Abstract:
An apparatus and method for powering opposing corona chargers for discharging moving receiver sheets. The discharging capacity of the chargers is expanded by adding a high voltage DC offset to a current regulated, AC voltage limited power supply.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to corona chargers, and more particularly, to a power supply for a corona charger utilized in an electrophotographic reproduction apparatus to discharge receiver members.  
       BACKGROUND OF THE INVENTION  
       [0002]     In electrophotographic reproduction apparatus and printers, an electrostatic latent image is formed on a photoconductive imaging member by first uniformly charging the imaging member and then image-wise exposing the imaging member using various light sources such as a scanned laser, LED array, optical flash, or other suitable, known methods. The electrostatic latent image is then developed into a visible image by bringing the imaging member into close proximity with an electrostatic developer that includes charged marking particles. In a 2-component developer, marking particles are mixed with larger, magnetic particles called carrier particles, where the marking particles become triboelectrically charged by contact with the carrier particles. The developer is contained in a development station that typically includes a roller with a magnetic core. The carrier particles transport the marking particles into contact with the imaging member bearing the electrostatic latent image. The development station is suitably biased and the marking particles suitably charged so that the proper amount of marking particles, are deposited in electrostatic image the regions of the imaging member.  
         [0003]     After the electrostatic latent image on the imaging member has been developed, the developed image is generally transferred to a receiver member, such as sheets of paper or transparency stock. This is generally accomplished by applying an electric field in such a manner to urge the marking particles from the imaging member to the receiver member. In some instances, it is preferable to first transfer the developed image from the imaging member to an intermediate member and then from the intermediate member to the receiver member. Again, this is most commonly accomplished by applying an electric field to urge the developed image toward the appropriate receiver member. The receiver member bearing the developed image is then passed through a fusing device to permanently affix the developed image to the receiver member by heat and/or pressure. The marking particles are typically a thermoplastic polymer that is electrically non-conductive. The process of transferring the developed image to the receiver results in a polar electrostatic charge on the surfaces of the marking particle image and the receiver member. The polar charge will dissipate through a conductive receiver member such as moisture-containing paper, but will not migrate through the insulating marking particle image layer or the insulating receiver member. The result is trapped electrostatic charge on the imaged receiver member. On a conducting receiver member, the amount of trapped charge is dependent on the coverage of the marking particle image. The trapped charge can inconvenience the user of the printed receiver members, as the member sheets will tend to stick together by the electrostatic forces of attraction between the charges on adjacent sheets.  
         [0004]     It is well known to use an ac corona discharge to dissipate electrostatic charges on the surfaces of moving webs. For example, two AC corona chargers, facing each other on opposite sides of the web are typically used. Current regulated, high voltage AC power supplies are typically used to power the corona chargers in these exemplary configurations. In attempting to use a pair of corona chargers in this way to discharge receiver members, new problems are encountered due to the interframe spaces between successive receiver members. The resistivity between the two opposing chargers changes significantly when a receiver member is between the two chargers, versus during an interframe when no receiver member is between the two chargers. With pure current regulation the corona wire voltage can increase to critical, high values when a receiver member is in between the two chargers. The voltage would also vary with different types of receiver members because of the variation in receiver member resistivity. When a highly resistive member exits the area between the opposed chargers it is possible for an arc to develop between the opposing corona wires. This can happen before the current regulation control of the power supply can reduce the output voltage of the supply to react to the change in resistance between the corona wires.  
         [0005]     Arcing results in undesired electrical noise radiated into the control system of the reproduction apparatus and possibly to the environment around the machine. Arcing can also be damaging to the apparatus hardware and materials. If, to address this problem, a pure peak to peak voltage regulating power supply is used, the current would reach critical, high levels when the interframe, between successive receiver member, is in between the two chargers. In this mode, the chargers will be operating at an unnecessarily high power level and generate excessive heat in the power supply. At the corona wire, the corona emission and the resultant chemical emissions will also be unnecessarily high.  
         [0006]     In co-pending U.S. patent application Ser. No. 09/866,182 (in the name of Hasenauer), a novel power supply is disclosed, hereafter referred to as the Hasenauer power supply, for powering a pair of opposed AC corona chargers for dissipating the trapped polar charge on the receiver members, that solves these problems. The Hasenauer power supply uses a combination of both output control methods, current regulation, and peak to peak voltage regulation, to provide a solution that prevents arcing and over-current loading for receiver member sheet fed applications. Driven by the resistance between the two chargers, the power supply changes automatically from current regulation to voltage limit mode. However, due to limited output current capacity, the Hasenauer power supply becomes less effective at very high receiver member speeds.  
