EP1106357A1 - Method and printer with fault masking - Google Patents

Abstract

Ink droplets are charged electrically by passage between two electrodes. Each of a set or salvo of ink droplets forced by pressure from the print head is aligned effectively parallel to the direction of advancement of the print media. In addition to the normal applied voltage a small random voltage is applied to each of the droplets which is less than one times the nominal voltage difference between successive drops. An Independent claim is made for an inkjet printer in which a small random voltage can be applied to successive ink drops in an ink salvo originating from the print head.

Description

Field of the invention

The invention lies in the field of inkjet printers in which drops ink are formed and electrically charged then deflected to strike a printing substrate. It relates to a process intended to mask or reduce line faults and the printer applying a such process.

Technological background

It is known that an ink jet under pressure ejected from a print nozzle can be broken into a succession of individual drops each drop being charged individually, in a controlled manner. Sure the path of these drops so individually charged, electrodes of constant potential deviate more or less the drops depending on the charge they own. If a drop should not reach the printing substrate, its charge is controlled in such a way so that it is diverted to an ink collector. The operating principle of such printers inkjet is well known and is described for example in patent US-A-4,160,982. As described in this patent and shown in Figure 1, such a printer has a reservoir 11 containing ink electrically conductive 10 which is distributed by a distribution channel 13 to a drop generator 16. The role of the drop generator 16 is to train from the pressurized ink contained in the channel dispenser 13 a set of drops individual. These individual drops are electrically charged by means of an electrode load 20 supplied by a voltage generator 21. Charged drops pass through a space between two deflection electrodes 23, 24 and according to their load are more or less deviated. The the least or not deviated drops are directed towards a 22 ink recuperator while the deflected drops are directed towards a substrate 27. The drops successive bursts reaching substrate 27 can thus be deflected to an extreme position low, an extreme high position and positions successive intermediaries, all of the drops of the salvo forming a vertical line of height ΔX substantially perpendicular to a direction of advance relative of the print head and the substrate. The print head is formed by the generator drops 16, the charging electrode 20, the electrodes of deflection 23, 24 and the recuperator 22. This head is generally enclosed in a casing not shown. The deflection movement imprinted on the drops charged 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. Time 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, we can consider that the substrate does not have moved relative to the print head during time for a salvo. The bursts are fired at intervals regular space. If all the drops of each salvo were directed towards the substrate we would print a succession of lines of height ΔX. In general only certain drops of a salvo are directed towards the substrate. Under these conditions, the combination of relative movement of the head and the substrate, and the selection of the drops of each burst which are directed to the substrate allows to print any pattern such as that shown at 28 in Figure 1. If the line that you draw with the drops of a salvo is in a direction X, the relative movement of the head and of the substrate is, in the plane of the substrate in a direction Y perpendicular to X. The drops not diverted are directed to the recuperator according to a trajectory Z perpendicular to the x, y plane of the substrate. The printed drops arrive on the substrate in following paths slightly deviated from in direction Z.

If the relative movement of the head and substrate is carried out continuously according to the dimension larger of the substrate, there will usually be several printheads printing parallel strips the to each other. An example of such use is shown in Figures 1 and 2 of the patent issued to IBM under number FR 2 198 410.

If the relative movement of the print head and of the substrate in the Y direction is carried out according to the smallest dimension of the substrate, the impression is carried out strip by strip the substrate having a movement intermittent advance in direction X after each scanning. The relative movement of the print head and the substrate is called the sweeping motion. The sweep movement thus consists of a movement back and forth between a first edge of the substrate and a second edge of the substrate. The movement between a edge and the other edge of the substrate allows to print to the stolen a strip of height L or quite often a part of the height band ΔX, ΔX being the most often a sub-multiple of L. The set of bands successively printed thus constitutes the motif to print on the substrate. After each impression of a strip or part of the strip, the substrate is advanced the space between two strips or part of a strip for printing the strip or part of the strip next. Printing can be done on the way simply or back and forth from the movement of the print head relative to the substrate.

When the graphics to be printed are colored, the multiple shades of colors are the result of the overlay and juxtaposition of impacts ink from nozzles supplied with inks of different colours. The relative displacement system of the substrate relative to the printheads is produced in such a way that a given point on the substrate is presented successively under the ink jets of each of the colors. The printing system presents usually multiple jets of the same ink operating simultaneously, either by juxtaposition multiple heads, either by the use of heads multijets, or finally by the combination of these two types of heads in order to achieve rates high impressions. In this case, each inkjet prints a limited part of the substrate. Drops can be produced, continuously, as described above in conjunction with Figure 1. They may also be produced "on demand", that is to say only when necessary for them printing needs. In this case, a recovery of unused ink is not necessary. Known means for controlling the different jets will now be described with reference to FIG. 2.

