EP3705295B1 - Method for operating a cij printer with optical monitoring of printing quality, cij printer with optical monitoring of printing quality and method for teaching a cij printer with optical monitoring of printing quality - Google Patents

Description

Inkjet printers are a popular class of printers. A family of this class, which is particularly suitable for industrial applications and has therefore achieved a high degree of acceptance in this field, are the so-called continuous inkjet printers (CIJ printers).

A continuous inkjet printer prints with an ink that contains a variable component of solvent. Accordingly, there is a mixing tank in which solvent from a solvent tank and the concentrated ink from an ink tank are mixed together to obtain the ink used for printing. When the term "ink" is used hereinafter, it means the liquid used for printing; the term "concentrated ink" is used for the liquid provided in the ink tank.

From the mixing tank, the ink is fed under pressure to a nozzle on the print head, where the drops required for the actual printing process are created from the ink jet according to the basic principle of Rayleigh's decomposition of laminar liquid jets. The formation of drops and in particular the size of the drops is controlled by a modulation which is impressed on the ink jet, for example by suitably excited piezo elements.

The droplets thus produced are electrically charged in a suitable manner and set on a desired trajectory by deflection electrodes directed, which leads them either to a desired position of a substrate to be printed or, if no printing process is to take place, interception at the print head, for example on a catcher tube, and recycling of the ink droplet, ie its return to the mixing tank, allowed.

The element to be printed, e.g. a letter or a number, is realized in this way by a matrix or bitmap of ink drops, e.g. in many cases by a 7x5 matrix, whereby one dimension, i.e. lines or Columns of the matrix or bitmap are realized and the other dimension is realized by a material feed of the material to be printed. The CIJ printer therefore usually prints a sequence of so-called "strokes", i.e. rows of ink droplets arranged side by side; In its controller, the character to be displayed is converted into a matrix or bitmap corresponding to the resolution, which is then processed line by line or column by column.

Obviously, an essential goal is to ensure that the ink drops land in the right place on the substrate to be printed as reproducibly as possible by activating the deflection electrodes, so that the print image on a given substrate is neither distorted nor does the position of the print image change significantly on substrates that are printed one after the other becomes. Such significant changes can occur as a result of fluctuations in operating parameters, and it is desirable to determine them as quickly as possible in order to be able to produce the desired print image again by appropriately adjusting the settings on the one hand and on the other hand to pull products that have misprints out of the production line in good time to be able to

A known way of approaching this goal is to carry out camera monitoring of the printed image, with the camera preferably being in signal communication with the CIJ printer, so that data determined by the camera and images recorded can be shown on a display of the CIJ printer .

Especially with systems optimized for printing speed, the time interval between two consecutive printing processes, in which different copies of the product to be printed are printed, is often very short, so that the aim is to identify misprints as quickly as possible in order to reduce the number of defective prints printed products as low as possible.

A further significant difficulty with camera monitoring of the printed image is that a learning process must be carried out, at least if this is to be (also) automated.

This learning process is particularly complicated by the fact that with CIJ printers, at least when they are operated with optimized printing speed, fluctuations in the position of individual drops (ie dots of the printed image) also occur in trouble-free, regular operation, for example depending on whether and/or where the previously generated drop has been printed/deflected. As a result, CIJ printers with a camera connection or integration do not represent real plug-and-play systems, but instead a complex commissioning procedure must first be carried out, which may have to be repeated each time the printing conditions, e.g. the material to be printed, are changed must become.

From the US2001/0040599 A1 a CIJ printer with camera monitoring and an operating method are known in which errors during printing, in particular adjustment errors, are compensated.

The object of the invention is therefore to improve CIJ printers with optical monitoring, in particular with regard to the reaction time during monitoring and/or with regard to the time required for teaching the optical monitoring system.

This object is achieved by a method for operating a CIJ printer with an optical monitoring means having the features of claim 1, by a CIJ printer with an optical monitoring means for carrying out such a method with the features of claim 6 and by a teaching method an optical monitoring system of such a CIJ printer with the features of claim 9. Advantageous developments of the invention are the subject matter of the respective dependent claims.

