Alignment of ink dots in an inkjet printer

Details Alignment of ink dots in an inkjet printer Alignment of ink dots in an inkjet printer

1998-11-24

2002-04-09

Barlow, John (Department: 2853)

Incremental printing of symbolic information

Ink jet

Controller

C347S019000

Reexamination Certificate

active

06367903

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to inkjet printers and, in particular, to a technique for improving the alignment of dots printed by an inkjet printhead.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,638,101, entitled High Density Nozzle Array for Inkjet Printhead, by Brian Keefe et al., and U.S. Pat. No. 5,648,806, entitled Stable Substrate Structure for a Wide Swath Nozzle Array in a High Resolution Printer, by Steven Steinfeld et al., are assigned to the present assignee and incorporated herein by reference. These two patents describe examples of an inkjet printer, incorporating an inkjet print cartridge, whose operation may be improved by the present invention. The below description of primitives used in printheads is taken from those two patents.
FIG. 1
is a simplified example of an inkjet printer
10
. This will be used to illustrate the problem with prior art printers and, later, will also serve as a printer whose operation has been improved after being modified to incorporate the present invention. Inkjet printer
10
includes an input tray
12
containing sheets of paper
14
which pass through a print zone
15
for being printed upon. The paper
14
is then forwarded to an output tray
16
. A moveable carriage
20
holds print cartridges
22
,
24
,
26
, and
28
, which respectively hold yellow, magenta, cyan, and black inks. The carriage
20
is moved along a scan axis by a conventional belt and pulley system and slides along a slide rod
30
.
Printing signals from an external computer are processed by printer
10
to generate a bit map of the dots to be printed. The bit map 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 print cartridge to be selectively fired at the appropriate time during a 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
are shown. The primitives P
1
-P
14
(to be described later) are labeled on the nozzle plate
34
. The print cartridge
22
has contact pads
36
formed on a TAB 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 a rectangular piece of silicon having formed on it ink channels
40
, ink ejection chambers
42
, and heater resistors
44
using photolitographic techniques. The various ink channels
40
and chambers
42
are formed by a barrier layer
45
of photoresist. Ink flows into each chamber
42
via an associated ink channel
40
. When current passes through a heater resistor
44
, ink is vaporized to cause a droplet of ink to be ejected by an associated nozzle. Each ink channel
40
is designed to reduce cross-talk between the ink chambers
42
when fired.
To further reduce cross-talk, and to simplify the firing electronics and wiring, the heater resistors
44
are divided into primitives.
FIG. 4
illustrates a single primitive having
22
heater resistors
44
.
FIG. 5
illustrates firing circuitry on the substrate for a single heater resistor
44
. To fire resistor
44
, an address pulse is provided on address select line
46
to turn on drive transistor
47
, and a primitive select pulse is provided on primitive select line
48
to cause a current to flow through resistor
44
sufficient to heat the resistor to a temperature needed to vaporize ink within the ink ejection chamber. Electrostatic discharge protection FETs
50
drain unwanted electrostatic charges, and a pull-down resistor
52
places all unaddressed select lines
46
in an off state.
All heater resistors
44
within a primitive receive the same primitive select signal, but only one of the resistors in a primitive at a time receives an address signal. This is illustrated in
FIG. 6
where address signals A
1
through A
22
are generated in sequence for associated heater resistors
44
within each primitive during a single firing cycle while the printhead is scanning across the medium.
More particularly, the address select lines
46
(
FIG. 5
) are sequentially energized according to a firing order counter located in the printer from A
1
to A
22
when printing from left to right and from A
22
to A
1
when printing from right to left. The print data retrieved from the printer memory causes the print engine to energize any combination of the primitive select lines at the appropriate times during the firing cycle. The primitive select pulses rather than the address select pulses are preferably used to control the resistor current pulse width, as shown in FIG.
7
. This is more desirable than using the address select pulses to control the pulse width since terminating an address pulse while the drive transistors
47
(
FIG. 5
) are conducting high current can cause avalanche breakdown and consequent physical damage to the MOS drive transistors. Accordingly, the address select lines are set before power is applied to the primitive select lines, and, conversely, power is turned off before the address pulse is removed. To provide uniform energy per heater resistor
44
, only one resistor is energized at a time per primitive. However, any number of the primitives may be enabled concurrently. Each enabled primitive select pulse thus delivers both power and one of the enable signals to the drive transistor. Each address select line is tied to a corresponding address select line in all the other primitives.
Modern print cartridges may print on the order of 300 or 600 dots per inch (DPI), and the width of a printhead along the direction of the column of nozzles may be ½ inch or greater.
Due to various factors, it is extremely difficult to print precisely aligned dots on the medium as the printhead is scanning across the medium.
FIG. 8A
illustrates an ideal vertical line
54
of connected dots printed during a single scan of the printhead across the medium.
FIG. 8B
is an exaggerated example of the actual line
56
printed during a single scan which was intended to convey a vertical line. The primitives from
FIG. 3
used to print the line are identified in FIG.
8
B. The skewing of the line at an angle with respect to the vertical axis
57
is due to the tilting of the print cartridge within the carriage in combination with the tilting of the printhead substrate with respect to the print cartridge. The wavering of the line
56
is due to a number of factors. One of the factors is the variation in the directionality of the ink droplets ejected from the nozzles. Another factor is that the paper may not be perfectly parallel to the plane of the nozzle plate. Another factor is that the nozzle plate may not be perfectly planar. Another factor is the different parasitic capacitances associated with the primitives.
Also, nozzles are formed in two offset columns, as shown in
FIG. 3
, to increase the density of dots in the direction perpendicular to the scan direction. To print a solid vertical line, the nozzles in the two columns must be fired so as to print dots which partially overlap on the medium. Thus, if the dots printed by the two columns of nozzles are not aligned precisely, distortion of the vertical line will result.
If the vertical line is made up of dots from different printheads, as would be for a composite color line, a blurring of the line would result by the nozzles in the printheads of the various print cartridges not being aligned with respect to each other.
The dot placement due to printhead misalignment gets worse when the printhead length is increased. Longer printheads enable higher throughputs but the manufacturing processes are not a

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