US3667036A

US3667036A - Electrometer amplifier circuits

Info

Publication number: US3667036A
Application number: US5441A
Authority: US
Inventors: Ned Jay Seachman
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1970-01-23
Filing date: 1970-01-23
Publication date: 1972-05-30
Anticipated expiration: 1989-05-30

Abstract

Electrometer amplifier circuits for producing a voltage signal which is proportional to the electrostatic potential on an insulator. In a first embodiment, the electrometer amplifier circuit is controlled such that the output voltage is equal to the sensed electrostatic potential or multiples thereof. In a second embodiment, the circuit output nulls about the surface potential on the insulator.

Description

[451 May 30, 1972 United States Patent Seachman 2,881,266 4/1959 Miller................................324/l23X 3,440,525 4/1969 Cardeiro............................324/l23X Ned Jay Penfield Primary Examiner-Alfred E. Smith [73] Assignee: Xerox Corporation, Rochester, NY.

[22] Filed: Jan. 23, 1970 Attomey-James J. Ralabate, John E. Beck and Irving Keschner ABSTRACT [21] Appl. No.: 5,441

Electrometer amplifier circuits for producing a voltage signal [52] U.S. 324/32, 324/123 I which is proportional to the electrostatic potential on an insu- -.G0lr 27/00, GOlr 5/28, GOlr 1/30 lator. In a first embodiment, the electrometer amplifier circuit is controlled such that the output voltage is equal to the sensed mh m l m d Ld MF 1] .l 8 6 w References Cited electrostatic potential or multiples thereof. In a second embodiment, the circuit output nulls about the surface potential on the insulator.

UNITED STATES PATENTS 3,448,291 Burk et al. .........................328/127 X 12 Claims, 5 Drawing Figures l-IIGH VOLTAGE SUPPLY PATENTEDMY 30 I972 SHEET 10F 4 INVENTOR.

NED J. SEACHMAN BY *M ATTORNEY PATENTEUMM 30 I972 SHEET t [IF 4 Xerography, as pertinent to the present invention, comprises an image reproduction method wherein an electrostatically charged photoconductive insulating plate is exposed to a light image and the resulting electrostatic latent image is developed or made visible through the selective deposition of electrostatically attractable particles. The latent image may optionally be transferred or fixed in image configuration to a sheet of paper or other support material.

One of the methods of developing the latent electrostatic image is by means of the two component development technique as disclosed by Wise, in U.S. Pat. No. 2,618,552. Two component development is based upon the phenomena of triboelectrification. By rubbing together two triboelectrically dissimilar materials, an opposite electrostatic charge is induced in each of the materials. In xerography, finely divided toner particles are mixed with relatively coarser carrier beads so that the toner particles are charged to a polarity opposite that of the latent electrostatic image. The two component material is then brought into contact with the exposed xerographic plate where the carrier beads give up their toner particles to the more highly charged image areas retained on the plate surface thus making the images visible. The two component developer material has been used in cascade development systems, as disclosed by Walkup, US. Pat. No. 2,638,4l6. In conventional cascade development, the developer material is allowed to flow over an image retaining plate surface where the image is first developed in the manner disclosed by Wise. However, after image development, the toner depleted carrier beads, still retaining a toner attracting charge, are allowed to clean or scavenge weakly held toner particles from the background or non-imaged areas on the plate.

A developement technique which produces a high quality image is generally characterized in that the xerographic plate, or photoconductive insulating member, is brought into contact with the electrostatically attractable particles while spaced adjacent to an equipotential member known as a development electrode. This configuration causes an electrostatic field to be formed between the plate and the equipotential member in proportion to the charge on the plate and is also effective to increase the electric field above large areas of uniform charge density. It is these electric fields which cause the electrostatically attractable particles to move to and adhere to the plate for purposes of development. In this way, large solid areas may be developed. However, the potential on the development electrode must be accurately matched to the minimum potential on the photoconductor if images are to be formed with clear backgrounds. Otherwise, the background potential produces an electric field between the plate and the development electrode and the electrostatically attractable particles are deposited in those areas giving a high background density in the areas which should be reproduced as white. Xerographic development is primarily dependent on the potential difference between background and image voltage, rather than on absolute values, and the biasing potential placed on the development electrode, is generally maintained at some level above or below one of these voltages. It has been found that the electrical characteristics of most xerographic plate materials, including the dark discharge rate, will change as the plate temperature changes or with extended plate usage thereby making it extremely difiicult to maintain a uniform quality of development in this type of system.

