Dot printer head with magnetic circuit through adjacent armatures

A dot printer head in which a radial array of armatures are disposed opposite to a yoke and cores equipped with coils, and the armatures are actuated together with needles by exciting the coils to perform a desired printing operation. In this arrangement, a magnetic flux generated from each core flows to the yoke via the associated armature and then returns to the former core while partially flowing through another magnetic path by way of the adjacent armature to the yoke and returning to the former core, whereby required magnetic paths are obtained without the necessity of increasing the areas of the mutually opposed surfaces of the armature and the yoke. Thus, the radial width of the yoke is reduced as well as the distance from the fulcrum of the armature to the core, so that the distance from the fulcrum of the armature to the fore end thereof can be sufficiently lengthened against the distance from the armature fulcrum to the core. Consequently, a great force of magnetic attraction is producible while the air gap between the armature and the core is maintained to be narrow, and furthermore the equivalent mass of the armature can be reduced to achieve high-speed printing with an economy of the power consumption.

FIELD OF THE INVENTION 
The present invention relates to a dot printer and, more particularly, to a 
dot printer head equipped with a radial array of armatures so actuatable 
as to selectively drive a multiplicity of needles. 
OBJECTS OF THE INVENTION 
It is a first object of the present invention to increase the force of 
magnetic attraction of each core to an armature. 
A second object of the invention resides in reducing the equivalent mass of 
each armature to perform a high-speed printing operation. 
And a third object of the invention is to decrease the amount of power 
consumption.

