Pen switching mechanism in drum plotters

A drum plotter has a pen carriage movable back and forth across a sheet of print paper on a platen which is rotatable about its own axis in opposite directions. The pen carriage supports thereon a pen holder unit rotatable for bringing one of pens with differently colored inks into a writing position. The pen in the writing position is pushed by a hammer toward the platen for writing desired characters, figures or the like on the sheet of print paper. The pen holder unit is rotatably actuatable by a ratchet wheel engaged by a pawl member driven intermittently by a cam mounted on a shaft supporting the pen carriage and drivable by a solenoid-operated mechanism.

BACKGROUND OF THE INVENTION 
The present invention relates to a drum plotter installed on a terminal 
device connected to a computer for drawing characters, figures, graphs and 
other patterns in multiple colors, and more particularly to a pen 
switching mechanism in such a drum plotter. 
There have recently been developed plotters having a pen with water-base or 
oil-base ink movable back and forth across the width of a sheet of print 
paper or in a direction normal to the direction in which the sheet of 
print paper is fed along. Characters, figures and the like can be drawn on 
the paper by controlling the movement of the print paper and the pen in X 
and Y directions. To enable the plotter of this type to draw characters 
and figures in multiple colors, a plurality of pens with differently 
colored inks are supported on a rotary pen holder rotatable to move a pen 
with a desired color into a writing position. One known mechanism for 
rotating the pen holder to switch the pens has a fixed lever for pushing 
the pen holder to rotate the same by utilizing the force with which the 
pen holder is moved across the sheet of print paper. This prior mechanism 
however is disadvantageous in that it is time-consuming to switch the pens 
since the pen holder needs to be moved back to the fixed lever each time a 
desired pen is to be selected. 
The plotter also has a hammer for pressing the pen selectively against the 
sheet of print paper. The hammer is driven by a solenoid fixed in a 
position different from a pen carriage supporting the pens and energizable 
to actuate the hammer through a shaft that guides the pen carriage in its 
movment. With this arrangement, however, the hammer actuating power is 
transferred through the shaft, and hence operating errors are accumulated 
to the point where the movement of the pens driven by the hammer cannot be 
adjusted easily. 
The drum plotter has pens supported horizontally in confronting relation to 
a platen. Since there is a tendency for no ink to come out of the pens 
when recording is just started, it is necessary that something be drawn on 
the paper on a trial basis to see if the ink flows out properly. This 
process delays the starting of desired recording, and produces a useless 
length of print paper. 
SUMMARY OF THE INVENTION 
It is a first object of the present invention to provide a drum plotter 
which will eliminate the foregoing prior problems and reduces the time 
required for switching pens by immediately rotating a pen holder no matter 
where a pen carriage installing the pen holder thereon may be positioned. 
A second object of the present invention is to provide a drum plotter 
having a hammer for pushing pens with a minimum of operating error 
achieved by shortening the path along which actuating power is transmitted 
from a solenoid to the hammer. 
A third object of the present invention is to provide a drum plotter in 
which ink is allowed to flow easily out of pens at all times to permit 
recording operation to be started smoothly and to make trial recording 
unnecessary. 
According to the present invention, a pen switching mechanism in a drum 
printer, comprises a platen rotatable about its own axis in one and 
opposite directions, a shaft extending parallel to the platen, and a pen 
carriage movably mounted on the shaft in confronting relation to the 
platen, the pen carriage being movable along the shaft in axial directions 
of the platen. The pen carriage has a pen holder unit supporting a 
plurality of pens for rotatable movement to bring one of the pens at a 
time into a writing position, a hammer for selectively pressing the pen in 
the writing position toward the platen, a ratchet wheel for rotation with 
the pen holder and a pawl member movably mounted on the pen carraige and 
operatively engaging teeth of the ratchet wheel for rotating the latter. A 
mechanism is coupled with the shaft for angularly moving the latter about 
its own axis, and has a cam held in engagement with the pawl member for 
actuating the latter. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which a preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the overall construction, in perspective, of a drum plotter. 
The construction and operation of the drum plotter shown in FIG. 1 will 
first be described. 
The drum plotter has a base 1 including a front plate 2 extending upwardly 
from a front edge of the base 1, the front plate 2 being a right-angularly 
bent extension of the base 1 as shown in FIG. 4. Left-hand and right-hand 
side plates 3, 4 are secured to lateral ends of the base 1. A guide plate 
5 is disposed above a rear edge of the base 1 and extending between the 
side plates 3, 4. 
