GB2462603A

GB2462603A - Light source variation compensating interferometric fibre optic sensor in a wind turbine component

Info

Publication number: GB2462603A
Application number: GB0814651A
Authority: GB
Inventors: Ib Olesen
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2008-08-11
Filing date: 2008-08-11
Publication date: 2010-02-17
Also published as: GB0814651D0

Abstract

An interferometric optical fibre sensor system for determining an operating parameter of a wind turbine component, such as the stress, temperature or pressure to which it is exposed, in which compensation is made for variations in the wavelength of light output by the light source. A sensing optical fibre 22b is attached to the component such that variations in the operating parameter cause the optical path length of the fibre to change. A reference optical fibre 32b is mounted in or on the wind turbine component in such a way that variations in the operating parameter do not affect the optical path length. Light is input into the optical fibres by the light source 10 and controller 12; the light is received from the optical fibres by the detectors 11, 16. Based on the detected light received from the reference optical fibre the controller 12 can determine whether the wavelength of the light source has drifted away from the desired value and correct for any drift in the subsequent measurement.

Description

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LIGHT SOURCE WAVLENGTH VARIATION COMPENSATING

INTERFEROMETRIC FIBRE OPTIC SENSOR SYSTEM AND SENSING

METHOD IN A WIND TURBINE COMPONENT

Background of the invention

The invention relates to a light source wavelength variation compensating interferometric fibre optic sensor system and method, and in particular to such a system for detecting deformation in a wind turbine component.

It is known to use fibre optic sensors to measure the operating properties of wind turbine components. Usually such sensors measure the strain or deformation of a wind turbine component. They may also measure temperature, or pressure for example.

The paper entitled "Fatigue strength of glass reinforced polyester (GRP) laminates with embedded optical fibres" by Alfredo Guemes and Jose M Menendez, published at the Third ICIM/ECSSM 96 at Lyon, ISBN 0-8194- 2165-0/96 discloses a technique for determining the deformation of a wind turbine component, based on interferometry. Incident light is input to two optical fibres and subsequently recombined to give an interference pattern based on a Michelson Interferometer arrangement. As one of the optical fibres is subject to strain, the resulting interference pattern depends on the strain.

The applicant's pending UK application GBO81 2037.0, discloses an alternative sensor and sensing technique for measuring the strain on a wind turbine component using interferometry. The sensor comprises a detector, a variable light source and a sensing optical fibre. The sensing optical fibre is attached to a wind turbine component in order to sense strain on the component. The fibre is mounted such that deformation of the wind turbine component causes a change in the optical length of the fibre, and detection of the optical length therefore provides a measure of the strain.

The optical length of the sensing optical fibre is measured as follows. Light from the tuneable light source is output and split into two optical paths before being recombined into an interference pattern and detected. In one optical path, the light travels along the sensing optical fibre, and in the second optical path it does not.

The wavelength of the light is varied across a known range, causing variations in the interference pattern at the detector. The variations arise from the number of waves accommodated in the sensing optical fibre changing. The number of variations observed from the known variation in output wavelength gives a value for the optical length of the sensing optical fibre.

In sensors that rely on interferometry techniques, it is therefore extremely important that the wavelength of the light output by the light source is determined accurately and precisely. The wavelength of the light is a central factor in the determination of the optical length and the strain, and errors will directly affect the sensor accuracy. This is not the case for sensors that rely on Fibre Bragg Gratings, for example, where it is the wavelength of light reflected by a grating in the fibre that is used to give a the measurement result, not the output wavelength of the light source.

All tuneable light sources are susceptible to wavelength drift, that is small undesired and uncontrol'able variations in their output wavelength that result from environmental changes, typically temperature, and current or voltage fluctuations in the power supply. Accurate tuneable lasers are often required if the sensor is to work accurately. However, such lasers can be expensive, and no matter how accurately and precisely the tuneable laser can be controlled, there may still be some drift.

We have appreciated that there is a need for an interferometry based fibre optic sensor in which variations in the wavelength of the light source are controlled, and which is more cost effective to implement than those employing existing highly precise tuneable lasers.

Summary of the Invention

According to the invention in a first aspect, there is a provided an interferometric fibre optic sensor for measuring an operating parameter of a wind turbine component, the sensor comprising: a light source for outputting light of a predetermined wavelength; a sensor optical fibre providing a sensing optical path, the sensor optical fibre operatively attached to the component such that variations in the operating parameter cause the optical length of the sensing optical path to change; a reference optical fibre providing a reference optical path for detecting changes in the wavelength of the light output by the light source, the second optical fibre operatively attached to the component such that the reference optical path is isolated from variations in the operating parameter; one or more light detectors for receiving light that has passed along the sensing and reference optical paths respectively, and for providing an output to the controller indicating the intensity of the received light; a controller coupled to the light detector for determining, based on the detected light from the reference path, any drift in the wavelength of the output light and a correction factor for the measurement of the operating parameter.

By compensating for variations in the wavelength of light output by the light source, the accuracy of the sensor is greatly improved. Additionally, determination of the compensation factor allows light sources that do not output light with great precision to be used in a working, accurate sensor.

Such light sources are generally less expensive, allowing the cost of the sensor to be reduced. The operating parameter may be strain, temperature of pressure for example that is experienced by the wind turbine component. In particular, in windmills, the temperature variations on wind turbine components can be significant.

