EP0714777A1 - Device for detecting inkjet print head malfunctioning in a franking machine - Google Patents

Abstract

The printhead (3) has about 256 nozzles in a row about 3 cm long at right angles to the direction (D) of advance of the label (2) unreeled from a dispenser (4) on to an endless belt conveyor (1). Its control unit (5) sends a sequence of binary ones followed by a sequence of zeros over a parallel line (30) for printing of a uniform rectangular dot pattern ahead of an empty rectangle. Gaps in the pattern and spurious dots in the empty rectangle are detected by a photodetector array (6) performing a diagonal scan of each area.

Description

The invention relates to a device for detecting a malfunction of at least one nozzle in an ink jet print head comprising a determined number of nozzles.

It applies more particularly to a postal franking machine having an ink jet printing head comprising a plurality of nozzles for printing on postal articles to be franked variable characters such as the figures of a postal amount.

An inkjet printhead is prone to failure. Failure of an inkjet printhead can result from ink drying in the nozzles which are then blocked or blocked. A prolonged interruption in the operation of the nozzles, the replacement of the ink cartridge supplying the nozzles, a defect in ink during the activation of the nozzles are still causes of failure of an ink jet print head.

In the event of failure of one or more nozzles in the print head of a franking machine, one or more digits of a postal amount printed by the print head on each item of mail processed by the machine may or may be illegible. However, the Postal Administration requires that the postage amounts be legibly printed on the mail items.

It is therefore important to be able to ensure, before undertaking franking operations with a franking machine equipped with an ink jet print head, that all of the nozzles of the print head either in working order.

The object of the invention is to solve this problem by proposing a device capable of detecting whether one or more of the nozzles of an ink jet print head is or are defective. In particular, the malfunction of a nozzle must be detected when the nozzle does not eject ink while it is activated or ejects ink when it is deactivated.

Another object of the invention is to provide a franking machine equipped with an ink jet print head in which the malfunction of one or more nozzles of the print head is detected automatically before printing a postal motif on a mail item processed by the machine.

Certain Postal Administrations, like the British Administration, require that bar codes (simple or modulated in length) appear in the postal motif printed on prepaid mail items. These barcodes are intended to be read automatically for the organization that delivers the mail items. It is important to ensure that the bar codes printed by an inkjet print head are machine readable before handing over the prepaid items to the carrier. Another object of the invention is to propose such a franking machine in which a part of the elements constituting the device for detecting the malfunction of the nozzles also serves to control the readability of bar codes forming part of the postal motif.

According to the invention, this device comprises means for advancing a print medium under the print head in a direction of movement. A control unit, such as a microprocessor, operates the print head according to a sequence of nozzle activations so that the nozzles are activated at the same time, a number of times equal to the determined number of nozzles in the print head. printing, to print ink dots in a first rectangular area on the print medium. This control unit then operates the print head according to a nozzle deactivation sequence so that the nozzles are deactivated at the same time a number of times equal to said determined number to prevent printing of ink dots in a second rectangular area on the print medium. A photo-detector means is controlled to photo-detect if a diagonal of the first rectangular area has a number of ink dots equal to the determined number and to photo-detect if a diagonal of the second rectangular area does not have d points 'ink. This photo-detector means delivers a signal having, in response to the detection of the determined number of ink dots on the first diagonal, a first level during a first time window corresponding to the time required to scan said first diagonal and having , in response to detecting the absence of ink dots on the second diagonal, a second level during a second time window corresponding to the time required to scan the second diagonal. Means is provided for processing the signal at the output of the photodetector means so as to detect a transition of the signal from the first level to the second level during the first time window, this transition being indicative that a nozzle has not printed of ink point in response to its activation, and so as to detect a transition of the signal from the second level to the first level during the second time window, this transition being indicative that a nozzle has printed an ink point in response to its deactivation.

Examples of embodiment of the device according to the invention are described below with reference to the figures.

Figure 1 schematically shows the device according to the invention forming part of a postage meter machine.

Figures 2A and 2B schematically show the arrangement of the ink dots printed by the print head and the arrangement of the nozzles of the print head.

FIG. 3 shows the shape of the signal supplied at the output of the photo-detector and representative of the reading of the arrangement of ink dots shown in FIG. 2.

