Multi-axis strain optical measurement device and method based on PDMS encapsulated micro-nano fiber ring cavity

Abstract

The application discloses a multi-axis strain optical measurement device and method based on a PDMS encapsulated micro-nano fiber ring cavity, and relates to the technical field of precise mechanics measurement and fiber optical sensing. The device comprises a first PDMS substrate layer, a second PDMS structure layer and a third PDMS protective layer arranged in sequence along the thickness direction. The second PDMS structure layer is formed by a plurality of microstructure units into a two-dimensional array and is divided into at least three sub-regions with different mechanical properties. Under the action of external three-axis load, each sub-region generates a linearly independent local strain mode. At least three micro-nano fiber ring resonators are embedded in different sub-regions, so that each ring cavity mainly responds to different local strain modes. An optical path unit is used for collecting the resonant wavelengths of the ring cavities, and a multi-axis strain solving unit is used for solving the three-axis strain. The application significantly reduces the cross-sensitivity, improves the condition number of the solving matrix, and realizes stable and high-precision optical measurement of multi-axis strain.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of precision mechanics measurement and fiber-optic sensing technology, and particularly relates to a multi-axis strain optical measurement device and method based on PDMS encapsulated micro-nano fiber ring cavity. BACKGROUND

[0002] In the field of precision equipment, aerospace structures, flexible electronic devices and intelligent manufacturing process monitoring, it is often necessary to obtain the strain state of components in three-dimensional space to evaluate the structural safety and service life. For this purpose, the industry has proposed a variety of multi-axis strain measurement schemes, among which fiber-optic sensors are widely used in structural health monitoring due to their small size, resistance to electromagnetic interference and suitability for long-distance transmission.

[0003] Existing multi-axis fiber-optic strain measurement devices usually adopt the following approach: multiple fiber Bragg gratings, Fabry-Perot cavities or micro-ring resonant cavities are arranged in the same substrate or encapsulation body, and by using different spatial positions or arrangement angles, each sensing unit corresponds to a different direction of strain component; in order to improve flexibility and adhesion, the sensing fiber is often encapsulated in a flexible elastomer such as silicone or polydimethylsiloxane (PDMS), and a mapping relationship between the output of multiple sensing units and multi-axis strain is established through calibration tests, and then matrix operation or regression algorithm is used for calculation.

[0004] However, due to the isotropy or weak anisotropy of the flexible encapsulation material in the macroscopic view, the external strain in the X, Y and Z directions is coupled into a complex spatial strain field inside the encapsulation body, and each fiber-optic sensing unit senses the mixed strain combination, resulting in obvious cross-sensitivity between their outputs; the multi-variable mapping matrix established on this basis is often large in condition number, sensitive to calibration errors, installation deviations and noise, and the stability of the calculation result is insufficient. SUMMARY

[0005] Technical purposes: In view of the problem that in the prior art, when a PDMS encapsulation micro-nano optical fiber ring cavity is used for multi-axis strain optical measurement, due to the fact that the flexible encapsulation material such as PDMS is approximately isotropic in macroscopic view, the strains in X, Y and Z directions outside are strongly coupled inside the encapsulation body, the cross-sensitivity between the outputs of each fiber resonant cavity is high, the condition number of the mapping matrix of multi-axis strain-output signal is large, and the solving result is sensitive to noise and installation error, so that stable and reliable measurement of three-axis strain is difficult to realize, the application discloses a multi-axis strain optical measurement device and method based on a PDMS encapsulation micro-nano optical fiber ring cavity, which is designed in cooperation from two aspects of encapsulation structure and analytical solving model under the premise of maintaining the PDMS flexible encapsulation and high sensitivity of the micro-nano optical fiber ring cavity, the coupling degree of strains in different directions inside the device is significantly reduced, the cross-sensitivity between each sensing unit is reduced, and the condition number of the solving matrix is improved, so that stable and analyzable three-axis strain measurement results are obtained.

[0006] Technical scheme: In order to achieve the above technical purposes, the application adopts the following technical scheme:

[0007] A multi-axis strain optical measurement device based on a PDMS encapsulation micro-nano optical fiber ring cavity, comprising:

[0008] A PDMS encapsulation body, which is in a sheet structure as a whole, comprises in sequence along the thickness direction: a first PDMS base layer, the lower surface of which is used to be fixedly attached to the surface of a measured object through an adhesive layer; a second PDMS structure layer, which is stacked on the upper surface of the first PDMS base layer; and a third PDMS protective layer, which covers the side of the second PDMS structure layer away from the first PDMS base layer;

[0009] The second PDMS structure layer is formed by a plurality of microstructure units into a two-dimensional array, the two-dimensional array is divided into at least three different sub-regions in the plane, the equivalent Poisson's ratio and the equivalent stiffness of each sub-region are different when compressed in the X direction and the Y direction and relative to the Z direction, and under the action of external three-axis load, a linearly independent local strain mode is formed in each sub-region;

[0010] An array of micro-nano optical fiber ring resonant cavities, comprising at least three micro-nano optical fiber ring resonant cavities, the micro-nano optical fiber ring resonant cavities are integrally embedded inside the second PDMS structure layer through a PDMS casting process, are located between the first PDMS base layer and the third PDMS protective layer along the thickness direction, and the loop center of each micro-nano optical fiber ring resonant cavity is located in a different sub-region, the loop plane is parallel to the plane of the first PDMS base layer, the included angle between the loop main direction and the first principal strain direction formed by the corresponding sub-region under the action of external load is less than a predetermined angle, and each micro-nano optical fiber ring resonant cavity produces a dominant response to different local strain modes;

[0011] An optical path unit is arranged outside the PDMS package and is optically connected with each micro-nano fiber ring resonator through an outcoming optical fiber, the optical path unit comprising a tunable laser source, a fiber coupler, a photodetector and a data acquisition module, for obtaining the resonance spectrum of each micro-nano fiber ring resonator and obtaining the resonance wavelength or its change amount;

[0012] A multi-axis strain calculation unit is electrically connected with the optical path unit and comprises a storage module and a processing module.

