Optical fiber shape sensor, measuring device and method for high-sensitivity torsion direction recognition

Abstract

The present invention relates to the field of optical fiber sensing, and discloses an optical fiber shape sensor, a measuring device and a method for high-sensitivity torsion direction recognition. The optical fiber shape sensor is formed by spirally winding a single-mode optical fiber on the surface of a multi-core optical fiber and encapsulating it. The measuring device includes an optical fiber shape sensor, a fan-in fan-out device, an optical switch, an optical frequency domain reflectometer and a data acquisition and processing device. The single-mode optical fiber is used as a high-sensitivity torsion measurement optical fiber, and is combined with an optical frequency domain reflectometer to achieve high-precision demodulation of torsion direction and torsion strain, which is used to compensate for the torsion error of the multi-core optical fiber shape and achieve high-precision reconstruction of the three-dimensional shape of the sensor. The present invention has the excellent characteristics of high torsion measurement sensitivity and the ability to identify torsion direction, which greatly reduces the production cost of high-precision optical fiber shape sensors. The present invention can be applied to the fields of intraoperative navigation of medical endoscopes, shape monitoring and navigation of interventional surgical catheters, posture tracking and control of aircraft and intelligent robots, and deformation monitoring of geotechnical structures.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to a low-cost, high-sensitivity optical fiber shape sensor with identifiable torsion direction, a measuring device and a method. Background Art

[0002] At present, the application of fiber optic shape sensing technology in the fields of medicine, aerospace, intelligent robots and structural health monitoring is becoming more and more mature. For example, in the medical field, the intraoperative shape navigation of continuum robots and endoscopes, the tracking feedback control of the shape of drone wings or robotic arms, and the deformation monitoring of geotechnical structures, etc. Compared with traditional visual imaging, X-ray imaging and electromagnetic positioning technology, fiber optic shape sensing technology has the advantages of high sensing resolution, anti-electromagnetic interference, small size, easy integration and suitability for harsh environments. Fiber optic shape sensing technology based on optical frequency domain reflectometer (OFDR) has the ability to resist electromagnetic interference, distributed, hundreds of microns sensing resolution, and monitor millimeter or even sub-millimeter shape errors, and has gradually become a research hotspot in this field.

[0003] However, this technology still has difficulties to overcome in practical applications. In most application scenarios, the axial torsion of the object being measured and the natural torsion of the multi-core optical fiber shape sensor when it is bent will cause the traditional optical fiber bending-strain model to be inaccurate. The torsional strain and torsion angle caused by the above torsion make the traditional shape reconstruction algorithm seriously mismatched, and the shape tracking accuracy is greatly reduced. In order to compensate for the errors caused by torsion, the sensitivity of torsional strain measurement in shape sensing is usually required to be high. At the same time, torsion is directional, and ordinary multi-core optical fibers or optical fiber bundles are not only difficult to achieve high-precision and high-sensitivity torsional strain measurement, but also unable to identify the torsion direction. Therefore, the key to achieving high-precision shape monitoring is to develop a high-sensitivity torsional direction-recognizable optical fiber shape sensor for accurate demodulation and compensation of torsional effects.

[0004] Currently, the publicly available fiber optic shape sensors that can identify the torsion direction include commercial spiral multi-core optical fibers and spiral fiber bundles. The former is obtained by adding axial torsion during the process of drawing multi-core optical fibers from preform rods; the latter is obtained by tightly winding multiple single-mode optical fibers in a spiral shape. Although the above two solutions can identify the torsion direction, their torsional strain measurement sensitivity is not high. In addition, the existing commercial spiral multi-core optical fibers and spiral fiber bundle shape sensors need to write continuous weak gratings on each fiber core to enhance the intensity of backward Rayleigh scattering, require special structural packaging, and may have a reflection spectrum splitting phenomenon caused by large curvature bending, resulting in a certain limitation on the minimum bending radius. At the same time, the price cost of a single shape sensor exceeds 10,000 yuan, which seriously limits the application scope of fiber optic shape sensing. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a high-sensitivity optical fiber shape sensor, a measuring device and a method that can identify the torsion direction, which can perform high-sensitivity, accurate measurement and compensation for torsion, while greatly reducing the production cost of high-precision optical fiber shape sensors.

