EP0553895B1 - Printing set for franking or obliterating machine or the like - Google Patents

Description

The invention relates to an inkjet printing assembly for printing on rapidly moving correspondence objects, in particular in a postage, franking or more generally stamping machine.

Such an assembly, known for example from document GB-A-2 110 854 or FR-A-2 158 636, comprises printheads provided with ink emission nozzles, guiding and driving means of the object or medium to be printed in front of said nozzles, and means for detecting the advance of the medium triggering the emission of ink by said nozzles.

In a postmarker, we want to print a standard postal imprint, dimensions 80 mm x 25 mm, in the upper right corner of postal items. In a franking machine, the width of the area to be printed can reach a greater value, without this changing the conditions and the solutions set out below. The speed of treatment must be able to reach several meters per second and, as such, the print heads using the technique "drops on demand" with piezoelectric actuator, allow to reach a linear speed of up to 2 m / s with a writing density of 4 to 6 points per mm [ie 100 to 150 dpi (where dpi is the abbreviation of "dots per inch")].

• la distance entre les buses qui produisent les gouttes et la surface du papier.
• L'émission des gouttes d'encre en fonction de l'avancement du papier afin d'éviter les distorsions d'image.

To function properly with a high-performance inkjet, it is necessary to precisely control:

• the distance between the nozzles that produce the drops and the surface of the paper.
• The emission of ink drops according to the progress of the paper in order to avoid image distortions.

This latter condition must be fulfilled in any case, but even more precisely with printing systems in which the nozzle spacing is greater than the dot spacing, which means that the nozzle line has to be inclined relative to the direction of movement of the paper.

It is understood that under these conditions, it is necessary to offset the times of emission of the different nozzles to compensate for their spatial offset due to the inclination. This spatial offset is of the order of a hundred elementary steps (distance between two adjacent drops), and to avoid visible distortion, the relative error on the offset must be less than one percent.

• il n'est pas possible de faire appuyer le papier sur un guide juste après l'impression car il y a un risque important de maculage,
• si l'épaisseur de l'objet n'est pas régulière, cela produit des irrégularités d'appui qui altèrent la qualité d'impression.

To control the distance between the nozzles and the paper, it was envisaged to use a support plate for flat objects projecting slightly from the nozzles and a system of belts holding the object on its rear face and pressing it on the support plate; such a system has two drawbacks:

• it is not possible to press the paper on a guide just after printing because there is a significant risk of smearing,
• if the thickness of the object is not regular, this produces support irregularities which deteriorate the print quality.

The object of the invention is to give a satisfactory solution to this fundamental problem.

According to the invention, as defined in claim 1, the means for guiding and driving the printing assembly comprise a device for keeping the object away from the nozzles by suction by means of compressed air jets operating in Bernoulli regime; thus an "air cushion" separates the printed surface of the paper and the surrounding objects, avoiding any risk of smudging. Such a solution is therefore completely different from known solutions where perforated belts connected to suction means create suction which presses the support against the belt, without self-regulation of a holding distance as in the present invention.

Advantageously, the support holding device consists of plates pierced with holes, arranged substantially on the periphery and in the vicinity of all of the nozzles, in a plane very close to that of said nozzles; alternatively the holes can be integrated in the printheads comprising the nozzles and it is advantageous to provide means for recovering the air emitted (for example collecting grooves and suction holes).

• la figure 1 est une vue schématique de face d'un ensemble d'impression, conforme à l'invention,
• la figure 2 est une vue schématique partielle de côté de l'ensemble de la figure 1,
• la figure 3 représente en perspective schématique, une première variante du dispositif de repérage de l'ensemble de la figure 2,
• la figure 4 représente en perspective schématique, une seconde variante du dispositif de repérage de l'ensemble de la figure 2,
• la figure 5 représente la face avant partielle d'une tête d'impression modifiée conformément à une variante de l'invention,
• la figure 6 représente une coupe transversale schématique de la tête de la figure 5.

