EP0156217A1 - Method and device for getting a predetermined value of potential by the illumination of electrostatically charged photoconductive layers - Google Patents

Abstract

The light-sensitive layer of a printing plate is charged to a given surface potential which is measured by means of a stationary or moving potential detector. The potential ratio, which changes during the exposure, is continuously compared with a given set value, and the exposure is terminated when the changing potential ratio is in agreement with the given set value. The measurement of the surface potential and of the potential ratio is carried out in a bright area of the light-sensitive layer, which area is located outside the area of the latent electrostatic image. The potential detector may be connected via a signal converter and an amplifier to a microprocessor control which actuates a shutter via a digital output. In the microprocessor, a program for controlling a corona electrode and a developing electrode is stored, which program actuates the corona control and the developing electrode control via a digital/analog output and high-voltage amplifiers.

Description

Method and arrangement for maintaining a predetermined potential ratio when exposing electrostatically charged photosensitive layers

The invention relates to a method for maintaining a constant potential ratio in the exposure of electrostatically charged photosensitive layers on supports on which an electrostatically latent image of an original is formed during the exposure.

A consistently uniform quality of printing plates, on the charged photosensitive layers of which electrostatic latent images are obtained by exposure and developed with toner, requires precise compliance with the potential relationships of charging, residual charge and counter-voltage to one another. In order to achieve this, the electrophotographic layer must be manufactured within narrow tolerances and an exposure device, for example a camera, which may have to be adapted to different working conditions from printing plate to printing plate. The exposure control of conventional cameras generally consists of a timer which switches off the shutter and the light source after a preselected time. Improved control is possible with so-called light dosimeters, of which the incoming light is measured in the area of the original and the exposure time is then corrected. So different Lam Pen performance, which is caused by aging or mains voltage fluctuations and dirt on the reflectors, partially balanced. The setting necessary for a desired residual tension of the exposed printing plate can only be found with the aid of sample plates which are exposed for different lengths and the exposure process is ended positively with different residual tensions of these sample plates. The one of the test plates, which enables a satisfactory printing, also provides the required setting for the. desired residual voltage at the end of the exposure. However, the setting obtained in this way cannot be maintained since the residual voltage resulting from the same setting changes with the charge and the sensitivity of the photosensitive layer of the printing plate. These parameters depend on manufacturing tolerances, plate type, air humidity, temperature, pre-exposure, corona contamination and the like.

From DE-OS 30 49 339 an electrostatic recording device with a measuring device in the form of an electrometer for measuring the surface potential of a light-sensitive layer is known. The surface potential of a latent image on the light-sensitive layer is measured, the surface potential being determined as an AC voltage signal, and on the basis of the measured surface potential, various conditions for image generation, such as the charging voltage and the development bias, controlled. For this purpose, a first control device contains a stored program for a sequential control of the latent image-forming device and a second controller contains a stored program for the conditions for forming the latent image by means of the latent image-forming device or for conditions for development by the developing device using output signals to be controlled by the surface potential measuring device.

From DE-PS 28 57 218 a method for keeping optimal conditions in electrophotographic reproduction is known, in which the carrier is electrostatically charged and exposed to form an electrostatic latent image on a photosensitive carrier and in addition to the electrostatic image of the original on the carrier an electrostatic latent reference image is formed, the potential of which is measured and an adjustable parameter for the reproduction is set in accordance with the measured potential of the reference image. The reference image is generated by forming light and dark areas on the light-sensitive support and accordingly has areas of low and high potential. The potentials of both areas are measured and if the potentials of the reference image deviate from specified target values, the setting parameter assigned to the respective area for the duplication is changed until the potentials of the electrostatic latent reference image are brought to the specified target values. If the exposure of the light-sensitive support is insufficient, then the potential is generally too high, signals are generated to automatically adjust the voltage of the lighting device, the width of the slot opening and so on. to correct or to provide a corresponding correction for the potential. If the charge of the light-sensitive carrier is insufficient, ie the potential is too low overall, signals are delivered in order to automatically increase the voltage of the charging device or to carry out a corresponding correction of the potential. If both the exposure and the charging are inaccurate, signals are delivered to correct both parameters, so that the specified setpoints can be reached after a few copying runs. There is thus a readjustment of the voltage of the charging device and / or an increase in the exposure intensity, the starting point for these corrections being the measurement of the surface potential of an electrostatic latent reference image on the photosensitive carrier.

