EP0600335B1 - Method for zonal open/closed loop control of ink supply in a printing machine - Google Patents

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 claim 1.

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 print 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. By comparison of the actual color location with the corresponding target color location, a possibly existing color difference is determined. The color difference is converted into changes in the layer thickness in the 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 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 color 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 color tolerance must be chosen unnecessarily small. So then the case occur that, for example, the greenishness disturbs the visual perception, that is, in extreme cases, waste is printed even though the actual color location is within the color tolerance around the target color location.

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

The object is achieved by a method according to the features of claim 1.

The measuring range selected for each color zone can either be in the subject of the printed product or in a print control strip. The selected area is close to the achromatic axis (L, 0.0). The selected area is therefore a gray field in which all printing-relevant colors are represented. The allowable range "no color change" satisfies the condition h ₀ ± α, where h _{0 is} the hue angle h of a desired color location E _to α and the maximum permissible change in the hue angle h features. As long as an actual color location is within this predetermined angular range, there is no risk that a color change will occur in the printed image.

• alle Punkte der Farbtoleranz E_tol'liegen innerhalb einer vorgegebenen Farbtoleranz E_tol um den Soll-Farbort E_soll,
• innerhalb der Farbtoleranz E_tol' erfolgt kein Farbumschlag,
• die Farbtoleranz 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, a color tolerance E ' _tol around the target color locus E set _is specified within the permissible range "no color change". This color tolerance E ' _tol 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 the color tolerance E _tol 'lie within a predetermined color tolerance E _tol around the target color location E _soll ,
• there is no color change within the color tolerance E _tol ',
• the color tolerance 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 c _{0 of} the target color locus E _soll .

If the chroma c _{0 of} the target color locus E target _is large, d. h, the predetermined color tolerance E _tol always lies within the allowable range h ₀ ± α, then there is no risk of color change and each actual color location _E, which is _intended within the color tolerance E _tol about the desired color point E, is of the controller treated as a good color locus.

As soon as the chromaticity c _{0 of} the target color locus E _should meet the condition c _min ≦ c ₀ <c _max , there is a risk of a color change when certain actual color loci E _{are accepted} , since these are still within the color tolerance E _tol , but are no longer within the permissible range. In this case, the chroma 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. To further ensure in this case that no color change occurs, the setpoint color locus E is _{intended 'to} replace a maximum of large color tolerance E' by a substitute desired color location allows E _tol. This replacement Sollfarbort E 'indeed is _tol even on the connecting line between the achromatic point and setpoint color locus E _should, but moves further away from the achromatic; the color tolerance E _tol 'is smaller than the color tolerance E _tol but larger than with a color tolerance around E _des without color change.

A third case occurs when the chroma c _{0 of} the target color locus E des _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. Here it makes little sense 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 because, for. B. machine fluctuations are too large. In addition, small fluctuations in the target value must not lead to large fluctuations in the replacement target color location.

The color tolerance is calculated in this case $${\text{E}}_{\text{tol}}{\text{'= (E}}_{\text{tol}}{\text{+ c}}_{\text{0}}\text{) * (sinα / (1 + sinα))}$$

Fig. 1

ein Blockschaltbild einer Ausführungsform einer Vorrichtung zur Durchführung des Verfahrens,

Fig. 2

eine Darstellung des zulässigen Bereiches im Farbraum und

Fig. 3

eine Darstellung des erfindungsgemäßen Verfahrens im Farbraum.

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

Fig. 1

2 shows a block diagram of an embodiment of an apparatus for carrying out the method,

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.

1 shows a block diagram of an embodiment of the device with which the method can be carried out. The printed product 2 created in a printing press 1 is scanned by a photoelectric detector 5 at certain image points 4a, 4b. Either these are Image areas 4a, 4b in the print control strip 3, or else there are image-relevant areas. 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. An actual color location E _{ist of} the respective measuring field 4a, 4b is determined in the computing / control device 6 and compared with a predetermined target value E _soll . According to the invention of the desired color location E _to shifted in the color space of both the desired color location E _to E and a predetermined tolerance _tol is redefined depending on the location.

The influencing of the coloring in the measurement fields 4a, 4b of the printed product 2 is performed by appropriate changes in layer thickness in the individual printing units, that is, the ink control elements 9 of the printing machine 1 are adjusted so that a measured actual color location E _{is intended} with a predetermined desired color location E the corresponding measuring point 4a, 4b agrees. 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 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 in such a way that a color change within the color tolerance E _tol around the target color location E Soll _is excluded.

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

A color coordinates, here, the target color point E, _is to be described in the color space by the coordinates L _0, C ₀ and h _0th C ₀ clearly identifies the chroma and hue angle h ₀ to the desired color location _{is to} e. The brightness L _{0 of} the target color locus E _soll is of no importance in the case of the method according _to the invention.

