Patent Publication Number: US-2005134872-A1

Title: System and method for closed-loop color control of printed media

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      The present application is claiming priority of U.S. Provisional Patent Application Ser. No. 60/530,587, filed on Dec. 18, 2003, the content of which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to printing systems, and more particularly, to color control and monitoring systems for commercial printers such as newspaper printers.  
      2. Description of the Related Art  
      Newspapers are produced predominantly according to the offset process. A plurality of paper webs are unwound from rolls, printed in the printing units and finally folded and cut. The inking can be set zone by zone in conventional inking systems. The system supplying ink to the printing plate and thereon to the web is divided into zones. For each zone there is a device controlling the amount of supplied ink. The device can be, for example, a pump (i.e., one pump per zone), one or more blades scraping a roller where the gap between the blade and the roller defines the amount of ink supplied together with the ink fountain roller speed. For each zone the opening of the pump or the size of the gap is controlled. The printer monitors the inking during the entire production and makes corrections in inking when necessary.  
      Current newspaper production has seen a dramatic increase in the use of color printing. Color printing requires the use of more complex printing techniques and color management that if not properly managed can result in waste and degraded quality.  
      Therefore, there is a need to provide both an on-line and off-line closed-loop color control management system for newspaper and commercial printing presses.  
     SUMMARY OF THE INVENTION  
      The closed loop color control system of the present invention provides a device that measures the colors on a printed product, presents color measurements to a user with respect to the current color targets, provides for automatic control of printing devices, and provides color quality reporting. The system is useful in newspaper production, however, it can also be used to advantage in other commercial and packaging printing applications.  
      The system helps to keep the colors of the printed product uniform throughout the production, decreases the start-up waste as well as the run-time waste, increases production efficiency and provides quality proofs in the form of reports on product quality.  
      The benefits of the system and method of the present invention are a higher and more even print quality, material savings in paper and ink waste, improved press output capacity and reduced quality assurance costs.  
      A printing control system and method is provided for control of printing devices. The printing control system embodiment of the present invention includes a measurement unit that provides an output signal indicative of a characteristic of an image on a media formed by a printing mechanism, and a controller that responds to the output signal to modify an operation of the printing mechanism to change the characteristic of the image.  
      In another embodiment of the present invention, the characteristic is a color density of the image.  
      In another embodiment of the present invention, the output signal is indicative of a measurement of the color density. The controller modifies an amount of ink supplied by the printing mechanism for printing on the media based on a deviation between the measurement of the color density and a target color density value.  
      In another embodiment of the present invention, the ink is of a color selected from the group consisting of: black, cyan, magenta, yellow and any combinations thereof.  
      In another embodiment of the present invention, the printing mechanism is a printing press. The measurement unit is in an on-press or off-press position.  
      In another embodiment of the present invention, the printing control system includes a user interface in communication with the controller. The interface can display the output signal and receive one or more target characteristics.  
      In another embodiment of the present invention, the printing control system includes a processor that performs at least one operation based on the output signal.  
      In another embodiment of the present invention, the operation includes providing a record of the output signal.  
      In another embodiment of the present invention, the characteristic is a color density of the image. The output signal is indicative of a measurement of the color density, and the operation includes providing a record of the color density measurement.  
      In another embodiment of the present invention, the printing control system includes a database that receives and stores the output signal.  
      In another embodiment of the present invention, the operation includes providing a record of all output signals made during a selected duration of a printing task.  
      In another embodiment of the present invention, the printing control system includes a user interface. A user can input a target color density value, and the operation includes providing a report including data representing the color density measurement, the target color density value, and/or a deviation between the color density measurement and the target color density value.  
      A method embodiment of the present invention includes measuring a color density of selected images printed on a media by a print mechanism, providing at least one output signal indicative of the color density, and, based on the output signal, modifying an amount of ink used to print the images so as to change the color density.  
      In another embodiment of the method of the present invention, modifying the color density is based on a deviation between the color density measurement and a target color density value.  
      In another embodiment of the method of the present invention, the color density is measured from a preselected portion of the image.  
      In another embodiment of the method of the present invention, the media is paper and the preselected portion is a gray bar.  
      In another embodiment of the method of the present invention, the printing mechanism is a printing press, and the measurement is performed on the printing press or off of the printing press.  
      In another embodiment of the present invention, the method includes providing a processor with the output signal to provide a record of color density measurements made during a selected duration of a printing task.  
      In another embodiment of the present invention, the method includes storing the color density measurements in a database.  