• Erzeugen einer Bitmap des gewünschten Druckbildes,
• sequentielles Ansteuern von Ladeelektroden und/oder Ablenkelektroden bzw. Ablenkplatten des CIJ-Druckers, um jeweils Punkte oder Gruppen von Punkten der Bitmap durch das Aufbringen von Tintentropfen auf ein zu bedruckendes Substrat zu realisieren und somit sequentiell ein reales Druckbild auf das Substrat aufzubringen,
• Erfassen des auf dem Substrat aufgebrachten realen Druckbilds mit dem optischen Überwachungsmittel, und
• automatisiertes Vergleichen der Bitmap des gewünschten Druckbilds und des mit dem optischen Überwachungsmittel erfassten, auf dem Substrat aufgebrachten realen Druckbilds

auf. Erfindungswesentlich ist, dass das automatisierte Vergleichen der Bitmap des gewünschten Druckbilds und des auf dem Substrat aufgebrachten realen Druckbilds auf der Basis von Zeilen oder Spalten des Druckbilds, die in der Regel durch Strokes gebildet werden, oder auf der Basis von Bestandteilen von Zeilen oder Spalten des Druckbilds, also der Position von Tintentropfen, erfolgt.The method according to the invention for operating a CIJ printer with an optical monitoring means has at least the steps to be carried out in this order, but not necessarily in immediate succession

• Creation of a bitmap of the desired print image,
• Sequential activation of charging electrodes and/or deflection electrodes or deflection plates of the CIJ printer in order to implement points or groups of points of the bitmap by applying ink drops to a substrate to be printed and thus sequentially applying a real print image to the substrate,
• detecting the real print image applied to the substrate with the optical monitoring means, and
• automated comparison of the bitmap of the desired print image and the real print image applied to the substrate and recorded with the optical monitoring means

on. It is essential to the invention that the automated comparison of the bitmap of the desired print image and the real print image applied to the substrate based on rows or columns of the print image, which are usually formed by strokes, or on the basis of components of rows or columns of the Print image, i.e. the position of ink drops, takes place.

It is no longer checked after the realization of the entire bitmap, e.g. a letter or a number to be printed as a real print image on the material to be printed, whether it matches the desired print image or the entire bitmap, but Stroke for Stroke, in particular after each stroke or possibly even during the execution of the stroke, checks whether this stroke or the individual ink drops that make it up have been placed correctly.

This approach has a number of advantages: The pattern to be recognized or checked is much simpler, which makes pattern recognition easier and more reliable and reduces the computing time and hardware requirements. In addition, the verification rate is significantly increased, so that an error can be detected more quickly.

According to an advantageous embodiment of the method, it is provided that at least one control signal for sequentially driving charging electrodes and/or deflection electrodes of the CIJ printer is used in the automated comparison of the bitmap of the desired print image and the captured with the optical monitoring means, applied to the substrate print image to determine the expected print image of the respective row or column.

In a further development of the embodiment just discussed, it is provided that at least one further control signal is used for the sequential activation of charging electrodes and/or deflection electrodes of the CIJ printer during the automated comparison of the bitmap of the desired print image and the print image recorded with the optical monitoring means and applied to the substrate is used to determine the expected print image of the respective row or column. What can be achieved in this way is an increase in the precision of the character comparison and thus greater sensitivity in the detection of emerging interference effects that lead to slight changes in the print image of a given stroke, which can be different depending on the previously executed stroke, for example.

If the charging electrodes and/or deflection electrodes of the CIJ printer are also driven sequentially in rows or columns, this control signal can be used directly to define a target position for monitoring.

It has proven to be particularly advantageous if the CIJ printer has multiple processors or one processor with multiple processor cores, with the bitmap of the desired print image being generated on one processor and the control signals for the sequential activation of charging electrodes and/or deflection electrodes being generated of the CIJ printer is controlled and the automated comparison of the bitmap of the desired print image and the print image applied to the substrate detected by the optical monitoring means takes place on the other processor. In this way, it is particularly easy to ensure that the image processing does not have a negative impact on the actual printing operation, even if a high level of CPU performance is required.

