Measurement method for detecting mechanical force acting on an object, and measurement device equipped with optical fiber sensor unit

Abstract

A measurement method for detecting a mechanical force acting on an object by means of an optical fiber sensor unit (1), the optical fiber sensor unit (1) having at least one measurement channel (4-1, . . . 4-2), the at least one measurement channel including a sensor fiber (7) having at least one sensor fiber Bragg grating (8) with a Bragg wavelength embedded therein and a sensor detection element (10), the sensor fiber (7) being attached to the object in the region of the sensor FBG (8); Coupling light from a light source (5) into a sensor fiber (7); The sensor detection element (10) detects reflected light and / or transmitted light of the sensor FBG (8). The measurement method includes The light source (5) has a wavelength-dependent intensity distribution that includes a slope, and the wavelength shift of the Bragg wavelength of the sensor FBG (8) is determined by evaluating a measurement signal that includes an intensity change of the detected light intensity of the entire reflected light of the sensor FBG (8) and / or the entire transmitted light of the sensor FBG (8). This provides a simple method that can be performed using an optical measurement device of simple structure.

Description

[Technical field]

[0001] The invention relates to a measuring method for detecting a mechanical force acting on an object by means of an optical fiber sensor unit, the optical fiber sensor unit having at least one measurement channel, the measurement channel comprising a sensor fiber in which at least one sensor fiber Bragg grating (sensor FBG) with a Bragg wavelength is embedded and a sensor detection element, the sensor fiber being attached to the object in the region of the sensor FBG. The invention further relates to a measuring device comprising an optical fiber sensor unit. [Background technology]

[0002] A measuring method in which mechanical forces acting on an object are detected by means of a fiber optic sensor unit is known, for example, from DE 10 2017 119 810 C1.

[0003] Fiber optic sensors are used in many technical fields to detect mechanical variables, such as to determine the load on components or the mechanical stress in structures. In the railway sector, fiber optic sensors are used in particular for axle counting.

[0004] A fiber optic sensor has a sensor fiber (optical fiber) with an embedded fiber Bragg grating (FBG). Each fiber Bragg grating has a reflection spectrum (spectrum at which the fiber Bragg grating reflects light) with a peak reflection at the Bragg wavelength. By coupling light into the sensor fiber (Einkopplung), light is incident on the fiber Bragg grating and wavelengths within the reflection spectrum of the fiber Bragg grating are reflected by the fiber Bragg grating. The Bragg wavelength is generally defined as λB=neff·2λ=neff·λ, where neff is the effective refractive index and λ is the grating period of the fiber Bragg grating. When the fiber Bragg grating is subjected to a load, the sensor fiber and thus the fiber Bragg grating are stretched or compressed, which changes the reflection or transmission wavelength of the fiber Bragg grating, and as a result, light of different wavelengths can be reflected into the evaluation and analysis unit depending on the stretching / compression of the fiber Bragg grating.

[0005] From DE 102014100653 A1 it is known to split the light beam leaving the sensor fiber into two partial beams for light evaluation, one of which passes through an edge filter before hitting a photoelectric element, e.g. a photodiode, and the other partial beam impinges unfiltered on another photoelectric element. The output signals of the photoelectric elements are correlated with each other and the reflected wavelength is determined. The light leaving the sensor fiber is split using a beam splitter. In the devices known from EP 3069952 A1 and DE 102012104874 A1, the beam splitter and the necessary filters together with the photoelectric elements are mounted on a plate to form an optoelectronic chip (OEC).

[0006] DE 102017119810 discloses a simplified OEC, in which the beam splitting into a first (filtered) and a second (unfiltered) beam part is not performed by a beam splitter, but by direct reflection at the filter, by reflecting the light leaving the optical fiber off the surface of the filter. That is, the filtered transmitted light intensity and the unfiltered reflected light intensity are measured. By dividing the transmitted light intensity by the reflected light intensity, an intensity ratio is obtained, from which the wavelength of the fiber Bragg grating in the optical fiber can be derived.

