Improvements relating to wind turbines

Abstract

The present invention relates to a method and to a wind turbine for determining the tip angle of a blade of a wind turbine rotor during rotation of the rotor. The method comprising: (a) transmitting a light signal from a first blade of the wind turbine rotor towards a second blade of the rotor; (b) receiving the light signal at the second blade of the rotor; and (c) calculating the tip angle of the first or second blade based upon characteristics of the received light signal.

Claims

1. A method of determining the tip angle of a blade of a wind turbine rotor during rotation of the rotor, the method comprising: (a) transmitting a light signal from a first blade of the wind turbine rotor towards a second blade of the rotor; (b) receiving the light signal at the second blade of the rotor; and (c) calculating the tip angle of the first or second blade based upon characteristics of the received light signal.

2. The method of claim 1, wherein step (a) comprises transmitting the light signal from a transmitter located near the tip of the first blade.

3. The method of claim 2, further comprising communicating light to the transmitter from a remotely-located light source via a first optical fibre extending longitudinally along the first blade.

4. The method of claim 1, wherein step (b) comprises receiving the light signal at a receiver located near the tip of the second blade.

5. The method of claim 1 further comprising communicating the received light signal to a remotely-located detector via a second optical fibre extending longitudinally along the second blade.

6. The method of claim 1, wherein: step (a) comprises transmitting first and second substantially identical light signals respectively from first and second transmitters, the first and second transmitters being located near the tip of the first blade and spaced apart in the chordwise direction of the first blade; and step (c) comprises calculating the tip angle of the first blade.

7. The method of claim 6, wherein step (c) comprises determining the optical path difference between the first and second signals and using the optical path difference to calculate the tip angle.

8. The method of claim 6, wherein: step (a) comprises continuously varying the frequency of the transmitted light signal from a first frequency to a second frequency; step (b) comprises detecting a blinking interference signal caused by constructive and destructive interference occurring between the first and second light signals as the frequency is varied between the first and second frequencies; and step (c) comprises calculating the blade tip angle based upon characteristics of the interference signal detected in step (b).

9. The method of claim 8, wherein step (c) comprises counting the number of blinks that occur in the interference signal when the frequency is varied from the first frequency to the second frequency and calculating the blade tip angle based upon the counted number of blinks.

10. The method of claim 8, wherein step (c) comprises determining the blinking frequency of the interference signal and calculating the blade tip angle based upon the blinking frequency.

11. The method of claim 1, wherein: step (b) comprises receiving the transmitted light signal at first and second receivers, the first and second receivers being located near the tip of the second blade and spaced apart in the chordwise direction of the second blade; and step (c) comprises calculating the tip angle of the second blade.

12. The method of claim 11, further comprising converting the received light signal into first and second substantially identical light signals.

13. The method of claim 1, wherein: step (a) comprises transmitting a spectrum of light from the first blade towards the second blade; step (b) comprises receiving one or more frequencies of the spectrum of light at the second blade; and step (c) comprises calculating the tip angle based upon the frequency of the detected light.

14. The method of claim 13, wherein the respective frequencies spread out spatially moving from the first blade towards the second blade.

15. The method of claim 13, wherein step (a) further comprises forming the spectrum of light by refracting white light using refracting means such as a prism.

16. The method of claim 1, further comprising: transmitting light from a plurality of transmitters spaced apart along the length of the first blade and/or receiving transmitted light at a plurality of receivers spaced apart along the length of the second blade; and determining the twist and/or load along the first blade on the basis of characteristics of the received light.

17. The method of claim 16, wherein the light from each transmitter is received by the same receiver.

18. The method of claim 16, wherein each transmitter transmits light having a unique frequency or a unique range of frequencies that is different to the frequencies transmitted by the other transmitters.

19. The method of claim 16, wherein each transmitter transmits light having a unique polarisation that is different to the polarisation of light transmitted by the other transmitters.

