Abstract:
This disclosure relates to a device for optically measuring the curvature of a rotor blade ( 18 ) of a wind power plant ( 1 ). The rotor blade is fixed at the blade root ( 23 ) to a rotor hub ( 8 ) or a rotor ( 6 ). The device includes at least one light-emitting external marking ( 33 ) fixed to the rotor blade ( 18 ) at a distance from the blade root ( 23 ), at least one camera ( 27 ) fixed to the rotor ( 6 ) for capturing the light ( 35 ) emitted from the external marking ( 33 ). The device provides first location information as a function of the relative position of the camera ( 27 ) with respect to the external marking ( 33 ). An evaluating device ( 37 ) is coupled to the camera ( 27 ), and a position detection means detects the relative location of the camera ( 27 ) with respect to the blade root ( 23 ) and provides second location information as a function of said relative location. The evaluating device ( 37 ) determines at least one variable characterizing the curvature of the rotor blade ( 18 ) while evaluating the position information.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 U.S. National Stage of International Application No. PCT/EP2011/056807, filed Apr. 28, 2011, and claims priority to German Patent Application No. 10 2010 017 749.0, filed Jul. 5, 2010, the disclosures of which are herein incorporated by reference in their entirety. 
     FIELD 
     The invention relates to a device for optically measuring the bending of a rotor blade of a wind turbine, which is secured to a rotor hub of a rotor with the blade root thereof, having at least one external marking which is secured to the rotor blade with spacing from the blade root and which emits light, at least one camera which is secured to the rotor and which receives the light emitted by the external marking and which provides first position information which is dependent on the relative position of the camera with respect to the external marking, and an evaluation device which is coupled to the camera. 
     BACKGROUND 
     Owing to the increase in the length and the flexibility of rotor blades in wind turbines, the control units thereof are increasingly intended to reduce loads and oscillations of the wind turbine which occur during operation. Loads of the rotor blades are primarily caused by aerodynamic effects. However, a direct measurement of the aerodynamic loads is not possible. Since the blade bending actions and the blade loads are closely related to each other, information relating to the blade bending actions constitutes good initial information for a control system of a wind turbine which such information can reduce undesirable, cyclical blade loads which can be attributed, for example, to incorrect orientation of the rotor shaft with respect to the wind direction or to vertical or horizontal wind shears. 
     In document NREL/TP-500-39253, published in January 2006, a method for measuring a rotor blade bending action is described according to which an infrared camera arranged in the vicinity of the blade root having an infrared radiation source and reflector strips secured to the rotor blade are used. The reflector strips which are illuminated by means of the infrared radiation source reflect the infrared radiation back to the camera so that the blade bending can be calculated by evaluating the image taken by the camera. 
     WO 2010/054661 A2 discloses a method for monitoring the loading of rotor blades of a wind turbine, which comprises a gondola and a rotor hub which is rotatably supported thereon by means of a rotor shaft and to which at least one rotor blade is secured. The rotor blade comprises a reflector whose position changes in accordance with the loading of the rotor blade. A radiation source for electromagnetic radiation and a radiation receiver are arranged in the gondola, a radiation path being provided from the radiation source to the reflector and back therefrom to the radiation receiver. By means of a monitoring device, a redirection of the loaded rotor blade can be determined on the basis of a modification of the radiation received from the radiation receiver. 
     In the above-mentioned methods, oscillations of the wind turbine lead to a measurement error since these oscillations cause a movement of the camera. Consequently, the camera movement of a movement of the reflector image caused by a blade bending is superimposed on the photo-sensitive surface of the camera. Furthermore, a measurement error is caused by the fact that the camera is displaced owing to an irreversible deformation of the camera retention member and/or the rotor blade. An example of such a deformation is the change of an originally circular cross-section of the rotor blade close to the blade root into an oval cross-section, which can be attributed to material creep under load. 
