EP1106357B1 - Method and printer with fault masking - Google Patents

Description

Field of the invention

The invention is in the field of ink jet printers in which ink drops are formed and electrically charged and then deflected to strike a printing substrate. It relates to a method for masking or reducing lineage defects and the printer applying such a method.

Technological background

It is known that a jet of pressurized ink ejected by a printing nozzle can be broken into a succession of individual drops each drop being individually loaded in a controlled manner. In the path of these drops thus individually charged, electrodes of constant potential deviate more or less drops according to the charge they have. If a drop is not to reach the printing substrate, its charge is controlled so that it is diverted to an ink recuperator. The operating principle of such inkjet printers is well known and is described for example in US-A-4 160 982. As described in this patent and shown in FIG. 1, such a printer comprises a reservoir 11 containing the electrically conductive ink 10 which is distributed by a distribution channel 13 to a drop generator 16. The role of the drop generator 16 is to form from the ink under pressure contained in the distribution channel 13 a set of individual drops. These individual drops are electrically charged by means of a charging electrode 20 supplied by a voltage generator 21. The charged drops pass through a space between two deflection electrodes 23, 24 and depending on their charge are more or less diverted. The least or no deviated drops are directed towards an ink recuperator 22 while the deviated drops are directed towards a substrate 27. The successive drops of a burst reaching the substrate 27 can thus be diverted towards an extreme low position, a extreme high position and successive intermediate positions, the set of drops of the salvo forming a vertical line of height Δx substantially perpendicular to a direction of relative advance of the print head and the substrate. The print head is formed by the drop generator 16, the charging electrode 20, the deflection electrodes 23, 24 and the recuperator 22. This head is generally enclosed in a not shown cowling. The deflection movement printed to the drops loaded by the deflection electrodes 23, 24 is completed by a movement along a Y axis perpendicular to the X axis, between the print head and the substrate. The time elapsed between the first and the last drop of a salvo is very short. As a result, despite a continuous movement between the print head and the substrate, it can be considered that the substrate does not have moved relative to the print head during the time of a salvo. The bursts are fired at regular space intervals. If all the drops of each burst were directed towards the substrate, a succession of lines of height ΔX would be printed. In general only certain drops of a burst are directed towards the substrate. Under these conditions, the combination of the relative movement of the head and the substrate, and the selection of the drops of each burst which are directed towards the substrate makes it possible to print any pattern such as that represented at 28 in FIG. the line drawn with the drops of a burst is in a direction X, the relative movement of the head and the substrate is, in the plane of the substrate in a direction Y perpendicular to X. The non-deflected drops are directed to the recuperator along a path Z perpendicular to the x, y plane of the substrate. The printed drops arrive on the substrate along paths slightly deviated from the Z direction.

If the relative movement of the head and the substrate is continuous in the largest dimension of the substrate, there will generally be several printing heads printing strips parallel to each other. An example of such use is shown in Figures 1 and 2 of the patent issued to IBM under the number FR 2 198 410.

If the relative movement of the print head and the substrate in the Y direction is made according to the smallest dimension of the substrate, the printing is done strip by strip the substrate having a motion intermittent feed in the X direction after each sweep. The relative movement of the print head and the substrate is called scanning movement. The scanning movement thus consists of a back and forth motion between a first edge of the substrate and a second edge of the substrate. The movement between one edge and the other edge of the substrate makes it possible to print on the fly a band of height L or quite often a part of the band of height ΔX, ΔX being most often a sub-multiple of L. the set of successively printed strips thus constitutes the pattern to be printed on the substrate. After each printing of a web or web portion, the substrate is advanced from the space between two webs or part of web for printing the next tape or web portion. Printing can be done just one way or the other way around the movement of the print head relative to the substrate.

When the graphic to be printed is colored, the multiple shades of colors are the result of the superimposition and juxtaposition of ink strikes coming from nozzles fed by inks of different colors. The system of relative displacement of the substrate relative to the printing heads is such that a given point of the substrate is presented successively under the ink jets of each of the colors. The printing system generally has several jets of the same ink operating simultaneously, either by the juxtaposition of multiple heads, or by the use of multijet heads, or finally by the combination of these two types of heads in order to achieve high print rates. In this case, each inkjet prints a limited portion of the substrate. The drops may be continuously produced as described above in connection with FIG. 1. They may also be produced "on demand", that is to say only when they are necessary for the needs of the patient. 'impression. In this case, an unused ink recovery circuit is not necessary. The known means for controlling the different jets will now be described with reference to FIG.

