Method and tool for detecting defects on wind turbine generator blades

Abstract

To provide an improved and cost-effective method and tool for performing a defect detection procedure aimed at detecting defects underneath an outer protective layer covering the leading edge of a wind turbine generator blade, a method is described during which a tool end is biased against the surface of the outer protective layer as it is guided over the outer protective layer and a uniformity deviation of at least one feedback value is sensed to identify potential defects. A tool and a detection unit suitable for use in the described method are also described.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method and a tool for detecting defects underneath a protective outer layer protecting the leading edge of a wind turbine generator blade, and a detection unit utilizing such a method and such a tool. BACKGROUND

[0002] When a wind turbine is rotating, the leading edge of a wind turbine generator blade cuts through the air surrounding it at very high speed, with the speed being highest at the outer end of the blade. Over time, collisions with particles in the air, such as raindrops, hail, dust, leaves, insects, etc. are unavoidable. Furthermore, over time, different weather conditions can put stress on the blade material as temperature, humidity, wind speed and UV radiation change. In order to protect the actual structural material of the blade, which is usually a fiber composite material, which imparts its structural strength to the blade, it is known in the art to provide the leading edge of a wind turbine generator blade with a protective layer, which covers at least a portion of the leading edge along its length, possibly from near the blade base close to the center of rotation, up to the end of the blade. Such a protective layer serves as an erosion barrier, can be provided in the form of a U-shaped profile or in the form of a sheet or strip, and will be attached or bonded, preferably glued, to the structural material of the blade, usually as one of the last steps of blade production or as part of a blade repair process. Such a layer of material can be attached or bonded to the structural material of the blade in one piece or in several adjacent segments. The material used for the protective layer can for example be a polyurethane material with high durability and high impact resistance, such as a thermoplastic polyurethane (TPU). Of course, other materials feasible for the intended purpose can also be used. The prior art documents WO 2018 / 114123 A1 and WO 2016 / 075619 A1 mention such protective layers of material, the invention described in the present application can also utilize such materials.

[0003] The published patent application document WO 2018 / 114123 A1 emphasizes the importance of a correct and continuous bond between the structural material of the blade and the protective layer serving as a protective cover or erosion barrier, and describes a method to avoid air pockets (so-called "voids") underneath the protective layer of material by drawing air through air outlet channels provided between the wind turbine generator blade and the protective layer during the bonding process. In areas of existing air pockets or voids, the bond between the protective layer and the structural material is not complete, so that the voids can lead to delamination, for example due to a notch effect at the defective bond points. The method described in WO 2018 / 114123 A1 can help to minimize the presence of voids, but will not completely eliminate them. Furthermore, the method does not provide a solution for the problem that even after performing the suggested method, a final quality control step needs to be performed to ensure that in fact no voids of relevant size are present.

[0004] Furthermore, methods and devices known from prior art documents EP 2 275 670 A2, CA 3 015 284, US 2010 / 0208247 Al and EP 3 453 874 Al for the inspection of wind turbine generator blades for detecting and identifying potential material defects, including material defects located underneath the outer surface of the blade. Some of these methods and devices can be applied as one of the last steps in the blade production process at the production site, or they can be applied after the final installation of the wind turbine, or even for inspection after the wind turbine has been in use for some time at the wind turbine site. However, these methods and devices do indeed require very special, technically highly complex equipment, and / or it is partly not feasible to use these methods and devices in a quick and uncomplicated manner during the production process of the wind turbine generator blade, just before the blade is finished and transported to the wind turbine site. Furthermore, the methods and devices known in the art can not have the ability to detect potential defects underneath a slightly thicker material layer, such as the protective layer of e.g. thermoplastic polyurethane described above. SUMMARY

[0005] It is therefore an object of the present invention to provide a method and a tool for detecting potential defects underneath an outer protective layer protecting the leading edge of a wind turbine generator blade as one of the final production steps, which is capable of detecting potential air pockets or voids in a simple, quick and easy to use manner. It is a further object of the present invention to make such a method and such a tool possible without the need for highly complex technical equipment.

