Patent Description:
Electrophotography is a useful process for printing images on a receiver (or "imaging substrate"), such as a piece or sheet of paper or another planar medium (e.g., glass, fabric, metal, or other objects) as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (i.e., a "latent image").

After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a toner image. Note that the toner image may not be visible to the naked eye depending on the composition of the toner particles (e.g., clear toner).

After the latent image is developed into a toner image on the photoreceptor, a suitable receiver is brought into juxtaposition with the toner image. A suitable electric field is applied to transfer the toner particles of the toner image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.

The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (i.e., "fuse") the print image to the receiver. Plural print images (e.g., separation images of different colors) can be overlaid on the receiver before fusing to form a multicolor print image on the receiver.

One problem that can occur in electrophotographic printing systems is that cross-track spacing variations can occur for various components. Such cross-track spacing variations are sometimes called "skew. " For example, the spacing between the charging subsystem and the surface of the photoreceptor can vary across the cross-track width of the photoreceptor. This can produce a gradient in the charge on the photoreceptor produced by the charging system, which can in turn produce non-uniformities in the printed images. Other subsystems such as the exposure subsystem and development subsystem can also be susceptible to image quality variations due to cross-track spacing variations. Such cross-track spacing variations can be difficult to detect and correct. For example, if a cross-track density gradation is detected in a uniform region of a printed image is observed, it can be difficult to troubleshoot which subsystem may have a cross-track spacing variation that is causing the artifact. This is particularly true for systems that are deployed at a customer location where specialized equipment may be unavailable.

Attention is drawn to <CIT>, which relates to producing correction data for a printer, wherein the correction data is produced for density errors in prints produced using a printer. While printing a test image, the periods of rotation of one or more rotatable imaging members arranged along a receiver feed path in the printer are measured using respective period sensors. The printed test image is measured along a selected measurement direction and a reproduction error signal representing deviation from aim density is determined. For each period sensor, the autocorrelation of the reproduction error signal for the corresponding period is determined. If the determined autocorrelation exceeds a selected threshold, the reproduction error signal is decomposed at the corresponding period to extract the variation from the measured component. The remaining error signal is separated by frequency terms. The variations from the data at measured periods and the remaining error signal are used to produce a correction signal. Furthermore, <CIT> relates to a printer having an automatic cross-track density correction. Printers are provided having a print engine having a print head that forms lines of picture elements on a receiver based upon lines of pixel values and a controller that causes the print engine to print a first print having a plurality of different areas along a cross-track direction with target densities and that receives data from which measured densities for different ones of the plurality of different areas can be determined. The controller determines a line density adjustment function based upon a functional relationship between a cross-track position of different ones of the areas and a difference between the measured density and the target density at the different ones of the areas and subsequently prints a production print according to lines of pixel values for the production print modulated by the line density adjustment function.

There remains a need for a method to reliably troubleshoot and characterize cross-track spacing variations in an electrophotographic printing system that can be performed without the need for specialized equipment.

The present invention represents a method for characterizing cross-track spacing variations for a plurality of printer subsystems of an electrophotographic printing system as defined by claim <NUM>, which includes among other features:.

This invention has the advantage that cross-track spacing variations in various printer subsystems can be characterized conveniently by printing and evaluating appropriate test patterns without the need to make physical measurements.

It has the additional advantage that the method can be performed by an unskilled system operator without the need for specialized equipment.

It has the further advantage that the detected cross-track spacing variations can be manually or automatically corrected to provide improved image quality.

It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures.

The invention is inclusive of combinations of the embodiments described herein. References to "a particular embodiment" and the like refer to features that are present in at least one embodiment of the invention. Separate references to "an embodiment" or "particular embodiments" or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated, or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the "method" or "methods" and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word "or" is used in this disclosure in a non-exclusive sense.

As used herein, the terms "parallel" and "perpendicular" have a tolerance of ±<NUM>°.

As used herein, "sheet" is a discrete piece of media, such as receiver media for an electrophotographic printer (described below). Sheets have a length and a width. Sheets are folded along fold axes (e.g., positioned in the center of the sheet in the length dimension, and extending the full width of the sheet). The folded sheet contains two "leaves," each leaf being that portion of the sheet on one side of the fold axis. The two sides of each leaf are referred to as "pages. " "Face" refers to one side of the sheet, whether before or after folding.

As used herein, "toner particles" are particles of one or more material(s) that are transferred by an electrophotographic (EP) printer to a receiver to produce a desired effect or structure (e.g., a print image, texture, pattern, or coating) on the receiver. Toner particles can be ground from larger solids, or chemically prepared (e.g., precipitated from a solution of a pigment and a dispersant using an organic solvent), as is known in the art. Toner particles can have a range of diameters (e.g., less than <NUM>, on the order of <NUM>-<NUM>, up to approximately <NUM>, or larger), where "diameter" preferably refers to the volume-weighted median diameter, as determined by a device such as a Coulter Multisizer.

"Toner" refers to a material or mixture that contains toner particles, and that can be used to form an image, pattern, or coating when deposited on an imaging member including a photoreceptor, a photoconductor, or an electrostatically-charged or magnetic surface. Toner can be transferred from the imaging member to a receiver. Toner is also referred to in the art as marking particles, dry ink, or developer, but note that herein "developer" is used differently, as described below. Toner can be a dry mixture of particles or a suspension of particles in a liquid toner base.

As mentioned already, toner includes toner particles; it can also include other types of particles. The particles in toner can be of various types and have various properties. Such properties can include absorption of incident electromagnetic radiation (e.g., particles containing colorants such as dyes or pigments), absorption of moisture or gasses (e.g., desiccants or getters), suppression of bacterial growth (e.g., biocides, particularly useful in liquid-toner systems), adhesion to the receiver (e.g., binders), electrical conductivity or low magnetic reluctance (e.g., metal particles), electrical resistivity, texture, gloss, magnetic remanence, florescence, resistance to etchants, and other properties of additives known in the art.