[0006] In order to solve the above technical problems, the present invention first proposes a high-sensitivity optical fiber shape sensor for identifying torsion direction, including a single-mode optical fiber and a multi-core optical fiber, the single-mode optical fiber is spirally wound on the surface of the multi-core optical fiber, the winding section of the single-mode optical fiber and the multi-core optical fiber in the winding section are encapsulated in a flexible material, the right ends of the spiral single-mode optical fiber and the multi-core optical fiber are both closed by flexible materials, and the left ends of the spiral single-mode optical fiber and the multi-core optical fiber are simultaneously used as the optical signal input and output ends of the sensor.

[0007] Furthermore, the middle core of the multi-core optical fiber is located at the central axis of the sensor.

[0008] Preferably, the smaller the pitch of the helical single-mode optical fiber, the higher the torsional strain measurement sensitivity of the optical fiber shape sensor.

[0009] Further preferably, the larger the distance between the central axes of the spiral single-mode optical fiber and the multi-core optical fiber, the higher the torsional strain measurement sensitivity of the optical fiber shape sensor.

[0010] Based on the above-mentioned optical fiber shape sensor, the present invention further provides a high-sensitivity measurement device for identifying torsion direction, comprising the optical fiber shape sensor, a fan-in and fan-out device, an optical switch, an optical frequency domain reflectometer, and a data acquisition processing and display device; the output end of the multi-core optical fiber is connected to the optical switch through the fan-in and fan-out device, the output end of the single-mode optical fiber is connected to the optical switch, the output end of the optical switch is connected to the optical frequency domain reflectometer, and the optical frequency domain reflectometer is connected to the data acquisition processing device through an acquisition channel and a trigger channel respectively;

[0011] The fan-in and fan-out devices are used to realize the optical coupling function from a single multi-core optical fiber to multiple single-mode optical fibers;

[0012] The optical switch is used to sequentially switch and interrogate each fiber core channel of the sensor;

[0013] The optical frequency domain reflectometer is used to measure the strain distribution of each core of a spiral single-mode optical fiber and a multi-core optical fiber;

[0014] The data acquisition and processing equipment is used to acquire data and reconstruct the shape of the optical fiber shape sensor after data processing.

[0015] Furthermore, the measuring device also includes a display device, which is connected to the output end of the data acquisition and processing device and is used to display the reconstructed shape of the optical fiber shape sensor.

[0016] The present invention also provides a measuring method based on the above-mentioned high-sensitivity measuring device for identifying torsion direction, comprising the following steps:

[0017] Step I: obtaining the torsional strain and torsional angle of each core of the spiral single-mode optical fiber and the multi-core optical fiber by measuring the strain curve of each core of the spiral single-mode optical fiber and the multi-core optical fiber;

[0018] Step II, performing torsion angle compensation and torsion strain compensation on the shape of the multi-core optical fiber before reconstruction, and obtaining the bending direction angle and bending curvature of the multi-core optical fiber after torsion compensation;

[0019] Step III: According to the bending direction angle and the bending curvature, a shape reconstruction algorithm is used to solve the spatial coordinates of each sensing point in turn, and reconstruct the three-dimensional shape of the sensor after torsion compensation.

[0020] Specifically, the step I specifically includes the sub-steps:

[0021] 1-1) When the entire sensor is in a straight state, the Rayleigh scattering beat frequency information of each core of the spiral single-mode optical fiber and the multi-core optical fiber is collected as the reference group signal; when the sensor is bent and twisted, the beat frequency information of each core of the spiral single-mode optical fiber and the multi-core optical fiber is collected as the measurement group signal;

[0022] 1-2) Using coherent demodulation or phase demodulation for the reference group signal and the measurement group signal, obtain the strain curve of each core of the helical single-mode optical fiber and the multi-core optical fiber;

[0023] 1-3) Accurately demodulate the distribution of torsional strain of helical single-mode optical fiber from the total strain curve of helical single-mode optical fiber;

[0024] 1-4) The torsional angle distribution of the helical single-mode optical fiber is obtained from the torsional strain distribution of the helical single-mode optical fiber.

