EP0098509A2

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Info

Publication number: EP0098509A2
Application number: EP83106300A
Authority: EP
Inventors: Harold J. Weber; Donald Richardson; John N. Poulakis
Original assignee: Coulter Systems Corp
Current assignee: Coulter Systems Corp
Priority date: 1982-06-28
Filing date: 1983-06-28
Publication date: 1984-01-18
Also published as: DK295383D0; IL69055A0; EP0098509A3; AU1632783A; JPS5946656A; CA1194092A; DK295383A

Abstract

@ For electrophotographic imaging in which a photoconductive surface is moved past a corona generating device, charged, exposed and toned, a method and apparatus for establishing a predetermined apparat surface voltage charge on the photoconductive surface at the start of the toning function and providing exposure control thereof. The photoconductive surface is charged with a ramped, staircase pattern of corona output levels. A selected region of the photoconductive surface is moved over an exposure lamp to provide exposed and unexposed regions, exposure taking place for a predetermined period of time to illuminate and at least partially discharge the selected, exposed region of the photoconductive surface. The electrostatic field charge on said surface is detected and related to the movement of the photoconductive surface. The detected charge levels for the exposed and unexposed regions are compared to a predetermined signal to provide a comparison signal for control tasks. The predetermined signal includes both the desired electrostatic field charge levels and a profile of the characteristics of the particular exposure lamp. The predetermined signal is stored in a control logic unit memory, the measured signal and the predetermined signal compared and control signals provided to regulate the a) corona output level, b) the exposure light intensity and c) the duration of exposure in accordance with both the detected signal and the predetermined stored signal.

Description

This invention relates generally to an electrophotographic imaging apparatus, and more particularly provides a method and apparatus for establishing a predetermined apparent surface voltage charge on the photoconductive surface of an electrophotographic member at the start of toning and providing exposure control thereof.
An electrophotographic member having an outwardly facing photoconductive surface is secured to a platen mounted on a linearly translatable carriage to bring said photoconductive surface past plural functional stations including a charging station, an exposure of imaging station, a toning or developing station and an optional image transfer station. A corona generating device at the charging station applies a surface voltage charge to the photoconductive surface, same then being moved to the exposure or imaging station. Light is projected in a pattern to the charged surface forming a latent electrostatic image on said photoconductive surface comprising exposed and unexposed areas. The latent image is developed (toned). At the start of toning it is desirable to have a predetermined apparent surface voltage charge on the unexposed areas of the photoconductive surface.
At the start of the toning function the apparent surface voltage charge of the respective exposed and unexposed areas of the photoconductive surface generally are deter- minded by a) the level to which the photoconductive surface was initially charged by the corona means; b) the exposure or imaging light intensity and duration; c) the elapsed time between the initial charging and the start of the toning function, and, d) the characteristics of the individual electrophotographic member employed. Among the characteristics of the photoconductive surface are the individual dark decay slope and aging. Other characteristics may be considered.
A prime factor for assuring acceptance of the electrophotographic processes is provision of consistent and repeatable imaging. If one is to gain the maximum benefit available through utilization of a medium such as disclosed in U.S. Patents 4,025,339 and 4,269,919, one must reduce the variable in the process, to obtain consistent and repeatable results independent of any particular electrophotographic medium selected or the particular electrophotographic machine employed, to reduce operator error and to reduce costs of manufacture and operation.
It is desirable to provide apparatus which can determine the best charging and exposure conditions for any one of a wide range of photoconductor members. The achievement of the dynamic selection of the best charge and exposure conditions enables the machine to be adaptable to a wide range of photoconductor performance characteristics thereby reducing the cost of selection for the photoconductor member while yielding more repeatable and consistent image results.
Accordingly, there is provided a method for controlling the electrostatic field charge on a photoconductive surface of anelectrophotographic recording member applied thereto by a corona generator, characterized by the steps of applying an electrostatic surface charge on the photoconductive surface, at least partially discharging regions of said charged surface by exposing same to radiation to provide exposed regions and blocking other regions of said surface to provide unexposed regions, generating signals representative of the comparison of said exposed and unexposed regions on the same photoconductive surface and controlling said corona generator in response to said comparison signals.
Further there is provided apparatus for the practicing of the above method characterized by a charging device for charging the photoconductive surface in a succession of levels extending from at least a lesser to a great level, an illuminating device for partially discharging by illuminating a region of successive charge levels of the surface to produce an exposed region and an unexposed region, a sensor for detecting the electrostatic field charge in each for the successive charge levels of the exposed and unexposed regions of the photoconductive surface, a signal generator producing a predetermined signal, a comparator for comparing at least said detected electrostatic field charge signal to a predetermined signal and providing said compared signals for control tasks in accordance with said detected signal and said predetermined signal.
The invention further provides for use in practicing the method above stated, an electrometer for detecting the electrostatic charge on a moving photoconductive surface characterized by a housing having an aperture therein and disposed adjacent the photoconductive surface, a sensor head enclosed in said housing and mounted to coincide with said aperture, a rotatable member having a plurality of equispaced apertures therein and mounted on said housing such that said member apertures coincide with said housing aperture und disposed between said sensor head and the photoconductive surface, a drive mechanism coupled to said rotatable member for repetitively interrupting and coupling said sensor head at a frequency related to the rotation speed of said member and the number of equispaced apertures therein and an amplifier circuit coupled to said sensor head.
The preferred embodiments of this invention now will be described, by way of example, with reference to the drawings accompanying this specification in which:

