Measuring method for detecting mechanical force acting on object, measuring device having fiber optic sensor unit

Abstract

The invention relates to a measuring method for detecting a mechanical force acting on an object by means of a fiber-optic sensor unit (1), in which at least one measuring channel (4-1,..., 4-2) is provided, which comprises a sensor fiber (7) and a sensor detection element (10), the invention relates to a method for producing a sensor fiber (7) having at least one sensor fiber Bragg grating (8) embedded therein and having a Bragg wavelength, the sensor fiber (7) being fastened to an object in the region of the sensor FBG (8), the method comprising: inputting a light coupling of a light source (5) into the sensor fiber (7), light reflected and / or transmitted by the sensor FBG (8) is detected by means of a sensor detection element (10), characterized in that the light source (5) has a wavelength-dependent intensity distribution with edges, and a wavelength change in the Bragg wavelength of the sensor FBG (8) is ascertained by evaluating the measurement signal, the measurement signal comprises a change in intensity of the detected light intensity of the entire light reflected by the sensor FBG (8) and / or the entire light transmitted by the sensor (8). A simplified method is thus provided, which can be carried out by means of a simply constructed optical measuring device.

Description

Background Art

[0001] The invention relates to a measuring method for detecting mechanical forces acting on an object by means of a fiber optic sensor unit, wherein the fiber optic sensor unit has at least one measuring channel, which comprises a sensor fiber and a sensor detection element, wherein the sensor fiber has at least one sensor fiber Bragg grating (sensor FBG) with a Bragg wavelength embedded in the sensor fiber, wherein the sensor fiber is fastened to the object in the region of the sensor FBG. The invention also relates to a measuring device having a fiber optic sensor unit.

[0002] For example, DE 10 2017 119 810 B4 discloses a measuring method for detecting mechanical forces acting on an object by means of a fiber optic sensor unit.

[0003] Fiber optic sensors are used for detecting mechanical quantities in many areas of technology, for example for checking loads on components and mechanical voltages in structures. In the railway sector, fiber optic sensors are used in particular for axle counting.

[0004] The fiber optical sensor unit has a sensor fiber (optical waveguide) in which a fiber Bragg grating (FBG) is embedded. Each fiber Bragg grating has a reflection spectrum with a reflection peak at the Bragg wavelength (within this spectrum the fiber Bragg grating reflects light). By coupling light into the sensor fiber, the light is guided to the fiber Bragg grating, wherein wavelengths in the reflection spectrum of the fiber Bragg grating are reflected by the fiber Bragg grating. The Bragg wavelength is usually defined as λB=neff·2λ=neff·λ. Therein, neff is the effective refractive index and λ is the grating period of the fiber Bragg grating. When a load acts on the fiber Bragg grating, the sensor fiber and the fiber Bragg grating are expanded or compressed, and the reflection wavelength or transmission wavelength of the fiber Bragg grating changes, so that depending on the expansion / compression of the fiber Bragg grating, light of different wavelengths is reflected and can be guided to the evaluation unit and the analysis unit.

[0005] It is known from DE 10 2014 100 653 B4 that the light beam emitted from the sensor fiber is divided into two partial beams for the evaluation of the light, wherein one partial beam passes through an edge filter before striking a photoelectric element, such as a photodiode, and the other partial beam is guided unfiltered to the other photoelectric element. The output signals of the photoelectric elements are proportional to one another and are used to determine the wavelength of the reflection. The light emitted from the sensor fiber is divided by means of a beam splitter. In the devices known from EP 3 069 952 A1 and DE 10 2012 104 874 B4, the beam splitter and the required filters with the photoelectric elements are assembled on a board and form an optoelectronic chip (OEC).

[0006] DE 10 2017 119 810 B4 discloses a simplified OEC in which the “division of the light beam into a first (filtered) light beam component and a second (unfiltered) light beam component” is not performed by a beam splitter, but by reflection of the light emitted from the optical waveguide on the filter surface, i.e., directly on the filter. Therefore, the filtered transmission intensity and the unfiltered reflection intensity are measured. By dividing the transmission intensity by the reflection intensity, an intensity quotient is obtained, from which the wavelength of the fiber Bragg grating in the optical waveguide can be inferred.

[0007] Known solutions are based on beam splitting and edge filtering of partial beams, for example using Fabry-Perot interferometers. Thus, although this enables a precise absolute measurement of the reflected wavelength, the OEC needs to be relatively complex to construct and is therefore too expensive to offer an attractive and competitive product, especially for applications within the railway range.

[0008] Invention Task

[0009] It is therefore an object of the present invention to specify a simplified method and an optical measuring device which are of simpler construction than those known hitherto and which are particularly well suited for use in axle counting methods. Summary of the invention

[0010] This object is achieved according to the invention by a method according to claim 1 and a measuring device according to claim 12 .

