Patent Publication Number: US-8983243-B2

Title: Wind turbine blade with optical sensor system

Description:
This is a National Phase Application filed under 35 U.S.C. §371 as a national stage of International Application No. PCT/EP2011/073249, filed on Dec. 19, 2011, claiming the benefit from European Patent Application No. 10197351.9, filed on Dec. 30, 2010, the content of each of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a wind turbine blade, and a method and apparatus for determining or estimating loads such as bending moment of a wind turbine blade, in particular root moment near or at the root end of a wind turbine blade. 
     BACKGROUND 
     Wind turbine manufacturers are constantly making efforts to improve the efficiency of their wind turbines in order to maximise the annual energy production. Further, the wind turbine manufacturers are interested in prolonging the lifetime of their wind turbine models, since it takes a long time and a lot of resources to develop a new wind turbine model. Systems for monitoring operating parameters of a wind turbine and its components such as the wind turbine blades have become an area of increased attention in order to optimize performance and prolonging the lifetime of different components. 
     Accordingly, there is a need for a wind turbine blade, methods and apparatus enabling accurate and efficient monitoring of one or more operating parameters of a wind turbine or wind turbine blade. An area of particular interest may be monitoring the loads and stresses applied on a wind turbine blade during operation of a wind turbine. 
     DISCLOSURE OF THE INVENTION 
     Accordingly, the invention provides a wind turbine blade extending along a longitudinal axis from a root end to a tip end and in a transverse plane perpendicular to the longitudinal axis, the transverse plane having a main axis extending through an elastic center point, the wind turbine blade comprising a blade shell forming a profiled contour including a pressure side and a suction side, as well as a leading edge and a trailing edge with a chord having a chord length extending there between, the main axis being parallel to the chord. The wind turbine blade may include a sensor system comprising a first sensor set, e.g. for measuring a first bending moment, in a first sensor position at a first distance from the root end, the first sensor set comprising a first primary sensor for measuring a primary component and a first secondary sensor for measuring a secondary component, wherein a first primary sensor axis in the transverse plane is oriented in a direction defined by the first primary sensor and the elastic center point, and a first secondary sensor axis in the transverse plane is oriented in a direction defined by the first secondary sensor and the elastic center point. An angle between the first primary sensor axis and the first secondary sensor axis may be in the range from 50° to 130°. 
     Accordingly, the wind turbine blade according to the invention allows determination of bending moment with only two sensors in a cross-section saving manufacture costs. 
     Also disclosed is a wind turbine blade extending along a longitudinal axis from a root end to a tip end and in a transverse plane perpendicular to the longitudinal axis, the transverse plane having a main axis extending through an elastic center point, the wind turbine blade comprising a blade shell forming a profiled contour including a pressure side and a suction side, as well as a leading edge and a trailing edge with a chord having a chord length extending there between, the main axis being parallel to the chord is provided. The wind turbine blade may comprise a sensor system including a plurality of sensor sets, each sensor set comprising a plurality of sensors including a primary sensor and a secondary sensor for measuring a primary component and a secondary component, respectively. The plurality of sensor sets includes a first sensor set for measuring a first bending moment in a first sensor position at a first distance from the root end, and a second sensor set for measuring a second bending moment in a second sensor position different from the first sensor position at a second distance from the root end. The first distance may be at least 1 m. The second distance may be at least 3 m. 
     Also disclosed is a method for estimating bending moment of a wind turbine blade, e.g. of a wind turbine blade as disclosed herein. The wind turbine blade extends along a longitudinal axis from a root end to a tip end and having a root region, a transition region and an airfoil region, the method comprising the steps of obtaining a first sensor set signal indicative of a first bending moment at a first sensor position different from the tip end along the longitudinal axis of the wind turbine blade, and estimating a bending moment at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis. 
     Further, a blade monitoring system for monitoring a wind turbine blade comprising a sensor system is disclosed, the blade monitoring system comprising a processing unit and an interface connected to the processing unit. The processing unit is configured for receiving a first sensor set signal indicative of a first bending moment at a first sensor position of a wind turbine blade extending along a longitudinal axis from a root end to a tip end. The processing unit is configured for estimating a bending moment at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis. 
     There is a need for a wind turbine blade having an optical sensor system with low loss. 
     Accordingly, a wind turbine blade is provided, the wind turbine blade extending along a longitudinal axis from a root end to a tip end and in a transverse plane perpendicular to the longitudinal axis, the transverse plane having a main axis extending through an elastic center point, the wind turbine blade comprising a blade shell forming a profiled contour including a pressure side and a suction side, as well as a leading edge and a trailing edge with a chord having a chord length extending there between, the main axis being parallel to the chord. The wind turbine blade comprises a sensor system with an optical path comprising a first optical sensor fiber, a second optical sensor fiber and optionally a patch optical fiber, the first optical sensor fiber including a first core with a first core diameter, wherein the first optical sensor fiber extends from a first end to a second end and comprising at least one sensor, the second optical sensor fiber including a second core with a second core diameter, wherein the second optical sensor fiber extends from a first end to a second end and comprising at least one sensor. The patch optical fiber includes a patch core with a patch core diameter, wherein the patch optical fiber extends from a first end to a second end. The patch optical fiber may connect the first optical sensor fiber and the second optical sensor fiber, and the first core diameter may be the same as the patch core diameter. 
     The sensor system with a patch optical fiber provides a high degree of design freedom in designing a wind turbine blade and provides an easily adaptable optical sensor system that may be used in different easily configurable configurations and wind turbine blade models. 
     Further, manufacture of the wind turbine blade may be facilitated since assembly of the sensor system does not require specialist knowledge or tools. 
     Further, the wind turbine blade according to the invention allows for easy reconfiguration of the sensor system after moulding and assembly of blade shell parts. 
     Wind turbine comprising a plurality of wind turbine blades including a first wind turbine blade as described herein, wherein the wind turbine comprises a blade monitoring system configured to estimate bending moment of the first wind turbine blade based on sensor set signals from the sensor system of the first wind turbine blade. 
     The present invention relates to a wind turbine blade, e.g. for a rotor of a wind turbine having a substantially horizontal rotor shaft, the rotor comprising a hub, from which the blade extends substantially in a radial direction when mounted to the hub, the blade extending along a longitudinal axis from a root end to a tip end and in a transverse plane perpendicular to the longitudinal axis, the transverse plane having a main axis extending through an elastic center point. The wind turbine blade comprises a blade shell forming a profiled contour including a pressure side and a suction side, as well as a leading edge and a trailing edge with a chord having a chord length extending there between. The profiled contour, when being impacted by incident airflow, generates a lift. The profiled contour is divided into: a root region having a substantially circular or elliptical profile closest to the root end with a root diameter being the chord length at the root end, an airfoil region having a lift-generating profile furthest away from the root end, and a transition region between the root region and the airfoil region, the transition region having a profile gradually changing in the radial direction from the circular or elliptical profile of the root region to the lift-generating profile of the airfoil region, and with a shoulder having a shoulder width and a shoulder distance and located at the boundary between the transition region and the airfoil region, wherein the blade has a blade length. By shoulder is meant the position at which the wind turbine blade has its largest chord length and the shoulder distance is the distance from the root end to the shoulder. The length interval is defined from the root end to the tip end, the root end thus being positioned at r=0 and the tip end being positioned at r=L along the longitudinal axis. 
     The blade may comprise a blade shell with a shell body. The shell body may for instance be assembled from a pressure side shell and a suction side shell, which are adhered or bonded to each other near the leading edge and near the trailing edge. In another embodiment, the shell is manufactured via a one-shot process, e.g. via a closed, hollow moulding method. 
     The shell body may comprise a longitudinally extending load carrying structure, such as a main laminate. Such a load carrying structure or main laminate is typically formed as a fibre insertion which comprises a plurality of fibre reinforcement layers, e.g. between 20 and 50 layers. On each side of the load carrying structure, the blade typically comprises a sandwich structure with a core material, such as balsa wood or foamed polymer, and with an inner and outer skin made of fibre reinforced polymer. 
     One or more sensors may be arranged in the main laminate or in edges thereof, e.g. the first sensor axis of one or more sensor sets may cross the main laminate or a main laminate edge. 
     The blade shell is typically made of a fibre reinforced polymer material. The reinforcement fibres may for instance be glass fibres, carbon fibres, aramid fibres, metallic fibres, such as steel fibres, or plant fibres, whereas the polymer for instance may be epoxy, polyester or vinylester. 
     The wind turbine blade comprises a sensor set comprising at least one sensor, e.g for measuring or determining bending moments. A sensor set may alternatively or in combination be configured for measuring other parameters. The at least one sensor set includes a first sensor set positioned at a first sensor position along the longitudinal axis. The first sensor set may be positioned at a first distance d 1  from the root end. 
     The wind turbine blade may comprise a plurality of sensors including the first sensor set and a second sensor set positioned at a second sensor position along the longitudinal axis. The second sensor set may be positioned at a second distance d 2  from the root end. 
     The plurality of sensor sets may include a third sensor set for measuring a third bending moment in a third sensor position at a third distance d 3  from the root end. 
     The plurality of sensor sets may include a fourth sensor set for measuring a fourth bending moment in a fourth sensor position at a fourth distance d 4  from the root end. 
     The plurality of sensor sets may include a fifth sensor set for measuring a fifth bending moment in a fifth sensor position at a fifth distance d 5  from the root end. 
     The first distance d 1  may be in the range from about 1 m to about 20 m and the second distance d 2  may be in the range from about 3 m to about 50 m. 
     Optionally, sensor position distances from the root end may depend on the length L of the wind turbine blade and/or the shoulder distance or position d 5 . 
     The first distance d 1  may be selected in the range from d 1,min  to d 1,max . The first distance d 1  may be in the range from about 4 m to about 15 m such as in the range from about 6 m to about 10 m, preferably about 8 m. 
     The first distance d 1  may depend on the shoulder distance d s , i.e. d 1  may be a function of the shoulder distance d s . For example, d 1  may be given by
 