     Conventional camera-based systems for measuring the blade bending at one or more locations along the rotor blade use a light source which illuminates one or more reflectors, which are arranged at predetermined positions along the blade. The light reflected by the reflectors is received by means of a digital camera which comprises a photo-sensitive surface for this purpose. By evaluating the position of the reflector images on the photo-sensitive surface, the displacement of the reflectors relative to the camera position is calculated and the blade bending is derived therefrom. In this instance, in order to maximise the measurement resolution in the camera, a lens is used with such a narrow viewing angle that a maximum displacement of the reflector image on the photo-sensitive surface is achieved when the reflector (or the reflectors) experience(s) a maximum displacement to be anticipated under blade loading. Therefore, the region of the blade root of the rotor blade is not generally located in the field of vision of the camera. 
     The following problems are in particular connected with conventional camera-based systems for measuring blade bending:
         Owing to oscillations of the camera, the reflector image moves on the photo-sensitive surface of the camera so that the system falsely interprets and signals this movement as a bending action of the rotor blade.   The position of the camera relative to the blade root changes in an irreversible manner with increasing time, for example, owing to a deformation of the rotor blade in the region of the camera position owing to material creep (for example, oval cross-section of the blade) so that a displacement of the reflector image on the photo-sensitive surface is brought about even without blade bending. Consequently, there is produced an error in the measurement of the blade bending.       

     SUMMARY 
     Based on the above, an object of the invention is to develop a device of the type mentioned in the introduction in such a manner that errors in determining the blade bending owing to camera vibrations and/or an irreversible displacement of the camera can be prevented or at least reduced. 
     This object is achieved according to the invention. The device according to the invention for optically measuring the bending of a rotor blade of a wind turbine, which is secured to a rotor hub of a rotor with the blade root thereof comprises:
         at least one light-emitting external marking which is secured to the rotor blade with spacing from the blade root,   at least a (first) camera which is secured to the rotor and which receives the light emitted by the external marking and which provides first position information which is dependent on the relative position of the camera with respect to the external marking,   an evaluation device which is coupled to the camera,   a position detection means which detects the relative position of the camera with respect to the blade root and which provides second position information which is dependent on this position,
 
the evaluation device determining the bending of the rotor blade and/or at least one variable which characterises this bending by evaluating the position information, that is to say, in particular both items of position information.
       

     Since the first position information characterises the relative position of the camera with respect to the external marking and the second position information characterises the relative position of the camera with respect to the blade root, the bending of the rotor blade and/or the at least one variable which characterises this bending can be determined independently of the position and/or the orientation of the camera. Consequently, the errors known from the prior art can be clearly reduced when determining the bending of the rotor blade (blade bending). 
     The evaluation device is preferably coupled to the position detection means. Furthermore, the position detection means may be provided separately from the camera or may comprise it so that, in the latter case, the camera forms part of the position detection means. 
     The evaluation device preferably derives the bending of the rotor blade from the at least one variable characterising the bending of the rotor blade. According to a development of the invention, the at least one variable characterising the bending of the rotor blade corresponds to the bending of the rotor blade. 
     According to an embodiment of the invention, there are secured to the rotor blade with spacing from the blade root a plurality of light-emitting external markings which preferably have a different spacing with respect to the blade root, the camera receiving the light emitted by the external markings and the first position information provided by the camera being dependent on the relative position of the camera with respect to the external markings. Owing to the use of a plurality of external markings, the measurement precision can in particular be increased. Furthermore, owing to the arrangement of the external markings with different spacings with respect to the blade root, a path of the blade bending along the blade axis can be determined. The first position information characterises in particular the relative position of the camera with respect to the external markings. 
     As the at least one variable which characterises the bending of the rotor blade, the evaluation device determines in particular the relative position of the blade root with respect to the external marking(s). In this instance, the relative position of the blade root with respect to the external marking(s) constitutes in particular position information which is independent of the position of the camera and which preferably characterises or describes the bending of the rotor blade. 
