Patent Publication Number: US-6707479-B1

Title: Apparatus and methods of printing on an electrically writable medium

Description:
TECHNICAL FIELD 
     This invention relates to apparatus and methods of printing on an electrically writable medium. 
     BACKGROUND 
     Many companies are developing electronic paper, which is a display system that retains images with little or no power. Images typically are generated on an electronic paper medium by selectively applying an electric field to switchable display elements (e.g., dichroic spheres) in localized regions of the medium. In a typical implementation, an electrically conductive backplane electrode is placed behind the electronic paper medium and a second electrically conductive front plane electrode is placed in front of the electronic paper medium. Applying an electric field of one polarity to the medium switches the display elements to one orientation (e.g., black-side-up), and reversing the polarity of the applied electric field switches the display elements to a second orientation (e.g., white-side-up). A two-dimensional electrode grid with individually addressable cells may be used to provide an electric field in selected areas of the electronic paper medium. Alternatively, a single electrode may be scanned across the electronic paper as the paper is advanced by a roller system. The electronic paper medium remains in the switched (or “printed”) state after the electric field is removed, until a new electric field is applied to change the orientation of the display elements. 
     One known electrode array printer for printing on rewritable electronic paper includes an array of independently addressable electrodes, each capable of applying a localized field to the rewritable media to rotate dichroic spheres within a given pixel area of a rewritable medium. In another known electrically writable media printing technique, a laser scanner is used to erase a uniform high-voltage charge that was deposited on the surface of a photoconductor drum or belt. The voltage swing between charged and discharged areas of the photoconductor is conventionally on the order of about 500-600 volts. When the rewritable medium is brought in contact with the charge-written photoconductor through a biased back electrode roller, electric fields that are generated between the photoconductor and back electrode cause color rotation of the dichroic spheres to develop a desired print image. 
     SUMMARY 
     In one aspect, the invention features a printer for printing on an electrically writable medium. The printer includes a print head and a biasing system. The print head has multiple solenoid-actuated print wires that are operable to reciprocate toward and away from the medium. The biasing system is coupled to the print head and is operable to apply through print wires extended toward the medium an electric field that is greater than a threshold electric field needed to reorient switchable display elements in a localized region of the medium. 
     In another aspect, the invention features a method of printing on an electrically writable medium in which multiple solenoid-actuated print wires are reciprocated toward and away from the medium. An electric field, which is greater than a threshold electric field needed to reorient switchable display elements in a localized region of the medium, is applied through print wires extended toward the medium. 
    
    
     Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is diagrammatic cross-sectional side view of an implementation of an electrically writable medium. 
     FIG. 2A is a diagrammatic front view of an embodiment of a print head that includes an array of two columns of print wires. 
     FIG. 2B is a diagrammatic front view of an embodiment of a print head that includes a linear array of print wires. 
     FIG. 2C is a diagrammatic cross-sectional side view of an embodiment of a print wire that is mounted to a solenoid-actuated plunger of a solenoid coil assembly. 
     FIG. 3 is a diagrammatic perspective view of an embodiment of a printer that incorporates the print head embodiment of FIG. 2A and a cylindrical platen. 
     FIG. 4A is a diagrammatic side view of an embodiment of a print wire that is retracted away from a region of an electrically writable medium. 
     FIG. 4B is a diagrammatic side view of an embodiment of a print wire that is extended toward and is in contact with the medium of FIG. 4A to apply an electric field through a localized region of the medium. 
     FIG. 5 is a diagrammatic perspective view of the printer embodiment of FIG. 3 with an inked ribbon cartridge holding an inked ribbon between the print head and the platen. 
     FIG. 6A is a diagrammatic side view of an embodiment of a print wire that is retracted away from an inked ribbon that is disposed between the print wire and a platen. 
     FIG. 6B is a diagrammatic side view of an embodiment of a print wire that is extended toward and is in contact with the inked ribbon of FIG. 6A to impress the inked ribbon against a localized region of a substrate that is disposed between the inked ribbon and the platen. 
     FIG. 7 is a block diagram of a printer that includes a substrate type detector, an erasing station, and a print head. 
     FIG. 8 is a diagrammatic view of an ink supply coupled to a print wire that has an ink supply channel for delivering ink to a substrate. 
