US3068356A

US3068356A - Xerographic charging apparatus

Info

Publication number: US3068356A
Application number: US36352A
Authority: US
Inventors: Joseph J Codichini
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1960-06-15
Filing date: 1960-06-15
Publication date: 1962-12-11
Anticipated expiration: 1979-12-11
Also published as: DE1214996B

Description

Dec. 11,1962 J. J. CEODICHINI 3,068,356

XEROGRAPHIC CHARGING APPARATUS Filed June 15, 1960 2 Sheets-Sheet 1 INVENTOR. JOSEPH J. CODICHINI A 7' TORNEY Dec. 11, 1962 J. J. CODICHINI 3,058,356

XEROGRAPHIC CHARGING APPARATUS Filed June 15, 1960 2 Sheets-Sheet 2:

8 ORONOD E/ SCREEN 5 CORONODE VOLTS x I03 SCREEN VOLTS x :0

Va l6 /4 %6 SPACING IN INCHES BAREPLA'TE CURRENT AMPS x 10' CORONODE VQLTAGE KV F/ 4 INVENTOR.

G JOSEPH J. CODICHINI BY fa/M A7 TORNE Y United States Patent Office 3,058,356 Patented Dec. 11, 1962 3,968,356 XEROGRAPHEC (IHARGHN G APPARATUS Joseph J. Codichiui, Fairport, N.Y., assignor to Xerox Corporation, a corporation of New York Filed .lune 15, 1960, Ser. No. 36,352 1 Claim. (Cl. 25049.5)

This invention relate to improved electric circuit means for controlling the application of electrostatic charge from a corona generating device to a xerographic plate. More specifically this invention relates to improved and simplified electric circuit means for controlling the output of a corona generating device whereby required surface potential on a xerographic plate is effected with a minimum of compensating adjustment.

In the process of xerography, for example, as disclosed in Carlson Patent 2,297,691 issued October 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject'matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material such as an electroscopic powder that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.

In the art of xerography it has been found that consistent reproductive quality is best effected when the uniform potential applied to the photoconductive surface of a xerographic drum is maintained at a constant value, considered optimum when in the range of approximately 600 volts. In xerographic machines currently in wide commercial use, this potential is applied by a corona generating device, commonly referred to as a scorotron, which is arranged in closely spaced relation to a xerographic plate which may be in the form of a cylindrical drum. It is known in the art that for any value of vol age supplied to the scorotron the resulting corona emission applied to the drum surface to eifect potential thereon is partially a function of the spaced distance between the surface to be charged and the scorotron, and for a given spacing higher supply potential effects higher surface potential. Because of manufacturing tolerances permitted for this spaced distance, and because of variables such as ambient atmospheric conditions and still others inherent in the scorotron itself which also influence corona charge imparted, compensating means are required for adjusting the scorotron supply potential to effect desired surface potential.

The scorotron currently is in wide commercial use and may be of a type disclosed in Walkup Patent 2,777,957 which generally consists of a plurality of regulating screen wires strung parallel to a conductive shield, and a plurality of coronode corona discharge wires arranged parallel to and between the screen wires and the conductive shield. Potential is supplied from a source to the coronode wires which then emits corona current. When arranged for charging a xerographic plate, a portion of corona emitted is imparted through the regulating screen wires to be applied to the plates photoconductive surface, and excess corona current is suppressed and drained off by the regulating screen wires. The magnitude of corona imparted from the scorotron is a function of both coronode potential and screen potential, the screen potential being derived from the corona emission, there existing a nonlinear resistance between the coronode wire and the screen.

Heretofore, it was usual practice to maintain potential on the regulating screen wires equal to potential on the plate surface. To maintain equal potentials, a form of electric voltage divider circuit having compensating variable means was employed whereby the screen potential could be maintained against variable potential supplied to the coronode by a balancing of respective circuits. Since coronode potential and screen potential are each interdependent for effecting surface potential, adjustment of one adversely affected the other, and ultimate adjustment of their respective circuits to effect screen potential equal to required surface potential was a delicate and cumbersome task. In addition, frequent. failure of the divider circuit was experienced because of excessive demands imposed on the circuit under extreme conditions of operation. Attempts to overcome the cause of circuit failure by increasing capacity of the circuit components encountered problems of physical limitation because of dimensional size of the components. Furthermore, the heretofore used power supplies included the divider resistor embedded in potting compounds and when the resistor failed, the power supply had to be scrapped.

Because of the above difficulties, extensive research was conducted from which it was found that reproductive quality of the plate is best when the regulating screen wire potential does not exceed surface potential and quality deteriorates when screen wire potential exceeds surface potential.

The principal object of this invention is to improve electric circuit means of a corona generating apparatus whereby screen potential does not exceed the potential applied on a xerographic plate.

It is a further object of this invention to improve electric circuit means of a corona generating apparatus whereby a single adjustment of coronode potential effects a desired potential on a xerographic plate which equals or exceeds screen potential.

