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

Abstract

The invention discloses an underwater intelligent cable spatial form reconstruction system based on fusion of fiber bragg grating sensing and underwater acoustic positioning, and belongs to the technical field of underwater measurement and sensing. A plurality of fiber bragg grating sensors are inscribed on a three-core fiber in the intelligent cable of the system along the length direction to measure multi-point strain data along the length direction. According to the strain distribution, the bending curvature and the torsion of each measuring point are calculated, a continuous curvature and torsion function is constructed, the tangent vector, the normal vector and the sub-normal vector of each point of the cable are solved in combination with a Freen-Serret algorithm, and the tangent vector is integrated to obtain the space curve form of the cable. And determining the absolute position of the cable terminal in the three-dimensional space according to the acoustic ranging result. And realizing spatial form and attitude recognition of the whole underwater cable through constraint fusion of a curve reconstruction result and an end point position. According to the system, an inertial measurement unit is arranged on a cable to obtain local attitude information, so that the accuracy and the stability of curve reconstruction are improved. The method can be widely applied to space 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, and the cable will form a dynamically changing three-dimensional bending posture under the influence of factors such as sea current, surge, self-weight, drag force, seabed topography and load movement. The spatial form of the cable not only directly determines the position accuracy of the observation equipment and the geometry 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: 1) Many existing schemes rely on discrete installation of positioning or attitude sensors such as several IMUs. The sparse measurement points make it difficult to accurately reflect the local small-scale bending and strain concentration area along the cable, affecting the resolution and reliability of the form reconstruction.

[0004] 2) The absolute positioning constraint of the current cable space form reconstruction system is insufficient. Only relying on relative form estimation or limited local acoustic positioning nodes, it is difficult to provide sufficient absolute coordinate constraint for the whole cable, and the reconstruction result has overall translation and rotation uncertainty, which is difficult to meet the engineering application demand in geographic coordinate system.

[0005] 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 realize online form update and real-time self-correction at high frequency at the same time, which cannot meet the rapid response demand in dynamic working condition.

[0006] 4) Current 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-of-cable point. This leads to the difficulty of accurately connecting the influence of actual seabed engineering deployment, affecting the accuracy and engineering practicability of shape reconstruction. SUMMARY

[0007] 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 grating sensing and underwater acoustic positioning. The underwater intelligent cable spatial shape reconstruction system combines fiber 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.

[0008] The technical scheme adopted by the present application is: an underwater intelligent cable spatial shape reconstruction system based on fusion of fiber 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. 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 for measuring the axial strain at each measurement point on the cable. 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. 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 in the cable cross section, combined with the strain data of each optical fiber core, and obtain the continuous curvature function along the cable arc length parameter s and the torsion function . The curve reconstruction module is used to reconstruct the continuous curvature function and the torsion function Substitute into the 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; 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; 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. 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 reconstruction result of the spatial form of the cable.

[0009] 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.

[0010] Further, the optical fiber demodulation and signal processing module comprises a broadband light source, an optical fiber demodulator and a photoelectric detector, 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:

[0011] 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.

[0012] 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; According to the cross-sectional linear strain model: ; wherein, represents the coordinate in the cross section of the cable axial strain at the position of the cross-section; denotes the axial average strain of the cross-section, denotes the axial average strain of the cross-section, denotes the curvature component of the cross-section in the axial direction; denotes the curvature component of the cross-section in the axial direction; denotes the curvature component of the cross-section in the axial direction; Combining the coordinates of the three-core fiber in the cross-sectional plane , three equations are constructed to solve: ; wherein, , , denote the axial strains of the three fiber cores of the three-core fiber at the same cross-section corresponding to the gratings; denotes the coordinate position of the i-th fiber core relative to the geometric center of the cross-section; is the axial average strain of the cross-section; , is the curvature component of the cross-section; Through matrix inversion, we get , , and calculate the bending curvature: ; wherein, denotes the magnitude of the bending curvature at the cross-section with arc length parameter ; is the arc length parameter along the length direction of the cable; The bending direction angle is: ; wherein, denotes the cross-sectional bending direction angle, which is used to represent the direction of the curvature vector in the cross-sectional coordinate system; atan2() is the four-quadrant arctangent function; The torsion is obtained by the arc length derivative of the bending direction: ; wherein, denotes the torsion at the arc length parameter , which is used to represent the rate of change of the rotation of the bending plane of the cable along the arc length direction; Using cubic spline interpolation on discrete , we obtain continuous curvature and torsion functions.

