Patent Publication Number: US-8991960-B2

Title: Compensation of bi-directional alignment error

Description:
BACKGROUND 
     Printing systems may include a print head disposed on a carriage that traverses a print zone in a forward direction and a reverse direction in a bi-directional printing mode. The print head may eject fluid such as ink drops onto the substrate in the print zone, while moving in the forward direction and the reverse direction in the bi-directional printing mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting examples are described in the following description, read with reference to the figures attached hereto and do not limit the scope of the claims. Dimensions of components and features illustrated in the figures are chosen primarily for convenience and clarity of presentation and are not necessarily to scale. Referring to the attached figures: 
         FIG. 1  is a block diagram illustrating a printing system according to an example. 
         FIG. 2  is a top view of a substrate illustrating bi-directional alignment error corresponding to misplacement of an ink drop on the substrate ejected from a print head in a bi-directional printing mode according to an example. 
         FIG. 3A  is a top view illustrating a test pattern formed on a substrate by a printing system according to an example. 
         FIG. 3B  is an exploded view illustrating a corresponding pair of vertical line patterns of the test pattern of  FIG. 3A  according to an example. 
         FIG. 4  is a graph illustrating a line of best fit including a slope and y-intercept corresponding to a data set of bi-directional alignment error at a plurality of carriage speeds of a printing system according to an example. 
         FIG. 5  is a flowchart illustrating a method of compensating for bi-directional alignment error in a printing system including a carriage, a print head disposed thereon, and a bi-directional printing mode according to an example. 
         FIG. 6  is a block diagram illustrating a computing device such as a printing system including a processor and a non-transitory, computer-readable storage medium to store instructions to operate a printing system including a carriage, a print head disposed thereon, and a bi-directional printing mode to compensate for bi-directional alignment error according to an example. 
     
    
    
     DETAILED DESCRIPTION 
     Printing systems may include a print head disposed on a carriage that traverses a print zone in a mono-directional printing mode and/or a bi-directional printing mode. Printing systems may be inkjet printing system in which ink droplets are ejected from the print head disposed on a movable carriage and onto a substrate. In a mono-directional mode, the print head may eject ink drops when the carriage is moving the print head in a same direction. The actual placement of the ink drop may be offset from its intended location on the substrate due to motion of the print head. However, because the offset is in the same direction with each successive pass of the print head the placement error is minimized. Alternatively, a bi-directional printing mode, the print head may eject ink drops while the print head is moving, for example, in a forward direction and a reverse direction allowing increased printing speeds. However, the changing of direction of the carriage and the resulting change in direction of the placement error may result in alignment errors between subsequent passes of the print head. That is, bi-directional alignment error corresponds to an offset distance in which fluid such as an ink drop is offset from an intended location on a substrate due to printing in a bi-directional printing mode. As bi-directional alignment error may be influenced by various carriage speeds, the offset distance may vary across the sweep due to acceleration of the carriage. Simple corrective measures based on error distance may not be effective in reducing bi-directional alignment error in the entire print zone. 
     In examples, a method of compensating for bi-directional alignment error in a printing system including a carriage, a print head disposed thereon, and a bi-directional printing mode includes determining a data set by a data set determination module corresponding to bi-directional alignment error at a plurality of carriage speeds. The method also includes determining a line of best fit of the data set by a best fit determination module and identifying a flight time of fluid ejected from the print head and a carriage position error of the carriage from the line of best fit by an alignment parameter identification module. Flight time may be an amount of time from when fluid such as an ink drop is ejected from the print head  12  and contacts a substrate. Carriage position error may correspond to the carriage stopping and/or starting its change in direction at an incorrect position. Additionally, the method also includes compensating for the bi-directional alignment error by an error compensation module based on the flight time and the carriage position error. The use of flight time and carriage position error resulting from a data set of bi-directional errors at various carriage speeds may be effective in reducing bi-directional alignment error. Further, such corrective bi-directional alignment error measures may be performed in real-time using the respective printing system also as a diagnostic tool. 
