Optic Fibre Sensing

Abstract

A fibre sensing apparatus (100) comprises an interrogation unit (104) to interrogate a sensing fibre with optical radiation, and to detect an optical signal returned from the fibre, and processing circuitry (114). A portion of the fibre may be excited with a test signal. The processing circuitry comprises an assessment module (115) to analyse the optical signal returned from the excited portion of fibre, and to determine at least one operational characteristic of the apparatus based on the detected optical signal.

Claims

1. Fibre sensing apparatus comprising: a. a sensing fibre; b. an actuator to excite a portion of the fibre with an acoustic test signal; c. an interrogation unit to interrogate the sensing fibre with optical radiation, and to detect an optical signal returned from the fibre, and d. processing circuitry comprising an assessment module to analyse the optical signal returned from the excited portion of fibre, and to determine at least one operational characteristic of the apparatus based on the detected optical signal.

2. Fibre sensing apparatus according to claim 1 in which the operational characteristic is at least one of: a. an indication that the sensing apparatus is functional, b. an indication that the sensing apparatus is non functional.

3. Fibre sensing apparatus according to claim 1 in which the operational characteristic is at least one of: a. an indication of the sensitivity of the apparatus over at least one operational range, b. an indication of the calibration of the apparatus.

4. Fibre sensing apparatus according to claim 1 in which the assessment module is arranged to compare at least one characteristic derived from the detected optical signal from the excited portion to at least one predetermined characteristic.

5. Fibre sensing apparatus according to claim 4 in which the assessment module is arranged to compare at least one characteristic derived from the detected optical signal from the excited portion to a signature characterising at least one acoustic signal or acoustic signal type.

6. Fibre sensing apparatus according to claim 4 in which the assessment module is arranged to compare at least one characteristic of the detected optical signal from the excited portion to one or more characteristics derived from the actuator test signal.

7. Fibre sensing apparatus according to claim 1 in which the assessment module is arranged to analyse the signal detected and, based on the analysis, optimize interrogation unit performance over one or more operational range.

8. Fibre sensing apparatus according to claim 1 in which the assessment module is arranged to analyse the signal detected and, if a determined characteristic does not meet at least one predetermined standard, the apparatus is arranged to vary at least one operational parameter based on the analysis.

9. Fibre sensing apparatus according to claim 1 in which the assessment module is arranged to produce an output indicative of at least one operational characteristic.

10. Fibre sensing apparatus according to any claim 1 in which the processing circuitry further comprises an alert module, and, if the determined operational characteristic(s) do not meet a predetermined standard, the alert module is arranged to produce an alert.

11. Fibre sensing apparatus according to claim 1 in which the actuator comprises a vibrational acoustic source.

12. Fibre sensing apparatus according to claim 1 in which the actuator comprises a controller arranged to control the acoustic signal generated thereby.

13. Fibre sensing apparatus according to claim 1 in which the interrogation unit is arranged to carry out sensing in relation to other fibre portions.

14. Fibre sensing apparatus according to claim 1 which comprises a Distributed Acoustic Sensor (DAS).

15. A method of assessing at least one operation characteristic of a sensing apparatus comprising: a. interrogating an optical fibre with optical radiation; b. detecting optical radiation returned from a portion of an optical fibre which is excited by an acoustic test signal; and c. analysing the detected optical radiation from the excited portion to determine at least one operational characteristic of the sensing apparatus.

16. A method according to claim 15 in which the test signal has one or more known characteristics and the step of analysing comprises determining if the detected optical signal is indicative of an acoustic signal having at least one known characteristic of the test signal.

17. A method according to claim 15 which comprises determining if the determined characteristic meets at least one predetermined standard.

18. (canceled)

19. A method according to claim 17 comprising, if a determined characteristic does not meet at least one predetermined standard, changing at least one operational parameter of the sensing apparatus.