In single-component or mono-component development systems, "developer" refers to toner alone. In these systems, none, some, or all of the particles in the toner can themselves be magnetic. However, developer in a mono-component system does not include magnetic carrier particles. In dual-component, two-component, or multi-component development systems, "developer" refers to a mixture including toner particles and magnetic carrier particles, which can be electrically-conductive or -non-conductive. Toner particles can be magnetic or non-magnetic. The carrier particles can be larger than the toner particles (e.g., <NUM>-<NUM> or <NUM>-<NUM> in diameter). A magnetic field is used to move the developer in these systems by exerting a force on the magnetic carrier particles. The developer is moved into proximity with an imaging member or transfer member by the magnetic field, and the toner or toner particles in the developer are transferred from the developer to the member by an electric field, as will be described further below. The magnetic carrier particles are not intentionally deposited on the member by action of the electric field; only the toner is intentionally deposited. However, magnetic carrier particles, and other particles in the toner or developer, can be unintentionally transferred to an imaging member. Developer can include other additives known in the art, such as those listed above for toner. Toner and carrier particles can be substantially spherical or nonspherical.

The electrophotographic process can be embodied in devices including printers, copiers, scanners, and facsimiles, and analog or digital devices, all of which are referred to herein as "printers. " Various embodiments described herein are useful with electrostatographic printers such as electrophotographic printers that employ toner developed on an electrophotographic receiver, and ionographic printers and copiers that do not rely upon an electrophotographic receiver. Electrophotography and ionography are types of electrostatography (printing using electrostatic fields), which is a subset of electrography (printing using electric fields). The present invention can be practiced using any type of electrographic printing system, including electrophotographic and ionographic printers.

A digital reproduction printing system ("printer") typically includes a digital front-end processor (DFE), a print engine (also referred to in the art as a "marking engine") for applying toner to the receiver, and one or more post-printing finishing system(s) (e.g., a UV coating system, a glosser system, or a laminator system). A printer can reproduce pleasing black-and-white or color images onto a receiver. A printer can also produce selected patterns of toner on a receiver, which patterns (e.g., surface textures) do not correspond directly to a visible image.

The DFE receives input electronic files (such as Postscript command files) composed of images from other input devices (e.g., a scanner, a digital camera or a computer-generated image processor). Within the context of the present invention, images can include photographic renditions of scenes, as well as other types of visual content such as text or graphical elements. Images can also include invisible content such as specifications of texture, gloss or protective coating patterns.

The DFE can include various function processors, such as a raster image processor (RIP), image positioning processor, image manipulation processor, color processor, or image storage processor. The DFE rasterizes input electronic files into image bitmaps for the print engine to print. In some embodiments, the DFE permits a human operator to set up parameters such as layout, font, color, paper type, or post-finishing options. The print engine takes the rasterized image bitmap from the DFE and renders the bitmap into a form that can control the printing process from the exposure device to transferring the print image onto the receiver. The finishing system applies features such as protection, glossing, or binding to the prints. The finishing system can be implemented as an integral component of a printer, or as a separate machine through which prints are fed after they are printed.

The printer can also include a color management system that accounts for characteristics of the image printing process implemented in the print engine (e.g., the electrophotographic process) to provide known, consistent color reproduction characteristics. The color management system can also provide known color reproduction for different inputs (e.g., digital camera images or film images). Color management systems are well-known in the art, and any such system can be used to provide color corrections in accordance with the present invention.

In an embodiment of an electrophotographic modular printing machine useful with various embodiments (e.g., the NEXPRESS SX <NUM> printer manufactured by Eastman Kodak Company of Rochester, NY) color-toner print images are made in a plurality of color imaging modules arranged in tandem, and the print images are successively electrostatically transferred to a receiver adhered to a transport web moving through the modules. Colored toners include colorants, (e.g., dyes or pigments) which absorb specific wavelengths of visible light. Commercial machines of this type typically employ intermediate transfer members in the respective modules for transferring visible images from the photoreceptor and transferring print images to the receiver. In other electrophotographic printers, each visible image is directly transferred to a receiver to form the corresponding print image. In other electrophotographic printers, each visible image is transferred sequentially onto an intermediate member such as an endless belt or cylinder and then the final, multilayered image is transferred to the receiver.

Electrophotographic printers having the capability to also deposit clear toner using an additional imaging module are also known. The provision of a clear-toner overcoat to a color print is desirable for providing features such as protecting the print from fingerprints, reducing certain visual artifacts or providing desired texture or surface finish characteristics. Clear toner uses particles that are similar to the toner particles of the color development stations but without colored material (e.g., dye or pigment) incorporated into the toner particles. However, a clear-toner overcoat can add cost and reduce color gamut of the print; thus, it is desirable to provide for operator/user selection to determine whether or not a clear-toner overcoat will be applied to the entire print. A uniform layer of clear toner can be provided. A layer that varies inversely according to heights of the toner stacks can also be used to establish level toner stack heights. The respective color toners are deposited one upon the other at respective locations on the receiver and the height of a respective color toner stack is the sum of the toner heights of each respective color. Uniform stack height provides the print with a more even or uniform gloss.

<FIG> are elevational cross-sections showing portions of a typical electrophotographic printer <NUM> useful with various embodiments. Printer <NUM> is adapted to produce images, such as single-color images (i.e., monochrome images), or multicolor images such as CMYK, or pentachrome (five-color) images, on a receiver. Multicolor images are also known as "multi-component" images. One embodiment involves printing using an electrophotographic print engine having five sets of single-color image-producing or image-printing stations or modules arranged in tandem, but more or less than five colors can be combined on a single receiver. Other electrophotographic writers or printer apparatus can also be included. Various components of printer <NUM> are shown as rollers; other configurations are also possible, including belts.

Referring to <FIG>, printer <NUM> is an electrophotographic printing apparatus having a number of tandemly-arranged electrophotographic imageforming printing modules <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, also known as electrophotographic imaging subsystems. Each printing module <NUM>, <NUM>, <NUM>, <NUM>, <NUM> produces a single-color toner image for transfer using a respective transfer subsystem <NUM> (for clarity, only one is labeled) to a receiver <NUM> successively moved through the modules. In some embodiments one or more of the printing module <NUM>, <NUM>, <NUM>, <NUM>, <NUM> can print a colorless toner image, which can be used to provide a protective overcoat or tactile image features. Receiver <NUM> is transported from supply unit <NUM>, which can include active feeding subsystems as known in the art, into printer <NUM> using a transport web <NUM>. In various embodiments, the visible image can be transferred directly from an imaging roller to a receiver, or from an imaging roller to one or more transfer roller(s) or belt(s) in sequence in transfer subsystem <NUM>, and then to receiver <NUM>. Receiver <NUM> is, for example, a selected section of a web or a cut sheet of a planar receiver media such as paper or transparency film.

