IMPACT DOT PRINTER

Abstract: An impact dot printer with a print head having a ring-like arrangement of electromagnets (3, 3a, 10) in cooperation with a leaf spring (7) and a ring-shaped permanent magnet (4), a substantially ring-shaped magnetic member (13) is provided radially inside of the arrangement of electromag­ nets. If an electromagnet is energized, a leakage flux along a closed magnetic path including said magnetic member is established and substantially decreases a leakage flux through the cores of adjacent electromagnets, thus, avoi­ ding the problem of magnetic interference between adjacent electromagents. The magnetic member is preferably provided with a gap giving it a C-shape, in order to avoid eddy currents to flow in the circumferential direction of said magnetic member.

Description: The present invention relates to an impact dot printer and in particular to an impact dot print head utilizing a coil magnet to effect printing. US-A-4,669,898 discloses a print head for an impact printer adapted to produce characters on the surface of a sheet of paper positioned on a platen. The characters are composed of a matrix of dots produced by a plurality of print wires impacting on an inked ribbon. The print wires are guided to the ribbon by a nose portion of the print head. The print wires of the print head are manipulated to contact the inked ribbon and to produce dots on a paper under control of electromagnetic means housed in the body of the print head. The mechanism for actually manipulating print wires comprises a plurality of armatures connected to driven ends of the print wires and supported within the print head by spring members which bias the armatures toward a print, or impact, position in which the printing ends of the print wires contact the ribbon. The armatures are drawn to a non-print, or cocked, position against the bias of said spring members by magnetic forces produced by a permanent magnet. The effect of the permanent magnet on individual armatures is selectively canceled by means of a plurality of electromagnets to allow a spring member to propel its armature and the print wire attached thereto to the print, or impact, position. In this prior art print head the electromagnets are arranged side by side along a ring inside of the ring-shaped permanent magnet. According to this structure, when a plurality of electromagnets are excited at the same time the magnetic flux generated from each of the excited electromagnets flows into the cores of the neighboring electromagnets in a direction opposite to the magnetic flux generated by those neighboring electromagnets themselves. This phenomenon of "magnetic interference", thus, affects the release of the armatures thereby deteriorating the printing quality because of an insufficient printing force or impact. In order to solve this problem the electric energy supplied to the electromagnets may be increased to compensate for said magnetic interference. However, this method has disadvantages since the printer becomes more expensive due to a corresponding large-sized power source and since more heat is generated by the electromagnets imposing a limitation on the allowable printing time. An impact dot printer according to the prior art portion of claim 1 is disclosed in JP-A-60-168663. In this printer a first and a second bypass magnetic member are provided, the second bypass member in one embodiment in the form of a closed ring disposed outside of the circumference of the arrangement of electromagnets. The bypass magnetic members serve to establish a first and a second closed loop magnetic path preventing the magnetig flux from an energized electromagnet to enter into the core of adjacent electromagnets. The invention as claimed is intended to remedy these drawbacks. It solves the problem of how to design an impact dot printer having an impact dot print head, said impact dot print head having the magnetic interference decreased without needing an increase of power consumption. This problem is solved with an impact dot printer as claimed. According to the invention a magnetic member of a C shape or the shape of a ring opened by a gap, is provided inside of the ring like arrangement of the electromagnets. The magnetic member is fitted onto a cylindrical member for positioning. With this additional magnetic member, even if the coils of a plurality of electromagnets are excited at the same time, a portion of the generated magnetic flux flows through the magnetic member instead of the cores of the neighboring electromagnets, whereby the influence of the magnetic interference is substantially reduced. Due to the C or open ring shape of the magnetic member, eddy currents in the circumferential direction of the magnetic member are prevented to occur. Therefore, the leakage fluxes from the excited electromagnets can flow through the magnetic member without being influenced by eddy currents, making the avoidance of the magnetic interference by the magnetic member more effective. The magnetic member is positioned and fixed by the cylindrical member so that the distance between the electromagnets and the magnetic member can easily be made the same with respect to all of the electromagnets. Therefore, any minor remaining influence of the magnetic interference is made