DOT matrix printer having an impact DOT printing head

Abstract: A dot matrix printer having an impact dot printing head is provided with a plurality of printing levers radially disposed with respect to guide holes arrayed at the end of a nose portion of the printing head. Printing wires (12) equal in number to the printing levers (10, 10F, 10R) are connected to the radially inner ends of the printing levers and guided by said guide holes (8, 8A). The printing levers are arranged such that their radially inner ends define a zigzag line. The printing head further includes plate springs for imparting biasing forces to the printing levers (10, 10F, 10R) and a spacer for pressing the plate springs to equalize the effective lengths thereof. The lever axial lines (10a) deviate from projected centers of the guide holes (8, 8A). Gaps (δ) are formed between adjacent levers to avoid a mutual interference between the printing wires.

Description: The present invention relates to a dot matrix printer. Both, enhancement in printing quality and speed-up in printing process are highly required for a dot matrix printer, conceived as a computer terminal equipment, for forming patterns of characters or the like on a recording medium by use of printing wires. The printing head, disclosed in US-A-4,669,898 and JP-A-29681/1983, has been developed to meet this requirement. In this known printing head, the printing ends of the printing wires are disposed in two parallel rows of guide holes provided at the front end of a nose portion of the printing head. A plurality of electromagnetically actuated armatures or printing levers have their movable ends connected to the driven ends of the printing wires. As illustrated in Fig. 5, all of the printing levers a are radially disposed with respect to said guide holes and have their movable ends positioned on an elliptical line, so as to minimize the curvatures of all of the printing wires. Thus, with this configuration, a projected distance l from the joining point c between the driven end of a printing wire b and the corresponding inner end of a printing lever a on the one hand to the printing end of the printing wire b on the other hand is shortened. The minimized curvature or deflection of the printing wires results in a reduced friction between the printing wires and the guide holes. Besides, the entire length of the printing wires b is also shortened, thus decreasing their inertia. Therefore, the known configuration serves to increase the velocity of the printing process. However, when the inner ends of the printing levers a are concentrated on an elliptical line at the head center, adjacent printing wires b come excessively close to each other in a region encircled by a circle in Fig. 5, namely a region where the curvature of the ellipse is large. As a result, it is impossible to provide a higher number of printing wires as would be required to enhance the printing quality. In addition, the problem may occur that contiguous printing wires come into contact with each other. To avoid this, a high accuracy of manufacturing and assembling the individual parts of the printing head is necessary and leads to increased manufacturing costs. It is a primary object of the present invention to remedy the foregoing problems of the prior art and to provide a printing head for a dot matrix printer which allows the printing quality to be enhanced and the printing speed to be increased to the greatest possible degree without necessitating a remarkable enhancing of the accuracy of its parts and of the assembly. This object is solved with a printer as claimed. Ways of carrying out the invention are described in detail below with reference to the drawings which illustrate only specific embodiments, and in which: Fig. 1 is a diagram explaining the positional relation between the printing levers and the printing wires according to a first embodiment of the present invention, Fig. 2 shows the arrangement of the printing levers, Fig. 3 is a sectional view showing one example of a printing head to which the present invention is applied, Fig. 4 is a diagram explaining the positional relation between the printing levers and the printing wires, Fig. 5 is a diagram explaining the positional relation between the printing levers and the printing wires according to the prior art, and Fig. 6 is a schematic plan view of a wire dot printer showing an embodiment in accordance with the present invention. Turning first to Fig. 3, there is shown one example of the printing head to which the present invention is applied. In Fig. 3, a plurality of cores, generally designated at 1, are protruded over the surface of an annular magnetic plate 2. Disposed on the respective end surfaces of cores 1 vis-á-vis with each other are armatures 11 fixed to printing levers 10. While no printing is performed, magnetic fluxes from permanent magnets 3 secured to the surface of magnetic plate 2 act to hold armatures 11 by attraction against the resilient forces of plate springs 5. During printing, the attraction of armatures 11 is released when electromagnetic coils 4 wound on cores 1 are selectively energized by electric current