Underwater intelligent cable space form reconstruction system based on fusion of fiber grating sensing and underwater acoustic positioning

Abstract

The underwater intelligent cable space form reconstruction system based on fusion of fiber grating sensing and underwater acoustic positioning belongs to the technical field of underwater measurement and sensing. A plurality of fiber Bragg grating sensors are written on a three-core optical fiber in the intelligent cable along the length direction to measure the strain data of multiple points along the length direction. The bending curvature and the flex rate at each measuring point are calculated according to the strain distribution, the continuous curvature and flex rate functions are constructed, the tangent vector, the normal vector and the binormal vector of each point of the cable are solved by combining the Frenet-Serret algorithm, and the spatial curve form of the cable is obtained by integrating the tangent vector. The absolute position of the cable terminal in the three-dimensional space is determined according to the acoustic ranging result. The spatial form and attitude of the whole underwater cable are recognized by fusing the curve reconstruction result and the terminal position constraint. An inertial measurement unit is arranged on the cable to obtain local attitude information, so as to improve the accuracy and stability of the curve reconstruction. The present application can be widely applied to the spatial state monitoring of underwater long cable operation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of underwater measurement and sensing technology, in particular to a kind of underwater intelligent cable space form reconstruction system based on fusion of fiber grating sensing and underwater acoustic positioning and its implementation method, which can be used for high-precision measurement and monitoring of the space form and position of underwater long cable. BACKGROUND

[0002] In the application scenarios of ocean observation, seabed resource development, underwater robot operation and towed array measurement, it is often necessary to lay a relatively long underwater composite cable to achieve energy supply, signal transmission and observation equipment deployment. With the development of marine equipment towards deep sea, long distance and high precision, the underwater operation environment becomes more and more complex. Influenced by factors such as sea current, surge, self-weight, drag force, seabed topography and load movement, the cable will form a dynamically changing three-dimensional bending posture underwater. The spatial form of the cable not only directly determines the position accuracy of the observation equipment and the geometric shape of the sensor array, but also affects the safety and reliability of the overall system. For example, in the application of towed array, cable type error will cause the decline of array imaging performance; in underwater robot operation, cable interference and drag force prediction error may cause equipment yaw or collision risk. Therefore, it is an urgent need in the field of ocean engineering, deep sea exploration and intelligent underwater equipment to obtain real-time and accurate three-dimensional spatial position and posture information of underwater cable.

[0003] The existing underwater cable space form reconstruction system has the following problems:

[0004] 1) Many existing schemes currently rely on discrete installation of positioning or attitude sensors such as several IMUs. The sparse measurement points make it difficult to accurately reflect local small-scale bending and strain concentration areas along the cable, affecting the resolution and reliability of the form reconstruction.

[0005] 2) The absolute positioning constraint of the current cable space form reconstruction system is insufficient. Relying only on relative form estimation or limited local acoustic positioning nodes, it is difficult to provide sufficient absolute coordinate constraints for the entire cable, and the reconstruction result has overall translation and rotation uncertainty, which makes it difficult to meet the engineering application requirements in geographic coordinate system.

[0006] 3) The current cable space form reconstruction system is difficult to realize real-time online monitoring and rapid self-correction. Limited by demodulation rate, positioning update frequency and fusion algorithm complexity, many schemes are difficult to simultaneously realize high-frequency online form update and real-time self-correction, which cannot meet the rapid response requirements in dynamic working conditions.

[0007] 4) Current most of the underwater cable shape reconstruction methods lack dynamic adaptability to the actual engineering environment of the cable starting end. The starting frame (initial position and attitude) of the reconstruction algorithm (such as the method based on Frenet-Serret coordinate system) is often fixed or preset, without considering the influence of underwater environment, platform motion or seabed topography on the real-time attitude of the cable out-cable point. This leads to the difficulty of the reconstruction model to accurately connect the influence of the actual seabed engineering deployment, affecting the accuracy and engineering practicability of shape reconstruction. SUMMARY

[0008] The present application aims to solve the problems in the prior art that the underwater cable shape monitoring means is mostly discrete measurement, the positioning information is limited and the error is accumulated seriously, and proposes an underwater intelligent cable spatial shape reconstruction system based on fusion of fiber Bragg grating sensing and underwater acoustic positioning. The underwater intelligent cable spatial shape reconstruction system combines fiber Bragg grating distributed sensing, underwater acoustic positioning, Frenet-Serret curve reconstruction and IMU. The strain distribution is obtained by the FBG array arranged along the cable, the curve reconstruction is carried out by combining the Frenet-Serret equation, and the USBL underwater acoustic positioning result and the IMU attitude information are fused to realize high-precision reconstruction of the three-dimensional spatial shape and absolute position of the underwater long cable.

