Serial printer

A serial printer wherein a print head is reversibly displaced transverse to a print medium by an open looped grooved rotating cylindrical cam is provided. A unidirectional drive shaft is operatively coupled to the cam by a drive gear engaged with planet gears mounted on a clutched rotary member for rotating the cam when the rotary member is stationary. An intermediate gear is intermittently engaged with the drive gear for changing the rotating direction of the cam upon engagement of the intermediate gear between the drive gear and planet gear. The rotary member includes geared regions adapted to be engageable with a print medium advancing assembly for advancing the recording medium during rotation of the rotary member when the drive of the print head is stopped and the direction of the cylindrical cam is about to be reversed. Advancing the print medium during change of direction of the print head when the cam is not being driven reduces the load placed on the motor, thereby permitting reduction in power consumption.

BACKGROUND OF THE INVENTION 
This invention relates generally to a printer, and particularly to a 
miniaturized serial printer wherein a print head is adapted to be 
displaced reversibly across a recording medium. In conventional serial 
printers, a print head is reversibly displaced across the recording medium 
by rotating a cylindrical cam. The cylindrical cam may include a closed 
loop spiral groove and the print head support includes a projection for 
following the closed loop sprial groove, in camming like fashion. The cam 
is rotated in one direction and displacement of the print head occurs as 
the print head follows the closed loop. 
Alternatively, a conventional serial printer may include a cylindrical cam 
having an open loop spiral groove wherein the direction is reversed by 
reversing the rotating direction of the cylindrical cam. The print head 
support in this construction also includes a projection for following the 
groove in camming like fashion. Displacement of the print head is reversed 
by reversing the rotating direction of the cylindrical cam. This reversal 
is usually accomplished by a two directional motor. 
In both types of conventional serial printers, the recording medium is 
advanced by a mechanical or electrical trigger driven by the printer 
motor. Both types of cylindrical cams always rotate while the motor 
rotates thereby displacing the print head. Thus, recording medium 
advancing also occurs while the print head is being displaced. For this 
reason, loads due to print medium advancing and displacing the print head 
are placed on the motor simultaneously. Accordingly, it is difficult to 
reduce the power consumption of the motor. Such operation also is a 
principal cause of mechanical problems as the print medium advancing 
mechanism tends to jam or the print medium tends to feed improperly. 
Additionally, the distance which the print head must be reversibly 
displaced includes the print range and an additional distance for the 
print medium advancing operation. Thus, the print heads are required to be 
displaced at a greater distance than merely the print range or margin. 
This latter point has been an obstacle to further miniaturization of 
serial printers. 
An example of a serial printer including a rotating cylindrical cam having 
a closed helical groove in the form of a continuous loop is shown in U.S. 
Pat. No. 4,046,246. An alternative type of rotating cam is a rotating disc 
as shown in U.S. Pat. No. 4,175,876 issued to Seiji Hanaoka on Nov. 27, 
1979 and assigned to the same assignees as the subject application. In the 
Hanaoka printer, the rotating disc drives the various mechanisms of the 
printer, including the print head and print tape advancing assembly. 
A cylindrical cam having a closed loop spiral groove is generally expensive 
to prepare. Alternatively, utilizing an open loop requires a more complex 
driving circuit in combination with a reversible motor in view of the need 
to reverse the direction of the rotation of the cam. This raises 
additional obstacles to reduction in manufacturing costs for a serial 
printer. Additionally, a reversible motor necessarily involves increased 
power consumption. Furthermore, when using a DC motor, the large power 
consumption generally leads to a decrease in the lifetime of the motor 
brushes. Accordingly, it would be desirable to provide a construction for 
a serial printer including a unidirectional motor and cylindrical cam 
having an open loop groove for displacing the print head. 
SUMMARY OF THE INVENTION 
Generally speaking, in accordance with the invention, a serial printer 
including a print head adapted for reversible transverse displacement 
across a print medium for effecting printing in at least one direction is 
provided. The printer includes a unidirectional drive for reversibly 
driving a cylindrical cam having an open loop groove for reversibly 
displacing a print head operatively engaged in camming fashion with the 
open loop groove. A print head includes a depending projection adapted to 
engage the open loop groove in camming fashion for displacing the print 
head across the print medium. A drive shaft rotates in only one direction 
and is selectively operatively engaged to the cam for rotating the cam by 
a rotary clutch assembly. 
