EP0600335A1 - Method for zonal open/closed loop control of ink supply in a printing machine - Google Patents

Abstract

The invention relates to a method and a device for zonal control / regulation of the ink flow in a printing press with several printing units. As long as an actual color locus Eist is within the tolerance range ETol around the target color locus Esoll, usually no regulation is carried out. If the target color location Esoll (Co, ho, Lo) is close to the achromatic axis (O, O, L), there is a risk that certain actual color locations within the tolerance range will show a color change that is no longer tolerable to the eye. As a result, waste is printed. To avoid the aforementioned color change, the admissibility of an actual color locus Eist is linked to the condition that the actual color locus Eist is in a range around the target color locus, in addition to the condition that it lies within the tolerance range ETol around the target color locus Esoll Esoll lies in which no color change occurs. Such an area free of color change can be described by a maximum permissible change ± α of the hue angle h in relation to the hue angle ho of the desired color location Esoll. <IMAGE>

Description

The invention relates to a method and a device for zonal control of the ink guidance in a printing press with a plurality of printing units according to the preamble of claims 1 and 13.

The control of the color guidance in the running printing process is the most important possibility to influence the coloring and thus the image impression. The aim of the color control is to achieve the best possible color match between an ok sheet and a printed product created in the production run.

In this context, the control of the color guidance according to colorimetric variables is to be regarded as a significant improvement, since a regulation in the sense of a comparison of target and actual color locations is adapted in a good approximation to the color perception of the human eye.

Spectral measurements of the emissions from color measuring fields, the mathematical conversion of these measured values into colorimetric quantities and furthermore into control data for adjusting the color guide elements of a printing press have already become known. In EP-PS 02 28 347, in addition to an appropriately equipped printing machine and a measuring device for such a printing machine, a method for controlling the ink flow of the printing machine is also specified. The spectral remissions of certain measuring fields are measured for the color balance between the ok sheet and the sheet created in printing / production. The corresponding color coordinates are calculated from the measured values. A possible color difference is determined by comparing the actual color location with the corresponding target color location. The color difference is reflected in changes in the layer thicknesses converted individual printing inks. The calculated changes in the layer thicknesses of the individual printing inks are forwarded to the ink guide elements of the printing press, so that the color difference between the actual color location and the target color location is minimal.

Although the colorimetric control is optimally matched to the color perception of the human eye, the quality of the color guide is subject to certain restrictions. These restrictions are related to the technical implementation of a control process:
A comparison between the actual color location and the target color location preferably takes place only if the actual color location lies outside a predetermined tolerance range around the target color location. On the one hand, this avoids constant rules that cause instabilities. On the other hand, the fact is taken into account that each measuring and control process can only be sensibly carried out within certain error limits.

The color tolerance E _Tol around the target color location is selected in the color guide control in a printing press in such a way that the target color location lies in the middle of the tolerance range and the eye usually does not perceive any disturbing color changes in the printed image within the tolerance. It may well be that an actual color location with a larger red cast is perceived as acceptable for a slightly reddish target color location. However, if the actual color locus has a slight green tint with the same color difference, the image cannot be accepted. Today's definition of the tolerance range either allows a color cast in green, blue and yellow or the tolerance must be chosen unnecessarily small. It can then happen that, for example, the green cast disturbs the visual sensation, ie in extreme cases it becomes waste printed, although the actual color location is within the color tolerance E _Tol around the target color location.

The present invention is based on the object of specifying a method and a device which rule out an intolerable color cast in the printed image.

The object is achieved in that an actual color locus can only be tolerated if it is within the range around the target color locus in which no color change occurs (permissible range) and the color difference is smaller than E _Tol .

According to an advantageous development of the method according to the invention, it is provided that the measuring range selected per color zone either lies in the subject of the printed product or else in a print control strip.

In an advantageous development of the method according to the invention, it is proposed that the selected area be in the vicinity of the achromatic axis (0,0, L). The selected area is therefore a gray field in which all printing-relevant colors are represented. The method according to the invention is advantageously further developed by the following configuration: The permissible range "no color change" satisfies the condition h₀ ± α, where h₀ denotes the hue angle h of the desired color locus E _Soll and α the maximum permissible change in the hue angle h. As long as the actual color location is within this predetermined angular range, there is no danger that a color change will occur in the printed image.

