WO1985001594A1

WO1985001594A1 - Method and apparatus for controlling charge on a photoconductor

Info

Publication number: WO1985001594A1
Application number: PCT/US1984/001551
Authority: WO
Inventors: George N. Tsilibes
Original assignee: Eastman Kodak Company
Priority date: 1983-10-03
Filing date: 1984-09-27
Publication date: 1985-04-11
Also published as: CA1229652A; JPH0525112B2; EP0158669B1; EP0158669A1; DE3467940D1; JPS61500690A

Abstract

In an electrophotographic reproduction apparatus and method a corona charger (20) is used for depositing an electrostatic charge of one polarity upon a moving photoconductor (16) prior to making an exposure. In a calibration phase, an electrometer (50, 50b) moves across the photoconductor (16) and measures the level of charge at selected locations across the photoconductor to provide data regarding the distribution of charge in a direction transverse to the photoconductor's direction of movement. This data is compared with a desired range of values. Where the level of charge is measured at some locations to be outside of the desired range of values, then the level of charge at such locations on the photoconductor is adjusted differentially.

Description

METHOD AND APPARATUS FOR

CONTROLLING CHARGE ON A PHOTOCONDUCTOR

Field of the Invention This invention relates to an improved method and apparatus for controlling the level of electrostatic charge on a photoconductive surface upon which an electrophotographic image is to be made.

Description of the Prior Art

In known electrophotographic reproduction apparatus such as copiers or duplicators, an electrostatic charge is deposited on an area of a photoconductor as the area is moved past a charging station. The photoconductor is then moved to an exposure station where the area is exposed to image-forming radiation to form a latent electrostatic image of a document to be copied. The latent image is thereafter developed and, in the case of plain-paper copiers and duplicators, subsequently transferred to paper upon which the copied image is to appear. Thereafter, the photoconductor is cleaned and otherwise made ready for the next copy cycle. In such apparatus, it is important to impart a generally uniform charge over the area upon which the latent image is to be formed. Too low a charge in portions of the area may result in weak, washed-out looking areas on copies, and too great a charge in portions of the area may result in areas on copies being too dark relative to other areas. Therefore, copy quality, particularly with pictorial subject matter, can be affected seriously where a non-uniform charge is placed on the photoconductor.

OMPI In the prior art, as exemplified by U.S. Patents 4,105,321 and 4,248,519, an apparatus is described for controlling the level of charge placed on a photoconductor. In this apparatus a corona wire is energized to deposit a positive electrostatic charge on a photoconductive belt. This level of charge imparted to the belt is measured by an electrometer that is located proximate the belt and a signal from the electrometer is compared with a reference signal that represents a maximum level to which it is desired to charge the belt. If there is an undercharge, the charge level is increased to the degree necessary to bring the charge level up to a reference charge. If the signal derived from the electrometer indicates that the charge is beyond this maximum level, an erase light source is illuminated to the brightness necessary to reduce the charge deposited on the belt to the level desired.

In electrophotography, it is known to charge a photoconductor with either a negative or a positive electrostatic charge, the particular charge chosen being a selection based on the type of photoconductor used. This charge is preferably deposited by a corona charging system comprising one or more corona wires which lie transverse to the direction of movement of the photoconductor past the charging station. In the case of employing positive corona charging, a generally uniform charge can be expected to be deposited on the photoconductor using known charging techniques. In the case of negative corona charging, control is more difficult, and a considerable amount of charge non-uniformity can result. One may observe the glow from a positively charged corona wire and note a uniform glow surrounding the entire length of the wire to indicate uniform current flow from the wire to the photoconductor. On the other hand, discrete glow spots often occur along a negatively charged corona wire. The glow spots are associated with creating non-uniformities in charging of the photoconductor. As the glow spots appear at different positions along the wire over the course of a day due to changes in humidity within the copier or because of other factors, the non-uniformity of charging changes with time (see R.M. Schaffert, Electrophotography, 1975 edition, pages 466-472). While minor amounts of non-uniformity may be tolerated, a large variance in non-uniformity as indicated above results in significantly affecting the quality of the image. The extent of the problem will depend upon the nature of the apparatus and the material to be copied. Obviously, continuous tone or halftone originals will be more of a problem than copying text. Color copiers demand even more uniformity in charging than ^• do monotone copiers.

