EP0767064A2 - Device for monitoring the operation of an inkjet printinghead - Google Patents

Abstract

Arrangement for monitoring the function of an ink print head. The aim is to increase functional reliability by increasing the monitoring accuracy with reduced effort. According to the task, continuous monitoring is to be carried out by evaluating the printed image, the brightness of the record carrier having no influence on the monitoring accuracy. According to the invention, a scanning head 2 with at least two grinding electrodes 21 is placed on the recording medium 4 on the one hand and connected to an evaluation circuit 3 on the other hand, which in turn is connected on the output side to the control circuit 5 for the ink printing head 1. The changes in the electrical properties of the recording medium 4 between the grinding electrodes are recorded in the evaluation circuit 3. The arrangement is suitable for both inkjet printheads based on the bubble jet principle and for ink printheads with piezo actuators. <IMAGE>

Description

• Verstopfung der Düsen- beziehungsweise Tintenkanäle durch Tintenpartikel,
• Verstopfung der Düsenöffnungen durch eingetrocknete Tinte oder/und Staub,
• Unterbrechung der Tintenkapillare durch Blasenbildung beziehungsweise Meniskusabriß,
• Gaseinschlüsse in der Tintenkammer,
• Fehler in der Ansteuerelektronik.

The invention relates to an arrangement for monitoring the function of an ink print head.
Such printheads are used in office printers and more recently also in franking machines and product labeling devices.
Failures of individual nozzles of an ink print head can result from:

• Blockage of the nozzle or ink channels by ink particles,
• Obstruction of the nozzle openings by dried ink and / or dust,
• Interruption of the ink capillary due to blistering or meniscus tear,
• Gas pockets in the ink chamber,
• Error in the control electronics.

These print failures are not only annoying in the typeface, but are particularly critical for safety-relevant print image data, such as value, date, machine number on franking machines. Since contamination of individual nozzles by dust is possible at any time during operation, malfunction of individual nozzles can occur even immediately after a functional test of the ink print head.
That is why there has long been a need for continuous monitoring of ink printheads.

It is known, compare EP 0 257 570 A2, EP 0331 352 A2 and EP 0 416 849 B1, to actuate all printing nozzles of an ink print head once per printing pass, so that a line arises transversely to the feed direction of the mail pieces. This line is then scanned using an optical sensor.
Usually, a CCD line sensor is used as the optical sensor, which is relatively expensive in itself with, for example, 200 nozzles - one photodiode per nozzle or pressure point, in addition there is a complex evaluation circuit, see also EP 0 397 810 B1. There is no constant monitoring. It must also be taken into account that franking is printed with red ink on recording media of very different brightness, so the difference in brightness between unprinted and printed recording media can also be very different from case to case.
In the case of a dark recording medium, it can be so small that the sensitivity of the optical sensor is difficult to meet.

In addition, a device for monitoring ink print heads is known, compare DE 40 23 390 A1, in which an ultrasonic sensor records the sound waves emitted during the printing process and sends them as a signal to an evaluation unit. The ultrasonic sensor is either arranged on a contact plate carrying the ink printhead or integrated in thin-film technology in the structure of the ink printhead - if it is built up in layers.
Piezo sensors, surface filters or polyphenyl foils can be used as ultrasonic sensors.

With this device, the function of the individual ink pressure chamber or nozzle can be determined, but not whether the ink drop also lands at the assigned point on the recording medium. This means that the quite common slate splashes are not covered by this. In addition, the fewer ultrasonic sensors one wants to use, the more extensive and complicated the evaluation unit must be.

The purpose of the invention is to increase the functional reliability of ink printheads by increasing the monitoring accuracy with reduced effort.

The invention is based on the task of continuously monitoring the function of an ink print head by evaluating the printed image, the brightness of the recording medium having no effect on the monitoring accuracy or the sensitivity of the test arrangement.

According to the invention, this object is achieved according to the patent claims.

The invention is based on two facts that have been known for a long time but have so far been ignored with regard to the inventive purpose.

