CA2415612A1

CA2415612A1 - Method of determining the relative position of a first and second imagesetting device with respect to each other

Info

Publication number: CA2415612A1
Application number: CA002415612A
Authority: CA
Inventors: Thomas Koehler; Bernhard Zintzen
Original assignee: Individual
Current assignee: Heidelberger Druckmaschinen AG
Priority date: 2002-01-31
Filing date: 2003-01-06
Publication date: 2003-07-31
Also published as: DE10259493A1; JP2003307854A; IL154061A0; IL154061A

Abstract

A method is presented of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device by setting images (58) on an associated imagesetting medium (54), said method comprising at least the following steps: an image of a group of mutually different reference patterns (16) and a basic pattern (18) is set by means of the second imagesetting device (58), which serves as a master, each reference pattern (16) from the group being assigned uniquely to one relative position. An image of at least one test pattern is set by means of the first imagesetting device (56), whose relative position is to be determined, over the basic pattern, so that a combination pattern (24) is produced. A reference pattern (16) whose area coverage coincides with the area coverage of the combination pattern (24) is determined. Since a relative position (510) is associated in a unique way with the identified pattern from the group, the relative position of the first with respect to the second imagesetting device (56, 58) is identified. The method according to the invention can advantageously be used in a printing form exposer or in a printing unit (52) of a printing press (50).

Description

METHOD OF DETERMINING THE RELATIVE POSITION OF A FIRST AND A
SECOND IMAGESETTING DEVICE WITH RESPECT TO EACH OTHER
The invention relates to a method of determining the relative position of a first imagesetting device with respect to a second imagesetting device by setting images on an associated imagesetting medium. zt further relates to a method of correcting the position of a point of projection of a first imagesetting device with respect to a point of projection of a second imagesetting device by changing the relative posii~ion of the point of projection of the first imagesetting device with respect to the point of projection of the second imagesetting device from an actual position to an intended position, in particular, in a printing form exposer or in a printing unit of a printing press.
In order to set an image on a two-dimensional surface of an imagesetting medium using one or more imagesetting devices, the surface is scanned in two linearly independent coordinate directions which cover the area, by a relative movement between the surface anal the imagesetting device or devices being produced by a suitable actuator mechanism. The scanning is typically carried out in a so-called fast scanning direction and a so-called slow scanning direction, in such a way that all the points to be imaged on the surface are swept over by the imagesetting devices, more precisely by a number of imagesetting beams. Here, an imagesetting beam can be a light beam, in particular a laser light beam, be it in the infrared, visible or ultraviolet spectra7_ range, a heat pulse, a gas jet or a droplet of a chemical substance or the like. An imagesetting device, also referred to as an imagesetting module, may have one or more imagesetting beams. In this case, imagesetting media comprise printing forms, printing plates, so-called printing form precursors, films or the like. For setting images on imagesetting media in the graphical industry, be it at the pre-press stage ~.n printing form exposers or in the printing stage in printing units (on-press imagesetting or direct imaging printing units), laser light sources in particular are particularly widespread in imagesetting devices. Laser light sources are frequently diode lasers or solid body lasers, such as lasers with reinforcing media of Ti: sapphire or Nd:YLF, preferably pumped by a diode laser. A plurality of laser light sources may be located on one or more assembled diode laser bars in one imagesetting device.
An imagesetting device can comprise one imagesetting channel or a group of imagesetting channels. A plurality of imagesetting devices can be integrated to form a block. For the purpose of imagesetting or labeling, the imagesetting channels are switched on and off (timed triggering). Depending on the imagesetting method selected, while at least one imagesetting is switched on, a relative movement may take place between the point of projection and imagesetting medium, or not. Using an imagesetting medium on which an image has been set or written, an image can be transferred to a printing material. Typical printing materials are paper, board, organic polymer films or the like, be they sheets or webs.
If a number of imagesetting devices are used, be it in a printing form exposer or a printing unit, it is very important that the relative positions of the imagesetting devices with respect to one another and, presupposing the case of a plurality of imagesetting beams from one imagesetting device, that the imagesetting beams from an imagesetting device are aligned with one another. If appropriately accurate imaging optics is used on the imagesetting medium, it is consequently then possible for the relative position of the imagesetting beams on the imagesetting medium to be set up or calibrated with great precision in relation to one another. As a result, without restricting the general number of imagesetting beams in an imagesetting device, merely to simplify the illustration and description, let it be assumed that each imagesetting device has one imagesetting beam each. Furthermore, without restricting the general number of the imagesetting devices, likewise to simplify the illustration, 1_et the description below be aimed at a first and a second imagesetting device, in the knowledge that the number of imagesetting devices can also be more than two.
The procedure for setting up or calibrating the relative position of a first with respect to a second imagesetting device (of a first with respect to a second imagesetting channel or of a first group of imagesetting channels with respect to a second group of imagesetting channels) can be required firstly in tree assembly of the imagesetting devices and secondly in the event of maintenance of the apparatus or the press with which the imagesetting devices are associated, be it in a workshop or at the customer. According to a widespread procedure, therefore, a considerable amount of effort is associated i.n particular with the installation of printing presses with on-press imagesetting printing units.
For each printing unit, test imagesetting of a printing form (imagesetting medium) is carried out, part of the printi~,g form with an image set on it is cut out and the part is examined by a reader, so that correction values for the relative position can be determined. The correction values are used to make changes to the relative position, for example, this information is made available to the control system, and the imagesetting devices are adjusted with respect to one another. The abovedescribed method for the relative position determination is iterated until the desired or required precision has been reached. The relative position deviation can be determined both for the fast scanning direction and for the slow scanning direction, and the relative position can consequently be adjusted. However, the high outlay on material and time is a drawback when this method is used.
For instance, the document DE 44 37 284 A1 discloses the fact that a calibration of a control system for the diffraction of a laser beam can be performed as follows. A light-sensitive medium is irradiated by the laser beam in order to produce a test image, and digitized image sections which are recorded by a CNC-controlled camera are then generated from said test image. A calculation of correction data for the control system for the diffraction of the laser beam is carried out on the basis of a comparison between the actual positions of the laser beam, measured by recording the image sections, with predefined intended positions. The drawback when this method is used is likewise tree outlay on material and the requirement to use a precise CNC control system for the camera, which is consequently expensive.
The document DE 197 32 668 A1 reveals that a calibration device for a beam scanning device can have a surface with defined markings. Using a detector device, the light reflected from the surface or transmitted and belonging to the beam.
scanning over the surface is registered. When the beam strikes a marking, only a low intensity is reflected or transmitted.
From the registered intensity variation of the radiation in reflection or transmission, the actual position of the laser beam is determined and, in a control system, is compared with the intended position envisaged for this point. From this comparison, a correction value can be determined and made available to the control system, for example stored as a tabular entry in a memory. One disadvantage of this calibration device is that a surface with precise markings must be made available. However, such a surface is sensitive and little suited to be moved to various locations or to be accommodated in different machines, frequently with changes in shape, which may possible lead to distortions of the coordinate system.
It is an object of the present invention to specify a method of determining the relative position of a first, with respect to a second imagesetting device which makes it possible in a simple way to detect maladjustment of the imagesetting devices with respect to one another, that is to say the deviation between an actual distance and an intended distance.
According to the invention, this object is achieved by a method of determining the relative position of a first imagesetting device with respect to a second imagesetting device having the features as claimed in claim 1 or as claimed in claim 17. Advantageous developments of the invention are characterized in the dependent and in the equivalent claims.
According to the invention, a determination of the deviation between the position of the first imagesetting beam, which is associated with the first imagesetting device, and the position of the second imagesetting beam, which is associated with the second imagesetting device, is carried out by means of a tonal value comparison or area coverage comparison between specific patterns, in particular grid or line patterns, directly on the imagesetting medium on which an image is set or on a test image printed with said imagesetting medium. According to the invention, the relative position of the first imagesetting device (imagesetting beam) with respect to a second imagesetting device {imagesetting beam) which functions as a so-called master, is determined. By means of a suitable selection of the orientation or. direction of the patterns, in particular line grids, registration of the position deviations or beam tolerances in the fast scanning direction and slow scanning direction is possible, the orientation of the patterns, in particular of the direction of the lines, in the case of line grids, preferably being at right angles to the direction of the dimensional tolerance, of the position deviation. A precondition for the use of the method according to the invention is precise generation of defined, pixel-accurate patterns, in particular line grids, on the imagesetting medium.
The method according to the invention of determining the relative position of a first imagesetting device with respect to a second imagesetting device by setting images on an associated imagesetting medium comprises at least the following steps: an image is set of a group of mutually different reference patterns and a basic pattern by means of the second imagesetting device which serves as a master, each reference pattern from the group being assigned uniquely to one relative position. An image of at least one test pattern is set by means of the first imagesetting device, whose relative position is to be determined, over the basic pattern, so that a combination pattern is produced. A reference pattern, from the group, whose area coverage coincides with the area coverage of the combination pattern is determined.
Since the identified pattern from the group is assigned in a unique way to one relative position, the relative position of the first with respect to the second imagesetting device is identified.