[0006] In order to achieve these objects, the present invention provides a method for detecting defects underneath an outer protective layer protecting the leading edge of a wind turbine generator blade, wherein the method comprises the following steps:

[0007] - guiding a defect detection tool over the outer protective layer in the direction of movement of the tool, and

[0008] - sensing at least one feedback value experienced by the tool when being guided over the outer protective layer for detecting potential defects,

[0009] wherein the defect detection tool comprises a tool base (12) and a tool end which, when guided over the outer protection layer in a tool movement direction, is biased against the surface of the outer protection layer in a tool biasing direction and detects potential defects under the outer protection layer as a uniformity deviation of at least one feedback value, and wherein the tool end is formed by a disc, a rim or a ball which is rotatably supported relative to the tool base such that, when the defect detection tool is guided over the outer protection layer in the tool movement direction and biased against the surface of the outer protection layer in the tool biasing direction, the tool end rolls over the surface of the outer protection layer at this time. This allows the tool to be moved along the surface of the outer protection layer tool and to sense the response to the biasing without transverse movement between the tool end and the outer protection layer in the measurement point between the tool end and the outer protection layer, which greatly increases the sensitivity of the measurement.

[0010] To meet these objects, the application also provides a tool for detecting defects under an outer protection layer protecting the leading edge of a wind turbine generator blade, the defect detection tool comprising a tool base and a tool end which is designed to be guided across the surface of the outer protection layer in a tool movement direction and biased against the surface of the outer protection layer in a tool biasing direction in order to sense at least one feedback value experienced by the tool end in the tool movement direction and / or in the tool biasing direction when the tool end is guided over the outer protection layer, wherein the tool end is formed by a rim or a disc which is rotatably supported relative to the tool base.

[0011] The application also provides a detection unit for detecting defects under an outer protection layer protecting the leading edge of a wind turbine generator blade, the detection unit utilizing at least one defect detection tool as described above and / or being used in a method as described above.

[0012] The present invention provides a method and a tool by which feedback experienced by the tool when in contact with and rolling over the surface of the outer protective layer can be sensed by biasing the end of the tool against the surface of the outer protective layer while moving the tool along the tool movement path in the tool movement direction along the leading edge. Any significant change in the uniformity of the sensed feedback can be seen as a potential defect underneath the outer protective layer, as the feedback or "feel" of a tool that is biased against and moved along the surface of the protective layer will be consistent if the outer protective layer is perfectly uniformly and continuously bonded and free of any defects. In particular, if the tool is moved across a gas pocket or void that exists underneath the outer protective layer, the end of the tool that is in contact with, biased against and rolling over the surface of the outer protective layer will, due to the sufficiently soft material properties of the material used for the protective layer (e.g. the above mentioned TPU), sink into the void at least to some extent when reaching the first edge of the void, while rising out of the void when coming into contact with the second edge of the void before leaving the area underneath which the void exists, as the void is softer than the adhesive that usually exists between the outer protective layer and the blade. It should be noted that the present method and tool make use of the tool end being rotatably supported by the tool base, allowing to sense the response of the mechanical forces applied to the outer protective layer by the tool, in particular changes in the response under lateral movement (where no lateral movement between the tool end and the outer protective layer in the measurement point is allowed, which allows to precisely observe the response of the tool end when moving over the surface). Another result of this is that the measurements can usually be carried out under dry conditions, i.e. without the supply of lubricants, vacuum gels or similar additive substances, which reduces costs, environmental impact and post-processing (clean-up) times after using the tool or method according to the present invention.

[0013] In this context, it is to be noted that the tool and method particularly relate to the detection of defects and gas pockets or voids in the adhesive layer underneath the outer protective layer. The adhesive layer will usually comprise or even consist entirely of an adhesive for bonding the outer protective layer to the structural material of the wind turbine generator blade, and will usually be positioned between the inner surface of the protective layer and the outer surface of the blade structural material.