A platen 6 is rotatably supported on upper central portions of the side 
plates 3, 4. The platen 6 has a shaft 7 having a right-hand end projecting 
outwardly beyond the right-hand side plate 4 and having a gear 8 secured 
thereto at a distal end thereof. A stepper motor 9 is placed outwardly of 
the right-hand side plate 4 and has a pinion (not shown) secured to the 
shaft of the motor and meshing with the gear 8. The platen 6 is rotatable 
about its own axis in both directions in response to normal and reverse 
rotation of the stepper motor 9. A sheet of print paper is supplied over 
the guide plate 5 into a position below the platen 6 and along a curved 
guide plate 10 underlying the platen 6, and is fed out in the direction of 
the arrow Xa. The sheet of print paper is pressed against the platen 6 by 
rollers 12 rotatably mounted on ends of a lower shaft 11 and rollers 14 
rotatably mounted on ends of an upper shaft 13, the lower and upper shafts 
11, 13 extending axially along and in confronting relation to the platen 
6. A pair of paper presser rollers 15 in the shape of abacus beads is 
rotatably mounted on the upper shaft 13 inwardly of the rollers 14. The 
sheet of print paper is rolled around a front surface portion of the 
platen 6 by these rollers, 12, 14, 15, and is fed out in the directions of 
the arrows Xa, Xb by rotating the stepper motor 9 in normal and reverse 
directions. 
Levers 16a, 16b are pivotably mounted on the left-hand and right-hand side 
frames 3, 4, respectively, on their outer sides. The levers 16a, 16b are 
operatively connected to the shafts 11, 13 via a mechanism such that when 
the levers 16a, 16b are pulled in the direction of the arrows, the rollers 
12, 14, 15 are moved out of contact with the platen 6, allowing the sheet 
of print paper to be loaded into the drum printer around the platen 6. The 
platen 6 has on one end thereof two rows of metal teeth 17a, 17b and on 
the other end one row of metal teeth 17c, the teeth 17a, 17b, 17c 
extending peripherally around the platen 6. When the sheet of print paper 
is wound around the platen 6, the side edges of the paper are firmly 
retained in place by the teeth 17a, 17b, 17c. Accordingly, there is no 
danger for the sheet of print paper to slip on the platen 6 and the sheet 
of print paper can reliably be fed out in the directions of the arrows Xa, 
Xb. Sprockets 18a, 18b are mounted on the platen 6 for corotation and 
located axially outwardly of the teeth 17a, 17b and 17c, respectively. 
Where the sheet of print paper has feed holes along lateral edges, the 
sprockets 18a, 18b engage in the feed holes for positive feeding of the 
sheet of print paper without any slippage on the platen 6. 
A pen carriage 20 is positioned in front of the platen 6. The pen carriage 
20 is movable along a shaft 21 extending between the left-hand and 
right-hand side plates 3, 4 while being guided by a guide member 2a bend 
from an upper edge of the front plate 2 for back-and-forth movement in the 
direction of the arrows Ya, Yb. The pen carriage 20 is driven to move back 
and forth by a wire 22 having ends fastened to side edges of the pen 
carriage 20. The wire 22 is trained around upper and lower pulleys 23a, 
23b and a drive pulley 24 which are mounted on the left-hand side plate 3 
at its outer surface, and also around pulleys (not shown) similar to the 
pulleys 23a, 23b and mounted on the right-hand side plate 4 at its outer 
surface. A gear 25 is integrally formed with the drive pulley 24 and held 
in mesh with a pinion 26 secured to the rotatable shaft of a stepper motor 
27 (only partly shown in FIG. 1) disposed below the guide plate 5. When 
the stepper motor 27 is driven to rotate in normal and reverse directions, 
the drive pulley 24 is rotated to cause the wire 22 to pull 20 back and 
forth in the directions of the arrows Ya, Yb. The pen carriage 20 support 
four pens P storing oil-base or water-base inks of different colors (only 
one pen shown in FIG. 1). Any one of the pens P with ink of a desired 
color can be brought to an uppermost position on the pen carriage 20 by a 
pen switching mechanism (described later on). 
For recording operation, the stepper motor 9 is rotated in normal and 
reverse directions to feed the sheet of print paper around the platen 6 in 
the directions of the arrows Xa, Xb. At the same time, the stepper motor 
27 is rotated in normal and reverse directions to move the pen carriage 20 
back and forth in the directions of the arrows Ya, Yb. A desired pen P is 
pressed against the sheet of print paper to draw characters, figures or 
the like on the sheet of print paper. 
The pen carriage 20 will be described in more detail with reference to 
FIGS. 2, 3 and 4. 
