Abstract:
A method and apparatus for an electrographic printer or copier provides a first layer of toner particles is deposited on a first toner receiving surface by a force field. The amount of toner actually applied is detected at at least one point in the first layer. The force field is modified according to changes in the actual amount of toner from the predetermined amount of toner.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to a method for the operation of an electrographic printer or copier, whereby a layer containing toner particles, referred to in brief as toner layer, is deposited on a toner acceptance surface under the influence of a force field. At least a part of the deposited toner particles is transferred onto a second toner acceptance surface under the influence of a second force field, the transferred toner particles forming a second layer thereon. 
     2. Description of the Related Art 
     Such a method is implemented in the developer unit that is set forth in European Patent EP 0 494 454 B1. Given this developer unit, the toner particles are electrically charged in a toner reservoir wherein a toner-air mixture is located and are subsequently deposited on the first toner acceptance surface that is grounded or provided with a potential, being deposited thereon as toner layer under the influence of an electrical force field. Due to the rotation of a developer drum whose generated surface forms the toner acceptance surface, the deposited toner particles are conducted past a development gap between the developer drum and a toner image carrier. 
     The second toner acceptance surface is formed by the generated surface of the developer drum. At the transfer gap, the toner particles are transferred from the transfer drum onto the developer drum, where they form the second layer. Due to the rotation of the drum, the toner particles of the second layer are conducted past a developing gap between the developer drum and a toner image carrier. The toner image carrier carries a latent charge image onto which toner particles are selectively applied at the development gap, whereby a toner image arises. The toner image is then applied onto an ultimate image carrier, for example onto paper, from the toner image carrier with or without employment of an intermediate image carrier. 
     What is disadvantageous about the known method is that a monitoring of the developing process and, in particle, of the amount of toner output by the developer unit is not possible. When the toner amount per surface on one of the toner acceptance surfaces or the charge of the toner particles during developing as well lies below or above a predetermined rated value or, respectively, rated range, then the error is not recognized until the finished print image. A reaction thus ensues relatively late. The disturbances in the print image are particularly noticeable given picture elements in the print image that are printed large-area. 
     In the known method, it is also not possible to exactly set the toner mass per surface section on the toner acceptance surface or the toner charge per surface section, to keep it constant during printing mode and to potentially adapt it to specific print jobs. 
     Both dry toner as well as liquid toner are employed as toner. Given liquid toners, for example, mechanical devices with electrical auxiliary potential are employed for applying the toner. The toners that are employed can still be divided into a single-component and multi-component toner. 
     What are understood by electrographic printer are, in particular, electrophotographic printers, ionographic printers and magnetographic printers. It is also known to employ bands on which the toner layer is deposited instead of the developer drum or, respectively, the transfer drum. 
     The Abstract of Japanese Patent Application JP-A-06067527 explains a developer unit wherein the actual toner mass per surface section of the developer drum is regulated dependent on the sensor signal of a sensor with the assistance of a force field between a transfer drum and a developer drum. 
     U.S. Pat. No. 5,339,140 explains a developer unit wherein the charge of a toner particle layer on the developer drum is acquired with the assistance of a sensor unit. Dependent on the output signal of the sensor unit, a charging device that is arranged along the toner path is controlled such in a control circuit that the actual toner charge reaches a rated toner charge. 
     U.S. Pat. No. 5,285,243 explains a developer unit wherein the developing rate is acquired with the assistance of a sensor that acquires the toner mass per surface unit. The charging potential, the illumination, the bias potential or, respectively, the toner concentration is modified dependent on the acquired developing rate. 
     The Abstract of Japanese Patent Application JP-A-58121050 explains a developer unit with a developer drum. The charge of the toner particles on the developer drum are [sic] acquired with the assistance of a sensor. A charge unit is driven such in a control circuit dependent on the output signal of the sensor that the toner on the developer drum has a predetermined charge or, respectively, a predetermined surface potential. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to specify a simple method for operating an electrographic printer or copier that enables developing with high quality. 
