WO1991000807A1

WO1991000807A1 - Process and device for monitoring the ejection of droplets from the output nozzles of an ink printing head

Info

Publication number: WO1991000807A1
Application number: PCT/EP1990/000659
Authority: WO
Inventors: Ernst Goepel; Andreas Kappel
Original assignee: Siemens Aktiengesellschaft
Priority date: 1989-07-07
Filing date: 1990-04-24
Publication date: 1991-01-24

Abstract

A heat sensitive ink droplet sensor (6) which uses the thermal energy carried by the ejected ink droplets (3) as a criterium for the functioning capacity of each output nozzle (3) monitors the ejection of droplets from the individual output nozzles (3) of a high resolution ink printing head. The thermal energy is detected by means of thermal conductivity or the heat radiation (11) emitted by the ink droplets.

Description

Method and arrangement for monitoring the droplet ejection from outlet nozzles of an ink print head

The invention relates to a method and an arrangement for monitoring the droplet ejection from outlet nozzles of an ink print head according to the features of claims 1 and 7.

Known methods for displaying characters or graphic patterns with ink printing devices are based on the fact that individual droplets are ejected in a controlled manner from outlet nozzles of an ink printing head. Such arrangements are referred to as so-called drop-on-demand (DOD) arrangements. Because of a relative movement between a record carrier and the ink print head, characters or graphic patterns in the form of a large number of individual points are thus built up in a grid pattern on the record carrier. One therefore speaks of a so-called matrix representation or a matrix printing process. The quality of the recordings made in this way, the so-called writing quality, essentially depends on the number of droplets by which a sign is formed. This has led to ink printheads with a larger number of outlet openings or nozzles, which are arranged, for example, in one or more rows. 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 in both the vertical and horizontal directions. In a DOD system, each outlet nozzle has its own drive element, for example in the form of an electrically controllable piezo element or a heating element (bubble jet principle) assigned. For an error-free operation, these drive elements, together with the ink channel and an ink supply system, must exhibit a behavior that is co-ordinated. If one also considers that each of these individual systems with a drop repetition frequency of up to approximately 4 kHz, an ink printing device must be in use almost continuously for a period of years and that an outlet nozzle has a diameter that is smaller than 100 μm, as can be seen the importance of monitoring the full functionality of such a facility. External influences in particular, such as dust, very small 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 so that we can react immediately.

Another problem with ink printing devices that are subject to the functional monitoring of the user is print image acceptance. The user of ink printing devices must recognize in good time whether the typeface generated is faulty or not. This is particularly difficult in the case of multi-nozzle ink heads of high print resolution, since the failure of one or more nozzles of the ink print head initially only results in a slight deterioration in the print image.

In general, the proper functioning of the print head is checked by the user himself by visual inspection of special print samples. This is not easy and, because of the relatively small droplet diameters, which are of the order of about 60 μm, requires a very strenuous observation, which often requires a magnifying glass. Especially when the failure is two relative As far as the outlet nozzles are concerned, the human eye is overwhelmed. Overall, such a review is unsatisfactory.

Various methods have therefore been proposed with which the functionality of ink print heads can be checked. They are based on the use of various physical effects, e.g. Resistance measurement, measurement of electrical charges or capacities.

From DE-33 10 365 AI it is known to detect malfunctions of the type mentioned with an ink droplet sensor. In this case, a collecting electrode is provided for the ink droplets ejected from the nozzles, and the ink droplets are electrically charged when they move to this collecting electrode. When the charged ink droplets strike the catch electrode, an electrical signal is formed which can be evaluated as a measuring 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 because of the high voltages, protective measures must also be taken to prevent contact with the live parts.

In order to be able to detect malfunctions due to the failure of individual nozzles, it is known from DE-36 34 034 AI to use an ink droplet detector which comprises a plurality of electrodes, at least the first electrode of which can be brought into a position in which it can Exit nozzles of the ink print head is opposite at a predetermined distance. The Ä ^'change of resistance between the first electrode and another electrode is detected, when ejected from the ink jet head, the conductive ink first electrode reached.

Another measure known from DE-37 32 395 A1 is to provide an ink droplet sensor for monitoring the droplet ejection from individual outlet nozzles of an ink-writing head, the surface of which faces the outlet nozzles is designed as an electrode comb with comb-like structured conductor tracks, which is formed on at least one of a non-conductive porous layer existing suction block are applied. Depending on the pitch of the comb structure, the resistance between adjacent conductor tracks changes when one or more droplets strike. An evaluation circuit connected to the conductor tracks monitors the change in resistance and emits a sensor signal corresponding to this change in resistance. The amount of liquid applied with the impact of one or more droplets is discharged capillary via the porous layer of the suction block.