Expressed in other words, the method according to the invention for determining the relative position of two imagesetting devices (imagesetting modules or imagesetting beams) with respect to each other comprises setting images over one another or writing patterns over one another, a number of reference patterns being written by the second imagesetting device, functioning as a master, and at least one test pattern being written by the first imagesetting device whose relative position deviation is to be determined, so that at least one combination pattern is produced for the purpose of comparing the area coverage or the tonal value by an optical route (measurement) either on the imagesetting medium or on a test image on a printing material printed by using the imagesetting medium.
The method according to the invention for determining the relative position of a first imagesetting device with respect to a second imagesetting device, the first imagesetting device being associated with a first imagesetting medium and the:
second imagesetting device being associated with a second imagesetting medium, comprises at least the following steps:
an image is set of a group of mutually different reference patterns and a basic pattern by means of the second imagesetting device on the second imagesetting medium associated with the second imagesetting device, each reference pattern from the group being assigned uniquely to one relative position. An image of at least one test pattern is set by the first imagesetting device on the first imagesetting medium associated with the first imagesetting device. The first and second imagesetting media on which images are set by the first and the second imagesetting device are printed off onto a printing material in such a way that the test pattern whose image is set by the first imagesetting device is printed off over the basic pattern,, so that a combination pattern is produced. A reference pattern, from the group, whose area coverage coincides with the area coverage of the combination pattern is identified. Since the identified pattern from the group is assigned a relative position in a unique way, the relative position of the first with respect to the second imagesetting device is identified.
It is particularly advantageous if the reference patterns from the group, the basic pattern and the test pattern are two-dimensional, but are uniform or translation-invariant in one of the two linearly independent directions. Expressed in other words, the patterns can have an orientation or direction. The uniformity, in particular for line patterns or line grids, can particularly advantageously extend at right angles to the direction of the relative position deviation to be determined, so that precise detection of the relative position in combination patterns is made possible, since even a small deviation in a direction not parallel to the uniformity can lead to a large change in the area coverage of the combination pattern.
The basic pattern can advantageously comprise a group of mutually identical control patterns. These can be arranged at various locations on the two-dimensional area of the imagesetting medium. These control patterns can, but do not have to be, overwritten by one and the same test pattern. In the case of a test pattern, the result is redundancy of the information, so that statistical or other error sources can advantageously be avoided. In the case of a plurality of test patterns, different combination patterns are produced, so that additional information can be obtained by optical comparison.
It is also advantageous if the number of reference patterns in the group and the number of control patterns is the same and in each case one reference pattern in the group is uniquely assigned just one control pattern. For example, it is additionally possible for the mutually uniquely associated control and reference patterns also to be arranged adjacently on the two-dimensional surface of the imagesetting medium. By means of a unique association or an adjacent arrangement, an optical comparison can be carried out simply, quickly and conveniently if the control patterns (basic patterns) are covered by test patterns to form combination patterns. In particular, the comparison can be carried out by means of a single measurement, a first part of the image field being assigned to the combination pattern and a second part of the image field being assigned to the reference pattern.
A further advantageous feature can be formed by the fact that the number of reference patterns in the group and the associated control patterns in the basic pattern can be uneven. It is then possible for the test pattern to cover a first part of the control patterns with a first sub-pattern, to cover a second part of the control patterns with a second sub-pattern and to cover a control pattern both by the first sub-pattern and with the second sub-pattern. As a result, at the location of the first sub-pattern, a first combination pattern is then produced, at the location of the second sub-pattern, a second combination pattern is produced and, at the location of the control pattern covered by the first and second sub-pattern, a third combination pattern is produced.
The first and the second sub-pattern can be selected in such a way that the first combination pattern is then exactly identical to the associated control pattern covered by it when the second combination pattern differs from the associated control pattern covered by the latter, anal that the second combination pattern is then exactly identical to the associated control pattern covered by it when the first combination pattern differs from the associated control pattern covered by the latter. The third combination pattern can coincide either with the first or with the second combination pattern. In this way, it is possible to carry out detection of the sign of the relative position between the first and second imagesetting device. For these additional features, it can also be true that the reference patterns from the group, the control patterns and the test pattern are two-dimensional, but are uniform or translation-invariant in one of the two linearly independent directions.
It is particularly advantageous if the basic pattern exhibits 50o area coverage, so that relative position deviations lead to small area coverage changes in the patterns starting from 50o area coverage. In this range, area coverage changes can be determined easily and well.
In one advantageous embodiment, the basic pattern and/or the control patterns, if the basic pattern comprises a group of control patterns, has a regular sequence of a number of exposed and of the same number of unexposed points in one direction of its extent. In other words, a pattern can be a line grid or a strip grid. The lines can be parallel to one another and in particular can run at right angles to the direction in which the relative position is to be determined.
Furthermore, it is also possible for each of the reference patterns in the group of reference patterns to have a regular sequence of exposed and unexposed points in one direction of its extent, the sequences in one period in each case having a first and a second number of exposed points and a third and a fourth number of unexposed points. The directions of the extents of the reference patterns in the group of reference patterns can advantageously also be parallel to one another.

