GB2043883A

GB2043883A - Measuring Colour Densities for Printing

Info

Publication number: GB2043883A
Application number: GB8001595A
Authority: GB
Original assignee: Grapho Metronic Mess und Regeltechnik GmbH and Co KG
Current assignee: Grapho Metronic Mess und Regeltechnik GmbH and Co KG
Priority date: 1979-01-19
Filing date: 1980-01-17
Publication date: 1980-10-08
Also published as: FR2446720B1; DE2901980C2; CH650200A5; DE2901980A1; GB2043883B; FR2446720A1

Abstract

A system for controlling the inking unit of a printing press uses the color densities of a color teststrip 2, printed on paper along with other material, making use of a measuring head 1 with color filters 20-23. The head is moved smoothly along strip 2. The jumps in density intensity on moving from one test target, forming part of the strip, to the next is used for detecting the target boundaries. There is a filter wheel 18 with the respective colors of the strip continuously turning in the path of light reflected from strip 2. Photosensor 24 feeds an electronic switch 26 synchronized so that density signals corresponding to the strip colors are stored in respective memories 30-33. The boundaries are detected using double- differentiation of the respective color density curves. False indication of boundaries is prevented by an indication only being given when the first derivative exceeds a threshold. <IMAGE>

Description

SPECIFICATION A System for Controlling the Inking Unit of a Sheet Feed Printing Press The present invention is with respect to a system for controlling a printing press by measuring the color densities of a color teststrip, printed with other material on paper or the like as printed in the printing press, making use of a measuring head which is designed to be moved along the color teststrip and has a light source and a light-sensing unit.

Such a system is to be seen, for example, in the U.S. patent 3,367,230. The measuring head of this system has a light source and a photo-cell, which, by way of an optical system and color filters answering to the colors in question, is acted upon light reflected from a target or measuring field. The separate color filters are turningly placed on a filter wheel and may be placed as desired in the light path between the light source and the photo-cell.

The measuring head is placed on a carriage able to be moved from side to side on a rail to and over the separate color test targets. The true position of the measuring head in relation to the separate testtargets is fixed by marks or locking holes placed along a shaft, and answering to locking holes or marks on a wheel and color marks on a turning head, locking pins on the measuring head and, in the other case, on the carriage of the measuring head being pushed by springs into the locking holes. The motion of the measuring head and the turning of the filter wheel is undertaken by hand.

In a further earlier system, the British patent 822,444, the measuring head is moved by a motor using a threaded shaft. At the separate testtargets in the teststrip the measuring head is stopped by the use of a switch, worked separateiy for the separate colors by adjustable stops, driving of the head being stopped by the switch.

In a further earlier system, see the German Offenlegungsschrift specification 2,150,319, use was to be made of a number of measuring feeiers for scanning testtargets or stripparts placed sideby-side. The system was designed for keeping the measuring operation in step with, that is to say synchronous to, the motion of the sheet.

For testing a multi-color print, testtargets for at least four colors and, as far as possible, further marks for testing certain qualities have to be present in a "zone breadth", which is normally about 30 mm in size, that is to say in breadth and is the same in size as the distance or spacing of the adjustment unit at the inking unit, with which the input of ink to the printing plate may be changed over the breadth.

Running over a great number of testtargets or testmarks of the normally used sort, as for example in the U.S. patent 3,367,230, with a filter change-over dependent on the color of the testtargets being tested, takes up much time and, because the printing press will be kept going at the time of testing of a sheet on a separate testing table, so that there is the chance of the printing press' producing waste, the use of this past process is uneconomic.

Furthermore, the separate marks of such a color control strip with such small divisions may not be moved to by an unmoving positioning system, because the breadth of the testmarks or testtargets will be changing all the time. Such changes are produced, for example, by shrinking of the printed paper.

Colors printed one on top of the other on a target may, generally speaking, not be measured at all because of the slow change-over of filters.

One purpose of the present invention, in the case of an automatic control system of the sort noted, is that of speeding up the measuring operation at the separate testtargets or testmarks of a color teststrip, more specially on measuring or testing a number of different colors and more exactly making out the position of measuring in each case.

For effecting these and other purposes a system of the sort noted is characterized in that the measuring head has a smoothly turning wheel, before the light-sensing unit with different optical filters, and intensity values, coming from the light-sensing unit, are changed into electrical density values and go by a change-over switch, to different memories, in step with the color filters, placed in the light path, on the wheel, and in that, using a further circuit the density values selected as the output signal of that memory, which is correlated with the filter answering to the color of the target being scanned, or the one after it, and in that the change in the output signal is timed in a further circuit as a condition of the limit of one target and the next one.

