Method and device for the physical measurement of environmental and operating conditions

Description

Technical field

[0001] The invention relates to the field of physical measurement of environmental and operational conditions, particularly in the field of monitoring the temperature and deformation of parts such as aeronautical structural parts using fiber optic technology. The invention relates in particular to the simultaneous measurement of the temperature and deformation of these parts. Prior art

[0002] It is known to carry out temperature or strain measurements using fiber optic devices and Bragg gratings. These measurements use either two lines of fiber optics glued to the same structural area or part, each line containing a Bragg grating. For these measurements, either the Bragg gratings are spatially close to each other so that this proximity allows both Bragg gratings to sense the same temperature variation and the same strain, or a fiber equipped with two Bragg gratings is used, one to detect the temperature and the other the strain, but in this case the Bragg gratings are distant from each other and do not measure the variations exactly at the same location.

[0003] Particularly for measuring physical quantities such as strain and temperature via fiber optic technology based on Bragg gratings, it is traditionally planned to have two fiber lines. One possibility is to have a first fiber that has a Bragg grating attached to a host structure and is sensitive to strain and temperature and a second fiber that has a Bragg grating not attached to the host structure. In the latter case, the second fiber can be slid into a capillary itself attached to the host structure and sensitive to temperature. The two fibers, each equipped with its grating, make it possible to discriminate the effect of temperature and strain at the level of the first fiber.

[0004] In the case of a mesh M, one fiber will have a plurality M of Bragg gratings and will be sensitive to deformation at the M points of the mesh while the other will have a plurality N of Bragg gratings and will be sensitive to temperature at the N points of the mesh, M possibly being equal to N.

[0005] Another example of a device comprising two fibers each equipped with a network is known for example from document US 2014326078 A1. In this document the fibers are twisted and the networks are arranged close to each other. In this case, the number of networks is multiplied by two in the case of use for meshing a room.

[0006] Single-fiber devices exist as described in the paper "Implementation of blind source separation for optical fiber sensing" Qiang Li, Zhi Wang, Zejia Huang, Kaili Guo, and Lanlan Liu, Optical Society of America APPLIED OPTICS Vol. 53, No. 9, 20 March 2014.

[0007] In the said publication, source separation tools are defined in the time domain in order to discriminate temperature and strain measurement using a single fiber optic line with two Bragg gratings inscribed. However, temperature and strain measurements are not made at the same location and lose precision.

[0008] Document EP 3 062 078 A1 relates to a measuring system for biological diagnosis comprising an optical fiber with a Bragg grating using a single interrogation frequency.

[0009] The paper MARS JEROME I ET AL: "Source separation and distributed sensing: The key for an efficient monitoring", 2013 5TH IEEE INTERNATIONAL WORKSHOP ON COMPUTATIONAL ADVANCES IN MULTI-SENSOR ADAPTIVE PROCESSING (CAMSAP), IEEE, December 15, 2013 (2013-12-15), pages 264-267, XP032553037, DOi: 10.1109 / CAMSAP.2013.6714058 concerns an improvement in measurement accuracy from an optical fiber by a method of source separation from backscattered light and signal processing at an interrogator detecting an optical fiber as an array of individual sensors.

[0010] Document WO2015 / 067292 A1 relates to a distributed Brillouin sensor device. Technical problem

[0011] It is desirable to reduce the number of fibers used, particularly in complex structures to be monitored, and to be able to use a single fiber combining temperature and strain measurements at the same points without increasing the number of Bragg gratings carried by the fiber in the case of a mesh. Statement of the invention

[0012] With this in mind, the present application proposes a method for measuring localized temperature and deformation.

[0013] To this end, the present invention relates to a method for measuring the temperature and deformation of a part or structure from a single optical fiber line having a single Bragg grating, comprising the following steps: an interrogation of the optical fiber based on two sampling frequencies; one being a low frequency ( f sl ) and the other being a high frequency ( f sh ), said low frequency f slbeing adapted to capture slow dynamics corresponding to the temperature variation, said high frequency f sh being adapted to capture rapid dynamics corresponding to vibration dynamics; An algorithmic brick based on a blind source separation (SAS) technique, comprising a frequency analysis including a Fourier transform, a multivariate analysis and Higher Order Statistics. This method allows using a single fiber and grating while making the measurements at the same location for a simple measurement and reducing the number of fibers and Bragg gratings while making the measurements at the same locations when a part is in use / operation to obtain a map of the temperatures and deformations of a part or structure

[0014] According to an advantageous embodiment, the high frequency f sh is a multiple of the low frequency f sl .

