Electrophotography – Diagnostics – Consumable
Reexamination Certificate
2001-01-18
2003-10-21
Grainger, Quana M. (Department: 2852)
Electrophotography
Diagnostics
Consumable
C399S049000, C399S055000
Reexamination Certificate
active
06636705
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to color laser imaging methods and apparatus and, in particular, to methods and apparatus for detecting a low toner condition in color laser imaging devices.
BACKGROUND OF THE INVENTION
Color printing by an electrophotographic printer is achieved by first scanning a digitized image onto a photoconductor. Typically, the scanning is performed with diodes which pulse a beam of energy onto the photoconductor. The diodes can be, for example, laser diodes or light emitting diodes (LEDs). The photoconductor typically comprises a movable surface coated with a photoconductive material capable of retaining localized electrical charges. In many cases, the movable surface is in the form of a revolvable cylindrical drum.
The surface of the photoconductor is divided into relatively small units called pixels. The photoconductor is generally configured to continuously revolve such that any given pixel is repeatedly moved past the diodes at a substantially regular cycle and at a substantially constant rate, and along a substantially fixed path relative to the diodes. Each pixel is capable of being charged to a given electrical potential, independent of the electrical charge of each surrounding pixel.
During operation of the printer, substantially all of the pixels are first charged to a base electrical charge as they move past a charging unit during each revolution of the photoconductor. Then, as the pixels move past a diode, a beam of energy, such as a laser, is either directed at, or not directed at, each of the pixels as dictated by the digital data used to pulse the laser. If the laser is directed at a given pixel, the given pixel can be electrically altered by changing (typically discharging) the base electrical charge to a second electrical charge.
Thus, after passing a laser during operation of the printer, a first portion of the pixels will remain at the base electrical charge because they were not exposed to the laser, while a second portion will have a different charge because of being altered by the laser. The first and second portions of unaltered and altered pixels thus form an image on the photoconductor. One portion of pixels will attract toner, while the other portion will not, depending on various factors such as the electrical potential of the toner. That is, the unaltered pixels will either attract or not attract toner, and vice versa with regard to the altered pixels.
In most electrophotographic printing processes, the altered, or electrically discharged, pixels attract toner onto the photoconductor. In this manner, toner is selectively transferred to the image made up of electrically discharged pixels on the photoconductor. This process is known as discharge area development (DAD). However, in some electrophotographic printing processes toner is attracted to the un-discharged (i.e., charged) pixels on the photoconductor. This latter type of electrophotographic printing is known as charge-area-development (CAD). The present invention is meant to encompass both DAD and CAD printers.
Once the toner has been applied to the photoconductor to form an image, the image is ultimately transferred to finished product medium, such as a sheet of paper. Although the finished product medium typically comprises paper, it can also comprise other materials such as plastic, as in the case of a transparency. Also, finished product medium can comprise individual sheets, such as a typical eight-and-one-half inch by eleven inch sheet of paper, or it can also comprise a long, continuous sheet.
The transfer of toner from the photoconductor to the finished product medium can be direct, or it can be indirect using an intermediate transfer device. That is, in the direct method, the toner is transferred directly from the photoconductor to the finished product medium. In the indirect method, the toner is transferred first to an intermediate transfer device, then transferred from the intermediate transfer device to the finished product medium. The intermediate transfer device typically comprises a revolvable endless belt. During operation of the printer, the intermediate transfer device typically moves by circulating, or revolving, past the photoconductor. The finished product medium, in turn, is caused to pass by the intermediate transfer device.
After the toner is transferred to the finished product medium, it is processed to fix the toner thereto. This last step is normally accomplished by thermally heating the toner to fuse it to the finished product medium, or applying pressure to the toner on the finished product medium. Any residual toner on the photoconductor and/or the intermediate transfer device is then removed by a cleaning station, which can comprise either or both mechanical and electrical means for removing the residual toner.
A variety of methods are known for selectively attracting toner to a photoconductor. Generally, each toner has a known electrical potential affinity. As described above, selected pixels of the photoconductor can be exposed by a laser from a base potential to a given potential associated with the selected toner, and then the toner can be presented to the photoconductor so that the toner is attracted only to the selectively exposed pixels. This latter step is known as developing the photoconductor.
In some processes, after the photoconductor is developed by a first toner, the photoconductor is then recharged to the base potential and subsequently exposed and developed by a second toner. In other processes, the photoconductor is not recharged to the base potential after being exposed and developed by a selected toner. In yet another process, the photoconductor is exposed and developed by a plurality of toners, then recharged, and then exposed and developed by another toner. In certain processes, individual photoconductors are individually developed with a dedicated color, and then the toner is transferred from the various photoconductors to a transfer medium which then transfers the toner to the finished product medium. The selection of the charge-expose-develop process depends on a number of variables, such as the type of toner used and the ultimate quality of the image desired.
Image data for an electrophotographic printer (which will also be known herein as a “printer”), including color laser printers, is digital data which is stored in computer memory. The data is stored in a matrix or “raster” which identifies the location and color of each pixel which comprises an overall image. The raster image data can be obtained by scanning an original analog document and digitizing the image into raster data, or by reading an already digitized image file. The former method is more common to photocopiers, while the latter method is more common to printing computer files using a printer. Accordingly, the invention described below is applicable to either photocopiers or printers.
Recent technology has removed the distinction between photocopiers and printers such that a single printing apparatus can be used either as a copier, a printer for computer files, or a facsimile machine. In any event, the image to be printed onto finished product media is provided to the printer as digital image data. The digital image data is then used to pulse the beam of a laser in the manner described above so that the image can be reproduced by the electrophotographic printing apparatus. Accordingly, the expression “printer” as used herein should not be considered as limited to a device for printing a file from a computer, but should also include any device capable of printing a digitized image in the general manner described herein, regardless of the source of the image.
The image data file is essentially organized into a two dimensional matrix within the raster. The image is digitized into a number of lines. Each line comprises a number of discrete points. Each of the points corresponds to a pixel on the photoconductor. Each point is assigned a binary value relating information pertaining to its color and potentially other attributes, such as densi
Grainger Quana M.
Hewlett--Packard Development Company, L.P.
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