Patent Publication Number: US-9851671-B2

Title: Method of controlling a printing process and controller therefor

Description:
BACKGROUND 
     Variations in the amount of ink applied to media can produce noticeable variations across a page, in particular, but not exclusively between the trailing edge of one page and the leading edge of the next. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a more complete understanding, reference is now made to the following description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates various components of an exemplary printing system including an example of a controller for controlling a printing process; 
         FIG. 2 a    is a flow diagram that illustrates an example of a method for controlling a printing process; 
         FIG. 2 b    is a flow diagram that illustrates, in more detail, the example of  FIG. 2 a   ; and 
         FIG. 3  illustrates various components of an exemplary printing device in which the printing system of  FIG. 1  can be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In a printing system, for example a printing system including a controller for voltage-controlling ink density, one or more of several developer voltages in the printing system can be adjusted to control a printing process such that a single printing station corresponding to a particular ink can print both standard and lighter versions of a color from the same ink source. In an implementation, a developer voltage for the printing station can be decreased such that fewer ink particles separate from the ink and a thinner layer, or less of a concentration, of the ink is transferred to appear lighter in color when printed as an image on a print media. Conversely, the developer voltage for the printing station can be increased such that more ink particles separate from the ink and a thicker layer, or more of a concentration, of the ink is transferred to appear darker in color when printed as an image on the print media. 
     Although the thickness of the ink layer, on each transfer and on the substrate is designed to be fixed, due to mechanical and physical characteristics of the printing system, the thickness of the ink layer does not remain constant across a page. As a result, the optical density (OD) of the printed ink of the image is non-uniform. In order to compensate for those changes in OD, the controller of the printing system controls the thickness of the ink layer that is printed on the substrate. 
     Although the controller may be implemented in various printing systems, voltage-controlled ink density is described with reference to the following printing environment. 
     Reference is made to  FIG. 1 , which illustrates an exemplary digital printing press  100 . The digital printing press  100  includes a Photo Imaging Plate (PIP) foil  110  wrapped around a PIP drum  112 , and a plurality of Binary Ink Development (BID) units  118  disposed about the PIP drum  112 . The PIP foil  110  includes photoconductive material. 
     Each BID unit  118  contains a single ink, but the different BID units  118  may contain inks of different colors. For example, the seven BID units  118  of  FIG. 1  contain a total of seven different inks. 
     The digital printing press  100  may produce a print as follows. The PIP foil  110  is charged by a Scorotron assembly  114 . As the PIP drum  112  is rotated, a writing head  116  produces a laser beam that discharges specific areas on the PIP foil  110 . These discharged areas define a latent image. 
     One BID unit  118  applies ink to the PIP foil  110  during each rotation of the PIP drum  112 . A BID unit  118  is moved near the PIP foil  110 . The BID unit  118  includes a developer roller  119 , which is charged to a lower potential than the charged areas on the PIP foil  110 , and a larger potential than the discharged areas on the PIP foil no. Charged ink in the BID unit  118  is attracted to the discharged areas on the foil  110 . Dots of the ink are transferred from the developer roller  119  to the discharged areas. Ink is not transferred to those foil areas having higher potential than the developer roller  119 . In this manner, ink is deposited on the PIP foil  110 . As the PIP drum  112  is rotated, a color plane of the image is formed on the PIP foil  110 . 
     With each additional rotation of the PIP drum  112 , the writing head  116  discharges specific areas on the PIP foil  110 , and another BID unit  118  applies ink to the discharged areas. In this manner, a developed image is formed on the PIP foil  110 . 
     The developed image is transferred from the PIP foil  110  to a blanket  120 , which is wrapped around an Intermediate Transfer Member (ITM)  122 . The transfer of the developed image is achieved through electrical and mechanical forces. The blanket  120  is charged and heated to raise the temperature of the ink on the blanket  120 . The increase in temperature causes the ink to swell and acquire a gelatin-like form. With the help of another drum  124 , the developed image is transferred from the blanket  120  to a substrate  126  (i.e., a print medium). 
     Various parameters such as ink density, ink conductivity, ink temperature, ink separation, imaging oil temperature, imaging oil dirtiness, ITM temperature, and ITM blanket counter (a measure of blanket age or usage, such as a number of impressions made by the blanket  120  since it was installed), corona voltage (the voltage of the corona in the Scorotron assembly  114 ), grid voltage (the voltage of a grid in the Scorotron  114  assembly), and vlight/vbackground (the voltage on the PIP foil  110  after/before the PIP foil  110  is discharged) and developer voltage are used to control the digital printing press  110 . Control hardware  128  of the digital printing press sets target values for the control parameters, and maintains the control parameters at or near their target values. These target values may be predetermined and defined by a print profile. 
