EP0142150A1 - Method and transducer for increasing the resolution in an ink mosaic recording device - Google Patents

Abstract

In order to be able to increase the resolution in an ink mosaic writing device simply and without additional transducers, the invention provides that the transducers (11-14) each consist of two contacted piezoelectric strips (20, 21; 30, 31) which are located between plates (26, 27; 36, 37) are included. Due to different voltage profiles on the strips (30, 31), the transducer (13) bends in addition to the change in cross section of the channel formed. This makes it possible to eject liquid drops in different positions of the transducer (13).

Description

The invention relates to a method for increasing the resolution in an ink mosaic writing device with a writing head having a number of piezoelectric transducers arranged in a row, in which writing fluid is ejected dropwise in the direction of a recording medium by piezoelectric deformation of the transducers.

In known ink mosaic writing devices, a resolution of about four ink drops per millimeter has been achieved (DE-PS 25 27 647). To improve the typeface, about ten such drops of ink per millimeter would be desirable. For this purpose, it has already been proposed to arrange a plurality of rows of piezoelectric transducers offset from one another in the paper transport direction. However, this requires the electronics to be duplicated and, under certain circumstances, can also lead to distortion in the typeface. A reduction in the distance between the piezoelectric transducers in a row - apart from the manufacturing difficulties - significantly increases the risk of hydraulic coupling between the transducers.

The present invention has for its object to provide a method and a transducer with which the resolution in an ink mosaic writing device of the type mentioned can be increased in a simple manner and without additional piezoelectric transducers.

This object is achieved according to the invention in that the individual transducers consist of two strips made of piezoelectric material exist, which are arranged at a distance from each other, contacted on both sides and covered on both sides by plates in such a way that an elongated, essentially rectangular channel is formed between the strips and the plates, from which the writing fluid is ejected, and that the two strips at least temporarily deformed to different degrees. In the transducers provided here, the ink channels formed directly represent the writing nozzles. A separate perforated matrix can therefore be dispensed with. For example, if an electrical voltage is applied to the strips, they become narrower and higher, so that the enclosed cross-sectional area of the channel is enlarged, which means that additional writing fluid is sucked into this channel. If the voltage at this contact is removed or even a pulse of reverse polarity is applied for a short time, the strips return to their original shape, which suddenly reduces the channel volume. This leads to ejection of writing fluid.

When the voltage is applied, the piezoelectric strips become shorter at the same time. If, according to the method according to the invention, the two strips of a piezoelectric transducer are deformed differently, for example due to different voltages, a sufficient cross-sectional enlargement can still be achieved in order to additionally draw writing fluid into the ink channel. At the same time, however, the difference in length of the two piezoelectric strips causes the transducer to bend somewhat, so that the end of the transducer facing the recording medium dodges sideways. If the deformation of the two strips of the transducer is now reversed so that the different linear expansion and thus the bending of the transducer are initially maintained, a drop of liquid is expelled, which is laterally offset for the transducer position in the unbent state. If the recording medium is guided past the ink channels as usual, this corresponds to the displacement of the point of impact of the ink drop approximately the displacement of the channel mouth.

This bending of the ink channel and the ejection of an ink drop in this position can be realized, for example, by superimposing a DC voltage that is different for the two strips on a voltage pulse applied to both piezoelectric strips, which practically causes and maintains the bending.

With the deflection, only the mass of a single transducer has to be accelerated. This makes it possible to carry out the deflection with an amplitude which corresponds to approximately a quarter of the distance between the liquid drops generated when the transducers are at rest. Taking a print head with the ink channels at a distance of approx. 250 µm as an example, this would result in a deflection amplitude of approx. 60 µm. In this case, the synchronization would have to take place in such a way that a drop of liquid is expelled in one direction or the other at the maximum deflection. In this way, the number of drops per mm could already be doubled. If the deflection amplitude is increased to 80 µm and if the non-curved rest position of the ink channel is also used to eject a drop, the number of drops per mm can already be tripled. Since the ink channels are practically at rest in the moment of ejection of a liquid drop, no speed component is superimposed on the drop transversely to the direction of flight.

On the other hand, however, it is also possible to perform the deflection of the transducer synchronously with the ejection of an ink drop, so that a velocity cross component is superimposed on the liquid drop. As a result, the point of impact of the drop on the recording medium is shifted further than it corresponds to the deflection of the transducer. In In this case, it is possible to work with a much smaller deflection amplitude.

A combination of the two deflection methods described above is also conceivable. In principle, the points of impact of the liquid drops on the recording medium can be as tight as desired.

The intermittent bending of the piezoelectric transducers can be achieved particularly easily by superimposing two voltage pulses different for the two strips on the voltage pulse applied to both piezoelectric strips to eject a drop of liquid. The voltage pulses additionally applied to the two strips can have different amplitudes or different polarities. The duration of the pulses only has to be selected so that the desired bending of the piezoelectric transducer is reached before the time of ejection and is maintained until after the drop of liquid is ejected, if the process is to be carried out without additional cross-speed components.