European patent application 0 098 509 describes a method for controlling the electrostatic charging of a photoconductor surface by means of a corona charging device. Here, the charged photoconductor surface is partially discharged by exposure and signals are measured that compare the exposed and unexposed areas enable the photoconductor surface. The charging device is controlled in accordance with these comparison signals. The comparison signals also serve to regulate the light intensity of an exposure _lamp Islam and the exposure duration. For this purpose, the corona charging voltage is regulated to a value corresponding to the corresponding levels of the measurement signal and the setpoint if the measurement signal matches a stored setpoint of the unexposed area of the photoconductor. The measurement signal belonging to this corona charging voltage in the exposed area of the photoconductor regulates the exposure lamp in accordance with stored data of the lamp characteristic, which indicate the aging of the lamp, non-linear influences due to voltage fluctuations, shift in the color temperature of the exposure lamp with respect to the photoconductor sensitivity, and the like. take into account in order to charge the exposed surface sections of the photoconductor to the desired surface tension. In this method, the corona charging voltage, the exposure intensity and the exposure duration are regulated in such a way that at the beginning of the concreting of the photoconductor there is a predetermined voltage in the exposed surface sections. A continuous measurement of the falling voltage of the photoconductor and the exposure process in the exposed surface sections is not carried out.

US Patent 3,438,705 describes an exposure and developing device in which the background density of a copy is automatically determined using a Controlled photosensitive device which scans the material to be copied. The potential obtained during the scanning of the background is applied to the developing plate during exposure to prevent the plate from being overloaded. In this device the exposure is measured which the plate to be developed receives in a background area. From this measured exposure value and the initial potential of the plate to be developed, a new potential is obtained which is equal to the potential which results from the exposure of the plate, and this potential is applied to the development electrode and the plate during development. A control of the exposure time due to the voltage contrast between exposed and unexposed areas of the plate to be developed is not provided. Rather, the potential of the latent image on the printing plate is determined and the conditions which are necessary for image generation are controlled by means of the signal corresponding to this potential. After the measurement, readjustment is carried out until the desired setpoints are reached.

The known methods and devices have in common that they do not take sufficient account of any changes in the light-sensitive layers from support to support during the exposure time and do not control the remaining charge of the exposed layer of the support or the exposed plate.

The object of the invention is to design a method of the type described in the introduction in such a way that the exposure time of a photosensitive layer of a support is determined on the basis of predetermined potentials or differences on the photosensitive layer discharged by the exposure in the bright areas. As part of this task, an arrangement for carrying out the method is also to be created.

This object is achieved according to the invention by a method according to the features of the preamble of claim 1 in such a way that the light-sensitive layer is charged to a predetermined surface potential, which is measured by means of an electrostatic potential detector, that the potential ratio, which changes during the exposure, continuously changes with a determined target value is compared and that if the changing potential ratio matches the predetermined target value, the exposure is ended.

The surface potential is expediently measured in a bright area of the light-sensitive layer outside the area of the latent electrostatic image. Various designs of electrostatic measuring probes are known for this measurement, but they generally have the disadvantage that they are not transparent probes and thus the ones beneath them during the exposure Cover part of the light-sensitive layer and it is not discharged and therefore the potential of a residual charge cannot be measured. There is also a method of charge measurement using a conductive coated glass plate and a coulomb meter, which has the disadvantage, however, that the distance between the measuring surface and the pressure plate directly influences the measurement result, that the reduced transmission compared to the pressure plate not covered by the measuring probe means that resulting result only provides a relative value, but not an absolute value, and that dusting of the glass plate causes a change in the transmission which influences the measurement result.

In an advantageous embodiment of the method according to the invention, the target value is specified in accordance with the residual potential of the light-sensitive layer after the discharge of the light areas of the latent image or in accordance with the predetermined potential difference between the light and dark areas of the exposed latent image. For each predetermined surface potential of a charge, a certain residual potential is advantageously stored or readable, and the surface potential that decreases during the exposure is continuously compared with the specific residual potential that is assigned to the predetermined surface potential at the beginning of the exposure.

Once the size of the sinking surface potential is equal to or less than the residual potential, the exposure is ended.

The arrangement for carrying out the method, which is equipped with a potential detector for measuring the surface potential of a light-sensitive layer of a support, is characterized in that the potential detector is connected to a signal converter which converts the AC output signals of the potential detector into DC signals and feeds them into a control device , which is connected to a shutter in the beam path of an exposure device and whose output signal closes the shutter when a predetermined target value for the potential conditions on the light-sensitive layer of the support is reached.

The further configuration of the arrangement results from the features of the other device claims.

The potential detector of the device is a detector which is equipped with oblique light incidence for transmitted light operation. Of course, a detector for vertical incidence of light can also be used, but with the disadvantage that the detector is reflected in the image area or, when used at the edge of the plate, the measured value is falsified by the shadow in the measuring field. The potential detector works according to the known compensation principle, which will be explained later.

With the invention, the advantages are achieved that the exposure time is controlled by monitoring the discharge curve of the photosensitive layer or the voltage contrast between the light and dark areas of the exposed photosensitive layer, whereby a uniform quality of the printed image of the developed photosensitive layer is obtained and larger Tolerances in the physical properties of the photosensitive layer of the printing plates can be compensated.

The invention is described below with reference to the drawings.