20. A wind turbine comprising: a rotor having a plurality of blades; a light source; a transmitter provided on a first blade of the rotor, the transmitter being arranged to transmit a light signal from the light source towards a second blade of the rotor; a receiver provided on the second blade, the receiver being arranged to receive the light signal transmitted from the first blade; a detector for detecting the received light signal; and a processor in communication with the detector and arranged to calculate the tip angle of the first or second blade based upon characteristics of the detected light signal.

21. The wind turbine of claim 20, wherein the transmitter comprises one or more lenses for directing the light signal towards the receiver, and the receiver comprises one or more lenses for receiving the light signal.

22. The wind turbine of claim 20, wherein the light source is located remotely from the transmitter, and the wind turbine further comprises a first optical fibre extending longitudinally along the first blade between the light source and the transmitter.

23. The wind turbine of claim 20, wherein the detector is located remotely from the receiver and the wind turbine further includes a second optical fibre extending longitudinally along the second blade between the detector and the receiver.

24. The wind turbine of claim 20 comprising first and second transmitters located near the tip of the first blade and spaced apart in the chordwise direction of the first blade, the first and second transmitters being arranged respectively to transmit first and second substantially identical light signals towards the receiver on the second blade, wherein the first and second light signals interact to form an interference signal.

25. The wind turbine of claim 24, wherein the first transmitter is located substantially at the leading edge of the blade and the second transmitter is located substantially at the trailing edge of the blade.

26. The wind turbine of claim 24, wherein the first optical fibre branches into first and second secondary optical fibres associated respectively with the first and second transmitters.

27. The wind turbine of claim 26, wherein the first optical fibre branches at a point close to the tip of the first blade.

28. The wind turbine of claim 20, comprising first and second receivers located near the tip of the second blade and spaced apart in the chordwise direction of the second blade.

29. The wind turbine of claim 28, wherein the first receiver is located substantially at the leading edge of the blade and the second receiver is located substantially at the trailing edge of the blade.

30. The wind turbine of claim 28, wherein the first and second receivers are arranged to convert the received light signal into first and second light signals which interact to form an interference signal.

31. The wind turbine of claim 20, wherein the frequency of light emitted by the light source can be varied between a first frequency and a second frequency.

32. The wind turbine of claim 31, wherein the interference signal comprises a series of flashes caused by constructive and destructive interference occurring between the first and second light signals when the frequency of the transmitted light signal is varied from a first frequency to a second frequency.

33. The wind turbine of claim 32, wherein the processor is configured to determine the blade tip angle on the basis of a determined optical path difference between the first and second signals.

34. The wind turbine of claim 32, wherein the processor is configured to determine the blade tip angle on the basis of a counted number of flashes associated with the interference signal when the frequency of the transmitted light is varied from the first frequency to the second frequency.

35. The wind turbine of claim 32, wherein the processor is configured to determine the blade tip angle on the basis of the frequency of flashes associated with the interference signal when the frequency of the transmitted light signal is varied from the first frequency to the second frequency.

36. The wind turbine of claim 20, wherein: the transmitter is arranged to transmit a spectrum of light from the first blade towards the second blade; the receiver is arranged to receive one or more frequencies of the spectrum of light at the second blade; the detector is arranged to detect the frequencies of the received light; and the processor is configured to calculate the tip angle based upon the frequencies of the detected light.

37. The wind turbine of claim 36, wherein the transmitter is configured to cause the respective frequencies to spread out spatially moving from the first blade towards the second blade.

38. The wind turbine of claim 36, wherein the light source is a source of white light and the transmitter comprises a prism for refracting the white light to produce the spectrum of light.

39. The wind turbine of claim 20, further comprising a plurality of transmitters spaced apart along the length of the first blade.

40. The wind turbine of claim 39, wherein each transmitter transmits light having a unique frequency or a unique range of frequencies that is different to the frequencies transmitted by the other transmitters.

41. The wind turbine of claim 39, wherein each transmitter transmits light having a unique polarisation that is different to the polarisation of light transmitted by the other transmitters.