[0014] For investigating the leading edge of a wind turbine generator blade, the tool or a detection unit comprising such a tool will preferably be moved in the longitudinal direction of the blade, such that the tool movement direction is preferably a direction substantially parallel to the course of the leading edge of the blade. However, particularly if the defect detection tool is designed as a tool to be used manually by a user (as further described below), the tool movement direction can of course be any direction in which the user decides to guide the tool over the surface of the protective layer. Being able to freely move the tool back and forth by hand will enable the user not only to detect the presence of potential defects or voids (or to verify the results obtained by using e.g. an automated detection unit), but also to identify the size of the potential defects or voids and allow for taking precise repair measures.

[0015] One of several advantages of the described method is that the method the defect detection tool can be designed as a hand-held tool and can be manually guided by a user, for example a worker at the production site, who wants to quickly control the correct adhesion of the protective layer to the structural material of the blade and / or who wants to find out not only the length of a detected void in the longitudinal direction of the blade, but also the width of the void in a direction substantially perpendicular to the longitudinal direction. To this end, the defect detection tool can be provided with a handle to be held by the hand of the user, so that the user will easily "feel" the voids under the protective layer by the feedback the tool gives to the user during its use. Via the handle, the user can easily exert pressure on the tool with his hand as needed to detect any voids under the protective layer.

[0016] Alternatively, the tool base can be part of a sensing portion, or can be connected to a sensing portion, which in turn is part of a sensing unit that senses the uniformity deviation of the at least one feedback value with the at least one sensor. The defect detection tool is then part of a detection unit that moves along the leading edge of a wind turbine generator blade, which detection unit comprises such a feedback value sensing unit that senses the uniformity deviation of the at least one feedback value with the at least one sensor. The at least one sensor is preferably one of a displacement measuring sensor and a pressure measuring sensor and a force measuring sensor. The feedback value to be sensed can thus preferably be the displacement experienced by the tool end portion with respect to the sensing portion as a result of sinking into or rising out of a void, or can be a change in pressure or force acting on the tool end portion during movement over the surface of the protective layer, so that the feedback value can also be described as a resistance value.

[0017] To allow easy use of the tool or any detection unit provided with such a tool, the rim, disc or ball forming the tool end portion can be supported with respect to the tool base by a rolling element arranged between the tool end portion and the tool base. In a preferred, very easy to implement embodiment, the tool end portion can be the outer rim of a roller bearing. Roller bearings are particularly useful to maintain a linear directional movement of the tool during its use. In another preferred embodiment, the tool end portion can be the ball of a ball transfer unit. Ball transfer units are omni-directional load-bearing balls mounted within a restraining clamp, also known as ball bearing universal joints or simply ball rollers. Ball transfer units are particularly useful to allow easy movement of the tool in all directions, for example in a circular or spiral direction, during its use.

[0018] Depending on the material properties of the outer protective layer used and the force that the user or tool is able to exert on the tool, the tool end can have many dimensions. However, when the method and tool are used to inspect the outer protective layer of a TPU, it is found to be very advantageous that the width of the tool end perpendicular to the tool movement direction is less than 10 mm, more preferably equal to or less than 6 mm. Preferably, the width of the tool end perpendicular to the tool movement direction is at least 4 mm. Furthermore, the outer diameter of the rim or disc forming the tool end can be less than 25 mm, preferably equal to or less than 20 mm. Preferably, the outer diameter of the rim or disc forming the tool end is preferably at least 15 mm. If the tool end is a sphere, it is preferred that the diameter of the sphere is between 6-20 mm. These preferred sizes allow for a reduction of the contact surface of the tool end with the surface of the protective layer during use in order to allow the tool end to be exposed to sufficiently high pressure, more easily sink into the voids underneath the protective layer without damaging the protective layer or the adhesive layer underneath. Furthermore, it also provides for a more accurate measurement of the size and location of potential defects underneath the outer protective layer.