• Fig. 1 eine schematische Seitenansicht eines ortsfesten Potentialdetektors für die Messung des Oberflächenpotentials der lichtempfindlichen Schicht einer Druckplatte,
• Fig. 2 eine Draufsicht auf einen schematisch dargestellten, entlang einer Kante einer Druckplatte verfahrbaren Potentialdetektor,
• Fig. 3 die Änderung des Oberflächenpotentials eines Hellbereichs im latenten elektrostatischen Ladungsbild auf einer Druckplatte in Abhängigkeit von der Belichtungszeit, gemessen mit einem ortsfesten Potentialdetektor,
• Fig. 4 die Entladungskurven und die Potentialdifferenz zwischen Dunkel- und Hellbereichen im latenten elektrostatischen Ladungsbild auf einer Druckplatte in Abhängigkeit von der Belichtungszeit, gemessen mit einem bewegten Potentialdetektor,
• Fig. 5 den Verlauf der Oberflächenpotentiale in einem Streifenmuster im Randbereich einer Druckplatte während der Belichtung
• Fig. 6 ein Blockschaltbild einer Anordnung zur Belichtungssteuerung aufgrund der Messung des Restladungspotentials einer belichteten Druckplatte,
• Fig. 7 ein Blockschaltbild einer Anordnung zur Belichtungs-, Koronaspannungs- und Entwicklerspannungssteuerung aufgrund der Messung der Potentialdifferenz zwischen Hell- und Dunkelbereichen einer belichteten Druckplatte,
• Fig. 8 ein Ablaufdiagramm für ein Programm zur Steuerung der Schaltungsanordnung nach Fig. 6,
• Fig. 9 ein Ablaufdiagramm eines in einem Mikroprozessor der Schaltungsanordnung nach Fig. 7 gespeicherten Programms, und
• Fig. 10 Schaltungseinzelheiten des Blockschaltbilds nach Fig. 6.

Show it:

• 1 is a schematic side view of a fixed potential detector for measuring the surface potential of the photosensitive layer of a printing plate,
• 2 shows a plan view of a schematically illustrated potential detector which can be moved along an edge of a pressure plate,
• 3 shows the change in the surface potential of a bright area in the latent electrostatic charge image on a printing plate as a function of the exposure time, measured with a stationary potential detector,
• 4 shows the discharge curves and the potential difference between dark and light areas in the latent electrostatic charge image on a printing plate as a function of the exposure time, measured with a moving potential detector,
• 5 shows the course of the surface potentials in a stripe pattern in the edge region of a printing plate during the exposure
• 6 is a block diagram of an arrangement for exposure control based on the measurement of the residual charge potential of an exposed printing plate,
• 7 is a block diagram of an arrangement for exposure, corona voltage and developer voltage control based on the measurement of the potential difference between light and dark areas of an exposed printing plate,
• 8 is a flowchart for a program for controlling the circuit arrangement according to FIG. 6,
• 9 is a flowchart of a program stored in a microprocessor of the circuit arrangement according to FIG. 7, and
• 10 shows circuit details of the block diagram according to FIG. 6.

Fig. 1 shows schematically a pressure plate 1, which rests on an exposure table 2 and is held, for example, by suction air. The printing plate 1 has previously been charged to a certain potential by a corona charging device (not shown) and conveyed onto the exposure table 2. An edge strip, for example 20 mm wide, of the printing plate protrudes above the exposure table 2 and lies below a shield 4 of a potential detector 3. This potential detector is generally arranged in a stationary manner on the edge of the exposure table 2, but it is also possible to pivotally mount the potential detector 3 and to pivot over the edge of the pressure plate 1 for each measurement. A measuring electrode 6 is shown schematically in the area of the shield 4 of the potential detector 3. The one side wall of the shield 4 is inclined with respect to the horizontal in order to take into account an oblique incidence of light 5 and thereby to avoid shadow formation by the side walls of the shield 4 on the measuring field to be exposed, which lies within the shield 4, which is open at the top and bottom . The walls of the shield 4 are expediently designed to be very thin, so that their possible imaging on the edge region of the printing plate hardly influences the measurement. The measuring area lying under the shield 4 is, for example, 11 mm × 15 mm, so that the ratio of the measured exposed area to the unexposed area immediately below the side walls of the shield 4 is one Has magnitude in which an error in the potential measurement, caused by the shadow formation of the side walls of the shield 4, can be kept to a minimum. Investigations have shown that the shadow zones, caused by the imaging of the side walls of the shield 4 of a conventional potential detector on the edge region of the printing plate 1 during the exposure, result in different charge zones below the shield 4, which together with that by sliders or spacers on the potential detector maintained distance between the measuring electrode and the printing plate 1, which is 0.2 to 1.5 mm, in particular 1.2 mm, in order to exclude contact during the transport of the printing plate 1 to its final position on the exposure table 2, the electric field between the shadow zones and the measuring electrode and thus falsify the measurement result. This can lead to the fact that when using a conventional potential detector on the market with a small shield opening, which is only about a third of the above-mentioned shield opening of 11 mm x 15 mm, instead of an actually existing residual charge potential of 65 V, starting from a surface potential of 400 V after charging and before exposure, a residual charge potential of 100 V is measured.

