EP0382741B1 - Device for monitoring droplet ejection from ejection nozzles of ink-printing heads - Google Patents

Abstract

In order to monitor droplet ejection from individual ejection nozzles of an ink-printing head, the face of the sensor plate (12) of an ink droplet sensor facing the ejection nozzle is shaped as a comb-like electrode with grooved conductive strips (18, 19) mounted on a suction block (17) composed of at least one non-conductive porous layer; according to the spacing of the comb-like structure, the resistance between adjacent conductive strips (18, 19) changes when hit by one or several droplets. An evaluation circuit (20) connected to the conductive strips (18, 19) monitors the changes in resistance and emits a sensor signal (SM). The amount of liquid applied by one or several droplets is removed by capillarity through the porous layer of the suction block (17).

Description

The invention relates to an arrangement for monitoring the droplet ejection from outlet nozzles of an ink writing head according to the preamble of patent claim 1.

The representation of characters or of graphic patterns with ink writing devices is known to be based on the fact that individual droplets are ejected in a controlled manner from outlet nozzles of an ink writing head. Such arrangements are referred to as drop-on-demand (DOD) arrangements. Because of a relative movement between a recording medium and the ink writing head, characters or graphic patterns in the form of a plurality of individual points are thus built up in a grid pattern on the recording medium. One therefore speaks of a so-called matrix representation or a matrix printing process. The quality of recordings made in this way, the so-called writing quality, depends essentially on the number of droplets by which a character is formed. This has led to ink heads with a larger number of outlet openings or nozzles, which are arranged in several rows, for example. It can thus be achieved that the individual droplets applied are so close to one another on the recording medium that they overlap and form lines which appear to be optically continuous both in the vertical and in the horizontal direction. In a so-called DOD system, each outlet nozzle is assigned its own drive element, for example in the form of an electrically controllable piezo element. This must have a co-ordinated behavior for an error-free operation together with the ink channel and an ink supply. You still pull Considering that each of these individual systems works with a droplet repetition frequency of up to about 4 kHz, an ink writing device has to be in use almost continuously for a period of years and an outlet nozzle has a diameter that is smaller than 100 μm, so one recognizes the importance of monitoring for full functionality. In particular, external influences, such as dust, minute paper rubbings, dried ink in the outlet nozzles or gas or air inclusions in the ink channel can already lead to the failure of an outlet nozzle and thus to a reduction in the font quality. It is therefore particularly important to recognize such malfunctions in operation in good time in order to be able to react to them in good time.

In general, the proper functioning of the print head is checked by the user himself by visual inspection of special print patterns. This is not easy, because of the relatively small droplet diameter, which is of the order of about 60 µm, requires a very strenuous observation, which often requires a magnifying glass. The human eye is particularly overwhelmed when the failure affects two outlet nozzles that are spaced relatively far apart. Overall, such a review is unsatisfactory.

It is already known (DE-OS 33 10 365) to determine malfunctions of the type mentioned with an ink droplet sensor. A falling electrode for ink droplets is provided and the ink droplets are electrically charged as they move to the falling electrode. When the charged ink droplets strike the catch electrode, an electrical signal is formed which can be evaluated as a measurement and signaling signal for faults.

This known method requires special charging electrodes, to which a relatively high voltage (up to 300 V) must be applied in order to charge the ink droplets. This not only results in additional design effort, but also protective measures against contact with the live parts must be provided due to the high voltages.

DE-36 34 034 A1 describes an ink drop detector for an inkjet printer with an ink ejecting nozzle, which comprises a plurality of electrodes, at least one of which can be brought into a position in which it is opposite the nozzle at a predetermined distance. The change in resistance between the first electrode and another electrode is detected when conductive ink struck by the nozzle reaches the first electrode. After each ink droplet ejected from the nozzle and struck the detector, the surface of the ink drop sensor is mechanically cleaned using a rubber wiper, thereby limiting the frequency for ink drop detection.

The invention has for its object to provide measures for monitoring the droplet ejection, with which the function of an ink writing head with a plurality of outlet nozzles by determining the impingement of ink droplets without a visual check of print patterns is determined, which without additional protective measures, a safe and clear evaluation of the Enable measurement results with little circuitry effort and which only require relatively simple constructive means, in particular the manufacture of the sensor plate, the assembly of the ink droplet sensor and the installation in the ink writing head, but also the use as a test or inspection device in production, e.g. as a calibration device is advantageously improved.