equal among the electromagnets whereby a uniform printing density is achieved. Ways of carrying out the invention are described in detail below with reference to diagrammatic drawings which illustrate only specific embodiments, and in which: Fig. 1 is a cross sectional view of an impact dot print head according to a first embodiment of the present invention, Fig. 2 is a top plan view showing the positional relation between a side yoke and an armature in accordance with the first embodiment of the present invention, Fig. 3 is a perspective view of the magnetic member in accordance with the first embodiment of the present invention and Fig. 4 is a perspective view showing the arrangement of coils of the electromagnets and the magnetic member in accordance with a second embodiment of the present invention. Fig. 5 is a top plan view of an impact dot printer in accordance with an alternative embodiment of the invention. Reference is now made to Fig.1 showing a cross-sectional view of an impact dot print head according to a first embodiment of the invention. Each of a plurality of print wires 1 (only one print wire being shown in Fig.1) is connected with its driven end to the distal end of a corresponding armature 2 (only one armature is shown in Fig.1), for instance by means of a silver solder. The print wires are made of a high-speed steel or a hard metal. In the vicinity of the print wire 1 circular holes 2a are provided in the armature 2 for reducing the weight of the armature. The print wires 1 are guided and held by a plurality of wire guides 11, 11' arranged in a nose portion of the print head. The distal or printing ends of the print wires 1 are held by a wire end guide 11' made of zirconia ceramics, alumina ceramics, titania ceramics or a similar material having a good abrasion resistance. The other end of each armature 2 opposite to the print wire 1 is fixed to a leaf spring 7 as by welding, and the leaf spring 7 is sandwiched between a spring pressing plate 8 and a side yoke 6. Fig.2 shows the configuration and the positional relation between the side yoke 6, the armature 2 and the leaf spring 7. As is clear from Fig.2, the armature 2 has a specific shape with a narrow portion at the side to which the print wire 1 is fixed and a wider portion at the side where it is fixed to the leaf spring 7. The side yoke 6 is substantially ring-shaped with radially extending tounges 6a spaced apart from each other by spaces 6b. The spaces 6b are shaped to be at least partly complementary with respect to the wider portion of an armature 1 to accomodate an armature with a gap of approximately from 0.05 to 0.3 mm formed between the circumferential face of at least the wider portion of the armature 2 on the one hand and the surrounding material of the side yoke 6 on the other hand. As shown in Fig.1, a core base 3 made of silicon steel or an iron cobalt alloy has a plurality of core portions 3a corresponding in number to the number of armatures and is arranged such that each core portion 3a is opposed to the lower side of a corresponding armature 2. A coil 10 is wound around each coil portion 3a to form the electromagnets. Radially inside of these electromagnets a substantially ring-shaped magnetic member 13 is provided. The magnetic member 13 is press-fitted onto a cylindrical member 12a provided on the inner side of a back lid 12. Thereby the magnetic member 13 is positioned and fixed. The magnetic member 13 may be made of pure iron, structural or tool steel. A perspective view of the magnetic member 13 is shown in Fig.3. According to Fig.3 the magnetic member of this embodiment is substantially in the shape of a ring having a cutout forming a gap of the width D which is approximately from 0.03 to 0.3 mm in the assembled condition of the print head. Due to this gap in view of which the magnetic member may also be called a C-shaped magnetic member electric current in the direction shown by an arrow E in Fig.3 is prevented from flowing. The width D of the gap is desired to be as small as possible. The inner diameter of the magnetic member 13 is a bit smaller than the outer diameter of the cylindrical member 12a which has a truncated cone shape. Due to its gap the magnetic member 13 is easily widened when it is press-fitted onto the cylindrical member 12a, so that it is an easy work to fix the magnetic member 13 to the cylindrical member 12a and the accuracy and the sizes of these parts are not so critical. Further, the cylindrical member 12a has a step portion 12b and the magnetic member 13 is pushed onto the cylindrical member 12a until it abuts against the step portion 12b. Thereby, the magnetic member 13 can be easily positioned in its axial direction when it is fixed on the cylindrical member 12a. Back lid 12 and core base 3 are positioned by a fitting jig (not shown) when back lid 12 is attached to core base 3. Accordingly, the distance between each of the plurality of electromagnets each having a coil 10 wound around a core portion 3a of the core base 3 and the magnetic member 13 can be made uniform. Turning now back to Fig.1 the effect of the magnetic member 13 in operation will be described. In a standby condition a magnetic flux generated from a permanent magnet 4 flows through yoke plate 5, side yoke 6, armature 2 and