       SUMMARY OF THE INVENTION  
       [0007]     In view of the foregoing discussion, an object of this invention is to provide improved apparatus for removing electrostatic charge from receiver members receiving marking particle images in electrophotographic reproduction apparatus. Accordingly, there is herein provided a high voltage power supply for electrostatically discharging marking particle bearing receiver members from a sheet fed reproduction apparatus that addresses the prior needs for a more effective power regulation system that can charge corona wires while preventing arcing and over current loading for sheet fed applications. The power supply has two high voltage outputs that are RMS current regulated with a regulated DC voltage offset and peak-to-peak voltage limited. Each corona wire is connected to one of the two high voltage outputs of the high voltage power supply. The current flow through ionized air neutralizes and reduces the electrostatic charge in the receiver member to acceptable (negligible non-adverse) values.  
         [0008]     The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention and its objects and advantages will become apparent upon reading the following detailed description and upon reference to the drawings, in which:  
         [0010]      FIG. 1  illustrates a system having opposing wire corona chargers within a sheet transport system; and  
         [0011]      FIG. 2  schematically illustrates the power supply circuit diagram of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     Referring now to the accompanying drawings,  FIG. 1  illustrates a receiver member (sheet) transport system within the field of electrophotographic color reproduction apparatus. Lower corona charger wire  22  and upper corona charger wire  23  are respectively contained within lower wire charger shell  20  and upper wire charger shell  21 . The opposing charger wires  22 ,  23  are paired together and positioned such that they are located downstream of a fusing apparatus (not shown). Receiver members  24  are guided seriatim through input paper guides  27 , and urged through the space between the two opposing charger wires  22 ,  23  by any suitable transport mechanism, such as nip roller pairs  28 ,  29 . The charger wires are driven by the high voltage power supply  26 . The two charger wires  22 ,  23  remove the electrostatic charge that is left over on the receiver members  24  once the reproduction copy has been made and after the fusing process is completed. If the left over charge is not removed from the receiver members  24 , the receiver members, when stacked downstream for operator retrieval, may stick to each other causing sheet-handling problems such as dishevelment in the stacking operation and/or difficulties in separating the sheets for subsequent finishing operations.  
         [0013]     The preset invention is directed towards the high voltage power supply  26  that is used for the electrostatic discharging of receiver members from sheet-fed reproduction apparatus. The power supply  26  has two high voltage outputs  30 ,  31 , each of which is RMS current regulated and peak to peak voltage limited with a regulated DC offset voltage. The two high voltage outputs  30 ,  31  of the high voltage power supply  26  are respectively connected to the corona charger wires  22 ,  23 . The output voltage is a trapezoidal waveform with a 400 Hz AC frequency. The voltage waveforms of the upper and the lower charger are synchronized at 180 degrees apart to provide maximum current flow between the wires  22 ,  23 . That current flow, through the ionized air, reduces the electrostatic charge in the receiver sheets to acceptable (negligible non-adverse) values.  
         [0014]     One incoming receiver member ( 24 ) is shown with a polar charge that results in a difference of electrical potential between the top and bottom surfaces of the member sheet. The outputs  30 ,  31  of power supply ( 26 ) are respectively labeled “+” and “−” to indicate the DC offset polarity of each output. The diagram further illustrates the connection of each output to the proper corona wire such that the polarity of the potential between the charger wires will be in opposition to the polarity of the charge on the receiver member.  
         [0015]      FIG. 2  schematically illustrates a circuit diagram for the power supply  26 . The preferred embodiment of power supply  26  includes two nearly identical circuits, one for driving each of the two AC output transformers  1   a,    1   b  for boosting a low voltage input to a high voltage (3-20 KVpp) AC output that energizes the corona wire chargers  22 , 23 . Each output transformer  1   a ,  1   b  has two primary windings  40   a ,  40   b ,  42   a ,  42   b , and secondary winding  44   a ,  44   b . Resistors  2   a ,  2   b  in series between the ground plane and the return  46   a ,  46   b  of the high voltage secondary windings  44   a ,  44   b  respectively of the transformers  1   a ,  1   b  function as current sense elements. The voltage developed across resistors  2   a ,  2   b  reflects the current sourced by the secondary windings  44   a ,  44   b  of transformers  1   a ,  1   b . Those voltage signals are AC coupled respectively to conditioning circuits  3   a ,  3   b  which are RMS to DC converters. The signals conditioned by the conditioning circuits  3   a ,  3   b  are then compared to reference signals from a regulation references  50   a ,  50   b  at comparators  4   a ,  4   b . The reference signals indicate the desired regulated current output set point. In the preferred embodiment these reference signals are analog DC voltage signals and comparators  4   a ,  4   b  are operational amplifiers. The signal conditioning circuits  3   a ,  3   b  reference signals and comparators  4   a ,  4   b  provide functionality that could be achieved with suitable alternate arrangements that will be readily apparent to those of ordinary skill in the art. Among these arrangements are the pulse width modulated signals, frequency modulated signals or digital techniques with parallel or serial reference signals delivered to the power supply, or some combination thereof. The reference signals may be generated internal to the power supply or provided by an external controller. An external controller is used in the preferred embodiment. The output of comparators  4   a ,  4   b  respectively control the low voltage DC-to-DC converters  5   a ,  5   b  which apply the voltage to the primary windings  40   a ,  40   b ,  42   a ,  42   b  of respective transformers  1   a ,  1   b  and adjust that primary voltage to provide a desired regulated current, sourced from the secondary windings  44   a ,  44   b  of transformers  1   a ,  1   b.    
