Method for compensating for disruption torques in the driving of a jetting cylinder

Abstract

A method for compensating for disruption torques in driving a jetting cylinder in an inkjet printing machine includes recording print image products for at least one color and at least one complete rotation of the jetting cylinder by using an image sensor and using a computer for a time parallel measurement of a driving torque of the jetting cylinder, generating an average gray value profile above a circumferential coordinate based on the recorded image data by using the computer, transforming the average gray value profile and the measured profile of the driving torque into a frequency range by using a computer-assisted Fourier transform and extracting order components from the frequency range. A periodic compensation torque is calculated based on the extracted order components, and the calculated periodic compensation torque is factored-in when actuating the jetting cylinder of the inkjet printing machine in the course of a printing operation.

Claims

1. A method for compensating for disruption torques in the drive of a jetting cylinder in an inkjet printing machine, the method comprising the following steps: using an image sensor to record print image products for at least one color and at least one complete rotation of the jetting cylinder, and using a computer for a time parallel measurement of a driving torque of the jetting cylinder; using the computer to generate an average gray value profile above a circumferential coordinate based on recorded image data; using a computer-assisted Fourier transform to transform the average gray value profile and a measured profile of the driving torque into a frequency range, and extracting order components from the frequency range; calculating a periodic compensation torque based on extracted order components; and factoring-in the calculated periodic compensation torque when actuating the jetting cylinder of the inkjet printing machine in a course of a printing operation.

2. The method according to claim 1, which further comprises carrying out at least two similar identification runs to record print image products and to measure the driving torque parallel in time, with measured driving torque data only differing in terms of a respective additive periodic disruption torque in the drive.

3. The method according to claim 2, which further comprises increasing error tolerance by carrying out at least 20 similar identification runs, using one-third of the identification runs to calculate the periodic compensation torque by using the computer and using two-thirds of the identification runs to validate data set by using the computer.

4. The method according to claim 3, which further comprises using the computer to influence the at least 20 similar identification runs by selecting amplitude and phase positions of the disruption torques relative to one another in such a way that conditioning of a resultant equation system of a respective additive periodic disruption torque in the drive is as easy to calculate for the computer as possible, and selecting at least two additive periodic disruption torques in the drive for which the conditioning of the resultant equation system is most favorable.

5. The method according to claim 1, which further comprises carrying out the calculation of the periodic compensation torque by the computer by using a machine learning approach with a repeated application of training data sets.

6. The method according to claim 1, which further comprises using the computer to separately mathematically process the average gray value profile in the frequency range, with orders below one treated differently from orders greater than or equal to one.

7. The method according to claim 1, which further comprises using an inline camera of an image recording system of the inkjet printing machine as the image sensor to record the print image products.

8. The method according to claim 1, which further comprises carrying out precisely two identification runs to record print image products and to measure the driving torque parallel in time, exclusively forming one disruption torque of pure sine terms and forming the other disruption torque of cosine terms of identical amplitude per frequency node.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a diagram indicating potential causes of additive disruption torques;

(2) FIG. 2 includes two diagrams illustrating the fundamental relationship between velocity changes on the jetting cylinder and resultant ink density fluctuations;

(3) FIG. 3 is a diagram illustrating different ink density fluctuations of specific orders in the frequency range; and

(4) FIG. 4 is a flow chart of the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) Referring now in detail to the figures of the drawings, in which mutually corresponding elements have the same reference symbols, and first, particularly, to FIG. 4 thereof, there is seen a preferred exemplary embodiment of the method of the invention illustrated in the form of a flow chart. FIG. 4 shows that upon an initial start-up of the printing machine, at least two identification runs need to be carried out in a similar way. In this process, an inline camera records images of print results 9 for at least one color and at least one complete rotation of a jetting cylinder and, in synchronism with the recordings, the driving torque of the machine is measured. The measurements only differ in terms of an additive periodic disruption torque in the drive, which includes all of the relevant order components. Potential causes 1 of this additive periodic disruption torque are shown in FIG. 1. The figure clearly shows a chain of effects from the disruption and the respective individual causes thereof to a resultant density profile.

(6) Based on the images of the print results, an average gray value profile 10 is then generated above the circumference coordinate in an image processing step. By using a Fourier transform, this profile 10 and the torque profile are converted to the frequency range where the order components indicated above are extracted from the gray value profile in a frequency range 11. FIG. 2 illustrates the correlation between jetting cylinder velocity changes and the resultant color density profile fluctuations. The first image of FIG. 2 represents the measured circumferential speed of the jetting cylinder 2. The second image illustrates established position errors 3 of ink drops that have hit the printing substrate. These errors cause the aforementioned color density fluctuation. The representation is in the time range and the illustrated signal corresponds to a signal 4 of the encoder on a jetting cylinder. Changes in surface velocity 8 cause defects in the drop positions and thus in a color density profile 7. The figure clearly illustrates a reverse correlation 5 between the two measurement curves. Conversely, FIG. 3 illustrates a frequency range 6 with the orders in question, indicating different order components such as color density 7 or surface velocity 8 for different order components.

(7) Due to the gap in the jetting cylinder in which no image information is generated, the gray value profile needs to be separately mathematically processed, resulting in a gray value profile with reduced orders 12. In this context, orders below one and orders greater than or equal to one are treated differently.

(8) These data and the knowledge about the additive periodic disruption torque are used to calculate a periodic compensation torque 13 that provides a fluctuation-free, i.e. constant color density profile in the circumferential direction.

(9) An aspect to be considered when the at least two additive periodic disruption torques are selected is that the conditioning of the resultant system of equations is as favorable as possible. This conditioning may be influenced by the amplitude and phase positions of the disruption torques relative to one another.

(10) Ideally, about 20 measurements with different disruption torques are recorded. One third of these measurements is used to calculate the periodic compensation torque 13, for instance in the form of a set of training data as in the machine learning approach, whereas the other two thirds are used to validate the data set. This causes the process to be sufficiently robust in the case of disturbances.

(11) If the periodic compensation torque 13 that has been calculated in this way is not valid for all modes of operation of the machine such as format adjustment, varnish on/off, precoat on/off, etc, the process needs to be repeated for every configuration that is of interest. The respective additive disruption torque would then be applied in accordance with the mode of operation.

(12) The error found in the surface velocity 8 and the image data path as described above are thus reduced and, in a best-case scenario, completely eliminated.

(13) Since most inkjet printing machines that are currently in use have all the required components and sensors, no additional hardware costs are incurred.

(14) Moreover, there is an alternative embodiment, which involves only two identification runs. In this context, the disruption torque of the first identification run is exclusively formed of pure sine terms of identical amplitude per frequency node, whereas the disruption torque of the second identification run is exclusively formed of pure cosine terms.

(15) An advantage of this approach is that it is faster and consumes less paper.

(16) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 1 causes of additive disruption torques 2 circumferential jetting cylinder speed 3 position error of an ink drop 4 encoder on the jetting cylinder 5 reverse correlation 6 frequency range with different orders 7 color density 8 surface velocity 9 recorded prints 10 determined gray value profile above circumference coordinate 11 Fourier-transformed gray value profile in the frequency range 12 gray value profile in the frequency range with reduced orders 13 periodic compensation torque