EP0337148A2 - Method for controlling the inking in a printing press - Google Patents

Abstract

In the control of the ink guiding members after the measurement of measuring fields on a printed sheet, maximum permissible layer thicknesses of the printing inks are often achieved before the intended inking site. …<??>In several process stages, inking sites are calculated which approximate the intended inking site as nearly as possible, and at which the maximum permissible layer thicknesses are not exceeded. If none of the calculated inking sites is located within a tolerance range around the intended inking site, proposals are passed on to the printer concerning the next procedure. …<??>The method is suitable for different printing presses, especially for three-colour printing. …<IMAGE>…

Description

The invention relates to methods for color control of a printing press, wherein measuring fields are optically scanned on the printed sheets printed by the printing press, the actual color location obtained for each measuring field with the aid of the scanning being compared with a predetermined target color location, and furthermore ink guiding members of the printing machine can be controlled in the sense of a reduction in color deviations.

In known methods for color control, the color guide elements are set in accordance with the measurement results without the limit values for the layer thicknesses or densities being observed in detail. However, it is also known to end the readjustment when limit values are reached, so that inadequate color rendering is accepted in favor of the maximum permissible layer thicknesses not being exceeded.

The layer thicknesses are ultimately controlled in the printing press. In known systems, however, inputs and outputs are usually made in density values, which are related to the values for the layer thicknesses through the Tollemaar function. Since in the method according to the invention Calculation, output and / or input based on density values or layer thicknesses, the term layer thickness / density is used in the following.

The object of the invention is to increase the print quality in an economical manner, in particular to allow the most accurate possible approximation to a target color location given predetermined maximum permissible layer thicknesses / densities.

A method according to the invention is characterized in that, using the actual color location and the target color location, the layer thicknesses / densities of printing inks required to achieve the target color location are calculated such that when the target color location or a color location is reached within a predetermined tolerance -Area around the target color location, the calculated layer thicknesses / densities are used to adjust the ink guide elements of the printing press, that a failure to reach the target color location or the tolerance range around the target color location is indicated, and that an adjustment of the ink guide elements and a subsequent print and / or a further calculation of layer thicknesses / densities takes place only after manual entry.

In addition to a largely automatic control, this method enables manual intervention if a weighing up between different quality parameters is required. Although the method is primarily intended for multi-color printing, it can also be used for single-color printing. The process according to the invention can also be carried out in a closed control loop by continuous scanning of printed sheets and correspondingly frequent readjustment of the ink guide members.

A further development of the method provides that values for expanding the tolerance range and / or for increasing one or more layer thicknesses / densities are output if the target color location or the tolerance range is not reached by the target color location. This output can also take the form of a proposal, to which the printer only has to enter one of several predefined answers. After the output, for example, an input can be queried as to whether the proposed extension of the tolerance range or whether the proposed increase of at least one of the layer thicknesses / densities should be carried out.

A further method according to the invention enables the color locus which is possible taking into account the maximum permissible layer thicknesses / densities to be largely approximated to the target color locus in that, in the event that the target color locus cannot be achieved within maximum permissible layer thicknesses / densities, on one a first color location (E1), for which one of the printing inks has its maximum permissible layer thickness / density, that an attempt is made by changing the layer thicknesses / densities of further printing inks, one within the smallest possible To achieve tolerance to the target color location, that when the maximum permissible layer thickness / density of a second printing ink is reached, the layer thickness (s) / density (s) of the remaining printing ink (s) is (are) changed to the smallest possible with the tolerances weighted distance to the target color location is reached, and that the color location thus calculated corresponds appropriate data are given to the ink guide elements of the printing press when the color location reached is within a predetermined tolerance.

A further development of this method provides that the tolerance is specified as a tolerance space, that after the calculation of the first color location, a level comprising the first color location and all color locations which can be achieved by changing the layer thickness / density of the further printing inks is calculated, that it is checked which point of the level requires a minimum tolerance space and this point is defined as the new target color location, that in the event that at the new target color location the layer thickness / density of at least one of the further printing inks is greater than the maximum permissible layer thickness / density, a second color location (E2) is calculated on a straight line connecting the first color locus (E1) and the new target color locus, at which one of the further printing inks reaches the maximum permissible layer thickness / density that a straight line is calculated on which those color locations lie which can be achieved by changing the layer thickness / density of a third printing ink, and that the point of the intersection line is determined as the new target color location at which the necessary tolerance space is minimal.

