Note: Descriptions are shown in the official language in which they were submitted.
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MICROCHANNEL MARKING ENGINE
BACKGROUND OF THE INVENTION
The present invention concerns printing. In
particular, it has application to portable color printers.
The printing art is quite mature. Vast amounts of
research and expense have been dedicated to optimizing the
quality and minimizing the cost of printers, particularly
those intended for the consumer market. Given the
difficulty of meeting the demands of the human eye, the
results of these efforts have in fact beers remarkable.
Still, the techniques employed to achieve these results have
tended to be complicated and expensive.
Printers early employed in offices for small-
computer output employed hammers and print-wire matrices.
These were noisy and slow and produced low-quality output.
Quality improved with the use of thermal printers, but these
required special paper and tended to be slow, too. A
greater quality advance accompanied the advent of laser
printers, but their mechanisms are complicated, and they
remain relatively expensive despite the high volumes in
which they have been produced. And none of these
technologies lend themselves well to color imaging.
Ink-jet and ink-bubble technologies have addressed
these shortcomings to a significant extent. Ink-jet
printers squirt charged ink at the paper, deflecting the ink
electrostatically to direct it to the desired location.
This approach is simple in comparison with, say, laser
printers, and it lends itself to color printing, since
successive jets of different-colored ink can be applied to
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the same locations. Ink-bubble approaches are similarly
direct: they employ explosive energy to propel ink drops to
the paper from an array of sources. But the ballistic
nature of the ink delivery in both of these approaches tends
to make the image quality quite dependent on the type of
paper or other image medium.
SUI~fARY OF THE INVENTION
Despite the apparent simplicity of a ink-jet and
ink-bubble approaches, I have devised a way of printing in a
manner that is even simpler and lends itself to embodiment
in printers that are more robust, faster, and less
expensive.
According to one aspect, there is provided for
marking on an image medium, an apparatus comprising: A) a
print head forming an ink capillary having a capillary
outlet and can be filled with ink that tends to remain in
the capillary by capillary action and has a dielectric
constant that exceeds the dielectric constant of air; B) a
positioning mechanism that places the image medium in
proximity to the capillary outlet; and C) a capillary driver
operable in response to image-data signals selectively so to
impose a potential difference across the capillary outlet,
in accordance with image data that the image-data signals
represent, as to create an electric-field gradient that
forces ink from the capillary through the capillary outlet
into contact with an image medium placed by the positioning
mechanism in proximity to the capillary outlet.
According to a second aspect, there is provided
for marking on an image medium, a method comprising the
steps of: A) providing a print head that forms an ink
capillary having a capillary outlet; B) positioning an image
medium adjacent the capillary outlet; C) filling the ink
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capillary with ink that tends to remain in the ink capillary
by capillary action and has a dielectric constant exceeding
the dielectric constant of air; and D) so imposing a
potential difference across the capillary outlet as to
create an electric-field gradient that forces the ink
through the capillary outlet into contact with the image
medium.
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In accordance with my invention, the printing surface is provided by a print
head
that forms a plurality of capillaries terminating in an array of respective
capillary outlets
on the printing surface. The capillaries contain ink that capillary action
ordinarily so re-
tains in the capillaries that it does not mark paper brought into contact with
the print sur-
face. But the ink's dielectric constant exceeds that of air, and electrode
pairs selectively
apply potential differences across respective capillary outlets at which marks
are to be
made. As a result, electrophoresis causes ink to bulge outward from the
associated capil-
lary outlets and into contact with the paper, leaving ink marks at the desired
pixel loca-
tions.
~o So the printer's marking mechanism does not itself require any moving parts
at
all; it requires only appropriate solid-state control circuitry and the print
head, which can
simply be a block that forms the capillaries and provides their associated
electrodes.
Moreover, the ink column in a given capillary is required to move only a
minute
distance in order to make a mark. This means that a single mark can be made in
an ex-
i s tremely brief period of time, and, since all that is required to make a
mark is a single
capillary and its associated electrode pair, the print head can readily be
provided with a
large number of capillaries and associated electrodes so that many pixels-
typically, a
whole row's worth or more-can be printed simultaneously. So the printing speed
can
be made relatively high with very little cost.
