Patent Application
20070071469
The electrophotography process used in some imaging devices, such as laser printers and copiers, utilizes electrical potentials between components to control the transfer and placement of toner. These electrical potentials create attractive and repulsive forces that tend to promote the transfer of charged toner to desired areas while ideally preventing transfer of the toner to unwanted areas. For instance, during the process of developing a latent image on a photoconductive surface, charged toner particles may be deposited onto latent image features (e.g., corresponding to text or graphics) on the photoconductive surface having a lower surface potential than the charged particles. At the same time, the charged toner particles may be prevented from transferring or migrating to more highly charged areas (e.g., corresponding to the document background) of the same photoconductive surface. In this manner, imaging devices implementing this process may simultaneously generate images with fine detail while maintaining clean backgrounds.
The precise magnitudes of these electrical potentials and the nature of the voltages (e.g., AC or DC) varies among devices and manufacturers. In general, however, a laser or imaging source is used to illuminate and selectively discharge portions of a photoconductive surface to create a latent image having a lower surface potential than the remaining, undischarged areas of the photoconductive surface. The toner is charged to some intermediate level between the discharge potential of the latent image and the surface potential of the undischarged photoconductive surface. The toner may be charged triboelectrically and/or via biased toner delivery control components, such as a toner adder roll, a doctor blade, and a developer roller. The developer roller supplies toner to develop the latent images on the photoconductive surface. The developed image is ultimately transferred onto a media sheet, typically by employing yet another surface potential that attracts the toner off of the photoconductive surface (or an intermediate transfer surface) and onto the media sheet where it is ultimately fused.
The various surface potentials may be optimized to strike a balance between maintaining clear backgrounds while producing quality images with fine detail. For example, the surface potential of a developer roller may be optimized to develop images with a desired toner density. Another variable termed a “white vector” may be optimized as well. White vector refers to the difference between the surface potential of the developer roller and the surface potential of undischarged portions of a photoconductive surface. An optimal white vector achieves certain desirable characteristics, one of which is to provide a clean media sheet with little or no appreciable background toner in areas other than where printing is desired. Very large white vector values may adversely affect the density of deposited toner and detail of a resulting image. This problem may be more apparent with fine, isolated features where the illumination energy applied to form such features may be insufficient to discharge the photoconductive surface. Conversely, as white vector values fall, unwanted background may begin to appear.
In addition, image quality may be affected by imaging power. Imaging power affects the formation of the latent image on a photoconductive surface. For instance, a low imaging power may be insufficient to discharge the photoconductive surface, particularly with a large white vector. One method of overcoming this problem is to locally control the background energy density on the surface of the photoconductor, particularly in the vicinity of isolated features or isolated clusters of features. The background energy or charge on the photoconductive surface may be controlled on a global basis through some combination of white vector control and discharge via illumination. However, print density variations may call for local control over background energy. As a result, improved image production may be obtained through local modifications of background energy density on the basis of feature density.
Embodiments of the present invention are directed to local control of photoconductive surface charge levels in the vicinity of image features having a predetermined image density. The embodiments are applicable in an image forming unit having a photoconductive unit, a charger unit to apply a charge to the surface of the photoconductive unit, an imaging unit forming one or more latent image features on the surface of the photoconductive unit, a developer member supplying toner to develop the latent image, and a controller to selectively control the various bias levels applied to these components.
A first charge is applied to bias the surface of the photoconductive unit to a first bias level. A window having multiple cells may be placed over image features and selected cells of the window may be discharged to modify the first bias level within the window to a second average bias level. The window may be centered over the image features. The individual cells of the window may be discharged by illuminating the cells with a first imaging power that is lower than a second imaging power that is used to illuminate the surface of the photoconductive unit to create a latent image of the image features. In one embodiment, cells in the window may be discharged upon identifying whether an image feature has a print density that is below a predetermined threshold. In general, more of the window cells may be discharged as the print density decreases. A third bias level may be established on a surface of a developer member, with the difference between the first and third bias levels termed a white vector value. More of the discrete cells may be discharged as the white vector value increases.
