The present disclosure relates to a system and method for monitoring by optical fiber an object/structure for the measurement and localization of even isolated, high-frequency vibrations.
The field of application is generally the monitoring of objects/structures subject to micro-crack phenomena such as composite tanks, pipelines for the transport of fluids, oil wells, wind turbines, concrete structures, historical buildings and other structures or similar objects.
Various systems and methods using acoustic sensors distributed along a transmission channel of optical fiber are known for the detection of vibration events. These systems employ methodologies of the type “acoustic sensing” or DAS acronym for Distribution Acoustic Sensing, as for example described in the book by A. Hartog, “An Introduction to Distributed Optical Fibre Sensors”, CRC Press, 2018.
Such known solutions are satisfactory in various aspects and used to detect medium or low frequency events. The detectable band or frequency is related to the time interval it takes for the light to travel back and forth in the transmission channel or optical connection. For example, for a 3 km long optical connection the detectable acoustic band is limited to about 17 kHz.
There is interest in increasing the measurement band to detect events that are considered at high or very high frequency, i.e. events that reach, for example, on the same optical connection lengths, of about 3 km, bands of not less than 500 kHz.
A known solution to increase the measurement band is described in the article by L. Marcon et al., “High-frequency high-resolution distributed acoustic sensing by optical frequency domain reflectometry” (Opt. Express, vol. 27, pp. 13923-13933, May 2019, doi: 10.1364/OE.27.013923). The solution involves using an OFDR scheme, acronym for Optical Frequency Domain Reflectometry with a high coherence laser that can be tuned on a band of a few tens of nanometers. This solution can be used for extremely short optical connection lengths and is therefore hardly applicable in the technical field of interest. In addition, continuous measurements over time require the use of two laser sources in parallel and high-capacity processing boards. These requirements entail extremely high costs, so the transportability in the field is very critical and of little interest.
Other known solutions are described in the article by P. Ma et al., “Probabilistic Event Discrimination Algorithm for Fiber Optic Perimeter Security Systems” (Journal of Lightwave Technology, vol. 36, no. 11, pp. 2069-2075, 1 Jun. 2018, doi: 10.1109/JLT.2018.2802324).
These solutions are based on interferometric schemes that require access from both ends of the optical connection and therefore cannot be used for measurements, for example in the well. This naturally reduces the applicability of the solutions described.
An other solution is described in the article by di C. Dorize et al: “An OFDM-MIMO Distributed Acoustic Sensing over Deployed Telecom Fibers”, 2021 Optical Fiber Communications conference and Exhibition (OFC), OSA, 6 Jun. 2021, pages 1-3, XP033947695, and in the European patent application EP3694117A1 “Multi-carrier coherent coded distributed acoustic sensing” filed by Nokia Technologies OY [FI] and published on 12 Aug. 2020. An other solution is described in the article by M. Wu et al: “Frequency Response Enhancement of Phase-Sensitive OTDR for interrogating Weak Reflector Array by Using OFDM and Vernier Effect”, published in the “Journal of Lightwave Technology”, IEEE, vol. 38, n. 17, 9 May 2020, pages 4874-4882, XP011806617.
For the above reasons, the systems and the methods of the prior art are not satisfactory for detecting high and very high frequency events by optical fiber with optical connection lengths over one kilometer and with a single input/output access to which the present disclosure relates.
The technical problem underlying this application is that of devising a system and a method having structural and functional characteristics such as to optimally satisfy the need to detect microcracks generated by high and very high frequency events in objects/structures, overcoming the drawbacks mentioned with reference to the prior art.
The idea of solution underlying the present disclosure is to consider a non-perturbed condition of the object/structure as a reference for the measurement of a perturbation.
Based on this solution idea, the technical problem is solved by a system for monitoring by optical fiber of an object/structure for the measurement of vibrations as defined by an independent claim.
Other preferred embodiments of the system are described by the dependent claims.
A monitoring method using optical fiber for the measurement of vibrations of an object/structure as defined by another independent claim and dependent claims is also an object of the disclosure.