[0019] In order to increase the potential feedback values that the defect detection tool is able to provide, the tool end is preferably rigidly supported relative to the tool base in the direction of the tool bias. By rigidly supported is meant that no damping element is provided between the tool end and the tool base for transmitting or communicating the feedback values to the user or to the sensors of the sensing unit.

[0020] It should be noted that if the defect detection tool as described above is used as part of a detection unit equipped with sensors, such a detection unit can make use of multiple such defect detection tools and can comprise multiple sensors in order to cover several paths in the tool movement direction, which will cover a more extensive area in a single "pass" of the detection unit along the leading edge.

[0021] In order to bias the tool end against the surface of the protective layer, the detection unit can be positioned on the leading edge of a wind turbine generator blade, wherein the tool end is biased against the surface of the outer protective layer by the weight of the detection unit, including any external weight that the detection unit can be equipped with. It can also be advantageous to use an external weight acting on the tool end via the detection unit, as the external weight can help lower the centre of gravity of the detection unit placed on the leading edge, thereby putting the detection unit in a statically stable position when being moved along the leading edge of the blade. Alternatively or in addition to any external weight added to the detection unit, pressure can of course also be applied to the detection unit by the user.

[0022] As mentioned above, the tool comprises a tool base and a tool end, wherein the tool end is formed, for example, by a disc or a rim rotatably supported relative to the tool base, such that the tool end rolls over the surface of the outer protective layer when the defect detection tool is guided over the outer protective layer in the tool movement direction and biased against the surface of the outer protective layer in the tool biasing direction. In particular if the defect detection tool or tools are part of a defect detection unit, the tool end can serve as a wheel for the detection unit.

[0023] Preferably, when the detection unit is moved along the leading edge of the wind turbine generator blade, the coordinates of the detected potential defects can be stored in a storage unit, and / or the detected potential defects can be marked on the wind turbine blade by a defect marking device, and / or the detected potential defects are communicated to a user by a signal unit by sending a signal. For example, the detection unit can comprise a control wheel of a known diameter, which rolls over the outer protective layer or another surface of the blade, by registering the rotation of the wheel and by storing the corresponding distance in a memory, data can be generated which shows the user on which positions of the covered distance potential defects were registered. Alternatively or in addition, when a potential defect is detected, a marking device, such as a paint spraying device, can be activated for marking the point or area on the surface where the defect was detected. Further, also alternatively or in addition, upon detection of a potential defect, a signal unit can emit, for example, a sound, a light signal or any other appropriate signal. This will enable the user to manually control and verify the points or areas with potential detected defects in a simple manner after having used the detection unit with a higher degree of automation in a first step. BRIEF DESCRIPTION OF DRAWINGS

[0024] Various additional features and advantages of the present application will become more apparent to those of ordinary skill in the art, upon review of the following detailed description of one or more illustrative embodiments in conjunction with the accompanying drawings. The drawings incorporated in this specification comprise one or more embodiments of the present application and, together with the general description of the application given above and the detailed description of the application given below, serve to explain the principles of the present application. In the drawings:

[0025] Figure 1 is a schematic perspective view of a horizontal axis wind turbine,

[0026] Figure 2 is a schematic perspective view of a part of a wind turbine generator blade and a protective layer to be bonded to the leading edge of the blade,

[0027] Figure 3 is a first schematic view of a part of a cross section of a wind turbine generator blade,

[0028] Figure 4is a second schematic view of a partial cross-section of a wind turbine generator blade,

[0029] Fig. 5 shows a view of a defect detection tool,

[0030] Fig. 6 is a schematic view of a defect detection tool being guided across a gap underneath an outer protective layer,

[0031] Figure 7 depicting a defect detection unit utilizing a defect detection tool equipped with a plurality of sensors, and

[0032] Figure 8 schematically illustrating sensor data retrieved by using Figure 7 the defect detection unit shown. DETAILED DESCRIPTION

[0033] Figure 1 A horizontal axis wind turbine 1 is shown with three blades 2 rotating in the direction indicated by the arrow. The blades 2 have a leading edge 3. At the outer region of the leading edge where the rotation is fast, an outer protective layer 4 is attached which serves as an erosion shield.