20. A method according to claim 15 in which the sensing apparatus is for use in monitoring a system and the test signal comprises at least one acoustic signal having at least one attribute of at least one anticipated acoustic signal for the system.

21. A method according to claim 20 in which at least one anticipated acoustic signal is a signal indicative of one or more of (i) a fault (ii) a safety critical scenario in the system.

22. (canceled)

23. A method according to claim 15 in which the step of analysing the detected signal comprises at least one of: a. determining if an acoustic signal has been received; b. comparing at least one characteristic of a detected signal to at least one characteristic based on the test signal; c. comparing a detected signal to one or more characteristic based on a signal type.

24. A method according to claim 15 further comprising carrying out acoustic sensing for portions of the fibre other than the portion of the fibre which is excited by the acoustic test signal by detecting optical radiation returned from such other portions of the fibre.

25. A method according to claim 15 in which the test signal comprises at least one of (i) a characteristic amplitude and/or frequency, (ii) a characteristic varying amplitude and/or frequency, (iii) a series of pulsed vibrations capable of providing a digital signal, (iv) a pseudo random signal, (v) a repeated sequence of acoustic impulses.

26. A method according to claim 13 which further comprises applying a test signal.

27. A method according to claim 26 which comprises varying at least one characteristic of the test signal over time.

28. A fibre sensing apparatus comprising: an interrogation unit to interrogate a sensing fibre with optical radiation, and to detect an optical signal returned from the fibre, and processing circuitry comprising an assessment module to analyse the optical signal returned from a portion of the fibre which is excited with a test signal, and to determine at least one operational characteristic of the apparatus based on the detected optical signal.

Description

[0037] Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

[0038] FIG. 1 shows an overview of a system according to one embodiment of the present invention;

[0039] FIG. 2 shows an example of an actuator arranged to act on a train track;

[0040] FIG. 3 shows and example of a monitored train track; and

[0041] FIG. 4 is a flow chart showing an example of a method according to one embodiment of the present invention.

[0042] Referring to FIG. 1, sensing apparatus 100 comprising an elongate length of standard single mode optical fibre 102 is connected to a distributed acoustic sensing (DAS) interrogation unit 104. The optical fibre 102 may be located along any path which it is desired to monitor, e.g. along a perimeter such as a border and fence line (buried or on the surface) or along linear assets such as pipelines, cable runs, roads or train tracks for example. The path need not be straight.

[0043] The interrogation unit 104 is adapted to launch light into the fibre 102 and detect light returned from the fibre 102 in such a way as to provide distributed sensing along the length of the fibre 102. In the present example, the unit 104 is substantially as described in GB 2442745, and uses Optical Time Domain Reflectometry (OTDR) to provide simultaneous independent sensing capability of approximately 4000 adjacent sensing bins 10 m in length. As described in GB2442745, the phenomenon of Rayleigh backscattering results in some fraction of the light input into the fibre being reflected back to the interrogation unit 104, where it is detected to provide an output optical signal which is representative of acoustic disturbances in the vicinity of the fibre 102. The interrogation unit 104 therefore conveniently comprises at least one laser 108 and at least one optical modulator 110 for producing a plurality of optical pulses separated by a known optical frequency difference. The interrogation unit 104 also comprises at least one photodetector 112 arranged to detect radiation which is Rayleigh backscattered from the intrinsic scattering sites within the fibre 102.

[0044] Other Rayleigh backscatter DAS sensor interrogation schemes are known and could also be used in carrying out embodiments of the invention. In addition, schemes based on Brillouin or Raman scattering are also known and could be used in embodiments of the invention, as could schemes based on heterodyne interferometry.

[0045] The photodetector 112 is arranged to pass a signal indicative of the detected optical signal to processing circuitry 114. The processing circuitry 114 is capable of analysing the signal, as set out below, and comprises an assessment module 115 having an output to an alert module 116. The processing circuitry 114 comprises a memory 120. The memory 120 is arranged to hold signatures of signals to be compared to the detected signals.

