EP0238926A1 - Print head for a dot matrix printer - Google Patents

Abstract

In a print head according to the invention, the electromagnets (58) which control the movement of needles (46) and which are disposed around their ends opposite those intended for printing comprise solenoid plungers (76) which are coupled at these ends of the needles by levers (82) made of non-magnetic material, preferably a synthetic material, which are fixed to them. On the other hand, in a preferred embodiment the levers are normally held in a rest position by elastic metal strips (90) which are fixed to their faces opposite the electromagnets and the needles carry overmoulded heads (80) which are simply held bearing against the strips by spiral springs (92) placed between these heads and the first guide device (40) into which the needles pass. <IMAGE>

Description

The subject of the present invention is a print head for a dot matrix printer.

In dot matrix printers, the printing of characters on paper is done by a certain number of needles whose ends aligned in the direction of the height of these characters strike perpendicularly and at a very high rate an inked ribbon interposed between them and the paper.

These needles are housed in a head which is moved step by step in the direction of the lines to be printed and which further comprises electromagnets for actuating them individually and several guide devices in which they pass successively and which have the role of concentrating the beam they form at the start and to ensure that the alignment of their ends is as perfect as possible.

Of course, speaking of alignment does not necessarily mean that these ends are all in the same row. There are also heads in which these are arranged in two parallel rows. This has the advantages of making it easier to print different types of characters and to represent these by a larger number of dots, and therefore to obtain better graphic quality.

There are currently printheads in which the electromagnets which control the movement of the needles are arranged in an arc of a circle and include plunger cores to which these needles are fixed directly by their ends opposite to those intended for striking. Consequently, the further one moves away from the plane of symmetry, the more the needles are curved, which means that if they were content to pass them through thin guides, most of them would exert forces on them. significant friction that the electromagnets should overcome. So to prevent heads like this from heating up too much, they would have to run at a low speed, which, of course, would be a big drawback. On the other hand, the wear of the guides by the most curved needles would be very rapid, which would very quickly cause a misalignment of their ends relative to those of the others and, therefore, a deformation of the printed characters.

This is why the needles are passed through very fine tubes which guide them over most of their length. Despite this, it is hardly possible to make these heads work at a frequency much greater than 1500 Hz, that is to say to make a needle or several needles perform at the same time more than 1500 movements of va- back and forth per second, without their temperature reaching and even exceeding 100 ° C.

Furthermore, their manufacture is relatively complicated and expensive, in particular because of the presence of capillary tubes, and they are always quite bulky because even with these tubes there is a limit not to be exceeded with regard to the curvature of the needles.

In other known heads the electromagnets, which have a U-shaped yoke surrounded by a coil, are arranged around the upper part of the needles and each of these is fixed to the free end of a thin blade of magnetic material which extends radially in front of one of the poles of an electromagnet and which has its other end linked and magnetically coupled to the other pole of the latter. Thus, as long as the coil of an electromagnet is not energized, the magnetic strip associated with it is kept away from the pole to which it is not fixed, due to its own elasticity or under the action of 'a spring, and the corresponding needle remains in a withdrawal or rest position. On the other hand, as soon as a current flows in the coil the blade comes to press against this pole and the needle is brought into printing position.

In this case, the needles only need to be slightly curved and they are almost all to the same degree. We can therefore dispense with passing them through tubes. In addition, these heads can be significantly smaller than the previous ones. Unfortunately, here again we can hardly exceed 1500 Hz and this for the following reason. Since the blades are made of magnetic material, they are relatively heavy and difficult to move and, in this sense, it is advantageous to reduce their dimensions as much as possible. However, the thinner and narrower they are, the more quickly they become saturated in magnetic flux when they are attracted to the electromagnets and the greater the iron losses in these, mainly due to the eddy currents. We are therefore obliged to be satisfied with a compromise which corresponds roughly to the value of the operating frequency which we have just indicated if we do not want the temperature of the heads in question to greatly exceed the 100 ° C.

Several solutions are currently available to avoid this saturation problem. One of them consists in replacing each electromagnet of which we have just spoken by another whose cylinder head still has the shape of a U but of which one of the vertical branches, or at least the end of -this consists of a permanent magnet and, preferably, to associate with it a blade made of magnetic material whose end is no longer fixed to the branch of the U furthest from the needles but at a point situated far beyond this one. In this way, when no current flows in the coil which surrounds it, the whole breech constitutes a permanent magnet by which the blade is attracted and the needle actuated by the latter remains in the rest position. On the other hand, when the coil is supplied so as to cancel the effect of the magnet, a spring provided for this purpose acts on the blade to bring the needle into the printing position.

The heads which are designed in this way make it possible to achieve a slightly higher working frequency, around 1800 Hz.

