Reference is made to commonly assigned U.S. patent application Ser. No. 12/568,762, filed Sep. 29, 2009 by John Saettel, entitled “Exposure Averaging”, commonly assigned U.S. patent application Ser. No. 12/568,13 filed Sep. 29, 2009 by John Saettel, entitled “A Calibration System For Multi-Printhead Ink Systems”, and commonly assigned U.S. patent application Ser. No. 12/568,750 filed Sep. 29, 2009 by John Saettel, entitled “Color to Color Registration Target” the disclosures of which are herein incorporated by reference.
The present invention generally relates to inkjet printing systems and, more particularly, to such inkjet systems that print and capture an image of a test registration target for determining speed compensation factor(s) due to speed variations of the media.
High-speed, multi-color inkjet printing systems need calibration for a variety of reasons including the need to accurately position the printed image at the proper position on the print media. For example, it is a well know fact that changes in transport speed of the media will result in color-to-color (C2C) registration changes. The reason for these changes can be attributed to two factors. The main contributing factor is “Time of Flight” (TOF)—the time required for an ink droplet to exit the print head and impact the substrate. The second contributing factor can be changes in substrate tension, with changes in transport speed. These factors produce a nearly linear relationship between the speed of the transport and registration error. Current technology allows an operator to manually adjust the TOF/Speed Compensation variable. This variable is expressed in units of time and defines the C2C/Speed slope relationship. This variable is then used by the system to automatically advance each colors print head timing depending on the current speed of the transport.
Although the above-described method is satisfactory, improvements are always desired. One such improvement is to automate the implementation of the TOF/Speed Compensation variable.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the invention, the invention resides in a method for determining a speed compensation factor in an inkjet printing system, the method comprising the steps of (a) transporting a media moving at first speed; (b) printing a first image on the media using at least one printhead while moving at the first speed; (c) capturing the first image in a first frame of an image capture device; (d) transporting the media moving at a second speed different from the first speed; (e) printing a second image on the media using the at least one printhead while moving at the second speed; (f) capturing the second image in a second frame of the image capture device; (g) determining using a change of position of the first image relative to the second image using automated image analysis; (h) inputting the determined change of position into a processor for computing the speed compensation factor from the determined change of position and a known difference in speed between the first and second speeds.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.
The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein:
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein:
Turning now to
An encoder 90 is used to monitor the motion (in the direction of the arrow) of the print media 20 through the printing system 10. Typically the encoder 90 is in the form of a rotary encoder that creates a defined number of pulses per revolution. The rotary encoder is connected to a roller or wheel (not shown) that is rotated by the moving paper. The circumference of the wheel or roller, in combination with the defined number of pulses per revolution of the rotary encoder 90, determines the number of encoder pulses per centimeter or inch of paper travel. The output of the encoder 90, in the form of an encoder pulse train, is used by the process controller 80 for controlling the placement of the print media 20 along the direction of print media travel. Typically the spacing of pixels in the in-track direction (along the direction of paper motion) corresponds to N times the spacing between encoder pulses, where N is a small (<10) integer. To properly print a multi-color document, the print data sent to each printhead T2-T4 downstream of the first printhead T1 must be delayed by increasing amounts relative to the data of first printhead. These delays are normally defined in terms of a delay count or the number of the encoder pulses that correspond to the spacing along the paper path of the printheads T2-T4 from the first printhead T1. For example, if the second printhead T2 is located 8.5 inches downstream of the first printhead T1 and the encoder 90 produces 600 pulses per inch, the print data to the second printhead T2 would be delayed by 5100 pulses relative to the data to the first printhead T1.
