Field of the invention

The present invention relates to a method of generating instructions for an additive manufacturing system to print a 3D object and a brim around the object. The invention also relates to a computing device with processing units arranged to perform the method and to a computer program product. Finally, the invention also relates to a method of printing a 3D object using an additive manufacturing system.

Background art

Fused filament fabrication (FFF) is a 3D printing process that uses a continuous filament of a thermoplastic material. Filament is fed from a filament supply through a moving, heated print head, and is deposited through a print nozzle onto an upper surface of a build plate. The print head may be moved relative to the build plate under computer control to define a printed shape. In certain FFF devices, the print head moves in two dimensions to deposit one horizontal plane, or layer, at a time. The work or the print head is then moved vertically by a small amount to begin a new layer. In this way a 3D printed object can be produced made out of a thermoplastic material.

The 3D printing process can result in part of a 3D-printed object curling and consequently losing adhesion to the build plate surface. The first layer of a 3D printed object can often determine the success or failure for the entire print. Detachment of the first layer from the build plate surface may cause print failures due to part or all of the 3D-printed object detaching from the build plate, or a first layer of the 3D object becoming curved instead of flat. This may especially be an issue with narrow or small features of a 3D object.

Nowadays, so-called slicing programs are used to create the print instructions for the devices performing the 3D printing process. A well-known slicing program is called Ultimaker Cura®. This program is arranged to transform an input file, e.g. an STL file, comprising a digital model of a 3D object, into print instructions for an additive manufacturing system such as a fused filament fabrication system. In order to produce the proper print instructions, the 3D model is virtually sliced (i.e. divided) into layers wherein each of the layers contain specific printing traces for producing the proper contour, infill and height of a layer. Around the first layer of the 3D object, a brim may be modelled. Alternatively, a raft may be created beneath a 3D object. A raft may be slightly larger than the 3D object, and is designed to be removed from the 3D object. Rafts may be difficult to remove, often requiring tools to cut and/or scrape the material away.

Brims are in fact continuations of the outer perimeter away from the 3D object. Brims are usually torn away from the model, often leaving behind some of the material. That residual material may then be cut, filed, or sanded away.

US2018056607 (A1) describes a technique to facilitate the removing of brims from the 3D objects, using so-called perforated brims. A perforated brim may include one or more perforations and be coupled to a perimeter of a 3D object. Techniques are described to provide the ability to remove brims after printing without using any special cutting tools. In order to avoid unwanted detaching of the brim from the build plate due to warping, it is suggested that the perforated brim may also include brim extension material to provide additional strength to the perforated brim. This however requires more material to be printed. Furthermore, strengthening the brim does not give any guaranties that the brim will not warp, since also in thicker brim part stresses may occur that will cause unwanted warping.

Summary of the invention

The aim of the present invention is to provide a method of generating instructions for an additive manufacturing system to print a 3D object, where the risk of a brim getting loose from the build surface during printing due to warping is minimized, or at least reduced as compared to the state of the art.

According to a first aspect of the present invention, there is provided a method of generating instructions for an additive manufacturing system to print a 3D object. The method comprises receiving a 3D model of a 3D object, and dividing the 3D model into a number of layers. The method further comprises determining a circumferential line of a first layer of the divided 3D model and generating a temporary outer brim line by performing a dilation operation on the circumferential line using a circle, which center scans the circumferential line.

The method also comprises looking for critical sections in the temporary outer brim line which have a length that is more than a predefined length threshold TL and have a radius of curvature R higher than a predefined radius threshold TR. If one or more critical sections are found, each one of the critical sections is divided into multiple subsections, wherein each of the subsections has (i) a radius of curvature R lower than the predefined radius threshold TR, and/or has (ii) a length less than the predefined length threshold TL, SO as to obtain a final outer brim line.

The method also comprises generating a brim model around the first layer of the 3D model by filling an area between the circumferential line and the final outer brim line with further brim lines, and finally generating, instructions for an additive manufacturing system to print the 3D object with a brim object based on the 3D model and the brim model.

By dividing the critical sections into smaller non-critical subsections, the outer lines of the brim object (below also simply referred to as ‘brim’) will build up less stress. Furthermore, by creating shorter and/or more curved subsections of the outer brim lines, stress in the outer brim lines is spread in every direction, thus decreasing a “peak” stress in one single direction.

