WO2012003828A1

WO2012003828A1 - Abrading arrangement to abrade a surface of an item and method of use thereof

Info

Publication number: WO2012003828A1
Application number: PCT/DK2011/000079
Authority: WO
Inventors: Keld Eriksen; Poul Lundum
Original assignee: Eltronic A/S
Priority date: 2010-07-09
Filing date: 2011-07-08
Publication date: 2012-01-12
Also published as: CN103118838A; EP2590781A1; US20130178134A1

Abstract

An abrasion arrangement is disclosed to abrade a surface of an item, the arrangement comprising a multiple axis robotic arm having at least five axes, an abrading cylinder mounted on the robotic arm and comprising abrasive means which comprise abrasive lamellae of an abrasive sheet, such as abrasive cloth, of which the front side has abrasive properties and which extend substantially radially from an elongated core and means for driving said core to rotate around a longitudinal axis of the core, and control means for controlling the operation of the robotic arm so as to control e.g. the position of the abrading cylinder on said surface, the force with which the abrading cylinder is pressed towards said surface and the velocity with which the abrading cylinder is moved with respect to said surface.

Description

ABRADING ARRANGEMENT TO ABRADE A SURFACE OF AN ITEM AND METHOD OF USE THEREOF

The present invention relates to an arrangement comprising a robotic arm carrying abrasive means for abrading the surface of an item and method of use of the arrangement.

BACKGROUND Abrasion of the surface of items of a reasonable size by means of a stationary abrading machine is well-known in the art from e.g. WO 2004/098831. However, the abrasion of the surface of larger items, where it is impractical to provide stationary abrasive means of dimensions sufficiently large is e.g. known to be provided by means of a robotic arm carrying abrasive means as disclosed in US 2002/0072297 for the surface treatment of panels for aircrafts and from the Spanish company ΕΓΝΑ which in the magazine Wind Systems of March 2010 disclosed surface grinding of a wind turbine blade by means of a 8-axis robot handling e.g. a sanding head having a plurality of abrasive disks arranged. However, improvements of the means for abrading the surface of large items in order e.g. to remove irregularities and prepare the surface for painting are requested and it is an object of the present invention to provide such improvements.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to an abrasion arrangement to abrade a surface of an item, the arrangement comprising a multiple-axis robotic arm having at least five axes, an abrading cylinder mounted on the robotic arm and comprising abrasive means which comprise abrasive lamellae of an abrasive sheet, such as abrasive cloth, of which the front side has abrasive properties and which extend substantially radially from an elongated core and means for driving said core to rotate around a longitudinal axis of the core, and control means for controlling the operation of the robotic arm so as to control e.g. the position of the abrading cylinder on said surface, the force with which the abrading cylinder is pressed towards said surface and the velocity with which the abrading cylinder is moved with respect to said surface.

The abrading cylinder in itself is well known from e.g. from WO 01/76824 and partly from the above-mentioned WO 2004/098831 and as well as from a number of other documents with various embodiments. The type of abrading cylinders disclosed in WO 01/76824 is preferred in the present invention.

With the term velocity herein understood a vector, i.e. the speed as well as the direction of movement. The cylinder is rotated so that the front side of the abrasive lamellae is moved across the surface to be abraded. The tangential movement of the front side of the lamellae over the surface to be abraded due to the rotation of the cylinder defines herein the abrading direction of the cylinder.

With the term robotic arm is herein understood a multiple-axis arm having at least 5 degrees of freedom, i.e. that it is able to move the abrading cylinder across the surface of the item, towards and away from said surface, to tilt the abrading cylinder in various planes and that it is able to rotate the abrading cylinder about an axis substantially normal to the surface of the item, so that the abrading direction of the cylinder may be reversed by a 180° rotation. The robotic arm is at least a 5-axis arm and most preferred a 6-axis arm, however robotic arms with higher degrees of freedom could also be employed. With a robotic arm having a degree of freedom of 5, 6 or even more, the arm with the cylinder becomes a more flexible abrading tool that can adjust better to a complex double curved surface of the item to be abraded and will be able to handle areas around edges more gentle.