[0025] Specifically, the step II specifically includes the sub-steps:

[0026] 2-1) Since the spiral single-mode optical fiber and the multi-core optical fiber in the same cross section will be twisted synchronously, the twist angles of the two are the same, so the twist angle of the multi-core optical fiber is compensated according to the twist angle of the spiral single-mode optical fiber;

[0027] 2-2) Infer the torsional strain of the multi-core optical fiber based on the torsional angle of the multi-core optical fiber;

[0028] 2-3) According to the obtained strain curves, torsional strains and torsional angles of each core of the multi-core optical fiber, multiple appropriate cores are selected to form a core triplet, and a bending, torsional-total strain decoupling model of the multi-core optical fiber core triplet is established;

[0029] 2-4) The torsional strain and torsional angle of the multi-core optical fiber are brought into the model for compensation, and the bending direction angle and bending curvature of the multi-core optical fiber after torsion compensation are obtained.

[0030] Optimally, after obtaining the bending direction angle and bending curvature of the multi-core optical fiber after torsion compensation, interpolation processing is performed on the bending direction angle and the bending curvature.

[0031] The high-sensitivity fiber shape sensor and measuring device capable of identifying torsion direction of the present invention adopts the composite packaging method of spirally winding the above-mentioned single-mode optical fiber on the surface of a multi-core optical fiber, and combines OFDR technology to measure and compensate the axial torsion strain and torsion direction of the fiber shape sensor, thereby realizing high-precision reconstruction of the three-dimensional shape of the sensor or the object to be measured. Compared with existing commercial shape sensors, this shape sensor not only improves the torsion measurement sensitivity, but also can accurately measure and compensate for torsion strain and torsion direction, while also greatly reducing the production cost of high-precision fiber shape sensors. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0033] Figure 1 This is a highly sensitive three-dimensional shape measuring device that can detect torsion.

[0034] Figure 2 Schematic diagram of the cross-sectional structure of an implementation case of an optical fiber shape sensor, wherein Figure 2 (a) is a schematic diagram of the cross-sectional position of the sensor when there is no torsion; Figure 2 (b) Schematic diagram of the cross-sectional position of the sensor after axial torsion.

[0035] Figure 3 Schematic diagram of torsional strain analysis when a helical single-mode optical fiber is twisted.

[0036] Figure 4 The simulation results of the distance-total strain of the No. 8 fiber core when the sensor is rotated axially by different unit length torsion angles.

[0037] Figure 5 Schematic diagram of the positional relationship between three adjacent sensing points on the sensor. DETAILED DESCRIPTION

[0038] like Figure 1 The figure shows a high-sensitivity measurement device for identifying torsion direction proposed by the present invention, which includes a high-sensitivity optical fiber shape sensor 1 for identifying torsion direction of the present invention, a fan-in and fan-out device 4, an optical switch 5, an optical frequency domain reflectometer 6, a data acquisition processor 9 and a display device 10.

[0039] The optical fiber shape sensor 1 includes a multi-core optical fiber 2 and a single-mode optical fiber 3, wherein the single-mode optical fiber 3 is spirally wound on the surface of the multi-core optical fiber 2, and the winding section of the single-mode optical fiber and the multi-core optical fiber in the winding section are encapsulated in a flexible material, and the flexible material adopts an existing general packaging material such as solid glue. The right ends of the multi-core optical fiber 2 and the spiral single-mode optical fiber 3 are both sealed by the flexible material, and the left ends of the multi-core optical fiber 2 and the spiral single-mode optical fiber 3 are simultaneously used as the optical signal input and output ends of the sensor.

[0040] Helical single-mode optical fiber has different strain responses for forward and reverse torsion, and the torsion angle of the sensor can be accurately demodulated according to the magnitude of the torsion strain. Multi-core optical fiber can be used to reconstruct the three-dimensional shape of the sensor. When the sensor is twisted, the cores of the helical single-mode optical fiber and the multi-core optical fiber in the same cross section are twisted synchronously. Therefore, the torsion angle demodulated by the helical single-mode optical fiber can be used to compensate for the torsion of the multi-core optical fiber to achieve high-precision shape reconstruction. It is worth noting that as the pitch of the helical single-mode optical fiber decreases or the distance between it and the center line of the sensor increases, the torsion measurement sensitivity of the helical single-mode optical fiber will also increase significantly.

