LASER ENGRAVING APPARATUS

Abstract: An apparatus for preparing a printing medium (2) such as an anilox roller is arranged to support and rotate the medium about a longitudinal axis. An engraving head (6) is mounted for movement substantially parallel to the longitudinal axis and engraves a series of cell sites on the printing medium. A control circuit (3, 40) controls the engraving parameters of the engraving head (6). The apparatus includes a monitoring device (30) which monitors the profile of the cells (9) engraved by the head (6) and produces a control signal depending on the monitored profile. The control circuit (40) varies the engraving parameters in response to the control signal so as to maintain a desired cell profile. The monitoring device may include a confocal optical system which samples the cross section of engraved cells at a selected depth. The optical system may be astigmatic and arranged to focus light reflected from the surface of the printing medium onto a photodetector (25) comprising at least two adjacent photosensitive elements (A, B). A servo mechanism driven by the control signal may be used to vary the height of the optical system (30) above the surface of the printing medium so as to track the surface.

Description: LASER ENGRAVING APPARATUS BACKGROUND TO THE INVENTION The present invention relates to the preparation of cylinders for use in printing, and in particular to the engraving or exposure of cells on an anilox roller. In a conventional system for preparing an αnilox roller, the roller is rotated about its longitudinal axis whilst a laser engraving head is moved along the roller parallel to this axis. The engraving head focuses laser light onto the surface of the roller to engrave a cell. The combination of the rotation of the roller and the longitudinal position of the laser engraving head is able to produce a helical pattern of cells in which each cell corresponds to a single pulse of the laser. Generally, the cells are arranged in a hexagonal, square or other pattern by synchronisation of the roller rotation with the speed of the engraving head. When engraving cells on an anilox roller, it is essential that each cell should have substantially the same volume and profile within a very tight tolerance. This is necessary to produce a uniform rate of ink delivery when the roller is used in a press. For a typical roller having a surface area greater than 10,000 cm 2 , and a cell density of 22,000/cm 2 , for an engraving rate of 10 kHz, such a roller takes at least six hours to engrave. During this long engraving period, the engraving conditions are liable to change, and drifts in any of the parameters could effect the cell's volume and profile significantly. In our earlier International Patent Application No. PCT/GB 91/01829 (WO 92/09399) , we disclosed a method and apparatus for improving the quality of the engraved cells, and the speed at which they can be engraved using an acousto-optic deflector. However, this process still takes some hours to engrave all the cells on a roller, and therefore there remains the possibility of drift in the engraving parameters. SUMMARY OF THE INVENTION According to the present invention, an apparatus for preparing a printing medium for use in a printing process comprises means to support the printing medium and rotate it about a longitudinal axis; an engraving head mounted to move substantially parallel to the longitudinal axis of the printing medium for engraving a series of cell sites on the printing medium and control means for controlling the engraving parameters of the head, characterised by means for monitoring the profile of the engraved cells and for producing a control signal depending upon the monitored profile, the control means being responsive to the control signal to vary the engraving parameters to maintain a desired cell profile. Preferably the means for monitoring includes a confocal optical system and is arranged to sample the cross-section of the engraved cell at a selected depth. Preferably, the means for monitoring is arranged to move substantially parallel to the longitudinal axis of the anilox roller to allow monitoring of the engraved cells. To build up a detailed picture of a cell, when using a simple confocal system, which provides height information only at the point of focus, it is desirable to scan the cell at a number of different depths, and therefore it is necessary to monitor a cell on each of a number of revolutions to determine the profile. Preferably not all of the engraved cells are monitored. It is not necessary to monitor each cell as the errors in the engraving that affect the final roller quality are generally due to a drift in the engraving parameters and therefore occur over a long period of time in relation to the engraving frequency. It is therefore only necessary to monitor one individual cell out of a group of cells which are engraved within a short period of time. By monitoring this cell at different depths over subsequent revolutions, an accurate profile of this cell can be determined which can be assumed to be characteristic of the cells engraved at about the same time. By repeating this cell monitoring for a number of cells engraved at different times during the engraving process, any drift in the engraving parameters can be identified and corrected when they