      In another embodiment of the method of the present invention, the method includes providing at least one target color density value. The record includes data representing the color density measurement, the target color density value, a deviation between the color density measurement, and/or the target color density value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram of a printing device control and monitoring system.  
       FIG. 2  is a diagram of an on-press color control and monitoring system.  
       FIG. 3  is a diagram of an off-press color control and monitoring system.  
       FIG. 4  is an on-press measurement bar as positioned over a web.  
       FIG. 5  is a plan view of a measurement bar.  
       FIG. 6  is an on-press control system web on which pages have been printed from a doublewide, double-around printing press.  
       FIG. 7  is an example of a page operation display interface in density control mode.  
       FIG. 8  is a block diagram showing the color control software functions.  
       FIG. 9  is a portion of the block diagram of  FIG. 8  showing the target handling function of the color control software.  
       FIG. 10  is a portion of the block diagram of  FIG. 8  showing the status handling function of the color control software.  
       FIG. 11  is a portion of the block diagram of  FIG. 8  showing the tuning parameter handling function of the color control software.  
       FIG. 12  is a portion of the block diagram of  FIG. 8  showing the control function of the color control software.  
       FIG. 13  is a portion of the block diagram of  FIG. 8  showing the output handling function of the color control software. 
    
    
     DESCRIPTION OF THE INVENTION  
      There is provided a closed-loop color control system for use in color printing devices such as a newspaper printing press. The main components of the closed-loop color control system include on-press measurement sensors that take color measurements, such as by reading color densities, of the ink printed on a web of materials. Also included is a control system that receives color measurement information and target density information, provides a display of such information to an operator, and can modify printing components based on measured density information. The system also includes a quality reporting system that collects and displays measured and target density information for review and analysis.  
      A color measurement device is integrated with a control system that receives color measurement data from the color measurement device and that controls ink keys, ink rollers, and other components of a printing press. An example of a preferred control system is the Printa™ Press Control and Printa Production Management System sold by Honeywell International Inc.  
      In a preferred embodiment, the production operator interface is completely integrated with a standard Printa page-oriented operation. The operator uses the standard operating displays that visually present both the measured color densities and the target color densities. The displays can also provide reports by the quality reporting system. Adjustments of the color densities are performed on the very same keyboard used for controlling ink zones or are performed directly on the display using the user-friendly touch screen. No separate monitors are required for the operation. The operator is able to run the system in automatic mode or disable the automatic control to operate the system in manual mode. In automatic mode, the control system automatically adjusts the ink zones and ink fountain rollers to achieve the desired target densities, while the operator is able to simultaneously make adjustments to the target densities. When the automatic mode is disabled then the operator interacts directly with ink zones.  
      Referring to  FIG. 1 , a printer apparatus includes a printing mechanism  102  and a control system  100  of the present invention. Printing mechanism  102  which, for example, may be a printing press, includes a reel stand  104 , a printing unit  106  including inking units  108  and a web  110 . Control system  100  includes a color measurement unit  112 , a printing control mechanism, i.e., controller  114 , a user interface  116 , and a control system bus  118 . A processor  120  is also provided. Press controller  114 , for example, may be the Printa Press Control System manufactured by Honeywell International Inc., and a quality reporting system.  
      Web  110 , for example, may be paper, plastic or any other material suitable for printing. Web  110  is advanced from reel stand  104 , through printing mechanism  106 , and to a folding and cutting mechanism (not shown) by a web advancement device (not shown). Printing unit  106 , in one example, is configured to print with various color inks, such as black, cyan, magenta, and yellow.  
      Color measurement unit  112  takes measurement data based on characteristics of one or more images printed on web  110  or a portion of web  110 , such as color density measurements, and sends measurement data in the form of one or more output signals to controller  114  via control system bus  118 . An interface (not shown) is provided to facilitate transmission of the output signal, and performs functions such as analog/digital conversion and signal shaping. The interface may be, for example, a PMD-controller. Controller  114  receives the output signal from measurement unit  112 .  
      Controller  114  receives input, such as preselected or preferred, i.e., “target”, measurement parameters, from user interface  116 , and modifies printing unit  106  based on a deviation between actual measurement data and target measurement parameters. In one embodiment, the output signal carrying actual measurement data is a measure of color density of the image, and controller  114  modifies an amount of ink supplied by printing mechanism  106  for printing on web  110  based on a deviation between the density measurement and the target color density value. The target color density is a value to which the color density measurement is compared to determine the deviation between the color density measurement and the target color density value. An error signal is generated based on the deviation to correct the amount of ink provided by printing unit  106 .  