[0011] In the method according to the invention, the detection of the light reflected and / or transmitted by the sensor FBG is carried out over the entire wavelength range of the light reflected and / or transmitted by the sensor FBG by means of the sensor detection element. The method according to the invention comprises determining the wavelength change of the Bragg wavelength of the sensor FBG by evaluating a measurement signal, the measurement signal comprising the intensity change of the light intensity detected by the sensor detection element (10). Thus, the light intensity of the light reflected and / or transmitted by the sensor FBG is evaluated over the entire wavelength range. In other words, the light reflected and / or transmitted by the sensor FBG is detected and evaluated without filtering.

[0012] The invention makes use of the fact that for a given application (eg axle counting in the railway sector) no exact wavelength measurement is required. Instead, it is sufficient to know whether the Bragg wavelength of the sensor FBG has changed.

[0013] Therefore, in the method according to the invention, the exact Bragg wavelength of the sensor FBG is not measured, but only the wavelength change. For this purpose, the reflected or transmitted light intensity is detected by the sensor detection element over the entire wavelength range (i.e., without filtering in terms of wavelength). Therefore, the sensor detection element detects over the entire wavelength range of the light coupled into the sensor fiber. Therefore, according to the present invention, edge filtering before the sensor detection element is abandoned. The light detected by the sensor detection element is only used to determine the wavelength change, without the need to specify the wavelength of the light reflected or transmitted by the sensor FBG. Since no filter is inserted between the sensor fiber and the sensor detection element, according to the present invention, no wavelength selection occurs before detection by the sensor detection element. Therefore, this simplifies the required measurement arrangement.

[0014] The light source has a wavelength-dependent or frequency-dependent intensity distribution (frequency diagram / frequency spectrum), i.e. the emitted light has different intensities for different wavelengths. According to the invention, a light source is used whose wavelength-dependent intensity distribution has an edge, which preferably has a steepness of at least 30 nW / nm and a length of at least 6 nm, preferably at least 8 nm. The edge can be a falling edge or a rising edge. If the analog amplification factor (i.e. the signal amplification factor, which is adjusted in the electronic circuit that converts the current of the detection element into a voltage) is increased accordingly, the edge steepness of the light source can be flatter.

[0015] This wavelength-dependent intensity distribution takes over the task of the edge filter known from the prior art in the method according to the invention. Due to the wavelength-dependent intensity distribution, wavelength changes cause changes in the overall intensity of the light transmitted or reflected by the sensor FBG, so that the load on the FBG or the load on the object to which the FBG is fixed can be inferred from the intensity changes measured by the sensor detection element. For this purpose, preferably as part of the data processing, an "equivalent signal" (Gleichsignal) is subtracted from the measured raw measurement data (intensity measured by the detection element), so that the value 0 is output when no force is applied.

[0016] When the wavelength change is detected using a sensor detection element, the mechanical force acting on the object is determined.

[0017] The detection element is preferably a photodetector, such as a photodiode.

[0018] When evaluating the light reflected by the sensor FBG (reflection variant), the entire spectrum of the light source is preferably coupled into the sensor fiber. Since the reflection spectrum of the FBG is finite, the light detected by the sensor detection element only includes a limited wavelength range.

[0019] In contrast, when evaluating the light transmitted by the sensor FBG (transmission variant), the entire non-reflected light is coupled out of the sensor fiber, which usually covers a very large wavelength range, so that a relatively large equivalent signal must be subtracted from the raw data, which reduces the detection accuracy. In order to increase the detection accuracy, in this method variant, the light of the light source is band-filtered by means of a filter element before the light is coupled into the sensor fiber. For example, a bandpass filter or a wideband FBG can be used for the band filtering. The bandwidth is preferably between 15 and 20 nm.

[0020] In a particularly preferred variant, the light source and the sensor FBG are coordinated with one another in such a way that the Bragg wavelength of the sensor FBG lies in a wavelength range in which the wavelength-dependent intensity distribution of the light source has an edge, preferably in the middle of the edge. In the range of the edge of the wavelength-dependent intensity distribution, the selection of the Bragg wavelength of the sensor FBG causes a wavelength shift (e.g. due to the action of forces on the FBG) that leads to particularly pronounced intensity changes.

[0021] The position of the Bragg wavelength relative to the edge is preferably selected such that the maximum expected load (maximum lift) plus the temperature influence does not shift the Bragg wavelength beyond the maximum (or minimum) of the wavelength-dependent intensity distribution. The stationary Bragg wavelength of the sensor FBG, i.e. the Bragg wavelength of the sensor FBG in the unaffected state, i.e. the state in which the sensor FBG is exposed to no external influences (in particular without the action of forces on the object and at a predetermined temperature), is preferably selected such that it is in the middle range of the edge. Otherwise, the edge should be selected to be large enough to avoid a shift of the Bragg wavelength beyond the maximum (or minimum) of the intensity distribution. The edge of the wavelength-dependent intensity distribution should extend over a correspondingly large wavelength range for this purpose.

