05/452023

06/24/1975

03/18/1974

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Xerox Corporation (Stamford, CT)

399/48

355/69, 324/455

G03G15/00; G03G15/00

355/3R,3DD,14,69 96/1R 324/32

Moses, Richard L.

Fleischer, Ralabate Green H. J. J. C. A.

What is claimed is

1. An electrophotographic printing machine of the type having a plurality of processing stations, including:

2. A printing machine as recited in claim 1, further including means, in electrical communication with said simulating means and each of the processing stations, for generating an electrical control signal arranged to regulate the electrical output from each of the processing stations.

3. A printing machine as recited in claim 2, wherein said electrical signal generating means includes:

4. A printing machine as recited in claim 2, further including:

5. A printing machine as recited in claim 1, wherein the processing stations include:

6. A printing machine as recited in claim 5, wherein said charging means includes:

7. A printing machine as recited in claim 1, wherein the processing stations include:

8. A printing machine as recited in claim 7, wherein said exposing means includes:

9. A printing machine as recited in claim 1, wherein the processing stations include:

10. A printing machine as recited in claim 9, wherein said developing means includes:

11. A printing machine as recited in claim 1, wherein the processing stations include:

12. A printing machine as recited in claim 11, wherein said transfer means includes:

13. A printing machine as recited in claim 1, wherein the processing stations include:

14. A printing machine as recited in claim 13, wherein said regulating means includes:

15. A printing machine as recited in claim 1, wherein said probe is mounted movably on said photoconductive member.

16. An electrophotographic printing machine of the type having a plurality of processing stations, including:

BACKGROUND OF THE INVENTION

This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for controlling the various processing stations employed in the printing machine.

In the process of electrophotographic printing, a photoconductive surface is uniformly charged and exposed to a light image of an original document. Exposure of the photoconductive surface records thereon an electrostatic latent image corresponding to the original document. The electrostatic latent image is then rendered visible by depositing toner particles which adhere electrostatically thereto in image configuration. Thereafter, the toner powder image may be transferred to a sheet of support material. The toner powder image is, then, permanently affixed to the support material providing a copy of the original document. The foregoing process was originally disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942.

In electrophotographic printing, the electrical characteristics of the photoconductive surface at each processing station is critical. Preferably, the electrical characteristics of the photoconductive surface should be repeated. However, it has been found that the electrical characteristics of the photoconductive surface will vary with temperature changes or with continuous usage thereof, i.e. dark decay, etc. This causes difficulties in repeating the potential on the photoconductive surface for successive cycles at the various processing stations employed in electrophotographic printing. To this end, electrometers have heretofore been employed to detect the characteristics of the photoconductive surface.

The use of electrometers in electrophotographic printing is well known in the art. The major advantage of an electrometer is that it provides a direct measurement of the charge actually on a specific surface at the time the surface passes the electrometer probe. Thus, by positioning the probe after one of the processing stations, the signal from the probe may be employed to control the station. However, an electrometer system is frequently employed only to control one of the processing stations. If all of the foregoing processing stations were to be controlled, one probe would be positioned after each processing station. This would provide a rather cumbersome system which would not be readily adaptable for employment in a commercial machine.

Various types of electrometer systems, employed in the measurement of the electrical characteristics of the photoconductive surface, are known in the art. For example, U.S. Pat. Nos. 2,781,705 issued in 1957 to Crumline et al; 2,852,651 issued in 1958 to Crumline et al.; 2,956,487 issued in 1960 to Giamo, Jr.; 3,013,203, issued in 1961 to Allen et al.; 3,068,056 issued in 1962 to Codichini; 3,321,307 issued to Urbach in 1967; 3,406,334 issued in 1969 to Marquart et al.; 3,438,705 issued in 1969 to King; 3,611,982 issued in 1971 to Coriale; 3,654,893 issued in 1972 to Piper et al.; 3,674,353 issued in 1972 to Tractenberg and 3,749,488 issued in 1973 to Delorme all describe the advantages of utilizing an electrometer to measure photoconductor charge. In particular, Delorme describes the automatic control of the exposure system by using an electrometer to detect the average charge of a photoconductive film during exposure. A sheet of zinc-oxide coated paper is positioned between a transparent, electrically conductive sheet and a conductive grounded plate. The zinc-oxide coated paper functions as a photoconductive film. An electrostatic latent image is produced on that film. This sheet is then removed for developing by the standard electrostatographic printing process. In the foregoing process, after the sandwich structure heretofore described is charged, it is exposed to a light image of the original document. The sandwich structure together with a high input impedance amplifier functions as an electrometer to provide an output signal proportional to the average charge on the zinc-oxide paper. The signal from the amplifier, in conjunction with suitable electrical circuitry, is employed to vary the exposure time. Thus, in the foregoing patent, the length of time that the light image is projected onto the zinc-oxide plate is varied until the charge remaining thereon reaches a preselected level.

