1. Field of the Invention
This invention relates generally to methods and apparatus for web tracking, and more particularly to apparatus for controlling the alignment, in a lateral (cross-track) direction, of a web moving along a path in a travelling (in-track) direction.
2. Description of the Prior Art
In high speed electrostatographic reproduction apparatus, it is a common practice to employ an elongated photoconductive belt or web adapted to record transferable images while the web is moving in a path in operative relation with various process stations. Typically the web is supported by, and driven about, at least one roller. With a roller support, there is a tendency for the moving web to shift laterally, or cross-track, with respect to such roller. Various apparatus for correcting for such lateral shifting of roller-supported webs are known, such as crowned rollers, flanged rollers, servo actuated steering rollers, or self-actuated steering rollers. Crowned rollers generally are not suitable for use with a web in an electrostatographic reproduction apparatus because they force the web toward the apex of such rollers, cause distortion of the web, and produce local stresses in the web at the crown which can damage the web. Flanged rollers generally are also not suitable because they produce a concentrated loading at the edges of the web resulting in edge buckling, seam splitting, or excessive edge wear.
Electrostatographic reproduction apparatus therefore typically utilize servo-actuated or self-activated steering rollers. While such steering rollers generally correct in a gross manner the cross-track shifting of the web, they tend to produce significant lateral movement of the web at an uneven rate as it is realigned. Further, the rate of web drift is inconsistent from correction to correction. Such an uneven rate of change of the web position results in the production of image frames which do not match, or register, therebetween. Additionally, at the onset of web lateral movement during a correction, an abrupt lateral distortion or deformation of the web is experienced due to the rather large and abrupt rotation of the steering roller. The abrupt change arises in the drift reversing motion of the steering correction wherein the angular position of the roller changes with respect to the machine frame. The roller position change causes a corresponding abrupt lateral web distortion of a few thousandths of an inch.
In making monochromatic reproductions, such misregistration, within limits of course, is not problematic as only one discrete area, or image frame, of the web is used in generating any one reproduction. However, in an apparatus wherein either monochromatic (separation) or multichromatic (color copy) image reproductions are made, i.e., for composite printing, copying, or duplicating applications, there is a significant likelihood that the correction will degrade the image quality of the composite image. Hence, the inconsistency or instability of the rate of web lateral movement, or drift, following a correction becomes a significant limiting factor in obtaining quality reproductions from such an apparatus.
In the prior art, as exemplified by U.S. Pat. No. 4,429,985, a recording system is described which comprises a recording member of the endless belt form in which a series of information is recorded while the recording member is moved. A deviation sensing means senses a deviation of the recording member and a deviation correcting means corrects the deviation of the recording member. A control circuit is provided for actuating the deviation correcting means based on a signal generated by the deviation sensing means upon sensing the deviation.
As exemplified by U.S. Pat. No. 4,462,676, there is described a skew control apparatus for use in a recording apparatus, capable of detecting the skew of an endless-belt-shaped recording material, and reversing the direction of the skew during an on-recording period, even if such skew is detected during the recording operation period.
In making monochrome reproductions with an apparatus utilizing, for example, a moving charged photoconductive web, charge patterns corresponding to related color separation images are formed in successive image frames of the web. Such patterns are developed with pigmented marking particles to form transferable images. Each image is transferred sequentially to a respective receiver member whereby each image forms one of the several color separations for the multicolor reproduction. The sequential image transfer must take place in accurate register in order to obtain quality output of separations for faithful multicolor reproduction. In such color applications, transferable images generated from such successive "master" separations are aligned for accurate superimposed registration during the creation of a multicolor composite print. Alignment is typically accomplished while installing the receiver in a printing or duplicating machinery by registering the receiver edge to a respective portion of the machinery.
Therefore, during the production of such a separation or other such imaged receiver, any lateral movement, or drift, of the web during web rotation must be controlled to a uniform rate; any inconsistency in the rate of lateral movement will place successive image frames in differing positions relative to the exposure and transfer stations of the apparatus. The resulting misregistration of the image frames must be held within acceptable limits. The maximum misregistration between corresponding points of the color separations at a screen ruling of 150 lines per inch has been found to be two-thousandths of an inch for proper cross-track and in-track registration for some applications.
A further problem of known servo-actuated or self-activated steering roller tracking systems is that they react to the absolute lateral position of the edge of the web, rather than to the actual lateral position of the web. Because the web edge is typically irregular, any misregistration due to other mechanisms such as described hereinabove are worsened or compounded. Such compounding is exacerbated due to a typical increase in web edge irregularity over its operational life. The resulting detriment to the registration of the sequential transferable images causes the production of composite images which are not within the acceptable limits for accurate composite registration. In reproduction apparatus using known tracking systems, such problems force the apparatus user to initially reject many webs which would otherwise be useable, and further to replace webs too soon that otherwise could remain in use.
Moreover, there exists a need for a electrostatographic reproduction apparatus, operable for the production of monochrome and multichrome images, having a web tracking system which is continually predictive. That is, the tracking system must continually be aware of the near-future reproduction activity in the apparatus, and must plan accordingly to avoid a correction during the exposure or transfer of an image frame. The web in a typical apparatus may travel at least one revolution in order to image one or more sets of separations of a three color image. In other embodiments of such an apparatus, the web travels 3 or more revolutions in order to image one set of separations, and in some applications may transfer the separation set to a single receiver to produce a multicolor reproduction, or to a corresponding set of receiver members to produce a color separation set. Therefore, before the printing of the separations is begun, there must be certainty that the web will not reach the limits of the allowable drift during the creation of the developed image(s). If a correction is so anticipated, then the correction must be made before the image(s) are created. Otherwise, the aforementioned abrupt change of the web position in a steering correction made during an imaging exposure or transfer is likely to degrade the image to the point that the image will be unacceptable or unuseable for multicolor reproduction. Any correction applied to the web creates a lateral web distortion to some degree which propagates throughout the web path at the web transport speed and which will subside only after one full revolution, thus making mid-print corrections undesirable. Lacking the ability to predict a requisite correction, known web tracking systems typically make corrections on demand (without regard to ongoing image exposure or transfer operations) and such corrections are detrimental to image quality.
Multicolor reproduction apparatus useable for high volume reproduction work therefore present strict registration requirements. The production of a misregistered separation set is costly in that the subsequent production of a degraded composite image forces the printing process to be forestalled or halted while a new set of separations are made. In the color reproduction industry, any waste or increment or process time is significant and is to be avoided.
It is an object therefore of the invention to provide a method an apparatus for web tracking which overcomes the above referenced limitations of the prior art.