US3912989A

US3912989A - Method and apparatus for charging by corona discharge

Info

Publication number: US3912989A
Application number: US451008A
Authority: US
Inventors: Masanori Watanabe; Takaaki Konuma; Hirokazu Asano
Original assignee: Kip KK
Current assignee: Katsuragawa Electric Co Ltd
Priority date: 1973-03-30
Filing date: 1974-03-14
Publication date: 1975-10-14
Anticipated expiration: 1992-10-14
Also published as: DE2414907B2; CA1015023A; GB1467972A; FR2223732A1; JPS49123336A; DE2414907C3; FR2223732B1; JPS5541430B2; DE2414907A1

Abstract

In a corona discharge device for charging the surface of a body, for example a photosensitive element utilized in the art of electrophotography, the corona discharge current is controlled to vary at a predetermined rate at the commencement and termination of corona discharge so as to ensure uniform deposition of electric charge on the surface of the element.

Description

United States Patent Watanabe et al.

METHOD AND APPARATUS FOR CHARGING BY CORONA DISCHARGE Inventors: Masanori Watanabe; Takaaki Konuma; Hirokazu Asano, all of Tokyo, Japan Kabushiki Kaisha KIP, Tokyo, Japan Filed: Mar. 14, 1974 Appl. No.: 451,008

Assignee:

Foreign Application Priority Data Mar. 30, 1973 Japan 48-36293 US. Cl. 317/262 A; 250/325 Int. Cl. H01T 19/00 Field of Search 317/262 A;

IQ DRIVING CIRCUIT l2 CURRENT 5 REGULATOR j SOURCE Primary ExaminerJ. D. Miller Assistant ExaminerI-larry E. Moose, Jr. Attorney, Agent, or FirmBosworth, Sessions & McCoy [57] ABSTRACT In a corona discharge device for charging the surface of a body, for example a photosensitive element utilized in the art of electrophotography, the corona discharge current is controlled to vary at a predetermined rate at the commencement and termination of corona discharge so as to ensure uniform deposition of electric charge on the surface of the element.

8 Claims, 9 Drawing Figures l4 HIGH VOLTAGE a l l SYNCHRONIZING CIRCUIT 4 CORONA DISCHARGE DEVICE l3 TIME CONTROLLER U.S. Patent 001. 14, 1975 CORONA DISCHARGE Sheet 1 of 2 3,912,989

.F I G I 5 Q 7 8 1 I 1 9 z o 2 I 3 19 F I G .2 FIG .3

o I -TIME o l --T|ME 1- I' I Z a 5: 3 0: O 3 0 7 F I G .4

IQ DRIVING CIRCUIT I2 CURRENT l4 HIGH VOLTAGE I REGULATOR 1 SOURCE L 1/ ll SYNCHRONIZING 4 gi mg CIRCUIT b RI DEWCE M QwQ/R l3 TIME CONTROLLER METHOD AND APPARATUS FOR CHARGING BY CORONA DISCHARGE BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for depositing electric charge on a surface of relatively large area by means of corona discharge, and more particularly to a method and apparatus for uniformly depositing electric charge in a short time on the surface of the photosensitive element of an electrophotographic apparatus on which an electrostatic latent image is to be formed.

In the art of electrophotography or the like wherein it is necessary to charge the surface of a body to be charged, e.g. a photosensitive element having a relatively large area, it has been the practice to provide a corona discharge device including a plurality of corona discharge wires confronting the image forming surface of the element and to move the corona discharge device in parallel with and relative to the image forming surface. The corona discharge device utilized in this method is usually constructed such that at least two corona discharge wires pass by respective points on the image forming surface during the corona discharge interval so as to prevent non-uniform distribution of the deposited charge and to prevent the formation of the shadows of the discharge wires, but where a large quantity of charge is to be deposited in a short time there is a defect in that non-uniform distribution of the deposited charge in the form of stripes parallel to the corona discharge wires often results. We have found that this defect is caused for the following reason. More particularly, it has been well known that when a high voltage is impressed upon an ordinary image forming surface, a large charging current flows instantaneously through the photosensitive element at the time of commencement of charging irrespective of the position of the corona discharge device. For this reason, portions of the image forming surface which were confronting the corona discharge wires at the time of commencement of discharge receive more charges than those remote therefrom, thus resulting in nonuniform charge distribution. Although the operation at the time of termination of the deposition of charge on the image forming surface is not yet theoretically established, we have noted that there is a substantial difference between the quantities of the charges deposited on the portions immediately beneath the corona discharge wires and on the portions remote therefrom when the deposition of the charge is interrupted while the corona discharge device is moving along the image forming surface or is maintained stationary. We have noted that these phenomena of nonuniform charge distribution occurring both at the time of commencement and interruption of the corona discharge are enhanced when the voltage impressed upon the corona discharge device is increased for the purpose of increasing the quantity of the charge to be deposited.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a new and improved method and apparatus for uniformly depositing electric charge on a surface of a relatively large area, such as the image forming surface of a photosensitive element utilized in electrophotography.