The measurement of the surface potential of the exposed printing plate 1 takes place with the potential detector 3 outside the actual image area of the printing plate 1, so that a possible image of the contour is also shown Ren the shield 4 does not interfere within the edge area of the printing plate 1, since the fully developed printing plate 1 is usually clamped on a cylinder in a printing press and is usually on at least one side of the edge area of about 20 mm of the printing plate 1 outside the printing zone.

FIG. 2 schematically shows a movable potential detector 3 which is moved along an edge of a pressure plate 1. The potential detector 3 is arranged in a holder 7 which can be displaced along a guide 8 in the direction of the arrow A.

The bracket 7 is moved for example by means of a cable, not shown. Another possible solution provides a screw spindle for the guide 8 which rotates and thereby displaces the holder 7 which is in engagement with the spindle. When the exposure begins, the potential detector 3 is guided in any case by motor along the guide 8 over an edge region of the pressure plate 1. In the edge area, a surface potential distribution serving as the reference standard for the light-dark distribution in the template is indicated by the shaded dark areas 9 ^I to 9 ^X and the light areas 10 lying between them in a stripe pattern. The hatches of different densities in the dark areas 9 ^I to 9 ^X indicate the different surface potentials in the corresponding dark areas. The ratio of the widths of the dark range to the bright areas is 1: 1 and the area width is adapted to the resolution of the potential detector and is in practice between 1 and 8 mm. The stripe pattern is obtained by imaging a bar pattern together with the original, which is adjacent to the original and whose density densities are known in the dark areas.

The stripe pattern of light and dark areas is located in the edge area of the printing plate 1, which lies outside the printing zone when the printing plate is clamped onto the printing cylinder, so that this is not impaired by the stripe pattern. The stripe pattern makes it possible to control the exposure time or duration on the basis of a predetermined potential difference between light and dark areas, which is considered to be more favorable than the exposure time control by reaching a fixed residual charge potential of the surface of the printing plate, since the light-sensitive layers of the printing plates are in the dark areas , ie the unexposed areas, generally show a non-uniform behavior. This is due on the one hand to the uneven photochemical and / or physical properties of the light-sensitive layers of the printing plates themselves, which can also occur within the same batch of printing plates, such as different dark decay speeds and light sensitivity maxima of the layers, and on the other hand to the different densification densities of the

Dark areas of the originals, scattered light due to soiling of the imaging optics, fluctuations in the charging voltage and different long exposure times of the individual printing plates.

3 schematically shows the change in the surface potential of a bright area of a charged printing plate as a function of the exposure time. The potential is measured with a fixed potential detector. First the printing plate is charged to the surface potential U _a and the exposure is started at the switch-on time t _E. The bright area in the latent electrostatic charge image is discharged during the exposure according to the curve shown, the surface potential decreasing exponentially. The respective continuously measured surface potential is continuously compared with a predetermined nominal value of the surface potential U _{R of} the residual charge of the photoconductive layer of the printing plate, and as soon as the measured surface potential matches the predetermined surface potential U _R , that is at time t _A , the exposure is switched off Exposure source ended.

4 schematically shows bright discharge curves U _E1 , U _E2 , U _E3 , ... and the dark discharge curve U _{S of} an electrostatic charge image as a function of the exposure time, these measurements with a potential moving along the printing plate detector. Several discharge curves of the light areas for differently high charges of the printing plate are shown. For example, the printing plates can be charged to values U ₀₁ = -580 V, U ₀₂ = -520 V and U _{03 =} - 480 V. In the ideal case, no discharge of the dark areas should occur, so that the potential U _{D of} the dark areas should lie on a straight line parallel to the time axis through the surface potential U _{o of} the charge. In practice, however, there is also a certain discharge of the dark areas, so that the actual potential profile is given by the dark discharge curve U _{s of} the dark areas. This discharge in the dark areas is caused by scattered light on the optics that enters the dark area, by different density densities of the dark areas in the originals and different dark decay speeds. The potential drop in the dark area compared to the surface potential U _{o of} the charge is of the order of approximately 80 V. The surface _potentials U _{Ri of} the residual charges belonging to the charge _potentials U _0i are, for example, U _R1 = -160 V, U _{R2 =} -130 V and U _R3 = - 115 V.

At the switch-on time t _{E of} the exposure source, the decrease in the surface potential begins in the bright areas of the electrostatic charge image on the printing plate in accordance with the discharge curve associated with the respective charging potential. From The potential difference ΔU _i = U _S - U _{Ei is ideally} compared to the potential detector moving along the pressure plate at any time with a predetermined target value ΔU, which is explained in more detail in connection with FIG. 5.