[0013] Preferably, the second PDMS structure layer is a PDMS mechanical metamaterial layer formed by periodically arranging microstructure units composed of positive Poisson ratio frame units, negative Poisson ratio reentrant units and connecting units, and the combination modes of the microstructure units in different sub-regions are different:

[0014] The first sub-region is mainly composed of rectangular frame units developed along the X-axis direction, so that the equivalent Poisson ratio of the first sub-region in the X direction is greater than that in the Y direction;

[0015] The second sub-region is mainly composed of reentrant polygonal units, so that the equivalent Poisson ratios of the second sub-region in the X direction and the Y direction are different in sign;

[0016] The third sub-region is formed by combining positive Poisson ratio frame units, negative Poisson ratio reentrant units and vertical support units, and satisfies that the in-plane hoop strain gain of the third sub-region in the Z direction under compression is greater than that of the first sub-region and the second sub-region.

[0017] Preferably, the loop center of the first micro-nano fiber ring resonator is located near the geometric center of the first sub-region, and the included angle between the main hoop direction of the first micro-nano fiber ring resonator and the X-axis direction is not greater than 15°;

[0018] The loop center of the second micro-nano fiber ring resonator is located near the geometric center of the second sub-region, and the included angle between the main hoop direction of the second micro-nano fiber ring resonator and the Y-axis direction is not greater than 15°;

[0019] The loop center of the third micro-nano fiber ring resonator is located in the middle region of the third sub-region, and the loop plane of the third micro-nano fiber ring resonator is inclined to the X-Y plane, so that the included angle between the main hoop stretching direction of the third micro-nano fiber ring resonator under the Z direction compression condition and the main hoop direction thereof is not greater than 15°, and the three micro-nano fiber ring resonators are arranged in a spaced-apart manner in the plane and do not overlap with each other.

[0020] Preferably, the storage module stores:

[0021] An optomechanical conversion matrix K for characterizing the linear relationship between the equivalent hoop strain of each micro-nano fiber ring resonator and the relative wavelength change thereof;

[0022] a mechanical conversion matrix M, used to represent linear mapping of strains of the measured object in X, Y, Z directions to local equivalent hoop strains of each sub-region;

[0023] a temperature coupling parameter q and a volume constraint or equivalent Poisson's ratio constraint coefficient;

[0024] The processing module is configured to calculate a relative wavelength change vector η according to the resonance wavelength output by the optical path unit, and construct an inverse mapping matrix G based on the linear relationship obtained by calibration and a volume constraint or Poisson's ratio constraint relationship F( ) = 0 about triaxial strain, to analytically solve strain vectors of the measured object in X, Y and Z directions , and output the strain vectors , wherein ΔT is a temperature change amount.

[0025] Preferably, the multi-axial strain calculation unit is configured to, in a calibration phase:

[0026] Under a plurality of groups of known triaxial strain and temperature change working conditions, the optical path unit is controlled to collect resonance wavelength changes of each micro / nano fiber ring resonator, the optomechanical conversion matrix K, the mechanical conversion matrix M and the temperature coupling vector q are solved by a least squares method, and the volume constraint coefficients c x , c y , c z are obtained based on the overall volume change of the PDMS mechanical metamaterial, and then the inverse mapping matrix G is calculated, so that the condition number of the matrix K·M is not greater than a preset threshold.

[0027] Preferably, the PDMS mechanical metamaterial layer is doped with magnetic particles, and the device further comprises an adjustable magnetic field source arranged around the PDMS package, which is electrically connected with the control circuit and used to generate an adjustable magnetic field to change the equivalent stiffness and equivalent Poisson's ratio of the microstructure units in the PDMS mechanical metamaterial layer, thereby changing the elements of the matrix M.

[0028] Preferably, a temperature reference resonator is further included, which passes through the PDMS package through a draw fiber and is optically connected with the optical path unit, and is arranged on the PDMS package away from the strain concentration area and is sensitive to temperature change; the multi-axial strain calculation unit calculates a temperature change amount ΔT according to the resonance wavelength change of the temperature reference resonator.

[0029] Preferably, under a working condition in which any uniaxial strain value is , the multi-axial strain calculation unit determines, through calibration, that the device satisfies a performance index that a resonance wavelength change amount of a non-corresponding micro / nano fiber ring resonator is not greater than 20% of a resonance wavelength change amount of a corresponding micro / nano fiber ring resonator.

[0030] A multi-axis strain optical measurement method based on a PDMS encapsulated micro-nano fiber ring cavity, applied to a multi-axis strain optical measurement device based on a PDMS encapsulated micro-nano fiber ring cavity as described above, specifically comprising the following steps:

[0031] Step 1, the first PDMS substrate layer is fixed and attached to the surface of the calibration base through the adhesive layer, the plane of the PDMS encapsulated body is parallel to the surface of the calibration base, and the correspondence between the device coordinate system and the X, Y and Z three-axis loading directions is established;

[0032] Step 2, under a plurality of known single-axis and combined three-axis strain loading conditions, a predetermined strain and temperature change are applied respectively, the optical path unit is controlled to collect the resonance wavelengths of each micro-nano fiber ring resonator, and the relative wavelength change is calculated 、 、

[0033] Step 3, according to the plurality of data of step 2, the optical mechanical conversion matrix K, the mechanical conversion matrix M and the temperature coupling vector q are solved, and the volume constraint coefficient c x 、c y 、c z is obtained according to the overall volume change of the PDMS mechanical metamaterial, the inverse mapping matrix G is calculated and stored in the storage module of the multi-axis strain calculation unit;

[0034] Step 4, the PDMS encapsulated body is pasted on the surface of the measured object through the first PDMS substrate layer, and the leading optical fiber is kept in optical connection with the optical path unit, and the multi-axis strain calculation unit is electrically connected with the optical path unit;

[0035] Step 5, the optical path unit is controlled to inject light into each micro-nano fiber ring resonator and collect the resonance wavelength data, and the relative wavelength change vector η is calculated;

[0036] Step 6, the temperature change ΔT is determined according to the measurement result of the temperature reference resonator or the external temperature sensor;

[0037] Step 7, the stored inverse mapping matrix G is called, and the strain values of the measured object in the X direction, the Y direction and the Z direction are solved by using the formula , and the strain results are output.

