Apparatus For Controlling The Amount Of Charge Applied To A Surface

Abstract

Apparatus for automatically controlling the amount of electrostatic charge applied to a plate by controlling the potential applied to a corona wire.

Description

BACKGROUND OF THE INVENTION

By present techniques, the charging of a xerographic plate in preparation for the exposure step is accomplished by means of a corona-generating device whereby an electrostatic potential on the order of 500 to 600 volts is applied to the xerographic plate. A typical corona-generating utilized for this purpose is that described in Vyverberg U.S. Pat. No. 2,836,725, constructed generally of an electrode wire connected to a high-voltage source and supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. The shield generally surrounds the electrode wire, except for an opening through which charge is emitted, and is adapted to attract surplus emission emanating from the wire. When the wires are energized, corona is generated along the surface of the wires and ions are caused to be deposited on the adjacent xerographic plate surface, which is photoconductive. Suitable means are usually provided to effect relative movement of the surface to be charged and the corona-generating device.

As is well known, the corona threshold potential and the corona current from an energized wire are functions of the wire diameter, i.e., the corona threshold increases and the corona current for any given potential decreases as the wire diameter is increased. Variations in the potential applied to corona wires of a given diameter will cause relatively large changes in corona current with corresponding variations in the charging rate. In addition, the corona threshold potential and corona current are also affected directly by deposits of dust that may accumulate on the wire, by atmospheric conditions such as humidity, temperature, and pressure, and by variations of movement and ionized conditions of the air sheath surrounding the wire. Thus, when operating at the corona threshold, minute differences in wire diameter, slight accumulations of dust on the wire, and variations in air current, atmospheric conditions and the spacing between the wire and the xerographic plate drastically affect the corona-generating potential of the wire and cause a nonuniform amount of electrostatic charge to be deposited on the xerographic plate.

In the art of xerography, it has been established that consistently high-quality reproductions can best be effected when a uniform charge of the proper amount is applied to a xerographic plate to prepare the plate for the exposure step. If the xerographic plate is not charged to a sufficient potential, the electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of a developer material thereon will be correspondingly small and, if the xerographic plate is overcharged, the converse will occur, and if overcharged sufficiently, the photoconductive layer of the xerographic plate may be permanently damaged.

Since the contrast value of the electrostatic latent image is related directly to the potential or charge on the xerographic plate before exposure, it is apparent that if the plate is not charged uniformly over its entire area and if the amount of charge deposited on the plate is either less than or greater than the proper amount, the contrast value of the electrostatic latent image obtained upon exposure will vary in different areas of the plate, and a streaky effect will be visible on the image when developed.

SUMMARY OF THE INVENTION

This invention relates to the field of charging and, particularly, to an improved electrical circuit for controlling a corona-generating device for applying electrostatic charge on a xerographic plate.

The improved electrical circuit operates cooperatively with the corona-generating device which comprises a grounded shield, or backup plate, having an aperture extending along its longitudinal axis and an electrode wire extending parallel to the shield and adjacent the aperture to charge a xerographic plate by corona discharge. A wire approximately the length of the electrode wire and parallel to it is positioned adjacent the shield aperture for detecting a portion of the corona current attracted to the shield. The detected current is utilized to control an electrical circuit which in turn maintains the electrode wire-to-plate potential at a desired value.

In the first embodiment of the invention, the detected current controls the resistance of a variable impedance means connected in a voltage divider network which supplies voltage to the electrode wire.

In the second embodiment of the invention, the detected current is coupled to a comparator circuit, the output of which controls the output pulse width or duty cycle, of a variable frequency oscillator coupled thereto. The output of the variable frequency oscillator is utilized to modulate, or chop a DC voltage, the modulated voltage being filtered and applied to the electrode wire. Changing the modulating rate by varying the pulse width or duty cycle of the oscillator output controls the power applied to the corona wire.

