Method of reducing vertical banding in ink jet printing

Details Method of reducing vertical banding in ink jet printing Method of reducing vertical banding in ink jet printing

2001-06-04

2003-07-22

Nguyen, Lamson (Department: 2861)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

C347S014000

Reexamination Certificate

active

06595621

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to inkjet printers. More particularly, the invention relates to a technique for improving the quality of inkjet printing systems by introducing a controlled variance in a scan axis to thus hide vertical bands that may form during the printing process.
BACKGROUND OF THE INVENTION
U.S. application Ser. No. 09/199,882, filed Nov. 24, 1998, entitled “Alignment of Ink Dots in an Inkjet Printer,” by Paul D. Gast et al., is assigned to the present assignee and incorporated herein by reference in its entirety. That application discloses a technique for compensating for misalignment that may occur during printing operations. In this respect, according to the disclosed technique, a test pattern is printed to determine whether any compensation for misalignment is required during the printing of a plot. Offset errors are introduced during the printing of the plot to compensate for misalignment. Thus, that application pertains to the reduction of misalignment that may occur during printing and thus is not concerned with the reduction of vertical banding.
FIG. 1
is a simplified example of a conventional inkjet printer
10
. This example will be used to illustrate some of the problems associated with known ink jet printers. As illustrated in
FIG. 1
, conventional ink jet printer
10
includes an input tray
12
containing sheets of print medium (e.g., paper)
14
which pass through a print zone
15
for being printed upon. The print medium
14
is then forwarded to an output tray
16
. A movable carriage
20
holds print cartridges
22
,
24
,
26
, and
28
, which may hold various colored inks, in addition to black ink. The carriage
20
is traversed along a scan axis by a belt and pulley system and slides along slide rod
30
.
In use, printing signals from an external computer (not shown) are processed by inkjet printer
10
to generate a bitmap of the dots to be printed. The bitmap is then converted into firing signals for the printhead. The position of the carriage
20
as it traverses back and forth along the scan axis is determined from an optical encoder strip
32
, detected by a photoelectric element on carriage
20
, to cause the various ink ejection elements on each printer cartridge to be selectively fired at the proper time during the carriage scan.
FIG. 2
illustrates the printhead portion of a print cartridge, such as print cartridge
22
in
FIG. 1
, while
FIG. 3
is a top down detailed view of a nozzle plate
34
on the print cartridge. Three hundred nozzles
35
arranged in two vertical rows
38
are shown in FIG.
3
. The primitives P1-P14 are labeled on the nozzle plate
34
. The print cartridge
22
has contact pads
36
formed on a circuit which electrically contact electrodes in cartridge
20
for receiving power and ground signals as well as the firing signals for the various ink ejection elements.
FIG. 4
illustrates a portion of the printhead substrate, underneath nozzle plate
34
, associated with a single primitive. The printhead substrate is typically a rectangular piece of silicon having formed on it ink channels
40
, ink ejection chambers
42
, and heater resistors
44
using photolithographic techniques. The various ink channels
40
and chambers
42
are formed by a barrier layer
45
of photoresist. Each chamber
42
is operable to receive ink via an associated ink channel
40
. When current passes through a heater resistor
44
, the received ink is vaporized to cause a droplet of ink to be ejected by an associated nozzle
35
.
To accurately print onto a print medium, the printheads of the ink jet printer devices must be accurately positioned over that portion of the print medium to which it is to print. In addition, the heater resistor
44
must be fired at the correct moment as the printhead is moved along the scan axis. Small inaccuracies due to uncontrolled movements, oscillations, etc., may allow for any faults in the printing output to become visible. Moreover, because the printheads are generally mounted in a mechanical part that moves over the print medium, errors occurring due to the printhead movements over the print medium may additionally adversely affect the accuracy and thus the quality of the printed output.
One manner in which conventional ink jet printer devices attempt to address the above-stated inaccuracies is to utilize a multi-pass printing process. In a multi-pass printing process, each part of the printing output is printed using a different part of the printhead. That is, with reference to
FIG. 3
, each part of the printed output for a given pass is printed with a different nozzle
35
. As illustrated in
FIG. 5
, the multi-pass printing method makes multiple passes
46
-
52
, with the print medium making small advancing movements in the direction
54
between the passes, to generate the desired images. Thus, a desired image is printed with the printhead making a first pass
46
in a scan axis, a second pass
48
in a scan axis, etc. By virtue of the multi-pass printing technique, those areas in which malfunctioning nozzles are to print may be printed upon by at least one functioning nozzle.
However, the multi-pass printing method is not completely immune from defects which may occur during a printing process. By virtue of a plurality of factors, the directionality and placement of the nozzles may become somewhat skewed. For example, if there are any sudden changes in the friction force or defects in the surfaces over which the carriage
20
runs, some oscillations or mechanical shifts may give rise to directionality and placement errors. Additionally, the electrical cables that connect the carriage electronics with external electronics may change the electrical properties (e.g., RC time constants) when the carriage
20
runs along the scan axis. This changes the time delay of the signal transmitted through the cables, thus potentially shifting the time for firing the drops of ink and may give rise to placement errors. In this respect, the placement errors may be effectuated each time the printhead
22
-
28
makes a pass to print an image. Thus, as illustrated in
FIG. 6
, this may result in the formation of vertical bands
56
during a printing operation which implements the multi-pass process.
The vertical bands
56
illustrated in
FIG. 6
may result from the row of nozzles
38
being fired simultaneously. As illustrated in
FIG. 7
, reference numeral
38
represents a row of nozzles which are configured to travel in a scan axis direction
58
. When there is an error
60
, e.g., a fly time delay, mechanical shift, or the like, each of the nozzles
35
in the row of
38
typically fire ink drops
62
some time after the fire signal
64
is received by the printhead
22
-
28
indicated by arrow
66
. In this instance, a vertical row of ink drops may be fired according to the error
60
each time the printhead makes a pass across the scan axis. This may result in the formation of the vertical bands
56
throughout the printed image
55
as illustrated in FIG.
8
.
Accordingly, known multi-pass printing methods have been relatively inadequate to print images on print media without vertical bands when errors occur and therefore, known multi-pass printing methods suffer from a variety of drawbacks and disadvantages.
SUMMARY OF THE INVENTION
According to one aspect, the present invention pertains to a method of reducing vertical banding in a multi-pass printing process. The multi-pass printing process utilizes a printhead having a plurality of nozzles positioned in a substantially non-linear arrangement along a surface of the printhead. The nozzles are operable to fire an ink drop within a predetermined time in response to receipt of a fire signal during each pass of said multi-pass printing process. In the method, a controlled variance is introduced to vary the predetermined time of firing of the ink drop. The controlled variance is configured to vary the predetermined time of firing the ink drop for each pass of the multi-pass printing process.
In accordance with another aspe

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