[0038] Preferably, in step 3, when the condition number calculated according to the obtained matrix K·M is greater than a preset threshold, the equivalent Poisson's ratio tensor of the first sub-region, the second sub-region and the third sub-region is re-divided by changing the geometric size or arrangement of the micro-structure units in the PDMS mechanical metamaterial layer, and steps 2 and 3 are re-executed until the condition number of the matrix K·M is not greater than the preset threshold.

[0039] ​Beneficial effects: the multi-axial strain optical measurement device and method based on the PDMS packaged micro-nano fiber ring cavity has the following beneficial effects:

[0040] 1、The application constructs three sub-regions with different equivalent Poisson's ratio tensors in the PDMS, and embeds three micro-nano fiber ring resonant cavities in the corresponding sub-regions respectively and matches the ring direction with the principal strain direction, so that the device forms three obviously different strain eigenmodes under the three-axis loading condition. The sensitive characteristics of the micro-nano fiber ring resonant cavity to the path-averaged ring strain are coupled with the eigenmodes of the mechanical metamaterial, so that each ring cavity output mainly corresponds to one eigenmode, and the physical separation of multi-axial strain is realized at the structural level.

[0041] 2、When the single-axis X-direction tensile is 1000με, the wavelength shift of the non-corresponding ring cavity in the traditional isotropic PDMS packaging structure reaches 17%-30% of the main response ring cavity, and the wavelength shift of the non-corresponding ring cavity in the device under the same working condition is not more than 5%-10% of the main response ring cavity, and the cross-sensitivity coefficient is reduced by one order of magnitude.

[0042] 3、Through the joint optimization of the mechanical metamaterial structure and the ring cavity arrangement, the condition number of the matrix K·M is reduced from about 15 of the comparative structure to not more than 5, and the sensitivity of the analytical solution to the measurement noise and the calibration error is significantly reduced, and the root mean square error of the three-axis strain under the three-axis combined loading condition is reduced from about 20% to not more than 5%.

[0043] 4、The application explicitly introduces the equivalent volume constraint or Poisson's ratio constraint in the solving model, avoids relying on the black box algorithm, the solving process has clear physical meaning, can be recalibrated and traced according to the PDMS mechanical metamaterial parameters, and is convenient for engineering application and safety evaluation.

[0044] 5、By doping magnetic particles or integrating piezoelectric actuating structures in the mechanical metamaterial layer, the application can change the equivalent stiffness and Poisson's ratio of each sub-region through an external magnetic field or an electric field, thereby adjusting the K·M matrix and the G matrix online, adapting to different installation conditions, working temperatures and range requirements, and improving the versatility of the device. DETAILED DESCRIPTION

[0045] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description.

[0046] Figure 1 It is a whole structure schematic view of the multi-axial strain optical measurement device of the application.

[0047] Figure 2 It is a microstructure unit and sub-region division schematic view of the second PDMS structure layer of the application.

[0048] Figure 3 Schematic diagram of the relationship between the arrangement of the three micro-nano fiber ring resonators in different sub-regions and the main direction and the main strain direction of the ring;

[0049] Figure 4 Schematic diagram of the calibration test arrangement of the device and the triaxial loading process;

[0050] Figure 5 Comparison chart of the non-corresponding ring cavity response proportion of the device and the isotropic PDMS packaging device under uniaxial loading conditions;

[0051] Figure 6 Comparison chart of the condition number and solution error of the device and the comparative device.

[0052] In the drawings, the meanings of the reference signs are as follows:

[0053] 1, PDMS packaging body; 11, first PDMS base layer; 12, second PDMS structure layer; 13, third PDMS protective layer; 121, first sub-region; 122, second sub-region; 123, third sub-region; 21, first micro-nano fiber ring resonator; 22, second micro-nano fiber ring resonator; 23, third micro-nano fiber ring resonator; 3, optical path unit; 4, multi-axial strain solution unit. DETAILED DESCRIPTION

[0054] The present application will be described in more detail below by way of a preferred embodiment and with reference to the accompanying drawings, but the present application is not limited to the scope of the described embodiments.

[0055] A multi-axial strain optical measurement device based on a PDMS packaged micro-nano fiber ring cavity, comprising:

[0056] The PDMS packaging body has a sheet structure as a whole, and the first PDMS base layer, the second PDMS structure layer and the third PDMS protective layer are sequentially arranged along the thickness direction.

[0057] The lower surface of the first PDMS base layer is fixedly attached to the surface of the measured object through the adhesive layer, and is used to transmit the strain of the measured object in the X, Y and Z directions to the inside of the device;

[0058] The second PDMS structure layer is stacked on the upper surface of the first PDMS base layer, and the inside thereof is formed by a plurality of microstructure units to form a two-dimensional array, which is divided into at least three sub-regions in the plane, and each sub-region is different from each other in terms of equivalent Poisson's ratio and equivalent stiffness, so that a linearly independent local strain mode is formed in each sub-region under the action of external triaxial load;

[0059] A third PDMS protective layer covers the side of the second PDMS structure layer away from the first PDMS substrate layer, encapsulates the internal microstructure and the micro-nano fiber ring resonator, and provides environmental protection.

[0060] In a preferred embodiment, the second PDMS structure layer adopts a PDMS mechanical metamaterial form, which is composed of positive Poisson's ratio frame units, negative Poisson's ratio heavy entry units, and connecting units, and three sub-regions highly sensitive to X-direction stretching, Y-direction stretching, and Z-direction compression are formed by different combinations.

[0061] The micro-nano fiber ring resonator array includes at least three micro-nano fiber ring resonators embedded in the second PDMS structure layer by a PDMS casting process, and located between the first PDMS substrate layer and the third PDMS protective layer in the thickness direction.

[0062] The loop center of each micro-nano fiber ring resonator is located in a different sub-region, the loop plane is parallel to the plane of the first PDMS substrate layer, the included angle between the circumferential main direction of the loop and the first principal strain direction of the corresponding sub-region under the action of external load is less than a predetermined angle, and each micro-nano fiber ring resonator produces a dominant response to different local strain modes.

