Patent Application
20130186290
The present disclosure is related to marking and printing methods and systems, and more specifically to an automatic ink supply method for printing systems which includes variable data lithography printing systems.
Offset lithography is a common method of printing today. (For the purpose hereof, the terms “printing” and “marking” are interchangeable.) In a typical lithographic process a printing plate, which may be a flat plate, the surface of a cylinder, belt, etcetera, is formed to have “image regions” formed of hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. The image regions are regions corresponding to the areas on the final print (i.e., the target substrate) that are occupied by a printing or a marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by the marking material.
The Variable Data Lithography (also referred to as Digital Lithography or Digital Offset) printing process begins with a fountain solution used to dampen a silicone imaging plate on an imaging drum. The fountain solution forms a film on the silicone plate that is on the order of one (1) micron thick. The drum rotates to an exposure station where a high power laser imager is used to remove the fountain solution at the locations where the image pixels are to be formed. This forms a fountain solution based latent image. The drum then further rotates to a development station where lithographic-like ink is brought into contact with the fountain solution based latent image and ink develops onto the places where the laser has removed the fountain solution. The ink is hydrophobic. An ultra violet (UV) light may be applied so that photo-initiators in the ink may partially cure the ink to prepare it for high efficiency transfer to a print media such as paper. The drum then rotates to a transfer station where the ink is transferred to a printing media such as paper. The silicone plate is compliant, so an offset blanket is not used to aid transfer. UV light may be applied to the paper with ink to fully cure the ink on the paper. The ink is on the order of one (1) micron pile height on the paper.
One of the main differences between traditional lithographic offset printing and variable data lithographic printing is that every image can be different, as in all digital printing. This is often referred to as Variable Data printing. Traditional lithographic offset is inherently a reprographic process in that all images for each revolution of the image drum are the same. Thus the mean ink throughput for each drum revolution is the same, and the critical ink-to-fountain-solution ratio is the same for each revolution. The process is tuned manually by an operator to find the correct ink supply rate to match the ink load and the image content, and to also match the ink and fountain solution mixture. The operator makes ink supply adjustments and visually inspects the printed output to perform the manual tuning process.
Skilled operators perform this function in traditional lithographic offset printing. Note that this is for a static document, which is a document that is the same for every revolution of the imaging drum. For variable data documents the challenge is even greater because the ink load presented by the image content is varying because the image content, itself, is varying from page-to-page. If not enough ink is supplied, a starvation defect will occur similar to the xerographic defect known as “ghosting”, or “reload”. The final (or potentially intermediate) inker rollers cannot pick up enough ink from their upstream supply rollers to keep up with the demand presented by the image. Note that the ink trains and particularly the final inker rollers are somewhat robust to ink load variation in that the circumference of the ink rollers are typically many times smaller than the circumference of the image drum (which is the extent of the printed image). Thus the inker rollers are robust enough to handle local variations in the ink load, as long as the mean ink supply meets the mean ink load. Where the mean is taken, at least, over one imaging drum revolution. If too much ink is supplied to the inker rollers, then fine detail in the image will be washed out resulting in poor image quality. Again, the inker rollers are robust to this in that they can handle local variations in ink load for at least one revolution of the imaging drum. In traditional lithographic offset printing, once the operator has tuned the system for ink supply rate and fountain solution rate, the system reaches a quasi-equilibrium in that the mean ink load for one drum revolution is fixed and thus the required ink supply is fixed. The adjustments are made every 1 inch or so in the cross-process direction. Thus for a cross process location that does not have much image content such as text, a key setting associated with a low ink supply is chosen to match the ink load in that cross-process location. In a cross process location that has high image content such as a solid fill, a key setting associated with a high ink supply is chosen, again, to match the ink load in that cross-process location. Note that in all cross-process locations, whether they have high ink load or low ink load, the total ink load over an imaging drum revolution is the same (high, low, or in between) from revolution to revolution.
