Abstract:
The invention relates to a marking apparatus ( 10 ) for marking an object comprising a marking head ( 20 ) having a plurality of marking devices ( 40, 40   a,    40   b ) for applying a marking on the object and a driving mechanism for providing a relative movement of the object relative to the marking head in an advance direction ( 16 ) during a marking operation. The marking head comprises in addition to the plurality of marking devices a plurality of sensor devices and the sensor devices ( 50 ) are arranged down-stream of the marking devices in the advance direction, so that the marking applied by the marking devices is detectable by the sensor devices, when the object is moved relative to the marking head in the advance direction. The invention also relates to a method for marking an object.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a marking apparatus for marking an object. The invention also relates to a method for marking an object. 
     RELATED ART 
     The known marking apparatus comprises a marking head having a plurality of marking devices for applying a marking on the object and a driving mechanism for providing a relative movement of the object relative to the marking head in an advance direction during a marking operation. 
     In the known method, which can in particular be carried out with a marking apparatus as described above, a marking is applied by a plurality of marking devices and the object is moved relative to the marking devices in an advance direction during a marking operation. 
     In the known marking apparatus and marking method it is a general problem that one or more marking devices might fail or not be operated correctly during a marking operation. Such a failure or malfunction of one or more marking devices may adversely affect the applied marking, so that the marking may for example lack one or more pixels. 
     The defect in the marking might not be perceived by an operator during a certain runtime of the marking apparatus, so that a number of marked objects might have to be discarded due to an insufficient quality of the marking. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a marking apparatus and method for marking allowing for a high quality of the markings. 
     The object is solved according to the invention by a marking apparatus and a method. Preferred embodiments are given in the dependent claims. 
     The marking apparatus may be characterized in that the marking head comprises in addition to the plurality of marking devices a plurality of sensor devices and the sensor devices are arranged downstream of the marking devices in the advance direction, so that the marking applied by the marking devices is detectable by the sensor devices, when the object is moved relative to the marking head in the advance direction. 
     The method may be characterized in that the marking applied by the marking devices is detected by a plurality of sensor devices, which are arranged downstream of the marking devices in the advance direction. 
     One idea of the invention is to provide an integrated marking and sensor apparatus for applying a marking on the object and detecting the presence of the applied marking. The integrated marking and sensor apparatus, which may also be referred to as an integrated marking and scanning apparatus, may allow for a high quality of the marking, as a failed or damaged marking device will instantly be detected by one of the sensor devices, such that the marking apparatus may be stopped and/or an operator may be notified. 
     An idea of the invention is to arrange the sensor devices and the marking devices in one common marking head, which may also be referred to as an integrated marking and sensor head or an integrated marking and scanning head. In particular, the marking devices and the sensor devices may be arranged in a fixed relative position to each other in the marking head. 
     The marking devices may in particular be marking devices for marking, printing and/or engraving the object with at least one laser beam. In a preferred embodiment the marking devices comprise a ferrule with a fibre coupled to a laser device. However, the marking devices can also include other types of marking devices such as inkjet nozzles, thermal printing devices, needle printing devices, micro pad printing devices, water jets, and/or electrical discharge machining devices. It is also possible to include different types of marking devices in the marking head. 
     The sensor devices may for example comprise a ferrule with a fibre arranged therein, a PIN diode, a photodiode, a phototransistor, a micro antenna, a capacity sensor element, an inductive sensor element and/or a chemical sensor element. The sensor devices may in particular be optical sensors devices. The sensor devices may be configured to detect a colour profile on the object. 
     In a preferred embodiment, the marking apparatus is a printing apparatus for printing or engraving an object by means of at least one laser beam. 
     In a method for marking or printing the object, the object may be in particular marked or printed by successively operating the individual marking devices, that is, the marking is applied line by line or pixel by pixel. In the same manner, the sensor devices are also operated successively, so that a marking may be scanned line by line or pixel by pixel, while the object is moved relative to the marking head. 
     In a preferred embodiment of the invention the marking head comprises a plurality of receiving spaces, in which the marking devices and the sensor devices are arranged. The marking devices and the sensor devices may be preferably each configured to be engaged with the receiving spaces of the marking head. 
