FIBER OPTIC SENSOR UNIT, OPTICAL MEASURING SYSTEM, AND AXLE-COUNTING DEVICE AND METHOD

Abstract

A fiber optic sensor unit for detecting a mechanical force acting on a rail includes at least a first sensor fiber, a first elongated fiber optic strain sensor and a second elongated fiber optic strain sensor. The first sensor fiber includes the first strain sensor and is characterized in that the at least one sensor fiber is attached to a sensor plate. The first fiber strain sensor and the second strain sensor are arranged in an x-type or v-type geometry, wherein the first strain sensor and the second strain sensor are arranged in an angle of 60° to 120°, in particular of 90°, to each other. Measurements with increased amplification of the measurement signal and improved raw data can be made.

Claims

1. A fiber optic sensor unit for detecting a mechanical force acting on a rail, comprising: at least one first sensor fiber; a first elongated fiber optic strain sensor and a second elongated fiber optic strain sensor, wherein the at least one first sensor fiber comprises the first elongated fiber optic strain sensor, wherein the first and second elongated fiber optic strain sensors are fiber Bragg gratings, wherein either both fiber Bragg gratings are inscribed in one sensor fiber or each of the fiber Bragg gratings is inscribed in a separate sensor fiber; wherein the at least one sensor fiber is attached to a sensor plate; wherein the first elongated fiber optic strain sensor and the second elongated fiber optic strain sensor are arranged in an x-type or v-type geometry, wherein the first elongated fiber optic strain sensor and the second elongated fiber optic strain sensor are arranged in an angle of 60° to 120° to each other; and wherein the sensor plate comprises a recess, wherein the at least one fiber spans the recess where the first and second elongated fiber optic strain sensors are positioned freely within the recess without contact to the sensor plate.

2. The fiber optic sensor unit according to claim 1, wherein the first strain sensor and the second strain sensor are arranged in an x-type geometry, and wherein the strain sensors are at a distance to each other in a direction perpendicular to the longitudinal extensions of the strain sensors.

3. The fiber optic sensor unit according to claim 1, wherein the sensor plate comprises at least one groove in which the at least one sensor fiber is attached.

4. The fiber optic sensor unit according to claim 3, wherein the at least one groove is etched.

5. The fiber optic sensor unit according to claim 4, wherein the at least one groove comprises two grooves being a first groove and a second groove, wherein the first groove and the second groove are part of the same sensor plate, wherein the two grooves are at different height levels of the sensor plate.

6. The fiber optic sensor unit according to claim 5, wherein the at least one first sensor fiber comprises both the first fiber Bragg grating as well as the second fiber Bragg grating.

7. The fiber optic sensor unit according to claim 5, wherein the at least one first sensor fiber comprises two sensor fibers, each fiber Bragg grating being part of a separate sensor fiber.

8. The fiber optic sensor unit according to claim 1, wherein the sensor plate is attached to a base plate for mounting the fiber optic sensor on the rail, wherein the base plate has a continuous bottom plane.

9. The fiber optic sensor unit according to claim 1, wherein the sensor plate includes a mechanical amplifier, which transfers and multiplies the alternation of length from the rail to the fiber Bragg grating.

10. The fiber optic sensor unit according to claim 1, wherein the first elongated fiber optic strain sensor and the second elongated fiber optic strain sensor are arranged in an angle of 90° to each other.

11. An optical measuring system for measuring shear stress of a rail, the system comprising: the rail having a longitudinal extension and a neutral axis, which extends along the longitudinal extension; the fiber optic sensor unit according to claim 1 for detecting optical signals in dependence of the shear strain acting on the rail; wherein the fiber optic sensor unit is mounted at the rail such that the fiber Bragg gratings are oriented obliquely with respect to the neutral axis; a light source which is adapted for coupling light into the sensor fibers of the fiber optic sensor unit; and a signal-processing unit for processing signals detected by the fiber optic sensor unit.

12. The optical measuring system according to claim 11, wherein the signal processing unit comprises an edge filter with a falling edge and a raising edge, and that the first fiber Bragg grating has a Bragg wavelength at the raising edge and the second fiber Bragg grating has a Bragg wavelength at the falling edge of the edge filter.

13. The optical measuring system according to claim 12, wherein the strain sensors are arranged symmetrically to a plane comprising the neutral axis of the rail.

14. The optical measuring system according to claim 13, wherein the strain sensors are arranged symmetrically to a plane perpendicular to the neutral axis of the rail.

