Optical fiber sensor unit, optical measuring system, and axle-counting method of axle-counting device

Abstract

To perform measurements with amplification of a measurement signal enhanced, and raw data improved.SOLUTION: There is provided an optical fiber sensor unit (1a; 1b; 1c) for detecting mechanical force acting on a rail (15), and comprises: at least one first sensor fiber (2, 3; 10); a first elongated optical fiber strain sensor (4); and a second elongated optical fiber strain sensor (5). The optical fiber sensor unit (1a) is characterized in that the at least one sensor fiber (2, 3; 10) is attached to a sensor plate (6), and the first and second fiber strain sensors (4, 5) are arranged mutually at angles of 60° to 120°in an x-type or v-type geometric shape, the sensor plate (6) is provided with at least one groove (7, 8; 11) to which the at least one sensor fiber (2, 3; 10) is attached, and the first groove (7) and the second groove (8) are at different height levels of the sensor plate (6).SELECTED DRAWING: Figure 1a

Description

[Background technology]

[0001] The present invention relates to an optical fiber sensor unit for detecting mechanical forces acting on a rail, the optical fiber sensor unit comprising at least one sensor fiber, a first elongated optical fiber strain sensor and a second elongated optical fiber strain sensor, wherein one of the at least one sensor fiber comprises the first elongated optical fiber strain sensor.

[0002] A compliant optical fibre sensor unit is known from US Pat. No. 5,399,633 (reference

[01] ).

[0003] Fiber Bragg grating sensors have been reported in recent years for use in axle counter measurements. Older conventional axle counters, which use electromagnetic sensors to detect changes in magnetic permittivity caused by the flanges of wheels passing through the sensor device, are subject to stray magnetic fields from electric traction motors and also require electronics boxes to be in close proximity to each sensor. Fiber optic sensors use two or more fiber Bragg gratings to measure the shear strains experienced by the railroad line as train wheels pass over it. The measured strains can then be used to detect the passage of the wheels, their passing speed, direction of travel, and the load on the wheels. The primary use of the sensors is to determine whether a track block is occupied by counting the number of wheels passing the sensor location. By using fiber Bragg grating sensors, much smaller and more robust sensors can be placed on the track, and the processing devices can be placed several kilometers away. This can significantly reduce the installation and maintenance costs of the signaling system.

[0004] US Pat. No. 5,399,433 (reference document

[02] ) discloses a rail measurement system for measuring mechanical forces acting on the rail by means of an optical fiber sensor using two single fiber Bragg gratings. One of the fiber Bragg gratings is attached to the rail at an angle of +45° to the neutral fiber and the other one is attached to the rail at an angle of -45° to the neutral fiber, the fiber Bragg gratings being spaced apart from each other in the direction of the rail. A disadvantage of the arrangement known from US Pat. No. 5,399,433 (

[02] ) is that the required safety level for trains (SIL4) is not guaranteed, since the sensitivity is not sufficient to reliably detect each axle. This is because the signal resulting from the shear stress measurement is not suitable for a threshold assessment.

[0005] Patent document 1 (

[01] ) discloses an optical fiber sensor unit for axle counting, comprising a pair of FBG sensors mounted at 45° on a plate and parallel to each other. The FBG sensors are spaced apart from each other along the direction of the rail. The signals from the two fibers are subtracted to obtain a detection pulse for each wheel. This concept assumes and requires that the bending, compression and vibrations experienced by the rail due to the passage of leading and trailing wheels of a train are coherent over the entire length of the optical fiber sensor unit. Subtracting the signals from the two fibers should result in a detection pulse that is purely due to the change in shear strain when the wheels pass (see Fig. 1 below). Sensor testing shows that the key assumption is not entirely correct, and that the two fibers experience different undesirable strains that do not cancel out, resulting in distorted detection pulses and high background noise levels. The optical fiber sensor unit known from Patent document 1 (

[01] ) produces unreliable counting when trains pass at high speeds and with different loads. This is problematic in particular when counting axles of freight trains. This is because the load on the rails varies significantly due to the large weight difference between loaded and unloaded wagons. The disturbances acting on the rails are an order of magnitude larger than the signal to be measured (shear stresses). As a consequence, the axles are not counted correctly or at all, which must be avoided at all costs.

[0006] Non-Patent Document 1 (reference

[03] ) proposes a double X-style arrangement of Figure E in the context of a four-sensor bridge configuration. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] European Patent No. 3069952 (Reference

[01] ) [Patent Document 2] German Patent No. 102014100653 (cited document

[02] ) [Non-patent literature]

[0008] [Non-Patent Document 1] Omega: Positioning strain Gages to monitor bending, axial, shear, and torsional loads, https: / / www.omega.com / faq / pressure / pdf / positioning.pdf (cited in

[03] ) Summary of the Invention [Problem to be solved by the invention]

[0009] It is an object of the present invention to provide an optical fiber sensor which allows measurements with increased amplification of the measurement signal and improved raw data. [Means for solving the problem]

[0010] This object is achieved by an optical fiber sensor unit according to claim 1, an optical measuring system according to claim 8, an axle counting device according to claim 12 and an axle counting method according to claim 13.