[0046] The processing circuitry 114 may be co-located with the interrogation unit 104 or may be remote therefrom, and may comprise a user interface/graphical display, which in practice may be realised by an appropriately specified PC. Any user interface may be co-located with the processing circuitry 114 or may be remote therefrom.

[0047] An actuator 118 is provided towards the far end of the fibre 102 to the interrogation unit 104 (although it will be appreciated that, in practice, the fibre 102 may double back and the far end of the fibre 102 may be physically close to the interrogation unit 104). The actuator 118 comprises a movable member, capable of acting in the vicinity of the fibre 102 to acoustically excite a portion thereof. While the actuator 118 could be positioned elsewhere on a fibre 102, any portion of fibre 102 optically beyond the actuator 118 will not be tested by operation of the actuator 118, and therefore it may be preferred to place the actuator towards the end of the fibre 102.

[0048] FIG. 2 shows an example of a fibre 102 arranged in situ along a train track 200. In this example, the actuator 118 comprises a metal hammer 202 mounted in an electromagnetic controller 204 such that it can be controlled to strike the track 200 in a manner controlled by a processor 206. In this example, the actuator 118 is capable of producing vibrations in the region of 100 Hz to 1 kHz.

[0049] In other examples, the actuator may comprise an alternative electromagnetic actuator, a piezoelectric element, a motor element (for example a micro DC motor) or the like. In some examples, an actuator may be arranged as a fibre stretcher, for example a piezoelectric or PZT fibre stretcher. In further alternative examples, the actuator may be a ground vibration source and may be mounted on, or at least partially implanted in the ground. Implanting an actuator can provide good acoustic coupling.

[0050] In general, therefore, the actuator may comprise hammer, thumper or other arrangement arranged to be movable to create an impact to impart vibrations into the fibre 102, directly or via an intermediate element such as a plate, train track or the like. Various other arrangements of acoustic sources may be used however and anything that creates a distinctive signal that can be detected by the DAS sensor could be used, including an acoustic transducer. The actuator may be controlled according to instructions provided by the processor 206, which may in turn hold, generate or receive instructions specifying the signal to be generated.

[0051] The actuator 118 is arranged to induce an acousto-mechanical signal in the fibre 102.

[0052] The fibre 102 can additionally be used to sense disturbances other than those produced by the actuator 118. To continue the example of FIG. 2, this may comprise a train on a portion of the train track 200, which may be spaced from the actuator 118. In such an example, the actuator 118 and the train would produce signals in different channels of the fibre 102.

[0053] The direction, speed, length and integrity (i.e. whether all cars are securely and correctly coupled together) and location of a moving train on the track 200 is detectable via the acoustic signal it induces in the fibre. The distance between vehicles (known as headway) can also be determined, as can the time and distance to fixed points (for example, a safety critical incident location). Indeed, it has been found that a particular vehicle can be identified through its acoustic signature, and this in turn can be monitored to detect changes such as deterioration. Characteristic acoustic signatures may also be associated with signal types, i.e. there may be a characteristic of a signal which is indicative of faults such as wheel flats (misshapen portions of train wheels), hot axle boxes, or operation of trackside machinery such as points and barrier machines, along with generators, pumps and other machinery. Indeed, faults in such machinery may also have associated signatures, or departure from a particular signal pattern may itself be indicative of a fault.

[0054] In the context of track monitoring (although of course there could be analogous functions in other contexts), an interrogation unit 104 could be provided, for example, about every 50 km, perhaps capable of monitoring two fibres extending up to 25 km in either direction. A single actuator 118 may excite a portion (for example the end portions) of fibres 102 connected to different interrogation units 104. Alternatively or in addition to monitoring the position of vehicles on the track, apparatus could be provided to (i) detect unauthorised movement and/or activity trackside (this could address issues such as copper theft, vandalism and/or potential terrorist activity), (ii) safeguard trackside personnel (e.g. monitor location of authorised individuals such as work parties), (iii) safeguard public safety (e.g. monitoring unmanned level crossings, platforms, etc.) and/or (iv) monitor infrastructure (for example, detecting and generating alerts for rock fall, land slip, bridge and tunnel collapse/strike).