This object is achieved according to the means specified in the characterizing part of claim 1. Further refinements and advantageous developments are characterized in the subclaims.

• Fig. 1 eine Prinzipdarstellung zur Erläuterung der Erfindung,
• Fig. 2 und Fig. 3 ein Ausführungsbeispiel für den als Tintentröpfchensensor vorgesehenen Elektrodenkamm,
• Fig. 4 ein zweites Ausführungsbeispiel für den Elektrodenkamm,
• Fig. 5, Fig. 6 und Fig.7 jeweils Beispiele für eine Auswerteschaltung,
• Fig. 8 ein Ausführungsbeispiel für den Aufbau und die Herstellung einer Sensorplatte,
• Fig. 9 und Fig. 10 ein Ausführungsbeispiel für den praktischen Einsatz der Sensoranordnung.

The invention is described below with reference to the drawings. Show there

• 1 is a schematic diagram for explaining the invention,
• 2 and Fig. 3 shows an embodiment for the electrode comb provided as an ink droplet sensor,
• 4 shows a second embodiment for the electrode comb,
• 5, 6 and 7 each show examples of an evaluation circuit,
• 8 shows an embodiment for the construction and manufacture of a sensor plate,
• FIGS. 9 and 10 show an exemplary embodiment for the practical use of the sensor arrangement.

On the right in FIG. 1 an ink writing head 1 known per se is shown, which in the example consists of a nozzle plate 2 with nine outlet nozzles 3, a head part 4 with nine ink channels 5 and drive elements 6 assigned to them, and an ink supply part 7. This is connected via an ink feed 8 to an ink reservoir, not shown here. By individually controlling the drive elements 6, a single ink droplet 9 is ejected from the associated nozzle 3. 1 can also be arranged several times in several rows perpendicular to the plane of the drawing. Four such rows would then form a write head with 32 nozzles, the nozzles of the individual rows being offset from one another. The ink droplet sensor 11 according to the invention is arranged at a distance 10 from the write head 1. It essentially consists of a sensor plate 12, designed as an electrode comb, with two connecting electrodes 13 and 14 leading to the outside, and of a layer located behind or below it, which is referred to below as suction block 17 and which serves to absorb and discharge liquid. The electrode comb has, at least in the region of the point of impact of the ink droplets, a multiplicity of conductor tracks 18 and 19 running parallel in the outlet region of the ink droplets. The device for removing the liquid supplied by the impingement of ink droplets exists made of non-conductive porous material; it can be constructed in one layer or preferably from several sub-layers. The connection electrodes 13 and 14 are connected to an evaluation circuit 20 which, as will be discussed in more detail later, depending on the impact of one or more ink droplets on the electrode comb 12, emits a corresponding signal, the sensor signal SM.

The mode of operation of the invention is described below with reference to FIGS. 2 and 3, which show an exemplary embodiment of the electrode comb 12 of the ink droplet sensor in a top view (FIG. 2) and in a sectional view (FIG. 3). In the example, the electrode comb is formed by two comb parts 121 and 122, the tongue-shaped conductor tracks 18, 19 of which lie next to one another in the region of the points of impact for the ink droplets and form the comb structure. The comb parts 121 and 122 with the conductor tracks 18 and 19 are applied here to the suction block 17 consisting of the porous, non-conductive layer. Each of these comb parts 121 and 122 is electrically accessible from the outside via the connection electrodes 13 and 14.