Description

FIGURES

(1) FIGS. 1, 2a and 2b have already been described by way of background to the present invention in which:

(2) FIG. 1 is a perspective illustration of an exemplary wind turbine blade having a circular cross-section at the root, and an airfoil cross-section profile outboard from the root; and

(3) FIG. 2a is a schematic illustration of the cross-section of the tip of the blade of FIG. 1 having a blade tip angle of 0 radians; whilst FIG. 2b illustrates a blade tip angle of >0 radians.

(4) Embodiments of the invention will now be described by way of non-limiting example only with reference to the following figures, in which:

(5) FIG. 3 is a perspective front view schematic illustration of a rotor-hub assembly as used in a horizontal axis wind turbine, configured in accordance with an embodiment of the invention;

(6) FIG. 4 is a perspective side view schematic illustration of the rotor-hub assembly of FIG. 3;

(7) FIGS. 5a and 5b are schematic illustrations of the principal of optical interference used by the rotor-hub assembly of FIG. 3 to determine blade tip angle; FIG. 5a illustrates the conditions required for constructive interference and FIG. 5b illustrates the conditions required for destructive interference;

(8) FIGS. 6a and 6b are schematic illustrations showing how an optical path difference is introduced with respect to an optical receiver located on the second blade when the first blade tip of FIGS. 3 and 4 is rotated relative to the x-axis; FIG. 6b shows how the points A-B-B form a right angled triangle when certain specific approximations are made;

(9) FIGS. 7a, 7b and 7c illustrate the right angled triangle A-B-B of FIG. 6b presented within a circle having a diameter equal to the physical distance of separation of the two optical transmitters located on the first blade of FIG. 3 or 4, and illustrate how the blade tip angle is associated to the optical path difference by a sinusoidal trigonometric function; FIG. 7a illustrates the right angled triangle A-B-B formed when the blade tip angle lies within the range

(10) $0<<\frac{}{2}$
radians; FIG. 7b shows the horizontal chord A-B formed when the blade tip angle is 0 radians; and FIG. 7c shows the vertical chord A-B formed when the blade tip angle is

(11) $\frac{}{2}$
radians; and

(12) FIG. 8 is a schematic illustration of an alternative embodiment showing how the diffraction of white light is used to determine the blade tip angle of a first blade.

DETAILED DESCRIPTION

(13) FIG. 3 schematically illustrates a rotor-hub 22 assembly as featured in a horizontal axis wind turbine. The illustrated rotor-hub assembly 22 comprises three turbine blades 24a, 24b, 24c affixed to a central hub 26 via a pitch mechanism (not illustrated). The blades 24a, 24b, 24c have a cross-sectional profile 16 as illustrated in FIG. 1, and are arranged to cause an anti-clockwise rotation of the rotor-hub, as indicated by the directional arrows 28, when wind is incident on the blades 24a, 24b, 24c in a substantially planar direction perpendicular to and into the plane of the page.

(14) FIG. 4 is a side view of the rotor-hub assembly 22 of FIG. 3.

(15) Each blade 24a, 24b, 24c of the rotor-hub 22 assembly is configured with at least two optical transmitters 30a, 30b (also labelled A and/or B), and at least one optical receiver 32 (also labelled C). A first one of the optical transmitters 30a is arranged on the leading edge 15 of the blade 24a, 24b, 24c, and a second one of the optical transmitters 30b is arranged on the trailing edge 17. The first and second optical transmitters 30a, 30b are separated in a chordwise direction of the blade 24a, 24b, 24c, which is substantially perpendicular to the longitudinal axis L of the blade. The optical transmitters 30a, 30b are located substantially in the vicinity of the tip of the blade 24a, 24b, 24c, to enable accurate determination of the pitch angle of the blade tip.

(16) The at least two optical transmitters 30a, 30b located on a first blade 24a are each configured to emit a light signal 34a, 34b, which is subsequently received by the optical receiver 32 (C) located on a second adjacent blade 24b. The light signals 34a, 34b are coherent and monochromatic, such that the two emitted signals 34a, 34b interfere constructively and/or destructively resulting in an interference signal formed at the optical receiver 32 (C). The tip angle of the first blade 24a relative to the second blade 24b is calculated on the basis of the characteristics of the detected interference signal, as will now be explained in detail with reference to the remaining figures.