An advantage obtained by employing the abrading cylinder as described is that a more efficient abrasion of a surface may be obtained as compared to the use of abrasive disks and the abrading cylinder is in itself flexible to the shape of the surface and does not need to be perfectly aligned with the surface to perform the abrading of the surface satisfactory. It also readily abrades surfaces of complex shapes such as double-curved surfaces. A drawback of the abrading cylinder is that it is only efficient when the relative movement of the surface and the abrading cylinder causes the surface to move in the opposite direction of the movement of the abrasive lamellae caused by the rotation of the cylinder, because the movement of the object in that case enhances the movement of the abrasive lamellae with respect to the surface, whereas relative movement of the surface against the direction of the movement of the abrasive lamellae caused by the rotation of the cylinder weakens the abrasive effect on the surface. Thus, the abrading cylinder is generally applied in machines as shown in the above-discussed WO 2004/098831 with a uniform relative movement of the item with respect to the abrading cylinder.

Furthermore, the abrading cylinder is not particularly suitable for use near edges of items as the rotation of the cylinder may cause the abrasive lamellae to collide with the edge of the item in case the axis of rotation of the cylinder is not substantially perpendicular to the edge, if the abrading direction of the cylinder is towards the surface of the item and not towards the edge of the item. The abrasive lamellae will collide with the edge which has a damaging and life-shortening effect on the lamellae as well a damaging effect on the edge of the item. This may however be avoided by the present combination of an abrading cylinder and a robotic arm, which allow for advances control of the operational position of the cylinder. According to a preferred embodiment of the present invention, the control means are adapted to move the abrading cylinder across said surface to alternate between

first movement in a direction substantially perpendicularly to the longitudinal axis of the core and substantially in the same direction as an abrading direction of the rotated cylinder, the first movement being continued until an edge of said surface is reached,

translating the cylinder substantially in a direction perpendicularly to the direction of the first movement, second movement in a direction substantially perpendicularly to the longitudinal axis of the core and substantially against an abrading direction of the rotated cylinder,

rotation of the cylinder half a turn to reverse the abrading direction thereof with respect to said surface, and

third movement in a direction substantially perpendicularly to the longitudinal axis of the core and substantially in the same direction as an abrading direction of the rotated cylinder. This working pattern of the abrasion arrangement provides for a time-efficient abrading of the surface of the item as the cylinder is moved in the same direction as the abrading direction of the cylinder over a maximum of the surface, i.e. in the first and the third movement and at the same time can be taken to or close to the edges of the item without risking the above-discussed damages to the abrasive lamellae or the edges of the item. The third movement is in principle similar to the first movement and is preferably followed by a translation of the cylinder and a movement similar to the second movement etc. until the full surface of the item has been abraded.

The translating of the cylinder is in one embodiment a translation of a distance substantially equal to a width of the cylinder, so that the overlap between neighbouring strokes or movements is negligible or small, such as 1 to 3 centimetres. Alternatively, the translation is about half the width of the cylinder so that each part of the surface is abraded twice by the cylinder. It is preferred that the cylinder is lifted form contact with said surface prior to the rotation of the cylinder half a turn and thereafter is moved towards the surface for continued abrading of the surface.

The control means are in a preferred embodiment adapted to abrade a surface of an item by means of said abrading cylinder to approximate a predefined curved shape of said surface, in particular a curved shape comprising a plurality of double curved areas, such as a wind turbine blade. Thereby, the abrasion arrangement may be employed to produce items of a highly uniform shape, which is very difficult otherwise for large items such as wind turbine blades. The abrasive means comprise preferably an elastic support element, preferably support brushes, which support the backside of the abrasive lamellae, said support element substantially having almost the same length as the lamellae.

It is an advantage that the actual shape of the surface of the item is detected so that the deviation between an ideal shape of the surface of the item may be determined and/or the deflection of the item due to the gravitational force may be determined. Thus, it is an advantage that the abrasion arrangement comprises a set of sensors for providing an input to the control means in order for the control means to determine the actual shape of the surface of the item. Preferably, at least some of the set of sensors are arranged on the robotic arm. The set of sensors comprises advantageously contactless distance sensors, in particular optical sensors.

The abrasion arrangement comprises preferably means for correlating the determined actual shape of the surface of the item with data representing a predefined curved shape of the finished surface of the item to identify deviations between said ideal surface shape and the actual shape of the surface of the item. Said deviations may comprise angular displacement deviations of the item to be abraded, deformities in the surface of the item to be abraded and/or deflections of said blade to be abraded when the blade is arranged to be abraded by said abrasion arrangement.