[0034] Figure 2 A portion of a wind turbine generator blade 2 is shown as well as an outer protective layer profile 4 before being glued to the leading edge 3 of the blade.

[0035] For the sake of completeness, it should be mentioned that the outer protective layer can - in addition to what is depicted in Figure 1 and Figure 2 - also extend in the longitudinal direction along the leading edge of the blade, for example (almost) all the way to the outer end of the blade and / or (almost) all the way to the root of the blade, by which the blade is mounted to the hub.

[0036] Figure 3 and Figure 4 are schematic cross-sectional views of a leading edge region of a wind turbine generator blade, in which there is an outer protective layer Figure 3 : transverse to the longitudinal direction of the blade 2 Figure 4 : in the longitudinal direction of the blade 2). The leading edge is a layered construction comprising a structural material layer 6 (typically consisting of a fiber composite) for imparting the structural strength to the blade, the already mentioned outer protective layer 4 (serving as an erosion shield) and a glue layer 5 (typically consisting mainly or only of an adhesive for gluing the protective layer 4 to the structural material layer 6).

[0037] The outer protective layer 4 is preferably made of a plastic material that is resistant to erosion and impact, such as for example a polyurethane-based plastic, for example TPU. The thickness of the protective layer can be thickest in the middle region and decrease towards the side edges, to improve aerodynamic performance, and can be from about 4 mm in the middle region to only 0.1 mm thick at the side edges. The thickness of the outer protective layer is generally substantially constant in the longitudinal direction of the blade.

[0038] Figure 3 and Figure 4 The voids 7 formed by the air pockets in the adhesive layer 5 are illustrated. In the void regions, the outer protective layer 4 is not directly adhered to the inner structural material layer 6. In this region, delamination can occur over time, such that the potentially achievable lifetime of the outer protective layer (and effectively the blade) will be reduced.

[0039] Figures 5a and 5b depict two schematic side views of a tool 10 according to the present application, which tool 10 can be used to detect voids as depicted in Figure 3 and Figure 4 Figures 1 to 4. The tool has a tool end 11 and a tool base 12 in the form of a handle for the user to hold. The handle is preferably of metal, such as aluminium or steel, or of hard plastic. It is apparent from Figures 5a and 5b that the tool end 11 is formed by a member that can rotate relative to the tool base 12. The rotating member forming the tool end 11 in Figures 5a and 5b is, as an example, formed by the outer steel ring (rim) 13 of a roller bearing 14 that is supported relative to the tool base 12 by a fork 15 that defines and houses the axis A about which the tool end 11 can rotate. The fork 15 also forms part of the tool base. Due to this design, the tool lacks any damping elements and is extremely rigid in all directions of any forces that can act on the tool, such that the tool provides “unfiltered” feedback to the surface on which it rolls to the tool base. Figure 5c is a schematic side view of another example of a tool 10 that can be used to detect voids such as Figure 3 and Figure 4The tool has a tool end 11 and a tool base 12 in the form of a handle for the user to hold. The handle will preferably be of a rigid material, for example a metal such as aluminium or steel, or a hard plastic. As is apparent from Fig. 5c, the tool end 11 is formed by a member that is rotatable relative to the tool base 12. The rotatable member forming the tool end 11 in Fig. 5c is, as an example, formed by a ball of a gimbal ball unit, which is associated with the tool base 12 by a socket 16 in which the tool end 11 can rotate. The socket 16 also forms part of the tool base 12. Due to this design, the tool lacks any damping elements and is extremely rigid in all directions of any forces that can act on the tool, so that the tool provides "unfiltered" feedback to the tool base from the surface on which it rolls.