At the beginning of the exposure, the potential detector 3 is moved over the stripe pattern and the distribution of the surface potentials is measured, which roughly have the theoretical course shown in dashed lines in FIG. 5. The practical course approximated to this course is shown with continuous lines. In the drawing, the associated course of the surface potentials in the individual light and dark areas during the exposure is entered above the stripe pattern. In the first section, the printing plate is charged to a surface potential U _o and the discharge begins with the exposure in the first bright area of the printing plate. As soon as the first dark area 9 ^{1 is} scanned, the surface potential increases from U _E1 to U _S1 . Accordingly, the blackening in the dark area and the other, above-described influencing variables, the surface potential within the range 9 ¹ decreases only slightly to a value U _S2. In the subsequent second light portion a strong lowering of the surface potential to U _E3 is made to rise at the beginning of the subsequent second dark portion 9 ^II U _S3 and _S4 slightly drop to U at the end of the dark area. In the subsequent bright area, the surface potential drops to U _E5 to then climb back up to U _S5 in the dark area that follows. This course continues in an analogous manner through the remaining light and dark areas. The potential difference ΔU _i = U _S i -U _E i is formed from the surface potentials with the same indices for i and _ideally compared with an associated predetermined target value LU. If the potential difference and the setpoint match or if the setpoint is exceeded, the exposure is ended. The residual charge potential U _{R of} the last measured bright area before the end of the exposure provides the basis for determining the counter voltage for the developing electrode, while the charge potential U _{o is used} for determining the required charging voltage for the next printing plate.

Since the bar pattern is imaged as a stripe pattern during exposure under the same conditions as the original on the evenly charged printing plate, the discharge or the course of the surface potentials in the stripe pattern follows the course of the surface potentials in the electrostatically latent charge image, so that the measurement of the potential difference in the stripe pattern representative of the direct measurement of the potential difference in the latent electrostatic charge image can be made without the electrostatic latent charge image being influenced by the imaging of the potential detector during the measurement process.

The setpoint value ΔU _ideal is primarily dependent on the respective toner, whether liquid or dry toner, the constancy of its triboelectric charging and the concreting or development process, and secondly on the printing plate type. The speed of the concreting plays a crucial role in the concreting process. For a conventional printing plate of the Elfasol ^(R) type, the absolute value of the potential difference ΔU is _ideally 330 V ∓ 100 V for dry toner development and 230 V ∓ + 100 V for liquid toner development, for example. The potential difference ΔU _ideal for optimal contrast between the light and dark areas after development is determined in advance for the parameters toner, development and printing plate and can be called up and stored.

6 shows a basic block diagram of an arrangement for exposure control based on the measurement of the residual charge potential of exposed printing plates. The potential detector 3, the operation of which will be described later, supplies an AC voltage signal as an output signal, which is fed to a signal converter 11 and is converted therein into a DC voltage signal. This DC voltage signal is fed to an impedance converter, which ensures that the DC voltage signal is passed with a low impedance to the one input of a comparator 13, the other input of which is supplied with an adjustable comparison voltage, which is supplied by a device 15, such as a memory or a ten-turn potentiometer, and which equal the desired residual charge potential of the exposed printing plate is applied. The switching threshold of the comparator 13 can be freely selected and the actual charge after a certain exposure time is compared in the comparator with the desired value corresponding to the desired residual charge potential at the time of the end of the exposure. As long as the surface potential of the actual charge is greater than the target value as an absolute value, the exposure is continued. As soon as the measured surface potential is equal to or less than the target value, the comparator 13 outputs an output signal to a shutter 14 in the exposure path, which is closed and thus ends the exposure of the printing plate. The impedance converter 12, the comparator 13 and the setpoint adjustment device 15 form a control device 51, which is shown in broken lines.

If the printing plate is exposed in an exposure camera, the shutter 14 is the camera shutter. Instead of the shutter 14, a relay can also be actuated, which interrupts, for example, the supply voltage to an exposure source. After completion of the exposure, the printing plate is transported to a developer device, not shown, in which it is concrete.

In the arrangement according to FIG. 6, although this is not shown, a microprocessor control can be used, which then controls the impedance converter 12 Comparator 13 and the memory 15 replaced. The output signal of the signal converter 11 is then fed directly to the microprocessor, which supplies a digital signal via the digital output to the shutter 14 or to a switching transistor for actuating a relay, which interrupts, for example, the supply voltage to an exposure source.