This results in an ink mosaic writing device which is simple in construction and stable for operation, for carrying out the method. if the transducers in the end facing away from the recording medium are connected via a web. The clamping of the entire transducer arrangement is simplified considerably. Furthermore, there is otherwise no need for complex adjustment work to align the individual transducers with one another. Under certain circumstances, the electrical control can be simplified further in that the plate is made of metal on one side of the strips and serves as a common electrode for both or even all of the strips. In this case, the additional contacting that would otherwise be required on this side of the strips can be dispensed with.

An advantageous improvement in ink ejection results from the fact that the outlet opening of the ink channels is made smaller, for example by the channel tapering in this end region. A simple embodiment provides that the strips of a converter form an acute angle.

An advantageous transducer arrangement for carrying out the method can be produced particularly simply in that a channel matrix, which consists of a series of strips of piezoelectric material, which are arranged in parallel and at a distance from one another, makes electrical contact on both sides and on both sides with a plate covering all strips are covered, is cut after two strips parallel to these and at least over part of the length thereof. This results in a comb-like arrangement with a web holding all transducers together, the teeth of the comb consisting of the individual transducers with the ink channels.

• Fig. 1 eine schematische Ansicht einer Tintenmosaikschreibeinrichtung,
• Fig. 2 ein Beispiels eines Sehreibkopfes mit vier piezoelektrischen Wandlern,
• Fig. 3 einen Querschnitt durch den Schreibkopf gemäss Fig. 2,
• Fig. 4 a und b zeitliche Spannungsverläufe für die beiden piezoelektrischen Streifen eines Wandlers und
• Fig. 5 a bis e die Aufzeichnungspunkte relativ zu den Tintenkanälen zu verschiedenen Zeiten.

Further advantages are apparent from the below on the basis of five figures illustrated embodiments, mi ^* where the invention is explained in more detail and described. Similar parts are provided with the same reference numerals. It shows or show

• 1 is a schematic view of an ink mosaic writing device,
• 2 shows an example of a visual head with four piezoelectric transducers,
• 3 shows a cross section through the write head according to FIG. 2,
• Fig. 4 a and b voltage curves over time for the two piezoelectric strips of a transducer and
• 5 a to e show the recording dots relative to the ink channels at different times.

The outer basic structure of an ink mosaic writing device can be seen from FIG. The recording medium 3, for example normal registration paper, is drawn past the front face 6 of a housing 7 in the direction of arrow 4 via transport rollers 1 and 2. In the housing 7, the connecting line 8 is guided, which at its free end has a plug 9 for connection to a corresponding control device which contains the control signals for recording the desired courses, characters or images and for the synchronization between the time deflection of the respective transducers and the ejection of the individual liquid drops. The housing 7 contains the actual write head, a possible embodiment of which is shown in FIG. 2.

The writing head 10 consists of a series of piezoelectric transducers 11 to 14 and a reservoir 15 for writing fluid 16. In the transducer 11, the ink channel 17 is indicated by dashed lines, which is explained in more detail in connection with FIG. 3. For the sake of clarity, the electrical contacts of the piezoelectric strips have not been shown. As can further be seen from FIG. 2, the piezoelectric transducers 11 to 14 are connected to one another at the rear end via a common web 18. At this point it should be noted that in principle it is also possible to individually design each transducer and fasten it separately in the print head. The embodiment shown has the advantage of a considerably simplified production, since first a closed channel matrix can be produced in which a row of contacted piezoelectric strips on a first plate are arranged parallel to one another and with a second plate to be covered. After every two strips that enclose an ink channel, an almost continuous slot is sawn into the plate in the longitudinal direction. In this way, a comb-like transducer arrangement, as is used in the exemplary embodiment according to FIG. 2, is produced.

FIG. 3 shows a cross section through the converter arrangement according to FIG. 2. The two outermost converters 11 and 14 are only indicated by dashed lines. Likewise, the converter 13 is only hinted at in its rest position. The transducer 12 shown in more detail in cross section, like the other transducers, consists of two piezoelectric strips 20 and 21, which are provided on both sides with a contact 22 to 25 and which are enclosed between two plates 26 and 27. In this case, the two plates are made of a non-conductive material.

If, as assumed for the converter 13, a voltage is now applied to the two strips 30 and 31, these strips become higher and at the same time narrower and shorter. If, as assumed in FIG. 3, a voltage of different magnitude is applied to the two strips 30 and 31, there are different changes in the length of the strips in the three dimensions. The transducer element bends due to the different length changes of the strips. Despite the different changes in height and width, the cross-sectional area of the ink channel is enlarged, so that writing fluid is sucked into it.