[0040] Fig. 6 illustrates how the tool can be used when it is guided over the surface of a protective layer 4 covering at least part of the leading edge of a wind turbine generator blade, in order to detect defects underneath the protective layer. Fig. 6 shows Figure 4 The same cross-sectional view is shown. At the leftmost tool 10 it can be observed that when the tool is biased in the biasing direction B, this will result in a deformation of the outer protective layer 4, also when there is no gap underneath the outer protective layer 4. However, by tracing the path of the tool 10 along the tool movement direction T it can be seen that once the tool reaches the area where there is a gap 7 underneath, the tool will sink at least to some extent into the gap 7 when entering the area, and rise from the gap when leaving the gap area, because the adhesive layer is not present, and the relatively flexible outer protective layer cannot support the end 4, so the protective layer will deform towards the structural material layer 6 of the blade. This is indicated by the tool in dotted form for different positions in the direction T, the resulting height being indicated relative to the original unbiased height of the outer protective layer. It can be observed that in particular the edges of the gap are clearly indicated in the response by steps. Surprisingly, even gaps in relatively thin adhesive layers, for example 0.5-2 mm of adhesive, can be clearly sensed using the tool and method according to the invention.

[0041] It has been found that a tool designed with high stiffness in the tool biasing direction B (ensured by the rigid tool base and tool end support), with low resistance in the tool movement direction T (provided by the rotatably supported tool end with respect to the tool base to ensure no relative movement between the outer protective layer 6 at the test point and the tool end 11), and capable of exerting high local pressure on the surface of the protective layer (made possible by providing the tool with a tool end that forms only a small contact area with the surface it contacts and against which it is biased), provides very direct and unfiltered feedback of any irregularities and / or unevennesses located under the protective layer, even if the protective layer is only a few millimetres thick, even in the presence of some but insufficiently adherent layers at the voids. A user rolling the tool 10 along the path shown in Fig. 6 will easily feel the start of the void under the protective layer and the end of the void, thus being placed in a position to identify potential voids that the eye cannot see or the hand cannot feel.

[0042] Figure 7 A schematic diagram showing a possible design of a defect detection unit 20 with a plurality of sensors S1, S2, S3, S4 and S5 is shown. The tool as described above forms part of the sensor, or in other words, the sensor comprises the tool as described above, wherein the tool end acts as the sensor end. The detection unit 20 can be moved along the leading edge of the wind turbine generator blade 2, either by pulling or pushing the unit manually or by means of a drive device 21. The drive device can comprise electric motors 22 on either side of the unit, which drive friction wheels 23 that are biased against and in contact with the surface of the blade 2, preferably against and in contact with the surface portion of the blade that is not covered by the protective layer 4 (for which the defect detection process is being performed). In order to enable the drive device to adapt to the varying cross-section of the blade in the longitudinal direction of the blade along the tool movement path, the drive device 21 can be pivotably supported, for example, with respect to a frame or housing of the detection unit 20. This frame or housing forms a sensing unit that houses the sensors S1 -S5. In addition, in order to increase the weight of the detection unit, thereby increasing the force with which the tool end 11 is biased against the surface of the protective layer 4, and to improve the movement stability of the detection unit, an external weight 24 can be used.

[0043] Figure 7The depicted detection unit 20 has a housing or frame that accommodates the sensors S1, S2, S3, S4 and S5. These sensors include tools with a tool end 1 1 and a tool base 12 as described above. Since these sensors are not manually guided, the sensors are designed to sense the feedback experienced by the sensor during movement of the tool end 1 1 in the tool bias direction when the tool end is rolled over the surface of the protective layer, at the respective contact area of the tool end with the surface of the protective layer 4, the tool bias direction being substantially perpendicular to the surface of the protective layer 4. It will be appreciated that the rotatably supported tool ends of the plurality of tools and sensors act as wheels over which the detection unit is moved along the leading edge of the blade.

[0044] Figure 7 It is also shown that the sensors including the tools are spring biased against the surface of the protective layer 4 by means of a coil spring 25, it being noted that other means of biasing the tool ends against the surface can of course also be used.

