Eigenfrequency measuring device and system for determining an eigenfrequency of a wind turbine rotor blade

In the design phase of wind turbine rotor blades , it is common practice to perform extensive testing of wind turbine rotor blade prototypes in dedicated test stands . Such testing typically includes mounting the wind turbine rotor blade root to a fixed test stand at a dedicated wind turbine rotor blade testing facility . The wind turbine rotor blade is then equipped with various sensors and subj ected to di f ferent static and dynamic loads . However, it is very time-consuming and costly to apply such testing to every single wind turbine rotor blade during series production . In most cases quality control is therefore limited to visual inspection and measurement of a few simple parameters , such as the overall weight and the position of the center of gravity .

The document EP 3 034 861 Bl discloses a wind turbine rotor balancing method . The wind turbine rotor blades are manufactured and their masses and centers of gravity are measured . A balancing mass may be placed in a balancing chamber of at least one of the wind turbine rotor blades , and selected wind turbine rotor blades are combined to form a wind turbine rotor having an imbalance within tolerances .

The document EP 3 423 711 Bl discloses a method for weighing a wind turbine rotor blade supported at its root end and at its tip end . The hori zontality of the wind turbine rotor blade is adj usted before the tip load and the root load are recorded . The document WO 2015/ 046651 Al discloses a measurement method for a center of gravity of a wind turbine rotor blade . The wind turbine rotor blade is seated on first and second weight measurement units placed at the root and tip ends . Distances between an edge of the wind turbine rotor blade root and intermediate positions of the first and second weight measurement units are measured . A controller calculates the center of gravity based on measured weight and distance data .

The document WO 2012 / 054937 Al discloses a test stand for wind turbine rotors . In the field, wind turbine rotor blades are attached to a wind turbine rotor hub . Before the wind turbine rotor is mounted to a shaft of the wind turbine , a proof test is performed to validate the mass and sti f fness characteristics of the blades , possibly including a determination of the natural frequencies of the blade/hub assembly . Either the hub or the rotor blades are anchored to the ground . The position or motion of the non-anchored element ( s ) is/are controlled by one or more actuators .

Accordingly, it is an obj ect of the invention to provide a reliable and ef ficient concept to control the quality of a wind turbine rotor blade .

An eigenfrequency measuring device for determining an eigenfrequency of a wind turbine rotor blade is disclosed .

The eigenfrequency measuring device comprises a support frame to be placed on a ground . The eigenfrequency measuring device comprises a spring bearing, being mounted on the support frame . The eigenfrequency measuring device comprises a support basket for laying the wind turbine rotor blade therein . The support basket is supported on the support frame via the spring bearing, preferably only via the spring bearing . The eigenfrequency measuring device comprises an excitation device for applying a vibration to the support basket and thus ( i . e . via the support basket ) to the wind turbine rotor blade . The device comprises a measuring unit being configured to measure vibrations of the wind turbine rotor blade .

With the disclosed device it is possible to determine an eigenfrequency of a blade and compare it with a predetermined eigenfrequency, e . g . an expected eigenfrequency, in order to control the quality of a wind turbine rotor blade .

Determining the eigenfrequency is carried out at a manufacturing site of the wind turbine rotor blade , in particular as a routine control step, for example in a finishing process of the wind turbine rotor blade . Thus , at a late stage of production it is ensured that only wind turbine rotor blades that have been evaluated as to their eigenfrequency will leave the factory . In this regard, manufacturing the wind turbine rotor blade may be essentially completed before the wind turbine rotor blade is placed on the disclosed device , i . e . placed in the support basket . In particular, i f the wind turbine rotor blade is assembled from two separate hal f-shells , this step will be completed so that the wind turbine rotor blade will have its final shape . Other manufacturing steps having little or no influence on the mechanical properties of the wind turbine rotor blade , such as e . g . application of a surface finish and/or assembly of add-on equipment such as sensors and/or lightning protection systems can be completed before the wind turbine rotor blade is placed on the support , but these steps could also be carried out later . For determining the eigenfrequency, a wind turbine rotor blade is placed on the eigenfrequency measuring device, i.e. laid into the support basket. The device is placed between the root and the tip of the wind turbine rotor blade, e.g. in the middle of the blade, and defines a support for the wind turbine rotor blade. In particular, the support basket has one or more contact surfaces that are in contact with the wind turbine rotor blade. Particularly, the wind turbine rotor blade only has contact with the support basket or portions thereof, meaning that a resulting weight force of the wind turbine rotor blade (at least in the region of the device) is only transferred via the support basket and the spring bearing to the support frame. In other words, there is no weight force (or portions thereof) of the wind turbine rotor blade passing the support basket and being transferred via another coupling to the support frame.

