EP1106371B1 - Printer with simplified manufacturing and manufacturing method - 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 simplifying the mechanical assembly of the printheads 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 line of height ΔX substantially perpendicular to a direction of relative advance of the print head 25 and the substrate. The print head is formed by the drop generator 16, the charging electrode 20, the deflection electrodes 23, 24 and in general the recuperator 22. This head 25 is generally enclosed in a not shown casing. The deflection movement printed to 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 25 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 25 and the substrate, it can be considered that the substrate has not moved relative to the 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 25 and the substrate is continuous over the largest dimension of the substrate, there will generally be several print 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 at the smallest dimension of the substrate, the printing is done strip by strip, the substrate having an intermittent feed movement in the direction X after each scan. 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 strip of height L or quite often a portion of the height band ΔX _b , ΔX _b being most often a submultiple 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 to achieve at high print speeds. In this case, each inkjet prints a limited portion of the substrate. 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 for each ink. A part of the bit pattern is extracted from the file for each of the 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 strip-cut digital pattern is represented at 1, 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 voltages charge different drops that 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. to the 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 in 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.

It is understood that for printing, especially in color, the necessary superposition of the drops from the different nozzles delivering the different ink colors 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 results in the appearance of light or dark lines when printing by successive sweeps. 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 solution is based on fine mechanical adjustments of the jets by eccentric screws or the position of the printing heads by means of micrometric tables. This solution is both expensive, by the number of mechanical elements 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 drops nearby, 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 Pat. No. 4,604,631 issued to RICOH. 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.

With regard to lineage defects and other possible defects in printing, it was envisaged to use test patterns and to compare a real printed pattern with a reference pattern to deduce from it. Nozzle choices or changes to be made to certain printer setting parameters. The patent application EP 0 589 718 A1 assigned to HEWLETT PACKARD provides for the use of patterns consisting of a succession of lines offset with respect to each other. The user of the printer examines the various printed models and selects a suitable alignment by means of a control panel. The choices are then stored for later use.

A pattern of sight for correcting possible defects of the printer is described in the patent application No. EP 0 863 012 A1 assigned to HEWLETT PACKARD. This pattern of sight allows easy reading for example by a camera so that it can make corrections automatically by comparing the printed pattern to a reference pattern. Finally, in patent application WO 98/43817 issued to JEMTEX INK JET PRINTING LTD., It is intended to use a pattern to perform various parameter corrections. According to the description of this application, the test pattern makes it possible to recognize the different types of errors, that is to say ink drop speed errors, phase errors due to incorrect sequencing of the ink drop. application of load voltage, offset errors in an X direction, offset errors in a Y direction, and angular offset errors. Speed or offset errors in the X direction are corrected by changing the charging voltage of the drops. Phase errors due to incorrect sequencing of the application of the charging voltage are corrected by changing the sequencing of the drop charge pulse. Errors of offset in Y that is to say in the direction of the sweep are compensated by a restructuring of the data sequencing. The same is true for angular errors. For reasons that will be explained later, such a use of a pattern may lead to a good position of the drops on the substrate, but it causes other defects that are essentially color defects, and permanent adjustment difficulties of the printer.

No. 5,481,288 discloses a printer having servocontrols for adjusting the viscosity of the ink, the jet speed, the breaking distance of the jet, as well as the control of the means for charging the drops.

Brief description of the invention

The present invention has the main object of reducing the difficulties of mounting the printheads on a printer, while ensuring a good quality of printing. Good print quality requires good color reproducibility, size of drop impacts resulting from their impacts and their spread on the constant substrate, and a relative position of the drops on the well-defined substrate. It also aims to ensure good reliability and good availability of the printer. It also aims to limit the substrate losses printed during defects. It aims to simplify maintenance operations. Finally, it also aims to ensure good stability of the print quality, that is to say to avoid a drift of this quality.

• la caractéristique colorimétrique des encres,
• la taille des impacts de gouttes résultant de leur impact et étalement sur le substrat,
• et enfin, la position relative des gouttes sur le substrat.

The print quality of a color inkjet printer depends on a large number of parameters, some of which are interdependent: it is possible to define as explained above, three main phenomena that condition the print quality:

• the colorimetric characteristic of the inks,
• the size of the impacts of drops resulting from their impact and spreading on the substrate,
• and finally, the relative position of the drops on the substrate.

The colorimetric characteristic of the ink depends mainly on its composition, namely for the main elements: the concentration of coloring matter, the concentration of solvent, and that of resin. In patent No. FR 2,636,884 assigned to the applicant, there is described a system for measuring and maintaining the viscosity of the ink in order to maintain the jet speed conditions, the pressure being fixed. Viscosity corrections are made by addition of solvent or an ink of higher concentration than the nominal concentration. A variation in temperature can induce a change in viscosity while the composition of the ink is unchanged. Therefore, in a preferred embodiment of the invention described in this patent assigned to the applicant is provided a control and control of the viscosity η of the ink taking into account the temperature of the ink. The viscosity and the temperature T are determined at the same point of the ink, and the more concentrated solvent or ink additions are made as a function of the difference in viscosity Δη with respect to a target viscosity which depends on the measured temperature. With the process described in this patent, the concentration of dye in the ink is precisely maintained. If the temperature of the ink at the print head is also controlled, for example by controlling the ambient temperature, the viscosity of the ink in the nozzle is automatically controlled. Mastering the viscosity and the concentration of dye are necessary conditions to maintain a good colorimetry, and also to maintain a law of variation of the speed of a droplet at the outlet of a printing nozzle according to the pressure which is applied constant.

