Fan blade real-time monitoring system

Abstract

The invention discloses a fan blade real-time monitoring system, which comprises a plurality of sensing modules arranged on the surface of a fan blade, and is characterized in that each sensing module is provided with a plurality of sensing units and is used for monitoring the multidirectional linear strain of a single measuring point and calculating the direction alpha 0 and the size of the maximum main strain epsilon max and the minimum main strain epsilon min of the measuring point; the sensing module arranged at the root of the fan blade is composed of a plurality of desensitization type sensing units, and the sensing modules arranged at other parts of the fan blade are composed of a plurality of sensitization type sensing units; the sensing unit comprises a fiber bragg grating; the fiber bragg grating is adhered to the surface of the fan blade through a substrate; the substrate comprises a pasting section A and a non-pasting section B. The width of the section A of the desensitization type sensing unit is larger than that of the section B of the desensitization type sensing unit, and the width of the section A of the sensitization type sensing unit is smaller than that of the section B of the sensitization type sensing unit. The invention provides an optical fiber sensing monitoring system which is low in cost, long in service life and capable of realizing high-precision real-time monitoring on complex stress on the surface of a blade under the condition of complex external force.

Description

Technical Field

[0001] The present invention relates to the technical field of fan blade monitoring, in particular to a real-time monitoring system for fan blades. Background Art

[0002] Currently, fiber optic sensing technology is the preferred method for monitoring wind blades. Fiber Bragg grating (FBG) sensing technology has become a popular application in wind blade monitoring due to its high precision and sensitivity. Fiber Bragg grating (FBG) sensing is typically used to establish physical contact between the optical fiber and the blade by gluing or welding, thereby enabling strain measurement. Because optical fiber is primarily made of quartz glass, which has low shear strength and is easily broken, existing wind turbine products that directly embed FBGs and fiber optic sensors encapsulated with materials such as metal or carbon fiber reinforced polymer (CFRP) offer some protection for the optical fiber and provide high-precision measurement results. However, these embedded FBG sensors are only suitable for small-sized wind turbines, making the production of large-scale intelligent wind blades both costly and difficult. Metal-encapsulated FBG sensors have poor corrosion resistance and are susceptible to lightning strikes. FBG sensors encapsulated with materials such as CFRP are prone to peeling from the blade surface during service, leading to sensor failure and a reduced service life. Summary of the Invention

[0003] Purpose of the invention: In order to overcome the deficiencies in the prior art, the present invention provides a real-time monitoring system for wind turbine blades, providing a low-cost, long-life fiber optic sensing monitoring system that can achieve high-precision real-time monitoring of complex stresses on the blade surface under complex external forces.

[0004] Technical solution: To achieve the above-mentioned purpose, the present invention provides a real-time monitoring system for wind turbine blades, comprising a plurality of sensing modules arranged on the surface of the wind turbine blades. The sensing modules are provided with a plurality of sensing units for monitoring the multi-directional linear strain of a single measuring point and calculating the maximum principal strain ε of the measuring point. mαx and the minimum principal strain ε min direction α0 and size, a plurality of the sensing units are connected in series through a single optical fiber; the sensing module arranged at the root of the wind turbine blade is composed of a plurality of desensitization sensing units, and the sensing modules arranged at other parts of the wind turbine blade are composed of a plurality of sensitivity-enhancing sensing units; the sensing unit includes a fiber grating, a substrate and a protective sheet are respectively pasted on both sides of the fiber grating, the substrate is pasted and connected to the surface of the wind turbine blade, the substrate and the protective sheet constitute an encapsulation layer that wraps the fiber grating, and a protective shell is provided on the outer side of the encapsulation layer; the substrate includes a pasted section A and a non-pasted section B relative to the fiber grating, the pasted section A of the desensitization sensing unit has a width greater than the width of its non-pasted section B, and the pasted section A of the sensitivity-enhancing sensing unit has a width less than the width of its non-pasted section B.

[0005] Furthermore, each of the sensing modules is provided with three sensing units, which are respectively used to monitor the linear strain ε in any three directions α1, α2, and α3 of the measuring point. α1 , ε α2 , ε α3 , and by adding ε α1 , ε α2 , ε α3 Substitute into the following formula ε one by one α Calculate ε x , ε y , γ xy ;

[0006]

[0007] According to ε x , ε y , γ xy Calculate ε mαx , ε min and α0:

[0008]

[0009] Among them, ε x , ε y , γ xy are the three strain components on the surface of the wind blade.

