EP0257570A2 - Method to align print nozzles in an ink jet printer head of an ink jet printer and electronic circuit for carrying out this method - Google Patents

Abstract

For aligning the print nozzles of an ink jet printer head (DK), a defined line pattern (SCI) is printed on the recording carrier (P) for each print nozzle (D1 to D32) during a bidirectional printer advance, and is then scanned by a sensor (AB) during a scanning run. The scanned values (AW) which represent the "actual" position of the ink droplets are compared in the central control unit (ZS) of the ink jet printer with the "nominal" position determined by the drive pulses in the print column pattern (DSR), and a so-called droplet offset (VS) is determined. The droplet offset (VS), that is to say the difference determined, is stored for each individual print nozzle (D1 to D32) separately for both directions of printing (L,R), and in normal printing operation, a correction in accordance with the offset (VS) stored in the memory (VPS) is made for each print nozzle (D1 to D32). <IMAGE>

Description

The invention relates to a method for matching the writing nozzles of an ink print head according to the preamble of patent claim 1.

An essential component of an ink writing device is the ink print head. This contains a large number of writing nozzles, each of which is assigned an individually controllable converter element. The writing nozzles are connected to an ink supply system and end on the side facing a recording medium in outlet openings which are arranged there in one or more rows. To control the transducer elements and thus to eject individual ink droplets, character information is taken from a character generator and transferred to a pixel memory, in which the character to be displayed is electronically depicted, so to speak. Depending on the relative movement of the print head relative to the recording medium and in accordance with the information in the pixel memory, the converter elements are controlled and the characters are built up in a grid pattern on the recording medium via printer electronics. It is built up in a character matrix, the horizontally running grid columns of which are referred to as traces, the vertically running columns of which are referred to as print columns. For a printhead that e.g. B. has 32 writing nozzles, such a character matrix can have 32 tracks and 96 printing columns.

For the use of such ink writing devices it is of great importance that the ink droplets hit the recording medium as precisely as possible at the intended points of the character matrix. This becomes clear using the example of a vertical line formed by ink droplets from different writing nozzles, in which a large number of ink droplets always have to be applied partially overlapping one another in the same pressure column. Deviations in the points of impact lead to clearly recognizable shifts within the vertical line. This problem ultimately arises with any single-droplet ink printhead. The reasons for this are of various types. For example, component tolerances of the drive elements or changes in temperature of the ink result in a change in the ejection speed and / or the droplet size. Furthermore, even with the greatest care, minor inaccuracies in the manufacture of the writing nozzles can cause a change in the point of impact of ejected ink droplets. As a result, these influences lead to a deviation in the point of impact of the individual droplets, which is referred to as droplet offset and which leads to a deterioration in the typeface shown.

It is therefore already known (DE-AS 25 48 691) to set the droplet parameters by comparing the control electronics for the drive elements. However, such a comparison requires considerable effort. For example, the adjustment must be carried out individually for each writing nozzle, with additional aids such as stroboscopic measurement of the droplet ejection being necessary. Another known measure (DE-OS 31 29 015) for an adjustment of the writing nozzles is to change the effectiveness of the drive elements by changing the degree of polarization of a piezo transducer or its effective surface. However, such a comparison is irreversible. Besides, is In this case, too, an additional stroboscopic measurement of the output of the individual ink droplets is required.

The use of these known measures, which are based in each case on an individual comparison of the individual writing nozzles, for high-resolution ink writing devices with a very large number of writing nozzles leads to an expense which is no longer acceptable.

DE-OS 34 38 675 proposes a method for avoiding alignment errors when printing with multiple nozzles. This is to prevent registration errors from occurring when printing an image, which are due to the positional deviation between the color or ink output by the respective nozzles with respect to the direction of the carriage carrying the nozzles. For this purpose, the positional difference between the color droplets from the respective nozzles with respect to the direction of the carriage movement is determined, in order to determine the times for the start of the printing process with the ink from the respective nozzles, based on the determined difference.

The object of the invention is to provide a way of comparing the writing nozzles of an ink print head without the need for additional aids, in which the number of writing nozzles to be compared does not play a decisive role and with which not only the tolerances of the components and the transducer or the Parameters changing ink properties but also the parameters acting on the entire writing device, such as carriage drive, distance between recording medium and writing nozzles, inclined position of the printhead etc. are also recorded.

This object is achieved with the measures specified in the characterizing part of patent claim 1. Advantageous te refinements and developments of the invention are characterized in the subclaims.

• Fig. 1 ein Beispiel für die geometrische Schreibdüsenan­ordnung eines Tintendruckkopfes sowie ein Beispiel für eine dafür geeignete Zeichenmatrix,
• Fig. 2 ein Beispiel für den Aufbau einer aus vertikal un­tereinander angeordneten Tintentröpfchen bestehenden Li­nie,
• Fig. 3 ein Beispiel für die Erzeugung eines Strichmusters und dessen Abtastung gemäß der Erfindung und
• Fig. 4 eine vorteilhafte Anordnung zur Durchführung der Erfindung.