In a preferred example, the sensor optical fibre provides a first, sensing optical path, and a second optical path, and the reference optical fibre provides a first, reference path and a second optical path, and both the sensor and reference optical comprise an optical adder for combining the light from each path before the one or more light detectors. Thus, the detector can calculate the measurement of the operating parameter and the compensation factor based on the interference pattern produced by the two paths.

The controller may comprise: a light source controller, coupled to the light source, for varying the wavelength of light input to the optical fibre for measurement; a memory for storing the output received from the light detector; an analyser for detecting a cyclical value of the intensity of the received light, as the wavelength is varied: and a counting unit, coupled to the intensity detection circuit, for counting the number of times the cyclical value of intensity is detected.

This sensor allows for measurement of relatively large fibre differences (say 10mm or more) with a variation in wavelength of just a few nanometres. The sensor is also sensitive, as a long fibre (say 0.5m or more) gives a large number of interference cycles for each nanometre change. The same controller can be used to receive the light signals from both the reference and sensor optical fibres for ease of implementation.

In a preferred example, the controller is operable to determine apparent changes in the length of the reference optical path, and associate any changes in the length of the reference optical path with variations in the wavelength of light.

The number of waves of a certain wavelength that fit into the reference optical path length is known in advance, either because the actual length of the reference fibre is known from installation or because of an earlier measurement of its length. Thus after the wavelength is varied across the predetermined range it is possible to know in advance how many wavelengths should be accommodate in the reference optical fibre length. Variations from the expected number will appear as an apparent change in length of the reference fibre. However, as the reference optical fibre is operatively attached to the wind turbine component so that it is not affected by changes in the operating parameter, any changes in length can be associated with changes in the means of measurement, namely the wavelength.

Preferably, the cyclical value is a maximum and/or minimum in the intensity of the received light as this simplifies detection.

In a preferred example, the sensor comprises an optical splitter coupling an output of the light source to the sensor and reference optical fibres. In this way, a less expensive single output light source may be used, rather than a single source multiple output. The optical properties of the light will be identical in the reference and sensor paths.

In one example, the sensor comprises first and second light detectors, coupled to the controller, and to respective ends of the sensor and reference optical fibres. This has been found to simplify detection.

In one example, the controller is operable to determine, based on the light received from the sensor and reference optical fibres, the length of the sensing and reference optical paths. In one example, the lengths of the reference optical fibre and the sensor optical fibre are substantially the same, as this has been found to improve the accuracy of the sensor.

A number of different implementations of the sensor have been found to give good results in practice. The different implementations involve the same principles of operation but are arranged slightly differently in terms of optical elements. Depending on which elements are used and their configuration, different implementations may be appropriate for different applications.

In one example, the sensor comprises a light splitting device coupled to the sensor (or reference) optical fibre for splitting the light into the sensing (or reference) and second optical paths; a light adding device arranged to receive the light from the sensing (or reference) optical path and directly from the light splitting device.

In a further example, wherein in the sensing (or reference) optical fibre the second optical path comprises: a non-measurement portion of optical fibre, separate to the sensing (or reference) optical path; and a mirror terminating the non-measurement portion of optical fibre; and the sensing (or reference) optical path comprises: a sensing (or reference) portion of optical fibre longer than the non-measurement portion; and a mirror terminating the sensing (or reference) optical measurement portion of the optical fibre; and wherein the sensor comprises: an optical coupler for splitting the light received from the light source between the non-measurement and sensing (or reference) portions of the fibre, and for receiving and combining the light reflected from both the sensing (or reference) and non-measurement portions of the fibre.

In a further example, the sensing (or reference) optical fibre comprises: a sensing (or reference) portion of optical fibre for providing part of the sensing (or reference) optical path; a non-measurement portion optical fibre, a partial mirror for separating the sensing (or reference) and non-measurement portions of the fibre and for splitting the light into the sensing (or reference) and second optical paths; and a mirror terminating the sensing (or reference) portion of the optical fibre; wherein the partial mirror allows the light reflected from the sensing (or reference) portion of the optic fibre to pass through to the light detector.

The compensation technique may be used with further arrangements of sensor that compensate for environmental parameters such as temperature, pressure or strain, while one of these parameters is being measured..

In one example, therefore the non-measurement portion of the fibre is isolated from variations in the operating parameter of the component to provide for compensation of a second parameter, the lengths of the measurement and non-measurement portions of the optical fibre and the measurement portion of the fibre being long in comparison to other optical distances in the sensor In a further example of a temperature compensated interferometric fibre optic strain sensor and the sensor optical fibre includes: a first sensing optical fibre providing a deformation measurement optical path, the first optical fibre operatively attached to the component such that deformation of the component acts on the first optical fibre and causes the length of the deformation measurement optical path to change; a second optical fibre providing a non-deformation measurement optical path, the second optical fibre operatively attached to the component such that it is isolated from deformation of the component, and wherein the controller is operable to determine based on the light received from the sensing optical fibre, the difference in lengths of the deformation measurement and non-deformation measurement optical paths, A corresponding method is provided.

Brief Description of the Drawings

Preferred embodiments of the invention will now be described, by way of example, and with reference to the drawings in which: Fig. 1 illustrates a known wind turbine; Fig 2. illustrates a first example implementation of the invention; Fig 3. illustrates an example implementation of a controller shown in Fig 2.

Fig 4. illustrates a second example implementation; Fig 5. illustrates a third example implementation; Fig. 6 illustrates a fourth example implementation, and a preferred embodiment of a temperature compensated sensor.