 d   1 =α 1   d   s ,
 
where a 1  is in the range from 0.2 to 1.0.
 
     The first distance d 1  may depend on the length of the wind turbine blade, i.e. d 1  may be a function of the blade length L. For example, d 1  may be given by
 
 d   1 =β 1   L,  
 
where β 1  is in the range from about 0.05 to about 0.95.]
 
     The first distance d 1  may depend on the root diameter d root , i.e. d 1  may be a function of the root diameter d root . The first distance d 1  may be at least a diameter of the root. 
     For example, d 1  may be given by
 
 d   1 =γ 1   d   root ,
 
where γ 1  is at least 0.8.
 
     The minimum first distance d 1,min  may depend on the root diameter d root , e.g. be given by
 
 d   1,min =γ 1   d   root ,
 
where where γ 1  is at least 0.8, such as about 1.
 
     The maximum first distance d 1,max  may depend on the shoulder distance d s , e.g. be given by
 
 d   1,max =α 1   d   s ,
 
where α 1  is in the range from 0.5 to 1.0.
 
     The shoulder distances d s  may be in the range from 11 m to 15 m, e.g. about 13 m for a wind turbine blade having a length about 61.5 m. 
     The root diameter d root  may be in the range from 2 m to 5 m, e.g. about 2.5 m for a wind turbine blade having a length from 40 m-50 m, or about 3.5 m for a wind turbine blade having a length from 60 m-75 m. 
     The second distance d 2  may be selected in the range from d 2,min  to d 2,max . The second distance d 2  may be in the range from about 5 m to about 40 m, such as in the range from about 10 m to about 30 m such as about 12 m or about 23 m. 
     The second distance d 2  may depend on the shoulder distance d s , i.e. d 2  may be a function of the shoulder distance d s . For example, d 2  may be given by
 
 d   2 =α 2   d   s ,
 
where α 2  is in the range from 0.5 to 10.0.
 
     The second distance (d 2 ) may depend on the length of the wind turbine blade, i.e. d 2  may be a function of the blade length L. For example, d 2  may be given by
 
 d   2 =β 2   L,  
 
where β 2  is in the range from 0.1 to 0.8.
 
     The second distance d 2  may depend on the root diameter d root , i.e. d 2  may be a function of the root diameter d root . For example, d 2  may be given by
 
 d   2 =γ 2   d   root ,
 
where γ 2  is at least 0.8.
 
     The minimum second distance d 2,min  may depend on the root diameter d root , e.g. be given by
 
 d   2,min =γ 2   d   root ,
 
where γ 2  is at least 0.8, such as about 2.
 
     The maximum second distance d 2,max  may depend on the shoulder distance d s , e.g. be given by
 
 d   2,max =α 2   d   s ,
 
where α 2  is in the range from 0.5 to 3.0.
 