[0007] Known solutions are based on beam splitting and edge filtering of the partial beams, for example by means of a Fabry-Perot interferometer, which allows for a precise absolute measurement of the reflected wavelength, but the relatively complex construction required for the OEC makes them too costly to offer an attractive and competitive product, especially for rail applications. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] German Patent No. 102017119810 [Patent Document 2] German Patent No. 102014100653 [Patent Document 3] European Patent Application Publication No. 3069952 [Patent Document 4] German Patent No. 102012104874 Summary of the Invention [Problem to be solved by the invention]

[0009] SUMMARY OF THE DISCLOSURE The object of the invention is therefore to propose an optical measuring device which is particularly suitable for use in axle counting methods and which has a simple method and a simpler construction than those known up to now. [Means for solving the problem]

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

[0012] The present invention exploits the fact that for certain applications (e.g. axle counting in the railway field) precise wavelength measurement is not essential: rather, it is sufficient to know whether a change has occurred in the Bragg wavelength of the sensor FBG.

[0013] Therefore, in the method according to the invention, only the wavelength change is measured, and not the exact Bragg wavelength of the sensor FBG. For this purpose, the intensity of the reflected or transmitted light is detected by the sensor detection element over the entire wavelength range (i.e. without filtering with respect to wavelength). That is to say, the sensor detection element detects over the entire wavelength range of the light coupled into the sensor fiber. According to the invention, edge filtering in front of the sensor detection element is omitted. The light detected by the sensor detection element is only used to determine that the wavelength has changed, without determining the wavelength of the reflected or transmitted light of the sensor FBG. Since no filter is interposed between the sensor fiber and the sensor detection element, according to the invention no wavelength selection is performed before detection by the sensor detection element. This simplifies the configuration of the required measuring device.

[0014] The light source has a wavelength-dependent or frequency-dependent intensity distribution (frequency profile / 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 a slope, preferably with a gradient of at least 30 nW / nm and a length of at least 6 nm, preferably at least 8 nm. The slope can be a downward or upward slope. If the analog amplification factor, i.e. the signal amplification factor set in the electronic circuit that converts the current of the detector element into a voltage, is increased, the slope of the light source slope can instead be made gentler.

[0015] In the method according to the invention, this wavelength-dependent intensity distribution plays the role of an edge filter known from the prior art. The wavelength-dependent intensity distribution causes a change in the overall intensity of the light transmitted or reflected by the sensor FBG when the wavelength changes, so that from the intensity change measured by the sensor detection elements it is possible to infer the load on the FBG or on the object to which the FBG is attached. For this purpose, in the course of data processing, a "constant signal" is preferably subtracted from the measured raw data (intensity measured by the detection elements), so that the value 0 is output when no force is acting.

[0016] A mechanical force acting on an object is determined when a wavelength change is determined using the sensor sensing element.

[0017] The detection element is preferably a photodetector, for example a photodiode.

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

[0019] On the other hand, when evaluating the light transmitted through the sensor FBG (transmission variant), the non-reflected light is coupled out of the optical fiber, but this light is usually in a very wide wavelength range, which results in the necessity to subtract a relatively large constant signal from the raw data, which reduces the detection accuracy. Therefore, in order to increase the detection accuracy, in this method variant, bandpass filtering is performed with a filter element before the light from the light source is coupled into the sensor fiber. For example, bandpass filters or wideband FBGs can be used for bandpass filtering. The bandwidth is preferably 15-20 nm.

[0020] In a particularly preferred variant, the light source and the sensor FBG are adjusted to 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 a slope, preferably in the middle region of the slope. Selecting the Bragg wavelength of the sensor FBG in the region of the slope of the wavelength-dependent intensity distribution has the effect that a wavelength shift (e.g. by applying a force to the FBG) leads to a particularly pronounced intensity change.

[0021] The position of the Bragg wavelength relative to the tilt is preferably selected such that the Bragg wavelength is not shifted beyond the maximum (or minimum) of the wavelength-dependent intensity distribution under the influence of the maximum expected load (maximum shock) and temperature. It is preferably selected such that the rest Bragg wavelength of the sensor FBG, i.e. the Bragg wavelength of the sensor FBG in an uninfluenced state, i.e. when the sensor FBG is not exposed to external influences (in particular when no force acts on the object and at a given temperature), is included in the intermediate region of the tilt. Otherwise, a tilt large enough must be selected so that the Bragg wavelength is not shifted beyond the maximum (or minimum) of the intensity distribution. The tilt of the wavelength-dependent intensity distribution must therefore extend over a correspondingly wide wavelength range.