A technique for minimizing the background density is to measure the electrostatic potential on the xerographic plate and adjusting the development electrode potential to the minimum measured potential. In order to measure the electrostatic potential, an electrometer is required. The electrometer ideally should be accurate, reliable, simple and economical. However, the prior art electrometers have certain deficiencies associated therewith. For example, one of the difficulties encountered in the use of the prior art electrometers is that the electrometer contains components having parameters which vary with temperature and aging. In addition, the capability of adjusting the output signal to multiples of the electrostatic potential, as well as being equal to it, is lacking. Slow changes in circuit parameters effect the accuracy of the output potential to electrostatic surface potential ratio of prior an electrometers. Finally, the prior art electrometers utilize a large number of components with a corresponding increase in cost.

SUMMARY OF THE INVENTION The present invention provides new electrometer amplifier circuits for measuring the potential of the electrostatic charge formed on an insulating surface. In the first embodiment. the circuit includes a probe assembly comprising probe and guard electrodes. The output of the probe electrode is connected to a high input impedance circuit of approximately unity gain. The output of the high impedance circuitis connected to a clamping circuit which clamps the signal portion of the output to a stable zero reference level. The output of the clamping circuit is coupled to a peak detector via an amplifier. The output of the peak detector is connected to a high voltage amplifier, the output of which is proportional to the electrostatic sur face potential. The output of the high voltage amplifier is controlled by a voltage divider circuit such that the output voltage is equal to the sensed electrostatic potential or multiples thereof.

In the second embodiment, the high voltage amplifier output is fed back to the guard electrode on the probe assembly and to a high gain differential amplifier so that the circuit output nulls about the surface potential on the insulator.

It is an object of the present invention to provide improved apparatus for developing a latent electrostatic image.

It is a further object of the present invention to provide improved electrometer amplifier circuits for producing a voltage signal which is proportional to the electrostatic potential on an insulator surface, and in particular, for utilization with apparatus for minimizing background on a developed xerographic image. 7

It is still a further object of the present invention to provide novel apparatus for sensing electrostatic potential on an insulating surface.

It is a further object of the present invention to provide novel, economical, simple and accurate electrometer amplifier circuits.

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

FIG. I is a schematic drawing of one embodiment of the novel electrometer amplifying circuit of the present invention;

FIG. 2is a block diagram of a second embodiment of the present invention;

FIG. 3 illustrates schematically a xerographic reproducing apparatus adapted for high speed automatic operation which incorporates the novel circuitry of the present invention;

FIG. 4 is a front elevation and partial section of the development system illustrated in FIG. 3 showing the development electrode and the control apparatus which includes the novel apparatus of the present. invention; and

FIG. 5 is a partial side elevation of the sensing probe and shutter mechanism taken along line 5-5 in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Refen'ing now to FIG. 1, there is shown a schematic diagram of one embodiment of the present invention. A probe assembly 12, positioned adjacent to an insulating surface 10, such as a photoconductor, generates the input voltage which is proportional to the electrostatic potential on insulating surface 10. The probe'assembly 12 comprises a probe electrode 13 positioned about 0. l 25 inches from insulating surface 10, a metal guard electrode 14 which surrounds the probe and which in operation, is maintained at a potential substantially equal to that of the probe electrode 13 to minimize leakage current, and, a grounded metal member surrounding the probe-guard assembly to shield the assembly from external electrical fields. The output of the probe assembly 12, which is proportional to the electrostatic potential on the insulated surface 10, is coupled to a high-impedance circuit 16. A high-impedance circuit is required to prevent discharge of the voltage induced on capacitor C1 during the probe-sense period. As shown schematically in FIG. 1, the high impedance circuit comprises a MOS (metal-oxide-semiconductor) field effect transistor Q1 in a source-follower configuration. Q2 is a passive device (i.e., a resistor) to maintain a high input impedance between the gate-to-source terminals of Q1 while also establishing the bias for Q1. Q2 is an N-channel enhancement type MOS field effect transistor which biases the N- channel depletion-type MOS field effect transistor Q1 so that charge carriers are present in the channel with no signal voltage applied to the gate electrode. A reverse gate voltage is applied to Q2 by the potential drop across R1 such that charge carriers in the channel are depleted, thereby reducing the channel conductivity. In the common-drain arrangement of Q1, known as a source-follower, the input impedance is high, there is no polarity reversal between input and output, and the voltage gain is always less than unity, and distortion is low. The input signal is effectively injected between the gate and drain electrodes, and the output is taken between the source and drain. For a more detailed explanation of MOS field effect transistors, reference may be made to the RCA Transistor Manual, published by RCA, Princeton, New Jersey, 1966, pages 93'through l09. It should be noted that the Q1, Q2 configuration functions essentially as a vacuum tube cathode follower and may be replaced by'a tube cathode follower or a transistorized emitter-follower stage.