It is therefore the principal object of this invention to provide an improved electrical circuit utilized in a corona-generating device whereby a uniform electrostatic charge of the proper amount may be deposited on a xerographic plate.

A further object of this invention is to provide an improved corona-generating device control circuit for use in automatic xerographic machines wherein it is desirable to continuously charge a xerographic plate to a uniform potential regardless of variations in the supply line voltage, or changes in the surrounding atmospheric conditions, structural variations of the control circuit elements, uneveness in a xerographic plate or variations in the spacing between the xerographic plate and the charging devices.

It is still a further object of the invention to provide an improved corona-generating device control circuit, the control circuit in a first embodiment including a variable impedance means connected to the electrode wire, the value of the impedance being adjustable so that it is proportional to the portion of the corona current detected by a wire or probe positioned adjacent the aperture of the corona-generating device. In the second embodiment of the invention, the control circuit includes a variable frequency oscillator, the pulse width or duty cycle of the output pulse train being proportional to the detected corona current, and a DC voltage modulator, or chopper, coupled to the oscillator, the modulator output including a DC voltage component coupled to the electrode wire for controlling the potential applied thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings and wherein:

FIG. 1 is a view in side elevation of a preferred corona-charging device:

FIG. 2 is a bottom view of the same:

FIG. 3 is a cross-sectional view taken on line 3--3 of FIG. 1;

FIG. 4 is a schematic diagram of the first embodiment of the control circuit utilized in the present invention;

FIG. 5 is a second embodiment of a schematic diagram of the control circuit utilized in the present invention; and

FIG. 6 is a pulse train waveform for defining the duty cycle of the variable frequency oscillator shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the subject matter of the invention, the electrostatic charging of a xerographic plate in preparation for the exposure step is accomplished by means of a corona-generating device whereby an electrostatic charge is applied to the plate surface as it moves relative to the charging device. The potential applied to the plate is dependent upon the particular print contrast desired. Higher print contrast requires higher initial plate potentials while the converse is true for lower print contrast.

To effect charging of the plate there is provided a corona-generating device and a charging circuit to supply electrical power to the corona-generating device. The corona generating device shown in FIG. 1 includes a grounded shield and a wire electrode called the coronode. A wire, positioned adjacent the device, is utilized to detect a portion of the corona current attracted to the shield. The coronode wire by corona discharge, charges the photoconductive surface of the xerographic plate. In the specific discussion recited hereinbelow, the corona-generating device will be referred to as the corotron, a term commonly used in the charging and xerographic art.

Referring more particularly to the drawings, in which the like reference numerals refer to the same parts in FIGS. 1, 2, and 3, the corotron structure shown is exemplary of one practical embodiment and consists of a grounded conducting shield housing preferably of aluminum or stainless steel. The shield is of generally inverted U-shaped cross section and includes a top wall 10 and sidewalls 12 in perpendicular relation to the top wall 10.

In the corotron illustrated in FIG. 3, the sidewalls 12 terminate in converging positions 14, each arranged in an angle of approximately 45.degree. to its sidewall 12 and spaced apart to afford a corona discharge opening or aperture 16 at the bottom of approximately 1/2 inch in width while 18 designates a high-voltage wire, or coronode, or any suitable noncorosive material such as stainless steel, platinum alloy, etc. having a uniform exterior and a diameter of approximately 0.0035 inches. The high-voltage wire 18 is stretched between and attached to blocks 20 and 22 of suitable insulating material which are arranged between the sidewalls 12 and attached at the ends of the shield by means of suitable fastening screws 24 extending through the top wall and into the insulating blocks, or in any other suitable fashion, the insulating block 22 having attached thereto a conducting finger 26 to which one end of the high-voltage wire 18 is attached at 28, as shown, the finger 26 being disposed for engagement with a suitable conducting bar or source carrying a high-voltage supply as usual in this class of device. The opposite end of the conducting wire 18 is attached to an insulating block 20 by a pin 30, while 32 designates a conducting plate secured to the adjacent end of the top wall 10 by means of a bolt 34, and provided with a terminal portion 36 which is grounded in any suitable fashion through a conductor with which the finger 36 contacts. A wire 38, approximately the length of the coronode wire 18 and parallel to it, is positioned near opening 16. The wire acts as a current, or corona detector, detecting a portion of the current, or corona discharge attracted to the shield. It has been determined experimentally that the current detected by wire 38 is related to the potential on the xerographic plate at various positions adjacent the opening 16. Wire current sensitivity to plate potential decreased while wire current magnitudes increased as the wire was positioned further into the opening 16. However, at the several positions tried, the wire did not alter the coronode-charging characteristics. The source of wire current appears to be the current which normally goes to the shield. Hence, no effect upon corotron-charging efficiency is anticipated.

