METHOD FOR OPERATING A CIJ PRINTER WITH OPTICAL MONITORING OF PRINTING QUALITY, CIJ PRINTER WITH OPTICAL MONITORING OF PRINTING QUALITY, AND METHOD FOR TEACHING-IN A CIJ PRINTER WITH OPTICAL MONITORING OF PRINTING QUALITY

Abstract

Provided are a method for operating a CIJ printer with an optical monitoring means (80) having the steps of generating a bitmap (90,180) of the printed image to be printed, sequential controlling of charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer, in order to generate dots or groups of dots of the bitmap (90,190) by applying ink droplets (12) to a substrate (100) to be printed and thus to sequentially apply a real printed image (195) to the substrate (100), capturing the real printed image (195) applied to the substrate (100) with the optical monitoring means (80), and automated comparing of the bitmap (90,190) of the desired printed image and of the real printed image (195) which has been applied to the substrate (100) and has been captured with the optical monitoring means (80), wherein the bitmap (90,190) of the desired printed image and the real image applied to the substrate (100) are automatically compared either on the basis of rows or columns of the bitmap (90,190) or on the basis of components of rows or columns of the bitmap (90,190), a CIJ printer for carrying out such a method and a method for teaching-in an optical monitoring means (80) of such a CIJ printer.

Claims

1. A method for operating a CIJ printer with an optical monitoring means (80) having the steps generating a bitmap (90,180) of the printed image to be printed, sequential controlling of charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer to realize dots or groups of dots of the bitmap (90, 190) by applying ink droplets (12) to a substrate to be printed (100) and to thus sequentially apply a real printed image (195) on the substrate (100), capturing the real printed image (195) applied to the substrate (100) with the optical monitoring means (80), and automated comparing of the bitmap (90, 190) of the desired printed image and the real printed image (195) applied to the substrate (100) and captured with the optical monitoring means (80), characterized in that the automated comparing of the bitmap (90, 190) of the desired printed image and the real printed image (195) applied to the substrate (100) is carried out either on the basis of rows or columns of the bitmap (90, 190) or on the basis of components of rows or columns of the bitmap (90, 190).

2. The method according to claim 1, characterized in that at least one control signal for the sequential controlling of charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer is used in the automated comparing of the bitmap (90, 190) of the desired printed image and the real image (195) applied to the substrate (100) and captured by the optical monitoring means (80) to determine the expected printed image of the respective row or column.

3. The method according to claim 2, characterized in that at least one further control signal for the sequential controlling of charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer in the automated comparing of the bitmap (90, 190) of the desired printed image and real printed image (195) applied to the substrate (100) and captured by the optical monitoring means (80) to determine the expected printed image of the respective row or column of the bitmap (90, 190).

4. The method according to claim 1, characterized in that the sequential controlling of charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer also takes place row by row or column by column.

5. The method according to claim 1, characterized in that the CIJ printer has a plurality of processors or a processor with a plurality of processor cores, wherein on the one processor the bitmap (90, 190) of the desired printed image is generated and the generating of the control signals for the sequential controlling of the charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer is monitored, and wherein on the other processor the automated comparing of the bitmap (90, 190) of the desired printed image and the real printed image (195) applied to the substrate (100) and captured by the optical monitoring means (80) is carried out.

6. A CIJ printer for carrying out a method according to claim 1, having a hydraulic module (5) for ink supply, a droplet generator having a nozzle (10) and an oscillator (20), which droplet generator is supplied with ink by the hydraulic module (5) and generates ink droplets (12), at least one charge electrode (25) for applying a defined charge to ink droplets (12) generated by the droplet generator, at least one deflection electrode (30) for influencing the trajectory of the ink droplets (12) generated by the droplet generator and charged by the charge electrode (25), a control configured to transform a bitmap to be printed (90, 190) by row or by column into a sequence of control signals with which the charge electrode (25) and/or the deflection electrode (30) are controlled in such a way that an image of that row or column is formed on a substrate to be printed (100) from droplets (12) of a droplet sequence, and an optical monitoring means (80) for monitoring the real printed image (195) formed on the substrate (100) to be printed, characterized in that the CIJ printer has a data processing device configured to carry out the step of the automated comparing.

7. The CIJ printer according to claim 6, characterized in that the CIJ printer has a first processor or a first processor core associated with the control and has a second processor or processor core associated with the data processing device.