15. An axle-counting device comprising at least one light source and at least one counting unit, wherein each counting unit comprises at least one fiber optic sensor unit according to claim 1, the at least one fiber optic sensor unit being adapted for mounting to a rail, and a signal-processing unit, wherein the light source is adapted for coupling light into the sensor fibers of the fiber optic sensor unit.

16. An axle-counting method for rail bound vehicles, comprising the following method steps: a) coupling, via at least one sensor fiber, light into a first and a second fiber optic strain sensor being fiber Bragg gratings of a fiber optic sensor unit which is attached to a rail; b) detecting light reflected by the first and the second fiber optic strain sensor, as a result of which a shear stress signal of the rail is received in each case, wherein each fiber optic strain sensor has a reflection spectrum having a reflection peak which is at a Bragg wavelength and has a full width at half maximum, c) generating a shear stress difference signal from the two received shear stress signals; d) generating a wheel signal within a signal-processing unit if the shear stress difference signal exceeds a predetermined upper limiting value or falls below a predetermined lower limiting value; wherein the fiber optic strain sensors which are used are arranged in an x-type or v-type geometry, wherein the first strain sensor and the second strain sensor are arranged in an angle of 60° to 120° to each other, wherein either both fiber Bragg gratings are inscribed in one sensor fiber or each of the fiber Bragg gratings is inscribed in a separate sensor fiber, wherein the sensor plate comprises a recess, wherein the at least one fiber spans the recess such that the strain sensors are positioned freely within the recess without contact to the sensor plate; and wherein the full width at half maximum of the reflection peak of the first fiber optic strain sensors and the second fiber optic strain sensor deviate from each other by a maximum of 200%.

17. The axle-counting method according to claim 16, wherein the sensor fiber which is used comprises both, the first and the second fiber optic strain sensors, the first and the second fiber optic strain sensors being arranged in a row and having different Bragg wavelengths, and that the shear stress difference signal is generated optically by a spectral overlap of the reflection peaks of the two fiber optic strain sensors during the transition from an unloaded state to a loaded state.

18. The axle-counting method according to claim 17, wherein method steps a) to d) are carried out with a further fiber optic sensor unit which is attached to another rail of the track wherein the two fiber optic sensor units are spaced apart from one another in the rail direction.

19. The axle-counting method according to claim 16, wherein the first strain sensor and the second strain sensor are arranged in an angle of 90° to each other.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1a shows a top view of a first embodiment of the inventive fiber optic sensor unit with two sensor fibers, each sensor fiber comprising a fiber Bragg grating (x-type geometry).

[0045] FIG. 1b shows an enlarged detail of area B of FIG. 1a.

[0046] FIG. 1c shows a cross sectional view along a longitudinal direction of the first embodiment of the inventive fiber optic sensor unit.

[0047] FIG. 1d shows an enlarged detail of area A of FIG. 1a.

[0048] FIG. 1e shows a perspective view of the first embodiment of the inventive fiber optic sensor unit.

[0049] FIG. 2a shows a top view of a second embodiment of the inventive fiber optic sensor unit with one sensor fiber, the sensor fiber comprising two fiber Bragg gratings with different Bragg wavelength (x-type geometry).

[0050] FIG. 2b shows an enlarged detail of area B of FIG. 2a.

[0051] FIG. 2c shows a cross sectional view along a longitudinal direction of the second embodiment of the inventive fiber optic sensor unit.

[0052] FIG. 2d shows an enlarged detail of area A of FIG. 2a.

[0053] FIG. 2e shows a perspective view of the second embodiment of the inventive fiber optic sensor unit.

[0054] FIG. 3a shows a third embodiment of the inventive fiber optic sensor with a v-type geometry.

[0055] FIG. 3b shows an enlarged detail of area A of FIG. 3a.

[0056] FIG. 4 shows a perspective view of the first embodiment of the inventive fiber optic sensor unit mounted at a rail.

[0057] FIG. 5 shows a perspective view of the second embodiment of the inventive fiber optic sensor unit mounted at a rail.

[0058] FIG. 6 shows an axle-counting device according to the invention using the second embodiment of the inventive fiber optic sensor unit for carrying out the inventive axle-counting method; different positions of a passing wheel are indicated.

[0059] FIG. 7 shows diagrams, which indicate the intensity of the light reflected by the fiber Bragg gratings of the axle-counting device shown in FIG. 6 in dependence of the wavelength, each diagram representing the intensity at a different wheel position indicated in FIG. 6.