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Detailed Description of Preferred Embodiments

Figure 1 illustrates a wind turbine 1, comprising a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. A wind turbine rotor 4 comprising at least one wind turbine blade 5 is mounted on a hub 6. The hub 6 is connected to the nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in Figure 1 may be a small model intended from domestic or light utility usage, or may be a large model used, such as those that are suitable for use in large scale electricity generation on a wind farm for example. In the latter case, the diameter of the rotor could be as large as 100 metres or more.

Figure 2 illustrates a first embodiment of the sensor. The sensor comprises a light emitting device 10, such as an LED, laser, halogen or metal halide source, an optical splitter 15, and first and second optical fibres 22 and 32 coupled to the optical splitter 15 to receive light from the light emitting device 10. First and second light collecting measuring devices 11 and 16, such as photo-sensors, are connected to the other end of the first and second optical fibres respectively to receive light transmitted along the fibre. A controller 12 is connected to both light emitting device 10 and light measuring devices 11 and 16, by connections 13, 14 and 17, such as wires or cables. Components 10 to 17 may be housed in a mounting box 18 for easy attachment to the inside or outside of a wind turbine component.

As will be appreciated from the following discussion, first optical fibre 22 is a sensing fibre for detecting and measuring a characteristic or operating parameter of the wind turbine component, such as strain, temperature or pressure, while second optical fibre 32 is a reference or compensating fibre for detecting undesired variations or drift in the output wavelength of the optical source.

The further effect of environmental changes on sensor accuracy can be dealt with by positioning the reference fibre optic close to the sensing optical fibre in S. use on the wind turbine component. In this way, any temperature or pressure changes will likely be experienced by both sensors, The operation of the sensing optical fibre will now be described in more detail with reference to a strain sensor. In Figure 2, the sensing optical fibre 22 is mounted on or in a wind turbine component (not shown in Figure 2) to measure the strain in the component. In one example, this may be achieved by mounts 40 attached to the outside or inside surface of the component.

Other mounting methods would be acceptable as would be known to the skilled person. If the sensor were to be installed in a wind turbine to measure the strain in the wind turbine blades, it is likely that the mounting box 18 would be situated in the hub 6, and the optic fibre 22 would extend internally within the blade from the hub to the relevant region of the blade to be assessed. In this way, the aerodynamic properties of the blades are not affected by the is presence of the sensor. In other locations, the optic fibre sensor may be mounted on the outside of the component.

In Figure 2, it will be appreciated that the spacing of mounts 40 from one another is determined by the dimensions of the turbine component or region of the turbine component that is to be monitored by the sensor. As the component experiences strain or deformation for example mounts 40 will move slightly, stretching the optical fibre 22 and increasing the length of the optical path. The optical fibre may therefore be wound around mounts 40 more than once, so that the stretch operates along a greater length of fibre.

This causes a greater increase in optical path length and greater sensitivity of the resulting sensor.

The sensor also comprises light splitting device 20 and light adding device 21 located in the path of the optical fibre 22. The optical fibre 22 is connected to input and output portions of the light splitting and adding devices, and is accordingly comprised of separate optical fibre portions 22a, 22b, 22c, and 22d. Although, these portions are separate optical fibres, it is helpful to think of them as a forming a single fibre 22 element 22 for the purpose of the present discussion.

Optical fibre 22b extends around the mounts 40 and is the portion of the fibre subject to strain in the component. It is therefore much longer than portions 22a, 22c and 22d that connect the splitter and the adder to the light emitting device 10 and to the light measuring device 11. It is assumed that the lengths of the fibres 22a, 22c, 22d and 22e result in an essentially negligible delay in the light received from optical source 10.

It will be appreciated that the splitter 20 and adder 21, as well as optical fibres 22a, 22c and 22d could be housed in the mounting box 18 for ease of installation.

The strain sensor operates by detecting the length of the optical fibre 22b.

First, light emitting device 10 inputs light having a predetermined wavelength into the optical fibre 22a via splitter 15.

First portion 22a connects the light emitting device 10 to the input terminal of the light splitting device or optical splitter 20. The optical splitter 20 divides the light received at its input terminal into two equal output signals. Second optical fibre portion 22b is connected to one output terminal of the splitter 20 and therefore receives a light signal 45, having a first phase, from the light emitting device 10 and fibre portion 22a. The second optical fibre portion 22b extends around the mounts 40 and is the portion of the fibre subject to strain in the component. Its other end connects to the input terminal of light adding device 21. It will be appreciated that optical splitter and adder could be provided in a single optical coupler, and are shown separately here for clarity.

The other input terminal of light adding device 21 is connected to optical fibre 22c, which in turn is connected to the other output terminal of splitter 20. At one terminal, the optical adding device therefore receives the light signal 45, and at the other it receives light signal 46, having a second phase. The second phase is different to the first phase, as the light signal 46 has travelled along the longer optical fibre portion 22b. In light adding device 21, the two light signals 45 and 46 are added together, and the resulting light signal 47 is sent to light measuring device 11, via optical fibre 22d connect to the output terminal of adder 21.

The light measuring device detects the intensity of the light signal 47 received at its input terminal. The intensity of the received light 47 will depend on the relative phases of the two light signals 45 and 47, and whether their relative phases result in constructive or destructive interference.

When the optical fibre 22b is in an unstrained state, the intensity of the light received at the input to light detector ills determined. This intensity may be considered a zero or a rest value for the purposes of calibrating the sensor.

Further, as the relative phase of the two light signals 45 and 46 is a function of the wavelength of light and the optical path length along fibre 22b, and to a lesser extent fibres 22c and 22d, the resting intensity value of the sensor may be tuned to a desired value by adjusting the wavelength of the input light signal 45.