The impact size of the drops on the substrate depends on the geometry of the nozzles, which are manufactured within tight tolerances and controlled during manufacture, their ejection speed and therefore impact, and the local conditions of the spreading drops on the substrate, namely the evaporation rate of the ink and its surface tension on said substrate, both of which depend on the temperature. For a given substrate and a given ambient temperature, the spreading depends on the physicochemical characteristics of the ink and the drop impact velocity.

The relative position of the drops on the substrate depends on the trajectory of the drops of each jet of the print head, the arrangement of the jets in the print head, as well as the relative position between the print head and the substrate. It has been seen that the drops are electrically charged, and then deflected more or less depending on their charge by deflection electrodes. As a result, the trajectory of the drops depends on their speed and their load. A good load of drops assumes that the drop is separated from the jet at a specific place and at the moment of this separation, the electric pulse defining the charge of the drops has been given. It has been seen above, that for a given viscosity, the speed depends on a pressure applied to the fluid. It is also known that the distance between the nozzle and the droplet formation site of a jet is a function of the amplitude of the oscillations applied for example to a piezoelectric crystal maintaining vibrations in the ink. A good charge of the drops therefore assumes a good control of the phase between the formation of the drops and the moment of charge of the drops, the phase being itself variable with the speed of the drops. Means for individually controlling parameters such as the viscosity of the ink as a function of its temperature, the speed of the drops by action on the pressure in the ink tank, the loading phase of the drops and the length of the jet before breaking it into drops by controlling the voltage of a piezoelectric crystal are individually known from the prior art. However, by lack of knowledge of the dependence of the various parameters with respect to each other for print quality, the printers of the prior art generally do not include control of each of these parameters. Thus, for example, the characteristics of the ink such as the viscosity can be controlled without simultaneously controlling the jet speed, maintaining the viscosity of the ink and a pressure being considered sufficient to ensure a constant speed. drops. This approach is faulted particularly when the nozzle orifice or Ink supply circuit filters are clogged. If the physico-chemical characteristics of the ink are enslaved, it is also important to ensure an ink drop speed and impact on the substrate within a predetermined tolerance. Often, also in the systems of the prior art, the drop positioning accuracy is considered the only factor influencing the print quality. Thus, in the patent application WO 98/43817 already mentioned, the position of the drops is measured on a test pattern and the defects are corrected in several ways. In particular, the defects of trajectory resulting from drop speed out of tolerance are corrected by an action on their electric charge. It has been seen that the speed of the drops influences the trajectory and size of the drop on impact. Print quality is therefore not guaranteed. A charge correction of the drops may eventually put these drops in their nominal trajectory, but their impact diameter has not been corrected, and the dye will be spread over a too large or too small surface thus changing the colorimetry.

The present invention aims at ensuring good print quality and simplifying the mounting of the printer. In a printer according to the invention, the phase of the drops, the length of the jet before breaking it into drops, the speed of the ink jet, the temperature, the viscosity and the composition of the ink are continuously controlled by loops. independent. All these parameters being controlled, then an error on the position of the drops results only from mechanical defects or tolerance margins of the electronic devices. Under these conditions, the printing of a test pattern and its comparison with a reference pattern will allow, by an appropriate modification of the charge of the drops, to modify this trajectory so as to restore its nominal value. The other parameters being controlled, this modification of the droplet load will not compensate for values outside the tolerance of the jet velocity or the composition of the ink, or the size of the drop on impact, and consequently the print quality will be preserved.

The method according to the invention aims to eliminate lineage problems without any impact on the printing speed.

• à des tensions nominales applicables aux électrodes de charge des gouttes en fonction du rang j de la position nominale du point imprimé par la goutte, ou encore
• au nombre de positions suivant le signal de détection de bord,

on applique les corrections déterminées aux valeurs nominales correspondantes.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. It also reduces the number of mechanical adjustment devices. According to the invention, before the initial start of the printer is carried out a step of electrical adjustments of the machine. This initial setting is made when the servo loops of the parameters are active, and will allow to adjust for example the position of the frame by correcting what will be called a translation deviation static and will also allow to adjust the height of the weft by modifying what we will call a dilation gap. For this, we will print with the machine a pattern representing a known pattern. We will compare it to a reference pattern representing this same pattern so as to derive values from the difference between the actual position of points of the printed pattern and the nominal position of the corresponding points on the reference pattern. The differences between corresponding points are memorized. Then, during the successive printing phases of patterns defined by a set D of digital data, the corrections to be applied are calculated from the stored differences:

• at nominal voltages applicable to the charging electrodes of the drops as a function of the rank j of the nominal position of the dot printed by the drop, or
• at the number of positions according to the edge detection signal,

the determined corrections are applied to the corresponding nominal values.