[0041] The output end of the multi-core optical fiber 2 is connected to the optical switch 5 through the fan-in and fan-out device 4, the output end of the single-mode optical fiber 3 is connected to the optical switch 5, the output end of the optical switch 5 is connected to the optical frequency domain reflectometer 6, the optical frequency domain reflectometer 6 is connected to the data acquisition and processing device 9 through the acquisition channel 7 and the trigger channel 8 respectively, and the data acquisition and processing device 9 is connected to the display device 10.

[0042] Among them, the fan-in and fan-out device 4 is used to realize the optical coupling function of a single multi-core optical fiber to multiple single-mode optical fibers; the optical switch 5 is used to switch and query the core channels of the sensor in sequence; the optical frequency domain reflectometer 6 is used to measure the strain distribution of each core of the spiral single-mode optical fiber and the multi-core optical fiber; the data acquisition and processing equipment 9 is used to collect data and reconstruct the three-dimensional shape of the optical fiber shape sensor after data processing; the display device 10 is used to display the reconstructed three-dimensional shape of the optical fiber shape sensor.

[0043] The present invention also provides a measuring method based on a measuring device for identifying torsion direction with high sensitivity, comprising the following steps:

[0044] Step I: by measuring the strain curves of each core of the spiral single-mode optical fiber and the multi-core optical fiber, the torsional strain and torsional angle of each core of the spiral single-mode optical fiber and the multi-core optical fiber are obtained. Specifically including:

[0045] When the entire sensor is in a straight state, the Rayleigh scattering beat frequency information of each core of the spiral single-mode optical fiber and the multi-core optical fiber is collected as the reference group signal; when the sensor is bent and twisted, the beat frequency information of each core of the spiral single-mode optical fiber and the multi-core optical fiber is collected as the measurement group signal;

[0046] Coherent demodulation or phase demodulation is used for the reference group signal and the measurement group signal to obtain the strain curves of each core of the spiral single-mode fiber and the multi-core fiber. When the torsion direction of the sensor is the same as the initial spiral direction of the spiral single-mode fiber, the torsion stretches the spiral single-mode fiber, and its torsion strain is positive, resulting in an increase in the total strain value. At this time, it can be judged that the sensor has undergone positive torsion; when the torsion direction of the sensor is opposite to the initial spiral direction of the spiral single-mode fiber, the torsion compresses the spiral single-mode fiber, and its torsion strain is negative, resulting in a decrease in the total strain value. At this time, it can be judged that the sensor has undergone reverse torsion. Utilizing this characteristic of the spiral single-mode fiber, the distribution of torsion strain can be accurately demodulated from its total strain curve. Combined with the theoretical formula, the torsion angle distribution of the spiral single-mode fiber can be obtained from the torsion strain distribution.

[0047] Step II: In order to accurately reconstruct the three-dimensional shape of the sensor, it is necessary to perform torsion angle compensation and torsion strain compensation on the multi-core optical fiber shape before reconstruction, and obtain the bending direction angle and bending curvature of the multi-core optical fiber after torsion compensation; specifically including:

[0048] Since the spiral single-mode optical fiber and the multi-core optical fiber in the same cross section will be twisted synchronously, the twist angles of the two are the same, so the twist angle of the multi-core optical fiber is compensated according to the twist angle of the spiral single-mode optical fiber obtained in step I;

[0049] When using a multi-core optical fiber to reconstruct the three-dimensional shape of the sensor, the torsional strain of the multi-core optical fiber is an error term that needs to be eliminated in the total strain model. Since the torsional strain measurement sensitivity of the multi-core optical fiber is not high, it is difficult to directly extract the torsional strain of the multi-core optical fiber from the strain curve of each core. However, the torsional strain of the multi-core optical fiber can be inferred from the torsional angle that has been obtained, and compensated in the subsequent total strain decoupling model.

[0050] According to the obtained strain curves, torsional strains and torsional angles of each core of the multi-core optical fiber, a plurality of appropriate cores are selected to form a core triplet, and a bending, torsional-total strain decoupling model of the multi-core optical fiber core triplet is established;

[0051] The torsional strain and torsional angle of the multi-core optical fiber are brought into the model for compensation, and the bending direction angle and bending curvature of the multi-core optical fiber after torsion compensation are obtained, and interpolation processing is performed on them.