The controller 12 controls the light emifting device 10, to slowly vary the wavelength of the input light signal 45 over a desired range for measurement.

It is preferred if the total variation in wavelength during the sensing process is small, say 1000 parts per million, or 0.1%. The variation may beto increase or decrease the wavelength. As the wavelength is varied, the controller monitors the intensity at the light detector 11, and detects the cyclical variation in intensity caused by the change in phase. The controller counts the number of times the relative phase of the two signals 45 and 46 changes by 360 degrees, a complete cycle. The controller 12 may count each cycle in phase by determining how many times the maximum (or minimum) intensity is reached.

After the controller 12 has completed varying the wavelength, the total number of phase cycles detected is used to determine the length of the optical fibre.

For example, if varying the wavelength of the input light 45 by 0.1% results in detection of 500 a complete phase cycles, then the length of the fibre can be determined, to a reasonable degree of accuracy, as: length = number of phase cycles x (variation)1 x wavelength = 500 x (0.00 1)1 x wavelength Thus, for red light in the fibre with a wavelength of 700nm, the length of the fibre can be determined as: =500000x7x i0' =0.35m.

This equation can be understood by considering that before the change in wavelength occurs, at any given instant in time, the number of light waves in the optical fibre 22b is equal to the length of the fibre divided by the wavelength. If the wavelength of the input light is increased by 0.1%, then the number of waves that can be accommodated in the optical fibre 22b will decrease slightly. That is to say some of the original waves will be pushed' out of the fibre as the wavelength changes. For example, if the length of the cable was I m and the wavelength of the original light signal was 1 pm, then I x 106 waves would have been accommodated originally in the fibre. An increase in the wavelength of the light of 0.1% would mean the number of waves accommodated after the increase is 1 x 106 divided by 1.001, or 999,001.

The number of cycles detected at detector 11 is a measure of the number of waves that can no longer be accommodated in the fibre 22b because of the change in wavelength.

It is known from the initial configuration of the sensor that the 500 waves correspond to 0.1% of the total number of waves in the fibre. Thus, to a good approximation, the total number of waves in the fibre before the wavelength increased was 500 x 0.11 or 500000. The length of the fibre is then calculated by multiplying the number of waves by the wavelength.

It may be advantageous to ensure that the polarisation of the light signals is substantially the same when they are added, to ensure that they are not

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orthogonal in polarisation and will therefore interfere strongly. Various techniques are known for achieving this in the art.

As will be appreciated from the above discussion, an accurate knowledge of the wavelength of the light used in the light source is critical, as variation or drift in the wavelength of just 0.1 % will significantly affect the output value.

For this reason, the sensor contains a reference or compensation optical fibre 32 which also receives the light from the light source, but which feeds the light back to second light detector 16. The second optical fibre is arranged between light emitter and detector in substantially the same way as the first sensing optical fibre 22, and therefore has portions 32a, 32b, 32c, and 32d, an optical splitter 30 and an optical coupler 31, as described above. However, the reference fibre measurement portion 32b is mounted on the wind turbine component in such a way as to be isolated from variations in the operating parameter to be determined, in this case strain on the wind turbine component.

For this reason, the reference fibre 32 is not shown as having mounting blocks 40. The different light components in the reference loop are given reference numbers 55, 56 and 57 to correspond to those of the sensing loop.

The desired variation in the input light is limited by the accuracy of the light emitting device. For very small changes such as 0.1% it can, in practice, be difficult to ensure that the input wavelength is exactly at the desired value, whereas small inaccuracies in the final wavelength variation can greatly affect the accuracy of the final calculation due to the inaccuracy being multiplied in the expression given. In the earlier application GB08120370, assuming a working range for the wavelength of the input light of between 200nm I 500nm, an advantageous range of variation in wavelength for measurement was from 0.5% to 5%. With the present invention, it is possib'e to vary the wavelength of the light for measurement over a range of just 0.1%. Light sources of higher frequency and lower wavelength are preferred for applications where a sensor with high resolution is required, as the wavelength of the light limits the resolution of the length, based on number of cycles.

A number of methods of mounting the reference fibre 32 so that it is isolated from variation in strain are known, such as those described in GB 2,440,954, which discusses housing the fibre in a separate tube which is subsequently attached to the component. Alternatively, the optical fibre may be sealed in a moulding attached to the wind turbine component via a smaller attachment portion, as disclosed in the applicant's pending application 0812258.2.

The controller 12 receives signals from detector 16 and operates as described above to calculate the length of reference fibre measurement portion 32b. As the length of the measurement fibre is not subject to strain, any changes in the detected length of the reference fibre portion 32b are a result of environmental changes such as temperature or pressure, or are a result of variations in the wavelength of the light emitter. Later embodiments discuss ways of compensating for temperature and pressure variations, so for the present discussion, any detected change in length in the reference fibre 32b is assumed to be due solely to drift in the source. The controller can use the measurement of this drift to adjust the measurement of length made of the sensing optical fibre measurement portion 22b. In this example, the length of the measurement portion 32b of the reference optical fibre is substantially the same length as the unstrained length of the sensing optical fibre portion 22b.

The light variation compensated operation of the sensor will now be described in more detail. Assuming that the apparent length of the measurement portion 32b at a time zero is known to be 500000, as above for example (the exact length is not required as only the apparent change in length due to drift will affect the measurement), the controller activates the light source to sweep through the desired range of wavelengths. As it does so detectors 11 and 16 count the number of detected waves and determine the lengths of the measurement portions 22b and 32b.