Other characteristics and advantages of the invention will emerge on reading the following description, made with reference to the appended drawings in which:

• FIG. 1 is a schematic front view of a printing assembly, in accordance with the invention,
• FIG. 2 is a partial schematic side view of the assembly of FIG. 1,
• FIG. 3 represents in schematic perspective, a first variant of the device for locating the assembly of FIG. 2,
• FIG. 4 represents in schematic perspective, a second variant of the device for locating the assembly of FIG. 2,
• FIG. 5 represents the partial front face of a printhead modified in accordance with a variant of the invention,
• FIG. 6 represents a schematic cross section of the head of FIG. 5.

The printing assembly according to the invention comprises printing heads 1 with ink jets arranged so as to be able to print on a support or correspondence object 2 by scrolling a standardized postal imprint represented by the dotted lines 3.

The object 2 scrolls through continuous drive means constituted for example by two endless belts 4 enclosing the object, or a belt and a pressure pad. The belt conventionally passes over free rollers and rollers driven by a motor system.

A horizontal guide edge 20 guides the lower edge of the object 2 to be printed.

According to the embodiment shown, use is made of printheads 1 manufactured by DATAPRODUCTS under the reference "Ultrajet 96/32". These heads have 32 ink spray nozzles 5 spaced 1.483 mm apart. By tilting the nozzles relative to the direction of movement of the paper it is possible to vary the distance between two adjacent points on the paper and therefore to simultaneously vary the pitch of the 32 traces and the total printing height in order to obtain a desired density of 128 points per inch (approximately 5 points per mm).

It is also easy to calculate that by assembling 4 inclined heads of 32 ° 03 ′ stacked one on the other, so that the traces of the 4 heads are offset by 0.195 mm, we can obtain a footprint of 160 mm x 25 mm thus compliant with the various postal regulations. The corresponding steps are indicated in FIG. 1. Each head 1 covers a height of 24.415 mm over which it is capable of individually addressing the 32 nozzles to form 32 points, in steps of 24.415 / 31 = 0.78 mm. The 4 heads are slightly offset in height by 0.195 mm relative to each other; they therefore complement each other to form by interlacing 128 lines of dots spaced 0.195 mm apart. As illustrated in FIG. 2, all of the print heads 1 are grouped in a support and holding member 22.

Around the print heads are arranged several plates or bars 6 pierced with holes 7, the planar front face of which is slightly above the plane of the ink projection nozzles 5; the holes 7 of these bars 6 are placed in communication with a source of compressed air, not shown, by means of conventional solenoid valves which make it possible to establish a jet or stream of air in the holes at the appropriate times. These air currents will, when a sheet of paper is located a short distance from the plates or bars 6 create a suction effect which stabilizes the sheet at a very short distance from the front face of the bars; this effect, and the equilibrium distance, depends on the speed of the air (the so-called Bernoulli effect), and can therefore be adjusted to the desired value. As can be seen in FIG. 2, the device of the invention maintains between the print side of the object 2 and the front plate of the nozzles and bars (substantially the front side of the support and holding member 22 ) a non-contact distance traveled by the ink jets 25 emitted by the nozzles 5.

   où V désigne la vitesse locale du fluide, dp la variation de pression, ρ la masse spécifique locale, et C une constante. Cet effet produit une force de succion qui, lorsque la distance papier/barrette est assez faible, dépasse largement l'effet en sens contraire du à la pression du jet d'air débouchant du ou des trous. En outre, cet effet de succion varie de façon inverse de ladite distance, ce qui stabilise cette distance et procure un moyen de guider le papier sans qu'il y ait de contact solide.More precisely, the air sent under pressure through the holes 7 is forced, in the presence of a sheet of paper 2, to suddenly change direction and to circulate along divergent paths in the space between the paper 2 and the surface of the bar 6. If the initial conditions are such that this space is small enough so that the section offered to the passage of air is sufficiently reduced, the increase of the speed of the air causes a drop in its pressure, in accordance with the so-called Bernoulli equation which, for a compressible fluid in the absence of volume or field force, takes the simplified form: $${\text{<figure-callout id="2" label="V" filenames="imgf0001.png,imgf0002.png" state="{{state}}">V</figure-callout>}}^{\text{2}}\text{/ 2 + ∫dp / ρ = C,}$$

where V denotes the local velocity of the fluid, dp the pressure variation, ρ the local specific mass, and C a constant. This effect produces a suction force which, when the distance between the paper and the strip is small enough, greatly exceeds the effect in the opposite direction due to the pressure of the air jet emerging from the hole or holes. In addition, this suction effect varies inversely with said distance, which stabilizes this distance and provides a means of guiding the paper without there being any solid contact.

