Measurement method for detecting a mechanical force acting on an object, and measuring device having a fibre-optic sensor unit
Patent Information
- Application Number
- EP2023725125
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-05-04
- Filing Date
- 2023-05-03
- Publication Date
- 2025-03-12
AI Technical Summary
Existing fiber-optic sensor systems for detecting mechanical forces, particularly in the railway sector, are complex and expensive due to their reliance on beam splitting and edge filtering, making them unsuitable for cost-effective applications like axle counting.
A simplified method using a fiber-optic sensor unit with a sensor fiber Bragg grating that detects light intensity changes over the entire wavelength range without filtering, employing a light source with a wavelength-dependent intensity distribution to determine wavelength shifts caused by mechanical forces, and optional band-filtering to improve detection accuracy.
This approach reduces system complexity and cost while maintaining accurate force detection, suitable for applications like axle counting, by focusing on wavelength changes rather than precise wavelength measurement, and enhances reliability through monitoring of disturbance parameters.
Smart Images
Figure 1.1
Abstract
Description
[0001] Measuring method for detecting a mechanical force acting on an object, measuring device with fiber optic sensor unit
[0002] Background of the invention
[0003] The invention relates to a measuring method for detecting a mechanical force acting on an object using a fiber optic sensor unit. The fiber optic sensor unit has at least one measuring channel comprising a sensor fiber with at least one sensor fiber Bragg grating (sensor FBG) embedded in the sensor fiber with a Bragg wavelength and a sensor detection element. The sensor fiber is attached to the object in the region of the sensor FBG. The invention further relates to a measuring device with a fiber optic sensor unit. A measuring method for detecting a mechanical force acting on an object using a fiber optic sensor unit is known, for example, from DE 10 2017 119 810 B4.
[0004] Fiber optic sensors are used to measure mechanical variables in many technical fields, e.g., to investigate loads on components and mechanical stresses in structures. In the railway sector, fiber optic sensors are used in particular for axle counting.
[0005] A fiber optic sensor has a sensor fiber (optical waveguide) in which a fiber Bragg grating (FBG) is embedded. Each fiber Bragg grating has a reflection spectrum (spectrum within which the fiber Bragg grating reflects light) with a reflection peak at the Bragg wavelength. By coupling light into the sensor fiber, light is supplied to the fiber Bragg grating, whereby wavelengths that lie in the reflection spectrum of the fiber Bragg grating are reflected by the fiber Bragg grating. The Bragg wavelength is generally defined as XB=neff-2X=neff-X. Here, neff is the effective refractive index and X is the grating period of the fiber Bragg grating. When a load acts on the fiber Bragg grating, the sensor fiber and thus the fiber Bragg grating is stretched or compressed, and the reflection or refractive index changes.The transmission wavelength of the fiber Bragg grating changes, so that depending on the stretching / compression of the fiber Bragg grating, light of different wavelengths is reflected and can be fed to an evaluation and analysis unit.
[0006] From DE 10 2014 100 653 B4, it is known to split the light beam emerging from the sensor fiber into two partial beams for light evaluation. One of the partial beams passes through an edge filter before striking a photoelectric element, e.g., a photodiode, and the other partial beam is directed unfiltered to another photoelectric element. The output signals of the photoelectric elements are related to each other to determine the reflected wavelength. The light emerging from the sensor fiber is split using 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 filter are mounted with the photoelectric elements on a board and form an optoelectronic chip (OEC). DE 10 2017 119 810 B4 discloses a simplified OEC in which the beam is split into the 1st (filtered) beam portion and the 2ndThe (unfiltered) beam portion is not separated by a beam splitter, but by reflection of the light emerging from the optical fiber at the surface of the filter, directly from the filter. Thus, a filtered transmission intensity and an 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 fiber can be determined.
[0007] The known solutions are based on beam splitting and edge filtering of a partial beam, for example, using a Fabry-Perot interferometer. While this enables precise absolute measurement of the reflected wavelength, it requires OECs, which are relatively complex and therefore too expensive to offer an attractive and competitive product, especially for railway applications.
[0008] It is therefore an object of the invention to propose a simplified method and an optical measuring device which is constructed more simply than previously known and is particularly well suited for use in axle counting methods.