[0015] High frequency f sh can be between a few kHz and a few hundred kHz depending on the type of vibrations or deformations to be measured.

[0016] The low frequency f sl can be between 1 Hz and f sh / 10 which is particularly suitable for measuring temperature variations.

[0017] According to a particular embodiment, the high frequency f sh is a multiple of the low frequency f sl which simplifies the calculations.

[0018] The method may further include a so-called "fixed point" optimization algorithm.

[0019] The method advantageously includes a return to the time domain by means of an inverse Fourier transform to obtain the estimated source(s).

[0020] The present application further relates to a computer program comprising instructions for implementing the method of the application according to one when this program is executed by a processor and a non-transitory recording medium readable by a computer on which is recorded a program for implementing the method of the application when this program is executed by a processor. Brief description of the drawings

[0021] Other characteristics, details and advantages of the invention will appear on reading the detailed description below, and on analyzing the attached drawings, in which: [ Fig. 1 ] schematically represents an optical fiber provided with a Bragg grating; [ Fig. 2 ] schematizes a problem of blind separation of sources; [ Fig. 3 ] diagrams the process steps of the present application; Description of the embodiments

[0022] The drawings and the description below contain, for the most part, elements of a certain character. They may therefore not only serve to better understand the present invention, but also contribute to its definition, if necessary.

[0023] There figure 1 schematically represents an optical fiber 1 provided with a Bragg grating 2.

[0024] When a Fiber Bragg Grating (FBG) is inscribed on an optical line (OL), glued to a structural area / part, it allows both temperature (environmental condition) and deformation (operational condition) stresses to be captured. However, it is not possible to discriminate the influence of temperature from the influence of deformation without additional measurement.

[0025] In the prior art, in order to discriminate / measure the contributions of said physical quantities, it is necessary to have two FO lines glued to the same structural area / part, each having a Bragg grating spatially close to each other so that this proximity would allow the two Bragg gratings to feel the same temperature variation and the same deformation: Either :

[0026] Two FO lines glued on a structural zone, each having an inscribed Bragg grating,λ B1 ,λ B2 , the characteristic wavelengths of said Bragg gratings.

[0027] When the structural area / part is simultaneously subjected to a variation in temperature (environmental condition), stress state and dynamic vibration (operational condition), the opto-thermomechanical interaction is written as follows: Δλ B 1 = K ε 1 ε + K T 1 ΔT Δλ B 2 = K ε 2 ε + K T 2 ΔT K ε 1 = 1 − ρ λ B 1 , K T 1 = α + η λ B 1 K ε 2 = 1 − ρ λ B 2 , K T 1 = α + η λ B 2 Or : Δλ B1 , Δλ B2 represent the variation of the wavelength of the two Bragg gratings λ B1 , λ B2 , due to the environmental and operational conditions, ε, ΔT represent respectively the deformation (relative elongation) and the variation of the temperature, α, η and ρ are respectively the coefficients of thermal expansion, thermo-optics and photoelasticity, intrinsic to the fiber, {K ε1 ,K ε2} are the parameters of sensitivity to the deformation of the two Bragg gratings, (K T1 ,K T2} are the parameters of sensitivity to the temperature of the two Bragg gratings.

[0028] In matrix form, equation (1) is written as follows: Δλ B 1 Δλ B 2 = K ε 1 K T 1 K ε 2 K T 2 ︸ T ε ΔT where the matrix T is called the "wavelength shift matrix"

[0029] Knowing the measurement emanating from the 2 Bragg gratings, i.e. Δλ B1 , Δλ B2 , and the sensitivity parameters {K ε1 ,K ε2 ,K T1 ,K T2}, the physical quantities ε and ΔT can be calculated by inverting the matrix T: ε ΔT = 1 K ε 1 K T 2 − K T 1 K ε 2 K T 2 K T 1 − K ε 2 K ε 1 Δλ B 1 Δλ B 2

[0030] However, when the two Bragg gratings are spatially close to each other, the strain and temperature sensitivity parameters are almost equal: K ε1 ≈K ε2 and K T1 ≈K T2 . This would lead to a non-solution of equation (4).