8. The CIJ printer according to claim 6, characterized in that the control is in signal communication with the data processing device, so that the respective sequences of control signals or control commands corresponding to these sequences are forwarded by the control to the data processing device.

9. A method for teaching-in an optical monitoring means (80) of a CIJ printer according to claim 6, characterized in that the CIJ printer in at least one pass generates a bitmap (90, 190) containing a sequence of control signals for controlling charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer when executing a stroke (40, 41), that a real printed image (195) of this bitmap (90, 190) is realized by applying ink droplets to a substrate (100) to be printed, that an image of the real printed image (195) is captured with the optical monitoring means (80) and evaluated such that the respective part of the real printed image (195) applied to the substrate (100) in response to a control signal for a given stroke (40, 41) is identified and is stored as the expected printed image associated with this stroke (40, 41) or this control signal.

10. The method according to claim 9, characterized in that the printer prints in a plurality of passes a bitmap (90, 190) containing a sequence of control signals for controlling charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer to turn dots or groups of dots of the bitmap (90, 190) into a real printed image by applying ink droplets (12) to a substrate (100) to be printed, and that the printed image applied to the substrate in each case in response to the control signal is captured and identified by means of the optical monitoring means and is stored as a printed image associated with the control signal.

11. The method according to claim 10, characterized in that the printer generates in each of the plurality of passes a sequence of control signals for controlling charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer, wherein the order in which the control signals for controlling charge electrodes (25) and/or deflection electrodes of the CIJ printer (30) are generated, varies from sequence to sequence.

12. The method according to claim 10, characterized in that the printer generates in each of the plurality of passes a sequence of control signals for controlling charge electrodes (25) and/or deflection electrodes (30) of the CIJ printer, wherein in the different passes printing parameters are varied that may fluctuate in the printing operation of the CIJ printer and lead to a change in the printed image (195).

Description

[0039] The invention is explained in more detail below on the basis of Figs. representing exemplary embodiments. In the drawings:

[0040] FIG. 1a shows a schematic representation of a character to be printed,

[0041] FIG. 1b shows a schematic representation of the breakdown of the printing process into individual strokes,

[0042] FIG. 1c shows a schematic representation of the writing of a stroke on the substrate by the CIJ printer,

[0043] FIG. 1d shows an example of a complex bitmap that can be created by the user,

[0044] FIG. 2a shows an example of a bitmap to be printed, which can also be used for teaching-in the camera,

[0045] FIG. 2b shows the printing result captured with the camera obtained during printing of the bitmap of FIG. 2a,

[0046] FIG. 3 shows a schematic flowchart of an exemplary procedure, and

[0047] FIG. 4 shows a schematic flowchart of an exemplary teach-in process.

[0048] First, the operating principle of a CIJ printer will be explained schematically based on FIGS. 1a to 1d. The characters to be printed are each defined as a group of dots on a matrix, with the dots then created by ink droplets. This can be represented as bitmap 90 for machine processing.

[0049] In FIG. 1a, the letter “E” on a 7×5 matrix 1 is shown as a simple example of such a bitmap 90. In reality, however, today a CIJ printer can usually illustrate more dots in one row, e.g. 32 dots, which allows the user to compile complex contents, as shown by way of example in FIG. 1d, as a desired printed image, which is then converted into the corresponding bitmap and processed.

[0050] When such a bitmap 90 is printed, one dimension of the matrix on which it is based, in the orientation of FIG. 1a the direction z of the rows, is realized by a different deflection of the ink droplets, while the other dimension, in the orientation of FIG. 1a, the direction s columns, is realized by a movement of the material to be printed. In particular, in the case of a different orientation, the role of the rows and columns can of course be reversed.

[0051] FIG. 1c shows schematically how the production and deflection of the ink droplets is realized by the CU printer. The ink is provided with defined properties, in particular defined pressure and defined viscosity, by a hydraulic module 5 shown only schematically in FIG. 1c and is supplied to the ink channel of the nozzle 10, which cannot be seen in FIG. 1c. The ink column in the ink channel of the nozzle 10 is modulated by means of an oscillator 20, which can be designed, for example, as a piezo actuator. With suitably selected jet conditions, which theoretically were derived from C. Weber in the journal of applied mathematics and mechanics, volume 11, 1931, constrictions are formed after exiting the nozzle 10, until there is a splatter-free separation of ink droplets 12 at tear-off point 11, which form an ink droplet jet. Typically, the ink droplets 12 of a jet that meets these conditions propagate at a speed of 20 m/s to 30 m/s, and high five-digit and even six-digit numbers of ink droplets 12 can be produced per second today.