[0011] According to the invention, at least one sensor fiber is attached to the sensor plate, and the first elongated optical fiber strain sensor and the second elongated optical fiber strain sensor are arranged in an x- or v-shaped geometry, and the first elongated optical fiber strain sensor and the second elongated optical fiber strain sensor are arranged at an angle of 60° to 120°, in particular 90°, to each other.

[0012] The detected optical signals generally include the signals to be detected (target signals), e.g. shear stresses generated by the passing wheels of a train, and undesired disturbances (interference signals), e.g. due to rail bending and vibrations or temperature.

[0013] According to the invention, the first and second elongated optical fiber strain sensors are arranged with no distance between them, overlapping (x-geometry) or adjacent to each other without overlapping (v-geometry). Thus, when mounted on the rail, the two elongated optical fiber strain sensors detect a specific disturbance acting on the same or nearly the same position on the rail. This is particularly advantageous in the case of disturbances that may vary in time, since the time between measurements by the first and second elongated optical fiber strain sensors is minimized.

[0014] The inventive arrangement allows for detection of signals with improved signal quality, which is particularly important when using the fiber optic unit to detect low signals, highly variable signals, or signals superimposed by disturbances of similar or higher amplitude, such as in the case of axle counting in railway systems.

[0015] In a highly preferred embodiment, the elongated optical fiber strain sensors are fiber Bragg gratings. Each fiber Bragg grating has a Bragg wavelength that depends on the mechanical strain acting on the fiber Bragg grating. Either both fiber Bragg gratings are inscribed in one sensor fiber (i.e. the sensor fiber comprises two FBG segments) or each of the FBGs is inscribed in a separate sensor fiber (i.e. each fiber contains one FBG segment).

[0016] According to a preferred embodiment of the optical fiber sensor unit of the present invention, the sensor plate has a gap (recess) and at least one fiber is spanned across the gap such that the first and second elongated optical fiber strain sensors are located within the gap. The gap in the sensor plate allows the FBGs to be positioned freely (i.e. without contacting the sensor plate). The optical fiber strain sensor that spans the gap is preloaded.

[0017] In a highly preferred embodiment, the first and second elongated optical fiber strain sensors are arranged in an x-geometry, with the elongated optical fiber strain sensors being spaced apart from each other in a direction perpendicular to the longitudinal extension of both elongated optical fiber strain sensors. That is, the first and second elongated optical fiber strain sensors are arranged in an x-geometry without contacting each other (due to the distance between the FBGs in a direction perpendicular to the longitudinal extension of the elongated optical fiber strain sensors). The first and second elongated optical fiber strain sensors extend along a skew line. The angle between the two elongated optical fiber strain sensors is the angle of the projection of the two elongated optical fiber strain sensors along a common vertical line. By "x-geometry" we mean that the projections of the strain sensors along a common vertical line intersect (i.e. the elongated optical fiber strain sensors overlap with respect to the longitudinal extension of the rail). Preferably, the x-geometry has two-fold rotational symmetry, in particular four-fold rotational symmetry, ie the centres of both strain sensors are located on a common vertical line.

[0018] Alternatively, the first and second elongated optical fiber strain sensors can be arranged in a v-geometry. In this case, the reaction of the two elongated optical fiber strain sensors is somewhat delayed, i.e. the processed sensor signal is temporally broadened and possibly has a somewhat lower maximum amplitude. The lateral forces do not have to be exactly the same, i.e. the suppression of disturbances is somewhat less effective compared to the x-geometry. However, the v-geometry is easier and cheaper to manufacture, since the two elongated optical fiber strain sensors can be arranged at the same height level.

[0019] The sensor plate may comprise at least one groove in which at least one sensor fiber is attached. By providing one or two grooves, the fiber can be easily positioned and fixed on the sensor plate. Furthermore, the path of the fiber can be determined by the groove. By providing at least one groove at different depths, the fibers can be arranged at a distance while crossing each other. Thus, in a special embodiment of the optical fiber sensor unit, the grooves provide gaps in the sensor plate from different directions and at different levels (i.e. the grooves have different depths), which allows the fiber segments to cross each other at different levels and thereby cross each other without contacting each other.

[0020] Preferably, two grooves of different depths are provided. Alternatively, the groove for one sensor may be on the top surface of the sensor plate with the groove for the other sensor on the bottom surface, so that they do not touch. The fiber for the bottom sensor can be led from the top of the plate through a hole in the plate to the bottom surface. The fiber can be attached to a mechanical amplifier, which can transmit the deformation from the rail to the strain sensor. It is a mechanical amplifier that is part of / integrated into the sensor plate.