The above efforts, in commercial rather than laboratory applications of electrometers to electrostatographic printing, have, as a practical matter, been hampered by the high cost, complexity, and instability of the systems. Most of the foregoing systems have required choppers or vibrating probes and expensive high voltage amplifiers and feedback circuits. However, these patents illustrate the highly developed nature of this art. Other patents which disclose electrometer systems are U.S. Pat. Nos. 3,370,225 issued in 1968 to Winder and 3,449,668 issued in 1969 to Blackwell et al. Examples of electrometer systems are disclosed in various tests. "Electrophotography" by Shaffert and "Xerography and Related Processes" by Dessauer and Clark both first published in 1965 by Focal Press, Ltd. London, England, pgs. 99 through 100, inclusive, and 213 through 216, inclusive of "Electrophotography" relate specifically to electrometers. However, none of the foregoing references describe a relatively simple, low-cost rugged and accurate electrometer system which readily enables all of the various processing stations within an electrophotographic printing machine to be controlled.

Accordingly, it is a primary object of the present invention to improve the process of electrophotographic printing by continuously controlling the various process stations employed therein with an electrometer system.

SUMMARY OF THE INVENTION

Briefly stated and in accordance with the present invention, there is provided an electrophotographic printing machine having a plurality of processing stations therein.

In the present instance, the printing machine includes a photoconductive member and means for simulating the photoconductive member. The photoconductive member is arranged to pass through each of the processing stations. The simulating means is mounted on the photoconductive member and moves therewith. As the photoconductive member passes through each of the processing stations, the simulating means produces an electrical signal indicative of the instantaneous condition of the photoconductive member after passing through the respective processing station. In this way, the electrical characteristics of the processing station may be readily controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:

FIG. 1 is a schematic perspective view of a color electrophotographic printing machine incorporating the features of the present invention therein;

FIG. 2 is a schematic perspective view of the photoconductive drum employed in the FIG. 1 printing machine and having the probe of the present invention mounted thereon;

FIG. 3 is a fragmentary elevational view, in section, depicting the probe employed to simulate the photoconductive member;

FIG. 4 is a schematic circuit diagram for periodically sampling the electrical signal from the probe;

FIG. 5 is a schematic circuit diagram for controlling the corona generating device;

FIG. 6 is a schematic diagram for regulating the intensity of light rays exposing the charged photoconductive drum;

FIG. 7 is a schematic diagram for regulating the electrical bias of the development system;

FIG. 8 is a schematic diagram for regulating the electrical bias of the transfer drum; and

FIG. 9 is a schematic diagram for controlling the dispensing of toner particles into the developer mix employed in the FIG. 6 development system .

While the present invention will be described in connection with a preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in conjunction with a color electrophotographic printing machine. For a general understanding of the printing machine continuous reference is had to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. Initially, the overall process for producing color copies will be described. Thereafter, the detailed structural configuration of the various sub-assemblies employed in the printing machine will be described in conjunction with the present invention. Although the present invention is particularly well adapted for use in a color electrophotographic printing machine, it should become evident from the following discussion that it is equally well-suited for other electrostatographic printing machines and is not necessarily limited to the particular embodiment shown herein.