[0010] Furthermore, the substrate and the protective sheet are provided with semicircular grooves adapted to the optical fiber Bragg grating at the longitudinal axis positions, the grating area of ​​the optical fiber Bragg grating is pasted at the center position of the semicircular groove of the substrate, and a first glue layer is formed between the optical fiber Bragg grating and the A section; the substrate is pasted on the surface of the fan blade, and a second glue layer is formed between the substrate and the fan blade; the protective sheet is pasted and connected to the substrate so that the grating area of ​​the optical fiber Bragg grating is located at the center position of the semicircular groove of the protective sheet; the protective sheet is pasted and connected to the inner wall of the protective shell; the A section is located in the middle of the substrate, and the B sections are respectively provided at both ends of the length direction of the A section.

[0011] Furthermore, a coordinate system is established with the center of the grating area as the origin O, the axial direction as the X axis, and the direction perpendicular to the blade surface as the r axis. The half-length of the bonding between the fiber Bragg grating and the substrate is set as L. The average strain transfer rate between the substrate and the fiber Bragg grating is

[0012]

[0013] Among them, k is calculated as follows:

[0014]

[0015] Where, E f is the elastic modulus of the core layer; r f , r1, r2, r R They are the radius of the core layer, coating layer, first glue layer, and conductive layer, that is, the distance from the outermost layer of each layer to point O; G1, G2, G R They are the shear moduli of the coating layer, the first glue layer, and the conductive layer respectively.

[0016] Furthermore, a coordinate system is established with the innermost center of the conductive layer as the origin Q, the axial direction as the X axis, and the direction perpendicular to the blade surface as the h axis. The half-length of the adhesion between the substrate and the fan blade is set as L1. The average strain transfer rate between the fan blade and the substrate is

[0017]

[0018] Among them, k1 is calculated as follows:

[0019]

[0020] Where, E3 is the elastic modulus of the second glue layer; h R , h3 is the distance from the outermost side of the conductive layer and the second glue layer to point Q; G3 is the shear modulus of the second glue layer.

[0021] Furthermore, the substrate has a sensitivity coefficient D of:

[0022]

[0023] Among them, D h is the ratio of the width of the substrate A and B segments, D L is the ratio of the lengths of segments A and B.

[0024] Furthermore, the base sheet and the protective sheet are made of the same rubber material, which is one of natural rubber, butyl rubber, and EPDM rubber.

[0025] Furthermore, the thickness d of the conductive layer is 3-6 mm; and the total length 2L of the optical fiber pasting is ≥6 cm.

[0026] Beneficial effects: A real-time monitoring system for wind turbine blades of the present invention calculates the magnitude and direction of the maximum and minimum principal stresses at the position of the measuring point by sensing the multi-directional linear strain at the measuring point, thereby accurately monitoring the actual strain magnitude and direction of each measuring point on the blade surface in real time to make an accurate early warning; through the selection of packaging materials, the design of the packaging structure and the calculation of dimensions, it has the characteristics of increasing and decreasing sensitivity to meet the monitoring needs of different positions of the wind turbine blades and adapt to the complex operating environment of the wind turbine blades; the installation process of the sensor relative to the wind turbine blade is calculated based on the material selection to ensure the maximum sensing accuracy; further, a specific rubber packaging material with low elastic modulus and corrosion resistance is selected, and the sensor is installed on the surface of the wind turbine blade through a pasting process, which has low cost, wide application range and long service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic structural diagram of an embodiment of a sensing module of the present invention;

[0028] Figure 2 This is a schematic structural diagram of two types of sensing units with increased and decreased sensitivity according to the present invention;

[0029] Figure 3 Schematic diagram of the cross-sectional structure of the sensing unit of the present invention;

[0030] Figure 4 This is a diagram showing the stress distribution in the encapsulated fiber Bragg grating strain transfer model;

[0031] Figure 5 This is a diagram showing the stress distribution relationship in the strain transfer model between the wind turbine blade and the rubber substrate;

[0032] Figure 6 is the relationship between strain transfer efficiency and optical fiber bonding length;

[0033] Figure 7 is the relationship between the average strain transfer efficiency and substrate thickness. DETAILED DESCRIPTION

[0034] The present invention will be further described below with reference to the accompanying drawings.

[0035] As attached Figure 1-7 The real-time monitoring system for wind turbine blades comprises a plurality of sensing modules arranged on the surface of the wind turbine blades. The sensing modules are provided with a plurality of sensing units 1 for monitoring the multi-directional linear strain of a single measuring point and calculating the maximum principal strain ε of the measuring point. mαx and the minimum principal strain ε min Each sensing module is provided with three sensing units 1, which are used to monitor the linear strain ε in any three directions α1, α2, and α3 of the measuring point. α1 , ε α2, ε α3 , and by adding ε α1 , ε α2 , ε α3 Substitute into the following formula ε one by one α Calculate ε x , ε y , γ xy ;

[0036]

[0037] According to ε x , ε y , γ xy Calculate ε mαx , ε min and α0:

[0038]

[0039] Among them, ε x , ε y , γ xy are the three strain components on the surface of the wind blade.