[0021] At least one of the grooves may be etched. Preferably, the grooves are produced using an etching process, where different etching depths are used for the different grooves. The etching process completely prepares the grooves for the gluing process to attach the fiber to the sensor plate. The two different etching depths are preferably achieved by double etching, although it is also possible to perform only one etching with different exposure times for the two grooves.

[0022] Preferably, the first and second of the at least one groove are part of the same sensor plate, and the two grooves are at different height levels of the sensor plate. This is particularly advantageous when using an x-geometry, since the fibers can be placed at a distance while crossing each other. In the case of a v-geometry, the sensors can be placed at the same depth level. In this case, different groove depths are not required.

[0023] In a special embodiment, one sensor fiber includes both the first and second elongated optical fiber strain sensors, i.e. both the FBG of the first and second elongated optical fiber strain sensors are attached to the same sensor fiber. Only one sensor fiber is required for the optical fiber sensor unit, which makes this embodiment of the optical fiber sensor unit of the invention cost-effective and easy to manufacture. This embodiment facilitates temperature compensation, since the operating ranges, and therefore the reflected wavelengths, of both FBGs shift in the same way when the temperature changes. No mechanical compensation means for temperature compensation are required. An embodiment with only one sensor fiber requires FBGs with different Bragg wavelengths.

[0024] In an alternative embodiment, the optical fiber sensor unit comprises two sensor fibers, and the FBG of the first elongated optical fiber strain sensor and the FBG of the second elongated optical fiber strain sensor are part of separate sensor fibers. By using different sensor fibers, each with one FBG, FBGs of the same Bragg wavelength can be used.

[0025] Preferably, both sensor fibers are attached to the same sensor plate, which makes it easier to mount the fiber sensor unit since the relative positioning of the FBGs is fixed.

[0026] The sensor plate can be attached to a base plate for mounting the optical fiber sensor unit to a rail, and the base plate has a continuous bottom surface. The sensor plate itself cannot be glued to a continuous area because it has a gap in the area where the FBG is located. By providing an additional base plate, a wide and continuous glued area can be provided.

[0027] To enable detection of light wheel loads, the sensor plate preferably includes a mechanical amplifier that transfers the length change from the rail to the fiber Bragg grating for multiplication. A corresponding amplifier is described in US Pat. No. 5,399,433 (

[01] ). In contrast to US Pat. No. 5,399,433 (

[01] ), no temperature compensation is required with the x-geometry of the present invention.

[0028] The present invention also relates to an optical measurement system for measuring shear stress in a rail, the system comprising: a rail having a longitudinal extension and a neutral axis extending along said longitudinal extension; an optical fiber sensor unit as described above for detecting an optical signal in response to a shear stress acting on the rail, the optical fiber sensor unit being mounted on the rail such that a fiber Bragg grating of the optical fiber sensor unit is oriented obliquely with respect to the neutral axis; a light source adapted to couple light into a sensor fiber of the optical fiber sensor unit; and a signal processing unit for processing the signal detected by the optical fiber sensor unit.

[0029] In a highly preferred embodiment, the fiber optic sensor unit is mounted to the rail such that the first and second elongated fiber optic strain sensors are oriented at an angle of 30° to 60° relative to the neutral axis, preferably at an angle of 45° relative to the neutral axis.

[0030] The light source couples light into at least one sensor fiber. The light is guided to the FBGs of the first and second elongated optical fiber strain sensors, each FBG having a reflection spectrum with a reflection peak at the Bragg wavelength and with a full width at half maximum. The light reflected by the two fiber Bragg gratings is detected and processed in the signal processing unit, resulting in a received rail shear stress signal.

[0031] The passage of the train wheels generates an optical signal in the fiber Bragg grating, which results in a wavelength change. Due to the x- or v-shaped arrangement of the two fiber Bragg gratings, the wavelength reflected by one of the fiber Bragg gratings in the first case decreases during the approach of the wheel, while at the same time the reflected wavelength of the other fiber Bragg grating increases. Furthermore, the reflected wavelengths of both fiber Bragg gratings behave in opposite directions while the wheel is passing the sensor and while it is moving away from the sensor. In contrast, a lateral disturbance of the rail, for example caused by a sinusoidal movement of the train axle or caused by temperature, changes the wavelengths of both fiber Bragg gratings in the same direction. This effect makes it possible to eliminate or at least substantially reduce interference signals in a post-signal processing by subtracting both detection signals (the signal detected by the first fiber Bragg grating and the signal detected by the second fiber Bragg grating) after the wavelength change has been converted into a change in light intensity. An optical filter preferably performs this conversion.

[0032] Preferably, the fibre optic sensor unit is mounted to the rail at the rail web.

