Abstract:
A method of attaching a load sensor comprising a support with a number of strain gauges on a surface of a rotor blade is proposed. The method comprises the steps of producing a set of holes with a predefined distance between the holes. Threaded inserts are provided in the holes. Bolts are inserted through the load sensor into the threaded inserts. A rotor blade with a load sensor is also proposed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European application No. 10000950.5 filed Jan. 29, 2010, which is incorporated by reference herein in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention concerns a method of attaching a load sensor to a surface of a rotor blade, the load sensor comprising a support with a number of strain gauges and a set of fixing holes with predefined distances. It also concerns a rotor blade with such load sensor attached to a surface of the rotor blade. 
       BACKGROUND OF THE INVENTION 
       [0003]    Rotor blades, more particularly rotor blades in wind turbine applications, need to be monitored with respect to the forces that put the blade under stress. The aim is to be able to control the static and/or dynamically oscillating load of the rotor blade in order to take countermeasures once the load or sum of dynamic load gets too big to find out if the unlikely event occurs that an exchange of the rotor blade (or parts of it) is necessary due to wear or preferably to control the load of the rotor blades of a wind turbine continuously avoiding the need for an exchange of a rotor blade or any other component of a wind turbine like e.g. a gearbox, pitch bearing, main bearing or similar. 
         [0004]    In order to measure the load, i.e. the deflection of a blade caused by the pressure or compression onto such blade, so-called load sensors or strain measuring transducers are used, as described e.g. in U.S. Pat. No. 3,853,000. A resistance strain gauge is mounted there on a thin strip of a plate-shaped support. 
         [0005]    Such load sensors can be positioned on a surface by means of small spikes which can be dug into the surface. The load sensor is firmly fixed to the surface by bolts passing through holes in the sensor which bolts are welded to the structure of the surface. It has to be taken into account, however, that welding bolts to a surface of a rotor blade weakens the structure of the surface and is also not always possible due to incompatibility of the materials of the surface and of the bolts. For example it is difficult or even impossible to weld glass fibre or similar non-metallic materials. Furthermore, the spikes or pins of the load sensor may damage the structure of the surface when the nuts of the bolts are tightened. 
         [0006]    Another possibility how to fix a load sensor on a rotor blade is by adhesion, e.g. by glue or other kinds of pressure sensitive adhesive. However, such adhesion method is time-consuming and also means that the surface has to be prepared. It has to be polished and acid corroded and still it is often difficult to make sure that the load sensor is properly attached. In addition, changing a load sensor once it has been firmly adhered to the surface is again time-consuming and expensive and may lead to injuries of the surface. 
         [0007]    Therefore it can be summed up that the currently available solutions of attaching load sensors of the principle described above and indeed of any other measure principle are still insufficient, in particular for an application like rotor blades of wind turbines. The surfaces of such rotor blades are of a special material and may suffer from damages to such an extent that the blades may become defect before or in operation. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an object of the invention to provide for an improved possibility of how to attach a load sensor to the surface of a rotor blade. Another object of the invention is to provide a rotor blade with a load sensor attached in such improved manner. 
         [0009]    According to the invention, this object is met by a method and a rotor blade according to the claims. 
         [0010]    Thus, the above-mentioned method comprises the steps of
   a) producing a set of holes with a predefined distance between the holes essentially equal to the predefined distances of the fixing holes,   b) providing threaded inserts in the holes,   c) inserting bolts through the load sensor into the threaded inserts.   
 
         [0014]    The set of holes is arranged in a predefined distance which corresponds with the distance of the fixing holes in the support of the load sensor. Thereby, the fixing holes in the load sensor may either be present in the workpiece as it comes from the manufacturer or may be drilled into it in the course of the method according to the invention. 
         [0015]    In this way the load sensor is firmly connected to the surface of the rotor blade while it may also be easily removed for the purpose of exchange if necessary. By producing not just holes but by additionally providing these holes with threaded inserts, a very precise arrangement and alignment can be achieved, making sure that the bolts are positioned and aligned exactly as wished for. At the same time, the injury to the surface of the rotor blade is such that no unnecessary forces are exerted which might lead to damage of the rotor blade in a way that disables its functionality. 
