System and Method for Controlling Multiple Light Sources of a Laser Scanning System in an Imaging Apparatus

An imaging device having a printhead unit which includes a plurality of independently controllable light sources, each light source generating a light beam when activated; a photoconductive surface operable at a plurality of image transfer rates; a scanning device having one or more deflecting surfaces, the scanning device arranged to direct the light beams so as to sweep in at least one scan direction across a surface such that, for each sweep, scan lines written by the light beams are spaced from one another on the photoconductive surface in a process direction that is nominally orthogonal to the scan direction; and a controller configured to selectively activate any number of the light sources for use during a print operation to write image data along scan lines on the photoconductive surface, wherein a rotational velocity of the scanning device is selected and the number of the light sources activated based upon at least one selected operating parameter for the imaging device.

DETAILED DESCRIPTION

Terms such as “first”, “second”, and the like, are used to describe various elements, regions, sections, etc. and are not intended to be limiting. Further, the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

Furthermore, and as described in subsequent paragraphs, the specific configurations illustrated in the drawings are intended to exemplify embodiments of the disclosure and that other alternative configurations are possible.

Referring now to the drawings, and particularly toFIG. 1, an electrophotographic device is illustrated in the form of a color laser printer10. The printer10includes generally, an imaging section12, a fuser assembly14and a paper path that moves a sheet of print media18through printer10. Briefly, a sheet of print media18is transported along the paper path so as to pass the imaging section12. At the imaging section12, cyan, yellow, magenta and black toner patterns (CYMK) are registered to form a color toner image, which is transferred to the print media18. The print media18then passes through the fuser assembly14, which causes the toner patterns to adhere to the print media18. After fusing, the print media18is transported outside the printer10.

To form the overlaid toner patterns, the imaging section12includes four printhead units24,26,28,30, four toner cartridges32,34,36,38, four photoconductive drums40,42,44,46and an intermediate transfer belt48. Printhead unit24generates a plurality of independently controllable laser beams50a-50nthat are modulated in accordance with bitmap image data corresponding to the color image plane to form a latent image on the photoconductive drum40. Similarly, printhead unit26generates independently controllable laser beams52that are modulated in accordance with bitmap image data corresponding to the magenta color image plane to form a latent image on the photoconductive drum42. Printhead unit28generates independently controllable laser beams54that are modulated in accordance with bitmap image data corresponding to the cyan color image plane to form a latent image on the photoconductive drum44. Similarly, Printhead unit30generates independently controllable laser beams56that are modulated in accordance with bitmap image data corresponding to the yellow color image plane to form a latent image on the photoconductive drum46.

Each photoconductive drum40,42,44,46continuously rotates clockwise such that toner is transferred to each photoconductive drum surface in a pattern corresponding to the latent image formed thereon by corresponding printhead unit24,26,28,30. Intermediate transfer belt48travels past each photoconductive drum40,42,44,46, as indicated by the directional arrow60, the corresponding toner patterns are transferred to the outside surface of the intermediate transfer belt48for subsequent toner transfer to the sheet of print media18.

It is understood that the photoconductive surfaces on which a latent image is formed is not limited to the photoconductive drums40,42,44,46shown inFIG. 1, and may include, for example, photoconductive belts or other structures.

In an alternative embodiment, a media transport belt (not shown) is used instead of intermediate transfer belt48for moving the sheet of media18to the nips formed in part by photoconductive drums40,42,44and46for direct transfer of toner from the photoconductive drums to the sheet of media18.

The timing of the laser scanning operations on each of the photoconductive drums40,42,44,46, the speed of intermediate transfer belt48and the timing of the travel of a sheet of media18along the paper path are coordinated such that a forward biased transfer roll62transfers the toner patterns from the intermediate transfer belt48to the print sheet of media18at a second transfer nip64so as to form a composite color toner image on the sheet of media18.

The print media18is then passed through fuser assembly14. Generally, heat and pressure are applied to the print media18as it passes through a fuser nip68of the fuser assembly14so as to adhere the color toner image to the print media18. The print media18is then discharged from the printer10along a media discharge path.

Referring now toFIG. 2, the printhead24includes laser sources70, e.g., a plurality of laser diodes71, each laser diode71generating an associated one of the laser beams50a,50b. . .50n.For sake of clarity, the example embodiments will be generally described below in terms of four laser beams50a-50dper photoconductive surface, i.e., n=4 for purposes of explanation. However, it is understood the number of laser beams is expandable to any reasonable number N of laser beams as indicated by the additional laser beam50nin phantom lines. Further, the description of the example embodiments will largely be directed to printhead unit24and it is understood that such description will apply equally to each of the other printhead units26,28and30which has a similar construction to printhead unit24.