Decreasing the stress in the outer brim lines will decrease the risk of warping of the outer lines of the brim, and decrease the risk the brim getting detached from the build surface.

Another advantage is that if the brim sticks better to the build surface, that less brim lines are actually needed. This could even mean that this method is faster than the naive approach (e.g. if we print 30% less lines, but print them 15% slower, the total is still faster).

In an embodiment, the dividing of each of the critical sections comprises adding one or more fictional parts to the first layer of model based on the 3D model, and generating a final outer brim line by performing the dilation operation, wherein the fictional parts are taken into account. In an embodiment, the fictional models are negative parts, which cover areas outside which the brim is to be generated. The placement of the negative parts will create indentations in the brim, which will cause the outer brim lines to have sections that are (more) curved, and are shorter as compared to the temporary brim lines.

In an embodiment, the fictional models are positive parts, which extend from the first layer of the divided 3D model, thereby causing the circumferential line to deform as compared to a situation without the positive parts. Once the outer brim line(s) is designed, the positive parts will be removed, and the brim will be finished by filling the area between the outer brim line and the object.

In an embodiment, the dividing of each of the critical sections comprises the super positioning of an alternating pattern on the critical sections of the outer brim line. Such a super positioning will for example convert a straight outer brim line into a wavy brim line having sections that are sufficiently curved. The alternating pattern may have all sorts of patterns, such as a sinusoidal pattern. A sinusoidal line can be printed in a continuous way without having to slow down the print process in sharp corners.

The outer brim lines may have a wavy pattern all along their revolution. Here, the term revolution means the revolution around the 3D object. Sometimes certain parts of a perimeter of a 3D object are more curved than other parts. Those curved parts of the objects will normally result in a curved brim in case the brim is printed in a way where the lines of the brim follow the perimeter of the object at the lowest layer. The outer brim lines may have a wavy pattern at those sections of their revolution, where the temporary outer brim line has a radius of curvature R higher than a predefined threshold TR. SO only in sections with slightly or non- curved outer brim lines the method may produce a wavy pattern to avoid unwanted stresses in the outer lines of the brim.

In an embodiment, the threshold TR is equal or higher than 20 cm. But other values of this threshold are possible, such as higher or lower than 20 cm.

The method may comprise receiving information on a type of printing material to be used, and wherein the threshold TR is depending on the type of printing material.

In another embodiment, the outer lines vary in width, wherein each of the outer lines comprises relatively thick sections and intermediate relatively thin sections. The relatively thick sections of the outer lines may be aligned so as to make the brim model thicker at those sections.

According to a further aspect, there is provided a computing device comprising one or more processing units, the one or more processing units being arranged to perform the method as described above.

According to a further aspect, there is provided a computer program product comprising code embodied on computer-readable storage and configured so as when run on one or more processing units to perform the described method.

According to a further aspect, there is provided a method of printing a 3D object using an additive manufacturing system, the method comprising printing a first layer of the 3D object on a build surface, and printing a brim around the first layer of the 3D object, wherein the brim is printed using instructions generated by the method as described above. In an embodiment, the additive manufacturing system is a fused filament fabrication system. Alternatively, the additive manufacturing system is a pellet printer.

Brief description of the drawings

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings,

Figure 1 shows a top view of a first layer of an object and a brim model according to the prior art;

Figure 2 shows a top view of a first layer of an object and a brim model created by a slicing program according to an embodiment of the invention;

Figure 3 shows a top view of a first layer of another object and a brim model created by a slicing program according to an embodiment of the invention;

Figure 4 shows a top view of a first layer of another object and a naive brim model wherein the first layer of the object is semi-disk shaped;

Figure 5 shows a first layer of the 3D model of Figure 4 and a number of added parts that extend from a straight edge of the 3D model;

Figure 6 shows a top view of a first layer of the object of Figure 4 and a brim model generated by a method according to an embodiment;

Figure 7 shows a top view of a first layer of the object of Figure 4, with an improved brim according to a further embodiment;

Figure 8 schematically shows a top view of a part of a brim model according to another embodiment;

Figure 9 is a flow chart of a method of generating instructions for an additive manufacturing system to print a 3D object, according to an embodiment, and

Figure 10 schematically shows a computing device according to an embodiment.