     For the printing system of  FIG. 1 , the developer voltage, which is applied to the developer roller  119  inside the MD unit  118 , controls the thickness of the ink dots that are deposited on the discharged areas of the PIP foil  110 . Increasing the developer voltage increases the thickness of the ink dots. In other types of printing system, the ink thickness may be controlled by adjusting other control parameters, for example, ink viscosity. 
     Color variations between the trailing edge of one page and leading edge of the next page may occur. This is caused by a lower OD on the first part of each page, for example, the few centimeters of each page, compared to the rest of the image. It would appear that after this point, the CD becomes stable and color variations for the remainder of the page do not occur. For web printing system, where the print media is provided by a continuous web of material and the print is produced in sequence, without any gap between pages, the color change is visible. This may be achieved by a method of controlling the printing process as illustrated in  FIG. 2 a   . An optical density of a printed image is increased,  200 , for a predetermined portion of a leading edge of a page to compensate for the lower OD on the leading edge and equal it to the same level of the rest of the image. For example, as shown in  FIG. 2 b   , a profile is selected,  201 , and applied,  203 , to control the printing process. 
     In the system of  FIG. 1 , for example, this may be achieved by adjusting at least one control parameter, for example, varying the BID&#39;s developer roller voltage, in order to vary the ink thickness and hence the OD. The developer roller is one of the main subsystems that controls the ink thickness of the system of  FIG. 1  and thus has an impact on color OD. The developer voltage is set during each Color Adjust calibration to provide a default developer voltage (default parameter value). This default developer voltage is set and used to print a page. For a predetermined portion of the page at the leading edge of the page, the default developer voltage is altered by adding a corrective developer voltage, á (corrective value). The corrective developer voltage, á, is applied on the default developer voltage for a predetermined portion of the leading edge of each page, and then the voltage returns to the default developer voltage set by the color adjust for the remainder of the page. Therefore, a profile is selected which alters the OD on the problematic area up to the point where the OD becomes stable and the default developer voltage alone can be applied. 
     The software infrastructure is very flexible. A profile for the developer voltage is created. The profile, for example, may be built using up to 16 bars, that is, 16 different set points along the page, each set point has its own developer voltage and period. If the developer voltage of one bar is lower than the previous bar, the profile generates a slope for a gradual change in the developer voltage. Therefore, as the developer voltage returns to its lower, default developer voltage, that is, the corrective developer voltage is no longer applied, the profile provides a gradual decrease in the developer voltage from its current value (for example the default value plus the corrective value) to the final default value, so that there is no sudden voltage drop and no sudden change in ink thickness and hence no sudden change in OD. 
     The color correction for the leading edge is achieved using pre-defined profiles. For example, 5 pre-defined profiles, each profile setting a corrective developer voltage to be added to the default developer voltage, and then return to the default developer voltage may be used. For example, a first, no-correction profile, á=0v; a second, low profile, á=5v; a third, medium profile, á=8v; a fourth, high profile, á=11v; and a fifth, rough profile, á=14v. 
     The 5 examples above are for illustrative purposes and it can be appreciated that any number of pre-defined profiles may be provided having different corrective developer voltages. 
     The profile may be selected by the user following color variations or, alternatively, it may be selected automatically based on OD measurements taken of the previously printed page. 
       FIG. 3  illustrates various components of an exemplary printing device  300  in which the printing system of  FIG. 1  can be implemented. As used herein, “printing device” means any electronic device having data communications, data storage capabilities, and/or functions to render printed characters, text, graphics, and/or images on a print media. A printing device may be a printer, fax machine, copier, plotter, and the like. The term “printer” includes any type of printing device using a transferred imaging medium, such as ink, to create an image on a print media. Examples of such a printer can include, but are not limited to inkjet printers, electrophotographic printers, plotters, portable printing devices, as well as all-in-one, multi-function combination devices. 
