EP0512448A1 - Device for reading pale colour marks in printing machines - Google Patents

Abstract

The device for reading a mark (15) printed on an element (10) moving across in front of a light source (20) comprises at least two parallel mark reading channels (32,34,40,50,60) generating as output an electrical pulse when the mark passes, each channel being sensitive to a particular colour. The device moreover comprises electronic means (70) selecting, among the electrical pulses generated by the channels, the pulse which is most representative of the mark. <IMAGE>

Description

The present invention relates, in a polychrome printing machine, to a device for reading marks of different colors printed by each printing station, these marks subsequently making it possible to determine the identification error of each color by relative to the color of the first station taken as a reference.

Known devices, such as that described in document EP 0 094 027, function to satisfaction as long as the yellow, blue or red marks are sufficiently contrasted to be recognized without ambiguity by the device. Some of these known devices may include a bundle of optical fibers carrying light on the printed element and conducting the reflected light to a reading photodiode which produces an electrical signal. In order to increase the contrast between the basic electrical signal corresponding to an area of the unprinted element and an electrical pulse caused by the passage of a mark, a filter, usually blue, is interposed between the optical fibers and the photodiode. .

However, as soon as the printing colors are pale, in particular in printing for packaging involving prepared inks of pastel yellow, cream or sky blue color, conventional devices can no longer safely detect each mark printed, which may prevent a tracking control system from working properly. Indeed, it would then be appropriate to try a first filter, to pass a light colored mark to observe the quality of the signal obtained and to repeat the test with one or more other filters in order to select the most suitable for reading all the marks. However, the most important phase of starting up the printing machine is precisely the search for the initially unknown position of a mark which cannot therefore be validly carried out without a reading device immediately operating. These many necessary tests quickly become prohibitive if you want to perform with the same printing machine many different jobs.

The object of the present invention is a device detecting a printed mark whatever its color, its intensity and its contrast with respect to the background color of the plate element on which it is printed.

These aims are achieved by means of a device for reading a printed mark because it comprises at least two parallel channels for reading marks, each generating an electrical pulse at the output when the mark passes in front of the light source, the photosensitive unit at the input of each channel being sensitive in a color band distinct from the others; as well as electronic means selecting, from the electrical pulses generated by the channels, the most representative pulse of the brand.

• une unité photosensible générant un signal électrique en tension, suivie, si désiré,
• d'un étage d'amplification à gain automatique fixant à une valeur prédéterminée la tension de base correspondant à une zone de l'élément non imprimée, suivi
• d'un étage convertissant l'impulsion électrique à flanc oblique, provoquée par le passage de la marque devant l'unité photosensible, en une impulsion électrique à flanc raide, chaque flanc raide correspondant au départ de la montée ou de la descente du flanc oblique associé,
• et des moyens électroniques sélectionnant, parmi les impulsions électriques issues des canaux à une période donnée, celle apparaissant et disparaissant la première.

Advantageously, each brand reading channel comprises:

• a photosensitive unit generating an electrical signal in voltage, followed, if desired,
• an automatic gain amplification stage setting the base voltage corresponding to an area of the unprinted element to a predetermined value, followed
• a stage converting the electrical impulse with an oblique flank, caused by the passage of the mark in front of the photosensitive unit, into an electrical impulse with a steep flank, each steep flank corresponding to the start of the ascent or descent of the oblique flank associate,
• and electronic means selecting, among the electrical pulses coming from the channels at a given period, that appearing and disappearing first.

Thus, thanks to this device, the most contrasted electrical pulse is systematically retained regardless of the quality of the other pulses considered.

A subsidiary problem may however complicate to some extent the design of the selection circuit since a color mark printed on a white plate element causes a negative pulse compared to the basic signal, whereas a highly reflective color, such as gold or silver, causes an inverted pulse, i.e. positive with respect to the basic signal. This problem is obviated in that each read channel further comprises, before the converter stage, a rectifier stage imposing on all the electrical pulses a variation in the same direction with respect to the basic voltage.

