Abstract:
A method and apparatus for a universal printhead is disclosed that functions independently of the diameter of a charge receiving dielectric drum, while optimizing print quality. The printhead includes two sets of electrodes mutually separated by a dielectric. Each of the electrodes from the first layer crosses each of the electrodes from the second layer forming a plurality of charge generating sites. The charge generating sites are generally disposed in only two rows.

Description:
FIELD OF THE INVENTION 
     The invention relates to a printhead suitable for use with image forming systems, and more particularly relates to an arrangement of electrode and dielectric layers within a printhead for optimizing print quality and performance. 
     BACKGROUND OF THE INVENTION 
     There are many different printing technologies utilized today for creating and reproducing images in an image forming system. Several of these technologies include a general process of charging a surface of a latent image receiving member, such as a drum, with a latent charge image. The term drum illustrates a common structure for support of the latent image-receiving member. The drum can also be one of several other architectures including a curved latent image receiving member, or a flexible dielectric belt, which moves along a predetermined path. A drum can also be an imaging member, such as a liquid crystal, phosphor screen, or similar display panel in which the latent charge image results in a visible image. The drum typically includes on an exterior surface thereof, a material that lends itself to receiving the latent charge image, such as a dielectric layer. Accordingly, the term drum used herein shall mean all such structures or devices. 
     A number of organic and inorganic materials are suitable for the dielectric layer of the drum. The suitable materials include glass enamel, anodized, flame or plasma sprayed high-density aluminum oxide, and plastic, including polyamides, nylons, and other tough thermoplastic or thermoset resins, among other materials. 
     The drum rotates past an image-forming device, such as a printhead, which produces a stream of accelerated electrons as primary charge carriers. The electrons reach the drum, landing in the form of a latent charge image. The latent charge image then receives a developer material to develop the image. The image is applied to a medium, e.g., a sheet of paper, by press or electrostatic transfer to form a printed document. 
     The printhead is most often a multi-electrode structure that defines an array of charge generating sites. Each of the charge generating sites, when the electrodes are actuated, generates and directs toward the drum a stream of charge carriers, e.g., electrons, to form a pointwise accumulation of charge on the drum that constitutes the latent image. A representative printhead generally includes a first collection of drive electrodes, e.g., RF-line electrodes, oriented in a first direction across the printing direction. A second collection of control electrodes, e.g., finger electrodes, oriented transversely to the drive electrodes, forms cross points or intersections with the drive electrodes constituting an array of charge generating sites at which charges originate. A dielectric layer couples to, and physically and electrically separates and insulates, the RF-line electrodes from the finger electrodes. 
     The printhead can also include a third electrode structure, often identified as a screen electrode. This screen electrode couples to the finger electrodes by an insulating structure, such as a spacer layer. The screen electrodes have a plurality of passages aligned with the charge generating sites, to allow the stream of charge carriers to pass through. The screen electrode can be a single conductive sheet having an aperture aligned over each charge generating site. Polarity of charge carriers passing through the passages, or apertures, depends on the voltage difference applied to the finger and screen electrodes. Polarity of particles accumulated on the drum to create latent image is determined by the voltage difference between the screen electrode and the drum surface. The charged particles of appropriate polarity are inhibited from passing through the aperture, depending upon the sign of their charge, so that the printhead emits either positive or negative charge carriers, depending on its electrode operating potentials. 
     One issue associated with current printing technology is that there is a significant size variation in dots landing on the drum. For example, conventional printheads have typically from 12 to 20 RF-line electrodes. Charge carriers that are generated from the outermost RF-lines deposit on the cylindrical drum in the form of a dot that is relatively smaller than those dots resulting from charge carriers emitted from more centrally located RF-line electrodes. This is because of a difference in distance between the outermost RF-lines and the curved surface of the drum, and the innermost RF-lines and the curved surface of the drum. Charge carriers emitted from the outermost RF-lines travel in a weaker electric field and must overcome greater distance to reach the drum surface than charge carriers emitted from the innermost RF-lines. The variation in the travel conditions causes this anomaly. The minimum air gap, and therefore the maximum electric field in between the screen and the dielectric drum, is normally directly beneath the more central RF-line electrodes. With decreasing drum diameters, the variations become increasingly severe because the curvature of the drum surface increases. 