In order to minimize the variation of the quiescent operating voltage of 01- with temperature and component aging, a clamping circuit a 18 is coupled to the output of high impedance circuit 16. The clamping circuit 18 clamps the signal portion of the output of circuit 16 to a stable zero reference level and removes the quiescent component from the output signal of high impedance circuit 16 which eliminates long term voltage drift from the electrometer amplifier. The output from the highimpedance circuit 16 is coupled to the inverting input of amplifier Al via clamping capacitor C2. The clamping voltage on C2 is maintained by the ideal" diode action of Al and CR1. The clamped output signal is connected to non-inverting gain circuit 20. The non-inverting gain circuit 20 comprises operational amplifier A2 in a configuration which achieves a voltage gain which is equal to the value (R4)/(R3) +1, and achieves the necessary impedance transformation between the high output impedance of the clamping stage and the low impedance drive required for subsequent stages. The output of amplifier A2 is coupled to diode peak detector 22 which comprises semiconductor diode CR2, resistor R6 and capacitor C3. The signal appearing at the output of amplifier A2, in the normal operation of the electrometer wherein the electrostatic potential is periodically sensed, is essentially a series of positive pulses and whose peak values are proportional to the voltage on surface 10. Capacitor C3 is charged by the positive pulses to the peak value thereof and provides a substantially constant output voltage equal to the peak input voltage. Re-

sistor R6 provides a discharge path for capacitor C3 to permit the peak detector to follow slow decreases in the peak value of the input pulses. The output across capacitor C3 is connected to the inverting input of differential amplifier 24. The output of differential amplifier 24 is coupled to the base electrode of transistor 03 via the parallel connection of resistor R7 and capacitor C4. The base electrode of transistor Q3 is shunted to ground via semiconductor diode CR3. Differential amplifier 24 and transistor 03 act essentially as a linear high voltage amplifier. The output voltage of amplifier 24 is applied to the base of transistor Q3 and an inverted, amplified signal E0 is produced at the collector of transistor Q3. The output E0 of transistor Q3 is coupled back to the other input of differential amplifier 24 via a feedback network comprising variable potentiometer 30. The variable potentiometer 30 comprises ages at the input of the differential amplifier are equal. The

output voltage E0 will thus always be a fixed multiple of the voltage on C3 determined by the setting of tap 32. The multiple may be selected such that the output E0 is equal to the sensed photoconductor electrostatic potential;

High voltage supply 34, with an output of approximately 650 volts, biases the collector circuit of transistor Q3 and provides the necessary supply voltage across potentiometer 30. The output voltage E0 varies from approximately zero volts (when O3 is saturated) to approximately 500 volts (when Q3 is operating close to cut-off)..

A more detailed description of the operation of differential amplifier 24, transistor Q3 and potentiometer 30 follows. If it is assumed that the voltage across capacitor C3, Vc3, is 50 volts and tap 32 has been set so that the voltage thereacross, V32, is 0.5 Eo which is greater than the voltage across C3, the positive difference in voltage at the input of differential ampli fier 24 is transmitted to the base of transistor Q3. Transistor Q3 is caused to conduct to a degree dependent on the magnitude of the positive voltage applied to its base. A greater portion of the current from high voltage supply 34 is directed to the collector of Q3, thereby decreasing the voltage across tap 32 so that the inputs to the differential amplifier 24 are equal. Therefore Since it has been assumed that Vc3 50 volts,

Eo=2 50= volts.

It should be noted that useful output signals may be obtained at the output of amplifier A2 and across capacitor C3.