Another possibility is to produce a printer head as described in US Pat. No. 4,531,848. In this case the electromagnets, which are always arranged in a circle around the upper part of the needles, comprise each a yoke, a coil and a plunger core to which is fixed, by its central part, an elastic blade, one of the ends of which is made solid of the yoke, on the side opposite to that where the needles are located. On the other hand, on this blade is molded a plastic stiffening piece which goes from the plunger core to the head of a needle which can be linked to it or simply kept in contact with it thanks to a spring acting on this needle.

In this type of head, the magnetic circuits of the electromagnets are never saturated. The latter therefore have a better yield than otherwise. Furthermore, since the blades are no longer part of these circuits, we can choose the appropriate material that we want to make them and we are much more free as regards the choice of their dimensions, in particular their thickness. The main thing is that these blades are rigid enough to be able to support each, without deforming, the load represented by a plunger, a stiffening piece and possibly a needle.

Thanks to this we can manage to manufacture a head capable of operating significantly faster than the previous ones and without it heating up as much.

The problem is that, due to their load, the blades can tire relatively quickly and are even likely to break fairly easily, all the more so since the stiffening parts that they carry force them to fail. deform only over a small part of their length.

The main object of the present invention is to provide a print head which can also work at a very high frequency and which does not have this drawback.

It also aims to ensure that this head is compact and can be manufactured easily, at a relatively low cost price.

These objects are achieved thanks to the fact that, in a printhead according to the invention which also includes a set of needles each having a first intended for printing and a second end, guide devices in which the needles are said to have their first ends aligned in at least one row, elastic means for making the needles normally occupy a first position, and a set of electromagnets arranged around the second ends of the needles and each comprising a yoke, a coil and a plunger core coupled to the second end of one of these needles to bring it into a second position when said coil is traversed by a current, the cores of the electromagnets are respectively coupled to the second ends of the needles by levers of non-magnetic material to which they are fixed.

In addition, these levers can advantageously be made of a synthetic material which allows them to be light and, consequently, to be able to be moved very easily by the electromagnets.

• - la figure 1 est une vue en perspective de la tête d'impres­sion selon l'invention, sous cette forme d'exécution;
• - la figure 2 est une vue en coupe longitudinale de la tête représentée sur la figure 1, suivant la ligne brisée II-II de la figure 3; et
• - la figure 3 est une vue de dessus de cette même tête lors­que le couvercle supérieur dont elle est munie est enlevé.

Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment. This description will be made with reference to the attached drawing in which:

• - Figure 1 is a perspective view of the print head according to the invention, in this embodiment;
• - Figure 2 is a longitudinal sectional view of the head shown in Figure 1, along the broken line II-II of Figure 3; and
• - Figure 3 is a top view of the same head when the upper cover which it is provided is removed.

In this embodiment that we have chosen to describe, the print head according to the invention has nine needles, which makes it possible to achieve a very good compromise between its volume and the graphic quality of the characters it is able to print.

As it appears in FIG. 1, this head comprises a main part 2 which would have the shape of a rectangular parallelepiped with a square base if its four lateral faces 4 were not slightly curved in the direction of their length and which is closed at its upper part by a cover 6, fixed by means of screws 8 arranged at its four corners.

At the front of this main part 2 is placed a rigid printed circuit board 10 which extends beyond the lower side face 4 in order to be able to carry a connection connector 12. Normally, this board is pierced with a number of holes and has conductive tracks on its front face but, for clarity of the drawing, neither these holes nor these tracks have been shown.

Finally, at the front of the plate 10 is a nose 14 which includes a part 16 in the form of a pyramid trunk with rectangular section, a flange 20 located at the base thereof which allows this nose to be fixed to the part main 2 by means of two screws 22, the printed circuit board being taken between the two, and two wings 24 each pierced with an oblong hole 26, by which the head can be mounted on the printer for which it is intended, all these parts being made in one piece of plastic.

It can also be seen in FIG. 1 that the pyramidal part has at its end a groove 28 in which is housed the last guide 30 in which the needles pass and which is the one which must ensure perfect alignment of their ends.

By cons, what we do not see in this figure is that the face of the trunk of the pyramid 16 which is between the two wings 24 is fully open. We will understand why later.

If we now look at Figure 2, we see that the side faces 4 of the main part 2 which we have spoken are, essentially, formed by those, 32, of a part 34 which has a circular central opening 36 and to which we will return later.

It is also noted that after having passed through a hole of the same diameter 38 in the printed circuit board 10, the nose 14 penetrates inside this opening 36 of the part 34, approximately to half the height of that -this, and forms a transverse wall 40 there, the upper face of which has nine countersinks 42 the utility of which will appear later, which are arranged in a circle around the center of the wall and the bottoms of which are pierced with respective holes 44, much smaller diameter, through which the needles pass 46.