[0052] Step III: According to the bending direction angle and bending curvature, a shape reconstruction algorithm is used to solve the spatial coordinates of each sensing point on the optical fiber shape sensor in turn, and reconstruct the three-dimensional shape of the sensor after torsion compensation. The shape reconstruction algorithm can be based on a homogeneous transformation matrix framework, a Frenet–Serret framework, a Bishop framework, or a rod model framework. Taking the shape reconstruction algorithm based on the homogeneous transformation matrix framework as an example, according to the obtained bending direction angle and bending curvature, the corresponding homogeneous transformation matrix of each sensing point is obtained, thereby solving the spatial coordinates of each sensing point in turn; according to the spatial coordinates of each sensing point, the three-dimensional shape of the sensor after torsion compensation is reconstructed.

[0053] In order to explain the present invention more clearly, the torsion measurement, compensation and shape reconstruction principles of the optical fiber shape sensor of the present invention are further described in detail below.

[0054] like Figure 2 It is a schematic diagram of the cross-sectional structure of an implementation example of the optical fiber shape sensor of the present invention. Figure 2 (a) and (b) are schematic cross-sectional views of the sensor before and after twisting, respectively. Figure 2 As shown in the figure, the types of multi-core optical fiber (seven-core optical fiber, four-core optical fiber, three-core optical fiber), spiral single-mode optical fiber (ordinary single-mode optical fiber, anti-bending single-mode optical fiber) and the core sensor element structure (equilateral configuration, right-angle configuration, axisymmetric two outer core configuration, etc.) are all exemplary, and the present invention is not limited to this. In this implementation case, the multi-core optical fiber is selected as a seven-core optical fiber. As shown in the figure, on the one hand, the torsion of the sensor will cause each core to produce corresponding torsional strain. On the other hand, the torsion will cause the outer core to be angularly opposite to the actual bending direction in the cross section. Deviating from an angle value, that is, a torsion angle is generated . When the sensor is twisted in different directions, the torsion angle can be positive or negative. It is necessary to solve the torsion strain and torsion angle at each position of the sensor and make corresponding angle compensation for the bending direction angle to achieve high-precision shape reconstruction of the sensor. A packaging method in which a single-mode optical fiber is spirally wound on the surface of a seven-core optical fiber is adopted. The spiral single-mode optical fiber is used to measure the torsional strain and torsional direction of the sensor with high precision, and compensate for the torsional strain and torsional direction of the seven-core optical fiber, thereby achieving high-precision reconstruction of the three-dimensional shape of the sensor. Figure 2 (a) In the cross section, select seven core optical fibers in an equilateral configuration. The arranged fiber cores 3, 5, and 7 are used as a sensing fiber core triplet. Since the diameters of the spiral single-mode fiber and the seven-core fiber are not much different, the cross-sectional diameter of the spiral single-mode fiber is scaled in the figure for ease of analysis. When the sensor is bent, is the bending direction vector of the sensor at this position, and its modulus is the bending curvature value, recorded as .

[0055] By measuring the axial strain of the triplet of fiber cores 3, 5, and 7 in each section, the bending curvature and bending direction of the optical fiber at each position are determined. By measuring the axial strain of fiber core 8, the torsional strain and torsion angle at each position of the sensor are obtained. Fiber cores 3, 5, 7, and 8 at each cross-sectional position of the sensor can be regarded as a sensing unit group. When the sensor is not torsionally twisted, R is the distance between the spiral single-mode optical fiber and the middle core of the seven-core optical fiber, r is the distance between the outer core and the middle core of the seven-core optical fiber on the cross section, and the angle between the fiber core 5 and the bending direction vector is the bending direction angle of the sensor, recorded as Fiber core No. 1 is always on the bending neutral line of the sensor. It is insensitive to bending and torsion and is only affected by external temperature changes, serving as a temperature compensation fiber core.

[0056] When the sensor is subjected to bending and axial torsion at the same time, Figure 2 (b) It can be seen that the fiber cores 3, 5, 7, and 8 are twisted around the sensor axis at the same time. , the measured strain values ​​of each fiber core in the torsion section are recorded as , , and , the strain measurement value of the middle fiber core is recorded as It should be noted that for ordinary seven-core optical fiber, no matter whether the sensor is twisted clockwise or counterclockwise, the axial twist always stretches the fiber core, and the torsional strain of the seven-core optical fiber is is always positive and small. Therefore, it is impossible to To determine the positive and negative direction of the twist, you need to subtract the total strain of each core of the seven-core optical fiber from the total strain of each core. .