[0009] This object is achieved according to the invention by a method according to claim 1 and a measuring device according to claim 12.
[0010] In the method according to the invention, the light reflected and / or transmitted by the sensor FBG is detected by means of the sensor detection element across the entire wavelength range of the light reflected and / or transmitted by the sensor FBG. The method according to the invention comprises determining a wavelength change in the Bragg wavelength of the sensor FBG by evaluating a measurement signal that includes a change in the intensity of the light intensity detected by the sensor detection element (10). The light intensity of the light reflected and / or transmitted by the sensor FBG is thus evaluated across the entire wavelength range. In other words, the light reflected and / or transmitted by the sensor FBG is detected and evaluated unfiltered.
[0011] The invention takes advantage of the fact that for certain applications (e.g., axle counting in the railway sector), precise wavelength measurement is not required. Rather, it is sufficient to know whether a change in the Bragg wavelengths of the sensor FBG has occurred.
[0012] In the method according to the invention, therefore, the exact Bragg wavelength of the sensor FBG is not measured, but merely a wavelength change. For this purpose, the reflected or transmitted light intensity across the entire wavelength range (i.e., unfiltered with respect to wavelength) is detected by the sensor detection element. The sensor detection element therefore detects across the entire wavelength range of the light coupled into the sensor fiber. According to the invention, edge filtering upstream of the sensor detection element is therefore dispensed with. The light detected by the sensor detection element is used solely to determine a wavelength change, but without determining the wavelengths of the light reflected or transmitted by the sensor FBG. Since no filter is interposed between the sensor fiber and the sensor detection element, no wavelength selection takes place according to the invention before detection via the sensor detection element.This simplifies the required measuring setup.
[0013] The light source has a wavelength-dependent or frequency-dependent intensity distribution (frequency image / 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 slope of at least 30 nW / nm and a length of at least 6 nm, preferably of at least 8 nm. The slope can be a falling or rising slope. The slope of the light source can be flatter if, in turn, the analog amplification factor is increased, i.e., the signal amplification factor set in the electronic circuit that converts the current of the detection element into a voltage.In the method according to the invention, this wavelength-dependent intensity distribution performs the function of the edge filter known from the prior art. Due to the wavelength-dependent intensity distribution, a change in wavelength causes a change in the total intensity of the light transmitted or reflected by the sensor FBG, so that a load on the FBG or the object to which the FBG is attached can be inferred from a change in intensity measured by the sensor detection element. For this purpose, a "DC signal" is subtracted from the measured raw data (intensity measured by the detection element) preferably during data processing, so that the value 0 is output without any force applied.
[0014] A mechanical force acting on the object is detected when a change in wavelength is detected by the sensor detection element.
[0015] The detection element is preferably a photodetector, e.g. a photodiode.
[0016] 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. Due to the limited reflection spectrum of FBGs, the light detected by the sensor detection element covers only a limited wavelength range.
[0017] When evaluating the light transmitted by the sensor FBG (transmission variant), however, all non-reflected light is extracted from the sensor fiber. This typically covers a very wide wavelength range, so that a relatively large DC signal would have to be subtracted from the raw data, which reduces detection accuracy. To increase detection accuracy, in this process variant, the light from the light source is band-filtered using a filter element before being coupled into the sensor fiber. A bandpass filter or a broadband FBG, for example, can be used for band-filtering. The bandwidth is preferably between 15 and 20 nm.
[0018] In a particularly preferred variant, the light source and the sensor FBG are matched to each other such that the Bragg wavelength of the sensor FBG lies in a wavelength range in which the wavelength-dependent intensity distribution of the light source exhibits the flank, preferably in the middle region of the flank. Choosing the Bragg wavelength of the sensor FBG in the region of the flank of the wavelength-dependent intensity distribution ensures that the wavelength shift (for example, due to the application of force to the FBG) causes a particularly significant change in intensity.
[0019] The position of the Bragg wavelength relative to the flank is preferably chosen so that a maximum expected load (maximum stroke) plus temperature influences do not shift the Bragg wavelength beyond the maximum (or minimum) of the wavelength-dependent intensity distribution. The rest Bragg wavelength of the sensor FBG, i.e. the Bragg wavelength of the sensor FBG in an unaffected state, i.e. in a state in which the sensor FBG is not exposed to any external influences (in particular without any force acting on the object and at a given temperature), is preferably chosen so that it lies in the middle region of the flank. Otherwise, the flank should be chosen large enough to avoid shifting the Bragg wavelength beyond the maximum (or minimum) of the intensity distribution. The flank of the wavelength-dependent intensity distribution should therefore extend over a correspondingly large wavelength range.
