Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-11-29
2002-06-11
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C327S111000
Reexamination Certificate
active
06404451
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a device for driving a print head of an image forming apparatus. More particularly, the present invention is directed to a circuit for generating an adjustable control voltage applied to electrodes in a print head of a charge deposition printing system.
In systems for electron beam imaging and charge deposition printing, a print head having several closely spaced RF electrodes with a number of overlapping, transverse control electrodes (fingers) is commonly used to deposit charges on an imaging member. The print head may be configured to deposit either positive or negative charge, and the negative charge may consist partly or entirely of either ions or electrons. Print heads of this type are described in several U.S. Patents including, for example, U.S. Pat. Nos. 4,160,257; 4,992,807; 5,278,588; 5,159,358 and 5,315,324.
Generally in systems using this type of print head the RF electrodes are selectively activated with a high-voltage RF drive signal which generates a localized plasma (that is, a localized charge source). The fingers, when maintained at a first potential, retain charge carriers within the charge source. Applying a control voltage to a finger electrode allows the charge carriers to escape from the charge source region at the crossing of the activated RF electrode and the finger. The charges gated from the charge source region are deposited on an imaging member, thereby forming a latent image that may be used to retain toner for transfer to a permanent recording media such as paper. By controlling the application of the high voltage RF drive signals along with the potential of the control voltage applied to the fingers, a specific pattern of charges can be deposited.
The accuracy with which the pattern of charges is deposited upon the imaging member depends, in part, upon the accuracy of the timing, duration and potential of the control voltage applied to the fingers and the accuracy of the RF signals energizing the RF electrodes. Assuming accurate application of drive signals to the RF electrodes, applying a control voltage to the individual fingers for a fixed period of time substantially co-extensive with the application of the RF drive signal produces a fixed amount of charge per activation of the finger. Varying the duration that a control voltage is applied to the finger varies the amount of charge deposited. Similarly, varying the potential applied to the finger modulates the amount of charge delivered by the print head to the imaging member. While it is necessary for some applications such as gray scale imaging to vary the total amount of charge deposited on the imaging member, any mechanism for generating and depositing charges must be precisely controlled to provide uniform imaging and ensure a faithful reproduction free of objectionable image artifacts.
Several methods and devices have been developed to precisely control the potential and timing of the control voltage supplied to finger electrodes, discussions of which can be found in U.S. Pat. Nos. 4,841,313; 4,992,807 and 5,239,318. While existing devices and methods accurately control the potential and/or timing of the voltage provided to the fingers, inherent characteristics of the print heads may limit the effectiveness of such devices. More specifically, charge deposition print heads can exhibit a significant variation in the amount of charge generated and supplied from different charge source regions (RF electrode/finger crossings) excited by the same RF drive signal and control voltage combination.
This deviation in charge output between charge source regions requires a mechanism to individually tune each charge source region output to calibrate the print head to ensure uniform imaging. Normalizing the charge source to charge source region output requires providing a specific control voltage to each finger/electrode crossing and/or supplying the control voltage for given time intervals for each finger/electrode crossing. With existing finger driver circuits, providing different control voltages to each finger requires multiple voltage supplies, each providing a specific voltage. Given the number and density of the fingers and RF electrodes which need to be tuned, a large number of voltage sources may be required making this option relatively expensive and complex. Modifying existing drivers to vary the length of time that the control voltage is applied is a rather inexpensive and simple solution to implement. However, implementing such a solution to normalize charge output with sufficient resolution in charge output to eliminate visual artifacts in the output image comes at the expense of reduced print speed (printer throughput).
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention there is provided a second circuit for driving control electrodes between a reset voltage and an adjustable extraction voltage. This circuit includes a reset switch including a first terminal and a second terminal, the first terminal being connected to a first high voltage source; a plurality of diodes, each one of the plurality of diodes connected between the second terminal of the reset switch and a corresponding one of the control electrodes; and a plurality of voltage control switches, each voltage control switch being capable of assuming an active state and an inactive state, each voltage control switch including first and second terminals, the first terminal being connected to a corresponding one of the control electrodes, and the second terminal being connected to a low voltage source, wherein the extraction voltage supplied to a control electrode is adjusted by adjusting the length of time that the corresponding voltage source is made active.
In accordance with one aspect of the present invention there is provided a method for driving a print head of an image forming device. The method includes (a) setting the voltage at electrodes in the print head to a nonprinting potential; (b) setting the voltage at a plurality of the electrodes to a first printing potential; and (c) drawing current from selected ones of the plurality of the electrodes to reduce the printing potential at the electrodes.
In accordance with one aspect of the present invention there is provided an imaging device comprising: a dielectric imaging member; a print head positioned to deposit charge on the imaging member, the print head including a plurality of RF electrodes and a plurality of control electrodes; an RF driver connected to the plurality of RF electrodes, the RF driver supplying an RF voltage to the RF electrodes; and a circuit for driving the plurality of control electrodes between a reset voltage and a control voltage, the control voltage being adjustable at each control electrode. The circuit includes a reset switch being capable of assuming a first state and a second state, the reset switch including a first terminal and a second terminal, the first terminal being connected to a first voltage source; a plurality of diodes, each one of the plurality of diodes connected between the second terminal of the reset switch and a corresponding one of the control electrodes; and a plurality of voltage control switches, each voltage control switch being capable of assuming an active state and an inactive state, each voltage control switch including first and second terminals, the first terminal being connected to a corresponding one of the control electrodes, and the second terminal being connected to a second voltage source, wherein the extraction voltage supplied to a control electrode is adjusted by adjusting the length of time that the corresponding voltage source is made active.
REFERENCES:
patent: 4841313 (1989-06-01), Weiner
patent: 4992807 (1991-02-01), Thomson
patent: 5239318 (1993-08-01), Vannerson
patent: 5687001 (1997-11-01), Shibuya et al.
patent: 5886723 (1999-03-01), Kubelik et al.
patent: 6015208 (2000-01-01), Wakahara et al.
Baker Wm Keith
Theodoulou Sotos M.
Eipert William
Pendegrass Joan
Xerox Corporation
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