     The invention is based on the perception that characteristic quantities that critically define the quality of the developing process and, thus, critically define the quality of the print image as well must already be acquired during developing and not after the developing process has been terminated, in order to be able to react quickly given deviations of these characteristic quantities from rated values. 
     In the invention, the average actual toner mass per surface section is therefore acquired at at least one location of the first toner layer and/or second toner layer. What per surface section thereby means is that a relatively uniform layer is assumed and the toner mass that is acquired is referred to a defined surface section of the toner layer of, for example, 1 cm 2 . The total area of the respective toner layer is employed as reference quantity. The average toner mass per surface section is also a criterion for the thickness of the layer. 
     In the invention, at least one of the force fields is modified in a first control circuit dependent on the deviation of the actual toner mass per surface section from a predetermined, average rated toner mass per surface section. When the actual toner mass is higher than the rated toner mass, then the force field is modified such that fewer toner particles are deposited on the toner acceptance surface. When the actual toner mass is smaller than the rated toner mass, then the force field is modified such that more toner particles are deposited on the toner acceptance surface. When actual toner mass per surface section and rated toner mass per surface section coincide, then the force field is not modified. As a result of these measures, the actual toner mass can be exactly set to the value of the rated toner mass and can be kept constant during printing operations. As required, the rated toner mass per surface section can also be modified during printing operations. 
     Which of the two force fields is modified is dependent on the specific parameters during printing. When, for example, the toner acceptance surfaces are made of an electrically conductive material and when the force fields are generated on the basis of difference in potential in which both toner acceptance surfaces participate or at least one of the toner acceptance surfaces participates, then the potential of the second toner acceptance surface cannot be arbitrarily selected when this toner acceptance surface is arranged on the developer drum. A change of the potential of the second toner acceptance surface would also influence the voltage in the development gap. However, it is not always possible or, respectively, expedient to modify this voltage. In this case, only the potential of the first toner acceptance surface, i.e. of the transfer drum, is modified during regulation. 
     Further, the average actual toner charge per surface section is acquired in the invention at at least one location of the first toner layer and/or second toner layer. The charge of the toner particles of the appertaining layer is then modified in a second control circuit dependent on the deviation between the actual toner charge per surface section and a predetermined rated toner charge per surface section. As a result of these measures, the toner charge per surface section is acquired as an important characteristic quantity of the developing process and is set to the value of the predetermined actual toner charge. During printing operation, the actual toner charge is kept constant at the value predetermined by the rated toner charge. The result is an improvement of the print images arising during developing. 
     The toner charge per surface section can be increased on both toner layers in order to in turn compensate charge losses during the transport of the toner particles on these layers. When the installation space in the developer unit is limited, then only the toner charge of the first toner layer or the toner charge of the second toner layer is modified by a single charging device. 
     For example, the control methods are combined in the fashion of a cascade control or of a relationship control. A control is also applied wherein the toner mass per surface section and the toner charge per surface section are placed in relationship, so that what is referred to as the mass-related toner charge is calculated as controlled quantity. According to a predetermined control strategy, the toner mass per surface section and/or the toner charge per surface section is modified given a deviation of the mass-related toner charge from a predetermined value. The mass-related toner charge is one of the most critical characteristic quantities of the developing process. When the mass-related toner charge is kept constant during the entire development process, then the same quantity of toner particles, whose charge lies in a predetermined range, is always available for developing. During the entire developing process, the toner particles thus deposit uniformly at the locations of the toner image carrier to be developed. The result is a high-quality print image. 
     When the predetermined rated toner charge per surface section is selected such that it is higher in amount than the toner charge per surface section immediately after the application of the toner particles onto the toner acceptance surface, then charge losses of the toner particle charges during transport on the toner acceptance surface can be compensated in that the toner charge is increased in any case in addition to the control of the toner charge. 