         [0016]    The holes can be produced in such a precise way that no unnecessary and uncontrollable stress is built up in the course of the operation of the rotor blade. Furthermore, the loads, i.e. static and/or dynamic oscillating loads that are to be measured by the load sensor are directly applied to the sensor coming from a few bolts at defined positions of the surface to be controlled. Therefore, unlike in the state of the art, the forces or loads are not applied from a multitude of pins or even from a rather undefined region of adhesion. The solution according to the invention makes measurement more reliable and more precise. In addition, the material and structure of the threaded inserts can be adopted to the specific demands of the structure of the surface into which they are inserted. They may be bolted or hammered into the holes of the surface. 
         [0017]    A “bolt” in the context of the invention is preferably a bolt with an external thread at least partially along its longitudinal extension. Therefore “to bolt” signifies in this context the action of introducing such bolt into a hole with a thread at its inside by rotating the bolt along an axis defined by its longitudinal extension. 
         [0018]    A rotor blade of the above-mentioned kind comprises
   a) a set of holes in the surface with a predefined distance between the holes equal to the predefined distances of the fixing holes,   b) threaded inserts inside the holes,   c) bolts going through the fixing holes of the load sensor into each hole and threaded insert and thus connecting the load sensor with the rotor blade.   
 
         [0022]    In such rotor blade, the load sensor is preferably connected to a turbine load control system. Such load control system derives control orders from the load values measured by the load sensor which control orders serve to reduce the load inflicted on the rotor blade in which the load sensor is placed and/or on all rotor blades of the rotor. For instance, the load control system may be programmed to derive control orders to initiate the reduction of rotation speed (e.g. signals to a break mechanism) and/or to change the pitch of the rotor blade or rotor blades and/or to activate additional devices that serve to reduce the load. 
         [0023]    The load control system may also accumulate load values from the load sensor over time and, once these accumulated values reach a certain threshold value, generate an alarm signal to a user and/or control orders to stop operation of the rotor in order to facilitate a checkup and/or exchange of the rotor or parts of it. 
         [0024]    In order to connect the load sensor with the load control system, it is particularly advantageous to use optical fibres for transmission as they are not affected by lightning strikes. The same applies to any other signal transmission connections in the context of this invention. 
         [0025]    Particularly advantageous embodiments and features of the invention are given by the dependent claims, as revealed in the following description. Thereby, features revealed in the context of the method may also be realized in the context of the rotor blade and the other way round. 
         [0026]    Concerning the threaded inserts used to accomodate the bolts, high standards have to be met concerning preciseness and mechanical stability. According to a first embodiment, at least one threaded insert is a helical insert. Such helical inserts are known under the trademark Heli-coil and are inserts made of coiled wire. They are usually over-sized so that they anchor themselves in the hole into which they are applied. In the context of the invention they offer stability and flexibility at the same time. 
         [0027]    In a second embodiment which may be used alternatively or in addition to the first embodiment at least one threaded insert is a bush comprising threads on its inner side. The stability of such bushes can be even higher than that of helical inserts so that first tests have proven that this second embodiment is particularly advantageous. 
         [0028]    Preferably, threaded inserts comprising corbels at their one longitudinal end are put into the holes in such way that the corbels come in direct or indirect contact with the surface of the blade. Such corbel functions like a stopper which indicates when the threaded insert has reached its final position in the hole. In such final position the insert is firmly connected to the hole and fills the hole to such an extent that a sufficient stability for the bolts is achieved. The use of a threaded insert with a corbel also guarantees that the insert is not inserted further into the surface of the rotor blade than wished for, thereby making sure that the hole is not extended by the insert which might lead to damage of the surface of the blade. Indirect contact of the corbel with the surface can be realized for example by washers or similar distance pieces firmly positioned in between the corbel and the surface. 
