Patent Application
20100080587
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 operating parameters, namely, bias voltages of charge and developer rolls, of the image forming device based on selected environmental conditions, namely, wet-bulb temperature values derived from dry-bulb temperature sensor and relative humidity sensor readings, to improve color registration.
2. Description of the Related Art
An EP image forming device, such as a single pass EP printer, typically employs four image forming stations, each one responsible for printing one of four primary colors, typically cyan, magenta, yellow, and black. The individual images, known as separations, produced by each of the four image forming stations are combined to produce the final output image. In tandem print engines, the four image forming stations are aligned in the sheet transport direction such that each separation is formed in succession on the copy sheet as the copy sheet is transported through each print station. Typically, a belt transports the copy sheet. In some printers, a belt serves as an intermediate transfer member (ITM). The image forming stations transfer the individual image separations onto the ITM to form a composite image on the ITM. The composite image is then transferred from the ITM to the copy sheet at a transfer station.
The alignment of the image separations produced by each image forming station is critical to producing a quality printed image. Various factors affect the proper alignment of the image forming stations, such as tolerances, wear, and thermal expansion and contraction. It can be expensive and impractical to control tolerances and wear in order to provide acceptable color registration. Therefore, many printers employ various other techniques to detect and correct for color registration errors.
One technique used to detect color registration errors, referred to as the reverse transfer process, is disclosed in U.S. Pat. No. 7,257,358 assigned to the assignee of the present application. The entire disclosure of this patent is hereby incorporated herein by reference. The basic idea underlying the reverse transfer process, as explained in this patent, is to print a registration mark at a first image forming station and to partially erase or remove the registration mark, printed by the first image forming station, at a second image forming station by reverse transfer of the toner. The registration mark may be printed, for example, on the media transport belt, on an ITM belt, on a media sheet, or some other substrate. The second image forming station does not print a registration mark, but instead partially erases the registration mark printed by the first image forming station to form the final registration pattern. A latent image of a second registration mark is formed by a laser as a discharged area on a photoconductive (PC) drum at the second image forming station, but is not developed. A controller controls the charge of the PC drum and a transfer device so that the PC drum attracts toner from the media transfer belt or ITM belt in areas when the latent image of the second registration mark overlap the first registration mark. When a registration sensor detects that certain specified portions of the first registration mark are completely or nearly completely erased by the overlapping latent image of the second registration mark, there is considered to be no registration error present.
In general, the reverse transfer of toner is maximized when the charge on the PC drum is as high as possible and the voltage of the discharged area is as low as possible. This allows for the largest contrast between charged and discharged areas. The high PC drum charge prevents reverse transfer to areas not discharged by the laser. The low discharge voltage is desirable to create the least negatively charged surface that acts in conjunction with the negative transfer voltage to best attract the negatively-charged toner. Therefore, high charge voltages and high laser energies are desirable.
However, because the developer roll bias in the “reverse transfer” station must be set sufficiently low in order to prevent the development of toner there may be a large difference in potential between the bias on the developer roll and the charge on the PC drum surface. If the potential difference is sufficiently large it may cause Paschen breakdown at the interface between the developer roll and PC drum. (Paschen breakdown voltage is one at which the insulation of air breaks down and an avalanche condition ensues allowing flow of ions.) If there is Paschen breakdown at this interface, some of the toner on the developer roll may become wrong-signed and then transfer to the PC drum and the belt. This wrong-signed toner can interfere with the registration sensor's ability to detect the registration pattern. In some embodiments black toner registration patterns are being sensed on a black belt. Because the reflectivity of the toner may be similar to that of the belt, a higher gain mode may be required. Under these circumstances cyan, magenta, or yellow wrong-signed toner can have a particularly detrimental effect on the sensor's ability to detect the registration patterns.
Paschen breakdown between the developer roll and PC drum also happens at a lower potential difference in certain environmental conditions of temperature and humidity. Although a majority of color laser printers operate 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 and developer roll biases. Cold start servo voltages are the servo values recorded when the printer is first powered on or after the printer has been idle. However, changes to the printer architecture have made servo algorithms less accurate for optimizing charge roll and developer roll biases to optimize registration operating parameters in all environments.
