Abstract:
A belt drive operative to rotate one or more photoconductor drums while carrying media sheets past the rotating drum or drums. For single pass color printers in which the different color planes are developed using a series of photoconductor drums, a substantially flat movable belt is positioned adjacent to and extends across each of the drums. The belt simultaneously engages each drum so that movement of the belt past the drums causes the drums to rotate together as the belt carries media sheets past the rotating drums. In one preferred version of the invention, grit applied to the ends of the drum(s) and/or to the edge of the belt is used to transfer driving force from the belt to the drums.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a system for reducing banding and color plane registration errors in an image forming device and, more particularly, to a belt drive operative to rotate one or more photoconductor drums while carrying media sheets past the rotating drum or drums. 
     BACKGROUND 
     To print in color, electronic data representing a desired print image is initially separated into four distinct color planes; one cyan, one magenta, one yellow, and one black. Single pass full color laser printers generally include a sequence of developer stations each responsible for producing one color plane image. As a sheet of paper or other suitable media passes through the first developer station, the first color plane of the separated print image is applied. The full color print image is formed as the paper passes through the other three developer stations with each of the remaining color plane images being superimposed over the first. 
     Each developer station includes an insulating photoconducting material usually placed on a drum and a light source such as a scanning laser. Repeatedly scanning a beam across the drum in a series of precise lines, the scanning laser creates a latent image corresponding to one color plane on the drum&#39;s surface by selectively exposing areas of the photoconductor drum to light. A difference in electrostatic charge density is created between the areas on the drum exposed and not exposed to light. The visible image is developed by electrostatic toners. The photoconductor drum may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. The toners are selectively attracted to the portions of the photoconducting surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor drum and the toner. A transfer roller is given an electrostatic charge opposite that of the toner and is rotated close to the photoconductor drum. The transfer roller pulls the toner from the surface of the photoconductor drum onto a sheet of paper or other print media in the pattern of the color plane image developed from the photoconducting surface. The full color image is produced as each color plane image is transferred and fused to the media sheet. 
     Since laser printers are designed to run very quickly, problems can arise with even slight variations in photoconductor drum speeds. Variations in photoconductor drum speed appear on the printed page as increased or decreased spacing between lines and visually appear as “bands.” Banding can be a particularly severe problem for laser printers when printing full color images such as photographs. In addition to banding, varying photoconductor drum speeds cause color plane registration errors. To produce accurate color prints, each successive color plane image must be precisely aligned and superimposed over the prior color plane. Color plane registration errors exceeding as little as about fifty microns, for example, produces a detectable degradation in print quality. 
     The principle cause of varying drum speeds is gear noise. Photoconductor drums are generally driven by a stepper motor or a brushless DC motor in connection with a gear array. Gear noise results from imperfect spacing of gear teeth, variances in flexing of gear teeth as forces are transferred from one gear to the next, and other intrinsic variations in gear force transfer. The stepper motor can also contribute to the problem because, as it drives the gear array in a laser printer, it may have slight variations in angular velocity due to the multiple magnet positions for each step. 
     Past solutions to banding and color plane registration errors include providing helical gears or gears made of better materials or with greater precision. These gears add significantly more expense to the final product. Solutions specific to correct banding include providing sensors to detect undesired variations in the photoconductor drum speed and additional circuitry directed to compensate the modulation of the laser accordingly. Solutions specific to registration errors include sensing variations in the speeds of the photoconductor drums and correspondingly adjusting the timing of the scanning lasers to correct the placement of each color plane image. Unfortunately, each of these solutions, requiring additional circuitry, sensing capabilities, and precision components and machining, substantially increase the manufacturing costs of an image forming device. 