         [0029]    The predefined distance between the holes can be achieved for example by simply measuring and marking the distance on the surface of the rotor blade and then producing the holes in those locations indicated by markings. In order to increase preciseness with respect to the predefined distance and possibly also to the positioning angle of the holes it is preferred to use a hole production template with template holes in the same predefined distances which template is put onto the surface. Through the template holes a hole producing tool such as a drilling machine is the guided onto the surface. 
         [0030]    Such template is preferably made up of a board or plate with the holes of which may be extended by cylindrical tubes projecting from the plane of the board, preferably in a perpendicular angle in at least one direction. In this way, a drill can be guided by the tubes which makes sure that the hole is produced in the correct angle. 
         [0031]    Therefore, the invention also comprises a hole production template for producing a set of holes on a surface of a rotor blade with a predefined distance between the holes in the context of the method according to the invention. It comprises a board with holes in an arrangement with the same predefined distances as the distances between the set of holes to be produced on the rotor blade. 
         [0032]    Preferably the template comprises at least three positioning legs projecting from a contact surface of the board. In this context, it has proven to be most advantageous to use a template with exactly three such legs as this guarantees that the template may be positioned very precisely on a surface even if it is slightly curved as is often the case with rotor blades. 
         [0033]    Preferably, the legs are realized as hollow tubes through which a fixing means such as a pin or the like can be guided. This construction makes it possible to pre-fix the template via these fixing means on the surface in order to make sure the template does not accidentally change its position during its application. 
         [0034]    The template may further comprise an indication of an orientation of the template which indirectly indicates the orientation of the load sensor when later attached to the surface. This indication preferably comprises a triangular shape of the template, whereby it is advantageous that the orientation of the load sensor, i.e. the direction in which load forces are measured is indicated by one angle of the triangle. 
         [0035]    According to a particularly preferred embodiment of the invention, the load sensor is attached on an inner surface of the blade. Rotor blades of wind turbines (unlike helicopter blades) are usually hollow and thus comprise an outer and an inner surface. Attaching the load sensor on the inner surface means that the sensor is protected from wind and from humidity at least to a much higher extent. It can also be easier accessed by the responsible staff from the inner side of the turbine through the tower, the nacelle and the hub. When operating from inside the rotor blade, the staff are much better protected than if they worked on scaffolds outside because the danger of falling down from the rotor is virtually eliminated. 
         [0036]    A single load sensor can already provide great insight into the loads inflicted upon the rotor blade, in particular in the context of the invention which delivers more reliable measurement values due to the precise transmission of load forces. However, it has proven to be even more reliable if a plurality of load sensors is mounted on the surface of the blade. This way, the load sensors can be orientated in more than one direction and thus measure load forces coming from different angles. In addition, the load sensors may be positioned in several locations along the longitudinal extension of the rotor blade and therefore may give a broader picture of which loads the rotor blade is confronted with along its length. 
         [0037]    That part of the rotor blade which is closest to the hub of the rotor is in highest danger of getting damaged. It is in this region that the load forces are highest and that all forces accumulate before being directed onto the shaft of the rotor. Therefore, this region is particularly prone to damage by loads which exceed a certain maximum threshold. Therefore, in order to closely monitor this region, the load sensor is preferably mounted in close proximity of an interface between the blade and a hub of a rotor. The close proximity is considered to be reached within the closest third, particularly in the closest fifth, and most particularly in the closest tenth of the rotor blade with respect to the hub. 
         [0038]    While attaching a load sensor using a method according to the state of the art is already difficult on a brand new rotor blade, these obstacles are even greater when it comes to a re-equipment of a blade that has already been in operation. Therefore, the method according to the invention is even more helpful if the load sensor is attached to a blade which has already been in operational service. Neither will it be necessary to weld any bolts onto the surface of such blade nor to prepare the surface—which is even more tedious to carry out because of the wear of the blade and because of its impurity due to operation. The method according to the invention therefore provides for a much easier possibility to equip existing wind turbines with a load sensor system for the first time or as a replacement of old load sensors. 
         [0039]    In order to protect the load sensor from outside influence such as forces which might damage it or influence measurement and such as dirt and humidity, it is preferred that a housing which protects the load sensor is attached to the surface of the blade. This housing may comprise a lid which can simply be put over the load sensor and fixed to the surface. In addition to simply covering the load sensor, the housing can also be made waterproof or even moistureproof in order to avoid corrosion of the load sensor. 
