Patent Application
20110061552
This disclosure relates generally to moving web printing systems, and more particularly, to moving web printing systems that use a double reflex system to register images from different printheads.
A known system for ejecting ink to form images on a moving web of media material is shown in
This system 10 also includes two load cells, one of which is mounted at a position near pre-heater roller 22 and the other is mounted at a position near the turn roller 30. These load cells generated signals corresponding to the tension on the web proximate the position of the load cell. Each of the rollers 22, 30, and 34 has an encoder mounted near the surface of the roller. These encoders may be mechanical or electronic devices that measure the angular velocity of a roller monitored by the encoder, which generates a signal corresponding to the angular velocity of the roller. In a known manner, the signal corresponding to the angular velocity measured by an encoder is provided to the controller 60, which converts the angular velocity to a linear web velocity. The linear web velocity may also be adjusted by the controller 60 with reference to the tension measurement signals generated by the load cells. The controller 60 is configured with I/O circuitry, memory, programmed instructions, and other electronic components to implement a double reflex printing system that generates the firing signals for the printheads in the marking stations 26. A double reflex printing process is described in U.S. patent application Ser. No. 11/605,735 entitled “Double Reflex Printing” and published as U.S. Publication Number 2008/0125158 and commonly owned by the assignee of the present document. The term “controller” or “processor” as used in this document refers to a combination of electronic circuitry and software that generates electrical signals that control a portion or all of a process or system.
The system 10 may also include an image-on-web array (IOWA) sensor 68 that generates an image signal of a portion of the web as it passes the IOWA sensor. The IOWA sensor 68 may be implemented with a plurality of optical detectors that are arranged in a single or multiple row array that extends across at least a portion of the web to be printed. The detectors generate signals having an intensity corresponding to a light reflected off the web. The light is generated by a light source that is incorporated in the IOWA sensor and directed toward the web surface to illuminate the surface as it passes the optical detectors of the IOWA sensor. The intensity of the reflected light is dependent upon the amount of light absorbed by the ink on the surface, the light scattered by the web structure, and the light reflected by the ink and web surface. The image signal generated by the IOWA sensor is processed by an integrated registration color controller (IRCC) to detect the presence and position of ink drops ejected onto the surface of the web at the IOWA sensor.
As noted above, the controller 60 uses the tension measurements from the two load cells along with the angular velocity measurements from encoders to compute linear web velocities at the rollers 22, 30, and 34. These linear velocities enable the processor to determine when a web portion printed by one marking station, station 26A, for example, is opposite another marking station, stations 26B, for example, so the second marking station can be operated by the controller 60 with firing signals to eject ink of a different color onto the web in proper registration with the ink already placed on the web by a previous marking station. When the subsequent marking station is operated too soon or too late, the ejected ink lands on the web at positions that may produce visual noise in the image. This effect is known as misregistration. Accurate measurements, therefore, are important in registration of different colored images on the web to produce images with little or no visual noise.
A method enables linear web velocities produced with reference to angular velocity signals generated by encoders at different rollers in a double reflex printing registration system to be equalized. The method includes identifying a low frequency component of a first linear velocity of a moving web, identifying a high frequency component of a second linear velocity of the moving web, and computing a linear velocity for the moving web at a roller in a print zone with reference to the identified high frequency component of the second linear velocity and the identified low frequency component of the first linear velocity.
A system for implementing the equalization method has been developed. The system includes a first converter configured to generate a first linear velocity for a moving web at a first roller that drives a web of printable media from a first angular velocity measurement signal generated by a first encoder mounted proximate the first roller, a second converter configured to generate a second linear velocity for a moving web at a second roller that drives a web of printable media from a second angular velocity measurement signal generated by a second encoder mounted proximate the second roller, a low pass filter coupled to the first converter to identify a low frequency component of the first linear velocity, a high pass filter coupled to the second converter to identify a high frequency component of the second linear velocity, and a controller configured to compute a linear velocity of the moving web at the second roller with reference to the identified high frequency component of the second linear velocity and the low frequency component of the first linear velocity.
The foregoing aspects and other features of a system and method that equalizes linear velocity measurements produced from angular velocity measurements obtained with encoders at different rollers driving a web of printable media are explained in the following description, taken in connection with the accompanying drawings.