[0063] Each micro-nano fiber ring resonator passes through the side of the PDMS encapsulant through the draw-out optical fiber and is connected with the external optical fiber.

[0064] The optical path unit is arranged outside the PDMS encapsulant and is optically connected with each micro-nano fiber ring resonator through the draw-out optical fiber. The optical path unit includes a tunable laser source, a fiber coupler, a photodetector, and a data acquisition module, which are used to inject light into each micro-nano fiber ring resonator, collect the transmission spectrum, and obtain the resonance wavelength and its change.

[0065] The multi-axis strain solving unit is electrically connected with the optical path unit and includes a storage module and a processing module.

[0066] The storage module stores the photo-mechanical conversion matrix K, the mechanical conversion matrix M mapping the strain of the measured object in the X, Y, and Z directions to the local equivalent circumferential strain of each sub-region, the temperature coupling parameter q, and the volume constraint or equivalent Poisson's ratio constraint related coefficient.

[0067] The processing module is configured to calculate the relative wavelength change vector η according to the resonance wavelength output by the optical path unit, and based on the linear relationship

[0068]

[0069] and the volume constraint or Poisson's ratio constraint relationship F( =0 to construct the inverse mapping matrix G, and analytically solve the strain vector of the measured object in the X direction, the Y direction and the Z direction , and output the strain vector , wherein ΔT is the temperature change.

[0070] The second PDMS structure layer is divided into at least three sub-regions with different mechanical characteristics in the plane by changing the geometric shape, arrangement mode or material composition of the microstructure unit, so that the three sub-regions generate linearly independent local strain modes under the action of external load.

[0071] In the preferred embodiment, the second PDMS structure layer is a PDMS mechanical metamaterial layer, which is constructed by the combination of positive Poisson ratio frame units, negative Poisson ratio heavy units and vertical support units:

[0072] The first sub-region has high equivalent stiffness in the X direction and low equivalent stiffness in the Y direction, and the strain mode is mainly X-direction stretching;

[0073] The second sub-region has high equivalent stiffness in the Y direction and low equivalent stiffness in the X direction, and the strain mode is mainly Y-direction stretching;

[0074] The third sub-region has significant in-plane circumferential strain amplification when compressed in the Z direction, and the strain mode is mainly Z-direction compression-induced in-plane stretching.

[0075] The above difference in mechanical response can be obtained by finite element simulation and test calibration, which provides a basis for subsequent ring cavity arrangement and matrix identification.

[0076] Three micro / nano fiber ring resonators are arranged in the above three sub-regions, so that the circumferential main direction of each micro / nano fiber ring resonator and the main strain direction of the corresponding region form a predetermined small angle. Since the micro / nano fiber ring resonator is sensitive to the average circumferential strain along the loop, when a uniaxial or three-axis combined load is applied externally, the different local strain modes in each sub-region are transmitted to the ring cavity through the structure, so that:

[0077] The first ring cavity mainly responds to the mode mainly in the X-direction stretching;

[0078] The second ring cavity mainly responds to the mode mainly in the Y-direction stretching;

[0079] The third ring cavity mainly responds to the mode of in-plane stretching induced by Z-direction compression.

[0080] Thus, the intrinsic decomposition of three-axis strain is realized at the structure level, and the mixing degree of strain in different directions in the ring cavity output is reduced.

[0081] Let the reference resonant wavelengths of the three micro / nano fiber ring resonators be , , , the wavelength variation amount in actual measurement is 、 、 , the relative wavelength variation is defined as:

[0082]

[0083] wherein is the relative wavelength variation amount of the i-th micro-nano fiber ring resonator, is the reference resonance wavelength of the i-th micro-nano fiber ring resonator, is the resonance wavelength variation amount of the i-th micro-nano fiber ring resonator;

[0084] The relative wavelength variation vector is defined as:

[0085]

[0086] wherein is the relative wavelength variation vector, 、 、 is the relative wavelength variation amount of the three micro-nano fiber ring resonators;

[0087] The three-axis strain vector of the measured object is:

[0088]

[0089] wherein 、 、 are the normal strains in X, Y and Z directions respectively, is the three-axis strain vector of the measured object. The equivalent hoop strain vector of the three micro-nano fiber ring resonators is :

[0090]

[0091] wherein 、 、 is the strain vector of the three micro-nano fiber ring resonators;

[0092] In the linear range of small deformation, there is a mechanical relationship:

[0093]

[0094] wherein M is a 3x3 mechanical conversion matrix, the element represents the contribution coefficient of the strain in the j-th direction to the equivalent hoop strain of the i-th micro-nano fiber ring resonator; q is a 3x1 temperature coupling vector, represents the influence coefficient of unit temperature change on the equivalent circumferential strain of the i-th micro-nano fiber ring resonator; ΔT is the temperature change amount relative to the calibration temperature.

[0095] The optical measurement relationship is written as:

[0096]

[0097] where K is a 3x3 optomechanical conversion matrix, generally a diagonal matrix, , is a diagonal function, k i is the optomechanical conversion coefficient of the i-th micro-nano fiber ring resonator, representing the relative wavelength change caused by unit equivalent circumferential strain.

[0098] By combining the above two formulas, we get:

[0099]

[0100] On the other hand, according to the simulation and experimental results of the overall volume change or equivalent Poisson's ratio of the second PDMS structure layer, the constraint relationship between the three-axis strains can be obtained, such as the linear volume constraint:

[0101]

[0102] where is the equivalent volume constraint or equivalent Poisson's ratio constraint coefficient. More generally, it can be written as a constraint function F(ε)=0 about the three-axis strain vector ε.

[0103] In the calibration stage, based on multiple sets of known strain and temperature data, the matrix K, the matrix M, the vector q and the constraint coefficient are solved, and the inverse mapping matrix G is constructed, so that:

[0104]

[0105] The multi-axis strain calculation unit calls the matrix G to analytically solve ε according to the measured η and ΔT in online measurement, avoiding the dependence on complex black box algorithms.

