This disclosure relates generally to inkjet printers, and more particularly, to printheads having inkjets that are operated with firing signals.
Inkjet printers include one or more printheads that are operated to produce ink images on substrates. The printheads typically have an array of inkjets, which include transducers that receive firing signals to activate the transducers and eject a drop of ink from an inkjet. The average volume of an ink drop ejected from an inkjet in a printhead may vary significantly over the life of the printhead. This inconsistency in drop volume can arise from a number of factors such as printhead aging, variations in the viscosity of the ink supplied to a printhead, and the like. The inconsistency in drop volumes can adversely impact image quality. For example, a reduction in the expected ink drop volume may result in images appearing less saturated and an increase in expected drop volume may result in defects such as graininess and mottle. Ejecting ink drops having volumes that are different than the nominal volume can cause some inkjets to operate intermittently, eject significantly smaller ink drops, or cease operating completely. Detecting changes in the nominal volume of ejected ink drops on substrates and restoring the ability of the inkjets to eject ink drops at the nominal volume would be beneficial.
A new printer is configured to detect changes in the nominal ink drop volume produced by a printhead outside a predetermined range and restore the ability of the printhead to eject ink drops having the nominal drop volume. The printer includes a plurality of printheads, each printhead being configured to eject ink drops onto a substrate as the substrate passes each printhead in a process direction, a plurality of printhead drivers, each printhead driver being configured to operate one of the printheads in the plurality of printheads in a one-to-one correspondence, an optical sensor configured to generate image data of the substrate after the substrate has passed the plurality of printheads, and a controller operatively connected to each printhead driver and the optical sensor. The controller is configured to operate each of the printheads using the printhead drivers to print a pattern of ink drops on the substrate, receive from the optical sensor the image data of the substrate, determine whether a density response for the pattern of ink drops for each printhead is within a predetermined range about a reference density response for a pattern of ink drops printed by each printhead at a predetermined time, identify a peak voltage for each printhead determined to have the density response outside the predetermined range, each peak voltage being identified using a peak voltage that was used to operate each of the printheads to print the pattern ink drops on the substrate, and store the identified peak voltage for each printhead having the density response outside of the predetermined range in the printhead driver corresponding to the printhead having the density response outside of the predetermined range so the printhead driver uses the identified peak voltage to generate firing signals for inkjets in the printhead operatively connected to the printhead driver.
A method of printer operation detects changes in the nominal ink drop volume produced by a printhead outside a predetermined range and restores the ability of the printhead to eject ink drops having the nominal drop volume. The method includes operating with a controller a plurality of printhead drivers that are operatively connected to a plurality of printheads in a one-to-one correspondence to operate each printhead in the plurality of printheads to print a pattern of ink drops on a substrate as the substrate passes each printhead in a process direction, generating with an optical sensor image data of the substrate after the pattern of ink drops has been printed on the substrate, determining with the controller whether a density response for the pattern of ink drops for each printhead is within a predetermined range about a reference density response for a pattern of ink drops printed by each printhead at a predetermined time, identifying with the controller a peak voltage for each printhead determined to have the density response outside the predetermined range, each peak voltage being identified using a peak voltage that was used to operate each of the printheads to print the pattern ink drops on the substrate, and storing with the controller the identified peak voltage for each printhead having the density response outside of the predetermined range in the printhead driver corresponding to the printhead having the density response outside of the predetermined range so the printhead driver uses the identified peak voltage to generate firing signals for inkjets in the printhead operatively connected to the printhead driver.
Another embodiment of the new printer includes a plurality of printheads, each printhead being configured to eject ink drops onto a substrate as the substrate passes each printhead in a process direction, a plurality of printhead drivers, each printhead driver being configured to operate one of the printheads in the plurality of printheads in a one-to-one correspondence, an optical sensor configured to generate image data of the substrate after the substrate has passed the plurality of printheads, and a controller operatively connected to each printhead driver and the optical sensor. The controller is configured to operate at a first time each of the printhead drivers to generate firing signals for each of the corresponding printheads in the plurality of printheads to form a first plurality of patches on the substrate for each printhead, each patch in the first plurality of patches formed by each printhead having a predetermined grayscale level, receive from the optical sensor image data of the first plurality of patches for each printhead on the substrate, store in a memory the image data for each patch in the first plurality of patches for each printhead on the substrate as a reference density response for each patch printed by each printhead at each predetermined grayscale level, operate at a second time that is subsequent to the first time each of the printhead drivers to generate firing for each of the corresponding printheads in the plurality of printheads to form a second plurality of patches on the substrate for each printhead, each patch in the second plurality of patches for each printhead being printed at the predetermined grayscale levels used to print the first plurality of patches for each printhead, receive from the optical sensor image data of the second plurality of patches on the substrate for each printhead, and determine whether a density response for each patch in the second plurality of patches for each printhead is within a predetermined range about the reference density response stored for each patch printed by each printhead at each predetermined grayscale level, determine a compensation parameter for each printhead that printed at least one patch in the second plurality of patches having the density response that is outside the predetermined range, and store the compensation parameter in the printhead driver corresponding to the printhead that printed the at least one patch in the second plurality of patches having the density response outside of the predetermined range.
