simulated annealing algorithm, or deep learning method.

[0015] A method for fabricating a shape sensor includes the following steps:

[0016] (1) Prepare several fiber grating strings or evenly and stably arranged multi-core optical fibers as fiber bundles, and prepare hollow tubes with an inner diameter slightly larger than the expected cross-sectional diameter of the fiber bundle, and flexible substrates of any diameter.

[0017] (2) Use adhesive to string fiber gratings into fiber bundles with a central rotational symmetry in cross-section, or write fiber gratings inside a uniformly and stably arranged multi-core fiber.

[0018] (3) Insert the prepared fiber bundle or multi-core fiber into the hollow tube, and use adhesive to fix one end of the fiber bundle or multi-core fiber to the hollow tube, while leaving the other end unfixed. The fixed end is usually used as the front end of the sensor, and the pigtail of the unfixed end is used to connect to the demodulation equipment.

[0019] (4) Attach the hollow hose to the outer wall of the substrate.

[0020] The fiber bundle is made using an adhesive that has a certain degree of flexibility after curing. The adhesive can tightly bond the fiber grating strings to form a fiber bundle. Multiple fiber gratings are distributed in the fiber grating strings. The fiber gratings near the front end have a smaller spacing, while the fiber gratings at other positions have a larger spacing.

[0021] Three fiber Bragg grating strings are bonded together to form an optical fiber bundle. When viewed from the cross-section, the three fiber Bragg grating strings are at an angle of 60° to each other. The gratings in the three fiber Bragg grating strings are aligned and then bonded together.

[0022] The beneficial effects of this invention are as follows:

[0023] (1) The shape sensing reconstruction system provided by this invention uses a spectral inversion method to inversely deduce the reflection spectrum containing strain information of multiple micro-elements on the FBG. Compared with the center wavelength demodulation method that only obtains one strain information, this method obtains more strain points without increasing the number of gratings used, thereby improving the spatial resolution and reconstructing a more accurate three-dimensional shape.

[0024] (2) In the shape sensing reconstruction system provided by this invention, the structure of the hollow shape sensor can effectively counteract the external strain of the thick-diameter, highly bent substrate. This extends the service life of the sensor, expands its measurement range, and enables it to handle measurement tasks in more complex environments. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall structure of the high-precision three-dimensional shape sensing and reconstruction system of the present invention;

[0026] Figure 2 This is a schematic diagram of the shape sensor structure of the present invention;

[0027] Figure 3 This is a schematic diagram of the fiber optic bundle and flexible tube in the shape sensor of the present invention;

[0028] In the attached diagram: 1 is a shape sensor, 2 is a demodulation device, 3 is a reconstruction device, 4 is a shape display, 5 is a hollow substrate, 6 is a fiber grating string, 7 is a hollow tube, 8 is a bundle of three fiber gratings, 9 is acrylic adhesive, and 10 is a fiber grating. Detailed Implementation

[0029] The specific embodiments of the present invention will be described in further detail below, but the present invention is not limited to these embodiments. Any improvements or substitutions based on the basic spirit of these embodiments shall still fall within the scope of protection claimed by the claims of the present invention.

[0030] A high-precision three-dimensional shape sensing and reconstruction system combining fiber optic grating inversion technology includes: a shape sensor 1 of arbitrary diameter, a real-time fiber optic spectral demodulation device 2, and a three-dimensional shape reconstruction device 3 combining spectral inversion technology, which can realize real-time sensing, reconstruction and display of three-dimensional shapes.

[0031] Preferably, the three-dimensional shape sensor 1 comprises a substrate of arbitrary diameter (with the neutral axis of the substrate as the object of reconstruction), a core sensing element placed outside the hollow substrate, and other components for fixing the sensor structure. This structure, by placing the sensing element outside the substrate, can be applied to shape sensing of solid substrates or substrates with reserved space in the center for other components.

[0032] Preferably, the core sensing element in the three-dimensional shape sensor is an optical fiber bundle 8 composed of three or more fiber optic grating strings 6, or a multi-core optical fiber with three or more cores. The optical fiber bundle or multi-core optical fiber used as the sensing element should have good flexibility, be able to bend freely, and have no length limit.