[0057] A spiral single-mode optical fiber is used to demodulate and compensate the axial torsion of the sensor. When the torsion direction of the sensor is the same as the initial spiral direction of the spiral single-mode optical fiber, the torsion causes the spiral single-mode optical fiber to stretch, the torsion strain is positive, and its total strain value increases. At this time, it can be determined that the sensor has undergone positive torsion; when the torsion direction is opposite to the initial spiral direction of the spiral single-mode optical fiber, the torsion causes the spiral single-mode optical fiber to compress, the torsion strain is negative, and its total strain value decreases. At this time, it can be determined that the sensor has undergone reverse torsion.

[0058] Figure 3 Schematic diagram of the analysis when twist is applied to a helical single-mode optical fiber to expand it within a pitch. Figure 3 middle, is the pitch of the helical single-mode fiber, is its inherent twist rate, S is the applied twist angle per unit length. L is the length of the helical single-mode fiber after it is unfolded within one pitch, As the length after torsion stretching or compression, the torsional strain generated by the helical single-mode fiber can be expressed as:

[0059] (1)

[0060] Ignore the higher-order terms in the formula and consider is very close to L, the above formula is simplified to:

[0061] (2)

[0062] From the above formula, we can see that when the pitch of the spiral single-mode fiber is smaller, the applied unit length torsion angle and torsion strain are closer to a linear relationship; when the spiral single-mode fiber is farther away from the geometric center of the sensor, the greater the torsion strain generated, that is, the greater the torsion measurement sensitivity. For example, when the pitch of the packaged spiral single-mode fiber is 20 mm and its distance from the geometric center of the sensor is 175 μm, the theoretical torsion measurement sensitivity of the optical fiber shape sensor is , which is nearly 105 times that of commercial ordinary multi-core optical fiber (core spacing 42μm) and nearly 25 times that of commercial helical multi-core optical fiber (pitch 20 mm, core spacing 35μm).

[0063] like Figure 4 The figure shows the total strain-distance simulation results of the No. 8 fiber core when the entire optical fiber shape sensor is rotated at different unit length torsion angles. The OFDR system sensing resolution is set to 1 mm, the sensor length is 1 m, and the sensor is bent into a circle with a radius of 10 cm. Figure 4 As shown in , when the sensor is only bent without external torsion, the strain curve changes periodically in a sinusoidal manner, and the mean value in each cycle is zero. When a positive torsion is applied to the tail end of the sensor, the torsion angle per unit length at the tail end reaches S=62.8 rad / m. As the distance increases, the torsion angle per unit length increases, resulting in a gradual increase in torsional strain, and the strain curve rises in the positive direction as a whole; similarly, when a reverse torsion is applied to the tail end of the sensor, the torsion angle per unit length at the tail end reaches S=-62.8 rad / m, and the strain curve drops in the negative direction as a whole.

[0064] The torsional strain value of the 8-helix single-mode optical fiber is recorded as ,but:

[0065] (3)

[0066] in, and are the adjacent peak values ​​and trough values ​​on the strain measurement curve of the spiral single-mode optical fiber, and the torsional strain at each position can be interpolated to obtain the torsional strain at each position of the sensor, such as Figure 4As shown by the bold lines.

[0067] By combining equations (2) and (3), the twist angle per unit length at each position of the helical single-mode fiber can be demodulated. Then the twist angle relative to the initial twist position is It can be calculated by integrating S along the length of the helical single-mode fiber, that is:

[0068] (4)

[0069] Since the seven-core optical fiber and the helical single-mode optical fiber in the same cross section have the same twist angle, the obtained After converting to the torsion angle per unit length and substituting it into Formula 3, the torsion strain at each position of the seven-core optical fiber can be inferred. , and this error term is compensated in the subsequent strain model.