This wall 40 therefore constitutes a first guide for the needles and, for this reason, it is necessary for the plastic material which constitutes the nose 14 to resist wear and temperature very well. This material may for example be a polycarbonate loaded with carbon fibers.

In addition, between this first guide and the last 30, which has already been discussed and which is made up of a block of very hard and self-lubricating material such as synthetic ruby, pierced of closely spaced and perfectly aligned holes, there are two intermediate guides 48 and 50 mounted in slides formed by pairs of rectilinear and parallel bosses 52, respectively 54, that are present inside each of the walls of the nose 14 to which the fixing wings 24 (see figure 1).

The first of these intermediate guides, that is to say the one, 48, which is closest to the transverse wall 40 consists of a plastic plate which is at least as resistant to wear and temperature as this wall and which is pierced with holes of diameter substantially equal to that of the needles. These holes can be arranged in a smaller circle than that formed by the holes 44 in the first guide 40 or better in an ellipse whose major axis is parallel to the direction of alignment of those in the last guide 30.

The second intermediate guide 50 has only a slit of width equal to or barely greater than the diameter of the needles and parallel to the row of holes in the guide 30. It can be constituted for example by two bars of hard material, as the synthetic ruby, linked together by plastic plates fixed at their ends by glue which also penetrates between these two bars to define the length of the slot.

It is now easily understood that if one of the faces of the nose 14 is open, it is to allow the establishment and, if necessary, the replacement of these intermediate guides. Of course, we could also have been content to provide slots in a wall for this.

Returning to part 34, it can be seen, with reference also to FIG. 3, that in addition to opening 36 it has nine cylindrical cavities 56 in which electromagnets 58 are housed and, like this part 34 its role is not only to carry these electromagnets but also to dissipate the heat they give off as best as possible, it is made of a material with good thermal conductivity, preferably cast aluminum. For the same reason, its faces 32 have longitudinal grooves 60 that could already be seen in Figure 1 and which increase its contact surface with the ambient air. Of course, we could just as easily have predicted grooves in the height direction of the part or any other kind of relief on the surface of it which would have obtained the same result.

As regards the electromagnets 58, each of these comprises a yoke 62, the bottom of which has a cylindrical internal boss 64, a coil 66 arranged around the latter and the wire 68 of which passes through holes 70, 72 and 74 drilled respectively in the bottom of the cylinder head, in the part 34 and in the printed circuit board, to have its ends fixed on conductive tracks not shown of the latter, a plunger core 76 placed above the boss 64 and a part of which comes out of the cavity in which the electromagnet is located, and a shunt 78 in the form of a washer situated above the coil and surrounding this core, all of these elements, except the coil, preferably being made of mild steel.

In this embodiment, each needle 46, which is advantageously made of tungsten and which has a molded head of plastic material 80, is actuated by one end of a lever 82 of synthetic material, for example of P.O.M. (polyoxymethylene) loaded with glass fiber and Teflon (registered trademark), which has been molded onto a head 84 which has the plunger core 76 of the electromagnet associated with this needle and the other end of which has, on the side of the part 34, a hemispherical or semi-cylindrical boss 86 which bears on a metal flange 88, for example of brass, placed on this part and surrounding all the cores at a distance.

On the other hand, under each lever 82 is fixed an elastic blade 90, for example made of steel or a beryllium-copper alloy, which tends to lift it by pivoting it around an axis perpendicular to its longitudinal direction, which passes through the top of its boss 86 and under each needle head is provided a coil spring 92 which bears on the bottom of the countersink 42 of the wall 40 in which this needle passes and which permanently keeps this head pressed against the blade 90 of the lever which corresponds to it.

The blades 90 therefore have two roles here. The first is to ensure a correct position of the levers 82 relative to the needles and guide them so that they can only rotate around the axis that has been indicated. The second is to cooperate with the springs 92 to force the needles to normally occupy a withdrawal position when the coils of the electromagnets are not supplied with current or to bring them there when they move away from it.

As shown in Figure 3, all these blades 90 are in fact attached to the same frame 94 with which they form a single piece and which has holes 96 at the location of the bosses 86 of the levers.

Returning to FIG. 2, it can be seen that this frame 94 is taken between the cover 6 and a spacer 98, for example made of brass, which rests on the flange 88 and whose thickness is substantially equal to the height of the bosses 86.

Note that, for fixing the blades to the levers, the ultrasonic welding technique can advantageously be used.

Note also that, for clarity of the drawing, only FIG. 2 shows the two levers, the two blades and the two needles which are in the broken section planes. Normally, we should see four more.