• alle Punkte von E_Tol' liegen innerhalb einer vorgegebenen Toleranz E_Tol um den Soll-Farbort,
• innerhalb von E_Tol' erfolgt kein Farbumschlag,
• E_Tol' wird bei gegebener Toleranzraumform maximal.

The choice of a permissible range in the form of an angle sector is difficult to implement mathematically for subsequent colorimetric calculations. Therefore, according to an advantageous development of the invention Method proposed that a color tolerance E _Tol 'around the target color locus E _{target is} specified within the permissible range "no color change". This color tolerance is a tolerance space that can take the form of a sphere of an ellipsoid or a cuboid, for example. Further geometric spaces are conceivable. This new color tolerance E _Tol 'fulfills the following conditions:

• all points of E _Tol 'lie within a predetermined tolerance E _Tol around the target color location,
• there is no color change within E _Tol ',
• E _Tol 'becomes maximum for a given tolerance space shape.

Further advantageous refinements of the method according to the invention take into account different method steps, depending on the chroma C₀ of the target color locus. Is the chroma C₀ of the target color location large, d. That is, the specified color tolerance is always within the permissible range h₀ ± α, so there is no risk of a color change and every actual color location that is within the color tolerance around the target color location is treated by the control as a good color location.

As soon as the colourfulness C₀ of the target color locus satisfies the condition C _min ≦ C₀ <C _max , there is a risk of a color change if certain actual color loci are accepted, since these are still within the color tolerance, but no longer within the permissible range . In this case, C _min characterizes the accuracy limit with which any measurements or adjustment processes can be carried out on the ink guide elements of the printing press. In order to ensure in this case that no color change occurs, the target color location is replaced by a replacement target color location, which enables a maximum color tolerance E _Tol '. This replacement target color location is still on the connecting straight line between the achromatic point and target color location, but is moving further away Achromatic point away; E _Tol 'is smaller than E _Tol but larger than with a tolerance around E _Soll without color change.

A third case to be treated separately occurs when the chroma C₀ of the target color locus is less than the minimum possible chroma C _min . As already mentioned above, this minimum chroma C _{min characterizes} the lower limit for the measuring accuracy when determining the color locations or for the setting accuracy of the ink metering elements in the individual printing units. It makes little sense here to place the condition on the tolerable actual color locations that they must be within the permissible range, since a color change can no longer be prevented within the tolerances (e.g. machine fluctuations are too great). In addition, small fluctuations in the target value must not lead to large fluctuations in the replacement target color location. The coordinates of the replacement target color location are:

${\text{((E}}_{\text{Tol}}\text{+ C₀) / (1 + sinα), h₀, L₀).}$

Die Erfindung wird anhand der nachfolgenden Zeichnungen näher erläutert. Es zeigt:

Fig. 1

ein Blockschaltbild einer Ausführungsform der erfindungsgemäßen Vorrichtung,

Fig. 2

eine Darstellung des zulässigen Bereiches im Farbraum und

Fig. 3

eine Darstellung des erfindungsgemäßen Verfahrens im Farbraum.

The color tolerance is calculated

${\text{E}}_{\text{Tol}}{\text{'= (E}}_{\text{Tol}}\text{+ C₀) * (sinα / (1 + sinα))}$

The invention is illustrated by the following drawings. It shows:

Fig. 1

2 shows a block diagram of an embodiment of the device according to the invention,

Fig. 2

a representation of the permissible range in the color space and

Fig. 3

a representation of the inventive method in the color space.