2o Additionally, since the ink merely bulges from the capillary-i.e., it is
not pro-
jected through the air the print quality is not as sensitive to paper type as
it is when,
say, ink jet printers are employed. Indeed, since the ink only bulges from the
capillary
and can thus the kept heated by the print head until the instant at which
contact with the
paper cools it, the invention lends itself to the use of hot-melt inks, which
are well known
2s for their lack of sensitivity to the type of image medium on which they are
used. And
hot-melt inks assume solid form when the printer is not in use, so they
contribute further
to the printer's operational robustness.
Another aspect of the invention also enables it to be embodied in particularly
compact printers. The paper or other image medium is advanced past the print
head by a
3o reciprocating electrostatic gripper. Electric fields generated by gripper
electrodes that an
advancement gripper includes draw the paper to the gripper, which then
advances by an
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incremental distance. advancing the paper with it. The advancement gripper
then re-
leases the paper, typically after another. retention gripper grips it to hold
it in place, and
returns, grips the paper again, and again advances the paper after the
retention gripper
releases it. The distance by which the advancement gripper advances is
typically so
small-the spacing of one or two image rows-as to be visually imperceptible,
and the
retention gripper remains stationary. So the printer needs essentially no
space to ac-
commodate feed-mechanism motion.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention description below refers to the accompanying drawings, of which:
io Fig. 1 is a perspective view, partly broken away, of a color printer that
employs
the present invention's teachings;
Fig. 2 is a cross-sectional view of the printer's print head taken at lines 2-
2 of
Fig. 1;
Figs. 3A-D are more-detailed views of one of the print head's capillary
outlets,
~s illustrating the mechanism by which the printer marks the image medium;
Fig. 4 is a cross-sectional view of one of the print-head modules that make up
the
print head;
Fig. 5 is an isometric view of the print head, illustrating the conductor
paths by
which control voltages are applied to the print head's capillary outlets;
20 Figs. 6A and 6B are footprint diagrams that illustrate the cooperation of
staggered
capillary rows to provide a rectangular pixel layout;
Figs. 7A and 7B are similar footprint diagrams illustrating the cooperation of
staggered capillary rows to provide a hexagonal pixel layout;
Fig. 8 is a cross-sectional view of the print head illustrating the ink-supply
ap-
zs proach that the printer employs;
Fig. 9 is a cross-sectional view of the printer showing the paper-feed
mechanism
that the printer uses;
Fig. 10 is an isometric view of a gripper surface illustrating the layout of
its elec-
trode fingers;
so Figs. 11 A-D are timing diagrams that illustrate the paper-feed mechanism's
op-
erating sequence;
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Fig. 12 is a simplified block diagram of capillary-driver circuitry for
driving the
capillary electrodes in a one-bit-per-pixel version of the present invention;
and
Fig. 13 is a simplified block diagram of capillary-driver circuitry for
driving the
capillary electrodes in a multi-bit-per-pixel version of the present
invention.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
Fig. 1 illustrates a printer 10 that employs the present invention's
teachings. In a
manner that will be described in more detail below, grippers 12, 14, and 16
advance pa-
per 18 from a paper supply 20 past a print head 22. With the aid of a
vibrating print
plate 24, the print head employs the present invention's teachings to apply an
image to
io the paper by marking it with hot-melt ink from a cartridge 26. To this end,
battery-
powered circuitry 28 receives image-data signals from a source not shown and
operates
the print head 22 in accordance with the image data thus received. It also
operates the
grippers and vibrating print plate and supplies the power to melt the hot-melt
ink.
Fig. 2, which is a cross-section taken at lines 2-2 of Fig. 1, partially
illustrates the
~s marking mechanism by which the illustrated embodiment operates. In a manner
that will
be described in more detail below, ink-supply channels 30, which extend the
length of
the print head 22, are filled with hot-melt ink that NiCr heating elements 32
keep molten.