Laser diodes71may have any known or future laser diode architecture, such as a vertical cavity surface emitting laser (VCSEL) diode architecture. Though the example embodiments are described herein as utilizing a plurality of laser diodes71, it is understood that components which generate light beams other than laser beams may be used instead of laser diodes71.

A controller74, e.g., a video processor or other suitable control logic, converts image data stored in memory72into a format suitable for imaging by the printhead24. The converted image data is communicated to the printhead24. The controller74may further designate whether each laser beam50a-50dshould be disabled or enabled to modulate image data for a particular print job as will be explained more fully herein. Each modulated laser beam50a-50dpasses through pre-scan optics76, and is reflected off of a rotating scanning device, e.g., a polygon mirror78. The polygon mirror78includes a plurality of deflecting surfaces, e.g., facets80(eight facets as shown) that reflect the laser beams50a-50dthrough post scan optics82so as to sweep generally in a scan direction SD across the corresponding recording medium, e.g., the photoconductive drum40.

Post scan optics82direct the laser beams50a-50nfrom the printhead unit24so as to form scan lines on the photoconductive drum40. The scan lines are spaced from one another in the process direction, which is generally orthogonal to the scan direction, by a beam scan spacing. That is, in a given sweep in which each laser beam50a-50dis turned on or is otherwise modulated, the respective beams will be spaced from one another on the surface of photoconductive drum40in the process direction by the predetermined distance. This distance between beams defines a “beam scan spacing” for the beams50a-50din the process direction. In an example embodiment, the beam scan spacing for beams50a-50nis about 42.33 microns, which corresponds to 600 dpi at a processing speed of 70 ppm and allows for 1200 dpi at a processing speed of 35 ppm, as discussed in greater detail below.

Multiple Speed Operation

In general, the image transfer rate of printer10defines a speed at which a toner image is transferred from the surface of photoconductive drums40,42,44and46to intermediate transfer belt48. Moreover, it is desirable in certain electrophotographic devices to provide several image transfer rates to support different modes of operation. Relatively slower image transfer rates generally result in the print media moving more slowly through the device, which may promote better fusing operations, e.g., to achieve translucence of color toners fused onto transparent media, improve adherence of toner when printing thick, gloss or specialty papers, or prevent fuser overheating. To this end, one approach is to slow down the image transfer rate by slowing down the intermediate transfer belt48and correspondingly slowing down the photoconductive drums40,42,44,46and the associated transport of the print media18. When slowing down the image transfer rate, either the laser output power, the rotational velocity of the polygon mirror, or both may be adjusted down in corresponding amounts to compensate for the new image transfer rate. As mentioned, relatively large variations in polygon motor velocity can also affect print quality, such as by causing jitter and otherwise unstable rotational velocity of the polygon mirror.

However, the speed of a brushless DC motor that is used to drive a photoconductive drum40,42,44,46may be adjusted over a relatively wide range and still maintain a robust phase lock to maintain a relatively constant rotational velocity. As such,FIGS. 3-5illustrate by way of illustration, and not by way of limitation, laser beam control for printhead unit24such that several speed modes can be realized.

Controller74controls the motor for polygon mirror78of each printhead unit24,26,28,30, the motor for rotating each photoconductive drum40,42,44,46, and laser diodes71appearing in each printhead unit so that printer10is able to print at several printing points. For example, controller74is configurable to print at low processing speeds, such as about 30 to about 40 ppm, more typical processing speeds, such as about 55 ppm to about 70 ppm, and high processing speeds, such as up to about 120 ppm. This relatively wide range of processing speeds may be accomplished while keeping the motor for polygon mirror78within a desired range between about 18 k rpm and about 38.5 k rpm to avoid instability and inducing jitter in the print output.

In an example embodiment, controller74individually activates laser diodes71of printhead unit24to provide for a wide range of printing performance. Specifically, during a print operation, each laser diode71may be activated to generate scan lines of image data that sweep across the surface of the photoconductive drum40, or deactivated in which the deactivated laser diode(s)71will not contribute to creating a latent image on photoconductive drum40during the print operation. A laser diode71that is activated may remain activated during the entire print operation so that the laser beam generated thereby is deflected from each facet80of the polygon mirror78and swept onto the surface of photoconductor drum40, or activated so that the laser beam is deflected from less than all facets80. In an example embodiment, though, an activated laser diode71will not be deactivated and unused during a portion of the print operation corresponding to certain one or more facets80of the polygonal mirror78. Controller74selects for activation any number of laser diodes71, from one diode71to all diodes71, for use in generating laser beams for sweeping across the surface of conductive drum40during a print operation.