     The distances d 1 , d 2 , d 3 , . . . of sensor sets from the root end may be at least 6 m in order to minimize or avoid undesired non-linearities from pitch bearings in sensor measurements. 
     In one or more embodiments, the distances d 1 , d 2 , d 3 , . . . of sensor sets from the root end may be less than 25 m, e.g. less than 20 m, to reduce or avoid excessive errors of superposition in sensor measurements. 
     The distances between sensor sets along the longitudinal axis are selected in order to facilitate desired measurements, e.g. for estimating bending moments along the wind turbine blade such as in one or more estimation positions. Distances between sensor positions are indicated as d ij , where i and j are index numbers for sensor sets and sensors thereof. 
     The distance d 12  between the first sensor set and optional second sensor set along the longitudinal axis may be in the range from about 1 m to about 30 m, such as from about 3 m to about 20 m, e.g. about 4 m, about 10 m, about 15 m. 
     The distance d 13  between the first sensor set and optional third sensor set along the longitudinal axis may be in the range from about 1 m to about 50 m, such as from about 10 m to about 40 m, e.g. about 15 m, about 25 m, about 35 m. 
     The distance d 14  between the first sensor set and optional fourth sensor set along the longitudinal axis may be in the range from about 1 m to about 60 m, such as from about 15 m to about 50 m, e.g. about 20 m, about 30 m, about 40 m. 
     The distance d 15  between the first sensor set and optional fifth sensor set along the longitudinal axis may be in the range from about 20 m to about L-d 1 , such as from about 20 m to about 60 m, e.g. about 30 m, about 40 m, about 50 m. 
     A sensor set may comprise one or more sensors. The one or more sensors of a sensor set may include a primary sensor and optionally a secondary sensor. Sensor(s) of a sensor set may be adapted to measure bending moment components, i.e. a sensor set may comprise a primary sensor for measuring a primary bending moment component M X  about a first axis perpendicular to the longitudinal axis, and/or a secondary sensor for measuring a secondary bending moment component M Y  about a second axis perpendicular to the longitudinal axis. In one or more embodiments, a sensor set consists of two sensors for measuring bending moment components, thereby allowing bending moment measurements on the wind turbine blade with only two sensors. 
     The wind turbine blade comprises a first sensor set, e.g. for measuring a first bending moment, in a first sensor position at a first distance from the root end. 
     The first sensor set may comprise a first primary sensor for measuring a primary component of the first bending moment (M X,1 ) about a first axis perpendicular to the longitudinal axis (first primary sensor) and a first secondary sensor for measuring a secondary component of the first bending moment (M Y,1 ) about a second axis perpendicular to the longitudinal axis (first secondary sensor). 
     The second sensor set may comprise a second primary sensor for measuring a primary component (M X,2 ) of the second bending moment about a first axis perpendicular to the longitudinal axis (second primary sensor) and a second secondary sensor for measuring a secondary component of the second bending moment (M Y2 ) about a second axis perpendicular to the longitudinal axis (second secondary sensor). 
     Sensors of a sensor set are positioned at the same distance from the root end, i.e. in the same transverse plane. In one or more embodiments, sensors of a sensor set, e.g. a primary sensor and a secondary sensor, may be displaced along the longitudinal axis. The distance between sensors of a sensor set along the longitudinal axis should be as small as possible. The maximum distance between sensors of a sensor set may be less than 1 m, such as less than 0.5 m. Larger distances may be employed. In case of displaced sensors of a sensor set, the sensor set distance to the root is the mean distance for sensors of the sensor set. 
     The length L of the wind turbine blade may be at least 40 m. 
     Root moments of a wind turbine blade are desired from a control perspective of the wind turbine, e.g. in order to control pitch and other operating parameters in order to optimize operation and power output of the wind turbine. However, measurement of the root moment with sensors positioned at the root end of the blade is largely affected by non-linear moment contributions from the pitch bearing. 
     Correct positioning of sensors in a wind turbine blade is important in order to obtain a precise measurement and in order to reduce or eliminate undesired effects such as nonlinear effects. It is from a cost and manufacture perspective desired to employ a low number of sensors. 
     A primary sensor is positioned on a primary sensor axis extending through the elastic center in the transverse plane with the primary sensor. A secondary sensor is positioned on a secondary sensor axis extending through the elastic center in the transverse plane with the secondary sensor. 
     The angle between the first primary sensor axis and the first secondary sensor axis may be in the range from about 85° to about 95°. The angle between the first primary sensor axis and the first secondary sensor axis may be about 90°. 
     In case of perpendicular first and second sensor axes, the first bending moment M 1  may be given as:
 
 M   1 =√{square root over ( M   X,1   2   +M   Y,1   2 )}
 
     An estimated primary component of the bending moment at the first estimation position M X,est , e.g. at the root end, may be given as a function of one or more sensor signals S 11 , S 12 , S 21 , S 22 , . . . , where S 11  is the first primary sensor signal, S 12  is the first secondary sensor signal, S 21  is the second primary sensor signal, S 22  is the second secondary sensor signal, etc. 
     An estimated secondary component of the bending moment at the first estimation position M Y,est , e.g. at the root end, may be given as a function of one or more sensor signals S 11 , S 12 , S 21 , S 22 , . . . , where S 11  is the first primary sensor signal, S 12  is the first secondary sensor signal, S 21  is the second primary sensor signal, S 22  is the second secondary sensor signal, etc. 
     Sensor signals may be multiplexed in time and/or in frequency/wavelength. Time multiplexing may be preferred for reducing the number of components in the reading unit and reduce the costs of the sensing system. 
     The estimated primary component of the bending moment at the first estimation position M x,est,1  may be estimated based only on primary sensor signals from primary sensors of one or more sensor sets. The estimated secondary component of the bending moment at the first estimation position M Y,est,1  may be estimated based only on secondary sensor signals from secondary sensors of one or more sensor sets. Additionally or alternatively, M x,est,1  may be estimated based on secondary sensor signals from secondary sensors of one or more sensor sets and/or M Y,est,1  may be estimated based on primary sensor signals from primary sensors of one or more sensor sets. 
     The bending moment M est,1  at a first estimation position may be estimated or given as:
 
 M   est, =√{square root over ( M   X,est,1   2   +M   Y,est,1   2 )}.
 
     The bending moment M est,1  at a first estimation position may be estimated or given as:
 
 M   est,1 =√{square root over (α x   M   X,est,1   2 +β Y   M   Y,est,1   2 )},
 
where α X  and β Y  are compensation factors for compensating for non-perpendicular first axis and second axis.
 