    
    
     DETAILED DESCRIPTION 
     In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
     Multiple embodiments of printers are described in detail below. Each of these printer embodiments is operable to print on electrically writable media. In general, these printer embodiments may print on any type of medium that includes display elements that are electrically switchable in localized regions of the medium to produce an image. Exemplary switchable display elements include bi-stable, dual-color microcapsules, dichroic spheres, and optically anisotropic colorant particles. 
     Referring to FIG. 1, in some embodiments, an electrically writable medium  10  includes at least one colorant layer  12  that is disposed between a pair of protective layers  14 ,  16 . In the illustrated embodiment, the colorant layer  12  is formed from a polymer binder and a plurality of switchable display elements that are implemented in the form of bi-stable, dual-color microcapsules  18 . Each microcapsule  18  includes a solid bi-colored sphere  20  housed in a microencapsulating shell  22 . Each microcapsule sphere  20  is coated with a lubricating fluid. Each sphere  20  is colored white on one hemisphere and colored black on the opposing hemisphere. The black colorant may be vapor-deposited, for example, on a solid white sphere that may be made of, for example, a pigmented glass, a polymer, or a ceramic. The vapor deposit contains charge species that give each of the spheres  20  an electric dipole for field alignment. The resulting charge on each bi-colored sphere allows the bi-colored spheres  20  to be oriented in accordance with an applied electric field so that each sphere  20  presents either the white hemisphere face or the black hemisphere face at the top surface of the electrically writable medium. The microcapsules  18  may be supported in a fixed polymer coating layer, while allowing each microcapsule sphere  20  to rotate within the microencapsulating shell  22 . The electrically writable medium  10  preferably contains a sufficient density of microcapsules  18  so that the electrically writable medium  10  appears completely white or completely black when all of the microcapsules  18  are oriented in the same direction. 
     In general, protective layer  14  may be formed of any flexible, fibrous or non-fibrous sheet material. In some embodiments, the protective layer  14  of electrically writable medium  10  has the look and feel of paper, but has far greater durability than most, commonly-used cellulose fiber papers. Such media are known in the art, and commonly consist of polymeric impregnated papers or polymeric fibers woven or assembled into films that have a paper appearance. Examples of such papers include Tyvek® (available from E. I. du Pont de Nemours and Company of Wilmington, Del., U.S.A.) and a series of Master-Flex™ papers (available from Appleton Papers Inc. of Appleton, Wis., U.S.A.). 
     Top protective layer  16  is optional and may be coated over the colorant layer  12  to increase the durability of electrically writable medium  10 . Protective layer  16  may be formed of a transparent polymer, such as PMMA (polymethylmethacrylate), or a blend of polymers. In some embodiments, the polymer binder and microcapsule shells  20  have matching refractive indices to minimize light scattering within the colorant layer  12 , improving image contrast. The gloss of the electrically writable medium  10  may be controlled by the characteristics of the colorant layer  12  or the optional protective layer  16 , or both. In some embodiments, the refractive indices of protective layer  16  and colorant layer  12  may be mismatched to enhance the “white paper” mode by inducing additional light scattering to enhance whiteness. 
     Referring to FIGS. 2A-2C, some printer embodiments may incorporate a serial print head  24  that is operable to “print” (or form an image) on electrically writable medium  10 . Other printer embodiments may incorporate a linear print head  26 . Serial print head  24  and linear print head  26  may correspond to conventional dot matrix print heads that each includes an additional biasing system that is operable to apply through print wires  28  that are extended toward electrically writable medium  10  an electric field that is greater than the threshold electric field need to reorient the switchable display elements of electrically writable medium  10 . In the illustrated embodiment, serial print head  24  has an array of eighteen print wires  28  that are arranged in two vertical columns; other embodiments may include a greater number (e.g., 24) or a lesser number (e.g., 7 or 9) of print wires  28 . The number of print wires in linear print head  26  also may be different from the eighteen print wires  18  in the illustrated embodiment. Serial print head  24  may be scanned across the width of electrically writable medium  10  so that discrete regions of electrically writable medium  10  may be printed in series; whereas linear print head  26  may be configured to simultaneously print on a linear region extending across the entire width of electrically writable medium  10   
     As shown in FIG. 2C, in the illustrated embodiment, each print wire  28  has a distal end  30  that is operable to apply an electric field to electrically writable medium  10  and a proximal end  32  that is connected to a plunger  34 . Plunger  34  is disposed along the central axis of a cylindrical coil assembly  35  that includes a solenoid coil  36 . A ring core  38  limits the outward extension of print wire  28  and a preloaded plunger-restoring disk spring  40  maintains the print wire  28  in a retracted position when the solenoid coil  36  is not energized. In operation, when a printing pulse is applied to the solenoid coil  36 , the plunger  34  is attracted to forwardly against the resiliency of disk spring  40 . As a result, print wire  28  is driven axially forward into an extended state. Upon termination of the printing pulse, the plunger  34  and the print wire  28  are restored to their initial, rest position under the action of the disk spring  40 . 
