Strain value measurement method, OFDR strain measurement method

Abstract

The invention relates to a strain value measurement method and an OFDR strain measurement method, and relates to the field of large strain measurement range. In order to solve the technical problem in the prior art that the existing OFDR strain measurement method cannot demodulate the signal with a spectral offset exceeding 50%, the technical solution provided by the present invention is: a strain value measurement method, the method comprising: a step of collecting the distance domain of the sensing optical fiber signal; a step of obtaining the scattered spectrum signal through the distance domain; a step of extracting the front and end local segments of the scattered spectrum signal through a rectangular window function, and obtaining two segment reference state signals; a step of performing a cross-correlation operation on the reference state signal and the measurement station signal; a step of comparing the values ​​of the cross-correlation operation corresponding to the two signals, and taking the larger value as the result. It is suitable for application in OFDR strain measurement work.

Description

Technical Field

[0001] The invention relates to the field of strain sensing and expanding the strain measurement range by using optical frequency domain reflectometry (OFDR). Background Art

[0002] Optical Frequency Domain Reflectometry (OFDR) is a technology that uses the interference between backscattered Rayleigh light and local oscillator light in optical fiber to generate and process beat signals. OFDR was first proposed by W. Eickhoff in 1981, drawing on the idea of ​​Frequency Modulated Continuous Wave (FMCW) technology. Its principle is based on heterodyne interferometry, interfering backscattered light with local oscillator light to obtain information in the sensing optical fiber. OFDR has broad application prospects and extremely high application value in many fields such as aviation, submarines, medical treatment, geological exploration, etc. At present, it has demonstrated its application value in the fields of pipeline deformation detection, bridge structure detection, geotechnical and geological monitoring, steel deformation and corrosion detection, etc.

[0003] Backscattered light in optical fiber is caused by the uneven distribution of refractive index inside the fiber. This unevenness causes the amplitude and phase of the scattered light to vary randomly. Once the fiber is manufactured, the unevenness of its refractive index will tend to stabilize and become a fixed characteristic of the fiber, and can also be regarded as the "fingerprint" of the fiber. Based on this characteristic, we can judge the fiber segments at different positions by spectral shift. Researchers have established a relationship between strain and spectral shift, allowing OFDR to be used for strain sensing. However, in strain sensing, if the spectral shift is too large, the similarity between the spectra will decrease, causing demodulation errors. Large strain leading to reduced similarity of time domain signals is a common problem faced in OFDR demodulation of strain.

[0004] In order to improve the strain measurement range, Heinze et al. proposed a method of using dynamic reference Rayleigh spectroscopy, using the previous measurement results as the reference state, and the current measurement state is the accumulation of the previous measurement results. However, this method has the problem of error accumulation, and the strain range of each solution is not really improved.

[0005] In order to solve this problem, researchers including Feng Kunpeng and Jiang Dong from Harbin Institute of Technology proposed a spectral offset solution method based on local spectral similarity. This method selects 10% of the reference spectrum as a local segment, and then calculates the Pearson correlation coefficient between the measured spectra of the same length, selects the local spectrum with the highest correlation as a reference, and then calculates the external offset based on the relative position between the two local spectra, and obtains the internal offset by cross-correlating the two segments. The spectral offset caused by strain can be expressed as the sum of the internal offset and the external offset. This method effectively avoids the problem of decreased similarity due to large strain.

[0006] Through the above improved methods, OFDR has achieved better performance in strain measurement. It can not only improve the strain measurement range, but also effectively avoid demodulation errors caused by large strains. This solution method based on local spectral similarity brings higher accuracy and reliability to OFDR in the field of strain sensing.

