US3638016A

US3638016A - Self-biasing grid control corona systems

Info

Publication number: US3638016A
Application number: US31717A
Authority: US
Inventors: Lee Fitzpatrick Frank
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1970-04-24
Filing date: 1970-04-24
Publication date: 1972-01-25
Anticipated expiration: 1989-01-25
Also published as: DE2119715B2; DE2119715A1; DE2119715C3; FR2090531A5; GB1352412A

Abstract

Contrast control in two-electrode electrographic apparatus is achieved by sensing changes in the charging current and automatically correcting the bias voltage or terminating the charge deposition on the receiver sheet. For bias voltage correction, a capacitor is connected between the photoconductive grid and the xerographic backing plate. For charge deposition termination, a signal indicative of charge buildup is compared with an external contrast control signal and the electrical circuit to the backing plate is disconnected when the charge buildup signal equals the external contrast signal.

Description

' Frank 1451 Jan. 25, 1972 5 SELF BIASING GRID CONTROL 3,062,956 11/1962 Codichini ..250/49.5 3,335,274 fill 967 Codichini et al ..250/49.5

United States Patent CORONA SYSTEMS Primary ExaminerWilliam F. Lindquist Attorney-W. H. J. Kline, P. R. Holmes and L. F. Seebach [57] ABSTRACT Contrast control in two-electrode electrographic apparatus is achieved by sensing changes in the charging current and automatically correcting the bias voltage or terminating the charge deposition on the receiversheet. For bias voltage correction, a capacitor is connected between the photoconductive grid and the xerographic backing plate. For charge deposition termination, a signal indicative of charge buildup is compared with an external contrast control signal and the electrical circuit to the backing plate is disconnected when the charge buildup signal equals the external contrast signal.

5 Claims, 3 Drawing Figures PATENIED m2 5:912

LEE E FRANK INVENTOR. 4M

' AGENT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to xerography and more particularly to an improved electric circuit to control the contrast of xerographic copies.

2. Description of the Prior Art In the art of xerography, a uniform negative charge is laid down on. a photoconductive grid, by means of a coronagenerating device. The grid can then be exposed to a radiation pattern derived from the subject matter to be reproduced by any conventional projection technique. Such an exposure will decrease the negative charge on the photoconductive grid in those areas or regions which correspond generally to the bright regions on the original and leave the negative charge substantially unaffected in those regions which are dark or substantially opaque. A latent electrostatic charge image is then produced upon a receiving sheet by directing an ion stream through the photoconductive grid. The ion stream will pass more readily through those regions which have been decreased in negative charge by exposure tothe radiation and will be blocked by the charged regions. The latent image on the receiving sheet can then be developed by any suitable xerographic means.

Multiple copies are made by replacing the receiving sheet and redirecting the ion stream through the charged photoconductive grid, without any necessity for reexposure of the subject material being copied. However, the charge pattern on the photoconductive grid will decay-with time after the exposure. In the prior art, the amount of. decay has been determined by merely guessing or averaged over a period of time. In addition, some compensation must be made for the dark current of the photoconductor. It is therefore desirable to provide means which will automatically compensate for varia tions in the photoconductor characteristics by controlling the current passing to the receiving sheet, or the time of charge buildup on the receiving sheet.

SUMMARY OF THE INVENTION The control system of this invention comprises a capacitor thatis connected between the photoconductive grid and the conductive backing plate of the electrographic system. The capacitor is charged to provide a voltage differential between the photoconductive grid and the receiving sheet. A current source connected in series with the backing plate sets the total current flowing out of the receiver sheet. If excess negative charges flow into the receiver sheet, the voltage across the current source becomes more negative, making the photoconductive grid more negative, and thereby reducing the current flow into the receiver sheet. A constant contrast is thereby achieved with a constant charge transfer time.

ilo control charge transfer time, a comparator circuit compares a signal derived from the voltage across the current source and a signal set by an external contrast control, the circuit to the backing plate being opened when the desired contrast has been achieved. The invention, its objects and advantages, will be apparent to those skilled in the art by the de led description of the preferred embodiments presented below.

BRIEF DESCRIPTION OF DRAWINGS electrode electrographic system in which the automatic bias control device according to this invention is incroporated;

FIG. 2 is a schematic electrical wiring diagram of one embodiment of the bias control device disclosed in FIG. I; and

FIG. 3 is a schematic electrical wiring diagram of a transfer time control device for use in the electrographic system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1,,the xerographic apparatus comprises support means, such as a backing plate 1 for carrying a receiver sheet 2. A constant current source 3, such as an illuminated vacuum photodiode, solid-state constant current diodes, a pentode vacuum diode, or a high-voltage transistor, is connected in series with backing plate 1. A conventional corona-generating means, such as corona source 4 is utilized to produce an ion stream by being held at a high DC potential by means of potential sources A and B when switch S is closed. A grid means is interposed between corona source 4 and backing plate 1 said grid means comprising a first grid 5 which is held at a DC potential, which is less than that of corona source 4, by means of a potential source B. A second grid 6 is completely coated with a photoconductive material and interposed between grid 5 and backing plate 1. Capacitor 7 is electrically connected between grid 6 and backing plate 1. The grids 5 and 6 can be formed of parallel wires as shown in FIG. I or of a wire mesh.