Further features and advantages of the disclosure will result from the following description of a preferred embodiment of the system and of the method and variants thereof provided by way of example with reference to the accompanying drawings, wherein:
With reference to
In the embodiment illustrated in
The optical fiber 3 is associated with the object/structure 2 and constitutes a waveguide with length L that is calculated between an end associated with a circulator 4 and a terminated opposite end. The opposite end of the optical fiber 3 is suitably terminated to obtain a maximum reflectivity of the backscattered signals generated by the optical fiber 3 by, for example, Rayleigh scattering.
The monitoring system 1 comprises a source of symbol 5 which emits in a continuous way, NC input signals sn(τ) n=[0, . . . , NC−1] defining an input vector s(τ). Each input signal sn(τ) comprises a repeating sequence of NS input symbols oi i=[0, . . . , NS−1]. The generated NS input symbols σi are preferably complex and with predefined values for each sequence of the 10 repeated sequences.
According to one embodiment, the repeated sequences in each signal of said NC input signals sn(τ) n=[0, . . . , NC−1] are equal to each other. Moreover, the predefined sequences comprise the same number NS of input symbols σi i=[0, . . . , NS−1]. In an embodiment, a same sequence with translated symbols σi is repeated in each signal of said Nc input signals sn(τ). Thus for instance, the symbols of the sequence of the input signal sn(τ) are translated with respect to the symbol of the sequence of the first input signal s0(τ) by a number equal to the index n—with n=[0, . . . , NC−1]—of the signal input.
In particular, each input symbol σi is generated for a duration T which is defined by the formula:
The monitoring system 1 further comprises a multi-carrier OFDM modulation block 6 that allows generating an OFDM sensing signal SIN(t) with NC subcarriers. The OFDM sensing signal SIN(t) is generated by modulating a light beam 13 emitted by a laser source 12 by means of an OFDM modulating signal x(m), multi-carrier. The OFDM digital modulating signal x(m) is generated on the basis of the input vector s(τ) using the OFDM technique, acronym for Orthogonal Frequency Division Multiplexing. Thus, the OFDM digital modulating signal x(m), is a complex-sampled multi-carrier signal with a number NC orthogonal subcarriers. The OFDM digital modulating signal x(m) comprises a continuous sequence of input symbols or samples wherein m is the sampling index determined with a sampling frequency equal to FS=1/T0=NC/T wherein T is the duration of the symbol σi and T0 is the duration of the OFDM sample.
The circulator 4 sends the multi-carrier sensing signal SIN(t) to the optical fiber 3 and receives a multi-carrier backscattered signal SOUT(t) generated by the optical fiber 3 as backscattering of the sensing signal SIN(t). Each Nc subcarrier of the backscattered signal SOUT(t) comprises an output sequence of backscattered symbols εi for i=[0, . . . , NS−1] which are subsequently extrapolated.
The optical fiber 3 is divided in length L into a number NZ of subsequent points which define NZ spatial discretization cells in which parameters of said optical fiber 3 are estimated, as will be clearer in the following description.
The number NZ can be correlated with some parameters of the optical fiber 3 according to the equation:
With the optical fiber 3 divided into NZ spatial discretization cells, the spatial resolution δZ is equal to:
δZ=L/NZ
An OFDM demodulation block 19 receives the light beam 13 from the laser source 12 to demodulate the backscattered signal SOUT(t) by extracting a demodulated signal, which may be analog (R[y(t)], I[y(t)]) or digital y(m)). The set of the demodulated signals extracted for each of said NC subcarriers determines an output vector u(τ) comprising NC output signals un(τ) n=[0, . . . , NC−1]. Each of the NC output signals un(τ) comprises sequences with backscattered symbols εi for i=[0, . . . , NS−1].
A processing unit 20 receives the output vector u(τ) with the NC output signals un(τ) n=[0, . . . , NC−1] and the input vector s(τ) with NC input signals sn(τ) n=[0, . . . , NC−1] and, on the basis of NC reference vectors,
The NC reference vectors,
The deformation vector v(τ) thus obtained makes it possible to determine the deformation of the object/structure 2 both with regard to the entity, i.e. the physical magnitude of the perturbation, deformation or vibration, and with regard to the position or zone of interest in the length L of the optical fiber 3. Furthermore, it is possible to determine the course of the perturbation over time τ by analysing the deformation vector v(τ).
In the embodiment of
The OFDM modulation block 6 further comprises an analog digital converter or DAC 10 and an I/Q electronic optical modulator 14.
