Dot printing with partial double scanning of raster lines

Details Dot printing with partial double scanning of raster lines Dot printing with partial double scanning of raster lines

1998-04-30

2001-02-20

Le, N. (Department: 2861)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06190001

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique of recording dots on a printing medium with a dot-recording head.
2. Description of the Related Art
Ink jet printers such as serial scan-type printers and drum scan-type printers are dot recording apparatus that record dots on a printing medium with a dot-recording head while carrying out scans both in a main scanning direction and in a sub-scanning direction. An ink jet printer has a plurality of nozzles formed on a print head to spout ink and thereby print characters and images on a printing medium. Each nozzle on a print head has a pressure chamber filled with ink and an electrical-to-mechanical conversion element. Application of an electric signal to the electrical-to-mechanical conversion element produces a pressure in the pressure chamber and causes ink droplets to jet out from the nozzle.
Picture quality improvement has been one of major issues for ink jet printers. One proposed technique is the “interlace printing” disclosed in U.S. Pat. No. 4,198,642.
FIG. 16
shows a conventional interlace printing scheme. A print head
1
has eleven nozzles #
1
-#
11
. A pitch k of the nozzles in the sub-scanning direction is set equal to 4 dots. Here the unit [dot] is defined as a minimum pitch P [inch] of dots in the sub-scanning direction recorded on the printing medium, and thus k dots correspond to k×P inches. In
FIG. 16
, the position of the print head
1
shown as pass
1
, pass
2
, or the like represents the position in the sub-scanning direction in each main scan. The term “pass” means one main scan. After each main scan, a sub-scan feed is carried out by a fixed amount F of 11 dots.
In the conventional interlace printing, the following two conditions are set to prevent skipping and overwriting of main scanning lines (hereinafter also referred to as “raster lines”):
[Condition 1] Number of used nozzles N and nozzle pitch k being relatively prime. (Two integers are said to be “relatively prime” if they have no common denominator other than 1.)
[Condition 2] Sub-scan feed amount F being identical with number of used nozzles N.
Printing speed increase and picture quality improvement are two major issues for the ink jet printers. The number of nozzles provided on a print head is to be increased to raise the printing speed In the interlace printing scheme, since the sub-scan feed amount F is set equal to the number of used nozzles N, the increase in the number of nozzles increases the sub-scan feed amount.
Mechanical accuracy of the sub-scan feed is, however, worsened substantially in proportion to the increase in sub-scan feed amount. The increase in the number of nozzles thus results in worsening the accuracy of the sub-scan feed. Especially when plural cycles of sub-scan feeds are carried out between recording of two adjacent raster lines, the feed errors due to the plural cycles of sub-scan feeds are accumulated and thereby significantly changes a pitch between the adjacent raster lines from a normal pitch. For example, in
FIG. 16
, three sub-scan feeds are carried out between the main scan of the second raster line and that of the third raster line. The pitch between these two raster lines is accordingly affected by the accumulated errors due to the sub-scan feeds.
FIG. 17
shows dots recorded in the conventional interlace printing scheme of FIG.
16
. The pitch between the second raster line and the third raster line is increased by the accumulated errors due to the sub-scan feeds. This causes observable strip-like deterioration of the picture quality, which is called the “banding”. Since the banding deteriorates the picture quality, it has been long demanded to reduce the occurrence of banding.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to reduces the occurrence of banding in a printed image.
In order to attain the above and other objects of the present invention, there is provided an apparatus for recording dots on a surface of a printing medium with a dot-recording head. The apparatus comprises: a dot-forming element array comprising a plurality of dot-forming elements which are arranged at a substantially constant pitch in a sub-scanning direction on the dot-recording head to face the printing medium and form a plurality of dots of the same color; a main scanning driver which drives the printing medium to carry out a main scan; a head driver which activates at least part of the plurality of dot-forming elements to form dots in the course of the main scan; and a sub-scanning driver which drives the printing medium to carry out a sub-scan every time when the main scan is complete. The sub-scanning driver carries out a sub-scan feed by a constant amount F×P (where P denotes a minimum pitch of dots in the sub-scanning direction and F is an integer). When the pitch of the plurality of dot-forming elements in the sub-scanning direction is expressed as k×P (where k is an integer of not less than 3), a number of dot-forming elements N used in one main scan (where N is an integer of not less than 3) and parameters Na, Nb, m, and L satisfy Equations (1)-(4)
N=Na+Nb
  (1)
Na=m×k±
1  (2)
Nb=Rd(L×Na÷k)
  (3)
F=Na
  (4)
where m is an integer of not less than 1, L is an integer satisfying a relation of 1≦L<k, and an operator Rd( ) denotes an operation of rounding a decimal fraction in parentheses.
In the dot recording apparatus of the present invention, there are two types of main scanning lines: first type of main scanning lines are recorded only by one nozzle and second type of main scanning lines are recorded by two nozzles. The two types of main scanning lines are arranged substantially in a regular manner to complete recording of dots with respect to one color. The second type of main scanning lines are recorded by two nozzles and thereby reduce the occurrence of “banding”. The second type of main scanning lines, however, require twice the scanning time of the first type of main scanning lines and thus halves the recording speed If the conditions of Equations (1) through (4) are satisfied, some main scanning lines are of the first type. This relieves the decrease in recording speed, compared with recording of all the main scanning lines as the second type.
The head driver may drive the dot-recording head to cause dots recorded by Nb dot-forming elements and dots recorded by Na dot-forming elements to have a complementary positional relationship on each main scanning line. Alternatively, the head driver may drive the dot-recording head to cause dots recorded by Nb dot-forming elements to overlap dots recorded by Na dot-forming elements on each main scanning line.
In a preferred embodiment, the dot-recording head comprises a plurality of the dot-recording element arrays which are used to record dots of plural colors, respectively; and the dot-recording elements in the plurality of dot-recording element arrays are arranged so that the plurality of dot-recording element arrays can record identical main scanning lines during one main scan. The head driver drives the dot-recording head to record dots in both ways of reciprocating main scan motion. This arrangement enables the difference in color between the main scanning lines recorded in the respective ways of the reciprocating motion to be inconspicuous.
The present invention is also directed to a method of recording dots and to a computer program product implementing the above scheme.
These and other objects, features, aspects, and advantages of the resent invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.


REFERENCES:
patent: 4198642 (1980-04-01), Gamblin
patent: 5070345 (1991-12-01), Lahut et al.
patent: 5359355 (1994-10-01), Nagoshi et al.
patent: 5614931 (1997-03-01), Koike et al.
patent: 5708463 (1998-01-01), Hirabayashi et al.
patent: 0 6

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