     The evaluation device calculates, in particular by evaluating the second position information, the relative position of the camera with respect to the blade root. Furthermore, the evaluation device, in particular by evaluating the first position information, calculates the relative position of the camera with respect to the external marking(s). Preferably, the evaluation device determines, from the relative position of the camera with respect to the blade root and from the relative position of the camera with respect to the external marking(s), the relative position of the blade root with respect to the external marking(s). 
     Preferably, a location of the blade root or a location in the region of the blade root forms a reference point which represents the position of the blade root. This reference point is preferably located in the blade axis of the rotor blade and/or preferably at the end of the rotor blade facing the hub. When referencing the position of the blade root, this position can consequently be replaced by the position of the reference point. It can thereby be taken into account that the blade root generally has an expansion. According to a development of the invention, the evaluation device determines, as the at least one variable which characterises the bending of the rotor blade, the relative position of the reference point with respect to the external marking(s). 
     The external marking(s) is/are preferably constructed as a light source or as a reflector which is illuminated by a light source. According to a development of the invention, the external marking(s) is/are each formed by the end of a light-guiding optical waveguide, such as, for example, a light-guiding optical fibre, in particular glass fibre, into which light is coupled preferably by means of one or more light sources. The light source or the light sources are, for example, formed by one or more light-emitting diodes. The reflector(s) is/are preferably constructed in a retro-reflective manner and can consequently also be referred to as retro-reflectors. 
     The camera is in particular adapted to the light with regard to the spectral sensitivity thereof. The light may be in the visible spectrum. Preferably, however, it is infrared light. 
     Preferably, the camera takes at least one image of the external marking(s) and provides this, in particular in the form of image data, as the first position information for the evaluation device. Preferably, the evaluation device determines from the image data, using at least one image evaluation method, the relative position of the camera with respect to the external marking(s). 
     The camera is secured to or in the rotor in particular by means of a retention member. Preferably, the camera is secured to the rotor at or in the region of the blade root by means of the retention member. For example, the camera is secured to a partition wall in the rotor blade in particular by means of the retention member. Such a partition wall serves, for example, to prevent service personnel and/or objects from falling into the rotor blade. There may be provided in the separation wall one or more through-openings, through which one or more optical signals or light beams extend which are used in connection with the device according to the invention. 
     According to an embodiment of the invention, the position detection means comprises at least one or more tension or expansion sensors which are secured to the retention member and by means of which a shape change of the retention member is detected. In this instance, the second position information provided by the position detection means is dependent on the detected shape change. Owing to the detection of the shape change of the retention member, the relative position of the camera with respect to the blade root can be determined. 
     According to an embodiment of the invention, the position detection means comprises at least one other camera which is arranged on or in the region of the blade root or in the rotor hub and by means of which a position change of the (first) camera which is secured to the rotor blade is detected. In this instance, the second position information provided by the position detection means is dependent on the relative position of the (first) camera with respect to the other camera. The relative position of the (first) camera with respect to the blade root can thereby be determined. The other camera preferably forms a portion of the position detection means. The (first) camera is in particular arranged in the field of vision of the other camera. The other camera is secured, for example, to the rotor blade, the blade bearing or the rotor hub. The other camera is preferably rigidly secured to the rotor hub, in particular to a portion of the blade bearing that is rigidly connected to the rotor hub. Consequently, using the other camera, a rotation of the (first) camera about the blade axis of the rotor blade relative to the rotor hub and/or the other camera can additionally be detected. Such a rotation occurs, for example, when the rotor blade is rotated about the blade axis thereof relative to the rotor hub (which is also referred to as “pitching”). In particular using the other camera, a rotation of the rotor blade about the blade axis thereof relative to the rotor hub can further be detected. Preferably, using the evaluation device, a rotation of the rotor blade about the blade axis thereof relative to the rotor hub is further determined. 
     The other camera preferably takes at least one image of the (first) camera and provides this, in particular in the form of image data, as the second position information of the evaluation device. Preferably, the evaluation device calculates, using at least one image evaluation method, the relative position of the (first) camera to the other camera from the image data. However, the other camera is preferably arranged on or in the region of the blade root or in the rotor hub. Consequently, the evaluation device calculates from the image data using at least one image evaluation method in particular the relative position of the (first) camera to the blade root. 