Referring now to FIG. 2, a'block diagram of the second embodiment of the present invention is illustrated. For illustrative purposes, the electrostatic potential is shown deposited on a photoconductive drum 40, such as that utilized in xerography. The probe electrode 42 is coupled to the input of a high input impedance, unity gain amplifier 44. As set forth in reference to FIG. I, amplifier 44 may comprise a cathode follower. The output of amplifier 44 is coupled to one input of a high gain differential amplifier 46. Amplifier 44 maintains a sufiiciently high input impedance to allow the remainder of the circuit to respond before capacitor C1 is discharged. It should be noted that Cl generally comprises stray capacitance and the input capcitance of amplifier 44. Capacitance C2 represents the stray capacitance between guard electrode 43 and ground. The ratio of C2 to C1 is generally about 100. If the ratio is significantly smaller, a physical capacitor may be added to the input of amplifier 44. The small positive output voltage from amplifier 44 is applied to one input of high gain differential amplifier 46 where it is compared with the voltage on the guard electrode line. Initially, a higher positive voltage will be induced on the probe electrode 42 than on guard electrode 43 because of the capacitance relationship between capacitor C1 and C2. The output of differential amplifier 46 therefore will initially be positive and will cause transistor O4 to conduct. The current through transistor Q1 causes a voltage drop across resistor R1 and the guard and output voltages begin to rise. As the guard voltage increases, the probe voltages will also continue to increase until the probe voltage matches the potential on the photoconductive surface. The guard voltage and thus the output voltage Ea will, at this point, be very nearly equal to the probe voltage. if the guard voltage tries to increase beyond that of the probe voltage, the output of differential amplifier 46 will go negative, reducing the current to Q4, thereby reducing the voltage across resistor R1. Should the guard voltage drop significantly below the probe voltage, the output of the differential amplifier will be more positive, increasing the current in transistor Q4 and thus increasing the guard voltage. A stable condition is therefore reached when the probe voltage and guard voltage are nearly equal to the electrostatic surface potential on the photoconductor. A diode peak detector for providing a relatively constant output voltage equal to the peak input voltage is provided at the emitter electrode of transistor Q4. The diode peak detector comprises semiconductor diode CR1, resistor CR2 and capacitor C3.

The above circuit has several advantages. Since it tends to null about the electrostatic potential on the photoconductor surface, slow changes in circuit parameters will have negligible effect on the accuracy of the output voltage to surface potential ratio. ln addition, the probe electrode and associated circuitry float above ground at the photoconductor surface potential. The circuit uses a limited number of components, therefore its cost is correspondingly low.

FIGS. 3, 4 and 5 illustrate apparatus in which the novel circuitry of the present invention may be utilized. in particular, a reproducing apparatus comprising a xerographic plate including a photoconductive layer of a light sensitive material placed on a conductive backing and formed in the shape of a drum generally designated 40, is shown. The drum is journaled for rotation in the machine frame (not shown) upon a horizontal support shaft 42. The xerographic drum is rotated in the direction indicated in FIG. 3 to cause the photoconductive surface to pass sequentially through a plurality of xerographic processing stations.

For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the drum surface may be described functionally as follows:

A charging station A, in which a uniform electrostatic charge is deposited on the moving photoconductive surface;

An exposure station B, wherein the light image or radiation pattern of an original document to be reproduced is projected on to the drum surface to dissipate the charge found thereon in the light exposed areas so as to form a latent electrostatic image which is retained thereon;

A developing station C, at which a two component xerographic developing material having toner particles possessing an electrostatic charge opposite to the image charge found on the drum surface are cascaded over the upwardly moving drum surface whereby the charged toner particles adhere to the electrostatic latent image areas making the images visible in the configuration of the original to be reproduced;

A transfer station D, in which the xerographic powder image is electrostatically transferred from the drum surface to a final support material; and

a drum cleaning and toner collecting station E, where the drum surface is first treated with a corona discharge to neutralize any residual charge found thereon and then cleaned with a flexible cleaning blade to remove residual toner from the drum surface. A reservoir for collecting and storing the removed residual toner and an incandescent panel to affect substantially complete the discharge of any residual electrostatic image remaining thereon is also included.