       FIG. 1  is a block diagram illustrating a printing system according to an example. Referring to  FIG. 1 , in some examples, a printing system  100  includes a carriage  11 , a print head  12  disposed on the carriage  11 , and an error compensation device  13 . The error compensation device  13  may include a data set determination module  14 , a best fit determination module  16 , an alignment parameter identification module  17 , and an error compensation module  18 . The data set determination module  14  may include a test pattern analyzer module  15  to analyze the test pattern and to provide a data set corresponding to the test pattern. In some examples, the test pattern analyzer module  15  may include at least one of a scanner and a sensor. For example, the test pattern analyzer module  15  may include a light emitting diode sensor to detect the test pattern. 
     Referring to  FIG. 1 , in some examples, the carriage  11  may move across a print zone in a forward direction and a reverse direction in a bi-directional printing mode. The print head  12  may eject fluid such as ink drops to a substrate to form an image in a print mode and to form a test pattern corresponding to the forward direction and the reverse direction in a test mode. A test mode may be a mode when the printing system  100  performs diagnostics on itself, for example, to determine and/or compensate for bi-directional alignment error. In some examples, the print head  12  may be an inkjet print head. The error compensation device  13  may compensate for bi-directional alignment error of the printing system  100 . The data set determination module  14  may determine a data set corresponding to the bi-directional alignment error at a plurality of carriage speeds. 
     Referring to  FIG. 1 , in some examples, the best fit determination module  16  may determine a line of best fit of the data set. For example, the best fit determination module  16  may perform a line fitting algorithm. In some examples, the best fit determination module  16  may be configured to perform a simple linear regression to determine the line of best fit of the data set and to identify a formula including a slope and y-intercept corresponding to the line of best fit. For example, the line may be a straight line represented by the formula, y=mx+b, where m is the slope of the line, b is the y-intercept, y is a respective value along the y-axis, and x is a respective value along the x axis. The slope m represents a respective change in y divided by a respective change in x. That is, the slope m represents the steepness of the line. 
     Referring to  FIG. 1 , in some examples, the alignment parameter identification module  17  may identify a flight time of fluid ejected from the print head and a carriage position error of the carriage from the line of best fit. The flight time may correspond to the slope m of the formula corresponding to the line of best fit and the carriage position error may correspond to the y-intercept b of the formula corresponding to the line of best fit. The error compensation module  18  may compensate for bi-directional alignment error based on the flight time and the carriage position error. For example, this may be accomplished by adjusting the drop ejection time based on the flight time and adjusting the drop ejection position based on the carriage position error. 
     In some examples, an error compensation device  13 , a data set determination module  14 , a test pattern analyzer module  15 , a best fit determination module  16 , an alignment parameter identification module  17 , and/or an error compensation module  18  may be implemented in hardware, software including firmware, or combinations thereof. The firmware, for example, may be stored in memory and executed by a suitable instruction-execution system. If implemented in hardware, as in an alternative example, the error compensation device  13 , the data set determination module  14 , the test pattern analyzer module  15 , the best fit determination module  16 , the alignment parameter identification module  17 , and/or the error compensation module  18  may be implemented with any or a combination of technologies which are well known in the art (for example, discrete-logic circuits, application-specific integrated circuits (ASICs), programmable-gate arrays (PGAs), field-programmable gate arrays (FPGAs), and/or other later developed technologies. In other examples, the error compensation device  13 , the data set determination module  14 , the test pattern analyzer module  15 , the best fit determination module  16 , the alignment parameter identification module  17 , and/or the error compensation module  18  may be implemented in a combination of software and data executed and stored under the control of a computing device. 
       FIG. 2  is a top view of a substrate illustrating bi-directional alignment error corresponding to misplacement of an ink drop on the substrate ejected from a print head in a bi-directional printing mode according to an example. Referring to  FIG. 2 , in some examples, a substrate S includes an ink drop  21  thereon ejected from a print head  12  and offset from its intended location  22  on the substrate S by a respective offset distance d o . For example, the print head  12  disposed on a carriage  11  moving the print head  12  in a reverse direction d r  in a bi-directional printing mode may have ejected the ink drop  21  there from. The print head  12  may also eject ink drops on the substrate S while moving in a forward direction d f . As illustrated in  FIG. 2 , bi-directional alignment error may correspond to an offset distance d o  in which fluid such as an ink drop  21  is offset from an intended location  22  on a substrate S due to printing in a bi-directional printing mode. 