     For a flexible arrangement of the marking devices and sensor devices, it may be preferred that the receiving spaces are adapted to selectively receive marking devices and sensor devices, that is, a receiving space may be selectively equipped with a marking device or a sensor device. In a preferred embodiment the receiving spaces have equal configurations, so that the positions of marking devices and sensor devices may be changed, in particular interchanged. 
     In another preferred embodiment, the marking devices and the sensor devices have corresponding connector sections for being variably or selectively coupled to or engaged with the receiving spaces of the marking head. In other words, both marking devices and the sensor devices are configured to be engaged with the receiving spaces of the marking head. This may provide flexible marking apparatus, because the arrangement of the marking devices and the sensor devices may be adapted to a given marking task. 
     It may be preferred according to the invention that the receiving spaces are arranged in a plurality of rows and columns, such that a two-dimensional array of receiving spaces is formed. When marking and scanning an object, the marking devices and sensor devices may be operated to apply a marking pixel by pixel and scan the marked object pixel by pixel. The plurality of rows and columns may in particular be employed for an enhancement of marking and/or scanning speed or an enhancement of marking and/or scanning resolution. The plurality of rows also allows for using at least one row exclusively for marking devices and a successive row exclusively for sensor devices. 
     In a preferable configuration the rows and columns, in which the receiving spaces are arranged, extend perpendicular to each other. In other words, it may be preferred that the receiving spaces are arranged in a two-dimensional array with a rectangular pattern of the receiving spaces. The rectangular pattern of receiving spaces, which may also be called an orthogonal pattern or arrangement of the receiving spaces, includes a plurality of rows and columns, in which the receiving spaces arranged, wherein the rows and columns extend perpendicularly to each other. The rows of the two- dimensional array preferably extend in a transverse direction, that is, a direction that extends transversely to the advance direction. It may be preferred that the receiving spaces have equal pitches, that is, equal distances between the central points between adjoining receiving spaces, in the row and/or in the column direction. Such a regular pattern provides a uniform marking and/or scanning resolution. 
     In another preferred embodiment of the invention the array of receiving spaces is tilted with regard to the advance direction such that the rows extend in a transverse direction relative to the advance direction and the receiving spaces of a successive row are offset with regard to the receiving spaces of a preceding row in the transverse direction, in particular in a direction perpendicular to the advance direction. With such a tilted or inclined position of the array the resolution of the marking head can be enhanced. 
     It may be particularly preferred that an array with a rectangular pattern of the receiving spaces is tilted. In the tilted position of the rectangular pattern array, the rows of individual receiving spaces extend transversely, but not perpendicularly, to the advance direction. Consequently, as the rows extend perpendicularly to the columns, the columns of receiving spaces also extend transversely to the advance direction. The array of receiving spaces is thus rotated or tilted from a position, in which the columns are aligned with the advance direction, to a position, in which the columns are at least slightly inclined or slanted with regard to the advance direction. A preferred tilting angle may be in the range of less than 10 degrees, preferably less than 5 degrees. 
     It may be preferred that the marking head is a page-wide marking head, that is, the marking head has a width corresponding to the width of an object to be marked, wherein the width of the object is defined as the dimension of the object in a transverse direction, in particular the direction perpendicular to the advance direction. The direction perpendicular to the advance direction may be referred to as the orthogonal direction. The object may therefore be marked by moving the marking head in the advance direction without overlaying a further movement of the marking head in the transverse direction and/or the orthogonal direction. The advance direction, which may also be called the object movement direction, may be in particular a linear direction. 
     In a preferred embodiment the receiving spaces are arranged in a regular rectangular pattern and the amount of offset of the receiving spaces of a successive row with regard to the receiving spaces of a preceding row is smaller than a pitch of the receiving spaces of one row. 
     The pitch of the receiving spaces, which is also called the device pitch, is the distance between the central points of two adjoining receiving spaces. The amount of offset corresponds to a marking line or scan line pitch, which is the distance between two adjoining marking or scan lines in the transverse direction, in particular the orthogonal direction. Moreover, the amount of offset can be described as the distance in the orthogonal direction between two corresponding receiving spaces of neighbouring or adjoining rows or as the distance in the orthogonal direction between two adjoining receiving spaces of one column. Thus, the receiving spaces are preferably arranged in a way, that the marking line pitch or scan line pitch is smaller than the device pitch. 