Usefully, the photosensitive unit includes a photodiode behind a colored filter and connected to the input of a current / voltage converter.
According to a preferred embodiment, the rectifier stage comprises a first stage for measuring the base voltage, followed by a stage for subtracting the base voltage leaving only positive or negative electrical pulses, followed by a stage rectification for only positive pulses into negative pulses, followed by a summation stage of all the pulses and followed by a readjustment stage of the basic voltage.

According to a preferred embodiment, the converter stage comprises a first peak detection stage followed by a second stage of subtraction from the input signal of the threshold detected by the first stage, the difference being applied to a tilting comparator as soon as the difference exceeds a predetermined threshold, as well as first electronic means resetting and reversing the direction of detection of the peak detection stage and second electronic means reversing the polarity of the comparison threshold applied to the comparator after a first switching of this comparator .

" étant branchées à l'entrée d'une seconde porte "ET" dont la sortie est reliée à l'entrée de réinitialisation "CL" de la première bascule, l'entrée de réinitialisation "CL" de chaque bascule secondaire étant branchée sur la sortie "Q" de la première bascule, une dernière ligne de contrôle des moyens électroniques étant reliée à l'une des entrées de la seconde porte "ET".According to a preferred embodiment, the electronic pulse selection means comprise a first "OR" gate receiving on each of its inputs one of the pulses and the output of which is connected to the clock input "CLK" d 'a first flip-flop, as well as as many secondary flip-flops as pulses to be analyzed, these pulses being received inversely on the clock input "CLK", all the outputs reversed of the secondary flip-flops " $\overline{\text{Q}}$

"being connected to the input of a second" AND "gate, the output of which is connected to the reset input" CL "of the first flip-flop, the reset input" CL "of each secondary flip-flop being connected to the output "Q" of the first flip-flop, a last control line of the electronic means being connected to one of the inputs of the second gate "AND".

According to an advantageous embodiment, the device further comprises an analog / digital and digital / analog converter connected to a microprocessor for receiving the measurement of the base voltage from the rectifier stage and for applying on the one hand to the stage amplification with automatic gain, when present, an electrical signal representative of the gain to be applied, and on the other hand at the converter stage an electrical signal representative of the optimum threshold for the comparator. Thanks to this latter device, the voltage is permanently maintained at a value of approximately 8 volts and the detection threshold of the comparator is fixed at a value between 200 and 400 millivolts above the average noise present on the base voltage. .

• la figure 1 est un diagramme schématique du dispositif selon l'invention,
• la figure 1a est une vue partielle d'une exécution particulière du dispositif selon l'invention,
• la figure 2 est un plan du circuit redresseur compris dans le dispositif de la figure 1,
• la figure 3 est un plan du circuit convertisseur inclus dans le dispositif de la figure 1,
• la figure 4 est un diagramme de l'opération effectuée par le convertisseur de la figure 3, et
• la figure 5 est un plan du circuit de sélection compris dans le dispositif selon la figure 1.

Tel qu'illustré sur la figure 1, le dispositif selon l'invention comprend un faisceau de fibres optiques 25 acheminant d'abord la lumière issue d'une source lumineuse 20 au-dessus de l'élément imprimé 10 portant des marques de couleur 15 imprimées sur la face supérieure. Ces éléments peuvent être des bandes de papier ou des plaques de carton en cours de fabrication. Ces marques 15 sont imprimées par chaque station d'impression à un endroit non contraignant de l'élément imprimé, et ce dans la couleur de la station. Le passage de ces marques 15 devant les fibres optiques modifie temporairement plus ou moins fortement la lumière réfléchie qui est acheminée, après dédoublement de ces fibres optiques, vers deux photodiodes séparées 32,33.The invention will be better understood on studying an embodiment taken by way of nonlimiting example and described by the following figures:

• FIG. 1 is a schematic diagram of the device according to the invention,
• FIG. 1a is a partial view of a particular embodiment of the device according to the invention,
• FIG. 2 is a plan of the rectifier circuit included in the device of FIG. 1,
• FIG. 3 is a plan of the converter circuit included in the device of FIG. 1,
• FIG. 4 is a diagram of the operation carried out by the converter of FIG. 3, and
• FIG. 5 is a plan of the selection circuit included in the device according to FIG. 1.