Figure 1 is a perspective view of a charge potential level sensing apparatus according to the invention herein;
Figure 2 is a top plan view of the apparatus of Figure 1, a panel being removed and portions broken away th show interior details;
Figure 3 is a schematic representation of the amplifier and motor circuit of the apparatus of Figure 1;
Figure 4 is a diagrammatic representation illustrating the method of the invention;
Figure 5 is a diagrammatic block diagram of the control logic circuitry according to the invention;
Figure 6 is a timing diagram illustrating the operation of the apparatus according to the invention; and
Figure 7 is an enlarged diagrammatic detail of the photoconductive surface illustrated in Figure 4.

Briefly according to the invention, in an electrophotographic imaging apparatus, a method and apparatus are provided for establishing a predetermined apparent surface charge on the exposed and unexposed areas of a photoconductive surface at the start of toning and providing exposure control thereof. A full range of optimum charge levels thereby can be provided at the instant of toning or developing a latent electrostatic image formed on the photoconductive surface of said electrophotographic member. The electrophotographic member is mounted on a platen which is secured to a linearly translatable carriage. The carriage is mounted for travel along a path sequentially from a home position through the respective functional stations for charging, imaging, toning, and, optionally, transfer and cleaning. A calibration techique provides an optimum corona level and a best level of light exposure whereby during electrophotographic imaging operation, the charging and imaging functions are controlled in accordance with the charge behavior characteristics of the photoconductive member employed.
Referring to Figures 1 and 2, an electrostatic field detector apparatus 1o is illustrated havinghousing 12. Adisk 14 is disposed over one face of detector 1o by ashaft 16 coupled to adrive motor 18. Thedisk 14 has a plurality ofapertures 24 formed therein and disposed concentric with the axis of thedisk 14, equispaced inwardly of the periphery thereof. As illustrated in Figure 1, fifteen equally spaced apertures. It was preferable that a finite relationship be maintained between the number of apertures and the power line frequency for reducing the deleterious effect of a.c. hum. The optimum arrangement of the apertures can be defined by:
where
where
Therefore, for 60 Hz power, 600 rpm speed, F_C = 150 Hz situated midway between the a.c. power second harmonic (120) Hz) and third harmonic (180 Hz). Thesensor electrode 20 is enclosed in thehousing 12 and disposed adjacent anaperture 26 that is formed inhousing 12. The sensor electrically is connected to an amplifier circuit shown as 22 in Figure 2. Theapertures 24 formed in thedisk 14 are coincident withaperture 26 in theenclosure 12. As thedisk 14 is rotated bymotor 18, theelectrode 20 alternately is blocked and exposed. Thedisk 14 can be rotated, for example, at a speed of 600 r.p.m., thereby providing a chopping frequency of 150 Hz that is removed from a harmonic frequency of 60 hertz. Theelectrode 20 conveniently can be provided as a flat screw, preferably plating the head 21 thereof with gold or silver. The size of the head 21 of theelectrode 20 approximately is equal to the size ofapertures 24 in the disk.
The rotation of thedisk 14 alternately couples and interrupts the capacitive coupling between theelectrode 20 and the electrostatic field of thephotoconductive surface 28, thereby inducing an A.C. signal on saidmeasurement electrode 20. Theelectrode 20 is disposed adjacent thephotoconductive surface 28. The A.C. signal induced onelectrode 20 is coupled toamplifier 22.
An example of oneuseful amplifier 22 is illustrated in Figure 3 wherein theamplifier 22 primarily comprises twooperational amplifiers 32, 34. Theoperational amplifiers 32, 34 are coupled through current-limitingresistors 36, 38 to a positive fifteenvolt power supply 40 and through current-limitingresistors 42, 44 to a negative fifteenvolt power supply 46. Theoperational amplifiers 32, 34 may have a field-effect transistor, FET input, such as RCA type CA3140E. The biasing resistors, and the by-pass and coupling capacitors are provided as follows:
Many variations could be made from the above example with the same results achieved.
Themotor 18 is coupled through aswitch 74 to anA.C. power supply 76.
The low level A.C. measured signal provided byprobe 20 is coupled to the input ofoperational amplifier 32. The output ofamplifier 32 is connected tovariable resistor 72 so that a portion of the output is coupled to the input ofoperational amplifier 34, for further amplification of the measured signal. Theoutput 80 ofoperational amplifier 34 is an employable electrostatic field signal for coupling to a contro-1 logic unit.