      User interface  116 , such as a control desk, is in communication with controller  114  via bus  118 . User interface  116  may include components such as a processor, memory, a screen and various inputs such as a keyboard, mouse or a touchscreen. User interface  116  allows a user to select image characteristics such as color density, and input preselected or preferred, i.e. target, image characteristics. Such target characteristics may be entered prior to commencement of a printing task, and new target characteristics may be added during the printing task.  
      System  100  also retains measurement data during the printing process for presentation to a user during and/or after the printing process. Processor  120  performs at least one operation in response to receiving the output signal of measurement unit  112 . Processor  120  keeps a record of measurement data represented by the output signals. Processor  120  can provide real time measurement data or a report of part or a whole of a printing task, i.e. a printing run, for evaluation and quality control. In another embodiment, processor  120  may be incorporated with controller  114  or user interface  116 .  
       FIG. 2  shows another embodiment of control system  100  of  FIG. 1 , having on-press measurement units  112  and including multiple printing units  106  and additional control system features. Multiple controllers  114  are connected to control system bus  118 . Controllers  114  are each in communication with one of printing units  106  for printing on webs  110 , and in communication with user interface  116 , such as one or more control desks. The printing press also includes turning bars  122  and folding/cutting mechanism  124 .  
      Also shown in  FIG. 2  are additional features of the system of  FIG. 1  for use in any control system. These features include one or more optional workstations  126 , quality database server  128 , pre-press interface  130 , and a production management server, i.e., a PMS server  132  that is in communication with control system  100  via bus  118 . A router  134  delivers measurement information to devices such as one or more workstations  126  or a quality database server  128 .  
      Workstations  126  are used, for example, for production planning, production monitoring and reporting. A pre-press interface  130  is used for pre-setting printing area density profiles for ink keys.  
      Upon activation, on-press measurement units  112  perform a search for measurement marks, such as gray bars, measure the color densities of the measurement marks, and provide the measure of color density data to press control system  100 . Measurement units  112  may provide an output signal that is converted by controller  114  or another device into density data, or may derive the density data and transmit the density data to controller  114 .  
      Press control system  100  performs numerous functions. Press control system  100  configures measurement units  112  and press  102  at the start of the production run for color density data collection. Press control system  100  also provides control software to adjust ink keys and ink fountain rollers (“ink rollers”) (not shown) of printing devices  106 , provides input/output operation (I/O) for user and/or operator interface  116 , and provides data to a quality reporting system.  
      The quality reporting system may be implemented in any of the computing devices that communicate with bus  118 . These computing devices include workstations  126 , database server  128 , pre-press interface  130 , PMS server  132 , user interface  116  as well as a remote computing device that communicates with control system  100  by a network. These computing devices, when implemented as the quality reporting system, each include processor  120  and an associated memory. By way of example for the present description, quality reporting system is implemented herein as a workstation  127 .  
      Quality reporting system  127  retrieves data representing actual and target image characteristics, such as actual measured color density and target color density, from quality database server  128  for presentation to a user, either during a printing task or after a printing task. The data may be presented in various forms to provide a user with desired quality information or specific quality reports.  
       FIG. 3  illustrates another alternative embodiment of control system  100  that has off-press measurement units  112  (not shown) that are integrated into a color measurement scanner  136 .  
      Color measurement scanner  136  is located off of printing press  102  and is used to read the color densities of images on web  110 . Printed pages are transferred to scanner  136  after they are taken off of press  102 . Measurement unit  112  in scanner  136  retrieves color measurement data and sends the data to controller  114  via bus  118 . Color measurement data may also be sent to workstations  126  or other components via router  134 . A personal computer may be included to retrieve data from off-press measurement units  112  and send the data to press controller  114 . The measurement data is also provided to quality reporting system  127 .  
      Quality reporting system  127  retrieves color density data and other data as required from press control system  100 , and provides the data to computing devices such as workstations  126 , quality database server  128 , controller  114  and user interface  116 . In another embodiment, quality reporting system  127  stores the data collected to quality database server  128 , and provides the collected data, and visualization and quality tools for viewing and analyzing the data, to workstations  126 .  
      Quality reporting system  127  can provide a user with a continuous record of actual color density measurements and density targets during the entirety of a printing run. This information can be used to evaluate a printing run and determine needed changes and/or improvements.  