Figure 4 schematically shows, seen from below, a first embodiment of a photo-detector for the device according to the invention.

Figure 5 shows schematically the photo-detector of Figure 4, seen from the front.

Figure 6 schematically shows a second embodiment of a photo-detector, seen from the front, for the device according to the invention.

Figure 7 shows an example of a postal imprint with bar codes.

The main components of the device according to the invention are shown in FIG. 1. These components are part of a machine for franking mail items such as envelopes or even labels intended to be affixed to mail items.

In the embodiment of the invention, a label 2 is moved by a conveyor system 1 in the direction D under the ink jet print head 3, the malfunction of which is checked automatically before undertaking operations postage.

The label 2 which serves as a printing medium during the control operation comes from an automatic label dispenser 4, conventional in itself, so that the control operation can be carried out automatically without the intervention of an operator . This control operation can take place, for example, when the franking machine is powered up, between two franking operations, or even after a prolonged stop of operation of the machine which follows its powering up.

The print head 3 has a plurality of nozzles B (10 are visible in FIG. 2B) which are arranged along a row (or more) extending transversely to direction D, in the case of example perpendicular to direction D. Note that in the embodiment considered, the print head has about 256 nozzles distributed over approximately 30 millimeters.

The device also comprises a control and processing unit indicated by 5 which is preferably a microprocessor under the control of a program recorded in a memory (not shown). The unit 5 controls the label dispenser 4 so that a label 2 is automatically supplied under the print head 3 to start the control operation.

The unit 5 also supplies the print head 3 via a parallel line 30 with a word composed of several bytes to control the activation or deactivation of each nozzle. In particular, each binary word comprises as many bits as there are nozzles in the print head. According to the logical state of a bit in the binary word, the nozzle which corresponds to this bit is activated or deactivated, ie ejects an ink droplet or not. Several binary words usually need to be sent in sequence to the print head to print a pattern such as a postal amount figure. Depending on whether all the bits of a binary word transmitted to the print head are in logic state 1 or in logic state 0, all the nozzles are activated at the same time or deactivated at the same time. To respect a constant resolution of printing of the ink dots in the direction D on the printing medium presented under the head 3, the sendings of binary words are synchronized with a signal proportional to the speed at which this medium is moved. printing by the system 1.

According to the invention, the unit 5 is programmed so as to send to the print head, a sequence of binary words of which all the bits are in logic state 1 to activate all the nozzles at the same time a number of times equal to the number of nozzles in the print head. If all the nozzles work correctly, ink dots, regularly spaced apart, are printed in a first rectangular area 20 on the label 2 as visible in FIG. 2A. It is understood that the ink dots P partially overlap on the lines L and transversely to these lines L although they are shown spaced from each other.

The unit 5 is also programmed so as to send to the print head, a sequence of binary words of which all the bits are in logic state 0 to deactivate all the nozzles at the same time a number of times equal to the number of head nozzles 3 to prevent them from printing an ink dot in a second rectangular area 21 on the label 2. If all the nozzles work properly, the second rectangular area does not have ink dots.

The nozzle deactivation sequence succeeds the nozzle activation sequence so as to keep the same printing resolution of the ink dots P in the direction D in the two rectangular areas 20 and 21 and between them as visible in Figure 2A.

The device also comprises a photo-detection member 6 which is mounted downstream of the print head 3 in the direction D and under which the label 2 with the ink dots P is moved according to the arrangement indicated above. above. This member 6 must be able to operate on a small analysis area shown in FIG. 2A by a small rectangle ZA inside the rectangular areas 20 and 21. In particular, when the nozzles print dots on the label ink along lines L parallel to the horizontal direction D, the resolution of the member 5 along the vertical in FIG. 2 must be at most equal to the distance between two successive lines L along the vertical. Two photo-detection circuits which are suitable for the device according to the invention are presented below with reference to FIGS. 4 to 6.

Generally, the member 5 includes a light source which is directed onto a particular analysis zone ZA on the label. The member 5 generates an output signal whose amplitude is proportional to the quantity of light reflected by this zone ZA. This quantity of light varying depending on whether an ink point is printed or not in the analysis area ZA, the amplitude of the output signal is therefore indicative of the presence or absence of a point ink in an area analyzed ZA.