With this method, too, it can be displayed if no color locus can be determined which lies within the existing tolerance range or tolerance range and for which the layer thicknesses / densities are not greater than the maximum permissible layer thickness / density. With an appropriate input, the printer can allow an increase in the layer thicknesses / densities or an extension of the tolerances.

In the methods according to the invention, the tolerance range or the tolerance space within the color space can have the shape of an ellipsoid, the longest axis of which lies in the direction of the brightness axis. However, other shapes are also conceivable and possible, such as that of a cuboid, the tolerances in the direction of the brightness axis L and the direction of the two color axes a, b are independent of one another. A tolerance space in which the tolerances in the direction of the two color axes depend on one another, but the tolerance in the direction of the brightness axis is independent of these, can be specified by a cylinder whose axis runs parallel to the brightness axis.

The measures listed in the further subclaims enable further advantageous developments and improvements of the invention specified in the main claims.

• Fig. 1 als Blockschaltbild eine Einrichtung zur Durchführung des erfindungsgemäßen Verfahrens,
• Fig. 2 einen Programmablaufplan und
• Fig. 3 bis Fig. 11 Darstellungen verschiedener Schritte des erfindungsgemäßen Verfahrens als Diagramme im Farbraum.

Exemplary embodiments of the invention are shown in the drawing using several figures and are explained in more detail in the following description. It shows:

• 1 is a block diagram of a device for performing the method according to the invention,
• Fig. 2 is a program flow chart and
• 3 to 11 representations of different steps of the method according to the invention as diagrams in the color space.

Identical parts are provided with the same reference symbols in the figures.

1, a printing press 1 is provided in a manner known per se with ink guide elements 2, with the aid of which the ink supply and thus the layer thickness / density can be controlled by control signals. These control signals are supplied to the color guide elements 2 by a color control unit 3, which generates the control signals on the basis of color control data, which are stored in one An input / output device 4 can be generated in connection with a measured value processing unit 5. An actual value feedback to the input / output unit 4 is provided by the ink guide elements 2.

A print control strip 7 with a plurality of color measurement fields is located on a print sheet 6 printed by the printing press. These are scanned by a measuring head 8, which is part of a color measuring device with a spectrometer 9. An electronic measuring head control unit 10 controls the position of the measuring head 8, whereby the actual position of the measuring head 8 can be forwarded to the measuring head control unit 10 and further to the measured value processing unit 5. Measured values are passed from spectrometer 9 to measured value processing unit 5.

With the device shown in FIG. 1, it is possible to control or regulate the layer thicknesses / densities in the sense of an optimization of the color rendering according to the inventive method explained in more detail below. In the case of corrections which make it necessary to weigh various quality parameters, the printer is made 4 suggestions via the input / output device, which he can follow or change by making appropriate entries. According to the program flow chart according to FIG. 2, the measured values of the spectrometer, ie the actual values, are queried at 21. At 22, the target values, the tolerances and the respective layer thickness / density Smax are taken from a memory. The differences between the measured values and the target values are formed by a subtraction 23, from which the layer thickness / density changes deltaSs at 24 are calculated. In program part 25, the existing layer thicknesses / densities Sist are calculated from the actual values. Adding Sist and deltaSs at 26 results in the setpoints for the layer thicknesses / densities Sneu. At 20, these are compared with the maximum values for the layer thicknesses / densities.

Depending on the result of this comparison, the program is branched at 27. If the target layer thicknesses / densities Sneu are not greater than the maximum permissible layer thicknesses / densities Smax, the data corresponding to the target layer thicknesses / densities Sneu are supplied to the color control unit 3 (FIG. 1) and the ink guide elements 2 of the printing press 1 are controlled accordingly.

If, however, the target layer thicknesses / densities Sneu are greater than the maximum permissible layer thicknesses / densities Smax, an optimal point in the color space is calculated at 28, at which the layer thicknesses / densities are smaller than the permissible layer thicknesses / densities Smax. At 29 a decision is then made as to whether this color locus lies within the tolerances. If this is the case, the output of the color control data and its transmission to the color control unit 3 (FIG. 1) is again initiated in the program part 30.