     In another exemplary embodiment of the invention, the voltage in a developing gap is taken into consideration given the prescription of the rated toner mass per surface section and/or of the rated toner charge per surface section. When an operator varies the voltage in the developing gap, for example when setting the contrast of the print image, then rated toner mass and rated toner charge are also to be automatically adapted in order to generate a high-quality print image. 
     A further aspect of the invention is directed to a developer unit for an electrographic printer or copier that, in particular, is utilized for the implementation of the above-explained methods. The technical effects cited above are thus also valid for the developer unit of the invention and the exemplary embodiments thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are explained below with reference to the drawings. Shown therein are: 
     FIG. 1 a schematic illustration of a developer unit with a developer drum and a transfer drum; 
     FIG. 2 a developer unit with a control means for controlling the mass-related toner charge on the developer drum with the assistance of a scorotron; 
     FIG. 3 a developer unit with a control means for controlling the mass-related toner charge on the transfer drum with the assistance of a corotron; and 
     FIG. 4 a developer unit with a control means for controlling the mass-related toner charge taking the potentials on the developer drum and the transfer drum into consideration. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically shows the structure of a developer unit  10  at which a photoconductor band  12  is conducted past in the direction of an arrow  14 . A latent charge image in which the charges are distributed according to the image information of the image to be printed is located on the photoconductor band  12  in the surface region facing toward the developer unit  10 . The conveyor means for the photoconductor band  12  was not entered in FIG. 1 for the sake of simplicity in the illustration. 
     The developer unit  10  contains a container  16  in which a toner-air mixture  18  is located. Toner particles and air are mixed in roughly the ratio 1:10 in the mixture  18 , as a result whereof the mixture  18  behaves like a liquid. A boundary surface  20  between the mixture  18  and the air contained in the developer unit  10  is relatively smooth. An ultrasound sensor  22  above the surface  20  acquires a filling level H of the mixture  18 . 
     The mixture  18  is thereby generated from solid toner particles having an average size of approximately 10 μm that are supplied to the toner-air mixture  18  in defined amounts by a toner metering means  24 . A toner particle supply  26  is located between inclined sidewalls  28  of the toner metering means  24 , so that the toner particles are supplied funnel-like to a metering wheel  30 . The metering wheel  30  has recesses along its circumference into which respectively identical quantities of toner particles are accepted. As a result of a rotational movement of the metering wheel  30 , toner particles from the inside of the toner metering means  24  are supplied to the toner-air mixture  18  as soon as the ultrasound sensor  22  registers a drop of the boundary surface  20  below a predetermined rated height. 
     An air-permeable plate  32  composed of a porous polyethylene material is arranged in the floor region of the developer unit  10 , air flowing into the toner-air mixture  18  large-area through said plate  32  from a chamber  34  lying below the plate  32 . 
     Two corona wires  38  and  40  are located in the developer unit  10 , these having a voltage of approximately −8 kV and negatively charging the toner particles of the mixture  18  in their proximity. The corona wires  38  and  40  transversely through the entire developer unit over a length that approximately corresponds to the expanse of the photoconductor band  12  transversely relative to the conveying direction  14  thereof. A transfer drum  42  whose axis  44  proceeds parallel to the corona wires  38  and  40  is arranged above the corona wires  38  and  40  and above the boundary surface  20 . A conductive surface layer  46  has a potential of approximately −0.9 kV, so that the negatively charged toner particles—due to the influence of the electrical field between the corona wires  38 ,  40  and the transfer drum  42 —are deposited on the surface layer  46  over the entire length of the corona wires  38  and  40 . Given a rotation of the transfer drum  42  in the direction of an arrow  48 , the deposited toner particles are in the direction of an opening  50  of the developer unit  10  for the delivery of toner particles. For the section shown in FIG. 1, the transport path of the charged toner particles proceeds through the developer unit  10  along the outside radius of the transfer drum  42  from a point A up to a point B. 