[0031] This leads to having to separate the networks or to making one of the fibers and its network insensitive to one of the parameters to be measured, which further complicates the positioning of the fibers.

[0032] The present application aims to overcome such complexity.

[0033] To do this, the present application proposes to simultaneously measure the temperature and deformation of a structural area / part from a single optical fiber line having a single Bragg grating.

[0034] The invention first uses a data acquisition measurement brick (optronic interrogator). During this acquisition, the interrogator measures the wavelength associated with the light reflected by the optical fiber through the Bragg gratings and converts it into an engineering unit (for example in Volt) so that it can be used by a processing unit. Said brick is interrogated successively according to two sampling frequencies: one being a low frequency (denoted f sl ) and the other being a high frequency (denoted f sh ), as represented in figure 1 .

[0035] According to the invention, the low frequency f sl is suitable for capturing slow dynamics which will highlight the temperature variation which is data varying relatively slowly compared to mechanical vibration dynamics while the high frequency f shis suitable for capturing faster dynamics highlighting vibrational dynamics and therefore deformation. It should also be noted that f sh is a multiple of f sl in order to have the same number of time samples while also playing on the acquisition times. This approach would avoid calculation errors in the part related to the multivariate analysis. For information, f sh can be between a few kHz and a few hundred kHz. As for f sl , it can be between 1 Hz and f sh / 10 The choice of these frequencies may depend on the chosen application and the dynamic difference between the temperature variation and the vibration frequency, the frequencies being chosen to be at least double the maximum variation frequency to be measured.

[0036] The invention secondly uses an algorithmic building block based on a blind source separation (BSS) technique, whose tools use frequency analysis, multivariate analysis and Higher Order Statistics in order to take advantage of the hypothesis of statistical independence associated with the physical information of temperature and deformation.

[0037] Higher Order Statistics (HOS) concerns moments and cumulants of order higher than 2. They are used in addition to second-order statistics and give a more complete description of the data and their properties.

[0038] The combination of these bricks aims to separate the measurement of temperature variation and the measurement of deformation to discriminate the effect of temperature from the effect of vibration.

[0039] Taking the matrix equation defined previously: Δλ B 1 Δλ B 2 = K ε 1 K T 1 K ε 2 K T 2 ︸ M ε ΔT

[0040] From an advanced signal processing point of view, the model defined in equation (5) is a blind source separation problem.

[0041] The scientific problem of source separation consists of extracting a set of unobservable signals, called "source signals", from a set of observable signals as schematized in figure 2 where the source signals S 1 , S 2 , ..., S p are mixed with noise or interference W 1 , W 2 , ... , W n to give mixed signals Y 1 , Y 2 , ...., Y n which must be processed by a source separation module 10 to obtain estimated sources Ŝ 1 , Ŝ 2 , ... , Ŝ p . These observations come from sensors, for example: microphones, antennas, cameras, piezoelectric transducers, etc... In the context of the present application, the source signals represent the variation of the temperature and the deformation of the structural zone / part and the observable signals emanate from a single Bragg grating inscribed on a FO line, successively interrogated with a low and high sampling frequency possibly with the same number of time samples. Taking into account the discrete time k and the transfer function of the medium, the model defined in equation (5) is a mixture model, and is expressed according to the following convolution model: y _ k = T k ∗ s _ k where as represented in the figure 3 : y _ k = y 1 k y 2 k = Δλ B 1 k Δλ B 2 k is the vector 100 of measurements observable at time k emanating from the Bragg grating, whose interrogation was carried out according to two sampling frequencies: low f sl and high f sh , s _ k = s 1 k s 2 k is the source vector at time k, reflecting the measurement emanating from the variation of temperature and deformation.

[0042] T(k) is the mixing matrix reflecting the impulse response of the medium, ie . transfer function of the structural part to be monitored.