     The wind turbine blade may comprise a second sensor set, e.g. for measuring a second bending moment, in a second sensor position at a second distance from the root end. The sensor set may comprise a second primary sensor for measuring a primary component and a second secondary sensor for measuring a secondary component, wherein a second primary sensor axis in the transverse plane is oriented in a direction defined by the second primary sensor and the elastic center point, and a second secondary sensor axis in the transverse plane is oriented in a direction defined by the second secondary sensor and the elastic center point. An angle between the second primary sensor axis and the second secondary sensor axis may be in the range from 50° to 130°, such as in the range from 85° to 95°. The angle between the first primary sensor axis and the first secondary sensor axis may be about 90°. 
     The primary sensor(s), e.g. the first primary sensor and/or the second primary sensor, may for instance be arranged at the pressure side or the suction side of the blade, advantageously at or embedded in a load carrying structure such as a main laminate of the blade shell structure. 
     The secondary sensor(s), e.g. the first secondary sensor and/or the second secondary sensor, may for instance be arranged at the leading edge or the trailing edge of the wind turbine blade 
     Positioning primary and secondary sensors on the pressure side and at the leading edge, respectively, may facilitate employment of strain gauge sensors. One or more of the sensors, such as the primary and/or the secondary sensors, may be strain gauge sensors. 
     One or more of the sensors, such as the primary and/or the secondary sensors, may be optical sensors, such as fiber Bragg gratings. 
     At least one of the sensors may be embedded in the blade shell. One or more sensors may be attached to or mounted on the inner surface of the blade shell. The wind turbine blade may comprise a beam attached to the shell and at least one of the sensors may in this case be mounted on the beam. One or more sensors may be mounted on or embedded in the main laminate of a shell body. 
     The wind turbine blade may comprise a web attached to the shell and at least one of the sensors may be mounted on the web. 
     The one or more sensor sets, e.g. the first sensor set, and/or the second sensor set may be arranged in the root region, the transition region or the airfoil region of the wind turbine blade. 
     The sensors of the wind turbine blade provide sensor signals indicative of a bending moment, e.g. in the form of strain/pressure signals indicating strain/pressure applied to the sensor. The strain/pressure signals of sensors can be transformed to bending moments or components thereof by use of sensor systems parameters, e.g. determined during calibration and/or design of the wind turbine blade. 
     A sensor signal indicative of a bending moment may be an optical signal from an optical sensor, e.g. a fiber Bragg grating, in an optical fiber, where the sensor signal is reflected light having a wavelength depending on the strain applied to the sensor. 
     The sensor signal may be fed to a reading unit positioned e.g. in the wind turbine blade or in the hub of a wind turbine. The reading unit may be configured to determine or derive the wavelength of one or more sensor signals and provide the wavelength(s) of sensor signal(s) to a processing unit in a blade monitoring system. The reading unit may be a separate device connectable to a sensor system and a blade monitoring system for receiving sensor signals and forwarding processed sensor signals to the blade monitoring system. In one or more embodiments, the reading unit may be embedded in the blade monitoring system, i.e. the blade monitoring system comprises the reading unit and is connectable to the sensor system via one or more sensor ports of the interface. 
     In one or more embodiments where the sensors are implemented as fiber Bragg gratings in an optical fiber, the sensor signals are optical signals with wavelengths λ 11  (first primary sensor), λ 12  (first secondary sensor), λ 21 , (second primary sensor), λ 22  (second secondary sensor), . . . , λ ij  where i and j are index numbers for sensor sets and sensors thereof. The sensor signals may be multiplexed in time and/or frequency/wavelength. The strain and/or pressure applied to the sensors may be derived from the wavelengths of the sensor signals. The processing unit may be configured to derive bending moments and/or components thereof from the sensor signals, e.g. based on sensor system parameters stored in a memory unit. 
     For a wind turbine with an optical sensor system including a second optical sensor fiber and a patch optical fiber, the second core diameter may be the same. The optical path in the sensor system may have the same diameter, i.e. different optical fibers in the system may have the same core diameter or mode field diameter. 
     The first optical sensor fiber may comprise a first primary sensor and optionally a second primary sensor for indicating strain at a first distance and optionally at a second distance, respectively, from the root end of the wind turbine blade. 
     The second optical sensor fiber may comprise a first secondary sensor and optionally a second secondary sensor for indicating strain at a first distance and optionally at a second distance, respectively, from the root end of the wind turbine blade. 
     Standard patch optical fibers have a core diameter of 9 μm. A sensing fiber typically has a core diameter of less than 7 μm. Having the same core diameter in the first optical sensor fiber and the patch optical fiber provides an optical path with low damping and thus a sensor system with low loss is provided. An optical sensor system with low loss, e.g. less than 3.5 dB, may be provided in order to meet sensor system requirements from the reading unit or in order to reduce the requirements to the reading unit. 
     The sensor system may comprise a plurality of patch optical fibers with the same patch core diameter. 
     The sensor system may comprise a number of optical fibers, each optical fiber comprising one or more sensors, including a third optical fiber and/or a fourth optical fiber. The third optical fiber may be positioned parallel and adjacent to the first optical sensor fiber in the wind turbine blade and/or the fourth optical fiber may be positioned parallel and adjacent to the second optical sensor fiber in the wind turbine blade. More than two optical fibers with sensors may allow for easy repair of the sensor system. For example, if the first optical sensor fiber breaks, the patch optical fiber is simply connected to the third optical fiber and the blade monitoring system is configured to estimate bending moments based on sensor signals from sensors in the second and third optical fiber instead of estimating bending moments based on sensor signals from sensors in the first and second optical sensor fiber. 
     Provision of one or more patch cables with the same core diameters as the optical fibers with sensor(s) facilitates a high degree of freedom for the blade designer. 
     An optical fiber with a relatively small core diameter, e.g. less than 7 μm, may be preferred for sensing in the wind turbine blade where a low bend-induced loss may be of importance. A core diameter of 4.2 μm may be used since a high photosensitivity may be desirable in order to facilitate fabrication of certain types of fiber Bragg gratings (FBGs). 
     Accordingly, the first core diameter, the second core diameter and/or the patch core diameter may be less than 7 μm, such as 6.4 μm, 5.3 μm or 4.2 μm. 
     The first and/or second optical sensor fibers may be single mode fiber(s) with a design wavelength of 1550 nm. 
     The patch optical fiber(s) may be single mode fiber(s) with a design wavelength of 1550 nm. 
     In one or more embodiments, the first and second optical sensor fibers and the patch optical fiber are optical fibers having MFD of 6.4 μm and outer diameter of 80 μm. 
     In one or more embodiments the first and second optical sensor fibers and the patch optical fiber are optical fibers having MFD of 4.2 μm and outer diameter of 125 μm. 
     The optical fibers may have a cut-off wavelength in the range from 1350 to 1500 nm. 
     The optical fibers may be SM 1500(4.2/125) optical fibers. 
     The distance between sensors of the sensor system may be at least 3 m along the optical path of the sensor system. In particular, the distance between the first primary sensor and the second primary sensor may be at least 3 m, e.g. at least 5 m, at least 8 m, along the first optical sensor fiber in order to enable sensing at distances along the longitudinal axis of the wind turbine. The distance between the first secondary sensor and the second secondary sensor may be at least 3 m along the second optical sensor fiber, e.g. at least 5 m, at least 8 m. 
     An inter-sensor distance of at least 3 m facilitates time multiplexing of sensor signals. The distances between sensors in an optical fiber are selected in order to arrange sensors in desired positions in the wind turbine blade. 
     The first optical sensor fiber and the second optical sensor fiber may be mainly or at least partly embedded in the blade shell. Preferably, one or more ends of the first optical sensor fiber and/or the second optical sensor fiber are provided with connector parts for coupling or connection to a patch optical fiber, a reading unit or another optical fiber with sensors. The connector part(s) may be embedded or accommodated in one or more connector boxes mounted on or moulded into the blade shell for allowing easy installation of one or more patch optical fibers. Connector parts at ends of the optical fibers provide optical coupling of the fiber cores of the optical fibers. 
     The first optical sensor fiber and the second optical sensor fiber may be at least partly adhered or mounted to the blade shell. 
     The wind turbine blade may comprise a reading unit comprising at least one sensor port including a first sensor port for optically coupling the sensor system, e.g. the first optical sensor fiber, to the reading unit. The reading unit may be configured for reading sensor signals of optical sensors in the sensor system as described above. The reading unit may be configured to derive a plurality of sensor signals indicative of strain on sensors of the sensor system in the wind turbine blade. 
     The reading unit may comprise a second sensor port, for example such that the second end of the second optical sensor fiber can be optically coupled to the second sensor port for reading sensor signals of optical sensors in the sensor system. This configuration in combination with optical coupling of the second end of the first optical sensor fiber and the first end of the second optical sensor fiber enables monitoring of sensors even in case of a damage at a single point in the optical path formed by the first optical sensor fiber and the second optical sensor fiber. 
     The temperature may affect operation and characteristics of sensors, i.e. the sensor signals from the optical sensors may depend on the temperature. A temperature-insensitive determination of bending moment may be desired. 
     Accordingly, the sensor system may comprise a first temperature sensor in the first optical sensor fiber and/or a second temperature sensor in the second optical sensor fiber. A fiber Bragg grating (FBG) of an optical fiber may be arranged in the wind turbine blade such that no changes in strain or pressure are applied to the FBG. Thereby the FBG may function as a temperature sensor. 
     The first core diameter of the first optical sensor fiber may be equal to the second core diameter of the second optical sensor fiber. 