     To compensate for the dot size variations, some prior art solutions have included enlarging the charge emitting sites, i.e., screen holes, or increasing the number of cycles incorporated in a single RF burst. Such compensation methods are unique to a particular drum/printhead combination, and do not compensate for blooming effects. 
     Blooming is essentially spreading the charged particles around the targeted area. Such spreading is a result of repulsive electrostatic forces between arriving and already deposited charge particles. The level of blooming depends on a ratio of these repulsive forces and attractive forces created by the electric field in the printhead/drum region. The resulting blooming effect has a substantial impact on dot geometry. 
     The surface charging effect also slightly deflects dots, which are aimed nearby. If charge dot is to be deposited in the proximity of one or more charged dots that have already been laid down, the interaction between the particle beam and previously deposited charge results in the dot lateral shift. Because the printing order of dots is constant, similar conditions and dot quality repeat in each printed line. Therefore, all deviations are organized in the process direction, which reveals itself as streaks of different intensity of print. This effect is known as Venetian blinding. 
     Therefore, charge density profiles of the dot latent images still depend on the screen hole positions. Further, the respective differences vary with charge level. Such issues significantly deteriorate the print quality of the printhead for grayscale or color images. 
     SUMMARY OF THE INVENTION 
     For the foregoing reasons, there exists in the art a need for a universal printhead that functions independent of the diameter of the charge receiving dielectric drum, while concomitantly optimizing print quality and lessening the effects of blooming or Venetian blinding (print quality descriptors that are well known in the art). The present invention is directed toward further solutions in this art. 
     In accordance with one aspect of the present invention, a printhead is provided having a first layer of electrodes covered and sealed by a dielectric material. Further layered upon the dielectric material is a second layer of electrodes. Each of the electrodes from the first layer intersects with each of the electrodes from the second layer and forms a plurality of charge generating sites. The charge generating sites are generally disposed in only two rows. 
     In accordance with another aspect of the present invention, the first plurality of electrodes includes two elongate electrode RF-lines. The second plurality of electrodes includes a plurality of finger electrodes that are arranged in a plurality of rows. Each finger electrode is coupled to a separate contact pad. 
     In accordance with still another aspect of the present invention, the first plurality of electrodes includes two rows of RF-line electrodes that are broken into sections or segments. The second plurality of electrodes includes a plurality of finger electrodes that are arranged in a plurality of rows. There is a single contact pad coupled to each of a subset of the second plurality of electrodes. 
     In accordance with yet another aspect of the present invention, the first plurality of electrodes includes a plurality of collector electrodes that are coupled to relatively shorter segments of said plurality of finger electrodes. The second plurality of electrodes includes a plurality of finger electrodes. Pairs formed from the plurality of electrodes are coupled to a single contact pad. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned features and advantages, and other features and aspects of the present invention, will become better understood with regard to the following description and accompanying drawings, wherein: 
     FIG. 1 is a diagrammatic illustration of an image forming system; 
     FIG. 2 is a diagrammatic cross section of a charge generating site in a printhead of the image forming system; 
     FIG. 3 is a schematic illustration of a collection of charge generating sites formed in the printhead of the present invention; 
     FIG. 4 is a schematic illustration of an alternate combination of charge generating sites formed in the printhead of the present invention; and 
     FIG. 5 is a schematic illustration of yet another alternate combination of charge generating sites formed in the printhead of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally relates to a universal printhead mounted in an image forming system. A characteristic of the universal printhead is that there exists a two-row arrangement of all charge generating sites. These rows of charge generating sites are substantially parallel to a dielectric drum axis. This geometry provides for printhead adjustment where all screen holes are evenly spaced from the dielectric surface. Therefore, electric fields inside the printhead cavities, as well as in the space between the screen and the dielectric drum, are substantially the same and homogeneous charge emission exists over the entire printhead area. Such an arrangement significantly reduces Venetian blinding effect that commonly arise during printing uneven dots. The structure is independent of the surface curvature of the drum, and of the charging level. 
     Drawings, throughout FIGS. 1 through 5, illustrate an image forming system and several example embodiments of a universal print head employed in accordance with the teachings of the present invention. Although the present invention will be described with reference to the example embodiments illustrated in the figures, it should be understood that many alternative forms can embody present invention. One of ordinary skill in the art will additionally appreciate different ways to alter the parameters of the embodiments disclosed, such as the size, shape, or type of elements or materials, in a manner still in keeping with the spirit and scope of the present invention. 