The charging station is preferably located at the bottom of the drum in the position indicated by reference A shown in FIG. 3. The charging arrangement consists of a corona charging device 43 which includes a corona discharge array of one or more corona discharge electrodes that extend transversely across the drum surface and are energized by a high potential source. The corona discharge device is substantially enclosed within a shielding member and is adapted to generate a positive charge confined'within this specific area.

Next subsequent thereto in the path of drum rotation is an exposure station B wherein a flowing light image of a stationary original is placed on the drum surface. Basically, the exposure station comprises an optical scanning and projecting assembly and a stationary transparent copyboard 44 adapted to support the original to be reproduced. A moving light source 45 is mounted below the copyboard and is arranged to move in timed relation with a lens element 48 to scan the original supported upon the copyboard thus creating a flowing light image of the original. The light image is projected by the lens through a folded optical system, including an object mirror 49 and an image mirror 50, arranged to focus the light image on the bottom of the drum.

Positioned adjacent to the exposure station is a developing station C in which is positioned a developer housing 52 having a reservoir area therein capable of supporting a quantity of two component developer material including negatively charged toner particles. A bucket-type conveyor 53 transports developer material from the lower reservoir area to the upper part of the developer housing where it is deposited in entrance chute 51. Any suitable drive means can be used to rotate the bucket conveyor in the direction indicated. As will be explained in greater detail below, the developer material moves downwardly in contact with the upwardly moving photoconductive drum surface through a completely electroded development zone wherein the latent electrostatic image on the drum surface is developed. The unused developer material passes from the development zone and is directed back into the reservoir area by means of a pick-off bafile. A toner container and dispensing apparatus 56 is affixed to the developer housing and is adapted to add fresh toner material into the reservoir area in proportion to the amount of toner deposited on the drum surface.

An image transfer station D is positioned adjacent to the developing station. Individual sheets of final support material are fed seriatim into the sheet registering and forwarding apparatus, generally referenced by numeral 57, from either of two supply trays 66 and 67. The individual sheets are properly registered and then forwarded into moving contact with the rotating drum surface and the developed electrostatic image transferred from the drum to the final support material by means of a transfer corotron 55. In operation, the electrostatic field created by corona discharge device electrostatically tacks or bonds the transfer material to the drum surface wherein the transfer material is caused to move in synchronous relation with the rotating drum surface.

A mechanical stripper finger 58 is pivotally mounted in close proximity to the drum surface immediately downstream from the transfer station. The stripper finger is arranged to move between the copy sheet and the drum surface breaking the electrostatic bond holding the sheet to the drum and to direct the support material into moving contact with the bottom surface of a stationary vacuum transport 59.

A combination of heat and pressure energy is employed in the present apparatus to fix the xerographic image to the final support material. The image bearing support material is guided into the fusing assembly 63 as it is moved along the bottom surface of transport 59. Fuser assembly 63 comprises an upper fuser roll 64 and a lower fuser roll 65 arranged to coact to deliver a pressure driving force to a sheet introduced therebetween. A radiant heat source 68 is positioned transverse to the lower fuser roll and applies heat energy to the surface of the roll. The roll, which is specially coated, stores the heat energy on its surface. As the rolls are rotated in the, direction indicated, both heat energy and pressure energy are delivered by the roll into the imaged areas thereby fixing the image to the final support material. After leaving the fuser assembly, the now fixed copies are transported through a circular paper path, as illustrated in FIG. 3, into a catch tray 69 where the copy can be conveniently collected by the machine operator.

Referring now to FIG. 4, the two component developer material is first transported from the reservoir or storage area in developer housing 52 and deposited in a hopper-like input chute 51 by means of a bucket conveyor system 53. A quantity of developer material is stored within the input chute and flows downwardly through a constrained opening 71 into the introductory region of development zone 70. The front wall of the development zone is formed by the movable drum surface 40 while the rear wall is formed by a series of downwardly extended electrodes running transversely across the photoconductive coating on the drum surface. The electrodes are supported in spaced parallel relation to the drum surface by means of an insulating support frame 73 secured to the walls of the developer housing by any suitable means. The individual electrodes are separated from each other by dielectric blocks 72 so that the rear wall of the development zone presents a substantially continuous surface to the developer material introduced therein. Although not shown, end seals are provided between the electrodes and the drum surface to substantially enclose the development zone thus providing a conduit through which the developer material gravity flows. The development zone extends from the introductory opening 71 opposite tothe upper drum surface to a point well below the horizontal center line of the drum.