As illustrated in FIG. 1, the device according to the invention comprises a bundle of optical fibers 25 first conveying the light coming from a light source 20 above the printed element 10 bearing colored marks 15 printed on the upper side. These elements can be strips of paper or cardboard plates during manufacture. These marks 15 are printed by each printing station at a non-binding location on the printed element, and this in the color of the station. The passage of these marks 15 in front of the optical fibers temporarily modifies more or less strongly the reflected light which is routed, after splitting of these optical fibers, towards two separate photodiodes 32,33.

According to the invention, the photodiodes 32,33 are respectively made sensitive to distinct colors by means of filters 30,31 interposed between the output of the optical fibers and the photodiodes. For example, the filter 30 can be a dark purple filter favoring the yellow marks, while the filter 31 is green promoting the blue marks. The electrical signals from each photodiode are first conditioned separately and in parallel by identical processing channels and constituted by the circuits 34, 40, 50 and 60 before being compared for selection by a circuit 70.

These identical and parallel conditioning channels each each firstly comprise a current / voltage converter 34 transforming the variation in intensity within the photodiode caused by the passage of the mark 15 in front of the optical fiber into a variation in voltage. As symbolized, this current / voltage converter is made so known by means of an operational amplifier and a negative feedback between its negative output and input. Jumpers symbolized at the output allow to implement a first or a second feedback circuit modifying in a ratio of 1 to 10 the gain of the amplification of this stage. This voltage signal is then amplified by an automatic gain amplifier circuit 40 so that the basic signal corresponding to an unprinted area of the element 10 is fixed at a value of the order of 8 volts. Indeed, depending on the background color of the printed element 10, the length of the fiber bundle 25, any dust that can alter the entry or exit of the fibers as well as the filters, the base voltage collected at the output of the current / voltage converter 34 can vary between 150 millivolts and 8 volts.

The electrical signal then passes through a rectifier circuit which has the function of bringing all the pulses caused by a color mark in the same direction, in this case negative, with respect to the basic voltage. Indeed, in the majority of cases, the marks 15 are printed with colors darker than the background color and thus cause a reduction in the light reflected in the optical fibers, ie an instantaneous reduction in the current passing through the photodiode 30 therefore a pulse of value lower than the basic voltage value. Conversely, if the marks appear lighter than the background color, or even are printed with particularly reflective colors such as gold or silver, the reflected light is momentarily greater than the base light and it in the same is true for the corresponding electrical pulse. This rectifier circuit makes it possible, by bringing all the pulses to the same side, to greatly simplify the subsequent selection circuit.

Figure la shows a device similar to that of Figure 1 in which the optical fiber 25 has not been split. A light diffusion member 25a has been arranged at the end of the optical fiber 25 so that the reflected light is routed indifferently on the filters 30 and 31. The construction of the other elements of the device comprising inter alia the photodiodes 32 and 33 as well as the current / voltage converters 34 and 35 remains unchanged compared to the arrangement described in FIG. 1.

With reference to FIG. 2, this rectifier circuit 50 first comprises a bottom measurement stage 51 followed by a bottom subtraction stage 53 followed by the effective rectification stage 55 followed by a stage for adding the pulses 57 which finally ends a floor of readjustment of the bottom 59.

As illustrated, the bottom measurement stage 51 essentially comprises the combination of a diode 513 and a capacitor 514, the other branch of which is connected to ground. Operational amplifiers 511 and 512 provide stage isolation. The switch 515 by temporarily short-circuiting the diode 513 makes it possible to periodically reset this bottom measurement.

The subtraction stage comprises, in a known manner, an operational amplifier 533 receiving on the one hand the complete signal through the resistor 531 on its positive input and on the other hand the background at subtract through resistor 532 on its negative input.

