Cathode filament assembly

Abstract

A cathode for an X-ray tube, an X-ray tube, a system for X-ray imaging, and a method for an assembly of a cathode for an X-ray tube include a filament, a support structure, a body structure, and a filament frame structure. The filament is provided to emit electrons towards an anode in an electron emitting direction, and the filament at least partially includes a helical structure. Further, the filament is held by the support structure which is fixedly connected to the body structure. The filament frame structure is provided for electron-optical focusing of the emitted electrons, and the filament frame structure is provided adjacent to the outer boundaries of the filament. The filament frame structure includes frame surface portions arranged transverse to the emitting direction, and the filament frame structure is held by the support structure.

Claims

1. A cathode for an X-ray tube, comprising: a filament; a support structure; a body structure; and a filament frame structure; wherein the filament is provided to emit electrons towards an anode in an electron emitting direction, and wherein the filament at least partially comprises a helical structure; wherein the filament is held by the support structure, which is fixedly connected to the body structure; wherein the filament frame structure is provided for electron-optical focusing of the emitted electrons; wherein the filament frame structure is provided adjacent to outer boundaries of the filament; and wherein the filament frame structure comprises frame surface portions arranged transverse to the electron emitting direction; wherein the filament frame structure is held by the support structure; and wherein the filament frame structure comprises at least one positioning device for at least one positioning direction of the filament in relation with the body structure, the at least one positioning device physically contacting a portion of the filament for positioning the filament in the at least one positioning direction of the filament in relation with the body structure, the at least one positioning device configured for removal from physical contact with the filament when the filament is secured to the support structure.

2. The cathode according to claim 1, wherein the filament is a straight end helical filament, and wherein connecting ends of the filament are aligned with a longitudinal axis, and a helical winding of the filament is provided around the longitudinal axis.

3. The cathode according to claim 1, wherein the filament frame structure comprises at least a second positioning device for a second positioning direction of the filament in relation with the body structure.

4. The cathode according to claim 1, wherein fitting members for receiving parts of a filament support arrangement is provided at the support structure, wherein the fitting members have through-holes through which a mounting bolt is configured to extend, and wherein, as the fitting members, the support structure comprises a reception for receiving two ends of a straight filament mounting-pin as the filament support arrangement, and wherein the mounting-pin is insertable within a coil opening of the filament for correct placing of the filament.

5. The cathode according to claim 4, wherein fitting members for receiving parts of a filament support arrangement is provided at the filament frame structure; and wherein, as the fitting members, the filament frame structure comprises a reception for receiving the two ends of the straight filament mounting-pin, and wherein the mounting-pin is insertable within the coil opening of the filament for correct placing of the filament.

6. The cathode according to claim 1, wherein in a non-operating state, the filament is arranged with a pretension provided by the support structure.

7. The cathode according to claim 1, wherein the body structure is provided as a cathode cup, wherein the cathode cup is provided as a ceramic cathode cup made from electrically non-conducting ceramic, and wherein a part of surfaces of the cathode cup is provided with a metallic coating.

8. The cathode according to claim 7, wherein the cathode cup is provided with a flat front side, and wherein the filament is arranged on the flat front side.

9. An X-ray tube, comprising: a cathode; an anode; wherein the cathode is provided as a cathode for the X-ray tube, the cathode comprising: a filament; a support structure; a body structure; and a filament frame structure; wherein the filament is provided to emit electrons towards an anode in an electron emitting direction, and wherein the filament at least partially comprises a helical structure; wherein the filament is held by the support structure, which is fixedly connected to the body structure; wherein the filament frame structure is provided for electron-optical focusing of the emitted electrons, wherein the filament frame structure is provided adjacent to outer boundaries of the filament; and wherein the filament frame structure comprises frame surface portions arranged transverse to the electron emitting direction; wherein the filament frame structure is held by the support structure; and wherein the filament frame structure comprises at least one positioning device for at least one positioning direction of the filament in relation with the body structure, the at least one positioning device physically contacting a portion of the filament for positioning the filament in the at least one positioning direction of the filament in relation with the body structure, the at least one positioning device configured for removal from physical contact with the filament when the filament is secured to the support structure.