If the apparent length of the reference measurement portion 32b at the end of the measurement period is 500600, for example, a difference of 600 or 0.12%, then it can be deduced that the wavelength of the source increased by a drift of 0.12% over the measurement period. The controller therefore adjusts the measured length of the sensing reference portion by a factor of 500000 I 500600, or 0.9988. Thus, in the above example, the length of the sensing optical fibre portion of 0.35m is multiplied to give a value of 0.3496m. The measured length of the reference measurement portion 32b is stored for the next measurement interval.

Figure 3 illustrates the controller in more detail. The controller comprises, a light source controller 30, coupled to the tuneable light source. A control signal from the light source controller is used to vary the wavelength of light input to the optical fibre, and to control the length of time over which the variation occurs.

The controller also comprises a memory 31 for storing the intensity output received from the light detector as the wavelength of the light is varied. An analyser 32, such as a processor, and coupled to the memory is provided to analyse the intensity variation stored in memory and determine how many full phase cycles are present. The analyser 32 does this by detecting a cyclical value in the intensity of the received light, as the wavelength is varied.

Preferably, the cyclical value is a maximum or minimum, allowing standard graphical processing techniques to be used. Alternatively, any predetermined value may be used providing that it is taken into account any value other than the maximum or minimum will occur twice each cycle.

A calculation unit 33 is coupled to the analyser to count and store the number of cycles detected for both fibres, and to calculate the resulting length of the fibre using the compensation factor. It will be appreciated that knowing the starting intensity of the light and the final intensity allows the calculation unit to use a fraction number of cycles in the calculation, not just an integer number.

The calculated values may be stored with previous values in the memory 31.

It is also possible that a fraction part of the intensity cycle be determined by the analyser, by having the processor apply a mathematical function such as a Fast Fourier Transform. This would give measurements with an increased resolution.

The controller comprises an input/output line 34 for receiving and transmitting instructions or data to and from a remote site, such as a monitoring station.

The input/output line may be wired or wireless.

The controller also comprises a timer 35 for defining respective measurement periods Although the controller has been described in terms of separate hardware components, this is solely to illustrate the functionality of the controller in a clear manner. It would be possible in practice to provide the hardware components as software or hardware, or as any combination of single or combined components.

In some applications it is required for the controller to calculate determine the length of the sensing optical fibre around 40 times a second. Other control systems in the wind turbine 1 can use this data to make real time adjustments to operating parameters, such as the direction of the hub, the yaw angle of the blades and so on. The input /output line 34 could be used for example to carry control signals from the controller 12 to a blade control system.

In applications where determining the fibre length is to be carried out many times a second, it is important to select the operational parameters to avoid limiting performance. For example, assuming a constant speed for varying the wavelength of the input light signal 45, larger variations will take more time to carry out, and will result in a larger number of detected phase cycles.

The first example above has been described in some detail in order to outline the overall sensing principle. However, the first example has a number of drawbacks, not least that it requires a two separate cables 22b and 22c to carry the light signals 45 and 46. This causes some difficulty of installation, and as the two cables will typically be located at different parts of the component may expose the cables to different environment effects leading to inconsistencies and errors in the calculations. The second and third example implementations therefore differ by having the two light signals 45 and 46 travel in the same optical fibre, after optical splitter 20.

Figure 4 illustrates a second example implementation. The elements of the drawing having the same function as those in Figure 2 have been given the same reference numbers. Again a first optical fibre 22 is used to make a strain measurement and a second optical fibre 32 is used to provide a compensation factor for variations in the wavelength of the light output from the light source.

In Figure 4, instead of light splitting device 20, the second example comprises first and second optical couplers 23 and 24. The first optical coupler is connected to optical fibre 22a to receive the input light 45, and has optical fibre 22c connected to its two way input/output terminal. Optical fibre 22d is connected between a further output termina' of the coupler and to the tight measuring device 11. Similarly, the other end of optical fibre 22c is connected to a two way input/output terminal of the second optical coupler 24. The second optical coupler splits the light received at from optic fibre 22c into two equal portions. Optical fibres 22b and 22e are connected to further two way input/output terminals of the second optical coupler and receive the two light portions. The end of the both optical fibres 22b and 22e are connected to a reflecting mirror 25. This may be a single Iwo sided reflecting mirror or two separate reflecting mirrors as desired. The reflectivity of mirror 25 is 100% or as close to 100% as practical. As before optical fibre 22b is mounted on mounts 40 and experiences the strain on the wind turbine component. Optical fibres 22b and 22e do not communicate with each other.

In the second example implementation, light signal 45 is received at the light measuring device by passing along a path from the light emitting device, though the first optical coupler 21, the optical fibre 22c, and the second optical coupler 24, to the optical fibre 22e. The light signal 45 is reflected from the mirror 25, and travels back along the same path to the first optical coupler 23 where it is passed to light measuring device 11 via optical fibre 22d.

Light signal 45 is also received at the input to the optical fibre 22b, and having passed along the fibre to be reflected at mirror 24 passes back along the fibre to the input/output terminal of the second optical coupler as light signal 46.

The second optical coupler combines the light signal 46 having the shifted wavelength with the signal input into the optic fibre 22c. The combined signal 47 then follows the same path back to the light measuring device as noted above.

The arrangement of elements in the reference fibre loop, including portion 33e, optical splitters 33, 34 and mirror 35, are substantially identical to those of the sensing loop. The different light components are given numbers 55, 56 and 57 to correspond to those of the sensing loop.