In the rectification stage 55, only the positive pulses are amplified and inverted by the operational amplifier 553 comprising two diodes 551,552 in its feedback circuit. The addition by the operational amplifier 573 of the summation stage 57, receiving on its negative terminal on the one hand the signal directly coming from the subtraction stage 53 through the resistor 571 and on the other hand the pulses amplified negatives counterbalancing the positive pulses, makes it possible to obtain at the output of this stage a series of pulses of the same amplitude as initially but now all in the negative direction.

The operational amplifier 593 of the summation stage 59 performs the addition of the background received directly from the first bottom measurement stage 51 through the resistor 591 and of the pulses received from the summation stage 57 through the resistor 592.

With reference to FIG. 1, the rectifier circuit 50 is followed by a converter circuit 60 of pulses with an oblique flank into pulses with a steep flank, pulses more capable of being processed logically thereafter.

Indeed, and as illustrated in FIG. 4, the pulses el and e2 generated by the photodiodes 32 or 33 have a first downward oblique slope corresponding to the progressive penetration of the mark in the reading field of the optical fibers, followed by a plateau during the passage of the body of the mark, and that ends a second rising oblique slope corresponding to the mark gradually leaving the reading field.
The detail of this converter 60 will now be described in relation to FIG. 3 from which one can distinguish four important stages: a first peak detection stage 61 followed by a subtraction stage 62 from the peak measured at the instantaneous signal followed by a comparison stage 63 of the difference with a predetermined threshold from a stage 64. The result of the comparison is shaped by the operational amplifier 632, an inverted signal of which is generated by the inverter 633. The output of the shaping amplifier 632 is also used in feedback to on the one hand reverse the direction of maximum detection of stage 61 and on the other hand modify the threshold value leaving stage 64.

The peak detection stage 61 essentially comprises a diode 614 (then 615) in relation to a capacitor 613, this device being isolated at the input by the conventional amplifier 611 and at the output by the operational amplifier 612. The direction of detection maximum, either up or down, is initially determined by the state of the relay 65 selecting the diode 614 or 615. This stage is reset, after a short delay introduced by the inverter 633 by means of the diodes 616 or 617 as the case may be, using relay 644.

The subtraction stage 62 receives on the one hand the signal coming from the peak detector stage 61 through the resistor 621 and on the other hand, through the resistor 622, the instantaneous signal previously amplified by a gain of 1 by the operational amplifier 619. The comparison is carried out by the amplifier 631 receiving the threshold signal on its positive input and the difference signal on its negative input.

As can easily be understood with reference to FIGS. 3 and 4, stage 61 first acquires the value of the base voltage, and the output of stage 62 first presents a zero signal which increases only at l appearance of the downward oblique slope of an impulse. When this oblique slope of this pulse has exceeded a predetermined threshold v1 with respect to the base voltage, the operational amplifier 631 switches over and there appears a first steep rise in voltage s11 at the output of the inverter 632. This rise in voltage s11 first causes the selection of the diode 615 allowing the capacitor 613 to be discharged through the diode 617 then the connection of the diode 616, after a delay determined by the inverter 633. The stage 61 is then ready to detect a new maximum but in the downward direction. This first rise in voltage also caused in stage 64 a modification of the threshold voltage v2 by grounding the positive input gate of an operational amplifier.

Stage 61 then detects the value of the lower plateau of the input pulse el and the output of the subtraction stage 62 remains at zero during the entire duration of this lower plateau. Again, as soon as the beginning of the oblique flank of the ascent of the input pulse appears, the difference at the output of stage 62 increases until it exceeds the new threshold v2 of comparator 631 which is inverted thus causing a sudden descent of the shaping amplifier 612 s12.

Thus, the rising steep flank of the output pulse s1 corresponds substantially to the start of the descending oblique flank of the input pulse el, and the steep flank falling from the output pulse s1 corresponds substantially to the start of the oblique flank rising from the input signal el.

