Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2002-03-08
2004-07-20
Pham, Hai (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S010000, C347S011000, C347S068000, C347S005000
Reexamination Certificate
active
06764152
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid jetting apparatus such as an ink jet printer and a method of driving the same. Particularly, the present invention relates to an apparatus and a method for driving piezoelectric elements provided with a print head in an ink jet printer, so that ink droplets are ejected from nozzle orifices formed with the print head.
An ink jet color printer of a type in which ink of several colors is ejected from a print head has spread up to now, and it has been widely used in order to print images processed by a computer with multi-colors and multi-tones.
For example, in an ink jet printer using a piezoelectric element as a drive element for ink ejection, plural piezoelectric elements associated with nozzles are selectively driven thereby to generate dynamic pressure to eject ink droplets from the nozzles. Printing is performed such that the ink droplets are landed on a print sheet to form ink dots thereon.
Each piezoelectric element is driven by a drive signal supplied from a driver circuit (driver IC) mounted in a printer body or a print head thereby to eject the ink droplets from the nozzles.
When the piezoelectric element is not driven (that is, when the printing is not performed), electric charges accumulated therein are discharged by inherent insulation resistance, so that a thus lowered potential of the piezoelectric element happens to affect the ink ejection.
In view of the above, Japanese Patent No. 3097155 discloses a head driving apparatus and a head driving method, in which charging voltage is applied to piezoelectric elements in accordance with charge signals when the piezoelectric elements are not driven, in order to keep a charged potential.
To drive the print head in such a way, a drive signal applied to each piezoelectric element is so configured as to have a high potential for deactivating the piezoelectric element and to have a lower potential for activating the same. Therefore, consumed power becomes large and the voltage applied to the piezoelectric element becomes relatively high, so that voltage drop due to the discharge (i.e., power loss) is also becomes large.
Increasing the number of piezoelectric elements arranged in a unit area is increased to improve the print quality, the distance between adjacent piezoelectric elements is accordingly reduced. In a case where an activated element and a deactivated element are juxtaposed, discharging between the adjacent elements would occur because of a potential difference caused by the voltage drop.
In the above case, the breakdown voltage of each element becomes low. Therefore, in a case where the drive signal having the maximum voltage higher than the breakdown voltage is applied to such an element, desired operation would not be attained. To avoid such a situation, it is necessary to apply insulation processing between the adjacent elements (e.g., filling an insulating material).
In a case where a charging voltage is suddenly applied to the piezoelectric element in which such voltage drop is occurred, there is a probability that the element happens to be driven so that ink drops are ejected unintentionally. To avoid such a situation, it is necessary to consider the timing of applying the charge signal when designing the drive signal.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide, with simple configuration, an apparatus and a method for driving a print head in an ink jet printer, which lowers a potential difference between electrodes of each piezoelectric element, and reduces a voltage drop occurring therein due to discharging, while eliminating erroneous operations thereof.
In order to achieve the above object, according to the present invention, there is provided a head driving apparatus, incorporated in an ink jet printer which comprises:
a print head, provided with a plurality of nozzles;
piezoelectric elements, each associated with one of the nozzles and provided with a drive electrode and a common electrode; and
a head driver, which generates a drive signal for driving the piezoelectric elements, and selectively supplies the drive signal to at least one of the piezoelectric elements to eject an ink droplet from at least one associated nozzle, the head driving apparatus comprising:
a bias power source, which applies a bias voltage having a predetermined potential to the common electrode of each piezoelectric element.
In this apparatus, by directly applying the bias voltage to the common electrode of the piezoelectric element from the bias power source, the potential of the piezoelectric element is held at the bias voltage. Consequently, since the voltage applied between both electrodes of the piezoelectric element becomes relatively low, consumed power is reduced.
Further, since the predetermined bias voltage is always applied to the common electrode of the piezoelectric element, leak current is reduced even if natural discharge of the piezoelectric element occurs, so that the voltage drop is reduced. Therefore, not only power loss is reduced, but also the steep voltage variation can be avoided when the piezoelectric element is charged so that the occurrence of the erroneous operation of the piezoelectric element can be eliminated. In addition, the restriction on the waveform design for placing the charge signal in the drive signal can be relaxed.
Further, since the voltage applied to the piezoelectric element becomes relatively low, occurrence of the discharge due to the voltage difference between the driven piezoelectric element and the non-driven piezoelectric element is also reduced. Even if the number of the piezoelectric elements per a unit area is increased while each size of the piezoelectric element is made small (the breakdown voltage becomes low), the piezoelectric element can normally operate without performing the insulation processing between the electrodes of the piezoelectric elements.
Preferably, the potential of the bias voltage is variable.
In this apparatus, the bias voltage can be controlled in accordance with the reference potential of the drive signal applied to the piezoelectric element which is inherent of each ink jet printer. Therefore, the voltage applied between both electrodes of each piezoelectric element can be set lower.
Preferably, the bias power source is provided as a logic power source.
In this apparatus, the bias power source can be constituted simply, readily and at a low cost.
Preferably, the bias power source generates the bias voltage based on a power supplied from a power source for driving the print head.
In this apparatus, since the bias voltage is generated using the existing head driving power source, it is not necessary to provide, for example, a logic power source, and the bias voltage can be obtained by the simple construction and at a low cost.
Here, it is preferable that the bias power source includes: a condenser, electrically connected to the common electrode; and a constant-voltage circuit, which applies the bias voltage to the condenser.
In this apparatus, the potential of the common electrode of the piezoelectric element is held at the bias voltage applied from the condenser.
Further, it is preferable that the constant-voltage circuit includes a Zener diode, a current limiting resistance and a coupling element. The Zener diode is electrically connected to the head driving power source through the current limiting resistance. The Zener diode is electrically connected to the common electrode through the coupling element.
In this apparatus, the condenser is charged by the stable bias voltage, and it is prevented by the coupling element that the electric charges discharged from the common electrode from flowing to the Zener diode.
Still further, it is preferable that the constant-voltage circuit includes a discharging diode electrically connected to the head driving power source in parallel with the current limiting resistance, such that a current is flowed to the head driving power source through the discharging diode.
In this apparatus, in a case that the potenti
Fukano Takakazu
Isamoto Hideyuki
Takamatsu Seiji
Umeda Atsushi
Nguyen Lam S
Pham Hai
Seiko Epson Corporation
Sughrue & Mion, PLLC
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