[0045] The detection unit 20 includes a data analysis and electronic storage or memory unit 26 to which the sensors are connected and which will record and store the data obtained by the sensors. The detection unit can also include an acoustic and / or optical signal unit 27 to signal to the user when a potential defect, such as a void, is sensed. Furthermore, a marking unit 28 is shown for leaving a paint mark on the surface of the protective layer 4 in the event that one of the sensors detects a potential defect. Finally, Figure 7 A control wheel 29 of known diameter is depicted, which will facilitate correlating the feedback values sensed by the sensors with the distance moved by the detection unit from a starting point when the sensor values indicative of a potential defect are sensed.

[0046] It should be noted that although Figure 7 Only an embodiment is shown in which all sensors are positioned in the same plane substantially perpendicular to the longitudinal direction of the edge of the blade, embodiments in which the sensors are mutually offset in the longitudinal direction are also possible, in particular to stabilise the detection unit by counteracting potential tilting of the detection unit about a horizontal transverse axis. The rotatably supported tool ends of such embodiments can form front and rear wheels.

[0047] Figure 8 It is shown that the detection unit is guided along the leading edge of the blade 2 by means of a guide rail 30, which is mounted on the blade 2 and which is arranged to be guided by the detection unit. Figure 7The graph of sensor data obtained by sensors S1-S5 when the detection unit 20 is in place. It can be seen that while the sensor values recorded from sensors S1, S2, S4 and S5 are fairly consistent and do not deviate much from the center line in distance, the sensor values recorded from sensor S3 show a significant deviation from the center line in distance and thus indicate a potential defect at the sensor position S3. Together with the data obtained from the control wheel and the known starting point, the user can easily find the area where the potential defect can be found, even if the detection unit is not equipped with the above-mentioned marking unit or signal unit.

[0048] While the application has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention that the application be limited thereto. The various features discussed herein can be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The application in its broader aspects is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Accordingly, departures can be made from such details without departing from the scope of the general inventive concept.

[0049] List of reference signs

[0050] 1 horizontal axis wind turbine

[0051] 2 wind turbine generator blade

[0052] 3 leading edge

[0053] 4 outer protective layer / outer protective layer profile

[0054] 5 adhesive layer

[0055] 6 structural material layer

[0056] 7 voids (defects)

[0057] 10 defect detection tool

[0058] 11 tool end

[0059] 12 tool base

[0060] 13 outer rim

[0061] 14 roller bearing

[0062] 15 fork

[0063] 16 spigot

[0064] 20 defect detection unit

[0065] 21 drive means

[0066] 22 electric motor

[0067] 23 friction wheel

[0068] 24 external weight

[0069] 25 coil spring

[0070] 26 storage unit

[0071] 27 signaling device

[0072] 28 marking device

[0073] 29 control wheel

[0074] A rotation axis of the tool end

[0075] T tool movement direction

[0076] B tool biasing direction

[0077] S 1-5 sensor

Claims

1. A method for detecting defects underneath an outer protection layer (4) protecting a leading edge (3) of a wind turbine generator blade (2), the method comprising the steps of: - guiding a defect detection tool (10) over the outer protection layer (4) in a tool movement direction (T), and - sensing at least one feedback value experienced by the tool (10) when guided over the outer protection layer (4) to detect potential defects (7), wherein the defect detection tool (10) comprises a tool base (12) and a tool end (11) which is biased against a surface of the outer protection layer (4) in a tool biasing direction (B) when guided over the outer protection layer (4) in the tool movement direction (T) and detects potential defects (7) underneath the outer protection layer (4) as a uniformity deviation of the at least one feedback value, characterized in that the tool end (11) is formed by a disc, a rim or a sphere (13) rotatably supported relative to the tool base (12) such that the tool end (11) rolls over the surface of the outer protection layer (4) when the defect detection tool (10) is guided over the outer protection layer (4) in the tool movement direction (T) and biased against the surface of the outer protection layer (4) in the tool biasing direction (B), and wherein in the tool biasing direction (B) the tool end (11) is rigidly supported relative to the tool base (12).