[0020] The resting Bragg wavelength is preferably adjusted by preloading the FBG before mounting it on the object. This ensures that the Bragg wavelength, in the unloaded state of the object, lies within the desired range of the flank of the wavelength-dependent intensity distribution of the light source, so that forces acting on the FBG from both directions cause a wavelength shift within the flank.
[0021] A C-band light source, in particular an ASE light source, is preferably used as the light source. A C-band light source has the advantage that light in this band is only slightly attenuated in typical glass fibers and therefore enables long ranges. An ASE-band light source has the advantage that it typically has a dominant maximum at approximately 1530 nm and thus has the required flanks. The method according to the invention should use a light source whose wavelength-dependent intensity distribution is stable over time or can be kept stable over time. The stability of the wavelength-dependent intensity distribution can be influenced, for example, by supplying the light source with a constant voltage.A "temporally stable wavelength-dependent intensity distribution" means that the intensities are kept stable, i.e., at a constant level, at least for a short time, depending on the wavelength for the operating range of the FBG (wavelength interval of the Bragg wavelengths that can occur due to the maximum intended influence on the FBG). "Short-term stable" means that the frequency pattern does not change during the expected duration of the force application. In the railway sector, where, for example, the force application of a train on a rail is to be measured, the duration of the force application would be, for example, the time required for a train to completely pass an axle counting sensor. In particular, the stability of the frequency pattern of the light source should be guaranteed for a period of at least a few seconds, preferably at least a few minutes.
[0022] Slow intensity deviations, however, are permissible and can be filtered out by a long-term average in the processing algorithm.
[0023] Since the measuring method according to the invention is sensitive to influences on the cable infrastructure, such as changes in light intensity between the sensor and the evaluation device due to fiber bending, poor connections, etc., it is advantageous to monitor changes in light intensity that are not attributable to the force to be 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 force to be detected. The influence of the disturbance parameter can then be calculated out of the measurement signal measured by the sensor detection element as part of signal processing following the detection of the measurement signal.This ensures that the detected change in the Bragg wavelength of the FBG is due to a load on the object and not to any environmental influences or external disturbances.
[0024] To determine disturbance parameters related to changes in the cable infrastructure, it is particularly advantageous to determine the change in the intensity of the light transmitted by the sensor FBG for monitoring the disturbance parameter. The light transmitted by the sensor FBG is directed, preferably via a bandpass filter, to a monitoring detection element (PDT). The monitoring detection element is arranged at the end of the sensor fiber opposite the light source. The bandpass filter filters in a wavelength range that lies outside the operating range of the sensor FBG. The bandpass filter therefore filters out the light whose intensity can be influenced by a wavelength shift of the Bragg wavelength of the sensor FBG, but not by a force acting on the object. A bandpass filter or a broadband FBG, for example, can be used for band filtering.The bandwidth is preferably 5-15 nm. The center frequency is preferably 1550 nm. The light to be detected by the sensor detection element is not filtered.
[0025] As an alternative to determining the change in intensity of the light transmitted by the sensor FBG (transmission monitoring), a monitoring FBG can be used to monitor the disturbance parameter. The change in intensity of the light reflected by the monitoring FBG is used to monitor the disturbance parameter (reflection monitoring). In this case, the monitoring detection element is arranged at the end of the sensor fiber facing the light source.
[0026] The monitoring FBG is preferably embedded in the same sensor fiber as the sensor FBG and has a Bragg wavelength that differs from the Bragg wavelength of the sensor FBG. In the sensor fiber, light from the sensor FBG and the monitoring FBG are reflected and redirected together within the sensor fiber. When the light is coupled out of the sensor fiber, the reflected light is split between the sensor detection element and a bandpass filter, which filters out the light reflected by the sensor FBG. The bandpass filter 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, a fault in the cable infrastructure can be assumed.The sensor detection element, on the other hand, measures the intensity of the light reflected by both FBGs.