[0022] The adjustment of the static Bragg wavelength is preferably carried out by pre-tensioning the FBG before assembly on the object. In this way, it can be ensured that the Bragg wavelength in the unloaded state of the object is in the desired range of the edge of the wavelength-dependent intensity distribution of the light source, so that forces acting on the FBG from two directions cause a wavelength shift inside the edge.

[0023] Preferably, a C-band light source, in particular an ASE light source, is used as the light source. A C-band light source has the advantage that the light in this band is only slightly attenuated in conventional glass fibers and thus enables long effective distances. An ASE-band light source has the advantage that it usually has a dominant maximum at about 1530 nm and thus has the required edge.

[0024] In the method according to the invention, a light source should be used whose wavelength-dependent intensity distribution is stable in time or can remain stable in time. The stability of the wavelength-dependent intensity distribution can be influenced, for example, by supplying an equivalent voltage to the light source. "Time-stable wavelength-dependent intensity distribution" should be understood as that for the working range of the FBG (i.e. the wavelength interval of the Bragg wavelength that may occur due to the influence of the maximum setting on the FBG), the wavelength-dependent intensity is stable at least for a short time, i.e. remains at an equivalent level. "Stable in a short time" means that the frequency diagram does not change during the expected duration of the force action. In the scope of railway technology, for example, the force action of a train on the rails is to be measured, and the duration of the force action is, for example, the time required for the train to completely pass the axle counter sensor. In particular, the stability of the frequency diagram of the light source should be guaranteed for a duration of at least several seconds, preferably at least several minutes. However, slow intensity deviations are allowed and are filtered out in the processing algorithm by long-term averages.

[0025] Since the measuring method according to the invention is sensitive to influences on the cable infrastructure, such as changes in the light intensity between the sensor and the evaluation device due to fiber bends, bad connections, etc., it is advantageous to detect light intensity changes which are not attributable to the forces to be detected on the object.

[0026] In a particularly preferred variant of the method according to the invention, at least one disturbance parameter is monitored, which has an influence on the wavelength-dependent intensity distribution independently of the force action to be detected. As part of the subsequent signal processing of the detected measurement signal, the influence of this disturbance parameter can be excluded from the measurement signal calculation, which is measured by means of the sensor detection element. In this way, it is ensured that the change in the Bragg wavelength of the FBG determined is due to the load on the object and not to possible environmental influences or external disturbances.

[0027] In order to determine the disturbance parameters related to changes in the cable infrastructure, it is particularly advantageous if the change in the intensity of the light transmitted by the sensor FBG is determined for monitoring the disturbance parameters, wherein the light transmitted by the sensor FBG is preferably conducted to the monitoring detection element (PDT) via a bandpass filter. The monitoring detection element is arranged at the end of the sensor fiber opposite to the light source. The bandpass filter filters the wavelength range outside the working range of the sensor FBG. Therefore, the bandpass filter filters out such light, whose intensity may be affected by the wavelength shift of the Bragg wavelength of the sensor FBG, but will not be affected by the force acting on the object. For example, a bandpass filter or a broadband FBG can be used for band filtering. The bandwidth is preferably 5-15nm. The center frequency is preferably 1550nm. On the contrary, no filtering is applied to the light to be detected by the sensor detection element.

[0028] As an alternative to obtaining the intensity change of the light transmitted by the sensor FBG (transmission monitoring), the monitoring FBG can be used to monitor the interference parameter, wherein the change in the intensity of the light reflected by the monitoring FBG is obtained for monitoring the interference parameter (reflection monitoring). In this case, the monitoring detection element is arranged at the end of the sensor fiber facing the light source. The monitoring FBG is preferably embedded in the same sensor fiber as the sensor FBG and has a Bragg wavelength different from the Bragg wavelength of the sensor FBG. In the sensor fiber, the light from the sensor FBG is reflected on the one hand, and the light from the monitoring FBG is reflected on the other hand, and is returned and guided together inside the sensor fiber.

[0029] When light is coupled out of the sensor fiber, the reflected light is distributed to the sensor detection element on the one hand and to the bandpass filter on the other hand, which filters out the light reflected by the sensor FBG so that only the intensity of the light reflected by the monitoring FBG is detected on the monitoring detection element. If the intensity detected by the monitoring detection element changes, it can be output that there is a disturbance in the cable infrastructure. Conversely, the sensor detection element measures the intensity of the light reflected by the two FBGs.

