Method for sheet registration using a single sensor

Abstract: A deskewing and registering device for an electrophotographic printing machine. A single sensor (132) determines the position and skew of a sheet (11) in a paper path and generate signals indicative thereof. A pair of independently driven nips (114,116) forward the sheet (11) to a registration position in skew and at the proper time based on signals from a controller which interprets the position signals and generates the motor control signals. An additional single sensor (134) can be used at the registration position to provide feedback for updating the control signals as rolls wear or different substrates having different coefficients of friction are used.

Description: This invention relates generally to a sheet registration system, and more particularly concerns an accurate, apparatus and method for registering sheets in a high speed printing machine using only a single sensor. In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet. High quality documents require registration of sheets of paper or other substrate to the photoreceptor for image transfer. Accurate registration control locates the image consistently with respect to the edge of the paper. This invention describes a sheet registration apparatus and method which senses the position of a sheet at a first location and generates a set of control signals to cause the sheet to arrive at a second location in proper registry and skew. Some portions of the foregoing disclosures may be briefly summarized as follows: EP 0814040 describes a method of sheet registration and a sheet stacker with a sheet registration device. A linear optical sensor which extends in a direction transverse to the sheet travel direction is used to derive information on the sheet length and on the sheet registration error. The detector output contains all required information on the initial skew error and side registration error of the sheet. It is the object of the present invention to improve registering and deskewing of a sheet along a paper path. This object is achieved by providing a method for registering and deskewing a sheet according to claim 1. Embodiments of the invention are set forth in the dependent claims. Figure 1 is a schematic elevational view depicting an illustrative electrophotographic printing machine incorporating a sheet registration device of the present invention; Figure 2 is a detailed plan view of the sheet registration device described herein. Figure 3 is a detailed plan view of a second embodiment OF the sheet registration device described herein. Figure 4 is a detailed plan view illustrating the operation of the first embodiment of the sheet registration device described herein. Referring to Fig. 1 of the drawings, the electrophotographic printing machine employs a photoconductive belt 10. Preferably, the photoconductive belt 10 is made from a photoconductive material coated on a ground layer, which, in turn, is coated on an anti-curl backing layer. The photoconductive material is made from a transport layer coated on a selenium generator layer. The transport layer transports positive charges from the generator layer. The generator layer is coated on an interface layer. The interface layer is coated on the ground layer made from a titanium coated Mylar®. The interface layer aids in the transfer of electrons to the ground layer. The ground layer is very thin and allows light to pass therethrough. Other suitable photoconductive materials, ground layers, and anti-curl backing layers may also be employed. Belt 10 moves in the direction of arrow 12 to advance successive portions sequentially through the various processing stations disposed about the path of movement thereof. Belt 10 is entrained about stripping roller 14, tensioning roller 16, idler roll 18 and drive roller 20. Stripping roller 14 and idler roller 18 are mounted rotatably so as to rotate with belt 10. Tensioning roller 16 is resiliently urged against belt 10 to maintain belt 10 under the desired tension. Drive roller 20 is rotated by a motor coupled thereto by suitable means such as a belt drive. As roller 20 rotates, it advances belt 10 in the direction of arrow 12. Initially, a portion of the photoconductive surface passes through charging station A. At charging station A, two corona generating devices indicated generally by the reference numerals 22 and 24 charge the photoconductive belt 10 to a relatively high, substantially uniform potential. Corona generating device 22 places all of the required charge on photoconductive belt 10. Corona generating device 24 acts as a leveling device, and fills in any areas missed by corona generating device 22. Next, the charged portion of the photoconductive surface is advanced through imaging station B. At imaging station B, a raster output scanner (ROS), indicated generally by the reference numeral 26, discharges selectively those portions of the charge corresponding to the image portions of the document to be reproduced. In this way, an electrostatic latent image is recorded on the photoconductive surface. An electronic subsystem (ESS), indicated generally by the reference numerals 28, controls ROS 26. E S S 28 is adapted to receive signals from a computer and transpose these signals into suitable signals for controlling ROS 26 so as to record an electrostatic latent image corresponding to the document to be reproduced by the printing machine. ROS 26 may include a laser with a rotating polygon mirror block. The ROS 26 illuminates the charged portion of the photoconductive surface. In this way, a raster electrostatic latent image is recorded on the photoconductive surface which corresponds to the desired information to be printed on the sheet. Other types of imaging systems may also be used employing, for example, a pivoting or shiftable LED write bar or projection LCD (liquid crystal display) or other electro-optic display as the "write" source. Thereafter, belt 10 advances the electrostatic latent image recorded thereon to development station C. Development station C has three magnetic brush developer rolls indicated generally by the reference numerals 34, 36 and 38. A paddle wheel picks up developer material and delivers it to the developer rolls. When the developer material