(17) FIGS. 5a and 5b are schematic illustrations of the principle of optical interference, which is used in order to determine the tip angle of the first blade. The two emitted signals 34a, 34b interfere constructively when the optical path difference d between the two emitted light signals 34a, 34b is a whole-integer multiple of the wavelength of the emitted signals 34a, 34b,

(18) i.e.

(19) $\begin{array}{cc}d=\frac{n}{2};n=2,4,6\mathrm{.Math.}& \mathrm{eq}.1.0\end{array}$
where is the wavelength of the two monochromatic signals 34a, 34b.

(20) In other words, the signals interfere constructively when the signals are in phase. Points of constructive interference are associated with intensity amplitude peaks or maximas in the detected interference signal. These intensity peaks are equivalent to the bright fringes present in optical interference patterns, and will interchangeably be referred to as such in the ensuing description.

(21) Similarly, the two emitted signals 34a, 34b interfere destructively when the optical path difference d between the two signals is a half-integer multiple of the wavelength, i.e.

(22) $\begin{array}{cc}d=\frac{n}{2};n=1,3,5\mathrm{.Math.}& \mathrm{eq}.1.1\end{array}$
where is the wavelength of the two monochromatic signals 34a, 34b.

(23) In other words, the signals interfere destructively when the signals are exactly out of phase. Points of destructive interference are associated with intensity amplitude troughs or minimas in the detected interference signal, and are equivalent to the dark fringes present in the optical interference patterns.

(24) An optical intensity maxima is measured by the optical receiver 32 when the position of an amplitude peak of the interference signal is coincident with the position of the optical receiver 32, as illustrated in FIG. 5a. An optical intensity minima is measured by the optical receiver 32 when an amplitude trough is coincident with the position of the optical receiver 32, as illustrated in FIG. 5b.

(25) The positions where amplitude peaks and troughs form relative to the optical receiver 32 vary as the optical path difference between the two emitted signals 34a, 34b varies. The optical path difference is dependent on the difference in distance of the two optical transmitters 30a, 30b from the optical receiver 32, which varies with the tip angle .

(26) FIG. 6a is a schematic illustration showing how an optical path difference is introduced when the first blade tip is rotated relative to the x-axis. The positions of the two optical transmitters 30a, 30b are illustrated with respect to the position of the optical receiver 32. The optical path difference is represented by the line segment B-B. The real physical distance of separation between the two optical transmitters 30a, 30b is represented by the line segment A-B. The line segments A-C and B-C are equal in length (i.e. A-C=B-C). The optical path difference B-B is the additional distance that the second signal 34b emitted from the second optical transmitter 30b travels to the optical receiver 32 with respect to the first signal 34a emitted from first optical receiver 34a. When this optical path difference is a whole integer multiple of the emitted wavelength, as indicated in equation 1.0, an amplitude peak is formed at the optical receiver 32. When the optical path difference is a half integer multiple of the wavelength, as indicated in equation 1.1, an amplitude trough is formed at the optical receiver 32.

(27) Since the line segments A-C and B-C are significantly larger than the line segment A-B, it is possible to assume that the line segments A-C and B-C are substantially parallel in orientation, since the angle of divergence of the two line segments is very small. FIG. 6b illustrates line segments A-C and B-C being substantially parallel in orientation. On the basis of this assumption, the lines A-B-B form a right-angled triangle having an angle of divergence from the x-axis of .

(28) FIGS. 7a, 7b, and 7c illustrate the right-angled triangle A-B-B plotted on a circle having a diameter equal to the line A-B (the diameter is equal to the physical distance of separation between the optical transmitters), for different values of the blade tip angle .

(29) FIG. 7a illustrates the right-angled triangle formed for a blade tip angle lying within the range 0<</2. The optical path difference B-B is then given by the following equation
optical path difference=AB sin()eq.2.0
where AB is the distance of separation of the optical transmitters 30a, 30b.