[0007] In summary, Optical Frequency Domain Reflectometry (OFDR) is a fiber optic sensing technology based on heterodyne interferometry, which uses the interference of backscattered Rayleigh light and local oscillator light for measurement and demodulation. In terms of strain sensing, OFDR faces the problem of demodulation errors caused by reduced spectral similarity. By adopting dynamic reference spectra and solution methods based on local spectral similarity, these problems can be avoided and the range and accuracy of strain measurement can be improved. OFDR has broad application prospects and value in many fields, and will provide effective solutions for monitoring and detection tasks in the fields of aviation, submarines, medical care, geological exploration, etc.

[0008] However, the above method cannot demodulate signals whose spectrum shift exceeds 50%. Summary of the invention

[0009] In order to solve the technical problem in the prior art that the existing OFDR strain measurement method cannot demodulate the signal with a spectral offset exceeding 50%, the technical solution provided by the present invention is as follows:

[0010] A strain value measuring method, the method comprising:

[0011] The step of collecting the distance domain of the sensing optical fiber signal;

[0012] A step of obtaining a scattered spectrum signal through the distance domain;

[0013] A step of extracting the front end and the end local segments of the scattering spectrum signal by a rectangular window function, and obtaining two segment reference state signals;

[0014] A step of performing a cross-correlation operation on the reference state signal and the measurement station signal;

[0015] The step of comparing the values ​​of the cross-correlation operations corresponding to the two signals and taking the larger value as the result.

[0016] Furthermore, a preferred embodiment is provided, wherein the distance domain is obtained by interpolating and zero-filling the sensing optical fiber segment of the distance domain.

[0017] Furthermore, a preferred embodiment is provided, wherein the scattering spectrum signal is a Rayleigh scattering spectrum signal.

[0018] Furthermore, a preferred embodiment is provided, wherein the scattering spectrum signal is obtained by inverse discrete Fourier transform.

[0019] Furthermore, a preferred embodiment is provided, wherein the two segment reference state signals are obtained by interpolating and zero-filling the front end and the end local segments.

[0020] Based on the same inventive concept, the present invention also provides a strain value measuring device, the device comprising:

[0021] A module for collecting distance domain of sensing fiber signals;

[0022] A module for obtaining a scattered spectrum signal through the distance domain;

[0023] A module for extracting the front end and the end local segments of the scattering spectrum signal through a rectangular window function, and obtaining two segment reference state signals;

[0024] A module for performing cross-correlation operation between the reference state signal and the measurement station signal;

[0025] A module for comparing the values ​​of the cross-correlation operations corresponding to the two signals and taking the larger value as the result.

[0026] Based on the same inventive concept, the present invention also provides an OFDR strain measurement method, the method comprising:

[0027] The step of collecting the optical fiber signal when no strain occurs as a reference state signal;

[0028] The step of collecting the signal of the optical fiber as the measurement state signal when the strain occurs;

[0029] Perform Fourier transform on the two signals to obtain the steps in the distance domain respectively;

[0030] A measuring step of measuring the distance domain by the method described;

[0031] The step of traversing the entire sensing optical fiber segment to be measured through the measuring step to obtain the strain distribution.

[0032] Based on the same inventive concept, the present invention also provides an OFDR strain measurement device, the device comprising:

[0033] A module for collecting optical fiber signals when no strain occurs as reference state signals;

[0034] When strain occurs, the signal of the optical fiber is used as a module for measuring state signals;

[0035] Perform Fourier transform on the two signals respectively to obtain modules in the distance domain;

[0036] A measuring module for measuring the distance domain through the device;

[0037] The entire sensing optical fiber segment to be measured is traversed through the measuring steps to obtain a module of strain distribution.

[0038] Based on the same inventive concept, the present invention also provides a computer storage medium for storing a computer program. When a computer reads the computer program, the computer executes the method described.

[0039] Based on the same inventive concept, the present invention also provides a computer, including a processor and a storage medium. When the processor reads a computer program stored in the storage medium, the computer executes the method described.

[0040] Compared with the prior art, the OFDR strain measurement method provided by the present invention proposes a strain measurement method based on double-segment Rayleigh scattering spectrum correlation, which uses the front and rear parts of the reference state signal as local segments, inserts zeros to fill the original length, and then cross-correlates them with the measurement state signal, and takes the cross-correlation with the larger maximum cross-correlation value as the final result.

