Abstract:
A single piece control grid electrode for a corona charging device for charging a curvilinear shaped dielectric support member with improved charging uniformity and consistency and with lower assembly and service cost. The single piece control grid is formed by a plurality of integral planar segments angularly oriented with respect to each other so that each segment may be equally spaced from the curvilinear shaped dielectric support member.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to corona charging devices for use in electrostatographic reproduction machines, and more particularly to a control grid electrode for connection to a corona charging device used in such machines. 
     BACKGROUND OF THE INVENTION 
     In typical commercial reproduction apparatus (electrographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon. 
     In such reproduction apparatus, the dielectric support member is typically initially charged by spraying its surface with ions produced by a corona charging device. The corona charging device has a plurality of fine wires positioned transversely to the direction of motion of the dielectric support member and energized by a high voltage power supply. Corona emission from the wires ionizes air molecules, which are drawn to the surface of the dielectric support member as the dielectric support member moves past the wires. The quality of the developed image, especially that of pictorial images, is very dependent on the uniformity and consistency of the initial charge on the surface of the dielectric support member. 
     To improve the uniformity and consistency of the initial dielectric support member surface charge, it is known to provide an electrically biased control grid between the corona wires and the dielectric support member. The control grid includes a series of parallel wires or narrow metal strips closely spaced to each other and positioned transverse to the direction of motion of the dielectric support member. The spacing of the control grid wires to each other and to the dielectric support member surface, and the electrical bias voltage applied to the control grid wires, are the parameters that control the amount and spatial uniformity of the ionic charge deposited onto the surface of the dielectric support member. The surface voltage potential created on the dielectric support member is directly related to the bias voltage applied to the control grid wires. 
     The surface voltage created on the dielectric support member also depends upon the velocity at which such member moves past the corona charging device. For slower reproduction apparatus in which the dielectric support member velocity might only be a few inches per second, a corona charger with only one corona wire may be adequate. However, for higher speed reproduction apparatus in which the dielectric support member velocity might be 15-20 inches per second or higher, a corona charger with a plurality of corona wires may be required to achieve the required surface charge amount and uniformity. 
     Optimum charging efficiency, as well as uniformity and consistency of the deposited charge, is achieved when the control grid is closely and uniformly spaced from the surface of the dielectric support member. If the dielectric support member is curvilinear shaped and a corona charger with a plurality of corona wires is required, this would require that the control grid be provided with a curvature that matched the curvature of the dielectric support member. It is however very difficult to give this curvature to the control grid and still maintain the uniform spacing of the control grid from the dielectric support member surface because the control grid is typically made up of a plurality of closely spaced, thin wires. Tensioning the wires of a curved control grid so as to keep them uniformly spaced to each other and to the surface of the dielectric support member is very difficult. 
     U.S. Pat. No. 5,206,784 (issued Apr. 27, 1993, in the name of Kimiwada, et al.) addresses uniformly spacing the control grid from a cylindrically shaped member by dividing the control grid into a plurality of separate and independent planar grids equally spaced from the cylindrical member. The use of separate and independent planar grid sections is an effective compromise to a single piece curved grid, but the charger is costly to manufacture and difficult to assemble and to service if the grid sections become contaminated and must be replaced. It is difficult and time consuming to precisely position and equally tension the separate sections relative to each other as is required to achieve optimum dielectric support member surface charge uniformity and consistency. 
     SUMMARY OF THE INVENTION 
     In view of the above, it is the object of the present invention to provide a control grid electrode for a corona charging device for charging a curvilinear shaped dielectric support member with improved charging uniformity and consistency and with lower assembly and service cost. The single piece control grid is formed into a plurality of planar segments angularly oriented with respect to each other so that each segment is substantially equally spaced from the curvilinear shaped dielectric support member. Attaching this single piece control grid to the corona charging device during initial assembly and service replacement is easier than with separate segments. Further, maintaining the required spatial relationship between each segment and between the segments and the surface of the curvilinear shaped dielectric support member is facilitated by this arrangement. Thus assembly, operation, and service cost are optimized. 
     The invention, and its objects and advantages, will become apparent in the detailed description of the preferred embodiment presented below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiment examples of a device according to the invention, from which other characteristics of the invention can also be gathered, but to which the invention is not limited in its scope, are shown schematically in the following drawings: 
     FIG. 1 is a schematic illustration showing a side elevation view of an electrographic reproduction apparatus, and showing a corona charging device having a control grid electrode in accordance with the present invention; 
     FIG. 2 is a view, in perspective, of the single piece formed control grid according to this invention; 
     FIG. 3 is a view, in cross-section of the single piece formed control grid according to this invention; 
     FIG. 4 is a view, in perspective, of the corona charging device with the single piece formed control grid of this invention attached thereto; and 
     FIG. 5 is a view, in cross-section, showing the spatial relationship between the corona charging device corona wires, single piece formed control grid of this invention, and the cylindrical dielectric support medium surface. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Electrostatographic reproduction apparatus and corona charging devices for such apparatus, generally are well known. Therefore the present description will be directed in particular to elements forming part of, or cooperating more directly with the present invention. While the invention will be described with reference to an electrographic apparatus, the invention can also be used in other electrostatographic systems known in the art. 