These and other objects of the invention are attained by the means of the apparatus of the invention which includes means to effect a variably preset potential supply to the coronode wires of a corona generating device and a selected resistance in the screen circuit by which magnitude of potential generated on the screen does not exceed plate potential.

A preferred form of the invention is shown in the accompanying drawings in which:

FIG. 1 is a schematic sectional view of a xerographic machine employing a scorotron for applying uniform electrostatic charge to a xerographic drum.

FIG. 2 is a wiring diagram of the scorotron control circuit of the invention.

FIG. 3 is a curve typically illustrating variation of coronode potential and screen potential to obtain plate potential of 600 volts at variable spacing in accordance with the invention; and

FIG. 4 is a curve typically illustrating in accordance with the invention the relationship of co'ronode voltage vs. plate current at a given spacing of 1 1 inch using a resistor in the screen circuit of approximately 1.5 megohm resistance.

Referring to the drawings there is illustrated in FIG. 1 an automatic xerographic apparatus which may be an adaptation of a type disclosed in copending application Serial No. 837,173, filed August 31, 1959, in the name of Cerasani and Lewis.

Xerographic drum 1 includes a cylindrical member mounted in suitable bearings in the frame of the machine and is driven at a constant rate in a clockwise rotation by motor 18. The drum surface 15 comprises a layer of photoconductivematcrial on a conductive backing 16 (FIG. 2) that is sensitized prior to exposure by means of a corona generating device 2 which may be of the type mentioned above.

The exposure of the drum to the light image discharges the photoconductive layer in the areas struck by light whereby there remains on the drum an electrostatic latent image in image configuration corresponding to the light image of copy to be reproduced. For exposing opaque original copy, apparatus 3 may be employed which may be of a type disclosed in the above cited copending application in the name of Cerasani and Lewis Whereas for exposing microfilm images, apparatus 4 is employed which may be an adaptation of a type disclosed in copending application Serial No. 796,561 filed March 2, 1959, in the name of S. R. Johanson.

For exposing opaque copy, copy to be reproduced is set on tray 22 wherefrom it is transported on belts 23 being driven by motor 24. The copy is illuminated by light 25 and the image is reflected from mirror 26 through lens 27 to mirror 28 thence through exposure control 29 normal onto surface 15.

In the alternative, for reproducing microfilm images, motor 34 effects transport of microfilm 35 from supply spool 36 to takeup spool 37. .At an intermediate transport position, film 35 is illuminated by light 3%; and its image is projected through lens 39 and exposure control 29 normal onto surface 15; mirror 28 being pivoted vertically by control arm 30 so as not to obstruct the optical path when projecting microfilm.

Thereafter the latent image is developed by means of developing unit which may be of the type disclosed in copending application Serial No. 393,058, filed November 19, 1953, in the names of Mayo et al. in which a twocornponent developing material 45 which may be of the type disclosed in Walkup Patent 2,63 8,416 is cascaded over the drum surface. Developing material, which is stored on the bottom of housing 46, is carried up by conveyor 47 driven by motor 51 and released into hopper 4-8. The developing material then gravitates down, adhering to the electrostatic latent image on the drum surface. Developer toner '44 consumed in the developing step is replenished from storage container 49 at a rate determined by the position of control gate 50.

After developing, the powder image is electrostatically transferred to a support surface web 17 by means of a second corona generating device 7 similar to corona generating device 2 mentioned above.

The support surface web to which the powder image is transferred may be of any convenient type and is transported by a handling mechanism 6 which may be an adaptation of the type disclosed in Crumrine et al. Patent 2,781,705. The support surface is obtained from supply roll 56 and is fed over suitable guide and tensioning rolls and directed into surface contact with the drum in the immediate vicinity of transfer corona generating device 7. After transfer, the support surface is separated from the drum surface and guided through a suitable fusing apparatus 8 which may be of the type disclosed in Crumrine et 'al. Patent 2,852,651 whereby the powder image is permanently aifixed to the support surface. Thereafter, the support surface is fed over a further system of guide and tensioning rolls and onto a take-up roll 54 that is driven by motor 55.

After transfer, the xerographic drum surface is cleaned by cleaning brush assembly having brushes 57 rotated by motor 58 whereby residual developing material remaining on the drum is removed. Suitable light traps are provided in the system to prevent any light rays from reaching the drum surface, other than the projected image, during the period of drum travel immediately prior to sensitization by corona generating device 2- until after the drum sur face is completely developed.