In electrophotographic image development, certain operating points may be varied and optimized to produce high quality images with little or no background noise (i.e., toner particles not intended to be transferred to the media sheet). Even with various surface bias levels and imaging power level optimized, some additional improvement to fine features may be obtained through localized optimization of background energy density. Optimization of the background energy density in a device such as the image forming apparatus 100 generally illustrated in
Within the image forming device housing 102, the image forming device 100 includes one or more removable developer cartridges 116, photoconductive units 12, developer rollers 18 and corresponding transfer rollers 20. The image forming device 100 also includes an intermediate transfer mechanism (ITM) belt 114, a fuser 118, and exit rollers 120, as well as various additional rollers, actuators, sensors, optics, and electronics (not shown) as are conventionally known in the image forming device arts, and which are not further explicated herein. Additionally, the image forming device 100 includes one or more system boards 80 comprising controllers (including controller 40 described below), microprocessors, DSPs, or other stored-program processors (not specifically shown in
Each developer cartridge 116 may include a reservoir containing toner 32 and a developer roller 18, in addition to various rollers, paddles and other elements (not shown). Each developer roller 18 is adjacent to a corresponding photoconductive unit 12, with the developer roller 18 developing a latent image on the surface of the photoconductive unit 12 by supplying toner 32. In various alternative embodiments, the photoconductive unit 12 may be integrated into the developer cartridge 116, may be fixed in the image forming device housing 102, or may be disposed in a removable photoconductor cartridge (not shown). In a typical color image forming device, three or four colors of toner—cyan, yellow, magenta, and optionally black—are applied successively (and not necessarily in that order) to an ITM belt 114 or to a print media sheet 106 to create a color image. Correspondingly,
The operation of the image forming device 100 is conventionally known. Upon command from control electronics, a single media sheet 106 is “picked,” or selected, from either the primary media tray 104 or the multipurpose tray 110 while the ITM belt 114 moves successively past the image forming units 10. As described above, at each photoconductive unit 12, a latent image is formed thereon by optical projection from the imaging device 16. In one embodiment, an imaging device 16 capable of producing an exposure level of about 1.1 micro-Joules per square centimeter at 100% power may be used. The latent image is developed by applying toner to the photoconductive unit 12 from the corresponding developer roller 18. The toner is subsequently deposited on the ITM belt 114 as it is conveyed past the photoconductive unit 12 by operation of a transfer voltage applied by the transfer roller 20. Each color is layered onto the ITM belt 114 to form a composite image, as the ITM belt 114 passes by each successive image forming unit 10. The media sheet 106 is fed to a secondary transfer nip 122 where the image is transferred from the ITM belt 114 to the media sheet 106 with the aid of transfer roller 130. The media sheet proceeds from the secondary transfer nip 122 along media path 38. The toner is thermally fused to the media sheet 106 by the fuser 118, and the sheet 106 then passes through exit rollers 120, to land facedown in the output stack 124 formed on the exterior of the image forming device housing 102. A cleaner unit 128 cleans residual toner from the surface of the ITM belt 114 prior to the next application of a toner image.
The representative image forming device 100 shown in
The latent image thus formed on the photoconductive unit 12 is then developed with toner from the developer roller 18, on which is adhered a thin layer of toner 32. The developer roller 18 is biased to a potential that is intermediate to the surface potential of the discharged latent image areas 28 and the undischarged areas not to be developed 30. In the embodiment depicted, the developer roller 18 is biased to a potential of approximately −600 volts. Negatively charged toner 32 is attracted to the more-positive discharged areas 28 on the surface of the photoconductive unit 12 (i.e., −300V vs. −600V). The toner 32 is repelled from the less-positive, non-discharged areas 30, or white image areas, on the surface of the photoconductive unit 12 (i.e., −1000V vs. −600V), and consequently, the toner 32 does not adhere to these areas. As is well known in the art, the photoconductive unit 12, developer roller 18 and toner 32 may be charged alternatively to positive voltages.