According to the invention, the member 6 is controlled by the unit 5 to successively photo-detect if an ink point P is present in each of the first analysis zones ZA situated on a diagonal of the rectangular zone 20. In response to the detection of an ink point in each of these zones ZA, that is to say a number of points equal to the number of nozzles having been actuated during the activation sequence, the member 6 outputs a signal whose amplitude is at a substantially constant level V1 during the time window T0 required to scan the diagonal of the area 20. In relation to FIG. 2A, the scanning begins in the example from an area ZA located in the upper right corner in the rectangular area 20.

The member 6 is also controlled by the unit 5 to successively photo-detect if an ink point is present in each of the second analysis zones ZA situated on a diagonal of the rectangular zone 21. In response to the detection of the absence of an ink point in each of the zones ZA situated on this diagonal, the member 6 outputs a signal whose amplitude is at a level V0, different from the level V1, during the time window T1 required to scan the diagonal of area 21.

Note that the scan on the diagonal of area 20 can just as well start from the bottom right corner in area 20 in Figure 2A but it is preferable that the scanning on the diagonal of zone 21 begins on the line L where the scanning of the diagonal of zone 20 ends to simplify the control of the member 5, in particular when the latter is mounted movable transversely to the direction D .

It should also be noted that the order according to which the zones 20 and 21 are analyzed may as well be the reverse to that presented above.

In the example which has just been described, the photodetector member 6 therefore delivers to the unit 5 a signal S shown in FIG. 3, the amplitude of which varies between a first level V0 indicative of the detection of the absence of an ink point in an analyzed area ZA and a second level V1 indicative of the detection of the presence of an ink point in this analyzed area. Maintaining the signal S during the time window TO at level V1 indicates that all the nozzles of the print head have correctly ejected ink droplets when they were activated by the unit 5. In the same way, maintaining the signal S during the time window T1 at level V0 indicates that all the nozzles of the print head did not eject ink droplets when they were deactivated by the unit 5.

FIG. 3 shows the shape of the signal S when a particular nozzle has not ejected an ink droplet during the time window T0 in response to its activation. In the event that a nozzle is faulty (it does not eject ink while it is activated), there will be a line L in the rectangular area 20 which will be empty of ink dots. When the member 5 sweeps across the diagonal of the area 20, it will restore a signal S which will have an amplitude transition 3A between the level V1 and the level V0. Likewise if a nozzle fails due to the fact that it constantly ejects ink droplets when it is not activated, there will be a line L in the rectangular area 21 which will be covered with ink dots. When the organ 5 will sweep across the diagonal of zone 21, it will restore a signal S which will have an amplitude transition 3B between level V0 and level V1. Therefore, on the basis of signal S, it is possible to detect that a nozzle of the print head does not eject ink while it is activated or ejects ink while it is deactivated .

Furthermore, according to the invention, the photo-detector occupies successive analysis positions which are identified by the unit 5 which controls it. Each analysis position corresponds to a nozzle and therefore a part of the signal S assigned to this particular nozzle so that the unit 5 is also able to identify the position of the faulty nozzle in the row of nozzles of the print head. 3.

The signal S at the output of the photo-detection member 6 is supplied to the unit 5 to be processed, in particular in digital form in order to detect a faulty nozzle and to identify this nozzle. In the context of the invention, there is provided a member such as a squeegee (not shown) which is mounted under the print head 3 for brushing the nozzles under the control of the unit 5 when the latter has detected that a or several faulty nozzles do not eject ink while they are activated. Alternatively, upon detection of such a failure, the unit 5 can initiate a purge cycle of the faulty nozzle or nozzles by a selective activation command of the latter.