It is particularly advantageous if the number of reference patterns in the group of mutually different reference patterns is uneven and if a reference pattern is assigned to the relative intended position, a first part of the group of reference patterns is assigned to relative positions which are greater than the relative intended position, and a second part of the group of reference patterns is assigned to relative positions which are smaller than the relative intended position. In a first embodiment, for one absolute value of the relative position, the reference pattern from the first part of the group can be identical to the reference pattern from the second part of the group. In a second embodiment, for one absolute value of the relative position, the reference pattern from the first part of the group with respect to the reference pattern from the second part of the group may exhibit mirror symmetry with respect to an axis at right angles to the extent of one of the reference patterns.
In a preferred embodiment of the method according to the invention, the group of mutually different reference patterns can be arranged on the imagesetting medium so as to be ordered in accordance with the associated relative intended position.
Also optional is the imagesetting or labeling of the reference patterns by means of a scale. Expressed in other words, the assignment of a reference pattern to a relative position or position deviation is also directly visible to the human eye.
An ordered arrangement therefore permits quick and simple optical evaluation of the imagesetting result.
In an advantageous development of the method according to the invention of determining the relative position of a first imagesetting device with respect to a second imagesetting device, if the imagesetting medium is accommodated in a printing unit for the purpose of imagesetting, provision is made for the imagesetting medium on which an image has been set by the first and second imagesetting device to be printed off onto a printing material before the identification of coincident area coverage, the identification being performed on the printing material, and the ident~_fication of the relative position associated with the identified reference pattern from the group are performed. In addition to the particularly convenient and simple possibility of being able to perform measurements of the area of coverage on the printing material and not on the imagesetting medium accommodated in the printing unit, this development of the method according to the invention also has the advantage that, if appropriate, influences of the printing unit on the intended position or relative intended position of the two imagesetting beams or imagesetting devices can be taken into account, since it is ultimately a question of the relative position of a pixel or printed dot placed by the first imagesetting device in relation to the position of a pixel or printed dot placed by the second imagesetting device in t:he image on the printing material.
The method according to the invention, its advantageous embodiments or its advantageous developments avoid high expenditure on labor and time, since processing of the imagesetting medium, printing form material, outside the printing form exposer or the printing unit, and examination under a reader or aids, such as magnifying glasses, imaging optics or the like and renewed loading of an imagesetting medium are no longer necessary, as opposed to the prior art.
The method according to the invention of determining the relative position or the relative position deviation of two imagesetting devices can be developed further to a method of correcting the position of a first imagesetting device with respect to a second imagesetting device, more accurately, to correcting the position of the points of projection of a first imagesetting device with respect to points of projection of a second imagesetting device, by changing the relative position of the first imagesetting device with respect to the second imagesetting device from an actual position to an intended position and/or by changing the timed triggering (of the switching on and off). In this method according to the invention for correcting the position of the points of projection, the area coverages are determined by a measuring device, for example a photodiode or a CCD array, by the intensity of the light reflected from the combination patterns and the reference patterns being detected and the measured values being compared in a suitable processing unit. As a result, the identification of combination patterns and reference patterns is made possible in electronic form, so that a signal for changing the relative position of the first imagesetting device with respect to the second imagesetting device and, consequently, also for changing the relative position of the associated points of projection can be generated. On the one hand, this signal can be in the form of a display for a human, on the other hand, provision can also be made for the relative position to be changed from an actual position to an intended position by an actuator mechanism. As an alternative to this, the timed triggering of the first imagesetting device relative to the second imagesetting device can take place later or earlier than in the uncorrected state.
The method according to the invention for correcting the position of the points of projection can be implemented in the following way in a device: a printing form exposer having at least a first and a second imagesetting device, which can be moved relative to the imagesetting medium and/or to one another by means of a controlled actuator mechanism, has a control unit. The printing form exposer is distinguished by the fact that the control unit comprises an electronic unit with a memory unit, in which a computer program for correcting the position of the first imagesetting device by means of the controlled actuator mechanism and/or by means of changed timed triggering is stored, the computer program having at least one functional section in which steps from the method according to the invention for correcting the position by means of the actuator mechanism and/or the timed triggering are performed.
As an alternative to this, an implementation can also be performed for a printing unit or a printing unit group: a printing unit having a first and a second imagesetting device or a printing unit group having a first printing unit, to which a first imagesetting device is assigned, and having a second printing unit, to which a second imagesetting dev~_ce is assigned, it being possible for the imagesetting devices to be moved relative to the imagesetting medium or to the imagesetting media and/or to one another by means of a controlled actuator mechanism, has a control unit. The printing unit or the printing unit group is distinguished by the fact that the control unit comprises an electronic unit with a memory unit, in which a computer program for correcting the position of the first imagesetting device by means of the controlled actuator mechanism and/or changing the timed triggering is stored, the computer program having at least one functional section in which steps from the method according to the invention for correcting the position by means of the actuator mechanism and/or changing the timed triggering a.re performed. A printing press according to the invention - be it a web-fed or a sheet-fed printing press - in particular.a planographic printing press, an offset printing press or the like, has at least one printing unit according to the invention and/or at least one printing unit group according to the invention.
Further advantages and advantageous embodiments and developments of the invention will be illustrated by using the following figures and their descriptions. In detail:
figure 1 shows a schematic representation of the structure of a pattern as can be used in the method according to the invention, figure 2 shows an advantageous embodiment of the patterns used and their arrangement for the method according to the invention, figure 3 shows combination patterns written one above another for a first relative position of the first with respect to the second imagesetting device an arranged patterns and reference patterns of the advantageous embodiment according to figure 2, figure 4 shows combination patterns written one above another for a second relative position of the first with respect to the second imagesetting device on arranged patterns and reference patterns of the advantageous embodiment according to figure 2, and figure 5 shows a schematic plan view of a. printing press having an embodiment of a printing unit according to the invention with a first and second imagesetting device, whose relative position is determined and corrected by the methods according to the invention.
Figure 1 serves to exp7_ain the structure of a pattern as can be used in the method according to the invention. A pattern 10 comprises a number of pattern units 12. The pattern units 12 correspond to pixels or printed dots, expressed in another way, a pattern unit 12 is an imagesetting point, which is produced on the imagesetting medium at least by an imagesetting beam or an imagesetting device. Adjacent pattern units 12 have a spacing which corresponds to the spacing of imagesetting points. In one advantageous embodiment of the method according to the invention, patterns 10 are used which are constructed by pattern units 12 lying on a regular and rectangular grid. By way of example, in the sequence of part images A, B and C of figure 1, patterns 10 are shown which are constructed in square form from eight by eight pattern units 12. Patterns 10 can also have a considerably larger number of pattern units 12, 112 by 112 pattern units 12 in the preferred embodiment which is described by using figures 2 to 4. In part A of figure 1, in order to simplify the representation, it is shown that the pattern 10 is built up from square pattern units 12 whose surface centers of gravity lie on a rectangular grid. For the method according to the invention, the pattern units 12 can be present in two states: a non-imaged and an imaged state. The imaged state can be achieved either by setting an image with a first imagesetting device or by setting an image with a second imagesetting device. In part B