It is an object of this invention to provide a generally uniformly charged image-forming area of a photoconductor, particularly where the charging source used or other operating conditions result in a tendency for a large viariance in the charge level to otherwise be deposited on the surface in a direction transverse to the direction of movement of the surface past the charging source. As used herein, the term, "generally uniform charge," implies that variation in charge level may occur from one part of the area to another; however, the various parts comprising the area have a level of charge falling within a desired narrow range of values.

It is a further object of the invention to provide a controlled imaging exposure of a charged

:7^:v photoconductor having a charge level varying from a desired reference level wherein the imaging exposure compensates for the variance of charge from the reference level.

Summary of the Invention

The objects of the invention are accomplished by an improved electrophotographic reproduction apparatus and method for controlling charge on a moving photoconductive member which is electrostatically charged by a charging means and on which developable electrostatic images are formed. The invention is directed to a means for measuring the level of charge on the photoconductive member at each of a plural number of locations lying in a direction transverse to the direction of movement of said member, to produce signals representative of such levels, and adjusting means responsive to such signals for differentially adjusting the charge level at transverse locations having a level of charge differing from the reference level.

The invention is further directed to a method and apparatus for exposing a photoconductive member with a narrow beam of radiation which is modulated with imaging information to form an electrostatic latent image and wherein data relating to the distribution of charge on the photoconductor are converted to one or more signals to also modulate said narrow beam during exposure at the latent electrostatic image forming area to compensate for a charge level on the photoconductor that was different from a reference level.

/ Brief Description of the Drawings

In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which:

Fig. 1 is a side elevational view in schematic form of a copier which embodies apparatus in accordance with the invention;

Fig. 2 is a schematic of a portion of the copier shown in Fig. 1 that is directed to the depositing of a generally uniform charge upon a photoconductor;

Fig. 3 is a flow chart illustrating the sequence of operations used by a control system for controlling the portion of the copier shown in Fig. 2; Figs. 4a-c and e present a hypothetical example of charge level readings across a photoconductor and are provided to facilitate understanding of the apparatus and method of the invention;

Fig. 4d presents a graph representing illumination of lamps in accordance with the hypothetical example;

Fig. 5 is a schematic of an arrangement of banks of lamps that may be used in accordance with the invention. Fig. 6 is a view similar to that of Fig. 2 but showing another embodiment of the invention;

Fig. 7 is a flow chart illustrating the sequence of operations used by a control system for controlling the portion of the copier shown in Fig. 6; Fig. 8 is a side elevation view in schematic form of still another embodiment of the invention;

Fig. 9 is a schematic showing additional details of elements forming a charge control means for the embodiment of Fig. 8.

OMPI Description of the Preferred Embodiment

Because apparatus of the type described herein are well known, the present description will be directed in particular to elements forming part of or cooperating more directly with the present invention.

For a general understanding of a web-type electrophotographic copier/duplic tor apparatus 10 wherein the invention has utility, reference is made to Fig. 1. As shown, a photoconductor member, in the form of a photoconductive web 16, is trained about rollers 4 through 9 for movement in the direction indicated by the arrow A. Roller 4 is driven by a drive mechanism 18 shown for simplicity to include a motor-pulley arrangement. An insulating layer or surface 16a of the web 16 is charged at a corona charge station (charger) 20. The charger 20 includes one or more corona generating wires 20a, a shield 20b, and a grid 20c for regulating the flow of negative corona current from the wires to the photoconductor member. Thereafter and at an appropriate time, an information medium 13 such as a document is illuminated at an image exposure station by radiation from flash lamps 14. Such radiation is reflected from the medium and projected by a lens 15 onto the charged insulating surface 16a of the web 16, to selectively dissipate charge and form an electrostatic latent image of medium 13 on a specific area of the web. For more specific disclosures of the web, see commonly assigned U.S. Patents Nos. 3,615,406 and 3,615,414, both issued October 26, 1971. A plurality of light sources 56 and an electrometer 50 are disposed between the exposure station and the charger 20 as will be described in detail later. The apparatus 10 further includes a development station 22 at which the moving electrostatic image is contacted with finely divided charged toner particles that adhere to the charged web surface in a configuration defined by the electrostatic image, to form a visible toner image; a transfer station 25 in which the toner image is transferred to a receiving surface of a copy sheet 26 on which it can be subsequently permanently fused; and a cleaning station 31 in which residual toner particles are removed from the web 16.

At the development station an electrostatic image on the insulating surface 16a of web 16 is moved past two magnetic brushes or rollers 24a and 24b mounted in a housing 27 of the development station 22. The housing 27 holds a supply of developer containing a mixture of toner and carrier particles. The brushes 24a and 24b can be constructed according to any one of a variety of designs known in the prior art. For a more complete description of the general organ¬ ization of a similar copier apparatus, reference may be made to commonly assigned U.S. Patent No. 4,025,186.