[0033] In a special embodiment, the signal processing unit comprises an edge filter having a falling edge and a rising edge, the fiber Bragg grating of the first elongated optical fiber strain sensor has a Bragg wavelength at the rising edge and the fiber Bragg grating of the second elongated optical fiber strain sensor has a Bragg wavelength at the falling edge of the edge filter. This is particularly useful when the two fiber Bragg gratings have different Bragg wavelengths. The initial wavelengths (Bragg wavelengths without extra load) of the fiber Bragg grating and the edge filter are selected such that the initial wavelengths of the fiber Bragg gratings are on different edges of the edge filter. Thus, the edge filter, which has an edge at the Bragg wavelength of the fiber Bragg grating, converts the change in the wavelength of the reflected light into a change in light intensity. This allows the signals from the two sensors to be subtracted in the edge filter mechanism, canceling out the undesired disturbances and making a pure measurement of the shear strain in the rail.

[0034] In a highly preferred embodiment, the first and second elongated optical fiber strain sensors are arranged symmetrically with respect to a plane containing the neutral axis of the rail, i.e. the first and second elongated optical fiber strain sensors have a perfect x-shaped geometry. The plane of symmetry of the x-shaped optical fiber strain sensors is perpendicular to the plane of symmetry of the rail. This ensures that both elongated optical fiber strain sensors are subjected to forces acting on the same rail segment, which optimizes the filtering of disturbances from the optical signal.

[0035] Due to the symmetry of the x-geometry about the horizontal neutral axis, a force acting vertically from the top towards the center of the X will not generate a signal, which is the case when each of the two FBGs is in the exact same position (perfect x-geometry).

[0036] Also, the first and second elongated fiber optic strain sensors are preferably positioned symmetrically with respect to a plane perpendicular to the neutral axis of the rail, the geometry relative to the plane perpendicular to the neutral axis ensuring that the two elongated fiber optic strain sensors are equally sized with respect to the neutral axis.

[0037] The present invention also relates to an axle counting device comprising at least one light source and at least one counting unit, each counting unit comprising at least one optical fiber sensor unit as described above adapted for mounting on a rail, and a signal processing unit, the light source being adapted to couple light into a sensor fiber of the optical fiber sensor unit.

[0038] The invention further relates to a method for axle counting of a rail-coupled vehicle, comprising the following steps: coupling light via at least one sensor fiber to first and second elongated optical fiber strain sensors of an optical fiber sensor unit mounted on a rail; detecting light reflected by the first and second elongated optical fiber strain sensors, as a result of which in each case a rail shear stress signal is received, each of the first and second elongated optical fiber strain sensors having a reflection spectrum with a reflection peak at the Bragg wavelength and with a full width at half maximum; generating a shear stress difference signal from the two received shear stress signals; generating a wheel signal in a signal processing unit if the shear stress difference signal exceeds a predetermined upper limit value or is below a predetermined lower limit value. According to the invention, the first and second elongated optical fiber strain sensors are arranged in an x- or v-shaped geometry, the first elongated optical fiber strain sensor and the second elongated optical fiber strain sensor are arranged at an angle of 60° to 120°, in particular 90°, to each other, and the full width at half maximum of the reflection peaks of the first elongated optical fiber strain sensor and the second elongated optical fiber strain sensor are shifted from each other by up to 200%.

[0039] In the method known from patent document 1 ([1]), the full width at half maximum of the first reflection peak is many times larger than the full width at half maximum of the second reflection peak. When the rail is loaded, the wavelengths of the two FBGs move in the same direction, albeit with a time delay, so that the two FBG signals temporarily overlap, which is supported by resulting different full widths at half maximum. Furthermore, if the first reflection peak is about four times wider than proposed in the current patent, the received power is reduced by up to one-fifth by a wheel crossing. Interference signals, for example caused by lateral forces, affect both FBG signals and are added. The signal-to-noise ratio is therefore very poor. A downstream evaluation allows the recognition of the direction, but is also susceptible to interferences. The wide pulse of the first reflection peak is also shifted by wavelength disturbances, so that, although it is about four times wider than the narrow pulse, disturbances act on it four times as much.

[0040] In contrast to the method known from patent document 1 ([1]), the method of the present invention uses FBGs whose reflection peaks have comparable full width at half maximum. The inventive X-arrangement shifts the wavelengths of the two FBGs in opposite directions. The distance between the operating points can be selected so that they "overtake" each other when the wheel passes over them. By providing two strain sensors with the same full width at half maximum, the intensity of the received light is halved in the ideal case of two wheel crossings. The full width at half maximum of the reflection peak is preferably selected so that the wheel load is in a predetermined range. The difference in the Bragg wavelengths of the two elongated optical fiber strain sensors is preferably selected so that a predetermined minimum sensitivity is obtained.

[0041] Each wheel generates a positive pulse followed by a negative pulse, or alternatively a negative pulse followed by a positive pulse, depending on the direction of the train. The conversion of the wavelength change into light intensity is performed within an OEC (optical chip), which limits the required optical signal processing to a minimum (one photodiode) and simplifies the electrical evaluation.