Basically, the control electrodes are biased so that the developer material perfonns a cleaning function in the upper development zone while a preponderance of image development takes place in the lower inverted development zone region thereof. By varying the charge potential and magnitude on the various electrodes, the concentration and positioning of toner in the flow stream can be controlled to regulate the degree of development and cleaning obtained in each of the electroded regions.

The first electroded region through which a latent electrostatic image is transported is the region influenced by a low potential electrode 75 physically located in the bottom of the development zone 70. The term low potential, as herein used, refers to a potential which is lower than the background potential on the xerographic plate surface. This term is broad enough to include a grounded electrode or even a floating electrode. Because of the unique control features of the present developing apparatus, carrier beads which are properly toned for optimum development are flowing through this lower development zone. In this preferred embodiment, the lowjpotential electrode is placed at a ground potential so that an extremely strong force field is established tending to force the negatively charged toner particles toward the plate side of the development zone. At the same time, the electrode acts as a conventional development electrode to enhance the latent electrostatic force fields, particularly the force field associated with solid imaged areas, so that extremely rapid and efficient image development is produced in this region.

The leading edge of the low potential electrode, that is, the edge that first presents itself to the developer flow, is chamfered to direct the developer flow upwardly into contact with the drum surface. In this manner, toner particles are both physically dislodged from the carrier beads and transported into contact with the plate surface. The airborne toner particles, because they are in a free state, are readily attracted into th'eimage areas so that extremely rapid development takes place in this region. Overdevelopment of the xerographic plate, in fact, may result. However, as will be explained below, an overdeveloped condition in this region can be tolerated by the present development system.

The next electrode positioned in the direction of drum rotation is the main development electrode 76. The main development electrode is biased at a potential somewhere between the image potential and the background potential found on the plate surface and preferably at some predetermined level above the background voltage. When an imaged area on the drum surface is transported through the main development electrode region, the force field associated with the imaged area, being of a higher magnitude than the electrode force field, predominates. The toner in the flow stream adjacent to the imaged surface is thus attracted into the imaged areas. However, when a non-imaged or background area is moved through the main developing region, the electrode force field dominates and the toner particles are pulled away from the plate surface towards the backside of the development zone. The developer material moving in contact with the nonimaged drum surface therefore tends to mechanically scrub the background areas to dislodge randomly dispersed,'weakly held, toner particles from the plate. This dislodged toner, coming under the influence of the stronger electrode force field, is similarly attracted towards the electroded side of the system. As can be seen, the main development electrode, in effect, acts as a self-regulating device to either complete image development or to clean up background areas in this region.

The now xerographically developed photoconductive surface next moves into the last development region in which an extremely strong toner attracting force field is produced by a clean-up electrode 77. A biasing source 74 is electrically connected to the clean-up electrode and electrically biases electrode at a potential greater than the image potential on the plate surface, preferably 300 volts abovethe image potential. The bias potential is sufficiently high enough to attract an extremely heavy concentration of toner in the flow stream to the backside of he development zone. The carrier beads moving in contact with the plate surface become toner depleted and therefore are capable of both mechanically scrubbing and electrostatically scavenging unwanted background development from the plate surface. Here again, the strong electrode force field attracts random toner particles from the vicinity of the plate surface so that a clearwell-defined developed xerographic image leaves the development zone. Clean-up electrode 77 is turned at a slight radius at the developer entrance 41 and extends outwardly and upwardly from the development zone to form the bottom wall of the input chute 51. The opposite wall of the input chute is formed by an electrically isolated baffle 78 secured to the developer housing wall by suitable means. The lower end of the bafilehas a lip formed thereon complementary to the tuming radius of the clean-up electrode so that a uniform opening 71 is provided through which the developer material enters the development zone in a relatively undisturbed flow. Bafile 78 is placed at a ground or toner repelling potential which, when combined with the toner attracting force field of electrode 77, forces a preponderance of the toner particles in the flow to the backside of the system. Because of the physical configuration of the input chute and the strong electrostatic force field associated therewith, the formation of toner powder clouds in and about the introductory region to the xerographic development zone is minimized thus preventing unwanted background develop ment from occurring. A strong toner concentration is thus established on the backside of the flow stream prior to the developer material entering the development zone so that toner depleted beads initially contact the drum surface as it leaves the development zone.