Referring now to Figure 4, the process according to the invention is illustrated diagrammatically. Figure 6 graphically illustrates the timing of the events involved.
Step 1 of Figure 4 illustrates theplaten 82 in a home position. A corona generating device 84-is shown positioned relative to thephotoconductive surface 28 of an electrophotographic member secured toplaten 82. Thecorona generating device 84 applies a surface voltage charge to thephotoconductive surface 28 when translated thereacross. Measuringelectrode 20 andamplifier 22 are shown positioned adjacent thephotoconductive surface 28. Theoutput 80 ofamplifier 22 is a signal proportional to the apparent surface voltage electrostatic field charge level of thephotoconductive surface 28. A section of thephotoconductive surface 28 is shielded bybaffle 88 from illumination provided by anexposure lamp 86.
Theplaten 82 is moved at a constant speed from left to right direction as viewed in Figure 4. Instep 2 of thecorona generating device 84 is shown the process of applying a charge to thephotoconductive surface 28 during movement thereof from left to right. The corona level output is varied in a sequence of levels synchronously with the movement ofsurface 28. A staircase pattern of corona level outputs is illustrated in Figure 6 starting at a minimum level at time TØ and increasing in equal steps to a maximum level at time T5 and decreasing in steps from time T7 to time T11.Step 2 of Figure 4 is represented in the chart of Figure 6 from time T0 to time T12. At the time T0, thecorona generating device 84 acts upon the leading edge of the movingphotoconductive surface 28. At time T6, thecorona generating device 84 acts upon the middle portion of thesurface 28. At time T12 the corona generating device acts upon the trailing edge of the moving photoconductive surface 28-and is translated pastcorona device 84.
AtStep 3 of Figure 4 theplaten 82 reverses and moves from right to left.Step 3 Figure 4 is represented in Figure 6 from time T12 to the time T24. The corona output level is varied in the same sequence of levels during movement of thephotoconductive surface 28 represented byStep 2 of Figure 4. The "double pass" charging acts to apply a relatively constant and uniform series of charge levels in staircase-like steps, or a ramp format, on thesurface 28.
Step 4 of Figure 4 illustrates theplaten 82 moved back to its home position. Theplaten 82 then is moved over thebaffle 88 to shield a predetermined portion of thephotoconductive surface 28 from light, such as one-half thereof as shown inStep 5 of Figure 4. Thebaffle 88 acts to shield or block the illumination of theexposure lamp 86 from the section of thephotoconductive surface 28 extending to the right of thebaffle 88.Step 5 of Figure 4 is shown on the chart of Figure 6 from the time T25 to the time T26. At time T25 theexposure lamp 86 is energized to achieve a predetermined intensity for a predetermined time duration. Theexposure lamp 86 is deenergized at the time T26. The effective exposure period from the time T25 to the time T26 typically is provided as a few seconds. The start of the exposure period at the time T25 typically is provided in the range of a few seconds to about twenty seconds after the completion of the charging function at the time T24.
From the time T25 to the time T26, theexposure lamp 86 emits illumination having a constant intensity, thereby uniformly discharging the exposed section of thephotoconductive surface 28 during this time period. The exposed section of the photoconductive surface is designated as region KA and the unexposed section of thephotoconductive surface 28 is designated as region KB.
Theplaten 82 is moved from left to right (in Figure 4) to a position on the right side of theelectrostatic field electrode 20 as represented instep 6. A time delay is effected that is equal to the time between the completion of the charging function and the start of the toning function in the normal operation of the electrophotographic imaging apparatus. The time delay between time T24 and time T27 (when the electrostaticfield detector apparatus 10 is activated), is provided generally in the range of thirty to fifty seconds.
The chart of Figure 6 shows the electrostatic field detector apparatus orelectrometer 10 activated at the time T27 through the time T28 as thephotoconductive surface 28 moves across theelectrode 20. Aplaten position encoder 110 synchronously defines each position of the movingplaten 82 with the amplified measuredsignal 80 and is coupled to acontrol logic unit 100. The measuredsignal output 80 fromelectrode 20 is illustrated for the partially exposed region KA and unexposed region KB. The resulting measuredsignal 80 has a triangular ramp-like staircase shape having a lesser leading staircase ramp due to the light exposure in the region KA and a greater trailing staircase ramp in the region KB representing the exposed and the unexposed apparent surface voltage charge levels.
