Method for ink jet printing

In moving carriage ink jet printers, the motion of the carriage may be imparted to droplets expressed offsetting the droplets' position on the record surface from one another where bidirectional printing is utilized. To eliminate print distortion resulting therefrom, lines are printed two at a time, each line thus being printed by the carriage moving in only one direction.

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.

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: 
______________________________________ 
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 
______________________________________ 
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.