Since the eigenfrequency measuring device supports the blade in a short section with respect to the length of the blade and not necessarily in the center of gravity, the wind turbine rotor blade is additionally supported at the root site, e.g. via one or more specific support elements fastened to the wind turbine rotor blade in the root area, e.g. a beam structure fastened to the wind turbine rotor blade, e.g. to a root section thereof, e.g. by one or more bolts. These specific support elements may simply rest on the ground or on a padding or may be held in a bearing. However, in contrast to a dedicated test stand, the further support will not be fixed completely in position relative to the ground, but will be able to move with the wind turbine rotor blade when the same vibrates, e.g. in the form of a floating bearing. Alternatively, the rotor blade can also be supported in a hanging manner in the root area. In other words, the wind turbine rotor blade is , on the one hand, supported on the eigenfrequency measuring device and on the other hand, supported in the root area .

We note that for ideal measuring results , in an embodiment a reproducible positioning of the root support and/or the eigenfrequency measuring device is provided . Thus , the measuring results are comparable , in particular i f the rotor blade is reproducibly inserted into the eigenfrequency measuring device at the same length . Therefore , one or more abutment elements for the root support and a fixed position for the eigenfrequency measuring device is provided . I f the rotor blade is hangingly supported at the root site , e . g . by loops , bands or the like , these also are to be precisely positioned .

Once the wind turbine rotor blade is placed, a dynamic load is applied via the excitation device in order to excite a vibration of the wind turbine rotor blade . In this regard and in contrast to known test stands , the load is not directly applied to the wind turbine rotor blade , but is applied to the support basket and thus - indirectly - to the wind turbine rotor blade . The dynamic load may be a single pulse trans ferring a defined momentum, e . g . applied by means of a colliding obj ect , but may also comprise several periodic excitations .

The dynamic load is applied such that the wind turbine rotor blade is subj ect to a certain deformation and is temporarily deflected from a rest position, i . e . the rotor blade swings or vibrates respectively . While the wind turbine rotor blade returns to its rest position, the vibration of the wind turbine rotor blade excited by the dynamic load is detected so that a frequency of the vibration corresponding to an eigenfrequency of the wind turbine rotor blade can be determined by the measuring unit . The determined eigenfrequency can then be compared to an expected eigenfrequency, that is to an eigenfrequency that the wind turbine rotor blade will have when manufactured according to its speci fications .

The expected eigenfrequency may have been obtained during the design phase of the wind turbine rotor blade , either by calculation or by a speci fic measurement . It is also possible to set the expected eigenfrequency based on an eigenfrequency that has been determined in accordance with the steps as described above and has been found to comply with the speci fications .

The disclosed eigenfrequency measuring device contributes to ef ficiently and reliably control the quality of a wind turbine rotor blade . The device allows for an easy and comparably quick placement of a wind turbine rotor blade in the support basket of the device and to determine the eigenfrequency of the blade . The described device provides for the vibration to be applied to the support basket and thus indirectly to the wind energy rotor blade . This means that no speci fic equipment needs to be mounted to the wind turbine rotor blade for excitation purposes . Furthermore , it is not necessary that the measuring unit , or parts thereof , is arranged directly on the wind turbine rotor blade itsel f or is attached to it . Rather, the measuring unit , or parts thereof , can be mounted separate to the eigenfrequency measuring device , on the support frame and/or the support basket . After placing the blade into the eigenfrequency measuring device , it is therefore not necessary to take additional measures , such as attaching sensors to the blade directly, in order to measure vibrations and determine the eigenfrequency . This particularly allows for short set-up times , since a wind turbine rotor blade is simply placed into the eigenfrequency measuring device and the above steps can be performed . Afterwards , the blade can be removed from the test stand, including the eigenfrequency measuring device , and another blade can be provided . As a result , comparatively short dwell times in the test stand can be achieved .

The inventive eigenfrequency measuring device allows for a relatively simple construction and contributes to very short test times , on a routine basis and at low cost per blade . In particular, the eigenfrequency measuring device can be used for multiple di f ferent types of wind turbine rotor blades . In many cases , only one system will be needed for a manufacturing site .