[0013] Further, the curve reconstruction module is based on the Frenet-Serret equation set: ; The tangent vector T(s), the normal vector N(s) and the binormal vector B(s) of the cable at each point are solved by using a numerical integral algorithm, and the tangent vector T(s) is integrated along the arc length parameter s to restore the three-dimensional spatial curve of the cable.

[0014] 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 locally reconstructed curve to the absolute curve in the geodetic coordinate system. 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 as follows: the start point position is , the end point position is , the start point attitude is , and the end point attitude is ; To obtain the optimal mapping of the local shape to the geodetic coordinate system, a rigid body transformation optimization model is constructed: The rotation matrix is , and the translation vector is ; The position constraint term is ; The attitude constraint term is ; The comprehensive optimization goal is ; wherein is the weight; The optimal solution is obtained by least squares or nonlinear optimization: ; The absolute cable shape in the geodetic coordinate system is obtained, and the final absolute spatial position is ; Further, the inertial measurement unit IMU includes a three-axis gyroscope, a three-axis accelerometer and a three-axis magnetometer, and the IMU is installed at a predetermined position of the cable 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 start point of the cable provides a starting frame for the Frenet-Serret equation.

[0015] 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.

[0016] Further, the system is used for state monitoring of underwater long cable which needs to monitor spatial form and position, and the underwater long cable includes a submarine observation cable, a towed array cable, and a cable-controlled underwater robot umbilical cable.

[0017] Compared with the prior art, the present application has the following beneficial effects: 1) The present application obtains multi-point strain data by using the fiber grating array arranged along the whole length of the cable, and solves the bending curvature and the flexure of the cable by the three-core fiber structure, so that the continuous spatial form perception of the underwater long cable along the length direction is realized, and the limitation of the traditional discrete measurement mode based on a small number of sensors is overcome.

[0018] 2) The present application introduces the Frenet-Serret curve reconstruction method, and converts the curvature and flexure information into a three-dimensional space curve, which has the advantages of clear geometric meaning, high reconstruction precision and strong applicability.

[0019] 3) The present application fuses the absolute coordinates of the cable terminal obtained by the underwater acoustic USBL positioning and the relative curve form obtained by the fiber sensing, so that the unified representation from the relative form to the absolute spatial position is realized, and the positioning accuracy of the underwater intelligent cable is effectively improved.

[0020] 4) The present application provides local attitude constraints of the starting point and the ending point by introducing the IMU, so that the noise and model uncertainty can be inhibited in the complex sea conditions and dynamic environment, the stability and robustness of the reconstruction result under the long-time and long-distance conditions are improved, and the engineering requirements are better met.

[0021] 5) The present application has compact system structure, high integration degree, and is suitable for the submarine observation cable, the towed array cable, the cable-controlled underwater robot umbilical cable and other underwater long cable engineering applications which need to monitor the spatial form and position in real time, and has a wide engineering popularization prospect. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a schematic diagram of the overall structure of the system of the present application.

[0023] Figure 2 It is a schematic diagram of the internal three-core fiber and fiber grating arrangement structure of the intelligent cable assembly of the present application.

[0024] Figure 3 It is a schematic diagram of the USBL underwater acoustic positioning and cable curve fusion positioning of the present application.

[0025] Figure 4 It is a scene diagram of the present application. DETAILED DESCRIPTION

[0026] The specific implementation manner of the present application will be further described in detail below with reference to the drawings.

[0027] The underwater intelligent cable space form reconstruction system 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.

[0028] As 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 inscribed with a plurality of fiber Bragg gratings (FBGs) 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. Meanwhile, inertial measurement units (IMUs) 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.

[0029] As preferred, the fiber demodulation and signal processing module includes a broadband light source, a fiber demodulator, and a photoelectric detection unit, which are 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: The strain sequence of the three-core optical fiber is obtained along the length direction of the cable . The signal processing module performs denoising, temperature compensation, and time synchronization processing on the original strain data to ensure the stability of subsequent form calculation.

[0030] As preferred, the curvature and torsion calculation module inversely solves the two-dimensional curvature components of the cable cross section using the strain difference of the three-core optical fiber. According to the linear strain model of the cross section: Combined with the coordinates of the three-core optical fiber in the cross section plane , three equations are constructed to solve: Through matrix inversion, we get , and calculate the bending curvature:

[0031] The bending direction angle is:

[0032] The torsion is obtained by the arc length derivative of the bending direction:

[0033] For discrete Using cubic spline interpolation, continuous curvature and torsion functions are obtained.

[0034] As preferred, the curve reconstruction module: according to the continuous curvature ​And the torsion The relative spatial configuration of the cable is reconstructed by using the Frenet-Serret differential equations. The equation set is as follows: The Frenet-Serret initial base is provided by the IMU at the starting point of the cable: All the vectors are expressed in the geodetic coordinate system.