The pattern to print is defined by a file digital. This file can be formed using a scanner, a graphic palette for assisted creation by computer (CAD), transmitted over a network data exchange computing, or, all simply read from a playback device of digital data storage medium (disc optical, CD-ROM). The digital file representing the colored pattern to print is first split into several binary patterns (or bitmap) for each of inks. It should be noted that the case of the pattern binary is a nonlimiting example; in certain printers, the pattern to be printed is of the "contone" type, that is, each position can be printed by a number of drops varying from 1 to M. Part of the binary pattern is extracted from the file for each of jets corresponding to the width of the strip that goes be printed. In Figure 2 where we are interested in the control electronics of a jet, there is shown in 1 a memory for storing the cut digital pattern in band, this storage memory containing the indications relating to a color. For printing of each band, an intermediate memory 2 receives the data required to print the tape by said color. The descriptive data of the band to print are then entered into a calculator 3 charge voltages of the various drops which go form the strip relative to this color. These data are entered into the calculator in the form a succession of descriptions of the frames which together will constitute the band. The voltage calculator 3 load of drops is often in the form a dedicated integrated circuit. This calculator 3 calculates in real time the sequence of voltages to be applied to charge electrodes 20 for printing a given frame defined by its frame description, as loaded at from intermediate memory 2. A circuit downstream electronics 4, called charge sequencer drops, ensures the synchronization of the voltages of load with on the one hand, the instants of formation of drops and, on the other hand, the relative advance of the head print and substrate. The advance of the substrate by report to the head is materialized by a clock of frame 5 whose signal is derived from the signal from an encoder incremental position of the printing unit relative to the substrate. The load sequencer 4 drops also receives a signal from a clock of drops 6. This drop clock is synchronous with the control signal of the drop generator 16. It allows to define the instants of transitions of different charge voltages applied to the drops to differentiate their trajectories. The data digital from the load sequencer 4 drops are converted to analog value by a digital to analog converter 8. This converter delivering a low voltage level generally requires the presence of a high voltage amplifier 21 which will supply the charge electrodes 20. The illustrations of the prior art given with reference Figures 1 and 2 are intended to do well understand the field and the contribution of the invention, but it is obvious that the prior art is not limited to descriptions made with reference to these figures. Other arrangements of electrodes and collectors for recovering unused ink drops are described in abundant literature. An arrangement electromechanical electrode printing nozzles charging and deflection electrodes as described in patent No. FR 2 198 410 issued to International Business Machine Corporation (IBM) reference to Figures 1 to 3 of this patent could perfectly be used in the present invention. Likewise, the electronic circuit for controlling the charge electrodes could be illustrated by the circuit described in connection with Figure 4 of the same patent. Also, the data to be printed could not be in the form of binary files, but as files containing words from several bits, to translate the fact that each position of the substrate can receive several drops ink of the same color. We understand that for a printing, especially in color, if necessary superimposition of drops from different nozzles delivering different colors of ink must be very specific. The main printing faults which are generated by all printing systems known, are the defects relating to lineages in the direction of relative movement of the print head by relation to the substrate. This defect results in the appearance of light or dark lines when printing by successive scans. These faults can be in the space between two bands which should in principle be equal to the interval between adjacent drops of a frame, or inside of the same band, in the space delimiting the zones printed by different jets, or even inside the pattern printed by a jet at the level of the space between two adjacent drops of the frame. These faults of lineage can come either from defects specific to some jets from the print head, these are then mechanical or electrical faults, either substrate positioning errors, or positioning error between printheads, or still between jets from the same printhead. Various solutions have been proposed to limit or eliminate lineage issues, but all are translate either by a limitation of the rate sometimes very high ratio vis-à-vis the nominal printing rate, either by redundancy of print heads and therefore a cost important. Examples of commonly known solutions implemented to limit the lineage are going to be succinctly set out below: a first type of solution relies on fine mechanical adjustments of the printhead position, thanks to tables micrometric. 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 overlap rate between neighboring drops, so as to avoid lineages whites. These white lines correspond to the absence substrate cover. Dark lineages are less visible and we prefer to have a lineage defect dark lines rather than a white line defect. The solution of increasing the rate of overlap between neighboring drops is effective for compensate for faults within the same band and to some extent the lineage defects between bands but it has the disadvantage of requiring a very high amount of ink per unit area of the substrate and generates difficulties in drying or deformation of the substrate.

