EP0098509A2

EP0098509A2 - Electrostatic field control method and apparatus

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: AU1632783A; JPS5946656A; EP0098509A3; DK295383D0; CA1194092A; DK295383A; IL69055A0

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 having housing 12. A disk 14 is disposed over one face of detector 1o by a shaft 16 coupled to a drive motor 18. The disk 14 has a plurality of apertures 24 formed therein and disposed concentric with the axis of the disk 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). The sensor electrode 20 is enclosed in the housing 12 and disposed adjacent an aperture 26 that is formed in housing 12. The sensor electrically is connected to an amplifier circuit shown as 22 in Figure 2. The apertures 24 formed in the disk 14 are coincident with aperture 26 in the enclosure 12. As the disk 14 is rotated by motor 18, the electrode 20 alternately is blocked and exposed. The disk 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. The electrode 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 the electrode 20 approximately is equal to the size of apertures 24 in the disk.
The rotation of the disk 14 alternately couples and interrupts the capacitive coupling between the electrode 20 and the electrostatic field of the photoconductive surface 28, thereby inducing an A.C. signal on said measurement electrode 20. The electrode 20 is disposed adjacent the photoconductive surface 28. The A.C. signal induced on electrode 20 is coupled to amplifier 22.
An example of one useful amplifier 22 is illustrated in Figure 3 wherein the amplifier 22 primarily comprises two operational amplifiers 32, 34. The operational amplifiers 32, 34 are coupled through current-limiting resistors 36, 38 to a positive fifteen volt power supply 40 and through current-limiting resistors 42, 44 to a negative fifteen volt power supply 46. The operational 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.
The motor 18 is coupled through a switch 74 to an A.C. power supply 76.
The low level A.C. measured signal provided by probe 20 is coupled to the input of operational amplifier 32. The output of amplifier 32 is connected to variable resistor 72 so that a portion of the output is coupled to the input of operational amplifier 34, for further amplification of the measured signal. The output 80 of operational 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 the platen 82 in a home position. A corona generating device 84-is shown positioned relative to the photoconductive surface 28 of an electrophotographic member secured to platen 82. The corona generating device 84 applies a surface voltage charge to the photoconductive surface 28 when translated thereacross. Measuring electrode 20 and amplifier 22 are shown positioned adjacent the photoconductive surface 28. The output 80 of amplifier 22 is a signal proportional to the apparent surface voltage electrostatic field charge level of the photoconductive surface 28. A section of the photoconductive surface 28 is shielded by baffle 88 from illumination provided by an exposure lamp 86.
The platen 82 is moved at a constant speed from left to right direction as viewed in Figure 4. In step 2 of the corona generating device 84 is shown the process of applying a charge to the photoconductive surface 28 during movement thereof from left to right. The corona level output is varied in a sequence of levels synchronously with the movement of surface 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, the corona generating device 84 acts upon the leading edge of the moving photoconductive surface 28. At time T6, the corona generating device 84 acts upon the middle portion of the surface 28. At time T12 the corona generating device acts upon the trailing edge of the moving photoconductive surface 28-and is translated past corona device 84.
At Step 3 of Figure 4 the platen 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 the photoconductive surface 28 represented by Step 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 the surface 28.
Step 4 of Figure 4 illustrates the platen 82 moved back to its home position. The platen 82 then is moved over the baffle 88 to shield a predetermined portion of the photoconductive surface 28 from light, such as one-half thereof as shown in Step 5 of Figure 4. The baffle 88 acts to shield or block the illumination of the exposure lamp 86 from the section of the photoconductive surface 28 extending to the right of the baffle 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 the exposure lamp 86 is energized to achieve a predetermined intensity for a predetermined time duration. The exposure 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, the exposure lamp 86 emits illumination having a constant intensity, thereby uniformly discharging the exposed section of the photoconductive surface 28 during this time period. The exposed section of the photoconductive surface is designated as region KA and the unexposed section of the photoconductive surface 28 is designated as region KB.
The platen 82 is moved from left to right (in Figure 4) to a position on the right side of the electrostatic field electrode 20 as represented in step 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 electrostatic field 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 or electrometer 10 activated at the time T27 through the time T28 as the photoconductive surface 28 moves across the electrode 20. A platen position encoder 110 synchronously defines each position of the moving platen 82 with the amplified measured signal 80 and is coupled to a control logic unit 100. The measured signal output 80 from electrode 20 is illustrated for the partially exposed region KA and unexposed region KB. The resulting measured signal 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, the electrode signal 80 is illustrated relative to the photoconductive 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 the photoconductive surface 28 moved thereacross. The shaded band regions 29 are extended below the photoconductive surface 28 to illustrate the stepwise change in the electrode signal 80 with the charge bands of the photoconductive 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 measured signal 80 and a predetermined apparent surface voltage charge level stored in memory for the unexposed region KB. The exposed line represents the correlated measured signal 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 the corona 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 the exposure lamp 86 in accordance with the exposure lamps characteristics to provide the predetermined apparent surface voltage charge in the exposed areas of the photoconductive surface 28.