     In point B, the toner particles—under the influence of a further electrical field—are transferred onto a conductive surface layer  52  of a developer drum  54  that rotates in the direction of an arrow  56 . The further electrical field lies between the surface layer  46  and the surface layer  52  charged to a potential of approximately −0.5 kV. The axis  58  of the developer drum  54  is arranged essentially parallel to the axis  44 . For the section shown in FIG. 1, the toner particles, following the transfer in point B, are transported by the developer drum  54  along the outside radius of the developer drum  54  to a point C in the opening  50 . 
     Individual toner particles that are not transferred from the transfer drum  42  to the developer drum  54  are removed from the surface layer  46  with the assistance of a stripper  60  before the respective region of the surface layer  46  is covered again with new charged toner particles. The stripper  60  proceeds over the entire length of the transfer drum  42  and is held by a stripper mount  62 . 
     The latent charge image of the photoconductor band  12  is developed in the region of the opening  50  in that toner particles from the surface layer  52  deposit in charged areas of the photoconductor band  12 . Toner particles remaining on the developer drum  54  are removed from the surface layer  52  by a further stripper  64  before new toner particles are again applied from the transfer drum  42 . The stripper  64  proceeds over the entire length of the developer drum  54  and is held by a further stripper mount  66  that, at the same time, is also a baffle means for the toner particles detaching from the developer drum  54 . The toner particles removed by the strippars  60  and  64  drop back into the mixture  18 . 
     Toner that replaces the toner particles consumed during developing is supplied to the developer unit by a toner delivery means  68 . 
     FIG. 2 shows the developer unit  10 , whereby, however, a latent charge image is developed on a photoconductor drum  12 ′ that is arranged at the developer unit  10  instead of the photoconductor band  12  (see FIG.  1 ). The photoconductor drum  12  [sic] rotates in the direction of an arrow  14 ′. A control means  100  to which a rated toner charge that, for example, refers to a specific surface section of a toner layer  104  on the developer drum  54  is prescribed via a line  102  is arranged at the developer unit  10 . A rated toner mass for the surface section of the toner layer  104  is also prescribed for the control means  50  via a line  106 . Via a line  108 , the control means  50  also receives signals from an optical sensor unit  110 . The optical sensor unit  110  contains a light transmitter, a light receiver as well as an evaluation unit. The light beamed out by the light transmitter is re-emitted to the receiver by the toner layer  104 . 
     With reference to the re-emission behavior of the toner layer  104  dependent on the actual toner mass per surface section, the actual toner mass per surface section in the toner layer  104  is identified in the sensor unit  110 . Via the line  108 , the momentary value of the actual toner mass proceeds to the control means  100  wherein the difference between rated toner mass and actual toner mass is formed in a subtracter  112 , whereby a toner mass error signal pends at the output of the subtractor  112 . Alternatively, the sensor unit  110  can also contain a capacitative sensor with whose assistance the actual toner mass per surface section is determined in that the change of the dielectric properties of the toner layer  104  are [sic] acquired given a change of the toner mass per surface section. 
     A potential sensor unit  114  that has its output side connected to the control means  100  via a line  116  is also arranged close to the surface  52  of the developer drum  54  covered with the toner layer  104 . The potential sensor unit  114  contains an electrode at which a potential that is determined by the potential of the developer drum  54  and by the totality of the toner charge that is located on the surface of the developer drum  54  in the field region of the electrode is influenced. The potential sensor unit  114  also contains an evaluation unit that determines the actual toner charge from the influenced potential. 
     The difference between rated toner charge and actual toner charge is formed in a subtractor  118  that is contained in the control means  50 . A toner charge error signal pends at the output of the subtractor  118 . 
     The two error signals of the subtractors  112  and  118  are supplied to a controller  120  that, for example, contains two PI regulators, whereof one generates a setting voltage USTELL 1  dependent on the toner mass error signal on an output line  122  of the control means  100 , said setting voltage USTELL 1  being adjacent at a controlled power pack part  124 . The controlled power pack part  124  generates a voltage U 3  at its output that determines the potential on the corona wires  38  and  40 . The voltage U 3  is set dependent on the setting voltage USTELL 1 . 
