Patent Application
20100080585
None.
1. Field of the Invention
The present invention relates generally to an electrophotographic (EP) image forming device and, more particularly, to a system and method for adjusting a selected operating parameter, namely bias voltage of the charge roll, of the image forming device based on selected environmental conditions, namely, wet-bulb temperature values derived from dry-bulb temperature and relative humidity sensor readings, to control white vector, the difference in surface potential of non-discharged areas of a photoconductive (PC) drum from surface potential of a developer roll, and thereby reduce background toner.
2. Description of the Related Art
The electrophotography (EP) process used in image forming 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 the surface of a PC drum, negatively charged toner particles deposit onto less negatively charged (positive relative to the toner's charge) latent image feature areas (e.g., corresponding to text or graphics) on the PC drum surface. At the same time, the negatively charged toner particles are prevented from transferring or migrating to more negatively charged areas (e.g., corresponding to the document background) of the same PC drum surface. In this manner, image forming devices implementing this process can simultaneously generate images with fine detail while maintaining clean backgrounds.
In general, a laser imaging source is used to illuminate and selectively discharge the desired areas of the PC drum surface to create the latent image so that it will have a lower surface potential than the remaining, undischarged areas of the PC drum surface. The developer roll, where a layer of charged toner is located, is biased to an intermediate level between the discharge potential of the latent image and the surface potential of the undischarged areas of the PC drum surface. The toner can be charged triboelectrically and/or via biased roll delivery control components, such as a toner adder roll, a doctor blade, and a developer roll. The developer roll supplies toner to develop the latent image areas on the PC drum 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 PC drum surface (or an intermediate transfer surface) and onto the media sheet where it is ultimately fused.
The difference between the surface potential of the developer roll and the surface potential of undischarged areas of the PC drum surface is referred to as a white vector. 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. The magnitude of the white vector needed to prevent background toner is a function of numerous factors, including developer material, environment, imaging device components, and age. Traditionally, image forming devices incorporating an EP process operate with a white vector that is fixed, but large enough to overcome the factors that contribute to unwanted background toner.
Very large white vector values are not necessarily the most desirable solution because the density of deposited toner and detail of the resulting image may be adversely affected and could lead to background toner if wrong-sign toner (toner charged positively) is present. Conversely, as white vector values fall, unwanted background toner may begin to appear. Determining an optimal white vector that is somewhere between these extremes and that accounts for the aforementioned factors and varying operating conditions is a legitimate problem that is not solved by setting a fixed operating point.
Although a majority of image forming devices, such as laser printers, operates in an air-conditioned office environment, such environment may not necessarily be controlled for humidity. It is important that a printer yields high print quality over a wide range of environments. As temperature and humidity of the ambient environment change, the electrical properties of printer components can also change which can have a significant impact on print quality. Heretofore, “cold start” servo voltage has been used to select or adjust charge roll bias. Cold start servo voltages are the servo values recorded when the printer is first powered on or after the printer has been idle. However, print quality requirements have made servo algorithms not accurate enough for optimizing charge roll bias to minimize background toner in all environments.
One approach to resolving this problem of controlling white vector is disclosed in U.S. Pat. No. 7,398,025 assigned to the assignee of the present application. The entire disclosure of this patent is hereby incorporated herein by reference. This patent proposes to control and adjust white vector by using one or more control circuits adapted to control the formation of a given image pattern on a substrate, such as a transport belt, transfer belt, or media sheet. The circuits utilize sensors to detect the coverage of the developed image pattern on the PC drum surface or on the substrate. White vector may then be adjusted in response to a comparison between the detected coverage of the developed image and the desired coverage of the developed image. In one embodiment, background noise is used as an indicator that white vector needs to be adjusted. In another embodiment, reflectance of a developed pattern is used to detect the coverage or bloom of the pattern relative to a predetermined standard. Iterative procedures also are used to determine an optimum operating point.
While the approach of this patent might represent a step in the right direction toward resolution of this problem, its implementation is not always feasible requiring an innovation that will provide an alternative approach to its resolution.
The present invention meets this need by providing an innovation that is directed to a more general or global approach to the resolution of the problem of controlling white vector. This approach dispenses with sensing the current condition of white vector in a developed pattern in comparison to a given desired pattern. Instead, this approach is directed toward adjusting an operating parameter, namely, the bias voltage of the charge roll, based on a wet-bulb temperature value derived from dry-bulb temperature and relative humidity readings from sensors of current environmental conditions, by application of an offset voltage value, correlated to the wet-bulb temperature value, instead of the currently-practiced cold start servo voltage. This offset adjustment will allow for better charge roll voltage optimization and optimal white vector control and thus better print quality, obviating the need to sense and then adjust the current condition of white vector in a developed pattern.
Accordingly, in an aspect of the present invention, a system for adjusting a selected operating parameter of an image forming device based on selected environmental conditions to control white vector includes a photoconductive unit, a charging unit having a surface biased to a voltage operative to charge a surface of the photoconductive unit, an imaging unit forming a latent image on areas of the surface of the photoconductive unit by selectively discharging the areas of the surface thereof by illumination thereof, leaving non-discharged areas of the surface different from the discharged areas thereof, a developer unit having a surface biased to a voltage operative to develop toner to the latent image on the discharged areas of the surface of the photoconductive unit, a sensor mechanism for measuring selected environmental conditions of dry-bulb temperature and relative humidity, a control mechanism for reading the sensor mechanism to adjust the voltage bias of the charging unit by applying an offset thereto based on a wet-bulb temperature value so as to minimize a white vector represented by the difference in potential between non-discharged areas on the surface of the photoconductive unit and the surface potential of the developer roll without enabling onset of development of toner background on the non-discharged areas of the surface of the photoconductive unit, and a memory connected to and accessible by the control mechanism and having stored therein lists of correlated values comprising a list of wet-bulb temperature values related to values of dry-bulb temperature and relative humidity measured by the sensor mechanism correlated to a list of voltage bias offsets related to the voltage bias of the charging unit to control the white vector.
