Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 12/777,463 (now U.S. Publication No. 2011/0278766), filed May 11, 2010, entitled THREE-DIMENSIONAL IMAGING OF FLEXOGRAPHIC MEDIA, by Vitaly Burkatovsky, the disclosure of which is incorporated herein. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for forming three-dimensional (3D) images on the surface of media cylindrical drum by laser ablation. 
     BACKGROUND OF THE INVENTION 
     Flexographic printing involves inking a raised image on a flexible media which then comes in contact with the print substrate, such as paper or plastic. The ink from the raised image onto the print substrate. The flexible plate is made of a rubbery material which has a somewhat pliant nature, the extent of which depends on the smoothness and fragility of the substrate. Contrary to other print processes such as offset lithography and gravure where high pressure is used during ink transfer, it is generally desirable to have a minimum of pressure between the raised inked image on the plate and the substrate. Too little pressure and no ink transfer or very uneven ink transfer will occur. Too much pressure and the pliant surface of the flexible plate will be squashed into the substrate blurring the image edges resulting in poor print quality. 
     Because of the requirement to work at minimal pressure for optimum quality, the distance between the plate surface and the substrate must be the same over the entire surface. While this depends on the uniformity of the press cylinder on which the plate is mounted, it also depends on the thickness uniformity of the flexible plate. 
     Methods for flexographic plate imaging by laser ablation with plates mounted on cylindrical drums is well known. The main application is in gravure and flexography printing industries where lasers are used to create ink carrying pits so the drums are able to transfer images directly or indirectly onto paper or polymer films. The techniques used are well developed and a wide range of lasers are used to create pits directly in metal drums or in drums coated with ceramic, rubber, or polymer layers. U.S. Pat. No. 5,327,167 (Pollard) describe a machine for ablating pits of variable density on the surface of a printing drum. 
     The lasers used are usually focused to spots on the drum surface with a diameter of 10 to 100 μm. Pits may be created by direct laser ablation or by ablation of a thin mask followed by chemical etching. 
     The drum or sleeve eccentricity as well as media thickness variations impact laser focusing and may lead to unacceptable defocusing. To eliminate this problem an autofocus system is required. The autofocus system described in WO 2009/115785 provides for measuring a distance to media just before imaging (engraving) and for subsequent corrections of the imaging lens position according to comparing of the resulted measurement with required focus distance. 
       FIG. 1  shows an example of an apparatus that can be used to ensure that the image created by the projection system remains in focus on the drum surface even if the drum varies in diameter, is not perfectly circular, or is mounted eccentrically on its axle. A cylindrical drum  104  is mounted on an axis about which it rotates. Cylindrical drum  104  is shown rotating in a clockwise direction  108 . A laser beam  128 , passes through lens  132  and creates an image on the surface of the cylindrical drum  104 . Lens  132  and additional imaging optics components are attached to a carriage on a servo motor driven stage  124 , to allow the optical components to move together along the Z direction  120  which is parallel to the projection system optical axis and perpendicular to the drum surface. 
     The stage that supports the carriage holding both the lens  132  mask is itself attached to second carriage. This second carriage is driven by a second servo motor driven stage which has a direction of motion parallel to the drum axis. This second stage, which is not shown in  FIG. 1 , has the function of moving the projection system and associated homogenizer along the length of the cylindrical drum  104 . An optical sensor unit  112  is attached to the second carriage so that it moves down the length of the cylindrical drum  104  with the projection optics. 
     The sensor  112  is mounted such that it measures the relative distance from the sensor  112  to the drum surface at a position on the surface that is about to be exposed to laser pulses. The distance data generated by the sensor  112  is processed by controller  116  and used to drive the servo motor on the projection system stage in order to maintain the distance from the lens  136  to the drum surface at the process point constant so that the imaging is always in focus. For this application, the cylindrical drum  104  is expected to be made with some precision so that as it rotates and the optical projection system traverses the full length of the cylindrical drum  104 , variations in the surface location and hence movement of the projection optics in the Z direction are expected to be small. 