[0055] Many of these functions are safety critical and therefore it is desirable to know that sensing apparatus is functioning, and/or that it is functioning to a desired standard. In particular for safety critical applications, it may be desirable to detect apparatus failure rapidly.

[0056] For example, if a monitoring failure is detected, a system may enter a failsafe mode, which is certified as safe absent the failed monitoring apparatus. To consider one example, in railway signalling, a moving block signalling system identifies blocks of safe track space around each train, allowing trains to be run closer together than it achievable using other systems. To operate as a moving block system, a railway operator needs a high degree of confidence that its train speed and train separation detectors are working properly. In the event of any failure, the system may revert to a fixed block system, which may result in trains slowing down, or even stopping, while the spacing between trains is resolved (trains are generally further apart in a fixed block mode, as only one train is permitted in each predetermined block of track at any one time).

[0057] An actuator 118 may be readily used to validate the operational status of the length of fibre 102 between the actuator and the interrogation unit 104. An actuator 118 may be readily retrofitted to an existing fibre sensor apparatus.

[0058] In some examples, the apparatus 100 may be arranged such that the actuator 118 operates substantially continuously (or substantially continuously while the apparatus 100 is used to perform monitoring functions). Such an arrangement would continuously test the integrity of the fibre 102 and, in some examples, the performance of the interrogation unit 104, and could therefore quickly generate an alert in the event that the processing circuitry 114 is unable to positively confirm proper operation of the monitoring function. In some practical examples, the result of such an alert may be that the monitored system operates in a failsafe mode.

[0059] The signal produced by the actuator 118 may be arranged for ease of detection. Providing such a signal may minimise the occurrence of false alarms. Such a signal is preferably readily distinguishable from other sources of acoustic noise which may occur at the same channel of the apparatus 100 (for example having a different frequency signature or range to that of anticipated background or other signals), and/or may have at least a threshold strength. In some examples, the actuator signal may be able to apply different signals over time to test different virtual sensors, i.e. different sensor functions. For example, it may be desirable to confirm the ability of the apparatus 100 to detect train speed and time, operation of trackside machinery and apparatus, specific configurations around level crossings or points, train length, etc., or any other sensor function of the apparatus. An anticipated signal may therefore be mimicked (or several signals mimicked in turn), or different actuators operated over a length of fibre to test such functions.

[0060] As such, the processor 206 may be arranged to control the actuator to produce an acoustic signal with has a characteristic amplitude and/or frequency, a characteristic varying amplitude and/or frequency, or may comprise a series of pulsed vibrations capable of providing a digital signal. In some examples, the signal may vary in a random or pseudo random manner. The test signal may be a repeating test signal. In some cases, providing a repeating test signal may assist with detectability and identification of the signal. The characteristics of the test signal may vary over time to test different monitoring functions.

[0061] A particular example is now discussed with reference to FIG. 3 and the flow chart of FIG. 4. FIG. 3 shows an intensity signal produced in various channels of a fibre 102 arranged along a train track 200. The fibre 102 is excited in the region of the actuator 118, and also in the region of a train 300.

[0062] As set out in FIG. 4, in a first step, the actuator 118 is controlled to emit an integrity test signal (block 402). The test signal comprises a continuous vibration with a predetermined repeated pattern being applied to the track 200 and is arranged to provide an indication of the operational status of the apparatus 100. To that end, in block 404, the interrogation unit verifies that an optical signal indicative of an acoustic signal is received from the portion of the fibre 102 which is near the actuator 118.