     The voltage USTELL 1  is prescribed such by the first PI regulator that the error signal of the subtractor  112  is reduced in amount and ultimately has the numerical value of “0”. A first control circuit I thus contains the optical sensor unit  110 , the control means  100 , the power pack part  124  and the corona wires  38  and  40 . The toner mass per surface section of the toner layer  104  is regulated with the assistance of the control circuit I in that, given too low a toner mass per surface section, the potential of the corona wires  38  and  40  is increased, so that more toner particles deposit on the surface  46  of the transfer drum  42 . Since these toner particles are transferred from the transfer drum  42  onto the developer drum  54  in the region of a transfer gap, the toner mass per surface section of the toner layer  104  also ultimately increases. When the actual toner mass lies above the value that is prescribed by the rated toner mass, then the potential of the corona wires  38  and  40  is lowered. The result is that fewer toner particles deposit on the surface  46  of the transfer drum  42 . Correspondingly fewer toner particles are then transferred onto the developer drum  54  at the transfer gap  125 . Ultimately, one succeeds in keeping the actual toner mass per surface section on the surface  52  of the developer drum  54  constant according to the predetermined rated toner mass per surface section with the assistance of the control circuit I. 
     Dependent on the toner charge error signal of the subtractor  118 , the second PI regulator contained in the controller  120  generates a setting voltage USTELL 2  on a line  126  and generates a setting voltage USTELL 3  on a line  128 , whereby it is essentially the setting voltage USTELL 3  that is modified during regulation. The setting voltage USTELL 2  is adjacent at a controlled power pack part  130  that, dependent on the value of the setting voltage USTELL 2 , generates a voltage U 1  that influences the charge behavior of a scorotron  132  at its output. The setting voltage USTELL 3  is adjacent at the input of a controlled power pack part  134  that, dependent on the value of the setting voltage USTELL 3 , generates a voltage U 2  at its output that is adjacent at a control grid of the scorotron  132 . The charge behavior of the scorotron  132  can be controlled better via the control grid  136  than via the voltage U 1 . 
     The second PI regulator therefore prescribes the setting voltage USTELL 3  essentially such that the error signal of the subtractor  118  is reduced in amount and ultimately has the numerical value of “0” until noise quantities lead to a new control procedure. A second control circuit II thus contains the potential sensor unit  114 , the control means  100 , the power pack part  130  or, respectively,  134  and the scorotron  132 . When the actual toner charge per surface section on the toner layer  104  decreases, then the voltage U 2  is set such, that the charge behavior of the scorotron  132  is boosted. When the actual toner charge per surface section of the layer  104  exceeds the predetermined rated value, then the voltage U 2  is modified such, that fewer charges are applied onto the toner layer  104  by the scorotron  132 . With the control circuit II, one succeeds in keeping the actual toner charge per surface section constant according to the predetermined rated toner charge per surface section during the developing process. 
     In a further exemplary embodiment, the control means  100  is given a mass-referred rated toner charge instead of the rated toner charge per surface section and instead of the rated toner mass per surface section. The mass-referred toner charge qT is calculated according to the following equation.          q                 T     =       Q                 T       M                 T                              
     whereby QT is the toner charge per surface section and MT is the toner mass per surface section. The prescribed, mass-referred rated toner charge is compared to a mass-referred actual toner charge that is determined from actual toner charge and actual toner mass with the above-recited equation. The mass-referred toner charge QT on the layer  104  is ultimately kept constant during the developing process on the basis of a predetermined control strategy of the control means  100 . 
     FIG. 3 shows the developer unit  10  with a control means  100 ′ for controlling the mass-referred toner charge qT on the transfer drum  42  with the assistance of a corotron  150 . The control means  100 ′ is constructed like the control means  100  (see FIG.  2 ), but contains a regulator  120 ′ instead of the regulator  120 , this having only the two output lines  122  and  126 . This is to be attributed thereto that the corotron, which is constructed significantly simpler that the scorotron  132  (see FIG.  2 ), has no control grid, so that the voltage U 2  generated by the power pack part  134  is eliminated. 