core base 3. Due to this flux the armature 2 is attracted to the end face of the corresponding core portion 3a of the core base 3 and retained there, bending and thus biasing the leaf spring 7. When the coil 10 is energized by a current flowing in a direction to cancel out the flux from the permanent magnet 4, the armature 2 is released from the attraction force keeping it at the end face of the core portion 3a and is rotated in the direction of an arrow B due to the energy stored in the bent leaf spring 7. Thereby the print wire 1 is propelled and strikes against a medium to be printed (not shown) to form a dot. After the striking and upon deenergizing of the coil 10 the armature 2 is again attracted to and held at the core portion 3a of the core base 3 by the attraction force of the permanent magnet 4 and a repulsion due to the striking. Then, one cycle of the printing process is completed. In the above-described printing process, when a current flows through the coil 10 in the direction to cancel out the magnetic flux of the permanent magnet 4, a portion of the flux generated from the coil 10 forms a loop A which passes through the magnetic member 13 located adjacent to the coil 10. Therefore, when the coils of a plurality of electromagnets are excited simultaneously corresponding loops A are formed and the portion of the flux of an electromagnet flowing into the cores of the neighboring electromagnets becomes extremely small in comparison with the case where the magnetic member 13 is not provided. The influence of the magnetic interference is thus reduced to achieve a good printing quality. Further, it is not necessary to increase the amount of electric energy to compensate for the magnetic interference and, therefore, the heat generation of the coils and the capacity of the power source can be small so that the printer becomes inexpensive. Further, when loops A are formed as mentioned above, an eddy current is likely to occur in the direction of the arrow E in Fig.3 in the magnetic member 13. However, due to the C-shape of the magnetic member 13 providing the gap of width D, an eddy current does not flow in the direction E. Accordingly, the effect of the magnetic member 13 to suppress the magnetic interference is further improved. The C-shaped magnetic member 13 has an effect on the magnetic interference which is by 30% higher then that of a comparable magnetic member having a closed ring shape. If the distance between the electromagnet consisting of coil 10 and core portion 3a on the one hand and the magnetic member 13 is not the same for all electromagnets a difference of the magnetic interference level among the electromagnets is caused. This would result in a non-uniform printing density. However, according to the explained embodiment, the distance between the electromagnets and the magnetic member 13 can be made constant to achieve a uniform printing quality because the magnetic member 13 is positioned and fixed in its radial and axial directions by the cylindrical member 12a of back lid 12. The degree of the effect on the magnetic interference can be changed by changing the thickness t of the magnetic member 13. Thus, by changing its thickness t the magnetic member 13 may be easily adjusted to obtain optimum conditions for each kind of print head. Fig. 4 is a perspective view of the magnetic member 13 and the arrangement of coils 10 of an impact dot print head according to a second embodiment of the invention. As shown in Fig.4, the outside circumference of the magnetic member 13 is formed to be substantially complementary to the opposing coils 10. Apart from this specific design of the magnetic member 13 the second embodiment is entirely the same as the first embodiment. In this second embodiment the same effects as with the first embodiment can thus be obtained and, in addition, the radiation property of the print head is improved since the heat generated from the coils can be easily released via the magnetic member 13. Fig. 5 schematically illustrates an impact dot printer in accordance with an alternative embodiment of the invention. Desired figures and characters are printed on printing paper P arranged between platen 27 and ink ribbon 25 by impact dot print head 20 mounted on carriage 26 movably supported in the printing direction.

Claims: An impact dot printer having an impact dot print head said impact dot print head comprising a ring-shaped permanent magnet (4), a plurality of electromagnets arranged in a ring form inside of said permanent magnet (4), each electromagnet having a coil (10) and a core (3a), armatures (2) connected to a leaf spring (7) for driving print wires (1), said armatures being disposed opposite to said electromagnets, respectively, and a member (13) of a magnetic substance arranged in the vicinity of said electromagnet (10, 3a), characterized in that said member (13) is in the form of a C having a cut out and is provided inside the circumference of said arrangement of electromagnets and in the vicinity of said armatures. The printer according to claim 1 wherein a cylindrical member (12a) for holding said member (13) of magnetic substance is provided.

Inventors: MIZUNO, SHIGEKI
Applicants: SEIKO EPSON CORPORATION
Application year: 1989
Main classification: B41J2
IPC codes: B41J2