However, if the color location is outside the tolerances, alternative suggestions for tolerances and layer thicknesses / densities are calculated in program part 31 and displayed to the printer at 32.

In program part 33 the printer input is queried. If this entry includes an expansion of the tolerances, the calculated color control data are output and transmitted after branching 34. If the printer has no new tolerances, but has entered larger values for the permissible layer thicknesses / densities, a new color location is calculated after branching 35 at 28. If this is within the tolerances, the color control data is output. If this is not the case, alternative suggestions are again calculated so that the printer has the opportunity to enter new tolerances or new permissible layer thicknesses / densities. The printer declares If there are no new tolerances or new permissible layer thicknesses / densities, then after passing through branches 34 and 35, the printer is asked at 36 whether printing should take place outside the permissible tolerances. If the printer affirms this by a corresponding input, corresponding color control data are output to the color control unit 3 after branching 37. However, if the printer does not allow printing outside the tolerances, this leads to an abort at 38.

Before individual steps of the method according to the invention are discussed with reference to FIGS. 4 to 11, the terms tolerance and tolerance range are explained with the aid of FIG. 3 using the example of a tolerance ellipsoid. In the origin of the coordinate system L, a, b representing the color space according to FIG. 3, the target value for the color to be printed is assumed. All actual values that lie within an ellipsoid with the semi-axes deltaL, deltaA and deltaB are tolerated. Since people tend to accept deviations in brightness as color casts, deltaL is larger than deltaA or deltaB. The tolerance values on which the respective print job is based depend on the print quality required in each case and can be entered by the printer. Since there are deviations between the measured and calculated data on the one hand and the printed image on the other hand due to measurement inaccuracies, linearization and machine fluctuations, a "calculation tolerance" which is smaller than the tolerance specified by the printer is preferably used for the calculations within the method according to the invention . This is shown in Fig. 3 as a dashed ellipsoid, which is similar to the ellipsoids due to the tolerances entered. In the following, however, the terms tolerance and tolerance ellipsoid are used for the sake of simplicity.

A first method step is explained with reference to FIG. 4. The color locus Eist measured with the spectrometer 9 and the target color locus Esoll are connected by a straight line, the equation of which is also shown in FIG. 4. Here deltaL, deltaa and deltab mean the differences between the coordinates of the target color location and the actual color location; deltaS1s, deltaS2s and deltaS3s the differences in the layer thicknesses / densities of the printing inks and A a constant. Lambda is the independent variable that identifies the location on the straight line.

As already explained in connection with the program flow chart according to FIG. 2, no further calculation steps are necessary if the layer thicknesses / densities for the color location Esoll lie within the maximum permissible layer thicknesses / densities. If this is not the case, however, the printing ink is determined which, with the same change in all three printing colors, is the first to reach the maximum permissible layer thickness / density in proportion to the required change in deltaSs. This printing ink is referred to below as F1.

After the definition of the printing ink F1, the color location E1 is calculated on the connecting line between Eist and Esoll at which the printing ink F1 has reached the maximum permissible layer thickness / density. The following step is explained with reference to FIG. 5. A level of those color locations is calculated which can be achieved by changing the two remaining printing colors. This level is hatched in FIG. 5 and in other figures.

In the method step shown in FIG. 6, the plane is cut with the tolerance ellipsoid. This determines the amount of color locations that match the both printing inks F2 and F3 can be achieved and are within the tolerance range. If the plane intersects the tolerance ellipsoids, a real ellipse is created. If the plane avoids the ellipsoids, the ellipse is imaginary.

The next step in the process is to determine the point for which the necessary tolerance space becomes minimal, here the center of the ellipse. This becomes the new setpoint Es'. In the case of an imaginary ellipse, It ′ is the point on the plane that would be the center point of the sectional ellipse in the case of an enlarged ellipsoid. It is always real, even with an imaginary ellipse.

After the determination of the new setpoint Es 'shown in FIG. 7, new setpoints for the layer thicknesses / densities are calculated and output, provided there is a real ellipse and the layer thicknesses / densities for Es' are not greater than Smax. If the latter is not the case, then starting from the color location E1, the printing ink is determined which, with the same change in the two printing inks F2 and F3 in proportion to their required change deltaSs, is the first to reach the maximum permissible layer thickness / density. The point E2 lies on the connecting line between E1 and Es' (Fig. 8).