(30) In practice, the blade tip angle is likely to be restricted within the range

(31) $0\frac{}{2}.$
This gives rise to two extreme scenarios, illustrated respectively in FIGS. 7b and 7c.

(32) FIG. 7b illustrates the scenario where the blade tip angle is 0, in which case the optical path difference, determined using equation 2.0 is zero, and both A and B are equidistant from the optical receiver C.

(33) FIG. 7c illustrates the scenario where the blade tip angle is

(34) $\frac{}{2},$
in which case the optical path difference is equal to the physical distance of separation of A and B.

(35) Equation 2.0 may be re-expressed as follows:

(36) $\begin{array}{cc}\sin \left(\right)=\frac{\mathrm{optical}\mathrm{path}\mathrm{difference}}{\mathrm{AB}}& \mathrm{eq}.2.1\end{array}$

(37) The tip angle , defined with respect to the direction of rotation R, is given by the inverse sine of the ratio of the optical path difference to the real physical distance of separation between the optical transmitters:

(38) $\begin{array}{cc}=\arcsin \left(\frac{\mathrm{optical}\mathrm{path}\mathrm{difference}}{\mathrm{AB}}\right)& \mathrm{eq}.2.2\end{array}$

(39) The physical distance of separation AB between the optical transmitters in the chordwise direction along a blade 24a, 24b, 24c is constant, and is accurately measured when the transmitters are installed on the blade.

(40) The optical path difference is determined empirically using the optical transmitters 30a, 30b and the optical receiver 32, by analysing the characteristics of the interference signal measured at the optical receiver 32 as will now be explained. In the first embodiment, the first blade 24a is arranged relative to the second blade 24b such that there is an optical path difference between the first and second optical transmitters 30a, 30b (e.g. as illustrated in FIG. 7a). The wavelength and/or frequency of the first and second emitted coherent light signals 34a, 34b is then varied by a known amount. Varying the wavelength and/or frequency causes the position of the interference fringes relative to the optical receiver 32 to be shifted. For example, the frequency of the first and second emitted coherent light signals 34a, 34b may be varied from a first known frequency to a second known frequency different to the first frequency. Assuming that the optical path distance is constant as the wavelength and/or frequency is varied, the total number of counted shifted bright or dark interference fringes is proportional to the change in frequency. This is illustrated in further detail in the below example, provided for illustrative purposes only. It is to be noted that since frequency and wavelength are proportional, a variance in the frequency will result in an inversely proportional variance in the wavelength. In other words, increasing the frequency by 1% results in a 1% decrease in wavelength, and vice versa.

(41) Assuming that the optical path difference between the two signals is 100 mm, and the wavelength of the emitted coherent light signals is 1 m, then there are 100,000 wave cycles (e.g. 100 mm/1 m=100,000 wave cycles) within the optical path difference, where each wave cycle relates to an amplitude oscillation over 27. In other words, there are 100,000 additional cycles between the second transmitter 30b and the optical receiver 32, compared to the number of wave cycles between the first transmitter 30a and the optical receiver 32. Therefore, a 1/100,000 (10 ppm) decrease in the wavelength of the first and second emitted signals 34a, 34b results in one additional wave cycle present in the optical path difference. Similarly, a 1/100,000 (10 ppm) increase in the wavelength of the emitted signals results in one less wave cycle present in the optical path difference.

(42) Similarly, a 1% (10,000 ppm) change in the frequency of the emitted first and second signals 34a, 34b, results in a change of 1,000 wave cycles between the first and second optical transmitters 30a, 30b. For example, if the frequency is increased by 1% (corresponding to a 1% decrease in wavelength), then an additional 1,000 wave cycles will be present within the optical path difference, whereas if the frequency is decreased by 1% (corresponds to a 1% increase in wavelength) this will result in 1,000 fewer wave cycles present within the optical path difference. Either way, and provided that the optical receiver 32 is configured to measure optical intensity (e.g. optical irradiance), 1,000 bright fringes will shift across the optical receiver 32. This manifests itself as a blinking or flashing interference signal, which would blink and/or flash 1,000 times as the frequency is varied from the first to the second frequency. The number of measured amplitude peaks (i.e. bright interference fringes) is proportional to the change in frequency (or equivalently the change in wavelength).