[0009] The technical scheme adopted by the present application is: an underwater intelligent cable spatial shape reconstruction system based on fusion of fiber Bragg grating sensing and underwater acoustic positioning. The system comprises an intelligent cable assembly, a fiber demodulation and signal processing module, a curvature and torsion calculation module, a curve reconstruction module, an underwater acoustic positioning module and a data fusion module.

[0010] A three-core optical fiber is arranged inside the intelligent cable assembly along the length direction of the cable. A plurality of fiber Bragg grating sensors are written on each optical fiber core of the three-core optical fiber along the length direction, for measuring the axial strain at each measurement point on the cable.

[0011] The fiber demodulation and signal processing module is used to provide a light source to the three-core optical fiber and receive the reflected light of each fiber Bragg grating, demodulate the reflected wavelength change of each grating, and calculate the discrete strain value at each grating position according to the relationship between strain and wavelength change.

[0012] The curvature and torsion calculation module is used to calculate the bending curvature and torsion at each measurement point along the length direction of the cable according to the relative geometric positions of the three optical fiber cores of the three-core optical fiber in the cable cross section, in combination with the strain data corresponding to each optical fiber core, and obtain the continuous curvature function along the cable arc length parameter s and the torsion function ;

[0013] The curve reconstruction module is used to reconstruct the continuous curvature function and the torsion function ​Substitute Frenet-Serret equation set, solve the tangent vector, normal vector and binormal vector of each point of the cable, and through the integral of the tangent vector along the arc length parameter, the three-dimensional space curve form of the cable under the local coordinate system is obtained;

[0014] The underwater acoustic positioning module comprises an ultra-short baseline transmitting transducer mounted on the mother ship and an underwater acoustic transponder mounted at the terminal of the intelligent cable assembly; the underwater acoustic positioning module is used for calculating the spatial position of the cable terminal in the absolute coordinate system according to the propagation time of the transmitting signal and the responding signal and the array receiving characteristics;

[0015] The data fusion module is used for constraint fusion of the spatial curve of the cable under the local coordinate system obtained by the curve reconstruction module and the absolute position of the cable terminal obtained by the underwater acoustic positioning module, so as to obtain the three-dimensional spatial position and attitude distribution of the whole underwater intelligent cable in the absolute coordinate system.

[0016] The system is connected with an inertial measurement unit (IMU) arranged at part of the nodes of the cable, and the inertial measurement unit (IMU) is used for providing local attitude information to correct the spatial form reconstruction result of the cable.

[0017] Further, the fiber Bragg grating sensors are arranged at a preset interval along the length direction of the optical fiber, and the preset interval is adjustable in the range of 5 to 10 meters according to the spatial resolution and the length requirement of the cable.

[0018] Further, the optical fiber demodulation and signal processing module comprises a broadband light source, an optical fiber demodulator and a photodetector, the optical fiber demodulator is used for extracting the reflection spectrum of each fiber Bragg grating and identifying the reflection peak wavelength, and the reflection peak wavelength change and the corresponding strain satisfy a preset strain-wavelength sensitivity relationship:

[0019]

[0020] Thus, the strain value at each grating is calculated; wherein, represents the axial strain of the i-th fiber Bragg grating on the optical fiber core where it is located; represents the Bragg center wavelength of the grating; represents the change amount of the Bragg center wavelength relative to the initial calibration wavelength; represents the preset strain-wavelength sensitivity coefficient, i.e. the strain sensitivity coefficient, which is used to represent the proportional relationship between the wavelength change and the strain; i is the grating number or the measurement point number.