In the illustrated embodiments, each receiver <NUM> can have up to five single-color toner images transferred in registration thereon during a single pass through the five printing modules <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to form a pentachrome image. As used herein, the term "pentachrome" implies that in a print image, combinations of various of the five colors are combined to form other colors on the receiver at various locations on the receiver, and that all five colors participate to form process colors in at least some of the subsets. That is, each of the five colors of toner can be combined with toner of one or more of the other colors at a particular location on the receiver to form a color different than the colors of the toners combined at that location. In an exemplary embodiment, printing module <NUM> forms black (K) print images, printing module <NUM> forms yellow (Y) print images, printing module <NUM> forms magenta (M) print images, and printing module <NUM> forms cyan (C) print images.

Printing module <NUM> can form a red, blue, green, or other fifth print image, including an image formed from a clear toner (e.g., one lacking pigment). The four subtractive primary colors, cyan, magenta, yellow, and black, can be combined in various combinations of subsets thereof to form a representative spectrum of colors. The color gamut of a printer (i.e., the range of colors that can be produced by the printer) is dependent upon the materials used and the process used for forming the colors. The fifth color can therefore be added to improve the color gamut. In addition to adding to the color gamut, the fifth color can also be a specialty color toner or spot color, such as for making proprietary logos or colors that cannot be produced with only CMYK colors (e.g., metallic, fluorescent, or pearlescent colors), or a clear toner or tinted toner. Tinted toners absorb less light than they transmit, but do contain pigments or dyes that move the hue of light passing through them towards the hue of the tint. For example, a blue-tinted toner coated on white paper will cause the white paper to appear light blue when viewed under white light, and will cause yellows printed under the blue-tinted toner to appear slightly greenish under white light.

Receiver 42a is shown after passing through printing module <NUM>. Print image <NUM> on receiver 42a includes unfused toner particles. Subsequent to transfer of the respective print images, overlaid in registration, one from each of the respective printing modules <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, receiver 42a is advanced to a fuser module <NUM> (i.e., a fusing or fixing assembly) to fuse the print image <NUM> to the receiver 42a. Transport web <NUM> transports the print-image-carrying receivers to the fuser module <NUM>, which fixes the toner particles to the respective receivers, generally by the application of heat and pressure. The receivers are serially detacked from the transport web <NUM> to permit them to feed cleanly into the fuser module <NUM>. The transport web <NUM> is then reconditioned for reuse at cleaning station <NUM> by cleaning and neutralizing the charges on the opposed surfaces of the transport web <NUM>. A mechanical cleaning station (not shown) for scraping or vacuuming toner off transport web <NUM> can also be used independently or with cleaning station <NUM>. The mechanical cleaning station can be disposed along the transport web <NUM> before or after cleaning station <NUM> in the direction of rotation of transport web <NUM>.

In the illustrated embodiment, the fuser module <NUM> includes a heated fusing roller <NUM> and an opposing pressure roller <NUM> that form a fusing nip <NUM> therebetween. In an embodiment, fuser module <NUM> also includes a release fluid application substation <NUM> that applies release fluid, e.g., silicone oil, to fusing roller <NUM>. Alternatively, wax-containing toner can be used without applying release fluid to the fusing roller <NUM>. Other embodiments of fusers, both contact and non-contact, can be employed. For example, solvent fixing uses solvents to soften the toner particles so they bond with the receiver. Photoflash fusing uses short bursts of high-frequency electromagnetic radiation (e.g., ultraviolet light) to melt the toner. Radiant fixing uses lower-frequency electromagnetic radiation (e.g., infrared light) to more slowly melt the toner. Microwave fixing uses electromagnetic radiation in the microwave range to heat the receivers (primarily), thereby causing the toner particles to melt by heat conduction, so that the toner is fixed to the receiver.

The fused receivers (e.g., receiver 42b carrying fused image <NUM>) are transported in series from the fuser module <NUM> along a path either to an output tray <NUM>, or back to printing modules <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to form an image on the backside of the receiver (i.e., to form a duplex print). Receivers 42b can also be transported to any suitable output accessory. For example, an auxiliary fuser or glossing assembly can provide a clear-toner overcoat. Printer <NUM> can also include multiple fuser modules <NUM> to support applications such as overprinting, as known in the art.

In various embodiments, between the fuser module <NUM> and the output tray <NUM>, receiver 42b passes through a finisher <NUM>. Finisher <NUM> performs various paper-handling operations, such as folding, stapling, saddle-stitching, collating, and binding.

Printer <NUM> includes main printer apparatus logic and control unit (LCU) <NUM>, which receives input signals from various sensors associated with printer <NUM> and sends control signals to various components of printer <NUM>. LCU <NUM> can include a microprocessor incorporating suitable look-up tables and control software executable by the LCU <NUM>. It can also include a field-programmable gate array (FPGA), programmable logic device (PLD), programmable logic controller (PLC) (with a program in, e.g., ladder logic), microcontroller, or other digital control system. LCU <NUM> can include memory for storing control software and data. In some embodiments, sensors associated with the fuser module <NUM> provide appropriate signals to the LCU <NUM>. In response to the sensor signals, the LCU <NUM> issues command and control signals that adjust the heat or pressure within fusing nip <NUM> and other operating parameters of fuser module <NUM>. This permits printer <NUM> to print on receivers of various thicknesses and surface finishes, such as glossy or matte.

Image data for printing by printer <NUM> can be processed by a raster image processor (RIP; not shown), which can include a color separation screen generator or generators. The output of the RIP can be stored in frame or line buffers for transmission of the color separation print data to each of a set of respective LED writers associated with the printing modules <NUM>, <NUM>, <NUM>, <NUM>, <NUM> (e.g., for black (K), yellow (Y), magenta (M), cyan (C), and red (R) color channels, respectively). The RIP or color separation screen generator can be a part of printer <NUM> or remote therefrom. Image data processed by the RIP can be obtained from a color document scanner or a digital camera or produced by a computer or from a memory or network which typically includes image data representing a continuous image that needs to be reprocessed into halftone image data in order to be adequately represented by the printer. The RIP can perform image processing processes (e.g., color correction) in order to obtain the desired color print. Color image data is separated into the respective colors and converted by the RIP to halftone dot image data in the respective color (for example, using halftone matrices, which provide desired screen angles and screen rulings). The RIP can be a suitably-programmed computer or logic device and is adapted to employ stored or computed halftone matrices and templates for processing separated color image data into rendered image data in the form of halftone information suitable for printing. These halftone matrices can be stored in a screen pattern memory.