     The sensor system may comprise a beam splitting/combining unit having a first, second and third port, wherein the first port is optically coupled to the first end of the first optical sensor fiber and the second port is optically coupled to the second end of the second optical sensor fiber, such that sensor signals from the first optical sensor fiber are combined with sensor signals from the second optical sensor fiber on the third port connected to a reading unit. 
     The processing unit may be configured to compensate for temperature variation, i.e. estimation of bending moments may comprise applying a compensation factor to the sensor signals, the compensation factor being based on one or more temperature signals from the first temperature sensor and/or the second temperature sensor. 
     Optical sensors are preferred in order to reduce or eliminate the damages of lightning strike. 
     The first optical sensor fiber may comprise a first end connector part and/or a second end connector part for connecting the first end and second end, respectively, to a reading unit, a blade monitoring system and/or other optical fibers. 
     The second optical sensor fiber may comprise a first end connector part and/or a second end connector part for connecting the first end and second end, respectively, to a reading unit, a blade monitoring system and/or other optical fibers. 
     The connector parts may be an E2000 connector. 
     In the method for estimating bending moment of a wind turbine blade, estimating a bending moment at a first estimation position may be carried out assuming a zero bending moment at the tip end of the wind turbine blade. 
     The distance between the first sensor position and the first estimation position along the longitudinal axis may be at least 1 m, such as at least 3 m, preferably in the range from 3 m to about 12 m. 
     During operation of a wind turbine, information on loads on the wind turbine blade may be of interest from a turbine control viewpoint. Accordingly, information on root moments of wind turbine blade may be a desired parameter. The first estimation position may be any position along the longitudinal axis, such as at the root end of the wind turbine blade. 
     The first bending moment may have a primary component about a first axis perpendicular to the longitudinal axis and/or a secondary component about a second axis perpendicular to the longitudinal axis. 
     The method may comprise obtaining a second sensor set signal indicative of a second bending moment at a second sensor position along the longitudinal axis and estimating a bending moment at a first estimation position may be based on the second sensor set signal. 
     In the method, the distance between the first sensor position and the second sensor position along the longitudinal axis may be at least 1 m. 
     The second bending moment may have a primary component about a first axis perpendicular to the longitudinal axis and a secondary component about a second axis perpendicular to the longitudinal axis. 
     The first axis and the second axis may be perpendicular or form an angle, e.g. the smallest angle may be in the range from 75° to about 90°. 
     The first axis may be perpendicular to the primary sensor axis. The second axis may be perpendicular to the secondary sensor axis. 
     In the method and in the blade monitoring system, estimating a bending moment may comprise estimating a primary component M X,est  about a first axis perpendicular to the longitudinal axis and/or a secondary component M Y,est  about a second axis perpendicular to the longitudinal axis at a first estimation position and/or at a second estimation position along the longitudinal axis. Estimating a bending moment may comprise curve fitting. 
     In the method and in the blade monitoring system, the bending moment M est,1  or components thereof M X,est,1  and/or M Y,est,1  at the first estimation position may be estimated by using a first approximation function from the tip end to the first sensor position and a second approximation function from the first sensor position to the first estimation position. The second approximation function may be based on the first approximation function. Different approximation functions may be employed for primary and secondary components, i.e. a first and second primary approximation function may be employed for the primary component and a first and second secondary approximation function may be employed for the secondary component 
     The first approximation function may be selected from a cubic spline function and a polynomial function. The polynomial function may be a first order, a second order, a third order, a fourth or higher order polynomial function. The second approximation function may be a linear interpolation. 
     The first sensor position may be located in the transition region or the airfoil region of the wind turbine blade. 
     The method may comprise transmitting the estimated bending moment or components thereof to a control system of a wind turbine, e.g. a blade monitoring system, pitch control system, a wind turbine controller, a wind park controller, alarm system or the like. 
     The blade monitoring system may be configured for implementing one or more parts or steps of the method described herein. The blade monitoring system enables determination and estimation of bending moments or components thereof applied to one or more wind turbine blades of a wind turbine, e.g. during operation of the wind turbine. Thus, the invention allows for a control system of the wind turbine to adjust operational parameters such as pitch angles in order to optimize power production and lifetime of the wind turbine and wind turbine blades. 
     The blade monitoring system may comprise a memory unit connected to the processing unit, the memory unit being configured for storing sensor system parameters of the sensor system. The processing unit may be configured for estimating a bending moment based on sensor system parameters stored in the memory unit, e.g. sensor system parameters derived during calibration or manufacture of the wind turbine blade. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which: 
         FIG. 1  illustrates a wind turbine, 
         FIG. 2  illustrates a wind turbine blade, 
         FIG. 3  is a cross section of a wind turbine blade, 
         FIG. 4  illustrates different views of a wind turbine blade, 
         FIG. 5  is a flow diagram of an exemplary method according to the invention, 
         FIG. 6  illustrates a cross section of a wind turbine blade, 
         FIG. 7  illustrates a cross section of a wind turbine blade, 
         FIG. 8  illustrates a cross section of a wind turbine blade, 
         FIG. 9  illustrates a cross section of a wind turbine blade, 
         FIG. 10  illustrates a cross section of a wind turbine blade, 
         FIG. 11  illustrates a wind turbine blade with sensor system according to the invention, 
         FIG. 12  schematically illustrates a first optical sensor fiber and a patch optical fiber, 
         FIG. 13  illustrates a blade monitoring system, 
         FIG. 14  illustrates a blade monitoring system, and 
         FIG. 15  illustrates estimated bending moment with curve fitting. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures are schematic and simplified for clarity, and they merely show details which are essential to the understanding of the invention, while other details have been left out. Throughout, the same reference numerals are used for identical or corresponding parts. 
       FIG. 1  illustrates a conventional modern upwind wind turbine according to the so-called “Danish concept” with a tower  4 , a nacelle  6  and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub  8  and three blades  10  extending radially from the hub  8 , each having a blade root  16  nearest the hub and a blade tip  14  furthest from the hub  8 . The rotor has a radius denoted R. 
       FIG. 2  shows a schematic view of a first embodiment of a wind turbine blade  10  according to the invention. The wind turbine blade  10  has the shape of a conventional wind turbine blade and comprises a root region  30  closest to the hub, a profiled or an airfoil region  34  furthest away from the hub and a transition region  32  between the root region  30  and the airfoil region  34 . The blade  10  comprises a leading edge  18  facing the direction of rotation of the blade  10 , when the blade is mounted on the hub, and a trailing edge  20  facing the opposite direction of the leading edge  18 . 
     The airfoil region  34  (also called the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region  30  due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade  10  to the hub. The diameter (or the chord) of the root region  30  may be constant along the entire root area  30 . The transition region  32  has a transitional profile gradually changing from the circular or elliptical shape of the root region  30  to the airfoil profile of the airfoil region  34 . The chord length of the transition region  32  typically increases with increasing distance r from the hub. The airfoil region  34  has an airfoil profile with a chord extending between the leading edge  18  and the trailing edge  20  of the blade  10 . The width of the chord in the airfoil region decreases with increasing distance r from the hub. 
     A shoulder  40  of the blade  10  is defined as the position, where the blade  10  has its largest chord length. The shoulder  40  is typically provided at the boundary between the transition region  32  and the airfoil region  34 . 
     The blade  10  has different airfoil profiles  41 ,  42 ,  43 ,  44 ,  45 ,  46  along the longitudinal axis of the blade. 
     As illustrated in  FIG. 4 , the wind turbine blade  10  comprises at least one sensor set including a first sensor set positioned at a first position along the longitudinal axis. The first sensor set comprises a first primary sensor  47 A and optionally a first secondary sensor  47 B positioned at a first distance d 1  from the root end. The sensor  47 A and the sensor  47 B may be displaced a distance d 1,12  along the longitudinal direction. The distance d 1,12  may be less than 1 m. 
     Optionally, the wind turbine blade  10  comprises a second sensor set positioned at a second position along the longitudinal axis. The second sensor set comprises a second primary sensor  48 A and optionally a second secondary sensor  48 B positioned at a second distance d 2  from the root end. The sensor  48 A and the sensor  48 B may be displaced a distance d 2,12  along the longitudinal direction. The distance d 2,12  may be less than 1 m. 
     In the wind turbine blade  10 , the sensors are optical sensors in the form of an optical fiber with fiber Bragg gratings embedded in the shell of the wind turbine blade. The sensors of the wind turbine blade may be a part of the same optical fiber and/or be a part of different optical fiber sections coupled by one or more optical connectors. 
     It should be noted that the chords of different sections of the blade normally do not lie in a common plane, since the blade may be twisted and/or curved (i.e. pre-bent), thus providing the chord plane with a correspondingly twisted and/or curved course, this being most often the case in order to compensate for the local velocity of the blade being dependent on the radius from the hub. 
     Table 1 below illustrates different suitable combinations of sensor positions (distances from the root end), optionally dependent on the length of the wind turbine blade. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Sensor positions. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 d 1 /m 
                 d 2 */m 
                 d 3 */m 
                 d 4 */m 
                 d 5 */m 
                 L/m 
               