     According to an embodiment of the invention, the position detection means comprises at least one internal marking which is secured to the rotor on or in the region of the blade root and which emits light, the (first) camera or a second camera which is fixed in position relative thereto receiving the light emitted by the internal marking. In this instance, the second position information provided by the position detection means is dependent on the relative position of the (first) camera with respect to the internal marking. The relative position of the (first) camera with respect to the blade root can thereby be determined. The (first) camera or the second camera in this instance preferably forms the position detection means or a portion of the position detection means. 
     According to a development of the invention, the position detection means comprises a plurality of internal markings which are secured to the rotor on or in the region of the blade root and which emit light, and which are preferably positioned at different locations, the (first) camera or the second camera which is fixed in position relative thereto receiving the light emitted by the internal markings. In this instance, the second position information provided by the position detection means is dependent on the relative position of the (first) camera with respect to the internal markings. The relative position of the (first) camera with respect to the blade root can thereby be determined. The (first) camera or the second camera in this instance preferably forms the position detection means or a portion of the position detection means. 
     According to a first variant of the invention, the camera is arranged in a region of the rotor hub opposite the rotor blade and is secured thereto. In the first variant, the camera preferably forms a portion of the position detection means. A second camera is not absolutely necessary in this instance. 
     According to a second variant of the invention, there is provided at least one mirror which is securely connected to the camera and which is preferably arranged in the field of vision thereof and by means of which the light emitted by the internal marking(s) is reflected in the direction towards the camera. According to a development of the second variant, there are provided a plurality of mirrors which are connected to the camera and which are preferably arranged in the field of vision thereof and by means of which the light emitted by the internal marking(s) is reflected in the direction towards the camera. In the second variant and/or the development thereof, the camera preferably forms a portion of the position detection means. A second camera is not absolutely necessary in this instance. 
     According to a third variant of the invention, the second camera is provided, the cameras in particular being arranged in close spatial proximity to each other. The cameras are preferably joined together to form a structural unit. In particular, the second camera is connected to the evaluation device. 
     The first, the second and/or the third variant may also be combined with each other. 
     The internal marking(s) is/are preferably each constructed as a light source or as a reflector which is illuminated by a light source. According to a development of the invention, the internal marking(s) is/are each formed by the end of a light-guiding optical waveguide, such as, for example, a light-guiding optical fibre, in particular a glass fibre, into which light is coupled, preferably by means of one or more light sources. The light source or light sources is/are formed, for example, by one or more light-emitting diodes. The reflector(s) is/are preferably constructed in a retro-reflecting manner and can consequently also be referred to as retro-reflectors. 
     Preferably, the (first) camera or the second camera takes at least one image of the internal marking(s) and provides this in particular in the form of image data as the second position information for the evaluation device. Preferably, the evaluation device, using at least one image evaluation method, calculates the relative position of the camera or the second camera with respect to the internal marking(s) from the image data. Since the two cameras are fixed in position relative to each other, using the evaluation device it is possible to derive from the calculated relative position of the second camera with respect to the internal marking(s) the relative position of the camera with respect to the internal marking(s). If the second camera is provided, the image obtained by means of the second camera of the internal marking(s) is preferably a different image from the image which is obtained by means of the camera of the external marking(s). Alternatively, the image obtained by means of the camera of the internal marking(s) and of the external marking(s) is preferably a common image if only one camera is used. 
     According to an embodiment of the invention, the rotor blade comprises a hollow space in which the external marking(s) is/are arranged. The camera is also preferably arranged in the hollow space. Alternatively, the camera may be arranged in the rotor hub. The other or second camera is preferably arranged in the hollow space. Alternatively, the other or second camera may also be arranged in the rotor hub. Preferably, the internal marking(s) is/are arranged in the hollow space. 
