EP0221453A1 - Electrostatic print head - Google Patents

Abstract

The invention relates to electrostatic print heads for gradually creating a latent electrostatic image on a moving support (18), of the special paper type or not, by ionic discharge by means of a set of aligned electrodes (20). , and a counter-electrode (1) facing them on the other side of the support (18) in movement.

The counter electrode (1) consists of a cylindrical or semi-cylindrical resistive layer (7) covering connection conductors (C₁ to C _{n + 1} ) constituted by rings or sectors of parallel rings having the same axis as the resistive layer and between which cylindrical or semi-cylindrical resistive portions are thus formed. The rings or ring sectors are individually connected to the outputs of an electronic selective switching circuit (CEC) placed at least partially in the concavity of said counter-electrode (1).

Description

The present invention relates to printing devices using a plurality of individual electrodes aligned in at least one row for printing a recording medium displaced in front of these electrodes.

It relates more particularly to an electrostatic print head by means of which a latent electrostatic image is gradually created on the recording medium, by ionic discharge obtained by carrying the high voltage electrodes, while the recording medium is moved in front of the head.

The individual electrodes used in the print head are of very small dimensions and in high number for a line of given length, if a good resolution is to be obtained; for example, there will be 1,728 for the printing of an A₄ format recording medium at 8 points per millimeter.

In practical embodiments, a counter-electrode brought to a high voltage of opposite polarity to that of the latter is associated with the electrodes, in order to avoid the application to the electrodes only of the high high voltage necessary for ionic discharge for printing. of the electrostatic image. Under these conditions, the high voltage required for the ionic discharge will be applied to each electrode / counter electrode pair, the high voltage applied to the electrode alone, or to the counter electrode alone, being less than the threshold value causing this ionic discharge and therefore unable to allow printing.

In indirect electrostatic printing, a thin insulating film, stored in a roll, is usually used as an intermediate recording medium on which the latent electrostatic image is recorded, which is then inked, for example using '' a magnetic brush carrying powder ink. The electrostatic image thus developed is then transferred from the intermediate support to a final support, usually made of paper and in the form of separate or separable sheets, for example by contact between intermediate support and final support, under high pressure.

In direct electrostatic printing, it is not necessary to use an intermediate support, the final support consisting of a base of conductive paper covered by a fine dielectric deposit allowing successively the formation of a latent image and the fixing of the image, revealed by inking, on the same support possibly using the means mentioned above.

European patent application No. 0124,856 describes an electrostatic printhead composed first of at least one row of individual electrodes arranged in regular pitch and organized in "n" groups, these groups being organized in successive sets of 'at least two groups and those of their electrodes, which have the same position in the groups and which belong to groups having the same ranks in the sets, being interconnected with one another. The electrostatic printhead mentioned above is secondly composed of a counter electrode facing the electrodes over at least the length of the row considered and this counter electrode is made of a resistive material with which they are in contact. "n + 1" conductors arranged at regular intervals, substantially equal to the pitch of the groups of electrodes in the row. The “n + 1” conductors define between them “n” resistive portions of substantially identical values which are connected in series, respectively arranged each opposite one of the groups of electrodes and each assigned to the selection of said group from the others .

In the embodiment presented in this application, it is necessary to provide a large number of connections both at the electrodes and against the electrodes, the number of the latter being for example 73 for an embodiment of a printhead comprising 1728 points.

The present invention therefore firstly proposes a counter-electrode arrangement making it possible to incorporate part of its control electronics therein in order to reduce the number of connections.

In addition, the movement of the printing medium (s) between the electrodes and the counter-electrode creates wear by friction of the latter and unnecessary braking of the moving medium.

The present invention therefore also provides an arrangement of counter-electrode as indicated above which aims to reduce the braking of the support and the wear of the counter-electrode.

According to a characteristic of the invention, the counter electrode is composed of a cylindrical or semi-cylindrical external resistive layer covering the conductors constituted by rings or sectors of parallel rings which have the same axis as the resistive layer and which are individually connected. at the outputs of an electronic circuit for selective voltage switching placed at least partially in the concavity of said electrode.

