DE102007016158B4 - Measuring arrangement and measuring method with fiber Bragg grating measuring points - Google Patents

Abstract

Measurement method for recording and evaluating several measuring points (1, 2, 3) with at least one optical fiber (10), in which several fiber Bragg gratings (1, 2, 3) with different focus points are used as measuring points (1, 2, 3) Fiber Bragg wavelengths λM are arranged into which a light beam with variable wavelength λs is periodically fed, the intensity maximas or peaks of the reflected light beam at the various intensity or intensity caused by the fiber Bragg grating (1, 2, 3) Peak wavelengths λM are detected and evaluated, characterized in that the light wavelength λs rises continuously from a lower light wavelength λu to an upper light wavelength and falls again within the period T, that the intensity maxima of the reflected light beam in time-spaced electrical measured values (M1, M2, M3) are converted so that the electrical measured values (M1, M2, M3) according to the time sequence of the meh rer measuring points (1, 2, 3) generated intensity maximas are supplied one after the other to an interpolation device (8) and that from at least two measured values (M10, M1; ...

Description

The invention relates to a measuring arrangement with fiber Bragg grating measuring points according to the preamble of patent claim 1 and to a measuring method with such fiber Bragg grating measuring points according to the preamble of patent claim 7.

Fiber Bragg grating sensors are robust fiber optic probes that are used in particular for the high-resolution detection of strains. The measuring principle of such fiber Bragg grating sensors is based on the fact that a periodic change in the refractive index is introduced in the direction of radiation of an optical waveguide as a so-called fiber Bragg grating (FBG). This fiber Bragg grating acts like a frequency-selective filter, the lattice constants of which are chosen such that, in the case of a broadband light, a very narrow-band intensity maximum is reflected as a so-called peak. Decisive for the position (focal Bragg wavelength) of the intensity maximum is the spatial distance within the periodic sequence of the zones with different refractive indices. This is used for the employment of measuring points with such fiber Bragg grating sensors preferably for the detection of strains. If an optical waveguide is stretched or compressed in the region of the periodic change in the refractive index, the position of the intensity maximum changes as a peak in its wavelength λ. This shift of the wavelength λ can be detected and evaluated in the sense of a determination of the size of the causative strain. There are several methods to maximally measure these wavelengths or wavelength changes of the reflected intensity and determine the strain values therefrom.

With the so-called edge filter measuring method, the reflected light is passed through an optical filter whose light transmittance changes with the wavelength. If a reflection peak changes its wavelength, this changes the light intensity behind the edge filter, which can be measured with photodiodes. This method is simple, but has the disadvantages that it can evaluate only a single fiber Bragg grating, and can not represent a spectrum, as well as showing large errors in a stray light exposure.

In the so-called CCD (charge coupled device) measuring method, the reflected maximum intensity is spectrally expanded and then projected onto a CCD line. The CCD line is read out and the wavelength of the reflection peaks is determined by an assignment to the pixels of the CCD. This method can measure many fiber Bragg gratings on a fiber or waveguide and also represent the spectrum of only one fiber. Another limitation is that this method is extendable at a reasonable cost only with silicon CCDs. This means a restriction to the wavelength range around 800 nm, whereby the widespread and inexpensive components from the telecommunications areas are not available, since it is preferably carried out in the wavelength range around 1550 nm.

Furthermore, a measuring method with a tunable laser is known for detecting the respective wavelength or wavelength change on fiber Bragg gratings. In this measurement method, no brand-band laser diode is used to generate light, but a laser that emits its entire radiant energy in a very small bandwidth (<0.005 nm). The wavelength of the laser is linearly varied over a measuring range of, for example, 1500 nm to 1600 nm and back again from 1600 nm to 1500 nm in the form of a triangular function. There are strong reflections at the respective peak wavelengths of the fiber Bragg gratings. The entire reflected spectrum of a fiber or waveguide can be determined with a photodiode, which measures the momentarily reflected back radiation energy during the tuning of the laser. Due to the laser's high spectral energy, which is concentrated on the very small bandwidth, it is possible to achieve very large S / N ratios, which allow the energy of the laser to be divided among many waveguides or fibers, so that the total number of connectable Measuring points can become very large. The measurement accuracy of this method is also very high. This measurement method has the disadvantage of using many measuring points with fiber Bragg gratings that a relatively wide wavelength range must be scanned sequentially, which can lead to wavelength changes during the sampling periods, especially in dynamic measurements with a time-varying strain curve. This results in time-dependent deviations, which can lead to errors in a comparative evaluation of the measurement results.