of figure 1, it is shown by way of example that the first, second, fifth and sixth columns from the left of the pattern have had an image set by the second imagesetting unit. The pattern 10 then has imaged pattern units 14. Further setting of an image of an already imaged pattern unit 14 leads to no qualitative state change in the sense of the method according to the invention. Expressed in other words, a pattern unit 14 already imaged by the second imagesetting device does not change qualitatively, that is to say remains imaged as a result of renewed imagesetting by the first imagesetting device. We assume that the first imagesetting device sets the image in the second and the third column from the left of the pattern 10. Part C of figure 1 then shows that the imaged pattern units 14 in the second column, which had already been imaged, have not experienced any qualitative change, while the pattern units 12 in the third column have now become imaged pattern units 14. Expressed in other words, part B of figure 1 shows an example of a basic pattern or control pattern, which becomes the combination pattern shown in part C of figure 1 as a result of overwriting in the second arid third columns of the pattern 10.
If the first and the second imagesetting~ devices have a relative intended position with respect to each other, then when the surface of the imagesetting medium is scanned in the method according to the invention, it is ensured that the pattern units 14 written by the first imagesetting device cover at least a subset of the pattern units 14 written by the second imagesetting device (master). Expressed in other words, combination pattern and basic pattern coincide; there is no requirement for manipulation. On the other hand, if the first and the second imagesetting device have a relative actual position with respect to each other which differs from the intended position, then when the surface of the imagesetting medium is scanned in the method according to the invention, there is at least one pattern unit 14 written by the first imagesetting device which does not cover a pattern unit 14 written by the second imagesetting device (master). Expressed in other words, combination pattern and basic pattern do not coincide; there is a requirement for manipulation.
Figure 2 shows an advantageous embodiment of the patterns used and their arrangement for the method according to the invention. Figure 2 has a left-hand and a right-hand part, the right-hand part being an enlargement of a detail from the left-hand part. In this advantageous embodiment, as illustrated in the left-hand part, the method according to the invention is based on fifteen reference patterns 16 and fifteen control patterns (basic patterns 18). In the method according to the invention, images of reference patterns 16 and basic patterns 18 are set by the second imagesetting device, the master. Each reference pattern 16 is assigned a control pattern 18, so that fifteen blocks exist. A scale 20 is provided, of which an image is likewise set, said scale labeling the fifteen blocks and visualizing the unique assignment of each reference pattern 16 to a relative position. This procedure is advantageous in particular for the measurement or checking of a printing material pulled off from the imagesetting medium. In this embodiment, the patterns have 112 by 112 pattern un~_ts 12. As viewed from area center of gravity to area center. of gravity, adjacent pattern units 12 have a spacing of 10 um. The patterns correspond to a grid of 60 lines per centimeter with addressing of 1000 lines per centimeter.
In this embodiment, the reference patterns 16 differ from one another in such a way that, as a result of the area coverage of the combination patterns produced by overwriting the .control patterns (basic patterns 18) by means of a test pattern, spacing changes of 5 um can be made visible or measurable. The blocks comprise reference patterns 16 and basic patterns 18 which are formed by line grids, which are uniform in the direction at right angles to the direction in which the relative position is to be determined. The line grid for the basic pattern 18, that is to say all the control patterns, is constructed regularly: it has columns of imaged and non-imaged pattern units 12. Figure 2 shows that, as viewed from left to right, alternately eight columns have an image set and eight columns have no image set. The basic pattern therefore exhibits 50% area coverage.
The reference patterns 16 are to be classified here into a first, upper part here and a second, lower part here, and a central reference pattern for the intended position 22. The central reference pattern for the intended position 22, as viewed from left to right, has alternately eight columns with an image set and eight columns with no image set, that is to say, like the basic pattern 18, exhibits an area coverage of 500. This central reference pattern 22 is assigned to the relative position 0 um, more accurately a deviation 0 um from the relative intended position, as can also be seen from the scale 20. In the first, upper part here, reference patterns 16 are arranged in an ordered fashion, are assigned the relative positions -5.0 um, -10.0 um, -15.0 um, -20.0 um, -25.0 urn, -30.0 um and -35.0 Vim, and are reproduced in the scale 20.. From adjacent reference pattern to reference pattern, the area coverage (tonal value) increases in steps of 3.1250 as far as 71.8750 for -35.0 um. This increase in area coverage is achieved as follows.
If the reference pattern 16 for a specific relative position is considered, then the reference pattern 16 for the relative position of the next step, that is to say the specific relative position minus 5.0 um, differs in the fact that a further column of pattern units 12 also has an image set by the second imagesetting device. The arranged sequence, starting from the central reference pattern 16 with 500 area coverage, is formed as follows in the advantageous embodiment shown in figure 2 by means of the variation in a 4-column periodicity: for 0 um, eight with an image set, eight with no image set, eight with an image set, eight with no image set;
for -5.0 um, eight with an image set, eight with no image set, nine with an image set, seven with no image set; -10.0 um, nine with an image set, seven with no image set, nine with an image set, seven with no image set; -15"0 Vim, nine with an image set, seven with no image set, ten with an image set, six with no image set; -20.0 um, ten with an image set, six with no image set, ten with an image set, six with no image set; -25.0 um, ten with an image set, six with no image set, eleven with an image set, five with no image set; -30.0 um, eleven with an image set, five with no image set, eleven with an image set, five with no image set; and -35.0 um, eleven with an image set, five with no image set, twelve with an image set, four with no image set. As a result of the four-column periodicity, in which only a proportion of the area coverage of the reference patterns 16 is changed by 3.125% from one relative position to the next relative position, it is possible to make a relative position deviation which is finer or smaller than the spacing of the pattern units 12 detectable, in particular here, half of the spacing of adjacent pattern units 12; the measured variable is the integral area coverage of the combination pattern, which is compared with the integral area coverage of the reference patterns 16.
The reference patterns in the second, lower part here are arranged in order and are assigned to the relative positions 5.0 um, 10.0 um, 15.0 um, 20.0 um, 25.0 um, 30.0 ~m and 35.0 pm, as also reproduced in the scale 20. In this advantageous embodiment, the two reference patterns 16 which are assigned to two relative positions whose absolute value coincides are the same and follow the four-column periodicity listed in more detail above for the first part.
With knowledge of the method according to the invention, it is clear to those skilled in the art that an increase in the sensitivity or a reduction in the size of the step width of relative positions can be achieved by increasing the grid frequency of the lines in the group of the reference patterns 16 and the control patterns 18 (basic patterns), expressed in other words the spacing of the pattern units 12. For example, therefore, four strips with an image set and four with no image set. The patterns then correspond to a grid of 120 lines per centimeter with addressing of 1000 lines per centimeter.
Furthermore, it is clear that, in the method according to the invention, a refinement in the step width of relative positions with a specific spacing of the pattern units 12 can be achieved by means of a different number of strips in the periodicity of the strip sequence; for example, by means of a six-strip periodicity of the reference patterns 16 which,. in a manner analogous to that described above for the four-strip periodicity, in each case in permutation broadens the strips with an image set, an ordered sequence of reference patterns 16 can be produced, which make it possible to visualize or make measurable relative position deviations of one third of the spacing of adjacent pattern units 12. For 2n-strip periodicities broadened in permutation, it is generally true that 1/n of the spacing of adjacent pattern units becomes measurable, n being a natural number greater than or equal to two.
By using figures 3 and 4, an explanation is to be given as to how, by covering the control patterns (basic patterns) with a test pattern, which produces combination patterns, relative positions of the first imagesetting device with respect to the second imagesetting device become measurable: in the event of different relative positions between the two imagesetting devices, different combination patterns are produced, whose area coverage is determined (measured) and compared with the area coverage of the reference patterns.