[0106] A multi-axis strain optical measurement method based on a PDMS packaged micro-nano fiber ring cavity, applied to a multi-axis strain optical measurement device based on a PDMS packaged micro-nano fiber ring cavity as described above, specifically comprising the following steps:

[0107] Step 1, the first PDMS substrate layer is fixed and attached to the calibration substrate surface through the adhesive layer, so that the plane of the PDMS packaging body is parallel to the calibration substrate surface, and the corresponding relationship between the device coordinate system and the X, Y, Z three-axis loading directions is established;

[0108] Step 2, under a plurality of known uniaxial and combined triaxial strain loading conditions, a predetermined strain and temperature change are respectively applied, the resonant wavelength of each micro-nano optical fiber ring resonator is collected by the optical path unit, and the relative wavelength change amount is calculated 、 、 ;

[0109] Step 3, according to the plurality of groups of data in step 2, the optomechanical conversion matrix K, the mechanical conversion matrix M and the temperature coupling vector q are solved, the volume constraint coefficients cx, cy and cz are obtained according to the overall volume change of the PDMS mechanical metamaterial, the inverse mapping matrix G is calculated and stored in the storage module of the multi-axial strain calculation unit;

[0110] Step 4, the PDMS package is pasted on the surface of the measured object through the first PDMS base layer, and the leading optical fiber is kept in optical connection with the optical path unit, and the multi-axial strain calculation unit is electrically connected with the optical path unit;

[0111] Step 5, the optical path unit is controlled to inject light into each micro-nano optical fiber ring resonator and collect resonant wavelength data, and a relative wavelength change vector η is calculated;

[0112] Step 6, the temperature change ΔT is determined according to the measurement result of the temperature reference resonant cavity or the external temperature sensor;

[0113] Step 7, the stored inverse mapping matrix G is called, and the strain values of the measured object in the X direction, the Y direction and the Z direction are solved by the formula , and the strain results are output.

[0114] Example 1

[0115] Unless otherwise specified, the second PDMS structure layer of the present application has the following common features:

[0116] First, the second PDMS structure layer is formed by a plurality of microstructure units to form a two-dimensional array, which is divided into at least three sub-regions in the plane, and each sub-region is different from each other in equivalent Poisson's ratio and / or equivalent stiffness, thereby forming linearly independent local strain modes under external triaxial load;

[0117] Second, at least three micro-nano optical fiber ring resonators are arranged in different sub-regions, and the circumferential main direction of each micro-nano optical fiber ring resonator forms a predetermined small angle with the first principal strain direction of the corresponding sub-region, so that each micro-nano optical fiber ring resonator produces a dominant response to different local strain modes.

[0118] On this basis, different second PDMS structure layers can be realized by changing the hole morphology, filling particle ratio, crosslinking density or using mechanical metamaterial form, etc. The application preferably adopts the mechanical metamaterial form, and the following embodiments mainly aim at the preferred form for detailed description.

[0119] Example one mainly describes the structure and working principle of the multi-axis strain optical measurement device based on the PDMS encapsulated micro / nano fiber ring cavity.

[0120] Referring to Figure 1 , the device comprises a PDMS encapsulant 1, a first micro / nano fiber ring resonant cavity 21, a second micro / nano fiber ring resonant cavity 22, a third micro / nano fiber ring resonant cavity 23, an optical path unit 3 and a multi-axis strain calculation unit 4.

[0121] The first PDMS substrate layer 11 of the PDMS encapsulant 1 has a thickness of 0.5 mm, and the lower surface is reliably bonded to the surface of the measured object through an adhesive layer. The second PDMS structure layer 12 has a thickness of 0.4 mm and adopts a mechanical metamaterial structure. The third PDMS protective layer 13 has a thickness of 0.3 mm and is used to protect the internal microstructure and fiber ring cavity.

[0122] As shown in Figure 2 , the second PDMS structure layer 12 is composed of periodically arranged microstructure units, each microstructure unit including a positive Poisson ratio frame unit, a negative Poisson ratio reentrant unit and a connecting unit. Three sub-regions are formed by combining these units:

[0123] The first sub-region 121 is formed by continuously arranging rectangular frame units along the X direction, and by adjusting the beam width and aspect ratio, the equivalent Poisson ratio in the X direction is greater than that in the Y direction;

[0124] The second sub-region 122 is formed by continuously arranging reentrant polygonal units, and by setting the internal angle and beam width, the equivalent Poisson ratio in the X direction and the Y direction is different in sign;

[0125] The third sub-region 123 adopts a positive Poisson ratio unit in the plane and a vertical support unit in the thickness direction, so that the ring strain gain in the plane of the third sub-region is significantly greater than that of the first and second sub-regions when compressed in the Z direction.

[0126] The three micro / nano fiber ring resonant cavities have a diameter of about 300 μm and are formed by a closed ring of fused taper optical fiber. Referring to Figure 3 :

[0127] The loop center of the first micro / nano fiber ring resonant cavity 21 is located near the geometric center of the first sub-region 121, and the angle between the ring direction and the X axis direction is not greater than 15°;

[0128] The loop center of the second micro-nano fiber ring resonator 22 is located near the geometric center of the second sub-region 122, and the angle between its main circumferential direction and the Y-axis direction is no greater than 15°.

[0129] The loop center of the third micro-nano fiber ring resonator 23 is located in the middle region of the third sub-region 123, and its loop plane is inclined to the XY plane, so that the angle between the main stretching direction formed during Z-direction compression and its main circumferential direction is no greater than 15°.

[0130] Each micro / nano fiber ring resonator extends from the side of the PDMS package 1 via an outgoing fiber and connects to the fiber port of the optical path unit 3. In the optical path unit 3, the light output from the tunable laser source is sequentially coupled to each ring cavity through fiber couplers. The transmitted signal is acquired by a photodetector, and the data acquisition module transmits the spectral line data to the multi-axis strain calculation unit 4.

[0131] When the measured object is subjected to external loads in the X, Y, and Z directions, the strain is transmitted through the first PDMS substrate layer 11 to the second PDMS structural layer 12, forming different local strain modes in the three sub-regions. This strain is further transmitted to their respective micro / nano fiber ring resonators, causing changes in the resonant wavelength. The multiaxial strain calculation unit 4 analytically solves for the triaxial strain vector using a pre-calibrated inverse mapping matrix G, based on the relative wavelength change vector η of each ring cavity and the temperature change ΔT. , , This enables multiaxial strain measurement.