The invention is explained in more detail below with reference to the drawings. There shows

• 1 shows an example of the geometric writing nozzle arrangement of an ink print head and an example of a suitable character matrix,
• 2 shows an example of the construction of a line consisting of vertically arranged ink droplets,
• Fig. 3 shows an example of the generation of a line pattern and its scanning according to the invention and
• Fig. 4 shows an advantageous arrangement for carrying out the invention.

1 shows on the right the front part of a print head DK, at which a total of 32 writing nozzles D1 to D32 end. The outlet openings of the writing nozzles D1 to D32 are arranged here in four vertical columns, each with eight writing nozzles. With the print head DK shown here, characters and / or patterns can be displayed in a grid pattern in the character matrix ZM shown on the left. The character matrix ZM also has 32 lines or tracks, corresponding to the 32 writing nozzles D1 to D32. 96 printing columns are provided in the horizontal direction. So that a very high resolution can be achieved for the representation of characters or patterns, so that, for. B. Text characters shown correspond in their quality to the characters printed with type printing units.

Fig. 2 shows an example of a row of ink droplets applied in the vertical direction. The illustration on the left shows the ideal case where all ink dots in the row are exactly in print column 1 of the characters matrix ZM ,, that is, they lie exactly below each other This case is ideal because all the writing nozzles, in the example D1 to D12, are involved in the construction of the row of dots shown and practically always more or less large tolerances, as mentioned at the beginning, cause a more or less large offset of the ink dots. In this case one speaks of droplet offset. The droplet row shown on the right in FIG. 2 shows a practical example of such a droplet offset. The writing nozzles D1 to D12 are activated taking into account the movement of the print head DK relative to a recording medium and the geometrical arrangement of the outlet openings of the writing nozzles D1 to D12 in the print head DK at precisely defined desired times, and ink droplets are also expelled at these times. However, only the ink droplets ejected by the writing nozzles D1, D5, D9 and D11 and thus the ink dots of the tracks 1, 5, 9 and 11 are located in the print column 1 of the character matrix ZM. All ink dots of the other tracks have a more or less large droplet offset. It can be seen that the typeface is considerably disturbed.

The method according to the invention with which this problem is solved is described with reference to FIG. 3. For the sake of clarity, only a section of a writing line is shown here. 3 shows the print column grid DSR in the first line, it being assumed here that the character matrix ZM comprises 96 print columns, the spacing of which is 1/960 ʺ in each case. This results in a character width of 1/10 ʺ. The DRM pressure pulse pattern is shown in the second line. Assuming that one and the same writing nozzle can eject a droplet of ink at a distance of 1/120 jeweils, the pressure pulses are at a distance of 1/120 ʺ. The third line shows the selected print times on a record carrier brought vertical lines of DRSC, as they would ideally occur. It has been assumed here that droplets are ejected from one and the same nozzle not with every pressure pulse but, for example, with the first, eighth, sixteenth, etc. pressure pulse. Finally, in the fifth line, the line patterns are shown in their actual position SCI on the recording medium.

The method according to the invention is described below with reference to FIGS. 3 and 4. It consists of the following steps. First, the line pattern SCI according to line 5 of FIG. 3 is printed out in a printer run for each individual nozzle D1 to D32. This line pattern SCI consists of a series of vertical rows of dots formed by ink droplets from one and the same writing nozzle. The writing nozzle in question is controlled for droplet ejection in a printing pulse raster DRM, which is made available by a clock disk T connected to the drive motor M for the movement of the printing head DK. The pressure column grid DSR is also synchronized with the pressure key grid DRM. If, as was assumed in the example shown, it is assumed that the pressure column grid DSR has a division of 1/960 ʺ and that a writing nozzle can be acted upon with a pressure pulse at a grid spacing of 1/120 ʺ, so in the section shown the pressure pulses are each delivered in the manner shown in line 2 of FIG. 3 within a character matrix ZM comprising 96 printing raster columns. However, droplets are not ejected with every pressure pulse in order to display the line pattern. In the example, as line 3 of FIG. 3 shows, droplet ejection is effected with the first, the eighth, the sixteenth, etc. pressure pulse. The line pattern is printed on the record carrier P by multiple passes and correspondingly finely divided feed of the record carrier.