[0060] FIG. 8 shows the signal detected at the photo diode of the axle-counting device shown in FIG. 6 in dependence of time during a wheel is passing the positions indicated in FIG. 6.

[0061] FIG. 9 shows diagram, which indicates the wavelength shift in dependence of the position of a passing axle with respect to a fiber optic sensor unit according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] FIGS. 1a-d show different views of a first embodiment of the inventive fiber optic sensor unit 1a. The inventive fiber optic sensor unit 1a according to the first embodiment comprises a first sensor fiber 2 and a second sensor fiber 3, wherein the first sensor fiber 2 comprises 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 in which the sensor fibers 2, 3 run. The sensor plate has a gap 9. The sensor fibers 2, 3 span the gap 9 such that the FBGs 4, 5 cross each other in a crossing area B. The crossing area B is shown in more detail in FIG. 1b. The cross sectional view shown in FIG. 1c and the detailed view of section A shown in FIG. 1d show that the two sensor fibers 2, 3 run at different height levels in order to cross each other without touching each other. A perspective view of the first embodiment of the inventive fiber optic sensor unit is shown in FIG. 1e.

[0063] A second embodiment of the inventive fiber sensor unit 1b is shown in FIGS. 2a-d. The inventive fiber optic sensor unit 1b according to the second embodiment comprises only one sensor fiber 10, wherein the sensor fiber 10 comprises the first FBG 4 and the second FBG 5. In this embodiment, the FBGs have different Bragg wavelengths λ1, λ2. The sensor fiber 10 is attached to the sensor plate 6. The sensor fiber 10 runs in a groove 11 of the sensor plate 6. As in the first embodiment, the sensor plate 6 has a gap 9. The sensor fiber 10 span the gap 9 two times from different directions such that the FBGs 4, 5 cross each other in the crossing area B. The cross sectional view shown in FIG. 2c and the detailed view of section FIG. 2d shows that the sensor fiber 10 runs at different height levels at different positions of the sensor plate 6 in order to cross each other in the crossing area without touching each other. A perspective view of the second embodiment of the inventive fiber optic sensor unit is shown in FIG. 2e.

[0064] Both embodiments show a cross-type geometry of the FBGs, wherein the FBGs are arranged in an angle 90° to each other.

[0065] A third embodiment of the inventive fiber optic sensor unit 1c is shown in FIG. 3. In the third embodiment, the FBGs are arranged in a V-geometry. FIG. 3 shows an embodiment where both FBGs, 4, 5 are inscribed in the same sensor fiber 10. Nevertheless, FBGs 4, 5 can also be inscribed in different sensor fibers 2, 3 (not shown). The sensor fibers 2, 3 are attached to the sensor plate 6. The sensor plate 6 has a groove 11 in which the sensor fibers 10 runs. The sensor plate has a gap 9a, 9b. The sensor fibers 10 span the gaps 9a, 9b such that the FBGs 4, 5 from a V in the crossing area A. The crossing area A is shown in more detail in FIG. 3b. The sensor fiber 10 runs at one height level, thereby allowing a simple construction of the fiber optic sensor unit.

[0066] FIGS. 4 and 5 show perspective views of the first embodiment and the second embodiment of the inventive fiber optic sensor unit 1a, 1b mounted at a rail web 14 of a rail 15. The fiber optic sensor unit is attached to the rail such that the FBGs 4, 5 are arranged symmetrically to the neutral axis 16 of the rail 15 and symmetrically to a plane which is orthogonal to the neutral axis 16.

[0067] The inventive fiber optic sensor unit 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 the first embodiment of the inventive fiber optic sensor unit 1b. The axle counting device 17 comprises a light source 18, and a counting unit 19, wherein the counting unit 19 comprises fiber optic sensor unit 1b a signal-processing unit 20 for processing of the light coming from the fiber optic sensor unit. Light is coupled from the light source 18 into the sensor fiber 10 of the fiber optic 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 photo diode 21 of the signal-processing unit 20. In dependence of positions a, b, c, d, e, f, g of a passing wheel 22 light of different wavelength is reflected from the FBGs. FIG. 7 shows diagrams in which reflection peaks P1, P2 of the 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 indicated in FIG. 6.