[0042] In a preferred variant of the method of the invention, the sensor fiber comprises both a first and a second elongated optical fiber strain sensor, the first and second elongated optical fiber strain sensors being arranged in line and having different Bragg wavelengths, and a shear stress difference signal is optically generated by the spectral overlap of the reflection peaks of the two elongated optical fiber strain sensors during the transition from the unloaded to the loaded condition.

[0043] In a special variant, method steps a) to d) are carried out with the optical fiber sensor unit and a further optical fiber sensor unit mounted on another rail of the track, where the two optical fiber sensor units are spaced apart from each other in the rail direction. This signal does not contain any directional information. In order to receive the directional information, a further optical fiber sensor unit is needed at a distance to the first optical fiber sensor unit in the longitudinal direction of the track. Ideally, it should be mounted on the other rail with a slight offset, since this allows an increased detection rate of the axle in case the wheels of the axle on one side of the track do not transmit the required force to the rail, for example via a flat spot.

[0044] Further advantages of the invention can be derived from the description and the drawings. The features mentioned above and those described below may be used individually by themselves or in any combination. The illustrated and described embodiments should not be understood as an exhaustive recitation, but rather as having an exemplary character for the explanation of the invention. [Brief description of the drawings]

[0045] [Figure 1a] FIG. 1 shows a top view of a first embodiment of an inventive optical fiber sensor unit having two sensor fibers, each containing a fiber Bragg grating (x-type geometry). [Figure 1b] 1 shows an enlarged view of region B of FIG. 1a. [Figure 1c]1 is a cross-sectional view taken along the longitudinal direction of a first embodiment of an optical fiber sensor unit of the present invention; [Figure 1d] 1 shows an enlarged view of area A of FIG. 1a. [Figure 1e] 1 is a perspective view of a first embodiment of an optical fiber sensor unit of the present invention; [Figure 2a] FIG. 2 shows a top view of a second embodiment of the optical fiber sensor unit of the present invention with one sensor fiber, which contains two fiber Bragg gratings (x-geometry) with different Bragg wavelengths. [Figure 2b] An enlarged view of region B of FIG. 2a is shown. [Figure 2c] FIG. 2 shows a cross-sectional view along the longitudinal direction of a second embodiment of the optical fiber sensor unit of the present invention. [Figure 2d] An enlarged view of area A of FIG. 2a is shown. [Figure 2e] FIG. 2 is a perspective view of a second embodiment of the optical fiber sensor unit of the present invention. [Figure 3a] 3 shows a third embodiment of the optical fiber sensor of the present invention having a v-shaped geometry. [Figure 3b] An enlarged view of area A of FIG. 3a is shown. [Figure 4] FIG. 1 shows a perspective view of a first embodiment of the optical fiber sensor unit of the present invention mounted on a rail. [Diagram 5] FIG. 2 shows a perspective view of a second embodiment of the optical fiber sensor unit of the present invention mounted on a rail. [Figure 6] 2 shows an axle counting device according to the invention using a second embodiment of the optical fiber sensor unit according to the invention for carrying out the axle counting method according to the invention, where various positions of the passing wheels are shown. [Figure 7] 7A-7C are plots of the intensity of light reflected by the fiber Bragg grating of the axle counting device shown in FIG. 6 as a function of wavelength, with each plot representing the intensity at a different wheel location shown in FIG. 6. [Figure 8]FIG. 7 shows a diagram illustrating the signal detected by the photodiode of the axle counting device shown in FIG. 6 as a function of time while the wheel passes through each of the positions shown in FIG. [Figure 9] FIG. 13 illustrates wavelength shift as a function of pass axis position for a fiber optic sensor unit according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] 1a to 1d show various views of a first embodiment of the optical fiber sensor unit 1a of the present invention. The optical fiber sensor unit 1a of the present invention according to the first embodiment comprises a first sensor fiber 2 and a second sensor fiber 3, the first sensor fiber 2 comprising a first fiber Bragg grating (FBG) 4 and a second FBG 5. The sensor fibers 2, 3 are attached to a sensor plate 6. The sensor plate 6 has grooves 7, 8 through which the sensor fibers 2, 3 pass. The sensor plate has a gap 9. The sensor fibers 2, 3 are spanned across the gap 9 such that the FBGs 4, 5 cross each other in a crossing region B. The crossing region B is shown in more detail in FIG. 1b. The cross section shown in FIG. 1c and the detailed view of cross section A shown in FIG. 1d show that the two sensor fibers 2, 3 pass at different height levels in order to cross each other without contacting each other. A perspective view of the first embodiment of the optical fiber sensor unit of the present invention is shown in FIG. 1e.