[0021] Further, in the curvature and torsion calculation module, the three optical fiber cores of the three-core optical fiber are arranged in an equilateral triangle in the cross section, and the geometric center coincides with the cable axis;

[0022] According to the cross-sectional linear strain model:

[0023] ;

[0024] wherein, denotes the axial strain at the position of the cross-section coordinate ; denotes the axial average strain of the cross-section, and denote the curvature components of the cross-section in the axis and axis direction, respectively; and are the cross-section coordinates with the cross-section geometric center as the origin;

[0025] The three equations are constructed in combination with the coordinates of the three-core fiber in the cross-section plane ;

[0026] ;

[0027] wherein, , , denote the axial strains of the three fiber cores of the three-core fiber at the same cross-section corresponding to the gratings, respectively; denotes the coordinate position of the i-th fiber core relative to the cross-section geometric center; is the axial average strain of the cross-section; , are the cross-section curvature components;

[0028] The matrix inverse is obtained by , , and the bending curvature is calculated:

[0029] ;

[0030] wherein, denotes the size of the bending curvature at the cross-section with the arc length parameter ; is the arc length parameter along the length direction of the cable;

[0031] The bending direction angle is:

[0032] ;

[0033] wherein, denotes the cross-section bending direction angle, used to represent the direction of the curvature vector in the cross-section coordinate system; atan2() is the four-quadrant arctangent function;

[0034] The torsion is obtained by the arc length derivative of the bending direction:

[0035] ;

[0036] wherein, denotes the torsion at denotes the torsion at

[0037] discrete Using cubic spline interpolation, continuous curvature function and torsion function are obtained.

[0038] Further, the curve reconstruction module reconstructs the cable curve based on the Frenet-Serret equation group:

[0039] ;

[0040] The tangent vector T(s), the normal vector N(s) and the binormal vector B(s) at each point of the cable are solved by using numerical integral algorithm, and the tangent vector T(s) is integrated along the arc length parameter s to recover the three-dimensional space curve of the cable.

[0041] Further, the data fusion module jointly constrains the cable curve reconstructed by the Frenet-Serret equation and the absolute position of the cable terminal obtained by the underwater acoustic positioning; and maps the local reconstructed curve to the absolute curve in the geodetic coordinate.

[0042] A local-global rigid body mapping is established, and the local reconstruction result is set as and the tangent vector ; the actual measurement of the start point and the end point is set as: the start point position: , the end point position: , the start point attitude: , and the end point attitude: ;

[0043] In order to obtain the optimal mapping of the local shape to the geodetic coordinate, a rigid body transformation optimization model is constructed:

[0044] the rotation matrix , and the translation vector ;

[0045] the position constraint term

[0046] ;

[0047] the attitude constraint term

[0048] ;

[0049] the comprehensive optimization target

[0050] ;

[0051] wherein is the weight;

[0052] The optimal solution is obtained by least square or nonlinear optimization:

[0053] ;

[0054] The absolute cable shape in the geodetic coordinate system is obtained, and the final absolute spatial position is:

[0055] ;

[0056] Further, the inertial measurement unit (IMU) includes a three-axis gyroscope, a three-axis accelerometer, and a three-axis magnetometer, the IMU is installed at a preset position of the cable, and is used to measure the attitude angle and acceleration information at the position, the data fusion module compares and corrects the attitude information output by the IMU with the local attitude vector calculated based on the Frenet-Serret equation, and the inertial measurement unit (IMU) installed at the starting point of the cable provides a starting frame for the Frenet-Serret equation.

[0057] Further, the output results of the system include a three-dimensional spatial coordinate curve of the cable in the absolute coordinate system along the length direction, an attitude vector at each discrete measurement point, and a spatial shape and attitude change sequence of the cable over time.

[0058] Further, the system is used for state monitoring of an underwater long cable that needs to be monitored in terms of spatial shape and position, and the underwater long cable includes a seabed observation cable, a towed acoustic array cable, and a cable-controlled underwater robot umbilical cable.

[0059] Compared with the prior art, the present application has the following beneficial effects:

[0060] 1) The present application uses a fiber grating array arranged along the entire length of the cable to obtain multi-point strain data, and solves the bending curvature and deflection of the cable through a three-core fiber structure, thereby realizing continuous spatial shape perception of the underwater long cable along the length direction and overcoming the limitations of the traditional discrete measurement method based on a small number of sensors.

[0061] 2) The present application introduces a Frenet-Serret curve reconstruction method to convert the curvature and deflection information into a three-dimensional spatial curve, which has the advantages of clear geometric meaning, high reconstruction accuracy, and strong applicability.

[0062] 3) The present application fuses the absolute coordinates of the cable terminal obtained by the underwater acoustic USBL positioning and the relative curve shape obtained by the fiber sensing, realizes unified representation from the relative shape to the absolute spatial position, and effectively improves the positioning accuracy of the underwater intelligent cable.