The object of the invention is to provide measures for monitoring the droplet discharge, with which the function of a high-resolution ink print head with a plurality of outlet nozzles can be checked automatically and in a simple manner and which enable a safe and reliable evaluation of the measurement results thus obtained.

This object is achieved in accordance with the means specified in claims 1 and 7. Further refinements and advantageous developments of the invention are characterized in the subclaims.

With a heat-sensitive ink drop sensor, it is easily possible to automatically determine the functionality of multi-nozzle ink printheads. As a result, the reliability of ink printing devices can be Grip of the printer user can be increased by the function of the ink print head being checked by the sensor before and cyclically during the printing operation and, if necessary, if a failure of one or more nozzles with the sensor is detected, measures to restore the so-called service station Functionality of the ink print head can be initiated.

According to the invention, the thermal energy carried by the ejected ink droplets is used to prove the functionality of the individual outlet nozzles. As a prerequisite for such a heat-sensitive sensor principle, it is necessary that the ink droplets and the detector are at different temperature levels. In the case of ink printing devices, this is already given to a certain extent by the fact that ink printing units are heated to temperatures which are generally above room temperature by heating devices which are already present, so that the requirements for the use of the sensor arrangement according to the invention are met in a simple manner without additional means.

This sensor principle can be used particularly advantageously in the case of ink printing devices which work with thermal converters, so-called bubble jet ink printing heads, since a considerable part of the electrical energy supplied is carried as heat by the ejected ink droplets. This eliminates the need for any measure to produce the required temperature difference between the detector and the ink droplets.

Optimal operational reliability and print quality are guaranteed by the automatic checking of the ink printing device before the start of the printing operation and by the automatic control procedure during the printing operation. The entire arrangement is inexpensive and inexpensive to manufacture and relieves the user of the printing device from its own, possibly imprecise controls, since the ink printing device controls itself at defined times.

The necessary components for the sensor arrangement are simple and easy to manufacture. If the thermal energy of the ejected ink droplets is detected by heat conduction, conventional and therefore inexpensive components such as PTC thermistor, thermistor or semiconductor components such as silicon sensors can be used as sensor elements. Infrared detectors, such as pyroelectric lithium tantalum (LiTa03), lithium niobate (LiNb0 ₃ ) or polyvinylidene fluoride (PVDF) detectors, are particularly suitable as receiving elements for the thermal radiation emitted by the ink droplets, which are sufficient for the evaluation ¬ give high output signal. A heat transmission film made of simple plastic can advantageously be connected between the nozzle plate of the ink print head and the sensor sensitive to heat radiation, since this enables a structure with relatively small dimensions to be realized.

If the thermal radiation of the ink droplets is used directly, for example when the fly by the active sensor surface is detected, it is advantageous to arrange a collecting device for the ink droplets after the sensor, from which the collected ink liquid e.g. is aspirated capillary. This makes it easy to dispose of this waste ink.

In addition to a few passive components, a simple evaluation device for the sensor signals generated by the ink droplet sensor that can be produced with little effort contains only two active components in the form of operational amplifiers convert and digitize the sensor signal so that a signal is available at the output of the evaluation device which can be used in a simple manner as a criterion for deactivating the ink printing device.

Since the thermal energy carried by the ink droplets includes also depends on the size of the ink droplets, it is also possible, in addition to the general statement as to whether a nozzle ejects ink droplets, also to obtain information as to whether a nozzle ejects ink droplets of small or large mass. It is only necessary for this, not only the presence of a sensor signal, but also to evaluate the magnitude of the sensor signal.

The invention is explained below using two exemplary embodiments, for which reference is made to the illustrations.

1 shows a first exemplary embodiment for the indirect use of the thermal energy carried by the ink droplets,

2 shows a second exemplary embodiment for the direct use of the thermal radiation emitted by the ink droplets, FIG. 3 shows a possible circuit arrangement of the evaluation electronics for the signals and generated by the ink droplet sensor

FIG. 4 waveforms when heat radiation caused by one or more ink droplets strikes the sensor.

A spray module 1 of an ink writing device, shown only schematically in a side view in FIG. 1, contains an ink print head 2 with a nozzle plate 16, at which a plurality of outlet nozzles 13 end. The energy for ejecting the ink droplets 3 is determined by thermal Generates energy (bubble jet technology). The spray module 1 is adjusted and locked on guide elements, not shown here, of a so-called printer block of the ink printing device with the aid of the locking elements 14, 15, which are only indicated in the drawing. In particular, for color printing, several such spray modules 1 can be arranged in a row perpendicular to the drawing plane, the ink print heads of which are then connected to different ink reservoirs containing the desired printing inks.