The CIJ printer according to the invention for carrying out the method according to the invention comprises a hydraulic module for ink supply, a droplet generator comprising a nozzle and an oscillator for pressure modulation, which is supplied with ink by the hydraulic module and generates ink droplets, at least one charging electrode for applying a defined charge to the droplet generator generated ink drops, at least one deflection electrode for influencing the trajectory of the ink drops generated by the drop generator, a controller that is set up to transform a bitmap to be printed line by line or column by column into a sequence of control signals with which the charging electrode and/or the deflection electrode be controlled so that an image of this row or column is formed from drops of a droplet sequence on a substrate to be printed, and an optical monitoring means, which can be embodied in particular as a CCD camera, for monitoring the image formed on the substrate to be printed.

It is essential to the invention that the CIJ printer has a data processing device that is set up to carry out the step of automated comparison according to one of claims 1 to 5.

According to a preferred development of the invention, it is provided that the CIJ printer has a first processor or a first processor core that is assigned to the controller and has a second processor or processor core that is assigned to the data processing device. In this way, an undesired influencing of the printing speed by the image analyzes to be carried out can be avoided.

In addition, it is advantageous if the controller 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 from the controller to the data processing device. The former corresponds to analog signal transmission, the latter to digital signal transmission. Both

In the method according to the invention for teaching an optical monitoring system of a CIJ printer with such an optical monitoring system, it is provided that the CIJ printer, in at least one pass, generates a bitmap that contains a sequence of control signals for driving charging electrodes and/or deflection electrodes of the CIJ printer the execution of a stroke is generated, that a real print image of this bitmap is realized by applying ink drops to a substrate to be printed, that an image of the real print image is recorded with the optical monitoring means and evaluated in such a way that the respective reaction to a Control signal for a given stroke on the substrate applied part of the real print image is identified and stored as this stroke or control signal assigned to expected print image. It is particularly advantageous if this sequence includes all control signals; but it can also suffice if it only includes specific, distinctive control signals for strokes whose printed image shows specific deviations to be expected.

It should be noted that this bitmap can also be generated stroke by stroke, i.e. the CIJ printer sequentially generates all control signals for driving the charging electrodes and/or deflection electrodes of the CIJ printer in at least one run, in order to generate dots or groups of points of the bitmap by applying ink drops to a substrate to be printed, and that the print image applied to the substrate in response to the control signal is recorded with the optical monitoring means and stored as the print image assigned to the control signal.

In the first case, a more complex bitmap is formed from the possible or selected "elementary strokes" and the image of this bitmap recorded by the optical monitoring means is evaluated, while in the second case each stroke is executed and analyzed individually. The advantage of the first approach is that interactions between consecutive strokes can already be taken into account, but the evaluation in the second case may be easier.

In both cases, storage can take place not only as an image file but also in the form of coordinates of camera pixels at which the signal from ink drops is to be expected. In this way, a library of in each case at least one image that is assigned to a stroke is created automatically, or a library of ink drop positions to be expected for specific strokes is created.

The great advantage of this method when teaching is that an immediate and automatic assignment between the result of the print command and the print command is made possible, while previously a more complex bitmap had to be classified by a user during teaching after it had been built up with several strokes.

Another advantage is that it is often easier to identify systematic deviations in the individual stroke and correct them if necessary. For example, if the medium to be printed is fed at too high a speed, the stroke and the bitmaps composed of it can tilt. What is characteristic of this is that the individual ink drops are offset systematically, independently of the specific print image of the stroke, and this becomes larger the closer the drop in question was produced to the end of a stroke.

In order to obtain a fluctuation range for the droplet positions obtained, it is advantageous if the printer generates sequences of—preferably, but not necessarily all—control signals for driving charging electrodes and/or deflection electrodes of the CIJ printer in several runs, in order to generate points or groups of points of the bitmap by applying ink drops to a substrate to be printed, and when the print image applied to the substrate in response to the control signal is recorded with the optical monitoring means and stored as the print image assigned to the control signal.

It should also be noted that the print image and in particular the size of the individual drops or that of a single droplet generated dots also depends on the ink used and the substrate.

In particular, with such a procedure, the differences in the print position, which are caused by different preceding strokes for a given stroke, can be detected and used when monitoring the print result. To do this, the printer is allowed to generate sequences of—preferably, but not necessarily all—control signals for driving charge electrodes and/or deflection electrodes of the CIJ printer in the multiple passes, with the order of the control signals for driving charge electrodes and/or deflection electrodes of the CIJ -Printer, are generated, is varied from sequence to sequence. In principle, it is also possible to do this for all possible combinations of strokes. In addition to a complete recording of the fluctuation ranges of the individual drop positions, which can then be used to check the printing success, there is also the possibility in this case to take into account the stroke printed before the stroke to be printed, for example, and thus the precision of the position information to increase.