A third type of solution to erase line faults on printers operating in scanning consists in partially printing the substrate during each scan. By multiplying the number of substrate scans we get full coverage of the substrate. This impression in several passages uses various interleaving strategies positions of the drops from the different jets. A example of interleaving of even and odd lines is given in patent no. US-A-4,604,631 issued to the RICOH company. An advantage of this solution often related to a high overlap rate is that it allows a drying time for the substrate, but it results in a reduced print rate of one factor ranging from 2 to 16.

The performance of printing systems colorful graphics naturally evolving towards increasingly higher resolutions and cadences it becomes critical to effectively limit problems lineage without compromising penalizing printing rates.

Brief description of the invention

The method according to the invention aims to mask some lineage issues with no impact on the printing speed.

The present invention does not require a rate high overlapping drops. She permits achieve high print rates with a relatively few print heads. When minimizing the overlap between drops adjacent there may remain a lineage defect, in particular a white line defect appearing from on a regular basis. This defect is very noticeable by the eye when it is regular. In order to decrease the perceptibility of this possible defect, we superimpose on a nominal charge voltage of the drops a voltage additional noise intended to give compared to the nominal position of each drop a position real with a random dispersion character. Thanks to this dispersion of the actual position of each drop around its nominal position, the default no longer appears as a straight line keep on going. It therefore becomes less noticeable to the eye

The invention therefore relates to a method for modifying the position of arrival on a substrate of ink drops electrically charged in an adjustable and sequential manner by charging electrodes, the drops coming from a print head, the trajectories of the drops being modifiable, by 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, a method characterized in that an additional random algebraic voltage is applied in superposition to a nominal voltage applied to the charge electrodes of drops, thus masking a possible defect in lineage by dispersion of the real position of each drop around from its nominal position.

The average amplitude of this noise voltage will depend on the rank j of the drop in the frame. Of preferably the maximum amplitude of the voltage additional noise 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 row j and the nominal voltage VJ + 1 or Vj-1 to be applied to one of the two drops immediately adjacent in the frame printed with a drop of row j, i.e. drops of row j + 1 and of row j-1.

As the differences in charging voltage applied to the adjacent printed drops have fairly close to each other, we can take for the maximum additional voltage value random a fraction of an average value, this mean value being the mean value of the differences nominal voltages between two adjacent drops printed in the frame.

Preferably the minimum amplitude of the voltage additional noise will be equal to the value of the voltage difference that can be obtained by doing vary the value of the least significant bit by one analog to digital converter whose output supplies a high voltage amplifier coupled to charge electrodes for drops.

Preferably the amplitude of the tension additional noise will correspond to a value random numeric generated by an algorithm of pseudo random number generation. The correspondence between the random numerical value and additional noise voltage will result from applying this numeric value to digital to analog converter. The regular defect of dark or white lineage will no longer 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 deviated continuous jet printer projecting in bursts of drops of rank 1 to N into the burst, the drops of '' a burst being directed or not towards a printing substrate as a function of data 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 electrode for charging the associated drops, means for deflecting part of the drops towards the printing substrate,
• printing control means having a means for fixing the charge of the drops to be directed towards the substrate as a function of their rank in the burst, coupled to the electrode for charging the drops,