[0052] After the separation of an ink droplet 12, it is provided with a target charge on the charge electrode 25, wherein the success of the charging process can be checked with a detector electrode, which is not recognizable in FIG. 1c, and is deflected at an energized deflection plate or deflection electrode 30 to different degrees depending on the charge, so that, as is shown by way of example in FIG. 1c, the charged ink droplets 12, when they hit the substrate 100 to be printed, land at a more or less well-defined position, at the present orientation row position, of the matrix defining the character, while unused ink droplets 12a, that are not charged, continue to fly into the catcher tube 35 and are returned to the ink mixing tank (not shown) in the hydraulic module 5.

[0053] The charge electrode 25 is controlled by a control unit, which converts a printed image which is produced directly or indirectly in a memory 60 by a user into a bitmap 90 in a grid image processor 65, and forwards the information about the rows or columns to be printed to a charging voltage computer 70 on the one hand, which is preferably designed as a separate processor. The charging voltage computer 70 generates a corresponding charging signal according to the calculated charge to be applied and passes it on as a control signal to the charge electrode 25.

[0054] The fact that the substrate 100 to be printed is moved makes it necessary, in particular if the printing speed is to be maximized, to print the rows (or columns) produced by different deflections of the droplets 12 as quickly as possible, since otherwise these are no longer on one row. Therefore, these are each processed by the CIJ printer as a common “stroke” 40, 41, as illustrated in FIG. 1b.

[0055] Specifically, the processing, as shown in FIG. 3 in the form of a schematic flow diagram, is accomplished in the CIJ printer in that a printed image predefined by the user in step 110, which, if it contains a counter information, for example, can change between printing processes to be carried out directly one after the other and is stored or cached in the memory 60, the bitmap 90 to be printed is obtained on a processor or processor core, the Raster Image Processor (RIP) 65, in a process referred to as ripping 120 and in particular the respective dot sequence, the current stroke 40,41, to be imaged next by the CIJ printer, is determined, which indicates, at which locations of the substrate 100 ink droplets 12 are to be applied in order to generate dots.

[0056] It is important for the invention that at this point there is already at least implicit information about the expected printed image, which is configured according to the invention as a target specification for success monitoring.

[0057] This information is then, on the one hand, in step 125 forwarded as input to the data processing system 75, which is here implemented with a separate processor, which carries out the comparison between the signal to be printed and an image of the printing carried out, which images is forwarded from the optical monitoring means 80, which here is executed as a CCD camera, to the data processing system 75.

[0058] On the other hand, the information is further processed by the charging voltage computer 70. The charging voltage computer 70 calculates from said information—preferably taking into account the information which stroke or which strokes were printed shortly before and, if applicable, also already which stroke or which strokes are printed immediately afterwards—in step 130, the charging voltage which has to be applied to the droplets associated with the stroke so that they land at the desired location of the substrate so that said charging voltage can be applied to the charge electrode 25 during flyby.

[0059] These calculations are particularly complex because, on the one hand, space charges and, on the other hand, aerodynamic effects such as the slipstream of other droplets can significantly influence the trajectory of the ink droplets and their point of impact on the substrate. Therefore, the process step 130 is also preferably carried out on a separate processor or processor core.

[0060] The charging voltage obtained in this way is then used to control the charge electrode 25 in step 140 during the execution of the actual printing process and charges droplets 12 of the continuous ink droplet stream so that said ink droplets are deflected by the deflection voltage applied to the deflection plate 30 from the stream of the uncharged ink droplets 12a traveling to the catcher tube 35 and are applied to the substrate 100.

[0061] In order to define the start time of the printing process for a printed image to be applied and to enable its timing, a “print GO” signal is generated, e.g. when an object to be printed, which passes through the CIJ printer and is to be printed while passing through, reaches a defined position relative to the CIJ printer. This then triggers the printing—possibly after an adjusted waiting time—starting with the first stroke 40, 41; it may be useful to wait for a prespecifiable waiting time between successive strokes 40, 41.