7 shows a block diagram of an arrangement which can also be used for exposure, corona and developing electrode control on the basis of the measurement of the potential difference between light and dark areas of an exposed printing plate. The surface potential measured by the potential detector 3 produces an AC voltage output signal which is fed to a signal converter 16 which converts the AC voltage signal into a DC voltage signal. The sign of the direct voltage depends on the phase position of the alternating voltage signal in the initial position of the measuring electrode of the potential detector. If the starting position of the measuring electrode is such that a maximum is passed through, ie the phase is positive, a positive DC voltage signal is generated in the signal converter. Conversely, if the starting position of the measuring electrode is such that a minimum of the AC voltage signal is run through, ie the phase is negative, a negative DC voltage signal is generated in the signal converter 16. The output signal of the signal converter 16 is fed to a control device 52, shown in broken lines, which is an amplifier 17 and an analog-digital converter 18, which is combined with a microprocessor 19, a digital output 20 and a digital / analog converter 21 to form a unit. The control of the microprocessor 19 is programmed in such a way that the measurement signal supplied by the potential detector 3, which corresponds to the potential difference between the dark discharge potential and the light discharge potential, is compared with a stored target value and, if the two values match, on the one hand the digital output 20 sends a signal to the closure 14 supplies, in order to close the latter and end the exposure, and on the other hand, the digital / analog converter 21 generates two output signals which are fed to a corona supply 24 and a developing electrode supply 25, respectively, in high-voltage amplifiers 22 and 23.

8 and 9 show the flow diagrams for the circuit arrangements according to FIGS. 6 and 7, it being assumed that both circuit arrangements are equipped with a microprocessor. After the start of the exposure, the surface potential U _{a of} the charged printing plate measured by the potential detector is stored in the memory with variable access by the microprocessor and the associated residual charge potential U _{R is} determined. Associated pairs of charge potentials U _o and residual charge potentials U _R are stored in tabular form in the microprocessor. In the next step, the surface potential U measured by the potential detector is included compared to the residual charge potential U _R. This comparison is continued as long as the potential U is greater than the residual charge potential U _R. As soon as the measured potential U is less than or equal to the residual charge potential U _R , the exposure is switched off. Further, the charging potential U ₀ is compared with a predetermined reference value U _0ideal and in the microprocessor at a combination of the two values in such a way that U _a U _0ideal is greater than controlled charging corona so that the corona voltage U _corona decreased by one step becomes. In the flow chart which is _tet angedeu- -1 by the expression U _corona. If the surface _potential U _{0 is} less than U _{0 ideal} , the voltage of the _{charging corona} is raised by one level, which is indicated in the flowchart by U _corona ⁺¹ . Further, the voltage U _{counter voltage,} the developing electrode in accordance with the _B e- ^{relationship U} reverse voltage ⁼ U _R ⁺ U _X determines, with the residual potential U _R and a size U _x, it is in an empirical value between 10 and 120 V. For example, if the residual charge potential -100 V, the amount U _x = -50 V is selected, so that the developing electrode a reverse voltage U _Ge g _en --spannung of -150 V is applied, which ensures that the development is carried out free of background. 8, the microprocessor outputs the counter voltage U _{counter voltage} for the developing electrode control and the corona voltage U _corona for the corona control. 8, the control is carried out with a fixed potential detector.

The flowchart shown in FIG. 9 is to be read in connection with the circuit arrangement according to FIG. 7 and relates to the microprocess control of a circuit arrangement with a moving potential detector. After starting, the exposure and the drive for the potential detector are switched on and the charging potential U _{o is} stored in the memory with variable access by the microprocessor. During the movement of the potential detector, the surface potentials measured by the potential detector along the dark discharge curve U _S and the associated surface potentials of the light discharge curve U _E are continuously read into the variable access memory. The potential difference ΔU = U _s - U _{E is} formed in the microprocessor and _ideally compared with a predetermined target value ΔU. As long as the potential difference ΔU is smaller than ΔU _ideal , this comparison is continued and each newly formed potential difference is compared _ideally with ΔU. As soon as the potential difference .DELTA.U is greater than or equal to .DELTA.U _ideal , the exposure is ended and the potential detector is moved into its initial position. At the same time the charge potential U _o of a predetermined value U _0ideal ver ^g li- Chen and is depending on whether U _o greater U _0ideal or smaller than U _0ideal, the ^{corona voltage U} corona reduced by one level or increased by one step, which in the flow chart is indicated by U _Korona -1 or U _Korona ⁺¹ . To be applied to the developing electrode Ge ^g ens ^p a ^{n g} U now _Ge g _ens p _Annun g results from U _R U ⁺ _x, with the remaining charge potential U _R of the printing plate at Completion of exposure and U _x , an empirical value in the range of 10 to 120 V, which is added to the residual charge potential to ensure that a background-free image is obtained during toner development. Last is performed the output of the counter voltage U _Ge g _ens p _Annun g and the corona voltage U _corona for controlling the development electrode and the Aufladekorona.