The clutch assembly includes a clutch plate fixedly mounted on the drive 
shaft and a rotary member rotatably mounted on the drive shaft. A clutch 
lever is intermittently engageable with the clutch plate for 
intermittently rotating the rotarty member. The rotary member includes a 
pair of rotatably mounted planet gears for transmitting rotation of the 
drive shaft through a first gear to the cylindrical cam. An intermediate 
gear is also mounted on the rotary member and engaged with one of the 
planet gears for rotating the first gear in an opposite direction than 
when rotated by the other planet gear. 
The clutch lever is held out of engagement by a change lever until the 
print head completes displacement and then engages the clutch plate for 
rotating the rotary member. At this time, the rotating rotary member 
actuates a printing medium advancing member. As the rotary member 
continues to rotate, the clutch lever is disengaged from the clutch plate 
by the change lever when the second planet gear engages the first gear for 
driving the cylindrical cam in the opposite print direction. 
The clutch assembly is disengaged at the end of a print displacement of the 
print head. This stops rotation of the cam and permits print medium 
advance with reduced load on the drive motor. Accordingly, of conventional 
serial printers are overcome by utilizing a unidirectional drive in 
combination with a cylindrical cam having an open loop groove. 
Accordingly, it is an object of the invention to provide an improved 
printer. 
Another object of the invention is to provide an improved printer wherein a 
print head is adapted to be reversibly displaced across a print medium for 
printing in each direction. 
A further object of the invention is to provide an improved printer 
suitable for miniaturization. 
Still another object of the invention is to provide an improved printer 
wherein a print head is displaced in rectilinear reciprocating motion 
without utilizing a two directional motor. 
Still a further object of the invention is to provide an improved serial 
printer of reduced power consumption. 
Another object of the invention is to provide an improved printer wherein 
the likelihood of interruptions caused by the print medium advancing is 
reduced. 
A further object of the invention is to increase the printing speed of a 
serial printer. 
Yet another object of the invention is to provide a serial printer wherein 
the print head is reversibly displaced by and open looped cylindrical cam 
in a unidirectional drive. 
Yet a further object of the invention is to provide an improved serial 
printer wherein the distance the print head is displaced is reduced. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specification. 
The invention accordingly comprises the features of construction, 
combination of elements, and arrangements of parts which will be 
exemplified in the construction hereinafter set forth, and the scope of 
the invention will be indicated in the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 2, the mechanism for driving a print head and 
paper advancing mechanism in a serial printer wherein printing occurs 
during the reversible rectilinear displacement of the print head is shown. 
A cylindrical cam 1 having a spiral groove in a closed loop 1a and a cam 
gear 2 fixedly mounted at one end thereof is supported on a printer frame 
(not shown). A guide shaft 4 is mounted on the printer frame adjacent to 
and parallel to cylindrical cam 1 and supports a print head 3 slideably 
mounted thereon. Print head 3 includes a projection 5 adapted to engage 
spiral groove 1a on cylindrical cam 1 in camming like fashion. 
A motor 8 rotates a motor shaft 8a in one direction only. A motor gear 7 is 
mounted on motor shaft 8a. A reduction gear 6 is rotatably mounted on the 
printer frame and engaged with motor gear 7. A paper feed gear 9 is 
fixedly supported on a paper feed shaft 10 for advancing a print paper 15 
through the printer. The ratchet 11, a paper feed roller 12 and a torsion 
spring 13 are mounted on shaft 10. Shaft 10, ratchet 11, paper feed roller 
12 and torsion spring 13 constitute a spring clutch mechanism for 
advancing recording paper 15 as a trigger lever 14 engaged with ratchet 11 
is intermittently released in a direction F to disengage from ratchet 11 
as will be described in more detail below in connection with operation of 
the drive mechanism. 
In the drive mechanism illustrated in FIGS. 1 and 2, motor 8 always rotates 
in the direction of arrow C. Rotation in direction of arrow C is 
transmitted to cam gear 2 by motor gear 7 which also rotates in direction 
C. Reduction gear 6 rotates in an opposite direction to motor gear 7 and 
cylindrical cam 1 thereby rotates in a direction of arrow D, which in this 
case is the same direction as arrow C. As cylindrical cam 1 is rotated in 
arrow direction D, print head 3 is reversibly displaced rectilinearly in 
arrow directions A and B as projection 5 follows spiral groove 1A in 
camming like fashion. Printing on recording paper 15 occurs as print head 
3 is displaced in arrow direction A or B. As print head 3 is displaced, 
paper feed shaft 10 is rotated in arrow direction E by motor gear 7 
engaged with reduction gear 6, cam gear 2 and paper feed gear 9. 
