Abstract:
A composite layup method is described in which strips of fibrous material are deposited progressively on a mould surface. The method involves feeding a strip of fibrous material to a placement device for placing the strip progressively on the mould surface, and imparting a shape to the strip upstream of the placement device. In this way, shape is imparted to the strip prior to the strip reaching the placement device. The shape imparted to the strip corresponds at least partially to a shape of the mould surface. This prevents or reduces air from becoming trapped between the strip and the mould surface as the strip is deposited in the mould. An end effector assembly suitable for use in the composite layup method is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(a) to GB Application No. 1101332.3, filed Jan. 26, 2011. This application also claims the benefit of U.S. Provisional Application No. 61/436,261, filed Jan. 26, 2011. Each of these applications is incorporated by reference herein in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to composite layup techniques suitable for laying up large composite structures such as modern wind turbine blades. 
       BACKGROUND 
       [0003]    Until recently, large composite structures such as the blades of modern wind turbines have generally been created using manual layup techniques. This involves arranging mats or plies of reinforcing fibrous material in large moulds by hand. Several layers of fibrous material are arranged in the mould. The mats typically comprise glass or carbon fibres. Once the mats have been arranged in the mould, resin is supplied to the mould using a technique such as resin transfer moulding (RTM) or vacuum-assisted resin transfer moulding (VARTM). Alternatively, the mats may be pre-impregnated with resin, i.e. prepreg, which dispenses with the need to supply resin to the mould. In any event, the layup is generally subjected to a vacuum-assisted and temperature-controlled consolidation and curing process. 
         [0004]    There is a continual drive to increase the power output of modern wind turbines. To this end, larger wind turbines are being produced, which require larger blades. As the size of the blades increases, manual layup techniques become less suitable. For example, due to the curvature of wind turbine blades, reaching the edges of very large moulds manually would require scaffolding or persons being suspended high above the factory floor. This can make manual techniques unfeasible. 
         [0005]    Presently, automated layup techniques are being considered for the production of modern wind turbine blades. Typically, this involves the use of robots arranged to move on tracks adjacent the moulds or upon gantries above the moulds. The robots are programmed to perform tasks such as applying gel coats to the moulds, laying up fibrous material in the mould, and painting the cured laminates. Robots are able to reach parts of the mould that are difficult to reach manually. In addition, the use of automated procedures can increase the speed, accuracy and repeatability of the layup process, and results in parts having a higher quality and consistency. 
         [0006]    One example of an automated layup technique involves laying a continuous strip of prepreg material in a mould using a deposition end effector assembly mounted on the end of a robotic arm or on a gantry. Whilst this technique works well on flat surfaces, the technique presents particular challenges when employed on curved surfaces such as the surface of a wind turbine blade mould. For example, applying a strip of material to a curved, or otherwise uneven surface can result in air becoming trapped beneath the material as it is laid. This often causes wrinkles to develop in the material during consolidation, which may compromise the structural integrity of the finished blade. 
       SUMMARY 
       [0007]    Aspects of the present invention provide a solution to the aforementioned problem. 
         [0008]    According to embodiments of the invention, there is provided a composite layup method in which strips of fibrous material are each deposited progressively on a mould surface, the method comprising: feeding a strip of fibrous material along a path extending from a supply of the fibrous material to a placement device for placing the strip progressively on the mould surface; and imparting a shape to the strip within the path while maintaining the length of the strip within the path substantially uniform across the width of the strip, wherein the shape imparted to the strip corresponds at least partially to a shape of the mould surface. 
         [0009]    In the case of fibrous material containing fibres which are directed along the length of the strip, by maintaining the length of the strip substantially uniform across its width, this will prevent or reduce variations in the tension of the strip across the width, which may cause distortions within the fibrous material, which, in turn, could cause buckling of the material within the mould. 
         [0010]    Embodiments of the invention also provide an end effector assembly for use in composite layup, the assembly being configured to deposit strips of fibrous material progressively on a mould surface, and the assembly comprising: a placement device for placing a strip of fibrous material on the mould surface; a feed mechanism for feeding the strip along a path extending from a supply of the fibrous material to the placement device; and a shaping device located within the path and configured to impart a shape to the strip while maintaining the length of the strip within the path substantially uniform across the width of the strip, wherein the shape imparted to the strip corresponds at least partially to a shape of the mould surface. 
         [0011]    Shaping the pre-deposited material to correspond to the shape of the mould surface ensures that the strip is placed flush against the mould surface across the full width of the strip, despite any curvature in the mould surface. This effectively prevents air from becoming trapped between the strip and the mould as the strip is laid. 