Referring particularly to FIG. 2, corona generating device 2 includes at least one and preferably three coronode wires 67 electrically series connected and arranged parallel to the drum axis of rotation. Potential is supplied thereto from an available V. AC. source and connected to the apparatus through switch 68. Through continuous variable volt transformer 69, potential is conducted from the secondary side thereof to stepup transformer 70 wherefrom the secondary voltage is connected to A.C.D.C. rectifier 71, which may typically provide half-wave rectification of alternating current. Rectified current of corona generating potential is then supplied by lead wire 72 to coronode wire 67 in the potential range of approximately 6000-9000 volts, the exact voltage supplied being a function of voltage output adjustably varied through variable transformer 60. Capacitor '79 maintains supply potential substantially ripple-free -at high voltage output of rectifier 71. To eliminate shock hazard that would otherwise exist in capacitor 79 after opening switch 68 to disconnect scorotron 2 from its supply potential, bleed wire 81 is provided to shunt capacitor 79 and includes a resistor 80.

With coronode wires 67 energized, corona emanating therefrom is partially dissipated to conductive shield 73 which is suitably grounded by lead wire 74. Regulating screen wires 75 suppress and drain off excess corona current thereby to limit and uniformly distribute corona current passing therethrough to surface '15, to apply a uniform potential thereon. On exposure to light the surface potential is electrically conducted through conducting plate 16, and lead wire 78 to ground.

As stated above, it heretofore was considered necessary that screen wire potential be maintained equal to surface potential. It is now known that reproductive quality of the plate is best when the screen wire potential does not exceed surface potential, with notable decrease in print quality at screen potentials exceeding surface potential. To ensure, therefore, that screen potential is limited in value at maximum anticipated potential supplied to the coronode wires, resistor 77 in the screen circuit is selected having an appropriate resistance. In a typical application of the invention it is sought to maintain approximately 600 volts on the drum surface requiring an application in the range of approximately 6000 to 9000 volts to the coronode wires which effect a correlated range of variation in current to the regulating screen wires as may be seen in FIG. 3. When employing drums of length most suitable for 11-inch web width, a resistor 77 having a resistance value of approximately 1.0 megohm has been found to give satisfactory results whereas for drum lengths most suitable for 24-inch web width an approximate 0.62 megohm resistor has been found to give satisfactory results. When employing these resistance values, the desired results have been achieved for equip ment manufactured within limits of manufacturing tolerances with screen potential dropping to as low as volts and volts respectively for drums of 11-inch and 24- inch web width at 6000 volt supply potential. In FIG. 4, bare plate current, which is correlated to plate potential is shown plotted against coronode voltage for a screen to plate spacing of inch.

In operation, with drum 1 in constant rotation, switch 68 is closed energizing coronode wire 67, and surface 15 is scanned by electrometer sensing device 83, which may be of a type commercially available. The potential sensed thereby is conducted by lead wire 84 to amplifier '85 thence to meter 86, where the magnitude of surface potential is indicated. By adjusting transformer 69, potential supplied to coronode wires 67 is varied until the desired potential on surface 15 is indicated on meter 86.

By the apparatus thus described, there is disclosed electric circuit means whereby a uniform potential of desired magnitude is effected on the photoconductive surface of a xerographic plate by a single adjustment means. By the circuitry of the invention, the resistance of the entire circuit is considerably reduced resulting in an overall reduction of power consumption, which may be as little as one-half of that previously consumed with the circuits employed heretofore. In addition, the resistors employed are physically smaller requiring less space than those used heretofore for voltage dividing and are considerably less expensive, as for example, small 10 percent carbon resistors used in ordinary radio circuits can be used. This effects a reduced cost of the power supply, especially since the resistor of the screen circuit can now be mounted external to the power supply. Furthermore, the circuit of the invention eliminates the need for separate adjustments for charging and screen potentials and simplifies testing of a power supply.

Whereas the description of the apparatus disclosed herein has been made with particular reference to a corona generating device 2 employed to effect desired potential on a xerographic drum, it is apparent that a similar control may be applied to the corona generating device 7 for applying a uniform potential to paper web 17 to etfect transfer of the xerographic powder image thereto.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

In a corona generating device for applying a uniform potential on a xerographic plate wherein the generating device is arranged in spaced charging relation to a plate surface and includes a plurality of regulating screen wires, a grounded conductive shield extending parallel to the regulating screen wires and at least one corona discharge wire supplied from a potential source and arranged parallel to and between the regulating screen Wires and the conductive shield; the improvement which comprises the combination of electric circuit means to apply a required potential on a plate surface, said circuit means including means to vary the magnitude of potential supplied to said corona wires including an adjustable first voltage transformer, a step-up transformer connected to the secondary of said first voltage transformer, an A.C.-D.C. rectifier connected to the secondary of said step-up transformer, a bleeder resistor and a filter capacitor shunting said rectifier from between which rectified potential is conducted to said corona wires, and a resistor connected to the regulating screen Wires whereby, within the adjustable potential range supplied to said corona Wires, the potential generated on said screen wires is of magnitude not greater than the magnitude of potential applied to the surface of a plate arranged to be charged.

References Cited in the file of this patent UNITED STATES PATENTS