               
                   
                   
               
               
                   
                 2-L  
                 3-L 
                 10-L  
                 20-L 
                 30-L 
                 ≧40 
               
               
                   
                 2-20 
                  3-40 
                 10-45 
                   
                   
                   
               
               
                   
                 4-15 
                  5-30 
                   
                   
                   
                   
               
               
                   
                 4-10 
                 10-30 
                   
                   
                   
                   
               
               
                   
                 6-10 
                 10-15 
                 15-20 
                   
                   
                   
               
               
                   
                 5 
                 10 
                   
                   
                   
                   
               
               
                   
                 8 
                 12 
                 23 
                 40 
                 50 
                 ≧60 
               
               
                   
                 1.5*d root   
                 0.9*d s   
                   
                   
                   
                   
               
               
                   
                 8 
                 d 1  + d 12   
                   
                   
                   
                   
               
               
                   
                 8 
                 12 
                   
                   
                   
                   
               
               
                   
                 8 
                 23 
                 40 
               
               
                   
                   
               
               
                   
                 *if present 
               
            
           
         
       
     
     Sensor position configuration may depend on the number of sensor sets available and estimation position(s). A sensor position near the root end may be desirable, however a sensor position too near the root end is not desirable due to contributions or noise from the pitch bearings. 
       FIGS. 3 and 4  depict parameters, which may be used to explain the geometry of the wind turbine blade according to the invention. 
       FIG. 3  shows a schematic view of an airfoil profile  50  of a typical blade of a wind turbine depicted with the various parameters, which are typically used to define the geometrical shape of an airfoil. The airfoil profile  50  has a pressure side  52  and a suction side  54 , which during use—i.e. during rotation of the rotor—normally face towards the windward (or upwind) side and the leeward (or downwind) side, respectively. The airfoil  50  has a chord  60  with a chord length c extending between a leading edge  56  and a trailing edge  58  of the blade. The airfoil  50  has a thickness t, which is defined as the distance between the pressure side  52  and the suction side  54 . The thickness t of the airfoil varies along the chord  60 . The deviation from a symmetrical profile is given by a camber line  62 , which is a median line through the airfoil profile  50 . The median line can be found by drawing inscribed circles from the leading edge  56  to the trailing edge  58 . The median line follows the centres of these inscribed circles and the deviation or distance from the chord  60  is called the camber f. The asymmetry can also be defined by use of parameters called the upper camber (or suction side camber) and lower camber (or pressure side camber), which are defined as the distances from the chord  60  and the suction side  54  and pressure side  52 , respectively. 
     Airfoil profiles are often characterised by the following parameters: the chord length c, the maximum camber f, the position d f  of the maximum camber f, the maximum airfoil thickness t, which is the largest diameter of the inscribed circles along the median camber line  62 , the position d t  of the maximum thickness t, and a nose radius (not shown). These parameters are typically defined as ratios to the chord length c. Thus, a local relative blade thickness t/c is given as the ratio between the local maximum thickness t and the local chord length c. Further, the position d p  of the maximum pressure side camber may be used as a design parameter, and of course also the position of the maximum suction side camber. 
       FIG. 4  shows other geometric parameters of the blade. The blade has a total blade length L. As shown in  FIG. 3 , the root end is located at position r=0, and the tip end located at r=L. The shoulder  40  of the blade is located at a position r=d s , and has a shoulder width W, which equals the chord length at the shoulder  40 . The diameter of the root is defined as d root . The curvature of the trailing edge of the blade in the transition region may be defined by two parameters, viz. a minimum outer curvature radius r o  and a minimum inner curvature radius r i , which are defined as the minimum curvature radius of the trailing edge, seen from the outside (or behind the trailing edge), and the minimum curvature radius, seen from the inside (or in front of the trailing edge), respectively. Further, the blade is provided with a pre-bend, which is defined as Δy, which corresponds to the out of plane deflection from a pitch axis  22  of the blade. 
       FIG. 5  illustrates an exemplary method according to the present invention. The method  100  comprises obtaining  102  a first sensor set signal indicative of a first bending moment at a first sensor position along the longitudinal axis of the wind turbine blade. Further, the method  100  comprises estimating  104  a bending moment at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis. The method may be employed on a wind turbine blade as described herein. Optionally, the method  100  comprises obtaining  106  a second sensor set signal indicative of a second bending moment at a second sensor position along the longitudinal axis, and estimating  104  a bending moment is based on the second sensor set signal. Sensor signals may be obtained serially and/or in parallel. 
     The first sensor set signal comprises a first primary sensor signal from a first primary sensor ( 47 A) and a first secondary sensor signal from a first secondary sensor ( 47 B). The first primary sensor signal indicates a primary component M x,1  of the first bending moment and the first secondary sensor signal indicates a secondary component M y,1  of the first bending moment. 
       FIG. 6  and  FIG. 7  are cross sections illustrating examples of sensor set positioning on a wind turbine blade. 
     In  FIG. 6 , the wind turbine blade  10 ′ comprises a first sensor set with a first primary sensor  47 A and a second primary sensor  47 B at a first distance d 1  from the root. The first primary sensor  47 A lies on a first primary sensor axis  74  extending through the elastic center  70  of the blade cross section (transverse plane at first distance d 1 ). The first secondary sensor  47 B lies on a first secondary sensor axis  76  extending through the elastic center  70  of the blade cross section. In  FIG. 6 , the elastic center lies on the chord  60  and thus the main axis  72  coincide with the chord  60 . The angle α 1  between the two sensor axes  74  and  76  is 90°. The angle β 1  between the main axis  72  and the first primary sensor axis  74  is 90°. 
     In  FIG. 7 , the wind turbine blade  10 ″ comprises a first sensor set with a first primary sensor  47 A and a second primary sensor  47 B at a first distance d 1  from the root. The first primary sensor  47 A lies on a first primary sensor axis  74  extending through the elastic center  70  of the blade cross section (transverse plane at first distance d 1 ). The first secondary sensor  47 B lies on a first secondary sensor axis  76  extending through the elastic center  70  of the blade cross section. In  FIG. 7 , the elastic center lies on the chord  60  and thus the main axis  72  coincide with the chord  60 . The angle α 1  between the two sensor axes  74  and  76  is 90°. The angle β 1  between the main axis  72  and the first primary sensor axis  74  is 75°. 