The pen carriage 20 has a carriage base 31 with a sleeve 32 mounted 
therebelow and fitted slidably over the shaft 21 for allowing the carriage 
base 31 to move in the directions of the arrows Ya, Yb. The carriage base 
31 has a slit 31a formed in a front portion thereof. While the carriage 
base 31 moves in the directions of the arrows Ya, Yb, the guide member 2a 
of the front plate 2 is inserted in the slit 31a in sliding relationship, 
as shown in FIG. 1. With the guide member 2a inserted in the slit 31a, the 
carriage base 31 is prevented from turning around the shaft 21 so that the 
pens 6 mounted on the carriage base 31 are directed at an angle 
(horizontally in the illustrated embodiment) to the platen 6. 
A shaft 33 is mounted on the carriage base 31. The shaft 33 has one end 
supported in a support hole 31b (shown by the broken line in FIG. 3) 
defined in the carriage base 31 and the other end supported by a support 
plate 35 through a sleeve 34. The support plate 35 has a lower end 
fastened by screws 36 to an end face 31c of the carriage base 31. The 
shaft 33 has opposite end portions 33a, 33b substantially ovel in 
cross-sectional shape. A pen holder 37 is mounted on the end portion 33a, 
while a pen holder 38 is mounted on the end portion 33b. Therefore, the 
pen holders 37, 38 are rotatable in unison with the shaft 33. The pen 
holder 37 has four angularly equidistant slots 37a, 37b, 37c and 37d 
defined in its outer peripheral edge, and likewise the pen holder 38 has 
four angularly equidistant slots 38a, 38b, 38c and 38d defined in its 
outer peripheral edge. The slots 37a and 38a, 37b and 38b, 37c and 38c, 
37d and 38d are positioned in axial alignment with each other. The paired 
pen holders 37, 38, jointly support the four pens P with inks of different 
colors such as red, blue, black and green. 
A pen switching mechanism for selecting the pens P and bringing the 
selected pen P into a writing position will now be described. 
A ratchet wheel 41 is mounted on the end portion 33a of the shaft 33, the 
ratchet wheel 41 being movable with the pen holders 37, 38 through the 
shaft 33. A stopper 42 is pressed against one tooth at a time of the 
ratchet wheel 41 (FIG. 5(B)). The stopper 42 comprises a leaf spring 
secured by a screw 43 threaded into a tapped hole 31d in the carriage base 
31. The stopper 42 allows the ratchet wheel 41, the shaft 33 and the pen 
holders 37, 38 to rotate clockwise as shown in FIG. 5(B) while preventing 
them from rotating counterclockwise. A pawl plate 44 is disposed in 
diametrically opposite relation to the stopper 42 across the ratchet wheel 
41 and engages one of the teeth of the ratchet wheel 41 at a time. The 
pawl plate 44 has a pair of tapped holes 44a, 44b in which there is 
threaded a pair of externally threaded pins 45, 46, respectively, 
extending through oblong holes 31e, 31f defined in the carriage base 31. 
The pawl plate 44 is thus slidably movable vertically along the oblong 
holes 31e, 31f. A spring 48 is connected between a pin 47 affixed to the 
carriage base 31 and the pin 45 for normally biasing the pawl plate 44 
downwardly as shown. 
A cam 49 is disposed below the carriage base 31 and has a hole 49a of a 
substantially oval shape. The shaft 21 extending through the sleeve 32 
beneath the carriage base 31 also extends through the hole 49a. The shaft 
21 is also substantially oval in cross-sectional shape. Therefore, the cam 
49 is angularly movable in unison with the shaft 21. The cam 49 has a 
distal end 49b held in abutment against a lower end face of the pawl plate 
44. 
As shown in FIG. 4, the shaft 21 has a right-hand end projecting outwardly 
through a hole 4a in the righthand side plate 4. A lever 50 is attached to 
the projecting end of the shaft 21 and has a bent portion 50aheld against 
a side surface of the right-hand end of the shaft 21 and fastened thereto 
by an attachment screw 51 threaded into a tapped hole 21a in the shaft 21. 
The base 1 has a front solenoid-attachment plate 1a positioned at the 
right-hand side plate 4, the plate 1a being an upward bent extension of 
the base 1. A solenoid 52 is mounted on the solenoid-attachment plate 1a 
by attachment screws 53. The solenoid 51 has a downward plunger 52a having 
a lower end coupled to the lever 50 by a pin 52b extending through a 
support aperture 50b in the lever 50. An eccentric stopper 54 is mounted 
on the right-hand side plate 4 at its outer surface. The lever 50 has a 
lower edge held in abutment against the eccentric stopper 54. As 
illustrated in FIGS. 5(A) and 6(A), a spring 55 is connected between the 
distal end of the lever 50 and the base for normally urging the lever 50 
into abutting engagement with the eccentric stopper 54. 