In order to obtain a background-free image after toner development, the printing plate and the measuring arrangement are also illuminated with a light intensity which corresponds to the background density under the most unfavorable conditions, such as are present, for example, in the shadow region of a cut edge of a part of a sentence. To simulate or create these conditions, a gray field of appropriate density in opaque or transparent form can be attached to or above the template edge or the head of the potential detector can be covered with a gray filter of the desired density. The density of the gray field is in the range of 0.05-0.5, in particular at a value of 0.26. If the counter-voltage of the developing electrode is then placed at the same voltage level as the residual potential of the printing plate in the exposed area measured by the potential detector or illuminated by the image of the gray field of the printing plate, the images of all image areas are shown up to this optical density not stressed. The lower on is also an advantage flow of various sources of error, such as the zero point drift and the detection of shadow potentials of the measuring probe, on the measurement result.

Circuit details of the circuit arrangement according to FIG. 6 are described with reference to FIG. 10. The potential detector 3 known per se is closed by a metal housing 35 with an opening 36 which surrounds the measuring device and shields it from external electrical fields. The opening 36 forms a measuring opening for the measuring electrode 6. A tuning or vibrating fork 31 in the metal housing is set in mechanical vibrations by an oscillator 28 via a frequency-adjustable drive 32, as indicated by the two double arrows B, B, and is electrically connected to connected to the metal housing 35. The arms of the tuning fork 31 swinging towards and away from each other work as a chopper which periodically opens and closes the measuring window 6. The electric lines of force emanating from the surface potential of the latent electrostatic charge image on the printing plate run through the measuring opening onto the measuring electrode 6 and are interrupted by the reciprocating arms of the tuning fork 31 which move transversely to the lines of force.

As a result, a chopped alternating voltage is induced in the measuring electrode 6, the amplitude of the voltage difference between the surface potential on the printing plate and the potential of the tuning fork is proportional, which is electrically connected to the metal housing 35. The phase of the induced alternating voltage is determined by the polarity of the direct voltage, which is applied to the measuring electrode 6 or to the metal housing 35 of the potential detector 3. The high-impedance AC voltage signal induced in the measuring electrode 6 is converted into a DC voltage signal in the signal converter 11, which is shown in broken lines in FIG. 10.

The induced AC voltage signal of high impedance is converted by an impedance preamplifier 26 into a signal of low impedance and fed to a signal amplifier 27, the output signal of which is fed to a phase detector 30 via an opto or photocoupler consisting of a light-emitting diode 29 and a phototransistor 37. The amplitude and polarity of the DC voltage output signal of the phase detector 30 are predetermined by the amplitude and phase of the induced AC voltage signal relative to the reference signal which is applied to the measuring electrode 6. A signal of the oscillator 28 is fed to the phase detector 30 via an optocoupler or photocoupler, consisting of a phototransistor 39 and a light-emitting diode 38, in contrast to which the starting position of the measuring electrode 6 is determined. If the tuning fork 31 opens the window between the measuring electrode and the pressure plate, the phase and the DC voltage output signal are positive, otherwise the phase and the DC voltage output signal negative. The DC voltage output signal of the phase detector 30 is integrated by an integrator 33 for low DC voltages. The polarity of the output signal of the integrator 33 is inverted to the polarity of the surface potential of the printing plate to be measured. The output signal of the integrator 33 is displayed on the one hand via a voltmeter V and on the other hand fed to a high-voltage amplifier 34. The output signal of the high-voltage amplifier 34 is fed back to the metal housing 35 of the potential detector 3 in order to bring it to the same potential as that of the plate surface to be measured and is, on the other hand, supplied via a voltage divider 40, which comprises a variable resistor and two fixed resistors, to the impedance converter 12 of the control circuit for determining the residual charge potential when the exposure of the printing plate has ended.

In the impedance converter 12, the impedance of the DC voltage measurement signal is reduced. The output of the impedance converter 12 is connected to the one input of a comparator 13, the other input of which is connected to a ten-turn potentiometer 41 for setting and feeding in the desired setpoint of the residual charge potential. A reference voltage of a light-emitting diode 42 is applied to the ten-turn potentiometer 41, which together with a resistor 43 forms a voltage divider. The light emitting diode 42 also serves to indicate whether a voltage is applied to the ten-turn potentiometer 41 or not. Between the output of the impedance converter 12 and the one input of the comparator 13 there is a filter capacitor 44, which filters out any AC voltage signal components that may still be present in the DC voltage measurement signal. The output of the comparator 13 is fed back to the input at which the target value of the residual charge potential is present. The setpoint signal is inverted compared to the measurement signal. A digital voltmeter 45 measures, depending on the position of a push button 46a, 46b, the measurement signal at one input or the setpoint signal at the other input of the comparator 13. If the measurement signal and the setpoint signal match, the comparator 13 supplies an output signal which a switching transistor 47 for actuates a switching source 48, which switches a switch 49, whereby, for example, the shutter 14 in the arrangement according to FIG. 6 is closed and the exposure is ended.

A light emitting diode 5C is connected to the switch as a display.

As already mentioned in connection with FIG. 6, a microprocessor control can be provided instead of the control circuit consisting of impedance converter <r 12, comparator 13, setpoint value setting device 15.