     The system as described above is limited to only one layer engraving. This is due to the fact that after the first engraving layer is completed, it limits the performance of an autofocus system for engraving of subsequent layers, both from the point of view of distance to media sensing as well as from dynamics of lens movement. 
     A first of this invention is to provide an autofocus system which is capable of maintaining constant focus distance between the drum or sleeve media surface and imaging lens in one and more than one engraving cycles. 
     A computer that supports engraving by multiple laser channels may be heavily loaded due to the need to process 3D image engraving data. In this case, additional autofocus tasks may affect the functionality by lowering the calculation speed. Hence a second purpose of this invention is to reduce the load of the machine computer while 3D image engraving process is performed. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a method and an apparatus for three dimensional precision imaging on a surface of a flexible media is disclosed. The media is mounted on a cylindrical drum and imaged by laser ablation. An imaging stage is adapted to move on a carriage in perpendicular and parallel direction relative to the drum. The imaging stage includes a displacement sensor configured to measure the surface structure of the flexible media, and imaging optics configured to image on the flexible media. The imaging optics is adapted to move in perpendicular direction relative to the drum. 
     A controller receives measurements of the surface structure from the displacement sensor. The measurements represent a media surface map. The computer will process the measurements and creates a command array structure for each of the drum revolutions. The computer will then transmit a relevant command array structure to the controller during or prior to imaging. The imaging optics will image on the media by controlling the distance from the imaging optics to each imaging spot on the flexible media according to the relevant command array structure and read outs from the media position encoders. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an autofocus prior art system; 
         FIG. 2  is a schematic illustration the autofocus system of the present invention; 
         FIG. 3  is an illustration of an array representing plate surface measurement; 
         FIG. 4  illustrating graphical interpretation of set focus commands; 
         FIG. 5  is an illustration of an array representing set focus commands; 
         FIG. 6  illustrating a graphical representation of the command arrays structure representing each drum revolution; and 
         FIG. 7  illustrating a cross section of an engraved area on a plate created by plurality of imaged layers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
     While the present invention is described in connection with one of the embodiments, it will be understood that it is not intended to limit the invention to this embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as covered by the appended claims. 
       FIG. 2  shows an embodiment of the present invention. An autofocus system for direct engraving of printing plates is depicted. Autofocus system  200  is built into an imaging device (shown partially). The imaging device includes a rotating cylindrical drum  104  or a printing sleeve (not shown). A printing plate  204  is mounted on cylindrical drum  104 . 
     An imaging stage  208  is placed on a carriage  210  coupled with screw  214  such that rotation of the screw driven by the carriage motor (not shown) forces carriage  210  to move in the horizontal (parallel to cylindrical drum  104  X-axis) X direction  248 . The imaging stage  208  is capable to move in a perpendicular Z direction  244  towards cylindrical drum  104 . The moving of imaging stage  208  in Z-direction is provided by the stage assembly drive  224 . Imaging stage  208  carries an imaging optics assembly  216 . The imaging optics assembly  216  is adapted to move relative to the imaging stage  208  in Z direction  244 , driven by an autofocus drive  220 . A displacement sensor  212  is positioned on imaging stage  208  to perform constant measurements of the distance to plate  204  in Z direction  244 . The movements in X and Y directions are measured by encoders  256  and  260  respectively. An ability of cylindrical drum  104  rotation as well as carriage  210  moving in conjunction with X, Y, and Z directions measurements allows the plate/drum surface scan. The plate  204  surface scan results are arranged by controller  228  and are further communicated to machine computer  236  via data link  232 . 