The sensing technique is the same as that described above for the first example implementation, except that the length of the fibre 22b and the compensation factor will now appear to be double the actual value.

The third example implementation, illustrated in Figure 5, is a simplified version of the second example, and advantageously requires fewer optical elements. The second optical coupler 24 is replaced by partial mirror 26, and the optic fibre 22e is omitted altogether. In this example, first light signal 45 is obtained by reflection of light in the optical fibre 22c at the partial mirror 26.

This light has not passed along fibre 22b and so is not phase delayed in comparison with the source or emitter 10. The partial mirror 26 advantageously reflects 50% of the incident light or less.

The light that is transmitted by partial mirror travels along the fibre 22b, and is reflected back at mirror 25. This reflected light is received at the partial mirror and is partially transmitted as phase delayed light 46. As not all of the phase delayed light is transmitted by the partial mirror, some will be reflected back again and again in the fibre 22b constituting a source of noise. For this reason the reflectivity of the two surfaces of the partial mirror is adjusted to ensure as equal a ratio of signal 45 to 46 as possible, and to minimise the amount of noise.

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As before, the reference optical fibre contains corresponding elements such as two way mirror 36.

None of the example strain implementations described above provide a way of addressing changes in temperature or other environmental factors that may affect the optical path length of the fibre 22b and the accuracy of the results.

Figure 6 therefore illustrates a sensor in which a further optic fibre 27 and 37 is provided in each loop, for compensating environmental effects, such as thermal expansion.

The arrangement is similar to that described above for the sensor of Figure 3, except that the optical fibre 22e is replaced by the second optic fibre 27.

Optical fibres 27 and 37 are mounted in the wind turbine component in such a way that they are not subject to strain. Optic fibres 27 and 37 end in mirrors and 35 respectively. The length of the second fibre optic cable 27 is known at installation.

In this arrangement, the light signals 45 and 46 are received at the light measurement device 11 in the same manner as described above for Figure 2.

However, in the earlier examples, light signal 45 was obtained by making the incident light travel along a much shorter path than the light signal 46 travelling along the longer measuring fibre 22b. It was assumed that the lengths of the fibres 22a, 22c, 22d and 22e were negligible in comparison.

In this example, however, both of the optical fibres 22b and 27 are similar in length and the two light signals will therefore travel along similar optical path lengths. The difference in the path lengths is then equal to the extension of the fibre optic 22b caused by strain on the wind turbine component. Although, thermal expansion will cause a change in length of the optic fibres, it should act on both equally, allowing the difference in length to be taken solely as indicating strain. This arrangement also advantageously allows compensation for any temperature induced change in the speed of light within the fibre.

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In this example, the number of cycles counted by the detector therefore indicates the difference in length of the two fibres rather than the total length of the measuring fibre, and can in turn be used to give a value for temperature compensated strain.

As before, the arrangement of the compensatory fibre 32 mirrors that of the sensing fibre.

The example implementations described above, in connection with Figures 2, 3, 4 and 5 all concern strain sensors. This is purely for illustration however, and the same embodiments could be used in fibre optic sensors for measuring other characteristics such as temperature or pressure. The skilled person will appreciate that the difference in configuration of such sensors will be the way in which the measurement portion 22b of the sensing fibre optic 22 is attached to the wind turbine component. In such sensors, the sensing fibre optic would also need to be isolated from variations in strain, and is therefore unlikely to be attached via mounts 40.

Other techniques may be used to measure the length of the optic fibre 22b or the difference in length of the optic fibres 22b and 27. In one example, a pulse of light may be input into optic fibre 22c, and the time taken for the pulse to travel along the two optical paths to the sensor would be recorded. The difference in time between the two pulses received at the detector could be used to determine either the path length of fibre 22b, or the difference in path length between 22b and 27. In this case, the controller illustrated in Figure 3 also comprises a timer 35.

A further implementation is to input white light into the optic fibre 22a. The optical path length travelled by the light in an optical fibre will be different according to the wavelength of the light. With the four different example embodiment proposed, the different wavelengths of light will be received at the detector at different phases according to the path travelled and will therefore interfere with each other constructively or destructively. The effect is like an interference pattern for a single wavelength of light, and has a sinusoidal shape. It is however a result of the interference of different wavelengths.

The separation between the peaks or troughs of the interference pattern indicates the relative difference in wavelengths that add destructively or constructively due to the different path lengths. Adding white light to input portion of the fibre is analogous to varying the wavelength of the input light over a very broad range, and the difference in wavelength can be expressed as a percentage variation between a first peak or trough and a second consecutive peak or trough.

Consider by way of example, light of wavelength I 000nm added to two optic fibres having lengths that differ by 1 mm. The 1mm difference in path length corresponds to 1000 waves, If the signals from both paths were added is together the waves would add constructively. A slightly longer or shorter wavelength however would add destructively. In this example destructive interference would require 999.5 or 1000.5 waves in the 1 mm path difference.

This number of waves corresponds to wavelengths of 1 mm / 999.5 = I.0005pm and 1mm / 1000.5 0.9995pm. This pattern will therefore repeat with a rate of I nm depending on the difference in length of the fibres.