         [0012]    In one embodiment, the method comprises imparting a curvature to the strip. In this respect, the shaping device may be suitably configured to impart the requisite curvature. The curvature may be imparted in a direction transverse to a direction in which the strip is laid in the mould, i.e., transverse to a direction of progression of the placement device. Shaping the strip in this way is suitable for laying up wind turbine blades which are formed in curved moulds. 
         [0013]    The shaping device may comprise one or more shaping rollers over which the strip of material is fed. In one embodiment, the shaping device includes a series of rollers. The rollers may be arranged in a curved formation, similar to troughing idlers, which are commonly found in conveyor belt systems, albeit for a different purpose. The rollers may rotate about a common shaft, which may be curved. In one embodiment, the shaping device comprises a set of garland troughing idlers. Garland troughing idlers generally take the form of a set of rollers that rotate on respective individual shafts, which are connected end-to-end in the form of a chain. Each end of the chain may be attached to a mounting point on the end effector assembly, and the chain may hang between the mounting points in a shallow U-shape. Alternatively, fixed roller mountings may be used. 
         [0014]    The strip of fibrous material may be fed over the rollers, i.e., through the U-shape, such that the rollers impart a curvature to the material immediately before it reaches the placement device. Alternatively, the shaping device may comprise any other suitable means or devices for imparting a desired shape to the strip within the end effector, such as, for example, a suitably shaped die through which the strip is fed. 
         [0015]    The placement device may comprise one or more placement rollers, and in an exemplary embodiment comprises a series of placement rollers. The rollers are configured to roll against the material as it is deposited. Hence, in use, the placement device is separated from the mould surface by the thickness of the deposited material. In one aspect, the placement device may be capable of displacement in a direction normal to the mould surface, akin to a vehicle suspension system. This enables the placement device to adapt to varying contours of the mould surface and ensures that an even pressure is applied across the width of the material as it is deposited in the mould. To this end, the placement rollers may comprise fluid-filled bags. The fluid may be gas or liquid. In an exemplary embodiment, it is gas, such as air. Rollers in the form of air-filled bags are flexible and can displace normal to the mould surface as required. Another way of achieving the desired normal displacement is for the rollers to be attached to suitable actuation devices or means. For example, each placement roller may be attached to a hydraulic or pneumatic actuator, or to a spring. With this arrangement, rigid placement rollers may be employed. 
         [0016]    The series of placement rollers may include one or more central placement rollers located between respective first and second sets of peripheral rollers, which may also be referred to as side rollers or ‘wing’ rollers. The first and second sets of peripheral rollers may each include one or more rollers. The placement rollers may be arranged in a chevron or V-shaped formation, in which the or each central placement roller is located ahead of the peripheral placement rollers in a direction of advancement of the placement device across the mould surface. Hence, the method may include advancing a central placement roller ahead of peripheral placement rollers. The effect of this is to smooth the deposited strip of fibrous material outwards from its centre to its edges as it is laid in the mould. This ensures that any air that does become trapped between the strip and the mould is expelled effectively. 
         [0017]    In one embodiment, the fibrous material is continuously supplied to the placement device. To this end, the fibrous material may be stored as a continuous length on a spool, i.e., as a roll, as part of the end effector assembly. Shaping of the fibrous material occurs on-the-fly, at a point between the roll and the placement device, immediately before the strip is placed in the mould. 
         [0018]    The shaping device and the placement device operate synergistically to prevent or reduce wrinkles developing in the deposited material. Shaping the material to conform to the contour of the mould surface immediately prior to deposition minimises the potential for trapped air, whilst smoothing the deposited material from the centre outwards expels any air that may nevertheless become trapped. 
         [0019]    The method and apparatus of the present invention are suitable for laying up any variety of fibrous reinforcing material, including dry fibres and prepreg. The fibres may be carbon, glass, aramid or any other suitable reinforcing fibre. In one embodiment, the fibrous material is a prepreg material, for example, a prepreg ply. Prepreg materials tend to be sticky, and hence generally include a backing layer on one or both surfaces. The shaping device may be configured to remove a first backing layer from the material prior to the material being deposited in the mould. The placement device may be configured to remove a second baking layer from the material as the material is deposited in the mould. 
         [0020]    The method and the end effector assembly may be employed in fully automated or semi-automated manufacturing schemes. In semi-automated schemes, at least one step is carried out manually. The end effector may be mounted on the end of a robotic arm or on a gantry arranged above the mould. The end effector may be computer controlled and move in accordance with a set of pre-programmed instructions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will now be described in further detail by way of example to the following drawings in which: 
           [0022]      FIG. 1  is a schematic side view of a deposition end effector assembly in accordance with an embodiment of the present invention, the assembly including a series of placement rollers and a series of shaping rollers; 
           [0023]      FIG. 2  is a perspective view of the placement rollers of the end effector assembly of  FIG. 1 ; and 
           [0024]      FIG. 3  is a perspective view of the shaping rollers of the end effector assembly of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Referring to  FIG. 1 , a deposition end effector assembly  10  in accordance with an embodiment of the present invention is shown schematically in side view. The assembly  10  comprises a housing  12 , indicated by the dashed line, which supports a placement device  14  comprising a series of placement rollers  16  for placing a strip of prepreg fibrous material  18  on a surface  20  of a mould  22 . The assembly  10  also includes a shaping device  24  comprising a series of shaping rollers  26  for pre-shaping the prepreg strip  18  prior to placement in the mould  22 . 