      An exemplary method is provided for conducting the workflow of a printing device such as a press using the closed-loop color control system.  
      Prior to printing, during the pre-press process, halftone gray bars are inserted in a printed product layout. Plates are then produced and page data is sent to the pressroom system for calculation of printing area coverage profiles, which will be used for ink key pre-setting. Alternatively, the ink key pre-setting may be accomplished prior to or at the start of a printing run.  
      Imposition plans are then created for the productions. This information includes the exact positions of each page and color on the press, thereby enabling the user to concentrate on the product. This information also includes the definition of the positions and types (3-color or black) of the gray bars for each plate.  
      Color density measurements are performed by color density measurement unit  112  on selected images printed by printing unit  106 . In a first embodiment, on-press measurement units  112  are utilized. When the printing of the selected images starts, on-press measurement units  112  immediately start measuring the color densities across the selected image printed on web  110 . In a second alternative embodiment, off-press measurement scanners  136  are utilized. The off-press measurement embodiment can be used in place of the on-press measurement embodiment. In the case of off-press measurement, the frequency of the measurement is dependent on the user&#39;s manual operation. The user can immediately see the measured densities on an interface such as control desk  116 .  
      Control system  100  can constantly calculate new values for the ink keys and/or ink fountain rollers to compensate for any detected deviation between color density measurements and target color density parameters. If the user has enabled the automatic mode, corrections are performed automatically by controller  114 . During the production run, the user can adjust target color density values using the standard press control desk interface  116 .  
      In one embodiment, control system  100  saves color density measurements and target color density values into database  128  for quality reporting and analysis purposes. The measured densities can be presented visually in real time on, for example, to workstations  126  for comparison with target densities. The data is also available for production reports to be sent to customers or for internal process improvement.  
       FIG. 4  and  FIG. 5  illustrate an exemplary embodiment of a measurement device  112  for use with control system  100 . Measurement device  112  includes color sensors for color density measurement on the press. Color may be measured, for example, by a densitometer.  
      As shown in  FIGS. 4 and 5 , measurement unit  112  includes a measurement bar  400  for each side  405  of web  110  requiring online color measurement. Measurement bar  400  includes a plurality of modules  410 . In one embodiment, each module  410  contains five measurement units  415 . Each measurement unit  415  includes three measurement channels  420 : one measurement channel for each of the colors Cyan, Magenta and Yellow. Thus, each module  410  contains 15 measurement channels. Each measurement channel  420  contains a light-emitting diode (LED) (not shown), emitting either Red, Green, or Blue light, and a photodiode (not shown) for reflectance measurement.  
      The sizes of modules  410  and measurement units  415  are fixed. The number of modules  410  per measurement bar  400  is defined by the width of web  110 . A typical exemplary number of modules  410  per measurement bar  400  is 11 modules.  
      In one embodiment, modules  410  can be configured from an external device and the data can be read by a processor  435  from measurement bar  400  and transmitted via a network, such as, an Ethernet  430 .  
      Various measurements may be performed by each measurement unit  415 . Such measurements include 1) intensity of each of C,M,Y in a 3-color gray bar, 2) color intensity in an area just prior to or after a 3-color gray bar, 3) black intensity of a 1-color gray bar, 4) color intensity in an area just prior to or after a 1-color gray bar, and 5) color intensity in a plate gap (area between pages, which should be plain paper).  
      Measurement bar  400  also includes an interface (not shown) to synchronize pulses, for synchronization of a printing cylinder  425 . As cylinder  425  rotates, measurement modules  410  receive pulses that indicate the angle of cylinder  425 . In one embodiment, there are a minimum of 2,000 pulses per cylinder revolution. In addition, one pulse is provided per revolution of cylinder  425  for reference. An exemplary power supply (not shown) is 220V.  
      Measurement unit  112  takes color measurements from selected images on each page of a printed web  110 . In a preferred embodiment, measurement unit  112  provides color density information from narrow halftone gray bars printed on each page, such as in a newspaper printing press. In additional embodiments, other printed portions of a page can be used as a reference to control color density. In the case of a newspaper printing press, two gray bars are typically used. A first type of gray bar is a 3-color Cyan, Magenta, Yellow (CMY) neutral gray bar, and second type of gray bar is a single gray bar for measuring black color. The narrow gray bars are hidden in the newspaper layout.  