[0022] The resting Bragg wavelength is preferably set by pre-stressing the FBG before attachment to the object, thus ensuring that the Bragg wavelength in the unloaded state of the object is within a desired range of the slope of the wavelength-dependent intensity distribution of the light source, such that forces acting on the FBG from both directions will result in a wavelength shift within the slope.

[0023] Preferably, a C-band light source, especially an ASE light source, is used as the light source. A C-band light source has the advantage that the light in that band is only slightly attenuated in a typical optical fiber, thus enabling long-distance transmission. An ASE band light source has the advantage that it usually has a main maximum around 1530 nm, so that the required slope can be obtained.

[0024] The method according to the invention uses a light source whose wavelength-dependent intensity distribution is stable in time or can be kept stable in time. The stability of the wavelength-dependent intensity distribution can be controlled, for example, by supplying a constant voltage to the light source. By "time-stable wavelength-dependent intensity distribution" it is meant that the respective intensities for the wavelengths in the operating range of the FBG (the wavelength range of the Bragg wavelengths that can occur when intentionally generating the maximum influence on the FBG) are stable at least in the short term, i.e. are kept at a constant level. By "short-term stable" it is meant that the frequency profile does not change during the expected duration of the force action. In the field of railway engineering, for example, where the force exerted by a train on the rail is measured, the duration of the force action is, for example, the time it takes for the train to completely pass an axle count sensor. In particular, the stability of the frequency profile of the light source must be guaranteed for at least a few seconds, preferably for at least a few minutes.

[0025] In contrast, slow intensity fluctuations are tolerated and can be filtered out by long-term averaging in the processing algorithm.

[0026] Since the measurement method according to the invention is sensitive to influences on the cable infrastructure, such as light intensity changes between the sensor and the evaluation device due to bending of the fiber or poor connections, it is advantageous to monitor light intensity changes that are not due to forces detected on the object. In a particularly preferred variant of the method according to the invention, at least one disturbance parameter is therefore monitored, which influences the wavelength-dependent intensity distribution independently of the action of the forces to be detected. The influence of the disturbance parameter can be calculated from the measurement signal measured by the sensor detection element as part of the signal processing following the detection of the measurement signal. In this way, it is ensured that the detected changes in the Bragg wavelength of the FBG are due to the load of the object and not due to environmental influences or disturbances.

[0027] To determine the disturbance parameters related to changes in the cable infrastructure, it is particularly advantageous to determine the change in the intensity of the transmitted light of the sensor FBG for monitoring the disturbance parameters, in which case the transmitted light of the sensor FBG is incident, preferably via a bandpass filter, on a monitoring detector element (PDT). The monitoring detector element is arranged at the end of the sensor fiber opposite the light source. The bandpass filter filters in a wavelength range outside the operating range of the sensor FBG. The bandpass filter therefore filters out light that may be affected in intensity by a wavelength shift of the Bragg wavelength of the sensor FBG, but is not affected by the forces applied to the object. For example, a bandpass filter or a wideband FBG can be used for the bandpass filtering. The bandwidth is preferably 5-15 nm. The central frequency is preferably 1550 nm. In contrast, the light detected by the sensor detector element is not filtered.

[0028] Instead of determining the intensity change of the transmitted light of the sensor FBG (transmission monitoring), the monitoring FBG can be used for monitoring a disturbance parameter, in which case the intensity change of the reflected light of the monitoring FBG is determined for monitoring the disturbance parameter (reflection monitoring), in which case the monitoring detection element is arranged at the end of the sensor fiber facing the light source.

[0029] 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, where light from the sensor FBG on the one hand and the monitoring FBG on the other hand are reflected back together in the sensor fiber.

[0030] When the light is extracted from the sensor fiber, the reflected light is split on the one hand to the sensor detection element and on the other hand to a bandpass filter, which filters out the light reflected by the sensor FBG, so that at the monitoring detection element only the intensity of the light reflected by the monitoring FBG is detected. If the intensity detected by the monitoring detection element changes, it can be assumed that a fault has occurred in the cable infrastructure. In response, the sensor detection element measures the intensity of the light reflected by both FBGs.

[0031] In a reflective variant for detecting a measurement signal in combination with reflective monitoring, the light reflected in the sensor fiber is preferably split into two light portions, one of which is incident unfiltered on the sensor detection element and the other is incident on the monitoring detection element via a bandpass filter.