Figures 4 and 5, the photo-detection device 6 is a read head of the "S3906" type from the company "HAMAMATSU" consisting of a battery 61 of photo-detection cells (in the case of example 1024 cells) . This read head is mounted with respect to direction D so that the cells are aligned transversely to this direction D. As shown in FIGS. 4 and 5, the battery of cells extends over almost the entire width of the label 2 which is moved here in a slide 64. Note that here, there are more photo-detection cells than of L lines. Consequently, it is not excluded that several photodetection cells participate in the detection of the presence or absence of an ink point on an L line. Each cell (or more exactly each group of cells assigned to a line L) outputs a signal whose level is indicative of the presence or absence of one or more ink dots on the line L in question. Each cell operates with a resolution in the direction perpendicular to D which is at most equal to the resolution of the print head in this perpendicular direction. For the type of photo-detection circuit indicated above, the resolution of a cell is approximately 1/40 of a millimeter while the resolution of the print head is approximately 1/8 of a millimeter. In this embodiment, the cells can operate in parallel and therefore each reproduce a signal having an amplitude varying between the level V1 and the level V0 depending on whether the zones of successive analyzes analyzed (due to the displacement of the label) have or have no ink dots. Each analysis zone can be addressed selectively from unit 5 by line 60, the corresponding information coming from the cells is transmitted by a connector 63 mounted on a printed circuit board 62 to unit 5 for processing.

Figure 6, the photo-detection member 6 is constituted by a read head 66 of the type "HEDS" from the company "Hewlett-Packard" which movably mounted on an axis 67 to move transversely to the direction D under the action of a motor 68 controlled step by step by the unit 5. This read head can be seen as a single cell head unlike the read head 61 insofar as it operates only on an analysis zone with a resolution of about 1/8 millimeter and outputs a signal indicative of the presence or absence of an ink point in this analysis area. Unlike the read head 61, it is not for unit 5 selects successively the outputs of several cells but moves a cell to successive analysis positions. To scan the diagonals of the rectangular areas 20 and 21, the unit 5 therefore sends via line 30 to the motor a succession of control data to the motor 68 which moves the read head 66 successively above each line L while the label moves in the slide 64 perpendicular to the direction of movement of the head 66.

In continuation of the invention, the franking machine has a print head 3 which functions to print postal imprints comprising a bar code as visible in FIG. 7. According to the invention, the photo- detection 6 is assigned to reading the code bars and the unit 5 to checking the readability of the code bars on the basis of the signal S. In particular, a read head like the head 61 is capable of supplying several signals S in parallel with unit 5 for checking the readability of height-modulated bar codes. Figure 7, the postal imprint includes under the postal stamp a bar code 70 with height modulation.

Claims (6)

A device for detecting a malfunction of at least one nozzle (B) in an ink jet print head (3) having a predetermined number of nozzles, comprising: means (1) for advancing a print medium (2) under the print head in a direction of movement (D); means (5) for controlling the print head (3) according to a sequence of activations of the nozzles so that the nozzles are activated at the same time a number of times equal to said determined number for printing ink dots in a first rectangular area (20) on the print medium and according to a sequence of deactivation of the nozzles so that the nozzles are deactivated at the same time a number of times equal to said determined number to prevent printing of ink dots in a second rectangular area (21) on the print medium; means (6) controlled to photo-detect if a diagonal of the first rectangle has a number of ink dots equal to the determined number and to photo-detect if a diagonal of the second rectangle does not have ink dots, this means photodetection (6) outputting a signal (S) having, in response to the detection of said determined number of ink dots on the first diagonal, a first level (V1) during a first time window corresponding to the time required to scan said first diagonal and having, in response to detecting the absence of ink dots on the second diagonal, a second level (V0) during a second time window corresponding to the time required to scan said second diagonal; and means (5) for processing said signal at the output of the photodetector means so as to detect a transition of said signal from the first level to the second level during the first time window, this transition being indicative that a nozzle has not printed an ink dot in response to its activation, and so as to detect a transition of said signal from the second level to the first level during the second time window, this transition being indicative that a nozzle printed an ink dot in response to its deactivation. The device according to claim 1, wherein the photodetector means (6) consists of a read head (66) mounted movable transversely to the direction of movement (D) of the printing medium. The device according to claim 1, in which the photodetector means (6) consists of a battery (61) of photodetection cells aligned transversely to the direction of movement (D) of the printing medium and in number as less equal to the number of print head nozzles. The device according to one of the preceding claims, in which the print medium (2) is a label supplied by an automatic label dispenser. A postal franking machine comprising a device according to claim 4, in which the label is supplied automatically when the machine is switched on or between two franking cycles. A postal franking machine comprising a device according to one of claims 1 to 4, in which the print head (3) also functions to print bar codes (70) on a printing medium entered in the machine , the photodetector means (6) is assigned to reading the code bars and the means of processing (5) is assigned to control the readability of the code bars printed on the printing medium.

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