Dot print head with restrained rebound of armature

A dot print head includes a spring plate having a plurality of integral tongues arranged successively along one longitudinal side edge thereof, a plurality of armatures each fixedly attached at a part at least excluding the rear end thereof to the upper surface of a respective tongue of the spring plate with the rear end of each armature located on the side of the fixed longitudinal side portion of the spring plate with respect to the fixed point of primary vibration thereof caused by the tongue of the spring plate, and a plurality of styluses each fixed to the free end of a respective armature. A yoke member is fixedly attached to the upper surface of the fixed longitudinal side portion of the spring plate and has a plurality of recesses for loosely receiving the armatures therein, to exert the effect of a magnetic field on the corresponding armatures. A restrictor is located on the respective path of the respective rear end of each armature to limit the movement of the respective rear end of the armature. A driver drives the armatures and a guide guides the styluses. A fastener fastens the spring plate, the yoke, the restrictor, the driver and the guide to a base plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a print head for a printer and, more 
specifically, to a dot print head comprising styli each attached to the 
nose of an armature. 
2. Description of the Related Art 
Various dot print heads of such type have been proposed. FIGS. 6 and 7 
illustrate respective dot print units of first and second conventional dot 
print heads, by way of example. 
Referring to FIG. 6, in the first conventional dot print head, a permanent 
magnet 2, a support plate 3 and a spacer 4 are placed one over another in 
that order on one end of a base plate 1. A spring plate 7, a yoke 8 and a 
guide member 5 are placed one over another on the spacer 4 in a cantilever 
fashion, and are held in place with a clamping member 6. An armature 12 is 
attached to the flexible free end of the spring plate 7 adjacent to the 
yoke 8 and opposite to the core 9 of an electromagnet 11. A stylus 13 is 
attached to the nose of the armature 12, and is guided by the guide member 
5 for movement relative to a platen. The spacer 4, the yoke 8, the 
armature 12, the core 9 and the base plate 1 form a magnetic path for the 
magnetic flux of the permanent magnet 2. Normally, the armature 12 and the 
spring plate 7 are attracted to and biased toward the core 9 by the 
magnetism of the permanent magnet 2. 
When the electromagnet 11 is energized to produce a magnetic force acting 
opposite to and exceeding that of the permanent magnet 2, the spring plate 
7 and the armature 12 are released from the core 9, to allow the stylus 13 
attached to the nose of the armature 12 to project from the guide member 5 
and to press an ink ribbon and a recording medium, not shown, against the 
platen for printing. 
FIG. 7 illustrates the constitution of the essential portion of a second 
conventional dot print head disclosed in Japanese Utility Model Laid-open 
No. 60-3042. The second conventional dot print head is substantially the 
same as the first conventional dot print head in constitution, except that 
the rear portion of an armature 12 is longer than that of the armature of 
the first related dot print head, and a fixed point C of primary vibration 
of the armature 12 between a restrained position indicated by continuous 
lines and a free position indicated by broken lines where an impact is 
applied to a stylus 13 during a printing operation coincides with the 
instantaneous center of rotation of the armature 12. 
As is obvious from FIG. 8 showing the motion of the free end of the 
armature 12 of the first conventional dot print head, the free end of the 
armature makes vibrations of higher degree immediately after a printing 
motion. Therefore, the armature is liable to break, both the contact time 
and the stabilizing time are long, and the motion of the armature is not 
converted into printing force efficiently. 
As shown in FIG. 9, the armature of the second conventional dot print head 
does not make vibrations of higher degree and hence this armature does not 
have the drawbacks of the foregoing armature. However, the high speed of 
the armature in the return stroke is liable to cause the armature to 
rebound after the armature is attracted to the electromagnet. If the 
attraction is increased to suppress the rebound of the armature, the time 
lag between printing motions is increased and, consequently, driving time 
is increased, thus reducing printing efficiency. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a dot print 
head remarkably reducing the rebounding motion of the armatures and 
capable of operating stably at a high printing efficiency for high-speed 
printing. 