The cross-sectional structure of the corona wire shield is not necessarily limited to a rectangular configuration. Such a form is preferred from the standpoint of ease of manufacture and assembly. Obviously, a circular of similar configuration can be employed so long as it permits suspending the corona wire substantially along its longitudinal axis, and is provided with a longitudinal slit of such width to prevent excessive ion deposition on the plate in accordance with the speed and spacing of the plate relative to the slit during a charging operation.

Referring now to FIG. 4, the first embodiment of the charging apparatus and its electrical controlling circuit is illustrated. A DC power source 40 is connected to a source of alternating current, such as a commercial outlet of 110 volts, for producing a DC potential in the range of approximately 6,000 to 11,000 volts. The positive terminal of source 40 is connected via conductor 42 to one terminal of resistor 44, the other terminal of which is connected to a terminal of variable resistor 46. The negative terminal of source 40 is connected to the other terminal of resistor 46 and to xerographic plate 50 via conductor 48. The wire or probe 38 is connected to one input of differential amplifier or comparator 52 via conductor 54. The junction point of resistors 44 and 46 is coupled to the other input of amplifier 52 via conductor 56. The shield is connected to ground via conductor 58. The positive terminal of power supply 40 is also coupled to one input of a variable impedance means 60 via conductor 42. The output of amplifier 52 is connected to the other input of variable impedance means 60 via the conductor 57. Resistors 44 and 46 and variable impedance means 60 form a voltage divider network supplying voltage to coronode wire 18. The variable impedance means 60 may take many forms, such as a Raysistor, manufactured by the Raytheon Company, Lexington, Mass., and schematically illustrated as comprising a photocell 62 whose resistance value is dependent upon the amount of light energy, produced by light bulb 64, impinging thereon. The light energy generated by light bulb 64 in turn is dependent upon the current output of amplifier 52.

In operation, variable resistance 46 is initially adjusted to a value to produce a coronode to plate potential which will produce the desired amount of electrostatic charge on plate 50. In this initial or quiescent condition, the current output form amplifier 52, by varying the light output from bulb 64, has adjusted the resistance value of photocell 62 such that the voltage drop across photocell 62 will be of a magnitude so that the proper wire-to-plate potential is attained. The above is true as long as all the conditions such as temperature, humidity, pressure, voltage potential on the plate, spacing between wire 18 and plate 50 and the evenness of the plate and wire 18 remain constant. In the event that there is any change in any of these conditions there will be a corresponding variation in the corona current produced by wire 18. This change is detected by wire or probe 38. It should be noted that the detector may take other forms than that recited hereinabove. This current variation changes the current appearing at the input to amplifier 52 from the initially zero, or balanced condition. The current in conductor 56 is the reference current produced by variable resistor 46, the value of resistor 46 determining the initial or quiescent wire-to-plate potential. If the current detected by wire 38 is less than the quiescent value determined by resistor 46, inverting amplifier 52 will generate a current which is larger than the quiescent current. This increased current will increase the light output from light bulb 64, thereby decreasing the resistance value of photocell 62. The voltage drop across the variable impedance 60 will therefore be less than the quiescent voltage drop and the resulting potential between wire 18 and the plate 50 will be increased until the initial quiescent potential value is attained. The converse is true if the current detected by wire 38 is greater than the initial quiescent current produced by resistor 46.