[0040] The formula is derived as follows: Due to the complex stress conditions experienced by wind turbine blades during operation, the direction of maximum strain can be uncertain. Therefore, sensors are placed in a single direction at the monitoring point. This direction of maximum strain may not always be along the sensor's direction, resulting in inaccurate monitoring results. Based on strain analysis of the wind turbine blade surface, it can be seen that the strain on the blade surface is not unidirectional, but rather two-dimensional plane strain. Material mechanics indicates that the maximum strain generally occurs at the outermost edge of a structure, and this is also true for wind turbine blades. Since the blade surface is a free surface, meaning one principal stress is zero, extracting a differential element from the blade surface approximates a biaxial stress state. That is, when the wind turbine is subjected to stress, two principal stresses are non-zero, while the third principal stress is zero. Assume that the plane with zero shear stress is the principal plane, and the force on the principal plane is the principal stress. In a plane strain state, the two non-zero principal stresses are the maximum and minimum principal stresses. By monitoring the maximum and minimum principal strains and their respective directions, the strain state at that location on the wind turbine blade can be accurately expressed. Based on this, the ultimate monitoring target of this scheme is the maximum principal strain ε at the measuring point. mαx and the minimum principal strain ε min The direction α0 and magnitude.

[0041] In plane strain analysis, there are three strain components ε on the surface of the fan blade x , ε y , γ xy , assuming that the three quantities are known, we can get the relationship between the measuring point and ε x The principal strain ε is at an angle αα and shear strain γ α Value:

[0042]

[0043] Since the shear strain is 0 in the principal strain direction, we can simplify Equation (7,2) to obtain:

[0044]

[0045] Based on this formula, the principal strain direction α0 can be calculated. However, in actual measurement, the three strain components cannot be directly obtained. Therefore, this scheme simultaneously deploys sensing units in three directions to monitor the linear strain in any three directions, which are marked as α1, α2, and α3 respectively. α1, α2, and α3 are angle values, referring to the angle between the measuring point and ε. x The direction of the linear strain is at angles of α1, α2, and α3, and the specific monitoring direction depends on the layout direction of the three sensing units.

[0046] Substituting the obtained principal strain direction into equation (7,1), the maximum and minimum principal strain values ​​can be obtained as follows:

[0047]

[0048] Where, ε x , ε y , γ xy By ε α1 , ε α2 , ε α3 Substituting into formula (7,1) we get:

[0049]

[0050] In actual monitoring, α1, α2, and α3 are preferably values ​​that are easy to calculate. As a preferred embodiment, α1 = 0°, α2 = 45°, and α3 = 90° are taken. According to the wavelength division multiplexing principle of fiber Bragg gratings, multiple sensing units 1 are connected in series through a single optical fiber and arranged as shown in the attached figure. Figure 1 As shown in the figure, the optical fiber is easily broken at the corners, so the proper curvature must be maintained during pasting. Use armored cable to protect the external optical fiber and connect the patch cords properly.

[0051] Substituting α1 = 0°, α2 = 45°, and α3 = 90° into equations (7, 6)-(7, 8), we can obtain the following results:

[0052] ε x =ε 0°

[0053] ε y =ε E0°

[0054] γ xy =ε 0° +ε E0° -2ε FG°

[0055] The magnitude and direction of the principal strain are further obtained:

[0056]

[0057] The maximum principal strain, minimum principal strain and their respective directions can be obtained by directly substituting the strain values ​​measured by the sensing module in the three directions into formulas (7, 9)-(7, 11).

[0058] Given the shortcomings of existing fiber Bragg grating (FBG) sensor packaging materials, the ideal packaging material requires the following characteristics: excellent corrosion resistance, including resistance to rain and salt spray erosion. Also, a matching elastic modulus is required. Since the surface material of wind turbine blades is relatively flexible and not completely flat, low-modulus packaging materials can better coordinate deformation with the blades, enabling real-time monitoring. Therefore, this solution uses rubber to encapsulate the fiber Bragg grating, creating a rubber fiber optic sensor.