To achieve this object, the present invention provides a dot print head 
having armatures each having a rear end extended beyond the fixed point of 
primary vibration and held by holding members, and a restrictor which 
restricts the movement of the rear end of each armature while the armature 
is being attracted. 
In the dot print head according to the present invention the armature turns 
about the fixed point of primary vibration when the rear end is free or 
unrestrained and turns on the rear end when the rear end is restrained. 
Accordingly, the spring constant of a spring plate resiliently biasing the 
armature varies between a high value when the rear end of the armature is 
restrained and a low value when the rear end is free. The resilient force 
of the spring plate varies along with the variation of the attraction of a 
permanent magnet. However, the rate of variation of the resilient force 
increases as the rear end of the armature approaches an electromagnet when 
the armature is attracted by the permanent magnet. Thus, the spring 
constant of the spring plate increases and the center of the turning 
motion is dislocated from the center or fixed point of primary vibration 
or instantaneous center of turning after the rear end of the armature has 
come into contact with the restrictor while the armature is being 
attracted by the permanent magnet immediately after a printing motion, and 
thereby the returning speed of the armature is reduced. When the armature 
is attracted by the permanent magnet, the rear end of the armature is in 
contact with the restrictor and a front portion of the armature is in 
contact with the core of the electromagnet, which suppresses the 
rebounding motion of the armature. During a printing stroke, the rear end 
of the armature is free and hence the armature turns about the 
instantaneous center of turning. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description taken 
in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, in which a stylus is shown retracted, the rear end M 
of an armature 14 extends rearward beyond the fixed point O (FIG. 3) of 
primary vibration. A restrictor 16 is placed on top of a yoke 8 with a 
shim 15 interposed therebetween so as to restrict the movement of the rear 
end M of the armature 14 after the front end of the armature 14 has been 
turned halfway toward an electromagnet 11. The arrangement and 
configuration of the rest of the parts are similar to those of the 
conventional dot print head. 
Referring to FIGS. 1 and 2, a spacer 4, a spring plate 7, the armature 14, 
the yoke 8, the shim 15 and the restrictor 16 are placed one over another 
in that order on top of a base plate 3. The spacer 4 is provided to form a 
gap between the spring plate 7 and the upper end of the core 9 of the 
electromagnet 11. The spring plate 7 is spot-welded to the upper surface 
of the spacer 4 so as to cantilever therefrom. The yoke 8 and the armature 
14 are spot-welded to the upper surface of the spring plate 7. The spring 
plate 7 acts against the attraction of a permanent magnet 2. The armature 
14 forms a part of a magnetic path and is attracted to the core 9 when the 
electromagnet 11 is not energized. A stylus 13 is secured to the front end 
of the armature 14. A recess 8a is formed in the yoke 8 to receive the 
rear portion of the armature 14 therethrough. A line of magnetic force 
extends from the inner surface of the recess 8a to the armature 14. The 
shim 15 is provided to form a predetermined gap between the rear end M of 
the armature 14 and the restrictor 16 when the armature is released from 
the core 9. The restrictor 16 restricts the further movement of the rear 
end M of the armature 14 after the armature 14 has been attracted halfway 
by the permanent magnet 2 so that the armature 14 will turn on the rear 
end M. The position of the rear end M of the armature 14 and the thickness 
of the shim 15 are decided so that the resilient force of the spring plate 
7 will vary along with the variation of the effective attraction of the 
permanent magnet 2 acting on the armature 14. Preferably, the restrictor 
16 is located so that the rear end M of the armature 14 will not be 
brought into contact with the restrictor 16 during the printing motion of 
the armature 14 shown in FIG. 4. A lubricant or thin films are applied to 
the upper surface of the rear end M of the armature 14 and a contact 
portion of the restrictor 16 to prevent wear and to extend the life of the 
armature 14 and the restrictor 16. 
The manner of operation of the dot print unit thus constituted will be 
described hereinafter with reference to FIGS. 3 to 5. 