(43) The total number of wave cycles present within the optical path difference may be determined knowing the wavelength and/or frequency of the emitted monochromatic light, the amount by which the frequency/wavelength was varied, and the resulting change in number of wave cycles. If a 1% change in frequency resulted in a change of 1,000 wave cycles, then the originally present number of wave cycles in the optical path distance was 100,000. The optical path difference may then be determined knowing the wavelengthspecifically, by dividing the total number of wave cycles by the wavelength.

(44) In summary, the optical path difference is determined by controllably varying the frequency (and therefore the wavelength) by a known amount, and observing the number of flashes and/or blinks in the received interference signal that occurs when the frequency of the emitted signals is varied. This provides a quantifiable association between the characteristics of the emitted signals 30a, 30b and the number of observed bright interference fringes, wherefrom the optical path difference is calculated.

(45) Once the optical path difference has been determined, then the blade tip angle may be determined using equation 2.2.

(46) In practice, the blade tip angle may be determined whilst the rotor is either in use or stationary. Since there is no relative motion between the first and second blades 24a, 24b, Doppler effects are not observed, and can be ignored for present purposes. The blade tip angle of the first blade 24a is determined by emitting a monochromatic light signal from respectively the first and second optical transmitters 30a, 30b, controllably varying the frequency/wavelength of the emitted light signals by a known amount, and counting the number of bright fringes measured at the optical receiver 32, in order to determine the optical path distance as described previously. The tip angle is then determined using equation 2.2.

(47) In use, the optical transmitters 30a, 30b may be configured to continuously emit monochromatic light signals 34a, 34b, in order to measure the blade tip angle continuously; or alternatively, the optical transmitters 30a, 30b may be activated when precise blade tip measurement is required.

(48) In order to avoid electrically conducting material present within the blades 24a, 24b, 24c, optical fibres are used to transmit optical signals from a coherent light source located inside the hub to the optical transmitters 30a, 30b located substantially in the vicinity of the blade tips.

(49) For example, each blade 24a, 24b, 24c is provided with an optical fibre 36 extending longitudinally along the blade in a direction substantially parallel to the longitudinal axis L, as illustrated in FIGS. 3 and 4. The optical fibre is operatively coupled to a coherent, monochromatic light source 38 located in the hub 26, at one end. In the vicinity of the optical transmitters 30a, 30b, the optical fibre is arranged to bifurcate into two separate component fibres 36a, 36b, with each component delivering a coherent light signal to a different one of the optical transmitters 30a, 30b. The bifurcation is arranged such that the two separate fibre components 36a, 36b do not introduce an optical path difference. This ensures that the two light signals 34a, 34b emitted from each optical transmitter 30a, 30b are coherent and in phase. As the rotor turns, any phase difference observed at the optical receiver 32 is therefore due to an optical path difference resulting from a difference in the relative distances of the optical transmitters 30a, 30b from the optical receiver 32.

(50) The optical receiver 32 located on each blade 24a, 24b, 24c is also operatively coupled to an optical fibre 40, such that a received optical signal, which signal will be the superposition of the first and second signals 34a, 34b, is transmitted to an optical sensor 42, located remotely from the optical receiver 32. As mentioned previously, the use of optical fibres avoids electrically conducting material located in the blades 24a, 24b, 24c, which would be susceptible to lightning strikes in adverse weather conditions. The remotely located optical sensor 42 is located within the hub 26 in this example, and is configured to measure optical irradiance (i.e. optical intensity). The optical sensor 42 is connected to a processor arranged to count interference fringes, in order to enable determination of the optical path distance as previously described.

(51) Alternatively, the processor may be arranged to calculate the Fast Fourier Transform (FFT) in order to determine a blinking frequency, which is defined as the number of counted bright interference fringes occurring within a unit of time. The blinking frequency can then be used to determine the total number of blinks which is associated to the change in optical path. The blade tip angle may then be calculated as described previously. Use of the FFT is particularly advantageous to measure weak and/or noisy signals emitted from the optical transmitters.