     Referring to FIG. 3, in one exemplary embodiment, a printer  23  includes a serial print head  24  that is mounted to a low friction slide  42 . Low friction slide  42  is mounted to a pair of carriage rails  44 ,  46  of a carriage assembly  48 . The print head  24  is moved across the length of a cylindrical platen  50  by a belt  52  that is connected to slide  42  and to a drive motor  54  through a drive pulley  56 . When the drive motor  54  turns in a clockwise direction the slide  42  is pulled to the right, and when drive motor  54  the motor turns in a counterclockwise direction the slide  42  is pulled to the left. Conventional limit switches may be used to prevent the slide  42  from being pulled too far in either direction. The rotation of feed platen  50  and the drive motor  54  may be controlled by conventional serial printer control electronics (not shown). 
     A biasing system  58  is coupled to the serial print head  24  through an electrical interface on slide  42  and to platen  50 . In the illustrated embodiment, the external surface of platen  50  is electrically conductive. Biasing system  58  is operable to generate between the external platen surface and the distal ends of print wires that are extended toward the platen  50  an electric field that is greater than the threshold electric field needed to reorient the switchable display elements of an electrically writable medium that is disposed between the print head  24  and the platen  50 . A wide variety of different voltage combinations may be applied by biasing system  58  to platen  50  and print wires  28  to achieve the necessary electric field strength. 
     In some embodiments, the print wires  28  are operable to contact an electrically writable medium that is disposed between print head  24  and platen  50 . In other embodiments, the print wires  28  are operable to apply the necessary electric field strength without contacting an electrically writable medium that is disposed between the print head  24  and the platen  50 . In some embodiments, the biasing system  58  is operable to maintain the print wires  28  in a biased (or “writing”) state during the entire printing process. In other embodiments, the biasing system  58  is operable to bias print wires  28  each time they are individually actuated for printing at respective localized regions of the electrically writable medium. 
     Referring to FIGS. 4A and 4B, in operation, electrically writable medium  10  is fed between print head  24  and platen  50  by a sheet feed system  60  that includes a drive roller  62  and an idler roller  64 . As the electrically writable medium  10  is being fed, the print wires  28  are in a retracted state (FIG. 4A) until they are actuated for printing. When a localized region of electrically writable medium  10  is to be printed by a print wire  28 , the printer control electronics transmits a printing pulse to the solenoid coil assembly corresponding to the print wire  28 . In response, the print wire  28  is driven toward and into contact with the electrically writable medium  10  (FIG.  4 B). As explained above, biasing system  58  may maintain the printing wire  28  in a biased state during the entire printing process; alternatively, the biasing system  58  may bias printing wire  28  to an appropriate voltage level only during the time that the printing pulse is applied. After the necessary electric field has been applied to the desired localized region of electrically writable medium  10 , the printing pulse is terminated and the print wire  28  is returned to the retracted state (FIG.  4 A). 
