Abstract:
A sheet transport system has an encoder roller in direct contact with the sheet media and driven by the sheet media to detect and compensate for any registration error particularly at lead and trail edges during transport of the sheet media as media enters and exits transport nips. The invention is well suited for use in controlling and monitoring paper movement in incremental advance and print systems, such as ink jet printers. A biasing member ensures that the sheet media and encoder roller are in intimate contact. Preferably, a material with a high coefficient of friction is provided on an outer periphery of the encoder roller to assist in mating of the roller with the sheet media.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an apparatus for precisely controlling the movement of sheet media between two transport nips of a media transport system. More particularly, the invention relates to the use of a rotary encoder roller in direct contact with the sheet media and driven by the sheet media to detect and compensate for any registration error at lead and trail edges as media enters or exits one of the nips. The invention is well suited for use in controlling and monitoring paper movement in incremental advance and print systems, such as ink jet printers. 
     2. Description of Related Art 
     Transport systems for printing apparatus, such as scanning ink jet printers, operate by incrementally advancing sheet media past a printhead. For example, some ink jet printers increment a paper 1″, print a 1″ swath with the printhead, increment the paper another 1″, etc. until an entire page is printed. Such movement needs to be precisely the same distance as the width of a printed swath in order to properly register the image to be printed and prevent visible image defects. 
     As the printing industry continues to push towards finer and finer levels of resolution, there is a need for increasingly higher levels of precision in the driving of the sheet media. For current 600 spot per inch (SPI) printers, observable defects can occur with misregistration of about 0.5 pixels. As this resolution, this 0.5 pixel error translates into a registration error of about 21 microns. 
     Many of today&#39;s low-cost printers have no feedback control, count motor steps to provide controllable incremental transport, or include a servo-controlled drive structure, such as that illustrated in FIG. 1, provided on a drive roller. In such printers, a sheet media P (such as a cut sheet of paper) is incrementally transported in direction R between first and second drive nip pairs  10 ,  20  across a platen  50  past a printhead  30 . In such conventional devices, the drive nip pair  10  exerts an entrance drive force on the sheet media P by contacting the top and bottom surfaces of the sheet media. Likewise, the drive nip pair  20  exerts an exit drive force on the sheet media P. Each of the nips  10 ,  20  may include a driven roller element and an idler element. The drive nip pairs  10 ,  20  may be driven by stepper motors or servo motors that include an encoder  40  that through conventional feedback control provides signals that control rotation of the drive roller of each drive nip pair to transport the sheet media P across platen  50  past the printhead  30 . Such transport structures assume that the sheet media P closely follows the rotation of the drive nip pairs  10 ,  20 . An example of such a system is co-pending U.S. Ser. No. 09/233,111 to Tellmer et al. filed Jan. 19, 1999, which is assigned to the same assignee as the present invention and incorporated herein by reference in its entirety. 
     Traditionally, engineers looking to improve positional accuracy would turn towards higher tolerance components in a drive system. That is, providing motors with higher positional accuracy and higher precision encoders. However, such higher tolerance components can be too costly to implement in low cost printing devices, such as ink jet printers. Moreover, it would be difficult for such systems to achieve high precision transport and high quality image production when such drive systems cannot necessarily identify misregistration of the sheet media being transported. 
     SUMMARY OF THE INVENTION 
     Applicants have found that such conventional transport structures can encounter problems in image registration, particularly near leading and trailing edges of the paper. This is primarily believed to be the result of handoff errors that are caused by a discontinuity, such as the paper transitioning from the exit of an input nip to the entrance of an exit nip. At such discontinuities, the paper may slip relative to the rotation of the nips, causing misalignment that cannot be properly sensed or compensated for using this conventional structure. Additionally, alignment errors can be caused by elastomeric nip microslip, which occurs when elastomeric rubber rollers are provided in the transport path and deform dependent on drag applied to the rollers. Such microslip can change as the paper passes through such rollers. 
     Accordingly, there is a need for other methods and apparatus that can achieve improved sheet media positioning so that observable print defects due to media misregistration can be substantially decreased. 