It should be noted that items which have the same reference numbers in different Figures, have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item has been explained, there is no necessity for repeated explanation thereof in the detailed description.

Detailed description of embodiments

Reference is made to Figure 1 to explain how a brim may be designed around a 3D object. In the example of Figure 1 , the object 80 has a rectangular shape at least at its first layer (i.e. the layer touching the build surface in a print process). A circumference 81 of a brim 82 is also shown. While the object 80 has sharp corners, the brim 82 has rounded corners. This is due to the way a brim is designed by present slicing software programs. Todays slicing software programs use a so-called dilation operation to find the circumference of a brim given a certain shape of the first layer of the object. This shape can be regarded as a binary image A. The binary image A can be viewed in mathematical morphology as a subset of a Euclidean space R ^d or the integer grid Z ^d , for some dimension d. Let E be a Euclidean space or an integer grid, A a binary image in E, and B a structuring element regarded as a subset of R ^d .

Then the dilation of A by B is defined by where Ab is the translation of A by b.

For example, if the structuring element B has a center on the origin, then the dilation of A by B can be understood as the locus of the points covered by B when the center of B moves inside A. The dilation of a square of size 10 cm, centered at the origin, by a disk of radius 2 cm, also centered at the origin, is a square of size 14 cm, with rounded corners, centered at the origin. The radius of the rounded corners is 2 cm.

Referring back to Figure 1 , the rectangle 80 represents the binary image A, while the disks 83, 84 represent the structuring element B. By letting the disk B follow the image A, the outer edge 81 of the brim 82 can be calculated. Once the outer edge 81 is calculated, the slicing software can fill the areas between the edge 82 and the object 80 as will be appreciated by the skilled person. The brim lines are usually designed as substantially parallel lines, following the shape of the edge of the brim, and getting smaller and having sharper corners towards the object.

In the example of Figure 1 , an outer brim line following the edge 81 will have two sections that have a length L0. Under certain conditions, using certain print materials, unwanted stress may build up in these brim lines if the length L0 is too large. The amount of stress depends on the material, the print process, and on the length and curvature of the brim lines. If too much stress builds up in the brim lines, the brim may get detached from the build surface. Once the brim gets detached, also the first layer of the 3D object may get detached from the build surface resulting in a failed print.

In order to decrease the risk of detachment of the brim, it is proposed to look forthose sections in the brim that have a critical length and curvature. If such sections are found, these sections are divided into multiple subsections, which do not have a curvature smaller than a critical curvature R or a length smaller than a critical length, or both. The method will be further explained below by describing some of the embodiments.

Figure 2 shows a top view of a first layer of an object 1 and a brim model 2 created by a slicing program according to an embodiment of the invention. In this example the first layer of the object 1 is rectangular. The bottom layer of the object 1 comprises a rectangular perimeter line 11 which is filled with parallel bottom lines 12. The brim model 2 comprises a number of inner lines 21 , 22, 23, 24 and a number of outer lines 25, 26, 27, 28. The inner lines substantially follow the perimeter (i.e. circumference) of the bottom layer of the object 1 . It is noted that here the word ‘substantially’ means that the inner lines 21 , 22, 23, 24 are parallel to the straight parts of the perimeter line 11 of the object 1 but have rounded corners instead of the sharp corners of the rectangular perimeter line 11 . The brim model 2 further comprises a number of intermediate lines 29 arranged to fill areas between on outer one of the inner lines, see line 24, and an inner one of the outer lines, see line 25. These areas are a result of the fact that the outer lines 25, 26, 27, 28 do not follow the perimeter line 11 of the object 1 but instead have a wavy pattern that deviates from the pattern of the inner lines 21 , 22, 23, 24. The number of the inner lines and outer lines may vary, also possibly depending on the dimension of the object, the materials used, and user needs. The number and form of the intermediate lines 29 may vary, also possibly depending on the size of the brim model 2, the form of the perimeter 11 of the object 1 , and the wavy pattern of the outer lines 25, 26, 27, 28.

