System and method for aligning images on media or platens

A printing system facilitates the registration of printheads in the system. The system includes a plurality of printheads, a planar member having at least three pins along a first edge of the planar member and a plurality of pins on a second edge of the planar member that is orthogonal to the first edge, a plurality of actuators, an optical imaging device, and a controller. The controller operates one of the printheads to form a registration target on the media sheet and processes image data of the planar member and the media sheet received from the optical imaging device to identify positions for the registration target and at least two of the three pins on the planar member. Error distances are identified from the positions for the registration target and the at least two pins on the edge of the planar member and are used to register the printheads.

TECHNICAL FIELD

This document relates generally to printing systems, and more particularly, to the alignment of ejected material on planar surfaces in printing systems.

BACKGROUND

A typical full width printer uses one or more printheads. The printheads are arranged in one or more arrays to enable a solid line at a predetermined resolution to be formed across the width of a planar member. The planar member can be a sheet of media in inkjet image printers or a platen in three-dimensional (3D) object printers. Each printhead typically contains an array of individual nozzles for ejecting drops of ink or material across an open gap to the planar member to form an image or layer on the member. In each printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink or material through an orifice or nozzle in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, frequency, or duration of the signals affects the amount of ink ejected in each drop. A printhead controller generates firing signals with reference to electronic image data to eject a pattern of individual ink drops at particular locations on the image receiving surface. The individual datum corresponding to a single drop in the electronic data is called a pixel and the locations where the drops land are sometimes called “drop locations,” “drop positions,” or “pixels.”

In order for the drop positions to correspond closely to the pixels in the electronic image data, the printheads must be registered with reference to the planar member and with reference to the other printheads in the printer. Registration of printheads is a process in which the printheads are operated to eject drops in a known pattern and then the printed image of the ejected drops is analyzed to determine the orientation of the printhead with reference to the planar member. In previously known printers, the registration process involved a human operator manually measuring a printed pattern position with reference to the edges of the planar member or by using an automated closed loop system that measured a distance of the printed pattern from a single fiducial mark on the planar member. The first registration method is susceptible to human error and the second method is subject to misidentifying the single fiducial mark as debris or other objects on the planar member that have an appearance that is similar to the single fiducial mark. Therefore, an image registration system and method that is not susceptible to human error or misidentification of the fiducial mark would be useful.

SUMMARY

A new printing system is configured with an image registration system that enables accurate registration of ejected material to a planar member without the limitations of previously known registration systems. The printing system includes a plurality of printheads, each printhead being configured to eject drops of material, a planar member having at least three pins along a first edge of the planar member and a plurality of pins on a second edge of the planar member that is orthogonal to the first edge, a plurality of actuators operatively connected to the printheads and to the planar member to enable the actuators to move the planar member bi-directionally in a process direction with reference to the plurality of printheads and to move the printheads bi-directionally in a cross-process direction, an optical imaging device configured to generate image data of the planar member as the planar member passes the optical imaging device, and a controller operatively connected to the plurality of printheads, the optical imaging device, and the actuators. The controller is configured to operate one of the actuators to move the planar member past the array of printheads and to operate one of the printheads in the plurality of printheads to eject drops of material onto a media sheet that has one edge against the plurality of pins on the second edge of the planar member and has another edge that is adjacent the one edge of the media sheet and proximate the first edge of the planar member as the planar member passes the printheads to form a collection of material drops having a circular shape on the media sheet, to receive image data of the planar member and the media sheet from the optical imaging device as the planar member and media sheet pass the optical imaging device, to identify from the image data received from the optical imaging device positions for the collection of material drops and for at least two of the pins on the first edge of the planar member, and to identify a process direction error distance and a cross-process direction error distance with reference to the positions for the collection of material drops and the at least two pins on the first edge of the planar member.