characterized in that the printing 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 charge voltage of each drop depending on the random value generated, the drops of each row being thus 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, the printer also includes a detector the position of a printed mark before each first frame of a band, this detector providing a value representative of a difference between the real and nominal positions of the substrate and that the printing control means include besides a calculator of a correction voltage of dynamic translation ϕ of the substrate in advance, this computer determining a correction voltage of dynamic translation ϕ of substrate advance for each drop of a salvo depending on its rank, this correction voltage taking into account a value deviation in 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 voltage ϕ in advance of the substrate being coupled to the means of fixing the load of the drops, the fixing means of the charge of the drops taking into account the value of the substrate advance correction voltage generated by the translation correction voltage calculator dynamic ϕ in advance of the substrate to modify the charge voltage of each drop depending on the dynamic translation correction voltage ϕ in advance 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;
• Figure 2 already described as Figure 1 in the context of the description of the prior art shows all of the calculation means necessary for the operation of the means shown in Figure 1;
• FIG. 3 is a diagram intended to explain the modifications of the 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 part A in their nominal positions,
  • in part B in positions with systematic errors,
  • in part C in positions with hidden systematic errors according to the invention;
• FIG. 5 is a diagram intended to explain the mode of correction of the displacement deviations of the substrate;
• Figures 6 and 7 are diagrams illustrating the hardware of a printer;
• FIG. 8 comprises parts A, B and C, each part corresponding to a phase of the kinematics of printing successive bands;
• FIG. 9 illustrates a case where a mark sensor is mechanically secured to a printing table supporting the substrate facing the printing heads;
• FIG. 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;
• FIG. 11 is a diagram representing the calculation means of a printer operating according to the method of the invention; and
• FIG. 12 is an illustration of the method of determining an exact position of the marking mark for the advance of the substrate from the calculation of the barycenter of the image of the mark on the detector.

Detailed description of an exemplary embodiment

Figure 3 is intended to explain what are the deviations caused by the algebraic tension additional noise. For this, we have represented in different configurations on the substrate plane materialized by XY axes, 9 different positions nominal drops of a frame drawn by a salvo drops. In the example shown and for simplify the explanation, we took nine drops, that one has represented it in an exaggeratedly spaced way.

In part A of Figure 3, three frames of nine drops numbered from 1 to 9 are represented in accordance with their nominal position by dots. These three frames are part of the same A band. suppose the real position of the drop number 4 is systematically shifted to drop number 5. This actual position is represented by a cross. The spacing d between the actual and nominal positions row 4 drops causes a lineage defect white materialized in part A of figure 3 by the gap between two straight lines, one joining the nominal positions of the drops, the other joining the actual positions. This white line defect is in general accompanied by a black line defect minus visible due to the more pronounced overlap, in the example here represented, drops of rank 4 and 5 compared to the overlap of the other drops.

It should be understood that the actual defect resulting from a positioning drop of two drops one over the other is not as important that what has been represented by the distance d, figure 3. A more realistic vision of a gap fault was shown in Figure 4. This figure contains parts A, B and C. In part A, we have depicted two successions of five frames comprising each nine drops numbered from 1 to 9. The drops are represented by circles whose surfaces are partially overlap between frames and between drops of the same frame.

One of the successions of five frames shown in part A is obtained during a first scan the other during a second scan for example, a forward scan and a return scan as materialized by arrows on the three parts of figure 4. In bet A, the positions of the nine drops conform to their nominal positions as it appears on the five consecutive frames and on a fictitious frame on which we wore the drop numbers.

In Part B, a single band is shown also on five consecutive frames and one frame fictitious bearing the numbers of the positions of the drops. In part B it was assumed that the drop of rank 5 was systematically displaced from its position nominal towards the drop of rank 4. Similarly we assumed that the drop of rank 6 was systematically displaced from its nominal position towards the drop of rank 7. Each of the actual and nominal positions of each of these two drops 5 and 6 is represented by a diamond in part B.

In the example shown, the differences d are such that the drops of rank 5 and 6 do not overlap more and are tangent to each other. We have there, the beginning of a visible defect which results, as represented figure B by a succession of points whites.

In part C of Figure 4, there is shown a succession of five frames for which drops 5 and 6 have the same defect as that commented on in link with part B. In part C, the position of drops of each frame is modified according to the invention by a random voltage added to the electrodes of charge. This results in a position sound effect. This sound effects breaks the regularity of the succession of points white so that the fault is less visible.

Returning to FIG. 3, in part B, we have shown two frames. These two frames are located in all the frames forming a band immediately consecutive to the strip of frames represented in part A. Normally bands A and B are spaced one of each other by a distance equal to the equal distance between two adjacent drops of a salvo.

If the distance between drop 1 of a frame of band B and drop 9 of a frame of band A is as a result of a systematic positioning defect of drop 1 or drop 9, too big or too small, as represented by crosses on the two frames in part B, there is also a line defect white or black respectively. We can see that the misalignment between consecutive bands or the interior of the same strip can have the same origin consisting of a systematic shift of one drop per relative to its nominal position, let this drop be a first or last drop of a salvo or a intermediate drop.