In this advantageous embodiment, the test pattern has the following property: the seven control patterns which are assigned to the reference patterns 16 of the first, upper part are overwritten by a test pattern of 112 by 112 pattern units by means of the first imagesetting device, which has a two-strip periodicity having four exposed and twelve unexposed columns. The central control pattern which is assigned to the reference pattern for the intended position is overwritten by a test pattern of 112 by 112 pattern units having a two-strip periodicity of eight exposed and eight unexposed columns. The seven control patterns which are assigned to the reference patterns 16 of the second, lower part, are overwritten by a test pattern of 112 by I12 pattern units by means of the first imagesetting device, which has a periodicity with four unexposed, four exposed and eight unexposed columns, that is to say has a two-strip periodicity which is offset by four columns as compared with the test pattern over the first, upper part.
The test pattern is written by the first imagesetting devices in such a way that the pixels (pattern units) are driven as though the relative position of the first with respect to the second imagesetting devices were the intended position.
Expressed in other words, if the relative actual position, that is to say the actual spacing between the two imagesetting devices, is the relative intended position, that is to say the intended spacing, then an image of a specific pattern element of the test pattern (specific column, specific row) is set at a time at which the projection of the imagesetting beam of the first imagesetting device comes to lie at that coordinate point on the surface of the imagesetting medium at which the setting of an image of the pattern unit of the control pattern (basic pattern) has been carried out at a different, preceding time in the same specific column and the same specific row.
The control patterns are therefore located in a region of the surface of the imagesetting medium which can be swept over both by the first and by the second imagesetting device or the projections of their imagesetting beams, so that test patterns can be written over the control patterns to produce combination patterns. As those skilled in the art can quickly imagine, firstly for the case of synchronous movement of the two imagesetting devices, if there is a fixed spacing, this overlapping region is reached by the first imagesetting device when the latter comes at a later time into a region which has previously been written by the second imagesetting device, or secondly, an overlapping region can be defined by overlapping amplitudes of the individual movements of the imagesetting devices for the case of mutually independent movements of the two imagesetting devices, that is to say a variable spacing when there is a suitable actuator mechanism during the imagesetting.
Figure 3 relates in schematic form to combination patterns written one over another for a first relative position, differing from the intended position, of the first with respect to the second imagesetting device on arranged patterns and reference patterns of the advantageous embodiment according to figure 2. Figure 3 has a left-hand and a right-hand part, the right-hand part being an enlarged detail of the left-hand part. In the left-hand part, the fifteen reference patterns 16 and the scale 20 which have already appeared in figure 2 are shown. The fifteen control patterns (basic patterns) 18 from figure 2 have now been covered by the test pattern, as described in more detail above, which has been written by the first imagesetting device whose relative position with respect to the second imagesetting device (master) is to be determined, so that fifteen combination patterns 24 have been produced. There therefore exist fifteen blocks each having a reference pattern 16 and a combination pattern 24 for an associated relative position. Since, in this example of figure 3, the first relative position of the imagesetting devices with respect to each other differs from the relative intended position, when the first imagesetting device is driven to write the pixels (pattern units) as though the relative position of the first with respect to the second imagesetting device were the intended position, the test pattern is offset with respect to the position of the control patterns (basic patterns) 18 on the imagesetting medium by the first relative position (deviation of the first position from the intended position).
In the example of figure 3, the results for the first position are the following combination patterns 24: a first subset of the combination patterns 210 remains unchanged, since the first imagesetting device has set an image only of pattern units of which an image has already been set by the second imagesetting device. As illustrated by using figure l, the result is no qualitative change in the pattern units as a result of renewed imagesetting. A second subset of the combination patterns 212, on the other hand, now has a changed two-strip periodicity: ten exposed and six unexposed strips.
In the right-hand part of figure 3, an enlargement of a detail of the blocks of reference pattern 16 and combination pattern 24 for the relative positions 15.0 um, 20.0 ~zm and 25.0 um, in addition to the scale 20, is shown. By means of optical determination or measurement of the area coverage, it is shown that the area coverage of a first identified combination pattern 26 agrees with the area coverage of a reference pattern 28 (ten exposed, six unexposed, ten exposed, six unexposed), so that it can be concluded or a signal can be generated within an electronic or information-technical processing unit that the reference pattern 28 corresponds with the first identified combination pattern 26. An identification of the reference pattern 28 has taken place and, on the basis of the unique assignment to the relative position (deviation), the latter is determined. In the example of figure 3, this is the relative position 20.0 um. By means of the specified method, for example in this advantageous embodiment, it is therefore possible to determine both the magnitude and the direction of the deviation.
Figure 4 describes combination patterns written above one another schematically for a second relative position, deviating from the intended position, of the first with respect to the second imagesetting device on arranged patterns and reference patterns of the advantageous embodiment according to figure 2. Figure 4 has a left-hand and a right-hand part, the right-hand part being an enlarged detail of the left-hand part. In the left-hand part, the fifteen reference patterns 16 and the scale 20 which have already appeared in figure 2 are shown. The fifteen control patterns (basic patterns) 18 from figure 2 have now been covered by the test pattern, as described in more detail above, which has been written by the first imagesetting device whose relative position with respect to the second imagesetting device (master) is to be determined, so that fifteen combination patterns 24, which differ from those for the first position according to figure 3, have been produced. Since, in this example of figure 4, the second relative position of the imagesetting devices with respect to each other differs from the relative intended position, if the first imagesetting device is driven to write the pixels (pattern units) as though the relative position of the first with respect to the second imagesetting devices were the intended position, the test pattern is offset with respect to the position of the control patterns (basic patterns) 18 on the imagesetting medium by the first relative position (deviation of the first position from the intended position).
In the example of fig. 4, the result for the second position is the following combination patterns 24: a third subset of the combination patterns 218 now has a changed two-strip periodicity: nine exposed and seven unexposed strips. A fourth subset of the combination patterns 220, on the other hand, remains unchanged, since the first imagesetting device sets images only of pattern units of which images have already been set by the second imagesetting device. As illustrated by using figure 1, the result is no qualitative change in the pattern units as a result of renewed imagesetting. In the right-hand part of figure 4, an enlargement of a detail of the blocks of reference pattern 16 and combination pattern 24 is now shown for the relative positions -15.0 Vim, -10.0 um and -5.0 Vim, together with the scale 20. By means of optical determination or measurement of the area coverage, it is shown that the area coverage of a second identified combination pattern 214 coincides with the area coverage of a reference pattern 216 (nine exposed, seven unexposed, nine exposed, seven unexposed), so that it can be concluded or a signal can be generated within an electronic or information-technical processing unit that the reference pattern 216 corresponds with the second identified combination pattern 214. An identification of the reference pattern 216 has taken place and, on the basis of the unique assignment to the relative position (deviation), the latter is determined. In the example of figure 4, this is the relative position -10.0 um. By means of the method specified, for example in this advantageous embodiment, it is therefore possible to determine both the magnitude and the direction of the deviation, in this case of figure 4, opposite to that of figure 3.