5, a plane of the color locations that could be achieved by changing the printing inks F2 and F3 was calculated, in a further method step a straight line is calculated, which is described by a further change in the printing color F3. This straight line is designated in Fig. 9 with S3 '. By cutting the straight line S3 'with the tolerance ellipse, the amount of those color locations is determined which can be achieved with the third printing ink and which are within the tolerance range. If the straight line avoids the ellipse, see above if the intersections are imaginary, if there is an intersection, they are real.

In a further process step, the point on the straight line is calculated, at which the necessary tolerance space becomes minimal - the center point of the chord formed by the straight line - and is adopted as the new setpoint Es˝. In the case of imaginary intersection points, Es˝ is the point that would result from a magnification of the ellipsoid as the center. It is always real, even at imaginary intersections. The formation of Es˝ is in Fig. 10 for the case that the straight line S3 'does not intersect the tolerance ellipsoids. If the layer thickness / density of the printing ink F3 is greater than the maximum permissible layer thickness / density Smax at Es˝, the color location Es ‴ is determined for which the layer thickness / density corresponds to the maximum permissible layer thickness / density Smax.

• 1. Es* wird als Sollwert zur Regelung verwendet, auch wenn Es* außerhalb des Toleranz-Ellipsoiden liegt.
• 2. Erhöhung der Toleranzen: Berechnen eines größeren Toleranz-Ellipsoiden gleicher Form (d. h. deltaL, deltaA und deltaB stehen im gleichen Verhältnis) derart, daß der Farbort Es* innerhalb der Toleranz liegt.
• 3. Berechnung der notwendigen Änderungen der maximalen Schichtdicken/Dichten derart, daß ein Farbort P auf der Verbindungsgeraden EISt-Esoll innerhalb bzw. am Rande des Toleranz-Ellipsoiden gedruckt werden kann. Dazu werden zunächst der Punkt P (Fig. 11) und die dort notwendigen Schichtdicken/Dichten berechnet. Danach wird geprüft, ob die Schichtdicke/Dichte einer oder zweier Druckfarben unterhalb des maximal zulässigen Wertes liegen. Ist dieses der Fall, so werden der Verfahrensschritt gemäß Fig. 4 und die darauf folgenden durchgeführt, um die beste Annäherung an den Farbort Esoll ohne weitere Erhöhung der maximalen Schichtdicken/Dichten zu erreichen.

Depending on the circumstances, some or all of the process steps explained so far are carried out. In the event that the target color location Esoll cannot be achieved without exceeding the maximum permissible layer thicknesses / densities, one of the color locations Es ′, Es˝ and Es ‴ is calculated as the target color location. This is referred to below as Es *. If Es * lies within the tolerance ellipsoid, It * is determined as the new setpoint for the control. However, if Es * lies outside the tolerance ellipsoid, the printer has the following options:

• 1. It * is used as the setpoint for control, even if It * lies outside the tolerance ellipsoid.
• 2. Increase the tolerances: Calculate a larger tolerance ellipsoid of the same shape (ie deltaL, deltaA and deltaB are in the same ratio) such that the Color locus It * is within tolerance.
• 3. Calculation of the necessary changes in the maximum layer thicknesses / densities such that a color location P can be printed on the connecting line EISt-Esoll within or on the edge of the tolerance ellipsoid. For this purpose, point P (FIG. 11) and the layer thicknesses / densities required there are first calculated. It is then checked whether the layer thickness / density of one or two printing inks is below the maximum permissible value. If this is the case, the method step according to FIG. 4 and the subsequent steps are carried out in order to achieve the best approximation to the color location Esoll without further increasing the maximum layer thicknesses / densities.