Each of the ink-supply channels forms a row of, say, 4000 capillaries 33 at
its base.
The illustrated printer is a color printer. It employs the conventional ink-
color
2o selection, namely, cyan, magenta, yellow, and black. Although Fig. 2 shows
only a sin-
gle capillary row for each color, more may be provided to speed printing or
for other rea-
sons, as will be explained below. None of these features is critical to the
present inven-
tion.
A piezoelectric actuator 34 causes the print plate 24 to reciprocate with a
fre-
zs quency of, say, 2 kHz through a vertical travel on the order of 75 pm
between extended
and retracted positions. In its extended position, the print plate's resilient
core 36 urges
the paper 18 into contact with the print head's bottom surface. There it is
marked by hot-
melt ink that selectively applied electric fields have caused to bulge from
selected capil-
laries' outlets despite the capillary action, as will now be explained by
reference to
3o Figs.3A-C.
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Fig. 3A diagrammatically illustrates a column of ink 38 held by capillary
action
in one of the capillaries 33. That drawing also shows two electrodes 40 and 42
disposed
at opposite sides of the capillary outlet. The electrodes are embedded in a
Teflon coat-
ing 44, which resists wetting by the ink in the capillary. Fig. 3A illustrates
the situation
s in which there is no difference in electrical potential between the two
electrodes 40
and 42. It can be seen that capillary action prevents the ink column from
effectively
marking paper that has been brought into contact with the bottom head surface.
To cause the ink to mark the paper, the printer of the present invention
applies a
voltage of, say, 200 V to electrode 40 while keeping electrode 42 at ground
potential.
~o The capillary outlet is on the order of only 40 ~m across, so the applied
potential differ-
ence causes electric fields on the order of millions of volts per meter at the
capillary out-
let. The attendant, similarly high field gradients cause the hot-melt ink,
which has been
chosen for its high dielectric constant, to bulge outward into the field thus
formed and
thereby mark the paper, as Fig. 3B illustrates. A hot-melt ink suitable for
this purpose is
i s Piccotex 75LC hot-melt ink, available from Hercules Incorporated of
Wilmington,
Delaware.
When the ink column comes into contact with the (relatively cool) paper, its
tip
solidifies in a matter of microseconds into a crust on the paper. As Fig. 3C
illustrates,
the printer then removes the electrodes' potential difference, so the (still-
liquid) ink col-
2o umn tends to withdraw back into the capillary. At the same time, the
vibrating print
plate 24 withdraws the paper into its retracted position with the help of a
further gripper
mechanism 46 (Fig. 2) embedded in its surface, as will be described in more
detail be-
low. This assists in breaking the contact between the crust thus formed and
the with-
drawing ink column.
2s Grippers 12, 14, and 16 then advance the paper 18 by a small advancement
dis-
tance. In a color version of the invention, the spacing between capillary rows
containing
different-colored inks is chosen to be an integer number of advancement steps
so that a
paper location at which a capillary in one row has deposited ink of one color
will even-
tually be positioned in registration with the corresponding capillary in the
row that con-
3o twins the next color so that ink of a different color may be deposited on
top of the ink
crust 47 that was deposited in the Fig. 3B operation.
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Fig. 4 depicts in more detail a single head module 48, which provides a single
row of ink capillaries. The module includes a body 50 of insulating material
such as an
A1203-powder matrix in which the capillary 33 has been formed by one of the
many
known microfabrication techniques. A NiCr resistor 52 deposited on one side of
the
head module 48 extends between the grounded electrode 42 and a similarly
deposited
power-supply rail 54. The ink that the head module 48 contains is in solid
form when the
printer is not in use, but turning the printer on applies power to the
resistor 52, which
thereupon heats the head module 48 and thus liquefies the hot-melt ink that it
contains.
Conductors 56 printed on the head module 48's opposite face connect the driven
~o electrode 40 to a printed-circuit board backplane (not shown) that leads to
drive circuits
in the printer's circuit module 28 (Fig. 1). Since similar conductors are
provided on the
corresponding face of an adjacent head module, an insulating layer 60
insulates resis-
tor 52 from the adjacent module's conductors.
