A printing head for an impact dot printer

Abstract: A printing head for an impact dot printer has a print wire (3) which is mounted on a lever (14) and which is selectively driven by means of an electro-­ magnetic arrangement. The electro-magnetic arrangement comprises a respective magnetic circuit (1, 10, 16; 2, 11, 17) arranged to apply a force on each side of the axis of rotation of the lever. The distance between the axis of the lever and the print wire is greater than the distance between the axis and the point of application of the force applied by the magnetic circuit on the opposite side of the axis.

Description: The present invention relates to a printing head for an impact dot printer. Prior art impact dot printers generally have an electro-magnetic circuit for controlling a lever for driving a print wire, arranged on one side of the axis of the lever. In order to achieve a high speed drive for the print wire in such a prior art printer, it is necessary to enlarge a plunger, which is located on the lever so as to be attracted by the magnetic force, so that the turning effort is made larger. However, this increases the moment of inertia, which in turn inhibits high speed driving of the print wire. A number of alternative proposals have been made to achieve high speed printing. For example in French patent publication 2547538 an arrangement is disclosed by which the lever is attracted to a permanent magnet in the rest position and an electromagnet is used to cancel the effect of the permanent magnet to drive the print wire. This is not efficient in that energy is wasted in overcoming the permanent magnet and the attraction force depends on the absolute value of the flux. Another example can be found in Japanese patent publication number 237656 which again is inefficient as current is supplied to both drive the print wire and to retract it. In accordance with the present invention, there is a printing head for an impact dot printer having one or more levers, the head comprising: each lever pivotally mounted on an axis; a print wire mounted at one end of a respective lever on one side of the axis; dual electromagnetic means provided respectively on opposite sides of a plane passing through said axis and perpendicular to the longitudinal extent of a respective lever for enabling at least one of said electromagnetic means to rotate and thereby drive the respective lever; a resilient biasing means arranged to rotate said respective lever in opposite relation to at least one of said dual electromagnetic means, at least one of said dual electromagnetic means and resilient biasing means co-operate to drive the lever to print and to return the wire to a rest position characterised in that each electromagnetic means belongs to different magnetic circuits. The present invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view through a first embodiment of impact dot printing head in accordance with this invention; Figure 2 is a sectional view through a second embodiment of impact dot printing head in accordance with this invention; Figure 3 is a sectional view through a third embodiment of impact dot printing head in accordance with this invention; Figure 4 is a sectional view through a fourth embodiment of impact dot printing head in accordance with this invention; Figure 5 shows graphs representing in Figures 5 (a) and 5 (b) the timing of electro-magnetic coils of the impact dot printing head, and in Figure 5 (c) the displacement of a print wire of the impact dot printing head in accordance with this invention; Figure 6 shows graphs representing in Figures 6 (a) and 6 (b) an alternative timing for the electro-magnetic coils, and in Figure 6 (c) the displacement of the print wire for certain embodiments of the impact dot printing head in accordance with this invention; and Figure 7 is a schematic plan view of an impact dot printer incorporating a printing head in accordance with this invention. Figure 7 is a schematic plan view of an impact dot printer incorporating the invention. An impact dot printing head 4 is slidably supported by a carriage 6 for movement axially of a platen 7 for printing drawings, characters or the like as desired on a sheet of printing paper P disposed between the platen 7 and an ink ribbon 5. Various embodiments of the printing head 4 will now be described with reference to Figures 1 to 4. Like parts are designated in these Figures by the same reference numerals. Figure 1 is a sectional view through a first embodiment of the impact dot printing head according to this invention. The lower half of the impact dot printing head as illustrated is in an initial or stand-by position, wherein no current is applied to electric coils 1 and 2, while the upper half of the impact dot printing head as illustrated is in a printing position, wherein a current is applied to the electric coils 1 and 2. A lever 14 is mounted for rotation about an axis 15 by means of a bearing (not shown) and has plungers 16 and 17 affixed thereto on both sides of the axis 15 so as to co-operate with cores 10 and 11, respectively, about which the coils 1 and 2 are wound. Further, a print wire 3 is fixed to one end of the lever 14, with the distance between the axis 15 and the print wire 3 being greater than the distance between the axis 15 and the plunger 17 on the opposite side of the axis 15 from the print wire 3. Thus, the supply of current to the coils 1 and 2 rotates the lever 14 and causes increased displacement of the print wire 3 for driving the print wire. In the stand-by condition, the lever 14 is held against an abutment 19 by means of a spring 8. The supply of current to the electric coils 1 and 2, as represented in Figure 5, generates a magnetic attraction between the plunger 16 and the core 10, and between the plunger 17 and the core 11, so that the lever 14 is rotated about the axis 15 causing the print wire 3 to strike the paper, and thereby effect printing. Since the plungers 16 and 17 are affixed to the lever 14 on both sides of the axis 15 according to the invention, the moment on the lever 14 is greater than in the case of the prior art in which a