For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that performs a print outputting function for any purpose, such as a digital copier, bookmaking machine, facsimile machine, a multi-function machine, or the like. Also, the description presented below is directed to a system for operating a printer that forms images on a moving web driven by rollers. The reader should also appreciate that the principles set forth in this description may be applicable to imaging systems that form images on sheets.
In one embodiment of a printing system that uses a double reflex technique to control the firing of the printheads in the marking stations, the marking stations are solid ink marking stations. Solid ink marking stations use ink that is delivered in solid form to the printer, transported to a melting device where the ink is heated to a melting temperature and converted to liquid ink. The liquid ink is supplied to the print heads in the marking stations and ejected from the print heads onto the moving web in response to firing signals generated by the controller 60. In such a continuous feed direct marking system, the print zone is the portion of the web extending from the first marking station to the last marking station. In some systems, this print zone may be several meters long. If the angular velocity of each encoder mounted proximate to a roller is converted to a linear speed for the web, the variations between the linear web velocities at the different rollers over time can accumulate and lead to misregistration of the images.
At steady state for such a printing system, the average web velocity times the web material mass per length must be equal at all rollers or other non-slip web interface surfaces. Otherwise, the web would either break or go slack. To account for the differences in instantaneous velocities at rollers in or near a print zone, a double reflex processor interpolates between linear web velocities at a pair of rollers, one roller on each side of a marking station with reference to the direction of the moving web, to identify a linear velocity for the web at a position proximate the marking station. This interpolation uses the linear web velocity derived from the angular velocity of a roller placed at a position before the web reaches the marking station and the linear web velocity derived from the angular velocity of a roller placed at a position after the web passes by the marking station along with the relative distances between the marking station and the two rollers. The interpolated value correlates to a linear web velocity at the marking station. A linear web velocity is interpolated for each marking station. The interpolated web velocity at each marking station enables the processor to generate the firing signals for the print heads in each marking station to eject ink as the appropriate portion of the web travels past each marking station. Any differences arising between the linear velocities for the web at the rollers arise from inaccuracies that may lead to linear web velocity errors at the marking stations. These errors may lead to misregistration between ink patterns ejected by different marking stations. In the double reflex control method, these errors may affect each station and roller differently because of the different distances separating them. Calibrating the encoders that generate the angular velocity signals used for the linear velocities computations is insufficient to address the variations in the linear velocities because small errors may eventually accumulate and cause misregistration. For example, a roller diameter miscalculation of only 5 μm, which is approximately a 0.002% error for one roller, would yield a continuously growing error of about 10 μm per meter of web travel.
To address this source of linear web velocity and position error, a method and system have been developed that approximates a base speed for the web at all of the rollers. The system 200 is shown in block diagram form in
With continued reference to
Again with reference to
While the low frequency component is obtained from a low pass filtering of the linear web velocity at a first roller in
An alternative embodiment of the system is shown in
The reader should understand that the use of the sequential terms “first”, “second”, and “third” do not specify a roller with reference to their order along the path of the web as it moves through the printing system. Thus, the linear velocity for the web at any of the rollers may be used to establish an average velocity for the web at all of the rollers. The high frequency components identified by the high pass filters may then be used for further refinement of the linear velocities at the rollers.
In another embodiment, the converters may receive a compensation value that is used to generate a linear web velocity. In one embodiment, this compensation value may correspond to a relatively small constant value for a predetermined web tension intended to be maintained in the web portion immediately preceding the roller at which the linear web velocity is generated. In another embodiment, this compensation value may correspond to an actual web tension measurement obtained from a load cell positioned immediately prior to the roller at which the linear web velocity is generated. These compensation values are used by the converters to produce the linear web velocity before the linear web velocity is low pass or high pass filtered.
The controller 60 that uses the filtered signals to compute the web velocities at the rollers and marking stations includes memory storage for data and programmed instructions. The controller may be implemented with general or specialized programmable processors that execute programmed instructions. The instructions and data required to perform the programmed functions may be stored in memory associated with the processor. The programmed instructions, memories, and interface circuitry configure the controller to perform the functions for computing web velocities at various locations and generating firing signals in correlation with those computed velocities. These components may be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits may be implemented with a separate processor or multiple circuits may be implemented on the same processor. Alternatively, the circuits may be implemented with discrete components or circuits provided in VLSI circuits. Also, the circuits described herein may be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits.
A method of establishing a common average linear velocity for the rollers in a double reflex printing system and computing web velocities is shown in
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. 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.