The print pattern is defined by a digital file. This file can be formed using a scanner, a computer-assisted graphic design (CAD) palette, transmitted by means of a computer network for data exchange, or simply read from a digital data storage device read device (optical disk, CD-ROM). The digital file representing the colored pattern to be printed is first split into several bit patterns (or bitmap) for each of the inks. It should be noted that the case of the binary pattern is a non-limiting example; in some printers, the pattern to print is of type "contone", that is to say that each position can be printed by a number of drops variable from 1 to M. Part of the binary pattern is extracted from the file for each jets corresponding to the width of the band that will be printed. In FIG. 2, where the control electronics of a jet is concerned, a storage memory of the cut digital pattern is represented in FIG. in tape, this storage memory containing the indications relating to a color. For the printing of each band, an intermediate memory 2 receives the data necessary for the printing of the band by said color. The descriptive data of the band to be printed are then introduced into a computer 3 of the charging voltages of the different drops which will form the band relative to this color. These data are introduced into the computer in the form of a succession of descriptions of the frames which together will constitute the band. The computer 3 drops charge voltages is often in the form of a dedicated integrated circuit. This calculator 3 calculates in real time the sequence of voltages to be applied to the charging electrodes 20 to print a given frame defined by its frame description, as loaded from the intermediate memory 2. A downstream electronic circuit 4, called a sequencer of drop charge, ensures the synchronization of the charging voltages with, on the one hand, the moments of drop formation and, on the other hand, the relative advance of the print head and the substrate. The advance of the substrate with respect to the head is embodied by a frame clock whose signal is derived from the signal of an incremental encoder of position of the printing unit relative to the substrate. The sequencer 4 for charging the drops also receives a signal from a drop clock 6. This drop clock is synchronous with the control signal of the drop generator 16. It makes it possible to define the instants of transitions of the different applied charge voltages. with drops to differentiate their trajectories. The digital data from the drop charge sequencer 4 are converted into analog value by a digital analog converter 8. This converter delivering a low voltage level generally requires the presence of a high voltage amplifier 21 which will supply the electrodes of the 20. The illustrations of the prior art given with reference to Figures 1 and 2, are intended to make clear the scope and contribution of the invention, but it is obvious that the prior art is not limited to the descriptions made with reference to these figures. Other arrangements of the electrodes and unused ink droplet collection collectors are described in extensive literature. An electromechanical arrangement of the charge electrode printing nozzles and the deflection electrodes as described in the patent No. FR 2 198 410 issued to International Business Machine Corporation (IBM) with reference to FIGS. of this patent could well be used in the present invention. Similarly, the electronic control circuit of the charging electrodes could be illustrated by the circuit described in connection with Figure 4 of this same patent. Also, the data to be printed may not be in the form of binary files, but in the form of files containing words of several bits, to reflect the fact that each position of the substrate can receive several drops of ink of the same color. We understand that for an impression, especially in color, the necessary superposition of the drops from the different nozzles delivering the different colors of ink must be very precise. The main printing defects that are generated by all known printing systems are the defects relating to lineations in the direction of the relative movement of the print head relative to the substrate. This defect is reflected by the appearance of light or dark lines when printing by successive scans. These defects can be in the space between two bands which should in principle be equal to the interval between adjacent drops of a frame, or within the same band, in the space delimiting the printed areas by different jets, or even inside the frame printed by a jet at the space between two adjacent drops of the frame. These lineage defects can come either from defects specific to certain jets of the print head, it is then defects of mechanical or electrical origin, either of errors of positioning of the substrate, or of error of positioning between heads. of printing, or between jets of the same print head. Various solutions have been proposed to limit or eliminate lineage problems, but all of them result either in a limitation of the print rate, sometimes in a very high ratio with respect to the nominal print rate, or a redundancy of printheads and therefore a significant cost. Examples of known solutions commonly used to limit the lineage will be briefly described below: a first type of The solution is based on fine mechanical adjustments of the position of the print heads, thanks to micrometric tables. This solution is both expensive, by the number of micrometric tables that are necessary, and often tedious, by trial and error it requires.

Another type of common solution is to use a very high rate of overlap between neighboring drops, so as to avoid white lineages. These white lineages correspond to the lack of substrate coverage. Dark lineages are less visible and it is preferred to have a dark lineage defect rather than a white lineage defect. The solution of increasing the rate of overlap between adjacent drops is effective to compensate for defects within the same band and to some extent the lineage defects between bands but it has the disadvantage of requiring an amount of very high ink per unit area of the substrate and generates difficulties of drying or deformation of the substrate.

A third type of solution for clearing lineage defects on scanning printers is to partially print the substrate during each scan. By multiplying the number of substrate scans, the total coverage of the substrate is obtained. This multi-pass printing exploits various strategies of interleaving the positions of the drops from the different jets. An example of interleaving of even and odd lines is given in US-A-4,604,631 issued to US Pat. RICOH company. An advantage of this solution often related to a high overlap ratio is that it allows a drying time of the substrate, but it results in the reduction of the printing rate by a factor ranging from 2 to 16.

WO 97/06009 shows a printer where the value of the charge applied to the drops is set according to the speed of the substrate.

The performance of color graphics printing systems naturally evolving towards resolutions and rates higher and higher, it becomes critical to effectively limit the lineage problems without compromising the penalty for printing rates.

Brief description of the invention

The method according to the invention aims to hide some problems lineage without impact on the printing speed.

The present invention does not require a high rate of overlapping drops. It achieves high print speeds with a relatively small number of print heads. When minimizing the overlap between adjacent drops, there may still be a lineage defect, in particular a white lineage defect appearing regularly. This defect is very perceptible to the eye when it is regular. In order to reduce the perceptibility of this possible defect, an additional voltage of noise is superimposed on a nominal charge voltage of the drops, intended to give, relative to the nominal position of each drop, a real position exhibiting a random dispersion character. Thanks to this dispersion of the real position of every drop around its nominal position, the lineage fault no longer appears as a continuous straight line. It becomes less noticeable to the eye

The invention therefore relates to a method of modifying the arrival position on a substrate of electrically charged drops of ink in a controllable and sequential manner by charging electrodes, the drops coming from a print head, the trajectories of drops being modifiable, by means of deflection electrodes, between N nominal positions a first position X ₁ , a last position X _N and N-2 intermediate positions, the N positions defining a frame in the form of a line segment parallel to a direction X of the substrate, characterized in that in superposition to a nominal voltage applied to the charging electrodes of the drops, an additional random algebraic voltage masking a possible defect lineage by dispersion of the actual position of each drop around of its nominal position.

The average amplitude of this noise voltage will be a function of the rank j of the drop in the frame. Preferably, the maximum amplitude of the additional noise voltage will be equal to a fraction less than 1 of the smallest difference between the nominal voltage Vj to be applied to the drop of rank j and the nominal voltage VJ + 1 or Vj-1 to apply to one of the two immediately adjacent drops in the weft printed pattern of rank j, ie the drops of rank j + 1 and rank j-1.