In this manner, the latent image on the photoconductive unit 12 is developed by toner 32, which is subsequently transferred to a media sheet 106 by the positive voltage of the transfer device 20, approximately +1000V in the embodiment depicted. Alternatively, the toner 32 developing an image on the photoconductive unit 12 may be transferred to an ITM belt 114 and subsequently transferred to a media sheet 106 at a second transfer location (not shown in
The above description relates to an exemplary image forming unit 10. In any given application, the precise arrangement of components, voltages, power levels and the like may vary as desired or required. As is known in the art, an electrophotographic image forming device may include a single image forming unit 10 (generally developing images with black toner), or may include a plurality of image forming units 10, each developing halftone images on a different color plane with a different color of toner (generally yellow, cyan and magenta, and optionally also black).
The difference in potential between non-discharged areas 30 on the surface of the photoconductive unit 12—that is, white image areas or areas not to be developed by toner—and the surface potential of the developer roller 18 is known as the “white vector.” This potential difference (with the white image areas 30 on the surface of the photoconductive unit 12 being less positive than the surface of the developer roller 18 in the embodiment depicted) provides an electro-static barrier to the development of negatively charged toner 32 on the white image areas 30 of the latent image on the photoconductive unit 12. A sufficiently high white vector is necessary to prevent toner development in white image areas; however, an overly large white vector detrimentally affects the formation of fine image features, such as small dots and lines. In exemplary embodiments of image forming devices, a white vector as low as 200-250V may result in acceptable image quality while preventing toner development in white image areas. Unfortunately, the optimal white vector for each image forming unit 10 within an image forming device may be different, due to environmental conditions, differing toner formulations, component variation, difference in age or past usage levels of various components, and the like. Controller 40, via sensor 126, monitors toner 32 formation on media sheet 106 or belt 114 and adjusts the surface potential of the surface of photoconductive unit 12 (via charging device 14) and the surface potential of developer roller 18. Thus, while exemplary voltages establishing a white vector of 400V (i.e., |−1000V-−600V|) are explicitly shown in
In an exemplary embodiment, controller 40 at least partially manages the formation of a predetermined pattern of toner 32 on a substrate, which may comprise a media sheet 106 or belt 114 (e.g., a transfer or ITM belt). A toner patch sensor 126 detects a reflectivity of the transferred pattern and controller 40 adjusts the bias voltage of the charging device 14 and/or developer roller 18 as needed to optimize image formation at least partly based on information provided by the toner patch sensor 126. The controller 40 may adjust the developer 18 bias accordingly to achieve a target reflectivity.
With the developer roller 18 bias established relative to the discharge bias of latent images 28 on the surface of the photoconductive unit 12, the white vector may now be determined relative to the developer roller 18 bias. That is, in this exemplary embodiment, the white vector is established by adjusting the charging device 14 bias level while maintaining a fixed developer roller 18 bias. A detailed description of various methods of optimizing white vector in an electrophotographic image forming device is provided in commonly assigned U.S. patent application Ser. No. 11/126,814 entitled “White Vector Feedback Adjustment” filed May 11, 2005, the relevant portions of which are incorporated herein by reference.
The white vector establishes the surface bias that is applied to the surface of photoconductive unit 12. This surface potential is discharged through illumination by an imaging device 16 to create a latent image that is subsequently developed. In certain instances, the white vector may be set relatively high (thus increasing the surface bias applied to the photoconductive unit 12) to prevent unwanted background toner. Unfortunately, the relatively high surface bias applied to the photoconductive unit 12 makes it difficult to effectively discharge the photoconductive surface by illumination thereof. This situation is particularly problematic for fine and/or isolated image features.