       FIG. 3A  is a top view illustrating a test pattern formed on a substrate by a printing system according to an example.  FIG. 3B  is an exploded view illustrating a corresponding pair of vertical line patterns of the test pattern of  FIG. 3A  according to an example. Referring to  FIGS. 3A and 3B , in some examples, a test pattern  30  includes a first set of rows  31   a ,  31   b ,  31   c ,  31   d  and  31   e  of printed vertical line patterns. Each one of the respective rows  31   a ,  31   b ,  31   c ,  31   d  and  31   e  of printed vertical line patterns may correspond to a different carriage speed. A respective pairing distance d p  between a corresponding pair of adjacent printed vertical line patterns  32  in the same row may correspond to bi-directional alignment error at the respective carriage speed. For example, for a corresponding pair  32  of vertical line patterns, a vertical line pattern  35   f  may be formed during a forward direction and a corresponding vertical line pattern  35   r  may be formed during a reverse direction in the bi-directional printing mode. The pairing distance d p  between the corresponding vertical line patterns  35   f  and  35   r  may correspond to the respective bi-directional alignment error. 
     Referring to  FIGS. 3A and 3B , in some examples, a warm up image  33  and a calibration image  34  may precede and follow each row  31   a ,  31   b ,  31   c ,  31   d  and  31   e  of the printed vertical line patterns. The respective warm up images  33  may establish a normal operating condition for the print head  12  to eject fluid to improve test pattern readability. Additionally, the respective calibration images  34  may enable a test pattern analyzer module  15  ( FIG. 1 ) to orient and/or synch itself with respect to the respective vertical line patterns. This test pattern  30  may correspond to a respective print head  12 . In some examples, a printing system  100  may include several print heads, and each one may have a corresponding test pattern. 
       FIG. 4  is a graph illustrating a line of best fit including a slope and y-intercept corresponding to a data set of bi-directional alignment error at a plurality of carriage speeds of a printing system according to an example. Referring to  FIG. 4 , in some examples, a graph includes offset (bi-directional alignment error) along the y-axis in units of 2400 th  seconds and carriage velocity (carriage speed) along the x-axis in units of inches per second (ips). The squared dots represent the respective offset (bi-directional alignment error)  42  at the respective carriage speeds. That is, squared dots are the data set corresponding to the bi-directional alignment error  42  at a plurality of carriage speeds. The graph also includes a line  41  such as a straight line corresponding to the best fit of the data set. The line  41  includes a slope m and y-intercept b. In some examples, the line  41  may be represented by the formula, y=mx+b, where m is the slope of the line, b is the y-intercept, y is a respective value along the y-axis, and x is a respective value along the x axis. The slope m represents a respective change in y divided by a respective change in x. 
       FIG. 5  is a flowchart illustrating a method of compensating for bi-directional alignment error in a printing system including a carriage, a print head disposed thereon, and a bi-directional printing mode according to an example. Referring to  FIG. 5 , in block S 510 , a data set is determined by a data set determination module corresponding to bi-directional alignment error at a plurality of carriage speeds. In some examples, the determining a data set corresponding to the bi-directional alignment error at a plurality of carriage speeds may include printing a test pattern by the print head corresponding to a forward direction and a reverse direction in a bi-directional printing mode. For example, the test pattern may include a first set of rows of printed vertical line patterns. Each one of the respective rows of printed vertical line patterns may correspond to a different carriage speed. The determining a data set corresponding to the bi-directional alignment error at a plurality of carriage speeds may also include analyzing a test pattern by a test pattern analyzer module to provide a data set corresponding to the test pattern. For example, the test pattern analyzer module may include at least one of a scanner and a sensor. The sensor may be a light emitting diode sensor. For example, the scanner and/or sensor may detect the various vertical line patterns and/or determine the spacing there between. 