A new method of operating a printing system configured with an image registration system enables accurate registration of ejected material to a planar member without the limitations of previously known registration systems. The method includes operating with a controller one of a plurality of actuators to move a planar member past an array of printheads, operating with the controller one of a plurality of printheads to eject drops of material onto a media sheet that has one edge against a plurality of pins on a first edge of the planar member and has another edge that is adjacent the one edge of the media sheet and proximate a second edge of the planar member that is adjacent to the first edge of the planar member as the planar member passes the array of printheads to form a collection of material drops having a circular shape on the media sheet, generating image data of the planar member and the media sheet with an optical imaging device, receiving with the controller image data of the planar member and the media sheet from the optical imaging device as the planar member and media sheet pass the optical imaging device, identifying with the controller positions for the collection of material drops and for at least two pins on the second edge of the planar member that is parallel to the cross-process direction, and identifying with the controller a process direction error distance and a cross-process direction error distance with reference to the positions for the collection of material drops and the at least two pins on the second edge of the planar member.

DETAILED DESCRIPTION

For a general understanding of the present embodiments, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements.

FIG. 1depicts a printing system that accurately registers ejected material on planar members in the printing system. The system100includes two arrays of printheads104, a full-width optical imaging device108, a planar member112, one or more actuators116, and a controller120that is operatively connected to the actuators116, the printheads124in the arrays104, and the optical imaging device108. Each of the depicted printhead arrays104is configured with two rows of printheads124that are stitched in a known manner to enable a continuous line to be printed across media carried by the planar member112. The printheads are configured to eject material drops at a predetermined number of drops per linear unit of measurement. For example, in one embodiment, the printheads in each array eject ink drops to form the continuous line across the media at a resolution of 300 drops per inch (dpi). The two arrays can be offset from one another across the width of the media by a distance that corresponds to one-half of the distance between adjacent nozzles in the printheads. This arrangement doubles the resolution of the continuous line that the arrays can form across media on the planar member112. For example, if each of the two arrays can form a line across the media at a resolution of 300 dpi, then offsetting the two arrays by one-half of the distance between adjacent nozzles in the printheads produces a resolution of 600 dpi for the continuous line formed by the two arrays across the media.

The optical imaging device108can be implemented with a linear array of photodetectors and a light source that extends the length of the linear array of photodetectors. The light source is oriented to direct light onto the media128on the planar member112as the controller120operates at least one actuator116to move the planar member from a position beneath the printhead arrays104to the position shown inFIG. 1. This direction of movement is called the process direction P in this document and the axis orthogonal to this movement direction in the plane of the media is called the cross-process direction C-P in this document. The directed light is reflected by the media128and material drops ejected onto the media128by the printheads124of the arrays104. Bare media reflects more of the light into the photodetectors and the ejected material drops absorb more of the light. The photodetectors generate electrical signals corresponding to the intensity of the light received by the photodetectors. Thus, photodetectors that receive light reflected from bare media generate electrical signals with a greater amplitude than the photodetectors that receive light that is absorbed by the ejected material drops. This contrast in electrical signal amplitude enables the controller120, which receives the electrical signals from the optical imaging device108, to analyze the signals to identify positions of ejected material drops on the media as well as the positions of fiducial marks on the planar member112.

The planar member112includes two edges132that are parallel to the cross-process direction and two edges136that are parallel to the process direction. One of the edges136is populated with a plurality of pins140that form a registration edge for the media sheet128. Similarly, the edge132that is a leading edge when the planar member112moves in the process direction from the printheads to the optical imaging device108is populated with at least three pins144to form a registration edge for a lead edge of the media sheet128. These pins140and144enable a media sheet128to be positioned on the planar member112at a known position so image data generated by the optical imaging device108of ejected material drops on the media sheet128can be correlated to the known positions of the pins140and144. This information enables the controller120, which operates at least one printhead124to eject material drops onto the media sheet128, to analyze the image data to identify whether the ejected material drops are located where they are expected to be on the media sheet. If they are not, the controller120identifies the process direction error distance and the cross-process direction error distance of the ejected material drops to enable correction of the printhead cross-process positions and the timing of the signals to the printheads that operate the ejectors in the printheads to eject material drops. In the embodiment shown inFIG. 1, the planar member112is configured with an array of apertures through the planar member that are arranged to be equidistant from adjacent apertures in both the process and cross-process directions. The rows are parallel to the process direction and the columns are parallel to the cross-process direction. These apertures152are pneumatically connected to a vacuum source148to draw a vacuum through the apertures. When a media sheet128is positioned on the planar member112and the vacuum source148is activated, the media sheet128is held against the planar member112by the action of the vacuum on the sheet.