In the event of a line-to-line defect consecutive, the lineage defect may have another origin. If the substrate advance in relation to the head is not equal to the nominal advance, a lineage defect 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 defect of lineage will now be explained with reference to the figure 5.

This complement of the invention relates to a band position deviation due to a deviation in advance of the substrate. This correction concerns printers in which the substrate is not advanced step by step after the printing of each strip. According to this aspect of the invention, we will print when printing a running tape a first mark represented at A in FIG. 5. This mark may consist of a single line printed using one or more drops of consecutive row.

After advance of the substrate the mark A is moved and occupies the position represented in B in FIG. 5. In order to materialize the error of deviation ε _x in advance of the substrate, the position in C of a mark has also been represented fictitious representing the nominal position that the mark A should have had in the absence of difference between the nominal position and the actual position. Mark C is not actually present on the substrate. The difference between the fictitious mark C and the mark in position B makes it possible to determine the difference ε _x between the nominal position marked in C and the real position marked in 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 following strip.

The next tape will print like printing the current tape, printing of a mark of the following strip printed while holding account of the actual advance of the substrate. It follows that the marks and bands will all be spaced between them from their nominal spacing.

The difference ε _x between the mark B and the nominal position C of the strip which is going to be printed will be detected by means of a sensor 12, for example a CCD sensor making it possible to measure this distance, for example by counting l 'number difference 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 in front of 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 supplemented by an emitter in the direction of the substrate of this determined wavelength.

Figures 6 and 7 are block diagrams inkjet printers showing some necessary features to the incorporation of the invention.

The system shown in Figures 6 and 7 corresponds to an architecture for printing wide formats chosen only as examples not limiting. Printing is done by scanning successive in direction Y. The system implements in a known manner a substrate 27 from a coil 28 which takes place upstream of a unit printing 29 by a pair 36 of cylinders 37, 38 contact training.

A first cylinder 37 is motorized, a second cylinder 38 provides back pressure to the point-of-contact. The two cylinders 37, 38 pinch the substrate and drive it without sliding. The advance of substrate 27 is controlled by an encoder, not shown because in itself known, angular positions mounted on the axis of one of the cylinders. After each advance intermittent substrate, the print area of it is kept flat on a printing table 30, located under the unit scan path printing 29. This holding flat is provided by a second drive system 39 located downstream of the printing unit.

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

The inkjet printing unit 29 is composed of several 25 printheads like those shown for example in Figure 1, each head being fed by one of the color inks primary, from tanks 11 thanks to an umbilicus or distribution channel 13.

The different print heads 25 print simultaneously the substrate while it is stationary. Printing of a strip is ensured by scanning in the Y direction of the printing unit. The scanning movement of the printing unit by relation to the substrate is ensured by a belt 40 integral with the printing unit and driven by a motorized pulley 41. Guiding the printing unit is provided in a known manner by a mechanical axis not represented.

Each print head prints a strip of constant width L. The print heads can be shifted in the X direction of advance of the substrate so that a head does not necessarily print the same strip at the same time as another head ink color different. 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 submultiple of L for multiple printing passes.

The distance between the printheads according to the direction Y and possibly along direction X on the one hand, sufficient drying time between deposition of different ink colors and allows on the other hand, to ensure an overlay order same colors same when printing is performed during the return and return of the head printing.

Compared to the known printing system such as shown in Figures 6 and 7, the invention according to this embodiment has the peculiarity to be 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 the heads is discussed below in connection with Figures 8 to 10.

Figure 8 includes parts A, B and C each corresponding to a phase of the kinematics printing a set of bands.

In the positioning mode described in connection with FIG. 8, the detector 12 is fixed, and fixed for example to an axis holding device for translating the printheads 16. On the Figures 8 to 10 four heads have been represented 25, one for each color, cyan marked C, magenta marked M, yellow marked Y and black marked K. The device for holding the axis of translation has not been represented because its geometry is specific to each printer. In addition, it is of an example. The skilled person will find or create a support for fixing the detector knowing that detector must fulfill the functions which are described below.

The detector must be able to detect a mark 51, printed by one of the printheads 25 between the left edge 52 or right edge 53 of the substrate 27 and the start or end of the printed pattern respectively.

In part A of Figure 8, there is shown a first strip marked 1 printed while the printheads 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 Y direction of scanning and perpendicular to the direction X in advance of substrate 27.