10. A system for X-ray imaging, comprising: an X-ray source; an X-ray detector; and a processing unit; wherein the processing unit is configured to control the X-ray source and the X-ray detector for providing X-ray image data of an object of interest; and wherein the X-ray source is provided as an X-ray tube comprising a cathode and an anode, wherein the cathode is provided as a cathode for the X-ray tube, the cathode comprising: a filament; a support structure; a body structure; and a filament frame structure; wherein the filament is provided to emit electrons towards an anode in an electron emitting direction, and wherein the filament at least partially comprises a helical structure; wherein the filament is held by the support structure, which is fixedly connected to the body structure; wherein the filament frame structure is provided for electron-optical focusing of the emitted electrons, wherein the filament frame structure is provided adjacent to outer boundaries of the filament; and wherein the filament frame structure comprises frame surface portions arranged transverse to the electron emitting direction; wherein the filament frame structure is held by the support structure; and wherein the filament frame structure comprises at least one positioning device for at least one positioning direction of the filament in relation with the body structure, the at least one positioning device physically contacting a portion of the filament for positioning the filament in the at least one positioning direction of the filament in relation with the body structure, the at least one positioning device configured for removal from physical contact with the filament when the filament is secured to the support structure.

11. A method for an assembly of a cathode for an X-ray tube, the method comprising acts of: providing a filament; wherein the filament is configured to emit electrons towards an anode in an electron emitting direction, and wherein the filament at least partially comprises a helical structure; aligning the filament with respect to a filament frame structure wherein the filament frame structure is configured for electron-optical focusing of the emitted electrons, wherein the filament frame structure is provided adjacent to outer boundaries of the filament, wherein the filament frame structure comprises frame surface portions arranged transverse to the electron emitting direction, and wherein the filament frame structure comprises at least one positioning device for at least one positioning direction of the filament in relation with the body structure, the at least one positioning device contacting a portion of the filament for positioning the filament in the at least one positioning direction of the filament in relation with the body structure; connecting the filament to the filament frame structure; connecting a support structure to a body structure; placing the filament frame structure on the support structure; fixing the filament frame structure and the filament to the support structure; and removing the at least one positioning device from physical contact with the filament when the filament is secured to the support structure.

12. The Method according to claim 11, wherein a support guide is provided for the assembly, and the method further comprises acts of: providing the filament on the support guide; and aligning the support guide with respect to the filament frame structure, wherein the support guide is removed after the filament is connected to the filament frame structure, or after the filament frame structure is fixed to the support structure.

13. The Method according to claim 11, wherein following the providing act, but before the placing act, the method further comprises an act of a total recrystallizing of the filament by applying external heat.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention will be described in the following with reference to the following drawings:

(2) FIG. 1 schematically shows an example for a cathode for an X-ray tube according to the present invention in a side view showing the longitudinal extension in FIG. 1A and in a top view in FIG. 1B;

(3) FIG. 2 shows a portion of a further example of a cathode according to the present invention in a perspective view in FIG. 2A and in a top view in FIG. 2B;

(4) FIG. 3 shows a portion of a further example of a cathode for an X-ray tube according to the present invention;

(5) FIG. 4 shows a further example of a cathode according to the present invention in a perspective view;

(6) FIG. 5 shows a further example of a cathode according to the present invention, wherein FIG. 5A shows a side view of a support structure and a filament, FIG. 5B shows a further side view of a support structure, and wherein FIG. 5C shows a further side view in the longitudinal direction of a filament frame structure, and FIG. 5D shows the frame structure in a perspective view;

(7) FIG. 6 shows an example of a filament in relation with a filament frame structure according to the present invention;

(8) FIG. 7 shows a further example of a filament frame structure according to the present invention in a perspective view in FIG. 7A and in a top view in FIG. 7B;

(9) FIG. 8 shows an example of a cathode for an X-ray tube according to the present invention in a perspective view;

(10) FIG. 9 shows the cathode of FIG. 8 in a further state after the mounting procedure;

(11) FIG. 10 shows a further example of a cathode for an X-ray tube in a perspective cross-section;

(12) FIG. 11 shows an example of an X-ray tube according to the present invention in a cross-section;

(13) FIG. 12 shows an example for a system for X-ray imaging according to the present invention, wherein FIG. 12A shows a medical imaging system and FIG. 12B shows an inspection apparatus, for example for scanning and screening of luggage pieces;

(14) FIG. 13 shows basic steps of a method according to the present invention; and

(15) FIG. 14 shows a further example of a method according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(16) FIG. 1A shows a side view, or elevation view of a cathode 10 for an X-ray tube. The cathode 10 comprises a filament 12, a support structure 14, a body structure 16, and a filament frame structure 18. For example, the support structure 14 comprises a first mounting bolt 20 and a second bolt 22.