Referring to Figure 7, theelectrode signal 80 is illustrated relative to thephotoconductive surface 28. The exposed region KA and unexposed region KB are shown as having bands comprising increments of charge variation according to the sequence corona level outputs as thephotoconductive surface 28 moved thereacross. The shadedband regions 29 are extended below thephotoconductive surface 28 to illustrate the stepwise change in theelectrode signal 80 with the charge bands of thephotoconductive surface 28. In practice, these charge bands appear more like a smooth transition than the discrete steps shown.
The coincidence unexposed line of the chart of Figure 6 shows a coincidence level between the measuredsignal 80 and a predetermined apparent surface voltage charge level stored in memory for the unexposed region KB. The exposed line represents the correlated measuredsignal 80 for the exposed region KB at the corona output level corresponding to the above coincidence level.
In the normal operation of the electrophotographic imaging machine thecorona generating unit 84 is controlled to provide a corona level output corresponding to the coincidence level. The correlated measured signal in the exposed region KA isused to control theexposure lamp 86 in accordance with the exposure lamps characteristics to provide the predetermined apparent surface voltage charge in the exposed areas of thephotoconductive surface 28.
Step 7 of Figure 4 illustrates theexposure lamp 86 illuminating thephotoconductive surface 28 in order to fully discharge thesurface 28.Steps 8 and 9 of Figure 4 illustrate the normal operation of the electrophotographic imaging apparatus. Instep 8 theplaten 82 is moving across thecorona generating device 84 from left to right.Step 9 shows theplaten 82 positioned to the left of thecorona generating device 84 after moving thereacross from right to left, completing charging in a double pass. Thesensing device 10 in the form of an electrometer measures the apparent surface voltage charge on thephotoconductive surface 28. This initial charge measurement is relatively meaningless in relation to the charge level at the start of the toning function; however, the initial charge measurement can be utilized to determine when the useful capability of thephotoconductive surface 28 has been exhausted.
Attention is now invited to Figure 5 which diagrammatically illustrates thecontrol logic unit 100 according to the invention. The amplifiedelectrode signal 80 is coupled to an analog-to-digital (A/D)converter 102. The A/D converter 102 produces a digital detectedcharge signal 103 in the form of a binary word, usually on the order of six bits. Thebinary word signal 103 is coupled to the date input of a random access memory (RAM) 104. Control signals KA, KB corresponding to the exposed and unexposed regions of the photoconductive surface, as shown inStep 5 of Figure 4, are coupled to amode control 106. The mode control unit is coupled to the most significant bit (MSB) input of therandom access memory 104.
The travel ofplaten 82 is encoded byposition detector 110, such as a tachometer or like device. Theplaten position encoder 110 is coupled to the input of a platentravel pulse generator 112. The platentravel pulse generator 112 produces a pulse train corresponding to the travel ofplaten 28. For example, each pulse produced by thepulse generator 112 may represent one tenth of an inch of travel of theplaten 82. The output of the platentravel pulse generator 112 is coupled to the clod input of amemory address counter 108. The reset line of thememory address counter 108 is connected to themode control function 106. A reset pulse having a brief, spike-like configuration is produced at the onset of either region KA, KB an resets thecounter 108 effectively to zero. The output of themode control 106 that is coupled to the MSB input ofRAM 104 can be provided, for example, as a binary LOW for the exposed region KA and as a binary HIGH for the unexposed region KB of thephotoconductive surface 28. The input from themode control 107 to the MSB input of theRAM 104 effects the addressing of two different files in theRAM 104. Thememory address counter 108 is coupled to the address input of theRAM 104 and scans the same remaining address lines ofRAM 104.
Thememory address counter 108 produces a most significant bit minus one (MSB - 1) signal that is coupled to acomplementor 114. Thecomplementor 114, when the active state of the significant bit of the address word which is equivalent to one bit less than the MSB occurs, will invert the relative sense of the binary words passing therethrough.