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

In a direction Z perpendicular to the plane of the substrate, detector 12 is at a height from to the substrate lower than the height of the lower parts of the print head so that they have the passage. The proximity of the substrate allows better reading accuracy.

The use of marks 51 and detector 12, in relationship with the print kinematics will now explained.

Before printing a marked first strip 1, the mark 51-1 is printed by the cyan head 25. This same cyan head then prints band 1 in the scanning direction indicated by an arrow in the direction Y. Before scanning, the heads 25 are are in the position represented by dotted lines in left part of figure 8 part A. At the end of scanning, the heads 25 are in the position shown in solid lines to the right of the substrate 27.

Then, chronologically, the substrate 27 is stepped forward. The 51-1 mark is found in the detector field 12. Detector 12 detects a deviation possible advance of the substrate relative to the advance nominal, and the calculation means 34, 35 calculate corrections to be made to the charge voltages of cyan head and magenta head drops, so that the modification of the trajectory of the drops compensates the advance difference of the substrate.

In the return movement of the heads, the head 25 magenta, prints the second color on strip 1 and the cyan head 25 prints the second strip then the mark 51-2. At the end of return scanning, the heads 16 are find 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 figure 8.

The detector detects a possible 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 strip are printed by the upstream cyan head. The magenta head 25 prints the second strip with corrections of charge voltage of the drops to take account of the value of the last difference ε _x , the yellow head Y prints the first strip.

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

The cycle thus continues modulo the number of print heads side by side, 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 sweep movement go and in the back sweep movement.

The kinematics would be the same in case print only by forward scan, advance of substrate taking place at the same time as the movement of heads return to the first edge 52.

We notice that the functioning which comes to be described, implicitly assumes that the sum cumulative algebraic of deviations in advance of the substrate, remains weak.

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

Reading brands, determining substrate advance deviation and correction of frames allows at all times to ensure good overlapping bands.

According to a software improvement, we are looking for to guard against an unexpected blocking of the advance of the substrate which is not due to non-functioning substrate unwinding and traction systems otherwise detected.

If the substrate is blocked, the mark printed when printing a current strip and which serves as a position reference for printing the following strip, does not arrive in the field of detector 12. Detector 12 will therefore reuse the brand used for printing the tape current with the same corrections, so that if no blocking or near blocking of the substrate the next strip will print in overlap on the previous strip.

To avoid this possible overlap, the printed pattern of even rank marks is different from that of marks of odd rank. Another case where the recognition of the current brand in relation to the next mark is interesting is where these two marks would appear simultaneously on the detector 12, for example one on an extreme part upstream of the detector and the other on an extreme part downstream with respect to the direction of movement of the substrate. This situation can arise in the event of accumulation difference in advance reaching a positive value or negative of a nominal half advance. In this case, the program will choose the benchmark brand for printing the next strip.

The program in case of blocking detection or quasi blocking may include a trigger of another substrate advance then the triggering of a alert if a blockage is detected again, or at otherwise the immediate triggering of an alarm.

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

If for example, the detector has only one array of detector elements, even patterns and will be distinguished from each other by the number of lines of one compared to the number of lines of the other, the distance 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 elements sensors detecting these lines. If the sensor 12 has sensor elements arranged so matrix, or if sensor 12 is, as it will be described below, mobile in the X direction of the scan, even or odd patterns may occur distinguish, moreover, by variations in the direction scanning for example points for one and traits for each other or different deviations of the same pattern.

Figure 8 was used to describe in the detail the principle of measurement and control of advance of the substrate. In practice, the substrate mark should be placed downstream of the head printing that prints the marks but in a place compatible with its size. So the positioning of the sensor in an area swept by printheads as in Figure 8 would require a very fine mechanical adjustment so that the print head can pass over the sensor during sweeps without risk of hitting it. Through elsewhere, this positioning can create difficulties at the level of the repeatability of the conditions of brand lighting at the sensor, depending on the head is located at the right edge or left edge of the substrate when detecting / measuring the brand. In practice, the printer has under the substrate in the area scanned by the heads printing a printing table that ensures good maintenance of the substrate. The sensor can therefore be fixedly positioned, downstream of the last head but in a place where the substrate is securely held by the printing table. This allows proper operation without constraint demanding on the size of the sensor and its lighting.

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

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

With this difference near the kinematics is the same as that described in relation with figure 8.