[0059] Generally, rubber sensors have a measurement range exceeding 5000 με, sufficient for strain monitoring at most locations on wind turbine blades. However, considering that the weak points at the blade root are prone to cracking, delamination, blade breakage, and other accidents that can generate large strains, a fiber Bragg grating rubber sensor with a larger range and desensitization properties is required. Conversely, strain variations at most locations on the blade are relatively small. To achieve higher-precision small-strain measurements, a fiber Bragg grating rubber sensor with enhanced sensitivity is required. Therefore, in this solution, the sensing module deployed at the blade root is composed of multiple desensitization sensing units, while the sensing modules deployed elsewhere on the blade are composed of multiple enhanced sensitivity sensing units.

[0060] The basic structure of the two types of sensing units 1 is as follows, including a fiber Bragg grating 2, which includes an internal fiber core 21 and an outer coating layer 22. A substrate 3 and a protective sheet 4 are respectively pasted on both sides of the fiber Bragg grating 2. The substrate 3 is pasted and connected to the surface of the wind turbine blade 10. The substrate 3 and the protective sheet 4 constitute an encapsulation layer that wraps the fiber Bragg grating 2. A protective shell 11 is provided on the outer side of the encapsulation layer. A semicircular groove that is adapted to the fiber Bragg grating 2 is opened on the longitudinal axis of the substrate 3 and the protective sheet 4.

[0061] During specific layout and installation, the grating area of ​​the fiber Bragg grating 2 is first adhered to the corresponding position of the center section A of the semicircular groove of the substrate 3, forming a first glue layer 5 between the fiber Bragg grating 2 and the section A; the substrate 3 is then adhered to the surface of the fan blade 10, forming a second glue layer 6 between the substrate 3 and the fan blade 10; finally, a protective shell 11 with a protective sheet adhered to it is placed on the substrate adhered to the optical fiber, and the protective shell 11 is used to press the two ends of the substrate 3 against the surface of the fan blade 10. The protective sheet 4 is adhered to the substrate 3 so that the grating area of ​​the fiber Bragg grating 2 is located at the center of the semicircular groove of the protective sheet 4; the protective sheet 4 is adhered to the inner wall of the protective shell 11.

[0062] Among them, the base sheet and the protective sheet are both made of rubber. In order to further cope with the complex working environment, the fiber grating is doubly protected by a 3D printed resin shell, which effectively blocks the contact of rain, ultraviolet rays, etc. with the sensor, avoids the interference of water absorption, expansion and aging of the sensor on strain monitoring, and plays a very important role in the stability of the sensor.

[0063] The base sheet 3 and the protective sheet 4 are made of one of natural rubber, butyl rubber and EPDM rubber. Preferably, EPDM rubber is used as the packaging material.

[0064] Based on the above structure, this solution mainly realizes the adjustment of its sensitivity increase and decrease characteristics by designing the structure and size of the packaging layer. Among them, the protective sheet mainly plays a role in protecting the fiber grating and has little effect on the sensitivity increase and decrease characteristics of the sensor. Therefore, this solution only considers the shape and size of the substrate. After determining the shape contour of the substrate, the protective sheet can adopt the same contour as the substrate.

[0065] In this solution, the substrate and optical fiber are connected by gluing, and the grating region of the optical fiber is relatively glued to the middle of the substrate. Therefore, the substrate can be segmented and analyzed based on the adhesion relationship between the two. The substrate 3 is set to include a glued segment A and an un-glueed segment B relative to the optical fiber grating 2. The segment A is located in the middle of the substrate 3, and the segments B are respectively provided at both ends of the length direction of the segment A. Analysis and deduction show that the width of the glued segment A of the desensitization sensing unit is greater than the width of its un-glueed segment B, and the width of the glued segment A of the sensitivity-enhancing sensing unit is less than the width of its un-glueed segment B. The specific analysis and deduction process is as follows:

[0066] When the sensor as a whole is subjected to a lateral load, the following can be obtained based on material mechanics:

[0067] E A S A ε A =E B S B ε B(8, 1)

[0068] Among them, E A 、E B Represents the elastic modulus of segments A and B, S A 、S B Represent the cross-sectional areas of segments A and B, ε A , ε B Represent the strains of segments A and B respectively.

[0069] The strain of the sensor is expressed as:

[0070]

[0071] in:

[0072]

[0073] Combining formulas (8, 1)-(8, 3) yields:

[0074]

[0075] Assuming the sensitivity coefficient is D, then:

[0076]

[0077] Among them, S A =h A δ A , h A Indicates the width of the rubber substrate A segment, δ A The thickness of the rubber substrate A section is expressed in the same way for section B. Since there are many ratios in the expression of the sensitivity coefficient, the following coefficients are added, including the ratio of the length of sections A and B, D L ; The ratio of elastic modulus of segments A and B D E ; The ratio of the cross-sectional area of ​​sections A and B is D S :

[0078]

[0079] Then the sensitivity coefficient D can be expressed as

[0080]

[0081] When the D value is less than 1, it means that the strain on section A of the substrate is greater than the strain on the entire sensor, and the substrate has a sensitivity-enhancing effect; when the D value is greater than 1, it means that the strain on section A of the substrate is less than the strain on the entire sensor, and the substrate has a sensitivity-desensitizing effect.