There is the useful effect that the measuring head does not have to be stopped at the position of measuring and, in fact, is run through the teststrip and the position of measuring in each case is worked out from the order of the values produced one after the other without stopping. It is more specially the zero transition, produced on double-differentiation of the values, which gives a true idea for the limits of the testtargets or testmarks and, for this reason, of the position of measuring. There is no cumulative addition of single errors produced, for example, on shrinking of the sheet, between one testtarget and the next one so that the breadth of the testtargets may be decreased and all testtargets of one inking unit control system with fine division may be taken into account.

The processing of the signals coming from the light-sensing unit may be undertaken by electronic components such as differentiation units, comparison units etc. Undesired outside effects may be stopped by changing the analog values into digital signals continuously, this i making possible further processing in a digital system. More specially, this may take place using a program in a small process computer or in a microprocessor.

In a form of the invention, which may more specially be used for multi-color printing, there are in the measuring head a smoothly turning filter wheel, placed in the light path, and a change-over switch, controlled in step with the filter wheel, by which the density pulses coming from the light-sensing unit may be made to go to different memory units in a way answering to the filters used.

The use of a filter wheel turning at a high speed in the measuring head makes it possible for any given color or a number of different colors to be measured at generally the same time (printing of one colored ink on top of an other one) at any time and without any loss in time and, furthermore, without the measuring head having to be stopped. When the measured color density or the differences with respect to the desired values are not used for control of the printing press in the form of figures which have to be seen by eye, and, in place of this, go to the printing press simply as electrical signals (possibly after processing in a small process computer) for control of the zone-by-zone input of ink, the time necessary for testing the sheets may be decreased to the lowest possible value.

An account will now be given of the invention making use of the figures.

Figure 1 is a view of part of a printed sheet having, as well as normal printed material, a color teststrip with its testmarks or testtargets; under the separate, different-colored testtargets the density values got and the signal D1 and signal D2 produced by differentiation, have been recorded.

Figure 2 is a diagrammatic view of the measuring head making clear the general teaching of the electronic processing circuit.

Figure 3 is a view of the measuring head making clear its relation to the measuring table.

Figure 4 is a diagrammatic view of an inking unit with a system for adjustment of ink input rate.

Recognition of Limits of Testtargets A measuring head 1, having a turning filter wheel, to be made clear presently, is moved smoothly along the color test-strip 2 of a printed sheet 3, this taking place on a measuring table T, on which the printed sheet is positioned by suction on a rail 5 (figure 3) having holes 4 so that the sheet may not be moved of the desired position. To the side of this suction rail 5 a carriage 7 is moved on a guide 6, the carriage 7 supporting the measuring head 1 and being run at an even speed, for example by way of a driving cord, a toothed belt or, in the present case, by a motor-powered threaded rod 8.

On measuring head 1 moving over the color teststrip 2 an unbroken or continuous density signal D (figure 1) is produced by it which is electrically monitored and further processed. As the reader will see, at the limits of the testtargets 9 to 14 of the color teststrip 2 there are sudden or high-speed changes in the density value D.

After one mathematical differentiation of the density value D or signal, this being undertaken in a normal way electrically, the outcome is the signal D,, this being representative of the size of the density change. After a second differention operation, the outcome is the signal D2, which, by way of its zero transitions, makes out position or time of the greatest change in density.

The processing of these signals is now undertaken by firstly making an observation as to if the signal D1 goes to a value higher than or less than a threshold values +D1O. If this takes place, it is possible to say that there has been a true jump in density (of for example Delta D=0.5) because of running over a testtarget limit and that it is not a question of lower-level changes in density within a testtarget or because of sudden changes in the color of the paper itself. It is only when this has turned out to be so that the zero transition N of the signal D2 is taken to be the time or position of the limit of the testtarget or teststrip unit.

With a given testtarget breadth it is now possible to make out, starting at the testtarget limit, the middle of the testtarget 1 by measuring the distance moved by the measuring head 1.

This is done in the present case by scanning a gearwheel 15 (fixed on the driving unit 8 of the threaded rod) electrically (with an inductive sensing unit) or optically (by a photo-electric system 16). Each scan pulse will, in this respect be answering to a certain motion of the measuring head up, for example, 0.1 mm. After the middle of the testtarget has been got to, the color density value D of the densitometer is electrically stored best in a digital form or is sent on at once to the inking unit for automatic control of the unit, Nextly, for the next testtarget, the limit is made out in the way which has been noted, and then the distance moved is counted out to get to the middle of the target, and then the density is measured and so on.

As a general teaching, the limits of the testtargets may be made out or recognized when the signal D, (figure 1) goes greater than or less than the values +D1O. As the reader will see, there is, for this reason, a broad range which is uncertain and which is dependent, in addition, on the size of the change in density. As part of the present invention the system now being noted makes use of the signal D2 got by further differentiation of the signal D1, this signal D2 giving a very exact or true position for the testtarget limit (without being dependent on the size of the change in density) because of the signal's zero transition.