FIGS. 5(A) through 5(C) show the position of the parts assumed when the 
solenoid 52 is de-energized. In this position, the lever 50 is held 
against the eccentric stopper 54 under the bias of the spring 55. The 
shaft 21 is not turned about its own axis, and the distal end of the cam 
49 mounted on the shaft 21 is in a lowered position as shown in FIG. 5(C) 
with the pawl plate 44 lowered under the force of the spring 48. The 
positional relationship between the pawl plate 44 and the ratchet wheel 41 
at this time is shown in FIG. 5(B). 
FIGS. 6(A) through 6(C) illustrate the parts position in which the solenoid 
52 is energized. The solenoid plunger 52a is pulled upwardly to turn the 
shaft 21 counterclockwise as shown in FIG. 6(A) through a predetermined 
angle. The cam 49 also turns with the shaft 21 to cause the distal end 49b 
to push the pawl plate 44 upwardly to the position of FIG. 6(B). When the 
solenoid 52 is de-energized at this time, the lever 50 and the shaft 21 
are brought to the position of FIG. 5(A) under the resiliency of the 
spring 55, and the pawl plate 44 is moved downwardly to the position of 
FIG. 5(B) under the biasing force of the spring 48. By intermittently 
energizing the solenoid 52, the pawl plate 44 turns the ratchet wheel 41 
clockwise about its own axis. The pen holders 37, 38 and the pens P 
supported thereby are also angularly moved about the shaft 33 in an 
intermittent steplike manner. 
As shown in FIG. 3, the carriage base 31 has a central slot 31g in which a 
stabilizing stopper 56 is mounted. The stabilizing stopper 56 has a 
support hole 56a through which extends a support pin 57 inserted through 
the carriage base 31. Therefore, the stabilizing stopper 56 is angularly 
movable about the support pin 57. A leaf spring 58 is fixed by a screw 59 
to the underside of the carriage base 31 and normally urges the 
stabilizing stopper 56 upwardly. When one of the four pens P supported by 
the pen holders 37, 38 is brought to a lowermost position while the 
solenoid 52 is being intermittently energized, a central lower surface of 
the pen P in the lowermost position is fitted in an upper recess 56b in 
the stabilizing stopper 56 to allow the pen P to be positionally 
stabilized in the lowermost position. Accordingly, the pen holders 37, 38 
are stabilized in the position in which the pens P are located in four 
positions, that is, upper, lower and two lateral positions (FIGS. 5(B) and 
5(C)). The uppermost one of the four pens P serves to write any desired 
characters, figures or the like on the sheet of print paper wound around 
the platen 6. 
A hammer drive mechanism for pressing the uppermost pen P against the sheet 
of print paper on the platen 6 will now be described. 
The hammer drive mechanism is mounted on the pen carriage 20 for movement 
therewith in the directions of the arrows Ya, Yb (FIG. 1). 
The hammer drive mechanism has a solenoid 61 serving as a drive source 
located below the carriage base 31 on its left-hand side. The carriage 
base 31 has an integral L-shaped solenoid attachment 31h on its left-hand 
side with a holder 61a made of a metal sheet screwed to the underside of 
the solenoid attachment 31h. A fixed iron core 61b is secured to the 
holder 61a and supports a coil bobbin 61c mounted thereon. A movable iron 
core 61d is positioned in front of the coil bobbin 61c and retained 
between the latter and the solenoid attachment 31h against dropping off 
the position therebetween. The movable iron core 61d can be attracted 
toward the fixed iron core 61b in response to energization of the coil 
bobbin 61c. The holder 61a has support holes 61e in its right-hand tabs 
with a shaft 62 (FIG. 3) inserted through the support holes 61e. The shaft 
62 also extends through support holes 31i, 31j defined in the carriage 
base 31. The shaft 62 is substantially oval in cross section, and has a 
lower end fitted in a substantially oval hole 63a in an actuator 63 
mounted laterally on the holder 61a. Therefore, the actuator 63 is 
angularly movable with the shaft 62. The actuator 63 is movable with the 
movable iron core 61d toward and away from the coil bobbin 61c, and is 
normally urged by a return spring 64 in a direction to move away from the 
coil bobbin 61c. A hammer 65 is mounted on the shaft 62 at its upper end. 
The hammer 65 has a substantially oval hole 65a in which the upper end of 
the shaft 62 is fitted for allowing the hammer 65 and the shaft 62 to move 
together. The hammer 65 also has a projection 65b held in abutting 
engagement with a rear end of the pen P as brought to the uppermost 
writing position. 