There comes a time when print head 3 is displaced out of a printing range 
or into the margin of print paper 15. The printing range is the distance 
within which print head 3 prints on print paper 15. After print head 3 is 
displaced out of the printing range in the direction of either arrow A or 
B, trigger lever 14 is rotated either mechanically or electrically in the 
direction of arrow F against a spring 16. This displacement of trigger 
lever 14 releases ratchet 11 from engagement with trigger lever 14 thereby 
permitting paper feed roller 12 to rotate in the direction of arrow E 
through torsion spring 13 by a given amount until the next tooth of 
ratchet 11 engages with trigger lever 14 which has been returned to its 
rest position by the biasing force of spring 16. During the time paper 
feed roller 9 has rotated, print paper 15 is advanced. 
Referring now to FIGS. 3 and 4, a drive mechanism for a serial printer 
wherein a cylindrical cam 17 includes a spiral groove in open looped 
configuration 17a is shown. A print head 18 includes a projection 18a 
which engages spiral groove 17a in camming like fashion and in the same 
manner as print head 3 and projection 5 engage spiral groove 1a in the 
mechanism illustrated in FIGS. 1 and 2. The remaining component parts in 
the drive mechanism illustrated in FIGS. 3 and 4 remain the same as in the 
mechanism illustrated in FIGS. 1 and 2. Accordingly, like elements are 
identified by the same reference numerals primed. 
When a two directional motor 8' is rotated in the direction of arrow C, 
cylindrical cam 17 is rotated in the direction of arrow D and print head 
18 is displaced in the direction of arrow A as projection 18a follows 
groove 17a. As print head 18 is displaced, printing occurs on a print 
medium. When print head 18 is displaced in arrow direction A so as to be 
out of the printing range, the paper feed mechanism (not shown), which may 
be of the type utilizing a trigger as illustrated in FIGS. 1 and 2, 
advances the recording paper. After the recording paper has been advanced, 
the direction of motor 8' is reversed to direction G and cylindrical cam 
17 is rotated in the direction of arrow H. When cylindrical cam 17 is 
rotated in the direction of arrow H, print head 18 is displaced in the 
direction of arrow B for printing on the recording medium in this opposed 
direction. 
The drive mechanisms of the printers described in FIGS. 1-4 are not 
completely satisfactory. Firstly, in each case cylindrical cams 1 and 17 
are rotated continuously as motors 8 and 8' are rotated for displacing 
print heads 3 and 18 in the direction of arrows A or B during the paper 
advancing operation. For this reason, the loads due to both paper 
advancing and displacement of print heads are placed on the motors 
simultaneously, making it difficult to reduce the power consumption for 
the motor. Such operation is often the main reason for problems in 
connection with paper advancing, such as paper jams and wandering of the 
paper, and the like. Additionally, the distance print heads 3 and 18 are 
reversibly displaced includes both the printing range and an additional 
distance into the margin for performing the paper advancing operation. 
Thus, print heads 3 and 18 are required to be displaced a distance greater 
than merely the printing range. Such additional distance prevents 
additional miniaturization of serial printers constructed and arranged in 
accordance with these constructions. 
The second shortcoming of these constructions is the manufacturing cost. A 
cylindrical cam having a spiral groove of the closed loop type as 
illustrated in the drive mechanism of the printer in FIGS. 1 and 2 is 
expensive to manufacture. Additionally, the drive mechanism of the printer 
illustrated in FIGS. 3 and 4 include a driving circuit which is expensive 
since the motor is rotated in a forward and reverse direction. Both 
factors are obstacles to reducing the manufacturing costs in either the 
printer of the type in FIGS. 1 and 2 or the printer of the type in FIGS. 3 
and 4. Additionally, in the drive mechanism of the printer illustrated in 
FIGS. 3 and 4 power consumption is increased when the motor is reversed. 
Further, when using a DC motor, this large power consumption often causes 
a further problem as it decreases the useful life of the motor brush. 
Referring now to FIGS. 5-8, the drive mechanism for a printer constructed 
and arranged in accordance with a preferred embodiment of the invention 
which overcomes many of the shortcomings is shown. Referring specifically 
to FIGS. 5 and 6, a cylindrical cam 20 having a spiral groove 20a of the 
open loop configuration is shown. A cam gear 21 is mounted at one end of 
cylindrical cam 20. A print head 22 is slideably mounted on a guide shaft 
23 and includes a projection 22a adapted to engage spiral groove 20a. 
Cylindrical cam 20 is driven by a drive shaft 24 which is rotated in one 
direction only by a drive source (not shown). A drive gear 25 is mounted 
at the drive end of drive shaft 24 and a clutch plate 26 is also fixedly 
mounted on drive shaft 24 adjacent to drive gear 25. A change wheel 27 
which functions as a rotary member is rotatably mounted on drive shaft 24 
and includes a first gear portion 27a and a second gear portion 27b on the 
periphery thereof with an integral between the gear portions of 
180.degree.. A plurality of pins 28, 29, 30 and 31 are mounted on change 
wheel 27. 