[0041] Compared with the prior art, the OFDR strain measurement method provided by the present invention is different from the traditional method. The traditional method may produce calculation errors, while the OFDR strain measurement method provided by the present invention can be well demodulated.

[0042] Suitable for OFDR strain measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a schematic diagram of the signal offset mentioned in the eleventh embodiment;

[0044] Figure 2 Schematic diagram of the strain demodulation method based on double-segment Rayleigh scattering spectrum correlation mentioned in the eleventh embodiment;

[0045] Figure 3 This is a schematic diagram of the normalized cross-correlation mentioned in the eleventh embodiment;

[0046] Figure 4 A schematic diagram comparing the conventional method mentioned in Embodiment 11 with the method provided by the present invention;

[0047] Figure 5 This is a schematic diagram of the bidirectional strain solution results of the method provided by the present invention mentioned in the eleventh embodiment. DETAILED DESCRIPTION

[0048] In order to make the advantages and benefits of the technical solution provided by the present invention more clearly reflected, the technical solution provided by the present invention is now further described in detail with reference to the accompanying drawings, specifically:

[0049] Embodiment 1: This embodiment provides a strain value measurement method, the method comprising:

[0050] The step of collecting the distance domain of the sensing optical fiber signal;

[0051] A step of obtaining a scattered spectrum signal through the distance domain;

[0052] A step of extracting the front end and the end local segments of the scattering spectrum signal by a rectangular window function, and obtaining two segment reference state signals;

[0053] A step of performing a cross-correlation operation on the reference state signal and the measurement station signal;

[0054] The step of comparing the values ​​of the cross-correlation operations corresponding to the two signals and taking the larger value as the result.

[0055] Embodiment 2: This embodiment further limits the strain value measurement method provided in embodiment 1, and the distance domain is obtained by interpolating and zero-filling the sensing optical fiber segment in the distance domain.

[0056] Embodiment 3: This embodiment further limits the strain value measurement method provided in embodiment 1, and the scattering spectrum signal is a Rayleigh scattering spectrum signal.

[0057] Embodiment 4: This embodiment further limits the strain value measurement method provided in embodiment 3, and the scattered spectrum signal is obtained by inverse discrete Fourier transform.

[0058] Embodiment 5: This embodiment further limits the strain value measurement method provided in embodiment 1, and the two segment reference state signals are obtained by interpolating and zero-filling the front and end local segments.

[0059] Embodiment 6: Embodiment 1 provides a strain value measuring device, the device comprising:

[0060] A module for collecting distance domain of sensing fiber signals;

[0061] A module for obtaining a scattered spectrum signal through the distance domain;

[0062] A module for extracting the front end and the end local segments of the scattering spectrum signal through a rectangular window function, and obtaining two segment reference state signals;

[0063] A module for performing cross-correlation operation between the reference state signal and the measurement station signal;

[0064] A module for comparing the values ​​of the cross-correlation operations corresponding to the two signals and taking the larger value as the result.

[0065] Embodiment 7: This embodiment provides an OFDR strain measurement method, the method comprising:

[0066] The step of collecting the optical fiber signal when no strain occurs as a reference state signal;

[0067] The step of collecting the signal of the optical fiber as the measurement state signal when the strain occurs;

[0068] Perform Fourier transform on the two signals to obtain the steps in the distance domain respectively;

[0069] A measuring step of measuring the distance domain by the method provided in the first embodiment;

[0070] The step of traversing the entire sensing optical fiber segment to be measured through the measuring step to obtain the strain distribution.

[0071] Specifically,

[0072] In order to solve the problem of large strain demodulation errors, this embodiment proposes a strain measurement method based on dual-segment Rayleigh scattering spectrum correlation, which uses the front and rear parts of the reference state signal as local segments, inserts zeros to fill the original length, and then cross-correlates them with the measurement state signal, and takes the cross-correlation with the larger maximum cross-correlation value as the final result.