In one embodiment, the value of the static translation difference is corrected on the one hand and the value of the expansion gap on the other hand. To correct the value of the translation deviation static will be added to each of the drops out of the printer nozzles, an algebraic electric charge to compensate for this translation error. The expansion error is due to a large or small difference in the load distributed between the most deviated drops and the least deviated drops forming the frame corresponding to a burst. The frame is too wide, when the gap between the high point of the frame and the low point of the frame is too large. This means that the drop corresponding to the highest point is not deviated enough while the drop corresponding to the lowest point is too deviated. To correct, it will therefore increase the load of the drop corresponding to the highest point and reduce the load of the drop corresponding to the lowest point. Equalization applied to the intermediate drops of the burst will correct the load applied to the intermediate drops as a function of the corrections made to the loads of the extreme drops of the weft. If on the other hand the frame is too narrow, which means that the gap between the highest point and the lowest point of a salvo is too narrow then we will reduce the load of the drop corresponding to the highest point of so that this drop is less deviated and we will increase the load of the drop corresponding to the lowest point so that this drop is further deviated. Equalization of the charge correction values applied to the intermediate drops between the last and the first drop will, as in the case of the wide frame, make it possible to refine the setting of the frame.

It is also possible to take into account the actual deviation of each drop from its nominal position to calculate the position correction applied to each drop.

• une viscosité de l'encre en fonction de sa température pour qu'elle garde une valeur dans une tolérance prédéterminée par addition de solvant ou d'encre plus concentrée en matières colorantes,
• une vitesse de jet par action sur une valeur de pression d'alimentation de l'encre,
• une distance de brisure du jet en gouttes par action sur un paramètre ajustable permettant de conserver une distance de brisure prédéterminée,
• une différence de phase entre des instants d'application d'impulsions électriques de charges des gouttes et le signal périodique appliqué au générateur de gouttes qui détermine la formation des gouttes par action sur un circuit de temporisation,
  et en ce que dans une phase préalable à des phases d'impression :
  • a) on imprime une mire,
  • b) on compare ladite mire imprimée à une mire de référence pour en déduire pour ladite tête d'impression et pour un nombre entier a de positions, a étant supérieur ou égal à 2 et inférieur ou égal à N, un écart algébrique ΔX_i entre une position réelle observée et une position nominale correspondante, ceci pour chacune des a positions choisies, avec i allant de 1 à a,
  • c) on détermine un écart de translation statique θ comme étant l'écart entre le barycentre des a positions réelles observées et le barycentre des a positions nominales correspondantes,
  • d) on détermine pour chacune des a positions de gouttes observées, un écart de position δ_i entre la position réelle de chaque goutte, corrigée de l'écart de translation, et la position nominale de ladite goutte,
  • e) on mémorise la valeur θ de l'écart de translation statique et les valeurs δ_i des écarts de position de goutte par rapport à leurs positions nominales respectives,
  • ensuite dans chaque phase d'impression d'un motif défini par un ensemble D de données numériques
  • on détermine pour chaque goutte, une valeur de correction de tension nominale conduisant à une valeur corrigée à appliquer aux moyens de charge des gouttes dirigées vers le substrat, ce calcul prenant en compte les valeurs mémorisées d'écart de translation et de position, des données extraites de l'ensemble D des données numériques définissant le motif à imprimer, et le rang j, j compris entre 1 et N de la position nominale d'impression visée.

In summary, the invention relates to a method for compensating for mechanical defects of an ink jet printer by adjusting the position of arrival on a substrate, of ink drops electrically electrically adjustable by electrodes. of charge, the drops from a print head, the trajectories of the drops being modifiable, by deflection electrodes, between N 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 substantially parallel to a direction X of the substrate, characterized in that permanently during the operation of the printer on enslaved:

• a viscosity of the ink as a function of its temperature so that it keeps a value within a predetermined tolerance by addition of solvent or ink more concentrated in coloring matter,
• a jet speed per action on an ink supply pressure value,
• a breaking distance of the jet into drops by action on an adjustable parameter making it possible to maintain a predetermined breaking distance,
• a phase difference between instants of application of electric pulses of drops charges and the periodic signal applied to the drop generator which determines the formation of the drops by action on a delay circuit,
  and in that in a phase prior to printing phases:
  • (a) print a pattern,
  • b) comparing said printed pattern with a reference pattern to deduce for said print head and for an integer a of positions, a being greater than or equal to 2 and less than or equal to N, an algebraic deviation ΔX _i between an observed actual position and a corresponding nominal position, this for each of a selected positions, with i ranging from 1 to a,
  • c) determining a static translation error θ as the difference between the centroid of the a actual observed positions and the center of gravity of a corresponding nominal positions,
  • d) determining for each of the a observed droplet positions, a position deviation δ _i between the real position of each droplet corrected by the translation error, and the nominal position of said drop,
  • e) storing the value θ of the static translation deviation and the values δ _i of the drop position deviations with respect to their respective nominal positions,
  • then in each print phase a pattern defined by a set D of digital data
  • for each drop, a nominal voltage correction value leading to a corrected value to be applied to the charge means of the drops directed towards the substrate is determined, this calculation taking into account the stored values of translation and position deviation, of the data. taken from the set D of the digital data defining the pattern to be printed, and the rank j, j between 1 and N of the target print position.