(17) The filament 12 is provided to emit electrons towards an anode (not shown) in an electron emitting direction 24. The filament 12 at least partially comprises a helical structure 26. The filament 12 is held by the support structure 14, which is fixedly connected to the body structure 16. The filament frame structure 18 is provided for electron-optical focusing of the emitted electrons. The filament frame structure is provided adjacent to the outer boundaries of the filament 12, which is also shown in FIG. 1B showing the cathode 10 of FIG. 1A in a top view. The filament frame structure 18 comprises frame surface portions 28 and 30 arranged transverse to the emitting direction 24. The filament frame structure 18 is held by the support structure 14.

(18) In FIG. 1A, a connection 33 of the support structure 14 to an electrical source 34 is schematically indicated. For example, the mounting bolts extending through the body structure 16 can have a respective connection at the side opposite to the side where the filament 12 is arranged. The electrical source provides the electrical potential to the filament 12 (not further shown).

(19) According to a further example, the filament 12 is a straight end helical filament, wherein the connecting ends 32 of the filament are aligned to a longitudinal direction around which the helical winding of the filament 12 is provided.

(20) It must be noted that although FIGS. 1A and 1B show the filament as a straight end helical filament, the filament of FIGS. 1A and 1B may also be provided with differently arranged ends, i.e. in another form as a straight end helical filament as shown. This is also the case for the further figures.

(21) As shown in FIG. 2A in a perspective view, and in FIG. 2B in a top view, the filament frame structure 18 may comprise at least one positioning device 36 for at least one positioning direction 38, indicated in FIG. 2B with a double arrow, of the filament 12 in relation with the body structure 16. As an example, a small leash is provided at the inner side of one of the surface portions, for example the portion 28, thus providing an abutting or stopping piece of the frame portion in order to properly position the filament 12. For example, the protruding leash can be removed or bent away once the filament 12 is fixedly connected to the support structure 14.

(22) As shown in FIG. 3 in a top view, at least a second positioning device 40 for a second positioning direction 42, indicated in FIG. 3 with a double arrow, of the filament 12 in relation with the body structure 16 can be provided. For example, a segment is provided at an inner side of one of the surface portions, for example portion 28, providing a longitudinal abutting surface for the proper alignment of the filament 12. As indicated with two little bars 44, the second positioning device may be removed, i.e. cut away or bent, away after the fixation of the filament 12 to the support structure 14.

(23) Of course, also other positioning means, i.e. other forms of abutting surfaces in one of the described directions can be provided, or also in a further direction, for example perpendicular to the second direction 42 and perpendicular to the longitudinal direction of the filament 12, for providing respective abutting or resting surfaces for the alignment of the filament 12. In other words, the positioning device provides a mechanical stop for abutting the filament's coil wounding in at least one direction, for example the linear direction of the filament.

(24) As shown in FIG. 4, according to a further example, fitting members 46 for receiving parts of a filament support arrangement, indicated in FIG. 4 with two dotted lines 48, are provided at the support structure 14. For example, the fitting members 46 are provided as through-holes through the mounting bolt 20, or 22, such that a straight filament mounting-pin 50 (shown in FIG. 6 et seq.), can be provided for the assembly and later removed.

(25) According to a further example (not further shown), the respective fitting members for receiving parts of a filament support arrangement are provided at the filament frame support alternatively or in addition to the fitting members for receiving parts of a filament support arrangement at the support structure.

(26) For example, as indicated in relation with FIG. 4, the support structure 14 comprises a reception 52, as also shown in FIGS. 5A and 5B, for receiving two ends of the straight filament mounting-pin 50, which is provided during the mounting of the filament, and which pin is insertable within the filament's coil opening for correct placing of the filament, and also for holding and supporting the filament 12.