A path difference of 2mm, would correspond to 2000 waves at a wavelength of l000nm. Again, light at this wavelength would add constructively, but light at slightly different wavelengths would add destructively. In this case, destructive interference would require 1999.5 or 2000.5 waves in the path difference, corresponding to wavelengths of 1.00025pm and 0.99975pm. This pattern repeats with a rate of 0.5nm, which is dependent on the difference in length. The difference between constructive and destructive interference is then wavelength / number of waves = repeat rate or, replacing number of waves',

I

wavelength / (fibre length / wavelength) = repeat rate which can be rearranged to fibre length wavelength2" repeat rate An interrogator is a light detector detects and measures light across a wide spread of wavelengths. Using an interrogator, the resulting interference pattern, in the received white light signal can be measured, and the spacing (repeat rate) between the constructive and destructive parts of the wavelength spectrum determined. These presently allow repeat rates from 2Onm down to 0.lnm at a wavelength of l500nm to be measured.

The above formula is simplified somewhat, as by definition the wavelength is not uniform across the spectrum. Accordingly the repeat rate at different wavelengths will be different. However, by approximating the wavelengths and repeat rates, by averaging over a plurality of values say, a reasonable degree of accuracy in the measurement of fibre length can still be obtained. It will be appreciated that it is not necessary to use white light, but light having a plurality of wavelengths across a sufficiently broad range for the necessary interference pattern to be produced.

The different techniques provide a number of different benefits. Wavelength variation for example allows relatively large fibre differences (say 10mm or more) with a variation in wavelength of just a few nanometres. Furthermore, the technique is relatively sensitive, as a long fibre (say 0.5m or more) gives a large number of interference cycles for each nm change.

Although in the above discussion the apparent length of the measurement portion 32b, is measured and stored, it may not always be necessary to measure the apparent length at every instant. For example, in an alternative implementation, the actual length of the measurement portion known from installation may used. In this case, recording the length of the measurement portion at the end of each measurement period may be omitted, and only the number of the waves resulting from the variation counted, and compared with the expected number of waves for that length of fibre given the wavelength.

Although, in all of the above examples the sensor optical fibre and reference optical fibre arrangements mirror each other in terms of optical elements and connections, in alternative embodiments the lay-out of the reference and sensing fibres may be different, and may be as desired selected from the alternatives shown in Figures 2, 4, 5 and 6, or their equivalents.

A non FBG sensor system is therefore provided for making accurate measurements of deformation on a wind turbine component In particular, based on the detected light received from the reference optical fibre, the controller can determine whether the wavelength of the light source has drifted away from the desired value and correct for such drift in any subsequent measurement. Due to the compensation effect, the sensor can use a light source that is less precise and is therefore less expensive, such as simple laser diodes that have an output wavelength dependent on the input supply voltage, rather than complicated tuneable lasers. The sensor is however easy to handle and install, and will be cost effective to operate and maintain over an intended life span of 20 years or more. The sensor works just as well, whether it is the whole fibre that is stretched, or just a small fraction that is stretched, as in all cases it is the total length that matters. This makes the mounting less demanding.

The invention has been described with reference to example implementations, purely for the sake of illustration. The invention is not to be limited by these, as many modifications and variations would occur to the skilled person. The invention is to be understood from the claims that follow.