Finally, with regard to the cover 6, it is noted that its internal wall, the shape of which is adapted to that of the upper face of the levers, has two concentric circular grooves 100 and 102 in which rings of the "O-" type are placed. ring "104, respectively 106, of a rubbery material. The first, 104, of these rings which is located above the bosses 86 of the levers has the role of keeping these pressed against the flange 88. The second, 106, larger, is placed above the heads of needles and serves as a stop for the ends of the levers and noise damper.

In addition, it is advantageous that this cover is made of the same material as the part 34 and that it also has grooves (not shown) because thanks to this it can participate effectively in the evacuation of the heat of the head.

The operation of this head, which has just been described, is obvious.

When the coil of an electromagnet is not supplied, the free end of the lever which it actuates is kept pressed against the ring 106 by the elastic blade which is fixed under it and, incidentally, by the coil spring. of the corresponding needle, so that the latter remains in the withdrawn position. This situation is shown on the left side of Figure 2.

On the other hand, as soon as a current flows through the coil, the plunger core of the electromagnet comes to press against the central boss of the cylinder head and the lever which then pivots slightly and quickly downwards pushes the needle into position as shown in the right part of the same figure.

Tests carried out have shown that with a head of this kind it is easy to reach a frequency of 2500 Hz without exceeding an operating temperature of about 65 ° and that one could seek to go up to 3000 Hz.

Furthermore, if we disregard the control block and the piece of printed circuit board on which it is placed, this head fits in a cube of only four centimeters on the side and its structure lends itself very well to automated assembly. , which reduces its cost price.

However, it is very clear that the invention is not limited to the embodiment which has been described.

For example, one could imagine attaching the ends of the needles to the blades or, better, to the levers, which would make it possible to remove the coil springs 92 and, possibly, the heads 80. In this case there would only be the blades to impose a withdrawal position on the needles.

Conversely, the blades could be replaced by other means for positioning and guiding the levers, located for example in the head cover, and leaving the coil springs to maintain and return the needles to this position. In this case it would obviously be necessary to use stronger springs.

We could also think of articulating the levers in another way and mounting them on the plunger cores in a different way from that which we have indicated.

As for the nose and the guides which are not really part of the invention, nothing obliges them to be produced and / or mounted as we have said and, as regards the guides, their number can vary.

Claims (8)

1. Printhead for dot matrix printer, comprising:
- a set of needles (46) each having a first end intended for printing and a second end;
- guide devices (40, 48, 50, 30) in which the needles pass to have their first ends aligned in at least one row;
- elastic means (90, 92) for making the needles normally occupy a first position, and
- a set of electromagnets (58) arranged around said second ends and each comprising a yoke (62), a coil (66) and a plunger core (76) coupled to the second end of one of the needles to bring the latter in a second position when said coil is traversed by a current, characterized in that the plunger cores of the electromagnets are coupled respectively to the second ends of the needles by levers (82) of non-magnetic material to which they are fixed. 2. Printhead according to claim 1, characterized in that the levers (82) are made of synthetic material and are overmolded on heads (84) that have said plunger cores (76). 3. Printhead according to claim 1 or 2, characterized in that said elastic means comprise metal blades (90) which are fixed on opposite faces of the electromagnets (58) that have the levers (82) . 4. Printhead according to claim 3, characterized in that the second ends of the needles (46) carry heads (80) and that said elastic means also comprise coil springs (92) placed between the first device guide (40) through which said needles and these heads pass to keep them permanently pressed against said blades (90). 5. Printhead according to claim 3 or 4, characterized in that said blades (90) are all attached to a frame (94) with which they form a single piece and which is caught between a piece ( 34) in which the electro- magnets (58) and a removable cover (6) which also includes said print head. 6. Printhead according to claim 5, characterized in that the faces of the levers (82) facing the electromagnets (58) have, on the side opposite the needles (46), almost hemispherical or semi-bosses - respective cylinders (86) which pass through said frame (94) and whose apices are held in abutment against a metal flange (88) which remotely surrounds said plunger cores (76) and which is also taken between said part (34) and said cover (6), so that each of said levers can pivot about an axis substantially perpendicular to its longitudinal direction and which passes through the top of its boss. 7. Printhead according to claim 6, characterized in that the tops of the bosses (86) are held in abutment against the metal flange (88) by an annular ring (104) of rubbery material, partially housed in a groove internal (100) that has the cover (6) and which is located substantially in line with said vertices. 8. A printhead according to claim 7, characterized in that the cover (6) has, approximately to the right of the second ends of the needles (46), another internal groove (102) in which is partially housed a another annular ring (106) of rubber material against which the levers (82) come to rest when said needles are in their first position.

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