A clutch lever 32 including an engaging tooth 32a is mounted on pin 28. 
Clutch lever 32 is free to be rotated about pin 28 from a first position 
out of engagement with clutch plate 26 as shown in FIG. 6 and a second 
engaged position as shown in FIG. 7. A small planet gear 33 is mounted on 
pin 29 which is always rotated by engagement with drive gear 25. A second 
large planet gear 34 is mounted on pin 30 which is always rotated by its 
engagement with drive gear 25. Large planet gear 34 is dimensioned to be 
engageable with cam gear 21. An intermediate gear 35 is mounted on pin 31 
and is always rotated by its engagement with small planet gear 33 which, 
in turn, is engageable with drive gear 25. Intermediate gear 35 is 
dimensioned to be engageable with cam gear 21 for driving cylindrical cam 
20. 
The paper advancing assembly of the drive assembly, illustrated in FIGS. 
6-8, includes a paper feed gear 36 which is fixedly mounted on a paper 
feed shaft 38 which is rotatably supported in the printer frame (not 
shown). A paper feed roller 37 is also mounted on paper feed shaft 38. 
Paper feed roller 37 is intermittently rotated a fixed amount by 
intermittent engagement with first gear portion 27a and second gear 
portion 27b of change wheel 27 when change wheel 27 is rotated as will be 
described in more detail in connection with operation of the drive 
mechanism. 
Referring now to FIG. 5, an elongated change lever 39 having a first 
engaging arm 39a and a second engaging arm 39b is arranged so that first 
and second engaging arms 39a and 39b are engaged by print head 22 as it 
approaches the end of travel in a print direction. Change lever 39 is 
mounted for displacement in the two directions of travel of print head 22 
and is pivotally engaged through a pin 41 to an engagement lever 40 which 
is free to be rotated about a fulcrum 42. Engagement lever 40 is 
engageable with clutch lever 32 through a first engagement portion 40a and 
a second engagement portion 40b. When change lever 39 is displaced by its 
engagement with print head 22, engagement between one of the engagement 
portions 40a or 40b and clutch lever 32 is released so that change wheel 
27 is free to rotate until clutch lever 32 engages a further engagement 
portion thereby stopping rotation of change wheel 25. 
Operation of the print drive mechanism shown in FIGS. 5-8 will now be 
described. Referring specifically to FIGS. 5 and 6, rotation of drive 
shaft 24 which is always rotated in the direction of arrow I is 
transmitted to cylindrical cam 20. Rotation is transmitted through drive 
gear 25, small planet gear 33 rotated in the direction of arrow J, 
intermediate gear 35 rotated in the direction of arrow K and cam gear 21 
fixed on cylindrical cam 20. Cylindrical cam 20 is rotated in the 
direction of arrow L and print head 22 is displaced along guide shaft 23 
in the direction of arrow M as projection 22a follows spiral groove 20a in 
camming like fashion. As print head 22 is displaced in the direction of 
arrow M, print head 22 prints on recording paper 44 within the printing 
range. 
As print head 22 is displaced in print direction M, clutch lever 32 is held 
in its first position out of engagement with clutch plate 26 by engagement 
of engaging portion 40a of engagement lever 40. When in this 
configuration, as shown in FIG. 6, rotation of change wheel 27 is 
suspended. As print head 22 enters out of the print range in print 
direction M, print head 22 engages first engaging arm 39a of change lever 
39 located within the locus of print head 22. Change lever 39 is displaced 
in arrow M thereby pivoting engagement lever 40 in the direction of arrow 
N at the same time. When engagement lever 40 is rotated by more than a 
fixed angle in the direction of arrow N, clutch lever 32 is disengaged 
from engaging portion 40a of engagement lever 40. This disengagement 
permits release of clutch lever 32 which is then rotated about pin 28 in 
the direction of arrow O by a spring 43. Engaging tooth 32a of clutch 
lever 32 then engages clutch plate 26. Change wheel 27 is then engaged 
with drive shaft 24 through clutch plate 26 and change wheel 27 together 
with clutch lever 32 is rotated in the direction of arrow I. When change 
wheel 27 begins to rotate in arrow direction I, intermediate gear 35 is 
disengaged from cam gear 21 and cylindrical cam 20 stops rotating. At this 
time, print head 22, change lever 39 and engagement lever 40 are in a 
stand-by state as illustrated by a two-dot-line in FIG. 5. 