to negate the permanent magnetic fluxes. Due to the resilient force of the plate springs 5, printing wires 12 connected to the radially inner ends of the printing levers 10 are then pushed through guide holes at the end of a nose portion 6, thus effecting the printing process. Fig. 4 is a plan view seen from the left side of Fig. 3 and illustrating on an exaggerated scale the positional relation between the printing levers 10 and the printing wires 12. The printhead of Fig. 4 forms the subject matter of a divisional application. Referring to Fig. 4, the reference numeral 7 denotes a guide plate provided with two parallel rows of guide holes 8. The guide plate 7 is fixed to the end of nose portion 6 as shown in Fig. 3. The guide holes 8 serve to guide the printing ends d of the printing wires 12 whose driven ends are fixed to the printing levers 10 as mentioned before. The printing levers 10 are positioned around these guide holes and, except for printing levers 10A, extend radially with respect to the corresponding guide hole 8 so that their axes pass through the projected centers of corresponding guide holes 8. The inner ends of the printing levers 10 are disposed outwards to have a projected distance l from the guide holes 8. The printing levers are arranged at spacings δ of, e.g. 0.2 mm, so that the printing wires 12 do not come into contact with each other. On the other hand, printing levers 10A corresponding to guide holes 8A provided at both ends of the parallel rows of guide holes are arranged in such a manner that their axial lines 10a are displaced inwards from the projected centers of the corresponding guide holes 8A. Furthermore, printing levers 10A are placed to keep at least spacings δ between contiguous printing levers 10B. The two levers 10A on each side of the parallel rows are spaced apart by a distance defined by the two rows of guide holes 8. As in the case of the other printing levers 10, their inner ends are positioned at given distances l from the corresponding guide holes 8A. As a result of this arrangement, angular deviations are caused between the axial lines 10a of the printing levers 10A and the projected axial lines 12a of the associated printing wires 12. In other words, an angle is formed between the axial line 10a of each printing lever 10A and the plane of the corresponding printing wire 12. It is, however, possible to reduce the curvature of the printing wires 12 connected to the printing levers 10A to the same value as that of the printing wires 12 connected to printing levers 10. The curvature or deflection corresponds to the projected distance l between the driven end corresponding to the joining point c with a printing lever 10A and the printing end d in Fig. 3. Hence, the printing can be surely performed under the same condition for all printing wires. In accordance with the described printing head, only the printing levers 10A are made to deviate from the projected centers of corresponding guide holes 8A to provide the necessary clearance δ between contiguous printing levers 10B and levers 10A themselves. However, when a considerable processing or assembling accuracy is required in order to secure necessary clearances δ between the printing levers 10 because of a high density with which the printing levers are disposed, necessary gaps may be provided between adjacent printing levers 10 by having them deviating from the alignment shown in Fig. 4 in a manner, corresponding to printing levers 10A. Referring to Figs. 1 through 3, an embodiment of the present invention will now be described. A group of printing levers 10 are disposed at spacings δ of, e.g. 0.2 mm, corresponding to the cores 1 provided on the annular magnetic plate 2, so that printing wires fixed to the ends of respective printing levers do not come into contact with each other. Different from the printing head shown in Fig. 4, in this case the inner ends of the printing levers 10 are arranged in what may be called a zigzag fashion, as to be seen from Fig. 2. With this arrangement, the inner ends of the printing levers 10F are disposed as close as possible to positions just above the respective guide holes 8. The inner ends of printing levers 10R are retracted with respect to those of printing levers 10F and disposed to approach positions just above the respective guide holes 8 to the greatest possible degree. As shown in Fig. 2, printing levers 10F and 10R are alternately arranged. Note that in the Figures the numeral 9 denotes a spacer for supporting plate springs 5. The inner peripheral edge of spacer 9 (Fig. 1) is formed with rugged plate spring support parts 9a corresponding to the position of the individual inner ends of printing levers 10 in order to equalize the effective lengths L of levers 10 through plate springs 5, viz., the distances from the inner ends of levers 10 to the inner peripheral edge of spacer 9. Fig. 1 schematically illustrates this situation with the armatures 11 and the plate springs 5 being omitted. The fundamental constructions, including dimensions, etc., of armatures 11 and printing levers 10 are respectively the same and, hence, the effective lengths A (see Fig. 