Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1998-11-25
2002-12-17
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S011000
Reexamination Certificate
active
06494554
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ink-jet printer for ejecting ink droplets through a droplet outlet orifice (a nozzle) and recording an image on paper and an apparatus and a method of driving a recording head for an ink-jet printer.
2. Description of the Related Art
Ink-jet printers for ejecting ink droplets through a droplet outlet orifice communicating with an ink chamber and recording on paper have been widely used. In such an ink-jet printer of related art, a single piezoelectric element is provided for each nozzle. The piezoelectric element is fixed to an oscillation plate forming an external wall of the ink chamber to which ink is fed through an ink duct. The piezoelectric element changes the ink chamber volume by bending in response to a voltage waveform of an applied drive signal so as to generate an ejection pressure. An ink droplet is ejected through the outlet orifice by the ejection pressure.
Since the ejection pressure is generated by changing the ink chamber in such an ink-jet printer as described above, ink ejected through the orifice flies in a columnar shape (in a trailing form). Differences in time and velocity result between the tip and the end of the flying ink droplet.
Consequently, the preceding main ink droplet is accompanied by unwanted minute droplets (called satellite droplets in the following description). Such satellite droplets landing on paper affect the printing result. Although satellite droplets do not have a great effect on the quality of a high-density image recorded with relatively large droplets, the image quality is expected to be significantly reduced by satellite droplets when the image is recorded with small droplets for representing a low-density image or a half-tone image. Satellite droplets generated when small droplets are ejected therefore cause a great problem.
Some methods have been proposed in order to cope with the problem. For example, a method is disclosed in Japanese Patent Application Laid-open Hei 7-76087 (1995) wherein a single piezoelectric element is provided for each nozzle and the velocity of changing ejection voltage applied to the piezoelectric element is switched between two levels for ejecting ink droplets. In the method, as shown in
FIG. 1
, the ejection voltage is initially increased at first voltage changing velocity ‘v1’. The ejection voltage is then increased at second voltage changing velocity ‘v2’ higher than v
1
. In
FIG. 1
, the vertical axis indicates voltage. The horizontal axis indicates time. According to the method, the next droplet is ejected to follow the tip of the preceding droplet. The difference in velocity between the tip and the end of the ink column is thereby decreased and satellite droplets are reduced.
Another method is disclosed in Japanese Patent Application Laid-open Sho 59-133067 (1984) wherein a single piezoelectric element is provided for each nozzle and an ink droplet is ejected by applying two independent voltage pulses to the piezoelectric element. In the method, as shown in FIG.
2
, first pulse P
1
is applied to the piezoelectric element to produce a first pressure fluctuation for starting ink droplet ejection through a nozzle. First pulse P
1
is then terminated and second pulse P
2
is applied to the piezoelectric element before the ejection of droplet through the nozzle is completed to produce a second pressure fluctuation. In
FIG. 2
, the vertical axis indicates voltage. The horizontal axis indicates time. According to the method, the ink column ejected through the nozzle ruptures at an early stage and generation of satellite droplets is suppressed.
An ink droplet ejection apparatus is disclosed in Japanese Patent Application Laid-open Sho 51-45931 (1976) wherein two pressure generating means are provided for each nozzle and an ink droplet is ejected by oscillating ink by combining oscillations produced by the two pressure generating means.
In the method disclosed in Japanese Patent Application Laid-open Hei 7-76087 (1995) described above, however, first voltage changing velocity v
1
is required to be lower than second voltage changing velocity v
2
. Consequently, the velocity of an ejected ink droplet is reduced when compared to the case wherein the voltage is changed at high velocity v
2
throughout the ejection cycle. A reduction in velocity of an ejected ink droplet results in unstable ejection affecting linearity of the droplet flying route and variations in droplet velocity. As a result, displacements of recorded dots may occur and printing quality may be reduced.
In the method disclosed in Japanese Patent Application Laid-open Sho 59-133067 (1984) described above, second pulse P
2
is applied after interval Ti, having terminated first pulse P
1
. If interval Ti is too long, a trail of an ink column becomes long and satellite droplets may be produced. On the other hand, if interval Ti is too short, the piezoelectric element does not follow the voltage change and the intended operation will not be achieved. This is because the piezoelectric element in general has its intrinsic oscillation characteristic and does not operate at a frequency above the intrinsic oscillation. Although this problem may be solved by fabricating a piezoelectric element having a high intrinsic frequency, this is not realistic since there is a limitation of the intrinsic frequency of the piezoelectric element obtained in practice. In addition, fabricating such a piezoelectric element is accompanied by technical difficulties and manufacturing costs are thereby increased. Furthermore, in the above-mentioned publication, although voltage V
1
of first pulse P
1
is lower than voltage V
2
of second pulse P
2
, voltage V
1
is required to be higher than voltage V
2
so that the trailing end of the ink column reaches the tip thereof and becomes integrated with the tip. However, an increase in the voltage applied to the piezoelectric element causes a reduction in the life of the piezoelectric element and the oscillation plate oscillated by the piezoelectric element. A residual oscillation is increased as well and the frequency characteristic may be affected.
The above-mentioned ink droplet ejection apparatus disclosed in Japanese Patent Application Laid-open Sho 51-45931 (1976) is provided for efficiently ejecting ink droplets with a small power input. In order to achieve the object, high-frequency drive signals are each applied to the two pressure generating means and the phase difference between the drive signals and the amplitude are changed so that the oscillations generated by the pressure generating means are successfully combined to oscillate ink. An ink droplet is thereby ejected. That is, the apparatus is not intended for preventing satellite droplets. The method of driving the pressure generating means and the configuration required for preventing satellite droplets are not disclosed, either. No suggestion about such a method or configuration is made in the publication, either.
As thus described, it is difficult to satisfactorily reduce satellite droplets in the related art without reductions in velocity of an ejected droplet, in the apparatus life, in the frequency characteristic and without a limitation of the intrinsic oscillation characteristic of the piezoelectric element.
The related-art ink-jet printers have further problems.
FIG. 3
is a schematic diagram of a recording head and a drive circuit thereof in a related-art ink-jet printer. As shown, a recording head
500
includes a nozzle
501
and a piezoelectric element
502
provided in correspondence with the nozzle
501
. The piezoelectric element
502
is fixed to a wall of an ink chamber (not shown) to which ink is supplied through an ink duct (not shown). A drive signal
504
of a specific waveform is selectively inputted to the piezoelectric element
502
through an on/off switch
503
. That is, the drive signal
504
is only inputted to the piezoelectric element
502
when the switch
503
is turned on. On the application of the drive signal
504
, the piezoelectr
Horii Shinichi
Ikemoto Yuichiro
Kishimoto Masaki
Nishi Shota
Suzuki Kenji
Barlow John
Dudding Alfred E.
Maioli Jay H.
Sony Corporation
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