     In  FIG. 6  and  FIG. 7 , the sensors are embedded in the shell body  78  of the wind turbine blade. 
       FIG. 8  is a cross section illustrating an example of sensor set positioning on a wind turbine blade. The wind turbine blade  10 ′″ comprises a beam  80  attached to the shell body  78  and the second primary sensor  47 B is attached to the beam  80  nearest the leading edge  56 . 
       FIG. 9  is a cross section illustrating an example of first sensor set positioning on a wind turbine blade at a first distance d 1 . The wind turbine blade comprises a shell body  78  and the sensors  47 A,  47 B are mounted on the inner surface of the shell body  78 . The angle α 1  between sensor axes  74  and  76  is 90°. 
       FIG. 10  is a cross section illustrating an example of second sensor set positioning on a wind turbine blade at a second distance d 2 . The wind turbine blade comprises a shell body  78  and the sensors  48 A,  48 B are mounted on the inner surface of the shell body  78 . The angle α 2  between sensor axes  74  and  76  is 90°. 
       FIG. 11  illustrates a part of a wind turbine. The wind turbine comprises a hub  8  from which the blades whereof a first wind turbine blade  10  is shown extend substantially in a radial direction when mounted to the hub  8 . The wind turbine blade  10  comprises a sensor system  82  with an optical path comprising a first optical sensor fiber  84 , a second optical sensor fiber  86  and a patch optical fiber  88 . Optical connector  90  couples the first optical sensor fiber  84  and the patch optical fiber  88  and optical connector  90 ′ couples the second optical sensor fiber  86  to the patch optical fiber  88 . The optical fibers  84 ,  86 ,  88  are SM1500 (4.2/125) fibers. The first optical sensor fiber comprises first primary sensor  47 A and second primary sensor  48 A in the form of fiber Bragg gratings and optionally first temperature sensor  98 A. The second optical sensor fiber comprises first secondary sensor  47 B and second secondary sensor  48 B in the form of fiber Bragg gratings and optionally second temperature sensor  98 B. The first end  85  of the first optical sensor fiber  84  is coupled to a reading unit  92  for reading sensor signals from the sensor system  82 . The reading unit  92  provides wavelength values of the sensor signals to a blade monitoring system  94  via a data cable  96 . The blade monitoring system is configured for estimating components of the bending moment at the root end of the wind turbine blade based on the sensor signals and configured for transmitting the estimated bending moment to a turbine controller (not shown). The second end  85 ′ of the first optical connector  84  is optically coupled to the first end  89  of the patch optical fiber  88  in connector or connector assembly  90 . The second end  89 ′ of the patch optical fiber  88  is optically coupled to the first end  87  of the second optical sensor fiber  86  in connector or connector assembly  90 ′. 
       FIG. 12  schematically illustrates the optical connectors or connector assemblies  90  between the first optical sensor fiber  84  and the patch fiber  88 . The first optical sensor fiber  84  comprises a first core  130  with a first core diameter d core,1 . The patch optical fiber  88  comprises a patch core  132  with a patch core diameter d core,p =d core,1 =4.2 μm. Fiber cladding material and sheet  134 ,  136  protect the cores  130 ,  131 ,  132 . The first optical sensor fiber  84  comprises a first end connector part at the first end (not shown) and a second end connector part  138  (e.g. female E2000 connector) at the second end  85 ′, and the patch optical fiber comprises a first end connector part  140  (e.g. male E2000 connector) for connecting the first optical sensor fiber  84  and the patch optical fiber  88 . The connector assembly  90 ′ is formed in the same way as the connector assembly  90  indicated by the reference numbers. 
     The second optical sensor fiber  86  includes a second core  131  with a second core diameter d core,2 =d core,p , wherein the second optical sensor fiber extends from a first end to a second end and comprising at least one sensor. The second optical sensor fiber  86  comprises a first end connector part  138 ′ (e.g. female E2000 connector) at the first end  87  and a second end connector part (not shown) at the second end, and the patch optical fiber comprises a second end connector part  140 ′ (e.g. male E2000 connector) for connecting the second optical sensor fiber  86  and the patch optical fiber  88 . 
       FIG. 13  schematically illustrates a blade monitoring system  94 . The blade monitoring system  94  comprises a housing  95  accommodating a processing unit  150  connected to an interface  152  and a memory unit  154  via connections  155 ,  155 ′, respectively. The interface  152  comprises a first connector port  156  and a second connector port  158 . The first connector port  156  is configured for connection to a reading unit for receiving data of sensor signals from a sensor system of a wind turbine blade. The second connector port  158  is configured for connection to a turbine controller for transmitting and/or sending data and/or control/alarm signals to a turbine controller. 
     The processing unit  150  is configured for receiving a first sensor set signal indicative of a first bending moment at a first sensor position of a wind turbine blade extending along a longitudinal axis from a root end to a tip end via the first connector port  156 . Further, the processing unit  150  is configured for estimating a bending moment or components thereof at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis. 
       FIG. 14  schematically illustrates a blade monitoring system  94 ′ wherein a reading unit  92  is integrated in the blade monitoring system and connected to the processing unit  150  via connection  155 ′″. The interface  152  comprises a first connector port  156  in the form of a first sensor port  160  for coupling the sensor system, e.g. the first optical sensor fiber  84 , (optionally via a patch optical fiber) to the reading unit  92  of the blade monitoring system  94 ′. 
       FIG. 15  schematically illustrates estimation of a primary component M X,est,1  of bending moment at the root end (first estimation position) of a wind turbine blade having L=53.2 m. The first distance d 1  is 7 m and the second distance d 2  is 10.5 m. The primary component M X,est,1  is estimated based on a first primary sensor signal S 11  from a first primary sensor at d 1  and a second primary sensor signal S 21  from a second primary sensor at d 2 . 
     It has been shown that using cubic spline functions from the first primary sensor to the tip end of the wind turbine blade and then using the gradient or derivative of the bending moment at the first distance to perform a linear extrapolation from d 1  to the root end may be preferred. Gradients of the bending moment at the tip end are zero. 
     Further, the bending moment gradient at d 1  may be estimated based on the bending moments at d 1  and d 2 , e.g. with a backward Euler method. A large distance between the first and second sensor sets may not be desirable. 
     The estimation as illustrated in  FIG. 15  comprises the following steps using measured bending moment components at d 1  and d 2 :
         A first cubic spline function with “correct” boundary conditions using the two measurement points and the tip end point where the bending moment is zero is fitted used to perform the interpolation of a point near the end of the wind turbine blade. Here “correct” means that the first derivatives at the two ends of the interval are correct,   the bending moment gradient at d 1  is estimated using the measured sensor data transformed to bending moments at d 1  (M X,1 ) and d 2  (M X,2 ) with a backward Euler method, and the derivative at the tip end is zero. The three points, i.e. d 1 , d 2  and the tip end point are denoted points A in  FIG. 15 . The interpolated point near the end of the wind turbine blade is denoted point B.   Then a second spline function, covering the interval from d 1  to the tip end is constructed using a “not-a-knot” method where the points A and B are used.   Then, the second spline function is extended to the root end with a linear extrapolation denoted app. A′ between d 1  and the root end.       

