Patent Publication Number: US-2011049886-A1

Title: Device and method for detecting the loading of pivoted rotor blades

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a device and method for detecting the loading of rotor blades when the latter are suspended in a rotary bearing arrangement. These pivoted rotor blades are used in wind power plants, and also in helicopters. 
     2. Description of Related Art 
     U.S. Patent Application Publication 2009/169357 A1 (European Patent Application EP 2 075 561 A2) discloses a method and a device with which rotor blades loads are measured. Here, proximity sensors or strain gauges are used. Attachment of these sensors to or in the vicinity of the main shaft or in the vicinity of the bearings is preferred for adjusting the angle of the rotor blades. This application relates to the problem of two coordinate systems for measurement with reference to the gondola of a wind power plant and with reference to the rotor hub. 
     International Patent Application Publication WO 2009/095025 A1 suggests measuring the loading of the rotor blades using FBG sensors in the rotor blades. 
     Commonly owned German Patent Application DE 10 2008 061 553.6 and corresponding U.S. Patent Application Publication 2010/0158434 A1 of one of the present inventors relates to a device with which the deformation of a bearing ring can be determined with a fiber optic sensor. 
     Therefore, the direct measurement of the loading of rotor blades is associated with problems because strain gauges are either unreliable or, if they are made with fiber optics, are very expensive. The direct measurement of the loading of the rotor blades should take place distributed over the length of the rotor blade; this is difficult at the current extensions of rotor blades of more than 50 m. For pivoted rotor blades there is the additional problem that power supply to the sensors and the delivery of their signals to the control unit which is ordinarily mounted in the gondola in wind power plants must take place wirelessly or with slip rings by way of two rotary joints, specifically the bearing of the rotor blade and the main bearing of the wind power plant; this is complex. 
     SUMMARY OF THE INVENTION 
     This invention circumvents these and other problems in the measurement of the loading of the rotor blades by measuring a quantity which is directly influenced by the rotor blade load, specifically measurement of the deformation of a bearing or a part of a bearing such as a bearing ring for rotation of the rotor blade for pitch adjustment. Here, deformation is defined not only as warping of a bearing ring, but can also be a misalignment, in which one bearing ring travels into a new position relative to another, for example, by its being tilted, or due to misalignment of roll bodies. This generally understood deformation can be measured especially easily and economically with proximity sensors. Alternatively deformation can also be detected by way of fiber optic sensors. 
     One special advantage of the invention is that the rotor blade loading to be detected is not determined in the rotor blade itself as in U.S. Publication No. 2009/0169357 A1, but in the vicinity of the rotor hub or in the rotor hub itself. It is not necessary to bridge distances of more than 10 m within the rotor blade or to transmit signals wirelessly or via slip rings from the rotor blade into the hub or the gondola of the wind power plant. 
     An electronic evaluation device establishes the relationship between the deformation of the bearing ring and the loading of the rotor blade. This electronic evaluation device can be integrated into other electronic controls, such as the control of rotor blade adjustment, which is often mounted in the rotor hub of the wind power plant, or the control of the entire wind power plant in the gondola. 
     In one advantageous embodiment of the invention, a one-dimensional or multidimensional inclinometer is additionally incorporated into the measurement device. The angular position of the rotor hub in the direction of rotation is detected via this inclinometer. If the inclinometer also detects a tilt of the direction which is axial with reference to the rotation of the rotor hub, the tilt of the gondola of the wind power plant is also detected. 
     With this measurement device, it is possible to detect both aerodynamic loads on the rotor blade and also mechanical ones which are caused, for example, by imbalances. These loads can then be used in the evaluation unit to intervene into the control of the wind power plant or the rotary adjustment of the rotor blades or the adjustment of parts of the rotor blades. Dynamic balancing of individual rotor blades is also possible, as is described in International Patent Application Publication WO 2009/033472 A2. 