[0068] At position a, the wheel 22 does not influence the FBGs 4, 5. The FBGs 4, 5 reflect light at the respective rest Bragg wavelength λ1, λ2 and the reflection peaks P1, P2 can be identified at rest Bragg wavelength λ1, λ2. Due to the inventive X arrangement of the FBGs, the wavelengths of the reflected light are shifted in opposite directions as soon as the sensor is subjected to a load. At position b both FBGs reflect light at the same wavelength—the reflection peaks overlap. The distances between the operating points (=Bragg wavelengths in the unloaded state) can be selected such, that the reflection peaks P1, P2 “overtake” each other (position of reflection peak P1 changes from left to right while position of reflection peak P2 changes from right to left, as shown in diagram for position c) when a wheel passes over the fiber optic sensor unit. As the wheel passes on the reflection peaks P1, P2 move to each other again (position d) return to their rest Bragg wavelength (position e) and pass over to lower wavelength in case of reflection peak P1 and higher wavelength in case of reflection peak P2 (position f). As the wheel stops influencing the FBGs, the reflection peaks return to their rest Bragg wavelength (position g).

[0069] The signal detected by the photo diode 21 of the axle-counting device 17 in dependence of time during the wheel 22 is passing the positions a-g is shown in FIG. 8. Two FBGs with the same half width are suggested, so that the detected light is reduced in intensity twice by the half (namely at positions b and d). I.e. each passing wheel produces two signal pulses. Please note, that these two pulses do not contain any direction information. With the width of the FBG the range can be influenced, with the distance of the wavelengths of the FBG the sensitivity of the sensor can be influenced. The signal pulses can be evaluated without an optical chip but directly by the photodiode 21. The conversion of the wavelength change into light intensities thus takes place in the fiber optic sensor unit 1b itself, which enables reducing the optical signal processing to a minimum (one photodiode 21) and keeps the electrical evaluation simple.

[0070] As shown in FIG. 9, the reflection peaks generated by the two FBGs due to the passing wheel move in opposite direction, whereas interfering signals, e.g. due to the bending and vibrations of the rails or temperature move in the same direction in case of both FBGs. The reason for this is the following: The wheel 22 compresses and bends the rail 15 as it passes but more importantly, it causes a localized shear strain in the rail 15. The shear strain is a result of a rail segment stretching in one direction whilst compressing in the orthogonal. The bending and vibration caused by the passing wheels (disturbances causing interfering signals) affects a large segment of the rail resulting in preceding or following wheels affecting the strains in the rail under the wheel being measured (nearly independent if the position of the wheel relative to the fiber optic sensor unit). The shear strain, however, is different in that it is localized and so only results from the wheel above the fiber optic sensor unit.

[0071] According to the invention, a pair of fiber optic strain sensors is attached to the rail at 45 degrees and orthogonal to each other. In the shown embodiment, both fiber optic strain sensors are located at the same longitudinal location of the rail, thereby forming an X-geometry. Thus, they will experience exactly the same vertical and horizontal strains but opposite components of the shear strain which are equal in amount. The difference between the signals detected by the two FBGs is therefore a direct measure of the shear strain without any of the distorting strains in the rail. An according diagram is shown in figure. The invention allows to reduce the size of the sensor and to measure directly the desired shear strain in the rail without requiring complex post processing.

LIST OF CITED REFERENCES

[0072] [01] EP 3 069 952 B1 [0073] [02] DE 10 2014 100 653 B4 [0074] [03] OMEGA: Positioning strain Gages to monitor bending, axial, shear, and torsional loads, https://www.omega.com/faq/pressure/pdf/positioning.pdf

LIST OF REFERENCE SIGNS

[0075] 1a, 1b, 1c fiber optic sensor units [0076] 2, 3 sensor fibers with one inscribed FBG [0077] 4, 5 FBGs [0078] 6 sensor plate [0079] 7, 8 grooves in the sensor plate at different height-levels [0080] 9 gap of the sensor plate [0081] 10 sensor fiber with two inscribed FBGs [0082] 11 groove in the sensor plate with varying height-level [0083] 12, 13 grooves in the sensor plate at the same height-level [0084] 14 rail web of the rail [0085] 15 rail [0086] 16 neutral axis of the rail [0087] 17 axle counting device [0088] 18 light source [0089] 19 counting unit [0090] 20 signal-processing unit [0091] 21 photo diode [0092] 22 wheel [0093] a-g positions of the wheel along the rail relative to the fiber optic sensor unit [0094] P1 light peak of light reflected by the first FBG [0095] P2 light peak of light reflected by the second FBG