From the DE 101 45 350 A1 a measuring method for detecting and evaluating a plurality of measuring points with at least one optical waveguide is known. In the optical waveguide, a plurality of fiber Bragg gratings with different centroid fiber Bragg wavelengths are arranged as measuring points, in which a light beam having a variable wavelength is periodically fed. The intensity maxima or peaks of the reflective light beams are detected and evaluated at the various intensity or peak wavelengths produced by the fiber Bragg gratings. It is not described, however To allow light wavelengths of the injected light beam to rise continuously, to convert the reflected intensity maxima into temporally spaced electrical measured values and to calculate simualtane measured values from the successively acquired measured values.

From the article by SH Yun, DJ Richardson and BY Kim "Interrogation of fiber grating sensor arrays wuth a wavelength-swept fiber laser", Optics Letters, Vol. 23, pp. 843-845, 1998, it is known to have a repetition frequency of one wavelength Light beam linearly increasing in the form of a measuring ramp to produce. In particular, a repetition frequency of the wavelength variations of 250 Hz is mentioned, which describes only one advantageous partial aspect in fiber Bragg grating sensors.

The invention is therefore based on the object to provide an improved measuring arrangement and measurement method for determining fiber Bragg grating reflection wavelengths, which has a simple structure, is suitable for dynamic measurements and simultaneous detection of the measurement results with a variety of fiber Bragg - allows lattices.

This object is achieved by the invention specified in claim 1 and claim 7 invention. Further developments and advantageous embodiments are specified in the dependent claims.

The invention has the advantage that, by means of a simple interpolation device, an optical measuring arrangement with a tunable laser can also be used for dynamic measurement with a time-varying strain profile with a large number of fiber Bragg gratings. The interpolation device calculates simultaneous measured values for all fiber Bragg grating measuring points from the measured values recorded in chronological succession, which then ensure a high measuring accuracy, particularly in the case of dynamic measurements. At the same time, this measurement method has the advantage that the measured values which are then present at the same time can be evaluated simultaneously or in a different manner at the same time.

In addition, the invention has the advantage that with a single tunable laser, a high number of measuring points can be evaluated, and compared to broadband light sources, a relatively high light intensity is available, which allows relatively high reflection energies even at a plurality of measuring points, due to the large Störsignalabstandes are easy to detect and even with time-varying strain curve highly accurate.

A particular embodiment of the invention with a linear interpolation device has the advantage that exact strain measurements in and on solids for comparative evaluation can be carried out thereby, in particular in the case of a continuous, temporally variable strain profile. This is particularly advantageous in the evaluation of a plurality of measuring points for strain measurement, since relatively large measuring time differences occur there and they can be mathematically very accurately interpolated due to the linear strain processes to a simultaneous reference time.

The invention will be explained in more detail with reference to an embodiment which is illustrated in the drawing. Show it:

1 an optical measuring arrangement with a tunable laser and three measuring points with fiber Bragg gratings, and

2 a graphical representation of the sweep of the tunable laser over time.

In 1 The drawing is an optical measuring arrangement with a tunable laser 4 and three measuring points with fiber Bragg gratings 1 . 2 . 3 represented by an optical fiber 13 and a fiber coupler 5 a laser light beam into the three fiber Bragg gratings 1 . 2 . 3 the measuring points initiates their reflected light beam as intensity maxima in an opto-electrical converter 6 converted according to the time intervals of the reflected wavelengths λ _M1 , λ _M2 , λ _M3 in corresponding voltage maxima or peaks and the via an A / D converter 7 an interpolation device 8th are fed from the sequentially recorded wavelengths (measured values) of each measuring point 1 . 2 . 3 simultaneous measurement results R _M the following in an evaluation device 9 at the same time further evaluable or displayable.

This is done first in a tunable laser 4 generates a light beam preferably having a wavelength λ _s 1500 nm to 1600 nm. Such lasers 4 are particularly inexpensive and are often used for telecommunications. However, lasers with other wavelengths can also be used for the invention. The tunable laser 4 generates a light beam with periodically continuous wavelength λ _s , which increases linearly with a lower wavelength λ _u before 1500 nm up to an upper wavelength λ _o of 1,600 nm and then linearly decreases again to λ _M = 1500 nm.