     The internal marking(s) is/are preferably secured to the rotor blade, to the blade bearing or to the rotor hub. According to a development of the invention, the internal marking(s) is/are rigidly secured to the rotor hub, in particular to a portion of the blade bearing rigidly connected to the rotor hub. Consequently, using the (first) camera or the second camera, a rotation of the (first) camera and/or the second camera about the blade axis of the rotor blade relative to the rotor hub and/or the internal marking(s) can additionally be detected. Such a rotation occurs, for example, when the rotor blade is rotated about the blade axis thereof relative to the rotor hub. In particular, using the (first) camera or the second camera, a rotation of the rotor blade about the blade axis thereof relative to the rotor blade can additionally be detected. Preferably, using the evaluation device, a rotation of the rotor blade about the blade axis thereof relative to the rotor hub is additionally determined. 
     The invention further relates to a wind turbine having
         a rotor which can be rotated about a rotor shaft by means of wind and which comprises a rotor hub and at least one rotor blade which is secured to the rotor hub and which extends away therefrom in the direction of a blade axis which extends transversely or substantially transversely relative to the rotor shaft,   at least one electrical generator which is mechanically coupled to the rotor and which can be driven thereby, and   a device according to the invention for optically measuring the bending of the rotor blade.       

     The wind turbine according to the invention may be developed in accordance with all the embodiments which have been explained in connection with the device according to the invention. 
     The invention also relates to a method for optically measuring the bending of the rotor blade of a wind turbine by means of a device according to the invention. 
     The invention further relates to the use of a device according to the invention for optically measuring the bending of the rotor blade of a wind turbine. 
     Preferably, the blade bending is determined in the following manner:
         in a first step, the relative position of the camera with respect to the blade root is calculated by evaluating the second position information;   in a second step, the relative position of the camera with respect to the external marking(s) is calculated by evaluating the first position information;   in a third step, the relative position of the blade root with respect to the external marking(s) is determined from the positions calculated in the first step and in the second step.       

     The above-mentioned steps are preferably carried out by means of the evaluation device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention is described below with reference to preferred embodiments and the drawings, in which: 
         FIG. 1  is a schematic view of a wind turbine, 
         FIG. 2  is a schematic and partial illustration of the rotor which can be seen in  FIG. 1  and which has a device according to a first embodiment of the invention, 
         FIG. 3  is a schematic and partial illustration of the rotor having a device according to a second embodiment of the invention, 
         FIG. 4  is a schematic and partial illustration of the rotor having a device according to a third embodiment of the invention, 
         FIG. 5  is a schematic and partial illustration of the rotor having a device according to a fourth embodiment of the invention, 
         FIG. 6  is a schematic and partial illustration of the rotor having a device according to a fifth embodiment of the invention, 
         FIG. 7  is a schematic and partial illustration of the rotor having a device according to a modification of the fifth embodiment, 
         FIG. 8  is a schematic illustration of an image taken by means of the camera, 
         FIG. 9  shows a light source and a reflector of a device according to a variant of the embodiments, and 
         FIG. 10  shows a light source and an optical waveguide of a device according to another variant of the embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a wind turbine  1  can be seen. The wind turbine  1  has a tower  3  which stands on a base  2  being connected to a machine housing  4  at the end thereof facing away from the base  2 . The machine housing  4  comprises a machine carrier  5 , on which a rotor  6  is rotatably supported about a rotor shaft  7  which has a rotor hub  8  and rotor blades  9  and  10  which are connected thereto and which are each rotatable about the blade axis  11  or  12  thereof relative to the rotor hub  8 . Each of the rotor blades  9  and  10  is mechanically coupled to a blade angle adjustment drive  13  or  14 , by means of which the respective rotor blade can be rotated about the associated blade axis. The rotor  6  is mechanically coupled to an electrical generator  16  which is arranged in the machine housing  4  and which is secured to the machine carrier  5 . The rotor  6  is rotated about the rotor shaft  7  thereof by means of wind  15 , the rotation energy of the rotor  6  largely being converted into electrical energy by means of the generator  16 . For controlled operation of the wind turbine  1 , there is provided a wind turbine control system  17  by means of which the blade angle adjustment drives are controlled inter alia. 