Fig. 1 shows a block diagram of an embodiment of the device according to the invention. The printed product 2 created in the printing press is scanned by a photoelectric detector 5 at certain image points 4a, 4b. Either these image points 4a, 4b lie in the print control strip 4, or they are image-relevant points. The positioning of the detector 5 takes place via a computing / regulating device 6 based on input data that are supplied by the input / output device 7. Depending on the specification of the input / output device 7, the computing / control device 6 positions the detector 5 at the selected measuring points 4a, 4b of the printed product created in the printing press 1. The measuring points 4a, 4b are selected either manually by the operating personnel or automatically, taking into account predetermined data. In particular, the measuring / regulating device 6 usually selects the measuring points 4a, 4b per color zone in such a way that their color locations in the color space lie near the achromatic axis, since color fluctuations are usually perceived as particularly disturbing. The actual color location E _{actual of} the respective measuring field 4a, 4b is determined in the computing / control device 6 and compared with a predetermined target value E _target . According to the invention, depending on the position of the actual color location in the color space, both the target color location is shifted and the predetermined tolerance E _{Toll is} newly defined.

The influencing of the coloring in the measuring fields 4a, 4b of the printed product 2 takes place via corresponding changes in layer thickness in the individual printing units, ie the ink guide elements 9 of the printing press 1 are adjusted so that a measured actual color location with a predetermined target color location of the corresponding measuring point 4a , 4b matches. A corresponding method is adequately described in EP 03 24 718 A1.

The calculation of the desired positions of the ink guide elements 9 of the printing press 1 is carried out in the ink control unit 8. The respective actual position of the ink guide elements 9 in the printing units of the printing press 1 is sent to the ink control unit 8 via potentiometer feedback.

In principle, the device according to the invention or the method according to the invention can be used “in-line” or “off-line”. The basic structure of a device with which an "in-line" measurement, i. H. A direct acquisition of measured values in selected measuring points 4a, 4b of the printed product 2 in the printing press 1 can be described for example in DE 24 16 009 A1.

The method according to the invention is designed such that a color change within the color tolerance E _Tol around the target color location E _{target is} excluded.

2, the permissible range within which an acceptable actual color locus E _{actual can} lie without the human eye noticing a disturbing color change is shown in the ab-plane of the color space Lab. Analogous considerations result for the windward area.

A color locus, here the target color locus E _target , is described in the color space by the coordinates C₀, h₀ and L₀. C₀ clearly indicates the chroma of the target color locus E _target and h₀ the hue angle of the target color locus. The brightness L₀ of the target color locus plays no role in the case of the method according to the invention.

The so-called permissible range around the target color locus E _Soll lies within the angular range h₀ ± α, ie if an actual color locus E _Ist lies within this permissible range, the eye does not perceive any disturbing color change. in the case shown, the angle α, which represents the maximum permissible change in the hue angle h, is 45 °. Only within this angular range are printing results acceptable to the human eye.

3 shows advantageous embodiments of the method according to the invention. As in FIG. 1, the position of the respective target color locations E _{target is shown} with the coordinates C₀, h₀, L₀ in the down and uv planes of the corresponding color space. Quadrant I of the down or uv plane was chosen as the permissible angular range. In the present case, h₀ corresponds to the angle α = 45 ° from FIG. 1. It should be mentioned again that the permissible range characterizes the angular range around the target color locus E _target , in which no color change disturbing the human color sensation occurs.

. Solange diese Bedingung erfüllt ist, kann ein Ist-Farbort an jeder beliebigen Stelle dieses Toleranzbereiches E_Tol liegen, ohne daß ein Farbumschlag im Druckbild auftreten würde. Ist α = 45°, so bedeutet dies, daß C_max mindestens den 1,4fachen Wert von der Farbtoleranz E_Tol haben muß.Depending on the position of the target color locus E _target , characterized by the changing coordinate C₀₁, C₀₂ or C₀₃, three cases can be distinguished. In the first case, the chroma C₀₁ of the target color locus ${\text{E}}_{\text{Debit1}}{\text{> = C}}_{\text{Max}}{\text{= 1 / sin α * E}}_{\text{Tol}}$

. As long as this condition is met, an actual color location can be anywhere in this tolerance range E _Tol without a color change occurring in the printed image. If α = 45 °, this means that C _max must have at least 1.4 times the color tolerance E _Tol .