[0027] In the reflection variant for detecting the measurement signal in combination with reflection monitoring, the light reflected in the sensor fiber is preferably split into two light components, one of which is passed unfiltered to the sensor detection element and the other via a bandpass filter to a monitoring detection element.
[0028] To improve availability, the measurement signal is acquired in several, in particular at least four, preferably eight, measurement channels. Providing multiple measurement channels increases redundancy and thus the availability of the system. The light from the light source is preferably distributed among the measurement channels using a splitter. Preferably, the light is distributed evenly across the measurement channels.
[0029] In a particularly preferred variant of the method according to the invention, only a single monitoring detection element is used, which detects light from all measurement channels. Alternatively, a separate monitoring detection element can also be used for each measurement channel.
[0030] In a special variant of the method according to the invention, temperature differences are determined using 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 using an evaluation algorithm. The temperature monitoring FBG is preferably arranged in the same temperature structure as the sensor FBG, but outside the mounting area of the sensor FBG. In particular, a relative temperature measurement can be performed using the temperature monitoring FBG.
[0031] In the process variants in which a disturbance parameter is determined, a mechanical force acting on the object is only detected if the monitoring detection element does not detect any wavelength change or detects a wavelength change that is below a predetermined limit value.
[0032] The method according to the invention is preferably used to determine a mechanical force acting on a rail (railway track). In particular, the method according to the invention can be used for axle counting.
[0033] The invention also relates to a measuring device for carrying out a measuring method according to one of the preceding claims, comprising a light source having a wavelength-dependent intensity distribution with a flank, and a fiber-optic sensor unit, wherein the fiber-optic sensor unit has at least one measuring channel comprising a sensor fiber with at least one sensor fiber Bragg grating embedded in the sensor fiber with a Bragg wavelength and a sensor detection element, wherein the sensor fiber is designed to be mounted on an object in the region of the sensor FBG.According to the invention, the measuring device is configured to determine a change in the Bragg wavelength of the sensor FBG by evaluating a change in the intensity of the detected light intensity of the light reflected by the sensor FBG and / or of the entire light transmitted by the sensor FBG across 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 (optionally via an optical distributor) directly to the sensor detection element and not, as in known measuring devices, to an OEC. The measuring device is thus constructed such that the light emerging 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.Preferably, the measuring device further comprises an evaluation device in which the detected intensity values are compared and evaluated.
[0034] The resting Bragg wavelength of the FBG in the mounted state is preferably in the region of the flank of the wavelength-dependent intensity distribution of the light source. A light source with an intensity maximum at 1530 nm is preferably used. The sensor FBG is preferably prestressed in the mounted state, in particular such that the Bragg wavelength in the unstrained state without external influences is approximately 1520 nm, and the operating range in the tensioned state is 1522-1530 nm. The resting Bragg wavelength of the sensor FBG in the prestressed state is preferably approximately 1526 nm.
[0035] In a particularly preferred embodiment of the device according to the invention, a 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.
[0036] 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 a fiber optic junction box.
[0037] The invention also relates to an axle counting device with a counting point comprising two previously described measuring devices.
[0038] In an axle counting device, the evaluation unit comprises evaluation cards (PCB boards) with which the signals from various measuring channels can be evaluated. The invention realizes a simplified structure of the measuring device so that the components for detecting and processing the signals from a large number of optical measuring channels can be accommodated within a single evaluation card. Preferably, in the axle counting device according to the invention, measuring channels from at least two axle counting points are evaluated using a single evaluation unit. Further advantages of the invention emerge from the description and the drawing. Likewise, the features mentioned above and those explained below can be used according to the invention individually or in groups in any desired combination.The embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for the description of the invention.
[0039] Detailed description of the invention and drawing
[0040] Figure 1 shows a structure of a measuring device according to the invention for carrying out the method according to the invention, wherein the sensor signal is measured in reflection (reflection variant).
[0041] Figure 2 shows a wavelength spectrum of an ASE C-band light source.
[0042] Figure 3 shows a structure of a measuring device according to the invention for carrying out the method according to the invention, wherein the sensor signal is measured in transmission (transmission variant).
[0043] Figure 4a shows the structure of the measuring device from Figure 1 with monitoring of the cable infrastructure in transmission (reflection variant with light monitoring in transmission) with separate monitoring of several measuring channels.