Preferably, and as described above, the integer number a of real positions observed is equal to 2, these positions being the first and the last position. One can also if one wants to get a finer correction measure the deviation of each of the N actual positions of drops from their nominal position. Naturally, if the printer has several nozzles distributed over one or more heads, the same operation will be applied for each of the nozzles. This does not mean that it will print a test pattern per nozzle, a single pattern that may be suitable for the operation of the jets of each of the nozzles. In particular, if the different nozzles correspond to jets of different colors, it is understood that it will be easy to constitute a single pattern for adjusting all the jets of all the nozzles.

Since, according to the invention, the overlap between consecutive drops is minimized, a lineage defect may remain, 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, a voltage of noise will be applied in superposition to the voltage applied to the charge electrodes of the drops. The average amplitude of this noise voltage will be a function of the rank j of the drop in the burst. Preferably, the maximum amplitude of the additional noise voltage will be equal to a fraction less than 1 of the difference between the nominal voltage to be applied to the drop of rank j and the nominal voltage to be applied to the drop of rank j + 1 or to the drop of rank j-1, that is to say to one of the two spatially adjacent drops of the drop of rank j. 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 feeds a high voltage amplifier coupled to the charging electrodes of the drops.

In this way, the position of the drops will be slightly noisy and the regular defect of dark or white lineage will not appear or will appear less.

The aspects of the invention which have been described so far make it possible to correct the lineage errors, ie the position errors of the different frames in successive bands or of adjacent jets and the errors of width of the different frames.

According to another aspect of the invention which will now be discussed, the position errors of the frames in a direction Y perpendicular to the printing direction of the frames may also be corrected.

Most current printers are equipped with a detection detector of the left edge or the right edge of the substrate. The beginning of the printing is triggered according to a difference between the instantaneous digital value present on a counter of position of the head relative to the substrate and the value of this same counter at the time of the detection of an edge of the substrate and also according to the data D relating to the printing of the substrate contained in the print data memory. The difference in number of positions is such that when this number of positions has been counted after detecting an edge of the substrate, the print head is at the location programmed by the data D to print the beginning of the tape. . It is possible that an offset in the Y direction is observed between the nominal position of a band and its actual position. According to one aspect of the invention, this defect, called the dynamic shift difference, can be corrected as follows. A comparison of the position of the first frame with respect to the nominal position of this first frame will define an algebraic deviation of the first frame from its nominal position. A dynamic offset correction α will be defined as the number of positions representing this deviation. A corresponding correction will be memorized and then used during successive frame prints to shift by this number of positions the printing of each frame of the band, the positions being counted with origin of the substrate edge detected at each scan. The printing of the frames is shifted, if the head goes from left to right relative to the substrate, to change the number of positions between the detection of the left edge and the beginning of the band. The print is shifted if the head goes from right to left relative to the substrate, to change the value of a counter representing the value of the position at which each frame of the strip is printed. The position of the last frame is in particular shifted by the same number of positions as the first frame and this should be taken into account when returning the print head. The correction thus takes into account the fact that the band is printed by a movement of the head from left to right and / or a return movement of the head from right to left.

It may be noted that the lineage corrections which have been applied up to now according to first aspects of the invention are not effective. only to the extent that the substrate is properly placed. This is not always the case. The absorption of the ink by the substrate, friction and other factors can lead to deviations in the actual advance of the substrate compared to the nominal advance and therefore lineages. According to a variant of the method of the invention, one goes for each band, print on the substrate by means of one of the printing heads a mark. This mark may be a simple line oriented in the Y direction. After advancing the substrate but before printing the next strip, the first mark will be positioned next to a substrate advance sensor. The optical sensor measures a distance between the first printed mark and a nominal position that should have this mark, if the substrate advanced its nominal advance. The actual distance used to define a real advance of the _actual substrate .DELTA.X that we will be able to compare the nominal value .DELTA.X _name. A difference between the actual feedrate and the nominal feedrate will be automatically corrected by a variation of the load voltage applied to the droplet loading means. This correction will be applied for all heads involved in writing the current tape. As has been seen, the various corrections according to the invention, which have just been defined, can be applied independently of each other in isolation. In particular if one of the corrections is not necessary given the quality of the printer, it will not be applied. They can be applied also in combination with each other different combination modes that result from their number.