Thecomplementor 114 is coupled to astaircase ramp generator 116. With thecomplementor 114 addressing thestaircase ramp generator 116, thegenerator 116 will count up, count down, count up and count down, corresponding to the corona level output illustrated in Figure 6 for regions KA, KB for both directions of the travel ofplaten 82. During the calibration function, thestaircase ramp generator 116 is coupled throughswitch 132 to the coronalevel control unit 130 while the predetermined sequence of corona output levels are produced by thecorona generating device 84.
Theoutput 105 ofRAM 104 is coupled to the DA input of acoincidence detector 118. Thecoincidence detector 118 conventiently may be a binary comparator. The output of mode contro-1 106 is coupled through theinverter 119 to the EN input of thecoincidence detector 118. The DB input of thecoincidence detector 118 is coupled to a predeterminedbinary word 120 equal to the desired apparent surface voltage charge at the start of the toning function. Thebinary word 120 can be provided from a manually operated switch or a databus of a separate digital system. The A = B output of thecoincidence detector 118 is coupled to the clock. input of alatch 122. When the DA input equals the DB input to thecoincidence detector 118, the A = B output of thedetector 118 clocks thelatch 122, thelatch 118 will latch onto the instantaneous count state of the word is addressing theRAM 104.
The output of thelatch 122 is coupled to the input of a digital-to-analog (D/A)converter 124 and the DB input of acoincidence detector 126. The D/A converter 124 produces an analog signal 128 corresponding to the digital coincidence word. The analog signal 128 is coupled to a corona level control uit 130 and is the control signal thereto when theswitch 132 is provided in the operate position, after the completion of the calibration function according to the invention.
The measuredsignal 80 that is sequentially stored in a second file position in thememory 104 corresponding to the exposed region KA is compared in thecoincidence detector 126 to the output of thelatch 122 that is coupled to the DB input ofdetector 126. The output ofmode control 106 is coupled to the EN input of thecoincidence detector 126. The coincidence output A == B ofdetector 126 corresponds to the measuredcharge signal 80 in the exposed region KA for the corona level output as determined by the value stored in thelatch 122. The A = B output of thecoincidence detector 126 is coupled to the clock input of alatch 134, The latched output of thelatch 134 is coupled to a least significant bit (LSB) input of amemory 136. Thememory 136 can be a programmable read only memory (PROM).
The latched, discharged signal of thelatch 134 generally is higher than the predetermined or desired apparent surface voltage charge for the exposed area. Thememory 136 couples to a digitallamp control unit 138 and atimed switch control 140. Thememory 136 functions as a look-up table of predetermined values which are used to determine a control word for coupling to thedigital lamp control 138 and thetimed switch control 140. An operator adjustlight level unit 142 is coupled to the most significant bit (MSB) input of thememory 136, thereby allowing for manual adjustment of the light level.
Thememory 136 stores the non-linear characteristics of theexposure lamp 86 relative to the changes in power applied thereto by the digitallamp control unit 138. Thememory 136 can include compensation data for such important factors as the shift in exposure lamp color temperature relative to the photoconductor sensitivity and non-linear lamp illumination output relative to changes in voltage applied to theexposure lamp 86. The profiling of the characteristics of theexposure lamp 86 provides for proper control of both the intensity of theexposure lamp 86 and the time duration that thelamp 86 is energized.
Thecontrol logic unit 100 can include the following:
Many variations could be made from the above example and the same results achieved, without departing from the invention.
In the practice of the invention a fully exposed, maxial- clear separation film can be provided in the optical path during the calibrate expose cycle, thereby compensating for the residual density of the separation film substrate. Thephotoconductive surface 28 acts as the light measuring device.
In conclusion, the method and apparatus according to the invention provide control for the charging and imaging fucntions thereby providing the desired charge levels on thephotoconductive surface 28 for the exposed regions KA and unexposed regions KB according to the image pattern at the start of the toning function in the normal operation of an electrophotographic imaging machine.