In 2 In the drawing, the periodic progression of wavelength tuning over time is graphed. Preferably, the passage of such a triangular amplitude in a period T of 1 s and Thus, with a frequency of 1 Hz. But it is also possible throughput frequencies of 0.5 to 10 Hz or sometimes up to 1 kHz. This tuned laser light beam is transmitted through an optical fiber 10 and a fiber coupler 5 the three spatially spaced measuring points with its fiber Bragg gratings 1 . 2 . 3 fed. At these measuring points 1 . 2 . 3 it can be three different strain sensors, each mounted on or in a solid or deformation body, in which an optical fiber 10 one fiber Bragg grating each 1 . 2 . 3 each with a different center of gravity Bragg wavelength is provided. The three measuring points 1 . 2 . 3 However, they can preferably also be an expansion rosette, which detects the strain on a solid or deformation body surface in three directions of elongation offset 120 °. The centroid Bragg wavelength is a designated wavelength λ at which an intensity maximum or peak of the reflected light occurs at rest or in the unstretched optical waveguide state. The three different center of gravity Bragg wavelengths lie on the linearly rising measuring ramp 11 between a preferably used wavelength range λ _M of 1500 nm to 1600 nm. The transit time of the laser 4 for the used wavelength range λ _s is for example 0.5 s.

Within this turnaround time, each fiber will be Bragg grating 1 . 2 . 3 a light beam each having an intensity maximum or peak at a wavelength λ _M1 , λ _M2 , λ _M3 reflected, which corresponds to the respective strain of the deformation or solid, at which the respective fiber Bragg grating 1 . 2 . 3 is attached as a measuring point. The deviation of the wavelength from the centroid Bragg wavelength corresponds to a value that is proportional to the strain or compression of the deformation or solid. This reflected light beam passes over the fiber co-op 10 to an opto-electrical converter 6 therein the light intensity maxima are converted therein into electrical voltage maxima or electrical current maxima or peaks whose time sequence corresponds to the reflected wavelength maxima.

In a subsequent analog-to-digital converter 7 This analog voltage or current value is converted into a digital value and an interpolation device 3 fed. Because all three measuring points 1 . 2 . 3 only during the ascending measuring ramp 11 in a measurement period T _M of 0.5 s with the entire measurement wavelength of λ _u at 1530 nm and λ _o are supplied with 1600 nm, the respective reflected fiber Bragg wavelength λ _M1 , λ _M2 , λ _M3 , as the maximum intensity only at different times t _M1 , t _M2 , t _{M3 occur} within these 0.5 s. If it is at the illustrated three measuring points 1 . 2 . 3 z. B. is a rosette arrangement, in which each fiber Bragg grating 1 . 2 . 3 detects a strain component at an angular distance of 120 °, the center of gravity Bragg wavelengths z. Eg at a wavelength distance of 10 nm. As a result, the measured values of the individual measuring points become 1 . 2 . 3 detected at a time interval of 50.0 ms, wherein between the first measuring point 1 and the third measuring point 3 even a detection difference of 100 ms exists.

For a time-varying strain curve, this measurement time difference would result in a measurement error that can not be negligible. Therefore, the invention proposes a measuring method with a measuring arrangement that is independent of the sampling times t _M1 , t _M2 , t _M3 , and for all measuring points 1 . 2 . 3 simultaneous measurement results R _M delivers. This is the interpolation device 8th which, in the present case, calculates the simultaneous measured values R _M by means of a linear interpolation. For this purpose, in the simplest case, the last two acquired measured values M1 ₀ , M1 ₁ ; M2 ₀ , M2 ₁ ; M3 ₀ , M3 ₁ from a measuring point 1 . 2 . 3 the simultaneous measured values R _{M are} calculated. This is possible because the measured values are basically wavelengths, and a wavelength in the given laser measuring system itself already determines how far the detection time t _M1₀ , t _M2₀ , t _M3₀ of the measurement time t _M1₁ , t _M2₁ , t _M3₁ subsequent _{tuning cycles} a medium wavelength.