Typically, each ink key may be adjusted by manually turning a knurled head or the like on the ink key. For Variable Data printing, the ink load will be different in both the cross-process and process directions. That is, the ink load will change in each cross-process location over time as the document is printed, since each page of the document can have different content. The ink supply system must automatically adjust to match ink supply with the dynamic ink load presented by the variable data image content. Traditionally, for keyed inking units in standard offset lithographic printing systems, the ink keys are typically adjusted manually by a human operator when the printing unit is in a maintenance mode. While the past practices have been reasonably satisfactory, there is a need for an improved ink key adjustment mechanism to automatically adjust the ink keys for a variable data lithography system.
A method and system to automatically control ink keys for a digital offset printing machine is provided. The ink keys being adjustable to control the ink feed to individual zones located adjacent to each other across the width of an ink roller. The system uses feedforward and feedback control loops to adjust the ink supply dynamically based on a pixel count of the image content. The pixel count looks ahead in the video stream far enough to allow for time for the adjustment at the inker keys to propagate through the ink train to affect ink output at the ink train onto the imaging drum. Feedback of the achieved ink density on control patches on the imaging drum is used in addition to the pixel count to command the inker key motors. Feedback is also used to update the inker propagation delay and dynamic model used to determine how much the inker keys need to be adjusted based on the pixel count stream.
The disclosed embodiment pertains to an automatic ink supply method for lithographic printing systems which includes variable data lithographic systems. The ink control system uses the video stream, measuring pixel count, to automatically adjust the ink supply and fountain solution supply rates to match the ink load of the image content. In addition to the predictive estimates from pixel count, closed loop control can be accomplished using control patches on the imaging drum to control ink supply and measure other key attributes such as inker propagation delay and the dynamic model used to determine ink supply versus pixel count. Automatic ink and fountain solution control is essential when a non-repetitive or variable data digital image stream is used and the ink and fountain solution demand varies with the changing digital image content.
Aspects of the disclosed embodiments relate to a system to control an inking unit for a variable data lithographic printing machine the inking unit comprising a plurality of ink keys, each ink key being adjustable to control the ink feed to individual zones located adjacent to each other across the width of a roller in a printing machine to which ink is being fed by the inking unit, the system comprising a feedforward controller responsive to an ink load demand for an image to provide an output in accordance to a first control function which is adaptable to control at least one of the plurality of ink keys; and at least one actuator to control the ink feed in response to the feedforward controller.
In yet further aspects of the disclosed embodiments relate a method to control an inking unit for a variable data lithographic printing machine the inking unit comprising a plurality of ink keys, each ink key being adjustable to control the ink feed to individual zones located adjacent to each other across the width of roller in a printing machine to which ink is being fed by the inking unit, the method comprising receiving a print job comprising at least one image; separating the at least one image into cross-process direction sub-images associated with each ink key; providing an output in accordance to a first control function which is adaptable to control at least one of the plurality of ink keys based on an ink load demand for each sub-image; and controlling the ink feed in response to the first control function.
Further aspects of the disclosed embodiments relate to a method to control an inking unit, the method further comprising adapting the first control function in accordance to an output from an inker dynamic model for the printing machine; wherein the ink load demand is based on a pixel count for each sub-image.
Further aspects of the disclosed embodiments relate to a method to control an inking unit wherein the ink load demand comprises ink developed onto the imaging cylinder of the printing machine and at least one of dynamics of the inking unit, delay in applying ink to the imaging cylinder of the printing machine.
Further aspects of the disclosed embodiments relate to a method to control an inking unit wherein the inker dynamic model is based on at least one of ink density measurement, ink density target, ink load at time of density measurement, feedback gain, or a combination thereof.
Further aspects of the disclosed embodiments relate to a method to control an inking unit wherein the inker dynamic model is updated with at least one of data obtained after printing of the image, data obtained before printing of the image using density patches at predetermined locations of the imaging cylinder, data obtained after printing a number of images.