The analysis options that can be created with the data obtained in the training routine can be increased even further if the printer varies print parameters in the multiple runs, which can fluctuate during the printing operation of the CIJ printer and lead to a change in the print image. For example, the viscosity of the ink can fluctuate during operation, which can lead to a change in the printed image. By changing this parameter in a targeted manner in a learning phase, its effects can be recorded and, on the one hand, to improve the print success control, but on the other hand can also be used for early detection of an impending malfunction.

Fig. 1a:

eine schematische Darstellung eines zu druckenden Zeichens,

Fig. 1b:

eine schematische Darstellung der Zerlegung des Druckprozesses in einzelne Strokes,

Fig. 1c:

eine schematische Darstellung des Schreibens eines Strokes auf das Substrat durch den CIJ-Drucker,

Fig. 1d:

ein Beispiel für eine komplexe, vom Benutzer erstellbare Bitmap,

Fig. 2a:

ein Beispiel für eine zu druckende Bitmap, die auch zum Einlernen der Kamera verwendbar ist,

Fig. 2b:

das mit der Kamera aufgenommene Druckergebnis, das beim Druck der Bitmap aus Fig. 2a erhalten wurde,

Fig. 3:

ein schematisches Ablaufdiagramm eines beispielhaften Verfahrens, und

Fig. 4:

ein schematisches Ablaufdiagramm eines exemplarischen Einlernprozesses

The invention is explained in more detail below with reference to figures that represent exemplary embodiments. It shows:

Figure 1a:

a schematic representation of a character to be printed,

Figure 1b:

a schematic representation of the breakdown of the printing process into individual strokes,

Figure 1c:

a schematic representation of the writing of a stroke on the substrate by the CIJ printer,

Figure 1d:

an example of a complex user-created bitmap,

Figure 2a:

an example of a bitmap to be printed, which can also be used to teach the camera,

Figure 2b:

the print result recorded with the camera, which is produced when the bitmap is printed Figure 2a was received

Figure 3:

a schematic flow diagram of an exemplary method, and

Figure 4:

a schematic flowchart of an exemplary learning process

Based on Figures 1a to 1d The function of a CIJ printer should first be explained schematically. The characters to be printed are each represented as a group of dots defined on a matrix, the dots then being created by ink drops. This can be represented as a bitmap 90 for machine processing.

In Figure 1a the letter "E" on a 7x5 matrix 1 is shown as a simple example of such a bitmap 90 . In reality, however, a CIJ printer today can usually display more dots in one line, e.g. 32 dots, which allows the user to print complex content, as exemplified in Figure 1d are shown to be assembled as the desired print image, which is then converted into the corresponding bitmap and processed.

When printing such a bitmap 90, one dimension of the matrix on which it is based, in the orientation of the Figure 1a the direction z of the lines, realized by a differently strong deflection of the ink drops, while the other dimension, in the orientation of the Figure 1a the direction s columns, is realized by a movement of the material to be printed. The role of the rows and columns can, of course, be reversed, particularly in the case of a different orientation.

Figure 1c shows schematically how the generation and deflection of the ink drops is realized by the CIJ printer. The ink is with defined properties, in particular defined pressure and viscosity defined by one in the Figure 1c Hydraulic module 5 shown only schematically provided and in the Figure 1c not recognizable ink channel of the nozzle 10 supplied. The column of ink in the ink channel of the nozzle 10 is modulated by an oscillator 20, which can be embodied as a piezo actuator, for example. After exiting the nozzle 10, with suitably selected jet conditions, the theoretical by C. Weber in the Journal of Applied Mathematics and Mechanics, Volume 11, 1931 were derived, to the formation of constrictions until it comes at the break point 11 for the satellite-free detachment of ink drops 12, which form an ink drop jet. Typically, the ink drops 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 drops 12 can already be produced per second today.