[0132] Example 2

[0133] Example 2 illustrates the calibration steps and analytical solution method of the multiaxial strain optical measurement device based on PDMS-encapsulated micro / nano fiber optic ring cavity of the present invention.

[0134] See Figure 4 The device from Example 1 is attached to the calibration substrate surface of the triaxial loading platform via the first PDMS substrate layer of the PDMS package 1, and the following calibration procedure is performed:

[0135] S1. Establish the correspondence between the device coordinate system and the X, Y, and Z axes of the loading platform.

[0136] S2. Apply multiple levels of uniaxial tensile strain (e.g., 0–2000 με) in the X direction, and collect the resonant wavelengths of three micro / nano fiber ring resonators under each loading level, and calculate the relative wavelength changes η1, η2, and η3.

[0137] S3. Apply multi-stage uniaxial tensile strain in the Y direction and repeat step S2.

[0138] S4, apply multi-stage vertical compression in Z direction, strain or pressure is converted to equivalent ε z Repeat step S2.

[0139] S5, apply several groups of triaxial combined loading conditions to enrich the data set.

[0140] S6, according to multiple groups of known , , , ΔT and measured η data, the least square method is used to solve the optomechanical conversion matrix K, the mechanical conversion matrix M and the temperature coupling vector q.

[0141] S7, fit the volume or equivalent Poisson's ratio constraint coefficient , , , construct the constraint relationship , solve the inverse mapping matrix G by synthesizing the matrix K, the matrix M and the constraint relationship, and store it in the storage module of the multi-axial strain calculation unit 4.

[0142] S8, calculate the condition number of matrix K·M, when the condition number is not greater than the preset threshold, the calibration is completed; if it is greater than the threshold, adjust the microstructure parameters of the second PDMS structure layer according to the simulation result, and execute the above steps again until the condition number meets the requirements.

[0143] When measuring online, the device is pasted on the surface of the measured structure and connected with the optical unit 3 and the multi-axial strain calculation unit 4. The optical unit continuously excites each ring cavity and collects the resonance wavelength data, and the multi-axial strain calculation unit calculates the relative wavelength change vector η and the temperature change ΔT in real time, calls the inverse mapping matrix G to solve the triaxial strain vector ε, and outputs to the host computer or the monitoring system.

[0144] Example three

[0145] In order to verify the effect of the device of the application in cross-sensitivity and calculation stability, a comparative example is designed.

[0146] The device of the comparative example is basically the same as example one in structure and parameters of the optical unit and the multi-axial strain calculation unit, only the traditional scheme is adopted in the structure of the PDMS package body and the arrangement mode of the ring cavity.

[0147] The PDMS package body of the comparative example device also includes a first PDMS substrate layer, a second PDMS encapsulation layer and a third PDMS protection layer, wherein the second PDMS encapsulation layer is an isotropic solid PDMS layer, and no mechanical metamaterial microstructure is arranged inside, nor is it divided into sub-regions with different mechanical properties.

[0148] The three micro-nano fiber ring resonators are integrally embedded in the isotropic PDMS layer by PDMS casting, the loop centers of the three ring resonators are arranged along the X direction, the ring direction of the three ring resonators is approximately directed to the X axis, the Y axis and the 45° direction respectively, but does not match any designed local principal strain mode.

[0149] The comparative example device adopts the same calibration process as that of the first embodiment, and corresponding optical-mechanical conversion matrix K ref , mechanical conversion matrix M ref and inverse mapping matrix G ref are obtained, which are used for subsequent comparison.

[0150] In the first embodiment and the comparative example, the same loading platform and optical system are used for finite element simulation and physical prototype experiment. Taking uniaxial X direction loading as an example, under the working condition of nominal strain , the resonant wavelength shift and the condition number of the solving matrix of the three micro-nano fiber ring resonators in the two devices are compared, and the results are shown in Table 1.

[0151] Table 1: Response comparison of two devices under uniaxial X direction loading

[0152]

[0153] Among them, the corresponding micro-nano fiber ring resonator refers to the micro-nano fiber ring resonator with the main response under the uniaxial X direction loading, that is, the first micro-nano fiber ring resonator; the total response proportion of the non-corresponding ring cavity R is defined as:

[0154]

[0155] From the above Table 1, it can be seen that:

[0156] (1) In the comparative example device, the wavelength shift of the second micro-nano fiber ring resonator and the third micro-nano fiber ring resonator is still in the same order of magnitude as that of the first micro-nano fiber ring resonator, and the total response proportion of the non-corresponding ring cavity is about 26%, which indicates that the different direction strain is highly mixed in the output of the three ring cavities under the isotropic PDMS packaging structure, and the cross-sensitivity is high;

[0157] (2) In the device of the present application, the wavelength shift of the second micro-nano fiber ring resonator and the third micro-nano fiber ring resonator is significantly reduced, and is much smaller than the response of the first micro-nano fiber ring resonator, and the total response proportion of the non-corresponding ring cavity is less than 5%, which indicates that through the mechanical partition of the second PDMS structure layer and the matching arrangement of the ring cavity and the intrinsic mode, the multi-axis coupling is effectively suppressed at the structural level;

[0158] (3) Under the same calibration process, the condition number of the solving matrix K·M of the comparative device is about 15, while the condition number of the device of the present application is about 3.5. The amplification of noise and calibration error of the device of the present application is significantly reduced, and the analytical solving process is more stable.

[0159] Figure 5 A comparison diagram of the total response proportion of the device of the present application and the isotropic PDMS packaging device under single-axis loading conditions is shown in the figure. The vertical axis represents the proportion (percentage) of the non-corresponding ring resonator to the total optical response under single-axis strain loading, and the horizontal axis represents the isotropic PDMS packaging device and the device of the present application. From Figure 5 It can be seen that under the same single-axis loading condition, the crosstalk response proportion of the non-corresponding ring cavity in the isotropic PDMS packaging device is obviously higher, while the device of the present application introduces a microstructure unit with direction selectivity in the second PDMS structure layer, so that the optical response of the non-corresponding ring cavity is significantly suppressed, and the total response proportion of the non-corresponding ring cavity is significantly reduced, which reflects the advantages of the device of the present application in crosstalk control and direction selectivity sensitivity between ring cavities, thereby providing a cleaner and separable measurement signal basis for subsequent multi-axis strain solving.