It was previously stated that a printer employing the present invention's
teachings
l: may provide more than one capillary row for each color. Clearly, such an
arrangement
can be used to increase printing speed. But Fig. 5 illustrates a head
arrangement that
provides two capillary rows per color for another purpose. As Fig. 5 shows,
the capillary
outlets 64 that module 48 provides are staggered with respect to the adjacent
module 62's
capillary outlets 66. The purpose of this arrangement is to enhance the
printer's spatial
2o resolution. If it proves inconvenient for a single capillary row to provide
the number of
capillaries per unit row length that the desired image resolution requires,
one solution is
to use different print-head modules to print different ones of a given row's
pixels. For
example, if head modules 48 and 62 contain the same ink color and their
respective capil-
laries are staggered as shown, module 48 can deposit, say, the odd-numbered
pixels in a
2s given row, and module 62 can deposit the even-numbered pixels in the same
row.
Fig. 6A illustrates this concept. Let us assume that a given capillary row
deposits
the nth row of image pixels at time t = n, where time is stated in paper-
advancement pe-
riods. That is, the paper is advanced at times t = 1, 2, . . . . Rectangles 68
represent the
module-48 capillaries' footprints on the pager at time t = n, while rectangles
70 represent
3o the module-62 capillaries' footprints on the paper at the same time. If it
takes N paper-
advancement steps for the row of marks made by module 62 to draw even with the
row
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of module-48 capillary outlets, then rectangles 72 represent the location at
time t = n of
the marks that the module-62 capillaries made on the paper at time t = n - N.
So if mod-
ule 48's electrodes receive signals for a given image row's odd pixels N paper-
advancement steps after module 62's electrodes receive signals for that image
row's even
pixels, the resultant resolution is twice that achievable by one module only.
The resul-
tant pixel arrangement is illustrated in Fig. 6B, in which the module-62
capillaries'
maarks are labeled A and the module-48 capillaries' marks are labeled B.
Such a staggered relationship between capillary rows can also be used to
achieve
a different, hexagonal effect, as Figs. 7A and 7B illustrate by diagrams
respectively cor-
io responding to those of Figs. 6A and 6B. To achieve the hexagonal effect,
not only are
the two capillary rows' footprints staggered "horizontally" (i.e., in the
direction trans-
verse to paper advancement), but their respective sequences of row marks on
the paper
also staggered "vertically" (i.e., in the direction parallel to paper
advancement). In such
an arrangement, the spacing between adjacent capillary rows is the product of
an odd in-
~s teger and the distance between printed rows on the paper, and the paper-
advancement
mechanism advances the paper by two row spacings at a time. Consequently, a
given
module's capillaries print all of the pixels in a row, but only on alternate
rows.
In Figs. 6 and 7, footprints are depicted respectively as rectangular and
hexago-
nal. These shapes reflect the conceptual pixels' shapes, and it may be
beneficial for the
zo capillaries' cross sections also to be so shaped. But some embodiments will
employ cir-
cular capillary cross sections for all pixel arrangements.
Just as capillary action retains ink in the capillaries, some of this
invention's em-
bodiments will also use capillary action to feed ink from the ink cartridge 26
(Fig. 1 ) to
the ink-supply channels 30 (Fig. 2). As Fig. 8 illustrates, the cartridge 26
is snap fit into
zs a receptacle 80 formed on the print head 22. It rests on a heater pad 82,
which heats the
cartridge 26 and thus the ink in longitudinally extending ink reservoirs 84.
These reser-
voirs communicate at the cartridge rear with respective tubes 86 (Fig. 1 ),
which fit into
respective print-head openings 88 that communicate with respective supply
channels 30.
The tubes 86 are formed in a connector 90 that additionally provides an
electrical con-
3o nection between the heater 82 (Fig. 8) and a power supply in the
electronics module 28.