[0044] Figure 4b shows the structure of the measuring device from Figure 1 with monitoring of the cable infrastructure in transmission (reflection variant with light monitoring in transmission) with joint monitoring of several measuring channels and temperature monitoring.
[0045] Figure 5 shows the structure of the measuring device from Figure 1 with monitoring of disturbance parameters in reflection (reflection variant with light monitoring in reflection).
[0046] Figure 6 shows schematically the principle of correcting slow intensity deviations.
[0047] Figure 1 shows a measuring device for implementing a reflection variant of the inventive method for detecting a mechanical force acting on an object (not shown). The measuring device comprises a fiber optic sensor unit 1 connected to a detection unit 3 via a fiber optic junction box 2. The fiber optic sensor unit 1 is attached to the object, whereas the detection unit 3 can be arranged remotely from the object to be monitored.
[0048] In the embodiment shown in Figure 1, several measuring channels 4-1, 4-2,..., 4-n are provided, each measuring channel 4-1,..., 4-n comprising a sensor fiber 7 and a sensor detection element 10. The analog measured values detected by the detection elements 10 are digitally converted (not shown in the figure) and evaluated by a processing element (e.g., CPU or FPGA) (not shown in the figure). The provision of several measuring channels 4-1,..., 4-n is not absolutely necessary, but is advantageous with regard to availability aspects. The detection unit 3 comprises a light source 5, the light of which is distributed among the various measuring channels 4-1,..., 4-n by means of a splitter 6. The splitter 6 is preferably a 1:n splitter which distributes the light from the light source 5 evenly to the n measuring channels 4-l,...,4-n and couples it into the sensor fibers 7.A sensor fiber Bragg grating 8 (sensor FBG) is embedded in each sensor fiber 7. The sensor FBGs 8 of all measurement channels 4-l,...,4-n have the same Bragg wavelength when mounted and unloaded. In each measurement channel 4-l,...,4-n, the light reflected by the sensor FBG 8 is returned in the sensor fiber 7, guided via a coupler 9 to a sensor detection element 10, where it is detected as raw data.
[0049] According to the invention, in contrast to the known methods, the entire wavelength range reflected by the sensor FBG 8 is detected by means of the sensor detection element 10, even if the Bragg wavelength of the sensor FBG 8 changes due to a load.
[0050] In order to be able to detect a wavelength change despite capturing the entire wavelength spectrum, a light source 5 is used which has a steep flank 11, 12 in the wavelength spectrum (wavelength-dependent intensity distribution). Figure 2 shows an example of the wavelength-dependent intensity distribution of an ASE light source with a steeply rising flank 11 and a steeply falling flank 12. The rest Bragg wavelength of the sensor FBG 8 in the mounted state is preferably selected in the middle of one of the flanks 11, 12. The operating range of the sensor FBG 7 should be within one flank, here for example between 1520 nm and 1530 nm. If the Bragg wavelength changes due to a force acting on the object, not only the wavelength changes, but also the intensity of the light reflected by the sensor FBG 8 due to the flank of the wavelength-dependent intensity distribution.In this way, although the absolute Bragg wavelength cannot be determined, a wavelength change can be determined, which is sufficient, for example, for applications in axle counting. Since the method according to the invention only concerns detecting a change in wavelength, not the wavelength itself, the intensity of the light reflected by the sensor FBG in the unloaded state of the object ("DC signal") is subtracted from the measured raw measurement data (intensity measured by the detection element) during data processing, so that without the application of force, the value 0 is output as the output signal. The DC signal to be subtracted is preferably the average of the measured intensity over a predetermined sliding period, in particular over the last few seconds, preferably on the order of the last 10 seconds.