At a distance 12 from the nozzle plate 16, a heat-sensitive ink droplet sensor 6 is arranged such that it is used indirectly to detect the thermal energy carried by the ink droplets 3. For this purpose, a relatively thin carrier film 5 is provided in the trajectory of the ink droplets 3 ejected from the outlet nozzles 13 between the nozzle plate 16 and the ink droplet sensor 6, and the ink droplets 3 impinge on the side facing the nozzle plate 16. The carrier film 5 acting as a heat transmission element can be realized as a thin film made of conventional plastic and emits the thermal energy of the sprayed-on ink droplets 3 from its rear side facing the ink droplet sensor 6 as heat radiation 11 onto an active sensor surface 7 of the sensor 6 . As a heat radiation receiver, the sensor 6 contains an infrared detector 8 which emits a sensor signal i corresponding to the heat radiation 11 received. An evaluation device 10 is connected downstream of the sensor 6 and, depending on the presence of the sensor signal i, either releases the ink printing device for printing operation or sets it in a fault state. In order to ensure trouble-free detection of the thermal energies of the ejected ink droplets 3 also over the entire service life of the ink print head 2, the ink liquid 4 which has accumulated on the carrier film 5 during the function test becomes capillary, removed by mechanical transport of the carrier film by a winding / unwinding device or by a wiping mechanism, not shown here. The waste ink thus discharged can then be disposed of in a manner known per se.

In a second exemplary embodiment of the sensor arrangement according to FIG. 2, the thermal radiation 11 emitted by the ejected ink droplets is used directly for function monitoring of the ink print head.

The upper half of FIG. 2 shows this sensor arrangement in a side view, the lower half in a top view. The ink droplet sensor 6, the construction of which corresponds to the construction of the sensor explained with reference to FIG. 1, is arranged outside the trajectory of the ink droplets 3 in such a way that its active sensor surface 7 lies at a short distance parallel to the trajectory of the ink droplets 3 . Thus, the heat radiation 11 is detected when the ink droplets 3 fly past the active sensor surface 7.

The ink droplet sensor 6 has a collar-like protruding edge 17 of the active sensor surface 7, which considerably reduces possible interference from the sensor environment and generates a sensor signal i with an evaluable amplitude. To evaluate the sensor signal i, an evaluation device 10 is connected downstream, analogous to the arrangement in FIG.

Seen in the ejection direction of the ink droplets 3 after the ink droplet sensor 6, a trap 9 is provided in the trajectory of the ink droplets 3 for the droplets 3 emitted during the function check. The ink liquid 4 applied in this case, like the ink liquid on the carrier film 5 according to the first exemplary embodiment, is, for example, suctioned off and disposed of. FIG. 3 shows a possible circuit arrangement for utilizing the sensox signals i generated by the ink droplet sensor 6. In addition to passive components, the evaluation device 10 contains two active electrical components, namely the operational amplifiers 0P1 and 0P2.

The two outputs of the sensor 6 are led to the inputs of the first operational amplifier 0P1, the inverting input of the operational amplifier 1 being connected to its output via an ohmic resistor Rp. The connections for the supply voltage of the operational amplifier 0P1 are omitted for reasons of clarity. The operational amplifier 0P1 converts the sensor signal i generated by the sensor 6 into a voltage U, so that a voltage U = i • Rp is present at the output of the operational amplifier 0P1 for further processing. The output of the operational amplifier 0P1 is connected via a resistor R once to the non-inverting input and once to the inverting input of the second operational amplifier 0P2. Furthermore, a capacitor C is connected to the inverting input, the other connection of which is at reference potential, for example 0 V. A resistor R _{v is} connected between the non-inverting input of the operational amplifier 0P2 and the positive operating voltage + U _{B of} the operational amplifier 0P2. -U _ß the negative writing tive operating voltage of the operational amplifier 0P2 is be¬ distinguished. An output signal U _{fi can be} tapped at the output A of the operational amplifier 0P2, which can be used to activate a fault display or to switch off the ink pressure device. The signal U output by the operational amplifier 0P1 is digitized in the operational amplifier 0P2, which operates as a comparator, with the aid of the integrating element, consisting of the resistor R and the capacitor C, which forms an average. The resistor R "serves to generate the necessary level shift when digitizing U on the comparator. FIG. 4 shows basic signal profiles as they can occur within the evaluation device 10 when the thermal radiation caused by one or more ink droplets 3 strikes the sensor 6. These are in detail the time profiles of the voltage U = i ^• Rp at the output of the operational amplifier 0P1, the instantaneous voltage U corresponding to the voltage U at the non-inverting input of the operational amplifier 0P2 operating as a comparator, the voltage U forming the mean value of U at the inverting input of the operational amplifier 0P2 and the output signal U _{ß of} the evaluation device 10.