Since the differences in charge voltage applied to the adjacent printed drops have values that are fairly close to each other, a value of the average value of the mean value of the difference in voltages can be taken as the average value of the additional random voltage. nominal between two adjacent drops printed in the frame.

Preferably, the minimum amplitude of the additional noise voltage will be equal to the value of the voltage difference that can be obtained by varying the value of the least significant bit of an analog-digital converter whose output supplies a high voltage amplifier coupled to the charging electrodes of the drops.

Preferably, the amplitude of the additional noise voltage will correspond to a random digital value generated by a pseudo random number generation algorithm. The correspondence between the random digital value and the additional noise voltage will result from the application of this digital value to the digital to analog converter. The regular default of dark or white lineage will not appear or will appear less.

• une tête d'impression, cette tête comportant des moyens de fractionnement en gouttes d'au moins un jet d'encre et une électrode de charge des gouttes associés, des moyens de déviation d'une partie des gouttes vers le substrat d'impression,
• des moyens de contrôle de l'impression disposant d'un moyen de fixation de la charge des gouttes à diriger vers le substrat en fonction de leur rang dans la salve couplés à l'électrode de charge des gouttes,

caractérisé en ce que les moyens de contrôle de l'impression comportent un générateur d'une tension aléatoire additionnelle, couplé aux moyens de fixation de la charge des gouttes, le moyen de fixation de la charge des gouttes prenant en compte la valeur de la tension aléatoire générée par le générateur de tension aléatoire additionnelle pour modifier la tension de charge de chaque goutte en fonction de la valeur aléatoire générée, les gouttes de chaque rang étant ainsi dispersées autour d'une position centrale correspondant à leur position en l'absence de tension additionnelle.The invention also relates to a printer equipped with means for carrying out the method according to the invention, it is a deflected continuous jet printer projecting in salvo drops of rank 1 to N in the salvo, the drops of d a burst being directed or not towards a printing substrate in data function defining a pattern to be printed, the printer having at least:

• a print head, this head comprising means for splitting into drops of at least one ink jet and an associated drop charge electrode, means for deflecting a portion of the drops to the printing substrate,
• printing control means having a means for fixing the charge of drops to be directed towards the substrate as a function of their rank in the burst coupled to the charge electrode of the drops,

characterized in that the print control means comprise a generator of an additional random voltage, coupled to the means for fixing the charge of the drops, the means for fixing the charge of the drops taking into account the value of the voltage random generated by the additional random voltage generator to modify the charging voltage of each drop as a function of the random value generated, the drops of each row thus being dispersed around a central position corresponding to their position in the absence of voltage additional.

In the preferred embodiment of the invention for a printer operating in scanning mode, the printer further comprises a detector of the position of a printed mark before each first frame of a tape, this detector providing a representative value of a difference between the real and nominal positions of the substrate and that the print control means comprise in in addition to a calculator of a forward translation dynamic correction voltage φ of the substrate, this calculator determining a substrate forward dynamic translation correction voltage φ for each drop of a burst according to its rank, this voltage of correction taking into account a difference value of the advance of the substrate delivered by means coupled to the detector and calculating a deviation value with respect to a nominal position, the dynamic translation correction correction voltage calculator φ of the substrate being coupled to the drop charge fixing means, the drop charge fixing means taking into account the value of the substrate advance correction voltage generated by the dynamic translation correction voltage calculator φ in advance of the substrate for modifying the charging voltage of each drop as a function of the forward dynamic translation correction voltage φ of the substrate.

Brief description of the drawings

• la figure 1 déjà décrite est une représentation schématique des moyens nécessaires à la création de gouttes d'encre et à leur déviation vers un substrat ;
• la figure 2 déjà décrite comme la figure 1 dans le cadre de la description de l'art antérieur représente l'ensemble des moyens de calcul nécessaire au fonctionnement des moyens représentés sur la figure 1 ;
• la figure 3 est un schéma destiné à expliquer les modifications de l'impression obtenues par le procédé de l'invention, elle comporte trois parties A, B et C ;
• la figure 4 donne une représentation physique agrandie de la position des gouttes :
  • dans une partie A dans leurs positions nominales,
  • dans une partie B dans des positions avec erreurs systématiques,
  • dans une partie C dans des positions avec erreurs systématiques masquées selon l'invention ;
• la figure 5 est un schéma destiné à expliquer le mode de correction des écarts de déplacement du substrat ;
• les figures 6 et 7 sont des schémas illustrant les éléments matériels d'une imprimante ;
• la figure 8 comporte les parties A, B et C, chaque partie correspondant à une phase de la cinématique d'impression de bandes successives ;
• la figure 9 illustre un cas où un capteur de marque est mécaniquement solidaire d'une table d'impression soutenant le substrat face aux têtes d'impression ;
• la figure 10 illustre le cas où deux capteurs sont montés de part et d'autre d'un chariot portant les têtes d'impression, l'un dans une direction amont du mouvement et l'autre dans une direction aval ;
• la figure 11 est un schéma représentant les moyens de calcul d'une imprimante fonctionnant selon le procédé de l'invention ; et
• la figure 12 est une illustration du mode de détermination d'une position exacte de la marque de repérage de l'avance du substrat à partir du calcul du barycentre de l'image de la marque sur le détecteur.