[0030] In the reflection variant combined with reflection monitoring for detecting the measurement signal, the light reflected in the sensor fiber is preferably divided into two light components, one of which is conducted unfiltered to the sensor detection element and the other is conducted via a bandpass filter to the monitoring detection element.

[0031] In order to improve the availability, the detection of the measurement signal is carried out in a plurality of, in particular at least four, preferably eight, measurement channels. The provision of a plurality of measurement channels increases the redundancy and thus the availability of the arrangement. The light of the light source is preferably distributed to the measurement channels by means of a beam splitter. Preferably, the light is distributed uniformly to the measurement channels.

[0032] In a particularly preferred variant of the method according to the invention, only a single monitoring detector element is used to detect the light from all measuring channels. As an alternative, a separate monitoring detector element can also be used for each measuring channel.

[0033] In a special variant of the method according to the invention, the temperature difference is determined by means of an additional optical fiber with an additional FBG (temperature monitoring FBG). In this way, the temperature influence on the sensor FBG can be determined and evaluated in the evaluation algorithm. The temperature monitoring FBG is preferably arranged in the same temperature configuration as the sensor FBG, but outside the fixed range of the sensor FBG. In particular, relative temperature measurements can be carried out by means of the temperature monitoring FBG.

[0034] In a method variant for ascertaining the disturbance variable, the mechanical force acting on the object is determined only if no wavelength change is ascertained using the monitoring detection element or the ascertained wavelength change falls below a predefined limit value.

[0035] The method according to the invention is preferably used to determine the mechanical forces acting on a rail (railway rail). The method according to the invention can be used in particular for axle counting.

[0036] The invention also relates to a measuring device for carrying out the measuring method according to one of the preceding claims, the measuring device comprising a light source having a wavelength-dependent intensity distribution with edges and a fiber optic sensor unit, wherein the fiber optic sensor unit has at least one measuring channel, the measuring channel comprising a sensor fiber with at least one sensor fiber Bragg grating (FBG) with a Bragg wavelength embedded in the sensor fiber, and a sensor detection element, wherein the sensor fiber is arranged to be assembled on an object in the region of the sensor FBG. According to the invention, the measuring device is arranged to determine the change in the Bragg wavelength of the sensor FBG by evaluating the intensity change of the detected light intensity of the light reflected by the sensor FBG and / or entirely transmitted by the sensor FBG over the entire wavelength range of the light reflected by the sensor FBG and / or transmitted by the sensor FBG. For this purpose, the sensor fiber is connected directly to the sensor detection element (possibly via an optical distributor) and not to an OEC as in known measuring devices. The measuring device is therefore designed in such a way that the light emitted from the sensor fiber (in the reflection variant from the side facing the light source and in the transmission variant from the side facing away from the light source) is guided unfiltered to the sensor detection element.

[0037] Preferably, the measuring device also has an evaluation facility in which the detected intensity values ​​are compared and evaluated.

[0038] In the assembled state, the stationary Bragg wavelength of the FBG is preferably in the range of the edge of the wavelength-dependent intensity distribution of the light source. Preferably, a light source with an intensity maximum at 1530 nm is used. The sensor FBG is preferably pre-stretched in the assembled state, in particular so that the Bragg wavelength in the unstretched state without external influences is approximately 1520 nm and the operating range in the stretched state is 1522-1530 nm. The stationary Bragg wavelength of the sensor FBG in the pre-stretched state is preferably approximately 1526 nm.

[0039] In a particularly preferred embodiment of the device according to the invention, the monitoring FBG is embedded in the sensor fiber, wherein the monitoring FBG has a Bragg wavelength that differs from the Bragg wavelength of the sensor FBG.

[0040] Preferably, the monitoring FBG is arranged outside the range in which the sensor FBG is fixed to the object. For example, the monitoring FBG may be arranged in a glass fiber connection box.

[0041] The invention also relates to an axle counter device with a counting point, which comprises two measuring devices as described above.

[0042] In the axle counting device, the evaluation facility comprises an evaluation card (PCB) with which the signals of the different measuring channels can be evaluated. The invention enables a simplified construction of the measuring device, so that the components for detecting and processing the signals of a large number of optical measuring channels can be accommodated in a single evaluation card. Preferably, in the axle counting device according to the invention, the measuring channels of at least two axle counting points are evaluated by means of a single evaluation unit.