     In another preferred version, permanent structural changes of the rotor blade can be recognized. These structural changes can be breaks or delaminations. For example, if part of the rotor blade on the front edge partially detaches, this part will lead the remaining rotor blade after top dead center is passed. When the rotor blade is in the lower part of rotation, there will be an instant at which the part which has detached from the front edge of the rotor blade again strikes the remaining rotor blade. Conversely, if a part on the back of the rotor blade partially detaches, striking of the part which trails the remaining rotor blade on the remaining rotor blade after passing top dead center, but still in the upper part of rotation, takes place. This change of load of the rotor blade is expressed in the deformation of the bearing ring of the rotor blade, in the form of irregularities in the deformation characteristic over time. For example, if the angular position of the rotor blade is detected at the same time, for example, by way of an inclinometer, the site of the damage and/or type of damage can be determined. 
     The invention is described in further detail below with reference to the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a rotor hub of a wind power plant which is intended for accommodating three rotor blades and with which device and method of the invention are implemented. 
         FIG. 2  is a cross-sectional view of a part of a rotor hub with the root of a rotor blade and part of the nacelle to which the rotor hub is connected. 
         FIG. 3  is a cross-sectional view of a wind energy plant. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a rotor hub  1  of a wind power plant which is intended, here, for accommodating three rotor blades. The rotor blades are pivotally mounted in outer bearing rings  2   a ,  2   b  and inner bearing rings  3   a ,  3   b  (the third bearing ring is not shown). The angular position of the rotor blades can be changed by way of a drive  7 . The connection to gearing in the gondola of the wind power plant takes place via the main bearing  8 . 
     The inner ring  3   a  is monitored using two proximity sensors  5 ,  6 . These proximity sensors are securely joined to the rotor hub  1  by way of holding devices (not shown). The proximity sensors detect deformation of the bearing ring  3   a . These proximity sensors  5 ,  6  can be inductive sensors. Here, the sensor  5  measures deformation in the radial direction relative to the axis of rotation of the bearing. The sensor  6 , conversely, measures deformation in the axial direction relative to the axis of rotation of the bearing. These proximity sensors are connected by way of cables to an evaluation unit (not shown in  FIG. 1 , reference numeral  17  in  FIG. 2 ). A second pair of sensors on the same bearing ring  3   a  can usefully measure when it is mounted at another peripheral position. In particular, the deformation of the bearing ring in the two dimensions of the radial plane of the bearing can be measured with a second radially attached sensor (not shown). 
     The evaluation unit  17 , which is shown mounted to the rotor blade in  FIG. 2 , can also be mounted within the rotor hub  1 . It can also be a component of a control unit for setting the rotary angle of the rotor blades. The evaluation unit can, in a further embodiment, be mounted in the gondola (nacelle) of the wind power plant. Even if the evaluation unit is mounted in the gondola, it can be a component of a control unit for the wind power plant there. The evaluation unit can detect and further process the results of measuring the deformation of the bearing rings  3   a ,  3   b . In this further processing, the loading of the rotor blade can be determined. Here, aerodynamic loads which lead to deformation of the rotor blade can be determined. These aerodynamic loads arise, for example, in stagnation in front of the tower of the wind power plant. Mechanical loads, for example, due to mechanical unbalances, can also be determined in this way. 
     Since the evaluation unit detects the deformation of the bearing rings, which also includes misalignments or tilting, monitoring of the bearing, bearing condition and the play in the bearing is possible here in exactly the same way. Thus, there is a linkage to the method of traditional condition monitoring in the monitoring of bearings. 
     For example, a fiber optic sensor  4  is peripherally attached in the second inner ring  3   b ; it detects the deformation of the bearing ring  3   b  and is connected to an evaluation unit within the rotor hub  1  or within the gondola. These fiber optic sensors can determine both the deformation of the bearing ring in the axial direction and also in the radial direction relative to the axis of rotation of the bearing. In one advantageous configuration, these fiber optic sensors can be made as FBG (Fiber Bragg Grating) sensors. These Bragg gratings are zones which have been introduced into the glass fibers with an altered index of refraction. It can be recognized on the transitions between zones with a different index of refraction whether there is deformation of the fibers. Depending on the distribution of these transitions between zones with a different index of refraction, the deformation of the fibers can be measured, resolved as to location. 