Further aspects of the disclosed embodiments relate to a method to control an inking unit wherein the controlling is performed with a servo motor or a stepper motor at each ink key being adjustable to control the ink feed to individual zones.
Further aspects of the disclosed embodiments relate to a method to control an inking unit wherein the first control function and the inker dynamic model are responsive to an ink density measurement obtained from the imaging cylinder.
Aspects of the disclosed embodiments relate to an apparatus for moving keys of a keyed inking train relative to an outer surface of an imaging cylinder of a printing machine, the apparatus comprising at least one movable ink key on the inking train, wherein the inking train comprises a plurality of ink keys, each ink key being adjustable to control the ink feed to individual zones located adjacent to each other across the width of a imaging cylinder; at least one actuatable motor for moving an associated ink key on the inking train to deposit ink on the imaging surface; at least one sensor for measuring ink density of the ink on the imaging cylinder; and a memory for storing an inker dynamic model for the inking train and for storing executable instructions to control the inking train, the executable instructions capable of directing a processor to perform receiving a print job comprising at least one image; separating the at least one image into cross-process direction sub-images associated with each ink key; providing an output in accordance to a first control function which is adaptable to control one of the at least one movable ink key based on an ink load demand for each sub-image; controlling the at least one actuatable motors to dispense ink by moving the associated ink key on the inking train in accordance to the first control function; updating the stored inker dynamic model for the inking train with the measured ink density.
Embodiments as disclosed herein may also include computer-readable media for carrying or having computer-executable instructions or data structures stored thereon for operating such devices as controllers, sensors, and electromechanical devices. Such computer-readable media can be any available media that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code means in the form of computer-executable instructions or data structures. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or combination thereof) to a computer, the computer properly views the connection as a computer-readable medium. Thus, any such connection is properly termed a computer-readable medium. Combinations of the above should also be included within the scope of the computer-readable media.
The term “print media” generally refers to a usually flexible, sometimes curled, physical sheet of paper, cloth, cardboard, plastic or composite sheet film, ceramic, glass, wood, sheet metal, or other suitable physical print media substrate for images.
The term “variable data printing” or “digital printing” generally refers to a system that can print or mark variable data documents, that is, documents that vary in image content from page-to-page. A “variable data lithographic printing machine” performs variable data printing.
The term “ink train” is used to describe a series of rollers or other mechanisms used to carry ink to an imaging member for printing of a print media.
The term “ink key” is intended to include any device which controls the amount of ink fed to a corresponding strip or zone of the imaging member with or without an ink train. It should be understood that the invention herein applies equally well to control of any ink metering devices, such as the ratchet for the ink ball and segmented ink keys, as well as other ink metering technologies such as ink jet or ultrasonic ink metering.
The term “inker” or “ink unit” is intended to comprise an ink train, ink keys, and one or more ink fountains for supplying ink to the ink train in proportion to settings of a plurality of keys aligned with a respective ink path along which ink is transferred from one ink fountain to a substrate or image on an imager member to be printed.
As used herein relational terms such as “first,” “second,” and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, relational terms, such as “offset”, “upstream”, “downstream”, “top,” “bottom,” “front,” “back,” “horizontal,” “vertical,” and the like may be used solely to distinguish a spatial orientation of elements relative to each other and without necessarily implying a spatial orientation relative to any other physical coordinate system. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “a,” “an,” or the like does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. Also, the term “another” is defined as at least a second or more. The terms “including,” “having,” and the like, as used herein, are defined as “comprising.”
The inked image from imaging member 12 may be applied to a wide variety of substrate formats, from small to large, without departing from the present disclosure. In one embodiment, imaging member 12 is at least 38 inches (38″) wide so that standard 4 sheet signature page or larger media format may be accommodated. The diameter of imaging member 12 must be large enough to accommodate various subsystems around its peripheral surface. In one embodiment, imaging member 12 has a diameter of 10 inches, although larger or smaller diameters may be appropriate depending upon the application of the present disclosure.