A printer comprising means for carrying out the method according to the invention and other details of the method according to the invention will now be described with reference to the appended drawings in which:

• Figure 1 already described is a schematic representation of the means necessary for the creation of ink drops and their deviation to a substrate;
• FIG. 2 already described as FIG. 1 in the context of the description of the prior art represents all the calculation means necessary for the operation of the means shown in FIG. 1;
• Figure 3 is a diagram for explaining the changes in printing obtained by the method of the invention, it comprises three parts A, B and C;
• FIG. 4 gives an enlarged physical representation of the position of the drops:
  • in a part A in their nominal positions,
  • in a part B in positions with systematic errors,
  • in a part C in positions with systematic errors masked according to the invention;
• FIG. 5 is a diagram intended to explain the mode of correction of the displacements of displacement of the substrate;
• Figures 6 and 7 are diagrams illustrating the hardware elements of a printer;
• FIG. 8 comprises the parts A, B and C, each part corresponding to a phase of the successive band printing kinematics;
• FIG. 9 illustrates a case where a mark sensor is mechanically secured to a printing table supporting the substrate facing the printing heads;
• Figure 10 illustrates the case where two sensors are mounted on either side of a carriage carrying the print heads, one in an upstream direction of movement and the other in a downstream direction;
• Figure 11 is a diagram showing the calculation means of a printer operating according to the method of the invention; and
• Fig. 12 is an illustration of how to determine an exact position of the substrate advance registration mark from the calculation of the center of gravity of the mark image on the detector.

Detailed description of an example embodiment

Figure 3 is intended to explain what are the deviations caused by the additional noise algebraic voltage. For this, there is shown in different configurations on the plane of the substrate materialized by XY axes, 9 different nominal positions of drops of a frame drawn by a salvo of drops. In the example shown and to simplify the explanation, nine drops were taken, which has been represented in an excessively spaced manner.

In part A of Figure 3, three frames of nine drops numbered from 1 to 9 are represented according to their nominal position by points. These three frames are part of the same band A. It is assumed that the actual position of the drop number 4 is systematically shifted to the drop number 5. This real position is represented by a cross. The distance d between the real and nominal positions of the drops of rank 4 causes a defect of white lineage materialized in part A of Figure 3 by the difference between two lines, one joining the nominal positions of the drops, the other joining the actual positions. This white lineage defect is in general accompanied by a defect of dark lineage less visible due to the overlap more accentuated, in the example shown here, drops of rank 4 and 5 relative to the overlap of the other drops.

It should be understood that the actual defect resulting from a positioning difference of two drops relative to each other is not as important as what was represented by the distance d, Figure 3. A vision more This figure comprises the parts A, B and C. In part A, there are shown two successions of five frames each comprising nine drops numbered from 1 to 9. The drops are represented by circles whose surfaces partially overlap between frames and between drops of the same frame.

One of the successions of five frames represented in part A is obtained during a first scan the other during a second scan for example, a forward scan and a reverse scan as shown by arrows on the three parts of Figure 4. Bet A, the positions of the nine drops are in accordance with their nominal positions as it appears on the five consecutive frames and on a fictitious frame on which we carried the numbers of the drops.

In part B, there is shown a single band also on five consecutive frames and a fictitious frame bearing the numbers of the positions of the drops. In part B it was assumed that the rank 5 drop was systematically displaced with respect to its position nominal to the drop of rank 4. Similarly it was assumed that the drop of rank 6 was systematically displaced with respect to its nominal position towards the drop of rank 7. Each of the real and nominal positions of each of these two drops 5 and 6 is represented by a rhombus partly B.

In the example shown, the differences d are such that the drops of rank 5 and 6 no longer overlap and are tangent to each other. Here we have the beginning of a visible defect which is translated as represented in figure B by a succession of white dots.

In part C of Figure 4, there is shown a succession of five frames for which the drops 5 and 6 have the same defect as that commented in conjunction with Part B. Part C, the position of the drops of each frame is changed according to the invention by a random voltage added to the charging electrodes. This results in a position sounding. This sound breaks the regularity of the succession of white dots so that the defect is less visible.

Returning to Figure 3, in part B, there is shown two frames. These two frames lie in the set of frames forming a band immediately following the band of frames represented in part A. Normally the bands A and B are spaced from each other by a distance equal to the equal distance. between two adjacent drops of a salvo.

If the distance between the drop 1 of a frame of the band B and the drop 9 of a frame of the band A is due to a systematic positioning error of the drop 1 or the drop 9, too large or too small, as represented by crosses on the two frames in part B, there is also a defect lineage white or black respectively. It is thus seen that the defect of lineage between consecutive bands or within the same band may have the same origin consisting of a systematic shift of a drop relative to its nominal position, whether this drop is a first or last drop of a salve or an intermediate drop.

In the case of a defect of lineage between consecutive bands, the lack of lineage can have another origin. If the advance of the substrate with respect to the print head is not equal to the nominal advance, a lineage fault may appear or be increased by the difference between the nominal position of the substrate and its actual position.

A possible complement to the present invention taking into account this possible origin of a lineage defect will now be explained with reference to FIG.

This addition of the invention relates to a positional deviation of a band due to a gap in the advance of the substrate. This correction concerns printers in which the substrate is advanced step by step after the printing of each strip. According to this aspect of the invention, a first mark represented at A in FIG. 5 will be printed when printing a current band. This mark may consist of a single line printed by means of one or more drops of consecutive rank.

After the substrate is advanced, the mark A is displaced and occupies the position shown in B in FIG. 5. In order to materialize the error of the spacing ε _x of the advance of the substrate, the position in C of a mark is also represented. fictitious representing the nominal position that should have had the mark A in the absence of difference between the nominal position and the actual position. The C mark is not present on the substrate in a real way. The difference between the imaginary mark C and the mark in position B makes it possible to determine the difference ε _x between the nominal position marked at C and the actual position marked at B. This difference in the advance of the substrate will be compensated according to this aspect. of the invention by modifying the charge of the drops printed during the next strip.