     While at a standby phase, the imaging stage  208  is parked in home position, which is typically in front of the drum left or right side and the imaging optics assembly  216  is positioned in a predefined focus position relative to cylindrical drum  104 . According to the start command of machine operator the computer  236  automatically controls the rotation of cylindrical drum  104  with plate  204  and movement of carriage  210  along cylindrical drum  104  from home position to the away position at the opposite to home position drum side. At the same time simultaneous capturing of X, Y, and Z measurements is performed by controller  228 . Captured data is further communicated to machine computer  236  via data link  232 . This take place up to the finish of scan happened for example at a moment when carriage reaches the away position. The measured data is transmitted to computer  236 , data is archived in the memory of computer  236  as a mapped surface structure  304  (shown in  FIG. 3 ). Index in parenthesis determines the measurements capture number and “k” is a maximum measurements capture number (last capture number) respectively. Processing the scanned data is performed by the computer only to generate a array of focus commands, each array is intended to be used for at least one drum revolution, and each array is different from the next one. The computer sends a new array of focus commands to the controller when it finds that the next revolution requires a different array of commands. The number of commands per each array are equal, and each array represents focus commands for a full drum revolution. 
     According to a predetermined algorithm, computer software creates and stores in computer memory the set command array. The explanation of the set command array creation may be simplified by graphical ( FIG. 4 ) representation of calculated set commands on the cylindrical drum  104  surface (X-Y axis). Each calculated set command (set point) characterized by Xi, Yi, and Zi values, where Xi and Yi are the cylindrical drum  104  and carriage  210  position where the autofocus drive  220  should update the position of imaging optics assembly  216  according to the desired calculated value Zi. It means that each set command may be shown graphically as a point on the drum surface shown in X-Y coordinates. For example, the first set command characterized by coordinates X(1),Y(1), and Z(1) is represented as point  404  having coordinates X(1),Y(1). The second set command characterized by coordinates X(2),Y(2), and Z(2) is represented as point  408  having coordinates X(2),Y(2) and Z(2). The other set commands, such as X(k), Y(k), and Z(k) representing point  412 , are represented respectively. 
     As it can be seen from  FIG. 4  the drum surface is virtually divided by slices  416 . Separation between the slices  416  is shown by the vertical dotted lines in  FIG. 4 . Each slice  416  is characterized by slice height  420  and slice width  424 . Slice height  420  is equal to the drum circumference and slice width  424  is equal to the carriage  210  displacement for time of one cylindrical drum  104  revolution. In case wherein cylindrical drum  104  and carriage  210  speeds are constant, all slices  416  will be equal in size. In order to simplify the algorithm it is also assumed that each slice will have a constant number of set points. At the same time there is no need for the distance between the adjacent set points within the slices  416  to be constant. 
     Assuming “m” is a slice number and “n” is a number of set points per drum revolution the set command array structure  504  may be represented as shown on  FIG. 5 . In other words the set command array structure  504  consists of commands characterized by X, Y, and Z coordinates and created for correction of focus position errors of imaging optics  216  by controlled the autofocus drive  220 . The goal of controller  228  in this case is to detect the appropriate time at which the autofocus drive  220  should be updated with a new command Zi for each upcoming set point, by comparing the actual and calculated (Xi and Yi) values. 
     Prior to the creation of set command array  504  the computer  236  software may additionally perform different tasks such as data filtering, resolution adjustment (sample resolution and set resolution may be different), compensations of control system components delays, and other. A set structure  504  may be communicated to controller  228  in different ways, for example by transmitting of whole command array structure  504  to controller  228  just after creation in the computer  236 . In this case controller  228  should store this array and use it for autofocus drive  220  control as long as imaging is executed. This option calls for large memory and logic requirements in controller  228 , but on the other hand such method reduces the load on computer  236 , which will need to perform fast 3D image calculations during the engraving process. 
     Another embodiment that will help to reduce the logic and memory requirements of controller  228  as well as reducing the amount of transmitted data from computer  236  to controller  228  may be preferable. According to this embodiment, in processing the array  304  computer  236  will estimate the differences between the desired trajectories of imaging optics assembly  216  per each drum revolution. In the case where the difference is small (set commands per drum revolution are substantially similar) the computer  236  will not include the set command data for current revolution into the set command array  504  thus making the array  504  representing only different drum revolution control trajectories. In order to support focus control in a continuous manner, the controller  228  should be capable to reuse the set control data of previous revolution up to the moment of an updated drum revolution control data is received from computer  236 . In this case controller  228  should finish the revolution with previous data and start with next (updated) drum revolution control data. It means also that at the moment of the updated revolution control data, transmission need to be in accordance with actual carriage position. 