The transit times t _M1₀ , t _M2₁ , t _M3 ; t _M1₀ , t _M1₀ , t _{M3₀ of} the individual from the laser 4 generated wavelengths λ _M1 , λ _M2 , λ _M3 become the interpolation device 8th from the laser 4 transmitted directly. Of the three measuring channels of the respective measuring points 1 . 2 . 3 the measured values M1 ₀ , M2 ₀ ; M1 ₁ , M2 ₁ , M3 ₁ on the ramp function 11 at different times t _M1₀ , t _M2₀ , t _M3₀ but recorded in a constant temporal _{tuning rhythm} Γ. The measured value of measuring point 1 is therefore always earlier than the measured value of the measuring point 2 and also earlier than the measured value of the measuring point 3 detected. If the measured variable increases with continuous elongation z. B. evenly, so is the measured value of the measuring point 2 greater than the measured value of the measuring point 1 and the measured value of measuring point 3 greater than the measured value of the measuring point 2 , For example, a simultaneous measured value R _M2 for the measuring point 2 to determine, which corresponds to a value at time t _s as a simultaneous measurement time, a linear interpolation between the measured values M _2₁ the measuring point 2 and the previously measured measured value M _{2₀ of} this measuring point. Accordingly, this also applies to the measuring points 1 and 3 executed in the interpolation device.

The correction calculation carried out by the interpolation is accurate only if the measured variable changes relatively slowly with a constant slope, as is often the case with strains on or in strainers or test bodies. In these cases, a linear interpolation between two measured values is sufficient. However, it is also possible to calculate very precise simultaneous measured values if the measured variable does not change uniformly, but runs according to a sine wave. In these cases, however, it is necessary to work with a higher order of interpolation. A third-order interpolation is z. For example, according to Lagrange or Newton, in which four measured values are used for the calculation and also provide very good simultaneous measured values.

wobei

C_M2

= ein Koeffizient als Funktion der gemessenen Peak-Zellenlänge λ_M2;

T_M

= die Zeit, in der der Messbereich (z. B. 1500–1600 nm) durchlaufen wird (hier z. B. 0,5 s);

T

= die Messperiode (z. B. hier = 1 s);

λ_M2

= die Wellenlänge des gemessenen Reflexions-Peaks;

λ_min

= 1500 nm, und

λ_max

= 1600 nm.

The calculation of the linear simultaneous measured values R _M in the interpolation device 8th z. B. for the measuring point 2 first follows the formula:

in which

C _M2

= a coefficient as a function of the measured peak cell length λ _M2 ;

T _M

= the time in which the measuring range (eg 1500-1600 nm) is traversed (here, for example, 0.5 s);

T

= the measuring period (eg here = 1 s);

λ _M2

= the wavelength of the measured reflection peak;

λ _min

= 1500 nm, and

λ _max

= 1600 nm.

R_M2

= der errechnete simultane Messwert;

M2₀

= der ältere gemessene Messwert, und

M2₁

= der neuste gemessene Messwert.

From the calculated coefficients C _M2 as a function of the measured reflection wavelength λ _M2 , for the measuring point 2 then becomes subsequent in the interpolation device 8th the searched simultaneous measured value R _{M2 is} calculated according to the following formula: R _M2 = (1-C _M2 ) * M2 ₁ + C _M2 * M2 ₀ ; in which

R _M2

= the calculated simultaneous reading;

M2 ₀

= the older measured value, and

M2 ₁

= the newest measured value.

To do this, first on the rising measuring ramp 11 for all measuring points 1 . 2 . 3 determined by their reflection maxima the associated wavelengths λ _M1 , λ _M2 , λ _M3 . The falling part of the trace 12 as return ramp 13 is not used and serves only for the return of the laser. A reflection maximum with the wavelength of 1500 nm is thus detected in each case 0.5 s earlier than a reflection maximum with the wavelength of 1600 nm. Since no reflection maximum can be earlier than at 1500 nm, it is advantageous to convert all measured values to this time t _s as simultaneous measurement results R _M.

The interpolation device 8th thus calculates T for the three measuring points at least after the passage of two consecutive tuning cycles T 1 . 2 . 3 three simultaneous measurement results R _M1 , R _M2 and R _M3 , which are related to the beginning of the second measurement cycle at λ _u = 1500 nm as a simultaneous measurement time t _s . These measurement results R _M can then be used in the following evaluation device 9 stored and further processed or displayed. In the evaluation device 9 may take into account the prior art centroid Bragg wavelength of each measurement site 1 . 2 . 3 From this, the previously performed stretching or compression on the deformation or solid body can be determined, for example, in the three detected strain directions of the rosette.