[0043] Further advantages of the invention are apparent from the description and the drawings. Likewise, the features mentioned above and further described can be applied according to the invention individually or in any combination of multiple features. The embodiments shown and described should not be understood as an exhaustive enumeration, but rather have an exemplary nature for the description of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 The embodiment of the measuring device according to the invention is shown for carrying out the method according to the invention, wherein the sensor signal is measured in reflection (reflection variant);

[0045] Figure 2 The wavelength spectrum of the ASE C band light source is shown;

[0046] Figure 3 shows the design of a measuring device according to the invention for carrying out the method according to the invention,

[0047] Therein, the sensor signal is measured in transmission (transmission variant);

[0048] Figure 4a Show Figure 1 Design of measuring device with monitoring of the cable infrastructure in transmission (reflection variant with optical monitoring in transmission) and with monitoring of individual to multiple measuring channels;

[0049] Figure 4b Show Figure 1 Design of a measuring device with monitoring of the cable infrastructure in transmission (reflection variant with light monitoring in transmission) and with joint monitoring of a plurality of measuring channels and temperature monitoring;

[0050] Figure 5 Show Figure 1 Design of a measuring device with monitoring of interference variables in reflection (reflection variant with light monitoring in reflection);

[0051] Figure 6 The principle of correcting slow intensity deviations is shown schematically. DETAILED DESCRIPTION

[0052] Figure 1A measuring device for carrying out the reflection variant of the method according to the invention for detecting mechanical forces acting on an object (not shown) is shown. The measuring device comprises a fiber optic sensor unit 1 which is connected to a detection unit 3 via a glass fiber connection box 2. The fiber optic sensor unit 1 is fixed to the object, while the detection unit 3 can be arranged remotely from the object to be monitored.

[0053] exist Figure 1 In the embodiment shown, a plurality of measuring channels 4-1, 4-2, ..., 4-n are provided, wherein each measuring channel 4-1, ..., 4-n comprises a sensor fiber 7 and a sensor detection element 10. The analog measurement value detected by the detection element 10 is digitally converted (not shown in the figure) and evaluated by a processing element (e.g., a CPU or FPGA) (not shown in the figure). It is not mandatory to provide a plurality of measuring channels 4-1, ..., 4-n, but it is advantageous from the aspect of usability. The detection unit 3 comprises a light source 5, the light of which is distributed to different measuring channels 4-1, ..., 4-n by means of a beam splitter 6. The beam splitter 6 is preferably a 1:n beam splitter, which evenly distributes the light of the light source 5 to n measuring channels 4-1, ..., 4-n and couples it into the sensor fiber 7. A sensor fiber Bragg grating 8 (sensor FBG) is embedded in each sensor fiber 7, wherein the sensor FBG 8 of all measuring channels 4-1, ..., 4-n has the same Bragg wavelength in the assembled and unloaded state. In each measuring channel 4 - 1 , . . . , 4 - n , the light reflected by the sensor FBG 8 is fed back in the sensor fiber 7 , guided via the coupler 9 to the sensor detection element 10 , and detected there as raw data.

[0054] According to the invention, in contrast to known methods, the entire wavelength range reflected by the sensor FBG 8 is detected by means of the sensor detection element 10 even when the Bragg wavelength of the sensor FBG 8 changes due to a load.

[0055] In order to detect wavelength changes even when the entire wavelength spectrum is being acquired, a light source is used as light source 5 which has steep edges 11 , 12 in the wavelength spectrum (wavelength-dependent intensity distribution). Figure 2The wavelength-dependent intensity distribution of an ASE light source with a steep rising edge 11 and a steep falling edge 12 is shown as an example. In the assembled state, the stationary Bragg wavelength of the sensor FBG 8 is preferably selected to be in the middle of one of the edges 11, 12. The working range of the sensor FBG 7 should be shifted within the edge, that is, here, for example, between 1520 nm and 1530 nm. If the Bragg wavelength changes due to the force acting on the object, not only the wavelength also changes, but also the intensity of the light reflected by the sensor FBG 8 changes due to the edge of the wavelength-dependent intensity distribution. In this way, although the absolute Bragg wavelength cannot be determined, the wavelength change can be determined, which is sufficient for applications in the axle counting range, for example. Since the method according to the invention only involves the detection of the wavelength change and not the wavelength itself, as part of the data processing, the intensity of the light reflected by the sensor FBG in the unloaded state of the object ("equivalent signal") is subtracted from the measured raw data (intensity measured by the detection element), so that when no force is applied, the output value is 0 as the output signal. The equivalent signal to be subtracted is preferably the average value of the intensities measured within a given sliding period, in particular within the last few seconds, preferably in the order of the last 10 seconds.

[0056] Figure 3 As an alternative method variant, a measuring arrangement is shown for the two measuring channels 4-1, 4-2, which performs the measurement in transmission (transmission variant). Figure 1 ) In contrast, in the transmission variant, the sensor detection element 10 is located at the end of the sensor fiber 7 opposite the light source 5. In this case, the light reflected by the sensor FBG 8 is not detected, but the light transmitted by the sensor FBG 8 is detected. However, this leads to an equivalent signal that is subtracted from the original measurement data that is relatively large compared to the intensity change caused by the force action. It is therefore advantageous in this variant if, before the light from the light source 5 is coupled into the sensor fiber 7, the wavelength range of the light to be coupled is defined by means of a bandpass filter 13. This bandpass filter should cover the working range of the sensor FBG 8. The bandwidth of the bandpass filter 13 is preferably 15 nm. The passband of the bandpass filter 13 is preferably at 1520-1535 nm.