       FIG. 2  is a schematic and partial illustration of the rotor  6  when viewed from direction A in  FIG. 1 . This shows a third rotor blade  18  being rotatably supported on the rotor hub  8  about the blade axis  20  thereof by means of a blade bearing  19  (in  FIG. 1 , the rotor blade  18  is concealed by the rotor hub  8 ). In the same manner as the rotor blades  9  and  10 , the rotor blade  18  is also mechanically coupled to a schematically indicated blade angle adjustment drive  58  and can be rotated thereby about the blade axis  20 . The blade bearings  21  and  22  for the rotor blades  9  and  10  are also illustrated. 
       FIG. 2  further shows a device (measurement device)  56  according to the invention which is completely or at least partially integrated in the rotor  6  according to a first embodiment which is explained below. The end of the rotor blade  18  that is secured to the blade bearing  19  is referred to as a blade root  23 , two light-emitting internal markings  24  and  25  being arranged in the region of the blade root  23  and being secured to the rotor blade  18 . Alternatively, the internal markings  24  and  25  may also be secured to the blade bearing  19 . With spacing from the blade root  23 , a camera  27  is arranged in a hollow space  26  of the rotor blade  18  and secured to the rotor blade  18 . There are securely connected to the camera  27  two mirrors  28  and  29  which reflect the light emitted by the internal markings  24  and  25  onto a photo-sensitive surface  30  of the camera  27 . The beam path of the light emitted by the internal marking  24  is designated  31  and the beam path of the light emitted by the internal marking  25  is designated  32 . 
     With spacing from the blade root  23  and the camera  27 , two light-emitting external markings  33  and  34  are arranged in the hollow space  26  and secured to the rotor blade  18 . The light emitted by the external markings  33  and  34  strikes the photo-sensitive surface  30  of the camera  27 , the beam path of the light emitted by the external marking  33  is designated  35  and the beam path of the light emitted by the external marking  34  is designated  36 . 
     The camera  27  is electrically connected to a schematically illustrated evaluation device  37 , which is secured in the rotor blade  18 , in the rotor hub  8  or to another suitable location of the wind turbine  1 . For example, the evaluation device  37  may also be formed by the wind turbine control system  17 . The image  45  taken by the camera  27  (see  FIG. 8 ) is supplied to the evaluation device  37  in the form of electronic image data  38 , the image data  38  comprising first position information which is dependent on the relative position of the camera  27  with respect to the external markings  33  and  34  and second position information which is dependent on the relative position of the camera  27  with respect to the internal markings  24  and  25 . 
     The evaluation device  37  calculates, using at least one image evaluation method, the relative position of the camera  27  with respect to the blade root  23  and the relative position of the camera  27  with respect to the external markings  33  and  34 . From the calculated positions, the evaluation device  37  then determines the relative position of the blade root  23  with respect to the external markings  33  and  34 , this relative position characterising the bending of the rotor blade  18 . 
     In  FIG. 3 , a schematic and partial illustration of the rotor  6  with a measurement device  56  according to a second embodiment of the invention can be seen, features which are similar or identical to the first embodiment being given the same reference numerals as in the first embodiment. In addition to the camera  27 , there is provided in the hollow space  26  a camera  39  having a photo-sensitive surface  40  which the light emitted by the internal markings  24  and  25  strikes. The fields of vision of the cameras extend in this instance in particular in or substantially in mutually opposing directions. Each of the cameras  27  and  39  is electrically connected to the evaluation device  37  and transmits electronic image data  38  or  41  thereto. The image data  38  of the camera  27  comprise first position information which is dependent on the relative position of the camera  27  to the external markings  33  and  34 , the image data  41  supplied by the camera  39  comprising second position information which is dependent on the relative position of the camera  39  with respect to the blade root  23 . The cameras  27  and  39  are fixedly arranged relative to each other and form a compact structural unit. 