         [0026]    Whilst not shown in  FIG. 1 , the assembly  10  is typically mounted at the end of a robotic arm or gantry arranged to move above the mould  22 . The end effector assembly  10  is traversed across the surface  20  of the mould  22  by the robotic arm or gantry, in accordance with a series of pre-programmed instructions. As it moves, the end effector  10  deposits the strip of prepreg fibrous material  18  on the surface  20  of the mould  22 , or on a previously deposited prepreg layer. 
         [0027]    The end effector assembly  10  includes a supply of prepreg material in the form of a roll  28 . The roll  28  comprises a continuous length of prepreg ply wound around a spool  30 . The prepreg material typically has a width of between 0.3 to 1.5 metres. In this example, the ply is pre cut to shape and length, however, in other examples the end effector assembly  10  may include a cutting device or means for cutting the prepreg material  18 , for example, when the end effector reaches the end of the mould, prior to placing the next strip. 
         [0028]    Referring to  FIG. 2 , as mentioned above, the placement device  14  comprises a series of placement rollers  16   a,    16   b,    16   c,  which may also be referred to in the art as compaction or consolidation rollers. The placement rollers  16   a,    16   b,    16   c  are arranged to roll across the surface  20  of the mould  22 , or over a previously deposited layer, and effect placement of the strip of prepreg material  18  in the mould  22 . 
         [0029]    Wind turbine blades are made in curved moulds. The blades, and hence the moulds, have a relatively complicated surface geometry, and typically exhibit a double curvature, i.e., they are curved both in a transverse direction between the leading and trailing edges of the blade, and longitudinally, between the root and tip of the blade. To ensure that the placement rollers  16   a,    16   b,    16   c  exert an even pressure across the width of the prepreg strip  18  despite this curvature, the rollers  16   a,    16   b,    16   c  are formed from air-filled bags. The rollers  16   a,    16   b,    16   c  are compressible to an extent, which allows them to be displaced in a direction normal to the surface  20  of the mould  23 , as represented by the arrow  32  in  FIG. 1 . This action is akin to the suspension system of a vehicle, and allows the end effector assembly  10  to accommodate curvature or undulations in the mould surface  20 . 
         [0030]    The placement rollers  16   a,    16   b,    16   c  comprise a central roller  16   a  located between first and second peripheral rollers  16   b,    16   c,  also referred to as ‘wing’ rollers. The rollers  16   a,    16   b,    16   c  are arranged in a chevron, or V-shaped formation, with the central roller  16   a  being located ahead of the peripheral rollers  16   b,    16   c  in the direction of progression of the assembly  10  as represented by the arrow  34  in  FIGS. 1 and 2 . The central roller  16   a  rotates about a central axis, whilst the first and second peripheral rollers  16   b,    16   c  rotate about respective first and second peripheral axes which are each transverse to the direction of progression  34  and transverse to the central axis. In this arrangement, the peripheral rollers slide over the strip to an extent to effect smoothing. This sliding action mimics the manual lay-up technique in which a deposited strip is smoothed out on the mould surface by a person&#39;s hands moving across the strip, from the centre outwards, in a sliding motion. In other examples, the peripheral rollers  16   b,    16   c  may be segmented and staggered and rotate about respective peripheral axes that are parallel to the central axis. 
         [0031]    Advancing the central roller  16   a  ahead of the peripheral rollers  16   b,    16   c  has the effect of smoothing the prepreg strip  18  outwards from a central region towards its edges. This reduces wrinkles when the strip  18  is laid upon regions of the mould having a curvature transverse to the direction of progression  34 . Wrinkles are further prevented or reduced by virtue of the shaping device  24 , which pre-shapes the strip  18  to conform to the curvature of the mould  22  immediately prior to the strip  18  being placed in the mould  22 . The shaping device  24  will now be described in more detail with reference to  FIG. 3 . 