In Figure 1, the guide bars 6 are arranged around the print heads 1 proper. In a variant of the invention (Figures 5 and 6) the holes 7 'are provided on the heads 1' themselves, near the holes 5 'for ejecting the ink, which makes the system both more compact and more efficient. In the case where the holes 7 'are very close together, it has been observed that the ejection of air parallel to the surface of the front plate of the head could more or less disperse the ink jet and disturb the quality of impression; to overcome this defect, recovery grooves 23 are formed in said plate; these grooves 23 are connected to orifices 24 formed in this plate, in turn connected to a pump which sucks up the jets of disturbing air.

The object detection device 2 shown in FIGS. 1 and 2 includes a row 8 of optical cells 9: these cells 9 are approximately 1,200 in steps of approximately 0.2 mm. The realization of this line of cells can be done in different ways, using components "to scale"; But the best way is to use an integrated sensor called CCD (charge coupled detector) which gathers on a single chip 1728 to 2432 cells mounted in shift register; taking into account the dimensions of the chip, an optical system will be used to project the image of the letter on the sensor; anyway, this network of cells aims to follow step by step the progress of the letter so as to precisely trigger the emission of each point. There is 80 mm of print length, and from top to bottom of the impression to be made, taking into account the offset between the different nozzles, an offset of 159.63 mm must be observed, i.e. a total tracking length of the letter equal to 240 mm, the resolution requested being 25/128 = 0.1953 mm. It therefore requires 240 / 0.1953 useful cells, or 1229.

Next to the receiving cell system 9 is a light source 10, constituted by a fluorescent tube, the axis of which is arranged parallel to the line 8 of the sensors 9.

Such a system, as described above, radically eliminates any print quality problem which could arise from the irregularity of the advancement speed or print medium, at least as long as one can neglect the incidence of the travel time of the ink drops. However, there are applications for which this system can be considered too expensive; in this case, a variant is preferred which consists in multiplying the step of the cells in an entire ratio. If, for example, there is only one cell every three steps (i.e. in the example given, one cell step equal to 3 x 0.1953 = 0.5859 mm), the moment of ejection of each drop d ink is determined successively and alternately by the detection of the front edge of the print medium by a cell, and by a time base interpolating between the signals emitted by two adjacent cells; this interpolation can call upon various principles well known in electronic technique: for example the technique of the phase-controlled oscillator making it possible to multiply the frequency of an almost periodic signal, or even the technique of the digital time base which delivers signals replacing those which would come from the deleted cells, this time base being permanently adjusted by measuring the time elapsed between two successive signals coming from the cells. The application of these methods makes it possible to reduce the cost of the sensors, but in return makes the system a little sensitive to variations in speed.

According to a variant of the object detection device 2, the location of said object is obtained by the pursuit of its front edge by means of a fine light brush.

In this case, instead of the row of optical sensors 9 shown in Figure 1 is a transparent window 12 behind which is placed the device which will be described.

• un laser 13 émettant un pinceau étroit de lumière 14 de très faible divergence dont le diamètre est de l'ordre de 0,2 à 0,5 mm.
• Associé étroitement à ce laser, un récepteur photoélectrique 15 (appelé capteur par la suite) dont l'axe optique est confondu avec celui du laser 13. En pratique, comme représenté sur la figure, la confusion théorique des axes optiques s'obtient par une séparation physique des faisceaux aller 14 et retour 28 grâce à un miroir semi-réfléchissant 26 interposé sur le pinceau lumineux émis par le laser 13 (et éventuellement d'autres miroirs de renvoi tels que le miroir 27).
• Un miroir tournant 16 entraîné par un moteur pas-à-pas 17 situé sur l'axe optique commun au laser 13 et au récepteur 15, disposé de façon à permettre au faisceau du laser de balayer, par la fenêtre transparente, la zone de passage de la lettre.
• Une lentille de correction 18 destinée à linéariser le déplacement du pinceau laser en fonction de la rotation du moteur pas-à-pas (relation angle miroir/déplacement linéaire), et un miroir de renvoi 19.
• Un système électronique 20 de commande du moteur pas-à-pas 17, des circuits d'amplification et de traitement du signal du récepteur photoélectrique, et des circuits d'alimentation du laser 13.