     Referring to FIG. 5, in some embodiments, printer  23  includes a conventional, removable inked ribbon cartridge  66  (shown diagrammatically in FIG. 5) that is configured to hold an inked ribbon  68  between platen  50 -and print head  24  when mounted within printer  23 . Inked ribbon cartridge  66  includes a supply reel  70  for supplying unused inked ribbon and a take-up reel  72  for taking-up used inked ribbon. These embodiments provide a two printing modes that enable printer  23  to print on a wide variety of different substrates, including electrically writable media and conventional paper-like substrates. As used herein, the term “substrate” encompasses any support material that can be printed on either by application of an electric field or by application of a marking substance (e.g., ink). Examples of substrate material can include but are not limited to paper, plastic (e.g., transparency), photographic paper, and electrically writable material. A substrate can come in the form of a sheet or can be a continuous substrate (e.g., paper rolls). In these embodiments, inked ribbon cartridge  66  preferably is mounted in a cartridge assembly that is operable to selectively move the inked ribbon cartridge  66  into and out of position with respect to the print head  24  and the platen  50  based on the type of substrate that is loaded into the printer. For example, if an electrically writable medium is loaded into printer  23 , then the cartridge assembly maintains the inked ribbon cartridge  66  in a “standby” position where the inked ribbon  68  is outside of the region between print head  24  and platen  50 . On the other hand, if a paper substrate, for example, is loaded into printer  23 , then the cartridge assembly moves the inked ribbon cartridge into an “active” position where the inked ribbon  68  is held between the print head  24  and platen  50 . 
     Referring to FIGS. 6A and 6B, in operation, a conventional substrate  74  (e.g., a conventional sheet of paper) is fed between print head  24  and platen  50  by sheet feed system  60 . As the substrate  74  is being fed, the print wires  28  are in a retracted state (FIG. 6A) until they are actuated for printing. When a localized region of substrate  74  is to be printed by a print wire  28 , the printer control electronics transmits a printing pulse to the solenoid coil assembly corresponding to the print wire  28 . In response the print wire  28  is driven toward and into contact with the inked ribbon  68 , impressing the inked ribbon  68  against substrate  74  (FIG.  6 B). Biasing system  58  maintains the printing wire  28  in an unbiased state during the entire inked-ribbon-based printing process. After the inked ribbon  66  has transferred a dot of ink onto substrate  74 , the printing pulse is terminated and the print wire  28  is returned to the retracted state (FIG.  6 A). 
     In sum, the above-described embodiments may be implemented in a printer system that leverages existing printer technology with improvements that enable printing on a wide variety of different types of electrically writable media. In addition, some embodiments provide dual modes of printing in which the printer system is operable to print on both electrically writable media and conventional paper-like substrates. 
     Other embodiments are within the scope of the claims. 
     For example, although the above embodiments are described in connection with one exemplary type of electrically writable medium, these embodiments readily may be used with other types of electrically writable media, including electrically writable media that incorporate optically anisotropic particles having one or more colors in addition to or replacing one or more of the black and white colors, and electrically writable media in which protective layer  14  is electrically conductive and forms an electrically conductive backplane. In some printer embodiments that are designed for use with electrically writable media that have electrically-conductive backplanes, the external surface of platen  50  may be electrically-insulating. 
     In addition, although the above embodiments are described in connection with exemplary print head designs, other embodiments may be used with different print head designs. 
     Referring to FIG. 7, some embodiments may include upstream of the print head  24  a substrate type detector  82  that is operable to detect whether an electrically writable medium or a conventional print medium has been loaded for printing. For example, substrate type detector  82  may include a test electrode  84  that applies a bias to mark (e.g., produce a discernable color change in a localized region) a substrate sheet  86  that is being fed through the printer  23 . A sensor  88  (e.g., a photodetector), which is positioned downstream of the test electrode  84 , may detect whether the applied bias produced a test mark on the substrate and produce a signal indicative of the type of substrate that is loaded into the printer for printing. If the test mark is detected, the inked ribbon cartridge  66  is moved to the standby position before the print head  24  is used to print on the substrate. If the test mark is not detected, the inked ribbon cartridge  66  is moved to the active position before the print head  24  is used to print on the substrate. 
     As shown in FIG. 7, some embodiments may include an upstream erasing station  80  that includes, for example, a charged-electrode that is biased to orient all of the switchable display elements of an electrically writable medium in the same direction (e.g., white sides facing up) before an image is printed on the medium. 
     Referring to FIG. 8, instead of being based on inked-ribbon ink delivery systems, some dual-mode printer embodiments may be implemented based on ink-supplied wire printing systems in which ink is supplied to distal ends  90  of print wires  92  through respective ink channels  94 . The ink that is delivered to the distal ends  90  of print wires  92  may be applied directly to a substrate by selectively reciprocating print wires  92  into and out of contact with the substrate.