[0032] To improve availability, the detection of the measurement signal is performed in a number of measurement channels, in particular at least four, preferably eight. The provision of a number of measurement channels increases redundancy and thus the availability of the device. The light from the light source is preferably distributed to the measurement channels by means of a splitter. Preferably, the distribution of light to the measurement channels is performed evenly.

[0033] In a particularly preferred variant of the method according to the invention, only a single monitor detector element is used which detects light from all measurement channels. Alternatively, a separate monitor detector element can be used for each measurement channel.

[0034] In a particular variant of the method according to the invention, an additional optical fiber with an additional FBG (temperature monitoring FBG) is used to determine the temperature difference. In this way, the influence of temperature on the sensor FBG can be determined and evaluated with an evaluation algorithm. The temperature monitoring FBG is preferably placed in the same temperature environment as the sensor FBG, but outside the area where the sensor FBG is attached. In particular, a relative temperature measurement can be performed using the temperature monitoring FBG.

[0035] In a variant of the method for determining the disturbance parameter, a mechanical force acting on the object is detected only if no wavelength change or a wavelength change below a predefined limit value is detected by the monitoring detection element.

[0036] Preferably, the method according to the invention is used for determining the mechanical forces acting on a rail (railroad track). In particular, the method according to the invention can be used for axle counting.

[0037] The invention also relates to a measuring device for carrying out the measuring method according to any one of the preceding claims, comprising a light source with a wavelength-dependent intensity distribution comprising a slope and a fiber optic sensor unit, the fiber optic sensor unit having at least one measurement channel, the at least one measurement channel comprising a sensor fiber with at least one sensor fiber Bragg grating with a Bragg wavelength embedded therein and a sensor detection element, the sensor fiber being configured to be attached to the object in the region of the sensor FBG. According to the invention, the measuring device is configured to determine the change in the Bragg wavelength of the sensor FBG by evaluating the intensity change of the detected light intensity over the entire wavelength range of the reflected light of the sensor FBG and / or the transmitted light of the sensor FBG. For this purpose, the sensor fiber is connected to the sensor detection element directly (possibly via an optical splitter) and not to an OEC as in known measuring devices. The measuring device is therefore configured such that the light emerging from the sensor fiber (on the light source side in the reflective variant and on the side remote from the light source in the transmissive variant) is incident on the sensor detection element unfiltered.

[0038] Preferably, the measuring device further comprises an evaluation device in which the detected intensity values ​​are compared and evaluated.

[0039] The resting Bragg wavelength of the FBG, in the mounted state, is preferably within the range of the slope of the wavelength-dependent intensity distribution of the light source. A light source with a maximum intensity at 1530 nm is preferably used. The sensor FBG, in the mounted state, is preferably prestressed, and in particular, the Bragg wavelength in the unstressed state without external influence is about 1520 nm, and the operating range in the stressed state is 1522-1530 nm. The resting Bragg wavelength of the sensor FBG in the prestressed state is preferably about 1526 nm.

[0040] In a particularly preferred embodiment of the device according to the invention, the monitoring FBG is embedded in the sensor fiber, the monitoring FBG having a Bragg wavelength different from the Bragg wavelength of the sensor FBG.

[0041] Preferably, the monitoring FBG is located outside the area where the sensor FBG is attached to the object, for example, the monitoring FBG can be located in an optical fiber connection box.

[0042] The invention also relates to an axle counting device with counting positions, comprising two measuring devices as described above.

[0043] In the axle counting device, the evaluation device comprises several evaluation cards (PCB boards), with which the signals of different measurement channels can be evaluated. The invention simplifies the structure of the measurement device, so that components for detection and signal processing of a larger number of optical measurement channels can be accommodated in a single evaluation card. Preferably, in the axle counting device according to the invention, the measurement channels from at least two axle counting positions are evaluated by a single evaluation unit.