Further advantages of the described eigenfrequency measuring device are for example :

- New quality criteria ( CTQs ) for eigenfrequency can be introduced .

- Automation of the measurement process and web connection to database contributes to prevention of misuse and to improvement of tracking of CTQs ( Industry 4 . 0/ digitali zation) .

- The quality of rotor blades can be improved, costs can be reduced and problems in the field can be avoided .

- Further, development of previously e . g . too heavy blades , is possible , i f eigenfrequency is correct .

- Further, avoidance of non-quality costs and sustainable use of resources can be ensured . - Even more, it is possible, even if according to the design of the rotor blade the eigenfrequency ranges of the rotor blade and tower of a wind turbine overlap (are similar or in a same range) , to ensure in individual cases that the eigenfrequencies of the tower and the rotor blade do not coincide .

The described device provides for the wind turbine rotor blade to be only spring-supported on the support frame. In this regard, different kinds of springs are possible, e.g. Belleville springs or spiral springs. Further, the number of springs for the spring bearing can be different, depending on the load and size of a wind turbine blade. Typically, four springs are provided, e.g. being arranged symmetrically.

At this point we would like to note that with the disclosed eigenfrequency measuring device, not only one or a single one eigenfrequency, but at least two eigenfrequencies can be determined, in particular the first two eigenfrequencies, for example edge-wise and flap-wise (see explanations further below) . Thus, at least four frequencies can be determined.

Even it is possible by the disclosed eigenfrequency measuring device to determine the third flap-wise eigenfrequency. For example in order to determine the eigenfrequencies, the support basket is to be located at a point along the blade (in longitudinal direction) where there is no vibration node of the eigenfrequencies to be determined and, seen from the root, outside the center of gravity.

According to an embodiment, the excitation device is configured to apply an impulse to the support basket to apply vibrations to the wind turbine rotor blade. For example, a mass is hit against or dropped on the support basket. As another example , a mass is guided to slide down on a plane and stopped at an abutment element in order to transmit an impulse to the support basket with the inserted wind turbine rotor blade . In other words , there is a direct mass impulse on the support basket . This enables a very simple and reliable setup . Alternatively, the excitation can also be done with an actuator/ shaker e . g . via a frequency sweep .

According to an embodiment , the excitation device is configured to deflect the support basket ( e . g . for pretensioning the support basket and/or the wind turbine rotor blade respectively) and release the support basket for applying vibrations to the wind turbine rotor blade . Thus , a so called " snap-back" solution is applied . For example vibrations are indirectly applied to the wind turbine rotor blade . Firstly, the support basket has to be moved out of a rest position into a deflected position and is then - secondly - released ( suddenly) in order that the wind turbine rotor blade vibrates in its eigenfrequencies . For example , the support basket is deflected in a flapwise and/or edgewise direction . The flapwise direction corresponds to a profile height , the edgewise direction corresponds to a profile chord . The vibration in any one or both of these directions can be important for quality control purposes . To detect the vibration in each direction, separate sensors or a combined sensor can be used . This , for example allows for a precise and defined excitation of the wind turbine rotor blade .

According to an embodiment , the excitation device comprises a pretensioning unit to bring the support basket from a rest position to a deflected position to pretension the support basket against a spring force of the spring bearing . The pretensioning unit is configured to provide a predetermined pretension to the support basket and thus deflects the wind turbine rotor blade . For example , the pretensioning unit provides a pretensioning force against a ( spring) force of the spring bearing . The pretensioning unit can be configured to hold the rotor blade in the deflected position . Optionally, a motor can be provided for an automated deflection of the support basket .

According to an embodiment , the excitation device comprises a locking device to hold the support basket in the deflected state . For example , the pretensioning unit pretensions the support basket and the locking device secures the deflected position of the support basket . Thus , the force provided by the pretensioning unit in order to deflect the support basket can be released, without releasing the support basket to vibrate .

According to an embodiment , the locking device comprises one or more magnets to magnetically hold the support basket in the deflected state . For example the locking device is an electromagnetic device comprising one or more electromagnets in order to magnetically hold the support basket in the deflected position . Thus , after having deflected the support basket , the latter can be easily and securely held in its pretensioned ( deflected) position .

According to an embodiment , the excitation device comprises a release unit to release the support basket from the pretensioned state . The release unit is configured to release , for example suddenly, the pretension in the deflected state of the support basket in order that the support basket and thus the wind turbine rotor blade vibrate back into the rest position . For example , the release unit switches off the electromagnet ( s ) the locking device in order that the support basket and the rotor blade snap back. Thus, a very accurate and sudden impulse can be generated. For example, the release unit is part of the locking device or integrally formed with the locking device.