occur. In another example, a number of cells which are engraved within a short period of each other are scanned on each rotation, and an average is taken of the profile of each of these engraved cells. This average profile is used as the cell characteristic for detecting a change in the engraving parameters. Preferably the confocal optical system includes a point illumination source, which may include an aperture, an objective lens positioned to focus light from the point illumination source onto the surface of the roller and a photodetector mounted behind an aperture and arranged to receive via the objective lens scattered and reflected light from the surface of the roller. The illumination source and the photodetector may share a common aperture in which case preferably the optical system includes a beam splitter on the opposite side of the aperture to the target. The illumination source for the optical system of this aspect of the invention, or for the optical system of the second aspect of the invention, may comprise a light source such as a laser diode, and a length of fibre optic, light being introduced from the light source to one end of the fibre optic and output to the confocal optical system from the other end of the fibre optic. Preferably, the light from the source is introduced into the fibre optic with an angular offset so that the output beam has a generally hollow conical configuration. The use of a beam with a hollow conical profile is found to further enhance the operation of the confocal optical system. Such a beam gives a generally annular region of illumination on the sample when the surface of the sample is out of focus as shown in Figure 2a discussed below. The fact that the central region is then unilluminated serves further to enhance the contrast between the level of illumination reaching the detector for in-focus and out of focus cases. The required offset can be produced simply by coupling the light source to the fibre optic in such a way that the source is off-axis. Preferably the angular offset is in the range from approximately 5 to approximately 10°. The use of a fibre optic in this manner to produce a conical beam for use in a confocal optical system is not limited in applicability to monitoring systems for engraving but may advantageously be used in any of a variety of confocal optical systems, such as otherwise conventional confocal optical microscopes. Other light sources including non-coherent sources such as a xenon lamp may be used. The illumination source may optionally be strobed. According to a second aspect of the present invention an apparatus for preparing a printing medium including means for engraving a plurality of cells on the medium, characterised by means for monitoring the depth of the cells, the means for monitoring including an astigmatic confocal optical system arranged to focus light reflected from the surface of the roller onto a photodetector. Preferably the photodetector comprises at least two adjacent photosensitive elements. The second aspect of the invention provides an alternative system for monitoring the engraving of cells on an anilox roller. While the system of the first aspect of the invention has the advantage that you can use all light returned from the target, both specular and diffuse, there is an associated problem in that spurious signals can be generated as a result of any variation in the reflectivity of the surface. As described in further detail below, the system of the second aspect of the invention, is less sensitive to variations in reflectivity, and is capable of providing more detailed information on the profile of the cell in the region of the level of focus. Since more height information is provided, it may not always be necessary with this arrangement, to sample a number of different depths. The confocal optical system includes an astigmatic element, so that changes in the height of the surface being imaged are converted in to change in the lateral position of the image. As well as providing an output signal to monitor the depth of the cells, the system in accordance with this aspect of the invention may be arranged to provide a servo signal to vary the height of the optical system above the surface so as to track the surface. Preferably the servo system is arranged to operate on low frequencies only and the higher frequency signals are used to monitor the cell depth. Although particularly advantageous in the context of monitoring the engraving of anilox rollers, the monitoring system of this aspect of the invention may also be used for other purposes. In particular, such a system may advantageously be adopted for the measurement and assessment of surface finishes. BRIEF DESCRIPTION OF THE DRAWINGS Examples of systems in accordance with the first and second aspects of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows a plan view of a laser engraving apparatus for an anilox roller; Figures 2a and 2b show a side elevation of a first example of an optical system for monitoring the engraved cells with the roller surface out of focus and in focus respectively; Figure 3 shows the development of a signal from the surface profile through to height information for a first example of an optical monitoring system; Figure 4 is a plot of a cell profile derived following the method of Figure 3; Figure 