In an example embodiment, for each printhead unit, controller74not only selects and activates a number of laser diodes71for a print operation, but also selects the speed for the motor of polygon mirror78as well as the speed of the motor for each of photoconductive drum40. The speed of the motor for photoconductive drum40, which corresponds to the processing speed of printer10, may be based, for example, upon a user selection of media type or media size. Media type and size may affect fusing time and thus affect processing speed accordingly. The processing speed itself, and thus the speed of photoconductive drum40, may also be selected by the user of printer10. The processing speed may be user selected simply by allowing printer10to print at a default speed, such as 70 ppm. The number of laser diodes71activated for a printer operation may be selected by controller74so that the resulting speed of polygon mirror78is maintained within an acceptable range of speeds.

In this way, the selection of the number of laser diodes71to use for creating the latent image in a print operation may be viewed similar to a motor vehicle transmission. The vehicle's engine, in this analogy corresponding to the motor for rotating polygon mirror78, has a limited range of operation but the vehicle, corresponding to printer10, has a much wider range of operating speeds, corresponding to the wide range of processing speeds of printer10. The motor vehicle's transmission is the mechanism that bridges the vehicle's engine's speed to the vehicle's speed, as the selection of laser diodes71serves to bridge or couple the speed of the motor for polygon mirror78to the processing speed of printer10.

FIG. 3illustrates the operation of printer10in four different operating modes in accordance with an example embodiment. Each operating mode illustrated utilizes a non-interlacing scheme for impinging the surface of photoconductive drum40with laser beams50. Printer10prints an image at 600 dpi at each operating mode. Each operating mode illustrated includes up to four columns, with each column corresponding to a facet80of polygon mirror78which intercepts laser beams50and deflects same towards photoconductive drum40. It is understood that polygon mirror78includes more than four facets80, each of which intercepts laser beams50a-50dand that only four facets80are shown for reasons of simplicity. The rows represent the process direction position of a laser scan sweep on the surface of photoconductive drum40. As illustrated, the first laser beam50a,designated beam A, is modulated in accordance with image data and deflected from every facet80of polygon mirror78. Depending upon the number of diodes activated for the print operation, beam A will scan across the photoconductive drum surface every 84.67 microns for the two diode mode, every 127 microns for the three diode mode, every 169.33 microns for the four diode mode, and every 211.67 microns for the five diode mode.

Similarly, the second laser beam50b,designated B, is modulated in accordance with image data corresponding to the facet resolution and is spaced about 42.33 microns from laser beam A from the same mirror facet, as described above. Third laser beam50c,designated C, is modulated with image data and spaced about 42.33 microns from laser beam B from the same mirror facet, and fourth laser beam50d,designated D, is modulated with image data and spaced about 42.33 microns from laser beam C from the same mirror facet. A fifth laser beam50e,designated E, generated from a fifth laser diode71according to an embodiment including at least five laser diodes71, is similarly modulated and spaced along the surface of the photoconductive drum about 42.33 microns from laser beam D. As can be seen inFIG. 3, activating a greater number of laser diodes71results in more scan lines impinging onto the surface of the photoconductive drum40with each mirror facet, thereby resulting in the print operation being completed faster than the time it takes for a print operation using a lesser number of diodes71.

FIG. 4illustrates the operation of printer10in three additional operating modes in accordance with an example embodiment. Each operating mode illustrated utilizes an interlacing scheme for impinging the photoconductive surface with laser beams50. The effective scanning resolution is increased to 1200 dpi in the process direction. Each operating mode illustration utilizes a different number of laser diodes71during the print operation. As can be seen inFIG. 4, activating a greater number of diodes71for the print operation results in more scan lines impinging the surface of the photoconductive drum40with each mirror facet, thereby resulting in the print operation being completed faster than the time it takes for a print operation using a lesser number of diodes.

FIG. 5illustrates the operation of printer10at both 600 dpi and 1200 dpi resolutions with different number of laser diodes71activated.FIG. 6is a table showing, for each process speed between 30 ppm and 85 ppm, the number of laser diodes71that may be selected by controller74and the speed of the motor for polygon mirror78. As can be seen, the speed of polygon mirror78remains within an acceptable range of speeds, between about 21 k rpm and about 35 k rpm. It is understood, however, that flexibility exists in the selection of the number of laser diodes71to be activated for a print operation to account for various user selections or device settings concerning a user's print operation.