• une tête d'impression, cette tête comportant des moyens de fractionnement en gouttes d'au moins un jet d'encre et une électrode associée de charge des gouttes, des moyens de déviation d'une partie des gouttes vers le substrat d'impression,
• des moyens d'asservissement de la viscosité de l'encre en fonction de sa température,
• des moyens d'asservissement de la vitesse des jets d'encre issus de la tête d'impression,
• des moyens d'asservissement de la distance de brisure du jet en goutte,
• des moyens d'asservissement de la phase entre des instants d'application d'impulsions de charge des gouttes et des instants d'application d'impulsions de formation des gouttes,
• 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 leurs rangs dans la salve, couplés à l'électrode de charge des gouttes,

caractérisée en ce que les moyens de contrôle de l'impression comportent :

• des moyens de mémorisation d'écarts entre une position nominale de points imprimés par la tête d'impression et une position réelle de ces points,
• des moyens de correction de translation statique θ,
• des moyens de correction de dilatation, les moyens de correction recevant des données en provenance des moyens de mémorisation d'écart et étant couplés aux moyens de calcul des tensions de charge des gouttes.

The invention also relates to a deviated continuous jet printer bursting drops of rank 1 to N in the burst, the drops of a burst being directed or not to a printing substrate according to 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 associated electrode for charging the drops, means for deflecting a portion of the drops to the printing substrate,
• means for controlling the viscosity of the ink as a function of its temperature,
• means for controlling the speed of the ink jets issuing from the print head,
• means for controlling the breaking distance of the jet in drop,
• means for controlling the phase between instants of application of drop charge pulses and instants of application of drop formation pulses,
• printing control means having a means for fixing the charge of the drops to be directed towards the substrate according to their ranks in the burst, coupled to the charge electrode of the drops,

characterized in that the print control means comprises:

• means for storing deviations between a nominal position of dots printed by the print head and a real position of these points,
• static translation correction means θ,
• expansion correction means, the correction means receiving data from the gap storage means and being coupled to the means for calculating the charge voltages of the drops.

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 ce que sont les erreurs de translation, de dilatation et leurs corrections ;
• la figure 4 est un schéma destiné à expliquer ce que sont les erreurs de décalage dynamique dans la direction du balayage et leurs corrections ;
• la figure 5 est un schéma destiné à expliquer le mode de correction des écarts d'avance du substrat ;
• les figures 6 et 7 sont des schémas illustrant les éléments matériels d'une imprimante ;
• la figure 8 est un schéma représentant les moyens de calcul d'une imprimante fonctionnant selon le procédé de l'invention.
• la figure 9 représente très schématiquement des asservissements d'une tête d'impression.

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 of the calculation means necessary for the operation of the means represented in FIG. 1;
• Fig. 3 is a diagram for explaining translation, expansion and correction errors;
• Fig. 4 is a diagram for explaining dynamic offset errors in the scanning direction and their corrections;
• FIG. 5 is a diagram intended to explain the mode of correction of the advance gaps of the substrate;
• Figures 6 and 7 are diagrams illustrating the hardware elements of a printer;
• Figure 8 is a diagram showing the calculation means of a printer operating according to the method of the invention.
• FIG. 9 very schematically shows servocontrols of a print head.

Figure 3 is intended to explain what are the differences in translation and expansion. For this, we have shown in different configurations on the plane of the substrate materialized by XY axes, 9 different positions and shapes 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, the frame of nine drops is represented in accordance with its nominal position defined by a line of symmetry axis MM '. This axis line passes perpendicular to the middle of the frame represented in A, so in nominal position. In part B, the frame has been represented as printed. It is seen on this frame that on the one hand, it is shifted which is materialized by the position of its central axis NN 'offset with respect to the position of the axis MM' and that, on the other hand, it is dilated , that is to say that the distance between the drop 1 and the drop 9 as shown in B is greater than the distance between the drop 1 and the drop 9 as shown in A.

In Figure 3 and for purposes of simplification, the N drops 1 to 9 of Part B are shown as equidistant. Obviously in the In reality, it may be otherwise, and the drops may have variable distances to each other. It follows that the position of the central drop materialized by the axis NN 'will not always be representative of the translation gap.

In the most general case, the best estimate that we can have of the offset in translation will be represented by the distance between the centroid of the drops in nominal positions as represented at A and the centroid of the drops in real positions as represented in FIG. B. The calculation of the position of these centers of gravity will be made by giving the drops the same coefficient, for example the coefficient 1.

It will also be possible for simplification purposes to simply compare the barycentres of an integer with drops on the one hand, of the frame represented on A and on the other hand, of the frame represented in B, these drops being in corresponding nominal positions. For example, if we take in drops 4, 5 and 7, we will take for the calculation of the center of gravity in B the same drops 4, 5 and 7.

Experience has shown that, as a general rule, it is sufficient to take the positions of the first and last drops, in the cases of FIG. 3, the drops 1 and 9. The offset in translation will then be equal to the difference between the points equidistant from the drops 1 and 9 as represented at A and drops 1 and 9 as shown in B. The static translation correction has the effect of reducing the axis NN 'of the frame as printed at position MM '. In this position, the axes MM 'and NN' are merged.

This static translation correction will be obtained by a modification of the load applied to each of the drops 1 to 9. The calculation of the magnitude of this change in the load applied to the drops 1 to 9 will be performed taking into account data acquired on machines of the same type. This data may include tables representing the displacement of the drop of rank j according to the correction made to the nominal load of this drop.