The print media 20 passes under and in the optical path of the image capture devices 50a-50c, such as a digital camera, in order to capture the printed test marks from the printheads T1-T4. Various digital cameras can be employed provided they have sufficient optical resolution and light sensitivity to capture images of the test marks. One such useful camera is the IMP-VGA210-L from Imperx. This is a black and white camera with a 640×480 pixel resolution. It is able to output images at a rate of 210 complete frames per second through a CameraLink™ interface to an image processing system. This camera also has an external trigger and an externally controllable electronic shutter so that acquisition of images and the shutter time for acquiring an image can be controlled by the process controller 80. This camera also allows a portion of the active pixels in the captured image frame to be defined as an area of interest. The camera sensor then uses only that portion of its active pixels for image capture, and only transfers the image data corresponding to that area of interest to the image system analyzer 70. By so doing, the camera is able to capture and transfer partial frame images at higher frame rates than its complete frame rate. An infinite conjugate micro-video lens from Edmund Optics, #56776, with a 25 mm focal length and a 1:1 magnification is an effective lens for use with this camera. In one embodiment, the strobe lights are light emitting diodes, two LED's each of red, green and blue, arranged circular around the lens of the camera. Light emitting diodes from Luxeon, such as LXHL-PH09, LXHL-PM09, and LXHL-PRO09, are examples of usable LED's. The image capture device may be mounted on a carriage downstream of each printhead so that the image capture device is adjustable in position in a cross-track direction. Alternatively, the image capture device may be mounted directly to the downstream side of the printhead so that it can capture the image of the test marks printed by that printhead and the first printhead.
Just as the process controller 80 can use the output of the encoder 90 in combination delay count values associated with each printhead to register the print from the various printheads, the process controller can control the flashing of the strobe lights and the capturing of a frame by the camera to a particular portion of the printed image by using a delay count associated with the image capture device and the output of the encoder.
The print media, which is moving at a velocity Vmedia, moves a time of flight displacement distance Dtof during this drop transit time; given by
Dtof=Vmedia*Ttr
To ensure proper placement of an inkjet printed image on the print media, requires accounting for this time of flight displacement distance Dtof. To hit the desired target location on the moving print media, it is necessary for the print drop to be fired or created when the target location on the print media has a distance of Dtof to travel to reach the drop impact position. In high speed inkjet printing systems that account for the time of flight displacement distance, typically the velocity of the print media is not a constant value established during product development, but rather is adjustable up to some upper speed limit. Therefore it is necessary to calculate the distance Dtof based on the print media speed Vmedia rather than use a fixed value for Dtof. As nominal values for the distance D and for the drop velocity Vdrop are defined or determined during product development, a nominal value for the transit time can be calculated and used in combination with the measured print media velocity Vmedia to calculate the distance Dtof. (The print media velocity is typically determined from the frequency of pulses from the encoder 90.) While nominal values of the D and drop velocity Vdrop, can be used in the calculation of Ttr and Dtof, the actual values for D and Vdrop can differ from the nominal value. For example, differences in the printhead mount and print media guide rollers can cause the actual value of D to differ from the nominal value. Similarly variations in nozzle diameter, ink pressure, ink properties such as viscosity, and in the amplitude of drop creation pulses can all cause the drop velocity to differ from the nominal value. This can cause the printed image to be misaligned relative to some pre-printed image on the print media or it can cause the different image planes printed by different printheads to be misregistered.
The printing system 10 of the present invention includes various components that permit more accurate TOF/Speed Compensation factors to be determined and implemented in the printing system. In this regard, the printing system prints two separate images. The images can include test patterns such as is shown in
In the printing of both the first and second images, the process controller calculates and employs the values for Dtof for the appropriate print media velocities to serve as compensation distances to try to place the printed images at the target position on the print media. If the proper value for the drop transit time is used in calculating the Dtof values at the first and second media speed, the images printed at the first and second media speeds will print at their target locations. The use of a nominal transit time that differs from the actual transit time will result in the computed compensation distance being different from the actual time of flight displacement distance, resulting in the test marks missing their intended locations. Furthermore the amount by which the test marks miss the target location will depend on the difference in the media speed between the first and second media speed. This result can be understood, by means of the following analysis. As mentioned above.
Dtof=Vmedia*Ttr
If the media speed is equal to V1, the actual time of flight displacement distance D1tof is given by
D1tof=V1*Ttr
If however a nominal value for the transit time, Tnom is used instead of the actual transit time Ttr, the compensation distance D1 used by the control to try to place the test mark at the target location with a media speed of V1 would be.