Thus, when using the reverse transfer process in correcting registration errors, there is a need for an innovation to compensate for environmental conditions of temperature and humidity in order to maintain the maximum potential difference between the charged and discharged areas on the PC drum while still avoiding Paschen breakdowns.
The present invention meets this need by providing an innovation directed toward adjusting certain selected operating parameters, namely, bias voltages applied to the charge roll and developer roll, based on wet-bulb temperature values derived from dry-bulb temperature and relative humidity readings from sensors of current environmental conditions, in place of cold start servo voltage values as practiced heretofore. These adjustments will allow for better charge roll voltage optimization and thus better print quality and color registration.
Accordingly, in an aspect of the present invention, a system for adjusting selected operating parameters of an image forming device based on selected environmental conditions to improve color registration includes a first image forming station to print a first registration mark on a substrate, a second image forming station having a photoconductive unit, a charging unit and a developer unit cooperable together so as to at least partially erase the first registration mark printed on the substrate by the first image forming station to form a registration pattern in a reverse transfer process of the color registration, 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 biases of the charging and developer units of the second image forming station based on a wet-bulb temperature value so as to maintain a predetermined potential difference between charged and uncharged areas of the photoconductive unit of the second image forming station that avoids Paschen breakdown and the development of toner at charged areas of the photoconductive unit during the reverse transfer process of the color registration, and a memory connected to and accessible by the control mechanism and having a lookup table stored therein containing 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 with lists of voltage values related to voltage biases of the charging and developer units to maintain the predetermined potential difference between the charged and uncharged areas of the photoconductive unit during the reverse transfer process of the color registration.
In another aspect of the present invention, a method for adjusting selected operating parameters of an image forming device based on selected environmental conditions to improve color registration includes at a first image forming station printing a first registration mark on a substrate, at a second image forming station at least partially erasing the first registration mark printed on the substrate by the first image forming station to form a registration pattern in a reverse transfer process of the color registration performed at the second image forming station, sensing selected environmental conditions of dry-bulb temperature and relative humidity so as to determine wet-bulb temperature values correlated with the dry-bulb temperature and relative humidity, and adjusting voltage biases of charge and developer rolls of the second image forming station based on the wet-bulb temperature value so as to maintain a predetermined potential difference between charged and uncharged areas of a photoconductive unit of the second image forming station that avoids Paschen breakdown and the development of toner at charged areas of the photoconductive unit during the reverse transfer process of the color registration. The method further includes storing a lookup table in memory containing lists of correlated values comprising a list of wet-bulb temperature values related to values of sensed dry-bulb temperature and relative humidity correlated with lists of voltage values related to voltage biases of the charging and developer units to maintain the predetermined potential difference between the charged and uncharged areas of the photoconductive unit during the reverse transfer process of the color registration.
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 controller 40 uses feedback from the registration sensor 44 to control latent image formation on the PC drum 102 to correct for registration errors. To detect registration errors, the controller 40 causes the image forming device 10 to print a registration pattern on a substrate. In one exemplary embodiment shown in
Referring now to
To carrying out the foregoing reverse transfer process so as to improve color registration, the controller 40 employs an auto-alignment adjustment algorithm to adjust charge and developer roll voltage biases based on wet-bulb temperatures from a lookup table set forth in
By way of further explanation, the values in
Also, it should be noted that the lookup table in
Wet-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 the 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=0.10035, 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
Additionally, environmental conditions of dry-bulb temperature and relative humidity may cause changes in the properties of EP components of the first station (the image forming or forward transfer station), resulting in more or less toner being developed. It is generally desired for the image from the first station to be as dense as possible, so environmental conditions at the first station may also be used to adjust the developer roll bias. This is especially useful in a system where the charge roll high voltage power supply (HVPS) is shared between the image forming and reverse transfer stations so that the bias at the image forming station is set as high as possible without developing right-signed toner for a given charge roll voltage level.
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.