[0044] Other advantages of the invention will become apparent from the following description and drawings. According to the invention, the above and following features can be used either alone or in any combination. The illustrated and described embodiments should not be understood as an exhaustive enumeration, but rather have an exemplary character for explaining the invention. [Brief description of the drawings]

[0045] [Figure 1] 3 shows the construction of a measuring device according to the invention for carrying out the method according to the invention, in which the sensor signal is measured in reflection (reflection variant). [Diagram 2] FIG. 2 is a diagram showing the wavelength spectrum of an ASE C-band light source. [Diagram 3] 3 shows the construction of a measuring device according to the invention for carrying out the method according to the invention, in which the sensor signal is measured in transmission (transmission variant). [Figure 4a] FIG. 2 shows a structure of the measurement device of FIG. 1 for separately monitoring multiple measurement channels and for monitoring the cable infrastructure in transmission (in a reflective variant with optical monitoring in transmission). [Figure 4b] FIG. 2 shows a structure of the measurement device of FIG. 1 for jointly monitoring multiple measurement channels and for monitoring temperature and for monitoring the cable infrastructure in transmission (in a reflective variant with optical monitoring in transmission). [Diagram 5] 2 shows the structure of the measuring device of FIG. 1 for monitoring a disturbance parameter in reflection (in a reflective variant for monitoring light in reflection). [Figure 6] FIG. 1 shows a schematic diagram of the principle of correcting gradual intensity variations. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] 1 shows a measurement device for implementing a reflective variant of the method according to the invention for detecting mechanical forces acting on an object (not shown). The measurement device comprises a fiber optic sensor unit 1 connected to a detection unit 3 via a fiber optic connection box 2. The fiber optic sensor unit 1 is attached to the object, whereas the detection unit 3 can be located remotely from the object to be monitored.

[0047] In the embodiment shown in Fig. 1, a number of measurement channels 4-1, 4-2, ..., 4-n are provided, each measurement channel 4-1, ..., 4-n comprising a sensor fiber 7 and a sensor detection element 10. Analogue measured values ​​detected by the detection element 10 are converted to digital form (not shown) and evaluated by a processing element (e.g. CPU or FPGA) (not shown). Providing a number of measurement channels 4-1, ..., 4-n is not necessary, but is advantageous in terms of availability. The detection unit 3 comprises a light source 5, the light of which is distributed by a splitter 6 to the several measurement channels 4-1, ..., 4-n. The splitter 6 is preferably a 1:n splitter, which distributes the light from the light source 5 evenly to the n measurement channels 4-1, ..., 4-n and couples them into the sensor fiber 7. A sensor fiber Bragg grating 8 (sensor FBG) is embedded in each sensor fiber 7, and the sensor FBGs 8 of all measurement channels 4-1,...,4-n have the same Bragg wavelength in the attached and unloaded state. In each measurement channel 4-1,...,4-n, the light reflected by the sensor FBGs 8 is returned into the sensor fiber 7 and enters the sensor detection element 10 via a coupler 9, where it is detected as raw data.

[0048] According to the invention, unlike known methods, the entire wavelength range reflected by the sensor FBG 8 is detected by the sensor detection element 10, even if the Bragg wavelength of the sensor FBG 8 changes due to the application of a load.

[0049] In order to be able to detect a change in wavelength while still detecting the entire wavelength spectrum, a light source with steep slopes 11, 12 in its wavelength spectrum (wavelength-dependent intensity distribution) is used as light source 5. FIG. 2 shows an example of a wavelength-dependent intensity distribution of an ASE light source with a steep upward slope 11 and a steep downward slope 12. The rest Bragg wavelength of the sensor FBG 8 in the mounted state is preferably selected in the middle of one of the slopes 11, 12. The operating range of the sensor FBG 7 will be within one slope, i.e. here for example between 1520 nm and 1530 nm. If the Bragg wavelength is changed due to the action of a force on the object, not only will the wavelength change, but also the intensity of the light reflected by the sensor FBG 8 will change due to the slope of the wavelength-dependent intensity distribution. In this way, it is not possible to determine the absolute Bragg wavelength, but it is possible to determine that the wavelength has changed, which is sufficient for applications in the field of axle counting for example. Since the method according to the invention detects changes in wavelength and not the wavelength itself, during data processing the intensity of the light reflected by the sensor FBG when the object is unloaded (the "constant signal") is subtracted from the measured raw data (the intensity measured by the detection elements), thereby giving the value 0 as the output signal when no force is acting. The constant signal that is subtracted is preferably the average value of the intensity measured over a given period of movement, in particular over the last few seconds, preferably of the order of the last 10 seconds.