         [0016]     Voltage limit comparators  6   a ,  6   b  respectively monitor the output of low voltage DC-to-DC converters  5   a ,  5   b . The voltages applied to the primary windings  40   a ,  40   b ,  42   a ,  42   b  of respective transformers  1   a ,  1   b  are compared to reference signals from voltage limit control references  52 a,  52   b . These comparators  6   a ,  6   b  and the voltage limit control reference signals are analog in the preferred embodiment. As discussed previously, suitable alternate arrangements may be used for this. Voltage limit comparators  6   a ,  6   b  impose a limit on the maximum output voltage of the respective DC-to-DC converters  5   a ,  5   b , which in turn limits the maximum voltage that can be applied to the corona wires  22 ,  23 . Alternately, the voltage limit comparison could be made by comparing the high voltage secondary voltage with the voltage limit control reference signal.  
         [0017]     As described above, the two high voltage AC outputs  30 ,  31  of power supply  26  are 180 degrees out of phase. This is accomplished with clock circuit  7 . Clock signal  8  and inverted clock signal  9  are connected to opposite sides of primary windings  40   a ,  40   b ,  42   a ,  42   b  of respective transformers  1   a ,  1   b  such that the voltages of the two outputs will be of opposite polarity. In the preferred embodiment, the circuit for transformer  1   a  is located on the same package as the circuit for transformer  1   b . An alternate arrangement places the two transformer circuits in different packages. In this separate arrangement, the clock signal  8  can be passed from one circuit package to the other via a wired connection. To allow both circuit packages to be the same, electrical connections will be provided for a clock output, a non-inverting clock input and an inverting clock input. The electrical wiring makes connection from the clock output of one circuit package to the non-inverting clock input of that same package, and to the inverting input of the second package. Alternately, the inverting and non-inverting clock inputs could be switched on both packages.  
         [0018]     The high voltage DC offsets for the two AC outputs  30 ,  31  of power supply  26 , are generated by DC-to-DC conversion. In the preferred embodiment high voltage DC-to-DC converter output stages  14   a ,  14   b  are respectively inserted between the current sense elements  2   a ,  2   b  and the high voltage secondary windings  44   a ,  44   b  of transformers  1   a ,  1   b . These DC-to-DC converter output stages  14   a ,  14   b  are inserted at these particular locations to avoid placing them at the higher voltage stress condition on the high side of the AC output transformers  1   a ,  1   b , and to avoid placing them between ground and the current sense resistors  2   a ,  2   b  which would complicate current sense circuitry. In the preferred embodiment the regulated DC offset voltage levels at the two outputs  30 ,  31  are equal in magnitude and opposite in polarity. The DC offset voltages are respectively conditioned by circuits  13   a ,  13   b  for comparison to offset reference signals from the offset references  54   a ,  54   b . The conditioning circuits  13   a ,  13   b  respectively include a voltage divider and buffer. The offset reference signals are DC voltages. As discussed previously, alternate arrangements may be used. The offset reference signals are respectively compared to feedback from the DC offset voltage at comparators  12   a ,  12   b . The output of the comparators  12   a ,  12   b  respectively adjust the power switching circuits  1   1   a ,  1   1   b , which adjust the voltage on the primary windings of the high voltage DC output stages  14   a ,  14   b . In the preferred embodiment the magnitude of the high voltage DC output stages can be adjusted in a range from 0 to 1000 V.  
         [0019]     The amount of trapped charge on marking particle developed receiver members may vary from sheet to sheet depending on a variety of factors such as: 
        1. Charge applied to the imaging member     2. Sheet material     3. Image content—percent marking particle coverage of sheet     4. Fusing temperature     5. Process speed        
 
         [0025]     The invention allows the reproduction apparatus control system  60 , based on operational parameters, to adjust the regulated AC current and DC offset voltage to provide optimum discharging performance through the reference control signals. Based on the various parameters, such as noted above, look up tables are defined for certified receiver member types, known process set points and analysis of digital image files, the reproduction apparatus control system may adjust the performance of the corona chargers such that the remaining charge on marking particle developed receiver members will be below adverse levels after discharging.  
         [0026]     The foregoing detailed description has detailed the best mode known to the inventors for practicing the invention. Other embodiment will be obvious to those skilled in the art; therefore, the scope of the invention should be measured by the appended claims.