This device shown in Figure 3, includes:

• a laser 13 emitting a narrow light brush 14 of very small divergence whose diameter is of the order of 0.2 to 0.5 mm.
• Closely associated with this laser, a photoelectric receiver 15 (hereinafter called sensor) whose optical axis is coincident with that of the laser 13. In practice, as shown in the figure, the theoretical confusion of the optical axes is obtained by a physical separation of the outward and return beams 28 by means of a semi-reflecting mirror 26 interposed on the light brush emitted by the laser 13 (and possibly other deflection mirrors such as the mirror 27).
• A rotating mirror 16 driven by a stepping motor 17 located on the optical axis common to the laser 13 and to the receiver 15, arranged so as to allow the laser beam to scan, through the transparent window, the zone of passage of the letter.
• A correction lens 18 intended to linearize the movement of the laser brush as a function of the rotation of the stepping motor (mirror angle / linear displacement relationship), and a deflection mirror 19.
• An electronic system 20 for controlling the stepping motor 17, amplification and signal processing circuits of the photoelectric receiver, and laser supply circuits 13.

The set works as follows:
Between two successive letters, the initial position of the laser spot is adjusted at the end of the transparent window 12, on the side where the letter 2 is expected; in this position, no obstacle is encountered by the beam and in return the sensor receives nothing. When a letter 2 travels, its front edge cuts the beam at a given time; then the sensor 15 receives the light scattered back, and the engine control system advances the engine 17 by one step; the sensor no longer receives anything, and the motor remains in the position thus acquired, until the letter again reaches the new position of the beam. In this way, it suffices to count the number of steps imposed on the motor 17 to pinpoint the position of the letter 2 with precision.

In another variant illustrated in FIG. 4, the plane rotating mirror is replaced by a helical reflecting surface 16 whose axis is that of the stepping motor 17. The laser system 13 ′ and sensor 15 ′ has its optical axis arranged parallel to this axis, so that by rotation of the motor 17 'the light rays sweep across the window for the passage of letters. In this variant, it is not necessary to have a linearity correction lens. An electronic system 21 'controls the motor 17' and the circuits of the laser 13 'and the sensor 15'. We have omitted, in FIG. 4, for simplicity, the beam separation and deflection system illustrated in FIG. 3 for the laser and its associated sensor.

Distance and step indications are given by way of illustration in FIG. 1.

Claims (7)

1. Printing assembly for a franking, cancelling or suchlike machine, of the type comprising ink emission nozzles, means for guiding and driving the article to be printed in front of the said nozzles and means for detecting the advance of the article, triggering the emission of ink by the said nozzles, characterized in that the guiding and driving means comprise a device (6, 7) for maintaining the article (2) at a distance from the nozzles (5) by suction by means of compressed-air jets functioning under Bernoulli conditions.
2. Assembly according to Claim 1, characterized in that the device for maintaining the article consists of plates (6) pierced with holes (7) and arranged substantially on the periphery and in the vicinity of the set of nozzles (5) in a plane closely adjacent to that of the said nozzles (5).
3. Assembly according to Claim 1, characterized in that the device for maintaining the support consists of holes (7') integrated in the printing heads (1') comprising the said nozzles (5').
4. Assembly according to any one of Claims 1 to 3, characterized in that it comprises means (23, 24) for recovering the emitted air.
5. Assembly according to any one of Claims 1 to 4, characterized in that the detection means comprise a device for the virtually permanent detection of the front edge of the article (2), triggering the emission of ink.
6. Assembly according to Claim 5, characterized in that the detection device comprises a row (8) of optical sensors (9) arranged in parallel with the movement of the article (2).
7. Assembly according to Claim 5, characterized in that the detection device comprises a generator (13, 16, 17; 13', 16', 17') of a light pencil scanning the space step by step along the path of the article (2).

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