2. The method of claim 1, wherein, The defect detection tool (10) is handheld and guided by a user, the at least one feedback value is sensed by the user.

3. The method of claim 1, wherein, The defect detection tool (10) is part of a detection unit (20) which is moved along the leading edge (3) of the wind turbine generator blade (2), the detection unit (20) comprising a feedback value sensing unit which senses the uniformity deviation of the at least one feedback value with at least one sensor.

4. The method of claim 3, wherein, The sensor is one of a displacement measuring sensor and a pressure measuring sensor and a force measuring sensor.

5. The method of claim 1 or claim 3, wherein, The detection unit (20) is moved along the leading edge (3) of the wind turbine generator blade (2) and - coordinates of the detected potential defects (7) are stored in a storage unit (26), and / or - the detected potential defects (7) are marked on the wind turbine generator blade (2) by a defect marking device (28), and / or - the detected potential defects (7) are communicated to a user by a signal unit (27) by sending a signal.

6. The method of claim 3, wherein, The detection unit (20) is positioned on the leading edge (3) of the wind turbine generator blade (2) with the tool end (11) biased against the surface of the outer protection layer (4) by the weight of the detection unit (20).

7. A defect detection tool (10) for detecting defects (7) underneath an outer protection layer (4) protecting a leading edge (3) of a wind turbine generator blade (2), the defect detection tool (10) comprising a tool base (12) and a tool end (11), the tool end (11) being designed to be guided across a surface of the outer protection layer (4) in a tool movement direction (T) and to be biased against the surface of the outer protection layer (4) in a tool biasing direction (B) in order to sense at least one feedback value experienced by the tool end (11) in the tool movement direction (T) and / or in the tool biasing direction (B) when being guided over the outer protection layer (4), characterized in that the tool end (11) being formed by a rim, a disc or a ball rotatably supported with respect to the tool base (12), and wherein in the tool biasing direction (B) the tool end (11) is rigidly supported with respect to the tool base (12).

8. The defect detection tool of claim 7, wherein, the tool base (12) comprises a handle to be held by a hand of a user.

9. The defect detection tool of claim 7, wherein, the tool base (12) forms part of and / or is connected to a sensing portion, the sensing portion being part of a sensing unit, the sensing unit sensing uniformity deviations of the at least one feedback value with at least one sensor.

10. The defect detection tool of claim 9, wherein, the at least one sensor is one of a displacement measuring sensor and a pressure measuring sensor and a force measuring sensor.

11. The defect detection tool of any one of claims 7 to 10, wherein, the rim (13) or disc forming the tool end (11) is supported with respect to the tool base (12) by rolling elements arranged between the tool end (11) and the tool base (12).

12. The defect detection tool of any one of claims 7 to 10, wherein, the tool end (11) is an outer rim (13) of a roller bearing (14).

13. The defect detection tool of any one of claims 7 to 10, wherein, the tool end (11) is a ball of a gimbal ball unit.

14. The defect detection tool of any one of claims 7 to 10, wherein, a width of the tool end (11) perpendicular to the tool movement direction is less than 10 mm and / or an outer diameter of the rim (13) or disc forming the tool end (11) is less than 25 mm.

15. The defect detection tool of claim 14, wherein, a width of the tool end (11) perpendicular to the tool movement direction is equal to or less than 6 mm and / or an outer diameter of the rim (13) or disc forming the tool end (11) is equal to or less than 20 mm.

16. A detection unit for detecting defects underneath an outer protection layer (4) protecting a leading edge (3) of a wind turbine generator blade (2), the detection unit comprising at least one defect detection tool (10) according to one of claims 7 to 15.

17. The method according to any one of the preceding claims 1 to 6, using a defect detection tool (10) according to any one of the preceding claims 7 to 15 and / or a detection unit (22) according to claim 16.

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