Line 4 shows the line pattern SCS without considering a droplet offset VS. Due to the tolerances described at the beginning of components, of mechanical parts of the writing device, etc., however, there are deviations between the positions determined by the pressure key pattern DRM and the actual impact points of the ink droplets. Line 5 shows the actually printed line patterns SCI. The distances can be called the difference between a target and an actual position. These distances can already have different values for one and the same writing nozzle for forward and backward running (in so-called bidirectional printing). The values continue to differ from writing nozzle to writing nozzle. If the printer advance described with reference to FIG. 3 is carried out for each nozzle in the forward and backward direction, such a line pattern is also available for each nozzle for both writing directions.

The measures provided for the adjustment of the individual writing nozzles provide for a scanning run. The line pattern is scanned with an optical sensor AB arranged on the print head DK during a pass through the print head DK, again separately for both writing directions R, L. At the start of a scanning run, the scanner AB, which works in the print column grid DSR, is synchronized with the print cycle grid DRM in order to obtain a common fixed point for determining the droplet offset VS. The sampled values AW determined by the scanner AB during a scan run represent the actual position, the values determined by the push-button matrix DRM represent the desired position. The values AW determined by the scanner are practically a sequence of "o" and "1" values These values enable a central control ZS to offset VS between the target and the actual position for each individual writing nozzle D1 to D32 exactly up to the subdivision of the printing column grid To calculate DSR.

Line 5 of FIG. 3 shows the actual printing position SCI of each vertical line printed during the printer advance. To determine a meaningful value via the droplet offset VS, the center of the ink dots forming the vertical line is assumed. In the example according to FIG. 3, this is designated with print position P1 (line 4) and print position P2 (line 5). In the example, it has been assumed that such an ink dot extends over eight printing raster columns, i. that is, in order to calculate the exact droplet offset VS in this example, the vertical line recognized as a dash by the scanner AB must be added to the value of four printing grid columns. The value VS thus calculated represents the exact droplet offset, which then z. B. is entered in tabular form for each writing nozzle separately in the forward and backward directions in a memory of the central control ZS of the ink writing device. The division of the information that continuously flows into the central control ZS due to the interaction between the pressure column grid DSR and scanner AB, which consists of a sequence of "0" or "1", is divided into blocks there and assigned to the individual writing nozzles D1 to D32.

The stored values of the droplet offset VS for each individual writing nozzle D1 to D32 separately in the forward and reverse writing directions are now available to the pixel memory BS of the ink writing device for a printing or recording operation carried out during normal operation of the ink writing device. When the pixels are entered in the character matrix ZM, a correction is made for each writing nozzle D1 to D32 in accordance with that in the memory VSP stored value for the droplet offset VS. When the characters are processed, predistortion takes place in accordance with the printing direction R, L and the measured droplet offset VS.

Line 6 of FIG. 3 shows the scanning areas ABB and the transition areas UEB between two “actual” positions of the ink droplets, the scanning beginning at the time TA and ending at the time TE. The transition area UEB, which lies between the scanning areas of two writing nozzles, is not included in the calculation of the nozzle specification / droplet offset.

An exemplary embodiment for carrying out the method according to the invention is shown in FIG. 4. The exemplary embodiment contains the essential parts of an ink writing device necessary for understanding the invention. Specifically, it is a matter of the print head DK, which is attached to a writing carriage (not shown here), which in turn is moved back and forth in the line direction in front of a recording medium P by means of a drive motor M. The printing of characters or the representation of patterns takes place bidirectionally, ie during the movement of the print head DK to the right R and to the left L. The ink writing device also has a central control ZS, a character generator ZG, a pixel memory BS and printer electronics DE. The central control ZS is provided with the push-button raster DRM via a clock disk T assigned to the drive motor M. The character generator ZG contains, for example, 256 different characters which can be addressed in accordance with the input data ED. The character information for a specific character to be printed then arrives in the pixel memory BS, where the character is electronically mapped, so to speak. From there, the character information is transferred to the printer electronics DE. This is done under the control of the pressure column grid DSR. The printer control unit DE is also supplied with the print key raster DRM by the central control ZS, so that the control of the individual writing nozzles takes place at the right time depending on the position of the print head DK. A barcode is also printed in the manner described. For this purpose, corresponding character information can be contained in the character generator ZG, it being ensured that the printing of this character information takes place in each case through a single writing nozzle.

An optical sensor AB, hereinafter called a scanner, is assigned to the print head DK. This is moved bidirectionally together with the print head DK. The bar code recorded during the printer run is scanned and the scanning values AW are transmitted to the central control ZS as the "actual" position SCI. There, the pressure cycle raster DRM emitted by the clock disk T is also available as the “target” position SCS. The value for the droplet offset VS is determined therefrom in a comparison circuit V. This value is entered for each individual writing nozzle in a memory VSP, which is preferably designed as RAM. With each subsequent printing process, be it in the creation of texts or in the recording of graphic patterns, the value for the droplet offset VS contained in the memory VSP for the individual writing nozzles is fed to the pixel memory BS as the distortion value VW. There, during the preparation of the characters, predistortion takes place in accordance with the printing direction and the measured droplet offset VS.