As shown in
After applying a precise and uniform amount of dampening solution, in one embodiment an optical patterning subsystem 36, see
Following patterning of the dampening solution layer 32, an inker subsystem 46 is used to apply a uniform layer 48,
In order for ink from inker subsystem 46 to initially wet over the reimageable surface layer 20, the ink must have low enough cohesive energy to split onto the exposed portions of the reimageable surface layer 20 (ink receiving dampening solution voids 40) and also be hydrophobic enough to be rejected at dampening solution regions 38. Since the dampening solution is low viscosity and oleophobic, areas covered by dampening solution naturally reject all ink because splitting naturally occurs in the dampening solution layer which has very low dynamic cohesive energy. In areas without dampening solution, if the cohesive forces between the inks are sufficiently lower than the adhesive forces between the ink and the reimageable surface layer 20, the ink will split between these regions at the exit of the forming roller nip. The ink employed should therefore have a relatively low viscosity in order to promote better filling of voids 40 and better adhesion to reimageable surface layer 20. For example, if an otherwise known UV ink is employed, and the reimageable surface layer 20 is comprised of silicone, the viscosity and viscoelasticity of the ink will likely need to be modified slightly to lower its cohesion and thereby be able to wet the silicone. Adding a small percentage of low molecular weight monomer or using a lower viscosity oligomer in the ink formulation can accomplish this rheology modification. In addition, wetting and leveling agents may be added to the ink in order to further lower its surface tension in order to better wet the silicone surface.
In addition to this rheological consideration, it is also important that the ink composition maintain a hydrophobic character so that it is rejected by dampening solution regions 38. This can be maintained by choosing offset ink resins and solvents that are hydrophobic and have non-polar chemical groups (molecules). When dampening solution covers layer 20, the ink will then not be able to diffuse or emulsify into the dampening solution quickly and because the dampening solution is much lower viscosity than the ink, film splitting occurs entirely within the dampening solution layer, thereby rejecting ink any ink from adhering to areas on layer 20 covered with an adequate amount of dampening solution. In general, the dampening solution thickness covering layer 20 may be between 0.1 μm-4.0 μm, and in one embodiment 0.2 μm-2.0 μm depending upon the exact nature of the surface texture. The thickness of the ink coated on roller 46a and optional roller 46b can be controlled by adjusting the feed rate of the ink through the roller system using distribution rollers, adjusting the pressure between feed rollers and the final form rollers 46a, 46b (optional), and by using ink keys to adjust the flow off of an ink tray (show as part of 46). Ideally, the thickness of the ink presented to the form rollers 46a, 46b should be at least twice the final thickness desired to transfer to the reimageable layer 20 as film splitting occurs. Typically, the final film thickness may be approximately 1-2 mm. Ideally, an optimized ink system 46 splits onto the reimageable surface at a ratio of approximately 50:50 (i.e., 50% remains on the ink forming rollers and 50% is transferred to the reimageable surface at each pass). However, other splitting ratios may be acceptable as long as the splitting ratio is well controlled. For example, for 70:30 splitting, the ink layer over reimageable surface layer 20 is 30% of its nominal thickness when it is present on the outer surface of the forming rollers. It is well known that reducing an ink layer thickness reduces its ability to further split. This reduction in thickness helps the ink to come off from the reimageable surface very cleanly with residual background ink left behind. However, the cohesive strength or internal tack of the ink also plays an important role.
