Printhead array compensation device designs

Details Printhead array compensation device designs Printhead array compensation device designs

1999-11-22

2002-07-09

Barlow, John (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06416163

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to acoustic printing, and more particularly to improving the off state of a column switch, in order to control the on/off switching ratio between ejectors of an acoustic printhead.
The fundamentals of acoustically ejecting droplets from an ejector device such as a printhead has been widely described, and the present assignee has obtained patents on numerous concepts related to this subject matter. In acoustic printing, an array of ejectors forming a printhead is covered by a pool of liquid. Each ejector can direct a beam of sound energy against a free surface of the liquid. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. When the radiation pressure is sufficiently high, individual droplets of liquid are ejected from the pool surface to impact upon a medium, such as paper, to complete the printing process. The ejectors may be arranged in a matrix or array of rows and columns, where the rows stretch across the width of the recording medium, and the columns of ejectors are approximately perpendicular.
Ideally, each ejector when activated ejects a droplet identical in size to the droplets of all the other ejectors in the array. Thus, each ejector should operate under identical conditions.
In acoustic printing, the general practice is to address individual ejectors by applying a common RF pulse to a segment of a row, and to control the current flow to each ejector using column switches. In some cases it is desirable to use one column switch for several rows in parallel in order to reduce the number of column driver chips and wire bonds, and hence cost, in the system. Unfortunately, this approach results in parasitic current paths which can cause undesired RF current to flow through ejectors that are not in an ON state.
In existing systems, the switching ratio is limited and will vary with the number of ejectors that are ON in a given row. A switching ratio is defined as the RF power in an OFF ejector, to the RF power in an ON ejector (i.e. P
OFF
/P
ON
).
FIG. 1
illustrates an acoustic switching array with a desired current path for a selected row and selected column for an existing system. Switching matrix
10
is a 4-row
12
a
,
12
b
,
12
c
,
12
d
by 64 column
14
a
,
14
b
,
14
zz
switching matrix. Rows are connected to the matrix via switching elements
16
a
,
16
b
,
16
c
,
16
d
, and columns are also connected through switching elements
18
a
,
18
b
,
18
zz
. At the intersection of the columns and rows are transducers
20
. Current paths of matrix
10
are terminated at RF ground
21
. It is to be appreciated that while the matrix of
FIG. 1
is a 4-row by 64-column matrix, the present invention may be used in other matrix designs.
Matrix
10
is supplied by a power source
22
which provides its output to an RF signal matching circuit
24
. By proper switch sequencing, a desired current path for a selected row and selected column is obtained. For example, in
FIG. 1
, by closing switch
16
a
and switch
18
a
, a current path is provided from the RF matching network
24
to transducer
20
a
via row
12
a
and column
14
a
. As the remaining rows and columns are unselected, only transducer
20
a
is intended to be activated to emit a droplet.
Unfortunately, the interconnect paths used to implement a low-cost acoustic printhead include unavailable, undesirable current paths, as shown and discussed for example in connection with
FIGS. 2-5
. One problem with the proposed printheads is that they used switches which are known as “leaky” or “lossy” switches which add to the existence of undesirable current paths. An example of the foregoing is depicted in FIG.
2
. In this figure, switches
16
a
and
18
a
are maintained in a closed position while the remaining switches are unselected, and current is provided to transducer
20
a
. However, undesired current will also flow through transducer
20
b
, which is in selected row
12
a
but unselected column
14
a
. Similarly,
FIG. 3
illustrates a situation where undesired current flows through transducer
20
c
, which is in selected column
18
a
and unselected row
12
c.
FIGS. 4 and 5
set forth similar simplified depictions of switching matrix
10
.
FIG. 4
illustrates a situation where 63 columns
26
and one row
12
a
are selected, i.e. are ON, and a single column
28
and remaining three rows
12
b
-
12
d
are unselected, i.e. are OFF. Under this arrangement, the inventors have calculated that there is approximately 514 &mgr;A flowing through transducer
30
, which represents the transducers in selected row
12
a
, and 63 ON columns
26
of matrix
10
. It was also determined by this analysis that 393 &mgr;A of current will flow in transducer
32
, located in selected row
12
a
and the 64th unselected column
28
of transducers. With this information, it is found that the switching ratio between these two currents is equal to:
 393 &mgr;A/514 &mgr;A=0.765=−2.32 dB.
FIG. 5
depicts an alternative arrangement where one column
34
, and one row
12
a
are selected, and remaining 63 columns
36
and 3 rows
12
b
-
12
d
are unselected. In this situation, the selected current path for transducer
38
has a current of 504 &mgr;A, whereas an unwanted current of approximately 368 &mgr;A exists through each of the unselected transducers connected to selected column
34
and unselected rows
12
b
-
12
d
. This results in a switching ratio equal to:
368 &mgr;A/504 &mgr;A=0.730=−2.73 dB.
The cumulative current through switch
18
a
is approximately 1607 &mgr;A (i.e. 504 &mgr;A from the transducer in column
34
, row
12
a
, and from the transducers in column
34
, rows
12
b
-
12
d
, at 368 &mgr;A each), and the voltage at switches
18
b
-
18
zz
is 741 mv.
When using aqueous inks for acoustic ink printing, the desired ejection velocity will be approximately 4 m/sec. This can be achieved using approximately 1 dB of power over the ejection threshold. Given that there are power non-uniformities in the aqueous printhead of approximately +/−0.5 dB, and the desire to maintain some margin of safety (e.g. −0.5 dB) to insure that ejectors which are unselected are truly OFF, an appropriate switching ratio may be found by the restrictions of: switching ratio (SR)>(overdrive for 4 m/sec)+(non-uniformity)+(margin to insure appropriate OFF state), which results in:
SR≧1+0.5+0.5=−2 dB.
Therefore, a switching ratio of −2.5 to −3.0 dB will be acceptable for printing of aqueous inks, when a −0.5 to −1,0 dB safety margin is added.
However, and more specifically related to the present invention, phase-change inks require more power over the threshold than aqueous inks. To achieve a necessary 4 m/sec ejection velocity, it has been determined that a −4 dB power over the threshold will be required. For phase-change inks, it is intended to use static E-fields to reduce this power requirement, however it is still necessary to eject the droplets at approximately 2 m/sec, i.e. −2 dB over threshold. Non-uniformities in the phase-change printhead are similar to those for aqueous ink printheads (i.e. +/−0.5 dB), and the margin for turning the switches fully OFF will also be similar (i.e. −0.5 dB). Therefore, the switching ratio for phase-change inks will require:
SR≧2+0.5+0.5=−3 dB.
Then, with a −0.5 to −1,0 dB safety margin added, a switching ratio of −3.5 to −4.0 dB is acceptable. Existing switching networks do not insure adequate switching ratios for phase-change printing when the foregoing requirements are taken into consideration.
It has thus been determined desirable to increase the switching ratio, and to control the switching ratio at a desired level, independent of the number of ejectors which are ON. It has also been determined desirable to provide such control in a circuit which is compact, manufacturable, and is functional with the general desig

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