The invention was explained on the basis of an example in which a writing line in the forward direction and a writing line in the backward direction are covered with a defined bar code and then scanned for the individual writing nozzles. Within the scope of the invention it is also possible to involve several writing nozzles in the printing of the bar code during a single pass. So it is z. B. possible to print three vertical lines within a character width, so that z. B. for a printhead with 32 writing nozzles, the bar code for all 32 writing nozzles is recorded in a writing line and the corresponding values for the droplet offset of each of the individual writing nozzles can then be determined and stored in a single scan.

Claims (11)

1. Method for comparing the writing nozzles (D1 to D32) of an ink print head (DK) in ink writing devices, which is moved bidirectionally in front of a recording medium (P) by means of a drive and from the individually controllable writing nozzles (D1 to D32) corresponding to that of a character generator (ZG ) individual droplets are ejected from the data taken at defined printing times during its movement, which build up the characters or patterns in a mosaic-like manner on the recording medium (P) in a character matrix provided for the character or pattern representation,
characterized, a) that a bidirectional printer advance takes place, during which a defined line pattern (SCI) is printed onto the recording medium (P) for each writing nozzle (D1 to D32) separately for each writing direction, b) that a scanning run then takes place during which the line pattern (SCI) is scanned by an optical sensor (AB) arranged on the print head (DK) and the scanning values (AW) in the grid of the printing columns of the character matrix (ZM) as "actual" Position (SCI) is transferred to a central control (ZS) of the ink writing device, c) that in the central control (ZS) a comparison of the "actual" positions (SCI) with the "target" positions (SCS) determined by the corresponding control pulses is carried out, the deviations between the two positions (SCI, SCS ) indicate the values for the so-called droplet offset (VS) in the grid of the printing columns, d) that the values for the droplet offset (VS) for each writing nozzle (D1 to D32) are stored separately for both writing directions (L, R), e) and that each time a writing nozzle (D1 to D32) is activated in normal writing or printing operation, the value of the droplet offset (VS) determined for the writing nozzle in question (D1 to D32) is taken into account depending on the writing direction (L, R). 2. The method according to claim 1,
characterized,
that the control of the individual writing nozzles (D1 to D32) takes place during the printer advance for one writing line by control pulses according to a predetermined information and that the line pattern (SCI) consisting of vertical bars is printed on the recording medium (P). 3. The method according to claim 1 or 2,
characterized,
that the sensor (AB) is synchronized to the pressure cycle grid (DRM) at the start of the scanning run. 4. The method according to claim 1 or claim 3,
characterized,
that the scanning run consists of an area (ABB) for evaluating the right (R) and left rotation (L), as well as a transition area (UEB), which lies between the scanning areas (ABB) of two nozzles (D1 to D32) and the not included in the calculation of the droplet offset (VS). 5. The method according to claim 1,
characterized,
that the values (AW) determined by the sensor (AB) during the scanning run represent a sequence of logical signals "0" and "1" and that these signals ("0", "1") are used to determine the droplet offset (VS) . 6. The method according to claim 5,
characterized,
that the information from the sensor (AB) consisting of the signals ("0", "1") is divided into blocks in the central controller (ZS) and that by counting the zeros or ones of the blocks and adding the droplet center of gravity, the droplet offset ( VS) is calculated separately for the writing directions (L, R). 7. The method according to claim 4,
characterized,
that for evaluating the scanning of the left-hand rotation (L) from the start (TA) or from the end of the transition area (UEB), the first arriving "1" is expected and the subsequent "1" are counted until a "0" occurs and that a quarter of the number obtained is determined and stored and that this result is added to the current meter reading. 8. The method according to claim 4, characterized in that for evaluating the scanning of the clockwise rotation (R) from the end of the previous scanning range (ABB) until the occurrence of the first "1" the number of incoming "0" are counted and stored, then the incoming "1" is counted and a quarter of it is determined and saved and this result is added to the previously stored value. 9. Circuit arrangement for performing the method according to claim 1, characterized in that the ink writing device in the central control (ZS) has a comparison circuit (VS), which consists of the "actual" position (SCI) and the "target" position ( SCS) of the ink droplets the value for the droplet offset (VS), determined separately for the writing directions (L, R). 10. Circuit arrangement according to claim 9,
characterized,
that the central control (ZS) has an additional memory (VSP) for recording the values for the droplet offset (VS), and that this droplet offset (VS), separately for the writing directions (L, R), as a distortion value (VW) a pixel memory (BS) is passed on, in which a pre-distortion takes place in accordance with the printing direction (R, L) and the determined droplet offset (VS) during the preparation of the characters. 11. Circuit arrangement according to claim 10,
characterized,
that the memory (VPS) is designed as a random access memory (RAM).

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