The above statements related to ink printing devices whose ink printing heads are equipped with thermal converters, that is to say with so-called bubble jet ink printing heads. In the case of these ink printing devices, the prerequisite for the heat-sensitive sensor principle described, namely that the ejected ink droplets and the detector (sensor) have different temperatures, is met to a particular degree, since in this case a considerable part (approx. 30%) of the electrical supply to the transducer elements Energy as heat is carried along by the ejected ink droplets.

However, the measures described can also be used for all other ink printing devices in which the droplet is ejected, for example, by electrical control of individual piezo elements. Due to the requirement of a uniform and reproducible droplet ejection, it is necessary to keep the temperature of the ink liquid at a constant value. It is therefore a customary measure to heat ink printing units and thus the ink liquid by means of various types of heating devices to temperatures which are generally above the room temperature. As a result, the requirements for the functional transmission according to the invention are also given.

Claims

1. A method for monitoring the droplet ejection from outlet nozzles (13) of an ink print head (2) in an ink writing device with an ink droplet sensor, which determines the output of the ink droplets (3), in which each outlet nozzle (13) of the ink print head (2) is subjected to a spray test comes with the following features:

- The thermal energy carried by the ejected ink droplets (3) is used to demonstrate the functionality of the individual outlet nozzles (13),

- Sensor signals (i) are generated as a function of the thermal energies of the individual ink droplets (3), - These sensor signals (i) are fed to an evaluation device (10) which, depending on the sensor signals (i), either releases the ink writing device for printing or put in a fault state.

2. The method according to claim 1, so that the entrained thermal energy of the ejected ink droplets (3) is transferred to the ink drop sensor (6) by means of heat conduction.

3. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the entrained thermal energy is detected via the thermal radiation (11) emitted by the ink droplets (3).

4. The method according to claim 3, characterized in that the heat radiation (11) is indirectly detected by the ink drop sensor (6),

5. The method of claim 3, d a d u r c h g e k e n n z e i c h n e t that the heat radiation (11) is detected directly by the ink drop sensor (6).

6. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the operability of the individual outlet nozzles (13) before each start of printing and / or cyclically at predetermined time intervals during printing is carried out.

7. Arrangement for monitoring the droplet ejection from outlet nozzles (13) of an ink print head (2) in an ink writing device with a in the movement area of the ink print head (2) at a distance (12) in front of the outlet nozzles (13), the ejection of the inks droplet-detecting heat-sensitive ink drop sensor (6), which is followed by an evaluation device (10) which evaluates a signal (i) generated by the ink drop sensor (6).

8. Arrangement according to claim 7, so that the ink drop sensor (6) is implemented as a sensor type responsive to thermal radiation (11).

9. Arrangement according to claim 8, characterized in that in the trajectory of the ink droplets (3) between the outlet nozzles (13) and an active sensor surface (7) of the ink drop sensor (6), a carrier film (5) is provided, on one side of which Impact ink droplets (3) and a portion of the thermal energy of the ink droplets (4) applied to the carrier film (5) is transferred from the opposite rear side to the active sensor surface (7) by heat radiation (11).

1 10. Arrangement according to claim 8, characterized in that the ink drop sensor (6) is arranged such that its active sensor surface (7) is outside the trajectory of the ink droplets (3), so that 5 the heat radiation (11) of the ejected ink droplets (3rd ) is detected when the active sensor surface (7) flies by.

11. The arrangement according to claim 7, d a d u r c h

10 g e k e n n e i c h n e t that the ink drop sensor (6) is realized as a sensor type responsive to heat conduction.

12. The arrangement according to claim 10, d a d u r c h

15 g e k e n n e i c h n e t that in the trajectory of the

Ink droplets (3) after the ink drop sensor (6) a collecting device (9) for the ejected ink droplets (3) is provided.

20 13. Arrangement according to claim 8 or 12, d a d u r c h g e k e n e z e c h n t that the ink liquid (4) sprayed onto the carrier film (5) and / or onto the collecting device (9) is discharged capillary.

f 25 14. Arrangement according to claim 1, characterized in that the evaluation device (10) contains switching means (0P1,0P2, R, R _V , C) which convert the signal (i) generated by the ink drop sensor (6) into a convert this signal (i) into proportional voltage (U) and digitize it 30 so that an output signal (U-) is available at an output (A) of the evaluation device (10), which serves as a criterion for deactivating the ink printing device h -εr is used.

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