In contrast,
Accordingly, there may be an optimal white vector WV value that prevents background toner while creating quality images in most situations. A problem arises when the image forming unit 100 is tasked with reproducing very fine details or very isolated details. These types of features are often characterized in that a small amount of toner is desired in an area that is otherwise free from toner. This situation may be represented by the bias levels shown in
Even with white vector WV optimized for given conditions, and imaging power optimized to produce quality latent images in most situations, there may still be problems reproducing fine or isolated details. This may be due, in part, to the fact that a relatively small amount of optical energy is used to create latent images 28 of these features. As a result, the latent image 28 of fine and isolated features may not be fully discharged. This is represented in
Reviewing the different scenarios illustrated in
A further enhancement of the image formation process considers the density of toner features that are being reproduced. The schematic illustrations provided in
In general, the illumination power from the imaging unit 16 may be distributed as a Gaussian curve with a peak at the center of the incident energy and tails on either side. While two dimensions are represented in
Therefore, in one embodiment, the localized background energy density may be altered as shown in
This natural drop 512 in photoconductor surface bias may improve image quality by lowering the latent image 28 bias levels. However, if the image features 500 are still somewhat sparse, some improvement may be gained by inducing a second bias drop 522 in the region surrounding the isolated image features 500. As above, this second bias drop 522 may be generated through illumination from the imaging device 16 and lowers the bias level on the surface of the photoconductive unit 12 to an intermediate level 524 that is below the charge level 516 established by charging unit 14. In the present example, the second bias drop 522 induced for a small cluster of features 500 may be less than the bias drop 412 induced for a single isolated feature 400. Similarly, other modifications to the background energy density 410, 510 may be induced in relation to the density of printed features.
It should be noted that the examples provided in
Illumination energy may be applied to discrete positions 806 in the window 804 in a manner that is analogous to halftoning of grayscale images. With respect to image reproduction, halftoning may produce a picture in which gradations of light are perceived as a result of the relative darkness and density of dots produced in varying numbers within a fine screen area. With regards to the present embodiment, halftoning may produce a desired background energy density by varying the number of illuminated dots in the window 804. For instance,
In one embodiment, the illumination energy applied to the discrete positions 806 in the window 804 may be some fraction of the illumination energy that is used to illuminate the feature of interest 802. For example, if full imaging power is applied to illuminate the feature of interest 802, then some intermediate imaging power (between on and off) may be applied at the discrete positions 806. As another example, if an imaging power that is 50% of the capacity of the imaging device 16 is used to illuminate the feature of interest 802, then some value between 5% and 45% may be used to illuminate the discrete positions 806. The energy used to illuminate the discrete positions 806 should not be so large as to create false latent image features that attract toner from the developer roller 18. Thus, lower illumination energy values may be appropriate. The total energy density of the area within the window 804 can be calculated as an average of the off background cells, the illuminated discrete positions 806, and the energy produced by illumination of the feature of interest 802. Alternatively, the energy density of the area within the window 804 may be calculated as a distance-weighted average of the illuminated discrete positions 806. In one embodiment, illuminated discrete positions 806 that are closer to the feature of interest 802 are assigned a greater weight. A greater modification to the background energy density is produced as more discrete positions 806 are illuminated. The size of the window 804 may be changed depending on the resolution of the image, the resolution of the imaging device, and the printing halftone screen frequencies. For instance, a 9×9 window 904 as shown in
Those skilled in the art should appreciate that the illustrated controller 40 shown in
Further, those skilled in the art of electrophotographic illumination should comprehend that application of the different illumination energy levels may be performed through pulse-width modulating the current to the imaging device 16. Pulse-width modulation is a technique well known in the art whereby the total current supplied to a load is controlled by altering the duration of time during each of a series of repetitive periods in which current is driven. In other words, by controlling the “duty cycle” of periodically driving current to the load, the net current received by the load may be precisely controlled. Pulse-width modulation may find particular utility in applications where the controller 40 is digital. In another embodiment of the present invention, the current received by the imaging device 16 is the sum of separate current sources. In another embodiment, the current received by the imaging device is controlled by a binary control string that establishes the current generated by a digital high voltage power supply.
The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. For example, the halftoning approach described above contemplated applying a lowered illumination energy at discrete points 806 in a window surrounding a feature of interest 802. In other embodiments, varying illumination energies may be applied at discrete points 806 in the window 804, 904. For instance, a larger illumination energy may be applied at discrete points 806 that are closer to or farther from the feature of interest 802. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
