Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2001-03-16
2003-06-24
Nguyen, Lamson (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06582043
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a driving device and a driving method for an ink jet printing head which conducts printing by discharging ink drops from nozzles by changing the volumes of pressure generation chambers filled with ink according to driving signals which are supplied to piezoelectric actuators etc., and in particular, to a driving device and a driving method for an ink jet printing head that improves the expressiveness of gradation and halftones in print images by changing the size of ink dots which are formed on paper etc. by varying the ink drip size based on print data indicating halftones.
DESCRIPTION OF THE RELATED ART
Printing of halftone images on paper etc. has been conducted generally by binary image printing by use of image processing techniques such as the “dot area modulation method” and the “screened half tone (screening)”. However, image printing of photographic-quality is being required in recent years and thereby printing by use of ink drop size modulation techniques is being required of ink jet printing heads.
A driving device for an ink jet printing head for meeting the request has been disclosed in Japanese Patent Application Laid-Open No.HEI9-11457 (hereafter, referred to as “document No.1”), in which two or more driving signals for the discharge of ink drops of different sizes are prepared, and the driving signals are selectively used based on image data.
FIG. 1
is a block diagram showing an example of a conventional ink jet printing head driving device for implementing the ink drop size modulation technique disclosed in the document No.1. The ink jet printing head driving device shown in
FIG. 1
includes a common waveform generation circuit
101
, an image memory
131
, a data transmission circuit
132
, a data reception circuit
143
, a decoder
142
, a level conversion circuit
141
and multiplexers
121
-
1
~
121
-
n
. The multiplexers
121
-
1
~
121
-
n
are provided corresponding to piezoelectric actuators
123
-
1
~
123
-
n
of the ink jet printing head. The common waveform generation circuit
101
includes a waveform generation circuit
111
and two or more voltage/current amplification circuits
112
. In the example of
FIG. 1
, the common waveform generation circuit
101
includes three voltage/current amplification circuits
112
A~
112
C. Each multiplexer
121
-
k
(1≦k≦n) which is composed of transfer gates selects (zero or) one of the voltage/current amplification circuits
112
A~
112
C according to image data (print data) indicating a halftone, and a driving signal (VD
1
, VD
2
or VD
3
) that is supplied from the selected voltage/current amplification circuit (
112
A,
112
B or
112
C) is transmitted by the multiplexer
121
-
k
and is applied to a corresponding piezoelectric actuator
123
-
k
(1≦k≦n), thereby printing of halftone images is conducted.
However, in the technique of the document No. 1, each voltage/current amplification circuit
112
(
112
A,
112
B,
112
C) of the common waveform generation circuit
101
is designed to drive a plurality of piezoelectric actuators
123
-
1
~
123
-
n
through the multiplexers
121
-
1
~
121
-
n
, thereby waveform distortion of the driving signal occurs considerably between the voltage/current amplification circuit
112
(
112
A,
112
B,
112
C) and the piezoelectric actuators
123
(
123
-
1
~
123
-
n
) due to wire resistance and inductance. Further, the distortion of the driving signal varies depending on the number of piezoelectric actuators
123
that are driven simultaneously.
The above problems come up since a driving signal of a very high slew rate (dV/dt) has to be supplied to the piezoelectric actuator
123
which is a relatively large capacitive load. The very high slew rate of the driving signal is necessary for the formation and discharge of minute ink drops of approximately 2 pl (picoliter) which is required for high quality printing by means of the ink drop size modulation.
If we assume that the capacitance of a piezoelectric actuator
123
is C
0
(pF) and the number of driven piezoelectric actuators
123
is n, the load on the voltage/current amplification circuit
112
becomes n×C
0
. In a typical case where the number n is 300 and the capacitance C
0
is 3000 (pF), the load on the voltage/current amplification circuit
112
becomes n×C
0
=0.9 (&mgr;F). In order to realize the discharge of the picoliter ink drops, application of high-frequency voltage between the voltage/current amplification circuit
112
and the piezoelectric actuators
123
is required as a matter of course, therefore, considerable signal distortion is caused due to the wire resistance and inductance between the voltage/current amplification circuit
112
and the piezoelectric actuators
123
. Further, the signal distortion varies depending on the number of simultaneously driven piezoelectric actuators
123
as mentioned above. Therefore, the sizes of discharged ink drops and printed ink dots are necessitated to vary and fluctuate.
For the printing of characters or letters, the ink jet printing head is required to discharge ink drops of large sizes. In such cases, a driving signal (waveform) having a large voltage variation has to be applied to each piezoelectric actuator
123
. When the voltage of the driving signal changes from V
0
(V) to V
1
(V), heat emission P (W) of the voltage/current amplification circuit
112
is calculated as: (½)×n×C
0
×(V
1
2
−V
0
2
). Therefore, the heat emission of the voltage/current amplification circuit
112
increases as the voltage variation (V
1
−V
0
) of the driving waveform gets larger.
The heat emission also changes proportionally to the number of simultaneously driven piezoelectric actuators
123
, therefore, the heat emission of the voltage/current amplification circuit
112
further increases when the number of nozzles (that is, the number of piezoelectric actuators
123
) of the ink jet printing head is set larger in order to realize high speed printing.
When the multiplexers
121
-
1
~
121
-
n
are implemented as an IC, each multiplexer
121
(
121
-
1
~
121
-
n
) is generally implemented as transfer gates each of which includes an N-MOSFET and a P-MOSFET. Therefore, when each multiplexer
121
includes three transfer gates as the example of
FIG. 1
, three N-MOSFETs and three P-MOSFETs become necessary in a multiplexer
121
. Current for driving the piezoelectric actuators
123
has to be passed through each MOSFET, therefore, channel resistance of each MOSFET has to be set as small as possible in order to reduce heat emission of the multiplexer
121
. However, in order to set the channel resistance small, chip size of the IC is necessitated to be large, and thus high integration of the IC becomes difficult.
A driving device for an ink jet printing head designed for eliminating the heat emission problem of the common waveform generation circuit and the waveform distortion of the driving signal supplied to the piezoelectric actuators has been disclosed in Japanese Patent Application Laid-Open No.HEI9-174883 (hereafter, referred to as “document No.2”).
FIG. 2
is a block diagram showing an example of a conventional ink jet printing head driving device which implements the techniques disclosed in the document No.2. In the ink jet printing head driving device of
FIG. 2
, each multiplexer (
221
-
1
~
221
-
n
) is provided with a corresponding voltage amplification circuit (
212
-
1
~
212
-
n
) before itself and a corresponding current amplification circuit (
222
-
1
~
222
-
n
) after itself.
By such composition of the ink jet printing head driving device for driving piezoelectric actuators
223
-
1
~
223
-
n
, the heat emission problem of the common waveform generation circuit
201
and the waveform distortion of the driving signals supplied to the piezoelectric actuators
223
-
1
~
223
-
n
are avoided.
However, considering the voltage amplification factor required of the voltage amplification circuit (
212
-
1
~
212
-
n
) amplifying the driving waveform
Fuji 'Xerox Co., Ltd.
Mouttet Blaise
Nguyen Lamson
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