With reference to FIGS. 1 and 4, the pulses s1 and s2 now in slots, originating respectively from the channel corresponding to the yellow color and from the channel corresponding to the blue color, are applied to a selection circuit 70 retaining the rising pulse s1 which will descend the first and which will correspond to the initial oblique and most contrasted impulse.

" de ces bascules secondaires sont branchées à l'entrée d'une seconde porte "ET" 75 dont la sortie est reliée à l'entrée de réinitialisation "CL" de la première bascule 72. Par ailleurs, la sortie "Q" de cette première bascule 72 est reliée également à l'entrée de réinitialisation "CL" de chacune des bascules secondaires 73,74. Enfin, une dernière ligne d'autorisation ou de blocage 85 du circuit de sélection 70 est branchée à l'une des entrées de la porte "ET" 75.The embodiment of circuit 70, as illustrated in FIG. 5, firstly comprises a first "OR" gate 71 receiving on each of its inputs one of the pulses in slots, and the output of which is connected to the clock input "CLK" of a first flip-flop 72. The selection circuit 70 also includes as many secondary flip-flops 73,74 as pulses to be analyzed, these pulses being received in reverse manner respectively on their clock input "CLK". All outputs reversed " $\overline{\text{Q}}$

"of these secondary flip-flops are connected to the input of a second" AND "gate 75, the output of which is connected to the reset input" CL "of the first flip-flop 72. Furthermore, the output" Q "of this first flip-flop 72 is also connected to the reset input "CL" of each of the secondary flip-flops 73,74 Finally, a last authorization or blocking line 85 of the selection circuit 70 is connected to one of the inputs of the "AND" gate 75.

" correspondante à l'état bas, créant ainsi le front descendant de l'impulsion de sortie. Cet état bas sur la sortie "Q" de la bascule 72 a également comme conséquence de réinitialiser toutes les bascules secondaires 73,74 remettant toutes les sorties inversées " $\overline{\text{Q}}$

" à l'état haut et bloquant par là-même ces bascules rendant inopérante la montée du signal inversé suivant. La porte "ET" 75 retourne à l'état haut ce qui libère à nouveau la bascule 72 prête pour une sélection suivante tant qu'une autorisation sur la ligne 85 est maintenue.In the initial state of the device, all the inputs of the "AND" gate 75 are in the high state which released the first flip-flop 72 whose output "Q" is initially low implying the blocking of the flip-flops 73 and 74. On the arrival of a pulse on one of the inputs of door 71, the output of this door is found high which results in the appearance of a high state on the exit door " Q "of flip-flop 72 constituting the rising edge of the output pulse and, also, unlocking flip-flops 73 and 74. The arrival of the rising edge of the second pulse then has no effect on circuit 70. By against, the arrival of the first rising edge of an inverted signal, corresponding in fact to the falling edge of this first pulse, changes the state of the corresponding flip-flop 73,74 which has the effect of immediately lowering the gate "Q "corresponding. The door "AND" 75 sees at least one of its entry doors put in the low state, and its exit also lowers, which resets the first flip-flop 72, therefore puts the door back " $\overline{\text{Q}}$

"corresponding to the low state, thus creating the falling edge of the output pulse. This low state on the output" Q "of the flip-flop 72 also has the consequence of resetting all the secondary flip-flops 73,74 resetting all the outputs reversed " $\overline{\text{Q}}$

"in the high state and thereby blocking these flip-flops rendering the rise of the next inverted signal inoperative. The" AND "gate 75 returns to the high state which again releases flip-flop 72 ready for a next selection as long as an authorization on line 85 is maintained.

With reference to FIG. 1, the device according to the invention further comprises an analog / digital and digital / analog converter 90 in relation to a microprocessor 80, this device being able to receive on line 81 a measurement of the value of the voltage of base in order to return on lines 82 an electrical signal corresponding to the gains to be applied to the automatic gain amplification circuits 40 and 41, and on the other hand on the lines 83 a threshold value for the comparator 63 of the circuits 60 and 61, threshold fixed between 100 and 400 millivolts above the noise of background measured on the basic signal. The microprocessor also sends on line 85 a control signal blocking the selection circuit when no mark is expected.