[0082] Since the base sheet 3 and the protective sheet 4 are made of the same rubber material, their elastic modulus is the same, and the thickness of the entire rubber base sheet is also exactly the same. Therefore, it can be concluded that:

[0083]

[0084] Further simplify the sensitivity coefficient D:

[0085]

[0086] Among them D h is the ratio of the width of the rubber substrate A and B segments, D L is the ratio of the lengths of segments A and B. It can be seen from this that the key to determining the sensitivity coefficient lies in the ratio of the width of the grating pasted segment to the unpasted end.

[0087] As attached Figure 2 As shown in A >h B When D h >1, the rubber substrate presents a "cross" shape, D>1, the rubber substrate has a desensitizing effect; when h A <h B When D h <1, at this time the rubber substrate presents an "I" shape, D<1, the rubber substrate has a sensitization effect.

[0088] Further analysis shows that when D>1, D L The smaller the D h The larger the value, the greater the D value, and the better the desensitization effect; when D<1, D L The smaller the D h The smaller the value, the smaller the D value, and the better the sensitivity enhancement effect. The effectiveness of the sensitivity enhancement and desensitization is directly related to the dimensions of the bonded and unbonded sections of the fiber Bragg grating (FBG). The smaller the length of bonded section A and the greater the width difference between bonded section A and unbonded section B, the better the sensitivity enhancement and desensitization effect of the rubber sensor. Based on this, when designing sensor dimensions, the preferred substrate structure should be determined based on the sensor's placement. The optimal bonding length range between the substrate and the fiber can then be calculated. Based on the desired sensitivity enhancement and desensitization effect relative to the placement, the length of bonded section A should be appropriately selected within this range.

[0089] For example, in a certain area within a certain distance from the root of the blade, a "X"-shaped rubber fiber optic sensor is selected, and the closer to the root, the shorter the length of its section A, and the larger the width difference between the pasted section A and the unpasted section B; in other areas, an "I"-shaped rubber fiber optic sensor is selected, and the farther away from the root of the blade, the shorter the length of its section A, and the larger the width difference between the pasted section A and the unpasted section B.

[0090] Among them, analysis shows that the bonding length of section A and the bonding length between the substrate and the blade also affect the overall strain transfer rate of the sensor, which in turn directly affects the sensor's sensing accuracy. Therefore, to ensure measurement accuracy, it is crucial to reasonably select the bonding length of section A and the bonding length between the substrate and the blade. Therefore, based on this research, this proposal also derives a method for determining the value range of the bonding length of section A and the bonding length between the substrate and the blade. The details are as follows:

[0091] In the longitudinal section of the sensor after pasting, a coordinate system is established with the center of the grating area as the origin O, the axial direction as the X-axis, and the direction perpendicular to the blade surface as the r-axis. A strain transfer model between the substrate and the optical fiber is constructed, and the half-length of the pasting between the optical fiber Bragg grating 2 and the substrate 3 is set to L, and the length of the A section is set to 2L.

[0092] As attached Figure 4 The figure shows the stress distribution relationship in the packaged fiber Bragg grating strain transfer model. r represents the distance from the origin O to any layer. f , r1, r2, r R Respectively represent the radius of the core layer, coating layer, first glue layer, and conductive layer (the distance from the outermost layer to point O), σ f ,σ1,σ2,σ R Respectively represent the axial stress of the core layer, coating layer, first glue layer, and conductive layer. x , the longitudinal stress σ of the fiber core can be obtained through the force balance equation f for:

[0093]

[0094] Where: τ f,1 represents the shear stress between the core layer and the coating layer, σ f is the normal stress on the core layer. According to the balance of stress, we can get:

[0095]

[0096]

[0097] By analogy, we can get:

[0098]

[0099] When x = r R When the boundary condition τ R (x,r R )=0 and substitute into formula (9.4) to obtain the following formula:

[0100]

[0101] It turns out that:

[0102]

[0103] Substitute formula (9.6) into formula (9.4):

[0104]

[0105] Combining formulas (9.1), (9.2), (9.3), and (9.7), we obtain:

[0106]

[0107] At this time, it is assumed that the deformation of the core layer, coating layer, first glue layer, and conductive layer occurs simultaneously and meets the following conditions:

[0108]

[0109] According to the stress-strain relationship σ=Eε, the elastic modulus of the core layer is much higher than that of the coating layer and the first glue layer, so the following relationship can be derived:

[0110]

[0111] Substituting formulas (9.8)-(9.10) and (9.12)-(9.13) into formulas (9.2)-(9.4), we can derive the following formulas:

[0112]

[0113]

[0114] Considering only the axial displacement, the following formula can be obtained:

[0115]

[0116] Where u(x) represents the axial displacement and γ(x) represents the interlaminar shear strain. The relationship between the axial strain and the interlaminar shear strain is:

[0117]

[0118] In the formula is the shear modulus, the integral of the above formula can be converted to:

[0119]

[0120] By integrating and simplifying the above formula, we get the following results:

[0121]

[0122] By superimposing formulas (9.24)-(9.26), we obtain the following formula:

[0123]

[0124] Simplifying formula (9.27) yields the following formula:

[0125]

[0126] Where,

[0127]

[0128] Solving formula (9.29) yields the following result:

[0129] ε(x,r f )=-αsinh(kx)-βcosh(kx)+ε(x,r R ) (9.30)

[0130] Among them, α and β can be solved by boundary conditions. Because both ends of the fiber core are axial stress free ends, ε(L,r f )=ε(-L,r f )=0 boundary condition, solve for α=0, Substituting into formula (4.30), simplifying it, we get:

[0131]

[0132] The strain transfer rate α of the packaged fiber Bragg grating model is obtained by calculating the ratio of the strain value of the core layer to the strain value of the conductive layer. Therefore, the strain transfer rate α of the packaged fiber Bragg grating model can be calculated as follows:

[0133]

[0134] The average strain transfer rate of the entire optical fiber can then be obtained for:

[0135]

[0136] Among them, k is calculated as follows:

[0137]

[0138] Where, E f is the elastic modulus of the core layer; r f , r1, r2, r R They are the radius of the core layer, coating layer, first glue layer, and conductive layer, that is, the distance from the outermost layer of each layer to point O; G1, G2, G R They are the shear moduli of the coating layer, the first glue layer, and the conductive layer respectively.

[0139] Then, a coordinate system is established with the innermost center of the conductive layer as the origin Q, the axial direction as the X-axis, and the direction perpendicular to the blade surface as the h-axis to construct a strain transfer model between the fan blade and the rubber substrate. The half-length of the adhesion between the substrate 3 and the fan blade is set to L1, and the adhesion length between the substrate and the blade is set to 2L1.

[0140] As attached Figure 5 The figure shows the stress distribution relationship in the strain transfer model between the fan blade and the rubber substrate. R ,h3,h m Respectively represent the distance from the outermost layer of the conductive layer, the second glue layer and the fan blade layer to point O, σ R ,σ3,σ m Represent the axial stresses of the conductive layer, the second glue layer and the fan blade layer respectively.

[0141] Based on the stress distribution, the longitudinal stress along the X-axis is expressed as:

[0142]

[0143] where τ R,3 (x,h R ) represents the shear stress between the rubber substrate and the second glue layer. Similar to the strain transfer model of packaged fiber Bragg gratings, it can be obtained that:

[0144]

[0145] Substituting formula (9.34) into formula (9.35), we obtain the following formula:

[0146]

[0147] According to Hooke's law, formula (9.36) can be converted into the following formula:

[0148]

[0149] Assume that the rubber layer and the second glue layer deform simultaneously and the following conditions are met:

[0150]

[0151] Formula (9.37) can be simplified and combined to obtain:

[0152]

[0153] Considering only the axial deformation u(x), the integral formula can be listed as:

[0154]

[0155] Because the sensor substrate designed in this paper is a rubber substrate, the elastic modulus is orders of magnitude different from that of the second glue layer, so (E R -E3) is simplified to (-E3) for integral calculation. The following formula is the result after solving the integral:

[0156]

[0157] This result can be further simplified into a solvable differential equation:

[0158]

[0159] Where:

[0160]

[0161] Similarly, it can be concluded that the strain transfer rate β between the rubber substrate and the fan blade can be expressed as:

[0162]

[0163] Then the average strain transfer rate between the entire rubber substrate and the fan blade can be obtained for:

[0164]

[0165] Among them, k1 is calculated as follows:

[0166]

[0167] Where, E3 is the elastic modulus of the second glue layer; h R , h3 is the distance from the outermost side of the conductive layer and the second glue layer to point Q; G3 is the shear modulus of the second glue layer.