Although a web-type copier/duplicator has been shown, it will be understood that the present invention is also particularly suitable with copier/duplicator apparatus that use drums and also sheet film photoconductors. In any case, it will be understood by those skilled in the art that a microcomputer having a stored program can be effectively used as the logic and control apparatus to control the operation of the copier/duplicator.

Turning now to Fig. 2, there is shown a schematic representation of apparatus that is assembled in

3MPI accordance with the invention. The electrometer or electrostatic voltmeter 50 is conventional and includes a module 50a including power supply, amplifiers and output circuitry and a probe 50b that is mounted on a rotatable lead screw 52. When a stepper motor 54 is energized, it rotates the lead screw 52 which translates the electrometer probe 50b in a transverse direction indicated by the arrow x across the width of the web 16. A plurality of light sources 56 are disposed between the charger 20 and the electrometer probe 50b. The light sources each emit light of a suitable spectral frequency(ies) to which the photoconductor is sensitive to cause the exposed portions of the photoconductor to become partially conductive to at least partially reduce the charge level appearing thereon. Such light sources may comprise a plurality of adjustable intensity lamps, each one of which delivers light to a respective light pipe which transmits it to a respective transverse location on web 16. Other light sources which may be used include plasma displays, LED arrays, digitally controlled electrolummescent panels, laser or halogen lamps or neon lamps with PLZT crystal apertures, and long arc lamps with PLZT crystal apertures. The preferred embodiment will be described with reference to a panel display. An example of such a panel display is Burroughs SELF-SCAN Panel Display, Model SSD0124-0039 which is described in further detail below. The apparatus shown in Fig. 2 will be described with reference to the hypothetical example shown in

Figs. 4a-e. In Fig. 4a, there is shown represented by a straight line a nominal or desired charge level _Q for the photoconductor of say -550 volts. In the transverse direction "X", the actual Vo in this example is shown in curve "C" to have some variability about this nominal level due to nonuniformity of charging for the reasons set forth above. A certain amount of variability from V , say +10 volts, is acceptable in this example, and this is labeled as the Band of Uniformity. To simplify matters, discussion of charge level will be in terms of absolute value or magnitude, thus a charge level of -550 volts is considered herein higher than a charge level of -540 volts. Depending on the circumstances, the band may be wider or narrower and perhaps even of no width so that the higher and lower levels are identical with nominal

It will be noted that certain portions of the photoconductor have charge levels that are outside this band. In accordance with the invention, the charger 20 and the light sources 56 are adjusted to vary the charge levels on the photoconductor 16 so that they are within the range of the preferred Band of Uniformity. See Fig. 4e.

Before the copier/duplicator is placed in a reproduction mode, once or twice a day or more frequently if necessary a calibration operation is taken to ensure that the actual charge levels V are within the Band of Uniformity. In this calibration, adjustments are made to the charger 20 and the light sources 56 in the manner to be described. After calibration, the copier/duplicator apparatus can be operated in the reproduction mode to produce production runs of copies. To start the calibration, an operator depresses a switch 45 which provides an interrupt signal to the microcomputer 47, which will be understood to also control the operation of the copier/duplicator apparatus 10. Prior to depressing the switch 45, a signal V, has been provided during the normal machine start-up to a summing port 48 which sets the low voltage control grid power supply 49 to adjust the charger 20 so that it will produce a nominal charge level V of say -550 volts. As shown in Fig. 4a, the actual V_Q as deposited on the photoconductor varies from the nominal level. Grid control programmable power supplies are well known in the art and are disclosed, for example, in U.S. Patent No. 4,166,690 and also U.S. Patent No. 4,294,536.