According to a complementary characteristic of the invention, the counter electrode which is of cylindrical shape of revolution is mounted to rotate about an axis perpendicular to the direction of movement of the recording medium which has just been inserted, pressed against it, between the individual electrodes and herself.

• La figure 1 présente le schéma électrique équivalent d'une tête d'impression électrostatique.
• la figure 2 présente un exemple de circuit électronique de commu­tation sélective de tensions pour une contre-électrode de tête d'impres­sion électrostatique.
• La figure 3 présente un exemple de contre-électrode selon l'invention selon deux coupes l'une transversale et l'autre longitudinale.

The invention, its characteristics and its advantages are explained in the following description in conjunction with the figures listed below.

• Figure 1 shows the equivalent electrical diagram of an electrostatic print head.
• FIG. 2 shows an example of an electronic circuit for selective voltage switching for an electrostatic print head counter-electrode.
• FIG. 3 shows an example of a counter electrode according to the invention in two sections, one transverse and the other longitudinal.

In Figure 1 is shown the equivalent electrical diagram of an electrostatic print head as provided in the European patent application mentioned above, this head has a counter electrode 1 associated with a set of electrodes 2 comprising at least a row of electrodes 20.

In this row 20, the individual electrodes 21 are arranged in regular pitch and organized into two networks of n identical groups denoted G₁ to G _n in each of which they occupy successive rows denoted P₁ to P _m . The electrodes of the same rank belonging to the odd groups G₁, G₃ .... are interconnected with one another. The electrodes of the same rank belonging to the even groups G₂, G₄ ..... are also interconnected with each other. These electrodes of rows P₁ to P _m are set to a potential -V or 0 volts, for the odd groups, by means of a first set of m individual switches E₁₁ to E _1m and, for the even groups, by l 'through a second set of m individual switches E₂₁ to E _2m , independent of the first set.

The counter electrode 1 is made of resistive material and comprises n + 1 connection conductors C, referenced C₁ to C _{n + 1} , associating each portion of counter electrode between two successive conductors with one of the different groups of electrodes at look of which it is placed.

On this resistive counter electrode 1, the n + 1 connection conductors define a set of n resistors in series at R₁ to R _n individually assigned to the groups of electrodes G₁ to G _n and arranged opposite these groups; the connection conductors C₁ to C _{n + 1} are set to a potential V / 2 or 0 volts by means of n + 1 individual switches CE₁ to CE _{n + 1} , V being the high voltage value necessary for printing.

An example of an electronic circuit for selective switching of voltages intended to serve the resistive counter-electrode 1, is presented in FIG. 2, this example should not be considered as limiting other known circuits such as for example shift register circuits with high voltage output is also likely to be used.

In the embodiment presented, the n resistors R are distributed into s groups each comprising at most q-1 resistors R which follow one another on the counter-electrode 1, such as the group of resistors R₁₁, R₁₂, ... R _{1 .q-1} .

A two-dimensional matrix control type command makes it possible to energize each of the n + 1 connection conductors C of these n resistors R.

Each connection conductor C is connected on the one hand by a diode D to a first switch arrangement, here called line 3, such as the connection conductor C₁₁ connected to the line switch arrangement 3₁ via the diode D₁₁, on the other hand by a resistor r to a switch arrangement, here called group 4, such as the resistor r₁₁ connecting the connection conductor C₁₁ to the switch arrangement group 4₁.

The switch arrangements 3 and 4 make it possible to apply alternately, according to printing needs, a ground potential or a + V / 2 potential to the connection conductors to which they are connected either by diodes D or by resistors r .

In the proposed embodiment, the connection conductors C of the successive resistors R of the same group are served by the same group switch arrangement 4, at the output S of which these connection conductors are each connected by a resistor r, such as the conductors of connection C₁₁ to C₁₉ respectively connected by resistors r₁₁ to r₁₉ at output S₄₁ of the group switch arrangement 4₁.

The examples of group switch arrangement 4₁, 4₂, 4₉ presented here each include a first switching transistor 5, of NPN type, the base of which receives an external control signal according to printing needs via a wire g, such as the wire g₁ for transistor 5₁, to alternately connect the output S of this arrangement to the ground potential or to the potential + V / 2.