     Further, the image forming system is illustrated solely to provide a general structure into which the present invention can fit. It is wholly anticipated that other systems or charge transfer apparati can be utilized in combination with different embodiments of the present invention. 
     Describing an image forming system  10  in detail shown in FIG. 1, generally a drum  12  mounts for rotation about an axis  13 . The drum  12  incorporates an electrically conductive core  14 , coated with a dielectric layer  16 . The dielectric layer  16  receives a charge image from a printhead  18 . A controller  20  drives the print head  18  as desired. As the drum  12  rotates in the direction of the arrow shown, charge generating sites  66  (see FIG. 3) within the printhead  18  generate charges projected to the dielectric layer  16  on the outer surface of the drum  12 . The drum  12  continues to rotate and the dielectric layer  16  becomes exposed to toner particles  23  supplied from a hopper  24  through a feeder  26 . The toner particles  23  electrostatically adhere to the charged image on the dielectric layer  16  to form a toner image. The rotating drum  12  then carries the toner image towards a nip formed with a pressure roller  28 . The pressure roller  28  has an outer layer  30  positioned in the path of a receptor, such as a paper sheet  32 . The paper sheet  32  enters between a pair of feed rollers  34 . The pressure in the nip is sufficient to cause transfer and affixation of the toner particles  23  to the paper sheet  32 . The paper sheet  32  continues through and exits between a pair of output rollers  36 . After passing through the nip between the drum  12  and the pressure roller  28 , a scraper blade assembly  38  removes any toner particles  23  that may remain on the dielectric layer  16 . An eraser  39  positioned between the scrapper blade assembly  38  and the printhead  18  removes any residual charge remaining on the dielectric layer  16  surface. 
     A portion of the printhead  18  (see FIG. 1) representing a single charge emitting site is illustrated in FIG.  2 . The printhead  18  includes a layer of first electrodes, the RF-line electrodes  52 , covered and sealed by a dielectric layer  54 . On the opposite side of the dielectric  54  is disposed a second set of electrodes, the finger electrodes  56 , according to one embodiment. The printhead  18  can also include a spacer layer  53  supporting a screen electrode  55 . The screen electrode  55  aids in the proper alignment of the emitted charge carriers as is understood by one of ordinary skill in the art. The drum  12  is shown disposed below the printhead  18 . It should be noted that the particular combination of electrodes as illustrated herein is merely one example embodiment of the present invention and additional combinations of layers, as understood by one of ordinary skill in the art, can be achieved. 
     The general arrangement of the electrode layers that form a portion of the printhead  18  is described with reference to FIG. 3, which is a top view of the printhead  18  of FIG.  1 . This illustration does not include any screen electrodes, or spacers supporting screen electrodes, in an effort to simplify the disclosure. RF-line electrodes  58  (positioned in a like manner to the first electrode layer  52  of FIG. 2, although this illustration shows the printhead  18  inverted) are provided underneath a dielectric layer  60  (similar to dielectric layer  54  of FIG.  2 ). The RF-line electrodes  58  terminate at portions (not shown) connected to contacts ultimately in communication with the controller  20 . The RF-line electrodes  58  extend in a substantially parallel manner across a length of the dielectric layer  60 . There is a substantially even separation distance between each of the two RF-line electrodes  58 . The dielectric layer  60  covering the RF-line electrodes  58  can be made of any number of materials, such as most typically mica, Si-based polymers, silicon oxide, aluminum oxide, and the like. Layered atop the dielectric layer  60 , and coupled thereto, is a layer of finger electrodes  62  (positioned in a like manner to the second electrode layer  56  of FIG.  2 ). Finger electrodes  62  terminate at finger contact pads  64 , ultimately in communication with the controller  20 . The finger electrodes  62  extend across the dielectric layer  60 , and both of the RF-line electrodes  58 . At locations where the finger electrodes  62  cross the RF-line electrodes  58 , intersections form. These intersections form the charge generating sites  66 , as shown in FIG. 2 required for the transmission of an electric charge image to the dielectric layer  16  of the drum  14  depicted in FIG.  1 . These intersections are generally disposed in two rows. 