In the calibration mode, the microcomputer 47 provides a series of pulse-type commands to the motor 54. The motor 54 is then energized to move the electrometer probe 50b in the transverse direction across the web 16. As indicated in Fig. 4b, for a specific example, there are eight transverse locations on the photoconductor for say an image area about 35cm wide. At each location (as determined by the number of pulses sent to the motor), the microprocessor activates analog/digital converter 57 which provides a digital representation of the charge level at a particular transverse location as an input to a memory associated with the microcomputer where it is stored in a predetermined one of eight locations. Preferably the electrometer remains at each location for a time sufficient for the photoconductor to complete one full revolution past the electrometer. This permits the electrometer to provide an averaged reading for that particular narrow strip portion of the photoconductor. After the first particular transverse location charge level is sensed and stored in memory, the measurements are taken for each of the next seven locations. Fig. 4b shows the digitized signal levels. After all eight locations have been measured and stored, the microcomputer 47 compares these digitized readings with the nominal level V_Q as sensed either directly by measuring V_Q or by using the low power grid supply voltage V, which bears a known relationship to nominal V . This may be accomplished by having analog to digital converter 62 convert the analog voltage V, into digital form which is stored in the computer's memory. The value of nominal V is then calculated or otherwise derived and compared with each of the eight measurements from the electrometer of the level of charge at each transverse location. If the level of charge at any transverse location is measured to be lower than the lower limit of the Band of Uniformity then, the computer determines the minimum additional charging needed to boost the charge level at the transverse location having the lowest charge level to bring this charge level up to the minimum value for the Band. A signal, V , representative of the additional charging needed is then delivered from the digital to analog converter 64 which in response thereto provides an offset signal to the summing port 48 so as to adjust the voltage provided to the grid of the corona charger 20. Advantageously, the computer may be programmed to have electrometer probe 50b make one or more additional crossings until as shown in Fig. 4C, the charger is adjusted so that the lowest measured charge level is just within the lower limit of the Band of Uniformity. After the last crossing determines that the charger is suitably adjusted, the computer also has data as to whether or not the level of charge at any transverse location exceeds the upper limit of the Band. If the level of charge at any location was measured to be larger than the upper limit of the Band, then the computer determines the difference between the charge level at each location that is higher than for example the midpoint of the band or nominal V_Q and a signal commensurate with this difference is delivered to the lamps controller 70. The lamps controller represents a control module that is adapted to light a particular number of lamps that are associated with a particular transverse location which they directly overlie. In Fig. 5, a preferred arrangement of banks of lamps is shown that comprises three panel displays that are mounted so as to be staggered as shown with the direction of movement of the photoconductor indicated by the arrow A so that taken together their active areas span the width of the photoconductor 16. The displays each have 17 rows with each row having 192 gas plasma display lamps 56a. Of the 17 rows of display lamps in each column there will be selected lamps that may be illuminated to erase charge from a respective transverse location on the photoconductor and over which the column lies. For example, one or more of the lamps in row 1, column 1, may be illuminated to remove a small overcharge (over nominal V ) from the first transverse location on the photoconductor. Should greater overcharge be present at this location more than one and up to 17 rows of lamps in column #1 may have its respective lamps illuminated to reduce the charge to nominal V_Q. Each of the transverse locations on the photoconductor of which a measurement is made of charge level has a respective group of lamps associated with a designated column, and one or more of these lamps may be illuminated row by row to reduce charge in the associated transverse photoconductor location for the respective column of lamps. Since the illumination from the lamps tends to flare out, it is desirious to avoid illuminating lamps adjacent the ends of each column to avoid light from lamps in one column affecting adjacent locations. Thus preferably only those lamps near the middle of the column need be used and their tendency to flare out is used to substantially cover the width of their respective location. The level of illumination emitted by each lamp is desirably preset to provide fine control over the charge-reducing effect of each lamp. Thus filters may be placed in front of the lamps and/or the current regulated thereto so that each lamp erases only a small amount of charge.

Returning now to the example shown in Fig. 4, the lamps controller 70 in response to the signal from the computer illuminates the number of rows of lamps in each column that are needed to reduce the charge at each location to nominal V . Thus, the charge at each location that is overcharged is reduced differentially in accordance with the difference between charge level measured at such location and a reference value which in this case is the nominal value V . The reference value could also be the upper boundary of the Band of Uniformity.

As shown in Fig. 4d, only those lamps which correspond to transverse locations on the photoconductor that are higher than the nominal level

V are illuminated, and only in numbers of rows selected to reduce the charge at the corresponding transverse photoconductor location to the nominal

V level. After the corresponding lamps are illuminated, an additional measurement is taken by the electrometer of each of the transverse locations on the photoconductor and each measurement is again

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^■ ■■-., V/IPO compared with nominal V to ensure that no locations have measured charge levels greater than nominal V . Should any still have greater charge levels, the computer, based on these new measurements, will signal the lamps controller to increase the number of rows of lamps illuminated for the respective transverse location. As shown in Fig. 4e, the charger is adjusted so that the lowest measured charge level is just within the lower limit of the Band of Uniformity and, with the proper erase lamps on, all the transverse locations on the photoconductor are within the Band of Uniformity. Now that the charging system is calibrated, it may be used in the reproduction mode with the appropriate charge erase lamps illuminated and with the charger properly adjusted. As may be noted from Fig. 3, the process of having the electrometer make a reading and then adjust the charger or the charge-reducing lamps is an iterative one. That is the cycle of measuring and adjusting of the charger or the lamps repeats based on the last taken measurements until the desired results are obtained.