         [0040]    Preferably, the load sensor is connected to a load controller via cable, be it optical or electric, or in a wireless way. The controller may be positioned within a housing in order to be protected from its environment. Between the controller and the load sensor, there may also be a transmitter which transmits signals from the load sensors to the controller and which may also collect signals from a plurality of load sensors. 
         [0041]    It is furthermore preferred to produce the holes into the surface of the blade in such way that the angle between the main extension of the holes and the surface is essentially perpendicular. The main extension of the surface can be considered to be a plane representing an average extension of the surface, e.g. in the case of a curved surface. The angle is still considered to be essentially perpendicular when it varies by +/−10% from 90°. This limited angle can be considered to be a good reference as to how the bolts are orientated. In addition, when using an essentially rectangular orientation for the bolts, the transmission of load forces is without any greater losses because otherwise a part of the forces might be directed in other directions than the one covered by the load sensor. 
         [0042]    Also for the purpose of clarity of measurement it is preferred that exactly two bolts are used to attach the load sensor. That also means that only two holes must be produced and only two threaded inserts put into these holes. Firstly, this means that the smallest possible damage is done to the surface. Secondly, this attachment method is not only more than sufficient to attach the load sensor, but also guarantees that as little other influences as possible will endager the exactness of the measurement. For example, if the load sensor is attached via more than two bolts, tension forces might play an additional role and tamper the measurement results. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. 
           [0044]      FIG. 1  shows a section view of a surface of a rotor blade while being subject to an embodiment of a method according to the invention, 
           [0045]      FIG. 2  shows the same view with a rotor blade completed according to an embodiment of the invention, 
           [0046]      FIG. 3  shows a shematic view into the inside of parts of a rotor of a wind turbine, 
           [0047]      FIG. 4  shows a schematic block diagramm of steps of an embodiment of the method according to the invention, 
           [0048]      FIG. 5  shows a top view of a template according to an embodiment of the invention, 
           [0049]      FIG. 6  shows a side view of the same template as in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    In the drawings, like reference numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale. 
         [0051]      FIG. 1  shows a surface  3  of a rotor blade  1 , which is the inner surface of the rotor blade  1 . In order to attach a load sensor, e.g. a load sensor working under the principle of a load sensor as shown in U.S. Pat. No. 3,853,000, a set of holes  7  is drilled into the surface  3  at a distance d 2  corresponding with the distance d 1  of fixing holes  8  in the load sensor (cf.  FIG. 2 ). Those holes  7  are furnished with threaded inserts  9 . The threaded inserts  9  are realized as bushes with threads on their inside  11  and comprise corbels  10  at their top longitudinal end. 
         [0052]    On the left hand side one can see a bush  9  while being inserted into a hole  7 , whilst on the right hand side a bush  9  is already positioned in a hole  7 , though not in its final position yet. This final position will be reached when the corbel  10  is in touch with the surface  3 . 
         [0053]    This situation can be seen in  FIG. 2  in which both bushes  9  are in their final positions within the surface  3  of the rotor blade  1 . In addition, a load sensor  5  is fixed to the surface  3  by bolts  13  that have been bolted or screwed into the inner side  11  of the bushes  9 . The load sensor  5  comprises a plate-shaped support  6  with two fixing holes  8 . It is lying on the corbels  10  of the bushes, pressed there by the bolts  13 . Exactly two bolts  13  are necessary and sufficient to fix the load sensor  5  to the surface  3 . 
         [0054]    In  FIG. 3  the location of load sensors  5   a,    5   b,    5   c,    5   d,    5   e  is shown in principle. On a rotor of a wind turbine the hub  15  has three interfaces  21  at which rotor blades  1  are connected, one of which rotor blades  1  can be seen in the figure. The rotor blade  1  has a diameter of  1 , 6  metres and a length of more than 40 metres, which is typical for rotor blades of the current generations of wind turbines. The load sensors  5   a,    5   b,    5   c,    5   d,    5   e  are positioned within that tenth of the longitudinal extension of the rotor blade  1  which is closest to the interface  21 , i.e. within the first 4 metres of the rotor blade  1  coming from the hub  15 . In this way it is made sure that loads are measured in that region which is most prone to be damaged due to overloading and where a damage can do most harm to persons or objects in the surrounding area of the wind turbine. 