[0043] In order to solve the separation problem: estimate the source vector s knowing only the measurement vector y , we define the following steps taken in figure 3 which represents an example of a method of application from mixed sources 100.

[0044] The different steps required to solve the source separation problem are: ( i ) the transition of the measured signals from the time domain to the frequency domain; ( ii) the use of a Higher Order Statistics tool in order to take advantage of the independence of the sources and pose the objective function and an optimization algorithm based on the "fixed point" in order to find a mixing matrix allowing the separation of the 2 source signals in the frequency domain; ( iii ) the passage of the source signals in the time domain by means of the inverse Fourier transform to obtain the estimated sources ŝ (k).

[0045] Step 1 : Passage into the frequency domain in order to transform the convolution product into a multiplication by a Fourier transform 200: Y _ f = T × S _ f Or : Y _ f ∈ ℂ is the Fourier transform of the measurement vector (complex vector), S _ f ∈ ℂ is the Fourier transform of the source vector that we are trying to determine, T is the mixing matrix. Step 2 :

[0046] Use of the following tools in three sub-steps: Linear algebra 300 applied to complex numbers, Higher Order Statistics to implement the hypothesis of statistical independence of sources 400, optimization algorithm called "fixed point" 450 to find the separation matrix (noted W).

[0047] These sub-steps allow to estimate the source vector in the frequency domain (denoted Ŝ (f)) 500: S ^ _ f = s 1 f s 2 f = W × Y _ f

[0048] Taking the mixing model in the frequency domain: Y _ f = T × S _ f

[0049] The separation model is defined by: S _ f = W Y _ f where W is the separation matrix.

[0050] We define an objective function noted JG (w) based on Higher Order Statistics: J G w = E G w H y 2 where: E is the mathematical expectation operator.

[0051] Find the matrix W by maximizing the objective function: maximizes ∑ j = n J G w j with respect to wj under constraint E{(wk H< x)(wj H< x) *<} = δ jk where: δ jk = 1 pour j = k 0 pour j ≠ k Step 3 :

[0052] Transition to the time domain via the inverse Fourier transform 600 to obtain the estimated sources ŝ (k) 700: s ^ _ k = s ^ 1 k s ^ 2 k = 1 N ∑ n = 0 N − 1 S ^ _ f e 2 πkn / N Or : N is the number of frequency points used in the calculation of the inverse Fourier transform: N = T × f sh , with T representing the acquisition time. n is the frequency index.

[0053] This allows us to find the 800 estimates of the deformation. ŝ (v) and temperature ŝ (t) with the corresponding frequency indices. Industrial application

[0054] The invention may be applied in particular to the monitoring of parts or structures of aircraft, space launchers or other systems where temperature and vibrations have an influence on the operation of the systems.

Claims

1. A method for measuring temperature and deformation of a part or structure on the basis of a single line of optical fiber having a single Bragg grating, characterized in that it comprises the following steps: a. - interrogating (50) the optical fiber based on two sampling frequencies; one being a low frequency (fsl) and the other being a high frequency (fsh), said low frequency fsl being suitable for sensing a slow rate of change corresponding to the temperature variation, said high frequency fsh being suitable for sensing a fast rate of change corresponding to vibrations; b. - implementing an algorithmic block based on a blind-source-separation (BSS) technique, comprising a frequency analysis (200), a multivariate analysis (300), higher-order statistics (400); c. - returning to the time domain via an inverse Fourier transform (600).

2. The measurement method according to claim 1, for which the high frequency fsh is a multiple of the low frequency fsl.

3. The measurement method according to claim 1 or 2, for which the high frequency fsh is comprised between a few kHz and a few hundred kHz.

4. The measurement method according to claim 1, 2 or 3, for which the low frequency fsl, is comprised between 1 Hz and fsh / 10.

5. The measurement method according to any one of the preceding claims, for which the high frequency fsh is a multiple of the low frequency fsl.

6. The measurement method according to any one of the preceding claims, comprising an optimization algorithm called "fixed point" optimization algorithm (450).

7. A computer program comprising instructions for implementing the method according to one of claims 1 to 6 when this program is executed by a processor.

8. A computer-readable non-transitory recording medium on which is recorded a program according to claim 7.