Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-02-19
2001-09-18
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S039000
Reexamination Certificate
active
06290319
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to machines and procedures for incremental printing of images (which may include text) in two-dimensional pixel arrays, and more particularly to a scanning-printhead machine and method that construct such images from individual colorant spots created on a printing medium, in row-and-column pixel arrays. The invention corrects small, systematic errors in colorant-spot placement that are important in regard to coordination of marks made by different printheads—e.g., in different colors. In some special cases these errors are also significant as to absolute positioning.
The problem solved by the invention, and also the invention itself, will be discussed in terms of thermal-inkjet printing. A person skilled in the art, however, will appreciate that both are applicable to certain other types of incremental printers.
BACKGROUND OF THE INVENTION
1. Importance of Placement Accuracy
Thermal-inkjet printing is based on accurate ballistic delivery of small ink droplets to exact locations on paper or some other printing medium. Ordinarily the droplet placement is with respect to a grid of specified resolution, most common grids nowadays being 12-by-12 or 24-by-24 dots per millimeter (300-by-300 or 600-by-600 dpi). Other possibilities are continuously being considered.
One key requirement for sharp, high-quality images is accuracy of the droplet placement. Drop-placement error (DPE) causes line discontinuity and roughness—especially important in plotters used primarily for computer-aided design (CAD)—as well as banding and color inconsistencies that are significant in printers mainly used to reproduce graphics or photos.
2. Previously Recognized Error Sources, and Solutions
There are several contributors to droplet-placement inaccuracies. Some of these arise in the printhead, and others in the printer mechanism proper; inaccuracies can occur along the scan-axis or paper-axis direction. Some inaccuracies are systematic, while others follow random patterns.
The previously mentioned Majette patent is representative of earlier innovations in encoder subsystems that enable basic determination and servocontrol of printhead position and speed. The Raskin patent teaches how to operate the timing of bidirectionally scanning systems to provide consistent dot placement independent of scanning direction.
The Cobbs and Sievert patents address a still more sophisticated problem, namely control of the mutual alignment of multiple printheads operating on a common scanning carriage. That challenge is met by printing and reading test patterns, to determine the mechanical relationship between the heads on the carriage—and then by, in effect, shifting the operational nozzle arrays on certain of the pens to obtain alignment within specifications.
To facilitate the shifting process, heads are provided with a few extra nozzles at each end, so that the shift is reduced to merely a selection and renaming process. The patents to Cobbs and to Sievert make use of relatively small test patterns automatically printed, and then automatically read.
Other reported efforts make use of laser-based measurements for interpen alignment. This approach likewise is based on measurements taken in a limited-width portion of the printer image space.
3. Newly Discovered Error Mode
Despite these advances, residual errors in interhead alignment have been detected in a current generation of printer/plotters. These errors have an adverse effect on print quality, most conspicuously taking the form of cyan-to-black misalignment in certain portions of standard test images—for instance, particularly where a cyan background appears at one side of a black area fill.
Appearance of these residual errors has been markedly erratic—not arising in every prototype unit but only some units, and also not consistent in all parts of the printed images but rather only within some regions. Furthermore these errors are more severe for some printheads (i.e., certain colors) than others.
On most papers the error appears where vertical lines change color from black to cyan. Also, in plots containing black area fills adjacent to green or violet areas, a certain yellow halo (when green) or magenta halo (when violet) can be seen on misalignments of two pixel columns. In addition these errors are believed to cause higher graininess, mainly in gray area fills.
Awareness of this peculiarity arose in late phases of a product development. As far as we know, no previous worker in this field has attempted to develop an understanding of these mysterious and stubbornly persistent error residuals.
Not only the correction of these defects but also a first recognition of their basic character has only now been revealed. Accordingly a description of the source of these errors is not properly a part of this background section in the present document, and therefore is reserved for a following section summarizing the invention.
A somewhat analogous or related problem of bringing pen-to-paper distance within specifications is treated in the Maher, Castano and Boleda applications mentioned earlier.
4. Conclusion
Small and seemingly erratic dot-placement errors have heretofore impeded achievement of uniformly excellent inkjet printing. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
SUMMARY OF THE DISCLOSURE
The present invention introduces such refinement. Before offering any relatively rigorous outline of the invention itself, however, this summary will begin with a brief informal indication of the nature and source of the elusive errors described above. It is to be understood that this preliminary presentation is not necessarily a statement of the invention as such.
The present invention has proceeded from discovery that the noted residual errors actually are consistent within respective different segments of the scanning sub-system. Furthermore it has been found that the errors are not strictly limited to differential errors between printheads but also extend to absolute errors, as measured by the encoder subsystem.
With these recognitions, the residuals have been traced to imperfections in the printhead-carriage support and guide subsystem. These imperfections directly cause rotations of the carriage relative to the printing medium, and the rotations degrade the relationships between the actual interhead distances and the encoder-measured carriage excursions.
Moreover these same rotations impair the absolute relationship between the actual and measured head positions. The absolute positioning error, since it does not make itself conspicuous in misalignment of marks made by different heads (i.e., marks of different colors) is less important in most applications—but can be significant in special cases when drawings are precisely scaled to provide dimensional analysis of illustrated features.
Specifically the support and guide subsystem includes a rod along which the printhead carriage slides, and a base or so-called “beam” that supports that rod. These components are subject to imperfections in straightness.
In particular the rod has very fine horizontal curvatures—that is, waviness in the horizontal plane, generally parallel to the plane of the printing medium in the printzone—and also in the vertical plane, perpendicular to the plane of the medium. The carriage when translated along the rod accordingly also undergoes very small rotations, respectively about a vertical axis z and about a horizontal axis parallel to the printing-medium advance direction x. (In engineering convention, in this field, the advance direction is more commonly designated “y”; however, for present purposes the notation x will follow that more often seen in the patent literature.)
Rotations in the third dimension (about the axis of the bar) are also possible. These “Theta-Y” (&thgr;y) rotations implicate the straightness and parallelism of yet another component—a follower bar—as well as the support/guide rod, and they have a different kind of significance.
Because all the pens are v
Boleda Miquel
Montaclair Guillaume
Pozuelo Francisco-Javier
Ashen & Lippman
Barlow John
Hewlett--Packard Company
Tran Ly T
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