After the static translation correction, the frame composed of the nine drops is, as shown in C, in a correct position relative to the axis MM 'but its height in the case shown in Figure 3 in C, is too large by compared to the nominal height as shown in Figure 3 A. This frame could also be too small. The expansion correction will consist in calculating the load modification to be made to the already corrected nominal load of the static translation error to return these drops to their nominal position.

In the case shown in FIG. 3, in which a uniform expansion of all the drops constituting the weft has been shown, it is conceivable that the correction of the position of the extreme drop 9 will require a larger change in charge than, for example, In the case shown in FIG. 3, the position of the central droplet 5 will not have to undergo any expansion corrections. In the most general case, it will be necessary to make a calculating the modification of the load to be applied to each of the drops to bring it back from its already corrected position by applying the static translation correction to its nominal position.

As in the case of the static translation error correction, this calculation of the expansion gap correction will be performed taking into account data acquired on the previous machines.

Figure 4 is intended to explain what is the dynamic shift error and its correction. In part A of Figure 4, there is shown in solid lines, the nominal position of a strip. This band is represented in the form of a rectangle whose height, the height of a frame made by a salvo comprising the N drops and its width is equal to the distance between the first and the last frame of the band. In the scanning direction, the print position of a field is determined by registering the position of the print head, for example, with respect to a position determination rule.

This rule has graduations, for example, magnetic or optical co-operating with means of the print head or a support of this head so that the position of the print head is permanently known to the control unit. of the printer. Knowing the position of an edge of the printing substrate and the head with respect to this rule, it is therefore possible to determine the position of the head relative to the substrate. The nominal position of the first frame is obtained by comparing the position of the head relative to the substrate to the predetermined position of this first frame relative to the edge of the substrate, depending on the data defining the pattern. This data will determine for example that the first frame must be at 2000 positions marked on the ruler, the edge of the substrate. When a position counter has been incremented by 2000 the printing of the first frame will be triggered. Suppose that the difference ΔY, between the actual position of the dotted band and its nominal position, is shifted to the right as shown in A, for example twenty positions.

According to the invention, the printing of each frame of the number α of positions necessary to change the frames from their actual positions to their nominal positions will be modified. In particular the first frame which materializes the beginning of the band will be brought back from its real position to its nominal position. In the numerical example chosen above, the printing of the first frame will start when the position counter has counted (2000 - 20) or 1980 positions after the detection of the left edge. All the frames of the band will be shifted by the same number of positions. In the case where the printing would also be done during the return movement of the print head, the printing of the last frame must start for example according to the digital data at the position 100,000, the value 100,000 will be replaced by the value 99,980 to account for the offset offset of twenty positions of the actual band. This correction will lead to a tape position as shown in FIG. 4, part B. It can be seen that the Dynamic offset correction applied to each of the frames will bring the position of the actual band back to the position of the nominal band.

Another possible aspect of the present invention 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 advancing the substrate but before printing the next strip, this mark will appear in position B in FIG. 5. In order to materialize the error of spacing ε _x in advance of the substrate, the position is also shown. in C of a fictitious mark representing the nominal position that should have had the mark present in B 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 B makes it possible to determine the difference ε _x between the nominal position marked in C and the actual position marked in B. This difference in the advance of the substrate will be compensated according to this aspect of the invention. invention by a modification of the charge of the drops printed during this band.

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 drag 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 submultiple of L for printing in several 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.

The synchronization between the jet of the ink drops and the scanning position of the printing heads 25 with respect to the substrate 27 is obtained by means of an optical detector (not represented by the width edge). The web edge detector is mounted on the print head or on a holder of this head to detect each of the two edges. This detector emits a detection signal from each edge of the width. The detection signal of a reference width edge, for example the left edge, is then used to trigger a position counter for synchronizing the position of each print head with the print data for that position, contained in the print memory. The position encoder may be in known manner an optical or magnetic rule mounted on the mechanical axis of the scanning guide.

Compared with the known printing system as represented in FIGS. 6 and 7, the invention may have the particularity of being equipped with one or more detectors 12 (FIG. 8) for detecting the actual advance of the substrate. There is a left substrate advance detector if printing is performed from left to right and a second right substrate advance sensor if printing is also performed from right to left. Also and in a known manner, a single advance detector of the substrate can be mounted on the print head or on a support of this head to detect the advance of the substrate when printing is carried out from left to right or right to the left.

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. 8 represents control means 31 according to the invention. In these control means 31, the elements having the same function as those shown in FIG. 2 bear the same reference number. With respect to the control means 26 shown in FIG. 2, the device according to the invention may comprise one or more of the following means.

The device according to the invention may comprise the detector 12 of difference between the actual advance of the substrate and its nominal advance, a calculator 34 of positional deviation of the substrate and a dynamic translational corrector 35 for correcting the charge of the drops so as to compensate for the difference found by the computer 34. The elements, detectors 12, calculator 34 position deviation and dynamic translation corrector 35 are connected in series with each other and the dynamic translation corrections φ calculated by the corrector 35 are applied to the computer 3 'drops of the charging voltages.