The printing of the next strip will, like the printing of the current strip, have the impression of a next strip mark printed taking into account the actual advance of the substrate. It follows that the marks and the bands will be spaced apart from each other by their nominal spacing.

Detection of the difference ε _x between the mark B and the nominal position C of the band that will be printed will be carried out by means of a sensor 12, for example a CCD sensor making it possible to measure this distance, for example by counting the the difference in number between a sensor element 12a which receives the mark when it is in the nominal position and a sensor element 12b which actually receives it. This sensor will preferably be placed facing the substrate and arranged so that its measurement field makes it possible to detect the mark with fairly wide tolerances. This sensor will preferably be a sensor of a determined light wavelength and will be completed by an emitter in the direction of the substrate of this determined wavelength.

FIGS. 6 and 7 are schematic diagrams of ink jet color pattern printers showing some of the features necessary to incorporate the invention.

The system shown in FIGS. 6 and 7 corresponds to an architecture for printing large formats chosen solely as non-limiting examples. The printing is carried out by successive scans in the Y direction. The system uses, in a known manner, a substrate 27 from a reel 28, the unwinding of which is carried out upstream of a printing unit 29 by a pair 36 of rolls 37, 38 for driving in contact.

A first cylinder 37 is motorized, a second cylinder 38 provides against pressure at the point of contact. The two cylinders 37, 38 pinch the substrate and drive it without slipping. The advance of the substrate 27 is controlled by an encoder, not shown because in itself known, of angular positions mounted on the axis of one of the cylinders. After each intermittent advance of the substrate, the printing zone thereof is kept flat on a printing table 30, located under the scanning path of the printing unit 29. This flat hold is ensured by means of a second drive system 39 located downstream of the printing unit.

This second drive system 39 maintains a constant voltage of the substrate 27. Intermittent depression of the printing table is sometimes performed to improve the flatness of the substrate 27 in the printing area.

The inkjet printing unit 29 is composed of several printing heads 25, such as those shown for example in FIG. 1, each head being fed by one of the primary color inks, from tanks 11 by means of an umbilicus or distribution channel 13.

The different print heads 25 simultaneously print the substrate while it is stationary. The printing of a band is ensured by a scan in the Y direction of the printing unit. The scanning movement of the printing unit relative to the substrate is ensured by a belt 40 integral with the printing unit and driven by a motorized pulley 41. The guiding of the printing unit is ensured by known by a mechanical axis not shown.

Each printhead prints a band of constant width L. The printheads can be shifted in the X direction of advance of the substrate so that a head does not necessarily print the same tape at the same time as another print head corresponding to a different ink color. After each scan, the substrate is advanced by a spatial increment ΔX at most equal to the bandwidth L but which is more generally a sub-multiple of L for printing in multiple passes.

The difference of the print heads in the Y direction and possibly in the X direction on the one hand, a sufficient drying time between the deposition of the different ink colors and allows on the other hand, to ensure an order the same superimposition of the colors themselves when the printing is carried out during the going and the return of the print head.

With respect to the known printing system as represented in FIGS. 6 and 7, the invention according to this embodiment has the particularity of being equipped with a detector 12 for detecting the actual advance of the substrate. The position of this detector 12 with respect to the substrate and to the printing heads is discussed below with reference to FIGS. 8 to 10.

FIG. 8 comprises parts A, B and C each corresponding to a phase of the print kinematics of a set of strips.

In the positioning mode described in connection with FIG. 8, the detector 12 is fixed, and fixed for example to a device for holding the translation axis of the printing heads 16. FIGS. 8 to 10 show four print heads 25, one for each of the colors, cyan marked C, magenta marked M, yellow marked Y and black marked K. The device for maintaining the translation axis has not been shown because its geometry is specific to each printer. In addition, this is an example. The skilled person will find or create a support for fixing the detector knowing that this detector must perform the functions that are described below.

The detector must be able to detect a mark 51 printed by one of the print heads 25 between the left or right edge 53 of the substrate 27 and the beginning or the end respectively of the printed pattern.

In part A of FIG. 8, there is shown a first marked strip 1 printed while the print heads 25 move between a first edge 52, in the figure the left edge, and a second edge 53, in the figure the right edge of the substrate, as indicated by an arrow parallel to the scanning direction Y and perpendicular to the direction X of advance of the substrate 27.

As shown in parts A, B and C of FIG. 8, the detector 12 is placed at the edge of the substrate 27, in the vicinity of the printing head 25 situated in the second position in the set of heads. The second position is understood by counting the heads in the direction Y of advance of the substrate 27. The first head is the one which is the most upstream relative to the direction of travel of the substrate.

In a direction Z perpendicular to the plane of the substrate, the detector 12 is at a height relative to the lower substrate at the height of the lower parts of the print head so as to leave them the passage. The proximity of the substrate allows a better reading accuracy.

The use of the marks 51 and the detector 12, in relation to the print kinematics will now be explained.

Before the printing of a first band marked 1, the mark 51-1 is printed by the cyan head. This same cyan head then prints the strip 1 in the scanning direction indicated by an arrow in the Y direction. Before the scanning, the heads 25 are in the position shown in dashed lines in the left-hand part of FIG. the heads 25 are in the position shown in solid lines to the right of the substrate 27.

Then, chronologically, the substrate 27 is advanced one step. The mark 51-1 is in the field of the detector 12. The detector 12 detects a possible deviation of the advance of the substrate from the nominal advance, and the calculation means 34, 35 calculate corrections to be made to the voltages Charging the drops of the cyan head and the magenta head, so that the change in trajectory of the drops compensates for the gap in advance of the substrate.