If the cross-sectional enlargement is subsequently reversed, but the bending is retained, a drop of liquid is practically expelled from a staggered channel opening.

4a and 4b exemplify the time profile of the voltage applied to the two strips of a converter posed. It is assumed that the tension on strip 30 is shown in FIG. 4a and that on strip 31 in FIG. 4b. At time t ₁ , an equally large voltage is applied to both strips 30 and 31, which is kept constant until time t ₂ and then suddenly returns to the initial value. With the help of this rectangular pulse on both strips, the normal effect of such a transducer element would result; the cross-sectional area increases as a result of the applied voltage and ink is sucked into the ink channel. At time t ₂ , the cross-sectional area and thus the volume of the ink channel suddenly decreases, so that a drop of liquid is expelled in the direction of the ink channel. In this case, there is no bending of the transducer element.

At time t _{3 it} is assumed that the two strips have a small voltage of the same, but opposite, voltage of z. B. 20 V is applied. This initially leads to a bending of the transducer 13, but not to an increase in the cross section of the ink channel. at time t ₄ this voltage is a voltage pulse of z. B. 100 V, as between the pages t ₁ and t ₂ , superimposed, which lasts until the time t ₅ . Finally, at time t ₆ , the voltage on both strips returns to the initial value. In the time interval between t ₄ and t ₅ , the cross-sectional area of the ink channel of the deflected transducer is increased. At time t ₅ , this cross-sectional enlargement is reversed, but the deflection is retained. The deflection is not reversed until time t ₆ .

5a-5e will be used as an example to explain how the higher resolution is achieved. 5a, for the sake of simplicity, only three outlet openings of transducers are shown side by side, from which liquid drops are to be ejected at the same time. 5b is assume that the transducers are not deflected, so that the locations of the recording points match the locations of the stationary transducers. 5c it is assumed that all transducers are deflected to one side before the ejection of a liquid drop, so that the recording points are also shifted to this side. In Fig. 5d the deflection is made to the other side. 5e finally shows the totality of the recording points possible as a result. With the help of the three ink channels, nine recording points can be achieved in this way.

The exemplary embodiments explained with reference to FIGS. 1 to 5 are not drawn to scale for the sake of clarity. On the basis of these figures, the essentials of the method according to the invention and an advantageous embodiment of the write head are to be shown. Without departing from the scope of this invention, it is also possible to further increase the number of recording points by deflecting the individual transducers to different extents.

Furthermore, it is also possible to synchronize the deflection of the transducers with the ejection of the liquid drops in such a way that a speed cross component is superimposed on them. This provides a further possibility of variation, by means of which the position of the points of impact of the drops can be changed further. A particular advantage of the method according to the invention and the ink mosaic writing device specified here for carrying out this method is that the additional deflection of the transducers and the higher resolution achieved thereby only have to be used when necessary and a lower paper speed only has to be accepted for these cases. For designs with lower resolution requirements, you can always work with a higher paper speed. For both cases, ie low or high resolution, there is always only one write head and thus one control electronics necessary. The additional effort for increasing the resolution is therefore limited to a minimum.

Claims (5)

1.Method for increasing the resolution in an ink mosaic writing device with a writing head having a number of piezoelectric transducers arranged in a row, in which writing fluid is ejected dropwise in the direction of a recording medium by piezoelectric deformation of the transducers, characterized in that the individual transducers (11 to 14 ) consist of two strips (20, 21; 30, 31) made of piezoelectric material, which are arranged at a distance from one another, contacted on both sides and covered on both sides with plates (26, 27; 36, 37) such that between the strips ( 20, 21; 30, 31) and the plates (26, 27; 36, 37) an elongated, essentially rectangular channel (17) is formed, from which the writing liquid (16) is ejected, and that the two strips (20, 21; 30, 31) are at least temporarily deformed to different extents. 2. The method according to claim 1, characterized in that the different deformation takes place by superimposing the voltage pulse provided for ejecting a liquid drop and applied to both piezoelectric strips (20, 21; 30, 31) with two voltage pulses different for the two strips. 3. ink mosaic writing device for performing the method, characterized in that the transducers (11 to 14) in the end facing away from the recording medium (3) are connected by a web (18). 4. Ink mosaic saw device according to one of claims 1 to 3, characterized in that the plate (27; 37) on one side of the strips (20, 21; 30, 31) consists of metal and serves as a common electrode for both strips. 5. A method for producing the transducer for carrying out the method for increasing the resolution according to one of the preceding claims, characterized in that a channel matrix, which consists of a series of strips of piezoelectric material, which are arranged and spaced apart, electrically contacted and on both sides are covered on both sides with a plate covering all strips, after two strips are cut parallel to these and at least over part of the length thereof.

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