reaches rolls 34 and 36, it is magnetically split between the rolls with half of the developer material being delivered to each roll. Photoconductive belt 10 is partially wrapped about rolls 34 and 36 to form extended development zones. Developer roll 38 is a clean-up roll. A magnetic roll, positioned after developer roll 38, in the direction of arrow 12 is a carrier granule removal device adapted to remove any carrier granules adhering to belt 10. Thus, rolls 34 and 36 advance developer material into contact with the electrostatic latent image. The latent image attracts toner particles from the carrier granules of the developer material to form a toner powder image on the photoconductive surface of belt 10. Belt 10 then advances the toner powder image to transfer station D. At transfer station D, a copy sheet is moved into contact with the toner powder image. First, photoconductive belt 10 is exposed to a pre-transfer light from a lamp (not shown) to reduce the attraction between photoconductive belt 10 and the toner powder image. Next, a corona generating device 40 charges the copy sheet to the proper magnitude and polarity so that the copy sheet is tacked to photoconductive belt 10 and the toner powder image attracted from the photoconductive belt to the copy sheet. After transfer, corona generator 42 charges the copy sheet to the opposite polarity to detack the copy sheet from belt 10. Conveyor 44 advances the copy sheet to fusing station E. Fusing station E includes a fuser assembly indicated generally by the reference numeral 46 which permanently affixes the transferred toner powder image to the copy sheet. Preferably, fuser assembly 46 includes a heated fuser roller 48 and a pressure roller 50 with the powder image on the copy sheet contacting fuser roller 48. The pressure roller is cammed against the fuser roller to provide the necessary pressure to fix the toner powder image to the copy sheet. The fuser roll is internally heated by a quartz lamp. Release agent, stored in a reservoir, is pumped to a metering roll. A trim blade trims off the excess release agent. The release agent transfers to a donor roll and then to the fuser roll. After fusing, the copy sheets are fed through a decurler 52. Decurler 52 bends the copy sheet in one direction to put a known curl in the copy sheet and then bends it in the opposite direction to remove that curl. Forwarding rollers 54 then advance the sheet to duplex turn roll 56. Duplex solenoid gate 58 guides the sheet to the finishing station F, or to duplex tray 60. At finishing station F, copy sheets are stacked in a compiler tray and attached to one another to form sets. The sheets can be attached to one another by either a binder or a stapler. In either case, a plurality of sets of documents are formed in finishing station F. When duplex solenoid gate 58 diverts the sheet into duplex tray 60. Duplex tray 60 provides an intermediate or buffer storage for those sheets that have been printed on one side and on which an image will be subsequently printed on the second, opposite side thereof, i.e., the sheets being duplexed. The sheets are stacked in duplex tray 60 face down on top of one another in the order in which they are copied. In order to complete duplex copying, the simplex sheets in tray 60 are fed, in seriatim, by bottom feeder 62 from tray 60 back to transfer station D via conveyor 64 and rollers 66 for transfer of the toner powder image to the opposed sides of the copy sheets. Inasmuch as successive bottom sheets are fed from duplex tray 60, the proper or clean side of the copy sheet is positioned in contact with belt 10 at transfer station D so that the toner powder image is transferred thereto. The duplex sheet is then fed through the same path as the simplex sheet to be advanced to finishing station F. Copy sheets are fed to transfer station D from the secondary tray 68. The secondary tray 68 includes an elevator driven by a bidirectional AC motor. Its controller has the ability to drive the tray up or down. When the tray is in the down position, stacks of copy sheets are loaded thereon or unloaded therefrom. In the up position, successive copy sheets may be fed therefrom by sheet feeder 70. Sheet feeder 70 is a friction retard feeder utilizing a feed belt and take-away rolls to advance successive copy sheets to transport 64 which advances the sheets to rolls 98 which feed the sheets to the registration device of the invention herein, described in detail below, and then to transfer station D. Copy sheets may also be fed to transfer station D from the auxiliary tray 72. The auxiliary tray 72 includes an elevator driven by a directional AC motor. Its controller has the ability to drive the tray up or down. When the tray is in the down position, stacks of copy sheets are loaded thereon or unloaded therefrom. In the up position, successive copy sheets may be fed therefrom by sheet feeder 74. Sheet feeder 74 is a friction retard feeder utilizing a feed belt and take-away rolls to advance successive copy sheets to transport 64 which advances the sheets to rolls 98 to the registration device and then to transfer station D. Secondary tray 68 and auxiliary tray 72 are secondary sources of copy sheets. The high capacity sheet feeder, indicated generally by the reference numeral 76, is the primary source of copy sheets. Feed belt 81 feeds successive uppermost sheets from the stack to a take-away drive roll 82 and idler rolls 84. The drive roll and idler rolls guide the sheet onto transport 86. Transport 86 advances the sheet to rolls 98 which, in turn, move the sheet through the registration device to transfer station D. Invariably, after the copy sheet is separated from the photoconductive belt 10, some residual particles remain adhering thereto. After transfer, photoconductive belt 10 passes beneath corona generating device 94 which charges the residual toner particles to the proper polarity. Thereafter, the pre-charge erase lamp (not shown), located inside photoconductive belt 10, discharges the photoconductive belt in preparation for the next charging cycle. Residual particles are removed from the photoconductive surface at cleaning station G. Cleaning station G includes an electrically biased cleaner brush 88 and two de-toning rolls. The reclaim roll is