The means for controlling the position and the deflection of the jets may also comprise a detector 14 deviating from the actual position of dots printed by a jet with respect to the nominal position of dots printed by said jet. The differences in the position of the dots printed by the jet are introduced firstly into a static translation corrector 17, into a dilation corrector 18 and finally into a dynamic shift corrector 19.

Finally, the ink drop charge control means may comprise a random noise generator 32 whose output is applied to the computer 3 'of the drop charge voltages so as to randomly modify the charge of each drop. 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 distance value ε _x in advance of the substrate 27. This difference is introduced into the corrector 35 of dynamic translation which will calculate corrections to be applied to the computer 3 'drops of charging voltages to correct this dynamic translation φ.

The difference calculator 14 on the position of the dots printed by each jet compares the position of the dots printed on a test pattern with respect to the position of the corresponding dots on a reference pattern. This calculation of the deviations can be performed automatically, for example by scanning the printed pattern and using the stored reference pattern. Using the calculated deviations, the static translation corrector 17 will calculate in one of the ways indicated above the displacement of the center of gravity of the points for which the position deviation has been measured. Likewise, the dilation corrector 18 will calculate the difference between a printed point and the corresponding nominal point.

Depending on this difference, a charge correction value applied to each of the ink drops will be calculated. The corrections θ _j calculated by the static translation correction calculator 17 and δ _ij by the dilation corrector 18 are both applied to the computer 3 'of the drop charge voltages. The computer 3 'of the drop charge voltage will calculate the algebraic sum of the voltages to be applied to the drop charge electrode as a function, on the one hand, of the nominal voltage resulting from the description of the frame coming from the memory 2, and on the other hand, of the correction θ _j static translation from the static translation corrector 17, the expansion correction δ _ij from the dilation correction corrector 18, the dynamic translation correction φ calculated by the computer 35 and finally, as a function of the value outputted by the random noise generator 32. The dynamic shift correction α calculated by the dynamic shift corrector 19 will it be applied to the sequencer 4 drops loading. In this way, the charge of the drops as provided by the computer of the charge voltages of the drops 3 'will be applied in coincidence with a position number of the position counter smaller or larger than the nominal number according to the algebraic value α of the dynamic shift, the positions being counted from the substrate edge.

Figure 9 is intended to very succinctly represent a print head 25 and the various servocontrols associated with it. Each of the servos that will be briefly discussed below is in itself known. However, the inventors do not know of printers that simultaneously present all of these servocontrols on the same printer. The inventors believe that this absence is due to a poor appreciation of the interference of the various parameters to be controlled to arrive at a good print quality as described above. The printer according to the invention has a control of the viscosity 61 as a function of the temperature, represented as the other servocontrols by a feedback loop in output of the head 25 returning to the input an error value. The viscosity correction that may be necessary is carried out by addition of solvent or by the addition of more concentrated dye ink so as to maintain a constant level of dyestuff. A jet speed servo 62 is obtained by acting on a value of the supply pressure of the ink. The breaking distance of the jet is maintained by a servocontrol 63 which acts on an adjustable parameter making it possible to maintain a predetermined breaking distance. It may be for example the supply voltage of a piezoelectric crystal causing vibrations in the ink. Finally, the printer according to the invention is equipped with a circuit 64 for controlling the phase between the instants of application of electric impulses for charging the drops and the instants for applying drop formation pulses. This phase can be set by action on a delay circuit.

Thus, in a printer according to the invention, the viscosity being kept constant for a reference temperature, the action on the pressure to modify the speed leads to truly known results so that this speed can be kept constant at a predetermined value. . Thus, the size of the drop impacts is very constant. The dye concentration is also kept constant, the color of each drop is a constant. Finally, the breaking distance of the jets and the phase being controlled, it is sure that each of the drops receives an electric charge which is function a supply voltage of the charging electrodes. In a printer in which all the print parameters are controlled as described above, the errors in positioning of the ink drops with respect to their nominal position only come from mechanical tolerances on the positioning of the printing heads and possibly on the diameter of nozzles for ejecting the ink. This is the reason why on such a printer it is possible to correct the positioning by action on the control electronics of the printer as described above.

In order to have good reproduction of the print quality, the ink ejection speed should be kept within limits around a set value. Obtaining this setpoint may correspond to an ink supply pressure that is variable depending on the print head, because of the tolerances on the ink outlet nozzles or the ink nozzle. environment of the printing machine. This is why a print head of a printer according to the invention will preferably comprise a memory in which the value of the speed setpoint for each jet, corresponding to a standard supply pressure, will be stored to obtain the speed of rotation. setpoint. This memory has been represented symbolically at 65 in FIG. 9. The speed control program will therefore provide a reading of this setpoint jet speed in the memory of the print head. In this way, during operation of the printer, the pressure being regulated in a value range close to the standard pressure, significant jet velocity defects, ie out of mechanical tolerance of the nozzles and specific to a single jet can be detected.

In the same way, the reference values of the control signal of the piezoelectric transducer are predetermined during manufacture and stored in the memory. Malfunctions specific to a single transducer may be detected.

Also, when replacing a printhead with another printhead, all the nominal operating parameters. being stored in memory, the program will not usually need to be changed.