5 is a side elevation of a second example of an optical monitoring system; Figures 6a to 6d show the signals derived using the system of Figure 5; Figures 7a and 7b show the height output signal from the system of Figure 5; Figures 8a and 8b show preferred configurations for the photodetector elements; Figure 9 shows an example of an engraving system including a servo system to automatically control the focusing of an optical monitoring system; and Figure 10 shows an example of the arrangement of the optical monitoring system in an engraving apparatus. DESCRIPTION OF EXAMPLES A plan view of a conventional laser engraving apparatus for engraving anilox rollers as shown in Figure 1 comprises a drive 1 to rotate the anilox roller 2 about its longitudinal axis. The angular position of the roller is monitored by an encoder 3 which provides an output to the driver 4 of a laser 5. The engraving head 6 includes a means 7,8 to direct and focus laser light onto the anilox roller 2. The engraving head 6 is mounted to be moveable parallel to the longitudinal axis of the roller 2 (see Figure 10) . The lateral displacement of the engraving head 6 is also monitored by the laser driver 4. From the information regarding the angular position of the roller 2 and the lateral position of the engraving head 6, the laser driver 4 drives the laser 5 to emit laser light which is directed by the engraving head 6 onto the roller 2 to engrave a cell site 9. The cells 2 engraved on the roller are engraved in a pattern, for example a hexagonal or diamond pattern. The present invention monitors the depth and profile of cells engraved on an anilox roller 2 using a confocal microscope. With a conventional microscope, if an object is not in focus, the image will be blurred. However, with a confocal microscope, an object not in focus will be very dim and blurred with minimal contrast. Using a confocal microscope, a point focus is scanned across the object. An output is produced whenever the surface is in focus, but only a very weak output when the surface is out of focus. Confocal systems are described in "Confocal Scanning Optical Microscopy", Kino and Corle, Physics Today 1989, pages 55-62. A first example of a confocal optical system suitable for use with an anilox roller engraving apparatus is shown in Figure 2. An illumination source such as a laser diode is mounted behind an aperture 10. Light from the source is reflected by the beam divider 11 and focused by an objective lens 12. Light returned from the surface of the sample 13 is focused by the same objective lens 12 onto a photodetector 14 via an aperture 15. With the arrangement shown in Figure 2a, the surface of the roller is out of the confocal plane and so only a weak intensity signal is received by the photodetector 1 . The dashed rays are from a point on the surface which intersects the optical axis 16 from the aperture 15 through the centre of the lens 12. These rays show that the aperture 15 is overfilled. Other points on the surface receiving illumination are also imaged by the lens either to overfill the aperture 15 substantially or to be too far off the optical axis 16 to overlap the aperture 15. Figure 2b shows a sample 13 whose surface is in the confocal plane so that the reflected light is focused to pass through the aperture 15 to the detector 14. When monitoring engraved cells on an anilox roller, the confocal microscope is focused on the surface of the roller 2. If the microscope is focused on the surface of the roller, then as the roller rotates, effectively scanning the surface of the roller, the output of the microscope will be high when scanning the unengraved roller surface and low when scanning an engraved cell since the surface of the engraved cell will be below the unengraved surface of the roller and therefore out of focus. If the focus of the confocal microscope is adjusted, for example, by moving the microscope closer to the roller's surface 13, the microscope is then focusing at a level slightly below the surface of the roller. When scanning the unengraved surface of the roller there will be only a very low output, since the unengraved surface is out of focus. However, there will be a point on the side wall of the engraved cell which is below the unengraved surface of the roller and in focus. Therefore, in scanning across a cell, there will be two outputs corresponding to two points on the side walls of the cell at the depth of focus. Figure 3 shows the cell depth information obtained from scanning an engraved cell 2 with the focal point of the objective lens 12 adjusted to lie in a plane 18 below the top surface of the cell. Figure 3a shows a cross-section of the engraved cell 2. Figure 3b shows the output from the detector 14. The peak outputs in Figure 3b correspond to points on the cell surface 17 which lie in the confocal plane of the lens 12. The signal may be passed to a threshold detector (not shown) which outputs the digital signal shown in Figure 3c. This information can be processed to obtain profile information across the width of a cell 2 at a height x. This is illustrated in Figure 3d. By repeating the scan of the engraved cell at a range of depths, the overall profile and depth of a cell can be found as shown in Figure 4. A comparison can be made by processing equipment (not shown) between the actual cell