electrically biased negatively relative to the cleaner roll so as to remove toner particles therefrom. The waste roll is electrically biased positively relative to the reclaim roll so as to remove paper debris and wrong sign toner particles. The toner particles on the reclaim roll are scraped off and deposited in a reclaim auger (not shown), where it is transported out of the rear of cleaning station G. The various machine functions are regulated by a controller 29. The controller 29 is preferably a programmable microprocessor which controls all of the machine functions hereinbefore described. The controller provides a comparison count of the copy sheets, the number of documents being recirculated, the number of copy sheets selected by the operator, time delays, jam corrections, etc. The control of all the exemplary systems heretofore described may be accomplished by conventional control switch inputs from the printing machine consoles selected by the operator. Conventional sheet path sensors or switches may be utilized to keep track of the position of the document and the copy sheets. In addition, the controller regulates the various positions of the gates depending upon the mode of operation selected. The invention herein has been illustrated in a high speed black and white printing machine. It is also very suitable for use in a high speed full color or highlight color printing machine where accurate sheet to image registration is critical. High quality documents require registration of sheets of paper to the photoreceptor for image transfer. Accurate registration control locates the image consistently with respect to the edge of the paper. In the registration systems described by these documents, each copy sheet 11 is delivered from the paper tray to the registration mechanisms by standard conveyance means. The registration mechanisms consist of two separately programmed pinch rollers 114, 116 laterally disposed with respect to the process direction. The position of the pinch rollers should always remain in control of the sheets while the distance between rollers should be maximized for best performance. When the copy sheet 11 comes in control of the pinch rolls 114, 116, one of its forward corners comes in the range of a linear position sensor 132 positioned with its long axis substantially transverse to the process direction designated by arrow 100 so as to always be partially covered by one of the lateral edges of the sheet 11. Two possible arrangements are shown in Fig. 2 and Fig. 3. In the former, the sensor 132 is in line with the pinch rollers 114, 116 and in the latter the sensor 132 is not in line with the rollers 114, 116. Referring to Figures 2 and 3, when the forward right corner of the sheet 11 first comes over the sensor 132 partially covering it, the resulting signal suddenly changes. The time at which this occurs is in indication of the relative forward position of the sheet 11 with respect to its travel schedule. The magnitude of the sensor signal measures the lateral position of the forward right corner of the sheet. The last datum to describe the sheet state of registration is the angle Θ which it forms with a reference straight line such as the process direction or a line parallel thereto. This can be evaluated by the following: a. since the independently controlled pinch wheel speeds V 1 and V 2 are known- at all times, the forward and lateral components of velocity, V f and V l respectively, of the sheet at the sensor can be calculated; b. the skew angle Θ of the sheet lateral side is computed as a ratio where: 1. the numerator is the difference between the rate of change of the sensor signal and the lateral velocity component of the sheet over the sensor. 2. the denominator is the forward velocity component of the sheet at the sensor. These are represented by the equations: V s = D s * V 1 /D 1 V f = D l *V 1 /D 1 V l = D f *V 1 /D 1 Θ = [(dP/dt) - V l ]/V f Fig. 4 graphically indicates the concept of skew determination where P is the desired registration position at the sensor. In performing the above-indicated calculation, the configuration of Fig. 2 offers the simplification of having a lateral velocity V l component of the sheet equal zero. This methodology allows complete knowledge of the state of sheet registration at the initial time of control and a continuous knowledge of the skew angle throughout the registration action. Proper motion for the wheels can then be synthesized to achieve the desired outlet registration which usually consists of: a. the coordinates of the forward right corner of the sheet must achieve a given value at a given time; b. the speed of the forward right corner must be of a given value; c. the skew angle of the sheet must be equal to zero. An additional single sensor 134 (Figs. 2 and 3) can be used at a downstream position to provide feedback for updating the control signals as rolls wear or different substrates having different coefficients of friction are used.

Claims: A method for registering and deskewing a sheet (11) along a paper path, comprising: sensing a lead edge position and a lateral edge position of said sheet (11) with a single sensor (132); determining a skew angle error and a registration position error of the sheet (11) and generating signals indicative thereof; driving a pair of drive nips (114,116) independently pursuant to a set of signals as a function of the skew angle error and registration position error so that the sheet (11) arrives at a registration position downstream in the paper path from the single sensor (132) at a proper time and in proper alignment position characterized in that determining the skew angle error comprises using data of drive nip speeds of each drive nip of the pair of the independently driven nips (114, 116) and sensing data of said single sensor (132). A method according to claim 1, further comprising checking a position of the sheet at the registration position with a second single sensor (134) and sending the position information to a controller to update a drive control function.

Inventors: CASTELLI VITTORIO R, POLATKAN OSMAN T, CASTELLI, VITTORIO R., POLATKAN, OSMAN T.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1999
Main classification: B65H9
IPC codes: B41J11, B65H9, G03G15, B41J13