A color change that has a negative impact can occur as soon as the chromaticity of the target color locus E _{Soll2 becomes} smaller than the maximum permissible chromaticity C _max . For this second case, it is additionally required that C₀₂ ≧ C _min , that is, in summary, the chroma C₀₂ of the target color locus fulfills the condition:
${\text{C.}}_{\text{min}}{\text{≦ C₀₂ <C}}_{\text{Max}}\text{.}$

.If the color control / color control would now accept all actual color locations E _Ist that lie within the color tolerance E _Tol around the target color location with the chroma C₀₂, then unacceptable printing results would still be achieved in the hatched area, since here the permissible range, in which there is no color change. In this case, the method according to the invention provides the following: In the circular section of the color tolerance E _Tol remaining within the permissible range 2 * h₀, a tolerance circle is placed which does not leave the permissible range at any point. The center of this tolerance circle becomes the replacement target color location with C₀₂ '. The new replacement target color location with C₀₂ 'has the coordinates ${\text{((E}}_{\text{Tol}}\text{+ C₀) / (1 + sin α), h₀, L₀)}$

.

Aus der Fig. 3 ist weiterhin die Vorgehensweise ersichtlich, die zur Anwendung kommt, wenn die Buntheit C₀₃ des Farb-Soll-Ortes E_Soll3 kleiner als die minimale Buntheit C_min ist. Wie bereits an vorhergehender Stelle beschrieben, kennzeichnet C_min die minimale Farbtoleranz. C_min liegt im Bereich der Meßgenauigkeit des Detektors bzw. der Eingabegenauigkeit der Stellgrößen für die Farbführungsorgane 9 der Druckmaschine 1. Würde auch jetzt, wie im vorhergehenden Fall, nur eine Farbtoleranz E_Tol' zugelassen werden die innerhalb des zulässigen Bereiches liegt, so würde eine Regelung stattfinden, obwohl die Regelgenauigkeit dies nicht zuläßt. Andererseits würde natürlich eine Farbtoleranz E_Tol um den Soll-Farbort mit der Buntheit C₀₃ im quergestreiften Bereich eindeutig zu Farbumschlägen im Druckbild führen.The calculated color tolerance E _Tol 'can be expressed mathematically as follows:

${\text{E}}_{\text{Tol}}{\text{'= (E}}_{\text{Tol}}\text{+ C₀) * (sin α / (1 + sin))}$

From Fig. 3, the procedure can be seen, which is used when the chroma C₀₃ of the target color location E _{Soll3 is} less than the minimum chroma C _min . As already described in the previous section, C _min indicates the minimum color tolerance. C _min is in the range of the measuring accuracy of the detector or the input accuracy of the manipulated variables for the ink guide elements 9 of the printing machine 1. If, as in the previous case, only a color tolerance E _Tol 'were allowed which is within the permissible range, then a Regulation take place, although the control accuracy does not allow this. On the other hand, of course, a color tolerance E _Tol around the target color location with the chroma C₀₃ in the striated area would clearly lead to color changes in the printed image.

ist. Der Ersatz-Soll-Farbort E_Soll3' hat die Koordinaten

${\text{(C₀₃' = C}}_{\text{Grenz}}{\text{' * C₀ / C}}_{\text{min}}\text{, h₀, L₀).}$

If the chroma C₀₃ of the target color locus E _{Soll3 is} less than C _min , the following procedure is proposed:
The calculation tolerance E _Tol 'is chosen to be as large as for the target color location at which ${\text{C₀ = C}}_{\text{min}}$

is. The replacement target color locus E _Soll3 'has the coordinates

${\text{(C₀₃ '= C}}_{\text{Border}}{\text{'* C₀ / C}}_{\text{min}}\text{, h₀, L₀).}$

Here, C _Grenz 'denotes the color locus for which the following applies:
${\text{C₀ = C}}_{\text{min}}\text{.}$

Aus der vorhergehenden Beschreibung läßt sich ablesen, daß sich das Problem eines Farbumschlages nur dann zeigt, wenn ein Soll-Farbort innerhalb eines gewissen Raumbereichs um die L₀ Achse liegt, d. h., daß Meßstellen, die zur Erkennung eines störenden Farbumschlages herangezogen werden, in der Nähe der Unbuntachse (L₀) liegen müssen.The color tolerance E _Tol 'of the replacement target color location with C₀₃' can be expressed by the following formula:

${\text{E}}_{\text{Tol}}{\text{'= (E}}_{\text{Tol}}{\text{+ C}}_{\text{min}}\text{) * (sin α / (1 + sin α))}$

From the preceding description it can be seen that the problem of a color change only shows up if a target color location lies within a certain spatial area around the L₀ axis, that is to say that measuring points which are used to detect a disturbing color change are nearby the achromatic axis (L₀) must lie.