[0057] Figure 4a , Figure 4b and Figure 5An embodiment of a measuring device according to the invention for carrying out a reflection variant of the method according to the invention is shown, in which, in addition to the actual measurement of the light reflected by the sensor FBG 8 to determine the wavelength change, a monitoring of interference parameters is also performed. This is particularly advantageous because the Bragg wavelength of the sensor FBG 8 can be influenced not only by the effects of forces on the object to which the sensor FBG 8 is fastened, but also by interference factors such as, for example, temperature changes or changes in the cable infrastructure (fiber bending, faulty connection, etc.), since these can cause changes in the intensity of the reflected light.

[0058] In order to determine whether the intensity change is due to the influence of the Bragg wavelength of the sensor FBG 8 or due to the influence on the cable infrastructure, an additional detection element (monitoring detection element 14) is provided in the detection unit 3. Using the monitoring detection element 14, the light from the sensor fiber 7 of the measuring channel 4-1, 4-2, ..., 4-n is detected, which has a wavelength outside the working range of the sensor FBG 8. A bandpass filter 15 can be selectively connected in front of the monitoring detection element 14, which allows a wavelength range outside the working range of the sensor FBG 8 (here: for example 1550nm) to pass. In this way, it can be ensured that the light intensity detected by the monitoring detection element 14 is not affected by the load acting on the object, but is convincing for the cable infrastructure. However, since the monitoring signal measured in transmission is much larger than the measurement signal measured in reflection, the influence of the shift of the Bragg wavelength on the monitoring signal due to the load acting on the object is very small. Therefore, it is also possible to abandon the bandpass filter 15. If the light intensity changes due to changes in the cable infrastructure, this can be identified based on the intensity changes detected by the monitoring detection element 14.

[0059] exist Figure 4a and Figure 4b In the embodiment shown, the monitoring detection element 14 is located at the end of the sensor fiber 7 opposite to the light source 5 and is used to detect the light transmitted by the sensor FBG 8 of the sensor fiber 7 of the different measurement channels 4-1, ..., 4-n. This detection can be performed separately for each measurement channel 4-1, ..., 4-n. For this purpose, Figure 4a In the embodiment shown, a separate monitoring detection element 14 is provided for each measuring channel 4-1, ..., 4-n. As an alternative, the transmitted light from all measuring channels 4-1, ..., 4-n can be combined by means of another beam splitter 16 and directed to a common monitoring detection element 14, such as Figure 4b shown.

[0060] Additionally, in addition to monitoring the cable infrastructure, Figure 4bTemperature monitoring is also provided in the embodiment shown. For this purpose, part of the light of the light source 5 is coupled into an additional optical fiber 17 with an additional FBG (temperature monitoring FBG 18). The light of the light source 5 can be distributed to the measurement channels 4-1, ..., 4-n on the one hand and to the additional fiber 17 on the other hand using an additional beam splitter 19, which is arranged between the light source 5 and the beam splitter 6 for distributing the light to the different measurement channels 4-1, ..., 4-n. The additional beam splitter is preferably a 90:10 or 80:20 beam splitter, so that only a small part of the light is coupled into the additional optical fiber 17, while the majority of the light is guided to the beam splitter 6. The light reflected by the temperature monitoring FBG 18 is detected by means of an additional monitoring detection element 20. In the example shown, the temperature monitoring FBG 18 is not part of the fiber optic unit 1 mounted on the object to be monitored, but is accommodated in a glass fiber connection box. However, it should be arranged in the vicinity of the object to be monitored so that the temperature monitoring FBG 18 is exposed to the same temperature fluctuations as the sensor FBG 8 .

[0061] Replacement Figure 4a and Figure 4b In an embodiment in which the monitoring of the cable infrastructure is determined by means of the light transmitted by the sensor FBG 8, Figure 5 1 shows an embodiment in which cable infrastructure monitoring is performed with the aid of a further FBG (monitoring FBG 21) embedded in the sensor fiber 7. For this purpose, the monitoring detection element 14 is located at the end of the sensor fiber 7 facing the light source 5 and is used to detect the light transmitted by the sensor FBG 8 of the sensor fiber 7 of the different measuring channels 4-1, ..., 4-n.