The so-called permissible range around the target color locus E _soll lies within an angular range h ₀ ± 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 ₀ , is 45 °. Printing results that are acceptable to the human eye can only be achieved within this angular range.

3 shows advantageous embodiments of the method according to the invention. As in Fig. 2, the location of the The respective target color locations E _{should be represented} with the coordinates L ₀ , C ₀ , h ₀ in the down or uv plane of the corresponding color space. Quadrant I of the down or uv plane was chosen as the permissible angular range. In the present case corresponds to h ₀ the angle α = 45 ° in Fig. 2. It should again be noted that the allowable range of the angle range around the desired color location E _to features, disturbing color change occurs in which no human color perception.

. Solange diese Bedingung erfüllt ist, kann ein Ist-Farbort E_ist an jeder beliebigen Stelle dieses Toleranzbereiches E_tol liegen, ohne daß ein Farbumschlag im Druckbild auftreten würde. Ist α = 45°, so bedeutet dies, daß die Buntheit c₀₁ mindestens den 1,4fachen Wert von der Farbtoleranz E_tol haben muß.Depending on the position of the target color locus E _soll , characterized by the changing colors C ₀₁ , C ₀₂ and C ₀₃ , three cases can be distinguished. In the first case, the chroma C _{01 of} a target color locus E _{soll1 is} greater than or equal to a maximum chroma C _max with ${\text{C.}}_{\text{Max}}{\text{= 1 / sinα * E}}_{\text{tol}}$

. As long as this condition is met, an actual color locus E _ist can be anywhere in this tolerance range E _tol without a color change in the printed image. If α = 45 °, this means that the chroma c ₀₁ 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 chroma c _{02 of} the desired color locus E _{soll2 becomes} smaller than the maximum permissible chroma c _max . For this second case, it is additionally required that the chroma c _{02 is} greater than or equal to the minimum chroma C _min , that is, in summary, the chroma C _{02 of} the target color locus E _{soll2 fulfills} the condition: $${\text{C.}}_{\text{min}}{\text{≦ C}}_{\text{02}}{\text{<C}}_{\text{Max}}\text{.}$$

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 E _soll2 with the _chroma C ₀₂ , then non-acceptable printing results would still be achieved in the hatched area 10, since here the permissible range in which no Color change occurs, is left. In this case, the method according to the invention provides the following: A tolerance circle 11 with a radius E ' _{tol2 is} placed in the circular section of the color tolerance E _{tol which} remains within the permissible range and does not _leave the permissible range at any point. The center of this tolerance _circle 11 becomes the replacement target color locus E ' _target2 with the _chroma C' ₀₂ . The new replacement target color locus E ' _soll2 has the coordinates: $${\text{(L}}_{\text{02}}{\text{, (E}}_{\text{Tol}}{\text{+ C}}_{\text{02}}{\text{) / (1 + sinα), h}}_{\text{02}}\text{).}$$

The calculated color tolerance E ' _tol2 can be expressed mathematically as follows: $${\text{E '}}_{\text{tol2}}{\text{= (E}}_{\text{tol}}{\text{+ C}}_{\text{02}}\text{) * (sinα / (1 + sinα))}$$

3 also shows the procedure which is used when the chroma C _{03 of} a 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 chroma. The minimum chroma C _min is in the range of the measuring accuracy of the detector 5 or the input accuracy of the manipulated variables for the ink guide members 9 of the printing press 1. Would, as in the previous case, only a color tolerance E ' _{tol be} permitted, which are within the permissible range a control would 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 locus E _soll3 with the _chroma C ₀₃ in the striated area 12 would clearly lead to a color change in the printed image.

If the chroma C _{03 of} the target color locus E _{soll3 is} less than the minimum chroma C _min , the following procedure is suggested:

The computing tolerance E _'TOL3 is chosen so large as _to at a desired color point E, wherein the variegation is equal to the minimum chroma C _min. The replacement target color locus E ' _soll3 has the coordinates $${\text{(L}}_{\text{03}}{\text{, C '}}_{\text{border}}{\text{* C}}_{\text{03}}{\text{/ C}}_{\text{min}}{\text{, H}}_{\text{03}}\text{).}$$

In this case, the _chroma c ' _{limit characterizes} the corrected _{chroma of} a target color locus E _soll in which the chroma c _{0 is} equal to the minimum chroma c _min .

The color tolerance E _'TOL3 the substitute desired color location E' _soll3 with the chroma C _'03 is expressed by the following formula: $${\text{E '}}_{\text{tol3}}{\text{= (E}}_{\text{tol}}{\text{+ C}}_{\text{min}}\text{) * (sinα / (1 + sinα))}$$

It is clear from the foregoing description that the problem of a color change is only shows when a desired color location E _should be within a certain spatial area around the achromatic axis, which means that measurement points which are used for the detection of a disturbing color change, in the Must be close to the achromatic axis.