As depicted in FIG. 1, the electrophotographic printing machine employs a photoconductive member having a drum 10 mounted rotatably within the machine frame (not shown). Photoconductive surface 12 is mounted on the exterior surface of drum 10. One type of suitable photoconductive material is disclosed in U.S. Pat. No. 3,655,377 issued to Sechak in 1972. In general, a suitable photoconductive member employs an aluminum substrate having a selenium layer adhering thereto. A series of processing stations are disposed about the circumference of drum 10. Thus, as drum 10 rotates in the direction of arrow 14 it passes sequentially therethrough. The present invention provides a closed loop system for controlling each of the processing stations. The operation of each processing station is substantially optimized for the specific characteristics of the photoconductive surface employed and the changes occurring thereto during the processing. The probe of the present invention, indicated generally by the reference numeral 16, may be mounted at a preselected location along the length of drum 10. However, one skilled in the art will realize that the probe may be mounted movably on drum 10 so as to determine any variations in the electrical characteristics of the drum in the lengthwise direction. Probe 16 is adapted to simulate the characteristics of the photoconductive surface and to produce an electrical signal indicative thereof. While probe 16 of the present invention is described hereinafter as being mounted on the photoconductive member, one skilled in the art will appreciate that the present invention is not necessarily so limited. For example, probe 16 may be mounted adjacent to the photoconductive member after the processing station being controlled rather than on the photoconductive member. The detailed structural configuration of drum 10 and probe 16 will be described hereinafter in greater detail with reference to FIGS. 2 and 3. A timing disc, mounted in the region of one end of the shaft of drum 10, cooperates with the machine logic to synchronize the various operations with the rotation of drum 10. In this manner, the proper sequence of events occurs at the respective processing station.

Initially, drum 10 rotates photoconductive surface 12 through charging station A. At charging station A, a corona generating device, indicated generally at 18, extends longitudinally in a transverse direction across photoconductive surface 12. This readily enables corona generator 18 to charge photoconductive surface 12 to a relatively high, substantially uniform potential. It should be noted that probe 16 is also similarly charged. Preferably, corona generator 18 is of the type described in U.S. Pat. No. 2,836,725 issued to Vyverberg in 1958. As is well known, this type of corona generator comprises a coronode wire connected to a high voltage source and supported in a conductive shield that is arranged in a closely spaced relation to photoconductive surface 12. The shield generally surrounds the coronode wire except for an opening through which the charge is emitted. Preferably, the shield is arranged to attract surplus emissions from the coronode wire. When the coronode wire is energized, corona is generated along the surface of the wire and ions are caused to be deposited on adjacent photoconductive surface 12 and probe 16. The detailed structural configuration of corona generating device 18 and its relationship with probe 16 will be described hereinafter in greater detail with reference to FIG. 5. Turning once again to FIG. 1, after photoconductive surface 12 and probe 16 are charged to a substantially uniform potential, drum 10 rotates to exposure station B.

At exposure station B, a colored filtered light image of original document 20 is projected onto the charged photoconductive surface 12 and probe 16. Exposure station B includes a moving lens system, generally designated by the reference numeral 22, a color filter mechanism shown generally at 24, and scan lamps, shown generally at 26. Original document 20, such as a sheet of paper, book, or the like is placed face down upon a transparent viewing platen 28. As shown in FIG. 1, lamps 26 are adapted to move in a timed relation with lens 22 and filter mechanism 24 to scan successive incremental areas of original document 20 disposed upon platen 26. It should be noted that the foregoing movement is in synchronism with the rotation of drum 10 in the direction of arrow 14. This creates a flowing light image of original document 20 which is projected onto photoconductive surface 12. In operation, filter mechanism 24 interposes a selected color filter into the optical light path. This color filter operates on the light rays passing through lens 22 to record an electrostatic latent image on photoconductive surface 12 and to partially discharge probe 16. The electrostatic latent image recorded on photoconductive surface 12 corresponds to a preselected spectral region of the electromagnetic wave spectrum, hereinafter referred to as a single color electrostatic latent image. The manner in which the exposure system is controlled will be discussed hereinafter with reference to FIG. 6.