After the detachment of an ink drop 12, it is provided with a nominal charge at the charging electrode 25, the success of the charging process being determined with a detector electrode which is shown in Figure 1c is not recognizable, can be checked and is deflected to different extents depending on the charge at a deflection plate or deflection electrode 30 which is set under voltage, so that, as in Figure 1c is shown as an example, the charged ink drops 12, upon their impact on the substrate to be printed 100, end up at a more or less well-defined position, in the present orientation line position, of the matrix defining the character, while unused ink drops 12a, are not charged, in the Fly catcher tube 35 and be returned to the ink mixing tank, not shown, in the hydraulic module 5.

The charging electrode 25 is controlled by a controller, which converts a print image generated directly or indirectly by a user in a memory 60 into a bitmap 90 in a raster image processor 65, and the information about the rows or columns to be printed on the one hand to a charging voltage calculator 70 forwards, which is preferably implemented as a separate processor. The charging voltage calculator 70 generates a corresponding charging signal according to the calculated charge to be applied and passes it on to the charging electrode 25 as a control signal.

The fact that the substrate 100 to be printed is moved results in the need, particularly if the printing speed is to be maximized, to print the rows (or columns) generated by the different deflection of the drops 12 as quickly as possible, since otherwise they would not more in line. Therefore, the CIJ printer prints them as a common "Stroke" 40,41, as in Figure 1b is illustrated, processed.

Specifically, the processing takes place as in figure 3 is shown in the form of a schematic flowchart, in the CIJ printer in that from a print image specified by the user in step 110, which, for example, if counter information is contained, can change between print processes to be carried out directly one after the other and is stored or temporarily stored in 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 sequence of points to be printed next by the CIJ printer, the current stroke 40, 41, is determined, which indicates at which locations of the substrate 100 ink drops 12 are to be applied in order to generate dots.

It is important for the invention that at this point there is already at least implicit information about the print image to be expected, which is set up according to the invention as a target specification for a success check.

This information is then forwarded on the one hand in step 125 as input to the data processing system 75, which is implemented here with a separate processor, which compares the signal to be printed and an image of the print that has taken place, which is carried out by the optical Monitoring means 80 is forwarded to the data processing system 75 performs.

On the other hand, the information from the charging voltage calculator 70 is further processed. The charging voltage calculator 70 uses it to calculate the charging voltage in step 130, preferably taking into account the information about which stroke or strokes were printed shortly beforehand and possibly also which stroke or strokes are printed immediately afterwards Drops must be applied so that they land at the desired location on the substrate so that they can be applied to the charging electrode 25 when flying past.

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 drops can significantly influence the trajectory of the ink drops 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.

With the charging voltage obtained in this way, the charging electrode 25 is then driven in step 140 when the actual printing process is being carried out and charges droplets 12 of the continuous stream of ink droplets, so that they can be separated from the stream of uncharged ink droplets 12a flying toward the catcher tube 35 are deflected and applied to the substrate 100.

In order to define the start time of the printing process for a print 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 the CIJ printer and is to be printed, reaches a defined position achieved relative to the CIJ printer. This then triggers the print, if necessary after an adjusted waiting time, starting with the first stroke 40,41; it can make sense to wait for a predefinable waiting time between successive strokes 40,41.

In order to check and monitor the printing process, a camera image is recorded as step 150, preferably with an optical monitoring means 80 designed here 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.

While this evaluation in the prior art is usually based on an evaluation of the entire print on the object compared to the bitmap 90 to be printed, this is done according to the invention by evaluating the individual lines or columns of the print image formed by a stroke 40,41. It should be explicitly pointed out that this is not automatically the case if the individual cells of the CCD chip of the optical monitoring means 80, designed here as a CCD camera, are read out in rows or columns during an image evaluation and the corresponding data are then processed further. what no evaluation of Rows or columns of the print image but an evaluation of rows or columns of the camera image. However, this cannot deliver the same results because it would be unsatisfactory for the achievable accuracy of the resolution if an ink drop on the substrate only corresponded to a set pixel in the camera image.

If the evaluation in step 160 indicates a malfunction or a printing error, an error warning or a printing stop can be triggered in step 170. Otherwise, processing can be continued by jumping back to step 120, particularly if the next stroke 40, 41 has not yet been calculated. However, when jumping back to step 120, another stroke that has already been calculated can also be read from a local memory, which is preferably managed according to the FIFO principle.