Step 7 of Figure 4 illustrates the exposure lamp 86 illuminating the photoconductive surface 28 in order to fully discharge the surface 28. Steps 8 and 9 of Figure 4 illustrate the normal operation of the electrophotographic imaging apparatus. In step 8 the platen 82 is moving across the corona generating device 84 from left to right. Step 9 shows the platen 82 positioned to the left of the corona generating device 84 after moving thereacross from right to left, completing charging in a double pass. The sensing device 10 in the form of an electrometer measures the apparent surface voltage charge on the photoconductive 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 the photoconductive surface 28 has been exhausted.
Attention is now invited to Figure 5 which diagrammatically illustrates the control logic unit 100 according to the invention. The amplified electrode signal 80 is coupled to an analog-to-digital (A/D) converter 102. The A/D converter 102 produces a digital detected charge signal 103 in the form of a binary word, usually on the order of six bits. The binary 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 in Step 5 of Figure 4, are coupled to a mode control 106. The mode control unit is coupled to the most significant bit (MSB) input of the random access memory 104.
The travel of platen 82 is encoded by position detector 110, such as a tachometer or like device. The platen position encoder 110 is coupled to the input of a platen travel pulse generator 112. The platen travel pulse generator 112 produces a pulse train corresponding to the travel of platen 28. For example, each pulse produced by the pulse generator 112 may represent one tenth of an inch of travel of the platen 82. The output of the platen travel pulse generator 112 is coupled to the clod input of a memory address counter 108. The reset line of the memory address counter 108 is connected to the mode control function 106. A reset pulse having a brief, spike-like configuration is produced at the onset of either region KA, KB an resets the counter 108 effectively to zero. The output of the mode control 106 that is coupled to the MSB input of RAM 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 the photoconductive surface 28. The input from the mode control 107 to the MSB input of the RAM 104 effects the addressing of two different files in the RAM 104. The memory address counter 108 is coupled to the address input of the RAM 104 and scans the same remaining address lines of RAM 104.
The memory address counter 108 produces a most significant bit minus one (MSB - 1) signal that is coupled to a complementor 114. The complementor 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.
The complementor 114 is coupled to a staircase ramp generator 116. With the complementor 114 addressing the staircase ramp generator 116, the generator 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 of platen 82. During the calibration function, the staircase ramp generator 116 is coupled through switch 132 to the corona level control unit 130 while the predetermined sequence of corona output levels are produced by the corona generating device 84.
The output 105 of RAM 104 is coupled to the DA input of a coincidence detector 118. The coincidence detector 118 conventiently may be a binary comparator. The output of mode contro-1 106 is coupled through the inverter 119 to the EN input of the coincidence detector 118. The DB input of the coincidence detector 118 is coupled to a predetermined binary word 120 equal to the desired apparent surface voltage charge at the start of the toning function. The binary word 120 can be provided from a manually operated switch or a databus of a separate digital system. The A = B output of the coincidence detector 118 is coupled to the clock. input of a latch 122. When the DA input equals the DB input to the coincidence detector 118, the A = B output of the detector 118 clocks the latch 122, the latch 118 will latch onto the instantaneous count state of the word is addressing the RAM 104.
The output of the latch 122 is coupled to the input of a digital-to-analog (D/A) converter 124 and the DB input of a coincidence 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 the switch 132 is provided in the operate position, after the completion of the calibration function according to the invention.
The measured signal 80 that is sequentially stored in a second file position in the memory 104 corresponding to the exposed region KA is compared in the coincidence detector 126 to the output of the latch 122 that is coupled to the DB input of detector 126. The output of mode control 106 is coupled to the EN input of the coincidence detector 126. The coincidence output A == B of detector 126 corresponds to the measured charge signal 80 in the exposed region KA for the corona level output as determined by the value stored in the latch 122. The A = B output of the coincidence detector 126 is coupled to the clock input of a latch 134, The latched output of the latch 134 is coupled to a least significant bit (LSB) input of a memory 136. The memory 136 can be a programmable read only memory (PROM).
The latched, discharged signal of the latch 134 generally is higher than the predetermined or desired apparent surface voltage charge for the exposed area. The memory 136 couples to a digital lamp control unit 138 and a timed switch control 140. The memory 136 functions as a look-up table of predetermined values which are used to determine a control word for coupling to the digital lamp control 138 and the timed switch control 140. An operator adjust light level unit 142 is coupled to the most significant bit (MSB) input of the memory 136, thereby allowing for manual adjustment of the light level.
The memory 136 stores the non-linear characteristics of the exposure lamp 86 relative to the changes in power applied thereto by the digital lamp control unit 138. The memory 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 the exposure lamp 86. The profiling of the characteristics of the exposure lamp 86 provides for proper control of both the intensity of the exposure lamp 86 and the time duration that the lamp 86 is energized.
The control 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. The photoconductive 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 the photoconductive 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.