festgelegt. Soll der Tintentröpfchensensor bereits einen einzigen Tintentropfen des Durchmessers D detektieren, so muß T ≦ D sein, um eine elektrische Widerstandsbrücke zwischen benachbarten Leiterbahnen 18 und 19 und damit zwischen den Kammteilen 121 und 122 zu bilden. Am Beispiel nach Fig. 3 erkennt man, daß ein Tintentropfen 9 beim Auftreffen auf die Oberfläche des Tintentröpfchensensors diese Bedingung erfüllt, also zwischen zwei benachbarten Leiterbahnen eine deutliche Widerstandsreduzierung herbeiführt, die an den Anschlußelektroden 13 und 14 durch die Auswerteschaltung bewertet werden kann. Nach dem Auftreffen des Tintentröpfchens 9 wird die Flüssigkeitsmenge zunächst von der oberen porösen Teilschicht 15 aufgenommen, nach unten transportiert und dringt schließlich in die zweite Teilschicht 16 ein. Die nichtleitenden porösen Teilschichten 15 und 16 wirken als eine Art Saugpumpe mit kapillarischem Effekt. Der Wirkungsgrad dieser Saugpumpe kann durch die Wahl der Porosität und/oder der Anzahl bzw. der Dicke S1, S2 der Teilschichten auf bestimmte Einsatzfälle eingestellt werden. Für das in Fig. 3 dargestellte Ausführungsbeispiel haben sich folgende Dimensionierungen als besonders vorteilhaft erwiesen:

$\text{A=B=40 µm}$

$\text{H (Goldelektrode)=1 µm}$

$\text{L=50 µm}$

$\text{S1=5 mm}$

$\text{S2=1,5 mm}$

Die Porosität P1 und P2 der beiden Schichten 15 und 16 ist unterschiedlich. Es ist vorteilhaft, wenn die Porosität der einzelnen Schichten mit zunehmendem Abstand vom Elektrodenkamm zunimmt ( P2 > P1). Dadurch wird gewährleistet, daß ein Flüssigkeitstransport bevorzugt von der oberen Teilschicht 15 zur unteren Teilschicht 16 stattfindet. Das hat den Vorteil, daß der Raum in der Nähe des Elektrodenkamms relativ rasch von Tinte entleert wird und daß damit eine in kurzen Zeitabständen eintreffende Folge von Einzeltröpfchen sicher detektiert werden kann. Als Materialien für die einzelnen Teilschichten 15 und 16 mit unterschiedlichen Porositäten eignet sich vorzugsweise Duran-Filterglas für die obere Teilschicht 15 und sog. Millipore-Filterpapier für die untere Teilschicht 16. Die Porenweiten der oberen porösen Teilschicht 15 können dabei zwischen 0,01 und 0,02, die Porenweiten der unteren porösen Teilschicht 16 zwischen 0,005 und 0,01 mm liegen. Die beschriebenen Kammstrukturen können vorteilhaft nach der an sich bekannten Dünnfilm- bzw. Dickschichttechnik hergestellt werden.Figure 3 shows the structure in detail. In the example, the suction block 17 consists of two partial layers 15 and 16 of absorbent material with the thicknesses S1 and S2. An insulating layer in the form of a gold-coated insulating film 21 is laminated onto the uppermost partial layer 15, which is then structured according to the division ratio T of the electrode comb and is provided with the conductor tracks 18 and 19. This creates the structure shown in Figs. 2 and 3. The comb parts 121, 122 with the conductor tracks 18 and 19 have a height L; the conductor tracks 18 and 19 each have a width A and are spaced apart B to each other. This is a division ratio $\text{T = A + B}$

fixed. If the ink droplet sensor is to detect a single ink drop of diameter D, then T ≦ D must be present in order to form an electrical resistance bridge between adjacent conductor tracks 18 and 19 and thus between comb parts 121 and 122. The example of FIG. 3 shows that an ink drop 9 meets this condition when it hits the surface of the ink droplet sensor, that is to say it brings about a significant reduction in resistance between two adjacent conductor tracks, which can be evaluated at the connection electrodes 13 and 14 by the evaluation circuit. After the ink droplet 9 strikes, the amount of liquid is first absorbed by the upper porous sub-layer 15, transported downward and finally penetrates into the second sub-layer 16. The non-conductive porous sub-layers 15 and 16 act as a kind of suction pump with a capillary effect. The efficiency of this suction pump can be adjusted to specific applications by selecting the porosity and / or the number or the thickness S1, S2 of the partial layers. The following dimensions have proven to be particularly advantageous for the exemplary embodiment shown in FIG. 3:

$\text{A = B = 40 µm}$

$\text{H (gold electrode) = 1 µm}$

$\text{L = 50 µm}$

$\text{S1 = 5 mm}$

$\text{S2 = 1.5 mm}$

The porosity P1 and P2 of the two layers 15 and 16 is different. It is advantageous if the porosity of the individual layers increases with increasing distance from the electrode comb (P2> P1). This ensures that a liquid transport preferably takes place from the upper sub-layer 15 to the lower sub-layer 16. That has the Advantage that the space near the electrode comb is emptied of ink relatively quickly and that a sequence of individual droplets arriving at short intervals can thus be reliably detected. Duran filter glass for the upper partial layer 15 and so-called Millipore filter paper for the lower partial layer 16 are preferably suitable as materials for the individual partial layers 15 and 16 with different porosities. The pore sizes of the upper porous partial layer 15 can be between 0.01 and 0 , 02, the pore sizes of the lower porous partial layer 16 are between 0.005 and 0.01 mm. The comb structures described can advantageously be produced by the thin-film or thick-film technique known per se.

When an ink droplet with a predetermined electrical conductivity strikes such a comb structure, the electrical resistance between the conductor tracks of the two comb parts changes abruptly. By removing the ink droplet by capillary suction of the liquid into the interior of the two layers, the resistance which can be measured between the comb parts 121 and 122, which are not galvanically connected, increases again in time, so that after one of the porosity P1, P2 of the partial layers 15 and 16 and a new suction time depending on the properties of the ink, e.g. ink drops ejected from another nozzle of the print head can be detected in the same way. With the dimensioning values given above, it is possible to reliably detect the impact of individual ink droplets at intervals of approximately 20 ms.

With the arrangement described above, in which the relationship T ≦ D exists between the division ratio T and the diameter D of an ink droplet, the impact of a single droplet is already certain measurable. It is within the scope of the invention to provide a division ratio T which is larger than the diameter D of a single droplet (T ≦ D). This makes it possible to reliably detect the arrival of several individual droplets ejected from one another of the writing head in quick succession. If the individual ink droplets hit the electrode comb within a period of time even before the liquid applied with a previously arrived ink droplet has been sucked off, the amount of liquid between two adjacent conductor tracks increases with each newly arriving ink droplet until the amount of liquid establishes an electrical connection between the two Produces conductor tracks. In this way it is possible that, for example, a significant abrupt change in resistance is only caused with the third droplet arriving. It is within the scope of the invention to adjust the porosity of the individual layers, which cause capillary suction of the ink liquid, accordingly. Practically, this means that with a division ratio T> D and a suction block consisting of several partial layers, the porosity of the individual partial layers is chosen to be larger from top to bottom, as was described in connection with FIG. 3.

In a second embodiment, the structures of the electrode comb arrangement are designed as bifilar conductor tracks, which results in the advantage that the conductor tracks of the comb structure are controllable, e.g. can be connected to each other during individual pauses in measurement. An example of this is shown in FIG. 4.

The conductor tracks 181 and 191 of the two comb parts 123 and 124 are meandered on the suction block 17 here. Its construction and the formation of the conductor tracks 181 and 191 can be done in the manner described with reference to FIG. 3. As before, the conductor tracks run parallel next to each other in the area where the ink droplets meet. In contrast to the previously described embodiment, the embodiment specified here offers the possibility of providing a second pair of connecting electrodes 23 and 24 in addition to the connecting electrodes 13 and 14 which lead to the outside and via which the conductor tracks 181 and 191 can be electrically connected to one another. The connection electrodes 23 and 24 are not connected to one another for the duration of a measurement process, that is to say for the duration during which the impact of ink droplets is detected. The mode of operation of the detection for the impact of ink droplets then takes place as described with reference to FIG. 2 and FIG. 3. The connections 23 and 24 can now be connected to one another via a switch (not shown here) which is actuated in the measurement pauses, that is to say when no ink droplets are detected. It is thus possible to use the conductor tracks 181 and 191 for heating and thus for evaporating the ink droplets during the measurement pauses with the aid of a current source (not shown here) which can be connected to the connections 13 and 14. This has the advantage that in addition to the capillary action of the suction block, there is also liquid removal by evaporation.