(27) FIG. 5A shows the mounting bolt 20 with the through-hole of the reception 52 and the filament 12. Further, a dotted line 54 indicates the longitudinal direction of the filament 12. Still further, the filament frame structure 18 is only indicated.

(28) Further, on an inner side of the mounting bolt 20, a first resting protrusion 56 is provided, which is also shown in FIG. 5B showing a view perpendicular to the view of FIG. 5A, viewing towards the mounting bolt 20. The protrusion 56 is provided with an adapted inner rounded surface 58, such that it is conforming to the through-hole of the reception 52. Thus, when mounting the filament 12, a support in form of the straight filament mounting-pin 50 can be inserted from above to be resting on the protrusion 56. Upon fixing the filament's end 34 to the mounting bolt 20, the straight filament mounting-pin 50 (not shown in FIG. 5A) can be pulled out to the left side for example, through the reception 52 of the final assembly and fixation of the filament.

(29) As shown in FIG. 5C, also the filament frame structure 18 may be provided with a reception 60 for receiving the mounting-pin 50. Thus, it is possible to provide the mounting-pin 50 for supporting the filament 12, and to rest the mounting-pin 50 on the frame structure. Thus, for the assembly, the filament frame structure 18 acts as a mounting tool. The mounting-pin 50 together with the filament 12 and the filament frame structure 18 can thus be positioned together such that the filament 12 is properly aligned.

(30) For example, the receiving reception 60 of the filament frame structure 18 can be provided by a bended portion 62, as shown in FIG. 5D. Further, it is also possible to remove the respective portion of the frame structure once the frame structure 18 is connected to the support structure 14 and the mounting-pin 50 is removed. For example, the small leashes connecting the bended portion 62 to the other portions of the filament frame structure can be cut away, as indicated with dotted lines 64.

(31) According to a further example (not shown), the pin may also be provided with a different shape resulting in, for example, a curved filament, instead of a straight and linear element for the pin.

(32) According to a further example (also not shown), instead of a pin, also other measurements can be provided, for example a support arrangement comprising several parts.

(33) The frame surface portions and the filament centre line may be arranged in a common layer.

(34) The support structure 14 comprises support pins, for example the mounting bolt, brazed to the body structure. The support pins, for example the mounting bolts, may be machined accurately by electron discharge machining (EDM) after being brazed.

(35) According to a further example, the support structure may be made integrally with a body structure, wherein the filament frame structure is brazed to the body structure representing the support structure.

(36) FIG. 6 shows the filament frame structure 18 together with the mounting-pin 50 and the filament 12. As can be seen, the filament frame structure 18 comprises two longitudinal frame surface portions 66 and 68 on either side of the filament 12. On each side, a small gap 70 is provided, thus separating the respective frame surface portions into two sub-portions, resulting in a first sub-portion 72 and a second sub-portion 74 for the side 66, and a third sub-portion 76 and a fourth sub-portion 78 for the opposite side 68.

(37) The respective sub-portions on one side are thus only connected by a small bar 80. Thus, it is possible to provide a one-piece filament frame structure 18 facilitating the mounting and handling procedure. However, after mounting and connecting the respective parts, i.e. the filament 12 and the frame structure to the support structure 14 (not shown in FIG. 6), the bars 80 can be removed.

(38) As FIG. 6 shows the bars in a middle portion, the bars 80 can also be provided on the side portion, as shown in FIG. 7A in a perspective view showing the frame only, and the respective top view in FIG. 7B.

(39) It must be noted that FIG. 6, and also FIGS. 7A and 7B, show the filament frame structure 18 with end portions bended outwardly, for example to rest on the outer sides of the mounting bolts 20, 22, whereas FIG. 5D shows an inwardly bended portion for providing a respective support reception for the mounting-pin 50. However, the provision of the gaps 70 and the bars 80 may also be provided for the inwardly bended version of FIG. 5D.

(40) It is further noted that FIG. 6 shows the mounting-pin 50 holding the filament 12 for the handling procedure.