[0062] The monitoring FBG 21 has a Bragg wavelength that is outside the operating range of the sensor FBG 8. Therefore, inside the sensor fiber 7, the light reflected by the sensor FBG 8 is guided back on the one hand, and the light reflected by the monitoring FBG 21 is guided back on the other hand. Via the coupler 9, the reflected light is distributed to the sensor detection element 10 on the one hand and to the monitoring detection element 14 on the other hand. In order to guide the light emitted by the light source 5 into the sensor fiber 8 and to guide the light component of the light reflected from the sensor fiber 8 to the bandpass filter 15, a further coupler or circulator 21 is provided, which connects the light source 5, the bandpass filter 15 and the sensor fiber 7 or the beam splitter 6 in series.

[0063] If by Figure 4a , Figure 4b or Figure 5The monitoring detection elements 14, 20 shown register intensity changes, which are then offset with the measurement signal of the sensor detection element 10 in order to exclude erroneous axis detection. In this way, it is possible to prevent short-term changes in the wavelength-dependent intensity distribution in the sensor fiber 7 and the intensity fluctuations associated therewith from influencing the actual measurement result, so that it can be safely determined whether a force is acting on the object to be monitored.

[0064] Figure 6 It is schematically shown how long-term changes in the wavelength-dependent intensity distribution can be corrected: In general, the raw data detected by the sensor detection element 10 (input signal I in ) is subtracted from the equivalent value (Gleichwert), so that in the absence of force on the object, the output signal I out Output value 0.

[0065] Preferably, the equivalent value is a moving long-term average value I of the light intensity detected by the sensor detection element 10. avl .

[0066] As long as the external environment, especially forces acting on the object or temperature changes, has no influence on the signal, the long-term average value I avl In order to ensure that the equivalent value does not contain any signal influence, it is preferred to additionally calculate the short-term average value I of the light intensity detected by the sensor detection element 10. avs , which is used to calculate the short-term average value I avs The time period used is used to calculate the long-term average I avl For example, the short-term average I avs can be calculated in a period of approximately 2.5 seconds, and the long-term average I avl Can be calculated in a period of about 10 seconds.

[0067] From the input signal I in Subtract the short-term average I avs If the result is smaller than the defined limit value I lim , the long-term average value I calculated in real time is used avl as an equivalent value. If this result exceeds the defined limit value I lim , then the long-term average value I calculated in real time is discarded avs In this case, it is preferred to use the previously calculated long-term average as the equivalent value, for which the corresponding short-term average I avs With input signal I in The difference between them does not exceed the limit value.

[0068] This method ensures that no measurement results (ie, measurement signals during loading of the object) enter into the long-term average value calculation.

[0069] By using the input signal I in Subtracting the equivalent value from the output signal I out , in the unaffected state, the output signal has the value 0. An algorithm for detecting forces on an object (eg, axle identification in an axle counting system) is then applied to the output signal I out .

[0070] Reference numerals list

[0071] 1 Fiber Optic Sensor Unit

[0072] 2 Fiberglass connection box

[0073] 3 Detection unit (counting board)

[0074] 4-1…4-n Measuring channels

[0075] 5 Light Source

[0076] 6 Beam splitter for dividing the light from the light source to be coupled into the measurement channel

[0077] 7 Sensor Fiber

[0078] 8 Sensor Fiber Bragg Grating

[0079] 9 A coupler for coupling light from the light source into the sensor fiber and for coupling reflected light out of the sensor fiber.

[0080] 10 Sensor detection element

[0081] 11 Rising edge of the wavelength-dependent intensity distribution

[0082] 12 Falling edge of the wavelength-dependent intensity distribution

[0083] 13 Bandpass filter with filtering bandwidth within the working range of sensor FBG

[0084] 14 Monitoring probe elements for monitoring cable infrastructure

[0085] 15 Bandpass filter with filtering bandwidth outside the working range of sensor FBG

[0086] 16 Beam splitter for combining the transmitted light from the sensor fibers of the measurement channels

[0087] 17 Additional optical fiber for temperature monitoring

[0088] 18 Attached fiber temperature monitoring FBG

[0089] 19 Additional beam splitter for splitting the light between the fiber for temperature monitoring and the sensor fiber

[0090] 20 Additional monitoring probe element for monitoring temperature

[0091] 21 Coupler / Circulator

[0092] I avs Short-term average

[0093] I avl Long term average

[0094] I in Input signal

[0095] I out Output signal

[0096] I lim Limit value of the deviation of the short-term average value from the input signal