[0063] 4) The present application provides local attitude constraints of the starting point and the ending point by introducing IMU, which can suppress noise and model uncertainty in complex sea conditions and dynamic environment, improve the stability and robustness of the reconstruction result under long time and long distance conditions, and meet the engineering requirements better.

[0064] 5) The system structure of the present application is compact and has high integration, which is suitable for submarine observation cable, towed array cable, cable-controlled underwater robot umbilical cable and other underwater long cable engineering applications that need to monitor the spatial form and position in real time, and has a wide engineering promotion prospect. BRIEF DESCRIPTION OF DRAWINGS

[0065] Figure 1 It is a schematic diagram of the overall structure of the system of the present application.

[0066] Figure 2 It is a schematic diagram of the internal three-core optical fiber and fiber grating layout structure of the intelligent cable assembly of the present application.

[0067] Figure 3 It is a schematic diagram of USBL underwater acoustic positioning and cable curve fusion positioning of the present application.

[0068] Figure 4 It is a scene diagram of the present application. DETAILED DESCRIPTION

[0069] The specific embodiments of the present application will be further described in detail below with reference to the accompanying drawings.

[0070] The underwater intelligent cable spatial form reconstruction system based on fusion of fiber grating sensing and underwater acoustic positioning includes an intelligent cable assembly, an optical fiber demodulation and signal processing module, a curvature and flexural rigidity calculation module, a curve reconstruction module, an underwater acoustic positioning module and a data fusion module.

[0071] As a preferred, the intelligent cable assembly includes a cable body sheath, an internal load-bearing structure and an embedded three-core optical fiber. The three-core optical fiber is engraved with a plurality of fiber Bragg gratings (FBG) along the length direction of the cable for obtaining axial strain information. The three-core optical fiber is arranged in an equilateral triangle in the cross section of the cable, and its geometric position is known, so that the strain of the three optical fiber cores can be used to calculate the bending deformation state of the cross section. At the same time, inertial measurement units (IMU) are installed at the starting point and the ending point of the cable respectively for collecting the attitude information (roll, pitch and yaw angles) of the cable endpoints in the earth coordinate system.

[0072] As a preferred, the optical fiber demodulation and signal processing module includes a broadband light source, an optical fiber demodulator and a photoelectric detection unit, which is used to obtain the reflection spectrum of each grating in real time and extract the center wavelength change. According to the linear relationship between the grating wavelength change and the strain:

[0073] ​Obtaining strain sequence of three-core fiber distributed along the length of the cable The signal processing module denoises, temperature compensates and time synchronizes the original strain data, ensuring the stability of the subsequent shape calculation.

[0074] As a preferred, the curvature and torsion calculation module inversely solves the two-dimensional curvature component of the cable section using the strain difference of the three-core fiber. According to the linear strain model of the section:

[0075] Combined with the coordinates of the three-core fiber in the section plane , three equations are constructed to solve:

[0076] Through matrix inversion, we get , and calculate the bending curvature:

[0077]

[0078] The bending direction angle is:

[0079]

[0080] The torsion is obtained by the arc length derivative of the bending direction:

[0081]

[0082] For discrete Using cubic spline interpolation, continuous curvature and torsion functions are obtained.

[0083] As a preferred, the curve reconstruction module: according to the continuous curvature and torsion , the relative spatial shape of the cable is reconstructed using the Frenet-Serret differential equation. The equation set is:

[0084] The Frenet-Serret starting base is provided by the starting point IMU of the cable:

[0085] All vectors are expressed in the geodetic coordinate system.

[0086] According to and the absolute coordinates of the starting point as the initial value, the preliminary three-dimensional shape of the cable is obtained: In order to maintain numerical orthogonality, periodic orthogonalization processing is performed on during integration.

[0087] As preferred, the underwater acoustic positioning module comprises a USBL transmitting transducer and an array receiving device installed on the mother ship, and an underwater acoustic transponder installed on the cable terminal. The mother ship transmits acoustic signals, and the cable terminal transponder responds after receiving the acoustic signals. The USBL system calculates the three-dimensional spatial position of the cable terminal in the absolute coordinate system according to the time difference of acoustic signal propagation and the phase / time delay difference of the receiving array , which is aligned with the geodetic coordinate system. In addition, the end point IMU provides an end point attitude matrix as a spatial attitude constraint.