[0070] in accordance with , , The position relationship of the fiber cores is used to establish a solution model for the bending direction angle and curvature of the seven-core optical fiber after twisting. At this time, the bending direction angle of the seven-core optical fiber after twisting is The demodulated torsion angle and torsional strain Bring it into the model and perform torsion compensation:

[0071] (5)

[0072] The bending direction angles at each position of the seven optical fibers before twisting can be obtained from formula (5): and the curvature value , that is, the sensor positions and :

[0073] (6)

[0074] Discrete values ​​for each sensor position and Interpolation processing is performed and it is brought into the homogeneous transformation matrix framework as a shape transfer parameter to reconstruct the three-dimensional shape of the sensor. In terms of shape reconstruction algorithms, the present invention is not limited to shape reconstruction algorithms based on the homogeneous transformation matrix framework, the Frenet–Serret framework, the Bishop framework, and the rod model framework. Taking the shape reconstruction algorithm based on the homogeneous transformation matrix framework as an example, Figure 5 Schematic diagram of the position conversion relationship between three adjacent sensing points on the optical fiber shape sensor.

[0075] like Figure 5 As shown, at the sensing point O iEstablish a Cartesian coordinate system x i -y i -z i , i is the sensor point count value of the sensor, and the value is 1, 2, 3... When there are enough sensing points on the optical fiber, O i -O i+1 or i+1 -O i+2 The arc segment is extremely small, the arc segment can be considered as a continuous curve on the osculating plane.

[0076] The sensor point coordinate transformation framework based on the homogeneous transformation matrix is ​​as follows:

[0077] (7)

[0078] (8)

[0079] (9)

[0080] in, is the tangent rotation angle of the arc segment, , P i is the bending angle of the sensor after torsion compensation The associated 3x3 rotation matrix, is a 3x1 position translation matrix, is the bending radius of the sensor arc segment. i Around z i Axis rotation angle , and then rotate around the new coordinate system The rotation angle of the axis along the arc length , and finally translate it Coordinates, can be realized from the sensor point O i To the sensing point O i+1 From the above analysis, we can see that the i-th sensing point on the sensor can be transformed by its transformation matrix M i Convert to the position of the i+1th sensor point, and multiply the coordinates of the i-th sensor point by M i M i+1 The coordinates of the i+2th sensing point can be obtained. By solving the spatial coordinates of each sensing point in turn using Equation 7, the 3D shape of the sensor or the object to be measured after torsion compensation can be reconstructed.

[0081] In summary, the present invention has the following outstanding features and significant improvements:

[0082] (1) By adopting a packaging method in which a single-mode optical fiber is helically wound on the surface of a multi-core optical fiber and setting appropriate pitch and core pitch parameters, the torsion measurement sensitivity can be greatly improved. The results show that the torsion measurement sensitivity of the optical fiber shape sensor is nearly 100 times that of commercial ordinary multi-core optical fibers and nearly 20 times that of commercial helical multi-core optical fibers.

[0083] (2) The packaging method of spirally winding a single-mode optical fiber into a multi-core optical fiber can achieve accurate identification and compensation of the torsion direction, overcoming the defect that ordinary multi-core optical fiber sensors cannot identify the torsion direction and achieving high-precision shape reconstruction.

[0084] (3) The packaging method of this sensor does not require changing the multi-core optical fiber structure, nor does it require engraving weak gratings or strong reflection points, which greatly reduces the production cost compared to commercial spiral multi-core optical fibers.

[0085] (4) The sensor and measuring device in the present invention have the advantages of distributed monitoring, anti-electromagnetic interference, no radiation, good medical biocompatibility, etc., and can simultaneously measure torsion and shape information with high precision. It can be applied to intraoperative navigation of medical endoscopes, shape monitoring and navigation of interventional surgical catheters, attitude tracking and control of aircraft and intelligent robots, and deformation monitoring of rock and soil structures.

[0086] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A high-sensitivity optical fiber shape sensor for identifying torsion direction, characterized in that: The invention comprises a single-mode optical fiber and a multi-core optical fiber, wherein the single-mode optical fiber is spirally wound on the surface of the multi-core optical fiber, the winding section of the single-mode optical fiber and the multi-core optical fiber in the winding section are encapsulated in a flexible material, the right ends of the single-mode optical fiber and the multi-core optical fiber are both sealed by the flexible material, and the left ends of the single-mode optical fiber and the multi-core optical fiber simultaneously serve as the optical signal input and output ends of the sensor; when the pitch of the spiral single-mode optical fiber is smaller, the applied unit length torsion angle and torsion strain are closer to a linear relationship, and the torsion strain measurement sensitivity of the optical fiber shape sensor is higher; when the spiral single-mode optical fiber is farther away from the geometric center of the sensor, the greater the torsion strain generated, that is, the greater the torsion measurement sensitivity.