The junction of the output S of the arrangement to one or the other of these two potentials is effected by means of a Darlington circuit with two transistors 6 and 7, of NPN type, and with two resistors 8 and 9, the emitter of each transistor 7 is connected to the corresponding output S, such as that of transistor 7₁ to output S₄₁, while the collector of each transistor 7 is connected to the potential + V / 2, as does the collector of the corresponding transistor 6 and one of the terminals of the corresponding resistor 9. The base of each transistor 6 is connected to the collector of the switching transistor 5 of the switch arrangement which comprises it so as to be brought to ground potential, via the emitter of this switching transistor 5, when this transistor conducts. The base of each transistor 7 is controlled by the emitter of transistor 6 of the switch arrangement which includes it.

The connection conductors C of successive resistors R of the same group are served by different line switch arrangements 3, such as arrangements 3₁, 3₂, ... 3 _y , each conductor of connection C being connected to the line switch arrangement 3 which serves it by an individual diode D such as the connection conductor C₁₁ connected by the diode D₁₁ to the line switch arrangement 3₁.

Each line switch arrangement 3 is usually connected by its output S₃ to the cathodes of the diodes D of several connection conductors C taken in different groups, such as the output S₃₁ of the line switch arrangement 3₁ to the diodes D₁₁, D₂₁ ... D _r1 of conductors C₁₁, C₂₁ ... C _r1 of rank 1 of groups 1, 2 ... r of resistances R.

Each output S₃ of line switch arrangement is also capable of being connected to ground potential or to potential + V / 2, here using a simplified assembly comprising a switching transistor 10 of NPN type, the transmitter is connected to ground, the collector of which is connected to the potential + V / 2, via a resistor 11, and the base of which receives an external control signal according to printing needs via a resistor 12 and a wire 1, such as the base of transistor 10 _y via resistor 12 _y and wire 1 _y .

A resistor 13 connected to a low potential + v and a capacitor 14 conventionally complete each line switch arrangement 10.

The two switching arrangements, one of group 4 and the other of line 3, which are connected to the same connection conductor C, such as the arrangements 3₁ and 4₁ for the conductor C₁₁, are connected simultaneously to the potential + V / 2 to bring this connection conductor C to this potential, in any other case the connection conductor concerned remains at ground potential.

According to a preferred variant, the connection conductors C are supplied sequentially two by two.

The electronic circuit for selective switching of the voltages defined above in connection with FIG. 2 is conventionally produced on a physical support constituted for example by a printed circuit board referenced 15 in FIGS. 3 and 4.

According to a preferred embodiment, this circuit is closely associated with the counter-electrode 1 which it serves, which has a cylindrical or possibly semi-cylindrical shape in the concavity from which the printed circuit board 15 is placed.

FIG. 3 shows a counter-electrode 1 chosen to be cylindrical and of revolution insofar as it is intended to be mounted for rotation, a semi-cylindrical shape which may prove to be advantageous in the case of a fixed counter-electrode.

The counter-electrode with circular section in FIGS. 3 and 4 comprises annular connection conductors C connected for example by pins P intended to provide electrical connections with the conductive tracks of the printed circuit board 15 which in particular carries the resistors r and the diodes D, such as the connection conductor C₁₁ connected by pins P₁₁ and P'₁₁ on the one hand to the diode D₁₁ on the other hand to the resistor r₁₁.

In one embodiment, the printed circuit board 15 is arranged in a median longitudinal plane of the circular section cylinder formed by the counter electrode between two semi-cylindrical shells 16, 16 ′ of insulating material which adjusts one on the other to form the assembly of said counter-electrode.

The rings or the sectors of rings which form the connection conductors C are placed for example in external circular grooves of the assembly formed by the two assembled half-shells.

The pins P are flush with the outside of the half-shells 16, 16 ′ for the connection of the rings or sectors of rings to which they are for example welded.

The pins P are here arranged in the plane of the rings they serve, and pass through the assembly formed by the half-shells assembled by not shown notches which are formed at the joint plane of these half-shells.