     In prior known arrangements for printhead configurations, there is most typically a plurality of intersections arranged in a plurality of rows or other array configurations. This leads to some potential issues with the charge generating sites being of unequal distances from the dielectric layer  16 , upon which the charges are being projected, due to its curvature, as previously discussed. With arrangement of the charge generating sites  66  into only two rows, the distance between the charge generating sites and the dielectric layer  60  is uniform, no matter how distally the two rows are spaced. More specifically, the distance between the charge emitting sites  66  and the dielectric  16  is uniform regardless of the dielectric curvature. 
     In FIG. 4, an additional configuration is illustrated. In this configuration, RF-line electrodes  72  are segmented with provided spaces  77  between them. The segmentation of the RF-line electrodes  72  allows for the option of multiplexing the electrodes either by outside circuitry or by circuitry placed directly in the printhead  18 . Covering the RF-line electrode  72  is again, a dielectric layer  75 . Layered upon the dielectric layer  75  are finger electrodes  74 . Corresponding to the RF-line electrode  72  segmentation, the finger electrodes are grouped into sections mutually interconnected with the finger collector lines  76 . Each of the finger collector lines  76 , representing many of finger electrodes, requires only a single contact pad to communicate with the controller  20 . In the particular embodiment shown, there are eight finger electrodes  74  for each segment of RF-line electrodes  72 . One finger electrode  74  of each segment  72  is connected together by a single collector line  76  (shown as A, B, C, D, E, F, G, and H). This results in each collector line  76  of lines A through H, having only a single finger electrode  74  crossing with each segment of RF-lines  72 . Such arrangement allows for second level multiplexing of the charge emitting sites. 
     Furthermore, reduction in number of the contact pads eases manufacturing constraints typically associated with forming print heads for high density print, when a large number of pads needs to be placed into a relatively small space. 
     Another embodiment of the present invention is illustrated in FIG.  5 . In this particular aspect of the invention, the RF-lines are broken into small sections  78 . Sets of RF-line electrode sections  78  are connected together by four collectors  80 ,  82 ,  84 , and  86 . Similar to structures shown in FIGS. 3 and 4, a dielectric layer  88  covers the RF-line sections  78 . Layered upon the dielectric layer  88  is a collection of finger electrodes in the form of finger electrode pairs  90 . The finger electrode pair  90  comprises a finger electrode that couples with a second finger electrode, and then couples with the contact pad  92 . Thus, signals sent to each contact pad  92  further convey to both electrodes of each finger electrode pair  92 . Each finger electrode pair  92  creates four intersections or charge generating sites  94  with four different RF-line electrode sections  78 , one from each of the collectors  80 ,  82 ,  84 , and  86 . 
     Again, the arrangement of crossings of the RF-line electrode sections  78  with the finger electrode pairs  90  results in two rows of charge generating sites  94 , in accordance with the aspects of the present invention. In this arrangement, the contact pads  92  can be ultimately placed on opposite sides of the printhead  18  to decrease their density. 
     It should be noted that the aforementioned configurations and embodiments are only examples of viable solutions. Advantages described herein apply for any kind of printhead where charge generating sites are organized into only two rows according to the present invention, regardless of the kind and shape of individual charge generators. The same principles are valid when considering modular printhead made from a series of small printhead modules. It should also be noted that similar results are attained with printheads where geometries of the finger electrodes are utilized for the RF-line electrodes and vice versa. 
     A significant advantage of the present invention is that in maintaining two rows of charge generating sites, the printhead  18  is not dependent on the shape of the image receiving dielectric layer  60 . With additional rows of charge generating sites beyond two, the distances between those sites and the dielectric drum surface continuously increases. However, with only two rows of charge emitting loci, the corresponding distances to the dielectric drum surface are substantially the same, thereby drastically reducing the problems associated with dielectric drum curvature. This is why there is a substantial elimination of so-called Venetian blinding affect for all charging levels, and an equalization of the blooming effect for all charge generating sites. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and for teaching those skilled in the art the best mode for carrying out the invention. Details of the structure may vary substantially without departing from the spirit of the invention, and exclusive use of all modifications that come within the scope of the appended claims is reserved. It is intended that the invention be limited only to the extent required by the appended claims and the applicable rules of law.