         [0055]    The outermost load sensor  5   a  has an angular orientation in comparison with the orientation of the other four load sensors  5   b,    5   c,    5   d,    5   e  which are positioned closer to the hub  15  and the orientation of which is along the longitudinal extension of the rotor blade  1 . Therefore, the latter load sensors  5   b,  Sc,  5   d,    5   e  will all measure load forces in the longitudinal direction of the rotor blade  1 , whilst the firstly mentioned load sensor  5   a  measures forces in an angular direction to the longitudinal direction. The measurement directions are indicated with arrows at the side of each of the load sensors  5   a,    5   b,    5   c,    5   d,    5   e.    
         [0056]    Signals from the load sensors  5   a,  S b,    5   c,    5   d,    5   e  are transmitted via wireless transmission connections  4  (indicated as lines) to a transmitter  17  located at the very end of the rotor blade  1  where it projects into the hub  15 . The transmitter  17  transmitts combined signals via an optical cable  33  further to a controller  19  of a turbine load control system which derives orders—if necessary—to reduce the load on rotor blade  1  and possibly on other rotor blades as well. 
         [0057]      FIG. 4  shows a schematic flowchart of an embodiment of the method according to the invention. This is explained with reference to the preceding figures. 
         [0058]    In a first step A the set of holes  7  is drilled into the surface  3  of the rotor blade  1  at a predefined distance d. In a second step B the bushes  9  are inserted into the holes  7 . In a third step C the bolts  13  are bolted into the bushes, thereby fixing the load sensor  5  to the surface. 
         [0059]      FIGS. 5 and 6  show an embodiment of a drilling template  23  according to the invention. 
         [0060]    As seen from above, the template  23  is triangularly shaped and thus indicates how a load sensor  5  will be orientated if aligned along holes  7  produced with the template  23 . The template  23  comprises a board or plate  29  with holes  27   a,    27   b.  These holes  27   a,    27   b  are distanced from each other at the same distance d 3  as shown in  FIGS. 1 and 2 , i.e. the distance d 2  of the holes  7  that are introduced into the surface  3  and the distance d 1  of the fixing holes  8 . In addition, as can be seen best in the side view of  FIG. 6 , the template  23  comprises three legs  25   a,    25   b,    25   c  which are realized as tubes projecting from a contact surface  31  of the board  29  which is directed towards the surface  3  of the rotor blade  1  when the template  23  is used. Through the tubes of the legs  25   a,    25   b,    25   c,  a nail or a pin can be pushed which helps to temporarily fix the template at a particular location on the surface  3 . 
         [0061]    The holes  27   a,    27   b  are also extended by tubes  35  which project out of the board  29  in the direction away from the contact surface  31 . The tubes  35  give a drilling tool inserted into them a certain guidance which guarantees that the drill is inserted into the surface  3  of the rotor blade  1  at a predefined angle, preferably essentially 90°, maybe with a little and tolerable variance. 
         [0062]    Because the template  23  has exactly three legs  25   a,    25   b,    25   c,  it can also be firmly put on curved surfaces  3 , even such surfaces which are curved in more than one direction with respect to a plane. Such is usually the case in rotor blade applications. 
         [0063]    Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. While the use of load sensors based on measurement of resistances was used as a basis for the description, other load sensor systems may be used to good effect in the context of the method and rotor blade according to the invention as well. Furthermore, the orientation of load sensors within the rotor blade, is purely shown as an example and may be altered in accordance with the relevant needs in their place of operation. Also, the use of a drilling template is not limited to the embodiment of template as shown in  FIGS. 5 and 6 . 
         [0064]    For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The expression “distances” also includes a single distance. A “unit” or “module” can comprise a number of units or modules, unless otherwise stated.