Referring now to FIG. 5, there is shown a block diagram and partial schematic of a second embodiment of a circuit for controlling the charging current applied to the plate 50 by controlling wire-to-plate potential. The blocks in the circuit are of conventional design and the schematic circuits within the dashed blocks are included to give a better understanding of the circuit operation.

The current detected by wire 38 is transmitted via conductor 70 to an amplifier 80. The output of amplifier 80 is connected to differential amplifier or comparator 90 via conductor 82. The differential amplifier 90 includes a pair of NPN transistors, Q1, and Q2, the emitters of which are coupled together. The current applied to the base electrode of transistor Q1 via conductor 82 is proportional to the current detected by the wire 38. The current appearing at the base of transistor Q2, representing the desired coronode wire-to-plate potential, is dependent upon the setting of contact 92 of a potentiometer 94. The operating characteristics of amplifier 90 is such that the difference of potential between the collector electrodes of Q1 and Q2, E.sub.o2 -E.sub.o1, is proportional to the differences of potential at the base electrodes of Q1 and Q2. The collector outputs of transistors Q1 and Q2 are connected to variable frequency oscillator 100. The variable frequency oscillator 100, in the embodiment illustrated, is an astable multivibrator. The output appearing at the collector electrodes of transistors Q1 and Q2 are coupled to the base of transistors Q3 and Q4, respectively, via resistors 106 and 108. The astable multivibrator in the present invention is used as a free-running oscillator for generating square waves of varying pulse width. The characteristic of the astable multivibrator utilized in the present invention is that the pulse width, or duty cycle, of the generated square wave is dependent upon the collector voltage difference, E.sub.o2 -E.sub.o1. Referring now to FIG. 6 the duty cycle of a pulse train is commonly defined as the ratio of the pulse width a to the pulse train period b or a/b. The terms "pulse width" and "duty cycle" have been used interchangeably hereinabove, since, for a constant period b, a change in pulse width causes an equal change in duty cycle. In the discussion to follow, however, only the term "pulse width" will be used. The output of the astable multivibrator 100 is coupled to modulator, or chopper, 110, the modulating or chopping rate thereof being dependent upon the pulse width of oscillator 100. A DC power supply source 120 is connected to a source of alternating current, such as a commercial outlet of 110 volts, for producing a DC potential in the range of approximately 6,000 to 11,000 volts. The DC output from source 120 is coupled to chopper 110 via conductor 122. The output appearing on conductor 112 is a series of positive pulses, produced by the chopping effect of chopper 110 on the DC voltage applied thereto. The output on lead 112 is connected to an averaging circuit 130, the DC output thereof being coupled to the coronode wire 18 via conductor 140.

In operation, assuming that the wire-to-plate potential has increased above a quiescent level, an increased current is detected by wire 38 and connected to amplifier 80, the amplifier output thereof being coupled via conductor 82 to the base of transistor Q1. The voltage difference, E.sub.02 -E.sub..1, initially zero, now changes to a value greater than zero. This voltage difference is coupled to astable multivibrator 100 via resistors 106 and 108. In the initial state, the pulse width of the square wave appearing at the collector of transistor Q3 is proportional to the product of resistor l06 (or 108) and capacitor 102 (or 104), assuming that the resistors and capacitors are equal. However, the voltage input to resistors 106 and 108, produced by the voltage imbalance at the base of transistors Q1 and Q2, causes the pulse width of the output wave to increase. The DC voltage generated by source 120 and coupled to chopper 110 by conductor 122, is now chopped at an increased rate. The output of chopper 110 is a series of positive pulses, the pulse width of which is dependent upon the chopping rate. The output of chopper 110 is averaged by averaging circuit 130 and then coupled to the coronode wire 18 via conductor 140. The DC voltage value appearing on conductor 140 is a direct function of the pulse width appearing on lead 112, and enables the wire-to-plate potential to increase to the value represented by the setting of potentiometer 94. The converse of the above is true when the wire-to-plate potential decreases below the value set by potentiometer 94.