In another aspect of the present invention, a method for adjusting a selected operating parameter of an image forming device based on selected environmental conditions to control white vector includes biasing a charging unit to a voltage operable to charge a surface of a photoconductive unit, discharging selected areas on the surface of the photoconductive unit to form a latent image thereon leaving non-discharged areas different from the discharged areas, biasing a developer unit to a voltage operative to develop with toner the latent image on the discharged areas of the surface of the photoconductive unit, sensing selected environmental conditions of dry-bulb temperature and relative humidity so as to determine wet-bulb temperature values correlated with said dry-bulb temperature and relative humidity, and adjusting the voltage bias of the charging unit by applying an offset thereto correlated to one of the wet-bulb temperature values so as to minimize white vector without enabling onset of development of toner background on the non-discharged areas of the surface of the photoconductive unit. The method further includes storing a lookup table in memory of a list of wet-bulb temperature values correlated to a list of voltage bias offsets, and accessing the lookup table from memory with a wet-bulb temperature value to determine a correlated value of voltage offset biases to apply to the charging unit.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numerals refer to like elements throughout the views.
Referring now to
Color printers typically include four image forming stations 100 for printing with cyan, magenta, yellow, and black toner to produce a four-color image on the media sheet. The media transport belt 20 conveys the print media with the color image thereon to the fuser roller 24, which fixes the color image on the print media. Exit rollers 26 either eject the print media to the output tray 28, or direct it into a duplex path 30 for printing on a second side of the print media. In the latter case, the exit rollers 26 partially eject the print media and then reverse direction to invert the print media and direct it into the duplex path. A series of rollers in the duplex path 30 return the inverted print media to the primary media path for printing on the second side. Also, the auxiliary feed 32 of the image forming device 10 may be utilized to manually feed media sheets into the device 10.
Turning now to
Referring to both
The non-discharged areas 114 of the surface 102A of the PC drum 102 that are not part of the discharged areas 116 forming the latent image and not to be developed by toner, which are referred to as “white” or “background” image areas, retain the potential induced by the charge roll 104, e.g., approximately −800 v in the illustrated embodiment. As a result, toner is repelled from these white image areas 114 on the surface 102A of the PC drum 102 and consequently toner does not adhere to these areas. The difference in potential between non-discharged areas 114 on the surface 102A of the PC drum 102, that is, white image areas or areas not to be developed by toner, and the surface potential of the developer roll 106 is known as white vector, as explained hereinabove. A sufficiently high white vector is necessary to prevent toner development in the white image areas 114; however, an overly large white vector affects detrimentally the formation of fine image features, such as dots and lines and could lead to a wrong-sign toner background. Typically, a white vector of 200-250 v results in acceptable image quality while preventing toner development in white image areas. Unfortunately, the charge roll bias voltage required for obtaining a certain white vector and the optimal white vector itself vary due to environmental conditions, such as dry-bulb temperature and relative humidity.
The controller 40 employs a charge roll environmental offset to minimize the white vector between the developer roll 106 and the surface 102A of the charged PC drum 102 so as to reduce wrong-sign toner background while preventing onset of development or background toner on the non-discharged areas 114. Previously the charge roll voltage bias was set based on the cold start servo voltage. Cold start servo voltages are the servo values recorded when the printer is first powered on or after the printer has been idle. In order to establish an algorithm for adjusting the charge roll voltage using dry-bulb temperature and relative humidity readings, different outputs were correlated to changes in EP performance over a range of environments. The intent was to minimize the white vector while preventing the onset of development background (i.e. toner development when the PC drum 102 is not charged high enough). The shift in the onset of development over a range of environments showed a correlation with wet-bulb temperature.
The charge roll environmental offset established by the algorithm for adjusting the charge roll voltage using wet-bulb temperature values are provided in a lookup table set forth in
By way of further explanation, web-bulb temperature is the temperature of a volume of air that is cooled to saturation at constant pressure by evaporating water into the air without adding or removing heat. A wet-bulb thermometer approximates wet-bulb temperature by measuring the temperature of the tip of the thermometer covered by a wet cloth. When the relative humidity is below 100%, water evaporates from the cloth and effectively cools the tip of the wet-bulb thermometer. Essentially, wet-bulb temperature is a quantity that combines temperature and humidity into a single value that can be used to differentiate one environmental condition from another. Though temperature and humidity measurements change significantly within the first several minutes of printing, wet-bulb temperature does not change significantly for a given environment, and serves as a quantity that can be used to determine ambient environmental conditions regardless of internal machine temperature. Iterative numerical-methods techniques were used to fit a quadratic surface to data taken from a psychrometric chart. The quadratic surface establishes an orthogonal relationship for dry-bulb temperature, relative humidity, and wet-bulb temperature. A best fit quadratic surface to approximate wet-bulb temperature as a function of dry-bulb temperature and relative humidity can be written in the following form: Z=AX̂2+BŶ2+CXY+DX+EY+F; where: A=−0.00079, B=−0.00047, C=0.00479, D=0.59473, E=0.1003, and F=−6.32789; and: X=Dry-bulb Temperature (° C.) read from a thermistor, Y=Relative Humidity (% RH), and Z=Wet-bulb Temperature (° C.).
Turning to
The foregoing description of several embodiments of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.