Initially, corona source 4 is activated by closing switch S with no receiving sheet 2 being present on plate 1. A uniform negative charge is applied to photoconductive grid 6, the magnitude of the charge being controlled by the potential of grid 5 and the duration of exposure time. Best results have been obtained when the level of charge on photoconductive grid 6 produces a voltage differential of about200 volts between grid 6 and backing plate 1. Capacitor 7 serves to maintain this voltage differential. Corona source 4 is deactivated by opening switch S and photoconduc'tive grid 6 is then exposed to a radiation pattern derived from the subject material to be copied. The negative charge onphotoconductor grid 6 is decreased in those areas where radiation strikes grid 6. A

. charge pattern is formed on grid 6 which is a reproduction of the subject material, the areas corresponding to dark areas on the subject material having a higher negative charge than the areas corresponding to bright areas. This exposure to a radia tion pattern can be accomplished in any manner; for example, by projecting on the grid 6, a light pattern generated by a film transparency (negative or positive) in a conventional manner,

' After the exposure to the radiation pattern as derived from the subject material to be copied, a receiving sheet 2 is placed upon backing plate 1. Corona source 4 is reactivated by closing switch S, thereby directing a stream of. ions through grid 5 and photoconductive grid 6 to receiving sheet 2. The ion stream is modulated by the double-grid structure so that the flow of ions to the receiving sheet is in accordance with the radiation pattern. A latent electrostatic image is thereby produced on receiving sheet 2, which is a reproduction of the subject material. Multiple copies of the subject material can be made without any additional exposure by placing a new receiver sheet 2 on backing plate I and modulating the flow of another stream of ions through the double-grid structure. A more complete and detailed description of such a double-grid electrographic system is disclosed in my copending application, Ser. No.492,988, filed Sept. 27, 1965.

After receiving sheet 2 has been processed by any conventional xerographic technique, adjustment can be made to current source 3 to correct for the desired contrast in the final copy. Current source 3 is electrically in series with backing plate 1 and thus sets the total current flowing out of the receiver sheet 2 and capacitor 7, that is, the ion current and the capacitive current. Because essentially no current flows to photoconductive grid 6, capacitor 7 does not discharge and, thus, current source 3 sets the current from receiver sheet 2. Should the copy prove too light, current source 3 can be adjusted to allow a larger charge to be deposited upon receiver sheet 2 during a given charge transfer time and thus darken the resultant copy. Alternatively, if the contrast of the copy is too great, current source 3 can be adjusted to produce a smaller total charge on receiver sheet 2 and, thus lighten the resultant copy.

When multiple copies of the subject material are made, changes in the charge on photoconductive grid 6, with time,

will affect the final copy. As the charge on grid 6 dissipates, a higher percentage of the ion stream from corona source 4 will penetrate through the double-grid structure to receiving sheet 2. Normally, this would result in darker copies because of the constant transfer time. However, in the practice of the invention, this will produce a larger current flowing out of backing plate 1 and will make the potential across current source 3 more negative. The potential across current source 3 is coupled to photoconductive grid 6 by capacitor 7, thereby making grid 6 more negative, and reducing the ion stream passing to the receiver sheet. Thus changes in the charge on photoconductive grid 6, with time, which produce a larger current to the receiver sheet 2, are automatically compensated for by increasing the negative bias on photoconductive grid 6, thereby reducing the current passing to receiver sheet 2.

While the embodiment described above automatically corrects for dissipation of the charge on grid 6 after exposure, it does not insure correct contrast on the first copy to be made. Contrast control for this and subsequent copies can be achieved by altering transfer control time as in the embodiment shown in FIG. 3, wherein corresponding numbers indicate the same elements as in FIGS. 1 and 2. A normally open switch 15 is connected in series with backing plate 1. A highimpedance voltage-sensing amplifier circuit 9 is connected between current source 3 and the common terminal of switch 15 and capacitor 7. The output of amplifier circuit 9 is connected to the input of a nonlinear response network 10. The output of network 10 is connected to the input of a standard integrator circuit 11, which provides, as an output signal, one input to comparator circuit 12. The second input or operating signal to comparitor 12 is derived from a variable external contrast control 13. The output or control signal from comparator circuit 12 is connected to ground through a relay 14 which actuates switch 15, thereby completing the series circuit through current source 3.

In operation, the desired contrast is initially set on external contrast control 13 which can be a variable resistor, such as a 'rheostat. Photoconductive grid 6 is charged to a desired potential by closing switch S and activating corona source 4.