In order to better understand the advantages of the procedure that result from such a row or column-based evaluation, the Figure 2a an example of a bitmap to be printed 190 and the in Figure 2b The corresponding print image 195 shown, as recorded by the optical monitoring means 80 embodied here as a CCD camera, is discussed. The image of an ink drop 12 in the printed image 195 recorded by the optical monitoring means 80, implemented here as a CCD camera, typically comprises between 10 and 20 pixels; the exact value naturally depends on the resolution of the optical monitoring means 80 used in each case and its geometric arrangement relative to the substrate 100 to be printed.

In the Figure 2a Bitmap 190 shown, which can also be used in particular for a teaching process according to the invention, is formed by a sequence of all dot or ink drop combinations that can be written with a five-dot stroke 40,41, ie all possible strokes 40,41 that are actually executed by a printer that writes five drops wide.

Comparing the two Figures 2a and 2b together one can clearly see a number of systematic deviations of the real printed image 195 according to Figure 2b from the bitmap 190 according to Figure 2a .

For example, a slight tilting of the individual strokes 40, 41 to the left can be seen immediately, so that the respective uppermost drop of a stroke 40, 41 is the drop of the stroke 40, 41 arranged furthest to the left on the substrate. This effect is related to the speed at which the substrate 100 is moved.

In addition, however, one also recognizes that the position of the individual lines changes, in particular depending on whether an adjacent drop is present or not. This effect is particularly clear in the top line when comparing the group of drops of the last eight strokes 40,41 belonging to this with the group of drops of the ninth to last to sixteenth stroke 40,41 belonging to this, which compared to the first-mentioned group according to offset above. However, it also clearly results from the height offset of the drops that belong to the last row.

Another deviation from the ideal image created by the bitmap 190 of the Figure 2a is predetermined, in the generated printed image 195 as recorded by the optical monitoring means 80 was according to Figure 2b is that adjacent ink drops can coalesce. For example, this can be seen in some of the droplet pairs shown in the second-to-bottom row of the Figure 2b can be seen, for example in the fifth and eighth pair of drops in this line.

These deviations are not in each case an indication of a disruptive effect but also occur in the case of a print that takes place without disruption. In the hitherto customary comparison of the entire print image 195 with the bitmap 190 to be printed, deviations are accordingly taken into account which are actually not caused by newly occurring printing errors.

The print images of the individual strokes 40, 41 recorded by the optical monitoring means 80 can, however, be used as the target image that should arise in response to the print command for this stroke 40, 41 when using the teaching according to the invention, which results in a very rapid evaluation leads. First, it is not necessary to wait until the entire bitmap 190 has been printed in order to then compare it with the print result; instead, the comparison is possible immediately after a stroke 40,41 has been executed.

When evaluating the image, it not only pays off that the corresponding objects to be compared are much smaller, but also that one knows in advance approximately where on the CCD chip of the optical monitoring means 80 according to points of the stroke 40,41 just printed must be searched for, because from a camera image like the one in Figure 2b shown, one can on the one hand derive the characteristic ink drop positions for a stroke 40,41 in the Y-direction and on the other hand the offset in the X-direction between adjacent strokes 40,41.

This enables a very targeted comparison algorithm, in which the search for the printed ink droplet can begin directly in the correct area of the CCD chip and an expected position of the ink droplet image can be specified with a relatively high level of certainty.

If deviations between such expected positions and the positions at which the corresponding ink drop of the respective stroke 40, 41 is then found in the camera image are systematically logged, under certain circumstances changes can occur that are gradual and require corrections to printing parameters in the long term, for example changes the ink viscosity or the proportions of concentrated ink and solvent, can be derived from the corresponding changes in the printed image at an early stage and then corrected by initiating appropriate countermeasures before malfunctions or misprints occur at all.

In addition, the stroke-based approach enables an extremely simple learning process, which can ultimately even make it possible to operate an optical monitoring device 80 on a CIJ printer as a real plug-and-play module and the in figure 4 is shown schematically. In order to teach in an optical monitoring means 80 after installation, you only have to generate at least one defined sequence of all strokes 40,41, i.e. all possible combinations of written ink drop positions in a stroke 40,41, as a bitmap under the later operating conditions in step 210 and this sequence in the Print step 220 onto the substrate 100.