[0035] According to and taking the absolute coordinates of the starting point as the initial value, the preliminary three-dimensional configuration of the cable is obtained by integration: In order to maintain numerical orthogonality, the periodic orthogonalization processing is performed on during the integration process.

[0036] As a preferred embodiment, the underwater acoustic positioning module includes a USBL transmitting transducer and an array receiving device installed on the mother ship, and an underwater acoustic transponder installed at the terminal end of the cable. The mother ship transmits acoustic signals, and the terminal end transponder responds after receiving the acoustic signals. The USBL system calculates the three-dimensional spatial position of the terminal end of the cable in the absolute coordinate system according to the time difference of acoustic signal propagation and the phase / time delay difference of the receiving array The coordinates are aligned with the geodetic coordinate system. In addition, the terminal IMU provides the terminal attitude matrix as a spatial attitude constraint.

[0037] As a preferred embodiment, the data fusion module is used to map the locally reconstructed curve to the absolute curve in the geodetic coordinate system.

[0038] First, a local-global rigid body mapping is established, and the local reconstruction result is set as and the tangent vector . The actual measurements of the starting point and the terminal point are as follows: Starting point position: ; terminal point position: ; starting point attitude: ; terminal point attitude: ; To obtain the optimal mapping of the local configuration to the geodetic coordinate system, the present application constructs the following rigid body transformation optimization model: The rotation matrix , and the translation vector .

[0039] The position constraint term ; The attitude constraint term ; The comprehensive optimization objective , wherein is the weight.

[0040] The optimal solution is obtained by least square or nonlinear optimization (such as LM algorithm): .

[0041] The absolute cable shape in the geodetic coordinate system is obtained, and the final absolute spatial position is: .

[0042] The method uses the starting point IMU as an initial attitude anchor point, the terminal USBL as an absolute position calibration, and the terminal IMU to provide directional constraints, so that the reconstructed curve does not drift with the arc length integral, and the whole underwater cable is positioned in the geodetic coordinate system with high precision.

[0043] The present application is described in detail by means of examples, and it is necessary to point out here that the following examples are only used to further illustrate the present application, but are not limited thereto, unless otherwise stated.

[0044] The specific embodiments of the present application are described in detail in combination with the technical solutions: Example 1

[0045] Referring to Figure 1 , the underwater intelligent cable space shape reconstruction system based on fusion of fiber grating sensing and underwater acoustic positioning disclosed in the embodiment 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.

[0046] 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 shape monitoring.

[0047] 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 data processing real-time performance and redundancy capability.

[0048] 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 transponder is arranged at the end of the cable for receiving and responding to the acoustic pulse emitted by the USBL, so as to realize absolute position acquisition of the cable end.

[0049] In addition, multiple inertial measurement units (IMUs) can be optionally distributedly installed on the cable for acquiring local attitude information, so as to improve the robustness of the system under large bending and complex sea conditions.

[0050] Referring to Figure 2, three-core optical fiber is arranged along the length direction of the cable inside the intelligent cable assembly, each core is arranged in an equilateral triangle on the cross section, and the center is coincident with the cable axis. This arrangement has the following advantages: Strong cross-sectional geometric stability: effectively resist the influence of cable extrusion and torsion on the relative position of the three cores.

[0051] Uniform strain sensitivity: the three-point layout on the cross section can completely invert the two-dimensional curvature and bending direction.

[0052] A plurality of FBGs are written on each optical fiber core along the length direction, which are used as a quasi-distributed strain sensing array. According to the system requirements, the grating spacing can be set to 5 m-10 m, preferably 5 m, which can balance the shape reconstruction accuracy and system cost.

[0053] The FBGs and the cable material are fixed by embedded sheath structure to ensure that the optical fiber can truly reflect the stress state of the cable during underwater operation. For deep sea application, pressure-resistant oil filling structure or metal reinforcing layer can be used to improve the pressure stability.

[0054] When the cable is bent and deformed by sea current, hydrodynamic force, self-weight or external force dragging, the three-core optical fiber will produce different strains, which constitute the core basis for inverting the curvature.

[0055] The fiber demodulation and signal processing module injects a broadband ASE light source into the three-core optical 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.

[0056] According to the strain-wavelength sensitivity coefficient , the strain at each grating is calculated:

[0057] The position of the three-core optical fiber core relative to the cross-sectional center is , and the corresponding strain is , and a linear relationship between strain and curvature can be established:

[0058] The cross-sectional two-dimensional curvature components are calculated, and finally the calculation is:

[0059] The torsion is obtained by discrete arc length point difference or spline derivation:

[0060] To avoid noise amplification, the curvature and torsion sequence can be smoothed by Savitzky-Golay filter.

