Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2002-03-28
2004-06-29
Meier, Stephen D. (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C400S074000
Reexamination Certificate
active
06755499
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to printer devices, and particularly, although not exclusively, to a method and apparatus for determining and correcting misalignments between printheads in ink jet devices.
BACKGROUND TO THE INVENTION
It is known to produce paper copies, also known as “hard” copies of files stored on a host device, e.g. a computer using a printer device. The print media onto which files may be printed includes paper and clear acetates for use in lectures, seminars and the like.
Referring to
FIG. 1
, there is illustrated a conventional host device
1
, in this case a personal computer, linked to a printer device
2
via a cable
3
. Amongst the known methods for printing text or graphics and the like onto a print media such as paper it is known to build up an image on the paper by spraying drops of ink from a plurality of nozzles.
Referring to
FIG. 2
, there is illustrated schematically part of a prior art printer device comprising an array of printer nozzles
4
arranged into parallel rows. The unit comprising the arrangement of printer nozzles is known herein as a printhead. In a conventional printer of the type described herein, the printhead
5
is constrained to move in a direction
6
with respect to the print media
7
e.g. a sheet of A4 paper. In addition, the print media
7
is also constrained to move in a further direction
8
. Preferably, direction
6
is orthogonal to direction
8
.
During a normal print operation, printhead
5
is moved into a first position with respect to the print media
7
and a plurality of ink drops
9
a
,
9
b
are sprayed from a number of printer nozzles
4
contained within printhead
5
. This process is also known as a print operation. After the completion of a print operation the printhead
5
is moved in a direction
6
to a second position and another print operation is performed. In a like manner, the printhead
5
is repeatedly moved in a direction
6
across the print media
7
and a print operation performed after each such movement of the printhead
5
. In practice, modern printers of this type are arranged to carry out such print operations while the printhead is in motion, thus obviating the need to move the printhead discrete distances between print operations. When the printhead
5
reaches an edge of the print media
7
, the print media is moved a short distance in a direction
8
, parallel to a main length of the print media
7
, and further print operations are performed. By repetition of this process, a complete printed page may be produced in an incremental manner.
Since the advent of colour printing, printers with more than one printhead are typically used. Generally, four printheads are used, each storing and printing a different colour; for example: cyan; magenta; yellow; and black. The inks from the four printheads are mixed on the print media to obtain any other particular colour.
However, full colour printing requires that the inks from the individual printheads are accurately applied to the print media.
In order that this may be achieved, precise alignment of the various printheads is required. The mechanical misalignment of a printhead may result in an offset in the positioning of ink drops on the print media. Such offsets may occur in the X direction (in the media advance/media axis) or the Y direction (in the carriage/scan axis). Additionally, angular offsets may also arise. If each printhead in a printer is not sufficiently accurately aligned with the remaining printheads of the printer, a misregistration between the images formed by the different coloured ink drops on the print media may result. This may cause too much ink to be deposited in some areas and too little ink to be deposited in others. This often gives rise “grainy” appearance in the printed image. This type of print error is often particularly noticeable to the viewer. Consequently, such misregistrations are generally unacceptable, with colour printing typically requiring image registration accuracy from each of the printheads of {fraction (1/2400)} inch.
Various systems have been devised to address misregistration. In particular, systems have been devised in order to ensure that offsets in the X direction (media axis) are reduced to acceptable levels. One such known system employs a unitary colour printhead, which contains the nozzles of each ink colour: cyan; magenta; and yellow. Thus, the nozzles of each ink colour may be accurately aligned with those of the other colours on manufacture. Thus, when the printhead is mounted in the print carriage of a printer, the positions of the nozzles of each ink colour are constrained with respect to each other. In this way, the operator need only ensure that the colour printhead is correctly aligned with the black ink printhead.
In this system, this is achieved by printing two overlying alignment patches on the print medium, one with the black ink printhead and the other with the colour printhead. Each alignment patch consists of a series of parallel lines. However, the spacing of the lines of the two alignment patches is slightly different, thus giving rise to an interference pattern. When the alignment patches have been printed, the operator manually inspects them to determine the position in the overlying alignment patches of the maximum or minimum ink density. From this information, the relative offset between the two printheads in the media feed direction may be determined.
Once this determination has been made, the processor of the printer compensates for any offset in the media feed direction between printheads by avoiding using those nozzles in each printhead that extend in the media feed direction beyond the nozzles of the other printhead. The processor of the printer also resets the “logical zero” in terms of the nozzles' numbering in each printhead. That is to say that the nozzles which are to be used in each printhead are re-numbered, where necessary, such that the nozzles in each printhead which correspond in terms of their position along the media feed direction are allocated the same number, in order to ensure correct registration between the images printed by the different printheads. In this manner, the print output of the two printheads may be aligned at the expense of a slightly reduced number of usable nozzles.
This technique suffers from the disadvantage that it is relatively slow, being non-automated and reliant upon an operator. Furthermore, the process is less suitable for use in printers having more than two printheads, due to the increased difficulty of determining the relative offsets for a greater number of printheads.
A second type of known system is generally used on large format ink jet printers, which employ separate printheads for each ink colour. In order to ensure that no misregistration occurs between the images formed by the different coloured ink drops on the print medium, an alignment routine is performed.
In this routine, alignment patches are printed across the sheet of print media with each printhead so that they are approximately aligned along the scan axis; i.e. in a direction perpendicular to the media feed direction. The positions of the alignment patches in the media feed direction are then measured using an optical scanner, often referred to as a line scanner, which is mounted on the printer carriage. This is achieved for each alignment patch by positioning the line scanner at the appropriate point along the scan axis so as to be able to detect the alignment patch and then feeding the print media backwards (i.e. in a reverse feed direction) so that the position of the patch on the media in the media feed direction may be determined. The line scanner is then positioned at the appropriate point along the scan axis to detect the next alignment patch and the print media is fed forwards once again in readiness for determining the position of the next patch in the media feed direction. Once the position of each alignment patch in the media feed direction has be determined in this manner, the relative offsets in the media
Castano Jorge
Toussaint David
Hewlett--Packard Development Company, L.P.
Meier Stephen D.
Stewart, Jr. Charles
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