3) of the plate springs 5 become all equal. Since the inner ends of the printing levers 10F are near to positions just above the corresponding guide holes 8, the deflections of the corresponding printing wires 12 are nearly eliminated. The position of the inner ends of the printing levers 10R are spaced apart by e.g. 0.5 mm from positions just above the corresponding guide holes 8, thereby still minimizing the deflections of the printing wires 12 connected to printing levers 10R. Due to this arrangement of the printing levers 10, the axial lines 10a of some printing levers 10 may not pass through the projected centers of the corresponding guide holes 8 and are not properly aligned with but intersect the plane of the printing wires 12. However, these deviations are remarkably small as compared with the effective length L of the printing levers 10, so that there is almost no influence from those deviations. Due to the zigzag arrangement of the plurality of printing levers, explained above, the outwardly fan-shaped spaces between the radially disposed printing levers can be utilized as spatial portions in which the respective printing wires are placed. It is also feasible to concentrate the printing wires with high density at the center of the printing head without remarkably enhancing both, the processing accuracy of parts and the assembling accuracy thereof. Moreover, the inner ends of the printing levers which protrude forward toward the center can be disposed as close as possible to positions just above the corresponding guide holes. Similarly, the inner ends of the retracted printing levers are allowed to be placed still near to positions just above the corresponding guide holes. By virtue of these characteristics, the deflections of the printing wires can be reduced to the greatest possible degree, thereby decreasing the friction between the printing wires and the wire guide members. Consequently, the durability of the printing head can be ameliorated. Simultaneously, the entire lengths of the printing wires are reduced and the inertia thereof is also decreased. It is thus feasible to attain a high-speed printing by smaller driving forces. The printing levers used to drive those printing wires guided by the guide holes 8A at the end of the rows of guide holes are displaced from the projected centers of the guide holes, thus providing the necessary gaps between those printing levers and the adjacent levers. This arrangement permits to dispose the inner ends of these printing levers closest to positions just above the respective guide holes, still keeping the necessary clearance between the printing levers densely provided in the vicinity of the ends of the guide hole rows without considerably enhancing the accuracy with which the parts are processed and assembled. Also in this case, the deflections of the printing wires are minimized resulting in advantages already mentioned above. Fig. 6 is a schematic plan view of a wire dot printer showing an embodiment in accordance with the present invention. Desired figures and characters are printed on printing paper P arranged between platen 27 and ink ribbon 25 by impact dot head 20 mounted on carriage 26 which is supported movably in the printing column direction.

Claims: A dot matrix printer using a printing head comprising a plurality of printing wires (12), each having a printing end (d) and a driven end, said wires having their printing ends disposed in guide holes (8, 8A) of a guide plate (7), a plurality of printing levers (10) equal in number to the number of wires (12), each of said printing levers (10) having an inner end portion connected to the driven end of one of said wires, said printing levers (10) being substantially radially disposed with respect to said guide holes (8, 8A) in a plane substantially parallel to said guide plate (7), and driving means (3, 4, 5, 11) for driving said printing levers (10) to selectively move said wires between a printing position and a rest position, characterized in that in the plane defined by the inner end portions of said printing levers (10), one (10F) of at least two contiguous printing levers (10F, 10R) has its inner end portion positioned nearer to the projection of the corresponding guide hole (8) than has the other with respect to the projection of its corresponding guide hole. The printer according to claim 1, further comprising plate springs (5) fixed to said printing levers (10) for imparting biasing forces to said printing levers and a spacer (9) for pressing said plate springs (5) to equalize the effective lengths of said plate springs. The printer according to claim 1 or 2, characterized in that the axial lines (10a) of those of the printing levers (10A) connected to printing wires (12) guided by guide holes (8A) positioned in the vicinity of the end portion of the rows of guide holes, deviate from projected centers of said guide holes (8A), these printing levers being disposed to form gaps (δ) between adjacent ones of the other printing levers (10) and between themselves enough to avoid a mutual interference of said printing wires.

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