[0088] As preferred, the data fusion module is used to map the local reconstruction curve to the absolute curve in the geodetic coordinate system.

[0089] First, a local-global rigid body mapping is established, assuming that the local reconstruction result is and the tangent vector is . The actual measurements of the start point and the end point are:

[0090] The start point position is: ; the end point position is: ; the start point attitude is: ; and the end point attitude is: ;

[0091] To obtain the optimal mapping of the local shape to the geodetic coordinate system, the present application constructs the following rigid body transformation optimization model:

[0092] The rotation matrix is , and the translation vector is .

[0093] The position constraint term is ;

[0094] The attitude constraint term is ;

[0095] The comprehensive optimization objective is , wherein is the weight.

[0096] The optimal solution is obtained by least squares or nonlinear optimization (such as the LM algorithm): .

[0097] The absolute cable shape in the geodetic coordinate system is obtained, and the final absolute spatial position is:

[0098] .

[0099] This method uses the start point IMU as the initial attitude anchor point, the end point USBL as the absolute position calibration, and the end point IMU to provide directional constraints, so that the reconstructed curve does not drift with arc length integration, and the entire underwater cable is positioned in the geodetic coordinate system with high precision in three dimensions.

[0100] The present application is described in detail by examples, it is necessary to point out here that the following examples are only used to further illustrate the present application, but not limited thereto, wherein unless otherwise stated.

[0101] The specific embodiments of the present application are described in detail below in combination with the technical solutions: Example 1

[0102] Referring to Figure 1 The underwater intelligent cable space form reconstruction system disclosed in the embodiment based on fusion of fiber grating sensing and underwater acoustic positioning includes an intelligent cable assembly, a fiber demodulation and signal processing module, a curvature and torsion calculation module, a curve reconstruction module, an underwater acoustic positioning module, and a data fusion module.

[0103] The intelligent cable assembly is laid along an underwater operation path, one end of which is fixedly connected to a mother ship, a work ship or an offshore platform, and the other end of which can be connected to a towed acoustic array, a deep sea sensor, a laying device or other underwater bodies. The intelligent cable bears the functions of data transmission, force bearing and self-form monitoring.

[0104] The fiber demodulation and signal processing module, the curvature and torsion calculation module, the curve reconstruction module and the data fusion module can be centrally deployed in a mother ship cabinet, or can be distributedly deployed in combination with a cable-controlled robot, a seabed node and the like, so as to improve the real-time performance and redundancy capability of data processing.

[0105] The USBL array is fixedly installed at the bottom of the mother ship as a key part of the underwater acoustic positioning module, forming a known array element coordinate system. A hydroacoustic transponder is arranged at the end of the cable, which is used for receiving and responding to the acoustic pulses emitted by the USBL, so as to realize absolute position acquisition of the cable end.

[0106] In addition, multiple inertial measurement units (IMUs) can be optionally distributedly installed on the cable, which are used for acquiring local attitude information and improving the robustness of the system under large bending and complex sea conditions.

[0107] Referring to Figure 2 The three-core optical fiber is arranged inside the intelligent cable assembly along the length direction of the cable, and each core is arranged in an equilateral triangle on the cross section, with the center coinciding with the cable axis. This arrangement has the following advantages:

[0108] Strong cross-sectional geometric stability: can effectively resist the influence of cable extrusion and torsion on the relative position of the three cores.

[0109] Uniform strain sensitivity: the three-point layout on the cross section can completely invert the two-dimensional curvature and bending direction.

[0110] A number of FBGs are inscribed along the length of each fiber core, serving as a quasi-distributed strain sensing array. The grating pitch can be set to 5-10 m, preferably 5 m, to balance the reconstruction accuracy and system cost.

[0111] The FBGs are fixed to the cable material using an embedded sheath structure, ensuring that the fiber can accurately reflect the stress state of the cable during underwater operation. For deep-sea applications, pressure-resistant oil filling structures or metal reinforcing layers can be used to improve pressure stability.

[0112] When the cable is bent and deformed due to sea currents, water power, self-weight, or external force dragging, the three-core fiber will produce different strains, forming the core basis for inverting the curvature.

[0113] The fiber demodulation and signal processing module injects a broadband ASE light source into the three-core fiber, and the demodulator reads the grating reflection spectrum in real time. Through high-resolution wavelength measurement algorithms (such as polynomial fitting, centroid extraction, reflection peak fitting, etc.), the reflection peak center wavelength and its change are obtained.