Claims

1. A method for controlling the electrostatic field charge on a photoconductive surface of an electrophotographic recording member applied thereto by a corona generator, characterized by the steps of applying an electrostatic surface charge on the photoconductive surface, at least partially discharging regions of said charged surface by exposing same to radiation to provide exposed regions and blocking other regions of said surface to provide unexposed regions, generating signals representative of the comparison of said exposed and unexposed regions on the same photoconductive surface and controlling said corona generator in response to said comparison signals.

2. The method according to claim 1 characterized by the additional step of generating a predetermined signal, detecting the electrostatic field charge in each of the exposed and unexposed regions and comparing at least said electrostatic charge signal to said predetermined signal to provide the comparison signal and directing said comparison signal to the corona generator for controlling same.

3. The method according to claim 1 characterized in that the photoconductive surface is charged in a succession of levels extending from at least a lesser to a greater level, the exposed and unexposed regions produced by illuminating a region of successive charge levels of said surface, the electrostatic field charge being detected in each of the successive charge levels of the exposed and unexposed regions of the photoconductive surface, and the further step of generating a predetermined signal, at least said detected electrostatic field charge signal being compared to the predetermined signal to provide said comparison signals.

4. The method according to any one of claims 1, 2 or 3 characterized in that the step of applying the charge to the photoconductive surface includes moving one of the photoconductive surface and a corona generating device relative to the other and controlling the corona level output of said corona generating device in a predetermined sequence related to said relative movement.

5. The method according to any one of claims 1, 2 or 3 characterized in that the step of applying the charge to the photoconductive surface includes moving one of the photoconductive surface and a corona generating device relative to the other and controlling the corona level output of said corona generating device in a predetermined sequence related to said relative movement, said predetermined sequence of corona level output including a staircase pattern.

6. The method according to any one of claims 1, 2 or 3 characterized in that the step of producing unexposed regions includes shielding said unexposed region from illumination.

7. The method according to any one of claims 1 to 4 characterized in that the step of detecting the electrostatic field charge includes moving the photoconductive surface across sensing device.

8. The method according to claims 2 or 3 characterized in that the-step of generating a predetermined signal includes generating a signal for a predetermined charge level and which is representative of the profile of the characteristics of an exposure lamp.

9. The method according to any one of claims 1 to 6 characterized in that the comparison signals are employed to control the intensity of an exposure lamp.

10. The method according to any one of claims 1 to 6 characterized in that the comparison signals are employed to control the duration of operation of an exposure lamp.