[0160] Figure 6 A comparison diagram of the condition number of the solving matrix and the root mean square error of three-axis strain of the device of the present application and the comparative device in the multi-axis strain solving process is shown in the figure. The condition number of the solving matrix (for example, the matrix obtained by multiplying the optical mechanical conversion matrix and the mechanical conversion matrix) of the comparative device and the device of the present application is shown in the form of a column, and the corresponding root mean square error (percentage) of three-axis strain solving is also shown. Figure 6 It can be seen that the condition number of the solving matrix of the comparative device is larger and more ill-conditioned, and the corresponding three-axis strain solving error is also obviously higher; in contrast, the device of the present application optimizes the microstructure layout and the configuration of the main sensitive direction of the ring cavity, so that the condition number of the solving matrix is significantly reduced, the numerical stability is obviously improved, and the root mean square error of three-axis strain solving is also reduced, which proves that the device of the present application is superior to the comparative device in terms of robustness and precision of multi-axis strain solving, and is more suitable for high-precision multi-axis strain measurement in engineering scenarios.

[0161] Further three-axis combined loading experiments show that under typical working conditions, the root mean square error of three-axis strain solving of the device of the present application is significantly lower than that of the comparative device, which is in good agreement with the finite element simulation results, thereby verifying from the experimental level that the device of the present application realizes low cross-sensitivity and multi-axis high-precision measurement through structural intrinsic decomposition and analytical constraint solving.

[0162] Example Four

[0163] Example four is based on example one, magnetic particles are doped in the second PDMS structure layer, and an adjustable magnetic field source is configured. The adjustable magnetic field source is arranged around the PDMS package, and different intensity magnetic fields are generated by adjusting the current size through the control circuit.

[0164] The magnetic particles under the action of the magnetic field have an influence on the equivalent stiffness and the equivalent Poisson's ratio of the PDMS mechanical metamaterial unit, the mechanical response of each sub-region is changed, and thus the elements of the matrix M are changed. In the calibration stage, the calibration process of example two is performed under different magnetic field intensities, and a group of (M, G) corresponding to the magnetic field intensity data is obtained.

[0165] In the online measurement, the multi-axis strain calculation unit selects the corresponding M and G according to the preset working mode, and realizes the switching between the high-sensitivity mode and the large-range mode.

[0166] Example five

[0167] Example five illustrates the application of the device of the present application on the curved surface structure. The PDMS package is pasted on the outer wall of the cylindrical pipeline, and the device is well fitted with the curved surface by reducing the thickness of the first PDMS base layer, optimizing the modulus and thickness of the adhesive layer, etc. The calibration process of example two is performed on the calibration sample pipe, and the corrected conversion matrix and constraint coefficient are obtained.

[0168] Under the action of internal pressure and bending load, the device of the present application can simultaneously measure the axial, ring and radial equivalent strains. The test shows that under the internal pressure-bending coupling working condition, the axial and ring strain measured by the device of the present application has a small deviation from the strain gauge measurement result, and the error in the non-corresponding direction is significantly lower than that of the comparative structure, which indicates that the multi-axis measurement capability of the device of the present application on the curved surface structure is good.

[0169] The above only describes the preferred embodiments of the present application, and it should be pointed out that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should be considered as the protection scope of the present application.

Claims

1. A multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity, characterized in that, include: The PDMS package has a sheet-like structure and includes, along its thickness direction, the following components in sequence: a first PDMS base layer, the lower surface of which is used to be fixedly attached to the surface of the object under test by an adhesive layer; a second PDMS structural layer, which is stacked on the upper surface of the first PDMS base layer; and a third PDMS protective layer, which covers the side of the second PDMS structural layer away from the first PDMS base layer. The second PDMS structural layer is formed by a two-dimensional array of multiple microstructure units. The two-dimensional array is divided into at least three different sub-regions in the plane. Each sub-region has different equivalent Poisson's ratio and equivalent stiffness when compressed in the X and Y directions and relative to the Z direction. Under the action of external triaxial load, each sub-region forms a local strain mode that is linearly independent of each other. The micro-nano fiber ring resonator array includes at least three micro-nano fiber ring resonators. The micro-nano fiber ring resonators are integrally embedded inside the second PDMS structural layer through PDMS casting process. They are located between the first PDMS substrate layer and the third PDMS protective layer along the thickness direction. The loop center of each micro-nano fiber ring resonator is located in a different sub-region. The loop plane is parallel to the plane of the first PDMS substrate layer. The angle between its circumferential principal direction and the first principal strain direction formed by the corresponding sub-region under external load is less than a predetermined angle. Each micro-nano fiber ring resonator generates a dominant response to different local strain modes. The optical path unit is arranged outside the PDMS package and is optically connected to each micro / nano fiber ring resonator through an outgoing optical fiber. The optical path unit includes a tunable laser source, an optical fiber coupler, a photodetector, and a data acquisition module, which is used to acquire the resonance spectrum of each micro / nano fiber ring resonator and obtain the resonance wavelength or its variation. The multi-axis strain calculation unit is electrically connected to the optical path unit and includes a storage module and a processing module.

2. The multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity according to claim 1, characterized in that, The second PDMS structural layer is a PDMS mechanical metamaterial layer, formed by a periodic arrangement of microstructure units consisting of positive Poisson's ratio framework units, negative Poisson's ratio re-entry units, and connecting units. The combination of these microstructure units differs in different sub-regions. The first subregion is mainly composed of rectangular frame units that expand along the X-axis, making the equivalent Poisson ratio of the first subregion in the X-direction greater than the equivalent Poisson ratio in the Y-direction. The second subregion is mainly composed of re-entrant polygonal units, which makes the equivalent Poisson ratio signs of the second subregion different in the X and Y directions. The third sub-region is formed by combining positive Poisson's ratio frame elements, negative Poisson's ratio re-entry elements, and vertical support elements, satisfying that when compressed in the Z direction, the circumferential strain gain of the third sub-region in the plane is greater than that of the first and second sub-regions.