<FIG> shows additional details of printing module <NUM>, which is representative of printing modules <NUM>, <NUM>, <NUM>, and <NUM> (<FIG>). Photoreceptor <NUM> of imaging member <NUM> includes a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated. In various embodiments, photoreceptor <NUM> is part of, or disposed over, the surface of imaging member <NUM>, which can be a plate, drum, or belt. Photoreceptors can include a homogeneous layer of a single material such as vitreous selenium or a composite layer containing a photoconductor and another material. Photoreceptors <NUM> can also contain multiple layers.

In-track direction <NUM> refers to the direction of motion of the receiver <NUM> and the image-bearing components (e.g., photoreceptor <NUM> and surface <NUM>), and cross-track direction <NUM> refers to the direction which spans the width of the components which will be perpendicular to in-track direction and to the plane of <FIG>.

Charging subsystem <NUM> applies a uniform electrostatic charge to photoreceptor <NUM> of imaging member <NUM>. In an exemplary embodiment, charging subsystem <NUM> includes a set of one or more wires <NUM> operated at a high voltage (DC, AC or some combination of the two) to create and deposit electrostatic charge on the surface of the photoreceptor <NUM>. Additional necessary components provided for control can be assembled about the various process elements of the respective printing modules. Meter <NUM> measures the uniform electrostatic charge provided by charging subsystem <NUM>.

An exposure subsystem <NUM> is provided for selectively modulating the uniform electrostatic charge on photoreceptor <NUM> in an image-wise fashion by exposing photoreceptor <NUM> to electromagnetic radiation to form a latent electrostatic image. The uniformly-charged photoreceptor <NUM> is typically exposed to actinic radiation provided by selectively activating particular light sources in an LED array or a laser device outputting light directed onto photoreceptor <NUM>. In embodiments using laser devices, a rotating polygon (not shown) is sometimes used to scan one or more laser beam(s) across the photoreceptor in the fast-scan direction. One pixel site is exposed at a time, and the intensity or duty cycle of the laser beam is varied at each pixel site. In embodiments using an LED array, the array can include a plurality of LEDs arranged next to each other in a line, all pixel sites in one row of pixel sites on the photoreceptor can be selectively exposed simultaneously, and the intensity or duty cycle of each LED can be varied within a line exposure time to expose each pixel site in the row during that line exposure time.

As used herein, an "engine pixel" is the smallest addressable unit on photoreceptor <NUM> which the exposure subsystem <NUM> (e.g., the laser or the LED) can expose with a selected exposure different from the exposure of another engine pixel. Engine pixels can overlap (e.g., to increase addressability in the slow-scan direction). Each engine pixel has a corresponding engine pixel location, and the exposure applied to the engine pixel location is described by an engine pixel level.

The exposure subsystem <NUM> can be a write-white or write-black system. In a write-white or "charged-area-development" system, the exposure dissipates charge on areas of photoreceptor <NUM> to which toner should not adhere. Toner particles are charged to be attracted to the charge remaining on photoreceptor <NUM>. The exposed areas therefore correspond to white areas of a printed page. In a write-black or "discharged-area development" system, the toner is charged to be attracted to a bias voltage applied to photoreceptor <NUM> and repelled from the charge on photoreceptor <NUM>. Therefore, toner adheres to areas where the charge on photoreceptor <NUM> has been dissipated by exposure. The exposed areas therefore correspond to black areas of a printed page.

In the illustrated embodiment, meter <NUM> is provided to measure the post-exposure surface potential within a patch area of a latent image formed from time to time in a non-image area on photoreceptor <NUM>. Other meters and components can also be included (not shown).

A development station <NUM> includes toning shell <NUM>, which can be rotating or stationary, for applying toner of a selected color to the latent image on photoreceptor <NUM> to produce a developed image on photoreceptor <NUM> corresponding to the color of toner deposited at this printing module <NUM>. Development station <NUM> is electrically biased by a suitable respective voltage to develop the respective latent image, which voltage can be supplied by a power supply (not shown). Developer is provided to toning shell <NUM> by a supply system (not shown) such as a supply roller, auger, or belt. Toner is transferred by electrostatic forces from development station <NUM> to photoreceptor <NUM>. These forces can include Coulombic forces between charged toner particles and the charged electrostatic latent image, and Lorentz forces on the charged toner particles due to the electric field produced by the bias voltages.

In some embodiments, the development station <NUM> employs a two-component developer that includes toner particles and magnetic carrier particles. The exemplary development station <NUM> includes a magnetic core <NUM> to cause the magnetic carrier particles near toning shell <NUM> to form a "magnetic brush," as known in the electrophotographic art. Magnetic core <NUM> can be stationary or rotating, and can rotate with a speed and direction the same as or different than the speed and direction of toning shell <NUM>. Magnetic core <NUM> can be cylindrical or non-cylindrical, and can include a single magnet or a plurality of magnets or magnetic poles disposed around the circumference of magnetic core <NUM>. Alternatively, magnetic core <NUM> can include an array of solenoids driven to provide a magnetic field of alternating direction. Magnetic core <NUM> preferably provides a magnetic field of varying magnitude and direction around the outer circumference of toning shell <NUM>. Development station <NUM> can also employ a mono-component developer comprising toner, either magnetic or non-magnetic, without separate magnetic carrier particles.

Transfer subsystem <NUM> includes transfer backup member <NUM>, and intermediate transfer member <NUM> for transferring the respective print image from photoreceptor <NUM> of imaging member <NUM> through a first transfer nip <NUM> to surface <NUM> of intermediate transfer member <NUM> (which is biased by a power source), and thence to a receiver <NUM> which receives respective toned print images <NUM> from each printing module in superposition to form a composite image thereon. The print image <NUM> is, for example, a separation of one color, such as cyan. Receiver <NUM> is transported by transport web <NUM>. Transfer to a receiver is effected by an electrical field provided to transfer backup member <NUM> by power source <NUM>, which is controlled by LCU <NUM>. Receiver <NUM> can be any object or surface onto which toner can be transferred from imaging member <NUM> by application of the electric field. In this example, receiver <NUM> is shown prior to entry into a second transfer nip <NUM>, and receiver 42a is shown subsequent to transfer of the print image <NUM> onto receiver 42a.