     It may be particularly preferred that the array is tilted to a degree, in which at least a part of the receiving spaces of at least one row of the rectangular pattern is aligned with at least a part of the receiving spaces of at least one preceding row in the advance direction. With this embodiment, it is possible to place a marking device and a sensor device such that they are aligned in the product movement direction, although the array is tilted. This provides the possibility of detecting a marking applied by a marking device in conjunction with a tilted array having an enhanced resolution of marking as compared to a non-tilted array. 
     In order to variably adapt the resolution of the marking it may be preferable that the marking head is rotatable about an axis perpendicular to the advance direction, in particular perpendicular to a surface of the object to be marked. The rotational marking head may allow for a flexible adjustment of the resolution of the marking apparatus. Moreover, with the rotational marking head it is possible to set up a tilt angle of the marking head in which the sensor devices are aligned with the marking devices in the advance direction. 
     For a precise movement of the marking head it may be preferred that a motor, in particular a stepper motor, is provided for rotating the marking head, in particular at defined small angle steps in the range of 0 to 90 degrees. The small angle steps may be in particular steps of less than 1 degree, for example less than 0.1 degrees. The motor may in particular be an electrical motor. 
     It may be preferred that the marking head comprises a receiving plate with a plurality of receiving holes, in which the marking devices and the sensor devices are arranged. The receiving holes may in particular be through-holes. 
     In a further preferred embodiment the marking devices and the sensor devices may include ferrules with fibre ends arranged therein. The combination of marking and sensor devices each including ferrules may provide a flexible marking apparatus, in which the marking devices and sensor devices may be arranged in the receiving spaces in a flexible manner. The ferrules of the marking devices and sensor devices may be inserted into the receiving holes of the marking head, so that the ferrules are each coupled to the receiving plate in a defined position. 
     For holding the ferrules tight and removable in the receiving holes, it may be preferred that a capture pad is arranged at at least one surface of the receiving plate. It may be preferred that the capture pad includes an elastic polymer, in particular a rubber and/or an elastomer. The capture pad is preferably made of viton® (a registered trademark of E. I. du Pont de Nemours and Company, Delaware, USA) or includes the material viton®. The ferrules may be pushed through the capture pad and are then held in place by the capture pad as it closes the ferrule after insertion. The ferrules can be removed by simply pushing back through from one side of the receiving plate. 
     The ferrules of the marking devices and the ferrules of the sensor devices may preferably have equal or corresponding connector sections, so that a receiving space of the marking head may be selectively equipped with a marking device or a sensor device. 
     A ferrule of a marking device or a sensor device includes at least one fibre arranged therein. In case of a marking device, the at least one fibre may be coupled to a lighting element, for example a laser for marking or engraving the object by means of a laser beam. In case of a sensor device, the at least one fibre may be coupled to a sensor element for detecting light received through the fibre. 
     Another preferred embodiment of the invention may be characterized in that the plurality of marking devices and sensor devices includes at least one integrated marking and sensor device comprising a ferrule with at least one first fibre connectable to a marking element for marking the object and at least one second fibre connectable to a sensor element for detecting the marking on the object. 
     The marking element may in particular be a laser for emitting a laser beam for marking the object. The sensor element may in particular be an optical sensor element such as a photosensor or photodetector. The photosensor or photodetector can for example be a photodiode, a phototransistor, or a photoresistor. 
     The integrated marking and sensor device allows for an integrated marking and verification of the applied marking in one pixel, that is, in one receiving space of the marking head. The marking may be applied through the first fibre, which may be called a delivery fibre, and the presence of the marking may be detected by the second fibre, which may be called a receiving fibre. According to the invention, the receiving fibre may be arranged downstream of the delivery fibre in the advance direction. 
     The integrated marking and sensor device can in particular be employed as an online pixel monitor for recognizing failed pixels or a fibre break during a marking operation. 
     The second fibre can be used to detect a reflected laser beam from the first fibre to verify that a marking was made. 
     The integrated marking and sensor device can also be used as a power monitor for measuring the power of the laser beam. To this end, a mirror coating may be applied onto the end of one of the fibres. A portion of the power in the delivery fibre can be directed to a detector or sensor element for determining the power of the laser beam. The detected power can for example be used to provide feedback for constant power control and/or for code verification, that is, verification of the presence of a marking applied, in particular the presence of a plume or reflected power to verify that a marking was made or a spot was printed. 