3. The multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity according to claim 2, characterized in that, The loop center of the first micro-nano fiber ring resonator is located near the geometric center of the first sub-region, and the angle between its main circumferential direction and the X-axis direction is no greater than 15°. The loop center of the second micro-nano fiber ring resonator is located near the geometric center of the second sub-region, and the angle between its main circumferential direction and the Y-axis direction is no greater than 15°. The loop center of the third micro-nano fiber ring resonator is located in the middle region of the third sub-region. Its loop plane is inclined to the XY plane, so that the angle between the main circumferential tension direction and the main circumferential direction of the third micro-nano fiber ring resonator under Z-direction compression is no greater than 15°. Furthermore, the three micro-nano fiber ring resonators are arranged in the plane without overlapping each other and are spaced apart from each other.

4. The multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity according to claim 2, characterized in that, The storage module stores: The optomechanical transformation matrix K is used to characterize the linear relationship between the equivalent circumferential strain of each micro / nano fiber ring resonator and its relative wavelength change. The mechanical transformation matrix M is used to characterize the linear mapping of the strain of the measured object in the X, Y, and Z directions to the local equivalent circumferential strain of each sub-region; Temperature coupling parameter q and volume constraint or equivalent Poisson's ratio constraint coefficient; The processing module is configured to calculate the relative wavelength change vector η based on the resonant wavelength output by the optical path unit, and to base it on the linear relationship obtained from calibration. And the volume constraint or Poisson's ratio constraint relationship F( ) regarding triaxial strain. Given that )=0, construct the inverse mapping matrix G, and analytically solve for the strain vectors of the measured object in the X, Y, and Z directions. and output strain vector , where ΔT is the temperature change.

5. The multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity according to claim 4, characterized in that, The multiaxial strain calculation unit is configured during the calibration phase as follows: Under multiple known triaxial strain and temperature variation conditions, the optical path unit was controlled to collect the resonant wavelength changes of each micro / nano fiber ring resonator. The optomechanical transformation matrix K, mechanical transformation matrix M, and temperature coupling vector q were solved using the least squares method. The volume constraint coefficient c was then obtained based on the overall volume change of the PDMS mechanical metamaterial. x c y c z Then, the inverse mapping matrix G is calculated such that the condition number of matrix K·M is not greater than a preset threshold.

6. The multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity according to claim 4, characterized in that, The PDMS mechanical metamaterial layer is doped with magnetic particles. The device also includes an adjustable magnetic field source arranged around the PDMS package. The adjustable magnetic field source is electrically connected to the control circuit and is used to generate an adjustable magnetic field to change the equivalent stiffness and equivalent Poisson's ratio of the microstructure units in the PDMS mechanical metamaterial layer, thereby changing the elements of the mechanical transformation matrix M.

7. The multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity according to claim 1, characterized in that, It also includes a temperature reference resonant cavity, which extends out of the PDMS package through an optical fiber and is optically connected to the optical path unit. The temperature reference resonant cavity is located on the PDMS package away from the strain concentration area and is sensitive to temperature changes. The multi-axis strain calculation unit calculates the temperature change ΔT based on the change in the resonant wavelength of the temperature reference resonant cavity.

8. The multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity according to claim 1, characterized in that, Loading at any uniaxial strain value Under the operating conditions, the multi-axis strain calculation unit meets the performance index by calibration determination device: the change in resonant wavelength of the non-corresponding micro-nano fiber ring resonator is not greater than 20% of the change in resonant wavelength of the corresponding micro-nano fiber ring resonator.

9. A multiaxial strain optical measurement method based on a PDMS-encapsulated micro / nano fiber optic ring cavity, characterized in that, The multiaxial strain optical measurement device based on a PDMS-encapsulated micro / nano fiber optic ring cavity, as described in any one of claims 1-8, specifically includes the following steps: Step 1: Fix the first PDMS base layer to the surface of the calibration substrate using an adhesive layer, so that the plane of the PDMS package is parallel to the surface of the calibration substrate, and establish the correspondence between the device coordinate system and the X, Y, and Z three-axis loading directions; Step 2: Under multiple known uniaxial and combined triaxial strain loading conditions, apply predetermined strain and temperature changes respectively, control the optical path unit to collect the resonant wavelengths of each micro / nano fiber ring resonator, and calculate the relative wavelength change. , , ; Step 3: Based on the multiple sets of data from Step 2, solve for the optomechanical transformation matrix K, the mechanical transformation matrix M, and the temperature coupling vector q, and obtain the volume constraint coefficient c based on the overall volume change of the PDMS mechanical metamaterial. x c y c z The inverse mapping matrix G is calculated and stored in the storage module of the multi-axis strain solution unit; Step 4: Attach the PDMS package to the surface of the object under test through the first PDMS substrate layer, and keep the lead-out optical fiber optically connected to the optical path unit, and the multi-axis strain calculation unit electrically connected to the optical path unit. Step 5: Control the optical path unit to inject light into each micro / nano fiber ring resonator and collect resonant wavelength data, and calculate the relative wavelength change vector η; Step 6: Determine the temperature change ΔT based on the measurement results from the temperature reference resonant cavity or external temperature sensor; Step 7: Call the stored inverse mapping matrix G, and use the formula The strain values ​​of the measured object in the X, Y and Z directions are calculated analytically, and the strain results are output.

10. A multiaxial strain optical measurement method based on a PDMS-encapsulated micro / nano fiber optic ring cavity according to claim 9, characterized in that, In step 3, when the condition number calculated based on the obtained matrix K·M is greater than the preset threshold, the equivalent Poisson ratio tensors of the first sub-region, the second sub-region, and the third sub-region are re-divided by changing the geometric size or arrangement of the microstructure units in the PDMS mechanical metamaterial layer, and steps 2 and 3 are re-executed until the condition number of matrix K·M is not greater than the preset threshold.

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