If desired, the charger may be initially adjusted so that it would overcharge the photoconductor and thus it would be unlikely ever to have any transverse locations lower than the minimum value of the band. In such instance, controls for providing an offset would be unnecessary and only controls directed towards erasing charge would be useful. While one moving electrometer is shown in the discussion of the preferred embodiment it will be appreciated that a plurality of stationary electrometers may be provided and arranged across the width of the photoconductor and appropriate circuitry

O rl provided for taking their respective readings in a timed sequence.

Reference will now be made to a second embodiment of the invention shown in Figs. 6 and 7. In this embodiment, elements identical to that described with regard to the first embodiment of the invention are identified with the same indicia. Since these identical elements function in the same way as described previously discussion with regard to this embodiment need only be directed toward differences in the operation of this embodiment vis-a-vis that for the first embodiment. In the second embodiment a needle charging station 35 is substituted for the charge erase lamps of the embodiment of Fig. 1. At charging station 35 a plurality of rigid needle-like charging elements A-H are arranged transverse to the direction of movement A of photoconductor 16. During a calibration phase of the apparatus both primary charger 20 and needle chargers A-H are turned on so that these charging elements are emitting corona current. In this embodiment both charging stations are emitting negative corona current onto the photoconductor. The electrometer probe 50a is then moved in the direction X across the photoconductor and measures the charge level at each of eight locations across the width of the photoconductor. The electrometer module 50b then feeds these analog signals to an A/D converter 57 which in turn delivers digitized values of these signals to microcomputer 47. In addition to storing these values, the microcomputer has also stored in memory the grid supply reference voltage V^ and has calculated the limits of the desired uniformity band. With regard now to Fig. 7 and the description which follows immediately, terms shown in parentheses are intended

OMPI to describe an alternative example. This alternative example is described by substituting at each occurrence the word in the parentheses for the immediately preceding word. If the readings by the electrometer 50 indicate that all locations have a charge level lower (higher) than the lower (upper) level of the uniformity band then the charge output of the primary charging station 20 is increased (decreased) by providing a signal to summing port 48 which combines with signal V, to increase (decrease) the grid voltage V₀. The electrometer again takes a measurement at each of the 8 locations to determine if all of the locations have a charge level lower (higher) than the lower (upper) level of the uniformity band. The above is repeated iteratively until this condition is met. When this condition is met the microcomputer next determines what additional charging is needed from the needle chargers to have all the transverse locations be at charge levels falling within the desired uniformity band. Each of the charging needles is adjusted differentially with regard to the respective transverse reading by the electrometer. After successive additional readings by the electrometer with iterative adjustment of the voltage on the needle chargers 35 the charge levels at the respective locations beneath the needle chargers will be brought to levels within the desired uniformity band. A needle charger controller 36 coupled to the microcomputer stores the determined voltage setting for each charging needle A through H and maintains this setting during the reproduction mode of the copier. While this embodiment has been described with reference to 8 charging needles one for each of the measured transverse locations it will be

OMPI appreciated that many more needles may be located across the width of the photoconductor and grouped together so that say 4 needles are electrically at the same potential and used to adjust the charge at each of the eight transverse locations. This may be desirable since the effect of each needle is limited to adding a charge along a narrow band beneath the needle and there could be a need for say 4 of these bands to equal the width of each band comprising a transverse location.

The effect of the charge-adjusting means, items 56 and 35, described in the first and second embodiments are to differentially adjust the non-uniform charging by the corona charging station 20 to provide a resultant generally uniform charge upon the photoconductor. Thus, in the embodiments described above, a generally uniform electrostatic charge was formed over the area upon which an electrophotographic image is to be made by the selective enablement of either charge-reducing means or charge-increasing means. The invention further co templates that since in the context of electro¬ photography charge-reducing means as described herein and image exposure means both involve the use of light that an equivalent is considered to be the use of a charge-reducing means during the exposure operation. For example, where imaging exposure is made through use of a narrow beam the intensity of the beam or the duration of time spent at any "point" may be adjusted in accordance with information derived from the electrometer as well as from the source of image information.