In the embodiment presented, the resistor r and the diode D assigned to a ring connection conductor C are arranged at this ring in the counter-electrode and more precisely on the printed circuit board 15, the pins P and P 'serving this resistor and this diode being arranged symmetrically with respect to the axis of the counter electrode and in the plane of said ring.

The two joined half-shells 16, 16 ′, carrying the rings formed by the connection conductors C, are covered as well as these rings by a layer 17 of flexible resistive material of small thickness, the resistivity of which is for example of the order of 10⁷ or 10⁸ ohm / cm; the resistors R between connection conductors are formed by the zones of this layer 17 located between the rings.

The counter-electrode 1 thus formed is intended to cooperate with a set 2 of electrodes 20 seen in profile in FIG. 3, the electrodes being arranged, in at least one row and preferably two, parallel to a generator of the cylinder which the counter-electrode 1, so as to allow the passage of the intermediate support 18 between electrode 2 and counter-electrode 1.

If the counter-electrode 1 is not provided for rotation, it is of course possible that it is semi-cylindrical, the printed circuit board 15 remaining placed in its concavity for reasons of convenience.

If the counter-electrode is provided for rotating and for example idle around its axis XX ′ (FIG. 4), it is necessary to provide for the supply of the electronic voltage switching circuit carried by the printed circuit board; this is done for example using a conventional device with rings 19 integral with the rotating part, that is to say with the counter-electrode 1, and with fixed friction brushes 21 connected to a connector 23, also fixed, by different control wires 23 of the electronic voltage switching circuit.

Preferably and as indicated above the counter-electrode 1 is carried by bearings or bearings 25, only one of which is shown, so as to rotate under the action of the intermediate support 18 which rubs against it.

Claims (5)

1 / Electrostatic printhead first comprising at least one row of individual electrodes (21) arranged in regular pitch and organized into n groups (G), said groups being themselves organized in successive sets each of at least two groups, and the electrodes, having the same positions in the groups and belonging to groups having the same ranks in the assemblies, being interconnected with each other, comprising, secondly a counter-electrode (1) facing the electrodes on at minus the length of the row considered, said counter-electrode being made of a resistive material with which are in contact n + 1 connection conductors (C₁ to C _{n + 1} ) arranged at regular intervals substantially equal to the pitch of the groups of electrodes of this row, which define between them n resistive portions, each arranged opposite one of the groups of electrodes (G₁ to G _n ) and being assigned to it for its selection from the other groups, the said s resistive portions forming n electrical resistances (R₁ to R _n ), substantially identical and connected in series, characterized in that the counter-electrode is composed of a resistive layer (17), external, cylindrical or semi-cylindrical, covering the n + 1 connection conductors (C₁à C _{n + 1} ) constituted by rings or sectors of parallel rings, having the same axis as the resistive layer, which are individually connected to the outputs of an electronic circuit for selective voltage switching ( CEC) placed at least partially in the concavity of said counter-electrode. 2 / electrostatic print head according to claim 1, characterized in that each ring conductor or ring sector is symmetrically connected to the electronic circuit of selective voltage switch (CEC) by two connections located at its level on the side edges of the support (15) of this electronic voltage switching circuit. 3 / electrostatic print head according to claim 2, characterized in that one of the two connections of a conductor (C) in a ring or ring sector is connected by a resistor (r) to a selector switch for group (4) capable of applying one or the other of two voltages to it, depending on its control state, the other connection of the conductor (C) considered being connected by a diode (D) to a line selection switch (3) capable of applying to it one or other of the voltages previously considered, according to its own control state. 4 / electrostatic printing head according to at least one of the preceding claims, characterized in that the counter-electrode which is of cylindrical shape of revolution, is mounted to rotate about an axis perpendicular to the direction of movement of the recording medium ( 18) which is just inserted, pressed against it, between the individual electrodes and itself. 5 / electrostatic printing head according to one of the preceding claims, characterized in that the support (15) of the electronic circuit for selective voltage switching (CEC) is immobilized between two semi-cylindrical shells 16), through the joints diametrically opposite from which pass pins (P) for electrical connection with the ring conductors (C), a cylindrical resistive layer of revolution coming to surround the assembly formed by the support and the assembled half-shells which enclose it.

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