The electrostatic properties of many known photoconductive plates tend to change slightly with changes in temperature or with extended plate usage. This change or drifting" in the electrical plate parameters has little or no effect on the control features of the low potential electrode or the clean-up electrode. However, this is not the case in regard to the main development electrode. As noted, the main development electrode is held at some predetermined voltage between plate image voltage and plate background voltage, and preferably at some fixed voltage above the plate background voltage. Here the difference between the background,or reference, voltage and the desired electrode voltageis small and any electrical drifting in the plate voltage will normally be reflected in a change in the quality of the development produced.

The circuitry as described with reference to FIGS. 1 and 2 hereinabove may be utilized to regulate the bias potential on the main development electrode in order to compensate for changes in the plate of voltage so that images of uniform quality are produced by the present developing apparatus. Physically, the main development electrode control system comprises: a sensing probe assembly adapted to periodically sample the level of background voltage on the rotating drum surface; a signal generating device adapted to convert the sampled voltage into a continuous control signal and a fixed or adjustable power supply responsive to the control signal wherein the development electrode is maintained at a predetermined voltage level in regard to the sampled plate voltage.

. A sensing probe support housing 79 (FIG. is secured in the machine frame and is positioned between the xerographic exposure station and the developing station. The sensing probe assembly 80 is seated within the support housing in juxtaposition to one end of the drum surface and is arranged to sense a narrow sample strip on the photoconductive surface near the edge of the drum.

The sample strip is passed through the charging and exposing stations and is placed at the plate background potential. The strip is sufficiently offset to one side of the drum surface so that its presence does not interfere with the normal machine operations.

A solenoid actuated shutter 83 is slideably mounted within the guides provided in the upper portion of the support housing. The shutter is operatively connected to a solenoid SOL-1 by means of a crank arm 84. The crank arm is rotatably mounted upon a pivot pin 85 and the pin secured in the body of the housing. The lower end of the crank arm is pivotally affixed to the solenoid actuator arm 86 while the opposite end of the arm is similarly connected to a downwardly turned dependent flange 88 formed in the lower part of shutter 83. A pin 87, passing through the upper part of the crank arm, rides in a vertically aligned slotted hold (not shown) formed in flange 88 which permits the shutter to move in a horizontal direction as the crank arm is rotated. In operation, the solenoid is energized once during each copying cycle. Energization of the .solenoid pulls actuator arm 86 upwardly causing the crank arm to rotate in a counterclockwise direction. As the crank rotates, the shutter is moved back exposing the sensing probe to the sampling strip.

The machine logic system, generally referenced 90 in FIG. 4, is arranged to generate a mid-scan trigger signal during each xerographic copying cycle. In practice, the signal which energizes solenoid SOL-l, is generated as the scanning lens 48 physically passes the midpoint of its programmed path of travel. The trigger signal is passed to the novel electrometer circuitry 92 of the present invention. A voltage indicative of the plate background voltage is sensed by the probe assembly and amplifier circuits of the present invention and a continuous output control signal is generated which is applied to a fixed or adjustable power supply 94. The power supply is operatively connected to the main development electrode 76 and regulates the electrode potential at a predetermined level above the background voltage on the plate. The details of the machine logic system 90 are not set forth since the present invention is concerned with novel circuitry for generating an electrical signal which is proportional to the electrostatic charge or background voltage on the sampling strip. The description of FIGS. 3 to 5 is set forth to illustrate the type of apparatus in which the circuitry of the present invention may be utilized. It should be noted that the novel electrometer amplifier circuits of the present invention may be utilized in any system wherein it is desired to produce a voltage proportional to the electrostatic potential formed on an insulating surface.

What is claimed is:

1. Apparatus for generating a modified electrical signal proportional to the electrostatic potential formed on an insulating surface comprising:

a. means positioned adjacent to said insulating surface for producing an electrical signal proportional to said electrostatic potential,

b. first circuit means connected to said producing means, said first circuit means having a high input impedance and a low gain,

c. means connected to said first circuit means for providing a stable reference level for the output of said first circuit means, d. second circuit means connected to the output of said reference level providing means, said second circuit means providing a controlled voltage gain and impedance transformation between the reference level means and subsequent apparatus, e. a peak detector connected to the output of said second circuit means, f. means for generating a signal which is proportional to the difference between the signals applied to its input terminals, means for connecting the output of said peak detector to one input terminal of said generating means,

h. means for amplifying the output of said generating means, the output of said amplifying means being said modified electrical signal, and

i. means for coupling the output of said amplifying means to the other input of said generating means.