The foregoing aspects and other features of a printer that detects changes in the nominal ink drop volume produced by a printhead outside a predetermined range and restores the ability of the printhead to eject ink drops having the nominal drop volume are explained in the following description, taken in connection with the accompanying drawings.
For a general understanding of the present embodiments, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements.
A printing system 10 configured to detect changes in the nominal ink drop volume produced by a printhead outside a predetermined range and restore the ability of the printhead to eject ink drops having the nominal drop volume is shown in
Four printheads, each of which ejects a different color of ink, are shown in the print zone 26. Each printhead 50A, 50B, 50C, and 50D in the print zone 26 is operatively connected to a corresponding printhead driver 54A, 54B, 54C, and 54D and the controller 14 is operatively connected to these printhead drivers. A single printhead for a single color has been depicted to simplify the figure. Typically, each color of ink is printed by an array of printheads, which are arranged in a known manner, and each printhead in an array is operatively connected to a corresponding printhead driver.
The controller 14 can be implemented with general or specialized programmable processors that execute programmed instructions. The instructions and data required to perform the programmed functions can be stored in memory associated with the processors or controllers. The processors, their memories, and interface circuitry configure the controllers to perform the operations described below. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in very large scale integrated (VLSI) circuits. Also, the circuits described herein can be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits.
The controller 14 is operatively connected to an image source 70. Image source 70 can be a scanner, database, or other image generation or data source. An image that the controller 14 obtains from the image source 70 is used to operate the printer 10 to form an ink image on the web W corresponding to the obtained image. The controller 14 processes the image obtained from the image source in a known manner for control of the printhead drivers 54A to 54D. Specifically, a composite image is obtained from the image source 70. As used in this document, the term “composite image” refers to pixel data for each color and feature present in an image. The controller processes the composite image to produce color separation files that correspond to the colors of ink ejected by the printheads in the print zone. Additional processing can also occur in a known manner such as halftoning and the like. Each color separation file derived from the composite image is supplied to the printhead driver corresponding to the printhead in the print zone 26 that ejects the color ink corresponding to the color separation file. For example, the black color separation file derived from the composite image is delivered to the printhead driver MA, which operates the printhead 50A that ejects black ink. As used in this document, the term “print zone” means an area directly opposite a plurality of printheads that forms an ink image on a substrate using color separation files. The term “process direction” means the direction in which media moves through the print zone as the inkjets eject ink onto the sheets and the term “cross-process direction” means an axis that is perpendicular to the process direction in the plane of the media in the print zone.
In previously known printers, the inkjets in the printheads change over time and eject ink drops with volumes that are different than the nominal ink drop volumes ejected when the printer was put into service. The changes in the ink drop volumes can significantly affect the quality of the printed images. To detect these changes and restore the ability of the printheads to eject ink drops having the nominal ink drop volume, the controller is configured with new programmed instructions to operate the printheads to print test patterns and analyze the image data generated by the optical sensor of these patterns to detect whether the printheads are ejecting ink drops with volumes outside a predetermined range about the nominal ink drop volume. To restore the ability of a printhead to eject ink drops having the nominal value, a compensation parameter is identified and stored in the corresponding printhead driver to alter the operation of the printhead so it produces ink drops that correspond to grayscale values of patches printed at the manufacture of the printer. The compensation parameter can be a simple voltage change in the firing signal parameters for the printhead. For example, by changing Vpp, which is the peak voltage in the firing signal waveform, the entire waveform is scaled by a corresponding amount to adjust the volume of the ink drops being ejected.
For each printhead in a printer, the relation between firing signal peak voltage and the volume of the ink drops ejections is determined. A resulting graph for each of these relationships is shown in
To implement with the controller a control law for modifying the peak voltage in the waveform to restore the desired nominal drop size, a process shown in
Using a first order approximation that equal volumes produce near equal densities, the value of a can be used to determine how to change the ink drop size. For example, if α is 1.05, then to match the results obtained at the time of printer manufacture, on average 5% more inkjets need to fire. This increased number of inkjets results in 5% more ink on the page. This increase of 5% in ink volume can also be achieved by setting a new drop size that is 5% more than the current drop size and not changing the inkjet firing pattern. To the first order, these two different methods of correction are assumed to be similar in effect.
The graphs of drop volume versus voltage change shown in
The control law used in this determination is: V(k+1)=V(k)+K*(α(k)−1)*Ddes/dVslope, where V(k) is the peak voltage of the waveform, Ddes is the target drop size (e.g. 4.5 pl), dVslope is the local slope of change in drop volume per change in voltage for the printhead, and K is a controller gain on the drop error term. The parameter dVslope is determined for each printhead in the printer at the time the printer is manufactured and stored in a memory within the printer. From the graphs shown in
A process for operating the printer shown in
It will be appreciated that variations of the above-disclosed apparatus and other features, and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.
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