2. The optical fiber shape sensor for identifying torsion direction with high sensitivity according to claim 1, characterized in that: The middle core of the multi-core optical fiber is located at the central axis of the sensor.

3. A measuring device based on the optical fiber shape sensor with high sensitivity for identifying torsion direction according to claim 1, characterized in that: It includes the optical fiber shape sensor, fan-in and fan-out device, optical switch, optical frequency domain reflectometer, data acquisition processing and display equipment; the output end of the multi-core optical fiber is connected to the optical switch through the fan-in and fan-out device, the output end of the single-mode optical fiber is connected to the optical switch, the output end of the optical switch is connected to the optical frequency domain reflectometer, and the optical frequency domain reflectometer is connected to the data acquisition processing equipment through the acquisition channel and the trigger channel respectively; The fan-in and fan-out devices are used to realize the optical coupling function from a single multi-core optical fiber to multiple single-mode optical fibers; The optical switch is used to sequentially switch and interrogate each fiber core channel of the sensor; The optical frequency domain reflectometer is used to measure the strain distribution of each core of a spiral single-mode optical fiber and a multi-core optical fiber; The data acquisition and processing equipment is used to acquire data and reconstruct the shape of the optical fiber shape sensor after data processing.

4. The measuring device of the optical fiber shape sensor for identifying the torsion direction with high sensitivity according to claim 3 is characterized in that: The measuring device further comprises a display device, which is connected to the output end of the data acquisition and processing device and is used to display the reconstructed shape of the optical fiber shape sensor.

5. A measuring method based on the measuring device according to claim 3, characterized in that: The following steps are involved: Step I: obtaining the torsional strain and torsional angle of each core of the spiral single-mode optical fiber and the multi-core optical fiber by measuring the strain curve of each core of the spiral single-mode optical fiber and the multi-core optical fiber; Step II, performing torsion angle compensation and torsion strain compensation on the shape of the multi-core optical fiber before reconstruction, and obtaining the bending direction angle and bending curvature of the multi-core optical fiber after torsion compensation; Step III: According to the bending direction angle and the bending curvature, a shape reconstruction algorithm is used to solve the spatial coordinates of each sensing point in turn, and reconstruct the three-dimensional shape of the sensor after torsion compensation.

6. The measuring method according to claim 5, characterized in that: The step I specifically includes the following sub-steps: 1-1) When the entire sensor is in a straight state, the Rayleigh scattering beat frequency information of each core of the spiral single-mode optical fiber and the multi-core optical fiber is collected as the reference group signal; when the sensor is bent and twisted, the beat frequency information of each core of the spiral single-mode optical fiber and the multi-core optical fiber is collected as the measurement group signal; 1-2) Using coherent demodulation or phase demodulation for the reference group signal and the measurement group signal, obtain the strain curve of each core of the helical single-mode optical fiber and the multi-core optical fiber; 1-3) Accurately demodulate the distribution of torsional strain of helical single-mode optical fiber from the total strain curve of helical single-mode optical fiber; 1-4) The torsional angle distribution of the helical single-mode optical fiber is obtained from the torsional strain distribution of the helical single-mode optical fiber.

7. The measuring method according to claim 6, characterized in that: The step II specifically includes the following sub-steps: 2-1) Since the spiral single-mode optical fiber and the multi-core optical fiber in the same cross section will be twisted synchronously, the twist angles of the two are the same, so the twist angle of the multi-core optical fiber is compensated according to the twist angle of the spiral single-mode optical fiber; 2-2) Infer the torsional strain of the multi-core optical fiber based on the torsional angle of the multi-core optical fiber; 2-3) According to the obtained strain curves, torsional strains and torsional angles of each core of the multi-core optical fiber, multiple appropriate cores are selected to form a core triplet, and a bending, torsional-total strain decoupling model of the multi-core optical fiber core triplet is established; 2-4) The torsional strain and torsional angle of the multi-core optical fiber are brought into the model for compensation, and the bending direction angle and bending curvature of the multi-core optical fiber after torsion compensation are obtained.

8. The measuring method according to claim 7, characterized in that: After the bending direction angle and the bending curvature of the multi-core optical fiber after torsion compensation are obtained, interpolation processing is performed on the bending direction angle and the bending curvature.

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