[0050] FIG. 3 shows a measurement device in a variant of another method, for example with two measurement channels 4-1, 4-2, the measurement being carried out in transmission (transmission variant). Unlike the measurement device used for the reflection variant (FIG. 1), 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 it is not the light reflected by the sensor FBG 8 that is detected, but the light transmitted through the sensor FBG 8. However, this means that the constant signal to be subtracted from the raw measurement data is relatively large compared to the intensity changes caused by the action of a force. Therefore, in this variant, before coupling the light from the light source 5 into the sensor fiber 7, it is advantageous to limit the wavelength range of the light to be coupled by a bandpass filter 13. For this purpose, the bandpass filter must cover the operating 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 1520-1535 nm.

[0051] 4a, 4b and 5 show an embodiment of the measuring device according to the invention for carrying out a reflection variant of the method according to the invention, in which in addition to actually measuring the light reflected by the sensor FBG8 to determine the wavelength change, a disturbance parameter is monitored. This is particularly advantageous, since the Bragg wavelength of the sensor FBG8 can be influenced not only by forces applied to the object to which the sensor FBG8 is attached, but also by disturbance factors such as temperature changes or changes in the cable infrastructure (bending of the fiber, bad connections, etc.), which can cause changes in the reflected light intensity.

[0052] In order to check whether the intensity change is due to an influence on the Bragg wavelength of the sensor FBG 8 or due to an influence on the cable infrastructure, an additional detection element (monitoring detection element 14) is provided in the detection unit 3. With the monitoring detection element 14, light from the sensor fiber 7 of the measurement channels 4-1, 4-2, ..., 4-n is detected, which has a wavelength outside the operating range of the sensor FBG 8. If necessary, a bandpass filter 15 can be connected in front of the monitoring detection element 14, which passes a wavelength range outside the operating range of the sensor FBG 8 (here, for example, 1550 nm). In this way, the light intensity detected by the monitoring detection element 14 is not influenced by the load on the object and is meaningful with respect to 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 in the Bragg wavelength due to the load on the object on the monitoring signal is small. It is therefore also possible to omit the bandpass filter 15. If the light intensity changes due to changes in the cable infrastructure, this can be detected based on the intensity change detected by the monitoring detection element 14.

[0053] In the embodiment shown in Fig. 4a and Fig. 4b, a monitoring detector element 14 is located at the end of the sensor fiber 7 opposite the light source 5 and is used to detect the transmitted light of the sensor FBG 8 of the sensor fiber 7 for each measurement channel 4-1, ..., 4-n. Monitoring can be performed separately for each measurement channel 4-1, ..., 4-n. For this purpose, in the embodiment shown in Fig. 4a, a separate monitoring detector element 14 is provided for each measurement channel 4-1, ..., 4-n. Alternatively, the transmitted light from all measurement channels 4-1, ..., 4-n can be combined by a separate splitter 16 and injected into a common monitoring detector element 14, as shown in Fig. 4b.

[0054] In addition to the monitoring of the cable infrastructure, temperature monitoring is also provided in the embodiment shown in Fig. 4b. For this, 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). Splitting the light of the light source 5 into the measurement channels 4-1, ..., 4-n on the one hand and into the additional fiber 17 on the other hand can be done by means of an additional splitter 19, which is arranged between the light source 5 and the splitter 6 used to split the light into several measurement channels 4-1, ..., 4-n. The additional splitter is preferably a 90:10 or 80:20 splitter, so that only a small part of the light is coupled into the additional fiber 17, while the majority of the light enters the splitter 6. The light reflected by the temperature monitoring FBG 18 is detected by an additional monitoring detection element 20. In the example shown, the temperature monitoring FBG 18 is not part of the optical fiber unit 1 attached to the object to be monitored, but is housed in a fiber optic connection box. However, in order for the temperature monitoring FBG 18 to be exposed to the same temperature variations as the sensor FBG 8, it must be placed close to the object being monitored.

[0055] Instead of the embodiment shown in Figures 4a and 4b, in which the monitoring of the cable infrastructure is determined by the transmitted light of the sensor FBG 8, an embodiment is shown in Figure 5, in which the monitoring of the cable infrastructure is performed by another FBG (monitoring FBG 21) embedded in the sensor fiber 7. 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 transmitted light of the sensor FBG 8 of the sensor fiber 7 of each measurement channel 4-1, ..., 4-n.