There are two competing results desired at this point. First, the ink must flow easily into voids 40 so as to be placed properly for subsequent image formation. Furthermore, the ink should flow easily over and off of dampening solution regions 38. However, it is desirable that the ink stick together in the process of separating from dampening solution regions 38, and ultimately it is also desirable that the ink adhere to the substrate and to itself as it is transferred out of voids 40 (
With reference to
Reimageable surface layer 20 consists of a polymer such as polydimethylsiloxane (PDMS, or more commonly called silicone) for example with a wear resistant filler material such as silica to help strengthen the silicone and optimize its durometer, and may contain catalyst particles that help to cure and cross link the silicone material. Alternatively, silicone moisture cure (aka tin cure) silicone as opposed to catalyst cure (aka platinum cure) silicone may be used. Reimageable surface layer 20 may optionally contain a small percentage of radiation sensitive particulate material (not shown) dispersed therein that can absorb laser energy highly efficiently. In one embodiment, radiation sensitivity may be obtained by mixing a small percentage of carbon black, for example in the form of microscopic (e.g., of average particle size less than 10 μm or nanoscopic particles (e.g., of average particle size less than 1000 nm) or nanotubes, into the polymer. Other radiation sensitive materials that can be disposed in the silicone include graphene, iron oxide nano particles, nickel plated nano particles, and the like. Relative motion between imaging member or moving surface and inking subsystem, for example in the direction of arrow A, permits a process-direction inking.
One exemplary apparatus 100 for accomplishing heating over a minimal time is illustrated in
In a still further approach, a heat pipe system may be incorporated within heated nip roller 108. Heated nip roller 108 may itself comprise a heating mechanism and at least one sealed, fluid-filled cavity within a cylindrical housing (e.g., double cylindrical walls with an enclosed annular cavity forming the heat pipe structure). The cavity is maintained at a controlled internal pressure corresponding to the vapor pressure of the enclosed fluid near the temperature at which effective heat transfer is desired. Through constant phase change (vaporization) at a “hot” (i.e., heat source) portion of the cavity, followed by transfer of the vaporized fluid to a “cold” (i.e., heat sink) portion of the cavity, and its subsequent condensation near the heat sink portion, large amounts of heat can be quickly transferred due to the rapid phase change heat transfer effects. Low thermal mass is required, e.g., to enable a rapid and power-efficient temperature rise in ink 100. See, e.g., U.S. Pat. No. 3,677,329, incorporated herein by reference.
As shown in
As shown in
The variable data lithographic printing process is depicted in
A main difference between traditional lithographic offset printing and the variable data lithographic print process is that every image can be different, as in all digital printing. This is often referred to as Variable Data printing. Traditional lithographic offset is inherently a reprographic process in that all images for each revolution of the image drum are the same. Thus the mean ink throughput for each drum revolution is the same, and the critical ink-to-fountain-solution ratio is the same for each revolution. The process is tuned manually, an acceptable step since there is little change in the ink demand between printing cycles, by an operator to find the correct ink supply rate to match the ink load and the image content, and to also match the ink and fountain solution mixture. The operator makes ink supply adjustments and visually inspects the printed output to perform the manual tuning process. In variable data lithographic print process those limitations are overcome by using inker unit 140 with keys 150 automatically adjusted by a control system.
Prior to developing the ink onto the exposed imaging cylinder the development station formulates the ink demand for each of the ink keys by performing pixel counting. The sub-image stream is used by pixel counter module 210 to determine a pixel count which is indicative of the anticipated ink usage when actually printing the print job on the variable data lithographic printer. The pixel count can be determined by means of a simple algorithm, or by a table look up. In order to ensure precise prediction of ink usage, a weighing factor might be taken into consideration to take care of printer or job specific considerations. The pixel count is proportional to the number of pixels to be inked. A pixel counter counts the number of pixels to be imaged with ink on each stripe of vertical scanline data; for each color. The pixel count information is stored in memory.
The various rotating rollers in the ink train 115 cause a substantial lateral distribution of the ink, so that the amount of ink supplied to a given zone at the imaging member is not only dependent on the ink key associated with that zone, but also on adjacent ink keys. In other words, as the ink travels from the inker unit 145 to the imaging cylinder via several laterally rotating rollers, a certain amount of ink bleeds from one zone to another.
The ink is dispensed at an inker unit 145, which is provided with a flexible lower section composed of ink fountain blades operated by motors 240. Ink keys 150 control the opening of the ink fountain blades. The inker is divided into zones and each zone has one ink key. The disclosed embodiment will be described with thirty six ink zones, i.e., with thirty six ink keys along the lateral extent of the inker roll.