[0047] A second embodiment of the inventive fiber sensor unit 1b is shown in Fig. 2a to Fig. 2d. The inventive fiber optic sensor unit 1b according to the second embodiment comprises only one sensor fiber 10, which comprises a first FBG 4 and a second FBG 5. In this embodiment, these FBGs have different Bragg wavelengths λ1, λ2. The sensor fiber 10 is attached to a sensor plate 6. The sensor fiber 10 passes through a groove 11 in the sensor plate 6. The sensor plate 6 has a gap 9, as in the first embodiment. The sensor fiber 10 is spanned through the gap 9 twice from different directions so that the FBGs 4, 5 cross each other in the crossing region B. The cross-sectional view shown in Fig. 2c and the detailed view of the cross-section in Fig. 2d show that the sensor fibers 10 pass through the sensor plate 6 at different height levels in different positions in order to cross each other in the crossing region without contacting each other. A perspective view of the second embodiment of the inventive fiber optic sensor unit is shown in Fig. 2e.

[0048] Both embodiments show FBGs in a cross-type configuration, with the FBGs arranged at 90° angles to each other.

[0049] A third embodiment of the optical fiber sensor unit 1c of the present invention is shown in FIG. 3. In the third embodiment, the FBGs are arranged in a V-shape. FIG. 3 shows an embodiment in which both FBGs 4, 5 are attached to the same sensor fiber 10. However, the FBGs 4, 5 can also be attached to different sensor fibers 2, 3 (not shown). The sensor fibers 2, 3 are attached to a sensor plate 6. The sensor plate 6 has a groove 11 through which the sensor fiber 10 passes. The sensor plate has gaps 9a, 9b. The sensor fiber 10 is spanned across the gaps 9a, 9b such that the FBGs 4, 5 extend from the V of the intersection area A. The intersection area A is shown in more detail in FIG. 3b. The sensor fiber 10 passes at one height level, which allows a simple construction of the optical fiber sensor unit.

[0050] 4 and 5 show perspective views of a first and a second embodiment of the optical fiber sensor unit 1a, 1b of the present invention mounted on a rail web 14 of a rail 15. The optical fiber sensor units are mounted on the rail such that the FBGs 4, 5 are arranged symmetrically with respect to the neutral axis 16 of the rail 15 and with respect to a plane perpendicular to the neutral axis 16.

[0051] The inventive optical fiber sensor units 1a, 1b, 1c can be used for axle counting. As an example, FIG. 6 shows an axle counting device 17 according to the invention using a first embodiment of the inventive optical fiber sensor unit 1b. The axle counting device 17 comprises a light source 18 and a counting unit 19, which comprises the optical fiber sensor unit 1b, a signal processing unit 20 for processing the light coming from the optical fiber sensor unit. Light is coupled from the light source 18 into the sensor fiber 10 of the optical fiber sensor unit 1b. The light source 18 can be integrated in the signal processing unit 20. The light reflected by the FBGs 4, 5 is detected by a photodiode 21 of the signal processing unit 20. Depending on the position a, b, c, d, e, f, g of the passing wheel 22, light of different wavelengths is reflected from the FBG. FIG. 7 shows diagrams in which reflection peaks P1, P2 of light reflected by the two FBGs 4, 5 can be identified, each diagram representing one of the positions a, b, c, d, e, f, g shown in FIG. 6.

[0052] In position a, the wheel 22 has no effect on the FBGs 4, 5. The FBGs 4, 5 reflect light at their respective rest Bragg wavelengths λ1, λ2, and the reflection peaks P1, P2 can be identified at the rest Bragg wavelengths λ1, λ2. Due to the X-arrangement of the FBGs of the present invention, the wavelength of the reflected light is shifted in the opposite direction as soon as the sensor is loaded. In position b, both FBGs reflect light of the same wavelength, and the reflection peaks overlap. The distance between the operating points (=Bragg wavelength at no load) can be chosen such that the reflection peaks P1, P2 "overtake" each other as the wheel passes over the optical fiber sensor unit (as shown in the diagram for position c, the position of the reflection peak P1 changes from left to right, while the position of the reflection peak P2 changes from right to left). As the wheel passes reflection peaks P1 and P2, they move past each other again (position d) and return to their rest Bragg wavelengths (position e), passing either lower wavelengths in the case of reflection peak P1 or higher wavelengths in the case of reflection peak P2 (position f). When the wheel no longer influences the FBG, the reflection peaks return to their rest Bragg wavelengths (position g).

[0053] The signal detected by the photodiode 21 of the axle counting device 17 as a function of time during the passage of the wheel 22 from position a to g is shown in Fig. 8. Two FBGs with the same half-width are proposed, and the detected light intensity is reduced by half twice (i.e. at positions b and d). That is, one passage of the wheel results in two signal pulses. It should be noted that these two pulses do not contain any directional information. The width of the FBG may affect the range, and the distance of the wavelength of the FBG may affect the sensitivity of the sensor. The signal pulses can be evaluated directly by the photodiode 21, without the use of an optical chip. Thus, the conversion of the wavelength change to light intensity is performed in the optical fiber sensor unit 1b itself, which allows the optical signal processing to be kept to a minimum (one photodiode 21) and the electrical evaluation to be simplified.