Figure 5 is a schematic plan view of a printing press having an embodiment of a printing unit according to the invention with a first and a second imagesetting device, whose relative position is determined and corrected by the methods according to the invention. Provided within a printing press 50 is a printing unit 52, in which an imagesetting medium 54 can be written on or have an image set (direct imaging printing unit or on-press imagesetting). A first imagesetting device 56 and a second imagesetting device 58 are assigned to the imagesetting medium 54 and have a relative position 510 with respect to each other. Translation 514 of the first imagesetting device 56 and translation 516 of the second imagesetting device 58 are in each case symbolized by a double arrow. Expressed in other words, the first and the second imagesetting device 56, 58 can be moved both relative to the surface of the imagesetting medium and also with respect to each other in this embodiment. In order to produce these movements, the printing unit 52 according to the invention comprises a linear drive 516 with a suitable actuator mechanism 518 for the first and the second imagesetting device 56, 58. The actuator mechanism 518 is linked to a control unit 520. The control unit 520 has a memory unit 522 in which a computer program is stored which has at least one section in which method steps relating to the position correction, which comprises the steps of the method according to the invention for determining the relative position of the first with respect to the second imagesetting device 56, 58, are executed. As an alternative to a computer program in a memory unit 522, it is also possible to provide an electronic circuit which can carry out the same logical operations.
The imagesetting medium 54 can be rotated about an axis of rotation 524, for which purpose a suitable drive, not shown here, is provided. As a result of the rotation about the axis of rotation 524 and the translations 512, 514 substantially parallel to the axis of rotation 524, all the coordinate points at which images are to be set on the surface of the imagesetting medium 54 can be reached by at least one of the two imagesetting devices 56, 58. With the aid of a measuring device 526, which is translated along the travel path 528 by a drive not shown here, indicated by the double arrow 530, the area coverages of various patterns written by the imagesetting devices, for example directly on the imagesetting medium or on a printing material onto which the written image has been printed off in the printing unit 52, can be determined. The measuring device 530 is linked to the control unit 520, so that the information obtained about the relative actual position can be processed for the position correction, be it via the controlled actuator mechanism, via a change in the timed triggering or via both of these measures.