         [0032]    Referring to  FIG. 3 , as mentioned above the shaping device  24  comprises a series of shaping rollers  26   a - e . Each roller  26   a - e  rotates on a respective axle, and the axles are joined end to end to form a chain of rollers  36 . In this example, the chain  36  includes five rollers  26   a - e . A central roller  26   a  is arranged between first and second groups  38   a,    38   b  of peripheral rollers  26   b - e . Each group  38   a,    38   b  includes two peripheral rollers  26   b,    26   c  and  26   d,    26   e,  respectively. The ends of the chain  36  are mounted to respective mounting points  40   a,    40   b  on the housing of the end effector assembly  10 . The chain of rollers  36  hangs between the two mounting points  40   a,    40   b  and adopts a shallow U-shaped curvature, in which the central roller  26   a  occupies a lowermost position and the first and second groups  38   a,    38   b  of peripheral rollers  26   b - e , occupy relatively higher positions, closer to the mounting points  40   a,    40   b.  In this configuration, the shaping rollers  26   a - e  are known as ‘garland troughing idlers’, and are similar to those used in conveyor-belt systems, albeit for a different purpose. In other examples, fixed rollers may be employed in place of the garland troughing idlers. 
         [0033]    The end effector assembly  10  includes a feed mechanism for feeding the prepreg material  18  from the prepreg roll  28 , over the garland idlers  26   a - e , and to the placement rollers  16   a - c . The garland idlers  26   a - e , which are located between the prepreg roll  28  and the placement rollers  16   a - c , serve to pre-shape the length of material  18  immediately before it reaches the placement rollers  16   a - c , at which point it is deposited in the mould  22 . With such an arrangement, it will be appreciated that the path length of the prepreg material  18  between the prepreg roll  28  and the surface  20  of the mould  22  will remain substantially uniform across the with of the prepreg material  18 , and thereby the tension along the length of the prepeg material  18  will also remain substantially uniform across its width. The U-shape curvature adopted by the garland idlers  26   a - e  corresponds to the transverse curvature of the mould  22 . 
         [0034]    As the prepreg strip  18  is fed over the garland idlers  26   a - e , i.e., through the U-shape, the idlers  26   a - e  impart a curvature to the strip  18  corresponding to the transverse curvature of the mould surface  20 . The effect of this is to ensure that the full width of the prepreg strip  18  is placed flush against the mould surface  20  at substantially the same time. Without this pre-shaping, the edges of the strip  18  would come into contact with the curved mould surface  20  before the central region of the strip  18 , resulting in air being trapped beneath the strip  18 . 
         [0035]    Wrinkles are effectively prevented from developing in the laminate layup due to the synergistic action of the shaping device  24  and the placement device  14 . The shaping device  24  pre-shapes the prepreg strip  18 , which ensures that the full width of the strip  18  comes into contact with the mould surface  20  at the same moment, hence minimising the possibility of air becoming trap, whilst the placement rollers  16   a - c  arranged in the chevron formation ensure that the strip  18  is smoothed outwards from its centre, thus expelling any air that may have become trapped. 
         [0036]    In addition to the functions described above, the placement rollers  16   a - c  and the shaping rollers  26   a - e  also serve to remove protective backing layers from the prepreg ply  18  during layup. Prepreg plies are sticky and so generally include upper and lower protective backing layers, which protect the plies during storage and enable the plies to be wound on a roll and easily unwound when required. Referring again to  FIG. 1 , a lower backing layer  42  is removed as the prepreg strip  18  is fed over the garland troughing idlers  26   a - e . The lower backing layer  42  is looped around the garland troughing idlers  26   a - e , which peel the lower backing layer  42  away from the prepreg strip  18  before the strip reaches the placement rollers  16   a - c . An upper backing layer  44  is looped around the placement rollers  16   a - c , which peel the upper backing layer  44  from the prepreg strip  18  as the strip is placed in the mould  22 . 
         [0037]    It will be appreciated that the method and apparatus described above are suitable for use in fully automated composite layup techniques and semi-automated layup techniques. In semi-automated techniques, one or more steps in the fabrication of a composite structure are performed manually. 
         [0038]    Whilst the present invention has been described in connection with the manufacture of wind turbine blades, it will be appreciated that the invention may be employed in the manufacture of other composite articles, for example aircraft components including wings and fuselages. 
         [0039]    Many modifications may be made to the specific example described above without departing from the scope of the present invention as defined by the accompanying claims. For example, in other embodiments, the placement device  14  may have more than three rollers  16   a - c . These rollers are not necessarily air-filled bags, but may be otherwise flexible. For example, rigid rollers may be used in conjunction with a suitable suspension system such as hydraulic or pneumatic actuators or springs, or intrinsically flexible rollers could be used, such as rubber or foam rollers. Other suitable shaping devices may be substituted for the garland troughing idlers  26   a - e . For example, other types of troughing idler may be used in which the individual rollers are located on a common shaft. Also, the shaping rollers  26   a - e  could be substituted with a suitably-shaped die.