Electrophotography – Diagnostics
Reexamination Certificate
2002-06-05
2004-09-21
Tran, Hoan (Department: 2852)
Electrophotography
Diagnostics
C399S018000, C399S021000
Reexamination Certificate
active
06795658
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to printing mechanisms. More specifically, the present invention relates to apparatus and methods for diagnosing printer performance faults.
2. Description of the Related Art
Modern printers have evolved as fast and sophisticated machines that produce high quality text and images from a digital printer file. While modern printers are currently very reliable, that the fact they are complicated machines implies that consumption, wear, component failure, misuse, and environmental factors will cause degradation in performance, partial failure, and even complete failure from time to time over the life cycle thereof. When degradation, partial failure, or complete failure occurs, it is useful to follow a logical troubleshooting procedure, using tools and aids such as those described herein, to accomplish repairs in a timely and economical manner. Since computer printers are complicated machines subject to a large number of failure modes, each repair effort must begin with a diagnostic task to identify the failure mode and isolate the faulty component or components. Accordingly, efficient diagnosis of faults is necessary to reduce the amount of time needed to effect the repair and to add certainty to the determination of the faulty components.
The diagnostic process for a printer is specific to the printing technology employed, as well as the model of the machine being serviced. There are, however, certain procedural similarities in the diagnostic process, whether the technology is a monochromatic or color printer of the inkjet, laser, offset, or any other printer variety. One similarity is the need for an analysis of the printing process at various points in time as the process proceeds in order to identify contributing faults and the source of same. By way of example, consider the modern color laser printer. As is well known in the art, systems and methods for reducing the time needed to isolate faulty components with greater certainty and effect a repair facilitate control and minimization of repair costs.
Laser printing is accomplished by encoding an image as a sequence of light pulses generated by a semiconductor laser. The pulsed beam of light is directed onto a photoconductive drum, belt, or sheet, as a sequence of scanned lines, the spacing of which represents one dimension of the printer resolution; 600 scan lines per inch, for example. The other dimension of the resolution is dependent upon the modulation rate of the light beam, for example, 600 light pulses per inch of scan. The scanning process typically utilizes a rotating mirror, which reflects the pulsed beam of light onto the photoconductive drum, belt or sheet, in a rasterized sequence. Consider the example of a drum type photoconductor for which a photoconductive drum is electrostatically charged prior to the laser beam scan. Areas of the photoconductive drum that are exposed to light are enabled to conduct electricity and the local electrostatic charge is thereby drained to a grounded substrate in the photoconductive drum. The areas of the photoconductive drum that are not illuminated by the pulsed laser beam retain their electrostatic charge. In the case of a color laser printer, three or four rasterized images are typically written to the drum, one for each of the primary colors, yellow, magenta, and cyan, and often an additional one for the color black. Some printing systems may use six or eight primary colors. Individual photoconductive drums for each color can be utilized or a single drum can be utilized multiple times in sequence. The circumference of some photoconductive drums are a fraction of a page, and being seamless, it is common practice for the same area of the drum's surface to be reused as many as eight or more times per page. In the case of larger photoconductors, depending on the relative size of the drum and print media, the same area of the photoconductor may only be used every Nth (every other, every third, every fourth, etc.) page.
Once the photoconductive drum is exposed with the image by the aforementioned laser beam scan, it is subsequently “developed” by transferring toner to the areas with the desired electrostatically charged or discharged condition. Toner is supplied from a toner cartridge in which a reserve of toner is stored. Some of the toner in this reserve is mechanically or otherwise agitated and handled by mechanical, magnetic, electrostatic, and other forces. For transfer from the toner cartridge to the photoconductive drum to occur, a cloud of toner is produced in the vicinity of the photoconductive drum along a lateral region tangential to the drum's surface. The cloud of toner has a bias voltage applied so as to cause the toner particles to be electrostatically attracted to and repelled from the various charged and discharged areas of the photoconductive drum. In the case of common monocomponent toners, the particles typically have a ferrite core, or, in the case of dual component development systems, the toner is most commonly mixed with ferrite “carrier” particles, that allow the toner particles to be manipulated by both magnetic and electric fields.
While there are a number of ways to generate a cloud of toner, one common approach is to coat the exterior surface of a hollow cylindrical developer sleeve, possessing a suitably thin wall thickness and appropriate surface characteristics, with a thin layer of monocomponent toner particles, or a toner/carrier particle mixture, where the particles are held to the surface of the sleeve by the magnetic field of one or more multi-pole magnets carefully arranged within the cylinder. As the motor driven cylinder rotates about the fixed position interior magnets, the particles on the surface move in response to the varying magnetic field or fields. Sometimes a pair of magnetic poles with the same polarity are arranged a short distance apart within the sleeve. These magnetic poles are each aligned parallel to the axis of the cylinder and run almost the full length of the cylinder. As the rotating sleeve quickly moves, the particles on its surface between the small gap between the magnetic fields causes the various particles to vigorously flip end to end due to their interaction with the magnetic fields, thus producing a cloud of toner particles over the “gap” region between the two like magnetic poles. Usually the entire developing mechanism is electrically isolated from its surroundings and a voltage bias signal is applied so that the toner particles within the cloud are electrically charged to a controlled range of values. The resulting charge on the various particles and surfaces is a function of the bias signal applied, the motion of the various particles, the magnetic field, and the properties of the various materials in the developer sleeve, the toner, carrier particles and possibly other factors. The axis of the aforementioned photoconductive drum is also arranged parallel to the developer sleeve at such a distance and angle that the gap region of the developer sleeve and the surface of the photoconductive drum with the electrostatic image are in close proximity to, though usually not contacting, one another. Both the photoconductive drum and the developer sleeve are mechanically rotated, usually at vastly different surface velocities and in opposite directions (e.g. both drums are driven counter-clockwise so that their surfaces in the gap region are moving in opposite directions). As the portion of the photoconductive drum with the electrostatic image on its surface is rotated through the charged toner particle cloud, a visible image is formed on the surface of the photoconductive drum. Where there are multiple colors, there may be multiple photoconductive drums or a single photoconductive drum may be utilized multiple times in sequence, once for each color.
The next step is to transfer the toner image to the print media (paper, transparency, label, envelope, postcard,—whatever substrate gets printed upon and eventually delivered to an
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