According to an embodiment, the support basket is pretensioned in a direction oblique to the vertical direction and horizontal direction. In other words, the support basket is pretensioned in a flapwise and edgewise direction. Thus, eigenfrequencies of two directions can be determined, e.g. in X- and Z-direction (horizontal and vertical direction or f lapwise/edgewise directions respectively) .

According to an embodiment, the support basket comprises one or more support belts for accommodating the wind turbine rotor blade. The rotor blade is laid down into the one or more belts, which - due to their flexibility - adapt to the outer shape of the rotor blade. This ensures a very good transmission of the excitation pulse to the rotor blade.

According to an embodiment, the measuring unit comprises a sensor unit. The sensor unit is formed by or comprises one or more sensors which are configured and arranged to measure vibration in one, two or more directions. For example, a so- called geophone sensor or other highly sensitive sensors with a correspondingly low frequency range (from approx. 0.1 Hz) are used.

According to an embodiment, the sensor unit is coupled to the support basket. This contributes to the above effects and advantages, in particular no sensor needs to be mounted directly onto the wind turbine rotor blade. According to an embodiment , the eigenfrequency measuring device comprises an evaluation unit for calculating an eigenfrequency from the measured values of the measuring unit . For example , the evaluation unit compares the determined eigenfrequency with an expected eigenfrequency of the wind turbine rotor blade . Depending on the results of the comparison, the evaluation unit may also evaluate the quality of the wind turbine rotor blade . The measuring unit and the evaluation unit are - for example - part of a controller unit or form a controller unit .

According to an embodiment , the eigenfrequency measuring device comprises a scale to measure a mass of the wind turbine rotor blade being laid into the support basket . For example , the scale is provided in the area of the support frame . For example , the scale is integrated in the support frame . For example , the scale is configured to measure/determine a mass and a center of mass ( gravity) , as well as a moment of mass . This further improves the determination and control of the quality of the wind turbine rotor blade . Alternatively, the eigenfrequency measurement device is placed on a standard scale .

According to an embodiment , the eigenfrequency measuring device comprises a controller unit to automatically operate the excitation device . The controller unit is configured to provide one or several of the following :

- automatic triggering of the excitation ( e . g . pretensioning and releasing the support basket ) ,

- acquisition of sensor readings ,

- output of measurement results and/or creating of a protocol - transfer of data (eigenfrequencies, mass and/or center of gravity) to a database, e.g. a web-based database

According to a further aspect, a system for determining an eigenfrequency of a wind turbine rotor blade is disclosed, the system comprising the wind turbine rotor blade and an eigenfrequency measuring device according to any one of the above embodiments. The rotor blade is supported at the root and placed in the support basket of the eigenfrequency measuring device.

The system essentially enables the above-mentioned functions, effects and advantages. The above described features and embodiments with regard to the first aspect similarly apply to the described method.

According to an embodiment, the wind turbine rotor blade is supported at the root in a hanging manner. This ensures the above-mentioned effects and ensures an easy and efficient support of the blade. In particular, supporting the rotor blade at the root area requires little time, since, for example, no screw bolt connections are made in the flange area .

According to an embodiment, the wind turbine rotor blade is mounted with the root to a root support. As already indicated above, the root support may comprise a beam structure, e.g. which is used during the finishing work.

According to an embodiment, the system comprises a root scale to measure a mass of the wind turbine rotor blade. The scale provides the effects and advantages as mentioned above with respect to the scale of the eigenfrequency measuring device. In particular, the scale can be provided in addition to the scale of the eigenfrequency measuring device in order to improve the measurement and quality control of a blade . In this regard, the eigenfrequency measurement device is placed on a standard scale or includes an own scale , as described above .

Further advantages , features and functions are given in the following exemplary embodiments of the invention, which are explained in connection with the figures . Identical , similar or similarly acting elements are provided with the same reference signs in the figures . All shown and described elements are not necessarily provided with reference signs for the sake of clarity and visibility .

Figure 1 shows a schematic view of a wind turbine ,

Figure 2 shows a schematic side view of a system for determining an eigenfrequency of a wind turbine rotor blade ,

Figure 3 shows a perspective view of an eigenfrequency measuring device , and

Figure 4 shows a perspective partial view of the device according to figure 3 .