After exposure, the single color electrostatic latent image recorded on photoconductive surface 12 and partially discharged probe 16 are advanced to development station C. Development station C includes three individual units, generally indicated by the reference numerals 30, 32 and 34, respectively. Preferably, the developer units are all of a type generally referred to in the art as "magnetic brush developer units." A typical magnetic brush system employs a magnetizable developer mix which includes carrier granules and toner particles. Generally, the toner particles are heat settable. In operation, the developer mix is continually brought through a directional flux field to form a brush thereof. The electrostatic latent image recorded on photoconductive surface 12 and partially discharged probe 16 are brought into contact with the brush of developer mix. The toner particles are attracted from the developer mix to the latent image and probe 16. Each of the developer units contain appropriately colored toner particles. For example, a green filtered electrostatic latent image is rendered visible by depositing green absorbing magenta toner particles thereon. Similarly, blue and red latent images are developed with yellow and cyan toner particles, respectively. As the toner particles are depleted from the system, additional toner particles are furnished thereto. Each developer unit contains a toner particle dispenser which stores a supply of colored toner particles therein. The probe of the present invention is employed to regulate the dispensing of toner particles to each of the respective developer units so as to maintain the concentration of toner particles within the developer mix substantially constant. This insures that the copy quality is maintained at a satisfactory level. The development system employed in the FIG. 1 printing machine and its relationship with probe 16 will be discussed hereinafter with reference to FIG. 7. Similarly, the toner dispensing system and its relationship with probe 16 will be discussed hereinafter with reference to FIG. 9.

Drum 10 is next rotated to transfer station D where the powder image adhering electrostatically to photoconductive surface 12 and probe 16 is transferred to a sheet of final support material 36. Support material 36 may be plain paper, or a sheet of thermoplastic material, amongst others. A transfer roll, shown generally at 38, is electrically biased and recirculates support material 36 in the direction of arrow 40.

Transfer drum 38 rotates in synchronism with drum 10, i.e. at the same angular velocity. Transfer drum 38 is electrically excited by a variable voltage source. Inasmuch as support material 36 is secured releasably on transfer drum 38 for movement therewith in a recirculating path, successive toner powder images may be transferred thereto in superimposed registration with one another. Probe 16 is in electrical communication with the voltage source electrically biasing transfer drum 38. In this manner, the electrical bias applied thereto is suitably adjusted so as to optimize the transfer process. This feature of the present invention will be described hereinafter in greater detail with reference to FIG. 8.

Prior to proceeding with the remainder of the electrophotographic printing process, a brief description will be provided of the sheet feeding apparatus. Support material 36 is advanced from stack 42 disposed on tray 44. Feed roll 46 cooperating with retard roll 48 advances and separates successive uppermost sheets from stack 42. The advancing uppermost sheet moves into chute 50 which directs the sheet into the nip of register rolls 52. Thereafter, gripper fingers 54 mounted on transfer roll 38 secure releasably thereto support material 36 for movement therewith in a recirculating path. After a plurality of toner powder images have been transferred to support material 36 (in this case three powder images) gripper fingers 54 release support material 36. Support material 36 is then separated from transfer roll 38 by stripper bar 56 and advanced on endless belt conveyor 58 to fixing station E.

At fixing station E, a fuser permanently affixes the transferred multi-layered toner powder image to support material 36. One type of suitable fuser is described in U.S. Pat. No. 3,498,592 issued to Moser et al. in 1970. After the fusing process, support material 36 is advanced by endless belt conveyor 62 and 64 to catch tray 66 for subsequent removal therefrom by the machine operator.

The last processing station in the direction of drum rotation, as indicated by arrow 14, is cleaning station E. Although a preponderance of the toner particles are transferred to support material 36, frequently residual toner particles remain on photoconductive surface 12 and probe 16 after the transfer process. These residual toner particles are removed from photoconductive surface 12 as it passes through cleaning station E. Here the residual toner particles are initially brought under the influence of a cleaning corona generating device (not shown) adapted to neutralize the electrostatic charge remaining on photoconductive surface 12 and the residual toner particles, as well as on probe 16. The toner particles are then cleaned from photoconductive surface 12 and probe 16 by a rotatably mounted fibrous brush 68 in contact therewith. A suitable brush cleaning device is described in U.S. Pat. No. 3,590,412 issued to Gerbasi in 1971.