     With reference to the electrographic reproduction apparatus  10  as shown in FIG. 1, an imaging drum  12  is provided on which is coated a photoconductive dielectric support member  14 . The imaging drum  12  is selectively rotated, by any well known drive mechanism (not shown), in the direction indicated by the arrow A, so as to advance the photoconductive dielectric support member  14  past a series of subsystems of the electrographic reproduction apparatus. A primary charging device  20  is provided for the purpose of depositing a uniform electrostatic charge onto the photoconductive dielectric support member  14 . The primary charging device  20  includes a housing  22 , corona wires  24  energized by a high voltage power supply  26 , and a control grid  30 , representative of the present invention, electrically biased by power supply  38 . The amount of electrostatic charge deposited onto the photoconductive dielectric support member  14  by the primary charging device  20  is controlled by the bias voltage applied to the control grid  30  by the power supply  38 . 
     The uniform charge on the photoconductive dielectric support member  14  is subsequently selectively dissipated by, for example, a digitally addressed exposure subsystem  40 , such as a Light Emitting Diode (LED) array or a scanned laser, to form an electrostatic latent image of a document to be reproduced. The electrostatic latent image is then rendered visible by development subsystem  50 , which deposits charged, pigmented marking particles onto the photoconductive dielectric support member  14  in accordance with the electrostatic charge pattern of the latent image. The developed marking particle image is then transported to nip  52  where it is transferred to a blanket  54 , coated on an intermediate transfer drum  56 . An electric potential difference applied between the intermediate transfer drum  56  and the imaging drum  12  creates an electric field that attracts the charged marking particles from the imaging drum  12  to the blanket  54  of the intermediate transfer drum  56 . The image on the blanket  54  is then transported to nip  58  where it is transferred to a receiver sheet carried on transport belt  60 . An electric potential difference applied between intermediate transfer drum  56  and backup roller  62  creates an electric field that attracts the charged marking particles from the intermediate transfer drum  56  to the receiver sheet. Cleaners  66  and  68  clean any marking particles that are not transferred from the photoconductive dielectric support member and blanket respectively. The receiver sheet bearing the marking particle image is then transported to a fusing subsystem to be fixed Alternatively, the receiver sheet bearing the marking particle image may be transported past subsequent electrographic reproduction apparatus identical to apparatus  10 , to receive, for example, different color marking particle images in superimposed register. 
     The electrographic reproduction apparatus  10  described above is just one embodiment in which the present invention could be used. For example, the photoconductor dielectric support member could be coated on a flexible web entrained about a plurality of rollers or other supports and/or the developed marking particle image could be transferred directly to a receiver sheet rather than via an intermediate transfer drum. 
     With reference now to FIGS. 2-5, various views of the control grid  30  forming a preferred embodiment of the present invention are illustrated. A single piece formed control grid according to this invention is shown in perspective in FIG. 2 and, as noted above, designated generally by the numeral  30 . In this embodiment of the present invention the single piece control grid  30  is shown formed into three angularly oriented segments  30   a ,  30   b ,  30   c . It should, of course, be understood that the number of segments will depend upon the total width of the control grid which, in turn, will depend upon the number of corona wires in the corona charging device, the radius of the curvilinear shaped dielectric support member, and the desired predetermined spacing of the control grid from the peripheral surface of the curvilinear shaped dielectric support member. 
     Each segment  30   a ,  30   b ,  30   c  of the control grid  30  includes the center section  32 , having a plurality of substantially parallel, electrically conducting wires or thin metal strips  33  terminating at each end into respective planar end sections  34   a  and  34   b . As depicted in FIG. 3, the end sections of each segment are angularly oriented with respect to the end sections of adjacent segments by an angle α. The angle α is determined by the width of each segment, the radius of the curvilinear portion of the dielectric support member  14  in juxtaposition with the corona charging device  20 , and the desired predetermined spacing of the respective segments from the peripheral surface of the curvilinear shaped dielectric support member. In this preferred embodiment of the present invention the wires  33  and end sections  34   a ,  34   b  may be formed from the same planar metal sheet by selectively etching away material such as through any well known photoetching process, followed by die stamping the end sections into the desired predetermined angular relationship. 
     FIG. 4 is a perspective view of the single piece control grid  30  attached to the housing  22  of the corona charging device  20 . In this embodiment the control grid  30  is mounted on the corona charging device housing  22  by L-shaped sheet metal tabs  70  that are attached to the ends of the housing  22  with threaded fasteners  72 . Lugs  74  on the sheet metal tabs  70  extend through apertures  36   a ,  36   b  formed in the respective end sections  34   a ,  34   b  of the control grid  30 . Tension is selectively applied to the control grid  30  by screws  76 , which are threaded through holes in the sheet metal tabs  70  and press against the end of the housing  22 . Advancing the screws  76  through the threaded holes in the sheet metal tabs  70  pulls the tabs away from the ends of the housing  22 , thus increasing the tension in the control grid  30 . It should be understood that the above described method of mounting and tensioning the control grid of the present invention represents only one of any number of well known variations found in prior art. With the corona charging device  20  assembled as described, FIG. 5 represents a cross-section view showing the general spatial relationship of the high voltage corona wires  24 , the single piece control grid  30 , and the peripheral surface of the curvilinear shaped dielectric support member  14 . As is readily seen, according to this invention the sections  30   a ,  30   b ,  30   c  of the control grid  30  are respectively at desired predetermined substantially equal spacing from the dielectric support member  14 . Additionally the corona wires  24  may also be arranged to be at desired predetermined substantially equal spacing from the dielectric support member  14 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.