profile and depth and the preferred cell profile and depth and the parameters for engraving such as power and duration of the engraving pulse and speed of rotation of the roller, controlled to improve the quality of the engraved cells. If, rather than simply scanning across the centre of the cell, the cell is also scanned across positions either side of the centre of the cell to obtain a plurality of cell profiles similar to those shown in Figure 4, then these profiles can be used to obtain a three-dimensional representation of the engraved cell for comparison with the preferred cell profile. In another example of the present invention, the confocal microscope scans a number of engraved ceils and determines an average profile. As cells are engraved in a helix, it is preferable to monitor the cells in a helix. By finding the average of a number of cells engraved at about the same time, the analysis of cell profile is less likely to be incorrect due to abnormalities in individual cells, as can occur with inho ogeneous ceramic. The cells to be averaged need not be adjacent cells in the engraved helix but may be cells arranged adjacent to each other longitudinally, or any arbitrary arrangement of cells engraved at about the same time. Figures 5 and 6 show a further example of an optical system for monitoring the engraved cells. The system uses a point source of illumination 19, in this example a laser diode, as the light source. The light from the source is initially reflected towards the surface by a beam combiner 20, passes through a collimator 21, and subsequently through a focusing lens 22 before being directed onto the surface 23. In the example shown in the Figure, the surface 23 is positioned just beyond the focal point of the optical system. The light reflected from the surface passes back through the focusing lens 22, collimator 21 and beam combiner 20. It then passes through a wedge 24 so that the system is astigmatic and from the wedge 24 strikes a photodetector 25 which, in the example shown in this Figure, has two adjacent zones A,B. Ignoring any variation resulting from changes in the reflectivity of the surface, the signal intensity detected by A,B varies in each of the two zones as the surface height crosses the plane of focus. At the point of focus then, ideally, equal signals should be received from the two zones A,B. At heights away from the plane of focus, the outputs are generally flat. In practice, there is also variation in the signals according the sample reflectivity and the angle of the viewed surface. These factors can largely be eliminated by normalising the signal. This is done by dividing the difference signal from the two detectors A,B by the sum of both signals. Figure 6b shows the ideal output from the detector zones A,B of the detector 25 for the cell profile of Figure 6a with a plane of focus 26. Figure 6c shows the actual output from each zone. Figure 6d shows the difference signal 27 and normalized signal 28 of the detector 25. In common with the confocal system of the first example, this system has only a limited depth of focus, set by the convergence angle or numerical aperture of the focusing lens 22. However, unlike the first example, this system does nonetheless provide height information on either side of the point of focus. As well as improving the ability of the system to capture in detail information on the profile of the cell, this can also be used in a control loop to provide a focus tracking system. Such a system, an example of which is described in detail below, uses a servo to move the point of focus of the optical system so as to follow variations in the contour of the surface. The features of interest, in the present example the anilox cells, correspond to the high frequency output of the detector system. Accordingly the servo mechanism includes a low-pass filter so that the low-frequency variations are effectively eliminated from the output of the detector, whilst leaving the high frequency signals intact. This is shown in Figures 7a and 7b, with 7b showing the output when the servo mechanism is in operation to eliminate the low-frequency variation from the signal. The unwanted low-frequency variations will typically occur at a frequency corresponding to the rotational frequency of the roller and so the servo control loop may be optimized by tuning to this particular frequency. The depth of focus of the system may preferably be arranged to be at least as great as the depth of the cells to be monitored. To further optimize the height information obtained away from the point of focus, additional detector zones may be provided. The use of additional detector zones can also be used to form a hybrid system shown in Figure 8a in which, in common with the optical system of the first example discussed above, diffuse reflections contribute to the sensed data, as well as specular reflections. In the multi-element detector shown in Figure 8a, the central element 48 is equivalent to the small aperture for the detector in the confocal system of the first example. Accordingly the output from this central element gives a narrow peak as the point of focus is passed, generally similar to the output shown in Figure 3. The other elements 49 provide a signal which varies as the specular reflection is displaced laterally, as described above. In