The present invention also relates to the method of use of an abrasion arrangement as described herein for abrading the surface of wind turbine blades. Hereby, uniform wind turbine blades may be produced which is very advantageous as the control of the wind turbine becomes more simple and predictable when the aerodynamic properties of the individual blades of the wind turbine are substantially identical. BRIEF DESCRIPTION OF FIGURES

Embodiments of the present invention are illustrated in the enclosed drawing of which

Fig. 1 shows a robotic arm equipped with an abrading cylinder, Fig. 2 is an example of an abrading cylinder within a shielding housing, Fig. 3 is a working pattern for abrading of the surface of an elongated item, Fig. 4 shows a first view of the use for abrading a wind turbine blades, and Fig. 5 shows a second view of the use for abrading a wind turbine blade.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An abrasion arrangement of the present invention for abrading of a surface 1 is shown in Fig. 1 comprising a 6-axis articulated robotic arm 2 on which is mounted an abrasion head 3 having an abrading cylinder 4 enclosed in a shielding housing 5 provided with a suction outlet (not shown) for removal of dust and a motor (not shown) for driving the rotation of the abrading cylinder 4. The base 6 of the robotic arm 2 is in Fig. 1 mounted on a vertical column 7 equipped with parallel, vertical tracks 8 on which the base 6 is displaceable arranged in the vertical direction so as to enable abrasion of a surface 1 of a wider extent in the vertical direction than the robotic arm 2 itself allows for.

The abrading cylinder 4 shown in Fig. 2 has abrasive means which comprise abrasive lamellae 9 of an abrasive sheet, such as abrasive cloth, of which the front side 10 has abrasive properties and which extend substantially radially from an elongated core 1 1 of the cylinder. The abrasive lamellae 9 are supported on the back side 9 by an elastic support element comprising support brushes 12 having almost the same length as the lamellae 9. The cylinder 4 is during operation of the abrasion arrangement rotated so that the front side 10 of the abrasive lamellae 9 is moved across the surface 1 to be abraded, the direction of rotation are indicated by the curved arrows R on Fig. 2. The tangential movement of the front side 10 of the lamellae 9 over the surface 1 to be abraded due to the rotation of the cylinder 4 defines the abrading direction of the cylinder 4 indicated with straight arrow AD. The core 1 1 of the cylinder 4 shown in Fig. 2 is equipped with helical or spiral shaped undercut grooves for retaining the flexible sanding strips holding the abrasive lamellae 9 as well as the brushes 12 but the grooves may in another embodiment of the present invention be straight along the longitudinal direction of the core 1 1.

In a particular embodiment of the present invention, the control means for controlling the operation of the abrasion arrangement are adapted to let the abrading cylinder operated according to the working pattern illustrated in Fig. 3 for abrading the surface 1 of an elongated item 13 having a first edge 14 and a second edge 15 both extending generally in the longitudinal direction of the elongated item 13, such as a wind turbine blade. The first edge 14 and the second edge 15 are not necessarily parallel to each other but are substantially so as depicted in Fig. 3. The part of the working pattern described in details herein start at the letter "S" on Fig. 3. The abrading cylinder 4 is in abrading engagement with the surface 1 of the elongated item 13 starting at a position at the first edge 14 and moving towards the second edge 15 at a first velocity and where the cylinder 4 is oriented so that the abrading direction is against the direction of movement of the abrasion head 3, indicated with the straight arrows M in Fig. 3. The reason to have the direction of movement M to be opposite the abrading direction AD of the cylinder 4 is that it is in that way avoided that the abrasive lamellae 9 collide with the first edge 14 which have a damaging and life-shortening effect on the lamellae 9 as well a damaging effect on the edge 14 of the item. However, when the direction of movement M is opposite the abrading direction AD, the abrading action on the surface 1 is less efficient and the speed of the movement of the abrasion head 3 must be reduced to obtain a satisfactory finish of the surface 1 , for which reason the extent of these parts of the working pattern, generally referred to with the letter "B" in Fig. 3 has been minimised. When the abrasion head 3 has been moved away from the first edge 14, the abrasion head 3 is lifted away from the item 13 so that the cylinder 4 disengages the surface 1 of the item 13 and the abrasion head 3 is turned around at the position indicated in Fig. 3 with the letters "TU" so that the abrading direction AD of the cylinder 4 is reversed. Now, the abrasion head 3 is lowered towards the item 13 until the cylinder 4 engages the surface 1 with a sufficient force and the movement of the abrasion head 3 across the surface 1 of the item towards the second edge 15 is continued. In this part of the working pattern, generally referred to with the letter "A" in Fig 3, the direction of movement M and the abrading direction AD of the cylinder 4 is the same direction, the abrading action is therefore more efficient and the speed of the movement of the abrasion head 3 can be considerably higher than in the B parts of the working pattern. When the second edge 15 is reached by the abrasion head 3, the head is translated substantially one width of the cylinder 4 in the longitudinal direction of the elongated item 13, the translation being indicated generally by the arrows in Fig. 3 referred to with the letters "TL". The sequence of working patterns is now repeated starting from the second edge 15 and moving towards the first edge 14 of the elongated item 13, where a part B of the working pattern where the direction of movement M is opposite the abrading direction AD ends with a turning TU of the abrasion head and is continued with a part A where the direction of movement M is the same as the abrading direction AD, etc.