Between the pen holder 38 and the sleeve 34, there is disposed a pen return 
spring 71 (better shown in FIGS. 7 and 8) made of a sheet of phosphor 
bronz or springy material by pressing. The pen return spring 71 has a 
central attachment hole 71a fitted over the shaft 33. The pen return 
spring 71 is also provided with four arms 71b disposed angularly 
equidistant relationship and extending circumferentially around the 
central hole 71a, each arm having a support hole 71c in its distal end. As 
illustrated in FIG. 8, each arm has an integral flange 71d extending 
around the support hole 71c and directed toward the platen 6. The pens P 
have tip ends inserted and supported in the support holes 71c, 
respectively. The pens P are normally urged away from the platen 6 under 
the resilient force from the arms 71b of the pen return spring 71. 
When the solenoid 61 remains de-energized, the movable iron core 61d and 
the actuator 63 are biased by the spring 64 to move away from the coil 
bobbin 61c. The hammer 65 coupled through the shaft 62 to the actuator 63 
is angularly moved in the same direction as that of turning movement of 
the actuator 63. The projection 65b of the hammer 65 pushes the rear end 
of the uppermost one of the four pens P, forcing the distal end of the pen 
P against the sheet of print paper on the platen 6 so as to be ready for 
writing characters, figures or the like as shown in FIG. 10. When the 
solenoid 61 is energized, the movable iron core 61d is attracted to the 
fixed iron core 61b to turn the actuator 63 toward the coil bobbin 61c. 
The hammer 65 is turned with the actuator 63, releasing the pen P which is 
pushed back under the resiliency of the arm 71b of the pen return spring 
71 until the tip end of the pen P is spaced from the sheet of print paper. 
In this position, no characters, figures or the like are written on the 
sheet of print paper (FIG. 9). 
Overall operation of the pen carriage 20 of the foregoing construction will 
be described with reference to operation of the drum plotter. 
The sheet of print paper extending around the platen 6 is fed back and 
forth in the directions of the arrows Xa, Xb (FIG. 1) on normal and 
reverse rotation of the stepper motor 9. The pen carriage 20 is moved back 
and forth in the directions of the arrows Ya, Yb by the wire 22 trained 
around the drive pulley 24 driven by the stepper motor 27. 
During such movements of the sheet of print paper and the pen carriage 20, 
the solenoid 61 in the hammer drive mechanism is de-energized to allow the 
hammer 65 to push the uppermost one of the four pens P supported by the 
pen holders 37, 38 until the tip end of the uppermost pen P is pressed 
against the sheet of print paper. At this time, a character or figure or 
both can be drawn on the sheet of print paper in response to controlled 
movements of the sheet of print paper and the pen carriage 20 in the 
directions of the arrows Xa, Xb and Ya, Yb. When the solenoid 61 is 
energized, the hammer 65 is retracted to permit the pen P to be spaced 
from the sheet of print paper, thus interrupting writing operation on the 
latter. To change colors of characters or figures to be written on the 
sheet of print paper, the solenoid 52 (on the right-hand side plate 4) in 
the pen switching mechanism is repeatedly energized and de-energized. The 
shaft 21 is reciprocably turned about its own axis by the lever 50 to 
enable the cam 49 below the carriage base 31 to lift the pawl plate 44 in 
several successive strokes. The pawl plate 44 then causes the ratchet 
wheel 41 to rotate about its own axis and hence the pen holders 37, 38 to 
rotate therewith. When the pen P with a desired ink color is elevated to 
the uppermost position, the solenoid 52 is inactivated. Thereafter, the 
sheet of print paper is moved in the directions of the arrows Xa, Xb and 
the pen carriage 20 is moved in the directions of the arrows Ya, Yb while 
the pen P is pressed against the sheet of print paper by the hammer 65, 
thereby writing characters, figures or the like in the selected color. 
While in the illustrated embodiment the plunger solenoid 52 and the lever 
50 have been described as a mechanism for angularly moving the shaft 21 
through a given angle, other rotational drive mechanisms such as a rotary 
solenoid and a stepper motor may be employed for turning the shaft 21 
about its own axis. The pen holders 37, 38 may be arranged to support 
three, five or more pens P with different ink colors. 
Furthermore, the pens P supported by the pen carriage 20 may be tilted as 
shown in FIG. 11 so that the tip ends of the pens P are directed obliquely 
downwardly for abutment against a sheet of print paper on the platen 6. 
Although a certain preferred embodiment has been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.