[0097] References

[0098] DE 10 2017 119 810 B4

[0099] DE 10 2014 100 653 B4

[0100] EP 3 069 952 A1

[0101] DE 10 2012 104 874 B4

Claims

1. A measuring method for detecting mechanical forces acting on an object by means of a fiber optic sensor unit (1), wherein: There is at least one measuring channel (4-1, ..., 4-2), which comprises a sensor fiber (7) and a sensor detection element (10), wherein the sensor fiber has at least one sensor fiber Bragg grating (8) with a Bragg wavelength embedded in the sensor fiber, wherein the sensor fiber (7) is fixed to the object in the region of the sensor FBG (8), Wherein, the method comprises: coupling the light from the light source (5) into the sensor fiber (7), Detecting the light reflected and / or transmitted by the sensor FBG (8) by means of the sensor detection element (10), It is characterized in that The light source (5) has a wavelength-dependent intensity distribution with edges (11, 12), The detection of the light reflected by and / or transmitted through the sensor FBG (8) is carried out by means of the sensor detection element (10) over the entire wavelength range of the light reflected by and / or transmitted through the sensor FBG (8), and By evaluating a measurement signal comprising an intensity change of the light intensity detected by the sensor detection element (10), a wavelength change of the Bragg wavelength of the sensor FBG (8) is determined.

2. The measuring method according to claim 1, characterized in that: The light source and the sensor FBG are coordinated with one another such that the Bragg wavelength of the sensor FBG lies in a wavelength range in which the frequency diagram of the light source has the edge; preferably, the Bragg wavelength of the sensor FBG lies in the middle of the edge.

3. The measuring method according to any one of the preceding claims, characterized in that A C-band light source, in particular an ASE light source, is used as the light source.

4. The measuring method according to claim 1, characterized in that At least one disturbance variable is monitored which has an influence on the wavelength-dependent intensity distribution independently of the force acting on the object.

5. The measuring method according to claim 4, characterized in that: For monitoring the disturbance variable, a change in the intensity of the light transmitted by the sensor FBG is ascertained, wherein the light transmitted by the sensor FBG is preferably guided via a bandpass filter to a monitoring detection element (PDT).

6. The measuring method according to claim 4, characterized in that: Use monitoring FBG to monitor disturbance parameters, and For monitoring of the interference parameter, the change in the intensity of the light reflected by the monitoring FBG is determined.

7. The measuring method according to claim 6, characterized in that: The monitoring FBG is embedded in the same measuring fiber as the sensor FBG, The monitoring FBG has a Bragg wavelength different from the Bragg wavelength of the sensor FBG.

8. The measuring method according to claim 6 or 7, characterized in that: The light reflected in the measuring fiber is divided into two light components, one of which is guided to the sensor detection element without filtering, and the other light component is guided to the monitoring detection element via a bandpass filter.

9. The measuring method according to any one of claims 5 to 8, characterized in that The fiber optic sensor unit has a plurality of, in particular at least four, preferably eight, measuring channels, and Only a single monitoring detection element is used, which detects light from all measurement channels.

10. The measuring method according to any one of claims 5 to 8, characterized in that The fiber optic sensor unit has a plurality of, in particular at least four, preferably eight, measuring channels, and A plurality of monitoring detector elements are used, preferably a separate monitoring detector element for each measuring channel.

11. The measuring method according to any one of the preceding claims, characterized in that When a wavelength change is ascertained by means of the sensor detection element, in particular only when no wavelength change is ascertained by means of the monitoring detection element or a wavelength change is ascertained which lies below a predefined limit value, the force acting on the object is ascertained. 12 . The measuring method as claimed in claim 1 , for determining mechanical forces acting on a rail, in particular in a counting point of an axle counter.

13. A measuring device for carrying out the measuring method according to claim 1, comprising a light source having a wavelength-dependent intensity distribution with an edge and a fiber optic sensor unit, wherein: The fiber optic sensor unit has at least one measuring channel, which comprises a sensor fiber with at least one sensor fiber Bragg grating (S-FBG) with a Bragg wavelength embedded in the sensor fiber, and a sensor detection element, wherein the sensor fiber is arranged to be assembled on the object in the region of the sensor FBG, It is characterized in that The fiber optic sensor unit is configured to determine the change in the Bragg wavelength of the sensor FBG by evaluating the intensity change of the detected light intensity of the light reflected by the sensor FBG and / or entirely transmitted by the sensor FBG over the entire wavelength range of the light reflected by the sensor FBG (8) and / or transmitted by the sensor FBG (8).

14. The measuring device according to claim 13, characterized in that The Bragg wavelength of the FBG in the assembled state lies in the region of the edge of the wavelength-dependent intensity distribution of the light source.

15. The measuring device according to any one of claims 13 to 14, characterized in that The monitoring FBG is embedded into the sensor fiber. The monitoring FBG has a Bragg wavelength different from the Bragg wavelength of the sensor FBG, and The monitoring FBG may be arranged outside the area in which the sensor FBG is fastened to the object.

16. Axle counter with counting points, comprising two measuring devices according to one of claims 13 to 15.

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