D1=V1*Tnom
This compensation distance D1 differs from the actual time of flight displacement distance D1tof by an amount ΔD1 given by
ΔD1=V1*Tnom−V1*Ttr=V1*(Tnom−Ttr)
One can also calculate for the second media speed V2, values for the actual time of flight displacement distance D2tof, compensation distance D2 and the deviation of the test mark from the intended location ΔD2, as follows.
D2tof=V2*Ttr
D2=V2*Tnom
ΔD2=V2*Tnom−V2*Ttr=V2*(Tnom−Ttr)
It is clear that we could calculate the difference between the actual transit time and the nominal value at either media speed from a measurement of the test mark deviation distance ΔD. This, however, is not practical as there is no mark on the print media to show the location of the intended location for the test marks to allow such measurements to be made.
It must be noted that at each media speed, an image capture device 50, under the control of the process controller 80, captures in a frame an image of the target location. The process controller 80 doesn't employ the calculated values of Dtof for controlling the image capture devices as the capturing of an image by the image capture device is essentially instantaneous; image capture doesn't involve the drop transit time. Since an image capture doesn't involve a time of flight displacement distance, the target location for the test marks remains fixed, within the captured image frame independent of the media speed. While the location within the captured frame of the target location is not know, the fact that the target location remains fixed independent of media speed allows one to determine the deviation of the nominal transit time from the actual value. This is done by means of a measurement of the shift in the test mark location with the frames captured at media speed 1 and media speed 2, δD.
δD=ΔD1−ΔD2=(V1−V2)*(Tnom−Ttr)
From this we get
This allows the actual transit time to be determined
To improve the correction amount for the transit time, preferably, one of the first and second speeds is near the maximum speed of the media and the other speed is near the slowest speed at which the media moves smoothly through the printing system without significant velocity variation. This provides the largest velocity difference and the largest shift in test mark location δD. The transit time calculated in this manner can then be used as the new value for the Tnom used to calculate the compensation distances for printing at any media speed. This value may be stored in memory 110 for use by the printing system 10.
This analysis has shown that the actual drop transit time can be calculated from the measured shift in the test mark location and the two media speeds. From an implementation point of view, it must be recognized that each of the measurements may involve some scaling factors. For example the velocity may be determined from a measurement of the encoder pulse frequencies with a scaling factor to convert from frequency to a linear speed. Similarly the shift distance dD can be measured from a measurement of the shift measured in the pixels in the frame with a scaling factor from pixels to actual distance. As a result of these various scaling factors being used, it may be more convenient to calculate a compensation factor or term that deviates from the actual transit time by some scaling factor. Such a compensation factor can then be employed to determine appropriate compensation for the drop time of flight at any media speed.
As the actual transit time can drift as a result of drifting operation parameters and ink properties, it is appropriate to repeat this test after a period of time. As a result, a second speed compensation factor may also be computed that is compared to the original compensation factor for detecting a change in a variable of the inkjet printing system other than the speed compensation factor. The tracking of compensation factors determined in this manner may then be utilized by the process controller as part of its internal diagnostics.
Camera 50c captures each image when all four colors are desired in the captured image. However, the present invention is not limited to this design. Any of the cameras 50a-50c may be used to capture the two images printed at the first and second media speeds. Any camera that can capture the test marks printed by a particular printhead can be used to capture the two images printed at the first and second media speeds from which a TOF/speed compensation variable can be determined for that printhead. After image capture, the image system analyzer 70 converts the images into bit maps, identifies each of the test marks, and determines their locations within the image.
Although the analysis is preferably done in the image system analyzer 70, a separate processor 100 may also be used. The processor 100 receives input from each of the cameras 50a-50c, as does the image system analyzer 70, although the input lines are omitted from
It is also noted that, while the description above describes the printer in terms of four printheads each printing a separate color, the invention is not limited to printing systems having exactly four printheads.
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