     SUMMARY 
     The present invention is directed to a belt drive operative to rotate one or more photoconductor drums while carrying media sheets past the rotating drum or drums. For single pass color printers in which the different color planes are developed using a series of photoconductor drums, a substantially flat movable belt is positioned adjacent to and extends across each of the drums. The belt simultaneously engages each drum so that movement of the belt past the drums causes the drums to rotate together as the belt carries media sheets past the rotating drums. In one preferred version of the invention, grit applied to the ends of the drum(s) and/or to the edge of the belt is used to transfer driving force from the belt to the drums. Conventional single pass color printers that use a paper transport belt to carry paper through the developer stations can be readily adapted to use the invention. It is expected that the invention will provide a cost effective alternative to conventional methods for reducing banding and registration errors. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a representational side sectional view of a single pass color printer incorporating the present invention. 
     FIG. 2 is a schematic side sectional view of media sheet transport belt driven photoconductor drums according to one embodiment of the present invention. 
     FIG. 3 is schematic view showing grit applied to the ends of the photoconductor drum to transfer driving force from the belt to the drum. 
     FIG. 4 is a schematic view showing grit applied to the sides of the transport belt to transfer driving force from the belt to the drum. 
     FIG. 5 is a schematic view showing grit applied to ends of the photoconductor drum and to the sides of the transport belt. 
    
    
     DETAILED DESCRIPTION 
     Although it is expected that the invention will be most useful in electrophotographic printing devices such as the single pass color laser printer illustrated in FIG. 1, the invention may be suitable for use in any single or multiple drum printer, copier, or other image forming device in which it is necessary of desirable to closely coordinate drum rotation. 
     FIG. 1 illustrates a single pass color laser printer, designated by reference number  10 . Color laser printer  10  includes a media sheet transport system that includes media tray  12 , pick roller  14 , follower rollers  16 ,  18 , transport belt  20 , tensioning rollers  22 ,  24 , and drive rollers  26 . Pick roller  14  picks a media sheet  30 , typically a sheet of paper, from tray  12  and advances sheet  30  toward follower rollers  16  and  18 . Sheet  30  is grabbed between follower rollers  16 ,  18  and transport belt  20  and carried past developer stations  32 ,  34 ,  36  and  38  on upper run  20   a  of belt  20 . Drive rollers  26  or another suitable mechanism engage the lower run  20   b  of belt  20  urging belt  20  to circulate around tensioning rollers  22 ,  24 . 
     The desired print image is applied to each sheet  30  at developer stations  32 ,  34 ,  36 , and  38 . Each developer station is the same except that each contains a different color toner and is responsible for transferring a different color plane image to media sheet  30 . For instance, developer station  32  contains black toner (K), developer station  34  contains yellow toner (Y), developer station  36  contains magenta toner (M), and developer station  38  contains cyan toner (C). Each developer station includes a photoconductor drum  40 , a charge roller  42 , a scanning laser  44 , a developer roller  46  and a transfer roller  47 . Each drum  40  is placed adjacent to one transfer roller  47  with transport belt  20  passing between the two. The toner supply for each developer station is maintained within a reservoir  48 . 
     In operation, as belt  20  carries media sheet  30  toward the black developer station  32 , charge roller  42  places a relative and uniform electrical charge on photoconductor drum  40 . Repeatedly scanning a light beam horizontally across photoconductor drum  40  in a series of precise lines, scanning laser  44  creates a latent image of the corresponding color plane, in this case black, on the surface of photoconductor drum  40  by selectively discharging portions of photoconductor drum  40  according to the black color plane image. A difference in electrostatic charge density is created between the areas on drum  40  exposed and not exposed to the beam. Each color plane image is developed by electrostatic toners. As photoconductor drum  40  rotates the charged image, it passes by developer roller  46  enabling toner to be taken up from roller  46  onto the exposed or not exposed portions of photoconductor drum  40  depending upon the relative electrostatic charges of drum  40  and the toner. Thereafter, the toner image is rotated into contact with media sheet  30  which is pressed between photoconductor drum  40  and adjacent transfer roller  47 . Transfer roller  47  is given an electrostatic charge opposite that of the toner. As media sheet  30  passes between photoconductor drum  40  and transfer roller  47 , transfer roller  47  pulls the toner onto media sheet  30 . The desired full color image is created as media sheet  30  passes through the remaining developer stations  34 ,  36 ,  38 , each functioning in a substantially identical manner. Once each color plane image is transferred to media sheet  30 , the toner is fused to media sheet  30  as the sheet passes between heated fusing rollers  50 , and media sheet  30  is released to output bin  52 . 