      Typical gray bar compositions as halftone percentages are described below. An exemplary three color gray bar includes a Cyan halftone percentage of 30%, a Magenta halftone percentage of 22%, and a Yellow halftone percentage of 22%. An exemplary halftone percentage of black in a single color gray bar is 30%  
       FIG. 6  shows gray bars on a product&#39;s pages on a doublewide, double-around press. In this example, a plurality of page areas  600  each contain one of a 3-color gray bar  605  or a 1-color gray bar  610 .  
      During one cylinder revolution, two of pages 600 (either the same or different pages depending on the production mode) are printed on a web  615 . Content  620  represents page content. All pages 600 contain a gray bar  605  or  610  for color measurement. Every other page in the machine direction contains 3-color gray bar  605  and every other alternate page contains 1 color gray bar  610 .  
      In a single or double width, single around newspaper printing process, both types of gray bars would be found on the same page.  
       FIG. 7  is a representation of an example of a page operation display  700  in the density control mode, as may be displayed, for example, in user interface  116 . Display region  705  provides a visual representation of the measured color densities for each of printing zones  1 - 10  on a selected page on a web. Printing zones  1 - 10  each correspond to an ink key. The number or printing zones can vary between different printing presses. Region  705  also illustrates each zone&#39;s deviation from a target density. Bufton  710  allows a user to choose between color density values or ink key values, i.e., settings for the amount of ink flowing to the ink fountain rollers. Buttons  715  and  720  allow the user to switch between automatic and manual control of ink keys. Bar  725  denotes manual (“M”) control of ink keys.  
       FIGS. 8-13  illustrate a Color Control Block Diagram of an exemplary embodiment of software  800  that is utilized to receive density measurements and control ink keys and rollers.  
      Color control software  800  controls the ink rollers and ink keys of a printing press in response to various data input including sensor status, density measurements and tuning parameters for inking devices, which are provided to a control function that controls the output of the ink rollers and ink keys. These functions, achieved through a control function in software  800 , are: 1) Target density handling  802 , 2) Status handling  804  receives information on status of sensors, 3) Tuning parameter handling  806 , 4) Output handling  808 , and 5) Ink roller control  810 .  
      A control function  812  receives three general types of information. Target density handling function  802  receives input of actual ink density measurements and target densities, and provides control function  812  with data regarding the deviation of the actual densities from the target densities. Status handling function  804  allows an operator to set ink key modes and relays that information to control function  812  and the user displays (not shown). Tuning parameter handling function  806  calculates and sets desired tuning parameters for the ink keys  814 , rollers  810  and other components, and relays that information to control function  812 .  
      Control function  812  calculates a correction value (“CV”) array from the received data and provides the CV array to output handling function  808 , which controls the output of ink keys  814  and also directs ink roller control  816  to modify the speed of ink roller  810  in response to ink key values. Ink key values refer to the settings of the amount of ink flowing to ink fountain rollers  810 .  
      Referring to  FIG. 9 , target handling function  802  receives information in the form of a density and intensity measurement arrays/profiles from measurement handling function  832 , which receives density measurements from color sensors  818 , and also receives the pre-set density set point/target profile, i.e. pre-settings  820 . In the case of measurement bar  400  as described in  FIGS. 4-5 , the array contains all measurements from one measurement bar  400 . In one embodiment, the measurement values are running averages with a varying length (e.g. last two values, last four values). Target handling function  802  may also receive ink key set point modification instructions or target density updates from an operator represented by operator entries  822 .  
      Target handling function  802  calculates the Intensity ratio array/profile that is used in controlling inking devices  810  and  814 . An intensity ratio average is calculated, and is used in conjunction with information regarding sensor status and tuning parameters for ink roller control. The Intensity ratio is defined by the following formulas: 
 
Intensity ratio=Intensity measurement/Target intensity 
 
Target Intensity=10 Target density . 
 
      Target handling function  802  also calculates the density deviation, using target density average, density deviation array, density average, and density deviation average. Color density is equal to log(intensity for light reflected from paper/color intensity), and density deviation is equal to the difference between the measured density and the target density. This function is also responsible for updating target intensities/densities.  