(52) For clarity purposes, FIG. 4 illustrates the first blade 24a comprising the optical transmitters 30a, 30b and associated optical fibres 36, 36a, 36b; and the second blade 24b is illustrated as comprising only the optical receiver 32 and its associated optical fibre 40. However, it is to be appreciated, and as illustrated in FIG. 3, that each blade 24a, 24b, 24c in this example comprises both optical transmitters 30a, 30b and associated optical fibres 36, 36a, 36b; and an optical receiver 32, and associated optical fibre 40.

(53) In a further embodiment, each blade 24a, 24b, 24c is provided with more than one pair of optical transmitters, each pair of optical transmitters being located at a different position along the longitudinal axis L of a blade 24a, 24b, 24c.

(54) For example, FIG. 3 illustrates the first blade 24a comprising two pairs of optical transmitters 30a, 30b, 44a, 44b. The second pair of optical transmitters 44a, 44b are located at a different position along the longitudinal axis L of the first blade 24a, with respect to the first pair of optical transmitters 30a, 30b. This configuration of optical transmitters enables the pitch of the first blade 24a, to be determined at different longitudinal positions. This is advantageous when the pitch of the first blade 24a varies along its longitudinal axis L. This might occur when the first blade 24a is subject to high stresses, in which case the pitch or blade tip angle may vary along the blade's longitudinal axis L.

(55) Different methods may be used to distinguish between the light signals 34a, 34b emitted from the first pair of optical transmitters 30a, 30b, and the light signals 46a, 46b emitted from the second pair of optical transmitters 44a, 44b. For example, polarization effects can be used to distinguish between the two pairs of light signals 34a, 34b, and 46a, 46b. Each pair of emitted lights signals 34a, 34b and 46a, 46b is polarised by a different amount, and the emitted signals are distinguished on the basis of their polarisation. For example, the first pair of emitted light signals 34a, 34b may be linearly polarised in the vertical direction, whilst the second pair of emitted light signals 46a, 46b may be linearly polarised in the horizontal direction. A polarization filer located at either the optical receiver 32, or the optical sensor 42, is used to distinguish between the interference signals resulting from each different pair of received optical signals 34a, 34b, 46a, 46b.

(56) Alternatively, wave-plates may be used to introduce a relative phase difference between the two pairs of emitted light signals 34a, 34b, 46a, 46b. The relative phase difference is maintained in the resulting received interference signal, and is used to distinguish between the interference signal resulting from the first pair of emitted light signals 34a, 34b and the second pair of emitted light signals 46a, 46b. The use of wave-plates is advantageous since it does not reduce the intensity of the emitted light signals.

(57) Alternatively, each blade 24a, 24b, 24c may be provided with a plurality of different optical receivers, each different receiver being arranged to measure the interference signal generated by a different pair of optical transmitters. For example, each different receiver is provided with a polarisation filter enabling the required interference signal to be measured, whilst filtering out the other interference signals.

(58) In accordance with a further variant, each blade 24a, 24b, 24c is provided with a single optical transmitter, and two optical receivers. This variant may be envisaged with reference to FIGS. 3 and 4, in which the positions of the optical transmitters and the optical receivers are interchanged. The position of the optical transmitters 30a, 30b are now associated with first and second optical receivers; and the position of the optical receiver 32 is now associated with the position of an optical transmitter. A single signal is emitted from the single optical transmitter. This signal is received by both the first and second optical receivers, generating first and second measured signals at respectively the first and second optical receivers 30a, 30b. The first and second measured signals are associated with the signals 34a, 34b in this example. The measured signals will be in phase if no optical path difference is present between the receivers 30a, 30b relative to the transmitter 32, in which case both measured signals are identical. When the two measured signals 34a, 34b are superimposed at the junction of the first and second optical fibre components 36a, 36b, the first and second measured signals 34a, 34b will interfere constructively. If instead an optical path difference is present between the first and second optical receivers 11a, 11b with respect to the optical transmitter 32, then a relative phase difference will exist between the first and second measured signals 34a, 34b, which will result in constructive or destructive interference depending on the relative phase difference between the two signals when the two signals are superimposed at the junction of the first and second fibre components 36a, 36b. The resulting interference signal is measured at the optical sensor 42. The blade tip angle is calculated in the same manner as described previously.

