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The invention relates to moving carriage ink jet printing where printing
occurs while the carriage is being moved in both directions, i.e.,
bidirectionally. Either the carriage or the record member or both may be
moved; it is the relative motion that is pertinent to this invention. The
relative carriage velocity is imparted to the ink jet droplets resulting
in droplet offset. This offset can become noticeable when the
carriage-to-medium velocity becomes appreciable with respect to the
droplet velocity. Where a single figure, such as a vertical line, is being
formed by droplets expressed while the carriage is moving in both
directions, this velocity offset imparted to the droplets can give the
resulting character or figure a jagged appearance. This can cause a print
quality problem when more than one carriage pass is needed to print a
character or continuous figure. To eliminate this problem, the lines of
characters are printed two at a time in such manner that each line is
produced from the carriage while the carriage is moving in one direction
only.
The foregoing advantages and features of the present invention will be
apparent from the following more particular description of a preferred
embodiment as illustrated in the accompanying drawing wherein:
FIGS. 1A and 1B illustrate how the velocity of the moving carriage causes
droplet offset on the record surface.
FIG. 2 shows how the droplets can appear on a record surface where droplet
velocity offset is not compensated for.
FIG. 3 shows how two lines of print are formed alternately, each being
produced while the carriage is moving in only one direction.
FIG. 4 shows the direction of movement of the record surface and the moving
carriage.
FIGS. 5 and 6 show schematically other possible scanning combinations.
Referring to FIG. 1A, ink jet nozzle 1 is moving in the direction shown by
arrow R. When a droplet is ejected from nozzle 1 in response to an
electrical signal operating on a transducer, the droplet, instead of
moving directly to record surface 3 along path 5, follows a trajectory
represented by line 7 resulting in offset dR. Similarly, referring to FIG.
1B, which shows ink jet nozzle 1 moving in direction L resulting in offset
dL. Where a single figure is produced by droplets expressed from ink jet
nozzle 1 moving in both directions R and L, the resulting image will have
droplets offset from each other by a distance of as much as dR plus dL.
In FIG. 2 centerline 9 represents the center point of droplets where they
would be if the nozzle were not being moved. That is, the droplets follow
path 5 in FIGS. 1A and 1B. Dots R, however, represent the droplet
positions on record surface 3 where nozzle 1 is moving in the direction R
as shown in FIG. 1A when droplets are being ejected. Dots L show the
position of droplets on record surface 3 resulting from the direction L
movement of ink jet nozzle 1 being imparted to droplets ejected from
nozzle 1. dR and dL again represent the velocity imparted droplet offset.
It can be seen that where a single figure represented as a vertical line
in FIG. 2 is formed by an ink jet nozzle moving in both directions that a
jagged appearance can result. Of course, this velocity imparted droplet
offset can be compensated for electronically by properly programming the
pulse controller for ink ejection. Such systems are, however, relatively
expensive and might not be economically feasible for use in inexpensive
marking devices.
Referring now to FIG. 3, there is shown an example of how the characters
are formed in the present invention. Two lines of printing are shown; one
beginning with a "T" and ending with an "I", the second beginning with an
"E" and ending with an "F". The lines can be of equal length, as shown in
FIG. 3, but, more commonly, will be unequal. In this specific exemplary
embodiment and for purposes of explanation, it is assumed that only one
ink jet nozzle 1 is being utilized. The same principle of operation
obviously could apply to a multiple nozzle arrangement. The nozzle is
caused to scan record surface 3 from left to right and from right to left
as shown by arrows R and L in FIG. 4. As the nozzle is scanned over the
predetermined lines of print on the record surface, a transducer is
triggered by an electrical input to eject droplets as is well known in
drop-on-demand type ink jet systems. Assume the nozzle 1 is making a first
scan from left to right as shown in FIG. 3 along path 1R. When nozzle 1
finishes the scan across record surface 3 in a left to right direction, it
will have completely printed the first row of the line. During turnaround
of the nozzle, record surface 3 is caused to move in the direction shown
as U in FIG. 4 an amount that results in the ink jet nozzle tracing path
1L as it returns from right to left. This amount of movement of record
surface 3 is referred to herein as a complete line, which includes the
printed line 11 and the space between lines 13. It can be seen that when
nozzle 1 has completed its return trip 1L, record surface 3 must be moved
in a direction d an amount equal to a complete line minus the distance of
one droplet row so that nozzle 1 traces path 2R. Nozzle scanning and
record surface cycling is continued in the same manner through 7L. At the
end of path 7L, instead of cycling record surface 3 a distance of a
complete line minus one row in a direction d, it is instead cycled in a
direction U an amount of one line space, which is the distance between the
bottom of one line and the top of the next line, again 1R, at which point
two more lines are printed in the same manner as described above.