It is believed that the foregoing description is sufficient for purposes of the present application to depict the general operation of the electrophotographic printing machine employing the apparatus of the present invention therein.

Referring now to the specific subject matter of the present invention. FIG. 2 depicts drum 10 with probe 16 mounted therein. A portion of probe 16 includes the photoconductive layer i.e. the selenium surface and aluminum substrate. The detailed structural configuration of probe 16 will be described hereinafter with reference to FIG. 3. An aperture or bore is cut through the circumferential surface of drum 10 so as to locate probe 16 thereon. Probe 16 may be mounted slidably on drum 10, i.e. by cutting a slot in drum 10 so that it may be located anywhere along the length thereof, or in a fixed location as is shown in FIG. 2. Similarly, probe 16 may be mounted movably in the printing machine so as to measure the charge distribution in a lengthwise direction across photoconductive surface 12, when probe 16 is spaced therefrom. Shaft 70 of drum 10 is a tubular member permitting electrical wiring to pass through the hollow central core thereof and out therefrom to the associated electrical circuitry of probe 16. The foregoing electrical circuitry will be described hereinafter in greater detail with reference to FIG. 4. Slip ring 72 is adapted to transmit the electrical signals from probe 16 to the electrical circuitry shown in FIG. 4. The foregoing electrical circuitry processes the electrical signal from probe 16 and is in electrical communication with the various processing stations so as to produce a control signal for regulating the respective station.

Turning now to FIG. 3, there is shown a fragmentary sectional view depicting probe 16 and a portion of drum 10. As shown therein, a transparent, substantially conductive sheet 73 is secured to photoconductive surface 12. A bore has been formed in photoconductive surface 12 and conductive substrate 74 to which it adheres. Bore 76 has positioned therein a tubular insulating member 78 adapted to have an electrically conductive member or wire 81 passing therethrough and secured conductively to transparent sheet 73. Transparent, electrically conductive sheet 73 is secured to photoconductive surface 12 by an insulating cement 80. Wire 81 is secured to transparent electrically conductive sheet 73 by an electrically conductive cement 82. In this manner, the sum of the voltages across the two dielectrics, that is, the sum of the voltage across photoconductive surface 12 and insulating layer or cement 80 is continually monitored. It is evident that the changing characteristics of photoconductive surface 12 are the only voltage induced changes that the sensor reads. The voltage across insulating layer 80 remains substantially constant. Thus, the electrical circuitry may subtract this constant voltage across the insulating layer to determine instantaneously the voltage characteristics of the photoconductive surface. The probe of the present invention is a photosensitive capacitive type device having three thin layers on an aluminum substrate. The device is fabricated and mounted tangentially flush with the surface of the photoconductor. The output of the device is a changing voltage measuring the state of the photoconductive surface as it passes through each processing unit adjacent thereto. The instantaneous output of the probe is analogous to the photoconductive surface as it passes through the various unit processes. The output is processed electronically to derive control signals representing the desired state of each of the foregoing process stations. Feedback is implemented to each of the processing stations thus controlling the electrical characteristic thereof in accordance with the measured characteristic of the photoconductive surface.

By way of example, transparent, electrically conductive sheet 73 is electrically conductive glass made by the Pittsburgh Plate Glass under the trademark NESA or may be made by the Corning Glass Company under the trademark Electro-Conductive. Electrically conductive sheet 73 is preferably about 25 microns thick or less. Similarly, transparent bonding dielectric 80 is preferably also about 25 microns thick or less. Photoconductive surface 12 preferably is about 60 microns thick. Tubular member 78 has a flange 76a adapted to properly position it in aperture or bore 76. Tubular member 78 is preferably made from a suitable insulating plastic such as Teflon.