     By way of example, two different techniques for measuring the deformation of bearing rings have been described. However, in practice one and the same technique will more likely be used on a machine for all bearing rings of adjustable rotor blades. This means that, for a given plant, either all bearings will be monitored by inductive sensors for deformation or all bearings will be equipped with FBG sensors. It goes without saying that, like the inner rings, the outer rings of the rotary joints can be equipped with sensors in accordance with the invention. 
     In one advantageous exemplary embodiment, the evaluation unit mounted in the rotor hub contains an inclinometer or is connected to an inclinometer. This inclinometer detects the rotary angle position of the rotor hub  1  with reference to terrestrial gravitation. Thus, for example, it can be detected if a rotor blade is pointing vertically down and is, therefore, located in the stagnation in front of the tower of the wind power plant. Moreover, an inclinometer which is made multidimensional can also determine the angular position in the axial direction of the rotor hub  1 . The inclinometer can also be mounted in the vicinity of the sensors for the deformation of the bearing rings. 
     The result of measuring the deformation and the loading of the rotor blade determined therefrom are sensibly used as parameters to be taken into account in the control units of the wind power plant. This control unit is, for example, the control for the rotary position of the rotor blade angle. Another possibility is to use the determined loading of the rotor blade as a parameter for the control for adjusting the aerodynamically active parts of the rotor blades. It is also possible to dynamically change balance weights by way of the ascertained rotor blade loading, for example, by pumping of fluids into the rotor blades, as is described in International Patent Application Publication WO 2009/033472 A2. 
       FIG. 2  contains a more detailed view of how measuring of the deformation and the loading of the rotor blade can be effected. 
     The determination of the deformation and the loading of the rotor blade take place by checking the deformation of the bearing ring. If, for example, only the rotor blade  11  which is at the top dead center of rotation is being hit by a sudden gust of wind, this rotor blade will be pushed suddenly in the direction of the length of the nacelle  14  towards the back of the nacelle  14  away from the spinning hub  1 . If this rotor blade  11  is connected to the inner bearing ring  3   a , the movement of the blade which has been caused by the aforementioned sudden gust will cause a shift in the location of the inner bearing ring  3   a  for this particular blade and also an upward bending of the windward side of this ring. The outer bearing ring  2   a  is fixedly attached to the spinning hub  1  of the wind energy plant. 
       FIG. 2  also contains a schematic cross-sectional view of rollers  21 . A detector  6   a  measuring the distance from inner bearing ring  3   a  along the direction of the rotor blade which is mounted in parallel to sensor  6  in  FIG. 1 , but located towards the windward tip of the spinning hub  1 , will detect a strong increase of the distance because the root  12  of the rotor blade  11  which is connected to inner bearing ring  3   a  since the rotor blade  11  will be lifted on its front windward side out of the outer bearing ring and the ring  3   a  will be bent in this direction. This increase in distance is caused by the sudden increase of the load. Sensor  6  which is located at 90° from sensor  6   a  will register only small changes, while sensor  5 , assuming that the ring as a whole will also be elongated along the wind direction, will register a decrease in distance relative to ring  3   a , because the diameter of the ring will decrease. Due to the elongation of the ring  3   a  in the wind direction, sensor  5   a  will register an increase in distance just like sensor  6   a.    
     The increase in distance detected by sensor  6   a  is registered when the load added by the sudden gust is uniform over the length and width of the blade. Normally, however, the distribution of wind forces over the area of the blade is not uniform, but more complex, and the positional change of the rotor blade root  12  and the inner bearing ring  3   a  connecting it to the spinning hub will also be more complex than described above. If a numerical simulation has been performed for this particular type of rotor blade in connection with this particular pivoting bearing, or if the reaction of the rotor blade and corresponding bearing to different wind load distributions has been experimentally tested, the electronic evaluation unit  17  is able perform a calculation of the actual load state of the blade from the measured deformation of the bearing ring. 