It has been observed that example embodiments achieve a shorter time to first print (TTFP) and time to first copy (TTFC) when a greater number of laser diodes71are activated for a print operation. For a process speed of 70 ppm, for example, use of four laser diodes71for a print operation has been seen to provide TTFP/TTFC times of about four seconds and use of three laser diodes71has been seen to provide between about 5.5 seconds and about six seconds. Accordingly, controller74may select the number of laser diodes71for activation for a print operation based in part upon a user selection or printer setting of a TTFP/TTFC time, wherein the selection or setting of shorter TTFP/TTFC times may result in controller74selecting a larger number of laser diodes71for activation and the selection or setting of longer TTFP/TTFC times may cause controller74to select a smaller number thereof

It has been further observed that, at the same process speed and resolution, the printed image generated using a larger number of laser diodes71is darker than the printed image generated using a smaller number of laser diodes71. This observation may be utilized to address printed images becoming darker when process speeds are slowed and different operating points being sometimes needed to compensate for the increased darkness.

Accordingly, controller74may be configured to provide sufficient compensation to account for a change in image darkness by selecting the number of laser diodes71for a print operation based upon process speed, wherein relatively slower process speeds may result in controller74selecting a lesser number of laser diodes71for activation than the number of laser diodes71that may be selected for a print operation at a faster process speed. In addition or in the alternative, controller74may select the number of laser diodes71for a print operation based in part upon a user selected darkness setting, wherein a relatively dark setting selected may result in controller74selecting a greater number of laser diodes71for activation than the number of laser diodes71that may be selected for a print operation in which a lighter image is selected.

FIG. 3illustrates that for a fixed process speed, polygon mirror78must rotate at a higher speed when a relatively small number of laser diodes71are activated for a print operation than the mirror speed when a larger number of laser diodes71are so activated. A motor operating at higher speeds typically generates more noise than at slower speeds, and acoustic performance may be an important operating characteristic for some printing applications. Accordingly, controller74may be configured to select the number of laser diodes71for activation for a print operation based in part upon a desired level of acoustic performance by printer10, wherein a greater number of laser diodes71may be activated for the print operation when less noise is desired (i.e., a “quiet mode” of operation) than the number of laser diodes71selected when noise level is less of a concern. An acoustic performance setting may be, for example, selected by a user or a default printer setting for printer10.

The term “overscan” generally refers to a printhead unit24,26,28,30having multiple scan lines containing imaging data being written on the location of the surface of a photoconductive drum. Overscanning may be used to improve the print quality of a printed image. Controller74may thus be configured to select a number of laser diodes71for activation as well as the speed of polygon mirror78during a print operation based in part upon a determination of the need to perform overscanning in generating the corresponding printed image.

Printhead unit24is described above as including a plurality of laser diodes71, each of which generates a laser beam that is deflected from the facets80of polygon mirror78onto the surface of photoconductive drum40during a print operation. The particular angle at which a laser beam is incident upon the facet80of polygon mirror78in part determines the location of the beam's scan line on the surface of photoconductive drum40. In circumstances in which less than all of the laser diodes71are activated for creating a latent image for a print operation, controller74may be configured to select the particular laser diodes71for activation based in part upon the location and orientation of each diode71relative to polygon mirror78.

Further, in an example embodiment, a laser diode71that is unselected for activation in creating a latent image on the surface of photoconductive drum40during a print operation may be used to perform another function, such as a function at lower power than the level of laser power normally utilized in creating the latent image. For example, an unselected laser diode71may be used to provide an erase operation in which the surface of the photoconductive drum40is modified by passing one or more scan lines of a laser beam from an unselected laser diode71operating at reduced power. Accordingly, controller74may be configured to select less than all of the laser diodes71for use in creating the latent image on photoconductive drum40, and use at least one unselected laser diode71to perform an erase operation or other operation at lower laser power levels. The benefit of erase operations is known as described in U.S. Pat. No. 6,356,726, assigned to the assignee of the present application, the content of which is incorporated by reference herein it its entirety.

The foregoing description of several methods and an embodiment of the invention have been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise steps and/or forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, the example embodiments described herein utilize a polygon mirror for deflecting the laser beams for creating scan lines of image data. In an alternative embodiment, a torsion or galvanometer oscillator is utilized for deflecting the laser beams.

It is intended that the scope of the invention be defined by the claims appended hereto.