According to one aspect of this invention there is provided a method of charging the surface of a body to be charged by corona discharge wherein the surface is charged by means of a corona discharge device substantially covering the entire surface of the body and by relatively moving the body and the corona discharge device, characterized in that the current flowing through the body is varied at a predetermined rate during the period of commencement of the corona discharge until a predetermined current value is reached and that the current is gradually decreased from the predtermined current value during the period of termination of the corona discharge.

According to another aspect of this invention there is provided apparatus for charging a body to be charged by means of corona discharge of the type comprising a corona discharge device substantially covering the surface of the body, a source of power for energizing the corona discharge device, and means for moving the corona discharge device relative to the body in a direction substantially parallel to the surface of the body, characterizedin that there is provided control means for controlling the corona discharge current of the corona discharge device, and that the control means includes means for varying the corona discharge current at a predetermined rate at the time of commencement of the corona discharge until a predetermined current value is reached and means for gradually decreasing the corona discharge current from the predetermined current value at the time of termination of the corona discharge.

The control means may be an electrical control circuit or a mechanical device.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic representation showing an arrangement of a conventional corona discharge device and a photosensitive element;

FIG. 2 is a graph showing the waveform of the current flowing through the grounded plate of the corona discharge device shown in FIG. 1;

FIG. 3 is a graph showing the waveform of the current flowing through the photosensitive element shown in FIG. 1;

FIG. 4 is a block diagram showing one example of the control device utilized to carry out the invention;

FIG. 5 is a graph similar to FIG. 2 and shows the current waveform obtained by carrying out the method of this invention;

FIGS. 6, 7, and 8 are graphs similar to FIG. 3 and show current waveforms of the current flowing through the photosensitive element when the method of this invention is applied; and

FIG. 9 is a diagrammatic perspective view showing a modified embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To have better understanding of the invention, prior art method and apparatus will first be described with reference to FIG. 1. The body to be charged by corona discharge shown therein comprises a photosensitive element 30 utilized in electrophotography and including an insulative layer 2, a photosensitive photoconductive layer 3 and a grounded electrode layer 1 which are bonded together into an integral structure. According to certain types of electrophotography, the insulative layer 2 is used as an image forming surface and is deposited with electric charges by means of a corona discharge device 4 spaced apart a short distance from the insulative layer 2 and moved in parallel with the surface thereof. The corona discharge device 4 has a size sufficient to cover the entire surface of the insulative layer 2 for the purpose of depositing charge in an extremely short time and includes a grounded conductive substrate 5, a plurality of equally spaced corona discharge wires 6 insulated from the substrate and a plurality of parallel conductive vanes 7 each interposed between adjacent discharge wires 6 and electrically connected to the substrate 5. In some types of electrophotography, a light image is projected upon the photoconductive layer 3 through the corona discharge device 4 in which case the substrate 5 is made of a conductive glass plate such as Nesa Glass (registered trade mark) or takes the form of a rectangular frame. In another type of electrophotography, a light image is projected through the electrode layer in which case the electrode layer 1 must be transparent to light. The corona discharge wires 6 are connected to the negative or positive terminal of a DC source 9 via a switch 8 depending upon the conductivity type of the photosensitive element 30. The opposite terminal of the source 9 is grounded as shown.