[0033] Preferably, the core sensing element is attached to the outside of the substrate, but not tightly bonded, only point-constrained. Except for the constraint points, the sensing element is movable relative to the substrate, and experiences minimal friction and minimal additional stress during movement. This structure can be implemented using a hollow flexible tube 7: the sensing element is placed within the hollow flexible tube 7, with only one or a few constraint points fixed. These constraint points can be at arbitrary locations, allowing the sensing element to move freely except at the constraint points when the substrate bends. The hollow flexible tube 7 is tightly bonded to the outside of the substrate, controlling the range of motion of the sensing element. This range can be of any shape, but it is best to make it relatively parallel to the neutral axis of the substrate. This sensor structure can counteract the additional strain on the outside caused by substrate bending, reduce the strain experienced by the optical fiber when the substrate bends, expand the curvature measurement range of the sensor, prevent breakage, and increase the lifespan of the sensing element.

[0034] Preferably, the reconstruction algorithm used in the reconstruction device incorporates spectral inversion technology. The spectrum generated after the grating is bent contains non-uniform strain information of all grating regions. Spectral inversion can obtain the non-uniform strain on each grating through the spectral information. Methods include, but are not limited to, particle swarm optimization algorithms, simulated annealing algorithms, deep learning methods, etc. Compared with demodulating strain information using the grating center wavelength and then reconstructing it, this method has higher accuracy and solves the problem of difficulty in identifying the spectral center wavelength under strong bending.

[0035] The method for fabricating shape sensor 1 includes the following steps:

[0036] (1) Prepare several fiber grating strings 6 (the number and position of gratings are the same: the number of gratings is greater than or equal to 3, the gratings are unevenly distributed, depending on the shape complexity of the actual shape. If the shape complexity is high, the grating spacing is small, and if the complexity is low, the spacing is large), adhesive, hollow tube 7 with an inner diameter slightly larger than the expected cross-sectional diameter of the fiber bundle, and flexible substrate of any diameter; (or use a multi-core fiber with uniform and stable arrangement and fiber gratings engraved inside).

[0037] (2) Use an adhesive to form the fiber optic grating string 6 into an optical fiber bundle 8 with a central rotational symmetry in cross-section.

[0038] (3) Insert the prepared fiber bundle 8 or multi-core fiber into the hollow tube 7, and fix one end of the fiber bundle 8 to the hollow tube 7 with adhesive, leaving the other end unfixed. The fixed end is usually used as the front end of the sensor, and the pigtail of the unfixed end will be connected to the demodulation equipment.

[0039] The fiber bundle 8 is made using an adhesive that has a certain degree of flexibility after curing. The adhesive can tightly bond the fiber grating strings 6 to form a fiber bundle 7. When the fiber bundle 8 is bent, the inner fiber is compressed and the outer fiber is stretched. The strain of the corresponding grating can be obtained through the real-time spectrum of the fiber grating strings 6. The three-dimensional bending information can be obtained through the strain, thereby reconstructing the shape.

[0040] The sensor diameter can be arbitrarily selected, allowing for the fabrication of a large-diameter sensor. The core sensing element, the fiber optic bundle, is located on the outside of the sensor, its reconstructed shape aligned with the neutral axis of the substrate. Other devices can be integrated within the substrate, or the substrate can be solid. The fiber optic bundle 8 is not directly bonded to the substrate but is housed within a hollow flexible tube 7 bonded to it. Only one end of the fiber optic bundle 8 and the hollow flexible tube 7 is fixed; the other end can move as the substrate bends. When the substrate bends and the fiber optic bundle 8 is on the outside of the bend, the hollow flexible tube 7 is stretched, and the fiber optic bundle 8 does not bear redundant strain from the outside of the substrate, exhibiting a contraction towards the inside of the hollow flexible tube 7. Conversely, when the substrate bends and the fiber optic bundle 8 is on the inside of the bend, the hollow flexible tube 7 is compressed, and the fiber optic bundle 8 extends outwards from the hollow flexible tube 7. This structure can counteract the external strain generated by bending a large-diameter substrate. The fiber optic bundle 8 only needs to sense strain corresponding to its own shape, thus expanding the sensing range and increasing sensor lifespan. Furthermore, the fiber optic bundle mentioned here can be replaced with a multi-core fiber optic cable with corresponding characteristics.