(59) In accordance with a further variant, the principle of refraction is used to determine the blade tip angle of a first blade relative to a second blade. FIG. 8 is a schematic illustration of this embodiment. Cross sections through a blade tip taken in a plane perpendicular to the longitudinal axes L of the blades are illustrated. The optical transmitter 48 of the first blade 24a is configured with an optical refracting element, such as a prism, arranged to refract different wavelengths and/or frequencies of light by different angles relative to the second blade 24b. When substantially white light is transmitted through the optical transmitter 48, the refractive element refracts the different wavelength and/or frequency components of the incident white light by different angles relative to the second blade 24b, such that a spectrum (i.e. a rainbow) of different coloured light (each different wavelength and/or frequency component will appear as a different colour of light) is emitted from the optical transmitter 48. The blade tip angle of the first blade 24a is determined from the wavelength and/or frequency of the light signal measured by the optical receiver 32 located on the second blade 24b.

(60) As the orientation of the first blade 24a varies relative to the second blade 24b, and a blade tip angle is introduced, the position of the optical transmitter 48 relative to the optical receiver 32 changes. As a result of this relative change in position, the wavelength and/or frequency of the signal received by the optical receiver 32 changes.

(61) The positional relationship between the first and second blades 24a, 24b is calibrated to define a reference signal with respect to which subsequent blade angle tip calculations are defined. For example, when the blade tip angle of the first blade 24a is 0 radians, the second blade 24b measures a specific colour of light, which is used to define a reference signal with respect to which all subsequent blade tip angles are determined. As the blade tip angle changes, so too does the position of the optical transmitter 48 relative to the optical receiver 32. As a result of this, the optical receiver 32 measures a different wavelength and/or frequency component of the emitted light as a function of the blade tip angle . Since the diffraction characteristics of the diffraction element are known, the angular diffraction of the measured wavelength component is determined with respect to the reference signal. The blade tip angle is then directly proportional to the diffraction angle associated with the measured wavelength component.

(62) The herein described embodiments may be used in pitch control strategies and/or to control stress loads on the blade.

(63) It is to be appreciated that equations 2.0, 2.1 and 2.2 are valid for right-angled triangles. Use of these equations provides an approximate value for the blade tip angle, where the triangle A-B-B may be approximated as a right-angled triangle. This approximation does not introduce excessive errors in the calculated blade tip angle, when the angle of divergence between line segments C-B and C-B is very small. This occurs where the distance of separation between the two adjacent turbine blades 24a, 24b is much larger than the physical distance of separation between the optical transmitters positioned at A and B respectively. In practice, as can be seen from FIG. 6a, the triangle A-B-B is not a right-angled triangle.

(64) The present method may still be used to determine blade tip angle even where the approximations underlying equations 2.0, 2.1 and 2.2 do not hold. In these circumstances known trigonometric relationships applicable to non-right-angled triangles may be used. For example, any one or more of the law of sines, the law of cosines, the law of tangents and the law of cotangents may be used to determine the blade tip angle, or any other known trigonometric equation valid for non-right angled triangles. Since the aforementioned trigonometric laws are well known in the art, it is unnecessary to provide a detailed discussion thereof here.

(65) The present method may be used to calculate the blade tip angle for a wind turbine comprising any number of turbine blades. Whilst the herein described embodiments relate to a wind turbine comprising three blades, this is non-limiting for illustrative purposes only.

(66) All herein provided embodiments are provided for illustrative purposes only and are not to be construed as limiting to the invention. It is to be appreciated that alternative embodiments are envisaged comprising suitable combinations of features of the previously described embodiments, and such alternatives fall within the scope of the present invention.