It can readily be seen that the same principle would apply if two or more
nozzles were used except that the record surface would be moved further
for each nozzle carriage pass. Also there are situations where it is
desirable to make a second pass over a line of characters to, for example,
darken the characters by laying a second set of droplets on the character
or for interlacing or for multicolor printing. The present invention can
be used for those situations as well. For example, assume seven nozzles
are used spaced one droplet row apart vertically so that in a single pass
a complete line is printed. Record surface 3 would then be moved a
complete line in the direction U so that the next line is printed on the
return pass. The record surface 3 is then moved in the direction d a
complete line so that the nozzles traverse the same rows, again both left
to right and right to left, after which record surface 3 is moved in a
direction U a complete line so that two more lines may be printed. For
carrying out the method of the present invention, apparatus such as that
shown in U.S. Pat. No. 3,787,884 to F. M. Demer or U.S. Pat. No. 4,207,579
to R. L. Gamblin et al or the scanning carriage printer used in, for
example, commercial Siemens ink jet printers could be utilized.
The term "line" as used herein is not necessarily restricted to be a row of
alphanumeric characters. For example, one line may be interpreted to be a
graph or figure. This line is then paired with another line for printing.
This second line may contain a similar figure or may be rows of
alphanumeric characters or a combination of both. These two lines may even
be printed with an unequal number of carriage passes. They are paired
together only for as long as is required to print the first line of the
pair unidirectionally; then the second line of the pair becomes the first
line of a new pair for as many passes as is required to finish printing
that line unidirectionally. For example, in FIG. 5, the following sequence
occurs:
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1. Left to Right Line A
2. Right to Left Line B
3. Left to Right Line A
4. Right to Left Line B
5. Left to Right Line C
6. Right to Left Line B
7. Left to Right Line C
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This provides two scans of lines A and C and three scans of line B.
If multiple jet nozzles are utilized, each nozzle may or may not
participate in printing any particular line. One example of printing in
which such jet skipping may occur is in alphanumeric printing with an
array of jets that spans a vertical distance greater than at least one
line of the pair. A second example is in color printing where the colors
might be printed sequentially in each pair of lines, with one or more
scans required for each color. Lines requiring unequal numbers of carriage
passes can also be paired as needed, with a new member of a pair being
selected whenever either of the two previous members has been completed.
For example, in FIG. 6, line A is produced in four scans, line B in two
scans, line C in three scans and line D in one scan.
The scope of this invention is not limited to printing the "lines" in
sequence as they are to appear on the medium or to including the full
length of a line in each scan. For example, if several pictorial figures
are to be printed along with interspersed alphanumeric text, it may be
desirable to pair the figures for printing first in the manner as has
herein been described, then pair the text lines for printing wherever they
occur. As a second example, one large pictorial figure might be divided
into a "pair" for printing, with the top half of the figure becoming the
first line of the pair and the bottom half, the second line.
While the apparatus and methods described herein constitute the preferred
embodiment of the invention as presently contemplated by the inventor, it
is to be understood that the invention is not limited to these precise
forms and that changes may be made therein without departing from the
scope of the invention. For example, the same technique can be applied to
vertical scanning printing when the characters are positioned at a
constant pitch distance. In this case, the character positions in each
line are paired for printing, instead of pairing the lines as has been
previously described.
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