Claims (9)

1. Process for compensation of mechanical defects in an ink jet printer by adjusting the arrival position on a substrate of electrically charged ink droplets in an adjustable manner using charge electrodes, the droplets originating from a print head and the trajectories of the droplets being modifiable by deviation electrodes between N positions, between a first position X₁ and a last position X_N and with N-2 intermediate positions, the N positions defining a frame in the form of a straight line segment approximately parallel to an X direction of the substrate, the process being
   characterized in that the following parameters are servocontrolled at all times during operation of the printer:

   - an ink viscosity value that remains within a predetermined tolerance as a function of its temperature, by adding solvent or ink with a higher concentration of colouring agents,

   - a jet velocity by acting on the ink supply pressure,

   - a distance at which the jet is broken into droplets by acting on an adjustable parameter to maintain a predetermined breaking distance,

   - a phase difference between instants at which electrical droplet charge pulses are applied and instants at which droplet formation pulses are applied by action on a timer circuit,

   and in that the following steps take place during a phase prior to the print phases:

   a) a pattern is printed,

   b) the said printed pattern is compared with a reference pattern to deduce an algebraic difference ΔXi between a real observed position and a corresponding nominal position, for the said print head and for an integer number a of positions, where a is greater than or equal to 2 and less than or equal to N, for each of the chosen a position, where i varies from 1 to a,

   c) a static translation error θ is determined as being the difference between centre of gravity of the a actual observed positions and the centre of gravity of the corresponding a nominal positions,

   d) for each of the a observed droplet positions, a position error δr is observed between the real position of each droplet corrected by the translation error, and the nominal position of each droplet,

   e) the value θ of the static translation error and the values δ_I of the droplet position errors from their initial nominal positions, are memorized,

   - then, in each phase in which a pattern is printed defined by a set D of numeric data,

   - a correction value to the nominal voltage is determined for each droplet of row j to give a corrected value to be applied to the means of charging droplets directed towards the substrate, this calculation taking account of memorized values of static translation and position errors, the data extracted from the set D of numeric data defining the pattern to be printed, and row j, where j is between 1 and N, of the nominal target print position.
2. Process according to claim 1, in which the integer number a of observed real positions is equal to 2, these positions being the first and last positions.
3. Process according to claim 1, in which the integer number a is equal to N.
4. Process according to one of the claims 1 to 3, applicable to a printer provided with means of detecting the position of the print head along the direction of movement of this head with respect to the substrate and means of detecting the edge of the substrate, characterized in that a dynamic offset ΔY between the nominal position of a printed band and its real position is measured during the phase prior to the print phases, this offset is memorized, and the print positions of the print head are offset during the print phases to compensate for the measured dynamic offset.
5. Process according to one of the claims 1 to 4, applicable to a printer in which the substrate is advanced step by step and printed by band,
   characterized in that:

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

   - the substrate is advanced so that the next band can be printed,

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

   - for each droplet in a burst, a substrate advance correction is determined as being a dynamic translation correction voltage ψ to be applied to the value of the charge voltage to be applied to each of the droplets output from the head (25) to correct the deviation of the droplets and to compensate for the algebraic difference between the position of the substrate and its nominal position,

   - the calculated dynamic translation correction voltage ψ to correct the substrate position is applied to each of the droplets in the burst directed towards the substrate.
6. Process according to one of the claims 1 to 5, characterized in that a random additional algebraic voltage is superposed on the nominal voltage to be applied to the means of charging each droplet to be directed towards the substrate, the maximum amplitude of this additional voltage being a fraction less than one of the difference between the nominal voltage to be applied to the charge electrodes 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.
7. Continuous deviated jet printer projecting droplets in rows 1 to N in bursts, the droplets in a burst possibly but not necessarily being directed towards a print substrate (27) depending on data defining a pattern to be printed, the printer being equipped with at least:

   - a print head (25), this head comprising means of separating at least one ink jet into droplets, and an associated droplet charge electrode (20), means (23, 24) of deviating a proportion of the droplets to the print substrate,

   - means (61) of servocontrolling the ink viscosity,

   - means (62) of servocontrolling the velocity of ink jets output from the print head,

   - means (63) of servocontrolling the distance at which the jet is broken into droplets,

   - means (64) of servocontrolling the phase difference between the times at which droplet charge pulses are applied and times at which droplet formation pulses are applied,

   - means of controlling the printout consisting of means of injecting the charge of droplets to be aimed at the substrate (27) as a function of their rows in the burst, coupled to the droplet charge electrode,

   
   characterized in that the print control means (31) comprise:

   - means (14) of memorizing errors between a nominal position of dots printed by the print head and a real position of these dots,

   - means (17) of correcting the static translation θ,

   - dynamic expansion correction means (18), the correction means (17, 18) receiving data originating from error storage means (14) and being coupled to means (3') of calculating droplet charge voltages.
8. Printer according to claim 7, characterized in that the print control means (31) also comprise means (19) of correcting the dynamic offset, these means receiving data from difference storage means (14) and being coupled to droplet charge calculation means (3').
9. Printer according to either of claims 7 and 8, characterized in that the print head (25) comprises a memory (65).

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