[0051] Figure 3 shows a measuring device for an alternative method variant, for example for two measuring channels 4-1, 4-2, in which measurements are taken in transmission (transmission variant). In contrast to the measuring arrangement used for the reflection variant (Figure 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, therefore, it is not the light reflected by the sensor FBG 8 that is detected, but rather the light transmitted by the sensor FBGs 8. However, this leads to the DC signal subtracted from the raw measurement data being relatively large compared to the change in intensity caused by a force. In this variant, it is therefore advantageous if the wavelength range of the light to be coupled in is limited by means of a bandpass filter 13 before the light from the light source 5 is coupled into the sensor fiber 7.The bandpass filter should 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 between 1520 and 1535 nm. Figures 4a, 4b, and 5 show embodiments of the measuring device according to the invention for implementing the reflection variant of the method according to the invention. In addition to the actual measurement of the light reflected by the sensor FBG 8 to determine the wavelength change, interference parameters are monitored. This is particularly advantageous because the Bragg wavelength of the sensor FBG 8 can be influenced not only by the force acting on the object to which the sensor FBG 8 is attached, but also by interference factors such as temperature changes or changes in the cable infrastructure (fiber bending, faulty connections, etc.), since these can cause a change in the reflected light intensity.
[0052] In order to determine whether a change in intensity is caused by an influence on the Bragg wavelength of the sensor FBG 8 or by influences on the cable infrastructure, an additional detection element (monitoring detection element 14) is provided in the detection unit 3. With the monitoring detection element
[0053] 14 detects light from the sensor fibers 7 of the measuring channels 4-1, 4-2, ..., 4-n, which has a wavelength outside the working range of the sensor FBG 8. The monitoring detection element 14 can optionally be provided with a bandpass filter
[0054] 15, which allows a wavelength range (here: for example, 1550 nm) outside the operating range of the sensor FBGs 8 to pass through. This ensures that the light intensity detected by the monitoring detection element 14 is not influenced by a load acting on the object, but is meaningful with regard 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 a load acting on the object on the monitoring signal is small. Therefore, it is also possible to dispense with the bandpass filter 15. If the light intensity changes due to a change in the cable infrastructure, this can be detected by an intensity change detected by the monitoring detection element 14.
[0055] In the embodiments shown in Figures 4a and 4b, the monitoring detection element 14 is located at the end of the sensor fibers 7 opposite the light source 5 and serves to detect the light transmitted by the sensor FBGs 8 of the sensor fibers 7 of the various measuring channels 4-l,...,4-n. Monitoring can be performed separately for each measuring channel 4-l,...,4-n. For this purpose, in the embodiment shown in Figure 4a, a separate monitoring detection element 14 is provided for each measuring channel 4-l,...,4-n. Alternatively, the transmitted light from all measuring channels 4-l,...,4-n can be combined by means of a further splitter 16 and directed to a common monitoring detection element 14, as shown in Figure 4b.
[0056] In addition to monitoring the cable infrastructure, temperature monitoring is also provided in the embodiment shown in Figure 4b. For this purpose, a portion of the light from light source 5 is coupled into an additional optical fiber 17 with an additional FBG (temperature monitoring FBG 18). The division of the light from light source 5 into the measuring channels 4-1, ..., 4-n on the one hand and into the additional fiber 17 on the other hand can be achieved with an additional splitter 19 arranged between light source 5 and splitter 6, which serves to divide the light into the various measuring channels 4-1, ..., 4-n. The additional splitter is preferably a 90:10 or 80:20 splitter, so that only a small portion of the light is coupled into the additional fiber 17 and the majority of the light is guided to 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 fiber optic unit 1, which is attached to the object to be monitored, but is housed in the fiber optic junction box. However, it should be located near the object to be monitored so that the temperature monitoring FBG 18 is exposed to the same temperature fluctuations as the sensor FBG 8.
[0057] As an alternative to the embodiments shown in Figures 4a and 4b, in which the monitoring of the cable infrastructure is determined using the light transmitted by the sensor FBG 8, Figure 5 shows an embodiment in which the cable infrastructure monitoring is carried out using a further FBG (monitoring FBG 21) embedded in the sensor fiber 7. For this purpose, the monitoring detection element 14 is located on the end of the sensor fibers 7 facing the light source 5 and serves to detect the light transmitted by the sensor FBGs 8 of the sensor fibers 7 of the various measuring channels 4-1,...,4-n. The monitoring FBG 21 has a Bragg wavelength that lies outside the operating range of the sensor FBG 8. Within the sensor fiber 7, light reflected by the sensor FBG 8 on the one hand and light reflected by the monitoring FBG 21 on the other hand are thus returned.The reflected light is distributed via the coupler 9 to the sensor detection element 10 on the one hand and to the monitoring detection element 14 on the other. To guide the light emitted by the light source 5 into the sensor fiber 8 and one of the light components 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 upstream splitter 6.