In the return movement of the heads, the magenta head, prints the second color on the tape 1 and the cyan head prints the second tape then the mark 51-2. At the end of the return scan, the heads 16 are found on the side of the first edge as shown in part B.

The substrate is again advanced so that the mark 51-2 arrives in the field of the detector 12, as shown in part C in FIG. 8.

The detector detects any deviation of the mark 51-2 from its nominal position.

Then, during a scan from the first edge 52 to the second edge 53, the mark 51-3 and the third band are printed by the upstream head cyan. The magenta head prints the second band with drop charge voltage corrections to account for the last gap value ε _x , the yellow Y head prints the first band.

At the end of the third scan, the heads 25 are on the side of the second edge 53. The continuous cycle. The substrate is advanced. The detector detects any deviation of the 51-3 mark from its nominal position. A correction taking into account this difference is applied to load the drops of the black head which will print by superposition the first band, the yellow Y head which will print the second band and the magenta and cyan heads which will print respectively the third band and the mark 51-4 followed by the fourth band.

The cycle thus continues modulo the number of juxtaposed print heads, for example four in the case shown in connection with FIG. 8.

The kinematics which has just been described concerns an impression in which the heads print in the forward scan movement and in the reverse scan movement.

The kinematics would be the same in the case of printing only by forward scanning, the advance of the substrate being done at the same time as the return movement of the heads towards the first edge 52.

Note that the operation just described, implicitly assumes that the cumulative algebraic sum of the lead gaps of the substrate, remains low.

To overcome significant drifts of the substrate advance, the motor control advance of the substrate may include a servo which takes into account the substrate advance gaps. This enslavement, known to those skilled in the art, may be of the "proportional integral and derivative" type, that is to say that it takes into account the real differences, their accumulation and their variation over time in order to avoid drifts.

The reading of the marks, the determination of the gap in advance of the substrate and the correction of the frames makes it possible at any moment to ensure the good superposition of the bands.

According to a software improvement, it is sought to guard against an unexpected blocking of the advance of the substrate which would not be due to non-operation of the unwinding and pulling systems of the substrate detected elsewhere.

In case of blocking of the substrate, the mark printed when printing a current band and which serves as a position reference for printing the next strip, does not arrive in the field of the detector 12. The detector 12 will therefore reuse the mark that was used to print the current band with the same corrections, so that if we do not detect the blocking or near-blocking of substrate the next tape will print overlap on the previous tape.

To avoid this possible overlap, the printed pattern of even-rank marks is different from that of odd-numbered marks. Another case where the recognition of the current mark with respect to the next mark is interesting is the case where these two marks would be simultaneously visible on the detector 12, for example one on an upstream end portion of the detector and the other on an end portion downstream from the direction of movement of the substrate. This situation can arise if there is a cumulative difference in advance reaching a positive or negative value of half a nominal advance. In this case, the program will select the reference mark for printing the next tape.

The program in the event of detection of a blocking or quasi-blocking may include triggering another substrate advance and then triggering an alert if a blockage is detected again, or on the contrary the immediate triggering of an alarm.

The pattern of even and odd-rank band marks will be a function of the detector.

If, for example, the detector has only one array of detector elements, the even and odd patterns will be distinguished from each other by the number of lines of one compared to the number of lines of the other, the difference between lines being such that each line is detected by a different sensor element. It could also be the same number of lines but with different spacings between lines corresponding to different numbers of the sensor elements detecting these lines. If the sensor 12 comprises sensor elements arranged in a matrix manner, or if the sensor 12 is, as will be described later, movable in the X direction of the scan, the odd or even patterns may be distinguished, moreover, by variations in the scanning direction, for example, points for one and lines for the other or different distances from the same pattern.

Figure 8 has been used to describe in detail the principle of measuring and controlling the advance of the substrate. In practice, the substrate mark detector must be placed downstream of the print head which prints the marks, but in a place compatible with its size. Thus, the positioning of the sensor in a zone swept by the printheads as in FIG. 8 would require a very fine mechanical adjustment so that the print head could pass over the sensor during the sweeps without risk of to hit. Furthermore, this positioning can create difficulties in the repeatability of the lighting conditions of the mark at the sensor, depending on whether the head is located at the right edge or the left edge of the substrate during the detection / measure of the mark. In practice, the printer has under the substrate at the area swept by the printheads a printing table which ensures a good maintenance of the substrate. The sensor can therefore be positioned in a fixed manner, downstream of the last head printing, but in a place where the substrate is securely held by the printing table. This allows proper operation without demanding constraints on the size of the sensor and its lighting.

It is this position which is represented in FIG. 9. The detector 12 is mechanically coupled to the printing table 30 immediately downstream of the printing heads 25.

Instead of being printed by the upstream head, the mark is printed, in the example shown, by the black downstream head K.

With this difference, the print kinematics is the same as that described with reference to FIG.

When the advance of the substrate is delicate, or when the printing table is not large enough, it will be advantageous to use two sensors, mounted on either side of the print head. Each sensor, labeled "left" and "right", respectively, will detect the mark printed on the left (respectively right) edge of the substrate, when printing the even-index scanning mark which takes place from the right edge to the left edge (respectively odd for scanning from left edge to right edge).

This case is the one represented in FIG. 10. The detector 12 is carried by the movable mechanical assembly comprising the printing heads which will be called carriage later.

This figure shows the case of a printer printing in forward scan and reverse scan. The carriage comprises in this case two detectors, a detector 12-1 which is upstream of the printheads during a forward scan and a detector 12-2 which is upstream of the printheads during a sweep back. For this purpose the detectors 12-1, 12-2 are located on both sides of the printing heads 25.

With respect to a fixed detector located near one of the edges of the substrate the operation is slightly different.