      Referring to  FIG. 10 , status handling function  804  provides for control of inking devices in response to the status of printing press components or measurement sensors  818 , and also provides status information to a user interface (not shown). Status handling function  804  receives ink key mode information, measurement status, and interlock information. The function can control various ink key modes. An operator can set the mode of ink keys or inking zones, represented by operator entries  824 , via interfaces such as density information or page operation displays. For example, in Manual mode, the user changes the settings of ink keys  814  and ink fountain rollers  810  manually; control system  100  takes color measurements but does not control the inking devices. In Automatic mode, the control system measures and adjusts the settings of ink keys  814  and ink fountain rollers  810  automatically. The user can change the color measurement targets. Control of all ink keys  814  may be automatically interlocked, i.e, disabled, if the press speed drops below a minimum preset value, or while the press speed is ramped up or down. Control of an ink key  814  may also be interlocked based on interlock settings  826  and/or if measurement status is bad, i.e if one or more sensors  818  malfunction or fail. Status handling function  804  provides mode array information to Control function  812 , and status information to the page operation displays.  
      Referring to  FIG. 11 , tuning parameter handling function  806  provides information regarding the pre-set tuning parameters  820  of ink keys  814 , ink rollers  810 , and other aspects of the press settings to control function  812 . Control function  812  can adjust its operation based on the supplied parameters  820 . The function includes calculation of the tuning parameters. A process to be controlled can be characterized by parameters such as gain, and time constants such as up/down and transfer time. Ink and paper type can be taken in account in pre-setting tuning parameters. Ink key specific tuning parameters may be among the pre-set parameters. An additional tuning parameter, a tightness parameter, is used to dampen the control when control deviation is small. The tightness parameter defines how ‘tight’ or fast the controller is. The smaller the value the slower the controller.  
      Referring to  FIG. 12 , control function  812  provides printing device based control, i.e. each printing device has its own control. One control function  812  can control several printing devices. Measurement data is correlated to control function  812  using preset ink-key configuration data, so that measurements can be associated with the correct ink keys  814 .  
      Control function  812  receives the intensity ratio array/profile, intensity ratio measurement, mode information, and tuning parameters such as machine speed and ink key parameters.  
      The control cycle is variable, but in one example is about five seconds. The control cycle refers to the frequency of calculation of new set points for ink keys  814  or ink fountain rollers  810 . Control function  812  calculates a delta output, from which set points of each ink key  814  can be modified. Ink key set point modification is proportional to the immediately previous ink key set point.  
      Referring to  FIG. 12 , output handling function alters the ink roller and ink key control in response to the output of Control function  812 . The output handling function also receives pre-set ink key profile information, and mode array information from the status handling function. Changes to ink key set points are proportional to the immediately previous ink key set point. An operator may also directly modify the the output of ink key  814 , represented by operator entries  830 .  
      In one embodiment, if the ink key set point is less than a minimum value, then the proportional set point is calculated using the minimum value. If the ink key set point change in the control cycle is bigger than a defined maximum value, then the set point change is limited to the maximum value. If an ink key preset set point is zero then the key is not used in control.  
      Setting an ink key set point maximum and minimum limit is an option. One common maximum value may be set for all ink keys  814 . Individual minimum set point values may be set for each ink key  814 . Another optional embodiment includes an ink key profile shape limiting option. In this embodiment, the preset ink key profile is kept in a memory. If this option is selected, the control system can not change an ink key set point more than defined amount from the preset profile. An ink key profile smoothing option may be employed to remove unnescessary bending in the ink key profile in the long run. In another option, first and second order bending limits can be employed to protect the ink blade from bending too heavily. The first order bending limit tells how much the ink key set point for one ink key can  814  differ from an adjacent ink key  814 . The second order bending limit is the allowed set point difference sum to the left and right hand side ink keys  814 .  
      Ink roller control function  816  controls the ink roller speed. The function receives pre-set information including speed set point information in speed pre-settings  828 . Ink roller function  816  also receives an ink key profile from the output handling function.  
      The ink duct roller speed preferably follows a predefined speed curve. The curve specifies the duct roller speed in relation to the press speed. This curve is defined during pre-setting. The duct roller speed can be adjusted with the speed set point. Speed set point is indicated as percentage from the range defined by the duct roller curve, (0%=duct roller stopped, 100%=max. speed defined by the curve). Control system  100  can adjust the ink roller speed by changing a so-called Correction Value. The correction value is adjusted when ink roller control  816  is enabled and if a sufficient number of ink keys  814  that are in automatic mode have a set point greater than a defined maximum value or less than a defined minimum value. The ink roller correction value is increased only if the intensity ratio average is less than a defined limit and there are no ink keys  814  already closed. Respectively, the ink roller correction value is decreased only if the intensity ratio average is greater than a defined limit and there are no ink keys  814  already open.  
      It should be understood that various alternatives, combinations and modifications of the teachings described herein could be devised by those skilled in the art. The present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.