     By contrast to FIG. 2, the optical sensor unit  110 , the potential sensor unit  114  and the corotron  150  are now arranged close to the transfer drum  42 , so that characteristic quantities of a toner layer  152  on the surface  46  of the transfer drum are acquired or, respectively, influenced. 
     The regulation of the toner mass per surface section in the control circuit I ensues as explained above with reference to FIG.  2 . For regulating the toner charge per surface section, the control means  100 ′ only prescribes the setting voltage USTELL 2 . The setting voltage USTELL 2  is selected such that the actual toner charge per surface section of the toner layer  152  adjusts to the value prescribed by the rated toner charge per surface section. 
     FIG. 4 shows the developer unit  10  with a control device for the regulation of the mass-referred toner charge qT taking the electrical potentials on the developer drum  54  and the transfer drum  42  into consideration. The control ensues with a control means  100 ″ that is constructed essentially like the control means  100  (see FIG.  2 ). Instead of the regulator  120 , however, the control means  100 ″ contains a regulator  120 ″ that also takes the momentary potential of the developer drum  54  and of the transfer drum  42  into consideration when regulating. These potentials derive, for example, from the contrast value that an operator of the printer has selected. 
     The control of the developing process ensues essentially as explained above with reference to FIG.  2 . Additionally, the control means  100 ″ is connected via a line  170  to a controlled power pack part  172  and via a line  174  to a controlled power pack  176 . A bias signal BIAS 1  is communicated on the line  170 . Dependent on the value of the bias signal BIAS 1 , a voltage U 4  is generated in the power pack part  172 , this voltage being applied to the conductive surface of the developer drum  54 . The value of the potential on the developer drum  24  [sic] that is respectively selected influences both the control circuit I as well as the control circuit II since it co-determines the value of the rated toner charge and of the rated toner mass. 
     A further bias signal BIAS 2  that determines what voltage U 5  is generated at the output of the power pack part  176  is communicated on the line  174 . The voltage U 5  determines the potential on the surface  46  of the transfer drum  42 . The potential on the transfer drum  42  also influences both control circuits I and II. 
     The difference of the voltage U 5  and U 3  determines the toner mass per surface section deposited on the transfer drum  42  and also determines the toner charge per surface section to a certain extent. The difference of the voltages U 4  and U 5  determines the toner mass per surface section transferred from the transfer drum  42  onto the developer drum  54 . The difference between the voltage U 4  and the potential on the surface of the photoconductor drum  12 ′ determines the toner mass per surface section that is transferred from the developer drum  54  onto the photoconductor drum  12 ′ on regions in which toner particles are deposited during developing. Due to the image-wise charge distribution on the surface of the photoconductor drum  12 ′, the inking with toner particles for each picture element of the latent charge image is determined by the respective difference in potential between the voltage U 4  and the local photoconductor potential. 
     Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art. 
       100 , 100 ′, 100 ″ control means 
       102  line 
       104  tonerlayer 
       106 ,  108  line 
       110  optical sensor unit 
       112  subtractor 
       114  potential sensor unit 
       116  line 
       118  subtractor 
       120 , 120 ′, 120 ″ regulator 
       122  output line 
       124  controlled power pack part 
       125  transfer gap 
       126 ,  128  line 
       130  controlled power pack part 
       132  scorotron 
       134  controlled power pack part 
       136  control grid 
       150  corotron 
       152  toner layer 
       170 ,  174  line 
       172 ,  176  controlled power pack part 
     USTELL 1  setting voltage 
     USTELL 2  setting voltage 
     USTELL 3  setting voltage 
     I, II control circuit 
     qT mass-referred toner charge 
     QT toner charge per surface section 
     MT toner mass per surface section 
     U 1  through U 5  voltage 
     BIAS 1 , BIAS 2  bias signal