Turning now to FIG. 4, the voltage signal from probe 16 is processed by a unity gain amplifier 84. A suitable amplifier having a high impedance can be utilized in conjunction with the probe of the present invention. The electrical output from amplifier 84 is transmitted through two successive amplifier stages 86 and 88, and then applied to a hold circuit including a high impedance unit gain amplifier 90 and a capacitor 92. However, the signal is initially prevented from passing the hold circuit by a normally open contact 94. The machine logic, preferably, includes suitable circuitry adapted to close contact 94 at the appropriate time. Thus, a sample voltage is applied across the high impedance unity gain amplifier 90. Closing contact 94 causes two discrete conditions to occur. Initially, the probe potential is applied across high impedance amplifier 90 and secondarily capacitor 92, in the hold circuit, is charged to the probe potential. Termination of the signal from the machine logic after the probe has passed the respective processing station permits contact 94 to return to its normally open position. However, the probe potential is stored in capacitor 92 and continues to be impressed across amplifier 90. Because of the high impedance of amplifier 90, a relatively constant output is maintained during the whole period until the subsequent reclosing of contacts 94 provides a new potential. This potential is applied to the respective processing station holding the output voltage therefrom substantially constant until the next sample signal is received. If the probe potential of the next sample differs from that of the first sample, capacitor 92 is allowed to recharge to the new potential through contact 94 and through the circuitry of amplifier 88. Capacitor 92 is recharged to this voltage. The output voltage is applied to power supply high-voltage operational amplifier 96 which holds the voltage output from the respective processing stations substantially constant until the next signal is received. At the end of the sample period, contact 94 is again opened and the whole circuit waits for the next sample. It is evident, therefore, that this type of arrangement permits the probe of the present invention to detect both increases and decreases in potential while, substantially simultaneously therewith, generating a continuous control signal for regulating the potential applied to the respective processing stations. The foregoing circuitry heretofore described with regard to FIG. 4 will be referred to hereinafter by the reference numeral 98.

Referring now to FIG. 5, there is shown corona generating device 18 and the requisite circuitry associated therewith for regulating the charging voltage therefrom. The construction of corona generator 18 is exemplary of one practical embodiment that consists of a conductive shield 100, preferably made of aluminum or stainless steel. Shield 100 is of a generally inverted, U-shaped cross-section. A corona generator includes a coronode wire 102 functioning as a discharge electrode. Preferably, coronode wire 102 is made of any suitable non-corrosive material such as stainless steel, platinum or tungsten having a tungsten oxide coating thereon. The wire has a substantially uniform exterior diameter of approximately 0.0035 inches. Coronode wire 102 extends longitudinally along the length of shield 100 and is connected at either end thereof to suitable dielectric blocks which are made of insulating material and attached to opposed, spaced ends of shield 100. As hereinbefore indicated after probe 16 passes through charging station A, it generates an electrical output signal. The electrical output signal is processed by circuit 98 which in turn produces an output signal which is processed by logic elements 104. Logic circuitry 104, preferably, includes a discriminator circuit for comparing a reference with the electrical output signal from circuit 98. The discriminator circuit may utilize a silicon control switch adapted to turn on and effectively lock in after an electrical output signal having a magnitude greater than the reference level is obtained. The signal from the discriminator circuit changes the state of a flip-flop to develop an output signal therefrom. The output signal is amplified by a suitable amplifier and is employed to excite an input controller. The output signal from the input controller regulates high voltage source 106. By way of example, high voltage source 106, preferably, is a constant current source adapted to excite coronode wire 102 at 400 micro-amps and about 7000 volts. In this manner coronode wire 102 is adapted to substantially uniformly charge photoconductive surface 12 to about 900 volts. The output from coronode wire 102 is regulated to vary as a function of the electrical signal from probe 16. Thus, coronode wire 102 will produce a charge sufficient to maintain probe 16, preferably, at about 900 volts, irrespective of variations or changed conditions in the surrounding environment or aging effects of the photoconductive surface.