2. The apparatus as defined in claim 1 wherein said generating means is a differential amplifier.

3. The apparatus as defined in claim 2 wherein said coupling means comprises a variable potentiometer having an adjustable tap, the tap of said potentiometer being connected to the other input of said differential amplifier.

4. The apparatus as defined in claim 3 wherein the magnitude of said modified electrical signal is determined by the position of said adjustable tap.

5. The apparatus as defined in claim 4 wherein said modified electrical signal equals said electrostatic potential.

6. The apparatus as defined in claim 4 wherein said modified electrical signal is a fixed multiple of said electrostatic potential 7. The apparatus as defined in claim 1 wherein said insulating surface comprises a photoconductor.

8. Apparatus for generating an electrical signal which is proportional to the electrostatic potential formed on an insulating surface comprising:

a. a probe assembly positioned adjacent said insulating surface, said probe assembly comprising a probe electrode and a guard electrode, the probe electrode producing an electrical signal proportional to said electrostatic potential,

b. means for producing a signal which is proportional to the difference in magnitude between signals applied to its input terminals,

c. first means for coupling the electrical signal appearing at the output of said probe electrode to one input of said producing means,

d. amplifier means connected to the output of said producing means for amplifying the output thereof, the output of said amplifier means being proportional to the electrostatic potential on said insulating surface, and

e. second means for coupling the output of said amplifier means to said guard electrode and to the other input terminal of said producing means, whereby the output of said amplifier means is nulled about said electrostatic potential.

9. The apparatus as defined in claim 8 further including a peak detecting means connected to the output of said amplifier means, the output of said peak detecting means being a voltage proportional to the peak value of the output of said amplifying means.

10. The apparatus as defined in claim 9 wherein said first coupling means comprises an amplifier having a high input impedance and substantially unity gain.

11. The apparatus as defined in claim 10 wherein said producing means comprises a differential amplifier.

12. The apparatus defined in claim 8 wherein said insulating surface comprises a photoconductor.

Claims

1. Apparatus for generating a modified electrical signal proportional to the electrostatic potential formed on an insulating surface comprising: a. means positioned adjacent to said insulating surface for producing an electrical signal proportional to said electrostatic potential, b. first circuit means connected to said producing means, said first circuit means having a high input impedance and a low gain, c. means connected to said first circuit means for providing a stable reference level for the output of said first circuit means, d. second circuit means connected to the output of said reference level providing means, said second circuit means providing a controlled voltage gain and impedance transformation between the reference level means and subsequent apparatus, e. a peak detector connected to the output of said second circuit means, f. means for generating a signal which is proportional to the difference between the signals applied to its input terminals, g. means for connecting the output of said peak detector to one input terminal of said generating meaNs, h. means for amplifying the output of said generating means, the output of said amplifying means being said modified electrical signal, and i. means for coupling the output of said amplifying means to the other input of said generating means.

6. The apparatus as defined in claim 4 wherein said modified electrical signal is a fixed multiple of said electrostatic potential .

7. The apparatus as defined in claim 1 wherein said insulating surface comprises a photoconductor.

8. Apparatus for generating an electrical signal which is proportional to the electrostatic potential formed on an insulating surface comprising: a. a probe assembly positioned adjacent said insulating surface, said probe assembly comprising a probe electrode and a guard electrode, the probe electrode producing an electrical signal proportional to said electrostatic potential, b. means for producing a signal which is proportional to the difference in magnitude between signals applied to its input terminals, c. first means for coupling the electrical signal appearing at the output of said probe electrode to one input of said producing means, d. amplifier means connected to the output of said producing means for amplifying the output thereof, the output of said amplifier means being proportional to the electrostatic potential on said insulating surface, and e. second means for coupling the output of said amplifier means to said guard electrode and to the other input terminal of said producing means, whereby the output of said amplifier means is nulled about said electrostatic potential.