In the example of Figure 2, the brim model 2 comprises a number of cut-outs 31 resulting in a sawtooth shaped circumference. This is different from a state-of-the-art brim model having only a multitude of lines following the perimeter line 11 of the rectangular object 1. The cut-outs 31 are a result of placing negative parts, see e.g. disc-shaped part 14, that intersect with a temporary (i.e. naive) outer brim line, see dashed line 15 indicating a brim line (i.e. the temporary outer brim line) that would have been created using state-of-the art methods without placement of the negative parts 14. Due to the negative parts 14, each of the outer lines 25, 26, 27, 28 comprises a number of straight sections and a number of intermittent curved sections. Due to the segmentation, the curved sections have a curvature below the critical radius of curvature TR, and the straight sections, while having an infinite radius of curvature, all have a length below the critical length TL. AS a result, no excessive stresses can be built up in the outer lines 25, 26, 27, 28 and thus the risk of warping of the brim, once printed, is considerably decreased as compared to known brims.

Figure 3 shows a top view of a first layer of another object 4 and a brim model 5 created by a slicing program according to an embodiment of the invention. The brim model 5 is designed in a similar way as the brim model 2 of Figure 2. In the example of Figure 3, the bottom layer of the object 4 is triangular shaped. The bottom layer of the object 4 comprises a triangular perimeter line 41 and multiple parallel bottom lines 42. The brim model 5 comprises a number of inner lines 51 , 52, 53, 54 and a number of outer lines 55, 56, 57, 58. The inner lines substantially follow the perimeter (i.e. circumference) of the bottom layer of the object 4. It is noted that here the word ‘substantially’ means that the inner lines 51 , 52, 53, 54 are parallel to the straight parts of the perimeter line 41 of the object 4 but have rounded corners instead of the sharp corners of the triangular perimeter line 41 .

It is noted that the brim lines arranged between the outer brim line 28, 58 and the circumference/perimeter 11 , 41 of the object, are also referred to a the ‘further brim lines’. So in other words, the further brim lines may comprise the inner lines (e.g. lines 21 ,22,23,24), the intermediate lines (e.g. lines 29), and one or more of the outer lines (e.g. lines 25,26,27).

As in the example of Figure 2, the brim model 5 of Figure 3 further comprises a number of intermediate lines 59 arranged to fill areas between on outer one of the inner lines, see line 54, and an inner one of the outer lines, see line 55. These areas are a result of the fact that the outer lines 55, 56, 57, 58 do not follow the perimeter line 41 of the object 4 but instead have a wavy pattern that deviates from the pattern of the inner lines 51 , 52, 53, 54.

By placing multiple disc-shaped negative parts on the outer brim line (not shown), multiple cut-outs 34 are created in the side of the brim model. This way, too long straight lines are prevented from being printed. It is noted that, although these long lines created by the naive methods, can be printed faster (since less (de) acceleration is needed), they tend to warp resulting in a detached part of the brim.

It is noted that whereas the distance between the objects 1 , 4 and the inner lines of the brims (see lines 21 , 22, 23, 51 , 52, 53) stays substantially constant along the circumference of the objects, the distance between the objects 1 , 4 and the wavy outer lines (see lines 28, 58) varies along the circumference due to their wavy nature.

It is noted that at those sections where the temporary outer brim line has a radius of curvature R higher than a predefined threshold TR too much stress may build up, if those sections have a length higher than a critical length TL. An example of such a brim is shown in Figure 4.

Figure 4 shows a top view of a first layer of another object 6 and a naive brim model 7. In this example the first layer of the object 6 is semi-disk shaped. The bottom layer of the object 6 comprises a semi- circular perimeter line 61 which is filled with parallel bottom lines 62. The brim model 7 comprises an outer brim line 70, generated by performing the dilation method described above with reference to Figure 1 . The outer brim line 70 (also referred to as temporary outer brim line) can be divided in several sections, each section having a specific radius of curvature, see sections S1 , S2, S3, S4. Section S3 is a straight line, and thus has an infinite radius of curvature. Sections S1 has a radius of curvature R_S1 indicated in Figure 4. In this example, we assume that the radius of curvature R_S1 is smaller a predefined threshold TR. SO the radius of curvature of sections S2 and S4 will also be smaller than the threshold. Because of this, in this embodiment, only the section S3 is divided into subsections, assuming that the length L3 of this section is larger than the threshold TL.