The DAC converter 10 receives the OFDM digital modulating signal x(m) and generates the OFDM analog modulating signal, that comprises two signals R[x(t)] and I[x(t)], multicarrier with NC orthogonal subcarriers.
In each subcarrier NC of the OFDM analog modulating signal, R[x(t)] and I[x(t)], the predefined sequences comprising a number NS of said input symbols oi are repeatedly transmitted.
Preferably, the optical source 12 is equipped with a high coherence laser emitting the light beam 13.
The I/Q modulator 14 modulates the light beam 13, which is received from the optical source 12, by means of the OFDM analog modulating signals, R[x(t)], I[x(t)], to generate the sensing signal SIN(t) comprising NC subcarriers.
The probe signal SIN(t) is transmitted to the optical fiber 3 through the circulator device 4.
The optical fiber 3 being terminated generates a backscattered signal SOUT(t) which will be correlated to the probe signal SIN(t) by the transfer function of the optical fiber 3 itself.
In each of the Nc subcarriers of the sensing signal SIN(t) the sequences of NS symbols included in the respective input signals sn(τ) are repeatedly transmitted
According to an illustrative and non-limiting embodiment, the sequences of NS symbols may be represented by a constellation of symbols according to modulation systems of the PSK Phase-shift keying QAM or or Quadrature Amplitude Modulation digital phase modulation type.
The OFDM demodulation block 19 comprises an I/Q opto-electronic demodulator 15 that receives from the circulator 4 the backscattered signal SOUT(t), comprising Nc subcarriers, and it further receives a second light beam 13′ emitted by the laser source 12 to extract an OFDM demodulated backscattered signal, which comprises two analog signals [y(t)] and
[y(t)]. The second light beam 13′ has an intensity proportional to the intensity of the light beam 13.
By means of an ADC analog digital converter 16, the OFDM demodulated backscattered signal [y(t)] and
[y(t)] is digitised to generate a digital OFDM output signal y(m). The digital OFDM output signal y(m) comprises a sequence of samples in which m is the sampling index.
The ADC converter 16 has a sampling frequency equal to FS=NC/T where T is the sampling index of each input signal sn(τ) and NC is the number of OFDM subcarriers.
By means of an OFDM demodulation unit 18, comprising a series/parallel converter and an FFT (Fast Fourier Transformer) module, the OFDM output vector u(τ) is determined from the OFDM output signal y(m)
According to one embodiment, the processing unit 20 substantially module 21 and a comprises a processing calibration module 25. The two modules are activated for two distinct procedures:
According to the embodiment illustrated in
Each first sliding window block 23, receives the respective input signal sn(τ) from the source of symbols 5 and generates the corresponding vector of antecedent symbols
The perturbation estimation block 22 processes the inputs to output the deformation vector v(τ)
According to one embodiment, the deformation vector v(τ) is determined using the circular matrices i.e:
The calculation matrix N(τ) is a matrix NC×NZ in which the rows
Considering the vectors of NS antecedent symbols
Furthermore, to generate a matrix N(τ) with full rank, considering that the reference vectors
Thus, it is possible to exploit the properties that derive from the use of the calculation matrix N(τ) with full rank, with no null eigenvalues, in which the rows of the matrix are linearly independent of each other.
In this way, the deformation vector v(τ) has a structure given by:
Note that the element of the deformation vector corresponding to the spatial discretization cell (κ0+1) is
Thus, the deformation vector v(τ) makes it possible to determine the intensity or deformation of the perturbation or of the vibration to which the object/structure 2 is subjected. Furthermore, the position of the perturbation is obtained by analysing the discretization cells as well as the time course of the perturbation determined by analysing instead the perturbation vector v(τ) in the time intervals T.
Each reference vector
The calibration module 25 receives as input the NC input signals sn(τ) n=[0, . . . , NC−1] of the input vector s(τ) and by means of the NC first sliding window blocks 23n n=[0 . . . . NC−1], generates the NC vectors of antecedent symbols
Furthermore, the calibration module 25 receives the output vector u(τ) with the Nc output signals un (τ) n=[0, . . . , NC−1]
The calibration module 25 comprises NC estimation blocks 26n, with n=[0 . . . NC−1], adapted to estimate the reference vector
Each estimation block 26n n=[0 . . . NC−1] generates the corresponding reference vector Ry with n=[0 . . . NC−1]
In one embodiment, the calibration module 25 uses the Nc first sliding window blocks 23, of the processing module 21. Alternatively, in an embodiment not illustrated, the calibration module 25 can use further first sliding window blocks.