     One exemplary embodiment of the invention overcomes such problems by providing an apparatus for regulating sheet media position within a sheet media transport path formed between an upstream transport nip and a downstream transport nip that compensates for error in sheet media position between the upstream transport nip and the downstream transport nip, which are driven by at least one drive mechanism. The apparatus includes an encoder roller, a biasing member, a controller and a feedback mechanism. The encoder roller is rotatably mounted between the upstream transport nip and the downstream transport nip so as to contact one side of sheet media itself as the sheet media is fed through the sheet media transport path. The encoder roller has an outer peripheral surface at least partly formed from a high coefficient of friction material and further includes an encoder member that measures angular rotation of the encoder roller. The biasing member is positioned between the upstream transport nip and the downstream transport nip and is juxtaposed relative to the encoder roller to bias the sheet media against the encoder roller. A combination of the high coefficient of friction material on the encoder roll and a biasing force of the biasing member are selected so as to prevent relative slip between the sheet media and the encoder roller. As such, the encoder roller is driven solely by a driving force created by the sheet media. The controller determines sheet media misregistration, such as by comparing output from the drive system, which controls driving of the nips, with output from the encoder roller. A feedback mechanism can then adjust the drive system or other parameters or components to compensate for any misregistration. 
     In preferred exemplary embodiments of the invention, the biasing member can be a vacuum platen, a thin flexure pressure finger, or an idler roller. However, other biasing members can be substituted so long as they function to bias the sheet media against the encoder roller without relative slip. 
     In a particular embodiment of the invention, the apparatus is part of an incremental advance and print printing system, such as an ink jet printer having a printhead located between the upstream and downstream nips to provide an image (black, highlight or full color) onto the sheet media traveling through the nips. The encoder roller is preferably positioned closely adjacent the printing to monitor sheet media position at the point of printing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of this invention will be described in detail with reference to the following figures, wherein: 
     FIG. 1 is a side view of a conventional transport system for an ink jet printer; 
     FIG. 2 is a side view of a first exemplary embodiment of the invention; 
     FIG. 3 is a side view of a second exemplary embodiment of the invention; 
     FIG. 4 is a side view of a third exemplary embodiment of the invention; 
     FIG. 5 is a top view of the vacuum platen and drive nips of FIG. 4; 
     FIG. 6 is a bottom view of a carriage mounted printhead according to an exemplary embodiment of the invention; 
     FIG. 7 is a perspective view of an exemplary encoder roller according to the invention; and 
     FIG. 8 is a perspective view of an exemplary encoder roller according to an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIG. 2, a first exemplary embodiment of the invention will be described. An apparatus  100  for regulating sheet media position can form part of a printing system, such as an ink jet printer, and is provided with a sheet media transport path  110  formed between an upstream transport nip  120  and a downstream transport nip  130 . The upstream transport nip  120  is made up of a drive roller  170  and an idler roller  150  spaced in close proximity to drive roller  170  to allow a predetermined sheet media P to be driven therebetween. The downstream transport nip  130  is made up of a drive roller  160  and an idler roller  165 . 
     Drive roller  170  is rotated by a motor  180  suitably coupled to the drive roller  170 . Motor  180  is driven to feed sheet media P in a direction R past a platen  140  of a printing system, such as an ink jet printhead  200 , which is provided along the transport path  110  opposed to one side of sheet media P for printing an image thereon. The printing system is preferably an incremental advance and print system in which the sheet media P is advanced by a predetermined swath distance, which corresponds to the printable size of the particular printhead  200  used, such as 1″. However, the invention is not limited to incremental systems or particular swath advances. 
     The upstream transport nip  120  can be considered an input nip of the transport path  110 , although it does not necessarily have to be the beginning of the entire transport path of the printer and can comprise an intermediate portion thereof. The downstream transport nip  130  can be considered an exit nip of the transport path  10 . Similarly, the exit nip of transport path  110  does not have to be an end of the overall transport path of the printer. 
     An encoder roller  210  is rotatably positioned intermediate the input and exit nip  120 ,  130 . Most preferably, the encoder roller  210  is provided on a shaft  220  and positioned closely adjacent printhead  200  (along the direction R). While placement close to the printhead provides for more accuracy where printing is to occur, this placement may not be as critical in other applications that may not use a printhead. Encoder roller  210  is provided at a position so that an outer peripheral surface of the encoder roller  210  comes in contact with a surface of a sheet media P traveling along the transport path  110 . The encoder roller  210  preferably has a diameter of less than about 2 inches when space constraints are of concern; however, a larger diameter can result in better resolution. 