     In one advantageous configuration of the invention, the deformation of the rotor blade in several dimensions is determined from the determined loading of the rotor blade together with the rotary angle position of the rotor hub which can be determined with the inclinometer  18 , since the measurement of the deformation of the bearing ring takes place multi-dimensionally. Thus, it becomes possible to determine the operating deflection shape (ODS) of a rotor blade without high cost for sensors. Measurements of the natural frequencies of the rotor blade for purposes of experimental modal analysis (EMA) are also possible here. 
       FIG. 2  contains also inclinometer  18  which is connected like sensors  5   a  and  6   a  to an electronic evaluation unit  17 . This electronic evaluation unit  17  is mounted on a holder  16  to the rotor blade and is able to transmit measurement data wirelessly via an antenna  22  from the rotor blade which may be rotated relative to an antenna  23  which transmits these data to control unit  24  in the nacelle. Of course, control unit  17  may also be mounted in the spinning hub if it is connected by slip rings or wirelessly to sensors  4 ,  5 ,  5   a ,  6 ,  6   a  and can wirelessly exchange data with a further control unit in the nacelle. In the latter configuration with control unit  17  in the spinning hub, it is not necessary to have an inclinometer on each rotor blade. One inclinometer attached to the spinning hub will suffice if the relative angular positions of the rotor blades with respect to the angular position of this inclinometer are known. 
     While sensors  5 ,  5   a ,  6 ,  6   a  shown in  FIGS. 1 and 2  are shown as inductive proximity sensors, in a preferred embodiment, a fiber optic sensor  4  is attached to the inner side of bearing ring  3   b , as shown in  FIG. 1 . This fiber optic sensor for monitoring axial and radial deformation of a bearing ring is described in commonly owned, co-pending U.S. Patent Application Publication 2010/0158434 A1, which is incorporated in its entirety by reference. In this co-pending application, it is described that the deformation of a bearing ring can be detected in both its axial and radial directions by means of a single sensor shaped like a thread with multiple measurement zones distributed along its length. Thus, the deformation of the bearing ring can be detected not only in the positions given by the positions of single sensors, like the windward position in which sensors  5   a  and  6   a  ( FIG. 2 ) are mounted, or the direction at a right angle thereto in which sensors  5  and  6  ( FIG. 1 ) are mounted. A FBG sensor  4  can contain a great number of zones for the detection of axial or radial deformation of a bearing ring  3   b . The use of FBG sensor  4  has the advantage that there is the possibility of having a sensitive zone, e.g., in the windward direction or the direction at a right angle thereto. From the description above, it can be seen that these two areas of the ring—windward and at a right angle thereto—will show the largest effects. Given a sufficient number of sensitive zones in the FBG sensor  4 , there will always be a sensitive zone of the FBG sensor in these two directions independent of the pitch of the pivotable rotor blade. 
     When more than one rotor blade is attached to the rotor hub, it is of course useful to determine the loading of each individual rotor blade by means of sensors for deformation of the pertinent bearing ring, determination of the loading taking place from the deformation in an evaluation device which is common to all rotor blades. In this way, differences between these rotor blades can be detected during installation of the rotor blades or shortly afterwards. Thus, the change of behavior of individual rotor blades over time can be monitored. Comparison of the development of the state of the rotor blades over time between the blades of a wind power plant, but also between the blades of several different plants, is possible. 
     Tests have shown that, with the measurement of the deformation of the bearing ring for torsion of a rotor blade, not only the loading of the rotor blade itself, but also the loading of other components of a wind power plant, such as, for example, the main bearings, brake, gearing, become possible. The natural frequencies of these components of the wind power plant can be easily evaluated on the bearing ring for the rotary adjustment of the rotor blade. 
       FIG. 3  is a modified version of  FIG. 1  of commonly owned, co-pending U.S. Patent Application Publication US 2010/0209247A1 (which is hereby incorporated by reference) and shows a wind energy plant in cross section. Reference numerals with a value of 100 or less in  FIG. 3  are taken from this co-pending application, and while they do not have any meaning in this application, they have been retained due to the incorporation by reference of this earlier application. Horizontal lines  101 ,  102  and vertical lines  103 ,  104  delineate a rectangular area which is shown enlarged and together with details of the invention in  FIG. 2 .