While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the invention without departing from its essential teachings.

Claims

What is claimed is:

1. In a xerographic reproducing apparatus having a corona-generating device with at least one corona wire positioned in closely spaced relation to a xerographic plate for applying an electrostatic charge onto the xerographic plate, said corona wire being supported in and substantially enclosed by a conductive shield and wherein the charging current to the xerographic plate is produced by the potential applied to the corona wire whereby an increase in potential on this wire will effect an increase in potential charging current and the decrease in this potential will affect a decrease in charging current, apparatus for controlling the potential applied to said corona wire comprising;

corona-detecting means approximately the length of and parallel to said corona wire and positioned adjacent said conductive shield for detecting a portion of the charging current,

a comparator for comparing signals applied thereto,

first means for applying a reference current to said comparator,

second means for connecting the output of said corona-detecting means to said comparator,

voltage-producing means operatively connected to the output of said comparator and responsive to the variations in the charging current from said reference current for producing a voltage whose magnitude relative to an initial value changes in a direction inversely proportional to said charging current variations, and

means connecting the output of said voltage-producing means to said corona wire to correct for the variations in the charging current.

2. The apparatus as defined in claim 1 wherein said corona-detecting means comprises a wire approximately the length of and parallel to said corona wire.

3. In a corona-generating device having at least one corona wire positioned in closely spaced relationship to a recipient surface for applying an electrostatic charge onto said surface, said corona wire being supported in and substantially enclosed by a conductive shield, apparatus for controlling the potential applied to said corona wire comprising:

corona-detecting means positioned adjacent said conductive shield for detecting a portion of the generated corona,

comparator means for comparing the detected portion of the generated corona with a source of reference current, said comparator means generating an output signal proportional to the variations of the detected corona from said reference current,

means operatively connected to the output of said comparator means for producing a voltage whose magnitude varies in a direction which is inversely proportional to said detected corona variations, said voltage-producing means including a variable frequency oscillator operatively connected to the output of said comparator means, the outside pulse width of said oscillator being a function of the current input to said comparator means, and a modulator coupled to the output of said variable frequency oscillator, the modulator operating on a DC voltage supplied thereto for producing a voltage whose average value is proportional to the output pulse width of said variable frequency oscillator, and

means for connecting said voltage-producing means to said corona wire to correct for variations in the detected corona.

4. In a xerographic reproducing apparatus having a corona-generating device with at least one corona wire positioned in closely spaced relation to a xerographic plate for applying an electrostatic charge onto the xerographic plate, said corona wire being supported in and substantially enclosed by a conductive shield and wherein the charging current to the xerographic plate is produced by the potential applied to the corona wire whereby an increase in potential on this wire will effect an increase in charging current and the decrease in this potential will affect a decrease in charging current, apparatus for controlling the potential applied to said corona wire comprising,

corona-detecting means approximately the length of and parallel to said corona wire and positioned adjacent said conductive shield for detecting a portion of the charging current,

a comparator for comparing signals applied thereto,

first means for applying a reference current to said comparator,

second means for connecting the output of said corona-detecting means to said comparator,

voltage-producing means operatively connected to the output of said comparator and responsive to the variations in the charging current from said reference current for producing a voltage whose magnitude relative to an initial value changes in a direction inversely proportional to said charging current variations, said voltage-producing means including a variable frequency oscillator operatively connected to the output of said comparator, the output pulse width of said oscillator being a function of the current input to said comparator, and a modulator coupled to the output of said variable frequency oscillator, said modulator operating on a DC voltage applied thereto for producing a voltage whose average value is proportional to the output pulse width of said variable frequency oscillator, and

means connecting the output of said voltage-producing means to said corona wire to correct for the variations in the charging current.