[0041] This invention provides the application of the high-precision three-dimensional shape sensing and reconstruction system combined with fiber Bragg grating inversion technology in medical surgical robots, enabling real-time shape sensing and reconstruction of surgical robots with integrated surgical devices. Applying this system to surgical robots not only allows for direct shape sensing and reconstruction of surgical robots with integrated surgical devices but also significantly improves accuracy.

[0042] Example 1: Shape Sensor

[0043] like Figure 1 , Figure 2 and Figure 3 As shown, the three-dimensional shape sensing and reconstruction system includes: 1 a shape sensor, 2 a demodulation device, 3 a reconstruction device, 4 a shape display, 5 a hollow substrate, 6 a fiber grating string, 7 a hollow tube, 8 a fiber bundle made of three fiber gratings, 9 an acrylic adhesive, and 10 a fiber grating.

[0044] The demodulation device 2 acquires spectral information, the reconstruction device 3 has a built-in reconstruction algorithm that combines deep learning spectral inversion, the hollow substrate 5 is a plastic metal hollow substrate with a diameter greater than or equal to 4mm, the fiber grating string 6 is an optical fiber with a coating layer, 125μm quartz single-mode fiber containing 10 FBGs (6 of which are spaced 10mm apart and the 4 near the front end are spaced 2mm apart), the hollow tube 7 is a PTFE tube with an inner diameter of 6mm, and the fiber grating 10 is a 15mm FBG.

[0045] The shape sensor 1 is fabricated as follows: Three fiber optic grating strings 6 with the same number and position of gratings (one end of which has a dense grating distribution, serving as the sensor front end), acrylic adhesive 9, a hollow flexible tube 7 with an inner diameter of 0.6 mm, and a hollow substrate 5 are prepared. The three fiber optic grating strings 6 are then bound together with acrylic adhesive 9 to form an optical fiber bundle 8 with a centrally rotationally symmetrical cross-section. The fabricated optical fiber bundle 8 is inserted into the hollow flexible tube 7, and the densely grating end of the optical fiber bundle 8 is fixed to the front end of the hollow flexible tube 7 with adhesive, while the other end remains unfixed. The fixed end will serve as the strongly bent portion at the sensor's end, and the unfixed end's pigtail will be connected to a demodulation device.

[0046] Example 2: Application of a high-precision three-dimensional shape sensing and reconstruction system for coarse-diameter hollow substrates

[0047] The application of the shape sensor 1 prepared in Example 1 in the high-precision three-dimensional shape sensing and reconstruction system of the entire large-diameter hollow substrate: such as Figure 1 As shown, the three-dimensional shape sensing and reconstruction system includes the shape sensor 1 prepared in Example 1, the light beam enters the fiber optic grating string 6 from the demodulation system 2, the light beam reflected by the internal fiber optic grating 10 returns to the demodulation system 2 to obtain spectral information, the spectral information is input into the reconstruction system 3, and the shape is reconstructed by the reconstruction system 3 and displayed on the display 4.

[0048] like Figure 1 As shown, the experimental steps of a high-precision three-dimensional shape sensing and reconstruction system for a large-diameter hollow substrate are as follows:

[0049] (1) Install the shape sensor 1 prepared in Example 1 on the thick-diameter hollow substrate to ensure a stable connection;

[0050] (2) The light is introduced into the fiber grating string 6 from the demodulation system 2, and the light signal after being reflected by the fiber grating 10 returns to the demodulation system 2 and then enters the reconstruction system 3;

[0051] (3) The collected light signals are processed by the reconstruction system 3, including two processes: inversion and reconstruction, in order to obtain the three-dimensional shape information perceived by the shape sensor 1.

[0052] (4) Finally, the shape information is displayed on the display 4, allowing the operator to visually observe the shape changes of the thick-diameter hollow substrate.

[0053] It should be noted that the above embodiments are only for further elaboration and understanding of the technical solution of the present invention, and should not be construed as further limiting the technical solution of the present invention. Inventions and creations that do not have prominent substantive features or significant progress made by those skilled in the art still fall within the protection scope of the present invention.