To deposit a charge of negative polarity, for example, on the insulative layer, the switch 8 is closed to create corona discharge between corona discharge wires 6 and vanes 7 and or substrate 5 while the corona discharge device 4 is being moved at a constant speed in parallel with the surface of the insulative layer 2 thereby depositing negative charge thereon. A predetermined interval later, switch 8 is opened and the movement of the corona discharge device 4 is stopped thus completing the deposition of the charge. The current flowing through the grounded substrate 5 of the corona discharge device 4 during this interval is shown by the graph shown in FIG. 2 in which t represents an instant at which the corona discharge was commenced and t an instant at which the corona discharge was terminated. As shown, the current is maintained at a constant value between instants t and The graph shown in FIG. 3 shows the variation in the current flowing through the photosensitive element during the interval between instants t and As shown, a large charging current I flows through the photosensitive element 30 at the instant t when the corona discharge is initiated and the current gradually attenuates to a small value I, at the instant I at which the corona discharge is terminated. Thereafter, the current rapidly decreases to zero. The reason that the current does not instantly decrease to zero at instant is caused by the fact that it takes a certain time for deionization.

These curves show the result of the following experiment. In this experiment, the photosensitive photoconductive layer was omitted and a body to be charged was comprised by a polyester resin film having a thickness of 16 microns and an area of 300 X 300 mm and the electrode layer consisted of an aluminum layer vapour deposited on the rear surface of the polyester resin film. Each corona discharge wire had a diameter of 0.06 mm, the spacing between the corona discharge wires and the polyester resin film was selected to be 7 mm. The corona discharge device was moved at a speed of mm/sec, and operated for 0.5 sec. under an applied voltage of 4,60OV and a total current of 60 mA. When the charge deposited on the surface ur the insulative layer was developed or visualized by sprinkling a toner which is used to develop electrostatic images in the art of electrophotography over thhe surface of the polyester resin film, clearly defined stripes were noted on the portions of the polyester resin film which were positioned immediately beneath the corona discharge wires at the times of commencement t and termination of the corona discharge t showing nonuniform distribution of the deposited charge.

From the result of this experiment, we have noted that such a nonuniform distribution of the charge can be prevented by controlling the current flowing through the member to be charged at the times of initiation and termination of the corona discharge.

FIG. 4 shows a connection diagram of one example of a current control device that can be used in combination with the corona discharge device shown in FIG. 1. This control device comprises a driving circuit 10 which operates to move the corona discharge device 4, FIG. 1, at a constant speed in parallel with the surface of the insulative layer 2 in response to an input signal. During movement, the corona discharge device 4 operates a first microswitch, not shown, included in a synchronizing circuit 11 for energizing a current regulator 12 which is connected to a time controller 13 including a plurality of variable resistors R R and R Variable resistor R is used to control the build-up rate of the corona discharge current between an interval of I or the inclination angle 0, until a current of a definite value I (see FIG. 5) is reached at the commencement of the discharge. Variable resistor R is used to maintain the current I at the constant value until an instant I is reached, whereas the variable resistor R is used to control the rate of build-down of the corona discharge current between an interval t.,, that is the inclination angle 6 Thus, by the suitable adjustments of variable resistors R R and R it is possible to obtain any desired current waveforms as well as the discharge period.

The output from the current regulator 12 is applied to a source of high voltage 14 so as to apply a variable voltage to the corona discharge device 4 to deposit a charge of the desired polarity (in this case, negative) on the surface of the insulative layer. The current flowing through the photosensitive element 30, or a body to be charged, is shown in FIG. 6, thus assuring uniform charge on the surface of the insulative layer as will be described later. When the corona discharge terminates at t.,, a signal is fed back from the output of current regulator 12 to synchronizing circuit 11, thereby deenergizing current regulator 12, high voltage source 14 and corona discharge device 4. At this time the corona discharge device 4 actuates a second micro-switch, not shown, for deenergizing the driving circuit 10 for the corona discharge device 4.

In the control circuit shown in FIG. 4, the parameters for the corona discharge device and of the body to be charged were selected to be the same as the experiment described above and

angles

6 and 6 were varied variously by adjusting variable resistors R R and R In this experiment, the time scale on the abscissa was selected to be 0.1 sec/cm, the current scale on the ordinate to be 20 mA/crn, the interval between t and to be 0.08 sec, and'the' current during this interval was varied uniformly from zero to 60 mA. The interval between t and was selected to be 0.12 sec. and the current was maintained at a constant value during this interval. Further, the interval between and L, was selected to be 0.3 sec. and during this interval the current was varied uniformly from 60mA to zero. Under these conditions, it was noted that the surface of the insulative layer was charged uniformly when angle 0, was selected to be more than 15 and angle 6 to be less than 45, and that the current flowed through the insulative layer has varied as shown in FIG. 6. As can be noted from the graph shown in FIG. 6 there is no spike in the current wave. I I

In another experiment, the speed of the corona discharge device was set to l 50 mm/sec. and angles 6 and 0 were varied under the same conditions as the first mentioned experiment. With the same time and current scales as described above, no nonuniform charge distribution on the surface of the insulative layer was, noted where angle 0 was larger than about and angle 0 was smaller than 55. As can be noted from these experiments, it is possible to increase angle 0 where the speed of movement of the corona discharge device is increased.