[0114] According to the strain-wavelength sensitivity coefficient , the strain at each grating is calculated:

[0115]

[0116] Let the three-core fiber core relative to the cross-sectional center position be , and the corresponding strain be , a linear relationship between strain and curvature can be established:

[0117]

[0118] The two-dimensional curvature component of the cross-section is calculated, and finally:

[0119]

[0120] The torsion is obtained by discrete arc length point difference or spline derivation:

[0121]

[0122] To avoid noise amplification, the curvature and torsion sequences can be smoothed using the optional Savitzky-Golay filter.

[0123] The curve reconstruction module uses the continuous functions and obtained by FBG and substitutes them into the Frenet-Serret equation set:

[0124]

[0125] IMU mounted at the start of the cable provides the initial pose matrix , thus we have

[0126]

[0127] The position integral relationship is:

[0128]

[0129] This embodiment adopts the fourth-order Runge-Kutta (RK4) method for solving, and performs Gram-Schmidt orthogonalization at intervals to ensure that the tangent vector, normal vector and binormal vector remain orthogonal.

[0130] Finally, the three-dimensional curve of the cable in the local coordinate system is obtained .

[0131] Referring to Figure 3 , the underwater acoustic positioning module transmits acoustic signals to the underwater using an ultra-short baseline (USBL) array installed at the bottom of the mother ship, and the hydroacoustic transponder installed at the terminal of the cable responds after receiving the signals. The USBL array calculates the three-dimensional position coordinates of the cable terminal in the earth / absolute coordinate system according to the time difference between the transmitted and received signals, the phase difference or time delay difference of each array element . Among them represents the spatial position coordinates of the intelligent cable assembly starting end in the earth coordinate system (such as the mother ship out-cable point or fixed anchoring point position); represents the attitude direction at the start of the cable, which is the tangent vector (initial tangent vector) of the cable along the arc length direction at the start, which can be converted from the attitude information provided by the inertial measurement unit (IMU) installed at the start represents the spatial position coordinates of the cable terminal in the earth coordinate system, which is obtained by USBL positioning; represents the attitude direction at the terminal of the cable, which is the tangent vector (terminal tangent vector) of the cable along the arc length direction at the terminal position, which can be converted from the attitude information of the terminal IMU, or estimated from the curve reconstruction result near the terminal in the case where the terminal IMU is not configured. The above , , , serve as boundary constraint information for the spatial form and absolute positioning of the cable.

[0132] Referring to Figure 4 , the curve reconstruction module inversely obtains the curvature and the torsion and substituting it into the Frenet-Serret equation to solve the local attitude vector of the cable at the arc length parameter s, wherein T(s), N(s), and B(s) represent the tangent vector, normal vector, and binormal vector of the cable at the arc length position s, respectively; and taking the initial boundary condition T(0) = T0 and numerically integrating the corresponding initial N(0), B(0) to obtain the three-dimensional space curve r(s) (or denoted as r(s)) of the cable in the local coordinate system. Further, the tangent vector T(s) is integrated along the arc length to obtain the continuous space trajectory of the cable, thereby realizing the reconstruction link of the “strain-curvature / torsion-space curve” mode.

[0133] The data fusion module fuses the curve r(s) of the cable in the local coordinate system obtained by the curve reconstruction module with the absolute coordinates of the terminal end obtained by the USBL positioning and the absolute coordinates of the starting end , the end point attitude constraint , , and maps the local curve to the absolute curve in the geodetic coordinate system by solving the rigid transformation of the rotation matrix R and the translation vector t. Preferably, the fusion process can use least squares or extended Kalman filtering to suppress measurement noise and calculation error, thereby obtaining the space mode of the entire cable in the absolute coordinate system. If an IMU is further installed at a part of the cable, the data fusion module can also compare and constrain the attitude angle output by the IMU with the local attitude vector {T(s), N(s), B(s)} obtained by the Frenet-Serret equation, and make corresponding corrections to the reconstructed curve, thereby improving the stability and robustness of the reconstruction result under complex sea conditions, strong disturbance, and large curvature bending conditions.

