Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2000-10-20
2002-12-17
Hallacher, Craig A. (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06494558
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to incremental printing, with transversely scanning printheads, onto a printing medium that is also advanced along a longitudinal axis—and more particularly to marking-position errors in the printing-medium-advance axis. These errors are particularly important in that they create perceptible and in some cases conspicuous defects in printed images.
BACKGROUND OF THE INVENTION
(a) The printer mechanism—The invention is amenable to implementation in a great variety of products. It can be embodied in a printer/plotter that includes a main case
1
(
FIG. 1
) with a window
2
, and a left-hand pod
3
which encloses one end of the chassis. Within that enclosure are carriage-support and—drive mechanics and one end of the printing-medium advance mechanism, as well as a pen-refill station with supplemental ink cartridges.
The printer/plotter also includes a printing-medium roll cover
4
, and a receiving bin
5
for lengths or sheets of printing medium on which images have been formed, and which have been ejected from the machine. A bottom brace and storage shelf
6
spans the legs which support the two ends of the case
1
.
Just above the print-medium cover
4
is an entry slot
7
for receipt of continuous lengths of printing medium
4
. Also included are a lever
8
for control of the gripping of the print medium by the machine.
A front-panel display
11
and controls
12
are mounted in the skin of the right-hand pod
13
. That pod encloses the right end of the carriage mechanics and of the medium advance mechanism, and also a printhead cleaning station. Near the bottom of the right-hand pod for readiest access is a standby switch
14
.
Within the case
1
and pods
3
,
13
a cylindrical platen
41
(FIG.
2
)—driven by a motor
42
, worm
43
and worm gear
44
under control of signals from a digital electronic processor—rotates to drive sheets or lengths of printing medium
4
A in a medium-advance direction. Print medium
4
A is thereby drawn out of the print-medium roll cover
4
.
Meanwhile a pen-holding carriage assembly
20
carries pens back and forth across the printing medium, along a scanning track—perpendicular to the medium-advance direction—while the pens eject ink. The medium
4
A thus receives inkdrops for formation of a desired image, and is ejected into the print-medium bin
5
.
As indicated in the drawing, the image may be a test pattern of numerous color patches or swatches
56
, for reading by an optical sensor to generate calibration data. For present purposes, such test patterns are for use in detecting positioning errors.
A small automatic optoelectronic sensor
51
rides with the pens on the carriage and is directed downward to obtain data about pen condition (nozzle firing volume and direction, and interpen alignment). The sensor
51
can readily perform optical measurements
65
,
81
,
82
(FIG.
7
); suitable algorithmic control
82
is well within the skill of the art, and may be guided by the discussions in the present document.
A very finely graduated encoder strip
36
is extended taut along the scanning path of the carriage assembly
20
and read by another, very small automatic optoelectronic sensor
37
to provide position and speed information
37
B for the microprocessor. One advantageous location for the encoder strip
36
is immediately behind the pens.
A currently preferred position for the encoder strip
33
(FIG.
3
), however, is near the rear of the pen-carriage tray—remote from the space into which a user's hands are inserted for servicing of the pen refill cartridges. For either position, the sensor
37
is disposed with its optical beam passing through orifices or transparent portions of a scale formed in the strip.
The pen-carriage assembly
20
is driven in reciprocation by a motor
31
—along dual support and guide rails
32
,
34
—through the intermediary of a drive belt
35
. The motor
31
is under the control of signals from the digital processor.
Naturally the pen-carriage assembly includes a forward bay structure
22
for pens—preferably at least four pens
23
-
26
holding ink of four different colors respectively. Most typically the inks are yellow in the left-most pen
23
, then cyan
24
, magenta
25
and black
26
.
Another increasingly common system, however, has inks of different colors that are actually different dilutions for one or more common chromatic colors, in the several pens. Thus different dilutions of black may be in the several pens
23
-
26
. As a practical matter, both plural-chromatic-color and plural-black pens may be in a single printer, either in a common carriage or plural carriages.
Also included in the pen-carriage assembly
20
is a rear tray
21
carrying various electronics. The colorimeter carriage too has a rear tray or extension
53
(FIG.
3
), with a step
54
to clear the drive cables
35
.
FIGS. 1 through 3
most specifically represent a system such as the Hewlett Packard printer/plotter model “DesignJet 2000CP”, which does not include the present invention. These drawings, however, also illustrate certain embodiments of the invention, and—with certain detailed differences mentioned below—a printer/plotter that includes preferred embodiments of the invention.
(b) Relatively direct PAD-derived banding—In the images produced by a printer of this type, defects called “banding” appear where unprinted, lightly printed, or double-printed pixel rows occur repetitively. Relatively direct sources of such errors include:
Print-medium-Axis Directionality or Pen-Axis Directionality “PAD” (FIG.
4
), i.e. an angling of inkdrop trajectories particularly near the ends of the nozzle array
12
,
variations &Dgr;PAD in the latter error,
variations &Dgr;PH in the pen height above the printing medium, and
rotation &thgr;
z
of a printhead about the pen-to-print-medium axis z.
A dominant error source is PAD, which may be seen as analogous to camber. It can be either inboard or outboard.
It operates through the height PH of a pen above the print medium—including the height variations &Dgr;PH—to extend or contract the swath length
14
in the pen-length direction on the printing medium, the so-called “paper swath length”.
(In this document the term “height” is reserved for actual vertical height of the pen above the medium; hence the dimension of the swath in the long dimension of the medium, though commonly called “swath height”, is here instead termed “swath length”. Please do not confuse this dimension with the width of the swath, i.e. its dimension transversely across the medium in the carriage-scan axis.)
To reduce visible banding, operating strategies can compensate for such extension or contraction by a matching extension or contraction of the printing-medium-advance stroke. In other words, unprinted or double-printed pixel rows can be avoided by matching the stroke to the actual swath length.
This may be regarded as “compensation”, though in a sense it is the opposite—namely, accommodation—and actually results in image deformation. Both the actual swath length and its matched stroke are different from the nominal swath length that constitutes a basic unit of the image.
Small resulting differences in shapes within an image can sometimes be detected. Accumulated differences due to such deformation can be seen as variation of overall printed image length.
The drawing shows a platen of the roller type, with print medium wrapped around the roller. In this geometry, swath-length extension is aggravated by the fact that the print-medium surface progressively recedes from the nozzle array
12
at all points off-axis relative to the pen-to-medium axis z.
This aggravating effect, in turn, is greatly complicated by any departure of the roller and printing-medium surfaces from nominal conditions, still relatively direct:
radius error &Dgr;radius from the nominal,
thickness of the printing medium, or difference of that thickness from any assumed nominal thickness,
runout or out-of-round, i.e. departure of the platen from circularity, and
eccentricity of th
Doval Jose Julio
Serra Albert
Subirada Francesc
Ashen & Lippman
Hallacher Craig A.
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