[0054] As shown in Fig. 9, the reflection peaks generated by the two FBGs due to a passing wheel move in opposite directions, whereas the interference signals due to, for example, rail bending and vibration or temperature move in the same direction in both cases of FBGs. The reason for this is as follows: the wheel 22 compresses and bends the rail 15 as it passes, but more importantly it induces local shear strains in the rail 15. The shear strains are the result of the rail segment stretching in one direction while compressing in the orthogonal direction. The bending and vibrations caused by the passing wheel (the disturbances that cause the interference signals) affect a large segment of the rail, so that the leading or trailing wheel affects the strain of the rail under the wheel that is measured (almost independent of the position of the wheel relative to the optical fiber sensor unit). However, the shear strains are different in that they are local and therefore only originate from the wheel above the optical fiber sensor unit.

[0055] According to the invention, a pair of optical fiber strain sensors are mounted at a 45° angle on the rail, perpendicular to each other. In the illustrated embodiment, both optical fiber strain sensors are placed at the same longitudinal position of the rail, thereby forming an X-shape. They are therefore subjected to exactly the same vertical and horizontal strains, but shear strains of equal and opposite components. The difference between the signals detected by the two FBGs can therefore directly measure the shear strains, without distorting strains, in the rail. A corresponding diagram is shown in the drawings. The invention allows the size of the sensors to be reduced and makes it possible to directly measure the desired shear strains in the rail without the need for complex post-processing.

[0056] List of cited references

[01] European Patent No. 3069952

[02] German Patent No. 102014100653

[03] OMEGA:Positioning strain gauges to monitor bending, axial, shear, and torsional loads https: / / www.omega.com / faq / pressure / pdf / positioning.pdf [Explanation of symbols]

[0057] 1a, 1b, 1c Optical fiber sensor unit 2,3 Sensor fiber with one FBG 4.5 FBGs 6 Sensor Plate 7,8 Grooves at different height levels in the sensor plate 9 Sensor plate gap 10 Sensor fiber with two FBGs 11 Grooves at different height levels in the sensor plate 12,13 Level grooves in the sensor plate 14 Rail web of rail 15 Rail 16 Rail neutral axis 17 Axle counting device 18 light source 19 Counting Unit 20 Signal Processing Unit 21 Photodiode 22 wheels ag Position of the wheel along the rail relative to the optical fiber sensor unit P1 Optical peak of the light reflected by the first FBG P2 Optical peak of the light reflected by the second FBG

Claims

1. An optical fiber sensor unit (1a; 1b; 1c) for detecting mechanical forces acting on a rail (15), At least one sensor fiber (2, 3; 10); a first elongated optical fiber strain sensor (4) and a second elongated optical fiber strain sensor (5), wherein one sensor fiber (2; 10) of the at least one sensor fiber (2, 3; 10) comprises the first elongated optical fiber strain sensor (4), and both the first elongated optical fiber strain sensor (4) and the second elongated optical fiber strain sensor (5) are fiber Bragg gratings, and either one of the fiber Bragg gratings is attached to the one sensor fiber (10) or the fiber Bragg grating constituting the first elongated optical fiber strain sensor (4) is attached to the one sensor fiber (2) and the fiber Bragg grating constituting the second elongated optical fiber strain sensor (5) is attached to another sensor fiber (3) of the at least one sensor fiber (2, 3; 10); Including, said at least one sensor fiber (2, 3; 10) being attached to a sensor plate (6); the first elongated optical fiber strain sensor (4) and the second elongated optical fiber strain sensor (5) are arranged in an x- or v-shaped geometry, the first elongated optical fiber strain sensor (4) and the second elongated optical fiber strain sensor (5) being arranged at an angle of 60° to 120° to each other; the sensor plate (6) has a recess (9, 9a, 9b), the at least one sensor fiber (2, 3; 10) is bridged over the recess (9, 9a, 9b) such that the first and second elongated optical fiber strain sensors (4, 5) are freely positioned within the recess (9, 9a, 9b) without contacting the sensor plate (6); said sensor plate (6) comprising at least one groove (7, 8; 11) in which said at least one sensor fiber (2, 3; 10) is mounted, A first groove (7) and a second groove (8) of said at least one groove (7, 8; 11) are part of the same sensor plate (6), said first groove (7) and said second groove (8) being at different height levels of said sensor plate (6). An optical fiber sensor unit (1a; 1b; 1c) characterized in that

2. 2. The optical fiber sensor unit (1a; 1b) according to claim 1, characterized in that the first elongated optical fiber strain sensor (4) and the second elongated optical fiber strain sensor (5) are arranged in an x-geometry, the first and second elongated optical fiber strain sensors (4, 5) being spaced apart from each other in a direction perpendicular to the longitudinal extension of both the first and second elongated optical fiber strain sensors (4, 5).

3. 3. The optical fibre sensor unit (1a; 1b; 1c) according to claim 1 or 2, characterized in that said at least one groove (7, 8; 11) is etched.