     In block S 512 , a line of best fit of the data set is determined by a best fit determination module. In some examples, the determining a line of best fit of the data set by a best fit determination module may include performing a simple linear regression. For example, the best fit determination module may perform a line fitting algorithm. In some examples, the determining a line of best fit of the data set by a best fit determination module may include performing a simple linear regression. The determining a line of best fit of the data set by the best fit determination module may also include identifying a formula including a slope and y-intercept corresponding to the line of best fit. The flight time may correspond to the slope of the formula corresponding to the line of best fit. The carriage position error may correspond to the y-intercept of the formula corresponding to the line of best fit. 
     In block S 514 , a flight time of fluid ejected from the print head and a carriage position error of the carriage is identified by an alignment parameter identification module from the line of best fit. For example, the ejection of fluid from the print head may be in a form of respective ink drops. In block S 516 , the bi-directional alignment error is compensated for by an error compensation module based on the flight time and the carriage position error. For example, this may be accomplished by adjusting the drop ejection time based on the flight time and adjusting the drop ejection position based on the carriage position error. 
       FIG. 6  is a block diagram illustrating a computing device such as a printing system including a processor and a non-transitory, computer-readable storage medium to store instructions to operate a printing system including a carriage, a print head disposed thereon, and a bi-directional printing mode to compensate for bi-directional alignment error according to an example. Referring to  FIG. 6 , in some examples, the non-transitory, computer-readable storage medium  65  may be included in a computing device  600  such as a printing system. In some examples, the non-transitory, computer-readable storage medium  65  may be implemented in whole or in part as instructions  67  such as computer-implemented instructions stored in the computing device locally or remotely, for example, in a server or a host computing device considered herein to be part of the printing system. 
     Referring to  FIG. 6 , in some examples, the non-transitory, computer-readable storage medium  65  may correspond to a storage device that stores instructions  67 , such as computer-implemented instructions and/or programming code, and the like. For example, the non-transitory, computer-readable storage medium  65  may include a non-volatile memory, a volatile memory, and/or a storage device. Examples of non-volatile memory include, but are not limited to, electrically erasable programmable read only memory (EEPROM) and read only memory (ROM). Examples of volatile memory include, but are not limited to, static random access memory (SRAM), and dynamic random access memory (DRAM). 
     Referring to  FIG. 6 , examples of storage devices include, but are not limited to, hard disk drives, compact disc drives, digital versatile disc drives, optical drives, and flash memory devices. In some examples, the non-transitory, computer-readable storage medium  65  may even be paper or another suitable medium upon which the instructions  67  are printed, as the instructions  67  can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a single manner, if necessary, and then stored therein. A processor  69  generally retrieves and executes the instructions  67  stored in the non-transitory, computer-readable storage medium  65 , for example, to operate a computing device  600  such as a printing system having a carriage and a print head disposed thereon to compensate for bi-directional alignment error in accordance with an example. In an example, the non-transitory, computer-readable storage medium  65  can be accessed by the processor  69 . 
     It is to be understood that the flowchart of  FIG. 5  illustrates architecture, functionality, and/or operation of examples of the present disclosure. If embodied in software, each block may represent a module, segment, or portion of code that includes one or more executable instructions to implement the specified logical function(s). If embodied in hardware, each block may represent a circuit or a number of interconnected circuits to implement the specified logical function(s). Although the flowchart of  FIG. 5  illustrates a specific order of execution, the order of execution may differ from that which is depicted. For example, the order of execution of two or more blocks may be scrambled relative to the order illustrated. Also, two or more blocks illustrated in succession in  FIG. 5  may be executed concurrently or with partial concurrence. All such variations are within the scope of the present disclosure. 
     The present disclosure has been described using non-limiting detailed descriptions of examples thereof that are not intended to limit the scope of the general inventive concept. It should be understood that features and/or operations described with respect to one example may be used with other examples and that not all examples have all of the features and/or operations illustrated in a particular figure or described with respect to one of the examples. Variations of examples described will occur to persons of the art. Furthermore, the terms “comprise,” “include,” “have” and theft conjugates, shall mean, when used in the disclosure and/or claims, “including but not necessarily limited to.” 
     It is noted that some of the above described examples may include structure, acts or details of structures and acts that may not be essential to the general inventive concept and which are described for illustrative purposes. Structure and acts described herein are replaceable by equivalents, which perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the general inventive concept is limited only by the elements and limitations as used in the claims.