(41) FIG. 8 shows a perspective view of the filament aligned with the support structure 14. For example, the mounting bolts are provided with a longitudinal groove for facilitating the proper positioning of the filament 12. After connecting the filament frame structure 18 to the support structure 14, the bars 80 shown in FIG. 8 are removed, which is shown in FIG. 9.

(42) It is further noted that FIG. 8 shows the body structure 16 as a cathode with a surface profile, whereas FIG. 9 shows the body structure 16 with a flat surface in the area of the filament 12 and the filament frame structure 18.

(43) FIG. 10 shows a cross-section of a further example of the present invention. As can be seen, the filament 12 is connected to the mounting bolts 20, 22 of the support structure 14 and the filament frame structure 18 is also connected and thus held by the support structure 14. Further, the mounting bolts of the support structure 14 are extending through the body structure 16. Therefore, a stepped through-hole 82 is provided with an upper portion 84 having a larger diameter than the lower portion. Thus, the brazing, e.g., high temperature soldering, to the body structure 16 can be provided in the lower part, whereas at the upper part, the bolts are not coupled to the body structure 16, resulting in independent thermal expansion, thus supporting the proper alignment of the filament 12.

(44) As can be seen, the connecting of the filament 12 and the connecting of the filament frame structure 18 to the same support structure 14 allows an independent movement during the operation, for example due to a thermal expansion. Thus, a proper alignment in respect of the electron-optical focusing is provided throughout the operation.

(45) As also indicated above, according to a further example, the cathode 10 is provided with a cathode cup 86 as having a flat front side 88, wherein the filament 12 is arranged on the flat front side 88, as shown in FIG. 10 as an example.

(46) It must be noted that the feature of the cathode cup having a flat front side may in particular be combined with the features shown in FIGS. 1 to 9. According to an example of the present invention, the filament 12 is totally recrystallized inside to achieve required straightness. For example, the filament 12 is made of W or high temperature ceramics such as BN and the like. The pin may be provided inserted into the helical structure during the recrystallization.

(47) Further, the guiding pin 80 may also have a groove to fix the straight ends of the filament in a plane while the recrystallization process is conducted.

(48) By attaching the filament frame structure to the support structure, together with the filament also being attached to the support structure, high accuracy stability is provided during the assembly. Thus, no manual or other adjustment and tooling is necessary. Further, high focal spot stability is provided over lifetime, with smaller drift effect from thermal expansions since both parts, i.e. the filament and the filament frame structure so-to-speak move together with each other.

(49) Further, radiation cooling at temperatures with high infrared radiation is provided by the flat surface portions of the filament frame structure, but below thermionic emission level.

(50) The filament frame structure can be attached in one piece and, for example, laser-cut after assembly, in order to separate electrical potentials. Thus, one piece or multi-piece frames are possible. Further, the isolation of the frame or counter parts may also be facilitated.

(51) According to a further example (not further shown), the filament frame structure may be provided on a mounting support for the frame structure parts, thus a laser cutting process is not necessary after the assembly. However, an additional tool would be necessary for the mounting of the frame parts.

(52) According to a further example, the filament is provided with a pretension in a non-operating state, for example provided by the support structure 14. For example, during the mounting, the pretension can be provided by the filament frame structure being a one piece component. The pretension while mounting is provided, for example to compensate thermal expansion of the filament, e.g. the coil, due to heating during operation. The pretension is adjusted to minimize mechanical stresses in operation, reducing plastic deformation and distortion of the filament 12. Thus, thermal mechanical and electron-optic stability is improved. As mentioned above, the pretension can be applied by the filament frame structure, or also by an additional tool.

(53) With reference to FIGS. 7A and 7B, the pretension may also be applied by a controlled deformation of the support bars 80, after the filament welding, the curved bars are to be compressed by a tool, resulting in longitudinal stretching of frames. The bars are then, as mentioned above, laser-cut after assembly, to separate electrical potentials.

(54) With reference to FIGS. 8, 9, and 10, according to an example, the body structure 16 is provided as a cathode cup, for example the cathode cup 86. The cathode cup is provided as a ceramic cathode cup, made from electrically non-conducting ceramic. A part of the cathode cup's surfaces is provided with a metallic coating.

(55) For example, the metallization is provided on the surfaces for brazing and electrical purposes, e.g. to avoid surface charges.