11. The method according to any one of claims 1 to 10 characterized by the provision of ramp-like average surface voltage wrought by the charge and partial discharge cycle.

12. The method according to any one of claims 1 to 11 characterized in that the average surface voltage of the photoconductive surface is independent of the specific choice of photoconductive surfaces.

13. The method according to claim 1 in which the exposure is effected by an exposure lamp and the charge detected by an electrometer;
characterized in that the photoconductive surface being positioned in facing relationship with the corona generator means, the exposure lamp and the electrometer, the photoconductive surface is moved past said corona generator and a charge potential applied to the surface, the corona level output of said corona generator is controlled in a predetermined sequence related to the movement of the photoconductive surface, the photoconductive surface is moved over said exposure lamp and selected regions thereof are shielded from illumination to produce the unexposed and exposed regions, said exposure lamp being excited for a predetermined period of time to illuminate said exposed region of the photoconductive surface, the photoconductive surface is moved across said electrometer, the charge on the photoconductive surface related to said movement of the photoconductive surface is detected to provide the detected charge signal output, and further characterized by the steps of storing said detected charge signal in memory means, correlating the detected charge signals to said predetermined sequence of corona outputs, comparing the detected charge signal for said unexposed region of the photoconductive surface with the predetermined stored signal to provide a compared signal for a first control task, comparing said compared signal and the predetermined signal for said exposed region, and providing a second compared signal for a second control task.

14. The method according to claim 11 characterized in that one of the photoconductive surface and the corona generating means is reciprocated at least once relative to the other.

15. The method according to claim 11 characterized by the step of effecting a time delay prior to detecting the charge on the photoconductive surface.

16. Apparatus for controlling the electrostatic field charge of a photoconductive surface of an electrophotographic member for practicing the method according to claims 1 to 15 characterized by a charging device for charging the photoconductive surface in a succession of levels extending from at least a lesser to a great level, an illuminating device for partially discharging by illuminating a region of successive charge levels of the surface to produce an exposed region and an unexposed region, a sensor for detecting the electrostatic field charge in each of the successive charge levels of the exposed and unexposed regions of the photoconductive surface, a signal generator producing a predetermined signal, a comparator for comparing at least said detected electrostatic field charge signal to a predetermined signal and providing said compared signals for control tasks in accordance with said detected signal and said predetermined signal.

17. In an electrophotographic imaging apparatus which includes a platen having an electrophotographic member secured thereto, the electrophotographic member including an exposed photoconductive surface, a drive for moving the photoconductive surface, a charging device for charging the photoconductive surface and an exposure device for exposing the photoconductive surface to radiant energy pattern to form a latent electrostatic image of the pattern on the exposed surface;
an electrometer for detecting the electrostatic charge on a moving photoconductive surface characterized by:

A. a housing having a aperture therein and disposed adjacent the photoconductive surface,

B. a sensor head enclosed in said housing and mounted to coincide with said aperture,

C. a rotatable member having a plurality of equispaced apertures therein and mounted on said housing such that said member apertures coincide with said housing aperture and disposed between said sensor head and the photoconductive surface,

D. a drive mechanism coupled to said rotatable member for repetitively interrupting and coupling said sensor head at a frequency related to the rotation speed of said member and the number of equispaced apertures therein, and

E. an amplifier circuit coupled to said sensor head.

18. The apparatus according to claim 17 characterized in that said frequency is removed from a harmonic frequency of a power line freqaency.

19. The apparatus according to claim 17 characterized in that said sensor head produces signal for direction for a control task.

20. An electrometer for detecting the electrostatic charge on a moving photoconductive surface characterized by:

A. a housing having an aperture therein and disposed adjacent the photoconductive surface,

C. a rotatable member having a plurality of equispaced apertures therein and mounted an said housing such that said member apertures coincide with said housing aperture and disposed between said sensor head and the photoconductive surface,

E. an amplifier circuit coupled to said sensor head.

21. The apparatus according to claim 20 characterized in that said frequency is removed from a harmonic frequency of a power line frequency.

22. The apparatus according to claim 20 characterized in that said sensor head produces signal for direction for a control task.