[0168] In summary, the overall strain transfer rate γ of a fiber Bragg grating (FBG)-encapsulated rubber sensor for wind blade monitoring can be considered in two parts: the strain transfer rate α of the encapsulated FBG, and the strain transfer rate β between the rubber substrate and the wind blade. Theoretically, the overall strain transfer rate γ for the entire model is γ = α × β.

[0169] Based on the above formula, the strain transfer rate of an encapsulated fiber Bragg grating (FBG) sensor is related to the bonding length and k value. The k value is primarily determined by the shear modulus and thickness of the coating layer, the first adhesive layer, and the conductive layer. The shear modulus is related to the elastic modulus and Poisson's ratio μ. Because the shear modulus and thickness of the coating layer and the first adhesive layer are roughly the same during sensor fabrication, the factors affecting the strain transfer rate of a rubber-encapsulated fiber Bragg grating sensor are primarily the type and thickness of the rubber substrate, as well as the bonding length of the optical fiber to the rubber substrate. The strain transfer rate between the rubber substrate and the wind blade is related to the distance between the second adhesive layer and the outermost layer of the rubber substrate and point O, as well as the elastic modulus. Since the elastic modulus of rubber is lower than that of the second adhesive layer, it can be neglected for simplification. Therefore, the factors determining the strain transfer rate between the rubber substrate and the wind blade include: the thickness and elastic modulus of the second adhesive layer; the thickness of the rubber substrate; and the bonding length of the rubber substrate.

[0170] The thickness of the second adhesive layer and the rubber substrate can be optimized based on experimental analysis. Other factors, except for the bonding length, can be determined by material selection. The elastic modulus and shear modulus ranges for the corresponding structural layers can be determined based on the selection of the optical fiber, rubber substrate, and adhesive. Furthermore, based on the ideal range of the overall strain transfer rate γ, the optimal ranges for L and L1 can be deduced.

[0171] The thickness of the conductive layer refers to the thickness of the thinnest part of the rubber substrate. The thickness d of the conductive layer is 3-6mm. Experiments have found that the smaller the thickness of the base layer, the higher the average strain transfer rate. Therefore, the thickness of the base layer needs to be reduced as much as possible when making the sensor. However, when the base layer is too thin, the armor wire at the end will crack and fall off, seriously affecting the service life of the rubber sensor. Under normal circumstances, an armor wire with an outer diameter of 3mm can meet the tensile strength required for the packaged sensor. Considering the sensor manufacturing process, when the base layer is less than 4mm, the end is easily damaged during the actual application of the sensor, and when the base layer is greater than 4mm, the strain transfer efficiency will be reduced. Therefore, the relatively ideal thickness of the base layer of the rubber sensor is 4mm.

[0172] Provide a preferred embodiment, taking the conventional optical fiber sensor size as an example, the total length of the optical fiber pasting 2L ≥ 6cm. Figure 6Figure 7 shows the relationship between average strain transfer efficiency, substrate thickness, and fiber bonding length. The figure illustrates the variation in strain transfer efficiency at different substrate thicknesses and bonding lengths, using a conventional fiber optic sensor size as an example and employing the control variable method. The rubber base should be of moderate length. The base length should not be too long, as this can lead to unclear measurement point locations. It should also not be too short, as this can shorten the bonding length between the fiber and the base, reducing strain transfer efficiency. In this example, a base length of 11 cm is tentatively set. The EPDM base is assumed to have a thickness of 4 mm, a width of 1.5 cm, and a total base length of 11 cm. The strain transfer curves are calculated for optical fiber bonding half-lengths of 10 mm, 20 mm, 30 mm, 40 mm, and 55 mm, respectively. Each curve contains ten calculation points, representing ten equal divisions of the bonding half-length. Figure 6 The following graph shows the relationship between strain transfer efficiency and fiber bonding length. All five curves follow a parabolic shape, with the strain transfer rate decreasing more rapidly toward the end. As the bonding half-length increases, strain transfer improves at all locations. Therefore, increasing the fiber bonding half-length as much as possible can improve the average strain transfer rate.

[0173] When L = 10mm, the maximum strain transfer rate at the center is only about 70%. As the coordinate increases, the strain transfer rate decreases. When L = 20mm, the maximum strain transfer rate at the center increases to 95%, and it can meet 90% within the first 10mm. When L = 30, 40, and 55mm, the maximum strain transfer rate at the center can reach 99%, and the average strain transfer rate can exceed 80%, which basically meets the needs of wind turbine blade strain monitoring. Therefore, the total length of the optical fiber bonding cannot be less than 60mm.