Referring now to FIG. 6, there is shown probe 16 in electrical communication with scan lamps 26. Once again, the electrical circuit 98 develops an output signal after probe 16 is partially discharged at exposure station B. This output signal is processed by logic circuitry 108. Preferably, logic circuitry 108 includes a suitable discriminator circuit for comparing a reference with the electrical output signal from circuit 98. The discriminator circuit may employ a silicon control switch adapted to turn and effectively lock in after an electrical output signal having a magnitude greater than that of the reference level is obtained. The signal from the discriminator circuit changes the state of a flip-flop to develop an output signal therefrom. The output signal from the flip-flop is an error voltage corresponding to the requisite change in the lamp voltage in order to have probe 16 discharged to the desired level. The error signal is amplified by a suitable amplifier and is utilized to excite an input controller arranged to regulate a high voltage source 110 exciting scan lamps 26. In this manner, the intensity of light rays developed by lamps 26 is regulated as a function of the error signal. Preferably, lamp 26 is excited at a nominal value optimized for exposure. As an error signal is produced, the voltage applied to the lamps varies as a function thereof about the nominal value to compensate for deviations in the photoconductor characteristics.

With continued reference to the drawings, FIG. 7 will now be referred to in the discussion of the developer system. After probe 16 is developed with toner particles, the electrical output signal from circuit 98 once again varies so as to indicate the present state of the photoconductive surface. The output signal from circuit 98 is processed by logic circuitry 112. As shown in FIG. 7, each of the developer units 30, 32 and 34 include magnetic developer rolls 115, 117 and 119 which are electrically biased so as to develop only those regions of the electrostatic latent image on photoconductive surface 12 having a potential greater than that of the biasing potential applied to the respective roller. Circuit 98 processes the electrical signal from probe 16 after probe 16 passes through exposure station D. An output signal from circuit 98 is processed by logic circuitry 112. Logic circuitry 112 includes a suitable discriminator circuit for comparing a reference with the electrical output from circuit 98. The discriminator circuit may utilize a silicon control switch adapted to turn on and effectively lock in after an electrical output signal having a magnitude greater than the reference level is obtained. The signal from the discriminator circuit changes the state of flip-flop to develop an output signal therefrom. The output signal from the appropriate developer unit actuates an AND gate which, in turn, is amplified by a suitable amplifier and is utilized to excite an input controller. The input controller is arranged to regulate high voltage source 114 which excites developer rollers 115, 117 and 119. Logic circuitry 112 contains three channels, one channel for each of the developer units 30, 32 and 34. The machine logic provides the other signal for the AND gate so as to actuate the appropriate channel corresponding to the developer unit being excited. It would be obvious to one skilled in the art that a single color electrophotographic machine would only require one channel inasmuch as only one developer unit is employed. A suitable development system employing a plurality of developer units is disclosed in co-pending application Ser. No. 255,259 filed in 1970, the disclosure of which is hereby incorporated into the present application. It should be noted that voltage source 114 is adapted to electrically bias developer rolls 115, 117 and 119 to a normal voltage of about 500 volts.

Turning now to FIG. 8, there is shown in greater detail probe 16 operatively associated with transfer drum 38. Transfer drum 38 includes an aluminum tube 116, preferably, having about a 1/4 inch thick layer of urethane 118 cast thereabout. A polyurethane coating 120, preferably, of about 1 mil thick is sprayed over the layer of cast urethane 118. Preferably, transfer drum 38 has a drum hardness ranging from about 10 units to about 30 units on the Shore A scale. The resistivity of transfer drum 38, preferably, ranges from about 10 ⁸ to about 10 ¹¹ ohm-centimeters. Voltage source 122 applies a direct current voltage to aluminum tube 116 by suitable means such as a carbon brush and brass ring assembly (not shown). The biasing voltage may range from about 1,500 to about 4,500 volts nominally. This voltage is suitably adjusted depending upon the electrical signal generated by probe 16. Contact between photoconductive surface 12 of drum 10 and transfer drum 38 with the support material 36 interposed therebetween is, preferably, limited to a maximum of about 1 pound linear force. A synchronous speed main drive motor rotates transfer drum 38. This drive is coupled directly to transfer drum 38 by a flexible metal bellows which permits the lowering and raising of transfer drum 38. Synchronization of transfer drum 38 and drum 10 is accomplished by precision gears (not shown) coupling the main drive motor to both transfer drum 38 and drum 10. After the transfer process, circuit 98 processes the electrical signal from probe 16. This processed electrical signal goes to logic circuit 124. Logic circuitry 124, preferably, includes a suitable discriminary circuit for comparing a reference with the electrical output signal from circuit 98. The discriminator circuit may employ a silicon control switch adapted to turn on and effectively lock in after an electrical output signal having a magnitude greater than the rest is obtained. The signal from the discriminator circuit changes the state of a flip-flop to develop an output signal therefrom. This output signal is amplified by a suitable amplifier and the resulting amplified signal is employed to energize an input controller arranged to regulate voltage source 122 which electrically biases transfer roll 38. Thus, depending upon the state of probe 16, transfer roll 38 has the electrical bias thereof suitably adjusted so as to substantially optimize the transfer process. This is achieved by detecting the state of probe 16. The next subsequent transfer process has the electrical bias therefor adjusted to correct any errors indicated in the previous cycle. Thus, this process is closed loop and self-adjusting.