(56) FIG. 11 shows an X-ray tube 100 comprising a cathode 110, and an anode 112. The cathode is provided as a cathode according to one of the above mentioned and described examples.

(57) For example, the X-ray tube is provided with a rotating anode, indicated with a rotation axis 114. For this purpose, driving device 116 are indicated, whereas only the parts inside a tube housing 118 are shown, neglecting any parts being outside, for example a scatter of the driving means. Further, steering or deflection means device 120 is shown for deflecting an electron beam 122 from the cathode 110 towards a focal spot portion 124 on the anode 112. An X-ray transparent window 126 is shown such that an X-ray beam 128 is radiated towards a not further shown object. It must be noted that FIG. 12A is a schematic drawing of an X-ray tube.

(58) Further, according to the present invention, also a system 200 for X-ray imaging is provided, comprising an X-ray source 210, and X-ray detector 212, and a processing unit 214. The processing unit 214 is configured to control the X-ray source 210 and the X-ray detector 212 for providing X-ray image data of an object of interest 216. The X-ray source 210 is provided as an X-ray tube 100 according to the above mentioned example.

(59) For example, the X-ray system may be a medical imaging system as shown in FIG. 12A. As can be seen, the X-ray source 210 and the X-ray detector 212 are provided as a so-called C-arm arrangement 218, where a C-arm structure is movably mounted to a support arrangement in order to provide free arrangement of the source and detector around the object of interest. For example, a patient table 220 as well as monitoring devices 222 and lighting devices 224 are shown indicating an operational room in a hospital, for example.

(60) However, according to the present invention also an inspection apparatus 226 is provided, for example for scanning and screening of luggage pieces 228, or for material and construction inspection. This is shown in FIG. 12B as a further example for an X-ray system 200 for X-ray imaging, comprising an X-ray source which is provided as an X-ray tube according to the above mentioned examples. It is noted that the X-ray source is not further shown in FIG. 12B.

(61) FIG. 13 shows a method 300 for an assembly of a cathode for an X-ray tube, comprising the following steps. In a first step 310, a filament is provided, wherein the filament is configured to emit electrons towards an anode in an electron emitting direction, and wherein the filament at least partially comprises a helical structure. In a second step 312, the filament is aligned with respect to a filament frame structure, wherein the filament frame structure is configured for electron-optical focusing of the emitted electrons, wherein the filament frame structure is provided adjacent to the outer boundaries of the filament, and wherein the filament frame structure comprises frame surface portions arranged transverse to the emitting direction. In a third step 314, the filament is connected to the filament frame structure. In a fourth step 316, a support structure is connected to a body structure. In a fifth step 318, the filament frame structure is placed on the support structure. In a sixth step 320, the filament frame structure and the filament are connected to the support structure.

(62) The first step 310 is also referred to as step a), the second step 312 as step b), the third step 314 as step c), the fourth step 316 as step d), the fifth step 318 as step e), and the sixth step 320 as step f).

(63) The filament frame structure may be connected to the body structure, in addition or instead of being connected to the support structure, for example in case of a ceramic cathode head.

(64) However, it is advantageously, if the filament frame structure is provided such that a movement in combination with the filament is provided in case of thermal expansion.

(65) According to a further example, shown in FIG. 14, a support guide is provided in a first preparation step 322 for the assembly. Thus, the steps are provided as step a1) in which the filament is provided 324 on the support guide. In a further step b1), the support guide is aligned 326 with respect to the filament frame structure. The support guide is then removed in a further step 328, for example as a step i) after the filament is connected to the filament frame structure, or as step ii) after the filament frame structure is connected to the support structure. This is indicated by dotted lines 330 in FIG. 14 indicating the two possibilities as alternative options.

(66) According to a further example (not further shown), following step a), but before the placing of step e), a total recrystallizing of the filament is provided by applying external heat. The recrystallizing may be provided, for example, before step c).

(67) It has to be noted that embodiments of the invention are described with reference to different subject matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments are described with reference to the device type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters is considered to be disclosed with this application. However, all features can be combined providing synergetic effects that are more than the simple summation of the features.

(68) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

(69) In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. A single processor or other unit may fulfil the functions of several items re-cited in the claims. The mere fact that certain measures are re-cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.