[0174] The above description is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the above principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A real-time monitoring system for fan blades, characterized in that: The invention comprises a plurality of sensing modules arranged on the surface of a wind turbine blade, wherein the sensing module is provided with a plurality of sensing units (1) for monitoring the multi-directional linear strain of a single measuring point and calculating the maximum principal strain ε of the measuring point. max and the minimum principal strain ε min direction α0 and size, a plurality of the sensing units (1) are connected in series via a single optical fiber; the sensing module arranged at the root of the fan blade is composed of a plurality of desensitizing sensing units, and the sensing modules arranged at other parts of the fan blade are composed of a plurality of enhancing sensing units; the sensing unit (1) comprises an optical fiber grating (2), a substrate (3) and a protective sheet (4) are respectively adhered to both sides of the optical fiber grating (2), the substrate (3) is adhered and connected to the surface of the fan blade, the substrate (3) and the protective sheet (4) constitute a packaging layer that wraps the optical fiber grating (2), and a protective shell is provided on the outer side of the packaging layer; the substrate (3) comprises an adhesive section A and an unadhered section B relative to the optical fiber grating (2), the adhesive section A of the desensitizing sensing unit has a width greater than the width of its unadhered section B, and the adhesive section A of the enhancing sensing unit has a width less than the width of its unadhered section B.

2. A wind turbine blade real-time monitoring system according to claim 1, characterized in that: Each of the sensing modules is provided with three sensing units (1), which are respectively used to monitor the linear strain ε in any three directions α1, α2, and α3 of the measuring point position. α1 , ε α2 , ε α3 , and by changing ε α1 , ε α2 , ε α3 Substitute into the following formula ε one by one α Calculate ε x , ε y , γ xy ; According to ε x , ε y , ε xy Calculate ε max , ε min and α0: Among them, ε x , ε y , γ xy are the three strain components on the surface of the wind blade.

3. A wind turbine blade real-time monitoring system according to claim 2, characterized in that: The substrate (3) and the protective sheet (4) are provided with semicircular grooves adapted to the optical fiber Bragg grating (2) at the longitudinal axis positions; the grating area of ​​the optical fiber Bragg grating (2) is pasted to the center position of the semicircular groove of the substrate (3), and a first glue layer (5) is formed between the optical fiber Bragg grating (2) and the A section; the substrate (3) is pasted to the surface of the fan blade, and a second glue layer (6) is formed between the substrate and the fan blade; the protective sheet (4) is pasted and connected to the substrate (3), so that the grating area of ​​the optical fiber Bragg grating (2) is located at the center position of the semicircular groove of the protective sheet (4); the protective sheet (4) is pasted and connected to the inner wall of the protective shell; the A section is located in the middle of the substrate (3), and the B sections are respectively arranged at both ends of the length direction of the A section.

4. A wind turbine blade real-time monitoring system according to claim 3, characterized in that: A coordinate system is established with the center of the grating area as the origin O, the axial direction as the X axis, and the direction perpendicular to the blade surface as the r axis. The half-length of the bonding between the fiber grating (2) and the substrate (3) is set as L. Then, the average strain transfer rate between the substrate (3) and the fiber grating (2) is Among them, k is calculated as follows: In the formula, E f is the elastic modulus of the core layer; r f , r1, r2, r R They are the radius of the core layer, coating layer, first glue layer, and conductive layer, that is, the distance from the outermost layer of each layer to point O; G1, G2, G R They are the shear moduli of the coating layer, the first glue layer, and the conductive layer respectively.

5. A wind turbine blade real-time monitoring system according to claim 4, characterized in that: A coordinate system is established with the innermost center of the conductive layer as the origin Q, the axial direction as the X-axis, and the direction perpendicular to the blade surface as the h-axis. The half-length of the adhesion between the substrate (3) and the fan blade is set as L1. The average strain transfer rate between the fan blade and the substrate (3) is Among them, k1 is calculated as follows: Where, E3 is the elastic modulus of the second glue layer; h R , h3 is the distance from the outermost side of the conductive layer and the second glue layer to point Q; G3 is the shear modulus of the second glue layer.

6. A wind turbine blade real-time monitoring system according to claim 5, characterized in that: The sensitivity coefficient D of the substrate (3) is: Among them, D h is the ratio of the widths of the A and B segments of the substrate (3), D L is the ratio of the lengths of segments A and B.

7. A wind turbine blade real-time monitoring system according to claim 6, characterized in that: The base sheet (3) and the protective sheet (4) are made of the same rubber material, which is one of natural rubber, butyl rubber and EPDM rubber.

8. A wind turbine blade real-time monitoring system according to claim 7, characterized in that: The thickness d of the conductive layer is 3-6 mm; the total length 2L of the optical fiber pasting is ≥6 cm.

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