The Applicant has been able to observe that the monitoring system, according to the present disclosure, allows an accurate estimation of the perturbation as is evident from the graph of
Calculation Matrix N(τ) and Sn(τ) with Full Rank
By way of illustration and not limitation, bearing in mind that the eigenvalues of a circulating matrix associated with the vector sn(0) are the coefficients ŝn(0) of the IDFT of sn(0), to generate a calculation N(τ) or Sn(τ) full-rank matrix, with no null eigenvalues, NS coefficients în(0) are selected making sure that each of them is non-zero; the DFT of the vector of the coefficients ŝn(0) is calculated and the resulting vector sn(0) satisfies the full rank condition. This mode allows wide discretion in the choice of the coefficients ŝn(0). For example, the coefficients can be selected so that the elements of the resulting vector sn(0) have a module that is not too dissimilar to each other.
Alternatively, circulating simplex codes can be used as described for example in the article by Song, Golomb, “Some new constructions for simplex codes,” IEEE Trans. Inf. Theory, 1994. The simplex codes are a set of vectors at equal distances from each other that make up the vertices of a simplex.
Alternatively, the Zadoff-Chu sequences can also be used which are used in LTE mobile telephony systems and described in the article by Chu, “Polyphase codes with good periodic correlation properties,” IEEE Trans. Inf. Theory, 1972 and in the article by Song, Shen, Jia, “Evolved Cellular Network Planning and Optimization for UMTS and LTE” 2011.
The disclosure also refers to a monitoring method using optical fiber for the measurement of vibrations of an object/structure. The measurement of vibrations essentially defines a mapping of the optical fiber with the determination of the position and of the intensity of even isolated, very high-frequency vibrations. For example, frequencies higher than 500 kHz are considered for a length L of the optical fiber equal to 3 km.
In the following description of the method, details and co-operating parts having the same structure and function as the parts included in the system, described above, will be indicated by the same reference numbers and abbreviations.
The monitoring method provides:
The method also provides:
The method provides generating an OFDM analog modulating signal, which comprises two signals R[x(t)], I[x(t)], by means of an OFDM modulation of said Nc input signals sn(τ) n=[0, . . . , NC−1]. An OFDM digital modulating signal x(m) is obtained by means of an analogue digital sampling of the OFDM analog modulating signal, R[x(t)], I[x(t)]). The OFDM digital modulating signal x(m) comprises a sequence of samples in which m is the sampling index determined with a sampling frequency equal to FS=1/T0=NC/T wherein T is the duration of the symbol σi and T0 is the duration of the OFDM sample.
The method therefore provides:
Thus, the method provides:
Furthermore, the method provides for a calibration procedure that allows to estimate for each subcarrier Nc of the sensing signal SIN(t) a corresponding reference vector
The calibration procedure provides:
Calculating said reference vector
The length L of the optical fiber 3 and the specific value of the acoustic band BA fix the minimum period of time T of the predefined sequence comprising the number NS of input symbols σi i=[0, . . . , NS−1] of the input signals si(τ) according to the following equation:
Furthermore, the total band of the multi-carrier OFDM sensing signal SIN(t) is proportional to the square of the acoustic band of the optical fiber 3 according to the equation:
As an example, purely by way of illustration and not limitation, to obtain an acoustic band BA=1 MHZ on an optical fiber of length L=3 km it is necessary to have OFDM input symbols of duration T=100 ns which correspond to a spatial resolution δz=10 m and a minimum number of subcarriers NC=300.
The total band of the OFDM signal is therefore BTOT≈3 GHz.
Number | Date | Country | Kind |
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102021000024347 | Sep 2021 | IT | national |
This application is a 35 U.S.C. § 371 National Stage patent application of PCT/IB2022/058922, filed on 21 Sep. 2022, which claims the benefit of Italian patent application 102021000024347, filed on 22 Sep. 2021, the disclosures of which are incorporated herein by reference in their entirety.
Filing Document | Filing Date | Country | Kind |
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PCT/IB2022/058922 | 9/21/2022 | WO |