First, an ink is used for document-correct imprints, such as franking imprints, which is based both on water components and predominantly on solvents, such as glycol.
If ink droplets strike the recording medium, they are immediately picked up by the latter. This is known as "the ink strikes away in the paper". The pressure points are then smudge-proof, but the bound ink is still fluid in the recording medium. This means that the conductivity in the fresh pressure points is greater than in the unprinted areas.
Secondly, embodiments of thermal print heads are known which have a comb-shaped structure of electrodes on their front side, along which the ink ribbon is guided along with the record carrier, compare EP 0 067 969 B1 and DE 32 18 731 A1. It is known that between the usual thermal printing process Recording medium and thermal print head arranged a heat-sensitive ink ribbon, from the color parts are transferred to the recording medium by partial heat.
In another embodiment, compare DE 28 55 631 A1 and DE 31 43 135 C2, the thermal print head likewise has a writing comb in which electrodes are arranged side by side on a support and are connected to a control circuit. The ribbon and the recording medium are one. Particles of metal are burned out, exposing a layer of paint.

In the solution according to the invention, the electrodes are placed on the recording medium as sensors or grinding electrodes and the change in conductivity between two electrodes is recorded in a downstream evaluation circuit.
Instead of the change in volume resistance, a change in capacitance can also be evaluated.

Analogous to the thermal printhead, the grinding electrodes are combined to form a scanning head and provided with corresponding control circuits.

The scanning head is arranged after the ink print head in the transport direction of the recording medium. The distance between the scanning head and the ink printhead is chosen so that the ink in the recording medium has just struck away. This means that the pressure points are smudge-proof but not yet completely dry.
It is characteristic of the printed images to be checked that they create a closed connection between the sliding contacts.

The fault tolerance of the sensor can be modified by arranging the sliding contacts at different points - grinding electrode comb with different gaps - in different widths and in different numbers and at different distances from one another.

An advantageous solution is to provide as many sliding contacts as possible - a maximum of one sliding contact more than nozzles in the ink print head - and processor-controlled the time of the measurement between control very different pairs of sliding contacts depending on the triggered target pressure information. It is also conceivable to query several pairs of sliding contacts simultaneously.

Another advantage of the arrangement according to the invention is that, due to the resilient grinding electrodes, the scanning head is constantly self-cleaning and contamination of the contacts is avoided.

Before starting each measurement, an automatic zero adjustment is possible, based on the dry recording medium used.

The arrangement is equally well suited for all ink printing technologies such as bubble jet, piezo, continuous.

The invention is explained in more detail below using an exemplary embodiment.

Fig. 1

Ein Blockschaltbild einer Anordnung mit einem Tintendruckkopf und einer Druckbildauswertung,

Fig. 2

ein ausführliches Blockschaltbild der Druckbildauswertung.

Show it:

Fig. 1

A block diagram of an arrangement with an ink print head and a print image evaluation,

Fig. 2

a detailed block diagram of the print image evaluation.

For the sake of simplification and easier understanding, the illustration is carried out schematically.

1, an ink print head 1 is controlled by a control circuit 5. In accordance with the control data transferred from an output 52, the ink print head 1 generates print dots or a print dot pattern on a recording medium 4.

The record carrier 4 is scanned in the printing area after the ink printhead 1 by means of a scanning head 2 with grinding electrodes 21. The grinding electrodes 21 are placed non-positively on the recording medium 4. The changes in the electrical properties - conductance, dielectric constant - between two grinding electrodes 21 are transmitted as electrical signals to one with the scanning head Given 2 connected evaluation circuit 3 and evaluated in this. The evaluation result is fed to the control circuit 5 for the ink print head 1 and is compared in the same with the pressure pulse pattern that triggered the scanned print. If the comparison result shows deviations beyond a permissible level, the printing process is blocked and a cleaning cycle is initiated for the ink print head. Then the system switches back to print mode with monitoring. If there are still inadmissible large deviations, the printing device is completely blocked. The sequence control is carried out by the control circuit 5, which contains a master processor.