[0056] The monitoring FBG 21 has a Bragg wavelength that is outside the operating range of the sensor FBG 8. Thus, the light reflected by the sensor FBG 8 on the one hand and the monitoring FBG 21 on the other hand returns in the sensor fiber 7. The reflected light is distributed via a coupler 9 to the sensor detection element 10 on the one hand and to the monitoring detection element 14 on the other hand. A further coupler or circulator 21 is provided to couple the light emitted by the light source 5 into the sensor fiber 8 and to couple one of the light portions of the light reflected from the sensor fiber 8 into the bandpass filter 15, connecting the light source 5, the bandpass filter 15 and the sensor fiber 7 or the front splitter 6.

[0057] If an intensity change is detected by the monitoring detector element 14, 20 shown in Figure 4a, 4b or 5, this is offset with respect to the measurement signal of the sensor detector element 10 in order to eliminate erroneous axle axis detection. In this way, short-term changes in the wavelength-dependent intensity distribution in the sensor fiber 7 and the associated intensity fluctuations can be prevented from influencing the actual measurement result, so that it can be reliably determined whether a force has been applied to the monitored object.

[0058] A method for correcting for long-term changes in the wavelength-dependent intensity distribution is shown diagrammatically in Fig. 6. In general, a constant value is subtracted from the raw data (input signal Iin) detected by the sensor detection element 10 so that the value of the output signal lout is zero when no force is acting on the object.

[0059] Preferably, the constant value is the floating long-term average value Iavl of the light intensity detected by the sensor detection element 10 .

[0060] The long-term average value Iavl is constantly updated unless the signal is affected by the external environment, in particular the action of force on the object or temperature changes. In order to ensure that the constant value does not include the influence of the signal, it is preferable to further calculate a short-term average value Iavs of the light intensity detected by the sensor detection element 10, and a shorter period is used for calculating the short-term average value Iavs than that used for calculating the long-term average value Iavl. For example, the short-term average value Iavs can be calculated over a period of about 2.5 seconds, and the long-term average value Iavl can be calculated over a period of about 10 seconds.

[0061] The short-term average value lavs is subtracted from the input signal lin. If the absolute value of the resulting value is smaller than the defined limit value Ilim, the currently calculated long-term average value Iavl is used as a constant value. If the absolute value of the resulting value exceeds the defined limit value Ilim, the currently calculated long-term average value Iavs is discarded. Preferably, in this case, a previously calculated long-term average value, the difference between the associated short-term average value Iavs and the input signal Iin does not exceed the limit value Ilim, is used as a constant value.

[0062] This approach ensures that measurement events, i.e. measurement signals during which the object is under load, are not included in the calculation of the long-term average value.

[0063] By subtracting a constant value from the input signal lin, an output signal lout is generated which in its unaffected state has the value 0. An algorithm for detecting the action of forces on an object (e.g. axle detection in an axle counting system) is then applied to this output signal lout. [Explanation of symbols]

[0064] 1 Optical fiber sensor unit 2 Fiber Optic Connection Box 3 Detection unit (counting board) 4-1...4-n Measurement channel 5 light source 6 A splitter for splitting the light from the light source to be coupled into the measurement channels 7 Sensor Fiber 8 Sensor Fiber Bragg Grating 9 A coupler for coupling light from a light source into the sensor fiber and extracting the reflected light from the sensor fiber. 10 Sensor detection element 11. Upward slope of the wavelength-dependent intensity distribution 12. The downward slope of the wavelength-dependent intensity distribution 13 Bandpass filter with filter bandwidth within the operating range of the sensor FBG 14 Monitoring detection elements for monitoring cable infrastructure 15 Bandpass filter with filter bandwidth outside the operating range of the sensor FBG 16 Splitters for combining the transmitted light of the sensor fibers of each measurement channel 17 Additional optical fiber for temperature monitoring 18 Additional fiber temperature monitoring FBG 19 Additional splitter to split the light into the fiber for temperature monitoring and the sensor fiber 20 Additional monitoring detector for temperature monitoring 21 Coupler / Circulator Iavs short-term average Iavl Long-term average value, constant signal Iin Input signal Iout Output signal Ilim Limit value for deviation of short-term average value from input signal References German Patent No. 102017119810 German Patent No. 102014100653 European Patent Application Publication No. 3069952 German Patent No. 102012104874