Claims

Claims 1. An interferometric fibre optic sensor for measuring an operating parameter of a wind turbine component, the sensor comprising: a light source for outputting light of a predetermined wavelength; a sensor optical fibre providing a sensing optical path, the sensor optical fibre operatively attached to the component such that variations in the operating parameter cause the optical length of the sensing optical path to change; a reference optical fibre providing a reference optical path for detecting changes in the wavelength of the light output by the light source, the second optical fibre operatively attached to the component such that the reference optical path is isolated from variations in the operating parameter; one or more light detectors for receiving light that has passed along the sensing and reference optical paths respectively, and for providing an output to the controller indicating the intensity of the received light; a controller coupled to the light detector for determining, based on the detected light from the reference path, drift in the wavelength of the output light and a correction factor for the measurement of the operating parameter.
2. The sensor of claim 1, wherein the sensor optical fibre provides a first, sensing optical path, and a second optical path, and the reference optical fibre provides a first, reference path and a second optical path, and both the sensor and reference optical comprise an optical adder for combining the light from each path before the one or more light detectors.
3. The sensor of claim 2, wherein the controller comprises: a light source controller, coupled to the light source, for varying the wavelength of output light for measurement; a memory for storing the output received from the one or more light detectors; an analyser for detecting a cyclical value of the intensity of the received light, as the wavelength is varied: andSa calculation unit, coupled to the anatyser, for counting the number of times the cyclical value of intensity is detected.
4. The sensor of claim 3, wherein the controller is operable to determine apparent changes in the length of the reference optical path, and associate any changes in the length of the reference optical path with drift in the wavelength of the light.
5. The sensor of claim 3, wherein the cyclical value is a maximum and/or minimum in the intensity of the received light.
6. The sensor of any of claims 2 to 5, comprising an optical splitter coupling an output of the light source to the sensor and reference optical fibres.
7. The sensor of any of claims 2 to 6 comprising first and second light detectors, coupled to the controller, and to respective ends of the sensor and reference optical fibres.
8. The sensor of any of claims 2 to 7 wherein, the controller is operable to determine, based on the light received from the sensor and reference optical fibres, the length of the sensing and reference optical paths.
9. The sensor of claim 8, wherein the lengths of the reference optical fibre and the sensor optical fibre are substantially the same.
10. The sensor of any of claims 7 to 9, comprising: a light splitting device coupled to the sensor optical fibre for splitting the light into the sensing and second optical paths; a light adding device arranged to receive the light from the sensing optical path and directly from the light splitting device.
11. The sensor of claim 7 or 10, comprising: a light splitting device coupled to the reference optical fibre for splitting the light into the reference and second optical paths; a light adding device arranged to receive the light from the reference optical path and directly from the light splitting device.
12. The sensor of any of claims 2 to 9, wherein in the sensing optical fibre the second optical path comprises: a non-measurement portion of optical fibre, separate to the sensing optical path; and a mirror terminating the non-measurement portion of optical fibre; and the sensing optical path comprises: a sensing portion of optical fibre longer than the non-measurement portion; and a mirror terminating the sensing optical measurement portion of the optical fibre; and wherein the sensor comprises: an optical coupler for splitting the light received from the light source between the non-measurement and sensing portions of the fibre, and for receiving and combining the light reflected from both the sensing and non-measurement portions of the fibre.
13. The sensor of any of claims 2 to9, or 12, wherein in the reference optical fibre the second optical path comprises: a non-measurement portion of optical fibre, separate to the reference optical path; and a mirror terminating the non-measurement portion of optical fibre; and the reference optical path comprises: a reference portion of optical fibre longer than the non-measurement portion; and a mirror terminating the reference optical measurement portion of the optical fibre; and wherein the sensor comprises: an optical coupler for splitting the light received from the light source between the non-measurement and reference portions of the fibre, and for receiving and combining the light reflected from both the reference and non-measurement portions of the fibre.
14. The sensor of any of claims 2 to 9, wherein the sensing optical fibre comprises: a sensing portion of optical fibre for providing part of the sensing optical path; a non-measurement portion optical fibre, a partial mirror for separating the sensing and non-measurement portions of the fibre and for splitting the light into the sensing and second optical paths; and a mirror terminating the sensing portion of the optical fibre; wherein the partial mirror allows the light reflected from the sensing portion of the optic fibre to pass through to the light detector.
15. The sensor of any of claims 2 to 9, wherein the reference optical fibre comprises: a reference portion of optical fibre for providing part of the reference optical path; a non-measurement portion optical fibre, a partial mirror for separating the reference and non-measurement portions of the fibre and for splifting the light into the reference and second optical paths; and a mirror terminating the reference portion of the optical fibre; wherein the partial mirror allows the light reflected from the reference portion of the optical fibre to pass through to the light detector.
16. The system of any of clams 12 to 15, wherein the non-measurement portion of the fibre is isolated from variations in the operating parameter of the component to provide for compensation of a second parameter, the lengths of the measurement and non-measurement portions of the optical fibre and the measurement portion of the fibre being long in comparison to other optical distances in the sensor.
17. The sensor of any of claims 2 to 7, wherein the sensor is a temperature compensated interferometric fibre optic strain sensor and the sensor optical fibre includes: a first sensing optical fibre providing a deformation measurement optical path, the first optical fibre operatively attached to the component such that deformation of the component acts on the first optical fibre and causes the length of the deformation measurement optical path to change; a second optical fibre providing a non-deformation measurement optical path, the second optical fibre operatively attached to the component such that it is isolated from deformation of the component, and wherein the controller is operable to determine based on the light received from the sensing optical fibre, the difference in lengths of the deformation measurement and non-deformation measurement optical paths,
18. The sensor of any preceding claim wherein the operating parameter is one of more of strain, temperature or pressure.
19. A wind turbine comprising the sensor system of any preceding claim.
20. A method of measuring an operating parameter of a wind turbine component, using an interferometric fibre optic sensor, comprising: attaching a sensor optical fibre providing a sensing optical path to the component such that variations in the operating parameter cause the optical length of the sensing optical path to change; attaching a reference optical fibre providing a reference optical path to the component such that the reference optical path is isolated from variations in the operating parameter; inputting into the sensor and reference optical fibre light of a predetermined wavelength; detecting light that has passed along the sensing and reference optical paths respectively, and providing an output indicating the intensity of the received light; determining, based on the detected light from the reference path, the variation in the wavelength of the input light from the predetermined value; and a correction factor for the measurement of the operating parameter.
21. The method of claim 20, wherein the sensor optical fibre provides a first, sensing optical path, and a second optical path, and the reference optical fibre provides a first, reference path and a second optical path, and the method comprises: adding the light from each path before the detecting step.
22. The method of claim 21, comprises: varying the wavelength of output light; determining the intensity of the light detected by the detector, analysing the light intensity to detect a cyclical value of the intensity of the received light, as the wavelength is varied: and counting the number of times the cyclical value of intensity is detected.
23. The method of claim 22, comprising: determining apparent changes in the length of the reference optical path, and associating any changes in the length of the reference optical path with variations in the wavelength of light.
24. The method of claim 22, wherein the cyclical value is a maximum and/or minimum in the intensity of the received light.
25. The method of any of claims 21 to 24, comprising splitting the light received from a single source output to the sensor and reference optical fibres.
26. The method of any of claims 21 to 25 comprising determining, based on the light received from the sensor and reference optical fibres, the length of the sensing and reference optical paths.
27. The method of claim 26, comprising providing sensor and reference optical fibres with lengths that are substantially the same.S28. The method of any of claims 21 to 27, wherein the sensor is a temperature compensated interferometric fibre optic strain sensor and method comprises: providing as the sensor optical fibre a first sensing optical fibre providing a deformation measurement optical path, the first optical fibre operatively attached to the component such that deformation of the component acts on the first optical fibre and causes the length of the deformation measurement optical path to change; and a second optical fibre providing a non-deformation measurement optical path, the second optical fibre operatively attached to the component such that it is isolated from deformation of the component, and determining based on the light received from the sensing optical fibre, the difference in lengths of the deformation measurement and non-deformation measurement optical paths.