The ink is picked up from the inker unit 145 by an inking roller. An inker path consisting of a series of rollers passes and spreads the ink until it gets to the imaging cylinder.
The inking roll is divided into zones,
A feedforward/feedback controller 215 uses the required ink demand into the future, as determined by the pixel counting module 210. The feedforward controller 217 determines the ink load as a function of time into the future for what is about to be printed from the received data from the pixel counting module 210. The feedforward controller 217 is able to anticipate the required ink supply based on knowledge of the future ink demand from the pixel counting module. This ink demand can be used by the feedforward controller to generate a first control function which can be used to control at least one of the plurality of ink keys. However, the feedforward controller 217 needs to account for the delay and transient dynamics of the ink train115 as well as feedback from current ink density measurements to determine a current set point for the motors 240 which drive the keys 150 of a keyed inking train 115.
A signal from an ink density sensor 230 or densitometer is converted to an ink density value through known logarithmic techniques. The particular advantage of ink density measurement is the fact that the density value has a simple relationship with the ink layer thickness. It is possible for a large number of measured values to be obtained on a measurement field of given size over a short period of time. The density measurements are made available to both the feedforward controller 217 and the feedback controller 219 in real time.
The feedback controller 219 uses the results of an inker model 220 to modify the first control function. The inker model 220 models the inker dynamics. The model is updated over time using the ink density sensor 230. This process is performed for each of the cross process location associated with an inker key. This is typically on the order of one inch in the cross process direction. The feedback density sensor is also used to update the model on a periodic basis. Also feedback parameters 225 such as gain signals and process speed are used to increase the robustness of the model.
The feedforward control parameters are adjusted for the particular inker configuration to reduce the error and maintain stability, i.e., to make sure that the prediction of the pixel count and the inker model can reflect the behavior of the system under a myriad of conditions. The transient response of the inker system depends on the speed at which the rollers at the ink train 115 are driven, as well as the number of cooperating rollers. The main objective is to drive the control signals using the feedback signals to reduce the error signal to zero.
A command for the ink demand for each zone is entered into a feedforward/feedback controller 215. The feedforward/feedback controller 215 defines the ink key openings according to its transfer function or opening response. The closed-loop system for the presetting obtains its error signal from a measuring bar which measures the ink thickness of each of the thirty six zones at the imaging member from the feedback loop that includes the coverage input with reference to each zone. The coverage represents the desired zonal coverage determined by the “tone reproduction curve” (TRC) and ink load for the print job.
The controller 300 may be embodied within devices such as a desktop computer, a laptop computer, a handheld computer, an embedded processor, a handheld communication device, or another type of computing device, or the like. The controller 300 may include a memory 320, a processor 330, input/output devices 340, a display 330 and a bus 360. The bus 360 may permit communication and transfer of signals among the components of the computing device 300.
Processor 330 may include at least one conventional processor or microprocessor that interprets and executes instructions. The processor 330 may be a general purpose processor or a special purpose integrated circuit, such as an ASIC, and may include more than one processor section. Additionally, the controller 300 may include a plurality of processors 330.
Memory 320 may be a random access memory (RAM) or another type of dynamic storage device that stores information and instructions for execution by processor 330. Memory 320 may also include a read-only memory (ROM) which may include a conventional ROM device or another type of static storage device that stores static information and instructions for processor 330. The memory 320 may be any memory device that stores data for use by controller 300.
Input/output devices 340 (I/O devices) may include one or more conventional input mechanisms that permit a user to input information to the controller 300, such as a microphone, touchpad, keypad, keyboard, mouse, pen, stylus, voice recognition device, buttons, and the like, and output mechanisms such as one or more conventional mechanisms that output information to the user, including a display, one or more speakers, a storage medium, such as a memory, magnetic or optical disk, disk drive, a printer device, and the like, and/or interfaces for the above. The display 330 may typically be an LCD or CRT display as used on many conventional computing devices, or any other type of display device.