In the illustrated embodiment, the toner image is transferred from the photoreceptor <NUM> to the intermediate transfer member <NUM>, and from there to the receiver <NUM>. Registration of the separate toner images is achieved by registering the separate toner images on the receiver <NUM>, as is done with the NEXPRESS SX <NUM>. In some embodiments, a single transfer member is used to sequentially transfer toner images from each color channel to the receiver <NUM>. In other embodiments, the separate toner images can be transferred in register directly from the photoreceptor <NUM> in the respective printing module <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to the receiver <NUM> without using a transfer member. Either transfer process is suitable when practicing this invention. An alternative method of transferring toner images involves transferring the separate toner images, in register, to a transfer member and then transferring the registered image to a receiver.

LCU <NUM> sends control signals to the charging subsystem <NUM>, the exposure subsystem <NUM>, and the respective development station <NUM> of each printing module <NUM>, <NUM>, <NUM>, <NUM>, <NUM> (<FIG>), among other components. Each printing module can also have its own respective controller (not shown) coupled to LCU <NUM>.

As discussed earlier, a problem that can occur in electrophotographic printing modules <NUM> is that cross-track spacing variations (i.e., skew) can occur for various components such as the charging subsystem <NUM>, the exposure subsystem <NUM> and the development subsystem <NUM>. For example, the spacing between the charging subsystem <NUM> and the surface of the photoreceptor <NUM> can vary across the cross-track width of the imaging member <NUM> (e.g., the spacing can be smaller at one end of the charging subsystem <NUM> than at the other end). This can produce a gradient in the charge on the photoreceptor <NUM> produced by the charging system <NUM>, which can in turn produce non-uniformities in the printed images. Cross-track variations may also occur due to individual components within a subsystem. For example, within the development subsystem <NUM> there is typically a metering skive controlling the amount of developer loading onto a toning roller. If this skive is not set parallel to the toning roller then there will be a variation in developer thickness (and toner mass flow rate) on the toning roller. This can also be a source of cross-track density variations. The various types of cross-track spacing variations can be difficult to detect and correct. For example, if a cross-track density gradation is detected in a uniform region of a printed image is observed, it can be difficult to troubleshoot which subsystem may have a cross-track spacing variation that is causing the artifact.

Inventors have discovered that different types of test patterns have different sensitivities to cross-track spacing variations of different subsystems. Inventors have developed a method to leverage these sensitivity differences to diagnose which subsystem(s) are misaligned and to estimate the magnitude the cross-track spacing variations. The method can be conveniently performed without the need for specialized equipment so that it can be easily performed by unskilled operators at a customer site.

Aspects of the present invention will now be described with reference to <FIG>, which shows a flow chart of an exemplary embodiment. The method is based on printing a plurality of different test patterns that respond differently to cross-track spacing variations in different subsystems of an electrophotographic printing module <NUM>. In an exemplary embodiment, the method uses two different test patterns (first test pattern <NUM> and second test pattern <NUM>). However, in other embodiment more than two different types of test patterns can be used. In an exemplary embodiment, the first and second test patterns <NUM> use different types of halftone patterns (e.g., conventional halftone dot screen and a line screen).

<FIG> shows an exemplary first test pattern <NUM> having a series of uniform tone level regions 302a-<NUM> spanning the width of the media in the cross-track direction <NUM>. Each of the regions 302a-<NUM> has a different tone level produced by a corresponding region test pattern 300a-<NUM>. (The illustrated region test patterns 300a-<NUM> in <FIG> are scans of a printed first test pattern <NUM>. ) The illustrated first test pattern <NUM> also includes fiducial marks <NUM> to aid in determining the locations of the regions 302a-<NUM> in scans of the printed test target.

In the illustrated example, the region test patterns 300a-<NUM> are classic halftone dot patterns as illustrated in the enlarged insets. The halftone dot patterns vary from a light region test pattern 300a having a small dot size in region 302a to a dark region test pattern <NUM> having a large dot size in region <NUM>. In the illustrated example there are seven regions, where m is the region number, which ranges from m = <NUM> to m = <NUM>. The halftone dot patterns can be formed using any method known in the art. In some embodiments, the first test pattern <NUM> is a halftoned image which is predetermined and stored in a digital memory. In a preferred embodiment, the first test pattern <NUM> is determined at the time of printing by running a stored continuous tone image file through an image processing system that applies an appropriate halftoning algorithm. The different dot sizes associated with the different image regions can be formed using a variety of different methods. For example, the dot size can be varied by controlling the number of image pixels that make up the halftone dots or by controlling the exposure level provided to the image pixels that make up the halftone dots. Controlling the exposure level will control the charge on the photoconductor and thereby control the amount of developed toner, resulting in different pixel optical density levels and different average tone values. In a preferred embodiment, the dot size is varied by controlling both the number of image pixels in the halftone dot and the exposure level provided to those image pixels. Such halftoning methods are well-known and conventional in electrophotographic printing systems.

As will be discussed later, the printed test patterns are digitized to provide digitized test patterns which are sampled at a set of cross-track positions. In the figure, the cross-track position is given by a variable n, which ranges from n = <NUM> to n = N. In an exemplary embodiment, N = <NUM>,<NUM>. In other embodiments, the value of N could be smaller or larger, but should preferably be at least <NUM>, and more preferably <NUM>.

<FIG> shows an exemplary second test pattern <NUM> having a series of uniform tone level regions 307a-<NUM> spanning the width of the media in the cross-track direction <NUM>. Each of the regions 307a-<NUM> has a different tone level produced by a corresponding region test pattern 305a-<NUM>. In this example, the region test patterns 305a-<NUM> are vertical line screen patterns as illustrated in the enlarged insets, where each region has a different line density and line width. In some embodiments, the second test pattern <NUM> is a line screened image which is predetermined and stored in a digital memory. In a preferred embodiment, the second test pattern <NUM> is determined at the time of printing by running a stored continuous tone image file through an image processing system that applies an appropriate line screen halftoning algorithm. The different line widths associated with the different image regions can be formed using a variety of different methods. For example, the line width can be varied by controlling the number of image pixels across the width of the lines or by controlling the exposure level provided to the image pixels that make up the lines. In a preferred embodiment, the lines are a single pixel wide and the line density/line width is varied by controlling the exposure level provided to those image pixels.

Returning to a discussion of <FIG>, a print first test pattern step <NUM> is used to print the first test pattern <NUM> using the electrophotographic printing system. A scan first test pattern step <NUM> is the used to capture an image of the printed first test pattern to provide a first digitized test pattern <NUM>. In an exemplary embodiment, the scan first test pattern step <NUM> captures the image of the printed first test pattern using an image capture system (e.g., an image scanning system such as a flatbed scanner). In other embodiments, the scan first test pattern step <NUM> can use other types of image captures systems such as a digital camera system. In an exemplary embodiment, the image capture system digitizes the printed first test pattern at a spatial resolution of <NUM> dpi. However, in other embodiments other resolutions can be used. Preferably the spatial resolution should be in the range of <NUM>-<NUM> dpi to adequately resolve the test pattern characteristics.