In Fig. 3 the parts A, B of the working pattern are depicted with a minor distance in between for the sake of clarity. However, the parts A, B are in the present embodiment abutting or are overlapping e.g. 1 to 3 centimetres so that the whole of the surface 1 is abraded by the cylinder 4. In an alternative embodiment, the parts A, B are overlapping in the longitudinal direction of the item 13 with half the width of the cylinder 4 so that each area of the surface will be abraded twice by the cylinder 4. The areas around the edges 14, 15 may only be partly abraded by the cylinder 4 and require a manually controlled abrasion to obtain the correct finish. A particular embodiment and use of the present invention is shown in Figs. 4 and 5, where two robotic arms 2, 2' each carrying an abrasion head 3 are mounted on each their vertical column 7, 7' which are connected by a horizontal crossbar 16 and supported on wheels so as to be displaceable along an elongated item 13 on a set of tracks 17 laid out horizontally on the floor. This arrangement is particularly suitable for abrading the surface of a wind turbine blade 13 where the two sides between the leading edge 14 and the trailing edge 15 of the blade 13 can be abraded simultaneously by the abrasion heads 3 carried by the robotic arms 3, 3'. The arrangement is displaced along the blade on the tracks 17 so that the whole surface of the blade may be abraded. In order to facilitate a compensation for the twist of the blade 13 along its longitudinal axis 18, the blade 13 is supported so that it may be rotated around its axis 18 for the blade 13 to be substantially horizontally oriented at the position of the vertical columns 7, 7'.

The control system controlling the operation of the arrangement shown in Figs. 4 and 5 comprises a predefined set of data defining the ideal three-dimensional shape of the finished blade 13 and the control system is adapted for controlling by means of the robotic arms 2, 2' the position of the abrasion heads 3, the force with which they are pressed towards the surface 1 of the blade 13 and the speed with which they are moved across the surface 1 so as to process the surface 1 of the blade 13 to approximate the predefined ideal shape of the blade 13. A set of sensors (not shown) are arranged on the robotic arms 2, 2' and/or on the vertical columns 7, 7', in particular contactless distance sensors using laser light or ultrasonic means for providing an input to the control system in order for the control system to determine the actual shape of the blade 13.

When e.g. a wind turbine blade 13 has been manufactured to a state before being abraded by the abrasion arrangement, it most likely will deviate from the ideal blade shape/geometry of a finished blade. Such deviations may e.g. originate from angular displacements of the blade compared to ideal angular displacements of the blade. Preferred ideal wind turbine blade profiles comprise an ideal angular displacement to obtain an improved aerodynamic blade profile. However, the manufacturing process of the blade may result in blades that deviate from such ideal angular displacements. Also, the manufacturing process of the blade may result in deformities such as deformities in the surface of the blade which causes the blade to deviate from the ideal blade shape/geometry.

Likewise, the blade may be arranged in a plurality of ways to be abraded by the abrasion arrangement. Preferably, arranging the blade comprises that the root end of the blade is attached to a suitable fixation arrangement (not illustrated in any figures) so the blade extends substantially horizontally to be abraded by the abrasion arrangement as illustrated in fig. 5 while only being supported at the root end of the blade 13. As another example the blade may be supported by one or more supportive structures arranged underneath the blade in the longitudinal direction of the blade. It is however naturally understood that any suitable way of arranging the blade so it can be abraded by the abrasion arrangement may be utilized, and furthermore e.g. a combination of fixating the root end to a suitable fixation arrangement may be combined with supporting the blade, e.g. underneath the blade at further locations along the blade.