[0058] If an intensity change is registered by the monitoring detection elements 14, 20 shown in Figure 4a, Figure 4b, or Figure 5, this change is offset against the measurement signal of the sensor detection element 10 to rule out false axis detection. This prevents short-term changes in the wavelength-dependent intensity distribution and associated intensity fluctuations in the sensor fiber 7 from influencing the actual measurement result, allowing it to be reliably determined whether a force has been applied to the object to be monitored.
[0059] Figure 6 shows schematically how long-term changes in the wavelength-dependent intensity distribution can be corrected: In general, an equivalent value is subtracted from the raw data (input signal lin) detected by the sensor detection element 10, so that the value 0 is output as the output signal lout when no force is applied to the object.
[0060] Preferably, the equivalent value is a moving long-term average value lavl of the light intensity detected by the sensor detection element 10.
[0061] Long-term average values lavl are continuously updated as long as there is no signal influence due to external circumstances, in particular force acting on the object or temperature changes. To ensure that the equivalent value does not contain any signal influence, a short-term average lavs of the light intensity detected by the sensor detection element 10 is preferably also calculated. A shorter period of time is used to calculate the short-term average lavs than to calculate the long-term average lavl. For example, the short-term average lavs can be calculated over a period of approximately 2.5 seconds, and the long-term average lavl over a period of approximately 10 seconds.
[0062] The short-term average lavs is subtracted from the input signal lin. If the absolute value of the result is less than a defined limit value (Ilim), the currently calculated long-term average lavl is used as the equivalent value. If the result exceeds the defined limit value (Ilim), the currently calculated long-term average lavs is discarded. In this case, a previously calculated long-term average is preferably used as the equivalent value, for which the difference between the corresponding short-term average lavs and the input signal lin has not exceeded the limit.
[0063] This method ensures that no measurement events, i.e. measurement signals during a load on the object, are included in the long-term mean value calculation.
[0064] By subtracting the equivalent value from the input signal lin, an output signal lout is generated, which in the unaffected state has the value 0. The algorithms for identifying the force detection on the object (e.g., axis detection in an axis counting system) are then applied to this output signal lout.
[0065] List of reference symbols
[0066] 1 fiber optic sensor unit
[0067] 2 fiber optic connection boxes
[0068] 3 Detection unit (counting board)
[0069] 4-1...4-n measuring channels
[0070] 5 Light source
[0071] 6 splitters for dividing the light from the light source to be coupled into the measuring channels
[0072] 7 sensor fiber
[0073] 8 sensor fiber Bragg gratings
[0074] 9 Coupler for coupling the light from the light source into the sensor fiber and for coupling the reflected light out of the sensor fiber
[0075] 10 Sensor detection element
[0076] 11 rising edge of the wavelength-dependent intensity distribution
[0077] 12 falling edge of the wavelength-dependent intensity distribution
[0078] 13 bandpass filters with filter bandwidth in the working range of the sensor FBG
[0079] 14 Surveillance detection element for monitoring the cable infrastructure
[0080] 15 bandpass filters with filter bandwidth outside the working range of the sensor FBG
[0081] 16 splitters for combining the light transmitted from the sensor fibers of the measuring channels
[0082] 17 additional optical fibers for temperature monitoring
[0083] 18 Temperature monitoring FBG of the additional fiber
[0084] 19 additional splitters to split the light between the temperature monitoring fiber and the sensor fibers
[0085] 20 additional monitoring detection element for monitoring the temperature
[0086] 21 Coupler / Circulator lavs Short-term average lavl Long-term average, DC signal lin Input signal lout Output signal
[0087] Him Limit value for deviation from short-term average to input signal Literature list
[0088] DE 10 2017 119 810 B4
[0089] DE 10 2014 100 653 B4
[0090] EP 3 069 952 Al DE 10 2012 104 874 B4
Claims
Patent claims 1. Measuring method for detecting a mechanical force acting on an object by means of a fiber optic sensor unit (1), wherein at least one measuring channel (4-1, ...4-2) is provided which contains a sensor fiber (7) comprising at least one sensor fiber Bragg grating (8) embedded in the sensor fiber having a Bragg wavelength and a sensor detection element (10), wherein the sensor fiber (7) is attached to the object in the region of the sensor FBG (8), the method comprising: • Coupling of light from a light source (5) into the sensor fiber (7), • Detection of the light reflected and / or transmitted by the sensor-FBG (8) by means of the sensor detection element (10), characterized in that the light source (5) has a wavelength-dependent intensity distribution with a slope (11, 12), that the detection of the light reflected and / or transmitted by the sensor-FBG (8) by means of the sensor detection element (10) takes place over the entire wavelength range of the light reflected and / or transmitted by the sensor-FBG (8), and that a wavelength change of the Bragg wavelength of the sensor-FBG (8) is determined by evaluating a measurement signal which includes a change in the intensity of the light intensity detected by the sensor detection element (10).