Claims (12)

1. Method for color control of a printing press, measuring fields being optically scanned on the printing sheets printed by the printing press, the actual color location obtained for each measuring field with the aid of the scanning being compared with a predetermined target color location, and furthermore ink guiding elements of the printing press in Controlled in order to reduce the color deviations, characterized in that
that the layer thicknesses / densities of printing inks required to achieve the target color location are calculated using the actual color location and the target color location,
that when the target color location or a color location within a predetermined tolerance range around the target color location is reached, the calculated layer thicknesses / densities are used for setting the ink guide elements of the printing press,
- That the target color location or the tolerance range around the target color location is not reached, and - that the ink guide elements are adjusted and then printed and / or a further calculation of layer thicknesses / densities takes place only after manual input. 2. The method according to claim 1, characterized in that
- That if the target color location or the tolerance range is not reached by the target color location, values for expanding the tolerance range and / or for increasing one or more layer thicknesses / densities are output. 3. The method according to claim 1, characterized in
- That an input is queried whether the proposed extension of the tolerance range or whether the proposed increase of at least one of the layer thicknesses / densities should be carried out. 4. Method for color control of a printing press, measuring fields being optically scanned on the printing sheets printed by the printing press, the actual color location obtained for each measuring field with the aid of the scanning being compared with a predetermined target color location and furthermore ink guiding elements of the printing press in Controlled in order to reduce the color deviations, characterized in that
- That in the event that the target color location cannot be reached within the maximum permissible layer thicknesses / densities, a first color location (E1) is calculated on a straight line connecting the actual color location and the target color location, for which one of the printing inks has its maximum permissible layer thickness / density,
- that by changing the layer thicknesses / densities of further printing inks an attempt is made to achieve a color location that is within the smallest possible tolerance of the target color location,
that when the maximum permissible layer thickness / density of a second printing ink is reached, the layer thickness (s) / density (s) of the remaining printing ink (s) is (are) changed until the smallest possible distance, weighted with the tolerances, from the target color location is reached, and
- That the color location thus calculated is given corresponding data to the ink guide elements of the printing press when the color location reached is within a predetermined tolerance. 5. The method according to claim 4, characterized in
- that the tolerance is specified as a tolerance space,
that after the calculation of the first color location, a level comprising the first color location and all color locations which can be achieved by changing the layer thickness / density of the further printing inks is calculated,
- it is checked which point of the plane requires a minimal tolerance space and this point is defined as the new target color location,
- That in the event that at the new target color location the layer thickness / density of at least one of the other printing inks is greater than the maximum permissible layer thickness / density, on a straight line connecting the first color location (E1) and the new target color location, a second Color locus (E2) is calculated at which one of the other printing inks reaches the maximum permissible layer thickness / density,
- That a straight line is calculated on which those color locations lie which can be achieved by changing the layer thickness / density of a third printing ink, and
- That the point on the straight line is determined as the new target color location at which the necessary tolerance space is minimal. 6. The method according to claim 5, characterized in
- That in the event that the layer thickness / density of the third printing ink is above the maximum permissible layer thickness / density at the new target color location, it is checked whether a color location lies on the straight line and within the tolerance for which the layer thickness / density is not greater than the maximum permissible layer thickness / density, if applicable as new target color location is defined. 7. The method according to any one of claims 4 to 6, characterized in that
- That is displayed if no color locus is found which lies within the tolerance range and for which the layer thicknesses / densities is not greater than the maximum permissible layer thickness / density, and
- That an input is queried whether an expansion of the tolerance space or an increase in the layer thicknesses / densities is to be carried out. 8. The method according to claim 7, characterized in that
- That if the maximum permissible layer thickness / density is released, the intersection of the straight line between the actual color location and the target color location with the surface of the tolerance space is calculated and
- Starting from the intersection points, layer thicknesses / densities, which may not yet have reached or exceeded the maximum permissible layer thickness / density, are changed in the sense of minimizing the weighted distance with the tolerances to the target color location. 9. The method according to any one of the preceding claims, characterized in that
- That the tolerance range or tolerance space in the L, a, b color space has the shape of an ellipsoid, the largest axis of which runs parallel to the L axis. 10. The method according to any one of claims 1 to 8, characterized in
- That the tolerance range or tolerance space in the L, a, b color space has the shape of a cylinder, the axis of which runs parallel to the L axis. 11. The method according to any one of claims 1 to 8, characterized in
- That the tolerance range or tolerance space in the L, a, b color space has the shape of a cuboid, the axes of which run parallel to the L, a, b axes. 12. The method according to claim 7, characterized in that
- That the necessary changes in the tolerances or the maximum permissible layer thicknesses / densities are displayed to the printer.

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