Figure 1 shows a schematic view of a wind turbine 100 , which comprises a tower 102 . The tower 102 is fixed to the ground by means of a foundation 104 . At one end of the tower 102 opposite to the ground a nacelle 106 is rotatably mounted .

The nacelle 106 , for example , comprises a generator which is coupled to a rotor 108 via a rotor shaft (not shown) . The rotor 108 comprises one or more (wind turbine ) rotor blades 110 , which are arranged on a rotor hub 112 .

During operation, the rotor 108 is set in rotation by an air flow, for example wind . This rotational movement is transmitted to the generator via the rotor shaft and, i f necessary, a gearbox . The generator converts the kinetic energy of the rotor 108 into electrical energy .

Figure 2 shows a system 114 for determining an eigenfrequency of a wind turbine rotor blade 110 according to an embodiment of the invention . The wind turbine rotor blade 110 has a rotor blade root 116 , a rotor blade tip 118 , a leading edge 120 , a trailing edge 122 , a center of gravity 124 and an aerodynamic profile with a suction side and a pressure side (not shown) . The wind turbine rotor blade 110 is supported on a root support 126 and an eigenfrequency measuring device 128 (hereinafter also named "device" ) , forming a second support . Both supports 126 and 128 are portable and stand on a ground 130 at a manufacturing site . The root support 126 comprises a beam structure 127 with at least one beam fastened to the wind turbine rotor blade root 116 . It was found that the vibration can best be detected at some distance from the wind turbine rotor blade root 116 , in particular at the midsection, but not in the node of a of the natural frequency ( eigenfrequency) to be determined .

In the position shown, a longitudinal axis of the wind turbine rotor blade is arranged hori zontally and the leading edge 120 is pointing downwards . The edgewise direction 132 of the wind turbine blade is arranged essentially vertically, the flapwise direction 134 is arranged essentially hori zontally, orthogonal to the drawing plane . In the following, the eigenfrequency measuring device 128 according to an embodiment of the invention is described in more detail with respect to figures 3 and 4 .

The eigenfrequency measuring device 128 comprises a support frame 136 to be placed on the ground 130 . The support frame 136 is , for example , built by several beam-like structures and forms a rigid, stable basic framework . The eigenfrequency measuring device 128 comprises a spring bearing 138 , which in the example shown, comprises four symmetrically arranged spiral springs or coil springs 140 . The springs 140 are supported, i . e . mounted, on the support frame 136 .

Further, the eigenfrequency measuring device 128 comprises a support basket 142 for laying the wind turbine rotor blade 110 therein . The support basket 142 is supported on the support frame 136 only via the spring bearing 138 , this means the spiral springs 140 . Thus , the support basket 142 is spring loaded wherein ( in the area of the device 128 ) a resulting mass force of the blade 110 only is trans ferred from the support basket 142 via the spring bearing 138 to the support frame 136 . There is no flow of force from the blade 110 to the support frame 136 past the support basket 142 .

The support basket 142 is configured to accommodate the rotor blade 110 , wherein the rotor blade 110 is in direct contact with the support basket 142 . Optionally, the support basket 142 can comprise one or more belts 144 , as indicated in figure 4 , in order that an optimum adaptation to the outer contour of the rotor blade 110 is guaranteed . The eigenfrequency measuring device 128 comprises an excitation device 146 for applying a vibration to the support basket 142 and thus to the wind turbine rotor blade 110 . The excitation device 146 is explained in more detail with the help of figure 4 below .

The eigenfrequency measuring device 128 comprises a measuring unit 148 being configured to measure vibrations of the wind turbine rotor blade 110 , after it has been excited by the excitation device 146 . The measuring unit 148 comprises one or more sensor units 150 for measuring the vibrations , being mounted on the support basket 142 . The one or more sensor units 150 , e . g . geophone sensors , may be connected by wire or wirelessly to the measuring unit 148 .