Thus, when the blade 13 is arranged in the abrasion arrangement, it may result in the blade deflecting from its ideal shape due to e.g. gravity. For example, if the blade is arranged in the abrasion arrangement by the root being attached to a fixation arrangement, the tip end/free end of the blade will deflect downwards. In general, the deflection of the blade is dependent of among others, the elasticity of the blade, the size of the blade, the orientation of the blade (e.g. orientation of the leading and trailing edges of the blade) and others. Also, the blade 13 may deflect in different ways dependent on the blade orientation when e.g. rotating the blade around the longitudinal direction of the blade as illustrated in fig. 5. It is preferred that the abrasion arrangement facilitates taking such angular displacement deviations, such deviations due to blade deformities, and/or blade deflections into consideration during the abrasion process. That is, the blade 13 is preferably abraded to reduce or even remove angular displacement deviations and/or blade deformities compared to the ideal blade shape whereas deflections of the blade are preferably compensated for during the abrasion so that the blade is not erroneously abraded.

Thus, it is preferred that the abrasion arrangement comprises a deviation handling arrangement for automatically handling and/or compensating for deviations such as the above mentioned angular displacement deviation, the mentioned deformity deviations and deflections due to the arrangement of the blade to be abraded.

Such a deviation handling arrangement is described in more details in fig. 6. The deviation handling arrangement gathers information from at least two data sources 21, 23.

The first data source 21 comprises predefined data regarding the predefined ideal three-dimensional surface shape of the finished item, in particular a wind turbine blade as described above. This ideal item surface shape data may e.g. be provided by data from three-dimensional blade drawings such as for example three-dimensional CAD drawings, from a look up table and/or any suitable other data which in self or on combination with other data facilitates determining the ideal blade shape/geometry.

The second data source comprises an item surface shape determination arrangement gathering measurement data regarding the actual shape/geometry of the surface of the item to be abraded. The surface shape determination arrangement 23 preferably comprises an optical arrangement, e.g. arranged at the vertical column(s) 7 and/or crossbar 16, and the optical arrangement facilitates scanning the surface of the blade to detect the actual shape/geometry of the item surface. However, it is understood that the said shape determination arrangement 23 may be arranged at any appropriate location at the abrasion arrangement and that it may comprise any suitable number of optical scanners and possible accompanying scanner controllers. The optical arrangement preferably comprises one or more emitters for emitting electromagnetic radiation towards the surface of the item, e.g. a wind turbine blade 13, to be abraded. This radiation is reflected from the surface and a part of the reflected radiation is detected by the optical arrangement, converted into electrical signals representing the detected reflected radiation from the surface, and these electric signals may thus be processed by the surface shape determination arrangement to establish information regarding the actual geometry/shape of the item to be abraded.

In a preferred embodiment, the item surface shape determination arrangement 23 facilitates establishing data of the actual surface shape by establishing three- dimensional data of the surface.

The ideal surface shape data from the first data source 21 and the data from the surface shape determination arrangement representing the detected actual shape/geometry of the surface is accessed/transmitted 20, 22 to and correlated by data processing means 19 receiving the data. The data processing means 19 can thus, based on the said correlation, generate control signals 24 to the control means of the abrasion arrangement. Based on such control signals, the control means can control the operation of the robotic arm by e.g. controlling the position of the abrading cylinder on the surface of the item, the force with which the abrading cylinder is pressed towards said surface and/or the velocity with which the abrading cylinder is moved with respect to the surface, and hereby reduce/remove e.g. the said angular displacement deviations and surface deformities identified by the correlation to approximate the predefined ideal three-dimensional shape of the finished item. Also, the control signals preferably facilitates that the abrasion arrangement takes into consideration the deflection of the item to be abraded so the abrasion arrangement compensates for such deflections during the abrasion of the surface of the item 13.

In an aspect of the invention, the deviation handling arrangement facilitates continuously scanning the surface during abrasion of the item surface to establish information of the surface area of the item being abraded and/or to establish information of the surface area of the item to be subsequently abraded.