In still another experiment, the control circuit was set to obtain an attenuation curve a shown in FIG. 7. With the time and current scales described above, it was confirmed that substantially uniform distribution of the charge deposited on the surface of the insulative layer can be obtained when the inclination angle 6 during current attenuation was adjusted to lie between about 45 and 50. Further, no nonuniform distribution of the deposited charge was noted when the control circuit was set to obtain an attenuation curve b shown in FIG. 7.

Further, with the time and current scales described above, an experiment was performed in which the attenuation angle of the current wave flowing through the grounded substrate of corona discharge device was set to 45 and the current was interrupted at an intermediate point of attenuation, as shown in FIG. 8. It was found that when the ratio of current B at the time of interruption to the maximum current A was larger than l:3, a stripe shaped nonuniform distribution of the deposited charge was noted and that substantially uniform distribution of the deposited charge was obtained when the ratio was selected to be less than one-third.

Further, in accordance with this invention it is also possible to obtain a current waveform shown in FIG. 6 which is necessary to assure uniform charge distribution by varying the distance between the corona discharge device and the surface of the body to be charged while maintaining at a constant value as shown in FIG. 2, the current flowing through the grounded substrate of the corona discharge device.

FIG. 9 diagrammatically shows the charging device utilized in this modified method. As shown in this figure, opposite ends of a pair of stationary parallel shafts and 16 are pivotally connected to a corona discharge device 18 through parallel arms 17. The corona discharge device 18 may have the same construction as that of device 4 shown in FIG. 1. One end of a rope 19 is connected to one end of the corona discharge device .18 and the opposite end of the rope 19 is connected to a drum 21 rotated by a motor 20. A spring 22 is interposed between the stationary portion of the copying machine and the opposite end of the corona discharge device 18 for biasing the same to the left as viewed in FIG. 9.

After disposing a body to be charged or an insulative layer 23 beneath the corona discharge device 18 and spaced apart therefrom a little and parallel therewith, motor 20 and drum 21 are rotated in the counterclockwise direction. Then the corona discharge device will be swung about

stationary shafts

15 and 16 to bring the corona discharge device 18 closer to the surface of the body to be charged 23 while maintaining them in parallel with each other. When current of a definite value is passed through the corona discharge device 18 during this movement, it begins to discharge to commence deposition of charge of a given polarity on the surface of the insulative layer. Upon further rotation of the drum 21, the arms 17 assume the vertical position thereby decreasing to a minimum the distance between the corona discharge device 18 and the insulative layer 23. Further rotation of the drum 21 increases the distance. During this stroke, the operations of the corona discharge device and motor are terminated. Then, the spring 22 pulls the corona discharge device 18 back to the original position. In this method of charging, the charging current flowing through the insulative layer assumes a waveform as shown in FIG. 6, thus assuring substantially uniform charging.

With the arrangement shown in FIG. 9 an experiment was performed by using a minimum distance of 7 mm between the corona discharge wires and the surface of the insulative layer and a distance of 20 mm between them at the commencement and termination of the corona discharge, other conditions being the same as in the first experiment. The result of this experiment showed that current having a waveform similar to that shown in FIG. 6 flowed through the insulative layer and that the charge distribution was substantially uniform.

Although in the foregoing description, the corona discharge device was moved relative to the body to be charged, it will be clear that it is also possible to move the body to be charged relative to the corona discharge device as in the case of a rotary drum type electrophotographic copying machine. Further, although the body to be charged was shown as a three layered photosensitive element in one case and as an insulative layer in the other case it will be clear that the method and apparatus of this invention are applicable to any application requiring uniform charging by corona discharge. The current control circuit shown in FIG. 4 may by replaced by a voltage control circuit for controlling the voltage of a high voltage source for the corona discharge device. As has been pointed out hereinabove, the polarity of the charge is not limited to being negative.