In this regard, reference will now be made to Figs. 8 and 9 wherein an electrophotographic apparatus is shown having a scanning exposure system. Items indicated with a prime (') in these figures correspond to similarly numbered items shown in Figs. 1 and 2 which have similar functions. In this embodiment a photoconductor 16', illustratively shown mounted on a drum, rotates past and is charged by a charging station 20* having a negative corona current discharge device and grid suitably spaced from the photoconductor. An exposure station, more fully described below, has an exposure beam 80 reflected from a rotating polygon 81. The beam suitably image-wise modulated traverses the scan width of the photoconductor in synchronism with the drum to form an electrostatic image on the photoconductor. The photoconductor then passes through a conventional development station 84 which may be of the cascade type shown or of the type illustrated in Fig. 1. This station causes toner particles to adhere to portions of the photoconductor not sufficiently discharged by the exposure beam to develop the latent image. The developed image is then transferred to a web of copy paper that is passed in contact with the photoconductor. The copy paper receives charge from an electrostatic discharger 86 to induce transfer of the developed image from the photoconductor to the copy paper. The copy paper is supplied from a supply reel 88, passes around guide rollers 90 and is advanced by drive rollers 92 into receiving bin 94. A fusing device 93 fixes the images to the copy paper as it passes into bin 94.

Usable images are provided in that the information content of the scanning spot is represented by the modulated or variant intensity of light respective to its position within the scan width. As the spot traverses, the charged surface of the photoconductor through a given scan angle, the spot dissipates the electrostatic charge in accordance with its light intensity. The electro¬ static charge pattern thus produced is developed and then transferred to the copy paper as described above. The photoconductor is then cleaned by a cleaning device 98 before being recharged by charging device 20' for the next copy cycle. The polygon 81 is continuously driven by motor 100 and synchronized in rotation to a synchronization signal repre¬ sentative of the scan rate used to obtain an original video signal which signal may be stored in binary form in a binary storage device indicated by the device 95 labeled "Page Storage" in Fig. 9. The beam is also modulated by a signal stored in binary form in a storage device 91 labeled "Charge Uniformity to Beam Intensity Correction" and representing charge control information. This information is derived by an electrometer 50' which includes probe 50b' that traverses the width of the photoconductor and at specific transverse locations of the photoconductor provides readings of the voltage levels of the photoconductor which has been charged by the charger 20'. As indicated for the embodiment described in Figs. 1 and 2, the microcomputer 47' may be programmed to cause the electrometer to make these readings during a warm-up period of the copier or at fixed times during the day when the copier is not otherwise in a copy mode. The readings of the probe 50b¹ are detected by the electrometer module 50a' and converted from an analog signal to a digital signal by converter 57'. The microcomputer 47' then determines if there are any locations which have charge levels lower than the lower limit of the Band of Uniformity as discussed previously for the embodiment of Figs. 1 and 2. If additional charging is required, the grid voltage supply 49' is adjusted until the repeated measure¬ ments by the electrometer determine that all transverse locations are charged to at least the lower limit of the Band of Uniformity. The last set of measurements used in calibrating the charger are then used by the computer to calculate adjustments to the intensity of the exposure beam for those of the eight locations requiring compensation for having extra charge being present thereon. These adjust¬ ments are stored in digital form in the storage device 91. As may be noted in Fig. 9, a signal representing the optical information from the Page Storage device 95 is combined in a multiplier 96 with a signal representing charge adjustments. The resulting signal is then converted to an analog signal by digital to analog converter 97 and the analog signal is coupled to acousto-optic modulator 99 to modulate a beam that is focused by beam forming optics 105 onto the modulator. The source of the beam may comprise, for example, a helium-neon laser source 106. The zero order non-diffracted beam is absorbed by a beam stop and the diffracted first order beam which is modulated with both optical and charge control information is passed through a beam expander 101 a filter 102 and reflected from polygon 81 through field flattening optics 107 onto photo¬ conductor 16'. A light sensor 108 is located adjacent one edge of the photoconductor and senses light reflected from polygon 81 at the start of each page line. A signal from this sensor and a signal from a shaft encoder 103, which detects the drum's rotational position, are fed to the microcomputer 47' and used to synchronize movement of the drum with the scan exposure.