     In particular in connection with the integrated marking and sensor device it may be preferred that at least one lens is provided in front of the ferrule. The lens may reflect a part of the light emitted by the first fibre, so that the reflected light is receivable by the second fibre. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further described with reference to the attached figures, wherein: 
         FIG. 1  shows an inventive marking apparatus; 
         FIG. 2  shows a perspective view of an inventive marking head; 
         FIG. 3  shows an empty array of receiving spaces; 
         FIG. 4  shows an array of receiving spaces, which is equipped with a plurality of marking devices and a plurality of sensor devices; 
         FIG. 5  shows a tilted array of receiving spaces, which is equipped with a plurality of marking devices and a plurality of sensor devices; 
         FIG. 6  shows an object having been marked using the array according to  FIG. 5 ; 
         FIG. 7  shows a cross-sectional view of a rotatable marking head; 
         FIG. 8  shows the general principle of a multiple mark or scan option; 
         FIG. 9  shows a perspective view of a ferrule to be inserted into a receiving space of a marking head; and 
         FIG. 10  shows a perspective view of a ferrule with a delivery fibre and a receiving fibre. 
     
    
    
     In all figures, identical components are identified by identical reference signs. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A principle structure of a marking apparatus  10  is shown in  FIG. 1 . The marking apparatus  10  comprises a marking head  20  with a plurality of marking devices  40  and a plurality of sensor devices  50 . The apparatus  10  further comprises a control and driving unit  12  for controlling the marking devices  40  and the sensor devices  50 . The control and driving unit  12  is connected to the marking head  20  through an umbilical  14 . The umbilical  14  may have a plurality of fibres arranged therein. 
       FIG. 2  shows a general embodiment of a marking head  20 , which can in particular be a printing head. The marking head  20  comprises a housing  21 , which in the shown embodiment has a cylindrical outer shape. 
     The marking head  20  includes a plurality of receiving spaces  24  arranged in a two-dimensional array  22 . The receiving spaces  24  are equipped with individual marking devices  40  and sensor devices  50 . The sensor devices  50  may also be referred to as scanning devices. 
     An empty array  22  of receiving spaces  24  is shown in  FIG. 3 . The receiving spaces  24  are arranged in rows  30  and columns  32  extending perpendicularly to each other. In other words, the receiving spaces  24  are arranged in a rectangular or square pattern, which may also be called a matrix, in particular a two-dimensional matrix. 
     Moreover, the receiving spaces  24  have equal distances or an equal spacing, so that a regular pattern is formed. The spacing between to adjacent receiving spaces  24 , more particularly the distance between the central points of two adjacent receiving spaces  24  in one row  30  or column  32 , is called a device pitch  34 . The receiving spaces  24  have equal device pitches  34  in the row direction and in the column direction. The array  22  of receiving spaces  24  has a rectangular outer shape. 
     The marking head  20  includes a receiving plate  28  having a plurality of receiving holes  26  forming the receiving spaces  24 . The receiving plate  28  may for example be a metal plate, in particular a steel plate. The receiving holes  26  each have a substantially circular cross-section and may in particular be through holes. The receiving holes  26  have equal diameters  27 . 
     In addition to the array  22  of receiving spaces  24 , a plurality of spare receiving spaces  25  is provided for accommodating spare marking and/or spare sensor devices. The spare receiving spaces  25  are also formed as receiving holes in the receiving plate  28  and may in particular be positioned outside the array  22 , as shown in  FIG. 3 . 
       FIG. 4  shows a marking head  20  with an array of receiving spaces  24 , wherein the receiving spaces  24  are equipped with a plurality of marking devices  40  and a plurality of sensor devices  50 . 
     The marking devices  40  include a plurality of marking devices  40   a ,  40   b ,  40   c , which are arranged in individual sub-arrays  23 . The marking devices  40   a ,  40   b ,  40   c  may be of different types for performing different marking tasks. For example, the marking devices  40   a  may be ink jet nozzles, the marking devices  40   b  may be CO 2  laser ferrules and the marking devices  40   c  may be laser diode ferrules. The ink jet nozzles may for example be used to print on a PVC cap. The CO 2  laser ferrules may be used to print on an object  8  having varying distances to the marking devices  40 . The laser diode ferrules can for example be used to print on paper. 