[0134] The fiber Bragg grating (FBG) sensing technology has the advantages of high sensitivity, anti-electromagnetic interference, and strong multiplexing capability. By embedding an FBG array in the underwater cable and measuring the strain data at multiple points on the cable, the strain distribution of the cable along the length direction can be obtained. Combined with the theory of curve differential geometry, especially the Frenet-Serret equation, the strain information can be converted into curvature and torsion, thereby realizing the reconstruction of the three-dimensional space curve of the cable. By fusing the reconstruction result with the absolute position of the cable end point obtained by the ultra-short baseline (USBL) underwater acoustic positioning , and introducing the end point attitude constraint provided by the IMU , (and optional middle attitude constraint), the high-precision space mode and position reconstruction of the underwater intelligent cable can be realized, thereby solving the problem of continuous, accurate, and robust measurement of the space mode and position of the underwater long cable in the prior art.

[0135] The present application can obtain continuous strain distribution of the cable with meter-level resolution by collecting distributed strain along the length direction through the three-core optical fiber; the curvature is deduced through the three-core geometric relationship and the torsion , and the continuous three-dimensional curve shape of the cable can be obtained by solving the cubic spline interpolation and the Frenet-Serret equation, which is significantly better than the traditional discrete point scheme. Further, in order to realize the absolute positioning of the cable in the geographical coordinate system, in view of the problem that the traditional optical fiber sensing can only obtain the relative geometric shape and lacks global absolute coordinates, the present application uses the USBL to obtain the absolute spatial coordinates of the cable terminal , and uses the IMU to obtain the starting frame (the end point attitude constraint taking as the core), and combines the absolute coordinates of the starting point to realize the three-dimensional absolute positioning of the whole cable in the geographical coordinate system by taking , , , as the boundary conditions for coordinate transformation and registration of the reconstructed curve. The reconstruction result of the present application can directly give T(s), N(s) and B(s), and can be fused with the local IMU attitude, so as to accurately obtain the attitude information (roll, pitch and yaw) of any point of the cable, and meet the demand of each segment attitude information in the scene of towed array, underwater robot umbilical cable and deep sea observation.

[0136] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. An underwater intelligent cable spatial morphology reconstruction system based on fiber optic grating sensing and underwater acoustic positioning fusion, characterized in that, The system includes a smart cable assembly, an optical fiber demodulation and signal processing module, a curvature and deflection calculation module, a curve reconstruction module, an underwater acoustic positioning module, and a data fusion module. The smart cable assembly has a three-core optical fiber inside along the length of the cable. Each fiber core of the three-core optical fiber is engraved with several fiber Bragg grating sensors distributed along the length direction, which are used to measure the axial strain at each measuring point on the cable. The fiber demodulation and signal processing module is used to provide a light source to the three-core fiber and receive the reflected light from each fiber Bragg grating, demodulate the reflection wavelength change of each grating, and calculate the discrete strain value at each grating position based on the relationship between strain and wavelength change. The curvature and deflection calculation module is used to calculate the bending curvature and deflection at each measuring point along the length of the cable based on the relative geometric positions of the three fiber cores within the cable cross-section and the strain data corresponding to each fiber core. It also obtains a continuous curvature function along the cable arc length parameter s through cubic spline interpolation. and torsion function ; The curve reconstruction module is used to convert continuous curvature functions and torsion function Substituting into the Frenet–Serret equations, we solve for the tangent vector, normal vector, and binormal vector at each point on the cable. By integrating the tangent vector along the arc length parameter, we obtain the three-dimensional spatial curve shape of the cable in the local coordinate system. The underwater acoustic positioning module includes an ultra-short baseline transmitting transducer installed on the mother ship and an underwater acoustic transponder installed at the terminal of the smart cable assembly; the underwater acoustic positioning module is used to calculate the spatial position of the cable terminal in the absolute coordinate system based on the propagation time of the transmitted signal and the response signal and the array receiving characteristics. The data fusion module is used to constrain and fuse the spatial curve in the local coordinate system of the cable obtained by the curve reconstruction module with the absolute position of the cable terminal obtained by the underwater acoustic positioning module to obtain the three-dimensional spatial position and attitude distribution of the entire underwater smart cable in the absolute coordinate system. The system is connected to inertial measurement units (IMUs) deployed at some nodes of the cable. The IMUs are used to provide local attitude information to correct the cable spatial morphology reconstruction results.

2. The system according to claim 1, characterized in that, The fiber Bragg grating sensor is arranged at a preset spacing along the length of the fiber, and the preset spacing is adjustable within the range of 5 to 10 meters according to the spatial resolution and cable length requirements.