In order to evaluate the impact of ink droplets, which is associated with a sudden reduction in resistance in the course of the conductor tracks of the electrode comb, a circuit arrangement is provided which emits the sensor signal SM each time an ink droplet hits. An exemplary embodiment of this is shown in FIG. 5. The circuit shown there essentially consists of a voltage divider consisting of a fixed resistor 30 and the variable measuring resistor 31. This represents the respective current resistance value between the conductor tracks 18 and 19 (Fig.2) or 181,191 (Fig.4) of the electrode comb, ie the circuit shown is connected at this point to the connection electrodes 13 and 14 of the electrode comb. The tap between the resistors 30, 31 of the voltage divider is connected to the inputs of a comparator 32. This connection takes place in such a way that the voltage value Um which is established at the tap point of the voltage divider circuit 30.31 is supplied as a respective instantaneous value via a resistor 39 directly to one input and via an integrating element 35.36 as a mean value Umm to the other input of the comparator 32. Another resistor 34 serves to generate a bias voltage at one of the two comparator inputs, which establishes the interference voltage spacing necessary for the function of the comparator 32. A bistable circuit 37 connected downstream of the comparator 32 forms the sensor signal SM from the output signal of the comparator 32 for a subsequent printer control, which is no longer shown here. The circuit works as follows. If an ink droplet hits the electrode comb due to an ejection of an ink droplet stimulated by the printer control in the print head, this is associated with a sudden reduction in resistance. The measuring resistor 31 thus becomes smaller, which means that the instantaneous value Um briefly becomes smaller than the temporal mean value Umm. In the example according to FIG. 5, a brief change in level from 1 to 0 occurs at the output of the comparator 32. This transition is buffered in the bistable circuit 37 and further processed by the printer controller. After droplet detection, the bistable circuit 37 is reset via its reset input with the reset signal R and the sensor is thus activated for the impact and evaluation of a next ink droplet from another nozzle of the write head.

By monitoring the time period between the excitation for droplet ejection by the printer controller and the occurrence of the sensor signal, it is possible to check the functionality of the individual nozzles. If there is no sudden change in resistance after a certain period of time, which can be set depending on predetermined parameters, such as printer structure, flight time of the droplets, ink composition, etc., the printer controller recognizes that the excited nozzle is not working.

The circuit arrangement described works with direct current, i.e. the voltage divider circuit is connected between a positive voltage source and ground. When using certain ink liquids, this can lead to decomposition of the ink liquid especially when several ink droplets arriving in quick succession are necessary for the evaluation of ink droplets. In order to bring about a measurable reduction in resistance, the ink liquid in this case is exposed to a current flow for a period of t ≧ 100 ms, which can cause electrolytic changes. For example, the dye precipitates out of the solvent, which leads to solidification, which means that capillary suction is no longer possible.

According to one embodiment of the invention, this problem is solved in that the ink droplet sensor is operated with AC voltage. An exemplary embodiment of this is shown in FIG. 6.

The evaluation circuit shown there also has the voltage divider circuit, consisting of the fixed resistor 30 and a resistor 31 representing the current resistance value between the conductor tracks. However, the voltage divider circuit 30, 31 is here connected to an AC voltage generator 38. Besides, is a demodulator 33 is connected between the dividing point of the voltage divider circuit 30, 31 and the comparator 32 and operates in the circuit configuration selected in FIG. 7 as a so-called peak value rectifier. A voltage value is therefore available at its output which corresponds to the current peak value of the voltage at the dividing point.

This is fed directly to one input of the comparator 32 via the resistor 39 and to the other input via the integrator 35, 36 as an average over time. The comparison in the comparator 32, the reversal of the bistable circuit 37 and the output of the sensor signal SM in the printer control (not shown) then take place, as described with reference to FIG. 5.

7 shows a detailed circuit structure as an example of an embodiment for the evaluation circuit according to FIG. 6.

An embodiment of the construction of the sensor plate according to the invention is explained with reference to FIG. 8. This example is based on an arrangement of the conductor tracks according to the example shown in FIG. 2. To produce the sensor plate 25, an electrically insulating carrier plate 26 is provided with a metal layer. This is preferably done by evaporating a glass plate with a thickness of 0.1 to 0.8 mm with a base metallization of Ti, Cu.

A photoresist layer is applied to both sides of this. Subsequently, the pattern of the electrode comb structure desired later on the sensor plate 25 with the conductor tracks 18, 19 is generated on one side and this is galvanically reinforced to 10 ... 20 µm Ni. In a subsequent photo-technical step, the area of a spray window 28 is exposed on both sides and after the base metallization has been etched off, the glass is etched away in this area, so that the conductor tracks 18, 19 span the now glass-free spray window 28. In addition, so-called contacting windows 27 are etched free in this glass etching process.