As will be seen from Figure 1, the zero transition of the signal D2 only takes place a small distance d after the testtarget limit itself, if the change in density was positive; this is because, generally speaking, the full measuring aperture of the densitometer has to have run into the testtarget till the greatest change in the increase D, of the density signal D takes place. For this reason, in the case of negative changes in density, the zero transition of the signal D2 takes place even at the amount d short of the testtarget limit.

This is, however, not responsible for any trouble in general use of the system, because the distance d is fixed or made out exactly enough by half the breadth of the measuring aperture of the densitometer.

In printing works use it is best for the testtarget limits to be made out by only using density changes in one direction, for example positive density changes. The distance or change in position d is then always in the same direction and the middle of the testtarget may be made out, in the same way in all cases, by further counting of the measuring head position (with the gearwheel sensing system 1 5, 1 6).

On processing with positive changes in density, in- the invention, the density value D of that color filter undergoes selection in the densitometer, which is on hand for measuring the next color target. In this way the greatest possible and preferably positive density changes are produced, when the measuring head is run over the limit of the testtarget. The selection control in this respect is undertaken with the help of a program, stored in a memory of a computer beforehand, with respect to the order of colors of the color teststrip.

As the reader will see from figure 2, the signal D, D or D2 go by way of an interface to the computer 50, which, dependent on the stored order of colors is able to make out from the values D1 if there is in fact any testtarget limit at all, and then, using the values D2 makes out the true position of the limit and makes use of the D values for ink or color control. It will be possible for the computer only to get the density values at one point in time, that is to say instantaneous, density values, and for the limit to be made out and fixed from this completely in the computer.

For producing the best negative density changes, selection of the density value of the color filter is made, which will be answering to the color target or testtarget, with which, at the point in time in question, the measuring head is lined up. Because of the turning filter wheel, to be given a full account further on, in the measuring head, the selection of the right color filter is very simple (that is to say electronically) and takes place without any loss in time.

In cases, in which it is a question of full tone targets with a high enough density and continuously changing colors coming one after the other, the system as noted is able to make out every testtarget limit. If in the color teststrip there are furthermore screen testtargets with a lower density or if testtargets with the same color are placed one after the other, it will frequently be the case, dependent on the level of the threshold +D that no target limit may be made out.Because, however, with respect to a short distance moved of the measuring head, the shrinking of the printed sheet or small errors in positioning (in making up the work) with respect to the testtargets do not have any effects, the middles of the testtargets (middle positions) may readily be made out or determined by position counting with the gearwheel sensing unit 1 5, 1 6. Dependent on the placing of the testtargets in the color teststrip 2, however, a testtarget limit will be made out, at the very most after 10 to 20 targets and the count of the measuring head position may then be put right dependently. The only condition in this respect is that all testtargets at least ratedly have the same breadth, something which, however, is, in any case, true in general printing work because of the simpler "making up" of the color teststrip.

The system of the present invention for automatically making out or recognition of the limits of the testtargets furthermore makes possible the true scanning of color teststrips of unlimited length and making out the density values of the separate testtargets without any errors in measuring at wrong testtargets or at testtarget limits taking place because of any shrinking or, furthermore, expansion of the printed sheet Even in the case of testtargets not keeping to the true breadth or in the case of errors in "makeup" of the color teststrip the position in the middle of the testtarget in question will be made out truly enough in every case.

Recognition of the Start of a Sheet In the present invention the system noted here may be used with a very useful effect for making out, that is to say recognizing, the start of the printed sheet 3, that is to say of the edge 1 7 of the paper. For this purpose the face of the suction rail 5, on which the sheet 3 is fixed by suction will have to be dark, that is to say black, in color so as to have a high color density. On the measuring head 1 running from the suction rail 5 onto the paper, the outcome may be a great negative change in density (see figure 1) which will be processed or made use of in the way noted earlier and will make out the start of the sheet. Nextly, for the time being, only positive changes in density are taken into account.A first positive change in density will take place when the measuring head 1 is run from the white color of the paper onto the first color testtarget of the color teststrip 2; it is processed in the way noted earlier and will be something marking the start of the color teststrip. Later, dependent on what is desired and needed, positive or the negative density changes may be processed.

This making out of the start of the sheet is responsible for the specially useful effect that the sheet may be placed at any desired position on the suction rail and does not have to be truly placed in some desired position; in connection with the making out of the first testtarget of the color teststrip, this makes possible to make out the middles of the targets (position of measuring) without being dependent on the position of the sheet.

The Turning Filter Wheel For measuring different color testtargets of a measuring head 1, running smoothly or continuously over these fields, a high-speed change-over in the color filters would be necessary, something which would be hard, and in need of a high-price system, if electromechanical systems were to be used.