[0063] If a signal is received from the excited portion, an operator may have a high degree of confidence that the fibre sensor between the interrogation unit 104 and the actuator 118 is operational. In block 406, the signal detected is compared to an anticipated signal. If the signal is recognised (i.e. the signal detected by the interrogation unit 104 corresponds to the integrity test signal, having the predetermined repeated pattern), this allows an operator to have a high degree of confidence that the interrogation unit 104 is functioning correctly. The step of recognition may also comprise an estimation of system noise, spectrum, latency, or any other indication of the system's operational characteristics.

[0064] If however, either a signal is not received from the portion of fibre 102 near the actuator 118, or the signal is not recognised, an alert is generated (block 408). This alert may result in the train system being operated in a failsafe mode, e.g. reducing or stopping the movement of vehicles, and the like. This is because it can no longer be assumed that the apparatus 100 is operating as intended. The lack of a signal may be due to interrogation unit malfunction, sub-optimal operating parameters being used in interrogation unit 104, a break in the fibre 102, excessive system noise, a malfunctioning actuator 118 or for some other reason. However, in safety critical functions, a failsafe state may be assumed in the absence of an assurance of effective monitoring system. Such an alert may be triggered immediately, or following a period of time of failed detection, which could range from less than a second to minutes depending on the safety criticalness of operation.

[0065] In this example, the integrity test signal is applied until a train 300 is detected by another portion of the fibre 102.

[0066] When a train is detected by the interrogation unit 104 (block 410), the actuator 118 is controlled to change the test signal to one matching the signature for wheel flats (block 412), i.e. a localised flattened region of a train wheel which may indicate that maintenance or replacement of a wheel should be carried out or scheduled.

[0067] The interrogation unit 104 then attempts to detect the wheel flat in the optical signal (block 414). In this case, the interrogation unit 104 may have a number of signatures of different signal types which relate to possible events, including fault events and safety critical events. These could include the presence of wheel flats and others such as signal box switching, trackside personal, rock falls, operational machinery, etc.

[0068] Whilst in the case of real signals, the detection of any such signal could generate an alert, in the case of a test signal, it is the absence of signal recognition which is of concern. In this case, the optical signal from the exited portion of fibre 102 is compared to the stored signature and, if the signal is not recognised correctly, the interrogation unit 104 is recalibrated (block 416). This may comprise recalibrating any operational parameter. For example, the pulse width, pulse separation, pulse timing, detector sensitivity, detector gating signal, channel length (i.e. by changing the returned signal tin size in the processing of the signal), or the like could be varied in isolation or in combination. In a particular example, the received signal may demonstrate a characteristic of signal wrapping for signals mimicking wheel flats. This could result in the bin size being changed to reduce the sensitivity of the apparatus, reducing wrap, and increasing the ability of the apparatus 100 to detect wheel flats.

[0069] In this example, recalibration is attempted up to 10 times, although of course this number is simply by way of example. In other examples, recalibration may be carried out for a predetermined time period. If the interrogation unit 104 successfully indicates a wheel flat for the location of the actuator 118, this indicates that it is correctly calibrated to detect such an event. Therefore, if, in block 418, it is determined that no wheel flat signal is received from the location of the train 300, the operator may have a high degree of confidence that the train does not have a wheel flat, and the integrity test signal may be resumed. If however, the test signal is not recognised despite recalibration attempts, or if a wheel flat is detected in the signal produced by the train, a manual inspection of the train may be scheduled (block 420).

[0070] It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention.

[0071] Although in some examples, a test signal may be applied continuously, in other examples, the integrity test signal may be applied periodically (for example, with a frequency related to the anticipated events and/or the level of assurance required given a particular set of facts).

[0072] An integrity test signal may be arranged to vary to test some or all intended monitoring functions. For example, a test signal designed specifically to trigger each of a plurality of safety critical alerts may be generated, and the failure to recognise any of these signals may trigger an alert state.

[0073] The signal may vary randomly for at least a portion of time. Such a signal may still have predetermined desired parameters, allowing it to be recognised. In some examples, the signal may vary pseudorandomly, according to a sequence which is known by the interrogation unit 104.

[0074] Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.