REFERENCE SIGN LIST

1

Printing press

2nd

Printed product

3rd

Print control strips

4a, b

Measuring point

5

photoelectric detector

6

Computing / control device

7

I / O device

8th

Color control unit

9

Color guide elements

Claims (13)

Method for zonal control / regulation of the ink flow in a printing press with several printing units, a printed product created in the printing press being photoelectrically measured at at least one location per ink zone, the actual color location being determined from the measured values and the predetermined color location being determined by the the corresponding point is compared and, in the case of a color difference between the actual and the target color location, control data for the ink guide members of the printing press are calculated and sent to the individual printing units in such a way that the color difference between the actual color location and the target color location is minimal,
characterized,
that a tolerable actual color locus E Act _is within a given color tolerance E _Tol and within a permissible range around the target color locus E _Soll (C _0, h _0, L₀), in that no color change occurs. Method according to claim 1,
characterized,
that the measuring point (4b) selected per ink zone lies in the subject of the printed product (2). Method according to claim 1,
characterized,
that the measuring point (4a) selected for each ink zone lies in a print control strip (4) of the printed product. Method according to claim 1,
characterized,
that the selected measuring point (4a, 4b) is close to the achromatic axis (0, 0, L). Method according to claim 1,
characterized,
that the measured values are determined "in-line". Method according to claim 1,
characterized,
that the measured values are determined "off-line". Method according to claim 1,
characterized,
that the permissible range satisfies the condition h₀ ± α, where h₀ denotes the hue angle of the desired color locus E _Soll and α the maximum permissible change in the hue angle h. Method according to claim 1,
characterized,
that a tolerance range E _Tol 'around the target color location E _{Soll is} determined, which lies within the specified color tolerance E _Tol and within the permissible range h₀ ± α. A method according to claim 8,
characterized,
that in the event that the chroma C₀ of the target color locus E _Soll satisfies the condition: C₀ ≧ C _max , the color tolerance around the target color locus E _{target is} equal to E _Tol . A method according to claim 8,
characterized,
that in the event that the condition applies to the chroma C₀ of the target color locus E _target :
${\text{C.}}_{\text{min}}{\text{≦ C₀ <C}}_{\text{Max}}\text{,}$

where C _min denotes the accuracy limit, E _Tol '- with E _Tol '<E _Tol - is determined so that E _Tol 'is within the permissible range. A method according to claim 10,
characterized,
that a new target color locus E _{target1 is} determined, which lies in the center of the calculated color tolerance E _Tol '. A method according to claim 8,
characterized,
that in the event that for the chromaticity of the target color locus E _target applies:
C₀ <C _min , a new one
Target color point ( ${\text{C.}}_{\text{Border}}{\text{'* C₀ / C}}_{\text{min}}\text{, h₀, L₀}$

) with the color tolerance ${\text{E}}_{\text{Tol}}{\text{'= (E}}_{\text{Tol}}{\text{+ C}}_{\text{min}}\text{) (sinα / (1 + sinα))}$

is calculated, where C _limit 'is the chroma of the target color locus E _target at which ${\text{C₀ = C}}_{\text{min}}$

is. Device for zonal control / regulation of the ink flow in a printing press with a plurality of printing units, a detector being provided which photoelectrically measures a printed product created in the printing press at at least one location per ink zone, a computing / regulating device being provided which is based on the measured values determines the actual color location of this point and compares it with a predetermined target color location of the corresponding location and, in the case of a color difference between the actual color location and target color location, calculates control data for the ink guide elements of the printing press in such a way and sends them to the individual printing units, so that Color difference becomes minimal,
characterized,
that the computing / control device (6) checks whether the actual color location E _{Ist is} within a permissible range around the target color location E _Soll (C₀, h₀, L₀), in which no color change occurs.

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