     Printing device  300  may include one or more processors  302  (e.g., any of microprocessors, controllers, and the like) which process various instructions to control the operation of printing device  300  and to communicate with other electronic and computing devices. Printing device  300  can be implemented with one or more memory components, examples of which include random access memory (RAM)  304 , a disk drive  306 , and non-volatile memory  308  (e.g., any one or more of a ROM  310 , flash memory, EPROM, EEPROM, etc.). 
     The one or more memory components store various information and/or data such as configuration information, print job information and data digital print data, graphical user interface information, fonts, templates, menu structure information, and any other types of information and data related to operational aspects of printing device  300 . Printing device  300  may also include a firmware component  312  that is implemented as a permanent memory module stored on ROM  310 , or with other components in printing device  300 , such as a component of a processor  302 . Firmware  312  is programmed and distributed with printing device  300  to coordinate operations of the hardware within printing device  300  and contains programming constructs used to perform such operations. 
     An operating system  314  and one or more application programs  316  can be stored in non-volatile memory  308  and executed on processor(s)  302  to provide a runtime environment. Further, application programs  316  can facilitate user interface display and interaction, printing, scanning, and/or any number of other operations of printing device  300 . A user interface allows a user of printing device  300  to navigate a menu structure with any of indicators or a series of buttons, switches, or other selectable controls that are manipulated by a user of the printing device. 
     Printing device  300  further includes one or more communication interfaces  318  which can be implemented as any one or more of a serial and/or parallel interface, a wireless interface, any type of network interface, and as any other type of communication interface. A wireless interface enables printing device  300  to receive control input commands and other information from an input device, such as from an infrared (IR), 802.11, Bluetooth, or similar RF input device. A network interface provides a connection between printing device  300  and a data communication network which allows other electronic and computing devices coupled to a common data communication network to send print jobs, menu data, and other information to printing device  300  via the network. Similarly, a serial and/or parallel interface provides a data communication path directly between printing device  300  and another electronic or computing device. 
     Printing device  300  also includes a print unit  320  that includes mechanisms selectively applying an imaging medium such as ink (e.g., liquid toner), and the like to a print media in accordance with print data corresponding to a print job. The print media can include any form of media used for printing such as paper, card stock, plastic, fabric, Mylar, transparencies, film, metal, and the like, and different sizes and types such as 8½*11, A4, roll feed media, etc. 
     Printing device  300 , when implemented as an all-in-one device for example, can also include a scan unit  322  that can be implemented as an optical scanner to produce machine-readable image data signals that are representative of a scanned image, such as a photograph or a page of printed text. The image data signals produced by scan unit  322  can be used to reproduce the scanned image on a display device or with a printing device. Printing device  300  may also include a graphical display  324  that provides information regarding the status of printing device  300  and the current options available to a user through the menu structure. 
     Although shown separately, some of the components of printing device  300  can be implemented in an application specific integrated circuit (ASIC). Additionally, a system bus (not shown) typically connects the various components within printing device  300 . A system bus can be implemented as one or more of any of several types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, or a local bus using any of a variety of bus architectures. Printing device  300  may also include any form of control logic  326  which refers to hardware, firmware, software, or any combination thereof that may be implemented to perform the logical operations associated with a particular function or with the operability of the printing device  300 . Logic  326  may also include any supporting circuitry is utilized to complete a given task including supportive non-logical operations. 
     Prior to printing, the default developer voltage for each BID unit  118  is derived or predetermined and stored by the ROM  310  and this default developer voltage is provided to the processor(s)  302 . A plurality of pre-defined profiles are stored in the ROM  310 , or alternatively, the RAM  304 , or disk within the disk drive  306  or flash memory or the like and have a corrective developer voltage á, for example, the profiles mentioned above. A profile is selected,  201 , and applied,  203 , to control; the printing process of the print unit  320 . The developer voltage defined by the selected profile defines the ink thickness such that for a predetermined portion of the leading edge of the page is greater than the thickness of the ink for the remainder of the page. 
     As the PIP drum  112  is rotated, the writing head  116  discharges areas on the PIP foil  110  and, while being controlled at the developer voltage provided by the selected profile, the BID unit  118  deposits dots on the substrate at a desired thickness. 
     Although implementations of printing systems have been described in language specific to structural features and/or methods, it is to be understood that the subject of the appended claims is not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as exemplary implementations of printing systems. 
     Although various examples have been illustrated in the accompanying drawings and described in the foregoing detailed description, it should be understood that the disclosure is not limited to the examples disclosed, but is capable of numerous modifications without departing from the scope of the disclosure as set out in the following claims.