[0073] The method comprises the following steps:

[0074] Step 1: When the optical fiber is in its initial state, a set of signals is measured as reference state signals for measurement, and the optical fiber strain magnitude at this time is regarded as 0.

[0075] Step 2: Collect the signal when the strain occurs as the measurement state signal.

[0076] Step 3: Perform discrete Fourier transform on the two sets of beat signals to convert the signals into the distance domain (frequency domain).

[0077] Step 4: Use a moving window to select two distance domain signal sensing optical fiber segments.

[0078] Step 5: Perform interpolation and zero padding on the two distance domain segments processed in step 4 to fill them to the original distance domain length.

[0079] Step 6: Perform inverse discrete Fourier transform on the two range domain signals processed in step 5 to obtain two Rayleigh scattering spectrum signals (time domain).

[0080] Step 7 uses a rectangular window function to extract the front and end local segments of the reference state spectrum signal obtained in step 6, and interpolates and fills zeros to the original length to obtain a double-segment reference state signal. The obtained measurement state spectrum signal at the specified position is not processed.

[0081] Step 8 performs cross-correlation operations on the dual-segment reference state spectral signal obtained in step 7 and the measured state signal respectively, compares the maximum values ​​of the two cross-correlations, takes the cross-correlation with the larger maximum value as the result of this calculation, records the spectral offset corresponding to the maximum value, and calculates the strain value.

[0082] Step 9, repeat the above steps until the entire sensing optical fiber segment is traversed, and the strain distribution of the sensing optical fiber segment can be obtained.

[0083] Embodiment 8: This embodiment provides an OFDR strain measurement device, the device comprising:

[0084] A module for collecting optical fiber signals when no strain occurs as reference state signals;

[0085] When strain occurs, the signal of the optical fiber is used as a module for measuring state signals;

[0086] Perform Fourier transform on the two signals respectively to obtain modules in the distance domain;

[0087] A measuring module for measuring the distance domain by means of the device provided by implementation mode 6;

[0088] The entire sensing optical fiber segment to be measured is traversed through the measuring steps to obtain a module of strain distribution.

[0089] Embodiment 9: This embodiment provides a computer storage medium for storing a computer program. When a computer reads the computer program, the computer executes the method provided in embodiments 1 to 5 or 7.

[0090] Embodiment 10: This embodiment provides a computer, including a processor and a storage medium. When the processor reads a computer program stored in the storage medium, the computer executes the method provided in embodiments 1 to 5 or 7.

[0091] Implementation Method XI: Combination Figure 1-5This embodiment is described. This embodiment provides a specific example of the OFDR strain measurement method provided in Embodiment 7 to demonstrate its advantages and beneficial effects. Specifically:

[0092] When the optical fiber is pulled or compressed, its Rayleigh scattering spectrum (time domain) will shift. Figure 1 As shown, this will cause the signal after tension and compression to differ from the reference state signal without strain, reduce the signal similarity, reduce the cross-correlation signal-to-noise ratio, and even cause demodulation errors when the strain is too large. To address this problem, the present invention proposes a strain demodulation method based on double-segment Rayleigh scattering spectrum correlation, using two parts that deviate from the middle position of the reference state signal in different directions as zero insertion sequences, and performing cross-correlation operations with the measured signal respectively, comparing the two cross-correlation maxima, and using the operation result with the strongest correlation. The schematic diagram is shown in Figure 2 shown.

[0093] When the optical fiber is strained, a phase term related to the strain will be generated in the phase of the backscattered Rayleigh light signal:

[0094]

[0095] Where ε is the strain, n is the effective refractive index of the optical fiber, λ is the central wavelength of the scattered light, l is the distance from the starting point of the optical fiber to the scattering point, and P 11 and P 12 is the elastic-optical coefficient and μ is the Poisson's ratio.