plunger is provided on only one side of the lever's axis of rotation. Therefore, increased acceleration of the print wire 3 is achieved and high speed printing is promoted. Figure 2 is a sectional view through a second embodiment of the impact dot printing head according to this invention. Again, the lower half of the impact dot printing head as illustrated is in the initial or stand-by position, in which no current is applied to the electric coils 1 and 2, while the upper half is in the printing position, in which a current is applied to the electric coils 1 and 2. A lever 24 is provided to drive the print wire 3. The lever 24 is mounted for rotation about an axis 25 by means of a bearing (not shown). Plungers 26 and 27 are affixed to the lever 24 on both sides of the axis 25, and co-operate with cores 20 and 21, on which the coils 2 and 1 are wound. In the stand-by position, the plunger 26 is attracted towards the core 20 by magnetic flux generated by a permanent magnet 23, and the plunger 27 is attracted towards the core 21 by magnetic flux generated by a permanent magnet 22, thereby compressing the spring 8 as shown. By applying current to the electric coils 1 and 2, as represented in Figure 5, the magnetic flux of the permanent magnets 22 and 23, and hence the magnetic attraction between the plunger 26 and the core 20 and between the plunger 27 and the core 21, is cancelled so that the lever 24 is rotated about the axis 25 by the spring 8, causing the print wire 3 to strike the paper, thereby effecting printing. In this embodiment again, since the plungers 26 and 27 are affixed to the lever 24 on both sides of the axis 25, the attractive force generated by magnetic flux of the permanent magnets 22, 23 between the plunger 26 and the core 20, and between the plunger 27 and the core 21 is greater than in the prior art in which a plunger is provided on only one side of the axis of rotation of a lever. The spring force of the spring 8 can also advantageously be enlarged according to a desired increase in attractive force. And consequently, greater moment on the lever 24 results. Therefore, acceleration of the print wire 3 may be increased, which provides for high speed printing. A third embodiment of this invention is shown in Figure 3, which is a sectional view through an impact dot printing head showing the lower half of the impact dot printing head in the initial or stand-by position, wherein no current is applied to the electric coils 1 and 2, and the upper half in the printing position, wherein current is applied to the electric coils 1 and 2. A lever 34 is provided to drive the print wire 3. The lever 34 is mounted for rotation on an axis 35 by means of a bearing (not shown) and the plungers 36 and 37 are affixed to the lever 34 on both sides of the axis 35. A first magnetic circuit, composed of the plunger 36 and a core 30 carrying the coil 1 and without a permanent magnet, is located in the interior of the printing head while a second magnetic circuit, composed of the plunger 37 and a core 31 carrying the coil 2, a permanent magnet 32 and a yoke 31a is located at the periphery of the printing head. As shown in the lower half of Figure 3, the plunger 37 is attracted towards the core 31 in the stand-by position by the magnetic flux generated by the permanent magnet 32, and the lever 34 compresses the spring 8 in this position. When current is applied to the electric coils 1 and 2, as represented in Figure 5, the magnetic flux of the permanent magnet 32 is overcome and, at the same time, magnetic flux is generated between the core 30 and the plunger 36 so that the lever 34 is rotated about the axis 35, by means of the attractive force between the core 30 and the plunger 36 and the spring force of the spring 8, causing the print wire 3 to strike the paper and thereby print. In this embodiment, since the magnetic circuit including the permanent magnetic 32 is located at the periphery of the printing head, the cross sectional area of the permanent magnet can be enlarged, so that the plunger 37 can be attracted towards the core 31 with greater force. Therefore, the spring constant of the spring 8 can be made greater. The lever 34 can be rotated at a high speed by the combined force of the plunger 36 and the spring 8 and so the print wire 3 can be driven at a high speed. Figure 4 is a sectional view through the impact dot printing head in accordance with a fourth embodiment of this invention. The lower half of the impact dot printing head as shown is in the initial or stand-by position, wherein no current is applied to the electric coils 1 and 2, while the upper half of the impact dot printing head is in the printing position, wherein current is applied to the electric coils 1 and 2. A lever 44 is provided to drive the print wire 3. The lever 44 is mounted for rotation on an axis 45 by means of a bearing (not shown). Plungers 46 and 47 are affixed to the lever 44 on both sides of the axis 45. A first magnetic circuit, composed of the plunger 46 and a core 40 carrying the coil 1 and without a permanent magnet, is located at the periphery of the printing head, while a second magnetic circuit, composed of the plunger 47, a core 41 carrying the coil 2, a permanent magnet 42 and a yoke 48, is located in the interior of the printing head. As shown in the lower half of Figure 4, the plunger 47 is attracted towards the core 41 by the magnetic flux generated by the permanent magnet 42 in the stand-by position, and the lever 44 is stopped in a position wherein the spring 8 is compressed. When a current is applied to the coils 1 and 2, as represented in Figure 5, the magnetic flux of the permanent magnet 42 is overcome and, at the same time, magnetic flux is generated between the core 40 and the plunger 46 so that the lever 44 is rotated about the axis 45 by means of the attractive force between the core 40 and the plunger 46 and the spring force of the spring 8, causing the print wire 3 to strike the paper and thereby print. In this embodiment, since a magnetic circuit having a permanent magnet is located at the interior of the printing head, the space within the printing