Reference 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

10th

Area

11

Tolerance range

12th

Area

Claims (3)

1. Method for the control/regulation of the ink feed in a printing machine having a plurality of printing units,

   - in which the ink feed can be adjusted in inking zones by means of ink feed elements,
   in that a printed image created in the printing machine is measured photoelectrically at least at one measurement point selected per inking zone,
   the actual colour locus being determined from the measured values and compared with a desired colour locus of a prescribed colour space,
   and, in the event of a colour offset between actual and desired colour locus, control data for the ink feed elements being calculated and led to the individual printing units in such a way that the colour offset between actual colour locus and desired colour locus becomes a minimum,
   characterized

   a) in that a measurement point is selected, whose desired colour locus (E_sp2, E_sp3) having a chroma (c₀₂, c₀₃), a hue angle (h₀₂, h₀₃) and a lightness (L₀₂, L₀₃) lies in the vicinity of the neutral axis (L, 0, 0) of the colour space,

   b) in that a prescribed tolerance (E_tol) is assigned to the desired colour locus (E_sp2, E_sp3),

   c) in that for the case in which the chroma (c₀₂) of the desired colour locus (E_sp2) is greater than or equal to a prescribed minimum chroma (c_min) and less than a prescribed maximum chroma (c_max),

   - a new desired colour locus (E'_sp2) is determined, in that the chroma (c₀₂) of the desired colour locus (E_sp2) is increased to a value (c'₀₂) and the prescribed tolerance (E_tol) is reduced to a corrected tolerance (E'_tol2), both in such a way that no colour change occurs and the corrected tolerance (E'_tol2) becomes a maximum and lies within the prescribed tolerance (E_tol),

   d) in that for the case in which the chroma (c₀₃) of the desired colour locus (E_sp3) is less than the minimum chroma (c_min),

   - a new desired colour locus (E'_sp3) is determined, in that the chroma (c₀₃) of the desired colour locus (E_sp3) is increased to a value (c'₀₃) and the prescribed tolerance (E_tol) is reduced to a corrected tolerance (E'_tol3), both in such a way that the colour change becomes a minimum, and the corrected tolerance (E'_tol3) is set to a prescribed limiting value (E'_tol) and lies within the prescribed tolerance (E_tol), and

   e) in that the new desired colour locus (E'_sp2, E'_sp3) with the corrected tolerance (E'_tol2, E'_tol3) is henceforth used in the control/regulation of the ink feed.
2. Method according to Claim 1,
   characterized

   - in that for the case in which the chroma (c₀₂) of the desired colour locus (E_sp2) is greater than or equal to a prescribed minimum chroma (c_min),

   - a new desired colour locus (E'_sp2) is determined, in that the chroma (c₀₂) of the desired colour locus (E_sp2) is increased to a value (c'₀₂) which results from the relationship $${\text{c'}}_{\text{02}}{\text{= (E}}_{\text{tol}}{\text{+ c}}_{\text{02}}\text{) / (1 + sinα),}$$

   

   where the angle α is a predetermined maximum permissible change in the hue angle (h₀₂), and furthermore the prescribed tolerance (E_tol2) is reduced to a value (E'_tol2) which results from the relationship $${\text{E'}}_{\text{tol2}}{\text{= (E}}_{\text{tol2}}{\text{+ c}}_{\text{02}}\text{) * (sinα / (1 + sinα)).}$$

   
3. Method according to Claim 1,
   characterized

   - in that for the case in which the chroma (c₀₃) of the desired colour locus (E_sp3) is less than the minimum chroma (c_min),

   - a new desired colour locus (E'_sp3) is determined, in that the chroma (c₀₃) of the desired colour locus (E_sp3) is increased to a value (c'₀₃) which results from the relationship $${\text{c'}}_{\text{03}}{\text{= c'}}_{\text{limit}}{\text{* c}}_{\text{03}}{\text{/ c}}_{\text{min}}\text{,}$$

   

   where the chroma (c'_limit) results from the relationship $${\text{c'}}_{\text{limit}}{\text{= (E}}_{\text{tol3}}{\text{+ c}}_{\text{min}}\text{) / (1 + sinα)}$$

   

   and furthermore the prescribed tolerance (E_tol3) is reduced to a value (E'_tol3), which results from the relationship $${\text{E'}}_{\text{tol3}}{\text{= (E}}_{\text{tol3}}{\text{+ c}}_{\text{min}}\text{) * (sinα / (1 + sinα)).}$$

   

Images