[0096] Substituting the corresponding parameters of the quartz optical fiber material into the equation:

[0097]

[0098] Where f is the central frequency of the scattered light. According to the time-shift-phase shift theory, the phase shift can be regarded as the shift of the signal in the time domain. The cross-correlation results of discrete data using the cross-correlation strain demodulation method are as follows:

[0099]

[0100] In the above formula, n represents the nth point of the signal collected by the acquisition card, N represents the total number of points collected, m represents the number of signal offset points, and I ab It represents the reference state signal, which specifically means the sum of the Rayleigh scattering signals from the starting point a to the end point b of the optical fiber to be measured in the initial state of the optical fiber. s Indicates the measurement state signal corresponding to the same optical fiber position, and the value range of n is an integer from -(N-1) to (N-1). In the frequency modulation range of 1nm, the frequency modulation speed is 10nm / s, the sampling frequency is 400kHz, the sensing unit selects 40 points (32mm) in the distance domain, and the cross-correlation results of the strain position when the loading strain size is 0~400με are as follows Figure 3 As shown, it can be seen that the greater the strain, the weaker the correlation between the two signals.

[0101] The local reference state signal selected by this method corresponds to the n1th to n2th points of the original signal, and the expressions of the two fragments of the reference state signal can be written as:

[0102]

[0103]

[0104] The results of cross-correlation operation using the two fragments as reference state signals are:

[0105]

[0106]

[0107] Compare the maximum values ​​of the two equations and select the cross-correlation result with the largest correlation as the final demodulation result. It can be seen from the above equation that as long as the selected local reference state segment is also included in the measured state signal, there will be no problem of reduced signal correlation due to the new signal segment generated by strain. The delay corresponding to the maximum cross-correlation value is the delay offset, and the final strain value expression can be expressed as:

[0108]

[0109] In the formula, k is the frequency modulation speed, R p To compare the obtained results, T s It represents the sampling time interval of the acquisition card, f0 represents the initial optical frequency of the laser linear frequency modulation, and the corresponding relationship between the sampling time interval and the sampling frequency is T s =1 / F s .

[0110] The simulation results obtained by using the common cross-correlation method and the method provided in this embodiment to solve the fiber segment loaded with 500με strain are as follows: Figure 4 As shown in FIG. 1 , at this time, since the new spectral fragments generated by strain account for about 60% of the total signal length, the traditional method will produce calculation errors, while the method of the present invention can demodulate well. The final simulation results of strain demodulation in two directions using the method of the present invention are as follows: Figure 5 shown.

[0111] The technical solution provided by the present invention is further described in detail above through several specific implementation modes in order to highlight the advantages and benefits of the technical solution provided by the present invention. However, the several specific implementation modes described above are not intended to be used as limitations on the present invention. Any reasonable modification and improvement of the present invention, combination of implementation modes and equivalent substitution within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

