Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2000-11-28
2002-07-23
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06422685
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a driving circuit for an acoustic printer and an acoustic printer, and particularly to the driving circuit of an acoustic printer and the acoustic printer for realizing high speed operation and high image quality.
2. Description of the Related Art
As an apparatus for recording an image by making ink liquid into small grains (so-called liquid drop) and flying the same on a recording medium to form a dot, an ink jet printer has been put to practical use heretofore. A known acoustic printer uses the operation of an acoustic transducer in a device for flying the liquid drop on the recording medium.
As an example, Japanese Patent Application Laid Open No. 5-278218 corresponding to U.S. Pat. No. 5,191,354 discloses a technique. The acoustic printer using an acoustic transducer is adapted to produce periodic perturbation on the free surface of liquid ink with a suitable excitation frequency. When the amplitude of vibration pressure is equal to or higher than the critical rise amplitude level, one or more standing capillary waves are generated on the free surface of liquid ink, thereby flying a liquid drop on the recording medium. To produce such perturbation, the transducer is connected to a driver to be driven.
Further, Japanese Patent Application Laid Open No. 8-187853 corresponding to U.S. Pat. No. 5,589,864 discloses a technique where a piezoelectric device driven by an RF signal is used as a transducer. In this technique, a PIN diode or a varactor is connected in series to the piezoelectric device, and in the case of the varactor, the impedance is changed to switch on and off the RF signal, thereby controlling ink injection.
To control the RF signal, the applicant of the invention has proposed a technique (Japanese Patent Application Laid Open No. 11-72211) of generating an a.c. signal in a piezoelectric element without an a.c. signal source concerning the RF-controller and the RF driving circuit. In this technique, the inductance connected in parallel to the piezoelectric element constitutes a parallel resonance circuit, and electric charges from charge storage means and energy based on the resonance circuit are alternately supplied by switching means to discharge ink, so that it is not necessary to always supply an a.c. signal so as to reduce power consumption.
Generally, to obtain a print image of high image quality in a printer, improvement in resolution and gradient is demanded. In the acoustic printer, the resolution can be improved by making an ink drop minute, and the gradient can be improved by increasing or decreasing the diameter of an ink drop or superposing minute ink.
Concerning the improvement in resolution, there is direct proportionality between the wavelength of an ultrasonic wave propagating through an ink chamber filled with ink and the discharged ink drop, and the size of an ink drop can be varied by controlling the wavelength of an ultrasonic wave. Thus, the resolution can be improved.
Concerning the improvement in gradient, there have been proposed a technique where the frequency/amplitude/duration of the RF signal is changed to obtain ink drops different in diameter and a technique where plural ink drops discharged by starting the piezoelectric element plural times are superposed on the same pixel on a recording medium. Japanese Patent Application Laid Open No. 8-290587 discloses a technique where an ink drop group continuously discharged by one drive of the piezoelectric element is superposed On the same pixel on a recording medium.
The technique of superposition recording plural or one group of ink drops, however, has the problem that essentially the print speed is lowered.
For example, in the conventional superposition recording, the dot diameter after superposition is calculated by the following expression.
(Dot diameter of superposition)=(Unit dot diameter)×(The one third power of superposition frequency)
Accordingly, to obtain double dot diameter, the discharge time for eight times is required. Japanese Patent Application Laid Open No. 8-290587 discloses:
(Time for obtaining N-number of ink drop groups)<N×(Time for obtaining one ink drop),
however, at least the time 1.2 or more times as much as the time for obtaining one ink drop is required for obtaining two ink drop groups.
Then the technology for changing the size of an ink drop by RF frequency is desired to realize high speed and high image quality recording. However, in the conventional technology for changing the size of an ink drop, the driving mode of the piezoelectric element needs a high output RF amplifier. Therefore, the cost is increased and the apparatus is increased in size.
As another driving mode of the piezoelectric element, the applicant of the invention has proposed the technology disclosed in Japanese Patent Application Laid Open No. 11-72211. In the technology disclosed in Japanese Patent Application Laid Open No. 11-72211, the driving circuit can be reduced in size and the cost of the apparatus can be held down to the lowest by adopting TANK circuit where an inductance and a capacity component of a piezoelectric element constitute a parallel resonance circuit.
The TANK circuit, however, has the problem that the resonance frequency is single and the gain is remarkably lowered for a different RF frequency so that sometimes an ink drop can not be discharged.
SUMMARY OF THE INVENTION
The present invention has been proposed in view of the above circumstances and an object of the present invention is to provide a driving circuit for an acoustic printer which enables high speed and high image quality recording and reduction of size and cost.
Other object of the present invention is to provide an acoustic printer using a driving circuit.
The first aspect of the present invention is a driving circuit for an acoustic printer adapted to form an image on a recording medium by an ejector for discharging stored ink by an ultrasonic wave generated from a piezoelectric element in response to the supply of an a.c. signal. The driving circuit comprises a resonance circuit which includes an inductance connected in parallel to the piezoelectric element, has a plurality of predetermined resonance frequencies, and is set to one resonance frequency among the plural resonance frequencies; and setting means for setting the resonance frequency of the resonance circuit.
According to the first aspect of the present invention, the resonance circuit includes the inductance connected in parallel to the piezoelectric element, and a parallel resonance circuit (so-called TANK circuit) is constituted by the capacity component contained in the piezoelectric element and the inductance. In this arrangement, according to a supplied designated a.c. signal, an a.c. signal on a resonance frequency in the parallel resonance circuit is output. When the signal on the resonance frequency is input to the piezoelectric element, the ultrasonic waves are generated from the piezoelectric element and stored ink is discharged from an ejector to form an image on a recording medium.
In this aspect, the resonance circuit has plural resonance frequencies, and the plural resonance frequencies are set by setting means, whereby the resonance frequency of the resonance circuit is changed, the frequency of an a.c. signal supplied to the piezoelectric element is changed, and the frequency for driving stored ink is changed. Accordingly, the size of an ink drop discharged above the critical point of surface tension of ink is changed. In other words, the resonance frequency of the resonance circuit is set by the setting means to control the size of an ink drop discharged from an ejector.
That is, the resolution can be improved by controlling the size of the ink drop and also the gradient can be improved. Since an image is recorded by changing the frequency of an a.c. signal supplied to the piezoelectric element, an image can be recorded with good image quality without superposition of dots so as to record an image at high speed.
Furthermore
Inoue Nanao
Kondo Yoshinao
Morita Naoki
Takahashi Kunihiro
Barlow John
Fuji 'Xerox Co., Ltd.
Morgan & Lewis & Bockius, LLP
Tran Ly T
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