In another embodiment, the deviation handling arrangement may before starting the abrasion of the surface establish a substantially complete abrasion template for the control means to follow during abrasion of the item by scanning the surface prior to performing the abrasion of the item.

In a further embodiment, the deviation handling arrangement may be arranged to facilitate that scanning the surface of the item and performing the abrasion of the item is performed during the same movement of the vertical column(s) 7 along the item to be abraded. Thus, the data from the optical arrangement is continuously processed and correlated with the ideal item surface shape data to continuously generate an output for the control means during the movement along the item to be abraded.

Claims

1. An abrasion arrangement to abrade a surface (1) of an item (13), the arrangement comprising

a multiple-axis robotic arm (2, 2') having at least five axes,

an abrading cylinder (4) mounted on the robotic arm and comprising abrasive means which comprise abrasive lamellae (9) of an abrasive sheet, such as abrasive cloth, of which the front side has abrasive properties and which extend substantially radially from an elongated core (1 1) and means for driving said core to rotate around a longitudinal axis of the core, and

control means for controlling the operation of the robotic arm so as to control e.g. the position of the abrading cylinder on said surface, the force with which the abrading cylinder is pressed towards said surface and the velocity with which the abrading cylinder is moved with respect to said surface.

2. An abrasion arrangement according to claim 1, wherein the control means are adapted to move the abrading cylinder across said surface to alternate between

first movement (A) in a direction substantially perpendicularly to the longitudinal axis of the core and substantially in the same direction as an abrading direction of the rotated cylinder, the first movement being continued until an edge of said surface is reached,

translating (TL) the cylinder substantially in a direction perpendicularly to the direction of the first movement,

second movement (B) in a direction substantially perpendicularly to the longitudinal axis of the core and substantially against an abrading direction of the rotated cylinder,

rotation of the cylinder half a turn (TU) to reverse the abrading direction thereof with respect to said surface, and

third movement (A) in a direction substantially perpendicularly to the longitudinal axis of the core and substantially in the same direction as an abrading direction of the rotated cylinder.

3. An abrasion arrangement according to claim 2, wherein said translating (TL) of the cylinder is a distance substantially equal to a width of the cylinder. 4. An abrasion arrangement according to claim 2 or 3, wherein the cylinder is lifted from contact with said surface prior to the rotation of the cylinder half a turn (TU).

5. An abrasion arrangement according to any of the preceding claims, wherein the control means are adapted to abrade a surface of an item by means of said abrading cylinder to approximate a predefined curved shape of said surface.

6. An abrasion arrangement according to claim 5, wherein said predefined curved shape comprises a plurality of double curved areas. 7. An abrasion arrangement according to any of the preceding claims, wherein the abrasive means comprise an elastic support element, preferably support brushes, which support the backside of the abrasive lamellae, said support element substantially having almost the same length as the lamellae. 8. An abrasion arrangement according to any of the preceding claims, wherein said robotic arm comprises six axes.

9. An abrasion arrangement according to any of the preceding claims, wherein said item is a wind turbine blade.

10. An abrasion arrangement according to any of the preceding claims, comprising a set of sensors for providing an input to the control means in order for the control means to determine the actual shape of the surface of the item. 1 1. An abrasion arrangement according to claim 10, wherein at least some of the set of sensors are arranged on the robotic arm.

12. An abrasion arrangement according to claim 10 or 1 1 , wherein the set of sensors comprises contactless distance sensors, in particular optical sensors. 13. An abrasion arrangement according to any of claims 10 to 12, wherein said abrasion arrangement comprises means for correlating the determined actual shape of the surface of the item with data representing a predefined curved shape of the finished surface of the item to identify deviations between said ideal surface shape and the actual shape of the surface of the item.

14. An abrasion arrangement according to claim 13, wherein said deviations comprise angular displacement deviations of the item to be abraded.

15. An abrasion arrangement according to claim 13 or 14, wherein said deviations comprise deformities in the surface of the item to be abraded.

17. An abrasion arrangement according to any of claims 13 to 16, wherein said deviations comprise deflections of said item to be abraded when the item is arranged to be abraded by said abrasion arrangement.

18. Use of an abrasion arrangement according to any of claims 1 -17 for abrading the surface of wind turbine blades.