When the advance of the substrate is delicate, or when the printing table is not large sufficient, it would be beneficial to use two sensors, mounted on either side of the print head. Each sensor, respectively marked "left" and "right" will detect the mark printed on the edge left (respectively right) of the substrate, when printing the even index scan mark which runs from the right edge to the left edge (respectively odd for scanning the left edge towards the right edge).

This case is that shown in Figure 10. The detector 12 is carried by the mobile mechanical assembly with the print heads which will be called trolley thereafter.

In this figure the case of a printer printing in scanning and scanning return. In this case, the carriage has two detectors, a 12-1 detector which is located upstream printheads during a go scan and a detector 12-2 which is located upstream of the heads when scanning back. To this end the detectors 12-1, 12-2 are located on both sides print heads 25.

Compared to a fixed detector located proximity to one of the edges of the substrate the operation is slightly different.

The 51-1 mark is always printed at the end of scanning. As a result, the odd rank marks are all on the side of the second edge 53 and that the even rank marks are all on the first side 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 from the print heads 25 during the back scan. Corrections of the drops' charges are made and the strip number 2 is printed, then the mark 51-2 near the first edge. After advance of 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 strip 3 and of the mark 51-3 printed at the end of scanning. This solution has the advantage of easier positioning of the detectors, of a position distinction of the even and odd marks. The disadvantage is that an additional detector 12 is required. Switching to switch the input of the means 34, 35 on the detector 12-1 or 12-2 is necessary, and can be carried out at the software level by changing the address for reading the substrate deviation information. ε _x .

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

Figure 11 shows control means 31 according to the invention. In these means 31 of control of printing, the elements having the same function as those shown in Figure 2 have the same number of reference. In relation to the means of control of the impression 26 represented in FIG. 2, the device according to the invention comprises a generator 32 random noise whose output is applied to 3 'calculator for fixing the load voltages drops according to their rank so as to change from randomly charge each drop. This generator delivers a random numeric value according to a pseudo-random number delivery algorithm. A person skilled in the art knows how to manufacture such algorithms. Preferably, the algorithm will be designed to deliver in average for at least three quarters of the values generated for drops with a number of frames greater than one predetermined quantity, a value less than a third of the difference between the nominal voltage to be applied to the charge electrodes for said drop and the nominal voltage to be applied to the charging electrodes for one of the two immediately adjacent drops of the frame. Also preferably, the quotient of number of times the sign of the voltage value additional algebraic is positive, the number of total additional voltages will average on a large number of additional values equal to ½. That reflects the fact that, on average, a drop of rank j will be moved away from its central position corresponding to zero additional random voltage, with the same probability towards higher ranking gout or towards drop of lower rank. For the end drops it will be a deviation outwards or towards the drop closest to the frame.

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

Due to the dispersion of the drops around of the actual position they occupy when they are applies the nominal charging voltage which corresponds to their rank a possible lineage defect such as that materialized, figure 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 straight lines, breaking the linearity of the defect and rendering therefore its perception more difficult.

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

The operation is as follows.

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

Another function of the computer 34 is relating to brand recognition and processing of information transmitted by the sensor 12 to deduce a deviation of the mark from its nominal position. It was quickly reported more top that a simple treatment to determine the value of the substrate advance difference was to count the number of sensor elements between the element of sensor corresponding to the numbered nominal position 0 and the sensor element receiving the mark. This way of doing it implicitly assumes that the thickness of the brand is of the same order of magnitude as the sensor resolution. Under these conditions, the difference is determined by the number of the sensor element detecting the brand, if this element is unique. If the detection of the brand straddles two elements sensors the difference is calculated as a function the number of the nearest sensor element which perceives the mark, increased by an increment making intervene the distance between two sensor elements and the proportions for example of current coming from each of the two sensor elements concerned.

There is shown in Figure 12, on an example of realization, different cases likely to arise present and how they are treated when the sensor resolution is more important than the diameter of the drops. In the example shown figure 12, the mark is composed of several lines, three in the commented example, traced by different drops of a burst for example the drops corresponding to positions 2, 4 and 6 of a burst of nine drops.

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

This barycenter is determined based on sensor elements that see the mark. If like represented in figure 12 part A, the drops are normally positioned, the measurement will be exact. Yes as shown in part B, the drops of rank 5 and 6 are displaced from their nominal position, the error will be lessened. The same will apply if the random voltage generator 32, is not inhibited at when the brand was printed as shown in part C. It goes without saying that it is better to inhibit this generator 32 in order to minimize the error.