     Referring now to FIGS. 2 and 3, instead of being individually driven by a separate motor and gear array as in a conventional printer, photoconductor drums  40  are commonly rotated by transport belt  20 . Belt  20  engages each of the drums  40  simultaneously so that movement of belt  20  past drums  40  cause the drums to rotate while belt  20  carries media sheets  30  past the rotating drums. The transfer driving force from belt  20  to drums  40  is improved through grit  54  applied to one or both ends of drums  40 . Grit  54  increases friction between photoconductor drums  40  and transport belt  20 . It may be desirable in some operating environments that grit  54  extend in far enough to overlap media sheet  30 . The increased friction on sheet  30  allows photoconductor drum  40  to secure the margins of media sheet  30  against belt  20  to help prevent media sheet  30  from slipping as belt  20  carries it past drum  40 . The increased friction between photoconductor drum  40  and transport belt  20  also helps prevent photoconductor drum  40  from slipping as transport belt  20  circulates. As shown in FIG. 3, grit  54  is applied to contact and overlap transport belt  20  and, preferably, the margins of media sheet  30  as belt  20  carries media sheet  30  past photoconductor drum  40 . 
     In an alternative embodiment of the invention shown in FIG. 4, grit  56  is applied along one or both edges on the outer surface  58  of belt  20 . As above, grit  56  provides increased friction between photoconductor drum  40 , media sheet  30 , and transport belt  20 . Again, the size and placement of grit  56  can be selected to allow grit  56  to contact and overlap both media sheet  30  and photoconductor drum  40  as belt  20  transfers media sheet  30  past drum  40 . Grit  54 ,  56  can be formed by depositing and affixing a plurality of grit particles such as sand, metal flake, rubber bits, or other suitable material to selected surface areas of photoconductor drum  40  and transport belt  20 . Alternatively, grit  54 ,  56  can be formed by roughening, machine etching, grinding, or cutting those surface areas of transport belt  20  and photoconductor drums  40 . It is expected that a random distribution of grit particles applied to one or both ends of drum  40  in which the spacing between grit particles is less than ⅓ mm will be sufficient to transfer adequate driving force from belt  20  to drum  40 . A random distribution of grit particles is preferred to help prevent the engagement of the belt to the drums from creating continuous frequencies that might contribute to banding. Also, it has been observed that bandwidths ⅓ mm or less are not visible when the printed sheets are viewed from 20 inches or more. Hence, it is expected that grit particles spaced ⅓ mm or less should be sufficient to prevent banding that is visible from 20 inches or more. 
     Although the present invention has been shown and described with reference to the foregoing exemplary embodiments, other embodiments are possible. For example, the invention could be used in a monochrome printer having only one photoconductor drum. One of the tension rollers  22  or  24  could be used to drive the belt in lieu of separate drive rollers  26 . As shown in FIG. 5, grit may be applied to both drum  40  and belt  20  as may be necessary or desirable to transfer adequate driving force to drum  40 . The type, size and density of the grit particles will likely vary depending on the particular printing device and environmental conditions. Other friction enhancement/driving force transfer mechanisms might also be used. The extent of the engagement between belt  20  and drums  40 , the normal force exerted at the point of engagement between belt  20  and drums  40  and other factors may influence the specific characteristics of the engagement between belt  20  and drums  40 . It is to be understood, therefore, that other forms, details, and embodiments may be made without departing from the spirit and scope of the invention which is defined in the following claims.