practice, the ideal configuration shown in Figure 8a may be approximated by the combination of a conventional multi¬ channel photodiode array 50 and a slit aperture 51 as shown in Figure 8b. An example of an autofocus engraving system is shown in Figure 9. A servo system 29 comprises an analysing optical system 30 which includes the confocal optical system with its associated focusing lens (not shown) , the focal point of which is varied by a control signal from a focusing optics controller 31 which mechanically alters the position of the focusing lens by small amounts. The output from the optical system 30 is fed back to a comparator 33 via a low-pass filter 32. The comparator compares the output from the low-pass filter 32 with a voltage level input signal 34 which may be set by the operator. Thus, this control system utilises negative feedback to constantly vary the position of the focusing lens and hence the focal point of the focusing lens to automatically compensate for any variations in the cell surface contour to maintain a sharp focus. The output from the optical system 30 is also fed to a high pass filter 35 and the output from the high-pass filter 35 is fed to a peak detector 36 and trough detector 37 and thereon to a difference detector 38. The output from the difference detector 38 gives information on the cell profile which is compared with the required cell depth control signal 39 and the resulting signal fed to a laser output controller 40 which controls the output of the laser and/or the light incident on the ceramic. The process of ceramic removal to form a cell is represented by 48. A time delay between the process 48 and the measurement by the analysing optical system 30 corresponds to rotation of one point on the roller from the focus of lens 8 to the focus of lens 12 or 22. The laser output controller 40 adjusts the engraving parameters to achieve a required cell profile. The construction of the laser output controller 40 will depend upon the type of laser being employed, i.e. a high frequency pulsed laser or CW laser. Typically the engraving parameters may be the intensity of the engraving spot, modified by controlling the transmission efficiency of an acoustic-optic modulator or alternatively, the duration of the engraving pulse. Other possible engraving parameters for control by feedback from the analysing optical system 30 include spot focus or beam energy. Spot focus control requires automatic control of the fine focus of the engraving head. Feedback may be based on the entire cell profile, or on a parameter derived from that profile, such as the cell depth. Illumination for the analysing optical system 30 typically comes from a laser diode. This may be mounted with the optics. Alternatively, a fibre optic may be used to take the light to the optical system 30. Figure 10 shows a side elevation of an example of a practical set-up of the analysing optical system 30 in the engraving apparatus of Figure 1. The engraving head 6 comprises a mirror 7 arranged to direct laser light onto a lens 8 which focuses the light onto the cells of an anilox roller 1. The lens 8 is supported within a housing 41 which includes a pair of apertures 42,43. The engraving head 6 is mounted on a pair of guide rails 44 which lie parallel to the longitudinal axis of the roller 1 and the engraving head 6 is arranged to be capable of being transported along the guide rails 44. The housing 41 is arranged to be capable of movement along a bearing 47 in a direction perpendicular to the roller axis to adjust the focal point of the light from the lens 8. The housing 41 further comprises a bracket 45 which is mounted at 45° to a side wall of the housing facing the roller 1. The analysing optical system 30 is slidably mounted on the bracket 45 for viewing engraved cells on the upper surface of the roller 1. The analysing optical system 30 is capable of sliding up and down the bracket 45 and sliding small distances along an axis parallel to the roller axis. In this manner the system can accommodate various sizes of roller 1 whilst keeping the focal point of the focusing lens 46 (which is equivalent to either lens 12 or lens 22 above) at or near a required position. It will be appreciated that an engraved cell rotates through 90° before it is viewed by the analysing optical system 30. Accordingly, the optical system 30 is offset slightly along the longitudinal axis of the roller 2 relative to the laser engraving point to compensate for the helix angle of the cell pattern. As mentioned above, the position of the focusing lens 46 in this system may be adjusted slightly to fine tune the position of the focal point. In a manual system, in addition to movement of the optical system 30 up and down the bracket 45 and along an axis parallel to the longitudinal axis of the roller, the vertical position of the lens 46 may be varied by hand-turning a focusing ring. In one example of an automatic focusing system employing the feedback servo system described above, the lens 46 is arranged to be driven by a piezo-electric transducer (not shown) or a moving coil (not shown) . In these cases, an electrical signal output from the focusing optics controller 31 (see Figure 9) determines the position of the lens 46. The focusing lenses 12 and 22 in Figures 2 and 5, respectively, may be adjusted in this manner.

