Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2002-10-04
2004-09-21
Meier, Stephen D. (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06793311
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an ink jet recording apparatus.
BACKGROUND OF THE INVENTION
A type of ink jet recording apparatus known in the art discharges a plurality of ink droplets from the same nozzle of an ink jet head in one printing cycle so that a single ink dot is formed from these ink droplets. A recording apparatus of this type arranges a plurality of ink dots on recording paper so that the ink dots together form an image, etc., on the recording paper. The number of ink droplets to be discharged in one printing cycle is adjusted so as to adjust the gradation and the size of the dot, thereby realizing so-called “multiple gray level recording”.
However, when printing at a high speed, the carriage of the ink jet head is moved at a high velocity, whereby the landing positions of the ink droplets discharged from the same nozzle are likely to be shifted from one another in the carriage direction. Then, the ink dot formed from the ink droplets will have an oblong circular shape elongated in the carriage direction, thereby lowering the image quality.
In view of this, a method for enabling high-speed printing has been proposed in the art, in which two ink droplets are discharged from the same nozzle with the later discharged ink droplet being discharged with a higher discharging velocity than that of the previously discharged ink droplet so that the two ink droplets are allowed to merge in flight into a single ink droplet before landing, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 59-133066.
In recent years, the density of an ink jet head has been increasing, whereby the dimensional error in an actuator, or other elements, the change over time in the characteristics of an actuator, etc., have an increasing influence on the ink droplet discharging velocity. Specifically, if an actuator, a pressure chamber, etc., has a dimensional error, or the like, the degree of deformation of the actuator or the behavior of ink in the pressure chamber in response to a driving signal will be different from those in a case where there is no dimensional error, or the like, for the same driving signal. Thus, if the dimensional error, or the like, varies among different actuators, the ink droplet discharging velocity will also vary among different nozzles. Such variations in the ink droplet discharging velocity lead to variations in the landing position, thereby resulting in deteriorations in the image quality such as a white streak in a solid print, for example. Particularly, with an ink jet head that discharges a plurality of ink droplets that are merged together in flight, such as that disclosed in the above publication, the error in the discharging velocity among different ink droplets is amplified, whereby variations in the landing position are likely to occur.
By matching the vibration period of an actuator with the natural vibration period thereof, it is possible to bring the actuator into resonance and to bring the ink meniscus vibration into resonance. In this way, it is possible to drive the actuator with a smaller amount of energy. Thus, it is possible to improve the discharging performance by effectively utilizing resonance.
Note that the term “vibration period of an actuator” as used herein refers to the vibration period of the entire vibration system including the ink, etc., i.e., the vibration period of the actuator with the pressure chamber, etc., being filled with ink. Similarly, the term “resonance frequency, or the like, of an actuator” as used herein refers to the resonance frequency, or the like, of the entire vibration system including the ink, etc.
However, even with an ink jet head utilizing resonance, if the resonance frequency varies among different actuators, the discharging velocity or the discharged ink volume will vary among different nozzles, thereby leading to a shift in the ink droplet landing position or variations in the ink dot size. Such variations in the landing position or the ink dot size will cause irregularities in the arrangement of ink dots on the recording paper, thereby lowering the image quality. Particularly, with an ink jet head that discharges a plurality of ink droplets that are merged together in flight, the discharging velocity or the discharged ink volume easily vary due to a shift in the resonance frequency. Effective countermeasures have not been taken in the prior art against such variations in the discharging velocity or the discharged ink volume due to a shift in the resonance frequency.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above, and has an object to improve the recording quality by reducing the variations in the discharging velocity among nozzles.
Another object of the present invention is to provide an ink jet recording apparatus that utilizes resonance and discharges a plurality of ink droplets that are merged together in flight, in which the recording quality is improved by suppressing the variations in the discharging velocity or the discharged ink volume of a merged ink droplet.
An ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles and a plurality of pressure chambers, which are communicated to the respective nozzles and are filled with ink; a plurality of actuators provided in the head body each including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in one of the pressure chambers so as to discharge ink from one of the nozzles; and a driving circuit for supplying a signal to the electrode of each actuator, wherein: the driving circuit applies, in one printing cycle, a driving signal composed of a plurality of pulse signals for discharging a plurality of ink droplets so that the ink droplets are merged together in flight; and the driving signal includes a pulse signal applied with an interval that is shorter than a predetermined pulse interval being equal to a Helmholtz period of a head, and a pulse signal applied with an interval that is longer than the predetermined pulse interval.
Note that the term “Helmholtz period of a head” as used herein refers to the natural period of the entire vibration system including the ink (an acoustic element), the actuator, etc.
Theoretically, the degree of resonance of the ink meniscus vibration increases as the pulse interval of a pulse signal is closer to the Helmholtz period. Therefore, the ink droplet discharging velocity increases as the pulse interval is closer to the Helmholtz period.
In this ink jet recording apparatus, the plurality of pulse signals to be applied in one printing cycle include a pulse signal applied with an interval that is shorter than the Helmholtz period, and a pulse signal applied with an interval that is longer than the Helmholtz period. Therefore, if the Helmholtz period shifts due to various factors such as a dimensional error in the actuator, the pressure chamber, etc., or a change in the characteristics of the actuator, the discharging velocity of one ink droplet may be increased by such a shift in the Helmholtz period while the discharging velocity of another ink droplet may be decreased by the shift. As a result, the shift component that increases the discharging velocity of the merged ink droplet (i.e., an ink droplet whose discharging velocity increases due to the shift in the Helmholtz period) and the shift component that decreases the discharging velocity of the merged ink droplet (i.e., an ink droplet whose discharging velocity decreases due to the shift in the Helmholtz period) are canceled out by each other to some degree, thereby suppressing the shift in the discharging velocity of the merged ink droplet. Thus, variations in the discharging velocity among different nozzles are reduced. Therefore, variations in the ink droplet landing position are reduced, thereby improving the recording quality.
Another ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles a
Baba Koichi
Ikeda Koji
Matsuo Koji
Oyama Masaharu
Tatekawa Masaichiro
Harness & Dickey & Pierce P.L.C.
Matsushita Electric - Industrial Co., Ltd.
Meier Stephen D.
Stewart, Jr. Charles
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