4. The optical fiber sensor unit (1b, 1c) according to claim 2 or 3, characterized in that the one sensor fiber (10) comprises both the first elongated optical fiber strain sensor (4) and the second elongated optical fiber strain sensor (5).

5. The optical fiber sensor unit (1a) according to claim 2 or 3, characterized in that the optical fiber sensor unit (1a) comprises the one sensor fiber (2) and the other sensor fiber (3), the first elongated optical fiber strain sensor (4) being part of the one sensor fiber (2) and the second elongated optical fiber strain sensor (5) being part of the other sensor fiber (3).

6. The optical fiber sensor unit (1a; 1b; 1c) according to any one of claims 1 to 5, characterized in that the sensor plate (6) is attached to a base plate for mounting the optical fiber sensor unit (1a; 1b; 1c) on the rail (15), the base plate having a continuous bottom surface.

7. The optical fiber sensor unit (1a; 1b; 1c) according to any one of claims 1 to 6, characterized in that the sensor plate (6) includes a mechanical amplifier which transmits and multiplies the change in length from the rail (15) to the fiber Bragg grating.

8. An optical measurement system for measuring shear stress in a rail (15), comprising: said rail (15) having a longitudinal extension and a neutral axis (16) extending along said longitudinal extension; A fiber optic sensor unit (1a; 1b; 1c) according to any one of claims 1 to 7 for detecting an optical signal as a function of the shear stress acting on the rail (15), the fiber optic sensor unit (1a; 1b; 1c) being mounted on the rail (15) such that a fiber Bragg grating of the fiber optic sensor unit (1a; 1b; 1c) is oriented obliquely with respect to the neutral axis (16); a light source (18) adapted to couple light into the at least one sensor fiber (2, 3; 10) of the optical fiber sensor unit (1a; 1b; 1c); a signal processing unit (20) for processing signals detected by said optical fiber sensor units (1a; 1b; 1c); A system comprising:

9. 9. The optical measurement system of claim 8, wherein the signal processing unit (20) comprises an edge filter having a falling edge and a rising edge, the first elongated optical fiber strain sensor (4) having a Bragg wavelength at the rising edge and the second elongated optical fiber strain sensor (5) having a Bragg wavelength at the falling edge of the edge filter.

10. 10. The optical measurement system according to claim 8 or 9, characterized in that the first and second elongated optical fiber strain sensors (4, 5) are both arranged symmetrically with respect to a plane containing the neutral axis (16) of the rail (15).

11. 11. The optical measurement system according to claim 8, wherein the first and second elongated optical fiber strain sensors (4, 5) are both arranged symmetrically with respect to a plane perpendicular to the neutral axis (16) of the rail (15).

12. 8. An axle counting device comprising at least one light source (18) and at least one counting unit (19), the at least one counting unit (19) comprising at least one optical fiber sensor unit (1a; 1b; 1c) according to any one of claims 1 to 7, the at least one optical fiber sensor unit (1a; 1b; 1c) being adapted for mounting on a rail (15), and a signal processing unit (20), characterized in that the light source (18) is adapted to couple light into the at least one sensor fiber (2, 3; 10) of the at least one optical fiber sensor unit (1a; 1b; 1c).

13. 1. An axle counting method for a rail-coupled vehicle, comprising the steps of: a) coupling light via at least one sensor fiber (2, 3; 10) to first and second elongated optical fiber strain sensors (4, 5), which are fiber Bragg gratings of an optical fiber sensor unit (1a; 1b; 1c) mounted on a rail (15); b) detecting light reflected by the first and second elongated optical fiber strain sensors (4, 5), as a result of which in each case a shear stress signal of the rail (15) is received, each of the first and second elongated optical fiber strain sensors (4, 5) having a reflection spectrum with reflection peaks (P1, P2) at Bragg wavelengths λ1, λ2 and with full width at half maximum (FWHM), c) generating a difference shear stress signal from the two received shear stress signals; d) generating a wheel signal in a signal processing unit (20) if said shear stress difference signal exceeds a predefined upper limit or falls below a predefined lower limit. Including, Method steps a) to d) are performed with a fiber optic sensor unit (1a; 1b; 1c) according to any one of claims 1 to 7 and a further fiber optic sensor unit (1a; 1b; 1c) according to any one of claims 1 to 7 mounted on another rail of a railway line, said fiber optic sensor unit (1a; 1b; 1c) and said further fiber optic sensor unit (1a; 1b; 1c) being spaced apart from each other in the direction of the rail, 13. An axle counting method according to claim 12, wherein in each of the optical fiber sensor units (1a; 1b; 1c) and the further optical fiber sensor units (1a; 1b; 1c), the full width at half maximum (FWHM) of the reflection peaks (P1, P2) of the first elongated optical fiber strain sensor (4) and the second elongated optical fiber strain sensor (5) are shifted from each other by up to 200%.

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