As we have seen on reading this presentation, this device according to the invention inevitably detects a mark passing in front of the light flux coming from the source 20 by making an instant selection of the best reading channel, for yellow or for blue, taking into account the color, contrast and intensity of the brand sought. For machines that have to do delicate work, it is quite possible to add a third or fourth parallel reading channel for other very distinct colors. Many improvements can be made to this device in the context of this invention.

Claims (8)

Device for reading a mark (15) printed on a plate or strip element (10) moving past a light source (20) in a printing machine, characterized in that it comprises at least two parallel mark reading channels (32,34,40,50,60) supplied with reading information by at least one optical fiber (25), said channels (32,34,40,50,60) each generating an output electrical impulse when passing the mark in front of the light source, the photosensitive unit (30,32,34) at the entrance of each channel being sensitive in a frequency band of color distinct from the frequencies of the other channels, as well as electronic means (70) selecting, from the electrical pulses generated by the channels, the most representative pulse of the brand. Device according to claim 1, characterized in that each brand reading channel comprises: - a photosensitive unit (30, 32, 34) generating an electrical signal in voltage, followed, if desired, - an automatic gain amplification stage (40) fixing the base voltage corresponding to an area of the unprinted element at a predetermined value, followed - followed by a stage (60) converting the electrical impulse with oblique sides caused by the passage of the mark in front of the photosensitive unit into an electric impulse with steep sides, each steep side corresponding to the start of the ascent or of the descent associated oblique flank; and - Electronic means selecting, from the electrical pulses coming from the channels at a given period, that appearing and disappearing first. Device according to claim 2, characterized in that each read channel further comprises, before the converter stage (60), a rectifier stage (50) imposing on all the electrical pulses a variation in the same direction with respect to the voltage basic. Device according to one of the preceding claims, characterized in that the photosensitive unit comprises a photodiode (32) behind a color filter (30) and connected to the input of a current / voltage converter (34). Device according to claim 3, characterized in that the rectifier stage (50) comprises a first stage (51) for measuring the base voltage, followed by a stage for subtracting (53) from the base voltage leaving only positive or negative electrical pulses, followed by a rectification stage (55) for only positive pulses into negative pulses, followed by a summation stage (57) of all the pulses and followed by a readjunction stage of the basic voltage (59). Device according to one of the preceding claims, characterized in that the converter stage (60) comprises a first peak detection stage (61) followed by a second subtraction stage (62) from the input signal of the detected threshold by the first stage, the difference being applied to a comparator (63) tilting as soon as the difference exceeds a predetermined threshold, as well as first electronic means (644,65) resetting and reversing the direction of detection of the peak detection stage (61) and of the second electronic means (66) reversing the polarity of the comparison threshold applied to the comparator (63) after a first tilting of this comparator. Device according to one of the preceding claims, characterized in that the electronic pulse selection means (70) comprise a first "OR" gate (71) receiving one of the pulses on each of its inputs and the output of which is connected to the clock input "CLK" of a first flip-flop (72); as well as as many secondary flip-flops (73; 74) as pulses to be analyzed received inversely on their clock input "CLK", all the outputs inverted ( $\overline{\text{Q}}$

) being connected to the input of a second "AND" gate (75), the output of which is connected to the reset input (CL) of the first flip-flop (72), the reset input (CL) of each secondary flip-flop (73,74) being connected to the output (Q) of the first flip-flop (72), a last control line of the electronic means being connected to one of the inputs of the second "AND" gate (75). Device according to claims 2, 5, 6 and 7, characterized in that it comprises an analog / digital and digital / analog converter (90) connected to a microprocessor (80) to receive the measurement from the rectifier stage (50) of the basic voltage and to apply on the one hand to the amplification stage with automatic gain (40), when present, an electrical signal representative of the gain to be applied, and on the other hand to the converter stage (60 ) an electrical signal representative of the optimum threshold for the comparator (63).

Images