Similarly, the second test pattern <NUM> is printed using a print second test pattern step <NUM> to provide a printed second test pattern. A scan second test pattern step <NUM> is then used to capture an image of the printed second test pattern to provide a second digitized test pattern <NUM>. In an exemplary embodiment, the printed second test pattern is digitized at <NUM> dpi, which is a higher resolution than was used for the first digitized test pattern <NUM>. This is to enable the line width of the printed lines in the line pattern to be characterized.

In some embodiments, various image processing operations can be the first and second digitized test patterns <NUM>, <NUM>. For example, an alignment process can be used to remove any skew in the scanned image. (Here the tern skew refers to a misalignment between the orientation of the patches and the scan lines in the digitized image. The optional fiducial marks <NUM> can be used to assist in the alignment process. In a preferred configuration, the scanner code values are inverted so that dark image values correspond to higher code values, and the code value corresponding to the paper is subtracted so that the code values will be a representation of the optical density of the toner in the printed image.

A determine first set of parameters step <NUM> is now used to analyze the first digitized test pattern <NUM> to determine a first set of parameters <NUM> which characterize an attribute of the printed first test pattern as a function of cross-track position. Likewise, a determine second set of parameters step <NUM> is used to analyze the second digitized test pattern <NUM> to determine a second set of parameters <NUM> which characterize an attribute of the printed second test pattern as a function of cross-track position. The first and second sets of parameters <NUM>, <NUM> can each have one or more parameters.

The attributes of the printed first and second test patterns that are characterized by the first and second sets of parameters <NUM>, <NUM> can be the same or can be different. In an exemplary configuration, the attribute of the printed first test patterns which is characterized by the first sets of parameters <NUM> is the cross-track density non-uniformities in the printed test patterns, and the attribute of the printed second test patterns which is characterized by the second sets of parameters <NUM> is the cross-track variations of the line densities in the printed test patterns. In other embodiments, first and second sets of parameters <NUM>, <NUM> can characterize other types of image attributes that are found to vary with cross-track spacing variations of the printer components. Such attributes could include density uniformity attributes, spatial noise attributes, and image sharpness attributes.

In an exemplary embodiment, an aggregate cross-track profile is determined combining the cross-track profiles for the various regions in the corresponding test patterns. In some embodiments, the first and second sets of parameters <NUM>, <NUM> each have a single parameter which characterizes an average slope of the aggregate cross-track profile.

Additional details of the determine first set of parameters step <NUM> which is used to process the first digitized test pattern <NUM> to determine the first set of parameters <NUM> are shown in <FIG> according to an exemplary embodiment.

A determine aggregate cross-track profile step <NUM> is first used to analyze the first digitized test pattern <NUM> to determine an aggregate cross-track profile <NUM>. Additional details of this step according to an exemplary embodiment which uses a singular value decomposition approach are shown in <FIG>.

As shown in <FIG>, a determine region cross-track profiles step <NUM> is first used to determine a cross-track profile for each of the regions 302a-<NUM> in the first test pattern <NUM> (<FIG>). For the first digitized test pattern <NUM>, the region cross-track profile represents the image density as a function of the cross-track position. In some cases, the region cross-track profile can be represented in terms of the well-known optical density value. In other cases, the region cross-track profile can be represented in terms of some other quantity (e.g., scanner code values) that have a known relationship to the image density. The cross-track position can be represented by a value n, where n = <NUM> to N, N being the number of cross-track positions that are sampled in the first digitized test pattern <NUM>. In an exemplary embodiment, the pixel values of the pixels in the first digitized test pattern <NUM> corresponding to each cross-track position within one of the regions are averaged to provide an N×<NUM> vector representing the region cross-track profile.

<FIG> shows a graph <NUM> showing a set of exemplary region cross-track profiles for each of the regions in the first test pattern <NUM> (<FIG>). The pixel values of the vertical axis in this example are inverse scanner code values. It can be seen that the region cross-track profiles exhibit a side-to-side nonuniformity as a result of a cross-track spacing variation (which in this case included a +<NUM> cross-track spacing variation in the charging subsystem <NUM> and a +<NUM> cross-track spacing variation the development subsystem <NUM>.

Generally, the determine second set of parameters step <NUM> will use an analogous set of steps to process the second digitized test pattern <NUM> to determine the second set of parameters <NUM>. However, the determine region cross-track profiles step <NUM> can characterize the cross-track performance using a different image attribute. For example, rather than determining an average density value at each cross-track position, an average line width or average line density can be determined at each cross-track position for the lines in the region test patterns 305a-<NUM> (<FIG>). In a preferred embodiment, the cross-sections through the lines at a particular cross-track position are overlaid and averaged to determine an average line profile. A line density (i.e., the amplitude of the line) or a line width (i.e., the width of the line at a certain density) is then determined from the average line profile. In one embodiment, a reference line profile is determined that takes into account the scanner modulation transfer function (MTF), and a scale factor is determined for scaling the reference profile that provides a best fit to the average line profile. The scale factor will then serve as a measure of the line density/line width. In this case, the region cross-track profiles are represented by the scale factor as a function of cross-track position.

Continuing with a discussion of <FIG>, the region cross-track profiles for each of the M regions 302a-<NUM> are merged into a single N×M cross-track profile matrix (D) <NUM>, where each column corresponds to the region cross-track profile for a particular region: <MAT> where Dn,m is the averaged pixel value for a cross-track position n in region m.

An apply singular value decomposition step <NUM> can be used to apply the well-known mathematical technique known as singular value decomposition to factor the cross-track profile matrix <NUM> into its orthogonal components: <MAT> where U is an N×N basis vectors matrix <NUM> where the columns form a set of basis vectors, Σ is an N×M diagonal singular values matrix <NUM> containing the singular values of D, and VT is an M×M matrix where the columns form another set of basis vectors orthogonal to those of the U.

A determine aggregate cross-track profile step <NUM> is then used to determine the aggregate cross-track profile <NUM> responsive to the basis vectors matrix <NUM> and the singular values matrix <NUM>. In an exemplary embodiment, the aggregate cross-track profile <NUM> is an N×<NUM> column vector which is set equal to the first basis vector corresponding to the first column of the basis vectors matrix <NUM> (U) scaled by the first singular value (Σ<NUM>,<NUM>).