In the case of mobile position detectors as commented in connection with figure 10, the measurement position of brands may result of samples taken during the scanning of the print head, the measurement accuracy will be increased, and the influence of noise minimized.

Claims (7)

Method for modifying the arrival position on a substrate (27) of electrically and sequentially charged drops of ink, the drops coming from a print head (25) being charged by electrodes (20) of charge connected to a voltage generator, the trajectories of the drops being subjected to the action of deflecting electrodes (23, 24) deflecting the drops according to the value of their electric charge between N positions defined by their rank j (1≤j ≤N), a first position X ₁ , a last position X _N and N-2 intermediate positions, the N positions defining a frame obtained by a burst of drops in the form of a line segment substantially perpendicular to a direction of a relative movement of the head (25) and of the substrate (27), process characterized in that an algebraic voltage is superimposed on a nominal value of voltage to be applied by means of charge of each drop to direct towards the substrate an additional random whose maximum amplitude is a fraction less than 1 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 (20) for one of the two drops immediately adjacent to the frame. Method according to claim 1, characterized in that the value of the voltage additional random is generated by an algorithm of pseudo random generation, this algorithm generating on average for at least three quarters of the values generated for the drops of a number of frames greater than a predetermined amount, a value less than a third of the difference between the voltage nominal to be applied to the charging electrodes for said drop and the nominal voltage to be applied to charge electrodes (20) for one of the two drops immediately adjacent to the frame. Method according to claim 1, characterized in that the value of the voltage additional random is generated by an algorithm of pseudo random generation, this algorithm generating on average for at least three quarters of the values generated for the drops of a number of frames greater than a predetermined amount, a value less than a third of the average difference between the nominal voltages to be applied to the electrodes of charge for two immediately adjacent drops of the weft Method according to one of claims 1 to 3 applicable to a printer in which the substrate is advanced step by step and printed by strip, characterized in that: we print a current strip and a first mark on the substrate, we advance the substrate for printing the next strip, an algebraic difference is determined between a nominal theoretical position of the mark and the real position, a substrate advance correction is determined for each drop of a burst as being a dynamic translation correction voltage ϕ of the value of the charge voltage to be applied to each of the drops coming from the head (25) to correct the deflection of the drops and compensate for the algebraic deviation of the position of the substrate with respect to its nominal position, applying to each of the drops of the burst directed towards the substrate, in addition to the random voltage, the dynamic translation correction voltage ϕ of calculated substrate position. Method according to claim 4, characterized in that the random voltage is not applied to the electrodes (20) charge of the drops while the mark (51) is printed. Continuous deviated jet printer projecting in a burst drops of rank 1 to N in the burst, the drops of a burst being directed or not towards a printing substrate (27) according to data defining a pattern to be printed, the printer having at least: a print head (25), this head comprising means for splitting into drops of at least one ink jet and an associated electrode (20) for charging drops, deflection means (23,24) of part of the drops towards the printing substrate, printing control means having a means for fixing the charge of the drops to be directed towards the substrate as a function of their ranks in the burst coupled to the electrode for charging the drops, characterized in that the means (31) for controlling the printing comprise a generator (32) of an additional random voltage, coupled to the means (3 ') for fixing the charge of the drops, the means (3') for fixing of the charge of the drops taking into account the value of the random voltage generated by the generator (32) of additional random voltage to modify the charge 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 additional voltage. Continuous jet printer deflected according to the claim 6, characterized in that it comprises in addition to at least one detector (12) of the position of a mark (51), this detector providing a value representative of a difference between a nominal advance and a real advance of the substrate (27) and in that the printing control means (31) include in addition to a voltage correction calculator (35) dynamic translation ϕ of the substrate in advance, this calculator (35) determining for each drop of a salvo according to its rank, a tension of dynamic translation correction ϕ in advance of substrate this correction voltage taking into account a value of difference in advance of the substrate delivered by means (34) coupled to the detector (12) and calculating deviation values from a position nominal, the correction voltage calculator (35) of dynamic translation ϕ of advance of the substrate being coupled to the means (3 ') for fixing the load of the drops, the means of fixing the load of drops taking into account the value of the correction voltage of dynamic translation ϕ of substrate advance generated by the voltage correction calculator (35) dynamic translation ϕ of the substrate in advance for change the charge voltage of each drop in translation correction voltage function dynamic ϕ in advance of the substrate.

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