In the embodiment just described, the laser beam is modulated for each scan line at numerous discrete points across the width. of the photoconductor. For each such point, the modulation will involve an input comprising both optical information and charge control information. If necessary, the charge control information signal may be modified to correct for other factors such as variations in facet reflectivity of the polygon 81. When reproducing white background areas of an original document, the modulated beam will be of such intensity defined herein as full exposure intensity that it will reduce the level of charge on corresponding areas of the photoconductor to a level below which will cause no toner to adhere to such areas of the photoconductor. Areas of the photoconductor corresponding to image information will be suitably either not discharged or have its charge modified in accordance with signals from the "Page Storage" and "Charge Uniformity to

Beam Intensity Correction" devices so that the charge in these areas form a latent electrostatic image that will be developed when passed through the developer station 84. The signals from the "Charge Uniformity to Beam Intensity Correction " device will be related to the difference in charge between the charge level sensed by the electrometer 50' at the particular location and the desired level of charging as exemplified by a predetermined "Band of Uniformity" whose values are stored in the microcomputer's memory.

In general, exposure of a photoconductor with a laser beam can be conducted in the following fashion during a point-by-point scan of a photoconductor to provide contrast in a finished print. Light areas in

OMPI the print are made by providing full exposure of many or all the points in the corresponding areas of the photoconductor; grey areas can be made by providing full exposure of only some points in the corresponding areas of the photoconductor or more preferably by providing a suitable amount of exposure for each point commensurate with the density desired in the final print; and black areas are made by providing no exposure of points in the corresponding areas of the photoconductor. Since full exposure of any point on the photoconductor by a scanning laser beam can result in its charge level being reduced to a level below which no toner will tend to adhere thereto, it is normally of no concern that the initial charge on that point is higher or lower than nominal V . What is important is that points in the image area which would receive no or little image light should be charged to V_Q so that they develop appropriately. Also important is that points in the image area which correspond to grey areas of the print and which are to be charged to voltage levels less than V in accordance with desired density for such areas should have their charge levels modified in accordance with both density-image information and charge correcting information. With the above in mind, the multiplier may be constructed so that at each point where full exposure is called for by information stored in the Page Storage device the multiplier provides a signal representing this information to converter 96 and AOM 99 and suppresses the charge correction information. At each point where exposure is not called for by the Page Storage device the multiplier provides the charge correction information signal to cause modulation of the laser

OMPI

& ^■ ._{. %} IPO beam to a level less than its full exposure level to adjust charge levels at such points on the photoconductor to levels within the Band of Uniformity. At points where some but less than full exposure is called for by the Page Storage Device, the multiplier combines both the Page Storage Device information signals and Charge Correction Device information signals to cause modulation of the laser beam to levels appropriate for the densities desired at such points.

While description of the embodiment of Figs. 8-9 have been made with regard to a laser beam it will be appreciated that other devices are known for forming a narrow light beam which may be modulated with image information for exposing an image area on a photo¬ conductor. These devices, such as a light emitting diode (LED) array, which extends across the width of the photoconductor, may have the individual LED's modulated with both image information and charge control information to correct for variances of charge level from a reference level during the imaging exposure operation.

Many of the causes of charge non-uniformities are time dependent. Therefore, it is quite possible that the illumination profile determined at the beginning of the day may not be appropriate for the desired uniformity at the end of the day. A number of control algorithms can be used to have the copier/duplicator enter the calibration mode automatically from the reproduction mode. For example, depending on the magnitude of the non-uniformity that the copier/duplicator can tolerate, the calibration mode can be entered once a day, twice a day, and so on without the need for the

•?F operator to initiate the calibration mode. For example , the calibration mode may be entered automatically during a copier' s warm-up period or between the start of a job and the first copy.

Claims

1. Electrophotographic reproduction apparatus having a moving member, in particular a photo¬ conductive member which is electrostatically charged by a charging means and on which developable electrostatic images are formed, means for measuring the level of charge on the member and means for adjusting such level to a referenced level, characterized in that first means (47, 52, 54) are provided for controlling the measuring means (50) to cause the measuring means (50) to measure the level of charge at each of a plural number of locations lying in a direction transverse to the direction of movement of the member (16) , and to produce signals representative of such levels, and that second means (47, 70, 47, 36, 47', 91, 96, 97) are provided responsive to such signals and operatively associated with said adjusting means (56, 35, 99) for differentially adjusting the charge level at transverse locations having a level of charge differing from the referenced level.

2. Apparatus according to Claim 1, characterized in that said adjusting means and second means comprise means (99, 101, 102, 105, 106) for producing a narrow beam of radiation (80) and means (97, 99) for modulating the beam (80) for exposing points in each location with signals representing an image to be formed on the member (16') at the location and with signals representing a difference between the level of charge measured for said location and the referenced level.