<FIG> shows a graph <NUM> illustrating an exemplary aggregate cross-track profile <NUM> determined from the region cross-track profiles of <FIG>. It can be seen that the aggregate cross-track profile <NUM> exhibits a trend that is similar to that of the region cross-track profiles of <FIG>.

It will be obvious to one skilled in the art that in other embodiments the determine aggregate cross-track profile step <NUM> can determine the aggregate cross-track profile <NUM> using any other method known in the art. For example, an aggregate cross-track profile <NUM> can be determined by forming a weighted sum of a plurality of the basis vectors, or by taking the inner product of the basis vectors matrix <NUM> and the singular values matrix <NUM> to form an N×<NUM> column vector. In other embodiments, the cross-track profiles for each region can be shifted to have a mean value of zero, and then the shifted cross-track profiles can be averaged to characterize the average profile shape. In general, any method for determining an aggregate cross-track profile <NUM> that adequately characterizes the response variations associated with the cross-track spacing variations can be used in accordance with the invention.

Returning to a discussion of <FIG>, a fit parametric function step <NUM> is used to fit a parametric function <NUM> to the aggregate cross-track profile <NUM> to determine the first set of parameters <NUM>. In an exemplary embodiment, a least-squares fitting process is used fit a linear function to the aggregate cross-track profile <NUM>. This step is repeated for each of the test patterns to give linear parametric functions <NUM> of the form: <MAT> where p is the test pattern number, αp is an intercept parameter, and βp is a slope parameter. In the example shown in <FIG>, there are two test patterns so that the first test pattern <NUM> will correspond to p = <NUM> and the second test pattern <NUM> will correspond to p = <NUM>. <FIG> shows the parametric function <NUM> determined for the illustrated aggregate cross-track profile <NUM>.

In an exemplary embodiment, the slope parameter βp determined for each test pattern is used as the corresponding set of parameters. In this case, the first set of parameters <NUM> is given by the slope parameter β<NUM> determined for the first test pattern <NUM>, and the second set of parameters <NUM> is given by the slope parameter β<NUM> determined for the second test pattern <NUM>. In other embodiments, each set of parameters can include more than one parameter (e.g., slope and intercept, or coefficients for higher order polynomial functions).

As illustrated in <FIG>, it can be useful to group the steps involved in determining the first and second sets of parameters <NUM>, <NUM> into a determine parameter sets process <NUM>. This will enable the simplification of <FIG> which will be discussed below.

The method of the present invention relies on the observation that different test patterns respond differently to skew (i.e., cross-track spacing variations) in different printer subsystems. This is illustrated in <FIG> which shows graphs <NUM>, <NUM>, <NUM>, <NUM> illustrating the slope parameter determined for the test patterns of <FIG> and <FIG> as a function of skew in the charging subsystem <NUM> and the development subsystem <NUM> (<FIG>). It can be seen that the slope parameter determined for the classic halftone dot patterns of the first test pattern <NUM> (<FIG>) is sensitive to skew in both the charging subsystem <NUM> and the development subsystem <NUM>. On the other hand, the slope parameter determined for the line screen patterns of the second test pattern <NUM> (<FIG>) varies with different amounts of skew in the charging subsystem <NUM>, but is relatively insensitive to skew in the development subsystem <NUM>. As a result, determining the slope parameters for both the first and second test patterns <NUM>, <NUM> enables the amount of both types of skew to be estimated. This is embodied by the determine spacing variation(s) step <NUM> of <FIG> which uses one or more defect model(s) <NUM> to estimate one or more types of cross-track spacing variation. For example, a first defect model <NUM> can be used to estimate a first cross-track spacing variation <NUM> for a first printer subsystem (e.g., the charging subsystem <NUM>), and optionally a second defect model <NUM> can be used to estimate a second cross-track spacing variation <NUM> for a second printer subsystem (e.g., the development subsystem <NUM>).

In an exemplary embodiment, a defect model <NUM> is provided for each type of skew which predicts the amount of skew Si (i.e., the amount of cross-track spacing variation) for the ith printer subsystem as a function of the first set of parameters <NUM> (i.e., slope parameter β<NUM>) and the second set of parameters <NUM> (i.e., slope parameter β<NUM>): <MAT> where fi(·) is the defect model <NUM> for the ith type of skew. In an exemplary configuration, i = <NUM> corresponds to a cross-track spacing variation in the charging subsystem <NUM> (<FIG>) and i = <NUM> corresponds to a cross-track spacing variation in the development subsystem <NUM> (<FIG>), and the defect model <NUM> is a linear model of the form: <MAT> where b0i, b1i and b2i are experimentally determined parameters.

<FIG> shows an exemplary experimental process that can be used to determine defect model(s) <NUM> for estimating the amount of cross-track spacing variation for one or more printer subsystems. The method involves adjusting the skew values for a set of printer subsystems to a set of known skew values <NUM>, and then applying the determine parameter sets step <NUM> to determine corresponding first and second sets of parameters <NUM>, <NUM> for each of the known skew value settings. For example, the known skew values <NUM> can include nine skew value settings with a <NUM>×<NUM> matrix of skew variations (three different skew variations for the charging subsystem <NUM> and three different skew variations for the development subsystem <NUM>).

A determine forward models step is used to determine a forward model <NUM> that predicts each of the parameters in the first and second sets of parameters <NUM>, <NUM> as a function of the skew values. In an exemplary embodiment, a linear model is fit to the measured data of the form: <MAT> where S<NUM> is the known skew value for a first printer subsystem (e.g., the charging subsystem <NUM>), S<NUM> is the known skew value for a second printer subsystem (e.g., the development subsystem <NUM>), and βp is the predicted slope parameter in the corresponding sets of parameters (i.e., β<NUM> is the slope parameter for the first set of parameters <NUM> corresponding to the classic halftone dot patterns and β<NUM> is the slope parameter for the second set of parameters <NUM> corresponding to the line screen patterns). In an exemplary embodiment, the following forward models <NUM> were determined: <MAT> <MAT> where S<NUM> is the skew (i.e., cross-track spacing variation) of the charging subsystem <NUM> (in microns) and S<NUM> is the skew of the development subsystem <NUM> (in microns). The model fits for the forward models <NUM> were very good, with the R<NUM> correlation coefficients being <NUM> for the β<NUM> slope coefficient model, and <NUM> for the β<NUM> slope coefficient model.