In one embodiment of a method that operates the system100to register the positions of the printheads124in the arrays104, one printhead is designated a reference printhead156. The reference printhead156can be any printhead in any array, but a centrally located printhead in the arrays is preferred. The controller120operates the reference printhead156to eject a predetermined number of material drops to form a collection of material drops on the media sheet128that has a relatively large diameter. Imaging device108generates image data of this circular collection of material drops160, the media sheet128, and the planar member112as the controller120operates the actuators116to move the planar member112from being beneath the printheads124to the position shown inFIG. 1. The controller120receives the image data from the imaging device108and analyzes the image data to identify the cross-process direction error distance and process direction error distance of the material circle160.

A process200for producing the material circle160and analyzing the image data of the circle on the media and planar member is shown inFIG. 2. In the description of this process, statements that the process is performing some task or function refers to a controller or general purpose processor executing programmed instructions stored in a memory operatively connected to the controller or processor to manipulate data or to operate one or more components in the printer to perform the task or function. The controller120noted above can be such a controller or processor. Alternatively, the controller120can be implemented with more than one processor and associated circuitry and components, each of which is configured to form one or more tasks or functions described herein.

The operator positions the media sheet128on the planar member112so one edge of the media sheet covers the next-to-last column of apertures152, but leaves the column of apertures adjacent to the pins144exposed. The edge of the media sheet perpendicular to the leading edge of the media sheet covers a portion of the row of apertures152adjacent the pins140. The controller120activates the vacuum source148to hold the media sheet128against the planar member112while the registration process is performed. Once the position of the media sheet is fixed, the controller120operates the actuators116to move the planar member112beneath the printheads124(block204). The position of the media sheet enables the controller120to operate the reference printhead156to form the material circle160near the edge of the media sheet128close to the pins144and approximately in the middle of the media sheet in the cross-process direction (block208). The controller120then operates the actuators116to move the planar member112past the imaging device108, while the imaging device108generates image data of the media sheet128on the planar member112(block212). The controller120processes the image data received from the imaging device108to identify a center of the material drop160(block214). This identification is made by identifying a cluster of darker pixels in the image data that form a nearly circular shape having a predetermined size that is within a predetermined radial tolerance. The controller identifies the column of apertures152exposed between the pins144and the edge of the media sheet128closest to the pins (block216). This identification includes eliminating any candidate apertures that are not within a predetermined distance of the pins144in the process direction. The controller120identifies a straight line fit through the image data locations identified as apertures152(block220). The controller120processes the image data received to identify the three pins144(block224). This identification is made by identifying a cluster of darker pixels in the image data that form a nearly circular shape having a predetermined size that is within a predetermined radial tolerance. This identification also includes eliminating any candidate pin positions that are not within a predetermined distance of the straight line and those candidate pin positions that are not within a predetermined distance from one another in the cross-process direction. If all three pins144are identified (block228), then the center pin position is selected as a reference point for measuring distances to the material drop160(block232). If only two pins144are identified, then an interpolation is made with reference to the distances between the two pins to identify the reference point for measuring distances to the material drop160(block234). From the identified reference point, the controller120measures a distance in the process direction between the reference point and material drop160and a distance in the cross-process direction between the reference point and the material drop160(block238). From the identified reference point, the controller120measures a distance in the process direction between the reference point and an expected position164for the material drop and a distance in the cross-process direction between the reference point and the expected position164for the material drop160(block242). The relationships between these positions and distances are shown inFIG. 3. The controller120identifies differences between the distances measured for the material drop160and its expected position in both the process and cross-process directions (block246). The process direction difference is compared to a predetermined threshold (block250), and if it is greater than the threshold, then it is stored for use to correct the timing of the printhead firing (block254). Likewise, the cross-process direction difference is compared to a predetermined threshold (block258), and if it is greater than the threshold, then the controller120uses the difference in the cross-process direction to operate the actuators116to adjust the positions of the printhead arrays in the cross-process direction (block262). If neither difference is greater than their respective thresholds, the printhead arrays are identified as being in registered positions (block266). If either difference exceeded its threshold, then the process is repeated until the printheads are considered to be registered.