This print image is then recorded in step 230 with the optical monitoring means 80 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 drop positions of the individual strokes 40,41.

In concrete terms, for example, each stroke 40,41 or a control signal corresponding to this stroke 40,41 is the position of the ink droplets 12 on the CCD chip of the optical monitoring means (80) designed as a camera in a y-direction, which corresponds to the deflection direction of the Ink drop 12 corresponds, logically associated as expected ink drop 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 designed as a camera, information is obtained at which x-positions on the CCD chip of the optical monitoring means 80 designed as a camera ink drops an nth stroke 40,41 of a predetermined sequence of strokes 40,41 is to be expected.

If a bitmap 90,190 is then printed in real operation following the learning process, the output of the ripper 65 representing a specific stroke 40,41, optionally together with information about the number of strokes 40,41 for writing this bitmap 90,190, can directly as input for the data processing device 75, which analyzes the camera image.

This input can then be converted directly into a set of expected pixel positions for the ink droplets 12 belonging to this stroke 40, 41 and it can be checked whether the corresponding pixels are set in the camera image. Even if the Should the drop position have wandered slightly, this ensures that the newly added drop 12 can be found quickly, and by analyzing deviations, it is possible on the one hand to determine whether the imprint is still acceptable or not by comparing it with acceptance ranges to be defined, while on the other hand there may already be indications of the present problems causing a deviation from the target position can be recovered.

Reference List

5

hydraulic module

10

jet

11

demolition point

12

ink drops

12a

uncharged ink drop

20

oscillator

25

charging electrode

30

baffle plate

35

catcher tube

40.41

stroke

65

Raster Image Processor (Ripper)

70

Charge Voltage Calculator

75

data processing system

80

optical surveillance device

90

bitmap

100

substrate

110

Specification of a print image

120

ripping

125

Forwarding input to data processing system

130

Calculation of the charging voltage

140

Control of the charging electrode

150

Capture a camera image

160

Evaluation of the camera image

170

error warning

190

bitmap

195

print image

210

Generating a sequence of all possible strokes as a bitmap

220

Print the bitmap

230

Capture a camera image

240

Evaluation of the camera image

s

direction of the columns

e.g

direction of the lines

Claims (12)

1. Method for operating a CIJ printer having an optical monitoring means (80), with the steps

   - generation of a bitmap (90, 180) of the image to be printed,

   - sequential actuation of charging electrodes (25) and/or of deflection electrodes (30) of the CIJ printer in order to realize lines, or respectively, columns of the bitmap (90, 190) by applying rows of adjacently arranged ink droplets (12) onto a substrate (100) for

   printing, each of which forming a stroke (40, 41), and thereby sequentially applying an actual printed image onto the substrate (100),

   - detection of the actual printed image applied to the substrate (100) by means of the optical monitoring means (80), and

   - automated comparison of the bitmap (90, 190) of the desired printed image and of the actual image (195) applied on the substrate (100) and detected by the optical monitoring means (80),

   characterized in that

   the automated comparison of the bitmap (90, 190) of the desired printed image and of the actual printed image (195) applied to the substrate (100) is carried out, either on the basis of lines or columns of the bitmap (90, 190), or on the basis of components of the lines or columns of the bitmap (90, 190) such that, after every stroke (40, 41), or while the stroke (40, 41) is still being carried out, it is verified whether the ink droplets (12) of which the stroke is composed have been correctly placed.
2. Method in accordance with claim 1,
   characterized in that at least one control signal is used for the sequential actuation of charging electrodes (25) and/or of deflection electrodes (30) of the CIJ printer during the automated comparison of the bitmap (90, 190) of the desired printed image and of the actual printed image (195) applied to the substrate (100) and detected by the monitoring means (80) in order to determine the expected printed image of the respective line or column.
3. Method in accordance with claim 2,
   characterized in that at least one further control signal is used for the sequential actuation of charging electrodes (25) and/or of deflection electrodes (30) of the CIJ printer during the automated comparison of the bitmap (90, 190) of the desired printed image and of the actual printed image (195) applied to the substrate (100) and detected by the monitoring means (80) in order to determine the expected printed image of the respective line or column of the bitmap (90, 190).
4. Method in accordance with any of claims 1 to 3,
   characterized in that the sequential actuation of charging electrodes (25) and/or of deflection electrodes (30) of the CIJ printer is also carried out in lines or columns.
5. Method in accordance with any of claims 1 to 4,
   characterized in that the CIJ printer comprises numerous processors, or one processor having numerous processor cores, wherein, on the one processor, the bitmap (90, 190) of the desired printed image is generated and the generation of the control signal for the sequential actuation of charging electrodes (25) and/or of deflection electrodes (30) of the CIJ printer is controlled and wherein, on the other processor, the automated comparison of the bitmap (90, 190) of the desired printed image and of the actual printed image (195) applied to the substrate (100) and detected by the optical monitoring means (80) is carried out.
6. CIJ printer for carrying out a method in accordance with any of claims 1 to 5, having