DESCRIPTION OF THE PRIOR ART 
In the known dot printer head of this type, an armature is actuated by 
exciting a coil and thereby causes a needle to collide with a platen to 
perform a desired printing operation. In general, a mechanism employed for 
driving the armature has a structure illustrated in FIGS. 1 and 2. 
Coils (3) are wound individually around a plurality of cores (2) formed 
integrally with a yoke (1), and each of armatures (5) actuatable through 
excitation of the associated coil (3) for causing a needle (4) to collide 
with a platen is supported at a fulcrum (6) in such a manner as to be 
swingable upward and downward. And recesses (8) to be held by a guide 
member (7) are formed on the two sides of each armature (5). The guide 
member (7) is located within the surface opposed to the yoke (1). During 
the printing performed by exciting the coil (3) and attracting the 
armature (5) to the associated core (2), the magnetic flux generated from 
the core (2) flows to the yoke (1) via the armature (5) and then returns 
to the former core (2). Since it is necessary to maximize the force of 
magnetic attraction in the core (2) to carry out the intended printing, 
the surface of the armature (5) opposed to the yoke (1) needs to have a 
sufficiently great area to meet the requirement. However, in the structure 
of FIGS. 1 and 2 where each recess (8) formed on the surface opposed to 
the yoke (1) must be located in the armature (5) due to the positional 
relation to the guide member (7), it becomes a requisite to increase the 
radial width l3 of the yoke (1) for attaining a greater surface area of 
the armature (5) opposed to the yoke (1). With regard to the distance l1 
from the fulcrum (6) to the center of the core (2) and the distance l2 
from the fulcrum (6) to the striking point of a needle provided at the end 
of the armature (5), an increase of l3 brings about an increase of l1 to 
eventually widen an air gap G, whereby a sufficient force of magnetic 
attraction is rendered unattainable in the core (2). Furthermore, the 
lever ratio l2/l1 is lowered with an increase of the distance l1 to 
consequently augment the equivelent mass of the armature (5), so that 
high-speed printing is rendered impossible with another disadvantage of 
consuming a larger amount of electric power. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter a first exemplary embodiment of the present invention will be 
described with reference to FIGS. 3 through 9, in which needle guide 
members (12, 13, 14) for slidably holding a plurality of needles (11) are 
secured to a guide frame (10), and an annular yoke (15) is attached to the 
guide frame (10) with screws. A plurality of cores (17) equipped with 
coils (16) are disposed in a radial array integrally with the yoke (15). 
Each of armatures (18) located opposite to the core (17) and the yoke (15) 
has recesses (19) on the two sides thereof, and guide portions (20) formed 
integrally with the guide frame (10) are fitted into the recesses (19) so 
that each armature (18) is swingable upward and downward on a fulcrum (21) 
while being energized elastically by means of a spring (22) in the 
direction to return to the former position. The guide frame (10) further 
has guide portions (23) for preventing lateral deflection of the fore ends 
of the armature (18). The annular yoke (15) includes a disk-shaped region 
(24) along its inner circumference to hold an armature stopper (25) 
thereon. 
Each armature (18) has, on its two sides, surfaces (26) defined between the 
core (17) and the fulcrum (21) and opposed in parallel to the adjacent 
armatures (18) in a length l5 with a small space l4 maintained. 
When any coil (16) is energized in the structure mentioned above, the 
associated armature (18) is magnetically attracted to the core (17) and 
thereby causes the needle (11) to collide with a platen. Supposing now 
that one selected coil (16) is energized in FIGS. 6 and 7, the magnetic 
flux generated from the core (17a) corresponding to the energized coil 
(16) comes to flow partially via the armature (18a) to a portion of the 
yoke (15) opposed to the armature (18a) and reaches the core (17a), while 
the remaining flux component flows further from the armature (18a) via the 
adjacent armatures (18b, 18c) to the yoke (15) and then returns to the 
former core (17a). 
In the case of exciting all of the coils (16), the same effect is 
achievable by circumferentially alternating the directions of the 
respective magnetic fluxes induced by the coils (16), as illustrated in 
FIG. 8. This may be accomplished, for example, by alternating the 
direction of the core windings from one core to the next. In this case, 
the magnetic flux generated from the core (17a ) partially flows to the 
yoke (15) via the armature (18a) and then returns to the core (17a) while 
the remaining flux component having reached the armature (18a) further 
flows therefrom to the adjacent armature (18b) or (18c) and arrives at the 
adjacent core (17b) or (17c). 
Due to the above arrangement where the magnetic flux is allowed to 
partially flow to the yoke (15) via the adjacent armature (18) and returns 
to the former core (17), a satisfactory magnetic path is obtainable even 
though the surface of the armature (18) opposed to the yoke (15) has a 
small area. As a result, the radial width of the yoke (15) is reducible to 
lessen the distance l3, whereby the distance l1 from the fulcrum (21) to 
the center of the core (17) can be rendered smaller to eventually narrow 
the air gap G between the core (17) and the armature (18). Therefore, it 
becomes possible to produce a sufficiently great force of magnetic 
attraction. Since the distance l1 is thus decreasable in relation to the 
distance l2 from the fulcrum (21) to the needle (11), the ratio l2/l1 can 
be increased to bring about a reduction in the equivalent mass of the 
armature (18), so that the structure is rendered optimal for high-speed 
printing with an economic advantage regarding the power consumption. 
In a second exemplary embodiment of this invention illustrated in FIGS. 10 
and 11, the same reference numerals as those used in the foregoing 
embodiment denote the identical components, and a repeated explanation is 
omitted. Differing from the foregoing embodiment where the cores (17) are 
disposed along the outer circumference of the annular yoke (15), the 
second embodiment is so arranged that the cores (17) are disposed along 
the inner circumference of the yoke (15). Accordingly, the fulcrum (28) of 
each armature (27) is located outside of the associated core (17). Each 
armature (27) has surfaces (26) opposed to the adjacent armatures (27), 
and guide portions (29, 30) for guiding the two sides of the armature (27) 
are formed in the guide frame (10). 
Consequently, the magnetic flux generated from each core (17) is allowed to 
partially flow via the adjacent armatures (27) to the yoke (15). 
Therefore, it becomes possible to diminish the surface area of the 
armature (27) opposed to the yoke (15), hence reducing the radial width of 
the yoke (15) to shorten the distance l3 for decreasing the distance l1 
from the fulcrum (28) to the center of the core (17), whereby the air gap 
G between the core (17) and the armature (27) can be narrowed to 
eventually lessen the ratio l2/l1.