LIST OF DESIGNATIONS
Pattern 12 Pattern unit 14 Set image of a pattern unit 16 Reference pattern 18 Basic pattern Scale 22 Reference pattern for intended position 24 Combination pattern 26 First identified combination pattern 28 Reference pattern of the first actual position 210 First subset of combination patterns 212 Second subset of combination patterns 214 Second identified combination pattern 216 Reference pattern of the second actual position 218 Third subset of combination patterns 220 Fourth subset of combination patterns 50 Printing press 52 Printing unit 54 Imagesetting medium 5& First imagesetting device 58 Second imagesetting device 510 Relative position 512 Translation of the first imagesetting device 514 Translation of the second imagesetting device 516 Linear drive 518 Actuator mechanism 520 Control unit 522 Memory unit 524 Axis of rotation of the imagesetting medium 526 Measuring device 528 Travel path of the measuring device 530 Translation of the measuring device

Claims

1. A method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) by setting images on an associated imagesetting medium (54), characterized by - setting an image of a group of mutually different reference patterns (16) and a basic pattern (18) by means of the second imagesetting device (58), each reference pattern (16) from the group being assigned uniquely to one relative position;
- setting an image of at least one test pattern by means of the first imagesetting device over the basic pattern, so that a combination pattern (24) is produced;
- identifying a reference pattern (16), from the group, whose area coverage coincides with the area coverage of the combination pattern (24);
- identifying the relative position (510) that is associated with the identified reference pattern (16) from the group.

2. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 1, characterized in that the reference patterns (16) from the group, the basic pattern (18) and the test pattern are two-dimensional, but are uniform in one of the two linearly independent directions.

3. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 1, characterized in that the basic pattern (18) comprises a group of mutually identical control patterns.

4. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 3, characterized in that the number of reference patterns (16) in the group and the number of control patterns is the same and in each case one reference pattern (16) in the group is uniquely assigned just one control pattern.

5. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 4, characterized in that the mutually associated reference patterns (16) in the group and control patterns are arranged adjacently on the surface of the imagesetting medium (54).

6. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 5, characterized in that the number of reference patterns (16) in the group is uneven, and in that the test pattern covers a first part of the control patterns with a first sub-pattern, a second part of the control patterns with a second sub-pattern, and a control pattern is covered both by the first sub-pattern and by the second sub-pattern.

7. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 2, 3, 4, 5 or 6, characterized in that the reference patterns (16) from the group, the control patterns and the test pattern are two-dimensional, but are uniform in one of the two linearly independent directions.

8. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in one of the preceding claims, characterized in that the basic pattern (18) exhibits 50% area coverage.

9. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in one of the preceding claims, characterized in that the basic pattern (18) has a regular sequence of a number of exposed and of the same number of unexposed points in one direction of its extent.

10. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in one of the preceding claims, characterized in that each of the reference patterns (16) in the group of reference patterns (16) has a regular sequence of exposed and unexposed points in one direction of its extent, the sequences in one period in each case having a first and a second number of exposed points and a third and a fourth number of unexposed points.

11. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 10, characterized in that the directions of the extents of the reference patterns (16) in the group of reference patterns (16) are parallel to one another.

12. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in one of the preceding claims, characterized in that the number of reference patterns (16) in the group of mutually different reference patterns (16) is uneven, and in that a reference pattern (16) is assigned to the relative intended position, a first part of the group of patterns is assigned to relative positions which are greater than the relative intended position, and a second part of the group of patterns is assigned to relative positions which are smaller than the relative intended position.

13. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 12, characterized in that for one absolute value of the relative position, the reference pattern (16) from the first part of the group is identical to the reference pattern (16) from the second part of the group.

14. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in claim 12, characterized in that for one absolute value of the relative position, the reference pattern (16) from the first part of the group with respect to the reference pattern (16) from the second part of the group exhibits mirror symmetry with respect to an axis at right angles to the extent of one of the reference patterns (16).

15. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in one of the preceding claims, characterized in that the group of mutually different reference patterns (16) are arranged on the imagesetting medium (54) so as to be ordered in accordance with the associated relative intended position.

16. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) as claimed in one of the preceding claims, the imagesetting medium (54) being accommodated in a printing unit (52), characterized by - printing off the imagesetting medium (54) on which an image has been set by the first and the second imagesetting device (56, 58) onto a printing material before the identification of coincident area coverage, the identification being performed on the printing material, and of the relative position associated with the identified reference pattern (16) from the group.

17. The method of determining the relative position (510) of a first imagesetting device (56) with respect to a second imagesetting device (58) by setting an image on a first imagesetting medium (54) assigned to the first imagesetting device (56) and on a second imagesetting medium (54) assigned to the second imagesetting device (58), characterized by - setting an image from a group of mutually different reference patterns (16) and a basic pattern (18) by means of the second imagesetting device (58) on the second imagesetting medium (54) assigned to the second imagesetting device (58), each reference pattern (16) from the group being assigned uniquely to one relative position;

- setting an image of at least one test pattern by means of the first imagesetting device (56) on the first imagesetting medium assigned to the first imagesetting device (56);

- printing off the first and second imagesetting media (54) on which images have been set by the first and the second imagesetting device (5b, 58) onto a printing material in such a way that the test pattern of which an image was set by the first imagesetting device (56) is printed off over the basic pattern, so that a combination pattern (24) is produced;

- identifying a reference pattern (16) from the group, whose area coverage coincides with the area coverage of the combination pattern (24);

- identifying the relative position (510) which is associated with the identified reference pattern (16) from the group.

18. A method of correcting the position of a point of projection of a first imagesetting device (56) with respect to a point of projection of a second imagesetting device (58) by changing the relative position (510) and/or by changing the timed triggering of the first imagesetting device (56) with respect to the second imagesetting device (58) from an actual position to an intended position, characterized by a determination of the actual position by a method as claimed in one of the preceding claims, the area coverages being determined by a measuring device (526).

19. A printing form exposer having a first and a second imagesetting device (56, 58) which can be moved relative to the imagesetting medium (54) and/or to one another by means of a controlled actuator mechanism (518), and a control unit (520), characterized in that the control unit (520) comprises an electronic unit with a memory unit (522), in which a computer program for correcting the position of the first and the second imagesetting device (56, 58) by means of the controlled actuator mechanism (518) and/or by means of changing the timed triggering by means of a method as claimed in claim 18 is stored.

20. A printing unit (52) having a first and a second imagesetting device (56, 58) which can be moved relative to the imagesetting medium (54) and/or to one another by means of a controlled actuator mechanism (518), anal a control unit (520), characterized in that the control unit (520) comprises an electronic unit with a memory unit (522), in which a computer program for correcting the position of the first imagesetting device (56, 58) by means of the controlled actuator mechanism (518) and/or changing the timed triggering by means of a method as claimed in claim 18 is stored.

21. A printing unit group having a first printing unit, to which a first imagesetting device (56) is assigned, and having a second printing unit, to which a second imagesetting device (58) is assigned, which can be moved relative to the imagesetting media (54) and/or to one another by means of a controlled actuator mechanism (518), and a control unit (520), characterized in that the control unit (520) comprises an electronic unit with a memory unit (522), in which a computer program for correcting the position of the first imagesetting device (56, 58) by means of the controlled actuator mechanism (518) and/or changing the timed triggering by means of a method as claimed in claim 18 is stored.

22. A printing press (50), characterized by at least one printing unit (52) as claimed in claim 20, or a printing unit group as claimed in claim 21.