According to these measures, the sensor plate 25 can be produced to a great extent and can be connected and contacted with the suction block in a simple manner. Details will be described with reference to FIGS. 9 and 10.

The embodiment shown in Fig. 9 (in supervision) and Fig. 10 (in a sectional view) consists only of four different parts, namely a housing 29, the suction block 17, the sensor plate 25 and contact springs 42 arranged on both sides Housing 29, designed as an electrically non-conductive plastic injection-molded part, serves to accommodate these parts and is in turn fastened in printer chassis 41 with the aid of the latching tongues 40 belonging to the housing. The surface quality of the suction block 17 consisting of non-conductive, open-porous material, such as, for example, suction ceramic, filter glass or foam, is subject to certain requirements only with regard to the side facing the ink writing head 1. The flatness of this surface should be of the order of magnitude of the pore size of the porous suction block 17 in order to ensure that the flat sensor plate 25 is supported on it. As described with reference to FIG. 8, the sensor plate 25 has the comb parts 121, 122, the conductor tracks 18, 19, the spray window 28 and two contacting windows 27.

The mechanical assignment of the sensor plate 25 to the suction block 17, whose side provided with the conductor tracks 18, 19 faces the suction block 17, is done by the multifunctional contact springs 42 arranged on both sides. When the device is installed, the suction block 17 is inserted into the housing 29 and after the subsequent placement of the sensor plate 25 on the suction block 17, these metal contact springs 42 pressed into corresponding insertion openings 43 of the housing 29. The contact springs 42 have latching lugs 44 which engage securely in a recess 45 when inserted into the housing 29. This ensures that the three spring tongues 46 formed at one end of the contact springs 42 resiliently rest on the sensor plate 25. In the example, the two outer spring tongues 46 each press on the support of the sensor plate 25 and guarantee a gap-free support of the sensor plate 25 on the suction block 17. The respective middle spring tongue 46 lies in the area of the contacting window 27 and presses directly on the respective contact surface of the electrode comb structure 18 , 19 and thereby establishes the electrical contact. The respective other end of the contact springs 42 forms the connection electrode 13 or 14. The electrical connection from the electrode comb structure to the electronic evaluation circuit, not shown here, is established via a connection designed as a flat plug 47 for standardized plug sleeves.

As described, the ink sprayed onto the conductor tracks 18, 19 is drawn capillary into the suction block 17. The absorbency of the suction block 17 depends on its suction volume and its material, on the ink and on the frequency of the spray test. In order to increase the suction volume and shorten the time for suction, an opening 48 can be provided in the housing of the device for an additional ink disposal, which is filled with a suction material of higher porosity than that of the suction block 17.

The impact of ink droplets, which is associated with a sudden reduction in resistance in the course of the conductor tracks of the electrode comb, is evaluated in a circuit arrangement (20 in FIG. 1) which emits the sensor signal SM with each impact of one or more ink droplets.

The height of the splash window 28 is adapted to the vertical distance of the outer nozzles of the ink writing head. The width of the spray window 28 depends on the horizontal extent of the nozzle exit area of the ink writing head. In the case of a single-row nozzle arrangement, only a narrow, and in the case of multi-row, a correspondingly wider spray window 28 is required. It is also possible to orient the spatially separated rows of nozzles in succession to the spray window 28. This is more advantageous since the spray test of the individual nozzles is carried out only in succession and not next to one another and a narrow spray window 28 enables a narrow design of the device and thus less overall widening of the printer chassis.

For the application and use of the arrangement of the ink droplet sensor constructed according to the invention, it is advantageous to provide it outside the actual printing area of an ink writing device, for example on the left or right edge of the line. To monitor the droplet ejection or to determine the impact of droplets, the write head is moved into this position, in which it is at a constant distance from the ink droplet sensor described. It is possible and advantageous for the write head to assume this position, for example in the idle state or before each start of writing or printing, and for a monitoring process to precede the start of operation. If a failure of one or more outlet nozzles is found, then a manual rinsing with a cleaning effect provided in many currently known ink-writing heads can be carried out in a short time with a cleaning effect.

