Systems and methods for reducing image registration errors

Details Systems and methods for reducing image registration errors Systems and methods for reducing image registration errors

1999-01-19

2001-04-17

Grainger, Quana M. (Department: 2852)

Electrophotography

Image formation

Transfer

C399S299000

Reexamination Certificate

active

06219516

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
This invention generally relates to maintaining image registration in color image processing. More particularly, this invention relates to systems and methods in which image registration errors in color output images are reduced in image processing systems that include tandem print engines.
2. Description of Related Art
Electrophotography, a method of copying or printing documents, is performed by exposing a light image. representation of a desired original image onto a substantially uniformly charged photoreceptor substrate, such as a photoreceptor belt. In response to this light image, the photoreceptor discharges to create an electrostatic latent image of the desired original image on the photoreceptor's surface. Developing material, or toner, is then deposited onto the latent image to form a developed image. The developed image is then transferred to an image receiving substrate. The surface of the photoreceptor is then cleaned to remove residual developing material and the surface as recharged by a charging device in preparation for the production of the next image.
Color images can be produced by repeating the above-described recording process once for each differently-colored toner that is used to make a composite color image. For example, in a one-color imaging process, referred to herein as the Recharge, Expose, and Develop, Image (REaD IOI) process, a charged photoreceptor surface is exposed to a light image that represents a first color. The resulting electrostatic latent image is then developed with a first colored toner. The toner is typically of a subtractive primary color, including magenta, yellow, cyan, or black. The charge, expose and develop process is repeated for a second colored toner, then for a third colored toner, and finally for a fourth colored toner. The four differently-colored toners are placed in superimposed registration on the photoreceptor so that a desired composite color image results. That composite color image is then transferred and fused onto an image receiving substrate.
SUMMARY OF THE INVENTION
Tandem print engine systems include two print engines arranged in a series configuration. Each print engine includes a photoreceptor belt and imagers disposed at spaced positions along the length, i.e., the process direction, of the photoreceptor belt. Each imager comprises an image source that exposes the photoreceptor belt. Typically, the image source includes a light emitting device that emits a light beam that is moved laterally across the photoreceptor belt to expose the photoreceptor belt to create a latent electrostatic image on the photoreceptor belt. Each latent image is then developed as outlined above. Image receiving substrates, such as sheets of copy paper, are fed in a time-controlled manner to the print engines. The first print engine transfers its developed image to the simplex side of the image receiving substrate. The image receiving substrate is then inverted and presented to the second print engine. The second print engine then transfers its developed image to the duplex side of the image receiving substrate.
Each photoreceptor belt of the first and second print engines includes a seam where opposed end portions of the photoreceptor belt are joined together. The photoreceptor belts include pitch regions in which images can be satisfactorily formed. Images cannot be satisfactorily formed at the seams, because the images formed at seams are normally defective. Accordingly, it is important to control the locations of the seams of both of the first and second photoreceptor belts during print runs, to prevent forming images at the seams, and to ensure that images are formed only in the pitch regions.
In a tandem print engine configuration, there arc several technology issues involved with synchronizing two photoreceptor belt modules of two separate print engines in a manner that does not negatively impact the registration of either module. If the periods of revolution of the two photoreceptor belts are not matched, then the positions of the seams will also not be synchronized. The photoreceptor belts can have different lengths and, accordingly, in such configurations must rotate at different velocities (speeds) to maintain the same periods of revolution. If the periods of revolution are not synchronized appropriately to each other or with imager velocities, image registration errors will occur during printing. The image registration errors can be characterized as 1) simplex to duplex image registration errors if the photoreceptor and imager velocities for each print engine are not matched appropriately, or 2) image-on-image (IOI) registration errors from changes in the photoreceptor velocity or imager velocity while printing is occurring. Image-on-image registration errors occur during the building of color images on the photoreceptor belts. If, during stacking the multiple color separation layers of a color image on each other, the images are not aligned with each other, then image registration errors between the color separation layers will occur. These registration errors produce print defects such as color shifts and trapping errors.
Registration errors are caused generally by the motion quality of the photoreceptor belts and the manner that the imagers form the latent images on the photoreceptor belts. Regarding the motion quality of the photoreceptor belts, image registration errors can be caused by changes in the photoreceptor belt velocity, making it difficult to form images smoothly and to align lead edges of the images on the photoreceptor belt. Velocity changes can occur due to various different factors, including errors of the drive motor, errors in roller velocities and diameters, belt length changes during operation due to tension and thermal effects, and normal roller and belt tolerances.
Factors that can cause registration errors in the manner in which the imagers form the latent images, include errors in the lateral scan velocity, i.e., the exposure velocity, of the image sources across the photoreceptor belt, the scanning start and end points of the scanning light beam, and the length of the scan lines.
In simplex (single print engine) configurations, the image registration can be set up off-line. Thus, adjustments can be made at times when print runs are not being performed. In such configurations, the photoreceptor belt velocity is maintained as constant as possible to minimize registration errors. In addition, the imagers are set to a specific reference and their velocity is tightly maintained. If, during the course of producing an image, the velocity of the photoreceptor belt and the scan velocity of the image sources of the imager vary with respect to each other, either in position or velocity, then registration errors will occur.
Simplex print engine systems can include monitoring systems for measuring and compensating for image registration errors. Simplex print engine systems can calibrate themselves to the characteristics of the photoreceptor belt to achieve good image alignment for color images. If the photoreceptor belt rims either too fast or too slow, the scan velocity of the image sources can be automatically adjusted to counter the change in the photoreceptor belt velocity. As long as the photoreceptor belt velocity is maintained substantially constant, then only small image registration errors occur due to the self-correcting measures that are taken by the system.
For tandem print engine configurations, however, the synchronization requirements for the two print engines require that the photoreceptor belt velocity of the downstream print engine, i.e., the “slave print engine,” must be adjusted to keep it timed with the period of revolution of the photoreceptor belt of the upstream print engine, i.e., the “master print engine,” Otherwise, it is not possible to control the locations of the seams of the photoreceptor belts of the master and slave print engines. As explained, it is important to control the seams to prevent the formation of images

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