Claims

1. A measuring method for detecting a mechanical force acting on an object by means of an optical fiber sensor unit (1), the optical fiber sensor unit (1) having at least one measurement channel (4-1, . . . 4-2), the at least one measurement channel (4-1, . . . 4-2) comprising a sensor fiber (7) in which at least one sensor fiber Bragg grating (8) having a Bragg wavelength is embedded and a sensor detection element (10), the sensor fiber (7) being attached to the object in the region of the sensor FBG (8), - coupling light from a light source (5) into said sensor fiber (7); Detecting reflected light and / or transmitted light of the sensor FBG (8) by the sensor detection element (10); In a method comprising: the light source (5) having a wavelength-dependent intensity distribution including slopes (11, 12); The detection of the reflected light and / or the transmitted light of the sensor FBG (8) by the sensor detection element (10) is performed over a full wavelength range of the reflected light of the sensor FBG (8) and / or the transmitted light of the sensor FBG (8); determining a wavelength change of the Bragg wavelength of the sensor FBG (8) by evaluating a measurement signal comprising an intensity change of the light intensity detected by the sensor detection element (10).

2. 2. The measurement method according to claim 1, characterized in that the light source and the sensor FBG are mutually adjusted so that the Bragg wavelength of the sensor FBG is included in a wavelength range in which the frequency profile of the light source has the inclination portion, preferably in an intermediate region of the inclination portion.

3. 3. The method according to claim 1, wherein a C-band light source, in particular an ASE light source, is used as the light source.

4. 4. The measuring method according to claim 1, wherein at least one disturbance parameter influencing the wavelength-dependent intensity distribution independently of forces acting on the object is monitored.

5. 5. The measuring method according to claim 4, characterized in that for monitoring the disturbance parameter, a change in the intensity of the transmitted light of the sensor FBG is determined and the transmitted light of the sensor FBG is incident on a monitoring detection element (PDT), preferably via a band-pass filter.

6. A monitoring FBG is used for monitoring the disturbance parameter; and A change in the intensity of the reflected light of the monitoring FBG is determined for monitoring the disturbance parameter; and The method according to claim 4, characterized in that

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

8. 8. The measuring method according to claim 6 or 7, characterized in that the light reflected in the sensor fiber is split into two light portions, one of the two light portions being incident on the sensor detection element without being filtered, and the other being incident on the monitoring detection element via a bandpass filter.

9. the optical fiber sensor unit has a plurality of measuring channels, in particular at least four, preferably eight, Only one monitor detector is used to detect light from all measurement channels; and The measurement method according to any one of claims 5 to 8,

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

11. 11. The method according to claim 1, characterized in that the force acting on the object is determined only if a wavelength change is determined using the sensor detection element, in particular if no wavelength change or a wavelength change below a predetermined limit value is determined using the monitoring detection element.

12. The measuring method according to any one of the preceding claims, which is used for determining the mechanical forces acting on a rail, in particular at the counting positions of an axle counting device.

13. A measuring device for carrying out the measuring method according to any one of claims 1 to 12, comprising a light source having a wavelength-dependent intensity distribution including a slope and an optical fiber sensor unit, the optical fiber sensor unit having at least one measurement channel, the at least one measurement channel comprising a sensor fiber in which at least one sensor fiber Bragg grating (S-FBG) having a Bragg wavelength is embedded and a sensor detection element, the sensor fiber being configured to be attached to an object in the region of the sensor FBG, The optical fiber sensor unit is configured to determine the Bragg wavelength shift of the sensor FBG by evaluating the intensity change of the detected light intensity over the entire wavelength range of the reflected light of the sensor FBG (8) and / or the transmitted light of the entire sensor FBG.

14. 14. The measurement apparatus of claim 13, wherein the Bragg wavelength of the FBG in an installed state is within the slope of the wavelength dependent intensity distribution of the light source.

15. A monitoring FBG is embedded in the sensor fiber; and the monitoring FBG has a Bragg wavelength different from the Bragg wavelength of the sensor FBG; The monitoring FBG can be located outside the area where the sensor FBG is attached to the object; and The measuring device according to any one of claims 13 to 14, characterized in that

16. An axle counting device with counting positions, comprising two measuring devices according to any one of claims 13 to 15.

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