The 51-1 mark is always printed at the end of the scan. As a result, the odd-rank marks are all on the side of the second edge 53 and the even-rank marks are all on the side of the first edge 52.

Thus, for example, the mark 51-1 printed at the end of the first scan on the second edge 53 of the substrate 27 is detected by the detector 12-2 which is upstream of the printing heads 25 during the reverse scan. The drops of the charge corrections are made and the band number 2 is printed then the mark 51-2 near the first edge. After advancing the substrate 27, this mark 51-2 is detected by the detector 12-1. The difference observed is used for the correction of the printing of the band 3 and the mark 51-3 printed at the end of the scan. This solution has the advantage of easier positioning of the detectors, a distinction of position of even and odd marks. The disadvantage is that it takes a additional detector 12. Switching to switch the input of the means 34, 35 to the detector 12-1 or 12-2 is required, and may be performed at the software level by changing the read address of the substrate gap information ε _x .

Another important difference of a printer according to the invention with respect to a known printer comes from the means for controlling the voltage of the charging electrode of the drops. A device according to the prior art has been previously described in connection with FIG. 2.

FIG. 11 represents control means 31 according to the invention. In these printing control means 31, the elements having the same function as those represented in FIG. 2 bear the same reference number. With respect to the printing control means 26 shown in FIG. 2, the device according to the invention comprises a random noise generator 32 whose output is applied to the computer 3 'for fixing the charging voltages of the drops as a function of their rank so as to randomly change the load of each drop. This generator delivers a random numerical value according to a pseudo random number delivery algorithm. The skilled person knows how to make such algorithms. Preferably, the algorithm will be designed to deliver on average for at least three quarters of the values generated for drops of a number of frames greater than a predetermined amount, a value less than one third of the difference between the nominal voltage to be applied to the charging electrodes for said drop and the nominal voltage to be applied to the charging electrodes for one of two immediately adjacent drops of the frame. Also preferably, the quotient of the number of times the sign of the value of the additional algebraic voltage is positive, the number of additional total voltages will be on average over a large number of additional values equal to ½. This reflects the fact that, on average, a drop of rank j will be removed from its central position corresponding to an additional zero random voltage, with the same probability towards the upper rank drop or the lower rank drop. For the end drops it will be a gap to the outside or to the nearest drop of the frame.

In this way, the position of the drops will be slightly noisy. Three-quarters of the drops will be at a distance from their nominal position without random variation of voltage, less than 1/3 of the nominal distance separating two drops with an equal probability that this distance is towards the drop of higher or lower rank. the predetermined quantity of frames on which the average of the deviations of the drops will be calculated with respect to the actual position they occupy when the nominal voltage corresponding to their rank is applied to them may, for example, be equal to the number of frames contained in three bands. Naturally, it is possible to choose different generation algorithms or even to have several algorithms choices for example based on a local density of dots printed by the nozzle.

Due to the dispersion of the drops around the actual position they occupy when the nominal charging voltage corresponding to their rank is applied to them, a possible lineage defect such as that shown in FIG. 3, by the two lines separated from the distance d, no longer appears or appears less because drops will be in the space between these two lines, breaking the linearity of the defect and thus making its perception more difficult.

In the preferred embodiment the printer comprises the detector 12 of difference between the actual advance of the substrate and its nominal advance. The control means 31 of the printing therefore also comprise a calculator 34 of position deviation of the substrate. The elements, detectors 12, position difference calculator 34 are serially connected to each other and to a dynamic forward translation correction voltage calculator 35 of the substrate. The dynamic translation corrections φ determined by the computer 35 as a function of the value of the difference error ε _x of the actual position of the substrate relative to its nominal position and as a function of the rank j of the drop, are applied to the computer 3 'of the charging voltages of the drops. The calculation of the additional charge voltage to be applied to each drop of the burst according to its rank can use memorized values of additional voltage to be applied to correct deviations ε _x appearing in a table of deviations. These values will be interpolated based on the actual deviation. The calculation can also use an algorithm making use in addition to the difference ε _x , data known by the manufacturer of the printer such as the unit mass of the drops, the value of the electric field created by the voltage of the deflection electrodes, laws of variation of the position of the drops in function of the voltage applied to the charging electrodes 20.

The operation is as follows.

The detector 12 detects the difference between a mark relating to the current band that will be printed and the nominal position of this band. This difference is introduced into the difference calculator 34. this calculator calculates, as a function of the signal transmitted by the sensor 12, the value ε _x of the lead gap of the substrate 27. This difference is introduced into the dynamic translation calculator 35 which will calculate corrections to be applied to the computer 3 'of the charge voltages drops to correct this dynamic translation. The computer 3 'of the drop charge voltage will calculate the algebraic sum of the voltages to be applied to the drop charge electrode by adding the nominal voltage resulting from the description of the frame from the memory 2, the value output by the random noise generator 32, and finally the correction value resulting from the difference correction performed by the dynamic translation correction calculator 35 φ.

Another function of the computer 34 relates to the recognition of the mark and the processing of the information transmitted by the sensor 12 to deduce a deviation of the mark from its nominal position. It was pointed out earlier that a simple treatment to determine the value of the substrate lead gap was to count the number of sensor elements between the sensor element corresponding to the nominal position numbered 0 and the sensor element receiving the mark. This way implicitly assumes that the thickness of the mark is of the same order of magnitude as the resolution of the sensor. Under these conditions, the difference is determined by the number of the sensor element detecting the mark, if this element is unique. If the detection of the mark straddles two sensor elements, the difference is calculated as being a function of the number of the nearest sensor element that perceives the mark, increased by an increment involving the distance between two elements sensors and the proportions for example of current from each of the two sensor elements concerned.