[0062] For checking and monitoring the printing process, a camera image is captured at step 150, preferably with an optical monitoring means 80 here designed as a CCD camera. This can be triggered, for example using the print go signal as a time frame of reference. The image data of the camera image are then forwarded to a data processing system 75 and evaluated in step 160.

[0063] While this evaluation in the state of the art is usually carried out as an evaluation of the entire print on the object in comparison with the bitmap 90 to be printed according to the invention, this is done by an evaluation of the individual rows or columns of the printed image, each formed by a stroke 40, 41. It should be pointed out explicitly that this is not already the case automatically if the individual cells of the CCD chip of the optical monitoring means 80, which is here designed as a CCD camera, are read out row by row or column by column during an image evaluation and the corresponding data are then processed further, which is not an evaluation of rows or columns of the printed image but an evaluation of rows or columns of the camera image. However, this cannot provide the same results for the reason alone that it would be unsatisfactory for the attainable accuracy of the resolution if an ink droplet on the substrate would correspond only to a set pixel in the camera image.

[0064] If the evaluation in step 160 shows indications of a malfunction or a printing error, an error warning or a printing stop can be triggered in step 170. Otherwise, the processing can be continued by returning to step 120, especially if the next stroke 40, 41 has not yet been calculated. However, in the return to step 120, it is also possible to read out an already calculated further stroke from a local memory, which is preferably managed according to the FIFO principle.

[0065] In order to understand even more precisely the advantages of the procedure resulting from such a row- or column-based evaluation, an example of a bitmap 190 to be printed and the corresponding printed image 195 shown in FIG. 2b, as it is captured by the optical monitoring means 80, executed here as a CCD camera, is discussed here with reference to FIG. 2a. The imaging of an ink droplet 12 in the printed image 195 captured by the optical monitoring means 80, executed here as a CCD camera, typically comprises between 10 and 20 pixels; the exact value of course dependents on the resolution of the respective optical monitoring means 80 used and its geometric arrangement relative to the substrate 100 to be printed.

[0066] The bitmap 190 shown in FIG. 2a, which in particular can also be used for a teach-in process according to the invention, is formed by a sequence of all dots or ink droplet combinations that can be written with a five-dot stroke 40, 41, i.e., all possible strokes 40, 41 that are executed by a printer that writes five droplets wide.

[0067] When comparing the two FIGS. 2a and 2b with one another, a number of systematic deviations of the real printed image 195 according to FIG. 2b from bitmap 190 according to FIG. 2a can be clearly seen.

[0068] For example, one can immediately see a slight tilting of the individual strokes 40, 41 to the left, so that the uppermost droplet of a stroke 40, 41 in each case is the droplet of the stroke 40, 41 arranged furthest left on the substrate. This effect is related to the speed at which the substrate 100 is moved.

[0069] In addition, however, it can also be seen that the position of the individual rows changes, in particular depending on whether an adjacent droplet is present or not. This effect can be particularly clearly seen in the top row when comparing the group of droplets belonging to this group of droplets of the last eight strokes 40, 41 with the group belonging to this group of droplets of the ninth to sixteenth last stroke 40, 41, which are offset upwards as compared to the first group, but said effect clearly also results from the height offset of the droplets that belong to the last row.

[0070] A further deviation from the ideal image, which is specified by the bitmap 190 of FIG. 2a, in the generated printed image 195 as captured by the optical monitoring means 80 according to FIG. 2b, consists in that adjacent ink droplets can converge. For example, this can be seen in some of the droplet pairs that can be seen in the second-lowest row of FIG. 2b, for example, in the fifth and eighth droplet pairs of this row.

[0071] These respective deviations are not an indication of an interference effect, but also occur with printing that occurs without interference. In the previously customary comparison of the entire printed image 195 with the bitmap 190 to be printed, deviations are accordingly taken into account that are actually not caused by any newly occurring printing errors.

[0072] Instead, when using the teaching according to the invention, the printed images of the individual strokes 40, 41 captured by the optical monitoring means 80 can be used as the desired image which should be produced in response to the printing command for this stroke 40,41, which leads to a very rapid evaluation. Firstly, it is not necessary to wait until the entire bitmap 190 is printed in order to then compare it with the printed result, but the comparison is possible immediately after the execution of a stroke 40, 41.

