Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-10-20
2001-08-14
Nguyen, Judy (Department: 2861)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06273538
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for driving a piezoelectric ink jet head which selectively makes ink droplets adhere to an image recording medium.
BACKGROUND ART
Among nonimpact printers, which are rapidly expanding their market share, the one that is the simplest in principle and most suitable for color printing is the ink jet printer.
Regarding this type of printer, it can be said that the so-called drop-on-demand type that ejects ink droplets only when forming dots is predominant. Many types of drop-on-demand piezoelectric ink jet heads that use piezoelectric elements have been disclosed and are represented by the Kyser type disclosed, for example, in Japanese Examined Patent Publication No. 53-12138, the layered piezoelectric actuator type disclosed, for example, in Japanese Unexamined Patent Publication No. 6-4327, and the shear mode type disclosed, for example, in Japanese Unexamined Patent Publication No. 63-252750.
In such a piezoelectric ink jet head, a piezoelectric element deformable by the application of a pulse waveform is mounted at least on a portion of the wall surface of an ink chamber which communicates at one end with a nozzle and at the other end with an ink reservoir, and ink is ejected by deforming this piezoelectric element.
The piezoelectric ink jet head is usually driven in the following way. First, a pulse waveform is applied to the piezoelectric element to deform the portion of the wall surface of the ink chamber in such a manner as to cause the internal volume of the ink chamber to increase and thereby draw ink into the ink chamber. Next, by either removing the voltage from the piezoelectric element or applying a pulse waveform opposite in polarity to the first applied pulse waveform, the portion of the wall surface of the ink chamber is deformed in a direction opposite from the direction in which it was first deformed. This causes the internal volume of the ink chamber to decrease, and an ink droplet is ejected. This is the so-called draw-and-fire driving method.
With the above method, however, oscillations will remain on the ink meniscus after the ink droplet is ejected. The residual oscillations consist of pressure wave oscillations due to the mechanical structure of the ink chamber itself and hydrodynamic surface tension oscillations of the ink itself.
If the next ink eject action is attempted while these oscillations remain, the meniscus oscillations caused by its ink ejection driving are superimposed on the residual oscillations of the meniscus caused by the previous ejection. This causes a difference in meniscus position between the current ink ejection and the previous ink ejection, as a result of which the size and ejection speed of the ink droplet vary and stable ink ejection cannot be accomplished.
The higher the ink ejection speed, the more accurately can the ink dot be deposited. Further, if the voltage applied for ink ejection is the same, the shorter the ink ejection time, the higher the ink ejection speed.
The residual oscillations of the meniscus can be suppressed by setting the length of the ink ejection time equal to the cycle of the pressure wave oscillations occurring due to pressure changes in the ink chamber.
However, if the length of the ink ejection time is set equal to the cycle of the pressure wave oscillations to suppress the residual oscillations, this in turn imposes a restriction on the ejection speed. On the other hand, if a high ink ejection speed is to be obtained while suppressing the residual oscillations occurring after ink ejection, the natural frequency of oscillation of the ink chamber itself must be increased. Since this demands a change in head size, freedom in head design is limited.
DISCLOSURE OF THE INVENTION
It is accordingly an object of the present invention to provide an ink jet head driving method that can achieve the desired ink droplet ejection performance independently of the natural frequency of oscillation of the head, and that can deliberately control the residual oscillations of the meniscus caused by the ejection of an ink droplet.
To achieve the above object, according to the ink jet head driving method of the present invention, first the voltage applied to the piezoelectric actuator is lowered from a voltage value in an initial condition, thereby causing the volume of the ink chamber to increase and thus causing ink to be drawn into the ink chamber. Thereafter, the applied voltage is rapidly raised to a prescribed voltage higher than the initial condition voltage value, thereby causing the volume of the ink chamber to decrease and thus causing ink to be ejected from the ink chamber. Next, the prescribed voltage value is held for a prescribed length of time until the meniscus returns to its initial position, after which the applied voltage is lowered from the prescribed voltage value to the initial condition voltage value, thereby causing the volume of the ink chamber to increase. Here, the time over which the applied voltage is lowered from the prescribed voltage value to the initial condition voltage value is equal to one half the cycle of meniscus surface tension oscillations caused by the ink ejection, and the difference between the prescribed voltage value and the initial condition voltage value is such that oscillations corresponding to the amplitude of the meniscus surface tension oscillations are generated in the ink chamber.
Further, the step of lowering the voltage applied to the piezoelectric actuator from the initial condition voltage value consists of two steps wherein, in the first step, the voltage is rapidly lowered and, in the second step, the voltage is lowered more slowly than in the first step.
ADVANTAGEOUS EFFECT OF THE INVENTION
According to the ink jet head driving method of the present invention, since the residual oscillations of the meniscus after ink ejection can be quickly damped and the meniscus be brought back to its initial condition in a short period of time by lowering the driving voltage after the ink ejection, stable print quality, unaffected by the driving frequency, can be obtained.
Further, the voltage in the initial condition is set lower than the maximum application voltage required for ink ejection in order to suppress the oscillation of the meniscus. Accordingly, the meniscus drawing action performed to control the ink ejection amount is prevented from becoming excessive; as a result, the ink supply time required to obtain the desired amount of ink can be shortened, achieving efficient driving for ink ejection. Furthermore, since leakage current between the electrodes of the piezoelectric actuator can be reduced to a low level, the power consumption of the ink jet apparatus as a whole can be reduced.
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Komine Shinichi
Mitsuhashi Tadashi
Citizen Watch Co. Ltd.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Nguyen Judy
LandOfFree
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