We claim:

1. In a method of charging the surface of a body to be charged by means of a corona discharge device including a plurality'of spaced corona discharge wires and substantially covering the entire surface of said body by relatively moving said body and said corona discharge device substantially parallel with each other, and wherein charging current tends to increase at a high rate through said body at the commencement of the corona discharge and tends to decrease at a high rate at the termination of the corona discharge, the improvement which comprises the steps of controlling the rate of increase of said charging current to a rate lower than said high rate of increase at the time of commencement of the corona discharge until a predetermined current value is reached, and controlling the rate of decrease of said current from said predetermined current value to a rate lower than said high rate of decrease at the time of termination of the corona discharge so as to prevent non-uniform charging of said surface caused by said corona discharge wires.

2. The method according to claim 1 wherein said current is maintained at said predetermined value for a predetermined interval before it is decreased.

3. The method according to claim 1 wherein the rate of current increase is made to be higher at the time of commencement than the rate of current decrease at the time of termination of the corona discharge.

4. ln apparatus for charging the surface of a body to be charged by means of corona discharge of the type comprising a corona discharge device including a plurality of spaced corona discharge wires and substantially covering the surface of said body, a source of power for energizing said corona discharge device, and means for moving said corona discharge device relative to said body in a direction substantially parallel to the surface of said body, and wherein charging current tends to increase at a high rate through said body at the commencement of the corona discharge and tends to decrease at a high rate at the termination of the corona discharge, the improvement which comprises control means for controlling the rate of increase of said charging current to a rate lower than said high rate of increase at the time of commencement of the corona discharge until a predetermined current value is reached and for controlling the rate of decrease of said corona discharge current from said predetermined current value to a rate lower than said high rate of decrease at the time of termination of the corona discharge so as to prevent nonuniform charging of said surface caused by said corona discharge wires.

5. The apparatus according to claim 4 wherein said control means further includes means for maintaining said predetermined current value for a predetermined interval before said decrease of the corona discharge current.

6. The apparatus according to claim 4 wherein said control means comprises an electric control circuit connected to said source of power, said electric control circuit including a plurality of variable resistors, a first one thereof determining the rate of increase of said corona discharge current, a second one determining the rate of decrease of said corona discharge current and a third one determining said predetermined current value.

7. The apparatus according to claim 5 wherein said control means comprises mechanical means for driving said corona discharge device toward the surface of said body during the period of commencement of the corona discharge and awayfrom said surface during the period of termination of the corona discharge while said corona discharge device is being moved relative to said body in a direction substantially parallel to the surface of said body.

8. The apparatus according to claim 7 wherein said mechanical means comprises spaced parallel arms for suspending said corona discharge device.

Claims

1. In a method of charging the surface of a body to be charged by means of a corona discharge device including a plurality of spaced corona discharge wires and substantially covering the entire surface of said body by relatively moving said body and said corona discharge device substantially parallel with each other, and wherein charging current tends to increase at a high rate through said body at the commencement of the corona discharge and tends to decrease at a high rate at the termination of the corona discharge, the improvement which comprises the steps of controlling the rate of increase of said charging current to a rate lower than said high rate of increase at the time of commencement of the corona discharge until a predetermined current value is reached, and controlling the rate of decrease of said current from said predetermined current value to a rate lower than said high rate of decrease at the time of termination of the corona discharge so as to prevent non-uniform charging of said surface caused by said corona discharge wires.

4. In apparatus for charging the surface of a body to be charged by means of corona discharge of the type comprising a corona discharge device including a plurality of spaced corona discharge wires and substantially covering the surface of said body, a source of power for energizing said corona discharge device, and means for moving said corona discharge device relative to said body in a direction substantially parallel to the surface of said body, and wherein charging current tends to increase at a high rate through said body at the commencement of the corona discharge and tends to decrease at a high rate at the termination of the corona discharge, the improvement which comprises control means for controlling the rate of increase of said charging current to a rate lower than said high rate of increase at the time of commencement of the corona discharge until a predetermined current value is reached and for controlling the rate of decrease of said corona discharge current from said predetermined current value to a rate lower than said high rate of decrease at the time of termination of the corona discharge so as to prevent nonuniform charging of said surface caused by said corona discharge wires.

7. The apparatus according to claim 5 wherein said control means comprises mechanical means for driving said corona discharge device toward the surface of said body during the period of commencement of the corona discharge and away from said surface during the period of termination of the corona discharge while said corona discharge device is being moved relative to said body in a direction substantially parallel to the surface of said body.