Dividing the section S3 can be done in several ways. It can be realized by adding negative parts, as was done in the example of Figures 2 and 3. Alternatively, it can be done by adding positive parts to the 3D model, as will be explained with reference to Figure 5.

Figure 5 shows a first layer of the 3D model 6 and a number of temporary added parts 63, 64, 65. The added parts extend from a straight edge of the 3D model at least at the first layer. In this example, the added parts are triangular shaped, but it is noted that any other shape could be used to artificially change the circumference of the 3D model 6, such as spikes, or rectangular shaped extensions. After the parts are added, the dilation operation can be performed to create the outer brim line. Once the new outer brim line is modelled, the added parts are removed, and the brim is further completed (i.e. by adding the further brim lines). The result of a brim modelled according to this embodiment is shown in Figure 6. Figure 6 shows a top view of the first layer of the 3D model 6 surrounded by a brim comprising a number of inner lines 71 , 72, 73 and a number of outer lines 74, 75, 76. The inner lines substantially follow the perimeter (i.e. circumference) of the bottom layer of the 3D model 6. The brim model 7 further comprises a number of intermediate lines 77 arranged to locally fill areas between on outer one of the inner lines, see line 73, and an inner one of the outer lines, see line 74. These areas are a result of the fact that the outer lines 74, 75, 76 do not follow the perimeter line 61 of the object 6 but instead have a wavy pattern that deviates from the pattern of the perimeter line 61 and thus of the inner lines 71 , 72, 73. In this embodiment, only at the straight section of the object 6, the brim has wavy outer lines 74, 75, 76.

Because the radius of curvature R in the curved section S1 of the outer brim line 70 (i.e. R_S1) is lower than the threshold TR ,the method will not divide section S1 into subsections. As a consequence, the outer lines 74, 75, 76 of the created brim will follow the inner lines without creating local areas to be filled. So for example, if the value for R_S1 is 18 cm, and the threshold value TR is 20 cm, the brim model 7 will look like the one shown in Figure 6. But if the value for R_S1 is 25 cm, and the threshold value TR is still 20 cm, the brim model would comprise waved outer lines all over the circumference, also in section S1 due to temporary added parts placed at both section S3 and S1 .

In the example of Figure 6, the number of the inner lines and outer lines is 3, but as mentioned before this number may vary, also possibly depending on the dimension of the object, the materials used, and user needs. The number and form of the intermediate lines 77 may vary, also possibly depending on the size of the brim model 7, the form of the object 6, and the wavy pattern of the outer lines 74, 75, 76.

The method may comprise receiving information on a type of printing material to be used, wherein the threshold TR is depending on the type of printing material. This may be advantages since it is known that the risk of warping in certain materials is higher than in other materials. For example, a material like ABS will warp more often and/or easier than a material like PLA.

The inner lines shown in the embodiments of Figures 2, 3 and 6 could be absent in certain cases. This will leave brim models with only outer lines and intermediate lines filling the voids between the inner lines and the 3D objects. Leaving out the inner lines may avoid unwanted stresses in the brim at those parts where the inner lines would have been straight and long.

Figure 7 shows a top view of a first layer of the object of Figure 4, with an improved brim according to a further embodiment. In this embodiment, the wavy pattern of the outer brim line(s) in the critical section(s) is generated by super positioning of an alternating function on top of the temporary outer brim line (i.e. the outer brim line created by a slicing method according to the state of the art. It is noted that this super positioning is only useful at sections that are of sufficient length and have sufficiently straight (i.e. a large radius of curvature). If a section of an outer brim line is relatively short, but straight, the suggested wavy pattern can not easily be implemented and would be of little use since the risk of having too much stress in a short straight brim part is neglectable. In the example of Figure 7, a sinusoidal function is super positioned on top of a straight temporary outer brim lines to obtain the final outer brim line 79.

It is noted that all the embodiments described above, provide for a brim model having a varying width along the circumference of the object. This is a result of the outer brim line(s) having a more wavy pattern than the circumference of the object. In the embodiment described above, a width of each of the brim lines is constant, although different outer lines may have different widths. Alternatively, the wavy pattern of the brim could be produced by printing each of the outer lines with varying widths. An example is shown in Figure 8.