With regard to figure 4 , the excitation device 146 is configured to deflect the support basket 142 for pretensioning and release the support basket 142 for applying vibrations to the wind turbine rotor blade 110 . In this respect , the excitation device 146 comprises a pretensioning unit 152 to force the support basket 142 from a rest position ( i . e . no acting forces , relaxed state , only resulting gravitational force of blade 110 is acting on the spring bearing 138 ) to a deflected position in order to pretension the support basket 142 against a resulting spring force of the spring bearing 138 , i . e . against spring forces of the spiral springs 140 . In the illustrated embodiment , the pretensioning unit 152 comprises a rope pulling device , which has a crank unit 154 and one or more ropes 156 . The crank unit 154 is mounted on the support frame 136 . Ropes 156 connect the crank unit 154 with the support basket 136 , wherein wheels 158 are provided on the support basket 142 for guiding the ropes 156 . The crank unit 154 is configured to pretension the support basket 142 . In this regard the crank unit 154 has a lever 160 for manually tensioning the ropes 156 in order to pull the support basket towards the support frame 136 . In the shown embodiment the support basket 142 is pulled in a pulling direction 162 , which has direction components of both the edgewise direction 132 and the flapwise direction 134 . In other words , the pulling direction 162 runs oblique to the hori zontal direction (parallel to the ground 130 ) and the vertical direction .

After having pretensioned the support basket 142 , the excitation device 146 further comprises a locking device 164 to hold the support basket 142 in the deflected, i . e . pretensioned state . The locking device 164 comprises one or more magnets , in particular electromagnets 166 . The electromagnets 166 are provided to interact with the support basket 142 , which for this purpose has one or more plates 168 . In the deflected (pretensioned) state of the support basket 142 , the locking device 164 is actuated such that the electromagnets 166 attach to the plates 168 and hold the support basket 142 in the current , deflected position .

The excitation device 146 further comprises a release unit 170 , which is integrally formed with the locking device 164 in the shown embodiment . The release unit 170 is configured to release the support basket 142 from the pretensioned state , i . e . the locking device 164 is actuated such that the magnetic force of the electromagnets 166 is released . Thus , the electromagnets 166 do not attach to the plates 168 anymore and an impulse is applied to the support basket 142 forcing it to move and vibrate the rotor blade 110 . In other words , after deactivating the locking device 164 a resulting spring force of the spring bearing 138 forces the support basket 142 to swing back into its rest position and thus the wind turbine rotor blade 110 .

The measuring unit 148 is configured to determine one or more eigenfrequencies , in particular a first and second eigenfrequency in the vertical direction and hori zontal direction, based on the measured vibrations .

The eigenfrequency measuring device 128 further comprises an evaluation unit 172 , which is configured to compare the determined eigenfrequency ( ies ) with expected eigenfrequencies of the wind turbine rotor blade . Depending on the results of the comparison, the evaluation unit 172 may then evaluate the quality of the analyzed wind turbine rotor blade 110 .

The measuring unit 148 and the evaluation unit 172 can be part of a controller .

Optionally, the eigenfrequency measuring device 128 comprises a scale 174 to measure a mass of the wind turbine rotor blade 110 being laid into the support basket 142 . The scale 174 can comprise one or more mass sensors , e . g . being arranged in the region of the spiral springs 140 .

Further optionally, the root support 126 can comprise a root scale 176 for measuring a mass of the wind turbine rotor blade 110 in the root region .

Optionally, the eigenfrequency measuring device 128 can comprise a controller unit 178 , which is configured to automatically operate the excitation device 146 . Optionally, the measuring unit 148 and/or the evaluation unit 172 can be an integral part of the controller unit 178 . Exemplarily, the controller unit 178 is configured to automatically pretension the support basket 142 and to release it.

It is noted that the eigenfrequency measuring device 128 can be differently designed and constructed. Essentially, it is designed such that the rotor blade 110 is not directly excited, but indirectly via the support basket 142. For example, the pretensioning unit 152 can be designed differently, e.g. an electronic or automatic control pretensioning can be provided. Instead of ropes, other mechanisms like gear- or mesh-like mechanisms, are possible. Essentially, the pretensioning unit 152 must be configured to pull the support basket 142 in one or more predetermined directions against an acting, resulting spring force.

Reference signs

100 wind turbine

102 tower

104 foundation

106 nacelle

108 rotor

110 rotor blade

112 rotor hub

114 system

116 rotor blade root

118 rotor blade tip

120 leading edge

122 trailing edge

124 center of gravity

126 root support

127 beam structure

128 eigenfrequency measuring device

130 ground

132 edgewise direction

134 flapwise direction

136 support frame

138 spring bearing

140 spiral spring

142 support basket

144 belt

146 excitation device

148 measuring unit

150 sensor unit

152 pretensioning unit

154 crank unit

156 rope

158 wheel 160 lever

162 pulling direction

164 locking device

166 electromagnet 168 plate

170 release unit

172 evaluation unit

174 scale

176 root scale 178 controller unit