Change wheel 27 continues to rotate in arrow direction I by drive shaft 24 
through operations of clutch plate 26 and clutch lever 32. As rotation 
continues, first gear portion 27a of change wheel 27 engages paper feed 
gear 36 as shown in FIG. 7 and paper feed roller 37 thereby is rotated at 
a fixed angle for advancing recording paper 44. As change lever 27 
continues rotating for about 180.degree., clutch lever 32 is engaged by 
the other engaging portion 40b of engagement lever 40 and begins to be 
returned to its first at rest position in arrow direction P against spring 
43. This causes engaging tooth 32a to disengage from clutch plate 26 
thereby bringing the members to a halt as shown in FIG. 8. At this time, 
change wheel 27 suspends rotation after being rotated through 180.degree. 
from the previously suspended position illustrated in FIG. 5. At this 
time, cam gear 21 is engaged by large planet gear 34 as shown in FIG. 8 
and the change of the rotating direction of cylindrical cam 20 is 
completed by operation of the rotary clutch assembly. 
When the elements are in position as illustrated in FIG. 8, the rotating 
direction of drive shaft 24, which is always rotated in the direction of 
arrow I, is transmitted to cylindrical cam 20. This rotation in arrow 
direction I is transmitted through drive gear 25, large planet gear 34 
which is rotated in the direction of arrow R and cam gear 21. Cam gear 21 
is rotated in the direction of arrow S, contrary to the rotational 
direction illustrated in FIG. 6. Thus, print head 22 is displaced along 
guide shaft 23 in the direction of arrow T for printing on paper 44 within 
the printing range. When print head 22 is displaced to the end of the 
printing range, print head 22 engages second engaging arm 39b of change 
lever 39 which is in the position as illustrated by the two-dot-line in 
FIG. 5. At this time, change lever 39 is displaced in the direction of 
arrow T thereby causing engagement lever 40 to be rotated in the direction 
of arrow U. 
When engagement lever 40 is rotated more than the fixed angle, clutch lever 
32 is released from the engagement with clutch plate 26 and change wheel 
27 begins to rotate in the direction of arrow I in the same manner as 
described with respect to changeover from rotating cam gear 21 in arrow 
direction L to rotating cam gear 21 in arrow direction S. 
As the change wheel 27 begins to rotate further, large planet gear 34 is 
disengaged from cam gear 21 and cylindrical cam 20 stops rotating. At this 
time, print head 22, change lever 39 and engagement lever 40 come into the 
stand-by state. As change wheel 27 rotates, second gear portion 27b of 
change wheel 27 engages paper feed gear 36 and paper feed roller 37 is 
rotated by the fixed angle in the direction of arrow Q, as shown in FIG. 
7, for advancing paper tape 44. When change wheel 27 has been rotated an 
additional 180.degree. from the position illustrated in FIG. 8, it comes 
to rest as shown in FIG. 6 when cam gear 21 is engaged again with 
intermediate gear 35 and print head 22 commences to be displaced in the 
direction of arrow M as described above. 
In a printer including a drive mechanism, as illustrated in FIGS. 5-8, 
print head 22 is displaced reversibly and includes a temporary suspension 
of displacement at both ends of the printing range. The printing of 
successive lines on recording paper 44 is made possible by repeating the 
printing operation which occurs while print head 22 is reversibly 
displaced with paper advancing occurring during the suspension of 
displacement of print head 22. In other words, paper advancing occurs when 
the direction of displacement of print head 22 is being changed. 
Several advantages are obtained by constructing and arranging the drive 
mechanism for a printer in accordance with the preferred embodiment just 
described. First, the print head is forced into rectilinear reciprocating 
motion with inexpensive component parts without requiring a two speed 
motor. This reduces the manufacturing cost and extends the lifetime of the 
motor and the power consumption of the printer is also reduced. Second, 
since the print head displacement is suspended at the time of paper 
feeding, the load placed on the motor is reduced and the distance which 
the printer head need travel is also reduced. Therefore, there is less 
power consumed in further miniaturization and a high speed printer is 
achieved while avoiding the troublesome problems of paper feeding, such as 
a paper jam or wandering print are avoided. Third, as paper advance occurs 
when the print head is not being displaced, the overall distance of 
displacement is reduced leading to further reduction in size of the 
printer. It will thus be seen that the objects set forth above, among 
those made apparent from the preceding description, are efficiently 
attained and, since certain changes may be made in the above construction 
without departing from the spirit and scope of the invention, it is 
intended that all matter contained in the above description or shown in 
the accompanying drawings shall be interpreted as illustrative and not in 
a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention herein described 
and all statements of the scope of the invention which, as a matter of 
language, might be said to fall therebetween.