     For example, as depicted in  FIG. 6 , the moment of the first revolution control data transmission occurred at time t 1  shown on the time t axis and around the initial carriage position shown as X 1  on the X-axis. Immediately after receiving of this data, controller  228  will begin to send the set commands Zi to the autofocus drive  220  by comparing the actual Y position indicated by encoder  260  and the received Yi (received from structure  504 ) drum position. The set points  604  are representing points on the drum surface where for each Yi a focus correction command Zi is send to the autofocus drive  220 , provided it is coordinated with the respective encoder position  260  actual readings. 
     In the case where there were no significant changes on the drum geometry the trajectory of imaging optics  216  for the next drum revolution should be similar to the previous one. Due to this similarity the next revolution set control data will not be included into the structure  504  and computer  236  will not send this data to the controller  228 . In order to support an uninterruptable focus correction, the logic of controller  228  detects the end of revolution provided that no updated revolution data was send to the controller  228 . In this case controller  228  logic starts the next revolution focus control referring to the previous revolution control data stored in the memory (not shown) of controller  228 . Set points  608  indicate those points on the drum surface where Yi is equal to position encoder  260  read out and the respective focus correction command Zi is send to the autofocus drive  220 . Yi is already resident in controller  228  memory. 
     Therefore at the end of each revolution controller  228  detects the next revolution data update. In the case where data was updated than controller  228  will use it for control, otherwise controller  228  will use the data stored in controller memory. 
     The first significant change in the desired imaging optics  216  trajectory caused by the drum geometry deviation should be around the carriage position depicted on X-axis as X 2 , showing plate surface deviation  612 . Respectively the current revolution control data was included into the set command array structure  504  and transmission of this data will occur when computer  236  finds that actual carriage position is equal or close to X 2 . This moment is shown as t 2 . Behavior of controller  228  receiving the next revolution control data was described above. The same behavior will be at the moment t 3  around carriage position X 3 . 
     Note that computer  236  to controller  228  communication method it is sufficient to send just the Yi information for each revolution set control data (without carriage position Xi). In this case carriage position is defined by computer  236  timing of relevant drum geometry change as is indicated by set command array structure  504 . Imaging for engraving on plate  204  can be performed in more than one imaging cycle, thus in each cycle a layer of a pre-determined depth is engraved. 
       FIG. 7  shows an engraved area  704  on plate  204 . In the first imaging cycle layer  708  is engraved. For each subsequent layer  712  the imaging stage  208  is advanced towards plate  204  in the distance equal to the engraving depth of previous layer. The imaging stage  208  is advanced by the stage drive assembly  224 , coordinated by controller  228 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
           104  cylindrical drum 
           108  drum direction 
           112  sensor 
           116  controller 
           120  moving direction of stage 
           124  driven stage 
           128  beam 
           132  lens 
           136  distance stage to drum 
           200  autofocus system for a direct engraving imaging system 
           204  plate 
           208  imaging stage 
           210  carriage 
           212  displacement sensor 
           214  screw 
           216  imaging optics assembly 
           220  autofocus drive 
           224  stage assembly drive 
           228  controller 
           232  data link between controller and computer 
           236  computer 
           244  Z direction 
           248  X direction 
           252  Y direction 
           256  X position encoder 
           260  Y position encoder 
           304  mapped surface structure 
           404  point in a set point 
           408  point in a set point 
           412  point in a set point 
           416  slices 
           420  slice height 
           424  slice width 
           504  set command array structure 
           604  sets of points transferred from computer to controller 
           608  sets of points restored from controller memory 
           612  plate surface deviation 
           704  engraved area 
           708  first imaged layer 
           712  subsequent imaged layers

Technology Category: 3