The invention has been described above primarily with regard to its use in a printer for monitoring droplet ejection. However, it is within the scope of the invention to use the arrangement according to the invention for measuring and adjusting the flight speed of individual ink droplets. Since the distance between the exit nozzles and the surfaces of the ink droplet sensor is known, this only requires that the times of ejection and impact are detected, which e.g. is possible in the printer control without considerable electronic effort. This option offers considerable advantages, especially in the manufacture of ink-writing heads with a larger number of outlet nozzles, because in this case, because of the never completely avoidable tolerance of the individual electrical and ceramic (e.g. piezo elements) components, each one consists of a control circuit, drive element, ink channel and outlet nozzles System must be adjusted.

The described device for detecting ink drops is characterized by a compact, small design that is easy to use or replace in the printer; it also offers the possibility of fully automatic assembly and only requires four different components that can be created with cost-effective technology.

The arrangement can not only be used very advantageously in inkjet printers which work with multi-nozzle print heads, but is also suitable for economical quality assurance, since it can be used advantageously in the manufacture and long-term testing of multi-nozzle print heads.

Reference symbol list

1 =

Ink writing head

2 =

Nozzle plate

3 =

Outlet nozzles

4 =

Headboard

5 =

Ink channels

6 =

Drive elements

7 =

Ink supply part

8 =

Ink supply

9 =

Ink droplets

10 =

distance

11 =

Ink droplet sensor

12.25 =

Sensor plate

13,14,23,24 =

Connection electrodes

15.16 =

Sub-layers

17 =

Suction block

18,19,181,191 =

Conductor tracks

20 =

Evaluation circuit

121,122,123,124 =

Comb parts

21 =

Insulating film

26 =

Carrier plate

27 =

Contact window

28 =

Splash window

29 =

casing

30 =

Fixed resistance

31 =

Measuring resistor

32 =

Comparator

33 =

Demodulator

34.39 =

resistance

35.36 =

Integrator

37 =

bistable circuit

38 =

AC generator

40 =

Latches

41 =

Printer chassis

42 =

Contact element, contact spring

43 =

Insertion opening

44 =

Latches

45 =

Recess

46 =

Spring tongues

47 =

Flat connector

48 =

Disposal opening

A =

Wide track

B =

Distance conductor track

D =

Ink droplet diameter

L =

Height, comb parts and conductor tracks

P1, P2 =

Porosity of the sub-layers

R =

Reset entrance

S1, S2 =

thickness

SM =

Sensor signal

T =

Division ratio

t =

time

To =

Average voltage at the moment

Umm =

Stress average over time

Claims (15)