   - a hydraulic module (5) for supplying ink,

   - a droplet generator, which comprises a nozzle (10) and an oscillator (20), is supplied with ink by the hydraulic module (5), and which produces ink droplets (12),

   - at least one charging electrode (25) for applying a defined charge to the ink droplets (12) produced by the droplet generator,

   - at least one deflection electrode (30) for influencing the trajectory of the, from the droplet generator produced and from the charging electrode (25) charged, ink droplets (12),

   - a controller which is configured to convert, in lines or in columns, a bitmap (90, 190) intended for printing into a sequence of control signals with which the charging electrode (25) and/or the deflection electrode (30) are controlled such that an image of this line or column is formed from droplets (12) of a droplet sequence on a substrate (100) intended for printing, and

   - an optical monitoring means (80) for monitoring the actual printed image (195) formed on the substrate (100) intended for printing,

   characterized in that the CIJ printer comprises a data processing device which is configured to carry out the step of automated comparison in accordance with any of claims 1 to 5.
7. CIJ printer in accordance with claim 6,
   characterized in that the CIJ printer comprises a first processor or a first processor core which is assigned to the controller, and a second processor or processor core which is assigned to the data processing device.
8. CIJ printer in accordance with claim 6 or 7,
   characterized in that the controller is in signal communication with the data processing device such that the respective sequences of control signals, or the control commands which correspond to these sequences, are transmitted from the controller on to the data processing device.
9. Method for the teach-in of an optical monitoring means (80) of a CIJ printer in accordance with any of the claims from 6 to 8,

   characterized in that the CIJ printer generates, in at least one run, a bitmap (90, 190) which contains a sequence of control signals for controlling charging electrodes (25) and/or deflection electrodes (30) of the CIJ printer while a stroke (40, 41) is being carried out,

   in that an actual printed image (195) of this bitmap (90, 190) is realized by applying ink droplets onto a substrate (100) intended for printing,

   in that, by means of the optical monitoring means (80), an image of the actual printed image (195) is recorded and analyzed such that the respective part of the actual printed image (195) which is applied to the substrate (100) in response to a control signal for a given stroke (40, 41) is identified and saved as the expected printed image corresponding to the stroke (40, 41) or respectively, to the control signal.
10. Method in accordance with claim 9,
    characterized in that the printer prints, in numerous runs, a bitmap (90, 190) which contains a sequence of control signals for controlling charging electrodes (25) and/or deflection electrodes (30) of the CIJ printer in order to make either points or groups of points from the points of the bitmap (90, 190) into an actual printed image by applying ink droplets (12) onto a substrate (100) intended for printing, and in that the respective printed image applied to the substrate in response to the control signal is recorded, identified and saved, by the optical monitoring means, as the printed image corresponding to the control signal.
11. Method in accordance with claim 10,
    characterized in that the printer generates, in numerous runs, either a sequence of control signals for the actuation of charging electrodes (25) and/or of deflection electrodes (30) of the CIJ printer, wherein the sequence in which the control signals for the actuation of charging electrodes (25) and/or of deflection electrodes (30) of the CIJ printer (30) are generated varies from sequence to sequence.
12. Method in accordance with either of claims 10 or 11,
    characterized in that the printer generates, in numerous runs, either a sequence of control signals for the actuation of charging electrodes (25) and/or of deflection electrodes (30) of the CIJ printer, wherein, in the various runs, printing parameters are changed which can fluctuate during the printing operation of the CIJ printer and lead to an alteration in the printed image (195).

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