[0073] With the image evaluation, it is not only advantageous that the corresponding objects to be compared with one another are much smaller, but also that one knows in advance where to look for dots of the currently printed stroke 40, 41 on the CCD chip of the optical monitoring means 80, because, on the one hand, from a camera image such as that shown in FIG. 2b the ink droplet positions in the y-direction characteristic of a stroke 40, 41 can be derived and, on the other hand, the offset in the x-direction between adjacent strokes 40, 41.

[0074] This allows a very targeted comparison algorithm, in which the search for the printed ink droplet can begin immediately in the correct area of the CCD chip and an expected position of the ink droplet image can be specified with a relatively high degree of certainty.

[0075] If deviations between such expected positions and the positions at which the corresponding ink droplets of the respective stroke 40, 41 are then found in the camera image are systematically logged, then changes that are gradually emerging and in the long-term require corrections to print parameters such as changes in ink viscosity or in the proportions of concentrated ink and solvent, can potentially be derived at an early stage from the corresponding changes in the printed image and then corrected by initiating appropriate countermeasures before any malfunctions or misprints occur.

[0076] In addition, the stroke-based approach enables an extremely simple teach-in process which may ultimately even make it possible to operate an optical monitoring means 80 on a CU printer as a true plug-and-play module and which teach-in process is shown schematically in FIG. 4. In order to teach-in an optical monitoring means 80 after installation, it is, in step 210, only necessary to generate at least one defined sequence of all strokes 40, 41, i.e. all possible combinations of written ink droplet positions in a stroke 40, 41, as a bitmap and to print this sequence on the substrate 100 in step 220 under the operation conditions to be used later.

[0077] This printed image is then captured in step 230 with the optical monitoring means 80, which is designed as a camera, and at least one corresponding camera image is evaluated in step 240, preferably in order to obtain expected values for ink droplet positions of the individual strokes 40, 41.

[0078] Specifically, for example, each stroke 40, 41 or a control signal corresponding to this stroke 40, 41 is assigned or logically connected to the position of the ink droplets 12 on the CCD chip of the optical monitoring means (80), which is executed as a camera, in a y-direction, which corresponds to the deflection direction of the ink droplets 12, as expected ink droplet positions. On the other hand, by analyzing the distance between the images of the individual strokes 40, 41 on the CCD chip of the optical monitoring means 80, which is designed as a camera, information is obtained, at which x-positions on the CCD chip of the optical monitoring is to be expected by means of 80 ink droplets of an n-th stroke 40, 41 of a predetermined sequence of strokes 40, 41.

[0079] If a bitmap 90, 190 is then printed after the teach-in process in real operation, the output of the ripper 65 representing a specific stroke 40, 41 may be directly forwarded, if applicable, together with information about which stroke 40, 41 for writing this bitmap 90, 190 it is, as input for the data processing device 75 that analyzes the camera image.

[0080] This input can then be converted directly into a set of expected pixel positions for the ink droplets 12 associated with this stroke 40, 41 and it can be checked whether the corresponding pixels are set in the camera image. Even if the droplet position has moved slightly, quickly locating the newly added droplets 12 is ensured in this way, and by analyzing deviations it is possible to determine, on the one hand, whether the imprint is still acceptable or not by a comparison with the acceptance ranges to be determined, while, on the other hand, indications of the problems at hand that cause a deviation from the target position may already be obtained.

LIST OF REFERENCE NUMBERS

[0081] 5 Hydraulic module

10 Nozzle

[0082] 11 Tear-off point
12 Ink droplet
12a Uncharged ink droplet

20 Oscillator

[0083] 25 Charge electrode
30 Deflection plate
35 Catcher tube

40, 41 Stroke

65 Raster Image Processor (Ripper)

[0084] 70 Charging voltage computer
75 Data processing system
80 Optical monitoring means

90 Bitmap

100 Substrate

[0085] 110 Specifying a printed image

120 Ripping

[0086] 125 Forwarding input to data processing system
130 Calculating the charging voltage
140 Controlling the charge electrode
150 Capturing a camera image
160 Evaluating the camera image
170 Error warning

190 Bitmap

[0087] 195 Printed image
210 Generating a sequence of all possible strokes as a bitmap
220 Printing the bitmap
230 Capturing a camera image
240 Evaluating the camera image
s Direction of the columns
z Direction of the rows