[0061] The curve reconstruction module uses the continuous function obtained by the FBG With and substitute it into the Frenet-Serret equations:

[0062] The IMU installed at the start of the cable provides the initial pose matrix , thus we can get:

[0063] The position integral relationship is:

[0064] This embodiment uses 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.

[0065] Finally, the three-dimensional curve of the cable in the local coordinate system is obtained .

[0066] 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 underwater acoustic transponder installed at the terminal of the cable responds after receiving the signal. 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 of the cable point or the fixed anchoring point position); represents the attitude direction at the starting end of the cable, which is the tangent vector of the cable at the starting end along the arc length direction (initial tangent vector), which can be calculated from the attitude information provided by the inertial measurement unit (IMU) installed at the starting end 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 of the cable at the terminal position along the arc length direction (terminal tangent vector), which can be calculated 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 、 、 、 together as boundary constraint information of the spatial form and absolute positioning of the cable.

[0067] Referring toFigure 4 , the curvature inversion module obtains the curvature based on the strain data obtained by the three-core fiber Bragg grating and the torsion , and substitutes it into the Frenet-Serret equation to solve the local attitude vector of the cable under the arc length parameter s, wherein T(s), N(s) and B(s) represent the tangent vector, the normal vector and the binormal vector of the cable at the arc length position s, respectively; and the initial boundary condition T(0)= (and the corresponding initial N(0) and B(0)) is used for numerical integration to obtain the three-dimensional space curve r(s) (or r(s)) of the cable under 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, so as to realize the morphological reconstruction link of “strain-curvature / torsion-space curve”.

[0068] The data fusion module fuses the curve r(s) of the cable under the local coordinate system obtained by the curve reconstruction module , the absolute coordinates of the terminal 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 under 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 the measurement noise and calculation error, so as to obtain the space morphology of the whole cable under 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, so as to improve the stability and robustness of the reconstruction result under the conditions of complex sea conditions, strong disturbance and large curvature bending.

[0069] 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, the strain distribution of the cable along the length direction can be obtained by measuring the strain data at multiple points on the cable; by combining the curve differential geometry theory, especially the Frenet-Serret equation, the strain information can be converted into curvature and torsion, so as to realize the reconstruction of the three-dimensional space curve of the cable; and the reconstruction result is fused with the absolute position of the cable end point obtained by the ultra-short baseline (USBL) underwater acoustic positioning , , and the end point attitude constraint provided by the IMU is introduced , (And optional middle posture constraint), that is, the high-precision spatial shape and position reconstruction of the underwater intelligent cable can be realized, and the problem that the spatial shape and position of the underwater long cable are difficult to be continuously, accurately and robustly measured in the prior art is solved.

[0070] By collecting the distributed strain along the length direction through the three-core optical fiber, the continuous strain distribution of the cable can be obtained with a meter-level resolution; the curvature and the torsion are inversely deduced through the three-core geometric relationship , and the continuous three-dimensional curve shape of the cable can be obtained through the cubic spline interpolation and the Frenet-Serret equation, which is significantly better than the traditional discrete measurement 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 of the shape and lacks global absolute coordinates, the absolute spatial coordinates of the cable terminal are obtained by using the USBL , and the starting frame (the end point posture constraint with as the core) is obtained by using the IMU, and the starting absolute coordinates , , , are combined as boundary conditions for coordinate transformation and registration of the reconstructed curve, so as to realize the three-dimensional absolute positioning of the whole cable in the geographical coordinate system. 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 posture, so as to accurately obtain the posture information (roll, pitch and yaw) of any point of the cable, and meet the demand of each segment posture information in the scene of the towed array, underwater robot umbilical cable and deep sea observation.