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Any convenient method made be used to transport the ink from the cartridge to
the ink-supply channels. Preferably, however, the conduits are formed by
materials that
the ink tends to wet and are so sized that capillary action alone will cause
the ink to flow
to the supply channels and thereby to the marking capillaries.
s Fig. 8 also shows that the print head 24 includes a cover 92 that closes a
cavity 94
in which the individual head modules are mounted. The cover 92 forms a recess
96 that
communicates both with the print-head exterior and with air holes 98 formed at
the print-
head modules' upper ends to permit air to be displaced as the capillaries' ink
column ex-
tend and retract. The print-head modules' upper surfaces may also be provided
with a
~o Teflon coating 100 to discourage ink from bleeding out the air holes.
Fig. 9 illustrates the illustrated embodiment's paper-feed mechanisms. Embed-
ded in the upper surfaces of grippers 12, 14, and 16, as well as print pad 24,
which also
serves as a gripper, are gripper electrodes interdigitated in a manner that
Fig. 10 illus-
trates. A first set of elongated electrodes 102 is connected to a positive-
voltage supply
is pad 104 and interdigitated with a second set of elongated electrodes 106
connected to a
negative-voltage supply pad 108. The spacing between adjacent electrodes is on
the or-
der of - mm, so the potential difference between the two supply pads, which is
on the
order of 200 V when the gripper is activated, sets up substantial electric
fields above the
gripper. The gripper thereby draws the first paper sheet tightly to itself.
But the first
zo sheet acts to shield all sheets above it, so it is only one that the
gripper attracts.
When an image is to be printed on a new paper sheet, actuators 112 and 114 ad-
vance gripper plates 12 and 14 into engagement with the bottom sheet in the
paper sup-
ply 20. Those plates' gripper electrodes are energized and thereby draw the
bottom
sheet 18 to their upper surfaces. Actuators 112 and 114 then retract the
gripper plates
2s and thereby pull the bottom sheet past retention lips 116 and 118. The
printer then re-
moves power from gripper 12 but not from gripper 14, which therefore retains
its hold on
the paper.
While gripper 14 retains its hold on the paper, piezoelectric actuator 114 ad-
vances gripper plate 14 and thus the paper sheet one advancement step to the
right. The
3o advancement step is one pixel-row spacing in the case of the pixel
organization of
*rB
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Figs. 6A and B. In the case of Figs. 7A and B's pixel spacing, the advancement
step is
two pixel rows.
tripper 12's electrodes are then powered again to hold the paper sheet in
place,
and gripper 14's electrodes release the paper sheet. While gripper 12 holds
the paper in
place, gripper 14's actuator moves it back to the left, where it again grips
the paper.
tripper 12 then releases the paper again, and gripper 14 again advances the
paper sheet
to the right as before.
This advancing operation feeds the paper sheet into the space between the
print
head 22 and the print plate 24, which itself has gripper electrodes embedded
in its upper
io surface. The print plate 24's electrodes are energized in synchronism with
those of grip-
per 12 and so timed as to cooperate with the printing process, as Figs. 11 A-D
illustrate.
Fig. 11 A represents the energization state of the gripper electrodes on the
reten-
tion grippers, i.e., the electrodes on gripper plate 12 and print plate 34.
Fig. 11B repre-
sents the positions of the the print plate 24 and the activated capillaries'
ink columns.
Those drawings show that the capillaries' ink columns and the print plate 24
assume their
advanced positions, in which the ink column can mark the paper, at time time
t,, while
the retention grippers' electrodes are in the energized state. At time t2, the
print plate and
ink columns retreat to their retracted positions while the print plate's
gripper is still en-
ergized and thus pulls the deposited ink crust out of contact with the still-
liquid ink col-
2o umn.
The mark thus having been made, it is time for the advancement gripper 14 to
grip the paper, and it does so at time t3, as is illustrated by Fig. 11 C,
which represents the
energization state of the advancement gripper's electrodes. The retention
grippers then
release the paper at time t4 so that the advancement gripper can begin
advancing the pa-
2~ per to the right. Fig. 11 D, which represents the advancement gripper 14's
position,
shows that gripper 14 begins that advance at time t5. By time t6, the paper
has been ad-
vanced to the point where the next marking is to take place, so the retention
grippers
grasp the paper again at time t~. With the the retention grippers thus holding
the paper
sheet in position, the advancement gripper releases the paper at time t8, and
it returns to
3o the left at time t9.