Claims (18)

1. A method for maintaining a predetermined potential ratio in the exposure of electrostatically charged photosensitive layers on supports on which an electrostatic latent image of an original is formed during the exposure, characterized in that the photosensitive layer is charged to a predetermined surface potential, which by means of a electrostatic potential detector is measured, that the potential ratio during exposure to changing is continuously compared with a specified desired value _'and that in correspondence of the changing potential relationship with the predetermined set value, the exposure is terminated. 2. The method according to claim 1, characterized in that the measurement of the surface potential is carried out in a bright area of the light-sensitive layer outside the area of the latent electrostatic image. 3. The method according to claim 1, characterized in that the target value is predetermined according to the residual potential of the light-sensitive layer after the discharge of the light areas of the latent image or according to the potential difference between the light and dark areas of the exposed latent image. 4. The method according to claim 3, characterized in that a certain residual potential is stored or readable for each predetermined surface potential of a charge, and that the surface potential which decreases during the exposure is continuously compared with the predetermined residual potential associated with the predetermined surface potential at the beginning of the exposure is. 5. The method according to claim 4, characterized in that at a size of the sinking surface potential equal to or less than the residual potential, the exposure is ended. 6. The method according to claim 3, characterized in that for each predetermined surface potential of a charge a certain potential difference, formed from the surface potential of the exposed dark areas, reduced by the surface potential of the exposed bright areas of the latent image is stored or adjustable in a control device, and that the potential difference which changes during the exposure is continuously compared with the determined potential difference which is assigned to the predetermined surface potential at the start of the exposure. 7. The method according to claim 6, characterized in that at a size of the currently measured potential difference equal to or greater than the determined potential difference, the exposure is ended. 8. The method according to claims 1 to 7, characterized in that a gray field with a density in the range of 0.05 to 0.50 ar brought on or above the template edge and that the counter voltage of the developing electrode to the same voltage level as the residual potential the printing plate is placed in the area of the printing plate exposed by the image of the gray field. 9. The method according to claims 1 to 7, characterized in that the head of the potential detector is covered with a gray filter of predetermined density in the range of 0.05 to 0.50 and that the counter voltage of the developing electrode to the same voltage level as the residual potential of the printing plate after exposure in the area of the printing plate measured by the potential detector. 10. Arrangement for performing the method according to claims 1 to 7, with a potential detector for measuring the surface potential of a light-sensitive layer of a support, characterized in that de potential detector (3) is connected to a signal converter (11 6), which the AC voltage output signals Potential detector (3) converted into DC voltage signals and feeds into a control device 51; 52) which is connected to a shutter (14) in the radiation path of an exposure device and whose output signal closes the shutter (14) when a predetermined setpoint for the Potential relationships on the light-sensitive layer of the carrier closes. 11. The arrangement according to claim 10, characterized in that the control device (51) has an impedance converter (12), a comparator and a setpoint adjustment device (15), and that the output of the impedance converter (12) with one input of the comparator ( 13), the other input of which is acted upon by the output signal of the connected setpoint adjustment device (15), the size of the output signal being set in accordance with the setpoint of the potential conditions. 12. The arrangement according to claim 11, characterized in that the setting device (15) has a memory in which potential values are stored, the residual potentials of the exposed light-sensitive layer correspond to the bright areas and which correspond to those at the beginning of the exposure due to the height given surface potentials associated with charging :. 13. The arrangement according to claim 10, characterized in that the control device (51; 52) contains a microprocessor in which a sequence program for controlling the closure of the closure (14) is stored. 14. Arrangement according to claim 13, characterized in that the control device (52) is an amplifier (17), which is followed by an A / D converter (18), a microprocessor (19), a digital output (20) and a D / A converter (21) and that the D / A converter (21) has two outputs, which are connected via high-voltage amplifiers (22, 23) to a corona controller (24) for the voltage of a charging corona and to a developing electrode control (25) for the voltage of a developing electrode. 15. The arrangement according to claim 13, characterized in that in the control device (51) the potential values of predetermined surface potentials (U _Oi ) and the associated residual charge potentials (U _Ri ) are stored with i equal to an integer whose difference (ΔU _{i =} U _ni - U _Ri ) is formed and compared with the difference between the measured surface potential of the potential detector (3) and the associated residual charge potential in order to close the shutter (14) in the radiation path of the exposure device in the case of differences of equal size. 16. Arrangement according to claims 8 to 15, characterized in that the potential detector (3) outside the region of the latent electrostatic image at a distance of 0.2 to 1.5 mm from the photosensitive layer of the support stationary over the edge region of the printing plate (1) is arranged. 17. Arrangement according to claims 8 to 15, characterized in that the potential detector (3) in a distance of 0.2 to 1.5 mm from the photosensitive layer of the support is arranged in a holder (7) which can be moved along a guide (8) during the exposure over the edge region of the printing plate (1).

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