Typewriting machines – Including selection of type-face by... – Including error detection
Reexamination Certificate
1999-10-20
2002-01-01
Hilten, John S. (Department: 2854)
Typewriting machines
Including selection of type-face by...
Including error detection
C400S118200
Reexamination Certificate
active
06334720
ABSTRACT:
This application is based on patent application Ser. No. 10-302806 filed on Oct. 23, 1998 in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printing apparatus and a method for correcting print positions and more specifically to a method for registering print positions when a plurality of print heads print in both forward and backward scans.
2. Description of the Prior Art
An ink jet printing method, because of its advantages such as a low noise, a low running cost and ease with which to reduce an apparatus size and produce color images, has found a wide range of applications as in printers and copying machines. Such an ink jet printing method ejects ink droplets from nozzles to form ink dots on printing paper thereby producing an image. In producing a color image, a plurality of print heads are used and the printing is often performed in both directions of scan (hereinafter also referred to simply as a “bidirectional printing”) to enhance a printing speed.
In the bidirectional printing using the plurality of print heads, registration of print positions is relatively important for improving an image quality. In this case, the print position registration is performed for following positional deviations with respect to the scan direction of the print heads. It should be noted that there are two types of bidirectional printing: one performs a plurality of scans over the same area to complete an image and the other performs a single scan over any particular area to complete the printing and feeds paper between the forward scan and the backward scan. This specification assumes mainly the former type of the “bidirectional printing” in the following descriptions but also includes the latter type. In the latter case, the print position deviation becomes critical at a boundary between scanned areas.
1) Print position deviation between forward and backward printing.
2) Print position deviation among a plurality of print heads arranged in a scan direction.
In an apparatus other than the above-mentioned serial type printing apparatus, such as a full line type printing apparatus in which the printing paper is fed with respect to the print heads, the print position registration is performed to eliminate print position deviations among the print heads in the paper feed direction.
A reason for which the print position registration stated above is required is as follows. As for the positional deviation between the forward and backward printing, it is caused by mechanical factors such as backlash. As for the positional deviation among the print heads, particularly when a plurality of replaceable print heads are used, it is caused by variations of tolerance between the print-heads and the carriage holding them.
In the case of the print position registration between the forward and backward scans in the bidirectional printing for example, a practice to deal with the problems described above involves adjusting a print timing during respective forward and backward scans to change relative print position conditions for the forward and backward scans in a plurality of stages and performing forward and backward scans under these adjusted conditions to print on the printing medium an adjustment pattern consisting of a plurality of lines or the like. On the other hand, in the case of the position registration among the plurality of print heads, it is a practice to change the relative print position conditions of each head as the carriage is scanned in a particular direction and similarly print an adjustment pattern consisting of a plurality of lines with each head. The user visually checks these printed patterns, selects the print position condition under which the positions of the printed lines are best aligned, and sets data on the print position condition in a printing apparatus or host computer, thereby correcting the print positions.
The print position correction based on the conventional method described above, however, may pose the following problem when the bidirectional printing is done using a plurality of print heads.
Print positions may deviate due to varying factors, including variations in an ejection angle and a speed of ink droplets ejected from the print head, variations in parallelism between the printing medium and the print head or the carriage holding the print head, and variations in a distance between the print head and the printing medium (also referred to as “paper gap”). To deal with this problem, a conventional method corrects the printing positions as described above and strictly controls manufacturing errors of print heads and a precision of the carriage drive mechanism to minimize the occurrence of the print position deviations per se. This, however, has led to an increase in the cost of the printer and consumables.
The reason that the conventional print position correction method requires an increased precision as described above, in the case of bidirectional printing using a plurality of print heads, will be explained by referring to
FIGS. 1A
to
1
D.
In
FIGS. 1A
to
1
D, a reference numeral
200
denotes a carriage,
201
and
202
print heads,
203
a printing medium such as paper, and
2101
-
2104
ink dots respectively formed on the printing medium with an ink droplet ejected from the print heads.
FIGS. 1A and 1B
represent a relation between the print heads and the printed dots in the case that printing is performed while scanning the print head from left to right in the figure (the scan direction in this case will be taken as a “forward direction”).
FIGS. 1C and 1D
represent the head-and-dot relation in the case that printing is performed while scanning the print head from right to left (the scan direction in this case will be taken as a “backward direction”). Reference signs in the figure are as follows.
X: a physical distance between the nozzle sections of the heads
201
and
202
V
CR
: a moving speed of the carriage
h: a nozzle-to-paper distance or the paper gap
V
1
: a speed of the ink droplet ejected from the head
201
V
2
: a speed of the ink droplet ejected from the head
202
&thgr;
1
: an ejection angle of the ink droplet ejected from the head
201
with respect to a direction normal to a plane of the paper
&thgr;
2
: an ejection angle of the ink droplet ejected from the head
202
with respect to the direction normal to the plane of paper
Here, the ejected ink droplet speeds V
1
, V
2
and angles &thgr;
1
, &thgr;
2
are those when the carriage is at rest. Actual ejection speeds and angles are obtained by combining respective components of the carriage moving speed V
cr
with the respective speeds V
1
, V
2
and angles &thgr;
1
, &thgr;
2
. For the sake of simplicity, such representations are omitted in the figure.
In
FIG. 1A
, the ink droplet ejected when the head
201
reaches a position above the position A on the printing medium during the forward scan forms a dot
2101
. Similarly, in
FIG. 1B
, the ink droplet ejected when the head
202
reaches the position A during the forward scan forms a dot
2102
. These dots land on the paper, deviated by &Dgr;Xf in the case that the ejection speeds V
1
, V
2
and ejection angles &thgr;
1
, &thgr;
2
, characteristics of the heads, vary among the heads.
Similarly, in
FIG. 1C
, the ink droplet ejected when the head
202
is at the position A during the backward scan forms a dot
2103
. Similarly, in
FIG. 1D
, the ink droplet ejected when the head
201
is at the position A during the backward scan form a dot
2104
. In this case, also, these dots land deviated by &Dgr;Xb in the case that the head characteristics differ between the heads.
Amounts of &Dgr;Xf and &Dgr;Xb are affected not only by the print head characteristics V
1
, V
2
, &thgr;
1
, &thgr;
2
but also by the paper gap h and the carriage speed V
CR
. The parallelism between the carriage and the printing medium, also, is in effect reflected on &thgr;
1
, &thgr;
2
, influencing the &Dgr;Xf and &Dgr;Xb. Because of these influencing factor
Kato Masao
Saito Hiroyuki
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Hilten John S.
Nolan, Jr. Charles H.
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