Such a measurement and interpolation process and preferably repeated at periodic intervals, so that it then in the evaluation device 9 not only the respective strain at a given simultaneous measurement time t _s , but also the strain change between the different measurement times can be calculated.

Claims (8)

Measuring method for the acquisition and evaluation of several measuring points ( 1 . 2 . 3 ) with at least one optical waveguide ( 10 ), in which as measuring points ( 1 . 2 . 3 ) several fiber Bragg gratings ( 1 . 2 . 3 ) are arranged with different center of gravity fiber Bragg wavelengths λ _M , into which a light beam of variable wavelength λ _{s is} periodically fed, the intensity maxima or peaks of the reflected light beam at the different fiber Bragg gratings ( 1 . 2 . 3 ) Are detected caused intensity or peak wavelengths λ _M and evaluated, characterized in that the light wavelength λ _{s and} up to an upper light wavelength increases within the period T continuously from a lower light wavelength λ and drops again that the intensity maxima of the reflected light beam are converted into temporally spaced electrical measured values (M1, M2, M3) in such a way that the electrical measured values (M1, M2, M3) correspond to the time sequence of the measuring points ( 1 . 2 . 3 ) intensity temporally successively an interpolation device ( 8th ) and that from at least two measured values (M1 ₀ , M ₁ , M2 ₀ , M2 ₁ , M3 ₀ , M3 ₁ ) of each measuring point ( 1 . 2 . 3 ) in the interpolation device ( 8th ) by an interpolation calculation for each measuring point ( 1 . 2 . 3 ) simultaneous measured values R _{M are} calculated, wherein the simultaneous measured values R _{M of} all measuring points ( 1 . 2 . 3 ) are related to a common simultaneous measurement or reference time t _s . A method according to claim 1, characterized in that the light wavelength λ _s in a predetermined measurement period T _M within the period T linearly increases from the lower wavelength light λ _u up to the upper wavelength light λ _o . A method according to claim 1 or 2, characterized in that in a linear course of the measured value change a linear interpolation calculation and in a non-linear course of the measured value change the interpolation calculation for a higher order, preferably with the interpolation calculation according to Lagrange or Newton. A method according to claim 3, characterized in that in the linear interpolation calculation first for each measuring point ( 1 . 2 . 3 ) an associated coefficient C _M as a function of the measured intensity or peak wavelength λ _M according to the formula:

where: T _M = the measuring time in which the measuring range is traversed, T = the measuring period, λ _M = the wavelength of the measured reflection peak, λ _min = the lower used wavelength of light, λ _max = the upper wavelength of light used, and afterwards for each measuring point ( 1 . 2 . 3 ) a simultaneous measured value R _M according to the formula: R _M = (1-C _M ) * M ₁ + C _M * M ₀ calculated for each measuring point ( 1 . 2 . 3 ): M ₀ = an older measured value and M ₁ = a newer measured value. Method according to one of the preceding claims, characterized in that the change in the wavelength of light takes place with a repetition frequency in the range of 0.5 Hz to 1 kHz. Method according to one of the preceding claims, characterized in that from the simultaneous measured values R _M comparable elongation values for those with the measuring points ( 1 . 2 . 3 ) deformation, or solids are detected, displayed or further processed. Measuring arrangement for carrying out the method according to one of the preceding claims with a tunable laser ( 4 ) for generating a light beam, with an opto-electrical converter ( 6 ) for converting the intensity maxima into temporally spaced electrical measured values (M1, M2, M3) and with an evaluation device ( 9 ) for display and further processing, characterized in that the tunable laser ( 4 ) via at least one fiber coupler ( 5 ) to the at least one optical waveguide ( 10 ) is connected, that via the fiber coupler ( 5 ) the reflected light beams with their intensity maximum at least one series connection of the opto-electrical converter ( 6 ), an A / D converter ( 7 ), an interpolation device ( 8th ) and the evaluation device ( 9 ) and that in the interpolation device ( 8th ) The calculation of the measurement result R _M of at least two successively detected in time measurement values (M1 _0, M1 _{1 (0} M3, M3 ₁₎ of each measuring point; M2 _0, M2 ₁ 1 . 2 . 3 ) he follows. Measuring device according to claim 7, characterized in that in the evaluation device ( 9 ) the calculation of the comparable strain values from the simultaneous measured values R _M and their display or further processing takes place.

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