     At least a part of the outer peripheral surface  210 A includes a material having a high coefficient of friction. See FIGS.  7 - 8 . A suitable material is a silicon spray coating on the roller surface. Other materials that can achieve such a friction coefficient include, but are not limited to, rubber or a grit-blasted carburized steel. The high coefficient of friction is desirable to ensure that the encoder roller  210  is driven by the feeding of the sheet media P. 
     Returning back to FIG. 2, maintaining contact with the sheet media P is further ensured by providing a biasing member  230  that biases the sheet media P against the encoder roller  210 . Biasing member  230  can be a pressure finger, such as a Mylar flexure element, provided juxtaposed to encoder roller  210 , but on an opposite side of sheet media P to bias the sheet media P therebetween. Other exemplary biasing members can take the form of an idler roller  230  (FIG. 4) or can be achieved by the combination of a vacuum assisted platen  140  fed by a vacuum source  280 . The vacuum forces the sheet media P against the belt and subsequently against encoder roller  210 . 
     The preferred embodiment of FIGS.  4 - 5  will be described in more detail. The media transport system includes platen  140  positioned in the area between the upstream drive nip  120  and the downstream drive nip  130 . The platen  140  has a sheet media side with vacuum holes  290  and a vacuum force side that includes vacuum source  280 . The vacuum force is generated by vacuum source  280  and applied to sheet media P through the vacuum holes  290  to draw the sheet media P against the sheet media side of platen  140 . 
     Rollers  160  and  165  that form the downstream drive nip  130  are formed near lateral edges of the sheet, which are preferably, but not necessarily, outside of the printable region of the sheet region. This will prevent smearing of a printed image formed on the sheet media P by printhead  200 . As the upstream drive nip  120  is prior to printing, rollers  170  and  150  do not necessarily have to be outside of the printable region of the sheet media. 
     In this embodiment, the sheet media P is advanced by a predetermined swath by upstream transport nip  120  across vacuum platen  150  to printhead  200 . After advancement of the sheet media P, the printhead  200  is traversed laterally along a carriage  205  to print a swath of the image. After which, the sheet media P is advanced again until the entire image is printed. In a preferred embodiment using the latter example, the encoder roller  210  is positioned a small distance (such as, for example, 0.0020″) into the transport path above the surface of platen  140  so that the vacuum force from vacuum source  280  biases the sheet media P into contact with encoder roller  210 . The high friction material helps ensure that contact is maintained. 
     As the encoder roller  210  is to be driven by the sheet media P, which has very limited driving force, roller  210  should have minimal drag. This can be achieved by making the contact area small, such as by making the width T of the encoder roller  210  thin. The actual dimensions of the encoder roller will be determined based on the drive force generated by the sheet media P, which is dependent on several factors including the stiffness and frictional coefficient of the paper, the force generated by the upstream and downstream nips, drive force losses due to the vacuum, and other factors. The basic requirements for the encoder roller  210  are a low rotational drag, but a high coefficient of friction outer surface  210 A. 
     As exemplary encoder roller  210  is shown in FIG.  7 . Encoder roller  210  includes an encoder that can accurately detect the angular rotation of encoder roller  210 . This can take the form of a separate encoder wheel  240  (see FIG. 7) affixed to shaft  220  for rotation therewith along with encoder roller  210  or, alternatively, the encoder wheel can form encoder roller  210  itself (FIG.  8 ), with a high coefficient of friction material provided on the outer periphery  210 A thereof. An encoder sensor  250  is mounted relative to encoder wheel  240  to detect rotation thereof. A particularly advantageous encoder is an encoder wheel  240  having a 20 mm diameter shaft and an encoder sensor  250  that has the capability of detecting 500 lines/revolution, which in quadrature results in 2000 pulses per resolution of precision. A suitable encoder capable of achieving this is a Hewlett Packard Series 9000 HEDS encoder. With such an encoder, an accuracy of 30 microns/pulse can be achieved, which is suitable for obtaining positioning accuracy of better than 21 microns for 600 SPI printing without visual quality defects. While an optical encoder wheel and sensor are illustrated, the invention can be implemented using any suitable or later developed capacitive, magnetic hall effect, inductive or optical encoder device. 