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The cycle begins again at time too and repeats until the entire image has been
written on the paper sheet. In the process, the paper sheet advances beyond
the reach of
the first two gripper plates 12 and 14. To continue the advancement process, a
further
piezoelectric actuator 120 (Fig. 9) moves advancement gripper plate 16 to the
left and
right in synchronism with the left-and-right movements of advancement gripper
plate 14,
its gripper electrodes being energized in synchronism with that plate's.
Gripper plate 16
thus cooperates with print plate 24 just as advancement gripper plate 14
cooperates with
retention gripper plate 12. But a further retention gripper plate 122 may be
added to take
over for the print plate 24 in the last stages of the advancement process.
~o Fig. 12 is a simplified block diagram that illustrates the data flow
employed to
drive the print-head electrodes that Fig. 3A's electrodes 40 and 42 exemplify
. In a typi-
cal arrangement, the electronics module 28 includes an image memory 126, which
re-
ceives image data from the source of the image to be printed. The source will
often be a
personal computer or other device that can be supplied with driver software
for process-
i s ing the image data into the form most compatible with the hardware
organization de-
scribed above. Alternatively, the printer can itself be provided with
circuitry that per-
forms such processing.
Between the times at which the print-head electrodes are energized, one row of
image data (or, as was explained above, a subset thereof) is fetched for each
capillary
2o row and supplied to a respective one of several shift registers such as
shift registers 128
and 130, which are associated with respective capillary rows. Each shift
register receives
its share of the image data for a full row between, say, times t2 and too of
Figs. I lA-D,
and an ENABLE signal gates the shift registers' contents to respective
electrode rows, as
gates 132 indicate, with the timing at Fig. 11B illustrates. Of course, the
Fig. 12 repre-
2s sentation is merely conceptual; as was explained above, the voltages
applied to the print-
head electrodes ordinarily are nearly two orders of magnitude greater than
conventional
logic levels.
Additionally, Fig. 12 depicts the printer as employing single-bit pixels,
whereas
the present invention's teachings are readily adapted to mufti-bit pixel data.
The voltage
3o applied to a capillary's outlet electrodes determines the distance by which
the ink column
protrudes from it. That distance, in turn, determines the size of the
resultant printed dot.
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So mufti-bit pixel data can specify which of a set of predetermined voltages
to apply to a
given capillary's electrodes. A printer that employs the present invention's
teachings in
a mufti-bit embodiment may use an arrangement such as that which Fig. 13
illustrates.
Fig. 13 shows the shift register 134 for a single capillary row. One of its
stages 136 may contain the data used to specify the voltage to the applied to
electrode 40.
Stage 136's contents may be, say, a four-bit number, which a decoder 138 uses
to select
among sixteen electronic switches 140 by which electrode 40 can be connected
to a se-
lected line of an electrode-voltage bus 142. The voltages on these lines are
the outputs of
respective taps of a voltage divider 144 whose input is the output of a gated
voltage
~o source 146. Source 146's output is a repetitive pulse whose timing Fig. 11
B depicts and
whose amplitude at least equals the voltage corresponding to the digital image
data's
full-range value.
It is thus apparent that a printer embodying the present invention's teachings
can
be exceedingly simple and robust mechanically. The print head is a simple
manifold
i s structure that has no moving parts. Ink application is controlled by
arrays of electrodes,
which can be provided on a simple flex-print substrate. It is well suited to
use with hot-
melt inks, so the method is not sensitive to the type of paper being used-and
it contains
no liquid ink when it is not in use. Moreover, the use of reciprocating
electrostatic grip-
pers greatly contributes to the compactness of the resultant printer package;
since their
2o travel is microscopic, a full-color printer can be made that is only
slightly larger than the
paper supply that it includes. The present invention thus constitutes a
significant ad-
vance in the art.