Claims: CLAIMS 1. An apparatus for preparing a printing medium (2) for use in a printing process comprising means (1) to support the printing medium and rotate it about a longitudinal axis; an engraving head (5,6) mounted to move substantially parallel to the longitudinal axis of the printing medium (2) for engraving a series of cell sites (9) on the printing medium (2) and control means (3,4) for controlling the engraving parameters of the head (6) , characterised by means (30) for monitoring the profile of the engraved cells and for producing a control signal depending upon the monitored profile, the control means (3,4) being responsive to the control signal to vary the engraving parameters to maintain a desired cell profile. 2. An apparatus according to claim l, in which the printing medium (2) is an anilox roller. 3. An apparatus according to claim 1 or 2, in which the engraving head (5,6) is a laser engraving head. 4. An apparatus according to claim 1, 2 or 3, in which the means (30) for monitoring includes a confocal optical system which is arranged to sample the cross-section of the engraved cell (9) at a selected depth. 5. An apparatus according to claim 2, in which the confocal optical system is an astigmatic confocal optical system arranged to focus light reflected from the surface (23) of the printing medium (2) onto a photodetector (25) comprising at least two adjacent photosensitive elements (A,B). 6. An apparatus according to any preceding claim, in which the means (30) for monitoring is arranged to move substantially parallel to the longitudinal axis of the printing medium to allow monitoring of the engraved cells (9). 7. An apparatus according to any proceeding claim, in which the means (30) for monitoring are arranged to monitor only a selection of the total number of engraved cells (9) . 8. An apparatus according to any preceding claim, in which the confocal optical system comprises a point illumination source, an objective lens (12,22,46) positioned to focus light from the point illumination source onto the surface of the printing medium and a photodetector (14,25) arranged to receive via the objective lens (12,22,46) scattered and reflected light froΩ the surface of the roller. 9. An apparatus according to claim 4 or claim 6 when dependent on claim 4 , in which the confocal optical system further comprises a beam splitter (20) , a collimator (21) and a wedge (24) , wherein light is initially reflected towards the surface of the roller by the beam splitter (20) , passes through the collimator (21) , and subsequently through the objective lens (22) before being directed onto the surface (23) of the roller, the light reflected from the surface passing back through the objective lens (22) , collimator (21) and the beam splitter (20) and through the wedge (24) onto the photodetector (25) . 10. An apparatus according to claim 9, in which the illumination source and the photodetector share a common aperture and the optical system includes a beam splitter on the opposite side of the aperture to a target cell. 11. An apparatus according to any preceding claim, in which the illumination source for the optical system comprises a light source and a length of fibre optic, in use, light being introduced from the light source to one end of the fibre optic and output to the confocal optical system from the other end of the fibre optic. 12. An apparatus according to claim 11, in which the light from the illumination source is introduced into the fibre optic with an angular offset so that the output beam has a generally hollow conical configuration. 13. An apparatus according to claim 12, in which the angular offset is in the range from approximately 5° to approximately 10°. 14. An apparatus according to claim 11, 12 or 13, in which the light source comprises one of a laser diode or a xenon lamp. 15. An apparatus for preparing a printing medium (2) including means for engraving a plurality of cells (9) on the medium, characterised by means (30) for monitoring the depth of the cells (9) , the means (30) for monitoring including an astigmatic confocal optical system arranged to focus light reflected from the surface of the medium onto a photodetector (25) . 16. An apparatus according to claim 15, in which the photodetector (25) comprises at least two adjacent photosensitive elements (A,B) . 17. An apparatus according to claim 5, 15 or 16, including a servo system controlled in accordance with an output of the photodetector which varies the height of the optical system (30) above the surface so as to track the surface. 18. An apparatus according to claim 17, in which a servo system (29) is arranged to operate on low frequencies only and the higher frequency signals are used to monitor the cell depth. 19. A confocal optical system characterised by an illumination system comprising an illumination source and a fibre optic, light from the illumination source being introduced into the fibre optic with an angular offset so that the output beam has a generally hollow conical configuration. 20. An apparatus according to claim 19, in which the angular offset is in the range from approximately 5° to approximately 10°.

Inventors: PROCTOR MICHAEL, ROBERTS SIMON HUGH, PROCTOR, MICHAEL, ROBERTS, SIMON, HUGH
Applicants: PROCTOR MICHAEL, ROBERTS SIMON HUGH, ZED INSTR LTD, ZED INSTRUMENTS LTD
Application year: 1993
Main classification: B23K26
IPC codes: B23K26, B41C1