Typewriting machines – Sheet or web – Including friction-feed means
Utility Patent
1999-06-08
2001-01-02
Hilten, John S. (Department: 2854)
Typewriting machines
Sheet or web
Including friction-feed means
C400S579000, C400S582000
Utility Patent
active
06168333
ABSTRACT:
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'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.
REFERENCES:
patent: 4297716 (1981-10-01), Hirayama et al.
patent: 4892246 (1990-01-01), Steele
patent: 5075702 (1991-12-01), Castelli et al.
patent: 5642949 (1997-07-01), Yamamoto
patent: 5715514 (1998-02-01), Williams et al.
Donahue Frederick Allen
Gilliland W. Keith
Markham Roger Guy
Merz Eric Alan
Teumer Roger Glenn
Hilten John S.
Nolan, Jr. Charles H.
Oliff & Berridg,e PLC
Xerox Corporation
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