Claims

1. A high-precision three-dimensional shape sensing and reconstruction system combining fiber optic grating inversion technology, comprising a shape sensor (1), a real-time fiber optic spectral demodulation device (2), and a three-dimensional shape reconstruction device combining spectral inversion technology (3), characterized in that: The shape sensor includes a substrate (1), a hollow tube (7) and an optical fiber bundle (8). The hollow tube (7) is fixed to the outer wall of the substrate (1), and the optical fiber bundle (8) is located inside the hollow tube (7). The front end of the optical fiber bundle (8) is fixed to the front end of the hollow tube (7), and the rear end of the optical fiber bundle (8) is a free end and connected to a real-time optical fiber spectral demodulation device (2). The fiber bundle (8) is made of multiple fiber grating strings (6) bonded together in parallel; Each fiber grating string (6) contains multiple fiber gratings (10), which are pre-designed according to the shape that the substrate may have during application. The fiber gratings (10) in the more complex parts have smaller spacing, while the fiber gratings (10) in the rest parts have larger spacing. Three fiber grating strings (6) are bonded together with a flexible and bendable adhesive after curing. The three fiber grating strings (6) are at an angle of 60° to each other when viewed from the cross-section. Alternatively, three or more fiber grating strings can be bonded together, with one fiber as the center and the other fibers distributed around the center fiber.

2. The high-precision three-dimensional shape sensing and reconstruction system combining fiber optic grating inversion technology according to claim 1, characterized in that: The adhesive is an acrylic adhesive (9). Each fiber grating string (6) contains ten fiber gratings (10). The four fiber gratings (10) closest to the front end are spaced 1-3 mm apart, while the remaining fiber gratings (10) are spaced 8-12 mm apart.

3. The high-precision three-dimensional shape sensing and reconstruction system combining fiber optic grating inversion technology according to claim 1, characterized in that: The fiber bundle (8) is a multi-core fiber with uniform and stable arrangement, and multiple gratings are engraved inside the fiber.

4. The high-precision three-dimensional shape sensing and reconstruction system combining fiber optic grating inversion technology according to any one of claims 1-3, characterized in that: The reconstruction algorithm used in the reconstruction device combines spectral inversion technology. The spectrum generated after the grating is bent contains non-uniform strain information of all grating regions. The non-uniform strain on each grating can be obtained from the spectral information using spectral inversion. The spectral inversion method is particle swarm optimization algorithm, simulated annealing algorithm or deep learning method.

5. A method for fabricating a shape sensor, characterized in that... Includes the following steps: (a) Prepare several fiber grating strings (6) or evenly and stably arranged multi-core optical fibers as fiber bundles (8), and prepare hollow tubes (7) with an inner diameter slightly larger than the expected cross-sectional diameter of the fiber bundle, and flexible substrates of any diameter. (ii) The fiber grating string (6) is made into an optical fiber bundle (8) with a central rotational symmetry in cross section by using an adhesive, or the grating is engraved inside a uniformly and stably arranged multi-core optical fiber; the optical fiber bundle (8) is made using an adhesive with a certain degree of flexibility after curing. The adhesive can tightly bond the fiber grating string (6) to form an optical fiber bundle (8). Multiple fiber gratings (10) are distributed in the fiber grating string (6). The fiber gratings (10) near the front end have a smaller spacing, while the fiber gratings (10) at other positions have a larger spacing. (iii) Insert the prepared fiber bundle (8) or multi-core fiber into the hollow tube (7), and use adhesive to fix one end of the fiber bundle (8) or multi-core fiber to the hollow tube (7), while leaving the other end unfixed. The fixed end is usually used as the front end of the sensor, and the pigtail of the unfixed end is used to connect to the demodulation equipment. (iv) Adhere the hollow hose (7) to the outer wall of the substrate.

6. The method for fabricating a shape sensor according to claim 5, characterized in that: Three fiber grating strings (6) are bonded together to form an optical fiber bundle (8). The three fiber grating strings (6) are at an angle of 60° to each other when viewed from the cross section. The grating positions in the three fiber grating strings (6) are aligned before bonding.

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