     The sensor devices  50 , for example sensor ferrules, are arranged downstream of the marking devices  40  in an advance direction  16  of the object  8 . The sensor devices  50  are arranged in a way that a marking applied by the marking devices  40  can be verified by the sensor devices  50 . To this end, the sensor devices  50  are at least partly aligned with the marking devices  40  in the advance direction  16 . The sensor devices  50  may also be arranged in individual sub-arrays  23  corresponding to the sub-arrays  23  of the marking devices  40 . 
     In particular, a plurality of sensor devices  50  is positioned in a manner that each of the sensor devices  50  is aligned with one of the marking devices  40 . In other words, each of the marking devices  40  is provided with or related to at least one sensor device  50  for verification of the marking applied by the respective marking device  40 . The number of sensor devices  50  in the marking head  20  is therefore equal to or greater than the number of marking devices  40 . With this configuration, the marking of each of the marking devices  40  may be individually verified or monitored. 
     In addition to the marking devices  40  and the sensor devices  50  a measuring device  60  is arranged in one of the receiving spaces  24 . The measuring device  60  is configured to measure a speed of the object  8  in the advance direction  16 . 
     Moreover, the marking head  20  includes a detector device  70  for detecting the presence of the object  8  to be marked. 
       FIG. 5  shows a tilted array  22  of receiving spaces  24  similar to the array illustrated in  FIG. 4 . As in  FIG. 4 , the receiving spaces  24  are equipped with a plurality of marking devices  40  and a plurality of sensor devices  50 . 
     A difference between the configuration of  FIG. 4  and the configuration of  FIG. 5  is that the array  22  according to  FIG. 5  is tilted or inclined with regard to the advance direction  16 . The tilted position is defined in particular in that the rectangular pattern of rows  30  and columns  32  is tilted from a position in which the columns  32  are aligned with the advance direction  16  to a position in which the columns  32  are inclined or slanted with regard to the advance direction  16 . 
     The tilted position of the array  22  or marking head  20 , respectively, enhances the maximum possible resolution of the marking and/or scanning. In a preferred embodiment, as shown in  FIG. 5 , the array  22  is tilted to a degree, such that the resolution is defined by the number of rows  30  times the number of columns  32 , that is, by the mathematical product of the number of rows  30  and the number of columns  32 . To this end, the array  22  is tilted to a degree, where the receiving spaces  24  of a successive row  30   b  are slightly offset with regard to the receiving spaces  24  of a preceding row  30   a , in particular such that the receiving spaces  24  overlap in the transverse direction. 
     With the tilted array  22  of receiving spaces  24 , respectively marking devices  40  and/or sensor devices  50 , the resolution of the marking in the transverse direction is enhanced. In particular, a marking line pitch or scan line pitch, which is defined as a distance between two adjoining marking or scanning lines in the transverse direction, is smaller than the device pitch  34 . 
       FIG. 6  illustrates an example of an object  8  marked or printed by the marking head  20  according to  FIG. 5 . 
     In another preferred embodiment not explicitly shown in the figures, the marking head  20  is inclined to a position, in which a certain number of receiving spaces  24  is aligned with other receiving spaces  24  in the advance direction  16 . In order to verify the marking applied by the marking devices  40 , the sensor devices  50  are positioned such that they are aligned with the marking devices  40  in the tilted marking head  20 , respectively array  22 . The principle of such a tilted array  22  allowing for a multiple mark or scan option will be described later on with reference to  FIG. 8 . 
       FIG. 7  shows a cross-sectional view of a marking head  20  comprising a receiving plate  28  with receiving holes  26 , in which marking devices  40  and sensor devices  50  (not shown) are arranged. 
     The receiving holes  26  are formed as through-holes. The marking devices  40  each comprise a ferrule  42 , in which at least one fibre end of a fibre  56  is arranged. In a corresponding manner, the sensor devices  50  can also comprise ferrules  42  and can in particular each have a shape corresponding to the shape of the marking devices  40 , so that the receiving holes  26  may be selectively equipped with marking devices  40  and sensor devices  50 . 