head is effectively utilised and a smaller printing head can be produced compared with the printing head of the third embodiment. In the embodiments described above, the current is applied to the electric coils 1 and 2 simultaneously, as shown in Figure 5. However, it is also possible in the case of the third and fourth embodiments particularly for current to be supplied first to the electric coil 2, which is in the magnetic circuit including the permanent magnet, and then to the electric coil 1, which is in the magnetic circuit not including the permanent magnet, as shown in Figure 6. The plungers 37 and 47 are, respectively, part of the magnetic circuit which includes the permanent magnet and which is composed respectively of the plunger 37 or 47, the core 31 or 41, the permanent magnetic 32 or 42, and the yokes 31a or 48. In the stand-by position, the plungers 37 and 47 are attracted towards the cores 31 and 41. The air gap of the magnetic circuit is thus minimised while the inductance of the coil 2 is maximised. In printing, the plungers 37 and 47 are separated from the cores 31 and 41, so that the air gap of the magnetic circuit increases continuously and the inductance of the coil 2 decreases, until the print wire 3 strikes the paper. Therefore, the energy conversion efficiency of the coil 2 is a maximum when the print wire 3 starts to move and it decreases constantly until the print wire 3 strikes the paper. On the other hand, the plungers 36 and 46 are, respectively, a part of the magnetic circuit which does not include the permanent magnet and which is composed of the plungers 36 and 46 and the cores 30 and 40. In the stand-by position, the plungers 36 and 46 are at their most distant from the cores 30 and 40 and the air gap of the magnetic circuit is maximised while the inductance of the electric coil 1 is minimised. In printing, the plungers 36 and 46 approach the cores 30 and 40, so that the air gap of the magnetic circuit decreases continuously and the inductance of the electric coil 1 increases until the print wire 3 strikes the paper. Therefore, the energy conversion efficiency of the coil 1 is a minimum when the print wire 3 starts to move, and thereafter it increases constantly until the print wire 3 strikes the paper. As shown in Figure 6, the current may therefore be supplied to the electric coils at different timings, first to the electric coil 2, and then to the electric coil 1. Accordingly, it becomes possible to use only high efficiency parts for the electric coils 1 and 2 and to improve the energy conversion efficiency. As explained above, in a printer employing an impact dot printing head according to this invention, the moment on the lever for driving the print wire can be increased without any notable increase of its moment of inertia by employing a respective magnetic circuit on both sides of the axis of the lever for driving the print wire. Therefore, the acceleration of the print wire may be increased and high speed printing can be obtained. Moreover, the energy conversion efficiency can be improved by the following means: (a) providing soft magnetic members on each side of the axis of the lever for driving the print wire; (b) including one of the soft magnetic members as a part of the magnetic circuit which includes the permanent magnet; (c) including the other soft magnetic member as a part of the magnetic circuit which does not include the permanent magnet; and (d) applying current to the electric coils at different timings, first to the electric coil in the magnetic circuit which includes the permanent magnet, and then to the electric coil in the magnetic circuit which does not include the permanent magnet.

Claims: A printing head for an impact dot printer having one or more levers, the head comprising: each lever (14; 24; 34; 44) pivotally mounted on an axis (15; 25; 35; 45); a print wire (3) mounted at one end of a respective lever on one side of the axis; dual electromagnetic means (1, 2; 10, 11, 16, 17; 20-23, 26,27; 30-32, 36, 37; 40-42, 46-48) provided respectively on opposite sides of a plane passing through said axis and perpendicular to the longitudinal extent of a respective lever for enabling at least one of said electromagnetic means to rotate and thereby drive the respective lever; a resilient biasing means (8) arranged to rotate said respective lever in opposite relation to at least one of said dual electromagnetic means; at least one of said dual electromagnetic means and resilient biasing means co-operate to drive the lever to print and to return the wire to a rest position, characterised in that each electromagnetic means belongs to different magnetic circuits. A printing head according to claim 1, in which each of the magnetic circuits includes a coil (1,2), and in that means are provided for simultaneously supplying the coils with electric current. A printing head according to claim 1 in which each of the magnetic circuits includes a coil (1,2), and in that means are provided for successively supplying the coils with current in asynchronous phase relation to place a print wire in its printing position. A printing head according to claim 2 or 3 in which one of the magnetic circuits includes a permanent magnet. A printing head according to claim 4 when dependent from claim 3 in which the means for supplying electric current is arranged to supply the current first to the coil in the magnet circuit including the permanent magnet. A printing head according to claim 2 or 3 in which each of the magnetic circuits includes a respective soft magnetic member mounted on the lever on a respective side of the rotational axis. A printing head according to any preceding claim in which each of the magnetic circuits includes a respective plunger mounted on the lever on a respective side of the axis, and in that the print wire is attached to one end of the lever. An impact dot printer including a printing head according to any preceding claim.

Inventors: MITSUISHI AKIO, TAKEUCHI TAKASHI, MITSUISHI, AKIO, TAKEUCHI, TAKASHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1989
Main classification: B41J2
IPC codes: B41J2