     From the image data  41 , the evaluation device  37  calculates, using at least one image evaluation method, the relative position of the camera  39  with respect to the blade root  23 . Since the two cameras are fixed in position relative to each other, this position also characterises the relative position of the camera  27  with respect to the blade root  23  so that the relative position of the camera  27  with respect to the blade root  23  is calculated by means of the evaluation device  37 . Furthermore, by means of the evaluation device  37 , using at least one image evaluation method, the relative position of the camera  27  with respect to the external markings  33  and  34  is calculated from the image data  38 . Subsequently, the evaluation device  37  determines from the calculated positions the relative position of the blade root  23  with respect to the external markings  33  and  34 , this relative position characterising the bending of the rotor blade  18 . According to the second embodiment, no mirrors are consequently required. 
     In  FIG. 4 , a schematic and partial illustration of the rotor  6  having a measuring device  56  according to a third embodiment of the invention can be seen, features which are similar or identical to the previous embodiments being given the same reference numerals as in the previous embodiments. The third embodiment is a modification of the second embodiment. In place of the internal markings  24  and  25  and the second camera  39 , tension or expansion sensors  42  and  43  are arranged on a retention member  57  by means of which the camera  27  is secured to the blade root  23  or to the blade bearing  19 . Using the sensors  42  and  43 , a mechanical deformation of the retention member  57  can be detected so that the sensors  42  and  43  which are electrically connected to the evaluation device  37  supply deformation data  44  to the evaluation device  37 . The image data  38  of the camera  27  comprise first position information which is dependent on the relative position of the camera  27  with respect to the external markings  33  and  34 , the deformation data  44  provided by the sensors  42  and  43  comprising second position information which is dependent on the relative position of the camera  27  with respect to the blade root  23 . 
     From the deformation data  44 , the evaluation device  37  calculates the relative position of the camera  27  with respect to the blade root  23 . Using at least one image evaluation method, the evaluation device  37  calculates from the image data  38  the relative position of the camera  27  with respect to the external markings  33  and  34 . From the calculated relative positions, the evaluation device  37  then determines the relative position of the blade root  23  with respect to the external markings  33  and  34 , this relative position characterising the bending of the rotor blade  18 . 
     In  FIG. 5 , a schematic and partial illustration of the rotor  6  with a measuring device  56  can be seen according to a fourth embodiment of the invention, features which are similar or identical to the previous embodiments being given the same reference numerals as in the previous embodiments. The fourth embodiment forms a modification of the first embodiment. The camera  27  is arranged at a side of the rotor hub  8  opposite the rotor blade  18  and is secured thereto. The light emitted by the internal markings  24  and  25  strikes the photo-sensitive face  30  of the camera  27  directly so that no mirrors are required. In order to further describe the fourth embodiment, reference is made to the description of the first embodiment. 
     In  FIG. 6 , a schematic and partial illustration of the rotor  6  with a measuring device  56  according to a fifth embodiment of the invention can be seen, features which are similar or identical to the previous embodiments being given the same reference numerals as in the previous embodiments. In addition to the camera  27 , there is provided a camera  39  having a photo-sensitive surface  40  which is secured to the rotor blade  18  or to the blade bearing  19  in the region of the blade root  23 . In this instance, the camera  27  is arranged in the field of vision of the camera  39 . Each of the cameras  27  and  39  is electrically connected to the evaluation device  37  and transmits electronic image data  38  or  41  thereto. In this instance, the image data  38  of the camera  27  comprise first position information which is dependent on the relative position of the camera  27  with respect to the external markings  33  and  34 . The image data  41  supplied by the camera  39  comprises second position information which is dependent on the relative position of the camera  39  with respect to the camera  27 . The camera  39  is preferably fixed in position relative to the blade root  23 . 