2, the scanning head 2 is provided with n grinding electrodes 21 and n outputs 22. The outputs 22 are connected to first inputs 311 of a first “1 out of n” multiplexer 31 and to first inputs 321 of a second “1 out of n” multiplexer 32. Second inputs 312, 322 of the multiplexers 31, 32 are connected together to a multiplex control output 351 of a slave processor 35.
Controlled by the slave processor 35, a selected first input 311, 321 of each multiplexer 31, 32 is activated, thereby accepting the associated output signal or querying two of the 1 to n outputs 22 of the scanning head 2.
If there is a need to evaluate two or more pressure point lines simultaneously, then four or more "1 out of n" multiplexers are required.
A common impedance converter 33 is connected downstream of both multiplexers 31, 32, which in turn is connected to inputs 353 of the slave processor 35 via an A / D converter 34 and subsequent data lines. The output of the slave processor 35 is connected to a control input 51 of the control circuit 5. As can be seen from the above, a scanning head 2 can be minimally equipped with two grinding electrodes, which can be arranged according to the outside area of the print or according to the particularly relevant inside area of the print. This is a matter of discretion, since misaligned splashes can interrupt the pressure line between two electrodes and are then recorded as an error.

Reference symbols used:

1

Ink printhead

2nd

Readhead with grinding electrodes

21

Grinding electrodes

22

Outputs of the scanning head 2

3rd

Evaluation circuit

31

first "1 out of n" multiplexer

311

first control inputs of the first "1 out of n" multiplexer

312

second control input of the first "1 out of n" multiplexer

32

second "1 out of n" multiplexer

321

first control inputs of the second "1 out of n" multiplexer

322

second control input of the second "1 out of n" multiplexer

33

Impedance converter

34

Impedance converter

34

A / D converter

341

Data outputs of the A / D converter 34

35

Slave processor

351

Multiplex control output of slave processor 35

352

Data output of the slave processor 35

4th

Record carrier

5

Control circuit for the ink print head

51

Control input of the control circuit

52

Data output of the control circuit

d

Diameter of a pressure point

dma

maximum distance between two grinding electrodes 21

dmi

minimum distance between two grinding electrodes 21

m

Number of nozzles in an ink print head 1

n

Number of grinding electrodes in a scanning head

Claims (5)

Arrangement for monitoring the function of an ink print head by evaluating the print image on the recording medium, characterized in that after the ink print head (1) a scanning head (2) with at least two grinding electrodes (21) is placed on the one hand on the recording medium (4) and on the other hand with an evaluation circuit (3) is connected, which serves to detect changes in the electrical properties of the recording medium (4) between the grinding electrodes (21). Arrangement according to claim 1, characterized in that the minimum distance (dmi) between two adjacent grinding electrodes (21) is equal to the diameter (d) of a pressure point and that the maximum distance (dma) between two adjacent grinding electrodes (21) is the longest of pressure points line formed transversely to the direction of movement of the recording medium (4) in one print run. Arrangement according to claim 1, characterized in that the number (n) of grinding electrodes (21) is greater by one than the number (m) of nozzles of the ink print head (1). Arrangement according to claim 1, characterized in that the evaluation circuit (3) is connected on the output side to a control input (51) of a control circuit (5) for the ink print head (1). Arrangement according to Claims 1 and 5, characterized in that the evaluation circuit (3) consists of at least two "1 out of n" multiplexers (31, 32), a common impedance converter (33) connected downstream of the latter, and a downstream one A / D converter (34) and a subsequent slave processor (35),
that the outputs (22) of the scanning head (2) are connected to first control inputs (311, 321) of the multiplexers (31, 32) via n data lines,
that a multiplex control output (351) of the slave processor (35) is connected to second control inputs (312, 322) of the multiplexers (31, 32) and
that a data output (352) of the slave processor (35) is connected to a control input (51) of the control circuit (5) containing a master processor for the ink print head (1).

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