Figure 8 schematically shows part of a brim model according to another embodiment. In Figure 8, only a part of a perimeter line 90 of an object is shown. In this part, the perimeter line 90 is straight, which would result in a straight outer brim line when using the naive brim modelling method described with reference to Figure 1 . Assuming that the outer temporary brim lines do not satisfy the conditions (i.e. division is needed), the associated brim section will be divided into subsections that do satisfy the conditions. In this embodiment, the brim model 9 comprises a number of outer lines 91 , 92, 93, 94 an inner one of which following the contour of perimeter line 90. In this example the inner one of the outer lines, i.e. line 91 , varies in width so that the line 91 comprises sections where the line 91 is relatively thin and intermediate sections where the line 91 is relatively thick. In this example the width of the outer lines 91 , 92, 93, 94 varies gradually. This can be achieved by varying the print speed at a constant flow of material, of by adjusting the flow during printing of the brim lines. In this example, all of the outer lines 91 , 92, 93, 94 have a relatively thin width at a certain location, so that the brim model 9 in that location is also be thinner than its neighbouring regions. When properly printed, these outer lines may leave the intermediate lines redundant since there will not be any voids in between the perimeter line 90 and the most inner of the outer lines, i.e. outer line 91 . In this example, all the outer lines 91 , 92, 93, 94 vary in width in a similar way, resulting in a brim having varying width created by brim lines that each follow their inner neighbouring lines exactly leaving no voids (i.e. no open areas).

It is noted that in Figures 2-6 the lines of the 3D objects (see e.g. lines 61 , 62) are shown very thin as compared to the lines of the brim. It should be noted that in practice the resulting printed traces will be thicker and that the bottom traces will cover the whole bottom of the object once printed. It is further noted that the brim lines shown in Figures 2-6 seem to be spaced apart. This is the way slicing programs show the print lines in a preview. The distances between the brim lines should however not cause any voids in the brim after printing, as will be appreciated by the skilled reader.

Figure 9 is a flow chart of a method of generating instructions for an additive manufacturing system to print a 3D object, according to an embodiment. The method 130 comprises receiving 131 a 3D model of a 3D object, and dividing 132 the 3D model into a number of layers. Next, the method comprises determining a circumferential line of a first layer of the divided 3D model, see step 133, and generating a temporary outer brim line, see step 134, by performing a dilation operation on the circumferential line using a circle, which center scans the circumferential line.

Next, the method 130 comprises looking for critical sections in the temporary outer brim line which have a length that is more than a predefined length threshold TL and have a radius of curvature R higher than a predefined radius threshold TR. If one or more critical sections are found, see test 136, then a step 137 follows which entails the division of each of the critical sections into multiple subsections, wherein each of the subsections has (i) a radius of curvature R lower than the predefined radius threshold TR, and/or has (ii) a length less than the predefined length threshold TL, SO as to obtain a final outer brim line. Step 137 is followed by generating 138 a brim model around the first layer of the 3D model by filling an area between the circumferential line and the final outer brim line with further brim lines. Finally, instructions for an additive manufacturing system are generated, see step 139, to print the 3D object with a brim object based on the 3D model and the brim model. If the outcome of test 136 is negative, the step 137 is bypassed, and step 138 follows the test 136, see Figure 9.

Figure 10 schematically shows a computing device 100 according to an embodiment. The device 100 comprises one or more processing units 111 , an I/O interface 112 and a memory 113. The processing unit 111 is arranged to read and write data and computer instructions from the memory 113. The processing units 111 may also be arranged to communicate with sensors and other equipment via the I/O interface 112. The computing device 100 may also comprise an interface 114 arranged to communicate with other devices via a LAN or WAN (not shown). Figure 6 also shows a display 115 which may be connected to the interface 112 so as to show information regarding a slicing process of a 3D object. The memory 113 may comprise a volatile memory such as RAM, or a non-volatile memory such as a ROM memory, or any other type of computer-readable storage. The memory 113 may comprise a computer program product comprising code configured to make the processing units 111 perform one or more of the embodiments of the method as described above.

According to a further aspect, there is provided a method of printing a 3D object using an additive manufacturing system, such as an FFF system. The method comprises the printing a first layer of the 3D object on a build surface, and the printing a brim around the first layer of the 3D object. The brim may be printed using instructions generated in a way as described above.

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible and are included in the scope of protection as defined in the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.