The controller 300 may perform functions in response to processor 330 by executing sequences of instructions or instruction sets contained in a computer-readable medium, such as, for example, memory 320. Such instructions may be read into memory 320 from another computer-readable medium, such as a storage device, or from a separate device via a communication interface, or may be downloaded from an external source such as the Internet. The controller 300 may be a stand-alone controller, such as a personal computer, or may be connected to a network such as an intranet, the Internet, and the like. Other elements may be included with the controller 300 as needed.
The memory 320 may store instructions that may be executed by the processor to perform various functions. For example, the memory may store instructions to control the inking train, the executable instructions capable of directing a processor to perform: receiving a print job comprising at least one image; separating the at least one image into cross-process direction sub-images associated with each ink key; providing an output in accordance to a first control function which is adaptable to control one of the at least one movable ink key based on an ink load demand for each sub-image; controlling the at least one actuatable motor to dispense ink by moving the associated ink key on the inking train in accordance to the first control function; updating the stored inker dynamic model for the inking train with the measured ink density.
Action 515 processes each image of the print job so as to separate the images into cross process sub-images. Each image is separated into cross-process direction (Z1, Z2 . . . Zn) sub-images associated with each inker key.
Action 520 counts pixels in each sub-image over look-ahead time. A running pixel count is determined for each sub-image. The pixel count is performed for the look-ahead time, which is related to the delay and transient response of the ink train 115.
In Action 525 the method inverts dynamic model and incorporates feedback density measurements to determine motor setpoint values for next time segment. A simulation of the inker control model can be used to preset setpoint values for the ink keys before the printing begins. The preset setpoint values may be obtained by inputting the necessary parameters including the desired coverage and printing ink characteristics into a processor such as processor 330. Typically, the processor program will have the necessary data regarding machine-specific parameters, such as the number and width of the ink keys, the actuator information, and the like. The data can be obtained from previous runs or by an operator inputting values to impact parameters such as ink density, ink train delays, print media parameters. The pixel count 520 is then used as an input into the inker inverse dynamic model to determine the feedforward portion of the control signal. In an embodiment, given the pixel count, the feedforward control signal value for time segment t, may be given by: uff(t)=Cff pc(t+tff)+βff(1) uff(t−1)+βff(2) uff(t−2)+ . . . +βff(Nff) uff(t−Nff), where pc( ) is the pixel count, tff is the delay through the ink train, Cff is a model parameter relating pixel count to ink load, Bff( ) are model parameters related to the dynamics of the ink train, and Nff is the number of delay time segments to use in the model. The number of delay time segments to use in the model is dependent on the specific dynamics of the ink train. Note that the pixel count into the future (positive value of tff) is used in the equation. This makes sense since the ink load into the future is used to determine the present key position to account for the delay through the ink train. The most recent density measurements, the density target, and the ink load when those density measurements were made and feedback gains are used to define the feedback portion of the control signal. In an embodiment, the feedback control signal at time segment t, is given by: ufb(t)=afb(1) ufb(t−1)+Qfb(2) ufb(t−2)+ . . . +Qfb(Lfb) ufb(t−Lfb)+βfb(0) efb(t)+βfb(1) efb(t−1)+ . . . +βfb(Mfb) efb(t−Mfb), where afb( ) are parameters related to the dynamics of the ink train, Lfb is the number of terms used which is related to the dynamics of the ink train, βfb( ) are parameters related to the weighting of past and present density errors and the desired responsiveness of the controller, Mfb is the number of terms used in the error feedback portion of the controller and is related to desired responsiveness of the control, and efb(t) is the density error (density target minus density actual). The feedforward portion of the control signal and the feedback portion of the control signal are then combined to define the aggregate control signal which is used to command the motor 240 setpoint for the key 150 associated with a cross-process direction sub-image. In an embodiment, this is given by: u(t)=uff(t)+ufb(t). This is repeated for all the sub-images in the cross-process direction (Z1, Z2 . . . Zn).