     A motor  64 , in particular a stepper motor, is arranged for rotating the marking head  20  and/or the receiving plate  28 . A transmission  66 , which in the shown embodiment is a belt, is arranged between an output shaft of the motor  64  and the marking head  20  for transmitting a rotational motion of the output shaft to the marking head  20  and/or the receiving plate  28 . 
       FIG. 8  schematically shows different tilting angles of an array  22 . In the left representation of  FIG. 8  the array  22  is tilted to a degree in which each of the receiving spaces  24  of one column  32  are offset with regard to all other receiving spaces  24  of the same column  32 , so that only one single mark or scan per pixel is possible. That is, if the receiving spaces  24  of one column  32  are equipped with marking devices  40 , the marking devices  40  are offset with regard to any other marking devices  40  of the same column  32 . With this configuration the maximum resolution of a given marking head  20  may be achieved. 
     In the middle representation the array  22  is tilted to a degree in which a double mark or scan of any pixel is possible. That is, the receiving spaces  24  of one column  32  correspond to the receiving spaces  24  of another column  32  such that one and the same pixel may be marked by two different marking devices  40  arranged in different columns  32  or a sensor device  50  may be arranged in an aligned position with a marking device  40  for verification of a marking applied by the marking device  40 . 
     In the next representation the array  22  is tilted to a degree in which a triple mark or scan of any pixel is possible. That is, the receiving spaces  24  of one column  32  correspond to the receiving spaces  24  of two other columns  32  such that one and the same pixel may be marked by three different marking devices  40  arranged in different columns  32  or at least one sensor device  50  may be arranged in an aligned position with at least one marking device  40 . 
     The right representation shows the array  22  in a non-inclined position. 
       FIG. 9  schematically shows a perspective view of an embodiment of a ferrule  42 , which may form a part of a marking device  40  or a sensor device  50 . The ferrule  42  is configured for a mating engagement with the receiving spaces  24 , in particular the receiving holes  26 , of the marking head  20 . 
     The ferrule  42  has a substantially cylindrical body  43  and can for example include a metal, a ceramic, a plastic material or glass. It is particularly preferred that the ferrule  42  includes steal or zirconia. 
     The body  43  of the ferrule  42  has a connecting portion or a connector section  49  for engaging a receiving space  24  of the marking head  20 . The connector section  49  has a substantially cylindrical shape for a mating engagement with a cylindrical receiving hole  26  provided in the receiving plate  28  of a marking head  20 . The body  43  of the ferrule  42  further comprises a collar  44  with an abutment surface  45  for contacting a planar surface of the receiving plate  28 . 
     At least one optical fibre  56  is arranged in the ferrule  42  for receiving light reflected from the object  8  or transmitting light or radiation to it. The at least one fibre  56  is arranged along a longitudinal axis of the ferrule  42 . 
     If the ferrule  42  is used as a part of a marking device  40 , it may be preferred that the at least one optical fibre  56  is configured to transmit a laser beam onto a surface of the object  8  for a laser marking operation, in particular a laser engraving operation. To this end, the fibre  56  may be coupled to a laser so that a laser beam is transmittable through the fibre  56  onto a surface of the object  8  for marking the object  8 . 
     If the ferrule  42  is used as a part of a sensor device  50 , it may be preferred that the at least one optical fibre  56  is configured to receive light reflected by the object  8 . The light can be any kind of electromagnetic radiation such as for example visible light or infrared light. The at least one fibre  56  may be connected to a sensor element for detecting the light received by the fibre  56 . 
       FIG. 10  illustrates a ferrule  42  with two fibres  56 ,  57  arranged therein. The ferrule  42  with two fibres  56 ,  57  may form a part of an integrated marking and sensor device  90 , wherein one of the two fibres, a first fibre  56 , is a marking or delivery fibre and the other fibre, a second fibre  57 , is a sensor fibre. 
     The two-fibre ferrule  42  provides a marking element and a verification element in the same pixel, that is, in the same receiving space  24  of the marking head  20 . The marking may be applied through the first fibre  56 , which may be coupled to a laser, and the presence of the marking may be detected by the second fibre  57 , which may be preferably coupled to a sensor element. 
     When ferrules  42  are used both as marking devices  40  and as sensor devices  50 , it may be preferred that a fibre diameter of the sensor device  50  is greater than a fibre diameter of the marking device  40 , so that the marking can be detected even when the sensor device  50  is not exactly aligned with the marking device  40 .