     From the image data  41 , the evaluation device  37  calculates the relative position of the camera  39  with respect to the camera  27  using at least one image evaluation method. Since the camera  39  is arranged in the region of the blade root  23 , this position also characterises the relative position of the camera  27  with respect to the blade root  23  so that the relative position of the camera  27  with respect to the blade root  23  is calculated by means of the evaluation device  37 . Furthermore, by means of the evaluation device  37 , using at least one image evaluation method, the relative position of the camera  27  with respect to the external markings  33  and  34  is calculated from the image data. Subsequently, the evaluation device  37  determines from the calculated positions the relative position of the blade root  23  with respect to the external markings  33  and  34 . This relative position characterizes the bending of the rotor blade  18 . According to the fifth embodiment, no mirrors are consequently required. 
     In  FIG. 7 , a schematic and partial illustration of the rotor  6  having a measuring device according to a modification of the fifth embodiment of the invention can be seen, the blade bearing  19  comprising a blade bearing portion  66  which is rigidly connected to the rotor hub  8  by means of bolts  65  and a blade bearing portion  68  which is rigidly connected to the rotor blade  18  by means of bolts  67 . The blade bearing  19  is constructed in this instance as a ball bearing, the balls of the blade bearing  19  being schematically indicated and being designated  69 . A retention member  70  which carries the camera  39  is placed on the illustrated bolt  65  and securely screwed thereto by means of a nut  71  so that the camera  39  is rigidly retained on the blade bearing portion  66  and consequently also retained rigidly on the rotor hub  8 . Since a rotation of the rotor blade  18  about the blade axis  20  thereof also leads to a rotation of the camera  27  about the blade axis  20 , a rotation of the rotor blade  18  about the blade axis  20  thereof can also be observed by means of the camera  39 . The evaluation device  37  consequently preferably also calculates the torsion angle of the rotor blade  18  about the blade axis  20  relative to the rotor hub  8 . 
     In  FIG. 8 , an image  45  taken by the camera  27  according to the first and/or fourth embodiment is schematically illustrated, the regions  46  and  47  representing the images of the external markings  33  and  34  in the non-bent state of the rotor blade  18 . If the rotor blade  18  is bent, the images of the external markings in the image  45  are displaced, which is indicated by arrows  59  and  60 . In this instance, the regions  48  and  49  characterise the images of the external markings  33  and  34  in the bent state of the rotor blade  18 . The position of the images of the external markings in the image  45  consequently represents the first position information. Since the position of the regions  46  and  47  in the image  45  is known, the displacement of the regions  48  and  49  with respect to the regions  46  and  47  can also be determined by means of the evaluation device  37 . 
     The regions  50  and  51  of the image  45  represent the images of the internal markings  24  and  25  in an original position of the camera  27 . If the camera  27  is displaced relative to the blade root  23 , the images of the internal markings are displaced in the image  45  which is indicated by arrows  61  and  62 . In this instance, the regions  52  and  53  characterise the images of the internal markings  24  and  25  after the displacement of the camera  27 . The position of the images of the internal markings in the image  45  consequently represents the second position information. Since the position of the regions  50  and  51  in the image  45  is known, the displacement of the regions  52  and  53  with respect to the regions  50  and  51  can also be determined by means of the evaluation device  37 . 
     In the previous embodiments, the markings  33  and  34  and optionally the markings  24  and  25  according to a first variant of the invention are preferably each formed by a light source. According to a second variant of the invention, at least one, a plurality or all of the markings is/are formed by means of reflectors  54  which are illuminated by means of one or more light sources  55 , which can be seen schematically in  FIG. 9 , which shows the marking  33  in the form of a reflector  54 . The light emitted by the light source  55  is designated  63 . 
     According to a third variant of the invention, at least one, a plurality or all of the markings is/are formed by means of optical waveguides  64  in which by means of one or more light sources  55  light  63  is coupled, which can be seen schematically in  FIG. 10 , which indicates the marking  33  in the form of an optical waveguide  64 . 
     For each marking which is constructed as a reflector or an optical waveguide, a separate light source may be provided. However, it is also possible for several or all reflectors or optical waveguides to be supplied with light from the same light source. For example, for the external markings  33  and  34 , a common or separate light source may be provided, respectively. Furthermore, for the internal markings  24  and  25 , a common or separate light source may be provided, respectively.