In action 530 the image is printed over the next time segment. The image is then printed over a time segment which corresponds to a fixed process direction length such as the turning of the imaging member.
In action 535 a decision is made whether to make density measurements at the current time. Density measurements are periodically performed. If a density measurement is to be performed at the current time, density patches are printed in inter-document zones and measured with a densitometer in action 540. Results are used in the control signal calculation by the feedforward controller and the feedback controller.
In action 545 the method determines if the last time segment has occurred. If the print job is not completed, the process goes back (550) to action 520 to update the pixel count based on the image or sub-image segments to be printed in the next time segment. Action 545 forwards control to action 555 for further processing if it is determined that the print job has been completed. In action 555, it is determined if the inker dynamic model needs to be updated. If the inker model needs to be updated then control is passed (action 560) to method 600 for further processing. If the inker model is not to be updated or the update has been performed by action 600 control is passed to action 570 indicating that the print job is complete.
Method 600 begins with action 605, where the update dynamic inker model process is started. Control is then passed to action 610 for further processing.
In action 610, a print job that prints an image that spans all levels of the “tone reproduction curve” (TRC) is printed. Control is then passed to action 615 for further processing.
In action 615, motor setpoints are adjusted while the TRC print job is being printed. The setpoint adjustments include step changes of various amplitude, sinusoidal variations of various amplitude, and pseudo-random sequences for various embodiments of the invention. Control is then passed to action 620 for further processing.
In action 620, the developed ink density on the drum is measured with a densitometer sensor. Actions 610, 615, and 620 are repeated until all data is collected. Control is then passed to action 625 for further processing.
In action 625, data is used to fit a dynamic model using known techniques from the field of system identification as is well known to those in the art. Examples of fitting functions can be found from the system identification literature including time-domain, frequency domain, and non-linear techniques as disclosed in Ljung et al., “System Identification: Theory for the User, Second Edition”, New Jersey, Prentice Hall, 1999. pp. i-672. QA402.L59. They include fitting a delay plus first order parameterized model to the step responses, fitting Nth order matrix Ordinary Differential Equation models using least squares techniques, fitting describing function models to the data, and fitting non-linear dynamical system models to the data. Note that the model may be such that parameters from the old dynamic inker model may be updated by a certain percentage change as defined by the new data. That is, the parameters themselves may be updated in an infinite impulse response (IIR) model where the updated parameter value is formed by adding a certain percentage of the old parameter value to a certain percentage of the new value. Therefore the model does not change abruptly. Note that in another embodiment, the dynamic inker model is composed of separate models for each inker key zone. Control is then passed to action 630 for further processing.
In action 630, the method is completed and the updated dynamic inker model is ready for use.
In the preceding paragraphs, example embodiments of the invention were described. These embodiments are presented for purposes of illustration rather than of limitation, and minor changes may be made to the example embodiments without departing from the inventive principle or principles found therein. It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, and are also intended to be encompassed by the followings claims.
This application is related to the following co-pending applications, which is hereby incorporated by reference in its entirety: “IMAGE DATA BASED TEMPERATURE CONTROL OF A KEYLESS INKER FOR DIGITAL OFFSET PRINTING SYSTEMS”, Attorney Docket No.: 056-0424, U.S. patent Ser. No. ______, filed herewith, by Peter Paul et al. This application is related to the following co-pending applications, which is hereby incorporated by reference in its entirety: “IMAGE FEEDFORWARD LASER POWER CONTROL FOR A MULTI-MIRROR BASED HIGH POWER IMAGER”, Attorney Docket No.: 056-0422, U.S. patent Ser. No. ______, filed herewith, by Peter Paul et al. This application is related to the following co-pending applications, which is hereby incorporated by reference in its entirety: “VARIABLE DATA LITHOGRAPHY SYSTEM”, Attorney Docket No.:20091609-US-NP, application Ser. No. 13/095,714, U.S. patent Ser. No. ______, filed on 27 Apr. 2011, by Timothy D. Stowe et al.