A determine defect model(s) step <NUM> is used to determine defect model(s) <NUM> for one or more printer subsystems <NUM> responsive to the determined forward models <NUM>. In an exemplary embodiment, the determine defect model(s) step <NUM> determines the defect model(s) <NUM> by taking the set of equations for the forward models <NUM> and using conventional equation solving techniques to solve for the skew values as a function of the parameter values as in Eq. (<NUM>). For the case of the forward models of Eqs. (7a)-(7b), the following defect models <NUM> were determined: <MAT> <MAT>.

The forward models <NUM> and defect models <NUM> that have been described with reference to an exemplary embodiment have been simple linear models. It will be obvious to one skilled in the art that other types of models could also be used including polynomial, exponential and logarithmic models. The form of the model that is most appropriate for a particular application can be determined based on evaluating the experimental data using standard data modeling techniques. Depending on the form of the forward models <NUM> it may not be possible to determine defect models <NUM> by simply solving the equations defining the forward models <NUM> for the skew levels. In such cases, it may be necessary to use some other type of mathematical inversion process.

While the method of <FIG> shows the formation of forward models <NUM> as an intermediate step in the determination of the defect model(s) <NUM>, one skilled in the art will be recognize that the defect models could alternatively be determined directly by fitting appropriate mathematical functions to the experimental data that will predict the known skew values <NUM> as a function of the first and second sets of parameters <NUM>, <NUM>. For example, linear mathematical functions of the form shown in Eq. (<NUM>) can be fit directly to the experimental data. With this approach it is not necessary to perform a separate inversion step to determine the defect models <NUM>. This approach is illustrated in <FIG>, where the determine forward models step <NUM> and determine defect model(s) step <NUM> of <FIG> have been replaced by a single determine defect model(s) step <NUM>. Using this approach, slightly different defect models <NUM> are determined compared to those determined above using the method of <FIG>: <MAT> <MAT> However, the difference between the predictions of the two sets of defect models <NUM> are not statistically different. The model fits for the defect models <NUM> were very good, with the R<NUM> correlation coefficients being <NUM> for the S<NUM> defect model, and <NUM> for the S<NUM> defect model.

While the method of <FIG> shows determining sets of parameters for two test patterns (i.e., first and second test patterns <NUM>, <NUM>), it will be recognized by one skilled in the art that the method of can easily be generalized to include more than two test patterns or different types of test patterns. For example, one or more additional sets of test patterns can be printed, scanned and analyzed to determine one or more additional sets of parameters. The additional parameters can be used to provide additional inputs to the defect models <NUM>. The method can also be generalized to determine cross-track spacing variations for more than two printer subsystems. In general, the number of parameters that are input into the defect models <NUM> should be at least as large as the number of printer subsystems that are sensitive to cross-track spacing variations, where the functional relationship between a parameter βp and the cross-track spacing variations Si for a given pattern p should preferably not be a linear combination of any of the other functional relationships for the other parameters (i.e., the relationships need to be linearly independent in order to have a unique solution for the inverse model.

Once the cross-track spacing variations <NUM>, <NUM> have been determined by the determine spacing variation(s) step <NUM> of <FIG>, there are a variety of actions that can optionally be taken in various embodiments. In some embodiments, the determined cross-track spacing variations <NUM>, <NUM> are compared to predefined thresholds to determine whether the cross-track spacing variations <NUM>, <NUM> are significant, and if one of the cross-track spacing variations <NUM>, <NUM> exceeds the threshold an appropriate corrective action can be taken. Generally, the corrective action will involve adjusting a position of an element of the corresponding printer subsystem in order to provide a reduced cross-track spacing variation. For example, if it is determined that there is an estimated cross-track spacing variation of a certain size for the charging subsystem <NUM> (<FIG>), then a spacing between the charging subsystem <NUM> and the photoreceptor <NUM> can be adjusted at one or both of the cross-track ends of the charging subsystem <NUM>.

In some embodiments, when a significant cross-track spacing variation is detected, a message can be provided to a system operator to alert them that an appropriate correction is required. The message can include a recommended adjustment magnitude that will compensate for the estimated cross-track spacing variations <NUM>, <NUM>. For example, the system operator can be instructed to change turn a knob that controls the cross-track spacing of the relevant printer subsystem to a specified position.

In some embodiments, the printing module <NUM> (<FIG>) may include automated mechanisms (for example, a computer-controlled motor) for adjusting the cross-track spacing of the relevant printer subsystems. In such cases, the logic and control unit <NUM> can send appropriate control signals to the automated mechanisms to make appropriate adjustments to the cross-track spacing of the relevant printer subsystems.

Claim 1:
A method for characterizing cross-track spacing variations for a plurality of printer subsystems of an electrophotographic printing system, comprising:
printing a first test pattern (<NUM>, <NUM>);
printing a second test pattern (<NUM>, <NUM>) having different sensitivities to cross-track spacing variations of different printer subsystems than said first test pattern;
capturing an image of the printed first test pattern (<NUM>, <NUM>) to provide a first digitized test pattern (<NUM>) including a first array of pixel values;
capturing an image of the printed second test pattern (<NUM>, <NUM>) to provide a second digitized test pattern (<NUM>) including a second array of pixel values;
analyzing the first digitized test pattern (<NUM>) to determine a first set of parameters (<NUM>, <NUM>) that characterize an attribute of the printed first test pattern (<NUM>, <NUM>) as a function of cross-track position;
analyzing the digitized second digitized test pattern (<NUM>) to determine a second set of parameters (<NUM>, <NUM>) that characterize an attribute of the printed second test pattern (<NUM>, <NUM>) as a function of cross-track position; and
using a first defect model (<NUM>) to determine an estimated first cross-track spacing variation (<NUM>) for a first printer subsystem as a function of the determined first set of parameters (<NUM>, <NUM>) and the determined second set of parameters (<NUM>, <NUM>),
characterised in that determining the first defect model (<NUM>) comprises:
printing the first (<NUM>, <NUM>) and second (<NUM>, <NUM>) test patterns at a set of known cross-track spacing variation levels;
determining the first and second sets of parameters (<NUM>, <NUM>) for each of the known cross-track spacing variation levels; and
determining a first mathematical function that defines the first defect model (<NUM>) responsive to the known cross-track spacing variation levels and the determined first and second sets of parameters, wherein the first mathematical function predicts the first cross-track spacing variation level for the first printer subsystem as a function of the first and second sets of parameters.