     The “not-a-knot” method means that the third order derivative at the second and second last point of the domain used for interpolation is the same when looking from each side of the point. 
       FIG. 15  illustrates estimations of the bending moment. In the graph, app. A′ represents sensor positions at d 1 =7 m and d 2 =10.5 m, and app. B′ represents sensor positions at d 1 =7 m and d 2 =20 m. As can be seen on  FIG. 15 , app A′ provides a better estimation of primary component near the root end while app B′ provides a better estimation between 10 m and 50 m from the root end. 
     It should be noted that in addition to the exemplary embodiments of the invention shown in the accompanying drawings, the invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. 
     LIST OF REFERENCE NUMERALS 
     
         
           2  wind turbine 
           4  tower 
           6  nacelle 
           8  hub 
           10 ,  10 ′,  10 ″, 
           10 ′″ wind turbine blade 
           15  blade tip 
           16  blade root 
           18  leading edge 
           20  trailing edge 
           22  pitch axis 
           30  root region 
           32  transition region 
           34  airfoil region 
           40  shoulder 
           41 ,  42 ,  43 , 
           44 ,  45 ,  46  airfoil profile 
           47 A first primary sensor 
           47 B first secondary sensor 
           48 A second primary sensor 
           48 B second secondary sensor 
           50  airfoil profile 
           52  pressure side 
           54  suction side 
           56  leading edge 
           58  trailing edge 
           60  chord 
           62  camber line/median line 
           70  elastic center 
           72  main axis 
           74  primary sensor axis 
           76  secondary sensor axis 
           78  shell body 
           80  beam 
           82  sensor system 
           84  first optical sensor fiber 
           85  first end of first optical sensor fiber 
           85 ′ second end of first optical sensor fiber 
           86  second optical sensor fiber 
           87  first end of second optical sensor fiber 
           87 ′ second end of second optical sensor fiber 
           88  patch optical fiber 
           89  first end of patch optical fiber 
           89 ′ second end of patch optical fiber 
           90 ,  90 ′ optical connector 
           92  reading unit 
           94  blade monitoring system 
           94 ′ blade monitoring system 
           95  housing 
           96  data cable 
           98 A first temperature sensor 
           98 B second temperature sensor 
           99  beam splitting/combining unit 
           130  first core 
           131  second core 
           132  patch core 
           134  fiber cladding material and sheet 
           136  fiber cladding material and sheet 
           138  second end connector part of first optical sensor fiber 
           138 ′ first end connector part of second optical sensor fiber 
           140  first end connector part of patch optical fiber 
           140 ′ second end connector part of patch optical fiber 
           150  processing unit 
           152  interface 
           154  memory unit 
           155 ,  155 ′, 155 ″, 
           155 ′″,  155 ″″ connection 
           156  first connector port 
           158  second connector port 
           160  sensor port 
         c chord length 
         d t  position of maximum thickness 
         d f  position of maximum camber 
         d p  position of maximum pressure side camber 
         d s  shoulder distance 
         d root  root diameter 
         f camber 
         L blade length 
         P power output 
         r local radius, radial distance from blade root 
         t thickness 
         v w  wind speed 
         θ twist, pitch 
         Δy prebend 
         α 1  angle between first primary sensor axis and first secondary sensor axis 
         α 2  angle between second primary sensor axis and second secondary sensor axis. 
         β 1  angle between first primary sensor axis and main axis 
         β 2  angle between second primary sensor axis and main axis 
         d core,1  first core diameter 
         d core,2  second core diameter 
         d core,p  patch core diameter