Referring now to FIG. 9, there is shown the detailed structural configuration for regulating the dispensing of toner particles to the respective developer unit. As shown therein, developer unit 30 contains toner particle storage container 126. Similarly, developer unit 32 contains toner particle storage container 128 and developer unit 34, toner particle storage container 130. Each of the foregoing toner particle storage containers has the respective colored toner particles therein. The foregoing toner particle containers are all substantially the same, as such, only one thereof will be described. In this case, toner particle storage container 126 will be described hereinafter in greater detail. Toner particle storage container 126 is a tubular cylinder having both ends thereof closed. The botton portion of tubular cylinder 132 has a screen 134 therein. Toner particles 136 are stored in tubular member 132. Toner particle storage container 126 is rotated about its longitudinal axis. The rotation of the toner particle housing about its longitudinal axis by an oscillator motor 138 dispenses a discrete amount of toner particles therefrom into the developer mix of developer unit 30. The electrical output signal from probe 16 passes through the commutator and out therefrom into circuitry 98. Circuitry 98 produces an electrical output signal which is processed by logic circuit 140. Logic circuitry 140 comprises a suitable discriminary circuit for comparing a reference with the electrical output signal from circuit 98. The discriminator circuit may employ a silicon control switch adapted to turn on and effectively lock in after an electrical output signal having a magnitude greater than the reference level is obtained. The signal from the discriminator circuit changes the state of a flip-flop to develop an output signal therefrom. The output signal from the appropriate developer unit actuates an AND gate which, in turn, transmits a control signal to oscillator motor 138 of the corresponding toner particle container adapted to be actuated, i.e., the developer unit developing the output signal to the AND gate. This control signal also resets the flip-flop. In this manner, the electrical output signal from probe 16 is employed to determine the requisite concentration of toner particles within the developer mix. The electrical output signal of probe 16 will vary as the function of the density of toner particles deposited thereon as the resultant charge on probe 16 is dependent upon the charge remaining thereon after exposure as well as the charge of the toner particles. The output from probe 16 is suitably processed by the heretofore described electrical circuitry to produce an output signal which excites an oscillator motor rotating one of the toner particle containers about its longitudinal axis. This dispenses the appropriate toner particles into the developer mix so as to maintain the concentration thereof substantially constant. In this manner, the copy quality is maintained at the desired level.

In recapitulation, the apparatus of the present invention is adapted to be mounted on the photoconductive member and undergo all of the various processes that the photoconductive surface undergoes. The apparatus is adapted to simulate the photoconductive member and to produce an electrical signal indicative of the instantaneous condition thereof. In this manner, the electrical signal describes the state of the photoconductive surface as it passes through each processing station. This electrical signal is employed to regulate the respective processing station. In fact, this permits the complete control of each processing station throughout the electrophotographic printing machine enabling the various system parameters associated with copy quality to be substantially optimized.

Thus, it is apparent that there has been provided in accordance with the present invention an apparatus for simulating the photoconductive surface and producing an electrical output signal indicative of the state thereof at each processing station. Each electrical signal is employed in a closed loop control system to regulate the various processing stations throughout the printing machine. The present invention fully satisfies the objects, aims and advantages set forth hereinbefore. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.