[0112] In the description of this specification, only the preferred embodiments of the present invention are provided, and the scope of the rights of the present invention cannot be limited by this. In addition, the description of the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or N embodiments or examples in a suitable manner. In addition, in the absence of mutual contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples. In addition, the terms "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" can explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "N" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined. Any process or method description in the flowchart or otherwise described herein can be understood to represent a module, fragment or portion of code including one or more executable instructions for implementing the steps of a custom logic function or process, and the scope of the preferred embodiment of the present invention includes other implementations, in which the functions may not be performed in the order shown or discussed, including in a substantially simultaneous manner or in a reverse order according to the functions involved, which should be understood by a person skilled in the art of the art to which the embodiments of the present invention belong. The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a sequenced list of executable instructions for implementing the logic function, can be specifically implemented in any computer-readable medium for use by an instruction execution system, device or device (such as a computer-based system, a system including a processor or other system that can fetch instructions from an instruction execution system, device or device and execute instructions), or in combination with these instruction execution systems, devices or devices. For the purpose of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit programs for use by an instruction execution system, device or device or in combination with these instruction execution systems, devices or devices. More specific examples (a non-exhaustive list) of computer-readable media include the following: an electrical connection having one or N wirings (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), a fiber optic device, and a portable compact disk read-only memory (CDROM).In addition, the computer readable medium can even be paper or other suitable medium on which the program can be printed, because the program can be obtained electronically, for example, by optically scanning the paper or other medium, then editing, interpreting or processing in other suitable ways as necessary, and then storing it in a computer memory. It should be understood that the various parts of the present invention can be implemented with hardware, software, firmware, or a combination thereof. In the above-mentioned embodiment, N steps or methods can be implemented with software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented with hardware, as in another embodiment, any one of the following technologies known in the art or their combination can be used to implement: a discrete logic circuit with a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit with a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0113] A person of ordinary skill in the art can understand that all or part of the steps carried by the method of the above-mentioned embodiment can be completed by instructing the relevant hardware through a program, and the program can be stored in a computer-readable storage medium, and the program, when executed, includes one or a combination of the steps of the method embodiment. In addition, each functional unit in each embodiment of the present invention can be integrated into a processing module, or each unit can exist physically separately, or two or more units can be integrated into one module. The above-mentioned integrated module can be implemented in the form of hardware or in the form of a software functional module. If the integrated module is implemented in the form of a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium.

Claims

1. OFDR strain measurement method, characterized in that: The method comprises: The step of collecting the optical fiber signal when no strain occurs as a reference state signal; The step of collecting the signal of the optical fiber as the measurement state signal when the strain occurs; Perform Fourier transform on the two signals to obtain the steps in the distance domain respectively; A step of obtaining a scattered spectrum signal through the distance domain; The steps of extracting the front end and the end local fragments of the scattering spectrum signal of the reference state by a rectangular window function, and obtaining two fragment reference state signals; The two segment reference state signals are obtained by interpolating and zero-padding the front and end local segments to their original lengths; The steps of performing a cross-correlation operation on the two segment reference state signals and the measurement state signal; comparing the values ​​of the cross-correlation operation corresponding to the two segment reference state signals, taking the larger value as the result, and calculating the strain value; The step of traversing the entire sensing optical fiber section to be measured to obtain the strain distribution.

2. The OFDR strain measurement method according to claim 1, characterized in that: The distance domain is obtained by interpolating and zero-filling the sensing optical fiber segment of the distance domain.

3. The OFDR strain measurement method according to claim 1, characterized in that: The scattered spectrum signal is a Rayleigh scattered spectrum signal.

4. The OFDR strain measurement method according to claim 3, characterized in that: The scattering spectrum signal is obtained by inverse discrete Fourier transform. 5.OFDR strain measurement device, characterized in that, The device comprises: A module for collecting the optical fiber signal when no strain occurs as a reference state signal; a module for collecting the optical fiber signal when strain occurs as a measurement state signal; Perform Fourier transform on the two signals respectively to obtain modules in the distance domain; A module for obtaining a scattered spectrum signal through the distance domain; The front and end local segments of the reference state scattering spectrum signal are extracted by a rectangular window function, and two modules of the reference state signals of the segments are obtained; The two segment reference state signals are obtained by interpolating and zero-padding the front and end local segments to their original lengths; A module for performing a cross-correlation operation on the two segment reference state signals and the measurement state signal; a module for comparing the values ​​of the cross-correlation operation corresponding to the two segment reference state signals, taking the larger value as the result, and calculating the magnitude of the strain value; The module traverses the entire sensing optical fiber section to be tested and obtains the strain distribution.

6. A computer storage medium for storing a computer program, characterized in that: When a computer reads the computer program, the computer executes the method of claim 1.

7. A computer, comprising a processor and a storage medium, characterized in that: When the processor reads the computer program stored in the storage medium, the computer executes the method of claim 1 .

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