For this reason, a wheel 18, turning at a high speed, is placed in the light path of the measuring head 1 (figure 2) that is to say between a light source 1 9, the color teststrip 2 and a lightsensing unit 24 the wheel having the necessary color filters 20, 21, 22 and 23, which, with an angle spacing of 900 in each case take the form of filters for black, magenta, cyano and yellow. In this respect the light-sensing unit 24 will have as its output electrical intensity pulses, which are changed into electrical density pulses in a logarithmic amplifier 25 joined with its output.In the present invention these density pulses undergo distribution, by an electronic changeover switch 26, to four like electronic memory and filter units 30, 31, 32 and 33, at whose output a smooth or continuous electrical voltage is produced representative of the level of the density pulses. A photo-electric system 34 is used for scanning the holes 35 to 38 in the filter wheel 18 and so controlling the change-over switch 26 that the density pulses of one color go, n each case, to the same memory unit of the units 30 to 33.

With this system it is possible to make certain that at the output of the memory units 30 to 33 there is, for each filter color, a continuous density value D2...DG, which, as in Figure 1 (signal D) is changed, when the measuring head has run over a limit between two testtargets. The useful effect of the system is, however, that all density values D5.. .DG are on hand at one at the same time and may be used for further processing (working out D1 and D2, see figure 1) for making out or recognition of the limits of testtargets. For density measuring as well, this is not without useful effects, because it is possible, without stopping the measuring head, for the density values of a number of colors to be measured in one and the same field (colors printed one on top of the other).

Using normal electrical systems it is, however, possible to make selection, generally without any loss in time, of one of the density values, which may be used for making out the limits of the testtargets, something which is important if processing is only to take place with respect to density changes in one direction.

Such useful effects are not possible with mechanical filter change-over because, in all cases, it is only the density value of one color which is on hand, dependent on the color filter whose selection has been made and the changeover to an other color filter is responsible for a more or less great loss in time.

Figure 4 is a view of an inking unit 40, presented as a schematic, by which the ink goes from a fountain 41 by way of rolls 42 to 45 to the plate cylinder 46 of a printing machine, not figured. A part for adjustment of the ink rate is, in this case, made up of a slide 47, able to be moved by a servo motor 48 towards the roll 42.

Claims

1. A system for controlling the inking unit of a sheet feed printing press by scanning the color densities of a color test-strip printed on the sheets with other material, using a measuring head designed for being moved along the color teststrip evenly, and having a light source and a lightsensing unit with a group of optical filters, answering to the colors of the scanned targets of the teststrip and designed to be automatically changed over, output intensity pulses from the measuring head acting on ink control parts of the printing press for adjustment of the control parts, characterized in that the measuring head has a smoothly turning wheel, placed in the light path in front of the light-sensing unit, and in it different optical filters are placed, in that the intensity values, as produced by scanning and coming from the light-sensing unit, are able to be changed in an amplifier into electrical density values and go to a change-over switch, used for input of the density values to different memories, such input being in step with the color filters, placed in the light path, on the wheel, in that, by using a further circuit in each case the density values may undergo selection as the output signal of that memory, which is correlated with the filter answering to the color of the target which is at the time being scanned, or the one after it, and in that the change in the output signal is able to be timed in a further circuit as a condition of the limit of one target and the next one.

2. A system as claimed in claim 1, characterized in that the amplifier getting the intensity values from the light-sensing unit is a logarithmic amplifier.

3. A system as claimed in claim 1, characterized in that the output signals of the memories are continuous voltage values proportional to the size of the density value, which was last stored in each case.

4. A system as claimed in claim 1, characterized in that for making out the target limit the further circuit is designed for differentiating the output signal of the memories two times over and in that the zero transitions of the second derivations or second differential coefficients are made out in the further circuit.

5. A system as claimed in claim 1, characterized in that the further circuit has a memory storing within it the color order of the targets of the color teststrip.

6. A system as claimed in any one of claims 1 to 5, characterized in that a distance moved (or displacement) sensing unit is present for measuring the distance moved by the measuring head in relation to the color teststrip and after the end of a certain distance, run after each target limit which is made out, the color density value from the target, over which the measuring head is placed at the time, may be used for automatic control of ink input for the ink in question at this target.

7. A system as claimed in any one of claims 1 to 6, characterized in that the measuring head is designed for moving over the sheet edge as well, which comes before the color teststrip.

8. A system as claimed in anyone of claims 1 to 7, characterized in that the further circuit is a computer.

9. A system as claimed in anyone of claims 1 to 8, characterized in that the density values and the values produced as the output of the distance moved sensing unit may be processed using a program and the selection of the density values may be undertaken by a program.

10. A system as claimed in claim 1, substantially as described above with reference to and as illustrated in the drawings.