[0071] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but 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: they 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. A system for reconstructing the spatial configuration of an underwater intelligent cable based on the fusion of fiber grating sensing and underwater acoustic positioning, characterized in that, The system comprises an intelligent cable assembly, an optical fiber demodulation and signal processing module, a curvature and flexure calculation module, a curve reconstruction module, an underwater acoustic positioning module and a data fusion module; The intelligent cable assembly is internally provided with a three-core optical fiber along the length direction of the cable, and each optical fiber core of the three-core optical fiber is engraved with a plurality of fiber Bragg grating sensors distributed along the length direction, for measuring the axial strain at each measuring point on the cable; The optical fiber demodulation and signal processing module is used for providing a light source to the three-core optical fiber and receiving the reflected light of each fiber Bragg grating, demodulating the reflected wavelength change of each grating, and calculating the discrete strain value at the position of each grating according to the relationship between the strain and the wavelength change; The curvature and torsion calculation module is configured to calculate the bending curvature and torsion of each measuring point along the length direction of the cable according to the relative geometric positions of the three fiber cores of the three-core optical fiber in the cable cross section, in combination with the strain data corresponding to each fiber core, and obtain a continuous curvature function along the cable arc length parameter s through cubic spline interpolation and a torsion function ​ The curve reconstruction module is configured to substitute the continuous curvature function and the torsion function into a Frenet-Serret equation set, solve tangent vectors, normal vectors and binormal vectors of each point of the cable, and obtain a three-dimensional spatial curve form of the cable under a local coordinate system by integrating the tangent vectors along the arc length parameter. The underwater acoustic positioning module comprises an ultra-short baseline transmitting transducer installed on a mother ship and an underwater acoustic transponder installed 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; The data fusion module is used for constraint fusion of the spatial curve of the cable in 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, to obtain the three-dimensional spatial position and attitude distribution of the whole underwater intelligent cable in the absolute coordinate system; 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.

2. The system of claim 1, wherein, 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.

3. The system of claim 1, wherein, The optical fiber demodulation and signal processing module comprises a broadband light source, an optical fiber demodulator and a photoelectric detector, 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: ； from which the strain value at each grating is calculated; wherein, represents the axial strain of the ith fiber Bragg grating on the fiber core where it is located; represents the Bragg center wavelength of the grating; represents the variation of the Bragg center wavelength relative to the initial calibration wavelength; represents the preset strain-wavelength sensitivity coefficient, i.e., the strain sensitivity coefficient, for characterizing the proportional relationship between the wavelength variation and the strain; i is the grating number or the measurement point number.

4. The system of claim 1, wherein, In the curvature and flexure 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; According to the linear strain model of the cross section: ； wherein, represents the axial strain at the position of the cross-section coordinate ; represents the axial average strain of the cross-section, represents the curvature component of the cross-section in the direction; represents the curvature component of the cross-section in the direction; and are the cross-section coordinates with the cross-section geometric center as the origin. coordinates of the three cores in the cross-sectional plane three equations are constructed to solve for: ； wherein, , , respectively denote the axial strain of the corresponding grating at the same cross-section of the three-core fiber denotes the coordinate position of the i-th fiber core with respect to the geometric center of the cross-section; is the axial average strain of the cross-section; , is the curvature component of the cross-section; By matrix inversion we get , and calculate the bending curvature: ； wherein, represents the magnitude of the bending curvature at a cross-section with arc length parameter is the arc length parameter along the length direction of the cable.​ the bending direction angle is: ； wherein denotes the cross-sectional bending direction angle, which is used to characterize the direction of the curvature vector in the cross-sectional coordinate system; atan2() is the four-quadrant arctangent function; the flexure is obtained through the arc length derivative of the bending direction: ； wherein, denotes the curvature at the arc length parameter denotes the torsion at the arc length parameter s, which is used to characterize the rate of change of the rotation of the cable bending plane along the arc length direction; for discrete Using cubic spline interpolation, continuous curvature and torsion functions are obtained.

5. The system of claim 1, wherein, The curve reconstruction module is based on the Frenet-Serret equation group: ； The numerical integral algorithm is used to solve the tangent vector T(s), the normal vector N(s) and the binormal vector B(s) at each point of the cable, and the tangent vector T(s) is integrated along the arc length parameter s to recover the three-dimensional spatial curve of the cable.

6. The system of claim 1, wherein, 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; the local reconstruction curve is mapped to the absolute curve in the geodetic coordinate; A local-global rigid body mapping is established, where the local reconstruction result is and the tangent vector ; The actual measurements of the start and end points are: start position: , end position: , start attitude: , end attitude: ; In order to obtain the optimal mapping of the local form to the geodetic coordinate, a rigid transformation optimization model is constructed: Rotation matrix , translation vector ; the position constraint term ； the attitude constraint term ； the comprehensive optimization target ； wherein is a weight; The optimal solution is obtained by least squares or nonlinear optimization: ； the absolute cable form in the geodetic coordinate system is obtained, and the final absolute spatial position is: 。 7. The system of claim 1, wherein, The inertial measurement unit (IMU) comprises 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.

8. The system of claim 1, wherein, The output of the system includes a three-dimensional spatial coordinate curve of the cable in the absolute coordinate system along the length direction, an attitude vector at each discrete measuring point, and a spatial form and attitude change sequence of the cable changing with time.

9. The system of any one of claims 1 to 8, wherein, The system is used for state monitoring of an underwater long cable which needs to be monitored in spatial form 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.

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