     In the exemplary embodiments shown in FIGS.  2 - 5 , the encoder roller  210  (including encoder sensor  250 ) is connected to a servo controller  190  that receives the output from encoder roller  210 , which is used to determine the position of sheet media P relative to an intended position. In these exemplary embodiments, the transport path  110  is controlled to advance the sheet media P by a calculated advancement, such as 1″, by causing motor  180  to turn a predetermined number of steps, corresponding to 1″ linear travel of the sheet media P by use of a servo motor  180  or a stepper motor  180 . However, depending on the accuracy of the motor  180  and the adherence of the sheet media P relative to the nips  120 ,  130 , the portion of the sheet media P opposed to printhead  200  may have been actually moved by more or less than 1″. 
     As the encoder  210  is driven by the advancing sheet media P, the actual position of the advanced sheet media P can nonetheless be determined, regardless of whether the sheet media P encountered any slip relative to the advancement of the upstream or downstream drive nips  120 ,  130  during the advancement. That is, information from encoder  210  can be used as feedback to control image processing by an adjustment device  270 , which can control any of a number of components or parameters thereof to correct for any detected misalignment. 
     For example, adjustment mechanism  270  can correction data to the motor  180 , causing the motor  180  to further advance or retract, repositioning the sheet media P at an intended position. If a stepper motor  180  is provided, this can be achieved by sending a signal to advance the motor  180  by a certain number of steps in either direction (clockwise or counterclockwise). Alternatively, if the drive motor  180  is servo-controlled, the adjustment mechanism  270  can send a signal (feedback signal) overriding the signal from an encoder located at the drive roller  170  to cause the motor  180  to advance by a certain amount. 
     Alternatively, adjustment device  270  can be electrically connected to print controller  260 , which controls data flowing to printhead  200 , to compensate for any misalignment. For example, an exemplary printhead has a resolution of 600 SPI and includes a matrix of print nozzles designed to print in the 1″ swath as the printhead is advanced transverse to the transport direction of the device. Such a printhead is represented in FIG. 6, which is intended to be illustrative and not limited to a specific configuration. 
     Should the incremental advance be incorrect (i.e., slightly more or less than 1.0″ advancement), then corrective action can be taken by adjustment device  270  so as to control the print controller  260  to adjust the firing of ink jet nozzles. As shown, illustrative printhead  200  has a two-dimensional array of nozzles of a particular resolution in rows A-N, where N can be any integer number. If, for example, the sheet media P is under advanced (i.e., less than 1″), then 1 or more rows of nozzles (A, A+B, etc.) can be controlled to not fire during that print swath. As such, a reduced swath is printed so that potential overlap in print coverage due to the under advancement can be avoided. If the nozzle array is sized to be more than the swath size (i.e., slightly more than 1″), then extreme end rows, such as A and N or A, B, N−1 and N, can be prevented from firing if advancement of exactly 1″ is detected. With such a nozzle array that is longer than 1″, over advancement can also be controlled by firing the intermediate nozzles plus the extreme end nozzles, either B and N−1 or A, B, N−1 and N. P. Accordingly, banding of ink or white spots between swaths can be avoided. 
     Alternatively, any combination of drive control and print control can be implemented. However, it may be preferable to have adjustment device  270  control the print controller rather than further movement of the transport system  110  for several reasons. First, further positioning requires additional time and slows throughput of the system. Second, when lower precision motors and drives are provided, this further positioning may also have positioning error, although inherently smaller as the distance being controlled is smaller. As such, it may be desirable to have the image to be printed at a particular position adjusted to fit into the area advanced so as to avoid ruined pages and poor visible images due to omissions in coverage or overlap. 
     The invention has been described with reference to specific embodiments, which are meant to be illustrative and not limiting. Various modifications can be made without departing from the spirit and scope of the invention as defined by the appended claims.