' After corona source 4 is deactivated by opening switch S, grid 6 is exposed to a radiation pattern corresponding to the subject that is to be copied. Switch S is then closed and corona source 4 reactivated. Switch is normally open. However, the output or control signal from comparitor 12 will initially be nonzero and, thus, will energize relay 14 and hold switch 15 in a closed condition, thereby completing the circuit to plate 1. As current flows to plate 1, amplifier 9 senses the resultant voltage across current source 3. Network 10 is utilized to translate a voltage sensed across current source 3 into the control current which compensates for the nonlinearity of the current flowing through the grids 5 and 6 to backing plate 1. This control current is then integrated with respect to time by integrating circuit 11, the resulting output signal being indicative of the charge produced on receiver sheet 2 since charging began. In comparitor circuit 12, this output signal is compared with an operating signal which is indicative of the contrast desired in the final print, the operating signal being derived from the setting of the external contrast control 13. When the total charge received by receiving sheet 2 reaches that set on the external contrast control 13, the control signal from comparator 12 becomes zero, thereby deactivating relay l4 and allowing opening of switch 15, which has been held in a closed position during charging of receiver sheet 2.

When multiple copies are being made, charge decay in grid ,6 will produce a large ion flow to receiver sheet. 2. Contrast Econtrol is automatically achieved by the circuitry of FIG. 3 since a larger ion flow will produce a larger voltage across current source 3. The voltage is sensed and converted to the comparable plate current; the current is integrated and compared with the externally set signal in comparitor 12. A shorter transfer time will result as the charge buildup signal will reach e uality with the external contrast signal in a shorter time.

he invention has been describe in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. In particular, it is understood that the circuitry illustrated in FIGS. 2 and 3 is only one example of circuitry which could be utilized to achieve copy contrast control by use of signal feedback from a constant current source in the backing plate circuit. In addition, the signal feedback illustrated in the embodiments described above could be adapted to control uniform charging of a receiving sheet by maintaining a constant charging current to the plate or attaining a fixed total charge on the plate. Such a device would result from removal of the subject matter exposure step as described in connection with the preferred embodiments and thereby allow charging of a receiving sheet 2 to a uniform charge density for later discharge in accordance with well-known xerographic principals.

Iclaim:

1. Apparatus for charging a receiving sheet comprising an electrically conductive support means for said receiving sheet; corona-generating means arranged in spaced relation to said support means for directing a charging current towards said support means; grid means interposed between said support means and said corona-generating means, said grid means including a first grid of electrically conductive material and a second grid of electrically conductive material coated with a photoconductive material and arranged in spaced, generally parallel relation to and between said first grid and said receiving sheet; a capacitor electrically connected between said second grid and said support means, said capacitor being charged to some predetermined voltage before charging current is directed towards said support means for applying a bias voltage to said second grid; and a constant direct current supply electrically connected between said support meansand ground whereby any change in said charging current will vary the current passing to said current supply, thereby varying the bias voltage applied to said second grid.

2. Apparatus as in claim 1 including control means serially connected between said direct current supply and ground and including means for generating an operating signal indicative of a predetermined charge level on said receiving sheet to maintain said capacitor connected to said support means so long as said charging current is being directed toward said support means.

3. Apparatus as in claim 2 wherein said generating means includes switch means electrically connected in series with said support means, capacitor and second grid, said switch means being movable to a closed position in response to said operating signal and being maintained in said closed position thereby until said receiving sheet is charged to said predetermined level.

4. Apparatus as in claim 3 wherein said switch means, when in said closed position, electrically interconnects said direct current supply and said capacitor to said support means for automatically correcting said bias voltage in accordance with any change in said charging current. 1

5. Apparatus as in claim 4 wherein said control means, includes voltage-sensing means responsive to the voltage derived from said direct current supply for producing a first signal indicative of said charging current; integrating means responsive to said first signal for generating an output signal; and means for comparing said output signal and said operating signal and for generating a control signal, when said output and operating signals correspond generally in amplitude, to release said switch means, thereby indicating that said receiv ing sheet has been charged to said predetermined charge level.

Claims

1. Apparatus for charging a receiving sheet comprising an electrically conductive support means for said receiving sheet; corona-generating means arranged in spaced relation to said support means for directing a charging current towards said support means; grid means interposed between said support means and said corona-generating means, said grid means including a first grid of electrically conductive material and a second grid of electrically conductive material coated with a photoconductive material and arranged in spaced, generally parallel relation to and between said first grid and said receiving sheet; a capacitor electrically connected between said second grid and said support means, said capacitor being charged to some predetermined voltage before charging current is directed towards said support means for applying a bias voltage to said second grid; and a constant direct current supply electrically connected between said support means and ground whereby any change in said charging current will vary the current passing to said current supply, thereby varying the bias voltage applied to said second grid.

4. Apparatus as in claim 3 wherein said switch means, when in said closed position, electrically interconnects said direct current supply and said capacitor to said support means for automatically correcting said bias voltage in accordance with any change in said charging current.

5. Apparatus as in claim 4 wherein said control means, includes voltage-sensing means responsive to the voltage derived from said direct current supply for producing a first signal indicative of said charging current; integrating means responsive to said first signal for generating an output signal; and means for comparing said output signal and said operating signal and for generating a control signal, when said output and operating signals correspond generally in amplitude, to release said switch means, thereby indicating that said receiving sheet has been charged to said predetermined charge level.