2. Measuring method according to claim 1, characterized in that the light source and the sensor-FBG are matched to each other such that the Bragg wavelength of the sensor-FBG lies in a wavelength range in which the The frequency profile of the light source exhibits an edge, preferably in the middle region of the edge.
3. Measuring method according to 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. Measurement method according to one of the preceding claims, characterized in that at least one disturbance parameter is monitored which has an influence on the wavelength-dependent intensity distribution independently of any force acting on the object.
5. Measurement method according to claim 4, characterized in that the change in intensity of the light transmitted by the sensor-FBG is determined for monitoring the disturbance parameter, wherein the light transmitted by the sensor-FBG is directed, preferably via a bandpass filter, to a monitoring detection element (PDT).
6. Measurement method according to claim 4, characterized in that a monitoring FBG is used to monitor the disturbance parameter, and that the change in intensity of the light reflected by the monitoring FBG is determined for monitoring the disturbance parameter.
7. Measurement method according to claim 6, characterized in that the monitoring FBG is embedded in the same measuring fiber as the sensor FBG, and that the monitoring FBG has a Bragg wavelength that differs from the Bragg wavelength of the sensor FBG.
8. Measuring method according to claim 6 or 7 characterized in that the light reflected in the measuring fiber is split into two light components, one of which is passed unfiltered onto the sensor detection element and the other The light is directed via a bandpass filter to a monitoring detection element. A measurement method according to any one of claims 5 to 8, characterized in that the fiber optic sensor unit has several, in particular at least four, preferably eight measurement channels, and that only a single monitoring detection element is used which detects light from all measurement channels. A measurement method according to any one of claims 5 to 8, characterized in that the fiber optic sensor unit has several, in particular at least four, preferably eight measurement channels, and that several monitoring detection elements, preferably a separate monitoring detection element for each measurement channel, are used.A measuring method according to one of the preceding claims, characterized in that a force acting on the object is determined when a wavelength change is detected by the sensor detection element, in particular only when no wavelength change or a wavelength change below a predetermined limit value is detected by the monitoring detection element. A measuring method according to one of the preceding claims for use in determining a mechanical force acting on a rail, in particular for use in a counting point of an axle counting device. A measuring device for carrying out a measuring method according to one of the preceding claims, comprising a light source having a wavelength-dependent intensity distribution with a slope, and a fiber optic sensor unit, wherein the fiber optic sensor unit has at least one measuring channel comprising a sensor fiber with at least one... comprising a sensor fiber Bragg grating (S-FBG) embedded in the sensor fiber with a Bragg wavelength and a sensor detection element, wherein the sensor fiber is configured to be mounted on an object in the region of the sensor-FBG, characterized in that the fiber optic sensor unit is configured to detect a change in the Bragg wavelength of the sensor-FBG by evaluating a change in the intensity of the detected light intensity of the light reflected and / or transmitted by the sensor-FBG over the entire wavelength range of the light reflected and / or transmitted by the sensor-FBG (8).
14. Measuring device according to claim 13, characterized in that the Bragg wavelength of the FBG in the mounted state lies in the region of the flank of the wavelength-dependent intensity distribution of the light source.
15. Measuring device according to one of claims 13 to 14, characterized in that a monitoring FBG is embedded in the sensor fiber, that the monitoring FBG has a Bragg wavelength that differs from the Bragg wavelength of the sensor FBG, and that the monitoring FBG can be arranged outside the area in which the sensor FBG is attached to the object.
16. Axle counting device with a counting point comprising two measuring devices according to one of claims 13 to 15.