Referring to FIG. 3, the armature 14 is entirely free in a released state 
A, the rear end M of the armature 14 is in contact with the restrictor 16 
in a contact state B, the armature 14 is attracted to the core 9 by the 
permanent magnet 2 in an attracted state C, and the armature 14 would be 
in an unrestricted state D when the movement of the rear end M thereof 
beyond a limit is not restricted by the restrictor 16, i.e. if restrictor 
16 was not provided. Between the released state A and the contact state B, 
the armature 14 turns about the fixed point O of primary vibration, and 
turns on the rear end M thereof between the contact state B and the 
attracted state C. Accordingly, the spring constant of the spring plate 7 
while the armature 14 turns between the released state A and the contact 
state B is smaller than that of the spring plate 7 while the armature 14 
turns between the contact state B and the attracted state C. Consequently, 
the resilient force of the spring 7 varies along with the variation of the 
effective attraction of the permanent magnet 2, and the rate of variation 
of the resilient force of the spring plate 7 increases as the armature 14 
approaches the core 9 of the electromagnet 11. Thus, the spring constant 
of the spring plate 7 becomes large and the radius of rotation of the 
armature 14 becomes large while the armature 14 turns between the contact 
state B and the attracted state C, whereby the returning speed of the 
armature 14 is reduced. Accordingly, as is obvious from FIG. 5, the 
rebounding motion of the armature 14 is suppressed. During printing 
motion, the armature 14 is turned about the instantaneous center of 
turning by the resilient force of the spring 7. Accordingly, vibrations of 
higher degree are not generated in the armature 14, and hence both the 
contact time and the returning time are reduced. 
The present invention is not limited in application to the foregoing 
embodiment and many changes and variations therein are possible. For 
example, the thickness of the yoke 8 may be increased by a size 
corresponding to the thickness of the shim 15 to eliminate the shim 15. 
A guide 5 formed of a hard material, such as a metal or a ceramic material, 
may be employed and the rear portion of the guide 5 may be used as the 
restrictor 16 to eliminate the restrictor 16. 
Furthermore, the restrictor 16 may be formed of a magnetic material so that 
the restrictor 16 serves as part of the magnetic path to enhance the 
effective attraction of the permanent magnet 2. 
As is apparent from the foregoing description, according to the present 
invention, the rear portion of the armature is extended rearward beyond 
the fixed point of primary vibration which is caused by the spring plate, 
and the movement of the rear end of the armature while the armature is 
attracted to the core of the electromagnet is restricted by the restrictor 
after the armature has been turned halfway toward the electromagnet. Thus, 
the present invention provides the following effects. 
(1) The rate of increase of the resilient force of the spring plate 
increases as the armature approaches the core of the electromagnet and the 
resilient force of the spring plate increases to a maximum when the 
armature is attracted to the core; consequently the effective holding 
force of the permanent magnet is reduced by a degree corresponding to the 
increase in the resilient force of the spring plate, and hence the 
armature is able to reach the printing position in a shorter time after 
being released from the core of the electromagnet, thus increasing the 
printing speed of the dot print head, and the duration of energization of 
the electromagnet is reduced to save energy. 
(2) The spring constant of the spring plate becomes large when the rear end 
of the armature has come into contact with the restrictor after the 
armature has turned halfway toward the electromagnet to absorb the kinetic 
energy of the armature and to increase the moment of inertia of the 
armature about the center of rotation, whereby the returning speed of the 
armature is reduced and impact on the core is reduced, and, since the 
motion of the armature is restrained at two points thereon, the rebounding 
motion of the armature is suppressed, so that the repetitive reciprocatory 
motion of the armature is stabilized. 
(3) Since the armature is turned by the spring plate about the 
instantaneous center of rotation during the effective printing motion, any 
vibration of higher degree is not generated by the impact of the stylus on 
the platen, both the contact time and the returning time are reduced, and 
the turning motion of the armature is converted efficiently into printing 
motion for stabilized high-speed printing operation. 
Although the invention has been described in its preferred form with a 
certain degree of particularity, it is to be understood that many changes 
and variations are possible in the invention without departing from the 
scope and spirit thereof.