1. Arrangement for monitoring the droplet ejection from outlet nozzles (3) of an ink writing head (1) by means of an ink droplet sensor (11) which assesses the impact of ink droplets (9) and is arranged in front of the outlet nozzles (3), the said monitoring arrangement having electrodes (18, 19; 181, 191) structured in a comb-like manner and arranged with a pre-determined spacing ratio (T), whereby the change in resistance occurring between two adjacent electrodes (18, 19; 181, 191) due to the impact of at least one ink droplet (9) on the electrodes (18, 19; 181, 191), is detected and assessed in an evaluation circuit (20),
   characterised in that the ink droplet sensor (11) has a sensor board (12, 25), with printed conductors (181, 191), arranged in a bifilar manner, being formed on the surface of this sensor board (12, 25) facing the outlet nozzles (3), which printed conductors (181, 191) extend in a meandering shape and, in the impact region of the droplets, form the comb structure having the spacing ratio (T), and in that the evaluation circuit (29) (sic) can be inserted at two respective terminals (13, 14) of the bifilar printed conductors (181, 191), and the other two terminals (23, 24) are not connected together, and in that, joined to the sensor board (12, 25), an absorption block (17) formed by at least one non-conducting porous layer, is provided for the capillary drainage of liquid.
2. Arrangement according to claim 1,
   characterised in that, during measuring intervals of the evaluation circuit (20), a source of current can be connected to two respective terminals (13, 14) of the bifilar printed conductors (181, 191), and in this case the other two terminals (23, 24) are connected together.
3. Arrangement according to claims 1 and 2,
   characterised in that, for monitoring the impact of individual droplets, the spacing ratio (T) of the comb structure is less than or equal to the diameter (D) of an individual ink droplet (T ≦ D).
4. Arrangement according to claim 1 or 2,
   characterised in that, for monitoring the impact of several individual droplets ejected successively from a respective outlet nozzle (3), the spacing ratio (T) of the comb structure is greater than the diameter (D) of an individual ink droplet (T > D).
5. Arrangement according to claim 1,
   characterised in that the absorption block (17) consists of non-conducting absorbent material.
6. Arrangement according to claim 1,
   characterised in that the absorption block (17) consists of at least two partial layers (15, 16) of non-conducting absorbent material of different porosity and equal or different thickness, and in that the porosity of the individual layers (15, 16) increases (P2 > P1) with an increase in distance from the comb of electrodes.
7. Arrangement according to claims 1 to 4,
   characterised in that the printed conductors (18, 19; 181, 191) and the comb portions (121, 122) are formed by electrodes on an insulating sheet (21) laminated on to the absorption block (17; 15, 16) and structured in accordance with the spacing ratio (T).
8. Arrangement according to claims 1 to 4,
   characterised in that the printed conductors (18, 19) and the comb portions (121, 122) of the sensor board (12, 25) are constructed on a carrier board (26), whereby there is provided on the comb portions (121, 122) at least one respective contacting window (27) and, in the region of impact of the droplets, a spraying window (28), in that a housing (29) is provided for receiving the absorption block (17) and the sensor board (12, 25) covering the absorption block (17), and in that there are provided securing elements (42) which both connect the sensor board (12, 25) to the absorption block (17) in a mechanically secure manner and also contact the sensor board (12, 25) electrically by way of the contacting window (27).
9. Arrangement according to claim 8,
   characterised in that the housing (29) has elastic snap-in pins (40) arranged at the sides, by means of which it is detachably secured to the printer frame (41).
10. Arrangement according to claim 9,
    characterised in that the housing (29) is secured to the printer frame (41) outside the printing region, preferably at the left-hand or right-hand line edge of the printing region.
11. Arrangement according to claim 8,
    characterised in that an accommodation region (48) is provided for an additional ink waste disposal means in the housing (29) beneath the absorption block (17).
12. Arrangement according to claim 8,
    characterised in that the securing elements (42) are contact springs arranged on either side of the housing (29), the catches (44) of which contact springs engage in a recess (45) in the housing (29) when the contact springs (42) are introduced into the housing (29), and the elastic tongues (46) of which contact springs grip the surface of the sensor board (12, 25) under pressure, whereby at least one contact spring is located on each side of the housing (29) in the region of the contacting window (27).
13. Arrangement according to claim 12,
    characterised in that the portions of the contact springs (42) which are led to the outside are designed as a plug connection.
14. Arrangement according to claim 1,
    characterised in that the evaluation circuit (20) comprises a voltage divider circuit which consists of a fixed resistor (30) and the resistance (31) between the terminals (13, 14) of the printed conductors (18, 19; 181, 191) and is connected to a d.c. source (+U), a comparator (32) connected to the dividing point of the voltage divider circuit and a bistable circuit (37) which can be controlled by way of the output of the comparator, whereby the voltage value (Um) occurring at the dividing point of the voltage divider circuit (30, 31) is connected on the one hand directly to the one input and on the other hand by way of an integrating element (35, 36) as a temporal mean value (Umm) to the other input of the comparator (32), the bistable circuit (37) being reversed by way of the output of the comparator (32) in dependence upon a change in resistance between the terminals (13, 14), and emits the sensor signal (SM), and in that, after evaluation of the sensor signal (SM), the bistable circuit (37) is reset again.
15. Arrangement according to claim 1,
    characterised in that the evaluation circuit (20) comprises a voltage divider circuit which consists of a fixed resistor (30) and the resistance (31) between the terminals (13, 14) of the printed conductors (18, 19; 181, 191) and is connected to an a.c. voltage generator (38), and a demodulator circuit (33) connected to the dividing point of the voltage divider circuit (30, 31), in that the output of the demodulator circuit (33) is connected on the one hand directly to the one input and on the other hand by way of an integrating element (35, 36) to the other input of the comparator (32), the bistable circuit (37) being reversed by way of the output of the comparator (32) in dependence upon a change in resistance between the terminals (13, 14), and emits the sensor signal (SM), and in that, after evaluation of the sensor signal (SM), the bistable circuit (37) is reset again.

Images