3. The system according to claim 1, characterized in that, The fiber demodulation and signal processing module includes a broadband light source, a fiber demodulator, and a photodetector. The fiber demodulator is used to extract the reflection spectrum of each fiber Bragg grating and identify the reflection peak wavelength. The change in reflection peak wavelength and the corresponding strain satisfy a preset strain-wavelength sensitivity relationship. ； The strain value at each grating is then calculated. in, This represents the axial strain of the i-th fiber Bragg grating on its fiber core. This indicates the Bragg center wavelength of the grating; This indicates the amount of change in the Bragg center wavelength relative to the initial calibration wavelength; This represents the preset strain-wavelength sensitivity coefficient, which is used to characterize the proportional relationship between wavelength change and strain; i is the grating number or measurement point number.

4. The system according to claim 1, characterized in that, In the curvature and deflection calculation module, the three fiber cores of the three-core optical fiber are arranged in an equilateral triangle in the cross-section, and their geometric center coincides with the cable axis. Based on the linear strain model of the cross section: ； in, The coordinates within the cable cross-section are: Axial strain at the location; This represents the average axial strain of the cross section. and They represent the cross sections at... shaft and Curvature components in the axial direction; and The coordinates of the cross section are taken as the origin from the geometric center of the cross section. Combining the coordinates of the three-core optical fiber in the cross-sectional plane Solve by constructing three equations: ； in, , , These represent the axial strain of the grating corresponding to the three fiber cores at the same cross-section in a three-core optical fiber. This represents the coordinate position of the i-th fiber core relative to the geometric center of the cross section; The average strain along the cross section; , The curvature component of the cross section; Obtained by matrix inversion , And calculate the curvature: ； in, The arc length parameter is The magnitude of the curvature at the cross-section; This is the arc length parameter along the length of the cable; The bending direction angle is: ； in, The curvature direction angle of the cross section is used to characterize the direction of the curvature vector in the cross section coordinate system; atan2() is the arctangent function in the four quadrants; The torsion is obtained by the derivative of the arc length in the bending direction: ； in, The arc length parameter is The deflection at a point is used to characterize the rate of rotational change of the cable bending plane along the arc length direction; For discrete Using cubic spline interpolation, the continuous curvature function and torsion function are obtained.

5. The system according to claim 1, characterized in that, The curve reconstruction module is based on the Frenet–Serret equation system: ； The numerical integration algorithm is used to solve for the tangent vector T(s), normal vector N(s), and binormal vector B(s) at each point of the cable, and the three-dimensional spatial curve of the cable is recovered by integrating the tangent vector T(s) along the arc length parameter s.

6. The system according to claim 1, characterized in that, The data fusion module jointly constrains the cable curve reconstructed by the Frenet–Serret equation with the absolute position of the cable terminal obtained by underwater acoustic positioning; and maps the locally reconstructed curve to the absolute curve in geodetic coordinates. Establish a local-global rigid body mapping, and let the local reconstruction result be... tangent vector ; Let the actual measurements of the starting and ending points be: Starting point location: Destination location: Starting position: Final posture: ; To obtain the optimal mapping from local shape to geodetic coordinates, a rigid body transformation optimization model is constructed: Rotation matrix Translation vector ; Position constraint terms ； Attitude constraints ； Comprehensive optimization objectives ； in As weight; The optimal solution can be obtained through least squares or nonlinear optimization. ； The absolute cable shape in the geodetic coordinate system is obtained, and the final absolute spatial position is: 。 7. The system according to claim 1, characterized in that, The inertial measurement unit (IMU) includes a three-axis gyroscope, a three-axis accelerometer, and a three-axis magnetometer. The IMU is installed at a preset position on the cable to measure the attitude angle and acceleration information at that position. The data fusion module compares and corrects the attitude information output by the IMU with the local attitude vector calculated based on the Frenet–Serret equation. The inertial measurement unit installed at the starting point of the cable provides a starting frame for the Frenet–Serret equation.

8. The system according to claim 1, characterized in that, The system outputs include the three-dimensional spatial coordinate curve of the cable along its length in the absolute coordinate system, the attitude vector at each discrete measurement point, and the spatial morphology and attitude change sequence of the cable over time.

9. The system according to any one of claims 1 to 8, characterized in that, This system is used for status monitoring of long underwater cables that require monitoring of their spatial morphology and location. These long underwater cables include seabed observation cables, towed acoustic array cables, and cable-controlled underwater robot umbilical cables.

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