FIG. 12 shows, on an exemplary embodiment, various possible cases and their mode of treatment when the resolution of the sensor is greater than the diameter of the drops. In the example represented in FIG. 12, the mark is composed of several lines, three in the commented example, drawn by different drops of a salvo, for example the drops corresponding to the positions 2, 4 and 6 of a burst of nine drops. .

In the different cases the difference, with respect to the nominal position, will be calculated by the calculator 34, from the calculation of the position of the projection the center of gravity of the mark 51 on an axis X parallel to the advance of the substrate.

This center of gravity is determined by the sensor elements that see the mark. If, as shown in FIG. 12 part A, the drops are normally positioned, the measurement will be exact. If, as shown in part B, the drops of rank 5 and 6 are displaced relative to their nominal position, the error will be reduced. The same will be true if the random voltage generator 32 is not inhibited at the moment of printing the mark as represented in part C. It goes without saying that it is preferable to inhibit this generator 32 in order to minimize the error.

In the case of moving position detectors as discussed in connection with FIG. 10, the measurement of the position of the marks may result from sampling carried out during the scanning of the print head, the accuracy of the measurement will be increased, and the influence of minimized noise.

Claims (7)

1. Process for modification of the position at which electrically-charged ink droplets arrive on a substrate (27), in an adjustable and sequential manner, the droplets originating from a print head (25) after being charged by charge electrodes (20) connected to a voltage generator (32), the paths of the droplets being affected by deviation electrodes (23, 24) deviating the droplets depending on their electrical charge between N positions defined by their row j, a first position X₁, a last position X_N, and N-2 intermediate positions, the N positions defining a frame obtained by a burst of droplets in the form of a straight segment approximately perpendicular to a direction of relative movement between the head (25) and the substrate (27), process characterized in that an additional random algebraic voltage is superposed on a nominal voltage to be applied to the charge means on each droplet to be directed to the substrate (27), the maximum amplitude of the additional random algebraic voltage being a fraction less than 1 of the difference between the nominal voltage to be applied to the charge electrodes (20) for the said droplet and the nominal voltage to be applied to the charge electrodes (20) for one of the two immediately adjacent droplets in the frame.
2. Process according to claim 1, characterized in that the value of the additional random algebraic voltage is generated by a pseudo-random generation algorithm, this algorithm generating a value less than 1/3 of the difference between the nominal voltage to be applied to the charge electrodes (20) for each droplet and the nominal voltage to be applied to the charge electrodes (20) for one of the two immediately adjacent droplets in the frame, for at least three quarters of the values generated for droplets in a number of frames exceeding a predetermined quantity.
3. Process according to claim 1, characterized in that the value of the additional random algebraic voltage is generated by a pseudo-random generation algorithm, this algorithm generating a value less than 1/3 of the average of the difference between the nominal voltages to be applied to the charge electrodes (20) for each droplet and the nominal voltages to be applied to the charge electrodes (20) for the two immediately adjacent droplets in the frame, for at least three quarters of the values generated for droplets in a number of frames exceeding a predetermined quantity.
4. Process according to one of the claims 1 to 3 applicable to a printer in which the substrate (27) is advanced step by step and printed by band, characterized in that:

   - a current band and a first mark are printed on the substrate (27),

   - the substrate (27) is advanced to print the next band,

   - an algebraic difference is calculated between a nominal theoretical position of the mark and the real position,

   - for each drop in a burst, a substrate advance correction is calculated as being a dynamic translation correction voltage ϕ to the value of the charge voltage to be applied to each of the droplets output from the head to correct the deviation of the droplets and to compensate for the algebraic difference of the position of the substrate from its nominal position,

   - the calculated value of the dynamic translation correction voltage ϕ to correct the substrate position is applied to each droplet in the burst directed towards the substrate, in addition to the random voltage.
5. Process according to claim 4, characterized in that the additional random voltage is not applied to the droplet charge electrodes (20) while the mark is printed.
6. Printer with a continuous deviated jet projecting droplets in rows 1 to N in bursts, the droplets in a burst being directed or not directed towards a print substrate depending on data that define a pattern to be printed, the printer having at least:

   - a print head (25), this head comprising means of separating at least one ink jet into droplets and an associated charge electrode for droplets, means of deviating some of the droplets towards the print substrate,

   - means of controlling the printout including a means of injecting the charge into the droplets to be directed towards the substrate depending on their row in the burst, coupled with the droplet charge electrode,

   
   characterized in that the means of controlling the printout comprise a random additional voltage generator coupled to droplet charge injection means, the droplet charge injection means taking account of the value of the random voltage generated by the additional random voltage generator to modify the charge voltage of each droplet as a function of the generated random value, the droplets of each row thus being dispersed around a central position corresponding to the position that they would have had without any additional voltage.
7. Printer with a continuous deviated jet according to claim 6, characterized in that it also comprises at least one position detector outputting a value representative of a variation between a nominal advance and a real advance of the substrate, and in that the print control means also comprise a calculator (35) that calculates the dynamic translation correction voltage ϕ for the substrate advance, this calculator (35) determining a dynamic translation correction voltage ϕ for the substrate advance for each droplet in a burst as a function of its row, this correction voltage taking account of a value of a variation in the advance of the substrate output by means coupled to the detector (12) and calculating a value of the difference from a nominal position, the calculator (35) that calculates the dynamic translation correction voltage ϕ for the substrate advance being coupled to means of injecting the droplet charge, the means of injecting the droplet charge taking account of the value of the substrate advance correction voltage generated by the calculator (35) that calculates the dynamic translation correction voltage ϕ for the substrate advance to modify the charge voltage on each droplet as a function of the dynamic translation correction voltage for the substrate advance ϕ.

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