3. Apparatus according to Claim 1, characterized in that said adjusting means comprise light source means (56) arranged transversely to the direction of movement of the member (16) for providing exposure of said member (16) to non-imaging light at locations and in amounts needed to reduce the charge levels differentially at the locations to a level falling within a range between referenced minimum and maximum levels.

4. Apparatus according to Claim 1, characterized in that said adjusting means comprise a plurality of point charging means (35) arranged trans¬ versely to the direction of movement of the member (16) for providing additional corona charge current to said member (16) at locations and in amounts needed to add charge differentially at such locations to have the resultant charge of such locations fall within a range between referenced minimum and maximum levels.

5. Apparatus according to Claims 1 through 4, characterized in that third means (47, 48, 49, 64) are provided responsive to the signals produced by the measuring means (50) representing a charge level lower than a referenced minimum level, for adjusting the charging means (20) to increase the level of charge on the member (16) so that all portions thereof have a level of charge greater than the referenced minimum level.

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OMPI

&* ϋo" 6. Apparatus according to Claim 5, characterized in that first means (47, 52, .54) cause the measuring means (50) to again measure the level of charge at each of the plural number of locations after each adjustment of charge and if the level of charge at any location is measured to be beyond the respective referenced levels the second means (47, 70, 47, 36) or the third means (47, 48, 49, 64) respectively again cause the adjusting means (56, 35) or the charging means (20) respectively to adjust the level of charge on the member (16) so that all portions thereof have a level of charge within the referenced levels and wherein the first, second and third means respectively are operated iteratively until the charge level at each of the locations is within the referenced levels.

Method for depositing electrostatic charge on an area of a surface, in particular on a photo¬ conductive surface, so that the level of charge on the surface is suitable for forming an electrostatic latent image upon exposure to image-bearing radiation, by moving the surface in a first direction past a charging station and depositing charge on the surface, measuring the level of charge deposited on the surface and adjusting the level of charge on the surface to a referenced level, characterized in that measurements are made of the level of charge at each of a plural number of locations lying across the surface (16) in a second direction transverse to the first direction and in that the charge level is differentially adjusted at each of the locations having a charge level differing from the referenced level.

8. Method according to Claim 7, characterized in that charge adjusting is made by employing light to differentially reduce charge on the surface (16*) and in that the surface (16') is exposed simultaneously at the same location to light that is modulated in accordance with image formation for forming the latent image and with charge- reducing information for differentially reducing the charge at the location in accordance with the difference between the respective measurement of the level of charge for the particular location and the referenced level.

9. Method according to Claims 7 or 8, characterized in that for charge adjusting a narrow beam of light is used, in particular emissions from a laser, that is scanned across the surface (16').

10. Method according to Claims 7 or 8, characterized in that for charge adjusting an LED array is used.

11. Electrophotographic reproduction apparatus having an electrostatically charged member on which developable electrostatic images are to be formed on an image area of the member, means for measuring the level of charge on the member and adjusting means for providing signals for adjusting such level to a referenced level suitable for developing unexposed points of the

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OMPI

^' , WIPO ^' image area, characterized in that means (101, 102, 105, 106) for forming a narrow beam of radiation (80) and means (81) for causing the beam (80) to expose points upon the member (16') are provided and in that the beam (80) is modulated by modulating means (97, 99), while exposing the member (16'), with image information signals representing an image to be formed on the member (16') providing exposure at certain points in accordance with the image information signals and the modulating means (97, 99), at points on the image area of the member (16') where no exposure is required by the image information signals modulates the beam (80) with signals for adjusting the level of charge on the member (16') to the referenced level.

12. Apparatus according to Claims 1 through 4, 6, or 11, characterized in that a corona charging station (20) charges the image area of the member (16) with a negative electrostatic charge at least for a level suitable for forming developable electrostatic latent images.

13. Method for depositing electrostatic charge on a latent image area of a surface, in particular on a photoconductive surface, so that the level of charge on the surface is suitable for forming an electrostatic latent image upon exposure to image-bearing radiation by moving the surface in a first direction past a charging station and depositing charge on the surface, measuring the level of charge deposited on the surface to produce at least one signal for adjusting such level to a reference level suitable for de¬ veloping the latent image characterized in that the photoconductor is exposed at the latent image area to a narrow beam of radiation that exposes the image area and is modulated while exposing the image area with image information for forming an electrostatic latent image and by at least one charge-adjusting signal for differentially ad¬ justing the charge at the location in accordance with a difference between the charge level sensed and the reference level.