Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
1999-12-01
2001-10-23
Beatty, Robert (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
Reexamination Certificate
active
06307578
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to multiple-color printers wherein imaging stations are disposed along an image receptor belt and, more particularly, to compensation for a disturbance of image registration induced by eccentricity of an idler roll disposed between two of the imaging stations.
One form of multiple-color printers employs a sequence of imaging stations disposed along a common span of an image receptor belt. By way of example, the belt may be carried by rolls wherein one of the rolls is a drive roll, a second of the rolls is a tension roll, and a third of the rolls is an idler roll. In the construction of such printers, it is important to maintain registration among the various color separations of the final image, which color separations are provided by the respective ones of the imaging stations. Construction of such printers is disclosed in United States Patents: Rees et al. U.S. Pat. Nos. 5,229,787; Dastin et al. 5,287,160; and de Jong et al. 5,287,162. In Dastin et al., placing all imaging stations at a distance from each other equal to an integer multiple of all component perimeters causes the cyclic error in velocity at each imager to be equal and in-phase, thereby eliminating the relative registration error. In the case of an image device such as a raster output scanner (ROS), the ROS synchronization and writing time might lead to errors in registration. These errors may be corrected by use of the mark-on-belt (MOB) sensor as taught by de Jong et al. Therein, the relative locations of different color chevron-shaped marks produced by each of the imagers is measured by the MOB sensor to enable corrective timing.
Practical rolls have significant eccentricity and cause the photoreceptor belt to have a cyclic error component in its velocity. The wavelength of this motion disturbance is too long to produce a perceptible single-color visual effect known as banding. However, unless the photoreceptor belt has the identical in-phase error at each imager, the various color separations do not overlap exactly. Such registration errors will cause shifts in hue, which may also appear in regular patterns known as rainbow banding. These effects are deleterious to image quality.
An aspect in the construction of the printers of the prior art is the locating of the successive imaging stations along a single span of the belt. Such a construction is useful in attaining a high degree of registration among the component images produced by the respective imaging stations. A disadvantage of such construction is the relatively large size of such a printer, the large size being due to the linear array of the in stations along the single span of the belt. In order to reduce the physical size of the printer, it is advantageous to locate the imaging stations on more than one span of the image receptor belt. With such a construction, an idler roll would be located between two of the imaging stations.
A problem arises with a construction wherein the idler roll is located between two of the imaging stations, this problem arising from a relatively small eccentricity within the idler roll. Such eccentricity may be characterized, in part, by a bump or portion of the idler roll with enlarged radius. As the bump engages with the belt, the speed of the belt upstream of the roll is increased momentarily while the consequent geometric alteration in the belt is taken up by the tension roll. As a result, the location of the image produced by an imaging station, located on the opposite side of the roll, is displaced slightly on the belt. In high-quality imaging, displacement of component images on the order of microns produces a noticeable effect on the final image. Therefore, there is a need to provide some form of compensation for the eccentricity of the idler roll.
SUMMARY OF THE INVENTION
The aforementioned problems are overcome and other advantages are provided by a system and method which compensate for eccentricity in an idler roll of a multiple-color printer, and thereby preserve registration among the images of the various imaging stations. While the invention has applicability generally to conveyor systems wherein there is an eccentric element disturbing registration among workstations located at opposite sides of an idler roll, the invention is particularly applicable and advantageous to the case of the multiple-color printer having imaging stations disposed along a photoreceptor belt for producing images on the belt, and wherein the belt passes around an idler roll. In an embodiment of the invention wherein the physical size of the multiple-color printer is reduced, one or more of the imaging stations is disposed on one side of the idler roll, for example the right-hand side, and a plurality of the imaging stations is disposed on the opposite side of the idler roll. Also included within the printer is a belt driver which drives the photoreceptor belt around the idler roll and past the imaging stations.
In accordance with the invention, compensation for eccentricity in the idler roll to preserve registration among the images of the imaging stations is accomplished, in accordance with one embodiment of the invention, by providing a correction to the driver to alter the speed of the belt by an amount which counteracts the effects of the apparent shift in speed of the belt observed at the one or more right-hand imaging stations. Alternatively, in accordance with further embodiment of the invention, the correction is applied to the timing circuitry which drives a laser scanner or LED (light emitting diode) imaging array within each of the one or more right-hand imaging stations, such that the times of occurrence of the respective scans are altered to compensate for the apparent shift in the speed of the belt. Such compensation must be synchronized with rotation of the idler roll. Accordingly, the idler roll is provided with an index which is sensed by a sensor, the sensing of the index establishing a once-per-revolution synchronization signal by which a pattern of speed alteration is synchronized with the rotation of the idler roll.
Each of the imaging stations provides for identification marks on the belt. Measurement circuitry is provided for measuring spacing along the belt between the identification marks of the right-hand imaging stations and one of the plurality of imaging stations, and for establishing an error in their spacing. The measurement circuitry also computes a corresponding error in belt speed, the latter being used for computing the correction. The error varies with rotation of the idler roll and, accordingly, a succession of values of error are obtained for successive positions along the belt corresponding to successive rotational positions of the idler roll. There is a corresponding functional relationship of the correction function to rotational position of the idler roll. A succession of values of the correction function is stored in a memory addressed with a sequence of address values starting with the occurrence of the index of the idler roll. Therefore, by the aforementioned sensing of the idler roll index, the memory outputs the requisite sequence of values of correction function to alter the speed of the belt, in the first embodiment of the invention, or the timing of the scanning in the second embodiment of the invention. A phase-locked loop may be employed for synchronizing the addressing of the memory with the occurrences of the index.
REFERENCES:
patent: 5075702 (1991-12-01), Castelli et al.
patent: 5153644 (1992-10-01), Yang et al.
patent: 5164783 (1992-11-01), Taguchi et al.
patent: 5200782 (1993-04-01), Castelli et al.
patent: 5200791 (1993-04-01), Dastin et al.
patent: 5229787 (1993-07-01), Rees et al.
patent: 5287160 (1994-02-01), Dastin et al.
patent: 5287162 (1994-02-01), De Jong et al.
patent: 5473421 (1995-12-01), Maruyama et al.
patent: 5493385 (1996-02-01), Ng
patent: 5497225 (1996-03-01), Matsuzuki
patent: 5950051 (1999-09-01), Casella
patent: 08-137153 (1996-05-01), None
Beatty Robert
Perman & Green LLP
Xerox Corporation
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